Organic semiconductors are nowadays intensively explored for the use in many applications with the most prominent ones being photovoltaic cells and light emitting diodes. The great potential of organic electronics is the virtually unlimited design freedom offered by organic chemistry, constantly allowing the development of new molecular building blocks. In contrast to conventional inorganic electronics, the functionality of these applications are greatly dictated by the distinct singlet and triplet excited states of the molecules used. The excitonic transition in the triplet manifold is typically spin forbidden and thus, in practice, associated with very small oscillator strengths. Triplet excited state spectroscopy is typically carried out at cryogenic temperatures to reduce non-radiative losses, as the latter outcompete radiative processes at room temperature. The involved techniques are time-consuming, resource intensive, and not flexible. Consequently, the determination of the molecular triplet state of organic semiconductors - especially in view of the enormous material pool available - is a persistent challenge, often hindering rapid material and device development.
In this presentation, we discuss our recent progress towards effective, room temperature triplet state spectroscopy of purely organic semiconductors, alleviating constrains of cryogenic measurements. Two aspects have to be addressed to enable phosphorescence from the triplet state: (i) the radiative transition in the triplet manifold must be at least comparable to the non-radiative losses and (ii) triplet population must be possible upon optical excitation of the system under study. To meet (i), we use inert host polymers like Poly(methyl 2-methylpropenoate) or polystyrene combined with an optimized fabrication process to form rigid films. These samples show strong phosphorescence at room temperature (293 K) with intensities comparable to values at 77 K, indicating that temperature dependent non-radiative losses are effectively eliminated. Triplet population within the molecule is set by the intersystem crossing (ISC), which can be negligible. To optically generate triplets [cf. (ii)], we incorporate molecules such as benzophenone with ISC of 100% as energy transfer intermediates. In addition to these approaches, we discuss the use of Poly(4-bromostyrene) - a brominated polystyrene derivative - as an inert, rigid host polymer that additionally induces external heavy atom (Br) effects, ultimately leading to increased ISC and faster radiative decay rates, thus, simultaneously aiming at (i) and (ii).
We present examples of these techniques on a wide range of materials with functionalities spanning materials for singlet exciton fission (anthracene and tetracene), wide band gap materials, and thermally activated delayed fluorescence type emitters.

Organic solar cell is promising for low-cost flexible photovoltaic applications. To improve the efficiency and stability, it is important to understand the microscopic mechanism of electronic excitation, involving both the electronic and the ionic degree of freedom. We simulate charge transfer and dynamics of excited carriers using real-time electron-ion dynamics within time-dependent density functional theory. A promising design for efficient electron-hole separation is combining C60 and zinc phthalocyanine (ZnPC) with various substrates. We systematically study the total potential profile, energy alignment for electronic transitions, and time domain evolution of the electron-hole dynamics. The results suggest novel designs of the organic molecule systems that lead to efficient electron-hole separation and charge transfer.

Organic Semiconductor crystals are viable candidates for applications in solar cells. The mechanism of photocarrier generation is currently a subject of debate, particularly concerning the role of non-equilibrium charge-transfer (CT) states following ultrafast exciton dissociation at the heterojunction. In order to understand the phenomena behind charge separation and improve the efficiency of organic photovoltaic (OPV) devices, a fundamental understanding of photophysics at the interfaces between donor and acceptor is necessary. To conduct this study, we choose copper phtalocyanine (CuPc) and fullerene (C60) bilayer heterojunctions. CuPc:C60 of different thicknesses are thermally evaporated on sapphire substrates. A large number of groups have observed CT exciton signatures and light-to-current efficiency close to 5% was achieved in CuPc:C60 solar cells. In this work, we study the kinetics of dissociation and recombination of excitons using ultrafast spectroscopy techniques. Time-resolved photoluminescence (TRPL) at ps and ns time scales gives us information about the dynamics of recombination of excitons. Also, we implement coherent control techniques to probe the role of state-to-state quantum coherence between excitons and delocalized charge-transfer states in such system by using phase-locked femtosecond pulse sequences, probing photocurrent in OPV diodes as well as photoluminescence spectroscopy. This knowledge of the interaction between CuPc and C60 will then give us a better understanding of the role of ‘hot&’ CT states in photocarrier generation processes in OPV diodes.

New donor-acceptor type copolymers comprised of benzobisthiazole (BBTz) as a weak donor rather than acceptor are proposed.[1] This approach can simultaneously lead to deepening the HOMO and LUMO of the polymers with moderate energy offset against fullerene derivatives in bulk heterojunction organic photovoltaics. As a proof-of-concept, we synthesized BBTz-based random copolymers conjugated with typical electron acceptors: thienopyrroledione (TPD) and benzothiadiazole (BT) based on density functional theory calculations. Laser-flash[2] and Xe-flash[3] time-resolved microwave conductivity (TRMC) evaluations of polymer: [6,6]-phenyl C61 butyric acid methyl ester (PCBM) blends were conducted for screening the feasibility of the copolymers, leading to optimization of processing conditions for photovoltaic device application. According to the TMRC results, we designed alternating BBTz-BT copolymers, which exhibit extended photoabsorption up to ~ 750 nm, deep HOMO (-5.5 ~ -5.7 eV), good miscibility with PCBM, and inherent crystalline nature. Moreover, the maximized PCE of 3.8%, the top-class among BBTz-based polymers reported so far, was realized in an inverted cell using TiOx and MoOx as the buffer layers. This study opens up opportunities to create low bandgap polymers with deep HOMO, and presents how the device-less TRMC evaluation is of help for decision making on judicious molecular design.
[1] M. Tsuji, A. Saeki, Y. Koizumi, N. Matsuyama, C. Vijayakumar, S. Seki, Adv. Funct. Mater. 2013, in press, doi: 10.1002/adfm.201301371.
[2] A. Saeki, M. Tsuji, S. Seki, Adv. Energy Mater. 2011, 1, 661.
[3] A. Saeki, S. Yoshikawa, M. Tsuji, Y. Koizumi, M. Ide, C. Vijayakumar, S. Seki, J. Am. Chem. Soc. 2012, 134, 19035.

An analytical expression for the diffusion-driven current in metal-insulator-metal (MIM) diodes based on undoped organic semiconductors is derived. The derivation is based on the classical diffusion theory of Schottky for doped inorganic semiconductors, with adaptations to account for the absence of doping and the presence of a built-in voltage due to asymmetric metal or oxidic contacts. We show that the derived relation accurately describes both the temperature and voltage dependence of the diffusion current in organic MIM diodes and solar cells. In organic bulk heterojunction solar cells, a deviation of the ideality factor of the dark current from unity is commonly put forward as evidence for the presence of trap-assisted recombination .We show that the commonly observed deviation of the current ideality factor from unity (~1.2) is characteristic of diffusion-limited currents in undoped organic semiconductors. As a result, the non-ideality of the dark characteristics in BHJ solar cells is determined by the transport-dominating constituent of the donor:acceptor blend, rather than a trap-assisted recombination mechanism. The light-intensity dependence of the open-circuit voltage confirms the absence of trap-assisted recombination and demonstrates that the dominant recombination mechanism in the investigated polymer:fullerene solar cells is bimolecular.

Bimolecular recombination to charge-transfer (CT) states is known to be an important process in polymer:fullerene photovoltaics. If there is a lower-lying intramolecular triplet exciton then formation of this state from the CT state can be a terminal loss pathway, whereas if the CT states are long-lived they may be redissociated into free charges. We investigated the recombination pathways in a range of polymer:fullerene solar cells by using a magnetic field to control the population of the charge-transfer (CT) states formed at the bulk heterojunction interface. The systems investigated included P3HT:PC60BM blends, and MMDO-PPV, PCPDTBT, PCDTBT and PTB7 in blends with PC70BM, chosen to have different relative positions of the intramolecular triplet exciton with respect to the CT state. The dark current and CT state emission were measured as a function of magnetic field, allowing the CT state energy to be determined. We find that the sign of the magnetoresistance depends on whether the CT state or the intramolecular triplet state is lower in energy. We interpret these results within a model accounting for spin polarization [1] to describe the effect of high magnetic fields in these devices, with redissociation of the CT state only favored in the absence of a lower-lying triplet exciton.

[1] J. Wang, A. Chepelianskii, F. Gao, N. C. Greenham, Nature Communications 2012, 3, 1191.

We have studied the time-dependent charge transfer (CT) emission in blends of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclo-penta[2,1-b;3,4-b&’]-dithiophene)-alt-4,7-(2,1,3 benzothiadiazole)] : [6,6]-phenyl C61-butyric acid methyl ester. This material combination is a promising absorber for bulk-heterojunction solar cells. Assuming a recombination of photoinduced free charges via the charge transfer state, a long-lived component in the CT emission is expected. Experimentally we have observed only a short lived component (~1ns) and no evidence of a long-lived emission. We will discuss the implications of our findings on the ultimate efficiency limit of bulk-heterojunction solar cells and the importance of the charge transfer emission intensity on the performance of state of the art devices.

Poly hexylthiophene is a ubiquitous hole transporting material which has been thoroughly investigated and applied in photovoltaic devices for example. We use a well-defined P3HT which has a 100% regio-regularity, low polydispersity and low molecular weigth as a model system to study the delocalization of excitons and formation of charge transfer states. We end-functionalize these polymers with an acceptor end-group such as F8TBT. By changing the way of conjugated coupling of these acceptor units we can influence the formation of intramolecular charge transfer states between the donor polymer and the acceptor unit. This model system helps to obtain insights into the effective exciton delocalization in poly-crystalline conjugated polymers as well as to understand the charge separation dynamics between donor and acceptor units. The coupling of the acceptor unit through a thiophene unit is significantly different to the coupling through the F8 unit: a visible charge transfer state is formed in the first case. Ultra-fast pump-probe spectroscopy is used to investigate the underlying dynamics of the charge transfer state formation and charge separation. The respective morphology is investigated by X-ray scattering which proves the formation of fully chain extended crystalline thiophene chains where the acceptor units are localized within the amorphous domains.

The optoelectronic properties of organic and inorganic semiconductors are fundamentally different and offer complimentary characteristics for photovoltaic (PV) applications. Coupling the strong molecular absorption of organics with the high charge mobility of inorganics can provide an optimal platform to enable cost-effective and high-efficiency photovoltaic devices. Recently there is a renewed interest in hybrid semiconductor systems, such as small molecules used for surface passivation of InP PV cells1 leading to increased efficiency, and monolayer thick molecular films used for photon absorption in dye sensitized solar cells.2 To understand exciton and charge dynamics in hybrid junctions, we follow the approach by Giebink et al. for modelling organic/organic hetrojunctions,3 and develop a comprehensive model for charge transport and photocurrent generation based on charge dynamics at the organic/inorganic heterojunction. To test the model&’s theoretical assumptions, we fabricated an Indium Tin Oxide(150 nm) / TiO2(60 nm) / Tetraphenyldibenzoperiflanthene(DBP)(30 nm) / MoO3(15 nm) / Al(100 nm) planar PV cell. The device exhibits over 15% EQE originating from absorption in DBP, followed by dissociation at its interface with TiO2. We show that by coupling interface charge balance with a self-consistent drift diffusion simulation for current in the organic, the current density versus voltage (J-V) characteristics of the cell can be accurately fit from 140-300 K both in the dark and at 1-sun AM1.5G illumination. Our work identifies the princpal effects that play a role in governing current processes in hybrid organic-inorganic semiconductor heterojunctions, such as thermally activated mobility and diffusion length for DBP, and voltage dependent charge dynamics at the hybrid interface.

As higher demands for new sustainable energy source have been raised than ever, great research efforts have been devoted into this field. Among them, organic semiconductors have made their names in transparent and flexible electronic devices as photovoltaic application. However, for organic semiconductor materials, foresight of their potential in performance and guidance in degradation prevention still remain underexplored. Especially for the study of the oxidation degradation mechanism, systematic theoretical verification and experimental investigation are utterly required.
ab intio electronic structure calculations are applied to study such oxidation degradation in organic photovoltaic materials. Also a specific type of transparent organic semiconductor PBIQ (short for 2-methyl-1, 4, 6, 13-tetraphenyl-7:8, 11:12-bisbenzo-anthro[g]isoquinolin-3(2H)-one) was chosen as the study case. The electronic properties like bandgap and molecular orbitals of the material and its possible oxidation product are examined and also confirmed by experimental measurements. Oxidation reaction mechanisms are then further investigated by ab intio electronic structure calculations to identify the reaction mechanism and predict the possible products of the degradations.
As a result, the reasonable explanation for the favored reaction route, intermediates, and final products is drawn through showing solid evidence that which reaction mechanisms are both energetically favored with lowest product energy and kinetically favored with lowest energetic barrier. These preliminary results and methods could serve as guidance in exploring organic semiconductor reaction stability, especially in field of passivation protection against oxidation degrading.

PTB7 is an important photovoltaic material which has been used to fabricate high efficiency single junction solar cells. In this paper, we report on a systematic exploration of device physics of PTB7/PCBM solar cells fabricated in this material. The device conversion efficiencies were in the range of 8%. We measured interfacial defects between PTB7 and PCBM70, deep defects within PTB7, tail states in PTB7, interfacial recombination velocity in solar cells, and the influence of changing the acceptor from PCBM70 to ICBA. The techniques used were capacitance-frequency spectroscopy, quantum efficiency-voltage-wavelength spectroscopy, dark I-V vs. temperature, light I-V at various intensities, and subgap quantum efficiency vs. wavelength. We find that the dark current has two distinct regions, one corresponding to interfacial re combination, and one corresponding to bulk recombination. The interfacial recombination region in dark I-V is well correlated with the defect density. We also find that incorporating ICBA instead of PCBM70 as the acceptor increases the voltage by ~0.2V, and that this difference in voltage can be directly correlated with the shift in subgap QE spectrum to ~0.2 eV higher, showing that, indeed, the LUMO level of acceptor has moved 0.2 eV above the value when using PCBM70, i.e. the bandgap at the hetero-interface has increased by 0.2 eV. Stability measurements done under 2X sun intensity, full spectrum, xenon light source show that there are significant changes in both interfacial and defect densities upon illumination. These changes, in turn, are correlated with changes in fundamental device properties such as dark current, fill factor, interfacial recombination velocity, and quantum efficiency. Blue and uv photons do significantly more damage than red photons.

Thin metal oxide films have become important components in high-performance organic solar cells (OSC). Among these oxides, nickel oxide stands out as a material well suited for hole extraction layer at the anode of an organic photovoltaic device. Films formed in a solution based process (sNiOx) exhibit strong p-type character due to nickel vacancies and a comparably high work function. The low electron affinity of these sNiOx films (2.1 eV) leads to very effective electron blocking and thereby reduces unwanted carrier recombination at the interface. However, untreated sNiOx surfaces still yield unsatisfactory results in hole collection from the new generation, high ionization energy, hole-conducting polymers employed in bulk heterojunctions.
Here, we investigate the addition of ultra-thin layers of vacuum-evaporated molybdenum oxide (MoO3) to alleviate the shortcomings of the underlying sNiOx surface. Using photoemission spectroscopy (PES) we find that the MoO3 effectively p-dopes the nickel oxide and substantially increases the work function of the anode up to 6.6 eV, which leads to a reduction of the electronic barrier for hole extraction and increases of the built-in electric field while maintaining the electron blocking characteristics. Power conversion efficiencies of 5.5% with a fill-factor of 56% are obtained for OSC consisting of a single PC71BM/PCDTBT bulk heterojunction and sNiOx/MoO3 bi-layers for hole extraction.
In an alternative approach, we further explore the mechanism for charge selectivity in pulsed-laser deposited nickel oxide by systematically adjusting the deposition parameters such as the partial O2 pressure and the substrate temperature. Using PES and inverse photoemission spectroscopy, we demonstrate that the doping character of the film can efficiently be tuned with the most beneficial films for hole extraction found at high oxygen partial pressures and low substrate temperatures.

The morphology of organic bulk heterojunction (BHJ) solar cells, which are of major interest due to their potential application for flexible, light-weight and low-cost solar cells, strongly affects the electrical properties and the per-formance of the device. Therefore, understanding the inter-play between morphology and electrical properties of do-nor-acceptor blends will spur the improvement of material and device design and ultimately lead to enhanced power conversion efficiency.
We present a combined study of the structure-function relationship of polymer diodes and organic solar cells. For this purpose we use the well characterized model system poly(3-hexylthiophene)(P3HT)/[6,6]-phenyl-C61-butyricacid-methyl-ester (PCBM).
In a previous work, we investigated the microstructure of P3HT:PCBM blends using electron energy loss spec-troscopy (EELS) and electron spectroscopic imaging (ESI) in a transmission electron microscope (TEM). A mixed phase was observed at the interface between the PCBM-rich and P3HT-rich phase, which appears to be es-sential for efficient charge separation.
To gain a deeper understanding of the phase separation in the blend we studied the influence of the molecular weight of P3HT on the morphology and the electronic properties. For this purpose we prepared pure P3HT samples with different molecular weights, which were gradually annealed up to 120°C. In the bright-field high resolution TEM images we observed an increase in crystallinity with increasing temperature and decreasing molecular chain lengths. By applying electron diffraction we could confirm the formation of a long-range order in the annealed samples. We correlated our investigations to the electronic properties of P3HT diodes prepared accordingly. We determined the charge carrier mobilities by using charge extraction by linear increasing voltage (CELIV). In CELIV charge carriers are extracted by a linearly increasing voltage pulse in reverse bias over a non-injecting contact. From the resulting current transient charge mobilities and charge carrier densities were determined. A significant change in the charge transport properties, due to thermally driven recrystallization processes, could be demonstrated.
References
M. Pfannmöller et al., Nano Lett. 2011, 11, 3099-3107

In recent years there has been a revived interest in Schottky junctions for photovoltaic applications. This revival has been brought on by the availability of highly conductive transparent organic materials, which enable a cost-effective and simple manufacturing process for these devices [1, 2, 3]. Even though the reports of experimental structures are becoming more prevalent, the behavior of the organic material in these devices remains poorly understood. We have been investigating Metal-Insulator-Semiconductor (MIS) structures of a PEDOT-based organic conductor/oxide/n-type silicon for the use in solar energy conversion. To explore the effects of replacing a traditional metal with an organic conductor two types of devices have been manufactured in parallel: Au/SiO2/Si and PEDOT:PSS/SiO2/Si. If PEDOT is to be treated as a metal, which is routinely done in the literature at the present, the two devices should show very similar electrical characteristics, as the work functions of both Au and PEDOT:PSS are generally reported to be alike (~5.1-5.2eV). The measurements, however, reveal higher current densities in the traditional Au-topped junction as compared to the PEDOT-topped device. To understand the difference in operation of these otherwise identical structures a simulator was written in Matlab to find solutions for a discretized version of the continuity equations incorporating drift-diffusion, thermionic emission and tunneling models. The simulator enables investigation of the effects of the qualities of PEDOT:PSS that make it inherently different from an ideal metal. In particular, the following phenomena will be discussed: de-doping of PEDOT:PSS at the interface with the oxide due to electron injection, variable-range hopping conduction in the PEDOT, and the inclusion of the polaron and bipolaron bands in the band gap of PEDOT:PSS.
[1] S. Demtchenko, S. McGarry, P. Gordon, S. Barry and N. G. Tarr, "Characterization and assessment of a novel hybrid organic/inorganic metal-insulator-semiconductor structure for photovoltaic applications," in Proc. SPIE 7750, Photonics North 2010, 77502Y, 2010.
[2] T.-G. Chen, B.-Y. Huang, E.-C. Chen, P. Yu and H.-F. Meng, "Micro-textured conductive polymer/silicon heterojunction photovoltaic devices with high efficiency," Appl. Phys. Lett., vol. 101, p. 033301, 2012.
[3] J. Zhang, Y. Zhang, F. Zhang and B. Sun, "Electrical characterization of inorganic-organic hybrid photovoltaic devices based on silicon-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)," Appl. Phys. Lett., vol. 102, p. 013501, 2013.

Low hole mobility and high recombination rates severely limit the incident photon-to-current collection efficiency (IPCE) of organic photovoltaics. In this study, we use a combination of rational design and first-principles density functional theory computations to tailor the properties of new hybrid organic-inorganic photovoltaic materials in order to ameliorate these issues. We investigate hybrid materials in which transition metal chalcogenide-based charge carrier channels are separated by domains of organic absorber on the order of several nanometers thick perpendicular to the light absorption direction. We show that varying combinations of cations in the inorganic layer and functional groups on the organic layer can be used to effectively tune recombination rates and overall charge carrier mobility, potentially leading to improved IPCE in hybrid photovoltaics.

The role of ZnO as a selective interlayer for organic photovoltaic systems has long been recognized. We will present a detailed study of the role of the physical properties of thin ZnO layers on the performance of inverted-architecture bulk heterojunction (BHJ) organic photovoltaics. We have employed the flexibility of the ZnO system to modulate the mobility mu; and carrier concentration N to alter device performance. ZnO films are prepared under a variety of deposition conditions to obtain films with mobility and carrier concentration spanning several orders of magnitude (mu; ~ 0.3 - 30 cm²/s and N ~ 1x10¹⁶ - 10²¹/cm³). A study of performance in P3HT:PCBM devices with ZnO electron-transport layers, including both dark and light behavior, will be presented. We will also discuss the impact of these changes on the ZnO physical properties in conjunction with higher performing, lower bandgap BHJ devices (e.g. PCDTBT:PCBM) as the active layer. The experimental data will be presented with modeling to describe the electronic behavior of the devices and provide insight into the design rules for optimizing electron-transport layers in organic photovoltaics.

The dramatic efficiency gains in organic photovoltaics seen in recent years have been driven largely by development of novel active layer materials based on electron-rich donor moieties and electron-poor acceptor moieties combined into a single donor-acceptor (D-A) molecule or polymer. The flexibility afforded by combining different D/A building blocks allows optoelectronic properties of active layer molecules to be tuned to improve device performance. Unfortunately, active layer materials with favorable optoelectronic properties do not always produce efficient devices; in some cases this is due to poor packing among polymer chains that results in low charge mobility.
Properties of novel active-layer materials may be estimated with a variety of electronic structure methods on isolated molecules and the results can be used to identify promising candidate materials, but the morphology and intermolecular structure of candidate materials typically is not predicted prior to synthesis and experimental characterization. One avenue for predicting the morphology of candidate materials is via molecular dynamics (MD) simulations using classical force fields. However, accurate force fields for conjugated D-A systems are still being explored. Accordingly, we describe the development of novel torsional potentials based on MP2 ab initio calculations. We have created these potentials by direct fitting for specific D-A type materials. These potentials are described and their transferability and applicability across families of multi-ring systems are assessed.
With these new potentials, we performed atomistic MD simulations of polymer films for a variety of D-A copolymers. We discuss how intermolecular structure depends on the torsional potentials used. We also discuss the predicted packing motifs in the context of experimental results such as X-ray diffraction, time-resolved microwave conductivity, and device characteristics. Finally, we describe implications of these results for extending in silico design of active layer materials beyond molecular optoelectronic properties to include properties of films.

Influence of donor and acceptor domain sizes and their mesoscopic ordering on excitons and charge carriers dynamics are investigated as a function of energetic disorder in 3D blend morphologies of bulk heterojunction solar cells (BHJ-SC). Several BHJ-SC geometries, such as: bilayer, evenly distributed, graded, and ordered morphologies are used in this investigation to estimate the exciton&’s fundamental properties like dissociation probability and diffusion coefficient as a function of energetic disorder. Simulation results suggest that the exciton dissociation efficiency estimated using exponential lifetime model is over 13.5% smaller compared to that estimated using constant exciton lifetime model, specifically in blends with low-energetic disorder. Monte Carlo (MC) simulation results of exciton diffusion coefficient agree reasonably with the experimentally reported values. It is observed that higher energetic disorder increases exciton recombination in larger phase separated domains. Based on the simulation results, exciton dynamics can be classfied into low and high energetic disorder regimes. Current density and voltage (JV) characteristics are reproduced in terms of superposition of nanoscale processes (morphology distribution, energetic disorder, coulombic interaction, image charge and bias). This study uses an exponential distribution of exciton lifetimes to simulate realistic photocarriers dynamics. These simulation results on exciton dynamics provide guidance to engineer blend morphology to enhance exciton dissociation and charge collection efficiency.

Organic photovoltaic cells promise a low cost renewable energy source that is expected to be only a tenth of the cost of their silicon based counterparts. In our organic solar cells, we make use of squaraines, a class of synthetic near-IR active dyes that are robust and stable in air. Studies show that squaraines have a narrow absorption peak in liquid media but broaden significantly in the solid state from aggregation. Currently, we are exploring the use of Stern-Volmer solution based quenching, which is a simple low-cost method to screen for viability of new squaraines targeted for use in organic photovoltaic cells. Ultimately, we want to determine what is the crucial excited state for an efficient device and what environmental factors contribute to the exciton&’s behavior. By understanding the nature of the excited state, we hope to devise a more robust and effective method/approach for screening materials that are targeted for organic solar cells.

Despite the establishment of organic electronics in commercial products, the fundamental understanding of charge transport in organic semiconductors is still lacking predictive power. Surface potential measurements by scanning Kelvin probe microscopy (SKPM) offer the possibility to spatially resolve charge transport and reveal barriers at grain boundaries and interfaces. Conventional SKPM is limited to observations of charge carrier transport horizontal to the device surface whereas the transport, e.g. in bulk heterojunction (BHJ) solar cells, occurs vertically to the surface. Therefore we introduce a method to directly measure charge transport at the cross-sections of organic devices by milling with a focused ion beam (FIB) micrometer sized holes in the samples and analyzing the formed cross section with SKPM.
In this work we used poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM) BHJ solar cells. Maximum power conversion efficiencies of 2 % have been achieved. Even with P3HT:PCBM not being the most efficient material system for organic solar cells, P3HT:PCBM cells serve as a standard reference widely investigated by the scientific community and therefore are well suited for fundamental investigations, such as the presented work.
Sample preparation via FIB milling and measurements via SKPM were performed in-situ in a unique integrated system. This system consists of a commercial AURIGA Scanning Electron Microscope (SEM) and Ga+ FIB crossbeam system by CARL ZEISS MICROSCOPY where a Scanning Probe Microscope (SPM) by DME is incorporated.
Using the FIB a hole was milled in the solar cell and therefore the cross-section was laid open. With the SEM it was then possible to position the cantilever at the FIB-milled edge and perform any desired SPM measurements which in this case are topography (AFM) and surface potential measurements (SKPM). The device could be operated at any given bias voltage and under illumination during the measurement. Therefore we also found out that the solar cells were still functional after FIB milling.
From SKPM measurements without an applied bias we derived the contact potential differences (CPD) of the materials in the solar cell stack. We found the CPD difference of the contact materials to be around 0.5 V which corresponds to the built in potential derived from capacitance-voltage measurements and to the open circuit voltage.
Furthermore, we found out that the open circuit voltage was built up at the contact between the BHJ and the top contact, when we illuminated the solar cells during measurement.
We applied different bias voltages to the solar cells, in the range between -2 V and 2 V. We found that the major potential drop occurred at the interface between the BHJ and the contacts and there was nearly no potential drop along the BHJ. The results have been published (doi: 10.1002/adfm.201301315).

A current challenge for the OPV community is identifying the result of exciton dissociation at the bulk heterojunction interface and determining how the photophysics of the initial exciton impacts the resultant charge carrier generation. This work attempts to contribute to that discussion by utilizing the unique aggregation properties of our squaraine donor materials. Three types of donor species exist in the same material, and by manipulating the amount and type of those species at the bulk heterojunction it can be demonstrated that Marcus-Hush Theory offers a plausible explanation for changes in external quantum efficiency when the donor species are compared to each other. Additionally, the contributing electronic coupling matrix element will be calculated from steady state and time-resolved photophysical data for a variety of squaraine materials and each of those squaraine materials&’ aggregates. By changing the side groups of the squaraine we change their packing structure and the type of aggregate that will be formed. Through examining a variety of squaraines and comparing their electronic coupling matrix element terms we can determine which materials will be preferential for incorporation into an organic photovoltaic device. By choosing the electronic coupling matrix element as a metric for performance we can tie together both photophysics of the materials and the physics of device operation as quantified by short-circuit current and external quantum efficiency.

Recombination and low mobility contribute to poor charge collection in many next-generation thin-film photovoltaic devices. Measurement of recombination and mobility in PV devices enables optimization of nanostructures to improve charge collection. Impedance spectroscopy is a non-destructive AC technique that measures PV recombination rate, effective charge mobility, density of states and more. We propose several nanostructures in PV devices to increase fill factor (FF) via improved charge collection. Impedance spectroscopy measurements of recombination rate demonstrate that nanostructures reduce recombination in thin-film PV devices. We find that small-molecule organic photovoltaic devices with nanostructured electrodes achieve increased FF by reducing recombination in low-mobility films.

Nickel oxide (NiOx) has been found to be effective as a hole transport layer in standard architecture organic solar cells. 1 However, the organic/inorganic interfacial energy level alignment needs to be optimized for a given active layer. For example, the ionization energy of the PCDTBT polymer is over 0.4eV larger than the untreated NiOx work function, which leads to open circuit voltage loss in devices. To address this, an O2 plasma treatment is usually applied to increase the NiOx work function, but it is commonly seen that the plasma treated metal oxides surfaces can be unstable and the increased work function degrades rapidly over a short period of time.2,3 By contrast, molecular monolayer modification can provide a more stable and controlled alteration.2,3,4 In particular, the triethoxysilane (TES) chemistry provides a covalent attachment scheme and it&’s demonstrated that the TES attachment can tune the ZnO work function effectively.5 In this study, three TES based molecules with different terminal groups, namely phenyltriethoxysilane (PTES), octadecyltriethoxysilane (OTES) and 4-chlorophenyltriethoxysilane (4CPTES), are utilized to tune the energy level alignment at the NiOx/polymer interface by introducing dipoles that form a molecular layer and change the work function of the solution deposited NiOx. Contact angle (CA) measurements show that OTES treated surfaces are much more hydrophobic than the untreated surface, with the CA increasing from 42.5°± 1.4 to 89.5° ± 1.3. The other TES molecules also show a CA increase, which indicates the successful attachment of these molecules to the surface. Infrared spectroscopy shows that the coverage is sub-monolayer, consistent with our previous studies of other metal oxide surfaces. Kelvin probe measurements show that all three TES treatments increase the NiOx work function compared to bare, untreated NiOx in the following order: PTES (242mEV), OTES (302meV), 4CPTES (450meV). To test the impact of TES layers on organic/inorganic energy level alignment, standard bulk heterojunction devices were fabricated with PCDTBT/PC71BM blend as active layer. We find that the open circuit voltage improves with increasing work function of the TES-treated surfaces.
We acknowledge support of the NSF through grant DMR-0907409 and the Renewable Energy Materials Research Science and Engineering Center (REMRSEC) through DMR-0820518. SRC acknowledges funding from the Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Fellowship through the Sunshot Solar Energy Technologies Program.
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2. Sharma, A. et al, Appl. Phys. Lett. 2008, 93, 163308
3. Cook, R. M. et al, Adv. Energy Mater. 2011,1, 440-447
4. Allen, C.G. et al, Langmuir 2008, 24, 13393-13398
5. Allen, C. G et al, J. Phys. Chem. C 2012, 116, 8872-8880

Organic photovoltaics provide an excellent opportunity for low cost manufacturing but efficiencies are low and our group seeks to improve efficiencies through better mechanistic understanding. Squaraines provide an excellent opportunity to investigate the processes critical to efficient charge photogeneration. Exciton diffusion, exciton dissociation and charge transport can be affected by increasing the crystallinitity through chemical and process modification and electron transfer at the bulk heterojunction interface can be monitored as a function of H- and J-aggregation. The electronic changes due to aggregation also broaden the optical absorbance which allow for a broader portion of the solar spectrum to be absorbed. We will present photophysical property data demonstrating how the type of aggregation is associated with chemical structure and how it affects device efficiency. We will describe how the introduction of PCBM for working devices changes the extent of aggregation and how trade-offs between directed aggregation through chemical modification and disruption of crystal packing by PCBM can be managed.

We experimentally and theoretically characterize back-scattering and absorption caused by Ag nanoparticle arrays at the interface between a Si substrate and an organic poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) bulk-heterojunction tandem thin film coating [1]. A strong red-shift in back-scattered light wavelength occurs from uncoated Ag nanoparticle arrays on Si as a function of increasing mean nanoparticle diameter (ranging from 30 nm to 90 nm). Following addition of the organic layer coating, back-scattering from the Ag nanoparticle array on Si is notably quenched in the wavelength range of strong P3HT absorption. However, back-scattering is enhanced to a degree relative to the uncoated Ag nanoparticle array on Si at wavelengths greater than the absorption band edge of P3HT (~660 nm). For comparison, the optical properties of Ag nanoparticles on an optically-thick Ag substrate are reported with and without P3HT:PCBM thin film coatings. On the reflective Ag substrates, a significant enhancement (by a factor of 7.5) and red-shift of back-scattered light occurs upon coating of the Ag nanoparticles with the P3HT:PCBM layer. Additionally, red-edge extinction is enhanced in the P3HT:PCBM layer with the presence of the Ag nanoparticles on Ag compared to the planar Ag case.
Theoretical electromagnetic simulations were carried out to help validate and explain the scattering and extinction changes observed in experiment. Both increasing nanoparticle size and an increasing degree of contact with the Si substrate (i.e., effective index of the nanoparticle environment) are shown to play a role in increasing back- and forward-scattering intensity and wavelength, and in increasing absorption enhancements in both the organic and Si layers. It was found that Ag nanoparticles placed at the P3HT:PCBM/Si interface give rise to absorption enhancements in the P3HT:PCBM layer of up to 18 % in the 400 - 660 nm wavelength range, and absorption enhancements in the Si layer of almost 80 % in the 660 - 1100 nm wavelength range. These results provide insight into how nanoparticles placed near an organic/inorganic interface can be employed for light-management in tandem or hybrid organic/inorganic semiconductor configurations for solar energy harvesting applications.
[1] C. T. Nemes, D. K. Vijapurapu, C. E. Petoukhoff, G. Z. Cheung, D. M. O&’Carroll, J. Nanopart. Res., in press (2013).

Inverted polymer bulk-heterojunction organic photovoltaic (BHJ-OPV) device designs have enabled a breakthrough in operational lifetime through the use of stable electrode materials. Degradation of the transparent electrode from corrosive poly(3,4-ethylenedioxythiophene) (PEDOT) layers is avoided by replacing the hole-conducting PEDOT layer with an electron-conducting layer, such as ZnO, TiO2, or Cs2CO3 [1,2,3]. Further, air-stable high workfunction metals (Au, Pd, Ni) can be used as the top electrode instead of more reactive low workfunction metals (Ca, Mg, Ba) that are typically used in the conventional configuration, which, together with the removal of the PEDOT layer, can allow for air-fabrication and storage of inverted BHJ-OPVs [4]. Computational analysis of conventional BHJ-OPVs has greatly aided our understanding of the limits to device efficiency [5]. Although there have been many experimental studies showing that high workfunction metals are beneficial for inverted BHJ-OPV device performance, a detailed, systematic, computational study comparing the performance parameters of conventional and inverted BHJ-OPV devices for a range of different metals and for possible formation of native metal oxides is needed.
Here, we computationally compare the performance of nine different bare metal and six metal oxide/fluoride-coated metal electrodes with a range of surface workfunctions in both inverted and conventional device configurations to identify the most suitable metal (either with or without an oxide/fluoride-coating) in terms of both optical and electronic properties [6]. We quantitatively demonstrate that: (1) high-workfunction bare metal electrodes (Au, Pd, Ni) are ideal for high-efficiency inverted device performance (power conversion efficiency, PCE, up to 6.5 % for devices employing Au anodes); and (2) native metal oxide formation on metal electrodes (e.g., Ag2O/Ag, CuO/Cu, NiO/Ni), which dramatically reduce conventional device efficiencies, can result in highly efficient inverted BHJ-OPV devices (e.g., PCE of 6.7 % for Ag2O/Ag compared with PCE of 5.8 % for bare Ag anodes in the inverted configuration). This work is an important advance over prior studies as it predicts the electrode materials and configurations that can lead to both high efficiency and high stability BHJ-OPV devices.
1. M. S. White, D. C. Olson, S. E. Shaheen, N. Kopidakis, D. S. Ginley, Appl. Phys. Lett. 2006, 89, 143517.
2. C. Waldauf, M. Morana, P. Denk, P. Schilinsky, K. Coakley, S. A. Choulis, C. J. Brabec, Appl. Phys. Lett. 2006, 89, 233517.
3. H.-H. Liao, L.-M. Chen, Z. Xu, G. Li, Y. Yang, Appl. Phys. Lett. 2008, 92, 173303.
4. M. T. Lloyd, D. C. Olson, P. Lu, E. Fang, D. L. Moore, M. S. White, M. O. Reese, D. S. Ginley, J. W. P. Hsu, J. Mater. Chem. 2009, 19, 7638.
5. M. C. Scharber, D. Mühlbacher, M. Koppe, P. Denk, C. Waldauf, A. J. Heeger, C. J. Brabec, Adv. Mater. 2006, 18, 789.
6. C. E. Petoukhoff, D. K. Vijapurapu, D. M. O&’Carroll, submitted.

The size-dependent tunability of the electronic energy levels of colloidal nanocrystals (CNC) offers opportunities for solution-processed, flexible and compact electronic devices. In particular, lead chalcogenide (PbX, X = S, Se, Te) NCs are prospective for photovoltaic applications where high power conversion efficiency has been demonstrated [1-2]. By changing the size of the NCs, the bandgap can be varied and the absorption edge can be tuned. Among the other unique properties of these NCs are the possibility to have multiple exciton generation (MEG), utilizing the discrete higher energy sub-bands formed by the quantum confinement of CNCs [3]. Therefore, a complete understanding of the energy levels of CNCs is necessary, not only the size dependent bandgap but also the complete picture of the energy sub-bands beyond the valence and conduction band edge. The current state-of-the-art methods to investigate the energy levels of nanocrystals are still having many limitation and technical complications. Moreover, all the device applications use cross-linked CNCs and there is the question whether the quantum confinement is still persisting in these assemblies of CNCs.
Here we report a new method to probe the quantized energy levels of colloidal nanocrystal assemblies by utilizing a new type of field-effect transistor devices. Ambipolar FET of PbS CNCs by using ionic-liquid-based gating has been demonstrated to achieve high carrier mobility values (mu; > 1 cm2/V.s) despite driven with only 1.5 V gate, since this gating technique can fill virtually all carrier traps due to the very high accumulated carrier density [4]. Because of the effective trap filling achieved with this gating technique, the Fermi energy level can be shift deep into the valence and conduction band to observe the band fillings.
By this method, we successfully electrically probed the band gap of the PbS CNCs of different diameters and the results are consistent with TEM, optical absorption measurement, as well as ab-initio calculation of the energy level. Most importantly, the band gap measurement was achieve electrically, and by combining the observation of the valence band and the conduction band from the FET measurements. Moreover, selecting specific ionic liquid gate materials we can access the higher energy sub-bands up to the third sub-band beyond the conduction band. Finally, by correlating experimental and theoretical results we can provide a comprehensive understanding of the electronic energy structure of nanocrystal films.
Ref.: [1] K. Szendrei, M. A. Loi, et al. Appl. Phys. Lett. 97, 203501 (2010); [2] K. Szendrei, M. A. Loi, et al. Adv. Funct. Mater. 22, 1598 (2012); [3] A. J. Nozik, et al. Chem. Rev. 110, 6873 (2010) [4] S.Z. Bisri, M. A. Loi, et al., Adv. Mater. DOI: 10.1002/adma.201205041 (2013).

One limitation of organic semiconductors is their low exciton diffusion length. In organic solar cells, this limitation was overcome successfully by the formation of an extended and randomly oriented nanoscaled heterojunction between donor and acceptor phases. However, the disordered nature of the heterojunction results in additional junctions not present in a planar architecture, e.g. the donor/cathode junction. Recently the formation of a Schottky junction due to the accumulation of the polymer at the cathode in organic bulk heterojunction (oBHJ) solar cells has been proposed. The formation of this junction is a consequence of the Fermi level alignment between the semiconductor polymer and cathode material. This new observed phenomenon has motivated the question of how this Schottky junction influences the open circuit voltage Voc in oBHJ solar cells, since such junction has not been considered in previous models. Here we address such question by studying the saturation current J0 in both, P3HT/cathode Schottky diodes and P3HT:PCBM solar cells. Four different metals were used as cathodes for the two kinds of devices. We found that J0 can be modeled consistently using thermionic emission theory. On this basis, a new general Voc expression for when a Schottky contact is formed with the cathode and polymer in any bulk heterojunction is proposed and validated using the Al cathode case.
Acknowledgements to Hanse-Wissenschafskolleg for the Fellowship and CONACyT-SENER (Mexico) grant 153094.
*[email protected]

Over the last decade, organic photovoltaics (OPVs) have seen a very fast development and highest power conversion efficiencies (PCE) are now close to 10%. In the effort to reduce losses further, optical approaches are important because optical losses account for ~40% of total losses. Here we employed an optical structure-microlens array (MLA)-to increase light absorption inside the active layer, and enhance the PCE of OPV devices. The MLA structures were employed on OPVs based on two (post-P3HT) high efficiency material systems: Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]: Phenyl-C71-Butyric-Acid-Methyl Ester (PCDTBT:PC70BM), and Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]: Phenyl-C71-Butyric-Acid-Methyl Ester (PTB7:PC70BM). Devices based on PCDTBT showed around 15% performance (photocurrent) enhancement, whereas devices based on PTB7 showed less than 10% performance enhancement. These relative material-specific performances are intuitively ascribed to the typical active layer thicknesses - thinner films are more optically enhanced using MLA structures. Device performance with MLA was improved not only from the reduced surface reflection and increased light traveling path, but also periodic electric field concentration in the active layer. PCDTBT and PTB7 based devices were fabricated on MLAs with microlenses of different pitches (0.6mu;m, 1mu;m and 1.5mu;m), and their performances were compared. Angle dependence of incident light was also characterized. Simulations using scattering matrix approach supported the experimental performance enhancement by MLA and provided insights into the enhancement mechanism. Simulations predict up to 20% performance enhancement for certain pitches and heights in MLAs. The MLA layer was fabricated using a simple stamping technique, which can be scaled to larger areas. Moreover, MLA fabrication is on the side opposite to the active layer and thus it does not hinder the cell fabrication processing. Finally, MLA approach is a general and a non-destructive approach applicable to photovoltaic technologies based on all material systems.

Organic buffer layers play several important roles in organic photovoltaics (OPVs). For example, they confine excitons to the active layer and prevent damage from the electrode deposition while allowing for maximization of the optical field in the thin active region of the devices. Common wide energy gap materials such as bathophenanthroline (Bphen) suffer from low conductivity, with electron transport enabled by defect states induced during the metal cathode deposition. This damage-induced conductivity mechanism limits the thickness of the layers to the depth of damage from the buffer/cathode interface, reducing their use in tuning the optical field inside the device active region. Recent work has shown that mixing wide energy gap buffers with fullerenes significantly increases their conductivities, allowing for improved charge extraction and fill factor compared to neat buffer materials [1]. In this work, we present simulation and experimental results to understand the trade-offs between exciton blocking and electron conduction to optimize their effectiveness in blocking excitons while conducting charge. For example, an optimal 1:1 mixed C60:Bphen layer blocks 82 ± 5% of excitons in the acceptor layer. The blocking characteristic is predicted using Monte Carlo simulations of exciton transport, and has been confirmed using photoluminescence quenching. This analysis provides insight into the exciton dynamics for a range of blockers and blocking interfaces. Ultimately, this understanding can lead to optimization in our choice and design of blocking layers for use in high performance OPVs.
[1] A Bartynski, C Trinh, A Panda, KJ Bergemann, BE Lassiter, JD Zimmerman, SR Forrest, and ME Thompson. “A Fullerene-Based Organic Exciton Blocking Layer with High Electron Conductivity”, Nano Letters, in press (2013).
[2] RR Lunt, NC Giebink, AA Belak, JB Benziger, and SR Forrest, “Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching”, J. Appl. Phys, 105, 053711 (2009)

Light induced degradation is a critical downside in organic solar cells. It is experimentally found that prolonged light exposure creates defect states at mid-gap of interface between donor and acceptor leading to higher SRH recombination. In this work we studied the change in fundamental properties of small molecule (Penacene-C60) based solar cell such as defect density (measured using capacitance spectroscopy), dark current and quantum efficiency upon prolonged light exposure. Acceptor (C60) was found to be affected mostly due to this photo exposure as significant drop in quantum efficiency recorded in blue region corresponding to the drop in short circuit current. However drop in dark current & extracted Io after exposure is indicating to a different photo degradation dynamics as opposed to the photo degradation of conventional P3HT based system. We will discuss the various techniques used for studying degradation and the influence of preparation conditions on degradation.

Despite the relatively simple structure of organic planar heterojunction devices, the physics behind their operation is still under debate. Here we will explain the origin behind the commonly observed non-zero JV-slope at reverse voltages. Via voltage dependent spectral response measurements, previous reports had identified this effect to be associated with C₆₀. [1,2] This JV-slope was conventionally attributed to bulk dissociation in C₆₀. At MRS Fall 2012, we proposed an alternative explanation, involving polaron induced exciton quenching, and showed how this effect can be suppressed with an advanced bathocuproine (BCP)/ 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI)/Ag cathode. [2] This led to 5.7% efficient diindeno[1,2,3-cd:1',2',3'-lm]perylene/C₇₀ planar heterojunction cells. Here we will present additional evidence that polaron induced exciton quenching severely affects planar heterojunction performance.
The C₆₀ layer of a tris[4-(5-phenyl thiophen-2-yl)phenyl]amine (TPTPA)/C₆₀/ BCP/Ag solar cell was excited with laser light (lambda; = 532 nm) of various intensities. A capacitance measurement (CV) revealed charge injection in the device around open-circuit conditions in the dark and at low light intensities. At high light intensities, photogenerated charges accumulate, even at reverse voltages up to -2 V. A voltage dependent photoluminescence (PL) measurement (Fig. 1b) reveals that the C₆₀ exciton density decreases around open-circuit (~0.9 V), and at high light intensities the exciton density has a more pronounced voltage dependence. The negative correlation of the PL to the capacitance can be interpreted in terms of charges accumulated in C₆₀ that quench C₆₀ excitons. The voltage dependent exciton density in turn leads to a strong voltage dependent photocurrent. This full characterization has been repeated on solar cells with an advanced BCP/ PTCBI/Ag cathode. Such cathode leads to less charge accumulation, which leads to less exciton quenching, which in turn results in higher short-circuit currents and fill factors. The measured relations in between charge density (CV), exciton density (PL) and photocurrent establish the previously neglected mechanism of polaron induced exciton quenching as a major effect in planar heterojunction devices.
[1] W.-I. Jeong et al., Adv. Funct. Mater.2012, 22, 3089
[2] B. Verreet et al, Appl. Phys. Lett.2013, 102, 043301

The control of trap densities in ordered thin films of conjugated molecules is crucial for the improvement of organic electronic devices. We have successfully developed a new experimental approach which allows for imaging of trap densities by using essentially a pulsed laser scanning microscope to scan across the channel of an organic field-effect transistor during operation. Employing a lock-in technique, we detect the light induced change of the transistor current in dependence on the position and the modulation frequency of illumination. The basic concept of the experiment is first to fill the trap states inside the transistor channel by the gate voltage. Second, focused illumination with the wavelength that corresponds to the first S₀-S₁ excitation of the material in focus (pentacene) induces a local release of the trapped charge by exciton assisted trap clearing. Detection of the charge release occurs via frequency-resolved photoresponse measurements. The outcome of these experiments is striking; from careful analysis of the frequency response we identify a photoresponse component which allows direct imaging of the trap density within the transistor channel. We expect that the application of this local technique to a broader class of materials and device configurations will be very useful to optimize fabrication protocols, ultimately allowing for microscopic control of organic interfaces.
Mapping of trap densities and hotspots in pentacene thin-film transistors by frequency-resolved scanning photoresponse microscopy.
C. Westermeier, M. Fiebig, B. Nickel, Adv. Mater. 2013 (in press).

We will show tandem solar cells in a three terminal structure with an inverted polymer cell used as bottom cell and a small molecule cell as the top cell. The absorbing layers of both cells feature different absorption properties. As a result the absorption spectrum of the tandem cells is broadened and the power conversion efficiency is increased. We investigated theses cells and compared the results to optical simulations of the device structure.
For the experiments we used a parallel connection of two subcells with a transparent conductive multilayer middle electrode consisting of a thin silver layer sandwiched between two zinc tin oxide (ZTO) layers. Recently, we reported on the electrical and optical properties of this electrode and applied the electrode as bottom and top contact for semitransparent bulk hetero junction (BHJ) solar cells with inverted device architecture [1]. The electrode exhibits a very high transparency and very good conductivity comparable to an ITO electrode.
Our setup allows obtaining IPCE measurement directly. There is no need to bias one subcell while measuring the other subcell. With our IPCE measurements we were able to prove that the used materials do absorb in different wavelength regions. The overall efficiency of the tandem cell was higher than the efficiencies of single cells, but we were not able to get the exact sum of the single cell efficiencies.
For further investigation, we performed optical simulations and found that the absorbing materials do overlap to a certain degree. With the simulation we are now able to look for new materials with more separate absorbance areas. The simulation reduces the needed experimental effort thus increasing the speed of testing and prototyping.
[1] T. Winkler, H. Schmidt, H. Flügge, F. Nikolayzik, I. Baumann, S. Schmale, T. Weimann, P. Hinze, H.-H. Johannes, T. Rabe, S. Hamwi, T. Riedl, and W. Kowalsky, Org. Electron. 2011, 12, 1612

Organic solar cells based on a bulk-hetero structure composed of an organic donor and fullerene-derivative as an acceptor are currently considered as one of the most potential candidates for high power conversion efficiency. In this system, it is well known that the device performance is often improved by thermal annealing. It is plausible that the improved structure of nano-segregation may cause the improvement of carrier percolation and transport and enhancement of the efficiency of charge separation in the device.
We propose a model for describing charge transport escaping from Coulomb bound of counter-charge at the Donor-Acceptor interface in the molecular aggregates. These charges transport through random energetic states originating from the interaction between carrier and randomly oriented dipoles. We confirmed that these electrostatic-potential distribution forms Gaussian density of states for carriers. Thus the charge transport is dominated by Gaussian Disorder Model (GDM) proposed by Bassler and Correlation Disorder Model (CDM) by D.H. Dunlap et al considering with the idea of spatial correlation of the electrostatic-potential.
Previous model for the calculation of Gaussian density of states suppose random orientation and alignment of dipoles. We introduce orientational and translational order parameter in our model. Using Monte Calro simulation, our new model demonstrate how the order of molecular aggregation enhance the charge transport. Especially correlation of energetic states limits the charge transport route and forms the transport path. This path is not a geometrically formed path based on a percolation. Both factors (path limitation by GDM and by geometrical percolation) strongly relate to the charge separation and correction efficiency. Based on the model demonstration, we discuss optimized order structure of molecular aggregation in the device.
Our model will gives us an insight into the effect of ordered alignment and role of dispersion of Gaussian carrier states in bulk-hetrojunction structure and provide optimized organic solar cell structure with high performance in bulk-heterojunction type of organic solar cells.

In the operation of organic solar cells the charge carrier collection efficiency is limited by recombination losses in the active layer. In this presentation we propose an equivalent circuit with a specific recombination term to describe the behavior of organic solar cells. Experimentally we show that this recombination term determines the slope of the current-voltage characteristic at the short-circuit condition. An analytical model is presented that can be used to calculate the charge carrier collection efficiency of the device [1]. Measuring the current-voltage characteristics of the solar cell at different illumination levels allows us to estimate the charge carrier collection efficiency. This collection efficiency is determined by the charge carrier transport and recombination processes in the active layer of the device. Recently, our group has fabricated 4% efficiency small-molecule solar cells with the following structure: glass/ITO/MoO3/DBP+C70/BCP/Al. In this presentation we compare and discuss the differences observed in the collection efficiency of bilayer and co-evaporated organic solar cells based on DBP and C70.
[1] C. Voz, J. Puigdollers, J.M. Asensi, S. Galindo, S. Cheylan, R. Pacios, P. Ortega, R. AlcubillaOrganic Electronics, Vol. 14 (6) 2013, Pages 1643-1648

Highly efficient organic electroluminescent devices have been fabricated for several decades. In the pioneering years they typically consisted of an organic luminescent dye sandwiched between ITO, which serves as transparent anode, and a metal cathode (typically aluminum) which had the main drawback of poor electron injection. One way to overcome this drawback is to deposit a thin spacer layer of alkali halides between the metal cathode and the organic dye. This approach, using an insulating interlayer to increase the electron injection, which is at the first glance counter intuitive, has been successfully applied to countless organic light emitting devices (OLED). Even though this approach is commonly used, the origin of the enhanced electron injection efficiency is still under debate.
In the present study, we want to focus on the interface energetics for a model system consisting of a molecular semiconductor (C60) deposited on a NaCl layer (only a few atoms thick) on metal single crystals of different work functions. The aim of this study is to analyze in detail the influence of the metal electrode (acting as an electron reservoir with a given electron affinity), on the establishment of thermodynamic equilibrium and consequently on the level alignment of the semiconductor separated by a defined insulating layer. The results reported here demonstrate that, when in such systems the electron affinity of the semiconductor is higher than the work function of the metal supported insulator before contact, electron transfer from the metal to the semiconductor occurs accompanied by the formation of interface dipoles. Interestingly, the vacuum level is observed to shift well beyond the completion of the first interface layer which is clearly related to the occurrence of band bending. When the work function of the metal lies within the band gap of the metal supported insulator, flat band conditions are observed throughout the C60 film growth. In order to explain the mechanisms leading to band bending, we model the charge density in the C60 film as a function of the film thickness by solving the one dimensional Poisson equation using as key parameters only material constants and measured quantities without any free fitting parameters. The results from this calculation prove to be in excellent agreement with the experimental data and bring evidence on the physical mechanisms leading to band bending which is due to charge transfer from the metal substrate to the semiconductor. This phenomenon is expected to substantially influence the electrical properties of real devices, even though additional effects due to structural and/or chemical imperfections might in addition play a role. Nevertheless, these additional effects appear not necessary for understanding the role played by the interlayer to improve the interface characteristics.

The low cost and relative ease of fabrication for organic photovoltaic (OPV) devices motivates their investigation. Bulk heterojunction (BHJ) morphologies for OPVs have improved the internal quantum efficiency (IQE) as a result of better exciton dissociation due to the higher interfacial area between the donor and acceptor materials. However, there still remains the challenge of providing a holistic model that can predict the performance of such devices before fabrication [1]. Ongoing research has focused on the continuum model (Poisson equation along with drift-diffusion equation) and the discrete model (dynamic Monte Carlo (DMC) based on the first reaction method (FRM) for describing the processes occurring within OPV devices) [2,3].
The unique approach of our work is to use measured materials properties, namely the nanoscale morphology, to provide important input parameters for a semi-empirical DMC model of OPV devices based on P3HT-PCBM BHJs. The semi-empirical model comprises three distinct modules: a Monte Carlo morphology generator, an optical exciton generation rate calculator, and a DMC charge transport simulator. The Monte Carlo morphology generator enables the comparison of various P3HT-PCBM morphologies, especially BHJs with different average feature sizes. An enabling technology for this approach is emulsion-based resonant-infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE), which is a vacuum-based deposition technique that allows for control over nanoscale morphology [4]. In this work, we will investigate the relationship between morphology and OPV performance by using RIR-MAPLE to deposit P3HT-PCBM BHJ active regions with different average feature sizes of the donor and acceptor domains. The average domain size will be determined experimentally from optical images obtained by a Zeiss Axio Imager wide-field fluorescence microscope. This feature size will then set the target BHJ for the Monte Carlo morphology generator, which serves as input for the remaining two simulation modules.
The calculated output from the model includes the spectral external quantum efficiency and the short-circuit current density (Jsc). Thus, devices will be fabricated from the RIR-MAPLE grown P3HT-PCBM BHJ OPVs with different donor/acceptor domain sizes, and the measured spectral response (EQE) and short circuit current will be compared to the calculated device performance. In this way, the impact of domain sizes will be investigated explicitly. It is important to note that this is a unique study that is not possible using traditional, solution-based deposition, but is enabled by RIR-MAPLE.
References:
1. Li, G. Y. et al. (2012). IEEE Journal of Photovoltaics 2(3): 320-340.
2. Yang, F. and S. R. Forrest (2008). ACS Nano 2(5): 1022-1032.
3. Meng, L. Y., D. Wang, et al. (2011). Journal of Chemical Physics 134(12).
4.Pate, R., R. McCormick, et al. (2011). Applied Physics A-Materials Science & Processing 105(3): 555-563.

Organic solar cells are a promising alternative source of electrical energy because of their printable and flexible device structure. The durability of solar cells is an important problem for the practical use. The reversible deterioration of device performance without material degradation has been reported, which has been ascribed to the accumulation of photogenerated charge carriers in the cells under device operation. However, a more detailed study clarifying molecules and these sites where charge carriers are accumulated (trapped) without molecular degradation has not yet been conducted, which will be extremely important for further device performance and durability improvements.
Electron spin resonance (ESR) is one promising method for such a microscopic characterization of charge-accumulation sites because it is a highly sensitive and powerful approach that is capable of investigating organic materials at the molecular level.
In this presentation, we report on an ESR study of organic solar cells to investigate accumulated charge carriers in these devices under device operation from a microscopic viewpoint [1,2]. We measured light-induced ESR (LESR) signals and device performance (short-circuit current J_sc and open-circuit voltage V_oc) simultaneously using the same device under simulated solar irradiation. From the ESR analysis, the molecules where photogenerated hole carriers were accumulated are clearly identified as poly(3-hexylthiophene) (P3HT). Moreover, the simultaneous measurements of ESR and device performance demonstrate a clear correlation between the increased LESR intensity and deteriorated device performance. That is, the number of spins, N_spin, due to the accumulation of photogenerated hole carriers in P3HT monotonically increases and J_sc and V_oc concomitantly decrease as the duration of simulated solar irradiation increases. This clear correlation demonstrates that the accumulation of photogenerated hole carriers in P3HT deteriorates the device parameters J_sc and V_oc. To the best of our knowledge, this is the first instance in which such a clear correlation between the microscopic ESR characteristics and macroscopic device parameters has been observed. The charge accumulation affects an internal electric field in the device, which prevents current flow and creates additional potential in the cells. The sites of hole accumulation with deep trapping levels were identified as being formed in P3HT at the PEDOT:PSS/P3HT:PCBM interfaces from the study of organic layered films. The deep trapping sites can be ascribed as the main mechanism for the reversible deterioration of the device performance of organic solar cells.
[1] T. Nagamori and K. Marumoto, Adv. Mater.25 (2013) 2362.
[2] K. Marumoto, T. Fujimori, M. Ito and T. Mori, Adv. Energy Mater.2 (2012) 591.

Organic bulk heterojunction (BHJ) devices are recognized as potential energy sources and may have application in signal processing and optical sensing systems. The J-V characteristics of organic BHJ devices have been extensively studied in order to improve device performance. For most organic BHJ photovoltaic devices, the forward J-V characteristic is well documented and there is good agreement between numerical simulation and experimental results. However, the reverse J-V curve is not fully understood.
In this paper, breakdown mechanisms and reverse J-V characteristics of P3HT:PC₆₁BM and P3HT-DPP-10%:PC₆₁BM organic bulk heterojunction devices are studied. Contrary to the current saturation under large reverse bias predicted by the Onsager-Braun model, the dark current decreases significantly when the electrical field exceeds 6×10⁷ V/m. This behavior is analyzed in terms of the common breakdown mechanisms in semiconductors, and the tunneling effect is found to be the dominant breakdown mechanism in most devices.
In order to better analyze the reverse dark current, a band-to-band tunneling model is proposed. For density of states (DOS), based on previous studies, both Gaussian distribution and Exponential-Parabolic distribution are introduced. We find that the simulation results from an Exponential-Parabolic distributed DOS show good agreement with experiment data at high reverse voltage. We also find the current leakage due to shunt resistance dominates in the low reverse voltage region of the J-V characteristics.

Here we present a methodology for increasing the luminescence intensity of the triplet-fusion induced photon up-conversion process in solid state layers of solution-processable organic composites. We study the photon up-converting blend films of the triplet photosensitizer (2,3,7,8,12,13,17,18-octaethyl porphyrinato) platinum(II) (PtOEP) mixed with the blue emitter 9,10 diphenylanthracene (DPA). Despite the established consensus on the negative effect of PtOEP aggregates in the process of triplet-fusion driven photon up-conversion, no systematic work has yet been performed for elucidating the parameters that determine the extent of PtOEP aggregation in these composites. Hitherto another aspect of equal importance that has not been addressed is the quenching of the up-converted DPA luminescence intensity by the fluorscent DPA aggregates that are always formed in the DPA:PtOEP system. Based on experimental and theoretical studies we address the aggregation of the PtOEP triplet photosensitizer and we provide rational guidelines for the controlled formation of PtOEP aggregates in solid state organic composites such as DPA:PtOEP. We use time-integrated UV-Vis and photoluminescence (PL) spectroscopy. The results of the time-resolved PL study at room temperature and at 77K will be presented. Quantum chemical density functional theory (DFT) and time-dependent (TD-DFT) calculations are performed with the aim to investigate the ground and the excited state properties of PtOEP in the form of single molecule and molecular aggregate. Tuning of the PtOEP aggregation in the DPA:PtOEP composites is achieved either by casting the DPA:PtOEP layers from solutions of solvents with increasing boiling point or by varying the PtOEP content in the composite. For these systems, atomic force microscopy imaging reveals the evolution of the PtOEP aggregates. The suppression of the fluorescent DPA aggregate formation is achieved by dispersing the DPA:PtOEP system in the photophysically inert matrix of poly(styrene), which does not compromise the solution-processable character of the robust PS:DPA:PtOEP layers. In respect to the binary DPA:PtOEP system, the up-converted luminescence intensity of DPA in the ternary photon up-converting layers of PS:DPA:PtOEP is found greatly improved. We discuss on the utilization of our findings for the sensitization of organic solar cell devices at low photon energies.

Charge transfer (CT) states at a donor-acceptor heterojunction (DA HJ) are increasingly recognized as key in determining both the practical and thermodynamic limiting efficiency of organic photovoltaic cells. These states result from photoinduced charge transfer at the heterojunction and consist of a Coulombically-correlated hole and electron that reside on adjacent (or closely neighboring) donor and acceptor molecules, respectively. Although the important role of CT states in determining photocurrent generation and open-circuit voltage has been established, the means by which these states overcome an estimated binding energy ~10k_bT to achieve efficient charge separation at room temperature has been the subject of debate.
Here, we explore the dependence of CT energy, E_CT, on background dielectric constant directly via electroluminescence (EL), photoluminescence (PL), and absorption by varying the blend ratio of small molecule bulk heterojunction (BHJ) organic solar cells based on the donor N,Nprime;-bis(1-naphthyl)-N,Nprime;-diphenyl-1,1prime;-biphenyl-4,4prime;-diamine (NPD) and the acceptor C₆₀. Consistent with previous observations, we observe a red-shift of E_CT with increasing C₆₀ fraction, but find that modeling based on the accompanying change in dielectric constant via the solid-state solvation effect can only explain the data at high (>50%) C₆₀ loading. We attribute a higher than expected E_CT at low fullerene concentration to increased localization of the electron component of the CT state due to a reduction in average C₆₀ crystallite size below 4 nm. Using electroabsorption spectroscopy, we observe a substantial increase in CT state polarizability beyond this threshold crystallite size indicative of increasing delocalization, and find that this leads to rapid decay in the CT photoluminescence transient attributed to efficient long-range charge separation. These results support the emerging model of charge separation via delocalized CT states independent of excess heterojunction offset ‘driving&’ energy and indicate that local fullerene crystallinity is critical to the charge separation process.

Rapid migration of separated charges from the bulk heterojunction (BHJ) to the appropriate electrode is critical to efficiency within organic photovoltaics (OPVs). However, methods of characterizing charge transport within complex BHJ morphologies are limited. In order to address this challenge, we have developed a new approach to measure mobility using solid state NMR spectroscopy and carrier induced paramagnetic relaxation enhancement. In many organic semiconductors, the charge carriers are radical cations or radical anions which as a result of the unpaired electron are paramagnetic. These paramagnetic carriers are capable of enhancing nuclear spin relaxation. The rate of nuclear spin relaxation can be related to the mobility of the carrier within the organic solid. We have studied the effect of electron carriers within PCBM and determined the mobility of these carriers within this important acceptor material. In these studies we have n-doped PCBM with controlled carrier densities which we quantify using EPR spectroscopy. We then perform saturation recovery experiments on these doped PCBM samples to determine longitudinal relaxation times (T1) as a function of dopant concentration. The relaxation rates (T1-1) are found to be proportional to dopant concentration, and the proportionality constant, known as the relaxivity, can be related to the carrier mobility using the Solomon-Bloembergen equations. Using this method we determined electron mobility in polycrystalline PCBM to be 0.5 cm2/Vs which is in reasonable agreement with reported field effect mobilities for PCBM (Singh et. al., J. Appl. Phys. 2005, 97, (8)). This NMR approach is unique in that measured carrier mobility can be related back to chemical environment, since the resonance lines used in relaxation measurements are specific to specific chemical environments.
This material is based upon work supported by the Office of Naval Research under contract number N00014-12-M-0097. Any opinions, findings, and conclusions or recommendations expressed in this material are those of LGS Innovations LLC and do not necessarily reflect the views of the Office of Naval Research.

Organic photovoltaic (OPV) cells show great potential for use as a source of low cost renewable energy. Indium tin oxide (ITO) is commonly employed as the transparent window electrode in OPV cells due to favourable transparency and conductivity. However, the electronic characteristics of the ITO surface give poor energy level alignment with many of the organic materials typically used in OPV cells, leading to compromised cell performance.
We have therefore investigated modifying the properties of the ITO electrode and photo-active organic interface with the insertion of a thin (~5 nm) thermally evaporated vanadium oxide (V₂O_x) hole extracting layer. In-situ ultra-violet photoemission spectroscopy (UPS) studies reveal the metal oxide to be highly n-type with a high work function of 6.8 eV. Subsequent deposition of a bilayer OPV architecture consisting of the high ionisation potential donor boron subphthalocyanine chloride (SubPc) and fullerene (C₆₀) acceptor produced a large open circuit voltage (V_oc) of 1.10 V, compared to 0.81 V with fabrication on ITO. Overall cell power conversion efficiency (eta;_p) thus increased by 25 %. Additional UPS studies of the ITO/SubPc and V₂O_x/SubPc interfaces highlighted the differences in energy level alignment causing the change in cell V_oc.[1]
As an alternative to the thermally evaporated hole extracting layer, we additionally explored a method of solution processing V₂O_x films. The atmospheric processing conditions of film preparation have a critical influence on the stoichiometry of the V₂O_x, measured using x-ray photoelectron spectroscopy (XPS). These led to direct impact on SubPc/C₆₀ cell performances. The optimised V₂O_x layer was fully fabricated under a nitrogen atmosphere and exhibited high work function n-type character analogous to the thermally evaporated oxide. The V₂O_x solution processed layer displayed a similar performance (~3.3 %) to poly(ethyleneoxythiophene):poly(styrenesulfonate) (PEDOT:PSS) when utilised in poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) cells, but produced favourable cell stability attributes.[2]
[1] I. Hancox, L.A. Rochford, D. Clare, P. Sullivan, and T.S. Jones, Applied Physics Letters, 2011, 99, 0133044
[2] I. Hancox, L.A. Rochford, D. Clare, M. Walker, J.J. Mudd, P. Sullivan, S. Schumann, C.F. McConville, T.S. Jones, J Phys Chem C, 2013, 117, 49-57

Dye-sensitized solar cells (DSCs) have attracted worldwide attention due to their potential low production cost, power conversion efficiency higher than 10% and ability to work at low light intensities. Ultralow-cost DSCs could be achieved by constructing both PE and CE on cheap metal foils, without any TCO glass substrate. Due to the opacity of metal foils, one way to circumvent illumination issues can be an architecture that employs a substrate endowed with thousands of through holes (via holes or vias). In this work, vias allow an ionic pathway between PE and CE through the electrolyte. The aim of this work is to study the performance of DSCs based on metal foils for constructing both PE and CE.
Stainless steel (SS) foils (AISI 301) were used as substrates for both the PE and CE. The proposed device architecture in this work was composed of 5,628, 4,888 and 4,128 through holes with diameters of 0.1, 0.12 or 0.15 mm, respectively, which were laser-drilled into an area of 4.8 cm2. Solar cell assembly and device characterization have been described in details elsewhere1. SEM images of the porous TiO2 layer deposited onto a perforated SS foil evidenced the tilted walls of the laser-drilled holes and some cracks. Open-circuit voltage (Voc) values were practically the same (ca. 0.62 V), as well as the photocurrent densities (Jsc). The main differences were observed in the fill factor (FF) and thus, energy conversion efficiency (eta;). As the hole diameter increased from 0.1 to 0.15 mm, FF was improved significantly, probably due to better electrolyte mass transport properties. The performance of DSCs was improved by: treating the surface of the perforated SS foil with a gel chemical remover prior to porous TiO2 layer deposition, using a TiO2 BL, and optimizing porous TiO2 layer deposition and electrolyte injection. The surface treatment of the perforated SS foil with a gel chemical remover and a TiO2 BL (DSC 0.15A) provided a ca. two-fold increase in performance. Additional performance improvement was observed by optimizing porous TiO2 layer deposition and electrolyte injection. This preliminary optimization study provided DSCs (DSC 0.15B, 1.34% under 100 mW cm-2) with a ca. 3-fold increase in performance compared to the DSC 0.15 (0.43% under 100 mW cm-2). In summary, the use of metal foils to assemble both electrodes of DSCs was demonstrated, providing lightweight, thin and truly flexible devices.
The authors thank FAPESP for financial support (11/50933-8). ASG (12/08039-0) and JEB (11/080304-6) thank FAPESP for scholarships. CB and ADG thank the company Celgard® for providing Li-ion battery separator samples and CTI for technical support. JEB and AFN thank LNNano for technical support.
(1) Bonilha, C.; Benedetti, J. E.; Nogueira, A. F.; Gonccedil;alves, A. D. Ind. Eng. Chem. Res. 2012, 51, 9700.

In this work, we illustrate how gate-voltage modulated Seebeck coefficient measurements in organic semiconductors complement conventional measurements of charge transport like conductivity to shine light on the nature of carriers that mediate charge transport. A device with integrated on-chip architecture comprising micro-fabricated source-drain electrodes, temperature sensors, and a heater has been built to measure the Seebeck coefficients of field-effect transistors (FETs) with high accuracy. The active layers of the FETs comprised several solution-processed p-type and ambipolar organic polymers with field-effect mobilities greater than 0.01 cm²/V-s at room temperature. Since the Seebeck coefficient is probed using a steady-state current-less measurement, it can be used to probe the nature of charge transport within an FET without the spurious effects that contact resistance may introduce.
As gate voltages are increased the changing dependences of the measured Seebeck coefficients on gate voltage indicates the Seebeck coefficients becoming dominated by charge transport within the FET&’s accumulation layer. The Seebeck coefficients then fall as the accumulation region&’s carrier density is increased. Capacitance measurements indicate that these surface carrier densities are between 10¹¹ and 10¹³ cm^-2. The corresponding Seebeck coefficients are large, between 300 and 1000 mu;V/K. In addition, the measured Seebeck coefficients show no discernible temperature dependences over the measured temperature range, 240 - 340 K. A discussion on the possible physics that leads to such a universal temperature invariance of the Seebeck coefficient in high mobility organic semiconductors will be the focal point of this presentation.

Exciton diffusion is of great importance to the performance of organic optoelectronic devices, including organic photovoltaics (OPV) and solid-state lighting. The ability to control exciton diffusion in organic semiconductors is crucial to the design of efficient optoelectronic devices. Here we present a first-principles simulation framework that can predict exciton dynamics in organic semiconductors [1]. The framework is based on the time-dependent density functional theory to provide the energy and many-body wave functions of excitons. Non-adiabatic ab initio molecular dynamics is used to calculate phonon-assisted transition rates between localized exciton states. Using Monte Carlo simulations, we determine exciton diffusion length, lifetime, diffusivity, and harvesting efficiency for both polymers and small molecules, and the results agree very well with corresponding experimental values. An experimentally speculated exciton diffusion mechanism is confirmed from the simulations. We will discuss the contrasting exciton diffusion behavior between polymers and small molecules for OPV. The effects of the backbone length and alkyl chains on exciton diffusion are examined, and the connection between exciton diffusion and carrier mobilities is also explored.
[1] X. Zhang, Z. Li and G. Lu, Phys. Rev. B 84, 235208 (2011).

To be able to study the generation and recombination and more importantly separate the effects, we developed a technique that is based on sweeping the excitation intensity from ultralow intensity (0.001 sun) and up to high intensity (few sun). We have used this technique to analyze the Quantum efficiency of BHJ P3HT:PCBM devices subjected to different annealing times. Our detailed modeling shows, that in order to explain to full intensity range and different annealing time, we have to introduce a new recombination mechanism. This "new" mechanisms is not charge-recombination but rather exciton recombination or annihilation by the generated charges. While such a mechanism is well known, it has never been identified to play a role in working devices.
In order to design more efficient OPV's there is a need to have the ability to identify the physical processes that govern the operation of these devices, and understand how to manipulate and control them. By having this ability, one can decide which directions to follow and where to aim in order to achieve better devices. For now, the main obstacle is the ambiguity found in various reports. Using the same set of measurements different conclusions are drawn pointing to different physical processes as the limiting ones.
The ultralow intensity regime is often considered irrelevant to solar cells since at such low intensity the “problems” associated with charge recombination within the device and/or bad contacts (i.e. recombination at the contacts) do not show up. This is exactly why we can use the ultralow intensity to directly measure the charge generation efficiency. As we ramp up the intensity the “problems” start to kick in one by one and from their evolution as a function of light intensity we can deduce the nature of the “problem” or the mechanism driving the loss of efficiency.
Analyzing the measured Quantum efficiency of BHJ P3HT:PCBM devices subjected to different annealing times, different shapes of the intensity dependent efficiency in the recombination range were observed. This fact indicates that there are several recombination mechanisms at play and that their relative power may change in the course of annealing.
We find that both trap assisted (Shockley-Read-Hall type) and bimolecular losses coexist and that the relative magnitude of which is dependent on both the light intensity and the processing conditions. We suggest that the use of Langevin type charge recombination in conjunction with trap assisted recombination is not the best choice and conclude that the charge-polaron induced exciton annihilation is most likely to be the process appearing as a bimolecular loss in bulk hetero-junction organic photovoltaic cells. The relative strength of trap induced recombination and exciton annihilation will determine whether at one Sun the efficiency loss appears as monomolecular or bimolecular. We also found the trap related recombination loss to be activated by the internal voltage.

Organic-based electronics have increasingly become a research focus in part due to the potential of light weight, mechanical flexibility, large are fabrication as well as low cost production. The understanding of the corresponding interface properties, especially donor-acceptor (D-A) interface is a critical important to improve device efficiency and optimize device structures. D-A interfaces control exciton dissociation and charge transfer, and affect open circuit voltage in organic photovoltaic cells (OPV). Here, the integer charge transfer (ICT) model[1] are applied to predict the electronic structures of D-A heterojunctions, estimate interface dipoles and probe existence of ground state charge transfer (CT) complex.
Three different donors, rr-P3HT, TQ1, and TFB, in combination with the acceptor PC71BM, in bilayer HJ on PEDOT:PSS and ZnO nanoparticles corresponding to the interface in the normal and inverted OPVs, and bulk HJ on PEDOT:PSS were investigated. The donors are selected so that their respective donor pining level EICT+ is smaller, equal and larger than the PC71BM EICT-, and their tendency to de-mix with PC71BM varies in strength. Using these model systems, we judge the formation of a weak ground-state CT complex at BHJ interface, which in fact can enhance the transformation of excitions into free charge carriers to improve OPV performance and is related with the Voc, and we also explore the effect of mixing/de-mixing, dark state interface dipoles and compare with device performance.
[1] S. Braun, W. R. Salaneck, M. Fahlman, Advanced Materials, 2009, 21, 1450

Organic photovoltaics have attracted substantial interest in scientific and industrial research because they can provide an environmentally friendly, portable and potentially inexpensive energy source. In these systems, the conjugated polymer typically acts as the electron donor and a fullerene derivative is the electron acceptor. They are blended together to form a phase separated interpenetrating structure at the nanoscale creating the so called bulk-heterojunction (BHJ) solar cells.
From a previous study in our group, it has been found that nanoparticles in the polymer films are driven to aggregate at an interface by an entropic force since the nanoparticle loses only three degrees of freedom while a polymer loses many more when force to a hard substrate. In P3HT/PCBM system, the diameter of the PCBM molecule is ~1.4 nm. So with the addition of larger nanoparticles with a diameter of ~10 nm, it could mimic a hard substrate in which the small particles could assembly around large particles. It is hypothesized by us that in the case of the P3HT/PCBM solar cell, with the addition of large particles, small particles (PCBM) could form electron pathways around the large particles throughout the entire active layer to deliver better charge transport. In our experiment, oleic-acid coated Fe₃O₄ (OA-Fe₃O₄) nanoparticle with an average diameter of 10 nm was dissolved into the P3HT/PCBM solution. A wide range of Fe₃O₄ volume fraction from 0 to 0.2 was used in experiment. Up to 20% efficiency enhancement was obtained from the annealed sample with 4% volume fraction of Fe₃O₄ nanoparticles in the active layer with a 100 nm thickness.
Neutron scattering was used to investigate this three component system because the scattering length density (SLD) of P3HT (0.74x10^-6 Å^-2) is different to PCBM (3.6 x10^-6 Å^-2) and OA-Fe₃O₄ (2.9x10^-6 Å^-2). Analysis showed that all the data could be fitted to a polydisperse hard sphere model with an average PCBM agglomerate size about 7 nm and 15 nm for as cast and annealed samples. For the as cast samples, a higher level plateau in the low q range imply more PCBM agglomeration formed by adding the Fe₃O₄ nanoparticles leading to better charge transport. Both grazing incidence x-ray diffraction and absorption measurements showed that the P3HT crystallinity and crystalline size became lower, which resulted into lower charge transport, with the addition of iron oxide nanoparticle since the structure of the semicrystalline was disturbed by the large amount of nanoparticle agglomerate. Meanwhile, the smaller P3HT crystal had a shorter excitons diffusion length which resulted into lower excitons recombination rate delivering higher short current density. By carefully tuning the amount of the Fe₃O₄ nanoparticle, we could apply this morphology control method to considerably enhance the device performance of P3HT:PCBM solar cell.

Organic photovoltaic devices have been extensively studied mainly in bulkhetero-junction system where donor and acceptor are mixed and segregated. On the other hand, p-i-n layered structure has been studied mainly in vacuum-deposited films. The separated structure is ideal for efficient charge generation and extraction, but the layered structure is difficult for solution-processed devices. In this study, we fabricated solution-processed p-i-n structure using soluble photoprecursor of 2,6-dithenyl-anthracene diketone (DTAntDK). Acene diketone molecules are soluble photoprecursors that can be converted to parent acene by photoirradiation.
The p-layer of DTAntDK was spin-coated on a PEDOT:PSS-treated ITO glass substrate. After photoirradiation to be insolubilized, the i-layer was prepared from a mixed solution of DTAntDK and PC71BM. After photoirradiation again, the n-layer of PC71BM was spin-coated. We compared the device performances of p-n, p-i-n, and i structure using the same materials.
The three devices (p-n, p-i-n, i) showed high open circuit voltage (V_OC) around 0.9V due to the wide HOMO-LUMO energy gap of photoconverted DTAnt. The p-n hetero-layered device showed high short circuit current (J_SC) and fill factor (FF) with low series resistance; however, the power conversion efficiency (PCE) was a smaller value of 0.62%. The i device showed higher J_SC and lower FF compared to the p-n device, resulting in PCE of 0.68%. The p-i-n device showed the highest J_SC and PCE of 1.66 %. These results indicate that efficient photogeneration occurs in the i-layer, but p and n layer having high electric properties are required for efficient charge extraction. Thus, solution-processed p-i-n type devices were successfully fabricated using soluble photoprecursors.

Organic photovoltaic devices have been extensively studied mainly in bulkhetero-junction (BHJ) system where donor and acceptor are mixed. In BHJ system, suitable segregation is essential to enlarge donor-accepter interface for photogeneration, and ensure charge extraction paths to the electrodes. In this study, a new quarterthiophene compound bearing barbituric acid moiety (BAR-T-3H4T). This molecule forms nanorod structure based on hydrogen bonding in film. We investigated nanorod formation process of BAR-T-3H4T and applied it to BHJ photovoltaic devices with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).
These materials were mixed in a solvent mixture of toluene and chloroform (1:1) at 50/50 wt%, and the blend films obtained by spin-coating were used for the active layer of the solar cells with a device structure of ITO/PEDOT:PSS/BAR-T-3H4T:PCBM/Ca/Al. The photovoltaic performance was measured under AM1.5G illumination.
The pristine BAR-T-3H4T:PCBM film has amorphous granular morphology. The film structure was dramatically changed upon annealing at 80 °C, and nanorod structure with 5-10 nm in diameter and hundreds of nanometers in length was observed in AFM images. The BHJ photovoltaic devices without annealing showed the power conversion efficiency (PCE) of 0.97%. In contrast, when the device was fabricated with thermal annealing at 80 °C before depositing cathodic materials, the PCE was improved to 2.1%. To the best of our knowledge, this is the highest value reported for small-molecular materials that organize through specific hydrogen-bonding interactions. The nanorods formation improves the photovoltaic performance in terms of both enlargement of donor-acceptor interface and carrier extraction to the electrodes.

Organic photovoltaic devices have been intensely investigated as a promising candidate for achieving low-cost, flexible, and scalable solar cells. Among the organic photovoltaic devices, power conversion efficiency (PCE) of solution-processed bulk heterojunction (BHJ) solar cells has been rapidly increased through the precise molecular tuning of organic semiconductors, the control of nanostructures within the active layers, and the optimization of device structures. New donor materials composed of a central pyrene core, four oligothiopenes, and peripheral acceptor units were synthesized and characterized with respect to optical and redox properties in solution and in solid films. It was found that the lowest unoccupied molecular orbital (LUMO) energy levels were ideal for achieving efficient electron transfer to fullerene derivatives PC60BM and PC70BM, and that the synthesized donors can function as electron donor components in solution-processed bulk-heterojunction (BHJ) solar cells. Solution-processed BHJ solar cells using the synthesized molecules as electron donor materials and fullerene derivatives as acceptor materials were fabricated and investigated. The structures of peripheral acceptor units were reflected in the performance characteristics of solar cell devices fabricated using donors. Power conversion efficiency (PCE) of 3.7 % was achieved for small-molecule BHJ solar cells under one sun condition.

A critical challenge in the field of organic photovoltaics is the control of morphology of the bulk heterojunction at the nano-scale to achieve efficient charge separation and collection. While several methods have been developed to control the nanoscale morphology within the donor-acceptor blend, resulting in significant gains in the overall photovoltaic performance, the optimal processing conditions for different organic photoactive materials often vary widely due to the complex phase separation and percolation processes in the bulk heterojunction. Minor modifications of molecular structures could lead to drastic changes in the processing conditions needed to achieve optimized device performance.
Here we report a first attempt to “program” the nanoscale morphology of the bulk heterojunction through hydrogen-bonding-enabled supramolecular assembly. Small molecules fitted with a phthalhydrazide group have been designed to form cyclic trimeric discs through self-interaction by hydrogen bonding, which are expected to further result in a columnar pi-stacked architecture beneficial to device performance.
Using branched quarterthiophene as the donor unit, we confirmed the presence of H-bonding as well as pi-stacked aggregation in solution through variable temperature NMR . Bulk neat films of the material deposited by thermal vacuum evaporation, as well as blends with C60, show active hydrogen bonding as confirmed by FTIR measurements. When compared to control molecules with nearly identical structure but lacking the ability to form hydrogen bonds, films of the H-bonding molecule show enhanced, red-shifted absorption. When co-deposited with C60 to form a bulk heterojunction photovoltaic device our H-bonding molecule shows 2-3 fold enhancement in power conversion efficiency relative to identical device structures containing the control molecules, and maximum external quantum efficiencies well exceeding 50% have been obtained (with little optimization work on the devices).
Although a portion of the photovoltaic efficiency boost originates from absorption enhancement, the majority of the improvement is a result of the electrical properties of these blends. Fits to the device characteristics using a charge collection model give us a highly improved charge collection length of 42nm for blends incorporating the H-bonding molecule, as compared to 15 and 18 nm for blends incorporating our two control molecules. This proof-of-concept study demonstrates the validity of our modular approach which involves “retrofitting” a molecular donor with an H-bonding recognition group. Work to synthesize and characterize retrofitted molecules containing lower band-gap chromophores more suitable for high efficiency photovoltaics is underway.

A novel fullerene derivative (N- (3-methoxy propyl)-2- carboxy ethyl -5- (4-cyano phenyl) fulleropyrrolidine) [NCPF] was synthesized and characterized by ¹H NMR, ¹³C NMR, MALDI-TOF, UV-VIS, Cyclic Voltammetry, and TGA. The solubility and electronic properties of NCPF was found to be similar to that of Pheny-C61-Butyric Acid Methyl Ester (PCBM). The effect of thermal annealing of NCPF and Poly (3-hexyl thiophene) (P3HT), spin coated from ortho dichlorobenzene (ODCB) on the morphology and power conversion efficiency of thin blend films (50:50 wt %) has been investigated. GIWAXS shows that the alignment of P3HT lamellae is enhanced with thermal annealing, as evidenced by an increase in d spacing of the (100) peak, which indicates the formation of highly crystalline P3HT with more ordered packing of polymer chains. In addition, the degree of P3HT edge-on orientation and crystallite size was found to be increased. However, significant effect of thermal treatment on crystallinity of P3HT chains was not observed in the optical spectra. The effect of thermal treatment in inducing highly ordered P3HT chains were further substantiated by the observed enhancement in power conversion efficiency (PCE) of P3HT/NCPF devices upon post annealing as a result of high photocurrent. The superior photocurrent of annealed devices is likely to be attributed to the enhanced exciton dissociation efficiency.
Acknowledgment: DOE

Fluorination has been widely utilized in liquid crystals to modify and enhance their range of desirable physical properties. However, the SNAr reaction on perfluorinated aromatic substrates appears to be thus far largely overlooked as an expedient route for their synthesis. SNAr reactions on a range of functionalized pentafluorobenzenes are highly para-specific and thus ideal for the construction of calamitic (rod-like) molecules. Useful nucleophiles include (but are not restricted to) alcoholate, thiolate, and (formally) hydride. A wide range of the requisite monosubstituted perfluorinated precursors are themselves now commercially available or readily prepared by a number of procedures. Here we will demonstrate the utility of SNAr chemistry for the preparation of highly fluorinated liquid crystals and their intermediates. In this work, we report the molecular design, synthesis, photophysical and mesogenic properties of partially fluorinated symmetric and asymmetric p-terphenyl liquid crystalline materials with a variety of para terminal alkoxy tails. The highly fluorinated terphenyls have been characterized by polarized optical microscopy, differential scanning calorimetry and by proton and carbon NMR. The mesogenic properties are examined as a function of the number and location of the fluorinated rings in the p-terphenyl. In addition, the absorption and fluorescence electronic properties of these new materials are under investigation.

Regioregular poly(3-alkylthiophene)s (P3ATs) are well known to have high crystallinity and high hole mobility. Moreover, due to its high solubility, P3ATs are applicable to solution process organic electronics, such as organic field-effect transistors and bulk-heterojunction photovoltaic cells.
This report discusses the controlled synthesis and block copolymerization of poly(3-hexylthiophene) (P3HT) using zincate complex, tBu4ZnLi2. We successfully adopted tBu4ZnLi2 to establish a purification-free and regiocontrolled synthesis of P3HT. However, the living nature of the P3HT polymerization is incomplete based on the results of the post-polymerization and block copolymerization of P3HT.
In this work, we focused on the structure of the ligands of Ni catalyst. It is known that the ligand scaffold of Ni catalyst has a substantial influence on the chain-growth polymerization mechanism. By optimizing the phosphorus ligands of Ni catalyst, we found that Ni(dcpe)Cl2 (dcpe = 1,2-Bis(dicyclohexylphosphinoethane)) has superior ability than conventional phenyl substituted one. We intended to accelerate the reductive elimination by introducing more bulky substituents. Moreover, several researchers reported that electron donating ability of phosphorous ligands enhance the pi-pi interaction between thiophene ring and Ni catalyst, which may reduce the diffusion of Ni(0) catalyst in the polymerization media to minimize chain-transfer reaction. As a result of polymerization, P3HT with extremely low polydispersity (PDI < 1.11) were successfully obtained. Number average molecular weight of P3HT increased linearly with molar ratio of monomer and Ni(dcpe)Cl2. The calculated regioregularity from 1H NMR spectra was 96 - 99 % (Mn > 6.5 kDa). From MALDI-TOF MS spectra, most of the polymer chains have Br/H terminal unit, which suggests that the polymerization takes place in a complete living manner. Finally, we succeeded in the post-polymerization of P3HT with controlled molecular weight and low PDI without the residue of the first block.

Isoindigo is a kind of nature dye, and it is renewable and available from plants. Isoindigo-based low-band gap conducting polymers have high absorption coefficient, crystallinity, and power conversion efficiency(PCE). We focus on developing this type of polymers for solar cell application. The polymers(PnTI) are designed to have different length of thiophene(nT) as donor unit and iosindigo(I) as acceptor unit. We synthesize four polymers of P3TI, P4TI, P5TI, and P6TI by Stille coupling, characterizing their optical property, electrical property, crystallinity and PCE systematically. The Properties of polymers are affected by the amount of side chain, the length of thiophene, and the symmetry of donor unit. As the amount of side chain on donor unit decreases, the UV-Vis absorption of polymers increases. As the length of thiophene increases, the HOMO and LUMO of PnTI rise. The P4TI and P6TI contain centrosymmetric donor unit, which exhibits higher crystallinity than that of axisymmetry of P3TI and P5TI. When the amount of side chain on donor unit increases, the crystallinity of PnTI becomes lower. Thus, the order of crystallinity is P4TI>P6TI>P3TI>P5TI. The solar cell fabricated from the blend of P6TI and fullerene derivative PC71BM reaches the best PCE of 7.24% among PnTIs. This is the highest record in the field of isoindigo-based polymer solar cell, according to our best knowledge. The PCE is expected to increase to more than 10% of commercial viable value by optimizing the polymer compositions and device structures.

We report the time dependence of the separation of electron hole pairs following photogeneration across the donor-acceptor heterojunction in a model organic photovoltaic (OPV). We track the modulation of the optical absorption due to the electric field generated between the charges, and find a very rapid separation to 4 nm within 300 fs, an implied velocity of 10^5 m/s, beyond which separation slows down to reach 5 nm by 3 ps. We show that the early time behaviour is consistent with coherent electron transport through delocalised band states in ordered regions of the fullerene acceptor material. This rapid separation to 4 nm brings the Coulomb interaction between electron and hole down to a few kT, thus allowing long-range charge separation. This explains why some OPVs can avoid geminate recombination and demonstrate near-unity charge yields.

Tandem organic solar cells have recently demonstrated very high performance with conversion efficiencies above 10%. However, the rapid development of higher performance solution-deposited tandem devices has proven challenging due in part to the great number of processing steps required to fabricate such devices. Additionally, the recombination layers greatly influence device performance. Inefficient recombination due to the recombination layer can lead to substantial changes in IV characteristics of the tandem device, but a rubric for the recombination layer in tandem organic solar cells is under investigation.
Here we demonstrate recombination layer devices that include the recombination layer in combination with either the top or bottom absorber layer. Such systems are employed to quickly optimize the recombination layer for each junction individually, separating performance of the sub-cells from the combined tandem device. We show that the optimized recombination layer device can then be successfully combined to quickly develop optimized tandem organic solar cells. Furthermore, we show that recombination layer devices can be used to characterize the IV characteristics of the tandem devices. Such systems allow for the identification of failure points with better accuracy, and for the evaluation of the relative contribution of different mechanisms (including chemical interactions, templating, conductivity, etc.) with precision. Upon optimization of recombination layer devices, the initial tandem architecture solar cells can immediately perform as predicted with no further optimization of recombination layer characteristics.
Specifically, we investigate recombination layers comprised of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) / polyethyleninime (PEIE) and PEDOT:PSS / sol-gel zinc oxide (ZnO) in inverted tandem organic solar cells. We study the effect of recombination layer conductivity, independent of optical effects, on device performance, and conclude that sufficient conductivity is necessary to extract carriers from the sub-cells. Low conductivity in the recombination layer can lead to the formation of a double diode and a strong space-charge regime, as characterized by illumination intensity-dependent IV and spectral responsivity. Optical models are used to optimize the thickness of each layer in the tandem architecture resulting in initial inverted tandem architecture BHJ solar cells with 6.5% power conversion efficiency.
We gratefully acknowledge funding from U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the National Renewable Energy Laboratory for OPV device development. SRC acknowledges funding from the Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Fellowship through the SunShot Solar Energy Technologies Program.

Interfaces between organic electron donating (D) and electron accepting (A) semiconductors have the remarkable ability to generate charge carriers upon illumination. In order to use such D-A interfaces for the construction of efficient organic solar cells, a high yield for this photon-to-electron conversion process, combined with a minimum of energy losses is required. In this talk, we will discuss experiments probing the relevant electronic states at the D-A interface, or so-called charge transfer (CT) states. By performing highly sensitive measurements of the absorption, photocurrent and electroluminescence spectra of a variety of organic D-A photovoltaic devices, we show that the lowest energy and weakly emissive CT state (CT0) plays the most important role in the conversion of absorbed photons to charge carriers. For the most efficient D-A systems, we even find that CT0 generates charge carriers with a yield close to unity. This excludes the necessity of large energy losses associated with efficient charge carrier generation in organic materials. Furthermore, for the contemporary generation of organic solar cells, the energy of CT0 is the main determinant of their open-circuit voltage (Voc). We demonstrate successful strategies to increase Voc and further reduce energy losses by decreasing the recombination of charge carriers via CT states. This will open up possibilities to increase the power conversion efficiency of organic photovoltaic devices beyond their current values of ~10%.

The Einstein relation is an important characteristic of the kinetic behavior of charge carriers in disordered organic and inorganic materials, playing an important role in determining carrier distributions in devices and in the vicinity of interfaces under steady state conditions. A semi-analytic formulation is presented by which the Einstein relation of charge carriers performing thermally assisted hopping in an energetically disordered lattice of localized electronic states can be calculated for non-dispersive transport conditions. The mobility and diffusion constants appearing in the drift diffusion equation are shown to BOTH depend on the carrier density and the carrier density gradient.
Among the conclusions of the presented study we show that while the transport process can be divided into more than one unique representation of drift and diffusion, if a representation in which all carrier density gradient effects are incorporated into the carrier density gradient driven part of the drift-diffusion equation, the well-known General Einstein Relation between the diffusion coefficient and the mobility arises.
In order to establish our results the aforementioned semi-analytic formulation was employed to calculate the carrier density spatial distributions for a system at steady state under an applied bias and given boundary conditions. The calculated distributions were than compared with distributions that were obtained by Monte Carlo simulations and under the same conditions. The insight we report is founded on the fact that the distributions derived by the two methods fit perfectly.

Recently, a naphthalenediimde(NDI)-based copolymer was developed by Polyera which exhibits all the desired characteristics of an ideal electron accepting material: exceptionally high charge mobilities, strong red absorption, and a low-lying LUMO. However, after extensive optimization the polymer:polymer blend solar cells based on mixtures of this material with P3HT achieve power conversion efficiencies only up to 1.4%; significantly lower than that of P3HT blended with fullerene acceptors.
Despite the poor power conversion efficiency (caused by a low photocurrent), fill-factors of the device are exceptionally high (~70 %). In combination with results from time-delayed collection field experiments, this suggests that non-geminate charge recombination does not play a role in limiting the devices&’ efficiency. On the other hand, PL quenching studies indicate that over 90% of the excitons generated on P3HT are quenched in blends. Thus poor exciton quenching also cannot explain the poor performance. Furthermore, it was observed that the PL quenching remains constant for devices with greatly varying power conversion efficiencies.
To investigate the performance limiting processes in this polymer-polymer solar cell we conducted transient absorption measurements both on the short (1ps - 3ns) and long timescale (up to 1 ms) on samples prepared with different processing conditions. We find that recombination of short-lived CT states is an important loss process on the short timescale.
Furthermore, we present a model-free analysis (or soft modeling approach) of transient absorption data where no explicit knowledge of reaction kinetics is necessary. This analysis reveals long-lived species are created: free charges whose decay depends on excitation density, and long-lived CT states whose decay is intensity independent.
Our results, combined with X-ray scattering experiments, illustrate that the material order at the interface has great influence on charge separation.

We demonstrate that by modifying the dielectric constant of a given small molecule semiconductor film via the addition of high permittivity molecules, the exciton binding energy decreases. And, as a result, we observe an increase in the internal quantum efficiency of OPV devices. The high exciton binding energy of photogenerated Frenkel excitons in organic photovoltaic materials represents a major intrinsic barrier to high efficiency for organic solar cells. The low dielectric constant of organic materials (ε ~ 3) is generally believed to be one of the main contributing factors of this inherent limitation. It is therefore predicted that organic photovoltaic materials with a high dielectric constant have great potential to realize high efficiencies, comparable to inorganic photovoltaics. Herein, we have demonstrated, for the first time, that by increasing the dielectric constant of organic thin films upon blending high permittivity molecules, the exciton binding energy can be reduced, which leads to enhanced charge separation in operating organic solar cells. A model system that we have studied consists of B,O-chelated azadipyrromethene (BO-ADPM), a low bandgap (~1.46 eV) small molecule organic semiconductor, and the high dielectric constant camphoric anhydride (CA) molecule. By tuning the blend ratio of BO-ADPM and CA, we have been able to increase the film dielectric constant from ~ 4.5 to ~ 11. This increasing of dielectric constant resulted in reduced exciton binding energy, and subsequently enhanced charge separation (by ~ 30 %) between the donor layer and C60 acceptor layer in planar heterojunction solar cells. A variety of spectroscopic techniques and electronic measurements have been performed to confirm this novel approach to realizing highly efficient organic solar cells, by overcoming one of the most significant intrinsic limitations of organic semiconductors, the high exciton binding energy.

Organic bulk heterojunction solar cells have been steadily increasing in efficiency over the last five years. The vast majority of these devices use a fullerene as electron acceptor combined with a polymer or small molecule electron donating material. It has been shown recently that active layers consisting of a small amount (< 5%) of donor dispersed in a fullerene film can achieve higher open-circuit voltages than the corresponding bulk heterojunction cells using equal amounts of donor and acceptor, while still maintaining a remarkably high internal quantum efficiency.
In order to elucidate the working principle of such low donor content solar cells, we perform sensitive spectral measurements on a series of devices using several different small molecules and polymers. We detect optical transitions from and to the interfacial donor-acceptor charge transfer (CT) states at photon energies below the optical gap of the fullerene. The electroluminescence spectra of the devices are dominated by CT emission, while the corresponding CT absorption band can be resolved in the external quantum efficiency spectrum, indicating that holes are transported by the donor molecules despite the apparent absence of percolating pathways at such low concentrations.
We relate the magnitude and energy of the CT absorption band to the open-circuit voltage of the devices. Analysis of the CT state energy for ternary blends using a mixture of different fullerenes and one donor reveals that in the best low donor content devices, the CT state is delocalized over several fullerene molecules, though localized on isolated donor molecules. We conclude that the electronic coupling strength between the fullerene molecules plays a fundamental role in organic photovoltaic devices.
1. M. Zhang et al, Adv. Mater. 2011, 23, 4960-4964.
2. B. Yang et al, Adv. Mater. 2013, 25, 572-577.

Singlet exciton fission splits one singlet exciton into two triplet excitons. Because it should double the photocurrent per photon, fission is a promising approach to surpassing the single junction efficiency limit in solar cells. We analyze the loss processes of singlet fission based photovoltaics using magnetic field dependent photocurrent spectroscopy. We find that organic solar cells and photodetectors featuring singlet exciton fission materials are subject to two particular loss processes: singlet exciton dissociation at the donor-acceptor junction, and triplet-charge annihilation.
When devices are affected by singlet dissociation prior to fission, an external magnetic field changes the photocurrent by reducing the singlet fission rate relative to the rate of singlet dissociation into charge. We determine that the high field asymptotic value of the change in photocurrent is correlated the fission yield. Triplet-charge annihilation is another particular concern for fission in solar cells because the triplets are frequently formed close together, and if one is dissociated, that charge can annihilate the other exciton. This loss process can be alleviated by sweeping out carriers in a carefully designed solar cell.
We utilize our understanding of loss processes to fabricate a pentacene photovoltaic cell where the external quantum efficiency exceeds 100%, breaking the conventional limit on the number of electrons per photon that a solar cell can harvest.

As organic photovoltaic efficiencies exceed 10%, the science of device stabilization and lifetime gains importance. We have measured the performance of several kinds of polymer solar cells as they age. In some cases the open circuit voltage drops, while in others most of the loss in performance is due to a drop in the short-circuit current. We can distinguish between bulk and interface degradation, even if it occurs on the same timescale. Delaminating and replacing the electrodes after aging reveals how much of the performance loss was due to the electrode-organic interface. For some devices the performance can fully be restored by replacing the electrode, which indicates that the degradation only occurs at the interface of the electrode. In other cases there is also a permanent degradation as a result of trap formation in the bulk of the film. The presence of traps can be confirmed by transient photocurrent (TPC) techniques, as well as Fourier transform photocurrent (FTPS) and photo-thermal deflection spectroscopy (PDS).
To distinguish between different types of traps and determine the mechanism of trap formation, we age solar cells made from different materials at various temperatures and different wavelengths of light. We further adjust the oxygen concentration to see if oxygen is needed for the reaction, intentionally introduce impurities, and operate the devices in the dark with current to see if light is necessary for the degradation to occur.
With the polymer PCDTBT we see spectroscopic evidence for the formation of deep traps during burn-in, but only if oxygen in present. Interestingly, for our tests in an oxygen free environment, where deep traps are not observed, the device still degrades. We therefore think that multiple kinds of traps form during burn-in and that one of them impacts device performance much more than the other.
When testing cells with varying amounts of crystalline material, we find that the open circuit voltage drops substantially only for mainly amorphous materials. We suggest that this loss of open circuit voltage is caused by traps forming in a mixed phase consisting of amorphous polymer and fullerenes. It appears that traps in a crystalline polymer phase have little effect on the voltage since there are no electrons present in this region.

The charge transfer (CT) state is the intermediate between excitons and charge generation in organic photovoltaic devices. Despite recombination loss and significant binding energy, dissociation of the CT state is an efficient process in many donor-acceptor blends. Understanding the physics underlining this process can provide insight on device optimization. In this work, we report our findings on CT dissociation in a 3TPYMB/m-MTDATA exciplex system. Due to significant fluorescence from the CT state, the CT state population and charge generation can be monitored by simultaneous optical and electrical measurements. We demonstrate that for small exchange splitting the singlet state CT lifetime is dependent on intersystem crossing to the triplet CT state. We performed this experiment by modulating the distance between the electron and hole using externally applied mechanical pressure on the thin heterojuntion film of the constituent materials. The ensuing nanoscale mechanical motion within the thin film junction enables our measurement. Additionally, since the triplet CT protects against recombination losses by spin conservation, we observe through magnetic field experiments that photocurrent generation is more efficient from the triplet CT state. The recorded findings unambiguously demonstrate that the triplet CT state plays a significant role in allowing photo-generated charge dissociation to occur efficiently.

This report is on all-solid dye-sensitized solar cell (Hybrid thin film solar cells) consisting of a transparent conductive oxide layered glass/a dense TiO2 layer working as a hole blocking layer/a porous TiO2 layer working as a electron collector layer/a metal oxide thin layer (surface passivation layer)/a perovskite layer (CH3NH3PbI3) as light harvesting layer /2,2',7,7'-tetrakis(N,N-di-p-methoxyphenilamine)-9,9'-spirobifluorene (Spiro) working as a hole collection layer/Ag/Au layers. We have already reported the presence of various surface traps in a porous TiO2 layer, which was observed by thermally stimulated current. The surface traps have been reported to be charge recombination centers. We tried to passivate the surface of the porous TiO2 layers with metal oxide materials such as Al2O3 and Y2O3 to reduce surface traps. It was found that the surface traps at around 4.2 eV from vacuum level (0.2 eV below TiO2 conduction band level, trap density: 10(15)/cm3) disappeared by the surface passivation. Photovoltaic performance of the all-solid dye sensitized solar cells without TiO2 surface passivation was as follows; short circuit current (Jsc): 11.81 mA/cm2, open circuit voltage (Voc): 0.79 V, fill factor(FF): 0.71, Efficiency: 6.59%. The photovoltaic performance increased to Jsc: 16:55 mA/cm2, open circuit voltage (Voc): 0.79 V, fill factor(FF): 0.57, Efficiency: 7.53 % when the porous TiO2 layer was passivated with Y2O3 thin layer. The Y2O3 passivation was much more effective than that of Al2O3 and other metal oxide layers. We concluded that the perovskite layer was not continuous layer, but was dot like structures in our case. After the charge injection from the perovskite layer to TiO2 layer, electrons diffuse in TiO2 layer passivated with Y2O3 thin layer which buries surface traps and retard the charge recombination. This explanation was supported the fact that electron life time in TiO2 layer became longer after the Y2O3 passivation. The charge dissociation from perovskite layer to TiO2 surface may be accelerated by the high dielectric constant (130) which is much higher than that of Al2O3 (2.14). The electron injection and charge recombination kinetics measured by transient spectroscopic methods are reported.

It is widely recognized that the performance of organic bulk heterojunction (BHJ) solar cells is strongly dependent on the donor/acceptor morphology. The visualization of these morphologies in three dimensions has been challenging and has hampered progress in this area. Furthermore, a quantitative description of the effect of morphology on efficiency has been lacking. We study the relation between morphology and performance of hybrid organic/inorganic solar cells in unprecedented detail: Due to the high contrast in electron microscopy and the distinct phase separation, such systems are ideally suited to study the link between nano-scale morphology and device performance.
Bulk heterojunction solar cells consisting of poly(3-hexylthiophene) (P3HT) and ZnO have been made by spin casting a co-solution of P3HT and diethylzinc in ambient conditions and subsequent annealing. By changing the processing conditions both the performance as well as the morphology can be tuned [1]. To further increase the efficiency of these hybrid devices, it is necessary to fully analyse where efficiency is lost.
A three-dimensional description of the nano-morphology was obtained by using electron tomography [1]. The 3D morphology data are then used as direct input into a fully 3D optoelectronic model to calculate the current-voltage characteristics and overall device efficiency [2]. This model includes the effects of exciton diffusion and quenching; space-charge; interfacial charge separation and recombination; drift and diffusion of charge carriers; and the injection/extraction of carriers at the contacts. Given the experimental morphologies, the experimentally observed differences in performance could be reproduced with a single set of parameters. Based on the simulations, we subsequently analyze how these solar cells can be further improved. Several morphological aspects that determine the efficiency are discussed and compared to other organic solar cells.
[1] S. D. Oosterhout, M. M. Wienk, S. S. van Bavel, R. Thiedmann, L. J. A. Koster, J. Gilot, J. Loos, V. Schmidt, and R. A. J. Janssen, The role of three-dimensional morphology on the efficiency of hybrid polymer bulk heterojunction solar cells, Nature Mater. 8, 818, (2009).
[2] L. J. A. Koster, O. Stenzel, S. D. Oosterhout, M. M. Wienk, V. Schmidt, and R. A. J. Janssen, Morphology and Efficiency: The case of polymer/ZnO solar cells, Adv. Energy Mater. 3, 615 (2013).

Devices incorporating both organic and inorganic functional components are of considerable interest, but uncertainty remains as to what influences charge separation and recombination processes at hybrid interfaces. We synthesize and dope zinc oxide (with nitrogen) using an atmospheric atomic layer deposition technique and examine the influence of the metal oxide properties on the photophysics of the hybrid interface using a variety of spectroscopic and device characterization methods. We identify properties of the ZnO that influence charge generation and separation, and demonstrate the ability to improve the performance of hybrid devices by controlling these properties.

Throughout recent years the implementation of nanocarbons into dye-sensitized solar cells (DSSC) has resulted in important breakthroughs. Most relevant are in this context: i) the enhancement of charge transport and charge collection in nanocarbon-doped electrodes, ii) the introduction of nanocarbon interlayers that simultaneously reduce the charge recombination and the increase of charge collection efficiency, iii) the use of nanocarbon-based, iodine-free, solid-state electrolytes featuring excellent diffusion coefficients and catalytic efficiencies, and iv) the use of novel nanocarbon-based hybrid dyes. All of the aforementioned aspects will be thoroughly discussed in this presentation.

Many next-generation solar technologies utilize toxic and/or scarce materials, such as Cd, Te, Pt, Ru, Ga and In. In many cases, the amount of a material required to generate a significant fraction of our energy requirements exceeds its natural abundance by many orders of magnitude. Many researchers are therefore pursuing alternative technologies that use earth-abundant elements.
We have been pursuing technologies that are inherently sustainable, such as organic/inorganic hybrid solar cells, where only plentiful elements, such as Ti, Zn, Sn, O, C, S and Al are used. Unfortunately, to date the performance of these devices, such as P3HT/ZnO hybrid cells, have been disappointing due to recombination losses at the P3HT donor/ZnO acceptor interface. By passivating the ZnO surface using SAMs, we aim to reduce the density of recombination sites, while simultaneously modifying the donor/acceptor band offset to enhance the open circuit voltage.
A second are of interest is modifying existing technologies, such as dye-sensitized solar cells, by replacing components utilizing scarce materials, such as Pt catalysts, with structures composed of abundant materials, such as vertically-aligned carbon nanotubes. Our recent progress in these areas will be reviewed and discussed in the context of competing solar cell technologies.

A plethora of different photovoltaic (PV) technologies are being developed for large scale solar energy conversion. Beyond wafer based first generation PV, exist a second generation of thin-film concepts based on thin solid semi-conductor absorber layers sandwiched between two charge selective contacts, and an emerging generation of meso or nanostructured solar cells which rely on a distributed heterojunction to generate charge and transport positive (p) and negative (n) charge in spatially separated phases. Many thin-film materials have been employed in nanostructured concepts, nevertheless, to-date, no materials discovered from the field of emerging PV have worked their way back to deliver a high efficiency thin-film technology. Within the last year, organometal tri-halide based perovskites have risen to become a very promising PV material, primarily evolving from activities in dye sensitized solar cells. In previous embodiments the highest efficiency perovskite solar cells have been obtained by incorporating a mesostructured composite8,9,10,11. We have recently discovered that extremely efficient solar cells can be constructed in a thin film planar heterojunction architecture via both solution and vapour phase deposition methods. Here I will present our recent progress with both meso-superstructured and planar heterojunction perovskite solar cells. Specific attention will be paid to the critical aspects of uniform and continuous perovskite film formation and the nature of the n and p-type contact regions. Through a combined device and spectroscopic study we have estimated the charge and exiton diffusion length within the perovskite absorber film which will be discussed.

In excitonic solar cells, where the primary photoexcitation is a bound exciton, a hetorojunction is needed to provide enough driving force to generate free charges. Unfortunately, this results in intrinsic energetic losses, which although conceivably surmountable have lead to relatively slow progress in efficiency over the last decade. Noteworthy, in the last year, the scientific community involved in the development of ”emerging” solar cells have realised a succession of breakthroughs employing ionic crystalline assemblies assuming a perovskite structure. Perovskites have been reported replacing the dye in dye-sensitize solar cells (DSC) with liquid-electrolyte based (power conversion efficiency, eta;= 6.5%)[1] and solid state cells with spiro-OMeTAD and conjugated polymers as the solid-state hole conductor (eta; over 9 %) [2], or as hole-conductors (eta; = 8.5%)[3]. These devices have generally shown impressive photocurrent generation, while the photovoltages achieved still indicated some significant losses. One particular device concept, where the mesoporous TiO2-perovskite heterojunction is removed, eludes this trend: the “meso-superstructured solar cell” that sees an organometal mix-halide perovskite, CH3NH3PbI3-xClx, employed as light harvesting and electron transporting layer and a spiro-OMETAD as hole transporter. A mesoporous Al2O3 layer is employed as insulating “scaffold” upon which the perovskite is deposited. The device exhibits exceptionally high open-circuit photovoltages of over 1.1 volts, despite the relatively narrow absorber band gap, which lead to a record eta; of 10.9% under standard conditions [4]. As the operation of these cells is quite different to the standard DSC device, a number of questions need to be answered. Here we will examine the effect of Chlorine doping on the optoelectronic properties of the CH3NH3PbI3 compound, with a particular focus on the functionalities of the principal interface in the device, i.e. the perovskite/Hole Transporter juction (HTM). The nature of the primary photo-excitation will be unveiled and the effect of crystal structure and thin film morphology on the opotolectronic processes of the photovoltaic device will be elucidated.
[1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 6050 (2009).
[2] J. H.Heo, S. H. Im, J. H. Noh, T. N. Mandal, C-S Lim, J. A. Chang, Y. H. Lee, H. Kim, A. Sarkar, Md. K. Nazeeruddin, M Gratzel and S. Seok, Nature Photonics, DOI: 10.1038/NPHOTON.2013.80
[3] I. Chung, B. Lee, J. He, R. P. H. Chang, M. G. Kanatzidis, All-solid-state dye-sensitized solar cells with high efficiency. Nature 485, 486 (2012).
[4] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science, 2012, 338, 643-647.

Mesoscopic perovskite solar cells based on methylammonium lead halides have recently achieved power conversion efficiencies of over 14% after only a couple years of research by a few research groups. These devices have primarily used device architectures based on solid-state dye-sensitized solar cells where the hybrid perovskite material is used in place of a dye molecule as the light absorbing material. However, the optoelectronics properties of these perovskite materials are not well understood and consequently it is not yet clear what the optimal device structure for perovskite solar cells should be. We perform a spectroscopic and electrical study of the family of mixed lead and tin halide perovskites CH3NH3PbBrxI(3-x), and CH3NH3PbBrxI(3-x), where the bandgap of the material can be tuned by varying the bromine/iodine ratio. We find that stable single phase alloys exist for the mixed halide compounds. From time resolved photoluminescence measurements, we measure long charge carrier lifetimes of several 100s of nanoseconds for the lead perovskites, and find that the lifetime is dependent on templating substrates, processing conditions and halide stoichiometry. We also examine how the carrier mobility, lifetime and concentration depend upon extrinsic doping levels. These studies suggest that a planar p-i-n architecture with carrier selective contacts may be the best design for perovskite solar cells.

We reported an ionization potential (IP) dependent air exposure effect on the MoO₃/organic interface energy level alignment by carrying out in situ ultraviolet photoelectron spectroscopy and synchrotron light based X-ray photoelectron spectroscopy investigations. The electronic structures at MoO₃/organic interfaces comprising various π-conjugated small organic molecules with different IP on MoO₃ substrate have been systematically investigated. For the molecules with low IP, MoO₃/organic interface electronic structures remained almost unchanged after air exposure. In contrast, for the molecules with high IP, the highest occupied molecular orbital (HOMO) leading edge (or hole injection barrier) increases gradually with the increasing molecule IP after air exposure. For the MoO₃/copper-hexadecafluorophthalocyanine (F₁₆CuPc, IP: ~ 6.58 eV) interface, air exposure can induce a significant downward shift of the HOMO level as large as ~ 0.80 eV. Our findings could have great implications when using MoO₃ as an effective hole-injection-layer via low-cost device manufacturing under low vacuum or non-vacuum conditions.

One technique to harvest a wide spectral solar energy is to stack different band-gap materials in a so-called tandem cell geometry. Here, we demonstrate that aqueous solution graphene oxide can be incorporated as an intermediate layer in tandem solar cells. Using aqueous solution graphene oxide, a high open circuit voltage (Voc) of 1.62 V and a high short-circuit current density (Jsc) of 8.23 mA/cm2 could be obtained in series connection. The obtained value of Voc in the tandem cell is very close to the summation of Voc&’s from the front and rear cells. Moreover, our tandem cells also retain 80% of its efficiency under 2880 h continuous illumination.

Solid-state dye-sensitized solar cells (DSCs) have attracted a lot of attention owing to their advantages compared to liquid-electrolyte DSCs such as ease of fabrication and the absence of corrosive electrolytes that are typically used in liquid-electrolyte cells. However, incomplete electron injection from the dye, fast charge recombination between electrons in the titania nanoparticle films and dye/hole conductor cations, poor pore-filling, and the limited thickness of the titania films necessary for efficient charge extraction currently limit the power conversion efficiency. Here, we compare the charge generation and recombination processes of different acceptor type dyes based on the cyclopentadithiophene (CPDT) donor building block that we studied by steady-state photoinduced absorption (PIA) and ultrafast broadband transient absorption spectroscopy (TAS) covering the sub-picosecond to millisecond time range. Specifically, we report on the effect of additives such as Li-TFSI on the charge carrier injection and compare the exciton and charge carrier dynamics of the different dyes attached to titania and also in device-like structures including the solid-state hole transport Spiro-MeOTAD.

Organic solar cells are at the rim of becoming competitive with the standard silicon solar cells. They can be manufactured at presumably much lower energy input. The major scientific drawback at present is the still comparatively low efficiency of the system (even though 12% have been reached recently). This has three major reasons, first is the high binding energy of the exciton in the receiving dye, the next is the loss in energy per particle on its way out of the optically active layer of the device due to the respective work functions of the multiple layers in series (limiting voltage), and third are recombination mechanisms in the layers and at their interfaces (limiting current). In this presentation we demonstrate the usefulness of carbon nanotubes (CNT) as a charge channel and potential anode of the system. The multiple physical challenges in using them in organic solar cells will be discussed. Vertically aligned forests of CNT represent a feasible device structure as will be demonstrated.

Aerogels are attractive structures due to their high surface area, high porosity and particle interconnectivity. These structures are made via a sol-gel synthesis, where we use the epoxy-assisted method to produce a monolithic gel. Several metal oxide, conductive aerogels have been synthesized but all with relatively narrow bandgaps, which is undesirable for applications in the dye-sensitized solar cell due to electron-hole pair recombination. Wide bandgap, fast electron mobility materials such as ZnO or SnO2 have been synthesized as aerogels and are promising as photoanodes in DSCs. We synthesize fluorine-doped tin oxide (FTO) aerogels via epoxy-assisted sol-gel synthesis and introduce doping by means of a fluorine precursor in solution. FTO has been widely studied and is commonly used as a transparent conductive oxide for applications in solar cells. In order to make an aerogel, the gels are dried supercritically to avoid the collapse of the structure as the solvent is withdrawn. This is achieved by a solvent-to-liquid CO2 exchange, where the latter is brought above its supercritical point. Here we present a study on the effect of blocking layers and their thickness on the electron recombination in the DSC. Nanometer-sized layers of TiO2 and Al2O3 are deposited via atomic layer deposition, inhibiting charge recombination from the conductive FTO aerogel to the redox couple. Scanning electron microscopy images show the film&’s morphology. X-ray diffraction is used to obtain data relevant to the crystalline phases of the material and the crystallite size. X-ray photoelectron spectroscopy is used to quantify the amount of Sn, O and F in the aerogel film and a 4-microprobe system, aided by an optical microscope, is used to calculate the conductivity of the material with respect to doping levels and sintering temperatures. DSC performance is analyzed by current vs. voltage curves under one sun illumination. Transient measurements are used to confirm improved electron recombination kinetics by the addition of the blocking layers.

Hybrid photovoltaic devices consisting of inorganic and organic materials require a large surface area to maximize charge separation at the interface. Zinc oxide (ZnO) and silicon (Si) nanostructures were grown by pulsed laser deposition (PLD) at elevated temperatures. In addition to PLD metal induced etching of silicon was used to fabricate Si nanowires. The morphology and structural quality of the nanostructures were characterized by scanning electron microscopy, Raman backscattering spectroscopy, x-ray photoelectron spectroscopy, and photoluminescence measurements.
The nanostructures, and for reference planar samples, were spin-coated with poly-(3-hexyl-thiophen) (P3HT), poly-(3-[3,6-dioxaheptyl]-thiophen (P3DT), poly-(3-[2,5,8-trioxanonyl]-thiophen (P3TT), and Poly(9,9-dioctofluoren-2,7-diyl-co-N-(4-methylphenyl)-diphenylamin. To complete the solar cells, a top electrode consisting of transparent MoO3 and/or gold was deposited on the polymeric layer. The devices were characterized by I-V measurements in the dark and under illumination. For planar silicon with a P3HT, P3DT, and P3TT coating a conversion efficiency of 3.2 %, 5.8 %, and 8.5 % was achieved, respectively. Information on the homo and lumo energy levels were obtained from XPS and UPS measurements and compared to values determined from density functional theory calculations.

ZnO, a n-type wide bandgap semiconductor, is frequently used as electron transport layers in organic photovoltaic (OPV) devices and as acceptors in hybrid photovoltaic (HPV) devices. Various groups have shown that variations in ZnO work function or bandgap can significantly impact device performance, but efforts to understand the precise correlation between ZnO electronic properties and OPV/HPV device physics have been complicated by the fact work function and bandgap could not be controlled independently. Here, we present a method to independently control ZnO work function and bandgap, by microwave assisted synthesis of ZnO nanoparticles in the presence of two different additives, diethanolamine (DEA) and water, respectively. From UV-vis spectroscopy, work function measurement, and transmission electron microscopy, we demonstrate that DEA improves the dispersion of ZnO nanoparticles and lowers the work function from 4.4 to 3.9 eV through the formation of interfacial dipoles, while reducing water content increases bandgap from 3.3 to 3.6 eV by quantum confinement effect without affecting work function. ZnO work function was found to influence the performance of OPV devices using ZnO:DEA nanoparticles as the electron transport layer on P3HT:ICBA inverted OPV devices, whereas ZnO bandgap was found to affect the performance of P3HT:ZnO nanoparticle HPV devices. We will discuss how these results can be understood in terms of charge collection efficiency, characterized via white-light biased external quantum efficiency measurements, intensity modulated photocurrent spectroscopy, and intensity modulated photovoltage spectroscopy.

Electrons injected from dyes excited by light, especially in a visible light region, have to be transported to the current collector in dye-sensitized solar cells (DSCs). Unfortunately, a large number of the electrons are recombined or intercepted while diffusing or drifting through the TiO2 layer. The electron recombination occurs with either oxidized dyes or oxidized species of electrolyte such as I3- at semiconductor TiO2 interface, which is a loss of electrons to be minimized to increase the efficiency. In particular, electron interception by I3- of electrolyte is commonly retarded via several routes: 1) addition of additives in electrolyte such as 4-tert-butylpyridine (tBP) or lithium iodide (LiI), etc.; 2) direct passivation of TiO2 interface with co-adsorbent such as chenodeoxycholic acid (CDCA)[1] and bis-(3,3-dimethyl-butyl)-phosphinic acid (DINHOP)[2]; 3) build-up of shell passivation layer onto TiO2 interface via atomic layer deposition (ALD) or solution dipping.
Herein, three kinds of siloxanes are applied to passivate the TiO2 surface as coadsorbents with a simple solution dipping process and consequently prevent the electron recombination. This is because the polymerized layer formed by coadsorbents is thick enough to hinder an I3- contact with electrons in the TiO2 surface. Interestingly, the reduced recombination does not guarantee the improvement of overall power conversion efficiency in some cases. These phenomena have been interpreted by the dye-coadsorbent interactions, which presumably make the efficiency of dye reduced, as evidenced by the dye peak shift of the UV-vis spectra.
[1] A. Kay and M. Grätzel, J. Phys. Chem., 1993, 97, 6272.
[2] M. Wang, X. Li, H. Lin, P. Pechy, S. M. Zakeeruddin and M. Grätzel, Dalton Trans., 2009, 10015.

Quantum dots (QDs) have enormous potential for a variety of applications based on their unique properties such as slow cooling rate of hot carriers, multiple exciton generation, tunable absorption/emission and solution processability [1-4]. Of the many QD families studied to date, silicon QDs are one of the most promising choices because of their low cost, and non-toxic/environment friendly properties. Two of the challenges for Si QDs in solar cell applications are its weak absorption in visible spectrum and low charge mobility.
We have performed computational studies indicating that small (1-3 nm) Si QDs with strategic organic conjugated capping groups can result in strong, direct absorption in the visible and near infrared range involving the HOMO of the capping units and the LUMO of the Si QDs. An additional short conjugated bridging group can improve subsequent charge transport between the dots. In such a SiQD/organic hybrid system, both efficient energy absorption and energy transfer could be achieved.
This presentation will describe the interplay of our computational analysis, synthesis and characterization efforts to create to SiQD/organic hybrid systems with improved absorption and transport properties. We will show results for several alkyne functionalized conjugated groups that react with the silicon hydride periphery of Si QDs under hydrosilylation conditions. This results in an alkene group bridge that allows for electron delocalization between the organic moiety and Si QD.

Plastic substrate-based flexible DSSCs have been focus of several investigations over the past few years. The main challenge associated with the fabrication of flexible DSSCs is identifying techniques which reduce the processing temperature because polymeric electrode substrates undergo structural or chemical change at higher temperature. Also, long-term stability of photovoltaic device is just as important as the efficiency for practical applications. We prepared 0-D hierarchically structured TiO2 (HS-TiO2) spherical particle with an average diameter of 600 nm on a flexible ITO-PEN (polyethylene naphthalate) substrate via electrospray deposition using a binder-free dispersion of commercially available TiO2 nanocrystalline powder (P25, Degussa) in order to fabricate flexible photoelectrode under low-temperature (<150 °C) conditions. The HS-TiO2 film formed by this method yielded a large surface area and a highly porous structure. The conversion efficiency of the as-sprayed HS-TiO2 particles on the ITO-PEN substrate was poor due to poor inter-sphere connectivity among the as-sprayed HS-TiO2 spheres. The cell efficiency increased when using ITO-PEN substrates post-treated by either a mechanical compression treatment or a chemical sintering treatment using titanium n-tetrabutoxide (TTB) to enhance the inter-sphere connectivity. The mechanical compression treatment this treatment also improved the inter-sphere connectivity and physical adhesion between the HS-TiO2 and ITO-PEN substrate. The photocurrent density and electron diffusion coefficient of the as-pressed HS-TiO2 improved upon compression treatment, whereas the recombination lifetimes remained unchanged. An additional chemical sintering post-treatment involving TTB was tested for its effects on DSSC efficiency. The freshly coated TiO2 submitted to TTB hydrolysis in water at 100°C yielded an anatase phase. Additional TTB low-temperature post-treatment of the HS-TiO2 cell after compression treatment yielded faster electron diffusion, providing a maximum efficiency of 5.57% under 100 mW cm-2, AM 1.5 global illumination. Finally, we revealed the binding mode of N719 on low-temperature processed HS-TiO2 surface and demonstrated strongly bound of N719 dyes on HS-TiO2 surfaces using stearic acid (SA) as co-adsorbent. We found that the SA-treatment enabled the dye to anchor strongly on TiO2 surface and impeded desorption of anchored dye from TiO2 surface from the analysis of FT-IR and transient absorption spectroscopy. Also, SA-treated HS-TiO2 DSSCs extended the long-term stability. This simple, cost-effective method using different types of post-treatment shows promise for the fabrication of highly efficient as well as highly stable flexible DSSCs.

Dye-sensitized solar cells (DSSCs) appear to be a highly promising and cost-effective alternative device for energy conversion. Recently, many groups have developed metal-free organic sensitizers due to its own advantages: 1) facile design and synthesis, 2) high molar extinction coefficient, and 3) tunable electrochemical properties through molecular design. Moreover, some groups obtained power conversion efficiency of ~ 10%. Most metal-free organic sensitizers synthesized an electron donor-π-conjugated linker-electron acceptor (D-π-A) structure with an anchoring group to achieve broad light absorption and well matched energy level with conduction band of semiconductor. Here we have synthesized two different metal-free organic dyes having same moiety of hexyl thiophene and thienothiophene as π-conjugated linker in order to investigate the effects on device performance induced from configurational difference of dyes. We found that position of alkyl chain and thiophene moiety affect spectral response and degree of dye aggregation. The trend of spectral response is consistent with the trend of photocurrent density of assembled cells. The photovoltage of organic dye which contains alkyl chain near acceptor group showed higher value than the dye which contains alkyl chain near donor group. In order to rationalize photovoltage difference, recombination lifetimes of these two dyes have been calculated using intensity-modulated photovoltage spectroscopy (IMVS) measurement with different concentration of co-adsorbent. It is concluded that recombination reaction can be delayed by preventing dye aggregation using high concentration of co-adsorbent. Also, we found that alkyl chain near acceptor can hinder formation of dye aggregation effectively. Finally, we optimized the concentration of co-adsorbent and thickness of TiO2 layer. A power conversion efficiency of 6.48% was obtained under 100 mW cm-2, AM 1.5G illumination condition with a short-circuit photocurrent (Jsc) of 14.46 mA cm-2, open-circuir voltage (Voc) of 0.663 V, and fill-factor (ff) of 0.677.

Conjugated polymer-based photovoltaic (PV) device has attracted attention in the past decade due to its solution processability and easy fabrication. A nanoarchitectural hybrid polymer solar cell, integrating the ordered and the bulk heterojunction hybrid polymer solar cells, is fabricated by infiltrating a solution containing diethylzinc, Ag or Au nanoparticles (NPs), and poly(3-hexylthiophene) (P3HT) into the interstices of the TiO2 nanorod (NR) array. An inorganic network composed of tiny ZnO nanocrystals is in-situ formed within the interstice of the single-crystalline TiO2 NRs. The effects of Ag or Au NPs incorporation into the in-situ generated ZnO/P3HT hybrid layer on the solar cell performance are investigated in this work. Time-resolved photoluminescence (TRPL), electrochemical impedance spectroscopy (EIS), and external quantum efficiency (EQE) spectra were employed to characterize the carrier dynamics and the light harvest in the solar cell with metallic NPs. The results will be discussed in the presentation.

PEDOT:PSS - n Si hybrid solar cells have shown promise at reducing the costs of solar cell processing by removing the energy intensive dopant diffusion step in forming the emitter. Instead, the work function difference between PEDOT:PSS and n-type Si forms the electric field needed for charge separation. We show that, rather than behaving as a simple Schottky solar cell, as suggested in the literature, PEDOT:PSS - n Si solar cells are, in fact, inversion layer devices. We demonstrate that, for suitable substrate doping concentrations, the Si reaches strong inversion. The impact of this with respect to optimal device design, as well as the implications of the selectivity of the organic contact in this model, will be discussed.

Techniques for the synthesis of well-ordered hybrid materials with 10-nm length scales are highly desired but currently lacking. We make use of the natural length scale (typically 10-15 nm) of the alternating crystalline (lamellar) and amorphous layers that are generally found in semi-crystalline polymers to direct the growth of a semiconducting metal oxide. As semi-crystalline polymer we use a carboxylic acid end-functionalized poly(3-hexylthiophene) and we diffuse ZnO precursor vapours (diethylzinc and water) into the amorphous parts of the lamellar stack. Growth of the metal oxide nanostructure is initiated at the carboxylic acid groups, which reside in the amorphous domains. Sheets of ZnO up to 0.5 µm in size can be grown within the polymer film, spaced by the interlamellar distance of 12 nm. This P3HT-ZnO nanostructure functions as a donor-acceptor photovoltaic system, with length scales appropriate for light capture in the P3HT and charge photogeneration at the heterojunction, showing external quantum efficiencies up to 33% for a 30 nm active layer. This photovoltaic system exemplifies this new and general method for the controlled growth of 10-nm scale hybrid nanostructures.

The improvement of solar cell performance in the near-infrared (near-IR) region is an important issue to increase power conversion efficiency. Quantum dots (QDs) based on compound semiconductors are promising materials for solar cells since the position of their exciton absorption bands can be tuned by selecting suitable semiconductor compositions, QD sizes and so on, thereby covering the entire solar spectral region. PbS quantum-dot-based solar cells with high efficiency in the near-IR region were constructed by combining ZnO nanowire arrays with PbS QDs, which give a first exciton absorption band centering at wavelengths longer than 1 mu;m. The 150-nm-thick and dense ZnO seed layers were first formed on F-doped SnO₂ (FTO) glass substrates. Four different seed layers were deposited either by 1) spraying zinc acetate dihydrate in methanol (10mM) to the FTO glass substrate heated at 275 °C or 450 °C, 2) by spin-coating the zinc acetate dihydrate in methanol (0.5M) followed by thermal-treatment at 350 °C, or 3) by the combination of the two methods. The ZnO nanowire arrays with different thicknesses (from 500 to 1900 nm) were grown on the seed layers at 90 °C by a liquid-phase growth method. The entire inner space of the ZnO nanowire array was filled with PbS QDs by the spin-coating method. A flat PbS QD layer was then spin-coated onto the top surface of the PbS-QD:ZnO-nanowire hybrid layer, and the Au back contact was formed to complete the solar cell fabrication.
Incident photon-to-current conversion efficiency (IPCE) action spectra of solar cells were measured using a monochromator equipped with a 150-W Xe-arc lamp. The current density-voltage (J-V) characteristics were measured under simulated AM 1.5G irradiation (100 mW/cm²) using a Class-A solar simulator to obtain open circuit voltage (V_oc), short-circuit current density (J_sc), fill factor (FF), and power conversion efficiency. The J-V characteristic measurements were performed on the solar cells using a mask having a 2-mm-diameter aperture. All the measurements were carried out on the solar cells in air.
The diameter and density of ZnO nanowires in the array depended on the surface structures of the seed ZnO layers. High light harvesting efficiency and efficient exciton diffusion were achieved by employing the 1200-nm-thick arrays composed of densely-grown ZnO nanowires (estimated density of 57 wires / mu;m², and the average diameter of 26 nm) deposited on the surface of spin-coated ZnO seed layers. The QD-based solar cells gave the maximum IPCE of 58% in the near-IR region (@1020 nm), and the power conversion efficiency of 6.078% (J_sc = 34.47 mA/cm², V_oc = 0.361 V, FF = 0.488). Moreover, the solar cells were confirmed stable in air. The power conversion efficiency of the solar cells was found to be almost constant in the course of the 1000-h stability test carried out without illumination in air.

New semiconductor materials, manufactured by low cost, solution-based deposition of colloidally synthesized semiconductor nanocrystals (NCs) are of ever growing interest for optoelectronic device applications. Although the power conversion efficiencies of NC-based solar cell are steadily increasing [1], performance must still be increased for commercial application.
In traditional semiconductor research, it is well established that device performance is limited by the presence of electronic states within the band gap. These states act as charge carrier traps and - even in very small concentrations (<< 1ppm) - can alter the electrical properties of the material, such as mobility, carrier recombination rates, and the Fermi energy. The significance of trap states in devices incorporating NC solids is gaining recognition; however, systematic and quantitative understanding is still lacking.
We implement a multi-technique approach to characterize trap states in NC solids and relate our findings to the measured NC-based solar cell performance. Specifically, we implement deep level transient spectroscopy (DLTS) [2], impedance spectroscopy, and photocurrent spectroscopy on PbS NC solids, which one of the best characterized NC-based material for solar cell applications. We use these techniques to study the impact of aging and different chemical treatments on the number density of traps and the solar cell performance.
[1] A. H. Ip et al., Nature Nanotechnology (2012).
[2] D. Bozyigit, M. Jakob, O. Yarema, and V. Wood, ACS Applied Materials & Interfaces (2013).

Single-walled carbon nanotubes (SWCNTs) are dimensionally confined quantum wires that have the potential to impact a variety of next-generation energy storage and generation devices. The ability to incorporate SWCNTs into photovoltaics requires an understanding of how SWCNT electronic structure affects exciton dissociation and charge carrier recombination. One important consideration for SWCNT PV blends is the high degree of electronic poly-dispersity present in as-synthesized SWCNT samples. This poly-dispersity implies the presence of potentially deleterious SWCNT species (zero-gap metallic SWCNTs), as well as a wide variation in the diameter-dependent absorption energies, electron affinities, and ionization potentials for the semiconducting species present.
In this study, we utilize time-resolved microwave conductivity (TRMC) to follow exciton dissociation and charge recombination in systems where photoexcited SWCNTs act as the electron donors. In these blends, dissociation of photoexcited SWCNT excitons at the interface between the SWCNTs and fullerenes has been shown to generate high internal quantum efficiencies for solar conversion in thin-film PV devices. Microwave conductivity is used to follow charge generation in films containing one, or several, species of semiconducting SWCNTs. The range of samples allows us to explore the influence of energetic dispersion on exciton transport to the interface, exciton dissociation efficiency, and interfacial charge recombination.
In samples with several different semiconducting SWCNT species, we observe a free carrier yield that depends on SWCNT diameter. Above a SWCNT diameter of ~1.0 nm, the exciton dissociation yield decreases significantly, in agreement with previous EQE measurements on devices made with the same blends. This drop-off in exciton dissociation yield indicates a low energetic driving force for electron transfer from the larger diameter SWCNTs to C60. The lifetime of free charges produced by exciton dissociation of photoexcited large-bandgap SWCNTs is on the order of microseconds, but depends sensitively on the presence of small-bandgap SWCNTs that may serve as charge traps. Thickness-dependent measurements indicate an exciton diffusion length that is much shorter than typically observed for isolated SWCNTs. I will also discuss efforts aimed at quantitatively determining the yield for free charge carrier generation and mobility through studies utilizing controlled doping of dark carriers into the SWCNTs. These studies compliment the information obtained through device studies by providing information on charge carrier generation and recombination in the absence of carrier collection, and can be used to develop routes towards further device improvements.

Hybrid organic-inorganic (OI) semiconductor devices have been investigated extensively, including work on dye-sensitized solar cells (DSSCs), [1] as well as junctions between archetypal molecular organic and crystalline inorganic semiconductors. [2] They exhibit interesting fundamental physics and potentially enable new avenues for improved solar energy harvesting. Here we examine the photovoltaic behavior of a hybrid p-n junction between silicon and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) as a function of applied bias, illumination intensity, and temperature. Current density-voltage (J-V) measurements performed at large reverse bias show that breakdown occurs at 175 V (occurring first in silicon), indicating that PTCDA forms an excellent junction with silicon. We observe a short-circuit current, J_sc, in excess of 14.0 mA/cm², open-circuit voltage, V_oc, of 0.44 V, and power conversion efficiency, PCE, of 3% at one Sun illumination. We study how J_sc varies with respect to PTCDA thickness using a coupled absorption and transport model, validated by experiments. The performance as a function of PTCDA deposition rate was also studied, linking rate-dependent crystallinity with PCE. The EQE contributions of silicon and PTCDA are established by fitting EQE measurements of this device to the absorption and transport model. We observe uniform broadband enhancement of the EQE spectrum in reverse bias, possibly attributed to a shared dissociation step in the photocurrent generation process. Measuring the J-V response over the temperature range of 78-300 K shows an Arrhenius behavior over an illumination intensity-dependent temperature range, with an activation energy on the order of 50 meV. The current density-voltage (J-V) characteristics are fit to a model for hybrid devices that considers the electron-hole dynamics at the OI interface. Possible interpretations of the activation behavior are discussed.
REFERENCES
[1] B. O&’Regan et al., Nature, 353, 6346 (1991)
[2] S.R. Forrest et al., J. Appl. Phys. 55, 1492 (1984)

Photon management is critical for improving the optical absorption of thin film solar cells and their power efficiencies. The thin active layer is especially appropriate to low-mobility hydrogenated amorphous Si (a-Si:H) materials as it lessens the need for long-diffusion length for photogenerated charges, and consequently mitigating light-induced degradation. In addition, electron-hole recombination is strongly suppressed in the thinner active layer of intrinsic a-Si:H, allowing one to focus on studying the effect of various photon management schemes.
Conventional a-Si:H solar cells utilize p- and n- doped layers, which require high temperature process and minimum thickness for the active layer. The photons absorbed in the doped layers are quenched by the dopants, not contributing to the electrical current of the solar cells. To address these issues, we exploit ultra-thin intrinsic a-Si:H solar cells without any doped layer but utilizing inorganic and organic charge transport layers for the anode and cathode, e.g. V2O5 for holes/anode and PCBM for electrons/cathode.
Our ultra-thin a-Si:H solar cells have an intrisic a-Si:H layer of only 5 nm ~ 30 nm, which is an order magnitude thinner than the traditional a-Si:H active layer (180 nm). Optical simulation compared the optical absorption spectrum with external quantum efficiency (EQE) to understand the effectiveness of photon management in the EQE spectrum without any loss of the absorbed photons, benefiting current density. By optical design of cavity structure, a 15 nm a-Si:H active layer cell can absorb 13 % more photons than a 30 nm cell. The absorption increase is proved in experiment by the enhanced short circuit current density (Jsc) of 6.9 mA/cm2 in the 15 nm cell compared to 6.1 mA/cm2 in the 30 nm cell. Remarkably, the Jsc performance in the ultra-thin 15nm cell is around a half of Jsc (13.3 mA/cm2) in the much thicker 180 nm cell.
Interestingly, in the ultra-thin devices, open circuit voltage (Voc) was reduced from 0.8 V of the thick device (180 nm) with the increased series resistance while shunt resistance and fill factor were improved. The amount of the Voc decrease depends on the thickness of intrinsic a-Si:H active layer. In order to understand the cause of the Voc variation, we studied the effect of the interfacial defects upon the optical field intensity profile in the active layer. The interfacial defects were measured by capacitance-frequency characterization.
We also discovered that the thickness of intrinsic a-Si:H active layer can be optimized by managing the photon absorption in the ultra-thin a-Si:H layer (under 30 nm), eventually leading to over 3 % power efficiency. Based on these results, we will also present our studies to various photovoltaics integrated window applications.

Despite its long history of being a popular ‘lab rat&’ for over two decades, Poly(3 hexylthiophene) (P3HT) remains one of the most exciting/puzzling materials in the realm of organic semiconductors. Its optical and electronic properties are known to vary drastically under different processing conditions. The photoluminescence spectrum alone changes in different solvents as well as in different film morphologies due to interactions of the chromophores with their immediate environment. The formation of aggregate structures has been implied to have an important influence on the optical as well as electronical properties in that regard.
Whereas conventional optical spectroscopy leaves the role of individual polymer chains hidden in the ensemble, single molecule spectroscopy can be a powerful tool in revealing the properties of single entities comprising the bulk material.
P3HT, known for its relatively low photoluminescence quantum yield and a high charge formation yield makes optical detection of single chains an extraordinarily challenging matter. Single molecule spectroscopy carried out at low temperatures (4 K) provides stable conditions to accomplish this task. We employ the advantage of this technique to arrive at insight into individual building blocks of P3HT and unravel the disorder in the bulk.
Single chromophoric units of P3HT exhibit blinking as well as surprisingly stable temporal behavior with spectral linewidths narrowed down to only a few meV. Whereas organic dyes or polymer materials like polyfluorene show inhomogeneous broadening of about 20 nm, we observe a constant linewidth and vibronic coupling energy of single chains over an unprecedented spectral range of almost 200 nm (~0.7 eV). Our results lead to a better understanding of the morphology of single chains and may clarify the necessity of the aggregate picture.

Recently, Sb2S3-sensitized mesoporous-TiO2 solar cells using several conjugated polymers as hole-transporting materials (HTMs) are reported by us [Nano Letters, 10, 2609 (2010); Nano Letters, 11, 4789 (2011)]. However, the cells still suffer from a low open circuit voltage (Voc), responsible for the power conversion efficiency (PCE). Impedance spectroscopy results [ACS Nano, 6, 873 (2012)] showed that Sb2S3 themselves induces efficient recombination pathways between TiO2 and Sb2S3. Thus, this recombination has to be avoided in order to improve the Voc of these cells and consequently their PCEs. In this work, we introduce a new simple approach of surface treatment to minimize recombination pathways in Sb2S3 through the sulfurization of the surface of Sb2S3 with organosulfur solutions. The sulfurization is conducted by spin coating of organosulfur solutions on surface of crystalline Sb2S3 and subsequent annealing before the deposition of HTMs. We find that the sulfurization reduces the recombination pathways by improving the crystallinity of Sb2S3 and by compensating the sulfur loss. This leads to the increase of PCE up to > 7 % (the highest value thus far reported) under full sunlight by enhancement of VOC as well as short circuit current density JSC, which is 10-30% higher than that obtained from the cells without sulfurization. In addition, our approach can be applied to other metal chalcogenide-based solar cells, such as Sb2Se3-based cells. The origin on the PCE improvement will be discussed.

Bulk heterojunction polymer solar cells have attracted considerable attention because of their advantages in being low-cost, lightweight, flexible and environmentally friendly. We have previously studied electron collection layers such as titanium oxide, zinc oxide (ZnO), and zinc sulfide for inverted polymer solar cells. Herein, we discussed the UV effects based on the results of the alternating current impedance spectroscopy (IS) measurements in addition to the photocurrent-voltage (I-V) measurements. We examined the relationship between the heating temperature of the ZnO electron collection layer and the device performance under simulated sunlight irradiation while cutting the UV light. An increase in the crystallinity of the ZnO layer caused the decreased device performance under the irradiation of simulated sunlight without UV.
The cell fabrication was carried out in air, with the relative humidity controlled at less than 35%. A ZnO solid film with thickness of about 60 nm was prepared on the clean FTO substrate using the sol-gel method. The ZnO precursor solutions were prepared in 2-methoxyethanol, which contained Zn(acac)2 and acetylacetone. The solutions were spin-coated onto the FTO, and the precursor films were heated at 150 ~ 450 °C for 1 h. A mixed chlorobenzene or dichlorobenzene solution containing P3HT and PCBM (weight ratio = 5:4) was spin-coated onto the FTO/ZnO. A PEDOT:PSS dispersion in water containing 0.5 wt.% of Triton-X 100 was spin-coated onto the PCBM:P3HT layer. The film thicknesses were about 250 nm for the PCBM:P3HT layer and approximately 150 nm for PEDOT:PSS. The Au back electrode with a thickness of about 150 nm was vacuum deposited at 2×10-5 torr on the PEDOT:PSS layer. Finally, the cells were annealed at 150 °C for 5 min.
When the ZnO cells were irradiated with simulated sunlight, they exhibited a maximum power conversion efficiency (PCE) of over 3%, which hardly varied with the heating temperature of ZnO layers treated at 150 oC, 250 oC, 350 oC, and 450 oC. In contrast, when the ZnO cells were irradiated with simulated sunlight without UV content, their photovoltaic characteristics were very different. In the case of the cell with ZnO prepared by heating at 250 oC, PCE of 2.7% was maintained even under continuous irradiation with simulated sunlight without UV. However, for the cells with ZnO prepared by heating at 350 oC and 450 oC, the shapes of the I-V curves changed with the UV-cut light irradiation time, accompanying an increase in their series resistance. Overall, after UV-cut light irradiation for 1 h, the PCE of the cell with ZnO prepared by heating at 350 oC decreased to 1.80%, while that of the cell with ZnO prepared by heating at 450 oC fell to 1.35%. The photo IS investigations suggested that this performance change was responsible for the formation of charge-trapping sites at the ZnO/PCBM:P3HT interface which act as recombination centers for photo-produced charges in the PCBM:P3HT layer.

Silicon quantum dots (SiQDs) with diameters in the range of 1.2-2 nm are promising for the large-scale manufacture of new optoelectronic devices. This is due to their tunable optoelectronic character, nontoxicity, processing know-how and resource abundance [1,2] However, their optical gaps are too large, and current SiQD based solar cells perform poorly due to low carrier mobilities.[3,4] We propose two schemes that have been designed to remedy these deficiencies wherein a type-II interface is established between specially terminated dots and short bridge molecules; the approaches differing in whether or not the bridge molecules are involved in initial charge separation. An electron rich ligand reduces the optical gap and enhances absorption intensity, and high carrier mobility is achieved via a bridge assisted superexchange mechanism. As a proof of concept, we carried out ab initio calculations on two prototype structures for each scheme, and the results suggest that strong direct optical absorption, with redshifts greater than 1 eV, can efficiently generate spatially separated excitons at the dot/ligand interface. We also predict that both local and spatially separated excitons will result in free carriers because the radiative and nonradiative recombination rates are much lower than the charge transfer and carrier hopping rates. The right combination of dot size and externally applied electric field is shown to result in efficient polaron dissociation as well.
References
[1] Lin, Z. et al., ACS Nano 6, 4029, 2012.
[2] Li, H. et al., ACS Nano 6, 9690, 2012.
[3] Niesar, S. et al., Green 1, 339, 2011.
[4] Liu, C.-Y. et al., Nano Letters 9, 449, 2009.

Polymer/inorganic hybrid solar cells have drawn considerable attention as a promising renewable energy source. These hybrid solar cells have several advantages over their pure inorganic counterparts. Hybrid solar cells based on silicon wafers have received great attention due to the possibility of reducing its effective thickness when applied to the traditional photovoltaic devices.
In this work, we present the modification of Silicon/poly-3-hexylthiophene (P3HT) interface through the alquilation of the silicon surface for application in Al/Si/P3HT/Au device architecture. Results from X-ray photoelectron spectroscopy (XPS) showed that the attachment of thiophene monomers to the silicon surface was successful, evidenced by the signal attributed to the sulfur group onto the silicon surface. Photophysical experiments showed the reflectance decreased in the presence of the polymer coating. This result indicates that P3HT acts as an anti-reflective coating for silicon.
For solar cell applications, Schottky, Si/P3HT, and Si/modifier/P3HT devices were assembled in order to evaluate the photovoltaic properties of the Si/P3HT interface. A similar Voc was measured for all PV devices employing P3HT, indicating that this interface contributes to the photovoltaic response. Devices based on Si wafers modified by thiophene alquilation provided overall energy conversion efficiencies up to 5.7%.

Effective optical spacers are required not only to change the spatial distribution of the optical field in photovoltaic (PV) devices but also often to act as charge extraction layers or heterojunctions for charge separation in excitonic solar cells. When designing such spacers consideration of the structural, morphological and electronic properties are required in addition to designing a growth process that enables direct deposition onto functional organic layers and multilayer structures. In the solution phase this may require the use of solvents with orthogonal solubility to the active layers or implementing low-temperature processing routes. From the vapor phase more control of composition and properties can be obtained but control of the energetics of the deposited species are required in addition to any constraints e.g. thermal, which may be imposed owing to the active layers.
In this contribution, we outline our most recent work where we demonstrate the deposition of ZnO optical spacers directly onto poly(3-hexylthiophene-2,5-diyl)(P3HT). Using pulsed laser deposition (PLD) for oxide growth we deposit highly crystalline, transparent (>90 %) ZnO resulting in planar, damage free interfaces with the P3HT - importantly the lack of damage in the polymer layer after deposition is confirmed. The oxide layer also acts as a heterojunction for charge separation and extraction in these simple hybrid excitonic devices.
Our experimental studies reveal variations in the measured PV devices performance as the oxide layer thickness is altered, which we ascribe to field related phenomenon rather that transport issues. Our complementary modeling studies show a 3-fold increase in light absorption in the near interfacial region of the polymer can be achieved by altering the transparent layer thickness. In addition to oxide layer thickness our method allows precise control of the oxide film orientation and stoichiometry - which influence the electronic and charge transport properties. Microstructural and morphological characterization of the polymer and oxide layers are given to support the measured PV performance over a wide range of oxide thicknesses (5 - 150 nm). The simplicity of our device architecture allows a direct comparison of the experiment and the theory, which we hope will have wider implications for more complex geometries.

Due to the large band gap, large electron mobility and the high dielectric constant of zinc oxide
(ZnO), it promises potential in organic-inorganic hybrid solar cells. However, such devices display lower efficiency compared to fully organic devices. Upon photoexcitation, excitons dissociate at the organic-inorganic interface, with the positive charge located on the organic material and the electron on the inorganic semiconductor. These are highly bound due to their mutual Coulomb interaction, and recombination of bound pairs at the interface is considered to be a significant reason for the low efficiency. One possible way to optimize the performance of the hybrid devices and limit this loss mechanism is to modify the interface of organic-inorganic materials. We report the influence of N3 (a commercial dye) and 6TP (an oligothiophene phosphonic acid) as surface modifiers on P3HT:ZnO (P3HT: Poly 3-Hexylthiophene) interfaces by studying charge generation, recombination and charge separation kinetics in P3HT:ZnO, P3HT:ZnO-N3, P3HT:ZnO-6TP via quasi-steady-state photo-induced absorption (PIA) spectroscopy. We observe the signature of charges (polarons) at the interface. We further investigate the kinetics of long-lived polarons formed in hybrid P3HT:ZnO and with either surface modifiers by measuring the dependence of PIA signal on pump modulation frequency. The results show that longer-lived polarons do exist in P3HT:ZnO with surface modifiers molecules than in P3HT:ZnO itself, given that the longer-lived polarons exhibit faster fall off with increasing frequency. This is due to the fact that the surface area between the electron donor and the acceptor is increased and the charge separation is enhanced. We apply transient absorption measurement to study the dynamics of charge separation within ultrafast time scales. Moreover, time correlated single photon counting measurement reveals the more efficient charge separation at P3HT:ZnO-N3, and P3HT:ZnO-6TP interfaces compared with P3HT:ZnO, through the shortening of the measured photoluminescence life time. These results indicate that surface modification influences nonradiative processes.

Two of the most researched optoelectronic materials in recent years have been carbon nanomaterials (CNMs) and inherently conducting polymers (ICPs). Some attractive features of both these materials for their utilization in photovoltaic devices are their environmental stability and tunable electrical properties by doping. When carbon nanomaterials, such as carbon nanotubes (CNTs) or graphene (G), are incorporated with conducting polymers, such as polyaniline (PANI) or poly(3-hexylthiophene) (P3HT), beneficial effects on the conversion of light energy into electrical energy has been observed. In this presentation, we focus on how CNM-polymer composites influence the characteristics of photovoltaic devices.
Our approach has been on synthesis and processing techniques - spin-coating, airbrush deposition, and film transfer - of CNM-polymer composites for use in photovoltaic (PV) devices. We have found that the incorporation of carbon nanomaterials, e.g. single-walled carbon nanotubes (SWCNTs), almost doubles the power conversion efficiency of a hybrid organic/silicon device, which can be attributed to a higher short-circuit current density (~2.5 times greater than the control without ). This has been observed in a layer-by-layer assembly in which silicon substrates have been coated with a layer of PANI and SWCNTs in various configurations. In order to further take advantage of the excellent electronic transport properties of CNMs, G- and CNT-polymer composites were utilized as hole-transport layers and as active layer additives in organic photovoltaic devices. The devices were characterized by current-voltage characteristics for photovoltaic behavior. The use of these composite materials has resulted in promising improvements to PV device characteristics compared to using polymers without carbon nanomaterials.

We report on the fabrication and properties of organicP3HT andPTB7 solar cells on doped a-Si:H films. The work function of n+ a-Si:H matches reasonably with the work function of PCBM and ICBA and as a result, it can be a good electron accepting contact (ETL layer). A-Si:H was doped n-type using PH3 during PECVD deposition. Its bandgap, and electron affinity, can be changed by incorporating C or Ge during growth. The device structure consisted of glass/n+ ZnO/n+ a-Si:H [or n+ a-(Si,C):H] on which either P3HT-PCBM heterostructure, or PTB7-PCBM heterostructure was deposited. This was followed by a hole transport layer, generally MoO3, followed by Al. Excellent devices with very high fill factors were obtained for P3HTdevices. Detailed measurements showed that the devices had defect densities and interfacial recombination velocities similar to those obtained in inverted cells onCsCO3. When the cells were subjected to degradation forma full spectrum xenon simulator source, the devices with a-Si:H contact degraded much less than devices with CsCO3 contact. TheCsCO3 ETL contact devices showed significant changes in defect densities, dark current and interfacial recombination velocity. We examine why this is the case. We also show that pre-treatment of CsCO3, before depositing the organic heterojunction, is critical for achieving better stability.

Organic or organic-inorganic hybrid solar cells have attracted great attention because of their advantages of low-cost process, flexibility, and large-area fabrication. Recently in terms of device configuration, an inverted structure has been steadily investigated due to its potential for better ambient stability. In the inverted structure, electron transport layer having good electron extraction characteristics is an important issue, and there are several n-type transport layers, such as ZnO and TiO2 demonstrated to be capable of extracting electrons to an indium tin oxide (ITO) electrode. Here, we first proposed to employ CdSe tetrapod (TP), which is an inorganic nanocrystal showing high electron conductance, as nanostructured electron extraction layer, and investigated the performance of inverted nanocrystal-polymer:fullerene hybrid solar cells. CdSe TPs were spin-coated on top of In-doped ZnO layer, then low bandgap semiconducting polymer PTB7 and the fullerene derivative PC70BM blend film was prepared to fabricate hybrid solar cells. As a result, solar cell device with CdSe TP showed improved short circuit current density (JSC) of 17.48 mA/cm2, fill factor (FF) of 57.26 %, and PCE of 7.55 %, compared with only In-doped ZnO, and it was attributed to better electron extraction. We expect this approach can be extended to a general platform for improving charge extraction in solar cells.

Dye-sensitized solar cells (DSSCs) have attracted the scientific and technological interest of researchers as a high efficiency and low-cost alternative to conventional inorganic photovoltaic devices. In DSSCs, the counter electrode (CE) is an important component, as it plays a crucial role in the reduction of iodide to tri-iodide ions. Generally, Pt has been used as a counter electrode catalyst in DSSCs because of its good catalytic properties and high conductivity. Despite these benefits, the need for high temperature sintering and high vacuum deposition processes, as well as issues related to corrosion in the redox electrolyte and cost, has spurred research into alternative low-cost CEs with good catalytic properties and a simple fabrication process.
Many efforts have been made to replace Pt with low-cost carbon materials. However, the conversion efficiency is still lower than that obtained with a Pt CE. Another disadvantage of carbon-based CEs is that they are highly opaque and require high temperature sintering. The use of thick carbon films (10-20 µm) also increases the resistance of the device. To overcome these drawbacks, several groups have attempted to replace Pt with conductive polymers such as PEDOT:PSS (poly (3,4-ethylenedioxythiophene:poly (styrenesulfonate)), PEDOT, and PANI (polyaniline).
Typical Pt-less CE materials in DSSCs should be prepared on fluorine-doped tin oxide (FTO) coated glass substrates to have efficient electron transport. The development of a cost-effective counter electrode should simultaneously remove the use of Pt and FTO because of high cost. Previously, our group reported on a PEDOT-based Pt, TCO-free CE fabricated by an in situ chemical polymerization method for use in a DSSC. However, the efficiency obtained with the PEDOT CE was lower than that achieved with a Pt/FTO counter electrode. From a knowledge that the surface area of catalysts is important in the evaluation of their properties. A larger active surface area plays a crucial role in the catalytic activity for iodide to tri-iodide ions. In the case of a polymer counter electrode, the use of a nanoimprinting technique to make nanopatterns has never been used to increase the active surface area of Pt, TCO-free CEs.
The nanopatterned PEDOT CE for a Pt, TCO-free CE in DSSCs. The DSSCs with a nanopatterned PEDOT-based counter electrode exhibited a power conversion efficiency of 7.1%, which is comparable to that of conventional DSSCs with standard platinum Pt/FTO paired counter electrodes. With an increase in the active surface area of the PEDOT film from the simple nanopatterning process, the catalytic effectiveness also increased, which leads to a higher power conversion efficiency. This systematic study gives fundamental information on CEs and will broaden the use of conductive polymers in DSSCs.

We demonstrate chemically exfoliated hexagonal boron nitride (h-BN) nanoflakes as surface-passivating material on wide-bandgap semiconductor TiO2 to enhance the performance of dye-sensitized solar cell. The dominant performance-limiting factors leading to interfacial recombination are suppressed by incorporating h-BN layer on the surface of TiO2. We observe significant enhancement in the performance of solar cell with the use of h-BN surface passivation. It is observed that the ultra-thin nature of h-BN does not impede electronic transport at the interface, while it suppresses interfacial recombination process, leading to enhanced photoelectric conversion efficiency. The chemically exfoliated h-BN nanoflakes are characterized by Raman spectroscopy, confirming the presence of mixed monolayer/few layer stack on TiO2 surface to efficiently minimize the impacts of TiO2 surface states on carrier transport and recombination. Lifetime spectroscopy performed on the solar cells suggests that the lifetime of photoexcited electrons at the conduction band of TiO2 is significantly improved using h-BN passivated TiO2 as compared to TiO2 without surface passivation. The surface states present in the TiO2 bandgap become inactive after h-BN passivation, and hence the probability of electron/hole capturing processes is minimized at the interface. The improved carrier lifetime by h-BN passivation (and hence effective collection at the transparent electrode) is attributed to the considerable improvement in key photovoltaic performance metrics.

Resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) is an enabling technique for organic film deposition. Unlike solution-based deposition techniques, RIR-MAPLE is a vacuum-based deposition technique in which an organic thin film is deposited onto a substrate, without significant damage, by infrared laser ablation (targeting -OH bond) of a host emulsion matrix. It has been demonstrated that RIR-MAPLE offers nanoscale phase separation for multi-phase films[1] and is able to deposit multi-layered structures regardless of material solubility.[2] These two advantages motivate the use of RIR-MAPLE to fabricate hybrid organic/inorganic solar cells (polymer/quantum dot nanocomposite) in order to achieve an ideal nanocomposite morphology to optimize both exciton dissociation and charge transport; and enable a tandem structure of nanocomposite to absorb a wider range of solar radiation.
In this study, we will present the impact of RIR-MAPLE deposition on the performance of hybrid organic/inorganic solar cells. Specifically, we will investigate the influence of emulsion target preparation (choice of surfactant, emulsion type, and emulsion composition) on the resultant film morphology and device performance. The bright field optical images, scanning electron microscopy (SEM) images and atomic force microscopy (AFM) images are used to characterize the surface morphology of the deposited film. Small-angle X-ray scattering are used the study the internal morphology (polymer stacking) of the film. Solar cell performance will be characterized using I-V and external quantum efficiency (EQE) measurement. The emulsion parameters are expected to affect the solar cell performance for the following reasons. First, the introduction of surfactant (ionic vs nonionic) is necessary to make a uniform and stable emulsion target. However, the surfactant is also deposited as a third phase in the film, which affects the charge transport properties of the film. Second, the type of emulsion (oil-in-water vs water-in-oil) affects the deposition by changing the absorption depth of the infrared laser within the target, which determines if evaporative or ablative deposition is dominant. Furthermore, the physical state of the polymer (coiled vs relaxed) is influenced by the type of emulsion and could result in different polymer stacking (edge-on, face-on, or hybrid). Third, the composition of emulsion is studied because it determines how likely the frozen target is to sublimate under vacuum, which could change the target surface composition over time.
In summary, this study will provide general insight from fundamental principle of target chemistry into the capability of the RIR-MAPLE technique to improve performance of hybrid organic/inorganic solar cells.
1. R. D. McCormick, E. D. Cline, A. S. Chadha, W. Zhou and A. D. Stiff-Roberts, Proc. SPIE, 2012,825806.
2. R. Pate, R. McCormick, L. Chen, W. Zhou and A. Stiff-Roberts, Appl. Phys. A, 2011, 105, 555-563.

We have investigated the characteristics of platinum (Pt) catalyst layers, deposited by chemical reduced method, electrodeposition (ED) and sputtering on conductive indium-doped tin oxide coated polyethylene naphthalate (ITO-PEN) for flexible dye sensitized solar cells (DSSCs). Scanning electron microscope(SEM), electrochemical impedance spectroscopy(EIS) are performed to compare the catalytic activities of these counter electrodes(CEs) for the reduction of tri-iodide to iodide. The results demonstrate that the flexible DSSC using electrodeposited Pt on ITO-PEN have superior catalytic activities and electron transport properties than others two tyep CEs. The flexible DSSCs based on ITO-PEN CEs in conjunction with a AN-based nanocomposite gel electrolyte achieved the best efficiency of 6.8% under one sun illumination (AM1.5, Pin of 100 mW cm^minus;2).

Several ways to deposit platinum catalyst layer on titanium metallic sheets for flexible dye-sensitized solar cells (DSCs) have been investigated. Three ways to reduce platinum on the metallic sheets including chemical deposition with NaBh4, chemical reduction with ethylene glycol and pyrolysis are discussed in this paper. The catalytic activities of these counter electrodes for the reduction of tri-iodide to iodide are represented by the electrochemical impedance spectroscopy analysis. And the platinum morphologies on the titanium metallic sheets are also discussed by the scanning electron microscopy image. The results demonstrate the flexible DSCs using pyrolysis deposited platinum on Ti foil achieved the highest power conversion efficiency of 7.15%.

One limitation of traditional dye-sensitised solar cells is the spectral response of dye molecules. Solid-state dyes have been rapidly progressing from a viable alternative to being the state-of-the-art. We focus on a class of dye with the perovskite structure, based on the ABX3 stoichiometry, where X is a halide ion (Cl, Br or I).
Following our recent work on the earth abundant absorber materials Cu2ZnSnS4 [1,2], we performed a computational study to systematically characterize the inorganic derivatives related to the prototype compound CdSnI3, which can be obtained by changing the metals and the halide in the original composition. A route to engineering the electronic properties of this class of materials is identified. Furthermore, the preference for ferroelectric distortions is addressed through the relative stability of the cubic and tetragonal perovskite phases.
Secondly, we have investigated related hybrid perovskite materials, which can be derived from CsSnI3 by replacing Cs with molecular cations (e.g. ammonium). We assess the orientational disorder of molecular cations within the perovskite lattice. In general, the barriers to rotation are small, but coherent ordering of molecular dipoles results in a spontaneous ferroelectric distortion to a series of low symmetry phases.
Finally, we provide a consistent set of crystal properties (mechanical, elastic, dielectric and optical response functions) that will be of benefit to both experimental characterisation and the design of an optimal absorber materials.
1. “Kesterite Thin-Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4” A. Walsh, S. Chen, S.-H. Wei and X. G. Gong, Advanced Energy Materials 2, 400 (2012).
2. “Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Earth-Abundant Solar Cell Absorbers” S. Chen, A. Walsh, X. G. Gong and S.-H. Wei, Advanced Materials 25, 1522 (2013).

Hybrid solar cells based on an organometal halide perovskite absorber, organic hole conductor and metal-oxide are an upcoming new player in the field of third generation photovoltaics. Remarkably, these solar cells have surpassed dye-sensitized and organic solar cells in terms of efficiency on a very short timescale, and thereby position themselves as strong competitors that deserve further investigation.[1-3]
Architecturally, the active layer of a typical perovskite solar cell is very similar to that of a classical hybrid solar cell. A mesoporous metal-oxide structure is infiltrated with a light absorber that also acts as hole transporter, the latter now being represented by the perovskite. It has been suggested, though, that the mesoporous structure merely acts as a (necessary) scaffold to structure the perovskite, which in itself takes care of both hole and electron transport.[1]
In our work we demonstrate the possibility of highly efficient (eta; > 9%) perovskite solar cells without any porous or nanostructured scaffold. Using only a solution processed perovskite absorber, sandwiched between flat metal-oxide and hole conductor, our thin-film approach attains efficiencies comparable to current state-of-the-art devices while rendering the ubiquitous porous metal-oxide structure obsolete. This is a highly desirable asset facilitating large-scale fabrication. The achievement of adequate performance from a dense perovskite film, with selective contacts on either side, demonstrates both the presence and the exploitability of the alleged p-i-n character of such solar cells. Optical and electronic spectroscopic techniques are used to elucidate the working principles of these devices.
[1] Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Science 2012, 338, 643-647.
[2] Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Gratzel, M.; Park, N.-G. Scientific Reports 2012, 2, 591.
[3] Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Kim, H.-j.; Sarkar, A.; K., N.; Gratzel, M.; Seok, S. I. Nat. Photonics 2013, 7, 486-491.

Rapid improvements in efficiency of photovoltaics based on the organometallic perovskites have been recently reported in spite of relatively low (<10 cm^2/Vs) carrier mobilities. Single crystals of these materials have been reported to have mobilities 2-3 orders of magnitude higher than polycrystalline samples, suggesting that control of crystal size and/or orientation may be a key to the next improvements in device performance. However, reports of the fundamental crystallization dynamics and their impact on electronic properties have been scarce. To help bridge this knowledge gap we use in-situ wide-angle x-ray scattering and conductivity/mobility measurements to elucidate the kinetics of the crystal formation as a function of substrate material, substrate surface roughness, and annealing conditions, and correlate the structural evolution to transport properties.

Using pair distribution function analysis of X-ray scattering, we report the structure of methylammonium lead iodide perovskite in mesoporous TiO2, as used in high-performance solar cells. Our findings underscore importance of fully characterizing and controlling the nanostructured components for improved solar cell efficiency.

We reveal the nano-structure of porous perovskite solar cells using transmission electron microscopy and correlate our findings with the device performance.

Structure control in organic-inorganic hybrid perovskite materials is crucial for fabricating highly efficient solar cells. We utilize in-situ Xray scattering to determine and understand the crystallization kinetics and film morphologies of CH3NH3PbI3-xClx perovskite materials. We also demonstrate that incorporation of metal nanoparticles into the perovskite solar cell introduces an unexpected plasmonic effect that increases the current generation and enhances the overall power conversion efficiency.

We show that Al2O3 nanoparticle additions to a CH3NH3PbI3-xClx precursor solution, yields high efficiency organolead halide perovskite solar cells; proving it is possible to co-deposit the alumina/perovskite active layer in a single, low temperature deposition step.

Taking advantage of the unique ability of OIPC&’s to formed VdW layered structures, we describe a new approached to study the electronic properties of OIPC surfaces and interfaces. Through mechanical exfoliation, we isolate atomically thin layers of OIPC. Then, using optical spectroscopy and charge transport measurements we characterize their electronic properties and possible new applications.

We present direct measurements of perovskite valence and conduction band energies, and interface molecular level offsets, by photoemission and inverse photoemission spectroscopy. The interface electronic structure of perovskite / hole transport material systems is mapped in detail, and reveals the origin of hole-transfer mechanisms. These results provide a basis for tailoring material composition for optimized charge carrier extraction.

Methyl ammonium lead halide perovskite solar cells achieve their record-breaking performances by using a high electronic quality absorber (“i”) in a “p-i-n” junction configuration, as shown directly by electron microscopy measurement of electron-voltaic current transport across cell cross sections.

Perovskite halides (e.g., (CH3NH3)PbI3) have recently emerged as a new category of light absorbers with impressive solar cell efficiencies (15%). Despite the rapid progress demonstrated by these light absorbers, there is a lack of understanding of some fundamental physical and chemical properties of this new material system. In this presentation, we report on our recent studies on the effect of film thickness on charge transport, recombination, and device characteristics of perovskite (CH3NH3)PbI3 sensitized solar cells. These results and others are discussed.

Thanks to the low deposition temperature of trihalide perovskite solar cells, it is possible to apply these planar heterojunction cells in an inverted monolithic configuration on top of highly efficient and flexible Cu(In,Ga)Se2 (CIGS) bottom solar cells. We will present our first results on planar perovskite heterojunctions grown by physical vapor deposition and discuss the potential of perovskite-CIGS tandem devices.

Our work presents a depleted hole conductor free CH3NH3PbI3/TiO2 heterojunction solar cell using a thick CH3NH3PbI3 film. The CH3NH3PbI3 formed large crystals which function simultaneously as light harvesters and as hole transport materials. We performed capacitance voltage measurements, which show a depletion region which extends to both n and p sides. This depleted hole conductor free CH3NH3PbI3/TiO2 heterojunction solar cell provides a power conversion efficiency of 8% with a current density of 18.8 mA cm-2, the highest efficiency achieved to date for perovskite based solar cells without a hole conductor.

We describe our latest results of Pb perovskite solar cells with CuSCN as hole conductor. The conversion efficiency was over 10%. Although the light stability was not good, it was found that the modification of the structure improved the light stability.

To control the optoelectronic property, we developed novel hybrid copper-halide based perovskite. The electronic properties can be controlled by some doping methods.

In excitonic solar cells, where the primary photoexcitation is a bound exciton, a hetorojunction is needed to provide enough driving force to generate free charges. Unfortunately, this results in intrinsic energetic losses, which although conceivably surmountable have lead to relatively slow progress in efficiency over the last decade. Noteworthy, in the last year, the scientific community involved in the development of ”emerging” solar cells have realised a succession of breakthroughs employing ionic crystalline assemblies assuming a perovskite structure. Perovskites have been reported replacing the dye in dye-sensitize solar cells (DSC) with liquid-electrolyte based (power conversion efficiency, eta;= 6.5%)[1] and solid state cells with spiro-OMeTAD and conjugated polymers as the solid-state hole conductor (eta; over 9 %) [2], or as hole-conductors (eta; = 8.5%)[3]. These devices have generally shown impressive photocurrent generation, while the photovoltages achieved still indicated some significant losses. One particular device concept, where the mesoporous TiO2-perovskite heterojunction is removed, eludes this trend: the “meso-superstructured solar cell” that sees an organometal mix-halide perovskite, CH3NH3PbI3-xClx, employed as light harvesting and electron transporting layer and a spiro-OMETAD as hole transporter. A mesoporous Al2O3 layer is employed as insulating “scaffold” upon which the perovskite is deposited. The device exhibits exceptionally high open-circuit photovoltages of over 1.1 volts, despite the relatively narrow absorber band gap, which lead to a record eta; of 10.9% under standard conditions [4]. As the operation of these cells is quite different to the standard DSC device, a number of questions need to be answered. Here we will examine the effect of Chlorine doping on the optoelectronic properties of the CH3NH3PbI3 compound, with a particular focus on the functionalities of the principal interface in the device, i.e. the perovskite/Hole Transporter juction (HTM). The nature of the primary photo-excitation will be unveiled and the effect of crystal structure and thin film morphology on the opotolectronic processes of the photovoltaic device will be elucidated.
[1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 6050 (2009).
[2] J. H.Heo, S. H. Im, J. H. Noh, T. N. Mandal, C-S Lim, J. A. Chang, Y. H. Lee, H. Kim, A. Sarkar, Md. K. Nazeeruddin, M Gratzel and S. Seok, Nature Photonics, DOI: 10.1038/NPHOTON.2013.80
[3] I. Chung, B. Lee, J. He, R. P. H. Chang, M. G. Kanatzidis, All-solid-state dye-sensitized solar cells with high efficiency. Nature 485, 486 (2012).
[4] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science, 2012, 338, 643-647.

Mesoscopic perovskite solar cells based on methylammonium lead halides have recently achieved power conversion efficiencies of over 14% after only a couple years of research by a few research groups. These devices have primarily used device architectures based on solid-state dye-sensitized solar cells where the hybrid perovskite material is used in place of a dye molecule as the light absorbing material. However, the optoelectronics properties of these perovskite materials are not well understood and consequently it is not yet clear what the optimal device structure for perovskite solar cells should be. We perform a spectroscopic and electrical study of the family of mixed lead and tin halide perovskites CH3NH3PbBrxI(3-x), and CH3NH3PbBrxI(3-x), where the bandgap of the material can be tuned by varying the bromine/iodine ratio. We find that stable single phase alloys exist for the mixed halide compounds. From time resolved photoluminescence measurements, we measure long charge carrier lifetimes of several 100s of nanoseconds for the lead perovskites, and find that the lifetime is dependent on templating substrates, processing conditions and halide stoichiometry. We also examine how the carrier mobility, lifetime and concentration depend upon extrinsic doping levels. These studies suggest that a planar p-i-n architecture with carrier selective contacts may be the best design for perovskite solar cells.

A plethora of different photovoltaic (PV) technologies are being developed for large scale solar energy conversion. Beyond wafer based first generation PV, exist a second generation of thin-film concepts based on thin solid semi-conductor absorber layers sandwiched between two charge selective contacts, and an emerging generation of meso or nanostructured solar cells which rely on a distributed heterojunction to generate charge and transport positive (p) and negative (n) charge in spatially separated phases. Many thin-film materials have been employed in nanostructured concepts, nevertheless, to-date, no materials discovered from the field of emerging PV have worked their way back to deliver a high efficiency thin-film technology. Within the last year, organometal tri-halide based perovskites have risen to become a very promising PV material, primarily evolving from activities in dye sensitized solar cells. In previous embodiments the highest efficiency perovskite solar cells have been obtained by incorporating a mesostructured composite8,9,10,11. We have recently discovered that extremely efficient solar cells can be constructed in a thin film planar heterojunction architecture via both solution and vapour phase deposition methods. Here I will present our recent progress with both meso-superstructured and planar heterojunction perovskite solar cells. Specific attention will be paid to the critical aspects of uniform and continuous perovskite film formation and the nature of the n and p-type contact regions. Through a combined device and spectroscopic study we have estimated the charge and exiton diffusion length within the perovskite absorber film which will be discussed.

Abstract not available.

Abstract not available.

Lead chalcogenide quantum dot (QD) assemblies represent a promising semiconductor technology for next-generation photovoltaics. However, no device study to date has explored the effect of ordered QD arrays on carrier transport. In this talk, I discuss the use of an organic molecular wire to mediate the assembly of PbSe QD into polycrystalline superlattice thin films that result in significantly enhanced electrical transport. Specifically, we fabricate ordered QD arrays that are electrically coupled with oligophenylene ethynylene molecular wires. These conductive, organic/inorganic nanocomposite films possess a degree of order that has not been observed in conductive QD solids to date. For comparison, we have also fabricated glassy QD solids with the same molecular wire ligand used to make ordered QD solids. Although grazing incidence small angle X-ray scattering (GISAXS) reveals very similar inter-QD spacing (~2 nm) in both the ordered and disordered PbSe QD nanocomposite films, a 100-fold larger carrier mobility is observed in the ordered QD nanocomposites, making these molecular wire/QD hybrid materials promising candidates for QD photovoltaics.

Quantum confined semiconductor nanocrystals have been widely investigated as light harvesting and charge separation components in photovoltaic and photocatalytic devices. Although in these devices, the dissociation of excitons (bound electron-hole pairs) through interfacial charge (electron or hole) transfer to acceptor materials is one of the most fundamental and crucial steps, it remains poorly understood. In excitonic nanomaterials, both the electron-hole interaction and electron-phonon interactions fall between those of the bulk semiconductor materials and molecular chromophores and the appropriate models for describing photoinduced charge transfer (or exciton dissociation) remains unclear.
In this study, we investigate the size dependence of electron transfer (ET) processes from CdS, CdSe and CdTe QDs to three molecular acceptors. The combination of band edge of bulk materials, size tunable confinement energy, and acceptor redox potential enables an examination of ET rates over a wide range of driving force (0 ~ 1.3 eV). Our experiment shows that ET rates from QDs to these acceptors increase monotonically with decreasing QD size, regardless of QD compositions and acceptor molecule redox potentials. To account for the experimental observation, which is inconsistent with the inverted regime expected from Marcus ET theory, we propose an Auger-assisted model for ET from QDs. In the Auger-assisted ET process, the excess energy of the electron can be transfer to the hole, which overcomes the unfavorable Franck-Condon overlap in the Marcus inverted regime and facilitates the ET process. Because of a quasi-continuum of hole states in these QDs, a larger driving force leads to an increase in the number of Auger-assisted ET pathway, which accounts for the observed monotonic increase of ET rate with driving force. This model is analogous to the well-established Auger-assisted hot electron relaxation process in QDs and can be justified by the much stronger e-h interaction in QDs than other energy relaxation pathways. We believe that the proposed Auger-assisted ET model is generally applicable to many excitonic nanomaterials and has important implications for designing solar energy conversion devices.

Semiconducting polymers are actively under development for use in light-weight, flexible, disposable organic light-emitting diodes, and thin-film transistors. A key application which is currently attracting a lot of interest for semiconducting polymers is their use in organic photovoltaic devices (OPVs). The main drive for developing OPVs is the lower cost associated with their manufacturing, because of the fact that organic semiconducting polymers can be solution processed. Poly(3-hexylthiophene) (P3HT) remains one of the most commonly used polymers in organic photovoltaics due to its desirable electronic properties. Our group has been studying and developing techniques to grow semiconducting polymers using a living polymerization method. This has allowed us to synthesize polymer architectures that we haven&’t been able to access till now including polythiophene brushes, star-shaped P3HT, as well as hyperbranched P3HT. It also allows us to accurately control the molecular weights of P3HT and produce materials with a narrow molecular weight distribution. More recently, we have been using this technique to create multiblock copolymers to introduce in a controlled manner areas of high crystallinity and areas which are more amorphous. In this talk, I will talk about the synthetic strategies used, and the thin film morphologies these polymer architectures provide.

Inorganic acceptors such as quantum dots offer many advantages for use in hybrid organic/inorganic solar cells. For instance, their high dielectric constants could reduce bimolecular recombination, and their size-tailorable energy levels could be used to optimize energetic offsets to maximize extracted power. Nevertheless, hybrid polymer quantum dot/polymer have tended to lag behind purely organic donor/acceptor blends in terms of overall performance. We compare and contrast the fundamental operating mechanisms of polymer/fullerene and polymer/quantum dot blends. We seek to understand apparent differences in the driving force dependence on carrier generation yields for both forward (donor-to-acceptor) electron transfer, and reverse (acceptor to donor) hole transfer. In addition, we explore the role of energy transfer in polymer/quantum dot blends with low bandgap quantum dots, and consider the implications if hybrid cell were operating as bulk heterojunctions alongside networks of quantum dot Schottky diode cells.

Colloidal semiconductor nanocrystals (NCs) are solution processable semiconductors, which have shown in the last years great potential for optoelectronic device fabrication. Their remarkably broad absorption, their large tunability, their high dielectric constant, and their high stability under ambient conditions, are all qualities that makes of NCs solids ideal solar cells active layers.
Once the dichotomy between quantum confinement and the electronic wave function overlap, which allows electrical transport, has been solved, both high performances field effect transistors [1] and efficient solar cells have been reported [2,3].
In this presentation I will report how PbS NCs with benzene dithiols ligands can be use as active layer for efficient solar cells, with power conversion efficiencies approaching 5.5% and fill factors of 60% [3]. The role of the NCs` quality and trap density will be discussed together with peculiar features of the device functioning [4]. Finally, I will show as PbS NCs together with conjugated polymers can give new possibilities towards broad absorption of the solar spectrum.[5]
[1] S. Z. Bisri, C. Piliego, M. Yarema, W. Heiss and M. A. Loi, Adv. Mater. (2013). DOI: 10.1002/adma.201205041
[2] Zhijun Ning , David Zhitomirsky , Valerio Adinolf i , Brandon Sutherland , Jixian Xu ,
Oleksandr Voznyy , Pouya Maraghechi , Xinzheng Lan , Sjoerd Hoogland , Yuan Ren ,
and Edward H. Sargent, Adv. Mater. 25, 1719 (2013).
[3] C. Piliego, L. Protesescu, S. Z. Bisri, M. V. Kovalenko, M. A. Loi (submitted); K. Szendrei, W. Gomulya, M. Yarema, W. Heiss and M. A. Loi, Appl. Phys. Lett. 97, 203501 (2010).
[4] K. Szendrei, M. Speirs, W. Gomulya, D. Jarzab, M. Manca, O. V. Mikhnenko, M. Yarema, B. J. Kooi, W. Heiss, and M. A. Loi, Adv. Funct. Mater. 22, 1598 (2012).
[5] C. Piliego, M. Manca, R. Kroon, M. Yarema, K. Szendrei, M. Andersson, W. Heiss and M. A. Loi, J. Mater. Chem., 22, 24411 (2012).

Solution-processed materials are desirable for next generation optoelectronic devices due to the promise of low temperature, inexpensive and high throughput processing methods (e.g., inkjet and roll-to-roll printing) that are suitable for flexible substrates. In particular, the organic photovoltaic community has been motivated and encouraged by the development of novel solution-processable polymeric and small molecule materials that have demonstrated AM1.5G power conversion efficiencies that have approached (small molecules) and even exceeded (polymeric polymers) 10%. Similarly, a variety of solution-processed metal oxides (e.g., MoOx, WOx, NiOx, ZnOx, TiOx, etc.), small molecules (e.g., substrate-tethered self-assembled monolayers, doped small molecules, etc.) and polymers (e.g., PEDOT:PSS, aliphatic polyimines, charged conjugated polymers, etc.) have demonstrated preferential harvesting for either holes or electrons, enabling the realization of all solution-processed high efficiency devices for both normal and inverted device architectures.
Here, we utilize x-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy to interrogate the interface between solution-processed n-type (WOx nanocrystals) and p-type (NiOx or PEDOT:PSS) hole-selective contacts and model organic semiconductor materials (P3HT and PCBM). We probe chemical information related to the “buried” interface between thin (ca. 5 - 10 nm) active layers and the selective contacts via angle-resolved XPS measurements. The hole-harvesting contact/polymer interface is probed at a take-off angle of 0° (relative to surface normal), while the organic components away from the interface are probed at a take-off angle of 60°. Specifically, we observe partial oxidation of the P3HT polymer near the interface via examination of the high-resolution S 2p core-level spectra. Our results demonstrate that there is a degree of charge transfer between the selective contact and polymer component during interface formation. We complete the interfacial picture by measuring the frontier orbital energetics between the contacts and the organic semiconductors using UPS. These chemical and energetic interface measurements elucidate aspects related to the thermodynamics and kinetics of preferential hole extraction at device relevant contacts for optimization of solution-processed organic photovoltaic devices.

While the diffusion of singlet and triplet excitons in organic semiconductors has been studied extensively, exciton diffusion in colloidal quantum dot (QD) thin films remains largely unexplored. In QD excitonic solar cells, the efficient diffusion of excitons to charge-separating interfaces is central to device operation. On the other hand, exciton diffusion to quenching interfaces in QD-LEDs is a process that can limit luminescence efficiency. In this talk, I will detail our experimental efforts to obtain a deeper understanding of excitonic energy transport phenomena in colloidal QD materials. Using a combination of time-resolved optical microscopy and spectrally-resolved transient photoluminescence spectroscopy, we are able to track the migration of excitons in space, energy, and time over a distance of several tens of nanometers. This analysis reveals sub-diffusive transport behavior, characterized by kinetic trapping of excitons at low-energy sites within the inhomogeneously broadened ensemble. Further, we are able to tune the diffusion length through variation of ligand chemistry and inorganic shell thickness, in accordance with the predictions of Forster theory. We assess the validity of available theoretical models to explain our observations and comment on the implications of these findings for device design.

A discussion of the effect of intraphase microstructure on organic photovoltaic (OPV) device performance will be presented. In one example, bilayer devices where the small molecule donor, TIPS-DBC, is deposited by both spin-coating and by thermal evaporation in vacuum, will be discussed. This bilayer approach enables a direct comparison of device performance for donor layers in which the same material exhibits subtle differences in microstructure. The electrical performance is shown to differ considerably for the two devices. The bulk and interfacial properties of the donor layers are compared by examination with photoelectron spectroscopy in air (PESA), optical absorption spectroscopy, photo-CELIV and time-resolved photoluminescence measurements, X-ray reflectometry (XR), and analysis of dark current behavior. The observed differences in device performance are shown to be influenced by changes to energy levels and charge transport properties resulting from differences in the microstructure of the donor layers. Importantly, this work demonstrates that in addition to the donor/acceptor microstructure, the intraphase microstructure can influence critical parameters and can therefore have a significant impact on OPV performance. In a second example, the translation of D-A type polymers from lab-scale to large area devices will be presented. In particular, the properties of large area films and devices will be discussed.

Our group recently developed the use of high angle annular dark field (HAADF) scanning transmission electron tomography (STET) in combination with discrete area reconstruction technique (DART) for the measurement of polymer fullerene morphology. The resulting images can resolve multiple gray levels and have a voxel size of 1.4x1.4x1.4 nm, which makes them by far the most highly resolved and chemically detailed images of bulk heterojunction (BHJ) layers ever published. So are the images accurate? We explore this question by comparing the vertical concentration profile measured using HAADF-STET to measurements performed using neutron reflectometry and x-ray reflectometry. In addition we use advanced numerical techniques to examine the 5x10^7 volume elements from each image and make measurements of average exciton diffusion length, domains size, and tortuosity of change transport. Finally, ultrafast spectroscopic measurements of charge separation and recombination are re-examined given knowledge of the measured morphology.

Organic materials have become an important class of semiconductors for thin film solar cells. There has been significant progress in improving the power conversion efficiency of organic photovoltaics (OPVs) to 10%, but questions still remain about the details of their operation. In bulk heterojunction (BHJ) OPVs, generation and extraction of carriers depends on the nanoscale phase separation between an electron donating, usually a polymer, and electron accepting material, usually a fullerene. We will discuss our recent work towards understanding the complex morphology of BHJs and how charge is extracted from BHJs. Detailed studies of the miscibility and mass diffusion of fullerenes in semiconducting polymers reveal the miscibility of fullerenes in disordered polymer domains. Tuning the miscibility of donor-acceptor pairs is a useful strategy to improve the performance of BHJs, which we demonstrate by increasing the fill factor of OPVs formed with a series of structurally similar fullerenes and a low band gap selenophene polymer. We have also developed the means to measure rapid mass diffusion constants of fullerenes using a dynamic secondary ion mass spectrometry imaging method. These studies reveal the dynamic nature of the structure of BHJs and point to the complex electronic density of states in BHJs caused by the structural disorder in transport pathways that the carriers must traverse during extraction. We will report the results of our recent studies that help to connect measurements of transient photoconductivity to the density of states of BHJs.

In this presentation we will report the design, synthesis, and characterization of new molecular and polymeric materials for organic photovoltaic cells. We have now families of molecular donor semiconductors based on DPP cores achieving PCEs of ~ 5% and polymeric donor materials with certified efficiencies approaching 10% in inverted architectures. Furthermore, the design of new polymers comprising the bithiopheneimide unit achieving fill factors approaching 80% is reported. Finally, our polymer-polymer blends based on naphthalenediimide polymer shows maximum efficiency of 4.2% whereas new acceptor polymers now exhibits PCEs surpassing 6.5% upon additional optimization.

Donor-acceptor polymers have become high potential donor materials for organic solar cells since they were first introduced in the early nineties. Compared to the standard donor material poly(3-hexylthiophene) (P3HT) they show for polymer/PCBM solar cells a smaller band gap, better quantum efficiencies and lower lying HOMO, LUMO levels.(1) One outstanding donor-acceptor polymer is PCPDTBT (poly{[4,4-bis(2-ethylhexyl)-cyclopenta-(2,1-b;3,4-b&’)dithiophen]-2,6-diyl-alt-(2,1,3-benzo-thiadiazol)-4,7-diyl}), first introduced by Brabec et al. in 2006.(1) For blends of PCPDTBT and PCBM efficiency values of 4.5% are reported.(2) Optimizations of efficiency were mainly obtained in the area of morphology improvement using solvent additives as diiodooctane (DIO) or alkanedithiols during the production process.(4) No crystal structure or thorough investigation on the film formation of the homopolymer PCPDTBT has been performed yet.
In this contribution we will present a systematic study of the absorption properties of the homopolymer PCPDTBT in different solvents and thin films. In-situ spectroelectrochemical data give further information about the neutral and the charged states, the experimental data are supported by DFT calculations based on TDDFT (B3LYP/6-31G**). Regarding thin film structures we find a strong influence of the processing solvent on the morphology of thin films prepared by spincoating ranging from completely unordered to highly ordered fibers.(3) For investigating real structure function relationships it is further necessary to prepare films with large areas of highly ordered and aligned polymer chains. We used solvent vapor annealing in a variety of solvents to improve the crystallinity and reduce the number of nucleation points. This method is well established in our group and was already successfully applied to P3HT.(4,5) The morphologies after different preparation and annealing methods were investigated by AFM, TEM, and electron diffraction to gain a deeper insight into the chain orientations. Using absorption spectroscopy we found a correlation between the different morphologies and their corresponding UV/VIS absorption spectra.(3) Investigating the morphology and absorption properties of solutions and thin films using 2 w%. DIO we could further correlate the different morphologies and absorption spectra with and without DIO. These results show that not only the blend morphology in PCPDTBT/PCBM solar cells but also the crystallinity of the homo polymer is influenced by solvent additives.

(1) a) Mühlbacher, D. et al. Advanced Materials 2006, 18, 2884-2889.
b)Hwang, I.-W et al. Advanced Materials 2007, 19, 2307-2312.
(2) Albrecht, S. et al. The Journal of Physical Chemistry Letters 2012, 3, 640-645.
(3) Fischer, F. S. U. et al. Macromolecules 2013
(4) Crossland, et al., Advanced Materials 2012, 24, 839-44.
(5) Fischer, F. S. U. et al. Nanoscale 2012, 4, 2138-44.

Over the last decade different morphological aspects have been considered such as lateral and vertical phase separation, crystallinity, domain size, domain purity, interface sharpness etc. and their role on the steady state opto-electronic properties have been analyzed. In this study we have manipulated the nanoscale morphology of BHJ solar cells by using different additives and/or thermal annealing and co-related the morphological parameters with recombination mechanisms using transient photocurrent/photovoltage measurements and their morphology was characterized using Energy filtered TEM (EFTEM) and AFM.
The morphology of PDPP3T and PC60BM blend films were modified using different solvent additives namely 1-Chloronapthalene(CN), 1,8 Diiodooctane(DIO) and 1,8 Octanedithiol(ODT) and their role on steady state and transient optoelectronic properties were investigated. It is found that use of blend additive improves the domain purity and crystallinity which leads to significantly higher short circuit current density. However when the cells were processed with 1,8 Octanedithiol (ODT) additive the fill factor and open circuit voltage reduce dramatically. Films processed with ODT additive showed smaller domain size but higher domain aggregation compared to films processed CN and DIO additives. Transient photocurrent analysis indicates faster charge collection in the case of CN and DIO processed solar cells and slowest charge collection in ODT processed solar cells. Interestingly devices processed with ODT additive also show the slowest recombination time and lower recombination coefficient compared to other cells. This is attributed to the bundling nature of the donor polymer when processed with ODT additive. Such bundling this leads to higher phase segregation resulting in lower interfacial area between polymer and fullerene and hence leading to lower recombination rate and higher recombination time.
Similar measurements were done for P3HT:PCBM solar cells. The morphology in these cells were modified using thermal annealing and additives. It is seen that using solvent additives in the blend solution improve the donor domain purity , however domain purity/crystallinity only improved the fill factor rather than increasing the short circuit current density as seen in PDPP3T:PCBM system. Improvement in short circuit current density was dependent upon fullerene purity and fullerene domain size. Among all additives, nitrobenzene was determined as the ideal additive due to increase in donor and acceptor domain purity.
To summarize direct correlation between nanomorphlogy and charge transport is observed where higher domain interfaces lead to lower charge carrier life time and higher domain purity leads to higher charge carrier density. To obtain efficient BHJ solar cells domain purity and sharp domain interfaces are required but domain aggregation needs to be supressed.

Film morphology of organic thin films is of significant importance for the performance of organic solar cell devices. This becomes immediately clear by considering that all steps of charge generation from absorption via exciton diffusion and charge separation to charge transport as well as loss mechanisms, like recombination, depend on the nanostructure of the active layers within the photovoltaic cell.
In the present contribution we put a particular focus on molecular orientation. We investigated the film morphology and device performance of planar heterojunction solar cells based on the molecular donor material α-sexithiophene (6T). We prepared bilayers of 6T and two different acceptor molecules, C₆₀ and diindenoperylene (DIP). The growth temperature of the underlying 6T film was varied between room temperature and 100 °C for each acceptor.
By means of X-ray diffraction and near edge X-ray absorption fine structure spectroscopy (NEXAFS) we show that the crystallinity and the molecular orientation of 6T is not only influenced by the preparation conditions but may also be affected by subsequent layers.
The induced structural changes are accompanied by changes in the characteristic parameters of the corresponding photovoltaic cells. This is most prominently observed as a shift of the open circuit voltage (V_oc): In the case of 6T/C₆₀ heterojunctions, V_oc is reduced from 0.4 V to 0.3 V, approximately, if the growth temperature of 6T is increased from room temperature to 100 °C, while it increases from 1.2 V to roughly 1.4 V in the case of 6T/DIP solar cells. Temperature dependent measurements of the open circuit voltage give insight on the cause of these opposite trends: while in the C₆₀ case increased recombination reduces V_oc, the recombination rate remains virtually unchanged for the DIP device. Instead an increased charge transfer energy gap is observed in this case, which yields a larger V_oc.

Organic semiconductors are promising materials for low-cost, large-area electronic devices such as organic photovoltaics (OPVs). OPVs require an active layer that comprises of an intimate mixture of an electron donor, usually a conjugated polymer, and an electron acceptor, typically a fullerene. While solar cells with fullerenes have shown high efficiencies, the inability to tune the electronic levels of fullerene limits the open circuit voltage, so alternative acceptors are desirable. However, relatively little work has been done to correlate the fundamental differences between non-fullerene and fullerene acceptors regarding molecular structure and assembly and device performance. Here, we report on bulk heterojunction films consisting of a novel non-fullerene acceptor, decacyclene triimide (DTI), that can form highly ordered self-assembled organic nanowires. Devices fabricated from the common polymer donor, poly(3-hexylthiophene) (P3HT), and DTI exhibit solar power conversion efficiency (PCE) of 1.6% as spun-cast. However, thermal annealing leads to a significant decrease in both the electron current and PCE. This is in contrast to P3HT:fullerene devices that require thermal annealing to enhance performance. Soft X-ray absorption spectroscopy and grazing incidence wide angle X-ray scattering suggest that the thermally induced drop in PCE is largely a result of reorientation of DTI molecules to a packing motif with an in-plane π-π stacking direction, resulting in poor electron transport in the out-of-plane direction. Furthermore, in situ X-ray scattering experiments suggest that the temperature required for this reorientation to occur is relatively low. Additionally, unlike most non-fullerene acceptors, DTI performs well with other low-bandgap polymer donors, suggesting a potential degree of universality that is rarely found outside of fullerenes. These results reveal some crucial differences between DTI-based and fullerene-based blends, and are some of the first studies to demonstrate the critical link between active layer morphology and charge transport in non-fullerene systems.

The interpenetrating morphology formed by the donor and acceptor components is critical for the performance of polymer: fullerene bulk heterojunction (BHJ) solar cells. In detail, the domain size of each phase, the nature of the interface and the configuration of molecular packing would all affect exciton dissociation, charge separation and recombination, and charge transport. In this work we carried out a systematic investigation on a BHJ system consisting of a newly developed 5,6-difluorobenzo[c][1,2,5]thiadiazole-based copolymer, PFBT-T20TT, and PC71BM. Grazing incidence X-ray scattering (GIXS) measurement reveals that the PFBT-T20TT: PC71BM BHJ exhibits a low degree of crystallinity and a two-phase morphology, consisting of domains of inter-mixed PFBT-T20TT: PC71BM and pure PC71BM. Interestingly, the lamellar spacing of PFBT-T20TT increases from 25.1 Å (for pure polymer) to 37.82 Å (for BHJ), suggesting the intercalation of PC71BM between the PFBT-T20TT lamella. Different from the previously reported highly crystalline pBTTT:PCBM blend, which shows a similar intercalation behavior, the amorphous nature of PFBT-T20TT leads to formation of more effective percolation paths for charge carriers and thus allows lower loading ratio of fullerene and much higher power conversion efficiency (>6%) of solar cells. To further prove this argument, we performed spectroscopic and electrical measurements on the PFBT-T20TT: PC71BM system. Steady-state and time-resolved transient photo-induced absorption spectroscopy reveals instantaneous charge transfer (CT) at the PFBT-T20TT/PC71BM interface, indicating that the CT process is no longer limited by exciton diffusion. Furthermore, we extracted the hole mobility based on the space limited current (SCLC) model and showed that more efficient hole transport is achieved in the blend of PFBT-T20TT: PC71BM BHJ as compared to pure PFBT-T20TT. Our study provides a coherent model to explain the high photovoltaic performance of some of the recently developed amorphous polymer-fullerene BHJ systems.

The Schottky-junction structure has been broadly applied in inorganic photovoltaic devices but has not yet been successfully applied in organic photovoltaic devices. There was concern that the weak internal electric field in the Schottky-junction device could not provide high enough driving force for the separation of photo-generated Frenkel excitons in organic semiconductor materials. In this talk, we will report a solution-processed fullerene-based organic Schottky-junction device which showed a large open-circuit voltage of 0.85-0.95 V, a high short-circuit current of about 9 mA/cm2, and a power conversion efficiency of 5%. In particular, the obtained open-circuit voltage in the Schottky-junction device is significantly higher than that in most conventional organic bulk-heterojunction devices. The open-circuit voltage in Schottky-junction devices is determined only by the Fermi energy level difference between the fullerenes and anode, insensitive to the energy levels of the donor polymers, which is totally different from that of bulk-heterojunction devices. Since the work function of the anode is easily tuned with well-established surface modification, this study paved the road to achieve a high open circuit voltage by the combination of the high work function of the anode with the low lowest unoccupied molecular orbital of the organic semiconductors. In addition, such fullerene Schottky-junction devices donot need to have fixed morphology and thus showed much better stability than bulk-heterojunciton devices.
Reference: B. Yang, F. Guo, Y. Yuan, Z. Xiao, Y. Lu, Q. Dong, and J. Huang*, Advanced Materials, 25, 572-577 (2013)

Polymer-based organic photovoltaics are promising candidates for harnessing the Sun's energy. However, their efficiencies remain low compared to their inorganic counterparts and the design and synthesis of the polymers are often arbitrary and laborious respectively. Furthermore, their structure-property relationships are little understood. New high-performing materials are required, but also, crucially, a rationale for why these materials are high-performing. Thus we have a dual approach: both novel chemistry and novel characterization. The former focuses on Rozen's reagent - a powerful oxidizing reagent that can generate thiophene-S,S-dioxides. The latter focuses on single molecule conductance measurements.
Thiophene oxidation is an important reaction for organic electronics since molecules containig the thiophene-S,S-dioxide unit show a narrowed band gap and dramatically lowered LUMO. However, traditional oxidizing agents have failed to carry out this reaction efficiently. On the other hand, Rozen's reagent, a stable form of hypofluorous acid, can generate thiophene-S,S-dioxides at room temperature and in a matter of minutes. We are interested in developing this chemistry and incorporating these oxidized units into both small molecules and polymers.
The question then remains as to how we can relate molecular structure to electronic behavior. In the past decade, wiring an individual molecule into a circuit has been realized. One means of achieving this is the scanning tunneling microscope break junction (STM-BJ) technique. Our group is interested in how this remarkable technique can be used to measure the conductance of molecular analogs of semiconducting polymers, with the aim of relating single molecule electronic properties to those in the bulk. We can apply this technique to our family of thiophene-S,S-dioxide containing materials and select the best performing materials for further exploration. In this way, we aim to develop design principles from these STM-BJ measurements so that we can strategically develop novel polymers. The combination of novel chemistry with novel characterization leads to a powerful design tool that could dramatically impact the organic photovoltaic community.

Increasing the absorption of the photoactive layer is a rational strategy to enhance photon harvesting and thus to improve the photocurrent of organic solar cells. Here we present the application of a low-bandgap polymer as donor with a strongly absorbing perylene diimide (PDI) as acceptor in solution processed bulk heterojunction organic solar cells. The contribution of the PDI to the photocurrent generation is clearly demonstrated by EQE measurements.
Additionally, exciton and charge carrier dynamics as well as photocurrent loss mechanisms are investigated by sub-picosecond to millisecond pump-probe transient absorption spectroscopy (TA) and time-resolved photoluminescence (TRPL) spectroscopy. With the former we track the dynamics of non-radiative species, whereas the latter enables us to observe emissive decay channels.
Supplementary experiments like the evaluation of the morphology by AFM and time-delayed collection field (TDCF) experiments used to investigate the field-dependence of charge generation deliver valuables insights that allow to derive structure-property-relations for polymer:PDI solar cells.

Indium tin oxide (ITO) is a commonly used transparent electrode in organic solar cells and is one of the components which make the current solar cell expensive. We will present our recent advances in developing highly conductive nanostructured PEDOT:PSS layer as a transparent electrode for replacing ITO. Additives like glycerol or dimethyl sulfoxide were mixed with PEDOT: PSS to increase its conductivity. Furthermore, spin-on nanoprinting (SNAP) technique was used to print nanoarchitectured PEDOT:PSS transparent electrode. Dimensions of PEDOT:PSS nanopillars are about 50nm height and 150nm diameter (include center to center distance too). Nanostructured electrodes can enhance the charge collection efficiency of solar cells due to the enhanced surface area accomplished by nanoarchitecturing. We will discuss sheet resistance and transparency of planar and nanostructured PEDOT:PSS electrodes prepared by mixing with different additives. In order to further increase the conductivity, silver nanowire (AgNW) has been added to PEDOT:PSS electrodes. Results of power conversion efficiency measurements of planar and nanostructured PEDOT:PSS/AgNW will also be demonstrated.

All-conjugated block copolymers are an emerging class of materials that bring together distinct pi-conjugated polymers and enable improved control of the interface between donor and acceptor organic semiconductors. Recent work has shown that all-conjugated BCPs can give near 3% PCEs in non-fullerene photovoltaic devices. Here, we explore the nanoscale structure, optical properties, and performance of a series of all-conjugated block copolymers with a poly(3-alkylthiophene) (P3AT) donor block and a second polymer block comprising poly(fluorene-co-dithienylbenzothiadiazole) PFTBT, electron conductive poly(naphthalenediimide) pNDI or low bandgap thieno[3,4-b]thiophene and benzodithiophene PTB7 polymers. P3AT-block-PFTBT block copolymers exhibit exceptional performance in non-fullerene devices, and we show that the length and composition of the side-chains for both blocks impacts solubility, crystal packing, phase behavior, and performance. Grazing-incidence X-ray studies show preferential in-plane pi-pi stacking of P3AT crystallites when thermally annealed at high temperatures (above 220C), but under solvent annealing or low-temperature (150C) thermal annealing the pi-pi stacking direction lies primarily out-of-plane, preferred for increased charge transport to electrodes. As a comparison, all-conjugated P3HT block copolymers with an electron conductive pNDI block and p-type, low bandgap PTB7 block are investigated for use in all-polymer and polymer-fullerene devices. The use of an n-type polymer can increase performance through improved electron mobility, while a low bandgap, p-type polymer can give significantly broadened absorption. This study presents the properties, structure, and performance of a broad series of all-conjugated block copolymers and demonstrates the potential of this emerging class of materials for solution-processible OPVs and for fundamental studies of interfacial charge and energy transfer processes.

Although the interfaces between donors and acceptors are considered to be one of the most important elements determining the performance of organic solar cells, detailed characterization on the interfacial structure has remained lacking because there has been no clear-cut method to characterize such interfacial structure. Most of research has so far focused on nano-morphologies such as crystallinity, domain size, and domain distribution. A new scattering technique developed recently, called Polarized Soft X-ray Scattering, allows us to have compositional contrast among different organic domains as well as information on the local, in-plane molecular orientation relative to the complex donor:acceptor interfaces. In addition, there has been research studying the change in such correlated molecular orientations as a function of backbone chemical moieties, fluorine substitution, and solvent type and their effects on device performance. However, much remains unknown about the factors determining the local molecular orientation in samples that are globally isotropic, which is a missing link between the complex donor:acceptor morphology and device performance.
The effect of additives- with different solubilities for phenyl-C61-butyric acid methyl ester (PCBM) while keeping the solubility for poly[3-hexylthiophene]s (P3HT) low- on the molecular orientations as well as the domain morphologies of P3HT:PCBM active layers will be presented in this study. The solubility of solvent was controlled by mixing various additives with P3HT:PCBM blend solutions, which could then alter the association and solvation characteristics for both P3HT and PCBM in the mixed solvents. As a result, our new additive, 2-chlorophenol drove the P3HT to higher crystallinity and to reduced PCBM domain size, as confirmed by small angle neutron scattering (SANS) measurements. In addition, we could also alter molecular orientation of the P3HT with respect to the P3HT/PCBM interface by changing the processing conditions (additive type and thermal annealing), which were confirmed from strong anisotropic scattering measured with Polarized Soft X-ray. Based on the changes in both domain morphology and molecular orientation with different additives, P3HT:PCBM BHJ solar cell devices were fabricated and the maximum power conversion efficiency of 3.24% was achieved with the addition of our new additive, 2-chlorophenol. This is a 43% enhancement in PCE compared to the reference device without additives.

Among Organic Electronics, solution-processable π-conjugated polymers are proving particularly promising in bulk-heterojunction (BHJ) solar cells with fullerene acceptors such as PCBM.[1] In the past few years, we have found that varying the size and branching of solubilizing side-chains in π-conjugated polymers impacts their self-assembling properties in thin-films. Beyond film-forming properties, nanoscale ordering in the active layer governs material and device performance. For example, in benzo[1,2-b:4,5-b&’]dithiophene- and thieno[3,4-c]pyrrole-4,6-dione-based polymers (PBDTTPD), TPD substituents of various size and branching impart distinct molecular packing distances (i.e. π-π stacking and lamellar spacing),[2] varying degrees of nanostructural order in thin films,[2] and preferential backbone orientation relative to the device substrate.[3] These structural variations have been found to correlate with solar cell efficiency, with PCEs ranging from 4% to 8.5%.[2,3] (Fig. 1) In parallel, in diketopyrrolopyrrole polymers (PDPPnFmT), replacing some of the thiophene (T) motifs by furan (F) can significantly improve polymer solubility, allowing for the use of shorter branched side-chains, while maintaining high PCEs of ca. 5% in BHJ solar cells.[4] Using this design principle, we find that branched alkyl side-chains can also be swapped for linear ones in order to promote thin-film nanostructural order.[5] In particular, linear substituents are shown to shorten π-π stacking distances between backbones, and to increase the correlation lengths of both π-π stacking and lamellar spacing, leading to PCEs >6% in BHJ solar cells.
[1] P. M. Beaujuge, and J. M. J. Fréchet, JACS 2011, 133, 20009.
[2] C. Piliego, T. W. Holcombe, J. D. Douglas, C. H. Woo, P. M. Beaujuge, and J. M. J. Fréchet JACS 2010, 132, 7595.
[3] C. Cabanetos, A. El Labban, J. A. Bartelt, J. D. Douglas, W. R. Mateker, J. M. J. Fréchet, M. D. McGehee, and P. M. Beaujuge, JACS 2013, 135, 4656.
[4] C. H. Woo, P. M. Beaujuge, O. P. Lee, T. W. Holcombe, J. M. J. Fréchet JACS 2011, 132, 15547.
[5] A. T. Yiu, P. M. Beaujuge, O. P. Lee, C. H. Woo, M. F. Toney, J. M. J. Fréchet JACS 2012, 134, 2180.

The growth of the efficiency of polymer solar cells (PSC) in recent years can be accounted to continuing development of numerous new donor and acceptor components for the active layer. Optimization of the morphology of this layer is a very important but time-consuming task. One of the most studied optimization approach is to anneal the freshly prepared blended films thermally. In-situ monitoring the blend morphology could help to find the best annealing protocol. The Raman spectroscopy can be utilized for this purpose.
It was shown in several works that Raman spectrum is very sensitive to the molecular level morphology of conjugated polymer such as P3HT. Based on recent achievements in this field we propose in-situ technique for probing the degree of molecular order in organic solar cells. Raman spectra are measured during film annealing inside a temperature controlled stage and then fitted by a superposition of two components - amorphous and quasi crystalline. From these data the fraction of the quasi crystalline polymer phase is evaluated giving a dynamics of polymer ordering upon annealing.
Various fullerene derivatives (methanofullerenes, metal complexes of fullerenes, fluorine fullerene derivatives) were studied as an acceptor component for PSC. We show that several fullerene types act similarly to most-studied PCBM ([6,6]-phenyl-C61-butyric acid methyl ester), which decreases the polymer order by intercalating in its phase. Annealing is turned out to be efficient to achieve the partially crystalline polymer domains that are beneficial for solar cell performance. The developed technique is used to determine the optimum annealing parameters by in situ monitoring the Raman spectrum evolution of a sample under study. We compare the power conversion efficiency and external quantum efficiency of P3HT(poly(3-hexylthiophene))-based PSCs to show the effect of the annealing on their performance.
The dynamics of annealing the polymer-fullerene blends both at constant and slowly increasing temperature has been measured. Interestingly, the different fullerene derivatives results in very different annealing behaviour. We have also studied the dependence of the blend morphology on solvent and metallic contact coating. We also found out that the polymer crystallinity can depend significantly on the coordinate normal to the film surface.
We discuss the applicability of the proposed technique for optimization of various donor-acceptor blends based on new polymer and low-molecular-weight organic materials for solar cells.
This work was supported by M.V. Lomonosov Moscow State University Program of Development and by Russian Foundation for Basic Research grant #12-02-31599.

Solution processable hybrid thin film solar cells are highly desirable to convert energy at a reduced cost and in a sustainable manner. Co-assembly of block copolymer and inorganic nanoparticles provide a simple synthesis route to fabricate well-ordered nanostructured materials that have been demonstrated to harness multiple properties, for example, charge carrier transport and light management in solid-state dye sensitized solar cells. Here we fabricate a new type of hybrid solar cell device using block copolymer nanostructures with hybrid perovskite absorbers. The crystallization kinetics of the hybrid perovskites in the mesoporous superstructures will be elucidated.

Progress in the development of novel and higher-performing photoactive materials for organic photovoltaics (OPVs) have lead to the necessity to improve electrode-interfacing (charge transport, charge collection, charge-selective, charge-blocking) layers. Many recent studies have been devoted to the tuning of optical and electronic properties of the OPV hole-collecting layer, typically based on p-type NiO, conductive polymers (PEDT:PSS), or deep n-type materials such as MoO₃, V₂0₅, WO₃, etc. With increased lifetime and stability of the inverted OPV device structure, the tuning of electron-collecting layers (ECLs), commonly ZnO, will likewise be necessary to support further advancements.
We employee two methods for tuning the electronic properties in a sol-gel-deposited ZnO-based ECL. First, by substituting Mg into the sol-gel deposition, a MgZnO alloy is formed and analyses (Kelvin probe, photoemission and inverse photoemission spectroscopies) show changes in the bulk electronic structure of the ECL. Inverted OPV devices fabricated with bulk heterojunction (BHJ) photoactive layers of poly(3-hexylthiophene) (P3HT) with both [6,6]-Phenyl-C₆₁-butyric acid methyl ester (PCBM) and indene C₆₀-bis-adduct (ICBA) have dramatically improved power conversion efficiencies, most notably due to improved open-circuit voltage and increased fill factor, when the alloy is implemented. Second, a self-assembled monolayer (SAM) is formed on the ECL, with similar measurements showing improved OPV efficiency for both blends, but due to different mechanisms. These two modification methods of the ECL properties offer alternative and complimentary routes to improving power conversion efficiency for the two different P3HT:fullerene BHJs, and analyses of the energetics suggests that the modifications would apply equally as well to optimization of inverted device structures with lower-bandgap polymer electron donors, small-molecule electron donors and other non-fullerene electron acceptors.

The present work reports the fundamental understanding of the morphology of PTB7 based BHJ solar cells. PTB7 is an important photon-active polymer that is widely used in device physics. There has been strong debate on their morphologies in the current stage. This work presents findings from various high-power characterization techniques and provides an unequivocal description of the morphological details. The static thin film samples were first characterized, and then in-situ experiments were designed to explain the morphology evolution. In the end, bilayer diffusion experiments were performed to reveal the interaction of PTB7 and PCBM. The morphology we observed correlated well with device performance. We see that PTB7:PCBM blends processed from solvent mixture yielded a multi-length scaled morphology, which accounts for their high power conversion efficiency. In diffusion experiment, we see that PTB7 crystallites can be dissolved by PCBM during the thermal induced diffusion process. These findings will help the OPV community to better understand the morphology of low band gap polymer blends.

Fullerenes derivatives are prone to their unfavorable absorption as well as limited chemical and energy tunability. The fact that a vast majority of organic solar cells (OSC)s relies heavily on fullerene-based molecules as the acceptor components impose serious challenges for PSCs to reach wide applicability as high performance, cost-effective photon harvesting devices. Thus, research efforts in seeking fullerene alternatives in BHJ PSCs have attracted considerable attentions. Recently, strategies using fully polymeric OSC using n-type polymer acceptors have demonstrated remarkable progresses. However, all-polymer OSC performance still lags behind the extensively studied polymer:fullerene cells, mainly due to the difficulties in controlling the blend film morphology and the lack of experimental and theoretical understanding in charge transport and recombination mechanisms in these systems. Here, we demonstrate a series of relatively high performance all-polymer systems. Through extensive control of film formation environment, precisely controllable nanophase separation can be obtained. Blend film morphologies are directly probed by microscopic measurements and reproduced by molecular dynamic simulations. The morphology-electrical structure-device performance relationship is established in all-polymer OSCs.

The power conversion efficiency of organic solar cells can be increased by the use of the tandem cell concept consisting of two serial stacked sub-cells. Ideally the sub-cells contain spectral complementary absorber materials to harvest sun photons over a wide spectral range. To reach this goal near infrared (NIR) absorbers are needed.
We present new NIR absorbers of the aza-Bodipy class based on thiophene chemistry which are thermally stable, enabling layer deposition by evaporation in vacuum. These materials show a spectral broadband absorption due to their isomeric composition. In combination with C60 as acceptor molecule these aza-Bodipys work as donors in bulk hetero junctions with efficient charge separation and charge transport through the active layer. By using doped transport layers for charge transport to the contacts, we obtain with the thiophene based NIR absorber materials highly efficient organic solar cells which can act as sub-cells for the red part of the sun spectrum in organic tandem cell architectures.

Weak intermolecular interactions and disorder at junctions of different organic materials limit the performance and stability of organic interfaces and hence the applicability of organic semiconductors to electronic devices. We have demonstrated control of donor-acceptor heterojunctions through microphase-separated conjugated block copolymers. When utilized as the active layer of photovoltaic cells, block copolymer-based devices demonstrate efficient photoconversion well beyond devices composed of homopolymer blends. The 3% block copolymer device efficiencies are achieved without the use of a fullerene acceptor (Nano Letters, 2013, 13, 2957-2963). Resonant soft X-ray scattering and grazing-incidence X-ray diffraction results reveal that the remarkable performance of block copolymer solar cells is due to self-assembly into mesoscale lamellar morphologies with primarily face-on crystallite orientations. We find that charge recombination can be modulated via control of the molecular structure both through moieties at donor-acceptor junctions and self-assembly into mesostructured morphologies. Conjugated block copolymers thus provide a pathway to enhance performance in excitonic solar cells through control of donor-acceptor interfaces.

Polymer-based semiconducting materials are promising candidates for large-scale, low-cost photovoltaic devices. To date, the efficiency of these devices has been low in part because of the challenge of optimizing molecular packing while also obtaining a bicontinuous structure with a length scale of approximately 10nm. Here we demonstrate an alternative approach to this problem by packing nanoparticles of electron- and hole-transporting semiconductors into a two-component film. An evaporation-induced nanoparticle self-assembly method was developed, which yields cm-scale uniform thin films. In films of P3HT nanoparticles, the charge carrier mobility is comparable to that of cast polymer films and can be tuned by controlling the particle packing within the film. The results demonstrate a potential advance in fabrication of large-area, high efficiency organic solar cell with optimized molecular packing at the scale of 10-100nm. This work was supported as part of Polymer-Based Materials for Harvesting Solar Energy, an Energy Frontier Research Center funded by the U.S. Dept. of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0001087.

The Photovoltaic characteristics of low bandgap semiconducting polymer, thieno[3,4-b]thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM), based bulk heterojunction solar cells were investigated from 300K to 473K to evaluate the applications in harsh environments. The short circuit current density and fill factor were found to increase despite the decreased open circuit voltage (Voc). When temperature was decreased to 300K, the Voc of device could recover. Even the device was tested after four cycles temperature changing, the performance still could reach to around 80% compare to fresh device. The Raman microscopy of the active layer film was tested to study the molecular order. Impedance and transient photovoltage and photocurrent of the device was tested to investigate the bulk resistance and charge carrier decay lifetime changing under different temperature changing, respectively.

We report on a novel light management approach based on laminating nanostrucutured electrodes onto the active layer. This unusual fabrication method allows the gluing of the top electrical contact onto the active layer to form the hole-collection contact in OPV. In addition and most importantly, it offers a simple way to develop pre-patterned photonic electrodes that is usually incompatible with direct deposition onto a polymer-fullerene active layer. As such, we have carefully modeled, designed and fabricated photonic elements on the top metal contact specifically for trapping red and near-infra red photons in an OPV device. At this spectrum region, photoexcitation of the polymer in a typical polymer-fullerene bulk-heterojunction based on poly(3-hexylthiophene) (P3HT) leads to generation of free charge carriers via the population of an interfacial Charge Transfer Exciton (CTX) in the middle of the band gap. The direct photoexcitation of these mid band gap states lead to the generation of free charge carriers. Thus, the top contact is not only an electrical contact, but also a photonic element of our device for harvesting the additional charges in the mid band gap.
The synergy between light management optimization through modeling of the photonic nanostructures is discussed, as well the integration of the optimal photonic nanostructures onto devices. We will compare results for devices using flat electrodes and will show that our fabrication methodology is a promising approach to integrate light management schemes and easily scalable and applicable to a wide range of devices.

Solar cells based on organic materials offer a wide variety of new applications in the field of renewable energy conversion. However, there are still some drawbacks such as the high cost of ITO which is primarily used as the transparent electrode material. In addition, its weight and brittleness limits the roll-to-roll processing of organic solar cells by printing technology for easy installation. To address these challenges, an alternative polymer mixture called PEDOT:PSS which has promising properties such as transparency, high hole conductivity and flexibility is investigated in the present work to be used as an electrode in flexible organic electronics. However, for replacing the ITO electrode, it is lacking three orders of magnitude in conductivity. We investigate a recently developed post treatment method for enhancing the conductivity of PEDOT:PSS, reaching the order of ITO conductivity.^[1]
To explain the increased conductivity of solvent treated PEDOT:PSS films we investigate the inner film morphology, crystallinity and molecular orientation of the films by grazing incidence small and wide angle X-ray scattering (GISAXS and GIWAXS) performed at the P03 beamline of the PETRA III storage ring at DESY, Hamburg. Complementary to the surface sensitive information obtained from SEM and AFM, GI-scattering provides average statistical information over the illuminated film volume. The results from the morphological investigation are consequently related to the enhanced lateral conductivity of the film which is obtained by four point probe measurements showing a clear increase with doping and post treatment. With increasing lateral conductivity the films shows a change towards smaller and more densely packed PEDOT-rich domains. On the molecular length scale, GIWAXS measurements show a molecular reorientation of the stacking conjugated PEDOT planes towards edge on orientation for enhanced lateral conductivity as well as an increase in overall crystallinity. With this morphological study we give an explanation of the highly improved lateral conductivity of solvent treated PEDOT:PSS by three orders in magnitude.
Furthermore, for enhancing the light harvesting in the organic thin film and hence the efficiency, a novel structuring routine for PEDOT:PSS is introduced, namely, the plasticizer assisted soft embossing.^[2] Being able to control the interface between the transparent electrode and the active material, the device efficiency of OPVs under oblique light can be increased due to light scattering. Combining now highly conductive PEDOT:PSS as electrode with controlled structuring of the electrode, these results reveal new paths for flexible structured ITO-free solar cells of enhanced efficiency.
[1] J.Y. Kim, J.HJ. Jung, D.E. Lee, and J. Joo, Synthetic Metals, 126(2):311-316, 2002
[2] R. Meier, C. Birkenstock, C.M. Palumbiny and P. Müller-Buschbaum, Physical Chemestry Chemical Physics, 14(43):15088, 2012

The typical organic planar heterojunction solar cell consists of the structure indium tin oxide (ITO)/donor/fullerene/electron transport layer/Ag. While optimization is generally neglected because of the focus on bulk heterojunction devices, we here show this structure can benefit from a significant improvement by a combination of exciton blocking and reduced recombination.
Here, we start with an ITO/subnaphthalocyanine (SubNc)/C₆₀/bathocuproine (BCP)/Ag cell. The first improvement is made by inserting a thin diindenoperylene (DIP) layer in between ITO and SubNc. While the effect on the open-circuit voltage (V_oc) and fill factor (FF) of the device is minimal, external quantum efficiency and reflection measurement shows an increase in the photocurrent associated with subnaphthalocyanine. The main function of the DIP layer is thus preventing exciton quenching at the ITO contact. This first step improves the power conversion efficiency (eta;_p) from 2.8% to 4.3%.
Both of these devices show optimal performance for 5 nm thin SubNc layers, as the fill factor drops rapidly at higher donor thicknesses. We investigate this effect and understand it in terms of recombination at the D/A interface in contrast to low charge transport within the bulk of SubNc. By inserting a partially chlorinated subphthalocyanine (Cl₆SubPc) acceptor in the structure ITO/DIP/SubNc/Cl₆SubPc/C₆₀/BCP/Ag, high fill factors are obtained over a wider donor thickness range. Moreover, this structure improves V_oc from ~0.85 V to ~1 V. In this structure, C₆₀ does not contribute to the photocurrent and functions merely as electron transport layer. This loss in photocurrent is however compensated by a photocurrent contribution of the Cl₆SubPc and the fact that a thicker SubNc layer can be used. This leads to a planar heterojunction, non-fullerene acceptor based solar cell with a top efficiency of eta;_p = 6%.

The influence of backbone composition on the physical properties of donor-acceptor (D-A) copolymers comprised of varying amounts of benzodithiophene (BDT) donor with thienoisoindoledione (TID) acceptor is reported. First, well-defined alternating structures with repeating (D-A), (D-D-A), and (D-D-D-A) units were synthesized. For comparison, 5 semi-random D-A copolymers with a D/A ratio of 1.5, 2, 3, 4, and 7 were synthesized. All of the polymers have comparable HOMO values (-5.4 eV); however, a systematic blue shift in the absorbance spectra onset (by up to 200 nm) is observed with increasing donor content. Very little difference is observed between the absorbance spectra of a well-defined (D-D-D-A) copolymer, for example, and a semi-random copolymer with a D/A ratio of 3. Additionally, the decay of the photoconductance transients of copolymer:fullerene films, measured by time-resolved microwave conductivity, becomes dramatically slower with increasing BDT content, most notably for the semi-random copolymers with the highest D/A ratios. These observations, in conjunction with the ease of synthesis of the semi-random copolymers as compared to those with well-defined structures, make the former synthetic strategy a powerful technique for fine-tuning the optoelectronic properties of a given class of D-A materials. In the presentation, preliminary OPV device data is presented together with our overview of the above results.

Charge separation and recombination in polymer/fullerene solar cells is highly dependent on the microstructure of donor-acceptor interfaces. Unfortunately, the ubiquitous presence of mixed phases in mixtures of organic semiconductors utilized in the active layer of polymer/fullerene solar cells creates multiple morphologically distinct interfaces which are capable of charge photogeneration or recombination. We have examined the microstructure and domain compositions of poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]germole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PGeBTBT)/[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) mixtures using energy-filtered transmission electron microscopy and resonant soft X-ray scattering. PGeBTBT is a low band gap polymer (1.5 eV) belonging to cyclopentadithiophene-based polymer family that has been shown to produce high short-circuit currents when employed as the active layer of organic solar cells. Structural characterization shows that the microstructures of the mixtures do not change significantly with the overall composition of the film. Composition maps generated from energy-filtered electron microscopy experiments, however, demonstrate that fullerene domain compositions vary significantly with the overall film composition. Furthermore, the amount of PGeBTBT in the fullerene-rich domains is anti-correlated with device performance. Photo-induced absorption studies using ultrafast infrared spectroscopy (URIR) demonstrate that at 1 ps after photoexcitation the polaron concentration is highest when fullerene domains contain the least amount of polymer. Thus, the purity of the fullerene-rich domains is critical for efficient charge photogeneration. Our results suggest that controlling the composition of mixed phases in polymer/fullerene active layers is critical for the device performance of organic photovoltaics.

As an emerging processing protocol, supercritical carbon dioxide (scCO₂) annealing shows great potential as an effective low temperature processing method for organic photovoltaic devices.¹ Compared to thermally annealed devices, higher short circuit current, J_SC, in scCO₂ processed devices is mainly attributed to a fivefold increase in the initial charge carrier density in devices, characterized by photo-charge extraction by linearly increasing voltage (photo-CELIV). The relatively low open circuit voltage, V_OC, and fill factor, FF, are explained by the faster charge recombination measured by photo-CELIV and impedance spectroscopy (IS). These transport properties are readily attributed to the macro- and nanoscale morphological changes, revealed by UV-Vis spectroscopy and energy filtered TEM (EF-TEM). Larger donor/acceptor interfacial area that develops due to scCO₂ annealing is responsible for higher initial charge carrier densities. In addition purer donor and acceptor phases contribute to the higher charge mobility, whereas disconnected P3HT-rich domains in the active layer is generally responsible for faster recombination.
1. J.A. Amonoo, E. Glynos, X.C. Chen and P.F. Green: An Alternative Processing Strategy for Organic Photovoltaic Devices Using a Supercritical Fluid. The Journal of Physical Chemistry C 116, 20708 (2012).

The overall power conversion efficiency in an organic solar cell depends on the balance between the rates of exciton dissociation, recombination and separation at the donor acceptor interface. Inability to design, control and engineer these interfaces remains a key bottleneck in their widespread use for the next generation organic electronic devices. Here, we show that we can control the ordering at the P3HT/C60 interface in bilayer device geometry by inserting a monolayer of oligothiophenes, which leads to a complete suppression in the exciplex (or charge transfer state) recombination. We observe that the photocurrent increases by 300 times, which in turn results in an increase in the overall power conversion efficiency by an order of magnitude. Moreover, we find that the oligothiophene with an odd number of rings (ter and penta oligothiophene) exhibit a much higher increase in the photocurrent in comparison to the oligothiophene with an even number of rings (tetra oligothiphene). STM measurements reveal that the oligothiophene with odd and even number of rings differ in their ordering respectively, that has a big effect on the overall device performance. We also find that this ordering is highly dependent on the side functional groups in the oligothiophenes. The mechanism of photocurrent generation will be discussed and a simple transport model will be used to explain the change in the charge transfer and recombination rates and predict current-voltage curves.

The purpose of this study is to better understand the interaction that PCBM has with a variety of squaraine dyes targeted for photovoltaic applications in the solid state. PCBM appears to disrupt H- and J- aggregates formed by the squaraines in the solid state. Comprehending the extent of aggregate disruption is important because their formation is tied to the extent of crystallinity of the system which is related to the overall morphology of the active layer and has significant effects on device efficiency because of changes to exciton diffusion, charge transport and charge transfer at the bulk heterojunction interface. In this work, the effect of the alkyl side chain length on the squaraine with respect to PCBM interaction is investigated. Annealing and blend ratio studies will be conducted in order to get a better understanding of how PCBM affects the photophysical and crystallographic properties of the squaraines. UV-Vis spectroscopy and TFXRD techniques will be used in order to monitor these changes.

Organic photovoltaic devices are a promising route to lower costs via roll-to-roll manufacturing. The most promising device architecture involves a bulk heterojunction (BHJ) active layer in which nano-scale phase separation into nominally bicontinuous donor and acceptor rich regions enables both exciton dissociation and charge extraction. Recently, it has become clear that the local composition in polymer BHJs is not simple, specifically, there is strong evidence for mixed fullerene/amorphous polymer regions in addition to semicrystalline polymer regions and possibly either glassy or crystalline fullerene regions. The natural gradation of band levels in the mixed regions, relative to more pure and ordered regions has been proposed as critical in enabling both charge separation and suppressing charge recombination. We present results on the driving forces for material mixing and the apparent miscibility from two independent experiments: thermal stability of initially pure bilayers and FTIR studies of the solvachromatic shift of the fullerene carbonyl frequency. We characterize the behavior of four commonly used fullerenes: PCBM-61, PCBM-71, bis-PCBM and ICBA in two canonical polymers: P3HT (low glass transition temperature, high crystallinity) and PCDTBT (high glass transition temperature, low crystallinity). Correlations between the observed miscibility and both the fundamental materials properties (Tg, Hansen solubility parameters) and the ultimate device performance will be presented.

Metalloporphyrins have been attracting attention due to being able to control of optical, magnetic, and electrical properties through combining various metal atoms in the center of porphyrin molecule. In addition, they have wide applications for state of the art electronic devices such as dye sensitized solar cells, thin film transistor (TFT) and multi-functional sensor. In previous studies, several approaches have been employed for obtaining metalloporphyrin, such as chemical method in a solution, thermal/e-beam evaporation process. [1], [2] In this work, we demonstrated a facile and effective method for deposition of metal tetraphenylporphyrin (TPP) thin film by a combined a thermal evaporation (TE) and atomic layer deposition (ALD). For the deposition of Zn-TPP thin film, TPP and diethyl zinc (DEZ) were used as organic and inorganic materials, respectively. Optimum conditions for the deposition of Zn-TPP thin film were established systematically: (1) the exposure time of DEZ as inorganic precursor and (2) the substrate temperature were adjusted, respectively. Subsequently, we studied on the ALD process by self-limited surface reaction between organic semiconductor (TPP) and metal atom (Zn). The surface and interface reactions between TPP with Zn were investigated by X-ray photoelectron spectroscopy, Raman spectroscopy, and UV-vis spectroscopy. These results show a facile and well-controllable fabrication technique for the metal-organic thin film for future electronic applications.
[1] Y. Park, K. S. Han, B. H. Lee, S. Cho, K. H. Lee, S. Im, M. M. Sung, Organic Electronics, 12 348-352 (2011).
[2] J. M. Gottfried, K. Flechtner, A. Kretschmann, T. Lukasczyk, and H. -P. Steinrück, J. Am. Chem. Soc., 128, 5644-5645 (2006).

The nature of the mixed phase in donor-acceptor bulk heterojunction (BHJ), blends of conjugated polymers and fullerene derivatives, has become an area of intense interest in recent years, particularly with respect to the relationship(s) between the microstructure, photophysical properties and device performance. We use time-correlated single-photon counting (TCSPC) and flash-photolysis time-resolved microwave conductivity (FP-TRMC) to probe exciton generation and dynamics; and carrier generation, transport and decay in a model BHJ system consisting of the thiophene-based conjugated polymer poly(2,5-bis(3-alkyl-thiophene-2-yl)thieno[3,2-b]thiophene)s (pBTTT) blended with the prototypical electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The pBTTT:PC61BM BHJ is well known to form an ordered mixed phase and we employ molecular additives to control the extent of mixing between the polymer and fullerene derivative, and subsequently correlate the photophysics with the induced microstructure. We demonstrate that by controlling the morphology of the intimately mixed phase and the pure polymer and fullerene domains we can influence the dynamics of generation and decay of photoinduced electrons and holes. Finally, we will discuss the results within the context of organic photovoltaic (OPV) device performance.

The efficiency of polymer solar cells has been greatly improved with bulk heterojunction (BHJ) concept [1]. Existing approaches [2] to reveal the obtained nanometer-scaled trait however suffer from either insufficient image contrast (electron microscopy) or equivocal model-based fitting (x-ray scattering). Scattering-type scanning near-field optical microscopy (s-SNOM), representing the newest revolution in the SNOM technology, is capable of revealing optical characteristics with a sub-10 nm resolution [3]. Here, we present the near-field mapping of pristine P3HT and P3HT/PCBM films of three different mixing ratios (1:3, 1:1 and 3:1). The s-SNOM is composed of an interferometer and an atomic force microscope with which the scattered radiation induced by a 632.8-nm excitation laser at the dithering tip apex, while the surface topography of the sample was recorded, was collected and combined with the other reference beam for extracting its amplitude and phase via heterodyne detection. The local dielectric constant can then be retrieved by analyzing the obtained amplitude and phase signals.
The phase image of the pure annealed P3HT film exhibits distinct 30-nm domains, albeit no distinguishable feature in the concurrently recorded topographic and amplitude images. The calculation based on quasi-electrostatic dipole model [3] confirms that the phase contrast is a result of the minute dielectric constant difference between the amorphous and orderly packed P3HT aggregates. Two types of signatures were identified in the phase images of P3HT:PCBM films: the first type is correlated with the signatures in the topographic and amplitude images, while the second type shows no apparent correlation. Most prominently, the size of the first-type signature increases with the PCBM mixing ratio (ranging from 20 to 60 nm), indicating that it can be assigned to PCBM aggregates. The type-two signature is, alternatively, similar to that in the pure P3HT film. The tentative assignment is confirmed with the quasi-electrostatic dipole model. This study represents the first successful demonstration to reveal phase-separated nano-domains in the blended P3HT:PCBM layer and to identify the ordered and disordered molecular packing of the pure P3HT film with high-resolution near-field optical microscopy, which is impossible with conventional techniques. Therefore, s-SNOM can be a powerful tool to investigate the key nanoscaled phase-separation traits in the high-efficiency BHJ polymer solar cells.
[1] G. Li et al., Nature Photon. 6, 153 (2012).
[2] W. Chen et al., Energy Environ. Sci. 5, 8045 (2012).
[3] J.-Y. Chu et al., Appl. Phys. Lett. 95, 103105 (2009); J.-Y. Chu and J.-K. Wang, Chap. 13, in “Advanced Photonic Sciences” (InTech, 2012).
[4] C.-K. Lee et al., Energy Environ Sci., 6, 307 (2012).

Bulk heterojunction (BHJ) solar cells incorporation of conjugated polymers and fullerene acceptors are emerging as potential low-cost alternatives for renewable energy generation. Considerable progress has been made in recent years to pursue power conversion efficiencies (PCE) of 8-10% through the development of new polymer materials. For conjugated polymers containing unsymmetric structural units, it is well-known that molecular structure, especially in terms of regioregularity, influence relevant optoelectronic properties. Herein, Regioregular conjugated polymers containing alternating pyridyl[2,1,3]thiadiazole (PT) and indacenodithiophene (IDT) structural units were synthesized. In these copolymers, the pyridyl nitrogen atoms on PT precisely arranged along the backbone so that each one has an adjacent proximal and an adjacent distal counterpart across the two IDT flanking units. We find that despite the absence of obvious differences in orbital energy levels and optical bandgap, the regioregular materials exhibit larger carrier mobility, as determined by using field effect transistor devices, and can yield 50 % higher solar cell PCE, relative to counterparts that are not synthesized with specific control.

Interfacial layers play a crucial role in the fabrication of high efficient organic solar cells as they provide selective contacts for charge carriers, affect the open-circuit voltage of the device and can act as optical spacers. [1] In the past, solution processed materials such as metal oxides and polymers like PFN have been successfully introduced as hole and electron blocking layers for organic solar cells. However most of these materials have to be included as very thin layers to keep the fill factor of the solar cells high due to their poor conduction properties. New materials for thick layer and thus robust solution processing are highly desirable for industrial applications and represent one of the challenges for printed OPV. Here, intentional doping of ZnO and TiO2 nanoparticles is used to produce thick and efficient interlayers for normal and inverted solar cell structures in combination with low band gap polymers. We discuss the influence of the interfacial layer nature (doped or not doped) and thickness on the performance of the organic solar cells and use transient open circuit voltage measurements, a technique successfully applied to P3HT:PCBM solar cells recently, [3] to study recombination processes at the metal oxide interfacial layer.
[1] R. Steim , F. R. Kogler, C. J. Brabec, J. Mater. Chem., 2010,20, 2499-2512
[2] Z. Liang, Q. Zhang, O. Wiranwetchayan, J. Xi, Z. Yang, K. Park, C. Li, G. Cao, Adv. Funct. Mater. 2012, 22, 2194.
[3] A. K. Thakur, H. Baboz, G. Wantz, J. Hodgkiss, L. Hirsch, J. Appl. Phys. 2012, 112, 044502.

Photovoltaic devices were made using two types of squaraine, 2,4-Bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]squaraine (DPSQ) and 2,4 bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ). DPSQ is blended with (6,6)-phenyl C71-butyric acid methyl ester (PC71BM) in different ratio for studying the effect of mixing ratio on the performance of solar cells. Among devices with various blend ratios, DPSQ:PCBM in the ratio 1:3 gave a better power conversion efficiency (PCE) of 0.59%. It is also found that DPSQ based solar cell with a thin interfacial layer of BCP have a better PCE due to the increased open circuit voltage. Current density - voltage (J-V) characteristics and photocurrent measurements shows that inverted bulk hetero junction solar cells based on SQ:PCBM giving higher efficiency and more stable performance than that of the normal configuration. PCE values were calculated from J-V characteristics as 1.14% and 1.32% for the normal and inverted solar cell configuration respectively. The fabricated ITO/SQ:PCBM/MoO₃/Ag inverted solar cells have no interfacial electron transporting layer. Post annealing of the device showed an enhancement in photovoltaic performance which may be due to modification in the morphology resulted from the crystallanity of squaraine. The effect of interfacial layers zinc oxide (ZnO) and poly[(9,9-bis(3-N,N- dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl fluorine)] (PFN) on the performance of the inverted solar cell were also studied.

Electron deficient monomers are the core components of n-type semiconductors and they can be copolymerized with electron rich monomers to produce low band gap polymers for solar cells. This study will assess how the size of the acceptor unit employed can affect the polymer performance in OPVs. A larger unit may lead to an increase in the effective conjugation length and a more ordered microstructure resulting in improved charge transport. It may also act as a stronger acceptor helping to reduce the band gap of the donor-acceptor polymer and therefore enhance the light harvesting potential of the cell.[1]
This talk will introduce the novel anthradithiophene bisimide (ADI) acceptor unit and its copolymerization with electron rich units. The performance of the resulting polymers will be compared with the analogous dithienophthalimide (DTP) polymers.[2] These two units were chosen due to their similar structures: both contain electron withdrawing imide functionalities and flanking thiophenes but ADI is around twice the size of DTP. The polymers energy levels will be probed and their performances in OPV devices reported and compared. To help rationalise the differences in performance and conclude whether or not a larger electron poor comonomer has a beneficial effect, morphological and mobility studies will be presented and discussed.
[1] M. Wang, X. Hu, P. Liu, W. Li, X. Gong, F. Huang, Y. Cao, J. Am. Chem. Soc. 2011, 133, 9638-9641.
[2] H. Wang, Q. Shi, Y. Lin, H. Fan, P. Cheng, X. Zhan, Y. Li, D. Zhu, Macromolecules 2011, 44, 4213-4221.

Zinc oxide (ZnO) thin films present a viable path towards electron selective contacts in organic photovoltaic devices (OPV). With a large optical band gap (~3.3 eV), ZnO is well suited as an interlayer at the transparent conductive cathode in inverted cell geometries. The low work functions (~3.8 eV) and conduction band edge close to the Fermi level of this n-doped material lead to efficient electron injection/extraction. The large ionization energy (~7.3 eV) causes hole-blocking and thereby reduces charge carrier recombination at the interface.
In this study, we investigate the energetics of solution-processed ZnO layers as well as of ZnO films grown using the same precursor by plasma enhanced atomic layer deposition (PEALD). Photoemission (PES) and Inverse Photoemission Spectroscopy (IPES) are used to determine work function as well as valence and conduction band offsets at various ZnO/organic interfaces. The performance of the films as electron transport layers is tested via current-voltage characteristics of PC71BM/PCDTBT bulk heterojunction-based solar cells.
We identify ZnO films from diethyl-zinc precursors produced in a low temperature (~120 °C) sol-gel process to yield consistently high performance results as compared to films processed at higher temperature from zinc acetate precursors and films produced via PEALD. A further treatment of the ZnO surface with phosphonic acid-based self-assembled monolayers (SAM) leads to improved device characteristics and stability due to the tailored interface energetics. The alignment of electronic levels of acceptor-type molecules is probed with the example of evaporated ultrathin layers of C60 on the bare and SAM-treated ZnO surfaces. We find that Fermi level pinning of the C60 lowest unoccupied molecular orbital (LUMO) can be achieved because of the low work function of the ZnO surface, thus allowing efficient electron injection/extraction. Complementary Near Edge X-Ray Absorption Fine Structure (NEXAFS) and resonant photoemission experiments together with density functional theory calculations reveal that the alignment is correlated with the surface defect density in the ZnO film.

Low-bandgap polymers[1] are promising materials to serve as electron donors in the photoactive layer of bulk heterojunction solar cells. In combination with a suitable electron acceptor such as PC70BM the photoactive layer covers a broad absorption range spanning from the visible to the near-infrared spectral region leading to increased photon harvesting and thus a higher photocurrent compared to mid-bandgap polymers such as P3HT.
In this study the photovoltaic performance of a low-bandgap polymer based on one diketopyrrolopyrrole (DPP) unit and five thiophene units was evaluated in Organic Photovoltaic Cells (OPVs) prepared with different solvent mixtures. Depending on the preparation conditions maximum power conversion efficiencies between 1 and 5 % could be reached. In addition the photophysical properties of the photoactive layer of the OPVs were investigated by steady-state photoinduced absorption (PIA) and broadband transient absorption pump-probe spectroscopy (TA) to get further insight into the exciton and polaron dynamics as well as into the efficiency-limiting processes in these devices.
References
[1] Gevaerts, V.; Furlan, A.; Wienk, M.; Turbiez, M.; Janssen, R. Advanced Materials 2012, 24, 2130-4.

The adapted Su-Schrieffer-Heeger (aSSH) model Hamiltonian is extended in this work to incorporate the interchain π minus; π stacking and dynamical electron-phonon coupling effects so that the photoinduced charge transfer mechanism can be directly probed at the π-conjugated heterojunction interfaces. It is found that excitons generated in the bulk poly-(p-phenylene vinylene) (PPV) phase reqire an activation energy of 0.23 eV to reach the heterojunction in- terfaces before getting their charges separated. Electron transfers from the D1^* state of PPV to the t1u^* state of C60 follow the non-adiabatic mechanism, which is accelerated by three major factors including the 0.95 eV energy drop between the two states, close vicinity of the electron- donating D1^* state to the C60 phase, and suppressed inversion symmetry of C60 at the interfaces. The irreversible phonon relaxation energy associated with the non-adiabatic electron transfer is estimated to be 0.3 eV, which explains the widely accepted empirical energy offsets between the measured open-circuit voltage and the theoretical built-in potential.

Bulk heterojunction (BHJ) organic solar cells (OSCs) are expected to combine high power conversion efficiencies with low cost fabrication based on solution processing. However, charge transport phenomena within OSCs are still not completely understood, so that progress in device fabrication is often based on a trial-and-error approach. To shed some light on the OSC fundamentals, we used scanning Kelvin probe microscopy (SKPM) to image their potential distribution. As the layer structure of OSCs is parallel to their surface, the electric transport occurs in vertical device direction. This necessitates the exposure of the device`s cross section to make it accessible to SKPM studies. We developed and adopted different methods for the preparation of OSC cross sections and studied their potential distributions with SKPM.
We prepared the OSC cross sections on three different ways: cleaving, microtome cutting and milling with a focused ion beam (FIB). We applied an OSC standard stack consisting of ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al on ITO coated glass substrates for the FIB milling and the cleaving method. The microtome cutting requires soft substrates, so we replaced the glass through gold coated plastic substrates. We monitored the IV characteristics of our devices before and after the cleaving/microtome cutting/FIB milling to assure that there is no significant change in terms of the device performance. We contacted the OSC samples in a defined manner to be able to study their potential distribution under working conditions, i.e. under illumination and bias voltages. To study the influence of the contact materials, we fabricated OSCs with contact materials varying from those mentioned above.
We could show that with all of the methods it was possible to prepare samples for meaningful SKPM studies. We studied the potential distribution of the OSC samples under bias voltages and illumination. We could localize the major voltage drops in the cell and display the photovoltage generation in an illuminated OSC under open circuit conditions.
To better understand the functionality of the contact materials, we studied OSCs with modified contact materials. We characterized their potential distributions with SKPM and compared them to the ones of our OSC standard stack. Additionally, the device performances of the OSCs were compared. Thus we were able to correlate the OSC performance to microscopic modifications in the potential distribution of the cell.
With our results we could show that SKPM studies on cross sections of organic electronic devices can provide deep insights in the device fundamentals. The ability of SKPM to directly observe the potential distribution with very high lateral resolution makes it a powerful tool for the characterization of organic electronic devices.

In polymer solar cells, the morphology of the active layer is known to play a critical role on the power conversion efficiency. However, the semicrystalline nature of the polymer semiconductor and the application of a bulk heterojunction (BHJ) structure results in a complex multiple phase system and attributing losses in energy conversion processes to the various microstructural features is difficult.
In this study we investigate differences in charge photogeneration of the crystalline and amorphous P3HT while keeping the charge collection efficiency nominally the same. This is achieved through the application of a novel strain alignment approach that preferentially aligns the crystalline P3HT while keeping the amorphous phase largely isotropic in-plane. The microstructural alignment is characterized in detail using X-ray diffraction and optical spectroscopy. The microstructure is also modeled using newly developed finite-element computational techniques that are able to elucidate how the crystalline and amorphous phases are modified under large physical deformation. With the primary optical transition dipole in P3HT along the polymer backbone, incident polarized light is able to selectively excite the crystalline and amorphous P3HT, or primarily the amorphous P3HT. Using polarized light, the absorption, power conversion efficiency, external quantum efficiency, and internal quantum efficiency are measured providing a relatively complete picture of energy conversion in the solar cell. We find that charge photogeneration is independent of aggregate P3HT quality and location of initial photon absorption. The large improvement with device efficiency is thus solely attributed to changes in charge collection efficiency. In addition, this unique device structure provides insights into the influence of P3HT morphology on charge recombination mechanisms. These results provide an important step in understanding the role of the active layer morphology on energy conversion processes.

The microstructure of neat conjugated polymers is crucial in determining the ultimate morphology and photovoltaic performance of polymer/fullerene blends, yet until recently little work has been focused on controlling the former. We demonstrate that the domain size of the crystalline regions in neat poly(3-hexylthiophene) (P3HT) and poly(2,2prime;:5prime;,2Prime;-3,3Prime;-didecyl-terthiophene) (PTTT-10) can be tuned by varying the crystallization temperature, and that crystalline domain size has a powerful impact on the yield and dynamics of photogenerated charge carriers in the films. Time-resolved microwave conductivity (TRMC) measurements show that films with larger crystalline domains, as determined by x-ray diffraction (XRD), have longer charge carrier lifetimes and lower yields of carriers compared to films with small crystalline domains. Our results suggest that the classical polymer science description of temperature-dependent crystallization of polymers from solution can be used to understand film formation in organic photovoltaics

It is well known that in organic solar cells the active layer morphology has a significant influence on the power conversion efficiency. Of particular importance is the microstructure at the heterojunction interface due to the critical processes that take place at this location. However, in many polymer-based solar cells the active layer consists of a bulk heterojunction that is a blend of the donor and acceptor molecules making it difficult to probe the structure at the junction interface.
In this work, we fabricate sharp planar heterojunction (PHJ) P3HT/PCBM solar cells with a focus on the role of polymer orientation on device performance. The solar cells are fabricated using transfer-printing methods that have been shown to form sharp bilayers between the organic layers. During fabrication, the P3HT film is independently processed to alter the out of plane orientation of the conjugated ring plane from being highly edge-on to highly face-on. The orientation of the film is characterized in detail using grazing incidence X-ray diffraction and near-edge X-ray absorption fine structure spectroscopy. In the PHJ solar cell, we find that the out-of-plane orientation of the polymer chains significantly alters the photocurrent due to a change in charge transfer dissociation efficiency. In addition, the out of plane orientation results in a shift in the HOMO energy level providing a change in open-circuit voltage. Finally, detailed device measurements along with equivalent circuit models of the PHJ solar cell provide insights into energy conversion at the heterojunction interface in the more efficient bulk heterojunction solar cell architecture.

There has been a lot of interest in solution processed inverted device architecture bulk heterojunction (BHJ) organic photovoltaic devices (OPV) due to several advantages over regular device architectures, including improved operational device life times, ease of device fabrication and also the great potential for low cost manufacturing.
Amongst many applications Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) is a well-established hole-transport material in OPV devices due to its tailored workfunction for good contacts and solution processability. When applied in inverted architectures the interface between photo-active layer and PEDOT:PSS was identified as critical for device stability due to layer delamination as one contributor to device degradation but also during a manufacturing process.[1]
In this study a new type of non-aqueous PEDOT:PSS dispersions is presented which targets this particular critical interface demonstrating a clearly improved layer adhesion of PEDOT:PSS on the photo-active layer to aim towards enhanced device stability. The work not just comprises the development of this new PEDOT:PSS formulation but also focuses on the optimization of this system including effect enhancing additives and processing methods. Complementary to the enhanced adhesion good wettability and film formation properties of these dispersions on non-polar surfaces such as photo-active materials are essential to allow device fabrication.
Thin film layer and interface characterization including techniques such as adhesion tape test and spectroscopic methods are applied to compare the adhesion behavior as well as absorption changes upon dispersion and process impact. This allows gaining a deeper understanding of the structure/function relationship and the parameters influencing the interface as well as the layer adhesion.
The experimental data suggests that the enhanced adhesion properties of this PEDOT:PSS type originate from additive effects which lead to the formation of a mixed interlayer consisting of interpenetrating photo-active material and PEDOT:PSS domains. It can be seen as a one-step in-situ interpenetration process to form a mixed interface of greater strength.
Further, the new PEDOT:PSS formulation was tested in inverted OPV devices based on the model system ITO/ ZnO/Poly(3-hexylthiophene): Phenyl-C61-butyric acid methyl ester (P3HT:PCBM)/ PEDOT:PSS/Ag to translate this added functionality into the device application where it showed an improved OPV performance over the aqueous reference system. In a final step long-term device stability tests performed allow evaluating the effects of the different improvement solutions developed and show the impact on the operational device stability.
The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2011 under grant agreement ESTABLIS n° 290022).
[1] M. Joslash;rgensen et al, Adv. Mater. 24, 5 (2012)

Organic photovoltaic devices have a great potential for efficient harvesting of solar energy due to their tunability of optoelectronic properties along with their low-cost, light weight, flexibility and ease of fabrication. We present here a promising approach to significantly improve the power conversion efficiency of planar bilayer organic solar cells using the following three stage energy cascade donor structure [1]: glass/indium-tin-oxide/Donor 1/Donor 2/Donor 3/C60/BPhen/Ag, where donors are diphenyltetracene, rubrene, and tetraphenyldibenzoperiflanthene (BPhen is 4,7-diphenyl-1, 10-phenanthroline). The donor materials were chosen on the basis of their 1) optical gap (Donor 1>Donor 2>Donor 3), 2) relative transport energy level offsets, and 3) comparable hole mobilities. The power conversion efficiency of the cascade solar cell reaches 6.21 ± 0.06 %, which is twice that of bilayer devices. This improvement is primarily due to a significant increase in photocurrent along with an increase in open-circuit voltage and fill factor. In the cascade, Donor 1 is an exciton blocker for Donor 2 which blocks excitons from Donor 3 with complementary singlet exciton Foerster transfer from Donor 1 to Donor 2 to Donor 3. We analyze the transfer and blocking properties of this structure based on photoluminescence and modeling of the exciton distributions in the donor thin films.
[1] Cody W. Schlenker, Vincent S. Barlier, Stephanie W. Chin, Matthew T. Whited, R. Eric McAnally, Stephen R. Forrest, Mark E. Thompson, Chem. Mater. 2011, 23, 4132-4140

For organic solar cells to well exceed the 10% power conversion efficiency benchmark, efficient photon-to-electrical current conversion must occur with a minimum amount of energy losses. Identification of the loss mechanisms, as well as the electronic states involved in free charge carrier generation is of great interest for the future development of this fascinating type of solar cells.
In this work we determine the free charge carrier formation efficiency as a function of photon energy and electric field for photovoltaic devices comprising high performance donor polymers blended with different fullerenes. When the photon excess energy is lowered from excitations above the polymer band-gap to exclusive excitation of the interfacial charge transfer (CT) states, both yield and field-dependence of free charge generation are almost unaffected for all blends studied. However when reducing the energy (driving force) lost per photon by varying the fullerene LUMO level, we find a reduced free carrier formation yield with strongly increased field-dependence. We show that when different fullerenes are used, the reduced generation efficiency is not caused by changes in morphology such as domain size, orientation, or domain purity.
This result indicates that the free carrier generation mechanism proceeds via the CT state manifold, irrespective of the initial photon energy, and that the field-dependence and efficiency of free carrier formation is dictated by the energetic offset between the CT state energy and the energy of the spatially separated charge pair.

It has been reported that the electronic properties of semiconducting conjugated polymers (SCPs) can be easily changed by fine-tuning of molecular structure such as atomic substitution, optimization of alky chain length and position, introduction of bridge unit and type of chain end-group. Among these fine-tuning methods, the atomic substitution, especially, the introduction of fluorine atom has been strongly attracted for few years. Recently, it has been reported that the fluorine substituted SCP-based solar cells exhibit the power conversion efficiency (PCE) over 7%. Until now, the reason of the performance enhancement by fluorine substitution in OPVs is not clearly understood although it has been known that the fluorine substitution lowers the HOMO energy levels and induces strong dipole in active layer due to its high electro-negativity.
In this study, we synthesized fluorinated poly(3,4-dialkylterthiophenes) (P34ATs) by the Stille coupling between di-fluorinated bithiophene and 3,4-dialkylthiophene in order to understand the effect of fluorine substitution on the electronic properties of SCPs and device performances of OPVs. We also investigated the effect of side chain on aggregation behavior of the polymers by introducing two different 2-ethylhexyl and dodecyl groups in the polymers.
Compared to non-fluorinated P34ATs, fluorinated P34ATs showed deeper HOMO energy level while the optical bandgap remains unchanged and also enhanced molecular aggregation as evidenced by stronger vibronic shoulder. These changes led to higher open-circuit voltage, short-circuit current (JSC), and fill factor, and thus the PCE was largely improved. Fibril structure was well-developed in fluorinated P34AT:PCBM blend, and especially finer fibrils were developed when bulky side chain(2-ethylhexyl) was attached on the chain backbone. Since nanoscale phase-separated morphology with bi-continuous interpenetrating network is essential for efficient charge transport in OPVs, fluorinated P34AT with bulky side chain exhibited the highest JSC and PCE. Simulation results with X-ray measurements lead us to conclude that fluorination of SCPs contributes not only to electronic properties, but also critically to ordering behavior and morphological structure of the polymer.

A promising method for improving organic photovoltaic (OPV) device performance is utilizing interlayers that modify the metal work-function and enhance carrier extraction. For example, poly(ethylene glycol) (PEG) was found to enhance efficiency of a Poly(3-hexylthiophene-2,5-diyl) (P3HT):Phenyl-C61-butyric acid methyl ester (PCBM) devices by up to by 70%. Recently, few studies have suggested spontaneous segregation of PEG and other additives towards the film/air surface in OPV films during film processing and annealing. However, these results often contradict the fundamental concept of surface enrichment by the component with lowest surface energy. Because PEG&’s surface energy is higher than that of P3HT and PCBM, it is not expected to enrich the surface. In this study we suggest that additive segregation, and hence interfacial composition and possibly also film morphology, are induced by the metal evaporation, namely the formation of an organic/metal interface. Importantly, deposition of the metal contact on the organic layer introduces a new interface, and the effect of its energetics on additive distribution, film morphology and interface composition has been overlooked till know. By comparing X-ray Photoelectron Spectroscopy (XPS) measurements of bare and metal-covered surfaces of films spun from P3HT:PCBM:PEG blends we show that PEG segregation towards the interface is induced by metal-PEG interaction. Furthermore, the PEG/Al interaction results in lowering the contact&’s work function and an associated dramatic improvement in device preference. Our findings suggest that the organic/metal interfacial energy in combination with additive size and composition can be utilized to enhance charge extraction and perhaps charge transport for the improvement of device performance. They also clearly demonstrate that characterization of OPV films prior to metal deposition does not necessarily represent the actual morphology in the final device.

To analyze the morphological evolution of P3HT:PCBM bulk heterojunctions during thermal treatment we derived a coarse-grained forcefield for Molecular Dynamics (MD) simulations based on a dedicated atomistic forcefield and reported experimental data. This atomistic forcefield had been was modified from a literature forcefield to improve agreement with Density Functional Theory simulation results and experimental data.[1] The coarse-graining scheme was determined from rigidity analysis of the atomistic simulations and the forcefield was tuned to fit experimental density, melting temperature range and atomistic simulation. Simulated crystal structures and radial distribution functions of P3HT and PCBM using the coarse-grained forcefield were validated against reported experimental data and results of the atomistic simulations.
The actual timescale for P3HT:PCBM blend phase separation is from seconds to minutes, which is not computationally viable for dynamics simulation. To overcome this, we first determined the interfacial energy for P3HT:PCBM for the most stable interface configuration from the coarse-grained MD simulations considering 4 types of PCBM:P3HT bilayers with respect to the P3HT orientation (face-on, end-on, edge-on and amorphous). The structures were annealed at 450 K for 10 ns before the interfacial energy information is extracted. Face-on and amorphous were determined to be the most and least stable configuration with interfacial energy of -1.34(3) eV/nm² and +0.01(1) eV/nm², respectively.
This interfacial energy information was then used to derive the P3HT:PCBM bulk heterojunction phase separated morphology in Monte Carlo simulation considering a (100 nm)³ system and a grid with a 1 nm resolution. Each grid element is characterized by the weight fraction of P3HT and phase separation from an initial mixed state proceeds via exchange of mass fractions among randomly chosen grid elements. The interface area is then identified with interface of grid elements that differ by > 50 % in their P3HT weight fraction. The final structure is averaged over the last 10⁶ frames after equilibration of the system energy. Domain size and volume fraction of bi-continuous percolation paths are analysed and the evolution of the percolation path is monitored. The phase separated structure will be the basis of continuum-level device performance simulations.
[1] To, T. T.; Adams, S. Nanosci. Nanotechnol. Lett. 2012, 4, 703-711.

Small molecule organic photovoltaic cells have made remarkable progress over the last several years. The power conversion efficiency, however, is still below values necessary for practical use. In this work, we describe a hybrid planar-mixed heterojunction (PM-HJ) organic photovoltaic cell based on tetraphenyldibenzoperiflanthene (DBP) and C70 with a power conversion efficiency of up to 6.8% ± 0.3%. Optimized cells consist of a DBP:C70 mixed layer at a volume ratio of 1:8 and a 9-nm thick C70 cap layer on top as photoactive region. The external quantum efficiency (EQE) in the visible of the PM-HJ cell is > 10% larger than the mixed-HJ cell (the latter with an average 60% EQE from wavelengths between 400 nm and 600nm) that lacks a C70 acceptor cap layer. To understand the improvement in EQE, we calculated the absorption efficiency using transfer matrix method. While the absorption efficiency in the visible is similar for both PM-HJ and mixed-HJ cells, the internal quantum efficiency of the PM-HJ is >10% larger than that of the mixed-HJ, with values >90% across the visible, indicating reduced exciton loss. We examine exciton loss mechanisms inside the OPV cells and find that the MoO3 layer, which is used in our cells as an anode buffer layer, quenches rather than blocks excitons, contrary to previous expectations[1]. The C70 cap layer redistributes optical field inside the photoactive layers, leading to reduced exciton quenching at MoO3/organic interface. The power conversion efficiency under simulated AM 1.5G, 1 sun irradiation increases from 5.7% ± 0.2% for the mixed-HJ cell to 6.4% ± 0.3%for the PM-HJ cell, with a short-current density of 12.3 ± 0.3 mA/cm2, open circuit voltage of 0.91 ± 0.01 V, and fill factor of 0.56 ± 0.01.[2] In addition, we employ a mixed cathode buffer layer in the DBP:C70 PM-HJ cells and further improve the efficiency to 6.8% ± 0.3%under simulated AM 1.5G, 1 sun illumination with an improved FF that results from reduced exciton-polaron quenching in the neat C70 cap layer.[3]
[1] D. W. Zhao, X. W. Sun, C. Y. Jiang, A. K. K. Kyaw, G. Q. Lo, D. L. Kwong, Appl. Phys. Lett. 93,083305 (2008).
[2] X. Xiao, J. D. Zimmerman, B. E. Lassiter, K. J. Bergemann, S. R. Forrest, Appl. Phys. Lett. 102,073302 (2013).
[3] A. N. Bartynski, C. Trinh, A. Panda, K. J. Bergemann, B. E. Lassiter, J. D. Zimmerman, S. R. Forrest, and M. E. Thompson, Nano Lett., ASAP.

We discuss an integrated approach for optimizing organic photovoltaic (OPV) absorber materials. The approach utilizes theoretical calculations to design candidate materials and contactless time-resolved microwave conductivity (TRMC) to evaluate those materials for OPV applications prior to their optimization in devices. The approach is demonstrated for a class of “push-pull” copolymer absorbers based on cyclopenta[c]thiophene-4,6-dione (CTD); the electronic and structural tunability of this unit provides a vast array of potential candidate materials. Theoretical calculations indicate polymer LUMO levels can be tailored nearly 0.6 eV by varying the substituents on the CTD unit. Also, the size and density of alkyl group substitution on the CTD system can be adjusted and is demonstrated to directly affect structural order in the polymer films, as determined by X-ray diffraction. We illustrate how the free charge carrier generation at the copolymer:fullerene interface, as evaluated by contactless time-resolved microwave conductivity (TRMC), is correlated with OPV device efficiency and can be used to rapidly screen new active layer materials. The results demonstrate how this approach can accelerate device optimization work on active layers with intrinsically inefficient free carrier generation and/or low carrier mobilities.

In this research, we study for efficient charge extraction via electrode interfacial layers for high performance vertical and lateral organic photovoltaics(OPVs). We demonstrate that the photo-current and the power conversion efficiency of organic photovoltaic cells OPVs can be maximized by the oriented interfacial dipoles within the water soluble conjugated polyelectrolytes (CPE) layer. We synthesized cationic and anionic conjugated polyelectrolytes and then we employed them as an electron extraction layer in polymer photovoltaic cells. In contrast to the conventional belief, both the anionic and cationic CPE layers improved the electron injection and extraction at the cathode interface, which leads to highly efficient OPVs irrespective of the ionic groups of polyelectrolytes. Therefore, we investigated the role of the conjugated polyelectrolytes as the charge extraction layer by using surface photoelectron spectroscopy. We applied external electric field on the device which employ CPE interfacial layer to make that CPE interfacial layer has a net internal electric field without external bias.

The incorporation of two or more single photovoltaic cells into a series connected tandem architecture requires a virtual middle electrode, or recombination zone (RZ), that collects photogenerated holes from one subcell and photogenerated electrons from the other subcell.. Similarly to the charge generation, the efficiency of the recombination processes is dictated by the interfacial energy level alignment controlled by the material selection and/or interface architecture. Leveraging these two factors is the subject of this study. For evaporated organic solar cells (OSCs), existing RZs consist either of extremely thin metal layers (sub nm) or doped transport layers. In both cases, tunnel junctions are required in order to minimize voltage drops at the RZ.
In order to evaluate different RZs, we develop a highly efficient tandem structure (up to 7.6% from subcells with ~6% efficiency each), using 2 complementary absorbing material systems. This structure has a high open-circuit voltage of 1.7 V, together with optimized fill factors up to 67%, making it a good test-vehicle for RZ evaluation. We demonstrate that the use of light and voltage biased spectral response measurements together with transfer-matrix based modeling are useful tools to gain a deeper insight in the limits of the device. Depending on the case higher efficiencies can be obtained by in fact allowing leakage currents in the current limiting subcell. The excess photocurrent of the other cell will be pushed through the current limiting subcell, removing the need for perfect current matching of the two subcells.
In a next step, the RZ is selected from a list of (doped) organic materials (PTCBI, BCP, pristine or doped with Yb) and (doped) metal-oxides (molybdenum oxide (MoO3), pristine or doped with Ag), with an optional thin Ag layer between the two. We demonstrate that, although the use of an undoped organic / thin Ag / undoped metal oxide stack creates working tandem devices, the Ag thickness is very critical and it dependent on the used organic layer. In contrast, incorporation of doped organic layers relax the thickness requirements, and remove the need of the thin Ag entirely. The use of a doped MoO3:Ag layers also allow for a broader thickness variation of the RZ, but induces a small voltage drop, and still requires the thin Ag layer.
We suspect that the thin Ag layer induces defects in the underlying organic transport layer, creating a lossless tunnel junction for holes from the organic layer into the MoO3. For an inverted MoO3 / Ag / organic stack, the organic layer is not damaged, and hence this structure is shown not to work in a tandem configuration. For the doped organic layers, the tunnel junction is created by depletion of the carriers that are generated by electrical doping. We demonstrate that it is possible to fabricate inverted, all evaporated tandem cells with these doped transport layers, with efficiencies matching the ones of the regular architecture.

To date, organic tandem solar cells yield power conversion efficiencies (PCE) beyond 10% [1]. Based on calculations, however, PCEs up to 15% should be achievable by combining two single-junction solar cells of complementary absorption [2]. Most high-efficiency tandem cells rely on rather sophisticated device architectures (up to thirteen organic layers), requiring significant amounts of laboratory time and material for trial and error optimization. Hence, simulation-guided optimization is essential to closing the gap between theory and experimental performance.

In this work, we present small-molecule tandem cells based on a simplified device stack, comprising at most three organic layers. The merocyanine dye HB226 is chosen due to its unique strong absorption at short wavelengths around 550 nm, thus having minimum spectral overlap with the well-investigated zinc phthalocyanine (ZnPc) or other long-wavelength-absorbing merocyanine dyes. Moreover, the dye HB226 is thermally stable and soluble, allowing for processing flexibility in device fabrication [3].
We combine the dye HB226 with ZnPc in a fully thermally evaporated tandem configuration. The device optimization is guided by optical simulations based on a transfer-matrix model. We demonstrate that the resulting tandem solar cell is 50% more efficient than either one of the subcells in an optimized single-junction configuration. The optimized tandem device yields a PCE of 4.5% [4].
Furthermore, we demonstrate a fully merocyanine tandem cell, comprising two solution-processed subcells based on the dyes HB226 and EL86. This proof-of-concept tandem device highlights the unique processing flexibility as well as the tunable absorption spectrum of merocyanine dyes, thus making them promising candidates in tandem cell applications. Here, the best performing tandem device yields a PCE of 2.2%.
[1] L. Dou, et al. Nat. Photonics, 2012, 6, 180.
[2] T. Ameri, et al. Energ. Environ. Sci., 2009, 2, 347.
[3] N.M. Kronenberg, et al. Adv. Mater., 2010, 22, 4193.
[4] V. Steinmann, et al. Organic Electr., 2013, 14, 2029.

The organic photovoltaic devices have attracted great attention as the 3rd generation solar cell. During device operation, the charge transfer, dissociation and recombination rate will determine the overall device performance. One of the major losses is the electron hole recombination at the interface due to the high binding energy between electron-hole pair in the organic semiconductors. In our study, we carried out a systematic experiment on examining how exciplex recombination rate at the donor/acceptor interface influences the power conversion efficiency. We have designed a mono layer that can be inserted between donor/acceptor interface to suppress the electron back recombination and facilitate the charge transportation. As a result, the modified device showed two orders of magnitude improvement in photocurrent at short circuit condition.

We study the effect of hexagonal boron nitride (h-BN) surface passivation on cadmium selenide (CdSe) quantum dots based bulk heterojunction (BHJ) solar cells. The solar cells are fabricated with TiO2 and P3HT as electron acceptor and hole conductor, respectively. Chemically exfoliated h-BN nanoflakes by ultra-sonication process are spin coated on TiO2 before depositing CdSe quantum dots. The BHJ solar cell employing h-BN passivated TiO2 is compared with that without h-BN surface passivation, while CdSe acts as a major photoactive material in both device structures. The UV-visible absorption spectroscopic measurement performed on CdSe quantum dots exhibits effective photo-absorption in a wavelength range of 350 nm~750 nm, which is further confirmed by the external quantum efficiency of the BHJ solar cells. The photo-generated electrons in CdSe quantum dots are injected into the conduction band of TiO2. Major interfacial recombination process occurs at TiO2/CdSe interface due to the presence of TiO2 surface states. We observe h-BN on TiO2 surface helps to effectively reduce interfacial recombination, resulting in improved electron lifetime. Deposition of h-BN on TiO2 assists in passivating the surface states. Hence the photo-injected electrons from CdSe can diffuse into the TiO2 bulk and subsequently reach the electrode with significantly reduced interfacial recombination. We perform optical and electrical characterization, supported by theoretical analysis, to demonstrate h-BN as an effective surface passivation material for BHJ solar cells.

The ideal bilayer organic photovoltaic (OPV) cell has electron-donor and -acceptor materials sandwiched between two carrier-selective and exciton-blocking contacts. Two frequently employed hole-extracting contact materials are MoO₃ and NiO. It has recently been shown that MoO₃ quenches excitons on common donors with similar efficiency to C₆₀.¹ This quenching is expected, considering the very deep frontier orbital energy levels that form a staggered-gap heterojunction with the donor, analogous to a donor-acceptor interface. NiO, however, has favorable energy levels that straddle most donors;² nonetheless, we find that it also efficiently quenches excitons. For a bilayer architecture, the consequence of quenching is that approximately half of the photons absorbed in the donor can be collected as photocurrent; losses in bulk heterojunction cells are reduced, but still exist.¹
While vacuum-deposited organic blocking (i.e. non-quenching) materials are available, few such materials exist for solution-processed devices. We find that treating NiO with various phosphonic acids significantly reduces the exciton quenching, and consequently, improves device efficiency. Bilayer OPVs made with solution-cast diphenylanilino-functionalized squaraine donors (DPSQ)³ and vacuum deposited C₆₀ that employ MoO₃ buffers have a power conversion efficiency of PCE=4.8%⁴ while similar devices fabricated on the treated NiO buffers have PCE=5.6%. This increased efficiency arises from a ~30% increase in the external quantum efficiency of the donor without affecting the response from the acceptor, the fill factor, or the open circuit voltage. We will describe techniques for applying the various phosphonic acids to NiO, as well as the general applicability of this contact architecture.
1. X. Xiao, J.D. Zimmerman, et al., Appl. Phys. Lett. 102 (7), 073302 (2013).
2. M.D. Irwin, J.D. Servaites, et al., Chem. Mater. 23 (8), 2218 (2011).
3. S. Wang, L. Hall, et al., Chem. Mater. 23 (21), 4789 (2011).
4. J.D. Zimmerman, X. Xiao, et al., Nano Lett. 12 (8), 4366 (2012).

The built-in voltage is a key parameter for the solar cell performance. Its influence significantly differs for inorganic and organic solar cells: in inorganic solar cells it sets the upper limit for the open-circuit voltage [1] while for organic solar cells the underlying physical processes are still controversially discussed. However, the effect of the built-in voltage on the JV-curve is significant, leading to a considerable s-shape for small built-in voltages. [2].
We investigate the built-in voltage of small-molecule organic solar cells based on the pin-concept using electroabsorption spectroscopy. This technique analyzes the change in absorption due to an electric field (Stark effect). Changing the bias voltage applied to the device results in a variation of the Stark signal, which is probed adding an AC voltage on top of the DC bias enabling the detection using a lock-in amplifier. The simultaneous evaluation of several electroabsorption spectra for different bias voltages by a fitting routine provides information about the built-in voltage of complete organic solar cell devices. Due to the analysis of an entire spectral range detected at the first as well as at the second harmonic frequency, the influence of interface states can be evaluated and reduced.
As model systems, both flat and bulk heterojunction solar cells containing BF-DPB and C60 embedded between two doped charge carrier transport layers are investigated. The doping concentration of both the hole and the electron transport layer is modified and it is shown that their work functions directly control the built-in voltage.
Furthermore, we studied both the short-circuit current and the fill factor dependence on the built-in voltage.
References
[1] B. A. Gregg and M. C. Hanna, “Comparing organic to inorganic photovoltaic cells: Theory, experiment, and simulation,” Journal of Applied Physics, vol. 93, no. 6, p. 3605, 2003.
[2] W. Tress, K. Leo, and M. Riede, “Optimum mobility, contact properties, and open-circuit voltage of organic solar cells: A drift-diffusion simulation study,” Physical Review B, vol. 85, pp. 1-11, Apr. 2012.

The efficiency of single layer bulk heterojunction solar cells is heavily limited by the incomplete photon absorption of the rather thin photoactive layer. In particular the fullerene derivatives often used as electron acceptors in current state-of-the-art devices limit the maximum possible photocurrent due to the intrinsically low absorption of the fullerene molecules. Hence, substituting the ubiquitous fullerenes with strongly absorbing non-fullerene acceptor molecules is a promising way forward to overcome the present efficiency limits. However, at present the device performance of non-fullerene acceptors is still significantly lower than that of fullerene-based devices for reasons not understood at all.
In this contribution we present recent insights into the efficiency-limiting processes in photovoltaic blends that use electron-rich polymers or small molecules as donors in combination with non-fullerene acceptors. Specifically, we report the exciton and charge carrier dynamics in a blend of a prototypic low-bandgap polymer (PBDTTT-C) and a perylene diimide (PDI) acceptor that has an efficiency of around 1.2 % and in which the blend&’s components exhibit complementary absorption spectra thereby covering the entire visible wavelength range up to 800 nm. Using broadband Vis-NIR transient pump-probe spectroscopy over a time range from femto- to milliseconds we show that after selective excitation of the polymer exciton dissociation into free charges is much slower compared to fullerene blends using the same donor polymer and thus a significant photocurrent loss due to exciton recombination occurs. Furthermore, charge transfer at the interface results in a large pool of coulombically-bound charge-transfer states, which undergo fast geminate recombination and hence are lost for photocurrent generation. Selective excitation of the PDI leads on the one hand to delayed hole transfer to the polymer, however, on the other hand also to exciton trapping in PDI excimers which is identified as an additional loss channel. Using time-delayed collection field experiments we also investigate the field dependence of free charge generation, which we compare to common polymer:fullerene blends. Furthermore, we demonstrate that free charges generated in the blend undergo non-geminate recombination on a timescale comparable to fullerene-based blends showing that these charges are sufficiently long-lived to be extracted as photocurrent from the device. We have very recently extended these experiments to other novel donor materials including state-of-the-art polymers such as PTB-7 as well as the small donor molecule p-DTS(FBTTh2)2 which give 1.9 % PCE and 3.0 % PCE under optimized conditions, respectively. The importance of the individual photocurrent loss channels of the respective material systems is presented and potential solutions to overcome the efficiency limits are discussed.

Efficient charge carrier generation in organic bulk-heterojunctions is strongly related to dissociation of the primary singlet excitons at the donor-acceptor interfaces. Charge transfer states (CTS) are intermediate but essential states between free electrons and holes and strongly bound neutral excitons. To increase the power conversion efficiency of organic solar cells (OSC) it is essential to improve converting of singlet, triplet as well as charge transfer excitons into free charges and to suppress their wasteful recombination.
To unambiguously probe such states with different spin-multiplicity, we applied cw-, pulsed electron spin resonance (ESR) and optical detection of ESR. This allowed us to monitor transformations of CTS on a time scale from several hundreds of nanoseconds to tens of microseconds and the degree of their delocalization. [1] Using light-induced ESR spectroscopy to study the electronic structure of CTS in blends of polymers (P3HT, PCDTBT, and PTB7) and fullerene derivatives (C60-PCBM and C70-PCBM) we found that in all three systems the positive polaron is distributed over distances of 40 to 60Å on the polymer chain. This corresponds to about 15 thiophene units for P3HT, approximately three monomer units for PCDTBT, and about three to four units for PTB7. [2] Strong delocalization of the positive polaron on the polymer chain is an important prerequisite for an efficient charge separation in bulk-heterojunctions as it minimizes the field-dependence of charge separation and reduces charge recombination.
Triplet exciton formation may be considered as charge carrier loss channel; however, the fusion of two triplets may lead to a formation of singlet excitons instead. Thus, the generation of charges by utilizing the so far unused photons might be possible. We surprisingly found triplets even in highly-efficient organic photovoltaic systems and proposed a scenario how these triplet excited states were formed, namely via electron back transfer (EBT) from acceptor to donor. [3] In summary, the fundamental understanding of the transformation processes involving the CTS, triplet excitons, free electrons and holes and their dependence on nanoscale morphology and energetics of blends is essential for the optimization of the OSC performance.
References: [1] J. Behrends, A. Sperlich, A. Schnegg, T. Biskup, C. Teutloff, K. Lips, V. Dyakonov, R. Bittl, Phys. Rev. B 85, 125206 (2012). [2] J. Niklas, K. L. Mardis, B. P. Banks, G. M. Grooms, A. Sperlich, V. Dyakonov, S. Beaupre, M. Leclerc, T. Xu, L. Yu, O. G. Poluektov, Phys. Chem. Chem. Phys.15, 9562 (2013).[3] M. Liedtke, A. Sperlich, H. Kraus, A. Baumann, C. Deibel, M. Wirix, J. Loos, C. Cardona, V. Dyakonov, J. Am. Chem. Soc. 133, 9088 (2011).

ABSTRACT BODY:
A new generation of thin-film photovoltaic cells based on organo-metal halide perovskite absorbers has recently emerged with extraordinary power conversion efficiencies. We have recently shown that charge carriers are capable of travelling over distances of up to a micron in some perovskite absorbers [1], which exceeds their typical absorption depth, thus making these materials suitable for planar heterojunction solar cells. However, the mechanism causing such an extended diffusion range remains mysterious, given that long charge-carrier diffusion distances require both low recombination rates and/or high charge mobility. Satisfying both requirements simultaneously is generally difficult given the fundamental Langevin limit for kinetic recombination, which typically holds for conductors with charge mobilities below the order of 1-10#8239;cm²#8239;V^minus;1#8239;s^minus;1.
We demonstrate here [2], that both CH₃NH₃PbI₃ and CH₃NH₃PbI_3minus;xCl_x exhibit unexpectedly long charge carrier diffusion lengths because of non-Langevin charge carrier recombination. Using transient THz spectroscopy, we determine that bi-molecular recombination rates are abnormally low in these materials, defying the Langevin limit by at least four orders of magnitude. We also establish lower bounds for the high-frequency photoinduced charge mobility of 11.6#8239;cm²#8239;V^minus;1#8239;s^minus;1 for CH₃NH₃PbI_3minus;xCl_x and 8#8239;cm²#8239;V^minus;1#8239;s^minus;1 for CH₃NH₃PbI_3minus;x, which are remarkably high for solution-processed materials. The combination of high charge mobility and low bi-molecular recombination leads to carrier diffusion lengths that exceed one micron and are significantly longer for the mixed halide system. We propose that such reduced bi-molecular charge recombination arises from spatial separation of electrons and holes in the system, which may be tuned through substitutions affecting the electronic structure of the metal-halide system.
[1] S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 342 (2013) p. 341.
[2] C. Wehrenfennig, G. E. Eperon, M. B. Johnston, H. J. Snaith, L. M. Herz, Adv. Mater. (2013) DOI: 10.1002/adma.201305172

We have investigated the photophysics and morphology of blends of the polymer PTB7 with PC71BM to understand why these are such efficient materials for organic photovoltaics. We present a comprehensive study of nanoscale phase separation, charge transport and recombination. We report exciton dissociation dynamics into charge carriers, as well as a measurement of the exciton diffusion length in PC71BM. We find that in blends spin-coated from a solution (without additive) the fullerene forms pure 20-60 nm clusters which in turn form ~150 nm fullerene-rich domains. In these blends fullerene exciton dissociation into charge pairs occurs with an efficiency of 90%, yet the solar cell quantum efficiency (IQE) is only 40%, indicating that only a minority of charges are extracted. In contrast, blends spin-coated from a solution with a high boiling point additive form fibre-shaped alternating fullerene and polymer-rich domains which are <40 nm wide and 100-300 nm long in which fullerene excitons dissociate into charge pairs with an efficiency of 98%, enabling the device IQE to reach 80%. Our results indicate that charge extraction efficiencies with the additive is twice higher than without , and so we conclude that fibre-shaped phase domains are highly beneficial for organic solar cells because they give efficient charge extraction whilst minimising recombination.

In Dye Sensitized Solar cells it is well established that ultrafast transient absorption is a powerful tool for determining the efficiency of injection from the dye exciton into the titanium dioxide. Recently, it has been established that ultrafast transient absorption surfaces measured for Dye Sensitized Solar cells contain significant features arising from the Stark effect on ground-state dyes of the photoinduced fields created by separated charges. Here we present the formalism and model we have recently developed to extract important novel information from the Stark effect portions of the transient absorption surface [1].
We show that rather than providing information regarding excited state concentrations, this Stark region reveals how the photogenerated charges move in relationship to the TiO2 surface. This means transient absorption data can provide a direct observation of the drift and diffusion of charges in a solid state dye sensitized cell directly after their formation. This new experimental observable is presented for two benchmark organic dyes. Interestingly, we see, particularly in one case, that charges tend to drift back towards the interface after injection. Over and above incomplete pore filling, such a tendency of charges to return to the interface may help to explain the comparatively high rates of recombination in solid state dye sensitized cells.
1. Meister, M.; Baumeier, B.; Pschirer, N.; Sens, R.; Bruder, I.; Laquai, F.; Andrienko, D.; Howard, I. A., Observing Charge Dynamics in Surface Reactions by Time-Resolved Stark Effects. The Journal of Physical Chemistry C 2013, 117, 9171-9177.

Drift-diffusion simulations were performed to understand the role of charge carrier recombination in bulk-heterojunction devices.
For a typical PTB7/PC70BM device, at working voltages close to open circuit, the voltage-dependent transient photocurrent (TPC) shows photocurrent overshoot following illumination turn-on. For pristine and diiodooctane--treated devices, simulations successfully reproduced the experimental TPC measurements using the Shockley-Read-Hall (SRH) recombination formalism. The simulations revealed that trap-assisted recombination plays an important role in this class of device, providing an extra charge recombination loss channel. The reduction of photocurrent overshoot and improvement in I-V characteristics in diiodooctane-treated devices could be further reproduced by decreasing the number of electron traps. This result is indicative of a change in bulk morphology resulting in a reduction of isolated PCBM domains which can act as electron traps.
Measurements of P3HT/PCBM devices under magnetic fields have shown an increase in dark current with magnetic field under low-temperature and high-bias conditions. We modeled these experimental findings and could demonstrate that the observed increase in dark current is due to dissociation of long-lived triplet charge-transfer states. These charge-transfer states are formed through bimolecular charge recombination. The findings are consistent with previous reports [1] which suggest magnetic field reduces the intersystem crossing rate between singlet and triplet charge-transfer states thereby leaving triplet charge transfer states to be harvested into free charges.
[1] J. Wang, A. Chepelianskii, F. Gao, N.C. Greenham Nat. Commun. 2012, 3, 1191

We have studied the charge generation dynamics in diblock copolymer films based on 3-hexylthiophene and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT-PCBM) units by transient absorption spectroscopy. The P3HT-PCBM diblock copolymer films exhibit microphase-separated domains with a periodic structure of 10-20 nm in width, which are similar to crystalline fibrils of P3HT reported for P3HT/PCBM blend films. After thermal annealing, the absorption and photoluminescence spectra of P3HT-PCBM copolymer films are similar to those of blend films based on poly(3-hexylthiophene) (P3HT) and PCBM, suggesting similar crystallinity and domain size of the P3HT phase. Immediately after the laser excitation, polymer singlet excitons are generated first. Subsequently, polymer polarons are generated from singlet excitons efficiently. As in the case with P3HT/PCBM blend films, there are two pathways for the charge generation: one is the prompt generation at an interface of P3HT and PCBM domains and the other is the delayed generation after the exciton diffusion to the interface on a time scale of tens picoseconds. In other words, there is no distinct difference in the charge generation from singlet excitons between P3HT-PCBM diblock copolymer films and P3HT/PCBM blend films. This is consistent with similar phase-separated structures as mentioned above. A part of polarons generated in P3HT disordered domains are transferred to more stable ordered domains both in P3HT-PCBM diblock and P3HT/PCBM blend films with a time scale of a few hundred picoseconds. Some polarons in P3HT-PCBM diblock copolymer films geminately recombine to the ground state but no geminate recombination is observed in P3HT/PCBM blend films. This is probably due to some defects such as micelle structures in microphase-separated domains where fullerene units are isolated by surrounding P3HT chains. The geminate recombination is one of the major loss processes in P3HT-PCBM diblock copolymer films.

One approach to increase the efficiency of polymer solar cells is to enhance the open-circuit voltage (Voc) by using acceptor materials with a higher LUMO level than the most common acceptor PCBM. Indene-C60 bisadduct (ICBA) was shown to enable efficiencies in P3HT solar cells of 6.5% in contrast to less than 4% with PCBM.[1] When other polymers are blended with ICBA, however, a decrease of short-circuit current density (Jsc) and fill factor (FF) is often observed along with an increase of Voc.[2,3] Understanding the efficiency limiting mechanism in blends of amorphous polymers and fullerene multi-adducts could yield currently missing guidelines for material development and morphology optimization.
As relevant example, we study bulk heterojunction solar cells with the polymers HXS-1 and PTB7 as both reach efficiencies of over 5% with PCBM as well as Voc values of around 1V with ICBA. They are compared with P3HT cells where the efficiency is not deteriorated in blends with ICBA. The experimental focus here is on transient absorption spectroscopy (TAS) that covers a large time range from 100fs to 0.5ms and therefore allows studying all the crucial processes of exciton dissociation and the generation, recombination and transport of charge carriers. To our knowledge it is the first time that fs and mu;s TAS are combined to thoroughly analyze the decrease of Jsc and FF in blends of amorphous polymers and fullerene multi-adducts. Results from fs TAS suggest immediate exciton dissociation and charge carrier generation in the first picoseconds, indicating that the smaller LUMO-LUMO offset between polymer and ICBA does not prevent efficient exciton dissociation. Besides, the absorption of the ICBA containing blends is comparable to that of the PCBM containing blends and the PL quenching is very efficient in both cases, so the charge generation is expected to be similar. The low FF<0.5 and the high voltage dependence of the current under illumination suggest hindered charge carrier transport in polymer:ICBA cells, which is further supported by electrical simulations with low electron mobilities. The faster decay of polarons indicates losses by recombination that are probably caused by the finer morphology of ICBA blends that AFM phase images show. Charge carrier transport and bimolecular recombination are further studied with mu;s TAS.
A possible explanation for the suboptimal morphology and hence the deteriorated charge carrier transport in polymer:ICBA blends is increased intercalation of the ICBA and polymer molecules so that electron transport is blocked by polymer and fullerene side chains as well as polymer backbones. Improving the morphology and preventing intercalation could increase Jsc and FF of these polymer:ICBA solar cells, which together with the Voc of 1V could result in efficiencies up to 10%.
[1] G. Zhao et al., Adv. Mater., 2010, 22
[2] M.A. Faist et al., Adv. Energy Mater., 2013
[3] N.C. Miller et al., Nano Lett., 2012, 12

Solar cells based on blends of conjugated polymers with fullerenes, such as PC60BM and PC70BM, have been intensively investigated but there are far fewer reports on blends that incorporate non-fullerene electron acceptors. One interesting feature of non-fullerene acceptors is that they typically have higher absorption coefficients than fullerenes and can therefore make a greater contribution to the photocurrent through photoexcited hole transfer to the polymer. Furthermore, they also present a broader platform on which to investigate the effects of blend film structure on the photophysics of the system and ultimately device performance. We report the results of an investigation into the nature of photoexcitations in blends of P3HT with the small molecule electron acceptor K12[1] using photoinduced absorption (PIA) spectroscopy, photoluminescence quantum yield, steady-state and time-resolved photoluminescence measurements. K12 has a tendency to crystallize and the performance of devices incorporating blends with P3HT depends strongly on the blend ratio and processing conditions.[2] Also, since the absorption of K12 overlaps strongly with that of P3HT it is a model system for investigating the photophysics of a blend where photoexcitations are generated in both the donor and acceptor. Blends of varying blend ratio were prepared and characterized before and after thermal annealing. To assess the degree of crystallinity present in the film and the relative size of any domain structure, the blends were imaged using polarized light microscopy before and after annealing. Prior to annealing the P3HT-rich blends were less crystalline than blends that were K12-rich with strong quenching of both the P3HT and K12 photoluminescence. The photoluminescence lifetime of the K12 decreases with increasing P3HT content in the blend confirming that hole transfer to the P3HT is occurring. The PIA spectrum of the as-cast P3HT-rich films reveals the characteristic positive polaron absorption features of P3HT, although upon annealing, the strength of the signal decreases and an additional peak at ~1 eV emerges. This peak was also observed in both neat K12 and the K12-rich blends, increasing with the photoluminescence signal, and is thus assigned to triplet excitons. Therefore, although hole transfer from the K12 to the P3HT can occur the structure of the blend is not optimized for efficient harvesting of the K12 excitons. These results highlight the need to optimize the film structure for harvesting of both the donor and the acceptor excitons when both contribute directly to the photocurrent.
References
[1] P. E. Schwenn et al., Adv. Energy Mater. 2011, 1, 73-81.
[2] P. Wolfer et al., J. Mater. Chem. A 2013, 1, 5989.

Efficient operation of organic solar cell relies on the multifunctional interlayer materials that are placed between the active layer and metal electrodes. Recently, significant improvement of power conversion efficiencies of bulk heterojunction devices has been achieved at the expense of developing a new class of interlayer materials that share a common functionality - dipolar chemical groups. In this presentation, we use a series of conjugated polyzwitterionic (CPZ) materials with all-thiophene or thiophene-benzothiadiazole backbones and various pendant dipolar side chains and discuss the mechanism of operation of such materials as efficient electrode interlayers. Integration of CPZs into photovoltaic devices, composed of [3,4-b]-thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM), enabled a 6-fold increase of power conversion efficiencies up 5.78 % for the devices with Ag cathode, and up 7.36 % for the devices with Al cathode. We present a correlated study of the device performance, electronic properties of interlayer/cathode interfaces, and orientations of CPZs&’ molecular functional groups atop the active layer, accessed by the ultraviolet photoelectron spectroscopy (UPS) and the near-edge X-ray absorption fine structure (NEXAFS) technique, respectively. The UPS revealed that CPZs create an interfacial dipole (Δ) with Ag electrode, alternatively described as a metal work function modification. A systematic correlation between the increased magnitudes of Δ and improved PCEs, as a result of larger open circuit voltage, short-circuit current and fill factor in devices with different CPZs were found. A “face-on” orientation of CPZ backbones and no vertical alignment of dipolar side groups atop the active layer suggest that the macroscopic interfacial dipole Δ forms as a result of dipolar side groups-metal surface interaction. A model of electrostatic self-alignment of dipoles at metal surface, rationalizing the origin of interfacial dipole created by zwitterions, will be discussed.

Molybdenum Oxide (MoO3) is one of the promising anode buffer layer (ABL) for high performance organic photovoltaic (OPV) device. One of the current accepted understanding is that the high work function of MoO3 gives a significant shift in the vacuum level that enhanced band bending at the MoO3/organic interface, which favours the hole extraction in OPVs [1-3]. However, a competing hypothesis suggest that naturally occurring oxygen vacancy defects in MoO3 show up as filled states in the forbidden energy gap and provide additional pathways for the enhancement of holes extraction [4,5].
We studied both the work function and the defects of MoO3 in detail. We examined the stability of annealing and sputter generated defects, some of their re-oxidation kinetics, and how they affect the OPV performances. In addition, since defects affected the work function of the MoO3, a systematic study can show that these defect states forms an important if not dominating role in improving the OPV performances. While this similarly gives a conclusion of an enhanced hole extraction rate, this is shown not to be predominantly a field effect at the MoO3/organic interface. We have also ruled out enhancement in bulk conductivity through surface treatment. From these findings, we therefore conclude that one of the role of MoO3 to improve OPV device performance is to provide an additional pathway for holes extraction, through the interface defects.
However, field effect does aid OPV performance through electron blocking effect. This is largely dependent on the work function of MoO3 since the interface field will be affected. In this aspect, we studied how the work function evolves through annealing and the generation of defects. In addition, we examine spontaneous nitrogen adsorption on MoO3 and discussed how this affected its work function. This is further verified using atomic nitrogen surface treatment.
In conclusion, we examined in detail two major mechanism for improvements of OPV using MoO3 as an anode buffer. Namely the field enhancement effect and defects enhancement effect. We show that at larger work function ranges of MoO3 , the defects enhancement should dominate, but lower work functions will affect electron blocking capability at the anode interface.
[1] D. Y. Kim et al., Applied Physics Letters, vol. 95, p. 093304, 2009.
[2] S. W. Cho et al., The Journal of Physical Chemistry C, vol. 114, pp. 18252-18257, 2010.
[3] K. Kanai et al., Organic Electronics, vol. 11, pp. 188 - 194, 2010.
[4] S. Y. Chiam et al., Solar Energy Materials and Solar Cells, vol. 99, pp. 197-203, 2012.
[5] B. Dasgupta et al, The Journal of Physical Chemistry C, vol. 117, pp. 9206-9211, 2013

The primary function of the contacts in organic photovoltaics is efficient extraction of charges while maintaining the open circuit voltage (VOC) through Fermi level offsets. A number of loss processes can reduce photogenerated carrier collection as applied voltage approaches VOC: reduced internal electric fields; increased diffusion current; injected carriers and/or a buildup of carriers from slow charge collection; space charge effects; and increased bimolecular recombination. Solution processed materials have high defect density and hence, are poorly described by abrupt energy band diagrams, making elucidation of contact-induced effects complicated. In the case of the interface between the blended heterojunction and the contact, the physical defects, polymer/fullerene grain boundaries, impurities, etc. will possibly give rise to dipoles, recombination centers, and essentially, leakage pathways that are templated by the contact materials. These interfacial states, formed in the “dark” and induced by the contact, may alter the dynamics of the system and result in dramatic differences in device performance, prior to, and after, any photo-excitation event.
The implementation of the selective interlayers is hypothesized to mitigate leakage of undesired charges across the active layer/contact interface. Thermodynamic selectivity for holes (electron blocking) is obtained using a material with a work function at or greater than the charge transport level for holes and a conduction band closer to vacuum than the transport level for electrons in the active layer, thus blocking electrons from crossing the interface while preferentially extracting holes. However, little is understood about how the device performance is influenced by the hole-collection rates at the interlayer/active interface, relative to rates of recombination. The impact of the work function and thermodynamic selectivity of hole collection interlayers on the open circuit voltage, photocurrent, and shunt resistance in bulk heterojunction organic photovoltaics is examined for poly(N-9‘'-heptadecanyl-2,7-carbazole-alt-5,5-(4&’,7&’-di-2-thienyl-2&’,1&’,3&’-benzothiadiazole) (PCDTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) solar cells. Direct correlations are found between the observed dark current and shunt resistances in the presence and absence of hole-selective properties of the interlayer (electron blocking). Light-intensity dependent current-voltage measurements yield insight into recombination and space-charge contributions, with clear indications of multiple mechanisms influencing contact-controlled recombination and extraction. Finally, transient photocurrent measurements are used to evaluate charge extraction and recombination dynamics in the organic photovoltaics as a function of the contact. Contributions to open circuit voltage will be outlined, with clear delineations between selective and non-selective contacts and effects of parasitic resistances.

We have investigated carrier collection efficiency (eta;_cc) in organic bulk-heterojunction solar cells with self-organizing small molecules, i.e., liquid crystalline pyrrolopyrrole derivative and C₆₁PCBM. Here we assume that external quantum efficiency is described as multiplying efficiencies of (1) light absorption and exciton generation, (2) exciton diffusion to the D/A interface, (3) transfer from exciton state to charge transfer exciton state, and (4) charge collection (eta;_cc). In this definition, eta;_cc includes the processes of carrier dissociation from weak coulomb interaction in charge transfer exciton state to free carriers, and the carrier conduction from D/A interface to electrodes. To evaluate collection efficiency, external quantum efficiency under reverse bias was measured. When the applied reverse bias was increased, EQE was also increased because of increased carrier range, and EQE was finally saturated because eta;_cc reached nearly 100%. eta;_cc was calculated by EQE/(EQE with bias) assuming efficiency of (1)-(3) don't depend on applied electric field. We found that the eta;_cc of molecularly parallel-oriented thin-film was 70% at maximum, which was 7 times higher than that of perpendicularly-oriented cells; indeed, the power conversion efficiency of the cell was improved in parallel-oriented thin-film. We also measured eta;_cc under different incident light intensity. Judging from relatively insensitive eta;_cc to light intensity, we concluded that the carrier collection was dominated by mono-molecular recombination process supposedly deriving from carrier traps of defects. These results indicate control of molecular orientation in the bulk-heterojunction cells, i.e., the ordered bulk-heterojunction solar cells is promising for further improvement of the power conversion efficiency in organic solar cells.

Some of the best-performing organic photovoltaics (OPVs) use conjugated polyelectrolytes, CPEs, as interface modifiers between the photoactive layer and the cathode [1]. These CPEs are processed from polar solvents such as alcohols and thus can be easily deposited on the OPV active layer via solution processing, making them compatible with roll-to-roll processing. Improvements in short-circuit current, open-circuit voltage (Voc), and fill factor have been reported, however the mechanism by which CPEs and the associated polar solvent enhances the performance of OPVs is currently unclear [2].
We show that in an efficient OPV system, PTB7:PC70BM, the improvement in Voc by introducing a thin conjugated polyelectrolyte (CPE) layer at the electron-extracting electrode is attributed to the exposure of the buried PEDOT:PSS layer to the methanol (the solvent for the CPE). We used time-resolved optical absorption spectroscopy to track the evolution of photogenerated holes in these devices and found that, under short-circuit conditions, the time for 50% charge extraction is reduced by a factor of 4 for CPE and methanol-treated devices. These results show that the time spent by the photogenerated carriers within a relatively efficient OPV system is significantly controlled by the interface with the electrode. Therefore, the charge extraction - and thus internal recombination - is often limited by interfacial rather than bulk properties. The observed increase in Voc is due to a combination of suppressed internal recombination and deepening in PEDOT:PSS work function as observed upon exposure to methanol.
[1] He, Z. et al. Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells. Advanced Materials 23, 4636-4643 (2011).
[2] Zhou, H. et al. High-Efficiency Polymer Solar Cells Enhanced by Solvent Treatment. Advanced Materials 25, 1646-1652 (2013).

Cyclopenta[c]thiophene-4,6-dione (CTD) is employed as an electron accepting building block in donor-acceptor (D-A) polymers for organic solar cells. The acceptor strength can be fine-tuned by introducing substituents with different electron withdrawing/donating power onto this monomer unit. Copolymerization with the electron rich 4,8-di(2-ethylhexy-loxyl)benzo[1,2-b:4,5-b]dithiophene (BDT) in D-A and D-D-A configurations gives a variety of polymers, where the band gap is tuned by varying the substituent on the CTD unit, as well as incorporating two BDT units for every CTD.
Calculations were first performed to examine the possible effect of different side groups (cyano (CN), fluoro (F), hydro (H), and methyl (Me)) on the acceptor and different backbone configurations (D-A and D-D-A with BDT) on the HOMO level and band gap. This leads to a family of 8 polymers with different HOMO and LUMO levels. Cyano and fluoro groups have a strong electron withdrawing effect on the CTD unit, thereby lowering the HOMO level and band gap of the CTD-BDT copolymer. Introducing an extra BDT unit for every CTD, the HOMO level and band gap are raised. These modifications lead to a bandwith of 0.54 eV in HOMO level and 0.16 eV in band gap.
These polymers were subsequently synthesized and characterized using optical absorption, cyclic voltammetry (CV), time-resolved microwave conductivity (TRMC) and photoluminescence (PL). The optical band gap of the polymers follow the trend as predicted, and the PL shows more exciton quenching upon blending the polymer with a commonly used fullerene acceptor, as the LUMO-LUMO offset with this fullerene increases. TRMC data reveals that not all quenched excitons result into free charges, indicating that there are recombination processes in these blends that need further study. Photovoltaic devices are fabricated, and the internal quantum efficiency and light intensity dependent measurements of these devices give insight into the different recombination mechanisms in the devices.

Nanoscale morphology has long been recognized as an important controlling factor in organic photovoltaic (OPV) performance—however, it may also affect the long-term photochemical stability of OPVs. Using the blend Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] and 3'H-Cyclopropa[8,25][5,6]fullerene-C71-D5h(6)-3'-butanoicacid, 3'-phenyl-, methyl ester (PTB7:PC71BM) as a model system, we show that dissipation imaging using frequency-modulated electrostatic force microscopy (FM-EFM) can be used to probe local changes in electronic properties due to local photochemistry. Furthermore, we compare the evolution of nanoscale electronic properties in PTB7:PC71BM blends processed with and without the solvent additive 1,8-diiodooctane (DIO) and show that the use of DIO yields a preferable nanomorphology with less resistive loss over the surface of the film.

If a photovoltaic (PV) technology is assessed today in a technical framework, then efficiency is the most commonly addressed parameter, followed by service lifetime. Cost, as the third parameter of the "magic triangle", is even less often reported. However, if a new technology is prepared to enter a market, other important parameters have to be considered, especially if non-standard PV applications are targeted.
Organic photovoltaic (OPV) is a well known but young PV technology of the so called third generation, which offers unique advantages for integrated products such as building integrated photovoltaics (BIPV). In this contribution we would like to highlight some of the advantages and challenges which are specific to the application of OPV in the field of building integration.
Key properties of OPV such as semi-transparency, color and shape can be tuned easily compared to other PV technologies. This variability, along with its light weight and flexibility, renders OPV very attractive from an architectural view point for BIPV applications such as facades, windows, roofs, and shadings. Free-form OPV modules can be used to design completely new products that are otherwise barely accessible. Moreover, the electrical performance of OPV modules is rather insensitive to environmental conditions encountered by BIPV, such as elevated temperatures, low-light conditions and non-perpendicular irradiation. As a result, the average energy harvest per nominal watt peak of OPV modules exceeds that of classical PV technologies by 20 to 35%.
Glass-laminated PV modules play a major role in the BIPV market. We have thus chosen OPV glass elements to illustrate the challenges encountered by OPV on the way to market readiness. We show that OPV can be laminated into glass-glass sandwich structures using common lamination processes without significant loss in module efficiency. These glass-laminated OPV modules exhibit excellent durability by passing the thermal cycling test, the humidity freeze test and the damp-heat test in the style of the IEC 61646:2008 norm for thin film terrestrial PV modules. These tests were performed and accredited by the independent test laboratory of TÜV Rheinland. Also first fire resistivity tests have been conducted with OPV in glass laminates with regard to building safety regulations for BIPV.
A series of examples will be given to demonstrate how the special features of OPV can be utilized to meet the challenges of the BIPV market.

Photovoltaics (PV) hold tremendous potential as a technology to address the rising global demand for clean energy. One key for wide-spread success of this technology is to drive down the cost per Watt of electricity. This requires utilisation of less energy-intensive/lower cost production methods. One highly promising emerging technology for this is organic photovoltaics (OPVs). OPVs promise large-area manufacturing of future products by high-throughput roll-to-roll processing in ambient conditions. OPVs have also seen a rapid improvement in performance over recent years with certified devices reaching 10% power conversion efficiency. However there still remain fundamental questions related to the exact microstructure that needs to be realised within the active layer of such solar cells to optimise - and further improve - their power conversion efficiency. A structural picture is also necessary to target specific processing protocols and permit reliable device fabrication at high yield. The active layer of bulk-heterojunction (BHJ) OPVs is comprised of a blend of an electron-donating and an electron-accepting material, which until recently were generally believed to form relatively pure phases. However new studies have indicated that the BHJ microstructure may also contain a phase, where both materials can be mixed at much more intimate scales. Here, we discuss the role of this intermixed phase. We show that relative straight-forward techniques, such as photoluminescences and UV-vis spectroscopy provide highly valuable insight in the structural features of these systems and propose, based on a series of structurally well-defined blend architectures, a strong influence of the presence of such an intermixed phase on the opto-electronic processes of the system and provide structural guidelines for further improving the efficiency of BHJ OPVs.

Bulk heterojunction (BHJ) solar cells rely on a phase separated morphology between donor and acceptor for efficient charge separation and collection. Imaging the morphology is however challenging due to the simultaneous requirements of good contrast between the phases and nanometer spatial resolution. Transmission electron microscopy (TEM) is widely used for characterising thin-films at the nano-scale, but conventional imaging does not produce any appreciable contrast, due to both donor and acceptor being carbon-rich. In conventional imaging all transmitted electrons are used to form the image. If however only electrons that have excited a plasmon within the thin-film are selected the resulting image is found to have significant contrast. The contrast improvement is due to damping of the plasmon excitation in PCBM acceptor by interband transitions of similar energy, so that for the same material volume the PCBM plasmon intensity is lower than the donor (in this case P3HT polymer).
These so-called ‘plasmon-loss&’ images contain a wealth of information about the BHJ morphology. For example, (semi-) crystalline fibres of P3HT with length of several 100 nm and diameter of only a few nm are revealed; the fibre geometry is ideal for optimising both charge generation and collection, provided the fibre is aligned along the film thickness direction. The region around the fibres was filled with molecular PCBM, as required for an efficient device. The presence of PCBM can be deduced by analysing the shape of the plasmon peak (PCBM has a shoulder at smaller energies due to interband transitions). Residual chlorobenzene solvent pockets, only a few nm in diameter, were also observed throughout the thin-film, and showed a higher concentration of PCBM, due to its preferential solubility. The PCBM density within the solvent pockets was however lower, as revealed by the red shift of the plasmon peak. Electron tomography was used to visualise the morphology in 3D. A new method that takes into account the non-linear dependence of the plasmon signal w.r.t projected thickness of the specimen was developed for generating accurate tomography reconstructions. Plasmon-loss imaging will also be used to investigate the role of thermal annealing on morphology evolution in real time; preliminary results from in-situ annealing will be reported at the conference.

Here, we present an analysis of nominally bilayer small-molecule organic photovoltaic (OPV) cells made from blends of two functionalized squaraine donor materials¹ that show markedly different response to solvent vapor annealing (SVA). We find that the interface order created during SVA processing strongly affects the photovoltaic properties of such cells² and explain why the blended-donor cells have higher performance than cells made with either neat donor.
On SVA, OPVs made with the symmetric, NIR-absorbing squaraine, DPSQ, paired with C₆₀ exhibit a small gain in efficiency related to a gain in short circuit current (J_SC) while open circuit voltage (V_OC) and fill factor (FF) are essentially unchanged. This squaraine has a stable interface with C₆₀ that remains disordered on SVA, while the bulk of the squaraine film crystalizes.² In contrast, OPVs made with the asymmetric green-absorbing squaraine, DPASQ, crystalizes much more aggressively on SVA with several lattice coincidences with C₆₀. This results in a highly ordered interface and a large degree of intermixing between the DPASQ and C₆₀ phases. This coincident interface reduces the V_OC while the interdiffusion increases the J_SC. Both of these cells produce power conversion efficiencies around PCE=4.5-4.8%.
By blending these distinctive squaraine donors into a single OPV, the overall efficiency is improved to PCE=5.6%. Surprisingly, cells made with a 4:6 blend of DPASQ:DPSQ have a larger response from the DPSQ than a similar sample with pure DPSQ. We find the improved efficiency originates from phase separation of the squaraines followed by interdiffusion of the DPASQ and C₆₀ phases, increasing the contact area between the DPSQ and the C₆₀. Furthermore, we are able to maintain the large open circuit voltage of DPSQ in these blended cells. This suggests that blending a stable material into a less stable materials is a general method for preventing losses in V_OC that are observed on SVA.
1. S. Wang, L. Hall, et al., Chem. Mater. 23 (21), 4789 (2011).
2. J.D. Zimmerman, X. Xiao, et al., Nano Lett. 12 (8), 4366 (2012).

Despite the rapid and significant progress in polymer:fullerene blends for use as the light-harvesting active layer in Organic Photovoltaic (OPV) devices, there is still a lack of complete understanding of the actual phase morphology (i.e. the number of phases and the complexity of their microstructure) achieved in the active layer and its correlation to device performance. Clearly, if we want to reach the maximum performance within polymer:fullerene bulk heterojunction (BHJ) solar cells, we need to gain a more in-depth knowledge and control of these multi-component systems in order to correlate their optical and electronic properties with their solid-state microstructure and phase morphology. We will first present a versatile way to manipulate, and thus easily study, such functional two-component, multi-phase blend architectures using poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT): [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) blends (1:1 by weight) with the assistance of alkyl-chain methyl esters as additives.[1,2] This allows us to evaluate the effect of the phase morphology of such structures - from fully intercalated to partially and non-intercalated systems - on the exciton and carrier dynamics, and the efficiency of charge collection, with relevance for future device design and processing. Furthermore we will also show how we can extend this additive-assisted manipulation to other polymer:fullerene blends such as the well-studied poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) system and blends comprising higher efficiency polymers (e.g. diketopyrrole-based copolymer or Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)],commonly named DPP-TT-T and PCDTBT, respectively) which are less ordered.[3,4]
[1] a) Miller et al., Journal of Polymer Science Part B: Polymer Physics 2011, 49, 499; b) Jamieson et al., Chemical Science 2012, 3, 485.
[2] manuscript submitted.
[3] a) Bronstein et. al., Journal of the American Chemical Society 2011, 133, 3272; b) Beiley et al., Advanced Energy Materials 2011, 1, 954
[4] a) Chang et. al, J. Physical Review B 2006, 74, 115318; b) Theander et. al., The Journal of Physical Chemistry B 1999, 103, 7771;

Organic semiconductors exhibit unique physical and electronic properties suitable for applications in electronics. Many of the existing organic semiconductors are highly sensitive to ambient atmosphere and easily oxidized when exposed to e.g. oxygen and water. Accordingly, such exposure can lead to a loss in device performance.[1] For organic photovoltaic (OPV) such losses typically show up as decrease in short circuit current, open circuit, fill factor and sometimes by the formation of an “S”-type current-voltage curve. The C60-derivative [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) is the most well-known and so far dominates in terms of choice of electron acceptor in OPV application. Therefore understanding the degradation mechanisms of PCBM and their effect on the device operation are very important.
Here, the effects of in situ oxygen/water/UV exposure on the energetics of PCBM films are investigated. Upon exposure oxygen, a down shift of the work function appears both for pinned (EICT+/-)[2] and vacuum level aligned PCBM/substrate interfaces, which is incompatible with significant introduction of electron trap states or p-doping of the PCBM films. The degradation originates from the weak interaction (no covalent bonding) between the fullerene part of PCBM and oxygen, and can be reversible by thermal treatment in vacuum. However, water-induced degradation is completely different. The occupied valance electron structure undergoes irreversible strong modification (covalent bonding) illustrated by significant modification of XPS core level spectra, extensive broadening and blenching of the UPS spectra features, as well as substantial decrease of work function and ionization potential. The origins of the studied effects for the different types of exposure are explored and the expected effect on OPV performance is commented upon.
[1] A. Seemann, et al, Solar Energy, 2011, 85, 1238.
[2] S. Braun, W. R. Salaneck, M. Fahlman, Advanced Materials, 2009, 21, 1450

An essential aspect in designing efficient and stable organic solar cells (OSCs) is the engineering of interfacial carrier transporting layers between the organic layer and metal electrodes. Among various materials available for interfacial layers, transition metal oxides (TMOs) have great potential owing to their wide range of energy level aligning capabilities. Bearing the compatibility with large-area, low-cost, high-throughput production and all-solution technology, we propose a one-step method to synthesize low-temperature solution-processed TMOs such as molybdenum oxide and vanadium oxide for hole transport layers through the synthesis of hydrogen molybdenum bronze and hydrogen vanadium bronze. Interestingly, the hydrogen metal oxide bronzes (HMOs) are dispersed uniformly and stably into water-free solvents which are particularly beneficial to the device stability and processing. With low temperature treatment or even at room temperature, the TMO films with small amount oxygen vacancies exhibit high film quality and desirable electrical properties. Through the analysis of UPS and XPS results, we identify the importance of oxygen vacancies for TMOs as HTL. Notably, the synthesized HMOs can be dispersed uniformly and stably into water-free solvents. By using our TMOs to make OSCs with polymer blend of P3HT:PCBM, the power efficiency (PCE) reaches 4% (vs 3.7% for PEDOT:PSS control device) and PCE of 7.75% using polymer blend of PBDTTT-C-T:PCBM (vs 7.24% for the corresponding PEDOT:PSS control device) [1]. Consequently, the results of our newly-synthesized TMOs demonstrate that oxygen vacancy plays an essential role for TMOs as effective HTL for applications on organic electronics.
[1] F. Xie, W.C.H. Choy, C. Wang, X. Li, S. Zhang, J. Hou, "Low-temperature Solution-Processed Hydrogen Molybdenum and Vanadium Bronzes for Efficient Hole Transport Layer in Organic Electronics", Adv. Mat., DOI: 10.1002/adma.201204425.

This talk reviews work done in our group to determine the energetics of key interfaces involving transparent metal oxides and organic molecular and polymer films in hybrid photovoltaic cells. We begin with requirements for carrier selective contacts, e.g. hole extractor / electron blocker and vice versa, and look at the relevant energy parameters (ionization energy, electron affinity, work function) of materials such as NiO, ZnO and TiO2 prepared via various deposition techniques (vacuum evaporation, sol-gel, atomic layer deposition, chemical vapor deposition). The energetics of interfaces between some of these oxides and organic semiconductors, determined via electron spectroscopy and transport measurements, are reviewed. We describe how specific combinations of bi-layers of metal oxides can be advantageous for hole or electron extraction. Finally, we describe on-going work on the electronic structure of electron- and hole-blocking contacts on Si using P3HT and TiO2, respectively, for Si-based solar cells.