A full understanding of charge carrier generation and recombination in fullerene-doped conjugated polymer films remains topical, despite these systems producing effective solar cells with respectable solar power conversion efficiencies. This presentation will describe some recent experiments aimed at investigating this critical component of the solar cell function. Using not blends, but low concentrations of a variety of fullerene and macrocycle acceptors doped into conjugated polymer films, we will report studies of charge carrier generation and recombination using the electrode-less technique of flash photolysis, time-resolved microwave conductivity (fp-TRMC) to detect the photogenerated charges. Using additional time-resolved and steady-state photoluminescence studies, we will discuss a new mechanism by which carriers are both generated and recombine.

The mechanisms of excitonic dissociation into mobile charge carriers remain of intense scientific research. Recently, it was shown that hot CT excitons could enhance the photocurrent due to the re-excitation in the CT exciton manifold. However, most of these studies were focused on spectroscopic evidence, while the critical question regarding the impact of hot excitons to the performance of actual devices remains debatable.
We have studied the impact of excess photon energy on excitonic dissociation efficiency in operational and efficient organic solar cells. Our results indicate that hot excitons are not beneficial for optimized and efficient operational organic solar cells, because Internal Quantum Efficiency (IQE) is independent on photon energy. We demonstrate that methodology of IQE measurement is crucially important for unambiguous result interpretation. It is critical to account for the optical interference effects and light absorption in the non-active layers in thin operational devices. An attention is further directed to the importance of treating an OSC as a complex optical cavity.
Furthermore, hot photocarriers (in addition to hot excitons) offer a potential for improved photocurrent and photovoltaic device performance. Recently, a number of publications has reported a time-dependent charge carrier mobility in several organic photovoltaic systems. The loss of photocarrier excess energy with time is expected to negatively impact the device performance though the reduction in photocarrier mobility. In this work we aim to answer the question whether the excess energy can be utilized to improve the charge transport and reduce the recombination. Our experiments demonstrate that the photocarrier mobility is independent on photon energy. It is concluded that excitons lose their excess energy at times scales which are much shorter than the time scales of charge extraction therefore the time-dependent mobility is not observed. However, the experiment shows that the photocurrent is reducing with time. The time-dependent photocurrent losses originate from the charge carrier capture by the trap states instead of the loss of their drift velocity (mobility). The impact of these results in regards to the present knowledge of charge transport is discussed.
Finally, a number of novel techniques developed for the purpose of unambiguous electron and hole mobility measurements in operational devices is presented. A critical conclusion arising from our results demonstrates that in contrasts the commonly accepted knowledge, the efficient solar cells do not need balanced charge carrier mobilities. Most importantly, we propose a novel approach to characterise the charge transport lengths in organic photovoltaic of photodetecting systems. It is clarified why the product of and recombination coefficient adequately describes that charge transport scale in contrast to the presently used mobility-lifetime product.

We present a systematic study of the roles of crystallinity, interchain interaction, and exciton delocalization on ultrafast charge separation pathways in donor-acceptor copoloymer blends. Energy levels, excited state structures, and dynamics of the interchain species are characterized by combining ultrafast spectroscopy and computational quantum chemistry approaches. Structural analysis of molecular order and phase segregation was carried out with X-ray scattering and atomic force microscopy imaging. The alkyl side chain of a highly efficient donor-acceptor copolymer for solar cell applications, PBDTTT (poly(4,8-bis- alkyloxybenzo[1,2-b:4,5-bprime;]dithiophene-2,6-diyl-alt-(alkylthieno[3,4-b]thiophene-2- carboxylate)-2,6-diyl), is varied to tune the molecular packing and interchain interaction of the polymers in order to elucidate the charge separation pathways originating from intrachain and interchain species. Polymers with linear side chains result in more crystalline polymer domain that lead to preferential formation of interchain excitons delocalizing over more than one polymer backbone in the solid state. Our results demonstrate that the higher polymer crystallinity leads to slower charge separation due to coarser phase segregation and formation of the interchain excited states that are energetically unfavorable for charge separation. Low lying energetics feature of the interchain excitons in low-bandgap copolymers calls for optimized solar cell morphologies that are fundamentally different from those based on homopolymers such as P3HT (poly-3-hexylthiophene). A long-range crystalline polymer domain is detrimental rather than beneficial to solar cell performance for a low-bandgap copolymer which is in direct contrast to the observed behavior in P3HT based devices.

Charge generation is a fundamental yet heavily debated issue in in organic photovoltaics (OPVs). The photo-generated electron-hole pair, if bound in a charge-transfer (CT) state at the donor-acceptor interfaces, faces a strong Coulomb interaction (hundreds of meV). Consequently, dissociation at room temperature is almost unlikely. However, efficient OPVs work with high internal quantum yield over 90%.
In the past few years, several competing models have been proposed to explain efficient charge carrier separation at the interfaces in OPVs. Thermodynamic model addresses ‘cold&’ dissociation through balance between lowest CT states and charge-separated states. Kinetic model proposes ‘hot&’ separation through competition between thermalisation and dissociation. Recent quantum mechanical model highlights the importance of delocalization in helping charge separation. Different models predict different dependence of charge separation on temperature. Therefore, in principle temperature could be a very useful parameter to distinguish between different models. However, temperature-dependent charge separation is difficult to investigate, as at leastalso charge transport is also heavily affected by the temperature.
In this contribution, we aim to understand charge separation (especially as a function of temperature) by investigating temperature-dependent open-circuit voltage (Voc). At Voc, all charge carriers recombine and no transport issue is involved, making temperature-dependent Voc a unique approach to understandingng charge generation. In addition, compared with previous transient measurements, steady-state Voc measurements are more straightforward. We measure the Voc as a function of temperature for several polymer:fullerene and polymer:polymer blends, down to temperatures below the liquid nitrogen temperature, which was rarely investigated before. We find that the Voc bends and even decreases with decreasing temperature. This contradicts previous observations that Voc increased linearly with decreasing temperature (usually above 100 K). We can rationalize our results with decreased charge generation at low temperatures and exclude other possibilities (like electrodes or trapping/detrapping) which could potentially cause this effect. Based on our results, we are able to identify the effect of several key parameters on charge separation, e.g. annealing and stoichiometry. We also compare our results with predictions from different models, providing insights into the effectiveness of these different models.

Organic donor (D)- acceptor(A) bulk heterojunctions (BHJs) have been a recent subject of intensive research, due to potential applications in a wide variety of opto-electronic devices. However, there is still a lack of understanding of the processes pertaining to charge carrier photogeneration and transport, which determine the opto-electronic device performance in these systems. In particular, understanding the mechanisms of charge carrier photogeneration in BHJs is key to the design of donor and acceptor molecules with high charge separation efficiency at the D-A interface.
We present a study of charge generation mechanisms in novel small-molecule- and polymer-based D-A BHJs, for which we combine measurements of time-resolved photocurrent and photoluminescence with computational modeling. For our studies, we choose several functionalized anthradithiophene (ADT) derivatives as donors and various functionalized ADT, pentacene (Pn), indenofluorene (IF), and fullerene derivatives as acceptors. The ADT donor molecules form crystalline films characterized by high charge carrier mobilities. The acceptor molecules are selected to create various scenarios for D-A charge transfer: in particular, to vary the D-A LUMO offset, D-A separation distance, and D-A molecular packing and crystallinity. The same acceptors are also explored in BHJs with a P3HT polymer donor. In all BHJs studied, we quantify multiple carrier generation pathways active on nanosecond time scales depending on the type and concentration of acceptor and on the external parameters.
In our model, charge photogeneration may proceed via (i) ultrafast charge carrier separation (via, for example, band-to-band transitions or hot exciton dissociation), (ii) nanosecond time-scale Frenkel exciton (FE) dissociation, and (iii) nanosecond time-scale charge transfer (CT) exciton dissociation. In small-molecule BHJs, we establish that the ultrafast charge carrier separation efficiency is strongly dependent upon donor crystallinity and can be further enhanced by acceptor crystallinity. The contribution of the FE dissociation to the photocurrent does not considerably depend on the acceptor, which indicates that this charge generation process occurs inside the donor domains and is only slightly affected by the presence of the D-A interfaces. In all small-molecule D-A blends studied, the nanosecond time-scale CT exciton dissociation contributes a relatively small percentage of all photogenerated charge carriers. Instead, the formation of emissive CT states (exciplexes), which do not significantly contribute to the photocurrent at nanosecond time-scales, are observed. Dependence of charge carrier photogeneration pathways on the wavelength, temperature, and applied electric field are also studied and will be reported.

My talk will focus on charge separation and recombination in organic solar cells and their impact upon device photovoltaic performance. Experimental these studies will employ transient absorption and emission studies on timescales from 100fs - ms, complimented by small perturbation transient optoelectronic studies. Studies will be undertaken for a range of different donor and acceptor materials and blend compositions. These studies will be correlated with measurements of molecular structure, blend microstructure, material crystallinity, material energetics and device electrodes and electric field. A key focus of my presentation will be to quantify the impact of these factors upon geminate and non-geminate recombination losses, and how these limit device performance. Time permitting, I will also briefly consider some of materials design considerations impacting upon material and device stability.

Of the parameters that characterize a solar cell, the fill-factor (FF) is the least well understood, making targeted improvement difficult. Here, we show that the FF is determined by the competition between charge extraction and recombination. Moreover, we empirically demonstrate the precise relationship between this competition and the FF for a wide variety of donor/acceptor combinations. The results apply to polymer/fullerene and polymer/polymer combinations of different thicknesses, composition, temperatures, and light intensities. For the whole range of 0.26-0.74, all FFs follow from the competition between extraction and recombination.
We measure charge carrier mobilities and bimolecular recombination rates using a combination of steady-state and transient extraction techniques. These quantities are used to estimate the extraction and recombination times. The ratio between the extraction and recombination times, which we call theta, indicates whether the majority of charge carriers recombine or can be extracted from the solar cell. If all the FFs of the solar cells studied are plotted versus theta, the data collapse onto one universal curve. This shows that the main determinant of the FF is the balance between bimolecular recombination and charge extraction.
To explain the observed trend of FF versus theta, we perform drift-diffusion simulations of organic solar cells. Charge carrier mobilities, recombination rate, light intensity, energy levels, and active layer thickness were all varied over wide ranges to reproduce the experimental conditions. Again, when the resulting FF are plotted versus theta a universal curve is obtained that is in quantitative agreement with the experimental data.
Our results unify the fill-factors of several very different donor/acceptor combinations and show why FFs change so much with thickness, light intensity and materials properties, which opens up the possibility of targeted improvement.

We examine the effect of materials and processing parameters on the photophysics of bulk heterojunction organic photovoltaic devices, using a CW measurement of quantum efficiency over a large range of excitation intensity (10^-4 to a few suns). The quantum efficiency data as a functional of excitation intensity fit well to a model containing both monomolecular and bimolecular recombinations, allowing us to quantify the loss mechanisms and, more importantly, their relative strength. We show a direct electronic correlation between trap assisted recombination parameters and the charge generation efficiency as well as with the charge transfer state absorption strength. This correlation allows us to estimate the electrochemical potential in the bulk of P3HT:ICBA devices and we find it to be above 1V leading to the conclusion that the VOC in such device is limited by recombination at the electrode. By examining solar cells based on blends of poly(3-hexythiophene) and C₆₀ derivatives, where polymer molecular weight, regioregularity, C₆₀ derivative chemistry, and film drying speed were varied, we demonstrate the effect of material and processing parameters on the balance of loss mechanisms in the devices.

Recent studies have shown that dilution of an organic semiconductor in a wide energy gap host material is an effective technique to enhance the exciton diffusion length (L_D) and power conversion efficiency (eta;_P) in planar heterojunction organic photovoltaic cells (OPVs). Increased performance is observed despite an overall reduction in donor layer absorption upon incorporating the wide energy gap material. In this work, we demonstrate dilute donor OPV cells that instead contain a complementary photoactive host material capable of contributing to device photocurrent. Excitons generated on the host experience a strong driving force to quickly energy transfer to the dilute guest, along which they are efficiently transported to a dissociating donor-acceptor interface. This is in contrast to other multi-layer energy-cascade OPVs where efficient exciton transport must occur along both donor species. Boron subnapthalocyanine chloride (SubNc, E_g = 1.9 eV) serves as a dilute guest material in a photoactive host of boron subphthalocyanine chloride (SubPc, E_g = 2.2 eV). For OPVs constructed with an acceptor layer of C₆₀, the short-circuit current density (J_SC) increases from J_SC = (5.5 ± 0.05) mA/cm² at 100 mW/cm² AM1.5G solar illumination for the transparent host-dilute donor device to J_SC = (6.2 ± 0.05) mA/cm² for the optimized photoactive host-dilute donor device, a 13% enhancement. Along with an additional 10% enhancement in the open-circuit voltage, the power conversion efficiency (eta;_P) increases by 20%, leading to a eta;_P = (3.2 ± 0.1)%. The eta;_P is further increased to (4.3 ± 0.2)% for photoactive host-dilute donor OPVs that utilize an acceptor layer of C₇₀. Complementary measurements of the external quantum efficiency and L_D further confirm that excitons generated on the photoactive host, SubPc, can efficiently diffuse to the donor-acceptor interface. Interestingly, since excitons generated on the host quickly energy transfer to the guest, they can also take advantage of the efficient transport path the dilute donor structure provides, relaxing the requirement for a long L_D on the host material.

The power conversion efficiencies of polymer:fullerene solar cells have increased dramatically over the last decade from 2.5 % in 2001 to 9.2% for single junction solar cells and 10.6% for tandem devices in 2012. The development has been mainly driven by a better understanding of morphology, improved donor polymers and new device geometries like tandem solar cells.
However, the non-geminate recombination mechanism in polymer:fullerene solar cells is still not fully understood. Here, we apply drift-diffusion modelling combined with different experimental techniques to explain the impact of non-geminate recombination on device performance. To identify non-geminate recombination spatially, we deliberately control device configurations by using bilayer and blend architectures to manipulate the non-geminate recombination. The electroluminescence (EL) intensity as a function of injection current and open-circuit voltage (V_oc) measurements as a function of light intensity are used to identify the recombination mechanism in different polymer:fullerene bulk heterojunction solar cells. The results show that surface recombination is important in all our investigated polymer:fullerene systems- P3HT:PCBM, APFO3:PCBM and PCDTBT:PCBM. By understanding how and where the surface recombination occurs, we also show an optimized device architecture with a buffer layer at the contact can suppress the surface recombination and improve the device open-circuit voltage and fill factor.
Further, we investigate the impact of energy levels on the non-geminate recombination. In particular, we address the relationship between the LUMO-LUMO offset between polymers and fullerenes and the non-geminate recombination rate. We use EL of blend films and pure materials to determine the LUMO-LUMO offset within the polymer: fullerene blends, and use transient photovoltage (TPV) and charge extraction (CE) techniques to investigate the relationship of the low LUMO-LUMO offset to non-geminate recombination. Knowledge of this relationship is relevant to the design of higher efficiency devices.

This talk will describe an approach to create architecturally compatible and decorative thin-film-based hybrid photovoltaics [1]. Most current solar panels are fabricated via complex processes using expensive semiconductor materials, and they are rigid and heavy with a dull, black appearance. As a result of their non-aesthetic appearance and weight, they are primarily installed on rooftops to minimize their negative impact on building appearance. Recently we introduced dual-function solar cells based on ultra-thin dopant-free amorphous silicon embedded in an optical cavity that not only efficiently extract the photogenerated carriers but also display distinctive colors with the desired angle-insensitive appearances [1,2]. The angle-insensitive behavior is the result of an interesting phase cancellation effect in the optical cavity [3]. In order to produce the desired optical effect, the semiconductor layer should be ultra-thin and the traditional doped layers need to be eliminated. We employed the charge transport/blocking layers commonly used in organic solar cells to meet this demand. We showed that the ultra-thin (6 to 31 nm) undoped amorphous silicon/organic hybrid solar cell can transmit desired wavelength of light and that most of the absorbed photons in the undoped a-Si layer contributed to the extracted electric charges benefited by the suppressed electron-hole recombination in the ultra-thin a-Si layer. Reflective colored PVs can be made in a similar fashion. Light-energy-harvesting colored signage was demonstrated. Furthermore, a cascaded photovoltaics scheme based on tunable spectrum splitting can be employed to increase power efficiency by absorbing a broader band of light energy. Our work provides a guideline for optimizing a photoactive layer thickness in high efficiency hybrid PV design, which can be adopted by other material systems as well. Based on these understandings, we have also developed colored perovskite PV by integrating an optical cavity with the perovskite semiconductors. The principle and experimental results will be presented.
J. Y. Lee, K. T. Lee, S.Y. Seo, L. J. Guo, “Decorative power generating panels creating angle insensitive transmissive colors,” Sci. Rep. 4, 4192, 2014. DOI: 10.1038/srep04192.
K. T. Lee, J.Y. Lee, S.-Y. Seo, and L. J. Guo, “Colored ultra-thin hybrid photovoltaics with high quantum efficiency,” Light: Science & Applications, 2014. In press.
K. T. Lee, S.-Y. Seo, J.Y. Lee, and L. J. Guo, “Ultrathin metal-semiconductor-metal resonator for angle invariant visible band transmission filters,” Appl. Phys. Lett. 104, 231112, (2014); DOI: 10.1063/1.4883494. and “Strong resonance effect in a lossy medium-based optical cavity for angle robust spectrum filters,” Adv. Mater, 2014, DOI: 10.1002/adma.201402117.

Organic photovoltaic (OPV) represents a promising route toward lightweight, flexible, and low-cost renewable energy generation. Solvent additives have been shown to be as an effective technique to tailor bulk-heterojunction (BHJ) nanomorphology in many polymer-fullerene systems with promising improvements in device performance.^1,2 However, a detailed understanding on the underlying physical mechanisms including charge generation, transport, and recombination is still needed. We study poly(di(2-ethylhexyloxy)benzo[1,2-b:4,5-bprime;]dithiophene-co- octylthieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) BHJ solar cells made with different concentrations of 1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO) as solvent additives using current density-voltage (J-V) measurement as well as frequency-domain techniques including impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS). An equivalent circuit incorporating a recombination resistance and chemical capacitance is used in IS analysis. We investigate the effect of solvent additives on recombination mechanism in terms of carrier lifetime, carrier density, and recombination rate at open circuit condition. On the other hand, carrier transit time and charge collection at short circuit condition are studied to reveal information on carrier transport. In addition, we analyze the J-V response, both in the dark and under illumination, using a drift-diffusion model³ that contains a spatially-resolved carrier generation profile calculated based on realistic optical properties of different layers in the OPV devices. In particular, we will discuss the effect of additives on the charge generation in terms of exciton dissociation rate and exciton decay rate within the framework of drift-diffusion modeling.
Reference:
1. Aïch, B. R., Lu, J., Beaupré, S., Leclerc, M. & Tao, Y. Control of the active layer nanomorphology by using co-additives towards high-performance bulk heterojunction solar cells. Organic Electronics13, 1736-1741 (2012).
2. Bartelt, J. A. et al. Controlling Solution-Phase Polymer Aggregation with Molecular Weight and Solvent Additives to Optimize Polymer-Fullerene Bulk Heterojunction Solar Cells. Adv. Energy Mater. n/a-n/a (2014). doi:10.1002/aenm.201301733
3. Koster, L., Smits, E., Mihailetchi, V. D. & Blom, P. Device model for the operation of polymer/fullerene bulk heterojunction solar cells. Phys. Rev. B72, 085205 (2005).
This project is sponsored by National Science Foundation DMR-1305893.

We present a systematic analysis of charge carrier transport in organic photovoltaic (OPV) devices based on deterministic, phenomenological charge carrier transport models. The models are used to simulate time-of-flight (TOF) experiments and predict photocurrent evolution in devices with organic polymeric active layers. Active-layer materials examined include P3HT drop cast thin films and nanoparticle assemblies, P3HT nanoparticle assemblies with dispersed PMMA nanoparticles, PBTDV thin films, and P3HT/PCBM polymer blends forming bulk heterojunctions (BHJs). The models account for transient drift and diffusional transport of electrons and/or holes together with charge carrier trapping and detrapping kinetics in the active layer, coupled self-consistently with Poisson&’s equation for the electric field in the layer. Dynamics of free charge carrier bimolecular recombination, as well as charge pair dissociation and geminate recombination processes also are accounted for. Our models are validated by comparisons of their predictions with experimental measurements of transient photocurrents. For all the polymer layers studied, fitting the modeling predictions to the experimental data of photocurrent evolution upon excitation by short laser pulses is used to examine the propensity of the active-layer material to generate charge and form defects, determine the value of zero-field electron and hole mobilities and their Poole-Frenkel field dependence, analyze the kinetics of charge trapping and detrapping processes, and investigate charge pair dissociation as well as bimolecular and geminate recombination. By comparing the fitting outcomes for drop cast thin films and nanoparticle assemblies, we have analyzed the effect on charge carrier transport in nanoparticle assemblies of excess stabilizing surfactant molecules and insulating (PMMA) nanoparticles and demonstrated quantitatively that charge transport efficiencies in centrifuged nanoparticle assemblies are comparable with those in drop cast thin films. We have also studied the effects on charge carrier transport of tuning the nanostructure of individual nanoparticles in BHJs. The modeling predictions provide valuable input toward synthesis of organic active layers that lead to improved OPV device performance.

G^C bases are fused bicyclic rings of guanine and cytosine with self-complementary sets of hydrogen bonding sites. The modules undergo hierarchical self-assembly into rosette nanotubes (RNTs) in solvent via hydrogen bonding, π-π stacking and hydrophobic effects. Three types of porphyrins and a 3-butylthiophene oligomer with terminal amines were synthesized, and were coupled to mono or twin G^C bases to form G^C-Por 1, (G^C)₂-Por 2, (G^C)₂-Por 3 and (G^C)₂-H₆T. Under heating and sonication, G^C-Por 1 self-assembles into RNTs in MeNO₂ at a maximum concentration of 0.5 mM, while (G^C)₂-Por 2, (G^C)₂-Por 3 quickly form RNTs in a mix of 1,2-DCB and MeOH at a concentration up to 20 mM. (G^C)₂-H₆T can be freely dissolved in 1,2-DCB or chlorobenzene and self-assemble into RNTs. The diameters of these RNTs vary from 4.7 nm to 6.2 nm, and the lengths range from 100 nm to 10 mu;m, controlled by the process condition of the self-assembly. SEM and TEM revealed the 3 dimensional network structures of these RNTs alone or blended with PCBM. The two-phase nanoscale morphology of thin films is highly wanted when spin-casting the active layer of organic photovoltaic (OPV) devices.
The spectroscopic changes upon self-assembly of these molecules were investigated by time-dependent and variable temperature studies of electron absorption spectroscopy. For all the porphyrin-G^C modules, bathochromic shifts and hyperchromic effects were observed in the porphyrin Soret and Q bands upon self-assembly. These changes indicate that the RNTs can absorb the sunlight more efficiently, and collect the low energy photons in the visible and near IR area of the solar spectrum. For (G^C)₂-H₆T, no major changes were observed in the UV-Vis spectra after the self-assembly. Ultraviolet photoelectron spectroscopy (UPS) and electron absorption spectroscopy were used to determine the HOMO and LUMO energy levels for these RNTs, and they match well with those of PCBM. Photoluminescence (PL) quenching studies show the efficient photo-induced electron transfer from RNTs to PCBM in the blended thin films (molar ratio 1:1), indicating that these RNTs can serve as electron donors in the active layer of RNTs:PCBM. Two-electrode devices were fabricated by standard photolithography technique, and were used to measure the conductivity of thin films of these RNTs. Compare to the non-assembled building blocks, the self-assembled RNTs showed several orders of magnitude higher conductivity. The highest conductivity was observed (4.5 × 10^-2 S/m) in the film of (G^C)₂-H₆T RNTs, which is 67 times higher than that of a commercial grade of P3HT thin film processed under the same condition. We attribute the improved conductivity to the highly ordered nanostructure provided by the RNT scaffold.
These functionalized RNTs may contribute to the repertoire of donor materials in polymer based OPV devices and nanoscale organic semiconductors.

We report a facile synthetic approach for edge-gold-coated silver nanoprisms (GSNPs) and their comprehensive optical and morphological characterization. These GSNPs exhibit remarkable high chemical stability and excellent performance as optical antennae for light-harvesting applications. Our results indicate that when embedded into a photovoltaic bulk heterojunction film of poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM), plasmonic GSNPs act as optical antennae to substantially enhance light absorption in the active organic solar cell layer.
We measure a asymp;7-fold enhancement in the polaron generation yield through photoinduced absorption spectroscopy. Owing to the high stability, strong field enhancement effect and large sensitivity factors, the GSNPs exhibit great potential as optical probes for sensing and photovoltaic applications. The refractive index sensing figure of merit (FoM) of GSNPs can reach 4.05 RIU^-1 and finite-difference time-domain (FDTD) calculations suggests that the FoM of GSNPs could reach even higher values with better control of particle dispersity.

Polymeric solar cells (PSCs) have attracted considerable attention due to their unique characteristics, such as low cost, light weight, and possible flexibility and large-area coverage. In order to obtain high efficiency PSCs, both high performance active materials and interfacial materials are needed. Herein, we report our efforts on the development new conjugated polymer donor and interfacial materials for the interface engineering of high performance PSCs. A series of new Donor-Acceptor (D-A) conjugated polymers with alternating electron-rich and electron-deficient units along their backbone were synthesized and characterized, where absorption spectra and band gaps of the resulting polymers can be effectively tuned by controlling the intramolecular charge transfer between different donor and acceptor units. Besides, water/alcohol soluble conjugated polymers (WSCPs) were developed as highly efficient interfacial materials for PSCs, which can enhance short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF), thus improve power conversion efficiency (PCE) of the resulting PSCs. By combing our newly developed D-A conjugated polymers and WSCPs interlayers, high performance PSCs with PCEs more than 8% were realized.
References
[1] Duan, C. H.; Huang, F.; Cao, Y. J. Mater. Chem. 2012, 22, 10416.
[2] Duan, C.; Zhang, K.; Zhong, C.; Huang F.; Cao, Y. Chem. Soc. Rev. 2013, 42, 9071.
[3] Liu, S.; Zhang, K.; Lu, J.; Zhang, J.; Yip, H. L.; Huang, F.; Cao, Y. J. Am. Chem. Soc.2013, 135, 15326.
[4] Duan, C.; Cai, W.; Hsu, B. Y.; Zhong, C.; Zhang, K.; Liu, C.; Hu, Z.; Huang, F.; Bazan, G. C.; Heeger A. J.; Cao, Y. Energy Environ. Sci. 2013, 6, 3022.
[5] Hu, X.; Yi, C.; Wang, M.; Hsu, C.-H.; Liu, S.; Zhang, K.; Zhong, C.; Huang, F.; Gong, X.; Cao, Y. Adv. Energy Mater. 2014, DOI: 10.1002/aenm.201400378.

Interfacial energetics determines the performance of organic photovoltaic cells (OPVs) based on a thin film of organic semiconductor blends. The charge carrier recombination and internal field formation can be severely influenced by efficient charge carrier extraction at the desired electrode. Here, several approaches for tailoring charge collecting interfaces for highly efficient OPVs are discussed. The n-type metal oxide semiconductor, ZnO, and functional polymeric insulator blends are introduced as a cathode interlayer to promote carrier selective Ohmic contact at metal-organic interface. Along with the highly rectifying nature of wide-bandgap metal-oxide semiconductor, the addition of insulating polymer significantly reduced the diffusion and recombination of minority carrier in the vicinity of electrode, which, in this case, is the hole carrier near cathode. The resultant device performance showed significantly increased shunt resistance accompanied by shift in photovoltage. To quantitatively assess the carrier-selective contact limit on the charge carrier dynamics, dipole moment within the interfacial layer was gradually tuned by introducing a ferroelectric polymer, P(VDF-TrFE). The transient photovoltaic responses showed a significant variation of the non-geminate recombination rate depending on the degree of rectification of photogenerated carrier within the blend. These results show that the carrier selectivity at the electrode mainly decides the fate of diffusive charge carrier near open-circuit condition. In addition, the systematic study on the effect of deviation from the Ohmic contact regime by various photoactive materials was also performed. Due to the simplicity and the excellent performance, the introduction of the carrier selective interfacial layer could be a new platform for the tunable interfacial energetics for high performance printed OPV devices.

We present solution-processed p-n, BHJ, and p-i-n photovoltaic devices via the photoprecursor approach using 2,6-dithienylanthracene diketone (DTADK) as a precursor of p-type semiconductor 2,6-dithienylanthracene (DTA). DTADK can be converted to the poorly soluble DTA upon photoirradiation; thereby, the p-n and p-i-n structures could be constructed by sequential solution deposition. Furthermore, we focus attention on “hetero p-i-n” architecture by employing 2,6-bis(5'-(2-ehtylhexyl)-[2,2'-bithiophen]-5-yl)anthracene (EH-DBTA) instead of DTA for the i-layer. EH-DBTA is assumed to be a superior option to DTA for the i-layer because of the better photoabsorption capability owing to the π-extended structure and the higher miscibility with the n-type material (PC₇₁BM) owing to the alkyl substituents. In this presentation, we report the fabrication and properties of solution-processed OPV devices via the photoprecursor approach.
The deposition process of organic active layers: The p-layer was prepared by spin-coating of a chloroform solution of DTADK followed by photoirradiation to effect the in-situ conversion of DTADK to DTA, and the i (BHJ) -layer was deposited in the same manner by using a mixed solution (DTADK:PC₇₁BM or EH-DBTADK: PC₇₁BM). The n-layer was prepared by spin-coating of a chloroform solution of PC₇₁BM.
The device structures were [ITO / PEDOT:PSS / DTA / PC₇₁BM / Ca / Al], [ITO / PEDOT:PSS / DTA:PC₇₁BM / Ca / Al], [ITO / PEDOT:PSS / DTA / DTA:PC₇₁BM / PC₇₁BM / Ca / Al], and [ITO / PEDOT:PSS / DTA / EH-DBTA:PC₇₁BM / PC₇₁BM / Ca / Al] for p-n, BHJ, p-i-n, and hetero p-i-n, respectively.
Among our tested systems, the performance of p-i-n device was better than that of p-n and BHJ devices. The p-n device showed an ideal diode behavior with a high FF (53.6%), resulting in a PCE of 1.21%. The BHJ device showed the lowest FF (29.3%) and inferior electrical properties, resulting in a PCE of 0.90%. The p-i-n device gives a high J_SC leading to PCE of 1.50%. The p-layer with a crystalline film structure was suitable for charge extraction, whereas the amorphous i-layer was suitable for efficient photogeneration. By sandwiching the highly resistant i-layer between the p- and n-layers, the charge extraction was improved leading to the high J_SC. Furthermore, employment of the hetero p-i-n architecture leads to the best photovoltaic performance (PCE = 2.89%). This improvement was attributed to the combination of the higher photoabsorption capability and miscibility with PC₇₁BM associated with EH-DBTA. This proof-of-concept study clearly demonstrates the applicability of the photoprecursor approach to the fabrication of p-i-n type devices, and solution-processed hetero p-i-n devices fabricated via the photoprecursor approach provide an efficient way to achieve high photovoltaic efficiency.

Over the past several years, the internal quantum efficiency for state of the art organic photovoltaic (OPV) devices has approached unity. This has been in large part due to precise understanding and control of the morphology present and phenomena occurring within the devices. However, despite the gains achieved in efficient photocurrent production, the open-circuit voltage (V_OC) of organic devices are generally low and serves as a substantial limit to overall device performance. The poor open-circuit voltages can ultimately be traced back to limitations imposed by fullerenes which, despite their widespread use, typically between exhibit voltages of only 0.6 #8209; 0.8 V. To overcome this problem, alternative strategies have been developed to obtain high open circuit voltages in organic devices through the judicious selection of active layer materials. Here, as an example, I will discuss OPVs based on a α-sexithiophene (6T) donor and Tetraphenyldibenzoperiflanthene (DBP) acceptor and compare them to analogous devices utilizing DBP as the donor and C₆₀ as the acceptor. The 6T/DBP devices exhibit substantially enhanced open circuit voltages in excess of 1.2 V, without compromising absorption across the solar spectrum. The increase in V_OC is due to a higher energy intermolecular charge transfer state formed between the 6T and DBP due to the larger interfacial gap between the HOMO and LUMO of the donor and acceptor, respectively. The results presented here provide a guideline towards material selection for high V_OC OPVs.

Photoinduced electron transfer lies at the heart of organic photovoltaics. In particular understanding how the offset of energy levels between donor and acceptor affects the electron transfer is crucial for minimising voltage losses, and hence optimising efficiency. We have investigated this issue using time-resolved fluorescence measurements to determine how the rate of electron transfer depends on the energy level offset. We have applied this approach to the highly efficient photovoltaic polymer PTB7, in blends with a range of custom synthesised and commercial acceptors with a wide range of electron affinities (9 in all). The quenching of fluorescence requires a combination of diffusion and electron transfer. The picosecond time-resolution of our measurements enables us to quantitatively determine the rate of each of these processes and hence separate electron transfer from exciton diffusion. The results are striking: the rate of electron transfer depends strongly on the choice of acceptor. It increases with increasing driving force up to an offset of 0.4 eV and then decreases with a further increase of driving force. This behaviour is observed for both fullerene and non-fullerene acceptors. It can be described by Marcus theory, as reported in microwave conductivity measurements on wider gap fluorene-containing polymers by Coffey et al. The decrease in rate at high driving force is a Marcus inverted region, and shows that too much offset is bad for the electron transfer process as well as for open circuit voltage. A fit of Marcus theory to our results give a reorganization energy of 0.4+/-0.2 eV. This small reorganization energy is one of the reasons PTB7 is such a good material, and a further advantage of it over P3HT.
[1] D.C. Coffey et al, J. Phys. Chem. C, 116, 8916 (2012)

Organic solar cells are a promising new solar cell technology, offering flexible, light-weight and potentially transparent modules at low cost. They have seen a dramatic improvement in the past few years. The best devices based on vapor-deposited small-molecule organic semiconductors have reached certified efficiencies of 8.3% (TU Dresden) for single junction and 12% (Heliatek) for multijunction solar cells. This successful development of these molecular organic solar cells is challenged by the recent breakthroughs achieved for organometal trihalide perovskite solar cells, which have increased their efficiency within a few years from a few percent to certified efficiencies of over 17%. In this talk, I will review our recent work on both molecular organic solar cells and perovskite solar cells. The main challenge in the molecular organic materials are the exciton separation and limited transport properties of the materials, requiring donor-acceptor bulk heterojunctions. I will discuss recent work on temperature and field dependent exciton separation. The beneficial effects of highly doped transport layers on the cell properties will be discussed as well. In comparison, the influence of doped transport layers in vapor-deposited perovskite solar cells is discussed.

Tandem solar cells based on the efficient use of polymeric absorbers with distinct absorption spectra can improve power conversion efficiencies (PCEs) beyond the limits of single-junction devices. In this study, we developed a triple-junction tandem design employing three distinct organic donor materials having band gap energies ranging from 1.4 to 1.9 eV. Through optical modeling, we achieved balanced photon absorption rates and, thereby, matched the photocurrents among the three subcells. Accordingly, we obtained a triple junction tandem organic solar cell exhibiting a record-high PCE of 11.5%.

Plasmonic nanostructures have been employed in bulk-heterojunction organic photovoltaics (BHJ-OPVs) in almost every layer of the device. Each of the possible configurations have shown enhancements in the power conversion efficiency (PCE) of the resulting devices; however, in many cases, the mechanism of the enhancement in not clear, and the total enhancement factor is typically small (1-2). There are 4 dominant mechanisms that are possible: 1) increased generation of excitons through near-field enhancements; 2) increased optical path length through far-field scattering; 3) coupling into propagating surface plasmon polariton (SPP) modes; and 4) morphological effects arising from incorporating nanostructures into the BHJ layer. We have developed an understanding of the dominant morphological and optical changes that nanostructured metallic (i.e., “plasmonic”) electrodes induce in conjugated polymer BHJ active layers using a range of nanostructural and spectroscopic characterization methods and supporting electromagnetic theory.
In this study, we have fabricated plasmonic electrodes by thermally evaporating Ag through a nanoporous alumina template onto a thin Ag film. The plasmonic electrodes were coated with thin films of neat conjugated polymers (P3HT, PCDTBT, and PTB7) and BHJ blends (P3HT:PC₆₀BM, PCDTBT:PC₇₀BM, and PTB7:PC₇₀BM). We have studied the morphology of these coated plasmonic electrodes through a combination of grazing-incidence wide- and small- angle X-ray scattering (GIWAXS/GISAXS), Raman spectroscopy, and atomic force microscopy (AFM). We have demonstrated that, although the crystallinity of the neat polymers is decreased in the presence of the plasmonic electrode relative to the planar electrode, the crystallinity of the polymers in the BHJ blends is increased in the presence of the plasmonic electrode, which could lead to improved charge separation in BHJ-OPV devices.
To characterize the optical properties of the coated plasmonic electrodes, dark-field (DF) imaging spectroscopy, integrating-sphere reflection spectroscopy, and finite-difference time-domain (FDTD) simulations have been employed. We have observed two dominant modes present in the DF spectra and FDTD simulations: one short wavelength mode (~400 nm) corresponding to the localized surface plasmon resonance of the Ag nanoparticles, and one longer wavelength mode with a peak occurring just beyond the cutoff wavelength of the polymer coating. We have tentatively attributed this long wavelength mode to a contribution from an out-coupled SPP and scattering from inter-particle electromagnetic coupling. These effects can be useful for enhancing the electromagnetic field in the conjugated polymer films, leading to improved BHJ-OPV device performance. In this talk, we will discuss which factors - morphological effects or plasmonic effects - we believe have a stronger influence on the enhancement factor of the efficiency in BHJ-OPVs incorporating plasmonic electrodes.

The performance of organic photovoltaic devices is integrally connected to the number of charge carriers, i.e., holes and electrons, which can be photo-generated. The device efficiency could potentially be improved in the case of singlet fission, whereby a singlet excited state forms two triplet excitations of lower energy on neighboring chromophores [Chem. Rev., 110, 6891 (2010)]. Singlet fission offers indeed the possibility of overcoming the theoretical Shockley limit on the order of 31% for the conversion of sun light power into electrical power [J. Appl. Phys., 100, 074510 (2006)]. However, this phenomenon is not well understood and therefore, there is a crucial need for a deeper understanding of the physical parameters determining the viability of the process. Recent experiments have found that several low-energy Raman-active phonons with frequencies around 75 cm^-1 could facilitate singlet fission in rubrene single crystals [Phys. Rev. Lett., 109, 097403 (2012)]. We use density functional theory (DFT) to simulate the Raman spectrum of the rubrene single crystal. We show that good agreement between experimental and simulated spectra is obtained with the B3LYP functional; we then assign the corresponding phonons to the peaks in the experimental spectrum. We show that the low-frequency phonons significantly contribute to the lattice relaxation energy and play an important role in the modulation of the electronic coupling due to normal mode displacement. The largest contributions come from phonons with frequencies of 71 and 75 cm^-1 that are primarily described by inter-molecular motions of the tetracene cores (improving inter-molecular pi-overlap) and twisting of the phenyl side-groups. Next, we apply a restricted active space spin-flip formalism [Acc. Chem. Res., 46, 1339 (2013)] to evaluate the dependence of the excitations of interest (low-lying singlet and multiexciton states) on the solid-state vibrations. We show that for several phonons, the localized singlet-state energy can fluctuate by approx. 0.2 eV. We then employ a one-electron transition density formalism [J. Phys. Chem. Lett., 4, 3845 (2013)] to evaluate the nonadiabatic coupling (NAC) between localized singlet and multiexciton excited state due to phonons. For several phonons, a 100% increase in NAC is observed. Therefore, we identify several phonons around 75 cm^-1 that could be important not only for the electron-phonon coupling, but also for the coupling between singlet and multiexciton states. We estimate the variations in electronic couplings and NAC at room temperature by evaluating their distributions based on the geometries obtained from molecular dynamics simulations.

Organic Photovoltaics (OPV) have been studied for nearly 30 years and the technology is now on the verge of commercialization. With record power conversion efficiencies approaching 10 % for laboratory devices, efficiency of OPVs still lag behind other technologies such as Silicon-based photovoltaics that can provide 15 % in module. However, the benefits of OPV rely on the possibility to produce flexible and low weight products with a high degree of design freedom. In addition, it has been claimed that OPV can overperform silicon technology under low light conditions. However, this statement has not yet been proved as a general principle. These advantages and the ease of handling in subsequent product-integration processes will enable the development of consumer and portable electronics and building-integrated photovoltaic (BIPV) products. A value-added application in BIPV is the production of semitransparent windows that permit both natural illumination of a room and production of solar electricity. In this regard a search for semitransparent top electrodes has recently proved: a fruitful range of materials can now be used.
Proper bulk heterojunction morphology in polymer-fullerene solar cells is a significant condition for the efficient photogenerated exciton separation, however with a random distribution of donor and acceptor in the active layer charge carriers can be trapped causing efficiency decreasing. One of the solutions is the controlled vertical phase-segregation, where electron selective fullerene is predominantly covering the cathode in a consequence of determined device treatment. Different techniques, such as: X-ray diffraction, variable-angle spectroscopic ellipsometry, transmission electron microscopy etc. have been developed to study segregation of donor and acceptor molecules within the active layer. However, abovementioned techniques are complementary and each presents its own limitations that make them inaccessible to many research labs.
Here we present a direct method to use EQE measurement to observe the donor/acceptor concentration at a given interface by measuring its individual contribution towards the photocurrent. The technique relies on the differential penetration of the light depending on the wavelength. Importantly, we are able to discern the cases in which having a high concentration of donor or acceptor at the incorrect contact (selective to the opposite carrier) have an impact on the device performance. Moreover, using the impedance spectroscopy, we explained the importance of vertical segregation in the semitransparent bulk-heterojunction organic solar cells espessially at low light intensities, which his particularly important for the indoor applications.

Organic photovoltaics (OPV) is a promising candidate technology for the low-cost fabrication of modules to harvest solar energy. Although OPV technology has significantly matured over the past few years, there remain significant challenges in addressing the gap between lab-scale devices and real manufacturing. Structure-property-performance relationships for OPV devices are still underdeveloped, and relationships based on one system are not necessarily transferrable to new, higher-performance systems. This talk will describe our efforts to develop measurements that support OPV manufacturing. Using a blade coating process as a prototype for slot-die coating, we have developed several techniques to observe the structure of OPV films in-situ as they dry. Our measurements include synchrotron-based X-ray scattering and a variety of optical methods. We use these techniques to identify the mechanisms by which formulation and processing choices influence the nanoscale structure of the films. Several OPV systems will be described including polymer/fullerene, small-molecule/fullerene, and polymer/polymer. Throughout solidification, we can follow the number of phases, their composition, and the extent of order within them. In-situ techniques provide far more information about the solidification process than can be obtained by measuring already-dried films, providing a valuable tool to guide the selection of formulation and processing parameters.

The current efforts to improve the OPV device performance almost exclusively employ spin-coating of the active materials in a protected atmosphere, with efficiencies now reaching 12% for small area devices on the order of millimeter squares. The fabrication conditions by which these very high electrical efficiency cells are currently produced generally cannot be simply transferred to a large-scale roll-to-roll industrial production scheme. The efficiency discrepancy between organic solar cells produced in the laboratory and large-scale devices manufactured in a roll-to-roll printing process need to be addressed before the commercialization of large area printed OPV devices. Correlating the process conditions, the morphology and electrical performance of the OPV devices are critical for solving the challenge of mass-producing high efficient large area organic solar cells.
To solve the above mentioned grand challenge, we developed a mini roll-to-roll compatible printing setup for organic solar cells with the capability to follow the film formation during solvent dry in situ with small and wide angle X-ray scattering at Stanford Synchrotron radiation facility. By using this set-up, time resolution down to 10ms was achieved to probe the drying kinetic and crystallization process of the organic semiconductor materials. This set-up also allows to use multiple inks that being delivered to the roll-to-roll printer head with different composition of the active layer between the donor and acceptor materials. We printed a variety of different BHJ OPV materials (both polymer fluorine systems and all polymer systems) on the flexible ITO/PET substrates and characterized the print process using in situ small/wide angle X-ray scattering (SAXS, WAXS). Wide range process parameters were investigated in details during R2R coating process, including solvent quality, drying temperature, shearing speed, blend ratio of donor and acceptor materials, additives, in order to obtain the optimized OPV morphology. By extracting the peak intensity, full widths at half maximum (FWHM) and peak position, rich information about the drying process as well as dried film are obtained. Finally the morphology of the OPV devices are correlated to electrical performance of the device (e.g. V_OC , Jsc , FF, PCE), thus shed light on the best protocol for drying process to obtain the highest possible efficiency for the R2R coating of OPV materials on flexible substrates.

A new picture of the bulk heterojunction (BHJ) polymer solar cell morphology has emerged recently, which indicates that the BHJ is composed of at least three phases: a highly ordered polymer rich domain; a disordered domain of intimately mixed polymer and fullerene; and a fullerene rich domain. Recent theoretical and experimental work have indicated a connection between this "three-phase" morphology and charge transport in BHJ polymer solar cells by showing that offsets in the energy levels of carriers between BHJ domains may improve charge collection efficiencies.
In this work, we show that energetic offsets do exist in polymers and fullerenes and arise primarily due to two factors, (i) bandgap widening due to wavefunction delocalization, and (ii) intermolecular interactions occurring between the polymer and fullerene. Using a combination of photoelectron spectroscopy and spectroelectrochemistry, we characterize the energy levels in all phases of a polymer:fullerene BHJ, accounting for both bandgap widening and intermolecular interactions. We find that in several polymers and in PCBM there is an energetic offset greater than 100 meV between the amorphous and aggregated phases of a given material. We also find that blending a polymer and PCBM modifies the energy levels of the materials in the mixed phase, increasing the polymer ionization potential and reducing the PCBM electron affinity by more than 100 meV. Such energetic offsets may prove beneficial to charge separation by driving carriers away from recombination centers. The intermolecular energetic offsets are composition dependent, and change in magnitude depending on the polymer:fullerene ratio, suggesting local molecular environment plays a strong role in the BHJ energy landscape. These results show that intermolecular interactions can be as important as intramolecular disorder when determining the energy landscape. To properly characterize the carrier energy levels across all domains in a BHJ, intramolecular and intermolecular effects must both be accounted for, and the appropriate morphology must be characterized.

In this presentation we will report the molecular design by DFT computations, synthesis, and molecular and structural characterization of new molecular building blocks and polymeric materials for organic photovoltaic cells. We have now families of polymer donor-polymer acceptor semiconductors achieving PCEs ~7% and polymeric donor-fullerene blends with efficiencies approaching 10% in inverted architectures. New donor-acceptor polymers achieving exceptionally large fill factors (~ 80) based on BTI cores or with very large Voc (>0.9 V) based on the isoDPP core will also be presented. Furthermore, we will show recent studies on device performance achieved upon ambient processing and lifetime stability test under light soaking. Finally, we report results on using self-assembled interlayers and metal oxide blends to replace conventional spin-coated materials enabling improved performance or enable new functions. Our results demonstrate that single-junction OPV cell with efficiencies surpassing 10% are possible and that all-polymer blend cells can also compete with polymer-fullerene devices.

For organic photovoltaic (OPV), bulk heterojunction (BHJ) architecture is widely used to achieve high power conversion efficiency (PCE) due to the widely dispersed donor-acceptor interface. However, the random molecular growth of donor and acceptor materials in blended films can lead to a high recombination rate due to the poorly defined pathways for charge extraction. Methods for growing ordered blended films with phase-separated morphologies that guarantee interconnected networks for photogenerated holes and electrons towards the electrodes are crucial for overcoming this challenge. Besides, it remains difficult to regulate the crystallization of organic molecules in co-evaporation process, which is important for exciton diffusion and therefore for photocurrent generation. Here, we demonstrate a strategy to control the morphology and crystalline growth of co-evaporated zinc phthalocyanine (ZnPc) and fullerene (C₆₀) blended films for efficient OPV cells.
A 2,5-bis(4-biphenylyl)-bithiophene (BP2T) film or a ZnPc film on BP2T was deposited on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) smoothed indium tin oxide substrates as nanostructured templates. On BP2T or ZnPc templates, co-evaporated ZnPc and C₆₀ molecules constantly kept away from each other forming a phase-separated morphology. The templating growth not only realized the construction of interpenetrating networks in the blended films for carrier transport, but also enhanced the crystallinity of the ZnPc domains in the blended films for efficient exciton dissociation. Furthermore, such templating-growth-directed morphologies closely associated with the growth characteristics of the templating layers. Rough and adhesive surfaces of the templating p-layer significantly degraded the phase separation in the blended films due to the hindered templating growth, resulting in a decreased short-circuit current density (Jsc) and fill factor (FF). By optimizing the templating growth, the Jsc and FF were improved 60% and 40%, respectively, which resulted in a 125% increase in PCE from 1.85 to 4.15% under 1 sun.¹ Through this systematic study, we find a substantial opportunity for improving the performance of small-molecule photovoltaics by careful control of molecular growth in the blended films, and highlight the importance of analyzing the templating effects to enable an integrate optimization.
References
(1) Wang, Z.-P.; Miyadera, T.; Yamanari, T.; Yoshida, Y. ACS Appl. Mater. Interfaces2014, 6, 6369.

We reported the optical and electronic properties, structures, and photovoltaic performance of non-peripherally octalhexyl-substituted derivatives of tetrabenzoporphyrin (C6TBPH₂), tetrabenzomonoazaporphyrin (C6TBMAPH₂), tetrabenzodiazaporphyrin (C6TBDAPH₂), tetrabenzotriazaporphyrin (C6TBTAPH₂), and phthalocyanine (C6PcH₂) macrocycles. By replacement of aza links between the eighteen-π-electron cores of phthalocyanine species by methine links, the highest occupied molecular orbital level and the bandgap energy increased from -5.3 to -4.9 eV and from 1.6 to 1.7 eV, respectively. As a result, the Q-band and B-band of C6PcH₂ were red- and blueshifted relative to that of C6TBPH₂, respectively [1]. Otherwise, X-ray diffraction patterns exhibited that the mesogenic phthalocyanine comprises columnar stacks of cofacial molecules arranged within a classical two-dimensional hexagonal lattice symmetry (pseudohexagonal structures), and that by replacing aza links by methine links, the arrangement of tetrabenzo(aza)porphyrin was probably changed to be 2D rectangular [2]. The photovoltaic performance of bulk heterojunction (BHJ) organic solar cells (OSCs) were strongly dependence on the number of aza and methine links between the eighteen-π-electron cores. The BHJ OSC utilizing tetrabenzotriazaporphyrin mixed in [6,6]-phenyl C₇₁ butyric acid methyl ester exhibited the high power conversion efficiency exceeding 4.9% [2-5].
Acknowledgments
This work was partly supported by Grants-in-Aid (Grant Nos. 24246009 and 25107719) for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by Advanced Low Carbon Technology Research and Development Program from the Japan Science and Technology Agency (JST-ALCA). Quang-Duy Dao was supported by JSPS Postdoctoral Fellowship Program for Foreign Researchers (No. P14051).
[1] A. N. Cammidge et al., Chem. Eur. J. 17, (2011) 3136.
[2] Q.D. Dao et al., Appl. Phys. Express, 6 (2013) 122301.
[3] Q.D. Dao et al., Appl. Phys.Lett., 101 (2012) 263301
[4] Q.-D. Dao, et al., Org. Electron. 14, (2013) 2628 .
[5] Q.D. Dao et al., Submitted to Appl. Phys. Lett.

We have developed a series of novel S,N-heteroacenes, which combine the stability of oligothiophenes and the plashy;nashy;r extended pi-system of oligoacenes.¹ By multiple fusion of thiophene and pyrrole rings, a series up to nonacenes has been synshy;shy;thesized and structure-property relationships elucidated. Theoretical calculations and X-ray structure analyses identify bond-length equalization in the carbon backbone towards the cyanine limit. The small bond length alternation, planarity, optoelectronic and good charge transport properties qualify the S,N-heteroacenes for application in organic electronics.
First promising results for the application of this novel class of organic semishy;shy;conducshy;tors in efficient vacuum- and solution-processed bulk heterojunction solar cells will be presented.² Structural fine-tuning allows the use of S,N-heteroacenes as hole transport material in perovskite solar cells. Besides acting as hole transporter, the S,N-heteroacenes contribute to the light absorption in the low energy region of the solar spectrum forming a dual light absorbing system with the perovskite, which opens up new avenue for the fabrication and material selection of perovskite-based devices.³
1. C. Wetzel, A. Mishra, E. Mena-Osteritz, A. Liess, M. Stolte, F. Würthner, P. Bäuerle, Org. Lett.2014, 16, 362-365.
2. A. Mishra, D. Popovic, A. Vogt, H. Kast, T. Leitner, K. Walzer, M. Pfeiffer, E. Mena-Osteritz, P. Bäuerle, Adv. Mater. in print (adma.201402448).
3. A. Mishra, P. Qin, H. Kast, M. Nazeeruddin, S. Zakeeruddin, P. Bäuerle, M. Grätzel, Energy & Environmental Science in print (EE-COM-04-2014-001220).

Herein, we report on the latest results of our research on merocyanine (MC) based small-molecule organic solar cells (SM-OSC). We present results on tandem solar cells with complementary absorbing subcells in series connection, containing red and blue dyes, respectively. Due to the versatility of MCs, all possible combinations of solution- (SOL) and vacuum-processed (VAC) active layers can be studied. Therefore, tandem solar cells with VAC/VAC, SOL/SOL, SOL/VAC and VAC/SOL active layer combinations are fabricated and characterized. The results are compared to optical simulations and the respective single-junction solar cells. In the SOL-devices the influence of the casting from solvent mixtures is investigated in detail.
Furthermore, we will introduce a series of symmetrical donor-acceptor-donor (D-A-D) dyes and compare their properties with the respective D-A chromophores. Finally, new NIR-absorbing merocyanine dyes featuring an extended polymethine chain and bearing varied acceptor units and an aminothiophene donor moiety are introduced. The optical, electronic, and solar-cell properties of these new merocyanines have been studied in comparison with their corresponding lower homologues.

Organic photovoltaic (OPV) devices generally utilize fullerene molecules as acceptor because these materials enable efficient charge separation in combination with a broad range of donor materials. However, due to the energetic offset at the donor-acceptor interface required for exciton dissociation, the open-circuit voltage (V_OC) of fullerene-based OPV devices is commonly limited below 1 V. Furthermore, the small absorption overlap with the solar spectrum limits the photocurrent generation in fullerenes.
We present a series of subphthalocyanine derivatives as effective alternative acceptors in evaporated small-molecule OPV cells. Boron subphthalocyanine chloride (SubPc) is typically known as a donor material with strong absorption of visible light. Substituting the terminal hydrogen atoms with electron-withdrawing groups lowers the frontier molecular orbital energies, which makes the substituted molecules usable as acceptor. In a systematic study we compare four different SubPc derivatives as acceptor in planar heterojunction devices. By altering the peripheral substituent groups on the subphthalocyanine acceptor, the V_OC of these non-fullerene-based devices can be tuned by 0.3 V. The highest power conversion efficiency is obtained for the combination of hexachlorinated SubPc as acceptor with boron subnaphthalocyanine chloride (SubNc) as donor. The device performance is further improved by replacing the molybdenum oxide hole transport layer by a diindenoperylene exciton blocking layer, and by doping the bathocuproine electron transport layer (ETL) with C₆₀. The low optical absorption and high conductivity of this ETL ensures sufficient optical spacer thickness without compromising charge extraction. These adjustments result in a planar heterojunction OPV device with 6.9% efficiency and a V_OC above 1 V.
Additionally, we present OPV devices comprising non-substituted subphthalocyanines as acceptor. Besides high V_OC values, the combination of SubNc and SubPc as acceptor enhances the photocurrent in a three-layer device structure. A two-step exciton dissociation mechanism is active, whereby excitons generated in the remote SubPc acceptor first transfer to the SubNc acceptor by a long-range exciton energy transfer process, and subsequently dissociate at the donor interface. This results in a remarkable efficiency of 8.4% for a fullerene-free organic solar cell, exceeding current state-of-the-art evaporated single-junction OPV devices.
In conclusion, our results demonstrate that subphthalocyanine molecules can be employed as effective electron accepting materials in organic solar cells. The employment of these non-fullerene acceptors enables high V_OC values by improvement of the energy level alignment with the donor. Furthermore, photocurrent generation can be increased by exploiting exciton energy transfer. The development of new non-fullerene acceptors is therefore a promising route to further improve the performance of organic solar cells.

The performance of organic solar cells consisting of a donor/acceptor bulk heterojunction (BHJ) has rapidly improved over the past few years.¹ Major efforts have been focused on developing a variety of donor materials to gain access to different regions of the solar spectrum. On the other hand, the most utilized acceptors are still restricted to the fullerene family, which includes PC₆₁BM, PC₇₁BM and ICBA.² All-polymer solar cells, consisting of polymers for both the donor and acceptor, gained significantly increased interests recently, because of their ease of solution processing, potentially low cost, versatility in molecular design, and their potential for good chemical and morphological stability due to entanglement of polymers. Unlike small molecular fullerene acceptors, polymer acceptors can benefit from the high mobility of intra-chain charge transport and exciton generation by both donor and acceptor. Despite extensive efforts on all-polymer solar cells in the past decade, the fundamental understanding of all-polymer solar cells is still in its inceptive stage regarding both the materials chemistry and structure physics.³ Thus, rational design rules must be utilized to enable fundamental materials understanding of the all polymer solar cells.
We report high performance all-polymer solar cells employing polymeric donors based on isoindigo and acceptor based on perylenedicarboximide. The phase separation domain length scale correlates well with the J_SC and is found to be highly sensitive to the aromatic co-monomer structures used in the crystalline donor polymers. With the PS polymer side chain engineering, the phase separation domain length scale decreased by more than 45%. The PCE and J_SC of the devices increased accordingly by more than 20%. A J_SC as high as 10.0 mA cm^minus;2 is obtained with the donor-acceptor pair despite of a low LUMO-LUMO energy offset of less than 0.1 eV. All the factors such as crystallinity, surface roughness, charge carrier mobility, and absorptions of the polymers blends are found irrelevant to the performance of these all polymer solar cells. This work demonstrates that a better understanding of tuning polymer phase separation domain size provides an important path towards high performance all-polymer solar cells. The use of polymer side-chain engineering provides an effective molecular engineering approach that may be combined with additional processing parameter control to further elevate the performance of all-polymer solar cells. We obtained a record PCE of 4.8% (avarage from 20 devices), with an average J_SC of 9.8 mA cm^-2. The highest PCE shoots to 5.1%, with J_SC as high as 10.2 mA cm^-2, and V_OC of 1.02 V. It is the highest performance ever published for an all-polymer solar cell.
1. Li, G.; Zhu, R.; Yang, Y., Nat. Photon. 2012, 6 , 153-161.
2. Sonar, P.; Fong Lim, J. P.; Chan, K. L., Energy Environ. Sci.2011, 4, 1558.
3. Facchetti, A., Mater. Today2013,16 , 123-132.

Low-bandgap alternating copolymers comprising dithienosilole as a donor unit and dialkyl-substituted dioxocycloalkene-annelated thiophene as an acceptor unit have been synthesized to investigate the influence of the polymer molecular weight and the alkyl chain length in the acceptor unit on the polymer properties and photovoltaic performance. Both the increase of polymer molecular weight and variation of the alkyl side chains in the acceptor unit subtly affected molecular properties. However, these structural modifications showed significant impact on the photovoltaic performance in bulk hetero-junction (BHJ) solar cells based on copolymer/PC₇₁BM. Furthermore, the optimization of thin-film fabrication led to the appearance of partially crystallized states of the copolymer in the BHJ films. Consequently, conventional BHJ solar cells can achieve a PCE of 7.85%, which is the highest performance among the amorphous copolymers.

Designing new polymers for organic solar cells has been challenging mainly because ultimately performance hinges on large number of materials properties. Semiconducting polymers based on diketopyrrolopyrrole (DPP) have emerged as a promising class of materials for this application. Recent insights will be presented on how performance can be enhanced by eliminating adverse homocoupling reactions in the condensation polymerization reactions. With the new method it is possible to make DPP based copolymers and terpolymers with unprecedented properties in terms of extending the spectral coverage and reducing the energy loss between optical band gap and open-circuit voltage. New small band gap semiconducting polymers will be described that provide a high photoresponse at 1000 nm (EQE > 50%), extending even up to 1200 nm. In terms of spectral coverage, these organic solar cells are similar to crystalline silicon solar cells. Further we will discuss our first results on new materials that allow reducing the photon energy loss to below 0.6 eV. Lowering this energy loss, while maintaining high quantum efficiencies, is crucial step to further enhance the efficiency of OPV.

Cascade heterojunction (CHJ) organic solar cells have recently emerged as an alternative to bulk heterojunction and series-connected tandem devices due to their capacity for high internal quantum efficiency (IQE) and broad absorption across the visible spectrum. However, CHJ devices have also been hindered by poor fill factor (FF), limiting the possible enhancement of power conversion efficiency (PCE) compared to single heterojunction devices. In this study, we use a combinatorial approach to develop a generalized framework for understanding the FF and overall performance of planar CHJ cells consisting of multiple subjunctions connected electrically in parallel.¹ A transparent exciton dissociation layer (EDL)/interlayer/acceptor structure is employed,² such that each CHJ cell tested has absorption efficiency identical to its interlayer/acceptor SHJ counterpart. To achieve high FF and PCE, the voltages at the maximum power point of each subjunction are matched and the charge injection barriers intrinsic to CHJs are minimized. Using these established design rules, optimized CHJs with 3 or more photoactive layers are demonstrated with FF >70%, IQE >99% in the interlayer, and PCE >8%.
¹A. Barito, M. E. Sykes, B. Huang, D. Bilby, B. Frieberg, J. Kim, P. F. Green, M. Shtein. Universal Design Principles for Cascade Heterojunction Solar Cells with High Fill Factors and Internal Quantum Efficiencies Approaching 100%. Advanced Energy Materials, doi:10.1002/aenm.201400216 (2014).
²A. Barito, M. E. Sykes, D. Bilby, J. Amonoo, Y. Jin, S. E. Morris, P. F. Green, J. Kim, M. Shtein. Recovering lost excitons in organic photovoltaics using a transparent dissociation layer. Journal of Applied Physics113, 203110, doi:10.1063/1.4807416 (2013).

Multijunction (MJ, or tandem) cells are a key approach to enhancing photovoltaic power conversion efficiency and potentially advancing OPVs into the range of commercial viability. There has been much work in the field regarding this device architecture, primarily focusing on materials selection and architecture optimization to maximize short circuit current (J_SC). However, there has been little discussion on the potential utilization of the imbalance of subcell photocurrents for efficiency optimization. In this talk we will report on our recent efforts optimizing the power conversion efficiency (PCE) of a dual-junction device comprising a perylene-based subcell and a phthalocyanine-based subcell. For this materials set, FF gain from increasing subcell imbalance dominates over J_SC loss, and therefore the MJ cell with the highest J_SC does not correspond with the MJ cell with the highest PCE. We hypothesize that this is due to the significantly dissimilar fill factors (FF) of the comprising subcells (0.73 vs 0.55). To probe the dependence of device performance on subcell photocurrent imbalance, we apply a numerical technique to determine subcell photocurrent balance via the reconstruction of MJ cell current-voltage characteristics from those of representative single junction cells. Through this technique we determine that the highest PCE cell fabricated is significantly current-imbalanced (i.e. the low-FF subcell generates 20% more photocurrent than the high-FF subcell). Furthermore, we computationally simulate this MJ architecture under a broad range of subcell photocurrent balances. We show that within subcell thickness ranges of ±20%, the FF of the MJ cell monotonically increases with increasing subcell photocurrent imbalance (i.e. relatively increasing photocurrent from the low-FF subcell). This work demonstrates that subcell photocurrent imbalance can be a key factor in fully optimizing power conversion efficiency in organic photovoltaics.

The active layers of organic bulkheterojunction solar cells (BHJ-SCs) are conveniently processed using solution-based wet processing techniques. Typically, the donor polymer and acceptor (fullerene derivative), initially blended in a common fluid medium, phase separate upon solvent evaporation. The extent of phase separation and the geometrical features of the morphology determine device performance to a large extent. It is therefore highly desirable to predict how the BHJ morphology depends on material properties and processing conditions.
The current paper presents a continuum model based on the minimization of a free energy functional involving square gradient terms to describe the formation of donor-acceptor interface. The interplay between bulk-interactions and substrate-induced effects will be treated, providing insight into why and under what conditions the initial stages of de-mixing, such as substrate-induced stratification, may still be visible in the morphology of the final dry layers. Time-resolved numerical simulations on PDPP-5T/[70]PCBM/solvent and APFO-3/[60]PCBM/solvent ternaries will be presented, together with scaling relations obtained from a novel analytical approach that describes the effect of evaporation rate on characteristic length scales encountered in the final phase morphology. Where appropriate, results obtained through calculation and simulation will be benchmarked against experimental findings with special focus on gaining insight in the link between phase behavior and -purity and opto-electronic performance.

The polymer solar cell (PSC) is one of the low-cost prosessable photoelectric conversion devices utilizing a composite film as a photoactive layer with an interpenetrating network, namely, a bulk-heterojunction structure composed of donor and acceptor semiconducting organic materials. The PSC has received much attention due to the excellent film-forming property of polymeric materials, expecting the future flexible and stretchable devices. We have developed novel p-conjugated polymers with 1,3,4-thiadiazole (TD) units in the main chains, focusing on their high electron-withdrawing property and more compact structure than the benzothiadiazole (BT) unit. The TD units provide more densely packed structures, and hence show a high charge mobility. In fact, the TD-containing polymer, TD-T, showed the high hole mobility of 8.81 × 10^-2 cm²V^-1s^-1 based on an organic field-effect transistor device. The PSC based on TD-T/PC₆₁BM achieved a relatively high V_oc of 0.80 V and power conversion efficiency (PCE) of 3.0%. In addition, we reported the detailed morphological studies of TD-based polymer thin films, utilizing AFM, synchrotron GIWAXS and GISAXS in order to clarify the relationship between the molecular structure, crystalline orientation, morphology and device performance. The TD-containing polymers exhibited the characteristic 2D-GIWAXS patterns indicating a “face-on” orientation which was observed for the first time. This is probably caused by the strong interaction between TD units and substrates, that is, the polymer lays the planar TD units on the substrate favorably. Moreover, TD-T forms the densely packed crystalline structure having the short p-p stacking distance of 0.351 nm. It was found that the packing density and orientation of the polymer play an important role in the improvement of J_sc, FF and PCE values in the PSC device characterization. The GISAXS experiments revealed that there are PC₆₁BM clusters (R = 11-32 nm) in the TD-based polymer/PC₆₁BM thin films, and the phase separated morphology is also found one of the important factor for OPV performance. In addition, we have recently succeeded in the synthesis of novel bis(1,3,4-thiadiazole)(BTD)-based semiconducting polymers. Their pristine and PCBM-blend morphology as well as OPV performance were also investigated in detail.

Regioregular conjugated polymers typically exhibit better pi-stacking than their regiorandom counterparts. A classic example involves regioregular poly(3-alkylthiophene), for which higher crystallinity, red-shifted optical absorption, and larger charge carrier mobilities are observed when the monomers are arranged in a head-to-tail configuration. Other polymer architectures exist, such as poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT), in which centrosymmetric comonomer units naturally lead to symmetric backbone structures and thereby alleviate synthetic concerns. This presentation will cover the synthesis and design of regioregular narrow bandgap conjugated polymers with donor/acceptor structural units with asymmetric acceptor units. Specifically, regioregular polymers with the pyridyl[2,1,3]thidiazole (PT) unit can be readily accessed because of the preferential reactivity of substituents in the electron poor heterocycle. The resulting polymers have been used to fabricate thin film transistors with exceptionally high hole mobilities (> 30 V/cm s). Specific regioregular polymer systems have also been prepared that perform much better than their regiorandom counterparts with respect to the power conversion efficiencies (PCEs) of bulk heterojunction (BHJ) solar cells. Finally, a regioregular narrow-band-gap (E_g ~1.5 eV) conjugated polymer (PIPCP) comprised of CPDT-PT-IDT-PT repeat units (CPDT = cyclopentadithiophene, IDT = indacenodithiophene) and strictly organized PT orientations, such that the pyridyl nitrogen atoms point toward the CPDT fragment was prepared. Comparison of PIPCP with the regiorandom counterpart illustrates that the higher level of molecular order translates to higher PCE values when incorporated into bulk heterojunction (BHJ) organic solar cells. Examination of thin films via absorption spectroscopy and grazing incidence wide-angle X-ray diffraction (GIWAXS) experiments provides evidence of higher order within thin films obtained by spin coating. Most significantly, we find that PIPCP:PC₆₁BM blends yield devices with an open circuit voltage (V_oc) of 0.86 V, while maintaining a PCE of > 7%. Comparison against a range of analogous narrow band-gap conjugated polymers reveals that this V_oc value is particularly high for a BHJ system with band-gaps in the 1.4-1.5 eV range thereby indicating a very low E_g-eV_oc loss.

The efficiency of organic solar cells has greatly increased in the past few years, recently surpassing 10% [1]. This new technology promises thin and easily integrable modules fabricated using low-cost solution processing of the materials. Unfortunately, these economical methods are often offset by high material costs. While ITO is often the main culprit, the active layer still represents a high proportion of the total device cost [2]. In order to allow large scale applications, we tried to investigate new push-pull conjugated polymers requiring minimal synthetic steps and presenting ideal properties for photovoltaic applications.
In this work, the isoindigo monomer was investigated as it is very easy to synthesize, requiring only two simple synthetic steps from commercial products. This unit also demonstrated great potential for solar cells, yielding power conversion efficiencies approaching 8% [3]. When copolymerized with strongly electron-donation comonomers, low bandgaps are possible, promoting its use in tandem solar cells. This unit shows great compatibility with the newly developed palladium-catalyzed direct heteroarylation polymerization (DHAP), further simplifying the synthesis of these materials [4].
A mean to increase reproducibility and scalability of the synthesis of these materials was also investigated. Batch-to-batch variation has always been a major cause of concern for polymers. As performance is affected by the molecular weight and polydispersity of the polymer, it is imperative to develop scalable and reproducible methods to synthesize such materials. One such method is continuous flow synthesis, which uses a reactor with fixed reaction conditions (heating rate, internal volume, etc.). Reactants are then flowed in this reactor at a controlled rate, allowing great control over reaction parameters. In this study, continuous flow methods were applied to the synthesis of an isoindigo copolymer by DHAP.
References:
[1] J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li and Y. Yang, Nat. Commun, 2013, 4, 1446.
[2] B. Azzopardi, C. J. M. Emmott, A. Urbina, F. C. Krebs, J. Mutale and J. Nelson, Energ. Environ. Sci., 2011, 4, 3741-3753.
[3] Y. Deng, J. Liu, J. Wang, L. Liu, W. Li, H. Tian, X. Zhang, Z. Xie, Y. Geng and F. Wang, Adv. Mater., 2014, 26, 471-476.
[4] F. Grenier, P. Berrouard, J.-R. Pouliot, H.-R. Tseng, A. J. Heeger and M. Leclerc, Polym. Chem., 2013, 4, 1836-1841.

The solution processable small molecule 7,7&’-(4,4bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b&’]dithiophene-2,6-diyl)bis(6-fluoro-5-(5&’-hexyl-[2,2&’-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazaole), p-DTS(FBTTh₂)₂, has emerged as a promising organic photovoltaic (OPV) electron donor, achieving over 7% power conversion efficiency (PCE) in bulk heterojunctions (BHJs) with fullerene in both conventional and inverted device architectures. A low volatility additive, 1,8-diiodooctane (DIO), is necessary to yield optimal performance. The PCE of a p-DTS(FBTTh₂)₂ -based BHJ is exquisitely sensitive to DIO concentration, in stark contrast to the behavior of polymer-fullerene systems, which are relatively insensitive to DIO concentration.
We have studied the influence of the DIO additive concentration on film formation in blade-coated p-DTS(FBTTh₂)₂ \ [70]PCBM BHJs. In-situ grazing-incidence X-ray scattering experiments deliver a detailed picture of the solidification sequence, from initial film drying and evaporation of the main solvent to the final film morphology after removal of DIO. We find that the additive both induces increased crystallinity, probed by Grazing-incidence X-ray diffraction (GIXD), and coarsens the spatial scale of phase separation, probed by grazing incidence X-ray scattering (GISAXS). The phase separation length scale varies with additive loading, suggesting the origin of the extreme sensitivity. We conclude that the additive acts as both a solvent and a plasticizer, controlling the nucleation density and extending crystal growth.

Organic photovoltaic devices (OPVs) have been improved significantly in the last decades, with the power conversion efficiency now reaching 12%. This is achieved through fundamental understanding of the morphology and material, design and synthesis of novel donor and acceptor materials and optimize organic/electrode interface.
In this talk, I will discuss the recent progress in low bandgap polymer materials development, which forms solid foundation for OPV device innovation. The focused device areas are:
1. Tandem polymer solar cell - interconnecting layer and subcell design for achieving high efficiency. Tandem devices with spectrum complementary polymer subcells, as well as with identical polymer subcells both achieved over 10% efficiency, and will be discussed.
2. Morphology control and mechanism understanding in low bandgap IR polymer solar cells. GIXRD, SANS, TEM, UV-vis etc. have been used to elucidate the morphology formation mechanism in DPP based polymers. I will show the aggregation in both solution state and solvent removing stages play a role in optimizing morphology.
3. Visibly transparent polymer solar cell via solution process - Low bandgap polymers provide unique applications of organic PV, allowing wide range of spectrum tuning in transparent solar cell. I will report our recent progresses in single junction and tandem transparent solar cells.

Due to their inherent semitransparency, organic photovoltaics cells (OPV) are an ideal candidate for integration into glass windows or other applications in buildings. We will discuss the interdependence between transparency and efficiency for semi-transparent OPV, with respect to the choice of electrode materials, the absorption spectrum of the active layer and the light management architecture.
In order to compete with other solar cell technologies, OPV needs to be produced by cost-efficient printing technologies that can be up-scaled for large area production. Using silver nanowires as both the electron- and the hole-collecting electrode, we have demonstrated fully solution-processed OPV cells were all the layers were deposited by doctor-blading. These devices show an excellent transparency of 50% at a power conversion efficiency which is 72% of the opaque reference device. In a further step towards up-scaling, we have successfully fabricated semitransparent modules with an area of 8cm x 8cm employing ultra-fast laser patterning. Due to the high accuracy of the laser patterning steps, these modules have a very high geometrical fill factor of 95% and no performance losses when compared to the single cell references.
One-dimensional photonic crystals have been shown to be an effective way to increase the efficiency of a semitransparent OPV cell at the same transparency. We will show how such a light management structure can be fabricated by a printing process which is easily compatible with large-area fabrication and integrated with the organic solar cell module.

Recent developments in organic photovoltaics (OPV) combine the development of novel absorbing materials with clever stack design in order to maximize power production. A prevalent concept is the tandem stack, where two identical or complementary absorbing materials are electrically connected in series. This has been demonstrated both for solution processed and thermally evaporated stacks.
Here, we present a recombination zone (RZ) consisting of a doped organic material as electron transport layer (ETL) in combination with a metal oxide (MoO3). This novel layer stack avoids the use of a very thin and thickness sensitive metal layer. The new RZ permits the inversion of the structure (MoOx / doped ETL), as it does not rely on surface states on the organic material induced by thin metal depositions. A further benefit of the structure is the ability to optimize the thickness of the doped and transparent ETL. By increasing the thickness of the RZ, the second optimum of the interference pattern can be utilized, broadening the application potential of this RZ.
The RZ is applied in a variety of device architectures. First, we demonstrate it for a stack of two evaporated small molecule cells with complementary absorption, obtaining power conversion efficiencies (PCE) above 9%, a 50% increase compared to the single cells. Both standard and inverted structures are possible, employing either the first or the second optimum of the optical interference pattern. In all cases, fill-factor (FF) remains high (above 70%).
In a second application, we report a tandem stack implementing both solution processed and thermally evaporated subcells. We combine an efficient solution processed, low bandgap polymer:fullerene based solar cell with a high bandgap evaporated small molecule cell, effectively utilizing the strengths of both deposition techniques. Also in this case, an improved device efficiency is obtained, owing to the high FF, an open-circuit voltage (Voc) which is the sum of the individual single cell Voc&’s and complementary absorption.
Finally, we can exploit the difference in refractive index of the two RZ materials, on the order of 0.4, to fabricate distributed Bragg reflectors (DBR). This unique feature adds an additional degree of freedom, especially regarding complementary absorbing tandems. Utilizing a DBR stack of only 2 pairs, we demonstrate a 50% reflection in a selected wavelength range. This DBR facilitates the balancing of photocurrent production in the single cells of a tandem structure.

Over the last couple of years, intensive research in the field of organic photovoltaics has boosted device efficiencies beyond 10%. Major progress resulted from the development of new polymer absorber materials and intensive efforts on the optimization of lab-scale devices. For future large-area solar cell fabrication, however, the main challenge will be the transfer of lab-scale processes into an industrial production environment where the requirements of the fabrication procedures and processes are very much different. While state-of-the-art solar cells are typically processed from chlorobenzene (CB) or dichlorobenzene (DCB) solutions, hazardous chlorinated solvents cannot be used in a roll-to-roll industrial fabrication but have to be replaced by environmentally friendly alternatives. As the nanomorphology of the bulk-heterojunctions and hence the efficiency of the devices are determined by the solvents and the film forming conditions, the choice of solvents is of pivotal importance for a successful process transfer to industry-scale.
In this work we present a comprehensive study of environmentally friendly solvents such as xylene, anisole or dimethylanisole for the fabrication of PTB7:PC₇₁BM solar cells. This absorber material system has shown excellent power conversion efficiencies (PCEs) in the past. When intentionally omitting any additives, solar cells fabricated from dimethylanisole yield higher PCEs than the commonly used CB.
Often high boiling solvent additives are used to affect the bulk-heterojunction formation and hence to enhance the device PCE. It is often neglected that the most common additives such as diiodooctane (DIO) are also halogenated and therefore are non-suitable for industrial device fabrication. Accordingly, we have investigated different non-halogenated additives in combination with the non-chlorinated main solvents, among them octanedithiol and methylnaphthalene that are known to be most effective additives in various polymer/fullerene systems. Since these additives do not enable device performances exceeding efficiencies of DIO mediated bulk-heterojunctions, we have investigated new eco-friendly solvent-additive combinations that enable even higher efficiencies than the typical combination of CB and DIO.
To better understand the influence of the solvent-additive system on the solar cell performance, atomic force microscope surface analyses were carried out and recombination loss mechanisms were investigated

Heavy glass substrates and protective covers dominate the weight and form factor of today's thin-film solar photovoltaic (PV) modules. High module weights limit potential applications and increase the cost of PV manufacturing, transportation, and installation. In this work we show that ultra-thin, lightweight, flexible polymer films can be grown in-situ as both substrate and encapsulation layers for thin-film PVs. We demonstrate a molecular organic solar cell that is entirely vapor-deposited—from transparent superstrate to active layers to metal electrode—based on poly(chloro-p-xylylene) (Parylene C) membranes with thicknesses below 1 micron. PVs on parylene membranes achieve power conversion efficiencies equal to or exceeding that of conventional glass-based devices and are among the thinnest, lightest solar cells yet demonstrated. Chemical vapor deposition (CVD) of parylene enables in-line substrate and device fabrication without breaking vacuum, minimizing contamination and damage risk associated with transportation, handling, and cleaning of ultra-thin substrates. Solar cells on ultra-thin and flexible parylene membranes can be seamlessly adhered onto a variety of solid surfaces, providing for additive solar power on any surface.

The development of alternate transparent conductive electrodes to replace tin doped indium oxide (ITO) is necessary to meet the rapid developments in the optoelectronics industry. The required bench mark of the novel electrode performance is to have its optoelectronic properties comparable to ITO (>80% T & <20 Omega;/#9633; R_sh) with an added requirement of room-temperature processing. Evolving strategies in this regard include: oxide based sandwich structures with metallic mid layers; carbon nanowire networks; binary or ternary oxides; and graphene [1]. These technologies require a range of different deposition methodologies from solution processing to advanced vacuum evaporation [2, 3].
In the present work we have prepared aluminum doped zinc oxide (AZO) electrodes with state-of-the-art optoelectronic properties on glass and flexible substrates at room-temperature with an industrially viable sputtering deposition technique. We identified ‘target tilt&’ as the most sensitive parameter to establish the best optoelectronic properties of the AZO layers. Films were prepared at 2 mTorr with AZO ceramic targets (2 wt% Al doped ZnO) by applying 200 W (RF) / 4 in² power density using argon as the sputter gas. AZO films of ~400 nm thickness were prepared with ~80% optical visible transparency and 30 Omega;/#9633; sheet resistance (1.2 × 10^-3 Omega;-cm), with the performance dependent on the target tilt angle. The optimum performance was found at a target tilt angle of 20.8° by the systematic study of target tilt angle from 16.2° to 31.3°. Optical constants of these films were determined by Ellipsometry measurements; which were further applied to model the optical field distribution in-device. Finally, the performance of the AZO electrodes were demonstrated by preparing ‘glass|AZO|MoO_x|P3HT:PCBM|Al&’ organic solar cells.
REFERENCES:
[1] K. Ellmer, Nature Photon. 6, 809 (2012)
[2] H. Hagendorfer, K. Lienau, S. Nishiwaki, C.M. Fella, L. Kranz, A.R. Uhl, D. Jaeger, L. Luo,
C. Gretener, S. Buecheler, Yaroslav E. Romanyuk, and A.N. Tiwari, Adv. Mater. 26, 632
(2014).
[3] J.B. Franklin, J.B. Gilchrist, J.M. Downing, K.A. Roy, and M.A. McLachlan, J. Mater.
Chem. C, 2, 84 (2014).

A ternary blend bulk heterojunction organic solar cell (OSC) using a mixture of one acceptor and two donors has emerged as a viable approach to improve the photovoltaic performance because of its simpler fabrication process than tandem architecture devices. Here, we report a ternary blend OSC composing fullerene derivative acceptor (SIMEF2)^[1] and structurally similar porphyrin donors, tetrabenzoporphyrin (BP)^[1,2] and dichloroacenaphtho[q]tribenzo[b,g,l]porphyrin (CABP). A blending two porphyrins of BP and CABP in a 75:25 ratio forms a solid solution state that gives entirely different structural and optoelectronic properties from those of either pristine compounds or a mixture at other blending ratios. The use of this solid solution for OSC devices resulted in a power conversion efficiency (PCE) value higher by ca.40-75% than the PCE values obtained for the devices using the single donor component because of the increase in short circuit current density. This increase mainly originates from the increase in internal quantum efficiency, and hence by enhanced charge carrier generation efficiency at the donor/acceptor (D/A) interface, which was probably caused by a more intimate D/A contact for the solid solution donor than the one using high crystalline porphyrin donors. The results suggest that physical and chemical modulation in solid solution is beneficial as an operationally simple approach to improve the photovoltaic performance.
[1] H. Tanaka, Y. Abe, Y. Matsuo, J. Kawai, I. Soga, Y. Sato, and E. Nakamura, Adv. Mater., 24, 3521-3525 (2012)
[2] Y. Matsuo, Y. Sato, T. Niinomi, I. Soga, H. Tanaka, E. Nakamura, J. Am. Chem. Soc., 131, 16048-16050 (2009)

Metal nanoparticles (NPs) have been applied to enhance the performance of thin-film devices, such as organic photovoltaics, through making use of the plasmonic effects to enhance the light absorption of OSCs with ultrathin-active-layer configuration. In this paper, we propose and demonstrate electron extraction enhancement induced by charge accumulation effects of metal NPs. The metal NPs (Au NPs and Ag NPs) are embedded in titanium oxide (TiO₂) layer, functioning as highly efficient transport layer for improving the performances of inverted organic solar cells (OSCs), which lead to significantly increased photocurrent and power conversion efficiency reaching 8.20%. Importantly, our results show that the optical plasmonic effect of metal NPs (Au NPs and Ag NPs) in the electron transport layer is a minor factor in improving the OSCs efficiency. Instead, the charge extraction enhancement under solar illumination can be explained by the transfer of UV-excited electrons from TiO₂ electron transport layer to metal NPs and the enhanced accumulation of the electrons in metal NPs-TiO₂ composites. The electron accumulation reduces the work function of the electron transport composite layer after UV illumination. The redistribution of charges in the UV-irradiated metal NPs-TiO₂ system can assist the charge extraction in OSCs. Mutliphysics study is also conducted to explain the effects of the charge accumulation on device performances (i.e. improving short-circuit current without degrading the open-circuit voltage), which show that the electron stored NP-TiO₂ layer can lead to the reduction of the charge transport resistance and enhanced electron extraction. As a result, the photocurrent and thus the device performance of OSCs are increased considerably. Consequently, by incorporation of metal NPs, our experimental and theoretical results show that the NP-TiO₂ transport layer, which is different from the conventional doping effects in semiconductors, exhibits very good charge extraction and collection at electrode for efficient organic optoelectronic devices.

Photon harvesting in organic solar cell is highly dependent on the anisotropic nature of optoelectronic properties of photoactive materials. Here, we demonstrate an efficient approach to dramatically enhance the photon harvesting in planar heterojunction solar cells by using a highly oriented pentacene crystal film on graphene. A simple insertion of monolayer graphene at anode interface induced the quasi-epitaxial growth of pentacene crystals, of which the lying-down orientation was favorable for overall optoelectronic properties including light absorption, exciton diffusion, charge transport and interfacial energetics. Spectroscopic and crystallographic analysis revealed that the lying-down orientation persisted until 110nm in thickness, which, along with increased exciton diffusion length, significantly enhanced the light absorption within the photoactive layers. The resultant photovoltaic performance showed simultaneous increment in V_oc, J_sc, FF and consequently a 457% increment in maximum power conversion efficiency than the devices without graphene layers.

We have demonstrated the EHD (Electrohydrodynamic) printing of Ag-grid transparent electrodes. Ag-grid is pressed into plastic substrates during heat treatment. More than 90% of the metal grids were embedded inside the plastic substrates which leads to a relatively smooth surface of the transparent film. The width of the patterned grid was less than 10 um, which cannot be identified by the naked eyes. The electrical and optical properties of the EHD-printed Ag-grid electrodes are investigated. Ag-grid is carefully designed in order to maximize both conductivity and transparency of the EHD-printed transparent electrodes. Depending upon the grid pitch (distance between adjacent Ag lines), the resistivity varies accordingly. The optimized Ag-grid transparent electrode is employed to the fabrication of optoelectronic devices.

Lifetime is still a major problem of organic photovoltaic (OPV) cells. There are many reasons for solar cell degradation varying from shunts induced by impurities or electromigration over photoinduced oxidation of active layer materials to corrosion and delamination of the metal contact both induced by oxygen or water ingress.
One issue concerns so-called pinholes through the metal back electrode of the device. These pinholes offer pathways for ingress of water and oxygen which may attack the metal-organic interface by introducing delamination through formation of insulating metal oxides or hydrogen evolution. As charge injection and extraction is suppressed at delaminated areas, the active area taking part in power conversion - and hence the overall efficiency - becomes reduced.
We investigated the influence of different environmental conditions on the reduction of the active area of the OPV cell. Spatially resolved measurements give information on location and size of insulated areas induced by pinholes in the metal back contact. Time resolved measurements during degradation of the devices revealed the dynamics and rate of growth of these individual defects. Looking at different device structures provides conclusions for increasing the lifetime of organic photovoltaic cells.

It may seem, by virtue of the small bending radii thin polymeric films can accommodate, that all organic semiconductors are already sufficiently compliant for flexible and mechanically robust applications. An examination of the modest body of literature on the mechanical properties of modern organic electronic materials reveals that these properties are highly variable. In particular, many of the best-performing organic semiconductors are mechanically brittle. In the 2012 report of the Workshop on Key Scientific and Technological Issues for Development of Next-Generation Organic Solar Cells, sponsored by the US National Science Foundation and the Office of Naval Research, researchers asked, “What has been done to prevent solar cells from failing mechanically?” (http://web.utk.edu/~opvwshop/) Moreover, in a well-known paper by Krebs et al., in which the researchers deployed roll-to-roll fabricated OSC-powered LED lanterns in rural Zambia,(10.1039/B918441D) one of the main conclusions was that “hellip;mechanical failure mechanisms were dominant during the field test and therefore these would have to be improved significantly before the photochemical stability of the [semiconducting] polymer becomes a problem.” Mechanical stability is of critical importance not only for portable applications—for which accommodation of strain is a defining operational requirement—but also for roll-to-roll production itself, transportation, and for utility-scale applications. In large-scale solar farms and in portable applications, thin organic solar modules will be subject to a range of stresses due to environmental forces. This talk reviews the current state of knowledge of the mechanical failure of organic solar cells, and emphasizes recent work by our laboratory whose goal is to understand the molecular and microstructural determinants of the mechanical stability of organic solar cells. Our approach comprises design and synthesis of new conjugated copolymers, mechanical testing, and the fabrication of stretchable—highly elastic and ductile—devices. Our major conclusion is that mechanical robustness and electronic performance are not fundamentally at odds, and that it is possible, in principle, to achieve the “best of both worlds,” with rational co-design of bulk properties and electronic performance.

Organic inverted solar cells (OISCs) have demonstrated long lifetime as well as high power conversion efficiency (PCE) up to the threshold for commercial application, i.e. 10%. While the increased lifetime as compared to regular organic solar cells (OSCs) is associated with the oxidation-free indium tin oxide cathode, the mechanism for the improved PCE has so far been unclear. In this report, we found that the improved PCE of OISCs is due to the diffusion of the electron acceptor (EA) material into the electron transporting layer, poly [(9,9-bis(3prime;-(N,N-dimethylamino)propyl)-2,7- fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), and hence the increased interfacial contact between the EA material and PFN, resulting in higher short circuit current density. Neutron reflectometry and cross section transmission electron microscope (TEM) results clearly show the existence of smeared EA in the PFN layer. The smearing of EA into the PFN layer is thought to occur when electron donor (ED)/EA solution is spun-cast on top of the PFN layer, when the PFN layer is swollen by the solvent and nano-sized EA clusters diffuse into the swollen PFN layer along with the solvent. When 1,8- diiodoctane (DIO) is added to the casting solution as the solvent additive, more EA clusters diffuse into PFN layer resulting in higher device performance.
This research was conducted at the Center for Nanophase Materials Sciences (CNMS), Hiflux Isotope Reactor (HFIR) and Spallation Neutron Source (SNS) which are sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities Division, U.S. Department of Energy, managed by UT-Battelle, LLC, for the U.S. Department of Energy.

Recent advancements in organic photovoltaics have been enabled by organic-based electrode modification layers leading to robust device architectures compatible with large area processing. These developments benefit from multi-component polymer platforms that integrate electronically active structures into otherwise insulating polymers. Reported herein are the synthesis and characterization of novel azulene-substituted methacrylate polymers via chain-growth approach, in which the azulenes represent hydrophobic dipoles strung pendent to the polymer backbone and impart unique electronic properties to the polymers. These systems prove fundamentally interesting, as their optical and excitonic behavior can be tuned by varying azulene density, ranging from homopolymers (having one azulene group per repeat unit) to copolymers in which the azulene density is diluted with other methacrylates. Copolymers of azulene and zwitterionic monomers prove useful as orthogonally soluble cathode modification layers in bulk heterojunction solar cells, where the relative azulene content affects device metrics, and power conversion efficiency values of 7.9% are obtained.

In bulk-hetero junction (BHJ) polymer solar cells (PSCs), the interfacial energy barrier between the metal oxide and organic active layer is known to act as electron traps and recombination centers, resulting in recombination losses and energy level mismatch. Furthermore, the hydrophilic ZnO surface leads to undesired phase-separated domains of the upper BHJ active layer. Modification of the surface properties of ZnO could be crucial for improvement of the device performance, especially in inverted PSCs that employ metal oxide films for the electron transporting layer (ETL). Herein, we present the development of a novel conjugated polymer electrolyte (CPE) polymer (PBN) having a novel benzodithiophene (BDT)-based molecular structure. The coating of PBN on the ZnO layer to form a PBN/ZnO stacked structure improved physical contact between the ZnO and a mixture of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-bprime;]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) and PC₇₀BM based-active layers and improved the wettability of a mixture of PTB7:PC₇₀BM on the ZnO layer. The PBN/ZnO stacked structure reduced the interfacial energy barrier by generating an interfacial dipole moment between ZnO and the active layer. Finally, we employed PBN as an electron-transporting layer on a ZnO layer and constructed a highly efficient, inverted structure device consisting of PTB7:PC₇₀BM achieving a high power conversion of up to 8.6%, constituting a 21.1% improvement over the control device performance (7.1%) prepared without a PBN layer. This result was ascribed to the reduced interfacial resistance and the improved charge transport and collection through the PBN ETL.

Networks of silver nanowires are becoming accepted as transparent conducting electrodes for capacitive touch sensors and are under development by many groups for photovoltaic applications. Improved theoretical understanding of the films can provide important input for optimisation of transmission vs sheet resistance properties.
We present a novel application of continuum percolation theory to the transmission-sheet resistance relation of percolating conductive thin films, by considering different density metrics and using a simple Beer-Lambert model to describe film transmission.
For the first time we are able to extend existing understanding to account for the effects of nanowire length statistics on electro-optical film performance, and the optical extinction due to nanowire coverage close to and below the percolation threshold. The predicted effects of length variance on the film behaviour are interrogated through experiment and simulation studies. We derive a new expression for the percolative figure of merit, by comparison of the presented expression to the accepted expression in their limit of equivalence.
Once the length statistics of the material in question are quantified, two-parameter fitting to data for silver nanowire films indicates that the percolative exponent of sheet resistance is typically overestimated, and the figure of merit is typically underestimated. Correction for the limiting transmission due to material not connected to the percolating network at high sheet resistance leads both the present and accepted expressions to produce fits with values for the figure of merit that agree to within ~10%. The percolative exponent is still over-estimated in the case of the accepted expression, but to a significantly lesser degree. Also, if the percolative exponent is measured directly from an area fraction-sheet resistance plot then the new expression produces superior one-parameter fits to the data; goodness-of-fit is compared using statistical information criteria.

Inverted polymer solar cells have attracted significant attention due to their high stability and increasing power conversion efficiencies^[1]. Transparent electron transport layers play an important role in determining the performance of inverted polymer solar cells, where zinc oxide thin films are one of the most commonly used candidates. Here, polycrystalline ZnO thin films are fabricated from an aqueous solution under atmospheric conditions using a custom-built mist-CVD system^[2], in which precise control of the thin film thickness and morphology is achieved. Polycrystalline ZnO thin films are traditionally fabricated from vacuum based systems such as sputtering or CVD, rendering them incompatible with the solution-based and low-cost fabrication process of polymer solar cells. Mist-CVD deposition of polycrystalline ZnO addresses these challenges by adopting a water based and open-air process, which is readily scalable and provides excellent uniformity over large areas. The ZnO crystallites form a thin yet closely packed layer that follow the contours of the underlying substrates, making this process particularly advantageous for use with patterned or ridged surfaces. High efficiency inverted polymer solar cells have been successfully fabricated using this mist-CVD grown polycrystalline ZnO as the transparent electron transport material and a highest efficiency near 9% has been achieved, a value which is higher than those obtained for conventional sol-gel process, which generally yields amorphous thin films^[3]. These properties indicate that mist-CVD might provide a potential low cost manufacturing method for the preparation of high quality transparent electron transport layer in inverted polymer solar cells.
References
[1]. He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure. Nat. Photon. 2012, 6 , 591 - 595.
[2]. Kawaharamura, T. Physics on Development of Open-air Atmospheric Pressure Thin Film Fabrication Technique Using Mist Droplets: Control of Precursor Flow. Jpn. J. Appl. Phys. 2014, 53, 05FF08.
[3]. Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with a Low-Temperature-Annealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23 , 1679 - 1683.

The transformative potential of organic photovoltaics (OPV) rests in part on the ability to rapidly fabricate devices onto flexible, stretchable, or opaque substrates without vacuum steps or indium-containing electrodes. This work examines utility of self-assembled monolayers (SAMs) of aminoalkylthiols (NH₂(CH₂)_nSH) having various alkyl chain lengths (n = 3, 6, 11, 16) adsorbed to Ag as part of an indium-free, vacuum-free “flipped” device architecture where Ag acts as the cathode and a transparent conducting polymer blend serves as the anode. The amine functionality is shown to improve the wetting of Ag by poly(3-hexylthiophene-2,5-diyl):phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) solutions, enhance the concentration of PCBM at the BHJ/Ag interface, and increase the work function of Ag to better match the lowest unoccupied molecular orbital of PCBM. Moreover, the use of long-chain aminoalkylthiols (n ge; 11) improved the average power conversion efficiencies by as much as an order of magnitude relative to bare Ag and smaller aminoalkylthiols (n < 11) due to uniform coverage that inhibits carrier recombination at the BHJ/Ag interface. The optimization of aminoalkylthiol functionalization and subsequent development of ink solutions led to the fabrication of entirely printed OPVs deposited flexible polymer films. These results have direct application to the development of low-cost, printed, flexible, stretchable organic electronics on arbitrary substrates, which can facilitate the transition of high-efficiency laboratory-scale OPVs to high-throughput industrial processing.

Flexible organic and hybrid organic-inorganic solar cells require both the flexible photoactive media and flexible electrodes with good conductivity and transparency. Graphene has been proposed as a promising flexible electrode due to its mechanical and chemical robustness, excellent electrical and optical properties, and potentially low-cost processing. We have designed graphene electrode-based flexible polymer solar cells with thieno[3,4-b]thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) by resolving the issues occurring at the interface between graphene and charge transporting semiconducting materials. We demonstrate highly efficient flexible devices with power conversion efficiencies of 6.1% (anode) and 7.1% (cathode) achieved via thermal treatment of molybdenum trioxide (MoO₃) electron blocking layer, direct deposition of ZnO electron transporting layer on graphene. The advances in this work demonstrate graphene has great potential as a promising transparent electrode in a variety of flexible optoelectronic devices.

We investigate the performance enhancement of all-polymer bulk heterojunction solar cells with pristine graphene. Blends of a low bandgap polymer, PCPDTBT (poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-bprime;]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]) with high electron mobility, and a prototypical p-type polymer, P3HT (poly(3-hexylthiophene-2,5-diyl), were doped with small concentrations of graphene. The base system has wide spectral coverage, and absorbs light into the red and near infrared regions. Additionally, the absorption spectrum of PCPDTBT shows significant overlap with the emission of P3HT, allowing for resonance energy transfer from P3HT to PCPDTBT. The addition of small amounts of graphene to the blends allow for more efficient charge transport and results in higher short circuit current and better performance over the neat system. The increase in efficiency is attributed to the excellent charge transport properties of pristine graphene. Varying concentrations of P3HT, PCPDTBT, and graphene were used in the active layer blend to determine the composition yielding the highest performance. The effect of blend morphology --with and without graphene-- on efficiency was investigated small angle x-ray scattering, small angle neutron scattering, neutron reflectivity, and grazing-incidence x-ray diffraction experiments. Through these studies, we correlate all-polymer solar cell efficiency with the nanoscale structure of the active layer blend, particularly with respect to changes in graphene concentration, thermal annealing and polymer blend ratio. Our results demonstrate that the addition of pristine graphene addresses the fundamental drawback of poor charge transport in organic photovoltaics, and can lead to enhanced performance by influencing both current and morphology in a range of systems.

Here, we report our recent study on organic ternary bulk heterojunction solar cells, in which we employed prototype electron donor and acceptor materials Poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the hosts and another high-performance low bandgap polymer as the sensitizer. We found that with the addition of only a small amount of the sensitizer, the power conversion efficiency (PCE) of the ternary cell was greatly improved compared with the binary host system. Remarkably, this ternary system exhibited high PCE even with a thick active layer(~200nm), while the low bandgap polymer binary solar cell usually fails at this active layer thickness due to the drastic fill factor (FF) drop. Synchrotron based grazing incidence X-ray scattering measurements were carried out to investigate for the first time the unique ternary morphology that facilitated charge transport in thick-film devices and thus overcome the FF drop problem. Our results suggest a brand new route for thick-film device optimization, especially useful for future large-scale manufacturing.

New organic materials are continuously being introduced that are significantly advancing organic solar cell efficiency. However, there is limited understanding of the link between the microstructure of these systems and their mechanical behavior. Establishing the microstructural origin of the mechanical behavior and the relationship to optoelectronic performance will help ensure efficient and physically robust photovoltaic devices. In this study, we investigate the mechanical behavior of multiple polymer:fullerene bulk heterojunction material systems (e.g. P3HT and PSBTBT) experimentally and computationally. The computational approach is based on a constitutive representation of semi-crystalline polymers and fullerenes using dislocation density crystal plasticity to simulate crystalline domains, and a hypo-elastic formulation for the amorphous polymer phases. The model is used to identify the dominant morphological and microstructural characteristics that would affect the mechanical behavior of the active layer as well as determine the evolution of morphology under strain. The model is then compared to experimentally determined morphological changes over a range of applied strains as well as the onset of fracture and adhesive failure in the films. We find that the microstructure has a significant influence on mechanical behavior such as the ductility of the film. In addition, the role of strain on optoelectronic characteristics is investigated providing insight into the structural origin of trap states. The ability to accurately model the mechanical behavior of multiple polymer:fullerene material systems provides first steps towards establishing broad design guidelines for the development of efficient and physically robust organic solar cells.

In this paper, a facile and efficient approach to fabricate highly ordered ZnO nanorods (NRs) on top of rough silver nanowire (AgNW) surface is discussed. Prepared by an all-solution process, this nanostructure composite of ZnO-AgNW-ZnO film shows a 77% optical transmission at 550nm with 30 #8486;/#9633; sheet resistance when measured from the surface of the top ZnO layer. Moreover, this well-ordered ZnO NR layer on top of AgNW is highly beneficial for organic solar cells as a multifunctional layer since it provides protection for AgNW against oxidation and moisture. With the use of this nanostructured composite electrode, we have observed a 14% improvement of J_sc over the control device.

We suggest an unconventional fabrication route to high efficiency organic solar cells (OSCs) by adopting highly ordered hierarchical mesoporous CeO2 inverse opal (IO) as electron transport layer. Those structures were prepared by colloidal crystal template approach where polystyrene (PS) beads of diameter ~800 nm were used as template. The resulting colloidal crystals were dried at 90 °C for 3 hours to improve the interconnectivity between the macrospores. Then infiltration was carried out by pouring CeO2 sol-gel precursors into the colloidal template to fill the interstitial spaces in the PS colloidal crystals with CeO2 sol-gel precursors. PS colloidal crystals were removed by calcination at 450°C for 4 hours which lead to the formation of highly ordered hierarchical mesoporous CeO2 IO. The hierarchical CeO2 nanostructure exhibits synergistic properties of three dimensionally ordered macroporous structures comprising interconnected network and photonic band gap in the visible region. This provides increased light activated surface area leading to fast electron transport and ultimate enhancement in the power conversion efficiency (PCE). SEM, XRD, Raman spectroscopy and UV-vis transmission measurements were used to analyze the structure and investigate the effect of highly ordered mesoporous CeO2 IO on the cell performance. In this work, a significantly enhanced PCE was demonstrated for highly ordered mesoporous CeO2 IO in the comparison with the devices without ETL and with neat CeO2, respectively.

Plasmonic metal nanoparticles hold great promise to improve light absorption and charge generation of organic photovoltaics. Unfortunately, power conversion efficiency of the organic photovoltaics with plasmonic nanoparticles is still limited as randomly separated nanoparticles hardly exploit the effective plasmonic enhancement by interparticle coupling. Here we demonstrate single-junction high performance organic photovoltaics using gold nanoparticle clusters with controlled morphology as a plasmonic component. Near-field coupling at the interparticle gaps of nanoparticle clusters gives rise to strong enhancement in localized electromagnetic field, which led to the significant improvement of exciton generation and dissociation in the active layer of organic solar cells. A power conversion efficiency of 9.48% is attained by employing gold nanoparticle clusters at the bottom of the organic active layer. This is one of the highest efficiency values reported thus far for the single active layer organic photovoltaics.

Recently developed organic solar cells have attained power conversion efficiencies (PCEs) in excess of 10% and have attracted great attention for potential use in many kinds of electronic devices. In spite of the advanced PCEs, OSCs still demand long-term stability and flexibility to be integrated into mobile and wearable electronic devices and commercialized. However, ITO which has been the most widely used as electrodes of organic solar cells has critical drawbacks in terms of increasing cost and brittleness, making it unsuitable as a material for electrodes in flexible OSCs. Moreover, the most common hole extraction material of organic solar cells, PEDOT:PSS has problem with its moisture-sensitive and acidic properties. In this work, we replaced ITO with CVD-grown graphene as a cathode and introduced sol-gel derived ZnO as electron extraction layer material on the graphene cathode to develop flexible organic solar cells with inverted structure. During the process of fabricating devices, however, the low surface free energy and the hydrophobicity of graphene make poor wetting of sol-gel derived ZnO film on its surface. To enhance wettability of the ZnO precursor solution on graphene surface, we modified ZnO precursor with a surfactant and successfully spin coated it on graphene surface. We compared the morphology of pristine ZnO and modified ZnO film on graphene surface and confirmed difference of electron mobility by characterizing electron-only devices with each ZnO layer. Furthermore, we could find gap states at the interface between graphene and modified ZnO film using photoluminescence (PL) measurement. Finally, we could successfully fabricated inverted OSCs with developed electrode and electron extraction layer and obtain PCE of 6.2% with a device using PTB7:PC₇₀BM as a photo-active layer.

Polymer solar cells (PSCs) have been under intensive interesting due to their various advantages, which include the high energy conversion efficiency of over 10% and the low-cost manufacturing of flexible plastic solar modules by employing roll-to-roll coating processes at room temperature [1-2]. Although flexible PSCs based on plastic substrate such as polyethylene naphthalate (PEN), polyethylene terephthalate (PET) have been already developed, not much study has been done on highly flexible and stretchable PSCs [3]. Polydimethylsiloxane (PDMS) is a promising candidate for flexible substrate due to its easy fabrication via prepolymer coating, good transparency, flexibility, and robustness. As one of the most powerful conductive materials, Ag nanowires (AgNWs) have recently attracted a lot of attention for potential applications as transparent and flexible electrodes due to their simple solution processability, cost-effectiveness, and high conductivity. However, AgNWs are typically deposited on top of substrates and could delaminate under repeated mechanical deformation [4].
In this work, we report to the PSCs using PDMS substrate with embedded AgNWs electrode in the PDMS surface layer. First, we casted AgNW solution on the Si wafer and dried. The mixed and degassed PDMS prepolymer (Dow Corning Sylgard 184, with a ratio of base to cross-linker of 10:1 by mass) was coated on top of casted AgNW surface, and cured at 65 #8451; for at least 2 h. Then, the PDMS substrate with embedded AgNWs was peeled off from Si wafer, and spin-coated with PEDOT:PSS, followed by P3HT:PCBM. The fabricated PSCs are highly flexible. The flexible substrate and device performances including transparency, conductivity, and current-voltage characteristics will be discussed in detail.
[1] S. Woo, W. H. Kim, H. Kim, Y. Yi, H.-K. Lyu, Y. Kim, Adv. Energy Mater. 4, 1301692 (2014).
[2] J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C. Chen, J. Gao, G. Li, Y. Yang, Nat. Commun. 4, 1446 (2013).
[3] D. J. Lipomi, B. C.-K. Tee, M. Vosgueritchian, Z. Bao, Adv. Mater. 23, 1771 (2011).
[4] F. Xu, Y. Zhu, Adv. Mater. 24, 5117 (2012).

Graphene quantum dots (GQDs), a newly emerging 0-dimensional graphene based material, have been widely exploited in optoelectronic devices owing to their tunable optical and electronic properties depending on their functional groups. Moreover, the dispersibility of GQDs in common solvents depending on hydrophobicity or hydrophilicity can be controlled by chemical functionalization, which is particularly important for homogeneous incorporation into various polymer layers. Here we report that a GQD-incorporated organic photovoltaic device shows enhanced power conversion efficiency (PCE), where the tuned oxygen-related functionalization of GQDs enabled good dispersity in a PEDOT:PSS hole transporting layer (HTL) or a PTB7 and PC₇₁BM bulkheterojunction (BHJ) layer, leading to significantly improved short circuit current (J_sc) and fill factor (FF) values. GQDs with the sufficient functional groups and good dispersibility in polar solvent were incorporated into the HTL resulting in morphologycal changes in the PSS doped PEDOT grains, thereby improving the charge extraction from BHJ layer. In the BHJ layer, reduced GQDs (rGQDs) prepared by hydrothermal deoxidation process were incorporated, which enhanced the carrier conductance through the polymer layer. Due to the hydrophylic and hydrophobic properties of each GQDs and rGQDs, GQDs and rGQDs were well dispersed into HTL and BHJ layer respectively. To maximize the device performance, rGQDs were additionally incorporated in a bulk-heterojunction layer of the device prepared by incorporating GQDs in HTL simultaneously, which is found to promote a synergistic effect with the GQD-incorporated polyemr layers. Therefore, the enhancement of the J_sc and FF values coincide resulting in the significant improvement of PCE of polymer solar cells.

Anatase titania has been widely used in several applications such photocatalysis and solar cells. Sol-gel is a conventional route to obtain amorphous titania and, a post-annealing about 450°C or a post-hydrothermal treatment, are necessary to obtain anatase crystalline phase. In fact, the routes and conditions of synthesis influence in the particle size, surface area and grain size of titania. In this work regular nanoparticles of anatase titania were obtained directly by an easy one-step synthesis assisted with ultrasonic action during the hydrolysis process, in order to form hybrid bulk heterojunctions. For this, a mixture of deionized water (0.90mL), HNO₃ (0.090mL), isopropanol (5.4mL) and Ti(OPr)₄(4mL) were putted under ultrasonication (145W, 40kHz ± 6%) during 1h. Then the product was stored overnight and washed several times with ethanol by centrifugation. Nanoparticles about 4-6nm were observed by TEM images and the anatase crystalline phase was verified by X ray diffraction (PDF#21-1272), this crystallinity was improved with a thermal treatment at 450°C during 1h. The nanoparticles powder was dispersed in aqueous solution at different concentrations (5 to 20mg/mL) and, microfibers of poly (3-hexylthiophene) (P3HT) were immersed into the nanoparticles solution during 24h, in order to form the hybrid bulk heterojunction. The P3HT microfibers covered with nanoparticles were dried at 80°C during 1h and they were characterized by SEM, EDS and Confocal microscopy. According to SEM results, at low concentration there were zones in the P3HT microfibers without covering with nanoparticles, while at high concentration, the microfibers were seen cracked. EDS analysis corroborated the presence of titanium and oxygen atoms on the surface of microfibers. According to the SEM and Confocal results, the best homogeneous covering onto the P3HT microfibers was obtained at 10mg/mL of titania nanoparticles, indicative that it is the optimal and feasible concentration to develop hybrid bulk heterojunctions for solar cells.
Acknowledgements: Thanks to CONACyT-México (CB176450) and SENER-CONACyT (P27 CEMIE-Sol) for the financial support. Thanks to Rogelio Morán Elvira, Gildardo Casarrubias, Ma. Luisa Ramoacute;n and Marina Vega for their technical support.

A particular organic photovoltaic device configuration that is receiving much attention due to the promise of significantly improved device operational lifetimes is the inverted bulk-heterojunction (BHJ) device. In this configuration, the metallic electrode is employed as the anode allowing more stable, high-work-function metals or metal-oxides to be employed, and unstable PEDOT layers to be replaced with more stable lower work-function materials such as zinc oxide or tin oxide at the transparent cathode. Recently, we have shown that incorporation of nanostructured noble metal electrodes that support surface plasmons into an inverted BHJ device configuration can also increase light trapping in the conjugated polymer/fullerene BHJ active layer. As a result, we have demonstrated absorption enhancement factors between 5 and 10 in the 570 to 750 nm wavelength range at the red of the conjugated polymer absorption band. However, to translate the enhanced absorption and improved stability of this inverted plasmonic BHJ configuration into useful photovoltaic device performance, the BHJ active layer must be deposited onto the nanostructured plasmonic metal electrode and, subsequently, a transparent cathode must be applied directly to the BHJ active layer.
To that end, we have developed solution-processable transparent cathodes composed of multi-layers of zinc oxide nanoparticles (ZnO NPs) and silver nanowires (Ag NWs) for application directly to the active conjugated polymer layer. First, ZnO NP thin films were applied to the active polymer layer from ethanol dispersions at a spin coating speed of 2000 rpm to act as a suitable electron transport layer. Subsequently, Ag NWs films were applied to the ZnO NP films to lower the sheet resistance of the transparent cathode. The average diameters of the ZnO NPs and the Ag NWs was 100 nm and 90 nm, respectively, and the average length of the Ag NWs was 30 µm. UV-visible spectroscopy of the multi-layer films prepared under a variety of spin coating conditions revealed that the multi-layers had greater than 87% transmission over the wavelength range 450 - 900 nm. Preliminary AFM results showed the average thickness of the multi-layer films was 142 nm. Initial two-point conductivity tests on the multi-layer films indicated that they were conductive following UV-exposure and when the coverage of Ag NWs reached approximately 25*10⁶ NWs/cm². Future work will further investigate the conductivity of the multi-layers as a function of processing conditions using a four-point probe setup and determine the performance of the transparent multi-layer cathodes in inverted plasmonic BHJ devices.

Electrospun carbon nanofiber (ECN)/TiO₂ nanoparticle composite counter electrode (CE) for dye-sensitized solar cells (DSSCs) were successfully prepared by spray-coating ECNs/TiO₂ (1:1 by weight) mixture on fluorine doped tin oxide (FTO)-glass substrate. TiO₂ particles (Degussa P25) were used to bind carbon nanofibers and adhere them to the FTO-glass substrate. Electrochemical impedance spectroscopy (EIS) measurements revealed that the spray-coated ECN/TiO₂ composite CEs have lower charge transfer resistance (R_ct) and higher interfacial capacitance (Q) than those of Pt CEs. Cyclic voltammograms (CV) further indicated that ECNs/TiO₂ composite CEs have faster tri-iodide reduction rate than those of Pt CEs. DSSCs fabricated using ECNs/TiO₂ CEs showed a power conversion efficiency (eta;) of 7.25% under 100 mW/cm² light intensity, which is comparable to that of thermally deposited Pt based DSSCs (eta;=7.57%). Moreover, ECNs/TiO₂ composite CE based DSSCs demonstrated almost equal power conversion efficiency to that of Pt based cells by adding only 8 w.t% Pt, which unveiled a cost-effective alternative of costly Pt CEs in DSSCs.

We report a novel architecture to fabricate solution-processed foldable transparent conducting electrodes by using a combination of metal-grid embedded into flexible substrate and ultrathin transparent electrodes. The use of the silver metal grid and the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer in combination resulted in an increase in conductivity without a reduction in the transmission and helped in overcoming the trade-off between them, leading to highly transparent (optical transmittance asymp; >80% at a wavelength of 550 nm), highly conducting (sheet resistance asymp; 20 ohm/sqr.) and extremely flexible (bending radius asymp; 200 mm) electrodes with very smooth surface (root-mean-square roughness asymp; 0.5 nm). These electrodes were used to fabricate foldable organic devices that exhibited performances similar to that of devices fabricated on glass substrate. In addition, these fabricated flexible devices did not show degradation in their performance even after being folded.

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 general, DSSCs consist of mesoporous TiO₂ nanoparticles, sensitizer, redox couple, and platinum (Pt) as a counter electrode (CE). In DSSCs, the CE is an important component, as it plays a crucial role in the reduction of redox couple so as to complete the electrical circuit. Generally, Pt has been used as a CE 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 has spurred research into alternative CEs with good catalytic properties and a simple fabrication process. In the past, many efforts have been made to replace Pt with low-cost carbon materials or conducting polymers. While high efficiency was achieved for DSSCs with carbon-based CEs, the power 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.
To overcome these drawbacks, we replace to Pt with conductive polymer. Conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) was coated onto fluorine-doped tin oxide (FTO) glass as a CE in a DSSCs with a [Co(bpy)₃]^3+/2+ complex redox couple. A highly conductive PEDOT thin Film was fabricated on a FTO substrate via a chemical polymerization method. The PEDOT exhibited improvements in the electrochemical stability for the Co(bpy)₃^2+/3+ redox couple compared to the Pt-CE. The charge transfer resistance (R_CT), related to the interface between an electrolyte and a CE, was significantly reduced much lower than those of Pt-CE as analyzed by impedance spectroscopy. The DSSC based on the JK-303-sensitizer and PEDOT-CE displayed a higher power conversion efficiency of 10.29% than those with the Pt-CE of 9.64%.

Colloidal quantum dots(CQD) are a promising candidates in the future optoelectronic material due to the advantages of precise control over bandgap through the quantum size effect and solution-processing. These characteristics make that CQD photodetectors apply in wide range spectrum in the visible, the near infrared, and the short-wavelength infared. Recent Progress in a combination of organic and colloidal quatum dot material devices has led to the demonstration of various large area, low cost, flexible, portable sensors.
In our research, inorganic/organic hybrid solar cell shows higher efficiency of solar cell and responsivity of planar type of photodetector compared with inorganic PbS quantum dot (QDs) conventional solar cell. Inorganic/organic hybrid solar cell used C₇₀ for active bilayer with PbS quantum dot, PEDOT:PSS and BCP for hole transfer layer (HTL) and electron transfer layer (ETL) as organic material respectively. Enhanced power conversion efficiency (PCE) of ~1.41% is obtained from inorganic/organic hybrid structure compared with PbS inorganic conventional structure (~1.29%) under air mass (AM) 1.5 solar illumination. Responsivity (A/W) of 0.16(375nm) and 0.02(808nm) is achieved from inorganic/organic hybrid structure which is one-order higher than PbS inorganic conventional structure.

Organic solar cells are promising alternative energy owing to low fabrication cost and simple fabrication processes. Although organic solar cells have many advantages comparing with inorganic solar cells, the efficiencies of organic solar cells are still inferior to the one of inorganic solar cells. Increasing the device efficiency is essential for real commercialization. In this work, gold nanoparticles were incorporated in the hole injection layer, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). We found that the device efficiency was enhanced when 10% gold nanoparticles of 20 nm were blended with PEDOT:PSS. We also synthesized gold nanoparticles with various diameters by adjusting the experimental conditions. The device efficiency was enhanced by 29.4% (power conversion efficiency from 3.61% to 4.67%) through blending 10% gold nanoparticles of 30 nm. Further increasing the diameter resulted in poor efficiency.

There has been much attention towards dye-sensitized solar cells (DSSCs) for the past decades due to their attractive features such as reasonable energy conversion efficiency and low energy production cost. Even though a respectable energy conversion efficiency (~12%) has been achieved for photovoltaic cells employing organic liquid electrolyte that consists of I^-/I₃^- redox couples and volatile solvent (e.g. acetonitrile), the leakage and evaporation of liquid solvent from such cells has been suggested as the one of the critical factors limiting the long-term stability of the DSSC. Moreover, the use of I^-/I₃^- redox couples limits the energy conversion and even causes some problems such as the metal corrosion and visible light absorption etc. Recently, cobalt complexes (e.g. [Co(bpy)₃]²⁺/³⁺, bpy=2,2&’-bipyridine) have been suggested as one of the most promising candidates for replacing conventional I^-/I₃^- redox shuttles owing to their high voltage characteristics. Although a high energy efficiency exceeding 12% was reported by employing liquid electrolyte that consists of cobalt complex redox mediator and volatile solvent, the use of volatile liquid solvent cannot ensure the long-term stability of the DSSCs. In this work, therefore, novel polymer gel electrolytes (PGEs) including cobalt complex redox couples have been developed for efficient and long-term stable DSSCs. The physicochemical properties of electrolyte medium were adjusted by mixing two or more solvents with high boiling points. Polyethers with polar groups and polymers without polar groups were used as the polymer additives for the gelation of the liquid electrolytes with various blending ratios. The DSSC employing the PGEs showed the respectable energy conversion efficiency comparable with those of the DSSCs with liquid electrolytes. After the thermal aging tests at 60 ^oC for 800 hr, it was also proven that the long-term stability of the DSSC employing the PGE is much better than that of the cell filled with volatile liquid electrolyte.
Acknowledgements : This work was supported in part by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (No. 2012013811) and by the New & Renewable Energy R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry & Energy (MOITE) (No. 20123010010070).

Indium tin oxide (ITO) is the most used material for transparent electrode in organic electronic devices. But ITO has several disadvantages towards such devices commercialization including high cost and availability of indium, brittleness of ITO and work function.Single-walled carbon nanotube networks (SWCNT-NW) is a fascinating alternative to replace ITO due to the benefits including higher work function, bendability and range of possible deposition methods including printing/coating. One of the challenges to obtain SWCNT-NW with good optical and electrical properties is to avoid the formation of aggregates in large bundles.
In this contribution we explore the ultrasonic spray deposition technique to get SWCNT-NW for transparent electrodes in organic photovoltaics (OPV). The SWCNTs investigated had low and high functionalities and were beforehand dispersed in water and surfactant. The dispersions were characterized by UV-Vis-NIR absorption spectroscopy and demonstrate a high level of isolated tubes. After sprayed on different substrates (e.g. glass, PET and PDMS) the NW were evaluated with respect to their morphological (optical microscopy, SEM and AFM), electrical (c-AFM and Van der Pawn) and optical properties (by UV-Vis-NIR spectroscopy). The best sheet resistance value obtained was 145 #8486;/Sq for a 15 nm thick film at 85% of transparency. We investigated also, different subtractive patterning techniques to achieve locally patterned SWCNT-NW. By using lift-off photolithography with negative tone photoresist lines with 10 µm width were realized while by laser ablation the smallest pattern was 20 µm large.

Solution-processable organic semiconductor nanowires offer a potentially powerful strategy for producing large-area printed flexible devices. Here we report the fabrication of lateral-architecture organic solar cells (LOSC) using a solution-processed organic nanowire blend on a flexible substrate to produce a power source for use in flexible integrated microelectronics. A high photocarrier generation and an efficient charge sweep-out were achieved by incorporating self-assembled poly(3-hexylthiophene) nanowires into the active layer. A high work function interfacial layer was introduced to increase the built-in potential. Scanning photocurrent microscopy studies revealed that the carrier diffusion/drift length and overall generated photocurrent in the channel increased significantly. The utility of the LOSCs for high power-source applications was demonstrated by connecting multiple devices in parallel or in series using designed interdigitated electrode patterns. We successfully fabricated high photovoltage-producing LOSC modules on plastic substrates for use in flexible optoelectronic devices. The LOSCs described here offer a new strategic device architecture for use in highly flexible photoresponsive energy devices.

Organic photovoltaics (OPVs) are prone to degradation in atmosphere due to the presence of deterrents such as oxygen and moisture. Therefore, flexible, light weight, barrier materials are required for encapsulating OPV devices. Various atomic, physical and chemical deposition techniques have been used to deign barrier materials with inorganic layers over flexible substrates such as polymers. However, these techniques have not been proven to be economic and suitable for roll processing. Therefore, it is required to develop ultra-low moisture permeable (with water vapour transmission rate < 10^-6 g m^-2 day^-1) barrier material that is low cost processable, flexible, transparent with ease of scalability. Polymer based composites and blends are a suitable choice if the ultra-low permeability limit is achieved. Hence, Surlyn (poly ethylene-co-methacrylic acid) based blends, composites and layered materials are designed and evaluated for their water vapor barrier properties from calcium degradation tests.

Monte Carlo (MC) simulations have been used to fully model both conventional organic photovoltaics (OPVs)and water-based nanoparticle organic photovoltaics (NP OPVs). For solvent based OPVs, the quantum efficiency and short-circuit current of these virtual devices are in excellent agreement with experimental measurements. Simulations show that, contrary to expectation, indium tin oxide/ poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)/poly(3-hexylthiophene):[6,6]-phenyl- C61-butyric acid methylester (PCBM)/aluminium devices lack effective charge blocking layers at the electrode interfaces. X-ray photoelectron spectroscopy depth profiling shows that despite a PCBM-rich region near the cathode, interface intermixing at the electrodes combined with incomplete PCBM coverage leads to significant interface recombination. For NP OPVS, MC modelling shows that the core-shell structure can be optimised to produce efficient dissociation. This work highlights the effectiveness of MC simulations as a predictive tool and emphasizes the need to control electrode interface processes.

In this work, we explored the capacity of Cochineals (Dactylopius coccus Costa) insects in charge transport for use in organic electronic applications. The charge transport properties of the Cochineals were investigated by the current-voltage measurements using Al, Au and indium-tin-oxide as hole-injecting contact. The current density vs. voltage characteristics it has been obtained to indicated a hole-transport in cochineals is governed and has a combination of space-charge effects. The Cochineals was compared with the PEDOT in the analyses of the PHOLED electroluminescence and current vs. voltage measurements. We observed a decrease in your electroluminescence spectral in the region of 450nm and 600nm, in the same region where has the absorption spectral of Cochineals. The understanding of the device operation of PHOLED indicates directions for understand the characteristics than has of cochineals sample.

Metallic nanoparticles (NPs) in polymer solar cells (PSCs) have been widely studied to solve the problem of limited light absorption in PSC devices by plasmonic effects. Many researchers have used metallic NPs in PSCs by embedding them into the PSC interlayers or blending them into the bulk heterojunction (BHJ) active layer. In that cases, because of the different nature of the ligands and the amount of ligands on the metallic NPs, the dispersity of the metallic NPs in each layer may vary and result in different PSC performance parameters.
So, in this study, we controlled the amount of attached organic ligands on gold NPs systematically so as to control their dispersion behavior in the BHJ active layer of PSCs. By controlling the amount of attached organic ligands on the gold NPs, the dispersity of the NPs in the BHJ layer was controlled. As a result, we found that the electrical effects of the gold NPs is a more important factor for enhancing the PSCs efficiency compared to the plasmonic effect.

Polymer solar cells (PSCs) as a new green technology, resulting from their merits in low cost, feasible and large-scale fabrication and light weight, are becoming increasingly more and more interesting both academically and commercially. Until now, many high-performance polymer donors and photovoltaic devices have been developed, in which the power conversion efficiency (PCE) can exceed 9 %. In order to increase the photovoltaic properties, device engineering is indispensable. One way attracted intensively of approaching to this target relates to the interface research which corresponds to insert a cathode interfacial layer (CIL) between active layer and cathode. Through utilizing water/alcohol soluble natural pigment as CIL, the photovoltaic performance of PSCs can improve tremendously. Under the device configuration of ITO/PEDOT:PSS/PTB7:PC₇₁BM/natural pigment/Al, the PCE can reach to 7.56 %. Also, the inverted device shows the better performance with short circuit current density (J_sc) of 17.64 mA cm^-2, open circuit voltage (V_oc) of 0.76 V, fill factor of 65.53 % and PCE of 8.79 % under the device architecture of ITO/ZnO/ natural pigment/PTB7:PC₇₁BM/MoO₃/Al.

We have set a new bench mark for DSSC performances by fabricating a very efficient device with a high photocurrent density and good stability. This bench mark was accomplished by harmonizing the absorption spectrum of the dye with the average size of the aggregates in the TiO₂ electrode. The resultant resonant multiple scattering enhanced the light harvesting efficiency and charge collection yield. The high and robust photovoltaic performance (with an initial efficiency of 9.18% and an efficiency of 7.44% after 800 h of irradiation with a light intensity of 100 mW cm^-2) of the JH-1 DSSC prepared without an antireflecting layer demonstrated the promise of this novel sensitizer for large scale applications. Our result should stimulate further investigations into novel dyes with similar structures and we expect that our method may be readily applicable to practical DSSC systems.

Organic photovoltaics (OPVs) has attracted much attention due to potential light-weight, low cost, transparency and flexible alternative to traditional inorganic solar cells. However, the absorber thickness in single solar cells is constrained by the exciton diffusion length which is much smaller than the light absorption length. Therefore, maximum theoretical efficiencies are difficult to achieve on single photovoltaic devices. Tandem solar cells, which consist of two or more single cells with complimentary absorptions, have been considered as an effective approach to overcome this limitation. For the configuration of tandem solar cells, series connected architectures have been widely used. In these architectures, utilizing effective recombination zones (RZs), which make series connection between the sub-cells, is one of the key aspects to achieve high-efficient tandem devices.
In this work, we demonstrate an effective recombination zone consisting of an Mg:Ag (1:3) alloy layer and a MoO₃ thin film with nominal thicknesses of 1.2 nm and 5 nm respectively for application in tandem OPV devices based on Zinc phthalocyanine donor and fullerene C₆₀ acceptor. The Mg:Ag layer ensures an optimum electron selectivity, while MoO₃ layer effectively selects holes [1-3]. Maximum open circuit voltages have been achieved on tandem devices consisting of two sub-cells, while short-circuit current matching is currently under optimization. The recombination Mg:Ag/MoO₃ layer system is investigated with regard to the requirements of high optical transparency, work function compatibility, and facilitation of light absorption depending on Mg:Ag and MoO₃ layer thicknesses. The respective characterizations were carried out by UV-Visible spectroscopy, Kelvin probe force microscopy in ultrahigh vacuum, current-voltage and external quantum efficiency methods.
[1] M. Rusu,S. Wiesner, I. Lauermann, Ch.-H. Fischer, K. Fostiropoulos, J. N. Audinot, Y. Fleming, and M. Ch. Lux-Steiner, Appl. Phys. Lett. 97, 073504 (2010).
[2] M. Rusu, F. Kraffert, S. Wiesner, W. Schindler, K. Fostiropoulos, and M. Ch. Lux-Steiner, Energy Procedia 31, 96 (2012).
[3] W. Riedel, S. Wiesner, D. Greiner, V. Hinrichs, M. Rusu, and M.Ch. Lux-Steiner, Appl. Phys. Lett. 104, 173503 (2014).

Plasmonic metallic nanoparticles (NPs), especially highly conductive silver (Ag) nanoparticles (NPs), have recently been identified as a breakthrough route for enhancing the efficiency of organic photovoltaic devices. We demonstrate the improvement of power conversion efficiency in bulk heterojunction polymer solar cells based on blended poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester by introducing 50 nm Ag NPs. The NPs were deposited on indium-tin-oxide (ITO) substrates by spin coating from colloidal solution prior to deposition of poly (3,4- ethylene dioxythiophene: poly (styrene sulfonate) (PEDOT: PSS) buffer layer. This efficiency gain was attributed to enhanced light harvesting and subsequent exciton generation rate into the photoactive layer due to localized surface plasmon resonance and scattering effects. This argument was supported by the combinational study of spectroscopic analysis and device photon-to-electron conversion efficiency measurements. Optical measurements, Characterization (e.g., microscopy), and fabrication of the solar cells will be presented and discussed.

The organic solar cells has an inherent disadvantage such as low charge carrier mobility, limiting the thickness of active layer, resulting in optical losses of the incoming photons and decreasing photocurrent. In order to efficiently absorbs the light in active layer, silver and gold nanoparticles are used as plasmonic light scattering center in PEDOT:PSS layer. Although efficiency enhancement is achieved by plasmonic nanoparticles, the effect of plasmonic light trapping is under debate. Since the field produced by LSPR rapidly decades as the distance increases from the nanoparticle, the field enhancement cannot disperses into active layer. Consequently, the efficiency enhancement by the gold nanoparticles in the PEDOT:PSS is mainly ascribed to increase in conductivity of PEDOT:PSS. Therefore, the close proximity of nanoparticles to photoactive layer is highly required. In order to solve the problem, the metallic nanoparticles are mixed into the blend solution of active layer to enhance the near-field enhancement by LSPR. The various geometric parameters of nanoparticles, such as the size and shape, were demonstrated in the active layer in OSCs. However, since the performance of OSCs is highly dependent on the concentration, dispersion, and size of metal nanoparticles, the fine tuning of metallic nanoparticle is complicated. For example, the Au nanoparticles without fine-tuning in P3HT:PCBM can cause the efficiency loss by non-radiative decay, charge-carrier recombination, and quenching of excitons. Therefore, the plasmonic nanostructure, which embeds the metallic nanoparticles into active layer with facile fabrication method is highly required.
In this work, we report the novel light trapping nanostructure, composed of silver nanoparticles (Ag NPs) on ITO nano-branches (ITO BRs). The plasmonic light trapping by Ag NPs have been demonstrated with near-field enhancement by LSPR, light scattering and radiative energy transfer to active layer. When the Ag NPs are attached on surface of the ITO BRs, they can be embedded into active layer resulting in close proximity to photoactive layer. Since the Ag NPs, nearby the absorbing materials, can build up the strong near-field by localized surface plasmon resonance in the active layer, the light absorption in active layer is enhanced. In addition, the plasmonic Ag NPs are the effective subwavelength scattering elements to trap incident light by elongating the optical path length. The radiative energy transfer from Ag NPs to active layer also contributes to light absorption. Not only the optical properties but also the electrical properties are improved by incorporation of ITO BRs. The entangled ITO BRs provide a connected network for charge carrier transport, act as an efficient 3-dimentional conductive pathway. As a result, the nanostructures with Ag NPs on ITO BRs efficiently absorb the light and transport the charge carriers simultaneously, resulting in increase of 26 % photo conversion efficiency.

Porphyrins and their derivatives have been exhaustively explored in the last few decades due to their unique photophysical properties. Such molecules present a wide range of optical effects, such as photoluminescence, linear singlet absorption, triplet transitions and non linear optical processes. In the present work, we investigate the UV-Vis absorption spectrum of a free base cationic porphyrin derivative (5,10,15,20-tetrakis (1,3-dimethylimidazolium-2-yl) porphyrin tetraiodide) (H₂TDMImP) in polar solvents. Our results show that, for the present sample, the less energetic transitions belonging to Soret band are favored when compared to other ionic porphyrins and that the Q-band presents a rhodotype structure. Quantum chemical calculations suggest that such spectral feature is caused by the strong interaction between the cationic outlying group, present in this porphyrin, and solvent molecules. Increasing the porphyrin concentration, the spectral changes suggest that H-type porphyrin aggregates are formed and that such aggregate structures protect the outlying groups of porphyrin against interactions with solvent molecules.
ACKNOWLEGDMENTS: FAPEMIG, FAPESP, CAPES AND CNPq

Transparent Conducting oxides are widely used in many applications such as solar cells, etc. These thin film oxides can be deposited by physical vapour deposition, Chemical vapour deposition and chemical solution deposition techniques. One of the challenges in solution based process is to produce high quality films. In this paper, we report a novel chemical solution method, a polymer assisted solution-based thin film deposition technique to grow indium oxide and zinc oxide based transparent conducting films. We use a new strategy to control the distribution of metal ions in solution with desired viscosity. By actively binding the metal ions, the polymer encapsulates the metal ions to prevent chemical reaction while maintaining an even distribution of metal ions in solution. This ensures a homogenous metal distribution and avoids unwanted reactivity of metal precursors. The Successful growth of transparent conducting oxide illustrates that polymer assisted solution-based thin film deposition could be an alternative approach for synthesis of high quality thin films. The transparent conducting oxide films are also employed to the fabrication of organic solar cells to confirm their feasibility as electrodes for device applications.

As an alternative to classical thin-film solar cells hybrid photovoltaic devices consisting of inorganic and organic materials are gaining more interest. When planar single crystal silicon is spin coated with poly-(3-hexyl-thiophen) (P3HT), poly-(3-[3,6-dioxaheptyl]-thiophen (P3DOT), and poly-(3-[2,5,8-trioxanonyl]-thiophen (P3TOT) conversion efficiencies of h = 3.2, 7.2, and 9.6 % are obtained, respectively.
To elucidate the influence of the different polymers on the device performance first-principle density functional calculations were employed. The supercells contained up to 100 atoms. For all molecules a rotation of the conjugated planes around the polymer axis is observed that causes a reduction of the overlap of the molecular orbitals. Our calculations show that interdigitaion of the side chains is energetically not favorable. However, the presence and chemical nature of the side chains influences the energy gap between occupied and empty states. While poly-thiophen shows a gap of 1.0 eV the energy gap decreases to 0.75 eV for P3HT. On the other hand, the presence of dioxaheptyl or trioxanonyl side chains causes an increase of the energy gap to about 0.98 eV. The calculated band structures are used to estimate the carrier mobility in crystalline P3DOT, P3TOT, and P3HT by employing an acoustic deformation potential model.

P-type metal oxides are among the key ultra-thin film interfacial materials used in organic optoelectronic devices because of their high work functions which are energetically favorable for barrier-less hole collection/injection. For this reason, various types of thin-film metal oxides are been explored as a buffer layer to modify the metal or transparent electrode work function for optoelectronic applications. In this work, electrochemical oxidation and ultra violet ozone (UV-ozone) oxidation techniques are used to partially oxidize silver films to form native semiconducting silver (I) oxide (Ag₂O) on metallic silver substrates. 100-nm-thick Ag films were thermally evaporated onto either ITO-, chromium- or gold-coated glass substrates. In the electrochemical method, the Ag film was immersed in a 1 M NaOH solution and the applied potential was cycled between -0.2 V and +0.45 V. In the UV-ozone treatment, the Ag films were exposed to UV in the presence of ozone which reacts with the Ag to form Ag₂O. The exposure times were 0, 1, 2, 3 and 4 minutes. The oxide formed using both techniques was characterized with x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), atomic force microscopy (AFM) and grazing incidence wide and small angle x-ray scattering (GIWAXS/GISAXS) techniques. In the XPS data, the binding energies (E_b) for O1s and Ag3d_5/2 photoelectrons were shifted from E_b,O1s = 531.6 eV and E_b,Ag3d = 368.2 eV for un-oxidized Ag to E_b,O1s = 529.4 eV and E_b,Ag3d = 367.6 eV for electrochemically oxidized Ag, and E_b,O1s = 530.6 eV and E_b,Ag3d = 367.7 eV for UV-ozone oxidized Ag indicating formation of Ag₂O. The scattering vectors of the GIWAXS/GISAXS peaks occurred at 2.5 Å^-1 and 2.24 Å^-1 for untreated Ag and UV ozone formed Ag₂O, respectively, indicating the larger lattice repeat distance for Ag₂O. The SEM and AFM analyses showed a relatively smooth surface morphology for bare silver and increasingly rough morphologies for the oxidized films with Ag₂O regions appearing to grow as spherical crystallites via a nucleation and growth mechanism for the UV-ozone treated samples with exposure times greater than 2 minutes. For the electrochemically oxidized samples, the morphology was more irregular and porous indicative of an amorphous silver oxide. Hole-only devices made of Ag/Ag₂O/P3HT/Au were fabricated with the different treated oxide films to assess the effect of the silver oxides on hole injection. The implications of the results will be discussed to influence the selection of electrodes in conventional and inverted organic optoelectronic devices.

Small molecular organic thin film morphology is a major determinant of resulting electrical and optical properties, surface energy, and other properties. Nevertheless, it remains practically impossible to predict in advance what specific morphology may be obtained from a given set of film deposition conditions used to deposit a given molecule, due to the complex molecular structure and van der Waals forces dominating bonding in the film. We experimentally and computationally study the deposition of small molecular organic thin films using additive, solvent-free organic vapor jet printing (OVJP), which allows precise control over the surface temperature, growth pressure, admolecule flux and growth rate throughout a very wide range of values. Using a combination of measurable material parameters and finite element methods we demonstrate the evolution of different film morphologies, including interesting and novel surface features accessible through a specific combination of process conditions and molecular structure. We show how the highly kinetic aspects of OVJP result in a unique spherical microstructures for some materials (e.g., boron subphthalocyanine chloride (SubPc) and tris (8-hydroxyquinolinato) aluminum (Alq3)), and fibers and platelet morphologies in other materials (i.e. caffeine, pentacene, 3,4,9,10-Perylentetracarbonsäuredianhydrid (PTCDA)). The crystal structure, morphology and growth of the materials, are studied using in-situ X-Ray Synchrotron diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Possible applications of the resulting morphologies in are discussed and demonstrated.
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As reported earlier, the photovoltaic performance of PCDTBT:PCBM polymer solar cells drastically decreases upon thermal annealing. It was demonstrated in the literature, that thermal annealing leads to increased trap formation and as a consequence disturb solar cell performance, especially via a reduced fill factor. This has been demonstrated by space-charge-limited-current analysis and ellipsometry, as well as, structural changes analyse of PCDTBT upon annealing. However, we decided in addition to investigate morphological changes occurring within PCDTBT:PCBM photoactive blend layers upon thermal annealing, as these must have an impact on charge transport. By application of several characterizations techniques, and especially supported by results of Impedance Spectroscopy and Auger Electron Spectroscopy (AES), indeed the existence of an unfavourable compositional gradient within the photoactive layer could be revealed. This compositional gradient may be in part accounted for harming the transport of electrons and holes in either direction.

Molecular doping enhances the performance of state of the art organic solar cells and light emitting diodes. Furthermore, it allows for a precise tuning of the threshold voltage of organic field effect transistors. However, the physical mechanism of the doping process in organic semiconductors is still controversially discussed, in particular with respect to the doping efficiency, which is the ratio of free charge carriers per dopant molecules.
In this contribution, we systematically measure the Fermi level position of various p- and n-type doped organic semiconductor materials by ultraviolet photoelectron spectroscopy. Here, the precise control of molar doping ratios as low as 10^-5 is demonstrated for vacuum co-sublimated small molecule thin-films, allowing analysis of the doping properties in a much broader range as previously accessible. We show that the Fermi level can be tuned from its intrinsic position close to mid-gap over the whole semiconductor gap towards the highest occupied molecular orbital (HOMO) for p-type and towards the lowest unoccupied molecular orbital (LUMO) for n-type doping. In this context, kinks and slopes in the Fermi level vs. doping concentration diagrams characteristic for the host-dopant system are identified.
By numerically solving the charge neutrality equation using a classical semiconductor physics approach, the experimental findings are explained by trap-limitation, dopant saturation, and reserve regimes as known from inorganic semiconductor physics. In particular, we observe a transition from impurity saturation to impurity reserve for p-type doped systems, which is characterized by the Fermi level crossing of an effective dopant acceptor level at molar doping ratios in the order of 10^-3...10^-2. Strongly decreasing doping efficiencies down to a few percent are thus predicted by the model for higher concentrations, which agrees well with values experimentally determined by impedance spectroscopy. Furthermore, it is shown that at ultra-low concentrations (MR<10^-4), molecular doping is hindered by deep gap states whose density depend on the purification grade of the host material. Additionally, it is demonstrated that electron traps with defined density can intentionally be introduced in pentacene by co-evaporation of C₆₀ and gradually filled-up by n-doping, and thus control the Fermi level position until completely occupied.
Our results represent a significant step forward in the understanding of the physical mechanism of molecular doping. Furthermore, the novel approach of extremely low doping will allow for completely new design rules of organic transistors with improved long term stability and precise parameter control.

Despite the important improvement organic photovoltaic devices (OPVs) have shown in the past decade, there are still some fundamental issues in the device architecture that limit their performance. The conversion from bilayer to bulk heterojunction configuration has allowed reducing the effect of low exciton diffusion length of the absorbing layers, by increasing the donor-acceptor interface area and, subsequently, the total thickness of the active layer. As a consequence, the collection of carriers has been sacrificed, altering the continuous pathways for the electrons and holes to be collected at their respective electrodes. One approach to overcome this has been maintaining the bilayer configuration, by adding phosphorescent materials capable of extending the exciton diffusion length. The phosphorescent dopant should be able to populate the triplet level of the host (donor), by a triplet-triplet energy transfer process. Several studies have reported on the MEH-PPV:PtOEP (Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]:Platinum octaethylporphyrin) host-guest system. Though they all report some improvement, especially, in the short-circuit current density (J_sc), it is not clear if this is due to effective triplet-triplet energy transfer from the dopant to the host polymer or just to the absorbing properties of PtOEP. In addition, there are no thickness dependence studies. In this study, we have modified the device architecture by reducing the thickness of the acceptor fullerene layer to only 4 nm, in order to isolate the effective contribution of the dopant on the device performance. Simultaneously, the thickness of the cathode buffer layer bathophenanthroline (Bphen) is increased to assure proper contact between the active layer and the aluminum cathode. A significant thickness dependence of the polymer is observed with respect to the J_sc, as the current decreases as the MEH-PPV film thickness increases from 5 nm to 40 nm. This is, in part, due to the increased distance the excitons must travel before reaching the acceptor interface to be dissociated. The addition of the dopant results in an improvement on the J_sc, regardless of the polymer thickness, even to the point where the device performance of the thick films is comparable to the thin ones. Furthermore, an increase in the spectral response where MEH-PPV absorbs is observed. This improvement is supported by photoluminescence quenching of the donor by PtOEP, following the Perrin Formulation.

We study both the energetics and spatial distribution of charge transfer (CT) states in organic bulk heterojunction blends. Using a series of indacenodithiophene polymers and their selenium-substituted analogs as donors blended with fullerene acceptors, we explore how changes in donor/acceptor electronic coupling and morphology affect recombination and open circuit voltage. We use sub-gap photocurrent, transient photovoltage, electroluminescence, and time-resolved photoluminescence to characterize the CT state energies, densities, and lifetimes in model blend films and show that increased donor/acceptor coupling is consistent with increased recombination losses in these films. Structural studies provide morphological signatures consistent with increased electronic coupling. Finally, we apply novel scanning probe microscopy methods to detect local photocarrier generation by direct sub-gap excitation of charge transfer states, and show that the spatial distribution of below-gap CT states leading to carriers is distinct from that arising from above-gap excitonic transitions.

The morphology of the active layer of organic bulk heterojunction (BHJ) solar cells is crucial for efficient charge separation and extraction and thus strongly affects the performance of the device. However, imaging a BHJ with material specific contrast is challenging due to the similar chemical compositions of donor and acceptor materials and the required high spatial resolution. In previous works we demonstrated on different types of solar cells that spectral information obtained by electron energy loss spectroscopy (EELS) in a transmission electron microscope (TEM) can be combined with multivariate statistics and machine learning to yield contrast between the materials within the active layer [1,2]. Here, we extend these techniques to small molecule co-evaporated devices using fluorinated zinc phthalocyanine (F₄ZnPc) as donor and the fullerene C₆₀ as acceptor. As F₄ZnPc and C₆₀ exhibit different optical absorption features in EELS, we can map donor- and acceptor-rich domains after analysis of the monochromatic images. A significant change of the microstructure of F₄ZnPC:C₆₀ layers annealed at 110°C during co-evaporation was observed compared to layers deposited at room temperature. The non-annealed sample exhibits smooth layers with a homogenous distribution of the two materials. On the other hand, we observe pronounced C₆₀-agglomerates in the annealed BHJ. Additionally, an increase in crystallinity is measured by electron diffraction. The thermally driven phase separation and the crystallization processes are crucial for the charge transport and lead eventually to a better performance of the solar cells. In order to get further insights into the structure of the agglomerates we performed electron tomographie. The 3-dimensional structure of the active layer and its influence on the device performance will be discussed.
[1] M. Pfannmöller et al., Nano Lett. 2011, 11, 3099-3107
[2] D. Nanova et al., Nano Lett. 2014, 14 (5), 2735-2740

Replacing PCBM with a bis-adduct fullerene (i.e. ICBA) has been reported to significantly improve the open circuit voltage (Voc) and power conversion efficiency (PCE) in P3HT bulk heterojunctions. However, for the most promising low band-gap polymer (LBP) system, replacing PCBM with ICBA results in very poor short-circuit current (J_SC) and PCE although the V_OC is significantly improved. In this work, we exploit the selective solubility of the LBG:fullerene nanocomposite components to direct the assembly of these mixtures post-deposition by annealing in the vapor of a selective solvent. We also describe work that controls the morphology of LBP/bis-fullerene BHJs by the addition of a cosolvent to the polymer/fullerene solution prior to film deposition. Both processes, therefore, provide a unique tool to precisely tune the morphology of a broad range of CP:Fullerene BHJ systems, optimizing the morphology and performance of the active layer.

Printed organic electronics, a technology based on organic semiconductors that can be processed into thin films using conventional printing and coating techniques, has been the subject of active research and development over the past decades. Due to their ability to be processed at low temperature, over large areas, at low cost, organic semiconductors are experiencing an accelerated development that will lead to a new generation of products with thin and flexible form factors. While the organic semiconductor layer plays a central role, the interfaces that are formed between the organic semiconducting layer and adjacent oxide layers are very critical and often determine the overall electrical performance of the device.
In this talk, we will discuss novel device architectures that incorporate organic semiconductors and insulating or semiconducting oxide thin films processed by atomic layer deposition (ALD). The performance of a range of solid-state devices, including organic field-effect transistors (OFETs), sensors, and solar cells, will be presented. Our results show that ALD offers unique advantages over alternative thin-film deposition techniques that can yield devices with higher performance and longer lifetime. We will show that these advances are likely to accelerate the deployment of flexible printed electronic technologies.

Bathocuproine (BCP) is one of the most frequently used materials for interfacial layers in organic solar cells. It is inserted between electron-acceptor layer (e.g. fullerene and its derivatives) and metal cathode (Ag or Al) to improve power conversion efficiency [1]. Although much effort has been devoted, its mechanism has not yet been clarified. It is widely believed that BCP works as an exciton blocking layer because the HOMO (highest occupied molecular orbital) level of BCP lies 6.5 eV below the vacuum level (VL) which is significantly lower than the hole conduction levels of the other materials in the photovoltaic cell. By adding the optical gap of 3.5 eV to the HOMO level, the LUMO (loweset unoccupied molecular orbital) level is often assumed to lie at 3.0 eV below VL. Since this value is substantially higher than the workfunction of the cathode and the LUMO level of the fullerene derivatives, the electrons is believed to conduct through the gap state of BCP. On the other hand, recent photoemission studies suggest that the LUMO of BCP aligns with the electron conduction levels of the cathode and acceptor owing to the vacuum level shift [2]. In the latter scenario, electrons conduct through the LUMO level of BCP. Such confliction arises from the lack of reliable data on the LUMO levels of BCP because there were no proper experimental techniques.
Recently, we have developed a new experimental technique, low-energy inverse photoemission spectroscopy (LEIPS), to examine the LUMO levels of organic semiconductors [3]. In this method, electrons having kinetic energy below 4 eV is introduced to sample films. By detecting near-ultraviolet photons emitted due to the radiative transition, the density of unoccupied states including the LUMO levels are precisely examined. In contrast to the previous inverse photoemission spectroscopy (IPES), the damage to the organic samples is negligible because the electron energy is below the damage threshold. Since the near-ultraviolet photons can be analyzed using the interference bandpass filters, the energy resolution is improved to 0.25 eV which is a factor of two better than the conventional IPES.
Using LEIPS, the LUMO level of BCP was precisely examined. Surprisingly, the determined LUMO level is found to be more than 1 eV higher than that believed before urging to reconsider the previous discussions. LEIPS spectrum shows gap states when Ag is deposited on the BCP layer. With the aid of DFT calculation, the reaction product between BCP and Ag is identified. From these findings, we conclude that the electrons in the BCP layer conduct through the gap state which is formed by chemical reaction between the metal and BCP through the diffusion of the metal into the organic layer.
[1] Peumans, et al. Appl. Phys. Lett. 76, 2650 (2000).
[2] Sakurai et al, J. Appl. Phys. 107, 043707 (2010).
[3] H. Yoshida, Chem. Phys. Lett., 539-540, 180 (2012); Anal. Bioanal. Chem.406, 2231 (2014).14).

The presence of interlayers between the active layer and the electrode are known to modify the metal work-function and enhance OPV device performance. Spontaneous formation of interlayers eliminates separate processing steps and hence is technically advantageous and cost effective. However, surface enrichment during film processing of the interlayer material is limited to materials with low surface energy. Here we show that migration of the interlayer molecules to the organic/electrode interface can be induced by interlayer molecule-metal interactions. This is demonstrated by blending polyethylene oxide (PEG), a known interlayer material with a surface energy higher than that of P3HT and PCBM, into the active layer. XPS analysis reveals that, as expected, PEG is not present on the surface of the organic spun film. However, Ca or Al evaporation induces a significant migration of PEG to the organic/metal interface. In contrast, Au evaporation does not induce such migration. The comparison between Al, Ca and Au, metals with significantly different reduction potentials revealed that the driving force for PEG migration is its chemical interaction with the deposited metal atoms. The extent of PEG migration was also found to depend on the type of the underlying substrate, ITO/PEDOT:PSS or ITO. Finally, the PEG interlayer results in a reduction the metal work function confirming that spontaneous additive migration induced by metal-additive interactions could be harnessed for improved charge extraction in organic electronic devices.

Because of the optical transparency and n-type conductivity, ZnO is commonly used as the electron collecting contact interlayer in inverted organic photovoltaics (OPVs). Thin ZnO interlayers can be easily fabricated in the laboratory from sol-gel precursor methods. Diethylzinc (deZn) has been shown to rapidly convert to ZnO (deZn-ZnO) with low thermal annealing requirements (120 °C or less). Zinc acetate (ZnAc) typically requires annealing temperatures above 280 °C to convert to ZnO (ZnAc-ZnO), although some recent reports suggest that temperatures in the range of 130 to 200 °C may be sufficient. We report here on a comparison of the effectiveness of using these two precursors as low temperature routes to forming ZnO interlayers in inverted OPVs.
We found that deZn-ZnO annealed at 120 °C performed similarly to ZnAc-ZnO annealed at 300 °C, when initially exposed to solar simulator irradiation, while ZnAc films annealed below 300 °C did not perform as well. Characterization of these films by X-ray diffraction (XRD) shows that the lower temperatures are insufficient for fully converting ZnAc to ZnO. When the two initially similar device architectures are exposed to long-term illumination under resistive loading the performance diverges significantly, with the deZn-based ZnO being more stable. Photoluminescence (PL) spectroscopy suggests that the two types of ZnO formed with the sol-gel precursors have different defects, and XRD shows different preferential orientation. We attribute the differences in defects and orientation, between the two types of ZnO, to the difference in precursor oxygen coordination to zinc, and the relative rates of reactions in the precursors.
Further, we report on simultaneous enhancements to stability and performance of devices with ZnAc-ZnO that are surface modified with a dipolar phosphonic acid molecule. Differences in PL of unmodified and surface modified ZnAc-ZnO also suggest differences in defect types within or at the surface of the film. We therefore propose that the difference in stability observed by using the two different sol-gel precursors and by using surface modifiers is due to the presence of defects which may form reactive centers for degradation of ZnO/organic interfaces withing OPV devices.

The wide range of work function (WF) tunability of metal electrodes has been desired for high performance organic electronics, in which precise energy level matching at the metal/organic interface is crucial. We report unprecedented WF modifiers based on conjugated polyelectrolytes (CPEs) that can bilaterally shift ‘upward&’ or ‘downward&’ the effective WFs of various metal electrodes with a maximum ‘net&’ variation up to ΔWFsim;1.2 eV (+ 0.4 eV for n-type and minus; 0.8 eV for p-type). The new ‘downward&’ CPEs, p-type CPEs, originate from the reversed dipoles that are induced by a facile oxidative process of conventional ‘upward&’ n-type CPEs, in which ionic side-chains develop molecular dipoles and shift WF of the metal electrodes. Moreover, because our p-type CPEs can be applied to various metals, including indium tin oxide, Ag, Au, Cu, and even graphene, our approach is universal and promises versatile interface engineering in organic electronics (organic solar cells and light-emitting diodes) with substantially enhanced device efficiency and lifetime.

Surface modification of metal oxide contacts with self-assembled monolayers (SAMs) has been used to adjust interfacial properties in thin-film applications for energy conversion, light emission and organic logic: including control of chemical composition, surface free energy, work function and the stability of the oxide surface. More specifically, in organic photovoltaics (OPVs), self-assembled monolayers have been used to modify contact work function to control built-in electric fields near open circuit conditions, where charge carrier extraction (and transport) often competes with recombination events. One proposed strategy to enhance carrier extraction in OPVs is to utilize surface modifiers with redox functionality at a metal oxide contact. This strategy utilizes tethered materials with (first-oxidation or first-reduction) redox potentials close to the transport energy levels relevant for either hole- or electron-harvesting from the donor or acceptor components of the OPV active layer. The orientation and distance to the oxide interface of the redox-active moiety is expected to play a critical role in: i) the efficiency of electron injection and extraction from the organic into valence and conduction bands of the oxide; ii) the rate of hole-hopping between adjacent chromophores; and iii) recombination efficiency, which controls the photo-potential. This talk will focus on well-characterized donor molecule, ZnPc, with controllable orientation, either in plane or perpendicular to the oxide layer, and variable linker length. Waveguide-based spectroscopic methods are used to probe differences in interface dynamics, as a function of molecular orientation and sub-populations with distinct optical absorption bands. Likewise, we can measure the rate of charge transfer for a specific orientation and sub-population as a function of applied potential to investigate the dependence of rates of electron transfer on over-potential (eta; = E - E⁰&’), with respect to the redox potential of the monolayer. Finally, waveguide-based transient absorption measurements of a single monolayer of ZnPc on planar oxides (RMS < 5 nm) varying density of states will be discussed.

The importance of charge transfer (CT) complexes to the device efficiency in bulk heterojunction (BHJ) solar cells is a topic of recent high interest.[1] These studies have focused on the photogenerated CT complexes but the presence and effect of integer charge transfer (ICT) states[2] present in the dark and created as a consequence of Fermi level equilibrium has all but been ignored so far, despite indications of their significance.
Here we show that for a series of poly(3-hexylthiophene):fullerene organic photovoltaic devices with pinned electrodes, the ICT states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage.[3] The donor ionization potential to acceptor electron affinity energy difference is thought to provide an upper limit to the open circuit voltage, but it is the effective energy gap including possible potential steps at the BHJ that is the relevant parameter. The ICT state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in the fullerene series&’ electron affinity. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of variations in trap-assisted recombination via ICT states. We show that this holds true regardless if one assumes a pn-junction or metal-insulator-metal based description of BHJ solar cells. EPR measurements confirm the creation of ICT states at the BHJ in the dark and together with PA measurements show that the D/A coupling strength and recombination-induced loss can be estimated using the so-called pinning energies (E_ICT+,-). Based on the results, novel design rules for the donor/acceptor materials to optimize open circuit voltage and performance of BHJ solar cells are proposed, and the design rules are tested against a series of high performing donor polymer: fullerene solar cell devices available in literature, and excellent correlation is obtained.
[1] K. Vandewal, et al. Nat. Mater.2014, 13, 64
[2] S. Braun, et al. Adv. Mater. 2009, 21, 1450
[3] Q. Y. Bao, et al. Adv. Funct. Mater. (Accepted)

Organic photovoltaics may be a promising solution for the growing energy demand all over the planet. Their easy fabrication and installation, flexibility and recyclability are advantageous in solar cell development. Even though the efficiency of organics solar cells still needs to be improved, their stability and cost are already approaching commercially used types of photovoltaics. In our work we focus on the monitoring of the transient behavior of organic solar cells, for example in order to obtain a better understanding of temperature-induced degradation processes. We employ impedance spectroscopy and current-voltage characterization, together with laser-beam-induced current measurements to get a picture of the degradation processes within the device. The first two techniques enable the measurement of area averaged key parameters used for the evaluation of overall solar cell performance, like internal resistance and fill factor. The latter technique reveals the distribution of the local photocurrent across the solar cell area, providing additional information on the degradation path. When combined, the three techniques yield valuable information on solar cell degradation processes.

Inserting a (n- or p-) doped transport layer at an electrode-organic semiconductor contact can significantly improve the efficiency of the device.^[1-4] However, this concept is difficult to implement on polymer-based devices because of difficulties in fabricating polymer multilayers and in spatially confining dopants at interfaces in solution-deposited films. In our work, we combine solution-doping and soft contact lamination of polymer films, and successfully demonstrate interface doping with several polymer systems, i.e. poly(3-hexylthiophene-2,5-diyl (P3HT),^[4] poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] (poly-TPD), poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b&’]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)]-2,6-diyl) (PBDTTT-C). P-doping of the doped transport layers is achieved by spin-coating a co-solution of the polymer and soluble oxidizing molecule molybdenum tris-[1-trifluoroethanoyl-2-trifluoromethylethane-1,2-dithiolene] (Mo(tfd-COCF₃)₃).^[4] The electronic structure of the doped layers is investigated via ultra-violet photoemission spectroscopy, which shows the expected Fermi level shift toward to the polymer HOMO levels, and current-voltage measurements, which show an enhancement of the conductivity of the host polymer layers by several orders of magnitude. We modify the hole-collection electrode of polymer-based inverted solar cells by inserting via soft-contact lamination a p-doped polymer interlayer. The inverted polymer-based solar cells comprising (i) a PEIE-modified ITO substrate, (ii) a bulk heterojunction (BHJ) active layer ,(iii) a laminated interlayer with doping concentration of 4 wt%, and (iv) a Ag top contact. The cells yield the following characteristics: V_oc= 0.8 V; J_sc= 7.7mA/cm²; FF=63% and mu;#65309;3.9% for P3HT:6,6-phenyl-C₆₁-butyric acid methyl ester (PCBM) BHJ and V_oc= 0.56V; J_sc= 8.0mA/cm²; FF=64% and mu;=2.9% for P3HT:Indene-C₆₀ Bis-Adduct (ICBA) BHJ.^[4] The same method is also been applied to the low bandgap polymer PBDTTT-C system. Devices with the structure ITO/ZnO/PEIE/PBDTTT-C:PC₇₀BM/8.6wt% doped PBDTTT-C/Ag achieve mu;=5.6% with Voc = 0.70V; Jsc= 13.50mA/cm² and FF =59%. The stability of the dopants as a function of temperature and time is also investigated with both I-V and SIMS measurements. Results indicate that dopants diffuse spontaneously into pure P3HT films, while gradually diffuse into PBDTTT-C matrix over time. The diffusion of the dopant into the BHJ is considerably reduced and the performance of the solar cells is stable over time in inert environment. We conclude that the combination of solution-based doping with soft-contact lamination is a promising technique to fabricate all-solution processible devices, in particular polymer-based photovoltaic devices.
1. Qi, Y., et al., Chem. Mat., 2009, 22, 524
2. Tress, W. et al., Adv. Funct. Mat., 2011, 21, 2140
3. Uhrich, C., et al., J. Appl. Phys., 2008, 104, 043107.
4. An, D. et al., Adv. Funct. Mat., 2014, 24, 2197

Most currently available solar cells, in particular, organic solar cells, are only able to strongly absorb "visible" photons, while leaving about 50% of the solar energy in the near infrared (NIR) region untapped. Harvesting NIR photons thus represents an attractive approach to improve the power conversion efficiency of organic photovoltaics (PVs). Herein, I will present some of our most recent development in the NIR quantum dots (QDs), such as PbS QDs and PbS@CdS core@shell QDs, and demonstrate their beneficial effect, after appropriate structural design and integration, on polymeric solar cells [1-7]. One example is about the controlled hybridization of NIR PbS QDs with carbon nanotubes (CNTs) and their further integration into poly(3-hexylthiophene), which is a hole-transporting polymer. The nanohybrid cells show considerably enhanced power conversion efficiency, which is attributed to the significantly extended absorption in NIR by QDs and the effectively enhanced charge transportation due to CNTs. On the other hand, plasmonic nanostructures have been recently explored for enhancing the efficiency of solar cells. Our recent work on some interesting plasmonic nanostructures that have strong resonances in the NIR regime [8-9], and their application in organic solar cells will also be briefly highlighted.
References
1. D. Wang, (et al.), D. Ma, Adv. Funct. Mater, 21, 4010 (2011).
2. H. Zhao, (et al.), D. Ma, Nanoscale,6, 215 (2014); Inside Back Cover.
3. I. Ka, V. Le Borgne, D. Ma, M. A. El Khakani, Adv. Mater. 24, 6289 (2012); Frontispiece Cover.
4. H. Zhao, (et al.), D. Ma, J. Mater. Chem., 21, 8898 (2011); highlighted at http://blogs.rsc.org/jm/.
5. H. Zhao, (et al.), D. Ma, Chem. Commun. 46, 5301 (2010).
6. D. Wang, (et al.), D. Ma, J. Phys. Chem. Lett. 1, 1030 (2010).
7. F. Ren, (et al.), D. Ma, Nanoscale, 5, 7800 (2013).
8. H. Liang, (et al.), D. Ma, J. Am. Chem. Soc., 135, 9616 (2013).
9. H. Liang, (et al.), D. Ma, Chem. Mater. 24, 2339 (2012).

Plastic solar cell (OPV), as a next generation solar technology, with intrinsic characteristics of colorful, transparent, light, and high specific power, can be fabricated by high through-put, low cost, roll to roll manufacturing process, which has attracted broad attention in renewable energy and printed electronics fields. This presentation will discuss the development of OPV active materials and OPV manufacturing technology in Solarmer, meanwhile its potential applications, market size, and future development will be also discussed.

The power conversion efficiency (PCE) of 9.1% in organic photovoltaic (OPV) module (25 cm²) with inverted device structure has been demonstrated. This PCE of the module exceeded our previous world records of 8.5% (25 cm²) and 6.8% (396 cm²) that were listed in the latest Solar Cell Efficiency Tables ver.43 [1].
For the development of module, we firstly investigated the OPV cell structure whose active area was 1 cm². Using donor-acceptor type polymer synthesized by our group possessing on both lower highest occupied molecular orbital (HOMO) energy level and small energy band-gap, purified PC₇₀BM, and band matched metal oxide buffer layers, we obtained a high performance cell with PCE of 10.4%.
Next, the above mentioned experimental conditions were applied to the fabrication of module. In order to achieve highly efficient modules, we increased the ratio of photo-active area to designated illumination area to 94% without any scribing process and placed insulating layers in order to decrease the leakage current. Meniscus coating method was used for the fabrication of both buffer and photoactive layers. This technique ensures the fabrication of uniform and nano-scaled thickness layers with thickness variation less than 3%.
Furthermore, the PCE of the OPV under indoor illumination was found to be higher than that of the conventional Si type solar cells. We have also developed several prototypes for electronics integrated photovoltaics (EIPV) embedded with our OPV modules, which can be operated by indoor lights.
Acknowledgments
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) project of “Solar power system next generation high-performance technology development”.
References
[1] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, Solar cell efficiency tables (version 43), Prog. Photovolt: Res. Appl. 22, 1 (2014).

The success of organic photovoltaics (OPVs) as a future energy source is entirely dependent on the cost of the electricity produced by the modules. Firstly, an engineering study estimate has been performed to determine the projected cost of mass-manufactured OPV modules. The materials, production capital and operating costs have been calculated and sensitivity analyses performed to determine the parameters of greatest economic influence. Significantly, the model includes a calculation of the costs required to establish bulk manufacturing of the current high cost speciality materials components, encompassing synthesis and associated chemical plant design. The economic modelling reveals that the calculated mass-manufactured OPV module costs are considerably lower than current literature estimates, with OPV modules costed at $7.85 per square metre with an uncertainty of ± 30 %. Secondly, this cost of materials calculation is then used to provide the first commercial scale levelised cost of electricity (LCOE) estimates for OPVs by integrating the OPV-specific measured and calculated data into the estimates. The impacts of physical and financial variables are also investigated. The study shows that OPVs will become equivalently priced with current conventional solar technologies when efficiencies of 2 % and lifetimes of 3 years are achieved. At efficiencies of 5 % and lifetimes of 3 - 5 years the LCOE for OPVs will be competitive with that of current coal-based electricity generation.

Spray deposition is a promising technique for the solution processing of plastic electronic devices, owing to the potential for high-speed, large-scale device fabrication using low capital cost equipment. To date most reports of spray-deposited electronic devices have used ultrasonic systems for film deposition to minimise surface roughness [1], but such systems suffer from low materials throughput, and are consequently ill-suited to industrial manufacturing. Pressure-driven spray heads enable higher throughput materials delivery but typically result in an unacceptably rough film due to the hard-impact nature of the deposition system.
Here we describe a new automated pressure-driven spray coater for the controlled deposition of a broad variety of solution processed electronic materials, which overcomes many of the disadvantages of conventional pressure-driven spray coaters. Key features of the system include: a novel, high performance 3D-printed spray-head for the generation of ultrafine sprays; full-three dimensional position and velocity control of the spray-head; integrated temperature control, and independent control of solution and gas rates.
The system yields high quality, large area (10 x 10cm) metal oxide thin films, allowing for the controlled fabrication of injection and extraction layers in hybrid organic/inorganic devices. It is also suited to the deposition from non-chlorinated solvents of thin film semiconducting polymers, specifically P3HT and F8BT. We investigate the influence of deposition variables such as solution carrier gas pressure, solvent composition and substrate temperature as well as post deposition treatments on the properties of the polymer films. We also demonstrate sequential deposition of oxide and organic materials, solvent annealing of polymer films and gradient doping of both organic and inorganic films.
[1] C. Girotto, D. Moia, B. Rand and P. Heremans, Adv. Funct. Mater. 2011, 21, 64-72

Low band-gap (LBG) polymer-based Bulk-Heterojunction (BHJ) organic solar cells (OSCs) have gained significant attention in the last few years, since the possibility to reach a power conversion efficiency (PCE) more than 9% [Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure, Nat. Photonics. 6 (2012) 591-595.DOI: 10.1038/NPHOTON.2012.190] for a single layer device, and to overcome the breakthrough of 10% [J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, et al., A polymer tandem solar cell with 10.6% power conversion efficiency, Nat. Commun. 4 (2013) 1446. DOI: 10.1038/ncomms2411] in a tandem structure.
Such high efficiency values, together with the native opportunity to scale up the realization process on large area at relatively low costs, due to the many applicable printing techniques (screen, spray, gravure and ink-jet printing, blade and slot-die coating) implemented on roll-to-roll (R2R) mass production systems, push the BHJ-based photovoltaics to the top of the “future feasible ” renewable energy technologies.
The scaling up of organic photovoltaics based on thin-films deposited via solution-processing passes however from a key issue that is to avoid toxic and harmful solvents such as chlorinated ones, at moment largely used in polymer solar cell laboratory research, and move to more environmental friendly ones (green-chemistry) [J.E. Carlé, M. Helgesen, M. V. Madsen, E. Bundgaard, F.C. Krebs, Upscaling from single cells to modules - fabrication of vacuum- and ITO-free polymer solar cells on flexible substrates with long lifetime, J. Mater. Chem. C. 2 (2014) 1290. DOI: 10.1039/c3tc31859a].
In this work, we found high solubility and miscibility of the low band gap polymer 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]] (PBDTTT-E-F, commonly known as PTB7), blended with [6,6]-Phenyl-C71-butyric acid methyl-ester (PC₇₁BM fullerene derivative) in a non-chlorinated solvent (dimethylbenzene). The solar cells realized depositing blend solutions based on various xylenes (ortho, para and isomeric mixture) show high power conversion efficiencies up to 8.7% under 100mW/cm² solar irradiation (AM1.5G). The device has also been scaled up with a solution based process obtaining a module with an active area of 24 cm² with an efficiency of 2%.

Water-based dispersions of nanoparticulate polymer blends offer the exciting prospect of solvent-free fabrication of OPV modules with nanoscale morphology control. We report a morphological study of the poly(3-hexylthiophene) (P3HT) : phenyl-C61-butyric acid methyl ester (PCBM) material system in nanoparticle organic thin films. The morphology and chemical composition of unannealed and annealed P3HT:PCBM nanoparticles has been investigated using scanning transmission x-ray microscopy (STXM) for a range of P3HT molecular weights (M_w = 5 - 72 kg mol^-1). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been used to support the STXM data. We find that unannealed P3HT:PCBM nanoparticles (NPs) exhibit a common core shell morphology, with a PCBM-rich core and P3HT-rich shell. The morphology of the thermally annealed NP films is highly dependent upon the molecular weight of the P3HT and is determined by PCBM diffusion through the P3HT matrix. Two PCBM diffusion mechanisms operate within this system: (1) at high molecular weights diffusion of molecular PCBM dominates whilst (2) at low molecular weights diffusion of the PCBM cores is significant. The Stokes-Einstein continuum model for diffusion has been used to determine a threshold molecular weight at which the diffusion of PCBM cores is activated in these films. The calculated value (M_w ~38 - 25 kg mol^-1) is shown to agree very well with experimental observations. Finally, we develop a model for the morphological evolution of annealed P3HT:PCBM NP films.

Squaraines are a class of materials keenly investigated for applications such as bio-imaging, photo-dynamic therapy and, in particular Organic Photovoltaics (OPV). Yet, the rate of synthesis development, characterization and implementation in devices reveals a need for better predictive modeling of structure-property relationships.
This work describes the development and utilization of predictive theory based on the Holstein Hamiltonian and a multiparticle basis set to describe the excited states of squaraines in the solid phase where intermolecular interactions play a major role in vital device characteristics. Results of theory will be confirmed through comparisons with presented measurements (e.g. UV-VIS-NIR optical absorption, I-V curves).
For OPV application, data suggests that exciton dissociation is the critical step bridging exciton generation and charge transport that limits device efficiency, consistent with Marcus-Hush theory for blended aggregates of squaraine donors and fullerene-based acceptors.
Significantly we will demonstrate effective theoretical modeling of the excited states in squaraine crystals and we will explain the observed changes in efficiency of our DiPSQ(OH)2 devices as we anneal these devices to control the aggregation. The device efficiency changes are consistent both with a Marcus-Hush theory interpretation and the validated modeling of squaraine aggregate spectroscopy.
We highlight that a complete understanding of the excited states in squaraine crystals will lead towards a prescription for derivatives that can be tailored for optimized exciton generation and charge transport in an OPV device.

Singlet fission is a process known to occur in a very limited set of molecules at high efficiencies only when the excited first singlet state [E(S₁)] is twice of the energy of the triplet state [E(T₁)] in both ordered and, typically to a lesser extent, in disordered media. Therefore, we investigate the single-molecule energy levels and solid-state packing in tetracene, 5,12-diphenyltetracene (DPT), rubrene, and several recently-synthesized rubrene derivatives (with substitutions on the arylene rings of rubrene) to understand how various structural choices alter the important parameters that control the ability of a system to undergo singlet fission. We use density functional theory (DFT) to compute the energy levels important for singlet fission and we discuss the molecular orientations in molecular dynamics-derived amorphous films for tetracene, DPT, rubrene, and two rubrene derivatives in order to gain an understanding of the key aspects of packing that are important for singlet fission. From our quantum mechanical and molecular dynamics calculations, we demonstrate that the p-stacking interactions, which help to planarize the rubrene derivatives in the solid state, are also advantageous for singlet fission in these systems. Molecular dynamics simulations also reveal that tetracene shows a crystalline-like packing, whereas DPT, rubrene, and rubrene derivatives all show various degrees of disorder with a different number of sites that are strongly coupled in the ground and excited states. This analysis has important consequences for understanding the key microscopic parameters for efficient singlet fission. Therefore, we identify promising molecular systems for singlet fission and suggest future experiments.

The effects of different polar solvents on the performance of solvent vapor annealing treated polymer solar cell (PSC) with a structure of ITO/ZnO/PTB7 : PC₇₁BM/MoO₃/Ag was systematically investigated by applying different polar solvents, including methanol, ethanol, dimethylsulfoxide, acetone and isopropanol. By analyzing the variation of PSC performance and the morphology of active layer, we found that both the solubility parameters (δ) and viscosity of solvent were playing an important role in controlling the morphology of PTB7 : PC₇₁BM blend. Especially, the PSC treated by methanol with high δ and low viscosity exhibited a remarkable enhancement of power conversion efficiency from 6.55 % to 8.13 %. The performance improvement was mainly due to the formation of the nanoscale crystallization of PTB7 : PC₇₁BM blend and the moderated aggregation of PC₇₁BM, resulting in efficient charge separation, balanced charge transport and suppressed charge recombination.

Significant improvements in power-conversion efficiency (PCE) of polymer photovoltaics have been made with the syntheses of various polymers. Among them, thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7) is one of the donor materials having highest PCE because of its superior electronic properties, so called “push-pull” mechanism. This push-pull alternating polymer enhances the intra-molecular charge transfer between the electron-donating (push) and electron-withdrawing (pull) moieties. For these reasons, PTB7:PC₇₁BM bulk heterojunction photovoltaics have attracted great attentions as a next generation renewable energy source [1]. For high performance PTB7:PC₇₁BM cells, interfacial electronics structures between them should be understood clearly. However, a “pristine” interfacial electronic structure between PTB7 and PC₇₁BM has not been studied thoroughly. It is due to the incompatibility between ultra-high vacuum (UHV) system for photoemission measurements and solution process for the formation of polymer film. Moreover, the initial interface formation between PTB7 and PC₇₁BM cannot be analyzed directly with conventional method due to the cross-dissolving problem.
Recently, we proposed vacuum electrospray deposition to overcome such limitations [2]. Step-by-step deposition of ultrathin PTB7 and PC₇₁BM layer is possible in UHV, thus the initial interface formation can be studied with in situ photoemission measurements. In this study, ultraviolet photoelectron spectroscopy measurements were performed to figure out pristine valence density of state of PTB7, PC₇₁BM and interfacial electronic structures between PTB7 and PC₇₁BM. Theoretical calculations were also performed to understand the push-pull mechanism.
[1] Zhicai He, Nature Photon. 6, 591(2012)
[2] S. Park, Rev. Sci. Instrum. 83, 105106 (2012)

We report on a new series of non-fullerene acceptors based on star-shaped anthracene diimide molecules for use in polymer bulkheterojunction (BHJ) solar cells. Organic solar cells have gained attention as a future photovoltaic technology due to their rapidly improving power conversion efficiencies, their low cost, and the potential for solution processing. Extensive effort has been devoted to the design of novel donor materials, assuming that fullerene based acceptor materials will be the ideal acceptor molecules. However, this emphasis has left a void in our understanding of the specific properties an acceptor material needs to fulfill in order to form the highest efficiency solar cells. To fill this gap we are exploring a novel acceptor material based on a star-shaped anthracene diimide core. Using an alkyne bridge, we have attached several different arene moieties as antenna molecules to this anthracene diimide core. The power conversion efficiency in P3HT-based BHJ solar cells improves from 0.45% to>2.3% by simply increasing the size of the antenna moiety. Also, by systematic inclusion of electron deficient units to these attached antenna moieties, we are able to control the electron affinity of these novel acceptor materials and determine the impact of these changes on device performance. Separately, we are able to alter the solubilizing functional groups elsewhere on the molecule in order to control domain sizes and crystallization rate of the acceptor. Systematic studies have been carried out by varying the type (arene vs alkane), positioning, shape and electron affinity of the antenna moiety in order to acquire the broadest spectrum of structure-function relationships in this system. Single X ray crystal and GIXD (Grazing Incidence X ray Diffraction) studies have been performed to understand the impact of molecular packing on the morphology and micro structure of polymer small molecule blends.
Key Words:
Bulk heterojunction, Star-shaped anthracene di-imides, rr-P3HT, Antenna molecules, GIXD

The organic photovoltaics (OPVs) have attracted remarkable attention in the last decade due to the advantages of low cost, light weight, flexibility, and solution processability. To obtain a high power conversion efficiency (PCE) of OPVs, the amount of crystallized domains and intercalated zones should be balanced and a desirable vertical morphology segregation is required. However, the conventional morphology evolution treatment, such as thermal or solvent vapor, is inadaptable for the flexible substrate or requires a time consumption process respectively. Here, the fast and low temperature post treatment method is required for mass fabrication.
The fast morphology evolution treatment, which we named as microwave assisted solvent vapor annealing (MWSA), was achieved by internal heat generation in the active layer. Once vaporized polarized solvent molecules are smeared into an active layer, they are oscillated in the active layer remotely by microwaves. The oscillation motion of polarized molecules in the active layer eventually dissipated into frictional heat and induces the morphology evolution. MWSA treatment is analogous to conventional solvent vapor annealing but the oscillation of polarized molecules smeared in the active layer with microwave facilitates morphology regeneration in a short time.
The chlorobenzene and Poly(3-hexylthiophene-2,5-diyl) (P3HT):1-(3-methoxycarbonyl)-propyl-1,1-phenyl-(6,6)C61(PCBM) were used as a polar solvent and active layer. The power conversion efficiency (PCE) of MWSA film, which is microwave irradiated for 7 seconds for three times, showed 18% higher value than that of thermally annealed film. The MWSA films were characterized by X-ray diffraction, absorption spectroscopy, and time-of-flight secondary ion mass spectrometry to verify the polymer crystallization and vertical distribution of electron donor (P3HT) and acceptor (PCBM) components. The polymer crystallization of MWSA films was somewhat analogous to thermally annealed films but the significant difference of MWSA films was vertical distribution of donor and acceptor components. The fullerene-derivatives (PCMB) rich phase near the electron collecting electrode (cathode) facilitated electron transport and played a role of hole blocking layer. However, the conventional thermally treated film showed unfavorable vertical segregation such as polymer rich phase near cathode.
In this method, vertically favorable segregation of donors and accepters can be achieved due to the fact that molecular diffusion of fullerene-derivative towards the top of the active layer where polymers are swelled by solvent vapor. This segregation facilitates the charge transport and increases the short circuit current density. Also, the volumetric internal heat generation by penetrated vapor molecules through oscillation in the active layer induces polymer crystallization and diffusion of fullerene-derivatives in a short time.

Seeking for novel π-conjugated electron-donating materials with targeted optical bandgaps and novel electron-accepting materials with compatible orbital alignments is not only a grand scientific challenge, but also in great practical needs for systematically improving the performance of organic optoelectronic devices. In this work, we will present the first combinatorial bandgap design map over 780 different copolymer donor materials with their targeted optical bandgap values effectively covering the entire solar spectrum from the infrared, visible, to ultraviolet regions. Novel electron acceptor materials are constructed with full flexibility in both the orbital alignments and functional side-group additions beyond the commonly used fullerene-based structures. All of these new materials designs are made possible through breaking the charge conjugation symmetry (CCS) in the underlying electron-phonon couplings that are intrinsic to these π-conjugated systems. Combined with empirical arguments widely acknowledged in the literature, new π-conjugated structures are identified with the optimal power conversion efficiencies for both single- and tandem-cells.

Luminescent solar concentrators (LSCs) are strategic, cost-effective, and scalable solutions for the convenient incorporation of photovoltaic (PV) technology into building infrastructures or other electronic architectures. LSCs consist of transparent slabs of plastic or glass that guide the emission of sunlight-absorbing photoluminescent dyes to their edges, where small and efficient PV devices may be attached. Perylene bisimides (PBIs) are valued as LSC emitters because of their exceptionally high photoluminescence quantum efficiencies and photostability. However, due to their small Stokes shifts, these materials often suffer from non-radiative reabsorption processes that severely curb their optical efficiencies in LSCs of commercially relevant sizes. One approach to minimize reabsorption is to employ very low concentrations of PBI, albeit at the expense of the dye&’s overall absorbing ability. This presentation communicates the development of an LSC that employs two luminescent conjugated polymers as surrogate absorbers to amplify the emission of small amounts of a PBI (e.g. 0.5 - 1 wt% relative to the polymers) through an energy transfer cascade. The optical bandgaps of the polymers are designed such that their corresponding absorption spectra cover a significant fraction of the solar spectrum, but are entirely decoupled from the emission of the PBI. Upon excitation by sunlight, a thin film comprised of the polymers and minority PBI emits photons that are guided into a transparent glass slab and consequently directed to its edges through total internal reflection. A key structural feature of the polymers is a pentiptycene repeat unit that prevents solid-state photoluminescence quenching and hampering of energy transfer processes. The high internal free volume induced by this pentiptycene substituent also results in the polymers having high solubility in organic solvents for facile processing into thin films and relatively low refractive indices for waveguide compatibility with inexpensive glass substrates. The photophysical properties of the composite blend are presented along with its performance in a model LSC.

Push-pull polymers are attractive components of bulk heterojunction solar cells because of their tunable electronic properties, but the morphology of polymer-based active layers can be difficult to control and plays a key role in overall device efficiency. Scattering experiments provide a partial picture of morphology, but do not provide recipes for polymer chemistries which will self-assemble morphologies favorable for charge separation and transport. In this work we investigate benzodithiophene and cyclopenta[c] thiophene-4,6,-dione based copolymers with molecular dynamics simulations accelerated with graphics processing units (GPUs). We show how the architecture of side-chains extending from benzodithiophene sub-units impacts the thermodynamically stable morphologies. We provide direct comparison to pi-stacked and lamellar structures measured in diffraction experiments and discuss the degree to which different coarse-grained models reproduce these features.

Organic photovoltaic devices have progressed a lot in the past decade of research. Many polymer electron donors which yield high photovoltaic performance when combined with PCBM have been identified, such as poly-3(hexylthiophene) (P3HT) (4.4%[1]), poly[N-9”-hepta-decanyl-2,7-carbazole-alt-5,5-(4&’,7&’-di-2-thienyl-2&’,1&’,3&’-benzothiadiazole) (PCDTBT) (6.9%[2]), thieno[3,4-b]-thiophene/benzodithiophene (PTB7) (9.2%[3]) and 4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4,5-bprime;]dithiophene- thieno[3,4-c]pyrrole-4,6-dione (PBDT-TPD) (7.3%[4]).
However, although P3HT is a more classic polymer with lower efficiency than the other, newer polymers mentioned, it still exhibits the highest free carrier lifetime as observed in time-resolved microwave conductivity (TRMC), when compared to the other polymers which exhibit higher power conversion efficiency. It is known that crystallinity of the polymer plays a large role. For example, regiorandom P3HT and regioregular P3HT have a large difference in crystalline properties, and the free carrier lifetime is correlated (high crystallinity - high lifetime). The crystallinity of a polymer can also be altered by using different solubilizing side chains on the polymer backbone, for example linear dodecyl side chains show a longer lifetime than the same polymer with ethylhexyl side chains, on the bis-benzodithiophene- cyclopenta[c]thiophene-4,6-dione (BBDT-CTD) copolymer.
This study will show a large variety of different side chains on the polymer backbone, and the differences in crystallinity as observed in X-ray diffraction, and free carrier lifetime as observed in time-resolved microwave conductivity will be discussed. Finally, solar cell device data will be presented.
[1] G. Li et. al., Nature Materials 2005, 4, 864 - 868
[2] J.S. Moon et. al., Advanced Energy Materials 2012, 2, 304 - 308
[3] Z. He et. al., Nature Photonics 2012, 6, 591
[4] J. A. Bartelt et. al., Advanced Energy Materials 2013, 3, 364 - 374

The design of novel conjugated polymers with appropriate frontier orbital energy levels, low band gap (LBG) and suitable carrier transport properties are needed to improve the power conversion efficiency (PCE) of organic photovoltaic devices.^[1] Hence, in order to further enhance the PCE and to obtain the desired performances with polymeric semiconductors, different parameters at the molecular and supramolecular levels (such as electronic structure, regioregularity, purity, solubility, molecular weight, thermal transitions, crystallinity and morphology within the blend) should be carefully controlled. However the exact control of the HOMO and LUMO energy levels is not a very simple process. Despite the fact that donor-acceptor or “push-pull” is the preferred approach for the synthesis of a plethora of new LBG conjugated polymers for solar cell applications, many issues remain to be addressed including the relative strength, the placement and the ratio of the donor and acceptor moieties in the polymer backbone.^[2] In this presentation, it will be demonstrated how minor modifications to donor-acceptor polymeric semiconductors, which include the use of different electron deficient building blocks or the use of different isomeric structures optimize the optoelectronic properties of polymers, hence increasing the PCE. More specifically, it will be presented high performance dithienogermole-based LBG copolymers^[3] and quinoxaline-based copolymers^[4] as high band gap copolymers suitable for tandem solar cell fabrication.
References
[1] J. You, L. Dou, Z. Hong, G. Li, Y. Yang, Prog. Polym. Sci.2013, 38, 1909-1928.
[2] C. L. Chochos, S. A. Choulis, Prog. Polym. Sci.2011, 36, 1326-1414.
[3] C. P. Yau, Z. Fei, R. S. Ashraf, M. Shahid, S. E. Watkins, P. Pattanasattayavong, T. D. Anthopoulos, V. G. Gregoriou, C. L. Chochos, M. Heeney, Adv. Funct. Mater.2014, 24, 678-687.
[4] R. Singh, G. Pagona, V. G. Gregoriou, N. Tagmatarchis, A. Katsouras, A. Avgeropoulos, T. D. Anthopoulos, M. Heeney, P. E. Keivanidis, C. L. Chochos, 2014, manuscript in preparation.

Abstract
In the development of novel materials for organic photovoltaics (OPVs), much research has been focused on designing non-fullerene molecular acceptors for polymer and small-molecule based bulk heterojunction (BHJ) OPVs. Although fullerene derivatives continue to be the most investigated acceptors for BHJ OPVs due to their favorable electron accepting properties and isotropic charge transport, the light harvesting capabilities of solar cells based on these materials is limited by the poor absorption of the acceptor at longer wavelength (e.g. >600 nm). Recently, novel electron acceptors based on azadipyrromethene (aza-DPY) dyes and their complexes¹ have been studied as a potential replacement to fullerene derivatives due to their strong absorption at longer wavelengths and high electron affinity. Despite the potential for improved light harvesting in polymer BHJ solar cells employing these acceptors, it is unclear whether the strong long-wavelength absorption of these materials would contribute to significant photocurrent generation due to the energy level offset requirement for charge transfer and generation².
In this work, we study the charge transfer and generation processes in polymer BHJ blends and solar cells based on the solution-processable aza-DIPY complex bis[2,6-diphenylethylene - 1,3,7,9 tetraphenylazadipyrromethene]zinc(II) (Zn(WS3)₂) as acceptor. We elucidate the charge transfer and generation processes for light absorbed by the donor polymer P3HT (e.g. channel I) and for light absorbed by the acceptor Zn(WS3)₂ (e.g. channel II). The charge transfer and generation processes are studied by steady-state photoluminescence and transient-absorption spectroscopy measurements, respectively. It is shown that although Zn(WS3)₂ has a complementary absorption spectrum to P3HT due to its long-wavelength absorption, the channel II charge transfer is weak compared to the conventional channel I process. This leads to a lower fraction of the photogenerated charges coming from light absorption by the acceptor. Interestingly, polymer solar cells featuring optimized P3HT:Zn(WS3)₂ blends show comparable efficiency to devices based on P3HT:PC₆₀BM, despite the imbalanced photocurrent generation within the P3HT:Zn(WS3)₂ blend.
References
1. W. Senevirathna, G. Suavé, J. Mater. Chem. C 1 (2013) 6684.
2. Y. Zhang, et. al, Appl. Phys. Lett. 102 (2013) 223302.

Monte Carlo (MC) method was used to simulate exciton and charge transport kinetics in bulk heterojunction solar cells incorporating detailed photophysical models. Contrary to deterministic approach, MC method allows tracking of photogenerated excitons, monomolecular recombination, hot exciton dissociation, charge accumulation, injection, bimolecular recombination, charge collection in a variety of 3D nanoscale morphologies at varying energetic disorders. Simulation was conducted for domain sizes ranging from 2 nm to 45 nm to investigate the role of morphology on monomolecular and bimolecular recombination. New stochastic models for hot-carrier injection and bimolecular recombination (BR) were developed based on experimental observations. The model addresses BR&’s dependence on charge separation, diffusivity and trapping while hot exciton dissociation on band energetic offsets between donor and acceptor materials. Exciton dissociation efficiency decreased exponentially with increasing morphology domain size at 60 meV of energetic disorder. The dissociation efficiency decreased from nearly 100% for 2 nm morphologies to 33% for 45 nm morphologies; and bilayer morphologies showed 6.5% efficiency. On the other hand, a nonlinear increase in the charge transport efficiency with increasing morphology domain size was observed. The charge transport efficiency can be divided into two regions, first region spanning from 2 nm to 25 nm domain sizes in which charge transport efficiency increased from 17% to 60%. Second region from 25 nm to 45 nm domain sizes where charge transport efficiency stayed nearly 60% and bilayer morphology resulted charge transport efficiency of 75%. For homogeneous electrodes and active layer morphology interfaces, simulation results showed that the product of exciton dissociation and charge transport efficiency is highest for 10 nm domain size morphology. Current density versus voltage characteristics were calculated for all the morphologies. The cell efficiency increased with morphology domain size from less than 1% for 2 nm domain size to more than 2% for 10 nm domain size and stayed almost constant from 10 nm to 25nm domain size morphologies. The cell efficiency decreased with increasing morphology domain size from more than 2% for 2 nm domain size to nearly 1.5% for 45 nm domain size. Bilayer morphology was least efficient with resulting cell efficiency less than 0.5%. Active layer morphology of fabricated bulk heterojunction solar cell was analyzed using energy filtered transmission electron microscope and resultant domain size of 20 nm was estimated. Simulation results of short circuit current density and open circuit voltage values for 20 nm domain size morphology matched with experimental results.

Thiophene small branched structures have been proposed as candidates for dopant agents transporting holes-electron in organic solar cell (OSC). Low-band gap of these branched oligotiophene have been obtained to be used in organic solar cells.
Two branched thiophene oligomers, a sexithienylene vinylene (E)-Bis-1,2-(5,5''-Dimethyl-(2,2':3',2''-terthiophene)vinylene , (BSTV) and octathienylene vinylene (BOTV) (E)-Bis-1,2-(5,5'''-Dimethyl-(2,2':5',2'':3',2'''-tetrathiophene)vinyleneoligomers, have been synthesized and used as electron donor or dopant in a planar heterojunction poly(3-hexylthiophene) (P3HT), /[6,6]-phenyl C61-butyric acid methylester (PCBM), Organic Photovoltaic cell .
The electrochemical measurements, and particularly the differential pulse voltammetry (DPV) experiments, were carried out to calculate the energy levels of the compounds (HOMO and LUMO levels). The basic steady-state spectroscopy (absorption, emission) allowed determination of the main photophysical characteristics of the molecules under investigation, revealing useful indications about usability of these molecules in the photoactive part of the solar cell.
Several solutions of BSTV and BOTV with different concentrations were blended with P3HT and PCBM and a spin-coating technique was used as film formation for the organic solar cell. Cathode was aluminum film, while ITO glass was used as anode and their influence on the cell power conversion efficiency was investigated. The preferred solvent was dichlorobenzene (DCB). Each constituent of the blend was first dissolved in a separate tube and stirred at 50 ^oC overnight. The “P3HT: PCBM: PSTV(BOTV)” blend was spin-coated in different experimental conditions to optimize the film quality. Thin and reasonably smooth films were obtained when spin-coated at 600 rounds per minute (rpm) for 5 min under N₂ flow. The solar cells for the doping experiments were fabricated and characterized by using standardized and well-known procedures. The device architecture in all the experiments is an organic bulk heterojunction (BHJ) cell with an inverted configuration. An inorganic electron-collecting layer (ZnO) is introduced between the organic BHJ blend and the bottom electrode (Indium Tin Oxide, ITO), causing a drift of the electrons towards the cathode (ITO in this case), and of holes towards the anode (top-electrode metal, Ag in this case). Various concentration ratios of PSTV and POBT as dopants were tested against a reference standard cell made of P3HT: PCBM. From the obtained results, low percentages between 1 and 3% were able to improve the efficiency of the solar cell. The best result was a P3HT:PCBM: 1 % POBT with an efficiency of 3.26%. Experiments to modify the branched thiophene groups with cyanonitrile and cyanoacetic are in progress to be tested as donor or dopant components in an OSC.

Inspired by nature, specifically the efficacy of the photosynthesis process which utilizes cascade energy systems, we demonstrated high performance inverted solar cells consisting of co-evaporated symmetrical squaraine (SQ) and asymmetrical squaraine, (ASSQ). These squaraines synergize each other by broadening the photovoltaic spectral response and showing excellent spectral overlap between the emission of SQ and the absorption of ASSQ. Ultrafast time-resolved photoluminescence and transient absorption spectroscopy shows evidence on the presence of strong FRET from ASSQ to SQ. Our calculation indicates theoretical Förster radius as large as 8.3 nm is achievable and the energy transfer occurs rapidly in ~ 1 ps. We also demonstrated that the power conversion efficiency (PCE) increases by 46% to 6.2% at the optimal blending ratio. Open circuit voltage (V_oc) of SQ:C₆₀ and ASSQ:C₆₀ devices average at 0.81 V and 0.97 V respectively and our study reveals the compositional ratio of ASSQ:SQ impacts the V_oc of blended film device linearly. Furthermore, in comparison to a conventional architecture, the inverted structure small-molecule solar cells has prolonged durability in ambient condition.

We designed and synthesized two low-band-gap conjugated copolymers with alternating difluorinated benzoselenadiazole (DFDTBSe) and ethylhexyloxy (EH)- or octyldodecyloxy (OD)-substituted benzo[1,2-b:4,5-b&’]dithiophene (BDT) building blocks. PEHBDT-DFDTBSe and PODBDT-DFDTBSe have optical band-gap energies of 1.66 and 1.69 eV, respectively, and HOMO energy levels of minus;5.44 and minus;5.43 eV, respectively. The different alkyloxy side chains in the polymers affect the molecular packing and ordering in active-layer films blended with [6,6]-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM). The PEHBDT-DFDTBSe:PC₇₁BM film comprises predominantly “face-on” crystal structures with short π-π stacking distances (3.69 Å) while PODBDT-DFDTBSe:PC₇₁BM has mostly “edge-on” structures according to two-dimensional grazing-incidence X-ray diffraction analysis. Bulk heterojunction solar cells were fabricated with an inverted structure of ITO/PEIE/polymer:PC₇₁BM/MoO₃/Ag. The device fabricated using the PEHBDT-DFDTBSe:PC₇₁BM active layer shows a maximum power conversion efficiency (PCE) of up to 5.74%, which is the highest value reported for OPVs containing benzoselenadiazole and BDT-derivative polymers. A tandem solar cell was also fabricated using PEHBDT-DFDTBSe:PC₇₁BM and poly(3-hexylthiophene):indene-C₆₀-bisadduct as the top and bottom cell components, respectively; its maximum PCE was 7.15%.

A series of quinoxaline-based low band gap copolymers containing thiophene derivatives as an e-rich moiety were synthesized. Experimental results and theoretical calculations show that the coplanarity of the polymeric backbone and strong intermolecular packing with pronounced crystalline morphology. Consequently, the best performance of polymer solar cell was achieved based on PDFQx3T with an inverted device structure of ITO/ZnO/PDFQx3T:PC₇₁BM/MoO₃ /Ag, which showed an open-circuit voltage (Voc) of 0.73V, a short-circuit current (Jsc) of 13.87mA/cm², and a fill factor (FF) of 75%, leading to a high power conversion efficiency (PCE) of ~7%. Moreover, PDFQx3T could still maintain PCE of over 6% with film thickness of the active layer over 250 nm, which makes it a promising candidate for printed PSCs. This study provides an insight into molecular design for efficient semi-crystalline photovoltaic structures.

Organic photovoltaic (OPV) is a promising technology for photovoltaic cells. Compared with silicon based solar cell, it is relatively cheaper and more roll-to-roll processable since layers of solid thin films are deposited from solution. However, up to 80% of polymer solution is wasted using conventional methods such as spin or spray coating or using a doctor blade. In this work, we introduce a new rapid convective deposition to fabricate hole transporting, photoactive polymer, and electron transporting layers as well as to reduce material losses. Rapid convective deposition has been used previously to fabricate 2D crystal of nano/micro particles and enhance performance of light emitting diode (LED), dye-sensitized solar cell (DSSC) and solar cell panels. This method is applicable for both lab and pilot scales since it uses only 5 microlitters of solution per square inch of substrate. We use this method to fabricate PEDOT:PSS/PCDTBT:PCBM/TiOx thin film layers on ITO glass. An aluminum film is deposited on top of these layers as the negative electrode. Fabricating these cells in air with the risk of oxidation, our OPV achieves 5.8% efficiency which is comparable to conventional cells fabricated under nitrogen in a glove box. The efficiency of the OPV strongly depends on thin film morphology and thickness which is controlled by the deposition parameters.

In this work we use a three-dimensional Pauli master equation to investigate the charge carrier mobility of a two-phase system, which can mimic donor-acceptor and amorphous-crystalline bulk heterojunctions. Our approach can be separated in two parts: the morphology generation and the charge transport modeling in the generated blend. The morphology part is based on atomistic molecular dynamics of a binary mixture (donor/acceptor) using fully atomistic molecular simulations with the ReaxFF force field. The second part is carried out by numerically solving the steady-state Pauli master equation. By taking the energetic disorder of each phase, their energy offset and domain morphology into consideration, we show that the carrier mobility can have a completely different behavior when compared to a one-phase system. When the energy offset is equal to zero, the mobility is controlled by the more disordered phase. When the energy offset is non-zero, we show that the mobility electric field dependence switches from negative to positive at a threshold field proportional to the energy offset. Additionally, the influence of morphology, through the domain size and volume ratio parameters, on the transport is investigated.

Understanding how microstructure affects charge carrier dynamics and doping levels in neat conjugated polymers is important for engineering the next generation of organic photovoltaic devices. We demonstrate how varying the crystallite size in neat poly(2,2prime;:5prime;,2Prime;-3,3Prime;-didecyl-terthiophene) (PTTT-10) films modulates the doping density in these films. Introduction of photochemical probe molecules into the amorphous phase of the polymer provides insight into the dominant phase that dopants occupy and into the location of charge carrier generation in the neat films using photogenerated carrier dynamics measured by Time Resolved Microwave Conductivity (TRMC) as a proxy for local doping density.

Bulk Heterojunction (BHJ) polymer solar cells are an area of intense interest due to their flexibility and relatively low cost. The mixture of polythiophene derivatives (donor) and fullerenes (acceptor) is spin coated on substrate as the active layer, and are phase-separated into interconnected domains. However, due to the disordered inner structures in the active layer, donor or acceptor domains isolated from electrodes and long path conduction, the power conversion efficiency (PCE) of BHJ solar cell is low. Therefore, control of the inner structure within the active layer is required to enhance the efficiency.
Our research demonstrated a method to build up self-assembled vertical columns of the photoactive polymer, poly (3-hexylthiophene) (P3HT), within the active layer of BHJ solar cells via the spontaneous phase segregation in the polymer blends of P3HT, a non-active but well characterized polymer PMMA or PS and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) confined at the interface between PMMA/PS and P3HT. The as prepared devices were promising for exhibiting higher power conversion efficiency due to the shorter carrier transportation pathway and larger interface area between donor and acceptor. The columnar structured template is investigated under atomic force microscopy (AFM) and transmission electron microscopy (TEM). Neutron reflectometry was used to demonstrate the confinement of PCBM at the interface between P3HT and PMMA/PS in the active layer. The different morphological structures formed via phase segregation are correlated with the performance of the PEV cells fabricated at the BNL-CFN and significant enhancement for the efficiency is observed.

The polaron pair binding energy is a key factor controlling the exciton dissociation process and the transport properties of heterojunctions in organic electronics.¹ We have identified several measures that affect the polaron pair binding energy and thus device performance. These include interfacial polarization and induced charges. To quantify the effects that interfacial polarization has on the polaron pair binding energy, analytical expressions are derived using both classical and quantum electrostatic methods. We then used a mixture of experimental and ab initio data for several example systems as input for these formalisms, and compared to the results of previous calculations.² These examples demonstrate the non-negligible effects of interfacial electrostatics on polaron pair binding energies, and the importance of accounting for these effects when designing materials for organic electronic devices.
¹N.C. Giebink et al., Phys. Rev. B82 155305 (2010)
²S. E. Morris et al., Organic Electronics in review

Organic semiconductors are a highly tunable class of optically active materials that are promising as next-generation photovoltaics. Utilizing these materials for efficient solar energy conversion relies on an understanding their excited-state electronic structure, i.e. how light absorption, charge transfer, and charge transport relate to the properties of their molecular components and are influenced by solid-state morphology. While many organic materials have varying degrees of disorder, crystalline films with long-range order provide an opportunity to understand many fundamental physical properties relevant to solar energy conversion. Here, we combine theory and experiment to investigate the nature of low-energy excitons within 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-Pen) polycrystalline thin films. First-principles many-body perturbation theory and optical absorption spectro-microscopy on ordered domains reveal multiple low-energy absorption peaks that are composed of delocalized excitonic states. Moreover, predicted and measured angle-resolved absorption spectra reveal the exact inter-grain orientation within the TIPS-PEN film. We discuss the implications of this study for understanding exciton dynamics as measured by spatially-resolved transient absorption spectroscopy. This work was supported by the DOE, NSF and the US-Israeli BSF; computational resources were provided by NERSC.

Orientation of conjugated polymer affects significantly charge transport in thin films. Poly(3-hexylthiophene) (P3HT) is a prototypical conjugated polymer used for organic electronics and the oriented P3HT film has been prepared by various methods. Rubbing is a method to produce the uniaxial alignment of molecules and has been used to obtained “in-plane” orientation of P3HT thin film. The charge carrier mobility of in-plane direction was enhanced by rubbing of P3HT thin film [1]. Recently, the rubbed P3HT film has been used to a photovoltaic as an active layer. This photovoltaic has both the improved power conversion efficiency [2] and the functionality based on the polarized optoelectronic property [3]. In the photovoltaic, carriers migrate in “out-of-plane” direction, corresponding to the normal to cell. Thus, the enhancement of out-of-plane mobility is crucial for the photovoltaic.
Here, we investigated the out-of-plane mobility of holes in P3HT thin films before and after rubbing. The out-of-plane mobility was evaluated by time-of-flight method. It was found that the mobility showed 8-fold enhancement by rubbing. The degree of enhancement was examined by changing the regioregularity (RR) of P3HT. As a result, the lower RR gives the higher enhancement of mobility. To discuss the mobility with respect to the molecule structure, we measured the polarized absorption spectra, resonance Raman spectra, optical microscope image, atomic force microscope image, scanning electron microscope image, and grazing-incidence X-ray diffraction of the films. It was found that the rubbing produces the aggregate and planar molecular structure of P3HT. In particular, a subtle aggregation led to a great enhancement of mobility. The aggregate, given by rubbing, was face-on orientation whose structure provides the efficient carrier pathway in out-of-plane direction due to π-stacking. In addition, ππ distance of the stacking was decreased by rubbing. Their structural evolutions were significantly achieved by low-RR film. These findings are useful for the improvement of carrier transport property of conjugated polymer thin film.
References: [1] Hartmann et al. Adv. Funct. Mater. 2011, 21, 4047. [2] Vohra et al. J. Phys. Chem. Lett. 2012, 3, 1820. [3] Zhu et al. Adv. Mater. 2011, 23, 4193.

Currently, the interfacial modification of bulk heterojunction (BHJ) morphology is focus of considerable research towards optimization of photovoltaic performance of polymer solar cells. Block copolymers (BCPs) become prime candidates as compatibilizers in BHJ blends due to their ability to self-assemble into well-ordered structures, e.g. sphere, cylinders, bi-continuous gyroid and lamellae, when being subject to specific conditions. Here, we utilize the diblock copolymer, polystyrene-block -poly (3-hexylthiophene) (PS-b-P3HT), to control polymer/fullerene BHJ morphology thereby effectively improving power-conversion efficiency (PCE). In this regard, we report the synthesis of PS-b-P3HT block copolymer with various ratios by click chemistry and investigate the influence of different loading fractions of BCP on the morphology and photovoltaic performance of various polymer/fullerene systems. We used commercially available PCBM and newly synthesized fulleropyrrolidine derivative namely C60-fused N-(3-methoxypropyl)-2-(carboxyethyl)-5-(4-cyanophenyl) pyrrolidine (NCPF) and C60-fused N-(3-methoxypropyl)-2-(carboxyethyl)-5-(2,2-difluoro-benzodioxole) fulleropyrrolidine (FFNCPF) blended with P3HT to compare the effect of BCP compatibilizer on different polymer/fullerene systems. The synthesized fullerene derivatives show good solubility in common organic solvents such as chlorobenzene and 1, 2-dichlorobenzene which is important for uniform film formation. Moreover, their optical absorbance and electronic properties are comparable to PCBM. The AFM images of P3HT/NCPF/5wt% BCP and P3HT/FFNCPF/5wt% BCP show homogenous morphology with hill like texture compared to P3HT/NCPF and P3HT/FFNCPF thin films which exhibit several nano-size clusters. GIWAXS data shows that P3HT crystallinity improves for polymer:fullerene/BCP compared to pristine samples, which is beneficial for hole transporting in active layer. The increase in PCE is found to be dependent on BCP addition and type of polymer fullerene pair used. Future studies are underway to investigate the optimum loading fraction of BCP for different polymer/fullerene systems.
Acknowledgements: U.S. Department of Energy, Division of Basic Energy Sciences under contract No. DE-FG02-10ER4779

Isoindigo, a dye that can be isolated from the nature, is attracting great attention due to its electron-withdrawing properties as an acceptor in photovoltaics. The compatible energy levels with P3HT may increase open circuit voltage(V_oc), and the absorption complementary to P3HT helps light-harvesting, both of which increase the performance of solar cell. When isoindigo is copolymerized with thiophenes, the donor-acceptor copolymer shows broad absorption due to narrower band gap. Anderson&’s and Su&’s group independently developed a series of thiophene-isoindigo copolymers, varying the number of thiophenes to one isoindigo unit, abbreviated as P3TI, P4TI, P5TI, and P6TI. Among these polymers, the highest power conversion efficiency (PCE) was reported by Su&’s group to be 7.25% using P6TI, suggesting that P6TI is a highly crystalline polymer. In this research, we copolymerized the monomer of P4TI and P6TI at different feed ratio to adjust crystallinity for better polymer/PCBM interface and domain size. We found that, high PCE was still achieved over 7%, but with much better processibility and maintain good thermal stability. Low solubility, thus low processibility due to ring structure and conjugation nature, can easily overcome by incorporation of intermediate product of P6TI monomer (i.e. P4TI monomer) for polymerization, without significantly changing PCE.

Organic photovoltaics(OPVs) emerged few decades ago have drawn lots of attention due to their low cost, light weight, flexibility, and large area processibility. However, the power conversion efficiency is still lower than that of inorganic solar cells. The morphological control of active layer in OPVs is one of the most efficient ways to improve their performance. Rod-coil block copolymers that can self-assemble into nano-scale, bi-continuous ordered structure is a candidate. However, the insulating nature of coil segment will be detrimental to performance. The eliminating process for coil segment and morphological stability after coil removal become difficult issues. With the aid of Stille coupling and microwave irradiation, we can polymerize fully-conjugating rod-rod copolymer P3HT-b-TPD, where P3HT and TPD served as donor and acceptor respectively. Without etching process, the polymeric material can be directly used in devices. Due to the dissimilar chemical structure of the two segments, the copolymer is expected to self-assemble into two crystalline domains, which facilitate exciton dissociation and carrier transportation. As a result, better power conversion efficiency is expected.

Organic photovoltaics (OPVs) are formed from blends of electron donating and electron accepting materials in a bulk heterojunction (BHJ) architecture. Currently, fullerene derivatives are the most widely used acceptor materials in high efficiency OPVs; however, they have a high production cost, low absorption in the visible range and limited synthetic variability of electronic and optical properties. Small molecule acceptor materials have increased synthetic flexibility, which allows for fine-tuning of the optical and electronic properties of the molecules. In order to reach the full potential of this set of materials, a detailed understanding of the optimal morphology is needed. Non-fullerene acceptors generally underperform PCBM, but it is unclear in many cases if the reason is morphological or due to the electronic structure of the acceptor. Here we present a promising fullerene alternative - 4,7-bis(4-(N-hexyl phthalimide)vinyl)benzo[c]1,2,5-thiadiazole (HPI-BT). HPI-BT was blended with poly(3-hexylthiophene) (P3HT) and achieved a power conversion efficiency (PCE) of 2.1%. Using complimentary techniques of grazing incidence wide angle x-ray scattering (GIWAXS), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM), it is possible to develop a comprehensive understanding of the active layer microstructure. GIWAXS data indicates that the pure acceptor films are highly crystalline which should facilitate better charge transport; however, the blended films were highly textured in the 2-D x-ray patterns, which suggests that the HPI-BT crystals may be nucleating off the substrate. Acceptor nucleation on the anode limits overall cell performance because the electron acceptor is gathered around the electrode where holes are collected. Examination of the surface morphology with AFM showed a smooth surface before annealing and the presence of large acceptor crystallites after annealing. STEM was used to image the as-cast film and revealed buried acceptor crystallites on the order of 1 mu;m, demonstrating the need for analysis by multiple techniques for complex blends. This multi-technique characterization approach elucidates the relationship between the microstructure and electrical performance and may help to enhance the efficiencies of OPVs in the future.

While organic photovoltaics offer great promise as an alternative energy source, the challenges of reaching 20% efficiency are great. Our group addresses those challenges through unique materials design that balance the cost of charge recombination against the cost of charge separation at the bulk heterojunction (BJH). We accomplish this through the use of a series of small molecule squaraine dyes, which have extremely high extinction coefficients (>10⁵) and aggregation properties that allow for molecular probing of the optoelectronic nature of the BHJ. The molecular structure of the squaraines is described as a function of the presence or absence of hydroxyl groups around the central squaric acid moiety, as well as the length of the hydrocarbon side chains. Steady-state absorbance and thermal in-situ x-ray diffraction of thin films are used to characterize the type and quantity of aggregates that are present as a function of molecular structure. Both current-voltage and external quantum efficiency results are shown, and we demonstrate that in order to achieve the best device (>4% PCE and >900mV V_OC) the preferred molecular structure strikes a balance between the two competing thermodynamic interaction mechanisms that drive aggregate formation: hydrogen bonding and extent of side-chain disorder. We finish by prescribing ideal material properties for small molecule squaraine dyes that will result in the highest possible open circuit voltages and idealized exciton delocalization in order to maximize the efficiency while maintaining a panchromatic external quantum efficiency.

By utilizing first principles density functional theory (DFT) we examine the electronic properties of layered phosphate materials. Calculation of the electron and hole mobilities within the planar region of transition metal oxides provides useful information for advanced material designs. Utilizing this knowledge, a novel layered hybrid organic photovoltaic material is presented, exploiting the high mobilities of the phosphate layers; the overall efficiency of traditional organic photovoltaic materials can be increased.

Well-phase separation in polymer-fullerene bulk heterojunction (BHJ) composites is a major obstacle to achieving high power conversion efficiency (PCE) in BHJ polymer solar cells (PSCs). In this study, we demonstrate the high PCE of diketopyrrolopyrrole (DPP)-based BHJ polymer solar cells with a significant improved well-phase separated BHJ morphology by utilizing non-aromatic additive (NAA) solvents. The addition of a small volume of NAA to casting solution exhibited well-phase separation in the BHJ composite films with a fine fibrillar structure of polymer chains, whereas large size of phase segregations are observed in the BHJ composite films processed with aromatic additive (AA) solvents. The alkyl-side chain of NAA affects the side chain of polymers, forming the well-phase separated BHJ morphology by facilitating an inter-diffusion of fullerenes into the polymer chains. Therefore, a significant enhancement in internal quantum efficiency (IQE) of the BHJ solar cells from 30% (with AA) to 80% (with NAA) is achieved by balancing charge carrier mobility both of electron and hole. As a result, a high PCE of DPP-based BHJ solar cells of 7.9% is obtained in the thick BHJ film (sim;200 nm). Our results demonstrate the importance of considering the choice of processing additive on the BHJ composite films for achieving high-efficiency in BHJ PSCs.

Plasma polymerization is a simple but powerful technique for fabricating conducting as well as dielectric polymer films. However, the technology has rarely yielded narrow band gap semiconducting polymers suitable for solar cell application. We have successfully synthesized solvent free polythiophene film via plasma polymerization. Our studies showed that the film had a narrow band gap and it could absorb almost the entire visible region of the solar spectrum. The heterojunction solar cells fabricated using this material showed a high open circuit voltage of ~0.9 V that is not common for organic solar cells employing polythiophene. We will discuss in detail the fabrication of semiconducting polythiophene and its properties as an absorber material in organic solar cells.

Solution processed organic bulk-heterojunction solar cells have attracted considerable attention due to their strong potential advantages including low cost, flexible, easy and large area fabrication. Compared to the rapid developments of new electron donor materials, the development of novel electron acceptors was delayed. PCBM is still the only and best electron acceptor candidates for organic bulk-heterojunction solar cells. Recently, the develop non-fullerene electron acceptors have received considerable effort to overcome the insufficiency of PCBM, such as narrow absorption in the visible region. Perylene diimide (PDI) have good thermal, chemical, and light stability, strong electron-accepting abilities, and high electron mobility, which should be the most promising non-fullerene acceptor. However, the pristine PDIs possess high planarity and strong intermolecular interaction, leading to low power conversion efficiencies. The Mullen-type pentaphenylphenyl dendrimer is used as an efficient strategy to restrict planarity and avoid the crystalline in molecular design for optoelectronic devices.
Here, a pentaphenylbenzene dendrimer functionalized perylenediimide with twisted molecular structure has been designed and synthesized. Two pentaphenylbenzene functional groups were attached to the 1,7-bay positions of PDI units. Though it is hard to construct multi-components at the PDI bay positions due to the reactive activity and large steric hindrance, the introduction of substituent at the bay position would significantly work on electronic structures of perylene unit. The compound could be soluble in most regular solvents with high fluorescence quantum, and show strong fluorescence spectrum in solid film. HOMO and LUMO energies were estimated to be -5.52 eV and -3.66 eV by cyclic voltammograms calculation. After pentaphenylbenzene dendrimers were assembled to the PDI bay-positions, the planar structure were changed obviously, which supported by the theoretical structures (DFT at the B3LYP/6-31G (d) level). Further investigation of the photovoltaic properties of solution processed organic bulk-heterojunction solar cells with the PDI compound as acceptor is being preceded.

Boron subphthalocyanine (SubPc) is a promising donor material for organic photovoltaics, having one of the highest reported open circuit voltages among bilayer OPVs when coupled with C₆₀. Recently, C₇₀ has attracted more attention than C₆₀, largely due to a broader optical absorption spectrum, which leads to a higher current at relatively high voltages. The structure and electronic properties of SubPc derivatives on C₇₀-fullerene were explored using density functional theory (DFT) with added Van der Waals interactions. Total-energy calculations were used to elucidate the initial adsorption derivatives on low index surfaces of C₇₀. The dependence of the electronic and optical excitations on the interface morphology is studied within the Green&’s-function GW and Bethe-Salpeter approaches. Insights gained from these calculations, and how they can be used to improve device efficiency are discussed.

The macroscopic device performance of organic solar cells is governed by interface physics on a nanometer scale. We employ a nano-embossed bi-layer all-polymer system featuring a controlled enhancement in donor-acceptor interfacial area as a model system to investigate the fundamental processes of exciton separation and polaron recombination in excitonic solar cells. We fully characterized the different nanostructures, locally by SEM/AFM and the buried interdigitating interface of the final morphology statistically on a large area by an advanced grazing incidence X-ray scattering technique.
Our results show equally enhanced harvesting of photoexcitons in both the donor and the acceptor material directly correlated to the total enhancement of interfacial area. Apart from this beneficial effect the enhanced donor-acceptor interface leads to significantly increased polaron recombination losses both at the open circuit voltage and around the maximum power point, which we complementary determined with diode dark current characteristics, impedance spectroscopy and transient photovoltage decay measurements. Based on these findings we predict that the often postulated “ideal” comb-like donor-acceptor structure does not allow completely loss-free excitonic solar cells. Furthermore, the investigation outlines that care has to be taken about polaron recombination losses when increasing the interfacial area in order to improve exciton separation.

Plasmonic effects such as strong light trapping, large scattering cross-section, and giant enhancement of local electric fields, associated with localized surface plasmon resonances (LSPs), can be exploited to achieve efficient harvesting of solar energy. Notably, the incorporation of plasmonic effects can allow the light harvesting capability of a solar cell to be maintained even as the thickness of its active layer is significantly reduced. This is especially important for the development of organic solar cells where the short carrier diffusion length, low carrier mobility, and strong charge recombination in organic materials have favored very thin active layers(<100 nm), greatly limiting their light harvesting capability. In this work, we propose the use of plasmonic metallic nano particles (MNPs) of spheroidal geometry to overcome the weak optical absorption of ultra-thin film organic solar cells (OSCs). We show theoretically a broadband enhancement in optical absorption of ~60% and improvement in power-conversion efficiency (PCE) of ~20% when spheroid MNPs are introduced into a newly proposed OSC with ultra-thin active layer, as compared to an MNP absent reference. These improvements are explained according to spectral broadening of absorption resonances and enhanced directionality of light scattering into the active layer plane exhibited by LSPs of spheroidal MNPs with optimal eccentricity. Our proposed approach opens a new route for the application of MNPs toward efficient, ultra-thin photovoltaic systems, through the optimization of the in-plane directional scattering cross section of plasmonic MNPs.

Previously we have found that the efficiency of organic photovoltaics can be substantially increased by incorporating into the active layer a synthesized nanocomposite of gold-reduced graphene oxide (AuRGO) dispersed in sulfonated polystyrene (PSS). AFM imaging displayed self-assembled columnar structures in the polymer active layer, suggesting a more organized morphology which reduces charge recombination.
We have now additionally characterized AuRGO by Raman spectroscopy, KPFM, and HRTEM to further understand its properties and structure. We spun cast AuRGO for use as an electrode and tested its potential for replacing the easily oxidized aluminum cathode. And, since graphene oxide (GO) has been found to increase the efficiency of the organic polymer solar cell when it is incorporated into the PEDOT:PSS layer, we now test the effectiveness of gold-graphene oxide added to the PEDOT:PSS film, with the ultimate goal of increasing overall power conversion efficiencies of this cost-effective photovoltaic system.

Present day electronic devices are enabled by design and implementation of precise interfaces that control the flow of charge carriers. However, unlike the well-controlled interfaces in conventional electronics based on silicon and other inorganic materials, organic interfaces are poorly defined. Here, we describe how precise manipulation and control of organic-organic interface can increase power conversion efficiency by 2-5 times in a model bilayer system. Applications of these design principles to practically applicable architectures like bulk heterojunction lead to an enhancement in power conversion efficiency from 4.0% to 7.0%. These interface manipulation strategies are universally applicable to any organic donor-acceptor interface, making them both fundamentally interesting for organic electronics and for achieving high efficiency in organic solar cells.

Understanding the relationship between molecular structure, bulk-heterojunction morphology, and device performance in organic photovoltaics (OPV) is key to developing high performing materials. Toward this end, we have synthesized a series of diketopyrrolopyrrole-based small-molecule donors containing aryl-substituted benzodithiophene (BDT) cores. Alteration of the identity and position of the aryl side chains on the BDT core causes a large spread in OPV device performance, yielding power-conversion efficiencies (PCEs) between 0.5% and 5% when paired with fullerene acceptors. The spread in PCE is largely due to differences in the fill factor and short-circuit current. The synthesis and characterization of these molecules will be discussed with an emphasis on the physical origin of the PCE variation and its relation to the molecular structure.

It is common knowledge that polymer aggregation and phase separation in blends with fullerene derivatives is an important but often difficult to control issue and crucially impacts the photovoltaic parameters of polymer based solar cells. On the one side, strongly intermixed polymer:fullerene phases provide large interfacial area and consequently large exciton dissociation rates and thus charge carrier generation. On the other side, pristine, and well-ordered, polymer or fullerene domains support exciton delocalization and efficient charge transport.
Herein, we present versatile routes to control the morphology by applying side-chain modifications to the polymer and fullerene, tuning the polymer:fullerene blend ratio and controlling the order within the bulk heterojunction via ternary blends. On the basis of an anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) (PPE-PPV) copolymer backbone we investigated a number of these structure-property-relations. Our key finding, based on detailed device characterization and spectroscopic techniques, is that manipulation of ternary blend ratio along with molecular structure and processing parameters allows each of the processes of exciton generation, charge separation, and charge extraction to be addressed essentially separately.

Strongly electron withdrawing “end-cap” substituents, such as 1,3-indanedione and dicyanovinyl units, have been used to tune the electronic levels of a variety of small molecule organic optoelectronic materials, particularly those used in organic photovoltaics. A clear advantage of these end-caps is the fact that when they are attached in conjugation with an aryl group, they generally lie in plane with the aryl group, effectively increasing the area of the #7464; face. However, derivatization of the electron-withdrawing group to both solubilize and further tune the solid-state packing has been relatively unexplored. In the case of dicyanovinyl substituents, such functionalization invariably leads to disruption of the important planar junction. For 1,3-indanedione end-caps, the un-explored functionalization of the aryl moiety will not disrupt planarity, and has the potential to yield solubilized versions of heretofore insoluble organic semiconductors. Herein, we present a series of alkane-substituted indanedione units, and their impact on the electronic energy levels and solubility of notoriously insoluble benzodithiophene dimers. By examining the effect of an array of alkane substituents on the crystal packing and the device properties of extended benzodithiophene oligomers, elucidation of structure-function relationships will be undertaken.

In this work, the solvent field and temperature field are used to explore the mutual transformation process and mechanism between the α-conformation and β-conformation in Poly(9,9-dioctylfluorene) (PFO) precursor solution. The conformational transformation of PFO chain is researched by UV-vis absorption spectra and the proportions of β-conformation are quantitatively calculated. The corresponding variation trend of aggregation structure is researched by static/dynamic light scattering (SLS/DLS). It is found that the mutual transformation process between α-conformation and β-conformation are reversible in essence, and the mutual transformation mechanism between them is also clear. Especially in the transformation processes, the complicated relationship between the β-conformation and aggregation structure is clearly understood, while it was still on debate before. The above results give an insight into the β-conformation and aggregation structure of PFO in theory. Furthermore, under the temperature field, we find that both two transformation steps have good linear correlations, which indicates that using temperature field can be considered as a good method to accurately control the proportion of β-conformation in actual application, and it will help us to get the desired proportion of β-conformation in PFO precursor solution so as to increase the charge carrier mobility of optoelectronic films and the device efficiency.

Bulk heterojunction (BHJ) polymer solar cells (PSCs) became one of the most promising options for alternative energy sources. Especially, organic photovoltaic cells (OPVs) have attracted much interest due to their high possibility for solution processing and flexibility. In OPVs, a large interfacial area and vertically aligned charge pathways should be achieved for high efficiency. A BHJ with above characteristics might be constructed by tuning the interactions between donor and acceptor in a mixed solution. Such interactions could be controlled via the small amount of additives which can selectively interact with donor or acceptor and tune the surface energy of each material. Here, two additives, one is 2,2,3,3,4,4,4-heptafluoro-N-phenyl-butyr-amide (F-ADD) and the other is tetrabromothiophene (Br-ADD), which could selectively interact with donor or acceptor were used to increase the efficiency of bulk heterojunction OPVs. This kind of approach might open the road for general additives which could be applied to various polymers.

In this study, we propose a method for improving light absorption in the active layer using wavelength conversion luminescent dyes, which convert near ultra-violet to visible light. Typically, OPV materials have low absorption and external quantum efficiency in the near ultraviolet region. Therefore, we chose Coumarin 102 (C102) luminescent dyes which has a peak absorption wavelength at 389nm that is the weakest absorption region of Poly(3-hexylthiophene-2,5-diyl) (P3HT):1-(3-methoxycarbonyl)-propyl-1,1-phenyl-(6,6)C61 (PCBM) blends. When the C102 dyes absorb the near ultraviolet light, they re-emit visible light isotropically into the P3HT:PCBM active layer system, which corresponds to main absorption region of the P3HT. We put the C102 emitting dyes in the Poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) layer to prevent charge recombination at interface between C102 and active layer materials. By embedding C102 dyes into PEDOT:PSS layer, the incident ultra-violet lays are converted to visible light, thus increasing the light absorption in the active layer. Also, the C102 dyes re-emit the visible light isotropically, which increase light traveling distance. Through these mechanisms of enhanced light absorption, short circuit current density (J_sc) and external quantum efficienty (EQE) increase with the C102 concentration. The power conversion efficiency (PCE) increase from 2.64% to 2.77% with incorporating C102 dyes.

Organic photovoltaic cells (OPVs) present the potential to change the market of energy production. They have garnered advantages of low-cost process and mechanical flexibility. In recent years, the power conversion efficiency (PCE) of OPVs has steadily improved as high as 9.2%. Despite promising progress in device performance, PCE have to be enhanced for the commercialization. One of the crucial challenges for the high performance is the efficient charge transport at the polymer/fullerene active layers with bulk heterojunction (BHJ) structure. In this study, we demonstrate an ideal active layer employing various types of carbon nanotubes (CNTs), as they have excellent carrier mobility and chemical compatibility with the solution process. Unlike previously reported OPVs with CNTs significantly degraded the device performance than those without CNTs. Without charge selectivity, any small proportion of metallic CNTs present may build up undesired pathways for electron-hole recombination. Moreover, inhomogeneously dispersed CNT aggregates give rise to a fatal device short circuit. We present the remarkable device performance enhancement in BHJ solar cells employing N-doped (N-CNT), B-doped CNTs (B-CNT), quantum dot nanoparticle decorated CNTs (QD:CNT) or metal nanoparticle bound CNTs (Me:CNT) as highly selective electron- or hole-transport enhancement materials^1,2. We also emphasize that this study offers a general route to address the efficient charge transport for other organic optoelectronics, such as organic light-emitting diodes and organic transistors.
[1] J. M. Lee, J. S. Park, S. H. Lee, H-. Kim, S-. Yoo, S. O. Kim, Adv. Mater. 23(5), 629 (2011).
[2] J. M. Lee, B. H. Kwon, H. I. Park, H-. Kim, M. G. Kim, J. S. Park, E. Kim, S-. Yoo, D. Y. Jeon, S. O. Kim, Adv. Mater. 25(14), 2011 (2013).

Polymethacrylates bearing fullerene-C₆₀ pendants are promising materials with good solubility and processability for electron acceptor of thin layer organic photovoltaics (OPVs). Generally speaking, they have been prepared by free-radical polymerization and the subsequent polymer modification. On the other hand, anionic polymerizations of methacrylates are known to provide the polymers with well-controlled molecular weight and stereoregularity. The primary structures of macromolecular chain often affect the physical properties of polymers such as glass transition temperature, crystallization behavior, and solubility. Hence, we have great interest in the effects of the primary structures on the yield in polymer modification and physical properties of the C₆₀-containing polymers as well as their photoelectronic properties. In this work, precursor polymethacrylates with different stereoregularities were synthesized, and their C₆₀-modification and the properties of resulting polymers were investigated.
Stereospecific anionic polymerization of 6-chlorohexyl methacrylate and methyl methacrylate with a variety of feed ratios afforded a series of isotactic and syndiotactic copolymers, while atactic copolymers were also prepared by radical polymerization. After the substitution of pendant chlorine atoms to azide groups, C₆₀ was reacted with the azide groups. The complete consumption of azide was confirmed by ¹H NMR and IR spectra. The crude product was re-precipitated from tetrahydrofuran (THF) solution to hexane several times to isolate a soluble fraction. The highest C₆₀ content of the soluble polymer was 40 mass%, while the insoluble polymer might contain higher C₆₀ content. The differences in yields and solubilities among isotactic, syndiotactic, and atactic polymers were not significant.
Optical characterization by UV-Vis spectroscopy and photoelectron spectroscopy in air (PESA) and electrochemical characterization by cyclic voltammetry on syndiotactic and atactic polymers revealed that band gaps decrease as compare to pristine C₆₀ and its common derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Interestingly, LUMO levels increased significantly at 3.8 eV, providing a higher open-circuit voltage in the blend with poly(3-hexylthiophene) (P3HT) as observed on J-V curve (0.97 V). The miscibility with P3HT exhibited significant differences; atactic polymer (M_n = 10200, M_w/M_n = 2.77) showed coarse morphology in micrometer order, while syndiotactic polymer (M_n = 11200, M_w/M_n = 1.86) showed fine morphology. As a result, syndiotactic polymer quenched the fluorescence of P3HT up to 97% in blend, which was 3% higher than atactic polymer. These results strongly support that syndiotactic C₆₀-containing polymer is a potential electron acceptors for organic solar cell. The properties of isotactic polymer are now under characterization.

It is important to control the film crystallinity of the active layer and the interdiffusion between donor and acceptor materials in producing pseudo-bilayer polymer solar cells. The film crystallinity is essential for better charge transport, while the interdiffusion is essential for optimal bicontinuous networks. We investigated the effects of a doping solution-processable small molecule with high carrier mobility, 5,11-bis(triethylsilylethynyl) anthradithiophene (TES-ADT), on the performance of pseudo-bilayer polymer solar cells. The solar cell devices were made of an underlayer of poly(3-hexylthiophene) (P3HT) and an upper layer of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The TES-ADT was doped in P3HT underlayer. Through analysis of the X-ray diffraction and UVminus;vis absorbance spectra of P3HT:TES-ADT blend films, it was demonstrated that the film crystallinity was enhanced by TES-ADT doping. An intermixed photoactive layer was observed from scanning electron microscope image. With improved film crystallinity and interdiffusion, the optimal device performance was obtained when 5% TES-ADT was blended with P3HT and a thermal annealing treatment at 150 °C for 1 min was conducted.

Bulk heterojunction (BHJ) polymer solar cells are an area of intense interest due to their advantages such as mechanical flexibility, low costs, and the easiness of the fabrication. The active layer is typically spin coated from the solution of polythiophene derivatives (donor) and fullerenes (acceptor) and interconnected domains are formed because of phase separation. However, the power conversion efficiency (PCE) of BHJ solar cell is restricted by the disordered inner structures in the active layer, donor or acceptor domains isolated from electrodes and long path conduction. Here we report a self-assembled columnar structure formed by phase separation between poly[N-9Prime;-hepta-decanyl-2,7-carbazole-alt-5,5-(4prime;,7prime;-di-2-thienyl-2prime;,1prime;,3prime;-benzothiadiazole)] (PCDTBT) and poly(3-hexylthiophene) (P3HT) for the active layer morphology optimization. The BHJ solar cell device based on this structure is promising for exhibiting higher performance due to the shorter carrier transportation pathway and larger interfacial area between donor and acceptor. The surface morphology is investigated with atomic force microscopy (AFM) and the columnar structure is studied by investigation of cross-section of the blend thin film of PCDTBT and P3HT using the transmission electron microscopy (TEM). The influences of different parameters, including solvents and annealing time, on morphology are discussed. The different morphological structures formed via phase segregation are correlated with the performance of the BHJ solar cells fabricated at the Brookhaven National Laboratory.

Photovoltaic is hailed as one of promising green technology to generate electricity using an environmentally friendly source - the sun. In recent years, new materials such as quantum dot, perovskite, and organic semiconductor are being developed as emerging material for photovoltaic applications. Organic photovoltaics (OPV) have the advantage of being solution processable and hence can be a low cost option for this technology. There has been increasing interest on small molecular OPV because unlike polymers used for OPV, the small molecules are structurally well defined, easy to purify and can crystallize easily for good charge transport. Hence, the structure-property relationship for small molecules for OPV applications can be elucidated and this will provide the basis for an intelligent design for these molecules.
The criteria of a small molecule for efficient solar conversion are strong light absorption nature, suitable HOMO-LUMO level, good crystallinity, and balanced elctron/hole transporting properties. The design strategy for the molecule discussed in this work aims to address these. A novel donor-acceptor (DA) small molecule based on dithienocyclopentathieno[3,2-b]thiophene as a donor part is being synthesized. The structure has similarity with Indacenodithiophene, of which benzene core is replaced by thienothiophene, expecting better crystallinity and stronger electron-donating ability. This donor part was coupled with 5-hexylthiophene-2-yl-benzothiadiazole on the both side, to get the product having D-A-D-A-D architecture. This material has a high absorption coefficient of 3.7 x 10⁵ M^-1cm^-1 and a long range of absorption maximum matching that of the visible solar spectrum. The band gap estimated by both UV-vis absorption and cyclic voltammetry is 1.8 eV which is suitable for sunlight absorption. As a preliminary study, this new molecule was blended with PC₆₀BM to form the active layer and the device structure is ITO/PEDOT-PSS/active layer/Al. The unoptimized device showed a power conversion efficiency (PCE) of 1.3 %. With the optimization of the processing condition, the donor:acceptor ratio and use of additives, further improvement to the performance of the cells was achieved.
In this presentation, the effect of structural design of these molecules on the material and hence device performance will be discussed. In addition, the detailed device analysis obtained by X-ray diffraction, atomic force microscope, and charge carrier mobility measurement provides the solid understanding of material properties, which contribute to elucidate the effect of our novel donor material.

Semiconducting polymers, such as poly 3-hexylthiophene (P3HT), have attracted a substantial interest in optoelectronic devices application, such as field effect transistors, light emitting diodes, and solar cells, due to their simple and flexible processability.¹ As a p-type semiconductor, P3HT can be synthesized in a form of highly crystalline nanofiber, which are known to have higher mobility than randomly organized films.² However, P3HT is known to degrade under long term exposure to light and oxygen, so understanding the mechanisms of degradation as well as techniques for mitigation of the degradation will lead to a longer lifetime of the device using P3HT.
Here we show a dependence of the degree of degradation of P3HT films on the conductivity of the supporting substrate, by comparing the drop casted P3HT films on different substrates for a long term of degradation under various environments. The samples were subsequently characterized by UV-Visible, FTIR and X-ray photoelectron spectroscopy, for in-depth chemical analyses. We find that P3HT films on conductive substrates show significantly retarded degradation and retain their chemical and morphological features when compared to similar films on glass substrates. This ‘substrate effect&’ in retarding degradation of P3HT films is evident even upon prolonged exposure to air for up to five months.
1. Chen, D.; Nakahara, A.; Wei, D.; Nordlund, D.; Russell, T. P. Nano Lett. 2011, 11, 561.
2. Zhao, Y.; Sugunan, A.; Rihtnesberg, D. B.; Wang, Q.; Toprak, M. S.; Muhammed, M. Phys. Status Solidi C 2009, 9, 1546

In this contribution, we show that power conversion efficiencies (PCEs) in excess of 9% were obtained in single-junction solar cells with an inverted architecture, using a naphthobisthiadiazole-quaterthiophene copolymer (PNTz4T) as the p-type material and PC₇₁BM as the n-type material. It should also be noted that PNTz4T cells with PC₆₁BM also provided similar PCEs that reached 9%. Importantly, these efficiencies have been achieved by using thick active layers measuring around 300 nm, which is far thicker than the typical thickness for polymer-based solar cells and is beneficial for the practical use. In addition, PNTz4T cells do not require the use of solvent additives, such as 1,8-diiodooctane. The markedly high efficiency is most likely attributed to the highly ordered polymer structure in the active layer, in which the highly crystalline structure with the short π-π stacking distance (3.5 Å) and the favorable face-on orientation are achieved. We also found that the polymer crystallites with the face-on orientation were present in greater abundance on the ZnO surface than on the PEDOT:PSS surface, and that there was a “favorable” distribution of the polymer orientation through the film thickness. These characteristics would facilitate charge transport and reduce charge recombination particularly in the inverted architecture, resulting in the higher J_SC and FF. These results are evidence of the great promise of polymer-based solar cells, and indicate that further higher PCEs should be realized by careful molecular design using the NTz moiety.

Over the past few decades, polymer solar cells (PSCs) have made a significant progress, showing their potential in low-cost, flexible, lightweight, portable and large-area energy-harvesting devices. Considerable efforts have been dedicated toward the design of new materials, device architectures and processing techniques in order to improve the power conversion efficiency (PCE). To further improve the PCE value, first and foremost, the molecular structure of low bandgap (LBG) polymers should be carefully designed by considering its close relationship with the photovoltaic parameters, including short-circuit current density (J_SC), open-circuit voltage (V_OC) and fill factor (FF). Here, we present a series of crystalline low bandgap polymers which were designed by considering the backbone planarity, noncovalent intra- and interchain interactions (via H-bonding and dipole-dipole interactions, etc) and solution processibility, leading to highly ordered film morphologies, deep highest occupied molecular orbital (HOMO) level, ideally balanced electron and hole mobilities. The molecular design and resulting morphological, electrical and device properties will be discussed in detail.
Three different types of dialkoxyphenylene and benzothiadiazole (BT)-based low bandgap copolymers were designed by carefully considering the planarity, chain curvature and the resulting intermolecular orientations. The noncovalent attractive interactions between S (in thiophene) and O (in alkoxy groups), between S (in thiophene) and F, and between C-H (in thiophene) and N (in BT) minimize the torsional angle, thus maximizing the planarity of the polymer chain. The polymers formed well-distributed fibrillar networked morphologies with PC₇₀BM, showing well-balanced hole and electron mobilities. Notably, PSCs based on these polymers exhibited PCEs of up to ~9.4% (the highest amongst all reports so far) in a ~300 nm thick conventional single-cell device structure without any additional interfacial layer. The thick active layer in the PPDT2FBT:PC₇₀BM device enabled strong light absorption, yielding a high J_SC of 15.7~16.3 mAmiddot;cm^-2. It is not currently viable to fabricate uniform and defect-free films on the order of 100 nm thickness using industrial solution casting techniques. Most previous PSCs showed the performance degrades with concomitant decrease in FF, with increasing film thickness. This must be closely related to space charge accumulation and charge recombination losses which become stronger with thicker films. It is noteworthy to emphasize that ~300 nm thick active layer in the PPDT2FBT:PC₇₀BM device attenuates incident light almost completely without damages in FF. Furthermore, this work also demonstrates a high PCE of over 7% (without any post-treatments) with long-term thermal stability at 130 ^oC for ~200 h. These new polymers provide a great possibility to overcome the efficiency barrier of 10% and accelerate the commercialization of plastic solar cells.

Despite being over 20 years old, poly[3-hexylthiophene] (P3HT) remains one of the few commercially viable materials in the field of plastic electronics. Synthetically P3HT is straightforward to produce but has historically suffered from significant batch-to-batch variability resulting in inconsistent device performance. The root cause of this variance lies in the inability to reproduce material properties between batches both in terms of molecular weight distributions and purity, especially when working at high production volumes.
Here we will present our recent work on developing droplet flow reactors for the synthesis of semiconducting polymers.^1,2 Focusing specifically on P3HT we will demonstrate how these reactors enable on-demand tailoring of molecular weights and electronic energy levels (with an optically distinct co-monomer) for different application/processing needs, whilst maintaining narrow polydispersities. Furthermore we will show how these reactors have been scaled to produce industrially relevant quantities of material without loss of control over the chemistry, enabling continuous production of P3HT on the 100&’s-1,000 g/day scale.
In view of the need for high-performance processable materials for large-area solution-cast devices, we will discuss the importance of polymer purity and the role of the impurities on both device characteristics in P3HT:PCBM devices and solubility/gel formation in solution. We will conclude by proposing a stratagem for purifying this class of polymer, which will be shown to provide access to 7.0% P3HT:IC60BA devices.
1. J. H. Bannock, S. H. Krishnadasan, A. M. Nightingale, C. P. Yau, K. Khaw, D. Burkitt, J. J. M. Halls, M. Heeney, and J. C. de Mello, Adv. Funct. Mater., 2013, 23, 2123-2129.
2. J. H. Bannock, M. Al-Hashimi, S. H. Krishnadasan, J. J. M. Halls, M. Heeney, and J. C. de Mello, Mater. Horizons, 2014, 1, 214-218.

The success of benzodithiophene-alt-thienothiophene (PTB) based photovoltaic devices provides opportunity for realizing low cost solar energy resource. However, addition of solvent additive such as 1,8-diiodooctane (DIO) during device processing has been proven to be necessary for PTB based devices due to morphological challenges. This intrinsically complicates the fabrication process, making large-area module difficult to realize. Moreover, the kinetic controlled morphology obtained through DIO seems to be unstable towards thermal treatment. Herein, a selenophene modified PTB polymer, PBDTSe-TT is reported. The structure adjustment carried out by alkylselenophene substitution on the BDT building block is shown to slightly affect the polymer&’s electronic property, and an enlarged V_OC of the resulting photovoltaic device is observed. More importantly, the PBDTSe-TT:PC₇₁BM bulk-heterojunction thin film morphology can be optimized through this modification. As a result, an efficient PCE of 8.8% is achieved without using any solvent additive or special interfacial layer. In addition, the PBDTSe-TT based device shows relatively improved thermal stability compared to other PTB polymers, making it a good candidate for fabricating stacking cells. Finally, a ~ 10% PCE tandem device is demonstrated by using identical PBDTSe-TT:PC₇₁BM sub-cells. This work provides an opportunity to simplify the fabrication process of PTB based device without sacrificing its performance.

OPV technology has undergone a rapid development in performance and lifetime. Efficiencies of over 10 % were reported for single junction as well as tandem junction OPV cells. However, when going from small record cells to larger and more product relevant modules, most groups reported quite dramatic performance losses.
This presentation will discuss the various loss mechanisms during scale-up, and further will highlight the specific material and architecture challenges for printed tandem modules.

Polymer solar cell (PSC) technology has attracted much attention due to its promise as low-cost conversion of solar energy. Despite recent progress, several limitations are holding back PSC development. For instance, current high-efficiency (>9.0%) PSCs{Liao, 2013 #103} are restricted to materials combinations that are based on limited donor polymers and only one specific fullerene acceptor, PC₇₁BM. Furthermore, the PSC field lacks an effective approach to control the polymer:fullerene blend morphology that is critical in achieving high PSC performance. Here we show multiple cases of high-performance thick-film PSCs (efficiencies up to 10.8%, fill factors up to 77%) via the formation of a near-optimal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains. This morphology is controlled by the temperature-dependent aggregation behavior of the donor polymers during casting and is insensitive to the choice of fullerenes. Our comparative study show that the choice of alkyl chains is critically important in enabling optimal aggregation and morphology control. The uncovered aggregation and design rules yield three high-efficiency (>10%) donor polymers and will allow further synthetic advances, process optimizations, and matching of both the polymer and fullerene materials, potentially leading to significantly improved performance and increased design flexibility.

We have studied the degradation of a variety of organic photovoltaic (OPV) materials and find that materials with a higher crystallinity are more stable. Solar cells made from more crystalline OPV materials demonstrate both reduced burn-in loss and longer operating lifetimes. In materials with very high crystallinity, such as some solution processed small molecules, photo-induced burn-in can even be eliminated. Furthermore, crystalline thin films show substantially slower photobleaching than amorphous films under illumination in air.
In solar cells held under constant illumination and inert atmospheres, we find that amorphous polymers like PCDTBT and regiorandom P3HT show a severe, photo-induced efficiency loss of around 25%, while more crystalline materials like regioregular P3HT or the polymer ZZ115 show burn-in of less than 10%. We observe a similar correlation between crystallinity and photo-induced burn-in when solution processed small molecules are used as the OPV materials. This burn-in can even be eliminated when the materials are processed to have the highest crystallinity. The burn-in degradation observed in more amorphous materials is dominated by open-circuit voltage (Voc) and fill factor (FF) loss. We can isolate the burn-in loss to degradation of the bulk material by replacing the electrodes. The mechanism of this voltage loss is explored using a combination of charge extraction and photovoltage decay techniques.
To investigate the role of morphology on the intrinsic photochemical stability of OPV materials, we perform photobleaching experiments of thin films in air. We find that increasing a material&’s crystallinity increases its photochemical stability. For example, amorphous rubrene films photobleach in air ~25x faster than polycrystalline films, and amorphous TIPS-pentacene films photobleach ~5x faster than polycrystalline films. Crystallinity could improve a material&’s stability for a variety of reasons. For instance, crystalline materials are denser, reducing the solubility of photochemical reactants (O₂, H₂O, synthetic catalysts, organic impurities) and products. Interchain stabilization in crystallites could constrain the local chemistry and reduce the number of conformations available to form reaction products. Crystallinity can also increase backbone planarity, which could both reduce intersystem crossing rate (and reduce triplet concentrations) and increase the energy required to break conjugated bonds. We try to deconvolute these various effects by selecting materials with similar chemistries but differing crystallinity, density, and backbone planarity.

Luminescence Imaging has evolved to a versatile characterization method for studying the behavior of solar cells with lateral resolution. Amongst others we show its beneficial application within degradation studies of polymer and small molecule solar cells [1]. Qualitative understanding of degradation mechanisms can be achieved by tuning the excitation mechanism [2] or comparison to other measurement techniques within degradation studies [3]. A straightforward analysis of luminescence images allows quantification of the residual active area under accelerated ageing conditions and thus reliable testing of barrier and encapsulation techniques [4]. Apart from degradation studies we show how Luminescence Imaging can be used for calculating the current-voltage characteristics of the active layer of homogeneous solar cells as well as for determination of the involved resistances in an equivalent circuit network model [5]. In a very recent approach we extended the quantitative analysis for description of laterally inhomogeneous solar cells. Amongst a quantitative description of the laterally inhomogeneous progress of degradation this method will also allow a future quality control subsequently to large scale production processes.
References:
[1] M. Seeland, R. Rösch and H. Hoppe, Imaging Techniques for Studying OPV Stability and Degradation, in Stability and Degradation of Organic and Polymer Solar Cells, edited by F. C. Krebs (John Wiley & Sons, 2012).
[2] M. Seeland, R. Roesch and H. Hoppe, J. Appl. Phys. 109 (6) (2011).
[3] R. Rösch, D. M. Tanenbaum, M. Jorgensen, et al., Energy Environ. Sci. 5 (4), 6521-6540 (2012).
[4] H. Klumbies, M. Karl, M. Hermenau et al., Sol. Energ. Mat. Sol. Cells 120, 685-690 (2014).
[5] M. Seeland, R. Rösch and H. Hoppe, J. Appl. Phys. 111 (2), 024505 (2012).

Active-layer mechanical flexibility is a characteristic feature of organics-based electronic devices, which is considered as a main technological advantage over traditional inorganic electronics. Recent experimental studies quantifying the strength of materials derived from π-conjugated organic molecules and polymers, however, reveal that limited fracture toughness is a serious drawback, especially for multi-component polymer:fullerene blends used in photovoltaic applications.^1,2 Here, through a series of molecular dynamics simulations, we investigate the cohesion, mechanical properties, and fracture mechanisms of pure and blended films derived from poly-(3-hexyl thiophene) [P3HT] and chemically modified fullerenes. Comparison to experimental data reveals how differences in terms of the fullerene chemical substitution, P3HT molecular weight, and P3HT:fullerene derivative interactions influence these characteristics. The molecular-scale insights derived from the results provide an understanding that can be used to refine design principles and enhance material robustness and reliability.
(1) Dupont, S. R.; Oliver, M.; Krebs, F. C.; Dauskardt, R. H. Sol. Energy Mater. Sol. Cells2012, 97, 171.
(2) Savagatrup, S.; Makaram, A. S.; Burke, D. J.; Lipomi, D. J. Adv. Funct. Mater.2014, 24, 1169.
Financial support for this work is provided in part from by the Center for Advanced Molecular Photovoltaics (CAMP, Award KUS-C1-015-21) made possible by KAUST and the Office of Naval Research (Award No. N00014-14-1-0171)