We demonstrate a novel pathway to open an ultimate electroluminescence (EL) efficiency by using simple aromatic compounds displaying efficient thermally activated delayed fluorescence (TADF) having high photoluminescence (PL) efficiency. We found that a proper design of organic molecules enables the formation of a small energy gap between singlet and triplet excited states (ΔEST), resulting in the enhanced T1 to S1 reverse intersystem crossing (ISC). Such an excited state is attributable to the intramolecular charge transfer (CT) of the spatially separated donor and acceptor system. We discuss the molecular design of high efficiency TADF materials based on a quantum mechanical calculation. We also demonstrate very high efficiency EL using a new series of TADF materials.
References
[1]A. Endo, M. Ogasawara, A. Takahashi, D. Yokoyama, Y. Kato, C. Adachi, Adv. Mater. 21, 4802 (2009).
[2]A. Endo, K. Sato, K. Yoshimura, T. Kai, A. Kawada, H. Miyazaki, and C. Adachi, Appl. Phys. Lett. 98, 083302 (2011).
[3]K. Goushi, K. Yoshida, K. Sato, and C. Adachi, Nat. Photon. 6, 253 (2012).
[4]T. Nakagawa, S-Y. Ku, K-T. Wong, and C. Adachi, Chem.
Comm. 48, 9580 (2012).
[5]M. Gabor, H. Nomura, Q. Zhang, T. Nakagawa, and C. Adachi, Angewante Chem., 51, 11311 (2012).
[6]H. Tanaka, K. Shizu, H. Miyazaki, and C. Adachi, Chem. Comm., 48, 11392 (2012).
[7]Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki, and C. Adachi, J Am. Chem. Soc., 134, 14706 (2012).
[8]H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature, 492, 234 (2012).
[9]K. Sato, K. Shizu, K. Yoshimura, A. Kawada, H. Miyazaki, C. Adachi, Phys. Rev. Lett., 110, 247401, 2013

Unlike other luminescent, solid-state systems such as inorganic semiconductors and quantum dots, organic semiconductors are characterized by highly localized excitons, typically confined to a single molecule. Localization helps protect the efficiency of light emission even in the highly-disordered thin films that are compatible with cost-effective manufacturing. But localization in organic semiconductors also generates distinct bright and dark states, defined by the large exchange splitting between the spin 0 singlet and spin 1 triplet excited states. Recently, Adachi, et al. have demonstrated a new class of efficient emitters with much smaller exchange splitting, offering a new solution to the problem of dark states in OLEDs. By separating the constituent electron and hole, these materials exhibit excited states that are similar to those in inorganic materials. Dark state excitations can efficiently intersystem cross back to the bright state in a process known as thermally activated delayed fluorescence. Perhaps the key physics in these materials is the mechanism of intersystem crossing. In this presentation, we will describe experiments that directly modulate the exchange splitting by using pressure to squeeze together donor and acceptor molecules. The splitting is studied by transient analysis of prompt and delayed fluorescence. We also show that the process is modulated by a magnetic field. Together the experiments allow quantification of the key parameters in thermally activated delayed fluorescence.

Cyclometalated Ir(III) complexes have received intense interest as phosphorescent dopants in electroluminescent organic light-emitting diodes (OLEDs) for displays and organic solid-state lighting [1]. Recent work will be presented on the synthesis, photophysical properties and device performance of new homoleptic and heteroleptic Ir(III) complexes [2,3,4]. Colour tuning of phosphorescence and PhOLED efficiencies of 40 cd/A and external quantum efficiencies of 12% have been achieved. We will also report our latest studies on energy transfer and thermally-activated delayed fluorescence (TADF) in all-organic charge-transfer materials [5].
New design rules will be presented for colour tuning, enhanced efficiency and high stability of OLEDs.
[1] Review: Kamtekar, K. T.; Monkman, A. P.; Bryce, M. R., Adv. Mater. 2010, 22, 572-582.
[2] Al-Attar, H. A.; Griffiths, G. C.; Moore, T. N.; Tavasli, M.; Fox, M. A.; Bryce, M. R.; Monkman, A. P., Adv. Funct. Mater. 2011, 21, 2376-2382; Tavasli, M.; Moore, T. N.; Zheng, Y.; Bryce, M. R.; Fox, M. A.; Griffiths, G. C.; Jankus, V.; Al-Attar, H. A.; Monkman, A. P., J. Mater. Chem. 2012, 22, 6419-6428.
[3] Zheng, Y.; Batsanov, A. S.; Edkins, R. M.; Beeby, A.; Bryce, M. R., Inorg. Chem. 2012, 51, 290-297.
[4] Kozhevnikov, V. N.; Zheng, Y.; Clough, M.; Al-Attar, H. A.; Griffiths, G. C.; Abdullah, K.; Jankus, V.; Bryce, M. R.; Monkman, A. P., Chem. Mater. 2013, 25, 2352..
[5] Dias, F. B.; Bourdakos, K. N.; Jankus, V.; Moss, K. C.; Kamtaker, K. T.; Bhalla, V.; Santos, J.; Bryce, M. R.; Monkman, A. P., Adv. Mater. 2013, DOI:10.1002/adma.201300753.

In fluorescence-based organic light-emitting diodes (OLEDs), the internal electroluminescence (EL) quantum efficiency (eta;_int) is limited to the theoretical limit of 25%, while these OLEDs show high operational stability and high color purity. Here, we demonstrate an novel pathway for high-efficient fluorescence-based OLEDs by applying the general concept of thermally activated delayed fluorescence (TADF) [1-3] to a host layer with a small energy gap between singlet and triplet excited states.
After carrier recombination at a host molecule and formation of both singlet and triplet excitons, the triplet excitons are up-converted to the singlet state of TADF material through a reverse intersystem crossing (RISC) process because of the small energy gap between singlet and triplet energy levels (Δ_EST) of TADF molecules. Then, singlet excitons in the host layer are transferred to a fluorescent guest molecule via a Förster process, which then results in radiative decay from a singlet state of the guest fluorescence molecule. In this concept, the theoretical limitation of eta;int can be overcome using common fluorescent emitters, leading to eta;_int = 100 % in principle.
Here, we used tris(8-hydroxyquinolinato)aluminum (Alq₃) and 2-phenoxazine-4,6-diphenyl-1,3,5-triazine (PXZ-TRX) [2] as “conventional” and “up-conversion” host matrices, respectively. 2,8-Di[t-butyl]-5,11-di[4-(t-butyl)phenyl]-6,12-diphenylnaphthacene (TBRb) was used as a yellow fluorescent emitter. Based on this concept, the OLED containing the up-conversion host exhibited a significantly higher external EL quantum efficieny of 10.0 ± 1%, corresponding to 31 cd/A and 37 lm/W at 171 cd/m², compared with those of the OLED with an Alq₃ host (eta;_ext = 3.7 ± 1%, corresponding to 11 cd/A and 13 lm/W at 30 cd/m²).
References
[1] A. Endo, M. Ogasawara, A. Takahashi, D. Yokoyama, Y. Kato, C. Adachi, Adv. Mater. 21, 4802 (2009).
[2] H. Tanaka, K. Shizu, H. Miyazaki, and C. Adachi, Chem. Comm., 48, 11392 (2012).
[3] H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature, 492, 234 (2012).

Highly efficient ITO-free small molecule OLEDs (SMOLEDs) fabricated on multilayered highly conductive poly(3,4-ethylenedioxy thiophene): poly(styrenesulfonate) (PEDOT:PSS) as the anode are described. TheSMOLEDs&’ structure is glass/PEDOT:PSS/MoO3/MoO3 (10 wt.%):NPD/NPD/ Ir(ppy)3 (6 wt %):CBP/BPhen/LiF/Al, where NPD is N,N&’-diphenyl-N,N&’-bis(1-naphthyl-phenyl)-1,1&’-biphenyl-4,4&’-diamine, CBP is 4,4'-bis(9-carbazolyl)-biphenyl, Ir(ppy)3 is tris(2-phenylpyridine) iridium(III) and BPhen is 4,7-diphenyl-1,10-phenanthroline. The devices exhibited a peak power efficiency of 118 lm/W, which was 44% higher than that of similar ITO-based SMOLEDs, and without any outcoupling enhancing structures. The increase in the device performance is believed to be due to the significantly different weak microcavity effects in the different devices. Other ITO-free SMOLED architectures will also be discussed briefly.

Three novel carbazole-based host materials, 9&’-ethyl-9,3&’:6&’9”-(9CI)-Ter-9H-carbazole (ETC), 3,3”,6,6”-tetrakis(1,1-dimethylethyl)-9&’-(ethyl)-(9CI)-9,3&’:6&’9”-Ter-9H-carbazole (BETC) and 9&’-phenyl-9,3&’:6&’9”-(9CI)-Ter-9H-carbazole (PTC), are designed and synthesized. These hosts exhibit high triplet energy levels (2.90-3.02 eV) and high glass transition temperatures ( > 147 °C), which promises high efficiency and improved thermal stability compared with commonly used carbazole-based host material, 1,3-Bis(N-carbazolyl)benzene (mCP). Blue emitting phosphorescent organic lighting-emitting diodes (OLEDs) were fabricated with such host materials doped with the iridium(III) bis (4,6-difluorophenylpyridinato)-picolinate (FIrpic). The optimized device with ETC host showed a turn-on voltage of 2.9 V and a maximum efficiency of 48.4 cd/A at 260 cd/m² with a small reduction to 44.5 cd/A at 2600 cd/m². The PTC device showed a turn-on voltage of 3.1 V and a maximum efficiency of 35.4 cd/A at 200 cd/m², which only reduced to 33.9 cd/A at 2000 cd/m². Even though the turn-on voltage of BETC device was up to 5.1 V, the maximum efficiency reached 30.6 cd/A at 124 cd/m² with a reduction to 27.8 1240 cd/m². Device physics study indicated that not only the mobility of BETC is nearly 2 order magnitudes lower than that of ETC, but also the BETC device was carrier injecting limited.

Fluorenone based solution processable fluorophore with yellow emission was synthesized and spectroscopic studies on stable oligomers containing fluorenone and 9H-fluorin-9-ylidenemalononitrile showed a sharp change in emission and electrochemical properties. The fluorenone-containing trimer (FT) showed a high quantum yield and a complete quenching of fluorescence was observed for 9H-fluorin-9-ylidenemalononitrile containing trimer (CNFT). Theoretical investigations on the photophysical properties of these two oligomers were done to identify the origin of this remarkable change in the spectroscopic properties with the introduction of Dicyanovinylene (DCV) moieties in the system. The quenching of fluorescence of the DCV substituted analog is explained by spectral and computational studies. Further thin film of FT was characterized with AFM. Solution aggregation was studied using dynamic light scattering (DLS) and fluorescence life time studies. Further fluorophore FT was used to develop high quantum yield white light emitting polymers. Light emitting diodes (LEDs) fabricated with device configuration of ITO/PEDOT:PSS/polymer/Al showed good stability under ambient conditions.

New conjugated polymeric systems associating aryl imidazole structures (benzimidazole and thienoimidazole) with various common donors units like thiophene, fluorene, and benzodithiophene were synthesized. These systems are of particular interest due to the relative ease with which they may be converted to poly(ionomers) by treatment with acid (yielding p-doped cationic species) or base (yielding n-doped anionic species). This class of polymers presents the first opportunity to generate p- or n-type polymeric semiconductors from a single parent polymer, opening the door for universal conjugated polymer structures for plastic electronic applications. The majority of previously reported charged conjugated polymer structures are polyelectrolytes, containing an ionic component insulated on pendant chains.[1-3] Our work diverges from this motif to focus on the design of poly(ionomers) which bear their ionic character in direct conjugation with the polymeric backbone. Thus, we can explore the influence of delocalized charges on the properties of the materials.
Benzimidazole or thienoimidazole structures have been reported in a limited number of conjugated systems; generally, as an acceptor unit for electrochromic applications, charge transfer materials, or for photovoltaic materials.[4-6] We present the synthesis and the opto-electronic properties of the aforementioned series of neutral imidazole derivatives polymers; and the effect of the anionic or cationic conversion on the opto-electronic properties.
[1] Hoven, C. V.; Garcia, A.; Bazan, G. C. ; Nguyen, T. Q., Advanced Materials, 2008, 20, 3793
[2] Jian, H.; Taranekar, P.; Reynolds, J. R.; Schanze, K. S.; Angewandte Chemie International Edition, 2009, 48, 4300
[3] Schanze, K.S.; Shelton, A. H., Langmuir, 2009, 25, 13698
[4] Akpinar, H.; Balan, A.; Baran, D.; Ünver, E. K.; Toppare, L., Polymer, 2010, 51, 6123
[5] Balan, A.; Baran, D.; Toppare, L.; Polymer Chemistry, 2011, 2, 1029
[6] Sond, S.; Park,S. H.; Jin, Y. ; Park,J. ; Shim, J. Y.; Kim,I.; Lee, H.; Lee,K.; Suh,H. Journal of Polymer Science Part A: Polymer Chemistry, 2010, 48, 4567

Porphyrin molecules are a group of organic compounds that exist extensively in natural biological systems, and they manifest rich electronic and photonic properties, raising the possibility of optoelectronic device applications. In this research thiolated Zinc-porphyrin oligomer molecules were linked to Au(111) surface in a vertical device configuration, embedded within an alkanethiol self-assembled monolayer. Spectroscopic measurements were done to determine the electronic orbital structures of different Zinc-porphryin oligomer single molecules via scanning tunneling microscope. With lasers of different wavelength coupled to the tunneling junction, both ground state and excited state electronic orbital structures of Zinc-porphyrin single molecule were measured.

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

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

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

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

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

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

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

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

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

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

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

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

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

Nickel oxide (NiOx) has been found to be effective as a hole transport layer in standard architecture organic solar cells. 1 However, the organic/inorganic interfacial energy level alignment needs to be optimized for a given active layer. For example, the ionization energy of the PCDTBT polymer is over 0.4eV larger than the untreated NiOx work function, which leads to open circuit voltage loss in devices. To address this, an O2 plasma treatment is usually applied to increase the NiOx work function, but it is commonly seen that the plasma treated metal oxides surfaces can be unstable and the increased work function degrades rapidly over a short period of time.2,3 By contrast, molecular monolayer modification can provide a more stable and controlled alteration.2,3,4 In particular, the triethoxysilane (TES) chemistry provides a covalent attachment scheme and it&’s demonstrated that the TES attachment can tune the ZnO work function effectively.5 In this study, three TES based molecules with different terminal groups, namely phenyltriethoxysilane (PTES), octadecyltriethoxysilane (OTES) and 4-chlorophenyltriethoxysilane (4CPTES), are utilized to tune the energy level alignment at the NiOx/polymer interface by introducing dipoles that form a molecular layer and change the work function of the solution deposited NiOx. Contact angle (CA) measurements show that OTES treated surfaces are much more hydrophobic than the untreated surface, with the CA increasing from 42.5°± 1.4 to 89.5° ± 1.3. The other TES molecules also show a CA increase, which indicates the successful attachment of these molecules to the surface. Infrared spectroscopy shows that the coverage is sub-monolayer, consistent with our previous studies of other metal oxide surfaces. Kelvin probe measurements show that all three TES treatments increase the NiOx work function compared to bare, untreated NiOx in the following order: PTES (242mEV), OTES (302meV), 4CPTES (450meV). To test the impact of TES layers on organic/inorganic energy level alignment, standard bulk heterojunction devices were fabricated with PCDTBT/PC71BM blend as active layer. We find that the open circuit voltage improves with increasing work function of the TES-treated surfaces.
We acknowledge support of the NSF through grant DMR-0907409 and the Renewable Energy Materials Research Science and Engineering Center (REMRSEC) through DMR-0820518. SRC acknowledges funding from the Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Fellowship through the Sunshot Solar Energy Technologies Program.
1. Steirer, K. X. et al, Adv. Energy Mater. 2011, 1, 813-820
2. Sharma, A. et al, Appl. Phys. Lett. 2008, 93, 163308
3. Cook, R. M. et al, Adv. Energy Mater. 2011,1, 440-447
4. Allen, C.G. et al, Langmuir 2008, 24, 13393-13398
5. Allen, C. G et al, J. Phys. Chem. C 2012, 116, 8872-8880

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Recently, organic π-conjugated small molecules have been studied extensively for use in organic photovoltaics (OPVs). Simple chemical modifications to the parent compound has been shown to be an effective alternative to modifying their properties, which ultimately affect performance when these compounds are incorporated into functional devices. The isoindigo moiety has been introduced as an electron acceptor unit in low bandgap small molecules and polymers, resulting in high performance OPVs. As a promising electron acceptor moiety, it is important to understand its effects on the chemical, physical and electronic properties of the organic semiconductor. In this contribution, we systematically investigated the processing-structure-function relationships of a new series of model compounds containing the isoindigo moiety. Our study reveals that different placement of aryl substitutions (e.g., meta- versus para-) can give rise to dramatically different properties. Specifically, meta-substituted derivatives generally exhibit stronger charge-transfer bands in both solution and the solid state, likely due to the more extended π-conjugation spanning the isoindigo core and the meta-substituted aryl groups (benzothiophene and benzothiazole). In the solid state, para-substituted derivatives show a blue shift in its optical absorption compared to in solution, probably due to the formation of H-aggregates. Remarkably, the presence of a bulky ethylhexyl side chain on the meta-benzothiophene substituted derivative induces a significant red shift in its solid-state absorption upon thermal annealing, stemming from J-aggregation due to donor-acceptor interactions between benzothiophene and the isoindigo core. Grazing-incidence x-ray diffraction and near-edge x-ray absorption fine structure spectroscopy revealed that that the meta-substituted derivatives preferentially adopt a face-on orientation in the solid state, coinciding with the more desirable orientation for vertical charge transport between electrodes in OPVs. These isoindigo-based model compounds were incorporated into bulk-heterojunction solar cells as electron donors with [6,6]-phenyl C71 butyric acid methyl ester as the electron acceptor. Devices comprising the meta-benzothiophene substituted derivative, in particular exhibited a power-conversion efficiency that exceeded 1.4%, stemming from enhanced light absorption and preferential face-on orientation in the solid state. Based on this study, new extended systems were further designed to optimize for ease of processing and enhanced function. Particularly, alkyl chains were introduced to the extended conjugated core to induce self assembly. Devices comprising this second-generation derivative as electron donor exhibited improvements in device performance (Voc = 0.90 V, Jsc = 8.74 Am/cm-2, PCE = 3.40%) without the need for post-deposition processing (thermal/solvent annealing).

Here we present our recent work on the development of new electron-deficient structures that can be utilised not only as a component in push-pull type donor materials for organic photovoltaics, but also as the central component of non-fullerene electron acceptors for organic photovoltaics. We will discuss how the development of larger fused structural motifs comprising several well-studied units such as thiadiazole and pyrrolodione affects the electron-accepting properties and how these novel materials can be incorporated into promising donor and acceptor materials for organic solar cells.

This talk will discuss the effect of non-conjugated sidechains on the photophysics and solid-state assembly of π-conjugated oligomers and polymers. Bathochromic shifts in excess of 0.1 eV occur in the solution state spectra of terephthalate ester-containing donor-acceptor phenylene ethynylene oligomers upon increase of the inductive power of the ester moiety. This effect persists when applied to polymeric systems. Furthermore, the terephthalate ester provides appropriate geometry for backbone-sidechain aromatic interactions that can be manipulated to enforce or hinder backbone planarity in the solid state, resulting in bathochromic or hypsochromic shifts, respectively. Upon application to polymeric systems, the appropriate choice of ester sidechain hinders polymer aggregation. This work provides new techniques for band gap control and solid-state assembly in π-conjugated materials.

Hierarchically structured coordination oligomers that are linked covalently to a strongly electron-accepting moiety [e.g. fullerene (C60) or polyoxometalate (POM) entities] are in the focus of interest, since these photoactive materials are reminiscent of green plants photosystem (PS-II). Long-living charge-separated states play a crucial role in many natural processes, e.g. photosynthesis. Artificial systems mimicking nature&’s examples are of relevance for the development of applications related to light-to-energy conversion (i.e. photovoltaics, photocatalysis).1,2,3 In order to enable highly directed transfer processes within linear ensembles, we rely on established rigid terpyridine-based systems and the coordination chemistry associated with them.4,5 Upon light absorption energy and/or electrons will be transported along a hierachically structured coordination oligomer, functioning as a molecular antenna, to the electron sink. The linear coordination oligomers comprise combinations of terpyridine complexes with IrIII, RhIII, RuII, OsII and PtII ions. In an appropriate hierachical arrangement, e.g. in the triad IrIII - RuII - OsII, the transfer process will be highly directional. In order to fundamentally understand the transfer processes in such assemblies, the synthesis of appropriate molecular subunits and the spectroscopic characterization thereof is of utmost importance. In this contribution, we report on the synthesis and spectroscopic investigation of terpyridyl-functionalized C60 and POM derivatives as well as various mono-, di- and trinuclar model complexes.
1 L. Hammarström and O. Johansson, Coord. Chem. Rev., 2010, 254, 2546.
2 E. A. Medlycott and G. S. Hanan, Chem. Soc. Rev., 2005, 34, 133.
3 R. Sibert, A. Winter, M. Schmitt, J. Popp, U. S. Schubert, and B. Dietzek, Macromol. Rapid. Commun., 2012, 33, 481.
4 U. S. Schubert, A. Winter, and G. R. Newkome, Terpyridine-based Materials, Wiley-VCH, Weinheim, 2011.
5 A. Wild, A. Winter, F. Schlütter, and U. S. Schubert, Chem. Soc. Rev., 2011, 40, 1459.

Exciton fission is a process whereby one singlet exciton splits into two independent triplets. Because fission generates two triplet excitons from a single high energy photon, fission-based solar cells can produce quantum yields in excess of 100% and could lead to single junction photovoltaics with power conversion efficiencies above 40%. Here, we measure fission dynamics using ultrafast photoinduced absorption and derive a first principles expression that successfully predicts the rate of fission for a range of materials with vastly different structures. Our results show that the experimental rates are consistent with a nonadiabatic Marcus-like mechanism in weakly interacting systems and an adiabatic, coupling independent pathway at larger interaction strengths. For a range of electronic couplings covering almost three orders of magnitude, we predict near unit fission efficiency in any material where fission is energetically favored. This is confirmed experimentally, as we observe high fission yields even in materials where molecules are oriented orthogonal to one another at large separations (>5 Å). We conclude that singlet exciton fission in thin films is robust against variations in molecular structure. The success of this kinetic model simplifies the rational design of materials capable of fission. Crucial molecular properties such as solubility and energy level alignment can be safely tailored by functionalizing an active core while maintaining a high quantum yield.

Organic photovoltaics (OPVs) have become an active area of research in both academia and industry in recent years because of the demand for renewable and clean energy sources. Fullerene derivatives such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) have been employed as typical acceptor semiconductors in OPVs. It has been suggested that fullerene derivatives participate in not only electron transportation but also hole transportation in donor polymer/PC61BM blend films. However, the use of PC61BM as the donor material remains a challenge. To allow PC61BM to act as a donor material, the electron-transporting pi-conjugated system, which has a lowest unoccupied molecular orbital (LUMO) energy level much lower than that of PC61BM, was used as the acceptor material. As a result, for the first time, we successfully revealed the function of PC61BM as an electron donor in bulk heterojunction (BHJ) solar cells.

We have developed a great deal of chemistry around phosphorescent Iridium and Platinum complexes for monochromatic and white OLEDs. These materials give LEDs with efficiencies nearing the theoretical limits. In this talk will highlight our most recent results with Copper based phosphors for electroluminescence. Copper based materials offer the possibility of using lower cost materials, but are typically less stable than their Ir counterparts and have longer excited state lifetimes. By carful ligand and metal complex design we have achieved efficient emission that spans the visible spectrum. We will discuss the use of these materials in OLEDs as well as for white light production by down-conversion.

Triplet and singlet states, along with their interactions, are responsible for the emissive properties as well as their losses in organic light emitting devices (OLEDs). Among the various effects that employ these quantum states are triplet and singlet emission, triplet-triplet, triplet-singlet, and triplet-polaron annihilation. Indeed, triplet-triplet annihilation (TTA) has been identified as a significant source for efficiency roll-off at high brightness (> 1000 cd/m2) in phosphorescent organic light emitting diodes (PHOLEDs). To maintain the PHOLED efficiency at high brightness, the TTA rate must be minimized while maximizing the radiative decay rate. Furthermore, triplet losses lead to the quenching of laser emission in optically pumped organic lasers[1]. Finally, exciton interactions can lead to the reduction of operational lifetime in PHOLEDs, particularly for those devices that emit high energy (e.g. blue) photons[2]. In all cases, excitonic interactions significantly impact the performance and possibilities of OLEDs for use in high intensity applications, including in lighting and lasing. However, by appropriate design of emissive regions and choice of materials, these effects can be mitigated or even eliminated. We will discuss the basic concepts of exciton interactions with other excitons as well as charge, and how to engineer the materials and structures to fully exploit and optimize the emission in high intensity organic devices.
[1] Y. Zhang and S. R. Forrest, "Continuous-wave threshold exists for organic semiconductor lasers," Phys. Rev. B, vol. 84, p. 241301, 2011.
[2] N. C. Giebink, B. W. D&’Andrade, M. S. Weaver, P. B. Mackenzie, J. J. Brown, M. E. Thompson, and S. R. Forrest, "Intrinsic luminance loss in phosphorescent small-molecule organic light emitting devices due to bimolecular annihilation reactions," J. Appl. Phys., vol. 103, p. 044509, 2008.

Electroluminescent dendrimers are an important class of light-emitting materials, which can be both well-defined structure like small molecules and solution processing properties like macromolecules, so it can be as the third class of light-emitting materials for use in PLEDs. The electrophosphorescent polymers (PhPs), which the phosphors are incorporated into the polymeric main chain or side chain via covalent bonds, have attracted considerable interest since they can not only harvest both singlet and triplet excitons to achieve nearly 100% internal quantum efficiency, but also be fabricated by low-cost solution processes, for preventing the phase segregation observed in the physical blends of polymer hosts and phosphors.
This presentation will introduce recent advances in electroluminescent dendrimers and polymers towards the potential application in full color display (Polymer light-emitting devices-PLEDs).
The presentation will discuss the following topics with most updated results: (A) Bipolar polymer host for blue and white phosphorescence with fluorinated poly(arylene ether phosphine oxide) as backbone; (B) Red phosphorescent polymers using polyfluorene as polymer host; (C) Dendrimer host for blue and white electrophosphorescence based the carbazole-based dendron as building block.
References:
1. Zhihua Ma, Junqiao Ding, Baohua Zhang, Chongyu Mei, Yanxiang Cheng, Zhiyuan Xie, Lixiang Wang, Xiabin Jing, Fosong Wang, Adv. Funct. Mater., 2010, 20, 138.
2. Zhihua Ma, Lingcheng Chen, Junqiao Ding, Lixiang Wang, Xiabin Jing, Fosong Wang, Adv. Mater., 2011, 23, 3726.
3. Shiyang Shao, Junqiao Ding, Zhiyuan Xie, Lixiang Wang, Xiabin Jing, Fosong Wang, Adv. Mater., 2011, 23, 3570.
4. Shiyang Shao, Zhihua Ma, Junqiao Ding, Lixiang Wang, Xiabin Jing, Fosong Wang, Adv. Mater., 2012, 24, 2009.
5. Shiyang Shao, Junqiao Ding, Lixiang Wang, Xiabin Jing and Fosong Wang, J. Am. Chem. Soc., 2012, 134, 20290.
6. Shiyang Shao, Junqiao Ding, Lixiang Wang, Xiabin Jing and Fosong Wang, J. Am. Chem. Soc., 2012, 134, 15189.

We have designed and synthesized a carbazole based bipolar host material with thermally stable and high quantum efficiency in blue Phosphorecent organic light emitting diodes. Substitution of electron withdrawing groups at asymmetric position of carbazole is effective to improve the thermal stability and device performances of blue PHOLEDs. The (9-phenyl-9H-carbazole-2,5-dyil)bis(diphenylphosphine oxide) (PCPO25) showed high triplet energy of 2.81eV which has enough for transfer to blue emitting iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2 ]picolinate. It also showed good thermal and morphological stability up to 140 oC, world best high quantum efficiency of 31.4% and power efficiency 53.1 lm/W.

Over the last two decades, traditional inorganic semiconductor device physics has guided the OLED research community in device structure design and fabrication. Various layers of functional molecules have been progressively introduced, hoping to boost the efficiency. Many of these multilayered device structures have worked well, in particular for low luminance applications such as small size displays. However, efficiency roll off (similar to efficiency droop in LED) at high luminance and high injection current become a major challenge for OLED lighting applications. In this talk I will review fundamental problems in these multilayered devices and it correlation with efficiency droop. In order to reduce or eliminate this efficiency droop, considerations in design of device structure and in selection of materials will be discussed.

In recent years, organic light-emitting diodes (OLEDs) entered the commercial sector, especially for mobile display applications. For lighting applications, first commercial products have been launched with an emphasis on design and high-end luminaires. However, there is still much room for improvement in terms of efficiency and long-term stability during electrical operation. One important shortcoming is the lack of stable phosphorescent emitting systems in the deep-blue color range. Hence, alternative approaches such as fluorescent emitters using triplet-to-singlet up-conversion may serve as promising alternatives. Previous publications have demonstrated that it is possible to enhance the fluorescent radiative exciton fraction using effects such as triplet-triplet-annihilation (TTA) and thermally activated delayed fluorescence (TADF). However, the relevant singlet exciton fraction has only been extracted indirectly so far, mostly from an enhancement of the external quantum efficiency (EQE) beyond the value of 25% compatible with the spin statistical limit.
The external quantum efficiency (EQE) of an OLED is determined by four factors, namely the charge carrier balance (γ), the radiative exciton fraction (eta;r), the effective radiative quantum efficiency (qeff) and the light outcoupling factor (eta;out). While eta;out can be strongly enhanced by horizontally oriented transition dipole moments of the emitting species, eta;r of fluorescent dyes can exceed the limit of 25% by e.g. thermally activated delayed fluorescence.
Thus a method allowing for an independent determination of these four factors is needed to provide consistent results in a comprehensive efficiency analysis. This can be achieved by a systematic variation of the cavity strength at the emitter&’s position inside the OLED by layer thickness variation. A subsequent combination of EQE investigations with time resolved photo- and electroluminescence measurements gives self-consistent access to qeff and eta;out. Furthermore the product of γ and eta;r can be determined, which results in a lower limit of the additionally created singlet excitons by TADF without any assumptions of e.g. the reverse intersystem crossing rate of the emitting species.
For the fluorescent system under investigation the efficiency is boosted by two effects. First, due to a horizontal alignment of the transition dipole moments of the emitting molecules, the outcoupling factor is enhanced by a factor of 1.3. Second, the lower limit for the radiative exciton fraction was determined to 36%, resulting in an additional efficiency increase by a factor of 1.44. As a consequence of the combination of both effects the EQE almost doubles and values up to 5% are achieved for direct emission in spite of a comparatively low q value of 45% only.

We investigated the organic-organic interfaces in solution processed OLEDs by non-destructive neutron reflectometry[1] with angstrom resolution. The two-layer thin films, deuterated carbazole derivative (CBP) doped with Ir(ppy)3 on hole transport fluorene-containing polymer (TFB), were formed. d16-CBP was used in order to enhance the difference of the refractive indices between TFB and CBP doped with Ir(ppy)3 layers. d16-CBP doped with Ir(ppy)3 layers were spin-coated with using two solvents, 1, 4-dioxane or cyclopentanone. In UV absorption measurement, reduction of the thickness of TFB after spin-coating of the solvents was not observed. However, by neutron reflectometry measurements, we found the mixed interfaces of spin coated films formed by 1, 4-dioxane and cyclopentanone solvents, and their thicknesses were 6.0 nm and 10.5 nm, respectively. The thicker mixed interface by cyclopentanone is resulted from the stronger interaction to TFB than that of 1, 4-dioxane. Ir(ppy)3 and d16-CBP in solution may be soaked into the thin surface TFB layer although TFB itself is not soluble in the solution. We also investigated how these interface structures affect the performances of the OLEDs fabricated by solution-process.

In phosphorescent organic light emitting diodes (PHOLED), the emissive triplet can undergo triplet-triplet annihilation (TTA) and triplet polaron annihilation (TPA) [1], leading to high brightness efficiency roll-off. Previously, TTA and TPA were described by empirical triplet density dynamics based on triplet diffusion, leading to annihilation rates proportional to the square of the triplet density (for TTA) and the product of triplet and polaron density (for TPA). In this study, we develop an analytical model that considers both triplet diffusion and triplet-to-triplet capture (or annihilation). The model uses a triplet density function approach to accurately describe the spatially and temporally dependent triplet dynamics. The improvement of the new model from the empirical model is confirmed through Monte Carlo simulation of triplet diffusion and triplet-to-triplet capture.
Applying this model to PHOLED efficiency roll-off, we find the cumulative TTA dynamics is dominated by diffusion, i.e. triplets spend significantly longer time diffusing than capturing each other. Further, the diffusion results from a Dexter type energy transfer from the triplet state to ground state. As a result, we propose a new phosphorescent emitter design with a large Stokes shift between the absorption and emission, which can lead to mitigated TTA rate and thus improved PHOLED efficiency at high brightness.
[1] M. A. Baldo, C. Adachi, and S. R. Forrest, Phys. Rev. B 62, 10967 (2000).

Semiconductor quantum dots (QDs) are promising as emissive materials for light emitting devices because of their size-tunable band gaps, high photoluminescence (PL) quantum yield and saturated colors. Until now, almost all research related to QD based light emitting devices has been directed towards light emitting diodes (LEDs). However, because the QDs in LEDs have to be either blended with conjugated polymers or incorporated with hole and electron transport materials, the electroluminescence is usually accompanied by emissions from host polymers or cladding layers. Light emitting electrochemical cells (LECs) are an alternative choice that can circumvent those issues. The LECs typically use single layer structures and the single active layer contains a blend of luminescent materials and ion-transport electrolytes. In our work, size tunable QDs and the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) were used as luminescent materials and electrolytes, respectively. Polyvinylcarbazole (PVK) was used as a polymer matrix to make uniform and smooth active layers. The performance was evaluated using the device configuration: ITO/PEDOT:PSS/active layer/Al. Only color saturated electroluminescence from QDs was observed, without any other emissions from a polymer host or the electrolyte. By tuning the size of the QDs, blue, green and red QD-LECs were demonstrated by using the same device structure and procedures. The maximum brightness (> 300 cd/m2) and current efficiency (1.8 cd/A) are comparable to the polymer LECs and multilayer QD-LEDs. In addition, by utilizing silver nanowire composite electrode as cathode, transparent QD-LECs were demonstrated with average transmittance higher than 88 % over the whole visible range. Moreover, by replacing the glass substrate with flexible polyethylene terephthalate (PET) substrate, flexible and transparent devices were demonstrated, indicating its potential for unique applications.

Photochromic molecules provide an intriguing and relatively untapped alternative to traditional materials utilized in organic memory devices. We have recently reported on a new prototype of a nonvolatile light-emitting organic memory (LE-OMEM) that integrates a layer of crosslinkable dithienylethene photochromes (XDTE) into a solution-processed, multilayer OLED. The XDTE molecules undergo a change in both their UV-visible absorption and energy level position due to a photo- and/or electrically-induced ring-opening/-closing reaction. Exploiting the difference in HOMO and LUMO energies of both isomers and the subsequent change in hole-injection barrier we use this XDTE layer as an electrical switch within our OLED layer stack. Optimized devices have displayed ON/OFF ratios in both current and electroluminescence of greater than 10.000. We investigate both optical and electrical programming of the OMEM devices and show that precise control of the ratio of both isomers in the active layer enables access to a multitude of intermediate states demonstrating the potential of these devices for future multilevel memory applications. We also discuss the difference in the molecular-scale mechanisms that are responsible for the optically- and electrically-induced switching effect in these devices by in-situ monitoring of the fraction of closed molecules as a function of the external stimulus.

Light-emitting electrochemical cells (LECs) are one of the simplest type of molecular electroluminescent devices. Due to their peculiar mechanism LECs can be prepared from solution and operate with air stable electrodes making them suitable for low cost and large area lighting applications.1-3 In its simplest form they consist of a single active layer composed of an ionic transition-metal complex (iTMC) which supports all three processes of charge injection, charge transport and emissive recombination.2-5 Using iTMC LECs reasonable lifetimes in excess of 5000 hours at 600 cd mminus;2 combined with sub-second turn-on, making these devices interesting for lighting applications.6 Recently we showed that LECs based on polymers and those using ionic transition metal complexes are two members of one class of devices. Their behavior is controlled by the ionic conductivity in the active layers.7
Here we demonstrate tandem LECs based on a bottom polymer LEC and a top iTMC based LEC emitting white-light. Furthermore, we show that these devices are very suitable for the preparation of tandems as they do not require intentionally doped layer adjacent to- or as the charge generation layer.
1. Q. Pei, G. Yu, C. Zhang, Y. Yang and A. J. Heeger, Science 269, 1086-1088 (1995).
2. E. S. Handy, A. J. Pal and M. F. Rubner, J. Am. Chem. Soc. 121, 3525-3528 (1999).
3. J. D. Slinker, J. Rivnay, J. S. Moskowitz, J. B. Parker, S. Bernhard, H. D. Abruña and G. G. Malliaras, J. Mat. Chem. 17, 2976-2989 (2007).
4. K. M. Maness, R. H. Terrill, T. J. Meyer, R. W. Murray and R. M. Wightman, J. Am. Chem. Soc. 118 (43), 10609-10616 (1996).
5. R. D. Costa, E. Orti, H. J. Bolink, F. Monti, G. Accorsi and N. Armaroli, Angew. Chem. Int. Ed. 51, 8178-8211 (2012).
6. D. Tordera, S. Meier, M. Lenes, R. D. Costa, E. Orti, W. Sarfert and H. J. Bolink, Adv. Mater. 24 (7), 897-900 (2012).
7. S. van Reenen, T. Akatsuka, D. Tordera, M. Kemerink and H. J. Bolink, J. Am. Chem. Soc. 135 (2), 886-891 (2013).

Lighting accounts for more than 10% of the worldwide energy consumption which motivates the development of energy-saving solutions such as organic light-emitting diodes (OLEDs). However, OLED fabrication consists of a multi-layer process that involves evaporation steps and requires encapsulation.[1] In this context light-emitting electrochemical cells (LECs) are a promising alternative.[2-3] LECs can be prepared from solution and due to their operating mechanism can be operated with air stable electrodes making them suitable for low-cost lighting applications.[4]
However, despite the increase of performance in LEC devices,[5-6] the stability of green and blue LECs is still subpar.[7] In order to make LECs suitable for the lighting market the issue of the stability has to be addressed. In this work, the origins of the instability of green LECs are studied by means of a careful emitter design, theoretical calculations and device preparation and characterization. A strong dependence of the stability and the substituent groups attached to the emitter is found. Alternatives to chemically blue-shift the emission are proposed and an optimized driving method is employed in order to increase the stability of the devices.
[1] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem, K. Leo, Nature 2009, 459, 234.
[2] Q. B. Pei, G. Yu, C. Zhang, Y. Yang, A. J. Heeger, Science 1995, 269, 1086.
[3] J. D. Slinker, J. Rivnay, J. S. Moskowitz, J. B. Parker, S. Bernhard, H. D. Abruña and G. G. Malliaras, J. Mat. Chem. 2007, 17, 2976.
[4] R. D. Costa, E. Ortí, H. J. Bolink, F. Monti, G. Accorsi, N. Armaroli, Angew. Chem. Int. Ed. 2012, 51, 8178.
[5] D. Tordera, S. Meier, M. Lenes, R. D. Costa, E. Orti, W. Sarfert, H. J. Bolink, Adv. Mater. 2012, 24, 897.
[6] D. Tordera, A. Pertegás, N. M. Shavaleev, R. Scopelliti, E. Ortí, H. J. Bolink, E. Baranoff, M. Grätzel, M. K. Nazeeruddin, J. Mater. Chem. 2012, 22, 19264.
[7] D. Tordera, M. Delgado, E. Ortí, H. J. Bolink, J. Frey, M. K. Nazeeruddin, E. Baranoff, Chem. Mater. 2012, 24, 1896.

Light Emitting Electrochemical Cells (LEECs) from ionic transition metal complexes (iTMCs) are under development as low cost light sources. However, these devices have yet to achieve the stringent operational benchmarks required for lighting applications, particularly in the areas of luminance and response time. We used an archetypal iridium iTMC as the emissive material in LEECs and blended in alkaline additives to control ionic space charge effects and substantially improve performance. For lithium additives, the turn-on time was drastically reduced from hours to seconds, the maximum luminance was increased to practical lighting levels, and the lifetime was preserved. We have also studied other alkaline salts and justified their relative impact on device performance in view of double layer charging. These observations suggest that iTMCs from LEECs have the potential to serve as bright, long-lasting light sources.

Certain types of phenylene and thiophene based oligomers can self-assemble via surface growth into crystalline nanofiber structures. These nanofibers exhibit functional optical properties such as polarized luminescence and optical waveguiding. We have recently demonstrated that the nanofibers can be integrated in field-effect transistor devices via a roll-printing method [1] and can function as the active element in field-effect transistors [2] and light-emitting transistors [3]. In this work, we have investigated the use of nanofibers made from naphthyl end-capped bithiophenes (5,5-bis(naphthyl)-2,2prime;-bithiophene or ‘NaT2&’) [4] in organic phototransistors (OPTs). The device performance of OPTs based on crystalline NaT2 nanofibers has been compared with that of a thin film OPT with inferior molecular ordering both under white light and spectrally resolved illumination. The significantly better photoresponsivity of the nanofiber-based OPT demonstrates that the improved crystallinity leads to superior device performance. This combined with the possibility of tuning the band gap of the molecular constituents via chemical synthesis techniques therefore opens up the route to custom-designed, high-performance nanoscale light detectors.
[1] L. Tavares et al. Small, 7, 2460 (2011)
[2] L. Tavares et al. Nanoscale Res. Lett. 6, 319 (2011)
[3] L. Tavares et al. Nanotechnology, 23, 425203 (2012)
[4] X. Liu et al. Org. Electron. 11, 1096 (2010)

Organic thin-film electroluminescent (EL) devices, such as organic light-emitting diodes (OLEDs), typically operate using constant voltage or direct current (DC) power sources. Such approaches require power converters (introducing power losses) and make devices sensitive to dimensional variations that lead to run away currents at imperfections. Devices driven by time-dependent voltages or alternating current (AC) may offer an alternative to standard OLED technologies. However, very little is known about how this might translate into overall performance of such devices. Here, we demonstrate a solution-processed route to creating highly efficient AC field-induced polymer EL (FIPEL) devices. Such solution-processed FIPEL devices demonstrate maximum luminance, current efficiency, and power efficiency of 3,000 cd m-2, 15.8 cd A-1, and 3.1 lm W-1 for blue emission, 13,800 cd m-2, 76.4 cd A-1, and 17.1 lm W-1 for green emission, and 1,600 cd m-2, 8.8 cd A-1, and 1.8 lm W-1 for orange-red emission. The high luminance and efficiency, and solution process pave the way to industrial roll-to-roll manufacturing of solid state lighting and display.

Organic light-emitting devices (OLEDs) are expected to be adopted as the next generation of general lighting because they are more efficient than fluorescent tubes and are mercury free. The theoretical limit of operating voltage is generally believed to be equal to the energy gap, which is corresponding to the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for the emitter molecule divided by the electron charge, e. Here we demonstrate green OLEDs operating below a theoretical limit of the energy gap (Eg) voltage with high external quantum efficiency over 20% by using fac-tris(2-phenylpyridine)iridium(III) with a peak emission wavelength of 523 nm, which is equivalent to a photon energy of 2.38 eV. An optimized OLED operates clearly below the theoretical limit of the Eg voltage at 2.38 V showing 100 cd m-2 at 2.25 V and 5000 cd m-2 at 2.95 V without any light outcoupling enchancement techniques.

Organic light-emitting diodes (OLEDs) are promising large-area light sources on their way to commercialization. However, there is still room for improvement in terms of light outcoupling efficiency and long-term stability under electrical operation.
The external quantum efficiency (EQE) of OLEDs is significantly less than 100% since only a small fraction of the consumed electrical power is converted into visible light that is finally extracted to air. Most of the efficiency loss is caused by suboptimal radiative quantum efficiency (RQE) of the emitting guest-host system and by dissipating a huge part of the radiated energy to optical modes such as surface plasmons or waveguided modes, which cannot easily be extracted by common outcoupling schemes. In order to increase the EQE of OLEDs new approaches are needed [1].
Recent studies show that light outcoupling can be enhanced considerably by horizontally oriented emitters; a feature that is well known for fluorescent emitters [2] and has lately been demonstrated in phosphorescent state-of-the-art OLEDs [3]. We identify non-isotropic emitter orientation by a thorough efficiency analysis of OLED stacks with systematically varied thickness [4]. We show that in order to achieve a consistent analysis, it is indispensable to account for possible deviations from randomness. Ignoring these orientation effects leads to a significant misinterpretation of the RQE and other factors, which determine the EQE of a device. Furthermore, state-of-the art emitter-matrix combinations will be used to demonstrate the potential for improving the efficiency of OLEDs in this way.
Related publications:
(1) Device efficiency of organic light-emitting diodes: Progress by improved light outcoupling
W. Brütting, J. Frischeisen, T.D. Schmidt, B. J. Scholz, C. Mayr
phys. stat. sol. A 44 (2013) 44-65, DOI 10.1002/pssa.201228320
(2) Increased light outcoupling efficiency in dye-doped small molecule organic light-emitting diodes with horizontally oriented emitters
J. Frischeisen, D. Yokoyama, A. Endo, C. Adachi, W. Brütting
Organic Electronics 12 (2011) 809-817
(3) Oriented phosphorescent emitters boost OLED efficiency
M. Flämmich, J. Frischeisen, D.S. Setz, D. Michaelis, B.C. Krummacher, T.D. Schmidt, W. Brütting, N. Danz
Organic Electronics 12 (2011) 1663-1668
(4) Evidence for non-isotropic emitter orientation in a red phosphorescent organic light-emitting diode and its implications for determining the emitter&’s radiative quantum efficiency
T. D. Schmidt, D. S. Setz, M. Flämmich, J. Frischeisen, D. Michaelis, B. C. Krummacher, N. Danz and W. Brütting
Appl. Phys. Lett. 99 (2011) 163302

Green polymer light-emitting diodes (PLEDs) employing a polymer nanocomposite conductor as the transparent anode have been fabricated with high light extraction efficiency. The nanocomposite electrode is comprised of a layer of single wall carbon nanotubes and a layer of silver nanowire stacked and embedded in the surface of a polymer substrate. A small amount of barium titanate nanoparticles is dispersed in the substrate to suppress the total internal reflection of light, a major cause of the low external quantum efficiency (EQE) of organic light emitting diodes. The green PLEDs have a current efficiency of 118 cd/A at 10,000 cd/m2, measured in front viewing direction. The maximum EQE is 38.9%. The light extraction efficiency of the green PLEDs are 146% higher than those of the corresponding control devices fabricated on indium-tin oxide/glass. Finite-difference time-domain (FDTD) simulations indicate that the enhancement factor can be further increased to 246%. Both the silver nanowires and barium titanate nanoparticles in the composite electrode contribute to the enhanced light extraction efficiency.

We present polymer LEDs constructed onto ultrathin plastic foil substrates, with total thickness (substrate and device) of only 2 mu;m. The thin substrate reduces the bending strain on the active PLED layers, and allows for bending to radius of curvature below 10 mu;m without damaging the device. Thus, we demonstrate the extreme light-weight and flexibility often cited as advantages of polymer semiconductor technologies. They can be operated as free-standing ultrathin films allowing for crumpling during device operation. Or they may be applied to almost any surface whether rigid or elastomeric, and can withstand the associated mechanical deformation. In particular, we demonstrate 100% tensile strain when applied to an elastomeric membrane. The devices display brightness over 100 cd/m^2, and can be made in multiple colors. Such ultrathin light-emitting foils highlight many of the functional aspects of PLED materials, and constitute a step towards integration with malleable materials like textiles and artificial skin.

One of the ways to improve quantum efficiency, QE, of OLED is to use phosphorescent emitters such as iridium complexes that enable to achieve the internal QE of 100 percent. We have synthesized a variety of wide gap, or high T1 level, materials for host and charge-transporting layers and succeeded to fabricate extremely high efficiency OLEDs. High external QE of 25—30 percent was achieved for blue, green and red OLEDs, which correspond to the internal QE of nearly 100 percent. In addition, we have developed electron injection materials to reduce the drive voltages. Using Lithium complexes as electron injection layer combined with pyridine-containing electron transporting layer, extremely low drove voltages were achieved. In order to achieve long device lifetime at high luminance levels, which is required for general lighting applications, the multiphoton-emission (MPE) structures composed of multiple emissive units were developed. In this type of device, external quantum efficiency can be over 100 percent. For example, a green phosphorescent MPE OLED exhibited an internal QE of 220 percent. A blue OLED showed an external QE of over 40 percent without any out-coupling enhancement. We also developed MPE OLEDs having two emitting units by spin-coating. The charge generation layers were composed of MoO3 which were vacuum-evaporated. The MPE device exhibited doubly higher EQE than that of conventional unstacked solution-processed single OLED device. Recent progress in white OLEDs will be discussed.

The realization of high efficiency in organic light-emitting devices (OLEDs) requires the effective formation and confinement of excitons. Consequently, the position and spatial extent of exciton recombination in OLEDs is an important device parameter that can impact the efficiency of optical outcoupling as well as the prevalence of bimolecular exciton quenching processes under high injection. In this work, a technique is presented to directly measure the spatial extent and location of exciton recombination. This technique is applied to a series of OLED architectures as a means to better understand how emissive layer design and thin film composition impact exciton recombination. Structures based on mixed and continuously graded emissive layers are found to have significantly broader exciton recombination zones than conventional devices based on a double emissive layer. This is likely due to the fact that in the mixed and graded devices, exciton formation and recombination may occur over a large fraction of the emissive layer thickness, while in a double emissive layer structure, recombination occurs predominantly at the interface between the hole- and electron- transporting host materials. For devices with a graded emissive layer, the spatial recombination profile can be accurately modeled by considering the drift, diffusion, and recombination of charge carriers within the device. It is further found that the location and extent of exciton recombination may be tailored using the graded architecture, allowing for the realization of simple, single-layer devices with high-brightness, high-efficiency operation.

White organic light-emitting diodes (OLEDs) using a combination of phosphorescent blue, green, and red emitters are of great potential for general lighting due to their high efficiencies and light quality. However, developing phosphorescent emitters with saturated blue emission color and long lifetime has turned out to be challenging. To circumvent this issues, the use of a blue fluorescent emitter, combined with the concept of triplet harvesting (TH) is a promising approach while still maintaining high overall efficiency.[1]
We will present recent results for 4P-NPD as blue fluorescent emitter material in TH OLEDs. Firstly, we will discuss the investigation of the exciton diffusion length, which we derive from distance dependence measurements between exciton generation and exciton harvesting zone. The insights gained from these measurements are then used to develop highly efficient white OLEDs. By doping 4P-NPD with a yellow phosphorescent emitter, we achieve a two-color white TH OLED with an efficiency of 9.4% (27.1 lm/W, CIE (0.42/0.40), CRI 46) at a luminance of 1000 cd/m2.[2]
To increase the color quality of TH OLEDs further (i.e. obtain higher CRI), TH by green phosphorescent emitters would be beneficial. However, this is not possible when 4P-NPD is used as its triplet energy is below the green photon energy. With the aim of increasing the triplet energy, we developed two new blue emitters 8M-4P-NPD and 8M-4P-FPD. Here, small changes to the chemical structure of 4P NPD are used to tune the triplet energy in these emitters. We will show that both new blue emitters allow TH by a green phosphorescent emitter, thus paving the way to high-CRI TH OLEDs.[3] In addition, we will present time- and wavelength-resolved measurements of our white OLEDs which give direct proof of TH.
[1] G. Schwartz, S. Reineke, T.C. Rosenow, K. Walzer, K. Leo, Adv. Funct. Mat. 19, 1319 (2009)
[2] S. Hofmann, M. Furno, B. Lüssem, K. Leo, M.C. Gather , Phys. Stat. Sol. (a). (2013) DOI:10.1002/pssa.201329107.
[3] S. Hofmann, M. Hummert, R. Luschtinetz, C. Murawski, P.-A. Will, S.I. Hintschich, et al., (2013) submitted to Chemistry of Materials.

We demonstrate highly efficient organic light-emitting diodes (OLEDs) in which quadruple organic layers including a small molecule-based emission layer (EML) and electron transporting layer (ETL) are fully solution-processed. The key feature of the devices is the use of newly synthesized host molecules in the EML, which are highly resistant to the processing solvent (e.g. alcohols) of the upper ETL and thus enable the multilayer structure through subsequent solution-processing steps. While a robust polymer is typically required to realize the multilayer structure, we modified conventional host molecules simply by covalent dimerization or trimerization to afford sufficient resistance to alcohols, eliminating the need for the polymeric counterparts. With this approach, record high power efficiencies of 36, 52, 34 lm/W at 100 cd/m2 have been achieved for solution-processed blue, green and white OLEDs, respectively. The corresponding external quantum efficiencies reached over 20% without the use of any outcoupling enhancement. These efficiencies are considerably higher than that of the best polymer light-emitting diodes reported in the literature and comparable to that of evaporated multilayer OLEDs. We also show that the composition at the resulting EML/ETL interface is a critical factor in achieving these high efficiencies by time-of-flight secondary ion mass spectrometry depth profile analysis.

Most organic semiconductors and laser dyes are quasi four level optical systems. This allows stimulated emission from organic microcavities at room temperature and ~10-10 J/pulse laser excitations [1]. Micron-scale periodic structuring converts MC into organic photonic box, single photonic wire, and array of photonic wires (2D photonic crystal), results in forming extended Bloch waves. In the organic photonic wires, spontaneous and coherent emissions from ground and different excited discrete confined modes is observed experimentally at nonresonant above-threshold excitations at room temperature. Energy and angular distribution of spontaneous and stimulated emissions from discrete modes are described analytically and numerically, demonstrating excellent agreement with experimental observations. Tamm plasmon-polariton are observed in cavities with a metal layer [2]. Additional micron-size lateral structuring transforms parabolic continuous dispersion of organic plasmon and TPPs into discrete confined states, leading to optimization of lasing threshold and angular distribution of coherent emission.
[1] M. Koschorreck, et al., Appl. Phys. Lett. 87, 181108 (2005)
[2] R. Brückner et al., Nature Phot. 6, 322 (2012)

The talk presents examples of achieving and delivering photonic based devices, structures and environments based entirely on solution-processable materials. In the first instance, examples of general photonic structures, including as Bragg filters, microcavities and 2-D photonic structures, are presented. All made possible through the recent development of a versatile, low loss organic/inorganic hybrid material that is processable from an aqueous solution. The material&’s refractive index in solution-cast architectures may be tuned from 1.5 to 2.1 - without reducing the material&’s transparency (>98% over the spectral window 0.4-1.4µm). The second part of the talk considers a real molecular, bottom-up approach to the construction of metamaterials in which sub-wavelength physical structures, rather than variations in chemical composition, can be used to modify the nature of their interaction with electromagnetic radiation. Our distinct approach involves spatially patterning the physical geometry (conformation) of segments of a polymer chain and in this talk I will demonstrate an effective means to pattern conformation in poly(9,9-dioctylfluorene) (PFO) on length scales le; 500 nm. The resulting refractive index and optical gain patterns and their subsequent control offers the prospect of visible wavelength, conformational metamaterials and distinct highly-localized photonic environments.

π-conjugated systems in all possible shapes and variations have attracted researchers&’ interest ever since Kekulé&’s pioneering description of the benzene ring. Organic semiconductor materials based on π-conjugation have created whole research communities in the past decades. Most of these compounds are morphological derivatives of linear conjugated systems. We study an organic macrocycle comprising a fully conjugated ring stabilized by spokes with interrupted conjugation in its center. This particular molecular geometry allows us to observe photophysical processes in a shape-controlled model structure beyond randomly oriented π-conjugated polymer chains.
While the transition dipoles in a perfectly symmetric circular complex should cancel each other out and forbid an optical transition, the macrocycle molecules exhibit surprisingly efficient photoluminescence from toluene solution with a radiative lifetime of 850 ps at room temperature. The circular geometry of the electronic structure poses several fundamental questions: Where is the photoexcitation (exciton) going to be formed on the molecule? Does it always localize on the same part of the structure? For an extensive understanding of the chromophoric nature of the ring molecules, we perform single molecule spectroscopy giving us the ability to look at one macrocycle molecule at a time. Photoluminescence detection of single molecules, selected via horizontal and vertical polarization channels, shows temporal switching on a time scale of seconds: a strong indication of symmetry breaking in the system with dynamic chromophore formation. Single molecule spectra at 4 K show well defined narrow emission lines with the corresponding vibronic structure. Interestingly, no change in emission energy is observed accompanying the jumps in emission polarization, implying that the emission energy indicates that recombination occurs from only a partially delocalized exciton on the ring.

Transport of excitons is at the core of photosynthesis and a wide array of optoelectronic devices including solar cells, organic light emitting diodes, and excitonic transistors. Understanding and controlling the flow of excitons in such systems can enhance device performance and lead to development of next-generation nanoscale organic molecular devices. To date, the precise dynamics of exciton transport and how it relates to disorder, the defining characteristic of molecular and nanostructured materials, remains elusive. In this talk, we present in unprecedented detail a spatial, temporal, and spectral visualization of exciton transport in both single crystal and polycrystalline thin films. Using tetracane as an archetype organic semiconductor, we reveal that exciton transport occurs by random walk diffusion and that it transitions from normal to subdiffusive as exciton traps are filled. We also conclusively show that energy transport in tetracene is dominated by exciton hopping rather than by emission followed by re-absorption. The insights into exciton transport gained here, along with further application of the imaging technique, can guide the design of excitonic materials. In particular, the effect of traps, morphology, and the exciton energy landscape on transport dynamics can now be investigated.

A microcavity is a structure in which an optically active semiconductor is placed between two highly reflective mirrors. Such a structure quantizes the local electromagnetic field, and thus confines photons into a series of discreet optical modes. Within the so-called ‘strong-coupling regime&’ the confined cavity photons and the electronic excitations of the semiconductor (excitons) can no longer be thought of as separate eigenstates, but rather can couple together to form new quasi-particle states termed ‘cavity-polaritons&’. Polaritons necessarily have very different optical properties from their constituent parts and are a fascinating test-bed for fundamental physics, with non-equilibrium Bose-Einstein condensation observed in inorganic-semiconductor based structures suggesting potential applications in lasers, optoelectronics and ultra-fast optical switches.
We have fabricated strong-coupled microcavities using two high-reflectivity mirrors placed either side of thin film of an (organic) cyanine dye dispersed in an inert matrix material. Cyanine dyes are molecular materials that under appropriate processing protocols undergo self-organization to form J-aggregates. Such J-aggregates have intense, narrow electronic transitions that make them an ideal system to explore strong-coupling effects at room temperature. Using optical spectroscopy, we map the photoluminescence intensity along the upper and lower polariton branches as a function of temperature. We show that polariton states along the lower polariton branch can be populated by the scattering of reservoir-excitons that deposit excess energy in the various localised Raman-active modes of the molecular J-aggregates.
We also discuss recent work in which we have fabricated strongly coupled microcavities that contain two different cyanine dyes, whose excitonic transitions are both coupled to the same optical-mode. This coupling results in a 3-way anticrossing and an angle and energy-dependent mixing of cavity-photon and the two excitons. Using photoluminescence excitation spectroscopy to probe the efficiency of relaxation to low energy states, we show that that such mixed polariton states act as an energy relaxation pathway between reservoirs of uncoupled excitons.

The formation of triplet excitons in semiconducting organic materials plays an important role for the operation of organic optoelectronic devices. Triplet excitons are difficult to investigate spectroscopically at room temperature due to their non-radiative character. Here we show the measurement of the triplet decay dynamics by a highly sensitive time-resolved measurement of the triplet state absorption using pump and probe experiments within a waveguide configuration. Pump and probe pulse have to be separated spatially and in time to ensure a segregation of singlet and triplet excitons. The non-radiative triplet excitons are detected at room temperature by their absorption. A variation of the time delay between pump and probe pulse allows for the investigation of the dynamics of the excitons. Former experiments made use of the photoluminescence of the material under investigation itself as source of probe light. But since there is a spectral shift between the photoluminescence band and the triplet absorption band the spectral bandwidth of probe light has to be broadened for the identification of the triplet absorption. Here we show how a widening of the probe light band up to red part of the spectrum is achieved by an application of extra emission layers. With the help of this technique the triplet exciton dynamics of the hole transport material 2,2',7,7'-tetrakis(diphenylamino)-9,9'-spirobifluorene (SpiroTAD) was measured and analyzed.

Para-hexaphenylene (p6P) molecules can self-assemble via vapour deposition onto a muscovite mica substrate to form crystalline either straight nanofibers with length of hundreds of micrometers or nanofiber rings with diameters of a few micrometers. Such p6P nanofibers have peculiar optoelectronic properties such as a blue, polarized photoluminescence output upon UV excitation and the ability to act as optical waveguides and random lasers [1]. Waveguiding p6P rings would be of particular interest as they could be used in ring resonators as components in compact integrated photonic circuits. Compared to straight nanofiber growth, the formation of nanofiber rings requires an additional treatment of the mica substrate with water prior to deposition of the molecules. Here we demonstrate the formation of microrings with diameters larger than 10 µm. We find that the waveguided light through a nanofiber microring shows damping very similar to straight waveguides [2]. The spatially resolved spectra of the guided light through straight and curved sections (more than 90° bend) of a nanofiber show significant damping in the blue range of the spectrum due to reabsorption, while only very small losses are observed at below-gap photon energies. Current efforts focus on the spectral behaviour of the microrings.
[1] F. Quochi, J. Opt. (2010) 12, 024003.
[2] F. Balzer et al., Phys. Rev. B (2003) 67, 115408.

One-dimensional nanostructures are attracting considerable attention owing to unique optical interactions that arise from their subwavelength size, including light confinement, guiding, and amplification. These properties make the materials promising for applications in small optical devices such as light sources, waveguides, and sensors. Electrospun polymer fibers have nanometer diameters and high aspect ratios, making them well-suited for use in such optical devices. Several groups have reported optical waveguiding in electrospun nanofibers; however, these fibers were uncladded. In this work, we fabricate electrospun polymer nanofibers covered by cladding, and characterize the optical waveguiding properties of the fibers. Aligned electrospun nanofibers composed of poly(methyl methacrylate) (PMMA) and a small amount of Nile Blue A perchlorate (NBA) are prepared. The electrospun fibers, having mean diameters of 540±60 nm (mean ± standard deviation), are covered by a Cytop cladding and the ends of the fibers are cut. A 532 nm green laser beam is irradiated onto the fiber perpendicular to the fiber axis and the distance (d) between the end face of the fiber and irradiating point of the laser beam is varied. Photoluminescence (PL) spectra at the end face of individual fibers are then measured. The PL intensity at wavelengths shorter than 670 nm decreases with increased d because of reabsorption of PL that is guided in the fiber by an inner filter effect of NBA. We plot PL intensity at a wavelength of 700 nm as a function of d and evaluate the average propagation loss to be 17±10 dB/mm. We also evaluate the propagation loss of the nanofibers without cladding as 31±3 dB/mm. The lower loss is attributed to the cladding around the fibers that provides a uniform covering of a material of a single refractive index. To access the various contributions to the propagation loss, we evaluate the loss of PL from reabsorption by NBA molecules. From transmittance measurements of an NBA solution of the same concentration used to fabricate the PMMA electrospun fibers, the reabsorption loss at a wavelength of 700 nm is evaluated as 1.7 dB/mm. This reabsorption loss is almost one-tenth of the total transmission loss of the cladded fibers and we conclude the losses are mainly inherent to the PMMA fibers itself. We have demonstrated waveguiding in electrospun PMMA/NBA nanofibers covered by a Cytop cladding. The total propagation loss is evaluated as 17±10 dB/mm, which is lower than in fibers without cladding. Reabsorption losses from NBA molecules are evaluated as 1.7 dB/mm; thus we conclude the propagation losses in the fiber mainly result inherent in the PMMA fiber itself.

Plasmonics is concerned with the capacity of concentrating optical energy into regions lower than the diffraction limit together with enhancement and spatial confinement of the electromagnetic field at a metal-dielectric interface. Moreover, plasmonic-photonic coupled devices, funded on the interaction between surface plasmon based structures and active photonic media, represent a very promising field. In particular, organic emitters should reinforce the surface plasmon resonance in the near- infrared spectral region, a suitable effect for biochemical sensing.
We report on the development of a plasmonic-photonic coupled device based on the interaction between a plasmonic periodic nanostructure and an active photonic medium, e.g. a near-infrared fluorophore sprinkled into a thin film sol-gel glass matrix.
In this work, the activity has focused on one side in choosing, studying and optimizing a periodic surface able to support plasmonic resonances with tunable wavelengths; on the other side in defining and improving the sol-gel process to form and deposit thin films embedding the desired fluorophores.
Besides structural and optical characterization of the studied structures and films, theoretical modelling and finite-difference time-domain simulation (FDTD) have been developed in order to give a feedback on the structure quality and interpret its optical response.
Plasmonic surfaces consisting in a gold film embedding a two-dimensional array of polymeric pillars [1] have been used and an organic thin film based on rare earth organo-complexes (emitting in the 1mu;m spectral region [2]) has been developed as emitting medium. Several devices have been prepared and characterized by various microscopy (AFM, SEM) and spectroscopic (micro reflectance, ellipsometry and time resolved photoluminescence) techniques. Experimental results have been found to well agree with FDTD simulations. Preliminary results confirm that Yb-complexes photoluminescence establish a remarkable coupling with the plasmonic resonance (keeping its emission properties), due to a conformal deposition of the active medium on the plasmonic structures. Size and shape effects on the emission spectra have been observed, with varying the size and periodicity of the plasmonic pillars.
[1] S. Giudicatti et al., Interaction among plasmonic resonances in a gold film embedding a two-dimensional array of polymeric nanopillars, J.O.S.A.B 29, 1641-1647 (2012)
[2] F. Artizzu et al., Dual Emitting [Yb(5,7ClQ)2(H5,7ClQ)2Cl]: Chemical and Photophysical Properties, CHEM PLUS CHEM 77, 240-248 (2012)

Strongly electrophilic trifluoroacetyl (TFAc) functional groups are of particular interest due to their high reactivity with nucleophiles. We aim to use this interaction to develop resistance- and fluorescence-based sensors for nucleophilic species, such as amines, using trifluoroactylated- poly(3-hexylthiophene) (P3HT) materials. Accordingly, we have developed a 2-step post-polymerization modification procedure to install TFAc groups onto P3HT. Specifically, the procedure involves bromination of commercially available P3HT with N-bromosuccinimide (NBS) and subsequent lithium-halogen exchange, followed by quenching the reaction with trifluoroacetic anhydride. Analysis of 1H NMR, 19F NMR, and IR spectrometry indicates the production of TFAc-P3HT. The use of TFAc-P3HT for sensing applications is currently in progress.

Synthesis of π-conjugated polymers and applications of such polymers to electronic and optical devices have become of interests because of their processability, and excellent electronic and optical properties. Among many π-conjugated polymers, polythiophenes have been used in a variety of applications. The properties of polythiophene derivatives could be controlled by the functionalization of the side chains. Especially, polythiophene derivatives with ionic side chains have received much attention. Here, we present the synthesis of polythiophenes having a phosphonic acid group and applications as a metal ion chemosensor. The polythiophene derivative containing a phosphonate ester group, poly[3-(3-diethylphosphonate) propoxythiophene] (PEPPT), was synthesized by the Rieke method, and the optimum polymerization conditions were investigated. High molecular weight PEPPTs were obtained by using Ni(dppe)Cl₂ as a catalyst and tetrahydrofuran as a solvent. By increasing the amount of catalyst and solvent, the yields of high molecular weight PEPPT increased. PEPPT was soluble in dimethylsulfoxide (DMSO) and partly soluble in chloroform. The highest molecular weight of PEPPT was M_n = 4,200 g mol^-1 but solubility was low. Therefore, we characterized the other PEPPT whose molecular weight and average polymerization degree were M_n = 2,400 and 9, respectively. By removing the protecting group from PEPPT, phosphonate-functionalized polythiophene, poly[3-(3-phosphonic acid)propoxythiophene] (PPPT) was synthesized. PPPT was insoluble in chloroform but partly soluble in water. Especially, PPPT showed good solubility in alkaline aqueous solutions. These polymers were characterized by ¹H NMR spectroscopy and GPC measurements. The optical properties of PEPPT and PPPT were measured by UV-vis and fluorescence spectroscopy. The maximum absorption peaks of PEPPT and PPPT were observed at 559 nm and 565 nm in DMSO, and the fluorescence peaks were observed at 662 nm and 655 nm, respectively. The sensing properties of PEPPT to various metal ions were characterized in its 4 mu;M DMSO solutions. Upon addition of excess metal ions (36 mu;M), Ag⁺ , Ba²⁺ , Ca²⁺ , Mg²⁺ , no significant changes of the fluorescence spectra were observed. However, fluorescence quenching was observed upon addition of Co²⁺ , Fe³⁺ , Ni²⁺ , Pb²⁺ . When Fe³⁺ was added into the DMSO solution of PEPPT, the fluorescence intensity decreased with a 3-fold fluorescence quenching. In contrast, the addition of Co²⁺ , Ni²⁺ and Pb²⁺ only exhibited a 1.2-1.3-fold fluorescence quenching. The sensing properties of PPPT to different metal ions were also characterized in its 10 mu;M DMSO solution. Fluorescence quenching was observed only when excess Fe³⁺ (90 mu;M) was added into the PPPT DMSO solution and the fluorescence intensity decreased with a 8-fold decrease. These quenching was probably attributed to the aggregation of polymers.

Organic electronics often use conjugated polymers to enable optoelectronic activity. It is common to modify a polymer with alkyl side chains that improve its solubility so that the deposition of the polymer can be through solution processing. While this approach has proven useful, it introduces some additional issues: 1) if the device contains multiple layers, any solution based processing must not perturb already existing layers, and 2) the solubilizing side chains occupy space in the solid state with atoms that have no optoelectronic activity. Here we present conjugated oligomers and polymers that contain photocleavable solubilizing side chains, which enable direct spatiotemporal control over the solubility of the conjugated molecule. Both nitrobenzyl ester-linked chains and nitrobenzyl ether-linked chains are effective photocleavable linkers in this application. Our demonstrated results in photocleaving solubilizing chains from thin film samples demonstrate potential applicability in multi-layer solid-state devices and photolithographic applications.

We investigated the hole injection characteristics of triphenylamine (TPA) -containing polymer, named TPAPEK, doped with various acceptors, such as 4-isopropyl-4&’-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (PPBI), tris(4-bromophenyl)aminium hexachloroantimonate (TBPAH), tris(pentafluorophenyl)borane (PPB). We observed charge-transfer (CT) comlpex formation between TPAPEK and acceptors by means of UV-vis-NIR absorption spectroscopy. The CT absorption intensity around 1000 nm increases with increase of acceptor ratio. Then, we fabricated devices with structure [ITO/Buffer layer/NPD/NPD:Rubrene/Alq3:Rubrene/Alq3/Liq/Al] and evaluated the OLED performances. From the current density-voltage characteristics, the optimized ratio of TPA unit and acceptor is revealed to be 1:0.2~0.3. Although the CT absorption increases with increase of acceptor ratio, however, the current density decreases when the ratio is over 1:0.2~0.3. The optimized device showed superior performances and lifetime to the device without buffer and the devices with commercially available polymer buffers.

We investigated the effect of substituents on the carbazole host toward the performance in pure blue phosphorescent OLED using a carbene type blue emitter, Ir(dbfmi). As the substituents, we used three functionalities, such as triphenylmethane (CCz), triphenylsilane (SiCz) and diphenyl phosphine oxide (CzDPO) moieties. The electrochemical properties were determined by UV-vis, PL and photoelectron yield spectroscopy (PYS). The ionization potential (Ip) was observed at 5.95 eV for CCz, 6.04eV for SiCz and 6.24eV for CzDPO, respectively. While the Ea level was estimated at 2.45 eV for CCz, 2.60 eV for SiCz and 2.76 eV for CzDPO, by substraction of the energy gap (Eg) from the Ip level. The photophysical properties of 5 wt% Ir(dbfmi)-doped carbazole based hosts were estimated. The doped-film showed photoluminescent quantum yield of 45±1% for CCz, 60±1% for SiCz, 65±1% for CzDPO, respectively. A blue OLED with a structure of [ITO (130nm)/TAPC (40nm)/ Ir(dbfmi) 5 wt% doped Host (20nm)/Host (10nm)/B3PyPB (30nm)/LiF (0.5nm)/Al (100nm)] were fabricated and evaluated. SiCz-based device showed highest performances with 10.8 lm/W, 16.7% EQE at 100cd/m2. While CCz-based device showed lower performances of 1.2 lm/W at 100cd/m2. There are large defferences in the device performances depending on the used host. We also investigated the relationship between the device performances and the chemical structure of carbazole hosts, and revealed an effective molecular design for realizing an efficient blue OLED.

Heavy metal ions, such as Ni(II), Co(II), and Cd(II), are toxic compounds. They affect the human central nervous system and cause mental or neuropsychiatric disorders. Therefore, the concentration of heavy metals in water should be kept as low as possible. Routine analyses of these compounds in industrial effluents and in natural water require expensive and time consuming laboratory procedures. Therefore, development of low-cost monitoring methods is much desired.
We report on the development of a specially designed polymeric matrix that can be used as a simple strip test method for the detection of sub ppm concentrations of heavy metals. It is based on PVC-co-PVA and it contains a chelating agent, (1-(2-pyridylazo)-2-naphthol (PAN) that changes its color upon exposure to heavy metals. The presence of heavy metals can be visually detected and quantitative results can be obtained by spectral analysis of the exposed films.
Various morphologies of the polymeric matrix were tested: These include thin film casting and electrospinning. For the thin films, the resulted 95% confidence interval based detection limits were 92 ppb for nickel, 77 ppb of cadmium and 130 for cobalt. The electrospinning process resulted in strip tests composed of nanofibers, which possess a very high surface area and which are expected to improved sensitivity.

We have synthesized yellowish-green emitting material with a quinoline derivative as a main ligand and picolinic acid unit as ancillary ligand for white phosphorescent organic light-emitting diode. A high quantum efficiency of 21.2% and a current efficiency of 56.4cd/A with a color coordinate of (0.462, 0.508) could be achieved in yellowish-green phosphorescent organic light-emitting diodes. By combining yellowish-green emitter with a phosphorescent deep-blue emitting bis((3,5-difluoro-4-cyanophenyl)pyridine) iridium picolinate (FCNIrpic), and a red emitting Bis(1-phenylisoquinoline)(acetylacetonate)iridium (III) (Ir(piq)2acac), the fabricated white organic light-emitting diode showed a quantum efficiency of 11.3% and a current efficiency of 22.4cd/A at 1000cd/m2 with a color coordinate of (0.378, 0.404) and a high color rendering index of 84. The use of the new yellowish-green phosphorescent emitter could improve the color rendering index up to 84 and further optimization would give high color rendering index over 90.

Organic materials have been studied widely in the strong light-matter coupling due to their high dipole moments.[1] Rabi-splitting of organic polaritons as large as 700 meV at room temperature has been reported using photochromic spiropyrans inside optical microcavities or on plasmonic nanostructures.[2] The strong modification of the energy levels of molecules due to exciton-polariton formation has already been shown to affect relaxtion pathways in the coupled system[3], the rates of photochemical reactions[4] and the electrical properties of organic material such as work-function[5]. The efficiency of light-molecule coupling (Rabi-splitting) plays the main role in these fundamental changes.
It is already established that Rabi-splitting is determined by the square root of the molecular concentration within the optical mode volume under the resonance condition.[1-2, 4] Recently we also showed that larger Rabi-splitting can be achieved by using higher symmetry plasmonic arrays.[6] Here the efficiency of light-matter strong coupling is tuned by precisely arranging the spatial position of a cyanine dye J-aggregate layer in a Fabry-Perot micro-cavity. We use both static and fs transient optical spectroscopy to understand more about the emission and lifetime of the exciton-polariton as a function of Rabi-splitting.
[1] V. M. Agranovich et al., Chem. Rev. 2011, 111, 5179
[2] T. Schwartz et al., Phys. Rev. Lett. 2011, 106, 196405.
[3] T. Schwartz et al., ChemPhysChem 2013, 14, 125
[4] J. A. Hutchison et al., Angew. Chem. Int. Ed. 2012, 51, 1
[5] J. A. Hutchison et al., Adv. Mater. 2013, 25, 2481
[6] A. Salomon et al., ChemPhysChem 2013, in press

The importance of designing and synthesis of organic porous materials with enhanced thermal stability, storage capacity and controlled pore size that could replace conventional zeolite based materials is increasing from year to year. One of the potential alternatives could be metallated porphyrins based on weaker coordination bonds. In this work, we use zinc-nitro interaction to generate organic porous coordination polymers using porphyrin as a template. 4-nitrophenyl groups in trans-A₂D₂-porphyrin interact with zinc metal in the porphyrin core forming 4⁴ networks. Other aryl groups are between the layers forming the walls of pores. By changing the size and electron density of these aryl groups host-guest properties of the coordination polymer will change. It was shown that one of the networks selectively and reversibly intercalates only electron deficient guest molecules. Layered structure of the network enables formation of alternating layers of donor and acceptor domains. These kinds of networks have potential applications in molecular storage, explosive sensing and organic photovoltaics.

Self-assembly of bolaamphiphilic molecules offers soft platform for the fabrication of nanomaterials with diverse functionalities. Comparing to the previous studies where the complex structured self-assemblies have been used as simple templates for nanomaterial fabrication, exploitation of the surface functional groups on the self-assembly provide another way to use the molecular self-assembly as a active host matrix. In this study, we demonstrated that the self-assembly of designer bolaamphiphile molecule was used as a host matrix for the preparation of photoluminescent nanoparticle. A novel bolaamphiphile molecule containing tyrosine end groups were synthesized. This molecule self-assembled to generate photoluminescent nanospheres in an aqueous solution with the existence of photosensitizer and lanthanide ions. The photosensitizer and lanthanide ion were associated with the bolaamphiphile self-assembly through the π-π stacking and electrical attraction. Combinations of photosensitizers and lanthanide ions were tested to find out synergetic photoluminescent self-assembly. Due to the antenna effect by the phenol group of the tyrosine, the photoluminescence enhanced by a factor of two when the photosensitizer and lanthanide ions are incorporated with the bolaamphiphile self-assembly. From the photoluminescence study, the photon energy transfer was enhanced by the self-assembly of tyrosine-containing bolaamphiphile indicating the antenna effect. The outcomes of this study demonstrated that the bolaamphiphile molecule with proper functional groups could be exploited to generate a soft host matrix for the optical nanomaterials.

Much research on modern biotechnology is currently focused on near infrared (NIR) cyclic tetrapyrrole molecules to explore their NIR-absorbing, NIR-fluorescent and NIR-photosensitizing properties for potential applications. However, the photo-induced instability is a problem that hinders the extensive use for durable applications of many compounds of this class of molecules. In order to increase their utility, methods that slow the photo-induced instability must be develop. In this regard, the present study was conducted to evaluate the effect of silver nanoparticles (AgNPs) as a potential additive to remove the photo-induced instability of chlorophyll (Chl), which is known to be the most photosensitive and the most abundant NIR cyclic tetrapyrrole molecules on the earth. The photostability of Chl was characterized by UV-vis absorption spectroscopy. The results show that under red-light irradiation, the half-life time of Chla (4.5 µM) in the presence of AgNPs (4.8 nM) is 1471% longer relative to the Chla in the absence of AgNPs. Moreover, it was found that AgNPs may act as efficient photoprotective agent without losing the chlorophyll fluorescence emission signal and the reduction of singlet oxygen concentration. It is further seen that AgNPs are much more effective photoprotective agent of in vitro Chla than are the classic antioxidants molecules (β-carotene and lycopene). The protective ability of Chla by AgNPs is the result of their binding with Chla at its nitrogen sites, inhibiting, thus, the binging of reactive oxygen species (ROS) at these sites, known to cause the photodegradation of Chla and several other cyclic tetrapyrrole molecules.

Columnar liquid crystals are composed of disc-shaped aromatic molecules surrounded by flexible side chains, where the molecules self-assemble in columns that can form large surface-oriented domains [1]. This system is known to show good charge and exciton transport along the columns, with mobilities approaching those of aromatic single crystals [2]. Such semiconducting materials are promising for many device applications, where the efficiency of such devices is strongly dependent on the alignment of the columns [3].
In the work presented here, the charge mobility was tuned by five orders of magnitude after homeotropic columnar ordering induced by thermal annealing from the mesophase to produce state-of-the-art diodes. The diodes were constructed using spin-coated films, being the conductivity properties investigated by modeling the current-voltage characteristics, considering a trap-limited space-charge-limited current transport with a field dependent mobility for higher voltages. The homeotropic alignment with formation of hexagonal germs was observed by polarizing optical microscopy. The photophysical properties showed drastic changes at the mesophase-isotropic transition, which parallels the gain of order observed by X-ray diffraction.
[1] M. O&’Neill, S.M. Kelly, Adv. Mater. 23, 566 (2011).
[2] R.J. Bushby, K. Kawata, Liq. Cryst. 38, 1415 (2011).
[3] B.R. Kaafarani, Chem. Matt. 23, 378 (2010).

A family of small molecules with [1,2]dithiolo[4,3-c][1,2]dithiole (C₄S₄), a sulfur-rich heterocycle functional group, was synthesized with a variety of electron donating and withdrawing side groups to study the optical and electronic properties of the C₄S₄ moiety. By changing side groups, the optical band gap of these small molecules can be varied from 1.76 eV - 2.38 eV and extinction coefficients of up to 21000 M^-1cm^-1 can be achieved. Analogous small molecules with thiophene functional groups were synthesized for direct comparison. The small molecules with the C₄S₄ functional groups were found to have smaller band gaps and similar extinction coefficients compared to their thiophene counterparts, suggesting that the C₄S₄ functional group is a promising moiety for new low band gap organic semiconductors.

Colloidal quantum dot (CQD) is a promising candidate in the future optoelectronic material due to the advantages of solution-processing and precise control of bandgap through the quantum size effect. These characteristics are particularly advantageous for developing CQD photodetectors over a wide spectral range from the visible to the infrared region. Recent progress in organic and colloidal quatum dot device integration has led to the demonstration of large area, low cost, flexible, portable sensors. Here, we present photodetectors consisting of p-type lead sulfide (PbS) CQD as an active layer for absorbing an incident light and oganic materials which facilitate the collection of charge carriers and reduce dark current. We investigated various device structures for optimizing the device performance and characterized their responsivity and sensitivity in different wavelengh of light.

For optoelectronic applications, organic semiconductors exhibit distinct advantages over inorganic materials such as facile synthesis and tunability via chemical synthesis techniques combined with low temperature processing over large areas. Some phenylene and thiophene based oligomers can self-assemble via vapor deposition into molecular crystalline nanofibers, which exhibit a range of special optical properties including polarized photo- and electroluminescence, photonic and plasmonic waveguiding, and emission color tunability. One potential application is therefore that such organic nanofibers could therefore form ultra-small light-emitters and waveguides in future nanophotonic systems. Here we present our recent work towards a multicolored, electrically driven device by combining nanofibers made from two different molecules, parahexaphenylene (p6P) and 5,5'-Di-4-biphenyl-2,2'-bithiophene (PPTTPP), which emit blue and green light, respectively. The organic nanofibers are implemented on a bottom gate/bottom contact field-effect transistor platform using a simple roll-printing technique[1]. We compare electrical transport and electroluminescence (EL) properties and observe a correlation between the onset voltage for transport and EL emission.
[1]: L. Tavares, J. Kjelstrup-Hansen and H.-G. Rubahn, Nanotechnology, 23, 425203 (2012)

Polymer light-emitting diodes (PLEDs) are attractive for large-area lighting because of their solution processability that would lead to low-cost production. Herein, by introducing a solution-processed n-doped electron transport layer with and tris[3-(3-pyridyl)mesityl]borane (3TPYMB) and potassium borohydride (KBH4), we demonstrate bright, efficient, and low-voltage operating polymer light-emitting diodes (PLEDs). We investigated the n-type doping effect in 3 TPYMB films by doping with various concentrations of KBH4 using ultraviolet photoelectron spectroscopy (UPS). The KBH4 doping in the 3TPYMB layer improves electron injection and transport properties. As a result, the devices with the KBH4-doped 3TPYMB layer show much improved performances in terms of the driving voltage, maximum luminance and maximum external quantum efficiency. Based on these results, we can fabricate red, yellow, green and blue PLEDs by introducing the KBH4-doped 3TPYMB layer.

Conjugated polymers have a number of advantages in the field of developing responsive materials for detection and sensory applications. The development of new polymeric materials that respond to singlet oxygen (or 1O2) is one of the subjects of interest in our laboratory. This poster will focus on a series of furan-linked CPs that respond to 1O2 by fluorescence quenching upon cycloaddition with the furan units. The resulting oxidized products are more electron-poor than diarylfurans; they therefore quench the fluorescence of electron-rich fluorophores by photoinduced electron transfer. Among these derivatives, fluorescence quenching efficiency was increased by incorporating electron-withdrawing groups on the furan pendant in case of poly(fluorene-co-phenylene) backbone, whereas an analogous poly(phenylene-ethynylene) showed no quenching upon reaction with singlet oxygen.

The ability to tune the properties of organic semiconductors by chemical synthesis methods opens up the possibility to tailor the characteristics for a particular application. For example, thiophene-based semiconductors can be used as the active layer in organic optoelectronic devices such as light-emitting and light-sensing transistors with the band gap being determined by the molecular structure. In this work, we have investigated organic thin-film phototransistors (OPTs) based on two different molecules: alpha-sexithiophene (α-6T) and 5,5-bis(naphthyl)-2,2prime;-bithiophene (NaT2) [1]. The photoconductivity of both types of OPTs was characterized under both white light and spectrally resolved illumination. As expected from the optical absorption spectra, the devices respond to different wavelengths and the NaT2-based devices exhibit a significantly higher responsivity. These results demonstrate the large potential for organic transistors as photo detectors with the benefits of custom design of the molecular building blocks for specific applications.
[1] X. Liu et al. Org. Electron. 11, 1096 (2010)

Due to their application potential in lighting and display technology, organic light emitting diodes (OLEDs) have been attracting considerable attention. However, the lifetime of these devices is still a bottleneck for a broad application of the technology. Revealing the degradation processes and finding ways to avoid these is of great interest. We elucidate the chemical degradation pathways of the phosphorescent orange emitter Iridium(III)bis(2-methyldibenzo-[f,h]chinoxalin)(acetylacetonat) (Ir(MDQ)2(acac)) and red emitter Tris(1-phenylisoquinoline) iridium(III) (Ir(piq)3) and show that dual-doping of the emission layer is a suitable method to enhance operational lifetime. The analysis of current stressed OLEDs by laser desorption/ionization mass spectrometry shows that the emitter molecule degrades by a fission and complexation reaction with the adjacent electron transport material. Co-doping the emission layer with suitable molecules which perform exciton transfer to the emitter molecule hinder the degradation reaction and enhance the lifetime of the OLEDs by a factor of five.

Fluorescent organic dyes are used in a variety of scientific experiments ranging from biology and micro-rheology to protein formulations, where they are used as markers for activity when attached to entities such as proteins. They are also widely used in industrial applications ranging from signage to displays. However, dye photobleaching, a process where dye molecules degrade over time, is a main factor in limiting the lifetime of a dye and is therefore undesirable. In this work we show that the addition of small amounts of nanoparticles of rare earth (RE) oxides such as La2O3 and CeO2 to the dyes, in concentrations as low as 0.007 % by weight, significantly reduces dye photobleaching by factors of 2.5 to 4. Photobleaching is believed to occur as a result of chemical reaction between dyes and reactive oxygen species formed during the photoabsorption process. Rare earth (RE) oxides such as La2O3 and CeO2 are known to act as oxygen scavengers, a property likely related to their oxygen deficient compositions. We thus hypothesize that the addition of rare earth oxide nanoparticles to dyes will reduce photobleaching. We mixed La2O3 and CeO2 nanoparticles (with diameters ranging from 10 to 30 nanometers) with aqueous solutions of a Rhodamine 6G dye, as well as with fluorescently labeled bio-polymers of fibrin and collagen (with AlexaFluor 488 and Rhodamine B respectively). We compared the fluorescent polymeric samples to (i) “sans additive” control samples, (ii) samples infused with OxyFluor (laboratory reagent used in the prevention of photobleaching), and (iii) samples mixed with FeO (iron (II) oxide) nanoparticles. Unlike the oxygen-deficient RE oxides, iron oxide is known to have an oxygen-excess stoichiometry. We hypothesized that iron oxide nanoparticles would not be as effective as rare earth oxide nanoparticles in scavenging oxygen and suppressing photobleaching. In the experiments with aqueous solutions of Rhodamine 6G, the dye was excited using a 405 nm solid state laser, and then analyzed using an optical spectrometer. In the experiments involving polymeric matrices (namely, fibrin and collagen gels) confocal microscopy was used to image the fluorescently labeled polymers as function of time. From time dependent studies of fluorescence from these samples, we observed that the samples containing RE oxide nanoparticle additives exhibited significantly lower rates of fluorescence decay (by a factor of 2.5-4X) when compared to the control samples. In particular, the inability of the iron oxide nanoparticles in suppressing photobleaching supports the role of reactive oxygen in dye photobleaching. We posit that the observed suppression of dye photobleaching using RE oxide nanoparticles is likely related to the oxygen free radical scavenging properties of the rare earth oxides.

This project examines light matter interactions, particularly photocurrent and photogeneration, to examine organic FETs. In scanning photocurrent microscopy, a spatially resolved measurement, a diffraction limited laser beam separates electron-hole pairs within a biased device. The current generated is used to study band bending at interfaces, photoconductance and energy states, and film uniformity in OFETs. In this study, photocurrent measurements are used to probe pentacene bottom-contact and top-contact OFETs in order to investigate the disparate electrostatics in the two configurations. The results clearly show a difference in the shape of the photocurrent as a function of position in the channel, particularly at the source side, indicating that carrier-injection mechanisms within the two geometries are indeed dissimilar. We are also investigating organic photogeneration (fluorescence) and light/matter interactions in high-Q photonic cavities. We are exploring a new format for optically-pumped organic lasers and the possibility of an electrically-pumped organic laser. Our structure combines an organic light emitting transistor (LET) with an organic photonic resonator. For the gain medium, we are using poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), a well known fluorescent polymer semiconductor commonly used in OLEDs.

Controlling the luminescent properties of small organic molecules is of great interest for a variety of applications ranging from commercial light emitting devices (OLEDs), to biological imaging. In general, no methods exist whereby the luminescence intensity of an organic chromophore can be enhanced. Typically, organic chromophores are electron-rich aromatic molecules, and by extending the electronic conjugation, or oligomer length of these molecules both the intensity and color of luminescence can be controlled. However, larger aromatic molecules also become increasingly insoluble as the oligomer length increases. Ideally, the luminescence intensity of organic chromophores could be improved without affecting solution processability. We find that by symmetrically bridging a series of organic chromophores about a sulfur atom, the emission quantum yield systematically increases by successive oxidation of the bridging sulfur (i.e. S < SO < SO2). Furthermore, the enhanced fluorescence quantum yields were achieved without diminishing the solubility of these chromophores in common organic solvents. The photophysical characterization and properties, as well as potential applications of these molecules will be discussed.

In colloidal quantum dot (QD) thin films, the energy transfer efficiency between individual nanocrystals determines the length scale over which an excited electron-hole pair can diffuse. Therefore, understanding energy transfer in QD assemblies is essential for optimization of solar cells and light-emitting devices. We measure the energy transfer rate between donor and acceptor nanocrystals of different sizes using spectrally-resolved transient photoluminescence. Using this technique, we observe a distinct shortening of donor lifetimes in mixed films in conjunction with little change in the lifetime of the acceptor nanocrystals. We show that the rate of energy transfer between donor and acceptor nanocrystals can be tuned by varying ligand length or inorganic shell thickness, in accordance with the predictions of Förster resonant energy transfer (FRET) theory. The measured change in lifetime of donor nanocrystals in the presence of an acceptor allows calculation of both the energy transfer efficiency and the transfer rate, which has implications for the design of optoelectronic devices.

Quantum-dot excitonic solar cells rely on efficient diffusion of excitons to the charge-separating interface. To date, it remains unclear how inherent inhomogeneities in quantum dot assemblies affect the diffusion, largely due to a lack of good analytical tools. Here, we present a detailed study of exciton diffusion in these materials by combining transient optical microscopy with transient spectroscopy. In particular, the direct visualization of the diffusion through spatio-temporal imaging allows us to directly extract the diffusivity and the corresponding diffusion lengths. Importantly, we will show that the intrinsic presence of energetic disorder in inhomogeneously broadened QD films leads to subdiffusive behavior. Consistent with Forster theory, we can tune the diffusivity by changing the interparticle spacing, providing a route towards controlled diffusion in these materials.

Encapsulation of 4-[4-(dimethylamino)styryl]-1-octylpyridinium bromide (DASP) with amylose, and conformational transitions of amylose upon complexation with the dyes were investigated using UV-visible, fluorescence, circular dichroism (CD) spectra and two-photon absorption properties in various DMSO/H2O mixtures. Encapsulation of the dye was achieved by dilution of DMSO stock solutions of the chromophores (1×10-3 M) and amylose (1×10-1 M) with DMSO, followed by a dropwise addition of H2O with continuous stirring at room temperature. At less than 40% aqueous DMSO solutions, the chromophores had a stable encapsulation with amylose and showed over 100 times the fluorescence intensity of a non-encapsulated chromophores.
Both of the dye solution with and without amylose also exhibit two-photon absorption (2PA), as determined by nonlinear absorption measurements using 10 ns laser pulses at 800 nm. The respective values of two-photon cross-sections are 13.3 × 10-20 for encapsulated dye solution and 2.53 × 10-20 cm4/GW for non encapsulated dye solution. The data clearly indicate that the contribution of the encapsulated dye dominates the two-photon absorption cross-section of the non encapsulated dye at 800 nm.

High-brightness green phosphorescent hybrid inorganic-organic light-emitting diodes (HyLEDs) with conventional (bottom anode top cathode) and inverted (bottom cathode top anode) structures have been demonstrated. The HyLEDs comprised a transparent and conductive WO3 layer deposited by thermal evaporation, which improved both hole injection and hole transport, and led to significant performance enhancement. At 20 mA/cm2, the HyLEDs with a bottom anode had a low operation voltage of 6.1 V, 0.8 V lower than that of OLEDs with an organic hole transport layer (HTL). Inserting a 20 nm undoped CBP layer between the WO3 and CBP:Ir(ppy)3 emitting layer to broaden the exciton generation zone significantly enhanced the performance of the HyLEDs. A brightness of 104 cd/m2 was reached at 7.2 V. At this brightness level, the current efficiency of the HyLEDs was 55.2 cd/A, 57% higher than that of the OLEDs. HyLEDs with a WO3 layer thicker than 5 nm also exhibited markedly improved reliability under constant-current stressing due to a more stable ITO/HTL interface. In the inverted HyLEDs, facile hole injection and transport through WO3 balanced electron injection from the indium-tin-oxide (ITO) cathode overcoated with nanometer-thick Ca, leading to a low turn-on voltage of ~6 V. At 20 mA/cm2, they reached a luminance of 8133 cd/m2 and a current efficiency of 40 cd/A, which were 43% higher than a similar inverted HyLED with a conventional Al/LiF electron injection layer. These simplified hybrid device structures represent a viable design for efficient and durable OLEDs suitable for display and lighting applications.

Photorelease of biologically active compounds from polymeric nanocarriers may have broad impact on health sciences. However, in order for such technology to be practically feasible, the excitation wavelengths must penetrate dense biological tissue. As the high intensity, focused NIR laser beam required for two-photon processes likely does not remain focused in dense biological media, we developed a photodegradable polymer that photochemically degrades upon absorption of one-photon visible light. Nanoparticles formulated from this polymer rapidly release molecular cargo upon irradiation with blue visible light (400-500 nm) through mouse skin. Particles could also be disrupted by NIR pulsed laser light, albeit at a longer time scale. However, particles did not degrade when irradiated with NIR light through mouse skin, as the beam defocused by scattering. GPC demonstrates that the polymer degrades into smaller fragments upon irradiation; both the polymer and its degradation products are well tolerated by cells at concentrations up to 200 mu;g/mL as indicated by MTT assay. This novel polymer has the potential to be practically useful in drug delivery and translational work in models of rheumatoid arthritis is currently under way.

Manipulating excitons in organic materials at nanoscale is important to understand exciton dynamics, which will allow for designing efficient and high speed devices and also lay a platform for next generation organic devices. One way of achieving the above aim is by surface patterning of organic materials. Ability to pattern organic materials enables us to control exciton properties such as lifetime and spatial diffusion. Unlike the conventional microfabrication techniques, such as direct-write or lift-off processes, which can be detrimental to organic materials, we explore the possibility of using nano-imprinting technology for patterning j-aggregates thin film. A 4.5 molecular layer of J-aggregate is grown using layer-by-layer deposition of thiacyanine dye coadsorbed over ployallylamine hydrocholoride (PAH) polyelectrolyte on a polydimethylsiloxane (PDMS) substrate. A master stamp consisting of nanostructured patterns is fabricated using ebeam lithography on SU8 resist spun on a silicon substrate. A 100-nm oxide layer is sputtered on the stamp to increase the adhesion between the j-aggregate film and the stamp. The stamp is then pressed gently on the j-aggregate thin film to transfer the material onto the top surface of the patterned master. Fluorescence microscope imaging is used to confirm a clean transfer of j-aggregate films. This successful high-yield, large-area patterning process opens new possibilities of designing organic devices with controlled energy flow. Ongoing work on change in lifetime and directional diffusion due to patterning will also be reported.

Fluorescence of pyrene (Py) excimers in electrospun polystyrene (PS) films with an additive of tetrabutylammonium hexafluorophosphate (TBAPF6) - a three-component system can be effectively quenched by the vapor of nitro-aromatic and nitro-ester explosives.[1] The sensitivity is in the ppb level and the quenching by the nitro-explosives is much higher than other nitro non-explosive compounds, indicative of high sensitivity as well. The detection can be performed by naked eyes with a UV lamp since the fluorescence emits blue light (wavelength ~ 460 nm). In spite of all these promising properties, the fundamental understanding of the quenching mechanism remains very limited. For example, electrospinning appeared to be the only preparation method leading to the high Py excimer fluorescence and the fast quenching mechanism. Recently, it has been shown that high fluorescence of Py excimers can be also achieved under a high solvent vapor pressure,[2] providing important insights to the formation of Py excimers in such three-component system. Here, we will present an alternative system containing two components Py and PS which results in a similar quenching behavior as the films in presence of the vapor of 2,4-dinitrotoluene (2,4-DNT , a type of nitro-explosives). This study allows us to decouple the effects of TBAPF6 salt and PS on the excimer fluorescence and the quenching mechanism.[3] Different molecular weight (from 2500 to 900,000 g/mol) and molecular architectures (linear, hyper branched and centipede-like) of PS will be investigated in regards to the quenching efficiency to further understand the interaction between Py and PS during the process. The diffusion of 2,4-DNT into the solid films of different thicknesses will also be evaluated in order to understand the quenching mechanism at the molecular level. [4]
[1] Y. Wang, A. La, Y. Ding, Y. Liu and Y. Lei, Adv. Funct. Mater., 2012, 22, 3547-3555.
[2] H.-S. Jang, Y. Wang, Y. Lei and M.-P. Nieh, J. Phys. Chem. C, 2012, 117, 1428-1435.
[3] H-.S.Jang, J.Zhao and M.-P.Nieh, In preparation
[4] X. Lu and M. A. Winnik, Chem. Mater., 2001, 13, 3449-3463.

Semiconducting polymers, with their solution processability and flexible properties, offer exciting potential for high throughput, flexible electronic devices. However, electronically active polymers have historically experienced problems with consistent long-term performance and optoelectronic behavior. We have been exploring the rapid photo-induced thiol-ene formation of electroluminescent polymer networks. Networked materials display excellent device-level properties as good as their non-cross-linked analogues and allow for the solution processing of an arbitrary number of active layers without the need of orthogonal solvents. Additionally, the rigid network provides excellent color stability in situations (e.g., annealing) where the pristine polymer&’s optical properties normally bleach, quench, or undesirably evolve with time.
Using the stability of these networks to enhance the performance and longevity of electronic polymers requires deeper understanding of how charge transport is affected by the structure of the thin film semiconducting network. For these investigations, we turn to the fluorenone moiety - an oxidative defect in poly(fluorene) which is known to be a highly efficient charge trapping site, particularly under aggregation and solid state conditions. The deliberate introduction of small amounts of fluorenone defects into luminescent poly(fluorene) networks provides an excellent diagnostic handle due to its distinctly green emissive color. Our recent efforts have focused on determining the subtle influence of various network structures on the polymers&’ optoelectronic properties by systemically varying cross-linking density and the rigidity of cross-linking agents. We can thus identify the degree of interchain communication and charge transport depending on how the poly(fluorene) network is constructed, which has enabled us to tailor a wide range of electronic behaviors for a single conjugated polymer. We have also started investigating the use of surface-bound grafting agents for covalently linking active electronic polymers to material surfaces with good coverage.

Optical bandgaps of conducting co-polymers are not straightforward additive quantities of individual pi-conjugated units due to inevitable wavefunction hybridizations along the pi-conjugated backbones. This work explicitly addresses a few general bandgap tuning rules using the accurate and fully transferrable adapted-Su-Schrieffer-Heeger (aSSH) Hamiltonian. Our results indicate that five-membered conjugated carbon rings in the acceptor units are essential to break the electron-hole charge conjugation symmetry, so that the LUMO levels of the co-polymers can be significantly reduced while keeping the HOMO levels in the donor units unaltered. In contrast, by incorporating heteroatoms into the donors units, the co-polymer HOMO levels can be adjusted independently. Therefore, extremely small bandgap copolymers can be constructed by the combination of both strategies. Our results also indicate that 1) alkoxy side chains can increase both the HOMO and LUMO energy levels; 2) both alkoxycarbonyl and alkylcarbonyl side chains can reduce the LUMO energy levels; 3) thiophene side chains can push the HOMO levels up and the LUMO levels down; 4) side-chain substitutions on parallel fused-rings affect the HOMO/LUMO values more significantly than substitutions on perpendicular fused rings.

Simple and reliable methods for the detection of biomaterials such as DNA and protein in science research have been developed for bio-sensors and medical applications. In this study, optically direct (i.e., label-free) DNA sensing was described using a single strand of light-emitting poly (3-methylthiophene) (P3MT) nanowire including negative counter-ions from dopants. Reacting with probe DNA and target DNA, light emission color and intensity of a single strand of P3MT nanowire varied under dry conditions (green-slight red-deep red). Furthermore, thrombin protein was effectively detected by aptamer functionalized P3MT nanowires by the same mechanism. For statistical analyses, photoluminescence and Raman mapping technique with great sensitivity were used. We believe that these detection methods, based on light-emitting P3MT nanowires, will open a new area of investigation for label-free biomaterial detection.

The control of propagation and emission of light is one of the most interesting properties of photonic structures to be applied in chemical and biochemical sensing. Different methods of introducing ad-hoc defects, where light can be localized to increase the interaction with the photoactive material, are among the most studied subjects. Moreover, some conjugated polymers are good candidates as active elements in flexible optical sensor architectures, e.g. quenching their fluorescence upon exposure to vapours of nitroaromatic compounds [1].
Here we report on the preparation and optical study of polymer heterostructures, such as Distributed Bragg Reflectors (DBR) and planar microcavities. We demonstrate high accuracy and reliability of a cheap spin-casting technique for the realization of large-size and flexible polymer DBR. A defect layer of emitting F8BT - designed and synthesized ad-hoc - is sandwiched between two DBRs to create a microcavity, or on top surface of the DBR to create a Bloch Surface Wave (BSW) structure.
We first characterized each component material by spectroscopic ellipsometry. Then, using these material dielectric functions, we designed the complete heterostructures to have a photonic band gap centered around 550 nm wavelength, where the largest emission of the F8BT occurs. The complete structures have been then studied by means of ellipsometry, angle resolved reflectance and transmittance in the visible-NIR spectral range to determine their optical quality and performance. The results are in good agreement to the theoretical modelling, estimating a variation of the layer thickness (about 100 nm) of less than 5% from nominal parameters.
In the microcavity structures, the presence of a defect layer gives rise to a transmission peak conrresponding to the photonic cavity mode. We observe directional photoluminescence (PL) enhancement when the organic emitters of the F8BT layer, are resonantly and spatially coupled to the cavity mode [2]. Thus the procedure allows an easy tuning of the photonic gap position and of the microcavity resonance mode within the polymer emission spectrum.
The DBR structure supporting BSW is instead suitable for optical sensing of vapours and solutions. The emission of the thin F8BT film on top of the organic multilayer deposited on a transparent glassy prism was excited tuning the angle of incidence of the pumping laser beam (about 405 nm) impinging in ATR configuration. PL is instead collected from the front. The best coupling was found by matching the BSW absorption to the laser wavelength and a PL enhancement of one order of magnitude was successfully measured with respect to the emission of the same film deposited on a bare glass slide.
[1] A. Abbotto, N. Manfredi, D. Comoretto, F. Gagliardi, F. Marabelli, Patent MI2012A000637, deposited on April 17, 2012.
[2] L Frezza, M. Patrini, M. Liscidini, D. Comoretto, J. Phys. Chem. C 115, 19939 (2011).

The effective detection of chemical explosives is an important component of anti-terrorism applications. Chemical explosives cover a large range of compounds. In optical sensing, explosive molecules of similar electronic properties often lead to similar response. For example, nitro containing explosives generally induce fluorescent quenching. Thus it is challenging for optical sensors to further differentiate them. To address this problem, Fluorescence Active Metal-Organic Frameworks (FAMs) were introduced. As a new class of fluorescent functional material, FAMs are desirable candidates for explosive detection due to their versatile structures, tunable band gaps and easy synthesis. Two structurally related FAMs were investigated as explosives sensors. Besides changes in fluorescent intensity, the emission maxima of FAMs also shifted upon exposure to analyte vapor. Taking both factors into account, analyte recognition can be realized on a two dimensional (2D) level: an analyte can be pinpointed on a 2D color-coded map. The selectivity and sensitivity are significantly improved utilizing this method. Experimental and theoretical approaches were employed to study the detection mechanism. IR spectroscopy was used to study the electron transfer within FAM-analyte system. And changes in emission maxima were probed by DFT calculations. In summary, a new detection method, 2D mapping was developed for FAM based sensors. This strategy could significantly benefit the design of chemical sensors to improve their performance.

A series of carbazole-based dendritic host materials Cz-CCP, Cz-mCP and Cz-TCB for solution processed blue phosphorescent organic light-emitting devices (PhOLEDs) was designed and synthesized. The twisted and nonplanar molecular structure of the carbazole dendrimer Cz-CCP, Cz-mCP and Cz-TCB limited the effective extension of their π-conjugation, keeping the triplet energy at a very high level (>2.75 eV, to confine the electro-generated triplet excitons on the dopant molecules). All dendritic hosts have outstanding thermal stabilities with the decomposition temperature Td ( >420 oC), and the glass-transition temperatures Tg ( >240 oC). It is worth to note that no noticeable signals of Cz-TCB related to glass transition is observed in the DSC curves. Atomic force microscopy (AFM) measurements also indicate that excellent thermal and morphological stability of the dendrimer (with low root-mean-square (RMS) values less than 0.2 nm). The single layer device using Cz-CCP, Cz-mCP and Cz-TCB as the hosts for the phosphorescence emitter iridium(III) bis(4,6-difluorophenylpyridinato)-picolinate (FIrpic) showed the maximum luminance efficiencies of 13.6 cd A-1. By introducing a thin 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBI) electron-transporting and exciton-confining layer, the maximum efficiency of the solution processed double-layer device can be further improved to 25.7 cd A-1, and maximum external quantum efficiencies as high as 13.0%. Moreover, the best performance of iridium(III) tris(3,5-difluoro-4-cyanophenyl)pyridinato-N,C&’ (FCNIrpic) based deep-blue-emitting devices exhibits a maximum efficiency of 10.9 cd A-1, and a maximum external quantum efficiency 6.4 %. The performance of the novel host material based devices is far superior to those of the corresponding 1,4-bis(9-carbazolyl)benzene (CCP), 1,3-bis(9-carbazolyl)benzene (mCP) and 1,3,5-tri(N-carbazolyl)benzene (TCB) based devices, which is outstanding for a solution-processed blue PhOLED. Our results demonstrate a promising approach to well-designed dendritic host materials for solution-processed blue PhOLEDs.

The need for new energy harvesting techniques is increasing, and research in photovoltaics is becoming more and more essential. In particular, there has been a growing research effort focused on “green” manufacturing techniques, including the use of bacteria to precipitate semiconducting nanoparticles. We argue that E. coli has tremendous potential in the fabrication of patterned cadmium sulfide (CdS) thin films for solar cell applications. Here we capitalize on the ability of genetically engineered E. coli to precipitate CdS nanoparticles, through i) the expression of the Treponema denticola cysteine desulfhydrase gene, and ii) the variation of four parameters (supplemental cysteine, potassium sulfate, cadmium chloride concentrations, and IPTG). IPTG is necessary to induce expression of the cysteine desulfhydrase gene, which is under control of the plac promoter. Exogenous potassium sulfate is added to drive additional endogenous production of cysteine by E. coli&’s native serine acetyltransferase (SAT). Finally, cysteine and cadmium chloride are added as precursors for the reactions mediated by cysteine desulfhydrase. After bacterial precipitation of the nanoparticles we attempt several different methods of organic removal, including chemical, ultrasonicating, and heat treatments. We have analyzed the toxicity of the cadmium to the bacteria through growth monitoring experiments as well as found optimal particle precipitation conditions. We hope that we can combine both of these trends to optimize the entire process and production of thin films. We discuss the properties of the deposited CdS nanoparticles and films, where we have used several analytical techniques including X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). We determined CdS nanoparticle morphology by TEM and determined the compositions of the nanocrystals by XPS. We report on measuring the ratio of cadmium to sulfur within the particle using this technique. We also use Fourier transform infrared spectroscopy to ensure that after heat treatments we still have CdS and not a cadmium oxide derivative to ensure optimal semiconducting properties in the nanoparticles. We will also discuss electron diffraction experiments, used to determine the crystal structure of the particles from both the bacterially synthesized and chemically synthesized nanoparticles. Our objective is to determine if particle quality is being compromised for the efficiency of bacterial synthesis or if this method could be a similar but more waste efficient substitute for chemical synthesis. We show that these genetically engineered E. coli have the ability to form patterns and monolayers on silica, glass, and indium tin oxide.

In this paper, we investigate the limits in terms of performance (efficiency and energy) that can be reached with Vertical External Cavity Surface Emitting Organic Lasers (VECSOLs), a distinctive type of organic laser structure, developed as a counterpart of VECSELs with organic thin-film gain materials. The thermal nature of photodegradation is evidenced and solutions towards high-energy organic lasers are envisioned.
VECSOLs offer the promise to combine in a single device the advantages of thin-film organic lasers (low cost, ease of fabrication, broad emission spectra, easy chemical tuning and high gain) with those of inorganic semiconductor VECSELs, i.e., high conversion efficiency, single transverse-mode operation, power scaling capability, and a high versatility offered by the open cavity, which includes the possibility to easily tune the laser wavelength or to achieve frequency conversion to reach new spectral domains.
Organic solid-state lasers are not usually thought of as relevant sources when looking for pulse energies in excess of a few µJ or repetition rates above the kHz level because of harsh photodegradation issues (and triplet piling up issues for higher repetition rates/long pump pulses). However, VECSOLs offer this unique possibility for “energy scaling” (increasing the energy at constant photodegradation rate) through a simple increase of pump spot size at constant irradiance.
The limit in terms of conversion efficiency is first analyzed, and is treated both analytically with rate equations and experimentally. With a proper choice of output coupling and pump pulse duration, the conversion efficiency can be pushed theoretically to the quantum limit (~90%) but happens to be ultimately limited by reabsorption and scattering losses, as well as by fluorescence losses whenever the pump pulse is too short compared to the oscillation buildup time. In practice a maximum conversion efficiency of 60% has been obtained with a thin film of PMMA doped with Pyrromethene 597 as the active medium.
Energy scaling was then demonstrated by putting into evidence an invariance of the photodegradation rate upon varying the pump fluence at constant irradiance. We investigated the thermal origin of photodegradation at high pump fluences/high repetition rates with infrared thermal microscopy (in the 8-12 µm range.) We show that significant heating occurs even at low repetition rates: for a 100 µJ (20 ns) pump pulse focused onto a 100µm-in-diameter spot, the steady-state temperature at the center of pumped zone reaches 2 K above ambient at 10 Hz and 15 K above ambient at 1 kHz repetition rate. FEA thermal simulations are in accordance with experimental temperature profiles and reveal that the glass transition temperature is largely surpassed during a few µs after the pump pulse and may lead to laser ablation. These observations underline the importance of thermal management in the route towards high-energy organic lasers.

Organic materials have shown enormous potential in applications like organic light emitting diodes and transistors. Since the demonstration of lasing in organic solid-state thin films of both small molecular weight molecules and polymers many efforts have been devoted to this field. Dye doped matrices which are based on the Förster energy transfer from a host to a guest molecule have shown to be excellent organic laser materials. These guest host systems exhibit high efficiency and low threshold due to large stokes shifts and low waveguide losses. Since the Förster energy transfer depends strongly on the doping concentration a high variation of the gain properties can be observed. Here we present the results of waveguide measurements using the variable stripe length method providing information about waveguide losses and optical gain in different guest host systems. In such a guest host system the occupation of the upper laser level (dopant side) is due to Förster energy transfer. This energy transfer strongly depends on the donor acceptor distance and hence on the concentration of the laser dye. Therefore, the doping concentration is varied and the gain coefficients are measured at various excitation densities to analyze the stimulated emission cross section. Additionally small molecules systems and polymer based materials are compared concerning their properties like waveguide loss, material parameter and cross section determined by waveguide measurements. E.g. conjugated polymers are very suitable as laser active materials but with the help of waveguide measurements we show how the material treatment before the film preparation affects optical loss and gain in a thin film. And finally it discussed how the spectral position within the gain spectrum determined by waveguide measurement influences the threshold of an organic DFB laser.

The conversion of incoherent, spectrally broad solar energy into a coherent monochromatic energy beam opens the science and technology of solar energy to the field of nonlinear optics. The ideal optical gain media would efficiently absorb spectrally broad sunlight in a small mode volume for low solar pump concentration to achieve population inversion. By reducing the mode volume to reduce inversion threshold, the efficiency of pump absorption, for traditional gain media such as rare earths, is also reduced. These two parameters must be decoupled for low inversion threshold and efficient solar pumping of optical gain media. This can be done by adding an intermediate step between absorption and emission. Unlike rare earth materials, nanocrystal materials can absorb spectrally broad light in small volumes and have high photoluminescence efficiency with a spectrally narrow emission compared to sunlight. When the nanocrystal thin film is optically coupled to the gain media, it will act as a waveguide for the nanocrystal photoluminescence. The photoluminescence of the nanocrystal can then be tuned to the absorption peaks of the gain medium via the particle size for an indirect optical pump. This photoluminescence tuning and waveguiding allows long interaction distances between gain media and photoluminescence which results in sunlight absorption and energy transfer in small mode volumes. This novel solar pumped gain media scheme is realized through a 1064nm AR/AR coated Nd:YAG slab with a thickness of 750µm, width of 2mm, and length of 43mm. The slab is coated with a 100µm thick film of PLMA doped with 2% wt. CdSe/ZnS nanocrystals. The nanocrystal film absorbs broadly in the visible spectrum and emits at 580nm where neodymium has a strong absorption peak. This gain media is placed in a hemispherical cavity with a planar mirror of R = 99.999% and output mirror of radius 15cm and R = 98% to realize a solar powered laser. The nanocrystal coating is pumped with a 18W 532nm quasi-CW laser as a high intensity photon source. An equivalent solar threshold of approximately 1000 suns has been achieved. Integration of this solar gain media into an optical amplifier for solar energy applications will also be discussed.

Optical concentration is a powerful and near-term strategy to lower the cost of electricity produced from established solar cell technologies because it reduces the cell area needed to generate a given amount of power. Whereas passive geometric optical concentrators are invariably bound by the sine limit, luminescent solar concentrators (LSCs) can in principle achieve high concentration ratio (>100x) without tracking the Sun. These devices traditionally consist of a luminescent slab that absorbs sunlight and emits it into modes confined by total internal reflection, where it is subsequently absorbed by photovoltaic cells attached to the edges.
We are exploring opportunities to improve LSC performance and diversify their application by photonically controlling the luminescent etenduacute;e. Leveraging highly directional luminescence together with a generalized nonimaging optical framework, we demonstrate routes to both increase concentration ratio for photovoltaics and alternatively to optimize the distribution of light within closed photobioreactors for enhanced algal biofuel productivity. Our approach, which is based on the use of macroscopic freeform waveguides, leads to the formation of optical pseudo-potentials that act like in-plane graded index variations, channeling the flow of light and localizing it at particular points to dramatically boost concentration ratio.

Using our recently developed non-adiabatic excited-state dynamics simulations framework, we study ultrafast dynamics and exciton transport in several large molecular systems. These simulations reveal a fascinating interplay of conformational vibrational dynamics and internal conversion followed photoexcitation, which has specific spectroscopic signatures and can be observed using time-resolved pump-probe spectroscopies. Observed relationships between spatial extent/properties of electronic wavefunctions and resulting electronic functionalities allow us to understand and to potentially manipulate excited state dynamics and energy transfer pathways in a number of organic molecular materials suitable for a variety of technological applications.

In the frame of developing new active materials for laser and light amplifier applications large interest has been devoted in the last decades to organic conjugated polymers. These molecules are particularly interesting as they can combine high optical gain, good charge mobility, and good film forming properties when deposited from solution.
Amplified Spontaneous Emission (ASE) and lasing has been demonstrated in several classes of luminescent polymers, thus proposing these molecules as potential active materials for low cost light amplifying waveguides and solid state lasers.
Large research efforts have been devoted to date to the increase of the active material optical gain, in order to decrease the ASE and lasing threshold, recently allowing the remarkable demonstration of organic lasers pumped by inorganic Light Emitting Diodes, that evidences how close to real laser applications are organic lasing systems.
In the last years we focused our interest in the investigation of the physical aspects that affect the final ASE properties of active polymeric waveguides, beyond the intrinsic gain properties of the active molecule.
In this work we will present our recent results relative both to effects with a direct applicative interest, and to effects more related to the basic photophysics of the active molecules.
In particular we will investigate the extrinsic elements affecting the ASE threshold and the operational stability, beyond the intrinsic active material gain cross section and photostability.
Moreover we will investigate by confocal laser spectroscopy the correlation between the mascroscopic ASE properties and the microscopic non uniformities of the active layer emission properties and morphology.
Our results indicate, on one side, that some care as to be used in order to be able to directly relate the ASE properties of a polymeric thin film to the intrinsic emission and gain properties of the active molecules.
On the other side we will show that many parameters, when properly controlled, can be used to modify the ASE properties of a system, thus allowing the optimization of the sample emission for specific device configuration.

For achieving organic solid-state lasers, current-injected narrow linewidth emissions (NLEs) are indispensable because NLEs are thought to be a precursor phenomenon. For the laser devices, a combination of organic materials with high optical gain and optical cavities is prerequisite. We chose thiophene/phenylene co-oligomers (TPCOs) [1, 2] as the organic semiconductor materials, and observed optically-excited laser oscillations from crystals of various TPCOs with parallel crystal facets acting as a Fabry-Pérot resonator [3-5]. Thus TPCOs are regarded as one of the promising materials for the organic solid-state laser devices.
We have fabricated light-emitting field-effect transistors (LEFETs) using the TPCO crystals as an emitting layer. We equipped these LEFETs with diffraction gratings. These gratings functioned as parts of the gate insulators as well. We engraved these gratings by a focused ion beam [6] or fabricated them by using the interference exposure [7] or the nanoimprint lithography [8, 9]. The TPCO crystals were laminated or grown directly on the diffraction gratings. When we operated these devices with the alternating gate voltage method [10], the devices indicated current-injected NLEs. The full-widths at half-maxima of these narrow lines were less than ~10 nm.
[1] S. Hotta, H. Kimura, S.A. Lee, T. Tamaki, J. Heterocycl. Chem. 37 (2000) 281.
[2] S. Hotta, T. Katagiri, J. Heterocycl. Chem. 40 (2003) 845.
[3] T. Yamao, K. Yamamoto, Y. Taniguchi, T. Miki, S. Hotta, J. Appl. Phys. 103 (2008) 093115.
[4] S. Hotta, Y. Sakurai, Y. Okuda, T. Miki, K. Matsunaga, F. Hirato, T. Yamao, H. Jinnai, J. Nanosci. Nanotechnol. 10 (2010) 440.
[5] T. Yamao, Y. Okuda, Y. Makino, S. Hotta, J. Appl. Phys. 110 (2011) 053113.
[6] T. Yamao, Y. Sakurai, K. Terasaki, Y. Shimizu, H. Jinnai, S. Hotta, Adv. Mater. 22 (2010) 3708.
[7] Y. Makino, T. Hinode, A. Okada, T. Yamao, N. Tsutsumi, S. Hotta, Physics Procedia 14 (2011) 177.
[8] Y. Makino, A. Okada, S. Hotta, T. Yamao, Mol. Cryst. Liq. Cryst. 566 (2012) 8.
[9] A. Okada, Y. Makino, S. Hotta, T. Yamao, Phys. Status Solidi C 9 (2012) 2545.
[10] T. Yamao, Y. Shimizu, K. Terasaki, S. Hotta, Adv. Mater. 20 (2008) 4109.

Vacuum deposition of para-sexiphenyl (p-6P) on muscovite is known to yield linear aggregates of co-oriented nanocrystals, usually referred to as nanofibers [1-3], having precise epitaxial relationships to the muscovite substrate [4]. These epitaxial nanofibers exhibit important optical properties, such as highly polarized blue luminescence and laser action [5], which could be exploited for applications in photonic and sensing technologies. Recent advances in the field include (i) the achievement of highly polarized, red-green-blue (RGB) emission from epitaxially aligned bilayer nanofibers obtained upon deposition of sexithiophene (6T) on p-6P/muscovite templates, and (ii) the development of a roll-printing technique for high-yield and massive transfer of nanofibers from the native substrate to a receiving substrate for device applications.
We report successful tuning of laser wavelength from ~420 nm to ~600 nm in epitaxially aligned nanofibers grown by periodic deposition of p-6P and 6T on p-6P/muscovite templates. The nanofibers were photoexcited by subpicosecond pulses tuned to the lowest p-6P absorption band, and the emission of 6T, whose coverage was kept in the submonolayer regime, was efficiently sensitized through resonance energy transfer [6]. 6T lasing was achieved at room temperature with threshold fluences as low as 10 mu;J/cm2 per pulse. Transient photoluminescence measurements with picosecond resolution showed that at these pump fluences the decay dynamics of 6T emission is independent of the excitation density, thereby demonstrating the attainment of room-temperature monomolecular lasing from epitaxially oriented 6T submonolayer aggregates. Main lasing properties remained unaltered upon direct photoexcitation of 6T below the p-6P absorption edge [7].
[1] H. Yanagi, and T. Morikawa, Appl. Phys. Lett. 75, 187 (1999)
[2] A. Andreev et al., Adv. Mater. 12, 629 (2000)
[3] F. Balzer, and H.-G. Rubahn, Appl. Phys. Lett. 79, 3860 (2001)
[4] C. Simbrunner et al., J. Am. Chem. Soc. 133, 3056 (2011)
[5] F. Quochi et al., Appl. Phys. Lett. 84, 4454 (2004)
[6] C. Simbrunner et al., ACS Nano 6, 4629 (2012)
[7] F. Quochi et al., Adv. Opt. Mater 1, Adv. Opt. Mater. 1, 117 (2013)

The illumination of a film of a polymer containing azobenzene moieties by means of linearly polarized light, in the UV/Visible wavelengths region, leads to the orientation of the azobenzene units perpendicularly to the light polarization direction. This results, after many trans-cis-trans isomerization cycles of the azo-unit, from the reduction of the transition probability when the rod-like trans-isomer is perpendicular to the electric field vector. This mechanism has been proposed for holographic data storage on this polymeric support. However, in 1995 a new phenomenon has been observed, the material-displacement (mass-migration) on the free surface of azobenzene-containing polymers.
Here we report about a new feature in patterning azo-polymers deriving by light-matter interaction where the film is sensitive to the phase information carried out by optical vortexes of different topological charges.
In fact, we have found the unexpected experimental observation of spiral-shaped relief patterns on the surface of an azopolymer that has been illuminated with a vortex laser beam, that is a beam having a helical wavefront. The spiral handedness of the polymer pattern is determined by the vortex one. This result is quite surprising because the common understanding hitherto was that these surface patterns respond to the light intensity distribution and its gradients. The intensity pattern of a vortex beam is shaped as a “doughnut” and carries no information whatsoever about the vortex handedness. We found an explanation for our observations that links them to a peculiar interference effect occurring between longitudinal and transverse field components of the vortex beam. Furthermore, we have found out that the main features of the observed phenomenon can be predicted by a phenomenological theory that does not rely on a specific microscopic model and therefore, in this sense, is a model-independent interpretation.
Our finding [1,2] will benefit the development of new lithography schemes as well as the interpretation of the phenomenon driving the material-displacement and the imaging of phase-related information.
[1] A. Ambrosio, L. Marrucci, F. Borbone, A. Roviello, P. Maddalena, Light-induced spiral mass transport in azo-polymer films under vortex-beam illumination, Nature Communications, 3:989 (2012)
[2] A. Ambrosio, P. Maddalena, L. Marrucci, Molecular Model for Light-Driven Spiral Mass Transport in Azopolymer Films, Physical Review Letters 110, 146102 (2013)

We report two aspects of our recent research on OLEDs. The first concerns the control of their light emission. In particular we have explored the extent to which it is possible to control the spatial emission pattern - for example to direct emission in a particular direction. We show that wavelength scale microstructure can strongly modify the emission pattern of OLEDs, particularly in devices with very narrow linewidth emitters. We will discuss the possible uses of such devices for visible light communications and biomedical applications.
Organic optoelectronic devices have several attractive features for applications in biology and medicine. These arise from their compact shape, simple fabrication and potential for flexibility. Building on prior work in which we showed OLEDs are capable of treating many skin cancers, we have made wearable organic optoelectronic devices for sensing. By combining organic LEDs and photodiodes, we have made a sensor for tissue oxygenation and used to measure oxygenation of forearm muscles.

This talk will describe the singlet oxygen (1O2) -responsive conjugated materials based on conjugated polymers (CPs) in which acene pendants are covalently linked to the polymer backbone. Cycloaddition of 1O2 to the acenes yields endoperoxides, which interrupts energy transfer between the polymer backbone and the acene pendant. The resulting ratiometric photoluminescent response occurs in both CPs dissolved organic solution and in aqueous environments with presence of acene-doped CP films. A class of novel diethylnyltetrancenes was developed and additional features, such as the reversible response to 1O2 will be discussed.

Liquid Crystal Elastomers (LCEs) are a class of polymer materials capable of phase transitions leading to significant shape changes. Such actuation is possible to trigger with several stimuli, such as temperature changes and exposure to solvents, and reversibly cycling makes them very attractive for artificial muscles. Their use in engineering components, however, has been limited by a lack of scalability in the fabrication process, difficulties in maintaining liquid crystal order during synthesis, and designing custom shapes.
We demonstrate a scalable method for the fabrication of novel anisotropic, micro-scale objects from LCE polymers. These objects undergo reversible strain changes of ~ 30% and can be thermally actuated. Our method involves the massively parallel synthesis and simultaneous mechanical stretching, and polymerisation under UV light. We show control over the objects&’ shape and size. We envisage that these objects could find applications as micro-actuators, and have designed methods to make them responsive to additional stimuli, such as light.

Inorganic quantum dot (QD) nanoparticles have distinctive optical properties, such as high photostability, quantum yield and narrow symmetrical emission peaks that render them very valuable as probes for fluorescence-based detection methods. Because QDs of different sizes can emit at different wavelengths (from UV to IR) upon excitation with the same light, they are intrinsically suitable for multiplexing in immunological diagnostics, cell trafficking analysis or cell identification via multiple markers. Realizing this potential requires the stable and specific functionalization of QDs with biological molecules, such as antibodies or other cell membrane-binding units, like lectins. In our group, a stable surface chemistry strategy has been developed to enable the passage of inorganic QDs to aqueous media and their further functionalization with bioactive molecules. The work here presented focus on the various biofunctionalization strategies currently under investigation and on their promising applications in immunohistochemistry, cell identification and cell trafficking analysis. Antibody immobilization to CdSe/CdS/ZnS QD nanoparticles is carried out following random, partly-oriented and cleavable/coiled linker approaches. The biofunctional nanoparticles are characterized regarding total protein content, biological specificity and stability in biological media. Relevant applications of these nanoparticles in immunohistochemistry and rare cell separation via FACS are presented.

Carbon nanodots (CNDs), as one branch of luminescent carbon nanomaterials, are emerging as potential alternatives for traditional semiconductor nanocrystals because of their distinct benefits such as low cost, less toxicity, biocompatibility, and excellent photoresponses. They can also be readily obtained from abundant sources with relatively simple equipment under moderate reaction conditions. The key drawback associated with previous methods is that CNDs can hardly be isolated because their surfaces remain non-passivated to retain reactive surface functional groups such as carbonyls. Thus far, this has hampered investigation and understanding of size and surface effects on the energy structure. Although several organic-based surface-capping methods have been reported earlier, there are still challenges such as size-tunability, unraveling photoluminescence (PL) mechanism, and attaining high quantum yields ahead.
In this work, we report size-tunable soft-template synthesis of oleylamine-capped CNDs for optoelectronic devices. The size of the CNDs can be controlled by regulating the amount of an emergent. As the size increases, the CNDs exhibit blue-shifted photoluminescence or so-called an inverse photoluminescence shift. By using time-correlated single photon counting, ultraviolet photoelectron spectroscopy, and low-temperature PL measurements, we reveal that the CNDs are compose of the sp2 clusters with certain energy gaps and the oleylamine ligands act as auxochromes to reduce the energy gaps. The composition of the CNDs is controlled by incorporating nitrogen-containing molecules in the reaction. As the nitrogen content increases, the CNDs exhibit red-shifted photoluminescence and this phenomenon is resolved in terms of the auxochromic effect of nitrogen. We finally exploit our size and composition controlled CNDs in lighting devices to successfully demonstrate green, yellow, orange, and white color. We believe that this study could shed light on future applications of the CNDs and further expedite the development of other related fields.

The continuing development of colloidal quantum dot (QD) based light-emitting diodes (QD-LEDs) has been driven by QDs&’ exceptional color purity and tunability, along with amenability to scalable and inexpensive solution processing. Despite recent advances, QD-LEDs still suffer from poor stability and relatively low device efficiencies, particularly in terms of significant efficiency decreases at high current densities (often referred to as “efficiency roll-off”), which limits their practical application in displays or lighting. In this presentation, we examine the impact of carrier dynamics within a series of structurally-engineered cadmium chalcogenide QDs on QD-LED device performance. Detailed device characterization combined with spectroscopic analysis shows that overall device efficiency, and efficiency roll-off, are strongly influenced by the multicarrier dynamics of QDs rather than by the single exciton dynamics. Specifically, we show that the Auger recombination process plays a major role in electroluminescence as a consequence of a substantial imbalance in charge injection from neighboring charge transport layers. Finally, we will discuss how these insights are being applied to design new QDs and improved device architectures for achieving stable, high-efficiency QD-LEDs.

Over the last year of development, external quantum efficiencies (EQE) of the best electrically driven colloidal quantum-dot light-emitting devices (QD-LEDs) has risen to >18 %, with brightness in excess of 50,000 cd/m² at operating voltages of only a few volts, and turn on voltages just below the QD bandgap energy/q. Efficient performance has been demonstrated across the visible spectrum and into the near IR, exceeding the EQE of any other planar-emitting technology at emission wavelengths beyond 1µm. The high luminescence efficiency and uniquely size-tunable colour of solution-processable semiconducting colloidal QDs is the key enabler in development of QD-LEDs for use in energy-efficient, high-colour-quality thin-film display and solid-state lighting applications. The talk will highlight the next challenges in advancing the state-of-the-art visible-LEDs and the promise of near-infrared and heavy-metal-free devices. As QD-LED efficiencies approach those of molecular organic LEDs, the talk will catalog the key scientific and technology challenges facing QD-LED commercialisation and offer outlook on the ongoing strategies to overcome these challenges.

Porphyrins are promising antenna materials for artificial light harvesting systems that mimic natural photosynthesis. The high degree of chromophore ordering within crystalline porphyrins facilitates photon energy capture, transport and delivery. However, these porphyrin antennae display intense absorption in only a limited portion of the visible region. Unlike the porphyrins, semiconductor quantum dots (QDs) feature broad absorption spectra, large extinction coefficients, and excellent photostabilities. However, QDs show poor energy and charge transport capabilities that limit their applications in light-harvesting.
Here we report photophysics study of a series of porphyrin/QD complexes which are designed to combine the advantages of porphyrins and QDs as light harvesting materials. Two metal- prophyrins, namely, zinc 5,10,15,20-tetraferrocenylporphyrin (ZnTFcP) and Magnesium tetraferrocenyltetraazaporphyrin (MgTFcTAP) were synthesized and blended with Cadmium Selenite (CdSe) quantum dots with different sizes. It was found that the major excited state interaction between porphyrins and quantum dots are energy transfer. High energy transfer efficiency was obtained in both solution and thin film samples. It was also found that energy transfer rates can be regulated by the size of quantum dots. Increasing quantum dot size leads to higher energy transfer rate in MgTFcTAP/CdSe, but lower rate in ZnTFcP/CdSe.

Metal nanocrystals (MNCs) are gaining interest, particularly for bioimaging and sensing applications, due to their small size, environmental sensitivity, as well as facile and solution-based synthesis. Furthermore, luminescence quantum yields and photostability have improved considerably in recent years. By virtue of the fact that multiple metals have shown fluorescence (Ag, Au, Cu, Pt) and because quantum confinement effects are very strong at the subnanometer to nanometer size range, emission tunability throughout the visible and near infrared spectrum has been demonstrated.
Light emitting diodes (LEDs) are establishing themselves as an important element in emerging displays and lighting solutions. Materials such as III-V semiconductors are often used, but with growing popularity of both molecular- and quantum dot-based emitters. While the emission of MNCs has been demonstrated both in solution as well as solid-state, electroluminescence of MNCs within a thin film structure has never been shown. Here, we make use of a synthetic phase-transfer strategy for preparing glutathione-coated MNCs in organic solvents [Yuan et al., ACS Nano, 2011, 5, 8800] to demonstrate a thin film MNC-LED with either Au or Ag MNCs as the emitting element. In both cases, the electroluminescence peak of the LED corresponds with the photoluminescence of the MNCs in solution, and we are able to observe spectrally pure emission without parasitic emission from other materials making up the LED structure. Specifically, the Ag NCs feature an emission peak at 695 nm, whereas the Au NCs emit at 750 nm. The Au and Ag emission onsets at low voltages of 1.74 V and 2.21 V, respectively. This is therefore the first time that electroluminescence of a MNC thin film has been observed.
Ultimately, we show that MNCs represent an additional option for an emitter within the thin film LED research domain, and with continued improvements in spectral narrowing and quantum yield, that these emitters could find future applications.

The unique photoluminescence properties of graphene quantum dots (GQDs) underlie the intense and rapidly expanding research activity in this young field. We show that coating of GQDs with different molecular layers enables intimate modulation of the photophysical properties of the nanoparticles, particularly excitation/emission over a very broad spectral range. Furthermore, facile synthetic routes of hydrophobically-coated GQDs produced particles that could be assembled in monolayers, yielding additional tools for modifying the luminescence properties. Overall, coated GQDs constitute a promising platform for generating new luminescent material and photoactive devices.

We obtain a systematic study of photoluminescence (PL) intensity, photon fluctuations and biexciton (BX) quantum yields in single “giant” CdSe/CdS nanocrystal quantum dots (NQDs) as a function of NQD core sizes and shell thickness. Specifically, by recording PL blinking and second-order photon intensity correlations of individual core/shell NQDs we obtain statistically significant correlations between degrees of blinking suppression and BX quantum yields as a function of the nanocrystal core size for a given number of shell monolayers (MLs). We show that while large core sizes (d=5.5 nm) show large degree of blinking suppression and high values of BX QY, their small core counterparts (d=2.2, 3 nm) show increased blinking and significantly lower BX QYs. In addition, while large core NQDs show nearly perfectly Poissonian distribution of the photon emission statistics as characterized by Mandel Q parameter around one, smaller core dots show highly non-Poissonian emission statistics, suggesting a contribution of the multiple emissive states. We propose the model to explain our findings.