Organic/polymer and hybrid photovoltaics

Resume : Mesoscopic sensitized solar cells are one of the most promising low-cost and environmentally friendly photovoltaic solutions and represent a viable alternative to crystalline semiconductors and other thin films photovoltaic technologies. Recently, Kim et al. used a solution-processable hybrid organic-inorganic perovskite of the formula CH3NH3PbI3 as sensitizer for DSSCs reaching a PCE of 9.7% in conjunction with mesoporous TiO2 and spiro-MeOTAD1. Lee et al. showed that such a device using a mixed halide (CH3NH3)PbI3-xClx could achieve better photovoltage by replacing the TiO2 film by an insulating Al2O3 scaffold. 2-4 Lately, we reported on the application of a sequential deposition technique to produce a DSSC based on the deposition of PbI2 first on the oxide scaffold with subsequent transformation to the desired nanoscopic CH3NH3PbI3 perovskite pigment by dipping in a solution of CH3NH3I3.5 The use of this new procedure enabled stable performance and a new record PCE of 15.0 %.6 Finally, Snaith et al. have also demonstrated PCEs of over 15% using a vacuum evaporated planar heterojunction perovskite solar cell.7 Recent publications on transient absorption and photoluminescence-quenching measurements show impressive electron-hole diffusion lengths (100-1000 nm), diffusion constants, and lifetimes in triiodide perovskites.8,9 It has been speculated that these parameters and their impact on efficiency significantly depend on the deposition technique, the morphology, stoichiometry and crystallinity of the material. Herein, we will present our latest advances in perovskite-based solid-state solar cells. These include recent TEM characterization showing conformal coating of the TiO2 by the perovskite using the sequential deposition technique. Also, we will show direct evidence of “TiO2 sensitization” by the perovskite by femtosecond transient absorption.10 Finally, we will present the latest solution-based fabrication techniques that led to record breaking efficiencies above 17% at full AM1.5 sun intensity. 1. Kim, H. et al. Thin Solid Films 517, 2563–2580 (2009). 2. Lee, M. et al. Science 338, 643–647 (2012). 3. Crossland, E. J. W. et al. Sci. Rep. 495, 215–219 (2013). 4. Carnie, M. J. et al. Chem. Commun. (2013). doi:10.1039/c3cc44177f 5. Liang, K. N., et al. Chem Mater 10, 403–411 (1998). 6. Burschka, J. et al. Nature 499, 316–319 (2013). 7. Liu, M., Johnston, et al. Nature : Nature Publishing Group. Sci. Rep. 501, 395–398 (2013). 8. Stranks, S. D. et al. Science 342, 341–344 (2013). 9. Xing, G. et al. Science 342, 344–347 (2013). 10. Marchioro, A. et al. Nature Photonics 8, 250–255 (2014).

Resume : The performance of perovskite solar cells has rapidly surpassed those of both traditional dye-sensitised and organic photovoltaics. It has been demonstrated that high light-to-electricity conversion efficiency can also be realised on transition from mesoporous to thin-film architectures. We address the origin of this success in the context of the materials chemistry and physics from atomistic modelling [1,2]. In addition to the basic optoelectronic properties essential for a good photovoltaic device (band gap, optical absorption, carrier effective masses), the materials are structurally and compositionally flexible. While the crystals are weakly ferroelectric at room temperature, spontaneous polarisation in the presence of an electric field is predicted, which can be tuned through judicious choice of the organic cation. Further, the combination of high dielectric constant and low effective mass favours exciton separation and effective ionisation of donor and acceptor defects. A photoferroic effect may contribute to the high open-circuit voltages and result in the strong hysteresis found in perovskite based solar cells. 1. F. Brivio, A. B. Walker and A. Walsh, APL Materials 1, 042111 (2013). 2. F. Brivio, J. M. Frost, K. T. Butler, C. H. Hendon, M. van Schilfgaarde and A. Walsh, Under Review (2014).

Resume : Organometal halide perovskites are quickly rising as new low-cost, nature-abundant materials for thin film photovoltaic devices. Like any polycrystalline thin film solar cells, the crystal quality of perovskites is trivial in determining the performance of the perovskite solar cell devices, especially in the planar heterojunction devices. Here we will report our recent progress on improving the quality of perovskite material to achieve high efficiency of 15-17% in devices with a planar heterojunciton structure. The perovskite materials were formed by low temperature solution processes, spin-coated from either premixed precursor solution or by the interdiffusion of the precursor double layers. We will report the influence of substrate surface on the composition, electronic property, as well as the morphology of the perovskite films. Using a non-stoichiometric precursor solution was found critical to form stoichiometric pervoskite films because the compositions of the spun perovskite films are very sensitive to the surface of substrates, and can be very different from that in precursor solutions. We will also report how to passivate the defects in perovskite films to minimize the charge recombination which boosts the fill factor to record value of above 80% for perovskite solar cells.

Resume : Solution-processable metal-halide perovskites recently opened a new route towards low-cost manufacture of photovoltaic cells. [1-3] Meso-superstructured solar cells and solid thin-film planar hetero-junction made from lead-based hybrids show record efficiencies. Converting sunlight into electrical energy depends on several factors among which a broad absorption across the solar spectrum and attractive charge transport properties are of primary importance. Hybrid perovskites meet such prerequisites but, despite foremost experimental research efforts [4-5], their understanding remains scanty [6-8]. Here we show that the appropriate absorption and transport properties are afforded by the multi-bandgap and multi-valley nature of their band structure. We also discuss the nature of the excitation and evidence exciton screening related to collective orientational motion of the organic cations at room temperature. Chlorine doping hinders such collective motion. [1] Kojima, A., Teshima, K., Shirai, Y. & Miyasaka T J. Am. Chem. Soc. 131, 6050-6051 (2009). [2] Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Science. 338, 643-647 (2012). [3] Kim, H-S. et al. Sci. Rep. 2, 591-1-591-7 (2012). [4] Stranks, S. D. et al. Science. 342, 341-344 (2013). [5] Xing, G. et al. Science. 342, 344-347 (2013). [6] Even, J., Pedesseau, L., Dupertuis, M.-A., Jancu, J.-M. & Katan, C. Phys. Rev. B. 86, 205301 (2012). [7] Even, J., Pedesseau, L., Jancu, J.-M. & Katan, C. J. Phys. Chem. Lett. 4, 2999-3005 (2013). [8] Even, J., Pedesseau, L., Jancu, J.-M. & Katan, C. Phys. Status Solidi RRL.1 DOI: 10.1002/pssr.201308183 (2013),

Resume : High efficiency hybrid halide perovskite solar cells have been developed faster than the understanding of the device physics. Here we use electronic structure methods to understand the unique features of this system. We look at the interaction and dynamics of the organic cation [1,2], the role of its polarisation in creating ferroelectric domains within the active device, and the interaction of these domains with excitons and polarons. Electronic structure calculations provide parameters (elastic strain, and dipole strength) for an on-lattice Monte Carlo simulation of ferroelectric domains. Parameters from electronic structure calculations are used as inputs into Monte Carlo and drift-diffusion device models, to understand how the small scale structure relates to device operation. 1. F. Brivio, A. B. Walker and A. Walsh, APL Materials 1, 042111 (2013). 2. F. Brivio, J. M. Frost, K. T. Butler, C. H. Hendon, M. van Schilfgaarde and A. Walsh, Under Review (2014).

Resume : With rapidly rising efficiency values of organic-inorganic hybrid perovskite based solar cells, an anomalous hysteresis has also been reported in the current-voltage curves [1]. So far de-tailed explanations and qualitative models are still lacking, and the origin of this anomalous hysteresis still remains an open question. In this contribution we report a systematic study of strong hysteresis in the J-V curves in planar heterojunction perovskite solar cells, and propose a reorientation of ferroelectric dipole model to elucidate the underlying mechanism of the hys-teresis. Applying a positive initial bias before the backward measurement will further boost the efficiency considerably by increasing the Jsc, Voc and fill factor simultaneously. The effec-tiveness of this enhancement is heavily dependent on the value of applied bias, holding time, scan speed and illumination. The huge discrepancy between different measurement conditions is presumably primarily originated from the reorientation of the electric dipole of me-thylammonium ions and octahedral PbI6 under an externally applied electrical field. Further-more contributions from trap states and ion migration should also be examined to fully under-stand the origin of hysteresis. This study paves the way for further investigations of unraveling the physical mechanism behind this observed phenomenon, which is crucial for understanding the working mechanism of perovskite solar cells and ultimately for its further optimization. Meanwhile, it is urgent and imperative for the whole community to find a wide-ly accepted approach to assess the genuine power conversion efficiency to facilitate the effec-tive comparison of improvements made by different research groups and promote rapid ad-vancement in this field as well. In a second part we will discuss the application of CH3NH3PbI3-xClx based perovskite top solar cell together with thin film Cu(In,Ga)Se2 bottom solar cell in tandem devices. [1] H. Snaith et al., J. Phys. Chem. Lett., 2014, 5(9) , pp 1511-1515, DOI:10.1021/jz500113x, 2014

Resume : Molybdenum oxide (MoO3) and fullerene C60 thin films are ubiquitously used as hole extraction interfacial layer and electron acceptor material for the fabrication of organic photovoltaic (OPV) cells. It is well known that light exposure induces color changes in MoO3 (photochromism) and the formation of intermolecular bonds between C60 molecules (photopolymerization). The influence of these photoinduced reactions on the long-term stability of OPV cells, however, has not been studied in detail so far. We present a study and discuss the early (< 5 days) aging mechanisms occurring in illuminated ITO/MoO3/organic dye/C60/Ag bilayer solar cells under nitrogen atmosphere. With UV-vis spectroscopy we observed distinct changes in the absorption spectra of C60 and MoO3 thin films with kinetics of transformations that closely match the solar cell performance decay. Pre-processing experiments of individual layer stacks, light exposure at low temperatures and irradiations in selective wavelength regions were then used to separate the influence of the two photochemical transformations on solar cell degradation. We find that the major part of the open-circuit voltage decay stems from the MoO3 layer, and that changes in MoO3 and C60 contribute almost equally to the observed drop in the photocurrent.

Resume : As is known [1, 2], excitons are generated in the absorption of photons in all solar cells (SC) based on the polymeric compositions. The decay of excitons into electrons and holes is an essential stage in the functioning of such SC. In this work the study of some issues related to this topic has been carried out. 1. The possibility of tunnelling of excitons (as a whole) and electron-hole pairs, generated in quantum dot during absorption of high-energy photon, through the ?quantum dot ? matrix? interface has been studied. The analysis was carried out on the base of Feynman approach in quasi-classical approximation. It was shown that exciton tunnelling probability is greater than the one for the lone electron. 2. The process of exciton decay into electron and hole with the involvement of acceptor (fullerene molecule) was studied. The analysis is based on the second-order collision theory (Landau-Zener approach) [3]. The averaging of Landau-Zener probability has shown that charge-exchange cross-section is inversely proportional to the speed of exciton?s travel in the polymer chain. 3. The possibility of exciton decay into the electron and hole during passing through the regular cross-link of polymer chain has been studied. The conception of formation of compound state when exciton covers the cross-link was introduced, and a statistical theory of distribution of generated electron and hole along edges forming the cross-link was developed. It was found that the decay probability is symbate to the number of edges and antibate to the exciton binding energy. An alternative quantum-mechanical description (under the theory of quantum graphs) of the complex particle decay (exciton) into the components (electron and hole), consistent with the statistical model was proposed. REFERENCE: 1. M. Pope, Ch. Svenberg. Electronic processes in organic crystals, V.1, 2, M.: ?Mir?, 1985. 2. С. Deibel, V. Dyakonov, Rep. Progr. Phys.(2010), 73, 096401. 3. L.D. Landau, E.M. Lifshits. Quantum mechanics. M.: ?Nauka?, 1974

Resume : CuI nanoparticles were used as anode buffer layer with and without PEDOT:PSS in organic solar cell fabricated from anthracene ? containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) (PPE-PPV) (known as AnE-PVstat)3 based solar cells. AnE-PVstat blended with PCBM in bulk hetrojunction structure .The results showed an enhancement in the power conversion efficiency by using CuI as a buffer layer .

Resume : ITO, ZnO, ZnO:Al and NiO oxides were obtained by direct thermal oxidation of metallic thin films deposited onto optical glass substrates by magnetron sputtering. Two deposition configurations were taken into account: perpendicular and parallel to the plasma flow. The morphological features were determined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Roughness average and root-mean-square parameters were determined for metallic and oxide thin films, prepared in the both deposition configurations. The transmission spectra for oxide thin films, drawn in the UV-VIS range at room temperature, revealed a good transmittance, and they were used further to evaluation of the optical band gap for each prepared thin film oxide. The structural characterization by X-ray diffraction measurements showed that the obtained oxides have a well-defined crystalline structure. Taking into account the properties of the obtained thin film oxide we conclude that the direct thermal oxidation of metallic thin films is a low cost technique to prepare good quality thin film oxides for transparent electronics and optoelectronics applications. Keywords: oxide, photovoltaic cells, XRD Acknowledgments: S.I. is grateful to the Pays de la Loire and Angers University for the post-doc fellowship No 2012-12029 financial support.

Resume : 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. Alcubilla Organic Electronics, Volume 1 (6) 1643 (2013)

Resume : Organic?organic heterojunctions are at the basis of photovoltaic device operation. The interface between acceptor (A) and donor (D) materials provides the driving force for exciton splitting into a hole in the D and an electron in the A. However the understanding of charge photogeneration remains incomplete and certainly implies competing mechanisms for exciton recombination and charge separation. In this work we assess the impact of the relative molecular orientation at the D/A interface on the exciton dissociation. We focus on two small molecules, DIP (diindenoperylene) and PTCDI-C8 (N,N?-dioctyl-3,4,9,10-perylene tetracarboxylicdiimide) deposited on SiO2/Si substrates in ultra-high vacuum. The growth at submonolayer coverage results in two-dimensional islands of ordered molecules in a nearly standing upright orientation. Two type of model heterointerfaces can be grown in a controlled way: Adjacent packing of single component islands (horizontal configuration) or single component islands of A on top of an underlying D monolayer (vertical configuration). Aspects related to charge photogeneration have been studied by Kelvin probe force microscopy (KPFM) and photoluminescence spectroscopy (PL). We show that the dominant recombination mechanism for the horizontal heterostructures occurs through a CT state . These experiments provide a clear identification of molecular orientation as one of the factors governing recombination through interface states.

Resume : Controlling the morphology of bulk heterojunction (BHJ) polymer?fullerene solar cells is one of the keys for high performance. A complex interplay between thermodynamic and kinetic factors triggers the nanomorphology that forms dynamically during solvent evaporation. We report real-time grazing incidence x-ray scattering studies as solvent evaporates for blends of several polymers in combination with PCBM. An optical reflectometer allowed simultaneous measurement of the blend?solvent film thickness during solvent evaporation. We focus particularly on the effect of the substrate temperature during drying and the effect of additives.

Resume : Metallic diselenides (MSe2), such as MoSe2 or WSe2 have recently emerged as a new category of promising acceptor materials for third generation photovoltaic devices. Their low bandgap (1.42 eV) compared to organic semiconductors is expected to yield complementary absorption in the solar spectrum. Unlike II-VI metal oxides, MSe2 are shown to crystallize in hexagonal group, hence exhibiting conduction properties as performing as standard inorganic semiconductors. Furthermore, their chemical stability upon air exposure makes them very attractive for hybrid solar cells in which robustness is an important issue. In this work, single crystals of MoSe2 (intrinsic or Nb doped (n-type)) have been synthesized using chemical vapor transport. Upon cleavage, regular 0.6 nm steps were observed with atomic force microscopy (AFM). This typically corresponds to half the interplanar distance along the (0001) direction, as cleavage preferentially takes place along that axis due to the Van Der Waals packing of the MoSe2 sheets. Conductive AFM has been used to locally study the injection and transport properties of MoSe2, allowing specific discrimination of the doping contribution. MoSe2 was then exfoliated by sonication in dichlorobenzene; sheets consisting of around ten monolayers were observed by AFM and analyzed with C-AFM. Finally, the exfoliated material was blended with polythiophene in order to generate hybrid active layers for photovoltaic devices.

Resume : Nanocomposites materials have drawn much attention because of their interesting properties beyond what parent constituents possess [1]. Hence, characterization of hybrid thin films combining the high optical absorption of p-type conjugated polymer film and the n-type well crystallized ZnO nanoparticles has been reported in the aim to develop a low cost solar cells. The ZnO nanoparticles were successfully synthesized using a wet chemical route [2]. The structural and optical properties of PVK: ZnO nanocomposites with different mass ratios deposed on two different anodes (ITO and ITO: PEDOT: PSS) have been carried out by atomic force microscopy (AFM), scanning electron microscope (SEM), RAMAN spectroscopy, UV-Visible spectrometry and photoluminescence (PL). The roughness of the samples increased with the concentration of ZnO nanoparticles. AFM images confirm the amorphous nature of a PVK thin film and show that the role of PEDOT: PSS layer was to smooth the surface between the substrate and the active layer. SEM images showed that different extents of aggregates were formed for the different nanocomposites. The structure of PVK was not affected as revealed by Raman experiments. Results show that ZnO nanoparticles don?t affect the absorption features of PVK. Furthermore, a clear PL quenching was also observed upon the incorporation of ZnO nanoparticles in the PVK matrix indicating an efficient charge transfer along the

Resume : In the present work a vertically electrostatic field was applied to serve a more well order polymer chain accompany by pre-annealing to fix the oriented chains to provide well pathway for charge separation in Polymer solar cell with Glass/ITO/PEDOT:PSS/P3HT:PCBM/LIF/AL. For reaching to this purpose a device was designed to apply electrostatic field and vacuum simultaneously as well as temperature which named EVA (Electrostatic Vacuum Annealing). Two temperatures of 90°C and 120°C under 106 v/m electrostatic fields at vacuum atmosphere were selected; tremendous increase in crystallinity as well as final cell’s efficiency has been observed at 120°C. This significant enhancement might be attributed to highest crystallization rate of oriented polymers in this temperature, in other words, polymer chains have more chance to orient and order before freezing. The consequence of electrostatic field force inducing crystallization was well order nanomorphology in active layer and efficiency improvement in final samples; also, the observed reduction in the photoluminescence spectra (PL) for treated cells might be due to the formation of continues conductive pathway for more charge extraction. The optical and morphological characteristics are carried out via AFM and SEM. The current density-voltage (J versus V) curve of photovoltaic device under white light was shown significant improvement in efficiency of treated cells. The P3HTcrsytalinty was investigated by GIXRD. In addition, Uv-visible spectra provides information of degree on interaction order and reveals that there is substantial local ordering of the P3HTcomponent that have entailed great increase in active layer absorption.

Resume : Organic photovoltaic’s (OPVs) have been intensively investigated in recent years for generation of affordable, clean, and renewable energy due to their low-cost fabrication of large-area devices, lightweight, mechanical flexibility, and easy tunability of chemical properties of the polymer materials. PTB7/PC71BM solar cells were fabricated, characterized and the effect of DIO was studied. Morphology study was carried out by the Atomic Force Microscopy (AFM). The current density &voltage (J&V) characteristics of photovoltaic cells were measured under the illumination of simulated solar light with 100 mW cm-2 (AM 1.5G) by an Oriel 1000 W solar simulator. PTB7/PC71BM solar cell enhanced the power conversion efficiency (PCE) to 2.86% with % 0.5 DIO from %1.55 without DIO.

Resume : Combining both ultimately low cost materials and production with a high efficiency solar technology has thus far been elusive. Low cost materials, such as organics and oxides, tend to suffer from fundamental energy losses required to separate excitons and collect free charge carriers in electronically disordered semiconductors. Recently perovskite solar cells have emerged as a new material family capable to combine both extremely low cost and potentially the highest efficiencies in a single technology. Here I will present recent advances with perovkite based solar cells developed at Oxford, both in understanding of perovskite crystallization and film formation, and in understanding and enhancing solar cell performance. I will focus on both aesthetic applications for building integrated photovoltaics, and on understanding solar cell operation and maximising power conversion efficiency.

Resume : Organometal halide perovskites have recently attracted enormous attention since (CH3NH3)PbX3 can be successfully applied as photoactive material in photovoltaic devices, yielding solar cells with an efficiency exceeding 15%. Surprisingly, the exact mechanism how charges are generated and transported so well is unclear. In this work, we investigated the charge carrier mobility and recombination in (CH3NH3)PbI3 as an intrinsic layer on an inert quartz substrate or introduced into a meso-porous Al2O3 layer. To this end we recorded time resolved microwave conductance (TRMC) signals upon pulsed laser excitation at 410 nm at different temperatures ranging from 80 to 300 K. At 300 K we observed efficient formation of microsecond lived charge carriers at low laser fluences with a maximum charge carrier mobility of about 5 cm2/Vs yielding charge carrier diffusion lengths well above 1 μm. At higher laser intensities higher order recombination processes become operative lowering the mobile charge carrier population rapidly. At lower temperatures we see gradually higher charge carrier mobilities which we associate to a reduction in phonon scattering. Changes in the charge carrier dynamics at these temperatures are presented and discussed.

Resume : Among Organic Electronics, solution-processable π-conjugated polymers are proving particularly promising in bulk-heterojunction (BHJ) solar cells with fullerene acceptors such as PCBM.[1] In the past few years, we have found that varying the size and branching of solubilizing side-chains in π-conjugated polymers impacts their self-assembling properties in thin-films. Beyond film-forming properties, nanoscale ordering in the active layer governs material and device performance. For example, in poly(benzo[1,2-b:4,5-b’]dithiophene–thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD), side-chain substituents of various size and branching impart distinct molecular packing distances (i.e., π–π stacking and lamellar spacing),[2] varying degrees of nanostructural order in thin films,[2] and preferential backbone orientation relative to the device substrate.[3,4] While these structural variations seem to correlate with BHJ solar cell performance, with power conversion efficiencies ranging from 4% to 8.5%,[2,3,5] we believe that other contributing parameters – such as the local conformations between the polymer and the fullerene, and the domain distribution/composition across the BHJ (i.e., pure/mixed phases) – should also be taken into account.[6,7] Other discrete modifications of PBDTTPD’s molecular structure affect polymer performance in BHJ solar cells with PCBM, and our recent developments emphasize how systematic structure-property relationship studies impact the design of efficient polymer donors for BHJ solar cell applications.[8-10] [1] P. M. Beaujuge, and J. M. J. Fréchet, JACS, 2011, 133, 20009. [2] C. Piliego, T. W. Holcombe, J. D. Douglas, C. H. Woo, P. M. Beaujuge, and J. M. J. Fréchet, JACS, 2010, 132, 7595. [3] C. Cabanetos, A. El Labban, J. A. Bartelt, J. D. Douglas, W. R. Mateker, J. M. J. Fréchet, M. D. McGehee, and P. M. Beaujuge, JACS, 2013, 135, 4656. [4] J. Warnan, A. El Labban, O. Ratel, C. Cabanetos, C. Tassone, M. F. Toney, and P. M. Beaujuge, 2014, Submitted. [5] J. A. Bartelt, J. D. Douglas, W. R. Mateker, A. El Labban, C. J. Tassone, M. F. Toney, J. M. J. Fréchet, P. M. Beaujuge, and M. D. McGehee, Adv. Energy Mater., 2014, Accepted. [6] J. A. Bartelt, Z. M. Beiley, E. T. Hoke, W. R. Mateker, J. D. Douglas, B. A. Collins, J. R. Tumbleston, K. R. Graham, A. Amassian, H. Ade, J. M. J. Fréchet, M. F. Toney, and M. D. McGehee, Adv. Energy Mater., 2013, 3, 364. [7] K. R. Graham, C. Cabanetos, J. P. Jahnke, M. N. Idso, A. El Labban, G. O. Ngongang Ndjawa, B. F. Chmelka, A. Amassian, P. M. Beaujuge, M. D. McGehee, 2014, Submitted. [8] J. Warnan, C. Cabanetos, A. El Labban, M. R. Hansen, C. Tassone, M. F. Toney, and P. M. Beaujuge, 2014, Submitted. [9] J. Warnan, A. El Labban, C. Cabanetos, E. Hoke, C. Risko, J-L. Brédas, M. D. McGehee, and P. M. Beaujuge, Chem. Mater., 2014, Accepted. [10] J. Warnan, C. Cabanetos, R. Bude, A. El Labban, Liang Li, and P. M. Beaujuge, 2014, Submitted.

Resume : Intensive research performed during the last decade was focused on the design of novel material combinations for highly efficient organic solar cells. In spite of a certain success, existing materials failed to demonstrate simultaneously optimal optoelectronic properties, high efficiency in solar cells and excellent stability. One of very few conjugated polymers which showed advanced stability is a copolymer of carbazole, thiophene and benzothiadiazole named PCDTBT. Photovoltaic performance of PCDTBT is limited mainly by its wide optical band gap of 1.9 eV which prevents efficient harvesting of photons in the NIR region. In this talk we will describe our approach for lowering the band gaps of PCDTBT-type polymers via increasing the electron affinity (moving LUMO down) without affecting the ionization potential (HOMO). The desired properties were achieved by replacing TBT unit with extended TBTBT block in the main polymer chain. A family of conjugated polymers with Eg~1.6 eV was designed and investigated. The organic bulk heterojunction solar cells based on new materials showed power conversion efficiencies of >6.0%. Increase in the device performance up to 10% and beyond is theoretically feasible in the view of optimal optoelectronic properties of the designed conjugated polymers. New materials did not show noticeable photodegradation which suggests the possibility of their practical implementation in the production of efficient organic solar cells for outdoor applications.

Resume : Heliatek is the global technology leader in the field of organic photovoltaics (OPV) based on small molecules. We will show the advantages of vacuum deposition processes and introduce our pin tandem cell approach for high efficient solar cells. A-D-A-type conjugated absorber oligomers were in-house developed by Heliatek. In 2013, Heliatek has set a new world record for OPV with a cell efficiency of 12% on >1cm² (confirmed by SGS Fresenius). The high efficiency cells are very stable against light and temperature. In the meantime we have developed the first efficient small molecule NIR absorber with a significant absorption until 900nm, which will help us on our roadmap to 15% efficiency. Finally, we will show that the complexity of pin-tandem cell deposition and in situ laser structuring can be handled in a fully integrated vacuum R2R-process. Heliatek has successfully processed tandem modules in R2R production line with 5% efficiency on active module area.

Resume : We report on our recent progress in fabricating single-layer organic solar cells using hydrogen-bonded pigments as the active material. Hydrogen-bonded pigments are ambipolar organic semiconductors that form high crystal-lattice energy solids due to interplay of hydrogen bonding and pi-pi stacking. We have found that some neat films of H-bonded pigments can provide photocurrents in the milliampere range in the absence of a donor-acceptor interface. This corresponds to quantum efficiencies in the range of 10-20%, orders of magnitude higher than what is typically found for neat organic films. Herein we discuss spectroscopic evidence clarifying the intrinsic charge generation mechanism in H-bonded pigment films. Secondly, we explore the photovoltaic performance of single-layer diodes consisting of a homogeneous mixed crystal system of two H-bonded pigments enhancing photogeneration while retaining high photovoltage due to energetic proximity between frontier orbitals of the two pigments. Diverting from the well-established donor-acceptor concept frequently used in organic solar cells by applying only one absorber layer is potentially highly-advantageous: In addition facile fabrication of single-layer devices, the theoretical power conversion efficiency limit can be higher compared to conventional donor-acceptor type organic solar cells due to minimization of polarization losses.

Resume : We have recently predicted that when the dielectric constant of organic semiconductors is increased from 2-4 to about 10, a dramatic increase in (maximal) power conversion efficiency of organic solar cells, based on these materials as active layer constituents, can be achieved.1 Binding energies of initial excitons, of charge transfer excitons, and between charges in any type of recombination process are diminished with increasing electric permittivity of the medium. In order to design and develop new molecular materials with higher dielectric constant, the next challenge is to make the connection between the dielectric constant, as a macroscopic property, and molecular structure. One approach is to connect suitable substituents to known (opto)electronically functional molecular moieties. We report here on the design and preparation of molecules bearing substituents that are to enhance the dielectric properties without diminishing other crucial parameters (like charge carrier mobility, bandgap, stability, etc.). Through a computational chemistry approach, we investigate the influence of molecular moieties on the relative energies of various charge separated states resulting from photo-induced charge transfer between molecular donors and acceptors. We report on a system in which the energies of the charge separated states are tuned to a favorable situation by proper placement of dipolar substituents. 1. L. J. A. Koster, S.E. Shaheen, J.C. Hummelen, Adv. Energy Mater. 2012, 2, 1246.

Resume : Exciton fission, the spin-allowed conversion of a spin-singlet exciton to two spin-triplet ones, has been attracting increasing attention recently due to its potential for solar cell applications. However, despite numerous attempts to uncover the origin of fission’s ultimate speed and efficiency, the basic mechanisms at play are still debated. Particularly, in the core of the dispute lie the effects of coupling between electronic states and the role of the ‘multi-exciton’ (ME) intermediate. Here we employ sub-15 fs optical 2D photon-echo spectroscopy to elucidate the dynamics of excited states during fission process in films of pentacene and its derivatives. Our measurements demonstrate an abundance of long-lived oscillatory features in the spectroscopic observables which we attribute to ground-state vibrational coherences but not to the coherent beating between different electronic states. At the same time, the observed oscillations reveal the presence of ME intermediate state which have not been detected before in optical measurements due to transition selection rules. Our theoretical calculations indicate that the vibrational manifold of the ME intermediate is important for the coupling between different excitonic states and therefore may play a critical role in the singlet fission dynamics.

Resume : The nature of the donor/acceptor interface in an organic heterojunction solar cell plays a critical role in determining functional properties, but relatively little is known experimentally about how the interfacial arrangement of donor and acceptor molecules determines the photoexcited electron dynamics and carrier localization. We address this question by using resonant Auger electron spectroscopy on bilayers of copper phthalocyanine (CuPc) as the donor and C60 as the archetypal acceptor. We find that in the case where the CuPc donor and C60 CuPc donor and C60 acceptor are oriented face-on with respect to each other, corresponding to substantial C60-CuPc wavefunction overlap, we find that photoexcited electron transfer times are faster than 50 fs. This is in contrast to the mutual edge-on arrangement, where electron transfer is at least a factor of two slower. We explore the implications of these results for free charge carrier generation in blend organic photovoltaic devices.

Resume : The recent significant increase in power conversion efficiency (PCE) of polymer-fullerene solar cells largely originates from the successful development of new electron donor polymers. The donor-acceptor (D-A) or push-pull design, where electron rich and electron deficient units alternate along the copolymer chain is commonly used to tune the HOMO and LUMO energy levels and the optical band gap of these polymers. While structure-property relations for energy levels are well established, these are less clear for the actual photovoltaic performance. Creating morphologies in which nanometer-sized, interconnected, semi-crystalline domains of both polymer and fullerene exist seems crucial for high photovoltaic performance. These semi-crystalline domains optimize the conjugation along the polymer backbone and allow delocalizing the carrier wave functions to assist efficient charge separation. High molecular weight and a tendency to crystallize are important in achieving such morphologies. For a range of diketopyrrolopyrrole-based small band gap polymers it will shown how the molecular weight of semiconducting polymers and the nanomorphology are crucial parameters in obtaining high power conversion efficiencies in the range of 6-8% for single junctions, with optical band gaps down to 1.3 eV. When the new semiconductor materials are combined with a wide band gap material it is possible to make create tandem and multi-junction devices in tandem or triple layer configurations with efficiencies over 9%. The favorable efficiency of the tandem cell is achieved by an almost perfect complementarity of the absorption spectra of the different absorber layers that reduce thermalization losses. Because of their high voltages, triple junction solar cells can be in artificial leaves used for photo-electrochemical water splitting as shown below.

Resume : Solar cells based on quantum dots (QDs) are currently under intense investigation among the third generation solar cell technologies. Colloidal QDs, being solution-processable and possessing size-tunable electronic and optical properties, are particularly attractive for this application. Up to the present solar cells based on Lead Sulfide (PbS) QDs, capable to absorb from the visible to the near-IR spectra, have shown state-of-the-art record power conversion efficiencies of more than 8%. In this work, we report enhancement of the photovoltaic properties of TiO2/PbS QD-based depleted heterojunction solar cells by doping into the PbS QD matrix copper indium sulfide (CuInS2) QDs, which are of a larger band gap and reduced toxicity compared to PbS QDs. The discrete CuInS2 QDs form type-I heterjunctions with the surrounding PbS QDs, lowering the entire Fermi level of the QD layer and inducing a larger discrepancy between the electron and hole quasi-Fermi levels in the titania-PbS QD junction. Various PbS/CuInS2 QD ratios and different device configurations are experimented, aiming to simultaneously optimize the effects from band-alignment and the charge transport in these hybrid QD films. Combining structural and device characterizations, the mechanisms leading to the performance enhancement in the hybrid PbS/CuInS2 QD solar cells will be discussed.

Resume : Organic/inorganic hybrid solar cells are an interesting type of polymer based solar cells, which combine beneficial properties of inorganic semiconductors with them of polymer based materials, in particular, the easy processability on flexible plastic substrates. In this contribution we explore the possibility to prepare flexible copper indium sulphide polymer solar cells via the metal xanthate route. By adding hexylamine as additive, the conversion temperature of the metal xanthates to copper indiums sulphide nanoparticles directly in a matrix of the conjugated polymer can be lowered to temperatures compatible with low-cost plastic substrates i.e. polyethylene terephthalate (PET)-foils. The addition of hexylamine leads to a high crystallinity of the nanoparticles already at 140 °C. Hexylamine thereby facilitates the thermal decomposition by a direct chemical reaction with the xanthate precursor. Using this modified route, flexible hybrid solar cells with power conversion efficiencies of 1.6% could be realized using temperatures not higher than 140 °C in the whole fabrication process. Furthermore, we demonstrate that the metal xanthate route is also well suited for the fabrication of larger area solar cells and present hybrid solar cell modules on glass as well as on flexible PET substrates. In addition, the lifetime of the prepared solar cells was investigated.

Resume : Hybrid solar cells based on conjugated polymers and inorganic semiconducting nanocrystals combine beneficial properties of organic and inorganic semiconductors and are, therefore, an exciting solar cell technology. One key issue to further develop this type of solar cells is a better control of materials synthesis, in particular, a good control over nanomorphology formation of polymer and nanocrystals in the absorber layer, which critically influences charge separation and also charge transport. In this contribution, we present our results on the investigation of the formation of P3HT/CdS nanocrystal hybrid layers prepared via a ligand-free in situ route from cadmium xanthates with five different alkyl moieties. The absorber layer is prepared by mixing P3HT with the cadmium xanthate to get a precursor ink. After coating, the precursor layers are annealed at moderate temperatures (160 - 200 °C) to form the polymer/CdS absorber layers. The formation of the absorber layer morphology during annealing is thoroughly investigated on the nanometer scale by time resolved GISWAXS (grazing incidence small and wide angle X-ray scattering) using synchrotron radiation. The data reveal distinct influences of the alkyl moiety on the nanomorphology and also size and crystal structure of the nanoparticles. Moreover, charge generation in the prepared absorber layers is probed by transient absorption spectroscopy and also the influences on the characteristic solar cell parameters are discussed.

Resume : To fabricate long lifetime polymer solar cells, understanding the fundamental aspects controlling the performances, stability and degradation of the devices is essential. In this study, we compare the chemical and electrical degradation behaviors of polymer solar cells based on active layers of semi-crystalline p-type polymer poly(3hexylthiophene) (P3HT) and different fullerene derivatives as acceptors. Chemical stability study by UV-visible spectroscopy and photovoltaic performance study by current-voltage measurement were done to understand and compare air stability of P3HT blended with different fullerenes. The used acceptors are ICBA and PCBM. The obtained results show that solar cells fabricated from PCBM as acceptor is more stable than those based on ICBA. Based on the spectroscopic and electrical measurement results, it is safe to conclude that PCBM contributes more to stability of P3HT than ICBA, and the degradation behavior is compatible with recently proposed mechanisms. Chemical properties of ICBA such as the low electron affinity could be responsible for the relative low stability of the device.

Resume : To achive high power conversion efficency(PCE) of polymer solar cells, not only proper device structre but also good performance photo-acitve materials are required. Recent drastic improvement of the power conversion efficiency of polymer solar cells is mainly attributed to the development of new high performance materials. Application of newly developed materials to large area device is very important to judge the possibliliy of real application of newly developed materials, because the performance of large area device showed the average device performanc of each regions of the whole device. So the materials using for large area not only need to give high PCE but also need to have the small variation of PCE when the thickness of active layer changed within a certain range. Also, partial failure caused the deterioration of the whole device performance, so relyable property of the materials is crucial in large area device. Here we will present over 5% sub-module devices fabricated with new photo-acitve polymers using micro-slot die coater.

Resume : We implemented a low-temperature approach to fabricate efficient photoanodes for Dye Sensitized Solar Cells which combines three different nano-architectures, namely a highly conductive and highly transparent AZO film, a thin TiO2-blocking layer and a mesoporous TiO2 nanorods-based working electrode. All the components were processed at T≤200°C. Both the AZO and TiO2 compact layers were deposited by reactive sputtering, whereas the TiO2 nanorods were synthesized by surfactant-assisted wet-chemical routes and processed into photoelectrodes in which the native geometric features assured uniform mesoporous structure with effective nanocrystal interconnectivity suitable to maximize light harvesting and electron diffusion. Due to the optimised structure of the TiO2-blocking/AZO bilayer as well as to the good adhesion of the TiO2 nanorods over it, a significant enhancement of the charge recombination resistance was demonstrated, this laying on the basis of the outstanding power conversion efficiency achievable through the use of this photoanode’s architecture: a value of 4.6% (N719) was achieved with a 4m-thick electrode processed at T=200°C. This value noticeably overcomes the current literature limit got on AZO-based cells (N719) which instead use Nb-doped and thicker blocking layers, and thicker nanostructured photoanodes which have been even sintered at higher temperatures (450-500°C).

Resume : P-Type DSCs have received increasing attention since highly efficient photocathodes are necessary for the design of tandem DSCs, which could provide a pathway towards dye-sensitized solar cell efficiencies beyond 15%. Furthermore dye-sensitized photocathodes are relevant to solar fuel production. To date, however, p-DSCs lack behind in efficiency, predominantly due to their intrinsically low fill factor (FF). In this work we have investigated the origins of the low FF for state of the art p-DSCs and have developed a “toolbox” of characterization methods tailored for the investigation of p-DSCs. A multitude of effects has been identified which are overall limiting the FF of p-DSCs. The gained insights led to new approaches that may help to overcome the low FF and lead to higher p-DSC and tandem DSC efficiencies.

Resume : The goal of this research is to synthesize the hyperbranched polythiophene derivatives containing tetrachloroperylene bisimide as bridging moiety, and to investigate the thermal, electrochemical, and opto-electrical properties of those derivatives. The polymers containing soft alkyl spacer as bridging moiety and linear poly(3-hexylthiophene) (P3HT) were also synthesized for comparison in this study. Polymers with high regioregularity were synthesized via the Universal Grignard metathesis polymerization. The TGA experiments revealed a first-stage weight loss at about 300 oC for all polymers; besides, polymers containing rigid tetrachloroperylene bisimide groups possess less weight loss than P3HT after heating, indicative of enhanced thermal stabilities. The UV-vis absorption maxima of hyperbranched polymers are similar to that of P3HT in film state, while their absorption shoulder bands are stronger than that of P3HT, indicating stronger chain-chain interaction and shorter distance between backbones by the introduction of bridge architecture. Moreover, an attenuation of fluorescent intensity was found for those hyperbranched polymers, implying reduced recombination of excitons to emit light and more opportunity for carriers to migrate to both electrodes. Electrochemical analysis showed that introducing hyperbranched structure resulted in decreasing both LUMO and HOMO levels of polymers. Finally, all polymers were used for fabrication of organic solar cells with the configuration of ITO/PEDOT/polymer:PC60BM(1:1 w/w)/LiF/Al to evaluate their performance. The morphological study of polymer:PC60BM blend films were performed by AFM for interpretation of efficiency trend of devices.

Resume : Solution-processed small-molecule solar cells are emerging as promising alternatives to solution-processed polymer solar cells; they offer facile synthesis, high purity, crystallinity, well-defined molecular structures, high mobility, and batch-to-batch consistency. A new donor unit viz. 3,8-dialkoxynaphtho[3,2-b:7,6-b?]dithiophene (NDT) was developed for a novel donor-acceptor semiconducting polymer in our lab, and used in efficient solar cell applications. Many small molecules using NDT donor molecule have also been prepared. We systematically investigated the synthesis, thermal stability, optical and electrochemical properties, and photovoltaic characteristics of the resulting small molecules. In this presentation, I?m going to introduce the 3,8-dialkoxynaphtho[3,2-b:7,6-b?]dithiophene(NDT) monomer and NDT based new small molecules for organic solar cells in detail.

Resume : Organic solar cells (OSCs) based on soluble small molecules are very attractive because they possess both advantages of solubility of polymers and purity of small molecules. Photocurrent (Jsc) is an important parameter for a solar cell. To explore enhancing Jsc in OSCs via harvesting triplet excitons, two Ir complexes R1 and R2 were synthesized and utilized as solo electron donors in solar cells combined with PC71BM as the electron acceptor. Although the overlap between the absorption spectra of the two Ir complexes (with ~ 550 nm absorption onset) and the solar flux is relatively small, a decent Jsc of 6.5 mA/cm2 is achieved in the conventional structure OSCs without any solvent additives and thermal annealing treatment. With a Voc of 0.74 V and a FF of 0.42, an overall power conversion efficiency (PCE) of 2.0 % is obtained. Our results demonstrate the possibility to use phosphorescent dyes as solo electron donors in solution-processed OSCs, which provides an important guide for future new metal complexes design with low band gaps, desired HOMO and LUMO levels as well as molecular packing to enhance Jsc and FF for triplet OSCs.

Resume : Solution-processable bulk-heterojunction organic solar cells (OSC) based on conjugated small molecules (SM) are a promising alternative to conventional polymer OSCs. Small molecules have a number of attractive advantages over conjugated polymers like excellent batch-to-batch reproducibility, well-defined molecular structure and molecular weight, easy mass-scale production etc. Although SM-based solar cells are the emerging branch of OSCs, they have already shown outstanding efficiency of ~10% [1]. Competitive to the linear family of SMs, star-shaped push-pull SMs with triphenylamine donor core and dicyanovinyl acceptor end groups [2] are characterized by better solubility, higher degree of purity, and structural uniformity. In this contribution, we report on a series of novel star-shaped SMs with variable length of the thiophene conjugated arm (from 1 to 3 thiophene rings). Photovoltaic blends based on these SMs as a donor and PC70BM as an acceptor are studied by visible-pump IR-probe photoinduced absorption spectroscopy to interrogate ultrafast charge separation and recombination. Two different recombination channels (intra- and intermolecular) are identified with timescales of tens ps. The blends with optimal acceptor concentration of more than 50% provide up to 70% of long-lived separated charges which makes the star-shaped SMs perspective materials for OSCs. [1] Y. Liu et al., Sci. Rep., 3 (2013) [2] J. Min et al., Adv. Energy Mater. DOI: 10.1002/aenm.201301234

Resume : We report the design and investigation of >50 different fullerene derivatives whose electron affinity is lowered considerably due to the electron donating nature of a single organic addend attached to the fullerene cage. The electrochemical properties of the fullerene derivatives were investigated in solution and in thin composite films (with P3HT and PCDTBT). The best representatives of the designed families of compounds showed cathodic shifts of the first reduction potentials as large as 100-120 mV compared to [60]PCBM. It was revealed that the decrease in the electron affinity of the designed fullerene derivatives is related to the through-space electronic interactions of electron donating alkoxy groups with the fullerene cage. The application of the designed fullerene derivatives as n-type components in organic bulk heterojunction solar cells led to the increase in the open circuit voltage by 100-170 mV compared to the reference devices based on [60]PCBM. Few compounds provided even higher open circuit voltages (up to 770-780 mV with P3HT) than bis-PCBM bearing two organic addends on the fullerene cage. Moreover, using the best materials improved also the power conversion efficiency of organic solar cells where P3HT and PCDTBT were used as electron donor components. To our best knowledge this is one of the first examples of the fullerene derivatives with reduced electron affinity which outperform PCBM in solar cells based on conjugated copolymers such as PCDTBT.

Resume : Several conjugated PPV-PPE copolymers were studied as electron donor materials in bulk heterojunction organic solar cells in combination with a library of electron acceptor fullerene derivatives. It was shown that molecular structure and solubility of the fullerene counterpart affect significantly photovoltaic performance of both polymers. Use of [60]PCBM as electron acceptor material yielded quite moderate power conversion efficiencies. The best results were achieved when some alternative fullerene derivatives with better suiting molecular structures and solubility were applied [1]. In some cases the photovoltaic performance of the polymer/fullerene blends showed good correlation with the molecular structures of the materials [2]. The revealed correlations might be considered as the first step towards the development of a predictive model based on the material structure-composite morphology-photovoltaic performance relationships. This model might guide the material design for organic photovoltaics similarly to QSAR approach which has been guiding the drug design for years. [1] P. A. Troshin, O.A. Mukhacheva, O. Usluer, S. Rathgeber, A. E. Goryachev, A. V. Akkuratov, D. K. Susarova, N. S. Sariciftci, V. F. Razumov, D. A. M. Egbe. Adv. Energ. Mater. 2013, Adv. Energ. Mater. 2013, 3, 161 [2] P. A. Troshin, O. A. Mukhacheva, A. E. Goryachev, N. N. Dremova, D. Voylov, C. Ulbricht, D. A. M. Egbe, N. S. Sariciftci, V. F. Razumov, Chem. Commun., 2012, 48, 9477

Resume : Plasmonic solar cells were grown entirely by chemical spray pyrolysis using an extremely thin In2S3/CuInS2 as buffer/absorber to uniformly cover either planar ZnO or ZnO nanorod layer. Gold(III) chloride trihydrate (HAuCl4∙3H2O) was used as precursor for the synthesis of gold nanoparticles, grown either before or after absorber deposition. Au-nanoparticles were formed via thermal decomposition of HAuCl4∙3H2O solution at 0.01-0.1 mol/L concentrations. Current-voltage and quantum efficiency were used to evaluate cell output. Preliminary results show that a relative increase of 20% in JSC is achieved using plasmonic particles in the cell with planar layers, while 15% relative increase is obtained for cell with the corrugated junction. The effect of the size of Au nanoparticles and the location of Au particles in the solar cell will be discussed.

Resume : Nanostructured solar cells (NSSCs) have the potential to be produced at significantly lower costs than more established photovoltaics. They can use a wide range of visible-light sensitisers including organometallic dyes and semiconductor quantum dots (QDs). Sensitisers without rare or toxic elements are attractive to minimise costs and environmental impact; carbon QDs (CQDs) have been recently developed, which meet these requirements. We demonstrate ZnO nanorod NSSCs sensitised with CQDs produced using hydrothermal synthesis at <250?C from the biomass-derived materials chitosan (CS), chitin (CI) and glucose (G). The devices are completed with the hole transport material CuSCN. UV-Vis absorption spectra show CQDs sensitisation of the ZnO in the 400-500 nm range. CI-CQDs give the lowest light sensitisation attributed to poorer bonding by the alkyl functional group, giving lower Voc and efficiency (η) due to increased shunting. Despite higher light absorption, G-CQDs devices have the lowest η, linked to a high series resistance suggesting high recombination within the film. CS-CQDs give the optimum coating, potentially linked to the amine functional groups, leading to η of 0.052%. Layer-by-layer coating with CS/CI-CQDs increased light absorption and increased η to 0.065%. Although significant efficiency improvement is still required for these devices, this demonstrates the potential for CQDs as sensitisers for NSSC with properties controllable via the precursor functional groups.

Resume : The control of the active layer morphology of organic photovoltaic cells (OPV) is still a challenging issue and a major bottleneck towards stable and efficient devices. In the present study, a new series of pi-conjugated donor (D)-acceptor(A) block co-oligomers, designed to self-assemble into a stable network of interpenetrated D and A domains, are investigated, with a particular focus put on the charge carrier dynamics. The electron acceptor block is composed of a perylene diimide, while electron rich thiophene and fluorene units are used to form donor blocks of various lengths. Moreover, the chemical structure featuring DA spacers and electron donating groups remote from A were changed so as to modify the spatial extension and location of the HOMO and LUMO wave function, with the main aim to reduce unwanted recombination of charge transfer (CT) states. Experimental data obtained by ultra-fast pump-probe spectroscopy, transient photovoltage and field-effect mobility measurements will be reported. Long CT state lifetimes up to 2.5 ns were found in isolated molecules in solution for optimized molecular design parameters. The free charge carrier recombination rate as well as the electron and hole mobilities are found to be dependent on the donor block length. Finally, the impact of the molecular structure on the electrical characteristics and photovoltaic performances of mono-component OPV devices will be shown.

Resume : We have developed a hole-blocking layer for bulk-heterojunction solar cells based on cross-linked polyethylenimine (PEI). We tested five different ether-based cross-linkers and found that all of them give comparable solar cell efficiencies. The initial idea that a cross-linked layer is more solvent resistant compared to a pristine PEI layer could not be confirmed. With and without cross-linking, the PEI layer sticks very well to the surface of the indium–tin–oxide electrode and cannot be removed by solvents used to process PEI or common organic semiconductors. The cross-linked PEI hole-blocking layer functions for multiple donor–acceptor blends. We found that using cross-linkers improves the reproducibility of the device fabrication process.