Organic photovoltaics: material synthesis and characterization, device engineering, device physics and upscaling

Resume : Solution-processed bulk heterojunction (BHJ) polymer solar cells (PSCs) have exhibited great potentials for making large area and flexible solar panels through low-cost solution coating techniques. Here, we systematically studied the correlations between chemical structures of the photoactive materials including polymer donors and non-fullerene acceptors and their photovoltaic properties in fullerene free PSCs. We found that a conjugated polymer donor named as PBDB-T can form nanoscale aggregations in solid film and the aggregations are well miscible with a NF-acceptor named as IT-M, and hence the phase separation morphology that is favorable for photo-induced charge generation and transport can be obtained in the PBDB-T:IT-M blend film. Comparing to the PBDB-T:PC71BM blend, the PBDB-T:IT-M blend has broader absorption spectrum and also lower energy loss. As a result, the PBDB-T:IT-M PSCs show outstanding PCEs up to 12.1%, which is much better than the device based on PBDB-T:PC71BM. What is more, we also developed a few low band gap (LBG) non-fullerene acceptors by which the PSCs with photo response extending to 1000 nm can be fabricated. Benefiting from the broad absorption, the LBG PSC demonstrated an outstanding current density of 24 mA/cm2. Overall, we anticipate that these encouraging results will give a strong push to the study of fullerene-free PSCs from the aspects of fundamental research and practical applications.

Resume : In a bulk-heterojunction organic solar cell (BHJ-OSC) electron donor and electron acceptor materials are blended to form a bulk-heterojunction film which is sandwiched between a transparent and a back metal electrode. In this report, I will show you our group’s results on the molecular strategies towards high-efficiency BHJ-OSC devices, including (1) the design, synthesis and photovoltaic properties of small molecule photovoltaic materials and (2) engineering on the buffer layer between the organic layer and the electrode. References: 1. Weiping Li, Xinliang Zhang, Xin Zhang, Jiannian Yao and Chuanlang Zhan*, ACS Appl. Mater & Interfaces. 2016, acsami.6b12389. 2. Ailing Tang, Chuanlang Zhan,* Jiannian Yao, Erjun Zhou*, Adv. Mater. 2016, adma.201600013. 3. Jianhua Huang,* Shanlin Zhang, Yuxia Chen, Bo Jiang, Xinliang Zhang, Donghong Yu,* Zhiyong Lin, Jiannian Yao, Chuanlang Zhan*, TJ. Mater. Chem. A 2016, 4, 15688-15697. 4. Xin Zhang, Weiping Li, Jiannian Yao and Chuanlang Zhan*, ACS Appl. Mater. Interface 2016, 8 (24), 15415-15421. 5. Chuanlang Zhan* and Jiannian Yao, Chem. Mater. 2016, 28 (7), 1948-1964. 6. Xin Zhang, Jiannian Yao, and Chuanlang Zhan*, Sci. China Chem. 2015, 59(2), 209-217. 7. Ailing Tang, Chuanlang Zhan,* Jiannian Yao*, Chem. Mater. 2015, 27 (13), 4719-4730. 8. Ailing Tang, Chuanlang Zhan,* and Jiannian Yao, Adv. Energy Mater. 2015, 5(13), 1500059. 9. Xin Zhang, Chuanlang Zhan and Jiannian Yao, Chem. Mater. 2015, 27 (1), 166-173. 10. Zhenhuan Lu, Bo Jiang, Xin Zhang, Ailing Tang, Lili Chen, Chuanlang Zhan and Jiannian Yao, Chem. Mater. 2014, 26 (9), 2907-2914. 11. Xin Zhang, Zhenhuan Lu, Long Ye, Chuanlang Zhan, JianhuiHou, Shaoqing Zhang, Bo Jiang, Yan Zhao, Jianhua Huang, Shanlin Zhang, Yang Liu, Qiang Shi, Yunqi Liu, and Jiannian Yao, Adv. Mater. 2013, 25 (40), 5791-5797.

Resume : The power conversion efficiency of organic photovoltaic cells depends crucially on the morphology of their donor–acceptor heterostructure. Although tremendous progress has been made to develop new materials that better cover the solar spectrum, this heterostructure is still formed by a primitive spontaneous demixing that is rather sensitive to processing and hence difficult to realize consistently over large areas. Here we report that the desired heterostructure with built-in phase contiguity can be fabricated by acceptor doping into a lightly crosslinked polymer donor network. The resultant nanotemplated network is highly reproducible and resilient to phase coarsening. We obtained improvement in power conversion efficiency over conventional demixed biblend devices for a range of donors and acceptors.

Resume : The process of charge generation in organic photovoltaics proceeds through the formation of a charge transfer complex, where a hole on the donor molecule is found near an electron on the acceptor molecule. The energy of the charge transfer complex is strongly correlated to the open-circuit voltage of the solar cell. And yet, for all their importance, many questions remain about the role of the inherent disorder of donor-acceptor interfaces on the open-circuit voltage. Ternary blends (2 donors and one acceptor or 2 acceptors and one donor) have been shown to exhibit open-circuit voltage tuning. These experiments offer insight into the role of disorder at the donoe-acceptor interface on open-circuit voltage. We will use dilute ternary blends to shed light on the effect of interfacial disorder and morphological control of the open-circuit voltage. In addition to morphology effects, well-known electronic effects can be used to control the open-circuit voltage. We will show that judicious choice of dopant and dopant concentration allows to increase the open-circuit voltage and power-conversion efficiency. The effect of doping on other systems will be discussed as well.

Resume : The open circuit voltage (Voc) tunability with composition in ternary bulk heterojunction organic photovoltaics (OPVs) has been demonstrated by many groups in different donor:acceptor1:acceptor2 (D:A1:A2) devices but the underlying mechanism is still heavily disputed. Here we show experimentally that not only the energetic disorder of the acceptor LUMO varies considerably with A1:A2-ratio, but also the disorder in the donor HOMO. When accounting for this surprising phenomenon in a simple state filling model, a fully quantitative description of the continuous tunability of Voc in D:A1:A2 devices is obtained. In specific we find that the disorder of the donor HOMO is smaller in the presence of ICBA60 than in the same ratio with PC71BM. State filling modeling with variable disorder in a joint DOS confirms the composition dependence of own and literature Voc vs composition data for various material combinations. Gaussian disorder values for all ternary compositions are extracted from temperature dependent SCLC experiments on electron- and hole-only devices using the extended Gaussian disorder model. Our results demonstrate that the selection of the acceptor material not only influences Voc directly via the LUMO level, but also indirectly through its effect on the (donor) HOMO disorder. The implications of these findings for the performance limits of ternary OPV will be discussed.

Resume : -Preferably a contributed talk. Organic photovoltaics have surpassed 11% power conversion efficiency through the careful design of low bandgap polymer donors to be used with fullerene acceptors. Despite this impressive progress, most of these high performance devices exhibit poor stability. Additionally, P3HT remains the only cost-effective donor polymer and the limitations of using fullerenes in photovoltaic devices continue to persist. We have recently shown that the efficiencies of P3HT devices can reach as high as 6.4% by replacing PCBM with a non-fullerene small molecule acceptor.1 The use of these acceptors in multi-component heterojunctions can further increase the efficiencies achieved by the P3HT devices. We have developed a novel approach towards high efficiency organic photovoltaic devices; where polymer donors are combined with two non-fullerene acceptor molecules in a ternary blend. Ternary blends that make use of two polymer materials have been explored in the past, however the lack of entropic driving force and the potential for intermolecular interactions between polymer chains renders this approach problematic.2 The introduction of a newly synthesized small molecule acceptors into binary blends leads to efficiency up to 11.0 ± 0.4% and importantly a vast improvement in device stability. Tuning of the second acceptor’s LUMO allows a higher open circuit voltage, and through careful selection of the acceptors and their ratios a favourable blend morphology can be achieved, thereby reducing recombination in the blends. 1. S. Holliday et al. High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor, Nat. Commun., 2016, 7, 11585 2. C. B. Nielsen et al. Non-fullerene electron acceptors for use in organic solar cells, Acc. Chem. Res., 2015, 48, 2803-2812

Resume : In this talk the use of fullerene mixtures for polymer solar cells will be discussed. To realise the often intricate nanostructures that are necessary to optimise device performance, it is critical to tune the solubility of fullerenes in organic solvents as well as their tendency to crystallise. The use of fullerene mixtures, including both unsubstituted fullerenes as well as PCBMs, opens up a toolbox that can be employed to control aspects such as solubility, glass formation and crystal nucleation. Polymer solar cells based on mixtures of pristine C60 and C70 with a highly reproducible power-conversion efficiency of 6 % as well as a thermally stable active layer are reported.

Resume : Most of today’s donor-acceptor couples exhibit efficient and field-independent free charge generation, rendering the competition between free charge generation and extraction the main factor determining the performance of organic solar cells. This talk will summarize our recent work on the understanding of mobility and space charge formation on the fill factor of OPV devises. We will show how concepts developed for high mobility inorganic photovoltaic devices fail when applied to low mobility systems and how their application to organic solar cells result in erroneous values for the ideality factor, the serious and parallel resistances [1,2]. Different figure of merits will be evaluated with regard to their suitability to correctly describe the performance of organic solar cells [3-6]. In this context, the question is addressed to which extend non-thermalized carrier distributions contribute to free carrier recombination and extraction under solar cell working conditions [7], and which experiments are mostly suited to gain excess to the relevant physical parameters. We, finally, discuss recent results on the function of organic transport layers in hybrid perovskite devices. [1] U. Würfel, D. Neher, A. Spies, S. Albrecht, Nat. Comm. 6, 6951 (2015) [2] D. Neher, J. Kniepert, A. Elimelech, L.J.A. Koster, Scientific Reports 6, 24961 (2016) [3] D. Bartesaghi et al., Nat. Commun. 6, 7083 (2014) [4] M. Stolterfoht et al., Scientific Reports 5, 9949 (2015) [5] P. Kaienburg, U. rau, T. Kirchartz, Phys. Rev. Appl. 6, 024001 (2016) [6] M. Stolterfoht, A. Armin, B. Philippa, D. Neher, J. Phys. Chem. Lett. 7, 4716 (2016) [7] J. Kurpiers and D. Neher, Scientific Reports 6, 26832 (2016)

Resume : A great deal of research has been focused in understanding the morphology/performance correlation in bulk heterojunction (BHJ) solar cells. In polymer:fullerene BHJ devices, thermal degradation is known to induce morphological changes (i.e. phase separation processes) of the nano-structured blend. These strongly influence the charge separation and transport properties of the blend, affecting device efficiency. Here, laser scanning confocal (LSCM) and photocurrent (LSPM) microscopies were applied to simultaneously study, at the micro- and submicro- scale, the blend morphology and the electrical properties of BHJ active layers (in the region under the top electrode) of polymer:fullerene BHJ solar cells processed with standard chlorinated or environmental friendly solvents, which showed different evolutions with thermal degradation. Contrarily, for metal-oxide (MOx)/perovskite heterojunction solar cells, high efficiencies have been reported with both flat and mesoporous MOx scaffolds covered with the perovskite absorber. LSCM and LSPM techniques were applied then to study the correlation between the structure at the micro-scale of the active materials and the photovoltaic performance and air stability in complete devices with different MOx films or scaffolds. Then the use of two laser sources (at 405 or 488 nm) allowed to excite different electronic states of the active layers (perovskite or defects), aiming to correlate their distribution with the local device efficiency.

Resume : Numerous studies have shown that the open-circuit voltage in organic photovoltaic devices is determined by the energy of charge transfer states at the donor-acceptros interface, as well as the type of recombination. For this reason, recently major efforts have been directed towards reducing relaxation losses by minimizing the LUMO-LUMO offset between the donor and acceptor materials. However, this offset also determines the driving energy for charge separation and was shown to be critical for material photophysics. In this contribution, we summaries our recent findings on charge generation mechanism in organic donor:acceptor blends where driving energy is comparable or less than 0.1 eV. We show, that charge generation mechanism in these systems may be substantially different from previously studied large-driving-energy polymer:fullerene systems. For example, structural reorganisation may play substantial role in charge separation and recombination process leading to Marcus-type behavior in normal and inversed regimes.

Resume : Unique to the function of organic PVs are the creation of tightly bound excitons that can only be efficiently separated at a donor/acceptor (D/A) interface capable of providing the necessary energetic driving force for dissociation. At the D/A interface, the presence of charge transfer (CT) states, ground state complexes between the donor and acceptor materials, set the upper bound for the potential that can be extracted from a given D/A pair, but their role in photogeneration is not completely understood. Furthermore, the consequences of extreme levels of order in highly crystalline heterojunctions have not so far been revealed. We have recently been exploring pinhole-free organic semiconductor-based thin films that feature crystalline grains of up to 1 mm in extent. Because they are pinhole-free, we can apply them in vertical devices such as PV cells to explore various emergent phenomena. For example, we have found that CT states are more easily separated into free charge if they are delocalized, contributing to noticeably lower energy losses. Also, we have discovered that relative energies of CT states with respect to singlet and triplet energy levels are critical when considering devices that exploit multiple exciton processes such as singlet fission [1] and its complement, triplet-triplet annihilation (or triplet fusion). We will discuss these aspects and their implications for more efficient organic solar cell function. [1] Y.L. Lin, M.A. Fusella, O.V. Kozlov, X. Lin, A. Kahn, M.S. Pshenichnikov, B.P. Rand, Adv. Funct. Mater., 26, 6489 (2016).

Resume : Unveiling the photo-transport mechanisms is a key for the development of emerging photovoltaic (PV) technologies based on micro and nanostructured materials. Most experimental approaches used to address this challenge average sample properties over macroscopic scales, making the direct assess of the local heterogeneity impact on the recombination process a challenge. To overcome this, few teams began to develop time-resolved scanning probe microscopies (SPM) aimed at addressing the photocarrier dynamics. Kelvin Probe Force Microscopy (KPFM) under frequency modulated illumination has been used to investigate the surface photovoltage (SPV) decays, thus providing access to the recombination dynamics in organic blends. [1] However, all photo-modulated KPFM published works demonstrated only “point” measurements. Here, we demonstrate how non-contact atomic force microscopy combined with KPFM under frequency modulated illumination can be used to simultaneously image the SPV dynamics at different timescales. With this approach, images of the trap-filling, trap-delayed recombination and non-geminate recombination processes have been simultaneously acquired in nano-phase segregated organic donor-acceptor bulk heterojunction thin films with an achieved sub-10 nm lateral resolution, enabling the investigation of samples with optimized morphologies. [2] 1. G. Shao et al. ACS Nano 8, 10799 (2014). 2. P. Fernández Garrillo et al. ACS Appl. Mater. Interfaces 8, 31460 (2016).

Resume : I will focus on charge carrier dynamics in organic solar cells and their impact upon device performance, employing transient absorption and transient optoelectronic analyses on timescales from fs to ms. My talk will focus on three topics. First of all I will address the factors influencing charge separation in organic solar cells, including in particular the role of charge transfer / bound polaron pair states and how these are influenced upon film morphology and energetics. I will go on to focus on charge carrier dynamics in organic donor / acceptor blends employing non-fullerene solar cells, and particularly the challenge of minimizing geminate recombination losses for small energy offset devices. Finally I will consider the impact of inadvertent doping in limiting charge collection efficiency in organic solar cells, and the insights on this which can be gained on this from transient optoelectronic analyses.

Resume : Organic photovoltaic devices are one of the promising candidates for future energy conversion applications. It is known in general that the open circuit voltage is significantly lower than the built-in voltage, and the physical origin of this energy loss is currently under debate.1,2 One possible contribution is charge carrier relaxation, that photogenerated charge loses energy during their extraction process. In this presentation, we study this process using kinetic Monte Carlo simulations. With the state-of-the-art device simulation software Bumblebee (provided by Simbeyond B.V.), we show the importance of taking into account the 3D nature of the device. In time-resolved transient simulations, we show that carrier relaxation could be important during charge extraction. With material specific parameters, simulation results can well explain the results from time delayed collection field experiments on TQ1:PCBM devices. Depending on different parameter choices, the simulated recombination process could change from a first-order to a second-order process, consistent with experimental observations in different types of realistic systems. In the simulation, a uniform “effective medium” is assumed to describe the bulk heterojunction. As an outlook, we also show that it is well possible to include a more realistic 3D morphology, yielding a reasonable structure-performance relationship. 1 D. Neher et al., Sci. Rep. 6, 24861 (2016). 2 A. Melianas et al., Nat. Commun. 6, 8778 (2015).

Resume : In this contribution we show how drift-diffusion device modelling using SETFOS combined with temperature dependent, transient electrical characterizations using PAIOS can be used to get new insight into the device operating mechanisms and allows for parameter extractions. We apply this approach to planar MALI perovskite solar cells in order to distinguish between slow (likely ionic) and fast (electronic) charges carriers. We make use of dynamic current response to voltage steps with logarithmic time sampling in order to distinguish the different charge transport processes in perovskite films. We are therefore capable of extracting the ion mobility as well as their transport activation energy. In a second part, focusing on organic semiconductor devices, we make use of temperature dependent injection-CELIV (Charge Extraction using Linear Increasing Voltage) and impedance spectroscopy of MISM like devices (metal/insulator/semiconductor/metal). We observed a temperature dependent transition frequency in the impedance spectroscopy [1] signal that we can successfully reproduce in simulation [2] and attribute it to both the mobility activation energy as well as to the barrier for carrier injection from the metal to the semiconductor. Using CELIV we were able to deconvolute and extract these two parameters [3]. References: [1] Nowy et al. J. Appl. Phys., 107, 054501 (2010) [2] S. Altazin et al. Organic Electronics 39, 244-249 (2016) [3] S. Altazin et al. Proc. SPIE 9941, Organic Light Emitting Materials and Devices XX, 99410O (September 23, 2016)

Resume : A new green solvent, cyclopentyl methyl ether (CPME), has been applied to replace toxic halogenated solvents in the production of efficient molecular photovoltaics. We used a two-dimensional conjugated small molecule (SMPV1) as the donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor in organic photovoltaic devices. With CPME-only as the processing solvent, power conversion efficiency (PCE) of 3.13% was achieved. The low PCE arose from the low PC61BM solubility in this green solvent. Accordingly, we introduced toluene (Tol) in different amounts as a co-solvent for CPME. The greater solubility of PC61BM in these mixtures led to significant improvements in the short-circuit current density and fill factor of the device, achieving a PCE of 7% after processing in the optimized green solvent mixture of CPME:Tol. Thermal annealing (TA) of the active layers processed from the optimized green solvent mixture enhanced the PCE to 8.10%. This efficiency is the highest reported for a molecular solar cell processed from a green solvent mixture to date. Large-scale devices fabricated this way, having areas of 1 and 5.5 cm2, exhibited PCEs of 6.20 and 3.73%, respectively. The morphological changes that occurred when applying the co-solvent and TA played key roles in achieving such high PCEs. This work paves the way toward organic photovoltaics scaling up using green solvents.

Resume : Electronic skins and smart textiles are emerging applications integrating wearable displays, smart sensing systems, health-care monitors, and stretchable power systems. Although various deformable optoelectronic devices have been demonstrated, high-performance stretchable photovoltaic (PV) devices remain a significant challenge in device engineering and materials optimization. In this paper, we demonstrate how a ubiquitous material—commercially available tape—can be used as a transparent substrate for highly efficient stretchable organic PV (OPV) devices. We further reported the stretchable transparent electrodes for OPV with a visibly transparent (>70% in the range from 400 to 900nm), a sheet resistance of <50 ohmic square−1 and a high stretchability (retained 100% of its original conductivity after 100 cycles of stretching at 20% strain without the pre−strain treatment). Indeed, we constructed a stretchable OPV device displaying a power conversion efficiency (PCE) of 6.0% (under AM 1.5G 1000 W m–2)—the highest reported to date; in addition, this device retained 80% of its original PCE after 50 cycles of stretching at 20% strain. This study paves the way toward fully deformable OPVs for integration in wearable electronics.

Resume : Polymer solar cells are ‘thin-film’ solar cells that use a blend of two organic semiconductors (an electron donor and an electron acceptor) to achieve efficient photovoltaic operation.Since the blend active layer is deposited via solution-processing (eg spin-coating) and relies on the phase-separation processes to create the internal nanostructure it is very hard to control and characterise film morphology and microstructure. Furthermore due to recombination as the active layer is increased above ~ 100 nm in thickness optimum devices do not maximise light absorption by the organic semiconductor film. Nano-imprint lithography is a new and emerging technique by which the active layer morphology can be greatly controlled and optimized. However the creation of nano-structured master mold by complicated and long fabrication procedures, such as e-beam lithography, is extremely expensive and it is very difficult to make molds with large feature areas. In this work, inexpensive routes is realised to create nano-structured soft molds for nano-imprinted solar cells.Soft PDMS molds with average aluminium thickness varying from 20 nm to 40 nm have been fabricated via evaporation. The roughness of PDMS molds significantly increases after aluminium evaporation with nano-scale feature depth ranging from 128 nm to 200 nm and peak-to-peak distance ranging from 300nm to 800nm. The influence of the different amount of solvent additive on imprinting is also studied in this project. Nano-scale features cannot be transferred to PCE-10:PC71BM film with less than 2 vol% of DIO solvent additive, and starting from 2 vol% of DIO, the nano-structures can be successfully transferred. The reflection of PCE-10:PC71BM films containing more than 2 vol% of DIO significantly reduced after imprinting over a wide range of wavelength from 350 nm to 900 nm. The improved absorption from 400 to 650 nm, which is particularly useful for PCE-10:PC71BM system due to the relatively low EQE within this region. The device performance characterization has also been carried out. One example for PCE-10:PC71BM blend with 3 vol% DIO imprinted with flat PDMS mold (flat device) and 25 nm Al PDMS mold (imprint device) is shown in Fig. 10. The efficiency of the imprint device increased to about 10.9% from 10.2% (control device) due to the improved FF (0.69 for the imprinted device and 0.66 for control device) and improved VOC (0.02 V higher than the control device). Surprisingly the efficiency of the flat device increased as well due to the improved VOC and FF, the origin for this improvement still needs to be clarified. The observed EQE curves also agree well with the UV-Vis result with a maximum 8 % increase observed from 400 to 650 nm for the imprinted device.

Resume : The photovoltaic properties of a series of diketopyrrolo[3,4-c]pyrrole (DPP) copolymers containing 3,4-ethylenedioxythiophene (EDOT) as a comonomer are reported. With use of different aryl flanking units on the DPP core, namely thiophene, pyridine or phenyl, optical gaps ranging from 1.91 eV to 1.13 eV are achieved. When blended with the fullerene derivative [6,6]-phenyl C71- butyric acid methyl ester (PC71BM), the thiophene-flanked copolymer PDPP[T]2-EDOT with an optical gap of 1.13 eV was found to have the best photovoltaic performance, with an efficiency of 2.5% in an inverted device architecture. Despite having the lowest open circuit voltage of the three polymers studied, PDPP[T]2-EDOT-based devices were able to achieve superior efficiencies due to the high short circuit current of up to ~ 15 mA/cm2. PDPP[T]2-EDOT-based devices also exhibit higher external quantum efficiencies which are associated with a superior microstructure as revealed by transmission electron microscopy (TEM) and grazing incidence wide-angle X-ray scattering (GIWAXS), which are associated with the enhanced aggregation tendency of PDPP[T]2- EDOT chains. In particular PDPP[T]2-EDOT:PC71BM blends were found to have a finer phase separated morphology with superior thin-film crystallinity. Surface morphology was also investigated with atomic force microscopy and near-edge X-ray absorption fine-structure spectroscopy.

Resume : Polymer solar cells (PSCs), which have many advantages of cost, easy roll-to-roll processibility and light-weight, have been attracted attention.1-2 We designed pyrimidine-based conjugated polymers consisting 2-{4-[4,6-bis-(4-hexyl-thiophen-2-yl)-pyrimidin-2-yl]-phenyl}-thiazolo[5,4-b]pyridine (pPTP) which has strong electron withdrawing ability and applied the PSCs. By Stille polymerization, we synthesized the new conjugated polymers, pPTPBDT-12, pPTPBDT-EH, pPTPBDTT-EH and pPTPTTI. The HOMO energy levels of four polymers (pPTPBDT-12, pPTPBDT-EH, pPTPBDTT-EH and pPTPTTI) were at -5.61 ~ -5.89 eV, their LUMO energy levels were at -3.95 ~ -4.09 eV. The device comprising pPTPBDT-12 and PC71BM (1:2) showed a VOC of 0.67 V, a JSC of 1.33 mA/cm2, and a fill factor (FF) of 0.25, giving a power conversion efficiency of 0.23%. The device comprising pPTPBDT-EH and PC71BM (1:2) showed a VOC of 0.72 V, a JSC of 2.56 mA/cm2, and a fill factor (FF) of 0.30, giving a power conversion efficiency of 0.56%. The device comprising pPTPBDTT-EH and PC71BM (1:2) showed a VOC of 0.72 V, a JSC of 3.61 mA/cm2, and a fill factor (FF) of 0.29, giving a power conversion efficiency of 0.74%. The device comprising pPTPTTI and PC71BM (1:2) showed a VOC of 0.83 V, a JSC of 4.41 mA/cm2, and a fill factor (FF) of 0.31, giving a power conversion efficiency of 1.13%. References [1] X. Wang, P. Jiang, Y. Chen, H. Luo, Z. Zhang, H. Wang, X. Li, G. Yu and Y. Li, Macromolecules, 2013, 46, 4805 [2] Y. Ma, Q. Zheng, Z. Yin, D. Cai, S.-C. Chen and C. Tang, Macromolecules, 2013, 46, 4813.

Resume : We compared the electrical, optical, structural, and morphological properties of RF magnetron sputtered ZnO and solution-processed ZnO film to substitute solution process ZnO with directly sputtered ZnO film on ITO cathode for inverted polymer solar cells (IPSCs). By continuous sputtering process, ZnO-integrated ITO cathodes (ZnO/ITO, ZnO/Ag/ITO) were prepared and applied as transparent cathode for IPSCs. Although electrical, optical, morphological properties and work function of sputtered ZnO film were similar to those of solution-processed ZnO film, the power conversion efficiency (PCE) of IPSC with sputtered ZnO buffer layer is lower than that of IPSC with solution processed ZnO due to difference microstructure and interface of the ZnO buffer layer. Because the diffusion of PCBM through P3HT critically depends on the microstructure of the ZnO buffer layer, PCE of IPSC is closely related to the microstructure of the ZnO buffer layers. Based on transmission electron microscope examination, we suggested a possible mechanism to understand different behaviour of sputtered and solution-processed ZnO buffer layer in IPSCs

Resume : We have investigated the importance of the dispersion of charge carrier motion on performance of organic photovoltaics devices(OPV). In fact some reports based on pulse light experiment have shown a net dispersion of the transient mobilities (µ). However we believe that their experiment does not reflect the behavior of an operating OPV. To demonstrate that considering only steady-state µ is relevant to study OPV, we have set up an experiment to measure the charge extraction time. To do so we measure the current decay between two light intensities. The lifetime (τ) extracted from the decay, can be related to the µ by τ=8Vtµ/L2 where Vt is the thermal voltage and L is the device thickness. This experiment has been applied to different blends and shows that τ can be related to the geometrical average of the steady-state µ. This demonstrate that under real operating conditions the steady-state µ determines the charge extraction of OPV and gives a valuable insight on device performance.

Resume : Although zinc oxide (ZnO) is one of the most common electron transport materials in polymer solar cells technology the presence of defects at surface and grain boundaries significantly affects the efficiency and stability of the working devices. Here we investigate the performance, the ambient stability as well as the photostability under constant illumination of inverted polymer solar cells embedding pristine or hydrogen doped ZnO films as electron transport interlayers. It is found that devices using the hydrogen doped ZnO and based on photoactive layers composed of blends of poly(3-hexylthiophene) (P3HT) with electron acceptors possessing different energy levels, such as [6,6]-phenyl C70butyric acid methyl ester (PC70BM) or indene-C60 bisadduct(IC60BA) essentially enhanced their photovoltaic performance with maximum power conversion efficiency (PCE) reaching values of 4.40% and 6.33%, respectively, which are much higher than those of the cells with the pristine ZnO (3.08% and 4.53%, respectively). Most significantly, the degradation of non-encapsulated solar cells when exposed to ambient or under prolonged illumination is studied and it is found that hydrogen doping of ZnO is crucial for the overall stability of the resulting solar cell. We get fundamental insights into the beneficial effect of hydrogen intercalation within the lattice of ZnO by analyzing our experimental results and theoretical calculations and we propose that such doping restrains the charge traps thus inducing a chemisorbed oxygen removal, improves transparency, conductivity and surface energy of the interlayer while also provides for more efficient exciton dissociation, thus promoting efficient electron extraction/collection of photo-generated electrons.

Resume : Light upconversion (UC) via triplet-triplet annihilation remains extremely interesting topic over the decades because of relatively low power densities (~mW/cm2) required for the process to take place. The low power UC is considered to be very promising for enhancing performance of photovoltaic cells, since the power density of the sun light is sufficient to accomplish the conversion of otherwise unusable infrared radiation. The efficiency of UC in a solution, due to high molecular diffusion, exceeds 30%, however for the practical usage of UC in photovoltaics, the solid state is more desirable. Yet, the efficiency in the solid state is more than one order of magnitude lower as compared to solution. Although all the factors contributing to the efficiency drop are still unclear, most of them are mainly associated with the self-quenching of an emitter such as 9,10-diphenylanthracene (DPA). Therefore, to retain high UC efficiency, the new emitters with modified molecular structure capable of reducing self-quenching at high concentrations are needed. In this work, the UC properties of structurally modified DPA derivatives were investigated in polymer films. Quantum efficiency measurements of UC performed as a function of emitter concentration revealed that the modified DPA derivatives allow achieving almost twice as high concentration as compared to unmodified DPA without substantial quenching, which results in attaining over 2% maximal UC efficiency in the solid polymer films.

Resume : Organic-inorganic perovskite materials have gained lot of attention in photovoltics due to their several advantages such as broad light absorption from visible to the near-infrared region, high extinction coefficients, long charge-carrier diffusion lengths and tunable bandgap which leads to high efficiency and easy processability. Herein this report we demonstrate a novel strategy for morphology optimization of perovskite thin films using modified antisolvent coating method and devices were fabricated in ITO/PEDOT:PSS/MAPbI3/C60/BCP/Al architecture. The film morphology was further studied as an effect of annealing on prepared film to excess solvent vapor. The CH3NH3PbI3 film prepared in dimethylformamide (DMF):chlorobenzene (CB) mixture was exposed to different amount of residual DMF vapor and evolution of grain boundary and surface roughness was studied as a function of solvent vapor annealing. The effect of excess PbI2 concentration onto thin films morphology and crystal structure is investigated. The stability of the devices was also studied with different parametric conditions. Corresponding author: soumitrasatapathi@gmail.com

Resume : Electronic band structure of pentacene thin films depends on molecular orientations, which can be exploited to improve the performance of pentacene based optoelectronic devices. High carrier mobility and visible range absorption spectra of pentacene films can be tuned by its molecular orientations through growth dynamics and nature of substrates being used. Reports on making confined structures for charge transfer through substrate patterning showed enhanced carrier mobility due to the alignment of grains along the current direction. Optical properties of the thin film from a nontrivial fraction of nano-texture volume vary from that of flat surface, providing a key to enhance the efficiency. Dielectric substrate with a strong periodic texture could be a proper template for film deposition to understand the influence of lateral inhomogeneities to the self assembled molecules. We have fabricated 5μm,20μm,25μm and 50μm sized rectangular strips of SiO2 patterns on Si, by photolithography. Pentacene(50nm) deposited on the patterned substrates by thermal evaporation. The inplane and out-of-plane orientation of the molecules were investigated using Raman spectroscopy by analyzing polarization dependence of C−C short and long axes stretching modes. Morphology and the alignment of grains were studied by AFM. We will discuss the effect of the molecular alignments on optical properties of the films which may lead to more efficient coupling of light for solar cell and LED applications

Resume : In order for organic solar cells to fully mature from research and development into cost effective products, a continuous improvement in solar cell efficiency must be achieved. A fundamental comprehension of design principles of new polymer materials are necessary to push this area forward. We synthesized a series of conjugated polymers with a backbone of 5,6-difluoro-2,1,3-benzothiadiazole moieties and studied the effect of the polymer structure on photovoltaic properties. We demonstrated that modification of the polymer backbone affected the energy levels and the morphology of polymer/fullerene blend films, resulting in power conversion efficiencies (PCEs) of 7~10%. This work shows the effect of fine-tuning of the chemical structure on solar cell performance.

Resume : Inverted polymer solar cells based on P3HT/PCBM bulk heterojunction were prepared on flexible polyethylene naphthalate (PEN) substrate. The effect of annealing of the PEN/ITO/ZnO multilayer and ZnO/P3HT:PCBM on the structural, morphological, photophysical and photovoltaic properties was investigated and scrutinized directly on the OPV devices using essentially atom probe tomography (APT), scanning electron microscopy (SEM) and microfocus X-rays techniques. We carried out a 3D reconstruction of the interfaces of the multilayer containing PEN/ITO, ZnO/ITO and P3HT:PCBM/ZnO to address the interface micro-structure and its influence on the morphology of the photoactive film. The analyses show that the morphology of the interfaces is affected by the structure of each layer of the BHJ devices causing orientation of P3HT crystals with PCBM aggregates and ZnO, which in turn leads to a significant change of the charge transport across each layer and therefore photovoltaic performances

Resume : To commercialize the polymer solar cells many conditions should be satisfied such as efficiency, process and stability. Recently many scientists are interested in the stability of the polymer solar cells, but most of the researches are hovering at thermal stability. But real challenges are in photostability and the burn-in loss during exposed on the sun light is one of the biggest challenges to go to the market. Lost more than 40% of their initial efficiency in just 24 hours after exposing to 1 sun condition is common for polymer solar cell devices. This burn-in loss does not come from the decomposition of the photoactive materials but rather caused by the electro- and physical-properties of the photoactive materials. To overcome the burn-in loss, we tried two approaches. First one is inserting an appropriate interlayer for inverted solar cells. Second approach is design new photoactive materials which reduce the burn-in loss. Here we present the improved photo-stability results by applying both photo-stable interlayer and new-photoactive materials.

Resume : Surface-directed molecular orientations of pentacene thin films have a strong correlation with their optical and electronic properties. Control over molecular orientations can be achieved through growth dynamics and choice of substrates. Graphene has extended sp2-hybridized conjugated sheets with a strong directionality in the orientations of the π and σ orbitals. This controls the molecular alignment when pentacene is deposited over graphene. Devices fabricated from pentacene grown on graphene are found to have higher carrier mobility, due to larger π-π overlap. We have deposited pentacene(50nm) by thermal evaporation, on SiO2 substrates coated with CVD grown graphene. Quality and structure of the films were analyzed using XRD and AFM measurements. The XRD spectra showed signatures corresponding to in-plane orientation of molecules, which was further confirmed using polarized Raman spectroscopy. The polarized Raman signatures are sensitive to the C-C long and short axes stretching modes of pentacene molecules. Photoluminescence (PL) from pentacene was found to be quenched in presence of an underlying graphene layer. This may indicate an enhanced carrier transfer between the pentacene and graphene layers. Light coupling efficiency of the films was investigated using polarized absorption and PL measurements. We will discuss our results on the study of polarization anisotropy of optical responses of these films.

Resume : P-type polymer PBDTTPD were synthesized for photovoltaic applications. In this paper a structure with low band gap p-type polymer PBDTTPD and two electrodes were made and for the first time drift mobility of holes were measured by photo-CELIV method. PBDTTPD material were spincoated on transparent ITO electrode and after aluminum electrode were evaporated on top of the polymer layer. For photo-generation short pulse laser were used. Estimated value of holes’ drift mobility were about 10 in the power of -4 cm2/Vs.

Resume : Non-fullerene bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) have been studied extensively to replace the phenyl-C60 (or C70) butyric acid methyl ester (PC60BM or PC70BM) with electron-accepting small molecules because fullerene derivatives are expensive, provide a weak absorption over the solar radiation spectrum, and gradually lead to the phase separation in blend films due to the highly different surface energy between the donors and acceptors. In this study, we focused on the construction of electron-donating and –accepting small molecules that could enhance the photovoltaic performance. We expected that structural and functional similarities between the electron-donating (BDT3TR) and –accepting (O-IDTBR) materials would improve the miscibility of the blends as well as the molecular ordering. The absorption spectra, cyclic voltammetry, space charge limited current hole/electron mobility, and two-dimension grazing-incidence X-ray diffraction patterns were investigated for all materials. Bulk heterojunction devices comprising BDT3TR:O-IDTBR showed an excellent power conversion efficiency exceeding 7% and much better thermal stability than BDT3TR:PC70BM devices after ageing at 120 oC for 300 hours. Detailed synthetic scheme, optical, electrochemical, and photovoltaic properties of the oligomers will be presented.

Resume : Recently, organic single crystal materials with high carrier mobilities greater than 10 cm2V-1s-1 have been reported. Such high mobility suggests a possibility of the fabrication of organic solar cells having the comb-type electrodes for carrier collection. In this study, we demonstrate lateral electron transport and electron collection in organic photovoltaic cells comprising NTCDA single crystal. Hole collecting electrode underlying perylene derivative (DBP) was evaporated on NTCDA single crystal. Electron collecting electrode was evaporated at the 30 µm away from the hole collecting electrode. Electrons photogenerated at the NTCDA/DBP interface move the distance of 30 µm in lateral direction which is 1,000 times longer than that of 30 nm in vertical direction for holes. Clear photovoltaic characteristics showing the open-circuit voltage (VOC) of 0.4 V was observed for lateral electron-collecting-type organic solar cells. When the gap between electrodes was decreased from 50 to 30 µm, the photocurrent was suddenly increased. This result suggests that the effective electron transport occurred within 30 µm in the lateral direction, i.e., the electron range (Le) was around 30 µm. Based on the equation: Le = µeTeE and measured electron mobility of NTCDA (10-2 cm2/Vs), electron lifetime (Te) was determined to 0.26 ms. The large value of τe close to the order of a millisecond indicates the possibility of having electron ranges of the order of millimeters.

Resume : During last few years, Perovskite solar cells have shown a great potential both in the rapid efficiency improvement and in cheap material and production costs. The perovskite solar cell (PSC) is composed with an active layer, which is contacted with an n-type material for electron extraction and a p-type material for hole extraction. State of the art of perovskite cell architectures are fabricated using a mesoscopic porous layer based on TiO2 nanoparticles partially sintered as n-type material for electron transfer to front contact. Lead perovskite is grown in this mesoporous scaffold and leads to a highly efficient solar cell. We assume therefore that increasing the perovskite crystal size infiltrated within the porous n-type layer could have a positive effect on the cell efficiency because it allows the decrease of charge recombination effect at grain boundaries and the reduction of the conductivity path. The goal of this project is to increase the porosity of TiO2 scaffold to promote a better interface quality and enhance the size of perovskite crystal. This has been done by sol-gel moulding of self-assembled PMMA beads suspended in solution. The porosity obtained by this way is larger and improves the morphology of the perovskite materials and so could enhance the efficiency of complete cell.

Resume : The fulleroly Cmn derivatives were prepared by light illumination and ozonolysis of C60 gel solution. Experimental investigation was carried by UV-vis, IR, Raman spectroscopy, XPS and AFM. The structure of Cmn derivatives in gel solution (aggregates with hydrated shell) was studied. I present results from initial screening of the candidates based on informatics quantitative structure - property relationships, their comparison with results from density functional theory calculations about the effect of donor-acceptor architectures on the efficiency of the photovoltaic device. Fullerity and its derivatives can be photosensitizer due to a strong absorption of light throughout the UV and visible regions and the low energy gap between the excited singlet and triplet states facilitating efficient intersystem crossing. The comparison of spectral features for Cmn derivatives with the data for the adsorbed layers allowed detecting a series of Cmn hydroxyl group of derivatives.

Resume : Plasmonic particles can contribute via multiple processes to the light absorption process in solar cells. These particles are commonly introduced into organic solar cells via deposition techniques such as spin-coating or dip-coating. However, such techniques are inherently challenging to achieve homogenous surface coatings as they lack control of inter-particle spacing and particle density on larger areas. Here we introduce the interface assisted colloidal self-assembly as a concept for the fabrication of well-defined macroscopic 2-dimensional monolayers of hydrogel encapsulated plasmonic gold nanoparticles. [1] The monolayers showed a pronounced extinction in the visible wavelength range due to the localized surface plasmon resonance with excellent optical homogenity. Moreover this strategy allowed for the investigation of the potential of plasmonic monolayers at different interfaces of P3HT:PCBM based inverted organic solar cell. In general, for monolayers located anywhere underneath the active layer, the solar cell performance decreased due to the parasitic absorption. However with thick active layers, where the low hole mobility limited the charge transport to the top electrode, the plasmonic monolayer near to that electrode spatially redistributed the light and charge generation close to the electrode led to an improved performance. This work systematically highlights trade-offs that need to be critically considered for designing an efficient plasmonically enhanced organic solar cell. [2] References [1] T. Honold, K. Volk, A. Rauh, Fitzgerald, J. P. S. and M. Karg, J. Mater. Chem. C, 2015, 3, 11449–11457 [2] C. R. Singh, T. Honold, M. Karg and M. Thelakkat, Phys. Chem. Chem. Phys., 2016, 18, 23155

Resume : Developing materials with improved properties and discovering new organic electronic (OE) device concepts with enhanced performance remains a challenging task due to the complex interplay between charge transport and excitonic processes at the molecular scale. We have developed a three-dimensional kinetic Monte Carlo model which includes all these processes in an integral manner. The Bumblebee simulation tool, based on this model, enables researchers to effectively verify hypotheses on material and device level and to enhance understanding of the underlying processes, using a simulation-assisted approach. Here, we present an overview of various types of applications, including fundamental studies of physical processes, studies of material properties and studies of the performance of OE devices such as photovoltaic (OPV) devices, organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs).

Resume : Due to intensive research, the perfornance of organic solar cells has recently significantly improved. However, for a broad application, further improvements are needed. In this presentation, I will discuss recent work on small-molecule organic solar cells. One key research area is the search for novel absorber molecules which form the bulk heterojunction active layer. Recently, we have shown a novel strategy which reduces the synthetic work: By blending different molecules, we can continuously “tune” electronic properties, like it was done previously in inorganic semiconductors, e.g. III-Vs. The underlying physics is very different, though: Instead of electronic delocalization, the averaging of energies is obtained by long-range Coloumb interactions. As second topic, I will cover recent work in improved optical incoupling for organic solar cells: Using periodically patterned flexible substrates, efficienies can be improved. Finally, I will briefly discuss where preparation for mass manufacturing stands.

Resume : Efficiency of organic solar cells (OSCs) is determined by three parameters: the fill factor, short-circuit current and open-circuit voltage (Voc). The former two can be optimized by elaborate device engineering to reduce the recombination processes and maximize the amount of collected photons. Voc, in turn, fully relies on the design of the active layer materials, e.g. on matching the energy levels of the donor and acceptor, lowering the exciton binding energy, reducing energy disorder of the materials etc [1]. Typically, Voc in OSCs based on small molecules (SMs) as a donor, is considerably higher compared to the polymer-based devices with similar energy level alignment. Here we explore the reasons of surprisingly low (<0.5 V) Voc losses using four different SMs [2] as benchmark materials. Using ultrafast spectroscopies, we demonstrate that upon photoexcitation quasi-free charges rather than excitons are formed in the SM phase due to the strong intermolecular interactions. This decreases the Coulomb attraction in the electron-hole pair and diminishes filling-in of interfacial charge-transfer states which altogether reduces the Voc losses. Based on this scenario, we envision promoting exciton-to-charge conversion via enhanced intermolecular interactions in organic semiconductors be a promising way towards OSC efficiency optimization. [1] Elumalai et al., En Env Sci 2016 [2] Luponosov et al., Org Electron 2016

Resume : Organic photodiodes (OPD) are a promising complementary or alternative to inorganic based photodetectors for detection and imaging applications. In the context of OPD as a photodetector, there is much to gain in performance and one of the prerequisites for that, is a better understanding of the physical mechanisms and the loss factors that are at play. This is especially true in the context of dark current reduction. To allow for both high dynamic range and fast response, the dark current in a photodetector must remain negligible up to a few volts of reverse bias. In the organic photodetector or solar cell literature, it is often suggested that the dark leakage current is due to generation recombination across the junction. However, measuring the photo-response and applying the thermodynamic reciprocity relations results in a value orders of magnitude lower compared to those reported. To address this issue we fabricated several planar heterojunction (HJ) cells and studied the effect of donor materials, different hole injection layers (HILs) and active layer thicknesses on the dark current of the device. Dark current-voltage characteristics of all the devices were measured over the temperature range from 300 to 200 K. Focusing on planar HJ device with low dark current (~ nAcm-2 at -1V) we fitted their J-V data taken over a range of temperature using a current-voltage model for organic heterojunctions thus establishing a physical framework for interpreting J-V characteristics and understanding the source of dark current. In the presentation, we will describe the device design, the dark current characteristics, and the analytical model developed in the context of planar HJ cell.

Resume : Solution-processable small molecule (SM) donors are promising alternatives to polymer donors in bulk-heterojunction (BHJ) solar cells with fullerene acceptors, now reaching efficiencies >10% in single-cell devices. At this time, further efficiency improvements with SM donors require forging a better understanding of how molecular structure and functional substitutions direct the occurrence of molecular packing effects, specific BHJ morphologies and, in turn, carrier transport in thin films. Our recent studies with various sets of analogous SM donor systems indicate that main-chain substitutions[1,2] and donor-acceptor unit sequences[3] largely impact all of the aforementioned material and thin-film characteristics, yielding a broad range of efficiencies in BHJ solar cells (from ca. 1% to >8%).[4] Our parallel effort on SM acceptors that can serve as fullerene alternatives in efficient BHJ solar cells with polymer donors (4-10% efficiency achieved)[5] and SM donors (reaching >6% efficiency) indicates that examining the interplay between electron donor and acceptor components in the BHJ remains the important step to take in furthering the efficiency of nonfullerene acceptors. References: [1] J. Wolf, M. Babics, K. Wang, Q. Saleem, R.-Z. Liang, M. R. Hansen and P. M. Beaujuge, Chem. Mater., 2016, 28, 2058-2066. [2] K. Wang, M. Azouz, M. Babics, F. Cruciani, T. Marszalek, Q. Saleem, W. Pisula and P. M. Beaujuge, Chem. Mater., 2016, 28, 5415-5425. [3] K. Wang, R.-Z. Liang, J. Wolf, Q. Saleem, M. Babics, P. Wucher, M. Abdelsamie, A. Amassian, M. R. Hansen and P. M. Beaujuge, Adv. Funct. Mater., 2016, 26, 7103-7114. [4] R.-Z. Liang, K. Wang, M. Babics, P. Wucher, M. Al Thehaiban and P. M. Beaujuge, 2016, Submitted. [5] K. Wang, Y. Firdaus, M Babics, F. Cruciani, Q. Saleem, A. EI Labban, M. A. Alamoudi, T. Marszalek, W. Pisula, F. Laquai and P. M. Beaujuge, Chem. Mater., 2016, 28, 2200-2208.

Resume : Polymer Solar Cells (PSCs) continue to be a promising third generation, low energy-budget, lead-free PV technology. Efficiencies have now improved to over 12% and a new class of materials (small molecule acceptors) have recently provided rapid improvements that promise further advances. Significant effort in the field is being spent on synthetic efforts to tune the electronic structure and in understanding charge generation. In contrast, we explore the correlation of fill factor to the purity of domains as measured with soft x-ray scattering, and how said purity is controlled by thermodynamically metastable morphologies. We will argue that ideal materials systems will have a Flory Huggins interaction parameter χ that naturally leads to mixed domains that have a composition close the fullerene/SMA percolation threshold. Systems that are too miscible will have excessive bimolecular recombination. Systems too immiscible need to be quenched for best performance, are thus unstable and eventually produce fullerene islands that trap charges. Although some of these concepts are known and are indirectly referred to as “miscibility”, quantitative relations remained largely elusive. Understanding molecular interactions is even more important for conventional ternaries devices. In some cases, the two donor polymer used can have unfavorable thermodynamic interactions (χ = - 0.56 at 296 °C) that prevent improved performance due to lack of phase separation and alloying of the two donors. Overall, we advocate a program to measure χ(T) in model systems in order to develop a frame-work that will eventually lead to computational approaches that allow predictions of χ as a function of molecular tuning before synthesis of a targeted compound is attempted.

Resume : One of the main challenges in achieving commercial viability of organic solar cells is their poor stability to environmental conditions, in particular to the combination of light and oxygen. The active layer in a typical organic solar cell is a blend of a polymer electron-donor and a fullerene electron-acceptor. The photo-degradation of both components is likely to have a profound effect on device performance, yet most of the attention in the field has been on the polymer donor. Fullerenes have non-negligible reactivity: the curved surface of fullerenes induces a considerable strain on their carbon-carbon double bonds, and a number of (photo)oxidation pathways have been proposed.[1-3] In this work, we have studied the effect of photo-oxidation of PCBM on the performances of polymer:PCBM OPV devices. We have observed that small amounts (less than 5% of the total fraction) of PCBM, photo-oxidised in air under one sun illumination, dramatically affect the efficiency of a number of different device structures. In this talk, I will explore how the photo-oxidation of PCBM changes the distribution of electronic states available for charge separation and transport. We have combined electroluminescence and space-charge-limited-current measurements with simulations of the energy levels of a number of oxidised products, thought to be formed upon photo-oxidation of PCBM. Our results indicate the formation of energy states below the LUMO of PCBM, which may act as electron traps. Furthermore, through analysis of the sub-band gap external quantum efficiency, we have quantified the radiative and non-radiative losses to open-circuit voltage caused by these intra-band-gap states. Our analysis sheds light on the effects of PCBM oxidation on the fate of photo-generated charges in OPV devices, and supports the case for the development of more stable, non-fullerenes acceptors for commercial cells. [1] Taliani, C., et al., Journal of the Chemical Society, Chemical Communications, 1993(3): p. 220-222. [2] Xiao, Z., et al., Journal of the American Chemical Society, 2007. 129(51): p. 16149-16162. [3] Matsuo, Y., et al., Chemical Communications, 2012. 48(32): p. 3878-3880.

Resume : We present a systematic comparative study of the intrinsic photochemical stability of several fullerene-polymer systems under the natural outdoor conditions in the Negev desert. Analysis of our results challenges a general relevance of the widely accepted view that fullerene photodimerization controls the intrinsic operational stability of organic photovoltaics (OPV) and it is responsible for initial “burn-in” degradation of the devices. In particular, light-induced dimerization of the [60]fullerene derivatives was found to be irrelevant to the degradation behavior of OPV incorporating these materials in the blends with PCDTBT. The conventional [60]PCBM was shown to undergo rapid and severe photodimerization, when exposed to sunlight, which, however, does not affect noticeably the PCDTBT/[60]PCBM OPV efficiency. On the contrary, two novel fullerene derivatives, which were shown to be far more resistant towards the photodimerization, induced a dramatic failure of the OPV performance. The application of the electron paramagnetic resonance (EPR) spectroscopy allowed us to reveal an alternative photochemical degradation pathway of the fullerene derivatives resulting in the formation of persistent free radicals. The accumulation of free radicals in the OPV active layer was shown to be a critical degradation mechanism, ruining the photovoltaic performance of the devices. These findings strongly suggest that current understanding of the processes responsible for the “burn-in” failure of organic solar cells has to be reconsidered and additional studies should be performed to clarify the actual mechanisms of the relevant processes. Further research in that direction is expected to result in a considerable advancement of the OPV stability meeting the pragmatic benchmarks of a successful industrial technology.

Resume : Traditionally, progress in polymer based photovoltaics was based to a large degree on the reduction of the absorption onset of donor polymers and on the optimization of the interfacial energetics at the donor-acceptor heterojunction. However, the rapid progress in efficiencies has been slowed down in recent years, because the fairly obvious optimization of donor and acceptor energy levels has been successful, while further progress has to be achieved by other means. These can be improving the strength of light absorption, reducing recombination and improving transport. I will therefore review the figures of merit that have and will guide the efficiency progress in organic photovoltaics and explain how recent progress in achieving (i) higher fill factors at larger thicknesses, (ii) lower voltage losses (between absorption onset and open circuit voltage) as well as (iii) better absorption by using more highly absorbing molecules can lead to further improvements in efficiency. A special focus will be on the role of non-fullerene acceptors, which have shown to be crucial to achieve lower voltage losses and higher absorption.

Resume : The past few years have witnessed a rapid evolution of hybrid organic-inorganic perovskite solar cells as an unprecedented photovoltaic technology with both relatively low cost and high-power conversion. The fascinating physical and chemical properties of perovskites are benefited from their unique crystal structures represented by the general chemical formula AMX_3, where the A cations occupy the hollows formed by the MX_3 octahedra and thus balance the charge of the entire network. Despite a vast amount of theoretical and experimental investigations have been dedicated to the structural stability, electrical, and optical properties of hybrid halide perovskite materials in relation to their applications in solar cells, the thermal transport property, another critical parameter to the design and optimization of relevant solar cell modules, receives less attention. We evaluate the lattice thermal conductivity of a representative methylammonium lead triiodide perovskite (CH3NH3PbI3) with direct non-equilibrium ab initio molecular dynamics simulation. Resorting to full first-principles calculations, we illustrate the details of the mysterious vibration of the methylammonium cluster (CH_3 NH_3^+) and present an unambiguous picture of how the organic cluster interacting with the inorganic cage and how the collective motions of the organic cluster drags the thermal transport, which provide fundamental understanding of the ultralow thermal conductivity of CH3NH3PbI3.We also reveal the strongly localized phonons associated with the internal motions of the CH_3 NH_3^+cluster, which contribute little to the total thermal conductivity. The importance of the CH_3 NH_3^+ cluster to the structural instability is also discussed in terms of the unconventional dispersion curves by freezing the partial freedoms of the organic cluster. These results provide more quantitative description of organic-inorganic interaction and coupling dynamics from accurate first-principles calculations, which are expected to underpin the development of emerging photovoltaic devices.

Resume : Pi-conjugated organic molecules and polymers now provide a set of well-performing semiconductors that support devices, including light-emitting diodes (LEDs) as used in smart-phone displays and lighting, field-effect transistors (FETs) and photovoltaic diodes (PVs). These are attractive materials to manufacture, particularly for large-area applications where they can be processed by direct printing, so that the cost of materials and processing can be very low. This practical success is made possible by breakthroughs in the understanding and engineering of the underlying semiconductor science. The physics of organic semiconductors is often controlled by large electron-hole Coulomb interactions and by large spin exchange energies. Management of excited state spin is fundamental for efficient LED and solar cells operation. I will discuss in particular recent progress in the control of emissive spin singlet excited states and non-emissive spin triplet excited states.

Resume : We find that photoconductivity in pristine pentamethine cyanine films stems from efficient intrinsic photo-generation of charge carriers. Single layer cyanine diodes with selective hole and electron injecting contacts produce photocurrent above 20 mA/cm2 when driven at reverse bias under an irradiation intensity of 3 suns. By means of altering the electrodes contacting the single cyanine films as well as by doping electrode interfaces after poling the ionic semiconductor, we demonstrate a linear relationship between photocurrent and voltage as well as light intensity. Using photo-CELIV measurements we extract a charge density of 6.4·1016 cm-3 and average mobility of 5∙10-7 cm2/Vs under one sun. These numbers perfectly match the conductivity of 6.4·10-9 -1cm-1 obtained from the slope of the current-voltage curve. However, by carrying out numerical modeling studies, we show that the linear resistor behavior is rather induced by space charge limited current induced by mobility imbalance between holes and electrons, which we also reveal experimentally. Slow charge recombination kinetics as measured by time delayed photo-CELIV measurements deviate profoundly from Langevin behavior and points towards a different encounter mechanism. Electric field induced fluorescence quenching sets in at a field of 0.1 MV/cm but due to low fluorescence quantum efficiency of <10-4 in these disordered pentamethine cyanine films, electric field induced exciton quenching is only a minor contribution to photocurrent. Indeed we show that efficient quenching of radiant excitons occurs in the absence of electric field and is at the origin of photoconductivity in pentamethine cyanine films.

Resume : Organic conductors are being evaluated for potential use in waste heat recovery through lightweight and flexible thermoelectric generators manufactured by means of cost effective printing processes. Assessment of the potentiality of organic materials in real devices still requires a deeper understanding of the physics behind their thermoelectric properties, which can pave the way for further development of the field. In this contribution we will report a detailed thermoelectric characterization of a set of inkjet printed thin films of commercially available poly(3,4-ethylenedioxythiophene) polystyrene sulfonate formulations, displaying different electrical conductivities in the range from 1 to 1000 S/cm. The dependence of thermopower with respect to temperature is analyzed and discussed for all samples, and its correlation with the density of states close to the Fermi level is investigated and discussed in the framework of Mott’s relation. Finally, with the aim of extracting a reliable thermoelectric figure of merit zT, thermal conductivity is extracted thanks to a simplified pump-probe spectroscopy based thermal conductivity measurement. A quantitative assessment of the potentialities of organic thermoelectrics for micro-thermoelectric generators is finally derived outlining minimum thermoelectric properties requirements to be achieved in order to pave the way for real applications.

Resume : While it has been argued that field-dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). We have developed a simple approach to differentiate the contributions of GR and nongeminate recombination (NGR). We studied bilayer OSCs using either a PCDTBT-type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p-DTS(FBTTh2)2 layer as donor material and C60 as acceptor. We measure JV-characteristics as a function of intensity and charge-extraction-by-linearly-increasing-voltage-type hole mobilities. The experiments have been complemented by Monte Carlo simulations. We find that the fill factor (FF) decreases with increasing donor layer thickness even at the lowest light intensities where geminate recombination dominates. We interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor–acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor–acceptor interface. This effect is absent in the p-DTS(FBTTh2)2 diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s-shape of the JV-curves and the concomitant additional decrease of the FF with increasing layer thickness.

Resume : Spin-dependent exciton recombination and charge transport play important roles in determining efficiencies of optoelectronic devices based on organic semiconductors. This talk deals with the fundamental question of organic magnetoconductance (MC): How spin mixing changes charge transport process in organic semiconductors? This is still the most exciting question and also the most debated controversy in the field of organic MC. It is of importance for improving the performance of organic MC devices, and for fundamental insights into the electronic/optoelectronic properties of organic semiconductors. We investigate the origin of small-field MC effects in poly(3-hexylthiophene):[6,6]-phenyl- C61-butyric acid methyl ester (P3HT:PCBM) photovoltaic (PV) devices. By using photoinduced absorption spectroscopy, we show that the MC is due to the charge recombination effect. We simulate the device operation using a drift-diffusion model incorporating spin-dependent rate equations, which clearly demonstrates that the MC is a result of competition between the charge dynamics and spin mixing, and convincingly explains the microscopic origin of the effects of voltage, temperature, and injection barriers on the MC. Our results provide mechanistic understanding that can help to improve the performance of organic magnetoconductance devices, and underline the importance of spin mixing and spin-dependent recombination in organic LEDs and PVs.

Resume : With the advantage of high-efficiency and ease in material processing, perovskites demonstrate the great promise to be the next-generation solar-cell absorbers and have triggered enormous scientific efforts in the field of photovoltaics. Exploiting the fundamental physics beneath the optical absorption of perovskite absorbers is of crucial importance to engineer it for better photovoltaic performance. In this work, we mainly investigate the influence of electron-phonon interaction on the electronic band structure and optical absorption of perovskite absorbers CsPbI3 and CH3NH3PbI3 from the atomic level using first-principles. This work can provide insight into the optical absorption of perovskite absorbers at varying temperatures. Based on the density functional perturbation theory, the effect of lattice vibration (including the zero-point vibration) on the electron’s energy state and lifetime is investigated. Then with the thermally perturbed electronic structure, the effect of electron-phonon interaction on optical absorption can be analyzed. Moreover, by comparing the inorganic and organic-inorganic hybrid perovskite, the role of organic cations in altering the electronic and optical properties is further explored. This work aims at unrevealing the electronic structure and optical absorption of perovskites at finite temperature and providing insight to guide the perovskite material design for better performance.

Resume : Spin-polarized charge transfer at the interface between a ferromagnetic metal and a molecule can lead to ferromagnetic coupling[1] and to a high spin polarization[2] at room temperature[3-4]. The magnetic properties of these interfaces can not only alter those of the ferromagnet[5], but can also stabilize molecular spin chains[6-7] with interesting opportunities toward quantum computing. With the aim to enhance an organic spintronic device’s functionality[8], an external control over this spin polarization may thus be achieved by altering the ferromagnet/molecule interface’s magnetic properties. To do so, we utilize the magnetoelectric properties[9] of an underlying ferroelectric/ferromagnetic interface. Switching the ferroelectric polarization state of a PbZr0.2Ti0.8O3 (PZT) bottom layer within a PZT/Co/FePc-based device alters the X-ray-magnetic circular dichroism of the Fe site within the phthalocyanine (Pc) molecular top-layer. We thus demonstrate how to alter the magnetic properties of an interface with high spin polarization at room temperature[4]. This expands electrical control over spin-polarized FM/molecule interfaces, which was first demonstrated using ferroelectric molecules[10], to all molecular classes. [1] A. Scheybal et al Chem. Phys. Lett. 2005, 411, 214. [2] C. Barraud et al, Nat. Phys. 2010, 6, 615. [3] S. Lach et al, Adv. Funct. Mater. 2012, 22, 989. [4] F. Djeghloul et al, Sci. Rep. 2013, 3, 1272. [5] K. V. Raman et al Nature 2013, 493, 509. [6] M. Gruber et al Nat. Mater. 2015, 14, 981. [7] C. Barraud et al Phys. Rev. Lett. 2015, 114, 206603. [8] S. Sanvito, V. A. Dediu, Nat. Nanotechnol. 2012, 7, 696. [9] O. Vlašín et al, ACS Appl. Mater. Interfaces 2016, 11, 7553. [10] S. Liang et al, Adv. Mater. 2016, 28 1521.

Resume : We present our recent work in theoretical understanding and modeling of thermoelectric properties for organic materials. The charge transport is calculated by considering both electron-phonon scattering and carrier-impurities scattering. The heat transport is modelled by first-principles molecular dynamics compared with phonon Boltzmann equation considering the anharmonic terms. We find that for polymeric materials, it is possible to realize phonon glass electron crystal by engineering the crystallinity at the sub-micron scale by virtue of difference in the mean-free path. We further investigate the electronic structure and thermoelectric poweer factors for the one-dimensional organo-metallic coordinated polymers.

Resume : Since the isolation of free standing graphene in 2004, graphene research has experienced a phenomenal growth [1,2,3] Due to its exceptional electronic, optical and mechanical properties, it is believed to be the next wonder material for optoelectronics. The enhanced electrical conductivity, combined with its high transparency in visible and near-infrared spectra, enabled graphene to be an ideal low cost indium-tin oxide (ITO) substitute[4]. Solution-processed graphene combines the unique optoelectrical properties of graphene with large area deposition and flexible substrates rendering it compatible with roll-to-roll manufacturing methods. This work is focused in a laser based patterning technique compatible with flexible, temperature sensitive substrates method that allows the accurately control and enhance a rGO electrode's transparency, with a subsequent slight increase in the sheet resistance, and therefore improve the tradeoff between transparency and conductivity of reduced graphene oxide (rGO) layers [5]. We present our latest results regarding the laser patterning,using fs laser pulses, of a micro mesh on reduced graphene oxide films decorated with Ag metal nanoparticles. The film's transparency enhanced from 10% to 65% with a slight increase of sheet resistance from 210 to 600 Ωhms per square. The measured sheet resistance is superior compared to the sheet resistance reported by our group previously, where an rGO micro mesh electrode of similar transparency and mesh period, not decorated with nanoparticles, exhibited sheet resistances of the order of 1kΩ per square. The observed electrical superiority (40% lower sheet resistance) of the metal nanoparticle based micromesh electrode is attributed to the silver metal nanoparticles. It is strongly believed that the incorporation of these plasmonic films as semi-transparent electrodes in OSCs will lead to higher efficiencies that the 3.05% reported by our group and highlighted in Advanced Functional Material issue. The laser patterned Ag nanoparticle decorated rGO films are going to be applied also as transparent electrodes in flexible plane inverted perovskite semitransparent solar cells where the brittle and expensive ITO is incompatible. It is likely that the fabrication of mesh will increase the TCE transparency and at the same time the incorporation of metal NPs will boost its conductivity compared to pure rGO micromesh. References [1] E. Stratakis, K. Savva, D. Konios, C. Petridis, E. Kymakis, Nanoscale, 6, 6925, (2014). [2] G. Kakavelakis, D. Konios, E. Stratakis, E. Kymakis, Chemistry of Materials, 26, 5988, (2014). [3] D. Konios, G. Kakavelakis, C. Petridis, E. Stratakis, E. Kymakis, Journal of Materials Chemistry A, 4, 1612-1623 (2016) [4] C. Petridis, D. Konios, M. M. Stylianakis, G. Kakavelakis, M. Sygletou, K. Savva, P. Tzourbakis,, M. Krassas, N. Vaenas, E. Stratakis, E. Kymakis, Nanoscale Horizons, DOI: 10.1039/C5NH00089K (2016) [5] D. Konios, C. Petridis, G. Kakavelakis, M. Sygletou, K. Savva, E. Stratakis, E. Kymakis Advanced Functional Materials, (2015), 25, 2213-2221

Resume : Organic solar cells (OSCs) have approached efficiencies of around 11% in recent years. Recombination of charge carriers is an important efficiency limiting factor of OSCs. Two pathways of recombination are bulk recombination of photo-generated charges, and recombination influenced by the electrodes. To further improve the performance of OSCs, recombination must be understood and reduced. Here, we present a new approach to investigate the impact of electrodes on the recombination process. This is done by depositing an aluminum oxide transparent insulating layer on both sides of the active layer; therefore the electrodes have no contribution to the recombination process. Recombination lifetimes are measured from exponential decay of the displacement current density after discharging of the photo-capacitor due to recombination of excess charge carriers by small reduction of light intensity. The influence of electrodes on the recombination lifetime is then investigated using different techniques where injected charges are present in the active layer. We find that recombination near the electrodes considerably reduces net recombination lifetime in solar cells with smaller recombination rate constant and lower mobility of photo-generated charges. Low charge mobility might facilitate recombination of photo-generated charges next to the contacts with injected charges, which lowers net recombination lifetime.

Resume : The current record efficiencies for solution-processed organic solar cells (OSC) are beyond 11%. The absorption range has to be broadened to increase the current whereas (further) reduction of the offset between the LUMO levels of donor and acceptor will help to maximize the open-circuit voltage. Finally, improvement of the fill factor requires a mobility increase. However, the better the photoactive materials, the more crucial becomes the selectivity of the electrodes in order to avoid losses due to surface recombination. The selectivity of real electrodes is of course never ideal, but it is often difficult to determine whether or how severe it limits the device performance. In this contribution, experimental data and results from drift-diffusion simulations of the temperature and intensity dependence of the open-circuit voltage (Voc) will be shown. Within a certain range the Voc increases with decreasing temperature or increasing illumination intensity as can be expected. However, for many OSC the open-circuit voltage starts to saturate for higher illumination intensities or lower temperatures. This effect has often be observed but thus far not been analyzed thoroughly. By means of charge carrier extraction experiments and numerical simulations these findings can be explained consistently. The effects are caused by non-sufficient electrode selectivity for the above mentioned conditions which causes increasing losses due to enhanced surface recombination.

Resume : Recently, much attention has been paid to the preparation and investigation of organic–inorganic hybrid materials owing to their various properties used in optoelectronics. Among these compounds, organic–inorganic halometalates (III) (Sb, Bi..) presents a group of interesting materials thanks to their various properties that could lead to using in solar cells. While much effort has been placed on determining structural properties, little work of application studies related to this kind of metal complexes have been reported. Thus, in this paper we report for the first time the synthesis of three new hybrid materials using the slow evaporation method by dissolving aqueous solution of metal and organic entities in a concentrated HCl (37%) for solar cells application. We demonstrate that these materials can be used in solar cells and especially as interfacial layers in normal device structure. In fact, the energy band gap of these materials was found to be closed to that used in interfacial layers [1] of some organic solar cells using low band gap polymers. We show that even if there is no increase in optical absorption inside the device, all parameter determining the performance of the solar cells are strongly improved by the interlayer. More importantly, the morphology and especially the surface roughness of these hybrid layers is crucial to obtain hole blocking behavior leading to fill factor up to 70 %. [1] Sadok Ben Dkhil et al., Adv. Energy Mater. 2014, 4, 1400805

Resume : Electronic processes at organic hetero-interfaces between electron donating and electron accepting molecules determine the photocurrent, photovoltage and ultimately, the power conversion efficiency of organic solar cells. Solar cells with incident-photon-to-extracted-charge conversion yields of over 85%, and absorbed photon-to-extracted-charge conversion yields of 90-100% have been achieved, using organic materials. However, the difference between the optical gap of main absorber and open-circuit voltage (Voc) is much larger than for inorganic and perovskite based solar cells. The main improvements of the Voc of organic solar cells have so far been made by tailoring donor-acceptor interfacial energetics, taking advantage of well-known principles of molecular design. Nevertheless, for most material systems we consistently find a large, almost constant difference (~0.6 eV) between eVoc and the energy of the intermolecular charge transfer (CT) state, ECT. Added to this, electron transfer losses are very often larger than 0.1 eV, resulting in overall voltage losses larger than 0.7 eV. We present experimental evidence that the ECT-eVoc difference can be reduced by reducing the physical interfacial area available for free charge carrier recombination. We further discuss the influence of measurable molecular properties, such as the electronic coupling, molecular reorganization as well as non-radiative recombination pathways on free carrier recombination and the Voc of organic solar cells.

Resume : Impedance based techniques have been frequently used to study doping concentration, charge recombination and density of states in various thin-film solar cells. The most basic impedance based technique is the capacitance voltage (CV) measurement in the dark that allows one to derive the doping concentration for sufficiently thick absorber layers and gives some information about the amount of band bending in the device. While the CV measurement in the dark is a well understood method, CV measurements under illumination have so far been much more difficult to interpret and have shown features that were not straightforward to explain with analytical equations. Here, we present experiments and simulations to show which physical mechanisms affect the Mott-Schottky analysis on bulk-heterojunction solar cells under illumination. Provided that the device is sufficiently thin and not highly doped, we show that the CV curves can be approximately described by a theory of photocapacitance developed by Crandall for amorphous silicon solar cells. However, for practical cases we observe differences between the theory of Crandall and the experimental results which we attribute to the asymmetry of the system considered and the consequent non-uniform electric field. Using numerical simulations and analytical estimates of the space charge limited photocurrent, we show that the presence of a non-uniform electric field plays a major role in determining the shape of CV curves under illumination.

Resume : Luminescence spectroscopy is widely used and well established in the characterization of semiconducting materials. In the field of organic solar cells (OSC) luminescence spectroscopy was used for e.g. the investigation of radiative recombination of excitons in pure materials and the charge transfer state (CT) or the analysis of recombination dynamics. However, it is not common to consider optical interference effects in the organic thin-film structures which influence the optical outcoupling. In a comprehensive study we investigate the electroluminescence spectra of OSC with different layer stacks and cell architectures. We show that thin film interference has a large impact on the detected luminescence signal. The spectra show a major dependence on layer thickness, cell architecture and the angle of vision. Dependent on the layer stack and the wavelength of the emitted light these effects can be crucial. In case of P3HT:PCBM solar cells in an ITO based devices architecture both the peak position and shape of the CT emission are dominated by interference within the layer stack. Hence the luminescence in the near infrared (NIR) range is governed by interference and in this case does not deliver reliable information about the actual energy of the radiative recombination. This leads to the conclusion that in NIR spectroscopy special attention has to be paid on thin film interference especially in devices with a transparent ITO electrode.

Resume : Polymer:fullerene blends have attracted attention as efficient organic photovoltaic (OPV) materials promising over 10% power conversion efficiency. In this talk, I present results obtained with a variety of ultrafast spectroscopic techniques (transient absorption, terahertz and electro-modulated differential absorption spectroscopy) that have allowed to correlate the mechanism of charge generation in donor:acceptor blends to the phase morphology, i.e. to the arrangement of the donor and acceptor into phase-pure (neat) and intermixed domains. In particular, I describe how varying the miscibility between the components (by changing the fullerene acceptor, or using ternary blends containing two different fullerenes), and replacing the polymer with the corresponding dimer, affect the phase morphology and charge generation.

Resume : This paper presents an original investigation method to highlight the differences of mobilities and transport mechanisms of charge carriers in organic materials solar cells. Two types of polymer:fullerene solar cells were investigated: ITO/ PEDOT:PSS/ P3HT:PCBM /Al and ITO/PEDOT:PSS/PCDTBT:PCBM/Al. The I–V characteristics were done under standard illumination (1000W/m2) using a mask placed at different distances from the collecting cathode position. The influence of the distance between the irradiation area and the position of the collecting electrode on the performances of solar cells was studied. It has been observed that the short-circuit current and the open-circuit voltage considerably increase with the decrease of the distance between the cathode and the irradiation area. This study method gives also the opportunity to find the suitable collecting grid layout (distance between electrodes) depending on the nature of the organic materials. Moreover it can also explain the differences observed between the reported values in literature for the conversion efficiencies of organic solar cells.

Resume : A major challenge in the design of next generation organic photovoltaics (OPVs) is to develop materials that provide high absorptivity over the spectral range corresponding to the solar irradiance spectrum. In this regard, synthetic porphyrins have been utilized as efficient light-harvesting (LH) center for a photovoltaic device such as a dye sensitized solar cell with a porphyrin (PSSC). Although the intrinsic advantages of porphyrin as OPVs are their rigid molecular structures with large absorption coefficient in the visible region and their feasibility of functionalization such as fine tuning of basic properties, their narrow spectral response and little NIR absorptivity pose significant limitations. A common approach to extend the range of absorption spectrum into NIR is to extend pi-conjugation of the dye. Addition of ethynyl moiety to the porphyrin macrocycle has been amongst effective strategies. Introduction of the thiophene group has also been widely utilized in a variety of photovoltaic device to enhance absorption coefficient of the dye and redshift its absorption spectrum. In this presentation, we synthesized a series of free bases of meso-substituted tetra- and bis-thiophenylethynyl porphyrins as a new class of OPVs and characterized their optical and photophysical properties based on spectroscopic and computational methods. In addition, we demonstrate that the protonation of thiophenylethynyl porphyrins enables Q-band transitions at lower energy up to ~850 nm with higher intensity than their parent free bases, allowing for enhanced light harvesting properties in the red region of the spectrum. We also show that the protonation to significant changes in the bond lengths between the macrocyle and thiophenyl group; reduced single bonds and an elongated ethynyl triple bond are observed, leading to cumulene character.

Resume : Poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) (Fig. 1) in the form of an water dispersion as colloidal gel particles is one of the most successful conducting polymers having hierarchical structures. Because of its processability, transparency, high electrical conductivity, and thermal stability, the PEDOT:PSS has potential applications to electrical and optical devices such as capacitor, hole transport layer, and transparent electrodes for touch panels and flexible displays1). However, the electrical conductivity of the PEDOT:PSS is still lower compared to the indium tin oxide (ITO) for the practical applications. In this study, we have synthesized PEDOT:PSS colloidal gels and applied to hybrid solar cells. The PEDOT:PSS with a composition ratio of 1:2.5 was synthesized at various polymerization temperatures between 0 and 40 °C. The electrical conductivity of the PEDOT:PSS cast film was measured by a four-point technique using Loresta-GP (MCP-T610, Mitsubishi Chemical Analytech). The hybrid solar cells were fabricated　by spin-coating the PEDOT:PSS dispersion containing 5% of ethylene glycol on n-Si wafer. The photoelectric conversion characteristics of the PEDOT:PSS/n-Si hybrid solar cells were evaluated with a solar simulator. It was found that the electrical conductivity increased with decreasing the polymerization temperature and the value reached as high as 1145 S/cm at 0 °C, which was higher than that of the high-conductive commercial PEDOT:PSS (Clevios PH1000, Heraeus). This is probably due to the higher molecular weight and/or crystallinity of the PEDOT molecules. Using the high-conductive PEDOT:PSS, hybrid solar cells were fabricated and the relation between current density and voltage was measured. It was found that the hybrid solar cell exhibited photoelectric conversion, where short-circuit current density (Jsc), open-circuit potential (Voc), fill factor (FF), and power conversion efficiency (PCE) of the PEDOT:PSS/n-Si solar cell were found to be 30.0 mA/cm2, 0.51 V, 0.68, and 10.3% respectively. References [1] H. Okuzaki (ed.), "PEDOT: Material Properties and Device Applications", Science & Technology (2012).

Resume : Organic thin film transistors (OTFTs) have attracted a lot of attention for its application in low cost and for large area devices like displays. Thus understanding the carrier transport becomes crucial in improving the device performance. Organic diodes are limited by Space Charge at higher fields which change the internal field and thereby making it difficult to study the transport. OTFT provides better way to study charge transport using gate induced channel charge accumulation with low lateral fields. OTFTs suffer from non idealities such as non ohmic contacts and traps present at the semiconductor-insulator interface. The interface quality affects the charge transport and macroscopic parameters like mobility, threshold voltage and on –off ratio. Owing to these non idealities, it becomes difficult to analyse the charge transport in organic semiconductor. In this work we explore ways to isolate the effects of non-idealities introduced by injection contacts and semiconductor-insulator interface. We identify surface treatments that will enhance injection efficiencies and reduce the effect of interfacial states at the organic-insulator interface. For the ideal device, we have studied how the mobility varies as a function of channel charge concentration from output characteristics. Using this, we will present a model based on microscopic analysis of charge carrier transport to determine average tunneling lengths and density of states.

Resume : Film thickness and donor/acceptor blend ratio are key parameters defining OPV performance. While different techniques exist to evaluate these parameters very locally (AFM and its derivatives for <20 micron) or in average (ellipsometry, SIMS, etc.), there are no fast non-destructive techniques to monitor thickness and composition over medium sized areas (cm). Imaging of these parameters at these length scales will allow monitoring and optimizing solution processing of OPVs in order to avoid undesired effects arising from Marangoni flows, coffee ring, wetting issues, etc. We present a methodology that exploits Raman scattering to quantify the thickness as well as the volumetric composition of blended films. Interestingly, the method also applies if the film is embedded in a multi-layered stack, which is the case of an active layer in a functional OPV device. We adopt the transfer matrix method to describe the distribution of the incoming and scattered fields, including their (re)absorption and the interference of both fields with themselves. The observed oscillations in scattered intensity as a function of film thickness are well described with our model. The film composition is determined according to a linear combination of the materials Raman fingerprint weighted by their corresponding cross-sections, which are determined using pure films with lateral thickness gradients. The implementation of the methodology in a Raman imaging setup enables the rapid and facile characterization of film thickness and composition over large areas (up to several square centimeters), with diffraction-limited resolution (200-300 nm) and in functional OPV devices.

Resume : In view of efficient polymer solar cells, multi-junction devices are promising candidates to further improve the photovoltaic performance by overcoming the absorption limits of single-junction cells. One of the most critical factors of solution-processed multi-junction devices is the sub-cells interconnection layer (ICL), typically composed by poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and ZnO, respectively as hole and electron transporting layers. Despite the inverted structures are preferred to the standard ones, the effective deposition of the aqueous PEDOT:PSS solution onto the hydrophobic organic layer represents a delicate step. Through the introduction of a surfactant in the commercial PEDOT:PSS solution, a robust and reproducible ICL is obtained for blade-coated and air-processed inverted tandem solar cells. However, this method alters the properties of the PEDOT:PSS layer thus affecting the tandem performance. Through the investigation of the PEDOT:PSS/ZnO interface, a simple solvent washing of the PEDOT:PSS film surface is demonstrated as an effective way to optimize its energetic and morphological properties. As a result, the improved PEDOT:PSS/ZnO interface leads to tandem devices with more than 50% enhanced power conversion efficiency. The reported method is an easy approach to optimize the PEDOT:PSS/ZnO ICL of inverted multi-junction devices, applicable onto any organic active layer and compatible with a roll-to-roll production line.

Resume : Polymer:fullerene bulk heterojunction solar cells (BHJ SC) are one of the most studied and promising structure in organic photovoltaic family. Exciton generation, dissociation, free carrier transport and charge extraction play an important role in the short circuit current (Jsc) and power conversion efficiency (PCE) of an organic BHJ SC. We study the impact of band offset at the interfacial layer and morphology of active layer on the extraction of free carriers. The effects are evaluated on inverted BHJ SC using Zinc oxide as a buffer layer, prepared via two different methods-ZnO nanoparticle dispersed in mixed solvents (ZnO A) and sol gel method (ZnO B). It is observed that devices with ZnO A buffer layer improves the charge extraction and Jsc, resulted in a PCE of (9.1±0.4)% for polythieno[3,4-b]-thiophene/benzodithiophene):[6,6]-phenyl C71 butyric acid methyl ester (PTB7:PC71BM) blend and a PCE of (5.6±0.2)% for poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C71 butyric acid methyl ester (P3HT:PC71BM) blend. The underlying process responsible for the enhanced performance of the devices with ZnO A layer has been evaluated using the exciton generation rate and exciton dissociation probability studies based on Onsager model. The improvement is due to the better morphology and band offset of the blend near to the ZnO A/Active layer interface. Further, the numerical analysis illustrates that the morphology at the interface has a dominant role in improving the BHJ SC performance.

Resume : Porous organic polymers (POPs) exhibit high surface areas in (hydro)thermally stable, covalently bonded backbone.[1] Besides their good performances in conventional application areas of porous materials (i.e. gas storage, heterogeneous catalysis), such compounds can be also utilized for (opto)electronic applications.[2, 3] The comparison of this class of materials with their corresponding linear analogues reveal the advanced properties of the high dimensional porous backbone in optical/electrochemical sensing and photocatalysis.[4, 5] In this context, POPs, especially the fully conjugated ones, namely conjugated microporous polymers (CMPs), are interesting materials for organic photovoltaics (OPVs). Besides their integration as hole-transport layer,[6, 7] use of CMPs in the active layer of OPVs attracting attention, where the electron rich backbone can absorb broader spectrum of light, open pores can host prospective guests (i.e. fullerene derivatives), and high dimensional nature can transport charges freely without orientation limitations (face-on, edge-on).[8-11] The major disadvantage of this class of materials is their non-soluble nature preventing the easy thin film formation, in order to be introduced in nanoelectronic devices. However, there have been several reports on formation of porous polymer thin films, in which possibility of uniform film formation was proven.[12-19] Here, we present novel porous polymeric networks to be applied in the active layer of organic photovoltaic devices. In our work, benzotrithiophene was used as the structure directing motive, and its co-polymerization with various linkers by using Stille coupling revealed highly interesting properties in the area of OPVs. References [1] A. Thomas and J. Weber in Nanoporous Polymers, Vol. (Ed. Q. Xu), Taylor & Francis, 2013, pp. 1. [2] R. Dawson, et al., Prog. Polym. Sci. 2012, 37, 530. [3] M. Rose, Chemcatchem 2014, 6, 1166. [4] X. M. Liu, et al., J. Am. Chem. Soc. 2012, 134, 8738. [5] A. Palma-Cando and U. Scherf, ACS Applied Materials & Interfaces 2015, 7, 11127. [6] C. Gu, et al., Adv. Mater. 2013, 25, 3443. [7] C. Gu, et al., Angewandte Chemie International Edition 2016, 55, 3049. [8] P. J. Skabara, et al., Adv. Mater. 2013, 25, 1948. [9] R. Gutzler and D. F. Perepichka, J. Am. Chem. Soc. 2013, 135, 16585. [10] B. A. Gregg, The Journal of Physical Chemistry Letters 2011, 2, 3013. [11] C. Gu, et al., Angewandte Chemie International Edition 2015, 54, 13594. [12] M. Dogru, et al., Angew. Chem.-Int. Edit. 2013, 52, 2920. [13] M. Calik, et al., J. Am. Chem. Soc. 2014, 136, 17802. [14] D. D. Medina, et al., J. Am. Chem. Soc. 2015, 137, 1016. [15] C. Gu, et al., Angewandte Chemie International Edition 2014, 53, 4850. [16] D. Becker, et al., Chem. Commun. 2015, 51, 4283. [17] A. Palma-Cando and U. Scherf, Macromol. Chem. Phys. 2016, 217, 827. [18] A. Palma-Cando, et al., Macromolecules 2015, 48, 6816. [19] R. P. Bisbey, et al., J. Am. Chem. Soc. 2016, 138, 11433.

Resume : The large-scale production of organic solar cells requires the development of new electrode materials that are flexible, stable and R2R compatible. For the evaluation of the electrode’s quality the selectivity is a crucial parameter that can limit the overall solar cell performance, yet it is challenging to determine. In the case of a poorly selective contact, electrons and holes recombine non-radiatively at its surface leading to a surface recombination current (Jsr) which requires gradients of the quasi-Fermi energy (q-EF) as driving force. We will show that depending on the charge transport properties of the photoactive material it can be distinguished whether the loss in Voc is provoked mainly by the gradient of the q-EF to drive Jsr or is caused by a reduction in the charge carrier concentrations.[1] For this analysis we present advanced luminescence and charge extraction results and compare them to drift-diffusion simulations with very good agreement. We find that for the investigated systems, the loss in Voc can be mainly attributed to the gradient in q-EF and that the reduction in charge carrier density is negligible. Further, we will point out the dependence between transport properties and the reduction in Voc for balanced and imbalanced charge carrier transport. From these insights we explain why it is more challenging for many organic photoactive materials to realize a selective hole rather than electron contact. [1] Spies et al., Adv. Energy Mater. 2016, 1601750.

Resume : Typical architecture of perovskite solar cells is generally realized using mesoporous TiO2 as electron transport material and fluorine tin oxide (FTO) as photoanode on glass substrates. However TiO2 has low charge transport mobility and requires high sintering temperature and FTO is not easy to etch. In the perspective to overcome these disadvantages, ZnO nanorods (ZnO NRs) are a good alternative to TiO2 due to lower temperature of growth, comparable energy levels to TiO2 and higher electron mobility. In the present work ZnO NRs are grown by wet chemical bath, starting from Al-doped ZnO (AZO) film, sputtered on glass. AZO layer is therefore used both as seed for nanorod growth and as photoanode of perovskite solar cells. ZnO nanorods on AZO film show a good alignment and high packing density as it is evident by X-ray analysis and SEM images. Sequential deposition of perovskite (CH3NH3PbI3) by spin-coating and dipping was made on the ZnO NRs/AZO/glass substrates. Solar cells were completed by Spiro-MeOTAD as hole transport material (HTM) and evaporated gold as back electrode. Solar cells were fabricated in air under conditions of relative humidity below 40% and then characterized by acquiring both external quantum efficiency spectra (EQE) and J-V curves. Aspect ratio of ZnO NRs and the HTM/CH3NH3PbI3 capping layer have been optimized to increase EQE spectra and device performance.

Resume : According to the intercalation research shows that in the low temperature and high pressure environment is the most favorable to guest molecules intercalate in host molecules. For example, hydrogen storage material is in the environment of low temperature and high pressure, hydrogen atoms intercalate/deintercalate in the surface of the metal to process the reversible exothermic/endothermic reaction. But in our study, we found unusual phenomena that using the crystals of hair-rod semiconductive poly[2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT) sandwiched in between crystalline platelets of hexamethylbenzene (HMB), stepwise increase of inter-lamellar spacing during heating process. Via the in-situ wavelength shift of ultraviolet absorption and in-situ X-ray experiment can identified each step of the expansion of layer-stacking framework. Accordingly, multi-step intercalation of HMB vapor molecules into PBTTT crystals and liquid crystals is main mechanism for observed thermal evolution. The intercalation of vapor HMB molecules will change the layer-stacking periodicity, and produce internal stress, but the inter-lamellar spacing is gradually increased according to stepwise evolution is due to the side chain interdigitation, this interaction known as anchoring effect. With the presence of internal pressure, are relative for the progressive expansion of layer-stacking framework. Therefore, this interaction between internal pressure of intercalated vapor molecules and synergistic anchoring effect of side-chain interdigitation provides a new mechanism of structural evolution of host crystals upon guest intercalation. This completely new mechanism might expand the perspective of the feasible design of host crystals and application in organic solar cell systems.

Resume : Series resistance (Rs) of organic bulk heterojunction solar cells (OSC) has a major impact on their performance. It is a loss mechanism and should be kept as low as possible. It can be connected with the active layer itself, the interfaces with the carrier selective layers, the interfaces with the metal contacts or the resistance of the transparent conducting layer. In order to properly characterize an OSC a reliable method for measuring Rs under operating conditions is necessary. A variety of such methods are known from the literature. Most of them are based on the J-V characteristics of the cell and reveal or estimate the overall series resistance. In our study we apply Impedance Spectroscopy (IS) measurements and use specific equivalent circuit models, taking into account not only the standard a.c. one-diode equivalent circuit of a solar cell, but also the possible inhomogeneities of the contributing layers. In this study we attempt to determine the different contributions to the series resistance by combining standard current-voltage measurements and impedance spectroscopy of different types of OSCs. Solar cells with a polymer based bulk heterojunction active layer are studied. The donor material is P3HT or PCDTBT and the acceptor either PC61BM or PC71BM. The solar cells are deposited on ITO covered glass on which a hole conducting layer, PEDOT:PSS or MoOx, is deposited before spin-coating the bulk heterojunction. The back contact is evaporated Al or sputtered Ag.

Resume : A library of linear and star-shaped donor-acceptor oligomers having triphenylamine (TPA)-based electron-donating unit linked through (oligo)thiophene π-conjugated spacers to 1, 2 or 3 terminal dicyanovinyl (DCV) or 3-ethylrhodanine electron-withdrawing groups was designed, synthesized and investigated [1-5]. Systematic variations of the number and chemical nature of the acceptor units, length of both the solubilizing alkyl chains and oligothiophene π-bridges allowed elucidating the structure-properties relationships in this series of organic semiconductors. We have shown that the analog of TPA with methoxy-substitutes increases solubility and crystallinity of the star-shaped molecules [5]. Oligomers with alkyl- or phenyl- substituted DCV groups demonstrate significantly better solubility, electrochemical and oxidation stability. Ultrafast photoinduced absorption spectroscopy allowed studying the charge separation dynamics [2]. As a result, organic solar cells achieving PCE of 4.0 – 5.4% with high (0.9 - 1.0 V) open circuit voltage, which are stable without any encapsulation, have been developed. This work was supported by Russian Science Foundation (grant 14-13-01380). [1] S.A. Ponomarenko et al., Faraday Discuss., 2014, 174, 313 [2] O.V. Kozlov et al., Adv. Energy Mater., 2015, 5, 1401657 [3] Y.N. Luponosov at al., Organic Electronics, 2016, 32, 157 [4] V.A. Trukhanov at al., Synthetic Metals, 2016, 215, 229 [5] Y.N. Luponosov at al., J. Mater. Chem. C, 2016, 4, 7061

Resume : Fluorinated polymers have contributed a lot to the recent increase in power conversion efficiency (PCE) of organic solar cells. Remarkable out-of-plane charge carrier mobilities, which allow high fill factors to be maintained in thick films, are partly responsible for these good performances. While the simultaneous existence of lying and standing crystalline lamellae has been shown to support 3D charge transport, the mechanism that pushes fluorinated polymers to adopt such a favorable conformation is still under debate [a]. Both, the presence of Fluorine atoms and the extended alkyl side-chains, that need to be introduced to maintain sufficient polymer solubility, may contribute to this effect. In this report, we present detailed investigations of morphology, charge transport and photovoltaic performances of D/A blends based on polymers composed of thiophene, thieno[3,2-b]thiophene and either 5,6-difluoro-2,1,3 benzothiadiazole or benzothiadiazole units, with identical side chains. The results are compared to a previous non-fluorinated derivative with shorter side chains [b]. While a significantly higher PCE of 9.8% was obtained on the fluorinated derivative only, X-ray diffraction demonstrate that both long side chain polymers adopt a mixed edge-on/face-on configuration, revealing side-chains to be a major driving force for the polymer orientation at the substrate interface. [a] Leclerc et al. Polymers 2016, 8, 11. [b] Biniek et al. Macromol. Rapid Commun. 2010, 31, 651.

Resume : Research into organic photovoltaics has led to a steady increase in performance with the latest efficiencies reaching 12%. This has been possible through the use of novel materials, primarily, and device engineering in coordination with electrical and structural examination of the thin films comprising the devices. With regards to structural examination, it can be carried out ex situ and in situ. While ex situ studies provide structural information after film formation, it, however, cannot provide insight into the dynamics of film formation during processing. Thus, to gain information about the structural changes during film growth in situ techniques are required. Here, we employed in situ spectroscopic ellipsometry. We use a high-efficiency solar cell (8%) as our model device and study the growth of its first layer – α-sexithiophene – thermally deposited under vacuum on a PEDOT:PSS/ITO/Glass substrate. The growth is characterized by studying the changes in the complex refractive index of the film in a wide spectral range (200 nm – 1700 nm). Data analyses reveal anisotropic refractive index during film growth; correlation studies with in situ X-ray data will be carried out to understand the molecular orientation. This is the first step towards developing a methodology for examining the growth of multilayers forming the active layers of high-efficiency vacuum deposited organic solar cells thereby helping to consolidate the understanding of structure-function relationships.

Resume : A large development in the field of organic semiconductors took place in recent years. Concerning applicability in organic electronics, new generations of solution processable materials were developed. These materials, such as polymers, were found to have desirable photophysical (optical and electrical) properties. Additionally, the photophysical properties can be easily modified by doping. Due to this, a broad band optical characterization of these semiconducting polymers in their pristine and doped form is important. Usually spectroscopy is performed either in the UV-Vis or in the IR regions based on transmission / reflection (T, R) measurements, which are both rather insensitive for the accurate determination of optical properties of thin layers, because the measured real quantities (T, R) are still dominated by the reflection at the substrate boundaries. Also for the investigation of the changes in spectra, e.g. when comparing spectra of doped samples to undoped ones, the strong transmission / reflection of the substrate can be detrimental. In order to assign physical (optical) properties due to doping, the variation of the complex dielectric function or equivalently, the variation of the refractive index, as a function of doping has to be measured using ellipsometry. According to this we present an application of NIR-UV-Vis ellipsometry in an optical study of various conducting polymers. The ellipsometric values ψ and Δ were measured as a function of the applied potential. The obtained data were modeled and the real and imaginary part of the dielectric function, or equivalently, the real and imaginary part of the refractive index were determined. The optical dispersion functions in the VIS-UV were analyzed in terms of the electronic transitions. Additionally upon chemical doping in the iodine vapors, the appearance of new infrared activated vibrations (IRAVs) and polaron induced broadband absorptions in the complex dielectric function / complex refractive indexwere measured, giving an insight the physical properties of the created radicals. A special setup was used in the UV-Vis spectral range allowing transmission ellipsometric studies during electrochemical measurements. In the system a semiconducting polymer, poly(3-hexylthiophene) (P3HT), which is the model donor material for organic solar cells, deposited on ITO/glass was used as working electrode. 0.1M tetra(n-butyl)ammonium hexafluorophosphate (TBAPF6) in propylene carbonate was used as electrolyte solution and Pt was used as counter electrode. Upon electrochemical doping, the appearance of new peaks at lower energy due to transitions in the gap is clearly resolved. In parallel, a clear variation of the P3HT thickness was found as a function of applied potential during electrochemical measurement. Moreover, ellipsometric results obtained from electrochemical studies during chemical doping in iodine vapor show fully consistent results.

Resume : Organic photovoltaics, in spite of its unique advantages, still have some drawbacks including the high costs, related to the active material preparation and high toxicity of tin-based compounds usually involved in the synthesis. In this context, we have recently synthesized a promising polymer for BHJ solar cells, (poly[(benzo[1,2-b:4,5-b’]dithiophene)-alt-(4H-thieno[3,4-c]pyrrole-4,6(5 H)-dione) PBDTTPD, via Direct (Hetero)Arylation Polymerization (DHAP) in the presence of the cheap Pd(PPh3)4 catalyst [1]. To investigate the potentialities of PBDTTPD obtained via DHAP for solar cells, this polymer blended with PCBM71 has been tested in BHJ. For the sake of comparison, devices have also been made with a PBDTTPD synthesized via Stille polycondensation and with a third commercially available PBDTTPD. The devices made with the polymer prepared by DHAP shows a power conversion efficiency up to 5.3%. This is higher than the Stille polymer reference and comparable with the commercially available PBDTTPD, under the same processing conditions [1]. Through the use of optical and morphological characterizations and charge recombination analysis, an insight is provided about the relationship between the molecular characteristics of these polymers and their functionality as active components in BHJ solar cells. [1] G. Marzano, F. Carulli, F. Babudri, A. Pellegrino, R. Po, S. Luzzati and G.M. Farinola, J. Mater. Chem. A, 2016, 4, 17163

Resume : Deoxyribonucleic acid (DNA) was successfully incorporated as a nano-layer between the bottom indium tin oxide (ITO) transparent electrode and the polymer blend film in inverted polymer solar cells. Via Kelvin probe, atomic force (AFM) and scanning tunneling microscopy (STM), we show that a 1-6 nm thick DNA nanolayer functions as an effective electron extracting layer, leading to lowering of the work function of ITO electrode (from 4.7eV to 4.4eV) via the formation of an interfacial dipole. Consequently, a strong improvement in rectifying behaviour (by two orders of magnitude with rectifying ratios reached larger than 103), and in photovoltaic parameters like open circuit voltage (VOC from 0.39V to 0.73V), and power conversion efficiencies (PCE from  2% to  5%) was obtained[1]. Importantly, we further improved the performance considerably, by developing metal oxide/DNA composites as electron extraction layers leading to solar cells that are better than those with only metal oxide (e.g. ZnO) or DNA interlayers, reaching state of the art efficiencies (PCE of 8.3%). DNA nanolayers also improve the surface roughness of the metal oxide layer, leading to decreased series resistance and increased shunt resistance. We have also succeeded in transferring this device structure on plastic substrates (PET/ITO), with similar results as well as in perovskite solar cells. Thus we demonstrate that DNA is a very efficient nano biomaterial for electronic purposes, opening the way for future investigations where its electron-extracting behaviour is coupled with its functional and self-assembly properties. References [1] J. Dagar, M. Scarselli, M. De Crescenzi, T.M. Brown, “Solar Cells Incorporating Water/Alcohol-Soluble Electron-Extracting DNA Nanolayers”, ACS Energy Lett.1, 510–515 (2016).

Resume : In this work, we performed a study on the recovery of the photovoltaic performance of an ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al solar cell after the hole transport layer (PEDOT:PSS) has been degraded by contact with the environment. A device that was fully built in an inert environment had a fill factor of 0.64, while the device whose HTL layer was exposed to the air presented a FF equal to 0.2, and in addition its characteristic curve JV exhibited the usual degenerated S shape. On the other hand, the elimination of this deleterious effect was achieved after the generation of gold nanoparticles at the PEDOT:PSS/P3HT:PCBM interface. The gold nanoparticles were formed by means of a low energy gold ion beam, using Metal Plasma Immersion Ion Implantation & Deposition (MEPIIID) technique. This S-kink effect was also simulated by using an equivalent circuit model constituted by two diode circuit, one of which plays the role of the undesirable potential barrier formed at PEDOT:PSS/P3HT:PCBM interface. Our analysis shows that deposition of gold nanoparticles next to the interface recovers the good hole injection condition into the active layer of the solar cell, restoring the fill factor and thus its efficiency.

Resume : Organic solar cells face stability issues over time. Modern devices lifetime are expected to be years which enforces to perform accelerated tests to predict if a device will fail. We report an accelerated lifetime test on OPV cells according to ISOS-D-2 protocol. Led lighting was used to characterize the devices before and after the thermal storage. The results were analysed according to the Arrhenius model described in literature. The analyzed devices showed a satisfatory T80 suggesting a long lifetime.

Resume : Electric power storage in high-energy chemicals, called ?energy carriers?, has received much attention for the efficient storage and on-demand supply of renewable electricity. Here, we demonstrate direct power charge using an alcohol/carboxylic acid redox couple [1, 2]. Highly transportable glycolic acid, an alcoholic compound, was successfully produced by electroreduction of oxalic acid, a dicarboxylic acid, on ubiquitous TiO2 catalysts with high efficiency and selectivity (70-95% Faraday efficiency and >98% selectivity) under mild conditions in the potential region of -0.5 to -0.7 V vs. the RHE at 50 ºC[3, 4, 5]. The most desirable characteristic of this electroreduction is the suppression of hydrogen evolution even in acidic aqueous media (Faraday efficiency of 70?95%, pH 2.1). The detailed observation of TiO2 catalysts using scanning TEM and eels techniques provided a mechanistic insight into this highly selective catalysis. Recently, we succeeded in electric power generation via the selective electrooxidation of glycolic acid to oxalic acid without CO2 emission?specifically, carbon-neutral power generation. References [1] T. Matsumoto, M. Sadakiyo, M. L. Ooi, S. Kitano, T. Yamamoto, S. Matsumura, K. Kato, T. Takeguchi, M. Yamauchi, Sci. Rep., 4, 5620 (2014). [2] T. Matsumoto, M. Sadakiyo, M. L. Ooi, T. Yamamoto, S. Matsumura, K. Kato, T. Takeguchi, N. Ozawa, M. Kubo, M. Yamauchi,. Phys. Chem. Chem. Phys., 17, 11359-11366 (2015). [3] R. Watanabe, M. Yamauchi, M. Sadakiyo, R. Abe, T. Takeguchi, Energy Environ. Sci., 8 1456-1462, (2015). [4] M. Yamauchi, N. Ozawa, M. Kubo, Chem. Rec., 2249 (2016). [5] S. Kitano, M. Yamauchi, S. Hata, R. Watanabe, M. Sadakiyo, Green Chem., 18, 3700 (2016).

Resume : This study investigates the effect of localized surface plasmon resonance of metallic nanoparticles (NPs) incorporating an organic solar cell based on a blend of Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as donor and indene-C60 bisadduct (ICBA) as acceptor. Plasmonic Au NPs is a promising way to optimize plastic photovoltaic performance by enhancing light-trapping. This work aims to evaluate the impact of Au NPs size on the performance of polymer solar cell based on two different structures, the first one incorporating Au NPs in PEDOT:PSS used as hole transporter layer HTL: Glass/ITO/PEDOT:PSS:AuNPs/MDMO-PPV:ICBA/Al, and the second structure using thin layer of Au NPs deposited by spray pyrolysis after the HTL: Glass/ITO/PEDOT:PSS/AuNPs/MDMO-PPV:ICBA/Al. We synthesized different sizes (10nm, 20nm and 40nm) of spherical gold nanoparticles following Turkevich method, by reduction by citrate. Optical and structural properties of Au NPs and of the other layers are performed via UV-visible spectroscopy, Scanning Electron Microscopy SEM and Atomic Force Microscopy AFM. The current-voltage characteristics and electrical parameters are measured by the solar simulator in dark and under AM1.5 illumination.

Resume : Oligothiophenes are stable compounds characterized by a large degree of pi-delocalization along their flexible backbone, whose optical properties can be finely tuned by changing the length of the conjugated chain. These systems have been widely investigated as promising donor partners in organic solar cells (OSCs) [1,2]. As an interesting example, we have investigated here the 1,3-di(2- thienyl)benzo[c]thiophene (DTBT), a small compound easily synthesized and quite stable at air and light. Our study was in particular aimed at elucidating the photophysical and dynamical properties of DTBT, key features of its possible applications as a donor compound in OSCs. To this purpose, we have performed time-resolved experiments on DTBT, for which no ultrafast experiments have been reported so far, to gain insight into photoexcitation processes and the subsequent conformational dynamics and de-excitation pathways. More specifically, femtosecond pump-probe experiments have been able to trace back the molecular isomerism induced by the evolution of the excited state during its intra-band relaxation. This fast process has been monitored by transient stimulated emission experiments from which a survey of the vibrational relaxation (hundreds of fs) associated to the excited state geometry and of the dynamics of the slower torsional rearrangement (few ps) of the molecular conformation [3] has been provided. The most important experimental finding is the occurrence of a dynamic red-shift within the first few ps following the excitation. A close comparison between such results with those obtained by using theoretical TD-DFT calculations confirmed that the red-shift is due to the fast isomerization of the molecule in its excited state. References: [1] F. Zhang, D. Wu, Y. Xua and X. Feng J. Mater. Chem., 2011, 21, 17590. [2] Y.-J. Cheng, S.-H. Yang and C.-S. Hsu Chem. Rev., 2009, 109 (11), 5868. [3] T. Benincori, G. Bongiovanni, C. Botta, G. Cerullo, G. Lanzani, A. Mura, L. Rossi, F. Sannicolò, and R. Tubino, Phys. Rev. B, 1998, 58, 9082.

Resume : The influence of ZnO:Al properties in the performance of inverted organic solar cell is investigated. AZO with different Al concentrations is prepared by non-vacuum method using sol-gel; this layer is used as electron transport layer in the inverted structure based organic solar cell. The effects of Al concentration on structural, optical and electrical properties of AZO and of the solar cell are carried out through X-ray diffraction, atomic force microscopy (AFM), scanning electron microscopy (SEM), UV-visible spectroscopy and Hall effect. We prepared inverted solar cell using an active layer of blended poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester PCBM1:1 ratio, used as donor and acceptor, respectively with different AZO films: ITO/AZO/P3HT:PCBM/PEDOT:PSS/Ag. The J-V characteristics of the inverted solar cells are measured in dark and under simulated sunlight illumination AM1.5 with 100mWcm−2.

Resume : The donor polymer characteristics like molecular weight, regioregularity and polydispersity index play a vital role in the device performance of the bulk heterojunction solar cells. P3HT is one of the most commonly used material in organic electronics because of its simple synthesis and semicrystalline lamellar microstructure. Although many studies have investigate the effect of regioregularity in the device performance, its effect on electronic states of the polymer blend is not investigated in detail. Here we report the performance of P3HT:PCBM solar cells that are based on 100% regioregular P3HT and compare it to the performance of cells made from commercially available rr-P3HT. In addition to photovoltaic performance, optical properties, charge carrier dynamics, and thin-film microstructure of P3HT:PCBM blends based on these materials are also compared and correlated with the device performance. Solar cells that are fabricated using more ordered defect free 100% regioregular poly(3-hexylthiophene) (DF-P3HT) as the donor polymer show ca. 10% increase in the average power conversion efficiency (PCE) when compared to that of the solar cell fabricated using 92% regioregularity P3HT. EQE and UV−vis absorption spectrum show a clear increase in the 607 nm vibronic shoulder of the DF-P3HT blend suggesting better interchain ordering which was also reflected in the less Urbach energy (Eu) value for this system. The increase in ordering inside the blend has enhanced the hole-mobility which is calculated from the single carrier device J−V characteristics. Electroluminance (EL) studies on the DF-P3HT system showed a red-shifted peak when compared to rr-P3HT-based devices suggesting low CT energy states in DF-P3HT. The morphologies of the blend films are studied using AFM and grazing-incidence wide-angle X-ray scattering (GIWAXS) suggesting increase in the roughness and phase segregation which could enhance the internal scattering of the light inside the device and improvement in the crystallinity along alkyl and π-stacking direction. Hence, higher PCE, lower Eu, red-shifted EL emission, high hole-mobility, and better crystallinity suggest improved interchain ordering has facilitated a more delocalized HOMO state in high regioregular P3HT-based BHJ solar cells. Keywords: Interchain order, defect free P3HT, charge carrier dynamics, morphology, organic solar cells.

Resume : In order to limit the degradation of flexible organic photovoltaic devices by moisture and oxygen, the solar cells are encapsulated between gas-barrier films. Despite the importance of the encapsulation process and its potential consequences on the devices performance, it is still scarcely referenced [1]. Furthermore, in several real-life ageing studies, mechanical failure is suspected to limit the devices’ lifetime even before their photo-chemical stability becomes an issue [2]. Adhesion between the layers composing the devices is thus a critical factor for obtaining reliable flexible devices along processing and use. In this work, two encapsulation processes were studied and the properties of the devices were monitored by imaging techniques before/after encapsulation and during ageing in damp heat conditions. Mechanical cohesion was then improved, and these enhanced devices were characterized in the same manner. [1] Gevorgyan et al., Adv. Energy. Mater., 2016, 6, 1501208 [2] Krebs et al., Energy & Environmental Science, 2010, 3, 512

Resume : The main problem of the development of photovoltaic elements is the spatial separation of electric charges and the obtaining of their unbalanced distribution caused by the photons absorption by the system. The photovoltaic elements that are used in solar energetic "works" on the effect of the violation of balanced distribution of electrons and holes (due to the absorption of photon) in the region of p-n-transition in semiconductors. The optical, dark electrical and photoelectrical properties of the novel organic dye (solutions in different solvents and films that consist of the dye layer deposited on both quartz substrate and gilded quartz substrate) were investigated. The investigation results prove the appearance of photovoltaic effect in single molecules are presented. To our opinion, this effect can be used on macroscopic level for the organic solar cells design and fabrication as well as in the nanoelectronics for molecular photovoltaic energy sources creation for nanoelectronics devices.

Resume : Conventional ellipsometry is well established in the field of thin film metrology due to the exceptionally high resolution in the z-axis, enabling very accurate thickness measurements for nano- and microfilms. However, conventional ellipsometry does not have a sufficient lateral resolution for a number of the described developments nor a direct microscopic visualization with highest contrast. Spectroscopic imaging ellipsometry is a technology that combines optical microscopy and spectroscopic ellipsometry to a unique tool that enable the detection of thickness variations with a microscopic lateral resolution. The detector of the instrument is a scientific camera so that pixels can be combined to a region of interest and be averaged or the complet set of individual pixels are used like in an image. Ellipsometric enhanced contrast mirco graphs, microscopic maps of Delta and Psi as well as microscopic maps of the elements of the Müller Matrix are examples for the use of the complete array. A benefit of an array based method is the higher statistical relevance of the data and the direct view on the distribution of the parameters of interest with a microscopic lateral resolution. Examples of microscopic imaging and mapping at different wavelength will be reported for different types of organic photovoltaic materials.

Resume : The purpose of this work is to study the influence of ZnO nanoparticles concentration incorporating in the active layer RR-poly(3-hexylthiophene) (RR-P3HT) based bulk heterojunction organic solar cell. ZnO nanoparticles are synthesized by chemical way, their average grain size is calculated first, using Debye-Sherrer formula, and then confirmed by scaning electron microscopy (SEM), it is found to be around 40nm. Structural and optical properties of the prepared films with and without ZnO NPs are performed by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV-visible, photoluminescence spectroscopy (PL) and thermo-gravimetric analysis (TGA). Enhancing ZnO NPs concentration lead to increase the interface ZnO-polymer that improve optical and electrical properties of the prepared films compared to pure films without NPs.

Resume : We investigate the photovoltaic response of new biologic/polymeric thin films structures deposited by spin-coating onto optical glass substrates covered with an indium tin oxide (ITO) thin film, used as transparent and conductive oxide (TCO) for holes’ collection. The active layer, as a mixture between regio-regular polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) and fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in 1:1 weight ratio, was deposited by spin-coating. To improve the holes’ collection to anode a buffer layer as a mixture between poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and chlorophyll-a (Chl-a) was deposited by spin-coating too, using different weight ratios (1:1, 1:2, 1:3, 1:4) for the two components. Chl-a was extracted from fresh spinach leaves and mixed with PEDOT:PSS from a stock solution. Collection of the electrons to cathode was improved by deposition of lithium fluoride electron transport layer LiF), by thermal evaporation (TVE). To complete the photovoltaic cell structures aluminum (Al) cathode was deposited by TVE, too. Electrical and photoelectrical measurements were performed in dark and AM 1.5 conditions, at room temperature. Parameters characterizing photovoltaic cells such as external quantum efficiency (EQE), open circuit voltage (VOC), short-circuit current (ISC) and fill factor (FF), were determined and compared with results obtained for glass/ITO/PEDOT:PSS/P3HT:PCBM(1:1)/LiF/Al conventional structures. Keywords: P3HT, PCBM, chl-a, photovoltaic structures Acknowledgements: This study was partially financially supported by 8SEE/2014 project.

Resume : The possibilities of efficiency increasing of inorganic photovoltaic devices, using organic composites were analyzed. Two approaches were made to the design of solar luminescent transformer using the organic semiconductors to expand spectral sensitivity range of inorganic solar cells:1. Aromatic containing polymer composites were used that are characterized by high rates of absorption in the short-wave range of solar radiation ranging from approximately 250 to 400 nm. This spectral region corresponds to the shortwave rapid decrease in sensitive characteristics semiconductors solar cells (eg, silicon). In these composites Frenkel excitons, which are generated by the absorbed light, transport excitation energy to specially designed molecular traps emitting light in the maximum of spectral sensitivity of the solar cell. 2. In longwave spectral range of solar emission it is also offered to use dyes based organic composites with absorption centers in the near infrared range that sufficiently compensate long wavelength sensitive "obstruction" of inorganic photovoltaic converters (for silicon absorption of composite is essential for wavelength λ> 1000 - 1100 nm). We consider the process scheme that includes the next steps: a photon absorption, singlet-triplet conversion, the emergence of Frenkel triplet excitons, capture and annihilation on special traps, delayed fluorescence - which results on these centers. Spectrum of delayed fluorescence (result of triplet excitons annihilation) of these traps must overlap with the curve of spectral sensitivity of solar cells (eg, silicon maximum range of delayed fluorescence should be λmax <1000 nm. A necessary condition for the effectiveness of the luminescent transformer is high efficiency of singlet-triplet conversion of absorbing components.

Resume : The understandings of interfacial phenomena are of paramount importance in organic electronics. During their operating time, organic solar cells (OSCs) are submitted to many stresses such as light, oxygen, temperature and humidity which can influence the interface properties and ultimately the device performances. Depending of the stresses and materials concerned, different mechanisms of degradation can occur at the interfaces: photochemical reactions, diffusion, delamination, energy level changes etc… Herein, we have investigated the role of various electron transporting layers (ETLs) on the degradation mechanism of OSCs, for different donor/acceptor bulk hetero-junctions. In particular, the influence of the UV component of the incoming light on the initial degradation step of OSCs (so called burn-in) is assessed. Using a combination of characterization tools (impedance and X-ray photoemission spectroscopy) we managed to correlate the changes in the electric characteristic of the devices to modifications at the AL/ETL interface. A specific degradation mechanism was identified for TiOx ETL, which result in a loss of the built-in potential in the devices. The obtained results enable us to provide strategies to improve the stability of AL/ETL interfaces in OSCs. In this context, we investigated the influence of self assembled monolayers (SAMs) to reduce the degradation at the AL/ETL interface.

Resume : We have investigated the effect of thermal annealing on the photovoltaic parameters of all-polymer solar cells based on a quinoxaline-thiophene polymer (TQ1) and an acceptor material (N2200). Annealed devices show doubling power conversion efficiency compared to non-annealed devices, due to higher short-circuit current (Jsc) and fill factor (FF), but with a lower open circuit voltage (Voc). On the basis of the morphology—mobility examination by several scanning force microscopy techniques, and by grazing-incidence wide-angle x-ray scattering, we conclude that the better charge transport is achieved by higher order and better interconnected networks of the bulk heterojunction in the annealed active layers. The annealing improves charge transport and extends the conjugation length of the polymers, which do help charge generation and meanwhile reduce recombination. Photoluminescence, electroluminescence, and light intensity dependence measurements reveal how this morphological change affects charge generation and recombination. As a result, the Jsc and FF are significantly improved. However, the smaller band gap and the higher HOMO level of TQ1 upon annealing causes a lower Voc. The blend of an amorphous polymer TQ1, and a semi-crystalline polymer N2200, can thus be modified by thermal annealing to double the power conversion efficiency.

Resume : There are many types of solar cells, 80% of which are produced from crystalline silicon wafers. Some cells are categorized as “emerging photovoltaic” like the organic photovoltaics (OPVs). Organic solar cells have gained much attention as they offer a potentially economic and viable way of commercially manufacturing lightweight, flexible and low-cost photovoltaics. While organic solar cells based on conjugated polymer are more widely studied, a competitive alternative had emerged by using small-molecule. Potential advantage associated with molecular systems are that they are monodisperse in nature and, due to having a higher solubility than polymeric analogs, can be purified and characterized using standard organic chemistry protocols. Organosilicas which are prepared upon sol-gel hydrolysis condensation of organo-alkoxysilanes can be simply described as three-dimensional silica network with bridging or cross-linking organic units. The material properties result from the design of molecular precursor with appropriate structure and properties. The tuning of the properties also required a control of the interaction between the organic substructures within the solid material and therefore a control of the arrangement and organization of the organic units in the hybrid network. We recently use the sol-gel process to generate nanostructured materials containing organic units with extended π-conjugation. The optical and electro-optical properties of the self-organised organosilica material have been tuned by controlling the stacking of the -conjugated units into J or H aggregates [1]. In this case, the silylated π-conjugated system acting as the electron donor and acceptor, the resulting devices exhibit very low efficiencies. In this context, we realised the synthesis of silylated donors and acceptors [2]. Our objective was not only to control the supramolecular aggregation properties of the π-conjugated systems but also to use the sol-gel process to stabilize the morphology of the active layer integrated in solar cells.

Resume : A novel method for extracting the built-in potential of organic solar cells from the dark current density-voltage (J-V) characteristics will be presented. Experimentally it is observed that the dark current varies exponentially with the applied voltage for low forward bias and follows a power law for high forward bias. Moreover, the current varies from exponential to linear at low temperature and from exponential to a square law (space charge limited current i.e. SCLC) at high temperature. The voltage corresponding to the transition of current from exponential to power law, which can be termed as transition voltage hereafter, enables to estimate the built-in potential from the dark J-V characteristics. The transition voltage increases with the decrease in temperature and saturates to the built-in potential at low temperature. We discuss the origin of built-in potential, the variation of transition voltage with temperature and the variation of current from exponential to linear or to SCLC in detail using relevant charge profiles that are obtained from numerical simulation. We also present a method for estimating the transition voltage. The built-in potential extracted for the experimentally fabricated poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) devices is in good agreement with the reported values in the literature and is validated for devices with different P3HT: PCBM thickness.

Resume : Despite their outstanding optical absorption and photo-stability, the use of boron dipyrromethene (BODIPY) as soluble small molecule for organic photovoltaic (OPV) applications remains scarce. Recently, we used a BODIPY derivative (TB2) as electron-donor in single bulk heterojunction (BHJ) solar cells. A promising power conversion efficiency (PCE=4.7%) was obtained but it remained limited by a modest fill factor (FF<50%) [a]. Adding planar π-stacking units as end-groups onto small molecules has been shown to increase intermolecular interconnectivity and enhance charge transport [b]. Applying this strategy to BODIPY derivatives may circumvent the FF bottleneck. In the present work, we accordingly decided to functionalize the TB2 dye with two planar trisindole-core units (triazatruxene: TAT) that were previously used as building block for OPV materials [c]. We developed new synthetic ways to design original BODIPY -based dumbbell-shape molecules, including a first 2,3,5,6-tetravinyl aromatic BODIPY molecule. The TAT-BODIPY-TAT molecule using the 2,6 positions of the BODIPY core as TAT-grafting positions and an ethynyl linker exhibit improved transport in blends with PC71BM. As a consequence a higher FF (65%) and a PCE of 5.8% could be reached, which is currently the highest reported value for solution processed BODIPY-based BHJ solar cells. [a] Bura et al. J. Am. Chem. Soc. 2012 134 17404. [b] Lee et al. Adv. Mater. 2011 23 5359. [c] Bulut et al. J. Mater. Chem. C 2016 4 4296

Resume : Dumbbell-shaped molecules containing large pi-stacking platforms at the molecular termini were found to be of high interest for solution-processable low bandgap donor molecules for efficient bulk heterojunction (BHJ) solar cell. Different types of pi-stacking platforms were already reported (eg. pyrene[1], perylenedimide[2], triazatruxene[3,4]…) leading to molecular self-assemblies, ultimately leading to good photovoltaic performances. Metal phthalocyanines are large and functionalizable aromatic platforms which constitute promising pi-staking units in the design of novel molecules for organic photovoltaic applications.[5] In this line, we have developed a series of dumbbell-shaped molecules, containing zinc phthalocyanine as the terminal pi-staking platforms, and a dithieno[2,1,3]benzothienothiophene derivative as the central connecting moiety. In this presentation will be described the synthesis and characterization of the molecules. It will be shown in particular, that the chains substituted to the phthalocyanine platforms is of high importance, as the chain density controls the molecular organization, and the nature of the linking groups (OR, SR, SO2R) can be used to control the absorption and energy levels of the molecules. [1] O. P. Lee et al, Adv. Mater. 23, 5359 (2011) [2] S. McAfee et al, Chem. Mater. DOI: 10.1021/acs.chemmater.6b04862 (2017) [3] I.Bulut et al, J. Mat. Chem C. 3, 6620 (2015) [4] I. Bulut et al, J. Mat. Chem C. 4, 4296 (2016) [5] D. Molina et al, Eur. J. Org. Chem. 4585 (2014)

Resume : We investigate the different microscopic pathways of photo-carrier generation in pristine organic materials. We discuss the various phenomena such as interaction of excitons with free or trapped polarons and the possibility of the existence of de-localized states in pentacene, which can lead to photo-generation. For this purpose, photocurrent measurements were performed on a bottom-gate bottom contact field effect transistor (FET) with pentacene as the active material. A long channel FET eliminates non-uniform field in the channel. The low source to drain electric field decouples carrier generation from transport. Moreover, the screening effect of gate field lines by accumulated polarons, as suggested by our simulation results, turns off exciton-dissociation due to large external electric-field. We perform phase sensitive photocurrent measurements to detect low efficiency photo-carrier generation processes. We investigate the effect of channel charge densities on the photocurrent spectral response and the photocurrent-voltage characteristics. We discuss how different occupancy levels of trap states in pentacene affect photo-generation rate and charge carrier life time. Finally, we show the dependence of photocurrent internal quantum efficiency on excitation energy in pristine pentacene and explore the presence of de-localized and charge transfer states.

Resume : The Organic Photovoltaic technology (OPV) offers a wide range of integration possibilities in contexts which are unthinkable with traditional solar cells. Printability, flexibility, lightness and optical semi-transparency are some of the most characteristic features for a technology, OPV, which promises to change the way we imagine solar energy harvesting. Project TranspEnergy, financed by the Regional Government of Catalonia, is focused on the development of organic photovoltaic modules with the possibility to predetermine their optical transparency and colour. The principle goal is to obtain modules that are perfectly integrable into existing products, limiting as much as possible the modification of their visual aspect. At the beginning, parameters like the optical constants of common materials for all the layers, their film thickness and the proportions between organic semiconductors composing the active layer play a central role in this study which is focused on data collection for chromatic prediction methodology. In a second phase, the reverse approach is assessed: to obtain a desired combination of semitransparency and/or colour by employing the adequate combination of the same parameters. Modules with pre-designed colour/transparency characteristics will be printed by roll-to-roll using slot-die technique and used to obtain a demonstrator able to power a GPS localization system. For this task, a specific electronic unit will be developed for performing efficient energy management.

Resume : This communication reports our researches on light-induced degradation mechanism of Poly[[2,3-bis(3-octyloxyphenyl)-5,8-Quinoxalinediyl]-2,5-thio- phenediyl](TQ1):[6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) based polymer solar cells. It is observed that the open-circuit voltage (Voc) of both conventional structure and inverted structure devices decline after exposed to the illumination. Fourier transform photocurrent spectroscopy (FTPS) and Electroluminescence (EL) measurements demonstrate that the decreased Voc stems from the increased non-radiative recombination energy loss. Fourier transform infrared spectroscopy(FTIR) and temperature depended mobility measurements indicate that the increased non-radiative recombination energy loss results from negative effect of the light on PC71BM domains. Finally we employ Extended Gaussian disorder model (EGDM) fitting to demonstrate that traps states in PC71BM domains increase after exposed to the light.

Resume : Terpolymer approach is the combination of two electron rich or electron poor building blocks with one electron poor or electron rich building blocks. This approach has been explored as a tool to control the optoelectronic properties of donor-acceptor type polymers. The three components in the main chain provide broadening of absorption spectra, which can be more beneficial to harvest photons in photovoltaics.1 Another opportunity is to alter the molecular packing by introducing components that favor π–π stacking.2 In literature, random copolymers based on terpolymer approach revealed the broadening of the absorption spectrum. Li and coworkers synthesized a random copolymer with a broad absorption and they reported a high PCE over 5 %. 2 In random copolymers, spectral broadening of the energy levels stems from the various sections with different chemical composition which may result in the formation of trap sites. Some studies showed that random terpolymers predominate corresponding alternating copolymers. There is no any limitation to synthesize either random copolymers or regular polymers to get high performance solar cells at the moment. 3 We have previously reported the synthesis of 2,1,3-benzooxadiazole, thiophene and benzodithiophene for photovoltaic application with a value of PCE of 4.48 %. The incorporation of selenophene into the polymer backbone results in low band gap polymers as compared to sulfur and oxygen containing counterparts. In literature, thiophene is mostly used as the electron donor in the conjugated polymers however its selenium counterpart lately draws attention as a novel class of donor unit in the conjugated materials. Selenium atom in the polymer backbone provides promising optoelectronic properties ranging from high conductivity to high hole mobility. Selenium has less electronegativity and more polarizability than sulfur atom therefore tuning the band gap, optical and electronic properties can easily be achieved by introducing selenophene unit into polymer backbone. 4 This inspired us to couple benzodithiophene, selenophene and benzooxadiazole units to obtain a polymer with an extended absorption and hence increased device efficiency. In this study, a benzooxadiazole, selenophene and benzodithiophene based conjugated polymer was synthesized. Electrochemical and spectroelectrochemical studies were performed. References 1. a) M. Zhang, F. Wu, Z. Cao, T. Shen, H. Chen, X. Li, S. Tan, Polym. Chem., 2014, 5, 4054; b) B. SambathKumar, E. Varathan, V. Subramanian, N. Somanathan, New J. Chem., 2016, 40, 1377. 18 P. P. Khlyabich, B. Burkhart, C. F. Ng, B. C. Thompson, Macromolecules, 2011, 44, 5079. c) P. P. Khlyabich, B. Burkhart, C. F. Ng, B. C. Thompson, Macromolecules, 2011, 44, 5079. 2. J. Li, K. H. Ong, S. L. Lim, G. M. Ng, H. S. Tan, Z. K. Chen, Chem. Commun., 2011, 47, 9480. 3. K. H. Hendriks, G. H. L. Heintges M. M. Wienk R. A. J. Janssen J. Mater. Chem. A, 2014, 2, 17899. 4. (a) A. Patra, M. Bendikov, Polyselenophenes, J. Mater. Chem. 20 (2010) 422; (b) A. Patra, Y.H. Wijsboom, S.S. Zade, M. Li, Y. Sheynin, G. Leitus, M. Bendikov, J. Am. Chem. Soc. 130 (2008) 6734; (c) P.B. Pati, S. Das, S.S. Zade, J. Polym. Sci., Part A: Polym. Chem. 50 (2012) 3996