Organic solar cells including polymers, dyes or perovskites

Resume : Emerging PV devices based on solution-processable materials offer opportunities for the production of low-cost solar cells. To obtain high efficiencies of exciton dissociation and high photocurrent, it is desirable to have an interpenetrating network of electron-donor and electron-acceptor components within the device, referred to as a bulk heterojunction (BHJ). However, current limitations of these devices are inefficient hopping charge transport through the discontinuous percolation pathways in the BHJ films, and therefore modest power conversion efficiencies or non-competitive cost. We have developed an alternative type of nanowire-based solar cells that are based on organic/inorganic hybrid device structures and demonstrated two distinct hybrid BHJ architectures with enhanced power conversion efficiencies. Furthermore, we have developed a simple method to grow high-quality ZnO nanowires on graphene via the hydrothermal method. Based on the graphene/ZnO nanowire structure, we have demonstrated graphene cathode-based hybrid solar cells by using two different solution-processed photoactive materials – PbS quantum dots (QDs) and poly(3-hexylthiophene) (P3HT) conjugated polymers – and ZnO nanowires as hole and electron transport layers, respectively. We have also identified several critical parameters to further boost the device efficiency and enable scalable, cost-efficient production, and these will be discussed. In another example, the impact of microscale film inhomogeneities on performance of organic-inorganic perovskite solar cells (PSCs) remains poorly understood, despite the recent astronomical success of this class of PV devices. We show that localized regions with increased luminescence in CH3NH3PbI3 perovskite films correspond to iodide-enriched regions. These observations constitute direct evidence that nanoscale stoichiometric variations produce corresponding inhomogeneities in film luminescence intensity. Moreover, we observe the emergence of high-energy transitions attributed to beam induced iodide segregation, which may mirror the effects of ion migration during PSC operation. Our results address the observed complex behavior in PSC devices.

Resume : Bulk-heterojunction organic solar cell (BHJ-OSC) utilizes a nanostructured electron donor and electron acceptorblended film to capture and convert solar photons into electrons. Recently, much attention has been focused on non-fullerene organic acceptors, which are used as acceptor materials to replace the traditional fullerene acceptor materials. The exciton-type photon-to-electron conversion efficiencies such as exciton generation and charge separation and transport are largely dependnet on the material absorption, frontier molecular orbitals and film-morphology. In this report, I will show our group’s results on the molecular strategies towards efficient small-molecule photovoltaic materials. The results include (1) the photovoltaic properties from the twisted perylene-dimide dimer acceptors, (2) the photovotaic properties from BODIPy based small molecule donor and acceptor, and (3) the photovoltaic properties from diketopyrrolopyrrole (DPP) and Quinoidal Methyl-Dioxocyano-Pyridine based small molecule donors, in particular, the effects of the end units capping on the DPP mainchain and the influences from the anchoring groups terminated on the flexible alkyl chains. References: 1. Chuanlang Zhan, and Jiannian Yao, Chem. Mater. 2016, 28 (7), 1948-1964. 2. Chuanlang Zhan, and Xinliang Zhang, and Jiannian Yao, RSC Adv. 2015, 5(113), 93002 - 93026 3. Ailing Tang, Chuanlang Zhan, Jiannian Yao, Chem. Mater. 2015, 27 (13), 4719-4730. 4. Ailing Tang, Chuanlang Zhan, and Jiannian Yao, Adv. Energy Mater. 2015, 5(13), 1500059. 5. Yuxia Chen, Xin Zhang, Chuanlang Zhan, and Jiannian Yao, ACS Appl. Mater. Interface, 2015, 7 (12), 6462 – 6471. 6. Xin Zhang, Chuanlang Zhan and Jiannian Yao, Chem. Mater. 2015, 27 (1), 166-173. 7. Wenxu Liu, Ailing Tang, Chuanlang Zhan, and Jiannian Yao, ACS Appl. Mater. Interface 2014, 6 (24), 22496 – 22505. 8. Zhenhuan Lu, Bo Jiang, Xin Zhang, Ailing Tang, Lili Chen, Chuanlang Zhan and Jiannian Yao, Chem. Mater. 2014, 26 (9), 2907-2914. 9. 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 use of selective charge extraction/blocking layers has allowed significant improvements of bulk heterojunction organic solar cells performance. We have been investigating the use of a phenantroline derivative, BPhen, as electron-extraction layer and found that it leads to improvements of power conversion efficiencies, for some polymers, in particular when LiF/Al low workfunction electrodes as used. In addition, we found that the deposition of BPhen by thermal evaporation over the polymer:PCBM films leads, in general, to nanostructured films, as shown by AFM analysis, with rather tall motives in comparison to the average thickness. In this communication we present results of efficiency and BPhen films morphology for various polymers and different surface PCBM contents. Acknowledgments: We thank FCT Portugal for financial support under the contracts M-ERA.NET/0001/2012, UID/EEA/50008/2013 and UID/NAN/50024/2013 and for a PhD grant to Joana Farinhas.

Resume : Polymer bulk heterojunction (BHJ) solar cells based on composites of semiconducting conjugated polymers and soluble fullerene derivatives have received much attention because of their potential application in low-cost, printable, portable, and flexible renewable energy sources even though low power conversion efficiency. Insufficient solar light absorption is usually considered to be a crucial hindrance for BHJ solar cells towards achieving higher efficiency. Several light trapping approaches, such as diffraction gratings, photonic crystals, and surface plasmon resonance (SPR), have been investigated and have been demonstrated to increase the light absorption of organic thin films. Among these, utilizing SPR effects with various metal nanoparticles (NPs) like Au or Ag NPs in a few tens of nm-meter size has been the most successful approach. In this work, we have incorporated glutathione (GSH) monolayer-protected Au nanoclusters (Au:SR) with sub-nm-sized Au38 cores on inverted BHJ solar cells, instead of tens of nm-sized metal NPs. Because of effective energy transfer, based on protoplasmonic fluorescence of Au:SR, the highest performing solar cells, ~20% increase in the efficiency, fabricated with the Au:SR cluster. [1] In addition, the solar cell with Au:SR nanocluster showed more stable performance under light soaking condition.

Resume : Organic semiconductors used in the photoactive layer of organic solar cells usually comprise rather low charge carrier mobilities. Under current flow this can lead to considerable accumulation of charge carriers causing enhanced recombination. An important consequence is that the voltage applied at the contacts does not equal the voltage inside the photoactive layer. For this reason the current-voltage characteristics of such a transport-limited solar cell differs significantly from the the well-known Shockley diode equation. A model will be presented which considers the effect of limited mobility explicitly and which allows to determine the correct value for the voltage required for the application of the Shockley equation [1]. In addition, the model can be used to evaluate efficiency potentials in a more realistic manner. Surface recombination is a loss mechanism in addition to the recombination (radiative and non-radiative) in the bulk of the photoactive layer [2]. To minimize it in organic solar cells charge carrier selective layers are used between the photoactive layer and the electrode. These layers (e.g. PEDOT:PSS, TiOx, ZnO, WoOx, MoOx) often have a rather large band gap and thus can block the “wrong” type of charge carrier quite efficiently. However, surface states within the band gap of these materials at the interface with the photoactive layer can still act as recombination centers. Recently, there are numerous examples of polar (organic) molecules providing a high degree of charge carrier selectivity, one prominent example being the work of He et al. on PFN [3]. Some of these materials are however rather expensive. We used simple, harmless, extremely cheap and easy-to-process organic molecules with permanent dipole moments. These molecules alter the effective work function of the electrode as confirmed by scanning Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy (UPS) which revealed a corresponding shift of the surface potential. This leads to a strong increase (decrease) of the electron (hole) concentration in the adjacent photoactive layer. It will be shown in detail that this is the main reason for the enhanced selectivity causing an increase of the open-circuit voltage (and fill factor) for different photoactive layers. A theoretical model was set up and the results of the numerical simulations are in full accordance with the experimental data. Interestingly, DFT-calculations prove that the energy levels of the dipole molecules used are not suited to conduct charge carriers from the photoactive layer to the electrode but that a tunneling mechanism is involved. Implementing this into our model, it is found that there is no necessity to assume preferential tunneling for electrons. Their accumulation and the depletion of holes due to the altered work function are sufficient to explain the observed behavior [4]. [1] U. Würfel, D. Neher, A. Spies, S. Albrecht: "Impact of charge transport on current-voltage characteristics and power conversion efficiency of organic solar cells", Nature Communications 2015, 6, 6951. [2] J. Reinhardt, M. Grein, C. Bühler, M. Schubert and U. Würfel: "Identifying the Impact of Surface Recombination at Electrodes in Organic Solar Cells by Means of Electroluminescence and Modeling", Advanced Energy Materials, 2014, 4, 1400081. [3] Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu, L. Chen, S. Su, Y. Cao, Advanced Materials 2011, 23, 4636 [4] U. Würfel et al. "How Molecules with Dipole Moments Enhance the Selectivity of Electrodes in Organic Solar Cells - a Combined Experimental and Theoretical Approach", under review.

Resume : High-performance organic solar cells have been developed by our group based on various nanotechnology in recent years. First, the efficiency of organic solar cells have been improved by introducing plasmonic nanoparticles, high mobility conjugated polymers or 2-D materials. Pronounced effects have been observed in the devices due to the improvement of the carrier mobility or light absorption in the active layer. Graphene has shown promising applications in photovoltaic devices for its high carrier mobility and conductivity, high transparency, excellent mechanical flexibility and ultrathin thickness and can be used in solar cells as transparent electrodes or interfacial layers. Package-free flexible organic solar cells are fabricated with multilayer graphene as top transparent electrodes, which show high power conversion efficiency, excellent flexibility and bending stability. Semi-transparent organic solar cells and perovskite solar cells were prepared by using graphene transparent electrodes. For the perovskite solar cells, the devices show high power conversion efficiencies (~12%) when they are illuminated from both sides. Considering the poor stability of perovskite solar cells in ambient air especially with high humidity, we have recently developed a novel technique to improve the device stability by introducing SCN- to partially replace I- in the perovskite material, which can dramatically improve the lifetime of package-free device in air. All of the techniques will be very useful for the practical applications of the novel photovoltaic devices.

Resume : Inorganic material such as MoS2, WS2, TaS2 (MeSx), WOx, and graphene oxide (GO) were successfully applied as hole extraction layer (HEL) for organic photovoltaic cells (OPVs). WOx thin film was obtained via thermal decomposition of (NH4)2WS4 under air ambient condition. Whereas MeSx was obtained through the ultra-sonication method and GO was fabricated by Hummer’s method. The WOx layer which was obtained by thermal decomposition at 160 oC shows smooth surface (~ 2 nm) as well as high work function (~ 5 eV), resulting OPVs with high power conversion efficiency of 3.14 %. On the contrary, the synchrotron radiation photoelectron spectroscopy data show that the work functions (WFs) of pristine MeSx and GO are relatively low (WFMoS2 = 4.6 eV, WFWS2 = 4.9 eV, WFTaS2 = 4.4 eV, WFMoS2 = 4.4 eV, WFGO = 4.8 eV), indicating that they are not suitable for the use as HELs in organic photovoltaic cells. The power conversion efficiency of OPVs with MoS2, WS2, TaS2 and GO as HELs are 1.08, 1.84, 0.01, 2.06 %, respectively. The WFs of MeS2 were then modified using UV-ozone (UVO) treatment while the WF of GO was modulated through the silane-functionalization. Upon UVO treatment for a certain time (15 – 30 min) for MeS2 or silane-functionalization for graphene (SF-Gr), the WF of MeSx as well as GO increased up to approximately 5 eV which are comparable to that of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The increment in WF of MeS2 and GO lead to increase of OPVs power conversion efficiency (PCE = 2.44, 2.4, 3.06, 3 % with UVO-MoS2, UVO-WS2, UVO-TaS2 and SF-Gr as HELs, accordingly). These results provide a versatile method for the fabrication HEL to replace highly hygroscopic PEDPT:PSS in terms of high efficiency and stability.

Resume : We will discuss the role of mesoscale order, electrostatic effects, defects, and roughness for charge splitting and detrapping at donor-acceptor interfaces. We will show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation. We predict open-circuit voltages of planar heterojunction solar cells in excellent agreement with experimental data, based only on crystal structures and interfacial orientation. We show how long-range molecular order and interfacial mixing generate homogeneous electrostatic forces that can drive charge separation and prevent minority carrier trapping across a donor-acceptor interphase. Comparing a variety of small-molecule donor-fullerene combinations, we illustrate how tuning of molecular orientation and interfacial mixing leads to a trade-off between photovoltaic gap and charge-splitting and detrapping forces, with consequences for the design of efficient photovoltaic devices. [1] C. Poelking, M. Tietze, C. Elschner, S. Olthof, D. Hertel, B. Baumeier, F. Wuerthner, K. Meerholz, K. Leo, D. Andrienko, Nature Materials, 14, 434-439, 2015 [2] C. Poelking, D. Andrienko, J. Am. Chem. Soc., 137, 6320-6326, 2015 [3] M. Schwarze, W. Tress, B. Beyer, F. Gao, R. Scholz, C. Poelking, K. Ortstein, A. A. Guenther, D. Kasemann, D. Andrienko, K. Leo, Science, 2016

Resume : Improvement of power conversion efficiencies (PCEs) is one of the largest issues in polymer-based solar cells (PSCs). A number of new semiconducting polymers with donor–acceptor (D–A) backbones have been developed in the past decade. In this presentation, I will introduce new D–A semiconducting polymers incorporating acceptor π-bulding units, naphthobisthiadiazole (NTz) and naphthobisoxadiazole (NOz). NTz gave polymers with narrow band gaps (1.5–1.6 eV) and relatively large ionization potentials (~5.2 eV). It is also mentioned that the NTz-based polymers showed quite high crystallinity along with the face-on backbone orientation suitable for solar cells. As a result, one of the NTz-based polymers, PNTz4T, demonstrated PCEs as high as 10% in single-junction solar cells. On the other hand, NOz gave polymers with larger ionization potentials of around 5.5 eV than the NTz analogues, while having the similar band gaps. Interestingly, an NOz-based polymer, PNOz4T, afforded high PCEs of up to 8.9% with very high open-circuit voltages of ~1 V in solar cells. Thus, the photon energy loss of the cells was found to be 0.52–0.55 eV, which was reduced by ca. 0.3 eV from that of the cells using the NTz-polymers and was very small compared with the typical value for PSCs (0.7–0.8 eV). These unconventional features in PNOz4T originate in the fact that the photoinduced electron transfer between the NOz-polymer and fulluerene occurs even though the offset energy of LUMOs is ~0.1 eV, which is significantly less than the widely referenced threshold value of 0.3 eV. These results indicate that NTz and NOz are indeed fascinating building units for the development of high-performance semiconducting polymers for PSCs.

Resume : Organic semiconductors have wide-ranging applications, from photovoltaics and solid-state lighting to biological imaging. But rational design of organic semiconductor materials and devices remains a challenge, as these materials are typically highly disordered, while structural and functional electronics are strongly coupled over many length scales in devices. This presentation discusses our recent work developing and applying accurate multi-scale computational models [1] that combine physical and electronic structure, in order to understand the role of physical structure and dynamics on fundamental processes in organic electronics, specifically energy transfer [2], singlet fission, and triplet-triplet annihilation. Model accuracy is verified by comparing predictions with time-resolved spectroscopic measurements. References: [1] K.N. Schwarz, T.W. Kee, D.M. Huang, Nanoscale 5, 2017-2027 (2013). [2] P.C. Tapping, S.N. Clafton, K.N. Schwarz, T.W. Kee, D.M. Huang, Phys. Chem. C 119, 7047-7059 (2015).

Resume : We demonstrate that, in the limit, of large exciton coherence lengths, spin-forbidden triplet excitons can form via non-traditional pathways, namely, charge transfer state intermediates. Electric field dependent, time-resolved single molecule spectroscopy approaches are used to interrogate the presence and roles of charge transfer states in self-assembled aggregate nanofibers of poly(3-hexylthiophene) (P3HT). Importantly, this process allows only the high molecular weight fraction to self-fold into aggregated structures which suppresses typical exciton localization on sub-ps time scales. The resulting delocalized intrachain singlet excitons have a greater likelihood of encountering interchain charge transfer states owing to their longer lifetimes. Once a charge transfer state is populated, separated, and uncorrelated, carriers, may recombine on longer time scales preferentially to triplet excitons. We further demonstrate that the density of charge transfer states depends on the size of the nanofiber (i.e., number of self-folded chains) suggesting interchain contacts as the structural origins of these states. By selectively controlling formation kinetics of hierarchical organic polymer assemblies, we have demonstrated the ability to direct photophysical outcomes in these intrinsically disordered and stochastic materials.

Resume : Solar photovoltaics (PV) are the fastest-growing energy technology today and a leading candidate for terawatt-scale, carbon-free electricity generation by 2050. Commercial PV module prices have declined rapidly, but balance-of-system (BOS) costs remain high. Further cost reductions are needed to compensate for the declining value of intermittent solar power with increasing grid penetration. Emerging thin-film PV technologies such as perovskites, organics, and colloidal quantum dots open the door to new deployment formats today and could ultimately reach materials, processing, and deployment cost floors inaccessible with conventional technologies. In this talk, I will discuss potential limitations to scaling up PV deployment, emphasizing constraints on the use of commodity and PV-critical materials. I will introduce material complexity as a framework for classifying commercial and emerging PV technologies, and identify strategies to realize the unique advantages of solar PV as a room-temperature, noiseless, modular, seamlessly integrated, and mechanically imperceptible power source.

Resume : Efficient charge extraction is a crucial point in many semiconductor devices. Here, we introduce a new device concept for flexible platforms; structured electrodes that can be used for improved charge extraction, enabled via charge funnelling. We further demonstrate how the concept can be used in multiple applications, such as solar cells and real-time solid state X-ray detectors. This study focuses on developing this novel concept using an organic solar cell as a representative application where the nanostructured back contact leads to an overall 30% efficiency gain. Insulating bismuth oxide (Bi2O3) nanoparticles are used to structure the cathode contact in a standard polymer solar cell, with the aim of creating a localized geometric electric field enhancement. As Bi has a high X-ray attenuation coefficient, this has a high significance on X-ray detectors as well. The presence of the insulating nanoparticles leads to the formation of a device which operates under short circuit conditions. This feature, when combined with the field enhancement, allows for the extraction of currents (which are at the short circuit condition) under maximum power point conditions. The efficient extraction of charges is determined by the size of the Bi2O3 nanoparticles, and the tuning of the dimensions is shown to be a requirement in order to achieve optimum performance. Compared to a non-structured metal back contact, this solution-based structuring of the metal back contact results in an electric field enhancement in charge extraction and leads to an efficiency improvement of 30%. The novel device concept proposed here forms a new route for efficient charge funnelling via the nanostructured contacts, which could be extended into many device platforms.

Resume : Tremendous progress has been made in the field of organic photovoltaics (OPV) in the last few years, and the power conversion efficiencies (PCEs) of organic solar cells (OSCs) were steadily improved to the 11% regime. To push the OPV technology towards commercial applications, the reliability and stability of champion OPV devices have to be examined. The performance of OSCs is determined by the delicate, optimized bulk-heterojunction (BHJ) microstructure, where the organic donor and acceptor are fine-mixed in the nano-meter regime to facilitate exciton dissociation at the donor / acceptor interface. In this contribution, we will examine the reliability and stability of BHJ microstructures for various state-of-the-art solution-processed OSCs and explore their potential for large-scale mass production. Strategies to design and develop BHJ microstructures with promising stability will be discussed. Moreover, novel organic acceptors based on fullerene derivatives and non-fullerene molecules are developed and analysed for state-of-the-art organic donors to simultaneously reduce their sub-transmission losses and bandgap to VOC losses. Recent results on novel organic acceptors-based OSCs will be in-depth discussed in this work.

Resume : Nowadays scientists from around the world are looking for solution of high energy consumption, which also will be friendly for environment and available for average earning people (low costs of production). Photovoltaics is a major area of research within the field of renewable energy sources. Organic Photovoltaic Cells (OPV) are an alternative for Si-cells, CIGS, CdSe-cells, and others including expensive and toxic materials, but their energy conversion is smaller. Kesterite Cu2ZnSnS4 can be functionalized with organic ligand and dedicated for selected semiconducting polymers, what allows created new hybrid solar cells, which works with better efficiency. The obtained materials have been characterized with X-ray methods, transmission electron microscopy and spectrophotometry UV/Vis/NIR.

Resume : Increasing molecular weight (Mw) of conjugated polymers as electron donor has been reported to improve solar cell performance in a number of polymer: PCBM bulk heterojunction systems, the reason of which has been attributed to the change in film morphology as a result of changing molecular weight. However, further study correlating such morphological change with photo-physics of operating devices, more specifically the bimolecular recombination kinetics of photo-generated charge carriers, is scarce. Here, bimolecular recombination in a low bandgap polymer DT-PDPP2T-TT: PCBM bulk heterojunction solar cell is found reduced (Langevin prefactor decreasing from 0.27 to 0.07) when the polymer Mw is increased from 33kDa to 80kDa. The bimolecular recombination in the high and low Mw DT-PDPP2T-TT: PCBM devices were compared using charge extraction using a nanosecond switch, combined with charge extraction by linearly increasing voltage and bulk generation time-of-flight techniques. The study revealed a high charge carrier mobility of 2.39×10-3 cm2V-1s-1 in high Mw device, which is over an order of magnitude higher than that in the low Mw device. The bimolecular recombination coefficient normalized to mobility is four times lower in the high Mw device compared to the low Mw device. The reduced bimolecular recombination, along with the fast charge carrier transport due to the high mobility, explains the high performance achieved in the high Mw device with active layer thickness of 250 nm. Such molecular weight dependence of reduced-bimolecular recombination could be explained by the presence of energetic barrier between the amorphous and crystalline phase of the polymer: PCBM blend, which could be tuned by increasing the molecular weight of the donor polymer. Grazing-incident wide-angle X-ray scattering (GIWAXS) and transient absorption spectroscopy will be performed to examine the hypothesis.

Resume : During the last decade, a new kind of donor polymers for photovoltaic application have been intensively studied, the low band-gap polymers. They are based on repeating units associating two different moieties, one electron-rich (ER) and one electron-poor (EP), which allow to finely tune the molecular orbitals and to induce a delocalization of the exciton generated upon the photo-excitation process. A large variety of devices are based on such low band-gap polymers, with a power conversion efficiency record around 10%, and, to increase the efficiency, it is necessary to have a better understanding of these polymers during the photo-absorption phenomenon. To do that, computational chemistry is a powerful tool that permits to predict different opto-electronic properties. For this work, we used Density Functional Theory and Time-Dependent Density Functional Theory to compute the optical properties of increasingly large oligomers involving various ER and EP subunits. The optical properties in the polymer limit were then estimated for the different systems by using an extrapolation scheme based on the Kuhn equation(1). This theoretical screening allowed us to select promising candidates for syntheses, which were performed by a Stille coupling. The obtained polymers and size-controlled oligomers were further characterized by UV-visible spectroscopy, fluorescence, size exclusion chromatography and NMR, in order to extract structure-properties relationships(2) and correlate experimental results to theoretical data. (1) Wykes, M.; Milián-Medina, B.; Gierschner, J. Frontiers in Chemistry 2013, 1, 35. (2) Oriou, J.; Ng, F.; Hadziioannou, G.; Brochon, C.; Cloutet, E. Journal of Polymer Science, Part A: Polymer Chemistry 2015, 53, 2059.

Resume : Materials for organic or Perovskite-based solar cells typically require multi-step expensive synthetic routes. Alternatively, we have been considering step-growth, or (poly)condensation, chemistries as they offer easy access to large quantities of photoactive material without the need for expensive catalysts and workup procedures. Here we present an overview of organic materials based on Schiff Base and imide chemistries studied in our group over the last 10 years. Schiff base chemistry is a cheap and environmentally benign route towards producing p-type materials for organic photovoltaic cells as well as hole conducting materials for perovskite cells. Polymers based on azomethine and triphenylamine monomers in combination with PCBM are shown to have efficiencies exceeding 0.2%. Using small molecule derivatives, azomethine-based materials may reach 2.2% efficiency in an organic solar cell. We also demonstrated that azomethine-based materials are suitable as hole-conducting material in Perovskite-based solar cells reaching similar performance as SpiroOMeTAD (~11%), but at a much lower price. In addition to azomethines, a series of small molecule diimides and their performance in organic photovoltaic cells will be discussed. Changing substituents on the diimide core also changes the solar cell from p-type to n-type. Efficiencies exceeding 0.2% were obtained for PCBM-based devices and 0.5% for P3HT-based devices. Our work shows that there is potential for small molecules azomethines and imides as materials for organic solar cells. The thermal and morphological properties are tunable and their synthesis is easy, cheap and amendable to kg quantity scale-up.

Resume : One of the biggest challenges for emerging, organic materials based, solar technologies is to utilise red and near infrared light in ways that don't have deleterious effects on the device open circuit voltage. The device reported here largely resembles a conventional Dye-Sensitised Solar Cell, however two different dyes are employed. One, a functionalised BPEA (BDCA), grafted to TiO2, absorbs high energy photons and injects charge, as per thousands of previous examples of DSCs. A Pt-porphyrin dye, also present, absorbs low energy photons. The energy from these photons is combined (as described below) to realise a novel pathway for current generation, in a two photon process. Photoexcited porphyrins undergo intersystem crossing (ISC) from S1 to T1, which is both fast and results in only a small energy loss. The T1 porphyrin can transfer its energy (Triplet Energy Transfer - TET) to the bound dye, allowing it to enter its T1 state. As two of these T1 state molecules come into proximity they can undergo Triplet-Triplet Annihilation (TTA), which can yield a high energy S1 molecule, capable of injecting an electron into TiO2.

Resume : Since 1999, Nickel(II)oxide (NiO) has been the electrode material of choice in p-type dye-sensitized solar cells (p-type DSSCs). However, drawbacks such as the inability to produce thick and mechanical stable films, low conductivity, unsuitable valence band energies hindering high open-circuit voltages (Voc) and parasitic light absorption by NiO call for a paradigm shift.[1] In particular, copper(I)delafossites (CuXO2) with X = Chromium (Cr), Gallium (Ga), or Aluminum (Al), emerged as a new class of materials due to their excellent transmission features, high conductivities, and lower valence band energies yielding a higher Voc. Major issues are the expensive synthesis methods, such as hydrothermal or high temperature solid-state synthesis, and the relatively large particle size produced by these methods, which limits dye loading.[2] As an alternative, we focused on the long overlooked copper(II)oxide (CuO), which shows comparable valence band energies, a higher conductivity and charge carrier mobility compared to NiO, and a rich selection of synthesis methods to prepare nanoparticles in all sizes and shapes.[3] Herein, we present p-type DSSCs consisting of CuO-based photocathodes in combination with a novel electron-accepting phthalocyanines sensitizer. By optimization of a number of key factors, such as the calcination temperature, electrode thickness, and iodine/iodide electrolyte ratio, efficiencies of 0.11% have been achieved.[4] The latter are one order of magnitude higher than state-of-the art CuO-based DSSCs of 0.011%.[5] The above-mentioned optimization is supported by electrochemical impedance spectroscopy assays monitoring the changes in injection and recombination processes. Overall, our current work demonstrates that CuO-based photocathodes can be implemented in p-type DSSCs and are competitive to the newly developed delafossite-based devices. References: [1] F. Odobel et al., J. Phys. Chem. Lett., 4, 2551 – 2564 (2013). [2] M. Yu et al., Phys. Chem. Chem. Phys., 16, 5026 – 5033 (2014). [3] Q. Zhang et al., Prog. Mater. Sci., 60, 208 – 337 (2014). [4] O. Langmar et al., Angew. Chem., Int. Ed., 54, 7688 – 7692 (2015). [5] S. Sumikura et al., J. Photochem. Photobiol., A, 194, 143 – 147 (2008).

Resume : To increase the efficiency of bulk heterojunctions for organic photovoltaic devices, the complicated photon-to-electron conversion process has to be understood in detail. To this aim, one challenge is to resolve the correlation between processing parameters of organic solar cells (OSC), the resulting nanoscale morphology of the absorber layer, and efficiency of the completed device. Here, we present the effect of substrate heating on the morphology of the OSC where the active layer is a blend of two small molecules; ZnPc (ZnC32H18N8) as donor and C60 as acceptor. Obtaining insights into the morphology of the active layer requires the spatial resolution and a contrast mechanism to discriminate two phases with the similar average atomic number. To tackle this challenge, we combine electron microscopy imaging with different analytical techniques; energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) in TEM, as well as Energy selective Backscattered (EsB) imaging in SEM. We imaged different phases of the donor and acceptor, forming ordered and non-ordered regions, depending on the way the heterojunction is fabricated. To this aim, we fabricated samples at substrate temperatures of 110°C and 150°C, each in two different configurations: 1) Focused ion beam prepared ultrathin lamella of a complete solar cell stack with glass substrate, Indium tine oxide (ITO) electrode, ZnPc:C60 blend of the active layer between electron and hole transport layers and aluminum top electrode. 2) Plane view sample as ZnPc:C60 blend deposited on a TEM grid coated with ITO layer. It was shown that at a substrate temperature of 110°C, the solar cell device has high efficiency [1]., so we consider this as the optimum substrate temperature. Since identification of the composition of each phase in the plane view sample is more straight forward, we use the plane view sample to attribute each structure to one component of the blend. SEM images recorded by using secondary electron detector show that the high temperature sample consists of rod-like features and cube-shaped material in between the rods. By combination of analytical microscopy techniques, we can attribute the rod-like structures to ZnPc. An energy-selective backscatter (EsB) electron detector in SEM is used to obtain backscattered contrast of the plane view sample. Due to the atomic number difference between the donor phase and the acceptor phase, (the average atomic number of 8.6 for ZnPc and 6 for the C60) sufficient contrast can be achieved [2]. A clear confirmation for the attribution of the ZnPc phase to the rod-like features and the attribution of C60 to the granular structure in between can be drawn from the investigation of the sample in SEM using EDX and with even higher precision in TEM using an improved EDX system. The chemical mapping of zinc and carbon, proves the correct phase assignment, in both plane view and ultrathin samples, prepared at high temperature. For the sample produced at optimum temperature, a much smaller roughness was observed because of the absence of large ordered regions. Even in the plane view sample, the imaging contrast is low due to less separated phases and smaller domains. To conclude, we clearly resolved the phase morphology of the interpenetrating network of ZnPc:C60 blends for high and optimum temperature samples in plane view and furthermore we were even able to reveal the morphology from TEM images of cross-section lamellas of the real solar cell stacks. Since the ideal active layer should have domain sizes at the range of the exciton diffusion length (10-20nm), the sample produced at high temperature does not show the desired microstructure. Due to the domain sizes of ~100nm there is no closed path for exciton dissociation. In addition to that, the sample shows a quite high roughness. Acknowledgement: Authors thank A. G. Cid for SEM images. This work was supported by the German Science Council Center of Advancing Electronics Dresden (cfaed). TEM-EDX results were achieved by funding (support code 03SF0451) through the Helmholtz Energy Materials Characterization Platform (HEMCP) initiated by the Helmholtz Association and the German Federal Ministry of Education and Research (BMBF). References: [1] S. Pfuetzner, C. Mickel, J. Jankowski, M. Hein, J. Meiss, C. Schuenemann, C. Elschner, A.A. Levin, B. Rellinghaus, K. Leo and M. Riede, “The influence of substrate heating on morphology and layer growth in C60:ZnPc bulk heterojunction solar cells” Org. Electron., 12 (2010), pp. 435–441 [2] A. G. Cid, M. Sedighi, M. Löffler, W. F. van Dorp and E. Zschech, “Energy-Filtered Backscattered Imaging Using Low-Voltage Scanning Electron Microscopy: Characterizing Blends of C60:ZnPc for Organic Solar Cells”. (DOI: 10.1002/adem.201600063)

Resume : Following the debate in the literature, we have investigated the importance of the dispersion of charge carrier motion (mobility) on device performance of organic solar cells. Recently, it has been suggested that steady-state mobilities do not reflect the extraction rates of charge carriers (see A. Melianas, Adv. Funct. Mater., 2014, 24, 4507-4514). Pulsed light experiment have shown a net dispersion of the extraction time, which charge carriers losing energy as they move to the electrodes. This would make mobility a time-dependent quantity. Recently, our group has shown that a clear correlation exists between steady-state quantities, such as mobility, and solar cell performance (see D. Bartesaghi, Nat. Comm., 6:7083) This begs the question whether charge carrier extraction can be described with steady-state mobilities. In this contribution, we demonstrate that the extraction of charge carriers from a solar cell is given by a unique steady-state mobility. We have set up a new experiment to measure the charge extraction time from a solar cell in steady-state. To do so we measure the current decay due to a small decrease in light intensity (close to 1 Sun). The lifetime (τ) extracted from the exponential decay, under well-defined conditions, can be related to the mobility (µ) by τ=8Vtµ/L2 (Eq. 1) where Vt is the thermal voltage and L is the device thickness. This new experiment has been applied to different polymer/fullerene blends. The so-obtained mobilities are very close to the values obtained by space-charge-limited mobility measurements. This proves that charge carrier extraction can be described by steady-state quantities.

Resume : Novel design of polymers has brought more attention to bulk heterojunction polymer:fullerene solar cells (PSCs) in the past ten years. A typical example is the synthesis, through chemical structure engineering, of the benzodithiophene-co-thieno[3,4-b]thiophene (BDT-TT) backbone based polymers leading to power conversion efficiency (PCE) of over 10%. In this work, we study both device PCE and stability for a set of PBDT-TT – based polymers (the class of PTB7 and its derivative polymers) and the effect of an additive on the solar cell performances. Our results show an enhancement in efficiency (0-84 % rise in PCE) for devices processed with additive. We also observed an accelerated UV-degradation for the same devices. For example, in the case of PTB7, for devices without additive there was ~24 % loss in PCE over 2 hours light exposure. Over the same period, we recorded ~44 % loss for the devices with an additive. Additionally, we conducted a systematic study of UV-degradation of these solar cells. We found that based on the polymer chemical structure, PSCs of polymers with alkoxy side chains are more stable (~25 % loss in PCE) than those with alkylthienyl side chains (~40% loss in PCE). These findings pave the way for new materials that yield efficient as well as stable organic solar cells.

Resume : The production of large area organic solar cells (OSCs) in the lab is becoming ever more important to allow the device properties to be analysed on a size scale relevant for future mass production. This requires inexpensive fabrication methodologies in a lab environment using simple coating equipment, to avoid the high investment and running cost of production scale coaters. We demonstrate how a cheap printing proofer can be converted into a sheet-to-sheet slot die coater, with the addition of a slot die head and syringe pump, to allow for production of OSC modules in ambient atmosphere. This coater is shown to produce homogeneous layers of each component of an OSC, where the thickness and morphology of the layers can be controlled by the head speed and temperature of the coating bed. Using a bulk heterojunction system of a Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT):PC70BM we produce air processed, series connected OSC modules with an efficiency of 4 % on an area of 40 cm2. The modules show a linear increase in open circuit voltage with the connection of the single device stripes, up to 4.4 V for six connected devices. The reproducibility of the stripe coating is confirmed by light beam induced current mapping, showing the small variation in photocurrent across the module and to outline any defects within the device. This is also confirmed by the current-voltage characterisation of each stripe of the module. Our results show how this simple coating process can be used to up-scale the device production in a lab environment for more realistic determination of the real world performance of OSC material systems

Resume : The demand for understanding the photophysical processes in perovskite solar cells has become more evident in view of the rapid rise in device efficiency. The lifetime and mobility of light induced-carriers in photoactive layers are about the most important parameters governing the efficiency of a solar cell. However, there is limited knowledge about the relationship between e.g. preparation route, morphology and precursors and these photo-physical properties. Information on these parameters is obtained by complementary microwave photo-conductance (TRMC) and photo luminescence (PL) measurements. We developed a kinetic model to describe both TRMC and PL experiments with one set of global kinetic parameters. In this kinetic model a short laser pulse leads to photoexcitation of electrons to the conduction band, from where they can recombine with holes in the VB (K2). In competition with second order recombination, electrons can be immobilized in intra-band gap trap states with total density NT. Finally, in case of a non-intrinsic semiconductor, there are additional holes (p0, for p-type) on top of the photo-generated holes. Solving the coupled differential equations yield the time dependent concentrations of holes (np), electrons (ne) and trapped electrons (nt) from which the TRMC and PL traces can be calculated. We could successfully describe the dynamics of different systems including planar and mesoprous layers of MAPI1 and of MAPI single crystals2 using this kinetic model. From the found rate constants and mobilities the corresponding diffusion lengths for minority and majority carriers were deduced. In addition a relation could be made between the synthetic route and po and NT. Interesting findings include: i) The trap density in polycrystalline films is three orders of magnitude higher than in single crystals. ii) The minority carrier diffusion length in MAPI perovskites could exceed 10 μm if it is not limited by the crystal domain size. More recently we extended these studies by introducing a specific electron or hole transport layer allowing us to derive the charge injection rates.3 In short, our methodology gives apart from fundamental insight into the dynamics in perovskites a versatile way to optimize the electronic properties of photoactive layers and their charge transport layers. References 1Hutter, E. M.; Eperon, G. E.; Stranks, S. D.; Savenije, T. J. Charge Carriers in Planar and Meso-Structured Organic-Inorganic Perovskites: Mobilities, Lifetimes, and Concentrations of Trap States. JPCL 2015, 6, 3082-3090. 2Bi, Y.; Hutter, E. M.; Fang, Y.; Dong, Q.; Huang, J.; Savenije, T. J. Charge Carrier Lifetimes Exceeding 15 Μs in Methylammonium Lead Iodide Single Crystals. JPCL 2016, 923-928. 3Ponseca, C. S.; Hutter, E. M.; Piatkowski, P.; Cohen, B.; Pascher, T.; Douhal, A.; Yartsev, A.; Sundström, V.; Savenije, T. J. Mechanism of Charge Transfer and Recombination Dynamics in Organo Metal Halide Perovskites and Organic Electrodes, Pcbm, and Spiro-Ometad: Role of Dark Carriers. J. Am. Chem. Soc. 2015, 137, 16043-16048.

Resume : The tunability of the ABX perovskite structure using mixed ions has been key to achieving high photovoltaic efficiencies, for example FA1−xMaxPb(I1−yBry)3. The mixed cation, mixed halide structure enables optimization of the optical band gap of the material whilst maintaining impressive optoelectrical properties. However the effect of mixed ion perovskites structures on the stability has not been critically examined. Here we have investigated the degradation of methyl ammonium lead iodide/bromide (MAPIxBrx-3, x = 0…3) under controlled environmental conditions. For MAPI, the two principal extrinsic degradation mechanisms that have been identified thus far involve sensitivity to humidity and exposure to light + oxygen. The degradation pathway by water vapor was investigated by monitoring films of varying I/Br ratio in an inert atmosphere with controlled humidity. Likewise, the light + oxygen degradation pathway was investigated in dry air and 1 sun equivalent conditions. Using image analysis and XRD we show higher bromine concentration can improve the stability of MAPIxBrx-3 under humid conditions, but a more complex effect is seen under light + oxygen. Compared to MAPI, MAPBr shows much improved material stability in light and oxygen but all mixed films start degrading at the same time. Superoxide analysis and morphological studies show no significance difference between MAPI and MAPBr, suggesting the increased intrinsic crystal stability of MAPBr is the reason for the improved stability seen. Remarkably, the factor limiting the stability of unencapsulated MAPBr solar cell devices is shown to be the organic charge extraction layer and not the perovskite material. Development of stable interlayers can enable fully stable perovskite solar cells.

Resume : In this work, we have demonstrated a reliable fabrication technique for HC(NH2)2PbI3 planar perovskite solar cells. This is done by controlling the nucleation and crystallization processes of the perovskite layer through a combination of adding HI additive [2] in the perovskite precursor and gas-assisted [1] spin coating of the precursor. An average power conversion efficiency (PCE) of 13% can be achieved with negligible hysteresis when measured at a scanning rate of 0.1V/s. The best performing device has a PCE of 16.0%. The narrow spread of PCE’s is due to consistent formation of dense, uniform, pinhole-free good crystalline HC(NH2)2PbI3 film without lead iodide impurities. The effect of process conditions on film growth, physical and electrical proertpies including charge extraction has been comprehensively characterized by scanning electron microscopy, x-ray diffraction, Kelvin probe force microscopy, photoluminescence, and electroluminescence. This is also the first time a combination of nitrogen purge and the addition of HI additive is used in the solution process of HC(NH2)2PbI3 film planar solar cells. [1] Huang, F. et al. Nano Energy (2014), 10, 10-18. [2] Eperon, G. E., et al. Energ Environ Sci (2014), 7 (3), 982-988.

Resume : Recent advances in inorganic-organic hybrid perovskite photovoltaic devices represent revolutionary steps in the exploitation of solar energy. However, the stability of perovskite based solar cells is one of the great challenges for the commercialization of this PV technology1,2. In addition to the degradation of the perovskite material under ambient conditions3, the standard hole transport material (HTM) Spiro-OMeTAD has additional issues (e.g., high cost and low conductivity) 4. In organic solar cells, the use of MoO3 as a HTM can considerably enhance the stability of the device5. MoO3 has been used as HTM in perovskite solar cells before, but did not yield high-efficiency devices6. In this contribution, we report a systematic study of the MoO3/CH3NH3PbI3-xClx interface using synchrotron-based hard x-ray photoelectron spectroscopy, scanning electron microscopy, as well as energy-dispersive x-ray, and Raman spectroscopy. A thorough understanding of the chemical and electronic structure of the MoO3/CH3NH3PbI3-xClx interface is crucial for understanding the underlying cause of the performance limitations of related solar cells. We find that the MoO3 (Mo6+) is significantly reduced in the proximity of the interface, and that the formation of Mo5+ induces defect states in the band gap. At the same time, the perovskite decomposes, resulting in PbI2 diffusing through and accumulating at the molybdenum oxide surface. All of these findings indicate that the MoO3/CH3NH3PbI3-xClx interface is inherently unstable, which provides an explanation for the lower than expected power-conversion efficiencies of corresponding devices7. In our contribution, we will discuss the underlying mechanism for the (decomposition) processes and suggest a clear path for avoiding these effects. References [1] M.A. Green, A. Ho-Baillie and H.J. Snaith. Nature Photonics, 8,506 (2014). [2] S.T. Williams, A. Rajagopal, C.C. Chueh and a. K.-Y. Jen. J. Phys. Chem. Lett., 7, 811 (2016). [3] Y. Rong, L. Liu, A. Mei, X. Li and H. Han. Adv. Energy Mater., 5, 1501066, (2015). [4] G. Sfyri, C. V. Kumar, G. Sabapathi, L. Giribabu, K.S. Andrikopoulos, E. Stathatos and P. Lianos. RSC Adv., 5, 69813 (2015). [5] X. Hu, L. Chen and Y. Chen. J. Phys. Chem. C, 118, 9930 (2014). [6] Y. Zhao, A.M. Nardes and K. Zhu. Appl. Phys. Lett., 104, 213906 (2014). [7] Q.K. Wang, R.B. Wang, P.F. Shen, C. Li, Y.Q. Li, L.J. Liu, S. Duhm, and J.X. Tang. Adv. Mater. Interfaces, 2, 1400528 (2015).

Resume : The energy gap of insulators and semiconductors can change with pressure and temperature, and can be corrected with the energy of the lowest vibrational level, but is expected to be constant in time. Contrarily to this belief, we have found that the band gap in the CH3NH3PbI3 perovskite varies widely, between 0.42 eV and 2.33 eV, depending on the relative arrangement of the freely rotating methylammonium (CH3NH3) molecules in the PbI3 cubic lattice. Since the efficiency of the solar cells in the photovoltaic process depends on the absorption window, these results have very consequences for practical applications. Indeed, CH3NH3PbI3 is a light absorber for low-cost solar cells. Its dynamical gap means that a large window of the Sun spectrum may effectively enter the photovoltaic process, which points to the design of highly efficient cells made upon this compound.

Resume : CH3NH3PbI3 crystal is built of a perovskite-like PbI3 cubic frame and methylammonium (CH3NH3) molecules occupying the central position in the cell. It is commonly believed that the MA molecules are cations, with net charge +1e. On the basis of ab initio calculations, we show that MA in CH3NH3PbI3 behaves instead as an anion with charge -3e. Three electrons transferred to MA from the PbI3 frame originate from the Pb atom (about 1e) and three I atoms (about 2e in total), and locate around C (0.5e), N (0.5e) and H (0.4-0.3e), with respect to the valences of the neutral atoms. As a result of this charge transfer, bands reordering occurs in the MA sublattice with respect to the band structure of the periodic lattice of MA molecules. The reordering is accompanied by a change of the bands character in the PbI3 sublattice with respect to those in the PbI3 frame. Half of the additional charge gained by MA and transferred from the PbI3 frame originates from bands in the energy region [-4.0, 0.0] eV and the rest of charge from deeper states around [-12.0, -11.0] eV.