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While multi-junction geometries have the potential to boost the efficiency of organic solar cells, the experimental gains yet obtained are still very modest. This work proposes an alternative spectral splitting device concept in which various individual semiconducting junctions with cascading band gaps are laid side by side, thus the name RAINBOW. Each lateral sub-cell receives a fraction of the spectrum that closely matches the main absorption band of the given semiconductor. Here, simulations are used to identify the important material and device properties of each RAINBOW sub-cell. Using the resulting design rules, three systems are selected, namely PBDB-T-2F:IO-4Cl, PBDB-T-2F:Y6 and PTB7-Th:COTIC-4F, and their potential as sub-cells in this geometry is experimentally investigated. With the aid of a custom built setup that generates spectrally spread sunlight on demand, the simulations are experimentally validated, showing that this geometry can lead to a reduction in thermalization losses and an improvement in light harvesting, which results in a relative improvement in efficiency of 46.6% with respect to the best sub-cell. Finally, a working proof of concept monolithic device consisting of two sub-cells deposited from solution on the same substrate is fabricated, thus demonstrating the feasibility and the potential of the RAINBOW solar cell concept. This article is protected by copyright. All rights reserved., The Spanish "Ministerio de Ciencia e Innovación (MICINN)" is gratefully acknowledged for its supportthrough grant No. CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo OchoaCentre of Excellence program and the AEI/FEDER(UE) grants PGC2018-095411-B-I00(RAINBOW),TED2021-131911B-I00and PID2021-128924OB-I00(ISOSCELLES). The authors also thank the Cata-lan agency AGAUR for grant2021-SGR-00444. MCV acknowledges a FPI fellowship (PRE2019-089855)from MICINN co-financed by the European Social Fund and MGR acknowledges the scholarship FPU16/02631 from the Spanish "Ministerio de Educación". MCV and MGR also thank the PhD programme in Materi-als Science from Universitat Autònoma de Barcelona in which both were enrolled. KV and QL acknowl-edge funding by the European Research Council (ERC, grant agreement864625)., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Organic photovoltaics (PV) is an energy-harvesting technology that offers many advantages, such as flexibility, low weight and cost, as well as environmentally benign materials and manufacturing techniques. Despite growth of power conversion efficiencies to around 19 % in the last years, organic PVs still lag behind inorganic PV technologies, mainly due to high losses in open-circuit voltage. Understanding and improving open circuit voltage in organic solar cells is challenging, as it is controlled by the properties of a donor-acceptor interface where the optical excitations are separated into charge carriers. Here, we provide an electrostatic model of a rough donor-acceptor interface and test it experimentally on small molecule PV materials systems. The model provides concise relationships between the open-circuit voltage, photovoltaic gap, charge-transfer state energy, and interfacial morphology. In particular, we show that the electrostatic bias generated across the interface reduces the photovoltaic gap. This negative influence on open-circuit voltage can, however, be circumvented by adjusting the morphology of the donor-acceptor interface. Organic solar cells, despite their high power conversion efficiencies, suffer from open circuit voltage losses making them less appealing in terms of applications. Here, the authors, supported with experimental data on small molecule photovoltaic cells, relate open circuit voltage to photovoltaic gap, charge-transfer state energy, and donor-acceptor interfacial morphology. Open Access funding enabled and organized by Projekt DEAL. This publication is based on work supported by the KAUST Office of Sponsored Research (OSR) under award nos. OSR-2018-CARF/CCF-3079 and OSR-CRG2018- 3746. D.A. also acknowledges the KAUST PSE Division for hosting his sabbatical in the framework of the Division’s Visiting Faculty program. D.A. acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for financial support through the collaborative research centers TRR 146, SPP 2196, and grant number 460766640. We thank Kun-Han Lin, Leanne Paterson, Wenlan Liu, Mukunda Mandal, and Naomi Kinaret for fruitful discussions and proof reading of the manuscript. Furthermore, the authors acknowledge Dr. Beatrice Beyer from Fraunhofer Institut FEP for supplying the donor molecule ZnF4Pc. J.B. and K.L. acknowledge the German Federal Ministry of Education and Research (BMBF) for funding through the projects “Pergamon” (16ME0012) and “Flexmonirs” (01DR20008A).

The origin of energetic disorder in organic semiconductors and its impact on opto-electronic properties remains a topic of intense controversy. Particularly the disorder at electron donor-acceptor interfaces for organic photovoltaics is pivotal to understand as it is expected to affect photo-carrier generation, recombination and consequently device efficiency parameters. In this work we evaluate the temperature dependence of the line-shape of the photoluminescence (PL) and electroluminescence (EL) spectra of small molecule:fullerene blend devices, with the ambition to disentangle dynamic and static disorder contributions. The EL emission spectra are dominated by charge-transfer (CT) state emission and are confirmed to be of Gaussian character and almost completely voltage independent. More importantly, a strong line-width narrowing is persistently observed upon cooling, down to a certain material specific low temperature, below which the line-width remains constant. It is consequently clear that the main portion of the line-width measured at operating conditions of room temperature or higher, is originating from thermally activated, or dynamic, disorder. The observed temperature dependence of the high-energy emission tail can be fully described by taking into account high and low frequency molecular vibrational modes, without having to rely on static disorder. The presence of low frequency molecular modes with large Huang-Rhys factors results in a Gaussian line-shape, which is additionally broadened at high temperature by thermal population of high frequency intra-molecular modes. We therefore cast strong doubts regarding the commonly used assumption that single temperature optical measurements of absorption or emission tails are able to provide meaningful information regarding the shape of a static density of states tail. K. T. acknowledges the German Research Foundation (DFG) through project 382633022 (RECOLPER) and the Experimental Physics VI chair of Prof. Vladimir Dyakonov at Wurzburg University. J. B. acknowledges the DFG project VA 1035/5-1 (Photogen) and the Sachsische Aufbaubank through project no. 100325708 (Infrakart). The authors also acknowledge discussion on the topic with Robert Street, Thomas Kirchartz, Frank Ortmann and Carsten Deibel. Tvingstedt, K (corresponding author), Julius Maximilian Univ Wurzburg, Expt Phys 6, D-97074 Wurzburg, Germany. ktvingstedt@physik.uni-wuerzburg.de

Single-material organic solar cells have recently attracted research attention due to their simplicity, morphological robustness and high yield of exciton dissociation. Using alpha-sexithiophene as a model system, we show that the single-event probability of the exciton dissociation at the boundaries of polycrystalline domains with different molecular orientation is extremely low (similar to 0.5%), while a high efficiency of charge generation is gained via hundred-fold crossings of the domain boundaries due to the long exciton diffusion length (similar to 45 nm). European UnionEuropean Commission [722651]

Stability is now a critical factor in the commercialization of organic photovoltaic (OPV) devices. Both extrinsic stability to oxygen and water and intrinsic stability to light and heat in inert conditions must be achieved. Triplet states are known to be problematic in both cases, leading to singlet oxygen production or fullerene dimerization. The latter is thought to proceed from unquenched singlet excitons that have undergone intersystem crossing (ISC). Instead, we show that in bulk heterojunction (BHJ) solar cells the photo-degradation of C60 via photo-oligomerization occurs primarily via back-hole transfer (BHT) from a charge-transfer state to a C60 excited triplet state. We demonstrate this to be the principal pathway from a combination of steady-state optoelectronic measurements, time-resolved electron paramagnetic resonance, and temperature-dependent transient absorption spectroscopy on model systems. BHT is a much more serious concern than ISC because it cannot be mitigated by improved exciton quenching, obtained for example by a finer BHJ morphology. As BHT is not specific to fullerenes, our results suggest that the role of electron and hole back transfer in the degradation of BHJs should also be carefully considered when designing stable OPV devices., The commercialisation of organic photovoltaic technology calls for research on material degradation mechanisms. Ramirez et al. show that triplet excitons produced by back charge transfer can significantly impact the photo-stability of fullerene-based devices even in the absence of water and oxygen.