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The past three years have witnessed rapid growth in the field of organic solar cells (OSCs) based on non-fullerene acceptors (NFAs), with intensive efforts being devoted to material development, device engineering, and understanding of device physics. The power conversion efficiency of single-junction OSCs has now reached high values of over 18%. The boost in efficiency results from a combination of promising features in NFA OSCs, including efficient charge generation, good charge transport, and small voltage losses. In addition to efficiency, stability, which is another critical parameter for the commercialization of NFA OSCs, has also been investigated. This review summarizes recent advances in the field, highlights approaches for enhancing the efficiency and stability of NFA OSCs, and discusses possible strategies for further advances of NFA OSCs. ; Funding Agencies|National Natural Science Foundation of ChinaNational Natural Science Foundation of China (NSFC) [21704021, 21805288]; European Commission Marie Skodowska-Curie action [843872]; Stiftelsen for Strategisk Forskning through a Future Research Leader program [FFL18-0322]; Swedish Research Council VRSwedish Research Council [2016-06146, 2018-05484, 2018-06048]; Swedish Energy AgencySwedish Energy Agency [43691-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

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Despite significant development recently, improving the power conversion efficiency of organic photovoltaics (OPVs) is still an ongoing challenge to overcome. One of the prerequisites to achieving this goal is to enable efficient charge separation and small voltage losses at the same time. In this work, a facile synthetic strategy is reported, where optoelectronic properties are delicately tuned by the introduction of electron-deficient-core-based fused structure into non-fullerene acceptors. Both devices exhibited a low voltage loss of 0.57 V and high short-circuit current density of 22.0 mA cm(-2), resulting in high power conversion efficiencies of over 13.4%. These unconventional electron-deficient-core-based non-fullerene acceptors with near-infrared absorption lead to low non-radiative recombination losses in the resulting organic photovoltaics, contributing to a certified high power conversion efficiency of 12.6%. ; Funding Agencies|Air Force Office of Scientific Research (AFOSR) [FA2386-15-1-4108, FA9550e15-1e0610, FA9550-15-1-0333]; UC-Solar Program [MRPI 328368]; National Key Research & Development Projects of China [2017YFA0206600]; National Natural Science Foundation of China [21875286]; Science Fund for Distinguished Young Scholars of Hunan Province [2017JJ1029]; Swedish Energy Agency Energimyndigheten [2016-010174]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

Organic solar cells are currently experiencing a second golden age thanks to the development of novel non-fullerene acceptors (NFAs). Surprisingly, some of these blends exhibit high efficiencies despite a low energy offset at the heterojunction. Herein, free charge generation in the high-performance blend of the donor polymer PM6 with the NFA Y6 is thoroughly investigated as a function of internal field, temperature and excitation energy. Results show that photocurrent generation is essentially barrierless with near-unity efficiency, regardless of excitation energy. Efficient charge separation is maintained over a wide temperature range, down to 100 K, despite the small driving force for charge generation. Studies on a blend with a low concentration of the NFA, measurements of the energetic disorder, and theoretical modeling suggest that CT state dissociation is assisted by the electrostatic interfacial field which for Y6 is large enough to compensate the Coulomb dissociation barrier. ; Funding Agencies|Alexander von Humboldt FoundationAlexander von Humboldt Foundation; National Natural Science Foundation of ChinaNational Natural Science Foundation of China [21875286]; Swedish Energy Agency EnergimyndighetenSwedish Energy Agency [2016-010174]; BMBF grant InterPhase [FKZ 13N13661, FKZ 13N13656]; MESOMERIE [FKZ 13N13661, FKZ 13N13656]; European Union Horizon 2020 research and innovation program "Widening materials models" [646259]; Sofia Kovalevskaya Award from the Alexander von Humboldt FoundationAlexander von Humboldt Foundation

Non-fullerene acceptors (NFAs) as used in state-of-the-art organic solar cells feature highly crystalline layers that go along with low energetic disorder. Here, the crucial role of energetic disorder in blends of the donor polymer PM6 with two Y-series NFAs, Y6, and N4 is studied. By performing tempera-ture-dependent charge transport and recombination studies, a consistent picture of the shape of the density of state distributions for free charges in the two blends is developed, allowing an analytical description of the dependence of the open-circuit voltage VOC on temperature and illumination intensity. Disorder is found to influence the value of the VOC at room temperature, but also its progression with temperature. Here, the PM6:Y6 blend benefits sub-stantially from its narrower state distributions. The analysis also shows that the energy of the equilibrated free charge population is well below the energy of the NFA singlet excitons for both blends and possibly below the energy of the populated charge transfer manifold, indicating a down-hill driving force for free charge formation. It is concluded that energetic disorder of charge-separated states has to be considered in the analysis of the photovoltaic properties, even for the more ordered PM6:Y6 blend. ; This work has been funded by the Alexander von Humboldt Foundation (Sofja Kovalewskaja-Award) and from the Deutsche Forschungsgemeinschaft, DFG, German Research Foundation through the project Fabulous (project number 450968074) and HIOS (project number 182087777 – SFB 951), as well as project number 3923306670. E.M.H. and F.E. acknowledge funding from SolarEraNet (No. NFA4R2ROPV). F.E. thanks the Elite Study Program Macromolecular Science within the Elite Network of Bavaria (ENB) for support. This research used the Soft Matter Interfaces Beamline (SMI, Beamline 12-ID) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No: OSR-CARF/CCF-3079. Q.W. and Y.Z. acknowledge National Natural Science Foundation of China (No. 52125306). A.A. acknowledges support from the Welsh Government's Sêr Cymru II Rising Star and Capacity Builder Accelerator Programs through the European Regional Development Fund, Welsh European Funding Office, and Swansea University Strategic Initiative in Sustainable Advanced Materials.