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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 C-60 via photo-oligomerization occurs primarily via back-hole transfer (BHT) from a charge-transfer state to a C-60 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. ; We thank Dr. Olaf Zeika for TPDP synthesis, Dr. Josue Martinez-Hardigree for his insights on morphology, and Professor Natalie Banerji for her valuable advice with TA analysis. A.P. thanks Dr. William Myers and the Centre for Advanced ESR (CAESR) located in the Department of Chemistry of the University of Oxford (supported by EPSRC EP/L011972/1). I.R. thanks TU Dresden technicians for help with sample production and Dr. Frederik Nehm for help with device degradation. This work was supported by European Union's Horizon 2020 research and innovation program under Marie Sklodowska Curie Grant agreement number 722651 (SEPOMO) and by the COST Action MP1307 (StableNextSol). M.R. acknowledges funding from an EU FP7 Marie Curie Career Integration Grant (number PCIG14-GA-2013-630864) and STFC Challenge Led Applied Systems Programe (CLASP, Grant number ST/L006294/1). This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under award number OSR-2018-CARF/CCF-3079. ; Ramirez, I (corresponding author), Heliatek GmbH, Treidlerstr 3, D-01139 Dresden, Germany. Riede, M (corresponding author), Univ Oxford, Dept Phys, Clarendon Lab, Parks Rd, Oxford OX1 3PU, England. ivan.ramirez@heliatek.com; moritz.riede@physics.ox.ac.uk

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]

Unveiling the correlations among molecular structures, morphological characteristics, macroscopic properties and device performances is crucial for developing better photovoltaic materials and achieving higher efficiencies. To achieve this goal, a comprehensive study is performed based on four state-of-the-art non-fullerene acceptors (NFAs), which allows to systematically examine the above-mentioned correlations from different scales. Its found that extending conjugation of NFA shows positive effects on charge separation promotion and non-radiative loss reduction, while asymmetric terminals can maximize benefits from both terminals. Another molecular optimization is from alkyl chain tuning. The shortened alkyl side chain results in strengthened terminal packing and decreased pi-pi distance, which contribute high carrier mobility and finally the high charge collection efficiency. With the most-acquired benefits from molecular structure and macroscopic factors, PM6:BTP-S9-based organic photovoltaics (OPVs) exhibit the optimal efficiency of 17.56% (certified: 17.4%) with a high fill factor of 78.44%, representing the best among asymmetric acceptor based OPVs. This work provides insight into the structure-performance relationships, and paves the way toward high-performance OPVs via molecular design. Understanding correlations between molecular structures and macroscopic properties is critical in realising highly efficient organic photovoltaics. Here, the authors conduct a comprehensive study based on four non-fullerene acceptors revealing how the extended conjugation, asymmetric terminals and alkyl chain length can affect device performance. ; Funding Agencies|National Natural Science Foundation of ChinaNational Natural Science Foundation of China (NSFC) [21734008, 21875216, 51803178, 61721005]; National Key Research and Development Program of China [2019YFA0705900, 2017YFA0207700]; China Postdoctoral Science FoundationChina Postdoctoral Science Foundation [2020M671715, 2017M621907, 2019T120501]; Zhejiang University; Swedish Government Strategic Research Area in Material Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2017-04123]; China Scholarship Council (CSC)China Scholarship Council [201708370115]; Research Grant Council of Hong Kong (General Research Fund) [14303519]; CUHK Direct GrantChinese University of Hong Kong [4053415]

Low energy loss and efficient charge separation under small driving forces are the prerequisites for realizing high power conversion efficiency (PCE) in organic photovoltaics (OPVs). Here, a new molecular design of nonfullerene acceptors (NFAs) is proposed to address above two issues simultaneously by introducing asymmetric terminals. Two NFAs, BTP-S1 and BTP-S2, are constructed by introducing halogenated indandione (A(1)) and 3-dicyanomethylene-1-indanone (A(2)) as two different conjugated terminals on the central fused core (D), wherein they share the same backbone as well-known NFA Y6, but at different terminals. Such asymmetric NFAs with A(1)-D-A(2) structure exhibit superior photovoltaic properties when blended with polymer donor PM6. Energy loss analysis reveals that asymmetric molecule BTP-S2 with six chlorine atoms attached at the terminals enables the corresponding devices to give an outstanding electroluminescence quantum efficiency of 2.3 x 10(-2)%, one order of magnitude higher than devices based on symmetric Y6 (4.4 x 10(-3)%), thus significantly lowering the nonradiative loss and energy loss of the corresponding devices. Besides, asymmetric BTP-S1 and BTP-S2 with multiple halogen atoms at the terminals exhibit fast hole transfer to the donor PM6. As a result, OPVs based on the PM6:BTP-S2 blend realize a PCE of 16.37%, higher than that (15.79%) of PM6:Y6-based OPVs. A further optimization of the ternary blend (PM6:Y6:BTP-S2) results in a best PCE of 17.43%, which is among the highest efficiencies for single-junction OPVs. This work provides an effective approach to simultaneously lower the energy loss and promote the charge separation of OPVs by molecular design strategy. ; Funding Agencies|National Key Research and Development Program of China [2019YFA0705900]; National Natural Science Foundation of ChinaNational Natural Science Foundation of China [21734008, 21875216, 51803178, 61721005]; China Postdoctoral Science FoundationChina Postdoctoral Science Foundation [2017M621907, 2019T120501]; S&T Innovation 2025 Major Special Programme of Ningbo [2018B10055]; Research Grant Council of Hong KongHong Kong Research Grants Council [N_CUHK418/17, 14303519, 4053304]; Swedish Government Strategic Research Area in Material Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]; Swedish Research CouncilSwedish Research Council [2017-04123]; China Scholarship Council (CSC)China Scholarship Council

Purpose: In this paper, we examine the status, prospects and organization of OPV research, innovation and governance in three major world regions: Northern America, Western Europe and East Asia through our constructed evolutionary cognitive-institutional framework of reference. Method: We gathered data from a 65-question internet-based survey conducted from February 2013 to April 2013 with OPV researchers and research managers around the world. A multi-method (investigative/exploratory, descriptive statistics) approach is used for analyses and discussions. Results: Overall findings show that the organization of OPV research, innovation and governance in Northern America, Western Europe and East Asia reflect similar aspects, patterns with their political economies surveyed in the literature: Northern America's neo-liberal market and finance orientation, Western Europe's orientation to sustainable development and policy-driven research, coordinated-regulatory inspirations and research-driven system, and East Asia's neo-developmental state view with international trade, technology-export orientation. Commercialization prospects in China are lowest and highest in the US but even there expectations of market sales are low. As a disruptive technology which is competing with older generations of PV and other energy technologies, OPV requires a coordinated effort involving international cooperation, the use of public and private money. Positive elements of the three world regions (availability of venture capital in the US, the meritocratic research system and ambitious goals for renewable energy in the EU, and the willingness of the Chinese government to back sunrise industries) could be usefully exploited. Keywords: Political Economy, Emerging Energy Technology, Research, Innovation, Governance, Organic Photovoltaics

AbstractA critical bottleneck for improving the performance of organic solar cells (OSC) is minimising non-radiative losses in the interfacial charge-transfer (CT) state via the formation of hybrid energetic states. This requires small energetic offsets often detrimental for high external quantum efficiency (EQE). Here, we obtain OSC with both non-radiative voltage losses (0.24 V) and photocurrent losses (EQE > 80%) simultaneously minimised. The interfacial CT states separate into free carriers with ≈40-ps time constant. We combine device and spectroscopic data to model the thermodynamics of charge separation and extraction, revealing that the relatively high performance of the devices arises from an optimal adjustment of the CT state energy, which determines how the available overall driving force is efficiently used to maximize both exciton splitting and charge separation. The model proposed is universal for donor:acceptor (D:A) with low driving forces and predicts which D:A will benefit from a morphology optimization for highly efficient OSC. ; N.G. acknowledges the Imperial College Research Fellowship scheme. C.J.B. gratefully acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Project no. 182849149-SFB 953. C.J.B. gratefully acknowledges financial support through the "Aufbruch Bayern" initiative of the state of Bavaria (EnCN and SFF) and the Bavarian Initiative "Solar Technologies go Hybrid" (SolTech) and funding from DFG project DFG INST 90/917. C.C.L. thanks the European Union for the financial support. G.C. acknowledges the support from the PRIN 2017 Project 201795SBA3—HARVEST.

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Microfluidic technologies are highly adept at generating controllable compositional gradients in fluids, a feature that has accelerated the understanding of the importance of chemical gradients in biological processes. That said, the development of versatile methods to generate controllable compositional gradients in the solid‐state has been far more elusive. The ability to produce such gradients would provide access to extensive compositional libraries, thus enabling the high‐throughput exploration of the parametric landscape of functional solids and devices in a resource‐, time‐, and cost‐efficient manner. Herein, the synergic integration of microfluidic technologies is reported with blade coating to enable the controlled formation of compositional lateral gradients in solution. Subsequently, the transformation of liquid‐based compositional gradients into solid‐state thin films using this method is demonstrated. To demonstrate efficacy of the approach, microfluidic‐assisted blade coating is used to optimize blending ratios in organic solar cells. Importantly, this novel technology can be easily extended to other solution processable systems that require the formation of solid‐state compositional lateral gradients. ; The authors would like to acknowledge financial support from the Spanish Ministry of Economy, Industry and Competitiveness through the "Severo Ochoa" Programme for Centers of Excellence in R&D (SEV‐2015‐0496) and project reference PGC2018‐095411‐B‐I00 as well as the European Research Council (ERC) under grant agreement no. 648901. J.P.‐L. acknowledges the European Research Council Starting Grant microCrysFact (ERC‐2015‐STG No. 677020) and the Swiss National Science Foundation (200021_181988) and ETH Zürich. R. R.‐T. acknowledges the support from Generalitat de Catalunya and the COFUND programme of the Marie Curie Actions of the 7th R&D Framework Programme of the European Union (BP‐B 00256). X.R.‐M. acknowledges the departments of Physics, Chemistry and Geology of the Autonomous University of Barcelona (UAB) as coordinators of the PhD programme in Materials Science. X.R.‐M. and C.F. acknowledge Nicole Kleger‐Schai from ETH Zürich for her valuable help in using the rheometer. X.R.‐M. and M.C.‐Q. acknowledge Dr. Joan M. Cabot from the University of Tasmania for fruitful discussions on 3D printing. D.B.A. thanks the University of Nottingham Beacon Propulsion Futures. ; Peer reviewed

Microfluidic technologies are highly adept at generating controllable compositional gradients in fluids, a feature that has accelerated the understanding of the importance of chemical gradients in biological processes. That said, the development of versatile methods to generate controllable compositional gradients in the solid-state has been far more elusive. The ability to produce such gradients would provide access to extensive compositional libraries, thus enabling the high-throughput exploration of the parametric landscape of functional solids and devices in a resource-, time-, and cost-efficient manner. Herein, the synergic integration of microfluidic technologies is reported with blade coating to enable the controlled formation of compositional lateral gradients in solution. Subsequently, the transformation of liquid-based compositional gradients into solid-state thin films using this method is demonstrated. To demonstrate efficacy of the approach, microfluidic-assisted blade coating is used to optimize blending ratios in organic solar cells. Importantly, this novel technology can be easily extended to other solution processable systems that require the formation of solid-state compositional lateral gradients. ; Funding Agencies|Spanish Ministry of Economy, Industry and Competitiveness through the "Severo Ochoa" Programme for Centers of Excellence in RD [SEV-2015-0496, PGC2018-095411-B-I00]; European Research Council (ERC)European Research Council (ERC) [648901]; European Research Council Starting Grant microCrysFact (ERC-2015-STG) [677020]; Swiss National Science FoundationSwiss National Science Foundation (SNSF) [200021_181988]; ETH ZurichETH Zurich; Generalitat de CatalunyaGeneralitat de Catalunya; COFUND programme of the Marie Curie Actions of the 7th R&D Framework Programme of the European Union [BP-B 00256]

Non-fullerene acceptors (NFAs) are excellent light harvesters, yet the origin of their high optical extinction is not well understood. In this work, we investigate the absorption strength of NFAs by building a database of time-dependent density functional theory (TDDFT) calculations of ∼500 π-conjugated molecules. The calculations are first validated by comparison with experimental measurements in solution and solid state using common fullerene and non-fullerene acceptors. We find that the molar extinction coefficient (εd,max) shows reasonable agreement between calculation in vacuum and experiment for molecules in solution, highlighting the effectiveness of TDDFT for predicting optical properties of organic π-conjugated molecules. We then perform a statistical analysis based on molecular descriptors to identify which features are important in defining the absorption strength. This allows us to identify structural features that are correlated with high absorption strength in NFAs and could be used to guide molecular design: highly absorbing NFAs should possess a planar, linear, and fully conjugated molecular backbone with highly polarisable heteroatoms. We then exploit a random decision forest algorithm to draw predictions for εd,max using a computational framework based on extended tight-binding Hamiltonians, which shows reasonable predicting accuracy with lower computational cost than TDDFT. This work provides a general understanding of the relationship between molecular structure and absorption strength in π-conjugated organic molecules, including NFAs, while introducing predictive machine-learning models of low computational cost. ; J. N., J. Y., D. P., M. A., F. E., and E. R. thank the European Research Council for support under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 742708 and 648901). The authors at ICMAB acknowledge financial support from the Spanish Ministry of Science and Innovation through the Severo Ochoa" Program for Centers of Excellence in R&D ...

Continuous flow chemistry has been shown to be a suitable method for the large-scale preparation of conjugated polymers with uniform structural and macromolecular characteristics, which is especially relevant when applying these materials in optoelectronic devices. The molecular weight and dispersity of conjugated polymers have a major effect on final device performance through a combination of processing and morphological considerations. In this work, the low bandgap polymer PffBT4T-2OD ('PCE-11'), which provides highly efficient bulk heterojunction solar cells, is synthesized by continuous flow chemistry using an easily mountable home-made apparatus. The influence of various reaction parameters on the material characteristics is investigated. Particular attention is devoted to tuning of the molecular weight, as this has a major impact on solubility and processability of the resultant polymer and, ultimately, solar cell performance. We find that temperature, monomer concentration, and injection volume of the polymerization mixture are significant parameters that can be used to optimize the control over molecular weight. The same protocol is then also applied to a structurally similar polymer with longer alkyl side chains, PffBT4T-2DT, affording important advantages in terms of processing due to its higher solubility. An average power conversion efficiency of 9.4% for bulk heterojunction solar cells using PC71BM as the acceptor phase is achieved based on this flow-synthesized polymer. ; The authors of the Georgia Institute of Technology thank the National Science Foundation for support through the CCI Center for Selective C-H Functionalization (CHE-1205646) and the Department of the Navy, Office of Naval Research for support through Award No. N00014-14-1-0580 (CAOP MURI). The Belgian co-authors thank Hasselt University for continuing financial support. This work is also supported by the IAP 7/05 project Functional Supramolecular Systems, granted by the Science Policy Office of the Belgian Federal Government (BEL-SPO). We are also grateful for financial support by the Research Foundation - Flanders (FWO) (projects G.0415.14N and G.0B67.15N). Most of the work was performed during the stay of G. Pirotte as a visiting researcher at Georgia Institute of Technology, funded by the FWO (V4.298.16N) and a personal grant from the District 1630 of Rotary International, supported by the Rotary Foundation.