Polymorphism in Non-Fullerene Acceptors Based on Indacenodithienothiophene
Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG ; [Abstract] Organic solar cells incorporating non-fullerene acceptors (NFAs) have reached remarkable power conversion efficiencies of over 18%. Unlike fullerene derivatives, NFAs tend to crystallize from solutions, resulting in bulk heterojunctions that include a crystalline acceptor phase. This must be considered in any morphology-function models. Here, it is confirmed that high-performing solution-processed indacenodithienothiophene-based NFAs, i.e., ITIC and its derivatives ITIC-M, ITIC-2F, and ITIC-Th, exhibit at least two crystalline forms. In addition to highly ordered polymorphs that form at high temperatures, NFAs arrange into a low-temperature metastable phase that is readily promoted via solution processing and leads to the highest device efficiencies. Intriguingly, the low-temperature forms seem to feature a continuous network that favors charge transport despite of a poorly order along the π–π stacking direction. As the optical absorption of the structurally more disordered low-temperature phase can surpass that of the more ordered polymorphs while displaying comparable—or even higher—charge transport properties, it is argued that such a packing structure is an important feature for reaching highest device efficiencies, thus, providing guidelines for future materials design and crystal engineering activities. ; This work was supported by the Ministerio de Ciencia e Innovacion/FEDER (under Ref. PGC2018-094620-A-I00 and PGC2018-095411-B-I00, CEX2019-000917-S, and PGC2018-095411-B-100) and the Basque Country Government (Ref. PIBA19-0051). S.M. is grateful to POLYMAT for the doctoral scholarship. The authors thank A. Arbe, A. Alonso-Mateo, and L. Hueso for their support and access to characterization tools. The authors also thank the technical and human support provided by SGIker of UPV/EHU and European funding (ERDF and ESF). GIWAXS experiments were performed at BL11 NCD-SWEET beamline at ALBA Synchrotron (Spain) with the collaboration of ALBA staff. J.M and E.F.-G. acknowledge support through the European Union's Horizon 2020 research and innovation program, H2020-FETOPEN 01-2018-2020 (FET-Open Challenging Current Thinking), "LION-HEARTED," Grant Agreement No. 828984. J.M and N.S. would like to thank the financial support provided by the IONBIKE RISE project, which received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 823989. N.S., A.K., and A.B. furthermore are grateful to the U.S. National Science Foundation (NSF) for support via Project No. 1905901 within NSF's Division of Materials Research. A.S. and M.C. acknowledge financial support by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program "HEROIC," Grant Agreement No. 638059. This work was partially carried out at Polifab, the micro- and nanotechnology center of the Politecnico di Milano. C.M. thanks the Knut and Alice Wallenberg Foundation for funding through the project "Mastering Morphology for Solution-borne Electronics." A.I. thanks MICINN for a Personal Técnico de Apoyo contract (PTA2017-14359-I) and gratefully acknowledge the financial support of the Basque Government (Research Groups IT-1175-19) and the MICINN (PGC2018-094548-B-I00, MCIU/AEI/FEDER, UE. Funding for open access charge: Universidade da Coruña/CISUG. ; Gobierno Vasco; PIBA19-0051 ; Gobierno Vasco; IT-1175-19 ; Estados Unidos. National Science Foundation; 1905901