Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic solar cells
Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges. However, the use of D-A blends intrinsically limits the photovoltage and introduces morphological instability. Here, we demonstrate that polycrystalline films of chemically identical molecules offer a promising alternative and show that photoexcitation of alpha-sexithiophene (alpha-6T) films results in efficient charge generation. This leads to alpha-6T based homojunction organic solar cells with an external quantum efficiency reaching up to 44% and an open-circuit voltage of 1.61 V. Morphological, photoemission, and modelling studies show that boundaries between alpha-6T crystalline domains with different orientations generate an electrostatic landscape with an interfacial energy offset of 0.4 eV, which promotes the formation of hybridised exciton/charge-transfer states at the interface, dissociating efficiently into free charges. Our findings open new avenues for organic solar cell design where material energetics are tuned through molecular electrostatic engineering and mesoscale structural control. ; The authors thank Maxim Pschenitchnikov for useful discussion. This work was supported by the German Federal Ministry of Education and Research (BMBF) through the Innoprofile project 'Organische p-i-n Bauelemente2.2' (FKZ 03IPT602X), UKRI Global Challenge Research Fund project SUNRISE (EP/P032591/1). Y.-C.C. acknowledges the President's PhD Scholarship funding by Imperial College London. A.A.B. is a Royal Society University Research Fellow. J.-S.K. and J.R.D. acknowledge the Global Research Laboratory Program of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017K1A1A2 013153). This project has also received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreements 639750 and 864625). The work in Mons was supported by the European Union's Horizon 2020 research and innovation programme under Marie Sklodowska Curie Grant agreement No. 722651 (SEPOMO). Computational resources were provided by the French GENCITGCC infrastructure, by the Belgian Consortium des E ' quipements de Calcul Intensif (CE ' CI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, and by the Tier-1 supercomputer of the Federation Wallonie-Bruxelles, infrastructure funded by the Walloon Region under Grant Agreement No. 1117545. D.B. is a FNRS Research Director. We acknowledge Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities and we thank Luisa Barba for assistance in using beamline XRD1. ; Durrant, JR (corresponding author), Imperial Coll London, Dept Chem, London W12 0BZ, England ; Imperial Coll London, Ctr Processable Elect, London W12 0BZ, England ; Swansea Univ, SPECIFIC, Coll Engn, Bay Campus, Swansea SA1 8EN, W Glam, Wales. Kim, JS (corresponding author), Imperial Coll London, Dept Phys, London SW7 2AZ, England ; Imperial Coll London, Ctr Processable Elect, London SW7 2AZ, England. D'Avino, G (corresponding author), Univ Grenoble Alpes, Grenoble INP, Inst Neel, 25 Rue Martyrs, F-38042 Grenoble, France. Vandewal, K (corresponding author), Hasselt Univ, Inst Mat Res IMO IMOMEC, Wetenschapspk 1, B-3590 Diepenbeek, Belgium. ji-seon.kim@imperial.ac.uk; gabriele.davino@neel.cnrs.fr; j.durrant@imperial.ac.uk; koen.vandewal@uhasselt.be