Organic solar cells demonstrate external quantum eciencies and fill factors approaching those of conventional photovoltaic technologies. However, as compared with the optical gap of the absorber materials, their open-circuit voltage is much lower, largely due to the presence of significant non-radiative recombination. Here, we study a large data set of published and new material combinations and find that non-radiative voltage losses decrease with increasing charge-transfer-state energies. This observation is explained by considering non-radiative charge-transfer-state decay as electron transfer in the Marcus inverted regime, being facilitated by a common skeletal molecular vibrational mode. Our results suggest an intrinsic link between non-radiative voltage losses and electron-vibration coupling, indicating that these losses are unavoidable. Accordingly, the theoretical upper limit for the power conversion effciency of single-junction organic solar cells would be reduced to about 25.5% and the optimal optical gap increases to (1.45-1.65) eV, that is, (0.20.3) eV higher than for technologies with minimized non-radiative voltage losses.
Bibliographical noteFunding Information:
This work was supported by the German Federal Ministry for Education and Research (BMBF) through the InnoProfille project ‘Organische p-i-n Bauelemente 2.2’. K.T. acknowledges the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7 under the REA grant agreement PIEF-GA-2012-327199. F.P. and D.N. acknowledge funding by the German Research Foundation (DFG) via the SFB 951 ‘HIOS’. The work of Georgia Tech was supported by the Department of the Navy, Office of Naval Research Award No. N00014-14-1-0580 (CAOP MURI), and through a State-Sponsored Scholarship for Graduate Students to Y.F. from the China Scholarship Council. M.T. thanks the Christ Church Oxford for financial support with a Junior Research Fellowship. M.K.R. acknowledges the UK Engineering and Physical Science Research Council (EPSRC) through grant EP/L026066/1. Additionally, we thank for the supply of the donor molecules: P. Bäuerle from University of Ulm for DH4T, DH6T and several DCV2-nT, M. Hummert for P4-Ph4-DIP and BP-Bodipy, and B. Beyer for ZnF4Pc. Furthermore, we acknowledge F. Holzmueller, C. Koerner, M. Saalfrank and R. Meerheim for providing OSC devices for this study.