Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.
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This publication was supported by the Center for Advanced Molecular Photovoltaics (Award No KUS-C1-015-21) and the Department of Energy, Laboratory Directed Research and Development funding, under contract DE-AC02-76SF00515. The PCDTBT used in this work was provided by St-Jean Photochemicals. M.K.R. acknowledges financial support by the BMBF through project 03IP602 and J.W. acknowledges support from the Heinrich-Böll-Stiftung. S.A. and M.S. acknowledge financial support by the BMBF within PVcomB (FKZ 03IS2151D) and the DFG (SPP 1355). D.N. thanks the DFG for financially supporting a travel grant. K.R.G. and A.A. acknowledge SABIC for a post-doctoral fellowship. The authors thank J. Kurpiers for technical assistance with the TDCF set-up.
The authors declare no competing financial interests.
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Vandewal, K., Albrecht, S., Hoke, E. et al. Efficient charge generation by relaxed charge-transfer states at organic interfaces. Nature Mater 13, 63–68 (2014). https://doi.org/10.1038/nmat3807
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