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Efficient charge generation by relaxed charge-transfer states at organic interfaces


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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Figure 1: Energetics of the relevant states at a D/A interface.
Figure 2: Current density and relative number of photogenerated charge carriers as a function of applied voltage.
Figure 3: Determination of IQE(E) in the spectral region of CT emission for polymer:fullerene photovoltaic devices.
Figure 4: Determination of IQE(E) in the spectral region of CT emission for small-molecule:C60 and polymer:polymer photovoltaic devices.


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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.

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K.V., D.N., S.A. and A. Salleo designed the experiments. S.A. prepared devices for TDCF experiments and performed the TDCF experiments. K.V., W.R.M., E.T.H., K.R.G., J.T.B., M.S., J.W. and M.K.R. prepared photovoltaic devices and optimized their processing parameters for photovoltaic performance. E.T.H. and J.T.B. adjusted the EQE and electroluminescence measurement set-ups for the detection of weak signals, crucial for this work. K.V., E.T.H. and K.R.G. measured the EQE and electroluminescence spectra. K.V. measured the PDS spectra. J.D.D. synthesized PBDTTPD. A. Sellinger, J.M.J.F., A.A., M.K.R. and M.D.M. supervised their team members involved in the project. D.N. and A. Salleo supervised the overall project. All authors contributed to analysis and writing.

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Correspondence to Koen Vandewal, Dieter Neher or Alberto Salleo.

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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).

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