Polaron pair mediated triplet generation in polymer/fullerene blends

S.D. Dimitrov, S. Wheeler, D. Niedzialek, B.C. Schroeder, H. Utzat, J.M. Frost, J. Yao, A. Gillett, P.S. Tuladhar, I. McCulloch, J. Nelson, J.R. Durrant
Nature communications 6, 6501, (2015)

Polaron pair mediated triplet generation in polymer/fullerene blends

Keywords

Polaron pair mediated triplet , solar cells

Abstract

​Electron spin is a key consideration for the function of organic semiconductors in light-emitting diodes and solar cells, as well as spintronic applications relying on organic magnetoresistance. A mechanism for triplet excited state generation in such systems is by recombination of electron-hole pairs. However, the exact charge recombination mechanism, whether geminate or nongeminate and whether it involves spin-state mixing is not well understood. In this work, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related polymer-fullerene blends with differing polymer fluorination and photovoltaic performance. Using time-resolved laser spectroscopic techniques and quantum chemical calculations, we show that lower charge separation in the fluorinated system is associated with the formation of bound electron-hole pairs, which undergo spin-state mixing on the nanosecond timescale and subsequent geminate recombination to triplet excitons. We find that these bound electron-hole pairs can be dissociated by electric fields.

Code

DOI: 10.1038/ncomms7501

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