Abstract
Recombination of free charge is a key process limiting the performance of solar cells. For low mobility materials, such as organic semiconductors, the kinetics of non-geminate recombination (NGR) is strongly linked to the motion of charges. As these materials possess significant disorder, thermalization of photogenerated carriers in the inhomogeneously broadened density of state distribution is an unavoidable process. Despite its general importance, knowledge about the kinetics of NGR in complete organic solar cells is rather limited. We employ time delayed collection field (TDCF) experiments to study the recombination of photogenerated charge in the high-performance polymer:fullerene blend PCDTBT:PCBM. NGR in the bulk of this amorphous blend is shown to be highly dispersive, with a continuous reduction of the recombination coefficient throughout the entire time scale, until all charge carriers have either been extracted or recombined. Rapid, contact-mediated recombination is identified as an additional loss channel, which, if not properly taken into account, would erroneously suggest a pronounced field dependence of charge generation. These findings are in stark contrast to the results of TDCF experiments on photovoltaic devices made from ordered blends, such as P3HT:PCBM, where non-dispersive recombination was proven to dominate the charge carrier dynamics under application relevant conditions.
Highlights
All efficient organic solar cells consist of at least two semiconducting components with distinctly different electronic structures: a donor with low ionization energy and an acceptor with high electron affinity
We employ the method of time-delayed collection field (TDCF) to follow non-geminate recombination (NGR) in an actual device, at charge carrier densities comparable to those found at 1 sun steady state illumination, and biases in the actual working range of a solar cell
TDCF is a powerful pump-probe technique to study the efficiency of free charge generation and the dynamics of NGR at application-relevant illumination conditions
Summary
All efficient organic solar cells consist of at least two semiconducting components with distinctly different electronic structures: a donor with low ionization energy and an acceptor with high electron affinity. Excitations (be it neutral excitations such as excitons or charged excitations such as polarons) may undergo thermalization, leading to a time-dependence of their basic properties on different timescales. Typical examples of this are spectral diffusion of excitons[5,6,7] or time-dependent charge carrier mobilities[8,9,10,11,12]. We demonstrate that NGR exhibits a continuous slow-down over several orders in time and that most carriers have recombined or are extracted prior to complete thermalization
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