Dispersive Non-Geminate Recombination in an Amorphous Polymer:Fullerene Blend

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.

Barrierless Free Charge Generation in the High‐Performance PM6:Y6 Bulk Heterojunction Non‐Fullerene Solar Cell

“…In order to elucidate the role of the pathways on the free carrier generation, we performed TDCF experiments as a function of the electric field and temperature with excitation at different photon energies. The experimental details on TDCF have been described elsewhere . In short, the device is excited with a short laser pulse (≈5 ns) while being held at a given pre‐bias ( V pre ).…”

“…This is because SCLC is by the motion of dark‐injected charges, which are situated in the tail of the DOS right from the beginning, while photogenerated charges may initially occupy states near the center of the DOS. We have recently shown that studies of dispersive recombination with TDCF provide direct access to the thermalization properties of photogenerated carriers in the actual device . For this measurement, the device is held at a given V pre , typically close to the maximum power point, while the delay time ( t del ) is now varied from few ns to µs (see Note S4, Supporting Information).…”

“…The experimental details on TDCF have been described elsewhere. [29,40] In short, the device is excited with a short laser pulse (≈5 ns) while being held at a given pre-bias (V pre ). After a delay time of 6 ns charges are extracted by applying a high reverse collection bias (V coll ).…”

Apparent Field‐Dependence of the Charge Carrier Generation in Organic Solar Cells as a Result of (Bimolecular) Recombination

“…Neher et al have used a model to determine a potential field‐dependence of the bimolecular recombination coefficient itself . For each value of V pre , a series of TDCF experiments with a variation of the delay time t d is performed.…”

“…Neher et al have used a model to determine a potential fielddependence of the bimolecular recombination coefficient itself. [12,17,31] For each value of V pre , a series of TDCF During the delay charge carriers may either recombine or be extracted at the contacts. The latter are referred to as n pre in the following and charge carriers extracted after switching to the collection bias V coll are referred to as n coll .…”