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

Abstract

Charge carrier generation in organic solar cells is often reported to depend on the electric field. This is, however, not measured directly but derived from transient charge carrier extraction experiments based on the time delayed collection field (TDCF) method. In this work, numerical simulations of TDCF experiments are presented which – when analyzed in the same way as reported in literature – result in a field‐dependence of charge carrier generation despite the fact that a field‐independent generation is used. This discrepancy is shown to be caused by recombination of photogenerated charge carriers occurring in the time range prior to and during extraction. This apparent field‐dependence becomes more pronounced for larger recombination coefficients and decreasing charge carrier mobilities, very much in accordance with experimental TDCF data from literature. Even an apparent voltage‐ and time‐dependence of the bimolecular recombination coefficient is reproduced in the simulations although a constant, voltage‐independent one is used. These findings strongly question whether TDCF is an appropriate method to detect a potential field‐dependence of the photocurrent generation and the recombination coefficient. Our study shows that all experimental results can consistently be explained without the assumption of a field‐dependence of the charge carrier generation and the bimolecular recombination coefficient.

P3HT Molecular Weight Determines the Performance of P3HT:O‐IDTBR Solar Cells

“…We note that the field dependence of charge generation deduced from TDCF experiments has very recently been questioned by Würfel and Unmüssig, and in some cases, an apparent field dependence of charge generation might be caused by fast nongeminate recombination of photogenerated and dark (injected) charge carriers. [37] At this point, we cannot entirely rule out that this is also the case for the P3HT:O-IDTBR systems studied here, but we stress that the bias dependence of the photocurrent cannot account for the performance differences that we observe.…”

Impact of Fullerene on the Photophysics of Ternary Small Molecule Organic Solar Cells

“…Under these conditions, Q tot can be interpreted as a measure of the amount of free charges generated by the excitation laser pulse as a function of the applied prebias . We note that the applicability of TDCF to measure the true field dependence of charge generation has recently been questioned . However, here we do not observe any (apparent) field dependence of charge generation and use TDCF to study the charge recombination/extraction dynamics on a timescale faster than can typically be accessed by other charge extraction techniques.…”

“…[20] We note that the applicability of TDCF to measure the true field depend ence of charge generation has recently been questioned. [21] However, here we do not observe any (apparent) field depend ence of charge generation and use TDCF to study the charge recombination/extraction dynamics on a timescale faster than can typically be accessed by other charge extraction techniques.…”

Reliability of charge carrier recombination data determined with charge extraction methods

“…Here, we like to comment on the conclusions from the recent study by Würfel and Unmüssig on the reliability of recombination studies with TDCF. 46 Their drift-diffusion simulation showed (Figs. 4 and 5 of that work) that the determination of the (time dependent) recombination with TDCF becomes unreliable for initial carrier densities of 5 × 10 21 m −3 and below.…”

“…It has been pointed out that the accuracy of TDCF to determine the order and coefficient of NGR can be falsified by the recombination of photogenerated charges with dark-injected charges. 46 A way out of this problem may be to use a prebias much smaller than the built-in potential, but this comes at the cost of accelerated extraction during the delay. Therefore, simulations have been performed at V pre = 0.6 V and V pre = 0.8 V, below and above the V OC of the cell, with the resulting differential decay plots shown in Fig.…”