Voltage-dependent photocurrent transients of PTB7:PC70BM solar cells: Experiment and numerical simulation

Transient photocurrent measurements on efficient polymer/fullerene solar cells based on a blend of the donor polymer PTB7 with the fullerene acceptor PC70BM are reported. In particular, we examine the light intensity dependence and voltage dependence of the turn-on and turn-off photocurrent dynamics of devices in response to a 200 mu s square light pulse. At short circuit, subtle changes in the turn-on and turn-off dynamics are observed consistent with charge-density-dependent transport phenomena. As the working voltage is moved from short circuit to open circuit, we observe the appearance of an initial transient photocurrent peak a few microseconds after turn-on before the device settles to steady state. Furthermore, we observe only a weak dependence of the charge extraction dynamics on the working voltage, with the amount of charge extracted monotonically decreasing as the working voltage is moved from short circuit to open circuit. This collection of features is interpreted with the aid of numerical simulations in terms of charge trapping, with increased trap-assisted recombination closer to open circuit. The operation of devices fabricated with and without the solvent additive di-iodooctane is also compared. Charge trapping features are reduced for optimized devices fabricated with the solvent additive compared to devices fabricated without. The use of the solvent additive di-iodooctane in this system is therefore important in minimizing trap-assisted recombination. (C) 2013 AIP Publishing LLC.