Transient Optoelectronic Analysis of Charge Carrier Losses in a Selenophene/Fullerene Blend Solar Cell

In this paper, we employ transient photovoltage, transient photo-current, charge extraction, and transient absorption measurements to analyze the current/voltage response of bulk heterojunction solar cells employing a poly(3-hexylselenophene) (P3HS)/[6,6]-phenyl C-61 butyric acid methyl ester (PC61BM) blend photoactive layer. These techniques are employed to determine the charge carrier densities and lifetimes observed in devices held at open circuit as a function of light intensity. Excellent agreement is obtained between charge densities and lifetimes determined by the different techniques, supporting the validity of these analyses. These analyses are employed to calculate the nongeminate recombination flux at open circuit as a function of light intensity, and therefore open circuit voltage. This nongeminate recombination flux is found to be approximately equal and opposite to the short circuit current density measured at the same light intensity, indicating that the dominating charge carrier loss pathway determining the device open circuit voltage is nongeminate recombination. This analysis is extended across the device current/voltage curve by using charge extraction to determine the average charge density in the device as a function of applied light intensity and bias voltage. Using this analysis, and assuming that the nongeminate recombination flux depends only upon this average charge density, we demonstrate that we are able to obtain a reasonable reproduction of the device current/voltage behavior both in the dark and for light intensities up to similar to 1 sun without the use of any fitting parameters. We thus conclude that a simple device model based upon a light intensity dependent charge photogeneration term and a charge density dependent nongeminate recombination flux is capable of describing the dominating factors determining the fill factor and open circuit voltage of these devices.