Shockley-Read-Hall recombination in P3HT:PCBM solar cells as observed under ultralow light intensities

We present light intensity dependent measurements of the quantum efficiency of P3HT:PCBM photovoltaic devices. Unlike previous studies we focus on ultralow light intensities down to 10^-3 mW/cm². We find that although when the devices are excited at intensities close to 1 Sun they exhibit very little bias or light intensity dependence, this is clearly not the case for light intensities below 1 mW/cm², where the cell's efficiency becomes highly dependent on the bias and light intensity. Using a simple model for the device efficiency we can fit the experimental data across a wide range of parameters and thus separate the effects of generation efficiency (geminate recombination) and charge recombination. Our finding suggests that recombination through trap (charge transfer) states is an important loss mechanism and we are able to quantify the density and depth of these states.