Photocurrent transients in all-polymer solar cells: Trapping and detrapping effects

We have studied photocurrent transients in all-polymer bulk-heterojunction solar cells based on poly(3-hexylthiophene) and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl). By illuminating devices with square pulses of light of varying intensity, we reveal nonlinear photocurrent transients on the timescale of tens of microseconds. These microsecond photocurrent transients are attributed to the effects of trapping and detrapping of charges on this timescale, in particular, electrons. The buildup of trapped electrons results in the appearance of a peak in the photocurrent at high intensities at ∼10 μs after turn on. This trapped charge produces a local reduction in the strength of the internal electric field near the anode resulting in a net decrease in charge separation efficiency and an increase in the likelihood of bimolecular recombination due to increased and overlapping electron and hole densities. After turn off, a long photocurrent tail is observed with charge still being extracted after 0.5 ms consistent with the detrapping of deeply trapped charges. We are able to reproduce the observed transient photocurrent features using a time-dependent drift-diffusion model incorporating the trapping and detrapping of electrons.