Implications of Trap-Assisted Nongeminate Charge Recombination on Time- and Frequency-Domain Photocurrent Degradation Signatures of Organic Solar Cells

Charge transport and collection in organic solar cells are heavily influenced by traps which ultimately limit the ability to harvest all photogenerated carriers. We investigate photocurrent responses of organic solar cells subjected to varying degrees of aging from time- and frequency-domain perspectives. Intensity-modulated photocurrent spectroscopy (IMPS) is primarily used here to resolve the effect of trap-assisted nongeminate charge recombination over a broad frequency range (e.g., similar to 1 mHz-1 MHz). We use a combination of IMPS and time-dependent photocurrent transients to understand characteristic degradation signatures (i.e., positive, low-frequency imaginary component and "gain peak" where the real photocurrent exhibits a characteristic maximum, I-max at high frequencies) unique to organic solar cells. As trap densities and occupation increase with aging and light intensity, the photocurrent contrast (i.e., maximum/steady-state photocurrent, I-max/I-DC) and the size of the low-frequency imaginary contribution increase. Substantial harmonic content underlies this trend which becomes more prominent as modulation frequencies and trap levels increase. We then use drift-diffusion simulations to describe IMPS responses and photocurrent transient signals over the entire frequency sampling window for aged devices that show excellent agreement with experiment. The results provide deeper insights into trap-related phenomena over a larger frequency bandwidth and further demonstrate the effectiveness of IMPS in its ability to identify mechanistic and kinetic details of degradation.