On the origin of the quadrant I semicircle in intensity-modulated photocurrent spectra of P3HT:PCBM Bulk heterojunction solar cells: Evidence of degradation-related trap-assisted recombination

Abstract

Intensity-modulated photocurrent spectroscopy (IMPS) characterizes the charge carrier transport in a solar cell by subjecting it to a sinusoidally varying illumination and measuring the resultant photocurrent. The IMPS spectra of certain P3HT:PCBM bulk heterojunction solar cells exhibit a poorly understood feature: A quadrant I semicircle, which implies that the sinusoidal variation of the photocurrent leads that of the illumination in a certain frequency range. To understand the mechanisms underlying this feature, we have adopted a mathematical framework based on drift-diffusion modeling instead of the conventional equivalent circuit approach. By incorporating the effects of traps into the mathematical model and conducting targeted experiments, we have demonstrated that the quadrant I semicircle arises from degradation-related trap states behaving as recombination centers. Our analysis of the device's intrinsic mechanisms indicates that the trap-assisted recombination can have a time scale that is at least one order of magnitude slower than that of the carrier extraction mechanism; such a mismatch can generate the observed quadrant I semicircle. © 2013 American Chemical Society.