Hole Relaxation in Polymer:Fullerene Solar Cells Examined by the Simultaneous Measurement of Time-of-Flight and Time-Resolved Microwave Conductivity

Abstract

To comprehensively examine charge carrier transport and relaxation, we report a new technique combining time-of-flight (TOF) and time-resolved microwave conductivity (TRMC) using a harmonic cavity. The TOF analysis affords long-range hole mobilities of 10–4–10–2 cm2 V–1 s–1 for polymer:methanofullerene bulk heterojunction (BHJ) films (P3HT, PffBT4T, and PCPDTBT blended with PCBM), while TRMC transients are simultaneously recorded under an external electric field. The latter exhibits the acceleration of decay with increasing bias voltage. By analyzing the transient photocurrents and TRMC decays based on a diffusion theory with bulk charge recombination, hole relaxation is observed as a function of time (nanoseconds to microseconds) or distance (∼micrometers). Contrasting results are found among the BHJ films, which are consistent with the optimal thickness of the organic solar cells and provide the basis to interpret charge carrier dynamics from the spatiotemporal viewpoint.