Abstract
Photocarrier recombination kinetics was studied in regioregular poly(3-hexylthiophene)/methanofullerene composites by measuring the photocurrent of surface-type photocells. The photocurrent exhibited fast and persistent components. The light-intensity dependence of the photocurrent and the analysis of decay curves of the persistent component indicated the dominance of quadrimolecular recombination (QR) between two positive carriers on regioregular polymers and two negative carriers on methanofullerenes in the time range of 0.1 to 100 s. No QR but bimolecular recombination was observed in cells containing regiorandom polymers, consistent with light-induced electron spin resonance studies. These results differ markedly from those of sandwich-type cells in which photocarriers flow out of the cell prior to the recombination, demonstrating an advantage of the surface-type photocell with larger electrode spacing in studying the recombination of long-lived photocarriers. At high methanofullerene concentrations above 50 wt %, the fast component predominates over the persistent one by annealing owing to percolation of the pathway for negative carriers.