Photocurrent transients in polymer-fullerene bulk heterojunction organic solar cells

By measuring the transient photocurrents we study the charge carrier dynamics in regioregular poly(3-hexylthiophene) (P3HT): 6,6-phenyl-C61-butyric acid methyl ester (PCBM), and poly(3-hexylthiophene) thiophene thienopyrazine (P3HTT-TP):PCBM bulk heterojunction (BHJ) solar cells. The latter is based on a novel “semi-random” copolymer and exhibits broad spectral absorption. We observe that the charge carrier transport in P3HTT-TP:PCBM is more imbalanced than that of P3HT:PCBM. Voltage dependent photocurrent transients indicate that P3HT:PCBM has longer carrier lifetimes than P3HTT-TP:PCBM. Light intensity dependent open-circuit voltage measurements show stronger monomolecular recombination in P3HTT-TP:PCBM. The unfavorable charge carrier transport may be one of the reasons for the low power conversion efficiency of P3HTT-TP:PCBM BHJ solar cells.