Absence of Postnanosecond Charge Carrier Relaxation in Poly(3-hexylthiophene)/Fullerene Blends

In this work, the relaxation of positive charges and charge carrier recombination in blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are studied by combining time-resolved photoinduced optical absorption and microwave photoconductance measurements. The decay kinetics of both techniques are identical at temperatures > 150 K over a broad time window (50 ns-100 mu s). An important conclusion from these observations is that the average mobility of the positive charges remains constant, and relaxation into deeper traps does not occur. This is in contrast to the idea that charges are rapidly localized in trap states and subsequent thermal release results in progressively deeper trapping, which reduces the mobility. At 300 K, a lower limit of the local hole mobility of 0.012 cm(2)/(V s) is found combined with a lifetime on the order of 1 mu s.