Subnanosecond Charge Recombination Dynamics in P3HT/PC61BM Films

Abstract

Ultrafast near-infrared absorption spectroscopy was used to investigate the influence of film morphology and excitation photon energy on the charge recombination (CR) dynamics in the initial nanosecond timescale in the P3HT/PC61BM blend films. With reference to the CS2-cast films, the solvent vapor annealed (SVA) ones show 2-3-fold improvement in hole mobility and more than 5-fold reduction in the polymer-localized trap states of holes. At Dt = 70 ps, the hole mobility (mh) and the bimolecular CR rate (gbi) of the SVA films are mh = 8.7 × 10-4 cm2×s-1×V-1 and gbi = 4.5 × 10-10 cm3×s-1, whereas at Dt = 1 ns they drop to 8.7 × 10-5 cm2×s-1×V-1 and 4.6 × 10-11 cm3×s-1, respectively. In addition, upon increasing the hole concentration, the hole mobility increases substantially faster under the above-gap photoexcitation than it does under the band-gap photoexcitation, irrespective of the film morphologies. The results point to the importance of utilizing the photogenerated free charges in the early timescales.