Primary Dynamics of Exciton and Charge Photogeneration in Solvent Vapor Annealed P3HT/PCBM Films

Abstract

The primary dynamics of exciton and charge photogeneration in the neat P3HT (poly(3-hexylthiophene)) and the blend P3HT/PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) films were investigated by the use of near-infrared femtosecond transient absorption spectroscopy with varying excitation photon energy and fluence. The effects of film morphology were examined by comparing the solvent vapor annealed (SVA) and the carbon disulfide (CS2) cast films. Spectroelectrochemistry was employed to characterize the cationic polaron P3HT•+ to facilitate the assignments of the transient spectra. Time-resolved spectroscopy revealed two different types of polarons, the delocalized (DP, absorbing over 630–830 nm) and the localized (LP, 750–1100 nm) ones inhabiting the crystalline and the disordered P3HT phases, respectively. In addition, the characteristic absorption of the long-sought anionic polaron P3HT•– is proposed to be a broad-band spectrum extending up to ∼1300 nm with a maximum at ∼1080 nm. For SVA neat P3HT films under the low-fluence photoexcitation, ∼1012 photons·cm–2·pulse–1, the overall polaron yield (DP + LP) at 1.45 ns was determined to be <7% after the blue-edge excitation at 460 nm, or to be negligible after the red-edge excitation at 620 nm. For the P3HT/PCBM blends under the low-fluence photoexcitation, the prompt and the delayed polaron formation with comparable contribution are found to be independent of the excitation wavelengths; the former is governed by the instantaneous charge separation at the donor–acceptor interfaces, whereas the latter relies on the exciton diffusion in P3HT crystallites with a time constant of 7–10 ps. The effects of film morphology and photon energy on the charge photogeneration dynamics are discussed in terms of the disordered intermediate between the P3HT crystallite and the PCBM aggregate, which is suggested to be in less dimension for SVA films than that for thermally annealed films, and thereby to be advantageous in minimizing the traps or blocks of charge carriers.