Charge Generation and Recombination Dynamics in Poly(3-hexylthiophene)/Fullerene Blend Films with Different Regioregularities and Morphologies

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Abstract
The charge generation and recombination dynamics in blend films of a poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM) were comprehensively studied by transient absorption spectroscopy in the wavelength region from 450 to 1650 nm under various excitation intensities and different excitation wavelengths. In homogeneously mixed blend films of regiorandom P3HT (RRa-P3HT) and PCBM, virtually all the excitons can reach the interface of RRa-P3HT/PCBM and then form bound radical pairs. However, two-thirds of them geminately recombine to the ground state, and only one-third of them can be dissociated into free polarons that survive up to milliseconds. In phase-separated blend films of regioregular P3HT (RR-P3HT) and PCBM, almost all the excitons can reach the interface of RR-P3HT/PCBM, where most of them can be dissociated into free polarons efficiently and the rest of them form bound radical pairs. There are two pathways for the polaron generation: the prompt formation from hot excitons generated near the interface on a time scale of <100 fs and the delayed formation via the exciton migration to the interface on a time scale of ∼10 ps. The thermal annealing improves the charge dissociation efficiency of bound radical pairs. On the basis of such spectroscopic data, a series of fundamental photovoltaic conversion processes are quantitatively analyzed. Consequently, it is concluded that there is not much difference in the charge generation yield between RRa-P3HT/PCBM(50 wt %) and RR-P3HT/PCBM(50 wt %) blend films. Rather, the charge dissociation and collection have a critical impact on the overall device performance of P3HT/PCBM solar cells, where the phase-separated blend structures have a high tendency to form free carriers and transport these free carriers to the electrode.

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