Low-bandgap conjugated polymers, which have an alternating structure of electron-donating and electron-withdrawing monomer units in their main chains, are expected to show high photovoltaic efficiency when the monomer units are appropriately designed. For understanding key factors of the photovoltaic efficiency, we have investigated the structure and dynamics of short-lived transients in films of a low-bandgap conjugated polymer with an alternating structure of thieno[3,4-b]thiophene and benzo[1,2-b:4,5-b′]dithiophene, PTB7, by time-resolved near-infrared stimulated Raman spectroscopy as well as time-resolved near-infrared absorption spectroscopy. Positive polarons of PTB7 are clearly distinguished from its singlet excitons in stimulated Raman spectra by the peak position of a CC stretch band of the thiophene rings in benzodithiophene and intensity of a C[sbnd]S[sbnd]C deformation band of them. These differences strongly suggest that the repeated structure of thiophene rings in the main chain predominantly contributes to the charge transfer characteristics of PTB7 in films. Time-resolved absorption and stimulated Raman spectra of PTB7 films blended with a fullerene derivative show that spectral changes associated with the initial charge separation complete within 1 ps while effects of conformational changes and intermediate species are absent. We suggest that alternating copolymers composed of electron-donating and electron-accepting units commonly undergo the initial charge separation process without conformational changes or intermediates.
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