Photogeneration of confined soliton pairs (bipolarons) in polythiophene

From photoinduced absorption and light-induced electron-spin resonance, we demonstrate that the dominant photocarriers generated in polythiophene with excitation above the energy gap (Eg) are charged bipolarons (spin 0). The observation of bipolarons (B^2±) rather than polarons (P^±) as the dominant photoexciations demonstrates that the Coulomb contribution to the bipolaron energy (UB) is sufficiently small that P^±+P^±{→}B^±2. From the analysis of the spectra, we find UB/Eg{}0.12. .AE