Photogeneration of confined soliton pairs (bipolarons) in polythiophene

Abstract

From photoinduced absorption and light-induced electron-spin resonance, we demonstrate that the dominant photocarriers generated in polythiophene with excitation above the energy gap (${E}_{g}$) are charged bipolarons (spin 0). The observation of bipolarons (${B}^{2\ifmmode\pm\else\textpm\fi{}}$) rather than polarons (${P}^{\ifmmode\pm\else\textpm\fi{}}$) as the dominant photoexciations demonstrates that the Coulomb contribution to the bipolaron energy (${U}_{B}$) is sufficiently small that ${P}^{\ifmmode\pm\else\textpm\fi{}}$+${P}^{\ifmmode\pm\else\textpm\fi{}}$\ensuremath{\rightarrow}${B}^{\ifmmode\pm\else\textpm\fi{}2}$. From the analysis of the spectra, we find ${U}_{B}$/${E}_{g}$\ensuremath{\simeq}0.12. .AE