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 E sub g are charged bipolarons (spin zero). The observation of bipolarons B superscript 2 + or - rather than polarons P superscript + or - as the dominant photoexcitations demonstrates that the Coulomb contribution ot the bipolaron energy U sub B is sufficeitly small that P superscript + or + P superscrit + or - yields B superscript + or -2. From the analysis of the spectra, we find U sub B E sub g yields 0.12. approx.