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 ($B^{2\pm }$) rather than polarons ($P^{\pm }$) as the dominant photoexciations demonstrates that the Coulomb contribution to the bipolaron energy ($U_B$) is sufficiently small that $P^{\pm }$+$P^{\pm }$→$B^{\pm 2}$. From the analysis of the spectra, we find $U_B$/$E_g$≃0.12. .AE

• Received 1 October 1985

DOI:https://doi.org/10.1103/PhysRevLett.56.671