Charge-induced dephasing in thin polythiophene films

The third-order nonlinear susceptibility X(3) normalized to the absorption coefficient a displays a dramatic drop at 0.4 eV above the S0-S1 transition in polythiophene derivatives. Steady-state photoconductivity data show the intrinsic photocrent starting exactly where the decrease in X(3)/α is seen. These results point to a change from bound excitons to free-electron hole pairs at 0.4 eV above the S0-S1 transition, and support the description of such disordered conjugated polymers using a molecular approach rather than as infinite one-dimensional semiconductors.