Optical spectroscopy of field-induced charge in poly(3-hexyl thienylene) metal-insulator-semiconductor structures: Evidence for polarons

We have fabricated metal-insulator-semiconductor structures with poly(3-hexyl thienylene) as the active semiconductor. Optical-absorption bands due to injected carriers seen near 0.4, 1.80, and 2.16 eV are assigned to the optical transitions of the singly charged polaron, which is expected to be the thermodynamically favored excitation under these experimental conditions. Additional absorption bands at 0.5 and 1.18 eV are assigned to optical transitions of doubly charged bipolarons. We contrast the strong confinement deduced for the polaron (gap states separated by 1.80 eV) with the weak confinement observed for bipolarons (gap states separated by 0.7 eV).