Photoexcited charge collection spectroscopy of two-dimensional polaronic states in polymer thin-film transistors

The polaronic structures associated with the interchain stacking in the polymer semiconductor bulk or polymer semiconductor-dielectric interface in fully operational thiophene-based thin-film transistors (TFTs) were investigated with a newly developed in situ photoexcited charge collection spectroscopy (PECCS). The in situ PECCS spectra of polymer semiconductors exhibited the pi-pi* absorption peaks between 2.05 and 2.45 eV, which were well-matched with those in the Ultraviolet-visible (UV-vis) absorption spectra of the corresponding polymer films. The spectra also showed associated charge-induced absorption peaks at 1.32, 1.39, 1.58, and 1.71 eV below the pi-pi* absorption edge. These charge-induced absorption peaks might be related to the delocalized polaron states located in the both polymer semiconductor bulk and semiconductor-dielectric interface of our polymer-based TFTs. From the spectral results, we were able to extract the polaronic density of states (DOS) of the polymer channel, and we confirmed that the denser interchain packing in the polymer semiconductor channel leads to a larger DOS near the band edge, resulting in higher field-effect mobility.