Probing bulk transport, interfacial disorders, and molecular orientations of amorphous semiconductors in a thin-film transistor configuration

Thin film transistors (TFTs) can be used to determine the bulk-like mobilities of amorphous semiconductors. Different amine-based organic hole transporting materials (HTs) used in organic light-emitting diodes have been investigated. In addition, the present study also measures the TFT hole mobilities of two iridium phosphors: Ir(ppy)3 and Ir(piq)3. These materials are grown separately on SiO2 and polystyrene (PS). On SiO2, the TFT mobilities are found to be 1–2 orders smaller than the bulk values obtained by time-of-flight (TOF) technique. On PS gate dielectric layer, the TFT mobilities are in good agreement TOF data. Only 10 nm of organic semiconductor is sufficient for TFTs to achieve TOF mobilities. Using the Gaussian disorder model, it is found that on SiO2 surface, when compared to the bulk values, the energetic disorders (s) of the HTs increase and simultaneously, the high temperature limits (µ∞) of the carrier mobilities decrease. Both s and µ∞ contribute to the reduction of the carrier mobility. The increase in s is related to the presence of randomly oriented polar Si-O bonds. The reduction of µ∞ on SiO2 is related to the orientations of the more planar molecules which tend to lie horizontally on the surface.