Intrinsic hole mobility and trapping in a regio-regular poly(thiophene)

TL;DR

This study investigates the transport properties of a regio-regular poly(thiophene) in thin-film transistors, revealing how structural order and temperature influence mobility and trapping, with a theoretical model explaining the observed behavior.

Contribution

It introduces a comprehensive analysis of charge transport in poly(thiophene) TFTs, highlighting the role of traps and structural disorder, and compares experimental data with a mobility edge model.

Findings

Room-temperature mobility varies with processing and structural order.

Mobility is thermally activated below 200 K, with activation energy depending on charge density.

A mobility edge model with exponential traps best explains the data.

Abstract

The transport properties of high-performance thin-film transistors (TFT) made with a regio-regular poly(thiophene) semiconductor (PQT-12) are reported. The room-temperature field-effect mobility of the devices varied between 0.004 cm2/V s and 0.1 cm2/V s and was controlled through thermal processing of the material, which modified the structural order. The transport properties of TFTs were studied as a function of temperature. The field-effect mobility is thermally activated in all films at T<200 K and the activation energy depends on the charge density in the channel. The experimental data is compared to theoretical models for transport, and we argue that a model based on the existence of a mobility edge and an exponential distribution of traps provides the best interpretation of the data. The differences in room-temperature mobility are attributed to different widths of the shallow localized state distribution at the edge of the valence band due to structural disorder in the film. The free carrier mobility of the mobile states in the ordered regions of the film is the same in all structural modifications and is estimated to be between 1 and 4 cm2/V s.