Oxygen-related traps in pentacene thin films: Energetic position and implications for transistor performance

TL;DR

This study investigates how oxygen exposure creates trap states in pentacene thin films, impairing transistor performance by reducing free carriers and mobility, and highlights the importance of understanding degradation mechanisms in organic transistors.

Contribution

It provides detailed characterization of oxygen-induced trap states in pentacene and links these traps to device performance degradation, supporting the mobility edge model for charge transport.

Findings

Oxygen creates trap states at 0.28 eV from the mobility edge.

Trap states reduce free carrier density and mobility.

Oxygen exposure degrades subthreshold performance and field-effect mobility.

Abstract

We studied the influence of oxygen on the electronic trap states in a pentacene thin film. This was done by carrying out gated four-terminal measurements on thin-film transistors as a function of temperature and without ever exposing the samples to ambient air. Photooxidation of pentacene is shown to lead to a peak of trap states centered at 0.28 eV from the mobility edge, with trap densities of the order of 10(18) cm(-3). These trap states need to be occupied at first and cause a reduction in the number of free carriers, i.e. a consistent shift of the density of free holes as a function of gate voltage. Moreover, the exposure to oxygen reduces the mobility of the charge carriers above the mobility edge. We correlate the change of these transport parameters with the change of the essential device parameters, i.e. subthreshold performance and effective field-effect mobility. This study supports the assumption of a mobility edge for charge transport, and contributes to a detailed understanding of an important degradation mechanism of organic field-effect transistors. Deep traps in an organic field-effect transistor reduce the effective field-effect mobility by reducing the number of free carriers and their mobility above the mobility edge.