Calculating the trap density of states in organic field-effect transistors from experiment: A comparison of different methods

TL;DR

This study compares various analytical methods and simulations to accurately determine the trap density of states in pentacene-based transistors, highlighting the impact of assumptions on the results.

Contribution

It systematically evaluates different calculation methods for trap DOS in organic transistors and identifies key assumptions affecting accuracy.

Findings

Most methods yield a trap DOS near the valence band edge with a slope of 50-60 meV.

Trap DOS is consistently slightly steeper than an exponential.

Method choice significantly influences the calculated trap DOS, with certain assumptions causing errors.

Abstract

The spectral density of localized states in the band gap of pentacene (trap DOS) was determined with a pentacene-based thin-film transistor from measurements of the temperature dependence and gate-voltage dependence of the contact-corrected field-effect conductivity. Several analytical methods to calculate the trap DOS from the measured data were used to clarify, if the different methods lead to comparable results. We also used computer simulations to further test the results from the analytical methods. Most methods predict a trap DOS close to the valence band edge that can be very well approximated by a single exponential function with a slope in the range of 50-60 meV and a trap density at the valence band edge of approx. 2x10E21 eV-1cm-3. Interestingly, the trap DOS is always slightly steeper than exponential. An important finding is that the choice of the method to calculate the trap DOS from the measured data can have a considerable effect on the final result. We identify two specific simplifying assumptions that lead to significant errors in the trap DOS. The temperature-dependence of the band mobility should generally not be neglected. Moreover, the assumption of a constant effective accumulation layer thickness leads to a significant underestimation of the slope of the trap DOS.