The trap DOS in small molecule organic semiconductors: A quantitative comparison of thin-film transistors with single crystals

TL;DR

This study compares trap densities in small molecule organic semiconductors across different sample types, revealing structural defects and surface effects as key factors influencing trap states and device performance.

Contribution

It provides a comprehensive quantitative comparison of trap DOS in thin-film transistors, single crystals, and bulk samples, highlighting the impact of structural defects and surface conditions.

Findings

Grain boundaries are main cause of fast hole traps in vacuum-evaporated pentacene TFTs.

Reducing water-induced dipolar disorder improves transistor performance.

Trap DOS near the mobility edge shows a steep increase with a characteristic slope of 10-20 meV.

Abstract

We show that it is possible to reach one of the ultimate goals of organic electronics: producing organic field-effect transistors with trap densities as low as in the bulk of single crystals. We studied the spectral density of localized states in the band gap (trap DOS) of small molecule organic semiconductors as derived from electrical characteristics of organic field-effect transistors or from space-charge-limited-current measurements. This was done by comparing data from a large number of samples including thin-film transistors (TFT's), single crystal field-effect transistors (SC-FET's) and bulk samples. The compilation of all data strongly suggests that structural defects associated with grain boundaries are the main cause of "fast" hole traps in TFT's made with vacuum-evaporated pentacene. For high-performance transistors made with small molecule semiconductors such as rubrene it is essential to reduce the dipolar disorder caused by water adsorbed on the gate dielectric surface. In samples with very low trap densities, we sometimes observe a steep increase of the trap DOS very close (<0.15 eV) to the mobility edge with a characteristic slope of 10-20 meV. It is discussed to what degree band broadening due to the thermal fluctuation of the intermolecular transfer integral is reflected in this steep increase of the trap DOS. Moreover, we show that the trap DOS in TFT's with small molecule semiconductors is very similar to the trap DOS in hydrogenated amorphous silicon even though polycrystalline films of small molecules with van der Waals-type interaction on the one hand are compared with covalently bound amorphous silicon on the other hand.