Effects of film microstructure on the bias stability of pentacene field-effect transistors

Abstract

In this report, the effects of film microstructure on the bias stability of pentacene field-effect transistors (FETs) were investigated. To control the microstructure of pentacene film, substrate temperature was changed from 25 to 90 °C during pentacene deposition. As the substrate temperature increased, pentacene grain size increased (or grain boundary (GB) decreased) because of the elevated surface diffusion of pentacene molecules. Accordingly, field-effect mobility increased up to 1.52 cm2/V. In contrast, bias stability showed totally different characteristics: samples prepared at high substrate temperatures exhibited the lowest degree of bias stability. This GB independent charge trapping phenomenon was solved by examining molecular scale ordering within the intragrain regions. The pentacene film grown at 90 °C showed the largest percentage of pentacene molecules with bulk crystalline structures. This inhomogeneity in the pentacene microstructure induces crystal mismatch within intragrain region, thereby providing deep trap sites for gate-bias stress driven instability. Our study shows that GB is not the main sites for bias stress related charge trapping, rather the molecular orientation within intragrain region is responsible for the charge trapping events. In this regard, the control of molecular scale ordering is important to obtain OFETs with a high bias stability.