(Invited) Electrical and Structural Origin of Self-Healing Phenomena in Pentacene Thin Film Transistors

Abstract

Despite the great potential for flexible large-area applications in organic electronics, electrical and structural instabilities have been widely regarded as an unavoidable weakness of organic semiconductors. Electronic components made of organic molecules are considered more fragile and sensitive to external conditions than their inorganic counterparts and the lifetime of them is shorter than that of conventional devices. Despite these shortcuts it is even more surprising that organic field-effect transistors (FETs) can undergo a self-healing process, i.e. their electrical characteristics can counter-intuitively improve over time, due to the unstable nature of weakly bound molecules. This remarkable phenomenon has never been understood completely. In this work, we demonstrate that the self-healing in pentacene FETs is induced by a structural phase transformation and can be triggered by various extrinsic or intrinsic influences. We evaluate in detail the necessary conditions for electrical self-healing in pentacene FETs and reveal the structure-property relation at the heart of this phenomenon: a structural phase transformation from the metastable monolayer phase to the so called “thin film phase”. The transformation basically involves a tilt of the molecules against the surface normal near both interfaces of pentacene thin films (pentacene-dielectric and metal-pentacene) and has a profound influence on the electronic properties of pentacene FETs, which we demonstrate by way of electronic structure calculations using DFT and semi-classical transport theory. Finally, possible external triggers able to initiate the self-healing process are discussed. A case study adopting various fabrication procedures reveals that external triggers such as nitrogen gas or other organic compounds seem to be necessary to initiate the self-healing process.