In situ and Ex situ Investigation of the Organic-Organic Interface Effect

Organic-organic heterostructures have been widely applied in various organic electronic devices, including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic solar cells. A thorough understanding of the interface effect in these heterostructures is of crucial importance for device design and optimization. However, owing to the diverse chemical properties and weak van der Waals interactions of organic semiconductors, interface charge transport is critically related to the organic-organic electronic structure and environmental atmosphere. Therefore, an in situ real-time investigation of the electrical properties in vacuum could efficiently avoid atmospheric influence and aid determination of the instinct interactions at the organic-organic interface. Herein, we report in situ real-time electrical property monitoring of the pentacene/vanadyl phthalocyanine (VOPc) heterostructure with top layer pentacene growth. The hole mobility of the heterostructure transistors decreases from 0.4 cm2 center dot V-1 center dot s-1 to approximately 0.2 cm2 center dot V-1 center dot s-1, while the electron mobility increases rapidly from 0.01 cm2 center dot V-1 center dot s-1 to approximately 0.9 cm2 center dot V-1 center dot s-1 as the pentacene thickness increases. This enhanced electron transport is attributed to the interface electron transfer from pentacene to VOPc, leading to filling of trap states in the VOPc layer and an improvement in the charge mobility and n-channel current. In contrast, the ex situ processing results indicate that atmospheric exposure will significantly suppress this charge transfer effect, resulting to a negligible improvement in the electron transport. The film morphology, Kelvin probe force microscopy, and X-ray photoelectron spectroscopy characterizations suggest electron transfer occurs from pentacene to VOPc. Additionally, density functional theory (DFT) calculations confirm that the interaction between pentacene and VOPc is strong and the pentacene molecule tends to transfer electrons to VOPc with a calculated charge transfer value of approximately 0.15 e. Moreover, this interface charge transfer is significantly suppressed with the presence of either O2 or H2O, which is highly consistent with our experiment results. In this paper, we provide a clear understanding of the instinct organic-organic interface charge transfer effect by using in situ characterization, which will be helpful for further device performance optimization and analysis.