An origin of the irreproducibility of hole injection barrier from Au top-contact electrodes and its influence on device performance in top-contact organic field-effect transistors.

To understand the origin of the irreproducibility of the hole injection barrier at the interface between Au top-contact electrodes and organic semiconductors, we performed a systematic study by focusing on the deposition rate of Au. As a benchmarking material system, we fabricated field-effect transistors with pentacene active layers and Au top-contact electrodes evaporated with the deposition rate from 0.5 to 10 Å/s. From the contact potential differences (Δ CPDs) between Au and pentacene measured with Kelvin-probe force microscopy, cross-sectional structures with scanning transmission electron microscopy, crystallographic structures with X-ray diffraction, and Au depth profiles with secondary ion mass spectrometry, we concluded that the variation of Δ CPD is due to embedded Au nanoclusters in pentacene with the average diameter of 4–6 nm. These Au nanoclusters, of which depth significantly varies by the Au deposition rate, modulate the Δ CPD and interfere with the reproducibility of hole injection barrier. As a result, gate threshold voltage and field-effect mobility also become irreproducible. Image 100 • Band alignment between Au electrodes and pentacene was studied to understand the variation of the hole injection barrier. • Diffusion of Au atoms into the pentacene layer and cluster formation is sensitive to the Au deposition rate. • Variation of the contact potential difference is concluded to be due to the Au clusters embedded in the pentacene layer. [ABSTRACT FROM AUTHOR]