Ultra-thin and graded sliver electrodes for use in transparent pentacene field-effect transistors

The authors report the fabrication of efficient and transparent pentacene field-effect transistors (FETs) using a graded structure of ultra-thin silver (Ag) source and drain (S-D) electrodes. The S-D electrodes were prepared by thermal evaporation with a controlled deposition rate to form Ag layer with a graded structure, leading to a reduced injection barrier and smoothing the contact surface between the electrode and the pentacene channel. The sheet resistance of such Ag electrode was found to be as low as 9 Omega/sq. In addition, a hole-only behavior of device with Ag electrode characterized by current-voltage measurement and conductive atomic-force microscopy shows the injection property of high current flowing as compared with device using Au electrode, resulting in an efficient injection condition existing at the interface of the graded Ag/pentacene. Device characterization indicates the transparent pentacene FET with a graded ultra-thin Ag electrode and organic capping layer of N, N'-di(1-naphthyl)-N, N'-diphenylbenzidine exhibits a high transmission rate of similar to 75% in the range of visible light from 400 to 550 nm, a threshold voltage of -6.0 V, an on-off drain current ratio of 8.4 x 10(5), and a field-effect mobility of 1.71 cm(2)/V s, thus significantly outperforming pentacene FETs with multilayer oxide electrodes or other transparent thin metal layers. (C) 2014 Elsevier B.V. All rights reserved.