Abstract
We analyze the influence of three combined effects on the contact resistance in organic- based thin film transistors: a) the active layer thickness, b) device architecture and c) semiconductor degradation. Transfer characteristics and parasitic series resistance were analyzed in devices with three different active layer thicknesses (50, 100 and 150 nm) using top contact (TC) and bottom contact (BC) thin film transistor (TFT) configurations. In both configurations, the lowest contact resistance (2.49 × 106 ?) and the highest field-effect mobility (4.8 × 10-2 cm2/V·s) was presented in devices with the thicker pentacene film. Top contact thin film transistors presented field-effect mobility values one order of magnitude higher (4.8 × 10-2 cm2/V·s) than bottom contact ones (1 × 10-3 cm2/V·s). Threshold voltage for top-contact thin film transistors was -3.1 V. After 2 months, performance in the devices degraded and presented an increase of one order of magnitude (105 - 106 ?) for BC-TFTs and two orders of magnitude (106 - 108 ?) for TC-TFTs in contact resistance.
Highlights
Organic thin film transistors (OTFTs) have attracted interest for large area electronic applications due to the compatibility with low cost and low temperature fabrication processes
ID is higher for BC-TFTs and the devices remained always on, which is the result of a less resistive path for the current between source and drain compared to TC-TFTs configuration
Calculated VT for BC-TFTs is more positive compared to TC-TFTs, due to the higher channel and bulk conductivity resulting from the higher carrier injection and the direct contact between the accumulation layer and the source-drain (S-D) contacts that does not allow to completely close the channel [19]
Summary
Organic thin film transistors (OTFTs) have attracted interest for large area electronic applications due to the compatibility with low cost and low temperature fabrication processes. TC-TFTs have exhibited contact resistance (Rc) of about one order of magnitude higher than BC-TFTs [8] and Rc dependence on the pentacene/Au thickness ratio [9] Another important effect limiting the OTFT performance is the degradation of the active layer when exposed to ambient, as a result of oxygen diffusion in the bulk of the material. Researchers have analyzed the influence of the pentacene degradation in the performance of the OTFTs, they only focus in the main parameters such as μ and drain current (ID) [13,14] In this reports, it has been neglected other important parameters of importance; for example, the parasitic resistance.
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