High‐Performance Bottom‐Contact Organic Thin‐Film Transistors with Controlled Molecule‐Crystal/Electrode Interface

Abstract

Organic thin-film transistors (OTFTs) are emerging as an inexpensive alternative to amorphous silicon devices because of their many attractive features, such as a simple fabrication process, low cost, and mechanical flexibility. Recently, the performance of OTFTs has been significantly improved by modifying the dielectric surface and/or source and drain (S/D) electrodes, [1] or by using novel dielectric materials. [2][3][4][5] However, high-performance OTFTs were mostly achieved in top-contact (TC) configurations [6] and not in bottom-contact (BC)configurations. As organic active materials are sensitive to chemical wet and ion-beam processes, shadow-mask evaporation is usually used to form S/D electrodes in TC configurations. Such a process, however, is incompatible with largescale integration and does not allow one to produce a channel length shorter than a few tens of micrometers. It is therefore desirable to use a BC configuration, which is compatible with fine lithography processing, for promising applications such as high-resolution flexible displays. The performance of OTFTs is mainly dependent on i) molecular ordering in the organic active layer, ii) charge-injection ability of the source electrode, and iii) charge transport at the interface between the organic active layer and the dielectric layer; the inferior performance of BC-OTFTs compared with TC counterparts is believed to mostly derive from factors (i) and (ii). The step formed by S/D electrodes, in particular thick S/D electrodes on a dielectric layer, disturbs the continuous growth of a single-phase domain in the organic active layer. [7] Additionally, electrode edges with a relatively irregular shape may cause disordered growth of organic crystals adjacent to the electrodes. Despite this, not much effort is required to overcome the inherent drawbacks. A simple approach to completely eliminate the electrode-induced effects in BC-OTFTs is to embed the S/D electrodes in the dielectric layer and planarize the S/D electrodes, making the S/D electrodes and dielectric layer coplanar.In this Communication, we report, for the first time, planarized BC-OTFTs (pBC-OTFTs) with S/D electrodes embedded in the dielectric layer (Fig. 1a); this led to a preferential orientation of organic crystals adjacent to the S/D electrodes and a continuous growth of organic crystals across the S/D edges. Our pentacene pBC transistors on SiO 2 /Si showed a superior performance in controlling conventional BC-OTFTs (cBC-OTFTs) (Fig. 1d) and TC-OTFTs, and the mobility was comparable with the best value ever reported for TC-OTFTs made with similar materials.We investigated bottom-gate pBC-OTFTs and cBC-OTFTs built on a SiO 2 /Si platform. In the present study, Pt (ca. 31 nm)/Cr (ca. 2 nm) were used as the S/D electrodes, and pentacene thin films, fabricated by using the molecularbeam-deposition technique, [8,9] were used as the active organic semiconductor. The width and length of our transistors was 5000 and 22.9 lm, respectively. We tested eight devices for each type of transi...

Enhanced performance of solution-processed regioregular poly(3-hexylthiophene) thin-film transistors using planar bottom-contact architecture

“…Very recently, to improve electrical contact ͑carrier injection͒ between source/drain electrodes and organic active layer of bottom-contact ͑BC͒ OTFTs, we have reported a planar bottom-contact ͑pBC͒ configuration for high-performance OTFTs. 6 By embedding and planarizing the source/drain ͑S/D͒ electrodes in a gate dielectric layer, our pentacene pBC transistors showed superior performance to the control conventional BC ͑cBC͒ devices, 6 and the mobility is comparable to the best reported value of top-contact transistors without surface modification owing to a reduced contact resistance in the pBC architecture.…”

Inkjet-printed copper electrodes using photonic sintering and their application to organic thin-film transistors

“…8d, the grain sizes of pentacene on the damaged BCB polymer layer were also relatively small, compared with that on the BCB polymer at distances further from the Cu electrode. These results indicate that the contact resistance of pentacene TFT was increased as a result of thermal damage to the BCB polymer layer near the edges of the electrode, because both the grain size of semiconducting pentacene layer and the roughness of the BCB polymer underlayer influence charge transport [22,23]. Therefore, thermal damage to the polymer underlayer must be further suppressed to maximize transistor performance.…”

Impact of semiconductor/metal interfaces on contact resistance and operating speed of organic thin film transistors

“…However, bottom contact OTFTs have better compatibility for large scale integration [40]; therefore, bottom contact OTFT devices with low contact resistance must be developed. For example, several studies report that irregularly shaped electrode edges might cause disturbances on the continuous growth of organic crystals adjacent to the electrodes [20,39,40], causing large contact resistance. In this work both OTFT experimental and numerical investigations predicting contact resistances between gold S:D electrodes and the pentacene channel are presented.…”

“…In general, top contact OTFTs have lower contact resistance than bottom contact OTFTs [39]. However, bottom contact OTFTs have better compatibility for large scale integration [40]; therefore, bottom contact OTFT devices with low contact resistance must be developed. For example, several studies report that irregularly shaped electrode edges might cause disturbances on the continuous growth of organic crystals adjacent to the electrodes [20,39,40], causing large contact resistance.…”