Achieving Ultralow Turn-On Voltages in Organic Thin-Film Transistors: Investigating Fluoroalkylphosphonic Acid Self-Assembled Monolayer Hybrid Dielectrics.

Rachana Acharya,Boyu Peng,Paddy K L Chan,Hagen Klauk,Guido Schmitz

doi:10.1021/acsami.9b04361

Abstract

The properties of organic thin-film transistors (TFTs) and thus their ability to address specific circuit design requirements depend greatly on the choice of the materials, particularly the organic semiconductor and the gate dielectric. For a particular organic semiconductor, the TFT performance must be reviewed for different combinations of substrates, fabrication conditions, and the choice of the gate dielectric in order to achieve the optimum TFT and circuit characteristics. We have fabricated and characterized organic TFTs based on the small-molecule organic semiconductor 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene in combination with an ultrathin hybrid gate dielectric consisting of aluminum oxide and a self-assembled monolayer. Fluoroalkylphosphonic acids with chain lengths ranging from 6 to 14 carbon atoms have been used to form the self-assembled monolayer in the gate dielectric, and their influence on the TFT characteristics has been studied. By optimizing the fabrication conditions, a turn-on voltage of 0 V with an on/off current ratio above 106 has been achieved, in combination with charge-carrier mobilities up to 0.4 cm2/V s on flexible plastic substrates and 1 cm2/V s on silicon substrates.

Highlights

The development of organic thin-film transistors (TFTs) is important for potential portable and wearable electronic applications.[1,2] In order to ensure portability and safe handling of the systems, it is necessary that their power consumption is sufficiently low so they can be powered by solar cells or small batteries
A striking difference is observed between TFTs fabricated on silicon and on polyethylene naphthalate (PEN) in the way the morphology of the DPh-BTBT layer and the TFT performance are affected by the substrate temperature during the semiconductor deposition
As seen in Figure 2a−c, the TFTs fabricated on silicon have carrier mobilities of 0.56 cm2/V s when the substrate is held at a temperature of 25 °C during the DPh-BTBT deposition and 1.1 cm2/V s when the DPh-BTBT is deposited at a substrate temperature of 100 °C

Summary

Introduction

The development of organic thin-film transistors (TFTs) is important for potential portable and wearable electronic applications.[1,2] In order to ensure portability and safe handling of the systems, it is necessary that their power consumption is sufficiently low so they can be powered by solar cells or small batteries. This requires that the TFTs employed in such devices have low operating voltages. The turn-on voltage of organic TFTs can be controlled by employing gate dielectrics based on mixed SAMs with different combinations of alkylphosphonic acids and fluoroalkylphosphonic acids.[10−14] Kraft et al used fluoroalkylphosphonic acids to shift the threshold voltage of both pchannel and n-channel organic TFTs by about 1 V due to their strong electron-withdrawing character as compared to the Received: March 14, 2019 Accepted: July 3, 2019 Published: July 3, 2019

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