Elsevier

Organic Electronics

Volume 13, Issue 12, December 2012, Pages 2997-3003
Organic Electronics

UV-curable organic–inorganic hybrid gate dielectrics for organic thin film transistors

https://doi.org/10.1016/j.orgel.2012.08.014Get rights and content

Abstract

UV-curable hybrid thin films were prepared from ZrO2 hybrid sols containing the acrylic monomer, DPHA, on substrates. The prepared ZrO2 hybrid sols showed long-term storage stability. Hybrid dielectrics were prepared by sol–gel process and UV cross-linking below 160 °C. Leakage currents of dielectric layers remained below 10−6 A in 2 MV/cm and dielectric constants were measured to be 3.85–4 at 1 kHz. In addition, organic–inorganic hybrid thin films have smooth and hydrophobic surface. Pentacene OTFTs with thin hybrid dielectrics exhibit of mobility as large as 2.5 cm2/V s, subthreshold swing as low as 0.2 V/decade, an on–off ratio of 105. These results demonstrated that UV-curable sol–gel hybrid systems are suitable for gate dielectrics in OTFTs.

Highlights

► Inorganic–organic hybrid materials were synthesized by a sol–gel method. ► The microstructure and dielectric properties of hybrid films were studied. ► The hybrid dielectric layer exhibited high performance characteristics. ► High mobility of 2.5 cm2/V s and on/off ratios (>105) with OTFT were achieved.

Introduction

Organic field-effect transistors (OFETs) are attractive building blocks for low-cost electronic devices such as radio-frequency identification (RFID) tags, sensors, electronic paper, and backplane circuits for active-matrix displays [1], [2], [3], [4], [5], [6]. Recent interest has also been directed towards the development of reliable solution processable gate dielectrics, since the electrical performances of OFETs are known to largely depend on the material properties of gate dielectrics and organic semiconductors [7], [8]. To this end, thermally curable polymer gate dielectrics such as poly(vinyl phenol) (PVP) [9] and poly(methylmethacrylate) (PMMA) [10] are expected to be promising candidates for this process. However, polymer materials are not appropriate for use in low voltage applications due to their low dielectric constants. The development of new low-temperature processable gate dielectrics is also crucially important for fabricating electrically stable and printable OFETs on flexible substrates.

Organic–inorganic hybrid material represents a new class of materials that may combine the desirable physical properties of both organic and inorganic components within a single composite. The gate dielectrics with inorganic/organic bilayer insulator [11], organic–inorganic composite materials [12], hybrid organic/inorganic materials [13], [14], [15] lead to gate capacitance enhancement for accumulating enough carriers in the channel at low voltage. But they suffer from a lack of uniform dispersion and stability which results in high leakage current.

Among various inorganic dielectric materials, ZrO2 is an attractive candidate as gate dielectric material since it has a high dielectric constant (15–22), a high breakdown field (15–20 MV/cm), and may be thermodynamically stable on silicon. With vacuum deposition technique, ZrO2 dielectric layer with self-assembled buffer layer was reported by Lee et al. [16]. Unlike TiO2, however, ZrO2 film by solution deposition using zirconium alkoxide is very difficult to control and suffers from large leakage current according to our preliminary result. To circumvent this problem, we previously synthesized zirconium-based inorganic–organic hybrid material through sol–gel condensation which may be cured by UV irradiation under ambient conditions, eliminating the need for a high temperature annealing [15]. However, despite of the novel design, pentacene-based OTFT performance was not as good as expected.

In this paper, acrylic monomer, dipentaerythritol penta-/hexa-acrylate (DPHA), was incorporated in zirconium-based inorganic–organic hybrid material, which may induce photo-crosslinking easily. The dielectric constant and leakage current behaviors of solution processed films with/without DPHA were examined in terms of their applications as gate insulators. Furthermore, using the corresponding film layers, the switching property of organic TFTs and the performance of hybrid gate materials were also examined in order to gain an insight of hybrid material design.

Section snippets

Materials

All reagents were analytical reagents. Aqueous hydrochloric acid (37%), 1-butanol (99.5%) and n-propanol were purchased from Duksan pure chemicals (Korea). Zirconium n-propoxide (ZPO), dipentaerythritol penta-/hexa-acrylate (DPHA) and methacrylic acid (MA) were purchased from Sigma–Aldrich (USA). Tetrahydrofuran (THF) was purchased from J.T. Baker. Irgacure 184 of photoinitiators was supplied by Miwon (Korea). All materials were used as received.

Preparation and characterization of films

The synthetic procedure of [Zr(OPrn)3

Results and discussion

The chemical structure of the hybrid inorganic–organic sol–gel solution is given in Scheme 1. It contains photosensitive, polymerizable organic groups upon UV light irradiation. This homogeneous solution is stable in ambient conditions, and easily applied in the fabrication of OTFTs. Compare to ZrM2HD solution, ZrM1HD solution has better self-life stability. This may be explained since the olefin moiety of ZrM1HD is half of that of ZrM2HD.

Fig. 1 presents IR spectra of ZrM1 on KBr pellet and the

Conclusion

We have fabricated pentacene organic TFTs with hybrid gate dielectric materials. The sol–gel derived hybrid dielectric insulator can be cured at sufficiently low enough temperature to be used on temperature-sensitive polymeric substrates, whilst maintaining good electrical properties. Tuning with crosslinking agent, DPHA, pentacene-based OTFT devices showed enhanced carrier mobility, low threshold voltage, low sub-threshold swing, and large on/off current ratio. These results demonstrate that

Acknowledgment

This study was supported by the Ministry of Knowledge Economy, Korea (Strategic Technology Project # 10031791).

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