Elsevier

Organic Electronics

Volume 13, Issue 12, December 2012, Pages 2786-2792
Organic Electronics

Synthesis and characterization of a fluorinated oligosiloxane-containing encapsulation material for organic field-effect transistors, prepared via a non-hydrolytic sol–gel process

https://doi.org/10.1016/j.orgel.2012.08.025Get rights and content

Abstract

The operation of stable organic field-effect transistors (FETs) over long periods of time requires that organic FETs are encapsulated. We synthesized an inorganic–organic hybrid non-hydrolytic sol–gel material (TPDt) containing fluoroalkyl functional groups to encapsulate organic FETs. Fourier-transform infrared spectroscopy, atomic force microscopy, UV–Visible spectroscopy, and water contact angle measurements demonstrated that the TPDt films displayed smooth surfaces, good hydrophobicity, and optical transparency. The gas barrier properties of the TPDt films were tested by fabricating FETs using an organic semiconductor, poly[9,9-dioctylfluorenyl-2,7-diyl]-co-(bithiophene)]. The organic FETs were operated at 38 °C in the presence of a 90% relative humidity air atmosphere. The field-effect mobility of the organic FET decreased only negligibly, even after 2500 h operation under these conditions.

Highlights

► We synthesized an inorganic–organic hybrid material (TPDt) to encapsulate OFETs. ► TPDt, prepared via a non-hydrolytic sol–gel process, contains fluoroalkyl groups. ► TPDt film showed smooth surface, hydrophobicity, transparency and low gas permeability. ► TPDt with fluorine showed better film properties than without fluorine. ► The field-effect mobility of the OFETs with TPDt decreased only negligibly.

Introduction

Organic field-effect transistors (FETs) have attracted considerable interest in recent years due to their potential utility for the fabrication of low cost, flexible, large-area electronic devices [1], [2]. Despite significant investigational effort, several practical challenges have persisted, presenting a barrier to the commercial application of organic FETs. The electrical characteristics of organic FETs can degrade significantly over time due to the presence of water vapor and oxygen species in ambient air [3], [4], [5], [6]. FET performance stability may be improved in the presence of water vapor and oxygen by introducing a passivation layer. Thus far, passivation layers have been applied using vacuum deposition processes. Although inorganic thin film passivation layers and polymer/inorganic multilayers prepared by atomic layer deposition or chemical vapor deposition processes provide good protection barriers against water vapor and oxygen, long processing times and high costs prohibit their widespread commercial use [7], [8], [9], [10].

Passivation via solution deposition processes, on the other hand, could potentially be simple, low-cost, and facile, requiring only a short processing time. Solution-processed passivation layers tend to provide poor barrier properties relative to vacuum-deposited films due to the large numbers of pinhole or defects present in organic films [11]. Because exposure to solvents is unavoidable during the sequential application of multiple passivation layers via solution-processes [12], [13], it is important to select a passivation material that will not damage the active layers in the presence of solvent and will yield relatively good barrier properties.

Recently, solution-processable passivation materials, such as poly (vinyl alcohol) [14], CYTOP [15], TiOx [16], and inorganic–organic hybrid materials [17], [18], [19], have been developed. Sol–gel processable inorganic–organic hybrid materials are attractive for use in solution-processing organic electronics passivation layers. In a previous report, we investigated the use of an inorganic–organic hybrid material as a passivation layer, synthesized using a non-hydrolytic sol–gel method. This layer yielded good barrier properties and could be deposited via a solvent-free low-temperature curing process [19].

Generally, the gas permeability of a film is proportional to the solubility and diffusivity of gas molecules through the film [20]. The formation of a highly dense film is essential for reducing gas permeability because it is difficult for gas molecules to penetrate dense structures. Reducing the solubility of gas molecules in a film is another good approach to improving the barrier properties of a passivation layer. In general, polymers containing fluorine groups, such as CYTOP, poly(vinylidene chloride), and poly(chlorotrifluoro ethylene), make good barriers, especially to water vapor, because the fluorine groups tend to repel water vapor [15], [21], [22].

In this study, we synthesized an inorganic–organic hybrid encapsulant containing fluorine groups via a non-hydrolytic sol–gel process and characterized the resulting films using Fourier transform infrared (FT-IR) spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), secondary ion mass spectroscopy (SIMS), and UV–Vis–NIR (UV–Vis) spectroscopy.

Section snippets

Synthesis and characterization of inorganic–organic hybrid materials

1H, 1H, 2H, 2H-Perfluorooctyltriethoxysilane (PFAS, purchased from Aldrich), tetraethyl orthosilicate (TEOS, reagent grade, 98%, purchased from Aldrich), and diphenylsilanediol (DPSD, 95%, purchased from Aldrich) were used without purification (see Scheme 1). Barium hydroxide monohydrate (BH, purchased from Aldrich) was added as a catalyst to accelerate the condensation reaction by promoting the direct condensation of the ethoxy groups of PFAS and TEOS and the diol groups of DPSD to form

Results and discussion

Fig. 1(a) shows FT-IR spectra of TPDt (black line) and TDt (red line) films. An intense peak was detected around 1100 cm−1 in both the TPDt and TDt films and was attributed to the asymmetric stretching vibrations of Si–O–Si bonds. Peaks at 950–930 cm−1 and 820 cm−1 were also detected in the films, due to the Si–O–Ti and Ti–O–Ti vibrations, respectively [23], [24], [25]. Although non-hydrolytic sol–gel materials do not contain hydroxyl groups, broad bands around 3400–3000 cm−1 were detected in the

Conclusion

We designed an inorganic–organic hybrid material for encapsulating organic FETs. TPDt was synthesized via a non-hydrolytic sol–gel process to include fluoroalkylsilane. TPDt was cured at low temperatures to form a very uniform and transparent film, and the surface of the TPDt was smooth and hydrophobic. The TPDt film containing fluorine had a low permeability toward water vapor and oxygen. The TPDt film effectively protected the F8T2-based FETs from the detrimental effects of the ambient air.

Acknowledgements

This work was supported by a Korea Science and Engineering Foundation (KOSEF) Grant funded by the Korean government (MEST) (2012-0000127) and a Grant (2011-0031639) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea.

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