Elsevier

Organic Electronics

Volume 54, March 2018, Pages 40-47
Organic Electronics

Uniform, high performance, solution processed organic thin-film transistors integrated in 1 MHz frequency ring oscillators

https://doi.org/10.1016/j.orgel.2017.12.005Get rights and content

Highlights

  • 5 stage solution processed p-type ring oscillators have been fabricated.

  • >1 MHz frequency, 93 ns stage delay performance has been achieved.

  • Low width-normalised contact resistance (300 Ohm cm) was measured.

Abstract

Organic electronics is one of the most promising technologies for creating flexible electronic devices using low temperature plastic compatible processes. However, contact resistances of organic transistors remain one of the most significant hurdles to achieving high performance circuits in this technology. Short channel devices (<10 μm), essential for industrial applications, typically exhibit only a fraction of the performance promised by the high mobility results achieved in laboratory research on longer channel lengths (a few tens of μm or even longer). In this paper, we present results demonstrating solution processed devices having width-normalised contact resistances of less than 300 Ohm cm and show how these can be made into 5-stage ring oscillator circuits with the highest frequency of 1.08 MHz, corresponding to a stage delay of 93 ns. This is achieved through use of a thin, uniform high performance organic semiconductor (OSC) film made possible through formulation of a small-molecule semiconductor in a high-k binder polymer. The OSC formulation is employed in a fabrication process compatible with mass manufacture, opening up the route to commercial products made from organic thin-film transistor (OTFT) devices with high performance.

Introduction

Organic thin-film transistors (OTFTs) have been researched for more than 30 years and are of interest for flexible display backplane applications requiring materials that can be repeatedly bent to a small radius of curvature (<1 mm) [1], [2]. Such display applications typically incorporate photolithographically defined source-drain (S-D) electrodes of transistors with channel lengths of 5 μm or less. The charge mobility requirements for pixel transistors in this application range from below 1 cm2/Vs for Electrophoretic (EP) and Liquid Crystal (LC) display backplanes to 5–10 cm2/Vs or above for Organic Light Emitting Diode (OLED) display backplanes [3]. For OLED the requirement is influenced by the size and resolution of display. A significant number of organic semiconductor (OSC) materials have been reported with charge mobility exceeding 1 cm2/Vs [4], [5], [6], [7] but often the results are for measured OTFT devices with channel lengths of longer than 20 μm or even longer than 50 μm. An exception to this was a study reporting the short channel behaviour of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), 2,9-diphenyl-DNTT (DPh-DNTT), 2,9-didecyl-DNTT (C10-DNTT) and pentacene [8]. They were processed in devices having channel lengths of 1 μm–100 μm and some materials demonstrated an order of magnitude reduction in apparent mobility over this range of lengths due to contact resistance effects. Such a study demonstrates a common problem with OSC materials in that it is difficult to maintain high apparent mobility in short channel devices.

Soluble OSC materials have the potential to enable low-cost electronics prototyping through the use of digital printing techniques [9]. Historically, additive printing techniques such as gravure [10] or ink-jet [11] typically exhibited low spatial resolution (>50 μm) and had a limited capability to provide the accurate layer-to-layer registration required for transistor devices and circuits. Recent progress in this area has shown promise to meet these needs, with improved gravure printing demonstrated to better than 5 μm resolution [12], [13], [14], sub-micron scale reverse-offset printing [15] and sub-micron scale ink-jet printing [16], [17] reducing the critical dimensions of printed transistor source-drain electrode gaps. Laser ablation and sintering are techniques that can be employed to fabricate 1.3 μm resolution features without the need for photomasks and have been used to create high frequency polymer OTFTs [18]. Additionally, a combination of image analysis and digital printing has demonstrated improved layer-to-layer registration of 2 μm (3σ) on a flexible substrate through compensations for distortion [19]. Making beneficial use of the progress in micron scale printing will therefore require an improvement in the performance of OSC materials in short channel devices. Therefore, for both applications of flexible displays and high resolution printed logic devices OTFTs should have high mobility at the length scale of <10 μm. This will enable channel length scaling to increase the frequency of operation of OTFT logic, thereby broadening the application uses of the circuits. Despite the recent progress in OSC materials, there have been only a few demonstrations of logic circuits at more than 100 kHz, far short of the theoretical frequency for such high mobility materials [20], [21]. High mobility semiconductors are therefore only one of the prerequisites for fast logic circuitry and other factors must be optimised to extract the highest device performance from these materials. Short channel effects, mainly due to contact resistance, can dramatically reduce the effective mobility of the device to a fraction of the highest reported values. The cut-off frequency fT, commonly used as a figure of merit for OTFT devices, is given by the equation [derived from equations (5) and (20) in Ref. [22]]:fT=gm2π(Cch+Cpara)=μappV12πL2CchCch+Cparawhere μapp is the apparent mobility, L is channel length, Cch is the channel capacitance, and Cpara is the parasitic capacitance. V1 is equal to VD in the linear regime and to VG – Vth in the saturation regime, VD is the drain voltage, VG is the gate voltage, and Vth is the threshold voltage respectively. Higher frequency of operation in OTFT can therefore be achieved through decreasing channel length, reducing overlap capacitance and ensuring a high apparent mobility. The last of these can be achieved by ensuring short channel device mobility is not compromised through contact resistance effects.

In this paper we report, for the first time, organic PMOS ring oscillators with frequencies in excess of 1 MHz and stage delays of less than 100 ns. This is achieved through high mobility devices made with a 6 μm channel length having low contact resistance with the OSC layer, hence optimising two of the most important factors for increasing the cut-off frequency. The OSC formulations used are based upon the concept introduced previously, where a high mobility small-molecule and high-k polymeric semiconductor material are blended in solvent [23], [24]. This approach has been shown to produce uniformly high mobility (>4 cm2/Vs) across a substrate by spin-coating and with good device bias stress stability. In this work we demonstrate that this formulation approach can be used to minimise contact resistance in top gate bottom contact (TGBC) OTFT devices and the effect is exemplified by using the small-molecule 1,4,8,11-tetramethyl-6,13-bis(triethylsilylethynyl)pentacene (TM-TES pentacene) [25] and high-k random copolymer 30: 70 2-[p-(diphenylamino)phenyl]-2-methylpropiononitrile: N,N-diphenyl(2,4-xylyl)amine [23].

Section snippets

Device fabrication

All fabrication was performed at the Centre for Process Innovation (CPI, Sedgefield, UK) using the Gen2 photolithography facilities (www.uk-cpi.com/pe-equipment/). OTFTs were fabricated in TGBC configuration on 8-inch square glass substrates (Corning Eagle XG) per the scheme shown in Fig. 1. Laminated plastic-on-glass substrates are compatible with the process but, due to the need to scribe the pattern accurately into several sections, a glass-only substrate was used in these trials. A

Device uniformity, contact resistance and short channel effects

Devices were electrically tested after the gate layer processing and at the end of the fabrication. Table 1 shows the linear mobility values for 8 substrates tested after the gate processing; the data is displayed for each transistor channel length. Devices are of a Corbino design (see Fig. 2a inset), which eliminates parasitic source to drain current flows, hence permitting on/off ratios of in excess of 5 × 107 to be demonstrated, as can be seen by the transfer curves displayed in Fig. 2a for

Conclusion

Results presented in this paper demonstrate that low contact resistances can be achieved in solution coated OTFT devices. The combination of small-molecule and high-k polymer blends of semiconductors enable thin, uniform films by spin-coating. Low contact resistance at low dielectric capacitance permits efficient charge injection and fast logic operation in circuits. These materials have been used to fabricate ring oscillators operating at over 1 MHz with stage delays of 93 ns. Such progress

Author contributions

S.O. wrote the manuscript, analysed the OTFT data for the contact resistance studies and project managed the substrate fabrication at CPI, (Sedgefield, UK). H.M. made the measurements on the OTFT ring oscillators and cut-off frequency, and S.T. made contributions to the measurement setup and data acquisition. L.F. made essential contributions to the paper drafting and interpretation of the data. M.S. devised the synthetic methods and supervised the synthesis of the active semiconductor

Competing financial interests

NeuDrive Limited (the employer of SO, LF and MS) owns pending and granted patent applications covering the formulation approach described for the OSC materials in this paper (a reference to the patent is included in the paper [23]).

Acknowledgements

This work was partly supported by Leading Initiative for Excellent Young Researchers (LEADER) program, MEXT, Japan and by the Spanish government project TEC2014-59679.

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