Elsevier

Organic Electronics

Volume 12, Issue 7, July 2011, Pages 1253-1257
Organic Electronics

Use of poly(3-hexylthiophene)/poly(methyl methacrylate) (P3HT/PMMA) blends to improve the performance of water-gated organic field-effect transistors

https://doi.org/10.1016/j.orgel.2011.04.006Get rights and content

Abstract

Poly(3-hexylthiophene)/poly(methyl methacrylate) (P3HT/PMMA) blends were used as the semiconducting layer in water-gated organic field-effect transistors (OFETs), which resulted in improving the electrical performance of the previously reported devices with pure P3HT. Topographic investigations by atomic force microscopy carried out on blends with various PMMA to P3HT ratio reveal a lateral phase separation of the two components. All transistors operate at very low voltage (below 1 V), with a threshold voltage ranging form 0.3 to 0.5 V. An optimum of the composition of the blend is found with 70% of PMMA, leading to a maximum on/off current ratio and a mobility comparable to that of pure P3HT.

Graphical abstract

Blending the semiconducting P3HT to the insulating polymer PMMA in water-gated organic transistors results in improving the on/off ratio without loss of the mobility, while keeping low voltage operation.

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Highlights

► Water-gated organic field-effect transistors operate in the field-effect mode at very low voltages. ► Blends of P3HT and PMMA have mobility similar to that of pure P3HT with concentration of PMMA up to 70. ► Drain current on/off ratio reaches an optimum value for a PMMA concentration of 70%.

Introduction

For the past 20 years, organic electronics has known a growing interest. Its low fabrication cost combines with a large scale manufacturing and integration to electrical circuits on flexible substrate capability. Although the performance of organic semiconductors will probably remain lower than that of crystalline silicon-based electronic devices, it is continuously improving. Highly ordered organic solids such as rubrene single crystals and polycrystalline pentacene can achieve a charge carrier mobility up to 20 cm2/V s [1] and 5 cm2/V s [2], respectively, while charge carrier mobility is limited to 1 cm2/V s in amorphous silicon. However, these compounds do not fit low cost processes and chemical modifications are needed to achieve this goal [3].

On the other hand, disordered polymeric semiconductors are easily processable, but suffer from low mobility (below 0.1 cm2/V s). Alongside with the synthesis of new materials, recent progress have been made at improving the quality of known materials. A widespread method for this approach is to blend several materials with the desired properties. It has been used for long in the case of non-conjugated polymers to improve their mechanical properties. For the past 15 years, the technique has been extended to conjugated polymers, especially in optoelectronic devices such as organic light emitting diodes [4], [5] and photovoltaic cells [6]. OFETs made of blends are less common. However, a few examples have been recently reported using inorganic nanoparticles [7], [8] especially with semiconductor/semiconductor [9], [10], [11] or semiconductor/insulator blends [12]. These works have shown that blending materials could improve both air stability [13] and mobility of charge carriers [14], [15]. A one-step procedure for the formation of the gate dielectric and semiconductor is even possible by taking advantage of the self-organization of the two components.

Another concern for low-cost organic electronics is to lower the operating voltage of OFETs. This can be achieved by increasing the capacitance of the gate dielectric. The strategies that have been developed to achieve low-voltage operation include cross-linked polymers [16], self-assembled monolayers [17], [18] and the use of electrolytes [19], [20]. The latter can achieve very low voltage ranges due to the very high electric field generated at the interface between the electrolyte and the organic semiconductor. Mobile ions inside the electrolyte move to form an electric double layer (EDL) at the interface, with a capacitance that can reach several hundreds of μF/cm2 [19], [21], [22]. These devices typically operate below 1 V. Furthermore, increasing the capacitance leads to an increase of the charge carrier mobility for polymer semiconductors and the subsequent increase in the output current [23].

In a previous work [24], we successfully used pure water to isolate the gate from the semiconductor. We managed to show that within the water stability potential range, our devices run in the field-effect mode with negligible electrochemical reactions, which is usually the main issue for electrolyte-gated OFETs [25].

Here, we investigate the use of semiconductor/insulator blends as active layer in water-gated OFETs in order to improve the performance of the device.

Section snippets

Materials

Poly(3-hexylthiophene) (P3HT) (MW = 37,000 g mol−1, 98% regioregular) and poly(methyl methacrylate) (PMMA MW = 120,000 g mol−1) were purchased to Sigma–Aldrich and used without further purification. Chlorobenzene was purified by distillation.

Solutions of P3HT/PMMA (1 wt.%) blends with different ratios were prepared as followed. First PMMA was dissolved in chlorobenzene by stirring overnight. Then P3HT was added to the PMMA solution and stirred until the P3HT is totally dissolved. Finally, solutions were

Results and discussion

Fig. 2 shows AFM 10 × 10 μm2 topographic images of the P3HT/PMMA films with various weight ratios. Blends with PMMA concentrations up to 30% (Fig. 2a and b) do not show any clear phase separation at this scale. However, an abrupt decrease of the RMS roughness (Fig. 2f) occurs, indicating an enhancement of the homogeneity due to the increase of PMMA content (RMS roughness of pure PMMA was found to be below 0.5 nm). For a ratio of P3HT/PMMA of 1:1 (Fig. 2c), lateral phase separation clearly occurs as

Conclusion

Used in a water-gated configuration, P3HT/PMMA blends can lead to transistors with reasonably high mobility (up to 0.15 cm2/V s) working at very low voltage (below 1 V). The mobility is not affected when increasing the proportion of PMMA up to 70%, while the off current is significantly lowered, thus resulting in an enhancement in the on/off ratio. AFM images showed that up to 70% of PMMA, a percolation path exists, thus allowing an efficient charge transport through the channel, in good agreement

Acknowledgments

One of the author (L.K.) thanks the French Ministry of Education and Research for a scholarship. We would like to thank Lars Herlogsson (Linköping University) for providing the photolithography patterned substrates.

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