Significance of the double-layer capacitor effect in polar rubbery dielectrics and exceptionally stable low-voltage high transconductance organic transistors

Abstract

Both high gain and transconductance at low operating voltages are essential for practical applications of organic field-effect transistors (OFETs). Here, we describe the significance of the double-layer capacitance effect in polar rubbery dielectrics, even when present in a very low ion concentration and conductivity. We observed that this effect can greatly enhance the OFET transconductance when driven at low voltages. Specifically, when the polar elastomer poly(vinylidene fluoride-co-hexafluoropropylene) (e-PVDF-HFP) was used as the dielectric layer, despite a thickness of several micrometers, we obtained a transconductance per channel width 30 times higher than that measured for the same organic semiconductors fabricated on a semicrystalline PVDF-HFP with a similar thickness. After a series of detailed experimental investigations, we attribute the above observation to the double-layer capacitance effect, even though the ionic conductivity is as low as 10–10 S/cm. Different from previously reported OFETs with double-layer capacitance effects, our devices showed unprecedented high bias-stress stability in air and even in water.

Introduction

Field-effect transistors (FETs) with both high gain and transconductance are crucial for a broad range of applications^1,2,3, including logic circuits, display drivers and sensing^4,5,6. High-performance FETs based on organic materials are of particular interest due to their compatibility with low-cost, high-throughput processing and mechanical compliance with soft tissues. However, it has been challenging to realize high transconductance with organic materials due to their relatively low charge carrier mobilities. One effective method is to develop dielectric layers with high capacitances^7,8. Halik et al. used an ultra-thin self-assembled monolayer (SAM) dielectric layer⁹ to achieve a high capacitance of 0.7 μF/cm² and a transconductance of 0.01–0.04 S/m in vacuum evaporated OTFTs. Ion-doped polymer electrolytes¹⁰ and ion gels^11,12 have been used as the dielectric layers for OFETs. Their capacitances are high due to the double-layer capacitor effect. The resulting OFETs have been shown to reach transconductances up to 0.5 S/m. However, challenges remain in using the aforementioned systems for practical applications due to the low yield of SAM fabrication, incompatibility of liquid/gel materials with standard manufacturing processes and the high moisture sensitivity of ionic dielectrics⁶.

Results

Here, we serendipitously discovered that a polar fluorinated PVDF-HFP elastomer dielectric, despite of a low ion concentration, is able to induce an electric double-layer charging effect under an applied gate voltage. This polymer dielectric is solution-processable with a high static capacitance of ~0.3 μF/cm², even at a thickness of several micrometers. Devices made from this thick polymer dielectric are capable of operating at low voltages with a transconductance as high as 0.02 Sm⁻¹ for polymer OTFTs and as high as 1.2 Sm⁻¹ for CVD-graphene FET. This polymer dielectric is highly compatible with solution processing of various organic semiconductors. Remarkably, the resulting devices showed both high current output and low bias stress in both ambient and aqueous conditions.

PVDF-HFP polymers are usually semicrystalline when a high molar fraction of PVDF segments are incorporated¹³. However, a higher molar ratio of the HFP units (45mol% determined by ¹⁹F-NMR shown in Supplementary Information Fig. S2) results in an elastic material with a glass transition temperature (T_g) of around −20°C (Fig. S1 and S3). Its dielectric constant is 11 as measured at 1 kHz, a value similar to the previously reported range of 8 to 13^{14,www.nature.com/nature.}