Highly conductive graphene films laminated with ferroelectric polymers

Transparent conductors are important for a wide variety of modern electronics and optoelectronics, including flexible displays, photovoltaics, solar cells, touch panels, organic light emitting diodes, and electrochromics.¹ Currently, indium tin oxide (ITO) is a dominant material for transparent conductors because of its excellent combination of transparency and sheet resistance (a measure of thin film resistance). However, ITO is expensive, fragile, and is complicated to fabricate. Thus, the need for low-cost alternative materials to ITO is pressing.

Among promising candidates, graphene is unique due to its exceptional optical, mechanical, and electrical properties.² Graphene is a single-atomic-layer-thick sheet of carbon atoms arranged in a hexagonal structure. Because it is a single-atomic-layer membrane, graphene is highly transparent (97.7% light transmission) over a wide range of wavelengths from the visible to the near-infrared.³ Owing to its covalent carbon-carbon bonding, graphene is one of the strongest known materials, with a remarkably high elastic modulus (Young's modulus) of ∼1TPa.⁴ The combination of its high transparency, tunability across a wide range of optical wavelengths, and excellent mechanical flexibility make graphene a very promising candidate for flexible transparent conductors.^5–7

To use graphene as a transparent electrode, the major challenge is to reduce the sheet resistance to values comparable with ITO. Chemical doping, the purposeful introduction of impurities, has been used to achieve a relatively low sheet resistance in graphene.^{8, 9} However, the doping mechanism in graphene is not yet fully understood, and the relationship between charge density and carrier mobility is still under debate.^10–12 Furthermore, the adsorption of moisture and other molecules after chemical treatment leads to more than a 40% increase of the graphene sheet resistance within a few days.^{13, 14} A carefully chosen thin polymer coating is necessary to prevent this increase in sheet resistance without compromising its high transparency. Therefore, simplified approaches of making electrodes, with improved performance and zero power consumption, are highly desired.

By laminating graphene with the ferroelectric polymer poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)), we created novel flexible transparent conductors with low sheet resistance, high transparency, and excellent mechanical flexibility.¹⁵ Such graphene-ferroelectric transparent conductors (GFeTCs) exhibit sheet resistances as low as 120Ω/square at ambient conditions due to a large electrostatic nonvolatile doping of up to 3×10¹³cm⁻² from ferroelectric dipoles (see Figure 1). This is one of the key advantages of using a ferroelectric material as the laminate. When the P(VDF-TrFE) thin film is fully polarized, the charge donation to graphene is extremely long-lasting, and neither damages the graphene nor reduces its carrier mobility.

Figure 1. Illustration of the electrostatic doping to graphene induced by the ferroelectric polymer P(VDF-TrFE).

Beyond having low sheet resistance values, the GFeTCs were highly transparent (>95%) in the visible wavelength range, making them suitable for optoelectronics applications where a combination of both transparency and low sheet resistance is required. This transparency is attributed to the large (∼6eV) band gap of P(VDF-TrFE). Our GFeTCs also exhibited excellent mechanical properties. While the resistance slightly increased during bending to a radius of 3.0mm, it fully recovered after unbending the GFeTC devices. Notably, with an appropriate thickness, the fully polarized P(VDF-TrFE) thin films simultaneously doped the graphene and provided excellent mechanical support (see Figure 1).

This technique is an improvement over previous approaches of chemically doping graphene, where the pre-doped graphene had to be combined with a polymer supporting layer in a separate process step. Thus, the combination of P(VDF-TrFE) with large-scale graphene is well suited for graphene-based transparent conductor applications. With the excellent mechanical support of P(VDF-TrFE), hybrid GFeTCs fabrication can be easily integrated with industrial-scale fabrication processes such as roll-to-roll techniques.

Our research has led to a new type of transparent conductor using graphene-ferroelectric hybrid films. The ferroelectric thin film does not compromise the high optical transparency of graphene. The film also provides nonvolatile doping induced by the transfer of charge, yielding sheet resistances as low as 120Ω/square even in low mobility samples. The ferroelectric polymer also serves as an excellent mechanical supporting layer, allowing GFeTCs to be easily transferred to and integrated with flexible electronics, optoelectronics, and photonics platforms. In addition, we showed that the limiting factor for further lowering the sheet resistance is not the ferroelectric polymer but rather the commonly known charged impurities originating from existing transfer processes. Therefore, with further improvements in the transfer process, a sheet resistance of 50Ω/square with an optical transparency of >95% seem feasible. As the next step, we would like to study the optimization of current device fabrication processes and the use of our GFeTCs in flexible displays devices, solar cells, and other devices.

This work is supported by the Singapore National Research Foundation [grants NRF-RF2008-07, NRF-CRP(R-144-000-295-281), NRF-POC002-038, NUS-YIA(R144-000-283-101), and NUS/SMF], IMRE/10-1C0109, U.S. Office of Naval Research (ONR and ONR Global), A*STAR SERC TSRP-Integrated Nanophoto-Bio Interface (R-144-000-275-305), NUS NanoCore, and the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Global Research Lab. 20110021972, 2011K000615, 20110017587, 20110006268, and Global Frontier Research Program 20110031629).