Monolayer organic film delivers ultra-high-gain transistors and circuits

Using just a monolayer of organic film as channel, we demonstrate flexible thin film transistors and amplifiers that show record-high gain, which allow us to monitor extremely weak signals such as human electrocardiograms powered by just a coin battery.

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Organic electronics is a promising technology for flexible and wearable applications. Over the past few decades, significant progresses have been made ranging from material design to devices and integrated circuits. Today, organic light-emitting diode (OLED) is commercialized in high-end mobile displays. Similar to 2D layered materials, many organic crystals, such as rubrene and pentacene, also have layered structure due to the anisotropy between interlayer and intralayer molecular interactions. Since 2014, we began to investigate this interesting class of 2D organic films down to the monolayer thickness limit. We can epitaxially grow monolayer organic crystals on BN and show improved device performance in terms of mobility and contact resistance [1-3]. However, the size of the films is limited by the BN flakes to tens of micrometers. In this work, we adopt solution shearing method and, after extensive optimizations, successfully obtain uniform monolayer organic films of wafer size (Fig. 1a). Importantly, the process is fully compatible with flexible substrates, which enables large-area fabrication of flexible transistors and circuits.

Figure 1. a, Photograph of uniform monolayer C10-DNTT film on 2-inch Si wafer. b, Optical microscope image of an enhancement-depletion mode amplifier. c, Av as a function of input voltage of the organic amplifier.

The key to reducing the power consumption of transistors is to reduce the operation voltage, which is ultimately controlled by the steepness of the transition between “on” and “off” state - referred to as subthreshold swing (SS). Due to the Boltzmann thermionic limit, the SS is limited to about 60 mV/dec at room temperature for normal transistors. To solve this problem, we introduce ferroelectric oxide into the transistor structure. The negative capacitance (NC) effect can break the Boltzmann limit and realize sub-thermionic (below 60 mV/dec) switching. In addition, the NC effect provides large output resistance. This is a key feature leading to very large intrinsic gain (proportional to output resistance and transconductance).

 It is well-known that conventional device fabrication processes can damage the organic films, especially for monolayer. To this end, we develop a solvent-free and low-energy process by simply laminating the pre-patterned metal electrodes onto the organic films. Different from the traditional patterning-and-metal-evaporation process, our process can protect the relatively fragile organic films from solvents, high-energy metal particles and electron beams irradiations. As one can imagine, monolayer films are particularly susceptible to these damages. This is another key improvement that helps us to obtain decent device performance.

Figure 2. a, Photograph of the amplifier module, with a coin battery, a voltage divider and amplifier circuit integrated on a flexible circuit board. b, Amplified ECG signal of a human subject.

By combining solution-processed monolayer organic film, NC effect and non-invasive integrated process, we have successfully obtained organic transistors with intrinsic gain of 5.3×104. In addition, we build functional circuits such as inverter and logic gates. We show an inverter with voltage gain of more than 1.1×104 (Fig. 1, b and c), which break the record among similar devices using different materials. Furthermore, we integrate the flexible amplifier on a flexible circuit board and power it by a 1.5 V coin battery (Fig. 2). We apply it to the amplification and detection of human electrocardiogram (ECG) signals. Surprisingly, using such a simple device, we are able to amplify the ECG signal by more than 300 times and detect extremely weak ECG signals such as atrial fibrillation. Importantly, our sensor shows excellent signal fidelity: the deconvoluted ECG matches the results taken by the commercial equipment. The results show that our sub-thermionic organic transistor technology is promising for battery-powered continuous health monitoring, and when integrated with wireless data transmission, real-time cloud-based diagnosis.

 For more information, please refer to our article published in Nature Communications:

 [1] He, D. et al. Two-dimensional quasi-freestanding molecular crystals for high-performance organic field-effect transistors. Nature Communications 5, 5162, (2014).

[2] Zhang, Y. et al. Probing Carrier Transport and Structure-Property Relationship of Highly Ordered Organic Semiconductors at the Two-Dimensional Limit. Physical Review Letters 116, 016602, (2016).

[3] He, D. et al. Ultrahigh mobility and efficient charge injection in monolayer organic thin-film transistors on boron nitride. Science Advances 3, e1701186, (2017).

Xinran Wang

Professor, Nanjing University