A Current Supply with Single Organic Thin-Film Transistor for Charging Supercapacitors

Abstract

Supercapacitors (also referred to as ultracapacitors, electric double-layer capacitors) bridge the gap between rechargeable batteries and electrolytic capacitors with moderate capacity and fast charging and are very promising for electric energy storage. Compared with lithium-ion batteries, supercapacitors are also superior regarding cycle life, specific power, and operating temperature. However, the cost per kWh of supercapacitors are tenfold to lithium-ion batteries. Printed electronics offers a route to a low-cost manufacturing platform that enables mass production of supercapacitors at low cost. Supercapacitors are usually charged with constant current supply, for example, Keithley 2200 programmable power supply. Here we present a current supply with single organic thin-film transistor (OTFT) to charge supercapacitors. The current supply takes power directly from the electric grid (115 V AC, US standard), converts the AC voltage to a quasi-constant current (~0.1 mA) regardless of the impedance of loads, and charges the supercapacitors. This work demonstrates steps towards all printable supercapacitor energy storage systems integrated with chargers that enable direct charging from the power outlet. Solution-processed OTFTs based on the popular polymer semiconductor P3HT have been developed to rectify 115 V AC voltage. The P3HT OTFTs were fabricated on glass substrates and Au electrodes for drain and source were patterned with a standard UV-lithographic lift-off process. The thickness of the spin-coated P3HT layer is around 35 nm. In order to operate the transistors at such high voltage, a rather thick (2500 nm) polymer PMMA was deposited on the P3HT as the gate dielectric. For gate electrodes, 100 nm Al was subsequently deposited by thermal evaporation through a shadow-mask. The channel length and width are 50 um and 55 mm respectively. The transistors delivers 0.7mA current when the drain voltage and the gate voltage are both at -200V. The average mobility is 2.6x10-2 cm2/Vs while the average threshold voltage is 2V. A diode-configured OTFT was used to rectify 115 V AC as a half-wave rectifier. Paralleling a 15-uF capacitor with the load, the ripples were smoothed and a DC output was obtained. With a 10-kΩ load, the output current is 0.1 mA at a voltage of 1 V. Varying the load from 10 kΩ to 100 kΩ, the voltage across the load increased to 8 V (eight times higher) while the output current dropped only 20% to 0.08 mA. The rectifier thus provides quasi-constant current output for different loads. In addition, no other voltage step-down devices, for instance, transformers, are required to take the 115 V AC voltage down to a few volts. During charging of the supercapacitors, the load resistor also works as a protection device that protects against overcharging. This single OTFT current source was demonstrated to charge a 430mF supercapacitors to 1 V directly using 115 V AC voltage from power outlet.