Key Issues With Printed Flexible Thin Film Transistors and Their Application in Disposable RF Sensors

This paper addresses the key issues that must be overcome to realize fully printed TFT-based flexible devices via commercially viable methods. In particular the threshold voltage ( <tex-math notation="TeX">Vₜₕ</tex-math> ) variation in printed TFTs is a serious impediment to the successful launch of fully printed TFT-based devices in the market. The underlying causes of the <tex-math notation="TeX">Vₜₕ</tex-math> variation in fully printed TFTs were analyzed by considering the misalignment of printed drain-source to gate electrodes, the rheology of electronic inks and effects from external sources of charge. By alleviating the influences of external sources of charge using a printed passivation layer, <tex-math notation="TeX">Vₜₕ</tex-math> variation is maintained below 30% using a fully printed process. Based on the attainable variation range, the required number of integrated TFTs was estimated to fabricate a fully printed TFT-based radio frequency (RF) sensor device. A practical compromise enables fully printed RF sensors to be realized via the scalability of printing processes that mitigate <tex-math notation="TeX">Vₜₕ</tex-math> variation by minimizing the level of TFT integration. Prototypes of fully printed RF sensors with human interactive capability—an RF sensor label, and an RF <tex-math notation="TeX">e</tex-math> -sensor (cyclic voltammetry) tag—are enabled with as few as 26 printed TFTs, demonstrating that low-cost and high throughput manufacturing of printed electronics is feasible.