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

Abstract

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 ( $V_{\rm th} $ ) 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 $V_{\rm th} $ 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, $V_{\rm th} $ 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 $V_{\rm th} $ 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 $e$ -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.