Flexible Light-to-Frequency Conversion Circuits Built with Si-Based Frequency-to-Digital Converters via Complementary Photosensitive Ring Oscillators with p-Type SWNT and n-Type a-IGZO Thin Film Transistors.

In this study, as system-level photodetectors, light-to-frequency conversion circuits (LFCs) are realized by i) photosensitive ring oscillators (ROs) composed of amorphous indium-gallium-zinc-oxide/single-walled carbon nanotube (a-IGZO/SWNT) thin film transistors (TFTs) and ii) phase-locked-loop Si circuits built with frequency-to-digital converters (PFDC). The 3-stage ROs and logic gates based on a-IGZO/SWNT TFTs successfully demonstrate its performance on flexible substrates. Herein, along with the advantage of scalability, a-IGZO films are used as photosensitive n-type TFTs and SWNTs are employed as photo-insensitive p-type TFTs for better photosensitivity in circuit level. Through the controlling a post-annealing condition of a-IGZO film, responsivities and detectivities of a-IGZO TFTs are obtained as 36 AW and 0.3 × 10 Jones for red, 93 AW and 3.1 × 10 Jones for green, and 194 AW and 11.7 × 10 Jones for blue. Furthermore, as an advanced demonstration for practical application of LFCs, a unique circuit (i.e., PFDC) is designed to analyze the generated oscillation frequency (f ) from the LFC device and convert it to a digital code. As a result, the designed PFDC can exactly count the generated f from the flexible a-IGZO/SWNT ROs under light illumination with an outstanding sensitivity and assign input frequencies to respective digital code.