Defect Restoration of Low-Temperature Sol-Gel-Derived ZnO via Sulfur Doping for Advancing Polymeric Schottky Photodiodes

Abstract

This study shows that the deep-level defect states in sol-gel-derived ZnO can be efficiently restored by facile sulfur doping chemistry, wherein the +2 charged oxygen vacancies are filled with the S2- ions brought by thiocyanate. By fabricating a solution-processed polymeric Schottky diode with ITO/ZnO as the cathode, the synergetic effects of such defect-restored ZnO electron selective layers are demonstrated. The decreased chemical defects and thus reduced mid-gap states enable to not only enlarge the effective built-in potential, which can expand the width of the depletion region, but also increase the Schottky energy barrier, which can reduce undesired dark-current injection. As a result, the demonstrated simple-structure blue-selective polymeric Schottky photodiode renders near-ideal diode operation with an ideality factor of 1.18, a noise equivalent power of 1.25 × 10-14 W Hz-1/2, and a high peak detectivity of 2.4 × 1013 Jones. In addition, the chemical robustness of sulfur-doped ZnO enables exceptional device stability against air exposure as well as device-to-device reproducibility. Therefore, this work opens the possibility of utilizing low-temperature sol-gel-derived ZnO in realizing high-performance, stable, and reliable organic photodiodes that could be employed in the design of practical image sensors. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.