Enhancing electrical stability for flexible ZnO nanowire UV sensor by textured substrates

Abstract

The major problem in the fabrication of electronic devices on plastic substrate arises from the mismatch and weak physical bonding between inorganic semiconductor crystal and the organic plastics, so the electrical performance stability under mechanical stress is an essential factor affecting the practical application of flexible electronic sensors. In this paper, a flexible ZnO nanowire (NW) UV sensor is presented as an exemplary verification for investigating the effects of substrate morphology on reliability of flexible electronic devices. Sensors on ordinary smooth polyimide (PI) substrate have cracked during the device fabrication process due to the residual stress caused by temperature, humidity, etc. These cracks were short pieces and randomly distributed, so they may not significantly affect the device performance, but after mechanical bending, cracks that penetrating the electrodes were generated and caused electric contact failure. Although improving the ZnO seed layer quality could reduce the cracking and buckling that formed during device fabrication, it would not prevent cracking during mechanical bending. In a 3 × 3 sensor array, only one sensor survived after bending. Device failed when the bending angle larger than − 40° or 60°. On the contrary, device fabricated on textured PI substrates exhibited much better electrical stability. All the sensors remained their original performances after mechanical bending in a 3 × 3 sensor array. The light–dark current ratios kept in the level of 105 under the 365 nm UV light of 15 mW/cm2. The bending angle varied from − 80° to 80°. The enhancement of electrical stability was because that textured substrate could tolerate more stress than smooth substrate to prevent the film from cracking and at the same time it could increase the contact area between ZnO film and PI substrate, which improved the interfacial bonding strength and delayed the local strain of the film.