Ultrathin Al-Driven Polarity Engineering in Polycrystalline ZnO Thin Films on Flexible Glass for Sign-Reversible Piezoresistive and Photoconductive Responses

Flexible transparent devices require materials whose mechanical and optoelectronic responses can be tuned in a coordinated manner through interface design. This study shows that an ultrathin Al underlayer enables crystallographic polarity switching in c-axis-oriented polycrystalline ZnO thin films on flexible glass, converting O-polar films to Zn-polar ones through interfacial-oxidation-mediated nucleation control. This polarity inversion reverses both the sign of the piezoresistive response and the bending-dependent ultraviolet (UV) photoconductive decay. Specifically, O-polar films exhibit a positive gauge factor (up to 11.2) and a shortened UV decay time under convex bending, whereas Zn-polar films show a negative gauge factor (-2.8) and a delayed photoresponse decay. These contrasting behaviors can be attributed to charge-carrier accumulation and depletion induced by the piezoelectric field. These results show that interface engineering at the ultrathin limit can program strain-sensitive optoelectronic functionality in flexible ZnO-based devices. This approach provides a simple route for the design of sign-controlled multifunctionality in oxide thin films through interfacial polarity engineering rather than bulk compositional modification.