Zinc oxide heterostructures: advances in devices from self-powered photodetectors to self-charging supercapacitors

Over the years, zinc oxide (ZnO) has become a well-studied material due its versatile properties including both wide bandgap and large room temperature excitonic binding energy. The thermally-stable phase of ZnO allows it to grow in various nanostructures with high surface to volume ratio. So far, ZnO-based ultraviolet photodetectors (PD) have been a popular application in various heterostructures (HSs), and its piezoelectric properties have been exploited for the self-powering capability of devices. Both optical and mechanical excitations in combination provide a unique capability for the generation of excitons with built-in potential that has currently developed a vast interest in self-powered PDs as well as the energy storage capability in supercapacitors (SCs). This review provides a comprehensive detail of ZnO-based HSs exploited for interface engineering to provide a zero bias photoresponse and further being utilized for optically-activated SC applications. The self-powered photoresponse in ZnO is achieved through bandgap engineering by combining various semiconducting interfaces for broadband responses, fast response time, flexible devices, etc. Moreover, the ZnO interface along with electroactive materials in SC electrodes not only provides electrochemical routes for charge generation and separation but also the optical response, which is further attributed to self-powered voltage generation for efficient charge separation. The review describes the fundamental structural properties of ZnO along with its optical, electronic, and mechanical properties. The characteristics and significance of HSs in the field of science and technology is discussed in detail with emphasis on ZnO-based HSs. Thereafter, the optical interaction in ZnO-based HS is extended for two prominent applications targeting its self-powering/charging capability, i.e., PDs and SCs. Apart from the optical response, the contribution of mechanical and thermal stimuli toward these self-operating systems is also presented.