## 1 Introduction  
The piezotronic and piezo-phototronic effects of non-centrosymmetric semiconductors, such as ZnO, GaN, and InN, have received tremendous attention because of their ability to enhance the electrical and optoelectrical performance of devices, such as field-effect transistors, photodetectors, light-emitting diodes, and solar cells [1, 2, 3, 4, 5, 6]. To date, among the non-centrosymmetric wurtzite semiconductors, ZnO has been regarded as the most potential candidate owing to its attractive advantages: facile preparation for a variety of nanostructures with different shapes, outstanding semiconducting characteristics, [7, 8] and the ease of hybridization with other materials to utilize its extraordinary physical properties in various applications [9, 10]. In particular, a one-dimensional (1D) ZnO NW array architecture, compared to single-nanowire (NW)-based devices and 2D thin films, exhibits additional advantages because of its ability to withstand a relatively large external mechanical strain within a large surface area, its excellent multichannel charge transport, and its enhanced light absorption [11; 12; 13; 14]. Nonetheless, the presence of a large number of surface states within ZnO NWs limits the performance of electronic and optoelectronic devices, and it affects their stability [12; 15]. Accordingly, to improve the performance of ZnO NW arrays, surface and/or interface modification engineering has been proposed as an effective strategy, such as the fabrication of core/shell NWs by the deposition of a shell layer on the ZnO NWs [16; 17; 18; 19]. Among various composite structures for ZnO-based core/shell NWs, a heterojunction with a type-II band alignment, where the valence and conduction band of the ZnO core are lower than those of the shell, can separate the electrons and holes into different spatial regions [16; 17; 20]. This type-II core/shell structure not only enables the reduced surface states of the intrinsic ZnO core but also improves the charge carrier transport caused by the spatial separation of electrons and holes in different conducting channels. Thus, numerous studies have reported the application of a type-II band structure to various optoelectronic devices by coating the ZnO NW array with shells, such as ZnS, ZnSe, and CdS [20; 21; 22; 23]. Above all, the type-II band alignment of ZnO/ZnS core/shell NWs has become the focus of intensive research with respect to the fast separation of photogenerated carriers due to the remarkable application potential for not only photovoltaic devices but also solar-driven photoelectrochemical water splitting, photocatalytic hydrogen production, and gas sensors [24; 25; 26; 27]. To date, by applying external strain to various core/shell NWs with such a unique type-II band structure, numerous studies have reported on the piezotronic, piezo-phototornic, and piezo-photocatalytic effects [28; 29]. Related core/shell NW devices have emerged for a broad range of applications, producing eco-friendly hydrogen fuel or detecting different sources in the environment, such as human motion/pressure and light. However, an in-depth discussion of the influence of the piezoelectric and piezo-phototronic effect of these NWs on the performance of each device through the quantitative calculation of Schottky barrier height (SBH) changes has not been reported. The core of piezotronics and piezo-phototronics is based on the use of the strain-induced piezoelectric potential generated inside non-centrosymmetric semiconductors that are in contact with the metal contact interface [30]. Therefore, it is essential to understand how the strain-induced piezoelectric potential affects the SBH of the devices and how it modulates the charge transport, separation, and recombination processes at the junction. Here, we fabricated ZnO and ZnO/ZnS core/shell NWs and evaluated the electrical and optoelectrical properties of these two devices on flexible substrates under applied external strain and under UV irradiation.