Piezoelectric generator based on centrosymmetric CdO film with (111) orientation and its atomic mechanism

The centrosymmetric CdO is not generally considered piezoelectric. Here, we demonstrate the observation of piezoelectric, photovoltaic, and rectification effects in (111)-oriented CdO films. The piezoelectric, photovoltaic, and rectification performances are enhanced with an increase in the (111)-texture coefficient of CdO films. Moreover, these effects are decreased by applying an external force and eventually disappear. An electric field is produced in the (111)-oriented CdO film by spontaneous polarization (spontaneous electric field [Es]). The piezoelectric, photovoltaic, and rectification effects originate from the Es. We thus present piezoelectric, photovoltaic, and rectification atomic mechanisms. The CdO film piezoelectric generator corresponds to a flat-plate capacitor, the compression induces discharge, and the release of pressure is a charging process. This work reveals the nature of the piezoelectric effect and the bulk photovoltaic effect and challenges the current piezoelectric theory. The polar-oriented films can be used to produce high-performance piezoelectric and optoelectronic materials and devices.