Depletion width engineering via surface modification for high performance semiconducting piezoelectric nanogenerators

Piezoelectric semiconductor materials have emerged as the most attractive material for nanogenerator (NG)-based prototype applications, such as piezotronics, piezophotonics and energy harvesting, due to the coupling of piezoelectric and semiconducting dual properties. Understanding the mechanisms for high power generation, charge transport behavior, energy band modulations, and role of depletion width in piezoelectric semiconducting p-n junction, through piezoelectric charges developed by external mechanical strains, are essential for various NGs. Here, we demonstrate enhancement of the output power of one-dimensional zinc oxide (ZnO) nanowires (NWs)-based NG using a p-type semiconductor polymer, by controlling their energy band at depletion width in the piezoelectric semiconducting p-n junction interface and native defects presented in as-grown ZnO NWs. The piezoelectric output performance from the P3HT-coated ZnO NWs-based NG was several times higher than that from the pristine ZnO NWs-based NG, under application of the same vertical compressive strain. Holes from the p-type P3HT polymer significantly reduced the piezoelectric potential screening effect caused by free electrons in ZnO. Theoretical investigations using COMSOL multiphysics software were also carried out, in order to understand the improvement in the performance of surface passivated ZnO NWs-based NG, in terms of free carriers concentration and holes diffusion, due to the formation of p-n junction at the interface of ZnO and P3HT, and depletion width change.