The energetically confined traps on a semiconductor surface as a potent energy harvester: case study for the ZnO nano-flowers

Abstract

Hydrothermally grown ZnO nano-crystals were deposited on nickel substrates in the form of ZnO nano-thorn-flower formations by the spin-coating technique. The resulting semiconducting films were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). According to XRD spectra the synthesized ZnO powder was mainly characterized by (m) side structures, with ends of regular hexagonal pyramid structures (+p). The deposited ZnO film with nano-thorn-flower formations on the Ni (111) substrate had an average crystalline size of 48 nm. The SEM images confirmed the nano-thorn-flower formations. A metal–ZnO–metal structure was formed by positioning a Au electrode on top of the ZnO surface and the I–V characteristics of the resulting device were investigated in the dark and under low intensity diffused illumination. The experimental I–V data imply that the device is capable of producing electric power in the dark due to electrochemical surface interactions with gaseous polarizing molecules or ions, and also under low-energy low-intensity (0.2 mW cm−2) diffused solar radiation. The advantages offered on energy conversion efficiency by the presence of deep electronic states that are spatially confined on the nano-crystal semiconductor surface are experimentally and theoretically investigated. A potential energy diagram is proposed to present the operation principles of the structure (under dark or illumination) and link the deep level parameters to the obtained I–V characteristics. The investigated structure might be practically implemented on top of 1st G photovoltaic (PV) cells to reduce heating of PV cells by converting low energy photons of the electromagnetic spectrum to electrical energy (instead of heat).