Improved optoelectrical performance of nanostructured ZnO/porous silicon photovoltaic devices

Abstract

This report introduced a novel technique for epitaxial bottom-up deposition of macroporous zinc oxide thin films on porous silicon (PSi) substrates. For achieving a higher surface-to-volume ratio and improving the photon trapping ability of the ZnO top layer, an optimized high-porosity PSi substrate was chosen as the template. Then, ZnO thin film was grown on the PSi substrate using the radio-frequency sputter-deposition method. The morphology of samples showed that the epitaxial layer followed the porous nature of the PSi substrate and generated a macroporous ZnO structure. The influence of post-sputtering thermal treatment at different temperatures on the structural and optical characteristics of synthesized macroporous ZnO nanostructures was studied. According to the planar surface morphologies, the average grain size of the sputtered ZnO layers heated at 300, 400, and 500 °C were 826, 1027, and 1195 nm, respectively. These results conform with the increased average crystallite size of ZnO nanostructures with higher annealing temperatures. The calculated crystallite size values were 4.14, 7.76, and 8.84 nm for the annealing temperatures of 300, 400, and 500 °C, respectively. The optical properties of the deposited thin film were improved due to the annealing process at higher temperatures. The highest light absorption coefficient was reported for the thin film annealed at 500 °C. Next, prepared macroporous n-ZnO/p-PSi samples were used to fabricate heterojunction solar cells, and their electrical properties were studied at room temperature. The optoelectrical results showed improved performance of the fabricated devices with higher annealing temperatures. The results showed an enhanced efficiency of 15.08 % for the optimized device with an annealing temperature of 500 °C, which is assigned to its increased surface and enhanced exciton generation. Moreover, this device showed lower series resistance of 4.74 Ω and higher shunt resistance of 2.98 KΩ compared to other devices with lower annealing temperatures.