ZnO-Based Nanostructuring Strategy Using an Optimized Solution Process in CuInS2 Superstrate Photovoltaics

Abstract

Nanostructuring strategies have received significant attention recently in the context of thin-film solar cell design. Superstrate-type CuInS2 (CIS) thin-film solar cells based on hydrothermally grown ZnO nanorod (NR) arrays have been prepared through dimensional optimization and comparative parametric characterization to yield a highly efficient device configuration. Solution-processed transparent ZnO nanostructures covered with a uniform CdS buffer layer were prepared using liquid processing methods to achieve efficient light harvesting and transport of photogenerated charge carriers. Molecular precursor solutions containing metal and chalcogen precursors of CIS light absorbers yielded interpenetrated radial p–n junctions across nanostructured CdS/ZnO NR arrays. The performances of solar cells could be improved by experimentally optimizing device configurations across certain experimental parameters, such as the CIS annealing temperature, the buffer-layer thickness, or the NR length, all of which affect charge conduction and charge recombination kinetics. The interfacial charge-transfer properties and recombination characteristics were investigated by observing the dark current, electrochemical impedance, and open-circuit voltage (Voc) decay. Impedance data were fit to a proposed equivalent circuit model consisting of resistor–capacitor (ZnO/CdS) and resistor–constant phase element (CPE) (CdS/CIS) components as a means for characterizing the interfacial properties. Recombination at the p–n interface increased in samples comprising a thin buffer layer and long NRs. A 250 °C process temperature and optimal CdS deposition and ZnO NR growth times yielded the best efficiency, 6.8%. Our results suggest that solution-processed superstrate structures prepared using nanostructuring approaches could provide highly efficient low-cost photovoltaic devices.