Microreactor chemical bath deposition of laterally graded Cd _1-xZn_xS thin films: A route to high-throughput optimization for photovoltaic buffer layers

Cd_1-xZn_xS (CdZnS) is a promising replacement for the CdS buffer layers in copper indium gallium (di)selenide (CIGS) solar cells because the wider band gap of CdZnS offers improved optical transmittance of blue light. Chemical bath deposition (CBD) is the state-of-the-art deposition method for CdS and CdZnS. However, CBD of CdZnS is poorly understood, and relationships between bath composition and stoichiometry, microstructure, and optoelectronic properties of the deposited film are lacking. We introduce CBD using a continuous flow microreactor as a new technique to rapidly explore a wide variety of deposition conditions on a single substrate using spatially dependent characterization. X-ray diffraction and X-ray absorption spectroscopy indicate that the film is a mixture of nanocrystalline CdZnS and amorphous Zn(O,OH,S). Over the length of a single substrate, films showed increasing Zn:Cd ratio in the nanocrystalline phase, increasing amorphous content, and increasing quantum confinement, and resultant monotonic increase in band gap from 2.42 to 2.75 eV. Microreactor CBD (μR-CBD) enables rapid identification of CdZnS compositions that are ideal candidates for thin film photovoltaics, as well as determination of the CBD conditions required to deposit them.