(34d) Combinatorial Chemical Bath Deposition of Cd1-XZnxs for Thin Film Photovoltaics

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 is poorly understood, and relationships between bath composition and stoichiometry, microstructure, and optoelectronic properties of the deposit are lacking.

We introduce combinatorial chemical bath deposition (CBD), using a continuous flow microreactor, as a new technique to deposit ternary semiconductor thin films with graded composition along the length of a single substrate.  The graded composition results in graded material properties; and spatially-dependent characterization of the substrate enables rapid and unprecedented understanding of the effect of growth conditions on optical, electronic, and structural properties of the material. The microreactor used behaves like a plug flow reactor whereby the bath composition changes as material is deposited on the substrate.  

CdZnS films showed increasing Zn:Cd ratio, quantum confinement, and resultant monotonic increase in band gap from 2.42 to 2.75 eV over the length of a single substrate. X-ray diffraction and x-ray absorption spectroscopy indicate that the deposit is a single-phase solid solution, rather than phase-separated CdS and ZnS.  Combinatorial 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.