Nanocomposite (CuS)x(ZnS)1-x thin film back contact for CdTe solar cells: Toward a bifacial device

Critical avenues to increase energy yield for CdTe photovoltaic (PV) technology, such as bifacial cells or tandem cells designs, rely on the need to develop transparent back contact approaches that enable high efficiency and high transparency. We report the properties of earth-abundant p-type transparent conducting nanocomposite (NC) thin film (CuS)(x)(ZnS)(1-x) deposited by low-cost chemical bath deposition method. Nanocomposite films of similar to 45 nm thickness transmit > 70% of the visible light and show compact grains of size similar to 10 nm. We investigate the photovoltaic performance of both opaque and semi-transparent CdTe solar cells using (CuS)(x)(ZnS)(1-x) NC thin films as a back contact interface layer. Introduction of the NC layer between the CdTe and a standard Cu/Au metallization process yielded the best device efficiency of 13%, representing a 3.2% relative improvement over the standard Cu/Au back contact. Semi-transparent CdTe devices finished with an indium tin oxide (ITO) layer allowed for comparison of front and back illumination schemes. Experimental and modeling results confirm that the bifacial device configuration leads to significant recombination losses under back illumination due to the absence of a charge separating field, a relatively high interface recombination rate, and an insufficient electron diffusion length. Once optimized, and with an improved interface and bulk CdTe carrier lifetimes, this NC earth-abundant transparent conductor based on Cu, Zn, and S may serve as an effective, inexpensive, low-toxicity back contact layer for a bifacial CdTe solar cell.