A parametric study of TiO2/CuInS2 nanocomposite solar cells:: how cell thickness, buffer layer thickness, and TiO2 particle size affect performance

3D CuInS2/TiO2 nanocomposite solar cell performance is strongly influenced by several structural factors, including cell thickness, buffer layer thickness, and the morphology of the TiO2 nanoparticulate matrix. To delineate the effect of these structural factors on photovoltaic performance, a series of parametric studies are performed where a single structural parameter is varied ( TiO2 nanoparticulate matrix thickness, In2S3 buffer layer thickness, or TiO2 particle size) while all other fabrication conditions are held constant. The best overall performance ( 3.0% efficiency at AM 1.5) is achieved from a device with TiO2 matrix thickness approximate to 200 nm, In2S3 buffer layer thickness approximate to 60 nm, and TiO2 nanoparticulate size = 300 nm. Notably, the film thickness in the best-performing cell ( 200 nm) is less than the TiO2 particle size ( 300 nm), corresponding to a discontinuous nanoparticulate film. Thicker TiO2 nanoparticulate films or smaller TiO2 particles sizes lead to decreased performance due to increased charge transport resistance. However, the performance from a planar cell ( where the TiO2 nanoparticulate layer is not used) is inferior to the performance from the better-optimized 3D cells, indicating that some degree of nanostructuring improves performance. Device performance is also observed to depend strongly on In2S3 buffer layer thickness, with optimal performance achieved for a buffer layer thickness of approximately 60 nm. The optimal buffer layer thickness is governed by two opposing factors: increasing the buffer layer thickness improves the interfacial characteristics ( as measured by decreasing leakage conductance, G) but also screens the incoming light and causes an increase in the charge transport resistance ( as measured by the cell series resistance, R-s).