Numerical modeling of CdS/CdTe and CdS/CdTe/ZnTe solar cells as a function of CdTe thickness

CdTe-based solar cells have long been of interest for terrestrial usage because of their high potential conversion efficiency (in the range of 18–24%) with low-cost manufacturability and concern over environmental effects. In order to conserve material and address environmental pollution concerns as well as to reduce carrier recombination loss throughout the absorber layer, efforts have been carried out to decrease the thickness of the CdTe absorption layer to 1 μm. As a result, to date, the experimental part of this study has realized cell efficiencies of 15.3% and 11.5% with 7 and 1.2-μm-thick CdTe layers, grown by close-spaced sublimation (CSS) [N. Amin, T. Isaka, T. Okamoto, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 38 (8) (1999) 4666; N. Amin, T. Isaka, A. Yamada, M. Konagai, Sol. Energy Matter. Sol. Cells 67 (2001) 195]. Since some problems remain with such thin 1 μm CdTe layers, possible methods to realize higher efficiency have been investigated using novel solar cell structures, with the help of numerical analyses tools. In the theory part of this study, numerical analysis with a 1-D simulation program named NSSP (Numerical Solar Cell Simulation Program) has been used to simulate these structures. We investigated the viability of CdTe thickness reduction to 1 μm together with the insertion of higher band-gap materials (i.e., ZnTe) at the back contacts to reduce carrier recombination loss there. The study shows potential results of the thickness reduction of CdTe absorption layer for a conventional CdS/CdTe/Cu-doped C structure with around 16% efficiency for cells below 3  μm CdTe. Decreases were found in spectral response that suggest from minority carrier recombination loss at the back contact interface. A higher band-gap material like ZnTe has been inserted to produce a back surface field (BSF) to inhibit the minority carrier loss at the back contact. An increase in the efficiency to about 20% has been found for a 1 μm-thin CdTe cell, which can be attributed to the increased BSF effect at the back contact of thinner CdTe-based cells.