Effect of ZnTe and CdZnTe alloys at the back contact of 1-μm-thick CdTe thin film solar cells

N-2-doped ZnTe was introduced onto 1-mum-thick CdTe absorbers in order to reduce the carrier recombination at the back contact of CdS/CdTe/C/Ag configuration solar cells. ZnTe films were grown by molecular beam epitaxy (MBE) on GaAs and Coming glass substrates to investigate the characteristics of the films. Epitaxial growth of ZnTe was realized on GaAs substrates and a hole concentration of 8 x 10(18) cm(-3) with a resistivity of 0.045 Omega.cm was achieved as a result of nitrogen doping. In contrast, polycrystalline ZnTe films were grown on Coming glass and CdTe thin films. Dark and photoconductivity of ZnTe films increased to 1.43 x 10(-5) S/cm and 1.41 x 10(-4) S/cm, respectively, while the Zn to Te ratio was decreased to 0.25 during MBE growth. These ZnTe films with different thicknesses were inserted into close-spaced sublimation (CSS)-grown 1-mum-thick CdTe solar cells. A conversion efficiency of 8.31% (V-proportional to: 0.74 V, J(se): 22.98 mA/cm(2), FF: 0.49, area: 0.5 cm(2)) was achieved for a 0.2-mum-thick ZnTe layer with a cell configuration of CdS/CdTe/ZnTe/Cu-doped-C/Ag. Furthermore, to overcome the problem of possible recombination loss in the inter-face layer of CdTe and ZnTe, the intermediate ternary CdZnTe is investigated. The compositional factor in Cd1-xZnxTe:N alloy is varied and the dependence of the conductivity is evaluated. For instance, Cd0.5Zn0.5Te:N, with dark and photoconductivity of 2.13 x 10(-6) and 2.9 x 10(-5) S/cm, respectively, is inserted at the back contact of a 1-mum-thick CdTe solar cell, A conversion efficiency of 7.46% (V-proportional to: 0.68 V, J(sc): 22.60 mA/cm(2), FF: 0.49, area: 0.086 cm(2)) was achieved as the primary result for a 0.2-mum-thick Cd0.5Zn0.5Te:N layer with the cell configuration of CdS/CdTe/Cd0.5Zn0.5Te:N/Au.