Prospects of Thickness Reduction of the CdTe Layer in Highly Efficient CdTe Solar Cells Towards 1 µm

Abstract

This study focuses on the technique for the stable growth of CdTe (1.44 eV) with thickness near its absorption length, 1 µm, by close spaced sublimation (hereafter CSS) process, in order to achieve high conversion efficiency. X-ray diffraction (XRD) spectroscopy was carried out to examine the microstructure of the films. Current-voltage (I–V) characteristics, spectral response and other features of the solar cells using these CdTe films were investigated to elucidate the optimum conditions for achieving the best performance in such thin (1 µm) CdTe solar cells. Thickness was found to be reduced by controlling the temperature profile used during CSS growth. The temperature profile was found to be an important factor in growing high-quality thin films. By controlling the growth parameters and optimizing the annealing temperature at different fabrication steps, we have succeeded, to date, in achieving cell efficiencies of 14.3% (open-circuit voltage (Voc): 0.82 V, short-circuit current (Jsc): 25.2 mA/cm2, fill factor (F.F.): 0.695, area: 1 cm2) with 5 µm, 11.4% (Voc: 0.77 V, Jsc: 23.7 mA/cm2, F.F.: 0.63, area: 1 cm2) with 1.5 µm and 11.2% (Voc: 0.77 V, Jsc: 23.1 mA/cm2, F.F.: 0.63, area: 1 cm2) with only 1 µm of CdTe layer thickness at an air mass of 1.5 without antireflection coatings. This is important for establishing a strong foundation before developing a new structure (e.g., glass/ITO/CdS/CdTe/ZnTe/Ag configuration) with a back surface field of wide-bandgap material (e.g., ZnTe).