Segregation of Te at the back contact in electrochemically deposited CdTe thin film solar cells

Abstract

One significant problem associated with thin film CdTe solar cells is the reverse biased Schottky barrier at the back contact (CdTe/metal interface), caused by the high electron affinity of p-CdTe, along with the non-availability of metals with adequate work functions. This Schottky barrier increases contact resistance, thereby reducing cell performance. In order to eliminate this barrier, attempts to create a tunnelling contact are being made by etching the CdTe surface, to form a Te rich degenerate semiconductor surface. The Te rich surface layer usually contains increased conductivity and is p+-type. Electrochemically deposited CdTe thin films are naturally Te-rich, but significantly, in this work, we report the segregated concentration of Te at the back contact resulting from the controlled addition of Te to the deposition electrolyte. This segregation of Te is notable as it minimises the use of acidic etchants to create a Te rich surface. Raman and XRD spectra confirm Te segregation at the surface of the CdTe, which one would expect to produce a good ohmic back contact. Effectively, high-quality layers and cells can be obtained from this process, which can be scaled up to an industrial level, inline and without the use of etching baths or external CdCl2 treatment. Photocurrent spectroscopy was undertaken on CdTe thin film solar cells to investigate their interfacial and structural properties, both of which play a crucial role in determining photovoltaic performance. High chopping frequencies were employed in order to obtain reliable incident photon to current conversion efficiency (IPCE) data, in the cases where surface recombination was rapid. The absorption edge at the low energy end of the IPCE spectrum corresponds to an interfacial bandgap of 1.45 eV, which in turn corresponds to interdiffusion of about 10% of S into CdTe, according to bowing equations found in literature.