Growth patterns of chemiplated Cu_2-xS layers in CdS/Cu₂S thin film solar cells

Abstract

The kinetics of growth of chemiplated Cu_2-xS films on thermally deposited and chemically etched CdS layers were investigated. Scanning electron microscopy examination of parent films revealed the usual columnar growth and pyramidal tops as in typical high efficiency cells, with an enhanced area factor of 2.5. During chemiplating, Cu_2-xS growth occurs conformal to the grain surface but deep penetrations result along the grain edges. In the present studies, quantitative estimates of such growth behaviour are obtained by varying the ion exchange reaction parameters, particularly the dip period and the pH of the CuCl bath. Results suggest that for a pH value of 4.6 the growth is parabolic in nature whereas in solutions of lower pH (about 3.4) a fast linear growth mechanism dominates. The textured morphology of the grain yields an expression for net Cu_2-xS growth from which grain surface thickness and grain boundary penetration depths are evaluated. Resultant changes in junction area are shown to affect the normalized open-circuit voltage of the CdS-Cu₂S junction. The spectral responses of encapsulated cells are given after growth patterns of the Cu_2-xS layers. These studies reveal that the pH of the solution plays an active role in controlling the Cu_2-xS growth processes and also in obtaining superior cell characteristics.