  We study by photoacoustic spectroscopy the band--gap shift effect of CdS
films. The CdS films were grown by chemical bath deposition and exposed to
different annealing atmospheres over a range of temperature in which the sample
structure is observed to change. We show the band--gap evolution as a function
of temperature of thermal annealing and determine the process which produces
the best combination of high band--gap energy and low resistivity. It allows us
to know a possible procedure to obtain low--resistivity CdS/CdTe solar cells
with high--quantum efficiency.
