Abstract
Ph.D.
The two-step growth process, involving the selenization and sulfurization of sputter
deposited CuInGa alloys has been identified as a commercially viable method to produce
large area Cu(In1-xGax)(Se1-ySy)2 absorber films for solar cell application. The success of
this method is however limited by insufficient control over the lattice parameters and band
gap of the compound due to phase segregation associated with non-uniform Ga and S
incorporation.
This study provides an approach to overcome this limitation by investigating the influence
of process parameters on the structural features of the Cu(In1-xGax)(Se1-ySy)2 films. In this
approach, films were partially selenized in optimum H2Se/Ar flow to produce composite
alloys comprising of a mixture of binary selenides (InSe, CuSe and GaSe) and at least one
group I-III-VI ternary alloy. The subsequent reaction step in H2S/Ar produced
homogeneous Cu(In1-xGax)(Se1-ySy)2 films. The lattice constants of the resulting films
varied linearly with an increase in the S/(S+Se) ratio in accordance with Vegard’s law. The
Raman spectra of the films were characterized by the presence of the A1-Se mode near
180 cm-1 and a low intensity, A1-S mode around 290 cm-1. With an increase in the
S/(S+Se) ratio of the films, the FWHM of the A1-Se mode increased and its frequency
shifted linearly towards that of A1-S mode. A corresponding increase in the value of the
Urbach energy, attributed to an increase in chalcopyrite crystal alloy disorder, was
observed from the analysis of the transmission and reflectance data. 0.45 cm2 area
devices with conversion efficiencies between 12% and 15%, were fabricated from absorber
layers with the (112) x-ray diffraction peak position between 27.1°and 27.2°, corresponding
to the S/(S+Se) ratio of about 0.18 to 0.20. The process scale up was demonstrated by the
fabrication of large area, (30 x 40) cm2 modules, with conversion efficiencies of 10%.