High speed mapping of Ga compositional profiles in CuIn1−xGaxSe2 solar cells by spectroscopic ellipsometry

The dielectric functions of CuIn
_1-x
Ga
_x
Se
₂
(CIGS) alloys as deduced by spectroscopic ellipsometry (SE) have been parameterized versus CIGS bandgap E
_g
and versus x. As a result, E
_g
can serve as a free parameter in regression analyses of SE data acquired on multilayer structures incorporating CIGS. This enables the determination of CIGS bandgap profiles in solar cell structures with SE measurement times <; 1 s. Such a capability further enables module scale mapping of Ga profiles with on-line SE instrumentation. This capability has been demonstrated by presenting maps of depth profile information on Ga composition for three stage co-evaporation of CIGS solar cells with thin absorbers. The Ga profiles have been simulated with two linear segments, and the compositions at the junction, at the minimum within the absorber, and at the back contact have been mapped. Spatial variations in x at the junction correlate with the cell's open circuit voltage, supporting the validity of the methods.