Non-destructive optical analysis of band gap profile, crystalline phase, and grain size for Cu(In,Ga)Se2 solar cells deposited by 1-stage, 2-stage, and 3-stage co-evaporation



Cu(In,Ga)Se2 (CIGS) thin films co-evaporated by 1-stage, 2-stage, and 3-stage processes have been studied by spectroscopic ellipsometry (SE). The disappearance of a Cu2-xSe optical signature, detected by real time SE during multistage CIGS, has enabled precise endpoint control. Band gap energies determined by SE as depth averages show little process variation for fixed [Ga]/([In] + [Ga]) atomic ratio, whereas their broadening parameters decrease with increasing number of stages, identifying successive grain size enhancements. Refined SE analysis has revealed band gap profiling only for 3-stage CIGS. Solar cells incorporating these absorbers have yielded increased efficiencies in correlation with phase control, grain size, and band gap profiling. Copyright © 2013 John Wiley & Sons, Ltd.