Investigation of factors limiting efficiency in Cu(In,Ga)Se2 thin film solar cells during rapid evaporation process

Rapid evaporation is crucial in the low-cost manufacturing of Cu(InxGa1-x)Se2 (CIGS) thin-film solar cells. This form of evaporation deteriorates cell performance in an open-circuit voltage and fill factor. Cell performance is strongly dependent on the deposition process and properties of the thin films. With respect to evaporation rates, analyses of electrical properties, film composition, grain structure, and phase characteristics were conducted to investigate limits on the efficiency of evaporated CIGS thin-film solar cells. CIGS solar cells evaporated at different deposition rates were compared. Raman spectroscopy was used to characterize the residual phases deteriorating the cell performance of these solar cells. Sodium (Na) profiles measured using secondary-ion mass spectrometry revealed that with higher CIGS evaporation rates, Na diffusion in the CIGS layers is lower. Rapidly evaporated CIGS led to two features, residual phases of the CIGS remained and Na concentrations near the surface were insufficient. This result implies that a shortfall in the p-type carrier density during rapid evaporation is a critical factor negatively affecting the efficiency of CIGS thin-film solar cells.