CuIn1-xGaxSe2 growth process modifications: Influences on microstructure, Na distribution, and device properties

Co-evaporation of Cu, In, Ga, and Se to form CuIn1-xGaxSe2 is known to yield high efficiency solar cells and modules. Highest efficiencies are achieved when using a multi-stage co-evaporation process, where the Cu-In-Ga-Se film enters a Cu rich composition during an intermediate stage of the growth. Furthermore, incorporation of sodium is crucial for high performance devices. We investigated the influence of varying the evaporation profile, especially the Cu excess during growth, on the microstructure of the final CIGS layer, on the distribution of sodium through the layer, and on the photovoltaic performance. Experiments were performed on CIGS layers grown at substrate temperatures of 450 and 600 degrees C. With increasing maximum [Cu]/[In+Ga] ratio during the deposition, an increase in grain size of the CuIn1-xGaxSe2 layer is observed. For high temperature grown samples, best efficiencies are achieved using minimal Cu excess during the second stage of the growth process. Results show a change in the sodium distribution across the absorber thickness for layers grown at high temperature when the duration of the Cu rich growth regime is changed. We observed that for layers exposed to a Cu rich regime for a short timeframe, Na accumulates at the surface of the layer while for longer exposure times it is more evenly distributed in the top region of the CIGS layer. Evidence for the suppression of the growth of a group III rich phase (ordered vacancy compound) by Na is found. (C) 2011 Elsevier B.V. All rights reserved.