Evaluation of CuInSe2 Materials and Solar Cells Co-evaporated at Different Rates Based on Real Time Spectroscopic Ellipsometry Calibrations

Copper indium diselenide (CuInSe2; CIS) thin film solar cells have been fabricated with absorber layers deposited in a one-stage co-evaporation process at a substrate temperature of T
S
= 570°C and at different rates, r. The parameters T
S
and r are the two key variables in a one-stage thermal evaporation process that can be varied for optimization of thin film properties. The role of rate in CIS absorber layer deposition is explored in this study by utilizing Cu and In source calibrations developed previously. These calibrations were obtained by real time spectroscopic ellipsometry analysis, which provided accurate deposition rates in terms of effective thickness (volume per planar area) for Cu and In2Se3 thin films deposited at different Cu and In source temperatures. Good agreement of the measured stoichiometric composition y = [Cu]/[In] and r values with those predicted from the calibration are obtained by assigning film density factors to the Cu and In2Se3 calibration depositions. Solar cells were fabricated from 1.6-1.9 μm thick CIS absorbers with y = 0.91 ± 0.02 deposited at rates in the range of ∽ 3 – 8 Å/s. The crystallite size of CIS increases as its deposition rate is reduced, and this appears to give rise to lower nucleation rates for chemical bath deposition of the overlying CdS. An absorber deposition rate for optimum solar cell performance is identified at ~ 3.3 Å/s, as reflected in both the open circuit voltage and fill factor. The ultimate goal of this study is to explore and optimize narrow bandgap CIS for use as an absorber in the bottom junction of tandem solar cells.