Effect of Cu excess on three-stage CuGaSe2 thin films using in-situ process controls

The objective of this work is to study the effect of Cu excess and compare the growth mechanism of CuGaSe2 (CGS) films co-evaporated using a bilayer and a three-stage process and evaluate the consequences of the latter for CGS on transparent back contacts. CGS thin films are prepared by co-evaporation in a three-stage process onto Mo/soda-lime substrates and onto FTO. In contrast to the bilayer process, Cu-Se phases are only observed on the surface at the end of the second stage, e2. This allows to work with a broader Cu-excess window. Atomic ratios (Cu/Ga)e2 of around 1.3 at the end of deposition phase 2 in the three-stage process show the better device efficiencies due to a larger grain formation. Increasing the Cu content leads to a slight decrease of the grain size and voids are observed in the film, reducing the FF of the device. The CGS morphology and the solar cells efficiency are dominated by the Cu excess more than by the T substrate between 535 deg. C and 500 deg. C. Similar results are obtained for CGS on FTO: (Cu/Ga)e2 ∼ 1.3 as best composition at T substrate = 500 deg. C