Spray deposited porous CIGS nanoparticle-derived thin films were selenized in a two zone rapid thermal annealing furnace and effects of various selenization parameters including Se evaporation temperature, flow rate of carrier gas, and substrate temperature on densification of the CIGS layers were investigated. It was found that higher Se supply to CIGS nanoparticles either by increasing Se evaporation temperature or by increasing the flow rate of carrier gas resulted in larger CIGS grains with higher degree of crystallinity, while it also induced formation of a thicker MoSe2 layer in-between CIGS and Mo which resulted in partial detachment of CIGS/MoSe2/Mo layers from the glass substrate. Densification of CIGS layer by growth of nanoparticles and formation of thick MoSe2 were explained by a liquid Se assisted reaction rather than by a vapor phase Se assisted reaction.

We report a new approach to fabricating Cu(In,Ga)Se₂ (CIGSe) light absorbing layers for thin film solar cells without Ga segregation using a sputtering and single-step selenization process. To mitigate Ga segregation at the CIGSe/back-contact region, which has frequently been observed in the selenization of metal/alloy precursor layers, we used Se-containing precursor layers (Cu-In-Ga-Se) to capture Ga in covalently bonded structures and investigated the effects of Se content in the precursor layers on the properties of the selenized CIGSe films and the devices. As the Se content in the precursor layer increased, Ga segregation was significantly mitigated, resulting in a completely homogenized Ga distribution when the Se/metal ratio of the precursor films is over 0.8. Finally, a thin CIGSe film (∼670 nm) with a uniform Ga distribution was processed to fabricate a solar cell, and the device exhibited a conversion efficiency of 11.7% with an open circuit potential of 0.6 V. An increase of the CIGSe film thickness to 1.55 μm resulted in a device efficiency of up to 13.16%.