Surface Modification of Cu(In,Ga)Se2 Grains

Nearly all of the world’s energy demand today is being met by the use of non-renewable energy sources. With the worldwide energy demand projected to increase in the coming years, it is vital to find alternative and renewable energy sources. Among the available renewable energy sources, solar energy is the most promising in meeting the worldwide energy demand. Recently, thin film solar cells have garnered attention due to their thinner architecture and relatively high optical absorption coefficients, as opposed to the conventional crytslline silicon solar cells. One of the most promising thin-film solar cell absorber materials is Cu(In,Ga)Se2 (CIGSe), achieving power conversion efficiencies approaching those of crystalline Si. The highest efficiency CIGSe devices were made via costly vacuum-based co-evaporation process. CIGSe devices made from solution-processed methods have also garnered attention due to their lower costs, and their efficiencies have increased considerably in recent years. In this thesis, CIGSe absorber layers are fabricated via the solution-processed from nanoparticle-based sulfide CIGS. The most important step in fabrication of a CIGSe solar cell absorber layer is the selenization step, which is the thermal sintering of a CIGSe precursor layer in the presence of selenium vapor to achieve large, dense selenide grains that are required for adequate PV performance. It is determined that maintaining adequate selenium vapor pressure on the substrate during the selenization and subsequent cooldown is crucial in producing high efficiency solar cell devices. Furthermore, exposing the CIGSe grains to a Se-deficient atmosphere causes Se to evaporate from the grains, and subsequently modifying the CIGSe grain surface. The modified grain surface adversely impacts the PV performance of the final solar cell device by forming defects due to the decrease in selenium concentration. These defects are manifested in increased current shunting, and decrease the overall efficiency of the device