High Efficiency Aqueous Solution Sprayed CIGSSe Solar Cells: Effects of Zr4+-Alloyed In2S3 Buffer and K-Alloyed CIGSSe Absorber

Even though solution-processed Cu(In,Ga)(S,Se)(2) (CIGSSe) solar cells have been developed for long time, their power conversion efficiency (PCE) is still much behind that achieved by vacuum-based process, and some solvents used in solution-based processes are not environmental friendly. High-efficiency CIGSSe solar cells made by using water-based spray deposition in an air environment are reported. Water is the most eco-friendly and cheapest solvent compared to the other solvents used in solution-based fabrication. In2S3-based buffer (eco-friendly Cd-free buffer) and CdS buffer are tested as a buffer layer. For higher PCE with In2S3-based buffer, a Zr4+-alloyed In2S3 is used and Zr4+ alloying effects are investigated. It is found that Zr4+ alloying increases the electron concentration in the buffer and passivates defect states in the CIGSSe absorber, which enhance PCE significantly. On the other hand, it is found that K-alloying into the CIGSSe absorber passivates defects in the CIGSSe absorber, which further increases PCE. Owing to the observed beneficial effects, highest PCE of 16.08% and 17.74% is achieved with Zr4+-In2S3 buffer and CdS buffer, respectively. In addition, since fabricated solar cells are made with bandgap below 1.15 eV, they can be utilized as a bottom cell for high efficiency tandem cell.