Annealing treatment of Cu(In,Ga)Se₂ absorbers prepared by sputtering a quaternary target for 13.5% conversion efficiency device

Copper indium gallium selenide (Cu(In,Ga)Se₂, CIGS) absorbers have been prepared by sputtering a CIGS quaternary target and imposing a subsequent annealing process. The influences of annealing temperatures on the morphology, chemical composition, microstructure and electronic properties as well as phase transformations have been investigated. Scanning Electron Microscope (SEM), X-Ray Fluorescence (XRF), Energy-dispersive X-Ray spectroscopy (EDX), X-Ray Diffraction (XRD), Raman spectroscopy and Hall analysis have been employed to analyze the morphology, chemical composition, microstructure and electronic properties, respectively. The results show that a phase separation process occurs in the CIGS absorbers when the as-deposited CIGS samples are annealed at the temperatures between 210 and 380°C. The surfaces are partly covered by regular shaped grains and platelets, which are corresponding to Cu-Se phases. When the absorbers are annealed at the temperature higher than 410°C, the CIGS absorbers recrystallize. This process improves the grains size and makes the absorbers become homogeneous and compact. Hall analysis shows the variation of electronic properties with the increase of annealing temperatures. The relationships between the electronic properties and the microstructure have been discussed in the paper as well. The solar cells have been fabricated with an optimized annealing temperature of 550°C. The highest efficiency of the CIGS solar cell devices reaches 13.5%.