Ultrathin Cu(In,Ga)Se2 Solar Cells with Ag/AlOx Passivating Back Reflector

Ultrathin Cu(In,Ga)Se-2 (CIGS) absorber layers of 550 nm were grown on Ag/AlOx stacks. The addition of the stack resulted in solar cells with improved fill factor, open circuit voltage and short circuit current density. The efficiency was increased from 7% to almost 12%. Photoluminescence (PL) and time resolved PL were improved, which was attributed to the passivating properties of AlOx. A current increase of almost 2 mA/cm(2) was measured, due to increased light scattering and surface roughness. With time of flight-secondary ion mass spectroscopy, the elemental profiles were measured. It was found that the Ag is incorporated through the whole CIGS layer. Secondary electron microscopic images of the Mo back revealed residuals of the Ag/AlOx stack, which was confirmed by energy dispersive X-ray spectroscopy measurements. It is assumed to induce the increased surface roughness and scattering properties. At the front, large stains are visible for the cells with the Ag/AlOx back contact. An ammonia sulfide etching step was therefore applied on the bare absorber improving the efficiency further to 11.7%. It shows the potential of utilizing an Ag/AlOx stack at the back to improve both electrical and optical properties of ultrathin CIGS solar cells.

Automated summary

This study focused on the growth of ultrathin Cu(In,Ga)Se-2 (CIGS) absorber layers on Ag/AlOx stacks, resulting in improved efficiency of solar cells to almost 12%, along with enhanced fill factor, open circuit voltage, and short circuit current density. The addition of the stack led to improvements in photoluminescence (PL), time resolved PL, and an increase in light scattering and surface roughness. Furthermore, it was found that Ag was incorporated throughout the CIGS layer, and residues of the Ag/AlOx stack were observed on the Mo back contact, affecting surface roughness and scattering properties. An ammonia sulfide etching step further enhanced the efficiency to 11.7%.