Ultrathin Cu(In,Ga)Se 2 Solar Cells with Ag/AlO x Passivating Back Reflector

Ultrathin Cu(In,Ga)Se 2 (CIGS) absorber layers of 550 nm were grown on Ag/AlO x 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 AlO x . 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/AlO x 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/AlO x 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/AlO x stack at the back to improve both electrical and optical properties of ultrathin CIGS solar cells.