Postdeposition Sulfurization of CuInSe2 Solar Absorbers by Employing Sacrificial CuInS2 Precursor Layers

Herein, a new route of sulfur grading in CuInSe2 (CISe) thin-film solar absorbers by introducing an ultrathin (<50 nm) sacrificial sputtered CuInS2 (CIS) layer on top of the CISe. Different CIS top layer compositions (Cu-poor to Cu-rich) are analyzed, before and after a high-temperature treatment in selenium (Se)- or selenium+sulfur (SeS)-rich atmospheres. An [S]/([S] + [Se]) grading from the surface into the bulk of the Se- and SeS-treated samples is observed, and evidence of the formation of a mixed CuIn(S,Se)(2) phase by Raman analysis and X-ray diffraction is provided. The optical bandgap from quantum efficiency measurements of solar cells is increased from 1.00 eV for the CISe reference to 1.14 and 1.30 eV for the Se- and SeS-treated bilayer samples, respectively. A approximate to 150 mV higher V-OC is observed for the SeS-treated bilayer sample, but the cell exhibits blocking characteristics resulting in lower efficiency as compared with the CISe reference. This blocking is attributed to an internal electron barrier at the interface to the sulfur-rich surface layer. The difference in reaction routes and possible ways to improve the developed sulfurization process are discussed.

Automated summary

A new method of sulfur grading in CuInSe2 thin-film solar absorbers is proposed by introducing an ultrathin sacrificial CuInS2 layer. The sulfur grading phenomenon is observed in the Se- and SeS-treated samples, and the formation of mixed CuIn(S,Se)2 phase is confirmed. The treated bilayer samples show higher optical bandgap and V-OC, but exhibit lower efficiency due to the blocking effect of internal electron barrier.