Reduced recombination in a surface-sulfurized Cu(InGa)Se2 thin-film solar cell

This study demonstrates surface sulfurization effects on Cu(InGa)Se-2 (CIGSe) thin-film solar cells with a single back-graded band gap. Single back-graded CIGSe thin films were prepared via a three-stage process in a high-vacuum molecular beam epitaxial growth chamber and were subsequently annealed in a tube furnace under environmental conditions with H2S gas. After sulfurization, an similar to 80-to similar to 100-nm-thick CuIn(SSe)(2) layer with significantly small Ga contents (CISSe: Ga) was formed on the CIGSe layer. The newly formed CISSe: Ga layer exhibited graded S contents from surface to bulk, thus resulting in a front-graded band gap. In addition, CISSe: Ga was covered with S-enriched CISSe region that was extended from the surface to a depth of a few nm and was depleted of Ga. A device with the sulfurized CIGSe showed reduced recombination at the buffer-absorber interface, in space-charge region and in bulk. Consequently, the open circuit voltage increased from 0.58V (in the non-sulfurized case) to 0.66V, and the conversion efficiency improved from 15.5 to 19.4%. This large improvement is caused by the front graded band gap at the surface and the hole-blocking barrier, which suppress recombination at the CdS/CISSe: Ga interface. In addition, sulfurization followed by KF post-deposition treatment (PDT) increased the efficiency to 20.1%. Compared to the untreated sulfurized device, the KF-PDT device delivered an increased carrier lifetime and reduced the recombination in bulk probably because the defects were passivated by the K, which penetrated into the bulk region. (C) 2018 The Japan Society of Applied Physics