Ordered Vacancy Compound Formation at the Interface of Cu(In,Ga)Se$_2$ Absorber with Sputtered In$_2$ S$_3$ ‐Based Buffers: An Atomic‐Scale Perspective

The design of a Cd-free and wider-bandgap buffer layer is stringent for future Cu(In,Ga)Se$_2$ (CIGSe) thin-film solar cell applications. For that, an In$_2$S$_3$ buffer layer alloyed with a limited amount of O (well below 25 mol%) has been proposed as a pertinent alternative solution to CdS or Zn(O,S) buffers. However, the chemical stability of the In$_2$S$_3$/CIGSe heterointerface when O is added is not completely clear. Therefore, in this work, the buffer/absorber interface for a series of sputter-deposited In$_2$S$_3$ buffers with and without O is investigated. It is found that the solar cell with the highest open-circuit voltage is obtained for the O-free In$_2$S$_3$ buffer sputtered at 220 °C. This improved open-circuit voltage could be explained by the presence of a 20 nm-thick ordered vacancy compound (OVC) at the absorber surface. A much thinner OVC layer (5 nm) or even the absence of this layer is found for the cell with In$_2$(O$_{0.25}$S$_{0.75}$)3 buffer layer where O is inserted. The volume fraction of the OVC layer is directly linked with the magnitude of Cu diffusion from the CIGSe surface into the In$_2$(O$_x$S$_{1−x}$)$_3$ buffer layer. ... mehrThe O addition strongly reduces the Cu diffusion inside the buffer layer up to complete suppression for very high O contents in the buffer. Finally, it is discussed that the presence of the OVC layer may lower the valence band maximum, thereby forming a hole barrier, suppressing charge carrier recombination at the In$_2$(O$_x$S$_{1−x}$)$_3$/CIGSe interface, which could result in an increased open-circuit voltage.