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. The 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.

Ordered Vacancy Compound Formation at the Interface of Cu(In,Ga)Se2 Absorber with Sputtered In2S3‐Based Buffers: An Atomic‐Scale Perspective