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

Abstract

The design of a Cd-free and wider-bandgap buffer layer is stringent for future Cu(In,Ga)Se₂ (CIGSe) thin-film solar cell applications. For that, an In₂S₃ 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 In₂S₃/CIGSe heterointerface when O is added is not completely clear. Therefore, in this work, the buffer/absorber interface for a series of sputter-deposited In₂S₃ 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 In₂S₃ 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 In₂(O_0.25S_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 In₂(O_xS_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 In₂(O_xS_1−x)₃/CIGSe interface, which could result in an increased open-circuit voltage.