Fingerprints Indicating Superior Properties of Internal Interfaces in Cu(In,Ga)Se₂ Thin‐Film Solar Cells

Abstract Growth of Cu(In,Ga)Se₂ (CIGS) absorbers under Cu‐poor conditions gives rise to incorporation of numerous defects into the bulk, whereas the same absorber grown under Cu‐rich conditions leads to a stoichiometric bulk with minimum defects. This suggests that CIGS absorbers grown under Cu‐rich conditions are more suitable for solar cell applications. However, the CIGS solar cell devices with record efficiencies have all been fabricated under Cu‐poor conditions, despite the expectations. Therefore, in the present work, both Cu‐poor and Cu‐rich CIGS cells are investigated, and the superior properties of the internal interfaces of the Cu‐poor CIGS cells, such as the p–n junction and grain boundaries, which always makes them the record‐efficiency devices, are shown. More precisely, by employing a correlative microscopy approach, the typical fingerprints for superior properties of internal interfaces necessary for maintaining a lower recombination activity in the cell is discovered. These are a Cu‐depleted and Cd‐enriched CIGS absorber surface, near the p–n junction, as well as a negative Cu factor (∆β) and high Na content (>1.5 at%) at the grain boundaries. Thus, this work provides key factors governing the device performance (efficiency), which can be considered in the design of next‐generation solar cells.