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The deposition of a Cu2ZnSnSe4 (CZTSe) thin film with a multi‐stage co‐evaporation process is investigated with time‐resolved in situ X‐ray diffraction (in situ XRD). For the experiment a novel setup intended for in situ analysis of thin film deposition processes was used. The in situ data confirm the former observation that CZTSe growth is delayed with deposition of only Cu2−xSe and ZnSe in the initial process stage and provide new insight into the evolution of the appearing phases. In Zn‐rich deposition conditions, ZnSe deposited at the beginning may not be consumed by the growing CZTSe but remain as an unreacted layer at the interface to the Mo back contact. Cu2−xSe growth starts with the formation of a Cu rich phase, which is reduced to a Cu poor phase in the process. Furthermore, our results show that in situ XRD at elevated temperatures is able to distinguish between ZnSe and CZTSe and that it can be used for the detection of ZnSe as secondary phase.

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Solar cells with Cu(In, Ga)Se 2 absorbers rely on the three-stage co-evaporation process with Cu-poor/Cu-rich/Cu-poor absorber deposition conditions for highest efficiency devices. During the three-stage process, the formation and evolution of different selenide phases with changing compositions throughout the process crucially determine the final absorber quality. In this contribution, we monitor the evolution of crystalline phases in real-time with an X-ray diffraction (XRD) line detector setup implemented into an evaporation setup. Using the common three-stage process, we prepare and compare samples covering the full alloying range from CuInSe 2 to CuGaSe 2 . The in situ XRD allows the detection of the crystalline phases present at all times of the process as well as an advanced analysis of the phase evolution through a closer look at peak shifts and the full width at half maximum. For samples with a Ga/(Ga þ In) ratio (GGI) < 0.5, distinct phase transitions associated with the transition to the reported vacancy compounds Cu(In,Ga) 5 Se 8 and Cu(In, Ga) 3 Se 5 are observed. No such indication was found for samples with a GGI > 0.5. For Ga-rich Cu(In, Ga)Se 2 phases with a GGI of 0.55, the XRD analysis evidenced a Ga-rich phase segregation before the stoichiometric point was reached. The above findings are discussed in view of their implication on wide gap solar cell performances.

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