Kesterite is an attractive material for absorber layers in thin film photovoltaics. Solar cells based on kesterite have shown a substantial progress over the last decade; nevertheless, further improvements in device efficiency are pending due to the open‐circuit voltage (Voc) deficit (i.e., difference between the maximum V oc that can be achieved according to Shockley–Queisser limit and actual V oc from the device). In this study, the optoelectronic properties of the author's internal record Cu2ZnSnSe4 solar cell, which shows a power conversion efficiency of 11.4%, are presented. The device measurements reveal a Voc deficit of 337 mV, which is one of the lowest V oc deficits in the literature. Moreover, an unusual behavior for kesterite is observed: (i) photon energy of the photoluminescence emission and (ii) the extrapolated V oc for 0 K are both matching the band gap region of the absorber. These results indicate a significant improvement in the recombination characteristics and absorber quality in comparison to other kesterite devices in literature.

In this paper, we present our route to fabricate Cu2ZnSnSe4 (CZTSe) thin films, which allows to achieve reproducible processing of kesterite absorber, that leads to efficiencies in the range of 10%. The article mainly focuses on the annealing process and demonstrates that controlling of the reactor pressure for selenization can be reliably used to tune the losses of volatile constituents in the absorber, enabling adjustments on the properties of the film and solar cell. The findings reveal a noteworthy resilience to small changes of the process parameters in the vicinity of optimum conditions. Interestingly, a certain pressure range for optimum Zn and Sn composition exists, which results in a broad and stable process window and enables reproducible processing of CZTSe with high power conversion efficiencies. The established process also allows simple upscaling of the device area and results in a power conversion efficiency of ≈ 8% on a large area of 2.85 cm2. The highest efficiencies achieved by our process are around 11% for smaller lab scale devices.