Influence of the Rear Interface on Composition and Photoluminescence Yield of CZTSSe Absorbers: A Case for an Al₂O₃ Intermediate Layer

The rear interface of kesterite absorbers with Mo back contact represents one of the possible sources of nonradiative voltage losses (ΔV_oc,nrad) because of the reported decomposition reactions, an uncontrolled growth of MoSe₂, or a nonoptimal electrical contact with high recombination. Several intermediate layers (IL), such as MoO₃, TiN, and ZnO, have been tested to mitigate these issues, and efficiency improvements have been reported. However, the introduction of IL also triggers other effects such as changes in alkali diffusion, altered morphology, and modifications in the absorber composition, all factors that can also influence ΔV_oc,nrad. In this study, the different effects are decoupled by designing a special sample that directly compares four rear structures (SLG, SLG/Mo, SLG/Al₂O₃, and SLG/Mo/Al₂O₃) with a Na-doped kesterite absorber optimized for a device efficiency >10%. The IL of choice is Al₂O₃ because of its reported beneficial effect to reduce the surface recombination velocity at the rear interface of solar cell absorbers. Identical annealing conditions and alkali distribution in the kesterite absorber are preserved, as measured by time-of-flight secondary ion mass spectrometry and energy-dispersive X-ray spectroscopy. The lowest ΔV_oc,nrad of 290 mV is measured for kesterite grown on Mo, whereas the kesterite absorber on Al₂O₃ exhibits higher nonradiative losses up to 350 mV. The anticipated field-effect passivation from Al₂O₃ at the rear interface could not be observed for the kesterite absorbers prepared by the two-step process, further confirmed by an additional experiment with air annealing. Our results suggest that Mo with an in situ formed MoSe₂ remains a suitable back contact for high-efficiency kesterite devices.