Phase and film formation pathway for vacuum-deposited C u2BaSn (S,Se ) 4 absorber layers

Earth-abundant copper barium thioselenostannate, C u₂BaSn (S_{,Se ) 4} , absorbers have recently demonstrated promising optoelectronic and defect resistance properties for solar harvesting applications. The highest photovoltaic device efficiencies have been achieved in vacuum-based co-sputter deposited films, yet there is a tendency for a multilayer formation consisting of large, plateletlike surface grains and a smaller grain-sized underlayer (often accompanied by voids). In this work we use a combination of in situ and ex situ x-ray diffraction and scanning electron microscopy to unravel the coupling of phase evolution to film morphology. We find that Cu surface migration and associated Cu-rich phases play a defining role in determining the overall film structure.