Crystallographic properties and elemental migration in two-stage prepared CuIn1−xAlxSe2 thin films for photovoltaic applications

Highlights: •The effect of the x = Al/(Al + In) ratio on the formation of Cu(In1−xAlx)Se2 is studied. •Focus on the effect of the selenisation process on the structure and depth profiles. •Al is incorporated into the chalcopyrite lattice for x ⩾ 0.38. •For x = 0.12, Al aggregates at the back for temperatures in the range 250 °C ⩽ T ⩽ 540 °C. •A post-selenisation anneal yields the incorporation of Al where it is present. -- Abstract: Two-stage fabrication of CuIn1−xAlxSe2 thin films for photovoltaic absorbers using sputtered Cu–In–Al metallic precursors has been investigated. Precursors containing different relative amounts of Al were selenised and their structural and chemical properties characterised. X-ray diffraction (XRD) analyses revealed that the Al was only incorporated into the chalcopyrite structure for precursor composition ratios x = [Al]/([Al] + [In]) ⩾ 0.38, while chemical analysis of the cross-section indicated partial segregation of Al near the back of the film. Precursor films in the range of compositions that yielded no Al incorporation were then selenised at four different temperatures. Glow discharge optical emission spectroscopy, plasma profiling time-of-flight mass spectrometry and XRD analyses provided an insight into the diffusion processes and reactions taking place during the selenisation stage. The effect of post-selenisation annealing without additional Se was also investigated, and led to partial incorporation of the Al into the CuInSe2 lattice but no rediffusion