Physical vapor deposition and analysis of copper indium aluminum diselenide thin films for high band gap solar cells

CuInSe₂ films and related alloys have been used to fabricate the highest efficiency thin film solar cells. Alloying CuInSe₂ with CuAlSe₂ provides a way to engineer the band gap of the resulting films from 1 to 2.7 eV, thereby providing a pathway for improving device performance. In this work, thin films of CuIn_1−xAl_xSe ₂ obtained by multi-source PVD were characterized and investigated for their potential use as high band gap solar cells. The band gap of the films was varied by controlling the [Al]/[Al + In] ratio. Deposition of these films with varying [Cu]/[Al + In] ratios and thickness (1–4 μm) was carried out at substrate temperatures from 350–530°C.; CuIn_1−xAl_xSe₂ based solar cells have been fabricated using the structure glass/Mo/CuIn_1−xAl _xSe₂/CdS/ZnO/grid. The effect of varying the band gap on device performance will be discussed. The highest efficiency obtained in this work is 11% using a film with E_g ≈ 1.3 eV. For high Al content, x > 0.3, device-performance decreases mainly due to poor FF similar to that observed in CuIn_1−xGa_xSe₂ devices and is attributed to poor minority carrier collection.; For CuIn_1−xAl_xSe₂ films with x = 1, data is analyzed and presented with respect to [Cu]/[Al] and Se to total metal flux ratio, R_Se/R_M. Phase analysis shows that the resulting films contain different phases that depend on these parameters. Several of these films also contain concentrations of oxygen varying from 12 to 60 at. % as the [Cu]/[Al] ratio decreases. For R_Se/R _M > 10, a new structure we label as Cu_xAl_ySe _z was observed. The oxygen content in all of the films obtained under R_Se/R_M > 10 vary between 1–3 at. %. Based on the Cu-Se, Al-Se, Cu-Al binary and the Cu₂Se-Al₂Se ₃ pseudo-binary phase diagrams, a phenomenological film growth model is presented showing that the film growth kinetics are controlled by the delivery of Se.