Unveiling the Role of In Situ Al2O3 Passivation in Molecular-Ink-Processed CuIn(S,Se)2 Photovoltaics

We report on the optimization of in situ passivation of ink-based CuIn(S,Se)2 thin-film solar cells via controlled incorporation of Al2O3 in CuIn(S,Se)2 films by the addition of Al(NO3)3 to the molecular ink precursor. For this purpose, the Al/(Al + In) (AAI) metal ratio was varied from 0.05 to 0.30. We observe that the efficiency of the cells made of Al2O3-incorporated CuIn(S,Se)2 is consistently higher than those without Al2O3, especially due to an improvement in open-circuit voltage (VOC) and fill factor (FF), for all tested AAI ratios. With an AAI of 0.05, a maximum efficiency of 11.2% and an average efficiency of 8.5% (measured across 18 cells) was achieved, compared to 8.5% maximum efficiency and 6.5% average efficiency for Al-free CuIn(S,Se)2. Furthermore, we find that cells made of Al2O3-incorporated CuIn(S,Se)2 with an AAI of 0.2 show a narrow distribution in the photovoltaic performance, indicating higher reproducibility and higher FF. Energy-dispersive X-ray spectroscopy shows that, at AAI = 0.2, Al2O3 is distributed more homogeneously at the surface of the Al2O3-incorporated CISSe. Capacitance-voltage measurements reveal a reduced defect density by incorporation of Al2O3, which could be partly responsible for the higher VOC. Furthermore, using detailed surface analysis with various X-ray and electron spectroscopy methods, we derive chemical and electronic structure information from the surface. With ultraviolet photoelectron (UPS) and inverse photoemission spectroscopies (IPES), the electronic band gap of the CuIn(S,Se)2 thin-film surface is found to increase from 1.22 to 1.88 eV (+-0.12 eV) with Al2O3 incorporation. This is accompanied by a clear reduction of the conduction band spike at the CdS/CISSe interface due to Al2O3 addition, as derived by both UPS and IPES as well as temperature-dependent VOC measurements.