Synergistic effect between molybdenum back contact and CIGS absorber in the degradation of solar cells

The stability of molybdenum (Mo) back contact and Cu (In_xGa_(1-x)Se₂(CIGS) absorber layers interfaces relevant for CIGS-based solar cells was investigated using accelerated aging test, considering humidity and temperature daily variations as well as atmospheric pollution. Different configurations of sputtered Mo and co-evaporated CIGS layers deposited on soda lime glass with or without ALD-Al₂O₃ encapsulation were investigated. They were exposed for 14 days to 24 h-cycles of temperature and humidity (25°C at 85% RH and 80°C at 30% RH) with and without solution of the pollutant salts (NaCl, Na₂SO₄, and (NH₄)₂SO₄) deposited as drops on the sample to mimic marine, industrial, and rural atmospheric conditions, respectively. ALD-Al₂O₃ encapsulation failed to protect the samples against the pollutants regardless of configuration. The evolution of the films was characterized by Raman spectroscopy, grazing incidence X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Unencapsulated Mo degraded forming a mixture of oxides (MoO₂, MoO₃, and Mo₈O₂₃). Unencapsulated CIGS on glass substrates was not altered, whereas dark spots were visible at the surface of Mo/CIGS configurations. Further characterization evidenced that even though the Mo layer was buried, its corrosion products were formed on top of CIGS. Mo corrosion products and copper selenide, Cu_2-xSe, were identified in dark spots. Their formation and evolution were further investigated by in situ Raman spectroscopy. A speculative mechanism explaining the interplay of molybdenum and CIGS layers during aging is proposed. In place of Mo oxides, detected on the open surface of bare Mo, soluble molybdates are expected in confined environment where alkalinity locally increases. The molybdate ions may then react with sodium ions accumulated at the grain boundaries of CIGS, forming Na₂MoO₄. The latter could form Na₂Mo₂O₇ during drying because of pH decrease by atmospheric CO₂ adsorption. High pH in confined zone, combined with relatively high temperature, is also believed to lixiviate gallium into soluble tetragallates [Ga (OH)₄]²⁻, which could precipitate into Ga₂O₃ with pH decrease leaving Ga depleted Cu_2-xSe.