Influence of Agricultural Environment on the Degradation of CIGS-Based Solar Cells

Current approaches to reliability in photovoltaics focus mainly on the effects of temperature, illumination, or mechanical load, while among the chemical factors, only humidity is generally considered [1]. In this work, we took into account the presence of ammonium sulfate (NH₄)₂SO₄, an atmospheric pollutant generated by agricultural activities [2], [3]. We sought to understand its impact on the stability of CIGS solar cell materials and interfaces, relevant technology for flexible applications. Our previous research demonstrated that thin Al₂O₃ encapsulation, efficient in damp heat test [4], was dissolved by (NH₄)₂ SO₄ [5], [6]. The current study therefore presents the tests obtained on non-encapsulated cells and representative stacks of layers. The samples were subjected to accelerated aging tests involving daily variations in humidity and temperature with and without (NH₄)₂ SO₄ deposition. Loss of performance was assessed with IV curves and EQE and chemical modification was evaluated by Raman spectroscopy, SEM-EDX and GD-OES. Unencapsulated samples aged without pollutant lost 37% loss of efficiency in just 9 days. The loss mainly due to a decrease in V_oc and R_sh, which could be explained by an increase in recombination in the CIGS and its interfaces. New species, Cu_(2-x)Se and Na_xMo_yO_z, which formation was confirmed by in situ Raman, could act as recombination centers or shunt paths. In the presence of (NH₄)₂ SO4, the reduction in efficiency was 60% after rinsing. The efficiency degradation was mainly due to a loss of J_sc and FF. The current was certainly affected by the transformation of transparent conductive oxide AZO in Zn4(SO₄)(OH)₆ · nH₂O. The FF loss could be linked to an increase in contact resistance, evidenced by R_sheet measurement of Mo and AZO. Raman evidenced the oxidation of Mo and local dissolution of AZO. In conclusion, fertilizers can significantly affect reliability of thin layer solar cells and their presence must be considered for agrivoltaics.