Solar cell degradation : the role of moisture ingress

Abstract

Moisture ingress is one of the key fault mechanisms responsible for photovoltaic (PV) devices degradation. Moisture and moisture induced degradation (MID) products can attack the solar cell and the PV module components which can lead to solar cell degradation (e.g., microcracks), corrosion, optical degradation, potential induced degradation (PID), etc. These MID mechanisms have dire implications for the performance reliability of PV modules. Understanding the influence of moisture ingress on solar PV device’s degradation will boost the interest in investing in solar PV power installations globally, especially in the Nordics. In this thesis, the effect of moisture ingress on 20-years old field-aged multicrystalline silicon (mc-Si) PV modules is investigated. The defective areas in the PV modules were identified using visual inspection, electroluminescence (EL), ultraviolet fluorescence (UV-F), and infrared thermal (IR-T) techniques. Scanning electron microscopy and energy dispersive Xray spectroscopy (SEM-EDS) analyses were used to elucidate the role of moisture on the observed degradation mechanisms. In addition, temperature coefficient profiling is used as a diagnostic tool to characterize different moisture induced defects. The ethylene vinyl acetate (EVA) front encapsulation was found to undergo optical degradation and the extracted cells show dark discolored Tedlar®/Polyester/Tedlar® (TPT) backsheets. Corrosion at the solder joint was dominant and is attributed to the dissolution of lead and tin (main components of solder) and the Ag grids in moisture and acetic acid due to galvanic corrosion. Degradation of the EVA encapsulation produces acetic acid, carbon dioxide, phosphorus, sulfur, fluorine, and chlorine. It was observed that under the influence of moisture ingress, leached metal ions e.g., Na, Ag, Pb, Sn, Cu, Zn, and Al migrate to the surface of the solar cells. This led to the formation of oxides, hydroxides, sulfides, phosphates, acetates, and carbonates of silver, lead, tin, copper, zinc, and aluminum. Also, other competing reactions led to the formation of stannates of copper, silver, sodium, and zinc. Similarly, migration of silver and aluminum to the surfaces of the TiO2 antireflection coating (ARC) nanoparticles (NPs) lead to the formation of titania-alumina and silver-titania complexes. Formation of these titania-metal complexes affects the opto-electrical efficiency of the TiO2 ARC in the PV module. Additionally, in the presence of moisture and acetic acid, Pb is preferentially corroded (to form lead acetate complexes) instead of the expected sacrificial Sn in the solder. In the EL and UV-F images, these degradation species appear as dark spots, and as hot spots in IR-T images. More importantly, these MID defects and fault modes lead to parasitic resistance and mismatch losses, and hence, degradation in the current-voltage (I-V) characteristics, temperature coefficients, and maximum power (Pmax) of the field-aged PV modules. The observed temperature sensitivities are characteristic of different moisture-induced defects. Taken together, this work has expounded on the understanding and detection of MID phenomenon in field-deployed solar PV modules.