Potential-Induced Degradation of Cu(In,Ga)Se2 Solar Cells: Alkali Metal Drift and Diffusion Effects

Potential-induced degradation (PID) of photovoltaic modules is due to transport of alkali metal ions (Na
⁺
) from the soda-lime glass (SLG) into the semiconductors. For the most efficient polycrystalline absorbers, Na and K in the Cu(In,Ga)(Se,S)2 (CIGS) absorber often benefit performance, so a more detailed mechanism for PID is needed. To that end, CIGS solar cells have been encapsulated, stressed with high voltage, and cored to extract the active regions for characterization. SLG substrates led to 14.9% efficiency on average, which reduced to 0% after 25 h of -1000 V bias. This was accompanied by increased Na in the CIGS layer, particularly at the buffer interface, as well as decreased carrier concentrations. On the other hand, borosilicate glass substrates with low Na and high K content led to 13.6% efficiency on average, which only reduced to 12.1% after 25 h of stress. This was accompanied by slightly increased K content in the CIGS layer. As K has led to recent world record efficiency CIGS devices, understanding Na and K similarities and differences may help to enhance initial efficiency and reduce PID.