The potential-induced degradation (PID) mechanism in Cu(In,Ga)(Se,S)₂ (CIGS) thin-film solar cells, which are alternative energy sources with a high efficiency (>23%) and upscaling possibilities, remains unclear. Therefore, the cause of PID in CIGS solar cells was investigated in this study at the cell level. First, an appropriate PID experiment structure at the cell level was determined. Subsequently, PID and recovery tests were conducted to confirm the PID phenomenon. Light current–voltage (I–V), dark I–V, and external quantum efficiency (EQE) analyses were conducted to determine changes in the cell characteristics. In addition, capacitance–voltage (C–V) measurements were carried out to determine the doping concentration and width of the space charge region (SCR). Based on the results, the causes of PID and recovery of CIGS solar cells were explored, and it was found that PID occurs due to changes in the bulk doping concentration and built-in potential at the junction. Furthermore, by distinguishing the effects of temperature and voltage, it was found that PID phenomena occurred when potential difference was involved.

Monolithic perovskite–silicon tandem solar cells with MoO_x hole selective contact silicon bottom solar cells show a power conversion efficiency of 8%. A thin 15 nm-thick MoO_x contact to n-type Si was used instead of a standard p⁺ emitter to collect holes and the SiO_x/n⁺ poly-Si structure was deposited on the other side of the device for direct tunneling of electrons and this silicon bottom cell structure shows ~15% of power conversion efficiency. With this bottom carrier selective silicon cell, tin oxide, and subsequent perovskite structure were deposited to fabricate monolithic tandem solar cells. Monolithic tandem structure without ITO interlayer was also compared to confirm the role of MoO_x in tandem cells and this tandem structure shows the power conversion efficiency of 3.3%. This research has confirmed that the MoO_x layer simultaneously acts as a passivation layer and a hole collecting layer in this tandem structure.