Impact of front contact layers on performance of Cu(In,Ga)Se-2 solar cells in relaxed and metastable states

We studied the impacts of front contact layers composed of transparent conducting oxide (TCO) and transparent oxide semiconductor (TOS) layers on the performance of Cu(In,Ga)Se-2 (CIGS) solar cells with glass/Mo/CIGS/CdS/TOS/TCO in relaxed and metastable states. After annealing under illumination, the CIGS solar cells with KF and NaF postdeposition treatments exhibited metastable increases in open-circuit voltage, fill factor, and the resulting conversion efficiency with a metastable decrease in short-circuit current density. These are partly attributed to an increase in metastable acceptor density in the CIGS layers, being stable for a day even at room temperature, with a reduced space charge width. We found that the net acceptor density evaluated by capacitance-voltage measurements largely varied depending on the TOS/TCO layers, whereas the open-circuit voltage was identical. On the other hand, no significant differences in both the net acceptor density and open-circuit voltage were observed regardless of the TOS/TCO layers in the relaxed state produced by annealing in the dark. The results suggest that the CdS/CIGS heterojunction can no longer be treated as an n(+)/p junction in the metastable state and that the evaluated acceptor density was apparent values with large difference originating from different potential distribution in the CdS layer, which is related to the TOS/CdS and TCO/CdS junctions. In addition, the TOS/TCO layers largely affected the short-circuit current density owing to their optical constants and film thicknesses. The presented results highlight the importance of the design of CdS/TOS/TCO layers in solar cells operated in metastable states.