Metastable Defects Decrease the Fill Factor of Solar Cells

Solar cells are made from semiconductors, which inevitably include defects. The defects in Cu(In,Ga)Se${}_{2}$ (CIGS) are known to be metastable: With excitation they change character, which can have a profound impact on device function. The authors show that these metastable defects reduce the efficiency of the solar cell due to an increased diode factor. It is possible to measure this quantity directly from the bare absorber (without needing to build the entire solar cell), yielding a lower limit for the whole device. Experiments and simulations indicate that metastable defects must be reduced to increase efficiency, and not just in CIGS-based solar cells.