Understanding Performance Limitations of Cu(In,Ga)Se2 Solar Cells due to Metastable Defects—A Route toward Higher Efficiencies

Thin‐film Cu(In,Ga)Se2 solar cells reach power conversion efficiencies exceeding 23% and nonradiative recombination in the bulk is reported to limit device performance. The diode factor has not received much attention, although it limits the fill factor, and therefore the efficiency, for state‐of‐the‐art solar cells. Herein, the diode factor of Cu(In,Ga)Se2 absorbers, measured by photoluminescence spectroscopy, and of solar cells, measured by current–voltage and capacitance–voltage characteristics, are compared, supported by simulations using rate equations of generation and recombination. It is found that the diode factor is already increased in the neutral zone of the absorber due to metastable defects, such as the VSe–VCu defect found in Cu(In,Ga)Se2, because of an increased net acceptor density upon minority‐carrier injection. The metastable and persistent increase of the net acceptor density has a detrimental effect on the device performance. Diode factors of 1 and efficiencies exceeding 24% are expected when, in current state‐of‐the‐art Cu(In,Ga)Se2 solar cells, the formation of metastable defects is suppressed. Semiconductor devices are the basis of many technologies with functionality strongly dependent on their defects, including metastable defects—common in semiconductors. It is shown that these metastable defects can significantly alter the diode current of a pn junction. A novel loss mechanism is revealed for state‐of‐the‐art Cu(In,Ga)Se2 solar cells and efficiency improvements beyond 24% are predicted by avoiding metastable defects.