Understanding Performance Limitations Of Cu(In,Ga)Se-2 Solar Cells Due To Metastable Defects-A Route Toward Higher Efficiencies

Thin-film Cu(In,Ga)Se-2 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)Se-2 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 V-Se-V-Cu defect found in Cu(In,Ga)Se-2, 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)Se-2 solar cells, the formation of metastable defects is suppressed.