Fill factor losses in Cu(In,Ga)Se2 based solar cells due to metastabel defects — the effect of Ag addition

The fill factor in state-of-the-art Cu(In,Ga)Se2 based solar cells is still relatively low as a consequence of diode factors greater than the ideal value of 1. We show that the increased diode factor results from metastable defects, also responsible for the persistent photoconductivity, which increase the net doping upon electron injection. We present measurements from photoluminescence, capacitance-voltage and current-voltage characteristics in corroboration of simulations including a metastable defect, which all consistently describe the observed diode factor greater than 1. It is demonstrated that the addition of Ag to Cu(In,Ga)Se2 decreases metastable defects and that fill factors as high as 81.0% are achieved.