Metastable Defects Decrease the Fill Factor of Solar Cells

Abstract

Cu(In,Ga)Se2-based solar cells exceed power conversion efficiencies of 23%. However, the fill factor of these solar cells, with best values around 80%, is relatively low (Si reaches 84.9%) mostly due to diode factors greater than 1. Recently, we proposed metastable defects, a general feature of the Cu(In,Ga)Se2 alloy, to be the origin of the increased diode factor even in low injection. Here, we measure the diode factor of the bare absorber layers using excitation-dependent photoluminescence. The increased diode factor above 1 can be well described by the model of metastable defects, as well as a slight excitation dependence within the experimentally accessible range of excitation intensities. We discuss how the excitation dependence of the diode factor depends on the parameters of the metastable defects. Within the same model, we can additionally describe the experimental diode factors of n- and p-type epitaxial Cu(In,Ga)Se2 films. We find that the diode factors measured optically by photoluminescence impose a lower limit for the diode factor measured electrically on a finished solar cell. Interestingly, the lowest diode factor (optical and electrical) and consequently highest fill factor of 81.0% is obtained by Ag alloying, i.e., an (Ag,Cu)(In,Ga)Se2 absorber. This finding hints to a pathway to increase fill factors and thus efficiencies for Cu(In,Ga)Se2-based solar cells.