Characterization of defects in 9.7% efficient Cu2ZnSnSe4-CdS-ZnO solar cells

We have fabricated Cu2ZnSnSe4-CdS-ZnO solar cells with a total area efficiency of 9.7%. The absorber layer was fabricated by selenization of sputtered Cu10Sn90, Zn, and Cu multilayers. A large ideality factor of the order of 3 is observed in both illuminated and dark IV-curves, which seems to point in the direction of complex recombination mechanisms such as recombination through fluctuating potentials in the conduction and valence bands of the solar cell structure. A potential barrier of about 135 meV in the device seems to be responsible for an exponential increase of the series resistance at low temperatures, but at room temperature, the effect of this barrier remains relatively small. The free carrier density in the absorber is of the order of 1015 cm−3 and does not vary much as the temperature is decreased.