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.

Thin-film solar cells consist of several layers. The interfaces between these layers can provide critical recombination paths and consequently play a vital role in the efficiency of the solar cell. One of the main challenges for polycrystalline semiconductor absorber materials is the absorber-buffer interface. The Cu(In, Ga)Se 2 system is particularly interesting in this context, since Cu-rich absorbers are dominated by recombination at the interface, while Cu-poor ones are not. This paper unveils the root cause of the challenge in the interface of Cu-rich solar cells in terms of a Se-related defect with an activation energy of 200 ± 20 meV. This defect causes interface recombination and is responsible for the deficiency of open-circuit voltage in Cu-rich cells. Moreover, this paper demonstrates that the origin of this defect is due to the etching step necessary to remove secondary phases. Postdeposition surface treatments or modified buffer layers are shown to passivate this defect, to reduce interface recombination, and to increase the efficiency.