Performance of Nanocrystalline Iron Pyrite as the Back Contact to CdS/CdTe Solar Cells

We describe the application of iron persulfide (pyrite, FeS2) NC thin films as a Cu-free back contact to CdS/CdTe solar cells. We synthesized cubic FeS2 NCs, and prepared NC-FeS2 films by drop-casting to investigate their use in thin film solar cells. Characterization of the FeS2 NC films revealed sub-band-gap optical absorption, holes as the majority carrier, free hole concentration of similar to 10(19) cm(-3), and Hall mobility of mu(h) << 1 cm(2) V-1S-1. CdS/CdTe solar cells completed by depositing hydrazine-treated NC-FeS2 back contacts directly onto CdTe displayed efficiencies comparable to those of standard Cu/Au back contact devices. Our best FeS2 device shows an AM1.5G efficiency of 12.5%, V-oc = 0.81 V, J(sc) = 23.0 mA/cm(2), and FF = 67%. Temperature dependent studies of solar cells reveal that the ' roll-over' effect observed in standard devices at low temperature is controlled by replacing Cu by hydrazinetreated NC-FeS2.