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 ~10¹⁹ cm^-3, and Hall mobility of μ_h <;<; 1 cm² 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/cm², 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 hydrazine-treated NC-FeS₂.