Performance of nanocrystalline iron pyrite as the back contact to CdS/CdTe solar cells

We describe the application of iron persulfide (pyrite, FeS
₂
) NC thin films as a Cu-free back contact to CdS/CdTe solar cells. We synthesized cubic FeS
₂
NCs, and prepared NC-FeS
₂
films by drop-casting to investigate their use in thin film solar cells. Characterization of the FeS
₂
NC films revealed sub-band-gap optical absorption, holes as the majority carrier, free hole concentration of ~10
¹⁹
cm
^-3
, and Hall mobility of μ
_h
<;<; 1 cm
²
V
^-1
S
^-1
. CdS/CdTe solar cells completed by depositing hydrazine-treated NC-FeS
₂
back contacts directly onto CdTe displayed efficiencies comparable to those of standard Cu/Au back contact devices. Our best FeS
₂
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 hydrazine-treated NC-FeS
₂
.