Ultra-thin photovoltaic (PV) devices were produced by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD) incorporating a highly absorbing intermediate sulphurised FeSx layer into a CdS/ CdTe structure. X-ray diffraction (XRD) confirmed a transitional phase change to pyrite FeS2 after post growth sulphur (S) annealing of the FeSx layer between 400 °C and 500 °C. Devices using a superstrate configuration incorporating a sulphurised or non-sulphurised FeSx layer were compared to p–n devices with only a CdS/CdTe structure. Devices with sulphurised FeSx layers performed least efficiently, even though pyrite fractions were present. Rutherford back scattering (RBS) confirmed deterioration of the CdS/FeSx interface due to S inter-diffusion during the annealing process.