Investigation of the selectivity-mechanism of copper (I) sulfide (Cu2S) as a dopant-free carrier selective contact for silicon solar cells

Carrier selective contacts based on dopant free materials are receiving increased attention. Copper(I) sulfide (Cu2S), earth-abundant and non-toxic, shows potential for use in silicon (Si) solar cells, though its carrier selectivity mechanism remains unclear. We present a detailed investigation of the selectivity mechanisms behind thermally evaporated Cu2S on Si. Deposited Cu2S layers are studied by X-ray photoelectron spectroscopy, Auger electron spectroscopy, X-ray diffraction and UV/VIS (ultraviolet?visible) methods. From X-ray photoelectron spectroscopy and Auger electron spectroscopy, only sulfur related Cu(I) bonds are observed in the deposited film. X-ray diffraction measurements indicate that Cu1.95S (djurleite) may also exist in the film. The indirect and direct bandgaps are determined to be 1.2 eV and 2.2 eV, respectively. Band bending of -0.6 eV is calculated for Cu2S/ n-Si (-5 ??cm), while it is significantly lower (-0.07 eV) for Cu2S/p-Si (-2.25 ??cm). Current-voltage measurements indicate that the carrier selectivity originates from an induced junction at the Cu2S/n-Si interface, and not from a large band-offset of the conduction band. The Cu2S/p-Si structure displays no carrier selectivity, a finding attributed to the insufficient band bending at the interface.