Copper deficiency induced varying electronic structure and optoelectronic properties of Cu2−xS thin films

Copper sulfide (Cu<inf>2-x</inf>S) is a class of low-cost, environment friendly p-type semiconductor, where electronic structure and the thus induced optoelectronic properties can be significantly varied through the creation of copper deficiency. To this end, varying composition of Cu<inf>2-x</inf>S (i.e., Cu<inf>2</inf>S, Cu<inf>1.96</inf>S, Cu<inf>1.8</inf>S, Cu<inf>1.8</inf>S+ Cu<inf>1.6</inf>S and CuS) films were grown here by using a low temperature molecular solution based deposition method, following which a wide range of characterization tools were used to understand their microstructure, electronic structure and optoelectronic properties. The hole concentration of these films are found to vary from 3.32 × 10¹⁹ cm⁻³ to 2.54 × 10²² cm⁻³ as Cu<inf>2-x</inf>S composition changes from Cu<inf>2</inf>S to CuS. This is because of the induced Cu deficiency in Cu<inf>2-x</inf>S films with decreasing Cu/S-molar ratio, which reduced the Cu d-band width in the valence band, thus pushing the Fermi level deep into the valence band. This leads the optical and transport gap to increase from 1.36 eV to 2.23 eV and 1.31 eV to 2.02 eV respectively with increasing copper deficiencies from Cu<inf>2</inf>S to CuS. Moreover, in this work, both the valence and conduction band edge positions are found to shift negatively with increasing Cu deficiency in these films.