Narrow and Intense Photoluminescence Emission: Manifestation of Cu Vacancies in CuGaS₂/ZnS Quantum Dots

The photoluminescence (PL) of environmentally benign chalcopyrites is still debatable, although it is now believed that Cu-related defects play a prime role. In the case of the most studied (chalcopyrite) CuInS2, its emission mechanism is believed to involve Cu2+ defect centers, which can be controlled by nanocrystal stoichiometry. In this work, Cu vacancies are introduced into CuGaS2/ZnS (CGZS) quantum dots to enhance the PL quantum efficiency. Interestingly, narrowing of the PL feature or full width at half-maximum (fwhm) along with the improvement in the PL efficiency is observed in contrast to the belief that the inhomogeneous broadening of the emission line is due to the defects and size dispersion. Narrowing of the emission line on incorporation of Cu vacancies is commensurate with the tendency of CGS to form an “ordered vacancy compound”. Two types of radiative transitions, one from conduction level to Cu vacancy-related level and the other from donor to acceptor level, are observed. Furthermore, the locations of the conduction band edge, valence band edge, and Cu-related defects generated due to the variation in the Cu-to-Ga ratio are mapped with cyclic voltammetry measurements along with optical measurements. This work also reiterates that the radiative recombination in CGSZ quantum dots is governed by Cu vacancies and is not related to the Cu valence, as conjectured in the published work.