Tunable Structural and Optical Properties of CuInS2 Colloidal Quantum Dots as Photovoltaic Absorbers

Facile phase selective synthesis of CuInS2 nanostructures has been an important pursuit because of the opportunity for tunable optical properties of the phases, and in this respect is investigated by hot-injection colloidal synthesis in this study. Relatively monodispersed colloidal quantum dots (3.8 – 5.6 nm) of predominantly chalcopyrite structure synthesized at 140, 180 and 210 oC for 60 minutes from copper(II) hexafluoroacetylacetonate hydrate and indium(III) diethyldithiocarbamate precursors exhibit temperature-dependent structural variability. The slightly off-stoichiometric quantum dots are copper-deficient in which copper vacancies (VCu′), indium interstitials (Ini•••), indium-copper anti-sites (InCu••) and surface trapping states are likely implicated in broad photoluminescence emission with short radiative lifetimes, τ1, τ2, and τ3 of 1.5 – 2.1, 7.8 – 13.9 and 55.2 – 70.8 ns and particle-size dependent tunable band gaps between 2.25 and 2.32 eV. Further structural and optical tunability (Eg between 2.03 and 2.28 eV) is achieved with possible time-dependent wurtzite to chalcopyrite phase transformation at 180 oC likely involving a dynamic interplay of kinetic and thermodynamic factors.