Simple yet Versatile Synthesis of CuInSe_xS_2–x Quantum Dots for Sunlight Harvesting

Common approaches to synthesizing alloyed CuInSe_xS_2–x quantum dots (QDs) employ high-cost, air-sensitive phosphine complexes as the selenium precursor. Such methods typically offer low chemical yields and only moderate emission efficiencies, particularly for selenium-rich compositions. Here we demonstrate that such hazardous and air-sensitive selenium precursors can be completely avoided by utilizing a combination of thiols and amines that is very effective at reducing and then complexing with elemental selenium to form a highly reactive selenium precursor at room temperature. The optical properties of the CuInSe_xS_2–x QDs synthesized by this new approach can be finely tuned for optimal sunlight harvesting through control of QD size and composition. In order to demonstrate the importance of such material tunability, we incorporate QDs into liquid-junction Grätzel solar cells and study correlations between varied QD size and composition and the resulting device performance. We also investigate charge transport in films of CuInSe_xS_2–x QDs by incorporating them into bottom-gate field effect transistors. Such films exhibit measurable p-type conductance even without exchange of the long native surface ligands, and the film’s conductance can be improved by more than 3 orders of magnitude by replacing native ligands with shorter ethanedithiol molecules. The results of this study indicate the significant promise of CuInSe_xS_2–x QDs synthesized by this method for applications in photovoltaics utilizing both sensitized and p–n junction architectures.