Thermal Annealing Effects of Plasmonic Cu1.8S Nanocrystal Films and Their Photovoltaic Properties

Colloidal Cu2-xS nanocrystals are potential abundant, low-cost, and environment-friendly candidates for photovoltaic and photothermal applications. The fabrication of high-quality nanocrystal films through a solution process is a key step toward the exploration of their applications. In this work, we fabricated high-quality Cu1.8S nanocrystal films, characterized their phase transformation under thermal annealing treatments, and investigated the evolution of the corresponding optical and electrical properties. It was demonstrated that the Cu1.8S nanocrystal films undergo a phase transformation from metastable rhombohedral phase to stable tetragonal phase (Cu2S) after annealing at a temperature higher than 240 degrees C, which is much lower than that of the bulk materials (544 degrees C). Along with the transformation, both optical and conductivity properties exhibit well-defined evolution from nonstoichiometric semiconductor to stoichiometric semiconductor, which can be interpreted through a combined electronic structure analysis and theoretical modeling. The correlations between the crystal structure, composition, optical and electrical properties enable us to gain further insights into the structureproperty relationship in Cu2-xS nanocrystals. More importantly, a highly conductive Cu2-xS nanocrystal film with electrical conductivity up to 6.7 S/cm was obtained, implying the potential to be used as conductive electrodes. We further integrated the annealed Cu2-xS nanocrystal films into a photovoltaic device by adopting a FTO/TiO2/Cu2-xS:CdS/MoO3/Au structure, and a preliminary power conversion efficiency of 0.24% was achieved.