Thermal annealing effects of plasmonic Cu_1.8S nanocrystal films and their photovoltaic properties

Colloidal Cu_2-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 Cu_1.8S 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 Cu_1.8S nanocrystal films undergo a phase transformation from metastable rhombohedral phase to stable tetragonal phase (Cu₂S) after annealing at a temperature higher than 240°C, which is much lower than that of the bulk materials (544°C). Along with the transformation, both optical and conductivity properties exhibit well-de fined 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 structure-property relationship in Cu_2-xS nanocrystals. More importantly, a highly conductive Cu_2-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 Cu_2-xS nanocrystal films into a photovoltaic device by adopting a FTO/TiO₂/Cu_2-xS:CdS/MoO₃/Au structure, and a preliminary power conversion efficiency of 0.24% was achieved. (Graph Presented).