Phase Transformations of Copper Sulfide Nanocrystals: Towards Highly Efficient Quantum-Dot-Sensitized Solar Cells

Owing to their high electrical conductivity, tunable plasmonic absorption spectra, low cost, and abundance in nature, Cu2-xS nanocrystals are of great interest as functional materials for photovoltaic and photothermal applications. With the aim of developing low-cost high-efficiency quantum-dot-sensitized solar cells, solution-processed Cu2-xS nanocrystal films are synthesized and their phase transformations upon thermal treatment are investigated. A combination of experimental results and theoretical analysis illustrates the thermodynamic evolution of the crystal structures and the composition caused by the thermal-annealing process. The use of Cu2-xS nanocrystal films as counter electrodes in quantum-dot-sensitized solar cells is also explored. The devices have an optimized power-conversion efficiency of 5.81% for tetragonal Cu2S nanocrystal films that are derived from annealed Cu1.8S nanocrystal films.