Stoichiometric Doping of Highly Coupled Cu_2-S Nanocrystal Assemblies

The hole density of individual copper sulfide nanocrystals (Cu_2-S NCs) is determined from the stoichiometric mismatch (x) between copper and sulfide atoms. Consequently, the electronic properties of the material vary over a range of x. To exploit Cu_2-S NCs in devices, assemblies of NCs are typically required. Herein, we investigate the influence of x, referred to as the stoichiometric doping effect, on the structural, optical, electrical, and thermoelectric properties of electronically coupled Cu_2-S NC assemblies. The doping process is done by immersing the solid NC assemblies into a solution containing a Cu(I) complex for different durations (0-10 min). As Cu⁺ gradually occupied the copper-deficient sites of Cu_2-S NCs, x could be controlled from 0.9 to less than 0.1. Consequently, the near-infrared (NIR) absorbance of Cu_2-S NC assemblies changes systematically with x. With increasing x, electrical conductivity increased and the Seebeck coefficient decreased systematically, leading to the maximal thermoelectric power factor from a film of Cu_2-S NCs at an optimal doping condition yielding x = 0.1. The physical characteristics of the Cu_2-S NC assemblies investigated herein will provide guidelines for exploiting this emerging class of nanocrystal system based on doping.