Thiol Modification Enables ZnO-Nanocrystal Films with Atmosphere-Independent Conductance

Films based on colloidal ZnO nanocrystals are promising electron-transporting layers (ETLs) widely used in solution-processed optoelectronic devices. However, adsorption of gas molecules in the ambient atmosphere, such as O-2 and H2O, onto the surfaces of ZnO nanocrystals may cause significant changes in the electrical conductance of the ZnO-nanocrystal films. Such ambient-dependent conductivities of the ZnO-nanocrystal films impose a challenge to overcome in practical applications. Here, we develop a thiol-ligand exchange strategy to passivate the surface-adsorption sites of the colloidal ZnO nanocrystals, resulting in thiolate-capped ZnO nanocrystals. Thin films consisting of thiolate-capped ZnO nanocrystals show unprecedented atmosphere-independent conductivities. We further demonstrate that the thiol-ligand exchange strategy is readily extended to ZnO nanocrystals with different sizes. In consequence, the atmosphere-stable conductivities of films based on the thiolate-capped ZnO nanocrystals can be tuned over 5 orders of magnitudes.