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 O2 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.