Colloidal Indium-Doped Zinc Oxide Nanocrystals with Tunable Work Function: Rational Synthesis and Optoelectronic Applications

Transition metal oxides are widely used in solution-processed optoelectronic devices as charge-transporting interlayers to improve contact properties and device performances. Here we show that the work function of oxide nanocrystal thin films, one of the most important parameters for charge-transporting interlayers, is readily tuned by rational design of material synthesis. Mechanism studies reveal that the combination of employing the reverse-injection approach and using zinc stearate and indium 2-ethylhexanoate as the cationic precursors ensures both controlled reaction pathways and balanced relative dopant-host precursor reactivity and hence high-quality indium doped zinc oxide nanocrystals. We find that the empirical rule of relative Lewis acidity fails to predict the relative reactivity of the cationic precursors and quantitative measurements are obligatory. The successful incorporation of indium dopants into host oxide nanocrystals accompanied by the generation of high density of free electrons leads to oxide thin films with lower work function. Polymer light-emitting diodes with electron-transporting interlayers based on the indium doped zinc oxide nanocrystals exhibit improved electron-injection properties and enhanced device characteristics, i.e., lower turn-on voltage, higher maximum luminance, and higher efficiency. Our study is an excellent example that new understanding on the chemical kinetics of doped nanocrystals leads to rational design of synthetic protocols and materials with tailored electronic properties, providing benefits for their optoelectronic applications.