Effect of surface adsorption and non-stoichiometry on the workfunction of ZnO surfaces: A first principles study

ZnO has been actively studied for potential usage as a transparent conducting oxide (TCO) for a variety of applications including organic light emitting diodes and solar cells. In these applications, fine-tuning the workfunction of ZnO is critical for controlling interfacial barriers and improving the charge injection (or outcoupling) efficiencies. We have performed plane wave periodic density functional theory calculations to investigate the effect of different surface absorbents and surface defects (including surface non-stoichiometry) on the workfunction of ZnO. The aim was to understand the underlying mechanism of workfunction changes, in order to engineer specific workfunction modifications. Accurate calculations of workfunctions of polar surfaces were achieved by introducing balancing pseudo charges on one side of the surface to remove the dipolar effect. It was found that increasing the surface coverage of hydrocarbons (-CH3) decreased the workfunction, while adsorption of highly electronegative-F and -CF3 groups and increases in surface O/Zn ratio increased the workfunction of ZnO. The increase of workfunction was found to be directly correlated to the enhancement variation of surface dipole moment due to adsorptions or other surface modifications. Introducing surface absorbents that increase surface dipole moment can be an effective way to increase workfunction in ZnO TCOs.