Adsorption Geometries and Electronic Properties of a Dipolar Phosphonate-Based Monolayer on the NiO Surface

Phosphonates have been verified experimentally to have substantial influence on the performance of solution-processed nickel oxide (sNiO)-based organic electronic devices. However, a fully atomistic understanding of the phosphonate/sNiO interface is still lacking. Therefore, based on first-principles calculations, the interface of 4-cyanophenylphosphonic acid (CYNOPPA) molecules and the sNiO surface was studied comprehensively to clarify the grafting process. sNiO was modeled by a variety of reconstructed NiO(111) surfaces with different amounts of hydroxylation, as well as by NiO(100) and β-Ni(OH)2(001) surfaces. We discuss the adsorption geometries and energies of CYNOPPA on these surfaces, as well as the evolution of the microstructures due to thermal energy, and show the impact on the work function and thermodynamic driving forces for hole transport. The results indicate that CYNOPPA molecules adsorb on the sNiO surface in a variety of binding modes. Independent of the binding mode, the adsorption process always leads to a positive charging of the sNiO surface, with the counter charges in the adsorbed CYNOPPA molecules, either by direct proton transfer or by H2O elimination from the interface during the adsorption process. Consequently, the work function of the sNiO surface increases upon CYNOPPA adsorption, partially enhanced by the inherent dipole moment of CYNOPPA. The highest occupied molecular orbital of CYNOPPA is always below the valence band maximum and thus facilitates hole injection.