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

Phosphonates have been verified experimentally to have substantial influence on the performance of solution-processed nickeloxide (sNiO)-based organic electronic devices. However, a fully atomistic understanding of the phosphonate/sNiO interface is stilllacking. Therefore, based on first-principles calculations, the interface of4-cyanophenylphosphonic acid (CYNOPPA) molecules and the sNiOsurface was studied comprehensively to clarify the grafting process. sNiOwas modeled by a variety of reconstructed NiO(111) surfaces withdifferent amounts of hydroxylation, as well as by NiO(100) and beta-Ni(OH)2(001) surfaces. We discuss the adsorption geometries andenergies of CYNOPPA on these surfaces, as well as the evolution of themicrostructures due to thermal energy, and show the impact on the workfunction and thermodynamic driving forces for hole transport. Theresults indicate that CYNOPPA molecules adsorb on the sNiO surface ina variety of binding modes. Independent of the binding mode, the adsorption process always leads to a positive charging of the sNiOsurface, 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 CYNOPP Aadsorption, 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