Does Fermi Level Alignment Hold Across Organic Interfaces? -- An Investigation Using a Rotary Kelvin Probe

TL;DR

This study tests the validity of a thermal-equilibrium model for energy level alignment at organic interfaces by monitoring quasi-continuous work function changes with film thickness using a rotary Kelvin probe (RKP) for a donor/acceptor pair (HAT-CN and $\alpha$-NPD) on Au. Numerical simulations based on a self-consistent thermal equilibrium framework are compared with experiments under dark and UV-irradiated conditions. The metal/organic HAT-CN/Au interface aligns with the model only when UV assistance is used to promote charge transfer, whereas the organic/organic $\alpha$-NPD/HAT-CN/Au interface exhibits strong substrate-driven carrier supply and orientation-polarization effects that disrupt equilibrium predictions. The findings demonstrate that Fermi level alignment across organic multilayers cannot be assumed under equilibrium and emphasize the importance of nonequilibrium phenomena (traps, in-gap states, SOP) and external stimuli in dictating interfacial energetics, with RKP proving a valuable tool for probing these effects.

Abstract

Understanding energy level alignment at organic interfaces is crucial for optimizing the performance of organic devices. Interface dipole and band bending significantly influence carrier recombination and generation mechanisms. A method of simulating energy level alignment at metal/organic and organic/organic interfaces by assuming a thermal equilibrium model has been proposed, but its validation against experimental methods is still limited. In this study, the work function change in the $α$-NPD/HAT-CN/Au interface was measured as a typical donor/acceptor system using a rotary Kelvin probe (RKP). Our findings demonstrate good agreement with simulations only at metal/organic interfaces which have "active" charge transfer. It is suggested that thermal equilibrium is not achieved simply by depositing the film under dark condition, and some treatment to supply carriers, such as exposure to UV light, is necessary for accurate evaluation. At the organic/organic interface, the the experimental results did not agree with thermal equilibrium model, highlighting the need to consider substrate-driven carrier supply and polarization effects when evaluating energy level alignment.

Figures (7)

• Figure 1: Work function (blue) and film thickness (orange) of HAT-CN films measured by the RKP system as a function of time.
• Figure 2: Time variation of work function (blue) and film thickness (orange) of HAT-CN films irradiated with UV during deposition.
• Figure 3: Time variation of work function (blue) and film thickness (orange) of $\alpha$-NPD films.
• Figure 4: Time variation of work function (blue) and film thickness (orange) of $\alpha$-NPD films on HAT-CN
• Figure 5: Work function within a 80 nm thick $\alpha$-NPD film deposited on the UV irradiated HAT-CN film.