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

TL;DR

This study investigates energy level alignment at organic interfaces using a rotary Kelvin probe, revealing that thermal equilibrium assumptions hold only at certain metal/organic interfaces with active charge transfer, and not at organic/organic interfaces.

Contribution

It provides experimental validation of a thermal equilibrium model at metal/organic interfaces and highlights the importance of carrier supply and polarization effects at organic/organic interfaces.

Findings

Good agreement with simulations at metal/organic interfaces with active charge transfer.

Thermal equilibrium is not achieved by simple film deposition in dark conditions.

Organic/organic interface results deviate from thermal equilibrium model, indicating other factors are involved.

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 $\alpha$-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.