Molecular orientation is key: a new perspective on revealing electronic behavior using two-photon emission spectroscopy

Organic electronics has aroused great interest in academia and industry due to its potential applications in OLEDs and organic solar cells, with advantages such as lightweight design, flexibility, and cost-effectiveness. These devices are made by depositing organic molecular thin films onto a substrate that serves as electrodes and exerting their effects by controlling electron transfer between the film and substrate. Therefore, understanding the electronic behavior at the interface between the substrate and the thin film, as well as the electronic properties of organic thin films, is crucial for the further development of organic electronics. In addition, simultaneous observation of photocarrier electrons and intramolecular photoexcitation will provide more insights into organic molecular thin films.

Although a technique called photoelectron spectroscopy has been used to study the static electronic states of organic molecule films in detail, accurately detecting the dynamic behavior of electrons attempting to express their functions in devices has always been challenging and hindering progress.

The research team led by Associate Professor Masahiro Shibuta from the Graduate School of Engineering at Osaka City University used two-photon emission spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction to observe the electronic behavior and surface structure of triphenyl molecular thin films deposited on graphite substrates. The results indicate that TP molecules exhibit a special structure, which adsorbs on the substrate in a standing structure. Under light irradiation, two electrons are injected into TP molecules from the substrate, and photoexcited electrons in the molecular thin film are successfully observed simultaneously in a single sample. In addition, strong photoluminescence was also observed on thin films with a special structure consisting of only one layer of molecules, where the molecules were diagonally adsorbed onto the substrate, similar to the case of TP molecules. It is expected that these results will contribute to the development of new luminescent materials and the further development of functional organic electronic devices.

"2PPE spectroscopy is still a new method for evaluating electronic states, but its drawback is that although well optimized measurements are time-consuming, electronic states are sometimes well observed and sometimes not," said Professor Shibuta. Our research findings emphasize that the visibility of electronic states is closely related to the adsorption mode and electronic properties of molecules on the substrate. In other words, not only the type of molecules, but also their arrangement must be appropriately controlled to create a device that can fully demonstrate their functions. I am pleased that our research provides insights for the development of functional materials for practical applications.

Source: Laser Net