Electron transfer is significantly improved by a minimal energy requirement at the interface of organic semiconductors in upconversion (UC) organic light-emitting diodes (OLEDs). This leads to the creation of effective blue UC-OLEDs that have very low turn-on voltages, as demonstrated by scientists. This research enhances our understanding of how electron transfer works in organic optoelectronic devices and could contribute to the creation of new, efficient optoelectronics that minimize energy loss.
Electron transfer is a fundamental process where an electron moves from a donor molecule or atom to an acceptor. This mechanism is crucial for chemical reactions, electronic devices, and biological systems. Gaining insights into the electron transfer processes at solid/solid interfaces is vital for boosting the efficiency of organic optoelectronic devices, such as OLEDs and organic photovoltaics. These devices are popular for digital displays and portable electronics due to their lightweight and flexible nature.
A critical phase in the operation of these devices involves the charge transfer (CT) state, which consists of a loosely bonded electron-hole pair at the donor/acceptor interface. Improving device performance hinges on understanding the energetic and structural aspects of electron transfer. This can be accomplished through experimental exploration of the electron transfer process from the CT state to other excited states based on Marcus theory, which describes electron transfer in relation to energetics and structures. However, analyses concerning electron transfer have faced limitations due to various influencing factors. Recent studies have introduced upconversion OLEDs (UC-OLEDs), which leverage electron transfer from the CT state to a triplet excited (T1) state through a process known as triplet-triplet annihilation (TTA). This mechanism can notably reduce the turn-on voltage for blue UC-OLEDs in comparison to traditional blue OLEDs, potentially mitigating challenges like high driving voltage and low stability.
To fill this knowledge gap, a research team in Japan, led by Associate Professor Seiichiro Izawa from the Laboratory for Materials and Structures at Tokyo Institute of Technology, examined the efficiency of electron transfer from the CT state to the T1 state within 45 UC-OLEDs. Dr. Izawa points out, “Unlike typical organic molecules, UC-OLEDs display distinct CT and TTA emissions at different wavelengths without any overlap, making them perfect for concurrently assessing device efficiency and electron transfer between the CT and T1 states. These assessments not only enable the development of effective blue UC-OLEDs but also enhance our understanding of other similar devices.” Their findings were published in the journal Angewandte Chemie International Edition on June 24, 2024.
The research team performed a thorough analysis of various UC-OLEDs, utilizing 45 different material combinations, mixing three anthracene derivatives as donors and 15 naphthalenediimide derivatives as acceptors, following the guidance of Marcus theory. Their findings showed that the electron transfer from the CT state to the T1 state is boosted by a minimal energy driving force of less than 0.1 eV. Furthermore, they identified a new donor-acceptor pair, PCAN/NDI-PhE, through Marcus plots, which they employed to create a highly efficient blue UC-OLED with an extremely low turn-on voltage of 1.57 V.
“Our results further clarify the mechanisms behind the electron transfer from the CT state to the T1, enabling more efficient UC-OLEDs,” stated Dr. Izawa, highlighting the significance of this research. Looking ahead, he concluded, “We are optimistic that fine-tuning the energetics and structural components at the donor/acceptor interface will greatly enhance electron transfer, paving the way for highly effective organic optoelectronic devices that operate without energy loss.”
