报告题目：Photophysics of TADF Emitters for Efficient Triplet Harvesting in OLEDs
报告人：Prof. Fernando Dias（Durham University, UK）
The electrical current used to drive organic light emitting diodes (OLEDs) creates excited states with spin 0 (singlet) and spin 1 (triplet) in a ratio of 1:3, due to the localized nature of excitons in organic molecules. Additionally, a fraction of the singlet states (S1) created upon charge recombination may also be converted to triplet states (T1), due to local hyperfine interactions that affect the initially created charge transfer state, and due to intersystem crossing (ISC) from S1 to T1, caused by spin-orbit coupling. Triplet states are thus a major loss in the luminescence efficiency of OLEDs, as they are mostly non-emissive. Therefore, improving triplet harvesting to contribute to the radiative mechanism is a key step to improve device efficiency.
Molecules showing thermally activated delayed fluorescence (TADF) have been introduced in recent years as alternative to heavy-metal complexes. TADFs are designed with electron donor (D) and electron acceptor (A) units covalently linked to produce a singlet-triplet energy gap of a few hundred meV. In these conditions, significant population of the triplet state occupies upper vibrational levels and are able to cross back to the singlet manifold due to reverse intersystem crossing (RISC). This process gives origin to delayed fluorescence, which raises the device efficiency. OLEDs using TADF emitters show impressive performances in the green region, sometimes with EQEs above 25%. However, for emission in the blue and red regions, the performance of TADFs is still weaker and further improvement is needed. In particular, a pronounced roll-off on device efficiency is often observed at high current densities. The causes for this loss in efficiency are still unclear, but triplet-triplet annihilation and triplet-polaron quenching processes are thought to be the most probable causes. As the influence of these quenching mechanisms is enhanced in long-lived excited states, obtaining deeper understanding on molecular structure-relationships of TADF emitters is crucial to promote faster ISC/RISC rates, thus decreasing luminescence lifetime, and to help solving the RISC-luminescence yield dilemma in TADFs. In this talk, we will discuss fundamental aspects concerning recent findings on the investigation of the TADF mechanism in our group, focusing on the influence of molecular structure, and energy alignment of electronic excited states.
Fernando B. Dias is an Associated Professor in Material Physics at Durham University. Dias has extensive experience on the investigation of molecular photophysics, in particular on compounds designed for application in light emitting diodes and solar cells. Dias’ current research is centred on the investigation of the excited state dynamics of molecular emitters, aiming to investigate mechanisms that underpin the observation of thermal activated delayed fluorescence in small molecules and polymers, room temperature phosphorescence in metal-free emitters, and efficient NIR emission in organic emitters and metal complexes. Dias combines spectroscopy methods with detailed kinetic analysis and device fabrication to investigate the interplay between the energy ordering of electronic excited states and structure-property relationships, aiming to maximize the performance of molecular materials in relevant technological areas.