Thermally activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) is a process through which surrounding thermal energy changes population of excited states of molecular compounds and thus, alters light emission. The TADF process usually involves an excited molecular species in a triplet state, which commonly has a forbidden transition to the singlet ground state, termed phosphorescence. By absorbing nearby thermal energy, the triplet state can undergo reverse intersystem crossing (RISC) converting the triplet state population to an excited singlet state, which then emits light to the singlet ground state in a delayed process termed delayed fluorescence. Accordingly, in many cases, the TADF molecules show two types of emission, a delayed fluorescence and a prompt fluorescence. This is found for specific organic molecules, but also for selected organo-transition metal compounds, such as Cu(I) complexes. Along with traditional organic fluorescent molecules and phosphorescent organo-transition metal complexes, TADF compounds belong to the three main light-emitting material groups used in organic light-emitting diodes (OLEDs).

History The first evidence of thermally activated delayed fluorescence in an organic molecule was discovered in 1961 investigating the compound eosin. The emission detected was termed "E-type" delayed fluorescence, but the mechanism was not completely understood. In 1986, this mechanism was further investigated and described in detail using aromatic thiones, but a practical use was only identified much later. Application of the TADF mechanism for efficient light generation in OLEDs was proposed in 2008 by Yersin and coworkers and subsequently intensively studied. Originally the corresponding mechanism was designated as "singlet harvesting mechanism". Since 2009, the mechanism was extensively investigated by Chihaya Adachi and coworkers as well as by other research groups. A series of papers were published, reporting effective TADF molecular design strategies focusing on different TADF compounds. Extensive studies of green, orange, and blue emitting OLEDs based on organic TADF materials spiked interest in the TADF field. This mechanism was soon considered as possible high efficiency alternative to traditional fluorescent and also phosphorescent compounds used in lighting displays so far. TADF materials are being considered the third generation of OLEDs following fluorescent and phosphorescent based devices.

Mechanism The steps of the TADF mechanism are displayed in the figure at right (where it is assumed that the ground state is a singlet state, which is usually but not always the case). In the electroluminescent process, which is observed in OLEDs, an electrical excitation leads to population of singlet and triplet states of the TADF molecules. From the singlet state an allowed transition can occur to the electronic singlet ground state on a time scale of 10 to 100 nanoseconds for organic TADF molecules. This emission represents the prompt fluorescence. Principally, the electron can undergo a forbidden de-excitation to the ground state as a radiative transition, called phosphorescence, or as a non-radiative process. However, this occurs on a much slower time scale, being on the order of microseconds to seconds. However, in suitable cases, thermal activation from the triplet to the excited singlet state, the reverse intersystem crossing, can populate the singlet state in a fast process by quenching the triplet state population. As a consequence, delayed fluorescence is observed. Accordingly, when a TADF material becomes electronically excited, it exhibits prompt fluorescence and delayed fluorescence, usually occurring at (almost) the same wavelength. Selected organo-transition metal compounds can show both TADF and relatively fast phosphorescence. In an OLED based on traditional fluorescent materials only harvesting of the singlet state population is possible. Thus, due to spin statistics only 25% of the excitation can be exploited. On the other hand, both specific phosphorescent and TADF materials have the ability to harvest the excitation from both singlet and triplet states, theoretically allowing these materials to convert close to 100% of the electrically generated excitons, giving them a large advantage over traditional fluorescent-based materials. However, due to light out-coupling losses in OLED devices, the external quantum efficiency (EQE) is, without employing specific out-coupling enhancement strategies, substantially lower, lying slightly above 5% and 20%, respectively.

OLEDs, excitons, spin statistics, and electroluminescence An OLED device consists of a cathode and an anode and in between a number of layers that guarantee optimized electron transport (from the cathode) and hole transport (from the anode) toward the emission layer, where the emitter molecules are doped in a host matrix. The figure shown displays schematically a simplified OLED device structure. Usually, other layers are applied, such as electron and hole injection layers as well as electron and hole blocking layers, respectively, to confine both electrons and holes to the emission layer (EML). For high-performance OLEDs, an optimized charge carrier balance should be achieved. The thickness of the different layers adds up to only several hundred nanometers.

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