Host‐Dopant Interaction between Organic Thermally Activated Delayed Fluorescence Emitter and Host Material: Insight into the Excited State

Abstract

rapidly, and the products based on OLEDs including smartphones, televisions, monitors, etc. are being commercialized.The utilization mechanism of the excitons determines the quantum efficiency of the devices. [13][14][15] OLEDs based on the conventional fluorescent materials can only harvest the singlet excitons for light emission, which results in an upper limit of only 25% for internal quantum efficiency (IQE). The phosphorescent OLEDs incorporating transition metals into the molecular framework of organic emitters represent a huge breakthrough, which demonstrated an IQE of nearly 100%. [16,17] However, phosphorescent emitters contain expensive transition metals, which increases the fabrication costs of the devices and limits their large-scale applications despite the high efficiency. Besides, the toxicity of heavy atoms is also a bothering issue. Thus, novel emission mechanisms are needed to improve both the efficiency and the cost issue. In this context, triplet-triplet annihilation (TTA, also known as P-type delayed fluorescence), [18] hybridized locally and charge transfer (HLCT, also known as the "hot exciton" mechanism), [19][20][21][22] and thermally activated delayed fluorescence (TADF) [23][24][25] are promising. Among them, TADF represents a successful mechanism for realizing high IQE up to 100% (Figure 1). In recent years, TADF emitters based on the multiresonance (MR-TADF) concept have drawn much research attention, which offer a perfect solution to achieving both high efficiency and high color purity. [1,3,6,26,27] Besides, TADF can be realized in purely organic emitters, which is merit for low-cost devices. In the TADF mechanism, the T 1 state can be converted to the S 1 state through a reserve intersystem crossing (RISC) facilitated by a small S 1 -T 1 energy gap (ΔE ST ). The fluorescence resulting from the S 1 on charge recombination is defined as prompt fluorescence with a short lifetime, while the fluorescence originating from the S 1 that is up converted from the T 1 state is defined as the delayed fluorescence with a long lifetime.TADF emitters are typically doped into a suitable organic matrix to address the quenching effects such as concentration quenching, triplet-triplet annihilation, singlet-triplet annihilation (STA), and triplet-polaron annihilation (TPA) to enhance the device efficiency and stability. [28][29][30][31][32][33] Thus, both host materials Organic light-emitting diodes (OLEDs) represent one of the most promising technologies for future displays and lighting sources, which have received extensive research attention. Purely organic thermally activated delayed fluorescence (TADF) emitters offer obvious advantages, including high efficiencies and low costs, and they are typically doped in host materials to achieve optimal device efficiencies. TADF emitters typically feature intramolecular charge transfer characteristics, and their excited states properties are sensitive to local environment, giving the implication that host materials can finely tune their emission properties...

Tuning the Charge Transfer in λ5-Phosphinines with Amino Substituents

“…These results illustrate that the intermolecular interactions with the matrix are key parameters to obtain TADF with this λ 5 -phosphinine. This phenomenon has already been observed with several other fluorophores but remains poorly understood in terms of structure property relationships . However, these results remain promising as the OLED is very stable and presents a brightness that increases until very high current densities (300 mA·cm –2 ).…”

High-performance three-coordinated organoboron emitters for organic light-emitting diodes

“…[124][125][126][127] Designing proper host materials for specific emitters is also an essential research topic for the development of high-performance OLEDs. [128][129][130] Some representative hosts used in PHOLEDs, such as 1,3-bis(N-carbazolyl)benzene (mCP), bis(9-carbazolyl)biphenyl (CBP), and bis [2-(diphenylphosphino)phenyl]ether oxide (DPEPO), have also been used in most studies of TADFOLEDs. [129][130][131][132][133][134][135]…”

Investigation on fluorescein derivatives with thermally activated delayed fluorescence and their applications in imaging

“…In 1961, Parker and Hatchard discovered that eosin has the TADF property with long fluorescence lifetime. 9 Compared with traditional rare earth and transition metal complexes, eosin not only has long fluorescence lifetime but also has better biocompatibility. Thus, this excellent property has greatly attracted the scientific researchers' attention.…”