Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

Abstract

The earliest account of electroluminescence, the process of converting electrical energy into light, using organic materials can be traced back to 1963 when Pope et al. applied a direct current to an anthracene single crystal under a bias of 400 V using silver-paste electrodes. [3] Although anthracene fluorescence was observed, a driving voltage of 400 V is evidently not viable in practical applications. The seminal breakthrough in the development of OLEDs appeared in 1987 when Tang and Van Slyke reported a double-layered device using tris(8-hydroxyquinoline)aluminum (Alq 3 ) as the emitting and electron-transporting layer. [1] The green-emitting device showed an external quantum efficiency (EQE) of about 1% when driven at less than 10 V. This marked the dawn of OLED development and tremendous interest and effort from both academia and industry have followed subsequent to this pioneering work, resulting in the ultimate wide-scale commercialization of OLEDs, particularly for display applications.In order to make OLEDs commercially viable for lighting applications, where the cost per unit must be competitive with presently used technology, there are a number of challenges that must be overcome aside from reducing the production cost. The organic emitters should have high photoluminescence quantum yields (PLQYs), which directly impact the device efficiency. The energy levels of the frontier molecular orbitals (i.e., highest occupied and lowest unoccupied molecular oribitals (HOMOs and LUMOs)) of each of the layers in the device should be optimally relatively aligned in order to: i) minimize the barrier to charge injection, and ii) control the recombination region within the device, which greatly affects both the device efficiency and lifetime. [4] The organic materials must demonstrate sufficient thermal stability to be compatible with their vacuum deposition during device fabrication or produce thin films of suitable morphology when spin-coated during solution processing. Regardless of the fabrication method, the organic material must be morphologically stable during device operation when Joule heat is produced in the device. [5] Aside from the aforementioned challenges, another key issue to address is the management of hole and electron recombination within the device, each possessing their own spin. Unlike photoexcitation, in which mainly singlet excited states are produced in the organic emitters, exciton formation through charge (hole and electron) recombination in OLED devices results in 25% singlets and 75% triplets, according to The design of thermally activated delayed fluorescence (TADF) materials both as emitters and as hosts is an exploding area of research. The replacement of phosphorescent metal complexes with inexpensive organic compounds in electroluminescent (EL) devices that demonstrate comparable performance metrics is paradigm shifting, as these new materials offer the possibility of developing low-cost lighting and displays. Here, a comprehensive review of TADF materials is presented, with a...

Recent Advances in Polymer Organic Light-Emitting Diodes (PLED) Using Non-conjugated Polymers as the Emitting Layer and Contrasting Them with Conjugated Counterparts

“…In principle, TADF polymers can also be used as host materials for conventional fluorescent dyes. 69 In this setting, the host is responsible for upconversion of dark triplet excitons, which then transfers its energy to the fluorescent dopant dye which has key merits of high PLQY and much narrower emission spectrum (i.e., enhanced color purity). TADF polymers for white light emission still remain as a challenge.…”

“…However, given the explosive development of TADF OLED materials in the recent years, 69,70,72 the research interest in fluorescent materials has been sharply diminished.…”

“…One main reason is their inability to utilize dark triplet excitons in the device, which constitute as large as 75% of total excitons. Thanks to the recent advances of thermally activated delayed fluorescence (TADF) mechanism [69][70][71][72] (vide infra) which allow purely organic materials to recruit dark triplets and ,hence, achieve 100% IQE, it is expected that the 1st generation fluorescent emitters will soon fade.…”

“…[69][70][71][72] The key merit of the TADF mechanism is to allow purely organic emitters made of light elements (C, H, O, N etc.) to achieve comparable EQE (20-30%) to the expensive second generation phosphorescent emitters made of rare heavy metals such iridium and platinum.…”

“…to achieve comparable EQE (20-30%) to the expensive second generation phosphorescent emitters made of rare heavy metals such iridium and platinum. 69,70,72 Currently, the vast majority of TADF emitters are small molecules with devices that were fabricated by the costly vacuum thermal evaporation technique 69,70,72 with a few exceptions. [73][74][75] Polymeric emitters, on the other hand, are solution-processable and enjoy more facile device fabrication procedure.…”