A new generation of organic light-emitting materials and devices

Abstract

Organic light-emitting diodes (OLEDs) have been a research focus for almost 30 years because of their great potential applications in flat-panel displays, solid-state lighting, and wearable electronics. The recombination of holes and electrons within the devices under electrical excitation typically generates 25% singlet excitons and 75% triplet excitons. However, only 25% singlet excitons can be utilized to emit light, and the other 75% triplet excitons are generally wasted through non-radiative transition to give a maximum internal quantum efficiency of about 25% for the first generation fluorescent OLEDs. To improve the internal quantum efficiency, phosphorescent materials have been utilized to exploit the non-radiative triplet state by introducing noble heavy metal atoms like iridium (Ir) and platinum (Pt) to increase spin-orbit interactions. Overall electroluminescence (EL) internal quantum efficiency has been successfully improved to 100% since both singlet and triplet excitons could be harvested due to their radiative triplet excitons and efficient intersystem crossing (ISC) from the singlet excited state to the triplet excited state. Nevertheless, noble metals are indispensable for those phosphorescent materials, which are expensive and nonrenewable. In the past few years, metal-free thermally activated delayed fluorescence (TADF) emitters that can also realize 100% exciton utilization through efficient reverse intersystem crossing (RISC) of triplet excitons are regarded as promising materials for next-generation OLEDs. Besides, the intersystem crossing of excitons between higher energy Tm and Sn with close energy levels may also happen in hybridized local and charge-transfer (HLCT) excited state, where the local excited (LE) state contributes to a high efficiency fluorescence radiative decay, while the charge transfer (CT) state ensures the generation of singlet excitons in high yield through the reverse intersystem crossing from high-lying CT-based triplet excited state back to the CT-based singlet excited state. 75% Non-luminescent triplet excitons can theoretically be transferred to singlet ones via reverse intersystem crossing to give a comparable internal quantum efficiency to that of the phosphorescent devices. In addition, an organic open-shell molecule with one unpaired electron in the highest singly occupied molecular orbital (SOMO) could also be used as an emitter to circumvent the transition problem of triplet. Intermixing of electron-donating and electron-accepting molecules as an emitting layer could also break the fluorescence internal quantum efficiency limit of 25% by using the high reverse intersystem crossing efficiency of the intermolecular excited state (that is, exciplex state). Most recently, a simple strategy towards high-performance fluorescent OLEDs was reported by stacking p-type hole-transport layer and n-type electron-transport layer, resulting in a planar pn heterojunction configuration similar to their inorganic counterparts, namely light emitting diodes (LEDs). Like this, there is a great and urgent demand for low-cost high-performance light-emitting materials, simple device structure and simple manufacturing process in the field of OLED display. This project will focus on the development of noble metal free organic light-emitting materials and their high performance devices, including development of a new generation of organic light-emitting materials and corresponding host materials with high efficiency and practical lifetimes, investigation of their light emitting mechanisms, structure-property relationships, excited states and their regulation processes, etc. Based on the developed materials and devices, full-color OLED display panels will also be designed and developed.