Recent progress of phosphine electroluminescent materials and devices

Abstract

Organic light emitting diodes (OLEDs) have been attracting much attention and have been studied extensively by scientists and engineers in both academic and industrial communities, due to their great advantages of self-emitting, fast response, wide view-angle, full color capacity, high efficiency, low power consumption, ultra-thinness, lightness and flexibility for the applications of new generation flat panel displays and solid-state lighting sources. With the full use of active-matrix OLED (AMOLED) screen in iPhone X, OLED technology has become the mainstream information display technology and has gained more than 40% of market share. In the era of information, liquid crystal display technology will be gradually replaced by OLED technology. However, OLEDs technology still faces many challenges, including low yield, high cost, short service life, environmentally unfriendly and so on, which urgently requires more in-depth and detailed research in material development and device design. Since the first OLED reported by CW Tang, OLED technology has developed from the first generation of fluorescence (FL) technology, the second generation of phosphorescence (PH) technology to the third generation of thermally activated delayed fluorescence (TADF) technology. In general, PH materials are heavy metal complexes, in which triplet excited state is the main radiative excited state. Since singlet exciton can translate into triplet exciton through intersystem crossing (ISC) process, therefore, in theory, 100% electrogenerated excitons can be used in phosphorescence materials. Compared with the first generation of fluorescence materials, which can only use 25% single state electrogenerated excitons, the electro-optic conversion efficiency is greatly improved in phosphorescence materials. However, heavy metal ions in PH materials not only increase the cost, but also bring some potential environmental pollution. TADF technology can realize the transformation of non-radiative triplet excitons into radiative singlet excitons through reverse ISC (RISC) process, and thus achieving 100% exciton utilization. TADF material is usually pure organic molecule based on a push-pull electron system, thus making up for the defects of phosphorescent material. Therefore, as a kind of molecular electroluminescent devices, innovation of material system is the foundation for the development of OLED technology. With the insulating effect, moderate electronic effect, large steric hindrance, multi-functionalizability and coordination effect, aryl phosphine (oxide) group is one of the few groups with multiple functions. Through comprehensively utilizing these functions, aryl phosphine (oxide) group reveal the unique advantage in selectively optimizing molecular optoelectronic properties, further realizing high-efficiency OLED, and providing a platform for selective investigation of the influence of single property variation on optoelectronic performance. Recently, phosphine optoelectronic materials have been gradually developed as one of the focuses in OLED field, whose excellent performance and clear structure-property relationship reveal the significant theoretical and application values regarding to enriching OLED material, suggesting the methods in material design and optimization and promoting the OLED technology innovation. Based on material design strategy, we developed phosphine host materials, emitters and electron transporting materials and deeply investigated the photophysical properties, excited-state characteristics, electrical properties and OLED performance. In this paper, the main works are summarized, with the expectation to provide reference for the subsequent study on phosphine optoelectronic materials.