Phosphorescent organic light-emitting diodes (PhOLEDs) unfurl a bright future for the next generation of flat-panel displays and lighting sources due to their merit of high quantum efficiency compared with fluorescent OLEDs. This critical review focuses on small-molecular organic host materials as triplet guest emitters in PhOLEDs. At first, some typical hole and electron transport materials used in OLEDs are briefly introduced. Then the hole transport-type, electron transport-type, bipolar transport host materials and the pure-hydrocarbon compounds are comprehensively presented. The molecular design concept, molecular structures and physical properties such as triplet energy, HOMO/LUMO energy levels, thermal and morphological stabilities, and the applications of host materials in PhOLEDs are reviewed (152 references).

Organic light-emitting diodes (OLEDs) are competitive candidates for the next generation flat-panel displays and solid state lighting sources. Efficient blue-emitting materials have been one of the most important prerequisites to kick off the commercialization of OLEDs. This tutorial review focuses on the design of blue fluorescent emitters and their applications in OLEDs. At first, some typical blue fluorescent materials as dopants are briefly introduced. Then nondoped blue emitters of hydrocarbon compounds are presented. Finally, the nondoped blue emitters endowed with hole-, electron- and bipolar-transporting abilities are comprehensively reviewed. The key issues on suppressing close-packing, achieving pure blue chromaticity, improving thermal and morphological stabilities, manipulating charge transporting abilities, simplifying device structures and the applications in panchromatic OLEDs are discussed.

Owing to the electron spin–orbit coupling (SOC) and fast intersystem crossing (ISC), heavy-metal complexes (such as iridium(III), platinum(II) and osmium(II) complexes, etc.) are phosphorescent emitters at room temperature. Since 1998, heavy-metal complexes as phosphors have received considerable academic and industrial attention in the field of organic light-emitting diodes (OLEDs), because they can harvest both the singlet (25%) and triplet (75%) excitons for emission during the electro-generated processes. Among all the visible colors (blue, green, yellow, orange and red), the yellow/orange heavy-metal complexes play an important role for realizing full-color OLEDs as well as high-efficiency white OLEDs, and thus the development of highly efficient yellow/orange heavy-metal complexes is a pressing concern. In this article, we will review the progress on yellow/orange heavy-metal complexes as phosphors in OLEDs. The general principles and useful tactics for designing the yellow/orange heavy-metal complexes will be systematically summarized. The structure–property relationship and electrophosphorescence performance of the yellow/orange heavy-metal complexes in monochromatic phosphorescent OLEDs (PhOLEDs) and white OLEDs (WOLEDs) will be comprehensively surveyed and discussed.