Considering the imminent global energy crisis and inefficient energy utilization, energy-efficient organic light-emitting diodes (OLEDs) are considered one of the most competitive candidates for displays and particularly for future energy-saving lighting sources. Full color displays require all primary colors: red, green, and blue (RGB). In recent decades, numerous phenanthroimidazole (PI) RGB-emitting materials have been developed for efficient OLEDs. In organic electronics, considerable interest is shown on PI, due to ease in fluorophore modification. This chapter focuses on the design and synthesis of PI-based materials and their applications in OLEDs. At first, some nondoped blue, green, and yellow fluorescent materials are comprehensively studied. Then attention has been paid for typical blue, green, yellow, orange, and red PhOLEDs of PI-based fluorophores as a host materials are briefly presented. The molecular design concept, general synthetic routes for PI materials, and the applications of fluorophores in fluorescent OLEDs and host materials in PhOLEDs are reviewed.