Red to near-infrared organometallic phosphorescent dyes for OLED applications

In modern research on organic light-emitting diodes (OLEDs), cyclometalated iridium(III) complexes represent one of the most studied class of compounds. The high emission efficiency caused by the strong spin-orbit coupling in the presence of heavy metals leads to the mixing of singlet and triplet manifolds so that both the singlet and triplet excitons can be harvested. For OLEDs to be useful in displays application, true red, green, and blue emissions of sufficient luminous efficiencies and proper chromaticity are required. In recent years, the development of materials for phosphorescent red OLEDs has indeed gone through several important evolutional stages. However, the luminescent quantum yields of red-emitting iridium(III) phosphors tend to be intrinsically low which are governed by the energy gap law for triplet states in which the luminescence quantum yields tend to decrease with an increase in the emission wavelength. Many red organic dyes currently in use do not show a good compromise between device efficiency and color purity. In general, a dilemma facing red OLEDs was realized in which efficient and bright dopants are not red enough, and red-enough dopants are not efficient and bright. In this review article, we highlight the recent progress and current challenges of efficient OLEDs based on cyclometalated iridium(III) dyes which exhibit saturated red and near-infrared electroluminescence. Optimization of the phosphorescent red OLED efficiency/color purity trade-off and extension of the work to other organometallic phosphors are also presented and discussed.