US Patent 9051344 Stability OLED materials and devices

Organic light emitting materials and devices comprising phosphorescent metal complexes comprising ligands comprising aryl or heteroaryl groups substituted at both ortho positions are described. An organic light emitting device, comprising: an anode; a hole transport layer; an organic emissive layer comprising an emissive layer host and an emissive dopant; an electron impeding layer; an electron transport layer; and a cathode disposed, in that order, over a substrate.