Evolution of emission manners of organic light-emitting diodes: From emission of singlet exciton to emission of doublet exciton

The emission manners of organic light-emitting diodes (OLEDs) have experienced almost three-decade evolution. In this review, we briefly summarized the emission manners of OLEDs including: (i) emission from singlet exciton; (ii) emission from triplet exciton; (iii) emission from singlet exciton converted from triplet exciton. Then we introduced a new type of OLEDs with the emission from doublet exciton, wherein organic neutral radicals are used as emitters. Due to the spin-allowed transition of doublet excitons, using neutral radicals as emitters is believed to be a new way to break the 25% upper limit of internal quantum efficiency of OLEDs. The progress of emissive stable neutral radicals is also shortly reviewed.