Spirobifluorene-based hole-blocking material with enhanced efficiency through hole and exciton confinement in blue fluorescent OLEDs

A new hole-blocking material, 2-(9,9′-spirobi[fluoren]-5-yl)-4-phenylbenzo [4,5]thieno[3,2-d]pyrimidine (SBF-BTPym), was designed and synthesized using spirobifluorene and electron transport moieties. The high triplet energy and high thermal stability of the spirobifluorene moiety were suitable for designing the hole-blocking material for blue fluorescent organic light-emitting diodes (OLEDs). The electron deficiency of benzothienopyrimidine allowed good electron transport properties, and a deep highest occupied molecular orbital energy level of −6.55 eV suppressed leakage of the holes from the emitting layer (EML) to the adjacent layer. Blue fluorescent OLEDs consisting of a triplet-triplet annihilation–induced host were fabricated using SBF-BTPym. The maximum external quantum efficiency of the blue fluorescent device was 7.2%, which was higher than that of the device without a hole-blocking layer by 10%. According to transient electroluminescence analysis, exciton confinement in the EML is mainly responsible for the improved efficiency.