Since TSPO1 has a dominant electron-transport character, the recombination zone inside the emission layer (EML) is believed to be located near the interface between the emission layer (EML) and the hole transport layer (HTL). To avoid exciton loss into the HTL, an exciton blocking layer of 10 nm thick as shown in Figure and Table . At the lowest doping level of 1.5 wt%, the EL peak (lEL = 402, FWHM = 22 nm) was nearly identical to the lPL in dilute THF solution (lPL = 399 nm, FWHM = 23 nm). Figure shows the EL spectrum of MesB-DIDOBNA-N, which is the bluest of all the MR-TADF OLEDs reported to date. In addition, the MesB-DIDOBNA-N devices exhibited the shortest lEL and narrowest FWHM among the reported purely organic molecule-based violet OLEDs. With increasing of cD, the lEL red-shifted slightly to 404 nm (3wt%) and 405 nm (5wt%) while the FWHM increased to 25 nm and 31 nm, respectively. The spectral broadening resulted mainly from the increase in intensity of the low energy shoulder peak at 430 nm. Despite these undesired spectral changes with increasing cD, devices showed extraordinary color purity, with CIE coordinates of (0.165, 0.044), (0.166, 0.045), and (0.166, 0.055) for the MesB-DIDOBNA-N OLEDs with cD of 1.5, 3, and 5wt%, respectively. Those values, especially that with cD of 3 wt%, are very close to the BT.2020 standard for the primary blue color of UHDTV [i.e., CIE coordinates of (0.131, 0.046)].