The broadening of emission for fused planar emitters at a higher concentration is common due to intermolecular interaction in the emitting layers. Due to its high ET, we also fabricated devices using DPEPO as the host at the same cD of 1.5 wt% for comparison purposes. Besides the lower EQE, the EL spectrum is broadened (FWHM = 44 nm) and the turn-on voltage is higher by 0.9 V in the DPEPO device compared to that in TSPO1. In addition, the device showed incomplete electron confinement within the EML as evidenced by the high-energy EL peak at 380 nm, which was assigned to emission from the TCTA layer.