Breaking the Efficiency Limit of Fluorescent OLEDs by Hybridized Local and Charge-Transfer Host Materials.

Hybridized local and charge-transfer (HLCT) states with "hot exciton" properties are effective in harvesting high-lying triplet excitons for electroluminescence in organic light-emitting diodes (OLEDs). Here, we propose a technique based on the HLCT mechanism at the high-lying excited states to develop HLCT-sensitized fluorescent (HLCT-SF) OLEDs using HLCT host molecules and metal-free fluorescent dopants for highly efficient OLEDs. A maximum external quantum efficiency (EQE) up to 6.3% and an exciton utilizing efficiency (EUE) of 64% were achieved, apparently exceeding the upper limits of the EQE (5%) and EUE (25%) in conventional fluorescent OLEDs. The HLCT-SF process via long-range Förster resonance energy transfer from the singlet excited states of the HLCT host to that of the fluorescent guest is efficient in harvesting "hot triplet excitons" by efficient high-lying reverse intersystem crossing, and the newly proposed HLCT-SF OLEDs represent an important advance in realizing high-performance OLEDs.