Does Through-Space Charge Transfer in Bipolar Hosts Affect the Efficiency of Blue OLEDs?

Additional energy loss pathways which occur in blue electrophosphorescent devices via through-space charge transfer (TSCT) states of bipolar hosts are identified for the first time in this study. Blue phosphorescent organic light-emitting diodes (PhOLEDs) with maximum external quantum efficiency from 22.7 to 30.5% are fabricated using new bipolar derivatives of benzimidazole, linked through phenyl spacer with different number of tert-butyl substituted carbazoles, as hosts. Applicability of the newly synthesized compounds as hosts for blue PhOLEDs is justified by their high triplet levels (2.94–2.98 eV), bipolar charge-transporting properties with charge mobilities up to 2.34 × 10⁻³ cm² V⁻¹ s⁻¹ at electric field of 2.7 × 10⁵ V cm⁻¹, and high ionization potentials (5.63–5.81 eV). Weak microsecond-lived emission, which is attributed to TSCT, is detected for the developed bipolar hosts. The different efficiencies of the fabricated PhOLEDs are partly related to energy losses via TSCT states, the presence of which is detected for the developed donor–acceptor hosts in both solution and solid-state. To prevent energy losses through TSCT in PhOLEDs, the excited state energy of emitter has to be lower than triplet levels of bipolar host. It also has to be lower than the TSCT energy level of bipolar host for the fabrication highly efficient PhOLEDs.