Impact of Dopant Aggregation on the EL of Blue Fluorescent Host-Dopant Emitters

We examined the characteristics of blue fluorescent organic light-emitting diodes (OLEDs) based on mCP:BCzVBi and CBP:BCzVBi systems which have Type-I and Type-II energy level alignments, respectively. As the BCzVBi concentration was increased from 5 wt% to 100 wt%, the OLEDs exhibited a 2.1-V voltage reduction and a 14-nm spectral redshift at 50 mA/cm$^{2}$ , indicating dopant aggregation which facilitates direct charge injection into the dopant. The mCP:BCzVBi OLED displayed a weak concentration dependence and attained a maximum luminance at a high concentration of 50 wt%. For CBP:BCzVBi, the luminance showed a stronger concentration dependence and peaked at a low concentration of 15 wt%. This discrepancy demonstrated a big impact of the energy level alignment on the charge and exciton behaviors in host-dopant systems. With a Type-I alignment, balanced charge injection and trapping in the dopant lead to suppressed singlet exciton-charge annihilation and mitigated quenching effects of dopant aggregation.