Insights into energy transfer pathways between the exciplex host and fluorescent guest: attaining highly efficient 710 nm electroluminescence

Energy transfer between the exciplex host and fluorescent guest is a demanding process for attaining high-performance organic light-emitting diodes (OLEDs), particularly in the near-infrared (NIR) region, and insight into the dynamics of energy transfer has been elusive. In this study, new deep-red/NIR chromophores, NOz-TPA and NOz-t-TPA where NOz and TPA denote naphthobisoxadiazole and triphenylamine, respectively, have been developed with an electron donor-acceptor-donor (D-A-D) configuration. The optimized 1 wt% doped films for NOz-TPA and NOz-t-TPA blended with the Tris-PCz:CN-T2T (1 : 1 in molar ratio) exciplex host showed similar deep red/NIR emissions with photoluminescence quantum yields (PLQY) of 42 (680 nm) and 28%, (709 nm), respectively. Comprehensive time-resolved measurements and dynamics analyses revealed significant differences in the energy transfer pathways, i.e. Forster versus Dexter-type energy transfer between the exciplex host and the fluorescent guest, in which the introduction of bulky tert-butyl groups in the NOz-t-TPA doped film greatly suppressed the Dexter-type energy transfer pathway. Despite the lower PLQY, the analytical simulation predicted NOz-t-TPA to be a better candidate for realizing highly efficient electroluminescence. Confirmation was provided by the performance of the NOz-t-TPA-doped OLED, showing an external quantum efficiency (EQE) of 6.6% with peak wavelength at 710 nm, which is among the best records for the metal-free NIR OLEDs around 710 nm. Insight into energy transfer pathways thus plays a pivotal role in achieving the high-performance OLEDs that incorporate the exciplex host and fluorescent guest.