Simultaneous harvesting of triplet excitons in OLEDs by both guest and host materials with an intramolecular charge-transfer feature via triplet-triplet annihilation

A red naphthalimide derivative with an intramolecular charge-transfer (ICT) feature, namely (E)-2-(4-(t-butyl)phenyl)-6-(2-(6-(diphenylamino)naphthalen-2-yl)vinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (NA-TNA), was designed and synthesized. Photophysical and magneto-electroluminescence (MEL) characterization results revealed that NA-TNA could harvest triplet excitons via a triplet-triplet annihilation (TTA) process in organic light-emitting diodes (OLEDs) due to the presence of a lower-lying triplet excited state with ³ππ∗ character. Consequently, using NA-TNA as a guest compound and CzPhONI, another ICT-featured naphthalimide derivative with triplet fusion delayed fluorescence (TFDF) character as host material, a high-performance orange OLED with 6 wt% NA-TNA doped CzPhONI film as the emitting layer was acquired. The maximum current efficiency (LE_max), brightness (L_max), and external quantum yield (EQE_max) of this OLED is 7.73 cd A^-1, 31940 cd m^-2 and 5.83%, respectively, while the theoretical EQE_max of this device should not exceed 3.34%. On the contrary, the reference device with a NA-TNA doping level of 1.4 wt% showed much inferior performance, with a LE_max, a L_max, and an EQE_max of 3.19 cd A^-1, 24 900 cd m^-2 and 2.49%, respectively. The high performance of the 6 wt% NA-TNA doped device was attributed to the efficient harvesting of triplet excitons by both the guest and host materials.