Abstract
Throwing light upon the structure-property relationship of the excited state properties for next-generation fluorescent materials is crucial for the organic light emitting diode (OLED) field. Herein, we designed and synthesized three donor-acceptor (D-A) structure compounds based on a strong spin orbit coupling (SOC) acceptor benzo[a, c]phenazine (DPPZ) to research on the three typical types of excited states, namely, the locally-excited (LE) dominated excited state (CZP-DPPZ), the hybridized local and charge-transfer (HLCT) state (TPA-DPPZ), and the charge-transfer (CT) dominated state with TADF characteristics (PXZ-DPPZ). A theoretical combined experimental research was adopted for the excited state properties and their regulation methods of the three compounds. Benefiting from the HLCT character, TPA-DPPZ achieves the best non-doped device performance with maximum brightness of 61,951 cd m−2 and maximum external quantum efficiency of 3.42%, with both high photoluminescence quantum efficiency of 40.2% and high exciton utilization of 42.8%. Additionally, for the doped OLED, PXZ-DPPZ can achieve a max EQE of 9.35%, due to a suppressed triplet quenching and an enhanced SOC.
Highlights
The hybridized local and charge-transfer (HLCT) excited state is decent for fast triplet utilization and high photoluminescent (PL) efficiency, which is a promising method for the next-generation organic light emitting diode (OLED) materials
The HLCT excited state contains a coexistence of LE and CT characters, and simultaneous high PL efficiency with high exciton utilization in OLED is expected to achieve through rational state regulation(Li et al, 2014b; Liu et al, 2015, 2017b; Zhou et al, 2017)
We have proved that the two sp2-hybridized nitrogen atoms greatly contribute to the enhanced spin orbit coupling (SOC) in DPPZ (∼10 cm−1) according to the El-Sayed rule(El-Sayed, 1963, 1964), and obviously, for the same reason, it can be a suitable acceptor for the donor-acceptor (D-A) material, which is expectable for realizing decent electroluminescent (EL) performances
Summary
Over the past few decades, organic light-emitting diodes (OLEDs) have attracted much attention from academia to industry due to their advantages for high quality flat panel display and lighting applications (Tang and VanSlyke, 1987; Cao et al, 1999; Xiang et al, 2013; Jin et al, 2014; Yao et al, 2014; Krotkus et al, 2016; Chen et al, 2017, 2018; Liu et al, 2017a). In this work, based on the DPPZ acceptor, we designed and synthesized three D-A compounds with three typical donor moieties (CZP, phenyl-carbarzole; TPA, triphenylamine; PXZ, phenoxazine), which corresponds to three typical excited states characteristics: the LE-dominated excited state (CZP-DPPZ), the HLCT (TPA-DPPZ) and CT state with TADF characters (PXZDPPZ) Among these three molecules, the HLCT material TPADPPZ obtains both high quantum efficiency of 40.2% and high exciton utilization of 42.8% in the non-doped OLED, benefiting from the HLCT character that arises the radiative transition rate and restrains the non-radiative transition, and eventually achieves the best non-doped OLED performance among the three materials with a maximum external quantum efficiency (EQE) of 3.42%. The TADF material PXZ-DPPZ demonstrates an efficient orange-red electro-fluorescence in doped OLED with an EQE of 9.35%
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