Abstract
Two sets of conjugated polymers with anthraquinone groups as pendant acceptors were designed and synthesized. The acceptor is tethered to an diphenylamine group via a phenylene bridge, constructing a thermally activated delayed fluorescence (TADF) unit, which is embedded into the polymer backbone through its donor fragment, while the backbone is composed of dibenzothiophene-S, S-dioxide and 2, 7-fluorene or 2, 7-carbazole groups. The polymers show distinct TADF characteristics, confirmed by transient photoluminescence spectra and theoretical calculations. The carbazole-based polymers exhibit shorter delay lifetimes and lower energy emission relative to the fluorene-based polymers. The non-doped organic light-emitting diodes fabricated via solution processing approach produce efficient red emissions with the wavelengths of 625–646 nm. The carbazole containing polymer with 2% molar content of the TADF unit exhibits the best maximum external quantum efficiency of 13.6% and saturated red electroluminescence with the Commission Internationale de l'Eclairage coordinates of (0.62, 0.37).
Highlights
As the new-generation luminescent materials, metal-free thermally activated delayed fluorescence (TADF) emitters have drawn great attentions in the field of organic light-emitting diodes (OLEDs) because they could theoretically achieve 100% internal quantum efficiency through reverse intersystem crossing (RISC) process of non-radiative triplet excitons (Tao et al, 2014; Im et al, 2017; Yang et al, 2017; Huang et al, 2018; Liu et al, 2018; Zou et al, 2018; Godumala et al, 2019)
As shown in Scheme 1, polymers PFSOTAQx and PCzSOTAQx were synthesized via Suzuki polycondensation of the corresponding monomers, 2-(4-(bis(4bromophenyl)amino)phenyl)-anthraquinone (M1), 3,7dibromo-2,8-dioctyldibenzothiophene-S,S-dioxide (M2) and alkyl substituted fluorene or carbazole diboronic ester (M3 or M4), using Pd(OAc)2/PCy3 as catalyst and tetraethylammonium hydroxide (Et4NOH) as an emulsifying base, followed by end-capping with phenyl boronic acid and bromobenzene (Liu et al, 2008a)
The carbazolebased PCzSOTAQ2 exhibits better hole transport capability, ascribed to the shallower highest occupied molecular orbitals (HOMOs) energy level of PCzSOTAQ2, relative to the fluorene-based polymer PFSOTAQ2, while they show similar electron transport behaviors due to the same acceptor group and the similar conjugation. This may be one of the reasons for better comprehensive device performance of PCzSOTAQ2 than PFSOTAQ2. These results indicate that the backbone structure has a crucial influence on the EL behaviors of the polymers, and confirm that the conjugated structure is more favorable for obtaining more efficient red TADF polymer
Summary
As the new-generation luminescent materials, metal-free thermally activated delayed fluorescence (TADF) emitters have drawn great attentions in the field of organic light-emitting diodes (OLEDs) because they could theoretically achieve 100% internal quantum efficiency through reverse intersystem crossing (RISC) process of non-radiative triplet excitons (Tao et al, 2014; Im et al, 2017; Yang et al, 2017; Huang et al, 2018; Liu et al, 2018; Zou et al, 2018; Godumala et al, 2019). The clearly intensified red emission, especially in PFSOTAQ5, indicates the occurrence of the intramolecular Förster energy transfer from the polymer backbone to the TADF unit.
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