Thermally Activated Delayed Fluorescence Conjugated Polymers with Backbone‐Donor/Pendant‐Acceptor Architecture for Nondoped OLEDs with High External Quantum Efficiency and Low Roll‐Off

Abstract

Most thermally activated delayed fluorescence (TADF) emitters have to be doped in the host for fabricating efficient organic light-emitting diodes (OLEDs) and always suffer from quick efficiency roll-off at high brightness, which severely affect their commercial application in display and lighting fields. In the work, a series of the polymers are synthesized by copolymerization of two carbazole monomers and one acridine derivative monomer containing benzophenone acceptor group. The obtained polymers therefore possess a conjugated backbone with carbazole/acridine moieties and benzophenone pendant to form the twisted donor/acceptor structure. Consequently, the TADF features inherited from the acridine derivative are maintained and improved by managing the content of acridine derivative monomer in the polymers. Solution-processed OLEDs obtained from using neat polymer films exhibit comparable performance with organic TADF small molecules, achieving a maximum external quantum efficiency (EQE) of 18.1% and a very slow roll-off with EQE of 17.8% at the luminance of 1000 cd m −2 .

Bridging Small Molecules to Conjugated Polymers: Efficient Thermally Activated Delayed Fluorescence with a Methyl‐Substituted Phenylene Linker

“…Thefirst one is based on the combination of an electron donor (D) and electron acceptor (A). [16][17][18][19][20][21] That is,both Dand Aare carefully aligned at different sites to form aT ADF polymer,s uch as Da nd A simultaneously in the main chain, [16] Dand Asimultaneously in the side-chain, [17,18] or Di nt he main chain and Ai nt he side-chain [19][20][21] etc. Unlike small molecules,i ti sn ot an easy task to realize TADF at am acromolecular level because the relative distance,strength, and location of Dand Aall need to be well controlled to tune through-bond or through-space charge transfer (CT).…”

Effect of a Pendant Acceptor on Thermally Activated Delayed Fluorescence Properties of Conjugated Polymers with Backbone‐Donor/Pendant‐Acceptor Architecture

“…Another noticeable characteristic is that the polymers have short τ DF values, which are of benefit to restraining the efficiency roll‐off at high current density or luminance in EL device. For example, τ DF values of the polymers PCzAB3Py5, PCzAB2Py5, and PCzA3PyB5 are 0.88, 0.95, and 1.13 μs, respectively, even less than 1.29 μs of the TADF polymer PABPC5 previously reported …”

“…Thus, a further consequence should be that the electron‐withdrawing ability of the acceptor would intensify when the pyridyl groups were introduced as a linker and/or an ortho ‐positioned group of the carbonyl group. In addition, by comparison with the similar precursor with BP as the acceptor group (Table S2 in the Supporting Information), several marked differences in the structural parameters imply that the replacement of benzene by pyridine is anticipated to modify the photophysical properties in the corresponding polymers …”

“…[43] In our recent work, the differences in TADF behavior were also observed owing to using different pendant acceptors in the polymers with backbone-donor/pendant-acceptor (BDPA) architectures. [55][56][57][58][59][60][61] The solution-processed device based on the polymers with BP as the pendanta cceptora chieved the higher EQE than those with formylphenyl pendant acceptor,a lthough the devices had similar emissive wavelength with low turn-on voltages ands low efficiency roll-offs. [59,60] The results provide an incentive to further develop the TADF polymer by meanso f replacing the pendant acceptor so as to furnish improved EL performance, especially with other emission colors to supplement the numerous green TADF materials.O nthe basis of this consideration, three isomeric benzoylpyridine derivatives are selected as pendanta cceptorst oc reate three sets of conjugated polymers,P CzA3PyB, PCzAB2Py,a nd PCzAB3Py,w ithi dentical backbones.…”

“…[55][56][57][58][59][60][61] The solution-processed device based on the polymers with BP as the pendanta cceptora chieved the higher EQE than those with formylphenyl pendant acceptor,a lthough the devices had similar emissive wavelength with low turn-on voltages ands low efficiency roll-offs. [59,60] The results provide an incentive to further develop the TADF polymer by meanso f replacing the pendant acceptor so as to furnish improved EL performance, especially with other emission colors to supplement the numerous green TADF materials.O nthe basis of this consideration, three isomeric benzoylpyridine derivatives are selected as pendanta cceptorst oc reate three sets of conjugated polymers,P CzA3PyB, PCzAB2Py,a nd PCzAB3Py,w ithi dentical backbones. As expected, the pendant acceptors definitively influencet he TADF features of these polymers, especially giving rise to the redshifted emissions compared with the reported polymers with benzophenoneo rformylphenyla st he pendanta cceptor.T he photophysical properties and EL performances of thesep olymers are fully characterized, indicative of their TADF behavior.C ombined with the crystal structures of the three precursors, the effect of the pendant acceptors on the TADF properties is discussed in detail.…”