Organic Light-Emitting Diodes Based on Conjugation-Induced Thermally Activated Delayed Fluorescence Polymers: Interplay Between Intra- and Intermolecular Charge Transfer States

Abstract

In this work, interactions between different host materials and a blue TADF polymer named P1 are systematically investigated. In photoluminescence, the host can have substantial impact on the photoluminescence quantum yield (PLQY) and the intensity of delayed fluorescence (ΦDF), where more than three orders of magnitude difference of ΦDF in various hosts is observed, resulting from a polarity effect of the host material and energy transfer. Additionally, an intermolecular charge-transfer (CT) emission with pronounced TADF characteristics is observed between P1 and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), with a singlet-triplet splitting of 7 meV. It is noted that the contribution of harvested triplets in monochrome organic light-emitting diodes (OLEDs) correlates with ΦDF. For devices based on intermolecular CT-emission, the harvested triplets contribute ~90% to the internal quantum efficiency. The results demonstrate the vital importance of host materials on improving the PLQY and sensitizing ΦDF of TADF polymers for efficient devices. Solution-processed polychrome OLEDs with a color close to a white emission are presented, with the emission of intramolecular (P1) and intermolecular TADF (PO-T2T:P1).

Sterically‐Locked Donor–Acceptor Conjugated Polymers Showing Efficient Thermally Activated Delayed Fluorescence

“…For instance, the EQE still remains to be 23.4 % at 100 cd m −2 and 18.2 % at 1000 cd m −2 . To the best of our knowledge, the performance is among the highest ever reported for TADF polymers, [37–46] and even comparable to TADF small molecules [14–18] …”

Through‐space charge transfer polymers for solution‐processed organic light‐emitting diodes

“…For example, polyfluorene [38][39][40][41][42] and polyspirofluorene derivatives [43,44] containing D-A structure with TBCT emission are proved to be a successful approach to realize full-color emission covering the whole visible region for solution-processed OLEDs. Recently, TADF polymers [45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] with finely controlled CT processes based on conjugated donor-acceptor architectures have been demonstrated to utilize nonemissive triplet excitons through RISC process. However, the strong through-bond charge transfer effect in conjugated architecture tends to induce a large redshift of emission, which is undesirable for realizing blue emission.…”

Polymer Featuring Thermally Activated Delayed Fluorescence as Emitter in Light-Emitting Electrochemical Cells

“…We also note that significant changes to the P1 emission capacity have been reported following the inclusion of other small-molecule host compounds, although the exact changes appear to be intimately linked to the detailed chemical and energy structure of the host compound. 42 , 49 Our conclusion is that the PYD-2Cz host and the P1 guest interact strongly in the host-guest blend, as manifested in the fact that the host-to-guest transfer is complete and the PLQY of P1 increases strongly, and we suggest that the majority PYD-2Cz host acts as a solid-state solvent for the minority P1 guest. 50 …”

“…We note that these results are essentially identical to those reported in a previous study on the P1 polymer. 49 …”