Tuning the emission and exciton utilization mechanisms of pyrazine-based multi-carbazole emitters and their use in organic light-emitting diodes
Date
14/10/2023Author
Grant ID
EP/P010482/1
ep/l017008/1
SRF\R1\201089
Metadata
Show full item recordAbstract
Thermally activated delayed fluorescence (TADF) and hot excitons are two distinct exciton harvesting mechanisms that can lead to 100% internal quantum efficiency in organic light-emitting diodes (OLEDs). Herein, we show that with judicious molecular engineering, the resulting structurally similar compounds emit via distinct photophysical mechanisms, which has a direct consequence on the OLED efficiency. When the pyrazine core is substituted with four carbazoles, the molecule 4CzPyz shows TADF in doped PPT film, with ΦPL of 75%, ΔEST of 0.23 eV and τd of 150 μs. The device based on 4CzPyz emits in the sky-blue (λEL = 486 nm) with an EQEmax of 24.1%. When one carbazole is replaced by an ortho-biphenyl, the ΔEST of 3CzBPz increases to 0.29 eV, the ΦPL decreases to 56%, and the TADF character is largely suppressed in the PPT film. However, a RISC process between higher-lying triplet excited states and the S1 state is hypothesized to be operational, supported by a combined photophysical and DFT study, to rationalize how the device with 3CzBPz shows an EQEmax of 9.6% (λEL = 464 nm), reflecting that greater than 86% of the excitons are converted into light in the OLED. When two ortho-biphenyl groups are connected to the pyrazine core, the ΔEST of 2CzBPz is further increased to 0.34 eV while the ΦPL is reduced to 45% in the PPT film. The DFT and photophysical studies indicate that 2CzBPz should act as a traditional blue fluorescence emitter. The OLED devices with 2CzBPz bear this out and exhibit an EQEmax 3.2% at a λEL of 446 nm. These results show how subtle structural changes modulate the efficiency of the triplet exciton harvesting mechanisms and provide new design directions for highly efficient blue emitters for OLEDs.
Citation
Chen , D , Zhang , L , Matulaitis , T , Cordes , D B , Slawin , A M Z , Zhang , X-H , Samuel , I D W & Zysman-Colman , E 2023 , ' Tuning the emission and exciton utilization mechanisms of pyrazine-based multi-carbazole emitters and their use in organic light-emitting diodes ' , Journal of Materials Chemistry C , vol. 11 , no. 38 , pp. 13095-13105 . https://doi.org/10.1039/d3tc02463f
Publication
Journal of Materials Chemistry C
Status
Peer reviewed
ISSN
2050-7526Type
Journal article
Description
The St Andrews team would also like to thank EPSRC (EP/P010482/1 and EP/L017008/1) for financial support. EZ-C is a Royal Society Leverhulme Trust Senior Research Fellow (SRF\R1\201089). D.C thanks the China Postdoctoral Science Foundation (No. 2022TQ0227).Collections
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