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Rational design of TADF polymers using a donor–acceptor monomer with enhanced TADF efficiency induced by the energy alignment of charge transfer and local triplet excited states.

Nobuyasu, Roberto S. and Ren, Zhongjie and Griffiths, Gareth C. and Batsanov, Andrei S. and Data, Przemyslaw and Yan, Shouke and Monkman, Andrew P. and Bryce, Martin R. and Dias, Fernando B. (2016) 'Rational design of TADF polymers using a donor–acceptor monomer with enhanced TADF efficiency induced by the energy alignment of charge transfer and local triplet excited states.', Advanced optical materials., 4 (4). pp. 597-607.

Abstract

The photophysics of thermally activated delayed fluorescence (TADF) in phenothiazine-dibenzothiophene-S,S-dioxide (PTZ-DBTO2) molecule is investigated in detail. First, it is shown that the proximity of local triplet excited states (3LE), e.g., 3D or 3A, above or below the DA charge transfer states (CT) is crucial for the efficiency of the TADF mechanism in PTZ-DBTO2. This TADF emitter is then used as a monomer unit to design polymer materials with efficient TADF. The reverse intersystem crossing mechanism (RISC) that supports TADF is able to compete with internal conversion and triplet–triplet annihilation (TTA) in the polymer chains and generates efficient TADF emission in the polymer pristine films. Prototype devices with PTZ-DBTO2 dispersed in 4,4′-bis(N-carbazolyl)-2,2′-biphenyl (CBP) host give excellent performance with EQE of ≈22% at low luminance (<100 cd m−2), for 100 cd m−2 the EQE is 19.4%. In the case of solution processed devices, using the novel TADF polymers, the performance is much lower, EQE ≈3.5% at 100 cd m−2, which is still the highest value for a polymer TADF system at useful brightness, yet reported. This results from a combination of weak charge transport properties in these materials and device fabrication methods that require further improvement. Nevertheless, these results pave the way to explore TADF in polymer light emitting diodes (PLEDs), using less costly deposition methods, such as spin-coating and inkjet printing, which are more appropriate for large area deposition.

Item Type:Article
Keywords:Conjugated polymers, dibenzothiophene-S,S-dioxide, OLEDs, Phenothiazine, TADF.
Full text:(AM) Accepted Manuscript
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Status:Peer-reviewed
Publisher Web site:http://dx.doi.org/10.1002/adom.201500689
Publisher statement:This is the peer reviewed version of the following article: Nobuyasu, R. S., Ren, Z., Griffiths, G. C., Batsanov, A. S., Data, P., Yan, S., Monkman, A. P., Bryce, M. R. and Dias, F. B. (2016), Rational Design of TADF Polymers Using a Donor–Acceptor Monomer with Enhanced TADF Efficiency Induced by the Energy Alignment of Charge Transfer and Local Triplet Excited States. Advanced Optical Materials, 4(4): 597-607 which has been published in final form at http://dx.doi.org/10.1002/adom.201500689. This article may be used for non-commercial purposes in accordance With Wiley-VCH Terms and Conditions for self-archiving.
Date accepted:12 January 2016
Date deposited:18 January 2016
Date of first online publication:12 January 2016
Date first made open access:12 January 2017

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