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Title: Efficient deep‐blue light‐emitting polyfluorenes based on 9,9 ‐dimethyl‐9 H ‐thioxanthene 10,10‐dioxide isomers
Abstract

We have designed and synthesized a series of deep‐blue light‐emitting polyfluorenes, PF‐27SOs and PF‐36SOs, by introducing electron‐deficient 9,9‐dimethyl‐9H‐thioxanthene 10,10‐dioxide isomers (27SO and 36SO) into the poly(9,9‐dioctylfluorene) (PFO) backbone. Compared with PFO, the resulting polymers exhibit an equivalent thermal decomposition temperature (>415 °C), an enhanced glass transition temperature (>99 °C), a decreased lowest unoccupied molecular orbital energy level (ELUMO) below −2.32 eV, a blue‐shifted photoluminescence spectra in solid film with a maximum emission at ~422 nm, and a shoulder peak at ~445 nm. The resulting polymers also show blue‐shifted and narrowed electroluminescence spectra with deep‐blue Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.07) for PF‐27SO20 and (0.16, 0.06) for PF‐36SO30, compared with (0.17, 0.13) for PFO. Moreover, simple device based on PF‐36SO30 achieves a superior device performance with a maximum external quantum efficiency (EQEmax= 3.62%) compared with PFO (EQEmax= 0.47%). The results show that nonconjugated 9,9‐dimethyl‐9H‐thioxanthene 10,10‐dioxide isomers can effectively perturb the conjugation length of polymers, significantly weaken the charge‐transfer effect in donor–acceptor systems, substantially improve electroluminescence device efficiency, and achieve deep‐blue light emission.

 
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NSF-PAR ID:
10453658
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Journal of Polymer Science
Volume:
58
Issue:
10
ISSN:
2642-4150
Page Range / eLocation ID:
p. 1380-1392
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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A 15-μm-diam-mesa device was defined by standard planar processing including a top annular ohmic contact with a 5-μm-diam pinhole in the center to couple out enough of the internal emission for accurate free-space power measurements [4]. The emission spectra have the behavior displayed in Fig. 1(b), parameterized by bias voltage (VB). The long wavelength emission edge is at  = 1684 nm - close to the In0.53Ga0.47As bandgap energy of Ug ≈ 0.75 eV at 300 K. The spectral peaks for VB = 2.8 and 3.0 V both occur around  = 1550 nm (h = 0.75 eV), so blue-shifted relative to the peak of the “ideal”, bulk InGaAs emission spectrum shown in Fig. 1(b) [5]. These results are consistent with the model displayed in Fig. 1(c), whereby the broad emission peak is attributed to the radiative recombination between electrons accumulated on the emitter side, and holes generated on the emitter side by interband tunneling with current density Jinter. 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Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981). [2] M. Feiginov et al., Appl. Phys. Lett., 99, 233506, 2011. [6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995). [3] Y. Nishida et al., Nature Sci. Reports, 9, 18125, 2019. [7] E.O. Kane, J. of Appl. Phy 32, 83 (1961). [4] P. Fakhimi, et al., 2019 DRC Conference Digest. [8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018). [5] S. Sze, Physics of Semiconductor Devices, 2nd Ed. 12.2.1 (Wiley, 1981). [6] L. Coldren, Diode Lasers and Photonic Integrated Circuits, (Wiley, 1995). [7] E.O. Kane, J. of Appl. Phy 32, 83 (1961). [8] T. Growden, et al., Nature Light: Science & Applications 7, 17150 (2018). 
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