| Summary: | 博士 === 國立清華大學 === 化學工程學系 === 94 === This study is for synthesizing the carbazole(Cz)-modified polyfluorenes (CzPFs) and the polymer structures are characterized by FTIR, Elemental Analysis, and XRD. From the results of FTIR and XRD, CzPFs are Keto-free and amorphous materials.
The content of Cz moiety in CzPFs could be adjusted by adopting the copolymerization method, and the compositions of copolymers are estimated from Elemental Alanysis. As the content of Cz moiety is 75, Cz75PF, the best device performance (ITO/PEDOT/EML/Ca/Al) could be accomplished, with maximum brightness of 4565 cd/m2 (8 V, 0.82 cd/A), maximum efficiency of 1.92 cd/A (3.8 V, 13 cd/m2), and external quantum efficiency of 0.82% (6.8 times of that of PFO). By CFx modified ITO (ITO/CFx/Cz75PF/CsF/Ca/Al), the device performance could be improved, with maximum brightness of 34912 cd/m2 (8.5 V, 2.35 cd/A), maximum luminous efficiency of 6.71 (4.2 V, 223 cd/m2), and external quantum efficiency of 3.38% (4.1 times of the device with PEDOT as hole-injection layer and Ca as cathode).
Photo-excited and Field-induced thermally stimulated current (PE- and PI-TSC) and molecular dynamics simulation studies indicated the presence of two kinds of excimers in CzPFs, the partially overlapped “secondary” excimer and the fully overlapped “sandwich” excimer, both acting as hole-trapping sites, similar to those reported for PVK. From the results of current density-electric field characteristic of single carrier devices, the introduction of Cz group improves the hole injection and the electron transport in CzPFs. The electron flux and the hole flux are not more balanced (under high electric field), but, the external quantum efficiency of devices however increases with the content of Cz group. Thus, Cz/Cz dimer should act as an efficiently combination site in CzPFs.
The device performance is improved by grafting the Cz moiety onto the side chain of polyfluorene, but, it accompanies a broad band emission (525 nm) in electroluminescence spectra. By time-resolved electroluminescence spectroscopy analysis, the green emission originates from the electroplex formed between the stacked Cz/Cz dimer and the main chain under the applied electric field.
The introduction of charge transport moiety Cz groups and iridium complexes in the side chain of polyfluorenes provides an efficient way to control the charge transport and light emission for high performance and color tuning. By this way, a low turn-on voltage (4.2 V), high brightness (5493 cd/m2) and high efficiency (5.21 cd/A, external quen efficiency of 4.03 %) for red light emission device is obtained.
To understand the effect of Cz group in CzPFRs, the standard Stern-Volmer analysis is carried out. The quenching rate constants for Cz100PF and PFO (2.73 × 105 and 2.15 × 107 M-1 �酨-1, respectively) differ by two orders of magnitude, consistent with the higher efficiency of Cz100PF-based devices. This observation clearly indicates that Cz groups in Cz100PF serve to suppress energy feedback from Ir-R to the triplet state of the PF backbone. As the bulky Cz groups are higher in triplet state energy and are covalently attached via flexible spacers, they reside close to the backbone and effectively shield it from direct contact with Ir-R. This results in effective suppression of the Dexter-type energy transfer for triplet excitons and the effect extends to the undiluted state, as indeed verified via MD simulation.
Doping fluorescent (F8BT and MEH-PPV) and phosphorescent (iridium complex) dopants into Cz-modified polyfluorene (Cz75PF or Cz100PF), the best white and red light emitting diode could be obtained. The maximum luminous efficiency and maximum brightness of the device with Cz100PF:1 wt% Ir(btp)2(acac) are 5.14 cd/A and 3408 cd/m2 (with 1.30 cd A-1), respectively. As Cz75PF doping with 0.12 wt% F8BT, the best white light emitting diode could be obtained, with turn-on voltage of 3.0 V, external quantum efficiency of 4.1 %, maximum luminous efficiency of 11.0 cd/A (4.6 V, 210 cd/m2), CIE (0.29, 0.40), and maximum brightness of 46000 cd/m2 (10 V). As Cz75PF doping with 0.25 wt% MEH-PP, the best white light emitting diode could be obtained, with turn-on voltage of 3.1V, external quantum efficiency of 1.63 %, maximum luminous efficiency of 4.1 cd/A (5.2 V, 1921 cd/m2), CIE (0.34, 0.37), and maximum brightness of 32471 cd/m2 (8 V). Form these studies, CzPFs are potential host for fluorescence or phosphorescence dopants.
Via time-resolved electroluminescence spectroscopy shows that the spacer-grafted charge transport moiety may actually play multiple roles: (1) as a hole-injection promoter for reducing turn-on voltage, (2) as a hole-trap (in the form of stacked dimer) to facilitate efficient use of holes (via exciton formation with nearby electrons to emit blue light), (3) as an electron transport moiety, (4) as a sharp-green emitter (upon interaction of the stacked dimer with main chain segments via reorientation under electric field), and (5) as a shield for the Ir-red dopant by blocking away main chain segments and thereby suppressing triplet energy back-transfer from Ir-red to main chain.
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