Copolyfluorenes Containing Bipolar Groups: Synthesis and Application To Enhance Electroluminescence of MEH−PPV

Abstract

Enhancing electroluminescence of conventional MEH−PPV {poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]} is desirable due to its popularity in polymeric emitting materials. The enhancement is much readily attained by simple blending with functional polymers instead of tedious structural modification. In response to this, three copolyfluorenes (P1−P3) containing bipolar groups (3.1−11.2 mol %), directly linked hole-transporting triphenylamine and electron-transporting 1,2,4-triazole, are synthesized by Suzuki coupling reaction. The bipolar groups not only suppress undesirable green emission of polyfluorene under thermal annealing but also increase hole and electron affinity of the resulting copolyfluorenes. Blending the bipolar copolyfluorenes with MEH−PPV effectively improves the emission efficiency of its electroluminescent devices [ITO/PEDOT:PSS/polymer blend/Ca(50 nm)/Al(100 nm)]. The maximum luminance and maximum luminance efficiency are significantly enhanced from 3120 cd/m2 and 0.49 cd/A (MEH−PPV device) to 19 560 cd/m2 and 1.08 cd/A (blend device with ca. 0.5 wt % of bipolar groups), respectively. Our results demonstrate the efficacy of the bipolar copolyfluorenes in enhancing electroluminescence of MEH−PPV.

Optical and electrochemical properties of polyfluorenes with pyridine–triphenylamine bipolar unit

“…The last few years have seen several examples in the literature of PFs containing such bipolar units. Three research groups have reported PFs bearing bipolar units that consist of electron‐deficient 1,2,4‐triazole and electron‐rich triphenylamine . In addition, PFs with bipolar units that consist of 1,2,4‐triazole acceptor and carbazole donor have also been reported .…”

Solution‐processable and thermally cross‐linkable fluorene‐cored triple‐triphenylamines with terminal vinyl groups to enhance electroluminescence of MEH‐PPV: Synthesis, curing, and optoelectronic properties

“…Moreover, the HOMO energy levels of PEDOT:PSS, cured‐ FTV , and MEH‐PPV are −5.0 eV, −5.19 eV, and −5.02 eV, respectively, whereas the LUMO energy levels of cured‐ FTV and MEH‐PPV are −2.64 eV and −2.70 eV, respectively. The lower HOMO level of cured‐ FTV (−5.19 eV) than MEH‐PPV (−5.02 eV) result in blocking of holes, whereas the higher LUMO level (−2.64 eV) than MEH‐PPV (−2.70 eV) induces electron‐blocking effect 24. Therefore, the cured‐ FTV can be inserted between PEDOT:PSS and MEH‐PPV to simultaneously reduce hole injection from PEDOT:PSS to MEH‐PPV and to block electron transport from MEH‐PPV to anode.…”

“…TPA and its derivatives have attracted considerable interest as hole transport materials for use in organic EL devices 23. However, the highest occupied molecular orbital (HOMO) level of TPA (−5.30 eV) is lower than that of MEH‐PPV (−5.02 eV) to form the hole‐blocking effect, whereas the lowest unoccupied molecular orbital (LUMO) level (−2.12 eV) is higher than that of MEH‐PPV (−2.70 eV) to induce the electron‐blocking effect 24. TPA derivatives are inserted between PEDOT:PSS and MEH‐PPV, which lead to more balanced charges recombination in the MEH‐PPV emitting layer because of holes are usually more readily transported than electrons in MEH‐PPV.…”

UV induced formation of transparent Au–Ag nanowire mesh film for repairable OLED devices

“…It is important to note that the work function of ITO, 4.7 eV, 28 is quite suitable for the organic layer of MEH-PPV used in the device, which has a HOMO of 5.02 eV. 29 The MNWM work function was measured using a Scanning Kelvin Probe 60 Microscope (SKPM) and found to be about 4.5 eV, which is less than the reported work function of gold, 5.1 eV (see Supporting Information), but is consistent with recent measurements of gold nanowire networks carried out by Lyons et al 30 The difference can be attributed to the adsorbed molecules of surfactant which PPV's LUMO of 2.7 eV, however the incorporation of other top electrodes such as those based on calcium or LiF adds complexity to the device and was thus left for future work on practical devices. The I-V curves for devices with both types of transparent 5 electrodes showed typical OLED behavior and similar luminance characteristics, proving that the MNWM electrode is a suitable alternative to transparent conducting oxides (see Figure 5).…”