Ruifeng He

Highly efficient green-emitting electrophosphorescent hyperbranched polymers using a bipolar carbazole-3,6-diyl-co-2,8-octyldibenzothiophene-S,S-dioxide-3,7-diyl unit as the branch

Green-emitting hyperbranched polymers with a bipolar 3,6-carbazole-co-2,8-dioctyl-dibenzothiophene-S,S-dioxide-3,7-diyl [Cz-co-DOSO] as the branch and fac-tris(2-phenylpyridine)iridium[Ir(ppy)3] as the core were synthesized by Suzuki polycondensation. The Cz-co-DOSO branch with a triplet energy level above 2.5 eV can effectively prevent energy feedback from the phosphorescent core. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of the copolymers reduce dramatically by over 0.6 and 0.8 eV with increasing DOSO content up to 45.5 mol% in the polymers, respectively. A maximum luminous efficiency of 50.5 cd A−1 and a maximum external quantum efficiency of 15.3% were obtained for PCzDOSO40Ir3 based on a single layer device of ITO/PEDOT:PSS/polymer/CsF/Al, which were improved by over 13 times compared with PCzIr3. The hole- and electron-only devices show that the hole and electron flux are well balanced, which demonstrates that PCzDOSO40Ir3 is a bipolar polymer with a balanced charge carrier transport.

Bipolar blue-emitting poly(N-9′-heptadecanyl-carbazole-2,7-diyl-co-dibenzothiophene-S,S-dioxide-3,7-diyl)s

Poly(N-9′-heptadecanyl-carbazole-2,7-diyl-co-dibenzothiophene-S,S-dioxide-3,7-diyl)s (PCz-SOs) were synthesized by Suzuki polycondensation. The glass transition temperatures (Tg) of the polymers increase gradually with the increase of SO unit content, and reach 119 °C for PCz-SO20. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels reduce gradually with increasing the content of SO unit in the polymers. Devices based on the polymers exhibit an excellent electroluminescent (EL) spectral stability with the variation of applied current densities in a large range of 12–360 mA cm−2. A peak luminous efficiency (LEmax) of 4.8 cd A−1 (corresponding to a maximum external quantum efficiency (EQEmax) of 3.6%) with 14 933 cd m−2 was obtained, and remained at 4.4 cd A−1 even at an applied current density of 100 mA cm−2 based on PCz-SO20 with a single-layer device of ITO/PEDOT : PSS/polymer/CsF/Al. The efficiencies were 12 times that of PCz, resulting from the balanced charge transportation in bipolar polymer PCz-SO20.

Improving the efficiency and spectral stability of white-emitting polycarbazoles by introducing a dibenzothiophene-S,S-dioxide unit into the backbone

Highly efficient and spectral stable white-emitting copolymers based on 2,7-carbazole (Cz), dibenzothiophene-S,S-dioxide (SO) and benzothiadiazole (BT) units (PCz–SO–BT)s were designed and synthesized via Suzuki polycondensation. High fluorescence quantum yield (∼70%) and balanced charge transport were achieved by introducing a SO unit into the PCz–BT backbone. A peak luminous efficiency (LEmax) of 8.2 cd A−1, a maximum power efficiency (PEmax) of 8.0 lm W−1 (corresponding to a maximum external quantum efficiency of 3.9%) and a maximum luminance (Lmax) of 13526 cd m−2 with the CIE coordinate of (0.32, 0.41) were obtained from a single-layer device of ITO/PEDOT:PSS/PCz–SO–BT7/CsF/Al. The electroluminescence (EL) efficiencies were improved about 4 times compared to the case of SO absent copolymer PCz–BT4. Moreover, the EL spectra of PCz–SO–BTs exhibited dramatic stability with the variation of driving current densities from 12 to 300 mA cm−2, which could be attributed to the electron detrapping by decreasing the difference of the lowest unoccupied molecular orbital (LUMO) levels (ΔELUMO) between the chromophores. The results showed that the SO unit could be an excellent building block for white-emitting copolymers toward high efficiency and spectral stability.

Color tuning in inverted blue light-emitting diodes based on a polyfluorene derivative by adjusting the thickness of the light-emitting layer

Inverted blue polymer light-emitting diodes (IPLEDs) were fabricated by using polyethylenimine ethoxylate (PEIE) as the interlayer and a polyfluorene derivative (PF-FSO10) as the emissive layer, with the device structure of ITO/ZnO/PEIE/PF-FSO10/MoO3/Al. Upon insetting the PEIE interlayer, the maximum luminous efficiency (LEmax) of the IPLEDs increases by about three orders of magnitude, compared with that without. The enhancement of device performance could be attributed to the effective electron-injecting/hole-blocking ability of PEIE, and less exciton quenching at the interface of ITO/ZnO/PEIE. As the thickness of PF-FSO10 is tuned, the color coordinates of the devices can change from sky blue (0.18, 0.26) to deep blue (0.16, 0.07), and the LEmax of the devices can also be improved from 1.77 to 5.7 cd A−1, which should contribute to the micro-cavity effects and the expansible recombination zone, respectively. These results provide an excellent method to improve the efficiency and color purity of the inverted blue-light emitting diodes.

Synthesis and properties of five ring fused aromatic compounds based on S,S-dioxide benzothiophene

Asymmetrical five ring fused compounds based on S,S-dioxide benzothiophene, 9,9-dioctylfluorene[2,3-b]benzo[d]thiophene-S,S-dioxide(BTOF) and N-(2-decyltetradecyl)carbazole[2,3-b]benzo[d]thiophene-S,S-dioxide (BTOCz) were synthesized and characterized. BTOF and BTOCz were readily soluble in common organic solvents and exhibited high thermal stability with the melting points (Tms) of 119 °C and 150 °C, and thermal decomposition temperatures (Tds) of 280 °C and 338 °C, respectively. The highest occupied molecular orbital energy levels (HOMOs), the lowest unoccupied molecular orbital energy levels (LUMOs) and the band gaps (Egs) of BTOF and BTOCz were −6.11, −2.66 and 3.45 eV, −5.86, −2.52 and 3.34 eV, respectively. The photoluminescence (PL) spectra of BTOF and BTOCz were peaked at 404 and 420 nm in film, and the photoluminescence quantum yields (QPLs) were 30% and 10% in toluene solution, respectively. BTOF and BTOCz showed strong intramolecular charge transfer (ICT) character, relatively high QPLs and deeper HOMO and LUMO levels compared with compounds BTF and BTCz. Compared with the symmetrical seven ring fused compounds FBTO and CzBTO, the asymmetrical five ring fused compounds BTOF and BTOCz showed relatively shallow HOMO levels, which can facilitate hole injection, and therefore, could be potential construction units in organic semiconductors.

Synthesis and optical and electrochemical properties of water-soluble cationic fluorophores based on bispyridinium and dibenzothiophene-S,S-dioxide

Two novel water-soluble π-conjugated small molecules based on dibenzothiophene-S,S-dioxide and bispyridinium salts, named FSOPyCl and FSOmiCl, were successfully synthesized. Both molecules showed deep blue light emission, with photoluminescence peaks at about 408 and 405 nm in aqueous solution, respectively. Their photoluminescence quantum yields (PLQYs) in aqueous solution were as high as 68.9% and 50.3%, respectively. Electrochemical measurements suggested that they have excellent electron affinity, with lowest unoccupied molecular orbital (LUMO) energy levels at −3.78 and −3.94 eV, respectively. Due to their unique properties including large planarity, good thermal stability, excellent electron affinity and high fluorescence efficiency, FSOPyCl and FSOmiCl may be promising construction units for preparing water/alcohol-soluble conjugated materials with good optoelectronic performance.