Shiyang Shao

A Novel, Bipolar Polymeric Host for Highly Efficient Blue Electrophosphorescence: a Non‐Conjugated Poly(aryl ether) Containing Triphenylphosphine Oxide Units in the Electron‐Transporting Main Chain and Carbazole Units in Hole‐Transporting Side Chains

A novel, bipolar polymeric host based on a poly(aryl ether) containing phosphine oxide units in the electron-transporting main chain and carbazole units in the hole-transporting side chains is designed and synthesized for blue electrophosphorescence. This polymeric host possesses a bipolar character and a high E(T) of 2.96 eV. The efficiency of blue-emitting PhPLEDs based on this polymeric host doped with Flrpic reaches 23.3 cd A(-1) (EQE = 10.8%).

Starburst 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine-based deep-blue fluorescent emitters with tunable oligophenyl length for solution-processed undoped organic light-emitting diodes

On the basis of a well-known hole transporting material, namely 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine (TCTA), a series of star-shaped deep-blue fluorescent emitters (2P-TCTA, 3P-TCTA, 4P-TCTA and 5P-TCTA) have been successfully developed via a simple extension of the oligophenyl chain between two N atoms. When the number of phenyl rings increases, it is found that both the absorption and emission for these TCTA-based starbursts are red-shifted and finally become saturated for 5P-TCTA consisting of a pentaphenyl bridge. Interestingly, on going from 2P-TCTA to 5P-TCTA, the film photoluminescence quantum yield is gradually enhanced from 11.4% to 35.5%. The same trend is also observed for their corresponding solution-processed undoped OLEDs. As a consequence, 5P-TCTA shows the best device performance, revealing a maximum luminescence of 7300 cd m−2, and a peak luminous efficiency of 2.48 cd A−1 (2.15 lm W−1; 2.30%) together with CIE coordinates of (0.15, 0.09).

Efficient Phosphorescent Polymer Yellow-Light-Emitting Diodes Based on Solution-Processed Small Molecular Electron Transporting Layer

Based on a solution-processed small molecular electron transporting layer, efficient multilayer solution-processed polymer yellow-light-emitting diodes were successfully fabricated. The maximum luminance efficiency and power efficiency reached 41.7 cd/A and 12.5 lm/W, respectively, which are comparable to and even over those from the PLEDs based on the vacuum-deposited electron-transporting layer. The solution-processed small molecular electron transporting layer is based on a mixture of three electron-transporting materials TmPyPB, TAZ, and TPBI. By utilization of this mixed system, not only the thickness of the electron-transporting layer can be easily adjusted, but also device efficiency can be improved because of their excellent synthetic properties.

Facile synthesis of self-host functional iridium dendrimers up to the fourth generation with N-phenylcarbazole-based polyether dendrons for non-doped phosphorescent organic light-emitting diodes

A facile synthesis has been demonstrated for the first time to construct self-host functional Ir-cored dendrimers up to the fourth generation on the basis of a newly developed polyether dendron, where the N-phenylcarbazole (NPC) moiety is used as the basic building block instead of benzene to improve charge transport whilst keeping the ease of preparation. With the growing generation number, the dendrimer size can be well tuned in a wide range of 4–10 nm. The obtained fourth generation dendrimer 45NPC-G4 is the largest Ir complex ever reported so far, having a diameter up to 10 nm and a molecular weight as high as 15.9 kDa. Most interestingly, the performance of non-doped phosphorescent organic light-emitting diodes (PhOLEDs) is found to be greatly dependent on the molecular size. For example, 9NPC-G2 (R ≈ 30 A) reveals the best luminous efficiency as high as 50.5 cd A−1 (56.6 lm W−1, 14.8%), whereas the efficiency of 45NPC-G4 (R ≈ 50 A) sharply drops to 10.5 cd A−1 (5.6 lm W−1, 3.4%). The results suggest that an appropriate size of 6 ± 2 nm is desirable to balance the dilemma between luminescence quenching and charge transport, and thereby realize highly efficient non-doped PhOLEDs.

Highly efficient red electroluminescent polymers with dopant/host system and molecular dispersion feature: polyfluorene as the host and 2,1,3-benzothiadiazole derivatives as the red dopant

By selecting polyfluorene as the polymer host, choosing 2,1,3-benzothiadiazole derivative moieties as the red dopant units and covalently attaching 0.3 mol% of the dopant units to the side chain of the polymer host, we developed a novel series of red electroluminescent polymers of dopant/host system with molecular dispersion feature. Their EL spectra exhibited predominant red emission from the dopant units because of the energy transfer and charge trapping from the polymer backbone to the dopant units. The emission wavelength of the polymers could be tuned by modifying the chemical structures of the dopant units. Single-layer devices (device configuration: ITO/PEDOT : PSS/polymer/Ca/Al) of these polymers emitted red light with a peak at 615 nm, a luminous efficiency of 5.04 cd A−1 and an external quantum efficiency of 3.47%, or emitted deep-red light with a peak at 650 nm, a luminous efficiency of 1.70 cd A−1 and an external quantum efficiency of 2.75%. Their high EL efficiencies were due to the energy transfer and charge trapping from the host to the dopant units as well as the molecular dispersion of the dopant units in the host. Increase of the dopant unit content led to increased turn-on voltages and decreased EL efficiencies of the resulting devices.

Spiro‐Linked Hyperbranched Architecture in Electrophosphorescent Conjugated Polymers for Tailoring Triplet Energy Back Transfer

A spiro-linked hyperbranched architecture has been incorporated into electrophosphorescent conjugated polymers for the first time, aiming at simultaneously tailoring the intra- and intermolecular triplet energy back transfer from the phosphorescent guest to the conjugated polymer host. Based on a prototype with this unique structure, slower decay of triplet excitons, and 5-8 fold enhancement of device efficiencies are obtained compared with the conventional blending counterpart.

Tunable charge transfer effect in poly(spirobifluorene)s with different electron-rich side chains

A series of blue-emitting poly(spirobifluorene)s (PSFs), namely, Cz-PSF, 4RO-PSF and DPA-PSF, have been designed and synthesized by incorporating 2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-fluorene (CzF), 2,3,6,7-tetraoctyloxyfluorene (4ROF) and N,N′,N′′,N′′′-tetrakis(4-butylphenyl)-9H-fluorene-2,7-diamine (DPAF) as their electron-rich side chains, respectively. Consistent with their measured HOMO levels (CzF: −5.40 eV; 4ROF: −5.17 eV; DPAF: −4.86 eV), the donor strength of the pendant is enhanced in the sequence CzF < 4ROF < DPAF. As a result, the charge transfer (CT) effect from the pendant to the backbone is found to be gradually intensified on going from Cz-PSF to 4RO-PSF and DPA-PSF. The emission maximum is accordingly red-shifted from 423 nm for Cz-PSF to 452 nm for 4RO-PSF and 482 nm for DPA-PSF, which is associated with a significant decrease in the photoluminescent quantum yields. Compared with 4RO-PSF, the copolymer Cz-PSF containing a weaker donor shows deep-blue emission with CIE coordinates of (0.16, 0.09–0.12) and a peak luminous efficiency of 2.21 cd A−1. These results indicate that the CT effect can be effectively tuned to realize high-performance deep-blue-emitting PSFs by the manipulation of the electron-donating ability of their spiro-conjugated side chains.

Self-host yellow iridium dendrimers based on carbazole dendrons: synthesis, characterization and application in solution-processed organic light-emitting diodes

On the basis of different generation carbazole dendrons, a series of self-host yellow Ir dendrimers (Y-G0, Y-G1 and Y-G2) have been successfully synthesized and characterized in detail. It is found that the peripheral dendrons can effectively reduce the intermolecular interactions between emissive Ir cores, as verified by the increased photoluminescence quantum yields and film lifetimes. Among these dendrimers, Y-G2 bearing the second generation dendrons shows the best non-doped device performance, revealing a peak luminous efficiency of 20.2 cd/A. The value is nearly twice that of Y-G0 without any dendrons, which could be further improved to 32.1 cd/A by dispersing Y-G2 into a host matrix. We believe that this work will shed light on the development of highly efficient yellow phosphorescent dendrimers with a self-host strategy.

Detailed studies on energy loss mechanism in phosphor-sensitized fluorescent polymer light-emitting devices

We studied the main energy loss mechanism in electroluminescent (EL) processes in phosphor-sensitized fluorescent polymer light-emitting devices. The used organometallic phosphor is fac-tris(2-phenyl-pridine) iridium [Ir(ppy)3] and the used red fluorescent dye is 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetrame-thyljulolidyl-9-enyl)-4H-pyran (DCJTB). The investigation found that due to the stronger electron trapping ability of DCJTB than that of Ir(ppy)3, the excitons prefer to form on DCJTB molecules. The charge trapping on the DCJTB molecules obviously restrain the function of the phosphor-sensitizer Ir(ppy)3. Moreover, the energy transfer from phosphorescent triplet state (Tp) to the fluorescent triplet state (Tf) also has great negative impact on the phosphor-sensitized fluorescent process. We clearly demonstrated these energy loss processes by steady-state and transient photoluminescence and comparison of device efficiency.