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Exciton (or spin) statistics is a physical principle based on the statistics of spin multiplicity. In electroluminescence, injected electrons and holes have randomized spin states, and usually form singlet or triplet excitons in the ratio of 1:3. Exciton statistics determines that the upper limit of internal quantum efficiency is 25% in fluorescent devices, since only singlet exciton can decay radiatively. However, both experimental and theoretical evidence indicate that the actual efficiency can exceed the exciton statistics limit of 25% by utilizing materials with special electronic structure and optimized device structures. These results bring light to break through the exciton statistics limit and develop new-generation fluorescent materials with low cost and high efficiency. Recently, the exciton statistics, which has attracted great attention in the past decade, is being rejuvenated due to the discovery of some fluorescent materials with abnormally high efficiencies. In view of their significance in theoretical research of organic semiconductors and developing new-generation OLED materials, such materials are widely investigated in both academic institutions and industry. Several key issues still require further clarification for this kind of materials, such as the molecular design concepts. Herein, we review the progress of the materials with efficiency exceeding the exciton statistics limit, and the routes to improve exciton utilization efficiency. In the end, we present an innovative pathway to fully harvest the excitons in fluorescent devices, namely, “hot exciton” model and relevant fluorescence material with hybridized local and charge-transfer (HLCT) excited state.

The basic characteristics of anthracene(AN) and dibenz(a,c)anthracene(D(a,c)A) derivatives, important semiconductors, were systematically studied. Differential scanning calorimetry(DSC) shows that D(a,c)A has the re- versibility of solid and liquid phases. Thermalgravimetric analysis(TGA) shows that D(a,c)A exhibits higher Td(decomposition temperature), which indicates D(a,c)A, as active layers, should be more suitable to being made into stable devices. D(a,c)A shows blue-shifted UV-Vis absorption and emission despite increased conjugation compared to anthracene due to different symmetries. Both the compounds are planar and show weak blue fluorescence in the tetrahydrofuran(THF) solution and higher fluorescence in the solid-state, due to different energy levels in solution and solid-state, explained via Jablonski diagram. Keywords Anthracene; Dibenz(a,c)anthracene; Molecular property; Thermal; Electrochemistry

A series of poly(9,9-dihexylfluorene)s (PDHFs) have been synthesized via microwave-assisted Suzuki and Yamamoto coupling reactions. Compared with the conventional oil-bath heating (48 h, M w 20100 g/mol by Suzuki reaction and M w 24000 g/mol by Yamamoto reaction), microwave-assisted polymerization can yield PDHFs with higher molecular weights (M w 37200 g/mol by Suzuki reaction and M w 43400 g/mol by Yamamoto reaction) in shorter time (14 and 60 min). However, sometimes formation of insoluble gels was observed together with PDHF in Suzuki coupling reaction. PDHF and insoluble gels were analyzed by XPS, elemental analysis, FTIR, TGA and DSC. Experimental results demonstrated that H2O might take part in the Suzuki coupling reaction under microwave condition, which could make the cross-linking reaction occur and form gels.

The syntheses of a series of oligo (phenylene vinylenes) (OPVs) were described. The extended π-chains were built up by sequences of Wittig reactions. Their electronic structures were systematically studied with absorption spectra, photo-luminescent spectra. The influences of the effective conjugation length on the performance of the light emitting devices were investigated.

To improve the carrier-injection of polyfluorene,oligomers of polyfluorene,fluorene-bipyridine,and fluorene-phenanthrolin copolymers were theoretically studied for their geometries,electronic structures,excitation energies,and reorganization energies by density functional theory at the B3LYP/6-31G* level.We evaluated the properties of the corresponding polymers by linear extrapolation.We found a large decrease of about 0.45/0.47 eV and 0.32/0.38 eV for the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital(LUMO) levels,respectively,after the introduction of electron-deficient bipyridine/phenanthrolin into polyfluorene.These molecules improved the electron injection ability and the balance of carrier-injection in polyfluorene systems.Additionally,the reorganization energy increased for both electron and hole transports upon the incorporation of bipyridine,which resulted in worse carrier mobility.However,the fluorene-phenanthrolin copolymer showed a similar charge transport property to polyfluorene.

Two series of bis(styryl)benzene derivatives (BSBD), namely the single-BSBD and the double-BSBD, were investigated. The equilibrium geometries and electronic structures were obtained by using the density functional theory B3LYP and 6-31G basis set. In succession, the one- and two-photon absorption properties of all the molecules were studied theoretically with a ZINDO-SOS (sum-over-states) method in detail. It can be seen that the double-BSBDs have larger two-photon absorption (TPA) cross sections in the visible-IR range than the corresponding single-BSBDs, demonstrating that increasing the molecular dimension is a very effective method to enhance the values of the TPA cross sections. On the other hand, it can be also noticed that the values of the TPA cross sections are correlative with the ability of donating (accepting) electrons of the terminal substituent groups R [N(CH3)2, CH3, H and CF3] in these molecules. That is, the intramolecular charge transfer is also a factor for the enhancement of the TPA efficiency. To sum up, the idea of increasing the molecular dimension to enhance the TPA cross section value is a helpful direction to explore better TPA materials for practical applications. And the double-BSBD molecules are promising TPA materials for the further investigation from the standpoint of the high transparency and the larger TPA cross sections.

Organic crystals constructed by pi-conjugated molecules have been paid great attention to in the field of organic optoelectronic materials. The superiorities of these organic crystal materials, such as high thermal stability, highly ordered structure, and high carrier mobility over the amorphous thin film materials, make them attractive candidates for optoelectronic devices. Single crystal with definite structure provides a model to investigate the basic interactions between the molecules (supramolecular interaction), and the relationship between molecular stacking modes and optoelectronic performance (luminescence and carrier mobility). Through modulating molecular arrangement in organic crystal, the luminescence efficiency of organic crystal has exceeded 80% and carrier mobility has been up to the level of 10 cm2·V−1·s−1. Amplified stimulated emission phenomena have been observed in many crystals. In this paper, we will emphatically introduce the progress in optoelectronic functional organic crystals and some correlative principle.

By placing a hole blocking layer between the polymer and the metallic electrode, we have achieved improved efficiencies for blue electroluminescent devices fabricated with poly (N-vinylcarbazole) doped with 1, 1, 4, 4, -tetraphenyl-1, 3-butadiene as the emitter layer and with aluminum and indium/tin-oxide as the electron and hole injecting electrodes. This bilayer device yields bright blue emission having intensity of 300 cd/m2, in same case the devices without hole blocking layer have luminance of only 44 cd/m2.

We have fabricated high-efficiency white organic light-emitting devices by using the phosphorescent material fac tris (2-phenylpyridine) iridium [Ir(ppy3)] as a sensitizer. Ir(ppy)3 and the fluorescent dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl) (DCJTB) are co-doped into 4,4'-N,N'-dicarbazole-biphenyl (CBP) host. N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,4'-diamine (NPB) acts as a blue light-emitting as well as hole-transporting layer. The chromaticity of white emission can be tuned by adjusting the concentration of Ir(ppy)3 and DCJTB. The maximum efficiency and luminance of the device with 3-wt.% Ir(ppy)3 and 2-wt.% DCJTB are 7.5 cd A-1 and 12020 cd m-2, respectively, which produces fairly pure white emission with the commission international De L'Eclairage coordinates of (0.33,0.32) at 10 V.

The bilayer heterojunction devices were fabricated successfully by using a novel oxadiazole derivative: 2, 2'-(2,5-thiophenediyl) bis (5-(4-methyl) phenyl-1,3,4-oxadiazole) (T-OXD) as the electron-transporting layer (ETL) and a pyrazoline derivative:1-phenyl-3-(dimethylamino)styryl-5-(p-(dimethylamino) phenyl)pyrazoline (PDP) as the light-emitting layer and the hole-transporting layer. The emission at 500 nm was derived from PDP layer. In comparison with the bilayer device of tris (8-hydroxyquinoline) aluminum (Alq) as the ETL, the luminous efficiency of the PDP/T-OXD heterojunction device was enhanced by 104 times.

The polymer electroluminescence (EL) device with PBD as carriers confinement layer yields bright blue emission having intensity of 300 cd/m2, in same case the device without PBD layer have luminance only 44 cd/m2. The effect of PBD layer on EL characteristic was studied. The results show that only in EL devices with PBD thickness over 30 nm, the holes are completely confined in emitting layer. The luminance over 2000 cd/m2 can be obtained by inserting an electron injecting layer between the negative electrode and PBD to increase the electron injection.