Phosphorescent organic light-emitting diodes (PhOLEDs) unfurl a bright future for the next generation of flat-panel displays and lighting sources due to their merit of high quantum efficiency compared with fluorescent OLEDs. This critical review focuses on small-molecular organic host materials as triplet guest emitters in PhOLEDs. At first, some typical hole and electron transport materials used in OLEDs are briefly introduced. Then the hole transport-type, electron transport-type, bipolar transport host materials and the pure-hydrocarbon compounds are comprehensively presented. The molecular design concept, molecular structures and physical properties such as triplet energy, HOMO/LUMO energy levels, thermal and morphological stabilities, and the applications of host materials in PhOLEDs are reviewed (152 references).

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Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead-based perovskite solar cells. Here, the first double perovskite (Cs AgBiBr ) solar cells using the planar structure are demonstrated. The prepared Cs AgBiBr films are composed of high-crystal-quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high-quality double perovskite films show long electron-hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.

Phosphorescent organic light-emitting diodes (PHOLEDs) continue to attract intense interest because they can, in theory, approach a 100 % internal quantum efficiency by utilizing both singlet and triplet excitons.[1] To achieve highly efficient electrophosphorescence by reducing competitive factors such as concentration quenching and triplet-triplet annihilation, phosphorescent emitters of heavy-metal complexes are usually doped into a suitable host material.[2] Thus the synthesis of host materials and dopants are equally important for the formation of efficient PHOLEDs. It is desirable that the host materials have a large enough bandgap for effective energy transfer to the guest, good carrier transport properties for a balanced recombination of carriers in the emitting layer, and energy-level matching with neighboring layers for effective charge injection.Recently, bipolar hosts have aroused considerable interests in the area of organic light-emitting diodes (OLEDs) because they can provide more balance in electron and hole fluxes and simplify device structure.[3] However, a compromise is required between the bipolar transporting property and band gap of the material, because the electron-donating and electron-withdrawing moieties in bipolar molecules unavoidably lower the band gap of the material by intramolecular charge transfer, while the low triplet energy of the host can cause reverse energy transfer from the guest back to the host, which consequently decreases the efficiency of PHOLEDs. To address this issue, most recent molecular designs focus on the interruption of the p conjugation between electron-donating and electron-withdrawing moieties by the incorporation of steric groups [4] and/or meta linkages [2,5] between the two moieties. Efficient blue (46 lm W À1 , 24 %), [2] green (27.3 cd A À1 ) [4b] and orange (22 cd A À1 , 7.8 %) [4a] electrophosphorescence from such small bipolar host molecules has been reported.Carbazole derivatives can be used as host materials because of their high triplet energy and good hole-transporting ability.[ [7] and 5.82 cd A À1 for deep red (a dendritic iridium complex).[8] Unfortunately, the CBP host is prone to crystallization, especially when the dopant concentration is too low. [8,9] Furthermore, red PHOLEDs containing a CBP host usually need high driving voltages because the poor energy match between CBP and adjacent hole-and electron-transporting layers can result in insufficient and/or unbalanced injection of holes and electrons.[10] It is a worthwhile target to develop host materials with good thermal stability and matching energy levels to replace CBP. Oxadiazole derivatives have been proven to be very effective in improving the injection and transport of electrons. For example, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) [11] and 1,3-bis[4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD7) [11b] are usually incorporated in OLEDs as electron-transport materials.Herein we report a novel carbazole/oxadiazole hybrid molecule o-CzOXD linked t...