Cheng Yanxiang

Enhancement of Stability of Polymer Light-Emitting Diodes by Post Annealing

We investigate the effect of thermal annealing before and after cathode deposition on the stability of polymer light-emitting diodes (PLEDs) based on green fluorescent polyfluorene derivative. The annealed PLEDs exhibit improved charge transport and red-shift emission compared to the as-fabricated device. The stability of the PLEDs is largely enhanced by post-annealing before and after Ca deposition, which is attributed to the enhanced charge transport and the intimate contact between the cathode and the emissive layer.

Red Electroluminescent Polyfluorenes Containing D-A Type Naphthothiadiazole and Benzoselenadiazole Derivatives in the Main Chain

Two novel D-A type red dopants containing naphthothiadiazole(NT) or benzoselenadiazole(BS) electron withdrawing unit were synthesized.Through covalently attaching red dopants to polyfluorene(PFO) backbones and tuning the doping concentration,we obtained a series of red polymers.All these red polymers exhibit similar absorption spectra as PFO.Their photoluminescent(PL) spectra show not only blue emission from the PFO backbone but also red emission from the dopant.The relative intensity of the red emission increases successively with doping concentration.Unlike the PL spectra,the electroluminescent(EL) spectra of all the polymers predominantly exhibit red emission,which is attributed to the red dopants,with a weak blue emission from PFO backbone.It realizes completely energy transfer from PFO host to red dopants at 1% molar fraction doping level.The single-layer devices with the configuration of ITO / PEDOT: PSS / polymer / Ca / Al(PEDOT: poly(3,4-ethylenedioxythiophene);PSS: poly(styrenesulfonate)) show saturated red emission at 632 nm with a luminous efficiency of 1.61 cd / A and CIE of(0.63,0.35) for PFR-10NT,and pure red emission at 620 nm with a luminous efficiency of 1.10 cd / A and CIE of(0.63,0.36) for PFR-10BS,respectively.

Phosphorescent Cuprous Complexes with 2-Pyridylmethyldiphenylphosphine:Synthesis,Photophysical Properties and Electroluminescence

Three novel cuprous complexes of [Cu(L)2 ](BF4)(a),[LCu(PPh3)2 ](BF4)(b) and [LCu(POP)](BF4)(c)(L=2-pyridylmethyldiphenylphosphine,POP=bis[2-(diphenylphosphino) phenyl] ether) have been prepared,and characterized by 1H NMR,31P NMR,elemental analysis,and X-ray single-crystal diffraction.X-ray diffraction studies have revealed that the central Cu(Ⅰ) ions of all complexes have a distorted tetrahedral coordination geometry.The complexes show intense π-π absorption bands in the range of 261～274 nm in deaerated CH2 Cl2 solution,while no characteristic metal-ligand charge transfer(MLCT) transition bands appear obviously.In rigid polymethylmethacrylate(PMMA) matrix,these CuⅠ complexes show intense blue emission at 515,476,and 481 nm,with a quantum efficiency of 16.0%,12.9% and 7.0%,respectively.Multilayer organic light-emitting diodes(OLEDs) using poly(N-vinylcarbazole)(PVK) and complex b were fabricated.A current efficiency of 0.36 cd/A and the maximum brightness of 217 cd/m2 have been achieved at current density of 1.0×10-3 A/cm2.