The degradation mechanism of high power InGaN/GaN blue light emitting diodes （LEDs） is investigated in this paper. The LED samples were stressed at room temperature under 350-mA injection current for about 400 h. The light output power of the LEDs decreased by 35% during the first 100 h and then remained almost unchanged, and the reverse current at-5 V increased from 10^-9 A to 10^-7 A during the aging process. The power law, whose meaning was re-illustrated by the improved rate equation, was used to analyze the light output power-injection current （L-I） curves. The analysis results indicate that nonradiative recombination, Auger recombination, and the third-order term of carriers overflow increase during the aging process, all of which may be important reasons for the degradation of LEDs. Besides, simulating L-I curves with the improved rate equation reveal that higher-than-third-order terms of carriers overflow may not be the main degradation mechanism, because they change slightly when the LED is stressed.
Iridium complexes with dicyanovinyl-grafted phenylpyridine/1-phenylisoquinoline as ligands are synthesized and their photophysical, electrochemical, and sensitization properties in DSSCs are investigated. The iridium complexes present significantly enhanced absorption from 400 to 525 nm. The 1-phenylisoquinoline-based iridium complex show bathochromic-shift emission in DMSO solution compared with their phenylpyridine-based counterpart, while their absorption response and photoluminescence peak in solid show little difference despite extension of the conjugated system. Using DSSCs, the conversion efficiency of 0.62% and open-circuit current of 1.4 m A/cm2 is achieved. The poor performance is attributed to the excited-state properties of iridium complexes according to the TD-DFT calculation.
Dongdong WangHua DongXiaoyu ZhangYong WuShaohua ShenBo JiaoZhaoxin WuMin GaoGeng Wang
In this article, we reported the synthesis and characterization of a novel silafluorene-based host material, 1,3-bis(5-methyl-5Hdibenzo[b,d]silol-5-yl)benzene(Me-DBSi B), for blue phosphorescent organic light-emitting devices(PHOLEDs). The MeDBSi B was constructed by linking 9-methyl-9-silafluorene units to the phenyl framework through the sp3-hybridized silica atom to maintain high singlet and triplet energy, as well as to enhance thermal and photo-stability. The calculated result shows that the phenyl core does not contribute to both the highest occupied molecular orbital and lowest unoccupied molecular orbital. Wide optical energy gap of 4.1 e V was achieved. When the Me-DBSi B was used as the host and iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2′]picolate(Firpic) as the guest, a maximum current efficiency was 14.8 cd/A, lower than the counterpart of 1,3-bis(9-carbazolyl)benzene(28 cd/A). The unbalanced barrier for electron and hole injection to host layer may be responsible for low efficiency. Even so, our results show that silafluorene moieties are promising building blocks for constructing wide-energy-gap host materials.
