Efficiency droop in InGaN/GaN-based LEDs with a gradually varying In composition in each InGaN well layer

Temperature-dependent and driving current-dependent electroluminescence spectra of two different InGaN/GaN multiple quantum well structures SA and SB are investigated,with the In composition in each well layer(WL)along the growth direction progressively increasing for SA and progressively decreasing for SB.The results show that SB exhibits an improved efficiency droop compared with SA.This phenomenon can be explained as follows:owing to the difference in growth pattern of the WL between these two samples,the terminal region of the WL in SB contains fewer In atoms than in SA,and therefore the former undergoes less In volatilization than the latter during the waiting period required for warming-up due to the difference in the growth temperature between well and barrier layers.This results in SB having a deeper triangular-shaped potential well in its WL than SA,which strongly confines the carriers to the initial region of the WL to prevent them from leaking to the p-GaN side,thus improving the efficiency droop.Moreover,the improvement in the efficiency droop for SB is also partly attributed to its stronger Coulomb screening effect and carrier localization effect.

Metal-free phthalocyanine single crystal： Solvothermal synthesis and near-infrared electroluminescence

A metal-flee purple H2Pc single crystal was synthesized by a facile solvothermal method, and its solubility and near-infrared （NIR） optical properties were also investigated due to its potential applications as a light-emitting layer for OLEDs. The H2Pc single crystal is insoluble in 1-chlorine naphthalene and other organic solvents. It gives a wide absorption in the range from 620 nm to 679 nm and a wide emission in near 922 nm. As an active light-emitting layer, H2Pc was employed to fabricate electroluminescent （EL） devices with a structure of ITO/NPB （30 nm）/Alq3：H2Pc （30 nm）/BCP （20 nm）/ Alq3 （20 nm）/Al. The emission center is at 936 nm when the H2Pc doping concentration is 20 wt%. The doping concentration strongly governs the emission intensity. When doping concentration decreases from 10 wt% to 1 wt%, the emission intensity remarkably fades, and simultaneously the emission center undergoes a blue shift.

Synthesis and red electroluminescence of a dimesityl-functionalized bistetracene

A novel dimesityl-decorated bistetracene derivative 8,16-dimesityltetraceno[2,1,12,11-opqra]tetracene（DMTA） has been synthesized and characterized. Its single crystal analysis demonstrates that the parent bistetracene backbone is almost in a plane without any intermolecular 7 r-stacking interaction. DMTA exhibited the low-energy absorption at 560/607 nm and emission spectra at 617/663 nm, respectively.The fabricated device based on DMTA doping into 2,6-bis（3-（9 H-carbazol-9-yl）phenyl）pyridine（1 %） as an emitter present a maximum brightness of 632 cd/m^2 at 14.7 V with the CIE coordinate of（0.623,0.349）.