Two bipolar materials,2,5-bis(2-(9H-carbazole-9-yl)phenyl)-1,3,4-oxadiazole(o-CzOXD)and 2,5-bis(2-(3',6'-di-tert-butyl-9H-carbazole-9-yl)phenyl)-1,3,4-oxadiazole(tBu-o-CzOXD),were synthesized according to reported methods.In parallel study,it was demonstrated that introduction of inert tert-butyl group improved material thermal stability,even though this modification only had a slight influence to the photophysical and electrochemical properties of these materials.A comparative study focusing on effects of heat treatment was carried out on the quartz glass substrates with vacuum deposited films containing one of the bipolar host doped with 6 wt%fac-tris(2-phenylpyridinato-N,C2’)iridium(Ir(ppy)3).Results show that when the two samples were heated,the absorption,emission,and photo images of the host:dopant system changed,with the o-CzOXD suffering more severe degradation under high temperature,which is consistent with their thermal stability.In addition,it was proved that the high temperature-annealed host:dopant system can enhance the emission of the dopant.This finding was used as a guideline to improve our device performance.We fabricated two types of phosphorescent organic light-emitting devices(PhOLEDs),one was based on o-CzOXD,the other was based on tBu-o-CzOXD.They had analogous structure.We investigated the effect of heat on device performance by selectively annealing.Although these two freshly prepared devices exhibited similar performance,when annealed at 90°C for 10 min,the OLEDs based on tBu-o-CzOXD showed significant performance enhancement,which can be attributed to the observation that annealing Ir(ppy)3 doped host can change film morphology and enhance the dopant emission.The maximum efficiencies of the freshly prepared tBu-o-CzOXD device were 25.8 cd A-1,23.1lm W-1,and 9.3%;whereas those for annealed device were 47.0 cd A-1,42.2 lm W-1,and 13.4%.
WANG HongJiaoLIU ChenMI BaoXiuSANG JieLI XinGAO ZhiQiang
This work presents the study of optical constants and film thickness of blended organic thin films, emphasizing on the modeling procedure with modified genetic algorithm aided by absorption or transmittance spectra of both pure materials and the blends. Taking the blending of copper phthalocyanine(Cu Pc) and fullerene(C60) as an example, a simple, convenient and low-cost method for the determination of the optical constants and film thickness of blended organic thin films was demonstrated. New scheme for optical modeling of blended organic thin film was proposed by introducing peak energies of Cody-Lorentz oscillators of the pure materials, which were determined by fitting the film absorption of pure materials. These oscillators of pure materials could be recognized in the transmittance spectrum of their blends, and were further used as the initial searching ranges in the simulation of blended films. As a result, the constraint bounds of the unknown parameters were significantly reduced and modeling efficiency as well as fitting accuracy was improved. For instance, the fitting of the transmittance curves of blended films with different blending ratios reached reliable results in comparison with extinction coefficients obtained from experiment.
Stable organic field effect transistors(OFETs)based on copper phthalocyanine(CuPc)are reported using MoO_(3)/Al as source-drain top contacts.By annealing the fabricated device at 130℃ in air,the mobility and the stability of the OFETs can be significantly improved in comparison with the untreated device.The heat-treated devices without encapsulation show a device storage stability of nearly 400 h while the untreated one only 183 h.This improvement is suggested to be mainly attributed to the reduction of the contact barrier between CuPc and the electrode,as well as the better alignment of CuPc molecules via post annealing.
Two new iridium complexes with C^N=N type ligand （i.e., Ir（BFPPya）3{tris[3,6-bis（4-fluorophenyl）pyridazine]iridium（III）} and Ir（BDFPPya）3{tris[3,6-bis（2,4-di-fluorophenyl）pyridazine]iridium（III）}） attaching with fluorine atoms, were synthesized and the effects of fluorination on the material properties and device performance were investigated. Compared with our previously reported fluorine-free analogue material, that is Ir（BPPya）3{tris[3,6-bis（phenyl）pyridazine]iridium（III）}, blue shifts in the emission spectra as well as in the long wavelength region of the absorptions were observed. The photoluminescence quantum yield （PLQY） （0.44 and 0.84 vs. 0.29）, phosphoresces lifetime （0.88 and 1.31 vs. 0.66 gs）, and oxidation potential （1.10 and 1.37 vs. 0.95 V） increased obviously after fluorinating the ligand. In contrast, the thermal stability of the iridium complexes decreased slightly （Td： 435 and 402 vs. 440 ℃）. In the density functional theory （DFT） calculations, by comparing the steric shape of the three ligands within one optimized molecule, orientational differences among the complexes were observed. In OLED device studies, bluish green electroluminescence with peak emission of 500 nm, using the electron-transporting host of TPBI [2,2＇,2＂-（1,3,5-benzenetriyl）tris（1-phenyl-lH-benzimidazole）] and the most fluorinated dopant of Ir（BDFPPya）3, was achieved with maximum efficiency of 20.3 cd/A. On one hand this efficiency is not satisfactory considering a high PLQY of 0.84. On the other hand with the similar device structure, that the （HOMO-LUMO）s of all the dopants are wrapped within that of the host TPBI, and all the triplet energies of the dopants are smaller than that of the host TPBI, it is abnormal that the ordering of device efficiencies is contradictory to that of PLQY. Assisting with the phosphorescent spectrum of TPBI and the absorption spectra of the dopant, the contradiction was interpreted reasonably.
An inexpensive material, i.e., tetranuclear zinc（Ⅱ） complex, （Zn40（A/D）6） [AID = 7-azaindolate], was utilized as a cathode buffer in organic photovoltaic （OPV） devices, leading to the improvement of device performance. Compared to OPV devices based on a conventional cathode buffer of TPBi （1,3,5-tris（2-N-phenylbenzimidazolyl）benzene）, although the freshly prepared devices showed similar performance, when heated to a series of high temperatures under air, the short circuit current and the open circuit voltage of the Zn40（AID）6 devices dropped more slowly, indicating the superiority of using Zn40（AID）6 as a cathode buffer over TPBi in OPV devices.
LI ZhiGangGAO ZhiQiangWANG HaiShanZHANG HuiZHAO XinYanMI BaoXiuHUANG Wei
