The UV-Vis absorption and photoluminescence (PL) spectra of 2,3-bis(9,9-dihexyl-9H-fluoren-2-yl) quinoxaline (Py), 2,3-bis(9,9-dihexyl-9H-fluoren-2-yl)-6,7-difluoroquinoxaline (F2Py) and 2,3-bis(9,9- dihexyl-9H-fluoren-2-yl)-5,6,7,8-tetrafluoroquinoxaline (F4Py), which are fluorene molecular derivatives with conjugated structure, were investigated. For further investigation of the influences of fluorine auxochrome in fluorene molecular electroluminescent material on optoelectronic property, the elec- troluminescence (EL) characteristics of materials were studied by double-layer organic light-emitting diodes (OLEDs) using N,N'-Di-[(1-naphthalenyl)-N,N'-diphenyl]-(1,1'-biphenyl)-4,4'-diamine (NPB) as a hole transporting layer (HTL) with the conventional vacuum deposition method. The results showed that the absorption and PL spectra of materials in film state red shifted with fluorine substituents in- creased in molecule configuration. The performance of OLEDs is as follows: at a bias voltage of 5 V, Py emitted a blue-green light at 508 nm with the Commission Internationale d'Eclairage (CIE) coordinates of (0.23, 0.43) and full width at half maximum (FWHM) of 100 nm. The device had a turn-on voltage (de- fined as the drive voltage at the luminance of 1 cd/m2) of 4.8 V, a luminance of 129 cd/m2 with a current density of 59 mA/cm2 at 10 V, and a maximum luminous efficiency of 0.18 lm/W at 5.4 V. F2Py and F4Py emitted a green light peaking at 544 nm and a yellow light at 570 nm at 5 V, with the CIE coordinates of (0.38, 0.56) and (0.44, 0.49), and FWHM of 103 and 117 nm, respectively. The F2Py and F4Py devices had a turn-on voltage of 4 and 2 V, a luminance of 557 and 3300 cd/m2 with a current density of 100 and 880 mA/cm2 at 10 V, and a maximum luminous efficiency of 0.22 lm/W at 7.6 V and 0.53 lm/W at 2 V, respectively.
Organic light-emitting devices（OLEDs） with the structure of indium-tin-oxide（ITO）/N,N＇-bis-（1-naphthyl）-N,N＇-diphenyl-（1,1＇-biphenyl）-4,4＇-diamine（NPB）/2,9-dimenthyl-4,7-diphenyl-1,10-phenanthroline（BCP）/tris（8-hydroxyquinoline）aluminum（Alq3）/Mg：Ag or that of ITO/NPB/1,2,3,4,5,6-hexakis（9,9-diethyl-9H-fluoren-2-yl）benzene（HKEthFLYPh）/Alq3/Mg：Ag were studied.White light emission was achieved with the two devices when the thicknesses of BCP and HKEthFLYPh were 1.5 nm（device B） and 5 nm（device Ⅱ）,respectively.The obvious difference was that the EL spectrum of device Ⅱ was not sensitive to the thickness of HKEthFLYPh compared to that of BCP layer.Moreover,the maximum luminance of device Ⅱ was about 1000 cd/m^2 higher than that of device B at a forward bias of 15 V,and it exhibited a maximum power efficiency of 1.0 lm/W at 5.5 V,which is nearly twice that of device B.The performance of device Ⅱ using a novel star-shaped hexafluorenylbenzene organic material was improved compared with that of BCP.
MA TaoYU Jun-shengLOU Shuang-lingTANG Xiao-qingJIANG Ya-dongZHANG Qing
White organic light-emitting diodes （WOLEDs） with a structure of indium-tin-oxide （ITO）/N,N＇-bis- （1-naphthyl）-N,N＇-diphenyl- （1, 1＇-biphenyl）-4,4＇-diamine （NPB）/1,2,3,4,5,6-hexakis（9,9-diethyl-9H-fluoren-2- yl）benzene （HKEthFLYPh）/5,6,11,12-tetraphenylnaphtacene （rubrene）/tris（8-hydroxyquinoline） aluminum （Alq3）/Mg：Ag were fabricated by vacuum deposition method, in which a novel star-shaped hexafluorenyl- benzene HKEthFLYPh was used as an energy transfer layer, and an ultrathin layer of rubrene was inserted between HKEthFLYPh and Alq3 layers as a yellow light-emitting layer instead of using a time-consuming doping process. A fairly pure WOLED with Commissions Internationale De L＇Eclairage （CIE） coordinates of （0.32, 0.33） was obtained when the thickness of rubrene was 0.3 nm, and the spectrum was insensitive to the applied voltage. The device yielded a maximum luminance of 4816 cd/m2 at 18 V.
