Studies On Organic Light Emitting Devices Of OPV Derivatives

Since C.W.Tang has reported high brightness organic light-emitting devices (OLEDs) with low operating voltages in 1987,OLEDs have received more and more attentions. As a new flat panel display technology, OLEDs have many merits such as: light weight ,thin thickness, low coast, broad visual angle, fast response speed, active emitting, low energy consume, high brightness and efficiency, broad operating temperature, more choice of materials, availability for full color display and flexible display, etc. The research of new materials is important for the potential application in OLEDs. Poly-phenylenevinylene (PPV) is a good electroluminescent material. Recently, as the model compounds for poly-p-phenylenevinylene (PPV), distyrylbenzene (DSB) and its derivatives exhibit nice blue fluorescence and deserve considerable interest. DSB is a blue light-emitting material and it has high fluorescent quantum efficiency in solution. In solid ,however, the crystallization tendency of DSB is very obvious because there is very strongπ-πeffect among the conjugated molecules. In order to solve the problem, our group designed and synthesized the oligomer of PPV, trans-DPDSB, 2,5-diphenyl-1, 4-distyrylbenzene with two trans-double bonds (trans-DPDSB) that exhibits intriguing and promising properties for pure-blue-light-emitting device. However, the film of trans-DPDSB is not steady, which limits its practical application in OLEDs. To overcome this shortcoming, methyl substituents were introduced to trans-DPDSB, which might suppress the intermolecular interactions and provide high quality deposited films. The resulting compound, 2,5-diphenyl-1,4-di-p-methylstyrylbenzene with two trans double bonds (trans-DPDMSB) was designed and synthesized. Compared with trans-DPDSB, trans-DPDMSB can form stable amorphous films by the vacuum deposition method, indicating the potential application in OLEDs. So OLEDs based on trans-DPDMSB were fabricated. After optimizing the devices structures by using 4, 4′, 4′′-tri(N-carbazolyl)-triphenylamine (TCTA) and 1, 3, 5-tri(phenyl-2- benzimidazolyl)-benzene (TPBI), pure blue emission with Commission International De L'Eclairage (CIE) coordinates of (0.15,0.10) which are almost identical to the coordinates of National Television System Committee (NTSC) blue standards (0.14, 0.08), and maximum luminous efficiency of 4.2 cd/A (corresponding to an external quantum efficiency of 3.9%) are obtained.It was well-known that cyanogen substituent can reduce the energy level of HOMO and LUMO. The injection of electron can be easy. To achieve this aim, cyanogen substituents were introduced to trans-DPDSB. The resulting compound, trans-CNDPDSB was designed and synthesized. We found that the electron injection ability of this compound can be improved, while the electron transmission ability of this compound is not good. Using trans-CNDPDSB as emitting layer, N, N-diphenyl-N, N"-bis(1-naphthyl)-(1, 1-biphenyl)-4,4'-diamine (NPB) as hole- transporting layer, 2 , 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP) as hole-blocking layer, tris-(8-hydroxyquinoline) aluminum (Alq 3) as electron-transporting layer , we obtained colour stable organic white light-emitting devices(LEDs). The white light comes from the mixing of red exciplex emission at the solid-state interface between CNDPDSB and NPB and blue emissions from NPB and CNDPDSB, respectively. The red exciplex emission might be induced by the interface and energy level, and we proved it by designing the appropriate device structure.Recently, white organic light emitting device (WOLEDs) with high brightness properties is expected, not only for a display device, but also for the back light of a liquid crystal display. The WOLEDs show excellent properties of low driving voltage, high efficiency, compact and bright emission. A varity of methods have been proposed to achieve the WOLEDs. In this thesis, we report a white light emitting device based on blue-emitting material trans-DPDMSB and orange- emtting material DPACNDPDSB. The thicknesses of the emitting layers are varied to obtain the desired emission color. The structure of the WOLEDs is ITO/NPB/ trans-DPDMSB /DPACNDPDSB/TPBI/LIF/AL. The luminance was 5000 cd/m2 and maximum efficiency is 7.3 cd/A at 6.5V .