Study Of Highly Efficient Phosphorescent Polymer Light-emitting Devices Based On Polyfluorene Host

Organic light-emitting diodes (OLEDs) include organic small molecule light-emittingdiode (OLED) and polymer light-emitting diode (PLED), and they have drawn great attentionby academic and industrial sections because of the attractive prospect of application in thelighting, display, backlighting and other fields. Although organic small molecule organiclight-emitting diodes have been commercially developed, due to the expensive equipmentinvestment, the product price is too high, so it sill needs to enhance its marketcompetitiveness. On the other hand, PLED based on polymer can be achieved bysolution-processed, such as spin-coating, inkjet printing and screen printing and otherlow-cost methods, which have drawn widespread attention. However, compared with OLED,PLED still need further improvement in luminous efficiency and other areas. In this article,we will mainly use the polyfluorene (PFO) as the host to improve the luminous efficiency ofPLED; besides, the effection of interface materials on the performance of optoelectronicdevices are also included.On the basis of previous work, we know that the polyfluorene and its dericatives havehigh fluorescence quantum efficiency, high electroluminescent efficiency anddeep-blue-emitting properties; therefore, it’s an ideal host material. But polyfluorene and itsdericatives have lower-lying triplet energy levels, so when the phosphorescent materials aredoped into the PFO host, if the triplet energy levels of the phosphorescent materials werehigher than PFO host, it would cause energy back transfer and could not get efficientlight-emitting. Therefore, the PFO itself is often used as the host for the red phosphorescentmaterials, but can not be used as the host for the green and blue phosphorescent materials.Recently, we find that if we insert a PVK layer which has a higher-lying triplet energy levelsbetween the PEDOT: PSS and the emitting layer as a buffer layer, we can get efficient greenphosphorescence emission by using the PFO as the host. Based on this idea, we get efficientblue light by doping the blue phosphorescent material FIrpic into the PFO host as theemitting layer and PVK as the buffer layer. Other researches about PLED are also included inthis article. The work is mainly divided into the following parts.In the first part, we get efficient blue light by doping the FIrpic into the PFO-POSS host, PVK as the buffer layer. Experiment shows that when the proportion of FIrpic is10%, we getefficient blue light with the luminous efficiency of16.8cd/A. In order to further improve thedevice efficiency, we dope electron transport material (OXD-7) into the PFO-POSS: FIrpichost. When the proportion of OXD-7is40%, we get efficient blue light with the luminousefficiency of26.4cd/A, power efficiency of17lm/W and a low turn on voltage (3.6V).Compared with the blue device using the PVK as the host, this device has higher efficiencyand lower turn-on voltage. Finally, we discuss the reasons why we get so high deviceefficiency by energy levels and exciton lifetime of this device.Based on the efficient blue light devices that we get in the first part, in this part, westudy the white light-emitting devices based on the PFO-POSS as the host by doping anorange phosphorescent material into the blue devices. By changing the doping proportion oforganic phosphorescent material, we get efficient white light with the luminous efficiency of40.9cd/A, power efficiency of31.4lm/W and a low turn on voltage (3.4V). Then, we studythe efficient white-light-emitting diodes based on bilayer structure. By doping the Ir(piq) intothe PVK host to emitting red light, FIrpic into the PFO-POSS host to emitting blue light, wefinally get efficient white light with a luminous efficiency of12.1cd/A with this bilayerstructure.As we know, efficient charge injection and transport are a basic requirement for highlyefficient light-emitting devices. Under the guidance of this idea, we study the character of anelectron injection material containing SO group in the light-emitting devices. By using thiselectron injection material PFN-SO as the electron injection layer and three different polymermaterials as the emitting layer, we find that the electron injection material PFN-SO canimprove the electron injection efficiently. Compared with the devices using CsF/Al as thecathode, the devices using PFN-SO/Al as the cathode can get similar results, far higher thanthe results using Al only as the cathode. We also find that this electron injection material haveefficient electron injection character even if we use the metal Au as the cathode. At last, weanalyze the reasons why this material can improve the electron injection efficiently.Recently, inverted devices have drawn a lot of attention because of its potentialadvantages. On the other hand, a variety of electron-injection conjugated polyelectrolyte,especially water/alcohol soluble polyfluorenes, have been reported to be able to realize efficient electron injection using ITO as the cathode and get efficient inverted devices. Here,we get efficient inverted light-emitting device with ITO/PFN-SO as the cathode andMoO3/Al as the anode. From the UPS analyze, we find that the PFN-SO layer can lower thework function of ITO, so it can inject electron efficiently.