Flexible Top Emitting Phosphorescent White Organic Light-emitting Diodes

White organic light-emitting diodes （WOLEDs） have been widely applicated in full colordisplays, backlights of laptop computers and portable panel light sources in recent years. Especiallytop emission WOLEDs （TWOLEDs） got great attention because of its aperture ratio, high contrast,no requirement with transparent substrate, and so on.In this paper we have manufactured the phosphor TWOLEDs based on glass substrates, whichwas based on others’ work in our group but the device structure has been improved in order toachieve better electroluminescent （EL） performance. Our work is based on a TWOLED structure ofAg （100nm）/MEO-TPD:1.8%F₄-TCNQ （30nm）/MEO-TPD （10nm）/Ir（ppz）₃（10nm）/CBP:0.6%Ir（MDQ）2acac （5nm）/SPPO1:10%FIrpic （20nm）/Bphen （30nm）/Sm （5nm）/Ag （16nm）.Based on the analysis of the energy band structure, we found that the main recombination region ofthis device was close to the interface between the red emitting layer （EML） and the10nm Ir（ppz）3layer. So the blue EML didn’t emit until the red light was fully saturated. If one would like to get anideal WOLED, they should lower the concentration of the red dye of Ir（MDQ）2acac to0.8%andincrease the concentration of the blue dye of FIrpic to10%. According to the experimental results,we couldn’t realize the white light if the concentration of Ir（MDQ）2acac was higher than0.8%. Themaximum current efficiency of this device was2.6cd/A with the red dye concentration of0.6%,and the International Commission on Illumination CIEx,y change was （0.01786，0.00316） within aluminance change of1237-1763cd/m². According to the above analysis, we used an improvedTWOLED structure of Ag （100nm）/MEO-TPD:1.8%F4-TCNQ （30nm）/MEO-TPD （10nm）/Ir（ppz）₃（10nm）/CBP:4%Ir（MDQ）2acac （5nm）/Ir（ppz）₃（2nm）/SPPO1:10%FIrpic （20nm）/Bphen （30nm）/Sm （5nm）/Ag （16nm）, in which a2nm of Ir（ppz）3layer was inserted into theinterface between red EML and blue EML, and the concentration of the red dye can be increased toas high as4%to get a better EL performance. The current efficiency was finally increased to5cd/A,and the color were more stable with an ignorable CIEx,y change of （0.00637，0.00171） within aluminance change of2685-1956cd/m². From the energy band structure, we could draw aconclusion that the recombination area occurred in MEO-TPD/Ir（ppz）3interface andIr（ppz）3/SPPO1interface compared to one recombination area of MEO-TPD/Ir（ppz）3without theIr（ppz）3layer. We also proved the emission mechanism of red light through an experiment of partdoping method in the EML by annalysing their EL spectranalysis in different devices with part doping. That is, the Ir（MDQ）₂acac emission does not come from the energy transfer from the bluelight of FIrpic but direct carrier injection into the red phosphorescent dye doped CBP layer, whichhas been further proved with a transient photoluminescence experiment. In addition, with theinserted Ir（ppz）₃thickness between red and blue EMLs was up to10nm, the red emissioncomponent disappeared, indicating the good block characteristics of Ir（ppz）3to both electron andexciton. From above analysis, we also deducted the main emission of the red dye is originated fromelectron tunneling from blue EML into the red one.In the last part of this paper, we transferred the improved TWOLED onto the treated flexiblePET （polyethylene terephthalate） substrate, which was treated with a hot-pressing method toimprove its surface roughness. From the measurement of Luminance-Voltage, Currentdensity-Voltage, Current efficiency-Voltage curves we found that the roughness of the PET surfaceand the performance of the TWOLED was improved after hot-pressing treatment Unfortunately, theflexibility of the flexible device became even worse, indicating some proper method should bechosen to protect both good EL performances and excellent mechanical performance.