Efficient, Color-stable Flexible Top-emitting White Organic Light-emitting Diodes

Flexible top-emitting white organic light-emitting diodes (FTEWOLEDs) have great prospect infull color displays, solid state lighting and other areas, due to the portability, efficiency,environmentally friendly, large aperture ratio and unique flexibility. In this paper, our main topic isFTEWOLED. Aiming at gradually improving the device performances, color quality, the devicestructure, carrier transporting, and the related working mechanism are analyzed by combining theresults from optical simulation, device fabrication, and characteristic test.Firstly, we study the TEWOLEDs with CBP and SPPO1hosts, and optimize the devicestructures by combining optical simulation with experiment data. The TEWOLEDs based on theelectrically balanced P-I-N structure, and the emitting layers with stable blue/red/blue sandwichedstructure are made onto the flexible substrates of PET. The the color quality and the mechanicalstability of the FTEWOLEDs are improved via the utilization of ZnS as the coupling layer andMoO3as the buffer layer.In order to increase the brightness and luminous efficiency of FTEWOLEDs, a wide band-gapmaterial TCTA is employed as the host of a single emitting layer. The brightness and the currentefficiency of devices exceed10,000cd/m2and10cd/A, respectively. Further, the mainrecombination zone and the trapping mechanism of red molecules are analyzed by changingpositions and graded doping of the red EML.The injection depth of electrons in the TCTA host andthe role of the1st blue EML in the spectra stability are also revealed by comparing the first EML ofdevices with or without the blue dopants.The emitting structure of FTEWOLEDs is further developed by combining the hole-transportinghost TCTA and the electron-transporting host SPPO1. The blue components in white spectra areeffectively enhanced through reducing the thickness and doping concentration of the red or orangeEML, and introducing the orange emitter PO-01and the electron blocking layer of10nm Ir(ppz)3.The recombination region moving towards the anode is systematically investigated via thedelta-doping method, the stable energy transfer from FIrpic to PO-01is proved via thetime-resolved transient photoluminescence lifetime measurement, and the increasedhole-transporting attributed to blue phosphor FIrpic is exhibited via the fabricated single-carrierdevices. It’s concluded that the FTEWOLED based on the P-I-N structure and TCTA/SPPO1dual-EML with blue/orange/blue sandwiched structure obtains warm white illuminations with the maximum brightness of23,340cd/cm2, the maximum current efficiency and power efficiency of24.49cd/A and15.39lm/W. The device’s chromaticity coordinates are around (0.465,0.45), withthe tiny shift of (-0.0087,0.0015) from the turn-on luminance to the maximum brightness. Weachieve FTEWOLEDs with highly luminous performances and extremely stable chromaticity,which are among the best performances reported so far in FTEWOLEDs.