The Application Of Crossfading Doping In Organic Light Emitting Diodes

Doping technology has been widely used in organic light-emittingdiodes (OLEDs). Doping emitters with a suitable concentration in matrixcan increase the quantum efficiency of electroluminescence significantly,while introducing p-tpye and n-type dopant into hole and electrontransporting layer, respectively, will boost the charge carrier injection andtransport, and improve the power efficiency of OLEDs. Normally, thedopant is uniformly doped in the matrix in such system. However, theuniform doping method still has some drawbacks. The thesis aims atstudying a new crossfading doping method in the design of OLED devicesto overcome the disadvantages of uniform doping and enhance deviceperformance. Focusing on the topic, the thesis is organized as following:Crossfading doping in light-emitting layerAs the first step, we have designed a simplified phosphorescentorganic light-emitting diode by using a common blue matrix material asboth light emitting layer (EML) and transporting layer. The simplified structure not only reduces the process complexity and the material cost, butalso decreases the exciton quenching caused by carrier accumulation nearthe recombination zone. Based on the simplified structure, we have studiedthe application of the crossfading doping in EML. The crossfading-hoststructure is to mix two matrix materials in a crossfading profile, ensuringthe doping concentration of the emitter is stable. Compared withconventional double emission and co-host structure, crossfading-hostbroadens the recombination zone distinctly, and further enhances deviceefficiency and reduces roll-off. We also introduce a gradient doping to fitthe doping profile of crossfading doping to simplify the process. Afteroptimizations, a device with gradient doping achieves external quantumefficiency of21.0%and19.3%at the luminance of1000cd/m2and10,000cd/m2, respectively. The efficiency roll-off is only8.0%.Crossfading doping in transporting layersWe have proposed a novel nonlinear crossfading doping technology intransporting layers for the first time. In the crossfading doping intransporting layers, the high doping concentration near electrodesimproves the efficiency of carrier injection, and the low dopingconcentration near EML prevents the dopant diffusion into EML.Meanwhile, intermediate region doping provides a smooth transport path for carriers as well. We have proved that crossfading doping in hole andelectron transporting layer can significantly increase the I-V characteristicsand power efficiency of OLED devices.