High‐Efficiency Solution‐Processed Small Molecule Electrophosphorescent Organic Light‐Emitting Diodes

Extensive research on organic light-emitting diodes (OLEDs) continues due to their promise in applications such as fl at panel displays and solid state lighting. [ 1–5 ] Commonly, thermal high-vacuum evaporation technology is used for fabrication of small molecule-based OLEDs (SMOLEDs) and solution processing technology is used for those based on polymers (PLEDs). Thermal evaporation deposition enables complicated multilayer device architectures and renders excellent devices with high effi ciencies. [ 6 , 7 ] In contrast, solution-based deposition limits fabrication of composite device structures because the solvent used for one layer can redissolve or otherwise damage the previous layers. [ 8 ] Therefore, thermally evaporated SMOLEDs are typically more effi cient and longer-lived than solution-processed PLEDs. However, thermal evaporation deposition has its own disadvantages. First, it requires high vacuum and is consequently much more costly. Second, making multidopant OLEDs, such as white OLEDs (WOLEDs), requires precise control of the doping concentration of each dopant in the emitting layer (EML) to obtain the desired emission. [ 9 , 10 ] These reasons usually lead to a fabrication process of greater complexity and higher cost. On the other hand, solution processing, such as spin-coating, inkjet printing, and screen printing, is advantageous over thermal evaporation processing, due to its low-cost and large area manufacturability. [ 10 , 11 ] Additionally, it is possible to realize co-doping of several dopants by mixing the dopants and host material in solution. Hence, the fabrication of SMOLEDs via a solution process is of great importance. To that end, we demonstrate high effi ciency (forward power and luminous effi ciencies up to 60 lm W − 1 and 69 Cd A − 1 , respectively) spin-coated electrophosphorescent SMOLEDs based on greenemitting tris[2-(p-tolyl)pyridine] iridium(III) (Ir(mppy) 3 ) doped into a 4,4’-bis(9-carbazolyl)-biphenyl (CBP) host, probably due to the materials and fi lm morphology. This is the highest reported effi ciency of any solution-processed OLED and among the highest of any OLED without outcoupling enhancement. The

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