Study Of Electrically Doped Organic Interfacial Layers And Their Applications In Solution Processed OLEDs

In recent years, organic light-emitting diode (OLED) with its own advantages such asfast response, wide viewing angle, light and thin, low power consumption, etc., has attractedextensive attention because of its potential application in flat-panel display and solid-statelighting. OLED fabrication process has two ways: vacuum evaporation and solutionprocessing. Solution processing (spin coating, inkjet printing, screen printing, etc.) is expectedto reduce the processing cost and realize easily large-scale production. Despite the advantageon cost, the performance of OLED based on solution-process is poor when compared withinorganic light emitting diode (LED) and OLED based on vacuum evaporation. Therefore, wehope to improve the efficiency and stability of the solution processed OLED by interfacemodification. To this end, we respectively introduced a new solution processable interlayer inthe cathode and anode interface using n-type and p-type doping to improve their carrierinjection abilities. Expectedly, luminous efficiencies of the devices are both enhanced owingto the balanced hole and electron recombination in the devices.At the cathode interface, utilizing low work function metal (barium, calcium, etc.) andalkali metal fluorides (LiF, CsF) to realize effective electron injection is the major trend, butthis method requires a high cost of evaporation process and can cause the interface instability(sensitive to the water, oxygen). To avoid these problems, we chose a kind ofwater/alcohol-soluble polymer,poly[(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN)as a host, a water-/-alcohol-soluble small molecule cesium carbonate (Cs2CO3) as a dopant.After using simple physical blending, an N-type doped interlayer as electron injection layer(EIL) by means of spin coating was obtained. First, we applied it to the green polymer lightemitting diode (PLED), after doped, the device has lower driving voltage, higher currentefficiency. Compared with the device using Ba/Al as cathode, the device using purealuminum as cathode in the presence of the doped EIL, can obtain a similar turn-on voltageand the maximum current efficiency increased by28.2%. Then we used this doping system in the white phosphorescent PLED, luminescent properties of the device have been improvedobviously, the maximum current efficiency (LE) can reach to36.2cd/A.At the anode interface, poly(3,4-ethylenedioxythiophene: poly(styrene sulfonate)(PEDOT:PSS) is widely used as hole injection layer (HIL) in solution processed OLED,which can effectively improve the device performance. However, the acidic PEDOT:PSS willetch indium tin oxide (ITO) causing unstable device performance. To avoid this problem, wechoose other hole-transport polymer as a host, a suitable acceptor molecule as a dopant, andutilize P-type doping to improve its hole-injection property. At the end, an effective P-dopedanode interfacial layer used as HIL is successfully obtained. First, we applied it to the greenPLED. Compared with the device using PEDOT:PSS as HIL, this device using the P-dopedHIL can obtain a similar turn-on voltage and the maximum current efficiency increased by44%. Then we prepared a double layer HIL consist of the P-doped system and PEDOT: PSS.We applied it to a variety of solution processed OLEDs. Some low driving voltage, highluminous efficiency devices were successfully obtained.