Development of hybrid inorganic-organic light-emitting devices with metal oxide charge transport layers

Organic light emitting diodes (OLEDs) are currently being considered as the next generation technology in flat panel displays and solid state lighting applications. Among which, phosphorescent organic light emitting diodes (PhOLEDs) with nearly 100% internal quantum efficiency including other properties such as self emitting, high luminescence efficiency, broad wavelength range, wide viewing angle, high contrast, low power consumption, low weight, and large emitting area are gaining popularity in both academic and industrial research. Although development and commercialization of OLED technology is growing, there are still several key issues that need to be addressed---the external quantum efficiency (EQE) needs to be improved and the biggest technical challenge is to increase the device operational lifetime. Balanced charge injection and transport is vital for improving the device efficiency which demands for selection of better charge injection and transport materials. In addition imbalanced charge injection also degrades the device via joule\u27s heating and charge accumulation thereby limiting the device lifetime. Sensitivity of organic materials to the ambient atmosphere, particularly oxygen and moisture impedes the device performance.;This thesis work attempts to address these issues in the PhOLEDs through selection of proper charge injection and transport material as well as device structure optimization. At first we prepared thin films of thermally evaporated zinc-tin oxide (ZTO) with various ZnO and SnO2 compositions and studied its optical, electrical and morphological properties. After optimization of transparency and conductivity, these ZTO films showed promising materials for alternate transparent conducting oxides and electron transport layer (ETL) functions. Similarly, thin films of thermally evaporated tungsten oxide (WO 3) were prepared and their optical and electrical properties were studied and evaluated as a hole transport layer (HTL) material. We then fabricated and characterized various hybrid light emitting diode (HyLED) structures comprising of---ZTO as an ETL, WO3 as a HTL, and MoO3 as a hole injecting layer (HIL). The device structures were optimized for better performance in terms of efficiency and operational lifetime. Significant enhancement in EQE and operational lifetime were obtained in HyLEDs having WO3 as a HTL than of PhOLEDs with organic HTL. This is because WO3 improved hole injection as well as enabled facile hole transport thereby maintaining the balance of charge injection into the device. Finally, we also prepared inverted HyLEDs using WO3 as HTL and several metals including Ca, Ca/LiF, and Al/LiF as a cathode and their electron injecting capability were studied. Balanced charge injection was observed when a nanometer thick Ca was used as a cathode and WO3 as a HTL. As a result, inverted HyLED with better EQE and operational lifetime were fabricated