White organic light-emitting diodes (WOLEDs) resemble light more naturally, with emission spectrum that is comfortable to the human eye. A lot of effort has been devoted to improve the performance of WOLEDs. This research work was aimed at studying the emission behavior of WOLEDs, improving the device performance, and thereby developing several novel device concepts for application in high performing transparent WOLEDs and organic proximity sensors. Emission behavior, in terms of color stability and injection characteristics of phosphorescent organic light-emitting diodes (OLEDs), was investigated systematically and optimized via the experimental optimization and optical simulation. The transparent WOLEDs can be almost invisible at daytime and can emit a pleasant diffused light at night, allowing the surface light source to shine in both directions. It is an exciting new lighting technology that could bring new device concepts. However, undesirable angular-dependent emission and asymmetrical emission characteristics are often observed in transparent WOLEDs. In this work, a pair of optically and electrically comparable transparent anode and cathode was introduced to form weak microcavity transparent WOLEDs, e.g., employing a pair of Ag (10 nm)/MoO3 (2.5 nm)-modified indium tin oxide anode and Al (1.5 nm)/Ag (15 nm)/NPB (50 nm) cathode. It is found that the avoidance of the spectral overlap between the peak wavelengths of the emitters and the resonant wavelength of the organic microcavity moderates the angular-dependent electroluminescence emission behavior, thereby improving the color stability of the transparent white WOLEDs over a broad range of the viewing angles. As a result, the transparent WOLEDs developed possess a visible-light transparency of >50%, a symmetrical bi-directional illumination with an almost identical power efficiency of 11 lm/W (measured at 100 cd/m2) and the similar CIE coordinates of (0.36, 0.43) and (0.38, 0.46) measured from both sides of the devices. Efficient charge injection is a prerequisite for achieving low turn-on voltage and improved hole-electron current balance in OLEDs. Metal oxide (e.g. MoO3) is a commonly used hole-injection layer (HIL) for reducing the energy barrier at the anode/organic interface for efficient charge injection. However, fluctuation in the quality of the metal oxide-based HIL, e.g., changes due to the MoO3 formulation, film fabrication and post-treatment conditions, often places a practical challenge limiting reproducibility of the device performance. In this work, an effective solution-processed HIL that consists of a mixture of PEDOT:PSS and MoO3 was developed for application in OLEDs. It is found that the presence of the solution-processed HIL at the interface between the anode and the organic improves the hole injection and the performance reproducibility of the phosphorescent OLEDs. The effect of the presence of the MoO3 in the solution-processed HIL on charge injection in phosphorescent OLEDs, with a configuration of glass/ITO/CBP/ CBP:Ir(ppy)2acac/TmPyPB/LiF(1.0 nm)/Al(70 nm), was examined. It is shown that solution-processed HIL has a superior hole injection characteristic at the HIL/hole transporting layer (HTL) interface compared to that in the devices fabricated with a pristine PEDOT:PSS or a pure MoO3 HIL, yielded phosphorescent OLEDs with an external quantum efficiency of ~25% and a power efficiency of ~75 lm/W @ 1000 cd/m2. The morphological and surface electronic properties of the hybrid HIL were also investigated by AFM, XPS and UPS measurements, revealing the formation of a good contact at the HIL/HTL interface in the phosphorescent OLEDs. Apart from improving the device performance, a new organic proximity sensor based on the monolithic integration of organic photo-detectors (OPDs) and OLEDs was also developed. A MoO3-modified thin silver interlayer, serving simultaneously as a transparent cathode for the OPDs and an anode for the OLEDs, is used to link the functional organic electronic components. In the integrated OLED/OPD-based proximity sensors, the OLED components function as an illumination source while the coupled OPD units enable a high absorption when light is reflected from objects to create an optical signal. The photosensitivity is enhanced using organic photosensitive bulk heterojunction in the OPDs, thereby realizing a high photosensitivity and the high external quantum yield at a low reverse bias. The signal to noise ratio, optical and frequency responses of the integrated organic proximity sensors were optimized and examined. The design and fabrication flexibility of the integrated OLED/OPD-based organic proximity sensors also have cost benefits, making it possible for application in wearable units and compact information systems.
|Date of Award||9 Oct 2015|
|Supervisor||Furong ZHU (Supervisor)|
- Light emitting diodes