Organic polymer light -emitting devices: Optical modeling, engineering and evaluation of opto-electronic properties.

Abstract

This thesis deals with issues to enable polymer light-emitting devices (PLEDs) for electronic display applications. These issues are related to the development of measurement setup for material characterizations, investigation of new materials for PLEDs, investigation of light transport process by a Monte Carlo optical modeling method, and development of characterization methods for evaluating the opto-electronic performance of the PLED display. Chapter 2 develops an integrated monochromatic excitation light source-integrating sphere based detection system to accurately characterize the absolute photo-luminescence quantum efficiency (PLQE) of polymer light-emitting films based on polyfluorene and its derivatives. We demonstrate that the PLEDs might be underestimated or exaggerated without appropriate calibration of the spectral variation of the measurement system. Chapter 3 and Chapter 4 investigate the opto-electronic properties of PLEDs based on polyfluorene (P(DOF)) and its derivatives. Chapter 4 presents the optoelectronic properties of bi-layer PLEDs fabricated with a new series of copolymers, P(DOF-DPO). For PPV based molecules, it has reported that higher device efficiency could be achieved by using an emissive layer that combines the fluorescent property (PPV) and electron transport property (DPO) of molecules. However, our experiments clearly indicate that this methodology does not apply for P(DOF-DPO) based PLEDs. Chapter 5 demonstrates that a light transport Monte Carlo code can be used for modeling the realistic geometry of the PLEDs. This method takes into account the absorption in the materials, multiple refractions within the device structure, back-reflection from the cathode, and thin-film effect of the optically ITO thin film. We show that the non-smooth interfaces between the polymer light-emissive and hole transporting layers increases the probability of the photons out-coupling and wave-guiding fractions of the internally generated light. Finally, we demonstrate that the origin of the Lambertian behavior of the PLEDs is not only due to simple refraction at different interfaces, but it is also determined by material absorption, and thin-film effect of the ITO thin film. Lastly, we develop characterization methods for evaluating the opto-electronic performance of a 4 a-Si:H TFT active-matrix polymer light-emitting display (AM-PLED) fabricated in our laboratory. The opto-electronic properties of this AM-PLED are characterized and evaluated.