Spectrally narrowed leaky waveguide edge emission and transient electroluminescent dynamics of OLEDs

In summary, there are two major research works presented in this dissertation. The first research project (Chapter 4) is spectrally narrowed edge emission from Organic Light Emitting Diodes. The second project (Chapter 5) is about transient electroluminescent dynamics in OLEDs. Chapter 1 is a general introduction of OLEDs. Chapter 2 is a general introduction of organic semiconductor lasers. Chapter 3 is a description of the thermal evaporation method for OLED fabrication. The detail of the first project was presented in Chapter 4. Extremely narrowed spectrum was observed from the edge of OLED devices. A threshold thickness exists, above which the spectrum is narrow, and below which the spectrum is broad. The FWHM of spectrum depends on the material of the organic thin films, the thickness of the organic layers, and length of the OLED device. A superlinear relationship between the output intensity of the edge emission and the length of the device was observed, which is probably due to the misalignment of the device edge and the optical fiber detector. The original motivation of this research is for organic semiconductor laser that hasn't been realized due to the extremely high photon absorption in OLED devices. Although we didn't succeed inmore » fabricating an electrically pumped organic laser diode, we made a comprehensive research in edge emission of OLEDs which provides valuable results in understanding light distribution and propagation in OLED devices. Chapter 5 focuses on the second project. A strong spike was observed at the falling edge of a pulse, and a long tail followed. The spike was due to the recombination of correlated charge pair (CCP) created by trapped carriers in guest molecules of the recombination zone. When the bias was turned off, along with the decreasing of electric field in the device, the electric field induced quenching decreases and the recombination rate of the CCP increases which result in the spike. This research project provides a profound understanding of the EL dynamics of OLED, and the theoretical model can fit and explain the experiment data quite well. For the edge emission, we focused on the spectrum and the relative intensity of the edge emission. In the future, more research can be done on the comparison of the intensity between the total edge emission and the surface emission which will give us a sense what fraction of light was trapped in the device. Micro structures can be integrated into the OLED such as DFB and DBR, the character of edge emission should be very interesting. For the transient spike, the CCP model can give a good explanation. But in the model, the effect of the electric field change is not included, because from the start point (t=0), we assume the mobility of carriers is a constant. If we consider the details of the change of the electric field, then when turning of the bias, the decrease of the electric field results in decrease of the carrier mobility and the dissociation rate. If we can add the electric field effect into the model, the whole theory will be more convincing.« less