Optical analysis of organic light-emitting diode and photovoltaic cells

Abstract

In this thesis, the characteristics of organic light-emitting diode (OLEDs) and organic photovoltaic cells (OPVs) are demonstrated by using well-known theoretical method such as the dipole model and transfer matrix method. Various optical characteristics such as the outcoupling efficiency, emission spectrum, location of emission zone and angle dependent emission pattern for OLEDs, and the short circuit current density, and the external quantum efficiency for OPVs are simulated for the devices with a planar multilayered structure. This thesis is composed of two parts: first part describes the theoretical analysis of OLEDs (chapter 1 ~ chapter 7), and second part illustrates the analysis of small molecule based OPVs (chapter 8), respectively. Chapter 1 and 2 introduce the operating principles of OLEDs and the method for the optical simulation of OLEDs. The theoretical method is based on the dipole model that was introduced by Chance, Prock, and Silbey (so called CPS model) in 1987, where a molecular excited state is treated as an oscillating dipole. We describe the governing equations and the procedure of the optical simulation. Chapter 3 describes the theoretical analysis of the outcoupling efficiency (OCE) and external quantum efficiency (EQE) of OLEDs. The first section illustrates the analysis of the outcoupling efficiency for blue and green phosphorescent bottom emission OLEDs. The analyses take into account many factors such as thickness of organic layer, position of emission zone and the thickness of the ITO layer. Maximum outcoupling efficiencies over 27% and 30% are achievable if thin ITO layers in the range of 50–70 nm and 50–100 nm are used for blue and green phosphorescent OLEDs, respectively. This result clearly indicated that the phosphorescent OLEDs can have much higher external quantum efficiency than 20%. ITO, the most commonly used transparent conducting electrode, is difficult to use as an electrode for large area flexible displays because ITO is brittle and easily generates cracks under bending stress. Recently, graphene has received great attention to replace ITO due to the fact that graphene is a material possessing high conductivity, high charge mobility and high transmittance (_97.7% per monolayer) in the entire visible range. Chapter 4 shows the outcoupling efficiency of OLEDs incorporating graphene as anode. The OCEs of the OLEDs with 3 and 4 monolayers of graphene are comparable to the one of ITO based device with the 150 nm-thick ITO, where both electrodes have a similar sheet resistance of about 24 . However, the OCEs of graphene based OLEDs are lower than the ITO based devices with the same sheet resistance in most cases. This limitation can be overcome by utilizing the graphene/IZO composite electrode, which can achieve high outcoupling efficiency, low sheet resistance and high transmittance at the same time. An emitter with a horizontal transition dipole moment results in much higher outcoupling efficiency than the vertically aligned dipole as demonstrated in polymers and vacuum evaporated organic molecules. Chapter 5 demonstrates the optical analysis of high efficiency phosphorescent organic light emitting diodes (OLEDs) doped with Ir(ppy)2(acac) [bis(2-phenylpyridine)iridium(III)-acetylacetonate] in an exciplex forming co-host. This emitter has a preferred orientation with the horizontal to vertical dipole ratio of 0.77:0.23 as compared to 0.67:0.33 in the isotropic case. Theoretical analysis based on the orientation factor (, the ratio of the horizontal dipoles to total dipoles) and the photoluminescence quantum yield ( ) of the emitter predicts that the maximum external quantum efficiency (EQE) of the OLEDs with this emitter is about 30% which matches very well with the experimental data. Based on the results, we claim that the maximum EQE achievable with a certain emitting dye in a host can be predicted by just measuring and  in a neat film on glass without fabricating devices and offer a universal plot of the maximum EQE as functions of and . Chapter 6 introduces a quantitative method to determine the emission zone in OLEDs with a thin emitting layer. The location of the emission zone (EZ) significantly influences the performance of organic light emitting diodes (OLEDs), such as their emission spectrum, internal and outcoupling efficiencies and stability. Therefore, the direct observation or estimation of the position of the EZ is of great importance in terms of realizing high efficiency and high stability OLEDs. Chapter 6 described the method of determination of EZ that two devices with different thicknesses of electron transporting layer (ETL), but exhibiting same current density-voltage characteristics were used for the purpose. Emission zone in the OLEDs were extracted from the comparison of the experimental luminous intensity ratio depending on the current density with the calculated intensity ratio as a function of emission zone. This simple calculation method can contribute to further understanding of exciton behavior related to the realization of high performance OLEDs and greater understanding of the internal efficiency. Chapter 7 demonstrates a polarizer free high contrast flexible top emission OLED (TOLED) with a little reduction of efficiency by employing a multilayered dielectric-metal anti-reflection (AR) structure integrated on the top semi-transparent cathode. Through the careful design of the AR structure is performed using optical simulation, the flexible AR-TOLED showed a sufficiently low luminous reflectance (6%), high efficiency (86% of the TOLED without the AR structure, 1.75 times higher than the TOLEDs with circular polarizer (CP-TOLED)) and extremely high durability upon repeated bending up to 10,000 times for the bending radius of 0.7 cm. Chapter 8 shows the optical analysis of the OPVs with graphene as the anode in the bilayer and bulk heterojunction (BHJ) OPVs using the optical model and compared them with ITO based devices. The graphene based OPVs can achieve higher Jsc in the bilayer OPVs and total number of absorbed photons in BHJ-OPVs than the ITO based ones due to the lower reflective loss of incident light by the graphene electrode, which can increase the absorption by the active layers. The thinness and flexibility of the graphene electrode clearly demonstrate its potential to replace ITO in OPVs.