RCWA and FDTD modeling of light emission from internally structured OLEDs

Numerical analysis of micro-optics based single photon sources via a combined physical optics and rigorous simulations approach

The use of 3D printed micro-optical components has enabled the miniaturization of various optical systems, including those based on single photon sources. However, in order to enhance their usability and performance, it is crucial to gain insights into the physical effects influencing these systems via computational approaches. As there is no universal numerical method which can be efficiently applied in all cases, combining different techniques becomes essential to reduce modeling and simulation effort. In this work, we investigate the integration of diverse numerical techniques to simulate and analyze optical systems consisting of single photon sources and 3D printed micro-optical components. By leveraging these tools, we primarily focus in evaluating the impact of different far-field spatial distributions and the underlying physical phenomena on the overall performance of a compound micro-optical system via the direct evaluation of a fiber in-coupling efficiency integral expression.

Fast and rigorous optical simulation of periodically corrugated light-emitting diodes based on a diffraction matrix method

Increasing the light extraction efficiency has been widely studied for highly efficient organic light-emitting diodes (OLEDs). Among many light-extraction approaches proposed so far, adding a corrugation layer has been considered a promising solution for its simplicity and high effectiveness. While the working principle of periodically corrugated OLEDs can be qualitatively explained by the diffraction theory, dipolar emission inside the OLED structure makes its quantitative analysis challenging, making one rely on finite-element electromagnetic simulations that could require huge computing resources. Here, we demonstrate a new simulation method, named the diffraction matrix method (DMM), that can accurately predict the optical characteristics of periodically corrugated OLEDs while achieving calculation speed that is a few orders of magnitude faster. Our method decomposes the light emitted by a dipolar emitter into plane waves with different wavevectors and tracks the diffraction behavior of waves using diffraction matrices. Calculated optical parameters show a quantitative agreement with those predicted by finite-difference time-domain (FDTD) method. Furthermore, the developed method possesses a unique advantage over the conventional approaches that it naturally evaluates the wavevector-dependent power dissipation of a dipole and is thus capable of identifying the loss channels inside OLEDs in a quantitative manner.

Design of a Structure for Optimized Optical Performance of a Full Colored Organic Light-Emitting Diode on a Parameter Space Map

In general, optical properties of a top-emitting organic light-emitting diode (OLED) are dependent on the cavity effect of the OLED structure. Therefore, the optical path length of the many thin solid films in the OLED, which is strongly affected by the refractive index and thickness of each material, controls the cavity effect of the cell. In previous research, a parameter space method for optimizing the inorganic layer thickness of a red OLED structure was introduced to achieve the required bandwidth and peak wavelength. This is a simple method with high accuracy and can also be applied to red, green, and blue OLED structures. To design an OLED cell with a practical approach, however, the RGB OLED device requires the thickness of each inorganic layer and organic layer in all three R, G, and B OLED structures to be same. In this study, we applied the parameter space method to an RGB OLED device to find out and optimize the thickness of three inorganic parameters: Indium Tin Oxide (ITO), cathode, and capping layer (CPL) using the finite-difference time-domain (FDTD) method. The parameters ITO, cathode, and CPL were scanned from 18 to 21 nm, 5 to 100 nm, and 10 to 200 nm, respectively. The peak wavelength and bandwidth lines of the three spectral colors were placed on a map of the three inorganic layer thickness parameters to find the optimized points that can provide the desired optical characteristics with the same film thickness in the cell.

Generalized Poynting vector model to calculate the spatial and spectral profiles of the electric field intensity, optical power flow, and optical absorption for all optical modes of organic light-emitting diodes

We propose a generalized Poynting vector model (GPVM) that can simultaneously calculate the spatial and spectral distributions of the electric field intensity, optical power flow, and optical absorption as well as the power dissipation spectrum for all optical modes of organic light-emitting diodes (OLEDs). The theoretical formulation of the GPVM with respect to the dipole orientation and light polarization is derived by combining the dipole source term and transfer matrix method as a function of the normalized in-plane wave vector u. In the GPVM, the theoretical expression of the spectral power density, derived from the time-averaged Poynting vector at the emission layer, proves to be identical to that presented by the currently-used point dipole model. In a bottom-emitting OLED, the electric field profiles of the waveguide (WG) and surface plasmon polariton (SPP) modes obtained by the GPVM are nearly same as those calculated by the boundary eigenvalue solver except the slight difference at the position of the dipole emitter, which only occurs in the case that the excitation efficiency of a WG or SPP mode is relatively small. Finally, two-dimensional plots of the internal optical power flow and optical absorption, providing physical and intuitive information on the internal emission process as well as the absorption loss of all the optical modes, are calculated as a function of the longitudinal position and normalized in-plane wave vector. Compared with the currently-used electromagnetic methods of the Green's function, dipole radiation, and point dipole models, the proposed GPVM has the advantage that it can provide all the spatial and spectral calculation results of the electric field intensity, optical power flow, and optical absorption with respect to the dipole orientation and light polarization, which are essential in the optical modeling and analysis of OLEDs.

Theoretical comparison of the excitation efficiency of waveguide and surface plasmon modes between quantum-mechanical and electromagnetic optical models of organic light-emitting diodes

We theoretically compare the excitation efficiency of waveguide and surface plasmon modes between quantum-mechanical and classical electromagnetic optical models of organic light-emitting diodes (OLEDs). A sophisticated optical model combining the two approaches is required to obtain an accurate calculation result and a comprehensive understanding of the micro-cavity effect in OLEDs. In the quantum-mechanical approach based on the Fermi's golden rule, the mode expansion method is used to calculate the excitation efficiency. In the classical electromagnetic approach, the spectral power density calculated by the point dipole model is fitted by the summation of the Lorentzian line shape functions, which provide the excitation probability of each waveguide and surface plasmon modes. The mode coupling efficiencies on the basis of the two approaches are calculated in a bottom-emitting OLED when the position of a dipole emitter is varied. By comparing the calculation results, we confirm the equivalence of two approaches and obtain the better optical interpretation to the calculated excitation efficiency of waveguide and surface plasmon modes. The ratio of mode excitation efficiencies calculated by two approaches agrees well with each other except the contribution of the near-field absorption component.

Polarization-Dependent Atomic Force Microscopy-Infrared Spectroscopy (AFM-IR): Infrared Nanopolarimetric Analysis of Structure and Anisotropy of Thin Films and Surfaces

Infrared techniques enable nondestructive and label-free studies of thin films with high chemical and structural contrast. In this work, we review recent progress and perspectives in the nanoscale analysis of anisotropic materials using an extended version of the atomic force microscopy-infrared (AFM-IR) technique. This advanced photothermal technique, includes polarization control of the incoming light and bridges the gap in IR spectroscopic analysis of local anisotropic material properties. Such local anisotropy occurs in a wide range of materials during molecular nucleation, aggregation, and crystallization processes. However, analysis of the anisotropy in morphology and structure can be experimentally and theoretically demanding as it is related to order and disorder processes in ranges from nanoscopic to macroscopic length scales, depending on preparation and environmental conditions. In this context IR techniques can significantly assist as IR spectra can be interpreted in the framework of optical models and numerical calculations with respect to both, the present chemical conditions as well as the micro- and nanostructure. With these extraordinary analytic possibilities, the advanced AFM-IR approach is an essential puzzle piece in direction to connect nanoscale and macroscale anisotropic thin film properties experimentally. In this review, we highlight the analytic possibilities of AFM-IR for studies on nanoscale anisotropy with a set of examples for polymer, plasmonic, and polaritonic films, as well as aggregates of large molecules and proteins.

Anisotropic materials in OLEDs for high outcoupling efficiency

We present the results of an optical study in which we evaluate the effect of anisotropic electron transport layers (ETL) and anisotropic hole transport layers (HTL) on the outcoupling efficiency of bottom emitting organic light emitting diodes (OLEDs). We demonstrate that optical anisotropy can have a profound influence on the outcoupling efficiency and introduce a number of design rules which ensure that light extraction is enhanced by anisotropic layers.