טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentEpstein Ariel
SubjectRigorous Electromagnetic Analysis of Optical Emission of
Organic Light-Emitting Diodes
DepartmentDepartment of Electrical Engineering
Supervisors Professor Pinchas Einziger
Professor Nir Tessler
Full Thesis textFull thesis text - English Version


Abstract

Organic light-emitting diodes (OLEDs) have been intensively investigated in recent years, holding promise for novel optoelectronic devices such as thin and flexible displays and efficient lighting instruments. Despite rapid technological progress in this area, device performance is still far from optimal, and great scientific efforts are being made to understand the fundamental physical processes determining the electrical and optical properties of the devices, to promote design of efficient OLEDs with wide viewing angles.

Although the canonical electromagnetic model of dipole sources embedded in planar stratified media is found to be suitable for OLED modeling, and is indeed intensively used, fundamental problems exist in the basic approaches presented in the literature. First, the transition between the canonical deterministic sources and the realistic statistical exciton ensemble is treated only qualitatively. Second, the exclusively-numerical approaches to the problem (and inverse problem) introduce undesirable complexity to design and verification procedures, obscuring the underlying physical phenomena. Third, no rigorous formulation was presented for curved devices, a configuration attracting growing interest due to the emerging field of flexible OLEDs (FOLEDs).

Our research addresses these gaps using a rigorous analytical approach. First, we have rigorously incorporated the exciton spatial and spectral distributions into the canonical model, quantitatively formulating the interplay between coherence length and prominence of weak-microcavity interference fringes. This fundamental optical interplay, later generalized to arbitrary ray-optical scenarios, renders common artificial treatments of thick layers unnecessary and facilitates derivation of simple closed-form expressions for OLED emission patterns. Next, we have utilized these simplified expressions to develop analytical methods to estimate the exciton spatial distribution mean position and width from optical measurements, introducing a unique alternative to the cumbersome numerical techniques available to date. This was done by recognizing that the main emission pattern features are related to the interference between the source and its reflection from the cathode, which obeys Bragg's condition. Finally, we have rigorously solved the electromagnetic problem of cylindrically curved FOLEDs, deriving closed-form expressions for their emission pattern, forming a complete analogy to the planar scenario. These expressions, coinciding with the geometrical optics solution, reveal a fundamental interplay between the radius of curvature and the substrate thickness which may be utilized to increase the device viewing angle and outcoupling efficiency.

Our work, dealing intimately with issues presented by rigorous electromagnetic modeling of novel nanometric spontaneous emission devices, introduces a comprehensive set of physically intuitive analytical engineering tools for efficient OLED design and verification.