UCLA

The transition dipole moment (TDM), a vector whose quantity describes the strength and the direction of the electronic transition between the emissive and ground state of an emissive material, determines the performance of all optoelectronic devices. Since the orientation of the TDM plays a major role in affecting directional optical properties, it is crucial to know accurate TDM orientation for creating OLEDs that efficiently outcouple light and luminescent concentrators that trap light into total internal reflection angles. Back focal plane (BFP) imaging is a commonly used method to reveal the orientation of the TDM. Because the emission pattern depends strongly on the TDM orientation, one can calculate the TDM from the BFP image. Despite how common this measurement has become for determining the TDM orientation, the error is often not reported even though many factors affect the accuracy of TDM angle-fitting results. In this thesis, the parameters in the TDM angle-fitting process, including theoretical data-generating, emission pattern centering, background subtracting, BFP signal normalizing, and BFP signal fitting-range deciding, are discussed and to see how they affect the angle-fitting outcomes. Then methods that could ensure the accurate determination of the orientation of TDM are summarized.

Additionally, the limit of the accuracy and uncertainty of fitting TDM angles under different conditions are analyzed, including different refractive indexes and thicknesses of the emission layer, different numbers of dipoles, different positions where dipoles are, and different amounts of background noise of optical instruments. For a single dipole, such as dyes or nanoparticles, the uncertainty increases when the angle of the TDM becomes smaller. Besides, the maximum uncertainty appears when the single dipole is close to the dielectric interface; the minimum uncertainty happens when the dipole is right at the middle of the emission layer. For multiple dipoles, such as thin-film semiconductors, the uncertainty is at the same level as a single dipole in the middle of the emission layer, which means one dipole can represent all dipoles. Finally, a model which could generate mimetic experimental BFP emission patterns is introduced, so that the accuracy and uncertainty of fitting TDM angles under certain amounts of background noise can be predicted. With the insights provided by this work, designing high-efficient optoelectronic devices can be achieved.