Photophysics of Thermally-Activated Delayed Fluorescence Emitters and its Impact on the Performance of Organic Light-Emitting Diodes

Abstract

Research in thermally-activated delayed fluorescence (TADF) emitters is gathering
momentum and rapidly progressing towards commercial application in display industry.
Successful TADF combines design strategies that result in thermal up-conversion of nonemissive
triplets into emissive singlet excitons, increasing the maximum internal efficiency
from 25 to 100 % in purely organic systems. Its performance can therefore compete with
current leading emitters used in industry, however it is not without its hurdles and a full
understanding of how to produce efficient TADF systems and stable organic light-emitting
diodes (OLEDs) is still elusive.
This thesis aims at illuminating strategies to achieve highly efficient and stable TADF.
By examining the photophysical aspects of different donor-acceptor systems (D-A, D2-A, D3-
A, D4-A and D-A-D) and, more importantly, establishing comparisons between different
subsets of molecules, subtle but important aspects of the performance of these emitters are
isolated to allow understanding of future design rules for better combinations. These
comparisons are then correlated with the emitters’ performance in devices.
In a multi-donor platform, the inherent TADF mechanism and in host are both
considered by comparing the effect of number and position of donors as well as rigidity and
polarity of the host environment to the D-A angles. A separate comparative study elucidates
the real heavy atom effect in an emitter with dual emission from two different conformations.
Furthermore, the application of a well-established spectroscopy technique novel to TADF tests
its physical mechanism by probing character and mixing of the excited states involved. Finally,
in a more application-driven approach, a combination of three different TADF molecules for
the production of white light is studied in simple device structures. In this sense, guidelines of
how to produce optimised TADF systems emerge, moving the technology ever close to its
industrial application.