Quantifying Interdopant Exciton Processes in Organic Light Emitting Diodes

Abstract

Interdopant exciton transfer processes play a critical role in engineering emission spectra from multiple emitters in white organic light emitting diodes (OLEDs). Developing experimental techniques for probing these energy transfer processes are thus vital to gain a better understanding of the device physics and to eventually engineer better white OLEDs. In this article, we present a simple method, based on electroluminescence spectra, to study and quantify the energy transfer mechanisms in a two-dopant (triplet−singlet) system. Through a combination of experimental spectra obtained from a collection of donor−acceptor separations and theoretical simulations of spectral variations, we demonstrate that the key physical parameters such as the Dexter van der Waals radii and the Forster radius can be quantified.

Unicolored phosphor-sensitized fluorescence for efficient and stable blue OLEDs

“…To determine the Förster radius of the emitting system, we measured extinction spectra of acceptor dissolved in toluene at concentration of 10 −6 M. The Förster radius of the donor, , was estimated from the spectral overlap integral J ( λ ) of the extinction spectrum of the acceptor and the normalized photoluminescence spectrum of the donor, which are shown in Supplementary Figure 1 . The UPSF system has a smaller FRET radius compared to a non-unicolored system, which are typically in the range of 4–5.5 nm 20 , 33 – 35 . This reduction is due to a smaller spectral overlap of donor’s emission and the acceptor’s absorption.…”

Are the rates of dexter transfer in TADF hyperfluorescence systems optically accessible?

“…This R FRET value is inferior compared to analogous ''hyperphosphorescent'' systems consisting of a phosphorescent iridium or platinum complex sensitizer and a redshifted FE acceptor, with typical values in the range of 4-5.5 nm. [46][47][48][49] However, for deep-blue FE emission, all known high efficiency FE materials will have relatively small FRET overlap due to the scarcity of suitable deep-blue PE or TADF sensitisers. For example the unicolor phosphor-sensitized fluorescent system introduced by Heimel et al also has a small R FRET of 2.4 nm.…”

Triplet-to-singlet exciton transfer in hyperfluorescent OLED materials

“…More generally, the estimation of the individual contributions from different exciton transfer mechanisms to overall device performance is fundamental and has been the subject of experimental investigation. 26,27 In this work, we analyze four systems composed of TADF and fluorescent emitter dopants dispersed in host matrices (Fig. 1) that were used by Adachi and collaborators.…”

“…More generally, the estimation of the individual contributions from different exciton transfer mechanisms to overall device performance is fundamental and has been the subject of experimental investigation. 26,27…”