Beyond the Static Approximation: Assessing the Impact of Conformational and Kinetic Broadening on the Description of TADF Emitters

TL;DR

This paper introduces the Gamma-Fit method, a gamma distribution-based analytical framework that accounts for conformational and kinetic heterogeneity in TADF emitters, improving the characterization of their complex decay kinetics in OLEDs.

Contribution

The study presents the Gamma-Fit method to better analyze multiexponential TADF decay data, incorporating conformational and kinetic heterogeneity for more accurate kinetic parameter extraction.

Findings

Gamma-Fit accurately models complex decay kinetics in TADF emitters.

Accounting for local environment influences OLED efficiency.

Conformational ensembles and multiple triplet states are crucial for transition kinetics.

Abstract

Thermally activated delayed fluorescence (TADF) is a promising route towards high-efficiency, metal-free organic light-emitting diodes (OLEDs). However, the characterization of TADF kinetics in solid-state thin films is often complicated by pronounced multiexponential photoluminescence decays that prevent standard biexponential modeling. In this work, we introduce the 'Gamma-Fit' method, a streamlined analytical framework based on the gamma distribution that accounts for the continuous distribution of decay rates inherent in disordered molecular ensembles. By treating the decay as a result of conformational and kinetic heterogeneity, we accurately extract kinetic parameters for the benchmark emitters 4CzIPN and 5CzBN, as well as a series of novel diphenylamine (DPA)-based systems. Our results reveal that accounting for the local environment in thin films remains an important part in determining OLED efficiency. The experimental findings are complemented by a semiclassical Marcus-like computational approach. We evaluate the reliability of this conventional single-conformation rate calculation method and highlight the presence of conformational ensembles and multiple RISC-active triplet states as important factors for accurately describing the transition kinetics.