Impact of light-matter coupling strength on the efficiency of microcavity OLEDs: A unified quantum master equation approach

Abstract

Controlling light-matter interactions is emerging as a powerful strategy to enhance the performance of organic light-emitting diodes (OLEDs). By embedding the emissive layer in planar microcavities or other modified optical environments, excitons can couple to photonic modes, enabling new regimes of device operation. In the weak-coupling regime, the Purcell effect can accelerate radiative decay, while in the strong-coupling regime, excitons and photons hybridize to form entirely new energy eigenstates with altered dynamics. These effects offer potential solutions to key challenges in OLEDs, such as triplet accumulation and efficiency roll-off, yet demonstrations in the strong-coupling case remain sparse and modest. To systematically understand and optimize photodynamics across the different coupling regimes, we develop a unified quantum master equation model for microcavity OLEDs. The model is then applied to estimate device performance in the different coupling regimes to determine which one is the best.

Figures (5)

• Figure 1: Schematic picture of the study. (a) A basic OLED. The mirrors (or their reflectivity) can be ignored. (b) A weakly coupled microcavity OLED. The singlet excitons emit through optical modes supported by the cavity, with the emission possibly enhanced and red-shifted. (c) A strongly coupled microcavity OLED. The number of molecules is increased (or the mode volume decreased) to enter a regime, where emission through polaritons might be even more enhanced and red-shifted. The figure illustrates the research question: Does stronger light-matter coupling translate into better device performance? To answer this, we first need to develop a unified master equation model.
• Figure 2: Jablonski diagram of the system of interest: an organic molecule inside an optical cavity, embedded in a phonon bath and experiencing electrical excitation, polariton transitions, inter-system crossing (ISC), reverse inter-system crossing (RISC), emission, and non-radiative losses. Although a single, strongly coupled molecule is shown, we consider an ensemble of $N$ molecules across all the coupling regimes: no coupling, weak coupling, and strong coupling. It is important to note that the polaritons are collective states of all the $N$ sites and not localized, as depicted here for simplicity. UP = upper polariton, ER = exciton reservoir, LP = lower polariton, T = triplet state, G = ground state.
• Figure 3: (a) Chemical structure of 3DPA3CN. (b) Parameters used in this article.
• Figure 4: IQE as a function of cavity thickness $L_c$ and the number of coupled molecules $N$. The white dashed curves separate regions, where the system is entirely in the weak-coupling regime (WC), strong-coupling regime (SC), or different regimes at different outcoupling angles (WC+SC).
• Figure 5: Polariton energies and steady-state populations as functions of the outcoupling angle $\theta$, shown for different numbers of coupled molecules $N$ and cavity thicknesses $L_c$. The insets also show the angle-averaged IQEs, where $K=313$. Light blue = $E_+(\theta)$, light red = $E_-(\theta)$, solid black = $E_c(\theta)$, dashed black = $E_s$, dotted black = $E_t$, dark blue = $\langle P_+(\theta)\rangle$, dark red = $\langle P_-(\theta)\rangle$, gray shading = weak-coupling regime, no shading = strong-coupling regime.