An international collaboration between Finnish and American institutions, with most of the authors being based at the University of Turku in Finland, is doing research on the fundamental limitation in OLEDs. Despite their advantages, OLEDs face a significant efficiency challenge rooted in quantum mechanics: only 25% of electronically excited states can emit light during electrical excitation, while the remaining 75% form non-emitting triplet states.
The researchers were motivated by the need to overcome this inherent limitation that affects OLED performance. The challenge arises from spin statistics in molecular materials, where electrical injection results in a 3:1 ratio of triplet to singlet states. Traditionally, singlet states are preferred because they can undergo fluorescence much faster than phosphorescence, which reduces losses from exciton-exciton and exciton-polaron collisions.
While optical excitation can create more singlet states and is simpler to study experimentally, electrical excitation remains essential for practical applications. This has led researchers to explore various strategies for improving device performance, primarily focusing on two approaches: converting triplets into singlets or finding ways to rapidly and radiatively depopulate triplet states.
One promising development has been the emergence of thermally activated delayed fluorescence (TADF) emitters, which have shown high internal quantum efficiency. These materials can efficiently convert triplet excitations into singlets through reverse inter-system crossing (RISC). However, achieving high RISC rates typically requires complex molecular design techniques that often compromise the molecule’s ability to emit photons effectively, creating a challenging trade-off between RISC efficiency and light emission.
The researchers propose a novel solution using microcavity polaritons – hybrid light-matter states that emerge when light is strongly coupled with matter inside an optical cavity. These polaritons could potentially resolve the RISC-efficiency trade-off by acting as artificially Stokes-shifted singlet states. This approach suggests that through straightforward cavity designs, emitters could achieve both high RISC rates and high internal quantum efficiency, leading to devices that combine simple architectures with superior performance.
While preliminary experimental results have shown promise, the researchers identified a critical bottleneck: existing theoretical models are rudimentary and limit our understanding of how polaritons interact with molecular processes. This gap in theoretical understanding hinders the effective harvesting of triplet states in actual OLED devices.
The more comprehensive theoretical model introduced by the researchers centers on the Tavis-Cummings Hamiltonian, which describes a system of N identical organic molecules that can carry 0-2 excitations in total, with all molecules coupled to a single cavity mode. This Hamiltonian incorporates terms for singlet states, triplet states, ground states, and photon creation/annihilation operators.
What makes this model more sophisticated than previous approaches is its ability to go beyond single-excitation states to include two-excitation states, while accounting for both strong light-matter coupling and weak phonon coupling. The model integrates Marcus theory for electron transfer rates and employs Fermi’s golden rule to calculate transition rates between states. This allows for a thorough examination of key processes like reverse inter-system crossing (RISC), triplet-triplet annihilation (TTA), and singlet-singlet annihilation (SSA).
The mathematical treatment derives rates for both polaritonic RISC and TTA, taking into account reorganization energies and the collective nature of polaritons. Importantly, the model considers how the number of molecules (N) affects the system dynamics. By combining quantum optical and molecular physics approaches, the researchers created a framework that can simultaneously handle multiple interaction mechanisms and account for collective effects in molecular ensembles.
This comprehensive approach enables the researchers to explore how different processes interact and influence each other, providing a more complete understanding of the physics involved in polaritonic OLEDs. The model’s ability to handle multiple processes simultaneously while accounting for collective effects represents a significant advance over previous theoretical frameworks that were more limited in scope.
Reference
O. Siltanen, K. Luoma, A. J. Musser, K. S. Daskalakis, Enhancing the Efficiency of Polariton OLEDs in and Beyond the Single-Excitation Subspace. Adv. Optical Mater. 2025, 2403046. https://doi.org/10.1002/adom.202403046
