Exciplex strength as a design principle for high-performance organic light-emitting diodes

초록

Exciplex-forming co-hosts provide an effective strategy for improving the optoelectrical performance of organic light-emitting diodes (OLEDs), yet the functional impact of exciplex interaction strength on device operation remains insufficiently understood. Here, exciplex formation is proposed as a bimolecular interaction that governs charge-transfer stabilization, spin-conversion kinetics, recombination pathways, and ultimately device efficiency. Through an analysis of literature-reported donor-acceptor systems combined with temperature-dependent photoluminescence, transient spectroscopy, and drift-diffusion device simulations, we develop a generalized framework that quantitatively distinguishes strongly-and weakly-stabilized exciplex regimes. Strongly-stabilized exciplexes are characterized by low charge-transfer energy (E-CT < 2.8 eV), small singlet-triplet splitting (Delta E-ST similar to 0.06 eV), low reorganization energy, fast reverse intersystem crossing (similar to 8.6 & times; 10(6) s(-1)), and a favorable high energy-transfer-to-nonradiative-loss ratio (k(ET)/k(nr)), enabling Langevin-dominant recombination. Weakly-stabilized exciplexes, by contrast, exhibit elevatedE(CT), larger Delta E-ST, slower spin conversion, and lowerk(ET)/k(nr), leading to trap-assisted recombination and polaron accumulation. Using TCTA:B3PYMPM and TCTA:TPBi as model strongly-and weakly-stabilized exciplex co-hosts, respectively, we demonstrate that the strong bulk exciplex-based device achieves a maximum external quantum efficiency (EQE) of 30.0% with low-efficiency roll-off at 10,000 cd m(-2), while the weak interfacial exciplex counterpart suffers a severe efficiency drop and roll-off under identical conditions. A single normalized dimensionless exciplex strength descriptor (Phi(ex)) classifies exciplex strength, yielding 1.33 for TCTA:B3PYMPM and significantly lower 0.53 for TCTA:TPBi. Thus, the rationale for the energy transfer mechanism and the boundary of descriptors establishes a state-of-the-art design principle for selecting efficient exciplex systems in high-performance OLEDs.

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