White organic light-emitting diodes (WOLEDs) can in principle convert every electrically generated exciton into light by harvesting singlet excitons on a blue fluorescent emitter and triplet excitons on green and red phosphorescent emitters. The architecture introduced by Sun, Giebink, Kanno, Ma, Thompson and Forrest (Nature 2006) demonstrated this channel separation and projected total external quantum efficiencies of roughly 34% and power efficiencies near 60 lm/W with ideal chromophores, together with an approximately 20% power-efficiency advantage over ideal all-phosphorescent devices, but it left open the emitter materials, the resonant host/dopant energetic alignment, a predictive model of high-current efficiency roll-off, and the operational lifetime. This problem asks for the completion of that program: a deep-blue singlet-channel emitter whose combination with the green and red phosphors reaches near-unity exciton utilization, together with lighting-grade stability, resonant host/dopant systems, a quantitative model linking device geometry and local exciton densities to triplet-triplet annihilation, exciton-polaron quenching, singlet-triplet annihilation and field-induced exciton dissociation, and demonstrated long-term lifetime. Later literature has removed the 25% fluorescent quantum-efficiency ceiling (thermally activated delayed fluorescence), realized the resonant design principle, and exceeded the efficiency projection (150.7 lm/W in hybrid tandem devices), but the deep-blue efficiency-lifetime bottleneck and the comprehensive predictive loss model remain open.

Background

An organic light-emitting diode (OLED) is a thin-film diode in which injected electrons and holes recombine within an organic emissive layer to form bound electron-hole pairs called excitons. Because of spin statistics, roughly one quarter of these excitons are created in the spin-symmetric singlet state and three quarters in the spin-antisymmetric triplet state. Conventional fluorescent molecules can emit only from singlets, so their internal quantum efficiency (IQE, the fraction of formed excitons converted to photons inside the film) is capped near 25%; triplet excitons recombine non-radiatively. Phosphorescent emitters, heavy-metal complexes such as iridium(III) compounds, use strong spin-orbit coupling to harvest triplets as well, allowing an IQE of 100%, and all-phosphorescent white OLEDs (WOLEDs) became the efficiency benchmark of the field. External quantum efficiency (EQE) is the IQE multiplied by the light out-coupling fraction, and luminous efficacy in lumens per watt measures the photometrically weighted power efficiency relevant to lighting. Two drawbacks of all-phosphor WOLEDs motivated the 2006 work: efficient blue phosphorescent emitters degrade quickly, and relaxing carriers into a phosphor triplet state costs the singlet-triplet exchange energy (order 0.5-1 eV for blue emitters), which directly reduces luminous efficacy. In the hybrid architecture of Sun et al. (2006), excitons are generated near the two edges of a wide emissive layer based on the conductive host CBP (4,4'-bis(N-carbazolyl)biphenyl): singlets transfer by Förster resonance energy transfer (a near-resonant dipole-dipole mechanism) to a lightly doped blue fluorescent emitter, while long-lived triplets diffuse through the host (measured diffusion length LD≈46L_{\mathrm{D}}\approx 46 nm in CBP) into a central green/red phosphor region, with spacers chosen to suppress unwanted singlet transfer from fluorophore to phosphor. The device reached total EQE and power efficiency of 18.7% and 37.6 lm/W, kept its color coordinates essentially constant with current, and showed a much weaker high-current efficiency roll-off than all-phosphor WOLEDs. Efficiency roll-off is the loss of quantum efficiency at high current density, driven in phosphorescent devices mainly by triplet-triplet annihilation (two triplet excitons mutually quenching) and exciton-polaron quenching (the non-radiative decay of excitons on charged molecules), and in these hybrid devices additionally by singlet-triplet annihilation (a singlet exciton quenched by a long-lived triplet) and field-induced exciton dissociation (the driving electric field pulling the bound electron-hole pair apart); the paper quantified it phenomenologically through the current density J0J_0 at which the EQE falls to half its peak. Thermally activated delayed fluorescence (TADF), introduced in 2012, is a later mechanism in which metal-free molecules with a very small singlet-triplet energy gap convert triplets into radiative singlets by reverse intersystem crossing, in principle harvesting both spin channels in a fluorescent emitter. This work leaves four directions open for completing the hybrid concept: emitter chromophores with the projected internal efficiencies (a blue singlet-channel emitter with high exciton utilization and long lifetime, plus near-unity IQE phosphors); host/dopant combinations in which the host singlet is resonant with the blue fluorophore singlet and the host triplet is resonant with the green phosphor triplet, which is what buys the roughly 20% power-efficiency advantage over ideal all-phosphor devices; a quantitative predictive account of the loss mechanisms at high current; and data on long-term operational stability, which the 2006 report did not include.

Problem Statement

Complete the realization of the fluorescent/phosphorescent hybrid white organic light-emitting diode, in which electrically generated singlet excitons are radiatively harvested by a blue singlet-channel emitter and triplet excitons by green and red phosphorescent emitters. The single objective is a white source whose performance, physical understanding and stability are all established, comprising four linked requirements from the source work: (1) emitter materials meeting the projected internal efficiencies, namely a blue singlet-channel emitter (fluorescent, delayed-fluorescent or otherwise) that harvests its singlet channel at near-unity internal efficiency, with a deep-blue emission (CIE 1931 chromaticity y <= 0.10) that combines with the green and red phosphors into white emission on the Planckian locus (CIE 1931), and an operational lifetime adequate for lighting (the LT70 criterion of requirement (4)), combined with approximately 100% IQE green and red phosphors, so that the blue emitter and the green and red phosphors together harvest both spin channels and the white device reaches near-unity exciton utilization (a property of the emitter-plus-phosphor stack, not of the blue emitter alone); (2) host/dopant energetic alignment in which the host singlet is resonant with the blue fluorophore singlet and the host triplet is resonant with the green phosphor triplet, so that the projected order-20% power-efficiency advantage over an otherwise ideal all-phosphorescent device is actually obtained under operating conditions; (3) a quantitative, predictive model of high-current efficiency roll-off that connects device geometry and local exciton densities to the relative contributions of triplet-triplet annihilation, exciton-polaron quenching, singlet-triplet annihilation and field-induced exciton dissociation across the operating current range, replacing device-specific phenomenological fits; and (4) demonstrated long-term operational lifetime of the hybrid device under realistic driving, quantified as LT70 (the time to 70% of initial luminance at 1000 cd/m^2) under a stated accelerated-aging and extrapolation protocol and meeting the practical-lighting target of at least 10,000 h, with identification of the dominant degradation pathways. The target is the projected performance envelope (order 34% total EQE and 60 lm/W with the emitter internal efficiencies of requirement (1)), together with a validated loss model and the lifetime evidence (the LT70 criterion of requirement (4)). Each requirement is established from measurement on the device, or from a predictive model validated against measured roll-off and lifetime data. The four requirements must hold jointly for one and the same single-junction channel-separated hybrid device, as described in the Background; results for materially different device classes, such as tandem or light-extraction-enhanced demonstrators, are not part of this target.

References

  1. Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, S. R. Forrest, 'Management of singlet and triplet excitons for efficient white organic light-emitting devices', Nature 440, 908-912 (2006). DOI 10.1038/nature04645, https://doi.org/10.1038/nature04645
  2. N. C. Giebink, S. R. Forrest, 'Quantum efficiency roll-off at high brightness in fluorescent and phosphorescent organic light emitting diodes', Physical Review B 77, 235215 (2008). DOI 10.1103/PhysRevB.77.235215, https://doi.org/10.1103/PhysRevB.77.235215
  3. H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, 'Highly efficient organic light-emitting diodes from delayed fluorescence', Nature 492, 234-238 (2012). DOI 10.1038/nature11687, https://doi.org/10.1038/nature11687
  4. D. Song, S. Zhao, Y. Luo, H. Aziz, 'Causes of efficiency roll-off in phosphorescent organic light emitting devices: Triplet-triplet annihilation versus triplet-polaron quenching', Applied Physics Letters 97, 243304 (2010). DOI 10.1063/1.3527085, https://doi.org/10.1063/1.3527085
  5. N. C. Erickson, R. J. Holmes, 'Engineering Efficiency Roll-Off in Organic Light-Emitting Devices', Advanced Functional Materials 24, 6074-6080 (2014). DOI 10.1002/adfm.201401009, https://doi.org/10.1002/adfm.201401009
  6. N. Sun, Q. Wang, Y. Zhao, Y. Chen, D. Yang, F. Zhao, J. Chen, D. Ma, 'High-Performance Hybrid White Organic Light-Emitting Devices without Interlayer between Fluorescent and Phosphorescent Emissive Regions', Advanced Materials 26, 1617-1621 (2014; published online December 2013). DOI 10.1002/adma.201304779, https://doi.org/10.1002/adma.201304779
  7. X.-L. Li, X. Ouyang, M. Liu, Z. Ge, J. Peng, Y. Cao, S.-J. Su, 'Highly efficient single- and multi-emission-layer fluorescent/phosphorescent hybrid white organic light-emitting diodes with ~20% external quantum efficiency', Journal of Materials Chemistry C 3, 9233-9239 (2015). DOI 10.1039/C5TC02050F, https://doi.org/10.1039/C5TC02050F
  8. D. Zhang, D. Zhang, L. Duan, 'Exploiting p-Type Delayed Fluorescence in Hybrid White OLEDs: Breaking the Trade-off between High Device Efficiency and Long Lifetime', ACS Applied Materials & Interfaces 8, 23197-23203 (2016). DOI 10.1021/acsami.6b07107, https://doi.org/10.1021/acsami.6b07107
  9. C. Huang, Y. Zhang, J. Zhou, S. Sun, W. Luo, W. He, J. Wang, X. Shi, M.-K. Fung, 'Hybrid Tandem White OLED with Long Lifetime and 150 Lm W^-1 in Luminous Efficacy Based on TADF Blue Emitter Stabilized with Phosphorescent Red Emitter', Advanced Optical Materials 8, 2000727 (2020). DOI 10.1002/adom.202000727, https://doi.org/10.1002/adom.202000727
  10. W. Yuan, T. Huang, J. Zhou, M.-C. Tang, D. Zhang, L. Duan, 'High-efficiency and long-lifetime deep-blue phosphorescent OLEDs using deuterated exciplex-forming host', Nature Communications 16, 4446 (2025). DOI 10.1038/s41467-025-59583-8, https://doi.org/10.1038/s41467-025-59583-8