The 2018 demonstration of an add-on gelled green-to-blue photon-upconversion layer optically pumped by an operating organic light-emitting diode (OLED) left open how to turn this below-threshold, moderately efficient assembly into a practically efficient, fully integrated solid-state device. The open problem is to realize solid-state triplet–triplet-annihilation (TTA) upconversion layers with lower thresholds, higher conversion efficiencies, and higher absorption coefficients, organized (e.g., templated or framework-like) so that triplet transport and annihilation survive in the solid state, and integrated monolithically with OLED fabrication so that an electrically driven OLED's emission is converted to higher-energy light at device operating brightness.

Background

Organic light-emitting diodes (OLEDs) are thin-film electroluminescent devices used in displays and lighting; deep-blue OLED emitters remain their weakest component because high-energy excitons degrade efficiency and lifetime. Photon upconversion via triplet–triplet annihilation (TTA-UC) offers an alternative route to blue light: a sensitizer molecule absorbs a lower-energy (e.g., green) photon and, after intersystem crossing (the spin-forbidden relaxation from the excited singlet into the long-lived triplet state), transfers its triplet energy to an emitter molecule by Dexter energy transfer (a short-range electron-exchange process); two diffusing emitter triplets then collide and annihilate, leaving one emitter in a high-energy singlet that fluoresces at shorter wavelength (e.g., deep blue). TTA-UC is characterized by a threshold excitation intensity (below which efficiency falls because triplets decay before meeting) and a maximum quantum yield of 50% (one upconverted photon per two absorbed, conventionally normalized to 100%); in liquids, efficiencies approaching this limit are routinely reached, but in rigid or quasi-solid matrices triplet diffusion is severely hampered. In 2018, von Reventlow, Bremer, Ebenhoch, Gerken, Schmidt, and Colsmann attached a thin gelled upconversion layer, containing tetraphenylporphyrin sensitizers and diphenylanthracene emitters, to a high-brightness green-emitting tandem OLED (two emitting units stacked in series), adding a long-pass and a short-pass dichroic filter (wavelength-selective mirrors transmitting only above or below a cutoff wavelength) to manage self-absorption, and obtained deep-blue emission from the green OLED (J. Mater. Chem. C 6, 3845–3848, 2018). The device, however, was manually assembled and glass-encapsulated, operated below its threshold intensity at the OLED's brightness, and suffered incomplete pump absorption, reflection losses, self-absorption, and re-absorption. The authors therefore posed as an open objective: upconverters with lower thresholds, higher conversion efficiencies, and higher absorption coefficients; templating of sensitizer–emitter and emitter–emitter arrangements (e.g., by self-assembly or metal–organic-framework-like organization, where a metal–organic framework is a crystalline coordination polymer whose pores and linkers can position chromophores) to improve excited-state transfer in the quasi-solid state; and, ultimately, fully integrated solid-state upconversion layers compatible with OLED fabrication that preserve triplet transport and annihilation. A mechanistically adjacent but distinct line of work, electrically driven 'upconversion OLEDs' in which triplet excitons are generated electrically and then annihilate in one device stack (e.g., Chen et al., Adv. Mater. 30, 1804850, 2018), does not answer this photon–photon question. Meanwhile, theory (Frazer, Adv. Theory Simul. 2, 1800099, 2018) shows that an LED-pumped upconverter in an optical cavity could in principle approach the 50% quantum-yield limit at Watt-scale output, and solid-state TTA-UC performance has advanced through dry-processed films (Lin, Perkinson, and Baldo, Adv. Mater. 32, 1908175, 2020, ≈2.5% at 238 mW cm−2^{-2}, identifying energy back-transfer, sensitizer aggregation, and triplet–charge annihilation as the solid-state losses) and 2D-sensitized plasmonic heterostructures (Lowe et al., Nano Lett. 26, 4220–4226, 2026, thresholds down to 0.9 mW cm−2^{-2} with EQE up to 3.6%). Yet field reviews as late as 2022–2023 (Alves et al., J. Mater. Chem. C 10, 7783–7798; Feng et al., Annu. Rev. Phys. Chem. 74, 145–168) still pose whether a solid upconverter can reach liquid-system performance, and no integrated OLED-pumped photon-photon device exists.

Problem Statement

Realize photon upconversion by triplet–triplet annihilation as a fully integrated, solid-state layer on an operating OLED, replacing the manually assembled, glass-encapsulated, below-threshold add-on architecture of von Reventlow et al. (2018) with a device of practical utility. The single objective—an electrically driven OLED whose emission is converted to higher-energy (e.g., deep-blue) light inside one integrated solid-state structure—comprises three jointly necessary, scientifically linked requirements: (1) solid-state upconverter materials with sufficiently low threshold intensity, high conversion efficiency, and high absorption coefficient to operate at OLED brightness, where pump irradiances are of order mW cm−2^{-2}, well below the thresholds of the 2018 gel; (2) Dexter-type triplet energy transfer and triplet–triplet annihilation remaining efficient in the quasi-solid or solid state, avoiding back-transfer, aggregation, and concentration quenching, toward which molecular organization of the sensitizer–emitter and emitter–emitter arrangements (e.g., templating, self-assembly, or framework-like structuring) is a source-suggested route among others; and (3) monolithic or fully integrated device integration compatible with OLED fabrication (solvent-free or OLED-safe processing, encapsulation, optical management of reflection, self-absorption, and re-absorption losses) that preserves triplet transport and annihilation. Completion is judged by the integrated device's upconversion threshold relative to OLED operating irradiance and its conversion efficiency (normalized to the 50% two-to-one photon limit), evaluated holistically by domain-expert review rather than a single mechanical criterion.

References

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