Thermally activated delayed fluorescence (TADF) can, in principle, convert the triplet excitons that dominate electrical excitation into singlet gain in organic laser media, and a 2018 Nature Photonics study demonstrated this concept's first ingredient by combining nearly 10% external quantum efficiency near-infrared (NIR) electroluminescence with pulsed amplified spontaneous emission (ASE) in a single boron difluoride curcuminoid emitter. The unresolved problem is whether triplet harvesting through reverse intersystem crossing (RISC) can actually be made to work for light amplification on technologically relevant timescales: RISC in these NIR emitters operates on microsecond-to-hundred-microsecond times, all TADF ASE and lasing demonstrations still use short optical pulses, and electrically pumped organic lasers remain unrealized.

Background

Organic light-emitting diodes (OLEDs) inject electrons and holes that recombine into excitons; by spin statistics, 75% form triplet excitons and only 25% singlets. In conventional fluorescent emitters the triplets decay non-radiatively, capping internal quantum efficiency at 25%. Thermally activated delayed fluorescence (TADF) overcomes this with emitters whose singlet and triplet excited states are nearly degenerate, so that triplets can be thermally upconverted to emissive singlets by reverse intersystem crossing (RISC). Amplified spontaneous emission (ASE) is the single-pass light amplification that appears in an optically pumped waveguiding film above a threshold pump fluence, and a distributed feedback (DFB) laser adds a grating resonator to select narrow lasing modes. External quantum efficiency (EQE) is the ratio of emitted photons to injected charges; 'roll-off' is the loss of EQE at high current density, caused mainly by annihilation of excitons by long-lived triplets (singlet–triplet annihilation) and by polarons. In the near-infrared (NIR, here beyond 700 nm), non-radiative decay accelerates as emission red-shifts (the energy-gap law), making high quantum efficiency and gain hard to combine; most efficient NIR OLEDs have therefore used phosphorescent platinum or iridium complexes. Nakanotani et al. (Adv. Opt. Mater. 2017) reported the first light amplification from a TADF material, motivated by the fact that a future current-injection organic laser would have to live with 75% triplets in its gain medium. Continuous-wave (cw) and long-pulse operation of organic lasers is blocked by triplet accumulation, and Zhang and Forrest (Phys. Rev. B 2011) analyzed the resulting cw lasing threshold; Sandanayaka et al. (Sci. Adv. 2017) reached quasi-cw (80 MHz) and 30 ms long-pulse DFB lasing in a gain medium whose triplet absorption was shifted away from the lasing wavelength — i.e. by avoiding triplets rather than using them. Against this backdrop, Kim et al. (Nature Photonics 12, 98–104, 2018) showed that a donor–acceptor–donor (D–A–D) boron difluoride curcuminoid dye, whose strongly charge-transfer-like excited state gives both large oscillator strength and small singlet–triplet splitting, delivered NIR TADF OLEDs with EQE near 10% (emission tunable 700–780 nm by the polarity of the active medium) together with low-threshold pulsed ASE (4.7–36.7 µJ cm^-2 at 738–798 nm, per the supplementary information). The same supplementary information shows two thermally activated delayed components of 28 µs and 183 µs, attributed to two RISC channels of monomeric and dimeric species. This combination of efficient NIR electroluminescence and gain in one metal-free TADF molecule is what made the triplet-harvesting question for cw and electrically driven NIR organic lasers concrete.

Problem Statement

Can the triplet excitons that dominate near-infrared TADF gain media be converted from a loss channel into a gain channel on technologically relevant timescales, so that metal-free TADF emitters — exemplified by the donor–acceptor–donor boron difluoride curcuminoid whose 2018 demonstration combined nearly 10% EQE NIR electroluminescence with pulsed ASE — can support continuous-wave and ultimately electrically driven NIR organic light amplification? Concretely, the problem has four linked components that share this single objective: (1) demonstrate directly that RISC-based triplet harvesting (with effective upconversion times currently in the microsecond-to-hundred-microsecond range, 28 µs and 183 µs for the monomeric and dimeric channels of the curcuminoid blend) reduces light-amplification thresholds under continuous-wave or quasi-continuous-wave (long-pulse) photoexcitation, rather than being bypassed through triplet scavenging or triplet avoidance; (2) establish molecular-design principles that substantially accelerate RISC in NIR emitters while preserving the high oscillator strength, high photoluminescence yield and low self-absorption that optical gain requires; (3) suppress the severe EQE roll-off of NIR TADF OLEDs at high current density through charge- and exciton-management and device architecture, so that electrically generated singlet densities can reach the population-inversion regime; and (4) determine whether triplet management, charge balance, optical feedback and thermal stability can be combined into an electrically pumped NIR organic laser based on TADF materials. Resolving the umbrella question affirmatively turns on whether a physically validated mechanism shows the delayed (RISC-derived) singlet population participating in stimulated emission under sustained excitation, and whether the same mechanism functions under electrical injection. Each of the four components admits either polarity: a rigorously evidenced negative finding resolves that component (and, with the others, the task) just as a validated positive result would; for example, establishing that RISC-based triplet harvesting does not reduce continuous-wave or long-pulse light-amplification thresholds resolves component (1).

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