Optically pumped continuous-wave (CW) lasing was first stabilized at room temperature in quasi-two-dimensional (quasi-2D) lead-halide perovskite films by Qin et al. (Nature 585, 53–57, 2020), who attributed the long-standing 'lasing death' phenomenon to accumulation of long-lived, oxygen-sensitive triplet excitons that quench the emissive singlet population via singlet–triplet annihilation (STA), and who mitigated it by 'triplet management' (environmental oxygen and transfer to low-triplet-energy organic cations) combined with a high-quality-factor distributed-feedback cavity. The work itself, however, left the underlying triplet physics and the generality of the cure open: the microscopic electronic structure, spin character, and spatial localization of the triplet states remain unsettled; triplet management has not been proven across compositions, architectures, and electrical injection; long-term operation beyond roughly an hour and the kinetics of oxygen/triplet quenching in encapsulated devices are unquantified; and no quantitative model predicts CW lasing thresholds from triplet kinetics, STA cross-sections, cavity modal properties, and exciton diffusion. Later work (2021–2026) has deepened parts of this picture — ultrafast spectroscopy locating triplet formation on quantum-confined grains, additional room-temperature CW and electrically driven (pulsed or polariton) perovskite lasers — but all four facets retain open cores, so the problem remains open.

Background

Metal-halide perovskites are solution-processable semiconductors of general formula ABX3ABX_3 (e.g. CsPbBr3\mathrm{CsPbBr_3} or methylammonium lead bromide) that combine high optical gain coefficients with wavelength tunability, making them attractive gain media for lasers. 'Quasi-2D' (Ruddlesden–Popper-type) perovskite films are mixtures of layered phases A′2An−1PbnX3n+1\mathrm{A'}_2\mathrm{A}_{n-1}\mathrm{Pb}_n\mathrm{X}_{3n+1} with varying inorganic layer number nn: large-bandgap, quantum-confined thin layers (n=1,2,…n=1,2,\dots) absorb excitons that funnel down an energy gradient to lower-bandgap, high-nn grains, which act as the emitting and gain sites. This energy funnelling suppresses trap-mediated losses but, as discovered in the source work, the quantum-confined slabs also stabilize triplet excitons.

A triplet exciton is a spin-forbidden, typically dark and long-lived bound electron–hole pair, as opposed to the optically allowed singlet exciton. In these films, triplets are created by intersystem crossing (ISC, the spin-flip conversion of a singlet into a triplet) and can return by reverse ISC; they are quenched by molecular oxygen. Because triplets live far longer than singlets, under continuous-wave (CW) pumping they accumulate and destroy population inversion (the condition of having more emitting than absorbing states required for optical gain) mainly through singlet–triplet annihilation (STA), a quenching process in which a triplet meets a singlet and drains the emissive population. The abrupt termination of lasing under CW pumping despite sufficient pump power is called 'lasing death'. Triplet energy can also be passed to an organic cation of lower triplet energy by Dexter energy transfer, a short-range, exchange-mediated transfer mechanism — the basis of one 'triplet management' strategy, along with deliberate use of environmental oxygen as a triplet quencher.

Before 2020, room-temperature CW lasing in lead-halide perovskites had not been convincingly achieved. An early CW claim in a 3D perovskite (Jia et al., Nat. Photon. 11, 784–788, 2017) was followed by an analysis by the same group (Jia et al., Adv. Opt. Mater. 8, 1901514, 2020) attributing CW lasing failure in MAPbI3\mathrm{MAPbI_3} distributed-feedback (DFB) lasers to pump-induced heating and accumulation of photoinduced defects, quantified by a rate-equation model — an explanation that does not involve triplets and concerns 3D, not quasi-2D, films. Separately, bright triplet excitons had been characterized in caesium lead halide perovskite nanocrystals (Becker et al., Nature 553, 189, 2018), and the source group had introduced triplet management for perovskite light-emitting diodes (Qin et al., Nat. Photon. 14, 70, 2020).

Qin et al. (2020) studied quasi-2D films with two organic cations — phenethylammonium (PEA, whose high triplet energy blocks triplet transfer so triplets stay in the inorganic lattice) and naphthylmethylammonium (NMA, whose lower triplet energy accepts triplets by Dexter transfer) — observed that long-lived triplets impede amplified spontaneous emission (ASE; stimulated emission without a resonant cavity) and pulsed/CW lasing, proposed STA as 'a possible intrinsic mechanism' of lasing death, and achieved stable green CW lasing at room temperature in air by combining a high-quality-factor (high-QQ) DFB grating cavity with triplet management. The reported continuous operation was roughly one hour at 2 kW cm−22\,\mathrm{kW\,cm^{-2}}, and the authors framed the result as a step toward current-injection (electrically pumped) perovskite lasers, which were and remain unrealized in the conventional, room-temperature CW sense.

Problem Statement

The problem is to place stable room-temperature continuous-wave lasing in quasi-2D lead-halide perovskite films on a predictive, general footing by resolving the triplet-exciton physics that the 2020 demonstration identified but did not settle. Concretely, building on Qin et al. (Nature 585, 53–57, 2020), in which singlet–triplet annihilation (STA) from accumulated long-lived triplet excitons was proposed as a possible intrinsic cause of 'lasing death' and triplet management (environmental oxygen; Dexter transfer to low-triplet-energy organic cations) enabled roughly one hour of stable green CW lasing in air, four linked questions remain: (1) determine the microscopic nature of the triplet states in lead-halide perovskites — their exact electronic structure, spin character, energetics, and spatial localization, and how these vary with quasi-2D layer thickness and composition; (2) establish whether the STA-based lasing-death mechanism and the triplet-management mitigation strategies generalize beyond the studied compositions and cavity architectures — a question on which a negative result refutes generality while a positive result on a bounded test on at least one further quasi-2D composition and one further cavity architecture supports it without establishing it — and, in particular, whether they carry over under electrical injection, judged in the electrically driven regimes that exist (pulsed or polariton) rather than against the not-yet-demonstrated room-temperature CW electrically pumped conventional lasing; (3) quantify the long-term operational stability of CW quasi-2D perovskite lasers under realistic device conditions — continuous operation beyond the reported hour, varying humidity and temperature, electrical pumping — including the kinetics of oxygen penetration and triplet quenching in encapsulated devices, and use this to optimize encapsulation; and (4) develop a quantitative model that predicts CW lasing thresholds in quasi-2D films from triplet kinetics, STA cross-sections, cavity modal properties, and exciton diffusion. Together they serve one objective: making triplet-managed room-temperature CW lasing in quasi-2D perovskites — and ultimately electrically injected perovskite lasing — robust, general, and predictable rather than a single demonstrated instance.

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