mixing or exchange of cations or anions, covering the complete visible and near-IR wavelength region; (iii) they show lasing at carrier densities as low as 10 16 cm −3 , which is two orders of magnitude lower than those in conventional semiconductor NWs. [9] However, all lead halide perovskite NW lasers demonstrated thus far are limited to pulsed operation. While continuous wave (CW) lasing is vital to achieving electrically injected NW lasers and to applications such as optical communication and spectroscopy, [14,15] achieving CW lasing in lead halide perovskite NWs was thought to be exceptionally challenging due to thermal damage and to screening of the exciton resonance. [16,17] The latter leads to a reduction in oscillator strength and gain at high excitation densities necessary for population inversion. [16,17] However, lasing in a NW cavity may not necessarily require population inversion. Coupling between the exciton resonance and light is enhanced in a NW due to the reduced mode volume of the photons and the cavity enhanced oscillator strength through the relation Ω ∝ / f V , where Ω is the vacuum Rabi splitting; f is the oscillator strength; and V is the mode volume. [18] In this regime, coherent light emission can originate from the steady-state leakage of an exciton-polariton condensate below the threshold for population inversion. [19] An exciton-polariton, or polariton for short, is a bosonic quasiparticle formed by the superposition of strongly coupled exciton and photon states, which generates an upper and lower polariton branch (UPB and LPB, respectively) from the avoided crossing of the two dispersions. [20,21] The LPB is exciton like at high momentum (k) and photon like at lower k (see Figure S1 in the Supporting Information). Consequentially, polaritons relax along the LPB by acoustic phonon emission and accumulate near the avoided crossing, i.e., the polariton bottleneck, due to the reduced lifetimes and density-of-states in the photon-like region at lower energies. [21] Polaritons undergo Bose-stimulated scattering, which surpasses spontaneousscattering at a critical density to produce the coherent condensate state [22] and the light leaking out of the cavity from such a coherent state has been called polariton lasing. [20,21] Condensation in the bottleneck region was observed in CdTe microcavities as a ring of emission in angle-resolved fluorescence measurements. [23,24] The NW cavity differs from the planar microcavity. While both are Fabry-Perot cavities defined by end facets in a NW [7,8,13] or distributed Bragg reflectors (DBRs) in Lead halide perovskite nanowires (NWs) have been demonstrated in pulsed lasing with high quantum yields, low thresholds, and broad tunability. However, continuous-wave (CW) lasing, necessary for many optoelectronic applications, has not been achieved to date. This is thought to be due to many-body screening, which reduces the excitonic resonance enhancement of the oscillator strength at high excitation densities necessary for population inversion. Here CW lasi...

In conventional semiconductor solar cells, carriers are extracted at the band edges and the excess electronic energy (E*) is lost as heat. If E* is harvested, power conversion efficiency can be as high as twice the Shockley-Queisser limit. To date, materials suitable for hot carrier solar cells have not been found due to efficient electron/optical-phonon scattering in most semiconductors, but our recent experiments revealed long-lived hot carriers in single-crystal hybrid lead bromide perovskites. Here we turn to polycrystalline methylammonium lead iodide perovskite, which has emerged as the material for highly efficient solar cells. We observe energetic electrons with excess energy ⟨E*⟩ ≈ 0.25 eV above the conduction band minimum and with lifetime as long as ∼100 ps, which is 2-3 orders of magnitude longer than those in conventional semiconductors. The energetic carriers also give rise to hot fluorescence emission with pseudo-electronic temperatures as high as 1900 K. These findings point to a suppression of hot carrier scattering with optical phonons in methylammonium lead iodide perovskite. We address mechanistic origins of this suppression and, in particular, the correlation of this suppression with dynamic disorder. We discuss potential harvesting of energetic carriers for solar energy conversion.

Lead halide perovskites (LHPs) are solution processable semiconductors characterized by long carrier lifetimes. Recent studies have suggested that electrons and holes in LHPs interact with phonons to form large polarons on subpicosecond time-scales and polaron formation may also slow down hot carrier cooling. Using femtosecond time-resolved two-photon photoemission (TR-2PPE) and transient reflectance (TR) spectroscopies, we follow the initial electron cooling and polaron formation dynamics in single-crystal CsPbBr 3 perovskite. We find that the hot electrons cool down initially (≤0.2 ps) with rates of −0.64 ± 0.06 eV/ps and −0.82 ± 0.08 eV/ps at 300 and 80 K, respectively. This weakly temperature-dependent rate is attributed to the initial relaxation of unscreened hot electrons by the emission of longitudinal optical (LO) phonons. On longer time scales, we observe dynamic changes in the photoemission cross-section and in the red-shift of the optical bandgap. We attribute these dynamic changes to large polaron formation from electron−LO phonon interaction, with temperature-dependent polaron formation time constants of τ p = 0.7 ± 0.1 and 2.1 ± 0.2 ps at 300 and 80 K, respectively. The increase in polaron formation rate with temperature is correlated with the broadening in phonon resonances, suggesting that phonon disorder and dephasing facilitate large-polaron formation. The large polaron formation rate is not competitive with the cooling rate of unscreened hot electrons in CsPbBr 3 , in contrast to hybrid CH 3 NH 3 PbBr 3 (or CH 3 NH 3 PbI 3 ) where the two rates are similar. This contrast explains the observation of long-lived hot carriers in the latter but not the former.

The radiationless recombination of electron-hole pairs in semiconductors is detrimental to optoelectronic technologies. A prominent mechanism is Auger recombination, in which nonradiative recombination occurs efficiently by transferring the released energy-momentum to a third charge carrier. Here we use femtosecond photoemission to directly detect Auger electrons as they scatter into energy and momentum spaces from Auger recombination in a model semiconductor, GaSb. The Auger rate is modulated by a coherent phonon mode at 2 THz, confirming phonon participation in momentum conservation. The commonly assumed Auger rate constant is found not to be a constant, but rather decreases by 4 orders of magnitude as hot electrons cool down by ∼90  meV. These findings provide quantitative guidance in understanding Auger recombination and in designing materials for efficient optoelectronics.

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