Femtosecond optical pump-probe spectroscopy with 10 fs visible pulses is employed to elucidate the ultrafast carrier dynamics of few-layer MoS2. A nonthermal carrier distribution is observed immediately following the photoexcitation of the A and B excitonic transitions by the ultrashort, broadband laser pulse. Carrier thermalization occurs within 20 fs and proceeds via both carrier-carrier and carrier-phonon scattering, as evidenced by the observed dependence of the thermalization time on the carrier density and the sample temperature. The n(-0.37 ± 0.03) scaling of the thermalization time with carrier density suggests that equilibration of the nonthermal carrier distribution occurs via non-Markovian quantum kinetics. Subsequent cooling of the hot Fermi-Dirac carrier distribution occurs on the ∼ 0.6 ps time scale via carrier-phonon scattering. Temperature- and fluence-dependence studies reveal the involvement of hot phonons in the carrier cooling process. Nonadiabatic ab initio molecular dynamics simulations, which predict carrier-carrier and carrier-phonon scattering time scales of 40 fs and 0.5 ps, respectively, lend support to the assignment of the observed carrier dynamics.

The thermal decomposition products and kinetics of two typical organic−inorganic halide perovskites, CH 3 NH 3 PbI 3 (MAPbI 3 ) and HC(NH 2 ) 2 PbI 3 (FAPbI 3 ), were investigated via simultaneous thermogravimetric analysis coupled with Fourier transform infrared spectroscopy. NH 3 and CH 3 I were verified as the major thermal decomposition gases of MAPbI 3 . Furthermore, for the first time, methane (CH 4 ) was observed as a thermal degradation product of MAPbI 3 at elevated temperatures. In contrast to conventional wisdom, (HCN) 3 (trimerized HCN) and NH 3 were demonstrated as the major gaseous decomposition products of FAPbI 3 at lower temperatures, while HCN and NH 3 became dominant at high temperatures (>360 °C). The hybrid experimental/theoretical results presented in this study will further our understanding of the perovskite decomposition mechanism and provide new insights into designing of long-term stable perovskite-based devices.

Light-emitting diodes (LEDs) based on lead halide perovskites demonstrate outstanding optoelectronic properties and are strong competitors for display and lighting applications. While previous halide perovskite LEDs are mainly produced via solution processing, here an all-vacuum processing method is employed to construct CsPbBr 3 LEDs because vacuum processing exhibits high reliability and easy integration with existing OLED facilities for mass production. The high-throughput combinatorial strategies are further adopted to study perovskite composition, annealing temperature, and functional layer thickness, thus significantly speeding up the optimization process. The best rigid device shows a current efficiency (CE) of 4.8 cd A −1 (EQE of 1.45%) at 2358 cd m −2 , and best flexible device shows a CE of 4.16 cd A −1 (EQE of 1.37%) at 2012 cd m −2 with good bending tolerance. Moreover, by choosing NiO x as the hole-injection layer, the CE is improved to 10.15 cd A −1 and EQE is improved to a record of 3.26% for perovskite LEDs produced by vacuum deposition. The time efficient combinatorial approaches can also be applied to optimize other perovskite LEDs.

Femtosecond optical pump-probe spectroscopy is employed to elucidate the band-selective ultrafast carrier dynamics of few-layer MoS 2 . Following narrowband resonant photoexcitation of the exciton A transition, the sub-picosecond to picosecond relaxation dynamics of the electron and the hole at the K valley are separately interrogated by a broadband probe pulse. The temporal evolution of the spectral first moment reveals nonexponential intravalley relaxation dynamics in the conduction band. Fluence dependence measurements suggest that this relaxation process is predominantly mediated by acoustic phonon emission. Intervalley scattering of carriers from the K valley to the extrema of the conduction and valence bands is also observed via the decay of the spectral zeroth moment. In addition, second-order Raman scattering leads to the emergence of sidebands in the normalized differential transmission spectra. The observed two-phonon energies and the fluence-dependent time constants suggest that the E 1g LO phonon and the LA phonon participate in intervalley scattering in the conduction and valence bands, respectively. Ab initio nonadiabatic molecular dynamics simulations yield time constants of 0.80 ps and 0.72 ps for intra-and intervalley electronic relaxation, respectively; the latter agrees well with experiment. Finally, the normalized differential transmission spectra reveal a two-electron shake-up satellite that originates from band-edge radiative recombination and the simultaneous excitation of a hole from K v1 to K v2 . From its spectral position, a K v1 -K v2 spin-orbit splitting of 1166  1 cm -1 is deduced. The observation of the two-electron transition points to the existence of strong electron correlation in photoexcited few-layer MoS 2 .

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