21 research outputs found
Direct Observation of Sub-picosecond Hole Injection from Lead Halide Perovskite by Differential Transient Transmission Spectroscopy
Efficient charge separation at the interfaces between the perovskite and with
the carrier transport layers is crucial for perovskite solar cells to achieve
high power conversion efficiency. We systematically investigate the hole
injection dynamics from MAPbI perovskite to three typical hole transport
materials (HTMs) PEDOT:PSS, PTAA and NiO by means of pump-probe
transmission measurements. We photoexcite only near the MAPbI/HTM interface
or near the back surface, and measure the differential transient transmission
between the two excitation configurations to extract the carrier dynamics
directly related to the hole injection. The differential transmission signals
directly monitor the hole injections to PTAA and PEDOT:PSS being complete
within 1 and 2 ps, respectively, and that to NiO exhibiting an additional
slow process of 40 ps time scale. The obtained injection dynamics are discussed
in comparison with the device performance of the solar cells containing the
same MAPbI/HTM interfaces.Comment: 5 pages, 5 figure
Ultrafast Electron-Phonon Decoupling in Graphite
We report the ultrafast dynamics of the 47.4 THz coherent phonons of graphite
interacting with a photoinduced non-equilibrium electron-hole plasma. Unlike
conventional materials, upon photoexcitation the phonon frequency of graphite
upshifts, and within a few picoseconds relaxes to the stationary value. Our
first-principles density functional calculations demonstrate that the phonon
stiffening stems from the light-induced decoupling of the non-adiabatic
electron-phonon interaction by creating the non-equilibrium electron-hole
plasma. Time-resolved vibrational spectroscopy provides a window on the
ultrafast non-equilibrium electron dynamics.Comment: 4 pages, 4 figure
Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals
The femtosecond optical pump-probe technique was used to study dynamics of
photoexcited electrons and coherent optical phonons in transition metals Zn and
Cd as a function of temperature and excitation level. The optical response in
time domain is well fitted by linear combination of a damped harmonic
oscillation because of excitation of coherent phonon and a
subpicosecond transient response due to electron-phonon thermalization. The
electron-phonon thermalization time monotonically increases with temperature,
consistent with the thermomodulation scenario, where at high temperatures the
system can be well explained by the two-temperature model, while below
50 K the nonthermal electron model needs to be applied. As the
lattice temperature increases, the damping of the coherent phonon
increases, while the amplitudes of both fast electronic response and the
coherent phonon decrease. The temperature dependence of the damping of
the phonon indicates that population decay of the coherent optical
phonon due to anharmonic phonon-phonon coupling dominates the decay process. We
present a model that accounts for the observed temperature dependence of the
amplitude assuming the photoinduced absorption mechanism, where the signal
amplitude is proportional to the photoinduced change in the quasiparticle
density. The result that the amplitude of the phonon follows the
temperature dependence of the amplitude of the fast electronic transient
indicates that under the resonant condition both electronic and phononic
responses are proportional to the change in the dielectric function.Comment: 10 pages, 9 figures, to appear in Physical Review
Ultrafast phonon dynamics of epitaxial atomic layers of Bi on Si(111)
Ultrathin bismuth (Bi) layers on Si(111)-7×7 undergo a structural phase transformation with reducing the number of atomic layers at 3 bilayers (BL). We investigate the phonon dynamics of the Bi films close to the phase transformation by pump-probe reflectivity measurements. Coherent A1g and Eg phonons at 3 and 2 THz are clearly observed for the Bi layers with thicknesses down to 3 BL, confirming their rhombohedral crystalline structure. The A1g frequency exhibits an abrupt redshift and splits into two components at 3 BL, which are attributed to the vertical motions of Bi atoms localized at the surface and subsurface bilayers. The Eg frequency, by contrast, shows a gradual blueshift with reducing the thickness, possibly due to the lateral compressive stress at the Bi/Si interface. Below 3 BL, no coherent phonon signal is detected, in agreement with the phase transformation to the black-phosphoruslike structure. Our observations indicate that the vertical vibrations are significantly softened at 3 BL, but become almost as hard as those in the bulk crystal by adding another bilayer
Post-processing noise reduction via all-photon recording in dynamic light scattering
Dynamic light scattering (DLS) is widely used for the characterization of the size distributions of polymers and nanoparticles in dispersions. The time correlation function of the scattered light intensity can be calculated from the intensity fluctuation with time and converted into the size of the scatterers. It has been difficult to apply DLS to a dispersion containing large pollutant particles, however, because the pollutants moving in the dispersion can give rise to intense noise signals from time to time during data acquisition. In conventional DLS, this type of noise renders the entire measurement useless. Here we report a novel software-based DLS system in which the arrival times of all the scattered photons are recorded using a time-to-digital converter, and the time correlation function is calculated exclusively from the uncontaminated parts of the data in post-processing. We demonstrate the validity of this noise reduction scheme by evaluating the silica nanoparticle size in a dispersion containing a small number of micrometer-sized PMMA particles as a model contaminant