In this work, we investigate the ultrafast charge carrier dynamics in the double-layered architecture mixed halide (DHA) perovskite photovoltaic devices by ultrafast pump-probe transient absorption spectroscopy (TAS). The measured TAS results show the perovskite solar cell consists of SnO2/(FAPbI3)1-x(MAPbBr3)x/HTAB has stronger transient absorbance with photoinduced bleaching at 750 nm and photoinduced absorption in the range of 550-700 nm. The lifetime of DHA perovskite observed from TAS is approximately 46 µs in conjunction with the electron injection discovered within the first 150 fs, indicating the charge carriers would be easily extracted. Besides, we measured a high power-conversion-efficiency of the DHA perovskite solar cell (PSC) of 21%. Hence, understanding the ultrafast charge carrier dynamics in PSC by pump-probe TAS provides detailed insights into the advanced working mechanism. The results open a door for the development of high-performance perovskite photovoltaics.
We present that a thin layer of phthalocyanine (Pc) molecules can efficiently improve the optical quality of defective MoSe2 grown by CVD method. Since MoSe2 flakes are coupled with Pc molecules through van der Waals force, the structural change can be minimized, but the pristine properties can be significantly recovered. This defect healing effect can be achieved via ultrathin coverage of Pc molecules using a simple solution-mediated process. We also observed that Pc molecules with nanometer-scale lateral dimensions is useful in suppression of point defects commonly observed in mechanically exfoliated 2D TMDCs when it is improperly annealed at sub-decomposition temperatures.
We report the wavelength-dependent nonlinear absorption (NLA) of InN film grown on anr-plane sapphire by molecular beam epitaxy technique. In order to understand the nonlinear optical properties of InN, the Z-scan measurement was performed at two different wavelengths, the photon energies of which are near (resonant excitation) and much higher (non-resonant excitation) than the bandgap of InN, respectively. Under non-resonant excitation, band-filling effect leads the dominant saturable absorption, while under resonant excitation, reverse saturable absorption dominates the nonlinear absorption. From the open-aperture Z-scan measurement under resonant excitation, we found that InN exhibits more than one nonlinear absorption process simultaneously. Particularly, under relatively weak resonant excitation, the transformation from saturable absorption to 2PA through the intermediate excitation state absorption was observed as the sample approaches the beam focus. The close-aperture Z-scan signals of InN show valley-peak response, implying that the nonlinear refraction in InN is caused by the self-focusing of the Gaussian laser beam. Using the Z-scan theory, the corresponding nonlinear parameters, such as saturation intensity, 2PA coefficient, and nonlinear refractive index, of InN were estimated.
We report the carrier density dependence of carrier dynamics of Mg-doped InN (InN:Mg) films. Recently, we have
demonstrated a significant enhancement of terahertz emission from InN:Mg, which is due to the temporal evolution of
drift and diffusion currents depending on the background carrier density. We studied the details of carrier dynamics of
InN:Mg which is crucial for the clarification of the terahertz emission mechanism by performing the time-resolved
optical reflectivity measurement on InN:Mg films grown with different Mg-doping levels. Experimental analysis
demonstrates that the initial sharp drop and recovery of reflectivity response of InN:Mg films are dominated by
photocarrier-dependent bandgap renormalization and band filling processes, whereas the slow decay time constant (τ2) of
reflectivity of InN:Mg has the strong dependence on the background carrier density. As the carrier density decreases
from that of undoped InN, τ2 of InN:Mg continuously increases and reaches the maximum value at a critical value of
~1x1018 cm-3. Interestingly, the strongest terahertz radiation was observed at this carrier density and it keeps decreasing
with the increase of carrier density. Intense terahertz radiation corresponds to the fast and large spatial separation of
charged carrier density through diffusion and drift. Large spatial separation results in the longer decay time for charged
carriers to reach equilibrium after strong emission of terahertz waves, and it explains the similar carrier density
dependence of terahertz emission and τ2.
We report terahertz (THz) emission from magnesium doped a-plane indium nitride (a-InN:Mg) films with different
background carrier density, relative to the Mg-doped InN films grown along the c-axis (c-InN:Mg). Due to its high
electron affinity, as-grown InN film is typically n-type and it has extremely high background carrier density, which
causes much weaker THz emission than that from other semiconductors, such as InAs. The background carrier density of
Mg-doped InN can be widely changed by adjusting the Mg doping level. For c-InN:Mg, THz emission is dramatically
enhanced (×500 than that of undoped c-InN) as the background carrier density decreases to a critical value of ~1×1018cm-3, which is due to the reduced screening of the photo-Dember field at the lower carrier density. For a-InN, however,
intense THz emission (×400 than that of undoped c-InN) is observed for both undoped and Mg-doped a-InN and the
enhancement is weakly dependent on the background carrier density. The primary THz radiation mechanism of the aplane
InN film is found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane
electric field perpendicular to the a-axis, which effectively enhances the geometrical coupling of the radiation out of
semiconductor. The weak dependence of THz radiation on the background carrier density for a-InN shows that in-plane
surface field induced-terahertz emission is not affected by the background carrier density. Small, but apparent azimuthal
angle dependence of terahertz emission is also observed for a-InN, indicating the additional contribution of nonlinear
optical processes on terahertz emission.
We report a comprehensive study on THz emission and spectroscopy of indium nitride (InN) films and its nanorod arrays
grown by plasma-assisted molecular beam epitaxy technique. For the enhancement of THz emission from InN, we
demonstrated two method; firstly using nanorod arrays, which have large surface area for optical absorption and THz
emission, and secondly using nonpolar InN film, of which the electric field is along the sample surface. We propose that
a "screened" photo-Dember effect due to narrow surface electron accumulation layer of InN is responsible for the
nanorod-size-dependent enhancement from InN nanorods. The primary THz radiation mechanism of nonpolar InN is
found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane electric field. THz
time-domain spectroscopy has been used to investigate THz conductivity and dielectric response of InN nanorod arrays
and epitaxial film. The complex THz conductivity of InN film is well fitted by the Drude model, while the negative
imaginary conductivity of the InN nanorods can be described by using a non-Drude model, which includes a preferential
backward scattering due to defects in InN nanorods, or a Coulombic restoring force from charged defects.
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