Ultrafast THz Probe of Photoinduced Polarons in Lead-Halide Perovskites (original) (raw)
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Terahertz Modulation and Ultrafast Characteristic of Two-Dimensional Lead Halide Perovskites
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In recent years, two-dimensional (2D) halide perovskites have been widely used in solar cells and photoelectric devices due to their excellent photoelectric properties and high environmental stability. However, the terahertz (THz) and ultrafast responses of the 2D halide perovskites are seldom studied, limiting the developments and applications of tunable terahertz devices based on 2D perovskites. Here, 2D R-P type (PEA)2(MA)2Pb3I10 perovskite films are fabricated on quartz substrates by a one-step spin-coating process to study their THz and ultrafast characteristics. Based on our homemade ultrafast optical pump–THz probe (OPTP) system, the 2D perovskite film shows an intensity modulation depth of about 10% and an ultrafast relaxation time of about 3 ps at a pump power of 100 mW due to the quantum confinement effect. To further analyze the recombination mechanisms of the photogenerated carriers, a three-exponential function is used to fit the carrier decay processes, obtaining three...
Advanced materials (Deerfield Beach, Fla.), 2018
Unusual photophysical properties of organic-inorganic hybrid perovskites have not only enabled exceptional performance in optoelectronic devices, but also led to debates on the nature of charge carriers in these materials. This study makes the first observation of intense terahertz (THz) emission from the hybrid perovskite methylammonium lead iodide (CH3 NH3 PbI3 ) following photoexcitation, enabling an ultrafast probe of charge separation, hot-carrier transport, and carrier-lattice coupling under 1-sun-equivalent illumination conditions. Using this approach, the initial charge separation/transport in the hybrid perovskites is shown to be driven by diffusion and not by surface fields or intrinsic ferroelectricity. Diffusivities of the hot and band-edge carriers along the surface normal direction are calculated by analyzing the emitted THz transients, with direct implications for hot-carrier device applications. Furthermore, photogenerated carriers are found to drive coherent teraher...
Ultrafast zero-bias photocurrent and terahertz emission in hybrid perovskites
Communications Physics, 2018
Methylammonium lead iodide is a benchmark hybrid organic perovskite material used for low-cost printed solar cells with a power conversion efficiency of over 20%. Nevertheless, the nature of light-matter interaction in hybrid perovskites and the exact physical mechanism underlying device operation are currently debated. Here, we report room temperature, ultrafast photocurrent generation, and free-space terahertz emission from unbiased hybrid perovskites induced by femtosecond light pulses. The polarization dependence of the observed photoresponse is consistent with the bulk photovoltaic effect caused by a combination of injection and shift currents. Observation of this type of photocurrents sheds light on the low recombination and long carrier diffusion lengths arising from the indirect bandgap in CH 3 NH 3 PbI 3. Naturally ballistic shift and injection photocurrents may enable third-generation perovskite solar cells with efficiency exceeding the Shockley-Queisser limit. The demonstrated control over photocurrents with light polarization also opens new venues toward perovskite spintronics and tunable THz devices.
Ultrafast THz photophysics of solvent engineered triple-cation halide perovskites
Journal of Applied Physics, 2018
Solution processed thin film organic-inorganic perovskites are key to the large scale manufacturing of next generation wafer scale solar cell devices. The high efficiency of the hybrid perovskite solar cells is derived mainly from the large carrier mobility and the charge dynamics of films, which heavily depend on the type of solvent used for the material preparation. Here, we investigate the nature of conduction and charge carrier dynamics of mixed organic-inorganic cations [methylammonium (MA), formamidinium (FA), and cesium (Cs)] along with the mixed halides [iodine (I) and bromine (Br)] perovskite material [Cs 0.05 (MA 0.17 FA 0.83) 0.95 Pb(I 0.83 Br 0.17) 3 ] synthesized in different solvents using optical pump terahertz probe (OPTP) spectroscopy. Our findings reveal that carrier mobilities and diffusion lengths strongly depend on the type of solvent used for the preparation of the mixed cation perovskite film. The mixed cation perovskite film prepared using dimethylformamide/dimethylsulfoxide solvent shows greater mobility and diffusion length compared to γ-butyrolactone solvent. Our findings provide valuable insights to improve the charge carrier transport in mixed cation perovskites through solvent engineering.
Nano Letters, 2017
Colloidal all inorganic CsPbX 3 (X = Cl, Br, I) nanocrystals (NCs) have emerged to be an excellent material for applications in light emission, photovoltaics, and photocatalysis. Efficient interfacial transfer of photogenerated electrons and holes are essential for a good photovoltaic and photocatalytic material. Using time-resolved terahertz (THz) spectroscopy (TRTS) we have measured the kinetics of photogenerated electron and hole transfer processes in CsPbBr 3 NCs in presence of benzoquinone (BQ) and phenothiazine (PTZ) molecules as electron and hole acceptors, respectively. Efficient hot electron/hole transfer with a sub-300 fs timescale is the major channel of carrier transfer, thus overcomes the problem related to Auger recombination. A secondary transfer of thermalized carriers also takes place with time scales of 20-50 ps for electrons and 137-166 ps for holes. This work suggests that suitable interfaces of
Fröhlich interaction dominated by a single phonon mode in CsPbBr3
Nature Communications
The excellent optoelectronic performance of lead halide perovskites has generated great interest in their fundamental properties. The polar nature of the perovskite lattice means that electron-lattice coupling is governed by the Fröhlich interaction. Still, considerable ambiguity exists regarding the phonon modes that participate in this crucial mechanism. Here, we use multiphonon Raman scattering and THz time-domain spectroscopy to investigate Fröhlich coupling in CsPbBr3. We identify a longitudinal optical phonon mode that dominates the interaction, and surmise that this mode effectively defines exciton-phonon scattering in CsPbBr3, and possibly similar materials. It is additionally revealed that the observed strength of the Fröhlich interaction is significantly higher than the expected intrinsic value for CsPbBr3, and is likely enhanced by carrier localization in the colloidal perovskite nanocrystals. Our experiments also unearthed a dipole-related dielectric relaxation mechanism...
The exploration of new physical properties for various THz-based applications, such as THz-wave sensing, modulation, and imaging devices, is a key challenge in the research on organic-inorganic hybrid perovskite materials. These THzbased applications require satisfactory, sensitive, and stable absorption properties with values between 0.5 and 3 THz. To achieve these properties, candidate materials should possess a purified structure that induces regular and fixed phonon modes without any defects or impurities. CH 3 NH 3 PbBr 3 , an organic-inorganic hybrid perovskite thin film produced by a sequential vacuum evaporation method on a flexible PET substrate, was investigated in this study. Although the thin film contains only molecular defects related to CH 3 NH 2 incorporated into the perovskite structure, our THz-wave absorption measurement and first-principles simulation confirmed that these molecular defects do not influence the three phonon modes originating from the transverse vibration (0.8 THz), the longitudinal optical vibrations (1.4 THz) of the Pb-Br-Pb bonds, and the optical Br vibration (2.0 THz). After spin-casting an ultrathin PTAA polymer protective layer (5 nm) on the hybrid perovskite thin film, it was additionally observed that there was no significant effect on the phonon modes. Thus, this novel flexible organic-inorganic hybrid perovskite material is a potential candidate for THz-based applications.
The Journal of Chemical Physics, 2020
Lead halide perovskites are promising materials for optoelectronic applications because of their exceptional performances in carrier lifetime and diffusion length; however, the microscopic origins of their unique characteristics remain elusive. The organic–inorganic hybrid perovskites show unique dielectric functions, i.e., ferroelectric-like phonon responses in the 0.1–10 THz region and liquid-like rotational relaxation in the 1–100 GHz range. To reveal the role of the dielectric responses is of primal importance because the dielectric screening is a key to understanding the optoelectronic properties governed by polarons in the perovskites. Here, we conducted comparative studies of broadband dielectric spectroscopy on both all-inorganic CsPbBr3 and organic–inorganic hybrid (CH3NH3)PbBr3 single crystals to uncover the origin of the liquid-like dielectric relaxation in the 1–100 GHz range. We confirmed the absence of the dielectric response in the range of 106–1010 Hz in CsPbBr3, whi...
Polaron mediated long-lived bandedge carriers in lead halide perovskite nanocrystals
NATIONAL CONFERENCE ON PHYSICS AND CHEMISTRY OF MATERIALS: NCPCM2020
The lead halide perovskites (LHPs), (APbX3, A = Cs, MA, or FA and X = I, Br, or Cl) are well known for moderate carrier mobility, static and dynamic structural disorder. Despite this, LHPs exhibited unusual fascinating properties, including long carrier lifetime, diffusion length and better defect tolerance. This aroused great interest in scientific community to exploit many-body interactions in LHP. It is neither free carrier nor exciton, however, large polaron that derive and manipulate many-body interaction in these systems. These large polarons screens the coulombic potential which is responsible for tuning the exotic properties for optoelectronic applications. A polaron is a carrier dressed with polarized soft lattice, formed due to structural distortion in inorganic framework PbX3. We employ ultrafast pump-probe spectroscopy to study the dynamics of polaron in FAPbI3 nanocrystals (NCs) at low fluence targeting its practical commercialization in solar cells.