Fast Trapping Process Research Articles

Probing the Genuine Carrier Dynamics of Semiconducting Perovskites under Sunlight.

Understanding the nature of photogenerated carriers and their subsequent dynamics in semiconducting perovskites is important for the development of solar cell materials and devices. However, most ultrafast dynamic measurements on perovskite materials were conducted under high carrier densities, which likely obscures the genuine dynamics under low carrier densities in solar illumination conditions. In this study, we presented a detailed experimental study of the carrier density-dependent dynamics in hybrid lead iodide perovskites from femtosecond to microsecond using a highly sensitive transient absorption (TA) spectrometer. From the dynamic curves with low carrier density in the linear response range, we observed two fast trapping processes that occurred in less than 1 ps and tens of picoseconds, attributed to the shallow traps, and two slow decays with lifetimes of hundreds of nanoseconds and longer than 1 μs, related to the trap-assisted recombination and trapping at deep traps. Further TA measurements clearly show that PbCl2 passivation can effectively reduce both shallow and deep trap densities. These results provide insights into the intrinsic photophysics of semiconducting perovskites with direct implications for photovoltaic and optoelectronic applications under sunlight.

Carrier Transport Limited by Trap State in Cs2AgBiBr6 Double Perovskites.

Understanding the photoinduced carrier dynamics in Cs2AgBiBr6 double perovskites is essential for their application in optoelectronic devices. Herein, we report an investigation on the temperature-dependent carrier dynamics in a Cs2AgBiBr6 single crystal (SC). The time-resolved photoluminescence (TRPL) measurement indicates that the majority of carriers (>99%) decay through a fast trapping process at room temperature, and as the temperature decreases to 123 K, the population of carriers with a slow fundamental decay kinetics rises to ∼50%. We show that the carrier diffusion coefficient (theoretical diffusion length) varies from 0.020 ± 0.003 cm2 s-1 (0.70 μm) at 298 K to 0.11 ± 0.010 cm2 s-1 (2.44 μm) at 123 K. However, in spite of the long diffusion length, the population of carriers that can perform long-distance transport is restricted by the trap state, which is likely a key reason limiting the performance of Cs2AgBiBr6 optoelectronic devices.

Charge-Carrier Dynamics of Lead-Free Halide Perovskite Nanocrystals.

Lead halide perovskite nanocrystals (NCs) have been widely studied for application in optoelectronic devices due to their excellent optical properties and low-cost synthesis. However, the toxicity of lead and the poor stability of the NCs hindered their practical applications. Sn2+-based perovskite with low toxicity was first developed; however, the Sn2+-based perovskite NCs are unstable in air and oxidize easily. Recently, air-stable lead-free perovskite NCs have been developed and received increasing attention. Unfortunately, the optical and optoelectronic properties of these lead-free halide perovskite NCs are generally far worse than those of lead-perovskite NCs. Understanding the charge-carrier dynamics of semiconductors is crucial to improve their optical properties. In this Account, we mainly review our recent research progress on the study of charge-carrier dynamics in air-stable lead-free perovskite NCs. The exciton trapping followed by nonradiative recombination was the major carrier relaxation pathway and resulted in a low photoluminescence quantum efficiency (PLQE). A feasible route for passivating surface traps and tuning the self-trapped excitons from "dark" (nonradiative) to "bright" (radiative) was proposed. Through this strategy, the PLQE could be increased over 100-fold. In addition, we have compared several photophysical properties of lead-free perovskite NCs with that of lead perovskite NCs, such as charge-carrier relaxation, exciton-phonon coupling, and hot-carrier cooling. In 2017, we reported the synthesis, optical properties, and charge-carrier dynamics of Cs3Bi2X9 (X: Cl, Br, I) NCs. The Cs3Bi2Br9 NCs exhibited clear exciton trapping processes with time scales in the range of 2-20 ps. The fast trapping processes could be passivated via the use of surfactants (such as oleic acid), and the PLQE increased over 20-fold (from 0.2% to 4.5%). The low PLQE may be due to the reduced dimensionality of Cs3Bi2Br9 (2D) compared with the 3D cubic perovskite structure of CsPbBr3. We next reported double perovskite Cs2AgSb1-yBiyX6 (X: Br, Cl; 0 ≤ y ≤ 1) NCs, which exhibited a similar 3D cubic perovskite structure to that of the lead-perovskite NCs. The charge-carrier dynamics indicated that the sub-band-gap exciton trapping processes were dominated by ultrafast (∼1-2 ps) intrinsic self-trapping and trapping at surface defects (∼50-100 ps). While trapping at surface defects can be passivated using surfactants, the self-trapping processes is due to the giant carrier-phonon coupling effect. By designing direct band gap double perovskite NCs to tune the sub-band-gap trapping processes, bright dual-color emission was achieved. Furthermore, the violet PLQE could be improved to 36.6%, which is comparable to that in lead halide perovskite NCs. We hope this Account will deepen the understanding of the charge-carrier dynamics in lead-free perovskite NCs and guide the design of high-performance lead-free perovskites.

Colloidal Synthesis and Charge-Carrier Dynamics of Cs2 AgSb1-y Biy X6 (X: Br, Cl; 0 ≤y ≤1) Double Perovskite Nanocrystals.

A series of lead-free double perovskite nanocrystals (NCs) Cs2 AgSb1-y Biy X6 (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs2 AgSbBr6 NCs is a new double perovskite material that has not been reported for the bulk form. Mixed Ag-Sb/Bi NCs exhibit enhanced stability in colloidal solution compared to Ag-Bi or Ag-Sb NCs. Femtosecond transient absorption studies indicate the presence of two prominent fast trapping processes in the charge-carrier relaxation. The two fast trapping processes are dominated by intrinsic self-trapping (ca. 1-2 ps) arising from giant exciton-phonon coupling and surface-defect trapping (ca. 50-100 ps). Slow hot-carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot-carrier relaxation are also discussed.

Colloidal Synthesis and Charge‐Carrier Dynamics of Cs2AgSb1−yBiyX6 (X: Br, Cl; 0 ≤y ≤1) Double Perovskite Nanocrystals

AbstractA series of lead‐free double perovskite nanocrystals (NCs) Cs2AgSb1−yBiyX6 (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs2AgSbBr6 NCs is a new double perovskite material that has not been reported for the bulk form. Mixed Ag–Sb/Bi NCs exhibit enhanced stability in colloidal solution compared to Ag–Bi or Ag–Sb NCs. Femtosecond transient absorption studies indicate the presence of two prominent fast trapping processes in the charge‐carrier relaxation. The two fast trapping processes are dominated by intrinsic self‐trapping (ca. 1–2 ps) arising from giant exciton–phonon coupling and surface‐defect trapping (ca. 50–100 ps). Slow hot‐carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot‐carrier relaxation are also discussed.

Lead-Free, Air-Stable All-Inorganic Cesium Bismuth Halide Perovskite Nanocrystals.

Lead-based perovskite nanocrystals (NCs) have outstanding optical properties and cheap synthesis conferring them a tremendous potential in the field of optoelectronic devices. However, two critical problems are still unresolved and hindering their commercial applications: one is the fact of being lead-based and the other is the poor stability. Lead-free all-inorganic perovskite Cs3 Bi2 X9 (X=Cl, Br, I) NCs are synthesized with emission wavelength ranging from 400 to 560 nm synthesized by a facile room temperature reaction. The ligand-free Cs3 Bi2 Br9 NCs exhibit blue emission with photoluminescence quantum efficiency (PLQE) about 0.2 %. The PLQE can be increased to 4.5 % when extra surfactant (oleic acid) is added during the synthesis processes. This improvement stems from passivation of the fast trapping process (2-20 ps). Notably, the trap states can also be passivated under humid conditions, and the NCs exhibited high stability towards air exposure exceeding 30 days.

Lead‐Free, Air‐Stable All‐Inorganic Cesium Bismuth Halide Perovskite Nanocrystals

AbstractLead‐based perovskite nanocrystals (NCs) have outstanding optical properties and cheap synthesis conferring them a tremendous potential in the field of optoelectronic devices. However, two critical problems are still unresolved and hindering their commercial applications: one is the fact of being lead‐based and the other is the poor stability. Lead‐free all‐inorganic perovskite Cs3Bi2X9 (X=Cl, Br, I) NCs are synthesized with emission wavelength ranging from 400 to 560 nm synthesized by a facile room temperature reaction. The ligand‐free Cs3Bi2Br9 NCs exhibit blue emission with photoluminescence quantum efficiency (PLQE) about 0.2 %. The PLQE can be increased to 4.5 % when extra surfactant (oleic acid) is added during the synthesis processes. This improvement stems from passivation of the fast trapping process (2–20 ps). Notably, the trap states can also be passivated under humid conditions, and the NCs exhibited high stability towards air exposure exceeding 30 days.

Photoinduced Electron Transfer from Nitrogen-Doped Tantalum Oxide to Adsorbed Ruthenium Complex

To understand the photoinduced electron-transfer process and the reaction mechanism of semiconductor/metal-complex hybrid CO2-reducing photocatalysts, the excited-state dynamics of nitrogen-doped Ta2O5 (N–Ta2O5) and that adsorbed with Ru complexes (Ru/N–Ta2O5) were investigated. Upon adsorption of the Ru complex ([Ru(dcbpy)2(CO)2]2+) on N–Ta2O5 powder, one of the CO ligands was exchanged to COOH ([Ru(dcbpy)2(CO)(COOH)]+), which resulted in absorption spectral changes in UV/visible and infrared regions (dcbpy: 4,4′-dicarboxy-2,2′-bipyridine). A detailed analysis of time-resolved emission measurements after excitation at 400 nm (Ta 4f ← N 2p transition) revealed a fast trapping process from shallow defect sites to deep defect sites with a time constant of 24 ± 1 ps in N–Ta2O5 powder. In Ru/N–Ta2O5, ultrafast electron transfer from the shallow defect sites of N–Ta2O5 to the adsorbed Ru complex occurred with a time constant of 12 ± 1 ps. The values of rate constant (ket) and quantum yield (Φet) of the electro...

Ultrafast Carrier Dynamics of Titanic Acid Nanotubes Investigated by Transient Absorption Spectroscopy

Carrier dynamics of titanic acid nanotubes (phase of H2Ti2O5.H2O) deposited on a quartz plate was examined by visible/near-IR transient absorption spectroscopy with an ultraviolet excitation. The carrier dynamics of titanic acid nanotubes follows the fast trapping process which attributed to the intrinsic tubular structure, the relaxation of shallow trapped carriers and the recombination as a second-order kinetic process. Transient absorption of titanic acid nanotubes was dominated by the absorption of surface-trapped holes in visible region around 500 nm, which was proved by the faster decay dynamics in the presence of polyvinyl alcohol as a hole-scavenger. However, the slow relaxation of free carriers was much more pronounced in the TiO2 single crystals, as compared with the transient absorption spectra of titanic acid nanotubes under the similar excitation.

FIR photovoltaic effect in a boron-doped silicon structure

A new FIR photovoltaic effect, which does not depend on incident radiation intensity, is found at low temperatures in modulated unipolar boron-doped silicon structure. It is shown that the effect originates from an asymmetric ballistic hole transport across the low doped layer due to fast trapping process within the upper Hubbard impurity band of the doped layer. The analysis of the temperature dependence of the photovoltage gives the Fermi energy in the impurity band.

Ultrafast processes in laser irradiated wide bandgap insulators

Optical methods will be presented which allow a study of the charge trapping processes occurring in wide bandgap insulators submitted to a high density of electronic excitation. Interferometric measurement of the density of conduction electrons, immediately following their injection in the conduction band by an intense subpicosecond laser pulse is used to measure the time evolution of the free carrier density with a resolution of the order of 100 fs. Another method consists in time resolved measurements of specific absorption lines, which are a signature of the appearance of point defects. In the case of SiO2 the initial step is always the trapping (in 150 fs) of an electron–hole pair on one Si–O bound, forming a self-trapped exciton in its triplet state, which can subsequently decay either radiatively or into a permanent E′ center. Strong differences exist however between apparently similar materials. For instance, the extremely fast trapping processes discussed for SiO2, are not observed in two other important oxides: Al2O3 and MgO. In these cases, the electrons either do not trap, or trap into states which are very close to the conduction band, yielding a quite different signature in the interferometry experiment. Comparison of the self-trapping kinematics in SiO2 and NaCl, combined with Monte-Carlo simulations shows that the electron–phonon coupling is a decisive parameter in determining the exact nature of the trapping process.

Exciton Self-Trapping in Rare-Gas Crystals

The problem of exciton self-trapping in rare-gas solids is investigated from a dynamical point of view. Experimental ultraviolet emission spectra from rare-gas solids and liquids have been interpreted as arising from the radiative decay of an excited-state dimer which becomes localized in the substance. This embedded dimer can arise from electron capture by a self-trapped hole (R2+), or by the dynamical trapping of an exciton into an excimer state (R2*). A study of the vibrational relaxation of a molecule embedded in a solid leads to a fast trapping process in which an exciton becomes localized for all the heavy rare-gas substances. This localization is achieved within 6 × 10−12 sec by means of a resonant transfer relaxation process in which two neighboring atoms lose sufficient energy (∼ 0.5 eV) to become stabilized in a lower vibrational state of the excimer.

Self-trapping of Radiation Produced Electrons

THE problem treated here is the trapping of free electrons produced by ionizing radiation in dielectric systems. Detailed theoretical descriptions of the equilibrium properties of the solvated electron are available1–3, but its formation is still under discussion. Electrons are formed and thermalized in the coulomb field of the positive ions, but not fully recaptured by the ions. Magee4 considered the auto-ionization of the H3O radicals as a possible secondary source of electrons; Freeman5–7 assumed the electrons to be trapped in the defect sites of the medium at the instant of thermalization, while we thought8,9 that in polar dielectrics the escape of the electrons from the ions can be a free-energy decreasing, and therefore spontaneous, process. In our approach, the electrons which escape are treated as quasi-free, and the question of the kinetics and mechanism of electron trapping arises. Diffusion kinetic calculations on the radiolysis of water10,11 suggest very high initial hydrated electron densities, which require fast trapping processes.