Nonradiative Recombination Of Electrons Research Articles

Trapped-Hydrogen-Induced Energy Loss in Tin-Based Hybrid Perovskite Solar Cells

Tin halide perovskites present outstanding optoelectronic properties and great application potential, without the toxicity of lead. Here a systematic first-principles investigation reveals that a high-density defect complex, consisting of a tin vacancy plus a hydrogen molecule (${V}_{\mathrm{Sn}}$--H${}_{2}$), is a highly effective center for nonradiative recombination of electrons and holes in this semiconductor. That would explain the experimentally observed significant nonradiative loss in devices based on formamidinium tin triiodide. Therefore, the passivation of this defect complex is expected to improve the performance of tin-based perovskite solar cells and other optoelectronic devices.

Thickness-dependent ultrafast hot carrier and phonon dynamics of PtSe2 films measured with femtosecond transient optical spectroscopy

We systematically investigate the ultrafast hot carrier and phonon dynamics of PtSe2 with various van der Waals (vdW) layer numbers using femtosecond transient optical spectroscopy. The hot carrier dynamics of PtSe2 is found to be strongly dependent on its thickness. For semiconducting PtSe2 films (≤4 vdW layers), electron–phonon coupling, interlayer charge transfer and nonradiative recombination of electrons and holes contribute to the hot carrier decay, while electron–phonon coupling dominates the carrier decay in metallic PtSe2 films (with vdW layers). The PtSe2 films with 2 vdW layers show the most pronounced interlayer charge transfer decay process and are thus most suitable for photoelectric sensors and energy harvesting devices. The phonon dynamics of PtSe2 is also thickness dependent; its LB mode phonon frequency decreases as the number of vdW layers increases, as a result of the thickness-dependent dielectric screening of the long-range Coulomb interaction.

Molecular Ferroelectrics-Driven High-Performance Perovskite Solar Cells.

The nonradiative recombination of electrons and holes has been identified as the main cause of energy loss in hybrid organic-inorganic perovskite solar cells (PSCs). Sufficient built-in field and defect passivation can facilitate effective separation of electron-hole pairs to address the crucial issues. For the first time, we introduce a homochiral molecular ferroelectric into a PSC to enlarge the built-in electric field of the perovskite film, thereby facilitating effective charge separation and transportation. As a consequence of similarities in ionic structure, the molecular ferroelectric component of the PSC passivates the defects in the active perovskite layers, thereby inducing an approximately eightfold enhancement in photoluminescence intensity and reducing electron trap-state density. The photovoltaic molecular ferroelectric PSCs achieve a power conversion efficiency as high as 21.78 %.

Molecular Ferroelectrics‐Driven High‐Performance Perovskite Solar Cells

AbstractThe nonradiative recombination of electrons and holes has been identified as the main cause of energy loss in hybrid organic–inorganic perovskite solar cells (PSCs). Sufficient built‐in field and defect passivation can facilitate effective separation of electron–hole pairs to address the crucial issues. For the first time, we introduce a homochiral molecular ferroelectric into a PSC to enlarge the built‐in electric field of the perovskite film, thereby facilitating effective charge separation and transportation. As a consequence of similarities in ionic structure, the molecular ferroelectric component of the PSC passivates the defects in the active perovskite layers, thereby inducing an approximately eightfold enhancement in photoluminescence intensity and reducing electron trap‐state density. The photovoltaic molecular ferroelectric PSCs achieve a power conversion efficiency as high as 21.78 %.

Temperature dependent optical characteristics of all-inorganic CsPbBr3 nanocrystals film

As one of the most promising optoelectronic materials in recent years, colloidal all-inorganic lead halide perovskite nanocrystals (NCs) have become a research focus. An in-depth understanding of electron behavior in the material is essential for advanced device application. Here, temperature-dependent photoluminescence (PL) spectroscopy is employed to study the optical properties of CsPbBr3 NCs film. It is found that the PL intensity decreased with temperature below 200 K, while reverse trend has been observed with temperature above 200 K. The mechanism of the PL unusual quenching phenomenon is attributed to surface states of NCs, which leads to non-radiative recombination of electrons at low temperatures. Above the critical temperature, electrons will escape from the defect level and participate in radiative recombination, which give rise to the enhanced emission. The model is further verified by comparison with nanoplatelets, surface passivation, as well as temperature-dependent PL decay experiments.

Magnetic field improved photoelectrochemical synthesis of 5,5′-azotetrazolate energetic salts and hydrogen in a hematite photoanode-based cell

The oxidative-coupling of 5-amino-1H-tetrazole (5AT) into 5,5′-azotetrazolate is the initial and critical step for the synthesis of 5,5′-azotetrazolate energetic salts. Unfortunately, there are several disadvantages of insufficient security and inconvenient purification in the conventional chemical-oxidation method of 5AT-coupling. Here, we report the solar simultaneous initiation of 5AT oxidative-coupling and hydrogen evolution in a photoelectrochemical (PEC) cell that consisted of a Ti doped Fe2O3 film (Ti–Fe2O3) photoanode and a Pt wire cathode. A stable Faradaic efficiency of 65% for the 5AT oxidative-coupling into 5,5′-azotetrazolate was achieved on the Ti–Fe2O3 film photoanode at room temperature. Simultaneously, hydrogen evolution was initiated on the Pt wire cathode by the photogenerated electrons of Ti–Fe2O3 film photoanode with about 99% Faradaic efficiency. Our investigations indictaed that the 5AT oxidative-coupling on the Ti–Fe2O3 photoanode is a holes-driven reaction which is kinetically favorable relative to water oxidation. Since the non-radiative recombination of electrons and holes on the Ti–Fe2O3 film photoanode could be reduced in the presence of a magnetic field, a higher synthesis efficiency of sodium 5,5′-azotetrazolate and hydrogen was achieved in the PEC cell under a magnetic field of 0.5 T. The present work provides a green approach to produce 5,5′-azotetrazolate salts and H2 simultaneously using solar energy with high efficiency.

Enhancing the photocatalytic hydrogen evolution of copper doped zinc sulfide nanoballs through surfactants modification

In this study, copper-doped zinc sulfide nanoballs were successfully synthesized in DI water and ethanol solvent by a sonochemical approach using surfactants in aqueous medium, such as citric acid (CIT), sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), cetylpyridinium bromide (CPBr), polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP), and polyvinyl butyral (PVB). Since surfactants are usually organic compounds that are amphiphilic molecules and surface-active agents with unique properties stemming from their exclusive structure with a hydrophobic tail and hydrophilic head, they can self-organize to form colloidal aggregates of different morphologies. TEM images show that copper doped ZnS crystallites without surfactant have a hollow-sphere-like self-assembled nanostructure. When 2.0Cu/ZnS was prepared with CTAB surfactant, it had a flower-like microspheres. The other surfactants would make copper-doped zinc sulfide exhibit the nanospheric structure. The surfactant plays an important role on the transfer of photogenerated electrons and holes and prevents non-radiative recombination of electrons and holes at surface sites. Therefore, surfactants could significantly improve the photocatalytic activity. Hydrogen evolution from an aqueous solution containing 0.1 M Na2S at pH 3 and 0.15 g L−1 of 2.0Cu/ZnS photocatalyst with PVB surfactant had the maximum of 1137.5 μmol h−1 g−1.

New Route for Synthesis of Fluorescent SnO₂ Nanoparticles for Selective Sensing of Fe(III) in Aqueous Media.

A simple new route for synthesis of fluorescent SnO2 and its application as an efficient sensing material for Fe3+ in aqueous media is reported. The fluorescent SnO2 nanoparticles were obtained by oxidation of SnCl2, which when used as reducing agent for the reduction of organic nitro compounds to corresponding amino compounds in ethanol. The SnO2 nanoparticles have been characterized on the basis of powder-XRD, IR, UV-Vis, TEM, FESEM and EDX analysis and found that this material is highly fluorescent in aqueous media. Detail study revealed that this material functions as a selective probe for Fe3+ out of a large number of metal ions used. The oxygen vacancies (defects) generated on the surface of the SnO2 during synthesis, are the source of emission due to recombination of electrons with the photo-excited hole in the valance bond. The quenching of emission intensity in presence of Fe3+ is due to the nonradiative recombination of electrons and holes at the surface. This material is used for estimation of Fe3+ in real samples such as drinking water, tap water and soil.

Effects of nitrogen impurities on the microstructure and electronic properties of P-doped Si nanocrystals emebedded in silicon-rich SiNx films

Phosphorus doped Si nanocrystals (SNCs) emebedded in silicon-rich SiNx:H films were prepared using plasma enhanced chemical vapor deposition technique, and the effects of nitrogen incorporation on the microstructure and electronic properties of the thin films have been systematically studied. Transmission electron microscope and Raman observation revealed that nitrogen incorporation prevents the growth of Si nanocrystals, and that their sizes can be adjusted by varying the flow rate of NH3. The reduction of photoluminescence (PL) intensity in the range of 2.1–2.6 eV of photon energy was observed with increasing nitrogen impurity, and a maximal PL intensity in the range 1.6–2.0 eV was obtained when the incorporation flow ratio NH3/(SiH4+H2+PH3) was 0.02. The conductivity of the films is improved by means of proper nitrogen impurity doping, and proper doping causes the interface charge density of the heterojunction (H-J) device to be lower than the nc-Si:H/c-Si H-J device. As a result, the proper incorporation of nitrogen could not only reduce the silicon banding bond density, but also fill some carrier capture centers, and suppress the nonradiative recombination of electrons.

In English

Oxidized macroporous silicon structures with CdS surface nanocrystals have been proposed to enhance the photoluminescence of CdS nanoparticles due to reducing the electron recombination outside the nanoparticle layer. It has been found that the resonance electron scattering on the Si–SiO 2 interface for samples with low concentration of Si–O–Si states transforms into scattering on ionized surface states for samples with high concentration of Si–O–Si in the structured oxide. The maximum intensity of photoluminescence was measured for a structure with the maximal strength of the local electric field at the interface of silicon matrix with the structured oxide. It indicates a significant decrease of non-radiative recombination of electrons generated in CdS nanocrystal layer due to the counter flow of electrons from the silicon matrix towards the nanocrystals layer. The quantum yield of photoluminescence increases with time due to evaporation of water molecules.

Model of the electrical breakdown channel in ionic crystals

A model is proposed for the propagation of the breakdown channel in a NaCl single crystal taking into account the mechanism of charge carrier generation via cascade Auger transitions. The formation of the gas phase of the breakdown channel based on the heat release during nonradiative recombination of electrons and holes is considered.

Some aspects of radiation resistance of wide-gap metal oxides

Wide-gap oxides drastically differ in radiation resistance against nonimpact mechanisms of defect creation depending on the ratio between the values of the energy gap E g and the formation energy of a pair of Frenkel defects (FD) E FD . Materials with E g > E FD are radiation-sensitive even at a low excitation density, while the efficiency of FD creation in the materials with E g < E FD is noticeable only under a high excitation density or in the presence of impurity centers serving as the promoters of radiation damage due to the nonimpact mechanisms. Novel experimental results on the FD creation in the bulk of MgO single crystals ( E g < E FD ) irradiated by swift uranium ions at 300 K and 5 keV electrons at 6 K are presented. The prospects of luminescent protection against radiation damage as well as of the decrease of the luminescence efficiency due to the suppression of nonradiative recombination of electrons and holes (both relaxed and nonrelaxed) by doping the material with a sufficient amount of luminescent impurity ions are considered on the example of spectral transformers for plasma display panels.

Non-linear optical response and relaxation dynamics in double-walled carbon nanotubes

Relaxation dynamics of photoexcited carriers in individually suspended double-walled carbon nanotubes in solution have been investigated using femtosecond pump–probe spectroscopy. The average diameters of inner and outer tubes are 1.4 and 2.2 nm, respectively. The absorption spectrum reveals distinct band-to-band transitions in nanotubes with different diameters and chiralities. The differential absorption spectra measured by the pump–probe experiments show absorption bleaching and two-component decay behavior. The fast and slow components indicate carriers relaxation between the inner and outer tubes and non-radiative recombination of electrons and holes at the bottoms of the bands, respectively.

Breakdown initiating mechanisms at electrode interfaces in liquids

Two hitherto neglected mechanisms, which occur at the interface between insulating liquids and metal electrodes under high electrical fields, are considered and shown to be significant for the initiation of breakdown. One involves the Lippmann effect in which the electrical fields of the double layers at the electrodes reduce the interfacial tension and lead to the generation of low-density microcavities on the electrode surfaces. The other is the Auger effect in which non-radiative recombination of electrons and positive holes across the large energy gap between these states leads to secondary electrons of high energy. The coupling between these two mechanisms is expected to be highly conducive to streamer initiation at the electrodes.

Nonradiative recombination and kinetics of optically oriented electrons at the GaAs/AlGaAs interface

It is shown that optical orientation of electron spins in semiconductors can be used as a basis to develop a high-sensitivity method for measuring the dependence of the lifetime of carriers on their concentration. Experiments performed in a stationary regime on a GaAs/AlGaAs heterostructure at low excitation levels provided insight into the nonradiative recombination of electrons and holes separated by an electric field built into the interface.

Hole-electron mechanism of F-H pair generation in RbCl crystals with impurity electron traps

Production of F, Cl 3 − , Ag0, and Tl0 centers in RbCl:Ag and RbCl:Tl crystals by photons having energies ranging from 5 to 10 eV has been studied at 295 and 180 K. It is shown that creation of near-impurity excitations is accompanied by formation of F centers localized in the vicinity of Ag+ and Tl+ ions. F centers are produced in direct optical generation of self-trapped excitons. In addition to the well-known mechanism of F-H pair production in nonradiative recombination of electrons with self-trapped holes, a hole-electron process has been revealed for the first time to operate in RbCl:Ag having deep electron traps. By this mechanism, F-H pairs appear in the following sequence of stages: thermally stimulated unfreezing of hopping diffusion of self-trapped holes (V K centers), tunneling electron transfer from Ag0 to the approaching V K centers, and subsequent nonradiative decay of triplet self-trapped excitons near Ag+ ions.

Light-Induced Annealing of Dangling Bonds in a-Si:H

We have investigated the kinetics of light-induced defect (dangling bond) creation and annealing processes in a-Si:H containing a large amount of hydrogen at 300 K and 77 K using the ESR technique. We have obtained direct evidence for the light-induced annealing of dangling bonds at 300 K. A model, in which nonradiative recombination of electrons and holes at hydrogen-related dangling bonds is taken into account, is presented to interpret the experimental results.

Spatial Distribution of EL2 in GaAs Wafer Determined by Photoacoustic Spectroscopy

Photoacoustic (PA) measurements were carried out at 95 K for semi-insulating (SI) GaAs. A broad peak at 0.92 eV and humps in the higher-energy region are observed. Onsets of the humps are at 1.04 and 1.25 eV. Among these structures in the PA spectra, the broad peak at 0.92 eV is partly photoquenched upon illumination of a secondary light (hν=1.12 eV) and thermal recovery occurs after heating to around 150 K. The spatial profiles of this photoquenching along the wafer diameter show a W-shaped distribution. These results indicate that the quenching component of the peak at 0.92 eV in the PA spectra is due to nonradiative recombination of electrons excited from the EL2 level to the Γ-conduction band minimum. The electron transitions involving the EL2 level can be resolved by the PA measurements for the first time.

Decay of Cathodoluminescence and Nonradiative Processes in Manganese Activated Phosphors

Cathodoluminescence emission spectra and decay processes have been investigated as a function of primary electron energy and density for single crystals, evaporated layers, and powder layers of manganese activated calcium fluoride and zinc silicate. The emission spectra were found to be unchanged by the conditions of excitation. The value of the decay constant for powder layers was also unchanged, but that for single crystals and evaporated layers increased as the primary energy was decreased. This behavior is explained in terms of an Auger recombination leading to an interaction between the luminescence centers and the high density of electrons in the conduction band at low primary voltages. It is shown that for such an effect to occur the rate of nonradiative recombination of electrons in the conduction band of the host lattice must be small. This criterion is satisfied in the case of single crystals and evaporated layers, but not in microcrystalline powder layers.

Theory of the non-radiative recombination of electrons at perturbation centres in polar crystals

Non-radiative recombination of electrons at a perturbation centre in a polar crystal is considered as a single phonon transition of a polaron in a non-adiabatic approximation from the continuous spectrum to the discrete excited state in a perturbation centre. The perturbation causing a transition is taken as the interaction of a polaron with a “free” phonon field leading to the production and disappearance of phonons in the neighbourhood of the perturbation centre. Calculation is carried out of the recombination coefficient and the effective cross-section for recombination as a function of the temperature for perturbation centres of the Coulomb type in crystals of ZnS.