Time-resolved Microwave Photoconductivity Research Articles

Analysis of defects dominating carrier recombination in CeO2 single crystal for photocatalytic applications

In photocatalytic water splitting and CO2 reduction, recently, cerium oxide (CeO2) has been widely investigated. Defects that can directly dominate carrier recombination are essential for the photocatalytic performance of CeO2. In this study, several photoluminescence (PL) peaks are observed on the (100) face of an undoped CeO2 single crystal, indicating the presence of defects. Moreover, we characterize carrier recombination using time-resolved photoluminescence (TR-PL) and microwave photoconductivity decay (μ-PCD) measurements. The temperature dependence of decay curves is the result of carrier trapping and emission at deep levels. These decay curves are observed separately using a 565 nm band-pass filter (BPF) based on the PL spectra. The trap energy level ( ET ) and majority carrier capture cross-section ( σT ) of each defect are also analyzed using rate equations to fit the experimental results. The temperature-dependent time constants are well reproduced by a recombination model using hole traps HTinfrared and HTvisible at ET of 0.76 and 0.55 eV from the conduction or valence band, with estimated σT of the order of 10−21 and 10−22 cm2 for without and with the BPF, respectively. These findings regarding the presence of multiple defects in a single crystal indicate the necessity to focus on defect control.

Synthesis and crystal and defect structures of polycrystalline Cu2SrSnS4 and Cu1.9SrSnS4 powders

The polycrystalline samples of Cu2 – δSrSnS4 with a trigonal structure were prepared by solid-phase ampoule synthesis with a long annealing time (1944 h), and their crystal lattice parameters were refined and Raman spectra were obtained. For the first time, the effect of defect structure on the lifetimes of photogenerated current carriers in the polycrystalline powders was examined by combining time-resolved microwave photoconductivity and cathodoluminescence methods. A significant increase in the lifetimes during the conversion of Cu2SrSnS4 into Cu1.9SrSnS4 was found, which was probably due to changes in the defect structure.

Charge Carrier Recombination Dynamics in MAPb(BrxCl1-x)3 Single Crystals.

Understanding carrier recombination processes in MAPb(BrxCl1-x)3 crystals is essential for their photoelectrical applications. In this work, carrier recombination dynamics in MAPb(BrxCl1-x)3 single crystals were studied by steady-state photoluminescence (PL), time-resolved photoluminescence (TRPL), and time-resolved microwave photoconductivity (TRMC). By comparing TRPL and TRMC, we find TRPL of MAPb(BrxCl1-x)3 (x < 0.98) single crystals is dominated by a hole trapping process while the long-lived component of TRMC is dominated by an electron trapping process. We also find both electron and hole trapping rates of MAPb(BrxCl1-x)3 (x < 0.98) crystals decrease with an increase in Br content. A temperature-dependent PL study shows there are shallow trap states besides the deep level trap states in the MAPb(Br0.82Cl0.18)3 crystal. The activation energy for holes in shallow trap states detrapped into the valence band is ∼0.1 eV, while the activation energy for free holes to be trapped into deep trap states is ∼0.4 eV. This work provides insight into carrier recombination processes in MAPb(BrxCl1-x)3 single crystals.

Trapping effects and surface/interface recombination of carrier recombination in single- or poly-crystalline metal halide perovskites

The lifetime of a carrier is a crucial parameter for solar cell materials, and metal halide perovskite materials are promising for solar cell applications. In this study, we observed carrier recombination using time-resolved photoluminescence (TR-PL) and microwave photoconductivity decay (μ-PCD) in metal halide perovskite materials: NH3CH3PbI3 (MAPbI3), NH3CH3PbBr3 (MAPbBr3), and CsPbBr3 with single- and poly-crystalline structures. By comparing the decay curves of TR-PL and μ-PCD, we found trap levels in the band gap for all the materials. We employed two excitation wavelengths for the μ-PCD measurements, and we observed faster μ-PCD signal decays for short wavelength excitation for MAPbBr3 and CsPbBr3. Additionally, we established that the poly-crystals exhibited faster decay compared with the single crystals for MAPbBr3 and CsPbBr3. Therefore, we concluded that there are significant contributions of the interface and surface recombination on carrier recombination for MAPbBr3 and CsPbBr3, but not for MAPbI3.

Dark matter event in nuclear emulsions: Comparative analysis by time-resolved photoconductivity of silver halide grains

Dark matter (DM) detection by new nuclear emulsions composed of 40 nm AgBrI grains (Lindhard–Scharff theory) has been compared with minimum ionizing particle (MIP) detection and radiation therapy (RT) by conventional nuclear emulsions composed of 200 nm AgBrI grains (Bethe-Bloch theory), and with picture-taking by color films composed of 500–1000 nm AgBrI grains (band-to-band transition), and characterized by its uniqueness: latent image formation in the presence of electron–hole pairs with extremely high concentration (∼1000/grain) and insufficient space for chemical sensitization centers on a grain (∼10/grain). While AgBrI grains in nuclear emulsions for RT are also associated with electron–hole pairs with high concentration, they have space available for chemical sensitizations. Then, we have studied the effect of these characteristics on latent image formation by analyzing the behaviors of electrons and positive holes with variation of their concentrations in 200 nm AgBr grains by means of time-resolved microwave and radio-frequency photoconductivity methods. It has been predicted from the results of the above-stated experiments that DM detection and RT by nuclear emulsions are disturbed by direct recombination and by reaction between Agn and Br2 owing to high activity of positive holes on the contrary to MIP detection by nuclear emulsions and picture-taking by color films, both of which are solely disturbed by indirect recombination. On the basis of the obtained results, discussions are made on ideas to enhance the latent image formation in the presence of electron–hole pairs with high concentration.

Charge Carrier Recombination Dynamics in MAPb(Br1−yIy)3 Single Crystals

Studying the carrier recombination process in MAPb(Br1−yIy)3 single crystals (SCs) is important for its application in the optoelectronic field. In this work, a series of MAPb(Br1−yIy)3 SCs with varied Br/I compositions have been studied. Steady-state photoluminescence (PL) spectra, time-resolved photoluminescence (TRPL) spectra and time-resolved microwave photoconductivity (TRMC) were used to understand the radiative and non-radiative recombination processes of MAPb(Br1−yIy)3 SCs. By comparing the dynamics of TRPL and TRMC, we conclude that the dynamics of TRPL is dominated by the electron trapping process, which is in accordance with the fast decay component of TRMC kinetics, whereas the slower decay component in TRMC is dominated by the hole trapping process. Moreover, we find both the electron and hole trapping rates in mixed-halide perovskite MAPb(Br1−yIy)3 (0 &lt; y &lt; 1) SCs are higher than that of mono-halide perovskite MAPbBr3 SCs and MAPbI3 SCs. This suggests mixed-halide crystals could introduce additional electron and hole trapping densities, which could be related to the fluctuation of Br/I compositions in the crystals. This work is helpful for understanding carrier recombination process in mixed-halide perovskite SCs.

Charge Carrier Dynamics in Co-evaporated MAPbI3 with a Gradient in Composition

Co-evaporation of metal halide perovskites by thermal evaporation is an attractive method since it does not require harmful solvents and enables precise control of the film thickness. Furthermore, the ability to manipulate the Fermi level allows the formation of a graded homojunction, providing interesting opportunities to improve the charge carrier collection efficiency. However, little is known about how these properties affect the charge carrier dynamics. In this work, the structural and optoelectronic properties of co-evaporated MAPbI3 films varying in thickness (100, 400, and 750 nm) with a gradient in composition are analyzed. The X-ray diffraction patterns show that excess PbI2 is only present in the thick layers. From X-ray photoelectron spectroscopy depth analysis, the I/Pb atomic ratio indicates methylammonium iodide deficiencies that become more prominent with thicker films, resulting in differently n-doped regions across the thick MAPbI3 films. We suggest that due to these differently n-doped regimes, an internal electric field is formed. Side-selective time-resolved microwave photo conductivity measurements show an elongation of the charge carrier lifetimes on increasing thickness. These observations can be explained by the fact that excess carriers separate under the influence of the electric field, preventing rapid decay in the thick films.

Probing the long-lived photo-generated charge carriers in transition metal dichalcogenides by time-resolved microwave photoconductivity

Abstract Understanding the dissociation of excitons into long-lived free charge carriers is a crucial issue when considering the applications of transition metal dichalcogenides (excitonic semiconductors) oriented toward the use of solar energy (such as photovoltaics or photocatalysis). In our work, long-lived carriers have been observed by time-resolved microwave photoconductivity (TRMC) for the first time in both atomically thin and bulk MoS2, MoSe2, WS2, and WSe2 crystals. The lifetime of majority carriers is close to microseconds and can even reach several microseconds due to different contribution of surface and defect states, as well as surface band bending (bulk). The three components depend on the material and vary from sample to sample, therefore determining the dynamics of the TRMC signal. The rise time of TRMC signal was found to be in the range of 0.1–0.2 μs and as it depends on the studied material it can be speculated that it is related to the dissociation time of excitons captured by traps.

The investigation of the kinetics of the losses of nonequilibrium current carriers in Cu-=SUB=-2-delta-=/SUB=-NiSnS-=SUB=-4-=/SUB=- (0≤delta≤0.2) quaternary copper compounds

By solid-phase synthesis from elemental Cu, Ni, Sn, and S samples Cu2-deltaNiSnS4 (0≤delta≤0.2) was prepared. The parameters of their crystal lattice have been refined. The lifetimes of photogenerated current carriers in Cu2-deltaNiSnS4 were estimated for the first time by the contactless time-resolved microwave photoconductivity method. This times turned out to be tau~7 ns, which is comparable with the literature data for the times in kesterites CZTS. Herewith, in kinetics of loses of photogenerated cureent carriers is observed predominance of bimolecular recombination processes over capture processes. Keywords: Cu2-deltaNiSnS4, onequilibrium current carriers, lifetimes, kinetics.

Carrier recombination in SrTiO3 single crystals: impacts of crystal faces and Nb doping

We observed the carrier recombination in SrTiO3 single crystals with several crystal faces and Nd doping concentrations using time-resolved photoluminescence and microwave photoconductivity decay methods. The shapes of the decay curves were independent of the excited carrier concentrations, and thus the carriers recombined through the Shockley–Read–Hall and surface recombination processes. From the measurements for different crystal faces, we found that, although the surface recombination is significant for the (100), (110), and (111) faces, they have similar surface recombination velocities of ∼106 cm s−1. Therefore, as photocatalysts, the shape of the materials has a minor effect on the energy conversion efficiency. Based on the dependence of the Nb doping concentration, a high doping concentration significantly enhances the carrier recombination, whereas a moderate Nb doping induces only a slight reduction in the time constants of the carrier decay. In addition, the time constant increases with temperature, and thus moderate Nb doping will have a positive effect on the energy conversion efficiency of SrTiO3 photocatalysts at high temperatures.

Исследование кинетики гибели неравновесных носителей заряда в четверных соединениях меди Cu-=SUB=-2-delta-=/SUB=-NiSnS-=SUB=-4-=/SUB=- (0&amp;le;delta&amp;le;0.2)

By solid-phase synthesis from elemental Cu, Ni, Sn, and S samples Cu2 - δNiSnS4 was prepared . The parameters of their crystal lattice have been refined. The lifetimes of photogenerated current carriers in Cu2 - δNiSnS4 were estimated for the first time by the contactless time-resolved microwave photoconductivity method. This times turned out to be τ ≈ 7 ns, which is comparable with the literature data for the times in kesterites CZTS. Herewith, in kinetics of loses of photogenerated cureent carriers is observed predominance of bimolecular recombination processes over capture processes.

Solid-State Infrared Upconversion in Perylene Diimides Followed by Direct Electron Injection.

In this contribution we demonstrate a solid-state approach to triplet–triplet annihilation upconversion for application in a solar cell device in which absorption of near-infrared light is followed by direct electron injection into an inorganic substrate. We use time-resolved microwave photoconductivity experiments to study the injection of electrons into the electron-accepting substrate (TiO2) in a trilayer device consisting of a triplet sensitizer (fluorinated zinc phthalocyanine), triplet acceptor (methyl subsituted perylenediimide), and smooth polycrystalline TiO2. Absorption of light at 700 nm leads to the almost quantitative generation of triplet excited states by intersystem crossing. This is followed by Dexter energy transfer to the triplet acceptor layer where triplet annihilation occurs and concludes by injection of an electron into TiO2 from the upconverted singlet excited state.

Influence of Synthesis Conditions on the Lifetime of Excess Charge Carriers in Monograin Powders with Kesterite Structure

By solid-phase synthesis, macrocrystalline Cu2 − δZn2 − xSnxS4 − ySey monograin powders have been prepared, the chemical composition of which was shown by XRD and Raman spectroscopy to be different in powder fractions of different sizes formed during the synthesis. The influence of synthesis conditions on the decay kinetics of photogenerated charge carriers in different fractions has been studied using the frequency–time-resolved microwave photoconductivity method. The characteristic photoresponse half-life, τ1/2, increased with increasing grain size. The smallest values of the lifetime were observed for the fraction with sizes of 50–70 μm (τ1/2 <5 ns), and the largest values were for the fraction with a grain size of 70–90 μm (τ1/2 ~ 12 ns).

Interplay between Charge Carrier Mobility, Exciton Diffusion, Crystal Packing, and Charge Separation in Perylene Diimide-Based Heterojunctions.

Two of the key parameters that characterize the usefulness of organic semiconductors for organic or hybrid organic/inorganic solar cells are the mobility of charges and the diffusion length of excitons. Both parameters are strongly related to the supramolecular organization in the material. In this work we have investigated the relation between the solid-state molecular packing and the exciton diffusion length, charge carrier mobility, and charge carrier separation yield using two perylene diimide (PDI) derivatives which differ in their substitution. We have used the time-resolved microwave photoconductivity technique and measured charge carrier mobilities of 0.32 and 0.02 cm2/(Vs) and determined exciton diffusion lengths of 60 and 18 nm for octyl- and bulky hexylheptyl-imide substituted PDIs, respectively. This diffusion length is independent of substrate type and aggregate domain size. The differences in charge carrier mobility and exciton diffusion length clearly reflect the effect of solid-state packing of PDIs on their optoelectronic properties and show that significant improvements can be obtained by effectively controlling the solid-state packing.

Lifetime of excess electrons in Cu–Zn–Sn–Se powders

The method of time-resolved microwave photoconductivity at a frequency of 36 GHz in the range of temperatures of 200–300 K is used to study the kinetics of the annihilation of charge carriers in Cu–Zn–Sn–Se powders obtained by the solid-phase method of synthesis in cells. The lifetime of excess electrons at room temperature is found to be shorter than 5 ns. The activation energy for the process of recombination amounted to Ea ~ 0.054 eV.

Enhanced Absorption and &lt;1% Spectrum-and-Angle-Averaged Reflection in Tapered Microwire Arrays

We report ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely low angular (0° to 50°) and spectrally averaged reflectivity of <1% of the incident 400–1100 nm illumination. After isolating the microwires from the substrate with a polymer infill and peel off process, the arrays were found to absorb 89.1% of angular averaged incident illumination (0° to 50°) in the equivalent volume of a 20 μm thick Si planar slab, reaching 99.5% of the classical light trapping limit between 400 and 1100 nm. We explain the broadband absorption by enhancement in coupling to waveguide modes due to the tapered microstructure of the arrays. Time-resolved microwave photoconductivity decay measurements yielded charge-carrier lifetimes of 0.75 μs (more than an order of magnitude higher than vapor–liquid–solid-grown Si microwires) in the tapered microwires, resulting in an implied Voc of 0.655 V. The high absorption and high aspect ratio in these ordered microwire arrays make them an attractive platform for high-efficiency thin-film crystalline Si solar cells and as well as for the photoelectrochemical production of fuels from sunlight.

Photoconductivity of CdTe Nanocrystal-Based Thin Films: Te(2-) Ligands Lead To Charge Carrier Diffusion Lengths Over 2 μm.

We report on photoconductivity of films of CdTe nanocrystals (NCs) using time-resolved microwave photoconductivity (TRMC). Spherical and tetrapodal CdTe NCs with tunable size-dependent properties are studied as a function of surface ligand (including inorganic molecular chalcogenide species) and annealing temperature. Relatively high carrier mobility is measured for films of sintered tetrapod NCs (4 cm(2)/(V s)). Our TRMC findings show that Te(2-) capped CdTe NCs show a marked improvement in carrier mobility (11 cm(2)/(V s)), indicating that NC surface termination can be altered to play a crucial role in charge-carrier mobility even after the NC solids are sintered into bulk films.

Time-Resolved Microwave Photoconductivity (TRMC) Using Planar Microwave Resonators: Application to the Study of Long-Lived Charge Pairs in Photoexcited Titania Nanotube Arrays

Steady-state (SRMC) and time-resolved microwave photoconductivity (TRMC) are key techniques used to perform the contact-less determination of carrier density, transport, trapping, and recombination parameters in charge transport materials such as organic semiconductors and dyes, inorganic semiconductors, and metal–insulator composites, which find use in conductive inks, thin film transistors, light-emitting diodes, photocatalysts, and photovoltaics. We present the theory, design, simulation, and fabrication of a planar microwave ring resonator operating at 5.25 GHz with a quality factor of 224, to perform SRMC and TRMC measurements. Our method consists of measuring the resonance frequency (f0) and Q-factor of the microwave resonator with the sample to be probed placed in a defined sensitive region of the resonator where the microwave field is highly concentrated. We also provide proof of concept measurements of the time-resolved microwave photoresponse of anatase-phase TiO2 nanotube array membranes (TNTAM...

Two advanced research methods: Frequency-time-resolved microwave photoconductivity and broadband photodielectric spectroscopy

Two known methods of research of electric properties were applied on semiconducting materials and their methodology was significantly improved to obtain quantitative data on elementary reactions of charge carriers. The resonator method of microwave photoconductivity included the measurements of the frequency dependences of photoresponse. The modified frequency- and time-resolved microwave photoconductivity method gives the opportunity to separate contributions to a photoresponse of the free electrons which did not experience capture by acceptors and structural defects, and the electrons which are again released in a free state from traps. The use of a light source (λ = 350 ÷ 2500 nm) together with a broadband dielectric spectrometer provides an effective tool (the broadband photodielectric spectroscopy—BPDS) for research of relaxation processes in semiconductors caused by excess current carriers. Examples of investigation of the photoelectric properties of CdS, CdTe, and Cu(In,Ga)Se2 received with modified methods are presented.

Free Carrier Generation and Recombination in Polymer-Wrapped Semiconducting Carbon Nanotube Films and Heterojunctions

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising for solution-processed, thin film photovoltaics due to their strong near-infrared absorptivity and excellent transport properties. We report on the generation yield and recombination kinetics of free charge carriers in photoexcited thin films of polymer-wrapped s-SWCNTs with and without an overlying electron-accepting C60 layer, using time-resolved microwave photoconductivity (TRMC). Free carriers are generated in neat s-SWCNT films, even without an obvious driving force for exciton dissociation. However, most carriers recombine in <10 ns. Adding C60 increases the yield and extends the lifetime of a significant fraction of free carriers to ≫100 ns via interfacial charge separation. Spectral dependencies indicate that the driving force for interfacial electron transfer vanishes for large-diameter SWCNTs, from which we approximate (9,7) s-SWCNT energetics. We estimate a free carrier generation yield of ∼6% in neat s-SWCNT films and 9 GHz...