Shallow Carrier Research Articles

Novel 2D/2D/2D heterojunction of ZnIn2S4/g-C3N4/MoS2 for enhanced photocatalytic hydrogen evolution reaction

Photocatalysts for hydrogen evolution reaction (HER) have widely been explored but fine-tuning and engineering multiphase photocatalyst components are still challenging. Among catalysts, ZnIn2S4 has shown promise but maximizing the use of ZnIn2S4-photogenerated carriers and enhancing quantum yield still require further investigation. One way to achieve this is by enhancing the photon utilization and reducing the recombination of shallow carriers in ZnIn2S4. Accordingly, novel ZnIn2S4/g-C3N4/MoS2 type-II heterojunction composites with 2D/2D/2D structures (abbreviated ZIS/CN/MS) were synthesized in the present work by simple thermal condensation-solvothermal method. The optimized ZIS/CN/MS exhibited a photocatalytic hydrogen evolution rate of 13.6 mmol g−1 h−1 under visible light (λ ≥ 420 nm) irradiation, a value much higher than those of pure-phases ZIS and CN, as well as the binary composite of ZIS/CN. The apparent quantum efficiency of the optimized ZIS/CN/MS at a wavelength of 420 nm reached 27.0 % coupled with excellent cycling stability. The significantly improved photocatalytic performance and the excellent stability can mainly be attributed to the unique 2D/2D/2D arrangement of the ternary components of ZIS, CN, and MS in ZIS/CN/MS photocatalyst, providing a large number of close interfacial contacts between the components for shortened migration distance of photogenerated charges and more pathways for electron separation and transport in type-II heterostructures. Overall, the proposed methodology and ZIS/CN/MS composites with type-II heterojunction and 2D/2D/2D structures have great potential for use in solar-to-hydrogen energy conversion and can be extended to the preparation of other photocatalysts.

Investigation into the carrier recombination in Sb2Se3: Photo thermal effect, trapped carrier absorption and hot carrier cooling

Understanding the carrier recombination processes in Sb2Se3 is essential for its optoelectronic applications. In this work, carrier recombination dynamics in Sb2Se3 were studied by broad band transient absorption spectroscopy. Firstly, the contribution of photothermal effect to the transient absorption spectrum was thoroughly discussed. It is confirmed that the excited state absorption (ESA) band with lifetime of several nanoseconds results from co-contribution of photo thermal effect and deep trapped carrier absorption. Secondly, the features of transient absorption spectrum on picosecond time scale were interpreted. The short-lived ESA band around 1000 nm was assigned to shallow trapped carrier absorption, while not band gap renormalization (BGR) or free carrier absorption. By globally fitting the transient absorption spectrum, the hot carrier cooling time and time constant for free carrier relax into deep trap state were determined to be 0.25∼0.45 ps and 3.1∼8.7 ps, respectively. Finally, we built up the carrier recombination model of Sb2Se3. The experimental results in this work will improve the understanding on the carrier recombination in Sb2Se3.

Photoexcited Carrier Dynamics in Iodine-Doped CH3NH3PbBr3 Single Crystals.

Iodine-doped bromide perovskite single crystals (IBPSCs) have important applications in optoelectronic fields, such as in solar cells. Currently, much research has aimed to study the phase separation phenomenon and device performance improvements in IBPSCs. However, important intrinsic photoexcited carrier dynamics are often overlooked in IBPSCs. Here, we explored the photoexcited carrier dynamics in typical iodine-doped MAPbBr3 single crystals using the excitation intensity-dependent steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) technique. We found that the trap state density changes with an increase in the amount of doped iodine. Further, we noticed that there is an influence of carrier diffusion on the photoexcited carrier dynamics, and then, we evaluated the carrier diffusion coefficients and recombination constants via numerical simulations of the PL kinetics. Consequently, we found that the electron shallow trap-related carrier behaviors substantially impacted the PL kinetics. Our results greatly facilitate a deeper understanding of the fundamental characteristics of mixed halide perovskite material.

Ultrafast Exciton Dynamics of CH3NH3PbBr3 Perovskite Nanoclusters.

Exciton dynamics of perovskite nanoclusters has been investigated for the first time using femtosecond transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy. The TA results show two photoinduced absorption signals at 420 and 461 nm and a photoinduced bleach (PB) signal at 448 nm. The analysis of the PB recovery kinetic decay and kinetic model uncovered multiple processes contributing to electron-hole recombination. The fast component (∼8 ps) is attributed to vibrational relaxation within the initial excited state, and the medium component (∼60 ps) is attributed to shallow carrier trapping. The slow component is attributed to deep carrier trapping from the initial conduction band edge (∼666 ps) and the shallow trap state (∼40 ps). The TRPL reveals longer time dynamics, with modeled lifetimes of 6.6 and 93 ns attributed to recombination through the deep trap state and direct band edge recombination, respectively. The significant role of exciton trapping processes in the dynamics indicates that these highly confined nanoclusters have defect-rich surfaces.

Compound Defects in Halide Perovskites: A First-Principles Study of CsPbI3.

Lattice defects affect the long-term stability of halide perovskite solar cells. Whereas simple point defects, i.e., atomic interstitials and vacancies, have been studied in great detail, here we focus on compound defects that are more likely to form under crystal growth conditions, such as compound vacancies or interstitials, and antisites. We identify the most prominent defects in the archetype inorganic perovskite CsPbI3, through first-principles density functional theory (DFT) calculations. We find that under equilibrium conditions at room temperature, the antisite of Pb substituting Cs forms in a concentration comparable to those of the most prominent point defects, whereas the other compound defects are negligible. However, under nonequilibrium thermal and operating conditions, other complexes also become as important as the point defects. Those are the Cs substituting Pb antisite, and, to a lesser extent, the compound vacancies of PbI2 or CsPbI3 units, and the I substituting Cs antisite. These compound defects only lead to shallow or inactive charge carrier traps, which testifies to the electronic stability of the halide perovskites. Under operating conditions with a quasi-Fermi level very close to the valence band, deeper traps can develop.

Enhancement in power conversion efficiency and stability of perovskite solar cell by reducing trap states using trichloroacetic acid additive in anti-solvent

Perovskite solar cells (PSCs) have shown significant commercial promise due to their fast development over the last decade and recent demonstration of a power conversion efficiency (PCE) almost equivalent to that of silicon-based solar cells. Shallow carrier trapping states within the bulk or surface of a perovskite layer are highlighted by researchers as a reason for the PSC efficiency losses. Suppressing the trap-induced shallow defects is a powerful strategy to improve the potential of perovskite materials for solar cell application. The current study planned on tailoring chlorobenzene (CB) anti-solvent with a trichloroacetic acid (TCAA) additive to prepare a favourable MAPbI3 layer with lower defects to record higher power conversion efficiency (PCE) in MAPbI3-based solar cells. Characterization results indicate the introduction of a few amounts of TCAA into CB affects the perovskite crystallization through reactions of the carboxyl group with the under-coordinated lead ions during the fabrication process. The TCAA-based PSCs show lower radiative and non-radiative charge recombination, leading to facilitated charge transport within the PSCs. It produces a champion PCE of 19.37% for the TCAA-based PSCs with a boosted stability behavior, which is greater than the control PSCs, which have a maximum PCE of 16.90%.

Femtosecond Exciton and Carrier Relaxation Dynamics of Two-Dimensional (2D) and Quasi-2D Tin Perovskites.

The dynamics of exciton and free-carrier relaxation of low-dimensional tin iodide perovskites, BA2FAn-1SnnI3n+1, where n = 1 (N1), 2 (N2), 5 (N5), and 10 (N10), were investigated with femtosecond transient absorption spectra (TAS). The absorption and photoluminescence spectra of N1 and N2 show exciton characteristics due to quantum confinement, whereas N5 and N10 display a free-carrier nature, the same as for bulk three-dimensional (3D) films. The TAS profiles were fitted according to a global kinetic model with three time coefficients representing the interactions of biexcitons, trions, and excitons for N1 and N2 and hot carriers, cold carriers, and shallow trap carriers for N5 and N10. The carrier relaxation dynamics of N5 and N10 were similar to those of 3D FASnI3 except for the absence of surface recombination in the deep-trap states due to passivation of the grain surfaces by the long alkyl chain for these quasi-2D samples (N5/N10 vs 3D).

A unique photo charge behaviour of bare-KTaO3

A deep understanding of UV light response of KTaO3 (KTO) has been explored by means of temperature dependent UV treatment. Two different temperature dependent measurement schemes have been utilized to differentiate between the shallow carriers and deep carriers. A fake transition at 220 K is found, which is actually due to the UV-induced oxygen sub-bands, resulting in the transition in photo-response from transient to persistent. Giant change in the resistance is observed for cooling as compared to heating of substrate, which is credited to the small recombination rate at low temperatures. This temperature dependent resistance response of KTaO3 under UV light provides an importance of the measurement way of photo-charge carriers in KTaO3.

Enhanced Electrical Properties of Polyethylene-Graft-Polystyrene/LDPE Composites.

Nanocomposite dielectrics show a great potential application in high voltage direct current cables for their obvious improvements in electrical properties. In the present manuscript, nanocomposite composed of low-density polyethylene and nanoscale polystyrene particles is studied by using low-density polyethylene grafted with polystyrene molecule. Fourier-transform infrared spectra reveal successful grafting of the polystyrene molecule onto the low-density polyethylene chain and the scanning electron microscope image shows the homogeneously dispersed nanoscale polystyrene particles. The presence of the polystyrene nanoparticles obviously improves the dielectric properties, such as the direct current breakdown strength and space charge inhibition. The conductivity and thermally stimulated current characteristics imply the deep traps in the composite increase obviously. Density functional theory calculation reveals that the grafted polystyrene can accommodate both shallow and deep electron carriers, and the depth of the hole traps are as deep as 2.07 eV.

Improved optical properties of InAs submonolayer quantum dots in GaAsSb/InGaAs double-well structure

In this study, multistacked InAs submonolayer (SML) quantum dots (QDs) were sandwiched in an InGaAs/GaAsSb dot-in-a-double-well (DDwell) structure to enhance the crystal quality and optical properties of QDs. The photoluminescence (PL) intensity of the InAs SML QDs with the DDwell structure was 5.5 times higher than that of conventional InAs/GaAs SML QDs because of the reduced number of nonradiative recombination centers and the enhanced carrier hole confinement. The PL results of the DDwell structure exhibit two peaks that represent the carrier overflow from SML QDs to InGaAs quantum wells (QWs) and hence the radiative recombination in InGaAs QWs because of the shallow carrier confinement of SML QDs. Among the compared samples, the DDwell structure exhibited the highest activation energy of 101.8 meV. Furthermore, the carrier thermal escape was suppressed in these InAs SML QDs. High-resolution transmission electron microscopy revealed that the microstructures of the InAs SML QDs demonstrated larger dots for the DDwell structure, thus verifying that the emission wavelength elongated in the PL measurement. These improved optical properties of the InAs SML QDs with the DDwell structure were attributable to the improved crystal quality because of the use of Sb surfactants and additional volume for carrier recombination provided by the InGaAs quantum well. The DDwell structure can thus be applied in optoelectronic devices to obtain advanced performance.

Screening of pair fluctuations in superconductors with coupled shallow and deep bands: A route to higher-temperature superconductivity

A combination of strong Cooper pairing and weak superconducting fluctuations is crucial to achieve and stabilize high-Tc superconductivity. We demonstrate that a coexistence of a shallow carrier band with strong pairing and a deep band with weak pairing, together with the Josephson-like pair transfer between the bands to couple the two condensates, realizes an optimal multicomponent superconductivity regime: it preserves strong pairing to generate large gaps and a very high critical temperature but screens the detrimental superconducting fluctuations, thereby suppressing the pseudogap state. Surprisingly, we find that the screening is very efficient even when the inter-band coupling is very small. Thus, a multi-band superconductor with a coherent mixture of condensates in the BCS regime (deep band) and in the BCS-BEC crossover regime (shallow band) offers a promising route to higher critical temperatures.

The control mechanism of micron and nano SiO2/epoxy composite coating on surface charge in epoxy resin

Charge accumulation on the surface of epoxy resin affects the insulation performance under DC voltage. In order to illustrate the effect of surface coating on the characteristics of surface charge accumulation in epoxy resin, micron and nano SiO 2 /epoxy composite coatings were deposited on the surface of epoxy resin by spraying, and the charge accumulation on the surface was studied by the electrostatic probe method. The surface trap parameters were analyzed by the method of isothermal surface potential decay combining with the carrier mobility and cross-section morphology of samples, the transport characteristics of surface charge and the respective control mechanism of coating layer were studied. The results suggest that the surface charge mainly accumulates near the electrodes, the main source of charges is injected from the electrodes. The surface charges near the anode are caused by charge trapping, and the surface charges near the cathode are caused by the adsorption due to the normal electric field. Surface charge accumulation can be suppressed by increasing the shallow trap density and carrier mobility on the surface of material, which is available by coating with micron and nano SiO 2 /epoxy composites. It was found that, surface charge of epoxy resin could be suppressed effectively by coating of micron SiO 2 particles with content of 1~3 wt%, or nano SiO 2 particles with content of 3 wt%.

Measurement of Relaxation Time of Excess Carriers in Si and CIGS Solar Cells by Modulated Electroluminescence Technique

Excess carrier lifetime plays a crucial role in determining the efficiency of solar cells. In this paper, we use the frequency dependence of inphase and quadrature components of modulated electroluminescence (MEL) to measure the relaxation time (decay) of excess carriers. The advantage of the MEL technique is that the relaxation time is obtained directly from the angular frequency at which the quadrature component peaks. It does not need knowledge of the material parameters like mobility, etc., and can be used for any finished solar cells which have detectable light emission. The experiment is easy to perform with standard electrical equipment. For silicon solar cells, the relaxation time is dominated by recombination and hence, the relaxation time is indeed the excess carrier lifetime. In contrast, for the CIGS solar cells investigated here, the relaxation time is dominated by trapping and emission from shallow minority carrier traps.

Band-edge oscillator strength of colloidal CdSe/CdS dot-in-rods: comparison of absorption and time-resolved fluorescence spectroscopy.

We studied the oscillator strength fgap of the band gap transition in heteronanocrystals (hNCs) with a spherical CdSe core embedded in an elongated CdS shell. A comparison with fgap of core-only CdSe NCs confirmed a reduction of the electron-hole overlap in hNCs with a band gap larger than 2.05 eV or smaller than 1.98 eV. However, the decrease in fgap is limited to about 50% when compared to CdSe NCs, suggesting that residual confinement still localizes the electron near the core. We correlated fgap with the radiative lifetime obtained from multiexponential photoluminescence (PL) decay traces. The different components were attributed to radiative decay, or deep and shallow carrier trapping, respectively, using the PL quantum efficiency (QE) as a guideline. Our data highlight the challenges associated when extracting the radiative decay, and demonstrate the added value of absorption spectroscopy to obtain the band-edge oscillator strength and the associated radiative recombination rate in colloidal hNCs.

Nonlinear Carrier Interactions in Lead Halide Perovskites and the Role of Defects.

The simple solution processability at room temperature exposes lead halide perovskite semiconductors to a non-negligible level of unintentional structural and chemical defects. Ascertained that their primary optoelectronic properties meet the requirement for high efficiency optoelectronic technologies, a lack of knowledge about the nature of defects and their role in the device operation currently is a major challenge for their market-scale application due to the issues with stability and reliability. Here, we use excitation correlation photoluminescence (ECPL) spectroscopy to investigate the recombination dynamics of the photogenerated carriers in lead bromide perovskites and quantitatively describe the carrier trapping dynamics within a generalization of the Shockley-Read-Hall formalism. The superior sensitivity of our spectroscopic tool to the many-body interactions enables us to identify the energetics of the defects. In fact, in the case of polycrystalline films, depending on the synthetic route, we demonstrate the presence of both deep and shallow carrier traps. The shallow defects, which are situated at about 20 meV below the conduction band, dope the semiconductor, leading to a substantial enhancement of the photoluminescence quantum yield despite carrier trapping. At excitation densities relevant for lasing, we observe breakdown of the rate-equation model, indicating a buildup of a highly correlated regime of the photocarrier population that suppresses the nonradiative Auger recombination. Furthermore, we demonstrate that colloidal nanocrystals represent virtually defect-free systems, suffering from nonradiative quenching only due to subpicosecond Auger-like interactions at high excitation density. By correlating the fabrication conditions to the nonradiative loss channels, this work provides guidelines for material engineering towards superior optoelectronic devices.

Time-resolved luminescence spectroscopy of structurally disordered K3WO3F3 crystals

Three emission centers of exciton-like origin, with distinct relaxation time, emission and excitation spectra were revealed in K3WO3F3 and described taking into account its structural disordering. Low-temperature monoclinic phase of K3WO3F3 features few anion sites with mixed oxygen/fluorine occupancy per [WO3F3] octahedron. Therefore, different kinds of distorted octahedra form, providing different luminescence centers. The time-resolved luminescence spectroscopy technique was applied to distinguish these centers. The simultaneous thermal quenching of them above ∼200 K was qualitatively explained involving dynamic structural disorder of the compound. The energy transfer mechanism between centers was found and tentatively described by the diffusion of excitons. Apart from intrinsic luminescence, the PL of defect-related centers was discovered and the role of shallow charge carrier traps in the low-temperature persistent luminescence was revealed.

Shallow carrier traps in hydrothermal ZnO crystals

Native and hydrogen-plasma induced shallow traps in hydrothermally grown ZnO crystals have been investigated by charge-based deep level transient spectroscopy, photoluminescence and cathodoluminescence microanalysis. The as-grown ZnO exhibits a trap state at 23 meV, while H-doped ZnO produced by plasma doping shows two levels at 22 meV and 11 meV below the conduction band. As-grown ZnO displays the expected thermal decay of bound excitons with increasing temperature from 7 K, while we observed an anomalous behaviour of the excitonic emission in H-doped ZnO, in which its intensity increases with increasing temperature in the range 140–300 K. Based on a multitude of optical results, a qualitative model is developed which explains the Y line structural defects, which act as an electron trap with an activation energy of 11 meV, being responsible for the anomalous temperature-dependent cathodoluminescence of H-doped ZnO.

Trapped State Sensitive Kinetics in LaTiO2N Solid Photocatalyst: With and without Cocatalyst Loading

In addition to the process of photogeneration of electrons and holes in photocatalyst materials, the competitive process of trapping of these charge carriers by defects which can both enhance photocatalytic activity by promoting electron-hole separation or can deteriorate the activity by serving as recombination centers is also very crucial to the performance of the photocatalyst [1]. Similarly, addition of cocatalyst is also known to enhance the photocatalytic activity of a material by promoting efficient charge separation.In this work we report on the charge carrier dynamics in a visible-light responsive (oxy) nitride photocatalyst: LaTi2ON with and without the presence of CoOx cocatalyst. For femtosecond diffuse reflectance measurements, an amplified 150 fs Ti:Sa laser with OPA for pump (λexc= 500nm & 580nm) and white light continuum and IR light for probes are used [1]. Synthesis of LaTiO2N and LaTiO2N/CoOx powder are described elsewhere [2].Figure 1a shows the effect of change of excitation wavelength on the dynamics of the energetically shallow trapped charge carriers of LaTiO2N powder (with and without CoOx) probed at 880 nm. As is evident from the figure, shifting the excitation wavelength to longer λexc 580 nm, the kinetics of the relaxation from the shallow to deep traps slows down for unloaded LaTiO2N. This hints at the existence of energetically distributed trapped states in unloaded LaTiO2N powder which play an interesting role in the charge carrier kinetics making it excitation wavelength dependent. Possibly, direct trap filling of deep traps for higher λexc= 580nm slows down the relaxation process of charge carriers from shallow trap states. Kinetics of mobile conduction band electrons, which are observed by the IR probe, however remains unaffected to excitation wavelength changes.Interestingly, co-catalyst loading is found to have a suppressing effect on this slowing down of this relaxation of shallow trapped electrons to deep-trapped states as well as a decrease in TA intensity of the shallow trapped carriers. As can be seen in Figure 1b, the larger the amount of the cocatalyst, the faster is the decay and smaller the TA intensity. Thus, it is evident that cocatalyst loading has an impact on the carrier dynamics but, a better knowledge of the cocatalyst distribution, size etc. is necessary to strongly comment on the observation. At this point we lack this information.However, based on literature we can very loosely comment at this stage that probably due to charge transfer from the photocatalyst to the cocatalayst (TA data implies in <1ps which is very fast), the decay becomes less sensitive to λexc. Infact, a previous study on Co modified single crystalline LaTiO2N reports ~27% quantum efficiency at 440nm [2]. CoOx deposition was found to prolong the lifetime of mobile carriers (2000 cm-1) to a time scale of 1 second. Acknowledgement: This work is supported by “Research Project for Future Development: Artificial Photosynthetic Chemical Process (ARPChem)” (METI, Japan: 2012-2022). Reference [1] Tamaki, Y. Hara, K. Katoh, R. Tachiya, M. Furube, A., J. Phys. Chem. C, 2009, 113, 11741. [2] Zhang, Y, Yamakata, A, Maeda, K, Moriya, Y, Takata, T, Kubota, J, Teshima, K, Oishi, S, Domen, K., J. Am. Chem. Soc. 2012, 134, 8348.

Development of highly sensitive mechanoluminescent sensor aiming at small strain measurement

This paper was focused on the elasticoluminescence (ELS) characteristics, especially a response to small strain (below 1000 μst), of mechanoluminescence (ML) sensor using strontium aluminate doped with small amount of europium ( SrAl 2 O 4: Eu ) synthesized by different methods. By using nitrate decomposition method as a synthetic method of SrAl 2 O 4: Eu , the response to small strain of the ML sensor was enhanced in comparison with using a conventional solid-state reaction method. Based on SEM observation and thermoluminescence (ThL) measurement, we proposed a hypothesis that the sensing characteristic of small strain affect the plate-like shape of SrAl 2 O 4: Eu grain and/or shallower carrier trap levels formed by nitrate decomposition method.

Impurity-related degradation in a prototype organic photovoltaic material: A first-principles study

The phenyl-C61-butyric acid methyl ester (PCBM) fullerene derivative is among the most widely used acceptor materials in organic photovoltaics (OPVs). Similar to the case of several other organic semiconductors and systems, experimental data show that the performance of OPV blends of PCBM with a polythiophene polymer degrades when the system is exposed to ambient molecules, such as O2 and H2O. Here we use first-principles calculations to identify physical mechanisms that can give rise to this type of degradation. We find that oxygen impurities can be incorporated in PCBM crystals and form several different types of configurations that range from intact molecules in crystalline voids to oxidized PCBM molecules. A number of O-related impurity structures generate states within the energy band gap of the PCBM crystal, creating thus shallow and deep carrier traps. Likewise, incorporation of H2O molecules gives rise to a shallow acceptor-like trap. The results are consistent with pertinent experimental observations of degradation paths for air-exposed PCBM samples and can aid in the optimization of PCBM-based OPV blends.