Search

Understanding the defect characteristics that occur near the space‐charge regions (SCRs) of kesterite (CZTSSe) solar cells is important because the recombination loss at the CZTSSe/CdS interface is considered the main cause of their low efficiency. CZTSSe surfaces with different elemental compositions were formed without polishing (C00) and with polishing for 20 s (C20) and 60 s (C60). For C60, a specific region near the SCR was excessively Cu‐rich and Zn‐poor compared to C00 and C20. Various charged defects formed where the elemental variation was large. As the main deep acceptor defect energy level (Ea2) near the SCR increased, the efficiency, open‐circuit voltage deficit, and current density degraded, and this phenomenon was especially rapid for large Ea2 values. As the Ea2 near the SCR became deep, the carrier diffusion length decreased more for the CZTSSe solar cells with a low carrier mobility than for the CuInGaSe2 (CIGSe) solar cells. The large amplitude of the electrostatic potential fluctuation in the CZTSSe solar cells induced a high carrier recombination and a short carrier lifetime. Consequently, the properties of the CZTSSe solar cells were more strongly degraded by defects with deep energy levels near the SCR than those of the CIGSe solar cells. [ABSTRACT FROM AUTHOR]

Abstract Understanding the defect characteristics that occur near the space‐charge regions (SCRs) of kesterite (CZTSSe) solar cells is important because the recombination loss at the CZTSSe/CdS interface is considered the main cause of their low efficiency. CZTSSe surfaces with different elemental compositions were formed without polishing (C00) and with polishing for 20 s (C20) and 60 s (C60). For C60, a specific region near the SCR was excessively Cu‐rich and Zn‐poor compared to C00 and C20. Various charged defects formed where the elemental variation was large. As the main deep acceptor defect energy level (Ea2) near the SCR increased, the efficiency, open‐circuit voltage deficit, and current density degraded, and this phenomenon was especially rapid for large Ea2 values. As the Ea2 near the SCR became deep, the carrier diffusion length decreased more for the CZTSSe solar cells with a low carrier mobility than for the CuInGaSe2 (CIGSe) solar cells. The large amplitude of the electrostatic potential fluctuation in the CZTSSe solar cells induced a high carrier recombination and a short carrier lifetime. Consequently, the properties of the CZTSSe solar cells were more strongly degraded by defects with deep energy levels near the SCR than those of the CIGSe solar cells.

Two frequency-domain techniques (PEIS and IMPS) were used to unravel the key positive and negative influence of vacancy oxygen on photocatalytic performance, and an organic hole transport coating was used to eliminate the negative influence of surface vacancy oxygen. [Display omitted] A consensus is yet to be reached on the effects of oxygen vacancy (V O) on the performance of TiO 2 for photocatalytic water splitting as contrasting viewpoints have been presented in the latest researches. Herein, a comprehensive set of spectroelectrochemical methods are deployed to clearly reveal the advantages and disadvantages of V O on the performance of TiO 2. The results indicate that surface V O improves the photocatalytic activity while bulk V O has a negative effect on the water reduction performance of TiO 2. Intensity-modulated photocurrent spectroscopy (IMPS) and UV–vis spectroscopy provide compelling evidence that the improvement of H 2 evolution can be attributed to the presence of defect level, while the low interface charge transfer efficiency caused by surface V O limits the further improvement of photocatalytical H 2 evolution, which can be alleviated by an organic hole transport coating. The density functional theory (DFT) and surface photovoltaic (SPV) analyses confirm that the built-in field between TiO 2 and hole molecules is the reason for the interface charge transfer efficiency improvement. Our findings provide a comprehensive understanding of V O in TiO 2 by carrier behavior analysis and a scheme to further promote the photocatalytic performance. [ABSTRACT FROM AUTHOR]

Surface defect engineering and heterojunction fabrication are considered to be effective strategies for modifying semiconductor photocatalysts. In this work, we synthesized a series of defect-rich Ti3C2/BiOIO3 nanocomposites for photocatalytic degradation of methyl orange (MO). The samples were characterized by various techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), ultraviolet–visible (UV–vis) spectroscopy, and photoluminescence (PL). The composite exhibited the best photocatalytic performance toward MO under simulated sunlight, which can degrade 95.2% MO in 0.5 h. The total organic carbon (TOC) analysis was also used to ascertain the mineralization degree of MO. The BiOIO3 light response is widened from UV to the visible light region by generating defect energy levels of I–. At the same time, the oxygen vacancy (OV) as the negative charge attraction center captures the excited electrons in the conduction band and doping level, and Ti3C2 as a cocatalyst transfers electrons. This work provides guidance for designing photocatalysts with high-efficient interfacial charge transfer heterostructures. [ABSTRACT FROM AUTHOR]

[Display omitted] • The performance of eco-friendly lead-free CsBi 3 I 10 -based perovskite solar cell has been numerically analyzed. • Enhancement of open circuit voltage (V OC) up to 360 mV has been observed after introduction of NiO x HTL. • The influence of key defect parameters of the CsBi 3 I 10 absorber and CeO x /CsBi 3 I 10 interface layer have been extensively studied. • Exploration of energy band alignment impact on the device performance. Bismuth-based halide perovskite (CsBi 3 I 10) is a promising absorber material for the fabrication of eco-friendly perovskite solar cells (PSCs). In this research, the performance of the CsBi 3 I 10 -based PSCs with different hole transport layers (HTLs) has been numerically analyzed. The open circuit voltage (V OC) has enhanced up to 360 mV after the addition of NiO x HTL in the heterostructure of the CsBi 3 I 10 -based PSC. A comprehensive numerical study of the role of band alignment, key defect parameters of the CsBi 3 I 10 absorber layer, and CeO x /CsBi 3 I 10 interface on the newly designed heterostructure (ITO/CeO x /CsBi 3 I 10 /NiO x /Au) performance of the CsBi 3 I 10 -based PSC has been conducted. A massive deterioration of the V OC has been initiated when defect concentration (N t) of CsBi 3 I 10 crosses above 1014 cm−3. Apart from the N t , defect energy level within the bandgap (E t), and holes capture cross-section (σ p) of the CsBi 3 I 10 layer have also significantly affected the V OC loss. Besides, the investigation indicates that the device performance is almost independent of E t of the CeO x /CsBi 3 I 10 interface and slightly decreases with the increase of N t and σ p. Finally, the photovoltaic performance of the PSC has been explored for various thickness and carrier concentration of the CsBi 3 I 10 , cerium oxide (CeO x), and nickel oxide (NiO x). Therefore, this research provides efficient guidelines for the fabrication of eco-friendly high-performance CsBi 3 I 10 -based PSCs. [ABSTRACT FROM AUTHOR]

The intrinsic characteristics of novel devices and materials are often misunderstood due to the characterization methods which are developed to analyze existing devices or materials. Even though graphene is a very well-known material, there hasn't been a proper method to assess the density and energy levels of defects in graphene non-destructively, especially after the device fabrication. Here, we report a new non-destructive defect analysis method, amplitude-modulated discharge current analysis (AMDCA). The validity of this method was confirmed using a graphene field effect transistor with physically predefined defect densities in the channel. Charge trap densities (N c t ) of the order of ∼1012 cm−2 were observed at the defect level in the range of 0.15–0.29 eV. This method can be very useful for the in-depth study of graphene devices as well as other two-dimensional materials that don't have a body contact. [Display omitted] [ABSTRACT FROM AUTHOR]

Environment-friendly tin-based perovskite solar cells have gained much research interest because of the non-toxic nature of tin. The broader visible absorption spectrum of methylammonium tin iodide and smaller band-gap value of 1.3 eV have made methylammonium tin iodide viable to be used as an absorber layer in place of methylammonium lead iodide having toxic lead as its central element. Lead being water soluble is hazardous for the environment and human health, when a panel breaks or gets exposed to moisture. In this study, hole transport layer (HTL) free and lead-free, tin-based perovskite solar cell with layered structure as Glass/TCO/WS 2 /CH 3 NH 3 SnI 3 /back-contact, and its variant is proposed and investigated. To realize an optimum set of performance parameters, extensive investigations are carried out by considering two variants of the proposed design viz. lead-free absorber PSC (LF-PSC) and lead-free absorber optimized PSC (LFO-PSC). The impact of varying defect density and position of defects in bulk and at the interface on the performance of PSC is investigated and optimized. Changing defect density of the absorber layer shows a direct influence on diffusion length and the lifetime of charge carriers. Performance evaluation parameters of the LFO-PSC obtained in this study are V oc = 1.024 V, J sc = 33.33 mA/cm2, FF= 80.13%, PCE= 27.33%. Higher efficiency is attributed to the presence of a single WS 2 /MASnI 3 interface and therefore reduced recombination of charge carriers. PCE of optimized lead-free absorber PSC has been reported to be 8.9% higher than its basic variant i.e. LF-PSC design. The influence of defect levels, back reflecting coating and metal work function of back contact on the performance of PSC is investigated. Recombination curves are also studied to understand the impact of defects on the proposed PSC structure. [ABSTRACT FROM AUTHOR]

Post deposition treatment (PDT) for Cu 2 ZnSn(S,Se) 4 (CZTSSe) was carried out by simply dipping the absorber into the KF solution at 80 °C. The dipping time of absorber in KF solution was found to be crucial to device parameters of CZTSSe solar cell. The K-doping improved the solar cell efficiency from 4.4% to 7.6% by 1 min dipping whereas the longer than 5 min dipping solar cells showed distorted kink J-V curves. The activation energy of CZTSSe solar cell was increased upto 1 min KF treatment from 0.83 eV to 0.92 eV which indicates interface recombination is reduced significantly. However, the activation energies of 5 min and 10 min dipping solar cells were found to be 0.81 eV and 0.63 eV where dominant recombination was interface recombination. Furthermore, trap energies of 49 meV and 298 meV of pristine CZTSSe solar cell were modified to 33 meV and 117 meV for 1 min treated CZTSSe solar cell. Trap energies of 5 min were calculated to be 112 meV and 147 meV. The proper KF doping passivated the shallow as well as deep defects of CZTSSe solar cell which is reflected in photovoltaic performances directly. [ABSTRACT FROM AUTHOR]

[Display omitted] • α -BaCu 2 Se 2 is a wide-band-gap semiconductor with orthorhombic structure. • Cu vacancy engineering was applied in Zintl α -BaCu 2 Se 2 aiming to improve the thermoelectric properties. • The Cu vacancy can realize the temperature-dependent hole concentration to the optimal one. • Cu vacancy engineering signifies the enlargement of average zT in a wide temperature range. Vacancy engineering offers an alternative route to aliovalent and interstitial doping for optimization of the carrier concentration in thermoelectric materials. For the wide-bandgap semiconductor BaCu 2 Se 2 , the Cu vacancy is dynamically stable; this feature can be rationally manipulated to maximize the thermoelectric figure of merit zT. In this work, we show that at room temperature, Cu-deficient BaCu 2− x Se 2 samples exhibit increased hole effective mass and mobility, attributed to the energy band modulation, which are favorable for improved electrical transport properties. More importantly, the defect energy level resulting from the Cu vacancies continually contributes holes at high temperature, thereby allowing the hole concentration to approach an optimal concentration. This effect leads to an increase of the power factors over a wide temperature range. The artificial reduction of the Cu content in BaCu 2 Se 2 results in the strengthened point-defect scattering, suppressing the lattice thermal conductivity. This strategy allows simultaneous optimization of the electrical and thermal transport properties, with a thermoelectric figure of merit zT = 1.08 achieved for BaCu 1.94 Se 2 at 823 K, which is 38% higher than that of stoichiometric BaCu 2 Se 2. Within the measured temperature range, the average zT value for BaCu 1.94 Se 2 is 0.494, which is 52.9% higher than that of BaCu 2 Se 2. [ABSTRACT FROM AUTHOR]

The change of atom configuration in GaAs, caused by intrinsic point defects (Ga and As vacancies, Ga and As antisites, Ga and As interstitials), is first calculated by a plane wave pseudo-potential method with the generalised gradient approximation in the frame of density functional theory, and the most stable structure is obtained. Then, the formation energy of each kind of the native defect is calculated, by which the possibilities of the six kinds of point defects to be formed during crystal growth are analysed. The defect energy levels corresponding to each kind of the native point defect and their electron occupancy are analysed from the aspect of density of states. Finally, the elastic constants of GaAs saturable absorbers with native point defects are calculated, and the impacts on the elastic properties brought by native point defects are studied. The values of defect energy levels obtained will be helpful in ascertaining the mechanism of the EL2 deep level in the GaAs saturable absorber, and the analysis of the elastic properties of a GaAs crystal with native point defects will be helpful in guiding the application of the GaAs crystal as a saturable absorber in passively Q-switched lasers. [ABSTRACT FROM AUTHOR]

TiO2 fims have been deposited on glass substrates using DC reactive magnetron sputtering at different oxygen partial pressures from 0. 10 Pa.to 0.65 Pa. The transmittance (UV-vis) and photoluminescence (PL) spectra of the films were recorded. The results of the UV-vis spectra show that the deposition rate of the films decreased at oxygen partial pressure P(O2)≥0.15 Pa, the band gap increased from 3.48 eV to 3.68eV for direct transition and from 3.27 eV to 3.34 eV for indirect transition with increasing the oxygen partial pressure. The PL spectra show convincingly that the transition for films was indirect, and there were some oxygen defect energy levels at the band gap of the films. With increasing the O2 partial pressure, the defect energy levels decreased. For the films sputtered at 0.35 and 0.65 Pa there were two defect energy levels at 2.63 eV and 2.41 eV, corresponding to 0.72 eV and 0.94 eV below the conduction band for a band gap of 3.35 eV, respectively. For the films sputtered at 0.10 Pa and 0.15 Pa, there was an energy band formed between 3.12 eV and 2.06 eV, corresponding to 0.23 eV and 1.29 eV below the conduction band. [ABSTRACT FROM AUTHOR]

A detailed characterization of deep-level defects induced by low energy proton irradiation in n-GaAs LPE layer using AlGaAs-GaAs mesa structure has been carried out for several proton energies (i.e., 50 100 and 290 kev) and fluences (i.e., 10, 10, and 10 p/cm), using DLTS, SEM-EBIC, I-V and C-V measurement techniques. Important defect and recombination parameters such as density and energy level of electron and hole traps, thermal emission rates and capture cross sections for electrons and holes in each trap level as well as hole diffusion lengths in the n-GaAs LPE layer were deduced from these measurements. Hole diffusion lengths determined by the EBIC, DLTS, and I-V measurements are found in good agreement. It is shown that dark current under forward bias condition was dominated by recombination of electron-hole pairs via deep level defects in the junction space charge region of the diode. Significant carrier removal occurs for proton fluence greater than 10p/cm. [ABSTRACT FROM AUTHOR]

Novel Datura inoxia-like Fe 3 O 4 @SiO 2 @ZnO/ZnS (FSZZ) core/shell nanocomposites were prepared via a simple ultrasound-assisted in situ surface sulfidation method. The samples displayed excellent uniformity and were highly dispersed. The effects of the sulfidation concentration on the morphological, structural, optical, photoelectrochemical and photocatalytic properties of the nanocomposites were systematically studied. Regulating the sulfidation concentration introduced abundant zinc defects into the nanocomposites, so the light absorption was successfully extended to the visible light region. The intrinsic tubular core/shell heterostructures and number of zinc defects at the heterointerface were the key elements for synergistically promoting the transfer and separation efficiency of photogenerated charge carriers and then improving the degradation efficiency. When the concentration of thioacetamide was 9 mM (FSZZ0.3), the prepared sample exhibited superior photocatalytic efficiency of up to 99% under ultraviolet light irradiation and higher than 80% within 180 min under visible light irradiation for antibiotics and organic dyes degradation. Type-II staggered band alignment formed in FSZZ0.3, and the probable photocatalytic mechanism was proposed and discussed in detail. This novel 3D tubular core/shell heterostructure is an effective and promising photocatalyst for the wastewater purification, and our synthesis process provides a new simple and rapid strategy for designing other hyper-dispersed materials for different practical applications. The novel hyper-dispersed 3D Datura inoxia-like Fe 3 O 4 @SiO 2 @ZnO/ZnS nano-photocatalysts with tubular ZnO/ZnS core/shell heterostructures were synthesized via a simple ultrasound-assisted in situ surface sulfidation method, which displayed excellent photocatalytic activity for antibiotics and organic dyes degradation under ultraviolet and visible light irradiation. Image 1 • Novel tubular Fe 3 O 4 @SiO 2 @ZnO/ZnS (FSZZ) core/shell heterostructure was prepared. • A defect energy level introduced in nanocomposites could narrow the band gap. • FSZZ0.3 showed excellent visible-light degradation of antibiotics and dyes. • Probable photocatalytic mechanism is proposed. [ABSTRACT FROM AUTHOR]

The integration of metal-organic frameworks (MOFs) with semiconductors has attracted mounting attention for photocatalytic applications. However, more efforts are needed to unravel the interface structure in MOF/semiconductor composites and its role in charge transfer. Herein, a MIL-100(Fe)/TiO 2 composite was synthesized as a prototypical photocatalyst and studied systematically to explore the interface structure and unravel the charge transfer pathways during the photocatalytic processes. The composite was fabricated by growing MIL-100(Fe) crystals on TiO 2 using surface-coated FeOOH as the precursor. The as-prepared MIL-100(Fe)/TiO 2 exhibited significantly improved photocatalytic performance over pristine TiO 2 , which was mainly because of the enhanced charge separation as confirmed by transient absorption spectroscopy analysis. This enhancement partially arose from the special chemical structure at the interface, where the Fe-O-Ti bond was formed. As verified by the density functional theory calculation, this distinct structure would create defect energy levels adjacent to the valence band maximum of TiO 2 . During the photocatalytic processes, the defect energy levels serve as sinks to capture excited charge carriers and retard the recombination, which subsequently leads to the increased charge density and promoted photocatalytic efficiency. Meanwhile, the intimate interactions between MIL-100(Fe) and TiO 2 would also help to improve the charge separation by transferring photo-induced holes through the ligands to Fe-O clusters. These findings would advance the fundamental understanding of the interface structure and the charge transfer pathways in MOF/semiconductor composite photocatalysts.