Carrier Lifetime Research Articles

First-principle study of the effect of Hf doping and VO-Hi co-existence on absorption spectrum, conductivity and carrier activity of β-Ga2O3

At present, some progress has been made in the study of Hf doping on the conductivity. However, few studies on the effects of Hf doping, and O vacancy and interstitial H co-existence on photoelectric properties of β-Ga2O3. O vacancy and interstitial H will inevitably exist during the preparation of β-Ga2O3. Given these problems, the effects of Hf doping, O vacancy and interstitial H co-existence on photoelectric properties of β-Ga2O3 were studied using first-principles calculations. Results show that doping and defects affect the photoelectric properties of β-Ga2O3 to some extent. With increased Hf doping concentration, the bandgap of the system decreases gradually, the absorption spectrum in the deep ultraviolet region red shifts, the carrier activity and carrier lifetime are enhanced. The mobility and conductivity of the system also increase with increased doping concentration. The Hf doping and interstitial H can effectively improve the conductivity of the system. In summary, Hf can be used as a powerful candidate material for designing and preparing β-Ga2O3 optoelectronic devices.

Photoelectrocatalytic degradation of organic contaminants by spray coated Z-scheme TiO2/Bi2WO6 heterostructure electrode under solar light

This study demonstrates that spray deposited layered TiO2/Bi2WO6 heterostructure photoelectrodes exhibit strong interfacial interactions, leading to significant enhancements in photoelectrochemical performance. Time-resolved photoluminescence (TRPL) spectra indicated a prolonged lifetime of photoinduced carriers. The red shift in the absorption edge of TiO2/Bi2WO6 improved the visible light harvesting capability of Bi2WO6. Nyquist plots showed effective charge separation, and the TiO2 layer at the bottom significantly influenced the flat band potential compared to pristine Bi2WO6, as evidenced by Mott-Schottky analysis. This configuration improved the separation of photogenerated charge carriers via a Z-scheme charge transfer mechanism. The TiO2/Bi2WO6 electrode demonstrated a higher efficiency of 94% for methylene blue degradation within 160 min, compared to pure Bi2WO6. Quenching experiments confirmed the Z-scheme charge transfer mechanism in the coupled TiO2/Bi2WO6 heterojunction during degradation reactions. This study highlights the benefits of semiconductor coupling with different band potentials, promoting large-scale applications of solar-driven TiO2/Bi2WO6 photoelectrodes for environmental remediation.

MoS2-CZTS: a 2D-3D nanocomposite to enhance the photocatalytic performance in degradation of methylene blue dye

Cu2ZnSnS4 nanocrystals (CZTS NCs), MoS2 nanosheet (NS) and MoS2-CZTS nanocomposite (NC) have been synthesized using solvothermal route. Structural characterization of samples have been done by XRD, Raman spectroscopy, HR-TEM. Samples have optically characterized by UV-Vis absorption, photoluminescence (PL) and time co-related single photon counting (TCSPC) study. The XRD, HR-TEM and Raman spectroscopy established tetragonal kesterite phase for both CZTS NCs and MoS2-CZTS NC. Enhancement of efficiency of CZTS NCs to degrade methylene blue (MB) dye, illuminated by visible light, have been observed by loading 1 wt.% MoS2 NS and found to be ~100% in only 15 minutes. This is due to efficent transfer of charge carriers at p-CZTS and n-MoS2 heterojunction interface, confirmed by quenching of PL intensity and decrease in average lifetime of carriers.

Fabrication of dye-sensitized solar cells with natural dye pigments derived from mustard green (Brassica juncea L) and turmeric (Curcuma longa L)

Abstract Natural dye molecules are good alternatives for the ruthenium-based sensitizer for use in dye-sensitized solar cell (DSSC) application. In this research, we have reported the potential application of natural pigment extracts as a dye-sensitizer in DSSC. Natural dyes were extracted using simple maceration technique from natural sources such as mustard green (Brassica juncea L) and turmeric (Curcuma longa L). UV-Vis and FT-IR characterization were employed to examined the optical characteristics and identify their functional groups of the dyes, respectively. The UV-Vis characterization of mustard green dye and turmeric dye show a wide absorption in visible region. The FT-IR spectrum of mustard green dye and turmeric dye show the presence the anchoring groups. The DSSC’s performance was studied using current-voltage (I-V) measurements and electrochemical impedance spectroscopy (EIS). It was observed that the DSSC with turmeric dye shows better photovoltaic performance than DSSC with mustard green dye correlated to its longer charge carrier lifetime.

A Spin‐Polarized Electron‐Induced MXene‐KBi0.9Co0.1Fe2O5 Photocatalyst for Enhanced Dye Degradation

In photocatalysts aimed to degrade environmental pollutants, photons are generally used as primary stimuli to generate electron–hole pairs to target the chemical bonds to disintegrate. Recently, electron spin has been reported as an essential degree of freedom to improve the performance of photocatalysts. In this work, spin‐polarized electrons and holes are introduced into a brownmillerite KBi0.9Co0.1Fe2O5 semiconductor and a two‐dimensional (2D) MXene composite system by application of an external magnetic field. The spin polarization properties are monitored by measuring the magnetoresistance effect of the MXene‐KBCFO device, which is related to the carrier transfer efficiency. Remarkably, the photocatalytic performance of MXene‐KBCFO is significantly enhanced by the simultaneous application of an external magnetic field and illumination due to the increased number of spin‐polarized photoexcited carriers caused by the synergy effect of the 2D MXene heterostructure together with the ferromagnetic spin ordering. This results in increased reaction hotspots, effective built‐in electric field, extended carrier lifetime, and reduced charge recombination owing to parallel alignment of electron spin orientation. The results show that the efficiency and stability of photocatalytic dye degradation can be effectively enhanced by manipulating the spin‐polarized electrons in the MXene‐based oxide‐perovskite heterostructure photocatalyst.

Tailoring sub-5 nm Fe-doped CeO2 nanocrystals within confined spaces to boost photocatalytic hydrogen evolution under visible light

This work aimed to study the efficiency of the reverse micelle (RM) preparation route in the syntheses of sub-5 nm Fe-doped CeO2 nanocrystals for boosting the visible-light-driven photocatalytic hydrogen production from methanol aqueous solutions. The effectiveness of confining precipitation reactions within micellar cages was evaluated through extensive physicochemical characterization. In particular, the nominal composition (0–5 mol% Fe) was preserved as ascertained by ICP-MS analysis, and the absence of separate iron-containing crystalline phases was supported by X-ray diffraction. The effective aliovalent doping and modulation of the optical properties were investigated using UV-Vis, Raman, and photoluminescence spectroscopies. 2.5 mol% iron was found to be an optimal content to achieve a significant decrease in the band gap, enhance the concentration of oxygen vacancy defects, and increase the charge carrier lifetime. The photocatalytic activity of Fe-doped CeO2 prepared at different Fe contents with RM preparation was studied and compared with undoped CeO2. The optimal iron load was identified to be 2.5 mol%, achieving the highest hydrogen production (7566 μmol L−1 after 240 min under visible light). Moreover, for comparison, the conventional precipitation (P) method was adopted to prepare iron containing CeO2 at the optimal content (2.5 mol% Fe). The Fe-doped CeO2 catalyst prepared by RM showed a significantly higher hydrogen production than that obtained with the sample prepared by the P method. The optimal Fe-doped CeO2, prepared by the RM method, was stable for six reuse cycles. Moreover, the role of water in the mechanism of photocatalytic hydrogen evolution under visible light was studied through the test in the presence of D2O. The obtained results evidenced that hydrogen was produced from the reduction of H+ by the electrons promoted in the conduction band, while methanol was preferentially oxidized by the photogenerated positive holes.

In-situ passivating surface defects of ultra-thin MAPbBr3 perovskite single crystal films for high performance photodetectors

Ultra-thin single crystal film (SCF) without grain boundary inherits low charge recombination probability as bulk single crystals. However, its low depth brings a high surface defect ratio and hinders the carrier transport and extraction, which affects the performance and stability of optoelectronic devices such as photodetectors, and thus surface defect passivation is of great practical significance. In this paper, we use the space confined method to grow MAPbBr3 SCF and selected BA2PbI4 for surface defect passivation. The results reveal that BA cation passivates MA vacancy surface defects, reduces carrier recombination, and enhances carrier lifetime. The carrier mobility is as high as 33.6 cm2 V−1 s−1, and the surface defect density is reduced to 3.4 × 1012 cm−3. Therefore, the self-driven vertical MAPbBr3 SCF photodetector after surface passivation exhibits more excellent optoelectronic performance.

Studying the Effect of Composition on the Crystal Structure, Optical Properties, and Photogenerated Current Carriers Lifetimes in AgxCu1 – xGaSe2 (0 ≤ x ≤ 1) Solid Solutions

A set of AgxCu1 – xGaSe2 (0 ≤ x ≤ 1) solid solution powders has been prepared by solid-state synthesis. Using a combination of X-ray diffraction analysis and Raman spectroscopy, it has been found that the samples have a single-phase tetragonal structure (space group I-42d). It has been shown that their crystal lattice parameters do not follow Vegard’s law up to x ≈ 0.4. It has been revealed that the band gap of the samples also changes nonlinearly: initially it decreases and then increases. Studies of the low-temperature luminescence and microwave photoconductivity decay spectra have shown that a set of samples with x of 0 to ~0.4 and further in the region with x > 0.4 is characterized by an increase in the photogenerated current carrier lifetime in AgxCu1 – xGaSe2 powders. The observed effect is apparently attributed to the replacement of deep charge carrier traps, such as selenium vacancies, by shallower cation vacancies.

Unravelling charge carrier dynamics for enhancement of photocatalytic performance in Pd/MoS2 nanocomposites for water remediation of real-world pollutants

In this study, Pd/MoS2 nanocomposites were successfully synthesized incorporating Pd nanoparticles into three-dimensional (3D) flower-like MoS2 nanostructures. The controlled introduction of Pd was aimed to optimize the photocatalytic performance of the resulting nanocomposites, which exhibited exceptional efficiency in degrading various organic pollutants and antibiotic tetracycline (TC) under simulated solar light irradiation. The nanocomposites with an optimized Pd concentration of 2.5 % demonstrated outstanding photocatalytic efficiency, achieving the degradation of Rhodamine B (RhB), Methylene blue (MB), TC by 98 %, 98 %, and 96 %, respectively, with specific interval of 40, 30 and 60 min. The reaction rate constant of the Pd/MoS2 nanocomposites was measured to be 0.7798 min−1 using pseudo first-order rate kinetics. The value is about 19 times higher than that of pure MoS2 (0.00414 min−1) for degradation of RhB. The improved photocatalytic performance of these nanocomposites can be attributed to several factors. Firstly, the incorporation of Pd nanoparticles substantially enhanced their light absorption capabilities, leading to an overall increase in photocatalytic efficiency. Moreover, the presence of Pd contributed to a large surface area, further enhancing the nanocomposite's photocatalytic potential. Importantly, the formation of a built-in electric field at Pd/MoS2 interface facilitated the efficient separation of the electron-hole pairs, extending the life time of these photoinduced charge carriers. This study offers valuable insights into development of MoS2-based photocatalyst, which hold significant promise for addressing water pollution challenges and making substantial contributions to the field of water purification and environmental remediation.

Nanosecond Carrier Lifetime of Hexagonal Ge.

Hexagonal Si1-x Ge x with suitable alloy composition promises to become a new silicon compatible direct bandgap family of semiconductors. Theoretical calculations, however, predict that the binary end point of this family, the bulk hex-Ge crystal, is only weakly dipole active. This is in contrast to hex-Si1-x Ge x , where translation symmetry is broken by alloy disorder, permitting efficient light emission. Surprisingly, we observe equally strong radiative recombination in hex-Ge as in hex-Si1-x Ge x nanowires, but scrutinizing experiments on the radiative lifetime and the optical transition matrix element of hex-Ge remain hitherto unexplored. Here, we report an advanced spectral line shape analysis exploiting the Lasher-Stern-Würfel (LSW) model on an excitation density series of hex-Ge nanowire photoluminescence spectra covering 3 orders of magnitude. The analysis was performed at low temperature where radiative recombination is dominant. We analyze the amount of photoinduced bandfilling to obtain direct access to the excited carrier density, which allows to extract a radiative lifetime of (2.1 ± 0.3) ns by equating the carrier generation and recombination rates. In addition, we leveraged the LSW model to independently extract a high oscillator strength of 10.5 ± 0.9, comparable to the oscillator strength of III/V semiconductors like GaAs or GaN, showing that the optical properties of hex-Ge nanostructures are perfectly suited for a wide range of optoelectronic device applications.

Asymmetric main-chain twisted small molecules for efficient polymer solar cells

Polymer solar cells (PSCs) based on small molecules with twisted backbones as electron acceptors, have many advantages over their planar counterparts, such as upshifted molecular energy levels, better charge extraction performance, enhanced extinction coefficient, extended carrier lifetime and reduced recombination rate, which are very helpful in improving the power conversion efficiencies (PCE). The present study was designed to synthesize two new small molecules with main-chain twisted structures that include an asymmetric electron donor core thiophene-phenylene-thieno[3,2-b]thiophene, namely i-T-TT and i-T-TT-4F, to investigate the “structure-property” correlation of main-chain twisted acceptors. Both asymmetric molecules exhibit bent geometric structures, and the fluorinated acceptor i-T-TT-4F possesses a more red-shifted spectrum, improved molar extinction coefficient, and deepened molecular energy levels. As a result, when combined with the middle bandgap polymer donor J52, there was a remarkable efficiency of 12.22 % for the device of i-T-TT-4F, higher than that of i-T-TT (9.51 %). Our research illustrates the importance of the main-chain twisted asymmetric electron-donating core and fluorinated end-capping group in the construction of efficient PSCs.

Tuning optical properties of sustainable inorganic bismuth-based perovskite materials with organic ligands

We report a facile and environmentally friendly ball milling method for synthesizing Cs3Bi2Cl9 nanocrystals at different ball milling times: 30 min (BM1), 120 min (BM2), and 240 min (BM3). Structural analysis confirmed the formation of crystalline nanostructures. Optical studies revealed a tunable bandgap influenced by particle size, with a blue shift observed for smaller nanocrystals. Importantly, Cs3Bi2Cl9 demonstrated potential for optoelectronic applications as indicated by its interaction with 2,3-diaminonaphthalene and reduced charge carrier lifetime. These findings contribute to the growing interest in lead-free perovskites as sustainable alternatives for light-harvesting and conversion technologies. Increasing ball milling time from 120 to 240 minutes improved thermal stability between 200 °C and 610 °C. BM1 perovskite crystals showed significantly lower thermal stability, suggesting that the synthesis process influences crystal structure and thermal properties. However, thermogravimetric analysis (TGA) showed that BM2 and BM3 perovskite crystals exhibited exceptional thermal stability, withstanding temperatures up to 438.6 °C and 439 °C, respectively, without significant weight loss. This superior stability expands potential applications to high-temperature environments like concentrated solar power systems.

Enhanced Carrier Lifetime and Mobility in Monolayer NbOI2.

The lifetime and mobility of hot carriers are critical parameters for assessing the performance of optoelectronic materials, as they directly impact the response speed and operational efficiency of devices. Combining first-principles calculations with nonadiabatic molecular dynamics (NAMD) simulations, we systematically investigated the electronic properties and carrier dynamics of monolayer NbOI2. Our findings indicate that, at room temperature, this material demonstrates a carrier lifetime of up to ∼13 ns and an electron mobility reaching as high as ∼9 × 103 cm2 V-1 s-1. The low Young's modulus makes it susceptible to deformation under external stress, and we found that the carrier lifetime extends to ∼40 ns under a 4% tensile strain, along with a significant increase in hole mobility. This study elucidates the carrier dynamics in monolayer NbOI2, facilitating its potential application in future flexible optoelectronic devices.

The regular octahedral CdS/K2Nb2O6 core-shell Z-Scheme heterojunction towards enhancing photocatalytic H2 evolution and degradation via synergism of carrier efficiency and light response

The regular octahedral CdS/K2Nb2O6 core-shell Z-Scheme heterojunction is synthesized via an approach of hydrothermal-chemical method. The CdS/K2Nb2O6 nano-heterojunction (CdS/KNO-3) exhibits arresting enhanced photocatalytic HER (∼634.39 μmol g−1 h−1)/degradation (∼0.05428 min−1) than the monomer K2Nb2O6 (∼27.8/14.4 folds) and CdS (∼20.3/10.0 folds), including a decent stability (average HER of ∼621.39 μmol∙g−1 h−1). It can be mainly attributed to the CdS/K2Nb2O6 Z-Scheme heterojunction with appropriate potential gradient can promote the interface carrier driving to optimize carrier efficiency, including increasing carrier transport, extending carrier lifetime and reducing recombination. Additionally, the octahedral core-shell nanostructure can provide numerous active sites for decreasing overpotential and promoting electron diffusion, while improving the solar efficiency (high solar response of CdS), including shorting carrier pathway for photocorrosion optimization and increasing structural stability.

Inhibiting the Appearance of Green Emission in Mixed Lead Halide Perovskite Nanocrystals for Pure Red Emission.

Mixed halide perovskites exhibit promising optoelectronic properties for next-generation light-emitting diodes due to their tunable emission wavelength that covers the entire visible light spectrum. However, these materials suffer from severe phase segregation under continuous illumination, making long-term stability for pure red emission a significant challenge. In this study, we present a comprehensive analysis of the role of halide oxidation in unbalanced ion migration (I/Br) within CsPbI2Br nanocrystals and thin films. We also introduce a new approach using cyclic olefin copolymer (COC) to encapsulate CsPbI2Br perovskite nanocrystals (PNCs), effectively suppressing ion migration by increasing the corresponding activation energy. Compared with that of unencapsulated samples, we observe a substantial reduction in phase separation under intense illumination in PNCs with a COC coating. Our findings show that COC enhances phase stability by passivating uncoordinated surface defects (Pb2+ and I-), increasing the formation energy of halide vacancies, improving the charge carrier lifetime, and reducing the nonradiative recombination density.

Charge dynamics in the 2D/3D semiconductor heterostructure WSe2/GaAs

Understanding the relaxation and recombination processes of excited states in two-dimensional (2D)/three-dimensional (3D) semiconductor heterojunctions is essential for developing efficient optical and (opto)electronic devices, which integrate van der Waals 2D materials with more conventional 3D ones. In this work, we unveil the carrier dynamics and charge transfer in a monolayer of WSe2 on a GaAs substrate. We use time-resolved differential reflectivity to study the charge relaxation processes involved in the junction and how they change when compared to an electrically decoupled heterostructure, WSe2/hBN/GaAs. We observe that the monolayer in direct contact with the GaAs substrate presents longer optically excited carrier lifetimes (3.5 ns) when compared with the hBN-isolated region (1 ns), consistent with a strong reduction of radiative decay and a fast charge transfer of a single polarity. Through low-temperature measurements, we find evidence of a type-II band alignment for this heterostructure with an exciton dissociation that accumulates electrons in GaAs and holes in WSe2. The type-II band alignment and fast photoexcited carrier dissociation shown here indicate that WSe2/GaAs is a promising junction for advanced photovoltaic and other optoelectronic devices, making use of the best properties of van der Waals (2D) and conventional (3D) semiconductors.

Degradation of electrical properties of subcells in multi-junction solar cells under neutron irradiation

This paper presents an analysis of the photovoltaic characteristics and parameters of individual subcells of space multi-junction solar cells after irradiation by high-energy particles. Dark currents, charge carrier lifetimes, and damage coefficients for wide-bandgap subcells were determined both theoretically and experimentally.

Polyacid-Modulated Carrier Dynamic Behavior at the Interface of 0D/2D Heterojunctions.

Heterojunctions formed by polyoxometalates and 2D materials draw attention owing to their remarkable photoelectric and catalytic properties. However, the intrinsic mechanisms of polyoxometalates regulating the heterojunction photoelectric properties are unclear. Herein, we constructed two types of heterojunctions by integrating polyoxometalates (Keggin-type H3PW12O40 and Lindqvist-type H2W6O19) on g-C3N4 monolayers, exploring photoexcited carrier dynamics in these heterojunctions by ab initio calculations combined with nonadiabatic molecular dynamics (NAMD) simulations. Our results show that electrons and holes in H3PW12O40 on g-C3N4 monolayers relax within 583 and 760 fs, respectively. The electron-hole recombination occurs at 342 fs, faster than carrier separation, aligning with the behavior of Z-type heterojunctions. Contrarily, the H2W6O19/g-C3N4 heterojunction exhibits the typical characteristics of type II heterojunctions, with a long photogenerated carrier lifetime reaching 652 fs. These findings show tunable band alignment in polyoxometalate-supported systems by modulating polyoxometalate type, influencing hot electron dynamics, and guiding 0D/2D heterojunction design.

Oxygen precipitation behavior and its influence on phosphorus gettering in Czochralski silicon

This study investigates the effects of single-step and two-step annealing processes on the formation of oxygen precipitates and their impact on metal impurity gettering in gallium-doped monocrystalline silicon. The research focuses on the competitive relationship between internal gettering by oxygen precipitates and external gettering through phosphorus diffusion. Experimental results show that two-step annealing generates larger and more abundant oxygen precipitates, enhancing the internal gettering effect and reducing the recovery of minority carrier lifetime after phosphorus gettering. Despite this, phosphorus diffusion gettering remains effective in improving minority carrier lifetime, although the degree of improvement depends on the size and quantity of oxygen precipitates. These findings offer valuable insights for optimizing the fabrication process of silicon-based solar cells.

Formation of High-Photoresponsivity BaSi2 Films by Radio-Frequency Sputtering and Evaluation of a BaSi2/TiN/Glass Heterojunction by Transmission Electron Microscopy.

Barium disilicide (BaSi2) is a thin-film solar cell material composed of abundant elements, and its application potential is further enhanced by its formation on inexpensive substrates, such as glass. The effect of the substrate temperature on the co-sputtering of BaSi2 and Ba targets to form BaSi2 films on Si(111) and TiN/glass substrates was investigated. Contrary to expectations, the photoresponsivity reached maximum values exceeding 5 and 2 A W-1, respectively, the highest value ever reported for as-deposited samples formed at 750 °C, more than 100 °C higher than those reported previously. Because the photoresponsivity is proportional to carrier lifetime, this result indicates that high-temperature growth can bring out the high performance of BaSi2 as a light-absorbing layer. Because amorphous SiC (a-SiC) has a larger forbidden band gap and electron affinity than BaSi2, it is considered suitable as an electron transport layer (ETL) material for BaSi2 solar cells. On the basis of this, the formation of BaSi2 (absorption layer)/a-SiC (ETL)/TiN (electrode)/glass heterojunctions was also attempted, and the layered structure was examined by cross-sectional transmission electron microscopy (TEM). Polycrystalline BaSi2 films were found to be even on the amorphous layer by TEM. A high photoresponsivity of over 2 A W-1 was obtained. Therefore, the BaSi2/a-SiC/TiN structure provides a guideline for the structural design of BaSi2-based thin-film solar cells on glass.