Charge Carrier Lifetime Research Articles

Mitigating Band Tailing in Kesterite Solar Absorbers: Ab Initio Quantum Dynamics.

Open-circuit voltage deficits are limiting factors in kesterite solar cells. Addressing this issue by suppressing band tailing and nonradiative charge recombination is essential for enhancing the performance. We employ ab initio nonadiabatic molecular dynamics to elucidate the origin of band tailing and charge losses and propose a mitigation strategy. The simulations show that Cu-Zn disorder, associated with antisite defect clusters [CuZn+ZnCu], is a significant source of band tailing in kesterites, as evidenced by the much larger Urbach energy in disordered than ordered kesterites. Cu-Zn disorder gives rise to new sulfur-centered coordination polyhedra, increases structural inhomogeneity, changes electrostatic potential at sulfur centers, and shifts the S(3p) orbital energy. Differences in the S(3p)/Cu(3d) and S(3p)/Sn(5s) hybridization strengths and the S(3p) orbital energy shift reduce the band gap by 0.37 eV. Furthermore, Cu-Zn disorder enhances vibrational motion of sulfur anions and surrounding cations, increasing band gap fluctuations by 15 meV. The stronger electron-phonon interactions reduce charge carrier lifetimes and limit the kesterite solar cell efficiency. Partial substitution of Zn with Cd facilitates structural ordering and significantly suppresses band tailing, particularly in disordered systems. The improvement can be attributed to the larger atomic radius and mass of Cd, which weakens bonding around the anion, suppresses S-related vibrations within the covalent tetrahedra, and reduces nonadiabatic coupling, thereby increasing charge carrier lifetimes. The reported results establish the key influence of cation disorder on band tailing and reduced charge carrier lifetimes in kesterites and highlight cation disorder engineering as a strategy to achieve high-efficiency kesterite solar cells.

Preparation of Bi2CrO6/CuO heterostructure nanocomposite to increase methylene blue decomposition under visible light irradiation

In this study, the Bi2CrO6/CuO nanocomposite was used for the photocatalytic degradation of methylene blue (MB) under visible light. Nanocomposites with different ratios (1:1, 1:2, 2:1) were synthesized under hydrothermal conditions and investigated for MB removal. Various characterization techniques, including XRD, FT-IR, FE-SEM, BET, DRS, PL, and EDS, were employed to elucidate the physicochemical aspects of the catalysts. The 2:1 nanocomposite ratio was selected as the photocatalyst with the highest removal efficiency (85 %). Four main factors, including initial concentration, solution pH, catalyst dose, and light intensity, were studied using the response surface methodology (RSM) and the Box-Behnken model. Under optimal conditions, the MB removal efficiency reached 90.06 %. Additionally, the effect of oxidizing agents (H2O2) on the enhanced removal of MB was investigated. The improved photocatalytic performance of the Bi2CrO6/CuO (2:1) nanocomposite is attributed to visible light absorption, the formation of a p-n heterostructure, efficient charge separation via the S-scheme mechanism, and increased charge carrier lifetime. Moreover, economic calculations showed that the estimated costs for the photocatalytic removal of MB from wastewater are cost-effective.

Exploring Ultrafast Carrier Dynamics in Photoelectrochemical Water Splitting Using Transient Absorption Spectroscopy

AbstractHydrogen fuel produced from water splitting using sunlight remains one of the cleanest and most sustainable energy sources. However, photoelectrochemical hydrogen production faces several challenges, including poor sunlight absorption, deficient charge carrier lifetime and transfer rates, and very sluggish kinetics of the water oxidation half‐reaction. To address these challenges, researchers have concentrated on enhancing the efficiency of photoelectrochemical water splitting. A critical factor in this enhancement is a deeper understanding of the fundamental processes involved in photoabsorption and the subsequent oxidation evolution reaction. The advent of ultrafast lasers has enabled the detailed tracking of charge carriers within materials after sunlight absorption, including their separation and migration to the surface for water oxidation. Ultrafast transient absorption spectroscopy is a powerful technique that allows real‐time observation of the behavior of excited states and charge carriers on femtosecond to nanosecond timescales. Recent studies utilizing ultrafast transient absorption spectroscopy are explored to investigate the dynamics of charge carriers in photoelectrochemical water splitting, providing insights into the mechanisms that lead to the design of enhanced photoanodes with improved efficiency.

Dynamics of Photoinduced Charge Carriers in Metal-Halide Perovskites.

The measurement and description of the charge-carrier lifetime (τc) is crucial for the wide-ranging applications of lead-halide perovskites. We present time-resolved microwave-detected photoconductivity decay (TRMCD) measurements and a detailed analysis of the possible recombination mechanisms including trap-assisted, radiative, and Auger recombination. We prove that performing injection-dependent measurement is crucial in identifying the recombination mechanism. We present temperature and injection level dependent measurements in CsPbBr3, which is the most common inorganic lead-halide perovskite. In this material, we observe the dominance of charge-carrier trapping, which results in ultra-long charge-carrier lifetimes. Although charge trapping can limit the effectiveness of materials in photovoltaic applications, it also offers significant advantages for various alternative uses, including delayed and persistent photodetection, charge-trap memory, afterglow light-emitting diodes, quantum information storage, and photocatalytic activity.

Proximity-Induced Exchange Interaction and Prolonged Valley Lifetime in MoSe2/CrSBr Van-Der-Waals Heterostructure with Orthogonal Spin Textures.

Heterostructures, composed of semiconducting transition-metal dichalcogenides (TMDC) and magnetic van-der-Waals materials, offer exciting prospects for the manipulation of the TMDC valley properties via proximity interaction with the magnetic material. We show that the atomic proximity of monolayer MoSe2 and the antiferromagnetic van-der-Waals crystal CrSBr leads to an unexpected breaking of time-reversal symmetry, with originally perpendicular spin directions in both materials. The observed effect can be traced back to a proximity-induced exchange interaction via first-principles calculations. The resulting spin splitting in MoSe2 is determined experimentally and theoretically to be on the order of a few meV. Moreover, we find a more than 2 orders of magnitude longer valley lifetime of spin-polarized charge carriers in the heterostructure, as compared to monolayer MoSe2/SiO2, driven by a Mott transition in the type-III band-aligned heterostructure.

As‐Doped Polycrystalline CdSeTe: Localized Defects, Carrier Mobility and Lifetimes, and Impact on High‐Efficiency Solar Cells

AbstractThe efficiency potential for single‐junction photovoltaics (PV) is described by the detailed balance model, which requires the elimination of nonradiative recombination and perfect minority carrier collection. Improvements in GaAs, Si, and perovskite PV follow this model. It might be more complex for CdTe, a leading thin‐film PV technology. While lifetime, passivation, and doping goals for 25% efficient CdTe solar cells are largely reached, voltage is ≈20% below the detailed balance limit. Why is that? In Se‐alloyed CdSexTe1‐x (Se is required for >20% efficiency) additional losses can occur due to electrostatic and bandgap fluctuations and due to electronic trap states. To understand mechanisms limiting CdSeTe solar cell performance and to suggest improvements, carrier dynamics, and transport in CdSexTe1‐x with variation in Se composition and as doping is analyzed. It is shown that trapping, likely due to anion‐site defects and their complexes, is correlated with low charge carrier mobility of 0.1–0.6 cm2 (Vs)−1. Even with 1000 ns charge carrier lifetimes, carrier diffusion length is less than the absorber thickness, reducing efficiency to ≈23%. Device simulations are used to analyze the performance of CdSexTe1‐x solar cells; thermodynamic models are not sufficient for absorbers with electronic disorder and trapping.

Interface properties study of black silicon solar cells with front surface a-Si:H/c-Si heterojunction

Abstract The exceptionally low reflectance of black silicon across a broad wavelength range makes it an intriguing surface texture for solar cell applications. Silicon heterojunction (SHJ) solar cells fabricated on black silicon (Si) formed by dry reactive ion etching (RIE) using inductive coupled plasma (ICP) on n-type Si are explored. The study is focused on the properties of the a-Si:H/c-Si interface, being a key issue for the photovoltaic performance of SHJ. Deep-level transient spectroscopy (DLTS) detected no radiation defect in Si after the etching. The surface of black Si was passivated with an intrinsic a-Si:H layer, followed by the deposition of p-type and n-type a-Si:H on the front and back sides, respectively. The SHJ solar cell photovoltaic performance based on black Si is strongly influenced by defect density at the a-Si:H/Si interface. Admittance spectroscopy and effective charge carrier lifetime measurements demonstrate that interface defect density decreases with the increase of (i)a-Si:H thickness. The value of 0.75 ms effective charge carrier lifetime was reached for single (i) a-Si:H layer passivation and 0.25 ms when (p)a-Si:H was deposited over the intrinsic a-Si:H layer. The measured open circuit voltage (VOC) values for the SHJ solar cells increase with the (i)a-Si:H layer thickness reaching 658 mV. However, the fill factor decreases with increasing (i) a-Si:H layer thickness, limiting the efficiency at the maximum value below 14% due to the thickness uniformity of the a-Si:H layer. The development of conformal growth of a-Si:H is a key issue for further improvement of black Si heterojunction solar cell photovoltaic performance. 

A-Site Doping Promotes CO2 Activation and Prolongs Charge Carrier Lifetimes in SrTiO3: Insight from Quantum Dynamics.

Using time-dependent density functional theory and nonadiabatic molecular dynamics, we systematically investigated the effect of A-site doping on the CO2 activation and charge carrier lifetimes in SrTiO3(STO). Our simulations revealed that A-site doping significantly enhances the chemical adsorption of CO2 on SrTiO3 surfaces, which is beneficial for promoting CO2 activation. Moreover, we found that A-site doping can efficiently stabilize the lowest unoccupied molecular orbital (LUMO) of CO2 near the conduction band minimum of STO, promoting the photogenerated electron transfer from the conduction band of STO to the CO2 LUMO. Importantly, A-site doping causes a significant nonadiabatic coupling reduction and prolongs the charge recombination time by a factor of 1.86 compared to the pristine STO. Our study clarifies the influencing mechanism of A-site doping on CO2 activation and charge carrier lifetimes and suggests important principles for the design of high-performance photocatalytic semiconductors.

PHOTOELECTRICAL PROPERTIES OF CdMnSe THIN FILMS

The studied semiconductor structure for photovoltaic cells is composed of SnO2-coated glass and CdMnSe thin film. A study is made by examining the photoluminescence from the surface of the CdMnSe thin film with laser power and sample temperature for an as-grown, then an air-annealed thin film and undergone CdCl2 treatment. Thin films of Cd1-xMnxSe (x=0.02) were grown on a glass substrate. The lifetime of charge carriers under pulsed illumination was determined from the kinetic decay of the photocurrent. The study of relaxation curves of nonequilibrium photoconductivity under the influence of laser radiation confirmed the presence of two recombination channels - intrinsic and impurity. Photocurrent relaxation occurs through fast and slow recombination channels. The fast relaxation time τ = 6 μs associated with the intrinsic transition and the slow relaxation time is due to impurity excitation and τ = 22 μs. The photoluminescence spectra of thin films of Cd1-xMnxSe (x=0.02) were studied. In the photoluminescence study, two maxima are observed due to donor-acceptor recombination and intracenter transition of Mn atom.

Two-dimensional Fullerene-Based Ferroelectric Materials: Assembly and Effects on Charge Carrier Lifetime.

Manipulating the symmetry of fullerene-based low-dimensional materials is crucial to the development of electronic devices and modern nonvolatile memories. However, there have been few reports on studying the physicochemical properties of fullerene and its derivatives by controlling the symmetries. Herein, we demonstrate ferroelectricity in Sc3N@Ih-C80-Pd/Pt adducts with relatively strong spontaneous polarization. Polarization originates from subtle molecular interactions between Sc3N@Ih-C80 and Pd/Pt atoms, breaking the structural symmetry. Additionally, we investigate how a temperature-dependent polar-nonpolar phase transition affects the corresponding nonadiabatic electron-hole recombination process in the ferroelectric fullerene adduct Sc3N@Ih-C80-Pd. The results indicate that polarization has a greater impact than temperature effects, resulting in an extension of the carrier lifetime in the polar phase by over 3 times compared to that in the nonpolar phase. Our findings unveil new members of the ferroelectric fullerene family and provide guidance for improving the performance of optoelectronic materials.

Customizing Aniline-Derived Molecular Structures to Attain beyond 22 % Efficient Inorganic Perovskite Solar Cells.

The characteristics of the soft component and the ionic-electronic nature in all-inorganic CsPbI3-xBrx perovskite typically lead to a significant number of halide vacancy defects and ions migration, resulting in a reduction in both photovoltaic efficiency and stability. Herein, we present a tailored approach in which both anion-fixation and undercoordinated-Pb passivation are achieved in situ during crystallization by employing a molecule derived from aniline, specifically 2-methoxy-5-trifluoromethylaniline (MFA), to address the above challenges. The incorporation of MFA into the perovskite film results in a pronounced inhibition of ion migration, a significant reduction in trap density, an enhancement in grain size, an extension of charge carrier lifetime, and a more favorable alignment of energy levels. These advantageous characteristics contribute to achieving a champion power conversion efficiency (PCE) of 22.14 % for the MFA-based CsPbI3-xBrx perovskite solar cells (PSCs), representing the highest efficiency reported thus far for this type of inorganic metal halide perovskite solar cells, to the best of our knowledge. Moreover, the resultant PSCs exhibits higher environmental stability and photostability. This strategy is anticipated to offer significant advantages for large-area fabrication, particularly in terms of simplicity.

Atomic Dispersed Co on NC@Cu Core-Shells for Solar Seawater Splitting.

With freshwater resources becoming increasingly scarce, the photocatalytic seawater splitting for hydrogen production has garnered widespread attention. In this study, a novel photocatalyst consisting of a Cu core coated is introduced with N-doped C and decorated with single Co atoms (Co-NC@Cu) for solar to hydrogen production from seawater. This catalyst, without using noble metals or sacrificial agents, demonstrates superior hydrogen production effficiency of 9080 µmolg-1h-1, i.e., 4.78% solar-to-hydrogen conversion efficiency, and exceptional long-term stability, operating over 340h continuously. The superior performance is attributed to several key factors. First, the focus-light induced photothermal effect enhances redox reaction capabilities, while the salt-ions enabled charge polarization around catalyst surfaces extends charge carrier lifetime. Furthermore, the Co─NC@Cu exhibits excellent broad light absorption, promoting photoexcited charge production. Theoretical calculations reveal that Co─NC acts as the active site, showing low energy barriers for reduction reactions. Additionally, the formation of a strong surface electric field from the localized surface plasmon resonance (LSPR) of Cu nanoparticles further reduces energy barriers for redox reactions, improving seawater splitting activity. This work provides valuable insights into intergrating the reaction environment, broad solar absorption, LSPR, and active single atoms into a core-shell photocatalyst design for efficient and robust solar-driven seawater splitting.

Self-passivation of Halide Interstitial Defects by Organic Cations in Hybrid Lead-Halide Perovskites: Ab Initio Quantum Dynamics.

Halide interstitial defects severely hinder the optoelectronic performance of metal halide perovskites, making research on their passivation crucial. We demonstrate, using ab initio nonadiabatic molecular dynamics simulations, that hydrogen vacancies (Hv) at both N and C atoms of the methylammonium (MA) cation in MAPbI3 efficiently passivate iodine interstitials (Ii), providing a self-passivation strategy for dealing with the Hv and Ii defects simultaneously. Hv at the N site (Hv-N) introduces a defect state into the valence band, while the state contributed by Hv at the C site (Hv-C) evolves from a shallow level at 0 K to a deep midgap state at ambient temperature, exhibiting a high environmental activity. Both Hv-N and Hv-C are strong Lewis bases, capable of capturing and passivating Ii defects. Hv-C is a stronger Lewis base, bonds with Ii better, and exhibits a more pronounced passivation effect. The charge carrier lifetimes in the passivated systems are significantly longer than in those containing either Hv or Ii, and even in pristine MAPbI3. Our demonstration of the Hv and Ii defect self-passivation in MAPbI3 suggests that systematic control of the relative concentrations of Hv and Ii can simultaneously eliminate both types of defects, thereby minimizing charge and energy losses. The demonstrated defect self-passivation strategy provides a promising means for defect control in organic-inorganic halide perovskites and related materials and deepens our atomistic understanding of defect chemistry and charge carrier dynamics in solar energy and optoelectronic materials.

Identifying Rare Events in Quantum Molecular Dynamics of Nanomaterials with Outlier Detection Indices.

Nanoscale and condensed matter systems evolve on multiple length- and time-scales, and rare events such as local phase transformation, ion segregation, defect migration, interface reconstruction, and grain boundary sliding can have a profound influence on material properties. We demonstrate how outlier detection indices can be used to identify rare events in machine-learning based, high-dimensional molecular dynamics (MD) simulations. Designed to order data-points from typical to untypical, the indices enable one to capture atomic events that are hard to detect otherwise. We demonstrate the approach with a nanosecond MD simulation of a grain boundary in a metal halide perovskite that is extensively studied for solar energy and optoelectronic applications. The method captures the initial grain boundary sliding and a spontaneous fluctuation half a nanosecond later, both events giving rise to persistent deep electronic trap states that impact charge carrier lifetime and transport and material performance. The approach offers a generalizable and simple method for identifying outlier events in complex condensed matter, molecular, and nanoscale systems.

Efficient photoreforming of plastic waste using a high-entropy oxide catalyst

Simultaneous catalytic hydrogen (H2) production and plastic waste degradation under light, known as photoreforming, is a novel approach to green fuel production and efficient waste management. Here, we use a high-entropy oxide (HEO), a new family of catalysts with five or more principal cations in their structure, for plastic degradation and H2 production. The HEO shows higher activity than that of P25 TiO2, a benchmark photocatalyst, for the degradation of polyethylene terephthalate (PET) plastics in water. Several valuable products are produced by photoreforming of PET bottles and microplastics including H2, terephthalate, ethylene glycol and formic acid. The high activity is attributed to the diverse existence of several cations in the HEO lattice, lattice defects, and appropriate charge carrier lifetime. These findings suggest that HEOs possess high potential as new catalysts for concurrent plastic waste conversion and clean H2 production.

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.

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.

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.

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.

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.