Optimum Band Gap Research Articles

Solution processed AgSbS2 film for efficient planar heterojunction solar cells

AgSbS2 is a promising absorber material for photovoltaic cells because of its optimum bandgap, strong optical absorption, and excellent stability. Here, we report a spin-coating and annealing approach for the fabrication of AgSbS2 solar cells, where Ag-Sb-thiourea complex solution was prepared as the precursor solution. We identified that the annealing temperature crucially affected the phase composition, crystallinity, and surface morphology of the AgSbS2 film. We also probed the electronic structures and established a FTO/TiO2/AgSbS2/Spiro-OMeTAD/Au device structure. This device finally achieved an encouraging power conversion efficiency of 2.25%, which is highest efficiency in AgSbS2 solar cells. Our research opens up another prospect for pursuing high performance AgSbS2 thin film solar cells by adopting a solution processing method and planar heterojunction device structure.

Exfoliation method matters: The microstructure-dependent photoactivity of g-C3N4 nanosheets for water purification

Exfoliation of carbon nitride (g-C3N4) into an ultrathin nanostructure significantly improves its photoactivity. However, the effects of the exfoliation method on the microstructure and photocatalytic performance of g-C3N4 nanosheets remain largely unknown. Herein, several typical strategies, such as thermal, chemical, ultrasonic and one-step exfoliation, were applied to exfoliate g-C3N4 nanosheets for photocatalytic applications. A procedure capable of controlling the morphology, microstructure, light-absorption property, and visible light photoactivity of g-C3N4 nanosheets was attempted. We found that nanosheets prepared from one-step exfoliation present superior photocatalytic efficiency under visible light than those fabricated by thermal exfoliation and ultrasonic exfoliation. The kinetic constants for bisphenol A (BPA) photodegradation over these samples were determined to be 6.5, 4.5 and 2.3 times higher than bulk g-C3N4, respectively. For chemical exfoliation, excessive oxidation by H2SO4 can lead to the structural defects and deactivation of urea-derived g-C3N4 nanosheets. Carbon nitride nanosheets synthesized by one-step exfoliation exhibited high specific surface area, optimal band gap energy structure, and high charge separation efficiency, thereby increasing visible-light photoactivity. Enabling cost-effective production of ultrathin and robust g-C3N4 nanosheets, one-step exfoliation offers a potential strategy to exploit high-performance g-C3N4 for water purification applications.

Deposition and characterization of stannite Cu2FeSn(S0·8Se0.2)4 thin film for potential absorber layer in solar cell application

This work investigates the properties of CFTSSe material synthesized by melt quench technique and subsequent thin film deposition using thermal evaporation method. To study the compositional, structural, morphological and optical properties of the material, various characterization techniques like XRD, SEM, EDS, Raman spectroscopy, FTIR and UV–Vis–NIR spectroscopy have been applied. These studies show that CFTSSe is crystalline in nature and consistent with the stannite structure. Whereas some large density electronic defects have been detected in the lattice. UV visible spectroscopy estimates a high absorption coefficient (>104 cm−1) and optimal band gap of 1.49 eV. Additionally, the properties of CFTSSe are also compared with our previous study of CZTSSe and appeared to have potential to be used in thin film solar cell applications.

Effect of copper precursor layer thickness on the properties of preferentially oriented Cu4SnS4 thin films for photovoltaic applications

Copper tin sulfide (Cu–Sn–S) thin films have gathered great attention in the research for earth-abundant, non-toxic, and low-cost thin film solar cells. In this investigation, preferentially oriented Cu 4 SnS 4 thin films have been fabricated via a combination of chemical bath deposition and thermal evaporation for the first time. Stacked precursor layers of thermally evaporated copper and chemically deposited SnS thin films were annealed in the sulfur atmosphere at a temperature of 823 K for 1 h. The copper layer thickness was varied from 150 to 200 nm. Various characterization techniques were employed to study the structure, morphology, optical and electrical properties of the Cu 4 SnS 4 films (XRD, Raman, FE-SEM, AFM, UV–Vis spectroscopy) formed at different conditions. The orthorhombic structured films formed at the best conditions showed highly oriented growth along (102) plane. An intense peak at 316 cm −1 in the Raman spectrum further confirmed the formation of the Cu 4 SnS 4 phase. The crystalline sizes and bandgap energies of deposited films varied between 33.6 and 52.7 nm and 1.0–1.6 eV, respectively. The sample with a copper precursor layer thickness of 185 nm exhibited an optimum bandgap of 1.4 eV and larger grains implies that a copper layer of 185 nm in thickness on 200 nm SnS is an optimal precursor layer thickness for achieving highly oriented Cu 4 SnS 4 thin film with excellent properties. • First attempt to fabricate Cu 4 SnS 4 thin films via a combination of physical-chemical deposition methods. • Cu 4 SnS 4 thin films with novel preferential orientation along (102) plane. • Optimization of copper precursor layer thickness resulted enhanced crystallinity. • The optimum bandgap of 1.4 eV for photovoltaic applications.

HTL-Free Sb2(S, Se)3 Solar Cells with an Optimal Detailed Balance Band Gap

Antimony chalcogenides are widely studied as a light-absorbing material due to their merits of low toxicity, efficient cost, and excellent photovoltaic properties. However, the band gaps of antimony selenide (approximately 1.1 eV) and antimony sulfide (approximately 1.7 eV) both deviate from the optimal detailed balance band gap (∼1.3 eV) for terrestrial single-junction solar cells. Notably, the band gap of Sb2(S, Se)3 can be tunable in the range from 1.1 to 1.7 eV, which can cover the detailed balance band gap. In this work, the vapor transport deposition method with two independent evaporation sources is used to deposit Sb2(S, Se)3 thin films. By carefully optimizing the evaporation temperature and the start evaporation time of the Sb2Se3 and Sb2S3 sources, a suitable band gap of 1.33 eV is obtained. Finally, on the basis of the optimal Sb2(S, Se)3 films, Sb2(S, Se)3 solar cells without a hole transport layer achieved an efficiency of 7.03%.

Photo-thermo catalytic selective oxidation of cyclohexane by In-situ prepared nonstoichiometric Molybdenum oxide and Silver-palladium alloy composite

The highly selective oxidation of cyclohexane to cyclohexanone and cyclohexanol (KA oil) is one of the most challenging issues in the chemical industry. However, the difficulty in attaining high selectivity and high conversion rate in parallel for the existing catalysts limits its practical application. In this paper, a novel photo-thermo synergistic catalyst was reported for the aerobic oxidation of cyclohexane. The uniform blue MoO3-x nanowires with small diameter stabilized by polyvinyl pyrrolidone (PVP) were synthesized by a hydrothermal method, and a series of MoO3-x-AgPd composite materials of different proportions were prepared by an in-situ reduction process. The morphology, crystalline structure, surface chemical bonding, photoelectrochemical properties of MoO3-x-AgPd composites are thoroughly characterized. The MoO3-x-AgPd composites present significantly increased catalytic performance than MoO3-x nanowires in the photo-thermo synergistic catalytic oxidation of cyclohexane under dry air. The high conversion rate of 11.3% with the KA oil selectivity of 99.0% was achieved by the MoO3-x-Ag20Pd20 composites under photo-thermo catalytic process at 120 ℃, which is 1.5 times of that by MoO3-x nanowires. Under photo-thermo catalytic process, a high cyclohexane conversion rate similar to that of higher temperature thermal catalysis can be obtained at lower reaction temperature, and more cyclohexanol can be produced with a ketone to alcohol (K/A) ratio of 0.254. The significantly enhanced catalytic activity can be attributed to the effective charge transfer in the AgPd alloy nanoparticles, the optimized band gap structure, the suppressed charge recombination, and the promoted photo-thermo synergetic catalytic effect. This work provides a new reference scheme for the design and preparation of high-efficiency photo-thermo catalysts for the selective oxidation of cyclohexane.

Synthesis and Characterization of Cu2ZnSnS4 Thin Films Obtained by Combined Magnetron Sputtering and Pulsed Laser Deposition.

Cu2ZnSnS4 (CZTS) is a complex quaternary material, and obtaining a single-phase CZTS with no secondary phases is known to be challenging and dependent on the production technique. This work involves the synthesis and characterization of CZTS absorber layers for solar cells. Thin films were deposited on Si and glass substrates by a combined magnetron sputtering (MS) and pulsed laser deposition (PLD) hybrid system, followed by annealing without and with sulfur powder at 500 °C under argon (Ar) flow. Three different Cu2S, SnS2, and ZnS targets were used each time, employing a different target for PLD and the two others for MS. The effect of the different target arrangements and the role of annealing and/or sulfurization treatment were investigated. The characterization of the absorber films was performed by grazing incidence X-ray diffraction (GIXRD), X-ray reflectometry (XRR), Raman spectroscopy, scanning electron microscopy, and regular transmission spectroscopy. The film with ZnS deposited by PLD and SnS2 and Cu2S by MS was found to be the best for obtaining a single CZTS phase, with uniform surface morphology, a nearly stoichiometric composition, and an optimal band gap of 1.40 eV. These results show that a new method that combines the advantages of both MS and PLD techniques was successfully used to obtain single-phase Cu2ZnSnS4 films for solar cell applications.

Phononic Bandgap Optimization in Sandwich Panels Using Cellular Truss Cores

The development of custom cellular materials has been driven by recent advances in additive manufacturing and structural topological optimization. These contemporary materials with complex topologies have better structural efficiency than traditional materials. Particularly, truss-like cellular structures exhibit considerable potential for application in lightweight structures owing to their excellent strength-to-mass ratio. Along with being light, these materials can exhibit unprecedented vibration properties, such as the phononic bandgap, which prohibits the propagation of mechanical waves over certain frequency ranges. Consequently, they have been extensively investigated over the last few years, being the cores for sandwich panels among the most important potential applications of lattice-based cellular structures. This study aims to develop a methodology for optimizing the topology of sandwich panels using cellular truss cores for bandgap maximization. In particular, a methodology is developed for designing lightweight composite panels with vibration absorption properties, which would bring significant benefits in applications such as satellites, spacecraft, aircraft, ships, automobiles, etc. The phononic bandgap of a periodic sandwich structure with a square core topology is maximized by varying the material and the geometrical properties of the core under different configurations. The proposed optimization methodology considers smooth approximations of the objective function to avoid non-differentiability problems and implements an optimization approach based on the globally convergent method of moving asymptotes. The results show that it is feasible to design a sandwich panel using a cellular core with large phononic bandgaps.

Effect of heat treatment on the characteristics and NH3 Sensing properties of Tin Dioxide SnO2 Thin Film

In this research, Spray pyrolysis technique used to prepared Tin dioxide (SnO2) thin films on glass substrate with thickness about 200± 8 nm by dissolved 2.2563 g of SnCl2.2H2O in 100 ml of ethanol then added 60 drops from concentrated hydrochloric acid (HCl). After that the films were annealed at different temperatures (300, 400 and 400 °C ). X-ray analyses shows that the structure of all SnO2 films is polycrystalline with tetragonal rutile crystalline structure with preferential orientation in the direction (200).The optical measurements show that the optical transition has been direct and the average band gap has been tendency to decreases from 3.98 eV to 3.73 eV with increasing of Ta. The extent and nature of transmittance and optimized band gap of the material assure to utilize it for sensor applications.From sensing measurements for NH3 gas at different operating temperature (100,200,250 and 300)°C and gas concentration 1(5%, 10%, 15%, 20%, 25%, 30%, and 35% ) can be seen that the sensitivity increases with increasing operating temperature and gas concentration, the response time decreases to smallest value (4s) at operating temperature 200 °C while the recovery time (22 s).

Use of metamodels for rapid discovery of narrow bandgap oxide photocatalysts

SummaryNew photocatalysts are traditionally identified through trial-and-error methods. Machine learning has shown considerable promise for improving the efficiency of photocatalyst discovery from a large potential pool. Here, we describe a multi-step, target-driven consensus method using a stacking meta-learning algorithm that robustly predicts bandgaps and H2 evolution activities of photocatalysts. Trained on small datasets, these models can rapidly screen a large space (>10 million materials) to identify promising, non-toxic compounds as candidate water splitting photocatalysts. Two effective compounds and two controls possessing optimal bandgap values (∼2 eV) but not photoactivity as predicted by the models were synthesized. Their experimentally measured bandgaps and H2 evolution activities were consistent with the predictions. Conspicuously, the two compounds with strong photoactivities under UV and visible light are promising visible-light-driven water splitting photocatalysts. This study demonstrates the power of machine learning and the potential of big data to accelerate discovery of next-generation photocatalysts.

Comparative Study of Recombination Dynamics in Optimized Composition of Sn- Versus Pb-Based Perovskite Solar Cells.

The energy band gaps of Pb halide perovskites are higher than the optimal band gap required for single-junction solar cells, governed by the Shockley-Queisser radiative limit. The pure Sn and Pb-Sn mixed-based perovskites have drawn significant attention due to their ability to lead to lower band gaps and open a new door for all perovskite tandem applications. There has been continuous progress toward the rapid improvement in the power conversion efficiency of Sn and Pb-Sn mixed-based perovskite solar cells (PSCs). Along with efforts for efficiency, it is worth analyzing the in-depth recombination dynamics for further development of Sn-based PSCs. The lower bimolecular recombination rate constant (k) is often attributed to the high performance of PSCs. Herein, we study the role of "B" cations in charge carrier recombination dynamics (CCRD) of ABX3 (A = MA+, FA+, and Cs+; B = Pb2+, Sn2+, and X = I-)-based PSCs. We fabricated p-i-n configuration-based FA0.95Cs0.05PbI3 (pure Pb), MA0.20FA0.75Cs0.05SnI3 (pure Sn), and (MAPbI3)0.4(FASnI3)0.6 (Pb-Sn mixed) PSCs and compared the CCRD of all the three PSCs. We optimized the Sn-based perovskite thin film (pure Sn) in terms of moisture and thermal stability in order to minimize the error due to perovskite degradation. We note that despite having lower open-circuit voltage (VOC), a pure Sn-based PSC shows lower k than that of Pb-Sn mixed and pure Pb-based PSCs, which is a contradictory result. This slow relaxation lifetime of the charge carrier in Sn-based PSCs can be correlated with recombination through the defect states without introducing the quasi-Fermi-level splitting. Furthermore, our results suggest that the rate law of charge carrier decay has nonlinear dependence of k on n in Sn-based PSCs, whereas it is linear in the other two cases.

Co-precipitation Synthesis with a Variation of the Sulphur Composition of Kesterite Phase Cu2ZnSnS4 (CZSS) without Annealing Process

Commercially available compound CuInGa (S, Se) can be replaced with emerging quaternary compound Cu2ZnSnS4 (Copper Zinc Tin Sulphur or CZSS) for photovoltaic applications due to the high absorption coefficient and optimum bandgap. Unstable sulphur and the co-existence of binary and ternary phases in CZSS are the main obstacles for a single-phase kesterite quaternary compound. To overcome these issues, the researchers are synthesising the CZSS in presence of sulphur and selenium environment. The sulphurization and selenization are the constraints for the synthesis of CZSS and these processes make it costlier. In the present work, the wet-chemical method (i.e., co-precipitation method) was used to synthesise CZSS without vacuum annealing where the sulphur constituent was controlled by changing the stoichiometric ratio. X-ray diffraction (XRD) and Raman analysis confirm that the synthesised CZSS was in polycrystalline and single-phase kesterite nature. The average crystallite sizes for thiourea 16, 18, 20 mmol were found 15 nm, 17 nm and 17 nm, respectively. Surface morphology of the as-prepared film was identified by scanning electron microscope (SEM) and optical bandgap of the film was obtained ~1.33 eV by UV-visible (UV-vis) analysis. The 18 mmol of thiourea with stoichiometric ratio 4:2:2:9 is found the best optimisation for synthesising the CZSS without vacuum annealing by the co-precipitation method. Thus, the thin film of such synthesised CZSS may be employed for the low-cost photovoltaic application.

Band gap Engineering of ZnO via transition metal Doping: An ab initio study

Ab initio density functional theory calculations are performed to calculate the electronic and optical properties of transition metal doped ZnO, in order to achieve optimum bandgap reduction required for a potential photocatalyst. Geometry optimization analysis reveals that Fe, Cr, Mn and Nb doped systems are the most stable structures among our studied materials, which are further investigated for their aforementioned properties. The analysis of the Cr doped system show the maximum decrease in the bandgap and a red shift toward the visible region due to the valence electrons, suggesting that Cr doped ZnO is substantially a promising photocatalytic material for dye degradation applications.

Solution-processed Cd-substituted CZTS nanocrystals for sensitized liquid junction solar cells

The Earth-abundant kesterite Cu2ZnSnS4 (CZTS) exhibits outstanding structural, optical, and electronic properties for a wide range of optoelectronic applications. However, the efficiency of CZTS thin-film solar cells is limited due to a range of factors, including electronic disorder, secondary phases, and the presence of anti-site defects, which is a key factor limiting the Voc. The complete substitution of Zn lattice sites in CZTS nanocrystals (NCs) with Cd atoms offers a promising approach to overcome several of these intrinsic limitations. Herein, we investigate the effects of substituting Cd2+ into Zn2+ lattice sites in CZTS NCs through a facile solution-based method. The structural, morphological, optoelectronic, and power conversion efficiencies (PCEs) of the NCs synthesized have been systematically characterized using various experimental techniques, and the results are corroborated by first-principles density functional theory (DFT) calculations. The successful substitution of Zn by Cd is demonstrated to induce a structural transformation from the kesterite phase to the stannite phase, which results in the bandgap reduction from 1.51 eV (kesterite) to 1.1 eV (stannite), which is closer to the optimum bandgap value for outdoor photovoltaic applications. Furthermore, the PCE of the novel Cd-substituted liquid junction solar cell underwent a four-fold increase, reaching 1.1%. These results highlight the importance of substitutional doping strategies in optimizing existing CZTS-based materials to achieve improved device characteristics.

Investigating optical, structural and morphological properties of polycrystalline CdTe thin-film deposited by RF magnetron sputtering

In this work, optimization of the physical properties of cadmium telluride (CdTe) thin films has been carried out to improve its performance by the influence of RF power. CdTe Nanocrystalline thin films were characterized by using various techniques such as atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD). XRD study revealed that CdTe films are polycrystalline and have preferential growth in (111) direction. FESEM images showed a continuous and dense morphology of CdTe films on glass substrates. The AFM result shows that the surface roughness of films increases with annealing. Optical properties were investigated with the help of the UV–visible absorption spectrum which showed that the bandgap decreases with an increase in Radio-frequency power. Further, density functional theory (DFT) calculations showed that the optimum bandgap i.e. 1.57 eV for CdTe films deposited at 100 W RF power with 2% N2 is in good agreement with the experimental result. This study showed that CdTe films deposited at 100 W RF power with 2% N2 can be used as an excellent absorbing material for the application of optoelectronic devices solar cells.

Universal Current Losses in Perovskite Solar Cells Due to Mobile Ions

AbstractEfficient mixed metal lead‐tin halide perovskites are essential for the development of all‐perovskite tandem solar cells, however they are currently limited by significant short‐circuit current losses despite their near optimal bandgap (≈1.25 eV). Herein, the origin of these losses is investigated, using a combination of voltage dependent photoluminescence (PL) timeseries and various charge extraction measurements. It is demonstrated that the Pb/Sn‐perovskite devices suffer from a reduction in the charge extraction efficiency within the first few seconds of operation, which leads to a loss in current and lower maximum power output. In addition, the emitted PL from the device rises on the exact same timescales due to the accumulation of electronic charges in the active layer. Using transient charge extraction measurements, it is shown that these observations cannot be explained by doping‐induced electronic charges but by the movement of mobile ions toward the perovskite/transport layer interfaces, which inhibits charge extraction due to band flattening. Finally, these findings are generalized to lead‐based perovskites, showing that the loss mechanism is universal. This elucidates the negative role mobile ions play in perovskite solar cells and paves a path toward understanding and mitigating a key loss mechanism.

Comparing and Quantifying Indoor Performance of Organic Solar Cells

AbstractWith increasing efficiencies of non‐fullerene acceptor‐based organic solar cells, thin‐film technology is becoming a promising candidate for indoor light harvesting applications. However, the lack of standardized comparison methods makes it difficult to quantify progress and to compare indoor performance. Herein, a simple method to calculate the efficiency of solar cells under any possible light source and illuminance with only using simple standard measurements (current–voltage curves and quantum efficiency) is presented. Thereby, equal evaluation conditions are ensured, so that indoor solar cells can be ranked and compared according to their efficiency. Efficiencies are shown to typically vary by ±20% when using different different light emitting diode spectra with color temperatures ranging from 2700 to 6500 K. Calculations based on a detailed balance model indicate that the optimal bandgap of the absorber material depends on the used light source and ranges between 1.75 and 2 eV. The approach is validated by comparison with literature data and many calculated efficiencies match well with experimental data obtained with a specific light source. However, some reported efficiencies cannot be reproduced with the model, which highlights the need to reassess low light measuring techniques. Furthermore, a script is provided for use by the community.

Gap Sensitivity Reveals Universal Behaviors in Optimized Photonic Crystal and Disordered Networks.

Through an extensive series of high-precision numerical computations of the optimal complete photonic band gap (PBG) as a function of dielectric contrast α for a variety of crystal and disordered heterostructures, we reveal striking universal behaviors of the gap sensitivity S(α)≡dΔ(α)/dα, the first derivative of the optimal gap-to-midgap ratio Δ(α). In particular, for all our crystal networks, S(α) takes a universal form that is well approximated by the analytic formula for a 1D quarter-wave stack, S_{QWS}(α). Even more surprisingly, the values of S(α) for our disordered networks converge to S_{QWS}(α) for sufficiently large α. A deeper understanding of the simplicity of this universal behavior may provide fundamental insights about PBG formation and guidance in the design of novel photonic heterostructures.

Assessment and prediction of band edge locations of nitrides using a self-consistent hybrid functional.

Due to their optimal bandgap size and large defect tolerance, nitrides are becoming pivotal materials in several optoelectronic devices, photovoltaics, and photocatalysts. A computational method that can accurately predict their electronic structures is indispensable for exploring new nitride materials. However, the relatively small bandgap of nitrides, which stems from the subtle balance between ionic and covalent bond characteristics, makes conventional density functional theory challenging to achieve satisfactory accuracy. Here, we employed a self-consistent hybrid functional where the Hartree-Fock mixing parameter is self-consistently determined and thus the empiricism of the hybrid functional is effectively removed to calculate the bandgaps of various nitride compounds. By comparing the bandgaps from the self-consistent hybrid functional calculations with the available experimental and high-level GW calculation results, we found that the self-consistent hybrid functional can provide a computationally efficient approach for quantitative predictions of nitride electronic structures with an accuracy level comparable to the GW method. Additionally, we aligned the band edge positions of various nitride compounds using self-consistent hybrid functional calculations, providing material design principles for heterostructures of nitride-based optoelectronic devices. We anticipate the wide use of the self-consistent hybrid functional for accelerating explorations and predictions of new nitride-based functional materials in various photoactive applications.

Obtaining the solid solution Sb2S3-xSex by selenization of Sb2S3 film and identifying the thermal processing parameters to achieve recrystallization while maintaining phase-purity

In this work, thin films of the ternary material Sb2S3-xSex with promising optical and electrical properties were developed for its application as absorber material in photovoltaic devices. It was found that 400 °C is the lowest recrystallization temperature at which phase-pure polycrystalline films of Sb2S3-xSex can be obtained. Films were recrystallized in a mixture of S/Se vapor by varying the proportion of Se and the film properties were studied as a function of Se content. The conductivity and photosensitivity were increased with the increase in the amount of Se in films. The activation energies associated with the defects or traps in the films decrease with the amount of Se. The morphology of the film changes significantly depending on the annealing pressure. Films recrystallized in an ambient of S/Se vapor generated from 10 mg each of S and Se at 400 °C and 460 Torr pressure are found to have the optimum band gap, highest photosensitivity, and phase-purity. Assuming an appropriate window layer, the modeling result shows that the obtained film Sb2S0.6Se2.4 has the suitable material properties that can result in solar cells with efficiencies in the range of 27%.