Optimum Band Gap Research Articles

Bandgap engineering of Na1-xAgxSbS2 alloys for photovoltaic applications

NaSbS2 has shown the potential to be a light absorber material for solar cells in recent years. The efficiency of NaSbS2 can be improved by reducing the band gap to the desired value. In this paper, a theoretical approach is used to explore the electronic and optical properties of the alloyed Na1-xAgxSbS2 compounds. The results show that NaSbS2 has good stability when the concentration of Ag doping is 50%. The monoclinic structure is not thermodynamically stable for AgSbS2. The PBE0 functional is used to reveal the electronic structures of the studied compounds. The indirect band gap nature remains unchanged for each compound. An optimum band gap value (∼1.4 eV) can be realized for Na0.50Ag0.50SbS2, and the light absorption intensity and width are increased. The results indicate that Na0.50Ag0.50SbS2 shows great potential for photovoltaic applications. Therefore, alloying is proposed to be an effective way to improve the photovoltaic efficiency of NaSbS2.

Pseudohalide Anions to Suppress Oxidative Degradation for Efficient Formamidinium-Based Sn-Pb Halide Perovskite Solar Cells.

Although binary Sn-Pb perovskites possess optimal band gap approaching to the Shockley-Queisser limit efficiency, the enhancement on power conversion efficiency (PCE) of Sn-Pb perovskite solar cells (PSCs) is impeded by the detrimental oxidation of Sn2+. Herein, a novel and effective strategy is developed to introduce pseudohalide anion thiocyanate (SCN-) with similar ionic radius to iodide to occupy the X-site of the perovskite lattice, thus restraining the rapid oxidation of Sn2+ to Sn4+. The incorporation of SCN- into perovskite stabilizes the perovskite crystal structure thermodynamically and increases the adsorption-energy-barrier of oxygen molecules. The coordination between Sn2+ and SCN- can reduce the defect density by healing the undercoordinated Sn2+ and suppressing the Sn and I vacancies. With the incorporation of SCN-, the ion migration behavior and lattice strain associated with the defects are remarkably relaxed. The study on carrier dynamics based on steady-state and time-resolved photoluminescence suggests that the carrier lifetime and non-radiative recombination rate of SCN- PSCs can be remarkably prolonged and depressed, respectively. As a result, FASn0.5Pb0.5I3-based PSCs achieve a 14.5% increase in PCE, reaching 13.74% under AM 1.5G illumination. This strategy takes a noteworthy step toward high efficiency and high stability FA-based Sn-Pb PSCs.

Stable α‐FAPbI3 in Inverted Perovskite Solar Cells with Efficiency Exceeding 22% via a Self‐Passivation Strategy

AbstractFormamidinium lead iodide (FAPbI3) has endowed power conversion efficiencies (PCEs) up to 25.5% in regular‐structured perovskite solar cells (PSCs) because of its optimal bandgap and enhanced thermal stability. However, the performance of FAPbI3‐based inverted‐structured PSCs is unsatisfactory. Herein, four kinds of commonly used hole transport materials (HTMs) are selected, including PEDOT:PSS, PTAA, NiOx, and MeO‐2PACz, to study their impact on the methylamine chloride (MACl)‐assisted one‐step deposition of FAPbI3 films. It is found that MeO‐2PACz is the optimal substrate for stabilizing black‐phase FAPbI3 and the corresponding inverted‐structured PSCs show the best photovoltaic performance. Nonetheless, the PCE is restricted by low open‐circuit voltage (VOC) due to non‐radiative recombination caused by MACl residues. Therefore, homologous PbI2 in situ passivation is implemented to passivate defects at grain boundaries. The addition of excess PbI2 in precursor solution remarkably decreases charge trap densities and elongates carrier lifetimes. As a result, the optimized device achieves an impressive PCE of 22.13%, which is the highest efficiency of FAPbI3 based on inverted‐structured PSCs. Moreover, the best device exhibits free hysteresis and excellent long‐term stability, maintaining 92% of the initial PCEs after 800 h aging under ambient conditions.

Optimization of a ring-like phononic crystal structure with bonding layers for band gap

In this paper, the band gap (BG) properties of the phononic crystal (PC) involving bonding layers described by the ring-like structure are investigated. Based on the generalized Maxwell model, the effect of the bonding layers on BGs dependence is analyzed. Combing the numerical simulation and physical experiment, BG characteristic of the ring-like PC structure involving bonding layers is proved to be more suitable for the practical application. The effects of several parameters such as the thickness of bonding layers, the height of cuboid scatterer, the height of the elastic beams, and the width of the elastic beams onto BGs are analyzed by numerical simulation, and the last three parameters are demonstrated to be the critical factors for BGs. Based on the response surface method (RSM), three factors and seven levels experiments are designed to determine the relationships between the three key factors and BGs. The optimal BGs of the proposed PC structure involving bonding layers can be opened from 60 Hz to 300 Hz. It is established that the research results can provide possibilities to boost the development and design of PCs involving bonding layers.

Theory-Guided Material Design Enabling High-Performance Multifunctional Semitransparent Organic Photovoltaics without Optical Modulations.

Semitransparent organic photovoltaics (ST-OPVs) have drawn great attention for promising applications in building-integrated photovoltaics, providing additional power generation for daily use. A previously proposed strategy, "complementary NIR absorption," is widely applied for high-performance ST-OPVs. However, rational material design toward high performance has not been achieved. In this work, an external quantum efficiency (EQE) model describing this strategy is developed to explore the full potential of material design on ST-OPV performance. Guided by the model, a novel nonfullerene acceptor (NFA), ATT-9, is designed and synthesized, which possesses optimal bandgap for ST-OPVs, achieving a record short-circuit current density of 30mA cm-2 and a power conversion efficiency of 13.40%, the highest value among devices based on NFAs with bandgaps lower than 1.2eV. It is notworthy that, at such a low bandgap, the energy loss of the device is only 0.58eV, which is attributed to the low energetic disorder confirmed by an ultralow Urbach energy of 21.6 meV. Benefiting from the optimal bandgap and low energy loss, the ATT-9-based ST-OPV achieves a high light utilization efficiency of 3.33% without optical modulations, and meanwhile shows excellent thermal insulation, exceeding the commercial 3M heat-insulating window film, demonstrating the outstanding application prospects of multifunctional ST-OPVs.

On the use of topology optimized band gap structures for the realization of second-order acoustic topological insulators with valley-selective corner states

On the use of topology optimized band gap structures for the realization of second-order acoustic topological insulators with valley-selective corner states

Bismuth Stabilizes the α-Phase of Formamidinium Lead Iodide Perovskite Single Crystals

α-FAPbI3 (FA = formamidinium) perovskite offers an optimal bandgap for single-junction solar cells but converts into a more thermodynamically stable photoinactive δ-polymorph at room temperature. FA- or I-site substitutional alloying stabilizes α-FAPbI3; however, it leads to compositional segregation in operational devices. Here, we stabilize α-FAPbI3 single crystals through Pb-site doping with a heterovalent metal–bismuth (Bi). We show that undoped α-FAPbI3 has an α- to δ-phase half-life transition of <0.15 h, while the optimum concentration of Bi extends it by 4 orders of magnitude. Differential scanning calorimetry (DSC) reveals that Bi has effectively decreased the δ- to α-phase onset transition temperature. Density functional theory (DFT) calculations suggest a relatively clean gap, supporting previous findings on the improved photovoltaic performance of Bi-doped α-FAPbI3-based solar cells.

Sub-ppb-Level Detection of Nitrogen Dioxide Based on High-Quality Black Phosphorus.

The development of gas sensors based on two-dimensional (2D) layered materials has received lots of focus attributing to their excellent gas sensitivity. Here, a black phosphorus (BP) gas sensor device is fabricated based on high-quality few-layered BP microribbons using a facile route. Although BP is well known to oxidize in ambient conditions, energy dispersive spectroscopy (EDS) mapping manifests that the few-layered BP microribbons undergo slight oxidation and contamination during the grinding process. It is interesting that the surface and side of BP microribbons have nanoscale thin films and step-like nanoscale thin films, respectively, owing to the in-plane slip of the few-layered BP microribbons in the process of grinding, which are different from the conventional BP bulk crystals. The layered BP microribbon gas sensor demonstrated a high response to low-concentration NO2 and a very low limit of detection (LOD) of 0.4 ppb of NO2 under N2 and air conditions, which is the lowest LOD for NO2 detection reported so far. The mechanisms for excellently sensitive detection of NO2 for the BP microribbons have been investigated by first-principles calculations combined with experiment results, revealing that the sensitization mechanisms of the BP microribbon sensor are abundant nanoscale thin films, an optimum bandgap range with optimal carrier concentration, a hierarchical homojunction structure, and strong adsorption energy to NO2. In addition, the BP microribbon sensor demonstrated high selectivity to NO2, a low LOD under a high relative humidity, and good repeatability. The reported results of the BP sensor may provide great promise for improving the performance of other 2D material-based gas sensors and may expand sensing applications.

The impact of Ga and S concentration and gradient in Cu(In,Ga)(Se,S)2 solar cells

As one of the leading photovoltaic materials, Cu(In,Ga)(Se,S)2 has two distinct advantages over other candidates. First, its conduction band and valence band could be independently tuned by adjusting the Ga/(Ga + In) and S/(S + Se), respectively. Next, its composition distribution could be feasibly tuned by controlling the thin film deposition process. These merits have been utilized in fabricating the Cu(In,Ga)(Se,S)2 solar cells with world record efficiency. However, these features have not yet been satisfactorily simulated. In order to understand the mechanism of high-performance Cu(In,Ga)(Se,S)2 solar cells, we obtained the tunable bandgap of absorber layer and suitable conduction band offset between absorber layer and buffer layer via Ga and S concentration and gradient, by combining both first-principle calculations and numerical simulations with Poisson equation and the continuity equation. In particular, compared with other photovoltaic materials, the bandgap of Cu(In,Ga)(Se,S)2 could be tuned flexibly across the layer thickness to realize spatial energy band optimization with double-concentration-grading absorber. The efficiency of 23.29% was obtained based on theoretical calculations and numerical simulations, which is almost the same as the reported result, in the considerations of (i) optimum bandgap of absorber, (ii) band energy offset, (iii) double (Ga- and S-) concentration grading, and (iv) low defect density in the absorber layer. In addition, we obtained a Cu(In,Ga)(Se,S)2 solar cell with a high S content uniform band gap efficiency of 23.53%. Therefore, we provided an understanding for tunable engineered band energy research in high-efficient Cu(In,Ga)(Se,S)2 solar cells.

Design and analysis of III-V two-dimensional van der Waals heterostructures for ultra-thin solar cells

Van der Waals (vdW) heterostructures consist of six group III-V monolayers MX (M = Ga, In, X = P, As, Sb) have been systematically investigated via hybrid functional HSE06 based on DFT calculations in order to evaluate their utilities in potential solar cell materials. 24 different stacking configurations for the 15 heterostructures each, with a total of 360 different structure models, are designed and discussed. These vdW heterostructures are found to be energetically favorable with formation energies, calculated using DFT-D3 method, vary from −1.5 eV to 0.5 eV. The energetic and electronic properties of these heterostructures are examined. Our results show that the stacking model BB-3 for most heterostructures holds suitable type-II bandgaps around 1.1 eV, which guarantee the potential for solar cell applications. Furthermore, the power conversion efficiencies (PCEs) for GaSb/InSb and GaAs/InAs are found to reach 22.88% and 20.65% respectively. The optimal bandgap size, efficient optical absorption with ultrahigh photoresponsivity enable these materials to be promising candidates in optoelectronic devices.

Effect of dopant gas pressure on the growth of magnetron sputtered CuO thin films for electrical and optical applications

Tunability offers in the optoelectronic properties of TCOs open up the way to advanced applications in the field of photovoltaics. However, the conductivity of p-type TCOs remains an unresolved problem. In this work, an advantage of RF magnetron sputtering is employed to obtain polycrystalline CuO film at room temperature with better conductivity and optimum bandgap by controlling nitrogen gas pressure. Doping of N2 gas pressure has a significant influence on the morphology of CuO films, resulting in improved electrical and optical properties of films at room temperature. The variation of structural characteristics with respect to N2 gas pressure is discussed in detail using XRD, AFM, SEM, and EDX measurements, and these results confirm nitrogen's impact in improving the property of the fabricated film. The results show that the crystalline size, as well as the crystalline quality of the film, was found to increase with the increase in N2 gas pressure at room temperature. Atomic force microscope (AFM) shows that all the deposited films are well adherent and grains are uniformly distributed over the surface of the substrate. The simultaneous effect of doping and grain boundary migration of nitrogen atom into CuO lattice increases crystallinity and the mobility of charge carriers at room temperature. This experimental strategy made CuO a good p-type material with mobility of 2.17 × 103cm2/Vs and bandgap of 2.44eV at N2 gas pressure of 2 × 10−2mbar and resistivity is decreased due to high mobility obtained by tuning N2 gas pressure. These results suggest that the electrical and optical properties of CuO thin films can be modulated by optimizing the carrier concentration and thus mobility by controlling the N2 gas pressure.

First-principles calculations to investigate the electronic and optical properties of Cu2ZnSnS4 with Ag and Se codoping

In recent years, Cu2ZnSnS4 has drawn great attention owing to its outstanding performance, such as nontoxic and earth-abundant elements, suitable band gap, and high absorption coefficient. In the present study, we have explored the impact of the partial substitution of Cu and S with Ag and Se on the optoelectronic properties of Cu2ZnSnS4. The results suggest that the calculated lattice constants of A2ZnSnX4 (A = Cu, Ag; X = S, Se) with the PBE functional are in good agreement with the experimental values. Furthermore, the band gap obtained by the PBE0 functional for A2ZnSnX4 is consistent well with the experimental data. (Cu0.5Ag0.5)2ZnSn(S0.5Se0.5)4 displays an optimum direct band gap with 1.40 eV and shows wide and high absorption coefficient. Our work demonstrates that the studied compound is proposed to be a promising candidate for environmentally friendly photovoltaic material.

Promising applications of wide bandgap inorganic perovskites in underwater photovoltaic cells

Power supply for underwater devices such as vehicles, sensors and autonomous systems is an important factor which limits their practical application. Solar energy is a promising energy source but high-efficient solar cells on land cannot maintain their original superiority for the change of underwater solar irradiance caused by absorption and scattering in water. Here, we investigate the solar irradiance for water with different clarity and discuss the potential applications of different materials in underwater photovoltaic cells. By calculating and comparing the device performance of underwater solar cells (UWSCs) with different bandgap, it is demonstrated that wide bandgap (1.8 eV-2.3 eV) inorganic perovskite solar cells (IPSCs) have significant advantages for underwater applications due to its outstanding photoelectric properties and suitable range of bandgap. The IPSCs can obtain the power conversion efficiency (PCE) of 41.9% and power density of 3.11 mW/cm2 at the depth of 50 m in water, which is obviously higher than silicon solar cells. No matter what types of water we choose, the optimal bandgap of UWSC’s active layer get wider with the increasing of water depth and the wide bandgap inorganic perovskites with 2.0–2.2 eV is an optimal choice. Our work reveals the significance of IPSCs in underwater photovoltaic applications.

Photo- and Electrocatalytic Reduction of CO2 over Metal-Organic Frameworks and Their Derived Oxides: A Correlation of the Reaction Mechanism with the Electronic Structure.

A Ce/Ti-based bimetallic 2-aminoterephthalate metal-organic framework (MOF) was synthesized and evaluated for photocatalytic reduction of CO2 in comparison with an isoreticular pristine monometallic Ce-terephthalate MOF. Owing to highly selective CO2 adsorption capability, optimized band gaps, higher flux of photogenerated electron-hole pairs, and a lower rate of recombination, this material exhibited better photocatalytic reduction of CO2 and lower hydrogen evolution compared to Ce-terephthalate. Thorough probing of the surface and electronic structure inferred that the reducibility of Ce4+ to Ce3+ was due to the introduction of an amine functional group into the linker, and low-lying Ti(3d) orbitals in Ce/Ti-2-aminoterephthalate facilitated the photoreduction reaction. Both the MOFs were calcined to their respective oxides of Ce1-xTixO2 and CeO2, and the electrocatalytic reduction of CO2 was performed over the oxidic materials. In contrast to the photocatalytic reaction mechanism, the lattice substitution of Ti in the CeO2 fluorite cubic structure showed a better hydrogen evolution reaction and consequently, poorer electroreduction of CO2 compared to pristine CeO2. Density functional theory calculations of the competitive hydrogen evolution reaction on the MOF and the oxide surfaces corroborated the experimental findings.

Computational Exploration Toward Tunnel Oxide Passivated Contact (TOPCon) Solar Cells: Tailoring Higher Efficiency

AbstractDespite the inescapable technical challenges and unclear operating mechanisms, the last few years have witnessed considerable advances in tunnel oxide passivated contact (TOPCon) structured solar cells, most of which are centered around improving the device performance so as to truly become a step forward from current mainstream technologies. However, a systematic numerical exploration is urgently required to gain a thorough understanding of the effect of the core parameters of the device on the final performance. Here, the numerical simulation way is used to explore the potential of TOPCon technology, focused on the pursuit of higher efficiency. An exhaustive analysis concerning tunnel SiOx and doped polysilicon (poly‐Si) with field passivation effect is carried out to tailor excellent surface passivation. The simulation also suggests that the ultra‐thin SiOx with extremely low pinhole density can suppress the recombination of carriers, thus promoting the passivation quality. Additionally, simulation research is conducted on the potential of using poly‐SiOx(n+) as the doped layer, and an efficiency of 27.60% is realized via adjusting the optimal band gap and dopant concentration. Briefly, this work presents a comprehensive computational analysis of the tunnel oxide, doped layer and their synergistic impacts on the final performance.

Additive‐Free, Low‐Temperature Crystallization of Stable α‐FAPbI3 Perovskite

AbstractFormamidinium lead triiodide (FAPbI3) is attractive for photovoltaic devices due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium‐based perovskites. Crystallization of phase‐pure α‐FAPbI3 conventionally requires high‐temperature thermal annealing at 150 °C whilst the obtained α‐FAPbI3 is metastable at room temperature. Here, aerosol‐assisted crystallization (AAC) is reported, which converts yellow δ‐FAPbI3 into black α‐FAPbI3 at only 100 °C using precursor solutions containing only lead iodide and formamidinium iodide with no chemical additives. The obtained α‐FAPbI3 exhibits remarkably enhanced stability compared to the 150 °C annealed counterparts, in combination with improvements in film crystallinity and photoluminescence yield. Using X‐ray diffraction, X‐ray scattering, and density functional theory simulation, it is identified that relaxation of residual tensile strains, achieved through the lower annealing temperature and post‐crystallization crystal growth during AAC, is the key factor that facilitates the formation of phase‐stable α‐FAPbI3. This overcomes the strain‐induced lattice expansion that is known to cause the metastability of α‐FAPbI3. Accordingly, pure FAPbI3 p–i–n solar cells are reported, facilitated by the low‐temperature (≤100 °C) AAC processing, which demonstrates increases of both power conversion efficiency and operational stability compared to devices fabricated using 150 °C annealed films.

Exploring the stability and physical properties of double perovskite Cs2SnI6 by doping the Cl/Br atom: A DFT study

In recent years, it is of great interest to develop stable and high efficiency lead-free perovskite materials. Double perovskite Cs2SnX6 (X = Cl, Br, I) shows excellent stability and tunable band gap. The stability, electronic, and optical properties of three mixed halide double perovskites Cs2SnI4Cl2, Cs2SnI4ClBr, and Cs2SnI4Br2 are studied in this work. The stability of three mixed halide compounds is slightly lower than that of Cs2SnI6. The trend of phase separation is found for the studied compounds. Compared with CsSnX3, the SOC effect is negligible for Cs2SnX6. Cs2SnI4Cl2 and Cs2SnI4ClBr have the same band gaps with 1.75 eV, while Cs2SnI4Br2 shows an optimum band gap with 1.56 eV. Although these mixed halide compounds display stronger absorption coefficients than Cs2SnI6 from 300 to 450 nm, the lower light absorption capacity is observed between 450 and 800 nm.

Influence of sulfurization time and Cu-ZnS-Sn stack order on the properties of thermally evaporated CZTS thin films

Cu2ZnSnS4 (CZTS) thin films were synthesized in a two-step procedure. Sulfurization of stacked thin films SLG/ZnS/Sn/Cu (S1) and SLG/Cu/Sn/ZnS (S2) after sequential deposition of Cu, ZnS, and Sn precursors was carried out. At 550 °C, two sulfurization periods were applied to both stack orders. Sample S1 sulfurized for 30 min (S1-T30) had a better crystallite size of roughly 50 nm, lower lattice strain, and lower dislocation density than other samples. The Cu/Zn cation ordering in the CZTS crystal system was significantly affected by stack sequence and sulfurization time, according to Q-factor calculation. The stack order of the S1 series allowed for homogenous and distinct particle development. From the elemental analysis, it is observed that the stack sequence and sulfurization used for sample S1-T30 permitted a near stoichiometric composition. The sample S1-T30 exhibited an optimal band gap value of 1.47 eV. To propose feasible alterations in the structural ordering, band gap calculations were used. In comparison to the stack order of the S2 series, the stack order SLG/ZnS/Sn/Cu with a sulfurization time of 30 min created a single-phase CZTS, implying less synthesis time to obtain an absorber quality CZTS layer for solar photovoltaic application.

Geometric parameters optimization of a two-dimensional phoxonic crystal with dual forbidden band characteristics

In this paper, a novel phoxonic crystal (PxC) structure composed of silicon, with optimal dual phononic band gap (PNBG) and photonic band gap (PTBG), is presented. Using the finite element analysis method, both the transmission characteristics and dispersion relation of PNBG and PTBG are calculated, and the existence of dual BGs is demonstrated by the means of the analysis of transmission for the PxC structure. The influences of structural parameters on the dual forbidden band characteristics are further explored, the sensitive structure parameters can be determined: the width of elastic beams, the length of square silicon, and the length of square hole. Using the orthogonal test, 25 experimental runs based on 3-factor and 5-level experiment are performed to finish the numerical experimental design and analysis. Four functional relationships can be acquired between the three sensitive parameters and dual BGs. Finally, the unified objective function method is employed to perform the construction of the single objective optimization model for the purpose of obtaining the optimal dual BGs and the corresponding optimal parameter combinations of the PxC structure. Such scheme can be used as the potential optimization way, which may find wide application in the development and design of PxCs.

Polymer Based Functional Materials: A New Generation Photo‐active Candidate for Electrochemical Application

AbstractThis article reviews the successive development of photo‐assisted electrochemical performances using functionalized polymer materials. The photoelectrochemical technologies have already been well‐established as a major tool for generating renewable energy harvesting solar power. In this regard, the research on the generation of new photoactive polymer supported units has drawn remarkable attention to improve the power conversion output and build up low‐cost advanced devices. So, starting from fundamental working principle, we have tried to overview comprehensively the key features of most of these studies, which involve the tactical schemes behind the appropriate selection of different substituents to functionalize the polymer skeleton to achieve an optimal bandgap, challenges faced specially in the index of performance in post synthetic phase, the possible routes to overcome the hurdle, so that it can harvest photons matched with the solar spectrum and of course their relevance in the broad range of application window. Apart from photovoltaic solar cell application, the idea has been logically extended to cover highly selective photoelectrochemical sensing devices,where we have disclosed briefly the role of polymers in designing innovative biosensors, the trend of development along with specific illustrative examples, and some of their successful utilization in real time assay. The overall description is entirely focused on how these strategies have emerged from the background concepts and finally shaped to a viable and significant outcome.