Strongly Enhanced Photovoltaic Performance and Defect Physics of Air-Stable Bismuth Oxyiodide (BiOI)

Review on synthetic approaches and PEC activity performance of bismuth binary and mixed-anion compounds for potential applications in marine engineering

Photoelectrochemical (PEC) technology in marine engineering holds significant importance due to its potential to address various challenges in the marine environment. Currently, PEC-type applications in marine engineering offer numerous benefits, including sustainable energy generation, water desalination and treatment, photodetection, and communication. Finding novel efficient photoresponse semiconductors is of great significance for the development of PEC-type techniques in the marine space. Bismuth-based semiconductor materials possess suitable and tunable bandgap structures, high carrier mobility, low toxicity, and strong oxidation capacity, which gives them great potential for PEC-type applications in marine engineering. In this paper, the structure and properties of bismuth binary and mixed-anion semiconductors have been reviewed. Meanwhile, the recent progress and synthetic approaches were discussed from the point of view of the application prospects. Finally, the issues and challenges of bismuth binary and mixed-anion semiconductors in PEC-type photodetection and hydrogen generation are analyzed. Thus, this perspective will not only stimulate the further investigation and application of bismuth binary and mixed-anion semiconductors in marine engineering but also help related practitioners understand the recent progress and potential applications of bismuth binary and mixed-anion compounds.

Halide Engineering in Mixed Halide Perovskite-Inspired Cu2AgBiI6 for Solar Cells with Enhanced Performance

Cu2AgBiI6 (CABI) is a promising perovskite-inspired absorber for solar cells due to its direct band gap and high absorption coefficient. However, the nonradiative recombination caused by the high extrinsic trap density limits the performance of CABI-based solar cells. In this work, we employ halide engineering by doping bromide anions (Br-) in CABI thin films, in turn significantly improving the power conversion efficiency (PCE). By introducing Br- in the synthetic route of CABI thin films, we identify the optimum composition as CABI-10Br (with 10% Br at the halide site). The tailored composition appears to reduce the deep trap density as shown by time-resolved photoluminescence and transient absorption spectroscopy characterizations. This leads to a dramatic increase in the lifetime of charge carriers, which therefore improves both the external quantum efficiency and the integrated short-circuit current. The photovoltaic performance shows a significant boost since the PCE under standard 1 sun illumination increases from 1.32 to 1.69% (∼30% relative enhancement). Systematic theoretical and experimental characterizations were employed to investigate the effect of Br- incorporation on the optoelectronic properties of CABI. Our results highlight the importance of mitigating trap states in lead-free perovskite-inspired materials and that Br- incorporation at the halide site is an effective strategy for improving the device performance.

Growth of BiSBr Microsheet Arrays for Enhanced Photovoltaics Performance

In this study, single-crystalline BiSBr is synthesized using a solution-based approach and conducted a systematic characterization of its photoelectric properties and photovoltaic performances. UV photoelectron spectroscopy and density functional theory (DFT) calculations reveal that BiSBr is an indirect p-type semiconductor, characterized by distinct positions and compositions of the valence band maximum and conduction band minimum. The BiSBr single crystal microrod features a significant electrical conductivity of 14 800 S m-1 along the c-axis, denoting minimal carrier resistance in this direction. For photovoltaic performance assessment, the authors successfully fabricated two homogeneous BiSBr films on TiO2 porous substrates: A microsheet array film via physical vapor deposition (PVD) and solvothermal treatment, and a BiSBr microsheet film via PVD and thermal treatment. The solar cell, comprising a BiSBr microsheet array film with an architecture of fluorine-doped tin oxide FTO/TiO2/BiSBr/(I3 -/I-)/Pt, demonstrated a power conversation efficiency of 1.40%, ≈11 times that of BiSBr microsheet film counterpart. These preliminary results underscore the potential of BiSBr microsheet arrays, producible through low-cost solution processes, as adept light absorbers, enhancing photovoltaic efficiency through effective light scattering and promoting efficient electron-hole separation and transport.

Potential application of bismuth oxyiodide (BiOI) when it meets light

Bismuth oxyiodide (BiOI) is a kind of typical two-dimensional (2D) material that has been increasingly developed alongside other 2D materials such as graphene, MXenes, and transition-metal dichalcogenide. However, its potential applications have not been widely whispered compared to those of other 2D materials. Using its distinctive properties, BiOI can be used for various applications, especially when it meets sunlight and other light-related electromagnetic waves. In this present review, we discuss the developments of BiOI and challenges in the applications for photodetector and light-assisted sensors, photovoltaic devices, optoelectronic logic devices, as well as photocatalysts. We start the discussion with a basic understanding and development of BiOI, crystal structure, and its properties. The synthesis and further development, such as green synthesis and its challenges in the synthesis-suited industry, as well as device integration, are also explained together with a plausible strategy to enhance the feasibility of BiOI for those various applications. We believe that the provided discussion and perspectives will not only promote BiOI to be one of the highly considered 2D materials but can also assist recent graduates in any materials science discipline and inform the senior scientists and industrial-based stakeholders of the latest advances in bismuth oxide and mixed-anion compounds.

Halide Perovskites and Their Derivatives for Efficient, High-Resolution Direct Radiation Detection: Design Strategies and Applications

The past decade has witnessed a rapid rise in the performance of optoelectronic devices based on lead-halide perovskites (LHPs). The large mobility-lifetime products and defect tolerance of these materials, essential for optoelectronics, also make them well-suited for radiation detectors, especially given the heavy elements present, which is essential for strong X-ray and γ-ray attenuation. Over the past decade, LHP thick films, wafers, and single crystals have given rise to direct radiation detectors that have outperformed incumbent technologies in terms of sensitivity (reported values up to 3.5 × 106 µC Gyair -1 cm-2 ), limit of detection (directly measured values down to 1.5 nGyair s-1 ), along with competitive energy and imaging resolution at room temperature. At the same time, lead-free perovskite-inspired materials (e.g., methylammonium bismuth iodide), which have underperformed in solar cells, have recently matched and, in some areas (e.g., in polarization stability), surpassed the performance of LHP detectors. These advances open up opportunities to achieve devices for safer medical imaging, as well as more effective non-invasive analysis for security, nuclear safety, or product inspection applications. Herein, the principles behind the rapid rises in performance of LHP and perovskite-inspired material detectors, and how their properties and performance link with critical applications in non-invasive diagnostics are discussed. The key strategies to engineer the performance of these materials, and the important challenges to overcome to commercialize these new technologies are also discussed.

Emerging BaZrS3 and Ba(Zr,Ti)S3 Chalcogenide Perovskite Solar Cells: A Numerical Approach Toward Device Engineering and Unlocking Efficiency

BaZrS3 chalcogenide perovskites have emerged as a promising absorber due to their exceptional properties. However, there are no experimental reports on the applicability of BaZrS3 in photovoltaics. Thus, theoretical knowledge of device structure engineering is essential for its successful fabrication. In this regard, we have proposed various BaZrS3 device configurations by altering 12 electron transport layers (ETLs) in combination with 13 hole transport layers (HTLs) using SCAPS-1D, wherein a total of 782 devices are simulated by tuning the thickness, carrier concentration, and defect density of BaZrS3, ETLs, and HTLs. Interestingly, the absorber's thickness optimization enhanced the absorption in the device by 2.31 times, elevating the generation rate of charge carriers, while the increase in its carrier concentration boosted the built-in potential from 0.8 to 1.68 V, reducing the accumulation of charge carriers at the interfaces. Notably, on further optimization of ETL and HTL combinations, the best power conversion efficiency (PCE) of 28.08% is achieved for FTO/ZrS2/BaZrS3/SnS/Au, occurring due to the suppressed barrier height of 0.1 eV at the ZrS2/BaZrS3 interface and degenerate behavior of SnS, which increased charge carrier transportation and conductivity of the devices. Upon optimizing the work function, an ohmic contact is achieved for Pt, boosting the PCE to 28.17%. Finally, the impact of Ti alloying on BaZrS3 properties is examined on the champion FTO/ZrS2/BaZrS3/SnS/Pt device where the maximum PCE of 32.58% is obtained for Ba(Zr0.96,Ti0.04)S3 at a thickness of 700 nm due to extended absorption in the NIR region. Thus, this work opens doors to researchers for the experimental realization of high PCE in BaZrS3 devices.

Influence of chemical interactions on the electronic properties of BiOI/organic semiconductor heterojunctions for application in solution-processed electronics

Bismuth oxide iodide (BiOI) has been viewed as a suitable environmentally-friendly alternative to lead-halide perovskites for low-cost (opto-)electronic applications such as photodetectors, phototransistors and sensors. To enable its incorporation in these devices in a convenient, scalable, and economical way, BiOI thin films were investigated as part of heterojunctions with various p-type organic semiconductors (OSCs) and tested in a field-effect transistor (FET) configuration. The hybrid heterojunctions, which combine the respective functionalities of BiOI and the OSCs were processed from solution under ambient atmosphere. The characteristics of each of these hybrid systems were correlated with the physical and chemical properties of the respective materials using a concept based on heteropolar chemical interactions at the interface. Systems suitable for application in lateral transport devices were identified and it was demonstrated how materials in the hybrids interact to provide improved and synergistic properties. These indentified heterojunction FETs are a first instance of successful incorporation of solution-processed BiOI thin films in a three-terminal device. They show a significant threshold voltage shift and retained carrier mobility compared to pristine OSC devices and open up possibilities for future optoelectronic applications.

Membraneless, self-powered immunosensing of a cardiac biomarker by exploiting a PEC platform based on CaBi2Ta2O9 combined with bismuth oxyiodides

This work describes the development of a membraneless, self-powered immunosensor exploiting a photoelectrochemical system based on two photoelectrodes for cardiac troponin I (cTn). An electrode based on CaBi2Ta2O9 combined with bismuth oxyiodides (BiOI/Bi4O5I2/Bi5O7I) was modified with the cTnI antibody (anti-cTnI) and applied in a photoelectrochemical cell as a photoanode. To perform the cTnI detection exploiting a self-powered photoelectrochemical setup, the immunosensor (anti-cTnI/BiOI/Bi4O5I2/Bi5O7I/CaBi2Ta2O9/FTO) was coupled to a photoelectrochemical cell containing a photocathode based on CuBi2O4 (CBO/FTO) for zero-biased photoelectrochemical immunosensing of cardiac troponin I (cTnI) biomarker. For comparison purposes, the photoanode was applied for cTnI detection in a three-electrode electrochemical cell. The spectroscopic, structural, and morphological characteristics of the photoelectrochemical (PEC) materials were evaluated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Electrochemical impedance spectroscopy (EIS) measurements were performed in the presence and absence of light to investigate the effects of photons on the charge transfer resistance of the photoanode. The influence of the cTnI biomarker on the photoelectrochemical response of the anti-cTnI antibody-modified photoelectrochemical platform (anti-cTnI/BiOI/Bi4O5I2/Bi5O7I/CaBi2Ta2O9/FTO) was evaluated by measuring the photocurrent of the system. The immunosensor presented a linear response ranging from 1 pg mL-1 to 200 ng mL-1 as well as a mean recovery percentage between 95.7% and 108.0% in real human serum samples for the cTnI biomarker.

Bandlike Transport and Charge-Carrier Dynamics in BiOI Films

Following the emergence of lead halide perovskites (LHPs) as materials for efficient solar cells, research has progressed to explore stable, abundant, and nontoxic alternatives. However, the performance of such lead-free perovskite-inspired materials (PIMs) still lags significantly behind that of their LHP counterparts. For bismuth-based PIMs, one significant reason is a frequently observed ultrafast charge-carrier localization (or self-trapping), which imposes a fundamental limit on long-range mobility. Here we report the terahertz (THz) photoconductivity dynamics in thin films of BiOI and demonstrate a lack of such self-trapping, with good charge-carrier mobility, reaching ∼3 cm² V^-1 s^-1 at 295 K and increasing gradually to ∼13 cm² V^-1 s^-1 at 5 K, indicative of prevailing bandlike transport. Using a combination of transient photoluminescence and THz- and microwave-conductivity spectroscopy, we further investigate charge-carrier recombination processes, revealing charge-specific trapping of electrons at defects in BiOI over nanoseconds and low bimolecular band-to-band recombination. Subject to the development of passivation protocols, BiOI thus emerges as a superior light-harvesting semiconductor among the family of bismuth-based semiconductors.

Perovskite-inspired materials for energy applications

Lead-halide perovskites have come to dominate the emerging photovoltaics research scene over the past decade. But whilst perovskite photovoltaics exhibit exceptional efficiencies, their limited stability, as well as the toxicity of their lead component remain challenges. This focus collection captures a snapshot of the efforts in the community to address these challenges, from modifications to the synthesis and device structure of perovskite photovoltaics to improve their stability, through to efforts to understand, develop, and improve lead-free perovskite-inspired materials (PIMs). PIMs range from direct perovskite-derivatives (e.g. CsSnI3or halide elpasolites) through to electronic analogs (e.g. BiOI). The collection discusses the application of these materials not only for solar cells, but also more broadly for photodetection, light emission, and anti-counterfeiting devices. This collection emphasizes the diversity of strategies and directions in this field, as well as its highly interdisciplinary nature.

Lead-free perovskite-inspired semiconductors for indoor light-harvesting - the present and the future

Are lead-free perovskite-inspired materials (PIMs) the wise choice for efficient yet sustainable indoor light harvesting? This feature article outlines how wide-bandgap PIMs can provide a positive answer to this compelling question. The wide band gaps can hinder sunlight absorption, in turn limiting the solar cell performance. However, PIMs based on group VA of the periodic table can theoretically lead to an outstanding indoor power conversion efficiency up to 60% when their band gap is ∼2 eV. Yet, the research on PIM-based indoor photovoltaics (IPVs) is still in an early stage with highest indoor device efficiencies up to 10%. This article reviews the recent advancements on PIMs for IPVs and identifies the main limiting factors of device performance, thus suggesting effective strategies to address them. We emphasize the poor operational stability of the IPV devices of PIMs being the key bottleneck for the vast adoption of this technology. We believe that this report can provide a solid scaffolding for further researching this fascinating class of materials, ultimately supporting our vision that, upon extensive advancement of the stability and efficiency, PIMs with wide bandgap will become a contender for the next-generation absorbers for sustainable indoor light harvesting.

Photobio-electrocatalytic production of H2 using fluorine-doped tin oxide (FTO) electrodes covered with a NiO-In2S3 p-n junction and NiFeSe hydrogenase

Clean energy vectors are needed towards a fossil fuel-free society, diminishing both greenhouse effect and pollution. Electrochemical water splitting is a clean route to obtain green hydrogen, the cleanest fuel; although efficient electrocatalysts are required to avoid high overpotentials in this process. The combination of inorganic semiconductors with biocatalysts for photoelectrochemical H₂ production is an alternative approach to overcome this challenge. N-type semiconductors can be coupled to a co-catalyst for H₂ production in the presence of a sacrificial electron donor in solution, but the replacement of the latter with an electrode is a challenge. In this work we attach a NiFeSe-hydrogenase with high activity for H₂ production with the n-type semiconductor indium sulfide, which upon visible irradiation is able to transfer its excited electrons to the enzyme. In order to enhance the transfer of the generated holes towards the electrode for their replenishment, we have explored the inclusion of a p-type material, NiO, to induce a p-n junction for H₂ production in a photoelectrochemical biocatalytic system in absence of sacrificial reagents.

Enhanced Charge Separation and Transfer Efficiency of BiOI with the Dominantly Exposed (102) Facet for Sensitive Photoelectrochemical Photodetection

Bismuth oxyiodide (BiOI) has attracted much attention as a kind of novel functional material because of its highly anisotropic crystal structure and promising optical properties. However, the low photoenergy conversion efficiency of BiOI highly limits its practical applications owing to its poor charge transport. Tailoring the crystallographic orientation has emerged as an effective way to modulate the charge transport efficiency, while there is nearly no report on BiOI. In this study, (001)- and (102)-oriented BiOI thin films were synthesized for the first time with mist chemical vapor deposition at atmospheric pressure. The photoelectrochemical response for the (102)-oriented BiOI thin film was much better than that of the (001)-oriented thin film, owing to the enhanced charge separation and transfer efficiency. The intensive surface band bending and larger donor density for (102)-oriented BiOI were the main origins of the efficient charge transport. Besides, the BiOI-based photoelectrochemical-type photodetector exhibited excellent photodetection performance with a high responsivity of 78.33 mA W^-1 and a detectivity of 4.61 × 10¹¹ Jones for visible light. This work provided fundamental insights into anisotropic electrical and optical properties in BiOI, which would be beneficial for the design of bismuth mixed-anion compound-based photoelectrochemical devices.

Heavy pnictogen chalcohalides for efficient, stable, and environmentally friendly solar cell applications

Solar cell technology is an effective solution for addressing climate change and the energy crisis. Therefore, many researchers have investigated various solar cell absorbers that convert Sunlight into electric energy. Among the different materials researched, heavy pnictogen chalcohalides comprising heavy pnictogen cations, such as Bi³⁺and Sb³⁺, and chalcogen-halogen anions have recently been revisited as emerging solar absorbers because of their potential for efficient, stable, and low-toxicity solar cell applications. This review explores the recent progress in the applications of heavy pnictogen chalcohalides, including oxyhalides and mixed chalcohalides, in solar cells. We categorize them into material types based on their common structural characteristics and describe their up-to-date developments in solar cell applications. Finally, we discuss their material imitations, challenges for further development, and possible strategies for overcoming them.

New bismuth oxyiodide/chitosan nanocomposite for ultrasonic waves expedited adsorptive removal of amoxicillin from aqueous medium: kinetic, isotherm and thermodynamic investigations

Amoxicillin (AMX) is a widely used antibiotic, which induces harmful effects to nature via bioaccumulation and persistence in the environment if discharged untreated into water bodies. In the current study, a novel bionanocomposite, bismuth oxyiodide-chitosan (BiOI-Ch), was synthesized by a facile precipitation method and its amoxicillin (AMX) adsorption capacity in the presence of ultrasonic waves has been explored. Multiple batch experiments were performed to achieve the optimum operational parameters for maximum adsorption of AMX and the obtained results were as follows: pH 3, 80 mg g^-1 AMX concentration, 1.7 g L^-1 adsorbent dose, temperature 298 K and ultrasonication time 20 min. Composite removed approximately 90% AMX from the solution under optimized conditions, while the maximal adsorption capacity was determined to be 81.01 mg g^-1. BiOI-Ch exhibited superior adsorption capacity as compared to pure BiOI (33.78 mg g^-1). To understand the dynamics of reaction, several kinetic and isotherm models were also examined. The adsorption process obeyed pseudo-second-order kinetic model (R² = 0.98) and was well fitted to Freundlich isotherm (R² = 0.99). The addition of biowaste chitosan to non-toxic bismuth-based nanoparticles coupled with ultrasonication led to enhanced functional groups as well as surface area of the nanocomposite resulting in superior adsorption capacity, fast adsorption kinetics and improved mass transfer for the removal of AMX molecules. Thus, this study demonstrates the synergistic effect of ultrasonication in improved performance of novel BiOI-Ch for potential application in the elimination of persistent and detrimental pollutants from industrial effluent after necessary optimization for large-scale operation.

Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination

I-V-VI₂ ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have slowly-rising absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS₂ nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >10⁵ cm^-1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS₂ nanocrystals. These unusual features arise because of the localised, non-bonding S p character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.

Long-term solar water and CO2 splitting with photoelectrochemical BiOI-BiVO4 tandems

Photoelectrochemical (PEC) devices have been developed for direct solar fuel production but the limited stability of submerged light absorbers can hamper their commercial prospects.^1,2 Here, we demonstrate photocathodes with an operational H₂ evolution activity over weeks, by integrating a BiOI light absorber into a robust, oxide-based architecture with a graphite paste conductive encapsulant. In this case, the activity towards proton and CO₂ reduction is mainly limited by catalyst degradation. We also introduce multiple-pixel devices as an innovative design principle for PEC systems, displaying superior photocurrents, onset biases and stability over corresponding conventional single-pixel devices. Accordingly, PEC tandem devices comprising multiple-pixel BiOI photocathodes and BiVO₄ photoanodes can sustain bias-free water splitting for 240 h, while devices with a Cu₉₂In₈ alloy catalyst demonstrate unassisted syngas production from CO₂.

Modulation of the Bi3+ 6s2 Lone Pair State in Perovskites for High-Mobility p-Type Oxide Semiconductors

Oxide semiconductors are key materials in many technologies from flat-panel displays，solar cells to transparent electronics. However, many potential applications are hindered by the lack of high mobility p-type oxide semiconductors due to the localized O-2p derived valence band (VB) structure. In this work, the VB structure modulation is reported for perovskite Ba₂ BiMO₆ (M = Bi, Nb, Ta) via the Bi 6s² lone pair state to achieve p-type oxide semiconductors with high hole mobility up to 21 cm² V^-1 s^-1 , and optical bandgaps widely varying from 1.5 to 3.2 eV. Pulsed laser deposition is used to grow high quality epitaxial thin films. Synergistic combination of hard x-ray photoemission, x-ray absorption spectroscopies, and density functional theory calculations are used to gain insight into the electronic structure of Ba₂ BiMO₆ . The high mobility is attributed to the highly dispersive VB edges contributed from the strong coupling of Bi 6s with O 2p at the top of VB that lead to low hole effective masses (0.4-0.7 m_e ). Large variation in bandgaps results from the change in the energy positions of unoccupied Bi 6s orbital or Nb/Ta d orbitals that form the bottom of conduction band. P-N junction diode constructed with p-type Ba₂ BiTaO₆ and n-type Nb doped SrTiO₃ exhibits high rectifying ratio of 1.3 × 10⁴ at ±3 V, showing great potential in fabricating high-quality devices. This work provides deep insight into the electronic structure of Bi³⁺ based perovskites and guides the development of new p-type oxide semiconductors.

Wide Bandgap Oxide Semiconductors: from Materials Physics to Optoelectronic Devices

Wide bandgap oxide semiconductors constitute a unique class of materials that combine properties of electrical conductivity and optical transparency. They are being widely used as key materials in optoelectronic device applications, including flat-panel displays, solar cells, OLED, and emerging flexible and transparent electronics. In this article, an up-to-date review on both the fundamental understanding of materials physics of oxide semiconductors, and recent research progress on design of new materials and high-performing thin film transistor (TFT) devices in the context of fundamental understanding is presented. In particular, an in depth overview is first provided on current understanding of the electronic structures, defect and doping chemistry, optical and transport properties of oxide semiconductors, which provide essential guiding principles for new material design and device optimization. With these principles, recent advances in design of p-type oxide semiconductors, new approaches for achieving cost-effective transparent (flexible) electrodes, and the creation of high mobility 2D electron gas (2DEG) at oxide surfaces and interfaces with a wealth of fascinating physical properties of great potential for novel device design are then reviewed. Finally, recent progress and perspective of oxide TFT based on new oxide semiconductors, 2DEG, and low-temperature solution processed oxide semiconductor for flexible electronics will be reviewed.

Ultrafast saturable absorption of BiOI nanosheets prepared by chemical vapor transport

The saturable absorption properties of BiOI nanosheets with exposed {110} facets prepared by the chemical vapor transport were investigated by Z-scan with femtosecond pulse laser. The nonlinear absorption coefficient at 400 nm is stronger and more sensitive to photoexcitation than its nonlinear response at 800 nm. The small saturation intensity could have been achieved, which is one order of magnitude smaller than that of black phosphorus nanosheets, while the Imχ⁽³⁾ are determined to be -4.35×10^-12esu close to theoretical prediction. According to time-resolved photoluminescence spectrum results, this strong saturated absorption at 400 nm may be attributed to the interband recombination process, whose lifetime was 230 ps.

Controlled hydrothermal synthesis of BiOxCly/BiOmBrn/g-C3N4 composites exhibiting visible-light photocatalytic activity

The first systematic synthesis of bismuth oxychloride/bismuth oxybromide/graphitic carbon nitride (BiO_xCl_y/BiO_mBr_n/g-C₃N₄) nano-composites used a controlled hydrothermal method. The structure, morphology and characteristic of BiO_xCl_y/BiO_mBr_n/g-C₃N₄ photocatalyst were measured by XRD, UV-vis-DRS, FT-IR, FE-TEM, FE-SEM-EDS, PL, BET, HR-XPS and EPR. Under visible light irradiation, the photodegradation activity was evaluated for the decolorization of crystal violet (CV) and 2-hydroxybenzoic acid (2-HBA) in aqueous solution. The catalytic performance showed that, when using sample BB2C1-4-250-30 wt% g-C₃N₄ composite as a photocatalyst, the best reaction-rate-constant (k) was 0.071 h^-1. It was 1.5 times higher than the k value of BB2C1-4-250 as a photocatalyst. From the scavenging effect of various scavengers, the results of EPR showed that reactive OH was the main scavenger, while O₂^-, h⁺ and ¹O₂ were the second scavenger in CV degradation. In this study, a possible photodegradation mechanism was proposed and discussed. In this work, our method of BiO_xCl_y/BiO_mBr_n/g-C₃N₄ preparation could be used for future mass production and the BiO_xCl_y/BiO_mBr_n/g-C₃N₄ composite materials could be applied to the environmental pollution control in future.

Enhanced Photovoltaic Performance of BiSCl Solar Cells Through Nanorod Array

BiSCl single-crystalline nanofibers were synthesized by a facile one-pot solvothermal approach for the first time. BiSCl possesses a double chain type structure and grows readily along the c-axis, resulting the fibrous morphology. UV/Vis absorption spectroscopy revealed that BiSCl nanofibers exhibit a strong light absorption in a wavelength range from UV to visible light, corresponding to a bandgap of 1.96 eV. Ultraviolet photoelectron spectroscopy and density functional theory calculations revealed that BiSCl is a direct n-type semiconductor with valence band maximum and conduction band minimum located at 6.04 and 4.08 eV below the vacuum level, respectively. To investigate the photovoltaic performance, the homogeneous thin film of BiSCl-nanorod array was fabricated on a TiO₂ porous film by a modified solvothermal process, where the nanorod array is oriented vertically to the surface of the TiO₂ porous film. A proper band alignment of BiSCl-based solar cells with an architecture of fluorine-doped tin oxide (FTO)/TiO₂ /BiSCl/(I₃ ^- /I^- )/Pt gave a PCE of 1.36 % and a relatively large short-circuit photocurrent density of 9.87 mA cm^-2 for the first time. The preliminary photovoltaic study result revealed a potential possibility of BiSCl-nanorod array as a light absorber for solar cells that can be fabricated by the low-cost solution process.

Alternative Lone-Pair ns2 -Cation-Based Semiconductors beyond Lead Halide Perovskites for Optoelectronic Applications

Lead halide perovskites have emerged in the last decade as advantageous high-performance optoelectronic semiconductors, and have undergone rapid development for diverse applications such as solar cells, light-emitting diodes , and photodetectors. While material instability and lead toxicity are still major concerns hindering their commercialization, they offer promising prospects and design principles for developing promising optoelectronic materials. The distinguished optoelectronic properties of lead halide perovskites stem from the Pb²⁺ cation with a lone-pair 6s² electronic configuration embedded in a mixed covalent-ionic bonding lattice. Herein, we summarize alternative Pb-free semiconductors containing lone-pair ns² cations, intending to offer insights for developing potential optoelectronic materials other than lead halide perovskites. We start with the physical underpinning of how the ns² cations within the material lattice allow for superior optoelectronic properties. We then review the emerging Pb-free semiconductors containing ns² cations in terms of structural dimensionality, which is crucial for optoelectronic performance. For each category of materials, the research progresses on crystal structures, electronic/optical properties, device applications, and recent efforts for performance enhancements are overviewed. Finally, the issues hindering the further developments of studied materials are surveyed along with possible strategies to overcome them, which also provides an outlook on the future research in this field.

Nb/Se Co-doped BiOI nanomaterials with exposed (110) facets for enhanced visible-light-driven photocatalytic activity

Herein, an efficient strategy is demonstrated to prepare a visible-light-driven Nb/Se co-doped BiOI photocatalyst with exposed (110) facets. The results show that its photocatalytic activity is around 17 times higher than that of pure BiOI. This work paves the way towards the fabrication of efficient photocatalysts that have tunable charge dynamics.

Efficient and Stable Antimony Selenoiodide Solar Cells

Although antimony selenoiodide (SbSeI) exhibits a suitable bandgap as well as interesting physicochemical properties, it has not been applied to solar cells. Here the fabrication of SbSeI solar cells is reported for the first time using multiple spin-coating cycles of SbI3 solutions on Sb2Se3 thin layer, which is formed by thermal decomposition after depositing a single-source precursor solution. The performance exhibits a short-circuit current density of 14.8 mA cm-2, an open-circuit voltage of 473.0 mV, and a fill factor of 58.7%, yielding a power conversion efficiency (PCE) of 4.1% under standard air mass 1.5 global (AM 1.5 G, 100 mW cm-2). The cells retain ≈90.0% of the initial PCE even after illuminating under AM 1.5G (100 mW cm-2) for 2321 min. Here, a new approach is provided for combining selenide and iodide as anions, to fabricate highly efficient, highly stable, green, and low-cost solar cells.

Perovskite-inspired materials for photovoltaics and beyond-from design to devices

Lead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform industry-standard multi-crystalline silicon solar cells, despite the fact that perovskites are typically grown at low temperature using simple solution-based methods. However, the toxicity of lead and its ready solubility in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temperature, facile fabrication methods. This Review discusses the computational and experimental approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compounds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials in other optoelectronic applications, namely light-emitting diodes, photocatalysts, radiation detectors, thin film transistors and memristors. Finally, the prospects and key challenges faced by the field in advancing the development of perovskite-inspired materials towards realization in commercial devices is discussed.

Annealing effects on structural and photovoltaic properties of the dip-SILAR-prepared bismuth oxyhalides (BiOI, Bi7O9I3, Bi5O7I) films

Bismuth oxyhalides are becoming a promising contender for photovoltaic applications due to its non-toxic nature and decent optical properties. This study mainly deals with clarifying the effects of phase transformations on the structure, optical, and electrical properties of BiOI thin film prepared via dip-successive ionic layer adsorption and reaction (SILAR) method at different annealing temperatures ranging from 100 to 400 °C. Therefore, significant phase transformations (i.e., the existence of Bi7O9I3 and Bi5O7I have been confirmed at 300 °C and 400 °C, respectively) appeared in the produced films, which were mainly due to the change of annealing temperatures. The experimental results confirmed that produced films achieved the maximum current density and efficiency and minimum current density and efficiency at 100 °C and 400 °C, respectively. Experimental results were also showed that with increasing the annealing temperature from 100 to 400 °C, the indirect bandgap risen from 1.77 to 2.96 eV while the crystallite size decreased from 17.62 to 12.99 nm. The energy band diagram with electrolyte explained the observed poor electrical properties during the phase transformation. Hence, this result will add positive impacts on the new information on findings for the dip-SILAR-prepared BiOI photovoltaic cells.

Optimizing the Electronic Structure of In2O3 through Mg Doping for NiO/In2O3 p-n Heterojunction Diodes

In₂O₃ is a wide bandgap oxide semiconductor, which has the potential to be used as an active material for transparent flexible electronics and UV photodetectors. However, the high concentration of unintentional background electrons existing in In₂O₃ makes it hard to be modulated by the electric field or form p-n heterojunctions with a sufficient band-bending width at the interface. In this work, we report the reduction of the background electrons in In₂O₃ by Mg doping (Mg-In₂O₃) and thereby improve the device performance of p-n diodes based on the NiO/Mg-In₂O₃ heterojunction. In particular, Mg doping compensates the free electrons in In₂O₃ and reduces the electron concentration from 1.7 × 10¹⁹ cm^-3 without doping to 1.8 × 10¹⁷ cm^-3 with 5% Mg doping. Transparent p-n heterojunction diodes were fabricated based on p-type NiO and n-type Mg-In₂O₃. The device performance was considerably enhanced by Mg doping with a high rectification ratio of 3 × 10⁴ and a remarkable high breakdown voltage of >20 V. High-resolution X-ray photoelectron spectroscopy was used to investigate the interfacial electronic structure between NiO and Mg-In₂O₃, revealing a type II band alignment with a valence band offset of 1.35 eV and a conduction band offset of 2.15 eV. A large built-in potential of 0.98 eV was found for the undoped In₂O₃ but decreased to 0.51 eV for 5% Mg doping of In₂O₃. The NiO/Mg-In₂O₃ diodes with an improved rectification ratio and wider depletion region provide the possibility of achieving photodetectors with rapid photoresponse.

p-n Heterojunction of BiOI/ZnO nanorod arrays for piezo-photocatalytic degradation of bisphenol A in water

p-n Heterojunctions of BiOI/ZnO nanorod arrays (BiOI/ZnO NRs) were prepared by loading the p-type BiOI nanosheets on the n-type ZnO nanorod arrays for efficient removal of organic contaminants in water during the piezo-photocatalytic degradation. Under concurrent visible-light irradiation and ultrasonic vibration, the bisphenol solution (50 mL, 10 mg/L) could be completely degraded within 30 min by 10 mg of 0.15 BiOI/ZnO NRs. It shows a dramatically-enhanced degradation efficiency under light irradiation and ultrasonic vibration, which is four times as high as that only under light irradiation. The excellent piezo-photocatalytic ability of BiOI/ZnO NRs could be attributed to the piezoelectric effect coupling with photocatalytic process. Under the irradiation of light, the electron-hole pairs were generated in BiOI nanosheets, and the piezoelectric potential is created inside the highly oriented one-dimensional ZnO nanorods by ultrasonic vibration, which can accelerate the migration of photogenerated carriers. It shows a strategy to effectively enhance the photocatalytic activity through utilizing the internal piezoelectric potential, which is generated by the one-dimensional nanorods with piezoelectric properties under ultrasonic vibration. So, it can promote the separation and prolong the lifetime of photogenerated carriers, and result in high-efficient degradation of organic contaminants.

Epitaxial growth of bismuth oxyhalide thin films using mist CVD at atmospheric pressure

We report the epitaxial growth of bismuth oxyhalide BiOX (X = Cl, Br, and I) thin films using mist chemical vapour deposition at atmospheric pressure. The thin films grown under optimum conditions possessed atomically flat surfaces and high crystallinity, where the lattice constants of BiOX were controlled by epitaxial strain.

Quantification of Surface Reactivity and Step-Selective Etching Chemistry on Single-Crystal BiOI(001)

To bridge the gap between the cleanliness of a freshly cleaved surface of 2D BiOI and that available from a purely chemical-etching means, we subjected single-crystal BiOI to a series of surface treatments and quantified the resulting chemical states and electronic properties. Vapor transport syntheses included both physical vapor transport from single-source BiOI, as well as chemical vapor transport from Bi₂O₃ + BiI₃ and from Bi + I₂ + Bi₂O₃. Surface treatments included tape cleaving, rinsing in water, sonication in acetone, an aqueous HF etch, and a sequential HF etch with subsequent sonication in acetone. X-ray diffraction, XRD, and X-ray photoelectron spectroscopy, XPS, probed the resulting bulk crystalline species and interfacial chemical states, respectively. In comparison with overlayer models of idealized oxide-terminated or iodide-terminated BiOI, angle-resolved XPS elucidated surface terminations as a function of each treatment. Ultraviolet photoelectron spectroscopy, UPS, established work-function, and Fermi-level energies for each treatment. Data reveal that HF etching yields interfacial BiI₃ at BiOI steps that is subsequently removed with acetone sonication. UPS establishes n-type behavior for the vapor-transport-synthesized BiOI, and surface work function and Fermi level shifts for each chemical treatment under study. We discuss the implications for processing BiOI nanofilms for energy-conversion applications.

Chemical Surface Reactivity and Morphological Changes of Bismuth Triiodide (BiI3) under Different Environmental Conditions

Layered metal halides like BiI₃ are of current interest in connection with both 2D materials and photovoltaics. Here, we present a facile new method for the preparation of millimeter-sized BiI₃ single crystals. We use these crystals to study the surface reactivity of their (001) cleavage planes toward various environmental conditions by measuring morphological changes using atomic force microscopy and analyzing the formed species by means of X-ray photoelectron spectroscopy and X-ray diffraction methods. We find that freshly cleaved samples show atomically flat surface regions extending over several micrometers and reveal steps corresponding to single BiI₃ layers. However, we also find that the surface deteriorates in air on a time scale of hours. By studying samples cleaved and stored under different conditions, we identify water as the agent initiating the changes in surface morphology, while under inert gas and dry oxygen, the surface stays intact. On the basis of the analysis of deteriorated long-term-stored samples we identify BiOI as the main product of hydrolysis. We also observe a second long-term decomposition route for samples stored under dynamic vacuum, where formation of BiI whiskers occurs. Overall, our findings emphasize the challenges associated with the surface reactivity of BiI₃ but also demonstrate that well-ordered BiI₃ surfaces can be obtained, which indicates that preparation of extended, atomically smooth BiI₃ monolayers by exfoliation from bulk crystals should be possible.

Bias-free solar syngas production by integrating a molecular cobalt catalyst with perovskite-BiVO4 tandems

The photoelectrochemical (PEC) production of syngas from water and CO₂ represents an attractive technology towards a circular carbon economy. However, the high overpotential, low selectivity and cost of commonly employed catalysts pose challenges for this sustainable energy-conversion process. Here we demonstrate highly tunable PEC syngas production by integrating a cobalt porphyrin catalyst immobilized on carbon nanotubes with triple-cation mixed halide perovskite and BiVO₄ photoabsorbers. Empirical data analysis is used to clarify the optimal electrode selectivity at low catalyst loadings. The perovskite photocathodes maintain selective aqueous CO₂ reduction for one day at light intensities as low as 0.1 sun, which provides pathways to maximize daylight utilization by operating even under low solar irradiance. Under 1 sun irradiation, the perovskite-BiVO₄ PEC tandems sustain bias-free syngas production coupled to water oxidation for three days. The devices present solar-to-H₂ and solar-to-CO conversion efficiencies of 0.06 and 0.02%, respectively, and are able to operate as standalone artificial leaves in neutral pH solution.

Development of BiOI as an effective photocatalyst for oxygen evolution reaction under simulated solar irradiation

In this study, crystalline BiOI powders were prepared for photocatalytic O₂ evolution in the presence of NaIO₃ as the electron mediator.

The Physics of Light Emission in Halide Perovskite Devices

Light emission is a critical property that must be maximized and controlled to reach the performance limits in optoelectronic devices such as photovoltaic solar cells and light-emitting diodes. Halide perovskites are an exciting family of materials for these applications owing to uniquely promising attributes that favor strong luminescence in device structures. Herein, the current understanding of the physics of light emission in state-of-the-art metal-halide perovskite devices is presented. Photon generation and management, and how these can be further exploited in device structures, are discussed. Key processes involved in photoluminescence and electroluminescence in devices as well as recent efforts to reduce nonradiative losses in neat films and interfaces are discussed. Finally, pathways toward reaching device efficiency limits and how the unique properties of perovskites provide a tremendous opportunity to significantly disrupt both the power generation and lighting industries are outlined.

Triple-Cation-Based Perovskite Photocathodes with AZO Protective Layer for Hydrogen Production Applications

Metal halide perovskites are actively pursued as photoelectrodes to drive solar fuel synthesis. However, currently, these photocathodes suffer from limited stability in water, which hampers their practical application. Here, we report a high-performance solution-processable photocathode composed of cesium formamidinium methylammonium triple-cation lead halide perovskite protected by an Al-doped ZnO (AZO) layer combined with a Field's metal encapsulation. Careful selection of charge transport layers resulted in an improvement in photocurrent, fill factor, device stability and reproducibility. The dead pixels count reduced from 25 to 6% for the devices with an AZO layer, and in photocathodes with an AZO layer the photocurrent density increased by almost 20% to 14.3 mA cm^-2. In addition, we observed a 5-fold increase in the device lifetime for photocathodes with AZO, which reached up to 18 h before complete failure. Finally, the photocathodes are fabricated using low-cost and scalable methods, which have promise to become compatible with standard solution-based processes.

Ultrathin-Layer Structure of BiOI Microspheres Decorated on N-Doped Biochar With Efficient Photocatalytic Activity

Bismuth oxyiodide (BiOI) is among the most potential photocatalysts due to its photocatalytic activity under visible light irradiation. However, the photoinduced carrier separation efficiency has limited the BiOI photocatalytic activity. Herein, we utilized the direct carbonation of sapless cattail grass to obtain N-doped hierarchical structure cattail-based carbon (NCC). The NCC not only served as an appropriate host but also as a self-sacrificing template for BiOI microspheres for the preparation of BiOI/NCC composite material. The acidic solutions (HCl or AcOH) were used as a solvent which helped to obtain a well-defined micro/nano hierarchical BiOI microspheres composed of ultrathin nanosheets. Thus, BiOI/NCC composites were successfully designed through the in-situ self-template rapid dissolution-recrystallization mechanism. Additionally, numerous well-contacted interfaces were formed between NCC and BiOI, which served as an electron-acceptor bridge function for ultrafast electron transfer process in order to hinder the electron-hole pairs recombination. On account of the multiple synergistic effects of micro/nano hierarchical microsphere structure, ultrathin nanosheets, and well-contacted interface, the as-prepared BiOI/NCC composites exhibit the superior degradation of rhodamine B (RhB) than pure BiOI under visible light irradiation.

Identifying and Reducing Interfacial Losses to Enhance Color-Pure Electroluminescence in Blue-Emitting Perovskite Nanoplatelet Light-Emitting Diodes

Perovskite nanoplatelets (NPls) hold promise for light-emitting applications, having achieved photoluminescence quantum efficiencies approaching unity in the blue wavelength range, where other metal-halide perovskites have typically been ineffective. However, the external quantum efficiencies (EQEs) of blue-emitting NPl light-emitting diodes (LEDs) have reached only 0.12%. In this work, we show that NPl LEDs are primarily limited by a poor electronic interface between the emitter and hole injector. We show that the NPls have remarkably deep ionization potentials (≥6.5 eV), leading to large barriers for hole injection, as well as substantial nonradiative decay at the NPl/hole-injector interface. We find that an effective way to reduce these nonradiative losses is by using poly(triarylamine) interlayers, which lead to an increase in the  EQE of the blue (464 nm emission wavelength) and sky-blue (489 nm emission wavelength) LEDs to 0.3% and 0.55%, respectively. Our work also identifies the key challenges for further efficiency increases.

Efficient Solar Cells Employing Light-Harvesting Sb0.67 Bi0.33 SI

Sb_{1- x} Bi_x SI, an isostructural material with the well-known quasi-1D SbSI, possesses good semiconductive and ferroelectric properties but is not applied in solar cells. Herein, solar cells based on alloyed Sb_0.67 Bi_0.33 SI (ASBSI) as a light harvester are fabricated. ASBSI is prepared through the reaction of bismuth triiodide in N,N-dimethylformamide solution with an antimony trisulfide film deposited on a mesoporous (mp)-TiO₂ electrode via chemical bath deposition at 250 °C under an argon or nitrogen atmosphere; the alloy exhibits a promising bandgap (1.62 eV). The best performing cell fabricated with poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] as the hole-transporting layer shows 4.07% in a power conversion efficiency (PCE) under the standard illumination conditions of 100 mW cm^-2 . The unencapsulated cells exhibit good comprehensive stability with retention of 92% of zjr initial PCE under ambient conditions of 60% relative humidity over 360 h, 93% after 1 sun illumination for 1254 min, and 92% after storage at 85 °C in air for 360 h.

Towards visible-light photocatalysis for environmental applications: band-gap engineering versus photons absorption-a review

A range of different studies has been performed in order to design and develop photocatalysts that work efficiently under visible (and near-infrared) irradiation as well as to improve photons absorption with improved reactor design. While there is consensus on the importance of photocatalysis for environmental applications and the necessity to utilized solar irradiation (or visible-light) as driving force for these processes, it is not yet clear how to get there. Discussion on the future steps towards visible-light photocatalysis for environmental application is of great interest to scientific and industrial communities and the present paper reviews and discusses the two main approaches, band-gap engineering for efficient solar-activated catalysts and reactor designs for improved photons absorption. Common misconceptions and drawbacks of each technology are also examined together with insights for future progress.