Construction of Compact Methylammonium Bismuth Iodide Film Promoting Lead-Free Inverted Planar Heterojunction Organohalide Solar Cells with Open-Circuit Voltage over 0.8 V

Breaking the bottleneck of lead-free perovskite solar cells through dimensionality modulation

The emerging perovskite solar cell (PSC) technology has attracted significant attention due to its superior power conversion efficiency (PCE) among the thin-film photovoltaic technologies. However, the toxicity of lead and poor stability of lead halide materials hinder their commercialization. In this case, after a decade of effort, various categories of lead-free perovskites and perovskite-like materials have been developed, including tin halide perovskites, double perovskites, defect-structured perovskites, and rudorffites. However, the performance of the corresponding devices still falls short of expectations, especially their PCE. The limitations mainly originate from either the unstable lattice structure of these materials, which causes the distortion of their octahedra, or their low dimensionality (e.g., structural and electronic dimensionality)-correlated poor carrier transport and self-trapping effect, accelerating nonradiative recombination. Therefore, understanding the relationship between the structures and performance in these emerging candidates and leveraging these insights to design or modify new lead-free perovskites is of great significance. Herein, we review the variety of dimensionalities in different categories of lead-free perovskites and perovskite-like materials and conclude that dimensionality is an important aspect among the crucial indexes that determine the performance of lead-free PSCs. In addition, we summarize the modulation of both structural and electronic dimensionality, and the corresponding enhanced optoelectronic properties in different categories. Finally, perspectives on the future development of lead-free perovskites and perovskite-like materials for photovoltaic applications are provided. We hope that this review will provide researchers with a concise overview of these emerging materials and help them leverage dimensionality to break the bottleneck in photovoltaic applications.

Simulation Design of Novel Non-Fluorine Polymers as Electron Transport Layer for Lead-Free Perovskite Solar Cells

Significant progress has been made in the advancement of perovskite solar cells, but their commercialization remains hindered by their lead-based toxicity. Many non-toxic perovskite-based solar cells have demonstrated potential, such as Cs2AgBi0.75Sb0.25Br6, but their power conversion efficiency is inadequate. To address this issue, some researchers are focusing on emerging acceptor-donor-acceptor'-donor-acceptor (A-DA'D-A)-type non-fullerene acceptors (NFAs) for Cs2AgBi0.75Sb0.25Br6 to find effective electron transport layers for high-performance photovoltaic responses with low voltage drops. In this comparative study, four novel A-DA'D-A-type NFAs, BT-LIC, BT-BIC, BT-L4F, and BT-BO-L4F, were used as electron transport layers (ETLs) for the proposed devices, FTO/PEDOT:PSS/Cs2AgBi0.75Sb0.25Br6/ETL/Au. Comprehensive simulations were conducted to optimize the devices. The simulations showed that all optimized devices exhibit photovoltaic responses, with the BT-BIC device having the highest power conversion efficiency (13.2%) and the BT-LIC device having the lowest (6.8%). The BT-BIC as an ETL provides fewer interfacial traps and better band alignment, enabling greater open-circuit voltage for efficient photovoltaic responses.

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.

Antisolvent Engineering to Enhance Photovoltaic Performance of Methylammonium Bismuth Iodide Solar Cells

High absorption ability and direct bandgap makes lead-based perovskite to acquire high photovoltaic performance. However, lead content in perovskite becomes a double-blade for counterbalancing photovoltaic performance and sustainability. Herein, we develop a methylammonium bismuth iodide (MBI), a perovskite-derivative, to serve as a lead-free light absorber layer. Owing to the short carrier diffusion length of MBI, its film quality is a predominant factor to photovoltaic performance. Several candidates of non-polar solvent are discussed in aspect of their dipole moment and boiling point to reveal the effects of anti-solvent assisted crystallization. Through anti-solvent engineering of toluene, the morphology, crystallinity, and element distribution of MBI films are improved compared with those without toluene treatment. The improved morphology and crystallinity of MBI films promote photovoltaic performance over 3.2 times compared with the one without toluene treatment. The photovoltaic device can achieve 0.26% with minor hysteresis effect, whose hysteresis index reduces from 0.374 to 0.169. This study guides a feasible path for developing MBI photovoltaics.

One-pot synthesis of novel ligand-free tin(II)-based hybrid metal halide perovskite quantum dots with high anti-water stability for solution-processed UVC photodetectors

Recently, lead-based halide perovskites have gained extensive attention due to their outstanding optoelectronic properties. However, the toxicity of lead would seriously limit its future application. To address these issues, in this work novel ligand-free organic-inorganic hybrid metal halide TBASnCl₃ (C₁₆H₃₆NSnCl₃) quantum dots are synthesized by a one-pot method at room temperature, and they showed high anti-water stability and high potential applications for high-performance UVC photodetectors. Our experimental data showed that the responsivity of the lateral photodetectors Au/TBASnCl₃/Au, in which the active layer (i.e. TBASnCl₃) was synthesized by further introducing SnF₂ as a precursor besides SnCl₂, reached 7.3 mA W^-1 with a specific detectivity of 1.67 × 10¹¹ Jones under 0.36 mW cm^-2 254 nm illumination at -5 V, and it showed a long lifetime even in an environment with an air humidity of 60%. Therefore, it laid a solid foundation for further fabricating lead-free metal halide optoelectronic devices.

Two-Step Synthesis of Bismuth-Based Hybrid Halide Perovskite Thin-Films

Lead halide perovskites have been revolutionary in the last decade in many optoelectronic sectors. Their bismuth-based counterparts have been considered a good alternative thanks to their composition of earth-abundant elements, good chemical stability, and low toxicity. Moreover, their electronic structure is in a quasi-zero-dimensional (0D) configuration, and they have recently been explored for use beyond optoelectronics. A significant limitation in applying thin-film technology is represented by the difficulty of synthesizing compact layers with easily scalable methods. Here, the engineering of a two-step synthesis in an air of methylammonium bismuth iodide compact thin films is reported. The critical steps of the process have been highlighted so that the procedure can be adapted to different substrates and application areas.

Quasi-Zero Dimensional Halide Perovskite Derivates: Synthesis, Status, and Opportunity

In recent decades, many technological advances have been enabled by nanoscale phenomena, giving rise to the field of nanotechnology. In particular, unique optical and electronic phenomena occur on length scales less than 10 nanometres, which enable novel applications. Halide perovskites have been the focus of intense research on their optoelectronic properties and have demonstrated impressive performance in photovoltaic devices and later in other optoelectronic technologies, such as lasers and light-emitting diodes. The most studied crystalline form is the three-dimensional one, but, recently, the exploration of the low-dimensional derivatives has enabled new sub-classes of halide perovskite materials to emerge with distinct properties. In these materials, low-dimensional metal halide structures responsible for the electronic properties are separated and partially insulated from one another by the (typically organic) cations. Confinement occurs on a crystal lattice level, enabling bulk or thin-film materials that retain a degree of low-dimensional character. In particular, quasi-zero dimensional perovskite derivatives are proving to have distinct electronic, absorption, and photoluminescence properties. They are being explored for various technologies beyond photovoltaics (e.g. thermoelectrics, lasing, photodetectors, memristors, capacitors, LEDs). This review brings together the recent literature on these zero-dimensional materials in an interdisciplinary way that can spur applications for these compounds. The synthesis methods, the electrical, optical, and chemical properties, the advances in applications, and the challenges that need to be overcome as candidates for future electronic devices have been covered.

Lead-Free Perovskite Materials for Solar Cells

The toxicity issue of lead hinders large-scale commercial production and photovoltaic field application of lead halide perovskites. Some novel non- or low-toxic perovskite materials have been explored for development of environmentally friendly lead-free perovskite solar cells (PSCs). This review studies the substitution of equivalent/heterovalent metals for Pb based on first-principles calculation, summarizes the theoretical basis of lead-free perovskites, and screens out some promising lead-free candidates with suitable bandgap, optical, and electrical properties. Then, it reports notable achievements for the experimental studies of lead-free perovskites to date, including the crystal structure and material bandgap for all of lead-free materials and photovoltaic performance and stability for corresponding devices. The review finally discusses challenges facing the successful development and commercialization of lead-free PSCs and predicts the prospect of lead-free PSCs in the future.

All-inorganic Sn-based Perovskite Solar Cells: Status, Challenges, and Perspectives

Recently, the power conversion efficiency (PCE) of perovskite solar cells (PSC) based on organic-inorganic hybrid Pb halide perovskites has reached 25.2 %. However, the toxicity of Pb has still been a main concern for the large-scale commercialization of Pb-based PSCs. Efforts have been made during the past few years to seek eco-friendly Pb-free perovskites, and it is a growing consensus that Sn is the best choice as Pb alternative over any other Pb-free metal elements. Among Sn-based perovskites, all-inorganic cells are promising candidates for PSCs owing to their more suitable bandgap, better stability, and higher charge mobility compared to the organic-inorganic hybrid counterparts. However, the poor phase stability of all-inorganic Sn-based perovskites (AISPs) and low PCE of their PSCs are most challenging in the field at present. Herein, recent developments on PSCs based on AISPs, including CsSnX₃ and Cs₂ SnX₆ (X=Br, I), are comprehensively reviewed. Primarily, the intrinsic characteristics of the two AISPs are overviewed, including crystallographic property, band structure, charge carrier property, and defect property. Sequentially, state-of-the-art progress, regarding the photovoltaic application of AISPs as light absorber, is summarized. At last, current challenges and future opportunities of AISP-based PSCs are also discussed.

A Review on Lead-Free Hybrid Halide Perovskites as Light Absorbers for Photovoltaic Applications Based on Their Structural, Optical, and Morphological Properties

Despite the advancement made by the scientific community in the evolving photovoltaic technologies, including the achievement of a 29.1% power conversion efficiency of perovskite solar cells over the past two decades, there are still numerous challenges facing the advancement of lead-based halide perovskite absorbers for perovskite photovoltaic applications. Among the numerous challenges, the major concern is centered around the toxicity of the emerging lead-based halide perovskite absorbers, thereby leading to drawbacks for their pragmatic application and commercialization. Hence, the replacement of lead in the perovskite material with non-hazardous metal has become the central focus for the actualization of hybrid perovskite technology. This review focuses on lead-free hybrid halide perovskites as light absorbers with emphasis on how their chemical compositions influence optical properties, morphological properties, and to a certain extent, the stability of these perovskite materials.

Anisotropic Photoconductivity and Long-Lived Charge Carriers in Bismuth-Based One-Dimensional Perovskite with Type-IIa Band Alignment

Bismuth-based perovskites are attracting intense scientific interest due to low toxicity and excellent moisture stability compared to lead-based analogues. However, high exciton binding energy, poor charge carrier separation, and transport efficiencies lower their optoelectronic performances. To address these issues, we have integrated an electronically active organic cation, naphthalimide ethylammonium, between the [BiI₅^2-]_n chains via crystal engineering to form a novel perovskite-like material (naphthalimide ethylammonium)₂BiI₅ (NBI). Single crystal analysis revealed a one-dimensional quantum-well structure for NBI in which inter-inorganic well electronic coupling is screened by organic layers. It exhibited anisotropic photoconductivity and long-lived charge carriers with milliseconds lifetime, which is higher than that of CH₃NH₃PbI₃. Density functional theory calculations confirmed type-IIa band alignment between organic cations and inorganic chains, allowing the former to electronically contribute to the overall charge transport properties of the material.

A Potential Checkmate to Lead: Bismuth in Organometal Halide Perovskites, Structure, Properties, and Applications

The remarkable optoelectronic properties and considerable performance of the organo lead-halide perovskites (PVKs) in various optoelectronic applications grasp tremendous scientific attention. However, the existence of the toxic lead in these compounds is threatening human health and remains a major concern in the way of their commercialization. To address this issue, numerous nontoxic alternatives have been reported. Among these alternatives, bismuth-based PVKs have emerged as a promising substitute because of similar optoelectronic properties and extended environmental stability. This work communicates briefly about the possible lead-alternatives and explores bismuth-based perovskites comprehensively, in terms of their structures, optoelectronic properties, and applications. A brief description of lead-toxification is provided and the possible Pb-alternatives from the periodic table are scrutinized. Then, the classification and crystal structures of various Bi-based perovskites are elaborated on. Detailed optoelectronic properties of Bi-based perovskites are also described and their optoelectronic applications are abridged. The overall photovoltaic applications along with device characteristics (i.e., V OC, J SC, fill factor, FF, and power conversion efficiency, PCE), fabrication method, device architecture, and operational stability are also summarized. Finally, a conclusion is drawn where a brief outlook highlights the challenges that hamper the future progress of Bi-based optoelectronic devices and suggestions for future directions are provided.

Recent Advancements and Challenges for Low-Toxicity Perovskite Materials

Lead-based organic-inorganic hybrid perovskite materials have been developed for advanced optoelectronic applications. However, the toxicity of lead and the chemical instability of lead-based perovskite materials have so far been demonstrated to be an overwhelming challenge. The discovery of perovskite materials based on low-toxicity elements, such as Sn, Bi, Sb, Ge, and Cu, with superior optoelectronic properties provides alternative approaches to realize high-performance perovskite optoelectronics. In this review, recent advances in the aspects of low-toxicity perovskite solar cells, photodetectors, light-emitting diodes, and thermoelectric devices are highlighted. The antioxidation stability of metal cation and the crystallization process of the low-toxicity perovskite materials are discussed. In the last part, the outlook toward addressing various issues requiring further attention in the development of low-toxicity perovskite materials is outlined.

Single-Source Vapor-Deposited Cs2AgBiBr6 Thin Films for Lead-Free Perovskite Solar Cells

Lead-free double perovskites have been considered as a potential environmentally friendly photovoltaic material for substituting the hybrid lead halide perovskites due to their high stability and nontoxicity. Here, lead-free double perovskite Cs2AgBiBr6 films are initially fabricated by single-source evaporation deposition under high vacuum condition. X-ray diffraction and scanning electron microscopy characterization show that the high crystallinity, flat, and pinhole-free double perovskite Cs2AgBiBr6 films were obtained after post-annealing at 300 °C for 15 min. By changing the annealing temperature, annealing time, and film thickness, perovskite Cs2AgBiBr6 solar cells with planar heterojunction structure of FTO/TiO2/Cs2AgBiBr6/Spiro-OMeTAD/Ag achieve an encouraging power conversion efficiency of 0.70%. Our preliminary work opens a feasible approach for preparing high-quality double perovskite Cs2AgBiBr6 films wielding considerable potential for photovoltaic application.

Causes and Solutions of Recombination in Perovskite Solar Cells

Organic-inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley-Queisser limit efficiency (30.5%). Obviously, trap-assisted nonradiative (also called Shockley-Read-Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post-treatment are explored. The focus herein is on the recombination at perovskite/electron-transporting material and perovskite/hole-transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap-assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

Potential Substitutes for Replacement of Lead in Perovskite Solar Cells: A Review

Lead halide perovskites have displayed the highest solar power conversion efficiencies of 23% but the toxicity issues of these materials need to be addressed. Lead-free perovskites have emerged as viable candidates for potential use as light harvesters to ensure clean and green photovoltaic technology. The substitution of lead by Sn, Ge, Bi, Sb, Cu and other potential candidates have reported efficiencies of up to 9%, but there is still a dire need to enhance their efficiencies and stability within the air. A comprehensive review is given on potential substitutes for lead-free perovskites and their characteristic features like energy bandgaps and optical absorption as well as photovoltaic parameters like open-circuit voltage (V OC), fill factor, short-circuit current density (J  SC), and the device architecture for their efficient use. Lead-free perovskites do possess a suitable bandgap but have low efficiency. The use of additives has a significant effect on their efficiency and stability. The incorporation of cations like diethylammonium, phenylethyl ammonium, phenylethyl ammonium iodide, etc., or mixed cations at different compositions at the A-site is reported with engineered bandgaps having significant efficiency and stability. Recent work on the advancement of lead-free perovskites is also reviewed.

Supercompliant and Soft (CH_{3}NH_{3})_{3}Bi_{2}I_{9} Crystal with Ultralow Thermal Conductivity

In this Letter, we show the phonon dispersion of (CH_{3}NH_{3})_{3}Bi_{2}I_{9} single crystals at 300 K measured by inelastic x-ray scattering. The frequencies of acoustic phonons are among the lowest of crystals. Nanoindentation measurements verified that these crystals are very compliant and considerably soft. The frequency overlap between acoustic and optical phonons results in strong acoustic-optical scattering. All these features lead to an ultralow thermal conductivity. The fundamental knowledge obtained from this study will accelerate the design of novel hybrid materials for energy applications.

Lead-Free Antimony-Based Light-Emitting Diodes through the Vapor-Anion-Exchange Method

Hybrid lead halide perovskites continue to attract interest for use in optoelectronic devices such as solar cells and light-emitting diodes. Although challenging, the replacement of toxic lead in these systems is an active field of research. Recently, the use of trivalent metal cations (Bi³⁺ and Sb³⁺) that form defect perovskites A₃B₂X₉ has received great attention for the development of solar cells, but their light-emissive properties have not previously been studied. Herein, an all-inorganic antimony-based two-dimensional perovskite, Cs₃Sb₂I₉, was synthesized using the solution process. Vapor-anion-exchange method was employed to change the structural composition from Cs₃Sb₂I₉ to Cs₃Sb₂Br₉ or Cs₃Sb₂Cl₉ by treating CsI/SbI₃ spin-coated films with SbBr₃ or SbCl₃, respectively. This novel method facilitates the fabrication of Cs₃Sb₂Br₉ or Cs₃Sb₂Cl₉ through solution processing without the need of using poorly soluble precursors (e.g., CsCl and CsBr). We go on to demonstrate electroluminescence from a device employing Cs₃Sb₂I₉ emitter sandwiched between ITO/PEDOT:PSS and TPBi/LiF/Al as the hole and electron injection electrodes, respectively. A visible-infrared radiance of 0.012 W·Sr^-1·m^-2 was measured at 6 V when Cs₃Sb₂I₉ was the active emitter layer. These proof-of-principle devices suggest a viable path toward low-dimensional, lead-free A₃B₂X₉ perovskite optoelectronics.

Lead-Free Perovskites: Metals Substitution towards Environmentally Benign Solar Cell Fabrication

Perovskite solar cells have attracted significant attention during the current decade owing to their efficacy and photovoltaics performance, which has reached a new milestone in the thin-film category. Perovskite solar cells have witnessed a remarkable 25.2 % light-to-electricity conversion efficiency; however, the toxicity of the commonly employed Pb counterpart towards humans as well as the environment, in addition to material instability, are current bottlenecks towards commercial application. The scientific community has explored other metal ions as substitutions for Pb, while preserving the unique properties of the material, to produce environment-friendly perovskites. In this Review, we highlight the recent developments and challenges of Pb-free halide perovskite-based light harvesters for solar cell applications. This summary is intended to aid in the further development of a materials library for this sustainable technology.

The Thermomechanical Properties of Thermally Evaporated Bismuth Triiodide Thin Films

Bismuth triiodide (BiI₃) has been studied in recent years with the aim of developing lead-free semiconductors for photovoltaics. It has also appeared in X-ray detectors due to the high density of the Bismuth element. This material is attractive as an active layer in solar cells, or may be feasible for conversion into perovskite-like material (MA₃Bi₂I₉), being also suitable for photovoltaic applications. In this study, we report on the thermomechanical properties (stress, hardness, coefficient of thermal expansion, and biaxial and reduced Young's moduli) of BiI₃ thin films deposited by thermal evaporation. The stress was determined as a function of temperature, adopting the thermally induced bending technique, which allowed us to extract the coefficient of thermal expansion (31 × 10^-6 °C^-1) and Young's biaxial modulus (19.6 GPa) for the films. Nanohardness (~0.76 GPa) and a reduced Young's modulus of 27.1 GPa were determined through nanoindentation measurements.

Chemical Vapor Deposition of Organic-Inorganic Bismuth-Based Perovskite Films for Solar Cell Application

Perovskite solar cells have shown a rapid increase of performance and overcome the threshold of 20% power conversion efficiency (PCE). The main issues hampering commercialization are the lack of deposition methods for large areas, missing long-term device stability and the toxicity of the commonly used Pb-based compounds. In this work, we present a novel chemical vapor deposition (CVD) process for Pb-free air-stable methylammonium bismuth iodide (MBI) layers, which enables large-area production employing close-coupled showerhead technology. We demonstrate the influence of precursor rates on the layer morphology as well as on the optical and crystallographic properties. The impact of substrate temperature and layer thickness on the morphology of MBI crystallites is discussed. We obtain smooth layers with lateral crystallite sizes up to 500 nm. Moreover, the application of CVD-processed MBI layers in non-inverted perovskite solar cells is presented.

Solution-Processed Bismuth Halide Perovskite Thin Films: Influence of Deposition Conditions and A-Site Alloying on Morphology and Optical Properties

Bismuth-based halide perovskites have been proposed as a potential nontoxic alternative to lead halide perovskites; however, they have not realized suitable performance. Their poor performance has been attributed to substandard film morphologies and too wide of a band gap for many applications. Herein we used a two-step deposition procedure to convert BiI₃ thin films into A₃Bi₂I₉ (A = FA⁺, MA⁺, Cs⁺, or Rb⁺), which resulted in a substantial improvement in film morphology, a larger band gap, and greater compositional tunability compared toresults when using aconventional single-step deposition technique. Additionally, we attempted to reduce the undesirably wide band gap in Rb₃Bi₂I₉ thin films by inducing chemical pressures through cation-size mismatch, with an underlying hypothesis that cation-size mismatch could induce compressive strain within the 2D Rb₃Bi₂I₉ lattice. However, we found that all A _xRb_{3- x}Bi₂I₉ compositions with x > 0 adopted the 0D structure, and no changes to the band gap were observed with alloy. These results imply that the band gap of A _xRb_{3- x}Bi₂I₉ is insensitive to A-site alloying.

Tuning Optical and Electronic Properties in Low-Toxicity Organic-Inorganic Hybrid (CH3NH3)3Bi2I9 under High Pressure

Low-toxicity, air-stable methylammonium bismuth iodide (CH₃NH₃)₃Bi₂I₉ has been proposed as a candidate to replace lead-based perovskites as highly efficient light absorbers for photovoltaic devices. Here, we investigated the effect of pressure on the optoelectronic properties and crystal structure of (CH₃NH₃)₃Bi₂I₉ up to 65 GPa at room temperature. We achieved impressive photoluminescence enhancement and band gap narrowing over a moderate pressure range. Dramatic piezochromism from transparent red to opaque black was observed in a single crystal sample. A structural phase transition from hexagonal P6₃/ mmc to monoclinic P2₁ at 5.0 GPa and completely reversible amorphization at 29.1 GPa were determined by in situ synchrotron X-ray diffraction. Moreover, the dynamically disordered MA⁺ organic cations in the hexagonal phase became orientationally ordered upon entering into the monoclinic phase, followed by static disorder upon amorphization. The striking enhancement of conductivity and metallization under high pressure indicate wholly new electronic properties.

Robust Stability of Efficient Lead-Free Formamidinium Tin Iodide Perovskite Solar Cells Realized by Structural Regulation

The instability issue of Pb-free Sn-based perovskite is one of the biggest challenges for its application in optoelectronic devices. Herein, a structural regulation strategy is demonstrated to regulate the geometric symmetry of formamidiniumtin iodide (FASnI₃) perovskite. Experimental and theoretical works show that the introduction of cesium cation (Cs⁺) could improve the geometric symmetry, suppress the oxidation of Sn²⁺, and enhance the thermodynamical structural stability of FASnI₃. As a result, the inverted planar Cs-doped FASnI₃-based perovskite solar cell (PSC) is shown to maintain 90% of its initial power-conversion efficiency (PCE) after 2000 h stored in N₂, which is the best durability to date for 3D Sn-based PSCs. Most importantly, the air, thermal, and illumination stabilities of the PSCs are all improved after Cs doping. The PCE of the Cs-doped PSC shows a 63% increase compared to that of the control device (from 3.74% to 6.08%) due to the improved quality of the Cs-doped FASnI₃ film.

A Zero-Dimensional Mixed-Anion Hybrid Halogenobismuthate(III) Semiconductor: Structural, Optical, and Photovoltaic Properties

In this contribution, we present the synthesis and characterization of the mixed-anion halogenobismuthate(III) (CH₃NH₃)₆BiI_5.22Cl_3.78 (MBIC) as an alternative lead-free perovskite-type semiconductor, and discuss its optical, electronic, and photovoltaic properties in comparison to the methylammonium bismuth iodide (CH₃NH₃)₃Bi₂I₉ (MBI) compound. The exchange of iodide with chloride during synthesis leads to the formation of an orthorhombic A₆BX₉-type crystal structure ( Cmma, No. 67) with isolated BiX₆ octahedra and methylammonium chloride interlayers. The experimentally found optical indirect band gap of 2.25 eV is in good agreement with the calculated value of 2.50 eV derived from DFT simulations. The valence band maximum and the conduction band minimum were determined to be at -6.2 eV and -4.0 eV vs vacuum. Similar to MBI, thin films of MBIC are composed of microcrystalline platelets. Time-resolved photoluminescence measurements show electron transfer of MBIC to mesoporous TiO₂. The photovoltaic behavior of both compounds is compared in solar cells with the following device architecture: glass/ITO/compact TiO₂/mesoporous TiO₂/MBIC or MBI/spiro-OMeTAD/Au. Despite the zero-dimensional structure of MBIC, a maximum power conversion efficiency of 0.18% and a high fill factor of almost 60% were obtained with this material as absorber layer. When stored under inert conditions, these solar cells show an excellent long-term stability over the investigated period of more than 700 days.

Synthesis of Easily Transferred 2D Layered BiI3 Nanoplates for Flexible Visible-Light Photodetectors

Bismuth triiodide, BiI₃, is one of the promising 2D layered materials from the family of metal halides. The unique electronic structure and properties make it an attractive material for the room-temperature gamma/X-ray detectors, high-efficiency photovoltaic absorbers, and Bi-based organic-inorganic hybrid perovskites. Other possibilities including optoelectronic devices and optical circuits are envisioned but rarely experimentally confirmed yet. Here, we report the synthesis of vertical 2D BiI₃ nanoplates using the physical vapor deposition mechanism. The obtained products were found easy to be separated and transferred to other substrates. Photodetectors employing such 2D nanoplates on polyethylene terephthalate substrate are demonstrated to be quite sensitive to red light (635 nm) with good responsivity (2.8 A W^-1), fast stable photoresponse (3/9 ms for raise/decay times), and remarkable specific detectivity (1.2 × 10¹² jones), which attest to high comparability of the assembled components with many latest 2D nanostructured light sensors. In addition, such photodetectors exhibit outstanding mechanical stability and durability under different bending strains within the theoretically affordable levels, suggesting a variety of potential applications of 2D BiI₃ for flexible devices.

High-Quality Cs2 AgBiBr6 Double Perovskite Film for Lead-Free Inverted Planar Heterojunction Solar Cells with 2.2 % Efficiency

All-inorganic double-metal perovskite materials have recently gained much attention due to their three dimensionality (3D) and non-toxic nature to replace lead-based perovskite materials. Among all those double perovskite materials, theoretical works have demonstrated that Cs₂ AgBiBr₆ shows high stability and possesses a suitable band gap for solar-cell applications. However, the film-forming ability of Cs₂ AgBiBr₆ is found to be the utmost challenge hindering its development in thin-film solar-cell devices. In this work, a high-quality Cs₂ AgBiBr₆ film with ultra-smooth morphology, micro-sized grains, and high crystallinity is realized via anti-solvent dropping technology and post-annealing at high temperature. After optimization, the first example of an inverted planar heterojunction solar-cell device based on Cs₂ AgBiBr₆ exhibits a power conversion efficiency of 2.23 % with V_OC =1.01 V, J_SC =3.19 mA/cm² , and FF=69.2 %. Besides, the device shows no hysteresis and a high stability.

The Effect of Dopant-Free Hole-Transport Polymers on Charge Generation and Recombination in Cesium-Bismuth-Iodide Solar Cells

The photovoltaic characteristics of CsBi₃ I₁₀ -based solar cells with three dopant-free hole-conducting polymers are investigated. The effect on charge generation and charge recombination in the solar cells using the different polymers is studied and the results indicate that the choice of polymer strongly affects the device properties. Interestingly, for the solar cell with poly[[2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl] (TQ1), the photon-to-current conversion spectrum is highly improved in the red wavelength region, suggesting that the polymer also contributes to the photocurrent generation in this case. This report provides a new direction for further optimization of Bi-halide solar cells by using dopant-free hole-transporting polymers and shows that the energy levels and the interaction between the Bi-halide and the conducting polymers are very important for solar cell performance.

Vapor Annealing Controlled Crystal Growth and Photovoltaic Performance of Bismuth Triiodide Embedded in Mesostructured Configurations

Low stability of organic-inorganic lead halide perovskite and toxicity of lead (Pb) still remain a concern. Therefore, there is a constant quest for alternative nontoxic and stable light-absorbing materials with promising optoelectronic properties. Herein, we report about nontoxic bismuth triiodide (BiI₃) photovoltaic device prepared using TiO₂ mesoporous film and spiro-OMeTAD as electron- and hole-transporting materials, respectively. Effect of annealing methods (e.g., thermal annealing (TA), solvent vapor annealing (SVA), and Petri dish covered recycled vapor annealing (PR-VA)) and different annealing temperatures (90, 120, 150, and 180 °C for PR-VA) on BiI₃ film morphology have been investigated. As found in the study, grain size increased and film uniformity improved as temperature was raised from 90 to 150 °C. The photovoltaic devices based on BiI₃ films processed at 150 °C with PR-VA treatment showed power conversion efficiency (PCE) of 0.5% with high reproducibility, which is, so far, the best PCE reported for BiI₃ photovoltaic device employing organic hole-transporting material (HTM), owing to the increase in grain size and uniform morphology of BiI₃ film. These devices showed stable performance even after 30 days of exposure to 50% relative humidity, and after 100 °C heat stress and 20 min light soaking test. More importantly, the study reveals many challenges and room (discussed in the details) for further development of the BiI₃ photovoltaic devices.

Rubidium Doping for Enhanced Performance of Highly Efficient Formamidinium-Based Perovskite Light-Emitting Diodes

Organometal halide perovskites (OHPs) have become the most promising optoelectronic material in the past few years with a myriad of applications in the photovoltaic, light-emitting, and laser fields. However, for light-emitting applications, the low photoluminescence quantum yield (PLQY) of OHP film is critical to hinder the efficiency improvement of OHP-film-based light-emitting diodes (PeLEDs). Herein, we study the effects of rubidium incorporation on the crystal growth, structure, and photoelectric and optical properties of formamidinium-lead-bromide-based (FAPbBr₃-based) perovskite films and light-emission performance of PeLEDs. It is found that rubidium incorporation can significantly enhance the PLQY of FAPbBr₃ film by suppressing the trap density and thus improve the withstand voltage as well as the performance of PeLEDs. When FAPbBr₃ film with optimal Rb doping ratio is employed as the light emitter of PeLEDs, the maximum luminance and current efficiency is enhanced by ∼10-fold and ∼5-fold to 66 353 cd/m² and 24.22 cd/A compared to the controlled device, respectively, the record performance based on FAPbBr₃ PeLEDs so far. The enhanced performance can be chiefly attributed to the increase of PLQY and decrease of trap defect density of perovskite film with rubidium incorporation. Our research is expected to stimulate the development of OHPs for the next-generation lighting and display fields.

Lead-Free Hybrid Perovskite Absorbers for Viable Application: Can We Eat the Cake and Have It too?

Many years since the booming of research on perovskite solar cells (PSCs), the hybrid perovskite materials developed for photovoltaic application form three main categories since 2009: (i) high-performance unstable lead-containing perovskites, (ii) low-performance lead-free perovskites, and (iii) moderate performance and stable lead-containing perovskites. The search for alternative materials to replace lead leads to the second group of perovskite materials. To date, a number of these compounds have been synthesized and applied in photovoltaic devices. Here, lead-free hybrid light absorbers used in PV devices are focused and their recent developments in related solar cell applications are reviewed comprehensively. In the first part, group 14 metals (Sn and Ge)-based perovskites are introduced with more emphasis on the optimization of Sn-based PSCs. Then concerns on halide hybrids of group 15 metals (Bi and Sb) are raised, which are mainly perovskite derivatives. At the same time, transition metal Cu-based perovskites are also referred. In the end, an outlook is given on the design strategy of lead-free halide hybrid absorbers for photovoltaic applications. It is believed that this timely review can represent our unique view of the field and shed some light on the direction of development of such promising materials.

Photovoltaic Performance of Vapor-Assisted Solution-Processed Layer Polymorph of Cs3Sb2I9

The presence of toxic lead (Pb) remains a major obstruction to the commercial application of perovskite solar cells. Although antimony (Sb)-based perovskite-like structures A₃M₂X₉ can display potentially useful photovoltaic behavior, solution-processed Sb-based perovskite-like structures usually favor the dimer phase, which has poor photovoltaic properties. In this study, we prepared a layered polymorph of Cs₃Sb₂I₉ through solution-processing and studied its photovoltaic properties. The exciton binding energy and exciton lifetime of the layer-form Cs₃Sb₂I₉ were approximately 100 meV and 6 ns, respectively. The photovoltaic properties of the layered polymorph were superior to those of the dimer polymorph. A solar cell incorporating the layer-form Cs₃Sb₂I₉ exhibited an open-circuit voltage of 0.72 V and a power conversion efficiency of 1.5%-the highest reported for an all-inorganic Sb-based perovskite.

Defects in metal triiodide perovskite materials towards high-performance solar cells: origin, impact, characterization, and engineering

The progress of defect science in metal triiodide perovskite is critically reviewed, including the origin, impacts, characterization, and engineering.

High-Quality (CH3NH3)3Bi2I9 Film-Based Solar Cells: Pushing Efficiency up to 1.64

Bismuth-based solar cells have exhibited some advantages over lead perovskite solar cells for nontoxicity and superior stability, which are currently two main concerns in the photovoltaic community. As for the perovskite-related compound (CH₃NH₃)₃Bi₂I₉ applied for solar cells, the conversion efficiency is severely restricted by the unsatisfactory photoactive film quality. Herein we report a novel two-step approach- high-vacuum BiI₃ deposition and low-vacuum homogeneous transformation of BiI₃ to (CH₃NH₃)₃Bi₂I₉-for highly compact, pinhole-free, large-grained films, which are characterized with absorption coefficient, trap density of states, and charge diffusion length comparable to those of some lead perovskite analogues. Accordingly, the solar cells have realized a record power conversion of efficiency of 1.64% and also a high external quantum efficiency approaching 60%. Our work demonstrates the potential of (CH₃NH₃)₃Bi₂I₉ for highly efficient and long-term stable solar cells.

High Photon-to-Current Conversion in Solar Cells Based on Light-Absorbing Silver Bismuth Iodide

Here, a lead-free silver bismuth iodide (AgI/BiI3 ) with a crystal structure with space group R3‾ m is investigated for use in solar cells. Devices based on the silver bismuth iodide deposited from solution on top of TiO2 and the conducting polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) as a hole-transport layer are prepared and the photovoltaic performance is very promising with a power conversion efficiency over 2 %, which is higher than the performance of previously reported bismuth-halide materials for solar cells. Photocurrent generation is observed between 350 and 700 nm, and the maximum external quantum efficiency is around 45 %. The results are compared to solar cells based on the previously reported material AgBi2 I7 , and we observe a clearly higher performance for the devices with the new silver and bismuth iodides composition and different crystal structure. The X-ray diffraction spectrum of the most efficient silver bismuth iodide material shows a hexagonal crystal structure with space group R3‾ m, and from the light absorption spectrum we obtain an indirect band gap energy of 1.62 eV and a direct band gap energy of 1.85 eV. This report shows the possibility for finding new structures of metal-halides efficient in solar cells and points out new directions for further exploration of lead-free metal-halide solar cells.

Fabrication of methylammonium bismuth iodide through interdiffusion of solution-processed BiI3/CH3NH3I stacking layers

Methylammonium bismuth iodide (MA₃Bi₂I₉) perovskite has been deposited on a mesoporous TiO₂ film through interdiffusion of solution-processed BiI₃/CH₃NH₃I stacking layers, and the resulting device was prepared and characterized.

Nontoxic (CH3NH3)3Bi2I9 perovskite solar cells free of hole conductors with an alternative architectural design and a solution-processable approach

Methylammonium iodide bismuthate ((CH₃NH₃)₃Bi₂I₉) (MBI) perovskite was introduced as a new lead-free and air-stable absorber for hole conductor-free perovskite solar cells.