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

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.

Advancements and Prospects in Perovskite Solar Cells: From Hybrid to All-Inorganic Materials

Hybrid perovskites, materials composed of metals and organic substances in their structure, have emerged as potential materials for the new generation of photovoltaic cells due to a unique combination of optical, excitonic and electrical properties. Inspired by sensitization techniques on TiO2 substrates (DSSC), CH3NH3PbBr3 and CH3NH3PbI3 perovskites were studied as a light-absorbing layer as well as an electron-hole pair generator. Photovoltaic cells based on per-ovskites have electron and hole transport layers (ETL and HTL, respectively), separated by an ac-tive layer composed of perovskite itself. Major advances subsequently came in the preparation methods of these devices and the development of different architectures, which resulted in an efficiency exceeding 23% in less than 10 years. Problems with stability are the main barrier to the large-scale production of hybrid perovskites. Partially or fully inorganic perovskites appear promising to circumvent the instability problem, among which the black perovskite phase CsPbI3 (α-CsPbI3) can be highlighted. In more advanced studies, a partial or total substitution of Pb by Ge, Sn, Sb, Bi, Cu or Ti is proposed to mitigate potential toxicity problems and maintain device efficiency.

Recent Advances in Wide Bandgap Perovskite Solar Cells: Focus on Lead-Free Materials for Tandem Structures

A tandem solar cell, which is composed of a wide bandgap (WBG) top sub-cell and a narrow bandgap (NBG) bottom subcell, harnesses maximum photons in the wide spectral range, resulting in higher efficiency than single-junction solar cells. WBG (>1.6 eV) perovskites are currently being studied a lot based on lead mixed-halide perovskites, and the power conversion efficiency of lead mixed-halide WBG perovskite solar cells (PSCs) reaches 21.1%. Despite the excellent device performance of lead WBG PSCs, their commercialization is hampered by their Pb toxicity and low stability. Hence, lead-free, less toxic WBG perovskite absorbers are needed for constructing lead-free perovskite tandem solar cells. In this review, various strategies for achieving high-efficiency WBG lead-free PSCs are discussed, drawing inspiration from prior research on WBG lead-based PSCs. The existing issues of WBG perovskites such as VOC loss are discussed, and toxicity issues associated with lead-based perovskites are also addressed. Subsequently, the natures of lead-free WBG perovskites are reviewed, and recently emerged strategies to enhance device performance are proposed. Finally, their applications in lead-free all perovskite tandem solar cells are introduced. This review presents helpful guidelines for eco-friendly and high-efficiency lead-free all perovskite tandem solar cells.

Tuning the Optoelectronic Property of All-Inorganic Lead-Free Perovskite via Finely Microstructural Modulation for Photovoltaics

Bismuth-based halide perovskite materials have attracted extensive attention for optoelectronic applications due to nontoxicity and ambient stability. However, limited by low-dimensional structure and isolate octahedron arrangement, the undesirable photophysical properties of bismuth-based perovskites are still not well modulated. Here, the rational design and synthesis of Cs3 SbBiI9 with improved optoelectronic performance via premeditatedly incorporating antimony atoms with a similar electronic structure to bismuth into the host lattice of Cs3 Bi2 I9 is reported. Compared with Cs3 Bi2 I9 , the absorption spectrum of Cs3 SbBiI9 is broadened from ≈640 to ≈700 nm, the photoluminescence intensity enhances by two orders of magnitude indicating the extremely suppressed carrier nonradiative recombination, and the charge carrier lifetime is further increased from 1.3 to 207.6 ns. Taking representative applications in perovskite solar cells, the Cs3 SbBiI9 exhibits a higher photovoltaic performance benefiting from the improved intrinsic optoelectronic properties. Further structure analysis reveals that the introduced Sb atoms regulate the interlayer spacing between dimers in c-axis direction and the micro-octahedral configuration, which correlate well with the improvement of optoelectronic properties of Cs3 SbBiI9 . It is anticipated that this work will benefit the design and fabrication of lead-free perovskite semiconductors for optoelectronic applications.

Recent Progress of Low-Toxicity Poor-Lead All-Inorganic Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have achieved an impressive certified efficiency of 25.7%, which is comparatively higher than that of commercial silicon solar cells (23.3%), showing great potential toward commercialization. However, the low stability and high toxicity due to the presence of volatile organic components and toxic metal lead in the perovskites pose significant challenges. To obtain robust and low-toxicity PSCs, substituting organic cations with pure inorganic cations, and partially or fully replacing the toxic Pb with environmentally benign metals, is one of the promising methods. To date, continuous efforts have been made toward the construction of highly performed low-toxicity inorganic PSCs with astonishing breakthroughs. This review article provides an overview of recent progress in inorganic PSCs in terms of lead-reduced and lead-free compositions. The physical properties of poor-lead all-inorganic perovskites are discussed to unveil the major challenges in this field. Then, it reports notable achievements for the experimental studies to date to figure out feasible methods for efficient and stable poor-lead all-inorganic PSCs. Finally, a discussion of the challenges and prospects for poor-lead all-inorganic PSCs in the future is presented.

Unraveling the Defect-Dominated Broadband Emission Mechanisms in (001)-Preferred Two-Dimensional Layered Antimony-Halide Perovskite Film

By adding molar-controlled SbCl₃ in a Cs₃Sb₂Cl₉ precursor, we employed a low-temperature solution-processed approach to prepare high-quality (001)-preferred Cs₃Sb₂Cl₉ thin film, which demonstrates a stable defect-dominated broadband emission at room temperature. Density functional theory calculations reveal that the defect emission originates from the donor-acceptor pair (DAP) recombination between chlorine vacancy (V_Cl) and cesium vacancy (V_Cs). Furthermore, V_Cl + V_Cs DAP is more stable on the (001) surface. The improved film quality and the more stable V_Cl + V_Cs DAP increase the activation energy related to defect states, resulting in an enhancement of the defect emission for the high-quality (001)-preferred film. This work provides deep insight into the key role of the (001) surface in defect emission and a feasible strategy to enhance the defect emission in 2D halide perovskites A₃B₂X₉ (A = CH₃NH₃, Cs, Rb; B = Bi, Sb; X = Cl, Br, I) by control of the thin film preferred orientation.

Environmental and health risks of perovskite solar modules: Case for better test standards and risk mitigation solutions

Perovskite solar cells (PSCs) promise high efficiencies and low manufacturing costs. Most formulations, however, contain lead, which raises health and environmental concerns. In this review, we use a risk assessment approach to identify and evaluate the technology risks to the environment and human health. We analyze the risks by following the technology from production to transportation to installation to disposal and examine existing environmental and safety regulations in each context. We review published data from leaching and air emissions testing and highlight gaps in current knowledge and a need for more standardization. Methods to avoid lead release through introduction of absorbing materials or use of alternative PSC formulations are reviewed. We conclude with the recommendation to develop recycling programs for PSCs and further standardized testing to understand risks related to leaching and fires.

Electronic Structures and Photoelectric Properties in Cs3Sb2X9 (X = Cl, Br, or I) under High Pressure: A First Principles Study

Lead-free perovskites of Cs₃Sb₂X₉ (X = Cl, Br, or I) have attracted wide attention owing to their low toxicity. High pressure is an effective and reversible method to tune bandgap without changing the chemical composition. Here, the structural and photoelectric properties of Cs₃Sb₂X₉ under high pressure were theoretically studied by using the density functional theory. The results showed that the ideal bandgap for Cs₃Sb₂X₉ can be achieved by applying high pressure. Moreover, it was found that the change of the bandgap is caused by the shrinkage of the Sb-X long bond in the [Sb₂X₉]^3- polyhedra. Partial density of states indicated that Sb-5s and X-p orbitals contribute to the top of the valence band, while Sb-5p and X-p orbitals dominate the bottom of the conduction band. Moreover, the band structure and density of states showed significant metallicity at 38.75, 24.05 GPa for Cs₃Sb₂Br₉ and Cs₃Sb₂I₉, respectively. Moreover, the absorption spectra showed the absorption edge redshifted, and the absorption coefficient of the Cs₃Sb₂X₉ increased under high pressure. According to our calculated results, the narrow bandgap and enhanced absorption ability under high pressure provide a new idea for the design of the photovoltaic and photoelectric devices.

The non-centrosymmetric layered compounds IrTe2I and RhTe2I

The previously unreported layered compounds IrTe₂I and RhTe₂I were prepared by a high-pressure synthesis method. Single crystal X-ray and powder X-ray diffraction studies find that the compounds are isostructural, crystallizing in a layered orthorhombic structure in the non-centrosymmetric, non-symmorphic space group Pca2₁ (#29). Characterization reveals diamagnetic, high resistivity, semiconducting behavior for both compounds, consistent with the +3 chemical valence and d⁶ electronic configurations for both iridium and rhodium and the Te-Te dimers seen in the structural study. Electronic band structures are calculated for both compounds, showing good agreement with the experimental results.

Two-Dimensional Cs3Sb2I9-xClx Film with (201) Preferred Orientation for Efficient Perovskite Solar Cells

All-inorganic Sb-perovskite has become a promising material for solar cell applications owing to its air stability and nontoxic lead-free constitution. However, the poor morphology and unexpected (001) orientation of Sb-based perovskite films strongly hinder the improvement of efficiency. In this work, two-dimensional Cs₃Sb₂Cl_xI_9-x with (201) preferred orientation has been successfully fabricated by introducing thiourea (TU) to the precursor solution. The presence of the C=S functional group in TU regulates the crystallization dynamics of Cs₃Sb₂I_9-xCl_x films and generates the (201) preferred orientation of Cs₃Sb₂Cl_xI_9-x films, which could effectively improve the carrier transport and film morphology. As a result, the Cs₃Sb₂I_9-xCl_x perovskite solar cells (PSCs) delivered a power conversion efficiency (PCE) of 2.22%. Moreover, after being stored in nitrogen at room temperature for 60 days, the devices retained above 87.69% of their original efficiency. This work demonstrates a potential pathway to achieve high-efficiency Sb-based PSCs.

Highly oriented quasi-2D layered tin halide perovskites with 2-thiopheneethylammonium iodide for efficient and stable tin perovskite solar cells

The addition of TEAI can induce oriented growth of perovskite films and enhance the efficiency with high stability.

Recent Advancements in Nontoxic Halide Perovskites: Beyond Divalent Composition Space

Since the inception of organic-inorganic hybrid perovskites of ABX3 stoichiometry in 2009, there has been enormous progress in envisaging efficient solar cell materials throughout the world, from both the theoretical and experimental perspectives. Despite achieving 25.5% efficiency, hybrid halide perovskites are still facing two main challenges: toxicity due to the presence of lead and device stability. Two particular families with A3B2X9 and A2MM'X6 stoichiometries have emerged to address these two prime concerns, which have restrained the advancement of solar energy harvesting. Several investigations, both experimental and theoretical, are being conducted to explore the holy-grail materials, which could be optimum for not only efficient but also stable and nontoxic photovoltaics technology. However, the trade-off among stability, efficiency, and toxicity in such solar energy materials is yet to be completely resolved, which requires a systematic overview of A3B2X9- and A2MM'X6-based solar cell materials. Therefore, in this timely and relevant perspective, we have focused on these two particular promising families of perovskite materials. We have portrayed a roadmap projecting the recent advancements from both theoretical and experimental perspectives for these two exciting and promising solar energy material families while amalgamating our critical viewpoint with a future outlook.

Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(ii)/Sn(iv) doping

Lead-free halides with perovskite-related structures, such as the vacancy-ordered perovskite Cs₃Bi₂Br₉, are of interest for photovoltaic and optoelectronic applications. We find that addition of SnBr₂ to the solution-phase synthesis of Cs₃Bi₂Br₉ leads to substitution of up to 7% of the Bi(iii) ions by equal quantities of Sn(ii) and Sn(iv). The nature of the substitutional defects was studied by X-ray diffraction, ¹³³Cs and ¹¹⁹Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations. The resulting mixed-valence compounds show intense visible and near infrared absorption due to intervalence charge transfer, as well as electronic transitions to and from localised Sn-based states within the band gap. Sn(ii) and Sn(iv) defects preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(ii) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(ii) within the double substitution complex contributes to this unusual stability. These results expand upon research on inorganic mixed-valent halides to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure-property relationships of lead-free vacancy-ordered perovskite structures.

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.

Two-Step Chemical Vapor Deposition-Synthesized Lead-Free All-Inorganic Cs3Sb2Br9 Perovskite Microplates for Optoelectronic Applications

Lead-based halide perovskites (APbX₃, where A = organic or inorganic cation, X = Cl, Br, I) are suitable materials for many optoelectronic devices due to their many attractive properties. However, the concern of lead toxicity and the poor ambient and operational stability of the organic cation group greatly limit their practical utilization. Therefore, there has recently been great interest in lead-free, environment-friendly all-inorganic halide perovskites (IHPs). Sb and Sn are common species suggested to replace Pb for Pb-free IHPs. However, the large difference in the melting points of the precursor materials (e.g., CsBr and SbBr₃ precursors for Cs₃Sb₂Br₉) makes the chemical vapor deposition (CVD) growth of high-quality Pb-free IHPs a very challenging task. In this work, we developed a two-step CVD method to overcome this challenge and successfully synthesized Pb-free Cs₃Sb₂Br₉ perovskite microplates. Cs₃Sb₂Br₉ microplates ∼25 μm in size with the exciton absorption peak at ∼2.8 eV and a band gap of ∼2.85 eV were obtained. The microplates have a smooth hexagonal morphology and show a large Stokes shift of ∼450 meV and exciton binding energy of ∼200 meV. To demonstrate the applications of these microplates in optoelectronics, simple photoconductive devices were fabricated. These photodetectors exhibit a current on/off ratio of 2.36 × 10², a responsivity of 36.9 mA/W, and a detectivity of 1.0 × 10¹⁰ Jones with a fast response of rise and decay time of 61.5 and 24 ms, respectively, upon 450 nm photon irradiation. Finally, the Cs₃Sb₂Br₉ microplates also show good stability in ambient air without encapsulation. These results demonstrate that the 2-step CVD process is an effective approach to synthesize high-quality all-inorganic lead-free Cs₃Sb₂Br₉ perovskite microplates that have the potential for future high-performance optoelectronic device applications.

2D-MA3 Sb2 I9 Back Surface Field for Efficient and Stable Perovskite Solar Cells

In perovskite solar cells (PSCs), a defective perovskite (PVK) surface and cliff-like energy offset at the interface always slow down the charge extraction; meanwhile, interface ion diffusion causes oxidation of the metal electrode, inducing device instability. Here, the in situ grown 2D-(CH₃ NH₂ )₃ Sb₂ I₉ (MA₃ Sb₂ I₉ ) on the back surface of MAPbI₃ results in a more robust interface. MA₃ Sb₂ I₉ changes the MAPbI₃ surface to p-type and thus acts like a back surface field to drive charge extraction and suppress recombination, resulting in an obviously higher fill factor (FF) = 0.8 and power conversion efficiency (PCE) = 20.4% of SnO₂ /MAPbI₃ /MA₃ Sb₂ I₉ /Spiro-OMeTAD (2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene) PSC than the pure MAPbI₃ device. More importantly, strong chemical bonding of SbI prohibits ion diffusion, largely enhancing the thermal stability and longtime stability. Here, special 2D-MA₃ Sb₂ I₉ constructs' robust band alignment and chemical environment at the interface are highlighted for efficient and stable PSCs.

Recent advances toward environment-friendly photodetectors based on lead-free metal halide perovskites and perovskite derivatives

Recently, metal-halide perovskites have emerged as promising materials for photodetector (PD) applications owing to their superior optoelectronic properties, such as ambipolar charge transport characteristics, high carrier mobility, and so on. In the past few years, rapid progress in lead-based perovskite PDs has been witnessed. However, the critical environmental instability and lead-toxicity seriously hinder their further applications and commercialization. Therefore, searching for environmentally stable and lead-free halide perovskites (LFHPs) to address the above hurdles is certainly a worthwhile subject. In this review, we present a comprehensive overview of currently explored LFHPs with an emphasis on their crystal structures, optoelectronic properties, synthesis and modification methods, as well as the PD applications. LFHPs are classified into four categories according to the replacement strategies of Pb²⁺, including AB(ii)X₃, A₃B(iii)₂X₉, A₂B(i)B(iii)'X₆, and newly-emerging perovskite derivatives. Then, we give a demonstration of the preliminary achievements and limitations in environment-friendly PDs based on such LFHPs and perovskite derivatives, and also discuss their applications in biological synapses, imaging, and X-ray detection. With the perspective of their properties and current challenges, we provide an outlook for future directions in this rapidly evolving field to achieve high-quality LFHPs and perovskite derivatives for a broader range of fundamental research and practical applications.

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.

Recent Progress and Challenges in A3Sb2X9-Based Perovskite Solar Cells

The recent trends and current state of perovskite solar cells (PSCs) suggested their potential for practical applications. Since their origin, organic-inorganic lead halide (MAPbX₃) perovskite material-based PSCs have been widely attractive to the scientific community due to their simple manufacturing process, high performance, and cost effectiveness. In spite of the high performance, the lead halide perovskite solar cells are still agonizing due to the long-term stability and toxic nature of Pb. In the last 4 years or so, many alternative perovskite or perovskite-like materials were explored for the development of Pb-free PSCs. However, antimony (Sb)-based perovskite-like materials have shown enhanced stability and average photovoltaic performance. In this mini-review, we discuss the fabrication, recent trends, and current state of the Sb-based PSCs.

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.

Layered perovskite materials: key solutions for highly efficient and stable perovskite solar cells

Metal halide perovskites having three-dimensional crystal structures are being applied successfully in various optoelectronic applications. To address their most challenging issues-instability and toxicity-without losing efficiency, lower-dimensional perovskites appear to be promising alternatives. Recently, two-dimensional (2D) perovskite solar cells have been developed exhibiting excellent photostability and moisture-stability, together with moderate device efficiency. This review summarizes the photophysical properties and operating mechanisms of 2D perovskites as well as recent advances in their applications in solar cell devices. Also presented is an agenda for the next-stage development of stable perovskite materials for solar cell applications, highlighting the issues of stability and toxicity that require further study to ensure commercialization.

Modulating Performance and Stability of Inorganic Lead-Free Perovskite Solar Cells via Lewis-Pair Mediation

Fully inorganic perovskites based on Bi³⁺ and Sb³⁺ are emerging as alternatives that overcome the toxicity and low stability of their Pb-based perovskite counterparts. Nevertheless, the thin film fabrication of Pb-free perovskites remains a struggle, with poor morphologies and incomplete conversions greatly inhibiting device performance. In this study, we modulated the crystallization of an all-inorganic dimer phase of a Sb perovskite (d-Cs₃Sb₂I₉) through gradual increase in the annealing temperature, accompanied by the use of Lewis bases for adduct formation. Here, the role of Lewis pairing in the crystallization of the resulting Pb-free Cs₃Sb₂I₉ thin films has been investigated. Both, "S-donor" (thiourea) and "O-donor" [N-methylpyrrolidone (NMP)] Lewis bases are examined for their abilities to form adducts with Cs⁺ and Sb³⁺ cations. Furthermore, density functional theory has been used to estimate the binding energies of these Lewis bases with the Cs₃Sb₂I₉ lattice. Temperature-dependent photoluminescence spectroscopy revealed the nature of the band gap of d-Cs₃Sb₂I₉. The efficiency of the resulting perovskite solar cells was enhanced to 1.8%, with excellent stability observed, when using NMP to form the adduct film. To the best of our knowledge, this is the best solar cell efficiency for the dimer phase of the inorganic Sb-based perovskite. The effects of both S- and O-donors are studied under various environmental stresses to reveal the stability responses of the devices.

Bandgap engineering in two-dimensional halide perovskite Cs3Sb2I9 nanocrystals under pressure

Halide perovskites have attracted great attention owing to their outstanding performance in optoelectronic applications and solar cells. Recently, two-dimensional (2D) Cs3Sb2I9 nanocrystals (NCs) have attracted sustained interest due to their potentially useful photovoltaic behavior. However, their practical application is impeded by the large bandgap. In this study, the bandgap of 2D Cs3Sb2I9 NCs is successfully narrowed from 2.05 eV to 1.36 eV by means of a high pressure with a measurable rate of 33.7%. Optical changes of 2D Cs3Sb2I9 NCs originate from Sb-I bond contraction and I-Sb-I bond angle changes within the [SbI6]3- octahedra, which determines the overlap of orbitals. Angle dispersive synchrotron X-ray diffraction spectra and Raman spectra of Cs3Sb2I9 NCs indicate that the structural amorphization gradually begins at about 14.0 GPa and the changes are reversible once pressure is completely released. The band gap is slightly smaller after decompression than that under the initial ambient conditions, resulting from the incomplete recrystallization process. First-principles calculations further elucidate that variations in band gaps are mainly governed by the orbital interactions associated with the distortion of the Sb-I octahedral network upon compression. The research enhances the fundamental understanding of 2D Cs3Sb2I9 NCs and is expected to greatly advance the research progress of perovskites in band gap interception at high pressures. Meanwhile, this study demonstrates that pressure processing can be used as a robust strategy to improve materials-by-design in applications.

[NH2CHNH2]3Sb2I9: a lead-free and low-toxicity organic–inorganic hybrid ferroelectric based on antimony(iii) as a potential semiconducting absorber

A novel room-temperature ferroelectric crystal with the complex sequence of phase transitions.

From Lead Halide Perovskites to Lead-Free Metal Halide Perovskites and Perovskite Derivatives

Despite the exciting progress on power conversion efficiencies, the commercialization of the emerging lead (Pb) halide perovskite solar cell technology still faces significant challenges, one of which is the inclusion of toxic Pb. Searching for Pb-free perovskite solar cell absorbers is currently an attractive research direction. The approaches used for and the consequences of Pb replacement are reviewed herein. Reviews on the theoretical understanding of the electronic, optical, and defect properties of Pb and Pb-free halide perovskites and perovskite derivatives are provided, as well as the experimental results available in the literature. The theoretical understanding explains well why Pb halide perovskites exhibit superior photovoltaic properties, but Pb-free perovskites and perovskite derivatives do not.

Nanostructured Perovskite Solar Cells

Over the past decade, lead halide perovskites have emerged as one of the leading photovoltaic materials due to their long carrier lifetimes, high absorption coefficients, high tolerance to defects, and facile processing methods. With a bandgap of ~1.6 eV, lead halide perovskite solar cells have achieved power conversion efficiencies in excess of 25%. Despite this, poor material stability along with lead contamination remains a significant barrier to commercialization. Recently, low-dimensional perovskites, where at least one of the structural dimensions is measured on the nanoscale, have demonstrated significantly higher stabilities, and although their power conversion efficiencies are slightly lower, these materials also open up the possibility of quantum-confinement effects such as carrier multiplication. Furthermore, both bulk perovskites and low-dimensional perovskites have been demonstrated to form hybrids with silicon nanocrystals, where numerous device architectures can be exploited to improve efficiency. In this review, we provide an overview of perovskite solar cells, and report the current progress in nanoscale perovskites, such as low-dimensional perovskites, perovskite quantum dots, and perovskite-nanocrystal hybrid solar cells.

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.

High-Pressure Band-Gap Engineering and Metallization in the Perovskite Derivative Cs3 Sb2 I9

Among photovoltaic materials, the antimony-based, perovskite-like structure Cs₃ Sb₂ I₉ stands out owing to its low toxicity and air stability. Here, changes in the optoelectronic properties and crystal structure of the lead-free perovskite derivative Cs₃ Sb₂ I₉ are reported, caused by pressure-induced lattice compression. At 20.0 GPa, Cs₃ Sb₂ I₉ with a wide band gap (2.34 eV) successfully broke through the Shockley-Queisser limit (1.34 eV), accompanied by clear piezochromism from orange-yellow to opaque black. Additionally, Cs₃ Sb₂ I₉ experienced completely reversible amorphization at 20.0 GPa. These optical changes could be attributed to atomic-orbital overlap enhancement caused by contraction of the Sb-I bond length and diminution of the Sb-I bond angle. In addition, Cs₃ Sb₂ I₉ underwent a transition from semiconductor to conductor upon compression and obtained metallic properties at 44.3 GPa, indicating new electronic properties. The obtained results may further broaden the research prospects of halide perovskite materials in the field of photovoltaics.

Divergent Optical Properties in an Isomorphous Family of Multinary Iodido Pentelates

Multinary organic-inorganic metal halide materials beyond the perovskite motif can help to address both fundamental aspects such as the electronic interactions between different metalate building units and practical issues like stability and ease of preparation in this new field of research. However, such multinary compounds have remained quite rare for the halogenido pentelates, as the formation of simpler side phases can be a significant hindrance. Here, we report a family of four new multinary iodido pentelates [PPh₄]₂[ECu₂I₇(nitrile)] (E = Sb, Bi; nitrile = acetonitile or propionitrile), including the first metalate with a Cu-I-Sb unit. The compounds can be obtained by facile solution or mechanochemical methods and display good stability up to 160 °C. A comparison with compounds containing binary anions [EI₆]^3- reveals that, unexpectedly, the addition of the iodido cuprate unit causes a blue-shift in the absorption of the antimonates but a red-shift in the bismuthates. Photoluminescence investigations at 10 K show that the compounds display broad luminescence bands that correspond well with the trend in their onset of absorption. Overall, the work highlights that multinary, non-perovskite halogenido metalates can be a valuable expansion of the chemistry of metal halide perovskites.

Two Heteromorphic Crystals of Antimony-Based Hybrids Showing Tunable Optical Band Gaps and Distinct Photoelectric Responses

Organic-inorganic hybrid perovskites, most markedly CH₃NH₃PbI₃, have attracted extensive interest because of their potential use in optoelectronic and photovoltaic applications. Nevertheless, the toxicity of lead restricts their further application. Here, we successfully synthesized two lead-free heteromorphic hybrids, (C₇H₁₈N₂O)₃Sb₄I₁₈·H₂O (1) and (C₇H₁₈N₂O)Sb₂I₈·H₂O (2, C₇H₁₈N₂O²⁺ is N-aminopropylmorpholinium), both of which belong to the zero-dimensional tetranuclear perovskite-like structure. However, the inorganic [Sb₄I₁₈] cluster of 1 adopts a tetragonal topology, while 2 features the distorted [Sb₄I₁₆] motif; this disparity leads to a significant distinction between their electronic structures as well as an optical band gap ( E_g). Their absorption cutoffs are measured to be 708 nm (for 1, E_g = 1.71 eV) and 578 nm (for 2, E_g = 2.11 eV), respectively. In particular, 1 exhibits a stronger photoelectric response in a wider optical region compared to that of 2, and the "on/off" ratio of conductivity of 1 is estimated to ∼300 under sunlight illumination. Density functional theory calculation discloses that different inorganic motifs make greater contributions to their electronic structure and photoelectric response. It is believed that the heteromorphic method allows a potential pathway for construction of new lead-free hybrid materials as light absorbers for photoelectric application.

Will organic-inorganic hybrid halide lead perovskites be eliminated from optoelectronic applications?

In the development of perovskite solar cells, a new version of Don Quixote is needed if scientists are to keep on seeking the most celebrated works of literature, according to the evaluation criterion of 'THE FIRST' and 'THE BEST'. Perovskite solar cells have developed rapidly in recent years due to several factors, including their high light absorption capability, long carrier lifetime, high defect tolerance, and adjustable band gap. Since they were first reported in 2009, solar cells based on organic-inorganic hybrid halide lead perovskites have achieved a power conversion efficiency of over 23%. However, although there are broad development prospects for perovskite solar cells, their lead toxicity and instability resulting from the use of organic-inorganic hybrid halide lead perovskites such as CH₃NH₃PbI₃ limit their application, which is further deteriorating progressively. Therefore, the development of environmentally friendly, stable and efficient perovskite materials for future optoelectronic applications has long-term practical significance, which can eventually be commercialized. In this case, the discovery and development of inorganic lead-free perovskite light absorbing materials have become an active research topic in the field of photovoltaics. In this review, we discuss the application of organic-inorganic hybrid halide lead perovskites. This application is further analyzed and summarized using the research progress on inorganic lead-free perovskite solar cells by restoring some relevant prospects for the development of inorganic lead-free perovskite solar cells.

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.

Thiocyanate Containing Two-Dimensional Cesium Lead Iodide Perovskite, Cs2PbI2(SCN)2: Characterization, Photovoltaic Application, and Degradation Mechanism

We explored thiocyanate (SCN)-based two-dimensional (2D) organometal lead halide perovskite families toward photovoltaic applications. Using an SCN axial ligand and various cation species, we examined AA'PbI₂(SCN)₂-type 2D perovskite by replacing the cation species (AA') between methylammonium (MA), formamidinium (FA), and cesium. Among various cation compositions, only all-inorganic cesium-based SCN perovskite, Cs₂PbI₂(SCN)₂, film showed high thermal stability compared to known 2D perovskites. Perovskite solar cells (PSCs) using the Cs₂PbI₂(SCN)₂ absorber yielded approximately 2% conversion efficiency on the mesoscopic device. Relatively low efficiency is attributed, in addition to optical properties (large band gap (2.05 eV) and exciton absorption), to the orientation of perovskite layer parallel to the layered structure, preventing carrier extraction from the light-absorber perovskite. In device stability, the Cs-based 2D perovskite was stable against oxygen (oxidation), whereas it was found to be unstable against humidity. X-ray diffraction and X-ray photoelectron spectroscopy measurements showed that, unlike long alkylammonium-based 2D perovskite families such as BA₂PbI₄ (BA = butylammonium), the Cs-based 2D perovskite can undergo hydrolysis due to the hydrophilic Cs cations.

Lead-free hybrid perovskites for photovoltaics

This review covers the state-of-the-art in organo-inorganic lead-free hybrid perovskites (HPs) and applications of these exciting materials as light harvesters in photovoltaic systems. Special emphasis is placed on the influence of the spatial organization of HP materials both on the micro- and nanometer scale on the performance and stability of perovskite-based solar light converters. This review also discusses HP materials produced by isovalent lead(II) substitution with Sn2+ and other metal(II) ions, perovskite materials formed on the basis of M3+ cations (Sb3+, Bi3+) as well as on combinations of M+/M3+ ions aliovalent to 2Pb2+ (Ag+/Bi3+, Ag+/Sb3+, etc.). The survey is concluded with an outlook highlighting the most promising strategies for future progress of photovoltaic systems based on lead-free perovskite compounds.

Low-Bandgap Cs4 CuSb2 Cl12 Layered Double Perovskite: Synthesis, Reversible Thermal Changes, and Magnetic Interaction

Double perovskites (DPs) with a generic formula A2 M'(I)MIII X6 (A and M are metal ions, and X=Cl, Br, I) are now being explored as potential alternatives to Pb-halide perovskites for solar cells and other optoelectronic applications. However, these DPs typically suffer from wide (≈3 eV) and/or indirect band gaps. In 2017, a new structural variety, namely layered halide DP Cs4 CuSb2 Cl12 (CCSC) with bivalent CuII ion in the place of M'(I) was reported, which exhibit a band gap of approximately 1 eV. Here, we report a mechanochemical synthesis of CCSC, its thermal and chemical stability, and magnetic response of CuII d9 electrons controlling the optoelectronic properties. A simple grinding of precursor salts at ambient conditions provides a stable and scalable product. CCSC is stable in water/acetone solvent mixtures (≈30 % water) and many other polar solvents unlike Pb-halide perovskites. It decomposes to Cs3 Sb2 Cl9 , Cs2 CuCl4 , and SbCl3 at 210 °C, but the reaction can be reversed back to produce CCSC at lower temperatures and high humidity. A long-range magnetic ordering is observed in CCSC even at room temperature. The role of such magnetic ordering in controlling the dispersion of the conduction band, and therefore, controlling the electronic and optoelectronic properties of CCSC has been discussed.

A new antimony-based organic–inorganic hybrid absorber with photoconductive response

We report a new lead-free organic–inorganic hybrid compound, which shows notable light-absorption, photoconductive properties and phase stability.

CsAg2Sb2I9 solar cells

A perovskite material CsAg₂Sb₂I₉ was developed via room-temperature solution processing. Its solar cells gave a PCE of 0.99%.