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

Unraveling the Role of 2D Ti3C2Tx MXene Nanosheets in Cu-Based Double Perovskite Active Layer for Enhanced Photovoltaic Performance

Although the atmospheric stability of lead-free inorganic double perovskite (DP) solar cells (PSCs) looks promising, their further development is hampered by inadequate film quality and non-radiative carrier recombination at the interfaces. Herein, the incorporation of a newly developed intriguing class of 2D material Ti3C2Tx MXene nanosheets with the photo-absorbing Cu2AgBiI6 (CABI) active layer of a fully inorganic solar cell is reported. The highly conductive Ti3C2Tx nanosheets work as a multi-functional additive by tuning the band gap, reducing the non-radiative carrier recombination, and inhibiting carrier accumulation. In addition, the presence of Ti3C2Tx MXene increases the surface free energy of the perovskite film, which elevates the energy barrier for nucleation and realizes a highly crystalline CABI perovskite film. Primarily, the MXene modification accelerates the charge extraction and transport at the interfaces of the active layer, utilizing energy level alignment with the charge transport layers. Consequently, the photo-conversion efficiency (PCE) of the device with MXene is substantially enhanced to 1.50%. Moreover, the 2D Ti3C2Tx nanosheets increased the long-term stability of the devices by retaining 70% of the initial PCE after 1680 h. With regard to relieving the severe carrier recombination at the interfaces, this work sets a new paradigm toward imminent solar energy conversion.

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.

Doping strategies for inorganic lead-free halide perovskite solar cells: progress and challenges

In recent years, remarkable advancements have been achieved in the field of halide perovskite solar cells (PSCs). However, the commercialization of PSCs has been impeded by challenges such as Pb leakage and the instability of hybrid organic-inorganic perovskites (HOIPs). Hence, the future lies in the development of environmentally friendly inorganic lead-free halide perovskites (LFHPs) based on elements like Sn, Ge, Bi, Sb, and Cu, which show great promise for photovoltaic applications. However, LFHP photovoltaic cells still face challenges such as low efficiency, poor film quality, and stability in comparison to HOIPs. These limitations significantly hinder their further development. To address these issues, element doping strategies, including cationic and anionic doping, as well as the use of additives, are frequently employed. These strategies aim to improve film quality, passivate defects, reduce the band gap, and enhance device performance and stability. In this paper, we aim to provide a comprehensive review of the recent research progress in doping strategies for LFHPs.

Improved photovoltaic performance of Pb-free AgBi2I7 based photovoltaics

Hybrid perovskites based on bismuth are good candidates for developing lead-free and air-stable photovoltaics, but they have historically been constrained by poor surface morphologies and large band-gap energies. Monovalent silver cations are incorporated into iodobismuthates as part of a novel materials processing method to fabricate improved bismuth-based thin-film photovoltaic absorbers. However, a number of fundamental characteristics prevented them from achieving better efficiency. We examine bismuth iodide perovskite made of silver with improvements in surface morphology and a narrow band gap, and we achieve high power conversion efficiency. AgBi₂I₇ perovskite was used in the fabrication of PSCs as a material for light absorption, and its optoelectronic proficiencies were also studied. We reduced the band gap to 1.89 eV and achieved a maximum power conversion efficiency of 0.96% using the solvent engineering approach. Additionally, simulation studies verified an efficiency of 13.26% by using AgBi₂I₇ as a light absorber perovskite material.

Low-Temperature Solution-Processed Cu2AgBiI6 Films for High Performance Photovoltaics and Photodetectors

Recently, Cu₂AgBiI₆ semiconductor has been investigated due to the high absorption coefficient, direct bandgap, and low exciton binding energy, which are promising for eco-friendly photoelectric devices. Herein, pyridine is introduced as solvent additive to completely dissolve the solutes and form clear Cu₂AgBiI₆ precursor solution, which results in high-quality films and may provide a general approach for high-quality film growth of other bismuth-based metal halide semiconductors. In addition, the electronic structure of Cu₂AgBiI₆ has been demonstrated for the first time and shows an intrinsically weak n-type semiconductor. Furthermore, phenethylammonium iodide for surface passivation significantly improves the film quality, slightly n-dopes the material, and shifts up the band level. Finally, the photovoltaics and photodetector performance for n-i-p planar heterojunction devices have been investigated. The efficiency is up to 1%, highest for Cu₂AgBiI₆ solar cells and comparable with other lead-free bismuth based metal halide solar cells. Moreover, photodetectors with fast speed of rising and decaying time, especially the excellent specific photodetectivity of ∼10¹² Jones within the wavelength of ∼350-600 nm, are achieved, which paves an alternative and promising strategy for the design of future commercial photodetectors that are self-powered, stable, nontoxic, etc.

Mixed-Halide Double Perovskite Cs2 AgBiX6 (X=Br, I) with Tunable Optical Properties via Anion Exchange

Lead-free double perovskites, A2 M+ M'3+ X6 , are considered as promising alternatives to lead-halide perovskites, in optoelectronics applications. Although iodide (I) and bromide (Br) mixing is a versatile tool for bandgap tuning in lead perovskites, similar mixed I/Br double perovskite films have not been reported in double perovskites, which may be due to the large activation energy for ion migration. In this work, mixed Br/I double perovskites were realized utilizing an anion exchange method starting from Cs2 AgBiBr6 solid thin-films with large grain-size. The optical and structural properties were studied experimentally and theoretically. Importantly, the halide exchange mechanism was investigated. Hydroiodic acid was the key factor to facilitate the halide exchange reaction, through a dissolution-recrystallization process. In addition, the common organic iodide salts could successfully perform halide-exchange while retaining high mixed-halide phase stability and strong light absorption capability.

Solvent Engineering for Controlled Crystallization and Growth of All-Inorganic Pb-Free Rudorffite Absorbers of Perovskite Solar Cells

Solution-processed Ag-Bi-I rudorffites with direct band gaps of <2 eV show promise for highly efficient and cost-effective Pb-free solar cells. However, relatively fast crystallization rates of Bi-based films and limited solubility of BiI₃ in many solvents result in poor film morphologies, inhibiting their device performance. Here, we conduct a solvent-engineering method to adjust the dynamics of nucleation and growth during film formation. We fabricate Ag₂BiI₅, AgBiI₄, and AgBi₂I₇ absorber layers using a mixed solvent of dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) and find that a volume percentage of 50% DMSO causes highly uniform and dense perovskite films via a BiI₃-DMSO-AgI intermediate phase formation, leading to solar cells with an improved power conversion efficiency of 0.62% for the Ag₂BiI₅ absorber. These results provide valuable insights into the optimization of the solution processing technique to realize low-toxicity and efficient perovskite solar cells.

Alkali-metal-ion-doping strategy to improve the photovoltaic properties of Ag2BiI5 solar cells

An efficient alkali-metal-ion-doping strategy is proposed to improve the photovoltaic properties of Ag₂BiI₅ solar cells.

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.

Assembling Polyiodides and Iodobismuthates Using a Template Effect of a Cyclic Diammonium Cation and Formation of a Low-Gap Hybrid Iodobismuthate with High Thermal Stability

Exploiting a template effect of 1,4-diazacycloheptane (also known as homopiperazine, Hpipe), four new hybrid iodides, (HpipeH₂)₂Bi₂I₁₀·2H₂O, (HpipeH₂)I(I₃), (HpipeH₂)₃I₆·H₂O, and (HpipeH₂)₃(H₃O)I₇, were prepared and their crystal structures were solved using single crystal X-ray diffraction data. All four solid-state crystal structures feature the HpipeH₂²⁺ cation alternating with Bi₂I₁₀^4–, I₃^–, or I^– anions and solvent water or H₃O⁺ cation. HpipeH₂²⁺ assembles anionic and neutral building blocks into polymer structures by forming four strong (N)H···I and (N)H···O hydrogen bonds per cation, with the H···I distances ranging from 2.44 to 2.93 Å and H···O distances of 1.88–1.89 Å. These hydrogen bonds strongly affect the properties of compounds; in particular, in the case of (HpipeH₂)₂Bi₂I₁₀·2H₂O, they ensure narrowing of the band gap down to 1.8 eV and provide high thermal stability up to 240 °C, remarkable for a hydrated molecular solid.

Perovskite-Inspired Lead-Free Ag2BiI5 for Self-Powered NIR-Blind Visible Light Photodetection

In recent years, solution-processible semiconductors with perovskite or perovskite-inspired structures have been extensively investigated for optoelectronic applications. In particular, silver-bismuth-halides have been identified as especially promising because of their bulk properties and lack of heavily toxic elements. This study investigates the potential of Ag₂BiI₅ for near-infrared (NIR)-blind visible light photodetection, which is critical to emerging applications (e.g., wearable optoelectronics and the Internet of Things). Self-powered photodetectors were realized and provided a near-constant ≈ 100 mA W^-1 responsivity through the visible, a NIR rejection ratio of > 250, a long-wavelength responsivity onset matching standard colorimetric functions, and a linear photoresponse of > 5 orders of magnitude. The optoelectronic characterization of Ag₂BiI₅ photodetectors additionally revealed consistency with one-center models and the role of the carrier collection distance in self-powered mode. This study provides a positive outlook of Ag₂BiI₅ toward emerging applications on low-cost and low-power NIR-blind visible light photodetector.

Performance and stability gain in zero-dimensional perovskite solar cells after >2 years when hybridized with silicon nanocrystals

We report highly stable zero-dimensional (CH₃NH₃)₃Bi₂I₉ photovoltaic cells which demonstrate a 33% increase in performance after 2 years when hybridized with silicon nanocrystals (SiNCs). The natural oxidation of SiNCs is expected to consume radical species and improve the SiNC/(CH₃NH₃)₃Bi₂I₉ interface and electronic coupling whilst also inhibiting defect-induced degradation.

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.

Structural and Functional Diversity in Lead-Free Halide Perovskite Materials

Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community holds different views toward the toxic lead in these high-performance perovskites, it is certainly preferred to replace lead with nontoxic, or at least less-toxic, elements while maintaining the superior properties. Here, the design rules for lead-free perovskite materials with structural dimensions from 3D to 0D are presented. Recent progress in lead-free halide perovskites is reviewed, and the relationships between the structures and fundamental properties are summarized, including optical, electric, and magnetic-related properties. 3D perovskites, especially A₂ B⁺ B³⁺ X₆ -type double perovskites, demonstrate very promising optoelectronic prospects, while low-dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic, and multifunctional applications. Furthermore, based on these structure-property relationships, strategies for multifunctional perovskite design are proposed. The challenges and future directions of lead-free perovskite applications are also highlighted, with emphasis on materials development and device fabrication. The research on lead-free halide perovskites at Linköping University has benefited from inspirational discussions with Prof. Olle Inganäs.

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.

Perylene Diimide-Based Electron-Transporting Material for Perovskite Solar Cells with Undoped Poly(3-hexylthiophene) as Hole-Transporting Material

Perylene diimide-based small molecules are widely used as intermediates of liquid crystals, owing to their high planarity and electron mobility. In this study, tetrachloroperylene diimide (TCl-PDI) was used as a small-molecule replacement for TiO₂ as electron-transporting material (ETM) for planar perovskite solar cells (PVSCs). Among hole-transporting materials (HTMs) for PVSCs, poly(3-hexylthiophene) (P3HT) gives the devices the highest stability and reproducibility. Therefore, PVSCs with the structure of indium tin oxide (ITO)/ETM/perovskite/P3HT/MoO₃ /Ag were used to evaluate the performances of new ETMs. A reference device with compact TiO₂ and P3HT gave a reasonable power conversion efficiency (PCE) of 12.78 %, whereas the PVSC with TCl-PDI as ETM gave an enhanced PCE of 14.73 %, which is among the highest reported values for PVSCs with undoped P3HT as the HTM. Moreover, TCl-PDI-based devices displayed higher stability than those based on compact TiO₂ , owing to the superior perovskite quality.

Performance enhancement of AgBi2I7 solar cells by modulating a solvent-mediated adduct and tuning remnant BiI3 in one-step crystallization

We modulated a solvent-mediated adduct for one-step crystallization of lead-free AgBi₂I₇ at a lower temperature (90 °C) and to obtain remnant BiI₃ by controlling the nature of the substrate and precursor concentration.

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.

Solution-Processed "Silver-Bismuth-Iodine" Ternary Thin Films for Lead-Free Photovoltaic Absorbers

Bismuth-based hybrid perovskites are regarded as promising photo-active semiconductors for environment-friendly and air-stable solar cell applications. However, poor surface morphologies and relatively high bandgap energies have limited their potential. Silver-bismuth-iodine (Ag-Bi-I) is a promising semiconductor for optoelectronic devices. Therefore, we demonstrate the fabrication of Ag-Bi-I ternary thin films using material solution processing. The resulting thin films exhibit controlled surface morphologies and optical bandgaps according to their thermal annealing temperatures. In addition, it has been reported that Ag-Bi-I ternary systems crystallize to AgBi2I7, Ag2BiI5, etc. according to the ratio of the precursor chemicals. The solution-processed AgBi2I7 thin films exhibit a cubic-phase crystal structure, dense, pinhole-free surface morphologies with grains ranging in size from 200 to 800 nm, and an indirect bandgap of 1.87 eV. The resultant AgBi2I7 thin films show good air stability and energy band diagrams, as well as surface morphologies and optical bandgaps suitable for lead-free and air-stable single-junction solar cells. Very recently, a solar cell with 4.3% power conversion efficiency was obtained by optimizing the Ag-Bi-I crystal compositions and solar cell device architectures.

Solution-Processed Air-Stable Copper Bismuth Iodide for Photovoltaics

Bismuth-based solar cells have been under intensive interest as an efficient non-toxic absorber in photovoltaics. Within this new family of semiconductors, we herein report a new, long-term stable copper bismuth iodide (CuBiI₄ ). A solutionprocessed method under air atmosphere is used to prepare the material. The adopted HI-assisted dimethylacetamide (DMA) co-solvent can completely dissolve CuI and BiI₃ powders with high concentration compared with other organic solvents. Moreover, the high vapor pressure of tributyl phosphate, selected for the solvent vapor annealing (SVA), enables complete low-temperature (≤70 °C) film preparation, resulting in a stable, uniform, dense CuBiI₄ film. The average grain size increases with the precursor concentration, greatly improving the photoluminescence lifetime and hall mobility; a carrier lifetime of 3.03 ns as well as an appreciable hall mobility of 110 cm²  V^-1 s^-1 were obtained. XRD illustrates that the crystal structure is cubic (space group Fd3m) and favored in the [1 1 1] direction. Moreover, the photovoltaic performance of CuBiI₄ was also investigated. A wide bandgap (2.67 eV) solar cell with 0.82 % power conversion efficiency is presented, which exhibits excellent long-term stability over 1008 h under ambient conditions. This air-stable material may give an application in future tandem solar cells as a stable short-wavelength light absorber.

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.

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.

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.

Pronounced exciton and coherent phonon dynamics in BiI3

Long-lived coherent oscillations in transient absorption signals of BiI₃ suggest strong electron–phonon coupling in this material for photovoltaics.

Photovoltaic Rudorffites: Lead-Free Silver Bismuth Halides Alternative to Hybrid Lead Halide Perovskites

Hybrid CPbX₃ (C: Cs, CH₃ NH₃ ; X: Br, I) perovskites possess excellent photovoltaic properties but are highly toxic, which hinders their practical application. Unfortunately, all Pb-free alternatives based on Sn and Ge are extremely unstable. Although stable and non-toxic C₂ ABX₆ double perovskites based on alternating corner-shared AX₆ and BX₆ octahedra (A=Ag, Cu; B=Bi, Sb) are possible, they have indirect and wide band gaps of over 2 eV. However, is it necessary to keep the corner-shared perovskite structure to retain good photovoltaic properties? Here, we demonstrate another family of photovoltaic halides based on edge-shared AX₆ and BX₆ octahedra with the general formula A_a B_b X_x (x=a+3 b) such as Ag₃ BiI₆ , Ag₂ BiI₅ , AgBiI₄ , AgBi₂ I₇ . As perovskites were named after their prototype oxide CaTiO₃ discovered by Lev Perovski, we propose to name these new ABX halides as rudorffites after Walter Rüdorff, who discovered their prototype oxide NaVO₂ . We studied structural and optoelectronic properties of several highly stable and promising Ag-Bi-I photovoltaic rudorffites that feature direct band gaps in the range of 1.79-1.83 eV and demonstrated a proof-of-concept FTO/c-m-TiO₂ /Ag₃ BiI₆ /PTAA/Au (FTO: fluorine-doped tin oxide, PTAA: poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], c: compact, m: mesoporous) solar cell with photoconversion efficiency of 4.3 %.

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.