Ion Exchange/Insertion Reactions for Fabrication of Efficient Methylammonium Tin Iodide Perovskite Solar Cells

Spontaneous Compositional-Interfacial Co-Modification Engineering via Ion Exchange Reaction Between Perovskite and Electron-Transporting Layer for Exceptionally Long-Term Stability of Photovoltaics

The long-term stability of perovskite solar cells (PSCs) is still challenging for commercialization and mainly linked to the life span of perovskite films. Herein, a spontaneous compositional-interfacial co-modification strategy is developed based on the ion exchange reaction by introducing ammonium hexafluorophosphate (NH4PF6) into antisolvent to form gradient structures through a simple one-step solvent engineering. With the assistance of the ion exchange reaction, NH4PF6 forms a multifunctional structure to protect perovskite films from both internal and external factors for the exceptionally long-term stability of photovoltaics. The reason for this is linked to the high hydrophobicity of NH4PF6 for preventing H2O invasion, suppressing ion migration by forming hydrogen bonding, and reducing perovskite defects. The resulting unencapsulated devices show exceptionally long-term stability under standardized the International Summit on Organic Photovoltaic Stability (ISOS) protocols, with over 94%, 81%, and 83% retained power conversion efficiencies after aging tests under N2 (ISOS-D-1I), ambient air (ISOS-D-1), and 85 °C (ISOS-D-2I) for 14016, 2500, and 1248 h, respectively. These performances compare well with the state-of-the-art stability of inverted PSCs. Further investigations are conducted to study the evolution of macroscopic morphology and microscopic crystal structure in aged perovskite films, aiming to provide evidence supporting the aforementioned improvements in stability.

Device modeling of high performance and eco-friendly FAMASnI 3 based perovskite solar cell

Developing environmentally friendly and highly efficient inverted perovskite solar cells (PSCs) encounters significant challenges, specifically the potential toxicity and degradation of thin films in hybrid organic-inorganic photovoltaics (PV). We employed theoretical design strategies that produce hysteresis-reduced, efficient, and stable PSCs based on composition and interface engineering. The devices include a mixed-organic-cation perovskite formamidinium methylammonium tin iodide ( FAMASnI 3 ) as an absorber layer and zinc oxide (ZnO) together with a passivation film phenyl-C61-butyric acid methyl ester (PC 61 BM ) as a double-electron transport layer (DETL). Furthermore, a nickel oxide (NiO) layer and a trap-free junction copper iodide (CuI) are used as a double-hole transport layer (DHTL). The optoelectronic characterization measurements were carried out to understand the physical mechanisms that govern the operation of the devices. The high power conversion efficiencies (PCEs) of 24.27% and 23.50% were achieved in 1D and 2D simulations, respectively. This study illustrates that composition and interface engineering enable eco-friendly perovskite solar cells, improving performance and advancing clean energy.

Thermally Evaporated Metal Halide Perovskites and Their Analogues: Film Fabrication, Applications and Beyond

Metal halide perovskites emerge as promising semiconductors for optoelectronic devices due to ease of fabrication, attractive photophysical properties, their low cost, highly tunable material properties, and high performance. High-quality thin films of metal halide perovskites are the basis of most of these applications including solar cells, light-emitting diodes, photodetectors, and electronic memristors. A typical fabrication method for perovskite thin films is the solution method, which has several limitations in device reproducibility, adverse environmental impact, and utilization of raw materials. Thermal evaporation holds great promise in addressing these bottlenecks in fabricating high-quality halide perovskite thin films. It also has high compatibility with mass-production platforms that are well-established in industries. This review first introduces the basics of the thermal evaporation method with a particular focus on the critical parameters influencing the thin film deposition. The research progress of the fabrication of metal halide perovskite thin films is further summarized by different thermal evaporation approaches and their applications in solar cells and other optoelectronic devices. Finally, research challenges and future opportunities for both fundamental research and commercialization are discussed.

Crystallization Kinetics of Hybrid Perovskite Solar Cells

Metal halide perovskites (MHPs) are considered ideal photovoltaic materials due to their variable crystal material composition and excellent photoelectric properties. However, this variability in composition leads to complex crystallization processes in the manufacturing of Metal halide perovskite (MHP) thin films, resulting in reduced crystallinity and subsequent performance loss in the final device. Thus, understanding and controlling the crystallization dynamics of perovskite materials are essential for improving the stability and performance of PSCs (Perovskite Solar Cells). To investigate the impact of crystallization characteristics on the properties of MHP films and identify corresponding modulation strategies, we primarily discuss the relevant aspects of MHP crystallization kinetics, systematically summarize theoretical methods, and outline modulation techniques for MHP crystallization, including solution engineering, additive engineering, and component engineering, which helps highlight the prospects and current challenges in perovskite crystallization kinetics.

Fullerene-C60 and PCBM as interlayers in regular and inverted lead-free PSCs using CH3NH3SnI3: an analysis of device performance and defect density dependence by SCAPS-1D

One of the challenges hindering the commercialization of perovskite solar cells (PSCs) is the presence of toxic metals such as lead in their composition. Simulation studies using SCAPS-1D have already been conducted on lead-free PSCs to find optimized solar cell parameters, having tin as the primary candidate for replacing lead in perovskites. Here, we used fullerene-C60 and its derivative PCBM as interlayers in a lead-free tin-based PSC between the ETL (ZnO) and the perovskite MASI in both regular and inverted configurations of PSCs using SCAPS-1D software. To the best of our knowledge, this is the first simulation study reporting the impact of using fullerene-C60 and PCBM as interlayers in lead-free PSCs. The defect density (Nt) of the perovskite material is varied, allowing us to observe its influence on the power conversion efficiency (PCE). Using an Nt value of 1017 cm-3 without the interlayer, the PCE was 6.90% and 3.72% for regular and inverted devices. Using PCBM as an interlayer improves the efficiency of both simulated PSCs, achieving a maximum PCE of 8.11% and 5.26% for the regular and inverted configurations, respectively. Decreasing the Nt from 1017 cm-3 to 1016 cm-3 caused a significant increase in efficiency, reaching 13.38% (n-i-p) and 10.00% (p-i-n). Finally, using the optimized parameters and an ideal Nt value (1013 cm-3), both PSCs achieved a PCE close to 30%.

Synergetic Regulation of Interface Defects and Carriers Dynamics for High-Performance Lead-Free Perovskite Solar Cells

Severe nonradiative recombination and open-circuit voltage loss triggered by high-density interface defects greatly restrict the continuous improvement of Sn-based perovskite solar cells (Sn-PVSCs). Herein, a novel amphoteric semiconductor, O-pivaloylhydroxylammonium trifluoromethanesulfonate (PHAAT), is developed to manage interface defects and carrier dynamics of Sn-PVSCs. The amphiphilic ionic modulators containing multiple Lewis-base functional groups can synergistically passivate anionic and cationic defects while coordinating with uncoordinated Sn2+ to compensate for surface charge and alleviate the Sn2+ oxidation. Especially, the sulfonate anions raise the energy barrier of surface oxidation, relieve lattice distortion, and inhibit nonradiative recombination by passivating Sn-related and I-related deep-level defects. Furthermore, the strong coupling between PHAAT and Sn perovskite induces the transition of the surface electronic state from p-type to n-type, thus creating an extra back-surface field to accelerate electron extraction. Consequently, the PHAAT-treated device exhibits a champion efficiency of 13.94% with negligible hysteresis. The device without any encapsulation maintains 94.7% of its initial PCE after 2000 h of storage and 91.6% of its initial PCE after 1000 h of continuous illumination. This work provides a reliable strategy to passivate interface defects and construct p-n homojunction to realize efficient and stable Sn-based perovskite photovoltaic devices.

Engineering Stable Lead-Free Tin Halide Perovskite Solar Cells: Lessons from Materials Chemistry

Substituting toxic lead with tin (Sn) in perovskite solar cells (PSCs) is the most promising route toward the development of high-efficiency lead-free devices. Despite the encouraging efficiencies of Sn-PSCs, they are still yet to surpass 15% and suffer detrimental oxidation of Sn(II) to Sn(IV). Since their first application in 2014, investigations into the properties of Sn-PSCs have contributed to a growing understanding of the mechanisms, both detrimental and complementary to their stability. This review summarizes the evolution of Sn-PSCs, including early developments to the latest state-of-the-art approaches benefitting the stability of devices. The degradation pathways associated with Sn-PSCs are first outlined, followed by describing how composition engineering (A, B site modifications), additive engineering (oxidation prevention), and interface engineering (passivation strategies) can be employed as different avenues to improve the stability of devices. The knowledge about these properties is also not limited to PSCs and also applicable to other types of devices now employing Sn-based perovskite absorber layers. A detailed analysis of the properties and materials chemistry reveals a clear set of design rules for the development of stable Sn-PSCs. Applying the design strategies highlighted in this review will be essential to further improve both the efficiency and stability of Sn-PSCs.

Photoluminescence Intensity Enhancement in Tin Halide Perovskites

The prevalence of background hole doping in tin halide perovskites usually dominates their recombination dynamics. The addition of excess Sn halide source to the precursor solution is the most frequently used approach to reduce the hole doping and reveals photo-carrier dynamics related to defects activity. This study presents an experimental and theoretical investigation on defects under light irradiation in tin halide perovskites by combining measurements of photoluminescence with first principles computational modeling. It finds that tin perovskite thin films prepared with an excess of Sn halide sources exhibit an enhancement of the photoluminescence intensity over time under continuous excitation in inert atmosphere. The authors propose a model in which light irradiation promotes the annihilation of VSn 2- /Sni 2+ Frenkel pairs, reducing the deep carrier trapping centers associated with such defect and increasing the radiative recombination. Importantly, these observations can be traced in the open-circuit voltage dynamics of tin-based halide perovskite solar cells, implying the relevance of controlling the Sn photochemistry to stabilize tin perovskite devices.

Sn-Based Perovskite Halides for Electronic Devices

Because of its less toxicity and electronic structure analogous to that of lead, tin halide perovskite (THP) is currently one of the most favorable candidates as an active layer for optoelectronic and electric devices such as solar cells, photodiodes, and field-effect transistors (FETs). Promising photovoltaics and FETs performances have been recently demonstrated because of their desirable electrical and optical properties. Nevertheless, THP's easy oxidation from Sn2+ to Sn4+ , easy formation of tin vacancy, uncontrollable film morphology and crystallinity, and interface instability severely impede its widespread application. This review paper aims to provide a basic understanding of THP as a semiconductor by highlighting the physical structure, energy band structure, electrical properties, and doping mechanisms. Additionally, the key chemical instability issues of THPs are discussed, which are identified as the potential bottleneck for further device development. Based on the understanding of the THPs properties, the key recent progress of THP-based solar cells and FETs is briefly discussed. To conclude, current challenges and perspective opportunities are highlighted.

Numerical Simulation of NH3(CH2)2NH3MnCl4 Based Pb-Free Perovskite Solar Cells Via SCAPS-1D

Recently, the design and fabrication of lead (Pb)-free perovskite or perovskite-like materials have received great interest for the development of perovskite solar cells (PSCs). Manganese (Mn) is a less toxic element, which may be an alternative to Pb. In this work, we explored the role of NH₃(CH₂)₂NH₃MnCl₄ perovskite as a light absorber layer via SCAPS-1D. A Pb-free PSC device (FTO/TiO₂/NH₃(CH₂)₂NH₃MnCl₄/spiro-OMeTAD/Au) was simulated via SCAPS-1D software. The simulated Pb-free PSCs (FTO/TiO₂/NH₃(CH₂)₂NH₃MnCl₄/spiro-OMeTAD/Au) showed decent power conversion efficiency (PCE) of 20.19%. Further, the impact of the thickness of absorber (NH₃(CH₂)₂NH₃MnCl₄), electron transport (TiO₂), and hole-transport (spiro-OMeTAD) layers were also investigated. Subsequently, various electron transport layers (ETLs) were also introduced to investigate the role of ETL. In further studies, an NH₃(CH₂)₂NH₃MnCl₄-based PSC device (FTO/TiO₂/NH₃(CH₂)₂NH₃MnCl₄/spiro-OMeTAD/Au) was also developed (humidity = ~30-40%). The fabricated PSCs displayed an open circuit voltage (Voc) of 510 mV with a PCE of 0.12%.

Coevaporation Stabilizes Tin-Based Perovskites in a Single Sn-Oxidation State

Chemically processed methylammonium tin-triiodide (CH₃NH₃SnI₃) films include Sn in different oxidation states, leading to poor stability and low power conversion efficiency of the resulting solar cells (PSCs). The development of absorbers with Sn [2+] only has been identified as one of the critical steps to develop all Sn-based devices. Here, we report on coevaporation of CH₃NH₃I and SnI₂ to obtain absorbers with Sn being only in the preferred oxidation state [+2] as confirmed by X-ray photoelectron spectroscopy. The Sn [4+]-free absorbers exhibit smooth highly crystalline surfaces and photoluminescence measurements corroborating their excellent optoelectronic properties. The films show very good stability under heat and light. Photoluminescence quantum yields up to 4 × 10^-3 translate in a quasi Fermi-level splittings exceeding 850 meV under one sun equivalent conditions showing high promise in developing lead-free, high efficiency, and stable PSCs.

Effect of electronic doping and traps on carrier dynamics in tin halide perovskites

Tin halide perovskites have recently emerged as promising materials for low band gap solar cells. Much effort has been invested on controlling the limiting factors responsible for poor device efficiencies, namely self-p-doping and tin oxidation. Both phenomena are related to the presence of defects; however, full understanding of their implications in the optoelectronic properties of the material is still missing. We provide a comprehensive picture of the competing radiative and non-radiative recombination processes in tin-based perovskite thin films to establish the interplay between doping and trapping by combining photoluminescence measurements with trapped-carrier dynamic simulations and first-principles calculations. We show that pristine Sn perovskites, i.e. sample processed with commercially available SnI₂ used as received, exhibit extremely high radiative efficiency due to electronic doping which boosts the radiative band-to-band recombination. Contrarily, thin films where Sn⁴⁺ species are intentionally introduced show drastically reduced radiative lifetime and efficiency due to a dominance of Auger recombination at all excitation densities when the material is highly doped. The introduction of SnF₂ reduces the doping and passivates Sn⁴⁺ trap states but conversely introduces additional non-radiative decay channels in the bulk that fundamentally limit the radiative efficiency. Overall, we provide a qualitative model that takes into account different types of traps present in tin-perovskite thin films and show how doping and defects can affect the optoelectronic properties.

Growing MASnI3perovskite single-crystal films by inverse temperature crystallization

Perovskite single-crystal films are promising candidates for high-performance perovskite optoelectronic devices due to their optoelectrical properties. However, there are few reports of single-crystal films of tin based perovskites. Here, for the first time, we realize the controllable growth and preparation of lead-free tin perovskite MASnI₃single crystals via inverse temperature crystallization (ITC) strategy with γ-butyrolactone (GBL) as solvent. The solubility characteristics of MASnI₃in GBL are clarified by quantitative analytical method. Highly repeatability experiments are further demonstrated using this unique solubility and ITC properties. Sequentially, using space limiting method, tin perovskite MASnI₃single-crystal thin films are fabricated with micron-scale thickness, which is highly desired for efficient tin perovskite solar cells. Our MASnI₃single-crystal thin films show typical single-crystalline features including strongly optical absorbance with sharp absorption edges, pure-phase x-ray diffraction patterns, and absence of Sn(IV) x-ray photoelectron spectroscopy. We believe that our findings will further broaden the application prospects of tin perovskite MASnI₃single crystals and cause a new upsurge in exploring the field of lead-free perovskite single-crystal growth.

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.

FA/MA Cation Exchange for Efficient and Reproducible Tin-Based Perovskite Solar Cells

Nontoxic tin-based perovskite solar cells (Sn-PSCs) as a promising alternative to toxic Pb-PSCs have drawn great attention in recent years for their environmental friendliness and unique optoelectronic properties. However, both the efficiency and long-term stability of Sn-PSCs are considerably inferior to those of Pb-based ones. One of the main reasons is the difficulty in obtaining high-quality Sn-perovskite films due to the rapid crystallization of Sn-perovskites, which also results in poor device reproducibility. Here, we report a novel cation exchange strategy to prepare high-quality formamidinium tin triiodide (FASnI₃) perovskite films with a better controlled crystallization process and improved reproducibility, which allows easy access to smooth and pinhole-free perovskite films with oriented crystal growth, enlarged grain size, and reduced trap-state density. The corresponding Sn-PSCs show excellent photovoltaic performance with a champion efficiency of 9.11%, comparable to the best results reported for FASnI₃-PSCs, and the devices also demonstrate outstanding long-term stability without encapsulation. Our results offer a practical strategy for fabricating Sn-PSCs with superb performance and stability.

2D WSe2 Flakes for Synergistic Modulation of Grain Growth and Charge Transfer in Tin-Based Perovskite Solar Cells

Tin (Sn)-based perovskites with favorable optoelectronic properties and ideal bandgaps have emerged as promising alternatives to toxic lead (Pb)-based perovskites for photovoltaic applications. However, it is challenging to obtain high-quality Sn-based perovskite films by solution process. Here, liquid-exfoliated 2D transition-metal dichalcogenides (i.e., MoS2 , WS2 , and WSe2 ) with smooth and defect-free surfaces are applied as growth templates for spin-coated FASnI3 perovskite films, leading to van der Waals epitaxial growth of perovskite grains with a growth orientation along (100). The authors find that WSe2 has better energy alignment with FASnI3 than MoS2 and WS2 and results in a cascade band structure in resultant perovskite solar cells (PSCs), which can facilitate hole extraction and suppress interfacial charge recombination in the devices. The WSe2 -modified PSCs show a power conversion efficiency up to 10.47%, which is among the highest efficiency of FASnI3 -based PSCs. The appealing solution phase epitaxial growth of FASnI3 perovskite on 2D WSe2 flakes is expected to find broad applications in optoelectronic devices.

Progress towards High-Efficiency and Stable Tin-Based Perovskite Solar Cells

Since its invention in 2009, Perovskite solar cells (PSCs) has attracted great attention because of its low cost, numerous options of efficiency enhancement, ease of manufacturing and high-performance. Within a short span of time, the PSC has already outperformed thin-film and multicrystalline silicon solar cells. A current certified efficiency of 25.2% demonstrates that it has the potential to replace its forerunner generations. However, to commercialize PSCs, some problems need to be addressed. The toxic nature of lead which is the major component of light absorbing layer, and inherited stability issues of fabricated devices are the major hurdles in the industrialization of this technology. Therefore, new researching areas focus on the lead-free metal halide perovskites with analogous optical and photovoltaic performances. Tin being nontoxic and as one of group IV(A) elements, is considered as the most suitable alternate for lead because of their similarities in chemical properties. Efficiencies exceeding 13% have been recorded using Tin halide perovskite based devices. This review summarizes progress made so far in this field, mainly focusing on the stability and photovoltaic performances. Role of different cations and their composition on device performances and stability have been involved and discussed. With a considerable room for enhancement of both efficiency and device stability, different optimized strategies reported so far have also been presented. Finally, the future developing trends and prospects of the PSCs are analyzed and forecasted.

Precursor Engineering of Vapor-Exchange Processes for 20%-Efficient 1 cm2 Inverted-Structure Perovskite Solar Cells

Due to mass diffusion issues, it is challenging to prepare black-phase thick formamidinium-based perovskite (FAPbI₃) films via vapor approaches. Precursor engineering is employed here to overcome the dilemma of thorough reaction and black-phase stabilization of FAPbI₃ in a sequential vapor approach. For the first time, FAPbBr₃ was used as an additive in the precursor to promote the formation of FAPbI₃ perovskite. To balance off the increased crystallization degree of precursor films due to the addition of FAPbBr₃, CsI dissolved in dimethyl sulfoxide (DMSO) was further added. It is indicated that the simultaneous incorporation of FAPbBr₃ and CsI-DMSO successfully accelerated the formation rate of perovskite and inhibited the formation of FAPbI₃ yellow phase. The power conversion efficiency of the as-prepared devices of different areas (0.1125 or 1 cm²) reached 20%, the first report of large-area 20%-efficiency PSCs based on a vapor approach, highlighting its applicability to large-area manufacture in the future. Furthermore, when blade coating is used in preparing the precursor film, the efficiency reached 19%. When the precursor film was prepared by dip coating, we could prepare conformal FAPbI₃ coatings on carbon fibers, suggesting possible future applications in fabricating wearable PSCs.