Reducing Agents for Improving the Stability of Sn-based Perovskite Solar Cells

Fabrication and Composition of MA1-y FA y SnI3-x Br x Thin Films for Lead-Free Perovskite Solar Cells

Perovskite solar cells have gained significant attention in recent years due to their lightweight nature, flexibility, and ability to generate power even in weak-light conditions. Despite these advantages, the current mainstream perovskite solar cells contain lead, raising concerns about their environmental and human health effects. Tin is expected to be a substitutional element for lead; however, tin-based perovskite solar cells currently have low power conversion efficiency. Altering the composition of the perovskite is crucial for enhancing its performance. In this study, perovskite solar cells with mixed MA/FA and I/Br components were designed and fabricated based on the calculation of the tolerance factor. The crystallinity and band gap of perovskite thin films were manipulated by changing the compositions of anions and cations. A suitable composition ratio for perovskite solar cells was proposed and discussed.

Synergistic Modulation of Sn-Based Perovskite Solar Cells with Crystallization and Interface Engineering

A high-quality Sn-based perovskite absorption layer and effective carrier transport are the basis for high-performance Sn-based perovskite solar cells. The suppression of Sn2+ oxidation and rapid crystallization is the key to obtaining high-quality Sn-based perovskite film. And interface engineering is an effective strategy to enhance carrier extraction and transport. In this work, tin fluoride (SnF2) was introduced to the perovskite precursor solution, which can effectively modulate the crystallization and morphology of Sn-based perovskite layer. Furthermore, the hole-transporting layer of PEDOT:PSS was modified with CsI to enhance the hole extraction and transport. As a result, the fabricated inverted Sn-based perovskite solar cells demonstrated a power conversion efficiency of 7.53% with enhanced stability.

Mechanistic Understanding of Oxidation of Tin-based Perovskite Solar Cells and Mitigation Strategies

Tin (Sn)-based perovskites as the most promising absorber materials for lead-free perovskite solar cells (PSCs) have achieved the record efficiency of over 14 %. Although suppressing the oxidation of Sn-based perovskites is a frequently concerned topic for Sn-based PSCs, many studies have given vague explanations and the mechanisms are still under debate. This is in principal due to the lack of an in-depth understanding of various and complex intrinsic and extrinsic factors causing the oxidation process. In this context, we critically review the chemical mechanism of facile oxidation of Sn-based perovskites and differentiate its detrimental effects at material- and device-level. More importantly, we classify and introduce the intrinsic factors (raw materials and solvent of perovskite precursors) and extrinsic factors (exposure to neutral oxygen and superoxide) causing the oxidation with their corresponding anti-oxidation improvement methods. The presented comprehensive understanding and prospect of the oxidation provide insightful guidance for suppressing the oxidation in Sn-based PSCs "from the beginning to the end".

Stable Sn-Based Hybrid Perovskite-Related Structures with Tunable Color Coordinates via Organic Cations in Low-Temperature Synthesis

Organic-inorganic Pb-free layered perovskites are efficient broadband emitters and thus are promising materials for lighting applications. However, their synthetic protocols require a controlled atmosphere, high temperature, and long preparation time. This hinders the potential tunability of their emission through organic cations, as is instead common practice in Pb-based structures. Here, we present a set of Sn-Br layered perovskite-related structures that display different chromaticity coordinates and photoluminescence quantum yield (PLQY) up to 80%, depending on the choice of the organic monocation. We first develop a synthetic protocol that is performed under air and at 4 °C, requiring only a few steps. X-ray and 3D electron diffraction analyses show that the structures exhibit diverse octahedra connectivity (disconnected and face-sharing) and thus optical properties, while preserving the organic-inorganic layer intercalation. These results provide key insight into a previously underexplored strategy to tune the color coordinates of Pb-free layered perovskites through organic cations with complex molecular configurations.

Stirring-Time Control Approach to Manage Colloid Nucleation Size for the Fabrication of High-Performance Sn-Based Perovskite Solar Cells

Colloidal nucleation is first observed to exist in a Sn-based perovskite solution, and its components and size are discovered to alter in real time with stirring time. Then, a simple stirring-time control approach is developed to manage the size of the colloids, and with continuous stirring for 3 days an optimal colloid with size <10 nm is confirmed to successfully form a continuous, dense, high-quality perovskite film. Herein, the colloids exist in the form of [SnI₆]^4- complete coordination compounds, with which a compact FA_0.75MA_0.25SnI₃ perovskite film with reduced defects and enhanced crystallinity concentration is obtained as nucleation sites, and a planar inverted perovskite solar cell with a champion power conversion efficiency of 10.74% is eventually realized. This work provides a novel perspective to enhance perovskite film quality and thus device performance via controlling high-quality nucleation in precursor solution.

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.

Physically Detachable and Operationally Stable Cs2 SnI6 Photodetector Arrays Integrated with µ-LEDs for Broadband Flexible Optical Systems

With the surge in perovskite research, practical features for future applications are desired to be secured, but the reliability of the materials and the use of hazardous Pb are longstanding problems. Here, an air-stable Cs₂ SnI₆ (CSI) is prepared via diluted hydriodic acid solvent-based precursor optimization during scalable hydrothermal growth. Materials characterization is performed using various elemental peak analyses and crystallographic identification. The resulting CSI exhibits long-term operating stability over 6 months, i) at elevated temperatures, ii) in ambient air, and iii) under light illumination from UV to near-infrared. More importantly, to demonstrate an intriguing class of applications up to system level, physically detachable CSI photodetector arrays (PD-arrays), integrated with micro-light-emitting-diodes (μ-LEDs) arrays, are successfully fabricated. In addition, 3 × 3 flexible CSI PDs are fully operational, even in air, and their spatial uniformity in pixels is quantitatively evaluated. The charge-transport mechanisms of the CSI PDs under light and elevated temperature are assessed via temperature-dependent characterization from 148 to 373 K, implying the involvement of 3D variable-range hopping. Multicycle evaluation of the CSI PD-arrays confirms their operational stability in AC and DC modes, demonstrating this platform's potential benefit for wireless optical interconnection in advanced Si technology.

Instability Issues and Stabilization Strategies of Lead Halide Perovskites for Photo(electro)catalytic Solar Fuel Production

Photo(electro)catalysis is a promising route to utilizing solar energy to produce valuable chemical fuels. In recent years, lead halide perovskites (LHPs) as a class of high-performance semiconductor materials have been extensively used in photo(electro)catalytic solar fuel production because of their excellent photophysical properties. However, instability issues make it arduous for LHPs to achieve their full potential in photo(electro)catalysis. This Perspective discusses the instability issues and summarizes the stabilization strategies employed for prolonging the stability or durability of LHPs in photo(electro)catalytic solar fuel production. The strategies for particulate photocatalytic systems (including composition engineering, surface passivation, core-shell structures construction, and solvent selection) and for thin-film PEC systems (including physical protective coating, A site cation additive, and surface/interface passivation) are introduced. Finally, some challenges and opportunities regarding the development of stable and efficient LHPs for photo(electro)catalysis are proposed.

Challenges in tin perovskite solar cells

Perovskite solar cells are the rising star of third-generation photovoltaic technology. With a power conversion efficiency of 25.5%, the record efficiency is close to the theoretical maximum efficiency of a single-junction solar cell. However, lead toxicity threatens commercialization efforts and market accessibility. In this context, Sn-based perovskites are a safe alternative. Nevertheless, the efficiency of Sn-based devices falls far behind the efficiency of Pb-based counterparts. This concise review sheds light on the challenges that the field faces toward making Sn-based perovskites the perovskite photovoltaic benchmark. We identified four key challenges: materials and solvents, film formation, Sn(II) oxidation, and energy band alignment. We illustrate every single challenge and highlight the most successful attempts to overcome them. Finally, we provide our opinion on the most promising trends of this field in the future.

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.