Sn Perovskite Research Articles

Advancing Efficiency and Stability of Lead, Tin, and Lead/Tin Perovskite Solar Cells: Strategies and Perspectives

Halide‐perovskite‐based solar cells (HPSCs) have established themselves as a promising photovoltaic (PV) technology in a remarkably short time. The rapid improvement in HPSCs can be attributed to the unique material and optoelectronic properties of metal halide perovskite semiconductors coupled with a very knowledgeable and experienced PV community. This review briefly summarizes the chemistry of halide perovskites, delving into the fundamental aspects of crystal structure and optical bandgap, followed by a more in‐depth report on the advancements in HPSCs efficiencies, thanks to structural regulation, interfacial modulation, and thin‐film engineering. It is mainly focused on three metal halide perovskites topics: 1) high‐performance Pb‐based perovskites, 2) Sn‐based perovskites and their associated challenges, and 3) emerging work on mixed composition Pb–Sn perovskites. For each of these domains, the effects stemming from the tuning of the monovalent A‐site and the halide site are examined. Additionally, various approaches aimed at passivating defects in the bulk film and at the interface, along with carrier transport engineering, are discussed. The discussions also encompass the broader implications for device performance, stability, and material toxicity. Finally, perspectives on the future directions and the commercial feasibility of perovskite photovoltaic technologies are provided.

Tailoring Tensile Strain in Pb–Sn Perovskite Film for Efficient and Stable Narrow‐Bandgap Perovskite Solar Cells

Pb–Sn perovskites (PSPs) with narrow bandgap and low toxicity show great promise in next‐generation perovskite‐based photovoltaics, especially in constructing all‐perovskite tandem solar cells as the rare subcell. However, partially replacing Pb by Sn with smaller ionic size in PSPs is shown to reduce their crystal lattice symmetry, which leads to intrinsic compressed lattice strain and reduced structural stability, resulting in the poor device performance of PSPs‐based solar cells. Herein, the effect of lattice tensile strain on the optoelectronic properties of FA0.7MA0.3Pb0.5Sn0.5I3 film is investigated and the tensile strain is finely adjusted by introducing monovalent‐cation chlorides with different cation sizes as additive. It is found that a moderate tensile strain in the film is beneficial for stabilizing the crystal structure, promoting oriented crystal growth and reducing the defect density. After optimization, FA0.7MA0.3Pb0.5Sn0.5I3‐based perovskite solar cells using FACl as additive can produce the device with champion power conversion efficiency (PCE) of 19.30% together with improved device reproducibility. Importantly, the FACl‐based device shows robust stability that exhibits no PCE loss after being stored in N2 for 1000‐h. This work provides new insight into the key role of tensile strain in PSPs, which can facilitate the development of PSP‐based optoelectronic devices.

Tautomeric mixture coordination enables efficient lead-free perovskite LEDs.

Lead halide perovskite light-emitting diodes (PeLEDs) have demonstrated remarkable optoelectronic performance1-3. However, there arepotential toxicity issues with lead4,5 and removing lead from the best-performing PeLEDs-without compromising their high external quantum efficiencies-remains a challenge. Here we report a tautomeric-mixture-coordination-induced electron localization strategy to stabilize thelead-free tinperovskiteTEA2SnI4 (TEAI is2-thiopheneethylammonium iodide) by incorporating cyanuric acid. We demonstrate that a crucial function of the coordination is to amplify theelectronic effects, even for those Sn atoms that aren't strongly bonded with cyanuric acidowing to the formation of hydrogen-bonded tautomeric dimer and trimer superstructures on the perovskite surface. This electron localization weakens adverse effects from Anderson localization and improves ordering in the crystal structure of TEA2SnI4. These factors result in a two-orders-of-magnitude reduction in thenon-radiative recombination capture coefficient and an approximately twofold enhancement inthe exciton binding energy. Our lead-free PeLED has an external quantum efficiency of up to 20.29%, representing a performance comparable to that of state-of-the-art lead-containing PeLEDs6-12. We anticipate that these findings will provide insights into the stabilization of Sn(II) perovskites and further the development of lead-free perovskite applications.

Sn Perovskite Solar Cells with Tin Oxide Nanoparticle Layer as Hole Transport Layer

Sn Perovskite Solar Cells with Tin Oxide Nanoparticle Layer as Hole Transport Layer

Component Distribution Regulation in Sn-Pb Perovskite Solar Cells through Selective Molecular Interaction.

Tin-lead (Sn-Pb) perovskite solar cells (PSCs) with near-ideal bandgap still lag behind the pure lead PSCs. Disordered heterojunctions caused by inhomogeneous Sn/Pb ratio in the binary perovskite film induce large recombination loss. Here, an Sn-Pb perovskite film is reported with homogeneous component and energy distribution by introducing hydrazine sulfate (HS) in Sn perovskite precursor. HS can form hydrogen bond network and coordinate with FASnI3 thus no longer bond with Pb2+ , which reduces the crystallization rate of tin perovskite to the level of lead analog. The strong bonding between SO4 2- and Sn2+ can also suppress its oxidation. As a result, the Sn-Pb PSCs with HS exhibit a significantly improved VOC of 0.91 V along with a high efficiency of 23.17%. Meanwhile, the hydrogen bond interaction network, strong bonding between Sn2+ and sulfate ion also improve the thermal, storage, and air stability of resulting devices.

Theoretical Study of the Molecular Passivation Effect of Lewis Base/Acid on Lead-Free Tin Perovskite Surface Defects.

Extensive research has been recently conducted to improve the power conversion efficiency (PCE) of perovskite solar cells. However, the charge carriers are easily trapped by the defect sites located at the interface between the perovskite layer and the electrode, which decreases the PCE. To reduce such defect sites, the passivation technique is frequently employed to coat small molecules on the perovskite surface during the manufacturing process. To clarify the passivation mechanism from a molecular viewpoint, we performed density functional theory calculations to target Pb-free Sn perovskites (CH3NH3SnI3). We investigated the passivation effect of Lewis base/acid molecules, such as ethylene diamine (EDA) and iodopentafluorobenzene (IPFB), and discussed behaviors of the defect levels within the bandgap as they have strong negative impacts on the PCE. The adsorption of EDA/IPFB on the Sn perovskite surface can remove the defect levels from the bandgap. Furthermore, we discuss the importance of interactions with molecular orbitals.

Synergic Electron and Defect Compensation Minimizes Voltage Loss in Lead-Free Perovskite Solar Cells.

Sn perovskite solar cells have been regarded as one of the most promising alternatives to the Pb-based counterparts due to their low toxicity and excellent optoelectronic properties. However, the Sn perovskites are notorious to feature heavy p-doping characteristics and possess abundant vacancy defects, which resulted in under-optimized interfacial energy level alignment and severe nonradiative recombination. Here, we reported a synergic "electron and defect compensation" strategy to simultaneously modulate the electronic structures and defect profiles of Sn perovskites via incorporating a traced amount (0.1 mol%) of heterovalent metal halide salts. Consequently, the doping level of modified Sn perovskites was altered from heavy p-type to weak p-type (i.e. up-shifting the Fermi Level by ~ 0.12 eV) that determinately reducing the barrier of interfacial charge extraction and effectively suppressing the charge recombination loss throughout the bulk perovskite film and at relevant interfaces. Pioneeringly, the resultant device modified with electron and defect compensation realized a champion efficiency of 14.02%, which is ~46% higher than that of control device (9.56%). Notably, a record-high photovoltage of 1.013 V was attained, corresponding to the lowest voltage deficit of 0.38 eV reported to date, and narrowing the gap with Pb-based analogues (~ 0.30 V).

Synergic Electron and Defect Compensation Minimizes Voltage Loss in Lead‐Free Perovskite Solar Cells

AbstractSn perovskite solar cells have been regarded as one of the most promising alternatives to the Pb‐based counterparts due to their low toxicity and excellent optoelectronic properties. However, the Sn perovskites are notorious to feature heavy p‐doping characteristics and possess abundant vacancy defects, which result in under‐optimized interfacial energy level alignment and severe nonradiative recombination. Here, we reported a synergic “electron and defect compensation” strategy to simultaneously modulate the electronic structures and defect profiles of Sn perovskites via incorporating a traced amount (0.1 mol %) of heterovalent metal halide salts. Consequently, the doping level of modified Sn perovskites was altered from heavy p‐type to weak p‐type (i.e. up‐shifting the Fermi level by ~0.12 eV) that determinately reducing the barrier of interfacial charge extraction and effectively suppressing the charge recombination loss throughout the bulk perovskite film and at relevant interfaces. Pioneeringly, the resultant device modified with electron and defect compensation realized a champion efficiency of 14.02 %, which is ~46 % higher than that of control device (9.56 %). Notably, a record‐high photovoltage of 1.013 V was attained, corresponding to the lowest voltage deficit of 0.38 eV reported to date, and narrowing the gap with Pb‐based analogues (~0.30 V).

Alumina Nanoparticle Interfacial Buffer Layer for Low‐Bandgap Lead‐Tin Perovskite Solar Cells

AbstractMixed lead‐tin (Pb:Sn) halide perovskites are promising absorbers with narrow‐bandgaps (1.25–1.4 eV) suitable for high‐efficiency all‐perovskite tandem solar cells. However, solution processing of optimally thick Pb:Sn perovskite films is notoriously difficult in comparison with their neat‐Pb counterparts. This is partly due to the rapid crystallization of Sn‐based perovskites, resulting in films that have a high degree of roughness. Rougher films are harder to coat conformally with subsequent layers using solution‐based processing techniques leading to contact between the absorber and the top metal electrode in completed devices, resulting in a loss of VOC, fill factor, efficiency, and stability. Herein, this study employs a non‐continuous layer of alumina nanoparticles distributed on the surface of rough Pb:Sn perovskite films. Using this approach, the conformality of the subsequent electron‐transport layer, which is only tens of nanometres in thickness is improved. The overall maximum‐power‐point‐tracked efficiency improves by 65% and the steady‐state VOC improves by 28%. Application of the alumina nanoparticles as an interfacial buffer layer also results in highly reproducible Pb:Sn solar cell devices while simultaneously improving device stability at 65 °C under full spectrum simulated solar irradiance. Aged devices show a six‐fold improvement in stability over pristine Pb:Sn devices, increasing their lifetime to 120 h.

FA/Cs-based mixed Pb–Sn perovskite solar cells: A review of recent advances in stability and efficiency

Perovskite solar cells (PSCs) have gained widespread attention due to their impressive optoelectronic properties, extraordinarily high efficiency, and low-cost manufacturing processes. PSCs are poised to usher in a new era of clean energy production. However, the use of methylammonium (MA) as the A-site cation in PSCs may result in poor thermal and atmospheric stability, limiting their potential for widespread commercialization. On the other hand, the most extensively studied perovskite systems with respect to B-site cations are pure Pb-based compositions due to their high device performance demonstrated to date. However, mixed Pb–Sn compositions can offer optimal bandgap energies for single-junction solar cells, suggesting the promise of future efficiency improvements. Thus, this review focuses on mixed Pb–Sn perovskite solar cells that use formamidinium (FA) or mixed cations of FA and cesium (Cs) without MA. These materials have bandgap energies suitable for use in either single-junction or multi-junction solar cells and show improved stability compared to their MA-based counterparts. We discuss the structural and optoelectronic properties of FA/Cs-based Pb–Sn perovskites and the efforts to improve their solar cell performance and stability through additive engineering, process optimization, and structural modifications. Our review highlights the potential for FA/Cs-based mixed Pb–Sn perovskites as a more stable and efficient alternative for future commercial applications of PSCs.

Additive Engineering for Mixed Lead–Tin Narrow-Band-Gap Perovskite Solar Cells: Recent Advances and Perspectives

Low-cost perovskite solar cells (PSCs) with high power conversion efficiencies (PCEs) of >25% are considered as the most promising replacement for commercial silicon-based solar cells to realize a sustainable future. To break the theoretical PCE limits of single-junction PSCs, all-perovskite tandem solar cells consisting of a narrow-band-gap bottom subcell and a wide-band-gap top subcell have attracted particular attention recently. Mixed Pb–Sn perovskites with narrow band gaps have received great attention as an efficient light harvester in the bottom subcell of all-perovskite tandem solar cells as a result of the reduced toxicity, high light-absorbing capability, and matched current with the wide-band-gap top subcells. However, mixed Pb–Sn narrow-band-gap PSCs suffer from low PCEs, inferior stability, and high open-circuit voltage (Voc) loss, owing to the high defect amount and inferior perovskite film quality induced by the detrimental oxidation of Sn2+ cations and the rapid crystallization of perovskite crystals. Herein, the recent advances about the additive engineering for mixed Pb–Sn narrow-band-gap PSCs are reviewed by demonstrating the origins and unique features of Pb–Sn narrow-band-gap perovskites. Additionally, several strategies to improve PCEs and durability of Pb–Sn narrow-band-gap PSCs through additive engineering are proposed, including Sn2+ cation stabilization, heterojunction construction, crystallization control, surface/grain boundary passivation, film morphology control, carrier dynamics modulation, and gradient-distributed film formation. Furthermore, the existing challenges and future directions are also presented, aiming to provide important insights for designing and developing efficient and stable single-junction narrow-band-gap PSCs and all-perovskite tandem solar cells.

Origin of Enhanced Nonradiative Carrier Recombination Induced by Oxygen in Hybrid Sn Perovskite

Oxygen ingression has been shown to substantially decrease the carrier lifetime of Sn-based perovskites, behind which the mechanism remains yet unknown. Our first-principles calculations reveal that in prototypical MASnI3 (MA = CH3NH3), oxygen by itself is not a recombination center. Instead, it tends to form substitutional OI through combining with native I vacancies (VI) and remarkably increases the original recombination rate of VI by 2-3 orders of magnitude. This rationalizes the experimentally observed sharp decline of carrier lifetime in perovskites exposed to air. The significantly enhanced carrier recombination is due to a smaller electron capture barrier of OI, resulting from lattice strengthening and the suppressed structural relaxation upon electron capture. These insights offer a route to further improve device performance via anion engineering in broad Sn-based perovskite optoelectronics operating in ambient air. Moreover, our results highlight the important role of lattice relaxation for nonradiative carrier capture in materials in general.

Lead-free perovskite solar cells, what's next?

Although lead halide perovskite solar cells (PSCs) have achieved an efficiency of 25.7 %, the highly toxic composition, element lead, will hinder the use of PSCs in indoor photovoltaics, building-integrated photovoltaics and wearable devices. Therefore, it is imperative to search and study the low-toxicity lead-free perovskites as back-up materials. Recently, encouraging progress is made on lead-free PSCs in terms of Sn perovskites and double perovskites. In this comment, we will summarize the present status and give future perspectives on lead-free perovskite photovoltaics.

Pseudohalide-Modulated Crystallization for Efficient Quasi-2D Tin Perovskite Solar Cells with Minimized Voltage Deficit

Regulating the crystallization dynamics and suppressing the oxidation of Sn2+ is imperative for the emerging lead-free tin perovskite solar cells (TPSCs). In this respect, quasi-2D tin perovskites exhibit relatively slow crystallization and crystal growth rate compared to the three-dimensional analogues. However, these dimensional-mixed tin perovskites suffer from random orientation, which limits the charge carrier transport for photovoltaic applications. Herein, we report a facile and feasible strategy to modulate the dimensionality and crystallization of tin perovskites with the incorporation of phenethylammonium thiocyanate (PEASCN), which is exploited to deliver a high-quality crystal growth and preferred alignment. The pseudohalide SCN– can effectively hinder Sn2+ oxidation by synergistically modulating the coordination and crystal growth of Sn perovskite films. Meanwhile, these interactions significantly suppressed the nonradiative charge recombination. Finally, a high efficiency approaching 12.88% with an open-circuit voltage (Voc) of 863 mV is achieved for the inverted TPSCs. The obtained Voc is among the highest reported values for indene-C60 (ICBA)-free TPSC devices. The PEASCN devices also exhibited high stability for over 2000 h under a N2 atmosphere.

Bidirectional Anions Gathering Strategy Afford Efficient Mixed PbSn Perovskite Solar Cells.

Mixed lead-tin (PbSn) perovskite solar cells (PSCs) possess low toxicity and adjustable bandgap for both single-junction and all-perovskite tandem solar cells. However, the performance of mixed PbSn PSCs still lags behind the theoretical efficiency. The uncontrollable crystallization and the resulting structural defect are important reasons. Here, the bidirectional anions gathering strategy (BAG) is reported by using Methylammonium acetate (MAAc) and Methylammonium thiocyanate (MASCN) as perovskite bulk additives, which Ac- escapes from the perovskite film top surface while SCN- gathers at the perovskite film bottom in the crystallization process. After the optoelectronic techniques, the bidirectional anions movement caused by the top-down gradient crystallization is demonstrated. The layer-by-layer crystallization can collect anions in the next layer and gather at the broader, enabling a controllable crystallization process, thus getting a high-quality perovskite film with better phase crystallinity and lower defect concentration. As a result, PSCs treated by the BAG strategy exhibit outstanding photovoltaic and electroluminescent performance with a champion efficiency of 22.14%. Additionally, it demonstrates excellent long-term stability, which retains ≈92.8% of its initial efficiency after 4000 h aging test in the N2 glove box.

Efficient Environment‐friendly Lead‐free Tin Perovskite Solar Cells Enabled by Incorporating 4‐Fluorobenzylammonium Iodide Additives

Development of tin (Sn)‐based perovskite solar cells (PSCs) largely lags behind that of lead counterparts due to fast crystallization process of Sn perovskite and numerous defects in both bulk and surface of Sn perovskite films. Herein, this work reports a facile strategy of introducing 4‐fluorobenzylammonium iodide (FBZAI) as additives into Sn perovskite precursor to synergistically modulate the roles of benzylamine and fluorine in Sn‐based PSCs. Incorporation of FBZAI can increase crystallinity, passivate defects, and inhibit the oxidation of Sn2+, leading to suppression of nonradiative recombination and enhancement of charge transport and collection in devices. As a result, the best‐performing Sn‐based PSC with the FBZAI additive achieves the maximum PCE of 13.85% with the enhanced fill factor of 77.8% and open‐circuit voltage of 0.778 V. Our unencapsulated device exhibits good stability by maintaining 95% of its initial PCE after 160 days of storage.

Bottom-up modification boosts the performance of narrow-bandgap lead–tin perovskite single-junction and tandem solar cells

GlyCl passivates the defects of narrow-bandgap Pb–Sn perovskites from bottom to up, boosting the solar cell efficiency from 14.28% to 22.07%.

Resolving Oxidation States and X–site Composition of Sn Perovskites through Auger Parameter Analysis in XPS

AbstractReliable chemical state analysis of Sn semiconductors by XPS is hindered by the marginal observed binding energy shift in the Sn 3d region. For hybrid Sn‐based perovskites especially, errors associated with charge referencing can easily exceed chemistry‐related shifts. Studies based on the modified Auger parameter α ′ provide a suitable alternative and have been used previously to resolve different chemical states in Sn alloys and oxides. However, the meaningful interpretation of Auger parameter variations on Sn‐based perovskite semiconductors requires fundamental studies. In this work, a comprehensive Auger parameter study is performed through systematic compositional variations of Sn halide perovskites. It is found that in addition to the oxidation state, α ′ is highly sensitive to the composition of the halide site, inducing shifts of up to Δα ′ = 2 eV between ASnI3 and ASnBr3 type perovskites. The reported dependencies of α ′ on the Sn oxidation state, coordination and local chemistry provide a framework that enables reliable tracking of degradation as well as X‐site composition for Sn‐based perovskites and related compounds. The higher robustness and sensitivity of such studies not only enables more in‐depth surface analysis of Sn‐based perovskites than previously performed, but also increases reproducibility across laboratories.

Two-Dimensional Layered Simple Aliphatic Monoammonium Tin Perovskite Thin Films and Potential Applications in Field-Effect Transistors.

Two-dimensional (2D) layered organic-inorganic perovskites have great potential for fabricating field-effect transistors due to their unique structure that enables the horizontal transport of charge carriers in metal-halide octahedra, resembling the transport behavior in semiconducting channels. Their electronic band structures are mainly dominated by the metal-halide octahedra, which eventually determine the optical and electrical characteristics, whereas organic cations have no direct contributions but would impact the electronic structures via distorting the octahedra. So far, high performance has been achieved in 2D Sn perovskites compared to their Pb counterparts because the intrinsic differences of Sn promote transport properties. The champion hole mobility has been obtained in single-ring aromatic phenylethylammonium tin iodide perovskite [(PEA)2SnI4]. However, simple aliphatic monoammonium tin perovskites and their device applications have rarely been reported. Herein, 2D layered n-butylammonium tin iodide perovskite [(BA)2SnI4] thin films have been synthesized by a spin-coating approach. A structural phase transition occurs at about 225 K in the films, accompanied by the changes in the photoluminescence peak and exciton binding energy. Longitudinal optical (LO) phonons are found to govern the scattering of charge carriers and excitons via the Fröhlich interactions in the temperature range 77-300 K. The first-principles calculations predict that the perovskite has excellent transport characteristics comparable to those of molybdenum disulfide (MoS2) and methylammonium lead iodide perovskite (MAPbI3). The (BA)2SnI4 thin film field-effect transistors constructed on polymer dielectrics with a maximum hole mobility of 0.03 cm2 V-1 s-1 in ambient conditions have been successfully demonstrated for the first time. Our findings not only offer a deep insight into the physical properties of 2D layered aliphatic monoammonium tin perovskite thin films but also provide important experimental and theoretical guidance for their potential applications in lateral-type flexible optoelectronic devices.

Additive Engineering for High‐Performance Two‐Dimensional Dion–Jacobson Pb–Sn Alloyed Perovskite Solar Cells

Two‐dimensional perovskite materials attract great attentions for the application of perovskite solar cells (PSCs) due to the regulable bandgap and high stability. However, the power conversion efficiency (PCE) of 2D PSCs is far behind in that of three‐dimensional PSCs. Herein, Dion–Jacobson (DJ) phase Pb–Sn perovskite of (PhDMA)MA4(Pb0.5Sn0.5)5I16 in 2D PSCs is introduced, which has suitable bandgap of 1.27 eV and offers the candidate for the absorber of bottom device in all perovskite tandem solar cells. The nonlinear bandgap behavior of 2D Pb–Sn perovskite materials is certified by adjusting the recipe of precursors to vary the bandgap of perovskite films. Moreover, MACl is employed to improve the film quality of 2D Pb–Sn perovskite, leading to the suppression of the defect density and nonradiative recombination for 2D PSCs. As a result, a high PCE of 12.2% is achieved for DJ phase 2D Pb–Sn PSCs.