Large-Grain Tin-Rich Perovskite Films for Efficient Solar Cells via Metal Alloying Technique

Alleviating nanostructural phase impurities enhances the optoelectronic properties, device performance and stability of cesium-formamidinium metal-halide perovskites

The technique of alloying FA+ with Cs+ is often used to promote structural stabilization of the desirable α-FAPbI3 phase in halide perovskite devices. However, the precise mechanisms by which these alloying approaches improve the optoelectronic quality and enhance the stability have remained elusive. In this study, we advance that understanding by investigating the effect of cationic alloying in CsxFA1-xPbI3 perovskite thin-films and solar-cell devices. Selected-area electron diffraction patterns combined with microwave conductivity measurements reveal that fine Cs+ tuning (Cs0.15FA0.85PbI3) leads to a minimization of stacking faults and an increase in the photoconductivity of the perovskite films. Ultra-sensitive external quantum efficiency, kelvin-probe force microscopy and photoluminescence quantum yield measurements demonstrate similar Urbach energy values, comparable surface potential fluctuations and marginal impact on radiative emission yields, respectively, irrespective of Cs content. Despite this, these nanoscopic defects appear to have a detrimental impact on inter-grains'/domains' carrier transport, as evidenced by conductive-atomic force microscopy and corroborated by drastically reduced solar cell performance. Importantly, encapsulated Cs0.15FA0.85PbI3 devices show robust operational stability retaining 85% of the initial steady-state power conversion efficiency for 1400 hours under continuous 1 sun illumination at 35 °C, in open-circuit conditions. Our findings provide nuance to the famous defect tolerance of halide perovskites while providing solid evidence about the detrimental impact of these subtle structural imperfections on the long-term operational stability.

A comprehensive review on the advancements and challenges in perovskite solar cell technology

Perovskite solar cells (PSCs) have emerged as revolutionary technology in the field of photovoltaics, offering a promising avenue for efficient and cost-effective solar energy conversion. This review provides a comprehensive overview of the progress and developments in PSCs, beginning with an introduction to their fundamental properties and significance. Herein, we discuss the various types of PSCs, including lead-based, tin-based, mixed Sn-Pb, germanium-based, and polymer-based PSCs, highlighting their unique attributes and performance metrics. Special emphasis is given to halide double PSCs and their potential in enhancing the stability of PSCs. Charge transport layers and their significance in influencing the overall efficiency of solar cells are discussed in detail. The review also explores the role of tandem solar cells as a solution to overcome the limitations of single-junction solar cells, offering an integrated approach to harness a broader spectrum of sunlight. This review concludes with challenges associated with PSCs and perspective on the future potential of PSCs, emphasizing their role in shaping a sustainable energy landscape. Through this review readers will gain a comprehensive insight into the current state-of-the-art in PSC technology and the avenues for future research and development.

Ternary-Metal Sn-Pb-Zn Perovskite to Reconstruct Top Surface for Efficient and Stable Less-Pb Perovskite Solar Cells

Though Sn-Pb alloyed perovskite solar cells (PSCs) achieved great progress, there is a dilemma to further increase Sn for less-Pb requirement. High Sn ratio (>70%) perovskite exhibits nonstoichiometric Sn:Pb:I at film surface to aggravate Sn2+ oxidation and interface energy mismatch. Here, ternary metal alloyed (FASnI3 )0.7 (MAPb1- x Znx I3 )0.3 (x = 0-3%) is constructed for Pb% < 30% perovskite. Zn with smaller ionic size and stronger ionic interaction than Sn/Pb assists forming high-quality perovskite film with ZnI6 4- enriched at surface to balance Sn:Pb:I ratio. Differing from uniform bulk doping, surface-rich Zn with lower lying orbits pushes down the energy band of perovskite and adjusts the interface energy for efficient charge transfer. The alloyed PSC realizes efficiency of 19.4% at AM1.5 (one of the highest values reported for Pb% < 30% PSCs). Moreover, stronger bonding of Zn─I and Sn─I contributes to better durability of ternary perovskite than binary perovskite. This work highlights a novel alloy method for efficient and stable less-Pb PSCs.

High-Efficiency Low-Lead Perovskite Photovoltaics Approaching 20% Enabled by a Vacuum-Drying Strategy

Lead-tin mixed perovskites are excellent photovoltaic materials that can be used in single- or multi-junction perovskite solar cells (PSCs). However, most high-performance Pb-Sn mixed PSCs reported to date are still Pb-dominant. It is highly demanding to develop environmentally friendly low-lead PSCs, but the poor film quality caused by the uncontrollable crystallization kinetics has been hindering the efficiency improvement of low-lead PSCs. Here, a vacuum-drying strategy in the two-step method to fabricate low-lead PSCs (FAPb0.3 Sn0.7 I3 ) with an impressive efficiency of 19.67% is employed. The vacuum treatment induces the formation of low crystalline Pb0.3 Sn0.7 I2 films containing less solvent, thus facilitating the subsequent FAI penetration and suppressing pinholes. Compared with the conventional one-step method, the two-step fabricated low-lead perovskite films with the vacuum-drying treatment exhibit a larger grain size, lower trap density, and weaker recombination loss, thus giving rise to a record-high efficiency near 20% with better thermal stability.

Development of Perovskite (MACl)0.33FA0.99MA0.01Pb(I0.99Br0.01)3 Solar Cells via n-Octylammonium Iodide Surface Passivation

The influence of n-octylammonium iodide (OAI, passive layer) on the types of phases formed in a (MACl)_0.33FA_0.99MA_0.01Pb(I_0.99Br_0.01)₃ perovskite film was studied using X-ray diffraction. Using UV spectrophotometric techniques, it was determined how varied OAI additive layer ratios affected the linear and nonlinear optical characteristics of glass substrates/FTO/compact TiO₂/mesoporous TiO₂/(MACl)_0.33FA_0.99MA_0.01Pb(I_0.99Br_0.01)₃ films. All films' direct optical bandgap energies were determined to be 1.54 eV. The effects of OAI addition on the films' photoluminescence intensity and emitted colors were also investigated. For the fabricated perovskite solar cells (PSCs) without an OAI passivation layer, the corresponding power conversion efficiency (PCE), open-circuit voltage (V_OC), short-circuit current density (J_SC), and fill factor (FF) values were 18.8%, 1.02 V, 24.6 mAcm^-2, and 75%, respectively. When the concentration of OAI reached 2 mg, the maximum obtained values of PCE, V_OC, J_SC, and FF were 20.2%, 1.06 V, 24.2 mAcm^-2, and 79%, respectively. The decreased trap density and increased recombination resistance were responsible for the improvement in solar cell performance.

Synthetic approaches for perovskite thin films and single-crystals

Halide perovskites are compelling candidates for the next generation of photovoltaic technologies owing to an unprecedented increase in power conversion efficiency and their low cost, facile fabrication and outstanding semiconductor properties.

Defect Healing of MAPbI3 Perovskite Single Crystal Surface by Benzylamine

Controlling the surface traps in metal halide perovskites (MHPs) is essential for device performance, stability, and commercialization. Here, a facile approach is introduced to passivate the methylammonium lead iodide (MAPbI3) perovskite single crystal (PSC) surface defects by benzylamine (BA) ligand treatment, and the natural crystallographic (100) facets surface of PSC is chosen as the research platform to provide a deeper understanding of the passivation process. The confocal photoluminescence (PL) results show that the pristine three-dimensional (3D) MAPbI3 PSC surface with a symmetric emission spectrum is normally converted to a pure two-dimensional (2D) BA2PbI4, and also forms a quasi-2D Ruddlesden–Popper perovskite (RPP) BA2MAn−1PbnI3n+1 (n = 2, 3, 4, … ∞) after BA exchange with cation defects. The blue shift in the PL peak, as well as the extended exciton lifetimes of time-resolved photoluminescence (TRPL), indicate the realization of surface defect passivation. Additionally, changes in surface morphology are also investigated. The reaction starts with the formation of small, layered crystallites over the surface; as time elapses, the layered crystallites spread and merge in contact with each other, eventually resulting in smooth features. Our findings present a simple approach for MAPbI3 PSC surface defect passivation, which aims to advance MHP optimization processes toward practical perovskite device applications.

A Selective Targeting Anchor Strategy Affords Efficient and Stable Ideal-Bandgap Perovskite Solar Cells

Mixed lead-tin perovskite solar cells (LTPSCs) with an ideal bandgap are demonstrated as a promising candidate to reach higher power conversion efficiency (PCE) than their Pb-counterparts. Herein, a Br-free mixed lead-tin perovskite material, FA_0.8 MA_0.2 Pb_0.8 Sn_0.2 I₃ , with a bandgap of 1.33 eV, as a perovskite absorber, is selected. Through density functional theory calculations and optoelectronic techniques, it is demonstrated that both Pb- and Sn-related A-site vacancies are pushed into deeper energetic depth, causing severe nonradiative recombination. Hence, a selective targeting anchor strategy that employs phenethylammonium iodide and ethylenediamine diiodide as co-modifiers to selectively anchor with Pb- and Sn-related active sites and passivate bimetallic traps, respectively, is established. Furthermore, the selectivity of the molecular oriented anchor passivation is demonstrated through energetic depth specificity of Pb- and Sn-related traps. As a result, a substantially enhanced open-circuit voltage (V_OC ) from 0.79 to 0.90 V for the LTPSCs is achieved, yielding a champion PCE of 22.51%, which is the highest PCE among the reported ideal-bandgap PSCs. The V_OC loss is reduced to 0.43 V.

Urbach Energy and Open-Circuit Voltage Deficit for Mixed Anion-Cation Perovskite Solar Cells

The Urbach energy indicating the width of the exponentially decaying sub-bandgap absorption tail is commonly used as the indicator of electronic quality of thin-film materials used as absorbers in solar cells. Urbach energies of hybrid inorganic-organic metal halide perovskites with various anion-cation compositions are measured by photothermal deflection spectroscopy. The variation in anion-cation composition has a substantial effect on the measured Urbach energy and hence the electronic quality of the perovskite. Depending upon the compositions, the Urbach energy varies from 18 to 65 meV for perovskite films with similar bandgap energies. For most of the perovskite compositions studied here including methylammonium (MA) + formamidinium (FA)-based Pb iodides, mixed Sn + Pb narrow-bandgap perovskites with low or intermediate Sn contents, and wide-bandgap FA + Cs- and I + Br-based perovskites, the correlation between the Urbach energy of the perovskite thin film and open-circuit voltage (V_OC) deficit for corresponding solar cells shows a direct relationship with reduction of the Urbach energy occurring with a beneficial decrease in the V_OC deficit. However, due to issues related to material quality, impurity phases and stability in laboratory ambient air, and unoptimized film processing techniques, the solar cells incorporating Cs-based inorganic and mixed Sn + Pb perovskites with a higher than optimum Sn content show a higher V_OC deficit even though the corresponding films show a lower Urbach energy.

High-Performance Tin-Lead Mixed-Perovskite Solar Cells with Vertical Compositional Gradient

Sn-Pb mixed perovskites with bandgaps in the range of 1.1-1.4 eV are ideal candidates for single-junction solar cells to approach the Shockley-Queisser limit. However, the efficiency and stability of Sn-Pb mixed-perovskite solar cells (PSCs) still lag far behind those of Pb-based counterparts due to the easy oxidation of Sn²⁺ . Here, a reducing agent 4-hydrazinobenzoic acid is introduced as an additive along with SnF₂ to suppress the oxidation of Sn²⁺ . Meanwhile, a vertical Pb/Sn compositional gradient is formed spontaneously after an antisolvent treatment due to different solubility and crystallization kinetics of Sn- and Pb-based perovskites and it can be finely tuned by controlling the antisolvent temperature. Because the band structure of a perovskite is dependent on its composition, graded vertical heterojunctions are constructed in the perovskite films with a compositional gradient, which can enhance photocarrier separation and suppress carrier recombination in the resultant PSCs. Under optimal fabrication conditions, the Sn-Pb mixed PSCs show power conversion efficiency up to 22% along with excellent stability during light soaking.

Room temperature synthesis of Sn2+ doped highly luminescent CsPbBr3 quantum dots for high CRI white light-emitting diodes

With a high photoluminescence quantum yield (PLQY) being able to exceed 90% for those prepared by the hot injection method, CsPbBr3 quantum dots (QDs) have attracted intensive attention for white light-emitting diodes (WLEDs). However, the whole process is carried out in a 3-neck flask via air isolation and at a relatively high temperature. In addition, CsPbBr3 QDs suffer from poor stability under ambient atmosphere. In this work, an effective strategy through doping of Sn2+ ions at room-temperature is proposed to improve the emission efficiency and stability of CsPbBr3 QDs. Compared with pure CsPbBr3 QDs, a higher PLQY and a better stability are obtained. The detailed physical mechanism for this performance enhancement is discussed and described. An optimum result is found at an Sn2+ doping amount of 20%, which shows a high PLQY of 82.77%. WLEDs based on these 20% Sn2+ doped CsPbBr3 QDs are also studied, exhibiting a high color rendering index of 89 and a correlated color temperature (CCT) of 3954. The method proposed here provides an effective strategy to enhance the fluorescence and stability of CsPbBr3 QDs, which might have promising potential in the lighting fields.

Why Hybrid Tin-Based Perovskites Simultaneously Improve the Structural Stability and Charge Carriers' Lifetime: Ab Initio Quantum Dynamics

Much effort has been dedicated to boost the development of lead-free perovskite solar cells. However, their performance and stability are still much less competitive to the lead-bearing counterparts. By exploiting a mixed Sn-Ge cation strategy for the development of lead-free perovskites, we perform ab initio electronic structure calculations and quantum dynamics simulations on MASn_0.5Ge_0.5I₃ and compare them to MASnI₃. The calculations demonstrate that the hybrid cation strategy can improve simultaneously the perovskite stability and the lifetime of charge carriers. The stability increases due to a larger space of possible structures within the favorable range of the structural parameters, such as the Goldschmidt tolerance and octahedron factors. By exploring the larger structure space, mixed perovskites find stable configurations with lower free energies and better fitting components that exhibit reduced fluctuations around the equilibrium geometries. Charge carriers live longer in mixed perovskites because cation mixing results in an additional and moderate disorder that separates electrons and holes, reducing their interactions while still maintaining efficient band-like charge transport. These general and fundamental principles established by the analysis of the simulation results are useful for the design of advanced materials for solar energy and construction of optoelectronic devices.

Improved Doping and Optoelectronic Properties of Zn-Doped Cspbbr3 Perovskite through Mn Codoping Approach

Reducing the toxic Pb component in perovskites is an important step to realize environment-friendly perovskite optoelectronic devices. Herein, the structural, electronic, and optical properties of Zn and Mn codoped CsPbBr₃ have been investigated based on first-principle calculations and experimental verifications. Although the Zn dopant could reduce the optical band gap and exciton binding energy and enhance the optical absorption and defect tolerance for CsPbBr₃, the maximum reduction of the toxic Pb component was just about 12.5% in the experiment because the Zn dopant enlarges the formation energy of CsPb_1-xZn_xBr₃. For the stable CsPb_1-x-yMn_xZn_yBr₃ perovskite, the largest y and corresponding (x + y) could reach up to 25% and 83% respectively, since the Mn dopant could reduce the structural disorder. Especially when (x + y) < 50%, CsPb_1-x-yMn_xZn_yBr₃ exhibits a comparable carrier lifetime and exciton binding energy with a lower band gap to those of the CsPbBr₃, since the Zn dopant supplies a charge to CsPbBr₃ to counteract the variation of Pb-Br bonds induced by the Mn dopant. Meanwhile, the d orbitals of the dopant increase the optical absorption. These suggest that a 50% reduction of toxic Pb could be realized for stable CsPb_1-x-yMn_xZn_yBr₃ with negligibly deteriorated optoelectronic properties. This work provides an alternative approach to achieve a Pb-less perovskite with a high performance.

Efficient and Stable Perovskite Solar Cells Enabled by Dicarboxylic Acid-Supported Perovskite Crystallization

Defect states at surfaces and grain boundaries as well as poor anchoring of perovskite grains hinder the charge transport ability by acting as nonradiative recombination centers, thus resulting in undesirable phenomena such as low efficiency, poor stability, and hysteresis in perovskite solar cells (PSCs). Herein, a linear dicarboxylic acid-based passivation molecule, namely, glutaric acid (GA), is introduced by a facile antisolvent additive engineering (AAE) strategy to concurrently improve the efficiency and long-term stability of the ensuing PSCs. Thanks to the two-sided carboxyl (-COOH) groups, the strong interactions between GA and under-coordinated Pb²⁺ sites induce the crystal growth, improve the electronic properties, and minimize the charge recombination. Ultimately, champion-stabilized efficiency approaching 22% is achieved with negligible hysteresis for GA-assisted devices. In addition to the enhanced moisture stability of the devices, considerable operational stability is achieved after 2400 h of aging under continuous illumination at maximum power point (MPP) tracking.

Recent Progress in Growth of Single-Crystal Perovskites for Photovoltaic Applications

The growth of high-quality single-crystal (SC) perovskite films is a great strategy for the fabrication of defect-free perovskite solar cells (PSCs) with photovoltaic parameters close to the theoretical limit, which resulted in high efficiency and superior stability of the device. Plenty of growth methods for perovskite SCs are available to achieve a maximum power conversion efficiency (PCE) surpassing 21% for SC-based PSCs. However, there is still a lot of room to further push the efficiency by considering new crystal growth techniques, interface engineering, passivation approaches, and additive engineering. In this review, we summarize the recent progress in the growth of SC-based perovskite films for the fabrication of high-efficiency and stable PSCs. We describe the impact of SC growth of perovskite films and their quality on the device performance and stability, compared with the commonly used polycrystalline perovskite films. In the last section, the challenges and potential of SCs in PSCs are also covered for future development.

Photon management to reduce energy loss in perovskite solar cells

Despite the rapid development of perovskite solar cells (PSCs) over the past few years, the conversion of solar energy into electricity is not efficient enough or cost-competitive yet. The principal energy loss in the conversion of solar energy to electricity fundamentally originates from the non-absorption of low-energy photons ascribed to Shockley-Queisser limits and thermalization losses of high-energy photons. Enhancing the light-harvesting efficiency of the perovskite photoactive layer by developing efficient photo management strategies with functional materials and arrays remains a long-standing challenge. Here, we briefly review the historical research trials and future research trends to overcome the fundamental loss mechanisms in PSCs, including upconversion, downconversion, scattering, tandem/graded structures, texturing, anti-reflection, and luminescent solar concentrators. We will deeply emphasize the availability and analyze the importance of a fine device structure, fluorescence efficiency, material proportion, and integration position for performance improvement. The unique energy level structure arising from the 4fn inner shell configuration of the trivalent rare-earth ions gives multifarious options for efficient light-harvesting by upconversion and downconversion. Tandem or graded PSCs by combining a series of subcells with varying bandgaps seek to rectify the spectral mismatch. Plasmonic nanostructures function as a secondary light source to augment the light-trapping within the perovskite layer and carrier transporting layer, enabling enhanced carrier generation. Texturing the interior using controllable micro/nanoarrays can realize light-matter interactions. Anti-reflective coatings on the top glass cover of the PSCs bring about better transmission and glare reduction. Photon concentration through perovskite-based luminescent solar concentrators offers a path to increase efficiency at reduced cost and plays a role in building-integrated photovoltaics. Distinct from other published reviews, we here systematically and hierarchically present all of the photon management strategies in PSCs by presenting the theoretical possibilities and summarizing the experimental results, expecting to inspire future research in the field of photovoltaics, phototransistors, photoelectrochemical sensors, photocatalysis, and especially light-emitting diodes. We further assess the overall possibilities of the strategies based on ultimate efficiency prospects, material requirements, and developmental outlook.

Choose Your Own Adventure: Fabrication of Monolithic All-Perovskite Tandem Photovoltaics

Metal halide perovskites (MHPs) have transfixed the photovoltaic (PV) community due to their outstanding and tunable optoelectronic properties coupled to demonstrations of high-power conversion efficiencies (PCE) at a range of bandgaps. This has motivated the field to push perovskites to reach the highest possible performance. One way to increase the efficiency is by fabricating multijunction solar cells, which can split the solar spectrum, reducing thermalization loss. Low-cost all-perovskite tandems have a real chance to soon exceed 30% PCE, which could transform the PV industry. Achieving this goal requires the identification of perovskite sub-cells that are both highly efficient and can be effectively integrated. Herein, it is discussed how to navigate the multiple-choice adventure in choosing between the myriad of options and considerations present when deciding what perovskite materials, contact layers, and processing tools to use. Some of the potential fabrication pitfalls often encountered in MHP based tandem PVs are highlighted, so that they can hopefully be avoided in the future.

Tin and Mixed Lead-Tin Halide Perovskite Solar Cells: Progress and their Application in Tandem Solar Cells

Metal halide perovskites have recently attracted enormous attention for photovoltaic applications due to their superior optical and electrical properties. Lead (Pb) halide perovskites stand out among this material series, with a power conversion efficiency (PCE) over 25%. According to the Shockley-Queisser (SQ) limit, lead halide perovskites typically exhibit bandgaps that are not within the optimal range for single-junction solar cells. Partial or complete replacement of lead with tin (Sn) is gaining increasing research interest, due to the promise of further narrowing the bandgaps. This enables ideal solar utilization for single-junction solar cells as well as the construction of all-perovskite tandem solar cells. In addition, the usage of Sn provides a path to the fabrication of lead-free or Pb-reduced perovskite solar cells (PSCs). Recent progress in addressing the challenges of fabricating efficient Sn halide and mixed lead-tin (Pb-Sn) halide PSCs is summarized herein. Mixed Pb-Sn halide perovskites hold promise not only for higher efficiency and more stable single-junction solar cells but also for efficient all-perovskite monolithic tandem solar cells.

Progress in Materials Development for the Rapid Efficiency Advancement of Perovskite Solar Cells

The efficiency of perovskite solar cells (PSCs) has undergone rapid advancement due to great progress in materials development over the past decade and is under extensive study. Despite the significant challenges (e.g., recombination and hysteresis), both the single-junction and tandem cells have gradually approached the theoretical efficiency limit. Herein, an overview is given of how passivation and crystallization reduce recombination and thus improve the device performance; how the materials of dominant layers (hole transporting layer (HTL), electron transporting layer (ETL), and absorber layer) affect the quality and optoelectronic properties of single-junction PSCs; and how the materials development contributes to rapid efficiency enhancement of perovskite/Si tandem devices with monolithic and mechanically stacked configurations. The interface optimization, novel materials development, mixture strategy, and bandgap tuning are reviewed and analyzed. This is a review of the major factors determining efficiency, and how further improvements can be made on the performance of PSCs.

From Metallic Lead Films to Perovskite Solar Cells through Lead Conversion with Polyhalide Solutions

Solutions of methylammonium and formamidinium polyhalides (AX_1+n, A = MA, FA, X = I, Br) in isopropanol are introduced as novel versatile precursors for the fabrication of APbX₃ hybrid perovskite thin films via oxidation of metallic Pb. The polyhalide solution with adjustable reactivity is distributed over a metallic Pb layer followed by iodine vapor postprocessing to tune the morphology and composition of the film using only the elements inherently present in the perovskite. This method is easily reproducible in any materials science laboratory with equipment commonly used for perovskite solar cell fabrication and resulted in power conversion efficiencies of 16.2 and 17.2% for planar solar cells using MAPbI₃ and MA_0.25FA_0.75PbI_2.75Br_0.25 perovskites, respectively, as a proof of concept. Implementation of metallic lead thin films as the single Pb-containing precursors reduces a number of in-lab handling hazards compared to classical PbI₂ powder and solutions and provides a variety of scalable deposition options.

Superior Stability and Emission Quantum Yield (23% ± 3%) of Single-Layer 2D Tin Perovskite TEA2 SnI4 via Thiocyanate Passivation

Tin-based perovskite, which exhibits narrower bandgap and comparable photophysical properties to its lead analogs, is one of the most forward-looking lead-free semiconductor materials. However, the poor oxidative stability of tin perovskite hinders the development toward practical application. In this work, the effect of pseudohalide anions on the stability and emission properties of single-layer 2D tin perovskite nanoplates with chemical formula TEA₂ SnI₄ (TEA = 2-thiophene-ethylammonium) is reported. The results reveal that ammonium thiocyanate (NH₄ SCN) is the most effective additive in enhancing the stability and photoluminescence quantum yield of 2D TEA₂ SnI₄ (23 ± 3%). X-Ray photoelectron spectroscopic investigations on the thiocyanate passivated TEA₂ SnI₄ nanoplate show less than a 1% increase of Sn⁴⁺ signal upon 30 min exposure to air under ambient conditions (298 K, humidity ≈70%). Furthermore, no noticeable decrease in emission intensity of the nanoplate is observed after 20 h in air. The SCN^- passivation during the growth stage of TEA₂ SnI₄ is proposed to play a crucial role in preventing the oxidation of Sn²⁺ and hence boosts both stability and photoluminescence yield of tin perovskite nanoplates.

Highly Efficient and Stable GABr-Modified Ideal-Bandgap (1.35 eV) Sn/Pb Perovskite Solar Cells Achieve 20.63% Efficiency with a Record Small Voc Deficit of 0.33 V

1.5-1.6 eV bandgap Pb-based perovskite solar cells (PSCs) with 30-31% theoretical efficiency limit by the Shockley-Queisser model achieve 21-24% power conversion efficiencies (PCEs). However, the best PCEs of reported ideal-bandgap (1.3-1.4 eV) Sn-Pb PSCs with a higher 33% theoretical efficiency limit are <18%, mainly because of their large open-circuit voltage (V_oc ) deficits (>0.4 V). Herein, it is found that the addition of guanidinium bromide (GABr) can significantly improve the structural and photoelectric characteristics of ideal-bandgap (≈1.34 eV) Sn-Pb perovskite films. GABr introduced in the perovskite films can efficiently reduce the high defect density caused by Sn²⁺ oxidation in the perovskite, which is favorable for facilitating hole transport, decreasing charge-carrier recombination, and reducing the V_oc deficit. Therefore, the best PCE of 20.63% with a certificated efficiency of 19.8% is achieved in 1.35 eV PSCs, along with a record small V_oc deficit of 0.33 V, which is the highest PCE among all values reported to date for ideal-bandgap Sn-Pb PSCs. Moreover, the GABr-modified PSCs exhibit significantly improved environmental and thermal stability. This work represents a noteworthy step toward the fabrication of efficient and stable ideal-bandgap PSCs.

An across-species comparison of the sensitivity of different organisms to Pb-based perovskites used in solar cells

Organic-inorganic perovskite solar cells (PSCs) are promising candidates as photovoltaic cells. Recently, they have attracted significant attention due to certified power conversion efficiencies exceeding 23%, low-cost engineering, and superior electrical/optical characteristics. These PSCs extensively utilize a perovskite-structured composite with a hybrid of Pb-based nanomaterials. Operation of them may cause the release of Pb-based nanoparticles. However, limited information is available regarding the potential toxicity of Pb-based PSCs on various organisms. This study conducted a battery of in vitro and in vivo toxicity bioassays for three quintessential Pb-based PSCs (CH₃NH₃PbI₃, NHCHNH₃PbBr₃, and CH₃NH₃PbBr₃) using progressively more complex forms of life. For all species tested, the three different perovskites had comparable toxicities. The viability of Caco-2/TC7 cells was lower than that of A549 cells in response to Pb-based PSC exposure. Concentration-dependent toxicity was observed for the bioluminescent bacterium Vibrio fischeri, for soil bacterial communities, and for the nematode Caenorhabditis elegans. Neither of the tested Pb-based PSCs particles had apparent toxicity to Pseudomonas putida. Among all tested organisms, V. fischeri showed the highest sensitivity with EC₅₀ values (30 min of exposure) ranging from 1.45 to 2.91 mg L^-1. Therefore, this study recommends that V. fischeri should be preferably utilized to assess. PSC toxicity due to its increased sensitivity, low costs, and relatively high throughput in a 96-well format, compared with the other tested organisms. These results highlight that the developed assay can easily predict the toxic potency of PSCs. Consequently, this approach has the potential to promote the implementation of the 3Rs (Replacement, Reduction, and Refinement) principle in toxicology and decrease the dependence on animal testing when determining the safety of novel PSCs.

Effects of intrinsic and atmospherically induced defects in narrow bandgap (FASnI3)x(MAPbI3)1-x perovskite films and solar cells

Narrow bandgap mixed tin (Sn) + lead (Pb) perovskites are necessary for the bottom sub-cell absorber in high efficiency all-perovskite polycrystalline tandem solar cells. We report on the impact of mixed cation composition and atmospheric exposure of perovskite films on sub-gap absorption in films and performance of solar cells based on narrow bandgap mixed formamidinium (FA) + methylammonium (MA) and Sn + Pb halide perovskites, (FASnI₃)_x(MAPbI₃)_1-x. Structural and optical properties of 0.3 ≤ x ≤ 0.8 (FASnI₃)_x(MAPbI₃)_1-x perovskite thin film absorbers with bandgaps ranging from 1.25 eV (x = 0.6) to 1.34 eV (x = 0.3) are probed with and without atmospheric exposure. Urbach energy, which quantifies the amount of sub-gap absorption, is tracked for pristine perovskite films as a function of composition, with x = 0.6 and 0.3 demonstrating the lowest and highest Urbach energies of 23 meV and 36 meV, respectively. Films with x = 0.5 and 0.6 compositions show less degradation upon atmospheric exposure than higher or lower Sn-content films having greater sub-gap absorption. The corresponding solar cells based on the x = 0.6 absorber show the highest device performance. Despite having a low Urbach energy, higher Sn-content solar cells show reduced device performances as the amount of degradation via oxidation is the most substantial.

Core/Shell Nanocrystal Tailored Carrier Dynamics in Hysteresisless Perovskite Solar Cells with ∼20% Efficiency and Long Operational Stability

The ambient stability, hysteresis, and trap states in organo-halide perovskite solar cells (PSCs) are correlated to the influence of interlayer interfaces and grain boundaries. Astute incorporation of Cu₂ZnSnS₄ (CZTS) and Au/CZTS core/shell nanocrystals (NCs) can realize the goal of simultaneously achieving better performance and ambient stability of the PSCs. With optimized Au/CZTS NC size and concentration in the photoactive layer, power conversion efficiency can be increased up to 19.97 ± 0.6% with ambient air stability >800 h, as compared to 14.46 ± 1.02% for the unmodified devices. Through efficient carrier generation by CZTS and perovskite, accompanied by the plasmonic effect of Au, carrier density is sufficiently increased as validated by transient absorption spectroscopy. NCs facilitate the interfacial charge transfer by suitable band alignment and removal of recombination centers such as metallic Pb⁰, surface defects, or impurity sites. NC embedding also increases the perovskite grain size and assists in pinhole filling, reducing the trap state density.

All-inorganic quantum dot assisted enhanced charge extraction across the interfaces of bulk organo-halide perovskites for efficient and stable pin-hole free perovskite solar cells

In spite of achieving high power conversion efficiency (PCE), organo-halide perovskites suffer from long term stability issues. Especially the grain boundaries of polycrystalline perovskite films are considered as giant trapping sites for photo-generated carriers and therefore play an important role in charge transportation dynamics. Surface engineering via grain boundary modification is the most promising way to resolve this issue. A unique antisolvent-cum-quantum dot (QD) assisted grain boundary modification approach has been employed for creating monolithically grained, pin-hole free perovskite films, wherein the choice of all-inorganic CsPbBr x I3-x (x = 1-2) QDs is significant. The grain boundary filling by QDs facilitates the formation of compact films with 1-2 μm perovskite grains as compared to 300-500 nm grains in the unmodified films. The solar cells fabricated by CsPbBr1.5I1.5 QD modification yield a PCE of ∼16.5% as compared to ∼13% for the unmodified devices. X-ray photoelectron spectral analyses reveal that the sharing of electrons between the PbI6 - framework in the bulk perovskite and Br- ions in CsPbBr1.5I1.5 QDs facilitates the charge transfer process while femtosecond transient absorption spectroscopy (fs-TAS) suggests quicker trap filling and enhanced charge carrier recombination lifetime. Considerable ambient stability up to ∼720 h with <20% PCE degradation firmly establishes the strategic QD modification of bulk perovskite films.

Causes and Solutions of Recombination in Perovskite Solar Cells

Organic-inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley-Queisser limit efficiency (30.5%). Obviously, trap-assisted nonradiative (also called Shockley-Read-Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post-treatment are explored. The focus herein is on the recombination at perovskite/electron-transporting material and perovskite/hole-transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap-assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

A Review of Perovskite Photovoltaic Materials' Synthesis and Applications via Chemical Vapor Deposition Method

Perovskite photovoltaic materials (PPMs) have emerged as one of superstar object for applications in photovoltaics due to their excellent properties-such as band-gap tunability, high carrier mobility, high optical gain, astrong nonlinear response-as well as simplicity of their integration with other types of optical and electronic structures. Meanwhile, PPMS and their constructed devices still present many challenges, such as stability, repeatability, and large area fabrication methods and so on. The key issue is: how can PPMs be prepared using an effective way which most of the readers care about. Chemical vapor deposition (CVD) technology with high efficiency, controllability, and repeatability has been regarded as a cost-effective road for fabricating high quality perovskites. This paper provides an overview of the recent progress in the synthesis and application of various PPMs via the CVD method. We mainly summarize the influence of different CVD technologies and important experimental parameters (temperature, pressure, growth environment, etc.) on the stabilization, structural design, and performance optimization of PPMS and devices. Furthermore, current challenges in the synthesis and application of PPMS using the CVD method are highlighted with suggested areas for future research.

Synergistic interface and compositional engineering of inverted perovskite solar cells enables highly efficient and stable photovoltaic devices

Recently, interface and compositional engineering have been effective approaches in the field of perovskite solar cells (PSCs). In this work, the surface of the nickel oxide layer and perovskite composition are modified by poly[(9,9-bis(30-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] dibromide (PFN-P2) and deionized (DI) water, respectively, which result in a device with 20.5% efficiency and great stability.

Room-Temperature Sputtered SnO2 as Robust Electron Transport Layer for Air-Stable and Efficient Perovskite Solar Cells on Rigid and Flexible Substrates

Extraordinary photovoltaic performance and intriguing optoelectronic properties of perovskite solar cells (PSCs) have aroused enormous interest from both academic research and photovoltaic (PV) industry. In order to bring PSC technology from laboratory to market, material stability, device flexibility, and scalability are important issues to address for vast production. Nevertheless, PSCs are still primarily prepared by solution methods which limit film scalability, while high-temperature processing of metal oxide electron transport layer (ETL) makes PSCs costly and incompatible with flexible substrates. Here, we demonstrate rarely-reported room-temperature radio frequency (RF) sputtered SnO₂ as a promising ETL with suitable band structure, high transmittance, and excellent stability to replace its solution-processed counterpart. Power conversion efficiencies (PCEs) of 12.82% and 5.88% have been achieved on rigid glass substrate and flexible PEN substrate respectively. The former device retained 93% of its initial PCE after 192-hour exposure in dry air while the latter device maintained over 90% of its initial PCE after 100 consecutive bending cycles. The result is a solid stepping stone toward future PSC all-vapor-deposition fabrication which is being widely used in the PV industry now.

The Role of Chloride Incorporation in Lead-Free 2D Perovskite (BA)2SnI4: Morphology, Photoluminescence, Phase Transition, and Charge Transport

The incorporation of chloride (Cl) into methylammonium lead iodide (MAPbI3) perovskites has attracted much attention because of the significantly improved performance of the MAPbI3-based optoelectronic devices with a negligible small amount of Cl incorporation. It is expected that the Cl incorporation in 2D perovskites with layered nature would be much more efficient and thus can greatly alter the morphology, optical properties, phase transition, and charge transport; however, studies on those aspects in 2D perovskites remain elusive up to date. Here, a one-pot solution method to synthesize the Cl-doped lead-free 2D perovskite (BA)2SnI4 with various Cl incorporation concentrations is reported and how the Cl incorporation affects the morphology change, photoluminescence, phase transition, and charge transport is investigated. The Cl element is successfully incorporated into the crystal lattice in the solution-processed perovskite materials, confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy measurements. The temperature-dependent photoluminescence studies indicate that the emission properties and phase transition behavior in (BA)2SnI4- x Cl x can be tuned by varying the Cl incorporation concentration. Electrical measurement suggests that the charge transport behavior can also be greatly altered by the Cl doping concentration and the electrical conductivity can be significantly improved under a higher Cl incorporation concentration.

Management of Crystallization Kinetics for Efficient and Stable Low-Dimensional Ruddlesden-Popper (LDRP) Lead-Free Perovskite Solar Cells

Low-dimensional Ruddlesden-Popper (LDRP) lead-free perovskite has great potential due to its improved stability and oriented crystal growth, which is mainly attributed to the effective control of crystallization kinetics. However, the crystallization kinetics of LDRP lead-free perovskite films are highly limited by Lewis theory. Here, the management of the crystallization kinetics of LDRP tin (Sn) perovskite films jointly controlled by Lewis adducts and the ion exchange process using a mixture of polar aprotic solvent dimethyl sulfoxide (DMSO) and ion liquid solvent methylammonium acetate (MAAc) (the process named as "L-I") is demonstrated. Homogeneous nucleated LDRP Sn perovskite films with average grain size close to 9 µm are achieved. Both low electron and hole defect density with a magnitude of 1016, high carrier mobility, and excellent electrical performance are obtained. As a result, the LDRP Sn perovskite solar cell (PSC) with power conversion efficiency (PCE) of 4.03% is achieved using a simple one-step method without antisolvents, which is one of the best LDRP Sn PSCs. Most importantly, the PSC exhibits excellent stability with no degradation in PCE after 94 d in a nitrogen atmosphere owing to the high-quality film and the inhibition of the oxidation of Sn2+.