Oxygen Vacancy Mediation in SnO2 Electron Transport Layers Enables Efficient, Stable, and Scalable Perovskite Solar Cells.

Previous findings have suggested a close association between oxygen vacancies in SnO2 and charge carrier recombination as well as perovskite decomposition at the perovskite/SnO2 interface. Underlying the fundamental mechanism holds great significance in achieving a more favorable balance between the efficiency and stability. In this study, we prepared three SnO2 samples with different oxygen vacancy concentrations and observed that a low oxygen vacancy concentration is conducive to long-term device stability. Iodide ions were observed to easily diffuse into regions with high oxygen vacancies, thereby speeding up the deprotonation of FAI, as made evident by the detection of the decomposition product formamide. In contrast, a high oxygen vacancy concentration in SnO2 could prevent hole injection, leading to a decrease in interfacial recombination losses. To suppress this decomposition reaction and address the trade-off, we designed a bilayer SnO2 structure to ensure highly efficient carrier transport still while maintaining a chemically inert surface. As a result, an enhanced efficiency of 25.06% (certified at 24.55% with an active area of 0.09 cm2 under fast scan) was achieved, and the extended operational stability maintained 90% of their original efficiency (24.52%) after continuous operation for nearly 2000 h. Additionally, perovskite submodules with an active area of 14 cm2 were successfully assembled with a PCE of up to 22.96% (20.09% with an aperture area).

Regulating Hetero-Nucleation Enabling Over 14% Efficient Kesterite Solar Cells.

Developing well-crystallized light-absorbing layers remains a formidable challenge in the progression of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. A critical aspect of optimizing CZTSSe lies in accurately governing the high-temperature selenization reaction. This process is intricate and demanding, with underlying mechanisms requiring further comprehension. This study introduces a precursor microstructure-guided hetero-nucleation regulation strategy for high-quality CZTSSe absorbers and well-performing solar cells. The alcoholysis of 2-methoxyethanol (MOE) and the generation of high gas-producing micelles by adding hydrogen chloride (HCl) as a proton additive into the precursor solution are successfully suppressed. This tailored modification of solution components reduces the emission of volatiles during baking, yielding a compact and dense precursor microstructure. The reduced-roughness surface nurtures the formation of larger CZTSSe nuclei, accelerating the ensuing Ostwald ripening process. Ultimately, CZTSSe absorbers with enhanced crystallinity and diminished defects are fabricated, attaining an impressive 14.01% active-area power conversion efficiency. The findings elucidate the influence of precursor microstructure on the selenization reaction process, paving a route for fabricating high-quality kesterite CZTSSe films and high-efficiency solar cells.

Chemical Reaction of FA Cations Enables Efficient and Stable Perovskite Solar Cells.

Organometal halide perovskite solar cells (PSCs) have received great attention owing to a rapid increase in power conversion efficiency (PCE) over the last decade. However, the deficit of long-term stability is a major obstacle to the implementation of PSCs in commercialization. The defects in perovskite films are considered as one of the primary causes. To address this issue, isocyanic acid (HNCO) is introduced as an additive into the perovskite film, in which the added molecules form covalent bonds with FA cations via a chemical reaction. This chemical reaction gives rise to an efficient passivation on the perovskite film, resulting in an improved film quality, a suppressed non-radiation recombination, a facilitated carrier transport, and optimization of energy band levels. As a result, the HNCO-based PSCs achieve a high PCE of 24.41% with excellent storage stability both in an inert atmosphere and in air. Different from conventional passivation methods based on coordination effects, this work presents an alternative chemical reaction for defect passivation, which opens an avenue toward defect-mitigated PSCs showing enhanced performance and stability.

Tailored Supramolecular Interactions in Host-Guest Complexation for Efficient and Stable Perovskite Solar Cells and Modules.

Host-guest complexation offers a promising approach for mitigating surface defects in perovskite solar cells (PSCs). Crown ethers are the most widely used macrocyclic hosts for complexing perovskite surfaces, yet their supramolecular interactions and functional implications require further understanding. Here we show that the dipole moment of crown ethers serves as an indicator of supramolecular interactions with both perovskites and precursor salts. A larger dipole moment, achieved through the substitution of heteroatoms, correlates with enhanced coordination with lead cations. Perovskite films incorporating aza-crown ethers as additives exhibited improved morphology, reduced defect densities, and better energy-level alignment compared to those using native crown ethers. We report power-conversion efficiencies (PCEs) exceeding 25% for PSCs, which show enhanced long-term stability, and a record PCE of 21.5% for host-guest complexation-based perovskite solar modules with an active area of 14.0 cm2.

Polymerization Strategies to Construct a 3D Polymer Passivation Network toward High Performance Perovskite Solar Cells.

The spontaneously formed uncoordinated Pb2+ defects usually make the perovskite films demonstrate strong n-type with relatively lower carrier diffusion length and serious non-radiative recombination energy loss. In this work, we adopt different polymerization strategies to construct three-dimensional passivation frameworks in the perovskite layer. Thanks to the strong C≡N⋅⋅⋅Pb coordination bonding and the penetrating passivation structure, the defect state density is obviously reduced, accompanied by a significant increase in the carrier diffusion length. Additionally, the reduction of iodine vacancies also changed the Fermi level of the perovskite layer from strong n-type to weak n-type, which substantially promotes the energy level alignment and carrier injection efficiency. As a result, the optimized device achieved an efficiency exceeded 24 % (the certified efficiency is 24.16 %) with a high open-circuit voltage of 1.194 V, and the corresponding module achieved an efficiency of 21.55 %.

Thermally Crosslinked F-rich Polymer to Inhibit Lead Leakage for Sustainable Perovskite Solar Cells and Modules.

High-performance perovskite solar cells have demonstrated commercial viability, but still face the risk of contamination from lead leakage and long-term stability problems caused by defects. Here, an organic small molecule (octafluoro-1,6-hexanediol diacrylate) is introduced into the perovskite film to form a polymer through in situ thermal crosslinking, of which the carbonyl group anchors the uncoordinated Pb2+ of perovskite and reduces the leakage of lead, along with the -CF2 - hydrophobic group protecting the Pb2+ from water invasion. Additionally, the polymer passivates varieties of Pb-related and I-related defects through coordination and hydrogen bonding interactions, regulating the crystallization of perovskite film with reduced trap density, releasing lattice strain, and promoting carrier transport and extraction. The optimal efficiencies of polymer-incorporated devices are 24.76 % (0.09 cm2 ) and 20.66 % (14 cm2 ). More importantly, the storage stability, thermal stability, and operational stability have been significantly improved.

Molecular Dipole Engineering of Carbonyl Additives for Efficient and Stable Perovskite Solar Cells.

Carbonyl functional materials as additives are extensively applied to reduce the defects density of the perovskite film. However, there is still a lack of comprehensive understanding for the effect of carbonyl additives to improve device performance. In this work, we systematically study the effect of carbonyl additive molecules on the passivation of defects in perovskite films. After a comprehensive investigation, the results confirm the importance of molecular dipole in amplifying the passivation effect of additive molecules. The additive with strong molecular dipole possesses the advantages of enhancing the efficiency and stability of perovskite solar cells (PSCs). After optimization, the companion efficiency of PSCs is 23.20 %, and it can maintain long-term stability under harsh conditions. Additionally, a large-area solar cell module-modified DLBA was 20.18 % (14 cm2 ). This work provides an important reference for the selection and designing of efficient carbonyl additives.

A Multifunctional Polymer as an Interfacial Layer for Efficient and Stable Perovskite Solar Cells.

Metal-cation defects and halogen-anion defects in perovskite films are critical to the efficiency and stability of perovskite solar cells (PSCs). In this work, a random polymer, poly(methyl methacrylate-co-acrylamide) (PMMA-AM), was synthesized to serve as an interfacial passivation layer for synergistically passivating the under-coordinated Pb2+ and anchor the I- of the [PbI6 ]4- octahedron. Additionally, the interfacial PMMA-AM passivation layer cannot be destroyed during the hole transport layer deposition because of its low solubility in chlorobenzene. This passivation leads to an enhancement in the open-circuit voltage from 1.12 to 1.22 V and improved stability in solar cell devices, with the device maintaining 95 % of the initial power conversion efficiency (PCE) over 1000 h of maximum power point tracking. Additionally, a large-area solar cell module was fabricated using this approach, achieving a PCE of 20.64 %.

Understanding the Role of Fluorine Groups in Passivating Defects for Perovskite Solar Cells.

Introducing fluorine (F) groups into a passivator plays an important role in enhancing the defect passivation effect for the perovskite film, which is usually attributed to the direct interaction of F and defect states. However, the interaction between electronegative F and electron-rich passivation groups in the same molecule, which may influence the passivation effect, is ignored. We herein report that such interactions can vary the electron cloud distribution around the passivation groups and thus changing their coordination with defect sites. By comparing two fluorinated molecules, heptafluorobutylamine (HFBM) and heptafluorobutyric acid (HFBA), we find that the F/-NH2 interaction in HFBM is stronger than the F/-COOH one in HFBA, inducing weaker passivation ability of HFBM than HFBA. Accordingly, HFBA-based perovskite solar cells (PSCs) provide an efficiency of 24.70 % with excellent long-term stability. Moreover, the efficiency of a large-area perovskite module (14.0 cm2 ) based on HFBA reaches 21.13 %. Our work offers an insight into understanding an unaware role of the F group in impacting the passivation effect for the perovskite film.

Atomically Resolved Electrically Active Intragrain Interfaces in Perovskite Semiconductors.

Deciphering the atomic and electronic structures of interfaces is key to developing state-of-the-art perovskite semiconductors. However, conventional characterization techniques have limited previous studies mainly to grain-boundary interfaces, whereas the intragrain-interface microstructures and their electronic properties have been much less revealed. Herein using scanning transmission electron microscopy, we resolved the atomic-scale structural information on three prototypical intragrain interfaces, unraveling intriguing features clearly different from those from previous observations based on standalone films or nanomaterial samples. These intragrain interfaces include composition boundaries formed by heterogeneous ion distribution, stacking faults resulted from wrongly stacked crystal planes, and symmetrical twinning boundaries. The atomic-scale imaging of these intragrain interfaces enables us to build unequivocal models for the ab initio calculation of electronic properties. Our results suggest that these structure interfaces are generally electronically benign, whereas their dynamic interaction with point defects can still evoke detrimental effects. This work paves the way toward a more complete fundamental understanding of the microscopic structure-property-performance relationship in metal halide perovskites.

Polyacrylonitrile-Coordinated Perovskite Solar Cell with Open-Circuit Voltage Exceeding 1.23†V.

In solution-processed organic-inorganic halide perovskite films, halide-anion related defects including halide vacancies and interstitial defects can easily form at the surfaces and grain boundaries. The uncoordinated lead cations produce defect levels within the band gap, and the excess iodides disturb the interfacial carrier transport. Thus these defects lead to severe nonradiative recombination, hysteresis, and large energy loss in the device. Herein, polyacrylonitrile (PAN) was introduced to passivate the uncoordinated lead cations in the perovskite films. The coordinating ability of cyano group was found to be stronger than that of the normally used carbonyl groups, and the strong coordination could reduce the I/Pb ratio at the film surface. With the PAN perovskite film, the device efficiency improved from 21.58 % to 23.71 % and the open-circuit voltage from 1.12 V to 1.23 V, the ion migration activation energy increased, and operational stability improved.

Reducing Defects Density and Enhancing Hole Extraction for Efficient Perovskite Solar Cells Enabled by π-Pb2+ Interactions.

Molecular doping is an of significance approach to reduce defects density of perovskite and to improve interfacial charge extraction in perovskite solar cells. Here, we show a new strategy for chemical doping of perovskite via an organic small molecule, which features a fused tricyclic core, showing strong intermolecular π-Pb2+ interactions with under-coordinated Pb2+ in perovskite. This π-Pb2+ interactions could reduce defects density of the perovskite and suppress the nonradiative recombination, which was also confirmed by the density functional theory calculations. In addition, this doping via π-Pb2+ interactions could deepen the surface potential and downshift the work function of the doped perovskite film, facilitating the hole extraction to hole transport layer. As a result, the doped device showed high efficiency of 21.41 % with ignorable hysteresis. This strategy of fused tricyclic core-based doping provides a new perspective for the design of new organic materials to improve the device performance.

A Scalable Methylamine Gas Healing Strategy for High-Efficiency Inorganic Perovskite Solar Cells.

An easy and scalable methylamine (MA) gas healing method was realized for inorganic cesium-based perovskite (CsPbX3 ) layers by incorporating a certain amount of MAX (X=I or Br) initiators into the raw film. It was found that the excess MAX accelerated the absorption of the MA gas into the CsPbX3 film and quickly turned it into a liquid intermediate phase. Through the healing process, a highly uniform and highly crystalline CsPbX3 film with enhanced photovoltaic performance was obtained. Moreover, the chemical interactions between a series of halides and MA gas molecules were studied, and the results could offer guidance in further optimizations of the healing strategy.

Methylammonium-Mediated Evolution of Mixed-Organic-Cation Perovskite Thin Films: A Dynamic Composition-Tuning Process.

Methylammonium-mediated phase-evolution behavior of FA1-x MAx PbI3 mixed-organic-cation perovskite (MOCP) is studied. It is found that by simply enriching the MOCP precursor solutions with excess methylammonium cations, the MOCPs form via a dynamic composition-tuning process that is key to obtaining MOCP thin films with superior properties. This simple chemical approach addresses several key challenges, such as control over phase purity, uniformity, grain size, composition, etc., associated with the solution-growth of MOCP thin films with targeted compositions.

Exceptional Morphology-Preserving Evolution of Formamidinium Lead Triiodide Perovskite Thin Films via Organic-Cation Displacement.

Here we demonstrate a radically different chemical route for the creation of HC(NH2)2PbI3 (FAPbI3) perovskite thin films. This approach entails a simple exposure of as-synthesized CH3NH3PbI3 (MAPbI3) perovskite thin films to HC(═NH)NH2 (formamidine or FA) gas at 150 °C, which leads to rapid displacement of the MA(+) cations by FA(+) cations in the perovskite structure. The resultant FAPbI3 perovskite thin films preserve the microstructural morphology of the original MAPbI3 thin films exceptionally well. Importantly, the myriad processing innovations that have led to the creation of high-quality MAPbI3 perovskite thin films are directly adaptable to FAPbI3 through this simple, rapid chemical-conversion route. Accordingly, we show that efficiencies of perovskite solar cells fabricated with FAPbI3 thin films created using this route can reach ∼18%.

Transformative Evolution of Organolead Triiodide Perovskite Thin Films from Strong Room-Temperature Solid-Gas Interaction between HPbI3-CH3NH2 Precursor Pair.

We demonstrate the feasibility of a nonsalt-based precursor pair--inorganic HPbI3 solid and organic CH3NH2 gas--for the deposition of uniform CH3NH3PbI3 perovskite thin films. The strong room-temperature solid-gas interaction between HPbI3 and CH3NH2 induces transformative evolution of ultrasmooth, full-coverage perovskite thin films at a rapid rate (in seconds) from nominally processed rough, partial-coverage HPbI3 thin films. The chemical origin of this behavior is elucidated via in situ experiments. Perovskite solar cells, fabricated using MAPbI3 thin films thus deposited, deliver power conversion efficiencies up to 18.2%, attesting to the high quality of the perovskite thin films deposited using this transformative process.

Thin-Film Transformation of NH4 PbI3 to CH3 NH3 PbI3 Perovskite: A Methylamine-Induced Conversion-Healing Process.

Methylamine-induced thin-film transformation at room-temperature is discovered, where a porous, rough, polycrystalline NH4 PbI3 non-perovskite thin film converts stepwise into a dense, ultrasmooth, textured CH3 NH3 PbI3 perovskite thin film. Owing to the beneficial phase/structural development of the thin film, its photovoltaic properties undergo dramatic enhancement during this NH4 PbI3 -to-CH3 NH3 PbI3 transformation process. The chemical origins of this transformation are studied at various length scales.

Methylamine-Gas-Induced Defect-Healing Behavior of CH3NH3PbI3 Thin Films for Perovskite Solar Cells.

We report herein the discovery of methylamine (CH3NH2) induced defect-healing (MIDH) of CH3NH3PbI3 perovskite thin films based on their ultrafast (seconds), reversible chemical reaction with CH3NH2 gas at room temperature. The key to this healing behavior is the formation and spreading of an intermediate CH3NH3PbI3⋅xCH3NH2 liquid phase during this unusual perovskite-gas interaction. We demonstrate the versatility and scalability of the MIDH process, and show dramatic enhancement in the performance of perovskite solar cells (PSCs) with MIDH. This study represents a new direction in the formation of defect-free films of hybrid perovskites.

One-step, solution-processed formamidinium lead trihalide (FAPbI(3-x)Cl(x)) for mesoscopic perovskite-polymer solar cells.

Formamidinium (FA) lead triiodide perovskite with chlorine addition (NH2CH=NH2PbI(3-x)Clx) is employed as a light harvester in mesoscopic solar cells for the first time. It is demonstrated that a phase-pure FAPbI(3-x)Clx perovskite layer can be synthesized using a one-step solution-process at 140 °C, and the resultant solar cells deliver a maximum power conversion efficiency of 7.51%, which is the most efficient formamidinium-lead-halide perovskite mesoscopic solar cell employing a polymer hole-transporting layer. The effects of the thermal annealing temperature on the quality/morphology of the perovskite layer and the solar cells performance are discussed. The advantages offered by the one-step solution-processing method and the reduced bandgap make FAPbI(3-x)Clx perovskites an attractive choice for future hybrid photovoltaics.

Enhanced Quasi-Fermi Level Splitting of Perovskite Solar Cells by Universal Dual-Functional Polymer.

Perovskite solar cells (PSCs) have attracted extensive attention due to their higher power conversion efficiency (PCE) and simple fabrication process. However, the open-circuit voltage (VOC) loss remains a significant impediment to enhance device performance. Here, a facile strategy to boost the VOC to 95.5% of the Shockley-Queisser (S-Q) limit through the introduction of a universal multifunctional polymer additive is demonstrated. This additive effectively passivates the cation and anion defects simultaneously, thereby leading to the transformation from the strong n-type to weak n-type of perovskite films. Benefitting from the energy level alignment and the suppression of bulk non-radiative recombination, the quasi-Fermi level splitting (QFLS) is enhanced.  Consequently, the champion devices with 1.59 eV-based perovskite reach the highest VOC value of 1.24 V and a PCE of 23.86%. Furthermore, this strategy boosts the VOC by at least 0.07 V across five different perovskite systems, a PCE of 25.04% is achieved for 1.57 eV-based PSCs, and the corresponding module (14 cm2) also obtained a high PCE of 21.95%. This work provides an effective and universal strategy to promote the VOC approach to the detailed balance theoretical limit.