Steric Mixed‐Cation 2D Perovskite as a Methylammonium Locker to Stabilize MAPbI3

The reduced dimension perovskite including 2D perovskites are one of the most promising strategies to stabilize lead halide perovskite. A mixed‐cation 2D perovskite based on a steric phenyltrimethylammonium (PTA) cation is presented. The PTA‐MA mixed‐cation 2D perovskite of PTAMAPbI₄ can be formed on the surface of MAPbI₃ (PTAI‐MAPbI₃) by controllable PTAI intercalation by either spin coating or soaking. The PTAMAPbI₄ capping layer can not only passivate PTAI‐MAPbI₃ perovskite but also act as MA⁺ locker to inhibit MAI extraction and significantly enhance the stability. The highly stable PTAI‐MAPbI₃ based perovskite solar cells exhibit a reproducible photovoltaic performance with a champion PCE of 21.16 %. Such unencapsulated devices retain 93 % of initial efficiency after 500 h continuous illumination. This steric mixed‐cation 2D perovskite as MA⁺ locker to stabilize the MAPbI₃ is a promising strategy to design stable and high‐performance hybrid lead halide perovskites.     

Tetra‐ammonium Zinc Phthalocyanine to Construct a Graded 2D–3D Perovskite Interface for Efficient and Stable Solar Cells

The crystallographic defects inevitably incur during the solution processed organic‐inorganic hybrid perovskite film, especially at surface and the grain boundaries (GBs) of perovskite film, which can further result in the reduced cell performance and stability of perovskite solar cells (PSCs). Here, a simple defect passivation method was employed by treating perovskite precursor film with a hydrophobic tetra‐ammonium zinc phthalocyanine (ZnPc). The results demonstrated that a 2D‐3D graded perovskite interface with a capping layer of 2D (ZnPc)_0.5MA_n ? 1Pb_nI_3n + 1 perovskite together with 3D MAPbI₃ perovskite was successfully constructed on the top of 3D perovskite layer. This situation realized the efficient GBs passivation, thus reducing the defects in GBs. As expected, the corresponding PSCs with modified perovskite revealed an improved cell performance. The best efficiency reached 19.6%. Especially, the significantly enhanced long‐term stability of the responding PSCs against humidity and heating was remarkably achieved. Such a strategy in this work affords an efficient method to improve the stability of PSCs and thus probably brings the PSCs closer to practical commercialization.     

The Role of Surface Termination in Halide Perovskites for Efficient Photocatalytic Synthesis

Halide perovskites have received attention in the field of photocatalysis owing to their excellent optoelectronic properties. However, the semiconductor properties of halide perovskite surfaces and the influence on photocatalytic performance have not been systematically clarified. Now, the conversion of triose (such as 1,3‐dihydroxyacetone (DHA)) is employed as a model reaction to explore the surface termination of MAPbI₃. By rational design of the surface termination for MAPbI₃, the production rate of butyl lactate is substantially improved to 7719 μg g^?1 cat. h^?1 under visible‐light illumination. The MAI‐terminated MAPbI₃ surface governs the photocatalytic performance. Specially, MAI‐terminated surface is susceptible to iodide oxidation, which thus promotes the exposure of Pb^II as active sites for this photocatalysis process. Moreover, MAI‐termination induces a p‐doping effect near the surface for MAPbI₃, which facilitates carrier transport and thus photosynthesis.     

Steric Mixed-Cation 2D Perovskite as a Methylammonium Locker to Stabilize MAPbI3

The reduced dimension perovskite including 2D perovskites are one of the most promising strategies to stabilize lead halide perovskite. A mixed-cation 2D perovskite based on a steric phenyltrimethylammonium (PTA) cation is presented. The PTA-MA mixed-cation 2D perovskite of PTAMAPbI₄ can be formed on the surface of MAPbI₃ (PTAI-MAPbI₃) by controllable PTAI intercalation by either spin coating or soaking. The PTAMAPbI₄ capping layer can not only passivate PTAI-MAPbI₃ perovskite but also act as MA⁺ locker to inhibit MAI extraction and significantly enhance the stability. The highly stable PTAI-MAPbI₃ based perovskite solar cells exhibit a reproducible photovoltaic performance with a champion PCE of 21.16 %. Such unencapsulated devices retain 93 % of initial efficiency after 500 h continuous illumination. This steric mixed-cation 2D perovskite as MA⁺ locker to stabilize the MAPbI₃ is a promising strategy to design stable and high-performance hybrid lead halide perovskites.     

Controlled Growth of Hybrid Halide Perovskites by Crown Ether Complexation for Perovskite Solar Cells

Host-guest complexation has demonstrated potential for controlling hybrid organic-inorganic metal halide perovskite materials. In particular, crown ethers have been used due to their capacity to interact with metal cations (e. g., Pb²⁺) and small organic cations (e. g., methylammonium (MA)), which can affect hybrid perovskite materials and their solar cells. However, this strategy has been underexploited in perovskite photovoltaics, and the underlying mechanisms are not well understood. In this study, we investigate the influence of 15-crown-5 ( 15C5 ) and its benzannulated derivative (benzo-15-crown-5, B15C5 ), as well as amino-functionalized analogues (15-crown-5)-2-methylamine, 2A-15C5 , and 4′-aminobenzo-15-crown-5, 4A-B15C5 , on MAPbI₃ perovskite crystallization and inverted solar cell performance. We demonstrate the propensity of crown ether modulators to interact with Pb²⁺ cations at the perovskite interface by density functional theory calculations. This has been shown to facilitate oriented crystal growth and homogeneous film formation, as revealed by X-ray diffraction analysis complemented by scanning electron microscopy. As a result, we demonstrate an increase in the power conversion efficiency of the solar cells of interest to advancing hybrid photovoltaics.     

In CH3NH3PbI3 Perovskite Film,the Surface Termination Layer Dominates the Moisture Degradation Pathway

Theoretical studies have shown that surface terminations, such as MAI or PbI layers, greatly affect the environmental stability of organic–inorganic perovskite. However, until now, there has been little effort to experimentally detect the existence of MAI or PbI terminations on MAPbI₃ grains, let alone disclose their effects on the humidity degradation pathway of perovskite solar cell. Here, we successfully modified and detected the surface terminations of MAI and PbI species on polycrystalline MAPbI₃ films. MAI-terminated perovskite film followed the moisture degradation process from MAPbI₃ to hydrate MAPbI₃⋅H₂O and then into PbI₂, with penetration of water molecules being the main driving force leading to the degradation of MAPbI₃ layer by layer. In contrast, for the PbI-terminated perovskite film in a humid atmosphere, a deprotonation degradation pathway was confirmed, in which the film preferentially degraded directly from MAPbI₃ into PbI₂, here the iodine defects played a key role in promoting the dissociation of water molecules into OH⁻ and further catalyzing the decomposition of perovskite.     

Wide-Bandgap Perovskite Solar Cell Using a Fluoride-Assisted Surface Gradient Passivation Strategy

Wide-band gap (1.68 eV) perovskite solar cells (PSCs) are important components of perovskite/Si tandem devices. However, the efficiency of wide band gap PSCs has been limited by their huge open-circuit voltage (V_oc) deficit due to non-radiative recombination. Deep-level acceptor defects are identified as the major killers of V_oc, and they can be effectively improved by passivation with ammonium salts. Theoretical calculation predicts that increasing the distance between F and −NH₃⁺ of fluorinated ammonium can dramatically enhance the electropositivity of −NH₃⁺ terminals, thus providing strong adsorption onto the negatively charged I_A and I_Pb anti-site defects. Characterizations further confirm that surface gradient passivation employing p-FPEAI demonstrates the most efficient passivation effect. Consequently, a record-efficiency of 21.63 % with the smallest V_oc deficit of 441 mV is achieved for 1.68 eV-band gap inverted PSCs. Additionally, a flexible PSC and 1 cm² opaque device also deliver the highest PCEs of 21.02 % and 19.31 %, respectively.     

A Layered Hybrid Perovskite Solar‐Cell Absorber with Enhanced Moisture Stability

Two‐dimensional hybrid perovskites are used as absorbers in solar cells. Our first‐generation devices containing (PEA)₂(MA)₂[Pb₃I₁₀] ( 1 ; PEA=C₆H₅(CH₂)₂NH₃⁺, MA=CH₃NH₃⁺) show an open‐circuit voltage of 1.18 V and a power conversion efficiency of 4.73 %. The layered structure allows for high‐quality films to be deposited through spin coating and high‐temperature annealing is not required for device fabrication. The 3D perovskite (MA)[PbI₃] ( 2 ) has recently been identified as a promising absorber for solar cells. However, its instability to moisture requires anhydrous processing and operating conditions. Films of 1 are more moisture resistant than films of 2 and devices containing 1 can be fabricated under ambient humidity levels. The larger bandgap of the 2D structure is also suitable as the higher bandgap absorber in a dual‐absorber tandem device. Compared to 2 , the layered perovskite structure may offer greater tunability at the molecular level for material optimization.     

Lead‐Free Silver‐Bismuth Halide Double Perovskite Nanocrystals

Lead‐free perovskite nanocrystals (NCs) were obtained mainly by substituting a Pb²⁺ cation with a divalent cation or substituting three Pb²⁺ cations with two trivalent cations. The substitution of two Pb²⁺ cations with one monovalent Ag⁺ and one trivalent Bi³⁺ cations was used to synthesize Cs₂AgBiX₆ (X=Cl, Br, I) double perovskite NCs. Using femtosecond transient absorption spectroscopy, the charge carrier relaxation mechanism was elucidated in the double perovskite NCs. The Cs₂AgBiBr₆ NCs exhibit ultrafast hot‐carrier cooling (<1 ps), which competes with the carrier trapping processes (mainly originate from the surface defects). Notably, the photoluminescence can be increased by 100 times with surfactant (oleic acid) added to passivate the defects in Cs₂AgBiCl₆ NCs. These results suggest that the double perovskite NCs could be potential materials for optoelectronic applications by better controlling the surface defects.     

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 %.     

Stabilization of Highly Efficient and Stable Phase‐Pure FAPbI3 Perovskite Solar Cells by Molecularly Tailored 2D‐Overlayers

As a result of their attractive optoelectronic properties, metal halide APbI₃ perovskites employing formamidinium (FA⁺) as the A cation are the focus of research. The superior chemical and thermal stability of FA⁺ cations makes α‐FAPbI₃ more suitable for solar‐cell applications than methylammonium lead iodide (MAPbI₃). However, its spontaneous conversion into the yellow non‐perovskite phase (δ‐FAPbI₃) under ambient conditions poses a serious challenge for practical applications. Herein, we report on the stabilization of the desired α‐FAPbI₃ perovskite phase by protecting it with a two‐dimensional (2D) IBA₂FAPb₂I₇ (IBA=iso‐butylammonium overlayer, formed via stepwise annealing. The α‐FAPbI₃/IBA₂FAPb₂I₇ based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. In addition, it showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress, that is, simultaneous exposure with maximum power tracking to full simulated sunlight at 80 °C over 500 h.     

Managing Multiple Halide-Related Defects for Efficient and Stable Inorganic Perovskite Solar Cells

Halide-related surface defects on inorganic halide perovskite not only induce charge recombination but also severely limit the long-term stability of perovskite solar cells. Herein, adopting density functional theory calculation, we verify that iodine interstitials (I_i) has a low formation energy similar to that of the iodine vacancy (V_I) and is also readily formed on the surface of all-inorganic perovskite, and it is regarded to function as an electron trap. We screen a specific 2,6-diaminopyridine (2,6-DAPy) passivator, which, with the aid of the combined effects from halogen-N_pyridine and coordination bonds, not only successfully eliminates the I_i and dissociative I₂ but also passivates the abundant V_I. Furthermore, the two symmetric neighboring -NH₂ groups interact with adjacent halides of the octahedral cluster by forming hydrogen bonds, which further promotes the adsorption of 2,6-DAPy molecules onto the perovskite surface. Such synergetic effects can significantly passivate harmful iodine-related defects and undercoordinated Pb²⁺, prolong carrier lifetimes and facilitate the interfacial hole transfer. Consequently, these merits enhance the power-conversion efficiency (PCE) from 19.6 % to 21.8 %, the highest value for this type of solar cells, just as importantly, the 2,6-DAPy-treated CsPbI_3−xBr_x films show better environmental stability.     

Stabilization of Highly Efficient and Stable Phase-Pure FAPbI3 Perovskite Solar Cells by Molecularly Tailored 2D-Overlayers

As a result of their attractive optoelectronic properties, metal halide APbI₃ perovskites employing formamidinium (FA⁺) as the A cation are the focus of research. The superior chemical and thermal stability of FA⁺ cations makes α-FAPbI₃ more suitable for solar-cell applications than methylammonium lead iodide (MAPbI₃). However, its spontaneous conversion into the yellow non-perovskite phase (δ-FAPbI₃) under ambient conditions poses a serious challenge for practical applications. Herein, we report on the stabilization of the desired α-FAPbI₃ perovskite phase by protecting it with a two-dimensional (2D) IBA₂FAPb₂I₇ (IBA=iso-butylammonium overlayer, formed via stepwise annealing. The α-FAPbI₃/IBA₂FAPb₂I₇ based perovskite solar cell (PSC) reached a high power conversion efficiency (PCE) of close to 23 %. In addition, it showed excellent operational stability, retaining around 85 % of its initial efficiency under severe combined heat and light stress, that is, simultaneous exposure with maximum power tracking to full simulated sunlight at 80 °C over 500 h.     

Low‐Energy Electron‐Induced Transformations in Organolead Halide Perovskite

Methylammonium lead iodide perovskite (MAPbI₃), a prototype material for potentially high‐efficient and low‐cost organic–inorganic hybrid perovskite solar cells, has been investigated intensively in recent years. A study of low‐energy electron‐induced transformations in MAPbI₃ is presented, performed by combining controlled electron‐impact irradiation with X‐ray photoelectron spectroscopy and scanning electron microscopy. Changes were observed in both the elemental composition and the morphology of irradiated MAPbI₃ thin films as a function of the electron fluence for incident energies from 4.5 to 60 eV. The results show that low‐energy electrons can affect structural and chemical properties of MAPbI₃. It is proposed that the transformations are triggered by the interactions with the organic part of the material (methylammonium), resulting in the MAPbI₃ decomposition and aggregation of the hydrocarbon layer.     

The Second Spacer Cation Assisted Growth of a 2D Perovskite Film with Oriented Large Grain for Highly Efficient and Stable Solar Cells

The fabrication of high‐quality film with large grains oriented along the direction of film thickness is important for 2D Ruddlesden–Popper perovskite‐based solar cells (PVSCs). High‐quality 2D BA₂MA_n?1Pb_nI_3n+1 (BA⁺=butylammonium, MA⁺=methylammonium, n=5) perovskite films were fabricated with a grain size of over 1 μm and preferential orientation growth by introducing a second spacer cation (SSC⁺) into the precursor solution. Dynamic light scattering showed that SSC⁺ addition can induce aggregation in the precursor solution. The precursor aggregates are favorable for the formation of large crystal grains by inducing nucleation and decreasing the nucleation sites. Applying phenylethylammonium as SSC⁺, the optimized inverted planar PVSCs presented a maximum PCE of 14.09 %, which is the highest value of the 2D BA₂MA_n?1Pb_nI_3n+1 (n=5) PVSCs. The unsealed device shows good moisture stability by maintaining around 90 % of its initially efficiency after 1000 h exposure to air (Hr=25±5 %).     

The CuII−CuI−Cu0(Hg) system in the presence of benzotriazole: Part I. A polarographic investigation

A polarographic investigation of Cu^II electroreduction from solutions of benzotriazole (BTA) of pH from 1 to 3 at a dropping mercury electrode shows the presence of two successive adsorption waves of equal height (waves Ic and IIIc) due, respectively, to the formation and to the subsequent dissolution of an adsorbed film of a Cu^I compound. The dependence of the half-wave potential of wave IIIc, which is polarographically reversible, upon pH and BTA concentration indicates that the adsorbed film has the composition [Cu^I(BTA^?)], where BTA^? denotes the deprotonated anionic form of benzotriazole. Moreover, the maximum height of wave IIIc indicates that this film is one monolayer thick. A third cathodic wave (wave IIc), lying between waves Ic and IIIc, stems from the reduction to the metal state of the Cu^II ions diffusing from the bulk solution. Copper(0) oxidation at dropping amalgam electrodes in BTA solutions yields two successive adsorption waves (waves Ia and IIa). Wave Ia is due to the same electrode process, Cu⁰ (Hg)+BTA?Cu^I(BTA^?)+H⁺+e, responsible for the cathodic wave IIIc. On the other hand, wave IIa is due to the formation of roughly two adsorbed monolayers of [Cu^I(BTA^?)] upon that formed along wave Ia.     

Dredging the Charge-Carrier Transfer Pathway for Efficient Low-Dimensional Ruddlesden-Popper Perovskite Solar Cells

Low-dimensional Ruddlesden-Popper (LDRP) perovskites still suffer from inferior carrier transport properties. Here, we demonstrate that efficient exciton dissociation and charge transfer can be achieved in LDRP perovskite by introducing γ-aminobutyric acid (GABA) as a spacer. The hydrogen bonding links adjacent spacing sheets in (GABA)₂MA₃Pb₄I₁₃ (MA=CH₃NH₃⁺), leading to the charges localized in the van der Waals gap, thereby constructing “charged-bridge” for charge transfer through the spacing region. Additionally, the polarized GABA weakens dielectric confinement, decreasing the (GABA)₂MA₃Pb₄I₁₃ exciton binding energy as low as ≈73 meV. Benefiting from these merits, the resultant GABA-based solar cell yields a champion power conversion efficiency (PCE) of 18.73 % with enhanced carrier transport properties. Furthermore, the unencapsulated device maintains 92.8 % of its initial PCE under continuous illumination after 1000 h and only lost 3 % of its initial PCE under 65 °C for 500 h.     

Near-Stoichiometric and Homogenized Perovskite Films for Solar Cells with Minimized Performance Variation

Mixed-cation, small band-gap perovskites via rationally alloying formamidinium (FA) and methylammonium (MA) together have been widely employed for blade-coated perovskite solar cells with satisfied efficiencies. One of the stringent challenges lies in difficult control of the nucleation and crystallization kinetics of the perovskites with mixed ingredients. Herein, a pre-seeding strategy by mixing FAPbI₃ solution with pre-synthesized MAPbI₃ microcrystals has been developed to smartly decouple the nucleation and crystallization process. As a result, the time window of initialized crystallization has been greatly extended by 3 folds (i.e. from 5 s to 20 s), which enables the formation of uniform and homogeneous alloyed-FAMA perovskite films with designated stoichiometric ratios. The resultant blade-coated solar cells achieved a champion efficiency of 24.31 % accompanied by outstanding reproducibility with more than 87 % of the devices showing efficiencies higher than 23 %.     

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.     

Highly Efficient,Reproducible, Uniform (CH3NH3)PbI3 Layer by Processing Additive Dripping for Solution‐Processed Planar Heterojunction Perovskite Solar Cells

A processing additive dripping (PAD) approach to forming highly efficient (CH₃NH₃)PbI₃ (MAPbI₃) perovskite layers was investigated. A MAPbI₃(CB/DIO) perovskite film fabricated by this approach, which included briefly dripping chlorobenzene incorporating a small amount of diiodooctane (DIO) during casting of a MAPbI₃ perovskite precursor dissolved in dimethylformamide, exhibited superior smooth, uniform morphologies with high crystallinity and large grains and revealed completely homogeneous surface coverage. The surface coverage and morphology of the substrate significantly affected the photovoltaic performance of planar heterojunction (PHJ) perovskite solar cells (PrSCs), resulting in a power conversion efficiency of 11.45 % with high open‐circuit voltage of 0.91 V and the highest fill factor of 80.87 %. Moreover, the PAD approach could effectively provide efficient MAPbI₃(CB/DIO) perovskite layers for highly efficient, reproducible, uniform PHJ PrSC devices without performance loss or variation even over larger active areas.