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.     

A thioacetamide additive-based hybrid (MA0.5FA0.5)PbI3 perovskite solar cells crossing 21 % efficiency with excellent long term stability

An additive in hybrid perovskite is playing a vital role in the increment of power conversion efficiency (PCE), stability, and reproducibility of perovskite solar cells (PVSCs). Although, single-phase α-FAPbI₃ perovskite has an ideal band gap but is continuously transforming to δ–FAPbI₃, which is non-photoactive. Here, we controlled the methylammonium (MA) and formamidinium (FA) ratio in the (MA_xFA_1-x)PbI₃ perovskite composition and tuned its morphology with the help of the thioacetamide (TAA) Lewis base additive. The optimum MA:FA ratio and fine-tuning of TAA additive result in a highly crystalline, large grain size and smooth surface of the (MA_0.5FA_0.5)PbI₃ perovskite film. These highly uniform thin films with 850 nm grain size offered a superior interaction between the perovskite material and the electron transport layer (ETL) and a longer lifetime yielding a high PCE. The (MA_0.5FA_0.5)PbI₃+1% TAA-based champion device exhibited the highest PCE of 21.29% for a small area (0.09 cm²) and 18.32% PCE for a large area (1 cm²). The TAA-assisted devices exhibited high stability with >85% retention over 500 h. These results suggest that the (MA_0.5FA_0.5)PbI₃ along with the 1% TAA additive is a promising absorber layer that can produce >21% PCE.     

Low-Dimensional Dion–Jacobson-Phase Lead-Free Perovskites for High-Performance Photovoltaics with Improved Stability

1,4-butanediamine (BEA) is incorporated into FASnI₃ (FA=formamidinium) to develop a series of lead-free low-dimensional Dion–Jacobson-phase perovskites, (BEA)FA_n−1Sn_nI_3n+1. The broadness of the (BEA)FA₂Sn₃I₁₀ band gap appears to be influenced by the structural distortion owing to high symmetry. The introduction of BEA ligand stabilizes the low-dimensional perovskite structure (formation energy ca. 10⁶ j mol⁻¹), which inhibits the oxidation of Sn²⁺. The compact (BEA)FA₂Sn₃I₁₀ dominated film enables a weakened carrier localization mechanism with a charge transfer time of only 0.36 ps among the quantum wells, resulting in a carrier diffusion length over 450 nm for electrons and 340 nm for holes, respectively. Solar cell fabrication with (BEA)FA₂Sn₃I₁₀ delivers a power conversion efficiency (PCE) of 6.43 % with negligible hysteresis. The devices can retain over 90 % of their initial PCE after 1000 h without encapsulation under N₂ environment.     

Low‐Dimensional Dion–Jacobson‐Phase Lead‐Free Perovskites for High‐Performance Photovoltaics with Improved Stability

1,4‐butanediamine (BEA) is incorporated into FASnI₃ (FA=formamidinium) to develop a series of lead‐free low‐dimensional Dion–Jacobson‐phase perovskites, (BEA)FA_n?1Sn_nI_3n+1. The broadness of the (BEA)FA₂Sn₃I₁₀ band gap appears to be influenced by the structural distortion owing to high symmetry. The introduction of BEA ligand stabilizes the low‐dimensional perovskite structure (formation energy ca. 10⁶ j mol^?1), which inhibits the oxidation of Sn²⁺. The compact (BEA)FA₂Sn₃I₁₀ dominated film enables a weakened carrier localization mechanism with a charge transfer time of only 0.36 ps among the quantum wells, resulting in a carrier diffusion length over 450 nm for electrons and 340 nm for holes, respectively. Solar cell fabrication with (BEA)FA₂Sn₃I₁₀ delivers a power conversion efficiency (PCE) of 6.43 % with negligible hysteresis. The devices can retain over 90 % of their initial PCE after 1000 h without encapsulation under N₂ environment.     

Enhanced Charge Transport by Incorporating Formamidinium and Cesium Cations into Two‐Dimensional Perovskite Solar Cells

Organic‐inorganic hybrid two‐dimensional (2D) perovskites (n≤5) have recently attracted significant attention because of their promising stability and optoelectronic properties. Normally, 2D perovskites contain a monocation [e.g., methylammonium (MA⁺) or formamidinium (FA⁺)]. Reported here for the first time is the fabrication of 2D perovskites (n=5) with mixed cations of MA⁺, FA⁺, and cesium (Cs⁺). The use of these triple cations leads to the formation of a smooth, compact surface morphology with larger grain size and fewer grain boundaries compared to the conventional MA‐based counterpart. The resulting perovskite also exhibits longer carrier lifetime and higher conductivity in triple cation 2D perovskite solar cells (PSCs). The power conversion efficiency (PCE) of 2D PSCs with triple cations was enhanced by more than 80 % (from 7.80 to 14.23 %) compared to PSCs fabricated with a monocation. The PCE is also higher than that of PSCs based on binary cation (MA⁺‐FA⁺ or MA⁺‐Cs⁺) 2D structures.     

Guanine-Stabilized Formamidinium Lead Iodide Perovskites

Formamidinium (FA) lead iodide perovskite materials feature promising photovoltaic performances and superior thermal stabilities. However, conversion of the perovskite α-FAPbI₃ phase to the thermodynamically stable yet photovoltaically inactive δ-FAPbI₃ phase compromises the photovoltaic performance. A strategy is presented to address this challenge by using low-dimensional hybrid perovskite materials comprising guaninium (G) organic spacer layers that act as stabilizers of the three-dimensional α-FAPbI₃ phase. The underlying mode of interaction at the atomic level is unraveled by means of solid-state nuclear magnetic resonance spectroscopy, X-ray crystallography, transmission electron microscopy, molecular dynamics simulations, and DFT calculations. Low-dimensional-phase-containing hybrid FAPbI₃ perovskite solar cells are obtained with improved performance and enhanced long-term stability.     

Dual-Interface Engineering in Perovskite Solar Cells with 2D Carbides

Passivating the interfaces between the perovskite and charge transport layers is crucial for enhancing the power conversion efficiency (PCE) and stability in perovskite solar cells (PSCs). Here we report a dual-interface engineering approach to improving the performance of FA_0.85MA_0.15Pb(I_0.95Br_0.05)₃-based PSCs by incorporating Ti₃C₂Cl_x Nano-MXene and o-TB-GDY nanographdiyne (NanoGDY) into the electron transport layer (ETL)/perovskite and perovskite/ hole transport layer (HTL) interfaces, respectively. The dual-interface passivation simultaneously suppresses non-radiative recombination and promotes carrier extraction by forming the Pb−Cl chemical bond and strong coordination of π-electron conjugation with undercoordinated Pb defects. The resulting perovskite film has an ultralong carrier lifetime exceeding 10 μs and an enlarged crystal size exceeding 2.5 μm. A maximum PCE of 24.86 % is realized, with an open-circuit voltage of 1.20 V. Unencapsulated cells retain 92 % of their initial efficiency after 1464 hours in ambient air and 80 % after 1002 hours of thermal stability test at 85 °C.     

Lead-Free,Water-Stable A3Bi2I9 Perovskites: Crystal Growth and Blue-Emitting Quantum Dots [A=CH3NH3+, Cs+, and (Rb0.05Cs2.95)+]

Despite the great success in the increase in the power conversion efficiency of lead halide perovskite solar cells, the toxicity of lead and the unstable nature of the materials are still major concerns for their wider implementation at the industrial level. Herein, large-size single crystals (SCs) are developed in HI solution by using a temperature lowering method and nanocrystals (NCs) of A₃Bi₂I₉ perovskites [where A=CH₃NH₃⁺ (MA)⁺, Cs⁺, and (Rb_0.05Cs_2.95)⁺] are formed in ethanol (EtOH) and toluene (TOL). The stability of A₃Bi₂I₉ perovskite is investigated by immersing the SCs for 24 h and pellets for 12 h in water. Moreover, the A₃Bi₂I₉ perovskite NCs displays a promising photoluminescence quantum yield of 17.63 % and a long lifetime of 8.20 ns.     

Manipulation of the Buried Interface for Robust Formamidinium-based Sn−Pb Perovskite Solar Cells with NiOx Hole-Transport Layers

Low band gap tin-lead perovskite solar cells (Sn−Pb PSCs) are expected to achieve higher efficiencies than Pb-PSCs and regarded as key components of tandem PSCs. However, the realization of high efficiency is challenged by the instability of Sn²⁺ and the imperfections at the charge transfer interfaces. Here, we demonstrate an efficient ideal band gap formamidinium (FA)-based Sn−Pb (FAPb_0.5Sn_0.5I₃) PSC, by manipulating the buried NiO_x/perovskite interface with 4-hydroxyphenethyl ammonium halide (OH-PEAX, X=Cl⁻, Br⁻, or I⁻) interlayer, which exhibits fascinating functions of reducing the surface defects of the NiO_x hole transport layer (HTL), enhancing the perovskite film quality, and improving both the energy level matching and physical contact at the interface. The effects of different halide anions have been elaborated and a 20.53 % efficiency is obtained with OH-PEABr, which is the highest one for FA-based Sn−Pb PSCs using NiO_x HTLs. Moreover, the device stability is also boosted.     

A Narrow Bandgap of 2D Ruddlesden-Popper Bilayer Perovskite with Giant Entropy Change and Photoluminescence

The multifunctional two-dimensional (2D) organic-inorganic hybrid perovskites have potential applications in many fields, such as, semiconductor, energy storage and fluorescent device etc. Here, a 2D Ruddlesden-Popper (RP) perovskite (IPA)₂(FA)Pb₂I₇ ( 1 , IPA⁺=C₃H₉NI⁺, FA⁺=CN₂H₅⁺) is determined for its photophysical properties. Strikingly, 1 reveals a solid reversible phase transition with T_c of 382 K accompanied by giant entropy change of 40 J mol⁻¹ K⁻¹. Further optical investigations indicate that 1 reveals a narrow direct bandgap (2.024 eV) attributed to the slight bending of I−Pb-I edge and inorganic [Pb₂I₇]_n layer and a superior photoluminescence (PL) emission with super long lifetime of 0.1607 ms. It is believed that this work will pave an avenue to further design multifunctional semiconductors that combines energy storage and photoelectric materials.     

A Lead Iodide Perovskite Based on a Large Organic Cation for Solar Cell Applications

Methylammonium (CH₃NH₃⁺) and formamidinium ((NH₂)₂CH⁺) based lead iodide perovskites are currently the two commonly used organic–inorganic lead iodide perovskites. There are still no alternative organic cations that can produce perovskites with band gaps spanning the visible spectrum (that is, <1.7 eV) for solar cell applications. Now, a new perovskite using large propane‐1,3‐diammonium cation (1,3‐Pr(NH₃)₂²⁺) with a chemical structure of (1,3‐Pr(NH₃)₂)_0.5PbI₃ is demonstrated. X‐ray diffraction (XRD) shows that the new perovskite exhibits a three‐dimensional tetragonal phase. The band gap of the new perovskite is about 1.6 eV, which is desirable for photovoltaic applications. A (1,3‐Pr(NH₃)₂)_0.5PbI₃ perovskite solar cell (PSC) yields a power conversion efficiency (PCE) of 5.1 %. More importantly, this perovskite is composed of a large hydrophobic cation that provides better moisture resistance compared to CH₃NH₃PbI₃ perovskite.     

All‐Inorganic CsPb1−xGexI2Br Perovskite with Enhanced Phase Stability and Photovoltaic Performance

Compared with organic‐inorganic perovskites, all‐inorganic cesium‐based perovskites without volatile organic compounds have gained extensive interests because of the high thermal stability. However, they have a problem on phase transition from cubic phase (active for photo‐electric conversion) to orthorhombic phase (inactive for photo‐electric conversion) at room temperature, which has hindered further progress. Herein, novel inorganic CsPb_1?xGe_xI₂Br perovskites were prepared in humid ambient atmosphere without a glovebox. The phase stability of the all‐inorganic perovskite was effectively enhanced after germanium addition. In addition, the highest power conversion efficiency of 10.8 % with high open‐circuit voltage (V_OC) of 1.27 V in a planar solar cell based on CsPb_0.8Ge_0.2I₂Br perovskite was achieved. Furthermore, the highest V_OC up to 1.34 V was obtained by CsPb_0.7Ge_0.3I₂Br perovskite, which is a remarkable record in the field of all‐inorganic perovskite solar cells. More importantly, all the photovoltaic parameters of CsPb_0.8Ge_0.2I₂Br perovskite solar cells showed nearly no decay after 7 h measurement in 50–60 % relative humidity without encapsulation.     

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 %).     

Ion‐Exchange‐Induced 2D–3D Conversion of HMA1−xFAxPbI3Cl Perovskite into a High‐Quality MA1−xFAxPbI3 Perovskite

High‐quality phase‐pure MA_1?xFA_xPbI₃ planar films (MA=methylammonium, FA=formamidinium) with extended absorption and enhanced thermal stability are difficult to deposit by regular simple solution chemistry approaches owing to crystallization competition between the easy‐to‐crystallize but unwanted δ‐FAPbI₃/MAPbI₃ and FA_xMA_1?xPbI₃ requiring rigid crystallization conditions. Here A 2D–3D conversion to transform compact 2D mixed composition HMA_1?xFA_xPbI₃Cl perovskite precursor films into 3D MA_1?xFA_xPbI₃ (x=0.1–0.9) perovskites is presented. The designed Cl/I and H/FA(MA) ion exchange reaction induced fast transformation of compact 2D perovskite film, helping to form the phase‐pure and high quality MA_1?xFA_xPbI₃ without δ‐FAPbI₃ and MAPbI₃ impurity. In all, we successfully developed a facile one‐step method to fabricate high quality phase‐pure MA_1?xFA_xPbI₃ (x=0.1–0.9) perovskite films by 2D–3D conversion of HMA_1?xFA_xPbI₃Cl perovskite. This 2D–3D conversion is a promising strategy for lead halide perovskite fabrication.     

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 %.     

Lead-free Perovskite Materials (NH4)3Sb2IxBr9−x

A family of perovskite light absorbers (NH₄)₃Sb₂I_xBr_9−x (0≤x≤9) was prepared. These materials show good solubility in ethanol, a low-cost, hypotoxic, and environmentally friendly solvent. The light absorption of (NH₄)₃Sb₂I_xBr_9−x films can be tuned by adjusting I and Br content. The absorption onset for (NH₄)₃Sb₂I_xBr_9−x films changes from 558 nm to 453 nm as x changes from 9 to 0. (NH₄)₃Sb₂I₉ single crystals were prepared, exhibiting a hole mobility of 4.8 cm² V⁻¹ s⁻¹ and an electron mobility of 12.3 cm² V⁻¹ s⁻¹. (NH₄)₃Sb₂I₉ solar cells gave an open-circuit voltage of 1.03 V and a power conversion efficiency of 0.51 %.     

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.     

Spontaneous Crystallization of Perovskite Nanocrystals in Nonpolar Organic Solvents: A Versatile Approach for their Shape‐Controlled Synthesis

The growing demand for perovskite nanocrystals (NCs) for various applications has stimulated the development of facile synthetic methods. Perovskite NCs have often been synthesized by either ligand‐assisted reprecipitation (LARP) at room temperature or by hot‐injection at high temperatures and inert atmosphere. However, the use of polar solvents in LARP affects their stability. Herein, we report on the spontaneous crystallization of perovskite NCs in nonpolar organic media at ambient conditions by simple mixing of precursor–ligand complexes without application of any external stimuli. The shape of the NCs can be controlled from nanocubes to nanoplatelets by varying the ratio of monovalent (e.g. formamidinium⁺ (FA⁺) and Cs⁺) to divalent (Pb²⁺) cation–ligand complexes. The precursor–ligand complexes are stable for months, and thus perovskite NCs can be readily prepared prior to use. Moreover, we show that this versatile synthetic process is scalable and generally applicable for perovskite NCs of different compositions.     

Sn‐Pb Binary Perovskite Films with High Crystalline Quality for High Performance Solar Cells

As an environmentally friendly perovskite material with low bandgap, Tin (Sn)‐based perovskite has drawn much attention. A simple and effective method for fabricating high‐quality Sn‐Pb binary perovskite film is highly desired. Here, with methylammonium chloride (MACl) post‐treatment to assist vertical recrystallization, we fabricated high quality FA_0.75Cs_0.25Pb_0.5Sn_0.5I₃ perovskite film via one‐step processing method. This recrystallization method was first used in Sn‐based perovskite. The obtained film consists of vertically aligned grains with high crystallinity, which contributes to a power conversion efficiency (PCE) of 14.03% in corresponding perovskite solar cell (PVSC). The cells maintained 80% of their initial PCEs after being stored for 30 d in glove‐box. This simple, effective method provides an easy way to fabricate high performance Sn‐Pb binary PVSC.     

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.