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.     

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.     

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.     

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.     

Homogenous Alloys of Formamidinium Lead Triiodide and Cesium Tin Triiodide for Efficient Ideal‐Bandgap Perovskite Solar Cells

The alloying behavior between FAPbI₃ and CsSnI₃ perovskites is studied carefully for the first time, which has led to the realization of single‐phase hybrid perovskites of (FAPbI₃)_1−x (CsSnI₃)_x (0<x <1) compositions with anomalous bandgaps. (FAPbI₃)_1−x (CsSnI₃)_x perovskites exhibit stable, homogenous composition/microstructure at the nanoscale, as confirmed by microscopic characterization. The ideal bandgap of 1.3 eV for single‐junction solar cell operation is achieved in the rationally‐tailored (FAPbI₃)_0.7(CsSnI₃)_0.3‐composition perovskite. Solar cells based on this new perovskite show power conversion efficiency up to 14.6 %.     

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.     

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.     

Volatile Perovskite Precursor Ink Enables Window Printing of Phase-Pure FAPbI3 Perovskite Solar Cells and Modules in Ambient Atmosphere

Despite the great success of perovskite photovoltaics in terms of device efficiency and stability using laboratory-scale spin-coating methods, the demand for high-throughput and cost-effective solutions remains unresolved and rarely reported because of the complicated nature of perovskite crystallization. In this work, we propose a stable precursor ink design strategy to control the solvent volatilization and perovskite crystallization to enable the wide speed window printing (0.3 to 18.0 m/min) of phase-pure FAPbI₃ perovskite solar cells (pero-SCs) in ambient atmosphere. The FAPbI₃ perovskite precursor ink uses volatile acetonitrile (ACN) as the main solvent with DMF and DMSO as coordination additives is beneficial to improve the ink stability, inhibit the coffee rings, and the complicated intermediate FAPbI₃ phases, delivering high-quality pin-hole free and phase-pure FAPbI₃ perovskite films with large-scale uniformity. Ultimately, small-area FAPbI₃ pero-SCs (0.062 cm²) and large-area modules (15.64 cm²) achieved remarkable efficiencies of 24.32 % and 21.90 %, respectively, whereas the PCE of the devices can be maintained at 23.76 % when the printing speed increases to 18.0 m/min. Specifically, the unencapsulated device exhibits superior operational stability with T₉₀>1350 h. This work represents a step towards the scalable, cost-effective manufacturing of perovskite photovoltaics with both high performance and high throughput.     

Benign‐by‐Design Solventless Mechanochemical Synthesis of Three‐, Two‐, and One‐Dimensional Hybrid Perovskites

Organic–inorganic hybrid perovskites have attracted significant attention owing to their extraordinary optoelectronic properties with applications in the fields of solar energy, lighting, photodetectors, and lasers. The rational design of these hybrid materials is a key factor in the optimization of their performance in perovskite‐based devices. Herein, a mechanochemical approach is proposed as a highly efficient, simple, and reproducible method for the preparation of four types of hybrid perovskites, which were obtained in large amounts as polycrystalline powders with high purity and excellent optoelectronics properties. Two archetypal three‐dimensional (3D) perovskites (MAPbI₃ and FAPbI₃) were synthesized, together with a bidimensional (2D) perovskite (Gua₂PbI₄) and a “double‐chain” one‐dimensional (1D) perovskite (GuaPbI₃), whose structure was elucidated by X‐ray diffraction.     

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.     

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.     

Dithieno[3,2‐b:2′,3′‐d]pyrrole Cored p‐Type Semiconductors Enabling 20 % Efficiency Dopant‐Free Perovskite Solar Cells

Organic p‐type semiconductors with tunable structures offer great opportunities for hybrid perovskite solar cells (PVSCs). We report herein two dithieno[3,2‐b:2′,3′‐d]pyrrole (DTP) cored molecular semiconductors prepared through π‐conjugation extension and an N‐alkylation strategy. The as‐prepared conjugated molecules exhibit a highest occupied molecular orbital (HOMO) level of ?4.82 eV and a hole mobility up to 2.16×10^?4 cm² V^?1 s^?1. Together with excellent film‐forming and over 99 % photoluminescence quenching efficiency on perovskite, the DTP based semiconductors work efficiently as hole‐transporting materials (HTMs) for n‐i‐p structured PVSCs. Their dopant‐free MA_0.7FA_0.3PbI_2.85Br_0.15 devices exhibit a power conversion efficiency over 20 %, representing one of the highest values for un‐doped molecular HTMs based PVSCs. This work demonstrates the great potential of using a DTP core in designing efficient semiconductors for dopant‐free PVSCs.     

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.     

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.     

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.     

High‐Purity Inorganic Perovskite Films for Solar Cells with 9.72 % Efficiency

All‐inorganic perovskite solar cells with high efficiency and improved stability are promising for commercialization. A multistep solution‐processing method was developed to fabricate high‐purity inorganic CsPbBr₃ perovskite films for use in efficient solar cells. By tuning the number of deposition cycles (n) of a CsBr solution, the phase conversion from CsPb₂Br₅ (n ≤3), to CsPbBr₃ (n=4), and Cs₄PbBr₆ (n≥5) was optimized to achieve vertical‐ and monolayer‐aligned grains. Upon interfacial modification with graphene quantum dots, the all‐inorganic perovskite solar cell (without a hole‐transporting layer) achieved a power conversion efficiency (PCE) as high as 9.72 % under standard solar illumination conditions. Under challenging conditions, such as 90 % relative humidity (RH) at 25 °C or 80 °C at zero humidity, the optimized device retained 87 % PCE over 130 days or 95 % over 40 days, compared to the initial efficiency.     

Mixed‐Organic‐Cation Perovskite Photovoltaics for Enhanced Solar‐Light Harvesting

Hybrid organic–inorganic lead halide perovskite APbX₃ pigments, such as methylammonium lead iodide, have recently emerged as excellent light harvesters in solid‐state mesoscopic solar cells. An important target for the further improvement of the performance of perovskite‐based photovoltaics is to extend their optical‐absorption onset further into the red to enhance solar‐light harvesting. Herein, we show that this goal can be reached by using a mixture of formamidinium (HN=CHNH₃⁺, FA) and methylammonium (CH₃NH₃⁺, MA) cations in the A position of the APbI₃ perovskite structure. This combination leads to an enhanced short‐circuit current and thus superior devices to those based on only CH₃NH₃⁺. This concept has not been applied previously in perovskite‐based solar cells. It shows great potential as a versatile tool to tune the structural, electrical, and optoelectronic properties of the light‐harvesting materials.     

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 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 %).