Fabrication of Efficient and Stable CsPbI3 Perovskite Solar Cells through Cation Exchange Process

Inorganic lead halide perovskites have attracted attention due to their tolerance to higher processing temperature and higher bandgap suitable for tandem solar cell application. Not only do they improve cell stability and efficiency, they also reveal many interesting and un‐anticipated material qualities. This work reports a simple cation exchange growth (CEG) method for fabricating inorganic high‐quality cesium lead iodide (CsPbI₃) by adding methylammonium iodide (MAI) additive in the precursor. X‐ray diffraction results reveal a multi‐stage film formation process whereby i) MAPbI₃ perovskite first formed that acts as a perovskite template for ii) subsequent ion exchange whereby the MA⁺ ions in the MAPbI₃ are replaced by Cs⁺ (as temperature ramps up) and iii) form g‐phase perovskite CsPbI₃. Optical microscopy, photoluminescence, and electrical characterizations reveal that the CEG process produces high‐quality film with better absorption, uniform and dense film with better interface, lower defects, and better stability. Using the CEG approach, the power conversion efficiency of the best CsPbI₃ solar cell is significantly increased up to 14.1% for the device fabricated using 1.0 m MAI additive. The outcome is beneficial for further improvement of inorganic perovskite solar cells and their application in perovskite‐silicon tandem devices.     

MACl‐Assisted Ge Doping of Pb‐Hybrid Perovskite: A Universal Route to Stabilize High Performance Perovskite Solar Cells

Interfacial engineering, grain boundary, and surface passivation in organic–inorganic hybrid perovskite solar cells (HyPSCs) are effective in achieving high performance and enhanced durability. Organic additives and inorganic doping are generally used to chemically modify the surface contacting charge transport layers, and/or grain boundaries so as to reduce the defect density. Here, a simple but tricky one‐step method to dope organic–inorganic hybrid perovskite with Ge for the first time is reported. Unlike Ge doping to all‐inorganic perovskites, application of GeI₂ in organic–inorganic perovskite precursors is challenging due to the extremely poor solubility of GeI₂ in hybrid perovskite ink, leading to failure in the formation of uniform films. However, it is found that addition of methylammonium chloride (MACl) into the precursor remarkably increases the solubility of GeI₂. This MACl‐assisted Ge doping of hybrid perovskites produces high‐quality crystalline film with its surface passivated with nonvolatile GeI₂ (GeO₂) and the volatile MACl additive also improves the uniformity of GeO₂ distribution in the perovskite films. The resulting Ge‐doped mixed cation and mixed halide perovskite films with composition FA_0.83MA_0.17Ge_0.03Pb_0.97(I_0.9Br_0.1)₃ show superior photoluminescence lifetime, power conversion efficiency above 22%, and greater stability toward illumination and humidity, outperforming photovoltaic properties of HyPSCs prepared without the Ge doping.     

Impact of PbI2 Passivation and Grain Size Engineering in CH3NH3PbI3 Solar Absorbers as Revealed by Carrier‐Resolved Photo‐Hall Technique

With power conversion efficiencies now exceeding 25%, hybrid perovskite solar cells require deeper understanding of defects and processing to further approach the Shockley‐Queisser limit. One approach for processing enhancement and defect reduction involves additive engineering—, e.g., addition of MASCN (MA = methylammonium) and excess PbI₂ have been shown to modify film grain structure and improve performance. However, the underlying impact of these additives on transport and recombination properties remains to be fully elucidated. In this study, a newly developed carrier‐resolved photo‐Hall (CRPH) characterization technique is used that gives access to both majority and minority carrier properties within the same sample and over a wide range of illumination conditions. CRPH measurements on n‐type MAPbI₃ films reveal an order of magnitude increase in carrier recombination lifetime and electron density for 5% excess PbI₂ added to the precursor solution, with little change noted in electron and hole mobility values. Grain size variation (120–2100 nm) and MASCN addition induce no significant change in carrier‐related parameters considered, highlighting the benign nature of the grain boundaries and that excess PbI₂ must predominantly passivate bulk defects rather than defects situated at grain boundaries. This study offers a unique picture of additive impact on MAPbI₃ optoelectronic properties as elucidated by the new CRPH approach.     

Scalable Fabrication of Efficient Perovskite Solar Modules on Flexible Glass Substrates

Perovskite materials are good candidates for flexible photovoltaic applications due to their strong absorption and low‐temperature processing, but efficient flexible perovskite modules have not yet been realized. Here, a record efficiency flexible perovskite solar module is demonstrated by blade coating high‐quality perovskite films on flexible Corning Willow Glass using additive engineering. Ammonium chloride (NH₄Cl) is added into the perovskite precursor solution to retard the nucleation which prevents voids formation at the interface of perovskite and glass. The addition of NH₄Cl also suppresses the formation of PbI₂ and reduces the trap density in the perovskite films. The implementation of NH₄Cl enables the fabrication of single junction flexible perovskite solar devices with an efficiency of 19.72% on small‐area cells and a record aperture efficiency of 15.86% on modules with an area of 42.9 cm². This work provides a simple way to scale up high‐efficiency flexible perovskite modules for various applications.     

Controllable Perovskite Crystallization via Antisolvent Technique Using Chloride Additives for Highly Efficient Planar Perovskite Solar Cells

The presence of surface and grain boundary defects in organic–inorganic halide perovskite films can be detrimental to both the performance and operational stability of perovskite solar cells (PSCs). Here, the effect of chloride additives is studied on the bulk and surface defects of the mixed cation and halide PSCs. It is found that using an antisolvent technique, the perovskite film is divided into two layers, i.e., a bottom layer with large grains and a thin capping layer with small grains. The addition of formamidinium chloride (FACl) into the precursor solution removes the small‐grained perovskite capping layer and suppresses the formation of bulk and surface defects, providing a perovskite film with enhanced crystallinity and large grain size of over 1 µm. Time‐resolved photoluminescence measurements show longer lifetimes for perovskite films modified by FACl and subsequently passivated by 1‐adamantylamine hydrochloride as compared to the reference sample. Impedance spectroscopy measurements show that these treatments reduce the recombination in the PSCs, leading to a champion device with power conversion efficiency (PCE) of 21.2%, an open circuit voltage of 1152 mV and negligible hysteresis. The Cl treated PSC also shows improved operational stability with only 12% PCE loss after 700 h under continuous illumination.     

Impact of Monovalent Cation Halide Additives on the Structural and Optoelectronic Properties of CH3NH3PbI3 Perovskite

The influence of monovalent cation halide additives on the optical, excitonic, and electrical properties of CH₃NH₃PbI₃ perovskite is reported. Monovalent cation halide with similar ionic radii to Pb²⁺, including Cu⁺, Na⁺, and Ag⁺, have been added to explore the possibility of doping. Significant reduction of sub‐bandgap optical absorption and lower energetic disorder along with a shift in the Fermi level of the perovskite in the presence of these cations has been observed. The bulk hole mobility of the additive‐based perovskites as estimated using the space charge limited current method exhibits an increase of up to an order of magnitude compared to the pristine perovskites with a significant decrease in the activation energy. Consequentially, enhancement in the photovoltaic parameters of additive‐based solar cells is achieved. An increase in open circuit voltage for AgI (≈1.02 vs 0.95 V for the pristine) and photocurrent density for NaI‐ and CuBr‐based solar cells (≈23 vs 21 mA cm^?2 for the pristine) has been observed. This enhanced photovoltaic performance can be attributed to the formation of uniform and continuous perovskite film, better conversion, and loading of perovskite, as well as the enhancement in the bulk charge transport along with a minimization of disorder, pointing towards possible surface passivation.     

Efficiency Enhancement of Polymer Solar Cells Based on Poly(3‐hexylthiophene)/Indene‐C70 Bisadduct via Methylthiophene Additive

Photovoltaic performance of polymer solar cells based on poly(3‐hexylthiophene) (P3HT) as the donor and indene‐C₇₀ bisadduct (IC₇₀BA) as the acceptor is improved by adding 3 vol% 3‐methylthiophene (MT) or 3‐hexylthiophene (HT) as processing additives. The results of UV‐vis absorption spectroscopy, X‐ray diffraction analysis and atomic force microscopy indicate that with the MT or HT processing additive, the active layer of the blend of P3HT/IC₇₀BA showed strengthened absorbance, enhanced crystallinity and improved film morphology. The power conversion efficiency (PCE) of the PSCs was improved from 5.80% for the device without the additive to 6.35% for the device with HT additive and to 6.69% with MT additive. The PCE of 6.69% is the top value reported so far for the PSCs based on P3HT.     

Simultaneous Contact and Grain‐Boundary Passivation in Planar Perovskite Solar Cells Using SnO2‐KCl Composite Electron Transport Layer

The performance of perovskite solar cells is sensitive to detrimental defects, which are prone to accumulate at the interfaces and grain boundaries of bulk perovskite films. Defect passivation at each region will lead to reduced trap density and thus less nonradiative recombination loss. However, it is challenging to passivate defects at both the grain boundaries and the bottom charge transport layer/perovskite interface, mainly due to the solvent incompatibility and complexity in perovskite formation. Here SnO₂‐KCl composite electron transport layer (ETL) is utilized in planar perovskite solar cells to simultaneously passivate the defects at the ETL/perovskite interface and the grain boundaries of perovskite film. The K and Cl ions at the ETL/perovskite interface passivate the ETL/perovskite contact. Meanwhile, K ions from the ETL can diffuse through the perovskite film and passivate the grain boundaries. An enhancement of open‐circuit voltage from 1.077 to 1.137 V and a corresponding power conversion efficiency increasing from 20.2% to 22.2% are achieved for the devices using SnO₂‐KCl composite ETL. The composite ETL strategy reported herein provides an avenue for defect passivation to further increase the efficiency of perovskite solar cells.     

Constructing Charge Bridge Path for High-Performance Tin Perovskite Photovoltaics

Tin-based perovskite solar cells (TPSCs) have attracted significant research interest due to their exceptional optoelectronic properties and environmentally friendly characteristics. However, TPSCs with ideal bandgap suffer from substantial current losses, necessitating the development of innovative interface engineering strategies to enhance device performance. In this study, an unprecedented approach constructing charge transfer path is presented by a simple post-growth treatment of 3-Aminomethylbenzo[b]thiophene (3-AMBTh) on the perovskite film. The selective reaction of 3-AMBTh with exposed FA+ on the perovskite surface suppresses the formation of iodine vacancy defects, leading to a reduction in trap density. Additionally, the residual aromatic rings on the surface form an effective π–π stacking interaction system with subsequently deposited ICBA, facilitating enhanced charge transfer at the interface. By harnessing the potential of the charge transfer path, the TPSCs exhibit remarkable device efficiency of up to 14.53%, positioning them among the top-performing TPSCs reported to date.     

Simultaneous Improvement of Photovoltaic Performance and Stability by In Situ Formation of 2D Perovskite at (FAPbI3)0.88(CsPbBr3)0.12/CuSCN Interface

To solve the stability issues of perovskite solar cells (PSC), here a novel interface engineering strategy that a versatile ultrathin 2D perovskite (5‐AVA)₂PbI₄ (5‐AVA = 5‐ammoniumvaleric acid) passivation layer that is in situ incorporated at the interface between (FAPbI₃)_0.88(CsPbBr₃)_0.12 and the hole transporting CuSCN is reported. Surface analysis using X‐ray photoelectron spectroscopy confirms the formation of 2D perovskite. Hysteresis is reduced by the interfacial 2D layer, which could be ascribed to improvement of interfacial charge extraction efficiency, associated with suppression of recombination. Moreover, introduction of the interface passivating layer enhances the moisture stability and photostability as compared to the control perovskite film due to hydrophobic nature of 2D perovskite. The unencapsulated device retains 98% of the initial power conversion efficiency (PCE) after 63 d under moisture exposure of about 10% in the dark. A PCE of the control device is boosted from 13.72 to 16.75% as a consequence of enhanced open‐circuit voltage (V_oc) and fill factor along with slightly increased short‐circuit current density (J_sc), which results from reduced trap states of (FAPbI₃)_0.88(CsPbBr₃)_0.12 as evidenced by enhanced carrier lifetimes and charge extraction. The perovskite/hole transport material interface engineering gives insight into simultaneous improvements of PCE and device stability.     

Benzylamine‐Treated Wide‐Bandgap Perovskite with High Thermal‐Photostability and Photovoltaic Performance

Mixed iodide‐bromide organolead perovskites with a bandgap of 1.70–1.80 eV have great potential to boost the efficiency of current silicon solar cells by forming a perovskite‐silicon tandem structure. Yet, the stability of the perovskites under various application conditions, and in particular combined light and heat stress, is not well studied. Here, FA_0.15Cs_0.85Pb(I_0.73Br_0.27)₃, with an optical bandgap of ≈1.72 eV, is used as a model system to investigate the thermal‐photostability of wide‐bandgap mixed halide perovskites. It is found that the concerted effect of heat and light can induce both phase segregation and decomposition in a pristine perovskite film. On the other hand, through a postdeposition film treatment with benzylamine (BA) molecules, the highly defective regions (e.g., film surface and grain boundaries) of the film can be well passivated, thus preventing the progression of decomposition or phase segregation in the film. Besides the stability improvement, the BA‐modified perovskite solar cells also exhibit excellent photovoltaic performance, with the champion device reaching a power conversion efficiency of 18.1%, a stabilized power output efficiency of 17.1% and an open‐circuit voltage (V _oc) of 1.24 V.     

Efficient Perovskite Solar Cells by Reducing Interface‐Mediated Recombination: a Bulky Amine Approach

The presence of non‐radiative recombination at the perovskite surface/interface limits the overall efficiency of perovskite solar cells (PSCs). Surface passivation has been demonstrated as an efficient strategy to suppress such recombination in Si cells. Here, 1‐naphthylmethylamine iodide (NMAI) is judiciously selected to passivate the surface of the perovskite film. In contrast to the popular phenylethylammonium iodide, NMAI post‐treatment primarily leaves NMAI salt on the surface of the perovskite film. The formed NMAI layer not only efficiently decreases the defect‐assisted recombination for chemical passivation, but also retards the charge accumulation of energy level mis‐alignment for vacuum level bending and prevents minority carrier recombination due to the charge‐blocking effect. Consequently, planar PSCs with high efficiency of 21.04% and improved long‐term stability (98.9% of the initial efficiency after 3240 h) are obtained. Moreover, open‐circuit voltage as high as 1.20 V is achieved at the absorption threshold of 1.61 eV, which is among the highest reported values in planar PSCs. This work provides new insights into the passivation mechanisms of organic ammonium salts and suggests future guidelines for developing improved passivation layers.     

Spontaneous Interface Ion Exchange: Passivating Surface Defects of Perovskite Solar Cells with Enhanced Photovoltage

Interface engineering is of great concern in photovoltaic devices. For the solution‐processed perovskite solar cells, the modification of the bottom surface of the perovskite layer is a challenge due to solvent incompatibility. Herein, a Cl‐containing tin‐based electron transport layer; SnO_x‐Cl, is designed to realize an in situ, spontaneous ion‐exchange reaction at the interface of SnO_x‐Cl/MAPbI₃. The interfacial ion rearrangement not only effectively passivates the physical contact defects, but, at the same time, the diffusion of Cl ions in the perovskite film also causes longitudinal grain growth and further reduces the grain boundary density. As a result, an efficiency of 20.32% is achieved with an extremely high open‐circuit voltage of 1.19 V. This versatile design of the underlying carrier transport layer provides a new way to improve the performance of perovskite solar cells and other optoelectronic devices.     

Understanding Molecular Structures of Buried Interfaces in Halide Perovskite Photovoltaic Devices Nondestructively with Sub‐Monolayer Sensitivity Using Sum Frequency Generation Vibrational Spectroscopy

As performance of halide perovskite devices progresses, the device structure becomes more complex with more layers. Molecular interfacial structures between different layers play an increasingly important role in determining the overall performance in a halide perovskite device. However, current understanding of such interfacial structures at a molecular level nondestructively is limited, partially due to a lack of appropriate analytical tools to probe buried interfacial molecular structures in situ. Here, sum frequency generation (SFG) vibrational spectroscopy, a state‐of‐the‐art nonlinear interface sensitive spectroscopy, is introduced to the halide perovskite research community and is presented as a powerful tool to understand molecule behavior at buried halide perovskite interfaces in situ. It is found that interfacial molecular orientations revealed by SFG can be directly correlated to halide perovskite device performance. Here how SFG can examine molecular structures (e.g., orientations) at the perovskite/hole transporting layer and perovskite/electron transporting layer interfaces is discussed. This will promote the use of SFG to investigate molecular structures of buried interfaces in various halide perovskite materials and devices in situ nondestructively with a sub‐monolayer interface sensitivity. Such research will help to elucidate structure–function relationships of buried interfaces, aiding in the rational design/development of halide perovskite materials/devices with improved performance.     

In Situ Analysis of Solvent and Additive Effects on Film Morphology Evolution in Spin‐Cast Small‐Molecule and Polymer Photovoltaic Materials

To elucidate the details of film morphology/order evolution during spin‐coating, solvent and additive effects are systematically investigated for three representative organic solar cell (OSC) active layer materials using combined in situ grazing incidence wide angle x‐ray scattering (GIWAXS) and optical reflectance. Two archetypical semiconducting donor (p‐type) polymers, P3HT and PTB7, and semiconducting donor small‐molecule, p‐DTS(FBTTh₂)₂ are studied using three neat solvents (chloroform, chlorobenzene, 1,2‐dichlorobenzene) and four processing additives (1‐chloronaphthalene, diphenyl ether, 1,8‐diiodooctane, and 1,6‐diiodohexane). In situ GIWAXS identifies several trends: 1) for neat solvents, rapid crystallization occurs that risks kinetically locking the material into multiple crystal structures or crystalline orientations; and 2) for solvent + additive processed films, morphology evolution involves sequential transformations on timescales ranging from seconds to hours, with key divergences dependent on additive/semiconductor molecular interactions. When π‐planes dominate the additive/semiconductor interactions, both polymers and small molecule films follow similar evolutions, completing in 1–5 min. When side chains dominate the additive/semiconductor interactions, polymer film maturation times are up to 9 h, while initial crystallization times <10 s are observed for small‐molecule films. This study offers guiding information on OSC donor intermediate morphologies, evolution timescales, and divergent evolutions that can inform OSC manufacture.     

Electrospray‐Assisted Fabrication of Moisture‐Resistant and Highly Stable Perovskite Solar Cells at Ambient Conditions

An electrospray deposition technique to fabricate a perovskite (CH₃NH₃PbI₃) layer for highly stable and efficient perovskite solar cells at ambient humidity (30%–50% relative humidity) conditions is demonstrated. A detailed study is conducted to determine the effect of different electrospray parameters on the device performance and to provide a mechanistic explanation of the superior stability of the films. Due to the controlled reactivity that results in the formation of a smooth perovskite film, these cells exhibit stability exceeding 4000 h, in contrast to much lower stability of those fabricated by conventional spin coating methods. Furthermore, the perovskite film deposited by electrospray methods exhibits a self‐healing behavior when exposed to moisture. The authors hypothesize the formation of an intermediate metastable phase and smooth morphology of the film as the reason for this enhanced stability. Electrospray is a scalable technique that provides precise control over the amount of material required for deposition, reducing significant material loss that occurs in conventional solution‐based methods. Overall, this work shows that stability of perovskite solar cells can be improved by fabrication using a well controlled and optimized electrospray technique, without the use of any additives or cell encapsulants.     

Zwitterions for Organic/Perovskite Solar Cells,Light‐Emitting Devices,and Lithium Ion Batteries: Recent Progress and Perspectives

Zwitterions, a class of materials that contain covalently bonded cations and anions, have been extensively studied in the past decades owing to their special features, such as excellent solubility in polar solvents, for solution processing and dipole formation for the transfer of carriers and ions. Recently, zwitterions have been developed as electrode modifiers for organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light‐emitting devices (OLEDs), as well as electrolyte additives for lithium ion batteries (LIBs). With the rapid advances of zwitterionic materials, high‐performance devices have been constructed with enhanced efficiencies by introducing them as interface layers and electrolyte additives. In this review, recent progress in OSCs, PVSCs, OLEDs, and LIBs by using zwitterions is highlighted. The authors also elaborate the role of various zwitterionic materials as interfacial layers and additives for highly efficient OSCs, PVSCs, OLEDs, and LIBs. This article presents an overview of device performance of zwitterionic materials. The structure–property relationship is also discussed. Finally, the prospects of zwitterion materials are also addressed.     

Progress of Surface Science Studies on ABX3‐Based Metal Halide Perovskite Solar Cells

ABX₃ type metal halide perovskite solar cells (PSCs) have shown efficiencies over 25%, rocketing toward their theoretical limit. To gain the full potential of PSCs relies on the understanding of the device working mechanisms and recombination, the material quality, and the match of energy levels in the device stacks. In this review, the importance of designing PSCs from the viewpoint of surface/interface science studies is presented. For this purpose, recent case studies are discussed to demonstrate how probing of local heterogeneities (e.g., grains, grain boundaries, atomic structure, etc.) in perovskites by surface science techniques can help correlate material properties and PSC device performance. At the solar cell device level with active areas larger than millimeter scale, the ensemble average measurement techniques can characterize the overall average properties of perovskite films as well as their adjacent layers and provide clues to understand better the solar cell parameters. How generation and healing of electronic defects in perovskite films limit the device efficiency, reproducibility, and stability, and induce the time‐dependent transient behavior in the current‐voltage curves are also the central focus of this review. On the basis of these studies, strategies to further improve efficiency and stability, as well as reducing hysteresis are presented.     

The Role of Charge Selective Contacts in Perovskite Solar Cell Stability

Lead halide perovskite solar cells have rapidly achieved high efficiencies comparable to established commercial photovoltaic technologies. The main focus of the field is now shifting toward improving the device lifetime. Many efforts have been made to increase the stability of the perovskite compound and charge‐selective contacts. The electron and hole selective contacts are responsible for the transport of photogenerated charges out of the solar cell and are in intimate contact with the perovskite absorber. Besides the intrinsic stability of the selective contacts themselves, the interfaces at perovskite/selective contact and metal/selective contact play an important role in determining the overall operational lifetime of perovskite solar cells. This review discusses the impact of external factors, i.e., heat, UV‐light, oxygen, and moisture, and measured conditions, i.e., applied bias on the overall stability of perovskite solar cells (PSCs). The authors summarize and analyze the reported strategies, i.e., material engineering of selective contacts and interface engineering via the introduction of interlayers in the aim of enhancing the device stability of PSCs at elevated temperatures, high humidity, and UV irradiation. Finally, an outlook is provided with an emphasis on inorganic contacts that is believed to be the key to achieving highly stable PSCs.     

Interface Engineering for Highly Efficient and Stable Planar p‐i‐n Perovskite Solar Cells

Organic‐inorganic halide perovskite materials have become a shining star in the photovoltaic field due to their unique properties, such as high absorption coefficient, optimal bandgap, and high defect tolerance, which also lead to the breathtaking increase in power conversion efficiency from 3.8% to over 22% in just seven years. Although the highest efficiency was obtained from the TiO₂ mesoporous structure, there are increasing studies focusing on the planar structure device due to its processibility for large‐scale production. In particular, the planar p‐i‐n structure has attracted increasing attention on account of its tremendous advantages in, among other things, eliminating hysteresis alongside a competitive certified efficiency of over 20%. Crucial for the device performance enhancement has been the interface engineering for the past few years, especially for such planar p‐i‐n devices. The interface engineering aims to optimize device properties, such as charge transfer, defect passivation, band alignment, etc. Herein, recent progress on the interface engineering of planar p‐i‐n structure devices is reviewed. This review is mainly focused on the interface design between each layer in p‐i‐n structure devices, as well as grain boundaries, which are the interfaces between polycrystalline perovskite domains. Promising research directions are also suggested for further improvements.