Stable High-Performance Perovskite Solar Cells via Grain Boundary Passivation

Seed-Assisted Growth of Methylammonium-Free Perovskite for Efficient Inverted Perovskite Solar Cells

The traditional way to stabilize α-phase formamidinium lead triiodide (FAPbI₃ ) perovskite often involves considerable additions of methylammonium (MA) and bromide into the perovskite lattice, leading to an enlarged bandgap and reduced thermal stability. This work shows a seed-assisted growth strategy to induce a bottom-up crystallization of MA-free perovskite, by introducing a small amount of α-CsPbBr₃ /DMSO (5%) as seeds into the pristine FAPbI₃ system. During the initial crystalization period, the typical hexagonal α-FAPbI₃ crystals (containing α-CsPbBr₃ seeds) are directly formed even at ambient temperature, as observed by laser scanning confocal microscopy. It indicates that these seeds can promote the formation and stabilization of α-FAPbI₃ below the thermodynamic phase-transition temperature. After annealing not beyond 100 °C, CsPbBr₃ seeds homogeneously diffused into the entire perovskite layer via an ions exchange process. This work demonstrates an efficiency of 22% with hysteresis-free inverted perovskite solar cells (PSCs), one of the highest performances for MA-free inverted PSCs. Despite absented passivation processes, open-circuit voltage is improved by 100 millivolts compared to the control devices with the same stoichiometry, and long-term operational stability retained 92% under continuous full sun illumination. Going MA-free and low-temperature processes are a new insight for compatibility with tandems or flexible PSCs.

Broad-Band-Enhanced Plasmonic Perovskite Solar Cells with Irregular Silver Nanomaterials

The localized surface plasmon resonance (LSPR) from noble metal nanomaterials (NMs) is a promising solution to approach the theoretical efficiency for photovoltaic devices. However, the plasmon resonance of metal NMs with particular shapes and sizes can only be excited within narrow spectral ranges, which can hardly cover the broad-band solar spectrum. To address this issue, in this article, Ag NMs with irregular shapes and sizes are synthesized and embedded in the electron transport layer of perovskite solar cells. With the outstanding conductivity of Ag NMs, the series resistance and charge transfer resistance of the devices are dramatically decreased. The Ag NMs with larger size could enhance the light-trapping of the devices owing to the far-field light scattering effect. The near-field enhancement by LSPR of Ag NMs with a small size mainly contributes to the promotion of carrier transport and extraction. As a result, broad-band improvements in photovoltaic performance are achieved due to the significant enhancement of light absorption and electrical features. The highest power conversion efficiency of the perovskite solar cells increases from 19.52 to 22.42% after the incorporation of Ag NMs.

Interfacial Energy Band Alignment Enables the Reduction of Potential Loss for Hole-Conductor-Free Printable Mesoscopic Perovskite Solar Cells

Perovskite solar cells (PSCs) have achieved high efficiencies with diversified device architectures. In particular, printable mesoscopic PSC has attracted intensive research attention due to its simple fabrication process and superior stability. However, in the absence of hole conductors, the unfavorable energy band alignment between the perovskite and the carbon electrode usually leads to the reduction of device performance, especially the open-circuit voltage (V_OC). Here, a p-type molecule, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is utilized to post-treat the perovskite/carbon interface, which benefits the charge transfer and suppresses the charge recombination within the device. As a result, the post-treated device delivers a power conversion efficiency of 18.05% with an enhanced V_OC of 1044 mV. This work provides a facile method for tuning the interfacial energy band alignment and improving performance of printable mesoscopic PSCs.

Synergy Effect of a π-Conjugated Ionic Compound: Dual Interfacial Energy Level Regulation and Passivation to Promote Voc and Stability of Planar Perovskite Solar Cells

Defects and energy offsets at the bulk and heterojunction interfaces of perovskite are detrimental to the efficiency and stability of perovskite solar cells (PSCs). Herein, we designed an amphiphilic π-conjugated ionic compound (QAPyBF₄ ), implementing simultaneous defects passivation and interface energy level alignments. The p-type conjugated cations passivated the surface trap states and optimized energy alignment at the perovskite/hole transport layer. The highly electronegative [BF₄ ]^- enriched at the SnO₂ interface featured desired band alignment due to the dipole moment of this interlayer. The planar n-i-p PSC had an efficiency of 23.1 % with V_oc of 1.2 V. Notably, the synergy effect elevated the intrinsic endothermic decomposition temperature of the perovskite. The modified devices showed excellent long-term thermal (85 °C) and operational stability at the maximum power point for 1000 h at 45 °C under continuous one-sun illumination with no appreciable efficiency loss.

Surface-Anchored Acetylcholine Regulates Band-Edge States and Suppresses Ion Migration in a 21%-Efficient Quadruple-Cation Perovskite Solar Cell

Although incorporating multiple halogen (bromine) anions and alkali (rubidium) cations can improve the open-circuit voltage (V_oc ) of perovskite solar cells (PSCs), severe voltage loss and poor stability have remained pivotal limitations to their further commercialization. In this study, acetylcholine (ACh⁺ ) is anchored to the surface of a quadruple-cation perovskite to provide additional electron states near the valence band maximum of the perovskite surface, thereby enhancing the band alignment and minimizing the V_oc loss significantly. Moreover, the quaternary ammonium and carbonyl units of ACh⁺ passivate the antisite and vacancy defects of the organic/inorganic hybrid perovskite. Because of strong interactions between ACh⁺ and the perovskite, the formation of lead clusters and the migration of halogen anions in the perovskite film are suppressed. As a result, the device prepared with ACh⁺ post-treatment delivers a power conversion efficiency (PCE) (21.56%) and a value of V_oc (1.21 V) that are much higher than those of the pristine device, along with a twofold decrease in the hysteresis index. After storage for 720 h in humid air, the device subjected to ACh⁺ treatment maintained 70% of its initial PCE. Thus, post-treatment with ACh⁺ appears to be a useful strategy for preparing efficient and stable PSCs.

Surface Passivation Using 2D Perovskites toward Efficient and Stable Perovskite Solar Cells

3D perovskite solar cells (PSCs) have shown great promise for use in next-generation photovoltaic devices. However, some challenges need to be addressed before their commercial production, such as enormous defects formed on the surface, which result in severe SRH recombination, and inadequate material interplay between the composition, leading to thermal-, moisture-, and light-induced degradation. 2D perovskites, in which the organic layer functions as a protective barrier to block the erosion of moisture or ions, have recently emerged and attracted increasing attention because they exhibit significant robustness. Inspired by this, surface passivation by employing 2D perovskites deposited on the top of 3D counterparts has triggered a new wave of research to simultaneously achieve higher efficiency and stability. Herein, we exploited a vast amount of literature to comprehensively summarize the recent progress on 2D/3D heterostructure PSCs using surface passivation. The review begins with an introduction of the crystal structure, followed by the advantages of the combination of 2D and 3D perovskites. Then, the surface passivation strategies, optoelectronic properties, enhanced stability, and photovoltaic performance of 2D/3D PSCs are systematically discussed. Finally, the perspectives of passivation techniques using 2D perovskites to offer insight into further improved photovoltaic performance in the future are proposed.

Interfacial Modification via a 1,4-Butanediamine-Based 2D Capping Layer for Perovskite Solar Cells with Enhanced Stability and Efficiency

Organic-inorganic perovskites face the issues of being vulnerable to oxygen and moisture and the trap sites located at the surface and grain boundaries. Integration of two-dimensional (2D) perovskites as a capping layer is an effective route to enhance both photovoltaic efficiency and environmental stability of the three-dimensional (3D) underlayer. Here, we employ 1,4-butanediammonium diiodide (BDADI), which has a short chain length and diammonium cations, to construct a 3D/2D stacking perovskite solar cells (PSCs). The introduction of BDA2+ could passivate surface defects in 3D perovskites by forming 2D Dion-Jacobson (DJ) phase perovskites and effectively suppressing nonradiative recombination, thus resulting in a longer carrier lifetime. The DJ 2D capping layer also regulate the energy level arrangement, enabling a better charge extraction and transport process. In addition, the water-resistance ability of 3D perovskite gets improved because of the hydrophobic characteristic of 1,4-butanediammonium cations. Consequently, the 3D/2D stacking PSCs yield an energy conversion efficiency of 20.32% in company with the enhanced long-term stability.

Unraveling Passivation Mechanism of Imidazolium-Based Ionic Liquids on Inorganic Perovskite to Achieve Near-Record-Efficiency CsPbI2Br Solar Cells

The application of ionic liquids in perovskite has attracted wide-spread attention for its astounding performance improvement of perovskite solar cells (PSCs). However, the detailed mechanisms behind the improvement remain mysterious. Herein, a series of imidazolium-based ionic liquids (IILs) with different cations and anions is systematically investigated to elucidate the passivation mechanism of IILs on inorganic perovskites. It is found that IILs display the following advantages: (1) They form ionic bonds with Cs⁺ and Pb²⁺ cations on the surface and at the grain boundaries of perovskite films, which could effectively heal/reduce the Cs⁺/I^- vacancies and Pb-related defects; (2) They serve as a bridge between the perovskite and the hole-transport-layer for effective charge extraction and transfer; and (3) They increase the hydrophobicity of the perovskite surface to further improve the stability of the CsPbI₂Br PSCs. The combination of the above effects results in suppressed non-radiative recombination loss in CsPbI₂Br PSCs and an impressive power conversion efficiency of 17.02%. Additionally, the CsPbI₂Br PSCs with IILs surface modification exhibited improved ambient and light illumination stability. Our results provide guidance for an in-depth understanding of the passivation mechanism of IILs in inorganic perovskites.

Growth and Degradation Kinetics of Organic-Inorganic Hybrid Perovskite Films Determined by In Situ Grazing-Incidence X-Ray Scattering Techniques

Organic-inorganic halide perovskite (OIHP) solar cells hold a great promise for commercial breakthrough since their power conversion efficiency has been pushed beyond the mark of 25%, making them capable of competing with traditional crystalline silicon solar cells. The key to achieve efficient and stable perovskite solar cells is inherently related to the film morphology. The understanding of the kinetic processes of film formation and degradation opens up possibilities to tailor the film morphology via the regulation of precursor and processing parameters. In situ grazing-incidence X-ray scattering (GIXS) techniques allow for tracking the morphology evolution of thin films at different length scales and with high temporal resolution. In this review, the selected examples for application of in situ grazing-incidence wide-angle X-ray scattering and grazing-incidence small-angle X-ray scattering techniques to the growth and stability of OIHPs are summarized after a brief introduction to both techniques, highlighting particularly the morphological evolution of perovskite films over time. Then the correlated mathematical models are reviewed to give a toolbox for analyzing the mechanisms of film formation and degradation. Thus, an overview on the in situ GIXS methods is linked to the research of OIHP kinetics.

A Systematic Review of Metal Halide Perovskite Crystallization and Film Formation Mechanism Unveiled by In Situ GIWAXS

Metal halide perovskites are of fundamental interest in the research of modern thin-film optoelectronic devices, owing to their widely tunable optoelectronic properties and solution processability. To obtain high-quality perovskite films and ultimately high-performance perovskite devices, it is crucial to understand the film formation mechanisms, which, however, remains a great challenge, due to the complexity of perovskite composition, dimensionality, and processing conditions. Nevertheless, the state-of-the-art in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) technique enables one to bridge the complex information with device performance by revealing the crystallization pathways during the perovskite film formation process. In this review, the authors illustrate how to obtain and understand in situ GIWAXS data, summarize and assess recent results of in situ GIWAXS studies on versatile perovskite photovoltaic systems, aiming at elucidating the distinct features and common ground of film formation mechanisms, and shedding light on future opportunities of employing in situ GIWAXS to study the fundamental working mechanisms of highly efficient and stable perovskite solar cells toward mass production.

Low-Bandgap Organic Bulk-Heterojunction Enabled Efficient and Flexible Perovskite Solar Cells

Lead halide perovskite and organic solar cells (PSCs and OSCs) are considered as the prime candidates currently for clean energy applications due to their solution and low-temperature processibility. Nevertheless, the substantial photon loss in near-infrared (NIR) region and relatively large photovoltage deficit need to be improved to enable their uses in high-performance solar cells. To mitigate these disadvantages, low-bandgap organic bulk-heterojunction (BHJ) layer into inverted PSCs to construct facile hybrid solar cells (HSCs) is integrated. By optimizing the BHJ components, an excellent power conversion efficiency (PCE) of 23.80%, with a decent open-circuit voltage (V_oc ) of 1.146 V and extended photoresponse over 950 nm for rigid HSCs is achieved. The resultant devices also exhibit superior long-term (over 1000 h) ambient- and photostability compared to those from single-component PSCs and OSCs. More importantly, a champion PCE of 21.73% and excellent mechanical durability can also be achieved in flexible HSCs, which is the highest efficiency reported for flexible solar cells to date. Taking advantage of these impressive device performances, flexible HSCs into a power source for wearable sensors to demonstrate real-time temperature monitoring are successfully integrated.

Phthalide and 1-Iodooctadecane Synergistic Optimization for Highly Efficient and Stable Perovskite Solar Cells

The carrier non-radiative recombination and instability of device caused by the inherent defects are main factors limiting development of perovskite solar cells (PSCs). During the fabrication process of a PSC device, perovskite films often produce Pb⁰ and I⁰ defects. This paper reports a strategy for synergistic optimization of perovskite films by defects passivation and surface modification. The doping of phthalide (PT) in the Pb-rich (CH(NH₂ )₂ )_1-x (CH₃ NH₃ )_x PbI₃ film can passivate lead cation defects, and the modification of 1-iodooctadecane (1-IO) can reduce halogen anion defects and improve stability of PSCs owing to its hydrophobicity. The PT and 1-IO optimized device achieves a power conversion efficiency (PCE) of 22.27%. The optimized PSCs remain 93.2% of the initial PCE when placed in air environment (relative humidity of 10%, 25 °C) more than 70 days. The PT and 1-IO synergistic optimization provides a novel strategy for improving the performance and stability of PSCs.

Interface chelation induced by pyridine-based polymer for efficient and durable air-processed perovskite solar cells

Polymer doping is a significant approach to precisely control nucleation and crystal growth of perovskites and enhance electronic quality in perovskite solar cells (PSC) prepared in air. Here, a brand-new self-healing polysiloxane (SHP) with dynamic 2,6-pyridinedicarboxamide (PDCA) coordination units and plenty of hydrogen bonds was designed and incorporated into perovskite films. PDCA units, showing strong intermolecular Pb 2+ -N amido , I - -N pyridyl , and Pb 2+ -O amido coordination interactions, were expected to enhance crystallinity and passivate the grain boundary. In addition, abundant hydrogen bonds in SHP afforded the self-healing of cracks at grain boundaries for fatigue PSCs. Significantly, the doped device demonstrated a champion efficiency of 19.50% with inconspicuous hysteresis, almost rivaling those achieved in control atmosphere. This strategy of heterocyclic-based macromolecular doping in PSCs will pave a way for realizing efficient and durable crystalline semiconductors.

Carrier Transport Layer-Free Perovskite Solar Cells

Power conversion efficiencies (PCEs) of up to 25.5 % have been reported for perovskite solar cells (PSCs). Thus, they have shown great potential for commercial applications. Therefore, simplifying technological process and reducing production costs have been a widespread concern among scientific and industrial communities. In this study, PSCs are prepared with the simplest device architecture (FTO/MAPbI₃ /carbon). A high-quality perovskite film with few interface defects and good carrier transport is obtained by tuning the p-n properties, matching energy levels, and enhancing carrier collection and transport. A PCE of 12.01 % is achieved, which is the best reported to date for this device structure. The device also shows excellent long-term stability, owing to the elimination of charge transport layers and the usage of hydrophobic materials. This study provides a new approach to reduce production costs and simplify production of PSCs.

Benefitting from Synergistic Effect of Anion and Cation in Antimony Acetate for Stable CH3 NH3 PbI3 -Based Perovskite Solar Cell with Efficiency Beyond 21

Both the film quality and the electronic properties of halide perovskites have significant influences on the photovoltaic performance of perovskite solar cells (PSCs) because both of them are closely related to the charge carrier transportation, separation, and recombination processes in PSCs. In this work, an additive engineering strategy using antimony acetate (Sb(Ac)₃ ) is employed to enhance the photovoltaic performance of methylammonium lead iodide (MAPbI₃ )-based PSCs by improving the film quality and optimizing the photoelectronic properties of halide perovskites. It is found that Ac^- and Sb³⁺ of Sb(Ac)₃ play different roles and their synergistic effect contributed to the eventual excellent photovoltaic performance of MAPbI₃ -based PSCs with a power conversion efficiency of above 21%. The Ac^- anions act as a crystal growth controller and are more involved in the improvement of perovskite film morphology. By comparison, Sb³⁺ cations are more involved in the optimization of the electronic structure of perovskites to tailor the energy levels of the perovskite film. Furthermore, with the assistance of Sb(Ac)₃ , MAPbI₃ -based PSCs deliver much improved moisture, air, and thermal stability. This work can provide scientific insights on the additive engineering for improving the efficiency and long-term stability of MAPbI₃ -based PSCs, facilitating the further development of perovskite-based optoelectronics.

Defects in CsPbX3 Perovskite: From Understanding to Effective Manipulation for High-Performance Solar Cells

The rapid development of all inorganic metal perovskite (CsPbX₃ , X represents halogen) materials holds great promise for top-cells in tandem junctions due to their glorious thermal stability and continuous adjustable band gap in a wide range. Due to the presence of defects, the power conversion efficiency (PCE) of CsPbX₃ perovskite solar cells (PSCs) is still substantially below the Shockley-Queisser (SQ) limit. Therefore, it is imperative to have an in-depth understanding of the defects in PSCs, thus to evaluate their impact on device performances and to develop corresponding strategies to manipulate defects in PSCs for further promoting their photoelectric properties. In this review, the latest progress in defect passivation in the CsPbX₃ PSCs field is summarized. Starting from the effect of non-radiative recombination on open circuit voltage (V_oc ) losses, the defect physics, tolerance, self-healing, and the effect of defects on the photovoltaic properties are discussed. Some techniques to identify defects are compared based on quantitative and qualitative analysis. Then, passivation manipulation is discussed in detail, the defect passivation mechanisms are proposed, and the passivation agents in CsPbX₃ thin films are classified. Finally, directions for future research about defect manipulation that will push the field to progress forward are outlined.

Alkali Metal Fluoride-Modified Tin Oxide for n-i-p Planar Perovskite Solar Cells

The practical applications of perovskite solar cells (PSCs) are limited by further improvement of their stability and performance. Additive engineering and interface engineering are promising medicine to cure this stubborn disease. Herein, an alkali metal fluoride as an additive is introduced into the tin oxide (SnO₂) electron transport layer (ETL). The formation of coordination bonds of F^- ions with the oxygen vacancy of Sn⁴⁺ ions decreases the trap-state density and improves the electron mobility; the hydrogen bond interaction between the F ion and amine group (FA⁺) of perovskite inhibits the diffusion of organic cations and promotes perovskite (PVK) stability. Meanwhile, the alkali metal ions (K⁺, Rb⁺, and Cs⁺) permeated into PVK fill the organic cation vacancies and ameliorate the crystal quality of PVK films. Consequently, a SnO₂-based planar PSC exhibits a power conversion efficiency (PCE) of 20.24%, while the PSC modified by CsF achieves a PCE of 22.51%, accompanied by effective enhancement of stability and negligible hysteresis. The research results provide a typical example for low-cost and multifunctional additives in high-performance PSCs.

A Green Lead Recycling Strategy from Used Lead Acid Batteries for Efficient Inverted Perovskite Solar Cells

Lead is widely used as a crucial elemental for lead acid batteries (LABs) and emerging halide perovskite solar cells (PSCs). However, the use of soluble lead will raise environmental concerns. For the purpose of Pb recycling, herein, we report a reactant-recycling strategy to extract Pb from used LABs and synthesize high-purity PbI₂. The recycled PbI₂ shows smaller grain size, higher crystallinity, and higher thermal stability compared to the commercial sources. Perovskite films deposited with the high-quality PbI₂ show larger grain size and fewer defects than the commercial ones. Consequently, the synthesized PbI₂ enables a power conversation efficiency of 20.45% for the inverted MAPbI₃ (MA= methylammonium) PSCs with excellent air stability. This work offers a novel strategy for lead recovery from LABs and a green path for the realization of high-performance PSCs with high defect tolerance.

Synthesis of Lead-Free CaTiO3 Oxide Perovskite Film through Solution Combustion Method and Its Thickness-Dependent Hysteresis Behaviors within 100 mV Operation

Perovskite is attracting considerable interest because of its excellent semiconducting properties and optoelectronic performance. In particular, lead perovskites have been used extensively in photovoltaic, photodetectors, thin-film transistors, and various electronic applications. On the other hand, the elimination of lead is essential because of its strong toxicity. This paper reports the synthesis of lead-free calcium titanate perovskite (CaTiO₃) using a solution-processed combustion method. The chemical and morphological properties of CaTiO₃ were examined as a function of its thickness by scanning electron microscopy, X-ray diffraction (XRD), atomic force microscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible spectrophotometry. The analysis showed that thicker films formed by a cumulative coating result in larger grains and more oxygen vacancies. Furthermore, thickness-dependent hysteresis behaviors were examined by fabricating a metal-CaTiO₃-metal structure. The electrical hysteresis could be controlled over an extremely low voltage operation, as low as 100 mV, by varying the grain size and oxygen vacancies.

Universal Strategy for Improving Perovskite Photodiode Performance: Interfacial Built-In Electric Field Manipulated by Unintentional Doping

Organic-inorganic halide perovskites have demonstrated significant light detection potential, with a performance comparable to that of commercially available photodetectors. In this study, a general design guideline, which is applicable to both inverted and regular structures, is proposed for high-performance perovskite photodiodes through an interfacial built-in electric field (E) for efficient carrier separation and transport. The interfacial E generated at the interface between the active and charge transport layers far from the incident light is critical for effective charge carrier collection. The interfacial E can be modulated by unintentional doping of the perovskite, whose doping type and density can be easily controlled by the post-annealing time and temperature. Employing the proposed design guideline, the inverted and regular perovskite photodiodes exhibit the external quantum efficiency of 83.51% and 76.5% and responsivities of 0.37 and 0.34 A W-1 , respectively. In the self-powered mode, the dark currents reach 7.95 × 10-11 and 1.47 × 10-8 A cm-2 , providing high detectivities of 7.34 × 1013 and 4.96 × 1012 Jones, for inverted and regular structures, respectively, and a long-term stability of at least 1600 h. This optimization strategy is compatible with existing materials and device structures and hence leads to substantial potential applications in perovskite-based optoelectronic devices.

Deep-Level Transient Spectroscopy for Effective Passivator Selection in Perovskite Solar Cells to Attain High Efficiency over 23

Most studies choose passivators essentially in a trial-and-error fashion in an attempt to attain high efficiency in perovskite solar cells (PSCs). Using deep-level transient spectroscopy (DLTS) measurements, the type of defects in perovskite films was determined to guide the passivator selection for PSCs. Three kinds of positively charged defects were found in the target PSC system. Fluorinated phenylethylamine hydroiodide (FPEAI) was chosen to passivate the surface defects due to the electronegativity and hydrophobicity of fluorine. Due to the decreased surface roughness, increased hydrophobicity, lowered defect density, and improved carrier dynamics as observed by ultrafast transient absorption spectroscopy (TAS), a PSC with meta-F-PEAI had the best efficiency over 23 % with open-circuit voltage of 1.155 V and fill factor of 80.15 %. In addition, the long-term stability of the PSC was significantly improved. The present work provides a new means to select the best passivator for different types of defects.

Embedding of Ti3 C2 Tx Nanocrystals in MAPbI3 Microwires for Improved Responsivity and Detectivity of Photodetector

Organic-inorganic hybrid MAPbI₃ microwires show unique optoelectronic properties for high performances of photodetectors (PDs). However, the defects-assisted nonradiative recombination is harmful for carrier transport, which limits the performances improvement of MAPbI₃ microwires PDs. Traditional organic passivation agents effectively combine the surface defects of microwires and also reduce the mobility of overall film based on the perovskite microwires. Therefore, the improvement of internal carrier transport of microwires and the mobility of integrated film simultaneously is a particular challenge for fabrication of performances enhanced perovskite microwires PDs. Here, the Ti₃ C₂ T_x NCs are fabricated by nonfocus laser irradiation in liquid environment, and hybrids the high conductive NCs in the MAPbI₃ microwires. The presence of Ti₃ C₂ T_x NCs renders defects passivation, enhancement of crystalline orientation, charge transport, and carrier extraction for MAPbI₃ microwires, and boots the mobility of microwires based film, leading to about tenfolds enhancement of performances of PDs than that of the control. The maximum responsivity and the detectivity of the Ti₃ C₂ T_x NCs embedded MAPbI₃ microwires PDs reach to 1.70 A W^-1 and 7.0 × 10¹¹ Jones in visible window, respectively. The findings suggest that the laser generated high conductive Ti₃ C₂ T_x NCs is an effective additive for perovskite microwires to fabricate performances enhanced optoelectronics.

A Sodium Chloride Modification of SnO2 Electron Transport Layers to Enhance the Performance of Perovskite Solar Cells

A sodium chloride modification was applied where different amounts of sodium chloride was physically blended in a tin oxide colloid solution to passivate the interface between the electron transport layer (ETL) and perovskite layer and improve the performance of perovskite solar cells. Sodium chloride-modified tin oxide was utilized as the electron transport material to fabricate perovskite solar cells. It was found that sodium chloride-modified tin oxide as an ETL could considerably enhance the performance of the device compared to pristine tin oxide. The power conversion efficiency of the perovskite solar cell displayed 8.8% remarkable improvement from 18.7 ± 0.4% to 20.3 ± 0.3% on average and 9.5% improvement from 18.9 to 20.7% in champion devices because of the considerable enhancement of the fill factor when 25 mM sodium chloride-modified tin oxide as the ETL was used in comparison with pristine tin oxide.

Multifunctional Crosslinking-Enabled Strain-Regulating Crystallization for Stable, Efficient α-FAPbI3 -Based Perovskite Solar Cells

α-Formamidinium lead triiodide (α-FAPbI₃ ) represents the state-of-the-art for perovskite solar cells (PSCs) but experiences intrinsic thermally induced tensile strain due to a higher phase-converting temperature, which is a critical instability factor. An in situ crosslinking-enabled strain-regulating crystallization (CSRC) method with trimethylolpropane triacrylate (TMTA) is introduced to precisely regulate the top section of perovskite film where the largest lattice distortion occurs. In CSRC, crosslinking provides in situ perovskite thermal-expansion confinement and strain regulation during the annealing crystallization process, which is proven to be much more effective than the conventional strain-compensation (post-treatment) method. Moreover, CSRC with TMTA successfully achieves multifunctionality simultaneously: the regulation of tensile strain, perovskite defects passivation with an enhanced open-circuit voltage (V_OC  = 50 mV), and enlarged perovskite grain size. The CSRC approach gives significantly enhanced power conversion efficiency (PCE) of 22.39% in α-FAPbI₃ -based PSC versus 20.29% in the control case. More importantly, the control PSCs' instability factor-residual tensile strain-is regulated into compression strain in the CSRC perovskite film through TMTA crosslinking, resulting in not only the best PCE but also outstanding device stability in both long-term storage (over 4000 h with 95% of initial PCE) and light soaking (1248 h with 80% of initial PCE) conditions.

Efficient and stable inverted perovskite solar cells with very high fill factors via incorporation of star-shaped polymer

Stabilizing high-efficiency perovskite solar cells (PSCs) at operating conditions remains an unresolved issue hampering its large-scale commercial deployment. Here, we report a star-shaped polymer to improve charge transport and inhibit ion migration at the perovskite interface. The incorporation of multiple chemical anchor sites in the star-shaped polymer branches strongly controls the crystallization of perovskite film with lower trap density and higher carrier mobility and thus inhibits the nonradiative recombination and reduces the charge-transport loss. Consequently, the modified inverted PSCs show an optimal power conversion efficiency of 22.1% and a very high fill factor (FF) of 0.862, corresponding to 95.4% of the Shockley-Queisser limited FF (0.904) of PSCs with a 1.59-eV bandgap. The modified devices exhibit excellent long-term operational and thermal stability at the maximum power point for 1000 hours at 45°C under continuous one-sun illumination without any significant loss of efficiency.

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

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

Carrier Diffusion and Recombination Anisotropy in the MAPbI3 Single Crystal

MAPbI₃, one of the archetypical metal halide perovskites, is an exciting semiconductor for a variety of optoelectronic applications. The photoexcited charge-carrier diffusion and recombination are important metrics in optoelectronic devices. Defects in grain interiors and boundaries of MAPbI₃ films cause significant nonradiative recombination energy losses. Besides defect impact, carrier diffusion and recombination anisotropy introduced by structural and electronic discrepancies related to the crystal orientation are vital topics. Here, large-sized MAPbI₃ single crystals (SCs) were grown, with the (110), (112), (100), and (001) crystal planes simultaneously exposed through the adjusting ratios of PbI₂ to methylammonium iodide (MAI). Such MAPbI₃ SCs exhibit a weak n-type semiconductor character, and the Fermi levels of these planes were slightly different, causing a homophylic p-n junction at crystal ledges. Utilizing MAPbI₃ SCs, the photoexcited carrier diffusion and recombination within the crystal planes and around the crystal ledges were investigated through time-resolved fluorescence microscope. It is revealed that both the (110) and (001) planes were facilitated to be exposed with more MAI in the growth solutions, and the photoluminescence (PL) of these planes manifesting a red-shift, longer carrier lifetime, and diffusion length compared with the (100) and (112) planes. A longer carrier diffusion length promoted photorecycling. However, excessive MAI-assisted grown MAPbI₃ SCs could increase the radiative recombination. In addition, it revealed that the carrier excited within the (001) and (112) planes was inclined to diffuse toward each other and was favorable to be extracted out of the grain boundaries or crystal ledges.

Durable Defect Passivation of the Grain Surface in Perovskite Solar Cells with π-Conjugated Sulfamic Acid Additives

Defect passivation has shown an essential role in improving the efficiency and stability of perovskite solar cells (PSCs). Herein, an efficient and low-cost π-conjugated sulfamic acid additive, 4-aminobenzenesulfonic acid (4-ABSA), is used to realize durable defect passivation of PSCs. The incorporation of 4-ABSA not only constructs a compact and smooth perovskite film but is also capable of passivating both negative- and positive-charged defects derived from under-coordinated lead and halogen ions. Besides, the π-conjugated system in 4-ABSA can induce preferred perovskite crystal orientation and stabilize the coordination effect between 4-ABSA and perovskite grains. As a result, the inverted planar PSC incorporated with 4-ABSA additives demonstrates an improved power conversion efficiency (PCE) from 18.25 to 20.32%. Moreover, this 4-ABSA passivation agent also enhances the stability of devices, which retains 83.5% of its initial efficiency under ambient condition at 60 °C after 27 days. This work provides a π-conjugated sulfamic acid for durable defect passivation of perovskite optoelectronic devices.

Interphases, Interfaces, and Surfaces of Active Materials in Rechargeable Batteries and Perovskite Solar Cells

The ever-increasing demand for clean sustainable energy has driven tremendous worldwide investment in the design and exploration of new active materials for energy conversion and energy-storage devices. Tailoring the surfaces of and interfaces between different materials is one of the surest and best studied paths to enable high-energy-density batteries and high-efficiency solar cells. Metal-halide perovskite solar cells (PSCs) are one of the most promising photovoltaic materials due to their unprecedented development, with their record power conversion efficiency (PCE) rocketing beyond 25% in less than 10 years. Such progress is achieved largely through the control of crystallinity and surface/interface defects. Rechargeable batteries (RBs) reversibly convert electrical and chemical potential energy through redox reactions at the interfaces between the electrodes and electrolyte. The (electro)chemical and optoelectronic compatibility between active components are essential design considerations to optimize power conversion and energy storage performance. A focused discussion and critical analysis on the formation and functions of the interfaces and interphases of the active materials in these devices is provided, and prospective strategies used to overcome current challenges are described. These strategies revolve around manipulating the chemical compositions, defects, stability, and passivation of the various interfaces of RBs and PSCs.

Molecularly Engineered Interfaces in Metal Halide Perovskite Solar Cells

Perovskite solar cells (PSCs) have emerged as a promising candidate for next-generation thin-film photovoltaic technology owing to their excellent optoelectronic properties and cost-effectiveness. To gain the full potential of device performance, an in-depth understanding of the surface/interface science is an urgent need. Here, we present a review of molecularly engineered studies on interface modifications of PSCs. We elaborate a systematic classification of the existing optimization techniques employed in molecularly engineered perovskite and interface materials and analyze the insights underlying the reliability issues and functional behaviors. The achievements allow us to highlight the crucial strengths of molecular design for further tailoring of the interfacial properties, mitigating the nonradiative losses, optimizing the device performance, and retarding the degradation process of PSCs. Finally, the remaining challenges and potential development directions of molecularly engineered interfaces for high-performance and stable PSCs are also proposed.

Functionalized Ionic Liquid-Crystal Additive for Perovskite Solar Cells with High Efficiency and Excellent Moisture Stability

Organic-inorganic hybrid perovskite solar cells (PSCs) have emerged as a promising candidate for next-generation solar cells. However, the limited stability of PSCs hampers their practical applications. In this work, for the first time, a functionalized π-conjugated ionic liquid crystal (ILC), 4'-(N,N,N-trimethyl ammonium bromide hexyloxy)-4-cyanobiphenyl (6CNBP-N), is developed as a novel chemical additive to obtain CH₃NH₃PbI₃ (MAPbI₃) PSCs with high efficiency and excellent moisture stability. This 6CNBP-N ILC possesses the characteristics of ionic liquids and liquid crystals. The inclusion of the 6CNBP-N ILC can effectively improve the quality and stability of perovskite films, reduce the trap-state densities, and promote the carrier transport induced by the cyano group (C≡N), a rod-like π-conjugated biphenyl mesogenic unit and quaternary alkylammonium cations (R₄N⁺) in 6CNBP-N. Through this functionalized ILC engineering strategy, the power conversion efficiency (PCE) of PSCs is greatly increased from 18.07% for the control PSC to 20.45% for the PSC with 6CNBP-N along with the depressed hysteresis effect and enhanced moisture stability of PSCs. Our work provides a new strategy for designing functionalized additives for high-performance PSCs.

A Porphyrin-Involved Benzene-1,3,5-Tricarboxamide Dendrimer (Por-BTA) as a Multifunctional Interface Material for Efficient and Stable Perovskite Solar Cells

Surface defects of perovskite films are the major sources of nonradiative recombination which limit the efficiency and stability of perovskite solar cells. Surface passivation represents one of the most efficient strategies to solve this problem. Herein, for the first time we designed a porphyrin-involved benzene-1,3,5-tricarboxamide dendrimer (Por-BTA) as a multifunctional interface material between the interface of the perovskite and the hole-transporting layer (spiro-OMeTAD) for the surface passivation of perovskite films. The results suggested that Por-BTA not only efficiently passivated the perovskite surface defects via the coordination of the exposed Pb²⁺ with the carbonyl unit and basic sites of pyrrole units in Por-BTA but also improved the interface contact and the charge transfer between the perovskite and spiro-OMeTAD ascribed to the strong intermolecular π-π stacking of Por-BTA. It was shown that the PSC devices with the Por-BTA treatment exhibited improved power conversion efficiency with the champion of 22.30% achieved (21.30% for the control devices), which is mainly attributed to the increased short-circuit current density and fill factor. Interestingly, the stability of moisture for the Por-BTA-treated device was also enhanced compared to those without the Por-BTA treatment. This work presents a promising direction toward the design of multifunctional organic molecules as the interface materials to improve the cell performance of PSCs.

Cementitious grain-boundary passivation for flexible perovskite solar cells with superior environmental stability and mechanical robustness

The power conversion effciency (PCE) of flexible perovskite solar cells (PSCs) has increased rapidly, while the mechanical flexibility and environmental stability are still far from satisfactory. Previous studies show the environmental degradation and ductile cracks of perovskite films usually begin at the grain boundaries (GBs). Herein, sulfonated graphene oxide (s-GO) is employed to construct a cementitious GBs by interacting with the [PbI₆]^4- at GBs. The resultant s-GO-[PbI₆]^4- complex can effectively passivate the defects of vacant iodine, and the devices with s-GO exhibit remarkable waterproofness and flexibility due to the tough and water-insoluble GBs. The champion PCE of 20.56% (1.01 cm²) in a device treated with s-GO is achieved. This device retains 90% of its original PCE after 180 d stored in the ambient condition, as well as over 80% retention after 10,000 bending cycles at a curvature radius of 3 mm.

Chemical passivation of the under coordinated Pb2+ defects in inverted planar perovskite solar cells via β-diketone Lewis base additives

Hybrid organic-inorganic perovskite solar cells (PSCs) are promising new generations of solar cells, which is low in cost with high power conversion efficiency (PCE). However, PSCs suffer from structural defects generated from the under coordinated ions at the surface, which limits their photovoltaic performances. Herein we report, two β-diketone Lewis base additives 2,4-pentanedione and 3-methyl-2,4-nonanedione within the chlorobenzene anti-solvent to passivate the surface defects generated from the under coordinated Pb²⁺ ions in CH₃NH₃PbI₃ perovskite films. The incorporation of the two β-diketone passivators could successfully enhance the open-circuit voltage of the PSCs by 52 mV and 17 mV for 3-methyl-2,4-nonanedione and 2,4-pentanedione, respectively, with improved PCE by 45% for 3-methyl-2,4-nonanedione compared to the pristine PSC. This enhancement in the photovoltaic performance of the PSCs can be attributed to passivation of the defects through the interaction between two carbonyl groups of the β-diketone Lewis base additives and the under coordinated Pb²⁺ defects in the perovskite film, which improved the PSCs PCE and stability.

High-Quality Ruddlesden-Popper Perovskite Film Formation for High-Performance Perovskite Solar Cells

In the last decade, perovskite solar cells (PSCs) have undergone unprecedented rapid development and become a promising candidate for a new-generation solar cell. Among various PSCs, typical 3D halide perovskite-based PSCs deliver the highest efficiency but they suffer from severe instability, which restricts their practical applications. By contrast, the low-dimensional Ruddlesden-Popper (RP) perovskite-based PSCs have recently raised increasing attention due to their superior stability. Yet, the efficiency of RP perovskite-based PSCs is still far from that of the 3D counterparts owing to the difficulty in fabricating high-quality RP perovskite films. In pursuit of high-efficiency RP perovskite-based PSCs, it is critical to manipulate the film formation process to prepare high-quality RP perovskite films. This review aims to provide comprehensive understanding of the high-quality RP-type perovskite film formation by investigating the influential factors. On this basis, several strategies to improve the RP perovskite film quality are proposed via summarizing the recent progress and efforts on the preparation of high-quality RP perovskite film. This review will provide useful guidelines for a better understanding of the crystallization and phase kinetics during RP perovskite film formation process and the design and development of high-performance RP perovskite-based PSCs, promoting the commercialization of PSC technology.

Dual Passivation of Perovskite and SnO2 for High-Efficiency MAPbI3 Perovskite Solar Cells

So far, most techniques for modifying perovskite solar cells (PSCs) focus on either the perovskite or electron transport layer (ETL). For the sake of comprehensively improving device performance, a dual-functional method of simultaneously passivating trap defects in both the perovskite and ETL films is proposed that utilizes guidable transfer of Eu3+ in SnO2 to perovskite. Europium ions are distributed throughout the SnO2 film during the formation process of SnO2, and they can diffuse directionally through the SnO2/perovskite interface into the perovskite, while most of the europium ions remain at the interface. Under the synergistic effect of distributed Eu3+ in the SnO2 and aggregated Eu3+ at the interface, the electron mobilities of ETLs are evidently improved. Meanwhile, diffused Eu3+ ions passivate the perovskite to reduce trap densities at the grain boundaries, which can dramatically elevate the open-circuit voltage (V oc) of PSCs. Finally, the mainly PSCs coated on SnO2:Eu3+ ETL achieve a power conversion efficiency of 20.14%. Moreover, an unsealed device degrades by only 13% after exposure to ambient atmosphere for 84 days.

CsPbBrI2 perovskites with low energy loss for high-performance indoor and outdoor photovoltaics

Over the years, the efficiency of inorganic perovskite solar cells (PSCs) has increased at an unprecedented pace. However, energy loss in the device has limited a further increase in efficiency and commercialization. In this work, we used (NH₄)₂C₂O₄·H₂O to treat CsPbBrI₂ perovskite film during spin-coating. The CsPbBrI₂ underwent secondary crystallization to form high quality films with micrometer-scale and low trap density. (NH₄)₂C₂O₄·H₂O treatment promoted charge transfer capacity and reduced the ideal factor. It also dropped the energy loss from 0.80 to 0.64 eV. The resulting device delivered a power conversion efficiency (PCE) of 16.55% with an open-circuit voltage (V_oc) of 1.24 V, which are largely improved compared with the reference device which exhibited a PCE of 13.27% and a V_oc of 1.10 V. In addition, the optimized treated device presented a record indoor PCE of 28.48% under a fluorescent lamp of 1000 lux, better than that of the reference device (19.05%).

Perovskite Solar Cells toward Eco-Friendly Printing

Eco-friendly printing is important for mass manufacturing of thin-film photovoltaic (PV) devices to preserve human safety and the environment and to reduce energy consumption and capital expense. However, it is challenging for perovskite PVs due to the lack of eco-friendly solvents for ambient fast printing. In this study, we demonstrate for the first time an eco-friendly printing concept for high-performance perovskite solar cells. Both the perovskite and charge transport layers were fabricated from eco-friendly solvents via scalable fast blade coating under ambient conditions. The perovskite dynamic crystallization during blade coating investigated using in situ grazing incidence wide-angle X-ray scattering (GIWAXS) reveals a long sol-gel window prior to phase transformation and a strong interaction between the precursors and the eco-friendly solvents. The insights enable the achievement of high quality coatings for both the perovskite and charge transport layers by controlling film formation during scalable coating. The excellent optoelectronic properties of these coatings translate to a power conversion efficiency of 18.26% for eco-friendly printed solar cells, which is on par with the conventional devices fabricated via spin coating from toxic solvents under inert atmosphere. The eco-friendly printing paradigm presented in this work paves the way for future green and high-throughput fabrication on an industrial scale for perovskite PVs.

Nanoscale Architecture of Polymer-Organolead Halide Perovskite Films and the Effect of Polymer Chain Mobility on Device Performance

The integration of polymer chains with organolead halide perovskite (MAPbI₃) films, leading to enhanced stability and electro-optical performance, is critically affected by the molecular weight of chains. The molecular weight determines the mobility and volume of the chains, which affects the crystallization kinetics and, hence, perovskite grain size. The insulating nature of the chains is another critical factor that affects both ion migration and conduction of electronic charge. The combined effect of these factors leads to optimal performance with the use of medium-length chains. A simple model integrating the two effects accurately fits the response of the polymer-perovskite composite. Further characterization results show that the polymer-perovskite films have a three-layer architecture consisting of nanoscale polymer-rich top and bottom layers. These combined results show that the optimization of performance in polymer-perovskite devices depends critically on the size of the chains due to their multiple effects on the perovskite matrix.

Efficient and Stable Perovskite Solar Cells Enabled by Dicarboxylic Acid-Supported Perovskite Crystallization

Defect states at surfaces and grain boundaries as well as poor anchoring of perovskite grains hinder the charge transport ability by acting as nonradiative recombination centers, thus resulting in undesirable phenomena such as low efficiency, poor stability, and hysteresis in perovskite solar cells (PSCs). Herein, a linear dicarboxylic acid-based passivation molecule, namely, glutaric acid (GA), is introduced by a facile antisolvent additive engineering (AAE) strategy to concurrently improve the efficiency and long-term stability of the ensuing PSCs. Thanks to the two-sided carboxyl (-COOH) groups, the strong interactions between GA and under-coordinated Pb²⁺ sites induce the crystal growth, improve the electronic properties, and minimize the charge recombination. Ultimately, champion-stabilized efficiency approaching 22% is achieved with negligible hysteresis for GA-assisted devices. In addition to the enhanced moisture stability of the devices, considerable operational stability is achieved after 2400 h of aging under continuous illumination at maximum power point (MPP) tracking.

Nanoscale light- and voltage-induced lattice strain in perovskite thin films

We report on localized nonlinear lattice deformation and nanoscale structural rearrangement in methylammonium lead triiodide films triggered by the combined action of light and voltage. These effects, revealed by second harmonic piezoresponse force microscopy, are connected with organic cation motion, implicating localized cation migration as a key contributor to perovskite optoelectronic device instability under operating conditions.

Direct Surface Passivation of Perovskite Film by 4-Fluorophenethylammonium Iodide toward Stable and Efficient Perovskite Solar Cells

Passivating the defective surface of perovskite films is becoming a particularly effective approach to further boost the efficiency and stability of their solar cells. Organic ammonium halide salts are extensively utilized as passivation agents in the form of their corresponding 2D perovskites to construct the 2D/3D perovskite bilayer architecture for superior device performance; however, this bilayer device partly suffers from the postannealing-induced destructiveness to the 3D perovskite bulk and charge transport barrier induced by the quantum confinement existing in the 2D perovskite. Hence, developing direct passivation of the perovskite layer by organic ammonium halides for high-performance devices can well address the above-mentioned issues, which has rarely been explored. Herein, an effective passivation strategy is proposed to directly modify the perovskite surface with an organic halide salt 4-fluorophenethylammonium iodide (F-PEAI) without further postannealing. The F-PEAI passivation largely inhibits the formation of the iodine vacancies and thus dramatically reduces the film defects, resulting in a much slower charge trapping process. Consequently, the F-PEAI-modified device achieves a much higher champion efficiency (21%) than that (19.5%) of the control device, which dominantly results from more efficient suppression of interfacial nonradiative recombination and the subsequent decreased recombination losses. Additionally, the F-PEAI-treated device maintains 90% of its initial efficiency after 720 h of humidity aging owing to the enhanced hydrophobicity and decreased trap states, highlighting good ambient stability. These results provide an effective passivation strategy toward efficient and stable perovskite solar cells.

Reconstruction of the (EMIm)xMA1-xPb[(BF4)xI1-x]3 Interlayer for Efficient and Stable Perovskite Solar Cells

Defective grain boundaries (GBs) and surface trap states are detrimental to the efficiency and stability of perovskite solar cells (PSCs). In this research, ionic liquid (IL) is used to control the defect states at the perovskite surface and GBs. The newly formed (EMIm)_xMA_1-xPb[(BF₄)_xI_1-x]₃ interlayer promotes secondary grain growth to diminish GBs; besides, EMIM⁺ and BF₄^- fill the vacancies of MA⁺ and I^- and also passivate undercoordinated Pb²⁺ trap states. The newly formed interface largely reduces the nonradiative recombination, thus enhancing the solar-cell performance to 19.0% (AM 1.5, 1 sun) with higher photovoltage and fill factor than the control device. Due to the hydrophobicity of the (EMIm)_xMA_1-xPb[(BF₄)_xI_1-x]₃ interlayer, the unencapsulated device stability in 30 days is much better than the control device under relative humidity (RH) = 20%. This work highlights IL-induced secondary grain growth and a defect passivation method for efficient and stable PSCs.

Effects of ion migration and improvement strategies for the operational stability of perovskite solar cells

The fundamental factor affecting the stability of perovskite solar cells, ion migration, has been reviewed, which is found to be closely related to the degradation of perovskite solar cells. Characterization methods like impedance spectroscopy and galvanostatic measurement to identify ion migration in perovskite films have been reviewed. The influence of light on ion migration was further discussed, which could largely explain the photo-stability decay in most perovskite solar cells. Finally, several solutions to inhibit ion migration for better operational stability of perovskite solar cells were summarized, including bulk passivation, interface passivation and grain boundary passivation. Several strategies have also been proposed to further improve the stablity of perovskite solar cells.