Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance

The Chemical Design in High-Performance Lead Halide Perovskite: Additive vs Dopant?

Metal halide perovskite solar cells (PSCs) have attracted enormous attention as one of the most promising candidates for next-generation photovoltaics in the past few years. During the development of PSCs, various chemicals have been added to improve film quality and device performance. However, there are still debates about whether these chemicals are additives as removed from the final film or dopants incorporated into the crystal lattice. It is important to clarify whether these added chemicals are additives or dopants when designed for high-quality perovskite films' fabrications. Herein, we summarized several commonly used chemicals for hybrid and all-inorganic perovskites, such as MACl, DMAI, MAAc, and alkali metal cations. The underlying mechanism and their roles during the formation of perovskite films were discussed. In the end, we proposed some conclusive important factors to clarify additives and dopants, which would be helpful for the further chemical design for improving high-performance perovskite devices.

Impedance Spectroscopy of Metal Halide Perovskite Solar Cells from the Perspective of Equivalent Circuits

Impedance spectroscopy (IS) provides a detailed understanding of the dynamic phenomena underlying the operation of photovoltaic and optoelectronic devices. Here we provide a broad summary of the application of IS to metal halide perovskite materials, solar cells, electrooptic and memory devices. IS has been widely used to characterize perovskite solar cells, but the variability of samples and the presence of coupled ionic-electronic effects form a complex problem that has not been fully solved yet. We summarize the understanding that has been obtained so far, the basic methods and models, as well as the challenging points still present in this research field. Our approach emphasizes the importance of the equivalent circuit for monitoring the parameters that describe the response and providing a physical interpretation. We discuss the possibilities of models from the general perspective of solar cell behavior, and we describe the specific aspects and properties of the metal halide perovskites. We analyze the impact of the ionic effects and the memory effects, and we describe the combination of light-modulated techniques such as intensity modulated photocurrent spectroscopy (IMPS) for obtaining more detailed information in complex cases. The transformation of the frequency to time domain is discussed for the consistent interpretation of time transient techniques and the prediction of features of current-voltage hysteresis. We discuss in detail the stability issues and the occurrence of transformations of the sample coupled to the measurements.

A review on two-dimensional (2D) perovskite material-based solar cells to enhance the power conversion efficiency

With perovskite materials, rapid progress in power conversion efficiency (PCE) to reach 25% has gained a significant amount of attention from the solar cell industry. Since the development of solid-state perovskite solar cells, rapid research development and investigation on structure design, device fabrication and fundamental studies have contributed to solid-state perovskite solar cells to be a strong candidate for next-generation solar energy. The promising efficiency with low-cost materials is the key point over the other material-based solar cells. The power conversion efficiency (PCE) of two-dimensional (2D) perovskite materials is yet to be enhanced in order to contest with the 3D perovskite-based solar cells. Their enormous variety compromises better prospects and possibilities for research. Two-dimensional (2D) perovskites play a multi-functional role within a solar cell, such as a capping layer, passivating layer, prime cell absorber, and in a hybrid 3D/2D perovskite-based solar cell absorber. This review summarizes the evolution of solar cells that are based on 2D perovskites and their prominent character in solar cells, along with the significant trends. The fundamental configuration and the optoelectronic characteristics, including the band orientation and the transportation of the charges, are discussed in detail. The 2D perovskites are analyzed to study the confined charges within the inorganic structure due to the dielectric and quantum confinement influence. Furthermore, the importance of cesium cation (Cs⁺) doped with 2D substance (BA)₂(MA₃) PbI₃ approach has been discussed to attain high power conversion efficiency (PCE). These attributes offer an efficient step towards air-stable and small-sized perovskites as a new group of renewable energy sources.

A review on advances in doping with alkali metals in halide perovskite materials

Recent progress in doping of halide perovskite materials (HPM) by using targeted elements has provided a dimension beyond structural and compositional modification, for achieving desired properties and resulting device performance. Herein doping of alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) in three-dimensional HPM is reviewed to lay a particular focus on advances in synthesis, doping-induced changes in optical and electrical properties, and their optoelectronic applications. The introduction of alkali metals in HPM shows an effective route for improved morphology, suppressed ion migration, reduction in non-radiative recombination, passivation of bulk and interface defects, and increased thermal stability. In the end, we provide our perspective that the effect of alkali metal incorporation on the efficiency and stability of HPM should be further investigated via in-situ characterization methods and doped HPM should be considered for more functional applications.

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Insight into the Interface Engineering of a SnO2/FAPbI3 Perovskite Using Lead Halide as an Interlayer: A First-Principles Study

The interfacial properties of the perovskite photovoltaic layer and electron transport layer (ETL) are critical to minimize energy losses of perovskite solar cells (PSCs) induced by interfacial recombination. Herein, the interface engineering of the SnO₂/FAPbI₃ perovskite using PbX₂ (X = Cl, Br, or I) as an interlayer is extensively studied using first-principles calculations. The results reveal that the thickness of the PbI₂ interlayer needs to be finely controlled, which may limit charge transport if there is a large amount of PbI₂ precipitation at the interface. The high lattice mismatch of the PbBr₂ with the SnO₂/FAPbI₃ interface makes PbBr₂ an unfavorable passivation material. Due to the strong coupling of the PbCl₂ with both SnO₂ and FAPbI₃, an efficient electron transport pathway could be built after applying PbCl₂ as an interlayer. Meanwhile, the PbCl₂ interlayer could also effectively passivate interface defects, therefore lowering the energy losses of PSCs.

Polycyclic Arenes Dihydrodinaphthopentacene-based Hole-Transporting Materials for Perovskite Solar Cells Application

In this paper, two D-π-D type compounds, C1 and C2, containing dihydrodinaphthopentacene (DHDNP) as a π-bridge, p-methoxydiphenylamine and p-methoxytriphenylamine groups as the donor groups were synthesized. The four 4-hexylphenyl groups at the sp³ -carbon bridges of DHDNP were acquainted with control morphology and improving solubility. The light absorption, energy level, thermal properties, and application as hole-transporting materials in perovskite solar cells of these compounds were fully investigated. The HOMO/LUMO levels and energy gaps of these DHDNP-based molecules are suitable for use as hole-transporting materials in PSCs. The best power conversion efficiencies of the PVSCs based on the C1 and C2 are 15.96% and 12.86%, respectively. The performance of C1 is comparable to that of the reference compound spiro-OMeTAD (16.38%). Compared with spiro-OMeTAD, the C1-based PVSC device showed good stability, which was slightly decreased to 98.68% of its initial efficiency after 48 h and retained 81% of its original PCE after 334 h without encapsulation. These results reveal the potential usefulness of the DHDNP building block for further development of economical and highly efficient HTMs for PVSCs.

On the Shape-Selected, Ligand-Free Preparation of Hybrid Perovskite (CH3NH3PbBr3) Microcrystals and Their Suitability as Model-System for Single-Crystal Studies of Optoelectronic Properties

Hybrid perovskite materials are one of the most promising candidates for optoelectronic applications, e.g., solar cells and LEDs, which can be produced at low cost compared to established materials. Although this field of research has seen a huge upsurge in the past decade, there is a major lack in understanding the underlying processes, such as shape-property relationships and the role of defects. Our aerosol-assisted synthesis pathway offers the possibility to obtain methylammonium lead bromide (MAPbBr₃) microcrystals from a liquid single source precursor. The differently shaped particles are aligned on several substrates, without using a directing agent or other additives. The obtained particles show good stability under dry conditions. This allows us to characterize these materials and their pure surfaces at the single-crystal level using time- and spatially resolved methods, without any influences of size-dependent effects. By optimizing the precursor for the aerosol process, we were able to eliminate any purification steps and use the materials as processed. In addition, we performed theoretical simulations to deepen the understanding of the underlying processes in the formation of the different crystal facets and their specific properties. The model system presented provides insights into the shape-related properties of MAPbBr₃ single crystals and their directed but ligand-free synthesis.

Interfacial Engineering of Perovskite Solar Cells with Evaporated PbI2 Ultrathin Layers

Perovskite solar cells are one of the most promising thin-film photovoltaic techniques, which have an unprecedented progress in the last decade. It is well-recognized in the perovskite community that nonradiative recombination losses and the open-circuit voltage deficit are the dominant limiting factors to further improve the device efficiency. Recently, multiple groups have reported that lead iodide can effectively passivate both perovskite grain boundaries and the interfaces between perovskite and charge transport layers. However, most of the excess PbI2 was processed with solution methods and formed PbI2 grains, which cannot cover perovskite layers completely. It is also very challenging to spin-coat PbI2 layers directly on perovskites, which requires orthogonal solvents. In this work, we deposit additional PbI2 thin layers directly on perovskite thin films via thermal evaporation. The impact of PbI2 layers on the perovskite thin films and devices is systematically investigated. It was found that the evaporated PbI2 thin films can effectively reduce the nonradiative recombination and enhance the device performance. The optimized thickness of the PbI2 layer was determined to be around 10 nm, which results in a relatively high Voc of 1.18 V and power conversion efficiency of 21.52%.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Progress in Perovskite Solar Cells towards Commercialization-A Review

In recent years, perovskite solar cells (PSCs) have experienced rapid development and have presented an excellent commercial prospect as the PSCs are made from raw materials that are readily and cheaply available depending on simple manufacturing techniques. However, the commercial production and utilization of PSCs remain immature, leading to substantial efforts needed to boost the development of scalable fabrication of PSCs, pilot scale tests, and the establishment of industrial production lines. In this way, the PSCs are expected to be successfully popularized from the laboratory to the photovoltaic market. In this review, the history of power conversion efficiency (PCE) for laboratory-scale PSCs is firstly introduced, and then some methods for maintaining high PCE in the upscaling process is displayed. The achievements in the stability and environmental friendliness of PSCs are also summarized because they are also of significance for commercialization. Finally, this review evaluates the commercialization prospects of PSCs from the economic view and provides a short outlook.

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.

Nanoscale interfacial engineering enables highly stable and efficient perovskite photovoltaics

We present a facile molecular-level interface engineering strategy to augment the long-term operational and thermal stability of perovskite solar cells (PSCs) by tailoring the interface between the perovskite and hole transporting layer (HTL) with a multifunctional ligand 2,5-thiophenedicarboxylic acid. The solar cells exhibited high operational stability (maximum powering point tracking at one sun illumination) with a stabilized T _S80 (the time over which the device efficiency reduces to 80% after initial burn-in) of ≈5950 h at 40 °C and a stabilized power conversion efficiency (PCE) over 23%. The origin of high device stability and performance is correlated to the nano/sub-nanoscale molecular level interactions between ligand and perovskite layer, which is further corroborated by comprehensive multiscale characterization. These results provide insights into the modulation of the grain boundaries, local density of states, surface bandgap, and interfacial recombination. Chemical analysis of aged devices showed that molecular passivation suppresses interfacial ion diffusion and inhibits the photoinduced I₂ release that irreversibly degrades the perovskite. The interfacial engineering strategies enabled by multifunctional ligands can expedite the path towards stable PSCs.

Strain analysis and engineering in halide perovskite photovoltaics

Halide perovskites are a compelling candidate for the next generation of clean-energy-harvesting technologies owing to their low cost, facile fabrication and outstanding semiconductor properties. However, photovoltaic device efficiencies are still below practical limits and long-term stability challenges hinder their practical application. Current evidence suggests that strain in halide perovskites is a key factor in dictating device efficiency and stability. Here we outline the fundamentals of strain within halide perovskites relevant to photovoltaic applications and rationalize approaches to characterize the phenomenon. We examine recent breakthroughs in eliminating the adverse impacts of strain, enhancing both device efficiencies and operational stabilities. Finally, we discuss further challenges and outline future research directions for placing stress and strain studies at the forefront of halide perovskite research. An extensive understanding of strain in halide perovskites is needed, which would allow effective strain management and drive further enhancements in efficiencies and stabilities of perovskite photovoltaics.

Encapsulation Strategies for Highly Stable Perovskite Solar Cells under Severe Stress Testing: Damp Heat, Freezing, and Outdoor Illumination Conditions

A key direction toward managing extrinsic instabilities in perovskite solar cells (PSCs) is encapsulation. Thus, a suitable sealing layer is required for an efficient device encapsulation, preventing moisture and oxygen ingression into the perovskite layer. In this work, a solution-based, low-cost, and commercially available bilayer structure of poly(methyl methacrylate)/styrene-butadiene (PMMA/SB) is investigated for PSCs encapsulation. Encapsulated devices retained 80% of the initial power conversion efficiency (PCE) at 85 °C temperature and 85% relative humidity after 100 h, while reference devices without SB (only PMMA) suffer from rapid and intense degradation after only 2 h, under the same condition. In addition, encapsulated devices retained 95% of the initial PCE under -15 °C freezing temperature after 6 h and retained ∼80% of the initial PCE after immersion in HCl (37%) for 90 min. Moreover, applying an additional aluminum metal sheet on the PMMA/SB protective bilayer leads to the improvement of device stability up to 500 h under outdoor illumination, retaining almost 90% of the initial PCE. Considering the urge to develop reliable, scalable, and simple encapsulation for future large-area PSCs, this work establishes solution-based bilayer encapsulation, which is applicable for flexible solar modules as well as other optoelectronic devices such as light-emitting devices and photodetectors.improvement of device stability up to 500 h under outdoor illumination, retaining almost 90% of the initial PCE. Considering the urge to develop reliable, scalable, and simple encapsulation for future large-area PSCs, this work establishes solution-based bilayer encapsulation, which is applicable for flexible solar modules as well as other optoelectronic devices such as light-emitting devices and photodetectors.

Stable Perovskite Solar Cells with Bulk-Mixed Electron Transport Layer by Multifunctional Defect Passivation

Multifarious electron transport layers (ETLs), especially fullerene derivatives, have been applied in organic-inorganic hybrid perovskite (OIHP) devices owing to their superior optoelectronic properties. However, a PCBM Lewis acid molecule can only passivate the iodine-rich defect sites, which cannot solve the problem of uncoordinated Pb²⁺ and water oxygen erosion due to the high volatility of halide I^- and the hydrophilicity of organic cation MA⁺. Herein, we introduce a Lewis base, TBA-Azo with an electron-donating Azo moiety, and hydrophobic long alkyl chains into the PCBM layer to form a multifunctional bulk-mixed electron transport layer (MBE). PCBM of MBE can combine with iodine-rich trap sites at the surface and grain boundaries of perovskite. TBA-Azo molecules of MBE can passivate uncoordinated Pb²⁺ by forming Lewis adducts and isolate water/oxygen at the perovskite surface with hydrophobic alkyl chains. It results in a decrease of trap densities with 1 order of magnitude, effectively inhibiting both bimolecular and trap-induced recombination and thus elongating the carrier lifetime. The passivation of MBE can effectively improve the open-circuit voltage from 1.05 to 1.10 V. Furthermore, three long carbon chain structures of TBA-Azo in MBE can improve the water-resistant ability of OIHP devices, which can maintain 90% of the original PCE after 500 h at the humidity of 50 ± 10%. We believe that the MBE with multifunctional defect passivation provides a strategy for simultaneously achieving high-performance and high-stability OIHP optoelectronic devices.

Perovskite indoor photovoltaics: opportunity and challenges

With the rapid development of the Internet of Things (IoTs), photovoltaics (PVs) has a vast market supply gap of billion dollars. Moreover, it also puts forward new requirements for the development of indoor photovoltaic devices (IPVs). In recent years, PVs represented by organic photovoltaic cells (OPVs), silicon solar cells, dye-sensitized solar cells (DSSCs), etc. considered for use in IoTs mechanisms have also been extensively investigated. However, there are few reports on the indoor applications of perovskite devices, even though it has the advantages of better performance. In fact, perovskite has the advantages of better bandgap adjustability, lower cost, and easier preparation of large-area on flexible substrates, compared with other types of IPVs. This review starts from the development status of IoTs and investigates the cost, technology, and future trends of IPVs. We believe that perovskite photovoltaics is more suitable for indoor applications and review some strategies for fabricating high-performance perovskite indoor photovoltaic devices (IPVs). Finally, we also put forward a perspective for the long-term development of perovskite IPVs.

Role of Alkali Cations in Stabilizing Mixed-Cation Perovskites to Thermal Stress and Moisture Conditions

Perovskite solar cells (PSCs) based on organic-inorganic hybrid perovskites containing a small fraction of substituted alkali-metal cations have shown remarkable performance and stability. However, the role of these cations is unclear. The thermal- and moisture-induced degradation of FA_1-xCs_xPbI₃ and (FA_1-xCs_x)_1-yRb_yPbI₃ (where FA represents formamidinium, x, y = 0.1, 0.05) is investigated using in situ photoelectron spectroscopy (PES). Both compositions exhibit superior moisture stability compared with methylammonium lead iodide under 9 mbar of water vapor. Ga Kα hard X-ray PES is used to investigate the composition of the perovskites at depths up to 45 nm into the surface. This allows more accurate quantification of the alkali-metal distribution than is possible using conventional X-ray PES. The addition of RbI results in a fairly homogeneous distribution of both Cs⁺ and Rb⁺ in the surface layers (in contrast to surface Cs depletion seen in its absence), together with a marked reduction in surface iodide vacancies. Overall, RbI is found to play a critical role in increasing the thermal stability of FA_1-xCs_xPbI₃ by providing a source of I^- that fills iodine vacancy sites in the perovskite lattice, while Rb⁺ is not substantially incorporated into the perovskite. We suggest that the concomitant increase in ion migration barriers in the surface layers is key to improved PSC performance and long-lasting stability.

Fundamental Flaw in the Current Construction of the TiO2 Electron Transport Layer of Perovskite Solar Cells and Its Elimination

The top-performing perovskite solar cells (efficiency > 20%) generally rely on the use of a nanocrystal TiO₂ electron transport layer (ETL). However, the efficacies and stability of the current stereotypically prepared TiO₂ ETLs employing commercially available TiO₂ nanocrystal paste are far from their maximum values. As revealed herein, the long-hidden reason for this discrepancy is that acidic protons (∼0.11 wt %) always remain in TiO₂ ETLs after high-temperature sintering due to the decomposition of the organic proton solvent (mostly alcohol). These protons readily lead to the formation of Ti-H species upon light irradiation, which act to block the electron transfer at the perovskite/TiO₂ interface. Affront this challenge, we introduced a simple deprotonation protocol by adding a small amount of strong proton acceptors (sodium ethoxide or NaOH) into the common TiO₂ nanocrystal paste precursor and replicated the high-temperature sintering process, which wiped out nearly all protons in TiO₂ ETLs during the sintering process. The use of deprotonated TiO₂ ETLs not only promotes the PCE of both MAPbI₃-based and FA_0.85MA_0.15PbI_2.55Br_0.45-based devices over 20% but also significantly improves the long-term photostability of the target devices upon 1000 h of continuous operation.

Stability of Perovskite Solar Cells: Degradation Mechanisms and Remedies

Inorganic–organic metal halide perovskite light harvester-based perovskite solar cells (PSCs) have come to the limelight of solar cell research due to their rapid growth in efficiency. At present, stability and reliability are challenging aspects concerning the Si-based or thin film-based commercial devices. Commercialization of perovskite solar cells remains elusive due to the lack of stability of these devices under real operational conditions, especially for longer duration use. A large number of researchers have been engaged in an ardent effort to improve the stability of perovskite solar cells. Understanding the degradation mechanisms has been the primary importance before exploring the remedies for degradation. In this review, a methodical understanding of various degradation mechanisms of perovskites and perovskite solar cells is presented followed by a discussion on different steps taken to overcome the stability issues. Recent insights on degradation mechanisms are discussed. Various approaches of stability enhancement are reviewed with an emphasis on reports that complied with the operational standard for practical application in a commercial solar module. The operational stability standard enacted by the International Electrotechnical Commission is especially discussed with reports that met the requirements or showed excellent results, which is the most important criterion to evaluate a device’s actual prospect to be utilized for practical applications in commercial solar modules. An overall understanding of degradation pathways in perovskites and perovskite solar cells and steps taken to overcome those with references including state-of-the-art devices with promising operational stability can be gained from this review.

NMR spectroscopy probes microstructure, dynamics and doping of metal halide perovskites

Solid-state magic-angle spinning NMR spectroscopy is a powerful technique to probe atomic-level microstructure and structural dynamics in metal halide perovskites. It can be used to measure dopant incorporation, phase segregation, halide mixing, decomposition pathways, passivation mechanisms, short-range and long-range dynamics, and other local properties. This Review describes practical aspects of recording solid-state NMR data on halide perovskites and how these afford unique insights into new compositions, dopants and passivation agents. We discuss the applicability, feasibility and limitations of ¹H, ¹³C, ¹⁵N, ¹⁴N, ¹³³Cs, ⁸⁷Rb, ³⁹K, ²⁰⁷Pb, ¹¹⁹Sn, ¹¹³Cd, ²⁰⁹Bi, ¹¹⁵In, ¹⁹F and ²H NMR in typical experimental scenarios. We highlight the pivotal complementary role of solid-state mechanosynthesis, which enables highly sensitive NMR studies by providing large quantities of high-purity materials of arbitrary complexity and of chemical shifts calculated using density functional theory. We examine the broader impact of solid-state NMR on materials research and how its evolution over seven decades has benefitted structural studies of contemporary materials such as halide perovskites. Finally, we summarize some of the open questions in perovskite optoelectronics that could be addressed using solid-state NMR. We, thereby, hope to stimulate wider use of this technique in materials and optoelectronics research.

Two-Step Chemical Vapor Deposition-Synthesized Lead-Free All-Inorganic Cs3Sb2Br9 Perovskite Microplates for Optoelectronic Applications

Lead-based halide perovskites (APbX₃, where A = organic or inorganic cation, X = Cl, Br, I) are suitable materials for many optoelectronic devices due to their many attractive properties. However, the concern of lead toxicity and the poor ambient and operational stability of the organic cation group greatly limit their practical utilization. Therefore, there has recently been great interest in lead-free, environment-friendly all-inorganic halide perovskites (IHPs). Sb and Sn are common species suggested to replace Pb for Pb-free IHPs. However, the large difference in the melting points of the precursor materials (e.g., CsBr and SbBr₃ precursors for Cs₃Sb₂Br₉) makes the chemical vapor deposition (CVD) growth of high-quality Pb-free IHPs a very challenging task. In this work, we developed a two-step CVD method to overcome this challenge and successfully synthesized Pb-free Cs₃Sb₂Br₉ perovskite microplates. Cs₃Sb₂Br₉ microplates ∼25 μm in size with the exciton absorption peak at ∼2.8 eV and a band gap of ∼2.85 eV were obtained. The microplates have a smooth hexagonal morphology and show a large Stokes shift of ∼450 meV and exciton binding energy of ∼200 meV. To demonstrate the applications of these microplates in optoelectronics, simple photoconductive devices were fabricated. These photodetectors exhibit a current on/off ratio of 2.36 × 10², a responsivity of 36.9 mA/W, and a detectivity of 1.0 × 10¹⁰ Jones with a fast response of rise and decay time of 61.5 and 24 ms, respectively, upon 450 nm photon irradiation. Finally, the Cs₃Sb₂Br₉ microplates also show good stability in ambient air without encapsulation. These results demonstrate that the 2-step CVD process is an effective approach to synthesize high-quality all-inorganic lead-free Cs₃Sb₂Br₉ perovskite microplates that have the potential for future high-performance optoelectronic device applications.

Accelerated design of promising mixed lead-free double halide organic-inorganic perovskites for photovoltaics using machine learning

Mixed double halide organic-inorganic perovskites (MDHOIPs) exhibit both good stability and high power conversion efficiency and have been regarded as attractive photovoltaic materials. Nevertheless, due to the complexity of structures, large-scale screening of thousands of possible candidates remains a great challenge. In this work, advanced machine learning (ML) techniques and first-principles calculations were combined to achieve a rapid screening of MDHOIPs for solar cells. Successfully, 204 stable lead-free MDHOIPs with optimal bandgaps were selected out of 11 370 candidates. The accuracy of ML models for perovskite structure formability and bandgap is over 94% and 97%, respectively. Moreover, representative MDHOIP candidates, MA₂GeSnI₄Br₂ and MA₂InBiI₂Br₄, stand out with suitable direct bandgaps, light carrier effective masses, small exciton binding energies, strong visible light absorption, and good stability against decomposition.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Crystal-Size-Induced Band Gap Tuning in Perovskite Films

A comprehensive picture explaining the effect of the crystal size in metal halide perovskite films on their opto‐electronic characteristics is currently lacking. We report that perovskite nanocrystallites exhibit a wider band gap due to concurrent quantum confinement and size dependent structural effects, with the latter being remarkably distinct and attributed to the perturbation from the surface of the nanocrystallites affecting the structure of their core. This phenomenon might assist in the photo‐induced charge separation within the perovskite in devices employing mesoporous layers as they restrict the size of nanocrystallites present in them. We demonstrate that the crystal size effect is widely applicable as it is ubiquitous in different compositions and deposition methods employed in the fabrication of state‐of‐the‐art perovskite solar cells. This effect is a convenient and effective way to tune the band gap of perovskites.

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.

Origin, Influence, and Countermeasures of Defects in Perovskite Solar Cells

Defects are considered to be one of the most significant factors that compromise the power conversion efficiencies and long-term stability of perovskite solar cells. Therefore, it is urgent to have a profound understanding of their formation and influence mechanism, so as to take corresponding measures to suppress or even completely eliminate their adverse effects on device performance. Herein, the possible origins of the defects in metal halide perovskite films and their impacts on the device performance are analyzed, and then various methods to reduce defect density are introduced in detail. Starting from the internal and interfacial aspects of the metal halide perovskite films, several ways to improve device performance and long-term stability including additive engineering, surface passivation, and other physical treatments (annealing engineering), etc., are further elaborated. Finally, the further understanding of defects and the development trend of passivation strategies are prospected.

Comparison of surface-passivation ability of the BAI salt and its induced 2D perovskite for high-performance inverted perovskite solar cells

Organic-inorganic lead halide perovskite solar cells have shown a rapid increase in power conversion efficiency (PCE) in the past decade. However, their performance still suffers from trap-assisted decline due to defects at the surface and grain boundaries in the polycrystalline active layer. In this study, a thin BAI layer was formed on the CH₃NH₃PbI₃ surface, which was then transformed into either a two-dimensional (2D) perovskite layer (BA₂PbI₄) or the organic salt itself by controlling the post-annealing process. A variety of characterization results show that the 2D perovskites could effectively reduce the trap-assisted charge recombination and increase the device stability. Therefore, a remarkably enhanced PCE of 20.6% was achieved in the modified inverted perovskite solar cells.

One-Step Synthesis of SnI2·(DMSO)x Adducts for High-Performance Tin Perovskite Solar Cells

Contemporary thin-film photovoltaic (PV) materials contain elements that are scarce (CIGS) or regulated (CdTe and lead-based perovskites), a fact that may limit the widespread impact of these emerging PV technologies. Tin halide perovskites utilize materials less stringently regulated than the lead (Pb) employed in mainstream perovskite solar cells; however, even today's best tin-halide perovskite thin films suffer from limited carrier diffusion length and poor film morphology. We devised a synthetic route to enable in situ reaction between metallic Sn and I₂ in dimethyl sulfoxide (DMSO), a reaction that generates a highly coordinated SnI₂·(DMSO)_x adduct that is well-dispersed in the precursor solution. The adduct directs out-of-plane crystal orientation and achieves a more homogeneous structure in polycrystalline perovskite thin films. This approach improves the electron diffusion length of tin-halide perovskite to 290 ± 20 nm compared to 210 ± 20 nm in reference films. We fabricate tin-halide perovskite solar cells with a power conversion efficiency of 14.6% as certified in an independent lab. This represents a ∼20% increase compared to the previous best-performing certified tin-halide perovskite solar cells. The cells outperform prior earth-abundant and heavy-metal-free inorganic-active-layer-based thin-film solar cells such as those based on amorphous silicon, Cu₂ZnSn(S/Se)₄ , and Sb₂(S/Se)₃.

A Review of Integrated Systems Based on Perovskite Solar Cells and Energy Storage Units: Fundamental, Progresses, Challenges, and Perspectives

With the remarkable progress of photovoltaic technology, next-generation perovskite solar cells (PSCs) have drawn significant attention from both industry and academic community due to sustainable energy production. The single-junction-cell power conversion efficiency (PCE) of PSCs to date has reached up to 25.2%, which is competitive to that of commercial silicon-based solar cells. Currently, solar cells are considered as the individual devices for energy conversion, while a series connection with an energy storage device would largely undermine the energy utilization efficiency and peak power output of the entire system. For substantially addressing such critical issue, advanced technology based on photovoltaic energy conversion-storage integration appears as a promising strategy to achieve the goal. However, there are still great challenges in integrating and engineering between energy harvesting and storage devices. In this review, the state-of-the-art of representative integrated energy conversion-storage systems is initially summarized. The key parameters including configuration design and integration strategies are subsequently analyzed. According to recent progress, the efforts toward addressing the current challenges and critical issues are highlighted, with expectation of achieving practical integrated energy conversion-storage systems in the future.

Reduced Defects and Enhanced Performance of (FAPbI3)0.97(MAPbBr3)0.03-Based Perovskite Solar Cells by Trimesic Acid Additives

A high-quality organolead trihalide perovskite film with large-sized crystalline grains and smooth surfaces is required to obtain efficient perovskite solar cells (PSCs). Herein, high-quality (FAPbI₃)_0.97(MAPbBr₃)_0.03 perovskite films were fabricated using trimesic acid (TMA) additives in a halide perovskite precursor solution to obtain efficient PSCs. The X-ray diffraction analysis and scanning electron microscopy of the films revealed that the TMA had a significant effect on the roughness of the films by acting as a surface link, thus reducing the surface defects and recombination at the grain boundaries. In addition, with the addition of the TMA additive, a smooth perovskite film with a flat surface and no pinholes was obtained. The perovskite film was used to fabricate a PSC device, and the device exhibited a high power conversion efficiency of 17.26%, which was higher than that of the control device (15.15%) under the same conditions. This study demonstrates a facile method to passivate defects on the perovskite layer via surface modification.

Performance Comparison between the Nanoporous NiO x Layer and NiO x Thin Film for Inverted Perovskite Solar Cells with Long-Term Stability

The development of hole-transport layers (HTLs) that elevate charge extraction, improve perovskite crystallinity, and decrease interfacial recombination is extremely important for enhancing the performance of inverted perovskite solar cells (PSCs). In this work, the nanoporous nickel oxide (NiO _x ) layer as well as NiO _x thin film was prepared via chemical bath deposition as the HTL. The sponge-like structure of the nanoporous NiO _x helps to grow a pinhole-free perovskite film with a larger grain size compared to the NiO _x thin film. The downshifted valence band of the nanoporous NiO _x HTL can improve hole extraction from the perovskite absorbing layer. The device based on the nanoporous NiO _x layer showed the highest efficiency of 13.43% and negligible hysteresis that was better than the one using the NiO _x thin film as the HTL. Moreover, the PSCs sustained 80% of their initial efficiency after 50 days of storage. This study provides a powerful strategy to design PSCs with high efficiency and long-term stability for future production.

Strong Electron Localization in Tin Halide Perovskites

Tin halide perovskites (THPs) have been established as a lower-toxicity alternative to lead halide perovskites. In spite of the increasing interest, the behavior of photoexcited charges has not been well understood in this class of materials. We here investigate the behavior of excess electrons in a series of tin halide perovskites by employing advanced electronic-structure calculations. We first focus on CsSnBr₃ and show that electron localization is favorable in this compound and that bipolaronic states are the most stable form of self-trapped electrons. We then extend the analysis to CsSnI₃, CsSnCl₃, MASnBr₃, FASnBr₃, and DMASnBr₃ and show that electron bipolarons are stable in all these compounds, thus indicating that strong electron localization is recurrent in THPs.

Synergistic Effect of RbBr Interface Modification on Highly Efficient and Stable Perovskite Solar Cells

Compact TiO₂ films are one of the most widely used electron transport layers (ETLs) in planar perovskite solar cells (PSCs). However, the performance of the PSC device is controlled by the comprehensive qualities of the functional layers and their bilateral surfaces. In this work, the alkali metal halide of RbBr as the interfacial modifier is introduced into the interface of the TiO₂ ETL and perovskite absorber. By spin-coating the proper content of RbBr, the surface of the TiO₂ film consisting of smooth morphology and low density of oxygen-deficiency defect is readily obtained. The perovskite layer successively fabricated on the RbBr-modified TiO₂ film demonstrates large grain size, low surface roughness, and low bulk defect density, which enhances the electron extraction and decreases nonradiation recombination. By virtue of the modulation of the perovskite crystal quality and the passivation of the interfacial defects, the light-harvesting efficiency of the corresponding device is increased to 21.15 from 19.21% for the PSC without a RbBr insertion layer. More importantly, the passivation strategy enables impressive device stability by retaining 90% of its initial efficiency in an ambient environment for 500 h. This study provides a promising and feasible strategy to regulate surface passivation engineering and simultaneously facilitate the perovskite crystal growth for the achievement of efficient and stable perovskite photovoltaics.

Halide Perovskites: A New Era of Solution-Processed Electronics

Organic-inorganic mixed halide perovskites have emerged as an excellent class of materials with a unique combination of optoelectronic properties, suitable for a plethora of applications ranging from solar cells to light-emitting diodes and photoelectrochemical devices. Recent works have showcased hybrid perovskites for electronic applications through improvements in materials design, processing, and device stability. Herein, a comprehensive up-to-date review is presented on hybrid perovskite electronics with a focus on transistors and memories. These applications are supported by the fundamental material properties of hybrid perovskite semiconductors such as tunable bandgap, ambipolar charge transport, reasonable mobility, defect characteristics, and solution processability, which are highlighted first. Then, recent progresses on perovskite-based transistors are reviewed, covering aspects of fabrication process, patterning techniques, contact engineering, 2D versus 3D material selection, and device performance. Furthermore, applications of perovskites in nonvolatile memories and artificial synaptic devices are presented. The ambient instability of hybrid perovskites and the strategies to tackle this bottleneck are also discussed. Finally, an outlook and opportunities to develop perovskite-based electronics as a competitive and feasible technology are highlighted.

Interfacial stabilization for inverted perovskite solar cells with long-term stability

Perovskite solar cells (PSCs) commonly exhibit significant performance degradation due to ion migration through the top charge transport layer and ultimately metal electrode corrosion. Here, we demonstrate an interfacial management strategy using a boron chloride subphthalocyanine (Cl₆SubPc)/fullerene electron-transport layer, which not only passivates the interfacial defects in the perovskite, but also suppresses halide diffusion as evidenced by multiple techniques, including visual element mapping by electron energy loss spectroscopy. As a result, we obtain inverted PSCs with an efficiency of 22.0% (21.3% certified), shelf life of 7000 h, T₈₀ of 816 h under damp heat stress (compared to less than 20 h without Cl₆SubPc), and initial performance retention of 98% after 2000 h at 80 °C in inert environment, 90% after 2034 h of illumination and maximum power point tracking in ambient for encapsulated devices and 95% after 1272 h outdoor testing ISOS-O-1. Our strategy and results pave a new way to move PSCs forward to their potential commercialization solidly.

Phthalocyanines: An Old Dog Can Still Have New (Photo)Tricks!

Phthalocyanines have enjoyed throughout the years the benefits of being exquisite compounds with many favorable properties arising from the straightforward and diverse possibilities of their structural modulation. Last decades appreciated a steady growth in applications for phthalocyanines, particularly those dependent on their great photophysical properties, now used in several cutting-edge technologies, particularly in photonic applications. Judging by the vivid reports currently provided by many researchers around the world, the spotlight remains assured. This review deals with the use of phthalocyanine molecules in innovative materials in photo-applications. Beyond a comprehensive view on the recent discoveries, a critical review of the most acclaimed/considered reports is the driving force, providing a brief and direct insight on the latest milestones in phthalocyanine photonic-based science.

Toward Commercialization of Stable Devices: An Overview on Encapsulation of Hybrid Organic-Inorganic Perovskite Solar Cells

Perovskite solar cells (PSCs) represent a promising technology for energy harvesting due to high power conversion efficiencies up to 26%, easy manufacturing, and convenient deposition techniques, leading to added advantages over other contemporary competitors. In order to promote this technology toward commercialization though, stability issues need to be addressed. Lately, many researchers have explored several techniques to improve the stability of the environmentally-sensitive perovskite solar devices. Challenges posed by environmental factors like moisture, oxygen, temperature, and UV-light exposure, could be overcome by device encapsulation. This review focuses the attention on the different materials, methods, and requirements for suitable encapsulated perovskite solar cells. A depth analysis on the current stability tests is also included, since accurate and reliable testing conditions are needed in order to reduce mismatching involved in reporting the efficiencies of PSC.