Caesium for Perovskite Solar Cells: An Overview

Recent Advances in Carbon Nanotube Utilization in Perovskite Solar Cells: A Review

Due to their exceptional optoelectronic properties, halide perovskites have emerged as prominent materials for the light-absorbing layer in various optoelectronic devices. However, to increase device performance for wider adoption, it is essential to find innovative solutions. One promising solution is incorporating carbon nanotubes (CNTs), which have shown remarkable versatility and efficacy. In these devices, CNTs serve multiple functions, including providing conducting substrates and electrodes and improving charge extraction and transport. The next iteration of photovoltaic devices, metal halide perovskite solar cells (PSCs), holds immense promise. Despite significant progress, achieving optimal efficiency, stability, and affordability simultaneously remains a challenge, and overcoming these obstacles requires the development of novel materials known as CNTs, which, owing to their remarkable electrical, optical, and mechanical properties, have garnered considerable attention as potential materials for highly efficient PSCs. Incorporating CNTs into perovskite solar cells offers versatility, enabling improvements in device performance and longevity while catering to diverse applications. This article provides an in-depth exploration of recent advancements in carbon nanotube technology and its integration into perovskite solar cells, serving as transparent conductive electrodes, charge transporters, interlayers, hole-transporting materials, and back electrodes. Additionally, we highlighted key challenges and offered insights for future enhancements in perovskite solar cells leveraging CNTs.

Perovskite Solar Cells: A Review of the Latest Advances in Materials, Fabrication Techniques, and Stability Enhancement Strategies

Perovskite solar cells (PSCs) are gaining popularity due to their high efficiency and low-cost fabrication. In recent decades, noticeable research efforts have been devoted to improving the stability of these cells under ambient conditions. Moreover, researchers are exploring new materials and fabrication techniques to enhance the performance of PSCs under various environmental conditions. The mechanical stability of flexible PSCs is another area of research that has gained significant attention. The latest research also focuses on developing tin-based PSCs that can overcome the challenges associated with lead-based perovskites. This review article provides a comprehensive overview of the latest advances in materials, fabrication techniques, and stability enhancement strategies for PSCs. It discusses the recent progress in perovskite crystal structure engineering, device construction, and fabrication procedures that has led to significant improvements in the photo conversion efficiency of these solar devices. The article also highlights the challenges associated with PSCs such as their poor stability under ambient conditions and discusses various strategies employed to enhance their stability. These strategies include the use of novel materials for charge transport layers and encapsulation techniques to protect PSCs from moisture and oxygen. Finally, this article provides a critical assessment of the current state of the art in PSC research and discusses future prospects for this technology. This review concludes that PSCs have great potential as a low-cost alternative to conventional silicon-based solar cells but require further research to improve their stability under ambient conditions in view of their definitive commercialization.

Perovskite/silicon tandem solar cells-compositions for improved stability and power conversion efficiency

Perovskite/Silicon Tandem Solar Cells (PSTSCs) represent an emerging opportunity to compete with industry-standard single junction crystalline silicon (c-Si) solar cells. The maximum power conversion efficiency (PCE) of single junction cells is set by the Shockley-Queisser (SQ) limit (33.7%). However, tandem cells can expand this value to ~ 45% by utilising two stacked solar cells to harvest the solar spectrum more efficiently. 33.9% PCE has already been achieved with PSTSCs. This perspective analyses recent advances in PSTSC technology, with an emphasis on optimal perovskite composition, the problem and mitigation of light-induced halide phase segregation, self-assembled hole transporting monolayers and additives that can improve and stabilise the perovskite. Top-performing compositions show three cationic components (Cs+, FA+, Pb2+) and three anionic (I-, Br-, Cl-) with a bandgap between 1.55 and 1.77 eV and a theoretical maximum of 1.73 eV (717 nm). Anionic additives such as (Br3)- and SCN- reduce trap states and segregation. 2D-perovskite grain boundary interfaces are created with cationic alkylammonium additives such as methyl-phenethylammonium (MPEA) and result in improved performance. 2-, 3- or 4-terminal devices with a (partly) textured silicon heterojunction (SHJ) bottom cell are ideal. An ultra-thin interfacial recombination layer (~ 5 nm) of indium tin oxide (ITO) or indium zinc oxide (IZO) containing a carbazole-based hole transporting self-assembled monolayer (Me-4PACz) is used for optimal 2-terminal tandem devices.

Green-Solvent Engineering for Depositing Qualified Phenyl-C61-butyl Acid Methyl Ester Films for Inverted Flexible Perovskite Solar Cells

Flexible perovskite solar cells (fPSCs) with the inverted structure (p-i-n structure) show a promising commercialization future, owing to their lightweight and high efficiencies. Phenyl-C61-butyric-acid methyl ester (PCBM) is widely used as the n-type material due to its excellent conductivity and solvent processability. However, the commonly used chlorobenzene (CB), as the solvent of PCBM solution, is well recognized as a halogenated contaminant in the environment and is harmful to human health. There is an imperative need to develop nonhalogenated green solvents to replace CB. This work discusses the selection of green solvents based on the Hansen solubility parameters (HSPs). It is found that 2-methylanisole (2-MEA) acts as an excellent alternative to CB, with which high-quality PCBM films could be deposited. The experimental and theoretical studies demonstrate that 2-MEA can suppress the formation of PCBM aggregations during the solvation process compared with CB. The more uniform PCBM film achieved from the 2-MEA solution benefits carrier extraction at the electronic transport layer (ETL)/perovskite interface. As a result, better efficiencies are received among fPSCs based on the 2-MEA-processed PCBM, superior to that of the fPSCs based on the CB-processed PCBM. Moreover, using 1,8-diiodooctane (DIO) as a solvent additive is proven to further increase the solubility of PCBM in the 2-MEA solution, resulting in enhanced efficiencies of the flexible PSCs by more than 5% (from 19.25 to 20.30%). The developed green-solvent strategy is of great importance for the future large-scale production of environmentally sustainable fPSCs.

Interfacial engineering of halide perovskites and two-dimensional materials

Recently, halide perovskites (HPs) and layered two-dimensional (2D) materials have received significant attention from industry and academia alike. HPs are emerging materials that have exciting photoelectric properties, such as a high absorption coefficient, rapid carrier mobility and high photoluminescence quantum yields, making them excellent candidates for various optoelectronic applications. 2D materials possess confined carrier mobility in 2D planes and are widely employed in nanostructures to achieve interfacial modification. HP/2D material interfaces could potentially reveal unprecedented interfacial properties, including light absorbance with desired spectral overlap, tunable carrier dynamics and modified stability, which may lead to several practical applications. In this review, we attempt to provide a comprehensive perspective on the development of interfacial engineering of HP/2D material interfaces. Specifically, we highlight the recent progress in HP/2D material interfaces considering their architectures, electronic energetics tuning and interfacial properties, discuss the potential applications of these interfaces and analyze the challenges and future research directions of interfacial engineering of HP/2D material interfaces. This review links the fields of HPs and 2D materials through interfacial engineering to provide insights into future innovations and their great potential applications in optoelectronic devices.

Machine Learning Approach to Delineate the Impact of Material Properties on Solar Cell Device Physics

In this research, solar cell capacitance simulator-one-dimensional (SCAPS-1D) software was used to build and probe nontoxic Cs-based perovskite solar devices and investigate modulations of key material parameters on ultimate power conversion efficiency (PCE). The input material parameters of the absorber Cs-perovskite layer were incrementally changed, and with the various resulting combinations, 63,500 unique devices were formed and probed to produce device PCE. Versatile and well-established machine learning algorithms were thereafter utilized to train, test, and evaluate the output dataset with a focused goal to delineate and rank the input material parameters for their impact on ultimate device performance and PCE. The most impactful parameters were then tuned to showcase unique ranges that would ultimately lead to higher device PCE values. As a validation step, the predicted results were confirmed against SCAPS simulated results as well, highlighting high accuracy and low error metrics. Further optimization of intrinsic material parameters was conducted through modulation of absorber layer thickness, back contact metal, and bulk defect concentration, resulting in an improvement in the PCE of the device from 13.29 to 16.68%. Overall, the results from this investigation provide much-needed insight and guidance for researchers at large, and experimentalists in particular, toward fabricating commercially viable nontoxic inorganic perovskite alternatives for the burgeoning solar industry.

Improvements in Efficiency and Stability of Perovskite Solar Cells Using a Cesium Chloride Additive

Perovskite films with few defects play a key role in preparing high-performance perovskite solar cells (PSCs). Here, cesium chloride (CsCl) was introduced as a modulator into a perovskite precursor for manipulating the crystallization of perovskite films. By introducing CsCl, dense homogeneous perovskite films with high crystallinity, preferential orientation, and a pure black perovskite phase were prepared. In addition, the carrier lifetime of perovskite films was significantly increased because of the suppressed nonradiative recombination. Correspondingly, the power conversion efficiency (PCE) of small-area devices using CsCl regulation was increased from 20.56 to 22.86%. The 1 cm2 PSCs present a PCE of 21.53%, demonstrating their reliability for mass production. Furthermore, the device showed excellent stability maintaining 93.8% of its initial PCE after 500 h of continuous irradiation. Also, 95.3% of its PCE was kept after storage in ambient air for 2100 h. This study demonstrates that CsCl doping is a reliable way to prepare PSCs for practical applications.

Improving the performance of inorganic perovskite solar cells via the perovskite quantum dot dynamically mediated film growth method

Perovskite quantum dots (PQDs) are promising interface modification materials for perovskite solar cells (PSCs). However, due to the limitation of the preparation method, it is hard to use PQDs as substrates for the growth of perovskite films by the common solution process. In this work, by introducing the rare earth element Ce into PQDs with the vacuum freezing and drying technology, we have successfully improved the solvent stability of PQDs. Moreover, we propose a technology, PQD dynamically mediated growth of perovskite film (PDMG), to prepare high-quality perovskite films, which can avoid the formation of PQD charge-blocking layers. Thanks to the improvement of perovskite crystallinity and the charge transport ability, the PCE is improved from 10.44% to 12.14% for CsPbI₂Br PSCs and from 14.43% to 16.38% for CsPbI₃ PSCs. Our work opens an avenue for using PQDs as substrates in the fabrication of highly efficient PSCs.

Surface Passivation of MAPbBr3 Perovskite Single Crystals to Suppress Ion Migration and Enhance Photoelectronic Performance

Recently, organometal halide perovskites (OHPs) have achieved significant advancement in photovoltaics, light-emitting diodes, X-ray detectors, and transistors. However, commercialization and practical applications were hindered by the notorious ion migration issue of OHPs. Here, we report a simple solvent-based surface passivation strategy with organic halide salts (methylammonium bromide (MABr) and phenylethylammonium bromide (PEABr)) to suppress the ion migration of MAPbBr₃ single crystals. The surface passivation effect is evidenced by the stronger photoluminescence (PL) intensity and extended PL lifetime. Using the pulse voltage and continuous voltage current-voltage measurements, we found that single crystals with surface passivation showed negligible hysteresis on the surface due to the suppression of ion migration. As a result, the dark current stability of coplanar structure devices was significantly improved. Moreover, the vertical structure X-ray detectors with PEABr treatment exhibited a high sensitivity of 15 280 μC Gy_air^-1 cm^-2 and a low detection limit of 87 nGy_air s^-1 under 5 V bias. The proposed technology would be a versatile tool to improve the performance of perovskite photoelectronic devices.

Effects of All-Organic Interlayer Surface Modifiers on the Efficiency and Stability of Perovskite Solar Cells

Surface imperfections created during fabrication of halide perovskite (HP) films could induce formation of various defect sites that affect device performance and stability. In this work, all-organic surface modifiers consisting of alkylammonium cations and alkanoate anions are introduced on top of the HP layer to passivate interfacial vacancies and improve moisture tolerance. Passivation using alkylammonium alkanoate does not induce formation of low-dimensional perovskites species. Instead, the organic species only passivate the perovskite's surface and grain boundaries, which results in enhanced hydrophobic character of the HP films. In terms of photovoltaic application, passivation with alkylammonium alkanoate allows significant reduction in recombination losses and enhancement of open-circuit voltage. Alongside unchanged short-circuit current density, power conversion efficiencies of more than 18.5 % can be obtained from the treated sample. Furthermore, the unencapsulated device retains 85 % of its initial PCE upon treatment, whereas the standard 3D perovskite device loses 50 % of its original PCE when both are subjected to ambient environment over 1500 h.

Strategies for High-Performance Large-Area Perovskite Solar Cells toward Commercialization

Perovskite solar cells (PSCs) have received a great deal of attention in the science and technology field due to their outstanding power conversion efficiency (PCE), which increased rapidly from 3.9% to 25.5% in less than a decade, comparable to single crystal silicon solar cells. In the past ten years, much progress has been made, e.g. impressive ideas and advanced technologies have been proposed to enlarge PSC efficiency and stability. However, this outstanding progress has always been referred to as small-area (<0.1 cm2) PSCs. Little attention has been paid to the preparation processes and their micro-mechanisms for large-area (>1 cm2) PSCs. Meanwhile, scaling up is an inevitable way for large-scale application of PSCs. Therefore, we firstly summarize the current achievements for high efficiency and stability large-area perovskite solar cells, including precursor composition, deposition, growth control, interface engineering, packaging technology, etc. Then we include a brief discussion and outlook for the future development of large-area PSCs in commercialization.

Low-cost air-stable perovskite solar cells by incorporating inorganic materials

Herein, we demonstrate a new fabrication strategy for low-cost and stable-operation perovskite solar cells (PSCs) suitable for commercialization.

Electronic and geometrical parametrization of the role of organic/inorganic cations on the photovoltaic perovskite band gap

Recent state-of-the-art analysis techniques have revealed the high sensitivity of new generation perovskite photovoltaics to the organic/inorganic A-cation's chemical composition and atomic configuration. Various studies have focused on an extensive list of potential candidates to find the best A-cation with optimum stability and efficiency output. Regarding the perovskite band gap, different characteristics such as cation size, constituent elements, atomic configuration, possible bonding potential and induced lattice distortion, have been considered to screen plausible A-cations. However, there is not a comprehensive and comparative framework for developing predictive models because of the strong correlation between governing parameters. In this research, we develop an innovative approach, using first principle methods, to parametrize the role of A-cation on the regulation of the well-known ABX3 perovskites band gap in a quantitative and comparative form. Parameters are introduced concerning the A-cation impact on the shared electrons of the B-X bonds, whose s and p states control the whole band structure. The A-cation induced geometrical distortion on the BX3 network, including the subsequent bond length and bond angle, are designated as indirect parameters; and its impact on the electronic states of B-X bonding through long range electronic interactions, is attributed as the direct role. Dissociation of correlative parameters is achieved by comparing the electronic properties of the BX3 network including and excluding their A-cation, as well as swapping different A-cations on the corresponding equivalent BX3 scaffolds. The governing mechanisms behind the direct/indirect contribution of Cs, methylammonium (MA) and formamidinium (FA) cations, regarding their impact on electronic charge density distribution and bonding tendency via the introduced parametrization, are investigated and discussed.

Solid-State NMR and NQR Spectroscopy of Lead-Halide Perovskite Materials

Two- and three-dimensional lead-halide perovskite (LHP) materials are novel semiconductors that have generated broad interest owing to their outstanding optical and electronic properties. Characterization and understanding of their atomic structure and structure-property relationships are often nontrivial as a result of the vast structural and compositional tunability of LHPs as well as the enhanced structure dynamics as compared with oxide perovskites or more conventional semiconductors. Nuclear magnetic resonance (NMR) spectroscopy contributes to this thrust through its unique capability of sampling chemical bonding element-specifically (1/2H, 13C, 14/15N, 35/37Cl, 39K, 79/81Br, 87Rb, 127I, 133Cs, and 207Pb nuclei) and locally and shedding light onto the connectivity, geometry, topology, and dynamics of bonding. NMR can therefore readily observe phase transitions, evaluate phase purity and compositional and structural disorder, and probe molecular dynamics and ionic motion in diverse forms of LHPs, in which they can be used practically, ranging from bulk single crystals (e.g., in gamma and X-ray detectors) to polycrystalline films (e.g., in photovoltaics, photodetectors, and light-emitting diodes) and colloidal nanocrystals (e.g., in liquid crystal displays and future quantum light sources). Herein we also outline the immense practical potential of nuclear quadrupolar resonance (NQR) spectroscopy for characterizing LHPs, owing to the strong quadrupole moments, good sensitivity, and high natural abundance of several halide nuclei (79/81Br and 127I) combined with the enhanced electric field gradients around these nuclei existing in LHPs as well as the instrumental simplicity. Strong quadrupole interactions, on one side, make 79/81Br and 127I NMR rather impractical but turn NQR into a high-resolution probe of the local structure around halide ions.

Chemically Stable Black Phase CsPbI3 Inorganic Perovskites for High-Efficiency Photovoltaics

Research on chemically stable inorganic perovskites has achieved rapid progress in terms of high efficiency exceeding 19% and high thermal stabilities, making it one of the most promising candidates for thermodynamically stable and high-efficiency perovskite solar cells. Among those inorganic perovskites, CsPbI₃ with good chemical components stability possesses the suitable bandgap (≈1.7 eV) for single-junction and tandem solar cells. Comparing to the anisotropic organic cations, the isotropic cesium cation without hydrogen bond and cation orientation renders CsPbI₃ exhibit unique optoelectronic properties. However, the unideal tolerance factor of CsPbI₃ induces the challenges of different crystal phase competition and room temperature phase stability. Herein, the latest important developments regarding understanding of the crystal structure and phase of CsPbI₃ perovskite are presented. The development of various solution chemistry approaches for depositing high-quality phase-pure CsPbI₃ perovskite is summarized. Furthermore, some important phase stabilization strategies for black phase CsPbI₃ are discussed. The latest experimental and theoretical studies on the fundamental physical properties of photoactive phase CsPbI₃ have deepened the understanding of inorganic perovskites. The future development and research directions toward achieving highly stable CsPbI₃ materials will further advance inorganic perovskite for highly stable and efficient photovoltaics.

Understanding KOtBu in atomic layer deposition - in situ mechanistic studies of the KNbO3 growth process

Functional coatings based on alkali metals have become increasingly attractive in the current shift towards sustainable technologies. While lithium-based compounds have a natural impact on batteries, other alkali metal compounds are important as replacements for toxic materials in a range of electronic devices. This is especially true for potassium, being a major component in e.g. KxNa1-xNbO3 (KNN) and KTaxNb1-xO3 (KTN), with hope to replace Pb(ZrxTi1-x)O3 (PZT) in piezo-/ferroelectric and electrooptic devices. ALD facilitates functional conformal coatings at deposition temperatures far below what is reported using other techniques and with excellent compositional control. The ALD growth of potassium-containing films using KOtBu has, however, been unpredictable. Untraditional response to the pulse composition and precursor dose, severe reproducibility issues, and very high growth per cycle are some of the puzzling features of these processes. In this article, we shed light on the growth behavior of KOtBu in ALD by in situ quartz crystal microbalance and Fourier transform infrared spectroscopy studies. We study the precursor's behavior in the technologically interesting KNbO3-process, showing how the potassium precursor strongly affects the growth of other cation precursors. We show that the strong hygroscopic nature of the intermediary potassium species has far-reaching implications throughout the growth. This helps not only to enhance the understanding of alkali metal containing compounds' growth in ALD, but also to provide the means to control the growth of novel sustainable technological materials.

Ab initio modeling of 2D and quasi-2D lead organohalide perovskites with divalent organic cations and a tunable band gap

We describe theoretically the structure and properties of layered lead organohalide perovskites, considering purely bi-dimensional (2D) PbI₄ layers, and quasi-2D systems where the inorganic layers are formed by more than one lead iodide sheet. The intercalating organic dications were designed to have low lying virtual orbitals (LUMO), so as to induce in the perovskite the appearance of virtual bands, localized in the organic layer, either close to the inorganic conduction band bottom or valence band top, or in some cases in the middle of the inorganic band gap. Such a feature is quite uncommon for this class of materials, and deserves attention since it allows one to tune the effective band gap of the material, possibly leading to the absorption of visible light and influencing the optical properties deeply. We discuss the effect of functional groups on the organic cations, and of the different symmetries used in geometry optimizations: a careful analysis of the contributions to the dispersion curves and band gaps was performed. The charge carrier mobility is also discussed, computing the conductivity over relaxation time and the effective masses for all the systems, with particular attention to the features related to the unusual organic intra-gap bands. All the structures were optimized at the DFT level, with inclusion of dispersion effects; dispersion curves were computed with full relativistic potentials, and the band gaps corrected for long range coulombic effects at the GW level. A semiempirical approach, based on the integration of charge carrier group velocities over a dense grid of k-points, was used to compute the conductivities and effective masses.

Stable perovskite solar cells with efficiency exceeding 24.8% and 0.3-V voltage loss

Further improvement and stabilization of perovskite solar cell (PSC) performance are essential to achieve the commercial viability of next-generation photovoltaics. Considering the benefits of fluorination to conjugated materials for energy levels, hydrophobicity, and noncovalent interactions, two fluorinated isomeric analogs of the well-known hole-transporting material (HTM) Spiro-OMeTAD are developed and used as HTMs in PSCs. The structure–property relationship induced by constitutional isomerism is investigated through experimental, atomistic, and theoretical analyses, and the fabricated PSCs feature high efficiency up to 24.82% (certified at 24.64% with 0.3-volt voltage loss), along with long-term stability in wet conditions without encapsulation (87% efficiency retention after 500 hours). We also achieve an efficiency of 22.31% in the large-area cell.

"Waste to Wealth": Lignin as a Renewable Building Block for Energy Harvesting/Storage and Environmental Remediation

Lignin is the second most earth-abundant biopolymer having aromatic unit structures, but it has received less attention than other natural biomaterials. Recent advances in the development of lignin-based materials, such as mesoporous carbon, flexible thin films, and fiber matrix, have found their way into applications to photovoltaic devices, energy-storage systems, mechanical energy harvesters, and catalytic components. In this Review, we summarize and suggest another dimension of lignin valorization as a building block for the synthesis of functional materials in the fields of energy and environmental applications. We cover lignin-based materials in the photovoltaic and artificial photosynthesis for solar energy conversion applications. The most recent technological evolution in lignin-based triboelectric nanogenerators is summarized from its fundamental properties to practical implementations. Lignin-derived catalysts for solar-to-heat conversion and oxygen reduction are discussed. For energy-storage applications, we describe the utilization of lignin-based materials in lithium-ion rechargeable batteries and supercapacitors (e.g., electrodes, binders, and separators). We also summarize the use of lignin-based materials as heavy-metal adsorbents for environmental remediation. This Review paves the way to future potentials and opportunities of lignin as a renewable material for energy and environmental applications.

Reducing Agents for Improving the Stability of Sn-based Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have aroused tremendous research interest for their high efficiency, low cost and solution processability. However, the involvement of toxic lead in state-of-art perovskites hinders their market prospects. As an alternative, Sn-based perovskites exhibit similar semiconductor characteristics and can potentially achieve comparable photovoltaic performance in comparison with their lead-based counterparts. The main challenge of developing Sn-based PCSs lies in the intrinsic poor stability of Sn²⁺ , which could be oxidized and converted to Sn⁴⁺ . Notably, introduction of SnX₂ (X=Cl, Br, I) additive becomes indispensable in the fabrication process, which highlights the importance of incorporating a reducing agent to improve the device stability. Additionally, efforts are made to utilize other reducing agents with different functions for the further enhancement of device performance. Currently, Sn-based PSCs could attain a record efficiency over 10% with great stability. In this review, we present the recent progress on reducing agents for improving the stability of Sn-based PSCs, and we hope to shed light on the challenges and opportunities of this research field.

Suppressing Vacancy Defects and Grain Boundaries via Ostwald Ripening for High-Performance and Stable Perovskite Solar Cells

As one kind of promising next-generation photovoltaic devices, perovskite solar cells (PVSCs) have experienced unprecedented rapid growth in device performance over the past few years. However, the practical applications of PVSCs require much improved device long-term stability and performance, and internal defects and external humidity sensitivity are two key limitation need to be overcome. Here, gadolinium fluoride (GdF₃ ) is added into perovskite precursor as a redox shuttle and growth-assist; meanwhile, aminobutanol vapor is used for Ostwald ripening in the formation of the perovskite layer. Consequently, a high-quality perovskite film with large grain size and few grain boundaries is obtained, resulting in the reduction of trap state density and carrier recombination. As a result, a power conversion efficiency of 21.21% is achieved with superior stability and negligible hysteresis.

Performance of WO3-Incorporated Carbon Electrodes for Ambient Mesoscopic Perovskite Solar Cells

The stability of perovskite solar cells (PSC) is often compromised by the organic hole transport materials (HTMs). We report here the effect of WO₃ as an inorganic HTM for carbon electrodes for improved stability in PSCs, which are made under ambient conditions. Sequential fabrication of the PSC was performed under ambient conditions with mesoporous TiO₂/Al₂O₃/CH₃NH₃PbI₃ layers, and, on the top of these layers, the WO₃ nanoparticle-embedded carbon electrode was used. Different concentrations of WO₃ nanoparticles as HTM incorporated in carbon counter electrodes were tested, which varied the stability of the cell under ambient conditions. The addition of 7.5% WO₃ (by volume) led to a maximum power conversion efficiency of 10.5%, whereas the stability of the cells under ambient condition was ∼350 h, maintaining ∼80% of the initial efficiency under light illumination. At the same time, the higher WO₃ concentration exhibited an efficiency of 9.5%, which was stable up to ∼500 h with a loss of only ∼15% of the initial efficiency under normal atmospheric conditions and light illumination. This work demonstrates an effective way to improve the stability of carbon-based perovskite solar cells without affecting the efficiency for future applications.

Metal Cations in Efficient Perovskite Solar Cells: Progress and Perspective

Metal halide perovskite solar cells (PVSCs) have revolutionized photovoltaics since the first prototype in 2009, and up to now the highest efficiency has soared to 24.2%, which is on par with commercial thin film cells and not far from monocrystalline silicon solar cells. Optimizing device performance and improving stability have always been the research highlight of PVSCs. Metal cations are introduced into perovskites to further optimize the quality, and this strategy is showing a vigorous development trend. Here, the progress of research into metal cations for PVSCs is discussed by focusing on the position of the cations in perovskites, the modulation of the film quality, and the influence on the photovoltaic performance. Metal cations are considered in the order of alkali cations, alkaline earth cations, then metal cations in the ds and d regions, and ultimately trivalent cations (p- and f-block metal cations) according to the periodic table of elements. Finally, this work is summarized and some relevant issues are discussed.

Cesium Oleate Passivation for Stable Perovskite Photovoltaics

Despite their emergence as promising materials for low-cost and efficient energy power generation technology, the instability of hybrid organic-inorganic lead-halide perovskites toward moisture and heat stress remains a serious obstacle that needs to be tackled for commercialization. Here, we show improved moisture and thermal stability through the use of cesium oleate to modify the perovskite/hole transporting material interface. Passivation using cesium oleate does not induce the formation of any low-dimensional perovskites, suggesting that the organic species only passivate the perovskite's surface and grain boundaries. As a result, enhanced hydrophobic character of the perovskite film is realized upon passivation, evidenced by a large water contact angle of 107.4° and improved stability at ambient conditions (a relative humidity of ∼70%, room temperature). Concomitantly, the proposed passivation strategy leads to an increased amount of cesium concentration within the films, resulting in beneficial enhanced thermal stability of the film at 85 °C. By maintaining the three-dimensional (3D) structure of the solar absorber while concurrently passivating the interfacial defects and vacancies, improved open-circuit voltage (V_oc) and unsacrificed short-circuit current density (J_sc) were obtained from the treated devices, leading to power conversion efficiencies of more than 18%. When stored in a humid environment (a relative humidity of ∼55%), devices with cesium oleate passivation maintain 88% of their initial power conversion efficiency after 720 h, degrading two times slower than those of the control. This work offers a strategy of coating 3D perovskites with a unique combination of inorganic cations and long-chain organics to provide hydrophobicity and moisture stability to the solar absorber layer while maintaining good device performances.

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

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

Cation Alloying Delocalizes Polarons in Lead Halide Perovskites

Recently, mixed-cation perovskites have promised enhanced performances concerning stability and efficiency in optoelectronic devices. Here, we report a systematic study on the effects of cation alloying on polaronic properties in cation-alloyed perovskites using first principle calculations. We find that cation alloying significantly reduces the polaron binding energies for both electrons and holes compared to pure methylammonium lead iodide (MAPbI₃). This is rationalized in terms of crystal symmetry reduction that causes polarons to be more delocalized. Electron polarons undergo large Jahn-Teller distortions (∼15-30%), whereas hole polarons tend to shrink the lattice by ∼5%. Such different lattice distortion footprints could be utilized to distinguish the type of polarons. Finally, our simulations show that Cs, formamidinium (FA), and MA mixtures can effectively minimize polaron binding energy while weakly affecting band gap, in a good agreement with experimental findings. These modeling results can guide future development of halide perovskite materials compositions for optoelectronic applications.

Controlling Orientation Diversity of Mixed Ion Perovskites: Reduced Crystal Microstrain and Improved Structural Stability

Formamidinium lead iodide (FAPbI₃)-based perovskite has attracted increasing attention of researchers due to its lower band gap and improved thermal stability. However, it is structurally unstable and easy to phase-transfer at room temperature. Here, we improve the structural stability of perovskite by controlling its orientation diversity. XRD results show that incorporating CsBr into FAPbI₃ is effective to adjust the crystal plane stacking. For the first time, an orientation diversity factor (ODF) is identified, and it is found that an increased ODF is propitious to decrease the lattice distortion and relax the microstrain in the crystal, boosting the efficiency and stability of the perovskite solar cells (PSCs). The optimized inverted PSC based on FA_0.85Cs_0.15PbI_2.85Br_0.15 achieves efficiency of 17.59% and presents ignored performance decline under continuous light-soaking for 500 h.

Flexible and Highly Durable Perovskite Solar Cells with a Sandwiched Device Structure

Flexible perovskite solar cells (PSCs) have been quickly developed as the most promising candidates for low-cost photovoltaic technology. However, the bendable and foldable properties of PSCs induce the decrease of their efficiencies. In this paper, we report the design of a new kind of flexible PSCs with a sandwiched structure. The critical layer of the flexible device is designed at a neutral layer of the sandwiched structure, which is stress-free, no matter how the device bending is. During the bending test, sandwich-structured flexible PSCs showed extremely long bending lifetime, which is at least 5-8 times higher than that of generally reported devices. At the same time, the sandwiched structure works as the encapsulation effect. The flexible device with a sandwiched structure greatly improves the device's long-term stability. Therefore, the designed sandwiched structure significantly promotes the bending ability and stability of flexible PSCs.

Enhanced Open-Circuit Voltage in Perovskite Solar Cells with Open-Cage [60]Fullerene Derivatives as Electron-Transporting Materials

The synthesis, characterization, and incorporation of open-cage [60]fullerene derivatives as electron-transporting materials (ETMs) in perovskite solar cells (PSCs) with an inverted planar (p-i-n) structure is reported. Following optical and electrochemical characterization of the open-cage fullerenes 2a–c, p-i-n PSCs with a indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS)/perovskite/fullerene/Ag structure were prepared. The devices obtained from 2a–b exhibit competitive power conversion efficiencies (PCEs) and improved open-circuit voltage (V_oc) values (>1.0 V) in comparison to a reference cell based on phenyl-C₆₁-butyric-acid methyl-ester (PC₆₁BM). These results are rationalized in terms of a) the higher passivation ability of the open-cage fullerenes with respect to the other fullerenes, and b) a good overlap between the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels of 2a–b and the conduction band of the perovskite.

TiO₂ Nanoparticles/Nanotubes for Efficient Light Harvesting in Perovskite Solar Cells

To enhance the light harvesting capability of perovskite solar cells (PSCs), TiO₂ nanoparticles/nanotubes (TNNs) were incorporated into the active layer of PSCs. The TNN-containing cells showed a substantial increase in photocurrent density (J_SC), from 23.9 mA/cm² without nanotubes to 25.5 mA/cm², suggesting that the TiO₂ nanotubes enhanced the charge conduction and harvested more sunlight, which was attributed to the Mie scattering effect. Compared to the power conversion efficiency (PCE) of TiO₂ nanoparticles in the active layer (14.16%), the TNN-containing cells with optimal loading of 9 wt % TiO₂ nanotubes showed a high PCE of 15.34%.

Atomic layer deposition of a SnO2 electron-transporting layer for planar perovskite solar cells with a power conversion efficiency of 18.3

High-efficiency planar type perovskite solar cells were fabricated by atomic layer deposition (ALD) of SnO2 and subsequent annealing at 180 °C. As-dep. SnO2 layers prepared by post-annealing at 180 and 300 °C, respectively, were used as electron transporting layers (ETLs). ALD-TiO2 layers were also prepared by post annealing at 400 °C, and the thicknesses of all ETLs were around 12 nm. PL quenching, optical band gap measurement, UPS, and conductive AFM results show that SnO2 can more appropriately be used as an ETL compared to TiO2.

Efficient modulation of photoluminescence by hydrogen bonding interactions between inorganic [MnBr4]2- anions and organic cations

The different hydrogen bond interactions in two organic-inorganic hybrid manganese halide compounds, namely [A]2[MnBr4] (A = N-butyl-N-methylpyrrolidinium ([P14]+) for (1) and N-butyl-N-methylpiperidinium ([PP14]+) for (2)), lead to distinct photoluminescence quantum yields (81% for 1; 55% for 2). Further applications of luminescent 1 are also developed.

Efficient and stable perovskite solar cells based on perfluorinated polymers

Novel perfluorinated semiconductor compounds were introduced into the perovskite layer as additives and stable and efficient perovskite-based devices were achieved.

On Modeling of Plasmon-Induced Enhancement of the Efficiency of Solar Cells Modified by Metallic Nano-Particles

We demonstrate that the direct application of numerical packets like Comsol to plasmonic effect in solar cells metallically modified in nano-scale may be strongly inaccurate if quantum corrections are neglected. The near-field coupling of surface plasmons in metallic nanoparticles deposited on the top of a solar cell with band electrons in a semiconductor substrate strongly enhances the damping of plasmons in metallic components, which is not accounted for in standard numerical packets using the Drude type dielectric function for metal (taken from measurements in bulk or in thin layers) as the prerequisite for the numerical e-m field calculus. Inclusion of the proper corrections to plasmon damping causes additional enhancement of the plasmon-induced photo-effect efficiency growth of a metalized photo-diode by ten percent, at least, in comparison to only effect induced by the electric field concentration near metallic nanoparticles. This happens to be consistent with the experimental observations which cannot be explained by only local increases of the electrical field near the curvature of metallic nanoparticles determined by a finite-element solution of the Maxwell⁻Fresnel boundary problem as given by a numerical system like Comsol. The proper damping rate for plasmons can be identified by application of the Fermi Golden Rule approach to the plasmon-band electron coupling. We demonstrate this effect including the material and size dependence in two types of solar cells, multi-crystalline Si and CIGS (copper-indium-gallium-diselenide) as idealized photo-diode semiconductor substrate modified by various metallic nano-particles, in comparison to the experimental data and Comsol simulation.

Ag@SiO₂ Core-shell Nanoparticles Embedded in a TiO₂ Mesoporous Layer Substantially Improve the Performance of Perovskite Solar Cells

In this study, Ag@SiO₂ nanoparticles were synthesized by a modified Stöber method for preparing the TiO₂ mesoporous layer of carbon counter electrode-based perovskite solar cells (PSCs) without a hole transporting layer. Compared with normal PSCs (without Ag@SiO₂ incorporated in the TiO₂ mesoporous layer), PSCs with an optimal content of Ag@SiO₂ (0.3 wt. % Ag@SiO₂-TiO₂) show a 19.46% increase in their power conversion efficiency, from 12.23% to 14.61%, which is mainly attributed to the 13.89% enhancement of the short-circuit current density, from 20.23 mA/cm² to 23.04 mA/cm². These enhancements mainly contributed to the localized surface Plasmon resonance effect and the strong scattering effect of Ag@SiO₂ nanoparticles. However, increasing the Ag@SiO₂ concentration in the mesoporous layer past the optimum level cannot further increase the short-circuit current density and incident photon-to-electron conversion efficiency of the devices, which is primarily ascribed to the electron transport pathways being impeded by the insulating silica shells inside the TiO₂ network.

A computational approach to interface engineering of lead-free CH3NH3SnI3 highly-efficient perovskite solar cells

Interface engineering in lead-free CH₃NH₃SnI₃ perovskite solar cells (PSCs) provides a viable path to realization of environmentally benign, low-cost, and high-efficiency PSCs.