Two-Dimensional Materials for Highly Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) offer low costs and high power conversion efficiency. However, the lack of long-term stability, primarily stemming from the interfacial defects and the susceptible metal electrodes, hinders their practical application. In the past few years, two-dimensional (2D) materials (e.g., graphene and its derivatives, transitional metal dichalcogenides, MXenes, and black phosphorus) have been identified as a promising solution to solving these problems because of their dangling bond-free surfaces, layer-dependent electronic band structures, tunable functional groups, and inherent compactness. Here, recent progress of 2D material toward efficient and stable PSCs is summarized, including its role as both interface materials and electrodes. We discuss their beneficial effects on perovskite growth, energy level alignment, defect passivation, as well as blocking external stimulus. In particular, the unique properties of 2D materials to form van der Waals heterojunction at the bottom interface are emphasized. Finally, perspectives on the further development of PSCs using 2D materials are provided, such as designing high-quality van der Waals heterojunction, enhancing the uniformity and coverage of 2D nanosheets, and developing new 2D materials-based electrodes.

Influence of F-Containing Materials on Perovskite Solar Cells

Perovskite solar cells (PSCs) are usually modified and passivated to improve their performance and stability. The interface modification and bulk doping are the two basic strategies. Fluorine (F)-containing materials are highly favored because of their unique hydrophobicity and coordination ability. This review discusses the basic characteristics of F, and the basic principles of improving the photovoltaic performance and stability of PSC devices using F-containing materials. We systematically summarized the latest progress in the application of F-containing materials to achieve efficient and stable PSCs on several key interface layers. It is believed that this work will afford significant understanding and inspirations toward the future application directions of F-containing materials in PSCs, and provide profound insights for the development of efficient and stable PSCs.

Band Structure Optimized by Electron-Acceptor Cations for Sensitive Perovskite Single Crystal Self-Powered Photodetectors

Low-dimensional perovskites afford improved stability against moisture, heat, and ionic migration. However, the low dimensionality typically results in a wide bandgap and strong electron-phonon coupling, which is undesirable for optoelectronic applications. Herein, semiconducting A-site organic cation engineering by electron-acceptor bipyridine (bpy) cations (2,2'-bpy2+ and 4,4'-bpy2+) is employed to optimize band structure in low-dimensional perovskites. Benefiting from the merits of lower lowest unoccupied molecular orbital (LUMO) energy for 4,4'-bpy2+ cation, the corresponding (4,4'-bpy)PbI4 is endowed with a smaller bandgap (1.44 eV) than the (CH3NH3)PbI3 (1.57 eV) benchmark. Encouragingly, an intramolecular type II band alignment formation between inorganic Pb-I octahedron anions and bpy2+ cations favors photogenerated electron-hole pairs separation. In addition, a shortening distance between inorganic Pb-I octahedral chains in (4,4'-bpy)PbI4 single crystal (SC) can effectively promote carrier transfer. As a result, a self-powered photodetector based on (4,4'-bpy)PbI4 SC exhibits 131 folds higher on/off ratio (3807) than the counterpart of (2,2'-bpy)2Pb3I10 SC (29). The presented result provides an effective strategy for exporting novel organic cation-based low-dimensional perovskite SC for high-performance optoelectronic devices.

Understanding the Behaviors of Plasmon-Induced Hot Carriers and Their Applications in Photocatalysis

Photocatalysis driven by plasmon-induced hot carriers has been gaining increasing attention. Recent studies have demonstrated that plasmon-induced hot carriers can directly participate in photocatalytic reactions, leading to great enhancement in solar energy conversion efficiency, by improving the catalytic activity or changing selectivity. Nevertheless, the utilization efficiency of hot carriers remains unsatisfactory. Therefore, how to correctly understand the generation and transfer process of hot carriers, as well as accurately differentiate between the possible mechanisms, have become a key point of attention. In this review, we overview the fundamental processes and mechanisms underlying hot carrier generation and transport, followed by highlighting the importance of hot carrier monitoring methods and related photocatalytic reactions. Furthermore, possible strategies for the further characterization of plasmon-induced hot carriers and boosting their utilization efficiency have been proposed. We hope that a comprehensive understanding of the fundamental behaviors of hot carriers can aid in designing more efficient photocatalysts for plasmon-induced photocatalytic reactions.

The Synergistic Effect of Additives for Formamidinium-Based Inverted Dion-Jacobson 2D Perovskite Solar Cells with Enhanced Photovoltaic Performance

Two-dimensional (2D) perovskite solar cells (PSCs) have attracted rapid growing attention due to their excellent environmental and operational stability. As an important type of 2D perovskite, Dion-Jacobson (DJ) 2D perovskites exhibit better structural integrity and more stable optoelectronic properties than those of Ruddlesden-Popper (RP) ones because of the elimination of weak van der Waals interactions. Random phase distribution, phase impurity, and weak crystallinity, however, can lead to severe nonradiative recombination losses in 2D perovskites and inferior device stability. Herein, formamidinium chloride (FACl) and lead chloride (PbCl2) are selected as additives to fabricate efficient and stable DJ 2D PSCs. The synergistic effect of additives could efficiently induce crystallization and suppress the low-n phase perovskites. The obtained 2D perovskites exhibit extended charge lifetime and enhanced charge transfer. The corresponding PSC device delivers an efficiency of 16.63% with a significantly improved open-circuit voltage (VOC) of 1.18 V and a fill factor (FF) of 81.65% than the control one. This PCE ranks the highest for inverted FA-based 2D DJ PSCs. Moreover, this device has exhibited exceptional long-term stability, which retains more than 95% of the initial efficiencies at about 50% relative humidity for 600 h.

2D Materials Beyond Post-AI Era: Smart Fibers, Soft Robotics, and Single Atom Catalysts

Recent consecutive discoveries of various 2D materials have triggered significant scientific and technological interests owing to their exceptional material properties, originally stemming from 2D confined geometry. Ever-expanding library of 2D materials can provide ideal solutions to critical challenges facing in current technological trend of the fourth industrial revolution. Moreover, chemical modification of 2D materials to customize their physical/chemical properties can satisfy the broad spectrum of different specific requirements across diverse application areas. This review focuses on three particular emerging application areas of 2D materials: smart fibers, soft robotics, and single atom catalysts (SACs), which hold immense potentials for academic and technological advancements in the post-artificial intelligence (AI) era. Smart fibers showcase unconventional functionalities including healthcare/environmental monitoring, energy storage/harvesting, and antipathogenic protection in the forms of wearable fibers and textiles. Soft robotics aligns with future trend to overcome longstanding limitations of hard-material based mechanics by introducing soft actuators and sensors. SACs are widely useful in energy storage/conversion and environmental management, principally contributing to low carbon footprint for sustainable post-AI era. Significance and unique values of 2D materials in these emerging applications are highlighted, where the research group has devoted research efforts for more than a decade.

Managing Excess Lead Iodide with Functionalized Oxo-Graphene Nanosheets for Stable Perovskite Solar Cells

Stability issues could prevent lead halide perovskite solar cells (PSCs) from commercialization despite it having a comparable power conversion efficiency (PCE) to silicon solar cells. Overcoming drawbacks affecting their long-term stability is gaining incremental importance. Excess lead iodide (PbI2 ) causes perovskite degradation, although it aids in crystal growth and defect passivation. Herein, we synthesized functionalized oxo-graphene nanosheets (Dec-oxoG NSs) to effectively manage the excess PbI2 . Dec-oxoG NSs provide anchoring sites to bind the excess PbI2 and passivate perovskite grain boundaries, thereby reducing charge recombination loss and significantly boosting the extraction of free electrons. The inclusion of Dec-oxoG NSs leads to a PCE of 23.7 % in inverted (p-i-n) PSCs. The devices retain 93.8 % of their initial efficiency after 1,000 hours of tracking at maximum power points under continuous one-sun illumination and exhibit high stability under thermal and ambient conditions.

Recent Advances in Wide-Bandgap Organic-Inorganic Halide Perovskite Solar Cells and Tandem Application

Perovskite-based tandem solar cells have attracted increasing interest because of its great potential to surpass the Shockley-Queisser limit set for single-junction solar cells. In the tandem architectures, the wide-bandgap (WBG) perovskites act as the front absorber to offer higher open-circuit voltage (VOC) for reduced thermalization losses. Taking advantage of tunable bandgap of the perovskite materials, the WBG perovskites can be easily obtained by substituting halide iodine with bromine, and substituting organic ions FA and MA with Cs. To date, the most concerned issues for the WBG perovskite solar cells (PSCs) are huge VOC deficit and severe photo-induced phase separation. Reducing VOC loss and improving photostability of the WBG PSCs are crucial for further efficiency breakthrough. Recently, scientists have made great efforts to overcome these key issues with tremendous progresses. In this review, we first summarize the recent progress of WBG perovskites from the aspects of compositions, additives, charge transport layers, interfaces and preparation methods. The key factors affecting efficiency and stability are then carefully discussed, which would provide decent guidance to develop highly efficient and stable WBG PSCs for tandem application.

Humidity-Insensitive, Large-Area-Applicable, Hot-Air-Assisted Ambient Fabrication of 2D Perovskite Solar Cells

2D layered perovskites (LPs) have shown great potential to deliver high-performance photovoltaic devices with long-term stability. Despite many signs of progress being made in film quality and device performance, LP films are mainly processed in strict conditions and through non-scalable techniques. Here, the hot-air-assisted ambient fabrication technique is introduced to prepare LP films for efficient and stable solar cells. The high-quality LP films with good crystallinity, preferable orientation and desirable morphology are obtained by balancing the crystal nucleation and growth processes. Employing the synchrotron-based in situ grazing-incidence X-ray diffraction technique, hot air induces the solidification of solutes and forms an intermediate at the air-liquid interface, which transforms into 3D-like perovskite, followed by the growth of the 2D species toward the substrate. The optimal LP film delivers a device power conversion efficiency of 16.36%, the best value for the LP-based solar cells prepared by the non-spin-coating techniques. The solar cell performance is insensitive to the film processing humidity and the device size is upscalable, which promises real-world deployment of LP-based optoelectronic devices.

Design of Pd-Zn Bimetal MOF Nanosheets and MOF-Derived Pd3.9Zn6.1/CNS Catalyst for Selective Hydrogenation of Acetylene under Simulated Front-End Conditions

Novel zinc–palladium–porphyrin bimetal metal–organic framework (MOF) nanosheets were directly synthesized by coordination chelation between Zn(II) and Pd(II) tetra(4-carboxyphenyl)porphin (TCPP(Pd)) using a solvothermal method. Furthermore, a serial of carbon nanosheets supported Pd–Zn intermetallics (Pd–Zn-ins/CNS) with different Pd: Zn atomic ratios were obtained by one-step carbonization under different temperature using the prepared Zn-TCPP(Pd) MOF nanosheets as precursor. In the carbonization process, Pd–Zn-ins went through the transformation from PdZn (650 °C) to Pd_3.9Zn_6.1 (~950 °C) then to Pd_3.9Zn_6.1/Pd (1000 °C) with the temperature increasing. The synthesized Pd–Zn-ins/CNS were further employed as catalysts for selective hydrogenation of acetylene. Pd_3.9Zn_6.1 showed the best catalytic performance compared with other Pd–Zn intermetallic forms.

Advances in Perovskites for Photovoltaic Applications in Space

Perovskites have emerged as promising light harvesters in photovoltaics. The resulting solar cells (i) are thin and lightweight, (ii) can be produced through solution processes, (iii) mainly use low-cost raw materials, and (iv) can be flexible. These features make perovskite solar cells intriguing as space technologies; however, the extra-terrestrial environment can easily cause the premature failure of devices. In particular, the presence of high-energy radiation is the most dangerous factor that can damage space technologies. This Review discusses the status and perspectives of perovskite photovoltaics in space applications. The main factors used to describe the space environment are introduced, and the results concerning the radiation hardness of perovskites toward protons, electrons, neutrons, and γ-rays are presented. Emphasis is given to the physicochemical processes underlying radiation damage in such materials. Finally, the potential use of perovskite solar cells in extra-terrestrial conditions is discussed by considering the effects of the space environment on the choice of the architecture and components of the devices.

Unveiling the effect of 2D silagraphene structural diversity on electronic properties: DFT, DOS, and ELF studies

Recently, fully π-functional two-dimensional (2D) materials have been reported for electronic device applications. Graphene is one of these 2D materials that is attributed to 2D electron confinement effects and exhibits an aromatic character; however, it is characterized by vanishing the bandgap energy. Hence, research was focused on the discovery of graphene-based 2D materials to reduce the bandgap energy. Herein, we investigate the silagraphene structures (Si_xC_y) using DFT calculations to undertake and improve structural, physico-chemical, and electronic properties. Various types of 2D networks have been investigated by considering C-C and C-Si bonds in relative positions. Both conjugation and hyperconjugation phenomenon have been deeply examined and it seemed that they take advantage of each other depending on the C-C and C-Si bond positions. Localized orbital locator (LOL) and electron localization function (ELF) were also performed to examine the electronic densities in the investigated 2D networks and unveil the electronic properties of the studied materials.

The evolution of triphenylamine hole transport materials for efficient perovskite solar cells

In recent years, the dramatic increase in power conversion efficiency (PCE) coupled with a decrease in the total cost of third-generation solar cells has led to a significant increase in the collaborative research efforts of academic and industrial researchers. Such interdisciplinary studies have afforded novel materials, which in many cases are now ready to be brought to the marketplace. Within this framework, the field of perovskite solar cells (PSCs) is currently an important area of research due to their extraordinary light-harvesting properties. In particular, PSCs prepared via facile synthetic procedures, containing hole transport materials (HTMs) with versatile triphenylamine (TPA) structural cores, amenable to functionalization, have become a focus of intense global research activity. To optimize the efficiency of the solar cells to achieve efficiencies closer to rival silicon-based technology, TPA building blocks must exhibit favourable electrochemical, photophysical, and photochemical properties that can be chemically tuned in a rational manner. Although PSCs based on TPA building blocks exhibit attractive properties such as high-power efficiencies, a reduction in their synthetic costs coupled with higher stabilities and environmental considerations still need to be addressed. Considering the above, a detailed summary of the most promising compounds and current methodologies employed to overcome the remaining challenges in this field is provided. The objective of this review is to provide guidance to readers on exploring new avenues for the discovery of efficient TPA derivatives, to aid in the future development and advancement of TPA-based PSCs for commercial applications.

Origin and physical effects of edge states in two-dimensional Ruddlesden-Popper perovskites

The edge region of two-dimensional (2D) Ruddlesden-Popper (RP) perovskites exhibits anomalous properties from the bulk region, including low energy emission and superior capability of dissociating exciton, which is highly beneficial for the optoelectronic devices like solar cells and photodetectors, denoted as “edge states”. In this review, we introduce the recent progress on the edge states that have been focused on the origin and the optoelectronic properties of edge states in 2D RP perovskites. By providing theoretical frameworks and experimental observations, we elucidate the origin of the edge states from two aspects, intrinsic electronic properties and extrinsic structure distortions. Besides, we introduce the physical properties of the edge states and current debating on this topic. Finally, we present an outlook on future research about the edge states of 2D RP perovskites.

Engineering the Defects and Microstructures in Ferroelectrics for Enhanced/Novel Properties: An Emerging Way to Cope with Energy Crisis and Environmental Pollution

In the past century, ferroelectrics are well known in electroceramics and microelectronics for their unique ferroelectric, piezoelectric, pyroelectric, and photovoltaic effects. Nowadays, the advances in understanding and tuning of these properties have greatly promoted a broader application potential especially in energy and environmental fields, by harvesting solar, mechanical, and heat energies. For example, high piezoelectricity and high pyroelectricity can be designed by defect or microstructure engineering for piezo- and pyro-catalyst, respectively. Moreover, highly piezoelectric and broadband (UV-Vis-NIR) light-responsive ferroelectrics can be designed via defect engineering, giving rise to a new concept of photoferroelectrics for efficient photocatalysis, piezocatalysis, pyrocatalysis, and related cocatalysis. This article first summarizes the recent developments in ferroelectrics in terms of piezoelectricity, pyroelectricity, and photovoltaic effects based on defect and microstructure engineering. Then, the potential applications in energy generation (i.e., photovoltaic effect, H2 generation, and self-powered multisource energy harvesting and signal sensing) and environmental protection (i.e., photo-piezo-pyro- cocatalytic dye degradation and CO2 reduction) are reviewed. Finally, the outlook and challenges are discussed. This article not only covers an overview of the state-of-art advances of ferroelectrics, but also prospects their applications in coping with energy crisis and environmental pollution.

Thiocyanate-Passivated Diaminonaphthalene-Incorporated Dion-Jacobson Perovskite for Highly Efficient and Stable Solar Cells

Two-dimensional (2D) metal halide perovskites have recently emerged as promising photovoltaic materials due to their superior ambient stability and rich structural diversity. However, power conversion efficiencies (PCEs) of the 2D perovskites solar cells (PSCs) still lag behind their three-dimensional (3D) counterpart, particularly due to the anisotropy in the charge carrier mobility and inhomogeneous energy landscape. A promising alternative is Dion-Jacobson (D-J) phase quasi-2D perovskite, where the bulky organic diammonium cations are introduced into inorganic frameworks to remove the weak van der Waals interactions between interlayers and to improve the open-circuit voltage (V_oc). Although the D-J phase 2D perovskite shows a homogeneous energy landscape and better charge transport, their poor crystallinity and existence of higher trap states remain a major challenge for the development of high-efficiency solar cells device. To address this issue, here, we report the eclipsed D-J phase 2D perovskite using 1,5-diaminonaphthalene cation and subsequently treated the film with ammonium thiocyanate (NH₄SCN) additive to further improve the film crystallinity, out-of-plane orientation, and carrier mobility. We observe that 2 mol NH₄SCN surface treatment in NDA-based D-J phase perovskite leads to better film morphology and improved crystallinity, as confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Time-resolved photoluminescence (TRPL) spectroscopy and steady-state space charge limited current (SCLC) mobility measurement reveal a significant reduction of trap-assisted nonradiative recombination and improvement of carrier mobility in the thiocyanate-passivated perovskite. Consequently, the PCE of the NH₄SCN-treated (NDA)(MA)₃(Pb)₄(I)₁₃ perovskite device enhanced nearly 46% from 10.3 to 15.08%. We have further studied intensity-dependent J-V characteristics, which demonstrate the reduction of ideality factor, confirming the effective suppression of trap-assisted nonradiative recombination, consistent with the transient PL results. Electrochemical impedance spectroscopy (EIS) confirms the improved charge carrier transport in NH₄SCN additive-treated devices. Interestingly, our additive-engineered unsealed perovskite devices retained 75% of their initial efficiency after 1000 h of continuous storage under 60% relative humidity. This study opens up the strategy for developing high-efficiency and stable 2D perovskite solar cells.

Atomically Thin 2D Photocatalysts for Boosted H2 Production from the perspective of Transient Absorption Spectroscopy

Excited state photophysical processes play the most important role in deciding the efficiency of any photonic applications like solar light driven H2 evolution, which is considered to be the next...

Colloidal Inorganic Ligand-Capped Nanocrystals: Fundamentals, Status, and Insights into Advanced Functional Nanodevices

Colloidal nanocrystals (NCs) are intriguing building blocks for assembling various functional thin films and devices. The electronic, optoelectronic, and thermoelectric applications of solution-processed, inorganic ligand (IL)-capped colloidal NCs are especially promising as the performance of related devices can substantially outperform their organic ligand-capped counterparts. This in turn highlights the significance of preparing IL-capped NC dispersions. The replacement of initial bulky and insulating ligands capped on NCs with short and conductive inorganic ones is a critical step in solution-phase ligand exchange for preparing IL-capped NCs. Solution-phase ligand exchange is extremely appealing due to the highly concentrated NC inks with completed ligand exchange and homogeneous ligand coverage on the NC surface. In this review, the state-of-the-art of IL-capped NCs derived from solution-phase inorganic ligand exchange (SPILE) reactions are comprehensively reviewed. First, a general overview of the development and recent advancements of the synthesis of IL-capped colloidal NCs, mechanisms of SPILE, elementary reaction principles, surface chemistry, and advanced characterizations is provided. Second, a series of important factors in the SPILE process are offered, followed by an illustration of how properties of NC dispersions evolve after ILE. Third, surface modifications of perovskite NCs with use of inorganic reagents are overviewed. They are necessary because perovskite NCs cannot withstand polar solvents or undergo SPILE due to their soft ionic nature. Fourth, an overview of the research progresses in utilizing IL-capped NCs for a wide range of applications is presented, including NC synthesis, NC solid and film fabrication techniques, field effect transistors, photodetectors, photovoltaic devices, thermoelectric, and photoelectrocatalytic materials. Finally, the review concludes by outlining the remaining challenges in this field and proposing promising directions to further promote the development of IL-capped NCs in practical application in the future.

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 multifunctional 2D black phosphorene-based platform for improved photovoltaics

As one of the latest additions to the 2D nanomaterials family, black phosphorene (BP, monolayer or few-layer black phosphorus) has gained much attention in various forms of solar cells. This is due largely to its intriguing semiconducting properties such as tunable direct bandgap (from 0.3 eV in the bulk to 2.0 eV in the monolayer), extremely high ambipolar carrier mobility, broad visible to infrared light absorption, etc. These appealing optoelectronic attributes make BP a multifunctional nanomaterial for use in solar cells via tailoring carrier dynamics, band energy alignment, and light harvesting, thereby promoting the rapid development of third-generation solar cells. Notably, in sharp contrast to the copious work on revealing the fundamental properties of BP, investigation into the utility of BP is comparatively less, particularly in the area of photovoltaics. Herein, we first identify and summarize an array of unique characteristics of BP that underpin its application in photovoltaics, aiming at providing inspiration to develop new designs and device architectures of photovoltaics. Subsequently, state-of-the-art synthetic routes (i.e., top-down and bottom-up) to scalable BP production that facilitates its applications in optoelectronic materials and devices are outlined. Afterward, recent advances in a diverse set of BP-incorporated solar cells, where BP may impart electron and/or hole extraction and transport, function as a light absorber, provide dielectric screening for enhancing exciton dissociation, and modify the morphology of photoabsorbers, are discussed, including organic solar cells, dye-sensitized solar cells, heterojunction solar cells and perovskite solar cells. Finally, the challenges and opportunities in this rapidly evolving field are presented.

Designs from single junctions, heterojunctions to multijunctions for high-performance perovskite solar cells

Hybrid metal-halide perovskite solar cells (PVSCs) have drawn unprecedented attention during the last decade due to their superior photovoltaic performance, facile and low-cost fabrication, and potential for roll-to-roll mass production and application for portable devices. Through collective composition, interface, and process engineering, a comprehensive understanding of the structure-property relationship and carrier dynamics of perovskites has been established to help achieve a very high certified power conversion efficiency (PCE) of 25.5%. Apart from material properties, the modified heterojunction design and device configuration evolution also play crucial roles in enhancing the efficiency. The adoption and/or modification of heterojunction structures have been demonstrated to effectively suppress the carrier recombination and potential losses in PVSCs. Moreover, the employment of multijunction structures has been shown to reduce thermalization losses, achieving a high PCE of 29.52% in perovskite/silicon tandem solar cells. Therefore, understanding the evolution of the device configuration of PVSCs from single junction, heterojunction to multijunction designs is helpful for the researchers in this field to further boost the PCE beyond 30%. Herein, we summarize the evolution and progress of the single junction, heterojunction and multijunction designs for high-performance PVSCs. A comprehensive review of the fundamentals and working principles of these designs is presented. We first introduce the basic working principles of single junction PVSCs and the intrinsic properties (such as crystallinity and defects) in perovskite films. Afterwards, the progress of diverse heterojunction designs and perovskite-based multijunction solar cells is synopsized and reviewed. Meanwhile, the challenges and strategies to further enhance the performance are also summarized. At the end, the perspectives on the future development of perovskite-based solar cells are provided. We hope this review can provide the readers with a quick catchup on this emerging solution-processable photovoltaic technology, which is currently at the transition stage towards commercialization.

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.

Recent progress in metal sulfide-based electron transport layers in perovskite solar cells

High-quality electron transport layers (ETLs) are essential for stable and efficient perovskite solar cells (PSCs). Metal sulfides (MSs) are considered potential candidates for ETLs due to their high carrier mobility, low cost, and favorable chemical and physical stability. The quality of the MS films plays important role in the photovoltaic performance of PSCs. However, few reports focus on the relative preparation, characteristics, and corresponding mechanisms of MS-based ETLs. In this review, MS-based ETLs are summarized according to their preparation strategies and the mechanism. We hope that this review can help others understand the intrinsic phenomena of MS-based ETLs and motivate further investigations.

Interface Engineering for Perovskite Solar Cells Based on 2D-Materials: A Physics Point of View

The last decade has witnessed the advance of metal halide perovskites as a promising low-cost and efficient class of light harvesters used in solar cells (SCs). Remarkably, the efficiency of lab-scale perovskite solar cells (PSCs) reached a power conversion efficiency of 25.5% in just ~10 years of research, rivalling the current record of 26.1% for Si-based PVs. To further boost the performances of PSCs, the use of 2D materials (such as graphene, transition metal dichalcogenides and transition metal carbides, nitrides and carbonitrides) has been proposed, thanks to their remarkable optoelectronic properties (that can be tuned with proper chemical composition engineering) and chemical stability. In particular, 2D materials have been demonstrated as promising candidates for (i) accelerating hot carrier transfer across the interfaces between the perovskite and the charge extraction layers; (ii) improving the crystallization of the perovskite layers (when used as additives in the precursor solution); (iii) favoring electronic bands alignment through tuning of the work function. In this mini-review, we discuss the physical mechanisms underlying the increased efficiency of 2D material-based PSCs, focusing on the three aforementioned effects.

2D Hybrid Halide Perovskites: Structure, Properties, and Applications in Solar Cells

2D metal-halide perovskites have attracted intense research interest due to superior long-term stability under ambient environments. Compared to their 3D analog, the alternate arrangement of organic and inorganic layers leads to forming a multilayer quantum well (MQW), which endows 2D perovskites with anisotropic optoelectronic properties. In addition, the spacer layer functions as a hydrophobic barrier to effectively prevent 2D perovskite films from ion migration and moisture penetrating, thus realizing outstanding stability. Recently, 2D perovskites have been widely developed with abundant species. The stunning photovoltaic performance with the coexistence of long-term stability and high-power conversion efficiency (PCE) has been realized in 2D perovskite solar cells (PSCs), which paves an avenue for commercialization of PSCs. This review begins with an introduction of crystal structure and crystallization kinetics to illustrate the unique layer characters in 2D perovskites. Then, electron structure, excitons, dielectric confinement, and intrinsic stability properties are discussed in detail. Next, the photovoltaic performance based on recent Ruddlesden-Popper (RP), Dion-Jacobson (DJ), and alternating cations in the interlayer (ACI) phase 2D-PSCs is comprehensively summarized. Finally, the confronting challenges and strategies toward structural design and optoelectronic studies of 2D perovskites are proposed to offer insight into the advanced underlying properties of this family of materials.

Ultrafast femtosecond pressure modulation of structure and exciton kinetics in 2D halide perovskites for enhanced light response and stability

The carriers’ transportation between layers of two-dimensional (2D) perovskites is inhibited by dielectric confinement. Here, for the first time, we employ a femtosecond laser to introduce ultrafast shock pressure in the range of 0~15.45 GPa to reduce dielectric confinement by modulating the structure and exciton dynamics in a perovskite single crystal (PSCs), e.g. (F-PEA)₂PbI₄ (4-fluorophenethylammonium, F-PEA). The density functional theory (DFT) simulation and experimental results show that the inorganic framework distortion results in a bandgap reduction. It was found that the exciton-optical phonon coupling and free excitons (FEs) binding energy are minimized at 2.75 GPa shock pressure due to a reduction in dielectric confinement. The stability testing under various harsh light and humid thermal conditions shows that femtosecond laser shocking improves the stability of (F-PEA)2PbI4 PSCs. Femtosecond laser shock processing provides a new approach for regulating the structure and enhancing halide perovskite properties.

Exciton in two-dimensional perovskite is strongly influenced by dielectric confinement of the organic components. Here, the authors employ femtosecond laser to induce ultrashock pressure to investigate how the structural changes and the reduction of dielectric confinement affects exciton behaviour and dynamic.

Gold Nanomaterials and Bone/Cartilage Tissue Engineering: Biomedical Applications and Molecular Mechanisms

Recently, as our population increasingly ages with more pressure on bone and cartilage diseases, bone/cartilage tissue engineering (TE) have emerged as a potential alternative therapeutic technique accompanied by the rapid development of materials science and engineering. The key part to fulfill the goal of reconstructing impaired or damaged tissues lies in the rational design and synthesis of therapeutic agents in TE. Gold nanomaterials, especially gold nanoparticles (AuNPs), have shown the fascinating feasibility to treat a wide variety of diseases due to their excellent characteristics such as easy synthesis, controllable size, specific surface plasmon resonance and superior biocompatibility. Therefore, the comprehensive applications of gold nanomaterials in bone and cartilage TE have attracted enormous attention. This review will focus on the biomedical applications and molecular mechanism of gold nanomaterials in bone and cartilage TE. In addition, the types and cellular uptake process of gold nanomaterials are highlighted. Finally, the current challenges and future directions are indicated.

Robust Molecular Dipole-Enabled Defect Passivation and Control of Energy-Level Alignment for High-Efficiency Perovskite Solar Cells

The ability to passivate defects and modulate the interface energy-level alignment (IEA) is key to boost the performance of perovskite solar cells (PSCs). Herein, we report a robust route that simultaneously allows defect passivation and reduced energy difference between perovskite and hole transport layer (HTL) via the judicious placement of polar chlorine-terminated silane molecules at the interface. Density functional theory (DFT) points to effective passivation of the halide vacancies on perovskite surface by the silane chlorine atoms. An integrated experimental and DFT study demonstrates that the dipole layer formed by the silane molecules decreases the perovskite work function, imparting an Ohmic character to the perovskite/HTL contact. The corresponding PSCs manifest a nearly 20 % increase in power conversion efficiency over pristine devices and a markedly enhanced device stability. As such, the use of polar molecules to passivate defects and tailor the IEA in PSCs presents a promising platform to advance the performance of PSCs.

2D Lead Iodide Perovskite with Mercaptan-Containing Amine and Its Exceptional Water Stability

Two dimensional (2D) hybrid perovskites have attracted a great deal of interest because of their appropriate photovoltaic efficiency and environmental stability. Although some 2D hybrid perovskites with sulfur-containing amines have been reported, the cation having the mercaptan group has not been well explored yet. In this work, cysteamine (Cya, HS(CH₂)₂NH₂), a mercaptan-containing amine, was introduced into 2D hybrid perovskite. Two 2D lead iodides with different structures, (HCya)₂PbI₄ (1) and (HCya)₇Pb₄I₁₅ (2), were isolated as a red low-temperature phase and a yellow high-temperature phase, respectively. X-ray single-crystal structural analysis showed that the red phase 1 is a single layered corner-shared perovskite and that the yellow phase 2 is a corner/edge-shared quasi-2D perovskite. A thermo-induced reversible 1 to 2 phase transition was found in this synthetic system. The configuration of HCya cation greatly influences the crystallization equilibrium, generating different structures of the lead halides. The single-crystal structure of 1 is discussed in comparison with that of (HAE)₂PbI₄ (AE = HO(CH₂)₂NH₂), an analogue of 1. The different effects of OH and SH groups on the 2D frameworks are studied based on their hydrogen bonding properties. More remarkably, although the two perovskites have similar structures, the (HCya)₂PbI₄ (1) has an intrinsic water stability that is much more stable than (HAE)₂PbI₄, which should be attributed to the affinity of the SH group with lead on the surface of the lead halide.

High-Efficiency of Bi-Functional-Based Perovskite Nanocomposite for Oxygen Evolution and Oxygen Reduction Reaction: An Overview

High efficient, low-cost and environmentally friendly-natured bi-functional-based perovskite electrode catalysts (BFPEC) are receiving increasing attention for oxygen reduction/oxygen evolution reaction (ORR/OER), playing an important role in the electrochemical energy conversion process using fuel cells and rechargeable batteries. Herein, we highlighted the different kinds of synthesis routes, morphological studies and electrode catalysts with A-site and B-site substitution co-substitution, generating oxygen vacancies studies for boosting ORR and OER activities. However, perovskite is a novel type of oxide family, which shows the state-of-art electrocatalytic performances in energy storage device applications. In this review article, we go through different types of BFPECs that have received massive appreciation and various strategies to promote their electrocatalytic activities (ORR/OER). Based on these various properties and their applications of BFPEC for ORR/OER, the general mechanism, catalytic performance and future outlook of these electrode catalysts have also been discussed.

Polymer-Inorganic Thermoelectric Nanomaterials: Electrical Properties, Interfacial Chemistry Engineering, and Devices

Though solar cells are one of the promising technologies to address the energy crisis, this technology is still far from commercialization. Thermoelectric materials offer a novel opportunity to convert energy between thermal and electrical aspects, which show the feasibility to improve the performance of solar cells via heat management and light harvesting. Polymer–inorganic thermoelectric nanocomposites consisting of inorganic nanomaterials and functional organic polymers represent one kind of advanced hybrid nanomaterials with tunable optical and electrical characteristics and fascinating interfacial and surface chemistry. During the past decades, they have attracted extensive research interest due to their diverse composition, easy synthesis, and large surface area. Such advanced nanomaterials not only inherit low thermal conductivity from polymers and high Seebeck coefficient, and high electrical conductivity from inorganic materials, but also benefit from the additional interface between each component. In this review, we provide an overview of interfacial chemistry engineering and electrical feature of various polymer–inorganic thermoelectric hybrid nanomaterials, including synthetic methods, properties, and applications in thermoelectric devices. In addition, the prospect and challenges of polymer–inorganic nanocomposites are discussed in the field of thermoelectric energy.

High-efficient liquid exfoliation of 2D metal-organic framework using deep-eutectic solvents

The exfoliation of bulk two-dimensional metal-organic framework (MOF) into few-layered nanosheets has attracted much attention recently. In this work, an environmental-friendly route has been developed for layered-MOF (MAMS-1) delamination using deep eutectic solvent (DES), which is more sustainable and efficient alternative than conventional organic solvents for MOF nanosheet preparation. Under sonication condition, DES as solvents, the highest exfoliation rate of MAMS-1 is up to 70% with two host layers via poly(vinylpyrrolidone) (PVP) surfactant-assisted method. The presence of tert-butyl exteriors and the atomically thickness endow the MOF nanosheets stable suspension for at least one month. Due to the 2D structure and excellent stability, MAMS-1 nanosheet (MAMS-1-NS) was chosen as a good candidate to encapsulate Eu3+ cations. The obtained Eu3+@MAMS-1-NS acts as a multi-responsive luminescent sensor through fluorescence quenching, and can specifically recognize Fe3+ (LOD = 0.40 μM, KSV = 1.05 × 105 M-l), Hg2+ (LOD = 0.038 μM, KSV = 5.78 × 106 M-l), Cr2O72- (LOD = 0.33 μM, KSV = 1.55 × 105 M-l) and MnO4- (LOD = 0.088 μM, KSV = 4.49 × 105 M-l). Compared with bulk Eu3+@MAMS-1, the sensitivity of Eu3+@MAMS-1-NS is greatly improved owing to its ultrathin nanosheet morphology and highly accessible active sites on the surface.

Photodetectors of 2D Materials from Ultraviolet to Terahertz Waves

2D materials are considered to be the most promising materials for photodetectors due to their unique optical and electrical properties. Since the discovery of graphene, many photodetectors based on 2D materials have been reported. However, the low quantum efficiency, large noise, and slow response caused by the thinness of 2D materials limit their application in photodetectors. Here, recent progress on 2D material photodetectors is reviewed, covering the spectrum from ultraviolet to terahertz waves. First the interaction of 2D materials with light is analyzed in terms of optical physics. Then the present methods to improve the performance of 2D material photodetectors are summarized, such as defect engineering, p-n junctions and hybrid detectors, and the issue of serious overestimation of the performance in reported photodetectors based on 2D materials is discussed. Next, a comparison of 2D material photodetectors with traditional commercially available detectors shows that it is difficult to balance the current 2D material photodetectors with regard to having simultaneously both high sensitivity and fast response. Finally, a possible novel EIW mechanism is suggested to advance the performance of 2D material photodetectors in the future.

2D materials for conducting holes from grain boundaries in perovskite solar cells

Grain boundaries in organic-inorganic halide perovskite solar cells (PSCs) have been found to be detrimental to the photovoltaic performance of devices. Here, we develop a unique approach to overcome this problem by modifying the edges of perovskite grain boundaries with flakes of high-mobility two-dimensional (2D) materials via a convenient solution process. A synergistic effect between the 2D flakes and perovskite grain boundaries is observed for the first time, which can significantly enhance the performance of PSCs. We find that the 2D flakes can conduct holes from the grain boundaries to the hole transport layers in PSCs, thereby making hole channels in the grain boundaries of the devices. Hence, 2D flakes with high carrier mobilities and short distances to grain boundaries can induce a more pronounced performance enhancement of the devices. This work presents a cost-effective strategy for improving the performance of PSCs by using high-mobility 2D materials.

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

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

Laser Processing Optimization for Large-Area Perovskite Solar Modules

The industrial exploitation of perovskite solar cell technology is still hampered by the lack of repeatable and high-throughput fabrication processes for large-area modules. The joint efforts of the scientific community allowed to demonstrate high-performing small area solar cells; however, retaining such results over large area modules is not trivial. Indeed, the development of deposition methods over large substrates is required together with additional laser processes for the realization of the monolithically integrated cells and their interconnections. In this work, we develop an efficient perovskite solar module based on 2D material engineered structure by optimizing the laser ablation steps (namely P1, P2, P3) required for shaping the module layout in series connected sub-cells. We investigate the impact of the P2 and P3 laser processes, carried out by employing a UV pulsed laser (pulse width = 10 ns; λ = 355 nm), over the final module performance. In particular, a P2 process for removing 2D material-based cell stack from interconnection area among adjacent cells is optimized. Moreover, the impact of the P3 process used to isolate adjacent sub-cells after gold realization over the module performance once laminated in panel configuration is elucidated. The developed fabrication process ensures high-performance repeatability over a large module number by demonstrating the use of laser processing in industrial production.

Cost-efficient, Effect of Low-Quality PbI2 Purification to Enhance Performances of Perovskite Quantum Dots and Perovskite Solar Cells

In modern society, high-quality material development and a large stable supply are key to perform frontier research and development. However, there are negative issues to address to utilize high-quality resources with a large stable supply for research, such as economic accessibility, commercialization, and so on. One of the cutting-edge research fields, perovskite-related research, usually requires high-quality chemicals with outstanding purity (>99%). We developed an economically feasible PbI2 precursor with around 1/20 cost-down for perovskite/perovskite quantum dots through recrystallization and/or hydrothermal purification. Following the methodology, the quantum dots from both as-prepared and purified PbI2 demonstrated identical photophysical properties, with a photoluminescence quantum yield (PLQY) of 52.61% using the purified PbI2 vs. 45.83% PLQY using commercial PbI2. The role of hydrothermal energy was also checked against the problematic PbI2, and we checked whether the hydrothermal energy could contribute to the hindrance of undesired particle formation in the precursor solution, which enables them to form enlarged grain size from 179 ± 80 to 255 ± 130 nm for higher photoconversion efficiency of perovskite solar cells from 14.77 ± 1.82% to 15.18 ± 1.92%.

Exploring Light Polarization Effects of Photovoltaic Actions in Organic-Inorganic Hybrid Perovskites with Asymmetric and Symmetric Unit Structures

Hybrid perovskites are known as electrically polarizable semiconductors based on theoretical prediction and experimental information. Here we show the light polarization effects on the photovoltaic actions in perovskite solar cells by optically generating directional and random electronic transition dipoles within asymmetric and symmetric unit structures. In an asymmetric tetragonal unit structure, we observed the stripes with different orientations onto perovskite grains and that the randomly polarized photoexcitation can generate a higher photocurrent (Jsc) by 6.5 ± 0.5% as compared to the linearly polarized photoexcitation at same intensity in the hysteresis-free perovskite solar cells [ITO/PEDOT:PSS/MAPbI₃/PC₆₁BM/PEI/Ag]. Clearly, switching the photoexcitation between linear and random polarizations leads to a ΔJsc, which provides an experimental indication that all-directional and one-directional transition dipoles generate higher and lower photocurrents in organic-inorganic hybrid perovskites (MAPbI₃) with an asymmetric tetragonal unit structure. This implies that all-directional and one-directional transition dipoles develop stronger and weaker dissociative interactions, consequently giving rise to more and less dissociation toward generating photocurrent. This is confirmed by the experimental observation that the ΔJsc almost disappears when the temperature increases up to 55 °C, where the asymmetric tetragonal structure is changed to a symmetric cubic structure. Furthermore, the ΔJsc is shown to decrease with increasing light intensity. This indicates that the electronic transition dipoles encounter a polarization relaxation caused by mutual interaction. We show that the polarization relaxation time in MAPbI₃ is comparable to exciton dissociation time (∼ps). This presents the necessary condition to demonstrate light polarization effects of photovoltaic actions in perovskite solar cells.

A kirigami-inspired island-chain design for wearable moistureproof perovskite solar cells with high stretchability and performance stability

A range of power generating approaches, such as integration with clothing, fashion accessories, or textiles, allow electronic devices to be charged in environmentally friendly ways. Stretchable, efficient, stable, and even washable solar cells are considered the next necessary component to supply continuous wearable energy. However, ultra-thin photo-active materials are often fragile, which inevitably raises challenges for electron conduction during stretching and washing processes, thus resulting in unsatisfactory output performance. Herein, we have removed the stumbling block by designing a kirigami-inspired unique island-chain structure with serpentine interconnects, which prevented the photo-active layer of subcells from being subjected to excessive strain. Notably, this is the first time perovskite solar cell arrays met the above wearable requirements simultaneously. The obtained devices exhibited a high yet stable power output (efficiency of 17.68%) accompanied by a robust cycling performance (87% of the initial PCE) even after 300 times of continuous stretching with a large ratio of 80%. The efficiency of the optimized PSCs maintains promising stability after being exposed in a harsh environment (80% humidity) for 10 days. As textile-compatible power sources, the successfully designed stretchable and moisture-resistant photovoltaics add power-generation functionality to clothing, opening a new avenue for applications as long-term power sources for wearable electronics.