Bilateral alkylamine for suppressing charge recombination and improving stability in blade-coated perovskite solar cells

Tailoring Defects Regulation in Air-Fabricated CsPbI3 for Efficient Inverted All-Inorganic Perovskite Solar Cells with Voc of 1.225 V

Air fabrication of CsPbI₃ perovskite photovoltaics has been attractive and fast-moving owing to its compatibility to low-cost and up-scalable fabrication. However, due to the inevitable erosions, undesirable traps are formed in air-fabricated CsPbI₃ crystals and seriously hinder photovoltaic performance with poor reproduction. Here, 3, 5-difluorobenzoic acid hydrazide (FBJ) is incorporated as trap regulation against external erosions in air-fabricated CsPbI₃. Theoretical simulations reveal that FBJ molecules feature stronger absorbance on CsPbI₃ than water, which can regulate trap formations for water erosions. In addition, FBJ with solid bonding interaction to CsPbI₃ can enlarge formation energy of various defects during crystallization and further suppress traps. Moreover, profiling to reductive hydrazine groups, FBJ inhibits traps for oxidation erosions. Consequently, a champion efficiency of 19.27% with an impressive V_oc of 1.225 V is realized with the inverted CsPbI₃ devices. Moreover, the optimized devices present superior stability and contain 97.4% after operating at 60 °C for 600 h.

Triarylamine/Bithiophene Copolymer with Enhanced Quinoidal Character as Hole-Transporting Material for Perovskite Solar Cells

Polytriarylamine is a popular hole-transporting materials (HTMs) despite its suboptimal conductivity and significant recombination at the interface in a solar cell setup. Having noted insufficient conjugation among the triarylamine units along the polymer backbone, we inserted a bithiophene unit between two triarylamine units through iron-catalyzed C-H/C-H coupling of a triarylamine/thiophene monomer so that two units conjugate effectively via four quinoidal rings when the molecule functions as HTM. The obtained triarylamine/bithiophene copolymer (TABT) used as HTM showed a high-performance in methylammonium lead iodide perovskite (MAPbI₃ ) solar cells. Mesityl substituted TABT forms a uniform film, shows high hole-carrier mobility, and has an ionization potential (IP=5.40 eV) matching that of MAPbI₃ . We fabricated a solar cell device with a power conversion efficiency of 21.3 % and an open-circuit voltage of 1.15 V, which exceeds the performance of devices using reference standard such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) and Spiro-OMeTAD.

Universal Surface Passivation of Organic-Inorganic Halide Perovskite Films by Tetraoctylammonium Chloride for High-Performance and Stable Perovskite Solar Cells

The power conversion efficiency (PCE) of perovskite solar cells has been showing rapid improvement in the last decade. However, still, there is an unarguable performance deficit compared with the Schockley-Queisser (SQ) limit. One of the major causes for such performance discrepancy is surface and grain boundary defects. They are a source of nonradiative recombination in the devices that not only causes performance loss but also instability of the solar cells. In this study, we employed a direct postsurface passivation strategy at mild temperatures to modify perovskite layer defects using tetraoctylammonium chloride (TOAC). The passivated perovskite layers have demonstrated extraordinary improvement in photoluminescence and charge carrier lifetimes compared to their control counterparts in both Cs_0.05(FAPbI₃)_0.83(MAPbBr₃)_0.17 and MAPbI₃-type perovskite layers. The investigation on electron-only and hole-only devices after TOAC treatment revealed suppressed electron and hole trap density of states. The electrochemical study demonstrated that TOAC treatment improved the charge recombination resistance of the perovskite layers and reduced the charge accumulation on the surface of perovskite films. As a result, perovskite solar cells prepared by TOAC treatment showed a champion PCE of 21.24% for the Cs_0.05(FAPbI₃)_0.83(MAPbBr₃)_0.17-based device compared to 19.58% without passivation. Likewise, the PCE of MAPbI₃ improved from 18.09 to 19.27% with TOAC treatment. The long-term stability of TOAC-passivated perovskite Cs_0.05(FAPbI₃)_0.83(MAPbBr₃)_0.17 devices has retained over 97% of its initial performance after 720 h in air.

Monolithic Perovskite-Silicon Tandem Solar Cells: From the Lab to Fab?

This review focuses on monolithic 2-terminal perovskite-silicon tandem solar cells and discusses key scientific and technological challenges to address in view of an industrial implementation of this technology. The authors start by examining the different crystalline silicon (c-Si) technologies suitable for pairing with perovskites, followed by reviewing recent developments in the field of monolithic 2-terminal perovskite-silicon tandems. Factors limiting the power conversion efficiency of these tandem devices are then evaluated, before discussing pathways to achieve an efficiency of >32%, a value that small-scale devices will likely need to achieve to make tandems competitive. Aspects related to the upscaling of these device active areas to industry-relevant ones are reviewed, followed by a short discussion on module integration aspects. The review then focuses on stability issues, likely the most challenging task that will eventually determine the economic viability of this technology. The final part of this review discusses alternative monolithic perovskite-silicon tandem designs. Finally, key areas of research that should be addressed to bring this technology from the lab to the fab are highlighted.

Halide Perovskite Solar Cells for Building Integrated Photovoltaics: Transforming Building Façades into Power Generators

The rapid emergence of organic-inorganic lead halide perovskites for low-cost and high-efficiency photovoltaics promises to impact new photovoltaic concepts. Their high power conversion efficiencies, ability to coat perovskite layers on glass via various scalable deposition techniques, excellent optoelectronic properties, and synthetic versatility for modulating transparency and color allow perovskite solar cells (PSCs) to be an ideal solution for building-integrated photovoltaics (BIPVs), which transforms windows or façades into electric power generators. In this review, the unique features and properties of PSCs for BIPV application are accessed. Device engineering and optical management strategies of active layers, interlayers, and electrodes for semitransparent, bifacial, and colorful PSCs are also discussed. The performance of PSCs under conditions that are relevant for BIPV such as different operational temperature, light intensity, and light incident angle are also reviewed. Recent outdoor stability testing of PSCs in different countries and other demonstration of scalability and deployment of PSCs are also spotlighted. Finally, the current challenges and future opportunities for realizing perovskite-based BIPV are discussed.

Homogeneous solution assembled Turing structures with near zero strain semi-coherence interface

Turing structures typically emerge in reaction-diffusion processes far from thermodynamic equilibrium, involving at least two chemicals with different diffusion coefficients (inhibitors and activators) in the classic Turing systems. Constructing a Turing structure in homogeneous solutions is a large challenge because of the similar diffusion coefficients of most small molecule weight species. In this work, we show that Turing structure with near zero strain semi-coherence interfaces is constructed in homogeneous solutions subject to the diffusion kinetics. Experimental results combined with molecular dynamics and numerical simulations confirm the Turing structure in the spinel ferrite films. Furthermore, using the hard-soft acid-base theory, the design of coordination binding can improve the diffusion motion of molecules in homogeneous solutions, increasing the library of Turing structure designs, which provides a greater potential to develop advanced materials.

From Structural Design to Functional Construction: Amine Molecules in High-Performance Formamidinium-Based Perovskite Solar Cells

Formamidinium (FA) based perovskites are considered as one of the most promising light-absorbing perovskite materials owing to their narrower band gap and better thermal stability compared to conventional methylammonium-based perovskites. Constant improvement by using various additives stimulates the potential application of these perovskites. Amine molecules with different structures have been widely used as typical additives in FA-based perovskite solar cells, and decent performances have been achieved. Thus, a systematic review focusing on structural regulation and functional construction of amines in FA-based perovskites is of significance. Herein, we analyze the construction mechanism of different structural amines on the functional perovskite crystals. The influence of amine molecules on specific perovskite properties including defect conditions, charge transfer, and moisture resistance are evaluated. Finally, we summarize the design rules of amine molecules for the application in high-performance FA-based perovskites and propose directions for the future development of additive molecules.

Manipulating Ion Migration and Interfacial Carrier Dynamics via Amino Acid Treatment in Planar Perovskite Solar Cells

Instability caused by the migrating ions is one of the major obstacles toward the large-scale application of metal halide perovskite optoelectronics. Inactivating mobile ions/defects via chemical passivation, e.g., amino acid treatment, is a widely accepted approach to solve that problem. To investigate the detailed interplay, L-phenylalanine (PAA), a typical amino acid, is used to modify the SnO₂/MAPbI₃ interface. The champion device with PAA treatment maintains 80% of its initial power conversion efficiency (PCE) when stored after 528 h in an ambient condition with the relative humidity exceeding 70%. By employing a wide-field photoluminescence imaging microscope to visualize the ion movement and calculate ionic mobility quantitatively, we propose a model for enhanced stability in perspective of suppressed ion migration. Besides, we reveal that the PAA dipole layer facilitates charge transfer at the interface, enhancing the PCE of devices. Our work may provide an in-depth understanding toward high-efficiency and stable perovskite optoelectronic devices.

A Review of Recent Developments in Preparation Methods for Large-Area Perovskite Solar Cells

The recent rapid development in perovskite solar cells (PSCs) has led to significant research interest due to their notable photovoltaic performance, currently exceeding 25% power conversion efficiency for small-area PSCs. The materials used to fabricate PSCs dominate the current photovoltaic market, especially with the rapid increase in efficiency and performance. The present work reviews recent developments in PSCs’ preparation and fabrication methods, the associated advantages and disadvantages, and methods for improving the efficiency of large-area perovskite films for commercial application. The work is structured in three parts. First is a brief overview of large-area PSCs, followed by a discussion of the preparation methods and methods to improve PSC efficiency, quality, and stability. Envisioned future perspectives on the synthesis and commercialization of large-area PSCs are discussed last. Most of the growth in commercial PSC applications is likely to be in building integrated photovoltaics and electric vehicle battery charging solutions. This review concludes that blade coating, slot-die coating, and ink-jet printing carry the highest potential for the scalable manufacture of large-area PSCs with moderate-to-high PCEs. More research and development are key to improving PSC stability and, in the long-term, closing the chasm in lifespan between PSCs and conventional photovoltaic cells.

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

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

Designing New Indene-Fullerene Derivatives as Electron-Transporting Materials for Flexible Perovskite Solar Cells

The synthesis and characterization of a family of indene-C₆₀ adducts obtained via Diels-Alder cycloaddition [4 + 2] are reported. The new C₆₀ derivatives include indenes with a variety of functional groups. These adducts show lowest unoccupied molecular orbital energy levels to be at the right position to consider these compounds as electron-transporting materials for planar heterojunction perovskite solar cells. Selected derivatives were applied into inverted (p-i-n configuration) perovskite device architectures, fabricated on flexible polymer substrates, with large active areas (1 cm²). The highest power conversion efficiency, reaching 13.61%, was obtained for the 6'-acetamido-1',4'-dihydro-naphtho[2',3':1,2][5,6]fullerene-C₆₀ (NHAc-ICMA). Spectroscopic characterization was applied to visualize possible passivation effects of the perovskite's surface induced by these adducts.

Functionalizing phenethylammonium by methoxy to achieve low-dimensional interface defects passivation for efficient and stable perovskite solar cells

Low dimensional interface passivation has been proved to be an efficient method to lessen the nonradiative recombination loss in perovskite solar cells. To overcome the limitation of Phenethylammonium (PEA⁺) for carrier transport and water molecule intrusion, we developed a modification strategy by functioning the typical PEA⁺with the 4-methoxy to optimize the interface defects and carrier transport performance, thus maximizing the synchronous improvement of device efficiency and stability. Our results indicate that the 2 mg ml^-14-methoxy-phenethylammonium (MeO-PEA⁺) modified device could achieve a best power conversion efficiency of 19.64% with improved shelf-life stability in ambient conditions. The new passivation molecule of MeO-PEA⁺could possess the capability of defect passivation, carrier transfer, and moisture blocking, demonstrating that rationally designed organic components for interface passivation could help to achieve efficient and stable PSCs.

Printing strategies for scaling-up perovskite solar cells

Photovoltaic technology offers a sustainable solution to the problem of soaring global energy demands. Recently, metal halide perovskite solar cells (PSCs) have attracted worldwide interest because of their high power conversion efficiency of 25.5% and great potential in becoming a disruptive technology in the photovoltaic industry. The transition from research to commercialization requires advancements of scalable deposition methods for both perovskite and charge transporting thin films. Herein, we share our view regarding the current challenges to fabrication of PSCs by printing techniques. We focus particularly on ink technologies, and summarize the strategies for printing uniform, pinhole-free perovskite films with good crystallinity. Moreover, the stability of perovskite solar modules is discussed and analyzed. We believe this review will be advantageous in the area of printable electronic devices.

Interfacial Linkage and Carbon Encapsulation Enable Full Solution-Printed Perovskite Photovoltaics with Prolonged Lifespan

Simplified perovskite solar cells (PSCs) were fabricated with the perovskite layer sandwiched and encapsulated between carbon-based electron transport layer (ETL) and counter electrode (CE) by a fully blade-coated process. A self-assembled monolayer of amphiphilic silane (AS) molecules on transparent conducting oxide (TCO) substrate appeals to the fullerene ETL deposition and preserves its integrity against the solvent damage. The AS serves as a "molecular glue" to strengthen the adhesion toughness at the TCO/ETL interface via robust chemical interaction and bonding, facilitating the interfacial charge extraction, increasing PCEs by 77 % and reducing hysteresis. A PCE of 18.64 % was achieved for the fully printed devices, one of the highest reported for carbon-based PSCs. AS-assisted interfacial linkage and carbon-material-assisted self-encapsulation enhance the stability of the PSCs, which did not experience performance degradation when stored at ambient conditions for over 3000 h.

Advances in Flexible Memristors with Hybrid Perovskites

Hybrid organic-inorganic metal halide perovskite (HOIP)-based memristors have captured strong attention not only as an emerging candidate for next-generation high-density information storage technology but also for use in healthcare technology and the Internet of Things (IoT) because of their unique properties: low weight, flexibility, compatibility, stretchability, and low power consumption. In this Perspective, we review the recent advances of various aspects of flexible memristors focusing on the selection of the flexible substrates, materials, interfaces, several resistive switching mechanisms, and different methodologies of perovskite growth. The current state of the art of the memristor as an artificial synapse, light-induced resistive switching, and logic gates is comprehensively and systematically reviewed. Finally, we briefly discuss the stability factors of perovskites and present the conclusion with a broad outlook on the progress and challenges in the field of perovskite-based flexible memristors.

Correlating the Active Layer Structure and Composition with the Device Performance and Lifetime of Amino-Acid-Modified Perovskite Solar Cells

Additive engineering is emerging as a powerful strategy to further enhance the performance of perovskite solar cells (PSCs), with the incorporation of bulky cations and amino acid (AA) derivatives being shown as a promising strategy for enhanced device stability. However, the incorporation of such additives typically results in photocurrent losses owing to their saturated carbon backbones, hindering charge transport and collection. Here, we investigate the use of AAs with varying carbon chain lengths as zwitterionic additives to enhance the PSC device stability, in air and nitrogen, under illumination. We, however, discovered that the device stability is insensitive to the chain length as the anticipated photocurrent drops as the chain length increases. Using glycine as an additive results in an improvement in the open circuit voltage from 1.10 to 1.14 V and a resulting power conversion efficiency of 20.2% (20.1% stabilized). Using time-of-flight secondary ion mass spectrometry, we confirm that the AAs reside at the surfaces and interfaces of our perovskite films and propose the mechanisms by which stability is enhanced. We highlight this with glycine as an additive, whereby an 8-fold increase in the device lifetime in ambient air at 1 sun illumination is recorded. Short-circuit photoluminescence quenching of complete devices is reported, which reveals that the loss in photocurrent density observed with longer carbon chain AAs results from the inefficient charge extraction from the perovskite absorber layer. These combined results demonstrate new fundamental understandings about the photophysical processes of additive engineering using AAs and provide a significant step forward in improving the stability of high-performance PSCs.

Stability of Perovskite Solar Cells: Degradation Mechanisms and Remedies

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

Interfacial Defects Change the Correlation between Photoluminescence, Ideality Factor, and Open-Circuit Voltage in Perovskite Solar Cells

The ideality factor (n_id ) and photoluminescence (PL) analyses assess charge recombination characteristics in perovskite solar cells (PeSCs). However, their correlations with open-circuit voltage (V_oc ) are often found to be complicated depending on the recombination types in the devices. Herein, the correlation of n_id , PL characteristics and V_oc is elucidated depending on the interfacial crystal quality in triple-cation mixed-halide perovskite, Cs_0.05 (MA_0.17 FA_0.83 )_0.95 Pb(I_0.83 Br_0.17 )₃ , deposited on different hole transport layers (HTLs). In the devices with low quality interfacial crystals, V_oc increases together with n_id , which originates from the light intensity-dependence of majority carrier at the interface. Meanwhile, a negative correlation between V_oc and n_id is observed for devices with high quality interfacial crystals. The authors discuss the cases that PL enhancement by the improvement of overall crystal quality can fail to correlate with a V_oc increase if interfacial crystal quality becomes worse. The study highlights that interfacial crystal quality evaluation can help to understand charge recombination via n_id and PL measurements, and more importantly provide information of which defect engineering between at the interface and in the bulk would be more effective for device optimization.

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

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

Impact of Amine Additives on Perovskite Precursor Aging: A Case Study of Light-Emitting Diodes

Amines are widely employed as additives for improving the performance of metal halide perovskite optoelectronic devices. However, amines are well-known for their high chemical reactivity, the impact of which has yet to receive enough attention from the perovskite light-emitting diode community. Here, by investigating an unusual positive aging effect of CH₃NH₃I/CsI/PbI₂ precursor solutions as an example, we reveal that amines gradually undergo N-formylation in perovskite precursors over time. This reaction is initialized by hydrolysis of dimethylformamide in the acidic chemical environment. Further investigations suggest that the reaction products collectively impact perovskite crystallization and eventually lead to significantly enhanced external quantum efficiency values, increasing from ∼2% for fresh solutions to ≳12% for aged ones. While this case study provides a positive aging effect, a negative aging effect is possible in other perovksite systems. Our findings pave the way for more reliable and reproducible device fabrication and call for further attention to underlying chemical reactions within the perovskite inks once amine additives are included.

Inclusion of 2D Transition Metal Dichalcogenides in Perovskite Inks and Their Influence on Solar Cell Performance

Organic–inorganic hybrid perovskite materials have raised great interest in recent years due to their excellent optoelectronic properties, which promise stunning improvements in photovoltaic technologies. Moreover, two-dimensional layered materials such as graphene, its derivatives, and transition metal dichalcogenides have been extensively investigated for a wide range of electronic and optoelectronic applications and have recently shown a synergistic effect in combination with hybrid perovskite materials. Here, we report on the inclusion of liquid-phase exfoliated molybdenum disulfide nanosheets into different perovskite precursor solutions, exploring their influence on final device performance. We compared the effect of such additives upon the growth of diverse perovskites, namely CH₃NH₃PbI₃ (MAPbI₃) and triple-cation with mixed halides Cs_x (MA_0.17FA_0.83)_(1−x)Pb (I_0.83Br_0.17)₃ perovskite. We show how for the referential MAPbI₃ materials the addition of the MoS₂ additive leads to the formation of larger, highly crystalline grains, which result in a remarkable 15% relative improvement in power conversion efficiency. On the other hand, for the mixed cation–halide perovskite no improvements were observed, confirming that the nucleation process for the two materials is differently influenced by the presence of MoS₂.

How to apply metal halide perovskites to photocatalysis: challenges and development

Semiconductor photocatalysts are widely used in environmental remediation and energy conversion processes that affect social development. These processes involve, for example, hydrogen production from water splitting, carbon dioxide reduction, pollutant degradation, and the conversion of raw organic chemical materials into high-value-added chemicals. Metal halide perovskites (MHPs) have become a new class of promising cheap and easy to manufacture candidate materials for use in photocatalytic semiconductors due to their advantages of high extinction coefficients, optimal band gaps, high photoluminescence quantum yields, and long electron-hole diffusion lengths. However, their unstable ion-bonded crystal structures (very low theoretical decomposition energy barriers) limit their widespread application. In this review, we introduce the physical properties of MHP materials suitable for photocatalysis, and MHP-based photocatalytic particle suspension systems, photoelectrode thin film systems, and photovoltaic-photo(electro)chemical systems. Then, numerous studies realizing efficient and stable photocatalytic water splitting, carbon dioxide reduction, organic conversion, and other reactions involving MHP materials were highlighted. In addition, we conducted rigorous analysis of the potential problems that could hinder progress in this new scientific research field, such as Pb element toxicity and material instability. Finally, we outline the potential opportunities and directions for photocatalysis research based on MHPs.

Mechanism and Timescales of Reversible p-Doping of Methylammonium Lead Triiodide by Oxygen

Understanding and controlling the energy level alignment at interfaces with metal halide perovskites (MHPs) is essential for realizing the full potential of these materials for use in optoelectronic devices. To date, however, the basic electronic properties of MHPs are still under debate. Particularly, reported Fermi level positions in the energy gap vary from indicating strong n- to strong p-type character for nominally identical materials, raising serious questions about intrinsic and extrinsic defects as dopants. ​In this work, photoemission experiments demonstrate that thin films of the prototypical methylammonium lead triiodide (MAPbI3 ) behave like an intrinsic semiconductor in the absence of oxygen. Oxygen is then shown to be able to reversibly diffuse into and out of the MAPbI3 bulk, requiring rather long saturation timescales of ≈1 h (in: ambient air) and over 10 h (out: ultrahigh vacuum), for few 100 nm thick films. Oxygen in the bulk leads to pronounced p-doping, positioning the Fermi level universally ≈0.55 eV above the valence band maximum. The key doping mechanism is suggested to be molecular oxygen substitution of iodine vacancies, supported by density functional theory calculations. This insight rationalizes previous and future electronic property studies of MHPs and calls for meticulous oxygen exposure protocols.

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

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

Synergistic Engineering of Conduction Band, Conductivity, and Interface of Bilayered Electron Transport Layers with Scalable TiO2 and SnO2 Nanoparticles for High-Efficiency Stable Perovskite Solar Cells

A simple, synergistic engineering of the conduction band (CB), conductivity, and interface of TiO₂-based bilayered electron transport layers (ETLs) via scalable TiO₂ and SnO₂ nanoparticles processed at low temperature (≤ 100 °C) for regular planar perovskite solar cells (PSCs) was developed. The bottom layer (Lt-TiO₂:SnO₂ nanocomposite film) was prepared by spin coating from the ethanol suspension of small ground TiO₂ nanoparticles with big ground SnO₂ nanoparticles as the additive. The top C-SnO₂ layer (spin-coated from the concentrated commercial SnO₂ nanoparticles (C-SnO₂ NPs, 20 wt %, 7 nm in size suspended in H₂O)) can be regarded as an interlayer between Lt-TiO₂:SnO₂ and perovskite (Psk) absorbers. Bilayered Lt-TiO₂:SnO₂/C-SnO₂ ETLs are dense films with a cascade CB, good conductivity, facile electron extraction/transport ability, and a highly hydrophilic surface for depositing high-quality Psk films. Regular planar PSCs based on Lt-TiO₂:SnO₂/C-SnO₂ ETLs combined with a (FAI)_0.90(PbI₂)_0.94(MABr)_0.10(PbBr₂)_0.10 absorber and a spiro-OMeTAD hole transporter achieved the highest power conversion efficiency of 22.04% with a negligible current hysteresis. The champion cell lost less than 3% of the initial efficiency under continuous room lighting (1000 lux) for 1000 h (lost 10% after 2184 h) without encapsulation under an inert atmosphere. Four related low-temperature-processed ETLs (Lt-TiO₂/C-SnO₂, Lt-C-SnO₂, Lt-TiO₂:SnO₂, and Lt-TiO₂) were fabricated using the same metal oxide nanoparticle suspensions and studied simultaneously to reveal the function of each metal oxide in the bilayered Lt-TiO₂:SnO₂/C-SnO₂ ETLs. In the bottom Lt-TiO₂:SnO₂ layer, small TiO₂ nanoparticles were needed for making a dense film, and highly conducting big SnO₂ nanoparticles are used to increase the conductivity of ETLs and a handy electron transport path for reducing the charge accumulation and series resistance of the cell. A top C-SnO₂ layer (regarded as an interlayer between Psk and Lt-TiO₂:SnO₂) was used to extract/transport electrons facilely, to form a bilayered ETL with a cascade CB, and to create a hydrophilic surface to deposit high-quality Psk films to enhance the photovoltaic performance of the PSCs. This study provides a blueprint for designing good-performance ETLs for high-efficiency, stable regular planar PSCs using various sized nanoparticles prepared in a very simple and low-cost way.

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

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

Compositional and Interfacial Engineering Yield High-Performance and Stable p-i-n Perovskite Solar Cells and Mini-Modules

Through the optimization of the perovskite precursor composition and interfaces to selective contacts, we achieved a p-i-n-type perovskite solar cell (PSC) with a 22.3% power conversion efficiency (PCE). This is a new performance record for a PSC with an absorber bandgap of 1.63 eV. We demonstrate that the high device performance originates from a synergy between (1) an improved perovskite absorber quality when introducing formamidinium chloride (FACl) as an additive in the "triple cation" Cs_0.05FA_0.79MA_0.16PbBr_0.51I_2.49 (Cs-MAFA) perovskite precursor ink, (2) an increased open-circuit voltage, V_OC, due to reduced recombination losses when using a lithium fluoride (LiF) interfacial buffer layer, and (3) high-quality hole-selective contacts with a self-assembled monolayer (SAM) of [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) on ITO electrodes. While all devices exhibit a high performance after fabrication, as determined from current-density voltage, J-V, measurements, substantial differences in device performance become apparent when considering longer-term stability data. A reduced long-term stability of devices with the introduction of a LiF interlayer is compensated for by using FACl as an additive in the metal-halide perovskite thin-film deposition. Optimized devices maintained about 80% of the initial average PCE during maximum power point (MPP) tracking for >700 h. We scaled the optimized device architecture to larger areas and achieved fully laser patterned series-interconnected mini-modules with a PCE of 19.4% for a 2.2 cm² active area. A robust device architecture and reproducible deposition methods are fundamental for high performance and stable large-area single junction and tandem modules based on PSCs.

Gentle Materials Need Gentle Fabrication: Encapsulation of Perovskites by Gas-Phase Alumina Deposition

Metal halide perovskites have attracted tremendous attention as promising materials for future-generation optoelectronic devices. Despite their outstanding optical and transport properties, the lack of environmental and operational stability remains a major practical challenge. One of the promising stabilization avenues is metal oxide encapsulation via atomic layer deposition (ALD); however, the unavoidable reaction of metal precursors with the perovskite surface in conventional ALD leads to degradation and restructuring of the perovskites' surfaces. This Perspective highlights the development of a modified gas-phase ALD technique for alumina encapsulation that not only prevents perovskites' degradation but also significantly improves their optical properties and air stability. The correlation between precise atomic interactions at the perovskite-metal oxide interface with the dramatically enhanced optical properties is supported by density functional theory calculations, which also underlines the widespread applicability of this gentle technique for a variety of perovskite nanostructures unbarring potential opportunities offered by combination of these approaches.

Printing High-Efficiency Perovskite Solar Cells in High-Humidity Ambient Environment-An In Situ Guided Investigation

Extensive studies are conducted on perovskite solar cells (PSCs) with significant performance advances (mainly spin coating techniques), which have encouraged recent efforts on scalable coating techniques for the manufacture of PSCs. However, devices fabricated by blade coating techniques are inferior to state-of-the-art spin-coated devices because the power conversion efficiency (PCE) is highly dependent on the morphology and crystallization kinetics in the controlled environment and the delicate solvent system engineering. In this study, based on the widely studied perovskite solution system dimethylformamide-dimethyl sulfoxide, air-knife-assisted ambient fabrication of PSCs at a high relative humidity of 55 ± 5% is reported. In-depth time-resolved UV-vis spectrometry is carried out to investigate the impact of solvent removal and crystallization rate, which are critical factors influencing the crystallization kinetics and morphology because of adventitious moisture. UV-vis spectrometry enables accurate determination of the thickness of the wet precursor film. Anti-solvent-free, high-humidity ambient coatings of hysteresis-free PSCs with PCEs of 21.1% and 18.0% are demonstrated for 0.06 and 1 cm2 devices, respectively. These PSCs exhibit comparable stability to those fabricated in a glovebox, thus demonstrating their high potential.

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

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

Large-area and efficient perovskite light-emitting diodes via low-temperature blade-coating

Large-area light-emitting diodes (LEDs) fabricated by mass-production techniques are needed for low-cost flat-panel lighting. Nevertheless, it is still challenging to fabricate efficient large-area LEDs using organic small molecules (OLEDs), quantum dots (QLEDs), polymers (PLEDs), and recently-developed hybrid perovskites (PeLEDs) due to difficulties controlling film uniformity. To that end, we report sol-gel engineering of low-temperature blade-coated methylammonium lead iodide (MAPbI3) perovskite films. The precipitation, gelation, aging, and phase transformation stages are dramatically shortened by using a diluted, organoammonium-excessed precursor, resulting in ultra-flat large-area films (54 cm2) with roughness reaching 1 nm. The external quantum efficiency of doctor-bladed PeLEDs reaches 16.1%, higher than that of best-performing blade-coated OLEDs, QLEDs, and PLEDs. Furthermore, benefitting from the throughput of the blade-coating process and cheap materials, the expected cost of the emissive layer is projected to be as low as 0.02 cents per cm2, emphasizing its application potential.

Zwitterions: promising interfacial/doping materials for organic/perovskite solar cells

This review summarizes the recent progress in zwitterionic materials through the concepts of interfacial dipoles and passivating defects.

Self-Driven Perovskite Narrowband Photodetectors with Tunable Spectral Responses

Narrowband photodetectors with tunable spectral responses are highly desirable for applications in image sensing, machine vision, and optical communication. Herein, a filterless and self-driven perovskite narrowband photodetector (PNPD) based on the defect-assisted charge collection narrowing (CCN) mechanism is reported, which is enabled by a high-quality thick perovskite film. By adjusting the halide component of the perovskite layer, the bandgap is successfully modulated and the corresponding narrowband photodetectors show a wide spectral response range from the red to the near-infrared (NIR), all with full-widths at half maximum (FWHMs) below 30 nm. Specifically, the methylammonium lead iodide (MAPbI₃ ) narrowband photodetector exhibits a characteristic detection peak at 800 nm with a very low noise current of ≈0.02 pA Hz^-1/2 , a high specific detectivity up to 1.27 × 10¹² Jones, and a fast response speed with rise/fall time of 12.7/6.9 µs. Impressively, these values are among the highest of their kind reported previously, and allow demonstration of narrowband imaging. The excellent performance of self-driven PNPDs lights up their prospect in high-efficiency optoelectronic devices without external power sources.

Inorganic material passivation of defects toward efficient perovskite solar cells

Surface passivation with organic materials is one of the most effective and popular strategies to improve the stability and efficiency of perovskite solar cells (PSCs). However, the secondary bonding formed between organic molecules and perovskite layers is still not strong enough to protect the perovskite absorber from degradation initialized by oxygen and water attacking at defects. Recently, passivation with inorganic materials has gradually been favored by researchers due to the effectiveness of chemical and mechanical passivation. Lead-containing substances, alkali metal halides, transition elements, oxides, hydrophobic substances, etc. have already been applied to the surface and interfacial passivation of PSCs. These inorganic substances mainly manipulate the nucleation and crystallization process of perovskite absorbers by chemically passivating defects along grain boundaries and surface or forming a mechanically protective layer simultaneously to prevent the penetration of moisture and oxygen, thereby improving the stability and efficiency of the PSCs. Herein, we mainly summarize inorganic passivating materials and their individual passivation principles and methods. Finally, this review offers a personal perspective for future research trends in the development of passivation strategies through inorganic materials.

Multifunctional Charge Transporting Materials for Perovskite Light-Emitting Diodes

Despite their low exciton-binding energies, metal halide perovskites are extensively studied as light-emitting materials owing to narrow emission with high color purity, easy/wide color tunability, and high photoluminescence quantum yields. To improve the efficiency of perovskite light-emitting diodes (PeLEDs), much effort has been devoted to controlling the emitting layer morphologies to induce charge confinement and decrease the nonradiative recombination. The interfaces between the emitting layer and charge transporting layer (CTL) are vulnerable to various defects that deteriorate the efficiency and stability of the PeLEDs. Therefore, the establishment of multifunctional CTLs that can improve not only charge transport but also critical factors that influence device performance, such as defect passivation, morphology/phase control, ion migration suppression, and light outcoupling efficiency, are highly required. Herein, the fundamental limitations of perovskites as emitters (i.e., defects, morphological and phase instability, high refractive index with poor outcoupling) and the recent developments with regard to multifunctional CTLs to compensate such limitations are summarized, and their device applications are also reviewed. Finally, based on the importance of multifunctional CTLs, the outlook and research prospects of multifunctional CTLs for the further improvement of PeLEDs are discussed.

Nanoparticle Wetting Agent for Gas Stream-Assisted Blade-Coated Inverted Perovskite Solar Cells and Modules

Lab-scale perovskite solar cells (PSCs) have recently reached power conversion efficiencies (PCEs) of up to 25.2%. However, a reliable transfer of solution processing from spin coating to scalable printing techniques and a homogeneous deposition on large substrate sizes is challenging also caused by dewetting of the perovskite precursor solution on highly hydrophobic subjacent materials. In this work, we report the utilization of blade-coated nonconductive silicon oxide (SiO₂) nanoparticles (NPs) as wetting agent for the precursor solution to enable the deposition of a homogeneous perovskite layer on the nonwetting hole transport layer (HTL). The NPs enhance the HTL surface energy, thus, wetting and homogeneous spreading of the precursor solution is strongly improved so that pinholes in the perovskite layer are avoided. In addition, we apply this concept for the first time for gas stream-assisted blade coating of PSCs and modules in the inverted (p-i-n) device architecture with poly(triaryl amine) (PTAA) as HTL on large-area substrates. To prevent void formation at the HTL interface of gas stream-assisted blade coated perovskite layers, the effect of blending small amounts of lead chloride (PbCl₂) in the perovskite precursor solution is investigated, which also improves reproducibility and device performance. Following these optimizations, blade coated PSCs with 0.24 cm² active area achieve up to 17.9% PCE. Furthermore, to prove scalability, we show enlarged substrates of up to 9 × 9 cm² and analyze the homogeneity of the perovskite layer in blade coating direction. Moreover, by implementing the blade coated NP wetting agent, we fabricate large-area modules with a maximum PCE of 9.3% on 49.60 cm² aperture area. This represents a further important step bringing solution-processed inverted PSCs closer to application.

High-Pressure Nitrogen-Extraction and Effective Passivation to Attain Highest Large-Area Perovskite Solar Module Efficiency

Slot-die coating holds advantages over other large-scale technologies thanks to its potential for well-controlled, high-throughput, continuous roll-to-roll fabrication. Unfortunately, it is challenging to control thin.film uniformity over a large area while maintaining crystallization quality. Herein, by using a high-pressure nitrogen-extraction (HPNE) strategy to assist crystallization, a wide processing window in the well-controlled printing process for preparing high-quality perovskites is achieved. The yellow-phase perovskite generated by the HPNE acts as a crucial intermediate phase to produce large-area high-quality perovskite film. Furthermore, an ionic liquid is developed to passivate the perovskite surface to reduce surface defect density and to suppress carrier recombination, resulting in significantly increased efficiency to 22.7%, the highest for large-area fabrication. The strategies are successfully extended to large-area device fabrication, making it possible to produce a 40 × 40 mm² module with stabilized PCE as high as 19.4%, the highest-efficiency for a large-area module to date.

Phenethylamine ligand engineering of red InP quantum dots for improving the efficiency of quantum dot light-emitting diodes

In this Letter, red-emitting multi-shelled indium phosphide (InP) quantum dots (QDs) were synthesized using the safe phosphorus precursor tris(dimethylamino)phosphine ((DMA)₃P). The long-chain ligands of oleylamine (OAm) in the (DMA)₃P phosphide source-based InP QDs were partially exchanged with short-chain ligands of phenethylamine (PEA) in the core formation process, and the resulting InP QDs were applied to quantum dot light-emitting diodes (QLEDs). The short-chain ligands of PEA with the π-conjugated benzene ring improved the charge transport and electrical conduction of the QLEDs with (DMA)₃P phosphide source-based InP QDs. The PEA-engineering of InP QDs improved their maximum quantum yield from 71% to 85.5% with the full-width at half-maximum of 62 nm. Furthermore, the maximum external quantum efficiency of QLEDs with the PEA-engineered InP QDs improved from 1.9% to 3.5%, and their maximum power efficiency increased from 2.8 to 6.0 lm/W. This Letter demonstrates that engineering the core formation process with the short-chain ligands of PEA provides an efficient and facile way to improve the charge transport and electrical conduction in (DMA)₃P phosphide source-based InP QLEDs for electroluminescent devices.

Roll-to-roll gravure-printed flexible perovskite solar cells using eco-friendly antisolvent bathing with wide processing window

Driven by recent improvements in efficiency and stability of perovskite solar cells (PSCs), upscaling of PSCs has come to be regarded as the next step. Specifically, a high-throughput, low-cost roll-to-roll (R2R) processes would be a breakthrough to realize the commercialization of PSCs, with uniform formation of precursor wet film and complete conversion to perovskite phase via R2R-compatible processes necessary to accomplish this goal. Herein, we demonstrate the pilot-scale, fully R2R manufacturing of all the layers except for electrodes in PSCs. Tert-butyl alcohol (tBuOH) is introduced as an eco-friendly antisolvent with a wide processing window. Highly crystalline, uniform formamidinium (FA)-based perovskite formation via tBuOH:EA bathing was confirmed by achieving high power conversion efficiencies (PCEs) of 23.5% for glass-based spin-coated PSCs, and 19.1% for gravure-printed flexible PSCs. As an extended work, R2R gravure-printing and tBuOH:EA bathing resulted in the highest PCE reported for R2R-processed PSCs, 16.7% for PSCs with R2R-processed SnO₂/FA-perovskite, and 13.8% for fully R2R-produced PSCs.

Driven by recent improvement in efficiency and stability of perovskite solar cells, the next step toward commercialisation is upscaling. Here, the authors demonstrate pilot-scale fully roll-to-roll manufacturing of flexible perovskite solar cells through gravure-printing and antisolvent bathing.