Steric Mixed-Cation 2D Perovskite as a Methylammonium Locker to Stabilize MAPbI3

Surface Termination on Unstable Methylammonium-based Perovskite Using a Steric Barrier for Improved Perovskite Solar Cells

Compared to widely adopted low-dimensional/three-dimensional (LD/3D) heterostructure, functional organic cation based surface termination on perovskite can not only realize advantage of defect passivation but also prevent potential disadvantage of the heterostructure induced intercalation into 3D perovskite. However, it is still very challenging to controllably construct surface termination on organic-inorganic hybrid perovskite because the functional organic cations' substitution reaction is easy to form LD/3D heterostructure. Here, we report using a novel benzyltrimethylammonium (BTA) functional cation with rational designed steric hindrance to effectively surface terminate onto methylammonium lead triiodide (MAPbI3 ) perovskite, which is composed of the most unstable MA cations. The BTA cation is difficult to form a specific 1.5-dimensional perovskite of BTA4 Pb3 I10 by cation substitution with MA cation, which then provides a wide processing window (around 10 minutes) for surface terminating on MAPbI3 films. Moreover, the BTAI surface terminated BTAI-MAPbI3 shows better passivation effect than BTA4 Pb3 I10 -MAPbI3 heterojunction. Finally, BTAI surface terminated solar cell (0.085 cm2 ) and mini-module (11.52 cm2 ) obtained the efficiencies of 22.03 % and 18.57 %, which are among the highest efficiencies for MAPbI3 based ones.

Manipulating Crystallographic Orientation via Cross-Linkable Ligand for Efficient and Stable Perovskite Solar Cells

The crystallographic orientation of polycrystalline perovskites is found to be strongly correlated with their intrinsic properties; therefore, it can be used to effectively enhance the performance of perovskite-based devices. Here, a facile way of manipulating the facet orientation of polycrystalline perovskite films in a controllable manner is reported. By incorporating a cross-linkable organic ligand into the perovskite precursor solution, the crystal orientation disorder can be reduced in the resultant perovskite films to exhibit the prominent (001) orientation with a preferred stacking mode. Moreover, the as-formed low-dimensional perovskites (LDPs) between the organic ligand and the excess lead iodide can passivate the defects around the grain boundaries. Consequently, highly efficient p-i-n structured perovskite solar cells (PSCs) can be made in both rigid and flexible forms from modified perovskites to show high power conversion efficiencies (PCE) of 24.12% and 23.23%, respectively. The devices also exhibit superior long-term stability in a humid environment (with T₉₀  > 1000 h) and under thermal stress (retaining 87% of its initial PCE after 1000 h). More importantly, the ligand enables the derived LDPs to be crosslinked (under 254 nm UV illumination) to demonstrate excellent mechanical bending durability in flexible devices.

Decoupled Water Electrolysis Driven by 1 cm2 Single Perovskite Solar Cell Yielding a Solar-to-Hydrogen Efficiency of 14.4

Solar water splitting by photovoltaic (PV) electrolysis is a promising route for sustainable hydrogen production. However, multiple PV cells connected in series are generally required to fulfil the practical electrolytic voltages, which inevitably increases the system complexity and resistance. Decoupled water electrolysis for separate hydrogen and oxygen evolution needs smaller voltage to drive each half-reaction, which provides a feasibility to achieve the single PV cell driven water electrolysis. Herein, by introducing sodium nickelhexacyanoferrate (NaNiHCF) as the redox mediator, decoupled acid water electrolyzer and amphoteric water electrolyzer were respectively constructed. The required voltages for the hydrogen or oxygen evolution steps matched with the output voltages of the perovskite solar cell (PSC). Impressively, by combining one 1 cm² FAPbI₃ -based PSC (efficiency: 18.77 %) with the decoupled amphoteric water electrolyzer, a solar-to-hydrogen (STH) efficiency of 14.4 % was achieved, which outperformed previously reported PSC-driven water electrolysis cells.

Interfacial engineering of halide perovskites and two-dimensional materials

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

Bottom-Up Templated and Oriented Crystallization for Inverted Triple-Cation Perovskite Solar Cells with Stabilized Nickel-Oxide Interface

Inverted-structure perovskite solar cells (PSCs) are known for their superior device stability. However, based on nickel-oxide (NiO_x ) substrate, disordered crystallization and bottom interface instability of perovskite film are still the main factors that compromise the power conversion efficiency (PCE) of PSCs. Here, 2D perovskite of thiomorpholine 1,1-dioxide lead iodide (Td₂ PbI₄ ) is introduced as a template to prepare 3D perovskite thin film with high crystal orientation and large grain size via a bottom-up growth method. By adding TdCl to the precursor solution, pre-crystallized 2D Td₂ PbI₄ seeds can accumulate at the bottom interface, lowering the barrier of nucleation, and templating the growth of 3D perovskite films with improved (100) orientation and reduced defects during crystallization. In addition, 2D Td₂ PbI₄ at the bottom interface also hinders the interfacial redox reaction and reduces the hole extraction barrier on the buried interface. Based on this, the Td-0.5 PSC achieves a PCE of 22.09% and an open-circuit voltage of 1.16 V. Moreover, Td-0.5 PSCs show extremely high stability, which retains 84% of its initial PCE after 500 h of continuous illumination under maximum power point operating conditions in N₂ atmosphere. This work paves the way for performance improvement of inverted PSCs on NiO_x substrate.

Surface-capping engineering for electrically neutral surface of perovskite films and stable solar cells

Metal halide perovskite solar cells (PSCs) have developed rapidly in recent years, due to their high performance and low-cost solution-based fabrication process. These excellent properties are mainly attributed to the high defect tolerance of polycrystalline perovskite films. Meanwhile, these defects can also facilitate ion migration and carrier recombination, which cause the device performance and the long-term stability of PSCs to deteriorate heavily. Therefore, it is critical to passivate the defects, especially at the surfaces of perovskite grains where the defects are most concentrated due to the dangling bonds. Here we propose a surface-capping engineering (SCE) method to construct 'dangling-bond-free' surfaces for perovskite grains. Diamine iodide (methylenediammonium diiodide, MDAI₂) was used to construct an electroneutral PbX₆-MDA-PbX₆(X = Cl, Br or I) layer at the perovskite surfaces. Compared to the monovalent FA⁺which can only coordinate one [PbX₆]^4-slab, the bivalent MDA²⁺can coordinate two [PbX₆]^4-slabs on both sides, thus realizing a dangling-bond-free surface. Solar cells based on SCE-perovskite films exhibited a higher power conversion efficiency (PCE) of 21.6%, compared with 19.9% of the control group; and maintained over 96% of its initial PCE after 13 h during the maximum power point tracking test under continuous AM1.5G illumination, whereas the control group only lasted 1.5 h. Constructing a dangling-bond-free capping layer on the grain boundary opens new avenues for the fabrication of ultralow-defect polycrystalline semiconductors, paving the way to further improve the PCE and lifetime of PSCs.

Temperature-Reliable Low-Dimensional Perovskites Passivated Black-Phase CsPbI3 toward Stable and Efficient Photovoltaics

Low-dimensional (LD) perovskites can effectively passivate and stabilize 3D perovskites for high-performance perovskite solar cells (PSCs). Regards CsPbI₃ -based PSCs, the influence of high-temperature annealing on the LD perovskite passivation effect has to be taken into account due to fact the black-phase CsPbI₃ crystallization requires high-temperature treatment, however, which has been rarely concerned so far. Here, the thermal stability of LD perovskites based on three hydrophobic organic ammonium salts and their passivation effect toward CsPbI₃ and the whole device performance, have been investigated. It is found that, phenyltrimethylammonium iodide (PTAI) and its corresponding LD perovskites exhibit excellent thermal stability. Further investigation reveals that PTAI-based LD perovskites are mainly distributed at grain boundaries, which not only enhances the phase stability of CsPbI₃ but also effectively suppresses non-radiative recombination. As a consequence, the champion PSC device based on CsPbI₃ exhibits a record efficiency of 21.0 % with high stability.

Multidimensional perovskite solar cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted extensive attention, and their certified power conversion efficiency (PCE) has reached 25.5%. However, the instability of the high-efficiency 3-dimensional (3D) perovskite against ambient conditions (moisture, light and thermal) and the existing defects severely limit its practical applications and commercialization. Unlike 3D perovskites, the large hydrophobic spacer cations in low-dimensional (2D, 1D, and 0D) perovskites are able to effectively improve the stability, but they also weaken the light absorption range and hinder charge transport. The construction of a low-dimensional/3D perovskite multidimensional structure, which can combine the advantages of the high stability of low-dimensional perovskites and the superior efficiency of 3D perovskites, is proposed to achieve high efficiency and ultrastability. Moreover, the proper incorporation of low-dimensional perovskite into 3D perovskite can passivate defects and inhibit ion migration. Herein, this article summarizes the recent research progress of low-dimensional/3D perovskite multidimensional structures for PSCs and provides some perspectives toward developing stable and efficient PSCs.

Advancing 2D Perovskites for Efficient and Stable Solar Cells: Challenges and Opportunities

Perovskite solar cells (PSCs) have rapidly emerged as one of the hottest topics in the photovoltaics community owing to their high power-conversion efficiencies (PCE), and the promise to be produced at low cost. Among various PSCs, typical 3D perovskite-based solar cells deliver high PCE but they suffer from severe instability, which restricts their practical applications. In contrast to 3D perovskites, 2D perovskites that incorporate larger, less volatile, and generally more hydrophobic organic cations exhibit much improved thermal, chemical, and environmental stability. 2D perovskites can have different roles within a solar cell, either as the primary light absorber (2D PSCs), or as a capping layer atop a 3D perovskite absorbing layer (2D/3D PSCs). Tradeoffs between PCE and stability exist in both types of PSCs-2D PSCs are more stable but exhibit lower efficiency while 2D/3D PSCs deliver exciting efficiency but show relatively poor stability. To address this PCE/stability tradeoff, the challenges both the 2D and 2D/3D PSCs face are identified and select works the community has undertaken to overcome them are highlighted in this review. It is ended with several recommendations on how to further improve PSCs so their performance and stability can be commensurate with application requirements.

Highly luminescent and stable quasi-2D perovskite quantum dots by introducing large organic cations

Herein, ultra-stable quasi-two-dimensional perovskite quantum dots (quasi-2D PQDs) are synthesized by introducing the butylamine cation (BA⁺) into the methylamine lead bromide perovskite (MAPbBr₃). By reducing the dimensionality of the perovskite structure, the quasi-2D perovskite (BA)₂(MA) _x-1Pb _x Br_3x+1 presents higher luminescence efficiency and better environmental stability than traditional 3D perovskites, which is mainly because the dimensionality-reduced perovskite has higher exciton binding energy and formation energy. Under an optimal MA : BA ratio of 1 : 1, the quasi-2D perovskite exhibits about four times higher luminescence efficiency (PLQY = 49.44%) than pristine MAPbBr₃; meanwhile it emits stable luminescence in an environment with 80% humidity for 50 days. Most importantly, carbon quantum dot (CQD) doping has also been applied in this work, which effectively passivates the defects of (BA)₂(MA) _x-1Pb _x Br_3x+1 via H-bond interaction, further improving the stability of the perovskite in water. Inspired by the superior performances of the proposed quasi-2D nanomaterial, a novel colorimetric method based on halide ion exchange has been developed for H₂O₂ detection, which also demonstrates that PQDs show significant potential in the field of environmental monitoring.

Highly Stable Inorganic Lead Halide Perovskite toward Efficient Photovoltaics

ConspectusOwing to the remarkable progress achieved over the past decade with research efforts from the perspectives of material synthesis, device configuration, and theoretical investigation, metal halide perovskites have emerged as a revolutionary class of light-absorbing semiconductors. The perovskite photovoltaic devices have demonstrated an impressive increase in power conversion efficiency. For single-junction perovskite solar cells, the value has risen from the initial one-digit maximum to the state-of-art record of 25.5%. Among various chemical and structural variations of perovskites, inorganic lead halides possess a more favorable operational stability and hold greater potential for perovskite/silicon tandem photovoltaics' top cells. At the initial stage of exploring all-inorganic perovskites for optoelectronic applications, many concepts, technical routes, and modification strategies were directly adopted from research on the more-developed field of organic-inorganic hybrid perovskite (OIHP). However, as understandings on inorganic perovskite deepen with research experience gained from both experimental and theoretical progression, it has been found that the difference between the asymmetric, volatile organic cations and the spherical, stable inorganic cations can lead to drastic changes on overall material properties and the subsequent device performances. In detail, the disparities reflect the crystalline and phase profiles of the material, the fabrication and passivation rationales of perovskite thin films, and the photophysics in the assembled optoelectronic devices. Therefore, the discussions of all-inorganic perovskites have their own exclusivity and are worthy of a specialized scrutinization.Here in this Account, the latest progress on the stabilization of inorganic lead halide perovskites for efficient photovoltaic applications is highlighted. A library of chemical compositions will be discussed with a focus on notable works about CsPbI₃, which possesses a more favorable bandgap as a tandem to commercialized silicon solar cells. To underscore the influence of the crystal phase on the stability of inorganic perovskites, fundamental investigations regarding the chemical and physical properties, including experimental and theoretical studies, will be summarized as a means of phase control. The stability of inorganic lead halide perovskites can also be improved by the strategic introduction of external components to the light-absorbing layer, such as the incorporation of inorganic halides, organic cations, OIHPs with low dimension, etc. These strategies can synergistically improve the stability and efficiency of the fabricated devices from the perspectives of compositional tuning, dimensional engineering, surface termination, and low-dimension capping. On the basis of a careful examination and an analysis of works achieved in these categories from our group and others, we will then discuss some promising approaches toward achieving more stable and efficient photovoltaics using inorganic lead halide perovskites.

Hole-Transporting Low-Dimensional Perovskite for Enhancing Photovoltaic Performance

Halide perovskites with low-dimensionalities (2D or quasi-2D) have demonstrated outstanding stabilities compared to their 3D counterparts. Nevertheless, poor charge-transporting abilities of organic components in 2D perovskites lead to relatively low power conversion efficiency (PCE) and thus limit their applications in photovoltaics. Here, we report a novel hole-transporting low-dimensional (HT2D) perovskite, which can form a hole-transporting channel on the top surface of 3D perovskite due to self-assembly effects of metal halide frameworks. This HT2D perovskite can significantly reduce interface trap densities and enhance hole-extracting abilities of a heterojunction region between the 3D perovskite and hole-transporting layer. Furthermore, the posttreatment by HT2D can also reduce the crystal defects of perovskite and improve film morphology. As a result, perovskite solar cells (PSCs) can effectively suppress nonradiative recombination, leading to an increasement on photovoltage to >1.20 V and thus achieving >20% power conversion efficiency and >500 h continuous illumination stability. This work provides a pathway to overcome charge-transporting limitations in low-dimensional perovskites and delivers significant enhancements on performance of PSCs.

Using steric hindrance to manipulate and stabilize metal halide perovskites for optoelectronics

The chemical instability of metal halide perovskite materials can be ascribed to their unique properties of softness, in which the chemical bonding between metal halide octahedral frameworks and cations is the weak ionic and hydrogen bonding as in most perovskite structures. Therefore, various strategies have been developed to stabilize the cations and metal halide frameworks, which include incorporating additives, developing two-dimensional perovskites and perovskite nanocrystals, etc. Recently, the important role of utilizing steric hindrance for stabilizing and passivating perovskites has been demonstrated. In this perspective, we summarize the applications of steric hindrance in manipulating and stabilizing perovskites. We will also discuss how steric hindrance influences the fundamental kinetics of perovskite crystallization and film formation processes. The similarities and differences of the steric hindrance between perovskite solar cells and perovskite light emission diodes are also discussed. In all, utilizing steric hindrance is a promising strategy to manipulate and stabilize metal halide perovskites for optoelectronics.

The 2D Halide Perovskite Rulebook: How the Spacer Influences Everything from the Structure to Optoelectronic Device Efficiency

Two-dimensional (2D) halide perovskites have emerged as outstanding semiconducting materials thanks to their superior stability and structural diversity. However, the ever-growing field of optoelectronic device research using 2D perovskites requires systematic understanding of the effects of the spacer on the structure, properties, and device performance. So far, many studies are based on trial-and-error tests of random spacers with limited ability to predict the resulting structure of these synthetic experiments, hindering the discovery of novel 2D materials to be incorporated into high-performance devices. In this review, we provide guidelines on successfully choosing spacers and incorporating them into crystalline materials and optoelectronic devices. We first provide a summary of various synthetic methods to act as a tutorial for groups interested in pursuing synthesis of novel 2D perovskites. Second, we provide our insights on what kind of spacer cations can stabilize 2D perovskites followed by an extensive review of the spacer cations, which have been shown to stabilize 2D perovskites with an emphasis on the effects of the spacer on the structure and optical properties. Next, we provide a similar explanation for the methods used to fabricate films and their desired properties. Like the synthesis section, we will then focus on various spacers that have been used in devices and how they influence the film structure and device performance. With a comprehensive understanding of these effects, a rational selection of novel spacers can be made, accelerating this already exciting field.

5-Ammonium Valeric Acid Iodide to Stabilize MAPbI3 via a Mixed-Cation Perovskite with Reduced Dimension

5-Ammonium valeric acid iodide (AVAI) has been widely known as a stabilizer to enhance the stability of MAPbI₃ perovskite, but its role and function is still under exploration. The typical 2D perovskites of AVA₂PbI₄ have been proposed as the capping layer for stabilization. Here, a novel AVA-MA mixed-cation perovskite of AVAMAPbI₄ is found to show a more even and compact coverage than the typical 2D perovskite of AVA₂PbI₄. A simple post-treatment on MAPbI₃ films by using AVAI isopropanol solution can fabricate such a mixed-cation 2D perovskite capping layer on the MAPbI₃ sample. This AVAMAPbI₄ capping layer effectively passivates surface defects of MAPbI₃ perovskite films and reduces the charge-carrier recombination, enabling AVAI-MAPbI₃ perovskite films to exhibit improved stability against thermal and moisture stress. Finally, the AVAI-MAPbI₃-based perovskite solar cells also show an enhanced photovoltaic performance with a champion PCE up to 20.05% with enhanced stability.

Graphdiyne: Bridging SnO2 and Perovskite in Planar Solar Cells

The matching of charge transport layer and photoactive layer is critical in solar energy conversion devices, especially for planar perovskite solar cells based on the SnO₂ electron-transfer layer (ETL) owing to its unmatched photogenerated electron and hole extraction rates. Graphdiyne (GDY) with multi-roles has been incorporated to maximize the matching between SnO₂ and perovskite regarding electron extraction rate optimization and interface engineering towards both perovskite crystallization process and subsequent photovoltaic service duration. The GDY doped SnO₂ layer has fourfold improved electron mobility due to freshly formed C-O σ bond and more facilitated band alignment. The enhanced hydrophobicity inhibits heterogeneous perovskite nucleation, contributing to a high-quality film with diminished grain boundaries and lower defect density. Also, the interfacial passivation of Pb-I anti-site defects has been demonstrated via GDY introduction.

Exfoliated Fluorographene Quantum Dots as Outstanding Passivants for Improved Flexible Perovskite Solar Cells

Flexible perovskite solar cells (PSCs) are currently one of the most attractive flexible thin-film photovoltaic technologies. Despite achieving remarkable progress in power conversion efficiencies (PCEs), flexible PSCs have not yet kept pace with rigid PSCs. Defect passivation is of crucial importance to further enhance the PCEs of flexible PSCs. Here, highly dispersed fluorographene quantum dots (FGQDs) are exfoliated from graphite fluoride with the aid of stirring and sonication and used to passivate the grain boundaries and surface of the perovskite films for high-performance flexible PSCs. Photoluminescence spectroscopy (PL) and time-resolved PL decays indicate that the FGQDs are beneficial for suppressing carrier recombination. Space-charge-limited current measurements prove that the passivated perovskite film exhibits reduced trap densities. As a result, a best PCE of 20.40% is achieved from the flexible PSCs, owing to significantly reduced charge recombination. Moreover, the champion device delivers an outstanding steady-state PCE exceeding 20%. The flexible PSCs with the FGQDs also exhibit enhanced thermal stability and environmental stability. Our work not only highlights the importance of passivating the defects within the perovskite films for high-efficiency flexible PSCs but also offers a promising future for the commercialization of flexible PSCs.