Quasi-2D metal halide perovskites have emerged as highly promising candidates for the emissive layer in lowcost and high efficiency light emitting diodes (LEDs). Despite tremendous efforts have been made to improve the performance of quasi-2D perovskite LEDs (PeLEDs), previous studies have mainly focused on enhancing the radiative recombination efficiency via phase distribution control or defect passivation of perovskite films. However, an in-depth understanding of the crystallization kinetics of quasi-2D perovskites, crucial for achieving high performance PeLEDs, is still lacking. Herein, we propose the introduction of a strategically designed intermediate phase to regulate the crystallization behavior of quasi-2D perovskite films. It is revealed that the intermediate phase of phenylphosphonic acid (PPA)-PbI2 composite can significantly lower the nucleation energy barrier, effectively manipulating the crystallization kinetics and drastically improving the overall quality of the perovskite films. As a result, the PPA-treated quasi-2D PeLEDs obtain a peak external quantum efficiency (EQE) of 22.74 % along with an ultrapure red emission at 650 nm with CIE coordinates of (0.708, 0.2919), which is among the closest point approaching the pure-red light coordinates of (0.708, 0.292) in display standard of REC.2100 reported to date. Meanwhile, the optimal devices show excellent spectral stability and prolonged operational lifetime compared to control devices. This work presents a novel approach to constructing highperformance quasi-2D PeLEDs by modulating the crystallization kinetics of perovskite films, opening new avenues for future advancements in this field.

In the quest for durable photovoltaic devices, 2D halide perovskites have emerged as a focus of extensive research. However, the reduced dimension in structure is accompanied by inferior optical-electrical properties, such as widened band gap, enhanced exciton binding energy, and obstructed charge transport. As a result, the efficiency of 2D perovskite solar cells (PSCs) lags significantly behind their 3D counterparts. To overcome these constraints, extensive investigations into materials and processing techniques are pursued rigorously to augment the efficiency of 2D PSCs. Herein, The cutting-edge delve into developments in 2D PSCs, with a focus on chemical and material engineering, as well as their structure and photovoltaic properties. The review starts with an introduction of the crystal structure, followed by the key evaluation criteria of 2D PSCs. Then, the strategies around solution chemical engineering, processing technique, and interface optimization, to simultaneously boost efficiency and stability are systematically discussed. Finally, the challenges and perspectives associated with 2D perovskites to provide insights into potential improvements in photovoltaic performance will be outlined.

2D/3D perovskite heterojunctions typically yield mixed-phase 2D perovskites, generating multiple quantum wells that impede charge transfer, thereby limiting the potential enhancement of solar cell efficiency. Here, we successfully fabricated phase-pure 2D (n = 2)/3D perovskite heterojunctions via introducing the gamma-aminobutyric acid (GABA) ligand, which minimized energetic inhomogeneity, thus favoring interfacial charge transfer through optimized energy band alignment. The ligation between the oxygen atoms in the ligand and the uncoordinated lead in the 3D perovskite triggered a structural transition from cubic to tetragonal at the 3D perovskite surface, ensuring a seamless lattice matching with the 2D perovskite (n = 2), resulting in this optimized configuration. Utilizing this innovative structural configuration, the carrier properties of 2D/3D perovskite thin films have been significantly enhanced, exhibiting diffusion lengths exceeding 1000 nm and a mobility of 3.35 cm2 V-1 s-1. Consequently, the fabricated small-area perovskite solar cells exhibited an impressive power conversion efficiency (PCE) of 25.06 %, while the mini-modules (10 cm x 10 cm) attained a maximum PCE of 17.27 %. Furthermore, the passivation of the 2D perovskite layers, coupled with their inherent superior resistance, enabled the unencapsulated target device to maintain outstanding long-term stability, even under challenging environmental conditions of light, heat, and humidity.

Inverted perovskite solar cells (PSCs) are a promising technology for commercialization due to their reliable operation and scalable fabrication. However, in inverted PSCs, depositing a high-quality perovskite layer comparable to those realized in normal structures still presents some challenges. Defects at grain boundaries and interfaces between the active layer and carrier extraction layer seriously hinder the power conversion efficiency (PCE) and stability of these cells. In this work, it is shown that synergistic bulk doping and surface treatment of triple-cation mixed-halide perovskites with phenylpropylammonium bromine (PPABr) can improve the efficiency and stability of inverted PSCs. The PPABr ligand is effective in eliminating halide vacancy defects and uncoordinated Pb2+ ions at both grain boundaries and interfaces. In addition, a 2D Ruddlesden-Popper (2D-RP) perovskite capping layer is formed on the surface of 3D perovskite by using PPABr post-treatment. This 2D-RP perovskite capping layer possesses a concentrated phase distribution ≈n = 2. This capping layer not only reduces interfacial non-radiative recombination loss and improves carrier extraction ability but also promotes stability and efficiency. As a result, the inverted PSCs achieve a champion PCE of over 23%, with an open-circuit voltage as high as 1.15 V and a fill factor of over 83%.

The stabilization of the formamidinium lead iodide (FAPbI(3)) structure is pivotal for the development of efficient photovoltaic devices. Employing two-dimensional (2D) layers to passivate the threedimensional (3D) perovskite is essential for maintaining the a-phase of FAPbI(3) and enhancing the power conversion efficiency (PCE) of perovskite solar cells (PSCs). However, the role of bulky ligands in the phase management of 2D perovskites, crucial for the stabilization of FAPbI(3), has not yet been elucidated. In this study, we synthesized nanoscale 2D perovskite capping crusts with = 1 and 2 RuddlesdenPopper (RP) perovskite layers, respectively, which form a type-II 2D/3D heterostructure. This heterostructure stabilizes the alpha-phase of FAPbI(3), and facilitates ultrafast carrier extraction from the 3D perovskite network to transport contact layer. We introduced tri-fluorinated ligands to mitigate defects caused by the halide vacancies and uncoordinated Pb2+ ions, thereby reducing nonradiative carrier recombination and extending carrier lifetime. The films produced were incorporated into PSCs that not only achieved a PCE of 25.39% but also maintained 95% of their initial efficiency after 2000 h of continuous light exposure without encapsulation. These findings underscore the effectiveness of a phase-pure 2D/3D heterostructure-terminated film in inhibiting phase transitions passivating the iodide anion vacancy defects, facilitating the charge carrier extraction, and boosting the performance of optoelectronic devices. (c) 2024 Science China Press. Published by Elsevier B.V. and Science China Press. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Sn-based perovskite solar cells (Sn-PSCs) have received increasing attention due to their nontoxicity and potentially high efficiency. However, the poor stability of Sn2+ ions remains a major problem in achieving stable and efficient Sn-PSCs. Herein, an in situ polymerization strategy using allyl thiourea and ethylene glycol dimethacrylate as cross-linking agents in the Sn-based perovskite precursor is proposed to improve the device performance of Sn-PSCs. The C=S and N-H bonds of the cross-linkers are able to coordinate with SnI2 and inhibit the oxidation of Sn2+, thereby reducing defect density and improving the stability of Sn-based perovskite films. The high quality of the perovskite film induced by the in situ polymerization strategy delivers an improved power conversion efficiency (PCE) from 7.50 to 9.22%. More importantly, the unpackaged device with cross-linkers maintained more than 70% of the initial PCE after 150 h of AM 1.5G light soaking in a nitrogen atmosphere and 80% of the initial PCE after 1800 h in dark conditions. This work demonstrates that the in situ polymerization strategy is an effective method to enhance the stability of Sn-based perovskite films and devices.

Although metal halide perovskites (MHPs) have demonstrated remarkable external quantum efficiencies (EQEs) in red and green light-emitting diodes (LEDs), the blue ones confront efficiency and stability problems due to the high defect density in the perovskite films. Large amounts of defect passivation strategies are successfully developed to improve the device performance. Nevertheless, the influence of the molecular configuration of the passivators on the perovskite crystallization process has not been comprehensively investigated so far. Here, we investigate the effect of the phenyl ring on the perovskite crystallization dynamics and the passivation effect. The additive with a phenyl ring performs the pi-pi stacking ability with phenethylammonium (PEA(+)) molecules, resulting in a deteriorated crystallinity and a weakened passivation ability. Conversely, the additive without the phenyl ring is helpful to promote the participation of PEA(+) molecules in the crystalline process, leading to a higher crystallinity and a stronger passivation effect. As a result, the EQE of the blue perovskite LED has increased from 4.72 to 11.06% by using the phenyl ring-free additive. Therefore, it is advisible to develop the conjugated nonplanar additives in the PEA(+)-assisted quasi-two-dimensional perovskites. This finding may enlighten the rational design of defect passivators for highly efficient perovskite LEDs.

Additive engineering plays a pivotal role in achieving high-quality light-absorbing layers for high-performance and stable perovskite solar cells (PSCs). Various functional groups within the additives exert distinct regulatory effects on the perovskite layer. However, few additive molecules can synergistically fulfill the dual functions of regulating crystallization and passivating defects. Here, we custom-synthesized 2-ureido-4-pyrimidone (UPy) organic small molecules with diverse functional groups as additives to modulate crystallization and defects in perovskite films via the Michael addition reaction. Theoretical and experimental investigations demonstrate that the -OH groups in UPy exhibit significant effects in fixing uncoordinated Pb2+ ions, passivation of lead-iodide antisite defects, alleviating hysteresis, and reducing non-radiative recombination. Furthermore, the enhanced C=O and -NH2 motifs interact with the A-site cation via hydrogen bonding, which relieves residual strain and adjusts crystal orientation. This strategy effectively controls perovskite crystallization and passivates defects, ultimately enhancing the quality of perovskite films. Consequently, the open-circuit voltage of the UPy-based p-i-n PSCs reaches 1.20 V, and the fill factor surpasses 84 %. The champion device delivers a power conversion efficiency of 25.75 %. Remarkably, the unencapsulated device maintained 96.9 % and 94.5 % of its initial efficiency following 3,360 hours of dark storage and 1,866 hours of 1-sun illumination, respectively.

The stability issue of Sn-based perovskite solar cells (PSCs) is expected to be resolved by involving a two-dimensional (2D) layered structure. However, Sn-based 2D PSCs, especially Dion-Jacobson (DJ)-phase ones with potentially good stability, have rarely been reported. Herein, superior DJ-phase Sn 2D perovskites with 3-aminobenzylamine (3ABA2+) or 4-aminobenzylamine (4ABA2+) π-conjugated short-chain ligands are reported to fabricate efficient 2D lead-free PSCs. Notably, the high dipole moment of the 3ABAI2 organic spacer is approved to possess faster charge transfer for forming (3ABA)FA4Sn5I16 2D perovskite with an extremely low exciton binding energy (only 84 meV). In combination with a diacetate partial substitution and methylamine iodide/bromide (MAI/MABr) post-treatment strategy to delay crystallization and improve compactness and coverage of the perovskite film, a record power conversion efficiency (PCE) of 6.81% and stability of 840 h (less than 5% degradation in a N2 atmosphere for unencapsulated devices) are acquired in eventual (3ABA)FA4Sn5I16 2D PSCs, which are among the highest PCE and the longest stability of Sn-based 2D PSCs reported to date. Our work provides a prospective molecule design and film preparation strategy of 2D Sn perovskites toward nontoxic high-performance tin-based PSCs, which pushes the almost stagnant research forward.

With the increasing demand of higher-quality display for Metaverse, pure-red metal halide perovskite light-emitting diodes (PeLEDs), as a fundamental optoelectronic element for achieving full-color displays, is highly desirable to satisfy the color standards of Rec. 2020 specification. However, the EQE performance and color purity requirement remain much room for improvement, and the operational stability is still an enormous challenge hindering them toward commercialization. In this review, the recent strategies toward efficient pure-red PeLEDs involving colloidal NCs, quasi-2D, 3D mixed-halide and lead-free materials have been summarized in detail. Meanwhile, the device architecture optimizations focusing on charge balance, interfacial engineering and light outcoupling are also highlighted. In addition, the operational stability challenge and improvement strategies for pure-red PeLEDs are discussed. Finally, an outlook is proposed regarding the future development for high-performance pure-red PeLEDs.

Deep-level traps at the buried interface of perovskite and energy mismatch problems between the perovskite layer and heterogeneous interfaces restrict the development of ideal homogenized films and efficient perovskite solar cells (PSCs) using the one-step spin-coating method. Here, we strategically employed sparingly soluble germanium iodide as a homogenized bulk in-situ reconstruction inducing material preferentially aggregated at the perovskite buried interface with gradient doping, markedly reducing deep-level traps and withstanding local lattice strain, while minimizing non-radiative recombination losses and enhancing the charge carrier lifetime over 9 µs. Furthermore, this gradient doping assisted in modifying the band diagram at the buried interface into a desirable flattened alignment, substantially mitigating the energy loss of charge carriers within perovskite films and improving the carrier extraction equilibrium. As a result, the optimized device achieved a champion power conversion efficiency of 25.24% with a fill factor of up to 84.65%, and the unencapsulated device also demonstrated excellent light stability and humidity stability. This work provides a straightforward and reliable homogenization strategy of perovskite components for obtaining efficient and stable PSCs.

2D Ruddlesden-Popper perovskites (RPPs) have attracted extensive attention in recent years due to their excellent environmental stability. However, the power conversion efficiency (PCE) of RPP solar cells is much lower than that of 3D perovskite solar cells (PSCs), mainly attributed to their poor carrier transport performance and excessive heterogeneous phases. Herein, the binary spacers (n-butylammonium, BA and benzamidine, PFA) are introduced to regulate the crystallization kinetics and n-value phase distribution to form uniform phase permutation of RPP films. The study then incorporates n = 5 BA2MA4Pb5I16 memory single crystal to achieve ultrafast stepped-type carrier transport from the low n-value phases to the high n-value phases in the high-quality (BA0.75PFA0.25)2MA4Pb5I16 films. These binary spacers and single-crystal-assisted crystallization strategies produce high-quality films, leading to fast carrier extraction and significant nonradiative recombination suppression. The resulting PSC presents a champion PCE of 21.15% with an impressive open circuit voltage (VOC) of 1.26 V, which is the record high efficiency and VOC for low n-value RPP solar cells (n ≤ 5).

Two-terminal (2T) perovskite-based tandem solar cells (TSCs) arouse burgeoning interest in breaking the Shockley-Queisser (S-Q) limit of single-junction solar cells by combining two subcells with different bandgaps. However, the highest certified efficiency of 2T perovskite-based TSCs (33.9%) lags behind the theoretical limit (42-43%). A vital challenge limiting the development of 2T perovskite-based TSCs is the transparent recombination layers/interconnecting layers (RLs) design between two subcells. To improve the performance of 2T perovskite-based TSCs, RLs simultaneously fulfill the optical loss, contact resistance, carrier mobility, stress management, and conformal coverage requirements. In this review, the definition, functions, and requirements of RLs in 2T perovskite-based TSCs are presented. The insightful characterization methods applicable to RLs, which are inspiring for further research on the RLs both in 2T perovskite-based two-junction and multi-junction TSCs, are also highlighted. Finally, the key factors that currently limit the performance enhancement of RLs and the future directions that should be continuously focused on are summarized.

alpha-CsPbI3 quantum dots (QDs) are ideal materials for pure red light-emitting diodes (LEDs) to achieve a wide color gamut display. Small alpha-CsPbI3 QDs have a large specific surface area and exhibit a near-spherical shape, discontinuous crystal faces with intense surface defects and crystal distortion, which accelerates phase transition and optoelectronic degradation. Additionally, the tolerance factor for alpha-CsPbI3 is too small to be stabilized. Herein, we report a Ca2+ doping strategy to tune the CsPbI3 tolerance factor to inhibit its phase transition, enhance its structural stability, decrease the lattice distortion and reduce the defects. The stability of the Ca2+-doped CsPbI3 QDs under harsh conditions are improved. The Commission Internationale de l'Eclairage coordinates of the perovskite LEDs are (0.706, 0.294), satisfying wide color gamut as well as Rec. 2020 requirements for pure red, and the LEDs exhibit a 1.67-fold enhancement in half-lifetime to 32.0 min at a voltage of 5.0 V (346 cd m(-2)) and 1.31-fold enhancement in the EQE to 10.3%.

Formamidinium lead triiodide (α-FAPbI3 ) has been widely used in high-efficiency perovskite solar cells due to its small band gap and excellent charge-transport properties. Recently, some additives show facet selectivity to generate a (001) facet-dominant film during crystallization. However, the mechanism to realize such (001) facet selectivity is not fully understood. Here, the authors attempted to use three ammonia salts NH4 X (X are pseudohalide anions) to achieve better (001) facet selectivity in perovskite crystallization and improved crystallinity. After addition, the (001) facet dominance is generally increased with the best effect from SCN- anions. The theoretical calculation revealed three mechanisms of such improvements. First, pseudohalide anions have larger binding energy than the iodine ion to bind the facets including (110), (210), and (111), slowing down the growth of these facets. The large binding energy also reduces nucleation density and improves crystallinity. Second, pseudohalide ions improve phase purity by increasing the formation energies of the δ-phase and other hexagonal polytypes, retarding the α- to δ-phase transition. Third, the strong binding of these anions can also effectively passivate the iodine vacancies and suppress nonradiative recombination. As a result, the devices show a power conversion efficiency of 24.11% with a Voc of 1.181 V.

Although metal halide perovskites are increasingly popular for the next generation of efficient photovoltaic devices, the inevitable defects from the preparation process have become the notorious barrier to further improvement of performance, which increases non‐radiative recombination and lowers the power conversion efficiency of solar cells. Surface passivation strategies have been affirmed as one of the most practical approaches to suppress these defects. Therefore, it is necessary to have a detailed review on the surface passivation to reveal the improvements of the devices. Herein, the mechanism and recent advances of surface passivation have been systematically summarized with respect to various passivation approaches, including the Lewis acid–base, the low‐dimensional perovskite, inorganic molecules, and polymers. Finally, the review also offers the research trend and prospects of surface passivation.

Flexible perovskite solar cells (f-PSCs) as a promising power source have grabbed surging attention from academia and industry specialists by integrating with different wearable and portable electronics. With the development of low-temperature solution preparation technology and the application of different engineering strategies, the power conversion efficiency of f-PSCs has approached 24%. Due to the inherent properties and application scenarios of f-PSCs, the study of strain in these devices is recognized as one of the key factors in obtaining ideal devices and promoting commercialization. The strains mainly from the change of bond and lattice volume can promote phase transformation, induce decomposition of perovskite film, decrease mechanical stability, etc. However, the effect of strain on the performance of f-PSCs has not been systematically summarized yet. Herein, the sources of strain, evaluation methods, impacts on f-PSCs, and the engineering strategies to modulate strain are summarized. Furthermore, the problems and future challenges in this regard are raised, and solutions and outlooks are offered. This review is dedicated to summarizing and enhancing the research into the strain of f-PSCs to provide some new insights that can further improve the optoelectronic performance and stability of flexible devices.

Halide perovskites have attracted much attention because of their excellent optoelectronic properties, such as high light absorption, long carrier diffusion length, and high defect tolerance. Ion migration induced device performance degradation, which is not yet fully understood, has become the key obstacle for commercialization of halide perovskites. Here, a general mechanism is proposed, which can build up the connection between the ion migration barrier and the electronic density of states, to clarify the origin of low barrier for ion migration. Density functional theory (DFT) simulation results show that the low barrier is caused by a significant energy difference in band centers between Pb 2+ and the isolating halogen anion or by the small number of density of states. Following the explored mechanism, two strategies are proposed to boost barriers via DFT combination CI‐NEB simulations: 1) halide double perovskites and 2) B‐site doping. Furthermore, the finding not only deepens the understanding of ion migration in halide perovskites but also paves a new path for the commercialization of halide perovskite optoelectronic devices.

The power conversion efficiency (PCE) of the state-of-the-art large-area slot-die-coated perovskite solar cells (PSCs) is now over 19%, but issues with their stability persist owing to significant intrinsic point defects and a mass of surface imperfections introduced during the fabrication process. Herein, the utilization of a hydrophobic all-organic salt is reported to modify the top surface of large-area slot-die-coated methylammonium (MA)-free halide perovskite layers. Bearing two molecules, each of which is endowed with anchoring groups capable of exhibiting secondary interactions with the perovskite surfaces, the organic salt acts as a molecular lock by effectively binding to both anion and cation vacancies, substantially enhancing the materials' intrinsic stability against different stimuli. It not only reduces the ingression of external species such as oxygen and moisture, but also suppresses the egress of volatile organic components during the thermal stability testing. The treated PSCs demonstrate efficiency of 19.28% (active area of 58.5 cm2 ) and 17.62% (aperture area of 64 cm2 ) for the corresponding mini-module. More importantly, unencapsulated slot-die-coated mini-modules incorporating the all-organic surface modifier show ≈80% efficiency retention after 7500 h (313 days) of storage under 30% relative humidity (RH). They also remarkably retain more than 90% of the initial efficiency for over 850 h while being measured continuously.

Two-dimensional layered metal-halide perovskites (2D-LMHPs) are a promising family of organic–inorganic hybrid semiconductor materials because of their superior electronic and optical properties and high stability. To date, solution-processed 2D-LMHP thin films have a multiple quantum wells (QWs) structure, which has seriously impeded further progress in optoelectronics. Compared with 2D-LMHPs with multiple QWs, 2D-LMHPs with phase-pure QWs have a flattened energy landscape, resulting in less energy or charge-transfer losses and making them less susceptible to degradation. They would thus be attractive to promote the development of perovskite-based devices. In this Perspective article, we first elucidate the structure and optoelectronic properties of phase-pure 2D-LMHP films. Second, we systematically discuss their precursor engineering, focusing on stoichiometry, ligand design, chemical compositional engineering and formation energy aspects. Third, we comprehensively summarize the intermediate phase growth mechanism, in situ dynamic transformation observation and methodologies for the formation of quasi-2D perovskites with phase-pure structures. Finally, we deliberate the prospects and challenges of phase-pure 2D perovskites as a new family of semiconductors.