Modulation on Electrostatic Potential of Passivator for Highly Efficient and Stable Perovskite Solar Cells

The perovskite layer contains a large number of charged defects that seriously impair the efficiency and stability of perovskite solar cells (PSCs), thus it is essential to develop an effective passivation strategy to heal them. Based on theoretical calculations, it is found that enhancing the electrostatic potential of passivators can improve passivation effect and adsorption energy between charged defects and passivators. Herein, an electrostatic potential modulation (EPM) strategy is developed to design passivators for highly efficient and stable PSCs. With the EPM strategy, 1-phenylethylbiguanide (PEBG) and 1-phenylbiguanide (PBG) are designed. It is found that the charge distribution and electrostatic potential of phenyl- and phenylethyl- substituent on the biguanide are significantly enhanced. The N atom directly bonding to the phenyl group shows larger positive charge than that bonding to the phenylethyl group. The modulated electrostatic potential makes PBG bind stronger with the defects on perovskite surface. Based on the effective passivation of EPM, a champion efficiency of 24.67% is realized and the device retain 91.5% of its initial PCE after ≈1300 h. The promising EPM strategy, which provides a principle of passivator design and allows passivation to be controllable, may advance further optimization and application of perovskite solar cells toward commercialization.     

Self‐Crystallized Multifunctional 2D Perovskite for Efficient and Stable Perovskite Solar Cells

Recently, perovskite solar cells (PSC) with high power‐conversion efficiency (PCE) and long‐term stability have been achieved by employing 2D perovskite layers on 3D perovskite light absorbers. However, in‐depth studies on the material and the interface between the two perovskite layers are still required to understand the role of the 2D perovskite in PSCs. Self‐crystallization of 2D perovskite is successfully induced by deposition of benzyl ammonium iodide (BnAI) on top of a 3D perovskite light absorber. The self‐crystallized 2D perovskite can perform a multifunctional role in facilitating hole transfer, owing to its random crystalline orientation and passivating traps in the 3D perovskite. The use of the multifunctional 2D perovskite (M2P) leads to improvement in PCE and long‐term stability of PSCs both with and without organic hole transporting material (HTM), 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) compared to the devices without the M2P.     

Supramolecular Aza Crown Ether Modulator for Efficient and Stable Perovskite Solar Cells

Molecular modulators have been demonstrated to be an effectual strategy for reducing the defect at the interface and in bulk of perovskite and ameliorating the performance and stability of perovskite solar cells (PSCs). Herein, 1-aza-15-crown 5-ether (A15C5), with a unique nitrogen heterocyclic structure as a molecular modulator is introduced at the interface between perovskite layer and hole transport layer of PSCs. Multiple supramolecular synergistic interaction between A15C5 and perovskite dramatically suppress and passivate defects, resulting in a 38% decrease in electron trap-state density in perovskite. The formation of two-dimensional/th3D perovskite heterojunctions induced by planar A15C5 releases residual strain of perovskite film, optimizes the match of energy level array and boosts the stability of devices. Consequently, A15C5-modulated PSC achieves an impressing efficiency of 24.13% along with excellent humidity, light and thermal stability. This work provides a typical strategy to utilize supramolecular crown ether in PSCs.     

Hierarchical Chiral Self-Assembly of Lead Halide Perovskite Nanocrystals by Oriented Phase Transition

Self-assembly of lead halide perovskite nanocrystals (NCs) into close-packed, long-range-ordered nanostructures can effectively modulate their photoelectronic properties, yet significantly challenging. Herein, an efficient approach is reported to induce the hierarchical self-assembly of perovskite CsPbBr₃ NCs by phase transition using chiral cysteine ligands, yielding asymmetric Cs₄PbBr₆ nanorods (NRs) with the circularly polarized luminescent response. An interfacial phase transition process is found during the conversion of CsPbBr₃ nanocubes to Cs₄PbBr₆ NCs initiated by cysteine molecules. Then the Cs₄PbBr₆ NCs aggregate sequentially to form nanoclusters, which further self-assemble into the chiral Cs₄PbBr₆ NRs. Molecular dynamics simulations reveal that the Cs₄PbBr₆ nanochains gradually approach each other to achieve an asymmetric structure, and the simulated circular dichroism spectrum further supports the formation of a chiral structure. This work offers a facile method for the hierarchical chiral self-assembly of lead halide perovskite nanostructures, which brings new insights to explore chiral nanostructures by modulating the surface chemistry and post self-assembly.     

Multifunctional Phosphorus‐Containing Lewis Acid and Base Passivation Enabling Efficient and Moisture‐Stable Perovskite Solar Cells

Multiple‐cation lead mixed‐halide perovskites (MLMPs) have been recognized as ideal candidates in perovskite solar cells in terms of high efficiency and stability due to decreased open‐circuit voltage loss and suppressed yellow phase formation. However, they still suffer from an unsatisfactory long‐term moisture stability. In this study, phosphorus‐containing Lewis acid and base molecules are employed to improve device efficiency and stability based on their multifunction including recombination reduction, phase segregation suppression, and moisture resistance. The strong fluorine‐containing Lewis acid treatment can achieve a champion PCE of 22.02%. Unencapsulated and encapsulated devices retain 63% and 80% of the initial efficiency after 14 days of aging under 75% and 85% relative humidity, respectively. The better passivation of Lewis acid implies more halide defects than Pb defects at the MLMP surface. This unbalanced defect type results from phase segregation that is the synergistic effect of Cs and halide ion migrations. Identifying defect type based on different passivation effects is beneficial to not only choose suitable passivators to boost the efficiency and slow down the moisture degradation of MLMP solar cells, but also to understand the mechanism of defect‐assisted moisture degradation.     

Core/Shell Metal Halide Perovskite Nanocrystals for Optoelectronic Applications

Core/shell structured metal halide perovskite nanocrystals (NCs) are emerging as a type of material with remarkable optical and electronic properties. Research into this field has been developing and expanding rapidly in recent years, with significant advances in the studies of the shell growth mechanism and in understanding of properties of these materials. Significant enhancement of both the stability and the optical performance of core/shell perovskite NCs are of particular importance for their applications in optoelectronic technologies. In this review, the recent advances in core/shell structured perovskite NCs are summarized. The band structures and configurations of core/shell perovskite NCs are elaborated, the shell classification and shell engineering approaches, such as perovskites and their derivative shells, semiconductor shell, oxide shell, polymer shell, etc. are reviewed, and the shell growth mechanisms are discussed. The prospective of these NCs in lighting and displays, solar cells, photodetectors, and other devices is discussed in the light of current knowledge, remaining challenges, and future opportunities.     

Metal Halide Perovskite Surfaces with Mixed A-Site Cations: Atomic Structure and Device Stability

Mixing cations in the perovskite structure has been shown to improve optoelectronic device performance and stability. In particular, Cs_xMA_1-xPbBr₃ (MA = CH₃NH₃) has been used to build high-efficiency light-emitting diodes. Despite those advantages, little is known about the exact location of the cations in the mixed perovskite film, and how cation distribution affects device properties and stability. By using scanning tunneling microscopy , the exact atomic structure of the mixed cation Cs_xMA_1-xPbBr₃ perovskite interface is revealed. In addition, X-ray photoelectron spectroscopy, ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy are used to study the stability and electronic properties of the Cs_xMA_1-xPbBr₃ perovskite film. Partial substitution of MA⁺ by Cs⁺ induces a modification of the perovskite surface structure, leading to improved device stability is shown. These results provide a better understanding of the key parameters involved in the stability of mixed cation perovskite solar cells.     

Mixed Halide Formation in Lead-Free Antimony-Based Halide Perovskite for Boosted CO2 Photoreduction: Beyond Band Gap Tuning

Photocatalytic conversion of carbon dioxide (CO₂) into value-added fuels is a vastly promising anthropogenic chemical carbon cycle to combat the greenhouse effect while meeting the ever-increasing energy demand. Recently, lead-based halide perovskites have demonstrated great potential in various applications including photochemical reduction of CO₂. However, in view of lead toxicity, the exploration of a lead-free alternative is crucial for long term application. Herein, a series of lead-free mixed halide perovskites Cs₃Sb₂Cl_xBr_9−x (0 ≤ x ≤ 9) is prepared via a facile antisolvent recrystallization technique, where the incorporation of a secondary halide enhances the charge transfer and separation while allowing precise tuning of bandgap between 2.59 and 2.90 eV. Theoretical calculations further reveal that the formation of mixed Cl/Br halides engenders favorable charge redistribution due to lower octahedral distortion, which in turn strengthens CO₂ adsorption and activation. Under visible light illumination, the optimal dual halide perovskite, Cs₃Sb₂Cl₄Br₅ manifests substantial twofold and fourfold enhancements of CH₄ yield over the single halide perovskite, Cs₃Sb₂Br₉ and Cs₃Sb₂Cl₉, respectively. In brief, this study provides a compelling demonstration of lead-free mixed halide perovskites for photocatalytic CO₂ reduction, and it is anticipated to drive further application of perovskite-based photocatalysts toward a diverse range of artificial photoredox reactions.     

Modifying Surface Termination of CsPbI3 Grain Boundaries by 2D Perovskite Layer for Efficient and Stable Photovoltaics

It is highly desirable for all-inorganic perovskite solar cells (PVSCs) to have reduced nonideal interfacial charge recombination in order to improve the performance. Although the construction of a 2D capping layer on 3D perovskite is an effective way to suppress interfacial nonradiative recombination, it is difficult to apply it to all-inorganic perovskites because of the resistance of Cs⁺ cesium ions in cation exchange reactions. To alleviate this problem, a simple approach using an ultra-thin 2D perovskite to terminate CsPbI₃ grain boundaries (GBs) without damaging the original 3D perovskite is developed. The 2D perovskite at the GBs not only enhances the charge-carrier extraction and transport but also effectively suppresses nonradiative recombination. In addition, because the 2D perovskite can prevent the moisture and oxygen from penetrating into the GBs and at the same time suppress the ion migration, the 2D terminated CsPbI₃ films exhibit significantly improved stability against humidity. Moreover, the devices without encapsulation can retain ≈81% of its initial power conversion efficiency (PCE) after being stored at 40 ± 5% relative humidity for 84 h. The 2D-based champion device exhibits a high PCE of 18.82% with a high open-circuit voltage of 1.16 V.     

Molecular Configuration Engineering in Hole-Transporting Materials toward Efficient and Stable Perovskite Solar Cells

The development of hole-transporting materials (HTMs) that can passivate defects in perovskite is of great significance in improving the efficiency and long-term stability of perovskite solar cells. To date, the investigation on HTMs mainly focus on exploring new structures, while molecular configuration is seldomly concerned. In this work, two small molecules are developed as HTMs with benzil and phenanthrene quinone as the core structure, respectively. With similar structure and the same defect passivation groups, whereas, the two molecules exhibit different configurations, thus distinct properties. Compared to 3,6-bis(3,6-bis(bis(4-methoxyphenyl)amino)-9H-carbazol-9-yl)phenanthrene-9,10-dione (PQ) with a rigid core structure, the benzil group in 1,2-bis(4-(3,6-bis(bis(4-methoxyphenyl)amino)-9H-carbazol-9-yl)phenyl)ethane-1,2-dione (DB) is flexible and can adjust molecular configuration to efficiently interact with the underlying perovskite material, which is confirmed from both experimental results and theoretical simulations. The DB-based device exhibits a high power conversion efficiency of 22.21% with excellent long-term stability, superior to the PQ-based device (20.22%). This study demonstrates that molecular configuration engineering will directly affect the properties of hole transport materials, as well as their interactions with perovskite, which should also be taken into consideration when devising HTMs.     

Efficient and Spectrally Stable Blue Perovskite Light‐Emitting Diodes Based on Potassium Passivated Nanocrystals

Metal halide perovskites have aroused tremendous interest in the past several years for their promising applications in display and lighting. However, the development of blue perovskite light‐emitting diodes (PeLEDs) still lags far behind that of their green and red cousins due to the difficulty in obtaining high‐quality blue perovskite emissive layers. In this study, a simple approach is conceived to improve the emission and electrical properties of blue perovskites. By introducing an alkali metal ion to occupy some sites of peripheral suspended organic ligands, the nonradiative recombination is suppressed, and, consequently, blue CsPb(Br/Cl)₃ nanocrystals with a high photoluminescence quantum efficiency of 38.4% are obtained. The introduced K⁺ acts as a new type of metal ligand, which not only suppresses nonradiative pathways but also improves the charge carrier transport of the perovskite nanocrystals. With further engineering of the device structure to balance the charge injection rate, a spectrally stable and efficient blue PeLED with a maximum external quantum efficiency of 1.96% at the emission peak of 477 nm is fabricated.     

Chemical Reduction of Iodine Impurities and Defects with Potassium Formate for Efficient and Stable Perovskite Solar Cells

The performance of perovskite solar cells (PSCs) is negatively affected by iodine (I₂) impurities generated from the oxidation of iodide ions in the perovskite precursor powder, solution, and perovskite films. In this study, the use of potassium formate (HCOOK) as a reductant to minimize the presence of detrimental I₂ impurities is presented. It is demonstrated that HCOOK can effectively reduce I₂ back to I⁻ in the precursor solution as well as in the devices under external conditions. Furthermore, the introduced formate anion (HCOO⁻) and alkali metal cation (K⁺) can reduce the defect density within the perovskite film by modulating perovskite growth and passivating electronic defects, significantly prolonging the carrier lifetime and reducing the J–V hysteresis. Consequently, the maximum efficiency of the HCOOK-doped planar n–i–p PSCs reaches 23.8%. After 1000 h of operation at maximum power point tracking under continuous 1 sun illumination, the corresponding encapsulated devices retain 94% of their initial efficiency.     

The Relationship between Chemical Flexibility and Nanoscale Charge Collection in Hybrid Halide Perovskites

Hybrid organometal halide perovskites are known for their excellent optoelectronic functionality as well as their wide‐ranging chemical flexibility. The composition of hybrid perovskite devices has trended toward increasing complexity as fine‐tuned properties are pursued, including multielement mixing on the constituents A and B and halide sites. However, this tunability presents potential challenges for charge extraction in functional devices. Poor consistency and repeatability between devices may arise due to variations in composition and microstructure. Within a single device, spatial heterogeneity in composition and phase segregation may limit the device from achieving its performance potential. This review details how the nanoscale elemental distribution and charge collection in hybrid perovskite materials evolve as chemical complexity increases, highlighting recent results using nondestructive operando synchrotron‐based X‐ray nanoprobe techniques. The results reveal a strong link between local chemistry and charge collection that must be controlled to develop robust, high‐performance hybrid perovskite materials for optoelectronic devices.     

Epitaxial Magnetic Oxide Nanocrystals Via Phase Decomposition of Bismuth Perovskite Precursors

The phase instability of bismuth perovskite (BiMO₃), where M is a ferromagnetic cation, is exploited to create self‐assembled magnetic oxide nanocrystal arrays on oxide supports. Conditions during pulsed laser deposition are tuned so as to induce complete breakdown of the perovskite precursor into bismuth oxide (Bi₂O₃) and metal oxide (M‐O_x) pockets. Subsequent cooling in vacuum volatizes the Bi₂O₃ leaving behind an array of monodisperse nanocrystals. In situ reflective high energy electron diffraction beam is exploited to monitor the synthesis in real‐time. Analysis of the patterns confirms the phase separation and volatization process. Successful synthesis of M‐O_x, where M = Mn, Fe, Co, and Cr, is shown using this template‐free facile approach. Detailed magnetic characterization of nanocrystals is carried out to reveal the functionalities such as magnetic anisotropy as well as larger than bulk moments, as expected in these oxide nanostructures.     

Low-Dimensional 2-thiopheneethylammonium Lead Halide Capping Layer Enables Efficient Single-Junction Methylamine-Free Wide-Bandgap and Tandem Perovskite Solar Cells

Wide-bandgap (WBG) perovskite solar cells (PSCs) have garnered significant attention for their potential applications in tandem solar cells. However, their large open-circuit voltage (V_OC) deficit and serious photo-induced halide segregation remain the main challenges that impede their applications. Herein, a post-treatment strategy without thermal annealing is presented to form a 2D top layer of 2-thiopheneethylammonium lead halide (n = 1) on WBG perovskites. This thermal annealing-free post-treatment method can more effectively passivate the defects of WBG methylamine (MA)-free formamidinium/cesium lead iodide/bromide perovskite films and suppress photo-induced perovskite phase segregation, as compared with the thermal annealing method that yields multi-2D phases. The resulting opaque and semi-transparent 1.66 eV-bandgap perovskite solar cells deliver maximum power conversion efficiencies of 21.47% (a small V_OC deficit of 0.43 V) and 19.11%, respectively, both of which are among the highest reports for inverted MA-free WBG PSCs. Consequently, four-terminal all-perovskite tandem cells realize a remarkable efficiency of 26.64%, showing great promise for their applications in efficient multi-junction tandem solar cells.     

Zero‐Dimensional Perovskite Nanocrystals for Efficient Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are able to efficiently harvest solar energy through large‐area photovoltaic windows, where fluorophores are delicately embedded. Among various types of fluorophores, all‐inorganic perovskite nanocrystals (NCs) are emerging candidates as absorbers/emitters in LSCs due to their size/composition/dimensionality tunable optical properties and high photoluminescence quantum yield (PL QY). However, due to the large overlap between absorption and emission spectra, it is still challenging to fabricate high‐efficiency LSCs. Intriguingly, zero‐dimensional (0D) perovskites provide a number of features that meet the requirements for a potential LSC absorber, including i) small absorption/emission spectral overlap (Stokes shift up to 1.5 eV); ii) high PL QY (>95% for bulk crystal); iii) robust stability as a result of its large exciton binding energy; and iv) ease of synthesis. In this work, as a proof‐of‐concept experiment, Cs₄PbBr₆ perovskite NCs are used to fabricate semi‐transparent large‐area LSCs. Cs₄PbBr₆ perovskite film exhibits green emission with a high PL QY of ≈58% and a small absorption/emission spectral overlap. The optimized LSCs exhibit an external optical efficiency of as high as 2.4% and a power conversion efficiency of 1.8% (100 cm²). These results indicate that 0D perovskite NCs are excellent candidates for high‐efficiency LSCs compared to 3D perovskite NCs.     

Interfacial Energy Level Alignment and Defect Passivation by Using a Multifunctional Molecular for Efficient and Stable Perovskite Solar Cells

Tin oxide (SnO₂) is currently the dominating electron transport material (ETL) used in state-of-the-art perovskite solar cells (PSCs). However, there are amounts of defects distributed at the interface between ETL and perovskite to deteriorate PSC performance. Herein, a molecule bridging layer is built by incorporating 2,5-dichloroterephthalic acid (DCTPA) into the interface between the SnO₂ and perovskites to achieve better energy level alignment and superior interfacial contact. The multifunctional molecular bridging layer not only can passivate the trap states of Sn dangling bonds and oxygen vacancies resulting in improved conductivity and the electron extraction of SnO₂ but also can regulate the perovskite crystal growth and reduce defect-assisted nonradiative recombination due to its strong interaction with undercoordinated lead ions. As a result, the DCTPA-modified PSCs achieve champion power conversion efficiencies (PCEs) of 23.25% and 20.23% for an active area of 0.15 cm² device and 17.52 cm² mini-module, respectively. Moreover, the perovskite films and PSCs based on DCTPA modification show excellent long-term stability. The unencapsulated target device can maintain over 90% of the initial PCE after 1000 h under ambient air. This strategy guides design methods of molecule bridging layer at the interface between SnO₂ and perovskite to improve the performance of PSCs .     

Reversible Photoinduced Phase Segregation and Origin of Long Carrier Lifetime in Mixed‐Halide Perovskite Films

Mixed‐halide hybrid perovskite semiconductors have attracted tremendous attention as a promising candidate for efficient photovoltaic and light‐emitting devices. However, these perovskite materials may undergo phase segregation under light illumination, thus affecting their optoelectronic properties. Here, photoexcitation induced phase segregation in triple‐cation mixed‐halide perovskite films that yields to red‐shift in the photoluminescence response is reported. It is demonstrated that photoexcitation induced halide migration leads to the formation of smaller bandgap iodide‐rich and larger bandgap bromide‐rich domains in the perovskite film, where the phase segregation rate is found to follow the excitation power‐density as a power law. Results confirm that charge carrier lifetime increases due to the trapping of photoexcited carriers in the segregated smaller bandgap iodide‐rich domains. Interestingly, these photoinduced changes are fully reversible and thermally activated when the excitation power is turned off. A significant difference in activation energies for halide ion migration is observed during phase segregation and recovery process. Additionally, the emission linewidth broadening is investigated as a function of temperature which is governed by the exciton–optical phonon coupling. The mechanism of photoinduced phase segregation is interpreted based on exciton–phonon coupling strength in both mixed and demixed (segregated) states of perovskite films.     

Conjugated Polymer–Assisted Grain Boundary Passivation for Efficient Inverted Planar Perovskite Solar Cells

Grain boundaries in lead halide perovskite films lead to increased recombination losses and decreased device stability under illumination due to defect‐mediated ion migration. The effect of a conjugated polymer additive, poly(bithiophene imide) (PBTI), is investigated in the antisolvent treatment step in the perovskite film deposition by comprehensive characterization of perovskite film properties and the performance of inverted planar perovskite solar cells (PSCs). PBTI is found to be incorporated within grain boundaries, which results in an improvement in perovskite film crystallinity and reduced defects. The successful defect passivation by PBTI yields reduces recombination losses and consequently increases power conversion efficiency (PCE). In addition, it gives rise to improved photoluminescence stability and improved PSC stability under illumination which can be attributed to reduced ion migration. The optimal devices exhibit a PCE of 20.67% compared to 18.89% of control devices without PBTI, while they retain over 70% of the initial efficiency after 600 h under 1 sun illumination compared to 56% for the control devices.     

Composition Engineering of All‐Inorganic Perovskite Film for Efficient and Operationally Stable Solar Cells

Cesium‐based inorganic perovskites have recently attracted great research focus due to their excellent optoelectronic properties and thermal stability. However, the operational instability of all‐inorganic perovskites is still a main hindrance for the commercialization. Herein, a facile approach is reported to simultaneously enhance both the efficiency and long‐term stability for all‐inorganic CsPbI_2.5Br_0.5 perovskite solar cells via inducing excess lead iodide (PbI₂) into the precursors. Comprehensive film and device characterizations are conducted to study the influences of excess PbI₂ on the crystal quality, passivation effect, charge dynamics, and photovoltaic performance. It is found that excess PbI₂ improves the crystallization process, producing high‐quality CsPbI_2.5Br_0.5 films with enlarged grain sizes, enhanced crystal orientation, and unchanged phase composition. The residual PbI₂ at the grain boundaries also provides a passivation effect, which improves the optoelectronic properties and charge collection property in optimized devices, leading to a power conversion efficiency up to 17.1% with a high open‐circuit voltage of 1.25 V. More importantly, a remarkable long‐term operational stability is also achieved for the optimized CsPbI_2.5Br_0.5 solar cells, with less than 24% degradation drop at the maximum power point under continuous illumination for 420 h.