Strain engineering in perovskite solar cells and its impacts on carrier dynamics

Efficient wide-bandgap perovskite photovoltaics with homogeneous halogen-phase distribution

Wide-bandgap (WBG) perovskite solar cells (PSCs) are employed as top cells of tandem cells to break through the theoretical limits of single-junction photovoltaic devices. However, WBG PSCs exhibit severe open-circuit voltage (Voc) loss with increasing bromine content. Herein, inhomogeneous halogen-phase distribution is pointed out to be the reason, which hinders efficient extraction of carriers. We thus propose to form homogeneous halogen-phase distribution to address the issue. With the help of density functional theory, we construct a double-layer structure (D-2P) based on 2-(9H-Carbazol-9-yl)ethyl]phosphonic acid molecules to provide nucleation sites for perovskite crystallization. Homogeneous perovskite phase is achieved through bottom-up templated crystallization of halogen component. The efficient carrier extraction reduces the Shockley-Read-Hall recombination, resulting in a high Voc of 1.32 V. As a result, D-2P-treated device (1.75 eV) achieves a record power conversion efficiency of 20.80% (certified 20.70%), which is the highest value reported for WBG (more than 1.74 eV) PSCs.

Insights into the pressure-dependent physical properties of cubic Ca3MF3 (M = As and Sb): First-principles calculations

Here, first-principles calculations have been employed to make a comparative study on structural, mechanical, electronic, and optical properties of new Ca3MF3 (M = As and Sb) photovoltaic compounds under pressure. The findings disclose that these two systems possess a direct band gap, showcasing a large tunable range under pressure, effectively encompassing the visible light spectrum. Adjusting various levels of hydrostatic pressure has effectively tuned both the band alignment and the effective masses of electrons and holes. Both compounds were initially identified as brittle materials at 0 GPa pressure; however, as the pressure increases, they transform, becoming highly anisotropic and ductile. Due to the material's mechanical robustness and enhanced ductility, as evidenced by its stress-induced mechanical properties, the Ca3MF3 (M = As and Sb) material shows potential for use in solar energy applications. Furthermore, as the influence of external pressure increases, the absorption edge seems to move slightly towards lower energy region. Optical properties show that the materials studied might be used from several optoelectronic devices in the visible and ultraviolet range area. Our findings show that pressure considerably influences the physicochemical properties of Ca3MF3 (M = As and Sb) compounds, which is a promising feature that can be useful for optoelectronic and photonic applications, for instance, light-emitting diodes, photodetectors, and solar cells.

Effect of Guanidinium Salt for Stress-Relaxation and Interfacial Engineering in Antisolvent Free Perovskite Solar Cells Fabricated Under Air Ambient

In perovskite solar cells, the presence of stress and defects at interfaces promotes performance degradation and poor stability of the devices. The formation of these defects is more prominent in two-step antisolvent-free perovskite film fabrication. This study addresses these challenges by introducing guanidine sulfate (Gua-S) at the tin oxide/formamidinium lead iodide perovskite interface, fabricated without antisolvent under ambient air. Interfacial Gua-S enhanced morphology by forming bonds between uncoordinated Pb2+ ions and I- vacancies at the interface and showed improvement in the crystallinity and quality of the perovskite film. Microstructural stress analysis indicated a substantial reduction in stress, decreasing from 50.6 to 20.72 MPa with the application of Gua-S. Moreover, the Gua-S treated solar cells showed significant improvements and achieved an open circuit voltage of 1.08 V and 22.34% efficiency. Further, electrochemical impedance spectroscopic analysis showed improved built-in potential, carrier lifetime, and charge recombination lifetime for treated devices. The devices retained over 87% of the initial power conversion efficiency after 2000 hours of operation. This comprehensive study addresses the fundamental issues of interfacial stress and defects in perovskite solar cells and demonstrates the efficacy of Gua-S salt in enhancing both the structural and functional aspects of the antisolvent-free device fabrication process.

Mesoporous structured MoS2 as an electron transport layer for efficient and stable perovskite solar cells

Mesoporous structured electron transport layers (ETLs) in perovskite solar cells (PSCs) have an increased surface contact with the perovskite layer, enabling effective charge separation and extraction, and high-efficiency devices. However, the most widely used ETL material in PSCs, TiO2, requires a sintering temperature of more than 500 °C and undergoes photocatalytic reaction under incident illumination that limits operational stability. Recent efforts have focused on finding alternative ETL materials, such as SnO2. Here we propose mesoporous MoS2 as an efficient and stable ETL material. The MoS2 interlayer increases the surface contact area with the adjacent perovskite layer, improving charge transfer dynamics between the two layers. In addition, the matching between the MoS2 and the perovskite lattices facilitates preferential growth of perovskite crystals with low residual strain, compared with TiO2. Using mesoporous structured MoS2 as ETL, we obtain PSCs with 25.7% (0.08 cm2, certified 25.4%) and 22.4% (1.00 cm2) efficiencies. Under continuous illumination, our cell remains stable for more than 2,000 h, demonstrating improved photostability with respect to TiO2.

A multifunctional phenylphosphinamide additive for stable flexible inverted perovskite solar cells

The multifunctional phenylphosphinamide additive is used in flexible inverted perovskite solar cells to release tensile strain and increase the toughness of the perovskite film, achieving enhanced device efficiency and stability.

Molecule Anchoring Strategy Promotes Vertically Homogeneous Crystallization and Aligned Interfaces for Efficient Pb-Sn Perovskite Solar Cells and Tandem Device

Narrow-bandgap (NBG) Pb-Sn perovskites are ideal candidates as rear subcell in all-perovskite tandem solar cells. Because Pb-Sn perovskites contain multiple components, the rational regulation of vertical structure and both interfaces of the film is primarily crucial to achieve high-performing NBG perovskite solar cells (PSCs). Herein, a molecule anchoring strategy is developed to in situ construct Cs0.1MA0.3FA0.6Pb0.5Sn0.5I3 perovskite film with vertically aligned crystals and optimized interfaces. Specifically, l-alanine methyl ester is developed as an anchoring additive to induce the vertical crystal growth, while PEA2PbI3SCN film is introduced to promote the homogeneous crystallization at the buried interface via SCN- anchoring with cations. Further ethylenediamine dihalides (EDA(I/Cl)2) post-treatment leads to the gradient energy level alignment on the film surface. Pb-Sn PSCs based on such film show efficient charge transport and extraction, producing a champion power conversion efficiency (PCE) of 22.3% with an impressive fill factor of 82.14%. Notably, combining with semitransparent 1.78 eV wide-bandgap PSCs, the four-terminal all-perovskite tandem device achieves a PCE of 27.1%. This work opens up a new pathway to boost the performance of Pb-Sn PSCs and their tandem devices.

Recent Advances in Piezoelectric Coupled with Photocatalytic Reaction System: Synergistic Mechanism, Enhancement Factors, and Application

The field of photocatalysis has demonstrated numerous advantages in the domains of environmental protection, energy, and materials science. However, conventional modification methods fail to simultaneously enhance carrier separation efficiency, redox capacity, and visible light absorption solely through light activation due to the intrinsic band structure limitations of photocatalysts. In addition to modification methods, the introduction of an external field, such as a piezoelectric field, can effectively address deficiencies in each step of the photocatalytic process and enhance the overall performance. The assistance of a piezoelectric field overcomes the limitations inherent in traditional photocatalytic systems. Hence, this review provides a comprehensive overview of recent advancements in piezoelectric-assisted photocatalysis and thoroughly investigates the interaction between the alternating piezoelectric field and photocatalytic processes. Various ideas for synergistic enhancement of the piezoelectric and photocatalytic properties are also explored. This multifield catalytic system shows remarkable performance in stability, pollutant degradation, and energy conversion, distinguishing it from single catalytic systems. Finally, an in-depth analysis is conducted to address the challenges and prospects associated with piezoelectric photocatalysis technology.

A Graded Redox Interfacial Modifier for High-Performance Perovskite Solar Cells

Perovskite solar cells have emerged as a potential competitor to the silicon photovoltaic technology. The most representative perovskite cells employ SnO2 and spiro-OMeTAD as the charge-transport materials. Despite their high efficiencies, perovskite cells with such a configuration show unsatisfactory lifespan, normally attributed to the instability of perovskites and spiro-OMeTAD. Limited attention was paid to the influence of SnO2, an inorganic material, on device stability. Here we show that improving SnO2 with a redox interfacial modifier, cobalt hexammine sulfamate, simultaneously enhances the power-conversion efficiency (PCE) and stability of the perovskite solar cells. Redox reactions between the bivalent cobalt complexes and oxygen lead to the formation of a graded distribution of trivalent and bivalent cobalt complexes across the surface and bulk regions of the SnO2. The trivalent cobalt complex at the top surface of SnO2 raises the concentration of (SO3NH2)- which passivates uncoordinated Pb2+ and relieves tensile stress, facilitating the formation of perovskite with improved crystallinity. Our approach enables perovskite cells with PCEs of up to 24.91 %. The devices retained 93.8 % of their initial PCEs after 1000 hours of continuous operation under maximum power point tracking. These findings showcase the potential of cobalt complexes as redox interfacial modifiers for high-performance perovskite photovoltaics.

Mapping Spatial Strain Distribution and Its Effects on Optoelectronic Properties in Wrinkled Perovskite Films

Organic-inorganic halide perovskite films, fabricated by using the antisolvent method, have garnered intense attention for their application in high-efficiency and stable solar cells. These films characteristically develop periodic wrinkled microstructures. Previous research has indicated that macroscopic residual strain significantly influences the optoelectronic behaviors of these films. However, the detailed interplay between the wrinkled morphology, strain distribution, and local photophysical properties at the micro- and nanoscale has not been fully elucidated. Here, we explore the microscopic morphology-strain-property relationship within wrinkled perovskite films employing correlative micro-optical and nanoelectrical microscopy techniques. Microphotoluminescence (PL) mapping supplemented by in situ strain PL measurements identifies a heterogeneous spatial strain distribution across the microstructural hills and valleys. Additionally, light-intensity-dependent photoconductive atomic force microscopy reveals that valleys experiencing less compressive strain exhibit a lower conductivity and a higher propensity for ion migration. The findings underscore the potential of targeted strain engineering to optimize the performance and longevity of perovskite solar cells.

Sculpting the Electronic Nano-Terrain on a Perovskite Film for Efficient Charge Transport

Nanopatterned halide perovskites have emerged to improve the performance of optoelectronic devices by controlling the crystallographic and optical properties via morphological modification. However, the correlation between the photophysical property and morphology transformation in nanopatterned perovskite films remains elusive, which hinders the rational design of nanopatterned halide perovskites for optoelectronic devices. In this study, we employed nanoimprinting lithography on a perovskite film to exert a precise control over grain growth and manipulate electronic structures at the level of individual grains. Surface-selective fluorescence lifetime imaging microscopy (FLIM) analyzes the spatiotemporally disentangled geometrical variations in carrier recombination rate and band structure modulation according to different pattern morphologies. Consequently, the stereoscopic mechanism of confined grain growth was unveiled, highlighting the quantitative grain size-based parameters that are crucial for nanoscale material engineering. Notably, the pattern-induced reduction of effective charge mass enabled exclusive control over the subdiffusive carrier transport dynamics on perovskite surfaces, ultimately realizing the surface-selective perovskite photodetectors. The implications of this study are expected to provide valuable guidelines, inspiring innovative design protocols for advancing the next-generation optoelectronic technologies.

Unraveling the Degradation Mechanisms of Perovskite Solar Cells under Mechanical Tensile Loads

The short longevity of perovskite solar cells (PSCs) is the major hurdle toward their commercialization. In recent years, mechanical stability has emerged as a pivotal aspect in enhancing the overall durability of PSCs, prompting a myriad of strategies devoted to this issue. However, the mechanical degradation mechanisms of PSCs remain largely unexplored, with corresponding studies mainly limited to perovskite single crystals, neglecting the complexity and nuances present in PSC devices based on polycrystalline perovskite thin films. Herein, we reveal the underlying mechanisms of the mechanical degradation of formamidinium-based PSCs, which are the most prevalent high-performance PSC candidates. Under uniaxial tensile loads, we found that the degradation is mainly attributed to the sequential increase in the density of micropores and halide defects within the perovskite films. This phenomenon is consistent across various perovskite compositions and environmental conditions. Our findings elucidate mechanistic insights for more targeted mitigation strategies aimed at addressing the mechanical degradation of PSC devices.

Material and Device Design of Flexible Perovskite Solar Cells for Next-Generation Power Supplies

This review outlines the rapid evolution of flexible perovskite solar cells (f-PSCs) to address the urgent need for alternative energy sources, highlighting their impressive power conversion efficiency, which increases from 2.62% to over 24% within a decade. The unique optoelectronic properties of perovskite materials and their inherent mechanical flexibilities instrumental in the development of f-PSCs are examined. Various strategies proposed for material modification and device optimization significantly enhance efficiency and bending durability. The transition from small-scale devices to large-area photovoltaic modules for diverse applications is discussed in addition to the challenges and innovative solutions related to film uniformity and environmental stability. This review provides succinct yet comprehensive insights into the development of f-PSCs, paving the way for their integration into various applications and highlighting their potential in the renewable energy landscape.

Regulating Surface Metal Abundance via Lattice-Matched Coordination for Versatile and Environmentally-Viable Sn-Pb Alloying Perovskite Solar Cells

Narrow-bandgap Sn-Pb alloying perovskites showcased great potential in constructing multiple-junction perovskite solar cells (PSCs) with efficiencies approaching or exceeding the Shockley-Queisser limit. However, the uncontrollable surface metal abundance (Sn2+ and Pb2+ ions) hinders their efficiency and versatility in different device structures. Additionally, the undesired Pb distribution mainly at the buried interface accelerates the Pb leakage when devices are damaged. In this work, a novel strategy is presented to modulate crystallization kinetics and surface metal abundance of Sn-Pb perovskites using a cobweb-like quadrangular macrocyclic porphyrin material, which features a molecular size compatible with the perovskite lattice and robustly coordinates with Pb2+ ions, thus immobilizing them and increasing surface Pb abundance by 61%. This modulation reduces toxic Pb leakage rates by 24-fold, with only ∼23 ppb Pb in water after severely damaged PSCs are immersed in water for 150 h.This strategy can also enhance chemical homogeneity, reduce trap density, release tensile strain and optimize carrier dynamics of Sn-Pb perovskites and relevant devices. Encouragingly, the power conversion efficiency (PCEs) of 23.28% for single-junction, full-stack devices and 21.34% for hole transport layer-free Sn-Pb PSCs are achieved.Notably, the related monolithic all-perovskite tandem solar cell also achieves a PCE of 27.03% with outstanding photostability.

Designing Surface Passivators Through Intramolecular Potential Manipulation for Efficient and Stable Perovskite Solar Cells

The conjugation of terminal ammonium salt groups with perovskite surfaces is a frequently employed technique that aims to enhance the overall performance of perovskite materials, encompassing both bulk and surface properties. Particularly, it exhibits heightened efficacy when applied to surface modification, due to its ability to mitigate defect accumulation and facilitate facile binding with the receptive sites inherent to the perovskite structure. However, the interaction of the bulk ammonium group with PbI2 has the potential to form a low-dimensional phase of perovskite, which may obstruct carrier extraction at the interface. Therefore, the surface passivators (MeO-PFACl) are designed through intramolecular potential manipulation. The combinations of the electron-donating methoxy group and π-π conjugation of the phenyl ring reduce the local potential at the reactive site of formamidinium group, making it less likely to form a low-dimension phase with perovskite. This surface passivation strategy effectively suppresses the surface nonradiative recombination and promotes the interface carrier extraction. The devices treated with MeO-PFACl have demonstrated exceptional performance, achieving a peak power conversion efficiency (PCE) of 25.88%, with an average PCE of 25.37%. These works offer a novel principle for enhancing both the efficiency and stability of PSCs using ammonium-incorporated molecules without the induction of an additional phase layer.

Tailoring component incorporation for homogenized perovskite solar cells

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.

Synthesis of Multifunctional Organic Molecules via Michael Addition Reaction to Manage Perovskite Crystallization and Defect

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.

Improving FAPbBr3 Perovskite Crystal Quality via Additive Engineering for High Voltage Solar Cell over 1.5 V

Lead bromide-based perovskites are promising materials as the top cells of tandem solar cells and for application in various fields requiring high voltages owing to their wide band gaps and excellent environmental resistances. However, several factors, such as the formation of bulk and surface defects, impede the performances of corresponding devices, thereby limiting the efficiencies of these devices as single-junction devices. To reduce the number of defect sites, urea is added to the formamidinium lead bromide (FAPbBr3) perovskite material to increase its grain size. Nevertheless, urea undesirably reacts with lead(II) bromide (PbBr2) in the perovskite structure, creating unfavorable impurities in the device. To solve this problem, herein, in addition to urea, we introduced formamidinium chloride (FACl) into FAPbBr3. Owing to the synergistic effect of urea and FACl, the FAPbBr3 film quality effectively improved due to suppression of the generation of impurities and stabilization of film crystallinity. Consequently, the FAPbBr3 single-junction solar cell constructed using FACl and urea as additives demonstrated a power conversion efficiency of 9.6% and an open-circuit voltage of 1.516 V with negligible hysteresis. This study provides new insights into the use of additive engineering for overcoming the energy losses caused by defects in perovskite films.

Low-Cost, Scalable Fabrication of Multi-Dimensional Perovskite Solar Cells and Modules Assisted by Mechanical Scribing

The performance and scalability of perovskite solar cells (PSCs) based on 3D formamidinium lead triiodide (FAPbI3) absorber are often hindered by defects at the surface and grain boundaries of the perovskite. To address this, the study demonstrates the use of pyrrolidinium iodide for the in situ formation of an energetically aligned 1D pyrrolidinium lead triiodide (PyPbI3) capping layer over the 3D FAbI3 perovskite. The thermodynamically stable PyPbI3 perovskitoids, formed through cation exchange reactions, effectively reduce surface and grain boundary defects in the FAPbI3 perovskite. In addition to improved phase stability, the resulting 1D/3D perovskite film forms a cascade energy band alignment with the other functional layers in PSCs, enabling a barrier-free interfacial charge transport. With a maximum power conversion efficiency (PCE) of ≈23.1% and ≈20.7% at active areas of 0.09 and 1.05 cm2, respectively, the 1D/3D PSCs demonstrate excellent performance and scalability. Leveraging this improved scalability, the study has successfully developed a mechanically-scribed 1D/3D perovskite mini-module with an unprecedentedly high PCE of ≈20.6% and a total power output of ≈270 mW at an active area of ≈13.0 cm2. The 1D/3D multi-dimensional perovskite film developed herein holds great promise for producing low-cost, high-performance perovskite photovoltaics at both the cell and module levels.

Crown Ethers with Different Cavity Diameters Inhibit Ion Migration and Passivate Defects toward Efficient and Stable Lead Halide Perovskite Solar Cells

Organic-inorganic hybrid metal halide perovskite solar cells have been considered as one of the most promising next-generation photovoltaic technologies. Nevertheless, perovskite defects and Li+ ionic migration will seriously affect the power conversion efficiency and stability of the formal device. Herein, we designed two crown ether derivatives (PC12 and PC15) with different cavity diameters, which selectively bind to different metal cations. It is found that PC15 in perovskite precursor solution can actively regulate the nucleation and crystallization processes and passivate the uncoordinated Pb2+ ions, while PC12 at the interface between the perovskite layer and hole-transporting layer can effectively inhibit the migration of Li+ ions and reduce nonradiative recombination losses. Therefore, PC12 and PC15 can act as "lubricant" and defect passivators, as well as inhibitors of ion migration, when they are synergistically applied at the surface and bulk of perovskite layer. Consequently, the optimized device achieved a champion efficiency of 24.8% with significantly improved humidity, thermal, and light stability.

Bi-Directional Modification to Quench Detrimental Redox Reactions and Minimize Interfacial Energy Offset for NiOX/Perovskite-Based Solar Cells

The quality of the buried heterojunction of nickel oxide (NiOX)/perovskite is crucial for efficient charge carrier extraction and minimizing interfacial non-radiative recombination in inverted perovskite solar cells (PSCs). However, NiOX has limitations as a hole transport layer (HTL) due to energy level mismatch, low conduction, and undesirable redox reactions with the perovskite layer, which impede power conversion efficiency (PCE) and long-term stability. In this study, para-amino 2,3,5,6-tetrafluorobenzoic acid (PATFBA) is proposed as a bifacial defect passivator to tailor the NiOX/perovskite interface. The acid group and adjacent fluorine atoms of PATFBA effectively passivate NiOX surface defects, thereby improving its Ni3+/Ni2+ ratio, hole extraction capability, and energy band alignment with perovskite, while also providing active sites for homogenous nucleation. Meanwhile, the amine and adjacent fluorine atomsstabilize the buried perovskite interface by passivating interfacial defects, resulting in higher crystalline perovskite films with supressed non-radaitive recombination. Furthermore, the PATFBA buffer layer prevents redox reactions between Ni3+ and perovskite.These synergistic bi-directional interactions lead to optimized inverted PSCs with a PCE of 20.51% compared to 16.89% for pristine devices and the unencapsulated PATFBA-modified devices exhibit outstanding thermal and long-term stability. This work provides a new engineering approach to buried interfaces through the synergy of functional groups.

Lattice Strain Regulation and Halogen Vacancies Passivation Enable High-Performance Formamidine-Based Perovskite Solar Cells

Formamidinium lead iodide (FAPbI3) perovskite has lately surfaced as the preferred contender for highly proficient and robust perovskite solar cells (PSCs), owing to its favorable bandgap and superior thermal stability. Nevertheless, volatilization and migration of iodide ions (I-) result in non-radiating recombination centers, and the presence of large formamidine (FA) cations tends to cause lattice strain, thereby reducing the power conversion efficiency (PCE) and stability of PSCs. To solve these problems, the lead formate (PbFa) is added into the perovskite solution, which effectively mitigates the halogen vacancy and provides tensile strain outside the perovskite lattice, thereby enhancing its properties. The strong coordination between the C═O of HCOO- and Pb-I backbones effectively immobilizes anions, significantly increases the energy barrier for anion vacancy formation and migration, and reduces the risk of lead ion (Pb2+) leakage, thereby improving the operation and environmental safety of the device. Consequently, the champion PCE of devices with Ag electrodes can be increased from 22.15% to 24.32%. The unencapsulated PSCs can still maintain 90% of the original PCE even be stored in an N2 atmosphere for 1440 h. Moreover, the target devices have significantly improved performance in terms of light exposure, heat, or humidity.

Impact of Residual Strains on the Carrier Mobility and Stability of Perovskite Films

Solution-based inorganic-organic halide perovskites are of great interest to researchers because of their unique optoelectronic properties and easy processing. However, polycrystalline perovskite films often show inhomogeneity due to residual strain induced during the film's post-processing phase. In turn, these strains can impact both their stability and performance. An exhaustive study of residual strains can provide a better understanding and control of how they affect the performance and stability of perovskite films. In this work, we explore this complex interrelationship between residual strains and electrical properties for methylammonium CH3NH3PbI3-xClx films using grazing incidence X-ray diffraction (GIXRD). We correlate their resistivity and carrier mobility using the Hall effect. The sin2(ψ) technique is used to optimize the annealing parameters for the perovskite films. We also establish that temperature-induced relaxation can yield a significant enhancement of the charge carrier transports in perovskite films. Finally, we also use Raman micro-spectroscopy to assess the degradation of perovskite films as a function of their residual strains.

Elimination of Intragrain Defect to Enhance the Performance of FAPbI3 Perovskite Solar Cells by Ionic Liquid

Ionic liquids have been widely used to improve the efficiency and stability of perovskite solar cells (PSCs), and are generally believed to passivate defects on the grain boundaries of perovskites. However, few studies have focused on the relevant effects of ionic liquids on intragrain defects in perovskites which have been shown to be critical for the performance of PSCs. In this work, the effect of ionic liquid 1-hexyl-3-methylimidazolium iodide (HMII) on intragrain defects of formamidinium lead iodide (FAPbI3) perovskite is investigated. Abundant {111}c intragrain planar defects in pure FAPbI3 grains are found to be significantly reduced by the addition of the ionic liquid HMII, shown by using ultra-low-dose selected area electron diffraction. As a result, longer charge carrier lifetimes, higher photoluminescence quantum yield, better charge carrier transport properties, lower Urbach energy, and current-voltage hysteresis are achieved, and the champion power conversion efficiency of 24.09% is demonstrated. These observations suggest that ionic liquids significantly improve device performance resulting from the elimination of {111}c intragrain planar defects.

Photophysical Properties of Submicrometer Ultrathin Perovskite Single-Crystal Films

Organic-inorganic hybrid perovskite (OIHP) has attracted a great deal of interest with respect to diverse optoelectronic devices. However, the photophysical properties of the OIHP require further understanding because most of the investigations have been conducted with polycrystalline perovskites, which contain high-density structural defects. Here, diverse photophysical properties, including structural characterization, spectroscopic features, and photoexcited products, are studied in submicrometer CH3NH3PbBr3 ultrathin single-crystal (UTSC) films. Unlike polycrystalline films and large single crystals, the UTSC film provides a unique platform for studying spectroscopic characteristics of single-crystal perovskites. Compared with the polycrystalline film, the UTSC film presents an atomically flat morphology and near-perfect lattice with a lower defect density, leading to an isotropic system that can be applied in the construction of high-performance optoelectronic devices. Furthermore, a long lifetime emissive channel assigned to the trion is indicated, which is scarcely found in perovskite polycrystalline films. Our results profoundly improve our understanding of their photophysical properties and expand the horizons for perovskite materials in photonic applications.

Spectrally Stable and Bright Red Perovskite Light-Emitting Diodes

Mixing iodide and bromide in three-dimensional metal-halide perovskites is a facile strategy for achieving red light-emitting diodes (LEDs). However, these devices often face challenges such as instability in electroluminescence spectra and low brightness due to phase segregation in mixed-halide perovskites. Here, we demonstrate spectrally stable and bright red perovskite LEDs by substituting some of the halide ions with pseudohalogen thiocyanate ions (SCN-). We find that SCN- can occupy halogen vacancies, thereby releasing microstrain and passivating defects in the perovskite crystals. This leads to the suppression of mixed-halide phase segregation under electrical bias. As a result, the red perovskite LEDs exhibit a high brightness of >35 000 cd m-2 with stable Commission Internationale de l'Eclairage (CIE) coordinates of (0.713, 0.282). This brightness surpasses that of the best-performing red perovskite LEDs, showing great promise for advancing perovskite LEDs in display and lighting applications.

Lattice Mismatch at the Heterojunction of Perovskite Solar Cells

Lattice mismatch significantly influences microscopic transport in semiconducting devices, affecting interfacial charge behavior and device efficacy. This atomic-level disordering, often overlooked in previous research, is crucial for device efficiency and lifetime. Recent studies have highlighted emerging challenges related to lattice mismatch in perovskite solar cells, especially at heterojunctions, revealing issues like severe tensile stress, increased ion migration, and reduced carrier mobility. This review systematically discusses the effects of lattice mismatch on strain, material stability, and carrier dynamics. It also includes detailed characterizations of these phenomena and summarizes current strategies including epitaxial growth and buffer layer, as well as explores future solutions to mitigate mismatch-induced issues. We also provide the challenges and prospects for lattice mismatch, aiming to enhance the efficiency and stability of perovskite solar cells, and contribute to renewable energy technology advancements.

Enhancing the Efficiency and Stability of Inverted Formamidinium-Cesium Lead-Triiodide Perovskite Solar Cells through Lewis Base Pretreatment of Buried Interfaces

Mixed components of formamidinium(FA) and cesium (Cs)-based perovskite solar cells are the most hopeful for commercialization owing to their excellent operational and phase stabilities, especially for devices with inverted structure. The nonradiative recombination of carriers can be effectively suppressed through interface optimization, therefore, the performance of devices can be improved. Notably, the buried interface emerges as critical aspects such as charge transport, charge recombination kinetics, and morphology of perovskite films. This study focuses on a straightforward yet effective approach to overcome buried interface challenges between organic polymers (poly(-triarylamine) (PTAA) and FACs-based perovskite films. The PTAA substrate is pretreated with a Lewis base known as 2-butynoic acid (BA) with a C═O functional group. First, it can be an interfacial buffering layer, harmonizing stress mismatch between the perovskite and PTAA layers, consequently optimizing crystallization and improving perovskite film quality. Second, Pb2+ defect can be passivated at the buried interface of the perovskite film through binding with the C═O group of the BA molecule. This dual-function strategy leads to a substantial enhancement in both photoelectric conversion efficiency (PCE) and stability of devices. Finally, the PCE of the device-modified buried interface with BA reaches an impressive 23.33%. Furthermore, unencapsulated devices with BA treatment maintain approximately 94% of their initial efficiency after aging at maximum power point tracking for 1000 h.

Application of Strain Engineering in Solar Cells

Solar cells represent a promising innovation in energy storage, offering not only exceptional cleanliness and low cost but also a high degree of flexibility, rendering them widely applicable. In recent years, scientists have dedicated substantial efforts to enhancing the performance of solar cells, aiming to drive sustainable development and promote clean energy applications. One approach that has garnered significant attention is strain engineering, which involves the adjustment of material microstructure and organization through mechanical tensile or compressive strain, ultimately serving to enhance the mechanical properties and performance stability of materials. This paper aims to provide a comprehensive review of the latest advancements in the application of strain engineering in solar cells, focused on the current hot research area-perovskite solar cells. Specifically, it delves into the origins and characterization of strain in solar cells, the impact of strain on solar cell performance, and the methods for regulating stable strain. Furthermore, it outlines strategies for enhancing the power conversion efficiency (PCE) and stability of solar cells through strain engineering. Finally, the paper conducts an analysis of the challenges encountered in the development process and presents a forward-looking perspective on further enhancing the performance of solar cells through strain engineering.

Ultrastable and efficient slight-interlayer-displacement 2D Dion-Jacobson perovskite solar cells

Stability has been a long-standing concern for solution-processed perovskite photovoltaics and their practical applications. However, stable perovskite materials for photovoltaic remain insufficient to date. Here we demonstrate a series of ultrastable Dion-Jacobson (DJ) perovskites (1,4-cyclohexanedimethanammonium)(methylammonium)n-1PbnI3n+1 (n ≥ 1) for photovoltaic applications. The scalable technology by blade-coated solar cells for the designed DJ perovskites (nominal n = 5) achieves a maximum stabilized power conversion efficiency (PCE) of 19.11% under an environmental atmosphere. Un-encapsulated cells by blade-coated technology retain 92% of their initial efficiencies for over 4000 hours under ~90% relative humidity (RH) aging conditions. More importantly, these cells also exhibit remarkable thermal (85 °C) and operational stability, which shows negligible efficiency loss after exceeding 5000-hour heat treatment or after operation at maximum power point (MPP) exceeding 6000 hours at 45 °C under a 100 mW cm-2 continuous light illumination.

The effective prolongation of the excited-state carrier lifetime of CsPbI2Br with applying strain

In recent years, all-inorganic perovskites CsPbX3 (X = Cl, Br, I) have emerged as excellent candidates for solar cells due to their remarkable thermal stability and suitable bandgaps. Among them, CsPbI2Br is a hotspot in perovskite material research currently. Non-radiative electron-hole recombination often leads to significant energy losses, impacting the efficiency of solar cells, so a thorough understanding of carrier recombination mechanisms is crucial. Our work investigated the carrier recombination dynamics in detail and proved that strains can effectively reduce nonradiative recombination. In this study, using first-principles calculations combined with nonadiabatic (NA) molecular dynamics (MD), we demonstrate that applying 2% tensile and 2% compressive strains to CsPbI2Br can modify the bandgap, induce moderate disorder, reduce the overlap of electron-hole wavefunctions, decrease NA coupling, and shorten decoherence time, thereby minimizing non-radiative recombination and extending the carrier lifetime. Especially the 2% tensile strain exhibits more effective control performance, significantly reducing non-radiative electron-hole recombination and extending the charge carrier lifetime to 14.59 ns, nearly five times that of the pristine CsPbI2Br system (3.12 ns). This study reveals the impact mechanism of strain on carrier behavior in perovskite solar cells, providing a new non-chemical strategy for modulating the lifetime of photo-generated carriers and enhancing the efficiency of all-inorganic perovskite solar cells.

Zwitterion Dual-Modification Strategy for High-Quality NiOx and Perovskite Films for Solar Cells

Nickel oxide (NiOx) has been limited in use as a hole transport layer for its low conduction, surface defects, and redox reactions with the perovskite layer. To address these issues, the incorporation of zwitterion L-tryptophan (Trp) is proposed at the NiOx/Trp interface. The carboxyl group of Trp effectively passivates the surface positive defects of NiOx, thereby improving its optical and electrical properties. The ammonium group of Trp not only passivates negative defects but modulates the growth of the perovskite layer, resulting in an improved perovskite film quality. Furthermore, the Trp layer acts as a buffer layer, suppressing adverse interfacial reactions between the perovskite and NiOx. Consequently, perovskite solar cells with 1.56 and 1.68 eV absorbers achieve the champion power conversion efficiency (PCE) of 23.79% and 20.41%, respectively. Moreover, the unencapsulated devices demonstrate excellent long-term stability, retaining above 80% of the initial PCE value after 1600 h of storage in the air with a humidity of 50-60%.

Non-Fullerene Acceptors Assisted Target Therapy for Interface Treatment Enable High Performance Inverted Perovskite Solar Cells

Targeted treatment of the interface between electron transport layers (ETL) and perovskite layers is highly desirable for achieving passivating effects and suppressing carrier nonradiative recombination, leading to high performance and long-term stability in perovskite solar cells (PSCs). In this study, a series of non-fullerene acceptors (NFAs, Y-H, Y-F, and Y-Cl) are introduced to optimize the properties of the perovskite/ETL interface. This optimization involves passivating Pb2+ defects, releasing stress, and modulating carrier dynamics through interactions with the perovskite. Remarkably, after modifying with NFAs, the absorption range of perovskite films into the near-infrared region is extended. As expected, Y-F, with the largest electrostatic potential, facilitates the strongest interaction between the perovskite and its functional groups. Consequently, champion power conversion efficiencies of 21.17%, 22.21%, 23.25%, and 22.31% are achieved for control, Y-H-, Y-F-, and Y-Cl-based FA0.88Cs0.12PbI2.64Br0.36 (FACs) devices, respectively. This treatment also enhances the heat stability and air stability of the corresponding devices. Additionally, these modifier layers are applied to enhance the efficiency of Cs0.05(FA0.95MA0.05)0.95PbI2.64Br0.36 (FAMA) devices. Notably, a champion PCE exceeding 24% is achieved in the Y-F-based FAMA device. Therefore, this study provides a facile and effective approach to target the interface, thereby improving the efficiency and stability of PSCs.

Defect Engineering at Buried Interface of Perovskite Solar Cells

Perovskite solar cells (PSC) have developed rapidly since the past decade with the aim to produce highly efficient photovoltaic technology at a low cost. Recently, physical and chemical defects at the buried interface of PSC including vacancies, impurities, lattice strain, and voids are identified as the next formidable hurdle to the further advancement of the performance of devices. The presence of these defects has unfavorably impacted many optoelectronic properties in the PSC, such as band alignment, charge extraction/recombination dynamics, ion migration behavior, and hydrophobicity. Herein, a broad but critical discussion on various essential aspects related to defects at the buried interface is provided. In particular, the defects existing at the surface of the underlying charge transporting layer (CTL) and the bottom surface of the perovskite film are initially elaborated. In situ and ex situ characterization approaches adopted to unveil hidden defects are elucidated to determine their influence on the efficiency, operational stability, and photocurrent-voltage hysteresis of PSC. A myriad of innovative strategies including defect management in CTL, the introduction of passivation materials, strain engineering, and morphological control used to address defects are also systematically elucidated to catalyze the further development of more efficient, reliable, and commercially viable photovoltaic devices.

Strain Heterogeneity and Extended Defects in Halide Perovskite Devices

Strain is an important property in halide perovskite semiconductors used for optoelectronic applications because of its ability to influence device efficiency and stability. However, descriptions of strain in these materials are generally limited to bulk averages of bare films, which miss important property-determining heterogeneities that occur on the nanoscale and at interfaces in multilayer device stacks. Here, we present three-dimensional nanoscale strain mapping using Bragg coherent diffraction imaging of individual grains in Cs0.1FA0.9Pb(I0.95Br0.05)3 and Cs0.15FA0.85SnI3 (FA = formamidinium) halide perovskite absorbers buried in full solar cell devices. We discover large local strains and striking intragrain and grain-to-grain strain heterogeneity, identifying distinct islands of tensile and compressive strain inside grains. Additionally, we directly image dislocations with surprising regularity in Cs0.15FA0.85SnI3 grains and find evidence for dislocation-induced antiphase boundary formation. Our results shine a rare light on the nanoscale strains in these materials in their technologically relevant device setting.

Compression of Organic Molecules Coupled with Hydrogen Bonding Extends the Charge Carrier Lifetime in BA2SnI4

Two-dimensional (2D) metal halide perovskites, such as BA2SnI4 (BA═CH3(CH2)3NH3), exhibit an enhanced charge carrier lifetime in experiments under strain. Experiments suggest that significant compression of the BA molecule, rather than of the inorganic lattice, contributes to this enhancement. To elucidate the underlying physical mechanism, we apply a moderate compressive strain to the entire system and subsequently introduce significant compression to the BA molecules. We then perform ab initio nonadiabatic molecular dynamics simulations of nonradiative electron-hole recombination. We observe that the overall lattice compression reduces atomic motions and decreases nonadiabatic coupling, thereby delaying electron-hole recombination. Additionally, compression of the BA molecules enhances hydrogen bonding between the BA molecules and iodine atoms, which lengthens the Sn-I bonds, distorts the [SnI6]4- octahedra, and suppresses atomic motions further, thus reducing nonadiabatic coupling. Also, the elongated Sn-I bonds and weakened antibonding interactions increase the band gap. Altogether, the compression delays the nonradiative electron-hole recombination by more than a factor of 3. Our simulations provide new and valuable physical insights into how compressive strain, accommodated primarily by the organic ligands, positively influences the optoelectronic properties of 2D layered halide perovskites, offering a promising pathway for further performance improvements.

Design and simulation investigations on charge transport layers-free in lead-free three absorber layer all-perovskite solar cells

The multiple absorber layer perovskite solar cells (PSCs) with charge transport layers-free (CTLs-free) have drawn widespread research interest due to their simplified architecture and promising photoelectric characteristics. Under the circumstances, the novel design of CTLs-free inversion PSCs with stable and nontoxic three absorber layers (triple Cs3Bi2I9, single MASnI3, double Cs2TiBr6) as optical-harvester has been numerically simulated by utilizing wxAMPS simulation software and achieved high power conversion efficiency (PCE) of 14.8834%. This is owing to the innovative architecture of PSCs favors efficient transport and extraction of more holes and the slender band gap MASnI3 extends the absorption spectrum to the near-infrared periphery compared with the two absorber layers architecture of PSCs. Moreover, the performance of the device with p-type-Cs3Bi2I9/p-type-MASnI3/n-type-Cs2TiBr6 architecture is superior to the one with the p-type-Cs3Bi2I9/n-type-MASnI3/n-type-Cs2TiBr6 architecture due to less carrier recombination and higher carrier life time inside the absorber layers. The simulation results reveal that Cs2TiF6 double perovskite material stands out as the best alternative. Additionally, an excellent PCE of 21.4530% can be obtained with the thicker MASnI3 absorber layer thickness (0.4 µm). Lastly, the highest-performance photovoltaic devices (28.6193%) can be created with the optimized perovskite doping density of around E15 cm3 (Cs3Bi2I9), E18 cm3 (MASnI3), and 1.5E19 cm3 (Cs2TiBr6). This work manifests that the proposed CTLs-free PSCs with multi-absorber layers shall be a relevant reference for forward applications in electro-optical and optoelectronic devices.

Residual Stress Mitigation in Perovskite Solar Cells via Butterfly-Inspired Hierarchical PbI2 Scaffold

Residual stress in metal halide perovskite films intimately affects the photovoltaic figure of merit and longevity of perovskite solar cells. A delicate management of the crystallization kinetics is critical to the preparation of high-quality perovskite films. Only very limited methods, however, are available to regulate the residual stress of a perovskite film in a controllable manner, particularly for a perovskite film fabricated by a two-step method. Here, we demonstrate the construction of a hierarchical PbI2 scaffold inspired by Archaeoprepona demophon butterfly by combining an interlayer guided growth of porous structure and nanoimprinting. The hierarchically structured PbI2 that emulates the physical structure of the butterfly wing scale permits unimpeded permeation of organic amine salts and sufficient space for volume expansion during the crystallization process, accompanied by preferential perovskite growth of a defectless (001) crystal plane. The optimized perovskite film outperforms the control with reduced residual stress and defect density. Consequently, perovskite solar cells with a respectable power conversion efficiency reaching 23.4% (certified 23%) and an impressive open-circuit voltage of 1.184 V can be achieved. The target device can maintain 80% of initial efficiency after maximum power point tracking under illumination for 700 h. This work expands the range of engineering toward PbI2 by exploring a simultaneously tailored morphology and crystallinity and highlights the significance of a hierarchical PbI2 scaffold as an alternative choice to mitigate residual stress in a two-step processed perovskite active layer and boost the longevity of perovskite solar cells.

Top-Down Induced Crystallization Orientation toward Highly Efficient p-i-n Perovskite Solar Cells

Crystallization orientation plays a crucial role in determining the performance and stability of perovskite solar cells (PVSCs), whereas effective strategies for realizing oriented perovskite crystallization is still lacking. Herein, a facile and efficient top-down strategy is reported to manipulate the crystallization orientation via treating perovskite wet film with propylamine chloride (PACl) before annealing. The PA+ ions tend to be adsorbed on the (001) facet of the perovskite surface, resulting in the reduced cleavage energy to induce (001) orientation-dominated growth of perovskite film and then reduce the temperature of phase transition, meanwhile, the penetrating Cl ions further regulate the crystallization process. As-prepared (001)-dominant perovskite films exhibit the ameliorative film homogeneity in terms of vertical and horizontal scale, leading to alleviated lattice mismatch and lowered defect density. The resultant PVSC devices deliver a champion power conversion efficiency (PCE) of 25.07% with enhanced stability, and the unencapsulated PVSC device maintains 95% of its initial PCE after 1000 h of operation at the maximum power point under simulated AM 1.5G illumination.

Dynamic Reversible Oxidation-Reduction of Iodide Ions for Operationally Stable Perovskite Solar Cells under ISOS-L-3 Protocol

Despite rapid advancements in the photovoltaic efficiencies of perovskite solar cells (PSCs), their operational stability remains a significant challenge for commercialization. This instability mainly arises from light-induced halide ion migration and subsequent oxidation into iodine (I2). The situation is exacerbated when considering the heat effects at elevated temperatures, leading to the volatilization of I2 and resulting in irreversible device degradation. Mercaptoethylammonium iodide (ESAI) is thus incorporated into perovskite as an additive to inhibit the oxidation of iodide anion (I-) and  the light-induced degradation pathway of FAPbI3→FAI+PbI2. Additionally, the formation of a thiol-disulfide/I--I2 redox pair within the perovskite film provides a dynamic mechanism for the continuous reduction of I2 under light and thermal stresses, facilitating the healing of iodine-induced degradations. This approach significantly enhances the operational stability of PSCs. Under the ISOS-L-3 testing protocol (maximum power point (MPP) tracking in an environment with relative humidity of ≈50% at ≈65 °C), the treated PSCs maintain 97% of their original power conversion efficieney (PCE) after 300 h of aging. In contrast, control devices exhibit almost complete degradation, primarily due to rapid thermal-induced I2 volatilization. These results demonstrate a promising strategy to overcome critical stability challenges in PSCs, particularly in scenarios involving thermal effects.

Room Temperature Crystallized Phase-Pure α-FAPbI3 Perovskite with In-Situ Grain-Boundary Passivation

Energy loss in perovskite grain boundaries (GBs) is a primary limitation toward high-efficiency perovskite solar cells (PSCs). Two critical strategies to address this issue are high-quality crystallization and passivation of GBs. However, the established methods are generally carried out discretely due to the complicated mechanisms of grain growth and defect formation. In this study, a combined method is proposed by introducing 3,4,5-Trifluoroaniline iodide (TFAI) into the perovskite precursor. The TFAI triggers the union of nano-sized colloids into microclusters and facilitates the complete phase transition of α-FAPbI3 at room temperature. The controlled chemical reactivity and strong steric hindrance effect enable the fixed location of TFAI and suppress defects at GBs. This combination of well-crystallized perovskite grains and effectively passivated GBs leads to an improvement in the open circuit voltage (Voc) of PSCs from 1.08 V to 1.17 V, which is one of the highest recorded Voc without interface modification. The TFAI-incorporated device achieved a champion PCE of 24.81%. The device maintained a steady power output near its maximum power output point, showing almost no decay over 280 h testing without pre-processing.

Multi-Functional Regulation on Buried Interface for Achieving Efficient Triple-Cation Perovskite Solar Cells

Mixed-cation perovskite solar cells (PSCs) have attracted much attention because of the advantages of suitable bandgap and stability. It is still a challenge to rationally design and modify the perovskite/tin oxide (SnO2) heterogeneous interface for achieving highly efficient and stable PSCs. Herein, a strategy of one-stone-for-three-birds is proposed to achieve multi-functional interface regulation via introducing N-Chlorosuccinimide (NCS) into the solution of SnO2: i) C═O functional group in NCS can induces strong binding affinity to uncoordinated defects (oxygen vacancies, free lead ions, etc) at the buried interface and passivate them; ii) incomplete in situ hydrolysis reactions can occur spontaneously and adjust the pH value of the SnO2 solution to achieve a more matchable energy level; iii) effectively releasing the residual stress of the underlying perovskite. As a result, a champion power conversion efficiency (PCE) of 24.74% is achieved with a device structure of ITO/SnO2/Perovskite/Spiro-OMeTAD/Ag, which is one of the highest values for cesium-formamidinium-methylammonium (CsFAMA) triple cation PSCs. Furthermore, the device without encapsulation can sustain 94.6% of its initial PCE after the storage at room temperature and relative humidity (RH) of 20% for 40 days. The research provides a versatile way to manipulate buried interface for achieving efficient and stable PSCs.

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

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

Fluorinated Polyimide Tunneling Layer for Efficient and Stable Perovskite Photovoltaics

Despite the remarkable progress of perovskite solar cells (PSCs), challenges remain in terms of finding effective and viable strategies to enhance their long-term stability while maintaining high efficiency. In this study, a new insulating and hydrophobic fluorinated polyimide (FPI: 6FDA-6FAPB) was used as the interface layer between the perovskite layer and the hole transport layer (HTL) in PSCs. The functional groups of FPI play a pivotal role in passivating interface defects within the device. Due to its high work function, FPI demonstrates field-effect passivation (FEP) capabilities as an interface layer, effectively mitigating non-radiative recombination at the interface. Notably, the FPI insulating interface layer does not impede carrier transmission at the interface, which is attributed to the presence of hole tunneling effects. The optimized PSCs achieve an outstanding power conversion efficiency (PCE) of 24.61 % and demonstrate excellent stability, showcasing the efficacy of FPI in enhancing device performance and reliability.

Regulating structural stability and photoelectrical properties of FAPbI3via formamidine cation orientation

Organic cations play a significant role in the structural stability and photoelectrical properties of organic-inorganic hybrid perovskites. The orientation of organic cations impacts its interaction with inorganic octahedrons [PbI]6-, subsequently modifying the atomic structure and electronic and optical properties of perovskite materials. However, it is still challenging to regulate the stability of perovskites with different orientations. In this work, density functional theory calculations were performed to investigate the effects of the formamidine cation (FA+) located at the angles of 0°, 45°, 90° and 180° (relative to the normal of the crystal plane) along the typical crystal directions ([001], [010], [110] and [111]) on the structural stability and photoelectrical properties of formamidine lead iodide (FAPbI3). The results show that when FA+ is located at 45° along the [111] direction, FAPbI3 achieves the highest stability and excellent photoelectrical properties. The energy evolution curves display that the system with the orientation of [111] has the minimum energy value, signifying stronger stability than the other orientations. Especially, when FA+ is located at 45° along the [111] direction. it exhibits a stronger hydrogen bond between H and I atoms, shorter Pb-I bond length and smaller [PbI]6- octahedral tilt bond angle. The band gap in the [110] direction changes from direct to indirect while FAPbI3 with other FA+ orientations still maintains the direct band structure located at the high symmetric R point. Furthermore, FA+ orientation drives the redshift of FAPbI3 towards the long wavelength region in the [111] crystal direction, which enhances the light absorption coefficient. This work can offer guidance in employing molecular regulation technology for the development of stable perovskite solar cells.

Strain engineering improves the photovoltaic performance of carbon-based hole-transport-material free CsPbIBr2 perovskite solar cells

Alkylamines with different chain lengths including n-butylamine, n-hexylamine, and n-octylamine, are applied to regulate the CsPbIBr2 perovskite film quality by strain engineering. The status of residual strains is controllably modulated, resulting in improved efficiency and stability of carbon-based hole-transport-material free CsPbIBr2 perovskite solar cells.

Buried Interface Optimization for Flexible Perovskite Solar Cells with High Efficiency and Mechanical Stability

The power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs) are significantly reduced by defect-induced charge non-radiative recombination. Also, unexpected residual strain in perovskite films leads to an unfavorable impact on the stability and efficiency of PSCs, notably flexible PSCs (f-PSCs). Considering these problems, a thorough and effective strategy is proposed by incorporating phytic acid (PA) into SnO2 as an electron transport layer (ETL). With the addition of PA, the Sn inherent dangling bonds are passivated effectively and thus enhance the conductivity and electron mobility of SnO2 ETL. Meanwhile, the crystallization quality of perovskite is increased largely. Therefore, the interface/bulk defects are reduced. Besides, the residual strain of perovskite film is significantly reduced and the energy level alignment at the SnO2/perovskite interface becomes more matched. As a result, the champion f-PSC obtains a PCE of 21.08% and rigid PSC obtains a PCE of 21.82%, obviously surpassing the PCE of 18.82% and 19.66% of the corresponding control devices. Notably, the optimized f-PSCs exhibit outstanding mechanical durability, after 5000 cycles of bending with a 5 mm bending radius, the SnO2-PA-based device preserves 80% of the initial PCE, while the SnO2-based device only remains 49% of the initial value.

Suppressed thermal transport in silicon nanoribbons by inhomogeneous strain

Nanoscale structures can produce extreme strain that enables unprecedented material properties, such as tailored electronic bandgap1-5, elevated superconducting temperature6,7 and enhanced electrocatalytic activity8,9. While uniform strains are known to elicit limited effects on heat flow10-15, the impact of inhomogeneous strains has remained elusive owing to the coexistence of interfaces16-20 and defects21-23. Here we address this gap by introducing inhomogeneous strain through bending individual silicon nanoribbons on a custom-fabricated microdevice and measuring its effect on thermal transport while characterizing the strain-dependent vibrational spectra with sub-nanometre resolution. Our results show that a strain gradient of 0.112% per nanometre could lead to a drastic thermal conductivity reduction of 34 ± 5%, in clear contrast to the nearly constant values measured under uniform strains10,12,14,15. We further map the local lattice vibrational spectra using electron energy-loss spectroscopy, which reveals phonon peak shifts of several millielectron-volts along the strain gradient. This unique phonon spectra broadening effect intensifies phonon scattering and substantially impedes thermal transport, as evidenced by first-principles calculations. Our work uncovers a crucial piece of the long-standing puzzle of lattice dynamics under inhomogeneous strain, which is absent under uniform strain and eludes conventional understanding.

Synergy of Front-Surface Energy-Level Gradient and Lattice Anchoring Effect for Enhancing Perovskite Solar Cell Performance

A front surface gradient of the absorber valence band can effectively reduce the open-circuit voltage (VOC) loss of perovskite solar cells by suppressing the minority carrier concentration near the front surface. However, the existing method is limited to the one-step fabrication process, resulting in underachieved photon harvesting and power conversion efficiency (PCE). To solve the problem, ZnCd-based alloy quantum dots (QDs) are utilized to create a valence-band-maximum gradient at the front surface of a two-step processed FAPbI3 absorber. This design significantly enhances VOC without requiring surface passivation. Furthermore, it is demonstrated that reducing the QD-perovskite lattice mismatch while maintaining QD's energy levels mitigates nonradiative recombination without compromising the front surface gradient effect. As a result, normal-structured perovskite solar cells achieve a VOC equivalent to 93% of the Schockley-Queisser limit and a PCE of 24.37%.

Understanding Defects in Perovskite Solar Cells through Computation: Current Knowledge and Future Challenge

Lead halide perovskites with superior optoelectrical properties are emerging as a class of excellent materials for applications in solar cells and light-emitting devices. However, perovskite films often exhibit abundant intrinsic defects, which can limit the efficiency of perovskite-based optoelectronic devices by acting as carrier recombination centers. Thus, an understanding of defect chemistry in lead halide perovskites assumes a prominent role in further advancing the exploitation of perovskites, which, to a large extent, is performed by relying on first-principles calculations. However, the complex defect structure, strong anharmonicity, and soft lattice of lead halide perovskites pose challenges to defect studies. In this perspective, on the basis of briefly reviewing the current knowledge concerning computational studies on defects, this work concentrates on addressing the unsolved problems and proposing possible research directions in future. This perspective particularly emphasizes the indispensability of developing advanced approaches for deeply understanding the nature of defects and conducting data-driven defect research for designing reasonable strategies to further improve the performance of perovskite applications. Finally, this work highlights that theoretical studies should pay more attention to establishing close and clear links with experimental investigations to provide useful insights to the scientific and industrial communities.

Modulation of Photoinduced Phase Segregation and Stress-Driven Nanoscale Cracking in Hybrid Halide Perovskite Solar Cells

The negative effect of photoinduced halide segregation (PIHS) on the properties of hybrid halide perovskites poses a major obstacle for its future commercial application. Therefore, the in-depth understanding of halide-ion segregation and its causes is an urgent and intractable problem. When PIHS reaches a certain threshold, it will aggravate the deterioration of the film surface morphology and form nanoscale cracks. Herein, the formation mechanism and types of cracks are revealed by exploring the stress distribution in the film. Using the femtosecond time-resolved transient absorption spectroscopy, the ultrafast formation of the iodine rich phase is observed, which appears earlier than the bromine rich phase. In addition, the introduction of organic ligand didodecyldimethylammonium bromide can significantly inhibit PIHS and improve the surface morphology of the film, which can promote the device efficiency from 9.63 to 11.20%. This work provides a novel perspective for the exploration of the PIHS.