Dynamical Transformation of Two-Dimensional Perovskites with Alternating Cations in the Interlayer Space for High-Performance Photovoltaics

Simultaneous Halides Oxidation Inhibition and Defects Passivation for Efficient and Stable Perovskite Solar Cells

Despite significant progress in improving the photovoltaic efficiency of perovskite solar cells (PSCs), achieving long-term operational stability remains challenging for their commercialization. Light-induced halide ion migration causes instability, oxidizing iodide into iodine. Elevated temperatures exacerbate this issue, resulting in irreversible device degradation. Here, ammonium oxalate (AO) is introduced as an additive to the perovskite precursor to prevent both the degradation of the perovskite precursor and the photo-induced degradation pathway to formamidinium iodide and PbI2 in perovskite films. AO stabilizes the precursor by inhibiting the oxidation of iodide ions (I-) and passivates charged traps through coordination and hydrogen bonding interactions, thereby enhancing crystallinity and reducing defects within the resultant perovskite films. This leads to the achievement of a higher-quality perovskite film with a low trap density and an extended carrier lifetime. In addition, the oxidation of I- within the perovskite film is inhibited, reducing the corrosion of I2 on the silver electrode and enhancing the long-term operating stability of the photovoltaic device. Consequently, the champion power conversion efficiency (PCE) of PSCs is increased from 22.19% to 24.82%. Meanwhile, the air, thermal, and light stability are also enhanced.

Constructing bottom-up n-p perovskite homojunctions for enhanced performance of electron transport layer-free solar cells

We constructed the high-quality perovskite films with bottom-up n-p homojunctions via a modified sequential two-step deposition technique, which optimized the energy level alignment between the perovskite layer and the adjacent contacts for promoting charge separation and extraction, as well as suppressing non-radiative recombination. The resultant n-p homojunction-based electron transport layer-free perovskite solar cells achieved improved efficiency, which was ∼13% higher than that of the control counterpart.

Improved Conductivity of 2D Perovskite Capping Layer for Realizing High-Performance 3D/2D Heterostructured Hole Transport Layer-Free Perovskite Photovoltaics

Perovskite solar cells (PSCs) have emerged as low-cost photovoltaic representatives. Constructing three-dimensional (3D)/two-dimensional (2D) perovskite heterostructures has been shown to effectively enhance the efficiency and stability of PSCs. However, further enhancement of device performance is still largely limited by inferior conductivity of the 2D perovskite capping layer and its mismatched energy level with the 3D perovskite layer. Here, we developed an effective surface modification strategy via synergically incorporating inorganic high valence-state niobium ion (Nb5+) metal dopants and organic ammonium halide salts to in situ construct a high-quality 2D perovskite capping layer on top of the underlying 3D perovskite layer. As a result, the conductivity of the 2D perovskite capping layer was effectively enhanced by 43%, the energy barrier between 3D and 2D perovskite layers was favorably reduced, and the built-in electric field of the 3D/2D heterostructured perovskite stacks has been enlarged. In addition, the 2D perovskite capping layer also effectively reduced the defect densities by up to 29%, as verified by the space-charge-limited-current (SCLC) tests. Benefiting from the facilitated charge extraction and suppressed non-radiative recombination, the blade-coated hole transport layer-free PSCs based on this optimized 3D/2D heterostructured perovskite film achieved an efficiency of 23.2%, ∼19% higher than that of the control device (19.5%), which represented one of the best-performing PSCs with simplified device architecture fabricated via a scalable fabrication technique. The modified 3D/2D perovskite-based device also exhibited improved operational stability.

The Effect of Antisolvent Treatment on the Growth of 2D/3D Tin Perovskite Films for Solar Cells

Antisolvent treatment is used in the fabrication of perovskite films to control grain growth during spin coating. We study widely incorporated aromatic hydrocarbons and aprotic ethers, discussing the origin of their performance differences in 2D/3D Sn perovskite (PEA0.2FA0.8SnI3) solar cells. Among the antisolvents that we screen, diisopropyl ether yields the highest power conversion efficiency in solar cells. We use a combination of optical and structural characterization techniques to reveal that this improved performance originates from a higher concentration of 2D phase, distributed evenly throughout the 2D/3D Sn perovskite film, leading to better crystallinity. This redistribution of the 2D phase, as a result of diisopropyl ether antisolvent treatment, has the combined effect of decreasing the Sn4+ defect density and background hole density, leading to devices with improved open-circuit voltage, short-circuit current, and power conversion efficiency.

Investigation of perovskite materials for solar cells using scanning tunneling microscopy

The issue of energy scarcity has become more prominent due to the recent scientific and technological advancements. Consequently, there is an urgent need for research on sustainable and renewable resources. Solar energy, in particular, has emerged as a highly promising option because of its pollution-free and environment-friendly characteristics. Among the various solar energy technologies, perovskite solar cells have attracted much attention due to their lower cost and higher photoelectric conversion efficiency (PCE). However, the inherent instability of perovskite materials hinders the commercialization of such devices. The utilization of scanning tunneling microscopy/spectroscopy (STM/STS) can provide valuable insights into the fundamental properties of different perovskite materials at the atomic scale, which is crucial for addressing this challenge. In this review, we present the recent research progress of STM/STS analysis applied to various perovskites for solar cells, including halide perovskites, two-dimensional Ruddlesden-Popper perovskites, and oxide perovskites. This comprehensive overview aims to inspire new ideas and strategies for optimizing solar cells.

Homogenizing The Low-Dimensional Phases for Stable 2D-3D Tin Perovskite Solar Cells

2D-3D tin-based perovskites are considered as promising candidates for achieving efficient lead-free perovskite solar cells (PSCs). However, the existence of multiple low-dimensional phases formed during the film preparation hinders the efficient transport of charge carriers. In addition, the non-homogeneous distribution of low-dimensional phases leads to lattice distortion and increases the defect density, which are undesirable for the stability of tin-based PSCs. Here, mixed spacer cations [diethylamine (DEA+) and phenethylamine (PEA+)] are introduced into tin perovskite films to modulate the distribution of the 2D phases. It is found that compared to the film with only PEA+, the combination of DEA+ and PEA+ favors the formation of homogeneous low-dimensional perovskite phases with three octahedral monolayers (n = 3), especially near the bottom interface between perovskite and hole transport layer. The homogenization of 2D phases help improve the film quality with reduced lattice distortion and released strain. With these merits, the tin PSC shows significantly improved stability with 94% of its initial efficiency retained after storing in a nitrogen atmosphere for over 4600 h, and over 80% efficiency maintained after continuous illumination for 400 h.

Exploring Label-Free Imaging Techniques with Copper Sulfide Microspheres for Observing Breast Cancer Cells

A single breast cancer is a prevalent form of cancer, affecting over 2.3 million women worldwide, as reported by the World Health Organization. Recently, researchers have extensively explored the utilization of biomaterials in breast cancer theranostics. One notable biomaterial being investigated is various structures of copper sulfide (CuS). In this work, a microsphere (MS) structure composed of CuS was employed for label-free imaging of MCF-7 breast cancer cells and normal Vero cells, respectively. Various label-free imaging techniques, such as bright field, dark field, phase contrast (PC), and differential interference contrast (DIC), were employed to capture images of CuS MSs, cell, and intact CuS MSs within a cell. The study compared the outcomes of each imaging technique and determined that DIC imaging provided the highest resolution for cells incubated with CuS MSs. Furthermore, the combination of PC and DIC techniques proved to be effective for imaging breast cancer cells in conjunction with CuS MSs. This research underscores the potential of CuS MSs for label-free cell detection and emphasizes the significance of selecting appropriate imaging techniques to attain high-quality images in the field of cell observation.

Rational Strategies to Improve the Efficiency of 2D Perovskite Solar Cells

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

A binary 2D perovskite passivation for efficient and stable perovskite/silicon tandem solar cells

To achieve high power conversion efficiency in perovskite/silicon tandem solar cells, it is necessary to develop a promising wide-bandgap perovskite absorber and processing techniques in relevance. To date, the performance of devices based on wide-bandgap perovskite is still limited mainly by carrier recombination at their electron extraction interface. Here, we demonstrate assembling a binary two-dimensional perovskite by both alternating-cation-interlayer phase and Ruddlesden-Popper phase to passivate perovskite/C60 interface. The binary two-dimensional strategy takes effects not only at the interface but also in the bulk, which enables efficient charge transport in a wide-bandgap perovskite solar cell with a stabilized efficiency of 20.79% (1 cm2). Based on this absorber, a monolithic perovskite/silicon tandem solar cell is fabricated with a steady-state efficiency of 30.65% assessed by a third party. Moreover, the tandem devices retain 96% of their initial efficiency after 527 h of operation under full spectral continuous illumination, and 98% after 1000 h of damp-heat testing (85 °C with 85% relative humidity).

Optoelectronic insights of lead-free layered halide perovskites

Two-dimensional organic-inorganic halide perovskites have emerged as promising candidates for a multitude of optoelectronic technologies, owing to their versatile structure and electronic properties. The optical and electronic properties are harmoniously integrated with both the inorganic metal halide octahedral slab, and the organic spacer layer. The inorganic octahedral layers can also assemble into periodically stacked nanoplatelets, which are interconnected by the organic ammonium cation, resulting in the formation of a superlattice or superstructure. In this perspective, we explore the structural, electronic, and optical properties of lead-free hybrid halides, and the layered halide perovskite single crystals and nanostructures, expanding our understanding of the diverse applications enabled by these versatile structures. The optical properties of the layered halide perovskite single crystals and superlattices are a function of the organic spacer layer thickness, the metal center with either divalent or a combination of monovalent and trivalent cations, and the halide composition. The distinct absorption and emission features are guided by the structural deformation, electron-phonon coupling, and the polaronic effect. Among the diverse optoelectronic possibilities, we have focused on the photodetection capability of layered halide perovskite single crystals, and elucidated the descriptors such as excitonic band gap, effective mass, carrier mobility, Rashba splitting, and the spin texture that decides the direct component of the optical transitions.

Manipulation of Crystal Orientation and Phase Distribution of Quasi-2D Perovskite through Synergistic Effect of Additive Doping and Spacer Engineering

The diammonium precursor 1,4-phenylenedimethanammonium (PDMA) was used as a large organic spacer for the preparation of Dion-Jacobson-type quasi-2D perovskites (PDMA)(MA)n-1PbnI3n+1 (MA = methylammonium). Films with composition ⟨n⟩ = 5 comprised randomly orientated grains and multiple microstructural domains with locally differing n values. However, by mixing the Dion-Jacobson-type spacer PDMA and the Ruddlesden-Popper-type spacer propylammonium (PA), the crystal orientation in both the vertical and the horizonal directions became regulated. High crystallinity owing to well-matched interlayer distances was observed. Combining this spacer-engineering approach with the addition of methylammonium chloride (MACl) led to full vertical alignment of the crystal orientation. Moreover, the microstructural domains at the substrate interface changed from low-n (n = 1, 2, 3) to high-n (n = 4, 5), which may be beneficial for hole extraction at the interface between perovskite and hole transport layer due to a more finely tuned band alignment. Our work sheds light on manipulating the crystallization behavior of quasi-2D perovskite and further paves the way for highly stable and efficient perovskite devices.

Halide Segregated Crystallization of Mixed-Halide Perovskites Revealed by In Situ GIWAXS

Mixed-halide perovskites of the composition MAPb(BrxI1-x)3, which seem to exhibit a random and uniform distribution of halide ions in the absence of light, segregate into bromide- and iodide-rich phases under illumination. This phenomenon of halide segregation has been widely investigated in the photovoltaics context since it is detrimental for the material properties and ultimately the device performance of these otherwise very attractive materials. A full understanding of the mechanisms and driving forces has remained elusive. In this work, a study of the crystallization pathways and the mixing behavior during deposition of MAPb(BrxI1-x)3 thin films with varying halide ratios is presented. In situ grazing incidence wide-angle scattering (GIWAXS) reveals the distinct crystallization behavior of mixed-halide perovskite compositions during two different fabrication routes: nitrogen gas-quenching and the lead acetate route. The perovskite phase formation of mixed-halide thin films hints toward a segregation tendency since separate crystallization pathways are observed for iodide- and bromide-rich phases within the mixed compositions. Crystallization of the bromide perovskite phase (MAPbBr3) is already observed during spin coating, while the iodide-based fraction of the composition forms solvent complexes as an intermediate phase, only converting into the perovskite phase upon thermal annealing. These parallel crystallization pathways result in mixed-halide perovskites forming from initially halide-segregated phases only under the influence of heating.

The Synergistic Effect of Additives for Formamidinium-Based Inverted Dion-Jacobson 2D Perovskite Solar Cells with Enhanced Photovoltaic Performance

Two-dimensional (2D) perovskite solar cells (PSCs) have attracted rapid growing attention due to their excellent environmental and operational stability. As an important type of 2D perovskite, Dion-Jacobson (DJ) 2D perovskites exhibit better structural integrity and more stable optoelectronic properties than those of Ruddlesden-Popper (RP) ones because of the elimination of weak van der Waals interactions. Random phase distribution, phase impurity, and weak crystallinity, however, can lead to severe nonradiative recombination losses in 2D perovskites and inferior device stability. Herein, formamidinium chloride (FACl) and lead chloride (PbCl2) are selected as additives to fabricate efficient and stable DJ 2D PSCs. The synergistic effect of additives could efficiently induce crystallization and suppress the low-n phase perovskites. The obtained 2D perovskites exhibit extended charge lifetime and enhanced charge transfer. The corresponding PSC device delivers an efficiency of 16.63% with a significantly improved open-circuit voltage (VOC) of 1.18 V and a fill factor (FF) of 81.65% than the control one. This PCE ranks the highest for inverted FA-based 2D DJ PSCs. Moreover, this device has exhibited exceptional long-term stability, which retains more than 95% of the initial efficiencies at about 50% relative humidity for 600 h.

Nucleation and Crystallization in 2D Ruddlesden-Popper Perovskites using Formamidinium-based Organic Semiconductor Spacers for Efficient Solar Cells

The conjugated organic semiconductor spacers have drawn wide attention in two-dimensional (2D) perovskites and formamidinium (FA) has been widely used as A-site cation in high-performance 3D perovskite solar cells (PSCs). However, the FA-based semiconductor spacers have rarely been investigated in 2D Ruddlesden-Popper (RP) perovskites. Here, we developed two FA-based spacers containing thieno[3,2-b]thiophene (TT) and 2,2'-bithiophene (BT) units, namely TTFA and BTFA, respectively, for 2D RP PSCs. The nucleation and crystallization kinetics of TTFA-Pb and BTFA-Pb from sol-gel to film were investigated using in situ optical microscopy and in situ grazing incidence wide-angle X-ray scattering (GIWAXS) measurements. It is found that the TTFA spacer could reduce the energy barrier of nucleation and induces crystal vertical orientation of 2D perovskite by forming larger clusters in precursor solution, resulting in much improved film quality. Benefiting from the enlarged crystal grains, reduced exciton binding energy, and decreased electron-phonon coupling coefficient, the photovoltaic device based on (TTFA)2 MAn-1 Pbn I3n+1 (n=5) achieved a champion efficiency of 19.41 %, which is a record for 2D RP PSCs with FA-based spacers. Our work provides deep understanding of the nucleation and crystallization process of 2D RP perovskite films and highlights the great potential of FA-based semiconductor spacers in highly efficient 2D PSCs.

Controllable Black-to-Yellow Phase Transition by Tuning the Lattice Symmetry in Perovskite Quantum Dots

The black-to-yellow phase transition in perovskite quantum dots (QDs) is more complex than in bulk perovskites, regarding the role of surface energy. Here, with the assistance of in situ grazing-incidence wide-angle and small-angle X-ray scattering (GIWAXS/GISAXS), distinct phase behaviors of cesium lead iodide (CsPbI3 ) QD films under two different temperature profiles-instant heating-up (IHU) and slow heating-up (SHU) is investigated. The IHU process can cause the phase transition from black phase to yellow phase, while under the SHU process, the majority remains in black phase. Detailed studies and structural refinement analysis reveal that the phase transition is triggered by the removal of surface ligands, which switches the energy landscape. The lattice symmetry determines the transition rate and the coexistence black-to-yellow phase ratio. The SHU process allows longer relaxation time for a more ordered QD packing, which helps sustain the lattice symmetry and stabilizes the black phase. Therefore, one can use the lattice symmetry as a general index to monitor the CsPbI3 QD phase transition and finetune the coexistence black-to-yellow phase ratio for niche applications.

In Situ Self-Elimination of Defects via Controlled Perovskite Crystallization Dynamics for High-Performance Solar Cells

Understanding and controlling crystallization is crucial for high-quality perovskite films and efficient solar cells. Herein, the issue of defects in formamidinium lead iodide (FAPbI3 ) formation is addressed, focusing on the role of intermediates. A comprehensive picture of structural and carrier evolution during crystallization is demonstrated using in situ grazing-incidence wide-angle X-ray scattering, ultraviolet-visible spectroscopy and photoluminescence spectroscopy. Three crystallization stages are identified: precursors to the δ-FAPbI3 intermediate, then to α-FAPbI3 , where defects spontaneously emerge. A hydrogen-sulfate-based ionic liquid additive is found to enable the phase-conversion pathway of precursors → solvated intermediates → α-FAPbI3 , during which the spontaneous generation of δ-FAPbI3 can be effectively circumvented. This additive extends the initial growth kinetics and facilitates solvent-FA+ ion exchange, which results in the self-elimination of defects during crystallization. Therefore, the improved crystallization dynamics lead to larger grain sizes and fewer defects within thin films. Ultimately, the improved perovskite crystallization dynamics enable high-performance solar cells, achieving impressive efficiencies of 25.14% at 300 K and 26.12% at 240 K. This breakthrough might open up a new era of application for the emerging perovskite photovoltaic technology to low-temperature environments such as near-space and polar regions.

Two-Second-Annealed 2D/3D Perovskite Films with Graded Energy Funnels and Toughened Heterointerfaces for Efficient and Durable Solar Cells

The 2D/3D perovskite heterostructures have been widely investigated to enhance the efficiency and stability of perovskite solar cells (PSCs). However, rational manipulation of phase distribution and energy level alignment in such 2D/3D perovskite hybrids are still of great challenge. Herein, we successfully achieved spontaneous phase alignment of 2D/3D perovskite heterostructures by concurrently introducing both 2D perovskite component and organic halide additive. The graded phase distribution of 2D perovskites with different n values and 3D perovskites induced favorable energy band alignment across the perovskite film and boosted the charge transfer at the relevant heterointerfaces. Moreover, the 2D perovskite component also acted as a "band-aid" to simultaneously passivate the defects and release the residual tensile stress of perovskite films. Encouragingly, the blade-coated PSCs based on only ≈2 s in-situ fast annealed 2D/3D perovskite films with favorable energy funnels and toughened heterointerfaces achieved promising efficiencies of 22.5 %, accompanied by extended lifespan. To our knowledge, this is the highest reported efficiency for the PSCs fabricated with energy-saved thermal treatment just within a few seconds, which also outperformed those state-of-the-art annealing-free analogues. Such a two-second-in-situ-annealing technique could save the energy cost by up to 99.6 % during device fabrication, which will grant its low-coast implementation.

Recent Progress of Layered Perovskite Solar Cells Incorporating Aromatic Spacers

Layered two dimensional (2D) or quasi-2D perovskites are emerging photovoltaic materials due to their superior environment and structure stability in comparison with their 3D counterparts. The typical 2D perovskites can be obtained by cutting 3D perovskites along  < 100 >  orientation by incorporation of bulky organic spacers, which play a key role in the performance of 2D perovskite solar cells (PSCs). Compared with aliphatic spacers, aromatic spacers with high dielectric constant have the potential to decrease the dielectric and quantum confinement effect of 2D perovskites, promote efficient charge transport and reduce the exciton binding energy, all of which are beneficial for the photovoltaic performance of 2D PSCs. In this review, we aim to provide useful guidelines for the design of aromatic spacers for 2D perovskites. We systematically reviewed the recent progress of aromatic spacers used in 2D PSCs. Finally, we propose the possible design strategies for aromatic spacers that may lead to more efficient and stable 2D PSCs.

Additive Engineering for Stable and Efficient Dion-Jacobson Phase Perovskite Solar Cells

Because of their better chemical stability and fascinating anisotropic characteristics, Dion-Jacobson (DJ)-layered halide perovskites, which owe crystallographic two-dimensional structures, have fascinated growing attention for solar devices. DJ-layered halide perovskites have special structural and photoelectronic features that allow the van der Waals gap to be eliminated or reduced. DJ-layered halide perovskites have improved photophysical characteristics, resulting in improved photovoltaic performance. Nevertheless, owing to the nature of the solution procedure and the fast crystal development of DJ perovskite thin layers, the precursor compositions and processing circumstances can cause a variety of defects to occur. The application of additives can impact DJ perovskite crystallization and film generation, trap passivation in the bulk and/or at the surface, interface structure, and energetic tuning. This study discusses recent developments in additive engineering for DJ multilayer halide perovskite film production. Several additive-assisted bulk and interface optimization methodologies are summarized. Lastly, an overview of research developments in additive engineering in the production of DJ-layered halide perovskite solar cells is offered.

Highly efficient and stable quasi two-dimensional perovskite solar cells via synergistic effect of dual additives

Recently, quasi two-dimensional (Q-2D) perovskites with alternating cations in the interlayer space (ACI) have attracted more attentions owing to their elevated stability compared with three-dimensional (3D) analogs. While the efficiency of the devices derived from Q-2D perovskites is much smaller than that based on 3D perovskites. Here, we utilized urea and methoxyamine hydrochloride (MOAH) dual additives to acquire high quality Q-2D ACI perovskite GA(MA)₅Pb₅I₁₆ (GA = guanidinium, MA = methylammonium) films. The efficiency of the perovskite solar cells (PSCs) derived from the Q-2D perovskite films induced by the synergistic effect of urea and MOAH dual additives increases to 20.32% from 17.21% for the devices without additive. This efficiency enhancement could be attributed to the enlarged grain size, improved crystallinity, optimized quantum well thickness distribution, and reduced trap states of the perovskite films. Moreover, the solar cells with dual additives present improved stability. The efficiency of devices with dual additives holds 95% of the original value after storage for 1600 h in ambient air. These results prove that the synergistic effect of urea and MOAH is an effective method to achieve highly efficient and stable Q-2D PSCs.

Recent progress in layered metal halide perovskites for solar cells, photodetectors, and field-effect transistors

Metal halide perovskite materials demonstrate immense potential for photovoltaic and electronic applications. In particular, two-dimensional (2D) layered metal halide perovskites have advantages over their 3D counterparts in optoelectronic applications due to their outstanding stability, structural flexibility with a tunable bandgap, and electronic confinement effect. This review article first analyzes the crystallography of different 2D perovskite phases [the Ruddlesden-Popper (RP) phase, the Dion-Jacobson (DJ) phase, and the alternating cations in the interlayer space (ACI) phase] at the molecular level and compares their common electronic properties, such as out-of-plane conductivity, crucial to vertical devices. This paper then critically reviews the recent development of optoelectronic devices, namely solar cells, photodetectors and field effect transistors, based on layered 2D perovskite materials and points out their limitations and potential compared to their 3D counterparts. It also identifies the important application-specific future research directions for different optoelectronic devices providing a comprehensive view guiding new research directions in this field.

Synergistic Surface Modification of Tin-Lead Perovskite Solar Cells

Interfaces in thin-film photovoltaics play a pivotal role in determining device efficiency and longevity. In this work, the top surface treatment of mixed tin-lead (≈1.26 eV) halide perovskite films for p-i-n solar cells is studied. Charge extraction is promoted by treating the perovskite surface with piperazine. This compound reacts with the organic cations at the perovskite surface, modifying the surface structure and tuning the interfacial energy level alignment. In addition, the combined treatment with C₆₀ pyrrolidine tris-acid (CPTA) reduces hysteresis and leads to efficiencies up to 22.7%, with open-circuit voltage values reaching 0.90 V, ≈92% of the radiative limit for the bandgap of this material. The modified cells also show superior stability, with unencapsulated cells retaining 96% of their initial efficiency after >2000 h of storage in N₂ and encapsulated cells retaining 90% efficiency after >450 h of storage in air. Intriguingly, CPTA preferentially binds to Sn²⁺ sites at film surface over Pb²⁺ due to the energetically favored exposure of the former, according to first-principles calculations. This work provides new insights into the surface chemistry of perovskite films in terms of their structural, electronic, and defect characteristics and this knowledge is used to fabricate state-of-the-art solar cells.

Manipulating the Formation of 2D/3D Heterostructure in Stable High-Performance Printable CsPbI3 Perovskite Solar Cells

Manipulating the formation process of the 2D/3D perovskite heterostructure, including its nucleation/growth dynamics and phase transition pathway, plays a critical role in controlling the charge transport between 2D and 3D crystals, and consequently, the scalable fabrication of efficient and stable perovskite solar cells. Herein, the structural evolution and phase transition pathways of the ligand-dependent 2D perovskite atop the 3D surface are revealed using time-resolved X-ray scattering. The results show that the ligand size and shape have a critical influence on the final 2D structure. In particular, ligands with smaller sizes and more reactive sites tend to form the n = 1 phase. Increasing the ligand size and decreasing the reactive sites promote the transformation from 3D to n = 3 and n < 3 phases. These findings are useful for the rational design of the phase distribution in 2D perovskites to balance the charge transport and stability of the perovskite films. Finally, solar cells based on ambient-printed CsPbI₃ with n-butylammonium iodide treatment achieve an improved efficiency of 20.33%, which is the highest reported value for printed inorganic perovskite solar cells.

Balanced change in crystal unit cell volume and strain leads to stable halide perovskite with high guanidinium content

Up-to-date studies propose that strain in halide perovskites is one of the key factors that determine a device's efficiency and stability. Here, we show a systematic approach to characterize the phenomenon in the standard methylammonium lead iodine (MAPbI3) perovskite system by: (i) the substitution of some MA by guanidinium (Gu); (ii) the incorporation of PbS quantum dot (QD) additives and (iii) addition of both Gu and PbS at the same time. We studied the effect of these incorporations on the film strain and crystal cell unit volume, and on the solar cell device efficiency and stability. Gu cations and PbS QDs affect the strain, the former due to the relatively large dimensions of Gu, and the latter due to the lattice matching parameters. With the control of Gu and PbS QD content, higher performance and longer solar cell stability are obtained. We demonstrated that the presence of Gu and PbS QDs alters the structure of perovskite, in terms of modification of the unit cell volume and strain. The greater size of Gu cations produces a MAPbI3 unit cell volume expansion as Gu is incorporated, modifying the strain from compressive to tensile. PbS QDs aid Gu incorporation, producing a unit cell volume expansion. In the case of 15% mol Gu incorporation, the addition of PbS QDs modifies strain from compressive to tensile, limiting the deleterious effect. At the same time the unit cell volume is less affected, increasing the solar cell stability. Our work shows that the control of compressive strain and the unit cell volume expansion lead to a 50% increase in T 80, the time in which the PCE decreases to 80% of its original value, increasing the T 80 value from 120 to 187 days under air conditions. Moreover it highlights the importance of exploiting not only the control of the strain induced by internal component, the cation, but also the strain induced by the external component, the QD, associated instead with critical volume variation of metastable perovskite unit cell volume.

Roles of Organic Ligands in Ambient Stability of Layered Halide Perovskites

The combination of organic ligands and inorganic Pb-I frameworks in layered perovskites has bestowed upon them high structural tunability and stability, while their microscopic degradation mechanism remains unclear. Here, we found the key role of ligands in intrinsic structural stability and the consequent morphological evolution in layered perovskites during long-term ambient aging based on (GA)(MA)nPbnI3n+1 (GA = guanidinium, = 4) and (BDA)(MA)n-1PbnI3n+1 (BDA = 1,4-butanediammonium, < n > = 4) perovskites. The BDA-based perovskites have a low intrinsic stability due to high crystal formation energy (ΔH), which are prone to hydration during ambient aging. We overserved changed crystal orientation from perpendicular to parallel, a delayed charge populating time from <1 ps to >50 ps, an inhibited carrier transfer kinetics between quantum wells, an increase of 0.9 μs of charge carrier transport time and a decrease of 1.2 μs of charge carrier lifetime in the BDA-based film during ambient aging, which accounts for a large power-conversion efficiency (PCE) loss (14.2% vs 11.2%). By contrast, the GA ligand increases the intrinsic structural stability of perovskites, which not only yields an initial PCE as high as 20.0% but also helps retain excellent optoelectronic properties during aging. Therefore, only a slight PCE loss (20.0% vs 19.1%) was observed. Our work reveals the key role of organic-inorganic interaction affecting the intrinsic structural stability and optoelectronic properties, and provides a theoretical basis for the future design of stable and efficient optoelectronic devices.

Ordered Element Distributed C3 N Quantum Dots Manipulated Crystallization Kinetics for 2D CsPbI3 Solar Cells with Ultra-High Performance

Two-dimensional (2D) CsPbI₃ is developed to conquer the phase-stability problem of CsPbI₃ by introducing bulky organic cations to produce a steric hindrance effect. However, organic cations also inevitably increase the formation energy and difficulty in crystallization kinetics regulation. Such poor crystallization process modulation of 2D CsPbI₃ leads to disordered phase-arrangement, which impedes the transport of photo-generated carriers and worsens device performance. Herein, a type of C₃ N quantum dots (QDs) with ordered carbon and nitrogen atoms to manipulate the crystallization process of 2D CsPbI₃ for improving the crystallization pathway, phase-arrangement and morphology, is introduced. Combination analyses of theoretical simulation, morphology regulation and femtosecond transient absorption (fs-TA) characterization, show that the C₃ N QDs induce the formation of electron-rich regions to adsorb bulky organic cations and provide nucleation sites to realize a bi-directional crystallization process. Meanwhile, the quality of 2D CsPbI₃ film is improved with lower trap density, higher surface potential, and compact morphology. As a result, the power conversion efficiency (PCE) of the optimized device (n = 5) boosts to an ultra-high value of 15.63% with strengthened environmental stability. Moreover, the simple C₃ N QDs insertion method shows good universality to other bulky organic cations of Ruddlesden-Popper and Dion-Jacobson, providing a good modulation strategy for other optoelectronic devices.

High efficiency and large optical anisotropy in the high-order nonlinear processes of 2D perovskite nanosheets

Nonlinear nanophotonic devices have brought about great advances in the fields of nano-optics, quantum science, biomedical engineering, etc. However, in order to push these nanophotonic devices out of laboratory, it is still highly necessary to improve their efficiency. Since obtaining novel nanomaterials with large nonlinearity is of crucial importance for improving the efficiency of nonlinear nanodevices, we propose the two-dimensional (2D) perovskites. Different from most previous studies which focused on the 2D perovskites in large scale (such as the bulk materials or the thick flakes), herein we studied the 2D perovskites nanosheets with thickness of ∼50 nm. The high-order nonlinear processes including multi-photon photoluminescence and third-harmonic generation (THG) have been systematically investigated, and it is found the THG process can have a high conversion efficiency up to ∼8 × 10-6. Also, it is observed that the nonlinear responses of 2D perovskites have large optical anisotropy, i.e., the polarization ratio for the incident polarization dependence of nonlinear response can be as high as ∼0.99, which is an impressive record in the perovskite systems. Our findings reveal the properties of high efficiency and huge optical anisotropy in the nonlinear processes of 2D perovskite nanosheets, shedding light on the design of advanced integrated nonlinear nanodevices in future.

Dual Functions of Performance Improvement and Lead Leakage Mitigation of Perovskite Solar Cells Enabled by Phenylbenzimidazole Sulfonic Acid

With the continuous improvement of performance of lead-based perovskite solar cells (PSCs), the potential harm of water-soluble lead ion (Pb²⁺ ) to environment and public health is emerging as a major obstacle to their commercialization. Herein, an amphoteric phenylbenzimidazole sulfonic acid (PBSA) that is almost insoluble in water is added to the perovskite precursor to simultaneously regulate crystallization growth, passivate defects, and mitigate lead leakage of high-performance PSCs. Through systematic research, it is found that PBSA can not only regulate the crystallization of perovskite grains to form the film, but also passivate the defects of annealed films mainly due to the strong interaction between the functional groups in PBSA and Pb²⁺ , which greatly improves the crystallinity and stability of perovskite films. Consequently, the highest power conversion efficiency of 23.27% is achieved in 0.09 cm² devices and 15.31% is obtained for large-area modules with an aperture area of 19.32 cm² , along with negligible hysteresis and improved stability. Moreover, the leakage of lead ions from unpackaged devices is effectively prevented owing to the strong coupling between PBSA molecules and water-soluble Pb²⁺ to form insoluble complexes in water, which is of great significance to promote the application of optoelectronic devices based on lead-based perovskite materials.

Zwitterion-Assisted Crystal Growth of 2D Perovskites with Unfavorable Phase Suppression for High-Performance Solar Cells

Despite two-dimensional (2D) Ruddlesden-Popper-phase layered perovskites (RPLPs) exhibiting excellent environmental stability, most solar cells based on 2D RPLP films are fabricated in a controlled inert atmosphere. Meanwhile, the poor charge transport of 2D RPLP films owing to the unfavorable phase arrangement and defects limits the efficiency of 2D RPLP solar cells. Here, we fabricate high-efficiency 2D RPLP solar cells in ambient air assisted by a zwitterion (ZW) additive. We show that the ZW additive suppresses the formation of the bottom 2D phases (n ≤ 2) and the top 3D-like phases in 2D RPLP films. These 2D phases usually grow parallel to the substrate and act as trap sites that inhibit charge transport in the vertical direction. The 3D-like phases, on the other hand, aggravate the long-term stability due to the intrinsic instability of MA⁺ cations. With improved phase distribution, crystal orientation, and reduced trap states in 2D RPLP films, efficient charge transport is obtained. Finally, a record-high open-circuit voltage (V_oc) of 1.19 V and a power conversion efficiency of 17.04% with an enhanced stability are achieved for (BA_0.9PEA_0.1)₂MA₃Pb₄I₁₃-based (n = 4) solar cells fabricated under high humidity (∼65% RH).

Interfacial Modification via a 1,4-Butanediamine-Based 2D Capping Layer for Perovskite Solar Cells with Enhanced Stability and Efficiency

Organic-inorganic perovskites face the issues of being vulnerable to oxygen and moisture and the trap sites located at the surface and grain boundaries. Integration of two-dimensional (2D) perovskites as a capping layer is an effective route to enhance both photovoltaic efficiency and environmental stability of the three-dimensional (3D) underlayer. Here, we employ 1,4-butanediammonium diiodide (BDADI), which has a short chain length and diammonium cations, to construct a 3D/2D stacking perovskite solar cells (PSCs). The introduction of BDA2+ could passivate surface defects in 3D perovskites by forming 2D Dion-Jacobson (DJ) phase perovskites and effectively suppressing nonradiative recombination, thus resulting in a longer carrier lifetime. The DJ 2D capping layer also regulate the energy level arrangement, enabling a better charge extraction and transport process. In addition, the water-resistance ability of 3D perovskite gets improved because of the hydrophobic characteristic of 1,4-butanediammonium cations. Consequently, the 3D/2D stacking PSCs yield an energy conversion efficiency of 20.32% in company with the enhanced long-term stability.

Growth and Degradation Kinetics of Organic-Inorganic Hybrid Perovskite Films Determined by In Situ Grazing-Incidence X-Ray Scattering Techniques

Organic-inorganic halide perovskite (OIHP) solar cells hold a great promise for commercial breakthrough since their power conversion efficiency has been pushed beyond the mark of 25%, making them capable of competing with traditional crystalline silicon solar cells. The key to achieve efficient and stable perovskite solar cells is inherently related to the film morphology. The understanding of the kinetic processes of film formation and degradation opens up possibilities to tailor the film morphology via the regulation of precursor and processing parameters. In situ grazing-incidence X-ray scattering (GIXS) techniques allow for tracking the morphology evolution of thin films at different length scales and with high temporal resolution. In this review, the selected examples for application of in situ grazing-incidence wide-angle X-ray scattering and grazing-incidence small-angle X-ray scattering techniques to the growth and stability of OIHPs are summarized after a brief introduction to both techniques, highlighting particularly the morphological evolution of perovskite films over time. Then the correlated mathematical models are reviewed to give a toolbox for analyzing the mechanisms of film formation and degradation. Thus, an overview on the in situ GIXS methods is linked to the research of OIHP kinetics.

A Systematic Review of Metal Halide Perovskite Crystallization and Film Formation Mechanism Unveiled by In Situ GIWAXS

Metal halide perovskites are of fundamental interest in the research of modern thin-film optoelectronic devices, owing to their widely tunable optoelectronic properties and solution processability. To obtain high-quality perovskite films and ultimately high-performance perovskite devices, it is crucial to understand the film formation mechanisms, which, however, remains a great challenge, due to the complexity of perovskite composition, dimensionality, and processing conditions. Nevertheless, the state-of-the-art in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) technique enables one to bridge the complex information with device performance by revealing the crystallization pathways during the perovskite film formation process. In this review, the authors illustrate how to obtain and understand in situ GIWAXS data, summarize and assess recent results of in situ GIWAXS studies on versatile perovskite photovoltaic systems, aiming at elucidating the distinct features and common ground of film formation mechanisms, and shedding light on future opportunities of employing in situ GIWAXS to study the fundamental working mechanisms of highly efficient and stable perovskite solar cells toward mass production.

Layered 2D Halide Perovskites beyond Ruddlesden-Popper Phase: Tailored Interlayer Chemistries for High-Performance Solar Cells

Layered halide perovskites (LHPs) with crystallographically 2D structures have gained increasing interest for photovoltaic applications due to their superior chemical stability and intriguing anisotropic properties, as compared to their conventional 3D perovskite counterparts. The mostly studied LHPs are Ruddlesden-Popper (RP) phases, which suffer from a carrier-transport bottleneck due to the van der Waals gap associated with their intrinsic organic interlayer structures. To address this issue, Dion-Jacobson (DJ) and alternating-cation-interlayer (ACI) LHPs have rapidly emerged, which exhibit unique structural and (opto)electronic characteristics that may resemble the 3D counterparts owing to the eliminated or reduced van der Waals gap. Improved photophyiscal properties have been achieved in DJ and ACI LHPs, leading towards better photovoltaic performance. Here we provide a comprehensive discussion on the merits and promise of DJ and ACI LHPs from a chemistry perspective. Then, we review recent progress on synthesis and tailoring of DJ and ACI LHP crystals and thin films, as well as their optoelectronic properties and photovoltaic performance. Finally, we discuss future directions to realize the full potential of DJ and ACI LHPs for high-performance solar cells and beyond.

2D Hybrid Halide Perovskites: Structure, Properties, and Applications in Solar Cells

2D metal-halide perovskites have attracted intense research interest due to superior long-term stability under ambient environments. Compared to their 3D analog, the alternate arrangement of organic and inorganic layers leads to forming a multilayer quantum well (MQW), which endows 2D perovskites with anisotropic optoelectronic properties. In addition, the spacer layer functions as a hydrophobic barrier to effectively prevent 2D perovskite films from ion migration and moisture penetrating, thus realizing outstanding stability. Recently, 2D perovskites have been widely developed with abundant species. The stunning photovoltaic performance with the coexistence of long-term stability and high-power conversion efficiency (PCE) has been realized in 2D perovskite solar cells (PSCs), which paves an avenue for commercialization of PSCs. This review begins with an introduction of crystal structure and crystallization kinetics to illustrate the unique layer characters in 2D perovskites. Then, electron structure, excitons, dielectric confinement, and intrinsic stability properties are discussed in detail. Next, the photovoltaic performance based on recent Ruddlesden-Popper (RP), Dion-Jacobson (DJ), and alternating cations in the interlayer (ACI) phase 2D-PSCs is comprehensively summarized. Finally, the confronting challenges and strategies toward structural design and optoelectronic studies of 2D perovskites are proposed to offer insight into the advanced underlying properties of this family of materials.

Stability of Perovskite Solar Cells: Degradation Mechanisms and Remedies

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

Deep-Level Transient Spectroscopy for Effective Passivator Selection in Perovskite Solar Cells to Attain High Efficiency over 23

Most studies choose passivators essentially in a trial-and-error fashion in an attempt to attain high efficiency in perovskite solar cells (PSCs). Using deep-level transient spectroscopy (DLTS) measurements, the type of defects in perovskite films was determined to guide the passivator selection for PSCs. Three kinds of positively charged defects were found in the target PSC system. Fluorinated phenylethylamine hydroiodide (FPEAI) was chosen to passivate the surface defects due to the electronegativity and hydrophobicity of fluorine. Due to the decreased surface roughness, increased hydrophobicity, lowered defect density, and improved carrier dynamics as observed by ultrafast transient absorption spectroscopy (TAS), a PSC with meta-F-PEAI had the best efficiency over 23 % with open-circuit voltage of 1.155 V and fill factor of 80.15 %. In addition, the long-term stability of the PSC was significantly improved. The present work provides a new means to select the best passivator for different types of defects.

Two-dimensional halide perovskites: synthesis, optoelectronic properties, stability, and applications

Halide perovskites are promising materials for light-emitting and light-harvesting applications. In this context, two-dimensional perovskites such as nanoplatelets or Ruddlesden-Popper and Dion-Jacobson layered structures are important because of their structural flexibility, electronic confinement, and better stability. This review article brings forth an extensive overview of the recent developments of two-dimensional halide perovskites both in the colloidal and non-colloidal forms. We outline the strategy to synthesize and control the shape and discuss different crystalline phases and optoelectronic properties. We review the applications of two-dimensional perovskites in solar cells, light-emitting diodes, lasers, photodetectors, and photocatalysis. Besides, we also emphasize the moisture, thermal, and photostability of these materials in comparison to their three-dimensional analogs.

Spontaneous Formation of Upper Gradient 2D Structure for Efficient and Stable Quasi-2D Perovskites

Highly efficient and stable quasi-2D hybrid perovskite solar cells (PSCs) using hydrophobic 4-(trifluoromethyl) benzylamine (4TFBZA) as the spacer cation are successfully demonstrated. It is found that the incorporation of hydrophobic 4TFBZA into MAPbI₃ can effectively induce a spontaneous upper gradient 2D (SUG-2D) structure, passivate the trap states, and restrain the ion motion. Meanwhile, the strong hydrogen bonding of F···HN between 4TFBZA ions and methylamine ions can effectively suppress the decomposition of perovskite, which gives the device a better thermal stability. Besides, due to the SUG-2D structure with hydrophobic 4TFBZA, the device also exhibits a better moisture stability. The SUG-2D-structure-based device exhibits a power conversion efficiency of 17.07% with a high open-circuit voltage of 1.10 V and a notable fill factor of 71%. This work provides a new strategy for constructing efficient and stable quasi-2D PSCs, and it is an inspiration for the packaging strategy of perovskites.

Sulfonate-Assisted Surface Iodide Management for High-Performance Perovskite Solar Cells and Modules

Owing to the ionic nature of lead halide perovskites, their halide-terminated surface is unstable under light-, thermal-, moisture-, or electric-field-driven stresses, resulting in the formation of unfavorable surface defects. As a result, nonradiative recombination generally occurs on perovskite films and deteriorates the efficiency, stability, and hysteresis performances of perovskite solar cells (PSCs). Here, a surface iodide management strategy was developed through the use of cesium sulfonate to stabilize the perovskite surface. It was found that the pristine surface of common perovskite was terminated with extra iodide, that is, with an I^-/Pb²⁺ ratio larger than 3, explaining the origination of surface-related problems. Through post-treatment of perovskite films by cesium sulfonate, the extra iodide on the surface was facilely removed and the as-exposed Pb²⁺ cations were chelated with sulfonate anions while maintaining the original 3D perovskite structure. Such iodide replacement and lead chelating coordination on perovskite could reduce the commonly existing surface defects and nonradiative recombination, enabling assembled PSCs with an efficiency of 22.06% in 0.12 cm² cells and 18.1% in 36 cm² modules with high stability.

Effective Phase-Alignment for 2D Halide Perovskites Incorporating Symmetric Diammonium Ion for Photovoltaics

New structural type of 2D AA'n -1 Mn X3 n +1 type halide perovskites stabilized by symmetric diammonium cations has attracted research attention recently due to the short interlayer distance and better charge-transport for high-performance solar cells (PSCs). However, the distribution control of quantum wells (QWs) and its influence on optoelectronic properties are largely underexplored. Here effective phase-alignment is reported through dynamical control of film formation to improve charge transfer between quantum wells (QWs) for 2D perovskite (BDA)(MA)n -1 Pbn I3 n +1 (BDA = 1,4-butanediamine, 〈n〉 = 4) film. The in situ optical spectra reveal a significantly prolonged crystallization window during the perovskite deposition via additive strategy. It is found that finer thickness gradient by n values in the direction orthogonal to the substrate leads to more efficient charge transport between QWs and suppressed charge recombination in the additive-treated film. As a result, a power conversion efficiency of 14.4% is achieved, which is not only 21% higher than the control one without additive treatment, but also one of the high efficiencies of the low-n (n ≤ 4) AA'n -1 Mn X3 n +1 PSCs. Furthermore, the bare device retains 92% of its initial PCE without any encapsulation after ambient exposure for 1200 h.

Insights into Microscopic Crystal Growth Dynamics of CH3NH3PbI3 under a Laser Deposition Process Revealed by In Situ X-ray Diffraction

The process dynamics for the vacuum deposition of methylammonium lead iodide (MAPbI₃) perovskite was analyzed by in situ X-ray diffraction using synchrotron radiation. MAPbI₃ was fabricated by alternatingly supplying PbI₂ and methylammonium iodide via a laser deposition system installed at the synchrotron beamline BL46XU at SPring-8, and in situ crystallization analysis was conducted. Microscopic insights into the crystallization were obtained, including observation of Laue oscillation during the PbI₂ growth and octahedral unit (PbI₆) rotation during the transformation into perovskite. On the basis of this analysis, conditions that favor the construction of atomically flat MAPbI₃ perovskite films were deduced.

In Situ Surface Fluorination of TiO2 Nanocrystals Reinforces Interface Binding of Perovskite Layer for Highly Efficient Solar Cells with Dramatically Enhanced Ultraviolet-Light Stability

Low-temperature solution-processed TiO2 nanocrystals (LT-TiO2) have been extensively applied as electron transport layer (ETL) of perovskite solar cells (PSCs). However, the low electron mobility, high density of electronic trap states, and considerable photocatalytic activity of TiO2 result in undesirable charge recombination at the ETL/perovskite interface and notorious instability of PSCs under ultraviolet (UV) light. Herein, LT-TiO2 nanocrystals are in situ fluorinated via a simple nonhydrolytic method, affording formation of Ti─F bonds, and consequently increase electron mobility, decrease density of electronic trap states, and inhibit photocatalytic activity. Upon applying fluorinated TiO2 nanocrystals (F-TiO2) as ETL, regular-structure planar heterojunction PSC (PHJ-PSC) achieves a champion power conversion efficiency (PCE) of 22.68%, which is among the highest PCEs for PHJ-PSCs based on LT-TiO2 ETLs. Flexible PHJ-PSC devices based on F-TiO2 ETL exhibit the best PCE of 18.26%, which is the highest value for TiO2-based flexible devices. The bonded F atoms on the surface of TiO2 promote the formation of Pb─F bonds and hydrogen bonds between F- and FA/MA organic cations, reinforcing interface binding of perovskite layer with TiO2 ETL. This contributes to effective passivation of the surface trap states of perovskite film, resulting in enhancements of device efficiency and stability especially under UV light.

Visualization of halide perovskite crystal growth processes by in situ heating WAXS measurements

We observed the crystallization dynamics of halide perovskite crystals (CH3NH3PbI3) by in situ heating wide-angle X-ray scattering measurements. As a result, we revealed that crystal growth occurs during the conversion of complexes to perovskite crystals.

From 1D to 3D: Perovskites within the System HSC(NH2)2I/CH3NH3I/PbI2 with Maintenance of the Cubic Closest Packing

This work describes crystalline phases of the system [HSC(NH₂)₂]I/(CH₃NH₃)I/PbI₂ and discusses the crystal structures in the context of a common cubic closest packing of organic cations and iodide anions with Pb²⁺ in all anionic octahedral voids. Ternary boundary phases were (CH₃NH₃)PbI₃ (3D perovskite), [HSC(NH₂)₂]₃PbI₅ (1D perovskite), [HSC(NH₂)₂]PbI₃ (NH₄CdCl₃ type), and [HSC(NH₂)₂]Pb₂I₅, with strands of edge-sharing octahedra of the NH₄CdCl₃ type, which are connected to 2D layers via common corners. Within the system, we identified ribbonlike structures of the general composition [HSC(NH₂)₂]_m+1(CH₃NH₃)_mPb_mI_4m+1 with m = 2 and 3, representing the transition from 1D to 2D structures. Layered structures with variable thickness were found for the series [HSC(NH₂)₂](CH₃NH₃)_nPb_nI_3n+1 with n = 1-3. The color and band gap correlate directly with the pattern of how the PbI₆ octahedra are linked. 1D structures are colorless or pale yellow to orange. Layered structures are red to black, depending on the layer thickness. A first laboratory-scale solar cell yielded an efficiency of ∼6% based on the compound with n = 3.