Hybrid halide perovskite solar cell precursors: colloidal chemistry and coordination engineering behind device processing for high efficiency

Crystal Growth and Dissolution of Methylammonium Lead Iodide Perovskite in Sequential Deposition: Correlation between Morphology Evolution and Photovoltaic Performance

Crystal morphology and structure are important for improving the organic-inorganic lead halide perovskite semiconductor property in optoelectronic, electronic, and photovoltaic devices. In particular, crystal growth and dissolution are two major phenomena in determining the morphology of methylammonium lead iodide perovskite in the sequential deposition method for fabricating a perovskite solar cell. In this report, the effect of immersion time in the second step, i.e., methlyammonium iodide immersion in the morphological, structural, optical, and photovoltaic evolution, is extensively investigated. Supported by experimental evidence, a five-staged, time-dependent evolution of the morphology of methylammonium lead iodide perovskite crystals is established and is well connected to the photovoltaic performance. This result is beneficial for engineering optimal time for methylammonium iodide immersion and converging the solar cell performance in the sequential deposition route. Meanwhile, our result suggests that large, well-faceted methylammonium lead iodide perovskite single crystal may be incubated by solution process. This offers a low cost route for synthesizing perovskite single crystal.

Structure-Property Relations of Methylamine Vapor Treated Hybrid Perovskite CH3NH3PbI3 Films and Solar Cells

The power conversion efficiency of halide perovskite solar cells is heavily dependent on the perovskite layer being sufficiently smooth and pinhole-free. It has been shown that these features can be obtained even when starting out from rough and discontinuous perovskite film by briefly exposing the film to methylamine (MA) vapor. The exact underlying physical mechanisms of this phenomenon are, however, still unclear. By investigating smooth, MA treated films based on very rough and discontinuous reference films of methylammonium triiode (MAPbI₃) and considering their morphology, crystalline features, local conductive properties, and charge carrier lifetime, we unraveled the relation between their characteristic physical qualities and their performance in corresponding solar cells. We discovered that the extensive improvement in photovoltaic performance upon MA treatment is a consequence of the induced morphological enhancement of the perovskite layer together with improved electron injection into TiO₂, which in fact compensates for an otherwise compromised bulk electronic quality simultaneously caused by the MA treatment.

Assembly of donor–acceptor hybrid heterostructures based on iodoplumbates and viologen coordination polymers

Two D–A hybrid heterostructures have been formed by the insertion of electron-rich iodoplumbates into viologen coordination polymers, which exhibit highly effcient photocatalytic degradation RhB under visible light irradiation.

Blended additive manipulated morphology and crystallinity transformation toward high performance perovskite solar cells

The morphology and crystallinity of MAPbI₃ thin films were regulated using blended-additive engineering for high performance perovskite solar cells.

Hybrid Perovskite Thin-Film Photovoltaics: In Situ Diagnostics and Importance of the Precursor Solvate Phases

Solution-processed hybrid perovskite semiconductors attract a great deal of attention, but little is known about their formation process. The one-step spin-coating process of perovskites is investigated in situ, revealing that thin-film formation is mediated by solid-state precursor solvates and their nature. The stability of these intermediate phases directly impacts the quality and reproducibility of thermally converted perovskite films and their photovoltaic performance.

Enhanced performance of perovskite solar cells by modulating the Lewis acid-base reaction

The Lewis acid-base reaction between PbI₂ and solvent molecules is popular in fabricating PbI₂ films by a two-step method for making perovskite solar cells. Here, we control the microstructure of PbI₂ films through modulating the Lewis acid-base reaction by adding a small amount of N-methyl pyrrolidone into PbI₂/DMF solution. PbI₂ films with excellent crystallinity and full coverage are fabricated by spin-coating the mixed solution on the substrate, which leads to high quality perovskite layers with low recombination rate and high efficiency for carrier transfer. As a result, the power conversion efficiency of the best perovskite solar cells increases from 13.3% to 17.5%.

Facet-Dependent Property of Sequentially Deposited Perovskite Thin Films: Chemical Origin and Self-Annihilation

Quantification of intergrain length scale properties of CH₃NH₃PbI₃ (MAPbI₃) can provide further understanding of material physics, leading to improved device performance. In this work, we noticed that two typical types of facets appear in sequential deposited perovskite (SDP) films: smooth and steplike morphologies. By mapping the surface potential as well as the photoluminescence (PL) peak position, we revealed the heterogeneity of SDP thin films that smooth facets are almost intrinsic with a PL peak at 775 nm, while the steplike facets are p-type-doped with 5-nm blue-shifted PL peak. Considering the reaction process, we propose that the smooth facets have well-defined crystal lattices that resulted from the interfacial reaction between MAI and PbI₂ domains containing low trap states density. The steplike facets are MAI-rich originated from the grain boundaries of PbI₂ film and own more trap states. Conversion of steplike facets to smooth facets can be controlled by increasing the reaction time through Ostwald ripening. The improved stability, photoresponsivity up to 0.3 A/W, on/off ratio up to 3900, and decreased photo response time to ∼160 μs show that the trap states can be annihilated effectively to improve the photoelectrical conversion with prolonged reaction time and elimination of steplike facets. Our findings demonstrate the relationship between the facet heterogeneity of SDP films and crystal growth process for the first time, and imply that the systematic control of crystal grain modification will enable amelioration of crystallinity for more-efficient perovskite photoelectrical applications.

Nonstoichiometric acid-base reaction as reliable synthetic route to highly stable CH3NH3PbI3 perovskite film

Perovskite solar cells have received worldwide interests due to swiftly improved efficiency but the poor stability of the perovskite component hampers the device fabrication under normal condition. Herein, we develop a reliable nonstoichiometric acid-base reaction route to stable perovskite films by intermediate chemistry and technology. Perovskite thin-film prepared by nonstoichiometric acid-base reaction route is stable for two months with negligible PbI2-impurity under ∼65% humidity, whereas other perovskites prepared by traditional methods degrade distinctly after 2 weeks. Route optimization involves the reaction of PbI2 with excess HI to generate HPbI3, which subsequently undergoes reaction with excess CH3NH2 to deliver CH3NH3PbI3 thin films. High quality of intermediate HPbI3 and CH3NH2 abundance are two important factors to stable CH3NH3PbI3 perovskite. Excess volatile acid/base not only affords full conversion in nonstoichiometric acid-base reaction route but also permits its facile removal for stoichiometric purification, resulting in average efficiency of 16.1% in forward/reverse scans.

Simple and Efficient Green-Light-Emitting Diodes Based on Thin Organolead Bromide Perovskite Films via Tuning Precursor Ratios and Postannealing Temperature

Organometal halide perovskites (OHPs) are becoming a hot topic in the field of display and lighting. Unlike the strategy used for solar cells, that is, using several hundred nanometers thick OHP film for fully absorbing solar light to convert electricity, thin-film OHPs (<50 nm) are advantageous to restrain its self-absorption drawback and thus beneficial for preparing efficient light-emitting diodes (LEDs). Here we manipulate the excess molar ratio of MABr/PbBr₂ precursors and the post-annealing temperature to obtain uniform thin-film OHPs and suppress the nonradiative defects. Using this simple process, high efficient green perovskite light-emitting diode (PeLED) was obtained, with a maximum luminance of 6124 cd m^-2, current efficiency of 15.26 cd A^-1, and external quantum efficiency of 3.38%, which is nearly three-fold enhancement with respect to the previous reported best PeLED based on thin perovskite films (<50 nm).

Retarding Crystallization during Facile Single Coating of NaCl-Incorporated Precursor Solution for Efficient Large-Area Uniform Perovskite Solar Cells

We demonstrated crystallization retardation of CH₃NH₃PbI₃ thin film during single coating of precursor solution by simple addition of NaCl. NaCl was codissolved into a precursor mixture solution containing PbI₂ and methylammonium iodide (MAI). Dissolved NaCl interacted with the PbI₂ in solution and produced a stable intermediate phase, which was converted to a full-coverage uniform perovskite absorber layer via reaction with MAI during a single spin-coating. The resulting planar-structure perovskite solar cell made from NaCl-supplemented precursor solution showed a 48% improvement in power conversion efficiency (PCE) (maximum value 15.16%) over the device fabricated without the additive. Our NaCl-supplemented single coating represents an easy approach to effectively obtain highly reproducible uniform performance at an overall position in 5 cm × 5 cm sized cells (divided into 20 subcells with an active area of 0.06 cm²) with average PCEs of 12.00 ± 0.48%.

The Additive Coordination Effect on Hybrids Perovskite Crystallization and High-Performance Solar Cell

The coordination effects of additives during perovskite crystal growth are investigated, and a novel technique to fabricate high-quality perovskite thin films by introduction of weak coordination additives (e.g., acetonitrile) in the precursors is demonstrated.

Stable α/δ phase junction of formamidinium lead iodide perovskites for enhanced near-infrared emission

Although formamidinium lead iodide (FAPbI₃) perovskite has shown great promise in the field of perovskite-based optoelectronic devices, it suffers the complications of a structural phase transition from a black perovskite phase (α-FAPbI₃) to a yellow non-perovskite phase (δ-FAPbI₃). Generally, it is pivotal to avoid δ-FAPbI₃ since only α-FAPbI₃ is desirable for photoelectric conversion and near-infrared (NIR) emission. However, herein, we firstly exploited the undesirable δ-FAPbI₃ to enable structurally stable, pure FAPbI₃ films with a controllable α/δ phase junction at low annealing temperature (60 °C) through stoichiometrically modified precursors (FAI/PbI₂ = 1.1-1.5). The α/δ phase junction contributes to a striking stabilization of the perovskite phase of FAPbI₃ at low temperature and significantly enhanced NIR emission at 780 nm, which is markedly different from pure α-FAPbI₃ (815 nm). In particular, the optimal α/δ phase junction with FAI/PbI₂ = 1.2 exhibited preferable long-term stability against humidity and high PLQY of 6.9%, nearly 10-fold higher than that of pure α-FAPbI₃ (0.7%). The present study opens a new approach to realize highly stable and efficient emitting perovskite materials by utilizing the phase junctions.

Facile and Scalable Fabrication of Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells in Air Using Gas Pump Method

Control of the perovskite film formation process to produce high-quality organic-inorganic metal halide perovskite thin films with uniform morphology, high surface coverage, and minimum pinholes is of great importance to highly efficient solar cells. Herein, we report on large-area light-absorbing perovskite films fabrication with a new facile and scalable gas pump method. By decreasing the total pressure in the evaporation environment, the gas pump method can significantly enhance the solvent evaporation rate by 8 times faster and thereby produce an extremely dense, uniform, and full-coverage perovskite thin film. The resulting planar perovskite solar cells can achieve an impressive power conversion efficiency up to 19.00% with an average efficiency of 17.38 ± 0.70% for 32 devices with an area of 5 × 2 mm, 13.91% for devices with a large area up to 1.13 cm(2). The perovskite films can be easily fabricated in air conditions with a relative humidity of 45-55%, which definitely has a promising prospect in industrial application of large-area perovskite solar panels.

Identifying the Molecular Structures of Intermediates for Optimizing the Fabrication of High-Quality Perovskite Films

During the past two years, the introduction of DMSO has revolutionized the fabrication of high-quality pervoskite MAPbI3 (MA = CH3NH3) films for solar cell applications. In the developed DMSO process, the formation of (MA)2Pb3I8·2DMSO (shorted as Pb3I8) has well recognized as a critical factor to prepare high-quality pervoskite films and thus accomplish excellent performances in perovskite solar cells. However, Pb3I8 is an I-deficient intermediate and must further react with methylammonium iodide (MAI) to be fully converted into MAPbI3. By capturing and solving the molecular structures of several intermediates involved in the fabrication of perovskite films, we report in this work that the importance of DMSO is NOT due to the formation of Pb3I8. The use of different PbI2-DMSO ratios leads to two different structures of PbI2-DMSO precursors (PbI2·DMSO and PbI2·2DMSO), thus dramatically influencing the quality of fabricated perovskite films. However, such an influence can be minimized when the PbI2-DMSO precursor films are thermally treated to create mesoporous PbI2 films before reacting with MAI. Such a development makes the fabrication of high-quality pervoskite films highly reproducible without the need to precisely control the PbI2:DMSO ratio. Moreover, the formation of ionic compound (MA)4PbI6 is observed when excess MAI is used in the preparation of perovskite film. This I-rich phase heavily induces the hysteresis in PSCs, but is readily removed by isopropanol treatment. On the basis of all these findings, we develop a new effective protocol to fabricate high-performance PSCs. In the new protocol, high-quality perovskite films are prepared by simply treating the mesoporous PbI2 films (made from PbI2-DMSO precursors) with an isopropanol solution of MAI, followed by isopropanol washing. The best efficiency of fabricated MAPbI3 PSCs is up to 19.0%. As compared to the previously reported DMSO method, the devices fabricated by the method reported in this work display narrow efficiency distributions in both forward and reverse scans. And the efficiency difference between forward and reverse scans is much smaller.

High-Efficiency Light-Emitting Diodes of Organometal Halide Perovskite Amorphous Nanoparticles

Organometal halide perovskite has recently emerged as a very promising family of materials with augmented performance in electronic and optoelectronic applications including photovoltaic devices, photodetectors, and light-emitting diodes. Herein, we propose and demonstrate facile solution synthesis of a series of colloidal organometal halide perovskite CH3NH3PbX3 (X = halides) nanoparticles with amorphous structure, which exhibit high quantum yield and tunable emission from ultraviolet to near-infrared. The growth mechanism and photoluminescence properties of the perovskite amorphous nanoparticles were studied in detail. A high-efficiency green-light-emitting diode based on amorphous CH3NH3PbBr3 nanoparticles was demonstrated. The perovskite amorphous nanoparticle-based light-emitting diode shows a maximum luminous efficiency of 11.49 cd/A, a power efficiency of 7.84 lm/W, and an external quantum efficiency of 3.8%, which is 3.5 times higher than that of the best colloidal perovskite quantum-dot-based light-emitting diodes previously reported. Our findings indicate the great potential of colloidal perovskite amorphous nanoparticles in light-emitting devices.

Coordination Chemistry Dictates the Structural Defects in Lead Halide Perovskites

We show the influence of species present in precursor solution during formation of lead halide perovskite materials on the structural defects of the films. The coordination of lead by competing solvent molecules and iodide ions dictate the type of complexes present in the films. Depending on the processing conditions all PbIS5 (+) , PbI2 S4, PbI3 S3 (-) , PbI4 S2 (2-) , PbI5 S2 (3-) , PbI6 (4-) and 1D (Pb2 I4 )n chains are observed by absorption measurements. Different parameters are studied such as polarity of the solvent, concentration of iodide ions, concentration of solvent molecules and temperature. It is concluded that strongly coordinating solvents will preferentially form species with a low number of iodide ions and less coordinative solvents generate high concentration of PbI6 (-) . We furthermore propose that all these plumbate ions may act as structural defects determining electronic properties of the photovoltaic films.

A highly photoconductive composite prepared by incorporating polyoxometalate into perovskite for photodetection application

A photoconductive perovskite-polyoxometalate composite was prepared for the first time by a facile low-temperature and solution-processed method, and this composite exhibited a significantly enhanced photoconductivity and photodetection performance due to introduction of polyoxometalate into the perovskite for fine energy-level matching and efficient charge transfer.

Recent Advances in Interface Engineering for Planar Heterojunction Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells are considered as one of the most promising next-generation solar cells due to their advantages of low-cost precursors, high power conversion efficiency (PCE) and easy of processing. In the past few years, the PCEs have climbed from a few to over 20% for perovskite solar cells. Recent developments demonstrate that perovskite exhibits ambipolar semiconducting characteristics, which allows for the construction of planar heterojunction (PHJ) perovskite solar cells. PHJ perovskite solar cells can avoid the use of high-temperature sintered mesoporous metal oxides, enabling simple processing and the fabrication of flexible and tandem perovskite solar cells. In planar heterojunction materials, hole/electron transport layers are introduced between a perovskite film and the anode/cathode. The hole and electron transporting layers are expected to enhance exciton separation, charge transportation and collection. Further, the supporting layer for the perovskite film not only plays an important role in energy-level alignment, but also affects perovskite film morphology, which have a great effect on device performance. In addition, interfacial layers also affect device stability. In this review, recent progress in interfacial engineering for PHJ perovskite solar cells will be reviewed, especially with the molecular interfacial materials. The supporting interfacial layers for the optimization of perovskite films will be systematically reviewed. Finally, the challenges remaining in perovskite solar cells research will be discussed.

Organic-inorganic hybrid lead halide perovskites for optoelectronic and electronic applications

Organic and inorganic hybrid perovskites (e.g., CH(3)NH(3)PbI(3)), with advantages of facile processing, tunable bandgaps, and superior charge-transfer properties, have emerged as a new class of revolutionary optoelectronic semiconductors promising for various applications. Perovskite solar cells constructed with a variety of configurations have demonstrated unprecedented progress in efficiency, reaching about 20% from multiple groups after only several years of active research. A key to this success is the development of various solution-synthesis and film-deposition techniques for controlling the morphology and composition of hybrid perovskites. The rapid progress in material synthesis and device fabrication has also promoted the development of other optoelectronic applications including light-emitting diodes, photodetectors, and transistors. Both experimental and theoretical investigations on organic-inorganic hybrid perovskites have enabled some critical fundamental understandings of this material system. Recent studies have also demonstrated progress in addressing the potential stability issue, which has been identified as a main challenge for future research on halide perovskites. Here, we review recent progress on hybrid perovskites including basic chemical and crystal structures, chemical synthesis of bulk/nanocrystals and thin films with their chemical and physical properties, device configurations, operation principles for various optoelectronic applications (with a focus on solar cells), and photophysics of charge-carrier dynamics. We also discuss the importance of further understanding of the fundamental properties of hybrid perovskites, especially those related to chemical and structural stabilities.

Perovskite Photovoltaics with Outstanding Performance Produced by Chemical Conversion of Bilayer Mesostructured Lead Halide/TiO2 Films

A new method of producing high-quality perovskite films via sequential deposition is presented, introducing a PbX2 capping layer that is endowed with a network of interconnected nanopores. The mesoporous lead halide architecture provides a powerful tool to accomplish rapid and complete transformation of lead halide into the perovskite for high-efficiency solar cells.

Ultrathin efficient perovskite solar cells employing a periodic structure of a composite hole conductor for elevated plasmonic light harvesting and hole collection

We developed a molecule/polymer composite hole transporting material (HTM) with a periodic microstructure for morphology replication of a corrugated Au electrode, which in combination plays a dual role in the optical and electronic enhancement of high performance perovskite solar cells (PSCs). The electro-optics revealed that perovskite couldn't readily extinct the red light even though the thickness increased to 370 nm, but we found that the quasi periodic microstructure composite (PMC) HTM in combination with the conformal Au electrode could promote the absorption through the enhanced cavity effects, leading to comparable absorption even using much thinner perovskite (240 nm). We identified that the cavity was the combination of Fabry-Pérot interferometer and surface plasmonic resonance, with light harvesting enhancement through surface plasmon polariton or waveguide modes that propagate in the plane of the perovskite layer. On the other hand, the PMC HTM increased hole conductivity by one order of magnitude with respect to standard spiro-OMeTAD HTM due to molecular packing and self-assembly, embodying traceable hole mobility and density elevation up to 3 times, and thus the hysteresis was greatly avoided. Owing to dual optical and electronic enhancement, the PMC PSC afforded high efficiency PSC using as thin as 240 nm perovskite layer, delivering a V(oc) of 1.05 V, J(sc) of 22.9 mA cm(-2), FF of 0.736, and efficiency amounting to 17.7% PCE, the highest efficiency with ultrathin perovskite layer.

Room-temperature solution-processed and metal oxide-free nano-composite for the flexible transparent bottom electrode of perovskite solar cells

The exploration of low-temperature and solution-processed charge transporting and collecting layers can promote the development of low-cost and large-scale perovskite solar cells (PVSCs) through an all solution process. Here, we propose a room-temperature solution-processed and metal oxide-free nano-composite composed of a silver nano-network and graphene oxide (GO) flawless film for the transparent bottom electrode of a PVSC. Our experimental results show that the amount of GO flakes play a critical role in forming the flawless anti-corrosive barrier in the silver nano-network through a self-assembly approach under ambient atmosphere, which can effectively prevent the penetration of liquid or gaseous halides and their corrosion against the silver nano-network underneath. Importantly, we simultaneously achieve good work function alignment and surface wetting properties for a practical bottom electrode by controlling the degree of reduction of GO flakes. Finally, flexible PVSC adopting the room-temperature and solution-processed nano-composite as the flexible transparent bottom electrode has been demonstrated on a polyethylene terephthalate (PET) substrate. As a consequence, the demonstration of our room-temperature solution-processed and metal oxide-free flexible transparent bottom electrode will contribute to the emerging large-area flexible PVSC technologies.

Easily accessible polymer additives for tuning the crystal-growth of perovskite thin-films for highly efficient solar cells

For perovskite solar cells (Pero-SCs), one of the key issues with respect to the power conversion efficiency (PCE) is the morphology control of the perovskite thin-films. In this study, an easily-accessible additive polyethylenimine (PEI) is utilized to tune the morphology of CH3NH3PbI3-xClx. With addition of 1.00 wt% of PEI, the smoothness and crystallinity of the perovskite were greatly improved, which were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). A summit PCE of 14.07% was achieved for the p-i-n type Pero-SC, indicating a 26% increase compared to those of the devices without the additive. Both photoluminescence (PL) and alternating current impedance spectroscopy (ACIS) analyses confirm the efficiency results after the addition of PEI. This study provides a low-cost polymer additive candidate for tuning the morphology of perovskite thin-films, and might be a new clue for the mass production of Pero-SCs.

Current Challenges and Prospective Research for Upscaling Hybrid Perovskite Photovoltaics

Organic-inorganic hybrid perovskite photovoltaics (PSCs) are poised to push toward technology translation, but significant challenges complicating commercialization remain. Though J-V hysteresis and ecotoxicity are uniquely imposing issues at scale, CH3NH3PbI3 degradation is by far the sharpest limitation to the technology's potential market contribution. Herein, we offer a perspective on the practical market potential of PSCs, the nature of fundamental PSC challenges at scale, and an outline of prospective solutions for achieving module scale PSC production tailored to intrinsic advantages of CH3NH3PbI3. Although integrating PSCs into the energy grid is complicated by CH3NH3PbI3 degradation, the ability of PSCs to contribute to consumer electronics and other niche markets like those organic photovoltaics have sought footing in rests primarily upon the technology's price point. Thus, slot die, roll-to-roll processing has the greatest potential to enable PSC scale-up, and herein, we present a perspective on the research necessary to realize fully printable PSCs at scale.

Crystal Structure Formation of CH₃NH₃PbI3-xClx Perovskite

Inorganic-organic hydride perovskites bring the hope for fabricating low-cost and large-scale solar cells. At the beginning of the research, two open questions were raised: the hysteresis effect and the role of chloride. The presence of chloride significantly improves the crystallization and charge transfer property of the perovskite. However, though the long held debate over of the existence of chloride in the perovskite seems to have now come to a conclusion, no prior work has been carried out focusing on the role of chloride on the electronic performance and the crystallization of the perovskite. Furthermore, current reports on the crystal structure of the perovskite are rather confusing. This article analyzes the role of chloride in CH₃NH₃PbI_3-xCl_x on the crystal orientation and provides a new explanation about the (110)-oriented growth of CH₃NH₃PbI₃ and CH₃NH₃PbI_3-xCl_x.

Making and Breaking of Lead Halide Perovskites

A new front-runner has emerged in the field of next-generation photovoltaics. A unique class of materials, known as organic metal halide perovskites, bridges the gap between low-cost fabrication and exceptional device performance. These compounds can be processed at low temperature (typically in the range 80-150 °C) and readily self-assemble from the solution phase into high-quality semiconductor thin films. The low energetic barrier for crystal formation has mixed consequences. On one hand, it enables inexpensive processing and both optical and electronic tunability. The caveat, however, is that many as-formed lead halide perovskite thin films lack chemical and structural stability, undergoing rapid degradation in the presence of moisture or heat. To date, improvements in perovskite solar cell efficiency have resulted primarily from better control over thin film morphology, manipulation of the stoichiometry and chemistry of lead halide and alkylammonium halide precursors, and the choice of solvent treatment. Proper characterization and tuning of processing parameters can aid in rational optimization of perovskite devices. Likewise, gaining a comprehensive understanding of the degradation mechanism and identifying components of the perovskite structure that may be particularly susceptible to attack by moisture are vital to mitigate device degradation under operating conditions. This Account provides insight into the lifecycle of organic-inorganic lead halide perovskites, including (i) the nature of the precursor solution, (ii) formation of solid-state perovskite thin films and single crystals, and (iii) transformation of perovskites into hydrated phases upon exposure to moisture. In particular, spectroscopic and structural characterization techniques shed light on the thermally driven evolution of the perovskite structure. By tuning precursor stoichiometry and chemistry, and thus the lead halide charge-transfer complexes present in solution, crystallization kinetics can be tailored to yield improved thin film homogeneity. Because degradation of the as-formed perovskite film is in many ways analogous to its initial formation, the same suite of monitoring techniques reveals the moisture-induced transformation of low band gap methylammonium lead iodide (CH3NH3PbI3) to wide band gap hydrate compounds. The rate of degradation is increased upon exposure to light. Interestingly, the hydration process is reversible under certain conditions. This facile formation and subsequent chemical lability raises the question of whether CH3NH3PbI3 and its analogues are thermodynamically stable phases, thus posing a significant challenge to the development of transformative perovskite photovoltaics. Adequately addressing issues of structural and chemical stability under real-world operating conditions is paramount if perovskite solar cells are to make an impact beyond the benchtop. Expanding our fundamental knowledge of lead halide perovskite formation and degradation pathways can facilitate fabrication of stable, high-quality perovskite thin films for the next generation of photovoltaic and light emitting devices.

Insight into the effect of ion source for the solution processing of perovskite films

Ion exchange is proved to happen in solutions made from different routes, which results in similar properties of perovskite films.

Design rules for the broad application of fast (<1 s) methylamine vapor based, hybrid perovskite post deposition treatments

While organo-metal halide perovskite photovoltaics have seen rapid development, growth of high quality material remains a challenge.

Rapid growth of high quality perovskite crystal by solvent mixing

The formation mechanism of perovskite crystal in different mixed solvents.

Multiscale morphology design of hybrid halide perovskites through a polymeric template

Hybrid halide perovskites have emerged as promising active constituents of next generation solution processable optoelectronic devices. During their assembling process, perovskite components undergo very complex dynamic equilibria starting in solution and progressing throughout film formation. Finding a methodology to control and affect these equilibria, responsible for the unique morphological diversity observed in perovskite films, constitutes a fundamental step towards a reproducible material processability. Here we propose the exploitation of polymer matrices as cooperative assembling components of novel perovskite CH3NH3PbI3 : polymer composites, in which the control of the chemical interactions in solution allows a predictable tuning of the final film morphology. We reveal that the nature of the interactions between perovskite precursors and polymer functional groups, probed by Nuclear Magnetic Resonance (NMR) spectroscopy and Dynamic Light Scattering (DLS) techniques, allows the control of aggregates in solution whose characteristics are strictly maintained in the solid film, and permits the formation of nanostructures that are inaccessible to conventional perovskite depositions. These results demonstrate how the fundamental chemistry of perovskite precursors in solution has a paramount influence on controlling and monitoring the final morphology of CH3NH3PbI3 (MAPbI3) thin films, foreseeing the possibility of designing perovskite : polymer composites targeting diverse optoelectronic applications.

Microstructures of Organometal Trihalide Perovskites for Solar Cells: Their Evolution from Solutions and Characterization

The use of organometal trihalide perovskites (OTPs) in perovskite solar cells (PSCs) is revolutionizing the field of photovoltaics, which is being led by advances in solution processing of OTP thin films. First, we look at fundamental phenomena pertaining to nucleation/growth, coarsening, and microstructural evolution involved in the solution-processing of OTP thin films for PSCs from a materials-science perspective. Established scientific principles that govern some of these phenomena are invoked in the context of specific literature examples of solution-processed OTP thin films. Second, the nature and the unique characteristics of OTP thin-film microstructures themselves are discussed from a materials-science perspective. Finally, we discuss the challenges and opportunities in the characterization of OTP thin films for not only gaining a deep understanding of defects and microstructures but also elucidating classical and nonclassical phenomena pertaining to nucleation/growth, coarsening, and microstructural evolution in these films. The overall goal is to have deterministic control over the solution-processing of tailored OTP thin films with desired morphologies and microstructures.

Enhanced optoelectronic quality of perovskite thin films with hypophosphorous acid for planar heterojunction solar cells

Solution-processed metal halide perovskite semiconductors, such as CH₃NH₃PbI₃, have exhibited remarkable performance in solar cells, despite having non-negligible density of defect states. A likely candidate is halide vacancies within the perovskite crystals, or the presence of metallic lead, both generated due to the imbalanced I/Pb stoichiometry which could evolve during crystallization. Herein, we show that the addition of hypophosphorous acid (HPA) in the precursor solution can significantly improve the film quality, both electronically and topologically, and enhance the photoluminescence intensity, which leads to more efficient and reproducible photovoltaic devices. We demonstrate that the HPA can reduce the oxidized I₂ back into I⁻, and our results indicate that this facilitates an improved stoichiometry in the perovskite crystal and a reduced density of metallic lead.

An imbalance in I/Pb stoichiometry is thought to lead to defects in metal halide films. Here, Zhang et al. show that the addition of hypophosphorous acid in the precursor solution can significantly improve the film quality and enhance the photoluminescence intensity, leading to improved photovoltaic devices.

Improvement of CH₃NH₃PbI₃ Formation for Efficient and Better Reproducible Mesoscopic Perovskite Solar Cells

High-performance perovskite solar cells (PSCs) are obtained through optimization of the formation of CH3NH3PbI3 nanocrystals on mesoporous TiO2 film, using a two-step sequential deposition process by first spin-coating a PbI2 film and then submerging it into CH3NH3I solution for perovskite conversion (PbI2 + CH3NH3I → CH3NH3PbI3). It is found that the PbI2 morphology from different film formation process (thermal drying, solvent extraction, and as-deposited) has a profound effect on the CH3NH3PbI3 active layer formation and its nanocrystalline composition. The residual PbI2 in the active layer contributes to substantial photocurrent losses, thus resulting in low and inconsistent PSC performances. The PbI2 film dried by solvent extraction shows enhanced CH3NH3PbI3 conversion as the loosely packed disk-like PbI2 crystals allow better CH3NH3I penetration and reaction in comparison to the multicrystal aggregates that are commonly obtained in the thermally dried PbI2 film. The as-deposited PbI2 wet film, without any further drying, exhibits complete conversion to CH3NH3PbI3 in MAI solution. The resulting PSCs reveal high power conversion efficiency of 15.60% with a batch-to-batch consistency of 14.60 ± 0.55%, whereas a lower efficiency of 13.80% with a poorer consistency of 11.20 ± 3.10% are obtained from the PSCs using thermally dried PbI2 films.

Facile route to freestanding CH3NH3PbI3 crystals using inverse solubility

CH₃NH₃PbI₃ was found to exhibit inverse solubility at high temperatures in γ-butyrolactone. Making use of this unusual, so far unreported phenomenon, we present a facile method for the growth of freestanding crystals of CH₃NH₃PbI₃ from solution without addition of any capping agents or seed particles. Large, strongly faceted crystals could be grown within minutes. This finding may aid in understanding the crystallization process of CH₃NH₃PbI₃ from solution that may lead to improved morphological control of film deposition for a range of device architectures. Our process offers a facile and rapid route to freestanding crystals for use in a broad range of characterization techniques.

Efficient perovskite solar cells fabricated using an aqueous lead nitrate precursor

A novel, aqueous precursor system (Pb(NO₃)₂ + water) is developed to replace conventional (PbI₂ + DMF) for fabricating methylammonium lead iodide (MAPbI₃) perovskite solar cells (PSCs).

Smooth CH3NH3PbI3from controlled solid–gas reaction for photovoltaic applications

Schematic graphic of solid–gas reaction to form perovskite.

Molecular dynamics simulations of organohalide perovskite precursors: solvent effects in the formation of perovskite solar cells

Molecular dynamics simulations towards the formation of perovskite embryonic units for solar cells.