Growth Of Perovskite Films Research Articles

The Effect of Methylammonium Iodide on the Supersaturation and Interfacial Energy of the Crystallization of Methylammonium Lead Triiodide Single Crystals.

It is very important to study the crystallization of hybrid organic-inorganic perovskites because their thin films are usually prepared from solution. The investigation on the growth of perovskite films is however limited by their polycrystallinity. In this work, methylammonium lead triiodide single crystals grown from solutions with different methylammonium iodide (MAI):lead iodide (PbI2 ) ratios were investigated. We observed a V-shaped dependence of the crystallization onset temperature on the MAI:PbI2 ratio. This is attributed to the MAI effects on the supersaturation of precursors and the interfacial energy of the crystal growth. At low MAI:PbI2 ratio (<1.7), more MAI leads to the supersaturation of the precursors at lower temperature. At high MAI:PbI2 ratio, the crystal growing plans change from (100)-plane dominated to (001)-plane dominated. The latter have higher interfacial energy than the former, leading to a higher crystallization onset temperature.

Efficient and High-Color-Purity Light-Emitting Diodes Based on In Situ Grown Films of CsPbX3 (X = Br, I) Nanoplates with Controlled Thicknesses

We report a facile solution-based approach to the in situ growth of perovskite films consisting of monolayers of CsPbBr3 nanoplates passivated by bulky phenylbutylammonium (PBA) cations, that is, two-dimensional layered PBA2(CsPbBr3)n-1PbBr4 perovskites. Optimizing film formation processes leads to layered perovskites with controlled n values in the range of 12-16. The layered perovskite emitters show quantum-confined band gap energies with a narrow distribution, suggesting the formation of thickness-controlled quantum-well (TCQW) structures. The TCQW CsPbBr3 films exhibit smooth surface features, narrow emission line widths, low trap densities, and high room-temperature photoluminance quantum yields, resulting in high-color-purity green light-emitting diodes (LEDs) with remarkably high external quantum efficiencies (EQEs) of up to 10.4%. The solution-based approach is extended to the preparation of TCQW CsPbI3 films for high-color-purity red perovskite LEDs with high EQEs of up to 7.3%.

High-Performance Flexible Photodetectors based on High-Quality Perovskite Thin Films by a Vapor-Solution Method.

Organometal halide perovskites are new light-harvesting materials for lightweight and flexible optoelectronic devices due to their excellent optoelectronic properties and low-temperature process capability. However, the preparation of high-quality perovskite films on flexible substrates has still been a great challenge to date. Here, a novel vapor-solution method is developed to achieve uniform and pinhole-free organometal halide perovskite films on flexible indium tin oxide/poly(ethylene terephthalate) substrates. Based on the as-prepared high-quality perovskite thin films, high-performance flexible photodetectors (PDs) are constructed, which display a nR value of 81 A W-1 at a low working voltage of 1 V, three orders higher than that of previously reported flexible perovskite thin-film PDs. In addition, these flexible PDs exhibit excellent flexural stability and durability under various bending situations with their optoelectronic performance well retained. This breakthrough on the growth of high-quality perovskite thin films opens up a new avenue to develop high-performance flexible optoelectronic devices.

Enhanced Photovoltaic Performance of the Inverted Planar Perovskite Solar Cells by Using Mixed-Phase Crystalline Perovskite Film with Trace Amounts of PbI2 as an Absorption Layer

It is widely known that solvent engineering has a significant effect on the growth of high quality perovskite film. However, the crystal evolution process and crystal compositions of perovskite film when applying the two-step method are multifaceted, and the effects on the photoelectrical properties for the perovskite layer still require further study. In this study, the above two issues are systematically studied. Combining the XRD and photoluminescence spectra results, it was observed that the residual PbI2 decreased with the increase of DMSO in DMSO/DMF solvents and that there were two types of local phase crystallites (disordered and ordered) in the perovskite films. The phase transitions of the crystals were investigated with the ratio of DMSO increasing in the mixed solvents. In addition, the interfacial defect states in the PbI2/perovskite interface which can act as charge carrier trapping centers were significantly affected by the photo(dark) conductivity of the perovskite film. It was also observ...

Manipulation on Crystallization of Perovskite Thin-films during Blade Coating Fabrication in Air

Organometal trihalide perovskite solar cells (PSCs) have drawn more and attention in the past few years due to the merits of low-cost preparation processes and rapidly improved power conversion efficiency (PCE) with latest certified efficiency of 22.1%.1-4 Commonly, solution processes, including one- or two-step spin-coating methods, are often employed to produce perovskite films.5, 6 Recently, the blade coating technique, with the advantages of large scale fabrication and mass production besides the well-accepted low-cost manner, has also attracted researchers’ interest for the rapid growth of perovskite films.7-8 However, nucleation and crystal growth during the film’s crystallization by blade coating are still lack of controlling, generally resulting in relatively poorer crystal quality and film uniformity. So, further strategies are required to improve the crystallization of perovskite films and ultimate device efficiencies. In this work, different strategies were introduced during the blade-coating process to control and manipulate the crystallization of perovskite films. In depth understanding in the nucleation mechanisms and growth dynamics of films were well accomplished. As a result, the rapid growth of perovskite films with high quality and good optical performances were successfully produced. Correspondingly, efficient PSCs were correspondingly fabricated.

Stitching triple cation perovskite by a mixed anti-solvent process for high performance perovskite solar cells

With the rapid development of organic-inorganic lead halide perovskite photovoltaics, increasingly more attentions are paid to explore the growth mechanism and precisely control the quality of perovskite films. In this study, we propose a “stitching effect” to fabricate high quality perovskite films by using chlorobenzene (CB) as an anti-solvent and isopropyl alcohol (IPA) as an additive into this anti-solvent. Because of the existence of IPA, CB can be efficiently released from the gaps of perovskite precursors and the perovskite film formation can be slightly modified in a controlled manner. More homogeneous surface morphology and larger grain size of perovskite films were achieved via this process. The reduced grain boundaries ensure low surface defect density and good carrier transport in the perovskite layer. Meanwhile, we also performed the Fourier transform infrared (FTIR) spectroscopy to investigate the film growth mechanism of unannealed and annealed perovskite films. Solar cells fabricated by using the “stitching effect” exhibited a best efficiency of 19.2%. Our results show that solvent and solvent additives dramatically influenced the formation and crystallization processes for perovskite materials due to their different coordination and extraction capabilities. This method presents a new path towards controlling the growth and morphology of perovskite films.

Polymer assisted growth of high-quality perovskite films by Lewis acid-base adduct for efficient planar-heterojunction solar cells

Long-chain insulating polymers dissolved in perovskite precursor solution can assemble the polymer scaffold acted as the function of TiO2 porous layer to improve the quality of the perovskite films. Here, perovskite films with high electronic quality were prepared by mediating nucleation and crystal growth. Polymer post treatment can induce Lewis acid-base reaction with PbCl2, which alleviates the reaction velocity between PbCl2 and CH3NH3I, and then retard perovskite crystallization. The interactions between the perovskite films and polymer with different concentration are addressed to interpret the function and evolution of the polymer long-chains during the annealing process. These solar cells exhibit efficiency more than 15% with small variation. Our work demonstrates the value of retarding nucleation and crystal growth by Lewis acid-base adducts for high-quality perovskite films.

Unveiling the Concentration-Dependent Grain Growth of Perovskite Films from One- and Two-Step Deposition Methods: Implications for Photovoltaic Application.

In order to achieve high-efficiency perovskite solar cells, understanding both the crystal structure and the optoelectronic properties of perovskite layers is of importance. This paper uses crystallization analysis and the modeling study of two different perovskite deposition methods (two- and one-step methods) and thereby shows that the one-step method embodies a film formation process that is dominated by crystal growth, while the dominant mechanism of the two-step procedure is nucleation. Our data based on experimental and theory shows that the one-step recipe is superior in terms of morphology control, and, hence, reproducibility, compared to the two-step recipe.

Strategies for Improving Efficiency and Stability of Perovskite Solar Cells

ABSTRACTPerovskite solar cells (PSCs) based on organometal halide light absorbers have hit the limelight in recent years owing to their low temperature solution processability, material abundance and rapidly rising efficiency. To rival the leading photovoltaic technologies, efficiency and long-term stability of PSCs represent two prime facets of the challenges currently facing the research community. Herein we summarize the strategies for improving efficiency and stability of PSCs by drawing on our recent work. Emphasis is given to the importance of perovskite film growth, electron/hole transport materials and interface materials in cell performance. We also discuss possible degradation mechanisms of PSCs.

Moisture-driven phase transition for improved perovskite solar cells with reduced trap-state density

We report a mechanistic understanding of a moisture-driven intermediate-phase transition that improves the quality of perovskite thin films based on a lead-acetate precursor, improving the power-conversion efficiency. We clarify the composition of the intermediate phase and attribute the transition of this phase to the hygroscopic nature of the organic product, i.e., methylammonium acetate. Thermal annealing aids in the coarsening of the grains. These decoupled processes result in better crystal formation with a lower spatial and energetic distribution of traps. Thermal annealing of the films without exposure to air results in a faster intermediate-phase transition and grain coarsening, which occur simultaneously, leading to disorder in the films and a higher deep trap-state density. Our results indicate the need for a humid environment for the growth of high-quality perovskite films and provide insight into intermediate-phase dissociation and conversion kinetics. Thus, they are useful for the large-scale production of efficient solution-processed perovskite solar cells.

Rigid Amino Acid as Linker to Enhance the Crystallinity of CH3NH3PbI3 Particles

Organic–inorganic hybrid perovskites have attracted extensive interest in recent years due to their remarkable properties in highly efficient solar cells. The quality of the perovskite thin films has been identified as a critical factor to affect the performances of the solar cells. To achieve uniform and dense perovskite films, many efforts have been attempted on controlling the crystallinity of the particles, grain size, as well as surface coverage. Using organic coupling agents to connect the mesoporous TiO2 with the perovskite film is an effective way to guide the growth of perovskite film. However, the influences of organic molecular configurations on the perovskite crystallization are still not being well studied. In this article, two amino acids, 4‐aminobenzoic acid and 4‐aminobutyric acid, are employed with similar molecular size and closed chain length to study the molecular rigidity of the organic coupling agents affecting the quality of perovskite films. Greatly improved perovskite film has been obtained with the template effect of rigid 4‐aminobenzoic acid and thus the cell performance has been significantly increased.

Highly Oriented SrTiO3 Thin Film on Graphene Substrate.

Growth of perovskite oxide thin films on Si in crystalline form has long been a critical obstacle for the integration of multifunctional oxides into Si-based technologies. In this study, we propose pulsed laser deposition of a crystalline SrTiO3 thin film on a Si using graphene substrate. The SrTiO3 thin film on graphene has a highly (00l)-oriented crystalline structure which results from the partial epitaxy. Moreover, graphene promotes a sharp interface by highly suppressing the chemical intermixing. The important role of graphene as a 2D substrate and diffusion barrier allows the expansion of device applications based on functional complex oxides.

Molten Salt-Assisted Growth of Perovskite Films with Submillimeter-Sized Grains

Single crystalline structures with less grain boundaries and longer carrier diffusion length are essential for high performance perovskite solar cells. In this paper, we report a molten salt-assisted method to gain a perovskite film with submillimeter-sized single crystal domains by using excess ammonium salt for the first time. The resulted crystal size can reach 0.5 mm, and an aspect ratio over 1000. A photovoltaic device based on this film exhibits a champion power conversion efficiency (PCE) of 10.12%. This synthetic strategy enables the formation of submillimeter-sized perovskite films first, which holds promise for the understanding of crystallization in organic–inorganic perovskite and paves the way to enhance the efficiency of perovskite solar cells.

Elucidating the effect of the lead iodide complexation degree behind the morphology and performance of perovskite solar cells.

The inclusion of iodide additives in hybrid perovskite precursor solutions has been successfully exploited to improve the solar cell efficiency but their impact on perovskite formation, morphology and photovoltaic performance is still not clear. Here an extensive analysis of the effect of iodide additives in the solution-phase and during the perovskite film formation, as well as their effect on device performance is provided. The results demonstrate that in the solution-phase the additives promote the formation of lead poly-iodide species resulting in the disaggregation of the inorganic lead iodide framework and in the formation of smaller nuclei inducing the growth of uniform and smooth perovskite films. Most importantly, the complexation capability of different iodide additives does not only directly affect film morphology but also influences the density of defect states by varying the stoichiometry of precursors. These findings demonstrate that the fine control of the interactions of the chemical species in the solution-phase is essential for the precise control of the morphology at the nanoscale and the growth of the perovskite films with a reduced density of defect states. Therefore, the in-depth understanding of all the processes involved in the solution-phase is the first step for the development of a facile and reproducible approach for the fabrication of hybrid perovskite solar cells with enhanced photovoltaic performance.

Large grain growth for hole-conductor-free fully printable perovskite solar cells via polyoxometalate molecular doping.

A large-grained perovskite film, with a crystal grain size of ca. 30-50 μm, was achieved by polyoxometalate-induced Ostwald ripening. Aiming at commercially-oriented fully printable hole-conductor-free perovskite solar cells, the power conversion efficiency of the device was significantly improved from 9.17% to 11.35% (average) via polyoxometalate molecular doping. This is because the large-grained perovskite film could effectively reduce bulk defects and charge recombination, thereby facilitating charge transport.

Highly Efficient and Stable Perovskite Solar Cells Based on Monolithically Grained CH3NH3PbI3 Film

The synthesis and growth of perovskite films with controlled crystallinity and microstructure for highly efficient and stable solar cells is a critical issue. In this work, thiourea is introduced into the CH3NH3PbI3 precursor with two‐step sequential ethyl acetate (EA) interfacial processing. This is shown for the first time to grow compact microsized and monolithically grained perovskite films. X‐ray diffraction patterns and infrared spectroscopy are used to prove that thiourea significantly impacts the perovskite crystallinity and morphology by forming the intermediate phase MAI·PbI2·SC(NH2)2. Afterward, the residual thiourea which coursed charge recombination is completely extracted by the sequential EA processing. The product has improved light harvesting, suppressed defect state, and enhanced charge separation and transport. The sequentially EA processed perovskite solar cells offer an impressive 18.46% power conversion efficiency and excellent stability in ambient air. More importantly, the EA postprocessed perovskite solar cells also have excellent voltage response under ultraweak light (0.05% sun) with promising utility in photodetectors and photoelectric sensors.

Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%

The past several years have witnessed the rapid emergence of a class of solar cells based on mixed organic–inorganic halide perovskites. Today’s state-of-the-art perovskite solar cells (PSCs) employ various methods to enhance nucleation and improve the smoothness of the perovskite films formed via solution processing. However, the lack of precise control over the crystallization process creates a risk of forming unwanted defects, for example, pinholes and grain boundaries. Here, we introduce an approach to prepare perovskite films of high electronic quality by using poly(methyl methacrylate) (PMMA) as a template to control nucleation and crystal growth. We obtain shiny smooth perovskite films of excellent electronic quality, as manifested by a remarkably long photoluminescence lifetime. We realize stable PSCs with excellent reproducibility showing a power conversion efficiency (PCE) of up to 21.6% and a certified PCE of 21.02% under standard AM 1.5G reporting conditions. Controlling the crystallization process of perovskite films is crucial to obtaining high efficiency in perovskite solar cells. Bi et al. propose the use of poly(methyl methacrylate) as a template for the controlled nucleation and growth of perovskite crystals achieving efficiency of 21.6%.

Growth Instabilities as a Source of Surface Chemical Structuration in Functional Perovskite Thin Films

The spontaneous formation of double chemical terminated surfaces of La0.7Sr0.3MnO3 due to deviations from ideal epitaxial growth during its deposition on SrTiO3(001) is presented. The development of surface regions with differentiated topmost composition leads to outstanding surface nanostructuration presenting notably distinct local properties, particularly in terms of electric transport and local work function. Correlation between combined atomic force microscopy and X-ray photoelectron spectroscopy data allows the chemical identification of the two terminations as corresponding to La0.7Sr0.3O and MnO2. The most likely atomistic process at the origin of the surface organization has been interpreted by Monte Carlo simulations that reveal the importance of stacking fault formation as growth proceeds. We present a physical understanding of the experimental results for surfaces in which different electronic responses coexist. The conclusions of the present work can be extended to the growth of other perovskite thin films, therefore broadening the field of functional materials nanostructuration.

Highly Efficient p-i-n Perovskite Solar Cells Utilizing Novel Low-Temperature Solution-Processed Hole Transport Materials with Linear π-Conjugated Structure.

Alternative low-temperature solution-processed hole-transporting materials (HTMs) without dopant are critical for highly efficient perovskite solar cells (PSCs). Here, two novel small molecule HTMs with linear π-conjugated structure, 4,4'-bis(4-(di-p-toyl)aminostyryl)biphenyl (TPASBP) and 1,4'-bis(4-(di-p-toyl)aminostyryl)benzene (TPASB), are applied as hole-transporting layer (HTL) by low-temperature (sub-100 °C) solution-processed method in p-i-n PSCs. Compared with standard poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) HTL, both TPASBP and TPASB HTLs can promote the growth of perovskite (CH3 NH3 PbI3 ) film consisting of large grains and less grain boundaries. Furthermore, the hole extraction at HTL/CH3 NH3 PbI3 interface and the hole transport in HTL are also more efficient under the conditions of using TPASBP or TPASB as HTL. Hence, the photovoltaic performance of the PSCs is dramatically enhanced, leading to the high efficiencies of 17.4% and 17.6% for the PSCs using TPASBP and TPASB as HTL, respectively, which are ≈40% higher than that of the standard PSC using PEDOT:PSS HTL.

Controlled growth of textured perovskite films towards high performance solar cells

Antisolvent precipitation method has been one of the favored strategies to fabricate compact, smooth and uniform perovskite films for high efficiency solar cells due to its dramatically accelerated crystallization process. However, the excessively fast crystallization restricted the further improvement of the photovoltaic performance. In this work, we introduced CH3NH3Cl into the pristine CH3NH3PbI3 precursor for antisolvent precipitation at low temperature and fabricated high quality perovskite films with desired morphology, crystallinity and optical properties. The X-ray diffractometry and ultraviolent-visible spectroscopy provided ample evidence that CH3NH3Cl exerted significant impacts on the perovskite crystallization process by controlling the delivery speed of PbI2 from the intermediate phase CH3NH3PbI2Cl. The possible reactions in the perovskite formation process were first elaborated. The resultant solar cells demonstrated an average power conversion efficiency around 16.63% and a best efficiency at 17.22% under the standard light illumination condition. In addition, the ion migration was first detected in the perovskite solar cells by an ordinary impedance measurement.