Growth Of Perovskite Thin Films Research Articles

Improvement of X-ray Photoelectric Conversion Performance of MAPbI3 Perovskite Crystals by Ionic Liquid Treatment

Although perovskite has great potential in optoelectronic devices, the simultaneous satisfaction of material stability and high performance is still an issue that needs to be solved. Most perovskite optoelectronic devices use quantum dot spin coating or the gas-phase growth of perovskite thin films as the photoelectric conversion layer. Due to stability limitations, these materials often experience a significant decrease in photoelectric conversion efficiency when encountering liquid reagents. The self-assembled growth of hybrid perovskite crystals determines superior lattice ordering and stability. There are three types of ionic liquids—[Emim]BF4, EMIMNTF2, and HMITFSI—that can effectively enhance the X-ray photoelectric conversion performance of hybrid perovskite crystal CH3NH3PbI3 (MAPbI3), and the enhancement in the photocurrent leads to an improvement in the sensitivity of X-ray detectors. We soak the perovskite crystals in an ionic liquid and perform two treatment methods: electrification and dilution with ETOH solution. It is interesting to find that MAPbI3 perovskite single crystal materials choose the same optimized ionic liquid species in X-ray detection and photovoltaic power generation applications, and the effect is quite the opposite. Compared with untreated MAPbI3 crystals, the average photocurrent density of Electrify-HMITFSI MAPbI3 increased by 826.85% under X-ray excitation and the sensitivity of X-ray detectors made from these treated MAPbI3 crystals significantly increased by 72.6%, but the intensity of the PL spectrum decreased to 90% of the untreated intensity.

Insights into the Growth Orientation and Phase Stability of Chemical-Vapor-Deposited Two-Dimensional Hybrid Halide Perovskite Films.

Chemical vapor deposition (CVD) offers a large-area, scalable, and conformal growth of perovskite thin films without the use of solvents. Low-dimensional organic-inorganic halide perovskites, with alternating layers of organic spacer groups and inorganic perovskite layers, are promising for enhancing the stability of optoelectronic devices. Moreover, their multiple quantum-well structures provide a powerful platform for tuning excitonic physics. In this work, we show that the CVD process is conducive to the growth of 2D hybrid halide perovskite films. Using butylammonium (BA) and phenylethylammonium (PEA) cations, the growth parameters of BA2PbI4 and PEA2PbI4 and mixed halide perovskite films were first optimized. These films are characterized by well-defined grain boundaries and display characteristic absorption and emission features of the 2D quantum wells. X-ray diffraction (XRD) and a noninteger dimensionality model of the absorption spectrum provide insights into the orientation of the crystalline planes. Unlike BA2PbI4, temperature-dependent photoluminescence measurements from PEA2PbI4 show a single excitonic peak throughout the temperature range from 20 to 350 K, highlighting the lack of defect states. These results further corroborate the temperature-dependent synchrotron-based XRD results. Furthermore, the nonlinear optical properties of the CVD-grown perovskite films are investigated, and a high third harmonic generation efficiency is observed.

Two-step perovskite crystallization assisted with F-containing additives enables over 23% efficiency in perovskite solar cells

The two-step preparation of perovskite solar cells (PSCs) has attracted the interest of many researchers because this method can better control the nucleation and growth of perovskite thin films. This work proposes a crystallization management strategy to improve the quality of active layer with the assistant of F-containing additives with low boiling point. Hexafluorobenzene (HFB) is introduced into lead iodide (PbI2) precursor solutions, and HFB aggregation forms due to its low surface energies. As the result, nano porous PbI2 films are obtained after the evaporation of HFB during annealing process, leading to pinhole free perovskite films with large grain size and low defect states. As the consequence, the best power conversion efficiency (PCE) of 23.01% is obtained for the PSCs with HFB assisted perovskite films. Meanwhile HFB modified unencapsulated device maintains 88% of its initial PCE after aging for over 1440 h at 25 °C and 30% relative humidity in ambient conditions.

Additive Engineering for Efficient and Stable Perovskite Solar Cells

AbstractPerovskite solar cells (PSCs) have reached a certified 25.2% efficiency in 2019 due to their high absorption coefficient, high carrier mobility, long diffusion length, and tunable direct bandgap. However, due to the nature of solution processing and rapid crystal growth of perovskite thin films, a variety of defects can form as a result of the precursor compositions and processing conditions. The use of additives can affect perovskite crystallization and film formation, defect passivation in the bulk and/or at the surface, as well as influence the interface tuning of structure and energetics. Here, recent progress in additive engineering during perovskite film formation is discussed according to the following common categories: Lewis acid (e.g., metal cations, fullerene derivatives), Lewis base based on the donor type (e.g., O‐donor, S‐donor, and N‐donor), ammonium salts, low‐dimensional perovskites, and ionic liquid. Various additive‐assisted strategies for interface optimization are then summarized; additives include modifiers to improve electron‐ and hole‐transport layers as well as those to modify perovskite surface properties. Finally, an outlook is provided on research trends with respect to additive engineering in PSC development.

Rapid crystallization and controllable growth of perovskite thin films via a seeded approach

Organic–inorganic hybrid perovskite solar cells (PVSCs) have attracted extensive attention due to high efficiency, easy fabrication, and low-cost solution processes. One of the keys to achieve high-performance cost-effective PVSCs is to attain rapid crystallization with controlled morphology of the perovskite films. Herein, the authors report a technique for the rapid crystallization of perovskite with tunable crystal grain size and morphology via a seeded approach. Specifically, a solution of lead iodide (PbI2) was spin coated on a substrate, and a low-concentration solution of methylammonium iodide (MAI) was dropped onto the PbI2 film to form perovskite seeds prior to introducing high-concentration solution of MAI. The seeded nucleation and growth lead to dense and uniform perovskite thin films with controllable crystal grains. This seeded crystallization technique offers an effective way to boost the low-cost manufacture of efficient and reproducible PVSCs.

Effects of Organic Cation Additives on the Fast Growth of Perovskite Thin Films for Efficient Planar Heterojunction Solar Cells.

The film quality of organometallic halide perovskite is very crucial to the performance of planar heterojunction solar cells. Previous methods have generally required a long-time and complex process to control the crystal growth for obtaining a compact and smooth perovskite film. Here, we demonstrate a novel method of growing the high-quality films via a simple and rapid process. Organic cations are used as additives in the solution of CH3NH3PbI3 (MAPbI3), which plays a key role in the film-forming process. These organic cations can enhance the film-forming ability and do not lead to a residue in the film end-product. On the basis of these characteristics, highly efficient perovskite solar cells with a simple planar heterojunction structure were achieved. Because the additives can simplify and accelerate the fabrication process and have no chance to induce any negative effect on the device, they will have a large potential in the production of various high-quality perovskite films and low-cost, large-scale, and high-performance devices.

Effect of Polar (111)-Oriented SrTiO3 on Initial Perovskite Growth

In crystalline thin film growth a prerequisite is substrate surfaces with a stable and uniform structure and chemical composition. Various substrate treatments were used to obtain atomically smooth, step-and-terrace (111)-oriented SrTiO3 with uniform cation layers at the surface, i.e., single termination. The surface control enables subsequent layer-by-layer epitaxial growth of perovskite thin films of La0.7Sr0.3MnO3, LaFeO3, and BaTiO3. Reflection high-energy electron diffraction and electron energy loss spectroscopy revealed that a single chemically intermixed (A,A′)BO3 perovskite layer formed at the interface. As the terminating layer of (111) SrTiO3 is polar, a surface reconstruction consisting of TiOx surface layers is expected, and the intermixing at the interface can be understood as A′-cations from the film material compensating an A-cation deficient substrate surface during initial growth. This finding has important consequences for engineered interfaces between perovskite thin films and polar su...

Tailoring the room temperature ferroelectric/paraelectric state in polycrystalline (Ba0.70Sr0.30)TiO3 thin films for silicon compatible integration

Polycrystalline (Ba0.70Sr0.30)TiO3 (BST) thin films were prepared by ion beam sputtering and postdeposition annealing method, and their crystallization kinetics was monitored in situ. It was found that 550°C is a critical temperature for the BST׳s perovskite phase formation, below which a long annealing time is needed to obtain good perovskite structure because of a slow crystallization process, while generating much lower inhomogeneous and thermoelastic strains. As a consequence, the ferroelectric/paraelectric state formation in BST films, being strain sensitive, was purposefully tailored through simple kinetics instead of pure thermodynamics. A long time annealing at 530°C induced a room temperature (RT) ferroelectric state, whereas the annealing at 550°C and above promoted much faster perovskite crystallization and tailored the film at RT in its paraelectric state, thereby giving rise to two distinct dielectric behaviors. These findings provide important implications for simply controllable state growth of perovskite thin films with desired electrical properties so as to meet their respective requirements in different application fields.

Templated microstructural growth of perovskite thin films via colloidal monolayer lithography

Highly ordered perovskite domains within metal oxide honeycomb structures for optoelectronic applications.

Properties of rare-earth scandate single crystals (Re=Nd−Dy)

Rare-earth (RE) scandate single crystals ranging from NdScO 3 to DyScO 3 were grown by the conventional Czochralski technique. The crystals were found to have a non-stoichiometric composition caused by RE-deficiency on the A-site of the distorted perovskite structure. Their pseudocubic lattice constants lie between 3.95 and 4.01 Å and their average thermal expansion coefficients are between 8.2×10 −6 and 10.9×10 −6 K −1. The pseudocubic lattice constants of the RE scandates vary in about 0.01 Å steps as the RE series is traversed, enabling the growth of substrates appropriate for the strain engineering of perovskite thin films. The crystalline perfection and homogeneity of the DyScO 3 and GdScO 3 single crystals are characterized by a full-width at half-maximum of the 220 reflection of the substrates of about 20–25 arcsec in ω and a very small fluctuation margin of about ±3 arcsec over the whole wafer surface. The average dislocation density of the RE scandate crystals was 10 3–10 5 cm −2. These characteristics make RE scandate single crystals some of the best available substrates for the epitaxial growth of perovskite thin films.

Reflection high-energy electron diffraction and atomic force microscopy studies on homoepitaxial growth of SrTiO 3(001)

This paper reports a systematic study on the homoepitaxial growth of SrTiO 3(001) conducted using reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) in order to improve our understanding of the basic processes in the epitaxial growth of perovskite thin films. Under certain growth conditions, the homoepitaxial growth of SrTiO 3 essentially proceeds in the layer-by-layer (2D nucleation) growth mode with the basic unit of a molecular layer. This cyclic process due to unit-cell-by-unit-cell (uc-by-uc) growth is confirmed by undamped RHEED oscillations and the corresponding temporal evolution of AFM images of the growth front. Furthermore, the surface mobility is controlled by growth temperature and oxidation condition. By changing growth temperature or oxidation condition, crossovers from layer-by-layer growth to step-flow like growth and from layer-by-layer growth to 3D island growth through Stranski–Krastanov growth are observed. On the basis of these results, a universal phase diagram is proposed for the growth mode of the epitaxial film growth of complex oxides.