Metal Halide Perovskite Thin Films Research Articles

Thermally Evaporated Metal Halide Perovskites and Their Analogues: Film Fabrication, Applications and Beyond.

Metal halide perovskites emerge as promising semiconductors for optoelectronic devices due to ease of fabrication, attractive photophysical properties, their low cost, highly tunable material properties, and high performance. High-quality thin films of metal halide perovskites are the basis of most of these applications including solar cells, light-emitting diodes, photodetectors, and electronic memristors. A typical fabrication method for perovskite thin films is the solution method, which has several limitations in device reproducibility, adverse environmental impact, and utilization of raw materials. Thermal evaporation holds great promise in addressing these bottlenecks in fabricating high-quality halide perovskite thin films. It also has high compatibility with mass-production platforms that are well-established in industries. This review first introduces the basics of the thermal evaporation method with a particular focus on the critical parameters influencing the thin film deposition. The research progress of the fabrication of metal halide perovskite thin films is further summarized by different thermal evaporation approaches and their applications in solar cells and other optoelectronic devices. Finally, research challenges and future opportunities for both fundamental research and commercialization are discussed.

Patterning of Hybrid Metal Halide Perovskite via Printed Molecular Templates

AbstractPatterning metal halide perovskite thin films is a necessary pathway to expand their optoelectronic applications. However, developing a suitable patterning technique is challenging as conventional lithographic processes are either detrimental to the perovskite layer or lack compatibility with scalable fabrication processes used in the manufacturing of printed electronics. In this work, a complete bottom‐up patterning method is proposed based on de‐wetting from a hydrophobic molecular template, which constrains the perovskite ink to directly crystallize in selected hydrophilic regions. Octadecylphosphonic acid molecules are used as the hydrophobic template and are effectively transferred onto metal‐oxide substrates via micro‐contact printing. This allows for a low‐cost, rapid, and scalable micron‐scale patterning of perovskite thin films. Facile control over light transmittance is demonstrated by adjusting the amount of exposed area and the corresponding spectral shape changes shift the film's appearance toward being color‐neutral. Finally, the method's scalability and versatility are shown by fabricating 25 cm2 patterned films over rigid and flexible substrates.

Defect‐Induced Rashba Effect in Metal Halide Perovskite Thin Films Deposited on a Flexible Substrate

AbstractMetal halide perovskites (MHPs) exhibit strong spin‐orbit coupling and structural inversion asymmetry, making them ideal for testbeds of Rashba effects and for use in spin‐related optoelectronic and spintronic applications. However, the Rashba band splitting is mainly observed in bulk MHPs so far and is rarely observed in thin films. In this study, a direct observation of Rashba band splitting in MHP thin films at room temperature is reported. By employing a silicone rubber substrate with a high thermal expansion coefficient, the out‐of‐plane tensile strain significantly reduces the activation energy of ion migration in these thin films and leads to a considerable amount of point defects. These point defects disrupt the inversion symmetry and ultimately give rise to the Rashba band splitting. These findings open up new possibilities for manipulating Rashba effects in MHP thin films through device engineering.

Crystallization Kinetics of Hybrid Perovskite Solar Cells

AbstractMetal halide perovskites (MHPs) are considered ideal photovoltaic materials due to their variable crystal material composition and excellent photoelectric properties. However, this variability in composition leads to complex crystallization processes in the manufacturing of Metal halide perovskite (MHP) thin films, resulting in reduced crystallinity and subsequent performance loss in the final device. Thus, understanding and controlling the crystallization dynamics of perovskite materials are essential for improving the stability and performance of PSCs (Perovskite Solar Cells). To investigate the impact of crystallization characteristics on the properties of MHP films and identify corresponding modulation strategies, we primarily discuss the relevant aspects of MHP crystallization kinetics, systematically summarize theoretical methods, and outline modulation techniques for MHP crystallization, including solution engineering, additive engineering, and component engineering, which helps highlight the prospects and current challenges in perovskite crystallization kinetics

Crystallization Kinetics of Hybrid Perovskite Solar Cells.

Metal halide perovskites (MHPs) are considered ideal photovoltaic materials due to their variable crystal material composition and excellent photoelectric properties. However, this variability in composition leads to complex crystallization processes in the manufacturing of Metal halide perovskite (MHP) thin films, resulting in reduced crystallinity and subsequent performance loss in the final device. Thus, understanding and controlling the crystallization dynamics of perovskite materials are essential for improving the stability and performance of PSCs (Perovskite Solar Cells). To investigate the impact of crystallization characteristics on the properties of MHP films and identify corresponding modulation strategies, we primarily discuss the relevant aspects of MHP crystallization kinetics, systematically summarize theoretical methods, and outline modulation techniques for MHP crystallization, including solution engineering, additive engineering, and component engineering, which helps highlight the prospects and current challenges in perovskite crystallization kinetics.

Revealing Unusual Bandgap Shifts with Temperature and Bandgap Renormalization Effect in Phase‐Stabilized Metal Halide Perovskite Thin Films

AbstractHybrid organic–inorganic metal halide perovskites are emerging materials in photovoltaics, whose bandgap is one of the most crucial parameters governing their light‐harvesting performance. This work presents the temperature and photocarrier density dependence of the bandgap in two phase‐stabilized perovskite thin films (MA0.3FA0.7PbI3 and MA0.3FA0.7Pb0.5Sn0.5I3) using photoluminescence and absorption spectroscopy. Contrasting bandgap shifts with temperature are observed between the two perovskites. Using X‐ray diffraction and in situ high‐pressure photoluminescence spectroscopy, it is shown that thermal expansion plays only a minor role in the large bandgap blueshift, which is attributed to the enhanced structural stability of the samples. The first‐principles calculations further demonstrate the significant impact of thermally induced lattice distortions on the bandgap widening. It is proposed that the anomalous trends are caused by the competition between static and dynamic distortions. Additionally, both the bandgap renormalization and band‐filling effects are directly observed for the first time in fluence‐dependent photoluminescence measurements and are employed to estimate the exciton effective mass. The results provide new insights into the basic understanding of thermal and charge‐accumulation effects on the band structure of hybrid perovskite thin films.

Plasma‐Driven Atomic‐Scale Tuning of Metal Halide Perovskite Surfaces: Rationale and Photovoltaic Application

The effective defect passivation of metal halide perovskite (MHP) surfaces is a key strategy to simultaneously tackle MHP solar cell performances enhancement and their stability under operative conditions. Plasma‐based dry processing is an established methodology for the modification of materials surfaces as it does not present the disadvantages often associated with wet treatments. This is becoming a fine tool to reach precise atomic‐scale engineering of the MHP surfaces. Herein is reported a comprehensive picture of the interaction between different plasma chemistries and MHP thin films. The impact of Ar, H2, N2, and O2 low‐pressure plasmas on MHP optochemical properties and morphology is correlated with the performance of photovoltaic devices and rationalized by density functional theory calculations. Strong morphological modifications and selective removal of the uppermost methylammonium moieties are deemed responsible for nonradiative surface defects suppression and higher solar cell performances. Ellipsometry and X‐ray photoelectron spectroscopies shine light on the subtle modifications induced by the different plasma environments, paving the way for the more effective engineering of plasma‐based (deposition) processing. Notably, for O2 plasma treatment, deep‐state traps induced by the formation of IO4− species are demonstrated and rationalized, highlighting the challenges in optimizing O2 plasma‐based solutions for MHP‐based devices.

The Role of SnO2 Processing on Ionic Distribution in Double-Cation-Double Halide Perovskites.

Moving toward a future of efficient, accessible, and less carbon-reliant energy devices has been at the forefront of energy research innovations for the past 30 years. Metal-halide perovskite (MHP) thin films have gained significant attention due to their flexibility of device applications and tunable capabilities for improving power conversion efficiency. Serving as a gateway to optimize device performance, consideration must be given to chemical synthesis processing techniques. Therefore, how does common substrate processing techniques influence the behavior of MHP phenomena such as ion migration and strain? Here, we demonstrate how a hybrid approach of chemical bath deposition (CBD) and nanoparticle SnO2 substrate processing significantly improves the performance of (FAPbI3)0.97(MAPbBr3)0.03 by reducing micro-strain in the SnO2 lattice, allowing distribution of K+ from K-Cl treatment of substrates to passivate defects formed at the interface and produce higher current in light and dark environments. X-ray diffraction reveals differences in lattice strain behavior with respect to SnO2 substrate processing methods. Through use of conductive atomic force microscopy (c-AFM), conductivity is measured spatially with MHP morphology, showing higher generation of current in both light and dark conditions for films with hybrid processing. Additionally, time-of-flight secondary ionization mass spectrometry (ToF-SIMS) observed the distribution of K+ at the perovskite/SnO2 interface, indicating K+ passivation of defects to improve the power conversion efficiency (PCE) and device stability. We show how understanding the role of ion distribution at the SnO2 and perovskite interface can help reduce the creating of defects and promote a more efficient MHP device.

How to GIWAXS: Grazing Incidence Wide Angle X‐Ray Scattering Applied to Metal Halide Perovskite Thin Films

AbstractThe frequency of reports utilizing synchrotron‐based grazing incident wide angle X‐ray scattering (GIWAXS) to study metal halide perovskite thin films has exploded recently, as this technique has proven invaluable for understanding several structure‐property relationships that fundamentally limit optoelectronic performance. The GIWAXS geometry and temporal resolution are also inherently compatible with in situ and operando setups (including ISOS protocols), and a relatively large halide perovskite research community has deployed GIWAXS to unravel important kinetic and dynamic features in these materials. Considering its rising popularity, the aim here is to accelerate the required learning curve for new experimentalists by clearly detailing the underlying analytical concepts which can be leveraged to maximize GIWAXS studies of polycrystalline thin films and devices. Motivated by the vast range of measurement conditions offered, together with the wide variety of compositions and structural motifs available (i.e., from single‐crystal and polycrystalline systems, to quantum dots and layered superlatices), a comprehensive framework for conducting effective GIWAXS experiments is outlined for different purposes. It is anticipated that providing a clear perspective for this topic will help elevate the quality of future GIWAXS studies—which have become routine—and provide the impetus required to develop novel GIWAXS approaches to resolve unsettled scientific questions.

Epitaxial inorganic metal-halide perovskite films with controlled surface terminations

Metal-halide perovskite (MHP) thin films for next-generation solar cells are typically fabricated by wet-chemical synthesis in which surface properties such as the orientation of surface facets and their termination cannot be controlled. MHP device efficiencies depend critically on those surface properties. We demonstrate the epitaxial growth of several nanometer thick purely (001)-orientated films of ${\mathrm{CsPbBr}}_{3}$ and ${\mathrm{CsSnBr}}_{3}$ MHPs on Au(001) by molecular beam epitaxy. The epitaxial films are in a cubic phase aligned with the substrate. Their surfaces are singly terminated and can be modified for the first time. While the CsBr and ${\mathrm{PbBr}}_{2}$ surface terminations differ in terms of their atomic structure and defect content, the films are intrinsic semiconductors irrespective of the termination. The work function of the ${\mathrm{PbBr}}_{2}$-terminated surface is increased by $0.7\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, which has drastic implications for the level alignment at MHP interfaces.

Incorporation of functional polymers into metal halide perovskite thin-films: from interactions in solution to crystallization

Incorporation of polymers represents a viable new strategy to solve the major challenges related to metal halide perovskite materials in view of future commercialization.

(Digital Presentation) Preparation of Metal Halide Perovskite Thin Films Via Laser Deposition

We have been preparing metal halide perovskite (hereinafter referred to as perovskite) thin films by laser deposition. In this study, we report the results of our investigation of the effects of laser wavelength and fluence on the deposition process. One of the most important features of perovskites, which have been rapidly developed in recent years as materials for high-efficiency and low-cost thin-film solar cells, is the applicability of a simple process using solution coating under atmospheric pressure [1]. Crystal growth of perovskite by the solution method occurs through equilibrium chemical reactions, enabling the formation of thin films on the order of μm thickness in less than a few seconds; however, it is difficult to systematically control the crystal growth process at the molecular layer level by the solution method. On the other hand, in the physical vapor deposition (PVD) method, although the deposition rate is relatively low, ranging from sub-nanometer to several nm/s, the deposition can be controlled at the molecular layer level, as well as allowing different atoms to be readily introduced into the film. Therefore, the PVD method allows the addition of elements that are difficult to achieve with solution reactions and is suitable for studies that investigate the effects of such elements on physical properties.In laser deposition (LD), one of the PVD methods, materials are irradiated with an infrared (IR) or an ultraviolet (UV) laser beam, and thin films are formed by evaporation through IR heating or ablation with plasma formation through UV electron excitation. The advantage of LD is that it can cause instantaneous heating or ablation by switching on and off laser irradiation only on the material surface, thus enabling the deposition of molecular layers of organic materials with high vapor pressure or high melting point materials such as inorganic oxides with minimal cross-contamination caused by evaporation. We have prepared perovskite thin films by alternate deposition of PbI2 and CH3NH3I (MAI) using an IR laser and solid-phase reaction at room temperature aiming at the detailed investigation of the crystal growth process and introduction of various dopants in perovskite materials. [2, 3]. In this study, we have also performed perovskite film deposition by UV pulsed laser and investigated the difference in the physical properties of the perovskite films compared to those deposited by IR laser.Fig. 1 shows a schematic structure and appearance of the laser deposition system used in this experiment. The experimental method is described below. A synthetic quartz substrate was introduced into an ultra-high vacuum chamber with a base vacuum of 2×10-5 Pa., Then a semiconductor continuous IR laser (wavelength: 808 nm) or a UV pulse laser beam generated by the fourth harmonic of an Nd:YAG Q-Switch laser (wavelength: 266 nm, 10 Hz) was irradiated alternately onto PbI2 and MAI targets to form CH3NH3PbI3.The optical band gap of the thin films deposited by continuous infrared laser deposition shows a change in optical band gap depending on the thickness of the alternately deposited PbI2 and MAI, which can be attributed to the different content ratios of the tetragonal and orthorhombic phases. Interestingly, in UV pulsed laser deposition, the deposition rate was about 0.35 nm/s under the high laser fluence condition with lens focusing, while a high deposition rate of 20 nm/s was obtained under the low laser fluence condition without lens focusing. This result suggests that the excess kinetic energy of the deposition precursor in the case of lens-focused deposition causes re-sputtering on the surface of the already deposited film, resulting in a reduction of the effective deposition rate.In the presentation, we will demonstrate and discuss the results of crystallinity and optical absorption properties.[1] T. Miyasaka, A. Kulkarni, G. M. Kim, S. Öz and A. K. Jena, Adv. Energy Mater. 2020, 11902500 (2020).[2] K. Kawashima, Y. Okamoto, O. Annayev, N. Toyokura, R. Takahashi, M. Lippmaa, K. Itaka, Y. Suzuki, N. Matsuki and H. Koinuma, Science and Technology of Advanced Materials 18, 307 (2017).[3] N. Matsuki, Y. Iida, T. Shimada, Yuta Abeand T. Sato, ECS Meeting abst. MA2020-02, 1860 (2020). Figure 1

Van der Waals Metal Contacts for Characterization and Optoelectronic Application of Metal Halide Perovskite Thin Films

The metal contacts on metal halide perovskite thin films are often formed through physical vapor deposition (PVD) processes for investigation of the film properties or construction of optoelectronic devices. However, the PVD processes generate high-energy metal atoms, directly bombarding the film surface, potentially causing unintended damage in the film. In this study, we performed systematic investigation on the impact of a PVD-processed metal contact on the optoelectronic properties of underlying organic–inorganic hybrid perovskite thin films. We adopted a physically laminated van der Waals metal contact for comparison to enable quantitative analysis. Through space-charge-limited current measurement, we demonstrated the defect density increases by 26–48% on average after formation of the metal contact by the PVD process. In-situ photoluminescence measurements unraveled that the generated defects easily migrate under the electric field to seriously deteriorate the performance and stability of photodetectors. This study highlights the importance of the intact junction between the perovskite and metal contacts for characterization and optoelectronic application of perovskite thin films.

Long-range charge carrier mobility in metal halide perovskite thin-films and single crystals via transient photo-conductivity

Charge carrier mobility is a fundamental property of semiconductor materials that governs many electronic device characteristics. For metal halide perovskites, a wide range of charge carrier mobilities have been reported using different techniques. Mobilities are often estimated via transient methods assuming an initial charge carrier population after pulsed photoexcitation and measurement of photoconductivity via non-contact or contact techniques. For nanosecond to millisecond transient methods, early-time recombination and exciton-to-free-carrier ratio hinder accurate determination of free-carrier population after photoexcitation. By considering both effects, we estimate long-range charge carrier mobilities over a wide range of photoexcitation densities via transient photoconductivity measurements. We determine long-range mobilities for FA0.83Cs0.17Pb(I0.9Br0.1)3, (FA0.83MA0.17)0.95Cs0.05Pb(I0.9Br0.1)3 and CH3NH3PbI3-xClx polycrystalline films in the range of 0.3 to 6.7 cm2 V−1 s−1. We demonstrate how our data-processing technique can also reveal more precise mobility estimates from non-contact time-resolved microwave conductivity measurements. Importantly, our results indicate that the processing of polycrystalline films significantly affects their long-range mobility.

Excellent Long‐Range Charge‐Carrier Mobility in 2D Perovskites

AbstractThe use of layered, 2D perovskites can improve the stability of metal halide perovskite thin films and devices. However, the charge carrier transport properties in layered perovskites are still not fully understood. Here, the sum of the electron and hole mobilities (Σμ) in thin films of the 2D perovskite PEA2PbI4, through transient electronically contacted nanosecond‐to‐millisecond photoconductivity measurements, which are sensitive to long‐time, long‐range (micrometer length scale) transport processes is investigated. After careful analysis, accounting for both early‐time recombination and the evolution of the exciton‐to‐free‐carrier population, a long‐range mobility of 8.0 +/− 0.6 cm2 (V s)–1, which is ten times greater than the long‐range mobility of a comparable 3D material FA0.9Cs0.1PbI3 is determined. These values are compared to ultra‐fast transient time‐resolved THz photoconductivity measurements, which are sensitive to early‐time, shorter‐range (tens of nm length scale) mobilities. Mobilities of 8 and 45 cm2 (V s)–1 in the case of the PEA2PbI4 and FA0.9Cs0.1PbI3, respectively, are obtained. This previously unreported concurrence between the long‐range and short‐range mobility in a 2D material indicates that the polycrystalline thin films already have single‐crystal‐like qualities. Hence, their fundamental charge carrier transport properties should aid device performance.

Solvent-Free Method for Defect Reduction and Improved Performance of p-i-n Vapor-Deposited Perovskite Solar Cells.

As perovskite-based photovoltaics near commercialization, it is imperative to develop industrial-scale defect-passivation techniques. Vapor deposition is a solvent-free fabrication technique that is widely implemented in industry and can be used to fabricate metal-halide perovskite thin films. We demonstrate markably improved growth and optoelectronic properties for vapor-deposited [CH(NH2)2]0.83Cs0.17PbI3 perovskite solar cells by partially substituting PbI2 for PbCl2 as the inorganic precursor. We find the partial substitution of PbI2 for PbCl2 enhances photoluminescence lifetimes from 5.6 ns to over 100 ns, photoluminescence quantum yields by more than an order of magnitude, and charge-carrier mobility from 46 cm2/(V s) to 56 cm2/(V s). This results in improved solar-cell power conversion efficiency, from 16.4% to 19.3% for the devices employing perovskite films deposited with 20% substitution of PbI2 for PbCl2. Our method presents a scalable, dry, and solvent-free route to reducing nonradiative recombination centers and hence improving the performance of vapor-deposited metal-halide perovskite solar cells.

Delineation and Passivation of Grain‐Boundary Channels in Metal Halide Perovskite Thin Films for Solar Cells

AbstractA simple, generic method for delineating grain‐boundary (GB) channels in metal halide perovskite (MHP) thin films for scanning electron microscopy (SEM) observation is demonstrated, thereby enabling more accurate grain‐size measurements. This entails dipping the film in dichloromethane (DCM), followed by a moderate heat‐treatment, resulting in the in situ hydrocarbon functionalization of the film surface. More importantly, the hydrocarbons appear to congregate preferentially at GB‐channels, providing enhanced secondary‐electron contrast in the SEM. The hydrocarbons also provide extra protection to the vulnerable GB‐channels against environmental attack, resulting in improved environmental stability of DCM‐treated films. Also, the resulting solar cells show improved efficiency and operational stability.

High-Throughput Study of Antisolvents on the Stability of Multicomponent Metal Halide Perovskites through Robotics-Based Synthesis and Machine Learning Approaches.

Antisolvent crystallization methods are frequently used to fabricate high-quality metal halide perovskite (MHP) thin films, to produce sizable single crystals, and to synthesize nanoparticles at room temperature. However, a systematic exploration of the effect of specific antisolvents on the intrinsic stability of multicomponent MHPs has yet to be demonstrated. Here, we develop a high-throughput experimental workflow that incorporates chemical robotic synthesis, automated characterization, and machine learning techniques to explore how the choice of antisolvent affects the intrinsic stability of binary MHP systems in ambient conditions over time. Different combinations of the end-members, MAPbI3, MAPbBr3, FAPbI3, FAPbBr3, CsPbI3, and CsPbBr3 (MA, methylammonium; FA+, formamidinium), are used to synthesize 15 combinatorial libraries, each with 96 unique combinations. In total, roughly 1100 different compositions are synthesized. Each library is fabricated twice by using two different antisolvents: toluene and chloroform. Once synthesized, photoluminescence spectroscopy is automatically performed every 5 min for approximately 6 h. Nonnegative matrix factorization (NMF) is then utilized to map the time- and compositional-dependent optoelectronic properties. Through the utilization of this workflow for each library, we demonstrate that the selection of antisolvent is critical to the intrinsic stability of MHPs in ambient conditions. We explore possible dynamical processes, such as halide segregation, responsible for either the stability or eventual degradation as caused by the choice of antisolvent. Overall, this high-throughput study demonstrates the vital role that antisolvents play in the synthesis of high-quality multicomponent MHP systems.

Use of Sodium Diethyldithiocarbamate to Enhance the Open‐Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells

The incorporation of additives to modulate the properties of metal halide perovskite thin films has become a successful approach in improving the power conversion efficiency of perovskite‐based solar cells. Herein, the beneficial use of sodium diethyldithiocarbamate (NaDEDTC) as processing agent in improving the open‐circuit voltage of methylammonium lead triiodide perovskite solar cells is reported. DEDTC reduces the rate of perovskite crystallization. Absorption and emission spectra show that the optical bandgap of the perovskite films remain essentially unchanged and X‐ray diffraction reveals the formation of preferentially oriented crystallites independent of the use of DEDTC. The use of DEDTC, however, results in a decrease in nonradiative decay as inferred from a two order of magnitude increase in electroluminescence efficiency, explaining the increased open‐circuit voltage. Fourier‐transform infrared spectroscopy, nuclear magnetic resonance, and X‐ray photoelectron spectroscopy show that the DEDTC ligand is not present after the film processing. Therefore, DEDTC modulates film formation but is not incorporated in the perovskite or present as a surface ligand. Sodium ions, on the contrary, are incorporated in the perovskite layer.

Potassium Thiocyanate‐Assisted Enhancement of Slot‐Die‐Coated Perovskite Films for High‐Performance Solar Cells

Slot‐die coating is highly promising for scaled deposition of metal halide perovskite thin films. However, the power conversion efficiencies (PCEs) of slot‐die‐prepared perovskite solar cells (PSCs) still lag behind their spin‐casted counterparts. To resolve this issue, the crystal size and quality of slot‐die‐coated methylammonium lead triiodide (MAPbI3) perovskite films are dramatically improved via additive engineering using potassium thiocyanate (KSCN). The modified micrometer‐thick films have an average grain size of ≈11 μm and charge‐carrier parameters that are comparable with single‐crystal perovskites, such as a 1.89 μs lifetime, 136.65 ± 31.52 cm2 V−1 s−1 mobility, and 25.15 ± 3.55 μm diffusion length. Exploiting these enhanced properties, planar inverted PSCs with negligible hysteresis are fabricated and an average and a maximum PCE of 20.14% and 21.38%, respectively, are achieved which are among the highest reported values for slot‐die‐coated PSCs. Notably, our devices have a narrow PCE distribution along the slot‐die coating axis, highlighting slot‐die coating's promise to fabricate large‐scale, high‐performance PSCs.