Progress in Theoretical Study of Metal Halide Perovskite Solar Cell Materials

Lead halide perovskites have recently emerged as promising absorbers for fabricating low‐cost and high‐efficiency thin‐film solar cells. The record power conversion efficiency of lead halide perovskite‐based solar cells has rapidly increased from 3.8% in 2009 to 22.1% in early 2016. Such rapid improvement is attributed to the superior and unique photovoltaic properties of lead halide perovskites, such as the extremely high optical absorption coefficients and super‐long photogenerated carrier lifetimes and diffusion lengths that are not seen in any other polycrystalline thin‐film solar cell materials. In the past a few years, theoretical approaches have been extensively applied to understand the fundamental mechanisms responsible for the superior photovoltaic properties of lead halide perovskites and have gained significant insights. This review article highlights the important theoretical results reported in literature for the understanding of the unique structural, electronic, optical, and defect properties of lead halide perovskite materials. For comparison, we also review the theoretical results reported in literature for some lead‐free perovskites, double perovskites, and nonperovskites.     

From Pb to Bi: A Promising Family of Pb‐Free Optoelectronic Materials and Devices

Lead‐based organic–inorganic hybrid perovskite materials are widely used in optoelectronic devices due to their excellent photophysical properties. However, the main issues which hinder its commercialization are the toxicity caused by lead and the intrinsic instability of the material. Recently, many lead‐free halide materials with good intrinsic stability have been reported, among which bismuth‐based halide materials have attracted extensive research due to their structure and promising optoelectronic properties. In this review, bismuth‐based materials are divided into binary BiX₃ (X = I, Br, Cl), ternary A_aBi_bX_a+3b (A = Cs, Rb, MA, Ag, etc.), and quaternary A₂AgBiX₆ (A = Cs, Rb, MA, etc.) according to its elemental composition. The structure and optoelectronic properties of bismuth‐based halide materials, which may be helpful for the development of bismuth‐based halide materials and lead‐free perovskites in the future, are summarized and highlighted.     

A Review and Perspective on Cathodoluminescence Analysis of Halide Perovskites

Halide perovskite solar cells have achieved a certified efficiency of 25.2%, surpassing CdTe and CuInGaSe₂, which have long been regarded as the most‐efficient thin‐film photovoltaic materials. As this exciting class of materials continues to mature, researchers will require characterization techniques capable of exposing the interplay among structure, chemistry, and optoelectronic properties to inform processing strategies and increase both device efficiencies and long‐term stability. Cathodoluminescence microscopy is an ideal technique to provide such information due to the high spatial resolution and robust optical information acquired. Here, the current body of work related to cathodoluminescence analysis of halide perovskite materials for optoelectronic applications is surveyed. This review demonstrates how cathodoluminescence can monitor degradation due to environmental stressors, phase segregation resulting from material processing, and other halide perovskite‐centric material issues. A persistent concern associated with e‐beam‐based analysis of halide perovskites is what effect the electron beam has on the material properties being probed. Addressing this, a detailed discussion is provided on the origin of the cathodoluminescence signal and a review of studies focused on revealing changes in the properties of halide perovskites resulting from e‐beam excitation. Finally, a perspective on future opportunities to expand the role of cathodoluminescence analysis for halide perovskites is provided.     

Tin Halide Perovskites: Progress and Challenges

The chemical composition engineering of lead halide perovskites via a partial or complete replacement of toxic Pb with tin has been widely reported as a feasible process due to the suitable ionic radius of Sn and its possibility of existing in the +2 state. Interestingly, a complete replacement narrows the bandgap while a partial replacement gives an anomalous phenomenon involving a further narrowing of bandgap relative to the pure Pb and Sn halide perovskite compounds. Unfortunately, the merits of this anomalous behavior have not been properly harnessed. Although promising progress has been made to advance the properties and performance of Sn‐based perovskite systems, their photovoltaic (PV) parameters are still significantly inferior to those of the Pb‐based analogs. This review summarizes the current progress and challenges in the preparation, morphological and photophysical properties of Sn‐based halide perovskites, and how these affect their PV performance. Although it can be argued that the Pb halide perovskite systems may remain the most sought after technology in the field of thin film perovskite PV, prospective research directions are suggested to advance the properties of Sn halide perovskite materials for improved device performance.     

Imaging and Mapping Characterization Tools for Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted much attention as efficiencies have gone beyond 24%. To achieve these impressive numbers, the PSC scientific community is working to improve the perovskite optoelectronic properties. Imaging and mapping characterization techniques have been widely used to understand the fundamental properties that allow lead halide perovskites to achieve high performance. In this review, these techniques are evaluated, from simple tools, such as electron microscopy, to more complex systems that include atomic force microscopy, synchrotron‐based X‐ray mapping, and ultrafast and photoluminescence mapping. These tools have helped understand lead halide perovskites and their impressive optoelectronic properties, which make them outstanding materials for solar cell applications.     

Polarons in Metal Halide Perovskites

The peculiar optoelectronic properties of metal‐halide perovskites, partly underlying their success in solar cells and light emitting devices, are likely related to the complex interplay of electronic and structural features mediated by formation of polarons. In this paper the current status of polaron physics in metal‐halide perovskites is reviewed based on a first‐principles computational perspective, which has delivered hitherto noaccessible insights into the electronic and structural features associated with polaron formation in this materials class. The role of organic (dipolar) versus inorganic (spherical) A‐site cations is extensively analyzed, these cations are related to modulation of the energetics and structural extension of polarons in lead‐halide perovskites. Further tuning of polaron energetics is achieved by individual variations in metal (e.g., Pb → Sn) and halide (e.g., I → Br), showing a transition from a semilocalized to a localized polaron regime in which charge holes can be trapped at isolated Sn centers. The vastly varying and tunable nature of charge lattice interactions represents a peculiarity of metal‐halide perovskites that should be taken into account when designing novel materials or targeting specific compositional engineering of existing perovskites.     

Making by Grinding: Mechanochemistry Boosts the Development of Halide Perovskites and Other Multinary Metal Halides

Mechanochemical synthesis has recently emerged as a promising route for the synthesis of functional lead halide perovskites as well as other (lead‐free) metal halides. Mechanochemical synthesis presents several advantages with regards to more commonly used solution‐based processes such as an inherent lower toxicity by avoiding organic solvents and a finer control over stoichiometry of the final products. The ease of implementation, either through the use of a simple mortar and pestle or with an electrically powered ball‐mill, and low amount of side products make mechanochemical synthesis appealing for upscaling the production of halide perovskites. Due to the defect tolerance of lead halide perovskites, they are ideally suited to be prepared by this solvent‐free method. However, the implementation of these semiconductors in high‐efficiency optoelectronic devices requires the transformation of synthesized powder into smooth thin films where still some hurdles remain to be cleared.     

The Impact of Atmosphere on Energetics of Lead Halide Perovskites

Solar cells based on metal halide perovskites have emerged as a promising low‐cost photovoltaic technology. In contrast to inert atmospheres where most of the lab‐scale devices are made to date, large‐area low‐cost production of perovskite solar cells often involves processing of perovskites in various atmospheres including ambient air, nitrogen, and/or vacuum. Herein, the impact of atmosphere on the energy levels of methylammonium lead halide perovskite films is systematically investigated. The atmosphere is varied to simulate the typical fabrication process. Through a comprehensive analysis combining the Fermi level evolution, surface photovoltage, photoluminescence properties, photovoltaic performance, and device simulation, an overall landscape of the energy diagram of the perovskite layer is able to be determined. The findings have direct implications for real‐world devices under typical atmospheres, and provide insights into the fabrication‐process design and optimization. Furthermore, a universal Fermi level shift under vacuum for lead halide‐based perovskites revealed in this study, urges a refreshed view on the energetics studies conducted without considering the atmospheric effect.     

Localized Traps Limited Recombination in Lead Bromide Perovskites

Traps exert an omnipotent influence over the performance of halide perovskite optoelectronic devices. A clear understanding of the origin and nature of the traps in halide perovskites is the key to controlling them and realizing optimal devices. Herein, the role of localized traps on the optical properties of lead bromide perovskite films is investigated. In the low‐temperature orthorhombic phase of CH₃NH₃PbBr₃ perovskite, band‐edge carrier dynamics exhibit a power‐law decay due to the presence of structural‐disorder‐induced localized traps, which has a depth of ≈40 meV. The continuous distribution of these localized traps gives rise to a broad sub‐band‐gap emission that becomes more prominent in thicker films with a larger trap density. The presence of this emission only from the hybrid organic–inorganic perovskites points to the vital role of organic dipoles in localized trap states formation. This study explicates the nature of these localized traps as well as their nontrivial role in carrier recombination kinetics, which is of fundamental importance in perovskites optoelectronics.     

X‐Ray Microscopy of Halide Perovskites: Techniques,Applications, and Prospects

X‐ray microscopy can provide unique chemical, electronic, and structural insights into perovskite materials and devices leveraging bright, tunable synchrotron X‐ray sources. Over the last decade, fundamental understanding of halide perovskites and their impressive performance in optoelectronic devices has been furthered by rigorous research regarding their structural and chemical properties. Herein, studies of perovskites are reviewed that have used X‐ray imaging, spectroscopy, and scattering microscopies that have proven valuable tools toward understanding the role of defects, impurities, and processing on perovskite material properties and device performance. Together these microscopic investigations have augmented the understanding of the internal workings of perovskites and have helped to steer the perovskite community toward promising directions. In many ways, X‐ray microscopy of perovskites is still in its infancy, which leaves many exciting paths unexplored including new ptychographic, multimodal, in situ, and operando experiments. To explore possibilities, pioneering X‐ray microscopy along these lines is briefly highlighted from other semiconductor systems including silicon, CdTe, GaAs, CuIn_xGa_1?xSe₂, and organic photovoltaics. An overview is provided on the progress made in utilizing X‐ray microscopy for perovskites and present opportunities and challenges for future work.     

Addressing Toxicity of Lead: Progress and Applications of Low‐Toxic Metal Halide Perovskites and Their Derivatives

Metal halide perovskites have been brought to the forefront of research focus in solution‐processable photovoltaics, with the device efficiency swiftly surging to over 22% over the past few years. The state‐of‐the‐art metal halide perovskites that have been intensively investigated include toxic lead, which potentially hampers their commercialization process. To address this toxicity issue, intensive recent research effort has been devoted to developing low‐toxic metal halide perovskites and their derivatives for photovoltaic applications. Herein, the recent research progress achieved so far in addressing the toxicity issue of lead halide perovskites in photovoltaics is summarized. By comparing the merits and drawbacks of different low‐toxic metal halide systems, the current challenges and opportunities in the photovoltaic field are highlighted. Potential low‐toxic metal halide perovskites and their derivatives are also discussed from the perspective of theoretical calculations. Furthermore, promising applications of low‐toxic metal halide perovskites beyond the photovoltaic sector are briefly discussed.     

Ion Migration‐Induced Amorphization and Phase Segregation as a Degradation Mechanism in Planar Perovskite Solar Cells

The operation of halide perovskite optoelectronic devices, including solar cells and LEDs, is strongly influenced by the mobility of ions comprising the crystal structure. This peculiarity is particularly true when considering the long‐term stability of devices. A detailed understanding of the ion migration‐driven degradation pathways is critical to design effective stabilization strategies. Nonetheless, despite substantial research in this first decade of perovskite photovoltaics, the long‐term effects of ion migration remain elusive due to the complex chemistry of lead halide perovskites. By linking materials chemistry to device optoelectronics, this study highlights that electrical bias‐induced perovskite amorphization and phase segregation is a crucial degradation mechanism in planar mixed halide perovskite solar cells. Depending on the biasing potential and the injected charge, halide segregation occurs, forming crystalline iodide‐rich domains, which govern light emission and participate in light absorption and photocurrent generation. Additionally, the loss of crystallinity limits charge collection efficiency and eventually degrades the device performance.     

Interface Engineering Driven Stabilization of Halide Perovskites against Moisture,Heat, and Light for Optoelectronic Applications

Moisture, heat, and light instabilities of halide perovskites (HPs) represent a serious Achilles' heel that must be overcome, to enable future advancements in perovskite‐based optoelectronic devices such as solar cells and light‐emitting diodes. The instabilities are attributed to the unavoidable fragile ionic bonding between cationic and anionic parts of HPs during their formation. Surface passivation of HPs by various surface‐passivating materials has proven to be an attractive approach to stabilize perovskites against moisture, heat, and light, keeping intact their structural integrity and ionic bonding. Herein, the experimental and theoretical background for degradation mechanisms of HPs is reviewed along with various surface passivating materials to stabilize HPs. Finally, the existing challenges associated with thin‐film and device fabrication and an outlook for improving the stability of perovskites in optoelectronics are presented     

Excitons in Metal‐Halide Perovskites

The unprecedented increase of the power conversion efficiency of metal‐halide perovskite solar cells has significantly outpaced the understanding of their fundamental properties. One of the biggest puzzles of perovskites has been the exciton binding energy, which has proved to be difficult to determine experimentally. Many contradictory reports can be found in the literature with values of the exciton binding energy from a few meV to a few tens of meV. In this review the results of the last few years of intense investigation of the exciton physic in perovskite materials are summarized. In particular a critical overview of the different experimental approaches used to determine exciton binding energy is provided. The problem of exciton binding energy in the context of the polar nature of perovskite crystals and related polaron effects which have been neglected to date in most of work is discussed. It is shown that polaron effects can reconcile at least some of the experimental observations and controversy present in the literature. Finally, the current status of the exciton fine structure in perovskite materials is summarized. The peculiar carrier–phonon coupling can help to understand the intriguing efficiency of light emission from metal‐halide perovskites.     

Processing‐Performance Evolution of Perovskite Solar Cells: From Large Grain Polycrystalline Films to Single Crystals

Solution‐processable halide perovskites have emerged as strong contenders for next‐generation solar cells owing to their favorable optoelectronic properties. To maintain the efficiency momentum of perovskite solar cells (PSCs), development of advanced processing techniques, particularly for the perovskite layer, is imperative. There is a close correlation between the quality of the perovskite layer and its photophysical properties: Highly crystalline large grains with uniform morphology of the perovskite layer and their interface with charge transporters are crucial for achieving high performance. Significant efforts have been dedicated to achieve perovskite films with large grains reaching the millimeter‐scale for high‐efficiency PSCs. Recent work showcases a transition from large grain polycrystalline to single‐crystalline (SC) PSCs made possible by the facile growth of perovskite single crystals. In this review, the recent progress of the large grain polycrystalline PSCs and grain boundary‐free SC‐PSCs is reported, particularly focusing on the recent approach of depositing large‐grained perovskite layers and single crystal growth technique, that have been adopted for fabrication of efficient PSCs. In addition, prospects of SC‐PSCs and their further development in terms of efficiency, device design, scalability, and stability are discussed.     

The Rise of Textured Perovskite Morphology: Revolutionizing the Pathway toward High‐Performance Optoelectronic Devices

Halide perovskite materials have achieved overwhelming success in various optoelectronic applications, especially perovskite solar cells and perovskite‐based light‐emitting diodes (P‐LEDs), owing to their outstanding optical and electric properties. It is widely believed that flat and mirror‐like perovskite films are imperative for achieving high device performance, while the potential of other perovskite morphologies, such as the emerging textured perovskite, is overlooked, which leaves plenty of room for further breakthroughs. Compared to flat and mirror‐like perovskites, textured perovskites with unique structures, e.g., coral‐like, maze‐like, column‐like or quasi‐core@shell assemblies, are more efficient at light harvesting and charge extraction, thus revolutionizing the pathways toward ultrahigh performance in perovskite‐based optoelectronic devices. Employing a textured perovskite morphology, the record of external quantum efficiency for P‐LEDs is demonstrated as 21.6%. In this research news, recent progress in the utilization of textured perovskite is summarized, with the emphasis on the preparation strategies and prominent optoelectronic properties. The impact of the textured morphology on light harvesting, carrier dynamic management, and device performance is highlighted. Finally, the challenges and great potential of employing these innovative morphologies in fabricating more efficient optoelectronic devices, or creating a new energy harvesting and conversion regime are also provided.     

Metal Halide Perovskite Materials for Solar Cells with Long‐Term Stability

Metal halide perovskite solar cells (PSCs) have emerged as promising candidates for photovoltaic technology with their power conversion efficiencies over 23%. For prototypical organic–inorganic metal halide perovskites, their intrinsic instability poses significant challenges to the commercialization of PSCs. Recently, the scientific community has done tremendous work in composition engineering to develop more robust light‐absorbing layers, including mixed‐ion hybrid perovskites, low‐dimensional hybrid perovskites, and all‐inorganic perovskites. This review provides an overview of the impact of these perovskites on the efficiency and long‐term stability of PSCs.     

Perovskite Chalcogenides with Optimal Bandgap and Desired Optical Absorption for Photovoltaic Devices

Solar cells with organic‐inorganic lead halide perovskites have achieved great success and their power conversion efficiency (PCE) has reached to 22.1%. To address the toxicology of lead element and some stability issues associated with the organic‐inorganic lead halide perovskites, inorganic lead‐free perovskites have gained more attentions from the photovoltaic research community. Herein, a series of chalcogenide perovskites are proposed as optical absorber materials for thin‐film solar cells. SrSnSe₃ and SrSnS₃ are predicted to be direct bandgap semiconductors with the bandgap value being within the optimal range of 0.9–1.6 eV. Both SrSnSe₃ and SrSnS₃ not only exhibit good optical absorption properties and carrier mobility, but also possess flexible bandgaps that can be continuously tuned within the grange of 0.9–1.6 eV via the element‐mixing strategy, thereby render both perovskites as promising candidates for photovoltaic applications.     

Perovskite Tandem Solar Cells

The meteoric rise of perovskite single‐junction solar cells has been accompanied by similar stunning developments in perovskite tandem solar cells. Debuting with efficiencies less than 14% in 2014, silicon–perovskite solar cells are now above 25% and will soon surpass record silicon single‐junction efficiencies. Unconstrained by the Shockley–Quiesser single‐junction limit, perovskite tandems suggest a real possibility of true third‐generation thin‐film photovoltaics; monolithic all‐perovskite tandems have reached 18% efficiency and will likely pass perovskite single‐junction efficiencies within the next 5 years. Inorganic–organic metal–halide perovskites are ideal candidates for inclusion in tandem solar cells due to their high radiative recombination efficiencies, excellent absorption, long‐range charge‐transport, and broad ability to tune the bandgap. In this progress report, the development of perovskite tandem cells is reviewed, with presentation of their key motivations and challenges. In detail, it presents an overview of recombination layer materials, bandgap‐tuneability, transparent contact architectures, and perovskite compounds for use in tandems. Theoretical estimates of efficiency for future tandem and triple‐junction perovskite cells are presented, outlining roadmaps for future focused research.