Progress of Lead‐Free Halide Double Perovskites

The field of halide metal perovskite photovoltaics has caught widespread interest in the last decade. This is seen in the rapid rise of power conversion efficiency, which is currently over 23%. It has also stimulated a widespread application of halide metal perovskites in other fields, such as light‐emitting diodes, field‐effect transistors, detectors, and lasers. Despite the fascinating characteristics of the halide metal perovskites, the presence of toxic lead (Pb) in their chemical composition is regarded as one of the major limiting factors preventing their commercialization. Addressing the toxicity issues in these compounds by a careful and strategic replacement of Pb²⁺ with other nontoxic candidate elements represents a promising direction to fabricate lead‐free optoelectronic devices. Such attempts yield a halide double perovskite structure which allows flexibility for various compositional adjustments. Here, the authors present the current progress and setbacks in crystal structures, materials preparation, optoelectronic properties, stability, and photovoltaic applications of lead‐free halide double perovskite compounds. Prospective research directions to improve the optoelectronic properties of existing materials are given that may help in the discovery of new lead‐free halide double perovskites.     

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.     

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.     

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.     

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     

Electron‐Beam‐Related Studies of Halide Perovskites: Challenges and Opportunities

Electron beam microscopy and related characterization techniques play an important role in revealing the microstructural, morphological, physical, and chemical information of halide perovskites and their impact on associated optoelectronic devices. However, electron beam irradiation usually causes damage to these beam‐sensitive materials, negatively impacting their device performance, and complicating this interpretation. In this article, the electron microscopy and spectroscopy techniques are reviewed that are crucial for the understanding of the crystallization and microstructure of halide perovskites. In addition, special attention is paid to assessing and mitigating the electron beam‐induced damage caused by these techniques. Since the halide perovskites are fragile, a protocol involving delicate control of both electron beam dose and dose rate, coupled with careful data analysis, is key to enable the acquisition of reliable structural and compositional information such as atomic‐resolution images, chemical elemental mapping and electron diffraction patterns. Limiting the electron beam dose is critical parameter enabling the characterization of various halide perovskites. Novel methods to unveil the mechanisms of device operation, including charge carrier generation, diffusion, and extraction are presented in scanning electron microscopy studies combined with electron‐beam‐induced current and cathodoluminescence mapping. Future opportunities for electron‐beam‐related characterizations of halide perovskites are also discussed.     

Photoemission Spectroscopy Characterization of Halide Perovskites

Controlling the surface and interface properties of halide perovskites (HaPs) materials is key to improve performance and stability of HaP‐based optoelectronic devices such as solar cells. Here, an overview is given on the use of different photoemission spectroscopy (PES) techniques as a tool kit to investigate chemical and electronic properties of surfaces and interfaces in research on HaP compounds. The primary focus of the article is X‐ray photoelectron spectroscopy (XPS), hard X‐ray photoemission spectroscopy (HAXPES), ultraviolet photoemission spectroscopy (UPS), and inverse photoemission spectroscopy (IPES), highlighting the importance of good practices during PES measurements. Starting from the working principles of PES, critical measurement conditions are discussed. In particular, the exposure of the HaP surface to vacuum and high energy radiation can cause accelerated ageing, degradation, and also ionic migration in the sample. The impact of these changes on the electronic and chemical properties is discussed, followed by an analysis of the specific challenges encountered when performing PES measurements of HaPs. These include the deviation from pristine surface conditions, determination of “soft” band edges, and assessment of band bending. The review concludes by emphasizing good practices for PES measurements of HaP samples and outlining the scope of operando type measurements to capture the transient behavior of HaPs in the experiment.     

Engineering Multiphase Metal Halide Perovskites Thin Films for Stable and Efficient Solar Cells

The intrinsic instability of lead halide perovskite semiconductors in an ambient atmosphere is one of the most critical issues that impedes perovskite solar cell commercialization. To overcome it, the use of bulky organic spacers has emerged as a promising solution. The resulting perovskite thin films present complex morphologies, difficult to predict, which can directly affect the device efficiency. Here, by combining in‐depth morphological and spectroscopic characterization, it is shown that both the ionic size and the relative concentration of the organic cation, drive the integration of bulky organic cations into the crystal unit cell and the thin film, inducing different perovskite phases and different vertical distribution, then causing a significant change in the final thin film morphology. Based on these studies, a fine‐engineered perovskite is constructed by employing two different large cations, namely, ethyl ammonium and butyl ammonium. The first one takes part in the 3D perovskite phase formation, the second one works as a surface modifier by forming a passivating layer on top of the thin film. Together they lead to improved photovoltaic performance and device stability when tested in air under continuous illumination. These findings propose a general approach to achieve reliability in perovskite‐based optoelectronic devices.     

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.     

Photoluminescence‐Based Characterization of Halide Perovskites for Photovoltaics

Photoluminescence spectroscopy is a widely applied characterization technique for semiconductor materials in general and halide perovskite solar cell materials in particular. It can give direct information on the recombination kinetics and processes as well as the internal electrochemical potential of free charge carriers in single semiconductor layers, layer stacks with transport layers, and complete solar cells. The correct evaluation and interpretation of photoluminescence requires the consideration of proper excitation conditions, calibration and application of the appropriate approximations to the rather complex theory, which includes radiative recombination, non‐radiative recombination, interface recombination, charge transfer, and photon recycling. In this article, an overview is given of the theory and application to specific halide perovskite compositions, illustrating the variables that should be considered when applying photoluminescence analysis in these materials.     

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.     

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.     

First‐Principles Simulation of Carrier Recombination Mechanisms in Halide Perovskites

In recent years, there have been remarkable developments in halide perovskites, which are used in highly efficient optoelectronic devices and exhibit intriguing materials physics. Detailed knowledge of carrier recombination mechanisms is essential for understanding their excellent performance and to further increase their efficiency. Obtaining such knowledge is challenging however, and different studies have reached divergent conclusions in some cases. This progress report outlines the critical developments in understanding the carrier recombination mechanisms in halide perovskites from a computational perspective. The primary focus is radiative and Auger recombination, since they have not been systematically assessed and discussed before, and a number of important issues have been actively debated. This comprehensive discussion of the carrier recombination mechanisms is aimed at establishing physically justified insights that can form the basis for better materials and devices design.     

Inorganic Halide Perovskite Solar Cells: Progress and Challenges

All‐inorganic perovskite semiconductors have recently drawn increasing attention owing to their outstanding thermal stability. Although all‐inorganic perovskite solar cells (PSCs) have achieved significant progress in recent years, they still fall behind their prototype organic–inorganic counterparts owing to severe energy losses. Therefore, there is considerable interest in further improving the performance of all‐inorganic PSCs by synergic optimization of perovskite films and device interfaces. This review article provides an overview of recent progress in inorganic PSCs in terms of lead‐based and lead‐free composition. The physical properties of all‐inorganic perovskite semiconductors as well as the hole/electron transporting materials are discussed to unveil the important role of composition engineering and interface modification. Finally, a discussion of the prospects and challenges for all‐inorganic PSCs in the near future is presented.     

Optical Absorption‐Based In Situ Characterization of Halide Perovskites

Halide perovskites have emerged as materials for high‐performance optoelectronic devices. Often, progress made to date in terms of higher efficiency and stability is based on increasing material complexity, i.e., formation of multicomponent halide perovskites with multiple cations and anions. In this review article, the use of in situ optical methods, namely, photoluminescence (PL) and UV‐vis, that provide access to the relevant time and length scales to ascertain chemistry–property relationships by monitoring evolving properties is discussed. Additionally, because halide perovskites are electron/ion conductors and prone to solid‐state ion transport under various external stimuli, application of these optical methods in the context of ionic movement is described to reveal mechanistic insights. Finally, examples of using in situ PL and UV‐vis to study degradation and phase transitions are reviewed to demonstrate the wealth of information that can be obtained regarding many different aspects of ongoing research activities in the field of halide perovskites.     

Alternative Perovskites for Photovoltaics

The discovery of unique optoelectronic properties of 3D ABX₃ perovskites has produced a great impact on the field of photovoltaics. In the initial years after the breakthrough, interest has focused on a limited number of 3D ABX₃ perovskite materials, including the archetypal CH₃NH₃PbI₃ and its counterparts. Undoubtedly, the main limitation of perovskite devices is their low stability due the fast degradation of the perovskite layer; however, the high toxicity of Pb also poses a concern. Herein, the recent increasing number of articles reporting the theoretical modeling, synthesis, optoelectronic characterization, and implementation of alternative perovskite materials in solar devices is summarized. The extensive variety of perovskite derivatives is classified according to the material dimensionality and the crystal structure. The particular strengths and weaknesses for each novel material are discussed, and the device performance and potential stability enhancements are also highlighted.