Emerging Chalcohalide Materials for Energy Applications

Semiconductors with multiple anions currently provide a new materials platform from which improved functionality emerges, posing new challenges and opportunities in material science. This review has endeavored to emphasize the versatility of the emerging family of semiconductors consisting of mixed chalcogen and halogen anions, known as "chalcohalides". As they are multifunctional, these materials are of general interest to the wider research community, ranging from theoretical/computational scientists to experimental materials scientists. This review provides a comprehensive overview of the development of emerging Bi- and Sb-based as well as a new Cu, Sn, Pb, Ag, and hybrid organic-inorganic perovskite-based chalcohalides. We first highlight the high-throughput computational techniques to design and develop these chalcohalide materials. We then proceed to discuss their optoelectronic properties, band structures, stability, and structural chemistry employing theoretical and experimental underpinning toward high-performance devices. Next, we present an overview of recent advancements in the synthesis and their wide range of applications in energy conversion and storage devices. Finally, we conclude the review by outlining the impediments and important aspects in this field as well as offering perspectives on future research directions to further promote the development of chalcohalide materials in practical applications in the future.

Data-Driven Materials Innovation and Applications

Owing to the rapid developments to improve the accuracy and efficiency of both experimental and computational investigative methodologies, the massive amounts of data generated have led the field of materials science into the fourth paradigm of data-driven scientific research. This transition requires the development of authoritative and up-to-date frameworks for data-driven approaches for material innovation. A critical discussion on the current advances in the data-driven discovery of materials with a focus on frameworks, machine-learning algorithms, material-specific databases, descriptors, and targeted applications in the field of inorganic materials is presented. Frameworks for rationalizing data-driven material innovation are described, and a critical review of essential subdisciplines is presented, including: i) advanced data-intensive strategies and machine-learning algorithms; ii) material databases and related tools and platforms for data generation and management; iii) commonly used molecular descriptors used in data-driven processes. Furthermore, an in-depth discussion on the broad applications of material innovation, such as energy conversion and storage, environmental decontamination, flexible electronics, optoelectronics, superconductors, metallic glasses, and magnetic materials, is provided. Finally, how these subdisciplines (with insights into the synergy of materials science, computational tools, and mathematics) support data-driven paradigms is outlined, and the opportunities and challenges in data-driven material innovation are highlighted.

Modelling Interfaces in Thin-Film Photovoltaic Devices

Developing effective device architectures for energy technologies—such as solar cells, rechargeable batteries or fuel cells—does not only depend on the performance of a single material, but on the performance of multiple materials working together. A key part of this is understanding the behaviour at the interfaces between these materials. In the context of a solar cell, efficient charge transport across the interface is a pre-requisite for devices with high conversion efficiencies. There are several methods that can be used to simulate interfaces, each with an in-built set of approximations, limitations and length-scales. These methods range from those that consider only composition (e.g. data-driven approaches) to continuum device models (e.g. drift-diffusion models using the Poisson equation) and ab-initio atomistic models (developed using e.g. density functional theory). Here we present an introduction to interface models at various levels of theory, highlighting the capabilities and limitations of each. In addition, we discuss several of the various physical and chemical processes at a heterojunction interface, highlighting the complex nature of the problem and the challenges it presents for theory and simulation.

Emerging Chalcogenide Thin Films for Solar Energy Harvesting Devices

Chalcogenide semiconductors offer excellent optoelectronic properties for their use in solar cells, exemplified by the commercialization of Cu(In,Ga)Se₂- and CdTe-based photovoltaic technologies. Recently, several other chalcogenides have emerged as promising photoabsorbers for energy harvesting through the conversion of solar energy to electricity and fuels. The goal of this review is to summarize the development of emerging binary (Sb₂X₃, GeX, SnX), ternary (Cu₂SnX₃, Cu₂GeX₃, CuSbX₂, AgBiX₂), and quaternary (Cu₂ZnSnX₄, Ag₂ZnSnX₄, Cu₂CdSnX₄, Cu₂ZnGeX₄, Cu₂BaSnX₄) chalcogenides (X denotes S/Se), focusing especially on the comparative analysis of their optoelectronic performance metrics, electronic band structure, and point defect characteristics. The performance limiting factors of these photoabsorbers are discussed, together with suggestions for further improvement. Several relatively unexplored classes of chalcogenide compounds (such as chalcogenide perovskites, bichalcogenides, etc.) are highlighted, based on promising early reports on their optoelectronic properties. Finally, pathways for practical applications of emerging chalcogenides in solar energy harvesting are discussed against the backdrop of a market dominated by Si-based solar cells.

Ti-Alloying of BaZrS3 Chalcogenide Perovskite for Photovoltaics

BaZrS₃, a prototypical chalcogenide perovskite, has been shown to possess a direct band gap, an exceptionally strong near band edge light absorption, and good carrier transport. Coupled with its great stability, nontoxicity with earth-abundant elements, it is thus a promising candidate for thin film solar cells. However, its reported band gap in the range of 1.7-1.8 eV is larger than the optimal value required to reach the Shockley-Queisser limit of a single-junction solar cell. Here, we report the synthesis of Ba(Zr_1-x Ti _x )S₃ perovskite compounds with a reduced band gap. It is found that Ti-alloying is extremely effective in band gap reduction of BaZrS₃: a mere 4 atom % alloying decreases the band gap from 1.78 to 1.51 eV, resulting in a theoretical maximum power conversion efficiency of 32%. Higher Ti-alloying concentration is found to destabilize the distorted chalcogenide perovskite phase.

Perovskites with d-block metals for solar energy applications

Pb2+ halide organic-inorganic perovskites are excellent semiconductors for use in solar energy applications, but at the expense of robustness and environmental compatibility. Tin (Sn), which sits just above lead in the periodic table, forms pure (or mixed with lead) perovskites when at the 2+ or 4+ oxidation state. It can act as a promising alternative; however, there are still some serious concerns regarding its suitability. This presents a major challenge; viable metal cations have to be identified. A good number of elements, originating from a large range of d-block metal ions, with adequate oxidation states, moderate toxicity, and relative abundance, seem ideal for this purpose. In this review, we present the most characteristic perovskites (conventional perovskites, layered, or double perovskites) that can be formed with the help of these metals. We focus on d-block metal ions with stable oxidation states, such as Ag+ or Ti4+, which have exhibited satisfactory photovoltaic properties until now. Further, we highlight the results involving compounds other than halide perovskites, such as oxides, chalcogenides, and nitrides (as well as oxyhalides, oxysulfides, and oxynitrides); a few of them are ferroelectric (based on Ti4+, Zr4+, Fe3+, and Cr3+) and can yield a photovoltage that exceeds the bandgap of the material. Finally, we present the critical challenges that currently limit the efficiency of these systems and propose prospects for future directions.