An assessment of silver copper sulfides for photovoltaic applications: theoretical and experimental insights

Room-Temperature Synthesis and Low Thermal Conductivity in Nanocrystalline Ag3CuS2

Noble-metal-based chalcogenide materials recently gained massive attention in the field of thermoelectrics. In most cases, materials are synthesized using (i) high-temperature solid-state reactions or (ii) soft chemical methods where temperature requirements are lower than those of solid-state reactions (generally below 400 °C). Herein, we present a simple, surfactant-free, room-temperature, and energy-efficient synthesis of Ag3CuS2 nanocrystals. The present synthesis technique is scalable and capable of gram-scale production. A spark plasma sintering (SPS) pressed sample exhibits ultralow thermal conductivity (∼0.31 W/mK at room temperature). We found that Ag3CuS2 exhibits low sound velocity, as well as a non-Debye-like behavior based on a low-temperature heat capacity measurement. A high degree of anharmonicity of bonding, soft vibrations modes, and nanoscale grain boundary scattering in Ag3CuS2 lead to ultralow thermal conductivity, which can be important for thermoelectrics, optoelectronics, and thermal barrier coating applications.

Environment-friendly copper-based chalcogenide thin film solar cells: status and perspectives

Copper chalcogenides (CuCh) have attracted considerable attention due to their promising potential as environmental-friendly photoactive material for lightweight and flexible thin film solar cells. Further, CuCh can be fabricated from simple to complex chemical compositions and offer a remarkable charge carrier mobility and excellent absorption coefficient with a desirable bandgap (up to ∼1.0 eV). Currently, they have demonstrated maximum power conversion efficiencies of over 23% for single-junction, around 25% and 28% for monolithic 2-Terminal (2T) and mechanically-stacked 4-Terminal (4T) perovskite/CuCh tandem solar cells, respectively. This article presents an overview of CuCh-based materials, from binary- to quaternary-CuCh compounds for single- and multi-junction solar cells. Then, we discuss the development of fabrication methods and the approaches taken to improve the performance of CuCh-based thin film itself, including chemical doping, the development of complement layers, and their potential application in flexible and lightweight devices. Finally, these technologies' stability, scalability, and toxicity aspects are discussed to enhance their current marketability.

Band Structure, Phonon Spectrum and Thermoelectric Properties of Ag3CuS2

Sulfides and selenides of copper and silver have been intensively studied, particularly as potentially efficient thermoelectrics. Ag3CuS2 (jalpaite) is a related material. However very little is known about its physical properties. It has been found that the compound undergoes several structural phase transitions, having the tetrahedral structural modification I41/amd at room temperature. In this work, its band structure, phonon spectrum and thermoelectric properties were studied theoretically and experimentally. Seebeck coefficient, electrical conductivity and thermal conductivity were measured in a broad temperature range from room temperature to 600 K. These are the first experimental data on transport properties of jalpaite. Ab initio calculations of the band structure and Seebeck coefficient were carried out taking into account energy dependence of the relaxation time typical for the scattering of charge carriers by phonons. The results of the calculations qualitatively agree with the experiment and yield large values of the Seebeck coefficient characteristic for lightly doped semiconductor. The influence of intrinsic defects (vacancies) on the transport properties was studied. It was shown that the formation of silver vacancies is the most probable and leads to an increase of hole concentration. Using the temperature dependent effective potential method, the phonon spectrum and thermal conductivity at room temperature were calculated. The measurements yield low lattice thermal conductivity value of 0.5 W/(m K) at 300 K, which is associated with the complex crystal structure of the material. The calculated room temperature values of the lattice thermal conductivity were also small (0.14-0.2 W/(m K)).

Theoretical Design of Inorganic Flexible Bulk Photovoltaic Materials

Current flexible photovoltaics (PVs) are usually based on low-dimensional structures of inorganic semiconductors and hybrid perovskites, as well as organic materials. Here, we propose a type of inorganic flexible bulk PV material, evaluating its structure flexibility, electronic structure, and PV efficiency in the framework of density functional theory, suggesting α-Ag₂S as the best candidate. It is found that the band structure and effective masses of α-Ag₂S can be significantly modulated by external strain, whereas leaving the high PV efficiency was not affected much. The flexibility of α-Ag₂S can be further enhanced by applying electron doping during stretching or applying hole doping during compression. We further studied the intrinsic defect properties of α-Ag₂S by using the Heyd-Scuseria-Ernzerhof hybrid functional, and the calculation results show that α-Ag₂S is a defect-tolerant semiconductor even when an external strain is applied. Our results open the door for searching inorganic flexible bulk PV materials for robust flexible solar cells.

Ca4Sb2O and Ca4Bi2O: two promising mixed-anion thermoelectrics

The environmental burden of fossil fuels and the rising impact of global warming have created an urgent need for sustainable clean energy sources. This has led to widespread interest in thermoelectric (TE) materials to recover part of the ∼60% of global energy currently wasted as heat as usable electricity. Oxides are particularly attractive as they are thermally stable, chemically inert, and formed of earth-abundant elements, but despite intensive efforts there have been no reports of oxide TEs matching the performance of flagship chalcogenide materials such as PbTe, Bi2Te3 and SnSe. A number of ternary X4Y2Z mixed-anion systems, including oxides, have predicted band gaps in the useful range for several renewable-energy applications, including as TEs, and some also show the complex crystal structures indicative of low lattice thermal conductivity. In this study, we use ab initio calculations to investigate the TE performance of two structurally-similar mixed-anion oxypnictides, Ca4Sb2O and Ca4Bi2O. Electronic-structure and band-alignment calculations using hybrid density-functional theory (DFT), including spin-orbit coupling, suggest that both materials are likely to be p-type dopable with large charge-carrier mobilities. Lattice-dynamics calculations using third-order perturbation theory predict ultra-low lattice thermal conductivities of ∼0.8 and ∼0.5 W m-1 K-1 above 750 K. Nanostructuring to a crystal grain size of 20 nm is predicted to further reduce the room temperature thermal conductivity by around 40%. Finally, we use the electronic- and thermal-transport calculations to estimate the thermoelectric figure of merit ZT, and show that with p-type doping both oxides could potentially serve as promising earth-abundant oxide TEs for high-temperature applications.

Environment-friendly Cu-based thin film solar cells: materials, devices and charge carrier dynamics

Cu-based thin films are ideal absorbing layer materials for new-generation thin-film solar cells, which have many advantages, such as environment-friendly components, abundant raw materials, low cost, simple manufacturing process, strong anti-interference, radiation resistance, high light absorption coefficient and suitable band gap. Copper indium gallium selenide (CIGS) thin-film solar cells, which have the highest photoelectric conversion efficiency (23.35%) among the various Cu-based materials, have been intensively investigated and exploited. To promote the progress of Cu-based thin-film solar cells, the rational design of efficient materials and devices and the in-depth understanding of their photophysical mechanisms are not only urgently required, but also have plenty of room for research. Accordingly, herein, we firstly define the concept of "Cu-based materials", and further present a comprehensive review on the materials (design and fabrication), devices (assembly and performances), and charge carrier dynamics of Cu-based thin-film semiconductor materials, including perovskites, oxides, chalcogenides (binary, ternary, quaternary and quinary) and perovskite-like iodides. In addition, the current challenges and prospects in this topic are critically concluded. Particularly, this review may help researchers focused on investigating thin-film solar cells.

Colloidal Synthesis and Optical Properties of Perovskite-Inspired Cesium Zirconium Halide Nanocrystals

Optoelectronic devices based on lead halide perovskites are processed in facile ways, yet are remarkably efficient. There are extensive research efforts investigating lead-free perovskite and perovskite-related compounds, yet there are challenges to synthesize these materials in forms that can be directly integrated into thin film devices rather than as bulk powders. Here, we report on the colloidal synthesis and characterization of lead-free, antifluorite Cs₂ZrX₆ (X = Cl, Br) nanocrystals that are readily processed into thin films. We use transmission electron microscopy and powder X-ray diffraction measurements to determine their size and structural properties, and solid-state nuclear magnetic resonance measurements reveal the presence of oleate ligand, together with a disordered distribution of Cs surface sites. Density functional theory calculations reveal the band structure and fundamental band gaps of 5.06 and 3.91 eV for Cs₂ZrCl₆ and Cs₂ZrBr₆, respectively, consistent with experimental values. Finally, we demonstrate that the Cs₂ZrCl₆ and Cs₂ZrBr₆ nanocrystal thin films exhibit tunable, broad white photoluminescence with quantum yields of 45% for the latter, with respective peaks in the blue and green spectral regions and mixed systems exhibiting properties between them. Our work represents a critical step toward the application of lead-free Cs₂ZrX₆ nanocrystal thin films into next-generation light-emitting applications.

The Cs2AgRhCl6 Halide Double Perovskite: A Dynamically Stable Lead-Free Transition-Metal Driven Semiconducting Material for Optoelectronics

A-Site doping with alkali ions, and/or metal substitution at the B and B'-sites, are among the key strategies in the innovative development of A 2BB'X6 halide double perovskite semiconducting materials for application in energy and device technologies. To this end, we have investigated an intriguing series of five halide-based non-toxic systems, A 2AgRhCl6 (A = Li, Na, K, Rb, and Cs), using density functional theory at the SCAN-rVV10 level. The lattice stability and bonding properties emanating from this study of A 2AgRhCl6 matched well with those that have already been synthesized, characterized and discussed [viz. Cs2AgBiX6 (X = Cl, Br)]. Exploration of traditional and recently proposed tolerance factors has enabled us to identify A 2AgRhCl6 (A = K, Rb and Cs) as stable double perovskites. The band structure and density of states calculations suggested that the electronic transition from the top of the valence band [Cl(3p)+Rh(4d)] to the bottom of the conduction band [(Cl(3p)+Rh(4d)] is inherently direct at the X-point of the first Brillouin zone. The (non-spin polarized) bandgap of these materials was found in the range 0.57-0.65 eV with SCAN-rVV10, which were substantially smaller than those computed with hybrid HSE06 and PBE0, and quasi-particle GW methods. This, together with the appreciable refractive index and high absorption coefficient in the region covering the range 1.0-4.5 eV, enabled us to demonstrate that A 2AgRhCl6 (A = K, Rb, and Cs) are likely candidate materials for photoelectric applications. The results of our phonon calculations at the harmonic level suggested that the Cs2AgRhCl6 is the only system that is dynamically stable (no imaginary frequencies found around the high symmetry lines of the reciprocal lattice), although the elastic moduli properties suggested all five systems examined are mechanically stable.

Defect Engineering of Earth-Abundant Solar Absorbers BiSI and BiSeI

Bismuth-based solar absorbers have recently garnered attention due to their promise as cheap, nontoxic, and efficient photovoltaics. To date, however, most show poor efficiencies far below those seen in commercial technologies. In this work, we investigate two such promising materials, BiSI and BiSeI, using relativistic first-principles methods with the aim of identifying their suitability for photovoltaic applications. Both compounds show excellent optoelectronic properties with ideal band gaps and strong optical absorption, leading to high predicted device performance. Using defect analysis, we reveal the electronic and structural effects that can lead to the presence of deep trap states, which may help explain the prior poor performance of these materials. Crucially, detailed mapping of the range of experimentally accessible synthesis conditions allows us to provide strategies to avoid the formation of killer defects in the future.

Chemical Vapor Deposition Synthesis and Optical Properties of Nb2O5 Thin Films with Hybrid Functional Theoretical Insight into the Band Structure and Band Gaps

Nb₂O₅ is an important material able to exist in many polymorphs with unique optical properties and morphologies that are dependent on the synthetic route. Here we report a novel ambient-pressure chemical vapor deposition route to Nb₂O₅ via aerosol-assisted chemical vapor deposition. The amorphous as-deposited films were annealed in air to obtain the the three most stable crystal structures: orthorhombic, tetragonal, and monoclinic. The films were thoroughly characterized for their material properties, and an in-depth study into the optical properties was carried out using state-of-the-art hybrid functional theory that allowed more insight into the optical properties of the materials.

Can Pb-Free Halide Double Perovskites Support High-Efficiency Solar Cells?

The methylammonium lead halides have become champion photoactive semiconductors for solar cell applications; however, issues still remain with respect to chemical instability and potential toxicity. Recently, the Cs₂AgBiX₆ (X = Cl, Br) double perovskite family has been synthesized and investigated as stable nontoxic replacements. We probe the chemical bonding, physical properties, and cation anti-site disorder of Cs₂AgBiX₆ and related compounds from first-principles. We demonstrate that the combination of Ag(I) and Bi(III) leads to the wide indirect band gaps with large carrier effective masses owing to a mismatch in angular momentum of the frontier atomic orbitals. The spectroscopically limited photovoltaic conversion efficiency is less than 10% for X = Cl or Br. This limitation can be overcome by replacing Ag with In or Tl; however, the resulting compounds are predicted to be unstable thermodynamically. The search for nontoxic bismuth perovskites must expand beyond the Cs₂AgBiX₆ motif.