Band Degeneracy and Anisotropy Enhances Thermoelectric Performance from Sb2Si2Te6 to Sc2Si2Te6.

The complex interrelationships among thermoelectric parameters mean that a priori design of high-performing materials is difficult. However, band engineering can allow the power factor to be optimized through enhancement of the Seebeck coefficient. Herein, using layered Sb2Si2Te6 and Sc2Si2Te6 as model systems, we comprehensively investigate and compare their thermoelectric properties by employing density functional theory combined with semiclassical Boltzmann transport theory. Our simulations reveal that Sb2Si2Te6 exhibits superior electrical conductivity compared to Sc2Si2Te6 due to lower scattering rates and more pronounced band dispersion. Remarkably, despite Sb2Si2Te6 exhibiting a lower lattice thermal conductivity and superior electrical conductivity, Sc2Si2Te6 is predicted to achieve an extraordinary dimensionless figure of merit (ZT) of 3.51 at 1000 K, which significantly surpasses the predicted maximum ZT of 2.76 for Sb2Si2Te6 at 900 K. We find the origin of this behavior to be a combined increase in band (valley) degeneracy and anisotropy upon switching the conduction band orbital character from Sb p to Sc d, yielding a significantly improved Seebeck coefficient. This work suggests that enhancing band degeneracy and anisotropy (complexity) through compositional variation is an effective strategy for improving the thermoelectric performance of layered materials.

Interface-Centric Strategies in Kesterite Solar Cells: Addressing Challenges, solutions, and Future Directions for Efficient Solar-Harvesting Technologies.

As reserves of non-renewable energy sources decline, the search for sustainable alternatives becomes increasingly critical. Next-generation energy materials play a key role in this quest by enabling the manipulation of properties for effective energy solutions and understanding interfaces to enhance energy yield. Studying these interfaces is essential for managing charge transport in optoelectronic devices, yet it presents significant challenges. This review emphasizes the critical role of interfaces in kesterite solar cells (KSCs), focusing on interfacial architecture, carrier losses, and non-radiative recombination. This review highlights the importance of addressing interface issues and utilizing advanced characterization tools to reveal interface properties. Current interface problems are addressed, recent advancements in interface engineering are summarized, and perspectives on future challenges and prospects are offered. The goal is to illuminate the nature of interfaces and tackle interface losses, which are crucial for improving device design and performance. Despite their pivotal role in device operation, comprehensive reviews on interfaces are lacking, underscoring the relevance of the work for researchers in material interfaces and device engineering. It is hoped that this article will spark interest and inspire further research into interface studies and the mitigation of interface losses.

Imperfections are not 0 K: free energy of point defects in crystals.

Defects determine many important properties and applications of materials, ranging from doping in semiconductors, to conductivity in mixed ionic-electronic conductors used in batteries, to active sites in catalysts. The theoretical description of defect formation in crystals has evolved substantially over the past century. Advances in supercomputing hardware, and the integration of new computational techniques such as machine learning, provide an opportunity to model longer length and time-scales than previously possible. In this Tutorial Review, we cover the description of free energies for defect formation at finite temperatures, including configurational (structural, electronic, spin) and vibrational terms. We discuss challenges in accounting for metastable defect configurations, progress such as machine learning force fields and thermodynamic integration to directly access entropic contributions, and bottlenecks in going beyond the dilute limit of defect formation. Such developments are necessary to support a new era of accurate defect predictions in computational materials chemistry.

Interplay of Static and Dynamic Disorder in the Mixed-Metal Chalcohalide Sn2SbS2I3.

Chalcohalide mixed-anion crystals have seen a rise in interest as "perovskite-inspired materials" with the goal of combining the ambient stability of metal chalcogenides with the exceptional optoelectronic performance of metal halides. Sn2SbS2I3 is a promising candidate, having achieved a photovoltaic power conversion efficiency above 4%. However, there is uncertainty over the crystal structure and physical properties of this crystal family. Using a first-principles cluster expansion approach, we predict a disordered room-temperature structure, comprising both static and dynamic cation disorder on different crystallographic sites. These predictions are confirmed using single-crystal X-ray diffraction. Disorder leads to a lowering of the bandgap from 1.8 eV at low temperature to 1.5 eV at the experimental annealing temperature of 573 K. Cation disorder tailoring the bandgap allows for targeted application or for the use in a graded solar cell, which when combined with material properties associated with defect and disorder tolerance, encourages further investigation into the group IV/V chalcohalide family for optoelectronic applications.

Correction: Lone pair driven anisotropy in antimony chalcogenide semiconductors.

Correction for 'Lone pair driven anisotropy in antimony chalcogenide semiconductors' by Xinwei Wang et al., Phys. Chem. Chem. Phys., 2022, 24, 7195-7202, https://doi.org/10.1039/D1CP05373F.

Impact of metastable defect structures on carrier recombination in solar cells.

The efficiency of a solar cell is often limited by electron-hole recombination mediated by defect states within the band gap of the photovoltaic (PV) semiconductor. The Shockley-Read-Hall (SRH) model considers a static trap that can successively capture electrons and holes. In reality however, true trap levels vary with both the defect charge state and local structure. Here we consider the role of metastable structural configurations in capturing electrons and holes, taking the tellurium interstitial in CdTe as an illustrative example. Consideration of the defect dynamics, and symmetry-breaking, changes the qualitative behaviour and activates new pathways for carrier capture. Our results reveal the potential importance of metastable defect structures in non-radiative recombination, in particular for semiconductors with anharmonic/ionic-covalent bonding, multinary compositions, low crystal symmetries or highly-mobile defects.

Lone pair driven anisotropy in antimony chalcogenide semiconductors.

Antimony sulfide (Sb2S3) and selenide (Sb2Se3) have emerged as promising earth-abundant alternatives among thin-film photovoltaic compounds. A distinguishing feature of these materials is their anisotropic crystal structures, which are composed of quasi-one-dimensional (1D) [Sb4X6]n ribbons. The interaction between ribbons has been reported to be van der Waals (vdW) in nature and Sb2X3 are thus commonly classified in the literature as 1D semiconductors. However, based on first-principles calculations, here we show that inter-ribbon interactions are present in Sb2X3 beyond the vdW regime. The origin of the anisotropic structures is related to the stereochemical activity of the Sb 5s lone pair according to electronic structure analysis. The impacts of structural anisotropy on the electronic, dielectric and optical properties relevant to solar cells are further examined, including the presence of higher dimensional Fermi surfaces for charge carrier transport. Our study provides guidelines for optimising the performance of Sb2X3-based photovoltaics via device structuring based on the underlying crystal anisotropy.

Perovskite-inspired materials for photovoltaics and beyond-from design to devices.

Lead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform industry-standard multi-crystalline silicon solar cells, despite the fact that perovskites are typically grown at low temperature using simple solution-based methods. However, the toxicity of lead and its ready solubility in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temperature, facile fabrication methods. This Review discusses the computational and experimental approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compounds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials in other optoelectronic applications, namely light-emitting diodes, photocatalysts, radiation detectors, thin film transistors and memristors. Finally, the prospects and key challenges faced by the field in advancing the development of perovskite-inspired materials towards realization in commercial devices is discussed.

Computational Prediction of an Antimony-Based n-Type Transparent Conducting Oxide: F-Doped Sb2O5.

Transparent conducting oxides (TCOs) possess a unique combination of optical transparency and electrical conductivity, making them indispensable in optoelectronic applications. However, their heavy dependence on a small number of established materials limits the range of devices that they can support. The discovery and development of additional wide bandgap oxides that can be doped to exhibit metallic-like conductivity are therefore necessary. In this work, we use hybrid density functional theory to identify a binary Sb(V) system, Sb2O5, as a promising TCO with high conductivity and transparency when doped with fluorine. We conducted a full point defect analysis, finding F-doped Sb2O5 to exhibit degenerate n-type transparent conducting behavior. The inherently large electron affinity found in antimony oxides also widens their application in organic solar cells. Following our previous work on zinc antimonate, this work provides additional support for designing Sb(V)-based oxides as cost-effective TCOs for a broader range of applications.

Oxygen Dimerization as a Defect-Driven Process in Bulk LiNiO2.

To explore the possibility of oxygen dimerization-particularly, the formation of molecular oxygen-like species-in the bulk of LiNiO2 lithium ion cathode materials at high states of charge, we conduct a redox-product structure search inspired by recent methodological developments for point-defect structure prediction. We find that (1) delithiated Li1-x NiO2 (x = 1) has good kinetic stability toward decomposition into molecular oxygen and reduced transition-metal oxides but (2) defects can act as nucleation sites for oxygen dimerization. These results help reconcile conflicting reports on the formation of bulk molecular oxygen in LiNiO2 and other nickel-rich cathode materials, highlighting the role of defect chemistry in driving the bulk degradation of these compounds.

Li-Site Defects Induce Formation of Li-Rich Impurity Phases: Implications for Charge Distribution and Performance of LiNi0.5- xMxMn1.5O4 Cathodes (M = Fe and Mg; x = 0.05-0.2).

An understanding of the structural properties that allow for optimal cathode performance, and their origin, is necessary for devising advanced cathode design strategies and accelerating the commercialization of next-generation cathodes. High-voltage, Fe- and Mg-substituted LiNi0.5Mn1.5O4 cathodes offer a low-cost, cobalt-free, yet energy-dense alternative to commercial cathodes. In this work, the effect of substitution on several important structure properties is explored, including Ni/Mn ordering, charge distribution, and extrinsic defects. In the cation-disordered samples studied, a correlation is observed between increased Fe/Mg substitution, Li-site defects, and Li-rich impurity phase formation-the concentrations of which are greater for Mg-substituted samples. This is attributed to the lower formation energy of MgLi defects when compared to FeLi defects. Li-site defect-induced impurity phases consequently alter the charge distribution of the system, resulting in increased [Mn3+] with Fe/Mg substitution. In addition to impurity phases, other charge compensators are also investigated to explain the origin of Mn3+ (extrinsic defects, [Ni3+], oxygen vacancies and intrinsic off-stoichiometry), although their effects are found to be negligible.

Understanding the Photocatalytic Activity of La5Ti2AgS5O7 and La5Ti2CuS5O7 for Green Hydrogen Production: Computational Insights.

Green production of hydrogen is possible with photocatalytic water splitting, where hydrogen is produced while water is reduced by using energy derived from light. In this study, density functional theory (DFT) is employed to gain insights into the photocatalytic performance of La5Ti2AgS5O7 and La5Ti2CuS5O7-two emerging candidate materials for water splitting. The electronic structure of both bulk materials was calculated by using hybrid DFT, which indicated the band gaps and charge carrier effective masses are suitable for photocatalytic water splitting. Notably, the unique one-dimensional octahedral TiO x S6-x and tetragonal MS4 channels formed provide a structural separation for photoexcited charge carriers which should inhibit charge recombination. Band alignments of surfaces that appear on the Wulff constructions of 12 nonpolar symmetric surface slabs were calculated by using hybrid DFT for each of the materials. All surfaces of La5Ti2AgS5O7 have band edge positions suitable for hydrogen evolution; however, the small overpotentials on the largest facets likely decrease the photocatalytic activity. In La5Ti2CuS5O7, 72% of the surface area can support oxygen evolution thermodynamically and kinetically. Based on their similar electronic structures, La5Ti2AgS5O7 and La5Ti2CuS5O7 could be effectively employed in Z-scheme photocatalytic water splitting.

Band versus Polaron: Charge Transport in Antimony Chalcogenides.

Antimony sulfide (Sb2S3) and selenide (Sb2Se3) are emerging earth-abundant absorbers for photovoltaic applications. Solar cell performance depends strongly on charge-carrier transport properties, but these remain poorly understood in Sb2X3 (X = S, Se). Here we report band-like transport in Sb2X3, determined by investigating the electron-lattice interaction and theoretical limits of carrier mobility using first-principles density functional theory and Boltzmann transport calculations. We demonstrate that transport in Sb2X3 is governed by large polarons with moderate Fröhlich coupling constants (α ≈ 2), large polaron radii (extending over several unit cells), and high carrier mobility (an isotropic average of >10 cm2 V-1 s-1 for both electrons and holes). The room-temperature mobility is intrinsically limited by scattering from polar phonon modes and is further reduced in highly defective samples. Our study confirms that the performance of Sb2X3 solar cells is not limited by intrinsic self-trapping.

Frenkel Excitons in Vacancy-Ordered Titanium Halide Perovskites (Cs2TiX6).

Low-cost, nontoxic, and earth-abundant photovoltaic materials are long-sought targets in the solar cell research community. Perovskite-inspired materials have emerged as promising candidates for this goal, with researchers employing materials design strategies including structural, dimensional, and compositional transformations to avoid the use of rare and toxic elemental constituents, while attempting to maintain high optoelectronic performance. These strategies have recently been invoked to propose Ti-based vacancy-ordered halide perovskites (A2TiX6; A = CH3NH3, Cs, Rb, or K; X = I, Br, or Cl) for photovoltaic operation, following the initial promise of Cs2SnX6 compounds. Theoretical investigations of these materials, however, consistently overestimate their band gaps, a fundamental property for photovoltaic applications. Here, we reveal strong excitonic effects as the origin of this discrepancy between theory and experiment, a consequence of both low structural dimensionality and band localization. These findings have vital implications for the optoelectronic application of these compounds while also highlighting the importance of frontier-orbital character for chemical substitution in materials design strategies.

Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination.

I-V-VI2 ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have slowly-rising absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS2 nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >105 cm-1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS2 nanocrystals. These unusual features arise because of the localised, non-bonding S p character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.