Complicated and Unconventional Defect Properties of the Quasi-One-Dimensional Photovoltaic Semiconductor Sb2Se3

Active Passivation of Anion Vacancies in Antimony Selenide Film for Efficient Solar Cells

Binary antimony selenide (Sb2Se3) is a promising inorganic light-harvesting material with high stability, nontoxicity, and wide light harvesting capability. In this photovoltaic material, it has been recognized that deep energy level defects with large carrier capture cross section, such as VSe (selenium vacancy), lead to serious open-circuit voltage (VOC) deficit and in turn limit the achievable power conversion efficiency (PCE) of Sb2Se3 solar cells. Understanding the nature of deep-level defects and establishing effective method to eliminate the defects are vital to improving VOC. In this study, a novel directed defect passivation strategy is proposed to suppress the formation of VSe and maintain the composition and morphology of Sb2Se3 film. In particular, through systematic study on the evolution of defect properties, the pathway of defect passivation reaction is revealed. Owing to the inhibition of defect-assisted recombination, the VOC increases, resulting in an improvement of PCE from 7.69% to 8.90%, which is the highest efficiency of Sb2Se3 solar cells prepared by thermal evaporation method with superstrate device configuration. This study proposes a new understanding of the nature of deep-level defects and enlightens the fabrication of high quality Sb2Se3 thin film for solar cell applications.

Analysis of Carrier Transport at Zn1-xSnxOy/Absorber Interface in Sb2(S,Se)3 Solar Cells

This work explores the effect of a Zn1-xSnxOy (ZTO) layer as a potential replacement for CdS in Sb2(S,Se)3 devices. Through the use of Afors-het software v2.5, it was determined that the ZTO/Sb2(S,Se)3 interface exhibits a lower conduction band offset (CBO) value of 0.34 eV compared to the CdS/Sb2(S,Se)3 interface. Lower photo-generated carrier recombination can be obtained at the interface of the ZTO/Sb2(S,Se)3 heterojunction. In addition, the valence band offset (VBO) value at the ZTO/Sb2(S,Se)3 interface increases to 1.55 eV. The ZTO layer increases the efficiency of the device from 7.56% to 11.45%. To further investigate the beneficial effect of the ZTO layer on the efficiency of the device, this goal has been achieved by five methods: changing the S content of the absorber, changing the thickness of the absorber, changing the carrier concentration of ZTO, using various Sn/(Zn+Sn) ratios in ZTO, and altering the thickness of the ZTO layer. When the S content in Sb2(S,Se)3 is around 60% and the carrier concentration is about 1018 cm-3, the efficiency is optimal. The optimal thickness of the Sb2(S,Se)3 absorber layer is 260 nm. A ZTO/Sb2(S,Se)3 interface with a Sn/(Zn+Sn) ratio of 0.18 exhibits a better CBO value. It is also found that a ZTO thickness of 20 nm is needed for the best efficiency.

Over 10% Efficient Sb2(S,Se)3 Solar Cells Enabled by CsI-Doping Strategy

Antimony selenosulfide (Sb2(S,Se)3) is an emerging quasi-1D photovoltaic semiconductor with exceptional photoelectric properties. The low-symmetry chain structure contains complex defects and makes it difficult to improve electrical properties via doping method. This article reports a doping strategy to enhance the efficiency of Sb2(S,Se)3 solar cells by using alkali halide (CsI) as the hydrothermal reaction precursor. It is found that the Cs and I ions are effectively doped and atomically coordinate with Sb ions and S/Se ions. The CsI-doping Sb2(S,Se)3 absorbers exhibit enhanced grain morphologies and reduced trap densities. The consequential CsI-doping Sb2(S,Se)3 based solar cells demonstrate favorable band alignment, suppressed carrier recombination, and improved device performance. An efficiency as high as 10.05% under standard AM1.5 illumination irradiance is achieved. This precursor-based alkali halide doping strategy provides a useful guidance for high-efficiency antimony selenosulfide solar cells.

Mechanistic Study of the Transition from Antimony Oxide to Antimony Sulfide in the Hydrothermal Process to Obtain Highly Efficient Solar Cells

Obtaining high-quality absorber layers is a major task for constructing efficient thin-film solar cells. Hydrothermal deposition is considered a promising method for preparing high-quality antimony sulfide (Sb₂ S₃ ) films for solar cell applications. In the hydrothermal process, the precursor reactants play an important role in controlling the film formation process and thus the film quality. In this study, Sb₂ O₃ is applied as a new Sb source to replace the traditional antimony potassium tartrate to modulate the growth process of the Sb₂ S₃ film. The reaction mechanism of the transition from Sb₂ O₃ to Sb₂ S₃ in the hydrothermal process is revealed. Through controlling the nucleation and deposition processes, high-quality Sb₂ S₃ films are prepared with longer carrier lifetimes and lower deep-level defect densities than those prepared from the traditional Sb source of antimony potassium tartrate. Consequently, a solar cell device based on this improved Sb₂ S₃ delivers a high power conversion efficiency of 6.51 %, which is in the top tier for Sb₂ S₃ -based solar devices using hydrothermal methods. This research provides a new and competitive Sb source for hydrothermal growth of high-quality antimony chalcogenide films for solar cell applications.

Fast near-infrared photodetectors from p-type SnSe nanoribbons

Low-dimensional tin selenide nanoribbons (SnSe NRs) show a wide range of applications in optoelectronics fields such as optical switches, photodetectors, and photovoltaic devices due to the suitable band gap, strong light-matter interaction, and high carrier mobility. However, it is still challenging to grow high-quality SnSe NRs for high-performance photodetectors so far. In this work, we successfully synthesized high-quality p-type SnSe NRs by chemical vapor deposition and then fabricated near-infrared photodetectors. The SnSe NR photodetectors show a high responsivity of 376.71 A W^-1, external quantum efficiency of 5.65 × 10⁴%, and detectivity of 8.66 × 10¹¹Jones. In addition, the devices show a fast response time with rise and fall time of up to 43μs and 57μs, respectively. Furthermore, the spatially resolved scanning photocurrent mapping shows very strong photocurrent at the metal-semiconductor contact regions, as well as fast generation-recombination photocurrent signals. This work demonstrated that p-type SnSe NRs are promising material candidates for broad-spectrum and fast-response optoelectronic devices.

Deep defects limiting the conversion efficiency of Sb2Se3 thin-film solar cells

Quasi-one-dimensional (Q1D) semiconductor antimony selenide (Sb₂Se₃) shows great potential in the photovoltaic field, but the photoelectric conversion efficiency (PCE) of Sb₂Se₃-based solar cells has shown no obvious breakthrough during the past several years, of which the intrinsic reasons are pending experimentally. Here, we prepare high-quality Q1D Sb₂Se₃ thin films via the vapor transport deposition technique. By investigating the bandedge electronic level structure and carrier relaxation/recombination dynamics, we find that (i) the optimized Se-rich growth conditions can highly improve the crystal quality of the Q1D Sb₂Se₃ thin films, the carrier lifetime of which is substantially increased up to ∼8.3 μs; (ii) the Se-rich growth conditions have advantages to annihilate the deep selenium vacancies V_Sei (i = 1 and 3 for non-equivalent Se atomic sites) but is not effective for the deep donor V_Se2, which locates at ∼0.3 eV (300 K) below the conduction band and intrinsically limits the PCE value of devices below ∼7.63%. This work suggests that further optimizing the Se-rich conditions to technically eliminate this kind of deep defect is still essential for preparing high-performance Sb₂Se₃ film solar cells.

Thermally Driven Point Defect Transformation in Antimony Selenosulfide Photovoltaic Materials

Thermal treatment of inorganic thin films is a general and necessary step to facilitate crystallization and, in particular, to regulate the formation of point defects. Understanding the dependence of the defect formation mechanism on the annealing process is a critical challenge in terms of designing material synthesis approaches for obtaining desired optoelectronic properties. Herein, a mechanistic understanding of the evolution of defects in emerging Sb₂ (S,Se)₃ solar cell films is presented. A top-efficiency Sb₂ (S,Se)₃ solar-cell film is adopted in this study to consolidate this investigation. This study reveals that, under hydrothermal conditions, the as-deposited Sb₂ (S,Se)₃ film generates defects with a high formation energy, demonstrating kinetically favorable defect formation characteristics. Annealing at elevated temperatures leads to a two-step defect transformation process: 1) formation of sulfur and selenium vacancy defects, followed by 2) migration of antimony ions to fill the vacancy defects. This process finally results in the generation of cation-anion antisite defects, which exhibit low formation energy, suggesting a thermodynamically favorable defect formation feature. This study establishes a new strategy for the fundamental investigation of the evolution of deep-level defects in metal chalcogenide films and provides guidance for designing material synthesis strategies in terms of defect control.

Photoreflectance and Photoluminescence Study of Antimony Selenide Crystals

Among inorganic, Earth-abundant, and low-toxicity photovoltaic technologies, Sb₂Se₃ has emerged as a strong material contender reaching over 10% solar cell power conversion efficiency. Nevertheless, the bottleneck of this technology is the high deficit of open-circuit voltage (V _OC) as seen in many other emerging chalcogenide technologies. Commonly, the loss of V _OC is related to the nonradiative carrier recombination through defects, but other material characteristics can also limit the achievable V _OC. It has been reported that in isostructural compound Sb₂S₃, self-trapped excitons are readily formed leading to 0.6 eV Stokes redshift in photoluminescence (PL) and therefore significantly reducing the obtainable V _OC. However, whether Sb₂Se₃ has the same limitations has not yet been examined. In this work, we aim to identify main radiative carrier recombination mechanisms in Sb₂Se₃ single crystals and estimate if there is a fundamental limit for obtainable V _OC. Optical transitions in Sb₂Se₃ were studied by means of photoreflectance and PL spectroscopy. Temperature, excitation intensity, and polarization-dependent optical characteristics were measured and analyzed. We found that at low temperature, three distinct radiative recombination mechanisms were present and were strongly influenced by the impurities. The most intensive PL emissions were located near the band edge. In conclusion, no evidence of emission from self-trapped excitons or band-tails was observed, suggesting that there is no fundamental limitation to achieve high V _OC, which is very important for further development of Sb₂Se₃-based solar cells.

Oxygen Content Modulation Toward Highly Efficient Sb2Se3 Solar Cells

Vapor-transport deposition (VTD) method is the main technique for the preparation of Sb₂Se₃ films. However, oxygen is often present in the vacuum tube in such a vacuum deposition process, and Sb₂O₃ is formed on the surface of Sb₂Se₃ because the bonding of Sb-O is formed more easily than that of Sb-Se. In this work, the formation of Sb₂O₃ and thus the carrier transport in the corresponding solar cells were studied by tailoring the deposition microenvironment in the vacuum tube during Sb₂Se₃ film deposition. Combined by different characterization techniques, we found that tailoring the deposition microenvironment can not only effectively inhibit the formation of Sb₂O₃ at the CdS/Sb₂Se₃ interface but also enhance the crystalline quality of the Sb₂Se₃ thin film. In particular, such modification induces the formation of (hkl, l = 1)-oriented Sb₂Se₃ thin films, reducing the interface recombination of the subsequently fabricated devices. Finally, the Sb₂Se₃ solar cell with the configuration of ITO/CdS/Sb₂Se₃/Spiro-OMeTAD/Au achieves a champion efficiency of 7.27%, a high record for Sb₂Se₃ solar cells prepared by the VTD method. This work offers guidance for the preparation of high-efficiency Sb₂Se₃ thin-film solar cells under rough-vacuum conditions.

Coupled Electronic and Anharmonic Structural Dynamics for Carrier Self-Trapping in Photovoltaic Antimony Chalcogenides

V-VI antimony chalcogenide semiconductors have shown exciting potentials for thin film photovoltaic applications. However, their solar cell efficiencies are strongly hampered by anomalously large voltage loss (>0.6 V), whose origin remains controversial so far. Herein, by combining ultrafast pump-probe spectroscopy and density functional theory (DFT) calculation, the coupled electronic and structural dynamics leading to excited state self-trapping in antimony chalcogenides with atomic level characterizations is reported. The electronic dynamics in Sb2 Se3 indicates a ≈20 ps barrierless intrinsic self-trapping, with electron localization and accompanied lattice distortion given by DFT calculations. Furthermore, impulsive vibrational coherences unveil key SbSe vibrational modes and their real-time interplay that drive initial excited state relaxation and energy dissipation toward stabilized small polaron through electron-phonon and subsequent phonon-phonon coupling. This study's findings provide conclusive evidence of carrier self-trapping arising from intrinsic lattice anharmonicity and polaronic effect in antimony chalcogenides and a new understanding on the coupled electronic and structural dynamics for redefining excited state properties in soft semiconductor materials.

Sb2 Se3 Thin-Film Solar Cells Exceeding 10% Power Conversion Efficiency Enabled by Injection Vapor Deposition Technology

Binary Sb₂ Se₃ semiconductors are promising as the absorber materials in inorganic chalcogenide compound photovoltaics due to their attractive anisotropic optoelectronic properties. However, Sb₂ Se₃ solar cells suffer from complex and unconventional intrinsic defects due to the low symmetry of the quasi-1D crystal structure resulting in a considerable voltage deficit, which limits the ultimate power conversion efficiency (PCE). In this work, the creation of compact Sb₂ Se₃ films with strong [00l] orientation, high crystallinity, minimal deep level defect density, fewer trap states, and low non-radiative recombination loss by injection vapor deposition is reported. This deposition technique enables superior films compared with close-spaced sublimation and coevaporation technologies. The resulting Sb₂ Se₃ thin-film solar cells yield a PCE of 10.12%, owing to the suppressed carrier recombination and excellent carrier transport and extraction. This method thus opens a new and effective avenue for the fabrication of high-quality Sb₂ Se₃ and other high-quality chalcogenide semiconductors.

Tunable Color-Variable Solar Absorber Based on Phase Change Material Sb2Se3

In this paper, a dynamic color-variable solar absorber is designed based on the phase change material Sb₂Se₃. High absorption is maintained under both amorphous Sb₂Se₃ (aSb₂Se₃) and crystalline Sb₂Se₃ (cSb₂Se₃). Before and after the phase transition leading to the peak change, the structure shows a clear color contrast. Due to peak displacement, the color change is also evident for different crystalline fractions during the phase transition. Different incident angles irradiate the structure, which can also cause the structure to show rich color variations. The structure is insensitive to the polarization angle because of the high symmetry. At the same time, different geometric parameters enable different color displays, so the structure can have good application prospects.

Heterojunction Annealing Enabling Record Open-Circuit Voltage in Antimony Triselenide Solar Cells

Despite the fact that antimony triselenide (Sb₂ Se₃ ) thin-film solar cells have undergone rapid development in recent years, the large open-circuit voltage (V_OC ) deficit still remains as the biggest bottleneck, as even the world-record device suffers from a large V_OC deficit of 0.59 V. Here, an effective interface engineering approach is reported where the Sb₂ Se₃ /CdS heterojunction (HTJ) is subjected to a post-annealing treatment using a rapid thermal process. It is found that nonradiative recombination near the Sb₂ Se₃ /CdS HTJ, including interface recombination and space charge region recombination, is greatly suppressed after the HTJ annealing treatment. Ultimately, a substrate Sb₂ Se₃ /CdS thin-film solar cell with a competitive power conversion efficiency of 8.64% and a record V_OC of 0.52 V is successfully fabricated. The device exhibits a much mitigated V_OC deficit of 0.49 V, which is lower than that of any other reported efficient antimony chalcogenide solar cell.

Does Sb2Se3 Admit Nonstoichiometric Conditions? How Modifying the Overall Se Content Affects the Structural, Optical, and Optoelectronic Properties of Sb2Se3 Thin Films

Sb₂Se₃ is a quasi-one-dimensional (1D) semiconductor, which has shown great promise in photovoltaics. However, its performance is currently limited by a high V_oc deficit. Therefore, it is necessary to explore new strategies to minimize the formation of intrinsic defects and thus unlock the absorber's whole potential. It has been reported that tuning the Se/Sb relative content could enable a selective control of the defects. Furthermore, recent experimental evidence has shown that moderate Se excess enhances the photovoltaic performance; however, it is not yet clear whether this excess has been incorporated into the structure. In this work, a series of Sb₂Se₃ thin films have been prepared imposing different nominal compositions (from Sb-rich to Se-rich) and then have been thoroughly characterized using compositional, structural, and optical analysis techniques. Hence, it is shown that Sb₂Se₃ does not allow an extended range of nonstoichiometric conditions. Instead, any Sb or Se excesses are compensated in the form of secondary phases. Also, a correlation has been found between operating under Se-rich conditions and an improvement in the crystalline orientation, which is likely related to the formation of a MoSe₂ phase in the back interface. Finally, this study shows new utilities of Raman, X-ray diffraction, and photothermal deflection spectroscopy combination techniques to examine the structural properties of Sb₂Se₃, especially how well-oriented the material is.

Searching for Band-Dispersive and Defect-Tolerant Semiconductors from Element Substitution in Topological Materials

Topological insulators and semimetal materials composed of heavy elements usually have inverted and dispersive band structures. It is interesting to notice that if lighter elements with reduced spin-orbit coupling are substituted for the heavy elements, the topological materials can be mutated into semiconductors with variable band gaps; for example, topological HgTe and Bi₂Se₃ can be mutated into CdTe and Sb₂Se₃, which are excellent optoelectronic semiconductors because the element substitution opens the band gap and meanwhile inherits the large band dispersion and high carrier mobility. Recently, many topological materials have been reported, and their databases have been built. Here, we demonstrate that these new topological materials can be used as the starting points to search for semiconductors with high carrier mobility and defect tolerance through element substitution. We take three recently discovered topological materials, Na₃Bi, Pb₂Bi₂Te₅, and EuCd₂Sb₂, as the benchmark systems to show the general validity of this strategy and find that the derived Na₃P, Na₃As, Sn₂Sb₂S₅, and CaZn₂N₂ are all band-dispersive and defect-tolerant semiconductors with potential optoelectronic applications. For Na₃P, Na₃As, and Na₃Sb, the new P3̅c1 structure derived from the topological Na₃Bi is found unexpectedly to be their ground-state structure, more stable than their well-known structures reported in the literature. This study not only gains new insights into the physical properties of these semiconductors but also proposes an effective strategy for the search of band-dispersive and defect-tolerant semiconductors that can be generalized to other topological materials.

Distinctive Deep-Level Defects in Non-Stoichiometric Sb2 Se3 Photovoltaic Materials

Characterizing defect levels and identifying the compositional elements in semiconducting materials are important research subject for understanding the mechanism of photogenerated carrier recombination and reducing energy loss during solar energy conversion. Here it shows that deep-level defect in antimony triselenide (Sb₂ Se₃ ) is sensitively dependent on the stoichiometry. For the first time it experimentally observes the formation of amphoteric Sb_Se defect in Sb-rich Sb₂ Se₃ . This amphoteric defect possesses equivalent capability of trapping electron and hole, which plays critical role in charge recombination and device performance. In comparative investigation, it also uncovers the reason why Se-rich Sb₂ Se₃ is able to deliver high device performance from the defect formation perspective. This study demonstrates the crucial defect types in Sb₂ Se₃ and provides a guidance toward the fabrication of efficient Sb₂ Se₃ photovoltaic device and relevant optoelectronic devices.

Crystal Growth Promotion and Defects Healing Enable Minimum Open-Circuit Voltage Deficit in Antimony Selenide Solar Cells

Antimony selenide (Sb₂ Se₃ ) is an ideal photovoltaic candidate profiting from its advantageous material characteristics and superior optoelectronic properties, and has gained considerable development in recent years. However, the further device efficiency breakthrough is largely plagued by severe open-circuit voltage (V_OC ) deficit under the existence of multiple defect states and detrimental recombination loss. In this work, an effective absorber layer growth engineering involved with vapor transport deposition and post-selenization is developed to grow Sb₂ Se₃ thin films. High-quality Sb₂ Se₃ with large compact crystal grains, benign [hk1] growth orientation, stoichiometric chemical composition, and suitable direct bandgap are successfully fulfilled under an optimized post-selenization scenario. Planar Sb₂ Se₃ thin-film solar cells with substrate configuration of Mo/Sb₂ Se₃ /CdS/ITO/Ag are constructed. By contrast, such engineering effort can remarkably mitigate the device V_OC deficit, owing to the healed detrimental defects, the suppressed interface and space-charge region recombination, the prolonged carrier lifetime, and the enhanced charge transport. Accordingly, a minimum V_OC deficit of 0.647 V contributes to a record V_OC of 0.513 V, a champion device with highly interesting efficiency of 7.40% is also comparable to those state-of-the-art Sb₂ Se₃ solar cells, paving a bright avenue to broaden its scope of photovoltaic applications.

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.

Quasi-Vertically Oriented Sb2Se3 Thin-Film Solar Cells with Open-Circuit Voltage Exceeding 500 mV Prepared via Close-Space Sublimation and Selenization

Sb₂Se₃, one of the most desirable absorption materials for next-generation thin-film solar cells, has an excellent photovoltaic characteristic. The [hk1]-oriented (quasi-vertically oriented) Sb₂Se₃ thin film is more beneficial for promoting efficient carrier transport than the [hk0]-oriented Sb₂Se₃ thin film. Controlling thin-film orientation remains the main obstacle to the further improvement in the efficiency of Sb₂Se₃-based solar cells. In this work, the controlled [hk0] or [hk1] orientation of the Sb₂Se₃ precursor is readily adjusted by tuning the substrate temperature and the distance between the source and the sample in close-space sublimation (CSS). Well-crystallized stoichiometric Sb₂Se₃ thin films with the desired orientation and large crystal grains are successfully prepared after selenization. Sb₂Se₃ thin-film solar cells in a substrate configuration of glass/Mo/Sb₂Se₃/CdS/ITO/Ag are fabricated with a power conversion efficiency of 4.86% with a record open-circuit voltage (V_OC) of 509 mV. The significant improvement in V_OC is closely related to the quasi-vertically oriented Sb₂Se₃ absorber layer with reduced deep-level defect density in the bulk and defect passivation at the Sb₂Se₃/CdS heterojunction. This work indicates that CSS and selenization show a remarkable potential for the fabrication of high-efficiency Sb₂Se₃ solar cells.

More Se Vacancies in Sb2 Se3 under Se-Rich Conditions: An Abnormal Behavior Induced by Defect-Correlation in Compensated Compound Semiconductors

It was believed that the Se-rich synthesis condition can suppress the formation of deep-level donor defect V_Se (selenium vacancy) in Sb₂ Se₃ and is thus critical for fabricating high-efficiency Sb₂ Se₃ solar cells. However, here it is shown that by first-principles calculations the density of V_Se increases unexpectedly to 10¹⁶ cm^-3 when the Se chemical potential increases, so Se-rich condition promotes rather than suppresses the formation of V_Se . Therefore, high density of V_Se is thermodynamically inevitable, no matter under Se-poor or Se-rich conditions. This abnormal behavior can be explained by a physical concept "defect-correlation", i.e., when donor and acceptor defects compensate each other, all defects become correlated with each other due to the formation energy dependence on Fermi level which is determined by densities of all ionized defects. In quasi-1D Sb₂ Se₃ , there are many defects and the complicated defect-correlation can give rise to abnormal behaviors, e.g., lowering Fermi level and thus decreasing the formation energy of ionized donor V_Se ²⁺ in Se-rich Sb₂ Se₃ . Such behavior exists also in Sb₂ S₃ . It explains the recent experiments that the extremely Se-rich condition causes the efficiency drop of Sb₂ Se₃ solar cells, and demonstrates that the common chemical intuition and defect engineering strategies may be invalid in compensated semiconductors.

Alternative Lone-Pair ns2 -Cation-Based Semiconductors beyond Lead Halide Perovskites for Optoelectronic Applications

Lead halide perovskites have emerged in the last decade as advantageous high-performance optoelectronic semiconductors, and have undergone rapid development for diverse applications such as solar cells, light-emitting diodes , and photodetectors. While material instability and lead toxicity are still major concerns hindering their commercialization, they offer promising prospects and design principles for developing promising optoelectronic materials. The distinguished optoelectronic properties of lead halide perovskites stem from the Pb²⁺ cation with a lone-pair 6s² electronic configuration embedded in a mixed covalent-ionic bonding lattice. Herein, we summarize alternative Pb-free semiconductors containing lone-pair ns² cations, intending to offer insights for developing potential optoelectronic materials other than lead halide perovskites. We start with the physical underpinning of how the ns² cations within the material lattice allow for superior optoelectronic properties. We then review the emerging Pb-free semiconductors containing ns² cations in terms of structural dimensionality, which is crucial for optoelectronic performance. For each category of materials, the research progresses on crystal structures, electronic/optical properties, device applications, and recent efforts for performance enhancements are overviewed. Finally, the issues hindering the further developments of studied materials are surveyed along with possible strategies to overcome them, which also provides an outlook on the future research in this field.

Revealing composition and structure dependent deep-level defect in antimony trisulfide photovoltaics

Antimony trisulfide (Sb2S3) is a kind of emerging light-harvesting material with excellent stability and abundant elemental storage. Due to the quasi-one-dimensional symmetry, theoretical investigations have pointed out that there exist complicated defect properties. However, there is no experimental verification on the defect property. Here, we conduct optical deep-level transient spectroscopy to investigate defect properties in Sb2S3 and show that there are maximum three kinds of deep-level defects observed, depending on the composition of Sb2S3. We also find that the Sb-interstitial (Sbi) defect does not show critical influence on the carrier lifetime, indicating the high tolerance of the one-dimensional crystal structure where the space of (Sb4S6)n ribbons is able to accommodate impurities to certain extent. This study provides basic understanding on the defect properties of quasi-one-dimensional materials and a guidance for the efficiency improvement of Sb2S3 solar cells.

Ab initio prediction of semiconductivity in a novel two-dimensional Sb2X3 (X= S, Se, Te) monolayers with orthorhombic structure

[Formula: see text] and [Formula: see text] are well-known layered bulk structures with weak van der Waals interactions. In this work we explore the atomic lattice, dynamical stability, electronic and optical properties of [Formula: see text], [Formula: see text] and [Formula: see text] monolayers using the density functional theory simulations. Molecular dynamics and phonon dispersion results show the desirable thermal and dynamical stability of studied nanosheets. On the basis of HSE06 and PBE/GGA functionals, we show that all the considered novel monolayers are semiconductors. Using the HSE06 functional the electronic bandgap of [Formula: see text], [Formula: see text] and [Formula: see text] monolayers are predicted to be 2.15, 1.35 and 1.37 eV, respectively. Optical simulations show that the first absorption coefficient peak for [Formula: see text], [Formula: see text] and [Formula: see text] monolayers along in-plane polarization is suitable for the absorption of the visible and IR range of light. Interestingly, optically anisotropic character along planar directions can be desirable for polarization-sensitive photodetectors. Furthermore, we systematically investigate the electrical transport properties with combined first-principles and Boltzmann transport theory calculations. At optimal doping concentration, we found the considerable larger power factor values of 2.69, 4.91, and 5.45 for hole-doped [Formula: see text], [Formula: see text], and [Formula: see text], respectively. This study highlights the bright prospect for the application of [Formula: see text], [Formula: see text] and [Formula: see text] nanosheets in novel electronic, optical and energy conversion systems.

Synergistically enhanced thermoelectric performance by optimizing the composite ratio between hydrothermal Sb2Se3 and self-assembled β-Cu2Se nanowires

Sb₂Se₃ and β-Cu₂Se nanowires were synthesized via hydrothermal reaction and a water-evaporation induced self-assembly method, respectively, and a 70%-Sb₂Se₃ and 30%-β-Cu₂Se disk pellet shows enhanced thermoelectric performance.

Efficient and Stable Planar n-i-p Sb2Se3 Solar Cells Enabled by Oriented 1D Trigonal Selenium Structures

Environmentally benign and potentially cost-effective Sb2Se3 solar cells have drawn much attention by continuously achieving new efficiency records. This article reports a compatible strategy to enhance the efficiency of planar n-i-p Sb2Se3 solar cells through Sb2Se3 surface modification and an architecture with oriented 1D van der Waals material, trigonal selenium (t-Se). A seed layer assisted successive close spaced sublimation (CSS) is developed to fabricate highly crystalline Sb2Se3 absorbers. It is found that the Sb2Se3 absorber exhibits a Se-deficient surface and negative surface band bending. Reactive Se is innovatively introduced to compensate the surface Se deficiency and form an (101) oriented 1D t-Se interlayer. The p-type t-Se layer promotes a favored band alignment and band bending at the Sb2Se3/t-Se interface, and functionally works as a surface passivation and hole transport material, which significantly suppresses interface recombination and enhances carrier extraction efficiency. An efficiency of 7.45% is obtained in a planar Sb2Se3 solar cell in superstrate n-i-p configuration, which is the highest efficiency for planar Sb2Se3 solar cells prepared by CSS. The all-inorganic Sb2Se3 solar cell with t-Se shows superb stability, retaining ≈98% of the initial efficiency after 40 days storage in open air without encapsulation.

Structure, Morphology, and Photoelectric Performances of Te-Sb2Se3 Thin Film Prepared via Magnetron Sputtering

Antimony selenide (Sb₂Se₃) has been widely investigated as a promising absorber material for photovoltaic devices. However, low open-circuit voltage (V_oc) limits the power conversion efficiency (PCE) of Sb₂Se₃-based cells, largely due to the low-charge carrier density. Herein, high-quality n-type (Tellurium) Te-doped Sb₂Se₃ thin films were successfully prepared using a homemade target via magnetron sputtering. The Te atoms were expected to be inserted in the spacing of (Sb₄Se₆)_n ribbons based on increased lattice parameters in this study. Moreover, the thin film was found to possess a narrow and direct band gap of approximately 1.27 eV, appropriate for harvesting the solar energy. It was found that the photoelectric performance is related to not only the quality of films but also the preferred growth orientation. The Te-Sb₂Se₃ film annealed at 325 °C showed a maximum photocurrent density of 1.91 mA/cm² with a light intensity of 10.5 mW/cm² at a bias of 1.4 V. The fast response and recovery speed confirms the great potential of these films as excellent photodetectors.

Fundamental Physical Characterization of Sb2Se3-Based Quasi-Homojunction Thin Film Solar Cells

A new type of solar cell based on Cu-doped (p-type) and I-doped (n-type) Sb₂Se₃ has been designed and fabricated using magnetron sputtering with two different thicknesses of absorber. The overall objective is for better understanding the charge recombination mechanism, especially at the interface region. The investigation has been specifically performed using IMPS (intensity modulated photocurrent spectroscopy), IMVS (intensity modulated photovoltage spectroscopy), and diode characterizations. It has been found that an increase of the absorber thickness leads to a shorter carrier lifetime, but longer diffusion length and lower trap density, resulting in significantly better performance. Furthermore, it is demonstrated that trap-assisted recombination does not affect the short-circuit current density (J_sc), but significantly decreases the open-circuit voltage (V_oc). As a result, an encouraging power conversion efficiency (PCE) of 2.41%, fill factor (FF) of 41%, J_sc of 20 mA/cm², and V_oc of 294 mV are obtained. Most importantly, key parameters for further increasing the PCE have been identified.

Antimony selenide/graphene oxide composite for sensitive photoelectrochemical detection of DNA methyltransferase activity

An Sb₂Se₃/graphene oxide composite was applied as both the photoelectrochemical probe and substrate for biomolecule conjugation for the construction of a “signal-off” sandwich-type biosensor for DNA methyltransferase activity detection.