Monolithic Two-Terminal Tandem Solar Cells Using Sb2S3 and Solution-Processed PbS Quantum Dots Achieving an Open-Circuit Potential beyond 1.1 V

Multijunction solar cells have the prospect of a greater theoretical efficiency limit than single-junction solar cells by minimizing the transmissive and thermalization losses a single absorber material has. In solar cell applications, Sb2S3 is considered an attractive absorber due to its elemental abundance, stability, and high absorption coefficient in the visible range of the solar spectrum, yet with a band gap of 1.7 eV, it is transmissive for near-IR and IR photons. Using it as the top cell (the cell where light is first incident) in a two-terminal tandem architecture in combination with a bottom cell (the cell where light arrives second) of PbS quantum dots (QDs), which have an adjustable band gap suitable for absorbing longer wavelengths, is a promising approach to harvest the solar spectrum more effectively. In this work, these two subcells are monolithically fabricated and connected in series by a poly(3,4-ethylene-dioxythiophene) polystyrene sulfonate (PEDOT:PSS)-ZnO tunnel junction as the recombination layer. We explore the surface morphology of ZnO QD films with different spin-coating conditions, which serve as the PbS QD cell's electron transport material. Furthermore, we examine the differences in photogenerated current upon varying the PbS QD absorber layer thickness and the electrical and optical characteristics of the tandem with respect to the stand-alone reference cells. This tandem architecture demonstrates an extended spectral response into the IR with an open-circuit potential exceeding 1.1 V and a power conversion efficiency of 5.6%, which is greater than that of each single-junction cell.

Solution-Processed Sb2S3-Based Heterojunction for Self-Powered Broad Band Weak Light Detection

Antimony sulfide (Sb2S3) has recently regained the attention of photovoltaic researchers as a promising solar absorber; however, its application for the detection of white light remains relatively unexplored. Herein, we report on the self-powered heterojunction photodetector based on a device-grade Sb2S3 film made at low temperature with solution processing. The Sb2S3 absorber film was prepared by single-step spin coating of a novel precursor ink using antimony trichloride as the antimony source along with the low melting thioacetamide as the sulfur source in 2-methoxyethanol, a low boiling environmentally friendly solvent. A simple TiO2/Sb2S3 heterojunction device made by using the film shows a power conversion efficiency of 1.22% without any hole transporting layer. Interestingly, the self-powered photodetector performance of the device under white light exhibits a high on/off ratio of 2.2 × 104 under 1 sun illumination. Moreover, this optical filter-free ultraviolet-visible absorbing near-infrared blind photodetector is equally capable of detecting both strong and weak white light, with a response time of 98 ms. Further, an example of the real-life application of the device is successfully demonstrated by constructing a weak light-detecting sunlight tracking system.

Space-Charging Interfacial Layer by Illumination for Efficient Sb2S3 Bulk-Heterojunction Solar Cells with High Open-Circuit Voltage

Solution-processed material systems for effective photovoltaic conversion are the key to low-cost and efficient solar cells. While antimony trisulfide (Sb₂S₃) is a promising photovoltaic absorber, solution-processed quality Sb₂S₃-based heterojunction systems for solar cells, particularly with an open-circuit voltage (V_oc) higher than 0.70 V, are challenging issues. Here, a cadmium sulfide (CdS) interfacial engineering method is developed for the Sb₂S₃-based bulk-heterojunction (BHJ) solar cells with an efficiency of 6.14% and a V_oc up to 0.76 V that is the highest one among solution-processed Sb₂S₃ solar cells. The prepared Sb₂S₃-based BHJ solar cells feature a Sb₂S₃ nanoparticle film interdigitated by a titania (TiO₂) nanorod array with a nanostructured CdS shell as an interfacial layer on each TiO₂ nanorod core. Upon understanding the interfacial interactions and band alignments in the TiO₂-CdS-Sb₂S₃ system, the function of the CdS interfacial layer as a band-bended spatial spacer interacting strongly with both the TiO₂ electron transporter and Sb₂S₃ absorber for increasing charge collecting efficiency is revealed; moreover, space-charging the band-bended CdS layer by illumination is found and a photogenerated interfacial dipole electric field model is proposed for understanding the high V_oc subjected to the presence of the CdS interfacial layer. This work provides a conceptual guide for designing efficient inorganic heterojunction solar cells.

4.9% Efficient Sb2S3 Solar Cells from Semitransparent Absorbers with Fluorene-Based Thiophene-Terminated Hole Conductors

Fluorene-based hole transport materials (HTMs) with terminating thiophene units are explored, for the first time, for antimony sulfide (Sb₂S₃) solar cells. These HTMs possess largely simplified synthesis processes and high yields compared to the conventional expensive hole conductors making them reasonably economical. The thiophene unit-linked HTMs have been successfully demonstrated in ultrasonic spray-deposited Sb₂S₃ solar cells resulting in efficiencies in the range of 4.7-4.9% with an average visible transmittance (AVT) of 30-33% (400-800 nm) for the cell stack without metal contact, while the cells fabricated using conventional P3HT have yielded an efficiency of 4.7% with an AVT of 26%. The study puts forward cost-effective and transparent HTMs that avoid a post-coating activation at elevated temperatures like P3HT, devoid of parasitic absorption losses in the visible region and are demonstrated to be well aligned for the band edges of Sb₂S₃ thereby ascertaining their suitability for Sb₂S₃ solar cells and are potential candidates for semitransparent applications.

N, Ru Codoped Pellet Drum Bundle-Like Sb2S3: An Efficient Hydrogen Evolution Reaction and Hydrogen Oxidation Reaction Electrocatalyst in Alkaline Medium

Though investigations have been made on several metal chalcogenides in hydrogen evolution reactions (HERs) and hydrogen oxidation reactions (HORs), antimony sulfide (Sb₂S₃) has not generated much attention. In this direction, the present work reports on the synthesis of N, Ru codoped pellet drum bundle-like antimony sulfide (Sb₂S₃) via a simple reflux method. Subsequent HER and HOR electrocatalytic investigations in 1 M KOH revealed their suitability as an efficient and inexpensive alternative to platinum, as is evident from the overpotential (72 mV at a current density of 10 mA cm^-2), Tafel slope (193 mV/decade), exchange current density (1.42 mA/cm²), and breakdown potential at ∼0.6 V vs RHE, respectively. Such remarkable HER and HOR performance of N, Ru codoped Sb₂S₃ could be ascribed to the presence of relatively larger active sites compared to Sb₂S₃ and N-doped Sb₂S₃ individually due to synergistic effects arising from N and Ru dopants. Further, N, Ru codoped Sb₂S₃ demonstrated high intrinsic catalytic activity as indicated by its turnover frequency (2.03 s^-1) and current loss, corresponding to 35% after 10 h of continuous amperometric i-t operation. Alternatively, such excellent catalytic performance of N, Ru codoped Sb₂S₃ arises due to geometric lattice defects with surface oxygen vacancy, and the availability of abundant edges and its pellet drum-like morphology also cannot be overruled.

Adjusting Interfacial Chemistry and Electronic Properties of Photovoltaics Based on a Highly Pure Sb2S3 Absorber by Atomic Layer Deposition

The combination of oxide and heavier chalcogenide layers in thin film photovoltaics suffers limitations associated with oxygen incorporation and sulfur deficiency in the chalcogenide layer or with a chemical incompatibility which results in dewetting issues and defect states at the interface. Here, we establish atomic layer deposition (ALD) as a tool to overcome these limitations. ALD allows one to obtain highly pure Sb₂S₃ light absorber layers, and we exploit this technique to generate an additional interfacial layer consisting of 1.5 nm ZnS. This ultrathin layer simultaneously resolves dewetting and passivates defect states at the interface. We demonstrate via transient absorption spectroscopy that interfacial electron recombination is one order of magnitude slower at the ZnS-engineered interface than hole recombination at the Sb₂S₃/P3HT interface. The comparison of solar cells with and without oxide incorporation in Sb₂S₃, with and without the ultrathin ZnS interlayer, and with systematically varied Sb₂S₃ thickness provides a complete picture of the physical processes at work in the devices.

Semitransparent Sb2S3 thin film solar cells by ultrasonic spray pyrolysis for use in solar windows

The integration of photovoltaic (PV) solar energy in zero-energy buildings requires durable and efficient solar windows composed of lightweight and semitransparent thin film solar cells. Inorganic materials with a high optical absorption coefficient, such as Sb₂S₃ (>10⁵ cm^-1 at 450 nm), offer semitransparency, appreciable efficiency, and long-term durability at low cost. Oxide-free throughout the Sb₂S₃ layer thickness, as confirmed by combined studies of energy dispersive X-ray spectroscopy and synchrotron soft X-ray emission spectroscopy, semitransparent Sb₂S₃ thin films can be rapidly grown in air by the area-scalable ultrasonic spray pyrolysis method. Integrated into a ITO/TiO₂/Sb₂S₃/P3HT/Au solar cell, a power conversion efficiency (PCE) of 5.5% at air mass 1.5 global (AM1.5G) is achieved, which is a record among spray-deposited Sb₂S₃ solar cells. An average visible transparency (AVT) of 26% of the back-contact-less ITO/TiO₂/Sb₂S₃ solar cell stack in the wavelength range of 380-740 nm is attained by tuning the Sb₂S₃ absorber thickness to 100 nm. In scale-up from mm² to cm² areas, the Sb₂S₃ hybrid solar cells show a decrease in efficiency of only 3.2% for an 88 mm² Sb₂S₃ solar cell, which retains 70% relative efficiency after one year of non-encapsulated storage. A cell with a PCE of 3.9% at 1 sun shows a PCE of 7.4% at 0.1 sun, attesting to the applicability of these solar cells for light harvesting under cloud cover.

Enhanced Electrical Conductivity of Sb2S3 Thin Film via C60 Modification and Improvement in Solar Cell Efficiency

Sb2S3 has attracted great research interest very recently as a promising absorber material for photoelectric and photovoltaic devices because of its unique optical and electrical properties and single, stable phase. However, the intrinsic high resistivity property of Sb2S3 material is one of the major factors restricting the further improvement of its application. In this work, the C60 modification of Sb2S3 thin films is investigated. The conductivity of Sb2S3 thin films increases from 4.71 × 10-9 S cm-1 for unmodified condition to 2.86 × 10-8 S cm-1 for modified thin films. Thin-film solar cells in the configuration of glass/(SnO2:F) FTO/TiO2/Sb2S3(C60)/Spiro-OMeTAD/Au are fabricated, and the conversion efficiency is increased from 1.10% to 1.74%.

Uniform Sb2S3 optical coatings by chemical spray method

Antimony sulfide (Sb₂S₃), an environmentally benign material, has been prepared by various deposition methods for use as a solar absorber due to its direct band gap of ≈1.7 eV and high absorption coefficient in the visible light spectrum (1.8 × 10⁵ cm^-1 at 450 nm). Rapid, scalable, economically viable and controllable in-air growth of continuous, uniform, polycrystalline Sb₂S₃ absorber layers has not yet been accomplished. This could be achieved with chemical spray pyrolysis, a robust chemical method for deposition of thin films. We applied a two-stage process to produce continuous Sb₂S₃ optical coatings with uniform thickness. First, amorphous Sb₂S₃ layers, likely forming by 3D Volmer-Weber island growth through a molten phase reaction between SbCl₃ and SC(NH₂)₂, were deposited in air on a glass/ITO/TiO₂ substrate by ultrasonic spraying of methanolic Sb/S 1:3 molar ratio solution at 200-210 °C. Second, we produced polycrystalline uniform films of Sb₂S₃ (E _g 1.8 eV) with a post-deposition thermal treatment of amorphous Sb₂S₃ layers in vacuum at 170 °C, <4 × 10^-6 Torr for 5 minutes. The effects of the deposition temperature, the precursor molar ratio and the thermal treatment temperature on the Sb₂S₃ layers were investigated using Raman spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and UV-vis-NIR spectroscopy. We demonstrated that Sb₂S₃ optical coatings with controllable structure, morphology and optical properties can be deposited by ultrasonic spray pyrolysis in air by tuning of the deposition temperature, the Sb/S precursor molar ratio in the spray solution, and the post-deposition treatment temperature.

Spin-coated planar Sb2S3 hybrid solar cells approaching 5% efficiency

Antimony sulfide solar cells have demonstrated an efficiency exceeding 7% when assembled in an extremely thin absorber configuration deposited via chemical bath deposition. More recently, less complex, planar geometries were obtained from simple spin-coating approaches, but the device efficiency still lags behind. We compare two processing routes based on different precursors reported in the literature. By studying the film morphology, sub-bandgap absorption and solar cell performance, improved annealing procedures are found and the crystallization temperature is shown to be critical. In order to determine the optimized processing conditions, the role of the polymeric hole transport material is discussed. The efficiency of our best solar cells exceeds previous reports for each processing route, and our champion device displays one of the highest efficiencies reported for planar antimony sulfide solar cells.

Antimony(III) Sulfide Thin Films as a Photoanode Material in Photocatalytic Water Splitting

For the first time, we present exploratory investigations on the performance of thermally evaporated Sb2S3 thin film photoanodes for solar-assisted water-splitting applications. With a band gap of 1.72 eV, a 250 nm thick Sb2S3 photoanode showed a saturation photocurrent density of ∼600 μA cm(-2) measured at 1.0 V reversible hydrogen electrode (RHE) in 0.1 M Na2SO4 under 1-sun illumination, with an onset potential of ∼0.25 V RHE. However, subsequent photodegradation studies revealed that the material dissolves relatively quickly with the application of both illumination and bias. Nonetheless, Sb2S3 does have the advantage of having a relatively low optimal fabrication temperature of 300 °C and thus may have utility as a top cell absorber of a tandem device where the bottom cell is temperature sensitive, if protected from corrosion. Therefore, we characterized relevant aspects of the material in an attempt to explain the large difference between the theoretical maximum and measured current density. From our characterization it is believed that the photocatalytic efficiency of this material can be improved by modifying the surface to reduce optical reflection and addressing inherent issues such as high electrical resistivity and surface defects.

Determination of Young's modulus of Sb2S3 nanowires by in situ resonance and bending methods

In this study we address the mechanical properties of Sb2S3 nanowires and determine their Young's modulus using in situ electric-field-induced mechanical resonance and static bending tests on individual Sb2S3 nanowires with cross-sectional areas ranging from 1.1·10(4) nm(2) to 7.8·10(4) nm(2). Mutually orthogonal resonances are observed and their origin explained by asymmetric cross section of nanowires. The results obtained from the two methods are consistent and show that nanowires exhibit Young's moduli comparable to the value for macroscopic material. An increasing trend of measured values of Young's modulus is observed for smaller thickness samples.

Toward High-Efficiency Solution-Processed Planar Heterojunction Sb2S3 Solar Cells

Low-cost hybrid solar cells have made tremendous steps forward during the past decade owing to the implementation of extremely thin inorganic coatings as absorber layers, typically in combination with organic hole transporters. Using only extremely thin films of these absorbers reduces the requirement of single crystalline high-quality materials and paves the way for low-cost solution processing compatible with roll-to-roll fabrication processes. To date, the most efficient absorber material, except for the recently introduced organic-inorganic lead halide perovskites, has been Sb2S3, which can be implemented in hybrid photovoltaics using a simple chemical bath deposition. Current high-efficiency Sb2S3 devices utilize absorber coatings on nanostructured TiO2 electrodes in combination with polymeric hole transporters. This geometry has so far been the state of the art, even though flat junction devices would be conceptually simpler with the additional potential of higher open circuit voltages due to reduced charge carrier recombination. Besides, the role of the hole transporter is not completely clarified yet. In particular, additional photocurrent contribution from the polymers has not been directly shown, which points toward detrimental parasitic light absorption in the polymers. This study presents a fine-tuned chemical bath deposition method that allows fabricating solution-processed low-cost flat junction Sb2S3 solar cells with the highest open circuit voltage reported so far for chemical bath devices and efficiencies exceeding 4%. Characterization of back-illuminated solar cells in combination with transfer matrix-based simulations further allows to address the issue of absorption losses in the hole transport material and outline a pathway toward more efficient future devices.