Chemical bath deposition of SnS thin films on ZnS and CdS substrates

We illustrate that Tin sulfide (SnS) thin films of 110–500 nm in thickness may be deposited on ZnS and CdS substrates to simulate the requirement in developing window-buffer/SnS solar cells in the superstrate configuration. In the chemical bath deposition reported here, tin chloride and thiosulfate are the major constituents and the deposition is made at 25 °C. In a single deposition, film thickness of 110–170 nm is achieved and in two more successive depositions, the film thickness is 450–500 nm. The thicker films are composed of vertically stacked flakes, 100 nm across and 10–20 nm in thickness. The Sn/S elemental ratio is ~1 for the films 110–170 nm in thickness, but it slightly increases for thicker films. The crystalline structure is orthorhombic, similar to the mineral herzenbergite, and with crystallite diameters 13 nm (110–170 films) and 16 nm (450–500 nm films). The Raman bands at 94, 172 and 218 cm^?1 further confirm the SnS composition of the films. The optical band gap of SnS is 1.4–1.5 eV for the thinner films, but is 1.28–1.39 eV for the thicker films, the decrease being ascribed to the increase in the crystallite diameter. Uniform pin-hole free SnS thin films were successfully grown on two different substrates and can be applied in solar cell structures.     

A Solution Processable High‐Performance Thermoelectric Copper Selenide Thin Film

A solid‐state thermoelectric device is attractive for diverse technological areas such as cooling, power generation and waste heat recovery with unique advantages of quiet operation, zero hazardous emissions, and long lifetime. With the rapid growth of flexible electronics and miniature sensors, the low‐cost flexible thermoelectric energy harvester is highly desired as a potential power supply. Herein, a flexible thermoelectric copper selenide (Cu₂Se) thin film, consisting of earth‐abundant elements, is reported. The thin film is fabricated by a low‐cost and scalable spin coating process using ink solution with a truly soluble precursor. The Cu₂Se thin film exhibits a power factor of 0.62 mW/(m K²) at 684 K on rigid Al₂O₃ substrate and 0.46 mW/(m K²) at 664 K on flexible polyimide substrate, which is much higher than the values obtained from other solution processed Cu₂Se thin films (<0.1 mW/(m K²)) and among the highest values reported in all flexible thermoelectric films to date (≈0.5 mW/(m K²)). Additionally, the fabricated thin film shows great promise to be integrated with the flexible electronic devices, with negligible performance change after 1000 bending cycles. Together, the study demonstrates a low‐cost and scalable pathway to high‐performance flexible thin film thermoelectric devices from relatively earth‐abundant elements.     

A new class of doped nanobulk high-figure-of-merit thermoelectrics by scalable bottom-up assembly

Obtaining thermoelectric materials with high figure of merit ZT is an exacting challenge because it requires the independent control of electrical conductivity, thermal conductivity and Seebeck coefficient, which are often unfavourably coupled. Recent works have devised strategies based on nanostructuring and alloying to address this challenge in thin films, and to obtain bulk p-type alloys with ZT>1. Here, we demonstrate a new class of both p- and n-type bulk nanomaterials with room-temperature ZT as high as 1.1 using a combination of sub-atomic-per-cent doping and nanostructuring. Our nanomaterials were fabricated by bottom-up assembly of sulphur-doped pnictogen chalcogenide nanoplates sculpted by a scalable microwave-stimulated wet-chemical method. Bulk nanomaterials from single-component assemblies or nanoplate mixtures of different materials exhibit 25-250% higher ZT than their non-nanostructured bulk counterparts and state-of-the-art alloys. Adapting our synthesis and assembly approach should enable nanobulk thermoelectrics with further increases in ZT for transforming thermoelectric refrigeration and power harvesting technologies.     

Thin Film Tin Selenide (SnSe) Thermoelectric Generators Exhibiting Ultralow Thermal Conductivity

Tin selenide (SnSe) has attracted much attention in the field of thermoelectrics since the discovery of the record figure of merit (ZT) of 2.6 ± 0.3 along the b‐axis of the material. The record ZT is attributed to an ultralow thermal conductivity that arises from anharmonicity in bonding. While it is known that nanostructuring offers the prospect of enhanced thermoelectric performance, there have been minimal studies in the literature to date of the thermoelectric performance of thin films of SnSe. In this work, preferentially orientated porous networks of thin film SnSe nanosheets are fabricated using a simple thermal evaporation method, which exhibits an unprecedentedly low thermal conductivity of 0.08 W m^?1 K^?1 between 375 and 450 K. In addition, the first known example of a working SnSe thermoelectric generator is presented and characterized.     

磁控溅射沉积制备SnSe薄膜及其热电性能研究

因为晶体结构以及热电性能各向异性,硒化锡(SnSe)沿b轴方向表现出优异的热电性能,受到业内的广泛关注。但关于SnSe薄膜研究的报道较少。本研究利用磁控溅射技术,将SnSe沉积到Si/SiO₂基底得到SnSe薄膜,分析了沉积温度对SnSe薄膜结构和热电性能的影响。结果显示:沉积温度升高,晶粒尺寸相应增加,薄膜的结晶质量也随之提高。在573 K的沉积温度条件下,能获得高结晶质量和良好化学计量比的(111)取向SnSe薄膜,该薄膜具有约为1.25μW/(cm·K²)的最大功率因子(PF)。当沉积温度升高至773 K时,可以得到具有超高迁移率和赛贝克系数的(400)织构SnSe薄膜,该薄膜在573 K的测试温度下,其最大PF为0.5μW/(cm·K²),实现接近于文献报道的相同温度下单晶SnSe沿a轴的PF。本研究的结果证明了高沉积温度对SnSe薄膜微观结构和热电性能调控的重要性,并且为通过设计和调控SnSe基薄膜有序结构来提升其热电性能提供了新的研究思路。     

Synergistic band convergence and defect engineering boost thermoelectric performance of SnTe

As an eco-friendly thermoelectric material,SnTe has attracted extensive attention.In this study,we use a stepwise strategy to enhance the thermoelectric performance of SnTe.Firstly,AgCl is doped into SnTe to realize band convergence and enlarge the band gap of AgCl-doped SnTe.AgCl-doping also induces dense point defects,strengthens the phonon scattering,and reduces the lattice thermal conductivity.Secondly,Sb is alloyed into AgCl-doped SnTe to further optimize the carrier concentration and simultaneously reduce the lattice thermal conductivity,leading to improved thermoelectric dimensionless figure of merit,ZT.Finally,(Sn0.81Sb0.19Te)0.93(AgCl)0.07 has approached the ZT value as high as～0.87 at 773 K,which is 272％higher than that of pristine SnTe.This study indicates that stepwise AgCl-doping and Sb-alloying can significantly improve thermoelectric performance of SnTe due to synergistic band engineering,carrier concentration optimization and defect engineering.     

Chemical synthesis, structural and optical properties of quantum sized semiconducting tin(II) selenide in thin film form

A chemical route to nanocrystalline photoconducting tin(II) selenide quantum dots in thin film form was developed and the structural and optical properties of the synthesized materials were studied. The synthesized SnSe nanocrystals deposited as thin films belong to the orthorhombic crystalline system. Unit cell parameters of the as-deposited and thermally treated semiconducting quantum dots in thin film form were determined from experimental X-ray diffraction data employing multiple regression analysis technique. An average crystal size of 14.8 nm was estimated for as-deposited SnSe quantum dots using the Debye-Scherrer approach which increases to 23.3 nm upon annealing. Average crystal size increase upon thermal treatment is accompanied by slight enlargement of the unit cell parameters. On the basis of optical absorption studies of the SnSe films, conclusions regarding the band structure of this material in reciprocal space were derived. The as-deposited films are characterized by indirect band gap energy of 1.20 eV which exhibits a slight red shift to 1.10 eV upon annealing. Additional electronic transition of a direct type was found to occur at 1.74 eV in the case of as-deposited films, shifting to 1.65 eV in the course of annealing. All these values are blue-shifted with respect to the macrocrystalline material ones, which along with the red shift detected upon annealing, is a strong indication of the three-dimensional confinement effects in the studied nanocrystals.     

Atmospheric pressure chemical vapour deposition of SnSe and SnSe2 thin films on glass

Atmospheric pressure chemical vapour deposition of tin monoselenide and tin diselenide films on glass substrate was achieved by reaction of diethyl selenide with tin tetrachloride at 350–650 °C. X-ray diffraction showed that all the films were crystalline and matched the reported pattern for SnSe and/or SnSe₂. Wavelength dispersive analysis by X-rays show a variable Sn:Se ratio from 1:1 to 1:2 depending on conditions. The deposition temperature, flow rates and position on the substrate determined whether mixed SnSe–SnSe₂, pure SnSe or pure SnSe₂ thin films could be obtained. SnSe films were obtained at 650 °C with a SnCl₄ to Et₂Se ratio greater than 10. The SnSe films were silver–black in appearance and adhesive. SnSe₂ films were obtained at 600–650 °C they had a black appearance and were composed of 10 to 80 μm sized adherent crystals. Films of SnSe only 100 nm thick showed complete absorbtion at 300–1100 nm.     

Synthesis and characterization of nanoplate-based SnS microflowers via a simple solvothermal process with biomolecule assistance

A biomolecule-assisted mild solvothermal process has been successfully developed to synthesize the SnS microflowers with nanoplates, in which l-cysteine was used as the sulfur source and complexing agent. The phase structure, morphology, composition and optical properties of the as-prepared product were characterized by XRD, FE-SEM, TEM (HRTEM), SAED, XPS, TGA and UV–vis spectrum. Results demonstrated that the as-synthesized product is comprised of microflowers with nanoplates, and the nanoplates are 50 nm in average thickness. And a possible mechanism for the growth of the SnS microflowers with nanoplates was put forward and briefly discussed. The proposed solvothermal method using l-cysteine as the sulfur source and complexing agent is very promising for the low cost and large-scale synthesis of other tin chalcogenide compounds.     

溅射沉积掺Ag的SnSe薄膜的微结构和热电性能

使用粉末烧结SnSe合金靶高真空磁控溅射制备掺杂Ag的SnSe热电薄膜,利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)和能谱仪(EDS)等手段分析薄膜的相组成、表面形貌、截面形貌、微区元素含量和元素分布,利用塞贝克系数/电阻分析系统LSR-3测量沉积薄膜的电阻率和Seebeck系数,研究了不同Ag含量SnSe薄膜的热电性能。结果表明,采用溅射技术可制备出正交晶系Pnma结构的SnSe相薄膜,掺杂的Ag在薄膜中生成了纳米Ag₃Sn。与未掺杂Ag相比,掺杂Ag的SnSe薄膜其电阻率和Seebeck系数(绝对值,下同)明显减小。并且在一定掺杂范围内,掺杂Ag越多的薄膜电阻率和Seebeck系数越小。未掺杂Ag的SnSe薄膜样品,其Seebeck系数较大但是电阻率也大,因此功率因子较小。Ag掺杂量(原子分数)为7.97%的样品,因其Seebeck系数绝对值较大而电阻率适当,280℃时的功率因子最大(约为0.93 mW·m^-1·K^-2),比未掺杂Ag的样品(PF=0.61 mW·m^-1·K^-2)高52%。掺杂适量的Ag能提高溅射沉积的SnSe薄膜的热电性能(功率因子)。     

Thin films of tin sulphide for use in thin film solar cell devices

SnS is of interest for use as an absorber layer and the wider energy bandgap phases e.g. SnS₂, Sn₂S₃ and Sn/S/O alloys of interest as Cd-free buffer layers for use in thin film solar cells. In this work thin films of tin sulphide have been thermally evaporated onto soda-lime glass substrates with the aim of optimising the properties of the material for use in superstrate configuration device structures. The thin films were characterised using energy dispersive X-ray analysis (EDS) to determine the film composition, X-ray diffraction (XRD) to determine the phases present and structure of each phase, transmittance versus wavelength measurements to determine the energy bandgap and scanning electron microscopy (SEM) to observe the surface topology and topography. These properties were then correlated to the deposition parameters. Using the optimised conditions it is possible to produce thin films of tin sulphide that are pinhole free and conformal to the substrate that are suitable for use in thin film solar cell structures.     

High‐Performance Flexible Photodetectors based on High‐Quality Perovskite Thin Films by a Vapor–Solution Method

Organometal halide perovskites are new light‐harvesting materials for lightweight and flexible optoelectronic devices due to their excellent optoelectronic properties and low‐temperature process capability. However, the preparation of high‐quality perovskite films on flexible substrates has still been a great challenge to date. Here, a novel vapor–solution method is developed to achieve uniform and pinhole‐free organometal halide perovskite films on flexible indium tin oxide/poly(ethylene terephthalate) substrates. Based on the as‐prepared high‐quality perovskite thin films, high‐performance flexible photodetectors (PDs) are constructed, which display a nR value of 81 A W^?1 at a low working voltage of 1 V, three orders higher than that of previously reported flexible perovskite thin‐film PDs. In addition, these flexible PDs exhibit excellent flexural stability and durability under various bending situations with their optoelectronic performance well retained. This breakthrough on the growth of high‐quality perovskite thin films opens up a new avenue to develop high‐performance flexible optoelectronic devices.     

Photovoltaic structures using chemically deposited tin sulfide thin films

Chemically deposited thin films of tin sulfide forms in two crystalline structures depending on the bath compositions used: orthorhombic, SnS(OR), and zinc-blende, SnS(ZB). These films posses p-type electrical conductivity and have band gaps of 1.2 and 1.7 eV, respectively. The photovoltaic structure: SnO₂:F/CdS/SnS(ZB)/SnS(OR) with evaporated Ag-electrode reported here shows an open circuit voltage (V_OC) of 370 mV, a short circuit current density (J_SC) of 1.23 mA/cm², fill factor of 0.44 and conversion efficiency of 0.2% under 1 kW/m² illumination intensity. We present an evaluation for improvement in the light generated current density when the two types of SnS absorber films are used. Different evaporated electrode materials were tested, from which Ag-electrode was chosen for this work. The results given above were obtained with SnS(ZB) film of 0.1 µm and SnS(OR) film of 0.5 µm in thickness.     

Single-Crystal SnSe Thermoelectric Fibers via Laser-Induced Directional Crystallization: From 1D Fibers to Multidimensional Fabrics

Single-crystal tin selenide (SnSe), a record holder of high-performance thermoelectric materials, enables high-efficient interconversion between heat and electricity for power generation or refrigeration. However, the rigid bulky SnSe cannot satisfy the applications for flexible and wearable devices. Here, a method is demonstrated to achieve ultralong single-crystal SnSe wire with rock-salt structure and high thermoelectric performance with diameters from micro- to nanoscale. This method starts from thermally drawing SnSe into a flexible fiber-like substrate, which is polycrystalline, highly flexible, ultralong, and mechanically stable. Then a CO₂ laser is employed to recrystallize the SnSe core to single-crystal over the entire fiber. Both theoretical and experimental studies demonstrate that the single-crystal rock-salt SnSe fibers possess high thermoelectric properties, significantly enhancing the ZT value to 2 at 862 K. This simple and low-cost approach offers a promising path to engage the fiber-shaped single-crystal materials in applications from 1D fiber devices to multidimensional wearable fabrics.     

A facile way to optimize thermoelectric properties of SnSe thin films via sonication-assisted liquid-phase exfoliation

In our work, SnSe nanosheets and nanostructured thin films were successfully synthesized via sonication-assisted exfoliation and coating process. The SnSe nanosheets respond to a uniform lateral size, with two to three single layers by 2.82 nm and 280 nm² of average thickness and average area, respectively. The results were confirmed by Scanning Electron Microscope, Transmission Electron Microscope, and Atomic Force Microscope. X-ray diffraction and Raman spectra indicate that the SnSe nanosheets have high crystalline quality along a-axis. The SnSe nanostructured thin films were prepared in various thicknesses from 350 to 650 nm. The highest power factor value is achieved at 450 nm in 375–600 K temperature range. A simple method of fabrication and controllable thermoelectric properties of SnSe nanostructured thin films as well as other two-dimensional (2D) materials are introduced.

     

Fabrications of SnS thin films and SnS-based heterojunctions on flexible polyimide substrates

The flexible polyimide substrates were utilized to realize the flexibility of SnS thin films and SnS-based heterojunctions. The SnS thin films and ZnO/SnS heterojunctions were deposited on polyimide substrates by magnetron sputtering. The properties of SnS thin films and ZnO/SnS heterojunctions were studied. The experimental results show that the post annealing can enhance the degree of crystallinity of flexible SnS thin films. The annealed SnS thin films present polycrystalline structure with preferential orientation along the (040) plane and grain size of 18 nm. The compositions of as-deposited and annealed flexible SnS thin films are close to the stoichiometry of SnS. The direct band gaps are 1.48 and 1.32 eV for the as-deposited and annealed SnS thin films, respectively. The fabricated flexible ZnO/SnS heterojunctions show rectifying properties with the rectifying ratio of 6.85 and the diode ideal factor of 1.23. The experimental results indicate the feasibility of using polyimide as the substrates of SnS thin films and SnS-based heterojunctions.     

PVD grown SnS thin films onto different substrate surfaces

Recent interests focus on tin mono sulphide as a potential candidate for an absorber layer in heterojunction solar cells. In the present investigation, SnS thin films have been deposited onto different substrates such as glass, ITO and Mo-coated glass substrate by thermal evaporation method. The compositional, microstructural and photoelectrochemical properties of the SnS films were analyzed depending upon the chemical nature of the substrates used. The SnS layers were polycrystalline with Herzbergite orthorhombic structure on all three substrates and had nearly stoichiometric elemental composition with a Sn/S ratio of ~1.01. The films grown on ITO and Mo-coated glass substrates exhibit (040) as preferred orientation whereas the films deposited on glass showed (111) plane as predominant. The layers were densely packed and well adherent to the substrate surface. The Raman spectra showed bands at 64, 163, 189 and 219 cm^?1, which corresponds to the single phase (SnS) composition of films. p-type conductivity of all the deposited films were determined by the photoresponse studies. The highest photoresponse for the films on the ITO substrate indicates their appropriateness for the solar cell application.     

Structural and optoelectronic properties of indium doped SnO2 thin films deposited by sol gel technique

Indium doped tin oxide (SnO₂:In) thin films were deposited on glass substrates by sol–gel dip coating technique. X-ray diffraction pattern of SnO₂:In thin films annealed at 500 °C showed tetragonal phase with preferred orientation in T (110) plane. The grain size of tin oxide (SnO₂) in SnO₂:In thin films are found to be 6 nm which makes them suitable for gas sensing applications. AFM studies showed an inhibition of grain growth with increase in indium concentration. The rms roughness value of SnO₂:In thin films are found to 1 % of film thickness which makes them suitable for optoelectronic applications. The film surface revealed a kurtosis values below 3 indicating relatively flat surface which make them favorable for the production of high-quality transparent conducting electrodes for organic light-emitting diodes and flexible displays. X-ray photoelectron spectroscopy gives Sn 3d, In 3d and O 1s spectra on SnO₂:In thin film which revealed the presence of oxygen vacancies in the SnO₂:In thin film. These SnO₂:In films acquire n-type conductivity for 0–3 mol% indium doping concentration and p type for 5 and 7 mol% indium doping concentration in SnO₂ films. An average transmittance of >80 % (in ultra-violet–Vis region) was observed for all the SnO₂:In films he In doped SnO₂ thin films demonstrated the tailoring of band gap values. Photoluminescence spectra of the films exhibited an increase in the emission intensity with increase in indium doping concentration which may be due structural defects or luminescent centers, such as nanocrystals and defects in the SnO₂.     

Effect of copper doping on physical properties of nanocrystallized SnS zinc blend thin films grown by chemical bath deposition

SnS: Cu thin films have been successfully prepared on Pyrex substrates using low cost chemical bath deposition (CBD) technique with different copper doped concentration (y = [Cu]/[Sn] = 5%, 6%, 8%, 9% and 10%). The structure, the surface morphology and the optical properties of the SnS:Cu films were studied by X-ray diffraction (XRD), atomic force microscopy (AFM) and spectrophotometer measurements, respectively. To obtain a thickness of the order of 780 ± 31 nm for absorber material in solar cell devices, a system of multilayer has been prepared. It is found that the physical properties of tin sulphide are affected by Cu-doped concentration. In fact, X-ray diffraction study showed that better cristallinity in zinc blend structure with preferential orientations (111)^ZB and (200)^ZB, was obtained for y equal to 6%. According to the AFM analysis we can remark that low average surface roughness (RMS)value of SnS(ZB) thin film obtained with Cu-doped concentrations equal to y = 6%, is about of 54 nm. Energy dispersive spectroscopy (EDS) showed the existence of Cu in the films. Optical analyses by means of transmission T(λ) and reflection R(λ) measurements show 1.51 eV as a band gap value of SnS:Cu(6%) which is nearly equal to the theoretical optimum value of 1.50 eV for efficient light absorption. On the other hand, Cu-doped tin sulphide exhibits a high absorption coefficient up to 2 × 10⁶ cm⁻¹, indicating that SnS:Cu can be used as an absorber thin layer in photovoltaic structure such as SnS:Cu/ZnS/SnO₂:F and SnS:Cu/In₂S₃/SnO₂:F, where ZnS and In₂S₃ are chemically deposited in a previous studies.     

Pulsed electrodeposition of oxygen-free tin monosulfide thin films using lactic acid/sodium lactate buffered electrolytes

In this study, tin monosulfide (SnS) thin films have been prepared on indium-tin-oxide-covered glass substrates from an acidic electrolyte containing a pH buffer of lactic acid/sodium lactate using pulsed electrodeposition method. Results from Auger electron spectroscopy confirmed that nearly oxygen-free and stoichiometric SnS thin films were attained. X-ray diffraction and scanning electron micrograph studies indicated the formation of a smooth α-SnS thin film with the orthorhombic structure. Moreover, optical transmission spectroscopy showed a direct optical band gap of 1.39 eV and the photoelectrochemical measurements revealed the p-type conductivity.