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.     

Aqueous bath process for deposition of Cu2ZnSnS4 photovoltaic absorbers

Chemical bath deposition and ion exchange were used to incorporate copper, zinc, tin and sulfur into a thin film precursor stack. The stack was then sulfurized to form the photovoltaic absorber material Cu₂ZnSnS₄ (CZTS). The morphology and elemental composition of the films at each process stage were analyzed by Auger electron spectroscopy and scanning electron microscopy, and the structural and optical properties of the sulfurized film were determined by a combination of X-ray diffraction, Raman scattering, and diffuse reflectance UV-Vis spectroscopy. Compositionally uniform microcrystalline CZTS with kesterite structure and a bandgap of 1.45 eV were observed. A preliminary solar cell device was produced exhibiting photovoltaic and rectifying behavior.     

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.     

Fabrication and characterization of Cu2ZnSnS4 thin films deposited by spray pyrolysis technique

We have investigated synthesis conditions and some properties of sprayed Cu₂ZnSnS₄ (CZTS) thin films in order to determine the best preparation conditions for the realization of CZTS based photovoltaic solar cells. The thin films are made by means of spraying of aqueous solutions containing copper chloride, zinc chloride, tin chloride and thiourea on heated glass substrates at various temperatures. In order to optimize the synthesis conditions of the CZTS films, two series of experiments are performed. In the first series the sprayed duration was fixed at 30 min and in the second it is fixed at 60 min. In each series, the substrate temperature was changed from 553°K to 633°K. The X-ray diffraction shows, on one hand, that the best crystallinity was obtained for 613°K as substrate temperature and 60 min as sprayed duration. On the other hand, these CZTS films exhibit the kesterite structure with preferential orientation along the [112] direction. Atomical Force Microscopy was used to determine the grain sizes and the roughness of these CZTS thin film. After the annealing treatment, we estimated the optical band-gap energy of the CZTS thin film exhibiting the best crystallinity as 1.5 eV which is quite close to the optimum value for a solar cell.     

Single-graded CIGS with narrow bandgap for tandem solar cells

Multi-junction solar cells show the highest photovoltaic energy conversion efficiencies, but the current technologies based on wafers and epitaxial growth of multiple layers are very costly. Therefore, there is a high interest in realizing multi-junction tandem devices based on cost-effective thin film technologies. While the efficiency of such devices has been limited so far because of the rather low efficiency of semitransparent wide bandgap top cells, the recent rise of wide bandgap perovskite solar cells has inspired the development of new thin film tandem solar devices. In order to realize monolithic, and therefore current-matched thin film tandem solar cells, a bottom cell with narrow bandgap (~1 eV) and high efficiency is necessary. In this work, we present Cu(In,Ga)Se₂ with a bandgap of 1.00 eV and a maximum power conversion efficiency of 16.1%. This is achieved by implementing a gallium grading towards the back contact into a CuInSe₂ base material. We show that this modification significantly improves the open circuit voltage but does not reduce the spectral response range of these devices. Therefore, efficient cells with narrow bandgap absorbers are obtained, yielding the high current density necessary for thin film multi-junction solar cells.     

Influence of thermal annealing on microstructural,morphological, optical properties and surface electronic structure of copper oxide thin films

In this study, effect of the post-deposition thermal annealing on copper oxide thin films has been systemically investigated. The copper oxide thin films were chemically deposited on glass substrates by spin-coating. Samples were annealed in air at atmospheric pressure and at different temperatures ranging from 200 to 600°C. The microstructural, morphological, optical properties and surface electronic structure of the thin films have been studied by diagnostic techniques such as X-ray diffraction (XRD), Raman spectroscopy, ultraviolet–visible (UV–VIS) absorption spectroscopy, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The thickness of the films was about 520 nm. Crystallinity and grain size was found to improve with annealing temperature. The optical bandgap of the samples was found to be in between 1.93 and 2.08 eV. Cupric oxide (CuO), cuprous oxide (Cu₂O) and copper hydroxide (Cu(OH)₂) phases were observed on the surface of as-deposited and 600 °C annealed thin films and relative concentrations of these three phases were found to depend on annealing temperature. A complete characterization reported herein allowed us to better understand the surface properties of copper oxide thin films which could then be used as active layers in optoelectronic devices such as solar cells and photodetectors.     

Properties of tin sulphide thin films electrodeposited in the presence of triethanolamine

Tin sulphide thin films have been synthesized in aqueous media in the presence of triethanolamine using electrodeposition method. The effect of deposition potential, triethanolamine concentration and deposition time towards the properties of tin sulfide films was studied. The presence of triethanolamine showed improvement in reproducibility, adherence and crystallinity of the films. The X-ray diffraction studies indicated formation of polycrystalline compounds. The properties of the films varied with the variation in the deposition parameters. The highest photoresponse was obtained for the film deposited at –0.80 V in the presence of 0.06 M triethanolamine. Films deposited at longer deposition time showed higher photoresponse. The absorbance study revealed that the bandgap energy was about 1.20 eV with indirect transition.     

CuInS2 thin films obtained by spray pyrolysis for photovoltaic applications

Copper indium disulphide, CuInS₂, is a promising absorber material for thin film photovoltaic which has recently attracted considerable attention due to its suitability to reach high efficiency solar cells by using low-cost techniques. In this work CuInS₂ thin films have been deposited by chemical spray pyrolysis onto glass substrates at ambient atmosphere, using different composition solutions at various substrate temperatures. Structural, chemical composition and optical properties of CIS films were analysed by X-ray diffraction, energy dispersive X-ray spectroscopy and optical spectroscopy. Sprayed CIS films are polycrystalline with a chalcopyrite structure with a preferential orientation along the <112> direction and no remains of oxides were found after spraying in suitable conditions. X-ray microanalysis shows that a chemical composition near to stochiometry can be obtained. An optical gap of about 1.51 eV was found for sprayed CIS thin films.     

Annealing studies and electrical properties of SnS-based solar cells

Thin films of SnS (tin sulphide) were thermally evaporated onto glass and CdS/ITO (cadmium sulphide/indium tin oxide) coated glass substrates and then annealed in vacuum with the aim of optimising them for use in photovoltaic solar cell device structures. The chemical and physical properties of the layers were determined using scanning electron microscopy, energy dispersive x-ray analysis, x-ray diffraction, and transmittance versus wavelength measurements. “Superstrate configuration” devices were also made using indium tin oxide as the transparent conductive oxide, thermally evaporated cadmium sulphide as the buffer layer and evaporated copper/indium as the back contact material. Capacitance-voltage data are given for the fabricated devices. Capacitance-voltage, spectral response and I-V data are given for the fabricated devices.     

Characterization of electrosynthesized iron diselenide thin films

Iron diselenide (FeSe₂) is an interesting p-type semiconductor with a band gap of 1 eV suitable for solar cell applications. Deposition of FeSe₂ thin films by electrodeposition from aqueous solutions is a low temperature and inexpensive technique. In the present work, FeSe₂ thin films were deposited onto tin oxide coated conducting glass substrates by cathodic electrodeposition technique. The deposited films were characterized by X-ray diffraction, Energy dispersive X-ray analysis, Scanning electron microscope and optical absorption techniques. The effects of electrolyte concentration and deposition potential on the structural, compositional, morphological and optical properties of FeSe₂ thin films are studied. The experimental observations are discussed in detail.     

Development of electrodeposited ZnTe layers as window materials in ZnTe/CdTe/CdHgTe multi-layer solar cells

Zinc telluride (ZnTe) thin films have been deposited on glass/conducting glass substrates using a low-cost electrodeposition method. The resulting films have been characterized using various techniques in order to optimize growth parameters. X-ray diffraction (XRD) has been used to identify the phases present in the films. Photoelectrochemical (PEC) cell and optical absorption measurements have been performed to determine the electrical conductivity type, and the bandgap of the layers, respectively. It has been confirmed by XRD measurement that the deposited layers mainly consist of ZnTe phases. The PEC measurements indicate that the ZnTe layers are p-type in electrical conduction and optical absorption measurements show that their bandgap is in the range 2.10–2.20 eV. p-Type ZnTe window materials have been used in CdTe based solar cell structures, following new designs of graded bandgap multi-layer solar cells. The structures of FTO/ZnTe/CdTe/metal and FTO/ZnTe/CdTe/CdHgTe/metal have been investigated. The results are presented in this paper using observed experimental data.     

Development of low temperature approaches to device quality CdS: A modified geometry for solution growth of thin films and their characterisation

Cadmium sulphide thin films have been grown using a modified chemical bath deposition method with four innovative features: i) ethylenediamine was used as the complexing agent, enabling the use of low cadmium concentrations, ii) a rectangular bath geometry with heated glass plate walls was employed, iii) a low deposition temperature (30 °C) was used and iv) nitrogen gas was flowed over the substrate surface during growth. The latter two features eliminate the formation and adherence of gas bubbles on the substrate during growth, hence reducing pinhole formation. On inspection, films were found to be specularly reflective and homogeneous with no visible pinholes. Characterisation was performed by atomic force microscopy, grazing incidence X-ray diffraction, optical transmittance and photoluminescence spectroscopy. It was shown that films possessed a low surface roughness value of 5.2 nm, were highly crystalline, textured, had a grain size of 15 nm and a bandgap of 2.42 eV. Preliminary results from CdTe/CdS thin film photovoltaic devices demonstrate a notable efficiency of 9.8%.     

Metal chalcogenide-oxide composite coatings prepared by spray pyrolysis

The spray pyrolysis technique has been employed to deposit composite coatings of chalcogenides of cadmium, zinc, lead and cobalt with oxides of aluminium, tin, lead, zinc and cobalt. Widely varying microstructural, electronic, optical and chemical properties have been obtained for such layers by monitoring the oxide composition, its spatial distribution and profile along the thickness. The large area chalcogenide-oxide composite films prepared by this technique are eminently suited for photovoltaic energy conversion, photothermal energy conversion and voltage-dependent resistor (Varistor) applications.In this paper we report our studies on co-pyrolytically deposited CdS:Al₂O₃ and CdS:SnO₂ layers and their application to improved thin film solar cells. Each of the oxides is insoluble in CdS and is segregated at the grain boundaries in the deposited films. Small amounts (less than 10%) of oxide in CdS are found to reduce its grain size negligibly and to make the film more compact, hard, adherent and less susceptible to chemical attack. The altered microstructure modifies the surface topography of the CdS film from a pebble-like roughness to an improved void-free serpentine texture. Segregated oxide in CdS does not affect the optical band gap of the films, although the composites exhibit enhanced diffuse optical scattering.Large area CdS films with a gradient profile of oxide have been utilized to fabricate thin film CdS/Cu₂S solar cells. The growth (length and distribution) of Cu₂S fingers and/or curtains deep into the top CdS layers during the topotaxial conversion reaction of chemiplating is controlled by the presence of oxide along the grain boundaries. This has not only resulted in improved interface topography for better carrier collection and reduced shunt losses but has also enabled us to decrease drastically the CdS film thickness necessary for the solar cells. Furthermore, the subsequent degradation of the junction via the well-known mechanism of the loss of copper from the Cu₂S layer by diffusion into CdS is expected to be considerably reduced by the presence of the oxide gradient in the CdS layer.     

Synthesis of radio frequency plasma polymerized non-synthetic Terpinen-4-ol thin films

Recent advancements in the area of organic polymer applications demand novel and advanced materials with desirable surface, optical and electrical properties to employ in emerging technologies. This study examines the fabrication and characterization of polymer thin films from non-synthetic Terpinen-4-ol monomer using radio frequency plasma polymerization. The optical properties, thickness and roughness of the thin films were studied in the wavelength range 200–1000 nm using ellipsometry. The polymer thin films of thickness from 100 nm to 1000 nm were fabricated and the films exhibited smooth and defect-free surfaces. At 500 nm wavelength, the refractive index and extinction coefficient were found to be 1.55 and 0.0007 respectively. The energy gap was estimated to be 2.67 eV, the value falling into the semiconducting E_g region. The obtained optical and surface properties of Terpinen-4-ol based films substantiate their candidacy as a promising low-cost material with potential applications in electronics, optics, and biomedical industries.     

Low-Cost Antimony Selenosulfide with Tunable Bandgap for Highly Efficient Solar Cells

About 10% efficient antimony selenosulfide (Sb₂(S,Se)₃) solar cell is realized by using selenourea as a hydrothermal raw material to prepare absorber layers. However, tailoring the bandgap of hydrothermal-based Sb₂(S,Se)₃ film to the ideal bandgap (1.3–1.4 eV) using the selenourea for optimal efficiency is still a challenge. Moreover, the expensive selenourea dramatically increases the fabricating cost. Here, a straightforward one-step hydrothermal method is developed to prepare high-quality Sb₂(S,Se)₃ films using a novel precursor sodium selenosulfate as the selenium source. By tuning the Se/(Se+S) ratio in the hydrothermal precursor solution, a series of high-quality Sb₂(S,Se)₃ films with reduced density of deep defect states and tunable bandgap from 1.31 to 1.71 eV is successfully prepared. Consequently, the best efficiency of 10.05% with a high current density of 26.01 mA cm⁻² is achieved in 1.35 eV Sb₂(S,Se)₃ solar cells. Compared with the traditional method using selenourea, the production cost for the Sb₂(S,Se)₃ devices is reduced by over 80%. In addition, the device exhibits outstanding stability, maintaining more than 93% of the initial power conversion efficiency after 30 days of exposure in the atmosphere without encapsulation. The present work definitely paves a facile and effective way to develop low-cost and high-efficiency chalcogenide-based photovoltaic devices.     

Synthesis and optical characterization of n-ZnO and p-Cu2ZnSnS4 nanocrystalline thin films for low cost solar cells

High quality ZnO/Cu₂ZnSnS₄ thin films as a window/absorber layers were successfully synthesized via spin coating the sol-gel precursor of each composition without using any vacuum facilities. In this study, the impact of annealing temperature (400 °C, 3 h) on the ZnO window layer and different thickness (3 and 5 layers) of the Cu₂ZnSnS₄ (CZTS) absorber layer were investigated. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM) and UV–vis–NIR spectroscopy were used for the structural, compositional, morphological and optical absorption analysis of each layer. ZnO exhibits wurtzite hexagonal crystal structure with particle size equals to 8.60 and 28.59 nm for fresh and annealed films, respectively. Micro-strain and dislocations density decreased with the annealing temperature. X-ray diffraction patterns for CZTS films show small peak at (112) according to the kesterite structure with particle size in nano-scale for the two thicknesses. ZnO films demonstrated direct optical band gap of 3.23 and 3.21 eV for fresh and annealed films, respectively. CZTS films (3 and 5 layers) also have direct optical band with optimum value (1.51 eV) for thickness of 5 layers. The J-V characteristics of the CZTS-based thin film solar cells (CZTS/ZnO/ZnO:Ag) were measured under air mass AM 1.5 and 100 mW/cm² illumination. The values of the short circuit current (J_sc), open circuit voltage (V_oc) and fill factor (FF) also have been obtained.     

Growth and characterisation of electrodeposited ZnO thin films

The electrochemical method has been used to deposit zinc oxide (ZnO) thin films from aqueous zinc nitrate solution at 80 °C onto fluorine doped tin oxide (FTO) coated glass substrates. ZnO thin films were grown between − 0.900 and − 1.025 V vs Ag/AgCl as established by voltammogram. Characterisation of ZnO films was carried out for both as-deposited and annealed films in order to study the effect of annealing. Structural analysis of the ZnO films was performed using X-ray diffraction, which showed polycrystalline films of hexagonal phase with (002) preferential orientation. Atomic force microscopy was used to study the surface morphology. Optical studies identified the bandgap to be ∼ 3.20 eV and refractive index to 2.35. The photoelectrochemical cell signal indicated that the films had n-type electrical conductivity and current-voltage measurements showed the glass/FTO/ZnO/Au devices exhibit rectifying properties. The thickness of the ZnO films was found to be 0.40 μm as measured using the Talysurf instrument, after deposition for 3 min. Environmental scanning electron microscopy was used to view the cross-section of glass/FTO/ZnO layers.     

Effect of nitrogen ion implantation on the sprayed ZnSe thin films

The ZnSe thin films were deposited onto glass substrates by the spray pyrolysis method using mixed aqueous solutions of ZnCl₂ and SeO₂ at the substrate temperature 430 °C. These films were implanted with 130 keV nitrogen ions to various doses from 1 × 10¹⁶ to 1 × 10¹⁷ ions/cm². We have analysed the properties of the nitrogen ion-implanted ZnSe thin films using X-ray diffraction and optical transmittance spectra. The values of optical bandgap have been determined from the absorption spectra. The bandgap of the N⁺ doped films decreased from 2.70 eV for undoped film to 2.60 eV for maximum doping probably due to band-tailing, whereas the absorption coefficient values increased with the increase of the implantation dose.     

Crystal structure and optical absorption investigations on β-In2S3 thin films

The crystal structure of annealed β-In₂S₃ thin films with different thickness was investigated by X-ray diffraction technique. Lattice parameters, crystallite size and microstrain were calculated. It was found that the lattice parameters are independent on film thickness, while annealing temperatures increase them. Crystallite sizes were increased with the increase of the film thickness and improved by annealing temperatures. In all cases, the microstrains were decreased gradually with the increase in both film thickness and annealing temperatures. Optical properties of β-In₂S₃ thin films were performed in the spectral range from 400 to 2500 nm to determine the optical constants (n and k), the high frequency dielectric constant, ε_∞, the lattice dielectric constant, ε_L, and the energy gap. The optical constants were found to be independent on film thickness in the range from 200 to 630 nm. The high frequency dielectric and lattice dielectric constants of the as-deposited film increased by annealing temperatures. The energy gap for the as-deposited In₂S₃ was found to be 2.60 eV and increased to 2.70 and 2.75 eV by annealing at 423 and 473 K for 1 h, respectively.     

Supermagnetron plasma CVD of highly effective a-CNx:H electron-transport and hole-blocking films suited to Au/a-CNx:H/p-Si photovoltaic cells

Hydrogenated carbon nitride (a-CN_x:H) films (0-500 nm) were deposited on p-Si wafers to make Au/a-CN_x:H/p-Si photovoltaic cells using i-C₄H₁₀/N₂ supermagnetron plasma chemical vapor deposition. At a lower electrode RF power (LORF) of 50 W and an upper electrode RF power (UPRF) of 50-800 W, hard a-CN_x:H films with optical band gaps of 0.7-1.0 eV were formed. At a film thickness of 25 nm (UPRF of 500 W), the open circuit voltage and short circuit current density were 247 mV and 2.62 mA/cm², respectively. The highest energy conversion efficiency was 0.29%. The appearance of the photovoltaic phenomenon was found to be due to the electron-transport and hole-blocking effect of thin a-CN_x:H film.