Initial experiments on dew collection in Sweden and Tanzania

Moisture in the air can be condensed as dew and used for drinking and irrigation. The radiative cooling properties of polymer foils can enhance the performance of dew collecting surfaces. The main restrictions in condensing water in warm and arid locations are climatic factors, the dew collector design, and the optically selective and adhesive properties of the condensing surface itself. This paper concerns observations of dew formation on radiatively cooled pigmented polyethylene foils. The experiments were carried out in Sweden and in arid Dodoma, Tanzania. The results are in agreement with thermodynamical calculations, though the variation is large in the daily measured dew water volumes. This variation is caused by the hourly and daily changes in wind-speed, cloud cover, dry bulb temperature, and dew point temperature. The results are compared with earlier outdoor observations in Tanzania.     

Nickel sulphide-lead sulphide and nickel sulphide-cadmium sulphide selective coatings for solar thermal conversion

Mixed NiSPbS and NiSCdS thin films deposited on commercial aluminium and galvanized iron substrates by spray pyrolysis have been extensively investigated for their solar-thermal performance over the complete composition range. Optical, thermal and structural properties of these films have been investigated to establish the optimized composition. NiSPbS (～60% nickel) films and NiSCdS (～67% nickel) films perform better than nickel sulphide films above 160°C due to their lower emissivity. The films are extremely adherent to the substrate and have stable operational characteristics under temperature cycling up to ～250°C in dry environments.     

Fabrication and characterisations of n-CdS/p-PbS heterojunction solar cells using microwave-assisted chemical bath deposition

n-CdS/p-PbS heterojunction solar cells were prepared via microwave-assisted chemical bath deposition method. A cadmium sulfide (CdS) window layer (340 nm thickness) was deposited on an indium tin oxide (ITO) glass. A lead sulfide (PbS) absorber layer (985–1380 nm thickness) with different molar concentrations (0.02, 0.05, 0.075, and 0.1 M) was then grown on ITO/CdS to fabricate a p–n junction. The effects of changing molar concentration of the absorber layer on structural and optical properties of the corresponding PbS thin films and solar cells were investigated. The optical band gap of the films decreased as the molarity increased. The photovoltaic properties (J–V characteristics, short circuit current, open circuit voltage, fill factor, and efficiency) of the CdS/PbS heterostructure cells were examined under 30 mW/cm² solar radiation. Interestingly, changing molar concentration improved the photovoltaic cells performances, the solar cell exhibited its highest efficiency (1.68%) at 0.1 M molar concentration.     

Ag-doped PbS thin films by nebulizer spray pyrolysis for solar cells

Silver (Ag)-doped PbS (PbS:Ag) thin films of 616 to 745 nm in thickness were prepared on glass substrates via cost-effective nebulizer spray method by adding different Ag levels from 2% to 8% at 200°C. For solar cell applications, the effect of Ag doping concentration on structural, morphological, optical, photoluminescence, and electrical chattels of PbS thin film has been studied. X-ray diffraction pattern confirmed the polycrystalline behavior of the prepared PbS:Ag films with cubic crystalline nature. The crystalline size and texture coefficient were increased by increasing Ag doping concentration. From the morphological studies by scanning electron microscope (SEM) and atomic force microscope (AFM), the grain size of the films and surface roughness values were increased for the increase in Ag doping concentration. EDS spectra confirmed the existence of Ag, Pb, and S elements in the select 6% Ag-doped PbS film. Peaks related to silver oxide started to emerge at 6% of Ag doping level. The optical direct band gap value was reduced from 1.51 to 1.17 eV for Ag doping from 2% to 6% and thereby slightly increased as 1.79 eV for 8% Ag doping level. For all PbS:Ag films, the photoluminescence spectrum emitted a strong near band edge (NBE) emission at approximately 580 nm, meaning better optical quality. Hall effect measurements evidenced that Ag doping provides enhancement on the characteristics of mobility, carrier concentration, and resistivity with p-type conducting nature. The observed high carrier concentration and low resistivity values were 4.32 × 10¹⁴ cm⁻³ and 80 Ωcm, for 6% Ag-doped PbS film. The FTO/CdS/PbS:Ag heterostructure solar cell was formed from 6% Ag-doped film.     

CdSe thin films as solar control coatings

CdSe thin films deposited by a physical vapour deposition method were investigated as solar control coatings on architectural glazings. The optical transmittance and the near-normal specular reflectance in the range 0.40−2.40 μm and spectral distribution of reflected and transmitted intesities in the same range showed that CdSe thin films have solar control characteristics comparable to commercially available metallic coatings and other materials such as PbS and Cu_ξS films. The solar control characteristics of CdSe films were found to be dependent on film parameters, including deposition rate and deposition temperature.     

PbS nanocrystal solar cells fabricated using microwave-assisted chemical bath deposition

n-CdS/p-PbS heterojunction solar cell was fabricated using microwave-assisted Chemical Bath Deposition (CBD). The CdS window layer (340 nm thickness) was deposited on ITO-glass. The PbS absorber layer (685–1250 nm thickness) with different molar concentration (0.02, 0.05, 0.075 and 0.1 M) was then grown on ITO/CdS to fabricate the p–n junction. X-ray diffraction analysis confirms the formation of pure and nanocrystalline CdS and PbS phases with a preferred orientation depending on molarity; (111) or (200). Scanning electron microscopy observations show a uniform surface morphology with gatherings. UV–Vis spectrophotometer and FTIR was used to estimate the optical properties. Optical measurements gave an energy gap of 2.6 eV for CdS whereas that for PbS thin films were found to vary in a narrow range 0.40–0.47 eV, depending on the molar concentration. The photovoltaic properties under 30 mW/cm² solar radiation including J–V characteristics, short-circuit current (I_sc), open-circuit voltage (V_oc), fill factor (ff), efficiency (η) of CdS/PbS heterojunction cells have been as well examined. The results show that changing the molar concentration improved the performances of the fabricated photovoltaic cells; a high efficiency was observed at 0.1 M. However, high series resistance and poor crystallinity of PbS lead to low efficiency at lower molarity.     

Light scattering coatings: Theory and solar applications

We report on the recent theoretical advances in the field of light scattering by coatings or foils. Methods for computing all the parameters in the four-flux and two-flux radiative transfer theories have been devised. The four-flux theory is applied to the optical properties of white paints and pigmented polymer foils. Nocturnal dew collection by pigmented foils has been investigated in Dodoma, Tanzania, over a period of five months.     

Microstructure and optical properties of silicon nitride thin films as radiative cooling materials

SiN_X thin films were prepared by the RF plasma-enhanced chemical vapor deposition method. Composition, structure, surface morphology and optical properties of the thin films were analyzed by X-ray fluorescence, IR transmittance, IR reflectance and SEM. The results show that the composition of the films is SiN_0.35. Nitrogen atoms take part in the reaction with silicon atoms and Si–N bonds are formed. There are also some Si–H and N–H bonds in the films. The films have very low hemispherical IR reflectance across the full 8–13 μm band and high hemispherical reflectance elsewhere, which indicates that silicon nitride films can be used as good radiative cooling materials. The surface morphology and growth mechanisms of the films were also explained.     

太阳光TiO_2多孔纳米薄膜光催化降解有机磷农药的研究

锐钛矿型 Ti O2 多孔纳米薄膜可以从含聚乙二醇的钛醇盐溶胶前驱体中通过浸渍提拉法制备 ;涂层的形貌如孔的大小和孔的分布可以通过聚乙二醇的加入量来控制 ,当聚乙二醇的加入量为 0— 2 .0 g时 ,孔径大小在 0— 40 0 nm范围内变化。可见光透过光谱分析表明 :随着 Ti O2 薄膜中孔径增大 ,光的散射增强 ,透光率减小 ,该 Ti O2 镀膜玻璃对于紫外线具有吸收作用。有机磷农药水溶液的太阳光催化降解实验表明 :在 Ti O2 薄膜中引入气孔增强了光催化活性 ,孔的大小和薄膜厚度对光解率有显著的影响。当孔径大小为 1 0 0— 2 0 0 nm,镀膜次数为 1 0— 1 5次时 ,Ti O2 薄膜显示光催化效率高。     

Temperature effect on the electrical properties of pyrolytic MnO2 thin films prepared from Mn(C2H3O2)2·4H2O

Spray deposited MnO₂ thin films onto glass substrate were subjected to a post-deposition heat treatment and the effects of temperature on electrical transport properties were studied in details. The heating and cooling cycles of the samples are reversible after successive heat-treatments in air and vacuum. The films were polycrystalline in structure and the oxygen chemisorption–desorption process was found to play an important role in controlling the electronic properties. Various grain-boundary and energy band parameters were calculated by taking conventional extrinsic semiconductor theory and grain boundary trapping models into account. The samples were non-degenerate n-type semiconductors. The transport properties are interpreted in terms of Seto's model which was proposed for polycrystalline semiconducting films. The inter-crystallite boundaries of the thin films play an important role in the transport properties.     

Sputter deposited transition metal nitrides as back electrode for CIGS solar cells

Transition metal nitrides such as TiN and ZrN have a potential use as back electrodes in Cu(In,Ga)S₂ or Cu(In,Ga)Se₂ solar cells. Important properties of the back electrode are its conductivity, reflection at higher wavelengths, mechanical and chemical stability, and its barrier properties for impurity diffusion when metal foils are used as flexible substrate for the solar cells. Here, TiN thin films were deposited with reactive magnetron sputtering and were characterized with Rutherford backscattering spectroscopy, X-ray reflection, ellipsometry and UV-Vis-NIR optical spectroscopy. The resulting thin film density and optical reflection were characterized and compared to the particle fluxes towards the substrate. Therefore, the total momentum flux, total energy flux and momentum flux of target species towards the growing film were measured or simulated. A distinct relation between the density and optical reflection of the TiN films and the momentum flux of target species towards the substrate is shown.     

Characteristics of indium zinc oxide thin films prepared by direct current magnetron sputtering for flexible solar cells

The In₂O₃-ZnO (IZO) thin films were prepared on polyethylene terephthalate substrate at room temperature by direct current (dc) magnetron sputtering. The properties of IZO thin films were studied in terms of O₂ concentration and deposition parameters. As the O₂ concentration in O₂/Ar gas increased, the transmittances of the films were increased up to 90% and the resistivities were decreased. The systematic variation of process parameters including dc power, gas pressure and target-to-substrate distance was performed to examine the properties of the deposited films. It was disclosed that there was an optimum O₂ concentration for high transmittance and low resistivity. With decrease in dc power and gas pressure and increase in target-to-substrate distance, the IZO films with high transmittance and low resistivity were obtained. The observation of the IZO films by atomic force microscopy indicated that the microstructure and surface morphology of the films were responsible for the transmittance. It was demonstrated that IZO films with a resistivity of 5.1×10⁻⁴ Ω cm and an optical transmission of 90% in the visible spectrum could be prepared at room temperature on flexible substrates.     

The effects of TiO2 pigmented coatings characteristics on temperature and brightness of a coated black substrate

The high visible reflectivity of the cool coatings made by typical white pigment particles produces high glare, which is unpleasant to the human eye and possibly distorts the view of coated objects. A new approach to optimizing pigmented coatings considering both thermal and esthetic effects was proposed in previous works. For an accurate thermal analysis, a full spectral evaluation of radiative properties of pigmented coatings from UV to far IR wavelengths is required. We made a full spectral analysis of TiO₂ pigment particles in polyethylene resin as the host medium in the wavelength range of 0.3–36 μm. To find the spectral transmittance and reflectance of the pigmented layer, we conducted a radiation analysis using the radiation element method by ray emission model (REM²). The effects of characteristics of the coating layer, including size and volume concentration of pigment particles and coating thickness on esthetic and thermal behaviors were studied. The results show that by using the proposed optimum particle size, i.e., 0.8 μm, it is possible to design a coating with reasonable temperature and moderate brightness.     

不同络合剂对化学水浴法制备ZnS薄膜性能的影响

研究了不同络合剂对化学水浴法制备太阳电池用ZnS薄膜性能的影响。研究指出,在相同的浓度下以EDTA为络合剂时,由于其对锌离子的络合能力最强,无法生成ZnS薄膜,而以肼与柠檬酸钠为络合剂时,成功制备成ZnS薄膜。结果还表明,采用柠檬酸钠为络合剂,在搅拌条件下制备出的ZnS薄膜适用于CIGS太阳电池的过渡层。最后实验利用FE—SEM、XRD、紫外—可见光吸收谱,透射谱和反射谱研究了ZnS薄膜的性能。     

Semiconductor thin films by chemical bath deposition for solar energy related applications

In this paper we present the basic concepts underlying the chemical bath deposition technique and the recipes developed in our laboratory during the past ten years for the deposition of good-quality thin films of CdS, CdSe, ZnS, ZnSe, PbS, SnS, Bi₂S₃, Bi₂Se₃, Sb₂S₃, CuS, CuSe, etc. Typical growth curves, and optical and electrical properties of these films are presented. The effect of annealing the films in air on their structure and composition and on the electrical properties is notable: CdS and ZnS films become conductive through a partial conversion to oxide phase; CdSe becomes photosensitive, SnS converts to SnO₂, etc. The use of precipitates formed during deposition for screen printing and sintering, in polymer composites and as a source for vapor-phase deposition is presented. Some examples of the application of the films in solar energy related work are presented.     

Condensation of water by radiative cooling

Atmospheric humidity can be condensed as dew and used for example in small-scale irrigation. In arid locations, the most favourable conditions for dew collection persist in the late night and around sunrise. We study the possibility to use a dew collector for condensing atmospheric water vapour by exploiting the effect of radiative cooling. In particular, we study pigmented polymer foils with high solar reflectance and high thermal emittance. Suitable pigments are a mixture of TiO₂ and BaSO₄ particles or a novel SiO₂/TiO₂ composite. We calculate the condensation rate under different climatic conditions and report on initial field tests.     

Maxwell-Garnett theory analysis of thick PbS---Al selective absorbers

Previous measurements of the solar absorptance and thermal emittance values of PbS---Al tandem solar selective surfaces have resulted in ratios significantly higher than predicted for thick (>200 nm) PbS layers, assuming optical constants equal to those of the bulk material. We report calculations of photothermal conversion efficiencies for Pbs---Al tandem absorbers using the Maxwell-Garnett theory to determine α and values for porous PbS films. We have also prepared samples with PbS thicknesses up to 550 nm and have determined their α and values using reflectance measurements. Thermal efficiencies in the range of 0.55 to 0.86 were obtained at 300 K and are consistent with values expected for films with moderate (0.15–0.35) void fractions.     

Transformation of GO to rGO due to 8.0 MeV carbon (C++) ions irradiation and characteristics performance on MnO2–NiO–ZnO@GO electrode

The effect of 8.0 MeV carbon ions (C⁺⁺) radiations on features and performances of MnO₂–NiO–ZnO@GO electrodes (thin films). MnO₂–NiO–ZnO@GO thin films were produced using the hydrothermal technique. 8.0 MeV carbon ions (C⁺⁺) with doses of 2.25 × 10¹⁵, 5.0 × 10¹⁵, 7.5 × 10¹⁵ and 1.0 × 10¹⁶ ions/cm² were irradiated on MnO₂–NiO–ZnO@GO thin films. The XRD spectra indicate crystalline nature of the films while SEM images show rod-like structures. The XRD calculated crystallite sizes varied from 1.24 to 5.58 nm. Energy-dispersive X-ray spectroscopy, Proton induced X-ray emission (PIXE) and Rutherford back scattering (RBS) analysis are used to evaluate the elemental compositions of samples. Optical studies show reduced bandgap energies of various oxides due to the addition of graphene oxide. The electrochemical studies obtained a specific capacitance of 1627 and 1960 F/g for electrodes illuminated with radiation doses of 5.0 × 10¹⁵ and 7.5 × 10¹⁵ ions/cm², respectively. Results indicate that carbon ion irradiation with low doses improved the performances of the nanostructured thin films while radiation with high doses induces adverse disorder and flaw to the MnO₂–NiO–ZnO@GO thin film properties. These results show that ion beam irradiation is a useful tool to enhance or damage the properties of nanostructured materials depending on the dosages radiation beamed on the material.     

Advances in monocrystalline Si thin film solar cells by layer transfer

The transfer of monocrystalline Si films enables the fabrication of efficient thin film solar cells on glass or plastic foils. Chemical vapor deposition serves to epitaxially deposit Si on quasi-monocrystalline Si films obtained from thermal crystallization of a double-layer porous Si film on a Si wafer. A separation layer that forms during this crystallization process allows one to separate the epitaxial layer on top of the quasi-monocrystalline film from the starting Si wafer after solar cell processing. Independently confirmed thin film solar cell efficiencies are 15.4% and 16.6% for thin film solar cells transferred to a glass superstrate with a total Si film thickness of 24.5 and 46.5 μm, respectively, and a cell area of 4 cm². Device simulations indicate an efficiency potential above 20%.     

Structural properties of CdTe thin films on different substrates

Cadmium telluride (CdTe) thin films were grown by thermal evaporation method on glass and thin metallic foils such as stainless steel (SS) and molybdenum (Mo). The as-deposited film structure was studied by X-ray diffractometer. Films coated on glass for lower thickness shows amorphous structure whereas films at higher thickness exhibit crystalline nature. The structure exhibits a mixture of both cubic and hexagonal phases for films on metallic substrates. The analysis shows that the preferential orientation and grain size depend on the type of substrate used and its surface smoothness. The lattice constant and the structural parameters viz., crystallite size (D), dislocation density (δ), and strain (ε) were calculated and the influence of the substrates was demonstrated in this paper.