CuGaSe2 thin films for photovoltaic applications

Of the I-III-VI₂ group chalcopyrites, CuInSe₂ has already proved its suitability for thin film solar cells owing to its excellent optical and transport properties. CuGaSe₂ is expected to exhibit comparable properties from this point of view. With its band gap of 1.7 eV it is a candidate for use in photovoltaic tandem systems.

The preparation of CuGaSe₂ thin films by means of the vacuum evaporation of the constituent elements (four-temperature method) is described. The structural, electrical and optical properties of these films were investigated. Secondary electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction examination and measurements of the optical transmission, resistivity and thermoelectric power were used to determine the film properties relative to the preparation parameters and stoichiometry. The growth conditions were optimized for solar cell applications. Heterojunctions were prepared by the in situ evaporation of Zn_xCd_1−xS onto the CuGaSe₂ films. The characteristic data of the cells are a short-circuit current of 6 mA and an open-circuit voltage of 620 mV at an illumination at air mass 1.5 on an area of 1 cm².     

Characterization of Zinc-phthalocyanine-CdS composite thin films for photovoltaic applications

The optical properties of ZnPc-CdS composite thin films have been measured. The composite layers were prepared by vacuum evaporation. The electrical conduction mechanism prevailing in these junctions was of Poole-Frenkel type and the activation energy was found to have a linear dependence with applied field. The maximum photoconductivity occurred at an energy gap around 1.5 to 1.7 eV. The photosensitivity is found to increase with increase in applied voltage.     

Chemical solution deposition of thin TiO2-anatase films for dielectric applications

Thin films of TiO₂ have been prepared using chemical solution deposition on 6 n-type Si(1 0 0) wafers. Thin film thickness in the range from 70 to 210 nm could be achieved via control of the number of deposition sequences. The final annealing temperature of 700 °C resulted in an anatase phase structure with fine elongated grains and smooth surface topography. The capacity of the thin films is shown to depend on thickness, and could be interpreted assuming a series capacity with an SiO₂ interfacial layer. The resulting dielectric constant of the TiO₂ thin film has been calculated to be 23. The leakage current behavior and the break-down field also depend on film thickness. It is shown that thinner films are higher break-down fields.     

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.     

Polythiophene thin films electrochemically deposited on sol-gel based TiO2 for photovoltaic applications

In this work, the influence of titanium dioxide (TiO₂) thin films on the efficiency of organic photovoltaic devices based on electrochemically synthesized polythiophene (PT) was investigated. TiO₂ films were produced by sol-gel methods with controlled thickness. The best TiO₂ annealing condition was determined through the investigation of the temperature influence on the electron charge mobility and resistivity in a range between 723 K and 923 K. The PT films were produced by chronoamperometric method in a 3-electrode cell under a controlled atmosphere. High quality PT films were produced onto 40 nm thick TiO₂ layer previously deposited onto fluorine doped tin oxide (FTO) substrate. The morphology of PT films grown on both substrates and its strong influence on the device performance and PT minimum thickness were also investigated. The maximum external quantum efficiency (IPCE) reached was 9% under monochromatic irradiation (λ = 610 nm; 1 W/m²) that is three orders of magnitude higher than that presented by PT-homolayer devices with similar PT thickness. In addition, the open-circuit voltage (V_oc) was about 700 mV and the short-circuit current density (J_sc) was 0.03 A/m² (λ = 610 nm; 7 W/m²). However, as for the PT-homolayer also the TiO₂/PT based devices are characterized by antibatic response when illuminated through FTO. Finally, the Fill Factor (FF) of these devices is low (25%), indicating that the series resistance (R_s), which is strongly dependent of the PT thickness, is too large. This large R_s value is compensated by TiO₂/PT interface morphology and by FTO/TiO₂ and TiO₂/PT interface phenomena producing preferential paths in which the internal electrical field is higher, improving the device efficiency.     

Optimization of PbS thin films using different metal ion sources for photovoltaic applications

An extensive study on the influence of metal ion sources on the properties of chemical bath deposited lead sulphide thin films is reported in this paper. Four different lead sources namely lead nitrate, lead acetate, lead chloride and lead sulphate in alkaline medium have been used for the synthesis along with thiourea as sulphur source. The influence of lead sources on structural, surface morphological optical and electrical properties is investigated for photovoltaic applications. According to X-ray diffraction studies, all the films are poly crystalline with face centered cubic structure. The average crystallite sizes are found to be in the range 13–24 nm. The SEM photographs showed diverse morphology. The optical band gap is found to be very sensitive to the metal sources used. The direct band gap energy values obtained are in the range of 1.862–2.609 eV. The electrical conductivity varies in the range 33.6 ?7.62 × 10^?9 (Ω cm)^?1. Photosensitivity is closely linked to surface morphology. In this work, we established that the cationic precursor sources have significant role in physical properties of as-deposited PbS thin films. Samples prepared using nitrate as metal source are found to be most suitable to be used as solar control coating and the samples with lead acetate can be used as absorber layers for solar cell fabrication.     

Microstructure and lattice bending in polycrystalline laser-crystallized silicon thin films for photovoltaic applications

Grain size, grain boundary population, orientation distribution and lattice defects of polycrystalline silicon thin films are investigated by electron backscatter diffraction (EBSD). The silicon thin films are produced by a combination of diode laser melt-mediated crystallization of an amorphous silicon seed layer and epitaxial thickening of the seed layer by solid phase epitaxy (SPE). The combined laser-SPE process delivers grains exceeding several 10 μm of width and far larger than 100 μm in length. Strong lattice rotations between 10 and 50° from one side of the grain to the other are observed within the larger grains of the film. The misorientation axes are well aligned with the direction of movement of the laser. The intragranular misorientation is associated both with geometrically necessary dislocations and low angle boundaries, which can serve as recombination centres for electron-hole pairs. Since the lateral grain size is up to two orders of magnitude larger than the film thickness, the high dislocation density could become an important factor reducing the solar cell performance.     

CuAlxGa1−xSe2 thin films for photovoltaic applications: Structural,electrical and morphological analysis

Wide-bandgap quaternary semiconductor chalcopyrites can be of interest for photovoltaic application. Novel CuAl_xGa_1?xSe₂ (CAGS) (0 ≤ x ≤ 1) thin films have been achieved by selenization of evaporated metallic precursor layers in a wide range of Al content and for several film thicknesses. Influence of Al incorporation, x, and film thickness on the structural, electrical and morphological properties of the samples have been studied. Polycrystalline CAGS thin films have been obtained reproducibly with chalcopyrite structure. The lattice parameters, a and c, and the average crystallite size decreased with the increase of Al amount, x. All films showed random orientation with the degree of randomness reduced for increasing x values. All samples show photoconductivity and resistivities ranging from 10¹ to 10⁴ Ω cm depending on the film thickness and x value. Resistivity values increase and thermoelectric coefficient decreases with the increase of Al proportion, x. Surface morphology was studied by SEM images and roughness measurements. Grain size and the arithmetic average roughness decreases with the increase of x and with the decrease of film thickness.     

用于紫外光电导探测器的TiO2薄膜研究

采用直流反应磁控溅射的方法,制备了TiO2薄膜。用X射线衍射,原子力显微镜(AFM)和紫外-可见光分光光度计分别测试了TiO2薄膜的晶体结构、表面形貌及其紫外-可见光吸收谱,采用C／TiO2／ITO三层结构制备了TiO2光电导型紫外探测器,研究了它的光响应。初步实验结果表明：TiO2薄膜在4W的紫光灯辐射下,光电流可达2．1mA,10min的辐射时间内,光电流基本保持稳定,可见TiO2薄膜对紫外光有较高的灵敏度和稳定性,可作为一种良好的紫外光探测材料。     

Electrospinning processed nanofibrous TiO(2) membranes for photovoltaic applications

We have recently fabricated dye-sensitized solar cells (DSSCs) comprising nanofibrous TiO(2) membranes as electrode materials. A thin TiO(2) film was pre-deposited on fluorine doped tin oxide (FTO) coated conducting glass substrate by immersion in TiF(4) aqueous solution to reduce the electron back-transfer from FTO to the electrolyte. The composite polyvinyl acetate (PVac)/titania nanofibrous membranes can be deposited on the pre-deposited thin TiO(2) film coated FTO by electrospinning of a mixture of PVac and titanium isopropoxide in N,N-dimethylformamide (DMF). The nanofibrous TiO(2) membranes were obtained by calcining the electrospun composite nanofibres of PVac/titania as the precursor. Spectral sensitization of the nanofibrous TiO(2) membranes was carried out with a ruthenium (II) complex, cis-dithiocyanate-N,N(')-bis(2,2(')-bipyridyl-4,4(')-dicarboxylic acid) ruthenium (II) dihydrate. The results indicated that the photocurrent and conversion efficiency of electrodes can be increased with the addition of the pre-deposited TiO(2) film and the adhesion treatment using DMF. Additionally, the dye loading, photocurrent, and efficiency of the electrodes were gradually increased by increasing the average thickness of the nanofibrous TiO(2) membranes. The efficiency of the fibrous TiO(2) photoelectrode with the average membrane thickness of 3.9?μm has a maximum value of 4.14%.     

Enhanced photovoltaic performance of quantum dot sensitized solar cells with Ag-doped TiO2 nanocrystalline thin films

Ag-doped titanium dioxide (TiO₂) nanocrystalline thin films have been prepared by the sol–gel dip coating method and used as photoanode to fabricate quantum dot sensitized solar cells. The X-ray diffraction studies reveal the formation of anatase phase without any impurity phase. The surface morphology studied using scanning electron microscope shows uniform distribution of particles. The optical band gap was found to be 3.5 and 3.4 eV for CdS quantum dot sensitized TiO₂ and CdS quantum dot sensitized Ag-doped TiO₂ thin film respectively. The Ag-doped TiO₂ based solar cell exhibited a power conversion efficiency of 1.48 % which is higher than that of TiO₂ (0.9 %).     

Morphological characterization of TiO2 thin films

Films prepared by reactive magnetron sputtering always present some structural and morphological heterogeneities.In this work, optical parameters, n(λ), k(λ) and E₀, of TiO₂ thin films were obtained, using only optical transmittance measurements. Films were described according to Abèles's model. Using a mono-oscillator type dispersion curve for the refractive index and a Lorentzian type curve for the absorption coefficient, we were able to demonstrate that the films were optically equivalent to a porous layer, with some dispersion in film thickness.The detailed analysis of the experimental transmittance data, fitted between 330 nm to 2200 nm, also enabled us to correlate the effective refractive index of each film with its deposition conditions.     

Solid phase epitaxy of silicon thin films by diode laser irradiation for photovoltaic applications

High temperature solid phase epitaxial crystallization of amorphous silicon layers prepared by electron beam evaporation is investigated. By using a continuous wave diode laser for heating the films rapidly (in milliseconds to seconds) this method is suitable on glass substrates with low temperature resistance. Therefore, the method is an economically advantageous technique of producing absorber layers for thin film solar cells. For the experiments 500 nm of amorphous silicon was deposited on two different configurations of substrates. In the first one monocrystalline wafers of three different crystallographic orientations were used. In the second one a polycrystalline seed layer prepared on borosilicate glass served as substrate. The crystallization process was monitored in situ by time resolved reflectivity measurements. Depending on the crystal orientation 2 s to 3 s was needed for complete solid phase epitaxial crystallization of the amorphous films. The evolution of temperature during crystallization was simulated numerically.     

Metal sulfide-polymer nanocomposite thin films prepared by a direct formation route for photovoltaic applications

Blends of conjugated polymers and inorganic semiconductors are an interesting class of materials with various applications in the field of plastic electronics. This work presents a direct approach to obtain composites consisting of a conjugated polymer, poly(3-(ethyl-4-butanoate)thiophene) (P3EBT), and a sulfur-based semiconductor (i.e. CdS, PbS or ZnS) using an in-situ formation route. The metal sulfide semiconductor is formed by reaction of the corresponding metal salt (cadmium acetate, zinc acetate or lead thiocyanate) dispersed within the conjugated polymer matrix with thiourea at temperatures below 200 °C. Nanoscaled networks are formed in the case of the CdS- and ZnS-P3EBT composites as shown by X-ray diffraction and transmission electron microscopy investigations, whereas the PbS-P3EBT blend exhibits inorganic structures on the μm-scale. The materials were used as active layer in bulk-heterojunction type hybrid solar cells. First photovoltaic devices containing an active layer of CdS- or ZnS-P3EBT show photovoltaic action, though efficiencies are low (≤ 0.06%).     

Growth and characterizations of dual ion beam sputtered CIGS thin films for photovoltaic applications

The growth of CIGS thin films on soda-lime glass substrates at different substrate temperatures by dual ion beam sputtering system in a single-step route from a single quaternary sputtering target with the composition of Cu (In_0.70 Ga_0.30) Se₂ was reported. The effects of the substrate temperature on structural, optical, morphological and electrical properties of CIGS films were investigated. Stoichiometry of one such film was investigated by X-ray photoelectron spectroscopy. All CIGS films had demonstrated a strong (112) orientation located at 2θ ~26.70^o, which indicated the chalcopyrite structure of films. The value of full-width at half-maximum of (112) peak was reduced from 0.58° to 0.19° and crystallite size was enlarged from 14.98 to 43.05 nm as growth temperature was increased from 100 to 400 °C. However, atomic force microscope results showed a smooth and uniform surface at lower growth temperature and the surface roughness was observed to increase with increasing growth temperature. Hall measurements exhibited the minimum film resistivity of 0.09 Ω cm with a hole concentration of 2.42 × 10¹⁸ cm^?3 and mobility of 28.60 cm² V^?1 s^?1 for CIGS film grown at 100 °C. Film absorption coefficient was found to enhance nominally from 1 × 10⁵ to 2.3 × 10⁵ cm^?1 with increasing growth temperature from 100 to 400 °C.     

Deposition of bismuth sulfide and aluminum doped bismuth sulfide thin films for photovoltaic applications

Journal of Materials Science: Materials in Electronics - Binary bismuth sulfide (Bi2S3) and ternary aluminum-doped bismuth sulfide (Al@Bi2S3) thin films were prepared by the chemical bath...     

Photoelectrochemical properties of plasma-deposited TiO2 thin films

Photoanodes were fabricated from TiO₂ films deposited onto titanium substrates by plasma-enhanced chemical vapor deposition. The photocurrent-wavelength and photocurrent-voltage properties of the anodes were determined and compared with those of thermally grown TiO₂ photoanodes. The plasma-deposited photoanodes displayed quantum efficiencies higher than those for the thermally grown films and comparable with those reported for single-crystal rutile. The microstructure of the plasma-deposited films appeared to be primarily responsible for the high quantum efficiencies.     

Role of thermal treatment on the luminescence properties of CdTe thin films for photovoltaic applications

The efficiency of CdTe based solar cells is strongly enhanced by a thermal treatment in HCF₂Cl ambient. CdTe thin films deposited on CdS/ZnO/ITO/glass by Closed Space Sublimation before and after the annealing are characterised. The CdTe morphology is studied by atomic force microscopy and scanning electron microscopy. In the treated films the non-homogeneous distribution of the grain size disappears, in addition an increasing of the dimensions of the grains is observed. Cathodoluminescence analyses show a remarkable difference in the spectra between the treated and untreated structures. A strong increase in the intensity of the 1.4 eV band is observed by increasing the HCF₂Cl content. A model of the electronic levels inside the CdTe band gap, due to incorporation of Cl (or F) is proposed.