All-chemically deposited Bi2S3/PbS solar cells

We report on solar cells with a cross-sectional layout: TCO/window/Bi₂S₃/PbS, in which a commercial SnO₂ transparent conductive oxide (TCO-PPG Sungate 500); chemically deposited window layers of CdS, ZnS or their oxides; n-type Bi₂S₃ (100 nm) and p-type PbS (360-550 nm) absorber films constitute the cell structures. The crystalline structure, optical, and electrical properties of the constituent films are presented. The open circuit voltage (V_oc) and short-circuit current density (J_sc), for 1000 W/m² solar radiation, of these solar cells depend on the window layers, and vary in the range, 130-310 mV and 0.5-5 mA/cm², respectively. The typical fill factors (FF) of these cells are 0.25-0.42, and conversion efficiency, 0.1-0.4%.     

Solar cells with Sb2S3 absorber films

CdS/Sb₂S₃/PbS structures were prepared by sequential chemical deposition of CdS, Sb₂S₃ and PbS thin films on TEC-8 (Pilkington) transparent electrically conductive SnO₂ (TCO) coatings. CdS thin films (100 nm) were deposited with hexagonal structure from Cd-citrate bath and of cubic structure from Cd-ammine/triethanolamine bath. Sb₂S₃ thin films were deposited at 40 °C from a solution mixture of potassium antimony tartrate, triethanolamine, ammonia and thioacetamide(TA) or at 1 to 10 °C from a mixture of antimony trichloride and thiosulfate (TS). These films were made photoconductive by heating at temperatures 250 to 300 °C. When heated in the presence of a chemically deposited Se thin film of 300 nm, a solid solution Sb₂S_1.8Se_1.2 resulted. PbS thin films of 100-200 nm thickness were deposited on the TCO/CdS/Sb₂S₃ or TCO/CdS/Sb₂S_1.8Se_1.2 structure. Graphite paint was applied on the PbS film prior to applying a silver epoxy paint. The cell structures were of area 0.4 cm². The best results reported here is for a cell: TCO/CdS(hex-100 nm)/Sb₂S₃(TS-100 nm)/PbS(200 nm) with open circuit voltage (V_oc) 640 mV, short circuit current density 3.73 mA/cm², fill factor 0.29, and conversion efficiency 0.7% under 1000 Wm^− 2 sunlight. Four series-connected cells of area 1 cm² each gave V_oc of 2 V and short circuit current of 1.15 mA.     

Antimony sulfide thin films in chemically deposited thin film photovoltaic cells

Antimony sulfide thin films of thickness ≈ 500 nm have been deposited on glass slides from chemical baths constituted with SbCl₃ and sodium thiosulfate. Smooth specularly reflective thin films are obtained at deposition temperatures from − 3 to 10 °C. The differences in the film thickness and improvement in the crystallinity and photoconductivity upon annealing the film in nitrogen are presented. These films can be partially converted into a solid solution of the type Sb₂S_xSe_3 − x, detected in X-ray diffraction, through heating them in contact with a chemically deposited selenium thin film. This would decrease the optical band gap of the film from ≈ 1.7 eV (Sb₂S₃) to ≈ 1.3 eV for the films heated at 300 °C. Similarly, heating at 300 °C of sequentially deposited thin film layers of Sb₂S₃-Ag₂Se, the latter also from a chemical bath at 10 °C results in the formation of AgSb(S/Se)₂ with an optical gap of ≈ 1.2 eV. All these thin films have been integrated into photovoltaic structures using a CdS window layer deposited on 3 mm glass sheets with a SnO₂:F coating (TEC-15, Pilkington). Characteristics obtained in these cells under an illumination of 850 W/m² (tungsten halogen) are as follows: SnO₂:F-CdS-Sb₂S₃-Ag(paint) with open circuit voltage (V_oc) 470 mV and short circuit current density (J_sc) 0.02 mA/cm²; SnO₂:F-CdS-Sb₂S₃-CuS-Ag(paint), V_oc ≈ 460 mV and J_sc ≈ 0.4 mA/cm²; SnO₂:F-CdS-Sb₂S_xSe_3 − x-Ag(paint), V_oc ≈ 670 mV and J_sc ≈ 0.05 mA/cm²; SnO₂:F-CdS-Sb₂S₃-AgSb(S/Se)₂-Ag(paint), V_oc ≈ 450 mV and J_sc ≈ 1.4 mA/cm². We consider that the materials and the deposition techniques reported here are promising toward developing ‘all-chemically deposited solar cell technologies.’     

Structural analysis of chemically deposited nanocrystalline PbS films

Nanocrystalline PbS films are deposited on glass substrates by chemical bath deposition technique at room temperature. The structural parameters of PbS nanoparticles are studied by X-ray line profile analysis using Williamson-Hall and modified Williamson-Hall plot. The values of average crystallite sizes are found to vary from 12 to 18 nm having very high dislocation density of the order of 10¹⁷ m⁻².     

Structural and chemical transformations in SnS thin films used in chemically deposited photovoltaic cells

Chemically deposited SnS thin films possess p-type electrical conductivity. We report a photovoltaic structure: SnO₂:F-CdS-SnS-(CuS)-silver print, with V_oc > 300 mV and J_sc up to 5 mA/cm² under 850 W/m² tungsten halogen illumination. Here, SnO₂:F is a commercial spray-CVD (Pilkington TEC-8) coating, and the rest deposited from different chemical baths: CdS (80 nm) at 333 K, SnS (450 nm) and CuS (80 nm) at 293-303 K. The structure may be heated in nitrogen at 573 K, before applying the silver print. The photovoltaic behavior of the structure varies with heating: V_oc ≈ 400 mV and J_sc < 1 mA/cm², when heated at 423 K in air, but V_oc decreases and J_sc increases when heated at higher temperatures. These photovoltaic structures have been found to be stable over a period extending over one year by now. The overall cost of materials, simplicity of the deposition process, and possibility of easily varying the parameters to improve the cell characteristics inspire further work. Here we report two different baths for the deposition of SnS thin films of about 500 nm by chemical deposition. There is a considerable difference in the nature of growth, crystalline structure and chemical stability of these films under air-heating at 623-823 K or while heating SnS-CuS layers, evidenced in XRF and grazing incidence angle XRD studies. Heating of SnS-CuS films results in the formation of SnS-Cu_xSnS_y. ‘All-chemically deposited photovoltaic structures’ involving these materials are presented.     

Chemically deposited thin films of PbSe as an absorber component in solar cell structures

PbSe thin films were prepared by chemical deposition using dimethylselenourea as a source of selenide ions. Depending on the duration (30 min to 4 h) and temperature (30-60 °C) of the deposition, and the substrate, the films show a high degree of preferred orientation for the (111) planes. The texture coefficients could be up to 5 for these planes. The crystallite diameters are in the 30-35 nm range, and optical bad gap, 0.4-0.7 eV. The electrical conductivity is p-type, 0.01-10 (Ω cm)^− 1. These films were deposited over CdS/Sb₂S₃ or CdS/Sb₂Se₃ solar cell structures as an additional absorber. In a CdS/Sb₂Se₃/PbSe cell, this addition increases the short circuit current density (J_sc) from 0.2 mA/cm² to 8.9 mA/cm² and conversion efficiency (η) from 0.04% to 0.99%. In a CdS/Sb₂S₃/PbSe cell, J_sc is 5.91 mA/cm²; η, 0.98%; and open circuit voltage, 560 mV.     

Annealing effect on cadmium in situ doping of chemical bath deposited PbS thin films

This paper describes the effect of annealing on PbS and Cd-doped PbS thin films prepared by chemical bath deposition at different bath temperatures (T_b). The X-ray diffraction (XRD), optical absorption, scanning electron microscopy, and energy dispersive X-ray (EDX) analyses have been performed to explore the properties of PbS and PbCdS films. From the XRD measurements, the particle size (D) of as-deposited PbS and PbCdS films is estimated to be 22 (27) and 12 (9) nm, respectively, for a T_b of 75 (85) °C. A reduction in D was noticed upon annealing the films at 200 °C, irrespective of the T_b and the doping. The optical band gap energy (E_g) of as-deposited PbS films grown at different T_b is found to be in the range of 1.22-1.42 eV. Doping of PbS with Cd and annealing have led to increase in E_g up to 2.61 (2.66) eV. Optical studies revealed prominent blue shifts in the E_g of as-deposited and annealed films due to quantum confinement effect. The addition of Cd into PbS was confirmed by EDX analysis.     

Study of CuInS2/buffer/ZnO solar cells, with chemically deposited ZnS-In2S3buffer layers

Thin film solar cells based on CuInS₂/buffer/ZnO have been prepared with different buffer layers of mixed ZnS-In₂S₃ composition. The buffer films are grown by chemical bath deposition (CBD) from acidic solutions of InCl₃, ZnSO₄ and thioacetamide (TA). A kinetics study of the growth of the buffer layers is carried out with the quartz crystal microbalance. The influence of bath conditions (solution, temperature and composition) on the growth rate is studied. Films are obtained with different physical, chemical and morphological properties. The absorption coefficient spectra of the films show a variation depending on the CBD conditions, with absorption edges between 2.6 and 3.35 eV. Surface morphology of the films, observed with scanning electron microscopy, reveals that the presence of Zn produces characteristic structures with microtubular form, compared with pure In₂S₃ buffer films. Solar cell results show a significant increase of Voc and FF upon introducing Zn²⁺ ion in the buffer layer.     

Characterization of Al2O3/ZrO2 nano multilayer thin films prepared by pulsed laser deposition

Microstructural characterization of pulsed laser deposited Al₂O₃/ZrO₂ multilayers on Si (1 0 0) substrates at an optimized oxygen partial pressure of 3 × 10⁻² mbar and at room temperature (298 K) has been carried out. A nanolaminate structure consisting of alternate layers of ZrO₂ and Al₂O₃ with 40 bi-layers was fabricated at different zirconia layer thicknesses (20, 15 and 10 nm). The objective of the work is to study the effect of ZrO₂ layer thickness on the stabilization of tetragonal ZrO₂ phase for a constant Al₂O₃ layer thickness of 5 nm. The Al₂O₃/ZrO₂ multilayer films were characterized using high temperature X-ray diffraction (HTXRD) in the temperature range 298–1473 K. The studies showed that the thickness of the zirconia layer has a profound influence on the crystallization temperature for the formation of tetragonal zirconia phase. The tetragonal phase content increased with the decrease of ZrO₂ layer thickness. The cross-sectional transmission electron microscope (XTEM) investigations were carried out on a multilayer thin films deposited at room temperature. The XTEM studies showed the formation of uniform thickness layers with higher fraction of monoclinic and small fraction of tetragonal phases of zirconia and amorphous alumina.     

The influence of annealing temperatures on the properties of Bi2VO5.5/LaNiO3/Si thin films

Bi₂VO_5.5 ferroelectric thin films were fabricated on LaNiO₃/Si(100) substrate via chemical solution deposition. Ferroelectric and dielectric properties of the thin films annealed at 500-700 °C were studied. The thin film annealed at 700 °C exhibited more favorable ferroelectric and dielectric properties than those annealed at lower temperatures. The values of remnant polarization 2P_r and coercive field E_c for the film annealed at 700 °C are 10.62 µC/cm² and 106.3 kV/cm, respectively. The leakage current of the film is about 1.92 × 10^− 8 A/cm² at 6 V. The possible mechanism of the dependence of electrical properties of the films on the annealing temperature was discussed.     

Tin sulfide thin films and Mo/p-SnS/n-CdS/ZnO heterojunctions for photovoltaic applications

Tin sulfide (SnS) is one of the most promising materials for photovoltaics. Here we report on the preparation as well as chemical, structural and physical characterization of the Mo/p-SnS/n-CdS/ZnO heterojunctions. The SnS thin films were grown by hot wall deposition method on the Mo-coated glass substrates at 270-350 °C. The crystal structure and elemental composition were examined by X-ray diffraction and Auger electron spectroscopy methods. The CdS buffer layers were deposited onto the SnS films by chemical bath deposition. The ZnO window layers were deposited by a two step radio frequency magnetron sputtering, resulting in a ZnO bilayer structure: the first layer consists of undoped i-ZnO and the second of Al-doped n-ZnO. The best junctions have an open circuit voltage of 132 mV, a short circuit current density of 3.6 mA/cm², a fill-factor of 0.29 and efficiency up to 0.5%.     

Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

CuInS2 thin films deposited by sol-gel spin-coating method

CuInS₂ thin films were prepared using a sol-gel spin-coating method. Copper acetate monohydrate (Cu(CH₃COO)₂·H₂O) and indium acetate (In(CH₃COO)₃) were dissolved into 2-propanol and 1-propanol, respectively. The two solutions were mixed into a starting solution. The solution was dropped onto glass substrate, rotated at 1500 rpm, and dried at 300 °C for Cu-In as-grown films. The as-grown films were sulfurized inside a graphite container box. A clear chalcopyrite phase was observed without a secondary phase. Surface roughness of the films sulfurized at 500 °C was 19.1 nm. A Raman spectra measurement confirmed that no Cu-S or In-S compounds were created in the thin films.     

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.     

Optimization of thermoelectric properties on Bi2Te3 thin films deposited by thermal co-evaporation

The optimization of the thermal co-evaporation deposition process for n-type bismuth telluride (Bi₂Te₃) thin films deposited onto polyimide substrates and intended for thermoelectric applications is reported. The influence of deposition parameters (evaporation rate and substrate temperature) on film composition and thermoelectric properties was studied for optimal thermoelectric performance. Energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the formation of Bi₂Te₃ thin films. Seebeck coefficient (up to 250 μV K^− 1), in-plane electrical resistivity (≈10 μΩ m), carrier concentration (3×10¹⁹-20×10¹⁹ cm^− 3) and Hall mobility (80-170 cm² V⁻¹ s^− 1) were measured at room temperature for selected Bi₂Te₃ samples.     

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.     

Spectroscopic ellipsometry studies on the optical constants of Bi4Ti3O12:xNa thin films grown by metal-organic chemical vapor deposition

We have deposited undoped and Na-doped epitaxial Bi₄Ti₃O₁₂ thin films on SrTiO₃(001) substrates using the liquid-delivery spin metal-organic chemical vapor deposition technique. High resolution x-ray diffraction and Raman spectroscopy measurements were employed to investigate the structural perfection and incorporation of Na ions into the film. The ellipsometric measurements were carried out in the energy range 1.49-2.75 eV and the corresponding experimental data were fitted. Two different dispersion relations, Cauchy's absorbent and Tauc-Lorentz, have been used to determine the optical constants of the films. It is observed that there is a decrease in optical band gap for increasing sodium content. Furthermore, it has been found that the refractive index and extinction coefficient also depend on the sodium content. The refractive index dispersion data obeyed the single oscillator of the Wemple-DiDomenico model, from which the dispersion parameters were determined. The optical constants tend to decrease with increasing doping content.     

Structural and electrical characteristics of chemical solution derived (Bi3.2La0.4Nd0.4)Ti3O12 thin films

(Bi_3.2La_0.4Nd_0.4)Ti₃O₁₂ (BLNT) thin films were prepared on Pt/Ti/SiO₂/Si substrates by using chemical solution deposition technique, and the effects of annealing temperatures in the range of 550-750 °C on structure and electrical properties of the thin films were investigated. X-ray diffraction analysis shows that the thin films have a bismuth-layered perovskite structure with preferred (117) orientation. The surface morphology observation by field-emission scanning electron microscopy confirms that films are dense and smooth with uniformly distributed grains. The grain size of the thin films increases with increasing annealing temperature; meanwhile, the structural distortion of the thin films also increases. It was demonstrated that the thin films show good electrical properties. The dielectric constant and dielectric loss are 191 and 0.028, respectively, at 10 kHz for the thin film annealed at 600 °C, and the 2Pr value of the thin film annealed at 700 °C is 20.5 μC/cm² at an electric field of 500 kV/cm.     

Blue shift of optical band-gap in LiNbO3 thin films deposited by sol-gel technique

Lithium niobate (LN) thin films were deposited on quartz substrates by sol-gel technique. The measured absorption and photoluminescence spectra show that the band structure of LN thin films is direct unlike indirect band-gap in bulk LN and the optical band-gap of these LN thin films was measured to be 4.7 eV which is ~ 1 eV greater than that for stoichiometric bulk LN. The dependence of the blue shift of band-gap on several parameters like quantum confinement, composition (Li:Nb ratios of LN thin films) and strain was also investigated. The results obtained show that the large blue shift in band-gap of LN thin films is primarily due to strain in the film.     

Studies on chemically deposited CuInSe2 thin films

Nanocrystalline thin films of CuInSe₂ have been prepared by chemical bath deposition technique at temperatures below 60 °C. X-ray diffraction of the films confirmed the identity of CuInSe₂ and with largely broadened peaks indicated the nanocrystalline nature of the films. Images from scanning electron microscope represented spherical nanoparticles.