Anomalous photovoltaic effect in vacuum-deposited CdTe films

CdTe films of thicknesses varying from 125 to 1250 nm were vapour deposited onto glass substrates at oblique incidence in vacuum at substrate temperatures varying from room temperature to 250 °C. The samples are irradiated with an He-Ne laser from the direction of the deposited layer and through the substrate. It was found that the film thickness and substrate temperature play an important role in changing the magnitude and polarity of the photovoltages generated in CdTe films. A reversal in the polarity of the photovoltage is obtained in certain films depending on the deposition parameters. An attempt is made to interpret the results.     

Nb-doped TiO2 thin films for photovoltaic applications

Polycrystalline titania and Nb:TiO₂ thin films were deposited by RF magnetron sputtering. The influence of post-deposition annealing in vacuum and hydrogen atmosphere on the structure, morphology, oxidation states and optical properties was studied by X-ray diffraction, atomic force microscopy, XPS and UV–VIS spectroscopy. The heat treatment of titanium dioxide thin films in vacuum and H₂ atmosphere induces structural and morphological changes. The band gap narrowing was observed for the transparent as-deposited Nb:TiO₂ films, while annealing at 420 °C in H₂ atmosphere resulted in an enhancement of the electrical conductivity. Further on, TiO₂/p-CdTe photovoltaic devices with efficiency of 1.8% were fabricated and their characteristic ‘enhancement’ is discussed.     

Organic photovoltaic cell employing organic heterojunction as buffer layer

Hexadecafluorophthalocyaninatocopper (F₁₆CuPc)/zinc phthalocyanine (ZnPc) heterojunction layer has been used as buffer layer in organic photovoltaic (OPV) cells based on ZnPc and C₆₀. The F₁₆CuPc/ZnPc heterojunction with highly conductive property decreased the contact resistance between the indium-tin-oxide anode and the organic layer. As a result, the short-circuit current density and fill factor were increased, and the power-conversion efficiency was improved by over 60%. Therefore, the method provides an effective path to improve the performance of OPV cells.     

Optical properties and photovoltaic device applications of InSe films

Films were formed by alternately evaporating InSe and Selenium from separate evaporation sources onto glass substrates kept at temperatures below 150°C with subsequent thermal annealing.The effects of the additional selenium on the properties of the films were investigated by structural investigations and photoconductivity measurements. It is found from the results of the present measurements that compositional changes occur in the films with increasing selenium/InSe ratio and that single-phase films containing only InSe are obtained at a certain critical ratio.The refractive indices and absorption coefficients of amorphous and crystalline InSe films in the spectral range 0.6–2.5 μm are determined by spectrophotometric reflectance and transmittance measurements. The minimum optical energy gap obtained from measurements of the absorption spectrum for the crystalline films is close to the value for single crystals.A heterostructure diode using crystalline InSe films as “window layers” was prepared and the fundamental photovoltaic properties were investigated. An absolute quantum efficiency of about 18% is obtained in the wavelength range 1.1–1.25 μm.     

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.     

Modular process control system for photovoltaic thin films manufacturing

A method of temperature control for selenization process conducted in vacuum quartz tube furnace with radial energy transfer and a movable sample holder has been described. Selenization of metallic precursors to obtain CuInSe₂ absorber layer of a photovoltaic cell requires a special approach to the temperature regulation, where two-stage heating process is necessary. The modular process control system was developed in which the first module playing a supervising role was built in LabVIEW environment and the second one, called a predictor, was built in MATLAB. Relatively quick movement of the holder from the cold to the hot regions of the furnace allows for a rapid change of sample temperature, typical of rapid thermal processing processes.     

Interface modification of ITO thin films: organic photovoltaic cells

In this paper we review our recent studies of the surface characterization of commercially available indium-tin-oxide (ITO) thin films, using photoelectron spectroscopies (XPS and UPS) and electrochemistry of chemisorbed probe molecules such as ferrocene dicarboxylic acid (Fc(COOH)₂). The modification of these ITO films through chemisorption of carboxylic acid-substituted small molecules, such as Fc(COOH)₂, 3-thiophene acetic acid (3-TAA), and the subsequent modification of these interfaces with electrochemically grown conducting polymer (CP) films is also introduced. We report preliminary results of our studies changes in performance of vacuum deposited organic photovoltaic (PV) cells as a result of these ITO substrate modification steps. The surfaces of as-received ITO films, and those cleaned by various solution and plasma-etching processes, are unavoidably hydrolyzed to In(OH)₃-like and InOOH-like surface species, which leaves the ITO surface with at most 40-50% of the electronically active sites available for electron transfer reactions. Modification of the ITO surface with electroactive small molecules such as Fc(COOH)₂ and 3-TAA provides for better wettability of organic layers to the polar ITO surface and enhanced electrical contact (lower series resistance, R_S) between the ITO anode, spin-cast or electrodeposited PEDOT:PSS layers and copper phthalocyanine (CuPc) layers in multilayer (CuPc/C₆₀/BCP) excitonic PV cells. Improvements in PV J/V (current/voltage) responses are noted mainly through increases in short-circuit photocurrent and lowered series resistances (R_S) when electroactive small molecules are chemisorbed to the ITO surface, prior to spin-casting of conducting polymer, PEDOT:PSS, layers.     

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.     

Organic photovoltaic solar cells based on some pure and sensitized dyes

Organic photovoltaic solar cells based on metal-dye or sensitized dye-SnO₂ junctions are formed. The electrical and photovoltaic characteristics of these Schottky junctions have been studied on two pure and sensitized dyes (Eriochrome Blue Black B and Rodamine B). From the photovoltaic action spectra the active region responsible for electric power generation was found to be confined to the SnO₂-dye interface. The effect of sensitization, electrode material and intensity on photovoltaic and electrical parameters has also been discussed in detail. Finally theC-V characteristics are discussed in detail.     

Organic photovoltaic devices based on an acceptor of solution-processable functionalized graphene

We prepared the exfoliation of graphite, which was necessary for the production of graphene sheets that are desirable for the fabrication of nano-composites. Then a Solution-Processable Functionalized Graphene (SPFGraphene) with functionalization groups doped with P3HT hybrid thin film-based organic photovoltaic cells (OPVCs) was systematically identified using a general device structure of, ITO/PEDOT:PSS/P3HT:SPFGraphene/LiF/Al. The effect of annealing on the photoelectric properties of the SPFGraphene was analyzed by Fourier transform infrared FT-IR spectroscopy and solar cell performance. After treatment at different annealing temperatures, with an increase in the SPFGraphene content, the short-circuit current density J(SC) and power conversion efficiency PCE of the hybrid devices increased first, reaching the peak efficiency for the 10 wt% SPFGraphene content, and then decreased. After annealing at 160 degrees C, the device containing 10 wt% SPFGraphene showed the open-circuit voltage V(OC) of 0.73 V, the J(SC) value of 3.98 mA cm(-2), fill factor (FF) value of 0.36, the PCE value of 1.046%. After thermal annealing at 210 degrees C, with the removal of the functional groups and recovery of the pi-conjugated areas, the conductivity of the graphene sheet and the charge carrier-transport mobility increased greatly, the J(SC) value of the 10 wt% SPFGraphene content device increased to 4.2 mA cm(-2), the V(OC) value decreased to 0.59 V, which may be attributed to the altered work-function value of the functionalized graphene and low quasi-Fermi levels for electrons and holes, the FF value was 0.27, and the PCE was 0.669%, which is lower than the former one. The results indicated that annealing at the appropriate temperature can improve the device performance greatly, and the functionalized graphene is expected to be a competitive candidate in organic photovoltaic applications because it is soluble, cheap, easily prepared, stable, and inert against the ambient conditions.     

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.’     

Studies on electrochemical photovoltaic cells formed with as-deposited in-doped CdS films

Indium doped and undoped CdS films have been deposited on conducting substrates by a chemical bath deposition technique. The doping concentration was varied from 0.005 to 1.0 wt. % of indium. Electrochemical photovoltaic (ECPV) cells are formed with these films and their electrical properties are studied. It is observed that the performance of the cell measured in terms of open circuit voltage (Voc), short circuit current (Isc), efficiency (η), and maximum power output (Pm), is optimum for the cells formed with 0.01 wt. % In-doped CdS photoanode.     

Preparation and photovoltaic properties of anodically grown Ag2O films

The semiconducting and photovoltaic properties of p-type Ag₂O films grown anodically on silver electrodes were studied, in view of possible applications in solar energy conversion. Films were grown in different alkaline solutions; the best results were obtained for 0.02M Ag₂SO₄ + 0.17M NH₄OH + 5.7 × 10^–3M Ba(OH)₂ saturated with Ag₂O powder, stirred mechanically at room temperature. Film thicknesses of up to 10m were thus obtained for the first time in anodically grown Ag₂O. Photovoltaic spectra taken at 300 K give a bandgap ofEg = 1.42 ± 0.04 eV. Evaporated gold on Ag₂O appears to be ohmic while aluminium and platinum are rectifying. The barrier height of Ag/Ag₂O is 0.90 ± 0.02 eV, that of Al/Ag₂O is 0.93 ± 0.02 eV, and that of platinum 0.94 ± 0.02 eV. The best cells give an open-circuit voltage,V _oc, of over 150 mV, and a short circuit current,I _sc = 100A cm^–2 under 50 mW cm^–2 illumination.     

Spin coated graphene films as the transparent electrode in organic photovoltaic devices

Many research efforts have been devoted to the replacement of the traditional indium-tin-oxide (ITO) electrode in organic photovoltaics. Solution-based graphene has been identified as a potential replacement, since it has less than two percent absorption per layer, relative high carrier mobility, and it offers the possibility of deposition on large area and flexible substrates, compatible with roll to roll manufacturing methods. In this work, soluble reduced graphene films with high electrical conductivity and transparency were fabricated and incorporated in poly(3-hexylthiophene) [6,6]-phenyl-C₆₁-butyric acid methyl ester photovoltaic devices, as the transparent electrode. The graphene films were spin coated on glass from an aqueous dispersion of functionalized graphene, followed by a reduction process combining hydrazine vapor and annealing under argon, in order to reduce the sheet resistance. The photovoltaic devices obtained from the graphene films showed lower performance than the reference devices with ITO, due to the higher sheet resistance (2 kΩ/sq) and the poor hydrophilicity of the spin coated graphene films.     

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.     

Characterization and photovoltaic device application of dip-coated and rapid thermal annealed CdS films

The structure, composition, electronic and optical properties of dip-coated CdS in the as-deposited condition and following rapid thermal annealing have been investigated. It has been shown that oxygen incorporated in the CdS film can be leached out following rapid thermal annealing. Strongly oriented CdS films with resistivity = 0.16 cm and free electron concentration = 2.65 × 10¹⁷ cm^–3 have been grown. Thin film heterojunction devices fabricated by non-aqueous electrodeposition of CdTe on a glass-ITO-CdS cathode have been shown to exhibit good rectification behaviour and photovoltaic activity.