Properties of RF sputtered zinc oxide based thin films made from different targets

The effect of deposition parameters on optoelectronic and properties of ZnO based thin films prepared by RF magnetron sputtering have been studied. Different targets (pure Zn, ZnO, Zn---Al (98/2 at 2%), ZnO---Al (98/2 at%), and ZnO---Al₂O₃ (98/2 wt%)) have been investigated to compare resulting samples and establish the best target composition. From reactive sputtering, using a Zn---Al target, transparent conductive zinc oxide has been obtained at 380°C with E_g = 3.25–3.35 eV and = 4.8 × 10⁻⁴ ω cm. Reduction of substrate temperature at 200°C has been possible by nonreactive sputtering from ZnO---Al and ZnO---Al₂O₃ targets. The values of the energy gap and resistivity under these conditions are 3.30–3.35 eV and 1 × 10⁻³ ω can respectively.     

Preparation of Cu(In,Ga)Se2 thin films from Cu–Se/In–Ga–Se precursors for high-efficiency solar cells

Improved preparation process of a device quality Cu(In,Ga)Se₂ (CIGS) thin film was proposed for production of CIGS solar cells. In–Ga–Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu–Se/In–Ga–Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF₂/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V_oc=0.634 V, J_sc=36.4 mA/cm², FF=0.756).     

Flexible and light weight copper indium diselenide solar cells on polyimide substrates

Thin film flexible CuInSe₂ (CIS) solar cells have been fabricated for the first time on light-weight polymeric substrates. Evaporated Cu---In alloy precursors were selenized in H₂Se atmosphere at around 400°C to grow the CIS absorber layers. Low temperature techniques which are compatible with the polymeric substrates were used to deposit the window layers of CdS and ZnO. The demonstrated active area conversion efficiency of 9.3% makes this light-weight device very attractive for many terrestrial and space power generation applications where high specific power and mechanical flexibility are needed.     

The role of the ZnO buffer layer in Al/Si interdiffusion in α-Si : H solar cells on flexible substrates

Results of a study on the interdiffusion of Al and α-Si : H, occurring in cells grown on a KAPTON/Al substrate, are presented. Cell parameters of a series of cells grown at temperatures ranging from 100°C to 200°C are presented for KAPTON/Al and KAPTON/Al/ZnO substrates, where a thin ZnO buffer layer was inserted for the latter series of substrates. It is demonstrated that the implementation of this buffer layer results in a significant increase in cell efficiency. With SIMS and SXPS depth profiles, it is demonstrated that cells grown on substrates without the ZnO layer are destroyed as a result of the interdiffusion of α-Si : H and Al, and that the presence of the ZnO buffer layer impedes this process. It is argued that, apart from the well-known enhanced reflectance and resulting improved cell characteristics caused by the insertion of this buffer layer, the main effect of the buffer layer (in cells grown on aluminized flexible substrates) is to impede the interdiffusion of Al and α-Si : H at the semi-conductor/metal interface.     

CIS/CdS/ZnO/ZnO:Al modified photocathode for enhanced photoelectrochemical behavior under visible irradiation: Effects of pH and concentration of electrolyte solution

In this paper, the CuInS₂ films were firstly modified with CdS and CdS/ZnO/ZnO:Al/Au layers in order to improve the photoelectrochemical (PEC) water splitting efficiency. The CuInS₂ photoelectrode was synthesized by electrodeposition method as a facial and green method, on the FTO substrate. The effects of pH and concentration of Na₂S electrolyte solution on the photocurrent density of photoelectrode samples were studied. As a p-n junction photocathode, the CIS/CdS/ZnO/ZnO:Al/Au photoelectrode indicates the enhanced PEC activity. The photocurrent density of CIS/CdS/ZnO/ZnO:Al/Au photoelectrode reaches to 1.91 mA/cm², while is about 2.5 times higher than that for CuInS₂ film at pH = 8 (−0.6 V vs Ag/AgCl). The formation of a p-n junction at the CuInS₂ photoelectrode surface not only reduces the recombination of electron-hole pairs but also increases the PEC response and water splitting performance of the as-prepared CIS/CdS/ZnO/ZnO:Al/Au photoelectrode.     

Degradation studies of transparent conducting oxide: a substrate for microcrystalline silicon thin film solar cells

Aluminium doped ZnO films have been developed by RF-magnetron sputtering at 350 °C substrate temperature on glass substrate and commercially available SnO₂-coated glass substrate. The developed ZnO and SnO₂/ZnO films can be used as the substrates of microcrystalline silicon based solar cell. The electrical, optical properties and surface morphologies of ZnO film and SnO₂/ZnO bi-layer films have been investigated and they are compared with the commercially available SnO_2-coated glass substrate. The resistivities of ZnO and SnO₂ films are comparable (10⁻⁴ Ω-cm). Surface morphologies of different transparent conducting oxide coated substrates before and after H-plasma exposure were studied by scanning electron microscopy. The optical transmission of ZnO, SnO₂/ZnO and SnO₂ films are comparable and varies from 85 to 90% in the visible region. The optical transmission reduces drastically to less than 20% in SnO₂ films and for ZnO film it remains almost unchanged after H-plasma exposure. For SnO₂/ZnO film transmission decreases slightly but remains considerably high (80%). The performance of microcrystalline silicon solar cells fabricated on different transparent conducting oxides as substrates (ZnO/glass, SnO₂/glass and ZnO/SnO₂/glass double layer) is investigated in detail.     

Three-stage growth of Cu–In–Se polycrystalline thin films by chemical spray pyrolysis

Structural, optical and electrical properties of polycrystalline Cu–In–Se films, such as CuInSe₂ and ordered vacancy compounds (OVC), prepared by three-stage process of sequential chemical spray pyrolysis (CSP) of In–Se (first stage), Cu–Se (second stage) and In–Se (third stage) solutions have been studied in terms of substrate temperature at the second stage (T_S2). The films grown at T_S2420 °C exhibited larger grains in comparison with the Cu–In–Se films grown by the usual CSP method. Optical gap energy was approximately 1.06 eV for 360 °CT_S2420 °C, but increased dramatically from 1.06 to 1.35 eV when the T_S2 rose from 420 to 500 °C. Conductivity type was p-type for T_S2<420 °C, but n-type for T_S2>420 °C.     

Thin CdS films prepared by metalorganic chemical vapor deposition

Polycrystalline CdS thin films have been deposited on borosilicate glass substrates coated with ITO film by metalorganic chemical vapor deposition using dimethyl cadmium and diethyl sulfide as source materials. The growth of CdS film occurred at substrate temperatures within the range of 280–360°C. The deposition rate increased with increasing VI/II molar ratio at any substrate temperature and showed a maximum value at the VI/II molar ratio of 4. The grain size of as-deposited CdS film prepared at substrate temperatures from 300°C to 360°C was about 0.1 μm. The CdS films consist of hexagonal form with a preferential orientation of the (0 0 2) plane parallel to the substrate. Thin CdS film with high optical transmittance was prepared at 350°C with the VI/II molar ratio of 4. The CdS film deposited by MOCVD may be used as a window layer for CdS/CdTe solar cell.     

Study of CuInSe2 thin films prepared by electrodeposition

Thin films of p-type CuInSe₂ prepared by a one-step electrodeposition method have been studied by constructing CdS/CuInSe2 junctions. After the electrodeposition, the CuInSe₂ films were treated either in vacuum or in Ar. Cells of the form CdS (high σ)/CdS (low σ)/CuInSe₂ were then fabricated for studying the electrodeposited films. Measurements were specifically carried out to determine the diffusion length of minority carriers in the p-type CuInSe₂. It was found that the minority carrier diffusion length in CuInSe₂ films treated in Ar was generally greater than that for films treated in vacuum under similar conditions. A small area cell (active area 0.11 cm²) with a conversion efficiency of about 7% (under 125 mW/cm² illumination) has been fabricated.     

Preliminary photovoltaic cells with single crystal CIS substrates

Using the Bridgman method, ingots of CuInSe₂ have been grown, which are microcrackfree, void-free and adhesion-free. From these, p-type substrates have been obtained for the fabrication of preliminary CIS/CdS/ZnO and CIS/CdS/CdO photovoltaic cells, where the window layers were deposited, respectively, by rf sputtering from a ZnO target and by dc reactive sputtering from a Cd target and where the CdS buffer layer was deposited by a chemical bath method. These cells have yielded approximate illuminated jj_sc, V_oc,η and FF values, respectively, up to 28 mA/cm², 0.42 V, 5% and 0.41 for effective areas of 7 to 22 mm².     

Highly textured ZnO thin films doped with indium prepared by the pyrosol method

Indium doped ZnO thin films have been prepared on heated Corning 7059 glass by the pyrosol spray method. It was found that indium doping has an important role in grain growth at high substrate temperature. Indium also was used to improve the electrical properties, acting as an N type dopant, and we obtained highly conductive ZnO:In thin films with a resistivity of 3.0 × 10⁻³ Ω cm. At substrate temperatures from 425°C to 475°C, the deposited ZnO:In thin films have clear hexagonal crystallites and, therefore, a highly textured surface showing optical haze phenomena due to the crystallites. The haze ratio of ZnO:ln thin films can be controlled from 10% to 50% at the wavelength of 550 nm by varying the substrate temperature from 375°C to 475°C.     

Structure, morphology and photoelectrochemical activity of CuInSe2 thin films as determined by the characteristics of evaporated metallic precursors

CuInSe₂ thin films have been obtained by the sequential evaporation of Cu and In layers, and subsequent reaction at 400°C with elemental selenium vapor. The individual metallic film thickness and the substrate temperature during evaporation have been varied in order to promote intermixing and alloy formation before the selenization. The structure, morphology and photoelectrochemical activity of the CuInSe₂ films have been determined by the characteristics of the evaporated metallic precursors. An improvement in the CuInSe₂ quantum efficiency, related mainly to the increased homogeneity and smoothing of the sample surface, can be gained by using as precursors multiple stacked Cu–In bilayers evaporated onto unheated substrates.     

An investigation into effect of cationic precursor solutions on formation of CuInSe2 thin films by SILAR method

SILAR deposition of CuInSe₂ films was performed by using Cu²⁺–TEAH₃ (cupric chloride and triethanolamine) and In³⁺–CitNa (indium chloride and sodium citrate) chelating solutions with weak basic pH as well as Na₂SeSO₃ solution at 70 °C. A separate mode and a mixed one of cationic precursor solutions were adopted to investigate effects of the immersion programs on crystallization, composition and morphology of the deposited CuInSe₂ films. Chelating chemistry in two solution modes was deducted based on IR measurement. The XRD, XPS and SEM results showed that well-crystallized, smoothly and distinctly particular CuInSe₂ films could be obtained after annealing in Ar at 400 °C for 1 h by using the mixed cationic solution mode.     

Impact of Zn1-xMgxO:Al transparent electrode for buffer-less Cu(In,Ga)Se2 solar cells

Buffer layers such as CdS and ZnS are used in high efficiency Cu(In,Ga)Se₂ (CIGS) thin film solar cells. Eliminating buffer layer is attractive to realize low-cost thin film solar cells by reducing fabrication process. However, the elimination of the buffer layers leads to shunting due to the interface recombination between transparent conductive oxide (TCO) and CIGS layers. To reduce the interface recombination, the control of conduction band offset (CBO) is effective. In this study, we fabricated Zn_1−xMg_xO:Al (ZMO:Al) as the TCO for the CBO control. ZMO:Al was prepared by co-sputtering of ZnO:Al₂O₃ (ZnO:Al) and MgO:Al₂O₃ targets. ZMO:Al shows high transmittance in visible region and the band gap energy widen with the addition of Mg to ZnO:Al. Buffer-less CIGS solar cells with an Al/NiCr/TCO/CIGS/Mo/soda-lime glass structure using ZMO:Al and ZnO:Al were fabricated. For comparison, ZnO/CdS buffered cell was also fabricated. Current density-voltage characteristics of the devices showed the cell with ZMO:Al film achieved higher efficiency compared to the buffer-less cell with ZnO:Al. This result suggested that the control of CBO is important to reduce interface recombination between TCO layer and CIGS absorber.     

Co-deposition of TiO2/CdS films electrode for photo-electrochemical cells

A densely packed TiO₂ thin film onto an indium doped–tin oxide (ITO) substrate was synthesized at room temperature by chemical deposition and a CdS thin film was deposited onto the pre-deposited TiO₂ film by a doctor blade route (powder of CdS was obtained from chemical deposition). TiO₂/CdS film was annealed at 300 °C for 1 h in air for crystallinity improvement. The first grown TiO₂ film was nanocrystalline, whereas the CdS film was polycrystalline as evidenced by X-ray diffraction (XRD) and selected area electron diffraction (SAED). Scanning electron microscopy (SEM) images show formation of mono-dispersed CdS spherical grains onto compact, densely packed spherical nanocrystalline grains of TiO₂. The TiO₂/CdS bilayer film was used in a photo-electrochemical cell as a working electrode, and a platinum electrode as a counter electrode (0.1 M lithium iodide electrolyte) under 80 mW/cm² light illumination intensity.     

Study of phases formed during production of copper indium diselenide by reacting copper, indium and selenium layers

CuInSe₂ films were grown by reacting stacked layers of Cu, In and Se in an atmosphere of Se vapor. Incremental growth of the various phases was followed at different temperatures until a single phase CuInSe₂ film was formed. Conventional X-ray diffraction was used in identifying the different phases formed. Along with the knowledge of different phases formed at increasing reaction temperatures, it was concluded that CuInSe₂ is formed at temperatures as low as 235°C, although a single phase film is obtained only at higher temperatures (≈350°C).     

Highly transparent and conductive Zn0.85Mg0.15O:Al thin films prepared by pulsed laser deposition

Zn_1−xMg_xO:Al thin films have been prepared on glass substrates by pulsed laser deposition (PLD). The effect of substrate temperature has been investigated from room temperature to 500 °C by analyzing the structural, optical and electrical properties. The best sample deposited at 250 °C shows the lowest room-temperature resistivity of 5.16×10⁻⁴ Ω cm, and optical transmittance higher than 80% in the visible region. It is observed that the optical band gap decreases from 3.92 to 3.68 eV when the substrate temperature increases from 100 to 500 °C. The probable mechanism is discussed.     

Polymer impinged CdS/CuInSe2 solar cell

In the present paper we report, effect of conjugated polymer (polyaniline) impinging in nanostructured CdS/CuInSe₂ heterojunction thin film solar cell. The heterojunction architecture for the solar cell is achieved by sandwiching the conjugated conducting polymer in n and p type of wide band gap semiconducting material by multilayer chemical deposition methods onto the ITO coated glass substrate at room temperature. The obtained multilayer thin film heterojunction of ITO/CdS/Polymer/CuInSe₂/Ag has been characterized for structural, compositional, optical and solar cell characteristics by illuminating it to 100 mW/cm² intensity light source. The X-ray diffraction pattern (XRD) confirms formation of CdS/CuInSe₂ phase while on polymer impinging the crystallite size observed to be increased from 13 to 19 nm. The compositional analysis by energy dispersive X-ray spectra (EDAX) supports presence of expected elements in the heterojunction. The energy band gap calculated from absorbance spectra shows significant shift in its value from polymer and CdS/CuInSe₂ band gap. I–V analysis shows increase in conversion efficiency from 0.26 in CdS/CuInSe₂ to 0.55% in CdS/Polymer/CuInSe₂ heterojunction upon illumination.     

High efficiency CuInSe2 based heterojunction solar cells: Fabrication and results

A process for fabricating CuInSe₂/CdS solar cells is described. A data set of 202 substrates each containing 12 cells from 129 separate deposition runs is used to examine the processing parameters for fabricating high efficiency cells. The data show a broad range of CuInSe₂ compositions over which high efficiency cells can be made. The use of a thin CdS layer, less than 2 μm in thickness, can increase the short-circuit current by more than 3 mA cm⁻². High efficiency devices have also been made on low cost sodalime glass. Air heat treatments at 200 °C for between 8 and 60 h are required to optimize the output of the CuInSe₂/CdS cells.     

Fabrication and electrochemical performance of thin-film solid oxide fuel cells with large area nanostructured membranes

Thin-film solid oxide fuel cells (SOFCs) with large (5-mm square) membranes and ultra-thin La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF) cathodes have been fabricated and their electrochemical performance was measured up to 500 °C. A grid of plated nickel on the cathode with 5–10 μm linewidth and 25–50 μm pitch successfully supported a roughly 200-nm-thick LSCF/yttria-stabilized zirconia/platinum membrane while covering less than 20% of the membrane area. This geometry yielded a maximum performance of 1 mW cm⁻² and 200 mV open-circuit voltage at 500 °C. Another approach toward realizing large area fuel cell junctions consists of depositing the membrane on a smooth substrate, covering it with a high-porosity material formed in situ, then removing the substrate. We have used a composite of silica aerogel and carbon fiber as the support, and show that this material can be created in flow channels etched into the underside of a silicon chip bonded to the top of the SOFC membrane. We anticipate these integrated fuel cell devices and structures to be of relevance to advancing low-temperature SOFCs for portable applications.