Thin film tandem solar cells based on amorphous and microcrystalline silicon (a-Si:H/μc-Si:H) are employed as the cathode in a photoelectrochemical converter for solar water splitting. It is setup in such a way that the silver back contact of the cell is directly connected to the electrolyte and the light enters the cell through the glass substrate. This arrangement offers a number of distinct advantages compared to the conventional designs. The cathode is further optimized by the deposition of platinum nanoparticles to achieve higher conversion efficiencies. The front contact of the photovoltaic cell is connected to a standard platinum counter electrode in a three-electrode arrangement. Photon to current conversion efficiencies can reach up to 3% for our design, which has not been optimized to the requirements of the water splitting reaction, yet. The optimization of such tandem devices made from abundant silicon in combination with nanoparticle catalysts offers an affordable pathway for direct solar-to-fuel conversion devices in form of an artificial inorganic leaf.

Sputter-deposited ZnO:Al thin films, used as front contact in thin-film Si solar cells, were etched in diluted HCl containing catalytic amounts of Fe(III) salts. The Fe(III) effectively catalyzes the HCl-based etch process, leading to a crater-like morphology that is qualitatively similar to the one generated by the uncatalyzed etching process at the same HCl concentrations. Utilizing this catalyzed process, an increase of the etching rate by a factor of approximately 1.8–2.6 was observed. This allows for a well-controllable tuning of the etch duration without changing HCl concentration or temperature. The process has been evaluated with a selection of Fe(III) salts at different concentrations of the acid and of the catalyst. Optical, electrical and scanning force microscopic characterization of such catalytically etched ZnO:Al films has shown that the catalytical process leads to slightly smaller morphological features on the film compared to the uncatalyzed etching, accompanied by a shift of light scattering intensity to higher angles. This etching behavior may be used beneficially for light trapping in thin-film Si solar cells. In addition to these application-oriented aspects, this approach provides a deeper insight into the mechanistical details of the ZnO thin-film dissolution.

The electrochemical anodic dissolution of sputter-deposited, aluminum-doped zinc oxide (ZnO:Al) was investigated. The dissolution proceeds mainly via a local oxygen evolution reaction and subsequent dissolution of the zinc oxide lattice. The present work focuses on the influence of the anion species in the used electrolyte. Several electrolytes (KCl, K2SO4, KNO3, KClO4) were applied. The anion species turned out to be a major factor that influenced the kinetics of the etching process. In K2SO4 solutions a strong limitation of the etching process to the close vicinity of the grain boundaries was observed. However, anodic dissolution in KCl, KNO3, and KClO4 showed an unexpected etching behavior, which lead to the formation of cavern-like structures within the ZnO:Al thin films. This was attributed to different local pH values at the ZnO:Al/electrolyte interface. (C) 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

The processes during chemical and electrochemical etching of Al-doped ZnO are investigated utilizing a scanning flow cell setup with online detection of dissolved Zn ions. The rate of chemical dissolution was found to be a linear function of buffer and proton concentration in near neutral pH solutions according to a transport limited reaction. In contrast, electrochemical etching is limited by the kinetics of the reaction and increases linearly with the imposed current density. Due to this fundamental difference, the dissolution of Zn can be either uniform over the whole surface or highly localized at active sites like grain boundaries. A combined approach of chemical etching and the well-controllable galvanostatic dissolution thus allows a fine adjustment of the ZnO:Al surface texture for applications in silicon thin film photovoltaic cells in order to improve their overall energy conversion efficiency.

An etching procedure was applied to microcrystalline silicon (μc-Si:H) thin films in order to obtain a wedge-shaped profile for depth-resolved characterization. A microfluidic flow cell that merges deionized water with a potassium hydroxide solution (KOHaq) was utilized. The samples consisted of texture-etched ZnO:Al on a Corning Glass substrate, a microcrystalline p-doped layer serving as seed layer and the investigated intrinsic microcrystalline or amorphous silicon (a-Si:H). Along the etched profiles, microscopic Raman spectroscopy was used to estimate the crystalline volume fraction Xc for samples deposited with intentionally varied silane concentration to investigate the a-Si:H/μc-Si:H and μc-Si:H/a-Si:H transition.

Chemical etching is widely applied to texture the surface of sputter-deposited zinc oxide for light scattering in thin-film silicon solar cells. Based on experimental findings from the literature and our own results we propose a model that explains the etching behavior of ZnO depending on the structural material properties and etching agent. All grain boundaries are prone to be etched to a certain threshold, that is defined by the deposition conditions and etching solution. Additionally, several approaches to modify the etching behavior through special preparation and etching steps are provided.

An etching method for ZnO:Al films deposited by radio-frequency sputtering is presented. The method is developed to achieve appropriate surface morphology for efficient light scattering. This etching method consists of a first step where the sample is dipped in standard diluted HCl (0.5wt.%) for 40s (the “standard Jülich” etch process) and a subsequent step where a NH4Cl aqueous solution with concentrations ranging from 2 to 20wt.% is used. The introduction of the second step leads to a slight modification of the surface feature shape and an increase in the surface roughness of up to around 37% in relation with that obtained using only the first step. High haze values are also obtained, reaching up to 93% at 550nm and strong light scattering into angles above 50° at 632nm. On the other hand, the resistivity of the textured films remains low enough for cell application, being ranged from 6 to 13Ω/sqr depending on the NH4Cl concentration used. Finally, in order to assess the role of the features obtained on the surface as effective light trapping, the textured films are applied as front contact in silicon thin film solar cells.

Separating the dependence of ZnO:Al thin-film texturization from deposition conditions is an important aspect for optimization of ZnO as light scattering, transparent contact in solar cells. The crater density and shape for a given polycrystalline ZnO: Al thin-film are shown to be controllable by altering the temperature and concentration of various acids. The crater densities generally increase with increasing temperature, decreasing concentration, or by etching in a weak acid with small molecular size. The etching trends observed for polycrystalline ZnO: Al films are confirmed on ZnO single crystals. We discuss in detail the etch process in terms of etch rates and crater formation statistics. Results are explained in accordance with a recently proposed ZnO etch model, and possible physical explanations are given. (C) 2012 The Electrochemical Society. All rights reserved.

Magnetron sputtered aluminum-doped zinc oxide (ZnO:Al) is used as a window layer in silicon-based thin-film solar cells clue to its high transparency, high conductivity, and ability to provide effective light trapping after etching in a dilute Ha solution. A challenge with this method is the strong influence of sputtering conditions on the density and shape of the ZnO:Al etch features. Here we present a novel chemical etch process based on HF that enables the modification of surface features through the etch process itself, thus allowing the separate optimization of ZnO:Al deposition and texturization. The different etch characteristics of HCl and HF are studied on single crystal ZnO. Using the different etch characteristics of HF we effectively texturize polycrystalline ZnO:Al films which previously exhibited only poor light trapping in silicon thin-film solar cells. The light trapping improvement is seen by an 18% increase in short circuit current density when used in microcrystalline silicon solar cells. Additionally, using mixtures of HCl and HF we are able to tune the feature size and shape on a given ZnO:Al sample.[GRAPHICS]. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A novel approach is presented for introducing a surface morphology with beneficial light scattering properties to sputterdeposited ZnO:Al films, being used as front contact in Si thin film photovoltaic devices. Electrochemical anodization was used to trigger local corrosion, leading to interfacial structures complementary to those commonly prepared by an etching step in diluted HCl. By systematic variation of electrochemical etching conditions and electrolytes, sensible experimental parameters were evaluated for the preparation of ZnO films that can be applied in Si thin film solar cells. The prepared films were characterized by scanning electron microscopy, four-point resistance and Hall measurements. Furthermore, the kinetics of the heterogeneous interfacial reaction during the corrosion process were studied utilizing electroanalytical techniques. This allowed the identification of the processes occurring at the solid/liquid interface. Application of such films in microcrystalline Si single junction solar cells has shown promising initial results.

Aluminium doped zinc oxide (ZnO:Al) films prepared by magnetron sputter deposition and surface textured by etching are widely applied in silicon thin film solar cells. Based on 15 years of experience in ZnO texturization for solar cell application in our institute we propose an etch model, that describes the interaction between ZnO preparation and etching in different processes. All grain boundaries have a certain potential to etch. The threshold of which is set by the acid.

Atomic force microscopy (AFM), conductive atomic force microscopy (CAFM) in air, and scanning electrochemcial microscopy (SECM) in 2 M H2SO4 have been used to investigate model composite electrodes obtained by pressing sieved MnO2 particles into a Pb matrix. These model electrodes shall resemble new composite electrodes produced by coldspraying and currently being tested for Zn electrowinning. CAFM showed a very uneven distribution of the current path through the matrix electrode with the highest currents measured at the MnO2/Pb domain boundary. SECM images in the substrate-generation/tip-collection mode in vertical and horizontal planes could show spatial concentration distribution of H+, O2 and H2O2 that could be evaluated qualitatively despite interfering turbulent convection due to raising gas bubbles. There is a concentration overvoltage due to deviations of the H+ and O2 concentration close to the surface from the bulk value. It amounts to about 40–50 mV for both compounds. H2O2 is formed as an intermediate and is consumed at the MnO2 catalyst particles.