Electrochemical Texturing and Deposition of Transparent Conductive Oxide Layers for the Application in Silicon Thin-Film Solar Cells

摘要

Doped zinc oxide layers are widely used in thin-film solar cells for several purposes,for instance as transparent contacts, as a source of light scattering and as part of theback reflector. Magnetron sputtered, aluminum-doped zinc oxide thin films providevery high transparency and conductivity, and are usually flat in the as-depositedstate. To introduce light scattering, a surface texture is conventionally introduced bypost-deposition etching in diluted hydrochloric acid. However, the ability to obtainsuitable surface morphologies by chemical dissolution is strongly dependent on thedeposition process. Thus, optimization of zinc oxide thin films requires a carefultrade-off between optical, electric, and morphological properties. This markedly limitsthe process window and excludes layers with excellent optical and electric propertiesdue to a lack of suitable texturing processes. Electrochemical methods can help toovercome these limitations by making novel zinc oxide structures accessible. Bothdeposition and dissolution can be achieved using electrochemical methods. In thiscontext, an advanced understanding of the stability of polycrystalline zinc oxide thinfilms in aqueous solutions is crucial. This work investigates the zinc oxide/electrolyteinterface under various conditions in order to further the understanding of theinterfacial reactions and the zinc oxide itself.Cathodic electrochemical deposition was used for the growth of zinc oxide filmsand nano-structures from aqueous solutions. This method utilizes specific manipulationof the interfacial pH at the substrate surface by reduction of a suitable precursorsuch as nitrate or molecular oxygen. The dependence of the zinc oxide precipitationand crystallization on several parameters such as the deposition potential, the bathtemperature, the substrate, and the composition of the electrolyte were investigated.Temperatures above 50 ◦C were found to be necessary for the crystallization of welldefined hexagonal crystals. The comparison of electrochemical deposition on indiumtin oxide and zinc oxide substrates revealed the fundamental influence of the substrateon the nucleation. While the growth on zinc oxide seed layers seemed to proceedepitaxially, conserving the preferential c-axis orientation and crystallite size of thesubstrate, the nucleation on indium tin oxide substrates depended largely on theapplied potential. With increasing cathodic potential the density of nucleation sitesincreased. The crystallite size decreased simultaneously. Due to a catalytic effectof Zn2+ ions in solution on the reduction of NO3–, the Zn2+ concentration plays adecisive role on the growth process and must be carefully adjusted. A Burstein-Mossshift was observed for electrochemically deposited zinc oxide suggesting degeneratedoping by intrinsic defects, hydrogen, or chloride incorporated in the film.The main determinant of the stability of zinc oxide in aqueous solutions is thepH. The chemical dissolution of polycrystalline zinc oxide thin films was investigatedfrequently. However, the dissolution rate and the film thickness were, so far, accessibleonly after the etching process. This work applies an electrochemical method whichallows for the in-situ measurement of the dissolution rate. Therefore an online processcontrol becomes feasible allowing to stop the process when a desired film thickness isdissolved. The evolution of the dissolution rate was monitored in several bufferedand unbuffered acidic solutions. The measured values were in good agreement withvalues found in literature.Electrochemical anodic dissolution of polycrystalline zinc oxide thin films wasfound to provide access to a novel type of surface textures. A unique surfacemorphology was obtained due to a pronounced selectivity of the anodic dissolutionprocess to the close vicinity of the grain boundaries. The obtained surface featureswere too small to provide efficient light scattering, however, a decreased absorptancein the near infrared spectral range was observed. The selectivity of the etching processto the grain boundaries could be varied by the application of different electrolytes.Sulfate solutions showed the highest selectivity, which was ascribed to a bufferingeffect on the interfacial pH. Enhanced dissolution under illumination suggestedthat the dissolution kinetics were limited by the abundance of holes at the zincoxide/electrolyte interface. The determination of the flat band potential by means ofMott-Schottky analysis provided access to the band alignment at the interface andrevealed band to band tunneling as the dominant charge transfer process. A fieldenhancement at the grain boundaries was assumed to be the reason for the preferreddissolution at these sites.To benefit from the increased transmittance of the electrochemically obtainedtexture and the light scattering provided by chemically introduced surface textures,both processes were combined in a two-step approach. Depending on the sequenceeither a superposition of both surface textures was obtained or a gradual adjustmentof the feature size was feasible. The application of both types of two-step texturedzinc oxide thin films as front contacts in microcrystalline silicon solar cells has proventheir usability for thin-film photovoltaic devices.