Ion beam treatment of functional layers in thin-film silicon solar cells

In silicon thin-film solar cells, transparent conductive layers have to fulfill the following requirements: high conductivity as effective contact, high transparency to transmit the light into the cell, and a textured surface which provides light scattering. Magnetron sputtered and wet-chemically textured aluminum doped zinc oxide (ZnO: Al) films are widely used as the transparent conductor. The technological goal of this dissertation is to develop an alternative to the wet etching process for light trapping in the thin silicon absorber layers through modification of the glass/ZnO: Al or ZnO: Al/Si interfaces by ion beam treatment. The study focuses on the textured growth of ZnO: Al films on ion beam pretreated glass substrates, and the preparation and application of textured glass for light trapping. The technological aspects such as the etch rates of the glass substrate and ZnO: Al films with different ion beam configurations were studied. The experimental etch rates are compared with simulated and theoretically predicted values. With regard to the ion beam treatment of glass substrate, the influence of the ion pretreated glass on the growth of ZnO: Al films was investigated. The ZnO: Al films grown on ion beam pretreated glass substrates exhibit self-textured morphology with surface roughness of 40 nm while remaining highly conductive. Silicon thin-film solar cells prepared on the as-grown rough ZnO: Al films show that this front contact can provide excellent light trapping effect. The highest initial efficiencies for amorphous single junction solar cells on as-grown rough ZnO: Al films was 9.4 %. The as-grown rough morphology was attributed to large conical ZnO: Al grains initiated from the ion pretreated glass surface. It was found that the roughness of the as-grown rough ZnO: Al film is proportional to the number of O dangling bonds created by ion beam treatment on the glass substrate. A growth model was proposed to explain the growth mechanism of ZnO: Al films on Zn- and O-polar ZnO single crystals, as well as on untreated and ion beam treated glass substrates. With regard to the ion beam treatment of ZnO: Al films, the influence of the ion treatment on the surface morphology, HCl etching, silicon growth, and additional ZnO: Al growth was investigated. Ion beam etching has a smoothening effect on the textured ZnO: Al films. Using sputtered and wet chemically etched ZnO: Al as ion beam etching mask, textured glass with features similar to the ZnO: Al films were obtained. Textured glass with a wide range of morphologies was prepared by varying the etching mask and the ion beam treatment conditions. Finally, as-grown textured ZnO: Al films prepared on ion beam treated textured glass, which exhibit ’double textured’ features, were produced and applied in solar cells. The ion beam treatment enabled the preparation of light scattering surfaces by textured glass and as-grown rough ZnO: Al films without the need of wet etching between TCO preparation and absorber deposition. Further, new ’double textured’ surface structures could be created by the combination of both techniques. Solar cells with efficiency of 11.9 % proved the applicability as-grown textured ZnO: Al for light trapping. The growth studies provided deeper insights and a new understanding of ZnO structure formation and will govern optimization of ZnO: Al film properties.