Self-Organized Nanoscale Roughness Engineering for Broadband Light Trapping in Thin Film Solar Cells

Carlo Mennucci,Christian Martella,Paola Delli Veneri,Iurie Usatii,Lucia V Mercaldo,Francesco Buatier De Mongeot

doi:10.3390/app7040355

Abstract

We present a self-organized method based on defocused ion beam sputtering for nanostructuring glass substrates which feature antireflective and light trapping effects. By irradiating the substrate, capped with a thin gold (Au) film, a self-organized Au nanowire stencil mask is firstly created. The morphology of the mask is then transferred to the glass surface by further irradiating the substrate, finally producing high aspect ratio, uniaxial ripple-like nanostructures whose morphological parameters can be tailored by varying the ion fluence. The effect of a Ti adhesion layer, interposed between glass and Au with the role of inhibiting nanowire dewetting, has also been investigated in order to achieve an improved morphological tunability of the templates. Morphological and optical characterization have been carried out, revealing remarkable light trapping performance for the largest ion fluences. The photon harvesting capability of the nanostructured glass has been tested for different preparation conditions by fabricating thin film amorphous Si solar cells. The comparison of devices grown on textured and flat substrates reveals a relative increase of the short circuit current up to 25%. However, a detrimental impact on the electrical performance is observed with the rougher morphologies endowed with steep v-shaped grooves. We finally demonstrate that post-growth ion beam restructuring of the glass template represents a viable approach toward improved electrical performance.

Highlights

Thin film solar cell technologies, with the limited physical thickness of the light absorber, suffer from ineffective absorbance of near-bandgap light
In order to further enhance the vertical amplification of transferred nanostructures, we developed an upgraded version of the process where the metal stencil mask lifetime is prolonged by co-evaporation of metallic atoms during ion irradiation [18]
The assisted-ion projection projection lithography (A-IPL) nanostructuring technique was carried out in a UHV chamber equipped with a thermal evaporator and a defocused ion beam extracted from a gridded multi-aperture Ar+ source (Tectra instruments, Frankfurt, Germany)

Summary

Introduction

Thin film solar cell technologies, with the limited physical thickness of the light absorber, suffer from ineffective absorbance of near-bandgap light. Ion irradiation of a self-organized metal nanowire (NW) stencil mask allows the projection of its morphological features into the supporting substrates which can be either amorphous, like glass [15], or crystalline, as GaAs and Silicon [16,17] In the latter approach, high aspect ratio nanostructures are formed on the substrates as a consequence of the lower ion erosion rate of metal (e.g., gold) nanowires with respect to the substrate material. We demonstrate that we can separately optimize the pattern either for anti-reflective purpose or for high haze functionality by changing the lateral size of the nanostructures and their vertical scale in a controlled way This has been achieved either by introducing a titanium adhesion layer between Au and glass during A-IPL process, or by prolonging the sample exposure to Au atom evaporation during. A possible approach for improving the electrical performance of the solar cells, seriously affected by the more roughened morphologies, is reported

Materials and Methods

Self-Organized Nanopatterning

Atomic

Optical Properties of Nanostructured Glasses

Photon Harvesting in Thin Film Solar Cells

Conclusions