Ryosuke Watanabe

Evaluation of a New Acid Solution for Texturization of Multicrystalline Silicon Solar Cells

Surface texturing methods using an alkaline solution for monocrystalline Si (c-Si) solar cells have been widely accepted to improve cell performance. However, multicrystalline Si (mc-Si) cells are difficult to be texturized by alkaline etching, because the grains in the substrates are randomly oriented. In this study, we considered a HF/HNO3/H2SO4 acid solution for texturing the mc-Si cells. We evaluated the morphology of the textured surfaces and the reflectance spectra from the surfaces. The deep dimple textured structures are formed on the surfaces for only 30 seconds of the acid texturing process. This behavior results from the effect of H2SO4 in the solution. This process obtains up to 14.7% conversion efficiencies of the acid textured cells. These conversion efficiencies are up to 1.3 times larger than those of the mirror-etched cells.

Optical Properties of Spin-Coated TiO2 Antireflection Films on Textured Single-Crystalline Silicon Substrates

Antireflection coating (ARC) prepared by a wet process is beneficial for low cost fabrication of photovoltaic cells. In this study, we investigated optical properties and morphologies of spin-coated TiO2 ARCs on alkaline textured single-crystalline silicon wafers. Reflectance spectra of the spin-coated ARCs on alkaline textured silicon wafers exhibit no interferences and low reflectance values in the entire visible range. We modeled the structures of the spin-coated films for ray tracing numerical calculation and compared numerically calculated reflectance spectra with the experimental results. This is the first report to clarify the novel optical properties experimentally and theoretically. Optical properties of the spin-coated ARCs without interference are due to the fractional nonuniformity of the thickness of the spin-coated ARCs that cancels out the interference of the incident light.

Random texturing process for multicrystalline silicon solar cells using plasmaless dry etching

The authors have investigated a process for randomly texturing single- and multicrystalline Si solar cells by plasmaless dry etching with a chlorine trifluoride gas. Although the reflectance of as-textured surfaces was easily reduced to below 10% at a wavelength of 600 nm in our previous study, the increase of the efficiency of the randomly textured solar cells was insufficient. This insufficient improvement was considered to be due to submicron structures formed by the dry texturing. In this study, the authors aimed to enlarge the textured structures and improve the electrical characteristics of single- and multicrystalline solar cells by modifying the texturing conditions. Surfaces with reflectance below 12% at 600 nm (corresponding to an estimated weighted reflection of 12%–13% at wavelengths between 300 and 1200 nm) were obtained, and electrical characteristics of multicrystalline solar cells were improved by using plasmaless dry texturing at a relatively high etch rate for around 1 min.