Nanocrystalline n-Type Silicon Oxide Front Contacts for Silicon Heterojunction Solar Cells: Photocurrent Enhancement on Planar and Textured Substrates

Abstract

The conversion efficiency of silicon heterojunction solar cells is limited by current losses mainly in the front layer stack. In order to minimize these losses, we implemented n-doped nanocrystalline silicon oxide (nc-SiOx:H) as front surface field to enhance both transparency and conductivity, thus improving the fill factor. Layers with refractive indexes (n) in the range 2.1-2.7 and conductivity of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> S/cm were applied. Both optical simulations and experimental results suggest different optimizing approaches for short-circuit current (JSC) enhancement depending on the surface morphology of the silicon wafer. While planar wafers benefit from an improved antireflection effect using less transparent (n ~ 2.7) and thicker films, textured wafers require thinner highly transparent layers (n <; 2.7) with less parasitic absorption, since the reflectance losses are already low due to the texture. Finally, a thickness optimization of the (n)nc-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> :H leads to a conversion efficiency of 21.6%, a fill factor of 80.0%, an open-circuit voltage of 729 mV, and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC-EQE</sub> = 40.0 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .