• Schmidt, Michael Stenbæk
• Boisen, Anja
• Hansen, Ole
• Schmidt Davidsen, Rasmus

Abstract

We use plasma etched Black Si (BS)[1][2] nanostructures to achieve low reflectance due to the resulting graded refractive index at the Si-air interface. The goal of this investigation is to develop a suitable texturing method for Si solar cells. Branz et al. [3]report below 3% average reflectance for their 16.8% efficient black Si cell using a metal-assisted, chemical etching method on FZ mono-crystalline Si substrates. Yoo et al. [4] use RIE similar to this work on large-area, multi-crystalline Si cells and achieve a 16.1% efficiency despite a relatively high reflectance of 13.3%. Despite several advantages such as; (i) excellent light trapping, (ii) dry, single-sided and scalable process method and (iii) etch independence on crystallinity of Si, RIE-texturing has so far not been proven superior to standard wet texturing, primarily as a result of lower power conversion efficiency due to increased surface recombination. This work shows promising potential of future improvements in power conversion efficiency, since excellent light absorption has been shown for large-area, industry grade CZ Si wafers with several identified areas of improvement. We show that the RIE nanostructures lead to superior light absorption independent of crystalline grade and incident angle. A texturing method which is applicable to all industrially relevant grades of Si and which yields improved performance at non-ideal incident angles has a major scientific and commercial relevance. The nanostructures were fabricated using maskless RIE in a O2 and SF6 plasma, and the surface topology was optimized for solar cell applications by varying gas flows, pressure, power and process time. The starting substrates were 156x156 mm p-type, CZ mono-, multi- and quasi-mono crystalline Si wafers, respectively, with a thickness of 200 μm. Reflectance measurements of the RIE-textured mono-, multi and quasi-mono Si surfaces were performed using a broadband lightsource (Mikropack DH-2000), an integrating sphere (Mikropack ISP-30-6-R), and a spectrometer (Ocean Optics QE65000, 280-1000 nm). The reference solar spectral irradiance for AM 1.5 was used to calculate the weighted average reflectance in the wavelength range from 280-1000 nm. Our mask-less, scalable RIE nanostructuring of the Si surface is shown to reduce the AM1.5-weighted average reflectance to a level below 1 % in a fully optimized RIE texturing, and thus holds a significant potential for improvement of solar cell performance compared to current industrial standards. The reflectance is shown to remain below that of conventional textured cells also at high angle of incidence. The process is shown to be equally applicable to mono-, multi- and quasi-mono-crystalline Si.<br/>1

Topics

• impedance spectroscopy
• surface
• laser emission spectroscopy
• Silicon
• additive manufacturing
• crystallinity
• power conversion efficiency
• plasma etching