Paper EN+PS-MoM4
Effective Light Trapping for Crystalline Silicon Solar Cells by Plasma Texturing

Monday, October 31, 2011, 9:20 am, Room 103

Currently, in the photovoltaic (PV) industry, wet chemistry based etching is used for saw damage removal and surface texturing. It is known that plasma based dry etching leads to an improved light trapping on multi-crystalline silicon material and, therefore, it has the potential to increase the solar cell efficiency. However, up to now plasma based texturing has not been implemented in the PV industry, because of the very low etch rate (<1 µm/min) and the high cost of ownership. In this study, different front surface textures obtained by means of the Linear Microwave Plasma (LMP, commercialized by Roth&Rau) technique and the high rate Expanding Thermal Plasma (ETP, commercialized by OTB-Solar) technique, are investigated to reduce the overall reflection losses of mono-crystalline silicon solar cells and compared to KOH/IPA (Potassium hydroxide/Isopropanol) which is the standard process in solar cell manufacturing industry. The created textures employing the different etching techniques are characterized by reflectometry (250-1200 nm) to determine the weighted reflection and by Atomic Force Microscopy (AFM) to measure the surface topography so as to determine statistical roughness parameters. We have found that the average scatter angle, determined from the AFM measurements, shows a clear correlation to the measured weighted reflection. Effective light trapping has been obtained for two typical textures based on the described etching techniques. A texture which leads to a successive hit of the incident light ray towards the solar cell surface, can result in effective light trapping, as for example, the KOH/IPA process which creates a pyramidal texture. Efficient light trapping can also be obtained by the creation of a diffuse front surface (resembles the topography of “black silicon”), as for example by using the LMP technique, under the conditions used, which creates micro roughness. This micro roughness can be described as an effective medium with a refractive index between air and silicon and a typical thickness of up to (60±10) nm [1]. The micro roughness leads to an effective light trapping of a broad range of wavelengths from 250 nm up to 1200 nm. The light trapping of the wafer etched by the ETP technique is smaller than the as cut wafer and can be explained by a smaller average scatter angle compared to the as cut wafer. A short post treatment of the ETP textured wafer by the LMP technique, creates a diffuse front surface and results, therefore, in an improved light trapping. Moreover this combination shows great promise for a cost-effective approach towards plasma based texturing.

[1] R.B. Stephens and G.D. Cody, Thin Solid Films 45 (1977) 19.