**1. Introduction**

38 Solar Cells – Thin-Film Technologies

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The flexible solar cells fabricated on a stainless steel substrate are being widely used for the building of integrated photovoltaics (BIPVs) in recent years. Because stainless steel has many advantages, such as low cost, high extension, ease of preparing etc. It was believed that the wide application of BIPVs especially rooftop applications, would be the biggest market for flexible PV technology (Kang et al. 2006, Otte et al. 2006, Chau et al. 2010, Fung et al. 2008). Until now, one of the main challenges of the BIPVs remains how to improve the conversion efficiency. Since, the path length of the photovoltaic effect is considerable shorter in a thin film solar cell resulting in reduced efficiency. Many researchers have focused on light trapping, and have adopted a different TCO technology, such as LP-CVD, PVD, to increase the path length of the incoming light, and improve the photovoltaic conversion efficiency of thin film solar cells (Selvan et al. 2006, Llopis et al. 2005, Söderström et al. 2008, Müller et al. 2004). Moreover, light trapping provides some significant advantages including, reduction of the cell thickness, reduced processing time and reduced cost, improved cell efficiency and the improved stability of amorphous Si (a-Si:H).

The idea of trapping light inside a semiconductor by total internal reflection was reported by John in 1965 (John 1965). It also indicated that the effective absorption of a textured semiconductor film could be enhanced by as much as a factor of 60 over a plane-parallel film (Yablonovitch and Cody 1982). It should be mentioned that a major limitation to thin film solar cell efficiency is the long absorption length of the long wavelength photons and the low thickness of the absorber layer. The absorption length of amorphous silicon (a-Si:H) with a bandgap of 1.6 eV, for red and infrared solar photons, exceeds 1 μm and 100 μm, respectively (Ferlanto et al. 2002, Zhou and Biswas 2008). However, for a-Si:H the hole diffusion length is ~300-400 nm, which limits the solar cell absorber layer thickness to less than the hole diffusion length (Curtin et al., 2009). This makes it exceedingly difficult to harvest these photons since the absorber thickness of a p-i-n single junction solar cell is limited to only a few hundred nanometers for efficient carrier collection. In addition, the low-cost approach of thin-film silicon solar cells is very sensitive to film thickness, since the throughput increases with the decrease in layer thickness. Thus, sophisticated light trapping is an essential requirement for the design of thin-film solar cells (Rech et al., 2002).

Enhanced light-trapping in thin film solar cells is typically achieved by a textured metal backreflector that scatters light within the absorbing layer and increases the optical path length of the solar photons. In our recent researches [Lee et al., 2009], various processing

Enhanced Diffuse Reflection of Light by

(a) (b)

(a) (b)

**2.2 Sand blasting process** 

surface with EP process.

surface with sand blasting process.

the TR and DR rates of the 430BA SS substrate.

**2.3 Photolithography process** 

Using a Periodically Textured Stainless Steel Substrate 41

The glass sand (#320) was used to form randomly textured surface with cave size of several μm to tens μm on the surface of stainless steel substrate. The average surface roughness (Ra) of 304 SS substrate increased from 0.277 μm to 6.535 μm after the sand blasting process. The OM images of raw 304 SS substrate and with sand blasting process were shown in Fig. 3.

Fig. 2. The OM images (x2000) of (a) raw 304 SS substrate surface and (b) 304 SS substrate

 Fig. 3. The OM images (x400) of (a) raw 304 SS substrate surface and (b) 304 SS substrate

The photo-mask patterns were designed by CAD. Photolithography is a process of using light to transfer a geometric pattern from a photo-mask to a photo-resist on a 430BA SS substrate. The steps involved in the photolithographic process are metal cleaning, barrier layer formation, photo-resist application, soft baking, mask alignment, exposure and development, and hard-baking. After the photolithographic process, the 430BA SS substrate is etched by aqua regia (HNO3 : HCl=1 : 3). There are two types of photo-mask patterns: one, different diameters but with the same interval, and two, the same diameters but with a different interval. They are both designed to study light trapping for the application of thin film solar cells. Finally, silver coating technique by e-beam evaporation was used to improve

techniques including, electro-polishing, sandblasting, photolithography, lift-off and wetchemical etching were used to create periodically textured structures on the different types of stainless steel substrates. The relationships between the surface morphology of textured stainless steel substrate and optical properties will be carefully discussed.
