Thin Film Si Photovoltaic Devices on Photonic Structures Fabricated on Steel and Polymer Substrates.
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Thin Film Si Photovoltaic Devices on Photonic Structures Fabricated on Steel and Polymer Substrates. S. Pattnaik1,N. Chakravarty1, J. Bhattacharya1, R. Biswas1, D. Slafer2,V.L. Dalal1 Electrical and Comp. Engr., Iowa State University, Ames, Iowa; 2 Lightwave Power, Cambridge, Massachusetts. 1
ABSTRACT In this paper, we report on the growth and fabrication of thin film Si photovoltaic devices on photonic structures which were fabricated on steel and PEN and Kapton substrates. Both amorphous Si and thin film nanocrystalline Si devices were fabricated. The 2 dimensional photonic reflector structures were designed using a scattering matrix theory and consisted of appropriately designed holes/pillars which were imprinted into a polymer layer coated onto PEN, Kapton and stainless steel substrates. The photonic structures were coated with a thin layer of Ag and ZnO. Both single junction and tandem junction (amorphous/amorphous and amorphous/nanocrystalline) cells were fabricated on the photonic layers. It was observed that the greatest increase in short circuit current and efficiency in these cells due to the use of photonic reflectors was in nanocrystalline Si cells, where an increase in current approaching 30% ( compared to devices fabricated on flat substrates) was obtained for thin (~ 1 micrometer thick i layers) films of nano Si deposited on steel structures. The photonic structures (which were nanoimprinted into a polymer) were shown to stand up to temperatures as large as 300 C, thereby making such structures practical when a steel (or glass) of kapton substrate is used. Detailed measurements and discussion of quantum efficiency and device performance for various photonic back reflector structures on steel, kapton and PEN substrates will be presented in the paper. INTRODUCTION Thin film silicon solar cells are a very important part of the solar industry, by decreasing the amount of material used as well as using inferior so cheaper quality material they pursue the ability to decrease the cost of manufacturing [1]. But, using lower quality material does not allow making devices thick as they lead to poor device quality. On the other hand, absorption is a strong function of thickness of the absorber layer, which creates a dilemma. Many groups try to overcome this issue by using increasing the light path in the absorber by using light trapping methods by introducing randomly roughened back reflectors namely annealed silver or hot silver(Ag), etched Zinc oxide(ZnO) and silica spheres[2-5]. All these structures use a metallic reflector which is mostly silver and sometimes aluminum in conjunction with a layer separating the active device to prevent diffusion of metal into the active device. ZnO is the most general choice for this layer. Most of these textured reflectors lead to light path length enhancement close to ~10, whereas theoretical limit is close to 49, which is calculated for a perfect lambertian diffuser which should then give an enhancement of 4n2, where n is the refractive index of Silicon. This clearly indi
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