1. Introduction

As solar energy is a kind of infinite and clean energy, it has been the focus of research on solar energy by reducing the reflectance to improve the photoelectric conversion efficiency. Silicon solar cell is a kind of widely used solar cell [1], and the surface texturization of silicon cells is one of major way to improve photoelectric conversion efficiency. Dominant preparation methods of light trapping structure in silicon solar cells are acid etching techniques [2], alkaline corrosion [3], mechanical grooving [4], laser grooving [5–7], reactive ion etching RIE [8–10],

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

mask etching [11], mask growing [12], electro-chemical corrosion [13], electrical discharge machining [14]. These methods have positive influence on improving photoelectric conversion efficiency.

In this chapter, numerical analysis method is developed to simulate the reflection path of rays in the different groove light trapping microstructures, and reflectance of ray at each reflection point is determined according to Fresnel equations. Then the weighted reflectance of each structure with different incident angle of light and structural parameters was calculated and precise results were obtained. The reflection law of rectangular groove structure is strictly deduced by formula, and the best angle and optimized parameters of rectangular groove

Effects of the Novel Micro-Structure on the Reflectance of Photovoltaic Silicon Solar Cell

http://dx.doi.org/10.5772/intechopen.74972

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The size-independent behavior of the reflectance was clearly explained by a simple mathematical model in the perspective of geometric-optical. A profile was shown schematically in Figure 2. The orange (second reflection) and green (third reflection) areas play the role of light trapping efficiency. When the model size changes, if the ratio of the orange area to the green area does not change, the reflectance will not change. This principle also applied to the threedimensional pyramid structure. Therefore, the height of the structure was defined as a unit length. The reflectance of the whole wafer was not affected by the change of the parameters in

The schematics of cross-sectional views for ideal model with no fillet on the included angle and the model with a fillet on the included angle were depicted in Figure 3(a) and (b), respectively. If there exists a fillet on the included angle between neighboring pyramidal structures fabricated by mechanical way, the second model was proposed. The height of structure was set to a unit length in the algorithm. The size and angle of the structure was shown in Figure 3.

For convenience of calculation, a simplified two-dimensional model of arc-shaped groove structure formed under ideal mechanical condition (without uncut part, and will be mentioned below) is depicted in Figure 4. As Figure 5 shows, θ<sup>p</sup> is the primary incidence angle of light projects on arc-shaped groove trap of cell, θi1, θi2, and θi3 are the incidence angle of rays at

2. Textured surface models and calculated methods

The normal pyramidal textured surface morphology depicted in Figure 1.

structure were proposed.

2.1. Textured surface models

the subsequent stage.

Figure 1. Normal pyramidal textured surface morphology.

At present, wet etching is the most mature method to fabricate the light trapping structure on the surface of silicon solar cells. Because of the isotropy of polycrystalline silicon, the morphology of porous silicon can be etched on the surface of polycrystalline silicon solar cells by acid etching. Compared with pyramidal textured surface, the light trapping efficiency of porous silicon is lower, and the optical reflectance is higher. As for monocrystalline silicon solar cells, pyramidal textured surface can be formed due to the anisotropy of material by alkaline etching. However, the limitation of crystal structure makes the largest base angle of the pyramid is 54.74, some experimental studies believe that the angle is closer to 50–52. The reflectance shown dependence on the base angle, with larger base angle size resulting in reduced reflectance. The lowest reflectance of the pyramidal textured surface formed by alkaline etching is about 10.5%.

Etching with alkaline solution such as NaOH, KOH, the texture obtained is a generally welldistributed pyramidal texture with solid top angle of 70 due to the different corrosion rate in different crystal orientation [13]. Reflectance of light when propagating from one media to another media with different refractive indices is determined by incidence, wavelength and polarization of light. Some researchers studied the effects of pyramidal structure with different top angles fabricated by mechanical way on weighted reflectance. The reflectance can reach 6% when the top angle is 60 and the reflectance is less than 10% when the incident angle of light is less than 30. When the incident angle of light is larger than top angle, the structure tends to be a primary reflection area [15], in the light of Fresnel equations, the results could be considerably not precise. Hua et al. [16] proposed four different structures including triangular pyramid, rectangular pyramid, hexangular pyramid and cone structure. Ray-Tracing technology was employed to investigate the influence of many factors like geometrical shape, density and the top angle of the unit structure on sunlight absorption. In addition, mechanical tools develop in a trend of miniaturization and facilitation presently, Dehong et al. [17], Choong et al. [18], Rusnaldy et al. [19] have investigated and proved that single-crystalline silicon can be removed by micro machining in the ductile mode under the condition of appropriate parameters of mechanical grooving. Because of isotropy of polycrystalline silicon surface, pyramidal structure with large ratio of depth to diameter cannot be fabricated by alkaline corrosion, which appear to be a limit of this dominant fabrication method. Recently, the metal assisted chemical etching method [20–22] has been utilized to prepare SiNW(Silicon Nanowire) or nanopore structure in silicon crystalline surface called "black silicon", which has a relatively low reflectance at all angles, but the trade-off between low reflectance and low PCE (Power Conversion Efficiency) resulting from surface recombination efficiency caused by high surfaceto-volume ratio of SiNW arrays is a problem [23]. Besides, preparing a homogeneous and dense light trapping structure is difficult for chemical etching way, and the acid etching and alkaline corrosion produce contaminative water and cause pollution on the surface of the finished products, the quality could be influenced. The aim of this paper is to investigate the reflectance of arc-shaped and rectangular-shaped groove structure prepared by common cutting tools, while mentioned by few papers(for our knowledge). The reflectance of light trapping structure on rectangular groove can reach 3% and even lower without ARC (Antireflection Coatings).

In this chapter, numerical analysis method is developed to simulate the reflection path of rays in the different groove light trapping microstructures, and reflectance of ray at each reflection point is determined according to Fresnel equations. Then the weighted reflectance of each structure with different incident angle of light and structural parameters was calculated and precise results were obtained. The reflection law of rectangular groove structure is strictly deduced by formula, and the best angle and optimized parameters of rectangular groove structure were proposed.
