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Photonic crystals are interesting nano-engineered periodic structures that allow control and manipulation of light at very small scales, in particular at frequencies inside or close to the photonic bandgaps. By engineering the photonic bandgap, different regimes of the photonic crystal can be achieved, enhancing solar collection angles, concentrating light to increase the interaction with photovoltaic material, or serving as spectral filters over a certain bandwidth of the solar spectrum. One of the interesting phenomena is the so-called self-collimation, where light is guided without a physical channel, exclusively due to the peculiar dispersion properties of the photonic crystal structure . PCs are formed by alternating high-to-low refractive index materials. They can be periodic in one, two or three dimensions. This means that their optical properties vary periodically in one, two or three directions. In photonic crystals the scale of the periodicity is of the same order as the wavelength of light. A simple 1D photonic crystal is the multilayer stack (better known as Bragg reflector), where an alternating layers of high and low refractive index materials with thicknesses of quarter of the wavelength is stack together (fig 1a). A good example of 2D periodic structures can be a set of identical parallel cylinders placed in a homogeneous host medium (fig1b). In addition, the spheres in a diamond lattice can be good examples of 3D photonic crystals (fig 1c).
C:\Users\melheno\Desktop\Master Thesis\fig1a.png C:\Users\melheno\Desktop\Master Thesis\fig1b.png C:\Users\melheno\Desktop\Master Thesis\fig1c.png
Figure1: Illustration of photonic crystals a) 1D photonic crystal b) 2D photonic crystal c) 3D photonic crystal 
Photonic crystals have found many applications in filters, waveguides, resonators and many more applications. The last years have seen an increasing interest in using photonic crystals for solar-related applications [4-8], especially in configurations that can increase efficiencies of existing solar photovoltaic cells. 1D PC structure in form of a Distributed Bragg Reflector (DBR) can be used as a back reflector due to its near-ideal reflecting characteristics in specific wavelengths , [9-14]. Two-dimensional photonic crystals are promising for integration with different components due to their compatibility with existing mature fabrication techniques [15-17]. Though 3D photonic crystals require complex fabrication techniques for solar applications, interesting work has been done to show the importance of such structures for light trapping in solar cells [18-20].
A significant challenge to the incorporation of photonic crystals in solar cells is fabrication. One way to fabricate such structures is using Electron Beam Lithography technique ,  another method is nanoimprint lithography , , as well as interference lithography [25-27] and , of course, photolithography ,  . However, the last option better applies to semiconductor industry rather than for solar application considering the cost related to fabrication. The fabrication method which is to be applied for solar applications need to have high throughput, large scale manufacturing and of course, they need to be affordable. These structures need to provide such gain to efficiency so that they not only can cover for the manufacturing cost but also provide something more to it, otherwise, simply, industry will not be interested in such structures.
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