Periodic hexagonal microtexture arrays (also known as honeycombs) are successfully implemented for the first time in a superstrate glass configuration. Hexagonal textures on glass demonstrate an anti-reflective effect when compared to flat glass. It is shown that light scattering increases at the honeycomb interfaces with an increase in texture height and periodicity. The performance of the textures is demonstrated using thin-film single-junction PV devices based on an indirect bandgap semiconductor material, nanocrystalline silicon (nc-Si:H), which requires light trapping in the infrared region of the spectrum. Inspecting the nc-Si:H bulk absorber suggests a conformal, crack-free growth of crystals on the hexagonal arrays. Short-circuit current density (J_SC) increases with an increase in the aspect ratio of the superstrate, without compromising voltage and fill factor. The J_SC enhancement is attributed to a combined benefit of (i) the anti-reflective nature of developed textures, (ii) trapping light within the absorbing layer through multiple order diffraction at the front and (iii) reflection from a back reflector with adapted hexagonal morphology. With the above observations, a J_SC of 28.6 mA/cm² (photovoltaic conversion efficiency of 9.3 %) is achieved for a 5μm periodic texture with a height of 1μm (aspect ratio = 0.21). This is the highest reported J_SC for a single-junction nc-Si:H solar cell in a superstrate configuration without an external anti-reflection coating. ... Read more

The characteristic feature of ”thin” film solar cells is that they are thin, ranging over thicknesses of a few micrometers. This results in a drop in the light trapping capability of the solar cell, owing to the Beer-Lambert law. Thus, there is a need to optimize light management to enhance the amount of light being trapped in the photoactive region. This can be done by imparting texture in the glass superstrate to scatter the light and thereby allowing the light to travel over a longer distance in the absorber.

In this thesis, the focus is mainly on developing a high-quality reproducable procedure for achieving periodic texturing in glass and understanding the enhancement in the optical impact of the surface. the first objective was to understand the influence of the different processes involved in the fabrication procedure of periodic textured glass superstrates. This was done by varying the parameters associated with the individual processes and observing the resultant impact on the deterministic characteristics of the texturing process. The influence of the mask design, employed in the photolithography process, on the dimensionality of the texture was also analyzed. Based on the aforementioned results, an optimized recipe was developed and successfully implemented to enhance the depth of the periodic hexagonal texturing imparted onto the glass. Additionally, an indicative parameter Re/P was developed to imply the effect the processes had on the quality of the texturing.

This project also ventured into studying the growth of nanocrystalline silicon on the deep textures developed using the optimized recipe. Following this, an extensive optical analysis of the honeycomb textured superstrates was carried out to realize the scattering effect induced by this texturing on interacting with light. This was done by studying the transmittance components and the angular intensity distribution of the periodic textured glass. The interaction mechanism of the textures was found to be dependent on the wavelength and
the size of the textures. In general, there was a linear increase in the diffuse transmittance with texture height, for all wavelengths of light. However, it was noticed that the slope of this relationship varied for the different wavelengths, since it was mainly dominated by different scattering mechanisms for different wavelengths. Finally, the influence of the optical impact of these hexagonal textures on solar cell performance was also studied by studying the Jsc of the micromorph tandem solar cells, developed on deep hexagonal textures of different periodicities. ... Read more