Fabrication of photonic nanostructures for light harvesting in solar cells. (2017)

Reducing optical losses in the solar cells has always been a key challenge in enhancing the power conversion efficiency of the solar cells without increasing significantly the cost. In order to enhance the power conversion efficiency of the solar cells, a number of light trapping schemes have been investigated to manipulate the light inside the absorber layer and to increase the effective optical path length of the light within the absorber layer of a solar cell. In this work, periodic nanopyramid structures were utilized as the light trapping nanostructures in order to improve the performance of the solar cells using low cost maskless laser interference lithography (LIL) and UV nanoimprint lithography (UV-NIL). In addition, a superhydrophobic property of the nanopyramids was explored to add a self-cleaning functionality to the front encapsulation. Firstly, the inverted nanopyramid structures were fabricated on Si substrate by laser interference lithography and subsequent pattern transfer by combined reactive ion etching and KOH wet etching. Maskless LIL was employed as a high-throughput, high resolution and low cost for the fabrication of large scale periodic nanostructures. The periodic inverted nanopyramid structures on a silicon substrate were used as a master mold substrate for the imprint process. In the first nanoimprint process, the upright nanopyramid structures with light harvesting and hydrophobic properties were fabricated on the glass substrate by simple, high throughput and low cost UV nanoimprint lithography using Si master mold with inverted nanopyramid structures. The upright nanopyramids structured glass substrates were tested for protective cover glass for solar cells applications and were utilized as a mold for the second imprint process. The diffuse transmittance and haze ratio values were significantly increased for the upright nanopyramid patterned glass, especially, in the wavelength range 300-600 nm compared to the bare glass. This indicates that antireflection and strong light scattering functions due to the upright nanopyramid structures were achieved. The use of upright nanopyramids as a cover glass, lead to the power conversion efficiency of the encapsulated monocrystalline Si solar cell and commercially made polycrystalline Si solar cell to substantially increased about 10.888% and 8.216%, respectively. This is mainly due to the scattering and prolong the optical path length caused by the upright nanopyramid structures compared to the reference cells with bare glass. In addition, the fluorinated upright nanopyramid structured cover glass exhibited larger contact angle (θCA ~132°) and excellent self-cleaning properties for dust particles. In the second nanoimprint process, the periodic inverted nanopyramid structures were fabricated on the monocrystalline solar cell and commercially made polycrystalline Si solar cell front surfaces using a UV nanoimprint lithography. The pyramid coating can be applied after cell fabrication to eliminate any losses due to surface damage by the etching processes. The inverted nanopyramid structures decreased the reflectance and increased the external quantum efficiency over a broad wavelength range. The periodic inverted nanopyramid structure has successfully reduced the Fresnel reflection and led to directing and trapping more incident light into the monocrystalline and polycrystalline Si solar cells, thereby improving the short circuit current density and enhancing the power conversion efficiency. The power conversion efficiency of the monocrystalline Si solar cell and the polycrystalline Si solar cell with inverted nanopyramid structures were improved by 11.733% and 6.869% compared to the planar solar cells, respectively. Moreover, the surface of the solar cells exhibited hydrophobic properties due to increased contact angle caused by the nanostructure patterns and the self-assembled monolayer coating. The enhanced hydrophobicity provided the solar cells with an added self-cleaning functionality. Finally, the surface morphological, optical and electrical properties of ITO thin films prepared by RF magnetron sputtering and the fabrication of ITO films onto upright nanopyramid structured glass substrate were studied. The ITO films were conformally deposited onto upright nanopyramid structured glass substrate at optimized parameters. These results suggest that the periodic inverted nanopyramid and upright nanopyramid structures with light harvesting and self-cleaning properties have considerable potential for various types of solar cells and optical systems in real outdoor environments.