Self-cleaned Solar Cells with Super-Hydrophobic Photonic Nano-structures

This work has investigated the light scattering effect and self-cleaning (i.e. water hydrophobicity) properties of various micro/nano-structured front coatings for solar energy applications. This was done through enhancement of surface texturing, following a colloidal lithography methodology, which comprised the reactive ion etching of the material with a pre-deposited mask of a self-assembled monolayer of polystyrene microspheres. This process culminated in the formation of a surface patterned with micro structures. Experimentally, the maximum water contact angles obtained were 140° and 167°, for PET and parylene-C materials, respectively. The results of scanning electron microscopy suggest the wettability properties change due to variations in surface roughness at micro and nano scales. Then, through x-ray photoelectron spectroscopy analysis, that change was associated with variations in surface chemistry. Droplet state transition from Wenzel state to Cassie-Baxter state for parylene was analytically demonstrated through examination of water advancing and receding contact angles. It was observed that increasing plasma exposure results in an increase of reflectance and in a slight decrease of total transmittance. However, diffuse transmittance increased from 5% and 2% to above 60% for PET and parylene, respectively, which reveals potential for light trapping (via optical path length amplification). These changes are directly related to surface roughness modification and they intensify with colloidal lithography. For proof-of-concept, a-Si solar cells were fabricated in superstrate (p-i-n) and substrate (n-i-p) configurations to evaluate the performance of the textured parylene as a photonic-structured encapsulant. The measured devices showed a photocurrent ( ) enhancement up to 16%, in the substrate configuration, due to the light trapping effects of the photonic-structured parylene. In addition, the structured parylene layer also acted as a pronounced superhydrophobic surface which strengthens the device robustness in outdoor operation