Abstract
Highly reflective coatings with strong light scattering effect have many applications in optical components and optoelectronic devices. This work reports titanium dioxide (TiO(2)) pigment-based reflectors that have 2.5 times higher broadband diffuse reflection than commercially produced aluminum or silver based reflectors and result in efficiency enhancements of a single-junction amorphous Si solar cell. Electrophoretic deposition is used to produce pigment-based back reflectors with high pigment density, controllable film thickness and site-specific deposition. Electrical conductivity of the pigment-based back reflectors is improved by creating electrical vias throughout the pigment-based back reflector by making holes using an electrical discharge / dielectric breakdown approach followed by a second electrophoretic deposition of conductive nanoparticles into the holes. While previous studies have demonstrated the use of pigment-based back reflectors, for example white paint, on glass superstrate configured thin film Si solar cells, this work presents a scheme for producing pigment-based reflectors on complex shape and flexible substrates. Mechanical durability and scalability are demonstrated on a continuous electrophoretic deposition roll-to-roll system which has flexible metal substrate capability of 4 inch wide and 300 feet long.
Highlights
Reflective coatings with strong light scattering effect have many applications
Cross-section scanning electron microscope (SEM) at a section of film lifted off of substrate further showed the compactness of the pigment-based film and uniformity of the film thickness, which was approximately 23 μm for a 20 second deposition time
The average pigment-based film thickness and deposition rate compared to deposition time are shown in the inset of Fig. 3, and indicates that the thickness and deposition rate began to saturate with increasing deposition times. This is due to the pigments being electrically insulating which increases the resistivity of the electrophoretic deposition (EPD) electrode, i.e. metal substrate and pigment-based film, with increasing film thickness resulting in decreased deposition rate
Summary
Reflective coatings with strong light scattering effect have many applications. Substrate configured thin film Si solar cells have additional advantages such as low-cost, roll-to-roll manufacturing using inexpensive metal web substrates, which are applicable to flexible, lightweight niche markets. High efficiency and low cost thin film Si solar cells have electrically “thin” and optically “thick” device structures, which is achieved by using back reflectors to maintain high light absorption while decreasing Si film thickness. This improves material stability, increases the electric field across pin junction, and lowers deposition times [2]. The organic medium of white paint cannot withstand high vacuum, high temperature and plasma processing conditions of Si absorber and transparent conductive oxide layer depositions. A pilot roll-to-roll process validated a pathway to realizing large-area fully functional components and devices, such as the direct deposition of solar absorber layer onto pigmentbased back reflector
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