Fabrication and Design of Sub-Wavelength Periodic Textures for Improving Light Harvesting in Multi-Junction III-V Photovoltaics
Wang, Wei 1987-
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Optimization of non-planar antireflective coating and back- (or front-) surface texturing are widely studied to further reduce the reflection losses and increase the sunlight absorption path in solar cells. In III-V concentrator applications sunlight is focused onto the surface of cell and consequently, light arrives with a wide angular distribution that limits the effectiveness of conventional thin-film antireflective coatings (ARCs). Furthermore, the transmission properties are generally degraded non-uniformly over the electromagnetic spectrum, which in the case of multi-junction solar cells, leads to additional sub-cell current matching-related losses. Here, and in an attempt to identify a better alternative to the conventional dual-layer ARCs, a systematic analysis is undertaken regarding the design of parameters and angular dependent antireflective properties of dielectric grating formed through the implementation of sub-wavelength arrays of 2D pyramidal and hemispherical textures. The study includes evaluation of these properties for several common dielectrics through a careful selection of dielectric material and design. These structures can significantly surpass the performance of planar dual-layer ARCs, and the total number of reflected photons over 380-2000 nm wavelength range can be reduced to less than 2%, by use of single-material textured dielectric. It is also shown that the implementation of these structures for a typical concentrated 3 or 4 junction solar cells with apertures ranging from 0-60 degrees reduces total losses of reflected photons for each sub-cell to less than 4%, and therefore reduces current degradation. Back reflectors have been developed from perfect mirror to textured mirror in order to further increase light path, which can significantly improve the efficiency and allow for much-thinner devices. A Lambertian surface, which has the most random texture, can theoretically raise the light path to 4n2 times that of a smooth surface. It’s a challenge however to fabricate ideal Lambertian texture, especially in a fast and low cost way. In this work, a method is developed to overcome this challenge that combines the use of laser interference lithography and selective wet etching. The approach allows for a rapid wafer scale texture processing with subwavelength (nano-) scale control of the pattern and the pitch. The technique appears as being particularly attractive for the development of ultra-thin III-V devices, or in overcoming the weak sub-bandgap absorption in devices incorporating quantum dots or quantum wells (QWs). The design and fabrication process on the application of the technique for the development of back reflectors for MQWs solar cells are presented. Depending on the growth order (inverted or up-right growth), the two-side device-metallization incorporated with lift-off process are designed differently due to the fragile ultra-thin (~ 2 µm) active layer, and the strain from embedded QWs. Another approach is done through thinning the substrate (~ 15-20 µm), texturing the substrate as an incoherent reflector, and metallization, which won’t affect active layers.