Optimization of non-planar antireflective coating and back- (or front-) surface texturing are widely studied as advanced light management approach to further reduce the reflection losses and increase the sunlight absorption path in solar cells. Rear reflectors have been developed from coherent mirrors to incoherent mirrors 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 (LIL) and selective wet etching. This approach allows for a rapid (10 min) wafer scale (3 inch wafer) texture processing with sub-wavelength (nano)-scale control of the pattern and the pitch. The technique appears as being particularly attractive for the development of ultrathin III-V devices, or in overcoming the weak sub-bandgap absorption in devices incorporating quantum dots or quantum wells. The structure of the device is demonstrated, without affecting active layers.
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. 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 we have developed a method to overcome this challenge that combines the use of laser interference lithography (LIL) and selective wet etching. The approach allows for a rapid wafer scale texture processing with sub-wavelength (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. Preliminary results on the application of the technique for the development of back reflector for 1-1.3 eV (MQW bearing) GaAs solar cells are presented.
In III-V concentrator applications, sunlight is focused with wide angular distribution that limits the effectiveness of conventional thin-film AR coatings. 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 planar layer ARCs for III-V multi-junction concentrator cells in case of with/without protective cover glass in conjunction with wide optical aperture angles, a systematic analysis of design parameters and angular dependent antireflective properties of dielectric gratings has been undertaken, through the implementation of sub-wavelength 2D pyramidal gratings of ZnS and TiO2. The study indicated limited improvement for devices operated with SiO2 like cover glass. In the absence of SiO2 like cover glass, the evaluation indicated that reflection-loss related current losses can be reduced by 2-3 fold compared to their doublelayer ARC counterparts. i.e. for a 3J metamorphic device this lead to a current improvement of 0.7 mA/cm2 per concentration for a 60 degree aperture angles
In high X III-V concentrator applications sunlight is focused onto the surface of cell with a wide angular distribution that limits the effectiveness of conventional thin-film AR coatings. 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, we have undertaken a systematic analysis of design parameters and angular dependent antireflective properties of dielectric grating formed through the implementation of sub-wavelength arrays of 2D pyramidal and hemispherical textures. The evaluation indicates that through a careful selection of the design and dielectric material these structures can significantly surpass the performance of planar double layer ARCs (i.e. MgF2/ZnS), and the total number of reflected photons over the 380-2000 nm wavelength range can be reduced to less than 2%. Finally it is shown that the implementation of these structures for a typical 3 or 4 junction solar cells (i.e. inverted metamorphic) and for acceptance angles ranging from 0-60 degrees, reduces total losses of reflected photons for each subcell (and to some extent the resulting current degradation) to less than 4%. Anti-reflection and angular tolerant properties of 2D TiO2 surface texturing made by nano imprinting technique were simulated and measured in this work. It has been proved that from both simulation and experimental work textured surface surpasses both antireflection and angular tolerant characters of planar ARC, which supplies a potential candidate AR structure for concentrated photovoltaic system.
In our simulation of reflection losses for 1D and 2D subwavelength dielectric grating, surface texturing was done
while comparing reflection losses with various incident angles for photovoltaic materials like Si and III-Vs GaAs.
Transfer matrix formalism is modeled by treating each grating's effective refractive index as being composed of
several layers of varying refractive indexes. Discrete parameterization on intervals with different profiles such as 1D
rectangles and triangle, as well as 2D pyramids and hemispheres are used to minimize power reflected for black
body radiation. This simulation treats each layer to be uniform, which requires the texturing to be in the
subwavelength region. We compared the reflection loss and incident angle dependence for dielectric layers, dielectric
gratings, and the combination of both dielectric layers and gratings, and found that with gratings, reflection losses
are less dependent on incident angle. By optimizing the texturing and design parameters, we can obtain reflection
losses around 1% for spectral range of solar cell with a very small increase in incidence angle.
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