The reflection loss in imaging devices is one of the major drawbacks, which degrades efficiency resulting in lower responsivity. Since the reflected light is no longer available for conversion into electrons, it is very important to reduce the reflection from the top surface of the device as much as possible. Quarter wavelength and two index antireflection (AR) coatings have been developed to reduce reflection; however, these AR coatings are wavelength dependent and have not performed effectively in a broadband range. Attempts to make AR coating for broadband wavelengths by stacking multiple index AR layers result in thicker and expensive solutions, which still do not provide proper antireflection at all desired wavelengths. Moreover, the usage of AR coatings escalates material and fabrication costs of the device. We propose a novel nanostructure, which matches the refractive index of the device to that of free space to reduce reflection from the top surface, eliminating the use of AR coatings and hence reducing the device cost. It is shown via simulation that the proposed nanostructure effectively eliminates the reflection loss over the broadband spectrum of desired wavelengths e.g. Visible, Mid-wave IR (MWIR), Short-wave IR (SWIR) spectrums, opening various application opportunities.
The use of Graphics Processing Unit (GPU) for computational work has revolutionized how complex electromagnetic problems are solved. Complex problems which required supercomputers in the past for analysis can now be tackled and solved using personal computers by channeling the computational work towards GPUs instead of the traditional computer Central Processing Unit (CPU). Finite-Difference Time-Domain (FDTD) analysis, which is a computationally expensive method of solving electromagnetic problems is highly parallel in nature and can be readily executed in a GPU. We have developed an algorithm for three dimensional FDTD analysis of optical devices with micro and nano-structures using Compute Unified Device Architecture (CUDA). The developed algorithm exploits the benefits of multiple cores of GPU chips and boosts the speed of simulation without sacrificing its accuracy. We achieved a 25-fold speed up of simulation using CUDA compared to MATLAB code in CPU.
The advent of ultrathin crystalline silicon (c-Si) solar cells has significantly reduced the cost of silicon solar cells by consuming less material. However, the very small thickness of ultrathin solar cells poses a challenge to the absorption of sufficient light to provide efficiency that is competitive to commercial solar cells. Light trapping mechanisms utilizing nanostructure technologies have been utilized to alleviate this problem. Unfortunately, a significant portion of light is still being lost even before entering the solar cells because of reflection. Different kinds of nanostructures have been employed to reduce reflection from solar cells, but reflection losses still prevail. In an effort to reduce reflection loss, we have used an array of modified nanostructures based cones or pyramids with curved sides, which matches the refractive index of air to that of silicon. Moreover, use of these modified nano-pyramids provides a quintic (fifth power) gradient index layer between air and silicon, which significantly reduces reflection. The solar cells made of such nanostructures not only significantly increase conversion efficiency at reduced usage of crystalline silicon material (e.g. thinner), but it also helps to make the c-Si based solar cell flexible. Design and optimization of flexible c-Si solar cell is presented in the paper.
Crystalline Silicon (c-Si) solar cell has presented itself as the ultimate solution for solving the cost and efficiency dilemma for the solar industry. We at Banpil have been working on the development of novel solar cells based on nanostructures to increase the conversion efficiency significantly over standard solar cells. These nanostructure based c-Si solar cells could potentially break the cost barrier that has thwarted the photovoltaic industry. In this work, we have designed ultrathin c-Si solar cells based on nanostructures, enabling a light trapping phenomenon to achieve a power density ranging from 0.91 W/g to 3.5 W/g, which is from 3.5 to more than 10 times over available standard c-Si solar cells.