Study Of Nanophotonic Structures-based Perfect Absorbers

With the decrease of fossil fuel and the constant aggravation of pollution, solar energy is regarded as one of the most plenty sources of renewable energy and much effort has been made to develop highly efficient photovoltaic devices. Photovoltaic(PV) cells can serve as a virtually unlimited clean source of energy by converting sunlight into electrical power. Their importance is reflected in the tireless efforts that have been devoted to improving the electrical and structural properties of PV materials. The first step of converting sunlight is light absorption. Conventional silicon thin film-based photovoltaic cells are inefficient even though nanostructures in silicon photovoltaic cells is of particular importance as these currently make up over 80% of the photovoltaic market in the world. Advantage of the silicon-based photovoltaic cells with short collection length for excited carriers results in significant improvement in carrier collection efficiency. Silicon, however absorbs poorly over the peak of the solar spectrum, requiring the use of over 100 μm thick Si wafers for sufficient absorption. Nanostructured photovoltaics promise simultaneously to increase the efficiency and decrease the cost of solar cells. Both dielectric and metallic bulk materials of PV cells offer strong light confinement. Their distinct materials properties translate into markedly different behavior and applicationsThree nanostructures are introduced in this paper: Nano-quadrangular frustum pyramid array, Nano-pyramid array and Nano-multiwall cylinders array. By using the finite difference time domain method to solve the Maxwell equations, we calculate the absorption efficiency. Excellent performances of light absorption are shown: perfect absorption at normal incidence; extraordinary optical absorption in ultra-broadband wavelength; incident angle insensitiveness; polarization of angle insensitiveness. In order to achieve the best structure of perfect light absorption, we use control variable method to optimize the structure parameters. And the slow light effect, leaky mode resonance effect is shown to explain the phenomenon of optical absorption.In order to verify the reliability of the simulation, focused ion beam technology(FIB) is introduced, including its composition and working mode. We select the focused ion beam etching samples,and milling a nano-quadrangular frustum pyramid array in 1 μm thick germanium film. Due to a three-dimensional structure, the processing is very difficult. We use ocean optical spectrometer to measure light absorption efficiency of smooth germanium film and nano-quadrangular frustum pyramid array under the condition of vertical incidence, and then make comparison of light absorption efficiencies. Due to the measurement of nanostructures, it is not high precision. At last, Near-field scanning optical microscope, including the working principle and working mode are shown in the paper. In confocal mode a probe scanning mode, we measure the intensity distribution of smooth germanium film and nano-quadrangular frustum pyramid array. It is clear to show the effect of light absorption. The intensity distribution results, simulation and spectrum measurement show agreement with each other.