Study Of Solar Cells Based On Nano Structures

The most abundant source of renewable energy is solar energy.The most promising way to use solar energy efficiently is solar cell. It has been more than two handred years since solar cell was invented. But solar photovoltaic cells are not competitive with more conventional energy technologies because of their relatively low photoconversion efficiency and high cost. A substantial body of recent work in photovoltaics is beginning to exploit intentionally engineered nano-and micro-scale structures and the physics of reduced dimensionality to increase device performance.One focus of this thesis is nanowire(NW) solar cells. NWs are rods with a length typically on the order of microns and a diameter on the order of10’s to100’s of nanometers. It is well known that NWs can exhibit anti-reflection properties that are less dependent on incident wavelength, polarization and angle as compared to conventional thin film dielectric coatings. The reflectance, transmittance and absorptance of GaAs nanowire (NW) arrays are calculated by solving Maxwell’s equations using the finite element method. The model is compared with measurement results from well-ordered periodic GaAs NW arrays fabricated by dry etching. The model results are also compared with the reflectance measured from NWs grown by the Au-assisted vapor-liquid-solid (VLS) method. The optimum NW diameter, periodicity (spacing between NWs) and length are determined to maximize absorptance of the AM1.5G solar spectrum and short circuit current density in a NW array solar cell. A gold nanoparticle at the top of the NWs (used in the vapor-liquid-solid NW growth process) substantially reduced the optimum photocurrent density, while a polymer filling the space between NWs and a planar ITO contact had a relatively minor influence. Numerical simulation of the photocurrent density is performed for a two-junction nanowire (NW) on silicon solar cell under AM1.5G illumination. The photocurrent density is determined for NW diameters from100-250nm, period (spacing) from250-1000nm, and length of5m. The dependence of photocurrent density on NW bandgap is also determined. For each NW bandgap, the optimum diameter and period are determined to obtain current matching between the top NW cell and the bottom Si cell. This thesis explores a method which combined with electron beam lithography(EBL) and inductively coupled plasma(ICP) to fabricate GaAs NWs.Another focus of this thesis is nano-scale Fresnel lens. The reflectance, transmittance and absorptance of nano-scale Fresnel lens arrays are calculated by solving Maxwell’s equations using the finite element method. This Fresnel lens is used on Ⅲ-Ⅴ multi-junction solar cell. The influence of Fresnel lens on multi-junction solar cell has been investigated. A method which combines the electron beam lithography(EBL) and inductively coupled plasma(ICP) etching is developed to fabricate nano-scale Fresnel lens arrays on Spire Ⅲ-Ⅴ multi-junction solar cell. Finally, the Ⅰ-Ⅴ curve and the performance of this solar cell are measured.