Light management in nanostructures for solar cell application.
Date of Issue2013
School of Electrical and Electronic Engineering
Solar cells have received great interest and tremendous efforts have been devoted in the last few decades. However, the main problem of the conventional solar cell is that the cost is too high to achieve relatively high efficiency. In order to make the solar cell affordable to public, scientists are now focusing on the design of the third generation solar cells named as nanostructured solar cell. By texturing the front, back surface or the active layer itself, the light absorption and thus the efficiency can be largely improved. The other way to improve the light absorption is to directly texture the active layer into nanostructures which is also promising to achieve high efficiency and low cost solar cells. Transparent electrodes are essential part of the solar cell device. Indium tin oxides (ITOs) are the most widely used transparent electrodes for solar cell nowadays. However, the price of indium mineral has soared to rocket high due to the huge demand and limited mineral reserves. Thus, scientists are now trying to seek for alternative materials to replace ITO electrodes. The work of this thesis mainly focuses on the design and investigation of three types of nanostructures: anti-reflective coating, metallic nanostructure electrodes and nanostructured active layer. Nanostructured anti-reflective coatings achieved by front surface texturing have been studied intensively. Zinc Oxide (ZnO) nanowire and nanohole structures are presented in this work. The designed nanostructures can act as an anti-reflective coating in the visible range and also serves as a UV blocking layer simultaneously. The important structure parameters such as period and filling ratio are varied and optimized to achieve high transmittance in the visible range and high absorptance in the UV range. The optical properties of nanowire and nanohole structures, as well as the angular response properties are investigated in detail. A nanopatterned aluminum thin film electrode is also presented. With the optimized parameters, the nanopatterned aluminum thin film exhibits a high transmittance in the range from 300 – 1000 nm and has a low sheet resistance, which makes it a potential electrode to replace ITO. In this work, the key parameters such as period and filling ratio are optimized to increase transmittance and in the meantime reduce sheet resistance. The optical properties are analyzed in detail and the role of surface plasma polariton (SPP) in the influence of transmission spectrum is particularly discussed. Hole filling materials, substrate materials and angular response are also taken into consideration in the nanopatterned aluminum electrode design. Several commonly used metals are optimized respectively aiming to obtain high performance nanostructured metallic electrodes. In order to further increase the absorption in the active layer, a new type of nano-cone-hole (NCH) array was brought into study. With the optimized NCH array, the absorptance is largely enchanced compared to the thin film and the optimized nanohole (NH) array. The absorptance enhancement is attributed to the lower surface reflectance; more supported resonant modes and enhanced modes interaction. The angular dependences of ultimate efficiency of transverse electric (TE) and transverse magnetic (TM) polarizations are also studied. The influence on absorptance by introducing randomness into nanowire (NW) arrays is also investigated. The absorptance in the active layer can be improved by randomizing the regular arranged NW array. The ultimate efficiency of the optimized position random case is improved by 15% as compared to the regularly arranged one. According to electric field distribution at two different wavelengths, the absorption enhancement of random structure is due to a better anti-reflective performance, additional resonances introduced by the structural disorder and existing resonances broadening. In summary, this dissertation addresses three types of nanostructure designs, including the nanostructured ZnO anti-reflective coating, the nanopatterned metallic electrode and the nanostructured active layer. The work done here will help to understand better the design rules for light management in nanostructures and the know-how can be applied to new type nanostructure design for solar cells.
DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics