Investigation Of Novel Applications Of Nanostructures In Solar Cells

Photovoltaics is a promising solution for the energy demand and sustainable development of human race. In the process of increasing solar cell efficiency, application of nanostructures has shown great potentials and become a research hotspot in recent years. The unique optical, electrical and material properties of nanostructures have opened up many new paths in optimizing anti-reflection, light-trapping, carrier transport, and cell structure designs. It is apparent that the on-going research of nanostructures will continue to dominate the advancement of third-generation high efficiency solar cells.Nevertheless, the application of nanostructures in solar cells is still a relatively new field, with new designs and concepts proposed and investigated on a daily basis. At present, most applications still merely focus on improving one aspect of the cell properties, thus can be considered as an extension within the traditional cell design framework. As a result, most nanostructured solar cells are still bound by the structural drawbacks and theoretical efficiency limits of conventional solar cells, which is even aggravated by the negative influence of nanostructures on factors such as surface recombination. Thus, further investigation and innovation is needed to take full advantage of nanostructures and make breakthroughs in cell efficiency.In this dissertation, we investigated possible novel applications of nanostructures in solar cells by means of simulation, hoping to deepen our understanding of their fundamental properties and expand the manner of their usage. First, we investigated the promotion of internal quantum efficiency of silicon heterojunction solar cells by introducing a periodic nanopillar array on the front surface which can modulate the energy distribution of short-wavelength light. We have found that by properly choosing the array dimensions, incident light can effectively excite the resonant mode of the pillars and the guided modes of the array, shifting the absorption front of the cell from the highly defectful amorphous silicon layer to the low-defect crystalline silicon region. Thus the carrier collection efficiency is significantly improved and the short circuit current in this wavelength range is enhanced by over 38%, showing a new pathway in cell efficiency promotion.Second, we investigated the new concept of single standing nanowire(single SNW) solar cells and examined their potential in breaking the fundamental efficiency limits of planar devices. We have found that their optical properties can be accurately described by an analogy to the dielectric resonator antennas, thus overcoming the inadequacy of previous theories in quantitative analysis. Based on this knowledge, we have chosen a suitable excitation wavelength and maximized the built-in concentration to 21-fold, leading to an open circuit voltage promotion of 124 m V beyond the Shockley-Queisser limit. On the other hand, the characteristic absorption process of the single SNW cell results in a concentration of photogenerated carriers in the intrinsic middle layer rather than the highly defectful surface layer as it would be in a planar device, giving it significant advantages in carrier extraction and defect tolerance. Finally, the efficiency of the single SNW cell exceeds the planar limit by over 33%.At last, we have successfully expanded the single SNW cell concept to macro-scale devices. We have identified two key issues in assembling them into an array: the preservation of built-in concentration in each unit cell, and the complimentary design between the cavity-like light-trapping of single SNWs and the photonic light-trapping of the array. For this, we have proposed the usage of a coaxial dielectric coating in tuning the resonances of single SNWs instead of the previous radius-dependent method. As a result, we have demonstrated a macro-scale device that performs 30% more efficient than the planar limit, opening up new possibilities in designing next generation high efficiency solar cells.