Antireflection and self‐cleaning structures for solar cells using laser interference nanolithography

A thesis submitted to the University of Bedfordshire, in partial fulfilment of the requirements for the degree of Doctor of Philosophy.This research comprehensively reviews the properties of regular micro and nano structures fabricated by laser interference lithography and reports on their applications in the antireflection and self‐cleaning surface. The research systematically investigates the laser interference lithography technology taking into account its advantages and abilities to realize various potential applications. Multiple‐beam interference lithography systems are constructed. Laser interference interaction with silicon wafer is analysed and the optical and hydrophobic properties are obtained via measurements. In order to fabricate the extremely low reflection and very large contact angle for solar cells, fabrication methods of antireflection and self‐cleaning are surveyed and their advantages and disadvantages compared. The research investigates the effect of heat transfer and the radiation of laser interference plasma on silicon wafer surfaces and proposes equations of heat flow and radiation effects of laser plasma of interfering patterns in a four‐beam laser interference distribution. Following the irradiation, the silicon wafer surface is covered with a periodic array of micrometer and nanometer‐sized structures, which have the shape of grating, cone and hole. The research also investigates the effect of different laser parameters on the optical and hydrophobic properties of the structured silicon wafer surface. The results of periodic hexagonally‐distributed hole structures fabricated by three‐beam laser interference reveals excellent design guidelines for obtaining an extremely low solar‐weighted reflection, (SWR, 1.86%) and relatively large contact angle (140°) which can provide a strong self‐cleaning capability on the solar cell surface. In addition, the research creates a novel dual structure with antireflection and superhydrophobic properties fabricated by three‐beam laser interference lithography. The fabrication method is three‐beam laser interference combined with focused laser processing interacting on the silicon wafer surface. This kind of structure has a very low SWR (3.6 %) and extremely large contact angle which is more than 150° in the wavelength range from 380 nm to 780 nm. The research shows that the laser interference lithography technology can be employed and further developed to fabricate micro and nano structures of strong antireflection and self‐cleaning functions for applications in solar cells

High-resolution and large-area laser interference nanomanufacturing technology

A thesis submitted to the University of Bedfordshire, in partial fulfilment of the requirements for the degree of Doctor of PhilosophyThe thesis systematically investigates the laser interference nanomanufacturing technology taking into account its advantages and abilities to realise various potential applications. The latest progresses have addressed the major issues hampering the cross-scale developments of structural applications, such as cost-ineffective fabrication, limited area, low efficiency and challenging integration. The studies carried out on high-resolution and large-area laser interference nanomanufacturing technology will complement the exploration of modern optical devices and extraordinary functional applications. With respect to classical interference theory and relevant references, there is still a lack of studies providing insight into the effects of polarisation on the multi-beam interference while it is found that the polarisation vector plays a key role in the formation, period and contrast of interfering patterns. Herein, the theory of multi-beam interference is developed through the integration of the polarisation vector and electric field vector. It is worth pointing out that based on the detailed analysis of the four-beam interference with the special polarisation modes, it is demonstrated that the modulation phenomenon in four-beam laser interference is the result of the misalignment of incident angles or unequal incident angles only in the case of the TE-TE-TM-TM mode. In the experiments, a straightforward method of generating various well-defined structures on material surfaces is proposed using the nanosecond laser interference system. The experimental results of two-, three- and four-beam interference show a good correspondence to the theoretical analyses and simulations. Artificial bio-structures are fabricated using the four-beam interference method with the TE-TE-TE-TE polarisation mode and the fabricated microcone structures exhibit excellent properties with both a high contact angle (CA=156.3°) and low omnidirectional reflectance (5.9-15.4%). In order to fabricate high-resolution structures, the 266nm nanosecond laser interference system is employed to treat the organic and metal-film materials. Nanograting structures with feature sizes of sub-100nm width and 2nm height are fabricated on the organic material surface. An attempt is successfully conduced to produce the nanoelectrode arrays by using laser interference lithography and chemical deposition. Finally, the advantages of the developed laser interference technology and contributions of the research are summarised, and recommendations of future work are given