Rapid Laser Scanning Based Surface Texturing for Energy Applications
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Extensive research in micro/nano scale surface engineering has developed a variety of energy applications, including the improvement of light trapping in solar cells. The motivation for this work is the issues that come with nonrenewable energy, the dominating source of energy in this country. Renewable energy is clean and quiet. In particular, solar energy can generate enough power on its own to sustain the country. The current issue with this technology is its high manufacturing costs and low efficiencies. A solution to these challenges is through laser induced surface texturing. Laser induced surface texturing is an effective tool to roughen the surface of silicon, minimizing reflection and subsequently enhancing absorption through the creation of spiked structures. Extensive research has been conducted in this field already. So-called black silicon has been created and demonstrates enhanced absorption characteristics. However, the black silicon created is not yet ideal. In some cases, spikes are created on the microscale, but the structure is not spatially uniform, which causes problems for adaptation to manufacturing processes. In other cases, uniformity is attained, but high ablation causes loose structures to be formed, leading to a decrease in current density, which is equivalent to a reduction in solar cell performance. In all cases, additional chemical etching steps are required in order to get the desired results. The focus of this thesis is on creating black silicon by coupling a raster scan from a X-Y scanning system with a nanosecond laser with a Gaussian beam profile. An arbitrary sized area can be processed uniformly. Secondary heating allows for annealing of the previously created spikes, increasing homogeneity and current density of the overall solar cell performance. This would eliminate the needs for additional steps to the manufacturing process. A parametric study was performed by varying: laser power, line spacing, scanning speed, and pulse frequency. The parametric study showed that there was a broad processing range for creating black silicon. The demonstrated results showed great promise for processing of arbitrary sample size and practical processing speeds. In addition, surface texturing is applicable to broader energy applications. A significant enhancement of emissivity was demonstrated by texturing the surfaces of silicon and copper in uniform and controllable fashion. Texturing these materials can lead to efficiency improvement of solar collectors and heat exchangers. Laser induced surface texture can be applied to energy application where it is desirable to increase the surface area. Increased surface area on the electrodes of batteries or fuel cells can improve the efficiency and lifetime of these devices. Preliminary results operated at ablation dominated regime showed in fact a decrease in performance, displaying the importance of optimal annealing of the structure. Another use of an increased surface area is a boost of algae growth for massive bio-fuel production. An increased surface area can also be used for adhesion/cohesion enhancement of thermal-sprayed thin film for thermo-electrical devices. In summary, rapid laser scanning based surface texturing results have been demonstrated. The samples created display superior process uniformity at arbitrary sample areas with simultaneous annealing for improved crystallinity. While this process was designed for solar cells, it has great potential for various other energy applications.