A role of lasers in energy materials and future perspectives (2017)

Lasers are widely used in industry because of precision of operation, local treatment, high processing speed, and low cost. In the application of high power laser processing, local heat generation on the irradiated surface gives rise to rapid temperature increase. Depending on the process, temperature in the laser irradiated region exceeds well above the melting temperature of the substrate material. In this case, a local phase change takes place, and material undergoes rapid cooling locally in the solidification cycle. The rapid phase change can initiate formation of new metallurgical phases or glassy structures in the irradiated region. This alters the thermal electrical properties locally, and, in some instances, it improves electrical and/or thermal conductivities. Laser repetitive pulses can generate the overlapping of irradiated surface. Therefore, conduction of 3 or localized property improvement in terms of thermal and mechanical properties can be developed within the substrate material. In some of applications, laser processing involves gas-assisted processing; in which case, assisting gas may have multi-fold effects on the process. Some of these effects include formation of new species or compounds, such as oxides, nitrides, and carbides,1 shielding the irradiated surface from exothermic high temperature reactions,2 and contributing to cooling process.3 Forming nitrate compounds becomes critical for lowering surface energy of ceramics, such as alumina.1 This, in turn, improves the hydrophobic characteristics of the surface, which is particularly important for self-cleaning applications of solar energy harvesting surfaces. In addition, nitride species can improve solar absorption at the surfaces, such as titanium nitride, which can be used as a selective surface for solar volumetric receivers. However, in some cases, nitride or carbide species can improve the wear resistance of treated surfaces, which becomes important when the energy material is subjected to harsh wear conditions. Laser machining towards shaping the energy materials is one of the other applications when precision of operation is required. Some of these applications include net shaping of thermoelectric pins, scribing of thin-film solar cells, edge isolation of crystalline silicon solar cells, laser doping of solar cells, etc. In the case of high temperature processing, thermal stresses generated in the machined section can influence the life span of the parts used in the solar energy harvesting devices. In some cases, stress levels exceed the yielding limit of the substrate material, which in turn causes crack formation in the machined section while limiting the practical application of machined parts. Nevertheless, the defects site and related machining parameters can be minimized via optimizing the processing parameters. Some of the typical applications of lasers in energy harvesting materials include scribing of silicon photocells, surface texturing for hydrophobicity and photon capturing, creating selective surfaces for photo-thermal applications in solar energy receivers, shaping and trimming of thermoelectric pins, doping applications in solar cells, and localized connections of solar active elements. Figure 1 shows the typical applications of laser in processing of energy materials. In the case of laser net shaping, the process is involved with precision trimming of the energy harvesting materials towards optimization of geometric configurations. Recent developments in thermoelectric generators utilize segmented pin configurations with various geometric configurations. In order to develop such configuration, net shaping of pins with desired geometries can be achieved by laser trimming. Although the process is costly with current laser machining, configurations need further improvement for cost-effective processing. High power lasers are one of the important tools to treat the surfaces achieving hydrophobic characteristics pertinent to self-cleaning applications of solar harvesting device surfaces.4 Laser texturing involves the formation of micro/nano size pillars at the surface, which in turn changes the surface hydrophilic state to hydrophobic state. In order to improve the water droplet contact angle hysteresis, further improvement is required towards reducing surface energy of the textured surfaces.5 This can be achieved through using an assisting gas such as nitrogen. This arrangement provides formation of nitride compounds such as aluminum nitride while lowering the surface energy. Figure 2A shows the scanning electron image of the laser textured alumina surface, which resulted in water droplet contact angle in the order of 140° with contact angle hysteresis of about 8°. It is evident from the figure that the surface is composed of micro/nano features (Figure 2A). Laser texturing of transparent surfaces made possible to use polycarbonate surface as a protective wafer for self-cleaning applications of photovoltaic cells.4 Figure 2B shows the micrograph of the laser textured surface obtained from the scanning electron microscope. The textured surface comprises micro/nano pillars, which causes hydrophobic properties at the surface. Formation of nitride specifies, through laser gas assisted processing of titanium surfaces, improves absorption characteristics of surface, which can then be used as the selective surface for solar thermal receivers.1 Figure 2C shows SEM micrographs of the laser textured surface. X-ray diffraction data reveal that titanium nitride is evident from the diffractogram peaks. Formation of nitride species can be extended to zirconium, which provided high absorption for solar radiation and extremely low thermal emission from the surface. This enables zirconium nitiride as one of the demanding selective surfaces for thermal receiver applications. Testing of thermal characteristics of silicon base thin films for solar energy application is critical in terms of energy diffusion and phonon transport from heated film to its surroundings. Low power laser texting of such film arrangements provides correct estimation of thermal conductivity of the film, because the thermal properties vary significantly with the film size,6 which in turn influences film performance in terms of energy transport. Figure 3 shows equivalent equilibrium temperature distribution inside the silicon film, where aluminum quantum dot is placed at the film center pair, when thermally disturbed by heat source generated by laser irradiation in aluminum quantum dot. Laser trimming of edges is important for net shaping of energy materials. The process involved precise cutting of semiconductor/dielectric materials. In this case, laser beam scanning speed, laser beam power, and laser beam wavelength play an important role in achieving quality trimmed edges. Similar arguments are applied for micro-drilling of electronic compounds used in energy sector. The film thickness and uniformity of film properties are also critical in producing asperity free trimmed surfaces. Laser-induced doping of energy materials is one of the applications of laser processing in energy material technology. For example, the depth of p–n junctions in silicon solar cells is typically less than a micrometer; therefore, the laser-induced diffusion becomes ideally suited for forming shallow p–n junctions in silicon. Short pulse length lasers remain suitable for such process. In most of the cases, silicon surface is coated by the absorbing layer to improve the absorbed laser intensity for the thermal process. The laser-induced doping has advantages over other conventional techniques, because the selective doping can be achieved with a direct laser write process without incorporating photolithography. In addition, during the doping process, impurities in silicon film may not be contributing to heat generation so that localized heating is achieved. One of the challenges of cost-effective texturing of transparent surfaces, such as polycarbonate sheets, towards achieving hydrophobic characteristics at the surface is minimization of melt flow on the surface during texturing. Uncontrolled texturing lowers the optical transmittance of the surface significantly.4 However, thermal effect can result in crystallization of the surface during the process, which in turn contributes to attainment of low transparency of the textured wafer. Although laser gas nitriding on metallic and ceramic surfaces forms nitride compounds at the surface, thermal stress levels formed in the treated layer may excess the yielding limit of the substrate material locally while resulting in crack formation in the treated layer. This situation limits the practical applications of the laser gas-assisted processing. In addition, uniformity of nitride compounds across the surface as well as thickness of the treated layer becomes critical to achieve desired selective surface characteristics. In this case, increasing nitride depth generates thermal barrier resistance between the solar heated selective surface and the underneath material. This in turn lowers the heat transfer rates to the working fluid. Laser trimming of solar material has many challenges during net shaping. Some of these challenges include sideways burning due to local excessive heating and dross attachment on the trimmed edge. In addition, striation causes additional pattern on the trimmed edge and alters the optical and thermal characteristics of the substrate material. Consequently, process optimization towards minimizing asperities at the edges becomes critical prior to the process applications. Assessment of heating effects of thin quantum dot located in silicon film via thermal analysis based on radiative phonon transport provides useful information on the transport characteristics and performance of the silicon film and quantum dot combination. However, similar assessment for various sizes and doped silicon film is challenging because of complexity in estimating the dispersion relation of the film material. In addition, various film shapes can bring challenges when formulating the energy transport inside the film during the thermal disturbance. Although laser-induced doping has several advantages, for large area, applications of laser doping have many challenges. In this case, throughput needs to be optimized with larger beam size using high-power lasers or multiple laser beams. Recent development in laser technology, such as utilization of fiber lasers, and optical systems provide opportunity to extend the current applications in energy materials. High speed trimming, uniform thin film nitriding of metallic and ceramic surfaces, large area doping of semiconductors, and surface texturing of transparent wafers offer new opportunities and future challenges for the birth of new development in energy materials. Lasers can be used as effective tools in energy materials, and it provides several advantages over the conventional processes. Some of these advantages include precision of operation, local treatment, fast processing, and low cost. Although lasers are currently used in various processing steps of energy materials, the recent development in laser technology and optical instruments enables to give birth to new processes and technologies in the energy sector. The authors acknowledge the financial support of King Fahd University of Petroleum and Minerals (KFUPM) through Project# RG1501 to accomplish this work.