Spectral and spatial tuning of absorption for enhanced solar energy conversion [electronic resource]
Keywords: Electrical Engineering

The search for sustainable energy resources has emerged as one of the most significant and universal concerns society will face in the 21st century. Solar energy conversion offers a cost-effective alternative to our traditional greenhouse gas emitting power plants. The most common solar energy conversion processes are solar photovoltaic conversion and solar photothermal conversion. In the first one, solar photons are converted directly into electrical energy, whereas in the latter solar photons are converted into heat. The heat can then be converted into electrical energy by a steam turbine or a thermophotovoltaic cell. In both types of solar energy conversion it is of outmost importance to optimize the absorption process by maximizing the trapping of solar photons inside the photoactive absorber, while minimizing the potential loss mechanisms, such as thermal emission in photothermal conversion or exciton recombination in photovoltaic conversion. In this dissertation coherent light trapping approaches are explored based on wave optics, thus in the regime where light needs to be treated as a wave phenomenon and can be made to interfere or diffract in order to reinforce the electric field in certain regions (spatial tuning) for a desired frequency range (spectral tuning). Photonic design can help spectrally and spatially tune the electric field distribution and consequently the absorption for a specific conversion process, device or application. In chapter 1 we first introduce the physical quantities that will be used throughout this dissertation in greater depth. This chapter covers the optical properties of materials, the physical quantities describing spectral radiance and the emissivity/absorptivity of blackbody and non-blackbody surfaces, as well as the modeling techniques used in the optimization of nanophotonic designs for enhanced absorption or spectral selectivity. In chapter 2 and chapter 3 we focus on spectrally selective absorbers and photon radiators (emitters) for concentrated solar photothermal and thermophotovoltaic applications, respectively. For both applications, spectral tuning of the absorber surface is important. An ideal solar absorber operating at elevated temperature has high absorptivity over the solar spectrum, while suppressing parasitic IR thermal emission from its surface. In chapter 2, we study the use of aperiodic metal-dielectric coatings both on planar as well as on nanostructured substrates, to achieve the desired spectral selectivity over a wide angular range. Based on our modeled results, our optimized aperiodic multilayer stacks have the potential to outperform current commercially available solar thermal coatings. Using a vacuum emissometer, an apparatus that is able to measure the spectral emission of coatings at elevated temperature, we experimentally demonstrate the excellent spectral selectivity of these aperiodic metal-dielectric solar-selective coatings for concentrated solar thermal applications. In chapter 3, we study the use of aperiodic metal-dielectric coatings as photon radiators to achieve high thermophotovoltaic energy conversion at practical temperatures of operation. In chapter 4 we review light trapping strategies for thin-film solar photovoltaic energy conversion. Thin-film solar photovoltaic devices have an enormous potential to reduce the cost of solar electricity. However, because thin photoactive layers are used, optical absorption is incomplete unless light trapping strategies are employed. Since conventional light trapping approaches, based on geometric scattering, are less effective in thin-film devices, coherent light trapping approaches that exploit the wave nature of light are reviewed that have the potential to significantly enhance optical absorption over a broad spectral range. In chapter 5 and chapter 6 we focus on thin-film organic solar cells, a thin-film technology that has great potential to reduce the cost of solar energy conversion by the use of low-cost photoactive materials and the compatibility with high-throughput roll-to-roll manufacturing. In organic photovoltaic cells, the optical absorption is incomplete unless light trapping strategies are employed. As will be shown, it is important to both spectrally and spatially tune the absorption inside the photoactive region to optimize the exciton creation and to minimize parasitic losses. Multilayer metal-dielectric stacks are investigated to enhance absorption efficiency in organic solar cells without sacrificing charge carrier collection efficiency. We have experimentally shown that these multilayer stacks can at the same time serve as a transparent contact as well as enhance photon harvesting, resulting in improved power conversion efficiency. This will be demonstrated on glass substrates in chapter 5, as well as using a roll-to-roll process on flexible substrates in chapter 6.