Photon management in thermal and solar photovoltaics

Abstract

Photovoltaics is a technology that directly converts photon energy into electrical energy. Depending on the photon source, photovoltaic systems can be categorized into two groups: solar photovoltaics (PV) and thermophotovoltaics (TPV). In solar photovoltaic systems, the photon source is the sun, whereas in thermophotovoltaic systems the photons are from artificially designed thermal emitters that operate at a lower temperature. The differences in the photon sources lead to different research emphases on the two photovoltaic systems in this work. This thesis investigates ways to control photon emission and absorption for solar energy and TPV applications. Several topics are discussed, including photon transport in multilayer structures, measurement of near-field thermal radiation, optical absorption in silicon nanowire structures, surface-plasmon enhanced near-bandgap optical absorption in silicon, and selective absorber surface for solar thermal applications. For thermophotovoltaic systems, the work is focused on thermal emission and photon transport. The study of photon transport in multilayer structures is presented. Results based on wave-optics and ray tracing methods are compared. The analysis shows that for structures contain a large number of layers, the coherence length of the emitting source is no longer a valid criterion to indicate whether ray tracing method is valid. Instead, wave inference effects always play a role. The effects of photon localization are also discussed. Surface-mode enhanced near-field thermal radiation is explored in this work as an effective way to tailor the thermal emission for TPV systems. Calculations based on fluctuation-dissipation theorem and Maxwell's equations are presented to study radiative heat transfer between two closely-spaced glass plates. The theoretical analysis shows that the radiative heat transfer between closely-spaced glass plates is enhanced by surface phonon polaritions and the flux can exceed the far-field upper-limit imposed by Planck's law of blackbody radiation.

(cont.) An experimental system was built to test near-field radiative heat transfer between two parallel glass plates, and the experimental results show good agreement with the theoretical predictions. For solar photovoltaics, the emphasis in this work is on improving optical absorption in silicon-based cells. Two nanostructures, silicon nanowire arrays and silicon embedded with small silver particles, have been analyzed as potential candidates for solar energy harvesting. The study on silicon nanowire structures reveals that nanowires have desirable antireflection characteristics. Several parameters, such as the length and diameter of the nanowires as well as the spacing between the wires, have been studied to provide the basis for the optimization of nanowire based solar cells. The study shows that nanowire structures have low reflectance over a broad spectrum and can absorb shortwavelength photons efficiently. However, the analysis also indicates that silicon nanowire is not efficient in absorbing long-wavelength photons. Longer wires in comparison to the thickness of dense films are generally required to compensate low absorption of the near-bandgap photons. The analysis of surface-plasmon assisted photon absorption is presented to address the problem of inadequate absorption of near-bandgap photons in silicon. Instead of increasing the optical path of photons for more absorption, surface plasmons are explored to enhance the local electromagnetic field and thus the optical absorption. An extended Mie scattering formulation is used to calculate the optical absorption around spherical silver particles embedded in silicon. It is found that local field enhancement by surface plasmon can lead to 50 times more absorption near the bandgap of silicon. An analytical model is developed to study the concentration effects of the surface plasmon field. It is shown that the net absorption gain reaches maximum when the spherical shell surrounding the particle has an outer diameter of 1.26 times of the particle diameter. The absorption loss in the metallic sphere, however, is a main obstacle to overcome.

(cont.) Finally, a different approach of solar energy utilization is discussed in this work. Selective absorber surfaces are studied for solar thermal energy harvesting. The surfaces consist of subwavelength periodic metallic structures. Finite-Difference-Time-Domain (FDTD) analysis is conducted on the metallic structures. The effects of lattice spacing and structure thickness are presented. The numerical simulation indicates that the metallic structures have good spectral selectivity: high absorptance in visible range and low emittance in infrared. Fabrication of the selective absorber surface is attempted. Preliminary experimental results are given in this work. As a proof of concept, nickel is plated in porous anodic aluminum. The resultant structure shows good spectral selectivity which is not found in bulk nickel or aluminum.