Nannan Ding

Date of Award

9-15-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Ian Hosein

Keywords

energy harvest;polymer optics;solar energy;structure fabrication

Subject Categories

Chemical Engineering | Engineering

Abstract

The key to increasing the light capture is to maximize the light collection for both normal and non-normal incidence due to non-optimal sunlight over the course of day and seasons. Creation of waveguide lattices consisting of higher polymer component and possessing cylindrical geometry proves quite successful and effective material-based approach to realize management of solar energy. Inscription of periodic waveguide lattices into thin film, surrounded by low index common cladding form core-cladding architecture, establishing high-low index profile, and imparting angular collection range of waveguide to enable light collection and light guiding. Single vertically aligned waveguide lattices realized by our previous work show cylindrical geometry has collection window of ±30o (for blend of NOA 65 and PDMS) and confine optical incidence into waveguide cylindrical region, transmitting along the waveguide core toward the other side of thin films, effectively circumventing the shading loss originating from the metallic front contact, effect exacerbated at increasingly incident angle. The advance is required to explore widen the collection range to much higher angles, e.g., 50o, 60o and 70o. The established waveguide lattices loss the capability at such high incidence. The novelty of this research is to widen the collection range through embedding multi-directional waveguide lattice into encapsulate. Each waveguide lattices have its own collection range, and combination enables them to collect higher incidence. In other words, we define the boundaries of angular collection range, interval of incidence with lower and upper boundary. Widened angular collection range is achieved via rotating the low boundary originally aligned with normal waveguide core axis (0o position) to orientation angle of slant waveguide, thereby the upper boundaries were raised up to incidence with addition of slant angle. My research achieves ±25o waveguide lattice, inscribe into encapsulant in the form of intersecting waveguide lattice, and allows for the collection range of ±70o, which enable incident light to be captured and transmitted through waveguide lattices, resulting in more generation of electricity in silicon solar cell. The first part of my projects conducts the theoretical formation of multi-directional waveguide lattices in thin film, which was achieved by my simulation work in Chapter 2. This work designs the dimensions of simulation cell and intersecting optical beam arrays, which undergo self-focusing nonlinearity and become self-trapped intersecting beams. Each beam initiates the photopolymerization and its induced phase separation, thereby driving the binary blend to evolve into intersecting binary phases consisting of high index polymer in the region of optical beams, surrounded by the low index polymer. This work also examines the dependence of the formation of binary phase morphologies on the blend and processing parameters. (i.e., volume fraction of high-low index polymer, polymerization rate, polymer miscibility, refractive index difference and optical beam angles). With accordance to this theoretical realization of intersecting phase formation, Chapter 3 experimentally fabricated three-dimensional cylindrical shaped interesting waveguide lattices in binary photoreactive blend comprising of high index NOA 65 and low index PDMS and demonstrate the widened angular collection range of -70o to 70o and corresponding enhancement in conversion efficiency and electrical output, as opposed to single vertically aligned lattices. The waveguide lattices were optimized through a variety of parameters, including weight fraction and photo-initiator concentrations. It turns out that higher content of NOA 65 and photo-initiator CQ give greatest quality of waveguide lattice, which directly correlated with higher enhancement in external quantum efficiency (EQE) and solar electricity in silicon solar cell. Chapter 4 elucidates the underlying process that entailed multiple phenomena that, the self-trapping of intersecting optical beams, growth kinetics, optical nonlinearity enhancement in well-confined waveguide lattices involved in the formation of waveguide lattices (i.e., conversion of polymer, refractive index difference).We identified the interesting binary phase morphologies consisting of high index component in the intersecting region and PDMS in the surroundings, possessing optical functionality, which provide strong evidence of waveguide lattice with occurrence of phase separation during their formation and possess higher index in the waveguide region allowing for light collection and guiding functionalities towards encapsulant silicon solar cell. The formation of each waveguide lattice was achieved through the dynamic balance between photo-polymerization and phase separation over the duration of photo-irradiation, varying with optical intensities, leading to waveguide lattice properties as function of optical intensity. Chapter 5 expands the degree of parameters in waveguide lattices by employing the mask pattern and optical intensities. The range of waveguide lattices was created in terms of waveguide size, inter spacing in an array and also indicates the nature of parameter dependent waveguide quality. This works provides informative insight of influence of waveguide density on the phase separation of waveguide lattice, their cylindrical geometry, light confinement, and electrical generation in silicon solar cell. The light confinement property was affected by spatial arrangement of intersecting waveguides at their ends which were determined by mask pattern dependent cylindrical geometry. The possibility of well-defined waveguide lattice is achieved through modulation of mask aperture to optical profile, which in turn inscribes the same pattern of waveguide lattices with arrays of intersecting optical beams. We systematically investigate ±25o waveguide lattices on their synthesis approach, underlying process of their formation associated with enhancement in waveguide properties, ranges of waveguide cylindrical geometries, and optical intensities which finely tuned the waveguide quality. To further achieve enhancement in light collection, the novelty is needed, which is to integrate light scattering element into waveguide lattice. Chapter 6 realizes the enhancement via employing dye as light scattering component in thin film. This project integrates all three types of thin film based on the number of waveguide lattices (0, 1 and 2) with incorporation of dye into the waveguide lattices to study the effect of dye & its content on the enhancement in optical properties of waveguide lattices. Dye characterization confirms dye is concentrated into the region of irradiation, namely, within the waveguide lattices. The unique property of down-converting fluorescent dye can significantly enhance effective spectrum of light, redirecting light from dye emission towards the potential enhancement in solar electricity. This chapter confirms two intersecting waveguide lattices are best option for light collection and transmission in encapsulated silicon solar cell, due to its realization of ultra-wide angular collection capability via combination of multi-directional waveguide lattices in encapsulant and more inclusion of dye within the waveguide latices. There is a direct correlation between dye concentration and electrical enhancement. In other words, greater dye content leads to greater enhancement in solar electricity for all thin films. To gain in-depth understanding of dye and its concentration on waveguide lattices, Chapter 7 conduct Raman characterization of all thin films, which show function of waveguide number and dye content, and kinetic growth. The dye itself is diacrylate monomer and higher index component and will take part into the photochemical reaction and change dynamic evolution of processes during the irradiation owing to increases in refractive index in the region of irradiation, which in turn, will influence properties of waveguide structures in terms of phase separation, geometry, and its quality and consequently leads to differ among the optical functionalities. With established insight of 25o waveguide lattices, Chapter 8 presents 15o waveguide structure and investigates its respective characterization and the structure-property relations of this novel lattice. It elucidates the waveguide lattices geometry, spatial organization at the end of thin film and effect of optical beam orientation on solar electrical performance. 15o waveguide lattices exhibit opposite trends of phase separation and optical performance with 25o waveguide as parameters changes. Chapter 9 concludes this dissertation and provides insight onto the potential study related to multi-directional waveguide lattices towards improved waveguide quality and enhancement in solar electrical performance.

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