Date of Award

May 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Ian D. Hosein

Keywords

encapsulation of solar cells, light-induced self-writing, polymer, shading loss, total internal reflection, waveguide

Subject Categories

Engineering

Abstract

Solar cells are the mainstream of renewable energy, since solar energy is abundant and accessible to most places on Earth. However, it has drawbacks, such as low energy conversion commonly less than 20 %, the power supply is intermittent and not stable. In industry, the energy conversion efficiency versus cost is the first priority to promote. Thus, implementing low cost but effective technique is attractive to industry. Based on geometric and wave optics, two simple-integrated and scalable techniques to improve the energy conversion of solar cells are developed. The first method is to make a v-groove structure on the encapsulant, which is able to guide light to the absorption surface, rather than impinging on the metal contacts and being wasted. I have simulated the performance of isosceles triangles (IT), right triangles (RT), and metal-coated isosceles triangles (MC). Three figures of merit are introduced to determine the optimized design for the seasonal and daily trajectory of sun scenarios. The optimized apex angle is 54 degree with IT structure for the scenario 1, and 15 degree with IT structure is the best for scenario 2 and 3. RT and MC do not outperform the IT; however, MC has some values for broadening the angular range. To fabricate the v-groove structure with an apex angle of 45 degree, I developed a molding process, and a wet-coating method to coat silver on the facets of v-groove. They are encapsulated on a solar cell and their EQE and J-V curves are measured. It carries out a gain of 4.3 % for the short circuit current; that means 68 % of the shading loss is recovered, and the cost is down by 5.6 % because of the saved area. However, the efficiency is enhanced in the limited incidence range of 40 degree.

The v-groove structure is limited to the incidence range of 40 degree. In order to expand the incident angles. I proposed a polymer waveguide array, which is able to bend the light from a large incident angle to the normal direction. I adopted photolithography with a blue LED to polymerize the waveguides. This is a high-speed, low-cost process to fabricate large area waveguides. The photopolymerization with oxygen inhibition is understood. The relation among pillar height v.s. light intensity and film thickness is derived from a kinetic model and fitted with the empirical data well. This relation is used to configure work parameters for growing desirable waveguides without doing a blanking search.

A waveguide array with a diameter of 40 um, height of 466 um and spacing of 200 um is fabricated for EQE and J-V measurement. The gain of EQE and energy conversion efficiency is 1.61 % and 1.71 %, The gain of 1.71 % implies the cost is down by 1.7 % owning to the saving area.

In conclusion, I have developed two encapsulation techniques for three application scenarios. For the incidence range smaller than 40 degree, a v-groove encapsulant is an option. For the incidence range more than 40 degree, a waveguide encapsulant is more suitable. The results reveal a new structure-property-performance relationship, whereby waveguide arrays opens a novel optical application on capturing light with a large incident angle. Therefore, the efficiency enhancement is no more limited to the normal incidence, but broaden to all angle of incidence. It indicates the absorption of diffuse light is improved as well. Furthermore, the synthesis of polymer waveguides with LISW utilized the non-linear optics to confine the wave propagation in photoreactive polymer and obtained a high aspect ratio of 8.6 for the waveguide. This is a scalable approach to create functional optical properties of a surface.

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