A ray tracing investigation of light trapping due to grooves in solar cells

Abstract

The biggest problem the world faces today is finding a renewable energy source as fossil fuel reserves being depleted, and the ongoing burning of fossil fuels is destroying environments all over the world. Solar energy is the most abundant energy source but is too expensive compete with non-renewable sources. A way to increase the efficiency of solar cells is to texture the cell surface so that it traps light better, allowing more light to be absorbed and converted to electrical energy. Professor Sachs and Dr. James Bredt have developed a texturing scheme that consists of trenches etched on the top surface of the silicon cell. The profile of the trenches will either be a simple semicircle or a parabola with a set aspect ratio. Our objective was to determine the optimal cross-sectional shape by using Zemax, a ray-tracing program that models light striking the cell. Solid Models of the solar cell with different texturing schemes were created, and then imported to Zemax where optical properties were modeled. Using a detector to measure how many times a ray struck the bottom surface of the cell, we were able to determine that a parabola with a unit aspect ratio was the optimal trench cross-section. The average number of detector hits for the unit aspect parabola was 3.68 ± 0.11 as oppose to 1 detector hit with no texture.

(cont.) Another objective was to determine how light behaves when it strikes the cell at an oblique angle parallel to the trenches. Using Zemax again, we varied the angle of incidence and measured the number of times a ray struck the bottom detector. Up to an angle of incidence of 300, the number of detector hits remains constant at 3.68 ± 0.05. After that however, the number of hits increases as the angle of incidence increases. Although this was not predicted, there are many explanations for it including the fact that the model cell is much shorter in width than the actual cell. Overall, the parabolic trench with unit aspect ratio should be used to better trap light in solar cells, and therefore, increase their overall efficiency.