I recently entered and won a competition to design and fabricate solar cells. First place was given to the team which produced the highest efficiencies. Our team manufactured 18% efficient single junction silicon solar cells (I know, I know, that sounds low, but this efficiency meets today’s industry standard!).
It was a year-long project spanning over my final two semesters at VCU. I had the opportunity to work with Samantha Hoang, Anthony Pedicini, and Sam McDonald; we were a team of electrical engineers and physicists. Congrats guys, and thanks for a great year!
We used a potassium hydroxide – isopropyl alcohol (KOH-IPA) solution in water to randomly etch the smooth and highly reflective silicon surface. This created a variety of texturing patterns, as is seen below, depending on the orientation of the silicon crystal with respect to the wafer surface. After rigorous experimentation, we finally optimized the solution mixture ratio and temperature. The interesting blue figures below show results from four different texturing experiments. The bottom right figure shows the randomized pyramid texturing pattern used for the final device wafers.
Our final texturing method created random pyramids ~1-10 um wide.
The figure below is a profile measurement of the final device cells (left figures are 30 minute texturing, right figures are 60 minute texturing). The benefits of light trapping are clear upon inspection of the figure. With a flat surface, the light would reflect immediately away from the device; however, a textured surface would allow light to bounce toward the device and be reused. The tall protrusions are where the metal contact would later be installed.
We also experimented with an anti-reflective coating (ARC). We thermally grew a silicon dioxide ARC on the surface of the cells at 1000 oC in a dry oxygen ambient. The idea was to grow ~125 nm thin-film (1/4 wave length) that would reduce the reflection coefficient (i.e. increase optical absorption) at the surface of the polished crystal. The final results for using ARC were similar to the results from the texturing experiments.