What Do Holiday Lights and Solar Panels Have in Common?

Fatima Toor

University of Iowa, USA

Semiconductor light-emitting diodes (LEDs), which make up both holiday lights and solar photovoltaic panels, have p–n junctions.

Recently, in my semiconductor devices lecture, a junior/senior-level, undergraduate electrical engineering course I teach every spring, I started the lecture by asking students what holiday lights and solar panels have in common? The answer was p–n junction diodes. We were starting to study a chapter on semiconductor junctions, namely, p–n junctions, which make up both LEDs and solar cells. In a p–n junction device, a p-doped semiconductor is put in contact with an n-doped semiconductor. At the metallurgical p–n junction, diffusion of carriers occurs, and a built-in electric field is induced. This built-in electric field can then be manipulated by external stimuli to the p–n junction, such as electric potential (in LEDs) and sunlight photons (in solar cells). I think the question at the beginning of the lecture definitely piqued my students’ interest, and once I shared the answer, I overheard a few ah-ha’s from the students.

My favorite part of teaching is connecting hard sciences to items we use in our daily life. My favorite teachers were also those who could explain to me the science behind the things around us. While I really enjoyed the technical rigor of an engineering educational career because it was intellectually challenging, to me the fact that engineering was really applied science made it fun—especially the science of light, which helped me understand why the sky is blue (Rayleigh scattering of blue light), why we see colors (reflection), how cosmetics are designed using optical science (diffused scattering), and how ancient architects developed color in glass, also known as stained glass, using metal nanoparticles (plasmonics).

Interior of St. Vitus Cathedral, Prague, Czech Republic, where sunlight shines through stained-glass windows loaded with plasmonic metal nanoparticles.

One of my favorite professors of nonlinear optics in graduate school at Princeton always taught some of the most complex principles of optics via simple, everyday examples, which made me enjoy his lectures a lot. For my PhD qualifying exam, the optics questions, designed by said professor, were in fact on (i) explaining the science behind the famous Lycurgus cup (localized surface plasmon resonance excitation), and (ii) the reason for the reddish color of the sky at sunset (Rayleigh scattering). I hope that I can inspire optical science research and exploration in my students by making connections to everyday life examples as I teach them about unfamiliar concepts.