WHAM-Gap

The WHAM-Gap

How does a solar cell turn light into electricity? Sunlight arrives as photons, and when a photon hits a semiconductor it can give its energy to an electron. But the electron only gets freed if the photon carries at least a certain minimum energy, called the band gap. This is why the choice of material matters for a solar cell, and why combining several materials can yield better results than a single one.

The WHAM-Gap is a 3D printed teaching model that shows how solar cells convert light into electricity. We built it at the Scienteens Lab (University of Luxembourg) for a science outreach event, the Science Festival, and have been using it ever since during our courses.

The concept

A spring-loaded launcher plays the photon (the plunger) and sends an electron (the marble) up a column. Each level of the column represents a different semiconductor material, visualised as a bridge: Silicon, Gallium Arsenide, Top layer tandem cell, CIGS, Gallium Nitride and Aluminium Nitride. The height of the column is a visual representation of the energy the electron needs to cross the band gap of a given material. If the electron has enough energy, it crosses the bridge and falls back down, which represents electricity production. It then returns to the initial position, ready for the next electron to be launched!

The colours are part of the story too. The black base is space, the yellow launcher is the sun, and the bridges follow the visible light spectrum for the wavelength of each band-gap.

Documentation

Everything you need to build it is in the attached files:

• Assembly guide: parts list, printing recommendations, step-by-step assembly, optional extras

• Science of the WHAM-Gap: the explanation behind the model, for using it in a demonstration

• Material List: All the necessary materials for the project

• Handout: A handout with explanations for the Model

Before you start

• It is a large print. We used a Prusa XL, and an MK3.9 only works if you cut some corners and adapt the base. We will do our best to release an adapted version for smaller print beds.

• Most of the materials used are PLA and PETG, some PC or stronger materials is recommended for some of the parts
  

The model has some impractical aspects. It came about because of interest and curiosity rather than because we had the capabilities for it. The project gave us a lot of experience working on something far more ambitious than what we are used to. While the execution is not perfect, it ticks the boxes of what we wanted to achieve, it has already seen a lot of use, and it might be useful to other people. It is the result of people from the Physics & Sustainability and Biology teams of the Scienteens Lab pitching in, and a lot of stubbornness to see it through. The main contributors to the model are Olivier Elixander and Sébastien Elixander.