Producing enough solar power for the energy transition is a challenge, especially in a small country such as Switzerland. More efficient solar cells, meaning cells that can convert a greater proportion of incident sunlight into electricity, could make a decisive contribution to this transition. It pays to invest in efficiency: prices for solar cells have fallen steadily over the past few years and now account for only a small part of the costs of a photovoltaic system. A gain in efficiency also translates into a gain in electricity yield despite identical or only slightly higher costs. Against this background, researchers at EPFL and EMPA are jointly developing the next generation of solar cells. In doing so, they hope to contribute to the continuation of the strong upward trend in the efficiency of solar cells.
To achieve this objective, the scientists decided not only to improve existing technology, but also to innovate. Due to the incomplete use of high-energy photons, the efficiency of conventional solar cells is close to the theoretical limit and can only be slightly increased. The amount of electricity these cells harvest from a single photon is fixed. It is equal to the energy a photon requires to overcome the so-called bandgap. Materials with a small bandgap make use of a large portion of the light spectrum, but the excess energy of high-energy photons in the yellow to ultraviolet spectral range is lost. Materials with a wide bandgap have a better yield in this range, but the photons in the red and infrared range are too weak to overcome the bandgap and thus do not yield electricity.
In solid-state physics, the bandgap is an energy range that cannot be occupied by electrons. The bandgap is a material characteristic of the semiconductors used for solar cells.