Research projects

In the SuGaR project we conduct fundamental reach to develop a new generation of highly efficient solar cells. The operation of a solar cell is based on electrons in a semiconductor material absorbing the energy of solar photons. However, in conventional devices electrons lose a great part of that energy when they repeatedly collide with the atoms that form the semiconductor material. In that process electrons quickly go from being “hot” to being in thermal equilibrium with the atomic lattice. If we could develop a so-called hot carrier solar cell, that is, a cell where electrons do not get cold before extracting them, the efficiency limit would rise from 41% to 85%. The objective of the SuGaR project is to produce a prototype of hot carrier cell using graphene and other two-dimensional materials. An additional advantage of two-dimensional materials is that they allow us fabricate flexible devices that are ultrathin and ultralight.

The first milestone in the project has been the development of a conventional solar cell structure (p-n junction) using two-dimensional semiconductors. We have produced solar cells composed of a MoS2 homojunction reaching 1 V open-circuit voltage under illumination equivalent to 40 times the solar irradiance. Then, we have improved the electrical contacts and front reflectivity of the solar cell to reach a solar energy conversion efficiency of 4% in a device with thickness 130 nm, which is a breakthrough in the development of ultrathin solar cells. Finally, we have worked on the development of a hot carrier solar cell based on the previous results, by inserting in the structure a graphene absorber and optimizing the energy selective contacts. We have developed a theoretical model to understand how this hot carrier structure can be optimized to reach efficiencies higher than those achieved by conventional p-n junctions.