The Third European Workshop on Optics for Solar Energy will be held on 05-06 October 2021. We aim to organize the workshop as a hybrid event, where the speakers can present on site in Berlin or online. In case the pandemic situation does not allow an on-site event in October, the workshop will be fully virtual. The workshop is organized by researchers from Helmholtz-Zentrum Berlin für Energie und Materialien and Zuse Institute Berlin, within the framework of the Helmholtz Excellence Network SolarMath. Current trends and developments in the field of optics and nanophotonics for solar energy conversion devices will be discussed. The workshop will be organized as part of Photonic Days Berlin Brandenburg 2021.