The transition towards a society based on 100% renewable energy requires massive deployment of photovoltaics of 30-70 TW until 2050. This requires huge amounts of resources, while their limited availability is already becoming apparent. A major lever to reduce resource consumption is to increase the solar cell efficiency. As best single junction solar cells approach their fundamental limits, higher efficiency can only be reached with so-called tandem solar cells, made of two or more subcells. All tandem technologies so far are based on relatively thick absorber layers, reducing resource demand compared to single junction devices by efficiency increase. There, light trapping strategies are used to maximize absorptance close to the band gap of the materials and improve efficiency by few percent relative. However, by applying advanced light trapping techniques such as nanophotonic metasurfac-es, ultrathin single junction devices with a 5-10-fold decrease in semiconductor material were realized. To reduce resource demand further, the concept of ultrathin solar cells must be extended to tandem devices. This introduces severe challenges, as not only absorption needs to be maximized within the active part, but a spectrally dependent light guiding strategy is required. Metasurfaces have shown the ability to manipulate light e.g. spectrally dependent; however, they have never been implemented into tandem solar cells. Thus, the overarching goal of PHASE is to generate a deep physical understanding of metasurfaces for ultrathin tandem solar cells and to develop process flows to implement nanopho-tonic structures into such devices with efficiencies above 30%. This will proof that the chosen tech-nology pathway can support the urgently needed energy transition. More specific, the goal of PHASE is to realize tandem solar cells, where the resource demanding semiconductor part is 10 times thinner (and thus needs 10 times less semiconductor material) than similar existing devices.