In SUBSTAINABLE, we develop multifunctional biomaterial composite substrates to increase efficiency and durability of photovoltaic devices. The optical tunability of these films, including their UV-light down conversion and filtration capacity, will be realized by doping and covalently modifying cellulosics. Our target is the 25% increase of the photocurrent (from the standard glass based device). Hindering the degradation by UV light and moisture is expected to extend the device lifetime, which will enhance the commercialization potential of these cellulose films. In addition to better performance, cellulose-based substrates may, e.g., reduce the embedded energy of photovoltaic devices leading to decreased energy payback time. The cellulosic materials allow light-weight devices, flexibilitiy in functionalization, roll-to-roll production, and easy disposal. These solutions will enable a new high value solar energy product family for the Nordic forestry industry.