We present examples of strategies to enhance the efficiency and stability of organic and perovskite solar cells using sustainable materials, in particular forest biomass such as lignin, betulin and cellulose. We explore how biomass additives/fillers can tune film growth and hence bulk morphology, reduce trap-induced recombination losses and improve stability of both lead-halide perovskite [1, 2] and donor:acceptor bulk heterojunction [3] photoactive layers. We also demonstrate that biomass additives can induce a Fermi-level position gradient in lead-halide perovskite films, creating a p-n homojunction that enhances power conversion efficiency [4]. Finally we describe a binary cathode interface layer material for organic and perovskite solar cells utilizing industrial solvent fractionated LignoBoost kraft lignin [5]. The phenol moieties in kraft lignin form hydrogen bonds with common organic semiconductor materials like bathocuproine, enabling tuning of film work function while retaining sufficiently high electron transport properties. The binary cathode injection layers demonstrate high power conversion efficiency and stability, surpassing traditional materials systems in organic and perovskite solar cells, offering a sustainable method to achieve high-efficiency organic and perovskite photovoltaics.