Efficient Ultrathin Organic Solar Cells with Sustainable β-Carotene as Electron Donor

beta-carotene (bCar) is an abundant natural organic semiconductor that can be extracted from tomatoes or carrots at extremely low costs. Using natural bCar as electron donor combined with a C-70 derivative (PC71BM) as electron acceptor in bulk heterojunction active layers, we successfully fabricated efficient inverted organic solar cells (OSCs) processed in air without encapsulation. Unlike conventional OSCs produced with synthetic materials, higher short-circuit current densities are achieved in ultrathin active layers (similar to 30 nm) compared to thicker ones (similar to 90 nm). This peculiar behavior can be ascribed to the low hole transport properties of bCar that limit the charge collection efficiency in 90 nm thick bCar:fullerene OSCs. Our results demonstrate that higher boiling point solvents induce crystalline transformation of bCar in thin active layers resulting in OSCs with fill factors around 35% and average power conversion efficiencies (PCEs) of 0.58%. These devices demonstrate stable operation under constant illumination and are the best performing bCar-based OSCs published to date. They exhibit a 4-fold increase in PCE compared to previously reported bCar:fullerene OSCs, thus opening the path to low-cost yet efficient bCar photovoltaic device fabrication.