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

β-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 C₇₀ 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 (∼30 nm) compared to thicker ones (∼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.