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Efficient Ultrathin Organic Solar Cells with Sustainable β‑Carotene as Electron Donor
journal contributionposted on 2019-01-17, 00:00 authored by Varun Vohra, Takayuki Uchiyama, Shusei Inaba, Yoshiko Okada-Shudo
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.