Natural Photosynthetic Carotenoids for Solution-Processed Organic Bulk-Heterojunction Solar Cells

In this work, we demonstrate utilization of natural carotenoids (Cars), namely, fucoxanthin, β-carotene, and lycopene, as electron-donor molecules together with the electron-acceptor fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) in organic solar cells (OSCs). Unlike fucoxanthin and β-carotene, which form amorphous films, lycopene readily forms aggregates through a simple spin coating process. A high carrier mobility of up to 2.1 × 10^–2 cm²/(V s) was observed for lycopene, which is three orders of magnitude greater than those of fucoxanthin and β-carotene, with values of (8.1 and 1.8) × 10^–5 cm²/(V s), respectively. OSCs with different Car:PCBM blend ratios were optimized for these Cars. The highest photovoltaic performance was obtained for lycopene with a blend ratio of 1:1, at which the film morphology and charge transport were optimized. Replacement of the acceptor molecule PCBM with a high-lowest-unoccupied-molecular-orbital fullerene derivative indene-C₆₀ bisadduct improved the overall conversion efficiency of lycopene-based OSCs by enhancing the open-circuit current (<i>V</i>_oc). Interestingly, further investigation on charge-separation dynamics revealed that photocurrent is generated only from the S₂ (1B_u⁺) state, and the others underwent ultrafast excitation relaxation through S₂ → S₁ (2A_g^–) → S₀ (ground state), leaving much room for further improvement