Natural Photosynthetic
Carotenoids for Solution-Processed
Organic Bulk-Heterojunction Solar Cells
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Abstract
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-C<sub>61</sub>-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<sup>–2</sup> cm<sup>2</sup>/(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<sup>–5</sup> cm<sup>2</sup>/(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-C<sub>60</sub> bisadduct improved the
overall conversion efficiency of lycopene-based OSCs by enhancing
the open-circuit current (<i>V</i><sub>oc</sub>). Interestingly,
further investigation on charge-separation dynamics revealed that
photocurrent is generated only from the S<sub>2</sub> (1B<sub>u</sub><sup>+</sup>) state, and the others underwent ultrafast excitation
relaxation through S<sub>2</sub> → S<sub>1</sub> (2A<sub>g</sub><sup>–</sup>) → S<sub>0</sub> (ground state), leaving
much room for further improvement