  We analyze a theoretical model for energy and electron transfer in an
artificial photosynthetic system. The photosystem consists of a molecular triad
(i.e., with a donor, a photosensitive unit, and an acceptor) coupled to four
accessory light-harvesting antennas pigments. The excitation energy transfer
from the antennas to the artificial reaction center (the molecular triad) is
here described by the F\"{o}rster mechanism. We consider two different kinds of
arrangements of the accessory light-harvesting pigments around the reaction
center. The first arrangement allows direct excitation transfer to the reaction
center from all the surrounding pigments. The second configuration transmits
energy via a cascade mechanism along a chain of light-harvesting chromophores,
where only one chromophore is connected to the reaction center. At first sight,
it would appear that the star-shaped configuration, with all the antennas
directly coupled to the photosensitive center, would be more efficient.
However, we show that the artificial photosynthetic system using the cascade
energy transfer absorbs photons in a broader wavelength range and converts
their energy into electricity with a higher efficiency than the system based on
direct couplings between all the antenna chromophores and the reaction center.
