Porphyrin dimers harvest more sunlight for next-generation solar cells

Abstract

Emulating photosynthesis—the process by which plants convert sunlight into the chemical energy that we and other living organisms call food—represents one of the great scientific challenges or ‘Holy Grails’ of the 21st century. Solar-energy collection by photosynthetic structures, and its conduction to reaction centers, is the foundation of almost all energy transfer and generation on the planet. At the heart of this incredible process are the chlorophyll antennae in plants and green algae, which contain from two to 300 closely spaced photon-absorbing molecules embedded in a matrix of proteins and fats.1, 2 Synthetic porphyrin molecules such as tetraphenylporphyrin— see Figure 1(A)—have optical properties similar to those of chlorophyll—see Figure 1(B)—but are much more easily manipulated in the laboratory.3 Dye-sensitized solar cells, in which a photosensitive pigment or dye is attached to a large-surface-area inorganic semiconductor (typically titanium dioxide), have been likened to photosynthetic structures.4 In contrast to photosynthesis, however, light harvesting in these solar cells relies on the photoactivity of a single dye layer, which makes the process 2D rather than 3D. Because light absorption in the monolayer is very small, the cells must use a rather thick (up to 20μm) electrode with a huge internal surface area. Significant research has been invested in creating multichromophoric dye arrays to get beyond this approach.5 But, to date, introducing the arrays into photovoltaic devices has yielded no major improvement.3 Here we describe a significant step toward emulating natural 3D light harvesting.6 We covalently bound two porphyrins in either a linear or angular fashion and attached them to titanium Figure 1. The structures of tetraphenylporphyrin (A) and chlorophyll (B). Ph: Phenyl. Mg: Magnesium. R: Side chain.