Porphyrin dimers harvest more sunlight for next-generation solar cells

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

Figure 1. The structures of tetraphenylporphyrin (A) and chlorophyll (B). Ph: Phenyl. Mg: Magnesium. R: Side chain.

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 dioxide using a cyanoacrylic acid linker as shown in Figure 2. Surprisingly, the same amount of each dimer (or pair) could be attached to the surface even though they have quite different molecular volumes. Since the dimer dyes effectively now afford double the amount of absorption, light harvesting by the porphyrin dimer-sensitized thin photoanode is significantly improved. Using both conventional solar-cell-efficiency measurements and ultrafast-spectroscopy techniques, we demonstrated efficient charge generation by photoexciting both porphyrins in the dimers. Porphyrin dimers incorporated in thin (2.5μm), dye-sensitized solar cells resulted in a 20% increase in light-harvesting efficiency and a 10% increase in the incident photon-to-current conversion efficiency when compared to single-porphyrin dyes. Overall, 70–80% photon-to-electron conversion efficiency was obtained, clearly indicating that light absorption by both porphyrin chromophores within the dimer contribute to photocurrent generation. The result was a more efficient dye-sensitized solar cell, which shows promise for a number of new thin-film photovoltaic applications.

Figure 2. Structures of porphyrin dimers showing relative molecular volumes. Zn: Zinc.

The success of this dimer sensitization also opens up the possibility of developing next-generation solar-cell devices using aligned nanostructures with reduced surface areas and larger porosity, allowing faster redox shuttles (a necessary step in closing the photovoltaic cycle) as a result of improved charge transport. In addition, because solid-state, dye-sensitized solar cells based on single porphyrins have shown comparable efficiency to those of traditional ruthenium dye devices, solid-state organic photovoltaics should also benefit from the improved light-harvesting ability of the dimers.⁷

As a result of this demonstration of enhanced device functionality, we are investigating the usefulness of larger arrays—including mixed systems with multicolor components—in increasing both the light-harvesting efficiency of devices and their spectral coverage. The use of dimers in other energy-harvesting photo-electrochemical applications, such as water splitting and photocatalysis, is also being explored.

This work was performed at the Australian Research Council (ARC) Centre of Excellence for Electromaterials Science at the University of Wollongong, Australia, with significant collaboration from our partners at Shinshu University and the National Institute of Advanced Industrial Science and Technology, Tsukuba, in Japan as well as at the University of Otago, New Zealand. Financial support from ARC, the Australian Department of Industry, Innovation, Science, and Research International Science Linkages Program, and the MacDiarmid Institute for Advanced Materials and Nanotechnology is gratefully acknowledged.