Porphyrins and phthalocyanines and their many variations have been rather popular as dye molecules for solar cells. For example, a porphyrin molecule is featured on the cover of a DOE study on basic research needs for solar energy utilization. These molecules mimic the active center of biomolecules, such as chlorophyll, hemoglobin, and cytochrome c. The latter is a small protein that plays a role in biological charge transfer processes, a very useful property when it comes to separating electrons and holes in a solar cell. These biomolecules have been optimized by nature for billions of years. We are trying to take advantage of their high performance and to replicate their capabilities in smaller, more robust molecules.
The key to the performance of a molecule in a solar cell are the energy levels of the molecular orbitals. Unoccupied orbitals are revealed by NEXAFS spectroscopy, and occupied orbitals by photoelectron spectroscopy. In NEXAFS, one can select a specific atom inside a molecule by selecting the appropriate absorption edge (here the carbon 1s, nitrogen 1s, and iron 2p edges). A more detailed look at this method and its application to biomolecules can be found here.
The oxidation state of the iron atom at the center of a Fe-phthalocyanine molecule is revealed by the multiplet structure of the NEXAFS spectra shown above. A vacuum-deposited thin film exhibits the native Fe2+ oxidation state (top). As the sample is exposed to air, the spectrum becomes a mixture of Fe2+ and Fe3+ multiplets. A dried ethanol solution of commercial Fe-phthalocyanine powder consists almost completely of Fe3+ within the probing depth of about 5 nm (bottom). This gives an example how NEXAFS spectroscopy might be able to help optimizing the fabrication of solar cells, in this case by making sure that the molecules have the right oxidation state.
The polarization dependence of the pi* and sigma* orbitals reveals the orientation of the molecules in a Fe-phthalocyanine film (above). The strong maximum of the pi* intensity at normal incidence (0 degrees) indicates that the molecules are well oriented and that the molecular plane is nearly perpendicular to the surface. Having all molecules oriented in the right direction optimizes the overlap of their orbitals and thus the mobility of the electrons. The observed perpendicular orientation would be good for lateral electron flow (such as in a field effect transistor), but not for a thin film solar cell (where electrons are collected perpendicular to the thin film). One needs to either find growth techniques that give parallel orientation or use nano-structured electrodes that collect electrons laterally.
The effect of radiation on the N 1s NEXAFS spectrum of the biomolecule cytochrome c can clearly be seen in the figure above. Pristine cytochrome c shows a strong pi* peak (c) which can be assigned to the peptide bonds that link the amino acids and thereby form the backbone of the protein. This feature shrinks with irradiation, indicating that peptide bonds are broken. Two new pi* peaks (a,b) appear, which represent the product of the photochemical reaction. Such radiation-induced bond breaking suggests that complex protein structures should be replaced by simpler molecules which contain only the active heme region (shown in blue, with an iron atom inside a nitrogen cage). Most of the radiation damage occurs in the surrounding protein (green).
Further reading:
P. L. Cook et al., X-ray absorption spectroscopy of biomimetic dye molecules for solar cells , J. Chem. Phys. 131, 194701 (2009).
P. L. Cook et al., Radiation Damage in Biomimetic Dye Molecules for Solar Cells , J. Chem. Phys., in press (2009).