Biomimetic Dye Molecules for Solar Cells

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Pressing energy problems provide opportunities for solid-state physicists and chemists to solve a major challenge: solar cell adoption. Though solar cells can use energy directly from the Sun to produce electricity that can be converted efficiently into other kinds of energy, they are currently too costly to compete with traditional (polluting) energy sources. The most cost-effective solar cells are not high-end, high-efficiency single-crystal devices, but rather low-end cells based on organic molecules or conducting polymers. Vital information for making organic solar cells more competitive for widespread implementation was obtained using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy performed at ALS Beamline 8.0.1. The relevant energy levels of biomimetic dye molecules were mapped out systematically by determining their unoccupied molecular orbitals and their orientation. Organic molecules in dye-sensitized solar cells exhibit great potential to increase the efficiency and reduce the cost of photovoltaic power generation by allowing a wide variety of chemical modifications and combinations with inorganic nanocrystals.

The "Chemla Loop"

A key challenge to the widespread adoption of solar cells is cost. To reduce cost and encourage implementation, each step in the conversion of the Sun’s photons into usable electricity needs to be analyzed systematically for weaknesses. Bottleneck steps must be identified and fixed before they can be incorporated for use in mass-produced solar cells. This research represents a first step towards establishing a feedback loop to do this, beginning with chemical synthesis, spectroscopic analysis, theoretical prediction, and back to synthesis. In ALS circles, this concept is known as the Chemla loop (after former ALS director Daniel Chemla). A Chemla loop began with this research by spectroscopically measuring the relevant energy levels in organic dye molecules.

Dye molecules have been studied here as a vehicle for solar cells because they are very colorful, sturdy, and absorb sunlight strongly. This is exactly what one needs to intercept sunlight and convert it into electrical energy in a solar cell. The class of dye molecules used in this research is related to the dye that gives blue jeans their color. By choosing organic molecules, an enormous repertoire of possible chemical variations can be used to optimize the performance of dye molecules for solar cells.

Researchers investigated the x-ray absorption spectra of two classes of candidate dye molecules for dye-sensitized solar cells: phthalocyanines and porphyrins. These cloverleaf shaped molecules have a metal atom at the center, surrounded by a ring of nitrogen atoms (in porphyrins, two rings in phthalocyanines). In each, the metal atom can be exchanged for other atoms among the transition metals. But, how does this substitution affect the energy levels of the molecule? Can these molecules be tailored to optimize their performance in solar cells?

The NEXAFS spectrum of the iron atom (red) in cytochrome c, a biomolecule with a characteristic heme structure involved in many charge transfer reactions. X-ray absorption spectroscopy is highly selective, and can distinguish energy levels of this single iron atom from amongst hundreds of hydrocarbons. (N is blue, S is yellow, C is black).

The highest energy efficiency is currently obtained with dyes based on ruthenium, a rare and expensive metal. In nature, the chemically similar but much more abundant element iron facilitates electron transfer, for example in hemoglobin and cytochrome c. The active center of these dyes—the heme—is similar to that of porphyrin molecules, but it contains additional atoms that form a three-dimensional cage around the central iron atom. It would be advantageous to understand such biomolecules, what makes them so efficient, and how to mimic them with smaller, more robust molecules for use in organic solar cells.

In an organic solar cell, sunlight is absorbed by an optical transition between a molecule's highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). In the molecules discussed here, the nitrogen cage and the central transition metal atom form the HOMO and LUMO, whose close proximity and significant overlap facilitate electron transfer, which can be controlled by adjusting the orbital energies.

Relevant energy levels of a molecular solar cell. A photon from sunlight (green arrow) generates an electron–hole pair (exciton) in a dye molecule, with the electron in the LUMO (blue dot) and the hole in the HOMO (red dot). The electron is pulled away by an acceptor molecule towards the negative electrode (upper blue arrows). The hole is filled by an electron from a donor molecule, which in turn comes from the positive electrode (lower blue arrows). At least four molecular levels are available for optimization, compared to only the band gap for solid-state semiconductors. Spectroscopy reveals where these energy levels are. While a large energy drop between the levels facilitates the separation of electron and the hole, it also reduces the voltage output and thus the energy output.

The absorption of sunlight produces electrons and holes, which are ultimately responsible for producing electrical power at the solar cells' electrodes. Biomolecules are particularly good at separating electron–hole pairs produced by sunlight in a dye molecule and pulling them towards their destination with minimal energy loss, enabling optimal use of solar energy.

X-ray absorption spectroscopy provides much more information than just that about energy levels: it monitors oxidation states, and when coupled with polarized synchrotron radiation, it reveals the orientation of the molecular orbitals.

The multiplet structure of the metal spectra reveals the oxidation state of the metal atom in a dye molecule, which is crucial for electron transfer. In the course of this work, researchers found that iron and manganese (two important transition metals in biomolecules) easily change between +2 and +3 during the preparation of thin films such as those used in solar cells. This requires close monitoring to obtain reproducible solar cells.

The polarization dependence of the spectra reveals the orientation of the molecules, which also varies depending on film preparation. This is critical for optimizing electron transport from dye molecules to electrodes. Molecular conductivity is commonly poor, but can be mitigated by orienting the molecular orbitals of neighboring molecules such that they overlap optimally.

X-ray absorption spectra of the nitrogen atoms that surround the iron atom and form the lowest unoccupied molecular orbital (LUMO) of a phthalocyanine dye molecule. This molecule mimics the center of a heme. The polarization of the synchrotron light shows how the molecules are oriented, which is important for ensuring efficient movement of electrons from one molecule to another towards the collector electrode.

Research conducted by P.L. Cook, F.J. Himpsel and X. Liu (University of Wisconsin Madison); and W. Yang (Berkeley Lab).

Research funding: U.S. Department of Energy and the National Science Foundation. Operation of the ALS and SSRL are supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: P.L. Cook, X. Liu, W. Yang, and F.J. Himpsel, "X-ray absorption spectroscopy of biomimetic dye molecules for solar cells," J. Chem. Phys. 131, 194701 (2009).

ALSNews Vol. 308