Porphyrins and phthalocyanines and their variations have been rather popular as dye molecules for solar cells. 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 solar cell are the energy levels of its components (shown above). The most generic solar cell involves not only the light-absorbing dye, where an incoming photon creates an electron-hole pair. It also requires an electron donor which refills the hole created in the HOMO (highest occupied molecular orbital), plus an electron acceptor which extracts the extra electron from the LUMO (lowest unoccupied molecular orbital). These four energy levels can all be varied and optimized.
The excited electron lands at the negative electrode on the far left, travels around the external electrical circuit (not shown), and ends up at the positive electrode on the far right. The energy difference eV between the two electrodes determines the output voltage V of the solar cell. For an open circuit it is equal to the HOMO-LUMO gap minus the sum of all energy drops between adjacent levels. Minimizing the energy drop maximizes the output voltage, but it also slows the separation of electrons from holes. That causes them to recombine or get lost at defects, thereby reducing the output current. Increasing the energy drop maximizes the current by separating electrons and holes more quickly. For example, a typical dye-sensitized solar cell loses about half of its voltage due to a large energy drop on the donor side. That is needed to refill the hole at the HOMO quickly enough to prevent electrons diffusing from the LUMO to the acceptor from recombining with the hole. A delicate compromise needs to be found for optimizing the output power, i.e., the product of voltage and current.
Our goal is to determine these energy levels by spectroscopic techniques and to use this knowledge for a systematic optimization of the performance of solar cells. Organic molecules can be modified in many different ways to tailor their energy levels. Unoccupied energy levels are revealed by X-ray absorption spectroscopy, and occupied orbitals by photoelectron spectroscopy. With X-ray absorption spectroscopy one can single out specific atoms inside a molecule by selecting the absorption edge of the appropriate element (here the nitrogen 1s, iron 2p, and carbon 1s edges). Furthermore, one can select transitions into a specific molecular orbital, for example either the orbital of a pi absorber versus that of a donor group inside a donor-pi-acceptor complex, as shown below. Even finer details can be discovered, such as the effect of a missing central Zn atom on the nearby N atoms.
Above: Donor-pi-acceptor complexes combine the three components of a solar cell in one molecule with atomic precision. They have enabled new efficiency records for dye-sensitized solar cells (see Yella et al., Science 334, 629 (2011). X-ray absorption spectroscopy makes it possible to select individual orbitals located at specific atoms, such as the donor orbital (d) at the N atom of the donor group.
The figure above reveals the oxidation state of the iron atom at the center of a Fe-phthalocyanine molecule via the shape of the iron 2p absorption spectrum. A vacuum-deposited thin film exhibits the multiplet structure of the native Fe2+ oxidation state (top curve). As the sample is exposed to air, the spectrum becomes a mixture of Fe2+ and Fe3+ multiplets. Preparing a Fe-phthalocyanine film in air from an ethanol solution produces almost completely Fe3+ (within the probing depth of about 5 nm). This gives an example how X-ray absorption spectroscopy can help optimizing the fabrication of solar cells, in this case by making sure that the molecules have the right oxidation state.
This figure illustrates how one can use the polarization of synchrotron radiation to determine the orientation of organic dye molecules, here for a thin film of Fe-phthalocyanine film. 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.
There are many other materials to be explored for solar cells. For example, we have studied highly p-doped diamond films as an electron donor material. It complements the popular n-type titanium dioxide acceptor material. We have also developed a concept to use molecular complexes with more than one absorber molecule for better coverage of the solar spectrum. These molecules need to be stitched together with pi-bonded molecular wires, which are currently under investigation.
This research is part of a bigger picture. We are aiming at a feedback loop where our spectroscopic analysis of solar materials provides input for collaborators in theory, who explain the observed energy levels and predict promising materials. Those are then synthesized by collaborating synthetic chemists and tested for their electronic properties by collaborators familiar with the design of solar cells. From there the ball goes back to the spectroscopists.
Tutorial on Solar Energy
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