Materials that absorb all light in the right wavelength range and transfer the electronic excitation energy via fluorescence-resonance energy transfer (FRET) to well-organized acceptors offer unique potential for developing dye-sensitized solar cells, luminescent solar concentrators (LSCs), and color-changing media (used, for example, in sensing devices). We have succeeded in producing artificial photonic-antenna systems by incorporating dyes into a nanoporous material.1,2,3 The material we chose was zeolite L, as it has proven an ideal host. Its crystals are cylindrically shaped porous aluminosilicates featuring hexagonal symmetry. The size and aspect ratio of the crystallites can be tuned over a wide range.
A nanometer-sized zeolite-L crystal consists of many thousands of 1D channels oriented parallel to the cylinder axis. These can be filled with suitable ‘guest’ molecules (see Figure 1). Geometrical constraints imposed by the host structure lead to supramolecular organization of the guests. Thus, very high concentrations of non- or only very weakly interacting dye molecules can be reached. The channel openings are plugged with a second type of fluorescent dye, called stopcock. The two types of molecules are precisely tuned to each other. The stopcocks accept excitation energy from the dyes in the channels, but cannot pass it back.
Schematic overview of an artificial photonic antenna.1,3
The chromophores are embedded in the channels of the host material. The green dyes act as donor molecules that absorb the incoming light and transport the electronic excitation energy via fluorescence-resonance energy transfer4
to the red acceptors shown at the ends of the channels on the right. The process can be analyzed by measuring the emission of the red acceptors and comparing it with that of the donors. (right) Top view of a bunch of such strictly parallel channels.
The supramolecular organization of dyes in the zeolite allows light harvesting within the volume of a dye-loaded zeolite-L crystal and radiationless energy transport to the cylinder ends. The second stage of organization involves the coupling to an external acceptor stopcock at the ends of the zeolite-L channels, which can then trap electronic excitation energy. The third stage of organization is attained by interfacing the material to an external device via a stopcock intermediate.1 Electronic excitation-energy transfer in dye-zeolite-L materials occurs mainly along the channel axis. This implies that macroscopically organized unidirectional materials can be prepared for optimized energy-transfer purposes. Therefore, we prepared parallel-oriented zeolite-L monolayers, filled them with luminescent dyes, and finally added a stopcock.2 The new materials offer unique possibilities as building blocks for optical, electro-optical, and sensing devices.3
This type of dye-sensitized solar cell works by first absorbing light over a broad spectral range in the zeolite-antenna material. The excitation energy migrates radiationless among the inserted dyes toward the stopcocks. From there, FRET to the semiconductor takes place across a very thin insulating layer. The injected energy can now be used to drive the charge-separation process in the active medium (see Figure 2).
Figure 2. New building blocks for solar-energy-conversion devices. (A) Schematic overview of the host material, consisting of nanochannels containing two types of dye molecules (blue and green) and stopcocks (red). Light absorbed by the blue and also by the green molecules travels to the stopcock heads radiationless, via fluorescence-resonance energy transfer (FRET). (B and C) Principle of dye-sensitized solar cells. Arranging crystals of 50nm to a few 100nm length with their channel axes perpendicular to the surface of a semiconductor allows transport of the excitation energy toward the zeolite-semiconductor interface by FRET. The semiconductor layer can be very thin, because the electron-hole (n-p) pairs form near the surface. The transfer of electrons from the antenna to the semiconductor is prevented by introducing a thin insulating layer. Scheme B shows the principle of the operation of thin-layer silicon devices, while scheme C represents that of organic or plastic solar cells. The white area on top of the head is an insulating part directly integrated into the stopcock. The zeolite material is enlarged with respect to the rest of the device. (D) Small fluorescent concentrator (2×1cm2).
Finally, an LSC is a transparent plate containing luminescent chromophores. Light enters the face of the plate and is absorbed and subsequently re-emitted. The luminescent light is trapped by total internal reflection and guided to the edges of the plate, where it can be converted to electricity by a solar cell. As the edge area of the plate is much smaller than the face area, the LSC operates as a concentrator of light. A major loss is caused by the overlap between absorption and emission spectra.5 A solution to minimize this is to use specific antenna material. Absorption and emission spectra are separated by employing an absorbing dye present in large amounts, and a monolayer of an emitting dye. The photostability of dyes is considerably improved by embedding them into the zeolite channels, which protects them because of their confinement.1,3
The new building blocks are now ready to be tested in devices. Their size, morphology, and optical properties will need to be tailored to the specific task envisaged. The remaining problems to be solved require efforts at the interface of chemistry, physics, and engineering.
Department of Chemistry and Biochemistry
University of Bern
Gion Calzaferri is emeritus professor of physical chemistry at the University of Bern (Switzerland). He spent significant research secondments at the National Renewable Energy Laboratory, CO (1984), and at Cornell University, Ithaca, NY (1990). As visiting professor he was associated with the École Normale Supérieure Cachan (France) in 2000–2001, and with Westfälische Wilhelms University in Münster (Germany) in 2007–2008. He is the 2007 recipient of the Theodor Förster lectureship, awarded jointly by the German Chemical Society and the German Bunsen Society for Physical Chemistry. His research focuses on supramolecularly organized molecules, clusters, and complexes in zeolites, and on artificial antenna systems for light harvesting.