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Posted: Feb 23, 2010
Improved design for dye-sensitized solar cells includes quantum dot antennas
(Nanowerk Spotlight) With an increased focus on alternative sources of cheap, abundant, clean energy, solar cells are all the rage and the dye sensitized solar cell (DSSC) has been one of the most important developments in photovoltaics the last two decades. Untreated titanium dioxide absorbs light only in the UV region, but when the surface becomes modified with dye molecules, these can absorb light in the visible range and then transfer the excited electron to the particle. Back in 1991, Grätzel et. al came up with the methodology to dye-sensitize colloidal titanium dioxide film as a way to fabricate low-cost, high-efficiency solar cells ("A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films").
While the dye sensitized photovoltaic cell is a fairly mature design, researchers are still trying to improve its efficiency with various techniques, including structuring nanoporous electrodes to provide higher surface area and better charge transport, replacement of the liquid electrolyte by a solid one in order to prevent the electrolyte evaporation, and ways to widen the narrow absorption spectra of molecular dyes.
In a standard DSSC, an organic molecule adsorbed on the surface of a porous electrode absorbs light and then initiates the charge separation process eventually leading to generation of photocurrent. One major difficulty in such cells is that very few dyes can absorb a broad spectral range, essentially covering the solar spectrum. In contrast, broad spectral coverage is an inherent property of semiconductor nanocrystals. The latter, however, turn out to do a rather lousy job in separating the charges.
Researchers in Israel have now presented a new configuration for quantum dot sensitized DSSCs via a FRET (Förster resonance energy transfer) process. Their results prove the general feasibility of enhancing light absorption and broadening the absorption spectrum by the addition of quantum dots, effectively increasing the number of photons harvested by the dye sensitized solar cell.
"Energy transfer between several molecular dyes has been used to broaden the absorption spectrum of a dye-sensitized solar cell" Arie Zaban, a professor in the Department of Chemistry at Bar-Ilan University, explains to Nanowerk. "In this realization, however, energy transfer was rather inefficient since the design did not force the donor dye molecules to be in close proximity to the charge separating dye molecules."
"Our innovation involves inorganic semiconductor nanocrystals that are built into the solid electrode" adds Dan Oron, a senior scientists at the Weizmann Institute of Science. "Consequently the antenna becomes part of the electrode, distances are fixed to achieve efficient energy transfer and photostability of the system is significantly improved."
Built-in quantum dot antenna in DSSC. This figure schematically describes the antenna process. (Image: Dr. Arie Zaban, Bar-Ilan University)
Zaban and Oron have been leading a team that has developed a system that combines the advantages of both dye-based and quantum dot based systems. Semiconductor nanocrystals built into the electrode absorb the light, but are designed so that the excitation energy is transferred to a nearby dye molecule for charge separation and generation of photocurrent proceeds as in a standard dye-sensitized solar cell.
"In other words" explain Zaban and Oron, "the semiconductor nanocrystals serve as antennas that harvest the light (photons) and the dye molecule is responsible for the charge-separation (photocurrent). The two functions are mediated by resonance energy transfer. This allows for optimization of the two processes – light harvesting and charge separation – since they occur on two different materials."
The researchers note that FRET is the dominant mechanism funneling energy from the quantum dots to the dye, and IPCE (Incident-Photon-to-electron Conversion Efficiency) measurements show a full coverage of the visible spectrum, limited only by the efficiency of charge injection from the dye to the titania electrode.
Zaban says that the utilization of a FRET to transfer energy from quantum dots to dye molecules introduces new degrees of freedom in the design of quantum dot sensitizers for photovoltaic cells.
"In particular" adds Oron, "it opens the way toward the utilization of new materials whose band offsets – relative to the titania electrode – do not allow direct charge injection. The fact that quantum dots donors are incorporated into the solid electrode will potentially result in a significant improvement to the stability of such systems."
"Most importantly" adds Zaban, "our new design provides several critical benefits: the isolation of the quantum dot 'antenna' from the electrolyte solution prevents donor quenching; the geometry resembling parallel donoracceptor layers with short distance increases the energy transfer efficiency; finally, quantum dots isolation significantly improves their photostability as compared with traditional designs that involve charge transfer between the iodine electrolyte and the quantum dots."
The team are confident that the efficiency of their system can be significantly improved upon optimization of both the dye molecule and of the spacer layer between semiconductor nanocrystals and dye molecules for both optimal energy transfer and charge injection.
"We will need an efficient near-infrared dye in terms of charge separation with no demands of broad absorption spectrum" says Zaban. "Prior to the antenna design these dyes were not relevant for dye cells. Now they become important."
The further challenge now is mostly in material science – researchers need to design, synthesize and assemble nanostructured materials that will enable them to approach the theoretical limit of these solar cells.