Justification for the transfer of one Stark Spectrometer to California Institute of Technology for use with DOE grant DE-FG03-88ER13932 to Dr. Nathan S. Lewis
The DOE grant of Nathan Lewis (DE-FG03-88ER13932) has a major section (section II-B-2) that proposes the study of charge transfer processes across semiconductor/liquid interfaces. It is noted that
“the use of interpenetrating networks relaxes the usual constraints on the charge carrier lifetime that are present in planar, conventional, solar cell device structures. Provided that recombination can be suppressed, the carrier lifetime need only be sufficient to effect charge injection into the two different phases that form the interpenetrating network, unlike conventional devices in which the lifetime needs to be long enough to sustain charge-separation for the entire physical device dimension between the contacting electrodes.”
Dr. Lewis proposes to undertake
“a systematic approach to understanding the charge-carrier transport processes in such systems, by studying regularly ordered mesoporous semiconductor structures with well-defined and systematically variable dimensions. In the case of TiO2, the approach will be combined with a study of the individual TiO2 building blocks that constitute the mesoporous TiO2 membrane.”
Due to the equipment on hand at the time the proposal focuses on the charge transport and the changes in the charge transport with changes in the physical structure of the TiO2. It is widely recognized that changes in the charge injection properties critically affect the efficiency of the solar cells. In Section II-B-2 of the proposal it is noted that the charge injection must be rapid with respect to the lifetime of the carrier (or excited state of the dye).
The use of Stark spectroscopy allows one to probe the charge injection into the TiO2 structure. At present the dominant method of charge injection is from an excited state of a metal complex that has a sufficiently long lifetime such that the rate of charge injection into the semiconductoris much faster than the relaxation. This aspect of the charge injection process has been one area of the continuing research of the Lewis group.1-10 The ability to directly probe the charge transfer excited state of the injecting metal complex by use of Stark spectroscopy will enhance the understanding of the injection processes. In some cases the charge injection takes place directly during the excitation process rather than from a preformed excited state. In this case Stark spectroscopy can also help to define this process.11 Thus the use of Stark spectroscopy will introduce a new method in the studies proposed in the grant application that will provide information about the details of the charge injection process, including the charge-transfer distance that are not currently accessible. Stark spectroscopy studies will allow the development of a fuller understand of the charge transport processes in semiconductor structures which is the central purpose of the proposed research.
In the current DOE proposal it is proposed to study the charge transport in various TiO2 structures of wires, monolayers and three-dimensional networks of wires or TiO2 tubes. In these various configurations the characterization of the charge injection will provide valuable information regarding the feasibility of using such structures.
In his original letter to Dr. Brunschwig about the Stark spectrometer Dr. Lewis mentioned some of the same ideas:
“This is to confirm our discussions of a collaboration on your studies of charge-transfer spectroscopy of dye-sensitized TiO2 nanoparticles. We are also involved in studies of TiO2 films used in liquid junction photoelectrochemical cells, specifically in the use of these systems to advance our understanding of how to exploit interpenetrating networks and nanostructured materials to achieve effective photo-induced charge separation for solar energy storage using very inexpensive light absorbing materials. Research into dye sensitization of TiO2 nanocrystalline films at Caltech is supported by two DOE contracts, one through NREL and the other through the Office of Basic Energy Sciences.
The availability of a Stark spectrometer would add a new method for studying the charge-transfer spectra of dyes attached to TiO2 nanoparticles. This research involves characterization of the electron injection into the TiO2. In certain systems Stark spectroscopy offers a unique method of determining the nature of the initially formed Franck-Condon charge-transfer state.”
The DOE funding of Dr. Lewis has been in effect for over a decade and he is expecting to submit a renewal proposal in 2006. In the next round of the proposal the existence of the Stark spectrometer at Caltech will allow him to propose new experiment that make specific use of this equipment. These new areas of work would be an extension of his current work as seen in the work published to date.1-10
(1)Kilsa, K.; Mayo, E. I.; Lewis, N. S.; Winkler, J. R.; Gray, H. B. Abstracts of Papers of the American Chemical Society2003, 225, U167.
(2)Mayo, E. I.; Kilsa, K.; Freund, M. S.; Gray, H. B.; Lewis, N. S. Abstracts of Papers of the American Chemical Society2003, 225, U168.
(3)Kilsa, K.; Mayo, E. I.; Kuciauskas, D.; Villahermosa, R.; Lewis, N. S.; Winkler, J. R.; Gray, H. B. Journal of Physical Chemistry A2003, 107, 3379.
(4)Kuciauskas, D.; Monat, J. E.; Villahermosa, R.; Gray, H. B.; Lewis, N. S.; McCusker, J. K. Journal of Physical Chemistry B2002, 106, 9347.
(5)Lewis, N. S. Nature2001, 414, 589.
(6)Kuciauskas, D.; Sauve, G.; Freund, M. S.; Gray, H. B.; Winkler, J. R.; Lewis, N. S. Abstracts of Papers of the American Chemical Society2001, 222, U237.
(7)Lewis, N. S. Journal of Electroanalytical Chemistry2001, 508, 1.
(8)Kuciauskas, D.; Sauve, G.; Freund, M. S.; Gray, H. B.; Winkler, D. R.; Lewis, N. S. Abstracts of Papers of the American Chemical Society2001, 221, U143.
(9)Villahermosa, R. M.; Kuciauskas, D.; Mayo, E. I.; Lewis, N. S.; Winkler, J. R.; Gray, H. B. Abstracts of Papers of the American Chemical Society2001, 221, U660.
(10)Lewis, N. S. Chemical & Engineering News2001, 79, 278.
(11)Khoudiakov, M.; Parise, A. R.; Brunschwig, B. S. Journal of the American Chemical Society2003, 125, 4637.