Charge Storage Based Hysteretic Negative-Differential-Resistance in Metal-Molecule-Metal

MAE 297 Seminar Series Fall 2011

Thursday, September 22, 2011 4:10-5:00pm 1062 Bainer Hall

Direct Flame Synthesis of Mesoporous Thin Films of Nanocrystalline Metal Oxide for Solar Energy Applications

Professor Hai Wang Department of Aerospace and Mechanical Engineering University of Southern California

Transforming solar energy from a niche energy resource into a competitive technology is an exciting yet incremental science. Major obstacles towards solar energy utilization include the high costs associated with cell fabrication and installation. While thin-film solar cells can potentially reduce the installation costs, their fabrication remains prohibitively expansive for a broad range of utilization. This talk will argue for a combustion-science based approach to fabricating critical components of thin-film solar cells at low costs, with examples given for Graetzels dye-sensitized solar cells (DSSCs).

In a dye-sensitized solar cell, a polycrystalline, mesoporous TiO2 film is used as the photoanode. The film acts as an electron-diffusion media. Upon photo-excitation of a chemical dye immobilized on TiO2 surfaces, the electron injects into the conduction band of TiO2 and diffuses into the anode. Anatase with crystal sizes around 10 nm is the preferred over larger, rutile particles.

Traditionally, the fabrication of mesoporous TiO2 films require particle synthesis, collection and film preparation in separate steps. We introduce a new method to fabricate these films. The method uses the laminar, premixed, stagnation flame approach, combining particle synthesis and film deposition in a single step. A rotating disc serves as a combination of substrate holder and stagnation surface that stabilizes the flame. Disc rotation repetitively passes the substrates over a thin-sheet, fuel-lean flame doped with an organometalic titanium precursor. Convective cooling of the disc keeps the substrate well below the flame temperature, allowing thermophoretic forces to deposit a uniform film of particles that are nucleated and grown via the flame stabilized just above the surface. Analysis shows that knowledge of the particle thermophoretic velocity is critical to controlled film deposition. For this reason and because the particles of interest approach the molecular size, a fundamental theory of thermophoresis beyond Waldmanns was developed from a gas-kinetic theory analysis. The theory predicts that the thermophoretic velocity becomes sensitive to the particle diameter below ~3 nm only. Above this size, the insensitivity of particle size to thermophoretic velocity ensures narrowly distributed particle sizes in the film prepared. Tests show that the resulting film is capable of producing >9 % photoconversion efficiency in a DSSC, close to its record at 11%.