An added advantage is the simple high-yield chemical syntheses which, coupled with their inherent chemical flexibility, offers considerable scope for tuning the HOMO and LUMO levels and thereby optimizing photovoltaic performance. We are continuing to prepare and test new acceptor materials that have provided very promising results.

2. Hole conductors for solid state dye sensitized solar cells (ssDSSCs)

-Ms. Jinhee Park

Nanostructured dye sensitized solar cells (DSSCs) have shown promise as low cost alternatives to conventional solar cells. For example, top power conversion efficiencies (PCE) of over 11% have been reported for DSSCs using liquid electrolyte hole conducting solutions based on the iodide/tri-iodide redox couple. Although this efficiency is quite high and approaching what is required for commercial solar cells, problems with scaling this technology to high manufacturing volumes, primarily due to electrolyte leakage concerns, have led researchers to focus on ssDSSC that use solid state hole conductors.

From a materials perspective, the key components to ssDSSC are the dyes, hole conductors and titania. To date, far more R&D has been devoted to new dye development for increasing the absorption of the solar spectrum into the near infrared (>700nm), increasing the dye extinction coefficient, building in features to reduce charge recombination, and to move away from ruthenium based dyes by designing all-organic dyes. Although work on new dyes is important and needed for the advancement of this technology, equally as important is the need to develop new hole conductors and titania, which is where our group is interested.

With respect to hole conductors, our group is designing materials paying close attention to properties such as:

• Morphology: Crystallization is believed to prevent effective pore filling, reduce hole transport due to crystal boundaries, and reduce dye regeneration by reducing the intimate contact between the hole conductor and the dye. Therefore the material must be amorphous.
• Solubility: Enhanced solubility is necessary to optimize infiltration into the titania from solution based coating processes.
• HOMO level: Enhanced electron donating properties for HOMO levels -(4.8-5.2 eV) compared that may enhance electron transfer to the oxidized dye.
• Polar properties: Increased polarity may provide better wetting and thus enhanced infiltration properties and contact to the dyes attached to the titania surface.
• Size: A relatively small molecular size of <3 nm could help to provide better infiltration into the dye covered titania network.

3. Nanostructured ceramics for application in solar cells

-Mr. Tomas Leijtens

The physical properties of titania plays a crucial role in the overall PCE of ssDSSC devices. The primary prerequisites for the titania are: 1) phase pure (or nearly phase pure) anatase for enhanced electron mobility; 2) pore size diameters in the range of 10-50 nm for optimal infiltration of the hole conductor; 3) high surface areas (>200 m2/g) for enhanced dye absorption; and 4) well-connected nano-particle network for charge percolation out of the device. To date, commercially available titania has been the main resource for the ssDSSCs research community. However, despite the tremendous progress made on ssDSSCs over the past 10 years, it has become evident that the current titania has limitations with regard to the important properties described above. Thus based on these limitations, many research groups have been working to develop new synthetic routes towards improving the properties of titania.

Our group is interested in preparing titania with surface areas exceeding 500 m2/g that will allow for greater absorption of solar irradiation per unit area leading to higher PCE.