Paper EM1+PV-TuM5
Phosphonate Self-Assembled Monolayers as Organic Linkers in Quantum Dot Sensitized Solar Cells

Tuesday, November 10, 2009, 9:20 am, Room A8

Narrow band gap nanostructures such as cadmium sulfide quantum dots (QDs) are known to show size quantization effects as well as multiple exciton generation. They are therefore beneficial for absorption of light in the visible and near infrared region of the solar spectrum and can be used to fabricate photovoltaic devices with high theoretical efficiencies. In quantum dot sensitized solar cells (QDSSCs), these QDs can be engineered to transfer the electron to a wide band gap semiconductor such as titanium dioxide (TiO₂). However, performance in such devices is reduced by charge recombination at the TiO₂ surface and hence use of organic linkers and electron conductors such as self-assembled monolayers (SAMs) on these devices could provide a means of eliminating recombination sites and lead to increased efficiency. In this study, we investigated the effects of different aliphatic and aromatic SAMs with phosphonic acid headgroups and varied tailgroups on the bonding and performance of cadmium sulfide (CdS) QDSSCs. Our studies focus on bonding of the CdS QDs on both planar and nanoporous TiO₂ with or without the SAM linkers. To study the SAM/QD growth on planar surfaces, TiO₂ was deposited on Piranha-cleaned Si or microscope glass via atomic layer deposition (ALD) and the resulting surfaces were characterized by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Next, different SAMs were attached to the TiO₂ substrates from solution, where the effects of chain length, aromaticity, tailgroup, solvent, and dip time on the quality of the SAMs were investigated by the same techniques as well as infrared (IR) spectroscopy, water contact angle (WCA) measurements, and ellipsometry. Finally, CdS QDs were grown on the SAM-passivated TiO₂ surfaces by the successive ionic layer adsorption and reaction (SILAR) process, and the bonding and performance of the resulting materials were evaluated by UV-visible and other spectroscopic techniques. The results were compared to the case of QDs grown on the non-passivated TiO₂ surfaces. Our results show promising differences in the bonding of the CdS QDs at the TiO₂ surfaces with the SAM linkers. We will also present results on the dependence of solar cell performance on the properties of the SAM.