Paper EN-MoA10
Solid-State Quantum Dot Sensitized Solar Cells: Atomic Layer Deposition versus Successive Ionic Layer Adsorption and Reaction

Monday, October 18, 2010, 5:00 pm, Room Mesilla

Narrow band gap nanostructures such as cadmium sulfide quantum dots (QDs) are known to show size quantization effects. In quantum dot sensitized solar cells (QDSSCs), these QDs can be engineered to transfer an 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 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) solid-state QDSSCs. TiO₂ was deposited on piranha-cleaned Si or microscope glass via atomic layer deposition (ALD) and the resulting surfaces were characterized by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Next, different SAMs were attached from solution to either ALD-deposited (planar) or doctor-bladed (nanoporous) TiO₂ substrates, and the effects of chain length, aromaticity, and tailgroup on the quality of the SAMs were investigated. CdS QDs were then grown on the SAM-passivated TiO₂ surfaces by either successive ionic layer adsorption and reaction (SILAR) from solution or by atomic layer deposition (ALD) from the gas phase, and the bonding and performance of the resulting materials were evaluated by UV-visible and other spectroscopic techniques. Our results show that CdS QDs with particle sizes in the range of 2 to 6 nm are grown by SILAR on TiO₂ both with and without SAMs, but more CdS can be deposited on samples with SAMs with the exception of the long-chain methyl-terminated monolayer. Furthermore, it is determined that the SAM chain length affects the SILAR CdS deposition at the TiO₂ surfaces more significantly than does the identity of the tailgroup. ALD also is effective for depositing CdS QDs, but less CdS is deposited by ALD than by SILAR at TiO₂ surfaces for the same number of cycles. QDSSC devices have been made using both SILAR and ALD, and we will present results on the dependence of solid-state QDSSC performance and efficiency on the deposition technique employed to grow the CdS QDs, as well as on the properties of the SAM. Overall, we observed higher efficiencies in devices with SAMs and we propose that this result can be attributed to the presence of a charge recombination barrier.