Interface engineering and characterization in dye and quantum dot-sensitized solar cells [electronic resource]
Keywords: Chemical Engineering.

The increasing demand for energy as standards of living and population sizes rise across the globe motivates the development of scalable resources to meet the forecasted doubling of energy consumption. This challenge is further compounded by the need to reduce the CO2 emissions associated with our current level of energy consumption in order to stave off costly changes in the global climate. Solar energy promises renewable, carbon-free energy and a resource that is orders of magnitude larger than alternative sources. Capturing this solar energy with photovoltaic devices has become an increasingly economical means of energy production, but further development is needed to make solar energy conversion inexpensive and ubiquitous. One particular class of photovoltaics—the dye-sensitized solar cell (DSSC)—is especially appealing because it can be constructed with abundant, inexpensive materials and be engineered in a modular fashion for a wide array of product applications. Challenges remain, however, in order to make DSSCs more efficient, more economical, and more practical. The DSSC architecture is quite different than traditional—i.e. silicon and thin film inorganic—photovoltaics in that light harvesting and charge collection are decoupled. This is achieved by depositing a thin nanometer-scale layer of light-absorbing dye molecules atop a high surface area nanostructured TiO2 anode. The consequence of this architecture, however, is an abundance of interfacial area at which deleterious charge recombination processes can occur. Further improvements in DSSC performance therefore require a thorough understanding of and high control over the dual-interface that exists between the dye layer and the electron-conducting anode and the dye layer and the hole-transporting material (HTM). In the first part of this work we describe the application of thin, sub-nanometer insulating metal oxide layers grown by atomic layer deposition (ALD) to the TiO2 anode for the purpose of slowing down the undesirable back-recombination of electrons injected into TiO2 by the light-absorbing dye molecules. We use the well-characterized insulator Al2O3 as a recombination barrier material and perform a comprehensive study of different parameters that impact how such barriers change device performance. In a solid-state DSSC we demonstrate the importance of the dye chemical structure and the anode fabrication process in dictating whether or not improvements achieved through the recombination suppression outweigh device current losses that result from the insertion of an insulating layer. We apply these lessons to a new barrier layer material, In2O3, that unlike Al2O3 has not been previously well-characterized but is less-insulating and can be grown at an extremely low growth rate, providing excellent control over the competing consequences of barrier layers. With In2O3 we are able to demonstrate some of the highest ever reported open-circuit voltages for this class of DSSC and we observe an unexpected change in behavior as the In2O3 barriers reach a certain thickness. In our third barrier layer study, we achieve a near-doubling in efficiency when Al2O3 is applied to a quantum dot-sensitized solar cell, a close analogue of the DSSC, wherein recombination more severely limits device performance. In the second part of this work, we combine experimental and computational techniques to study the dye/anode interface and the orientation of dye molecules adsorbed on a TiO2 surface. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to measure the angles of individual bonds and in turn deduce the full geometry of adsorbed dye molecules. This result is compared to computational simulations using density functional theory-molecular dynamics (DFT-MD) of the same dye/TiO2 system. Our results show remarkable correspondence between the experimental and computational approaches and signify important consequences for understanding the dye/anode interface in DSSCs as well as adsorbate/substrate studies more generally.