Small molecules in mesoporous oxide systems for energy applications [electronic resource]

The growth of the human population is placing increasing strains on our natural resources. The resulting rise in energy demand, global temperatures, and greenhouse gas emissions are becoming major concerns. The work presented in this thesis attempts to address these issues in two ways. One is through renewable energy generation using efficient, low-cost solar technology. In particular, organic-based solar technologies, such as the dye-sensitized and perovskite solar cells presented in this thesis, can be roll-to-roll printed in a similar fashion to newspapers and help to pave the way for the democratization of solar energy. A second way is by reducing energy demand through conservation-enabling technologies such as electrochromic "smart" windows. Electrochromics have the potential to cut the energy consumption of a building in half, representing a significant reduction given that buildings consume approximately 40% of the world's energy production. A major limitation of solid-state dye-sensitized solar cells is a short electron diffusion length due to fast recombination between electrons in the TiO2 electron-transporting layer and holes in the 2,2', 7,7'-tetrakis(N, N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) hole-transporting layer. This recombination process results in a lower operating voltage and limits the overall efficiency of the device. The sensitizing dye can be engineered to slow down recombination and increase device performance by acting as a robust barrier that can keep the TiO2 physically separated from the spiro-OMeTAD. Through the synthesis and characterization of three new organic D-π-A sensitizing dyes, WN1, WN3, and WN3.1, the quantity and placement of alkyl chains on the sensitizing dye were found to play a significant role in the suppression of recombination. These dyes achieved the following efficiencies: 4.9% for WN1, 5.9% for WN3, and 6.3% for WN3.1, compared with 6.6% achieved with Y123, the state-of-the-art dye at the time. Of the dyes investigated in this study, WN3.1 is shown to be the most effective at suppressing recombination in solid-state dye-sensitized solar cells by using transient photovoltage and photocurrent measurements. Though spiro-OMeTAD is the prevalent organic hole-transport material used in solid-state dye-sensitized solar cells and perovskite solar cells, the common way in which it is processed and integrated in to a device has a critical flaw in that it relies on an uncontrolled oxidative process to increase its conductivity to a functional level, causing device performances to be highly variable. This thesis presents the synthesis and use of a dicationic salt of spiro-OMeTAD, named spiro(TFSI)2, as a facile means of controllably increasing the conductivity of spiro-OMeTAD up to 10-3 S cm-1 without relying on oxidation in air. Spiro(TFSI)2 enabled the first demonstration of solid-state dye-sensitized solar cells fabricated and operated with the complete exclusion of oxygen after deposition of the sensitizer with higher and more reproducible device performance. Perovskite solar cells fabricated with spiro(TFSI)2 show improved operating stability in an inert atmosphere. Gaining control of the conductivity of the HTM in both dye-sensitized and perovskite-absorber solar cells in an inert atmosphere using spiro(TFSI)2 is an important step towards the commercialization of these organic solar technologies. Electrochromic smart windows currently available on the market have not gained wide market traction due in part to their high cost and sub-optimal color transition from colorless to blue. In this work, transmissive-to-black electrochromic devices using a single working electrode were designed and fabricated by assembling a very simple organic small molecule, p aminotriphenlyamine (pAT), on the surface of mesoporous tin-doped indium oxide (mITO) electrodes. These electrodes produce reversible electrochromics with a maximum contrast ratio (i.e. change in the transmission of light) of 64%. The broad absorption profile of oxidized pAT under cathodic bias results in a color change from nearly clear and colorless to black with switching times of 4.3 seconds (on) and 2.0 seconds (off). The switching speeds and electron transfer rates of the devices were diffusion limited and correlated with the thickness of the mITO layer. Due to electrochemical leaching of the indium and tin from the mITO scaffold, the contrast ratio performance decreased by half after 50 on/off switches. The use of a primary amine anchoring group of pAT was key to demonstrating the solution processable, high-contrast, black electrochromic device reported in this work and may help to further future research towards producing cost-effective, high performance, organic electrochromic devices.

Keywords: Chemistry.
Advisor: McGehee Michael
Du Bois Justin advisor. Thesis advisor
Karunadasa Hemamala