Date on Master's Thesis/Doctoral Dissertation

8-2015

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Zamborini, Francis

Committee Co-Chair (if applicable)

Alphenaar, Bruce

Committee Member

Alphenaar, Bruce

Committee Member

Baldwin, Richard

Committee Member

Liu, Jinjun

Subject

Dye-sensitized solar cells; Solar cells--Research; Solar energy--Research

Abstract

Solar energy is widely believed to be the most promising renewable energy source to fulfill the ever-increasing energy demand from human society now and into the future. Dye-sensitized solar cells (DSSCs) have been explored as a potentially low-cost alternative to silicon solar cell technology due to their lower fabrication costs compared to crystalline semiconductor photovoltaics. However, the optimized efficiency for DSSCs has not been achieved yet, and the chemical stability between dye and semiconductor has also not been addressed completely. In this research, we applied the following four main strategies to prepare photoanodes used in DSSCs with the aim of improving stability and efficiency: 1) utilizing surface chemistry to modify the porous semiconductor films, 2) linking covalently [Di-tetrabutylammoniumcis-bis (isothiocyanato) bis (2, 2’-bipyridyl-4, 4’dicarboxy-lato) ruthenium (II), N719] dye on the surface of semiconductor films, and study the charge injection dynamics by ultrafast transient absorption spectroscopy, 3) doping the semiconductor films with micro-sized neodymium oxide (Nd2O3) particles, and (4) incorporating metal nanoparticles through molecular linkers to the semiconductor films. The objectives of this work are to address the limitations of chemical stability existing between TiO2/N719 dye systems, to explore the charge dynamics in TiO2/N719 system for a better understanding of the fundamental mechanisms of DSSCs, and to prepare high efficiency DSSCs. We for the first time created a strong covalent amide bond between TiO2 mesoporous films and N719 by chemically modifying TiO2 with 3-aminopropyltrimethoxysilane. The results show the dye is air stable for more than 60 days and more resistant to UV light, thermal stress, acid, and water when compared to traditional PAs. The experiments led to another unexpected result, which was the dramatic preservation of the SCN ligand of N719 on the TiO2 surface. In most cases, there is a loss of the ligand which causes the instability of N719 on TiO2. This is clearly observed in the ATR-FTIR data, where the CN stretch of the SCN ligand remained present for covalently-linked dye for more than 6 months, while the CN stretch disappeared completely after 17-20 days from the surface of directly adsorbed N719 on TiO2. Similar results showing no degradation of the CN stretch in the ATR-FTIR data were obtained when TiO2 was chemically modified with an aromatic linker, p-aminophenyltrimethoxysilane (APhS), and covalently-linked with N719. The efficiency of these devices was low initially. When the charge injection dynamic at the interface of dye and TiO2 was investigated by ultrafast transient absorption spectroscopy for covalently- linked dye, the injection rate was slower than that of traditionally-linked dye, which occurs because the linkers (APTES or APhS) increase the distance between the dye and TiO2 surfaces. However, a large amount of dye injecting very slowly was observed in the case of covalently-linked dye, which results in a higher baseline in the time region of ps. The larger baseline base clearly suggests that there were multilayers of dye on the covalently-linked dye. When these multilayers of dye were removed by dipping in acid (or water), the efficiency of the device went back to values similar to traditional photoanodes, but with an improved fill factor. This is an important advance in DSSC technology, allowing us to prepare more stable devices while maintaining the same efficiency. To study the effect of plasmonic Au NPs on the surface of mesoporous TiO2, we synthesized 4 nm diameter Au nanoparticles (NPs) and electrostatically attached them to the mesoporous TiO2 film through APTES prior to sensitization with N719 dye. Results showed an overall improvement of all photoelectrochemical parameters (PEC): short-circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and percentage efficiency (%η). Injection dynamics performed by UTAS clearly showed the lowest baseline in the optical density and time plot suggesting that Au facilitates the monolayer dye coverage with an increased amount of adsorbed dye and assists all dye molecules to contribute to the injection dynamics. This is in agreement with the results that both Jsc and Voc were increased in these Au NP-modified photoanodes, leading to overall better PEC performance than conventional photoanodes. We fabricated TiO2-Nd2O3 nano-micro (20 nm TiO2, 400 μm Nd2O3) composite films to make PAs for DSSCs. This unique combination showed a 10-30% improvement compared to traditional TiO2 films only. The Nd-doping led to a high dye-loading capacity on the photoanodes that helped to increase the short circuit current and efficiency of the devices. Electrochemical impedance spectroscopy revealed that the impedance for charge transport through the composite anode is substantially reduced compared to TiO2 alone. This decrease in resistance is possibly due to the filling of trap states in TiO2 by the Nd2O3 f-states.

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