Date on Master's Thesis/Doctoral Dissertation

12-2020

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

Druffel, Thad

Committee Member

Ramezanipour, Farshid

Committee Member

Zhang, Xiang

Author's Keywords

dye-sensitized solar cell; rare-earth metal oxide; perovskite; electric polarization; ac photocurrent, 2D perovskite

Abstract

Solar energy is one of the most important alternative renewable energy sources to fulfill the increasing demand of energy in the world. Third-generation solar cells like dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic solar cells are extensively studied to increase their photoconversion efficiency, and ultimately for their large-scale implementation. A dye-sensitized solar cell consists of a photoanode of a mesoporous film of titania sensitized with dye sandwiched with a counter electrode, which is usually a platinum-coated transparent conducting oxide, and a redox couple injected between the photoanode and counter electrode. Doping titania with rare-earth metal oxides (REOs) has been an interesting approach to improve the conversion efficiency of dye-sensitized solar cells. REOs have been doped into titania paste to show an improvement in the photovoltaic performance of dye-sensitized solar cells, however, most of the reported cells are not efficient enough to conclude whether the enhancement is due to doping or it is because of the cell quality. We incorporated nanoparticles (NPs) of REOs in titania paste and built highly reproducible dye-sensitized solar cells using amphiphilic C101 dye and iodide/triiodide redox couple in nitrile-based solvent (Z960 electrolyte). The doping level for optimized cells was 2.0 % for neodymium oxide and 1.0 % for erbium oxide. We did the measurements of photocurrent, impedance, incident photon-to-electron conversion efficiency (IPCE), and dye loading to investigate the mechanism of enhancement of the photovoltaic performance by REO NPs. Electrochemical impedance spectroscopy measurements showed that doping with REO decreased the total impedance of the cell and IPCE measurements revealed enhanced photon absorption by the dye in REO-doped cells. In the same fashion, the REO-doped anodes showed larger dye loading compared to undoped anodes, which was maximum for 1.0 % doping of erbium oxide and 2.0 % doping of neodymium oxide. REOs not only enhance dye adsorption but also facilitate electron transport through the mesoporous layer, thereby increasing the collection efficiency of the photoexcited electrons. To further explore the mechanism for the interaction between REO NPs and titania, an electrical and electrochemical study of REO-doped nanostructured titania films was performed. Doped films were found to be 40-50 times more conductive than undoped films, with linear current-voltage characteristics. Cyclic voltammograms of doped samples showed an enhanced scan rate dependence in the deep trap regime due to a slower charge trapping rate. Finally, electrochemical impedance measurements revealed a decrease in space charge density and a shift in the flat-band potential. Taken together, these results suggest that charge transfer from the REO neutralizes the deep trap states in the nanostructured titanium dioxide (NTD) film, decreasing charge scattering, and improving the NTD performance as an electron acceptor and electron transport material. Perovskite solar cells (PSCs) were first made when the dye-loaded semiconductor of dye-sensitized solar cell was replaced by perovskite layer and liquid electrolyte by a hole transport layer. The light harvesting perovskite layer is sandwiched between electron-transport and hole transport layers. Organic-inorganic perovskites, also known as hybrid perovskites have fascinating optoelectronic properties for their applications in highly efficient solar cells. The stability in ambient conditions and hysteresis in current-potential curves are two main challenges. The ease with which the separation of photogenerated charge carriers, electron-hole pairs (excitons), takes place is very critical for the performance of PSCs. In addition to the work function difference of electron-transport and hole transport layers, the intrinsic built-in potential in the perovskite films can play a significant role in the separation of these excitons. The internal electric originates from the local polarization of the film due to non-centrosymmetric lattice and ionic polarization and can be measured through an AC photocurrent technique. The polarization of a pristine sample is strongly dependent on the size of grains and can be used to determine the quality of the film. After poling the film by applying a potential through interdigitated Au electrodes, the devices with different grain sizes behaved differently upon relaxation. We observed that the polarization of a mixed halide hybrid perovskite film strongly depends on the background environment. The Quartz Crystal Microbalance measurements reveal that the perovskite film adsorbs Ar gas in the presence of solar light. The combination of Ar gas and solar illumination results in the enhancement of the electric polarization of the mixed halide hybrid perovskite film. Consequently, the photocurrent is increased due to the stronger driving force for the separation of excitons. This observation is illustrated in an actual PSC where the photovoltaic enhancement is observed with Ar gas. Our results suggest that the contribution from the background environment should be taken into consideration when describing the photovoltaic performance of a PSC.

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