Photoactive nanostructured materials and devices: Pt photoanodes, Cu photocatalysts and SnS absorbers

Open Access
Author:
Lee, Hyeonseok
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 30, 2014
Committee Members:
  • Mark William Horn, Dissertation Advisor
  • Mark William Horn, Committee Chair
  • Jerzy Ruzyllo, Committee Chair
  • Joan Marie Redwing, Committee Member
  • Srinivas A Tadigadapa, Committee Member
  • Jeffrey Brownson, Committee Member
Keywords:
  • Sculptured thin films
  • CO2 conversion
  • solar cells
Abstract:
Growing demand of energy and global warming concern regarding CO2 gases urge researchers to seek an alternative source of energy. Among all candidates, solar energy is very promising owing to its clean and renewable nature. In order to convert the solar energy into useful form such as electricity or hydrocarbon fuel, photoactive materials and devices are commonly used. However, their conversion efficiency is always problematic. In this dissertation, utilization of the solar energy with three different devices incorporating sculptured thin films (STFs) was demonstrated: (1) electricity generation by dye-sensitized solar cells adopting sculptured Pt nanowire counter electrodes, (2) TiO2 photocatalytic film with columnar Cu cocatalyts, and (3) sculptured SnS thin film solar cells. First, Pt STFs were fabricated for dye-sensitized solar cells with 5° obliquely incident vapor flux by electron beam evaporator. Pt STFs exhibit more desirable features for more functional counter electrodes than that in conventional Pt planar counter electrodes. Higher root mean square surface roughness of 44.4 nm and lower charge transfer resistance of 0.121 Ω•㎠ were measured for Pt STF than the root mean square surface roughness of 19.1 nm and charge transfer resistance of 0.578 Ω•㎠ for Pt planar films .These improvement in morphology and electrochemical properties led enhanced photocurrent density and conversion efficiency (9.766 mA/cm2, 5.1 %). Second, Cu cocatalysts were sculptured directly on porous TiO2 catalyst films. Varied lengths of Cu columns were grown and methane production rate from carbon dioxide and water was evaluated under AM 1.5 illumination. The methane production rate showed a maximum of 124.3 ppm•cm-2•h-1 with 160 nm Cu column films. This maximum corresponds to the point where the absorption peak at ~600 nm that is attributed to plasmonic effect by Cu STFs. Third, SnS thin films were deposited by radio frequency magnetron sputter. Deposited SnS thin films were characterized in structural, optical, and electrical manner. Surface morphology and growth pattern of the films were varied, depending on variation of deposition parameters such as power, pressure, throw distance and substrate heating. This variation was related to the energy of deposition modes that is possibly explained by mean free path and mean speed of the vapor flux. Also, the deposition rate is related to diffused vapor flux. The optical properties of the SnS thin films were investigated by spectroscopic ellipsometry. Obtained absorption coefficient was >104 cm-1 even in the range of >1000 nm wavelength. The calculated direct bandgap and indirect bandgap of SnS thin films were ranged from 1.33 to 1.55 eV and from 1.07 to 1.39 eV, respectively. In the investigation of electrical properties by using transport length method, it is found that the SnS films with lower root mean square roughness exhibited lower resistivity while more structured SnS film showed higher resistivity. In addition, the photoresponse was observed under AM 1.5 illumination. SnS thin film solar cells were fabricated in the two different types of thin film solar cells and electrochemical solar cells. The SnS electrochemical solar cells showed photovoltaic behavior and improved voltage, fill factor and efficiency (Voc= 0.38, FF=0.26 and η=0.0025 %) were measured with the incorporation of the sculptured SnS thin films.