Author : Yinghong Miao
Publisher : ProQuest
Page : 128 pages
File Size : 54,5 Mb
Release : 2008
Category : Germanium
ISBN : 0549821643
Nanocrystalline Titanium Dioxide Solar Cells Sensitized with Germanium Quantum Dots by Yinghong Miao Pdf
Germanium quantum dots are a small band gap semiconductor. The band gap of bulk Ge is 0.66eV. It can absorb near infrared wavelength. Germanium has a large Bohr radius of 24.3nm. Therefore, the size of Ge can be easily tailored. Thus the absorption range of Germanium quantum dots could be broadened into the near infrared range and visible light range. In the solar cells, Germanium quantum dots are embedded in porous titanium dioxide wide band gap semiconductor. Ge quantum dots are used as sensitizers for the wide band gap semiconductor TiO 2 in the solar cells. RF magnetron sputtering is used for depositing TiO 2 -Ge nanocomposite thin films. The sputtering target is made by mixing the germanium and titanium oxide powders together. By X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy(TEM) and Energy Dispersive X-ray System(EDXS) characterization techniques, the microstructure and morphology of thin films are clearly observed. TiO 2 exists in the thin films as anatase titanium dioxide. Ge exists in the form of cubic germanium, not germanium oxide. Germanium dots are embedded in the titanium dioxide nanocrystals in thin film. The TiO 2 -Ge composite thin films are porous, as observed from TEM images. The Ge atomic concentration in the thin film is lower than that of the target studied by EDXS. Three sets of solar cells were made. First, the working electrode TiO 2 -Ge composite thin films were made from 45wt% Ge-55wt% TiO 2 target at 600°C. In the thin film microstructure, Ge quantum dots are embedded in the titanium dioxide nanocrystals matrix. The average particle sizes of Ge dots made at RF(radio frequency) power 100W, 125W and 150W are 13.11 nm, 24.9 nm and 21.29 nm, respectively. The concentration of germanium in the thin films made from these three different RF power is almost the same. It is about 20 atomic%. From the UV-VIS spectroscopy, Ge quantum dots in the thicker films absorb light in the visible range and near infrared range. Solar cells were successfully fabricated. The cells produced a small short circuit current. The open circuit voltage is between 0.34 V and 0.54 V. The photocurrent is very small. The highest efficiency is 0.0027% obtained. As the thickness of thin film increases, the cell performance improves. But the cell performance regresses with further thin film thickness increase. Second, the thin films were deposited at 600°C from the 9wt% Ge-91wt% TiO 2 target. In the thin films, Ge quantum dots are embedded in the titanium dioxide nanocrystals as observed by TEM. The average particle size of Ge dots at RF power 100W, 125W, and 150W are 12.2 nm, 9.45 nm and 10.99 nm, respectively. The average particle size of Ge dots is much smaller than the thin film deposited from 45wt% Ge-55wt% TiO 2 target. From the UV-VIS spectroscopy observations, the thicker thin films begin to absorb visible light by Ge quantum dots. The thinner films do not absorb light in the visible range. The cells made from the thin films successfully produced a small amount of photocurrent. The open circuit voltage is in the range of 0.18 V and 0.52 V. The photocurrent is very small. The highest efficiency is 0.0079%. As the thickness of thin film increases, the cell performance improves. But with further thickness increase, the cell performance regresses. Third, at 500°C, anatase titanium dioxide is still the main phase in the thin films. In the thin films produced at 450°C from 45wt% Ge-55wt% TiO 2, rutile titanium dioxide begins to form and becomes the dominant phase. At both 450°C and 500°C, Ge is in the form of germanium. Cells made from these thin films do not produce photocurrent. The I-V curve goes through the original point. Mostly, it is because the thin films are very porous which makes the electrolyte come in contact with ITO thin film. The Solar cells made from TiO 2 -Ge composite thin films gave a small current. Many of improvements are needed to get good efficiency from the cells. The energy in the infrared light comprises half the energy of the solar spectrum. A solar cell which effectively uses the near infrared light in the solar light will be very promising.