TiO2-Ge nanocomposites for photovoltaic applications
Date
2006
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Germanium TiO2 nanocomposite thin films were deposited using rf-planar magnetron sputtering to synthesize photoactive material for next generation photovoltaics. We used 33.33 wt%, 15.00 wt%, and 9.00 wt% Ge in TiO2 targets to deposit films of various Ge concentrations. These targets will henceforth be called target (A), target (B), and target (C), respectively. The films were studied using X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM), High Angle Annular Dark Field Imaging (HAADF), UV-Visible Spectroscopy (UV-Vis), Raman Spectroscopy, and photoconductivity measurements. XRD and XPS analyses show the presence of Ge, its various chemical states, and changes in its concentration and particle size. These variations were achievable by varying the deposition parameters like rf power from 100 to 200W, and deposition/annealing temperature, from 500 to 700°C. As expected, a pronounced change also occurs by varying the concentration of the Ge in the target, the particle size decreases with the Ge concentration. Ge dispersed in titania matrix in the form of nanoparticles of average size around 25 nm is obtained on deposition from target (A) and in form of quantum dots average size of 10-20 nm and 5- 10 nm from target (B) and target (C), respectively, as revealed by TEM images. The Quantum Confinement Effect (QCE) was utilized to alter the band gap of the TiO2-Ge nanocomposite system. The band gap, as calculated using the transmittance and reflectance data, was obtained to be ~0.66 eV for Ge and ~3.2 eV for TiO2 for target (A) indicating that the Ge and TiO2 existed in bulk-like form. The photoconductivity of target (B) samples is almost equal to its dark conductivity. The dark conductivity is close to the bulk Ge conductivity. This data also supports the absence of Ge as nanodots in samples obtained from target (A). However, it was observed on working with this target that the relative concentration of elemental Ge increases on increasing sputtering rf power and deposition temperature. The sample deposited at 200 W and 700°C shows the highest elemental Ge concentration. The samples from target (B) and target (C) showed a noticeable blue shift in the absorption edge of Ge, as observed by UV Vis absorption spectra. Changing the target concentration and sputtering parameters could tailor the effective band gap from infrared region up to the upper edge of visible region. This was possible due to quantum confinement effect related blue shifts from 0.66 eV to ~ 2 eV. Moreover a higher photoconductivity (of the order of 3.17 × 10-4) as compared to dark conductivity (of the order of 1.2 × 10-5) also manifests the effects of quantum confinement in the Ge quantum dots in TiO2 matrix. A solar cell device was fabricated with a heterojunction structure of TCO/metal grid /TiO2-Ge/metal contact (Pt)/quartz and the cell characteristics were obtained. These measurements indicated that the cell was operational and could be a potential contender amongst the next generation photovoltaics due to its flexibility in altering the cell characteristics by changing the particle size and density of the quantum dots.