Titania-German Nanocomposites for Quantum Dot Solar Cells

Several new photovoltaic semiconductor materials and technologies have been developed due to the increasing need for renewable energy sources. Quantum dot (QD) based solar cell is potentially one of the best contenders. We have developed a thermodynamically stable nanocomposite (stable up to 900°C) titania-germanium (TiO2-Ge) which shows promise as the active layer for QD solar cells. In TiO2-Ge nanocomposites Ge nanodots are distributed in a TiO2 matrix. Due to the 3-D quantum confinement effect, tailoring of the optoelectronic properties is relatively easily done by simply varying the Ge nanodots size. Ge is particularly advantageous since its Bohr radius is relatively large, 25 nm. In this paper results of the variation of the optoelectronic properties of TiO2-Ge nanocomposites as a function of the nanostructure parameters are presented. TiO2-Ge nanocomposite thin films were synthesized using RF magnetron sputtering. The structural studies (by XRD, HRTEM) established the fact that the Ge concentration in the composite target governs the size of the Ge nanodots whereas RF sputtering power controls the density of Ge nanodots in the nanocomposite films. The optical spectroscopic studies showed that the variation of the size of Ge nanodots in the TiO2-Ge films shifts the absorption edge from ∼ 0.66 eV (infrared) to ∼ 2.3 eV (blue-green). Similarly, dark conductivity also varies in a wide range of 10−7 - 10−2 Scm−1 by altering the concentration as well as the structural phase (amorphous or crystalline) of Ge.