Quantum Dot Solar Cell
The depleting reserves of conventional energy resources have created an increasing realization for the development of alternative form of energy in the past few decades. Particularly, energy from sun is expected to be a great contender, in form of photovoltaics cells, photochemical cells, etc., since it is the only long-term renewable energy provider.
Research in this area has led to develop a huge spectrum of solar cells, which are commonly classified as
1st generation solar cell – these comprise mostly of silicon wafer solar cells.
2nd generation solar cells – these comprise of the thin film solar cell technology.
3rd Generation solar cells- these aim to achieve photovoltaic efficiency of more than 80% exploiting 93% of solar energy. At present, highest conversion efficiency of single junction solar cells, which include the presently available Silicon Solar cells and thin film solar cells, is 33% fig 1 gives an overview of efficiency versus the cost of these classes of solar cells [Ref 1]. The 3rd generation approaches include tandem cells, hot carrier solar cells, solar cells producing multiple electron-hole pair per photon through impact ionization, multiband and impurity solar cells and thermophotovoltaic and thermophotonic solar cells.
Our Research is mostly focused on the development of new material (Titania-germanium quantum dots nanocomposites) and technology for the 3rd generation solar cells. Titania is a popular photoactive material even though it has a band gap of 3.2 eV and hence only absorbs UV light, which comprises only ~5% of the solar energy. As evidence, Titania has been used in Dye sensitized solar cells (DSSC). In the DSSCs the dye attached to the Titania molecule absorbs light in the visible region generating and electron –hole pair. However, charge transport and instability of the electrolyte and the organic complex in the dyes are still major issues in these cells. To overcome these problems we have synthesized a Titania/Ge nanocomposite film in which Titania is sensitized by germanium quantum dots. This sensitization is based on the band gap alteration due to the Quantum confinement effect (QCE). By exploiting quantum confined regime we can obtain dots ascending in size from top layer to bottom layer in the same cell and hence absorbing all wavelengths of the visible spectrum, similar to what the present tandem cells try to achieve. QCE will occur in particles when the physical dimension of the particle is comparable to the exciton Bohr radius. This particular nanocomposite structure is chosen because the thermodynamics of this composite system works in favor of synthesizing elemental Ge in a TiO2 matrix. This is due to the fact that enthalpy of formation of TiO2 is lower (Hf = -944 kJ/mole) as compared to that of GeO2 (Hf = -580 kJ/mole). Ge has relatively large Bohr radius of 25 nm, and that is why size tailoring of Ge quantum dots is easy. Optical properties of the nanocomposites can be varied in a wide range from the infrared to ultraviolet.
Titania/Ge nanocomposites have been synthesized using RF magnetron sputtering whereby it is possible to form compositionally modulated film by changing the sputtering parameters like RF power and annealing temperature using a single or multitarget system. Ge Quantum dots as small as 5 nm have been synthesized in Titania matrix as shown fig. 2 by HRTEM and HAADF. The films were analyzed using XRD, XPS, and TEM. Spectroscopic measurements made using UV-Vis spectrometry clearly indicate a band gap change with the change of Germanium concentration and particle size in the nanocomposite films. There is a blue shift in absorption edge of germanium due to 3 Dimensional QCE in the dots. IV measurements have been carried out, and have clearly indicated a diode behavior in the films. The present study indicates huge prospects to utilize the novel nanocomposite materials into the solar cell devices and therefore research to develop the same is underway.
HRTEM plane view BF image of Germanium quantum dots in Titania matrix.