Ismat Shah at the University of Delaware in the USA has been using quantum dots to improve Dye Sensitised Solar cells. DSSC cells uniquely tolerate low light levels and glancing angles of incidence of light and even polarised light and they are easier to make than other options and use low cost materials. Indeed, wide area reel to reel production is feasible. They can be transparent. This means that they - and only they - have been used successfully from Antarctica to Africa and they will command many niche applications, even with only 5-10 years life in many cases and inferior efficiency under vertical light compared with some other options. Life is a problem because liquids must be confined - an iodine based electrolyte and a ruthenium polypyridine dye are used on the titanium dioxide nanoparticle semiconductor in a typical DSSC.
Given that charge transport and instability of the electrolyte and the organic complex in the dyes are still major issues in these cells, Shah has synthesized a Titania/Ge nanocomposite film in which Titania is sensitized by germanium quantum dots. He writes in the Materials Science and Engineering review from the University that, "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 Hf = -944 kJ/mole as compared to thatΔenthalpy of formation of TiO2 is lower (Hf = -580 kJ/mole). Ge has relatively large Bohr radius of 25 nm, andΔof GeO2 (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 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."
Top image: HRTEM plane view BF image of Germanium quantum dots in Titania matrix