310 Chem Lecture Module 10
Some Applications of Inorganic Transition Metal Chemistry in the Real World
Traditional Solar Cells
Solar cells are photovoltaic devices - convert light into electricity. p g y Offer an inexhaustible and environmentally benign energy source. Solar cells are made of amorphous silicon, or better - CuInSe2 They have to be of a very high purity and be defect-free. Very expensive. The efficiency of such a cell is also very small, converts 13-18 % of s nlight to electricit The efficienc s ch er small con erts 13 18 sunlight electricity. Low efficiency wouldnt matter if huge arrays of cells could be produced cheaply. Natures solar cells, chloroplasts in plants, are less than 1 % efficient.
Most solar powered devices rely on the same principle: a photon of sunlight boosts an electron in the material into a mobile state so that it can be used to generate electricity. The problem with this? The electrons are negatively charged, will leave a charged positive charge. These opposite charges attract one another and will tend to recombine The absorbed energy is squandered as heat or as re-emitted light. Silicon or CuInSe2 solar cells use an electric field to push the negatively charged electrons and positive charges apart. Chloroplasts adopt a more subtle approach: separate charges by making a distinction between units that generate the electron and those that transport it. it
Traditional Solar Cells contd
Light hi i Li ht shining on crystalline silicon frees electrons within the crystal lattice. t lli ili f l t ithi th t l l tti For these electrons to do useful work they must be separated and directed into an electrical circuit. To separate the electrical charges, the silicon solar cell must have a built-in electric field.
Although both materials are electrically neutral, n-type silicon has excess , yp electrons and p-type silicon has excess holes (Chem 213). Sandwiching these together creates a p/n junction at their interface, interface thereby creating an electric field.
To create this electric field within a photovoltaic (PV) cell, two separate semiconductors are sandwiched together. P-type semiconductors have an abundance of positively charged holes, whereas n-type semiconductors have an abundance of negatively charged electrons. i d h b d f i l h d l When n- and p-type silicon come into contact, excess electrons move from the n-type to the ptype, creating the electric field.
Inorganic Dye- based Solar Cells
The Grtzel cell uses Ru complexes (a dye) to capture light energy, to inject an electron from the dye into a semiconductor such as titanium dioxide (TiO2). This very efficient charge separation reaction initiates current flow and the output of electrical energy by the cell.
dye
See Fig 22.21 Housecroft
http://www.konarka.com/
The Ru-bipy absorbs a photon, resulting in a MLCT excitation from the Ru (II) centre to the organic ligand. It is said to be in the excited state S*. g 2. . Injection of the excited electron from the organic ligand into the conduction band of the TiO2 occurs within a femtosecond timescale. This corresponds to oxidation of the Ru (II) to Ru (III) 3. The electron percolates through the porous TiO2 layer to the conductive support and passes the external ( ) load (electrical work) to the counter electrode. 4. At the counter electrode the electron is transferred to I2 to yield I3-. 5. The iodide reduces the oxidized Ru dye (S+, or Ru(III) ) to its original state (So, or Ru (II)). The device operates in a regenerative mode. 1.
Optical Materials: Ruby Lasers
The Cr:Al2O3, a solid-state laser. 3+ Cr3 is incorporated into a synthetic crystal with a corundum structure, Al2O3 to give a red crystal
Absorption spectrum of ruby. Ab ti t f b How it works: The 4A2 is the ground of state the ruby. The two transitions from this state to the 4T1g and 4T2g states are the two strong absorption bands in the ruby absorption spectrum Cr3+ remains in that state for only about 108 seconds - then vibrationally relaxes to one of the two lower energy states indicated in the diagram. Lasing occurs on the transition labeled R1 in the diagram at a wavelength of 694.3 nm. The fluorescent lifetime of the upper lasing state is the average time an atom remains in that state before dropping to a lower state by spontaneous emission. For ruby this is about 3 msec, which is among the longest durations for any laser.
http://www.llnl.gov/nif/library/aboutlasers/how.html
Optical Materials: Nd- YAG lasers
The Nd:YAG, a solid-state laser. Nd:YAG = Neodymium: Yttrium Aluminum Garnet. Nd3+ is incorporated into a synthetic crystal with a garnet like structure, YAG. garnet-like structure YAG YAG = Y3Al5O12 - the Nd substitutes for the Y in the structure to give a purple crystal.
How it works: The upper energy levels of Nd3+ ions absorb light near 0.73 m (720 nm) and 0.8 m (830 nm) (irradiation from lamps). High intensity light overpopulates the excited state creates a population inversion. The Nd3+ ions vibrationally relax from the upper energy levels to the upper laser level. From this level, the main lasing transition occurs emission at 1.06 m (1060 nm), which is green The ions return to the ground state from the lower laser level through vibrational relaxation. The lasing occurs from the transition from 4F3/2 to 4I11/2.
Electrochromic Windows
Smart Windows: Electrochemical Colour Cell
WO3 changes in colour from white to deep blue when reduced LixV2O5 changes in colour from black to colorless when oxidized
http://www.gizmag.com/go/8253/ http://www.ebuild.com/articles/sectionId.1062/513552.hwx
Li-ion batteries
Transition metal Oxide
The first cathode material: LixTiS2
Structure type is CdI2, hcp p yp p packing of g anions, octahedral Ti Li intercalates between the S2- layers Pure TiS2 is a semi-metal upon semi metal, insertion of Li (high conductivity) Lithium insertion varies from 1 x 0 10% expansion, TiS2 LiTiS2 Capacity ~ 250 mAh/g Voltage ~ 1.9V (this is the major limitation of the TiS2 cathode) Energy density ~...