Energy conversion materials

Energy Conversion Materials The solar spectrum o About 46% of the spectral energy is distributed in the visible region o About 49% in near IR o About 3% in UV region and rest in far IR region Solar energy conversion devices Methods of tapping solar energy A Photosynthesis Plants (Visible light ) η = 2-4% B Water heaters Flat plate, tube (IR radiation) C Photovoltaic cells D Chemical routes p/n Si, a-Si, GaAs (Visible light) η = 12-26% D.2 PEC cells D.1 Biomimetism Mimicking Photosynthesis via chemicals b Photoelectrosynthesis a LJSC (PES) cells (i) Photoassisted (i) Sc/Elect/M η= 13-14% (ii) Photogalvanic cells M/Elect/M η= 0.01% electrolysis cells η= 13.3% (ii) Photoassisted electrosynthesis cells eg CO2 N2 CH3OH NH3 Drawback Of The Present Devices These devices are quantum converters, in which a photon is absorbed resulting in an electron-hole pair or breaking of the chemical bond These can use only the relatively high energy photons and considerable portion of the IR radiation cannot be used The photovoltaic technology has very high efficiencies of the order of 26% (on a laboratory scale) but it is not completely realised Photovoltaics & Photoelectrochemical cells (PEC) Terminology o In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an electric field o The only elements left in the depletion region are ionized donor or acceptor impurities o The Fermi level is an energy pertaining to electrons in a semiconductor Devices of solar energy conversion Photovoltaic cells Photoelectrochemical cells Photogalvanic cells Solar thermal (eg water heater) Dye sensitized solar cells Photovoltaic cells  A solar cell is a device that converts the energy of sunlight directly into electricity by the photovoltaic effect  The photovoltaic effect involves creation of a voltage (or a corresponding electric current) in a material upon exposure to electro-magnetic radiation  Though the photovoltaic effect is directly related to the photoelectric effect, the two processes are different  In the photoelectric effect electrons are ejected from a material's surface upon exposure to radiation of sufficient energy The semiconductor / electrolyte interface Relation to Fermi level A flat band potential semiconductor B accumulation layer electrolyte semiconductor + - + - + - + + - + Ec Ef + - E conduction band + - - + - electrolyte + + + conduction band Ec Ef Eredox Eredox Ev Ev valence band valence band - conduction band electrons + positive charge carriers - electrolyte anions D inversion layer C depletion layer semiconductor + - + - + - + + - + conduction band Ec Ef semiconductor electrolyte + + + - - + + + conduction band E electrolyte E Ec Ef Eredox Eredox Ev valence band E valence band Photoelectrochemical cells  Photoelectrochemical (PEC) cell is a device in which a photoactive semiconductor material is in contact with an electrolyte  Irradiation of the SC/electrolyte junction with light of energy > Eg, the band gap of the semiconductor, produces electron - hole pairs  The electron-hole pairs are spatially separated (due to the junction potential) to drive oxidation and reduction reactions in the system Construction of Grätzel cell o o o In Grätzel cell a range of organic dyes are used Examples: ruthenium- Polypyridine, Indoline dye & metal free organic dye These dyes are extractable from simple foods such as hibiscus tea, tinned summer fruits, blackberries Construction: o Two transparent glass plates are perforated on one side with a transparent thin layer of a conducting material o Onto the conducting sides, one plate is coated with graphite and the other plate is coated with titanium dioxide (TiO2) o o o o o A dye is then adsorbed onto the TiO2 layer by immersing the plate into a dye solution for 10 min.(approx.) The plates are then carefully sandwiched together and secured using a paperclip To complete the cell a drop of iodide electrolyte is added between the plates Figure shows a Grätzel cell prepared from hibiscus tea The upper plate is the TiO2 plate, dyed with hibiscus tea and the lower plate is coated with graphite Working Principle of Grätzel Cell o Sunlight energy passes through the titanium dioxide layer and strikes electrons within the adsorbed dye molecules o Electrons gain this energy and become excited o The excited electrons escape from the dye molecules to become free electrons o These free electrons move through theTiO2 and accumulate at the –ve plate (dyed TiO2 plate) o The free electrons then start to flow through the external circuit to produce an electric current o This electric current powers the light bulb o To complete the circuit, the dye is regenerated o The dye regains its lost electrons from the iodide electrolyte o Iodide (I ) ions are oxidised to tri-iodide (I3 ) o The free electrons at the graphite plate then reduce the tri-iodide molecules back to their iodide state o The dye molecules are then ready for the next excitation/oxidation/reduction cycle Characteristics of Semiconductor electrodes  Oxidic semiconductors (OS) such as TiO2, ZrO2, etc are being widely used as electrodes for a) photoelectrochemical (PEC) conversion of solar energy b) as photocatalysts for decomposition of toxic pollutants c) for preparation of the practically important catalysts and for the last 25 years  To improve photochemical properties of the OS at λ = 400 nm, doping of the OS matrix with transition metal ions was usually applied  It should be mentioned that influence of various metal dopants on the OS properties is rather well known, whereas peculiarities of their structure are studied poorly TiO2 based cells A Structure of the Doped Polycrystalline TiO2 o The samples of the ceramic polycrystalline TiO doped electrodes were prepared by elaborate mixing the precise amounts of specially purified TiO 2, V2O5, Cr2O3 or Nb2O5 powders, pressed into bricks and heated in air at 1200 °C for h in inert atmosphere (He) o Then the stuffs were ground and treated at 1200 °C for h in inert atmosphere o Samples set contained in their matrix uncontrolled amount of oxygen vacancies o The samples of set were additionally treated at 900 C in air for h to obviate these vacancies o The X -ray analysis showed that all mixtures had the rutile structure o The bricks of this modified TiO2 were cut to plates of 1.0 mm thickness & both faces were polished o The back side was covered by In or Cu using the vacuum-deposition technique, to make the electrical contact Photoelectrochemical properties of doped TiO2 Fig presents the photocurrent Spectra of polycrystalline Ti1-xVxO2 electrodes at different x values Similar ones have been obtained for Ti1-xCrxO2 Samples Although there is a strong increase of the visible light absorption at x > 0.01 there is a tenfold (Fig 1) drop of the photocurrent with increasing of x (Fig 2) For better understanding of the causes of this drop the Spatial organization of the doped OS on a molecular level has been studied Photoconductivity  In certain materials, there is an increase in electrical conductivity which results from increase in the number of free charge carriers generated when photons are absorbed  Note: The photons must have quantum energy sufficient to overcome the band-gap in the material in question Basic principles of the photoconductive effect  Directly beneath the conduction band of the CdS crystal is a donor level and there is an acceptor level above the valence band In darkness, the electrons and holes in each level are almost crammed in place in the crystal and the photoconductor is at high resistance  When light illuminates the CdS crystal and is absorbed by the crystal, the electrons in the valence band are excited into the conduction band This creates pairs of free holes in the valence band and free electrons in the conduction band, increasing the conductance  Furthermore, near the valence band is a separate acceptor level that can capture free electrons only with difficulty, but captures free holes easily This lowers the recombination probability of the electrons and holes and increases the number for electrons in the conduction band for N-type conductance Cadmium Sulphide (CdS) Cell The processes of making the photoconductive layer a) sintered type b) single crystal type c) evaporated type  Sintered type offers high sensitivity, a large mass production effect and relatively superior production profitability  Impurities and a fusing agent for encouraging crystal growth are added to highly pure CdS crystal power and this mixture is dissolved in water  The resulting solution is applied to CdS ceramic substrate and dried  Then it is sintered in a high-temperature oven to form multiple crystals  Thus, a thick layer with the photoconductive effect is formed  Then, lead terminals are introduced to the CdS substrate and the CdS is packaged CdS Cell Working process of CdS cells Spectral response of CdS Cells  The relative Sensitivity of a CdS cell depends on the wavelength of the incident light  The sensitivity as a function of wavelength is called the spectral response characteristic  The maximum sensitivity wavelength (or peak wavelength) for CdS cell is 515 nm  But by controlling the composition ratio of CdS to CdSe, the maximum sensitively can be optimized at a wavelength between 515 and 730nm  So, photoconductive cells with spectral response close to that of the human eye are available  By using a CdS cell with a spectral response similar to the human eye, it can be widely and easily be used in applications as sensors substituting for the human eye Spectral Response Characteristics of CdS cell The relative sensitivity of a CdS cell is dependent on the wavelength of the incident light The sensitivity as a function of wavelength is called the spectral response characteristic CdS cells Vs Human Eye Examples of CdS cell configurations Plastic coated CdS cells [...]... Conditions for Efficient Solar Energy Conversion – Electrodes  The requirements for the electrode materials are: (1) Band gap should be optimum (see section on efficiency considerations) (2) The doping level should be optimum so that there will be a good spatial separation of the photogenerated carriers and hence, high quantum efficiency Conditions for Efficient Solar Energy Conversion – Redox couple... are Classified into two types according to their application 1 Liquid Junction Solar Cell (LJSC) – This cell is used to convert solar energy into electrical energy 2 Photoelectrosynthesis (PES) cells – In this class of cells, solar energy is converted into chemical energy in the form of fuels Major advantages of PEC cells over photovoltaic cells  Easy junction formation (mere dipping of the SC electrode... Photochemical cells used for the photoassisted electrolysis of H 2O Dye Sensitization - Grätzel cell 1 Sunlight energy (photon of light) passes through the titanium dioxide layer and strikes electrons within the adsorbed dye molecules Electrons gain this energy and become excited because they have the extra energy 2 The excited electrons escape the dye molecules and become free electrons These free electrons... Labs as a method to prepare high purity materials for manufacturing transistors Its early use was on germanium for this purpose, but it can be extended to virtually any solute-solvent system having an appreciable concentration difference between solid and liquid phases at equilibrium This process is also known as the float zone process, particularly in semiconductor materials processing Photochemical... resistance of the cell) Why Silicon Silicon is a very common element abundant in nature (it is the main element in sand and quartz) Silicon is considered as the most suitable material for solar energy conversion because of 1 its abundance 2 Optimum band gap of 1.23 eV at 300K 3 Cost effectiveness Production of Silicon 1 Metallurgical Grade Silicon: SiO2 +2C Si +2CO  Sand (SiO2) is heated with carbon... plate is the TiO2 plate, dyed with hibiscus tea and the lower plate is coated with graphite Working Principle of Grätzel Cell o Sunlight energy passes through the titanium dioxide layer and strikes electrons within the adsorbed dye molecules o Electrons gain this energy and become excited o The excited electrons escape from the dye molecules to become free electrons o These free electrons move through... Solar Cell (LJSC) Cell : n-CdSe / Na2S + S + NaOH / Pt At the anode: S 2- + 2+ 2h -> S2 At the cathode: 22S2 + 2e - > + S Net reaction : Nil Energetics of LJSC Energetics of LJSC Energetics of LJSC  Energy band representation of the operation of PAE cell (a) in dark, after equilibration (b) under illumination without applied bias (c) under illumination with applied bias to effect electrolysis Photo-assisted... Fermi level of the SC should be above (in the case of n-type semiconductors) the Eredox Working of LJSC  The non-equilibrium electrons in the valence band are produced by illumination of light with energy hν ≥ Eg  The minority carriers (h+ in the n-type SC’s) are transferred to the surface where they are used up for oxidation and the electrons are transferred to the surface where they are used up ... spectral energy is distributed in the visible region o About 49% in near IR o About 3% in UV region and rest in far IR region Solar energy conversion devices Methods of tapping solar energy A... cell is used to convert solar energy into electrical energy Photoelectrosynthesis (PES) cells – In this class of cells, solar energy is converted into chemical energy in the form of fuels Major... ionized donor or acceptor impurities o The Fermi level is an energy pertaining to electrons in a semiconductor Devices of solar energy conversion Photovoltaic cells Photoelectrochemical cells Photogalvanic