Highsurface area of metal oxides provides a unique environment for substrateassembly at an interface. The choice of DSSCs molecule, metal oxide and theassembly method has significant impact on the mechanism, rate and efficiency ofphoto-induced energy and electron transfer at the interface.6 For example, the semiconductor nanoporousTiO2 when used as an electron collector in conjunction with an acetonitrile orother ionic liquid based electrolyte highly improves the performance of DSSCs. Additionally,the interface offers a means of controlling the proximity and orientation ofthe molecules which are critical factors dictating the rate and efficiency ofsolar cells. Several reports have found that Nanocrystaline such as ZrO2 hasrelative high conduction band energy which inhibits the excited state electrontransfer to the substrate. In recent days TiO2 has been employed as the chargeseparation interface to extract charge.
6Nanocrystaline thickness of TiO2 film have tremendous effect in thephotovoltaic characteristics, particularly for an ionic-liquid-basedelectrolyte, owing to the limitation of the photocurrent by the diffusion ofTi-iodide ions.7Choice of Semiconductor nanoporousmaterialTheabsorption of light photon in DSSC takes place in the dye’s molecule layer.Nanocrystaline TiO2 provides the indispensable major surface areafor dye molecule absorption.
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The sensitizers collect photons and produces theexcited electrons (ex) from the highest occupied molecular orbital (HOMO) inthe ground state to the lowest unoccupied (LUMO) orbital in the excited state.The dye inject an excited electron into the conduction band of nanoporous TiO2film and ZnO or SnO2, which are used in solar cells. The dye isagain oxidized after has lost the electron. The injected electron through thenanoporous TiO2 thin film toward the anode (conductive electrode)and is simultaneously attracted to a load where the work performed is deliveredas electrical energy. The electron finally travels back and reaches the cathode(carbon plated counter electrode) and the TiO2 photoelectrode. Thusthe oxidized dye receive electron from iodide ion redox to replace the lostelectron.
Then, the iodide molecules are oxidized into tri-iodide ions. Thetri-iodide ions float around until they reach the cathode and the circuit iscompleted by the immigration of the electron through the external load. Theelectron motion in the conduction band of the wide band gap semiconductornanoporous TiO2 is escorted by charge compensation cation diffusionin the electrolyte relative to the semiconductor surface. Therefore, thegeneration of electric current in DSSC doesn’t permanent chemicaltransformation.5 Ionic liquid containing iodideion/tri-iodide ion have been for to be the medium of choice for outdoorapplication due their thermal stability, non-flammability, negligible vapourpressure and low toxicity.3aOperation Principle of DSSCs (iii) Significantreduction of charge recombination between electrons in the semiconductor and electrolyteacceptor species and oxidized dye molecules at the interface.
(ii) Dramatic performance due to efficientincrease in the photocurrent and photovoltage. (i) Improvement in the efficiency of lightharvesting of combined synthetic and natural dyes compared to individual dyes-Thisstudy intends to investigate theoretically the combination of metal freeorganic synthetic dyes and natural dyes for the improvement of the favourablecharacteristics such as: Dye-sensitizedsolar cells (DSSCs) have attracted much attention since the first discovery ofa simple device capable of converting sunlight into electricity aided by regenerative photochemical processes in the 1991by Michael Grätzel and Brian O’Regan.12The low cost, facile methods used in production and relatively high energyconversion efficiency, turnable optical properties such as colour andtransparency make them good candidates for renewable source of energy.3 The overall powerconversion has risen to over 12% when the liquid electrolyte is adopted andwith high achievement in the long term stability for practical applications.4The increasingly global energy demand, has led to investigation of cheap materialsfor conversion and storage of renewable and sustainable source of energy. Byemploying organometallic dyes has led to the best performances, however, thesematerials have been proven to be expensive and not environmental friendly. Toaddress this, there are several studies that have reported significant resultsby employing fully organic dye sensitizers for dye sensitized solar cells. Aremarkable limit lies in the absorption spectrum that typically show a singleand narrow absorption band in the visible region around (550 nm).
These limitsare imposed by building blocks used and molecular design. The extension of theabsorption to a much longer wavelength near infrared region (NIR) will resultinto fundamental breakthrough for the metal free organic dyes. The absorption to(NIR) for organometallic dyes is the main reason for their higher performances.
Several literature reports and have found that DSSCs offers promising resultsand so named fourth generation solar cell materials. MolecularDesign for metal free Organic and Natural Dyes Towards co-Sensitized SolarCells: Theoretical and Computational Studies