Chemistry and physics of dssc ppt
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Dye-sensitized solar cells (DSSCs) represent an innovative approach to solar energy conversion, utilizing a dye-coated titanium dioxide layer to generate electricity at efficiencies near 10%. The operational mechanisms differ from traditional solar cells, relying on charge separation and transfer processes involving a photo-excited dye, a redox electrolyte, and various conductive materials. While promising, challenges such as stability, efficiency optimization, and the replacement of volatile organic solvents (VOS) with more stable alternatives remain critical for broader implementation.
![SCHEMATIC REACTION
• In DSSCs, the basic photovoltaic principle relies on
the visible photo-excitation of dye molecule; the
schematic reaction of overall process is follows:
• TiO2/D + hƲ ↔ TiO2/D*LUMO (1)
• D*LUMO + CBTiO2 → TiO2/D+ + eCB- (2)
• Pt + [I3]- → 3I- (3)
• TiO2/D+ + 3I- → [I3]- + D (4)
• eCB- + DHOMO → D + CBTiO2 (5)
• eCB- + [I3]- → 3I- + CBTiO2 (6)](https://image.slidesharecdn.com/chemistryandphysicsofdsscppt-150202025820-conversion-gate02/85/Chemistry-and-physics-of-dssc-ppt-7-320.jpg)
Chemistry and physics of dssc ppt
- 1. CHEMISTRY AND PHYSICS OF DSSC
- 2. INTRODUCTION • DSSCs given born to new solar cells generation replaced classical solid-state homo and hetero- junction device by a new concept with a nano- working electrode in photo-electrochemical cell. • This technology offers a very low cost fabrication and easy industry introduction prospective; furthermore efficiencies near to 10% AM1.5 for DSSCs have been confirmed.
- 3. COMPONENTS OF DSSC • DSSC’s consists of three main components: • A dye-covered Nanoporous TiO2 layer on a glass substrate coated with a transparent conductive oxide (TCO) layer, • An redox electrolyte and • a electrical contact deposited on conducting glass.
- 4. MECHANISM OPERATION • All photovoltaic devices present two important steps to convert sunlight into electrical energy: 1. Radiation absorption with electrical excitation. 2. Charge carriers separation. • The way which radiation is absorbed and carriers are separated are two of the main differences between DSSCs and classical p-n junction. • Conventional photovoltaic principle relies on differences in work functions between the electrodes of the cell in which photo generated carriers could be separated, and an asymmetry through cell is necessary to obtain electrical power. • In classical p-n junction of solid state device the separation of photo generated carriers relies on separation through depletion region built at p-n interface materials.
- 5. DIFFERENT PROCESS IN DSSC – The working electrode of DSSCs is conventionally constituted by mesoporous network of TiO2 nanocrystalline (5-15m, thickness) covered with a dye monolayer (conventionally Ru complex). – This working electrode is supported on conducting glass (transparent conducting oxide, TCO). – Different materials as platinum, palladium and gold could be use as counter-electrode of the cell. – Finally the gap between the electrodes is typically filled with a molten salt which containing a redox couple (A/A-); this salt is a hole conductor
- 6. • Most DSSCs studied so far employ redox couple as iodide/tri-iodide (I-/I3-) couple as electrolyte because of its good stability and reversibility. • However others hole conductors as solid and ionic electrolytes also can be used. • In overall process, the DSSCs generate electric power from light without suffering any permanent chemical transformation
- 7. SCHEMATIC REACTION • In DSSCs, the basic photovoltaic principle relies on the visible photo-excitation of dye molecule; the schematic reaction of overall process is follows: • TiO2/D + hƲ ↔ TiO2/D*LUMO (1) • D*LUMO + CBTiO2 → TiO2/D+ + eCB- (2) • Pt + [I3]- → 3I- (3) • TiO2/D+ + 3I- → [I3]- + D (4) • eCB- + DHOMO → D + CBTiO2 (5) • eCB- + [I3]- → 3I- + CBTiO2 (6)
- 8. EXPLANATION ON REACTION Where CBTiO2 is TiO2 conduction band and D is dye molecule. First, an electron is photo excited from highest occupied molecular orbital (HOMO) level to lowest unoccupied molecular orbital (LUMO) level into dye molecule - (eq. 1). Then, electron injection from excited dye (D*) into TiO2 conduction band occurs . - (eq. 2) Excitation of dye usually is a transition-metal complex whose molecular properties are specifically for the task is able to transfer an electron to TiO2 by the injection process. After that, electron migrates through TiO2 network toward the TCO substrate . The physics of charge transfer and transport in molecular and organic materials is dominated by charge localization resulting from polarization of the medium and relaxation of molecular ions
- 9. • Electron does a electric work at external load after go out from solar cell and then, come back through counter electrode. • Electrolyte transports the positive charges (holes) toward the counter electrode and couple redox is reduced over its surface (eq. 3), • On same time couple redox reduce the oxidized dye and regenerate the dye (eq. 4).
- 10. Additionally In this process, some electrons can migrate from CBTiO2 to the HOMO level of the dye or electrolyte Due to electron trapping effects; this process results in electron recombination (eq. 5, 6). These processes decrease the cell performance for affecting all its parameters
- 11. DIFFERENT STRUCTURES • Titanium dioxide presents three mainly different crystalline structures: rutile, anatase, and brookite structures; and other structures as cotunnite has been synthesized at high pressures. • Despite their three stable structures, only rutile and anatase play any role on DSSCs.
- 12. WORKING ELECTRODE • TiO2 films are important optical films due to their high reflective index and transparency over a wide spectral range. • Despite the existence of other types of oxide semiconductor with high band-gap and band gap positions as SnO2 and ZnO, the TiO2 thin films are the most investigated material as photo- electrode to be used in DSSCs. • Because the efficiency DSSCs constructed with TiO2electrodes yield the highest values of Isc, Voc, and the IPCE
- 13. KINETICS OF CHARGE TRANSFER • It is broadly accepted that DSSCs efficiency is mainly governed by the kinetics of charge transfer at the interface between TiO2, the dye, and the hole transport material..
- 14. STEPS OF PROCESS • The initial steps of charge separation in a DSSCs are the injection of an electron from a photo excited dye to the conduction band of the TiO2 . • Subsequently, the transfer of an electron is from the hole transport molecule to the dye. • The first process is usually completed within 200 ps . • Latter, the regeneration of the oxidized dye, is completed within the nanosecond time scale for liquid electrolyte DSSCs containing an (I-/I3-) redox couple.
- 15. EIS METHOD • Electrochemical impedance spectroscopy (EIS) is an experimental method of analyzing electrochemical systems. • This method can be used to measure the internal impedances for the electrochemical system over a range of frequencies between mHz-MHz. • Additionally EIS allows obtaining equivalent circuits for the different electrochemical systems studied.
- 16. DRAWBACK OF EIS METHOD • One drawback with DSSCs is the high band gap of the core materials, e.g. TiO2 particles <50–70nm in size (3.2 eV, wavelength <385nm in the ultraviolet range), compared to other semiconductors, which is illustrated by its absorption of UV light but not visible light. • In the case of solar ultraviolet rays, only 2–3% of the sunlight (in the UV spectrum) can be utilized To solve this, it appears the main characteristic of DSSCs: the dye, one of the most important constituents of the DSSCs.
- 17. • A variety of transition-metal complexes and organic dyes has been successfully employed as sensitizers in DSSCs thus far. • However the most efficient photosensitizers are ruthenium(II) polypyridyl complexes that yielded more than 11% sunlight to electrical power conversion efficiencies. • The first two sensitizers used in DSSCs were N3 and N719. N3 and N719, affording DSSCs with overall power conversion efficiencies of 10.0% and 11.2 %respectively.. SENSITIZER
- 18. REQUIREMENTS OF EFFICIENT SENSITIZER • A broad and strong absorption, preferably extending from the visible to the near infrared. • Minimal deactivation of its excited state through the emission of light or heat. • Irreversible adsorption (chemisorption) to the surface of the semiconductor and a strong electronic coupling between its excited state and the semiconductor conduction band. • Chemical stability in the ground as well as in the excited and oxidized states, so that the resulting DSSCs will be stable over many years of exposure to sun light.
- 19. • A reduction potential sufficiently higher (by 150–200mV) than the semiconductor conduction band edge in order to bring about an effective electron injection. • An oxidation potential sufficiently lower (by 200–300mV) than the redox potential of the electron mediator species, so that it can be regenerated rapidly.
- 20. VOS • A typical DSSCs consists of a dye-coated mesoporous TiO2 nano-particle film sandwiched between two conductive transparent electrodes, and a liquid electrolyte containing the (I-/I3-) couple redox to fill the pores of the film and contact the nano-particle. • The electrolyte, as one of the key components of the DCCS, provides internal electric ion conductivity by diffusing within the mesoporous TiO2 layer and is an important factor for determining the cell performance . • Typical VOS are : • Acetonitrile, propionitrile, methoxyacetonitrile, and methoxypropionitrile.
- 21. DRAWBACKS OF VOS • Volatile organic solvents (VOS) are commonly used as electrolyte (hole conductor). • And nowadays, high efficiencies can be obtained for DSSCs using electrolytes based on VOS. • Despite the relative high efficiency values of VOS in DSSCs (about 10%), the usage of these VOS electrolytes seriously limits a large scale implementation of this technology. • Due to the poor long-term stability of the cells and the necessity of a complex sealing process.
- 22. REPLACEMENT OF VOS BY ELECTROLYTE • Today this is one of the major drawbacks of DSSCs to implementation in a factory production line. • New field research tries to replace the VOS in DSSCs by new type of electrolytes such as: • Ionic liquid electrolytes, p-type inorganic semiconductors and polymer electrolytes.
- 23. USE OF RTIL • As an alternative to VOS, the use of room temperature ionic liquids (RTIL) and low molecular weight organic solvents with polar ligand have received considerable attention..
- 24. PROPERTIES OF RTIL RTIL have several superior properties such as : • The non-volatility, • Chemical stability, • Not inflammability and • High-ionic conductivity at room temperature.
- 25. LIMITATIONS OF RTIL • Nevertheless, the viscosity of the non-volatile solvents is usually higher than those of conventional volatile solvents, leading the energy conversion efficiency, which does not reach those of DSSCs employing conventional VOS electrolytes. • The low mass transport of redox couples is a limiting factor of applicability of these solvents in DSSCs.
- 26. • Therefore, high concentration of redox couples is required to obtain considerable conductivity in the electrolyte medium. • Polyether groups improve the self organization of molecules in electrolyte which increase the charge transfer and the wetting capacity of the TiO2 surface.
- 27. P-TYPE SEMICONDUCTOR • The most common approach to fabricate solid-state DSSCs is by using p- type semiconductors. Several aspects are essential for any p-type semiconductor in a DSSCs: • It must be able to transfer holes from the sensitizing dye after the dye has injected electrons into the TiO2 (i.e) the upper edge of the valence band of p-type semiconductors must be located above the ground state level of the dye. • It must be able to be deposited within the porous nano crystalline layer. • A method must be available for depositing the p-type semiconductors without dissolving or degrading the monolayer of dye on TiO2 nano crystallites. • It must be transparent in the visible spectrum, or, if it absorbs light, it must be as efficient in electron injection as the dye.
- 28. LIMITATIONS AND OVERCOME • However, the familiar large-band gap p-type semiconductors such as SiC and GaN are not suitable for use in DSSCs since the high- temperature deposition techniques for these materials will certainly degrade the dye. • After extensive experimentation, a type of inorganic p-type semiconductor based on copper compounds such as CuI, CuBr, or CuSCN was found to meet all of these requirements
- 29. POLYMER ELECTROLYTE • According to the physical state of the polymer, there are two types of polymer electrolytes used in DSSCs: THEY ARE : • solid polymer electrolytes • gel polymer electrolytes.
- 30. SOLID POLYMER ELECTROLYTE • Compared with inorganic p-type semiconductors, organic p-type semiconductors (i.e. organic hole- transport materials) possess the advantages of having plentiful sources, easy film formation and low cost production. • However, the conversion efficiencies of most of the solid-state DSSCs employing organic p-type semiconductors are relatively low particularly under high light irradiation.
- 31. GEL POLYMER ELECTROLYTE • GPE have some advantages, such as low vapor pressure, excellent contact in filling properties between the nanostructured electrode and counter-electrode, higher ionic conductivity compared to the conventional polymer electrolytes. • Furthermore GPE possess excellent thermal stability and the DSSCs based on them exhibit outstanding stability to heat treatments. • There was negligible loss in weight at temperatures of 200°C for ionic liquid-based electrolytes of poly (1-oligo(ethylene glycol) methacrylate-3-methyl-imidazoliumchloride) (P(MOEMImCl). • Thus the DSSCs based on GPE have outstanding long-term stability.
- 32. REDOX COUPLE • It is well known that the iodide salts play a key role in the ionic conductivity in DSSCs. • However, despite of its qualities; (I-/I3-) couple redox has some drawbacks
- 33. DRAWBACKS • The corrosion of metallic grids (e.g., silver or vapor-deposited platinum) and • The partial absorption of visible light near 430 nm by the I3- species. • The (I-/I3-) couple is the mismatch between the redox potentials in common DSSCs systems with Ru based dyes, which results in an excessive driving force of 0.5~0.6 eV for the dye regeneration process.
- 34. OVERCOME OF DRAWBACKS • In order to minimize voltage losses, other redox couples have been also used, such as : • SCN-/(SCN)3; SeCN-/(SeCN) 3-, (Co2+/Co3+), (Co+/Co2+), coordination complexes, and • Organic mediators such as 2,2,6,6- tetramethyl-1- piperidyloxy , SCN-/(SCN)3; SeCN-/(SeCN) 3-, • (Co2+/Co3+), (Co+/Co2+), coordination complexes, • And organic mediators such as 2,2,6,6- tetramethyl-1- piperidyloxy (Min et al, 2010). • Best efficiency has been reached with (I-/I3-) couple redox
- 35. COUNTER ELECTRODE • In DSSCs, counter-electrode is an important component, the open-circuit voltage is determined by the energetic difference between the Fermi-levels of the illuminated transparent conductor oxide (TCO) to the nano-crystalline TiO2 film and the counter- electrode where the couple redox is regenerated
- 36. METAL COUNTER ELECTRODE • Metal substrates such as steel and nickel are difficult to employ for liquid type DSSCs because the I-/I3- redox species in the electrolyte are corrosive for these metals. • However, if these surfaces are covered completely with anti-corrosion materials such as carbon or fluorine-doped SnO2, it is possible to employ these materials as counter- electrodes. • Metal could be beneficial to obtain a high fill factor for large scale DSSCs due to their low sheet resistance. Efficiencies around 5.2% have been reported for DSSCs using a Pt-covered stainless steel and nickel as counter- electrode .
- 37. CARBON COUNTER ELECTRODE • First report of carbon material as counter electrode in DSSCs was done by Kay and Grätzel. • In this report they achieved conversion efficiency about 6.7% using a monolithic DSSCs embodiment based on a mixture of graphite and carbon black as counter electrodes. • The graphite increases the lateral conductivity of the counter electrode and it is known that carbon acts like a catalyst for the reaction of couple redox (I3-/I-) occurring at the counter electrode. • Recently, carbon nanotubes have been introduced as one new material for counter electrodes to improve the performance of DSSCs . • The possibility to obtain nanoparticles and nanotubes of carbon permits investigation of different configuration in synthesis of counter electrode fabrication, to improve the DSSCs efficiency.
- 38. DEGRADATION OF DSSCs • According to the operation principle, preparation technology and materials characteristics of DSSCs, they are susceptible to: • Physical degradation: The system contains organic liquids which can leak out the cells or evaporate at elevated temperatures. • This could be overcome using appropriate sealing materials and low volatiles solvents. • Chemical degradation: The dye and electrolyte will photo chemically react or thermal degrade under working conditions of high temperature, high humidity, and illumination. • The performance DSSCs will irreversibly decrease during the process causing the life time lower than commercial requirements (>20 years)..
- 39. CONCLUSION • DSSCs are considered one economical and technological competitor to pn junction solar cells. • However, the module efficiency of DSSCs needs to be improve to be used in practical applications. • It is necessary to achieve the optimization of the production process to fabricate photo electrodes with high surface area and low structural defects. • It is necessary to solve problems associated to encapsulation of (I-/I3-) redox couple in an appropriate medium such as: • Ionic liquid electrolytes, p-type semiconductors, Solid polymer electrolytes, Gel polymer electrolytes and the deposited stable and cheap counter electrode. • If this problems are solved is possible that in near future DSSCs technology will become in a common electrical energy source and widely used around the world.