Light Harvesting Efficiency, Absorption Cross Section, Surface Coverage, Langmuir
Adsorption Isotherm, Intramolecular Char...
Importance of Light Harvesting Efficiency
 Light harvesting efficiency is the electrical response of the photovoltaic dev...
Different Methods to Improve the L.H.E
L.H.E depend on various factor, important are follows-
 The number of sensitizing ...
 In one article Scientist show that, Mirror-like nanoparticles can use to boost the
efficiency of solar cells. Scientists...
Application of Light Harvesting Efficiency
 Light harvesting efficiency makes the plants, to able to convert water and ca...
Absorption Cross Section
 Total amounts of the dyes adsorbed on the TiO2 films were determined by measuring the
absorbanc...
Application of absorption cross section
 This amount of sunlight absorbed depends on the fraction of the dye which is ads...
Surface Coverage
 Surface coverage is the number of moles of sensitizer per squarre cm of projected surface area
of the f...
Application of Surface Coverage
Surface coverage value gives the idea, which solvent is beneficial for our solar cell
per...
Langmuir Adsorption Isotherm Model
Surface binding was monitored spectroscopically by measuring the change in film and so...
 The performance of the DSSCs is strongly dependent on the quality of the adsorption of the
dye molecules on the surface ...
Intramolecular Charge Transfer
 Intramolecular charge transfer referred to the migration of electron in which migrating g...
 Figure shows the emission spectra of a
Bodipy fluorophore that has been substituted
with a dimethyl amino group. In a lo...
Application Of Intramolecular Charge Transfer (ICT)
 ICT has been well established that organic π-conjugated donor-accept...
Photo-induced Electron Transfer
 Photoinduced electron transfer has been extensively studied to understand quenching and ...
 Upon excitation the electron donor transfers an electron to the acceptor with a rate kP(r),
forming the charge-transfer ...
 DP and AP do not form a complex when both are in the ground state because this is
energetically unfavorable. The energy ...
Conclusion
 By optimizing the surface concentration value or high extinction coefficient, high light
harvesting efficienc...
Investigation of extinction coefficient ( ) of PPIX film in DMF, THF, Mix solvents.
 Measure absorbance of PPIX film in ...
Reference
L.h.e, acs, sc, laim
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L.h.e, acs, sc, laim

  1. 1. Light Harvesting Efficiency, Absorption Cross Section, Surface Coverage, Langmuir Adsorption Isotherm, Intramolecular Charge Transfer, Photo-induced Electron Transfer Dhruv Sharma
  2. 2. Importance of Light Harvesting Efficiency  Light harvesting efficiency is the electrical response of the photovoltaic device to the solar spectrum projected on earth.  The light harvesting efficiency is directly determined by the surface concentration of the dye in the film, and the molar extinction coefficient of dye. Light harvesting efficiency is given by- L.H.E = 1-10-Γσ Where, Γ is the number of moles of the sensitizer per squarre cm of projected surface area of the film. σ is surface absorption cross section, unit cm2/mol.  More specifically, light harvesting efficiency relies on the process of electronic energy transfer moving electronic excitation energy which is stored fleetingly (nanosecond) by molecules in excited state.  The LHE, together with the quantum yield of charge injection (φinj) and the efficiency of collecting the injected charge at the back con‐tact (ηc), determines the incident monochromatic photon-to-current conversion efficiency (IPCE), defined as the number of electrons generated by light in the external circuit divided by the number of incident photons. IPCE (λ) = LHE (λ) ϕinj ηc
  3. 3. Different Methods to Improve the L.H.E L.H.E depend on various factor, important are follows-  The number of sensitizing dye molecules adsorbed on a photo electrode is one of the key parameter for enhancing the light harvesting efficiency. The electronic interaction between the dye and the TiO2 surface and the mechanism of adsorption of the anchoring group in the dye on the TiO2 surface affect the light harvesting efficiency.  Increasing the internal surface area of the electrode, which has the potential to improve the dye-loading capacity of the photo anode over a specific film thickness and area.  Increasing the optical path of the incident light in the electrode, by introducing scattering centers in the bulk film, by introducing the scattering layer on the top of nanocrystalline electrode, and by constructing hierarchical structures possessing strong light scattering effects.  Enhancing the absorption of dye molecules by introducing plasmonic metal-semiconductor structures.
  4. 4.  In one article Scientist show that, Mirror-like nanoparticles can use to boost the efficiency of solar cells. Scientists in Australia coated a solar cell’s TiO2 photoanode with cubic cerium oxide nanoparticles. The nanoparticles’ large mirror-like facets are good at scattering light back onto the TiO2 nanoparticles, resulting in a 17.8% improvement in the power conversion efficiency compared to regular dye sensitised solar cells. Cubic CeO2 Nanoparticles as Mirror-like Scattering Layer for Efficient Light Harvesting in Dye-Sensitized Solar Cells Lianzhou Wang Chem. Commun., 2012, DOI: 10.1039/c2cc32239k
  5. 5. Application of Light Harvesting Efficiency  Light harvesting efficiency makes the plants, to able to convert water and carbon dioxide into dioxygen and matter with a higher energy content which then serve as energy suppliers for other species.
  6. 6. Absorption Cross Section  Total amounts of the dyes adsorbed on the TiO2 films were determined by measuring the absorbance of the dyes. Absorption cross section is a measure for the probability of an absorption process. The probability that a photon passing through a molecule will be absorbed by that molecule multiplied by the average cross-sectional area of the molecule. The net absorption cross section ( σnet) is defined by- σnet = κ/NA where, κ is the molar absorption coefficient and NA is the Avogadro constant.  Absorption cross section is the ability of a molecule to absorb a photon of particular wavelength.  Absorption cross section units (cm2/mol) are always given as an area, it does not refer to an actual size area, because the state of the target molecule will affect the probability of absorption.  Absorption cross section determine by the molar extinction coefficient as- σ(λ) = Ɛ(λ) . 1000 (cm3L-1)
  7. 7. Application of absorption cross section  This amount of sunlight absorbed depends on the fraction of the dye which is adsorbed on the surface of TiO2 and the extinction coefficient of the dye. Absorption cross section particularly use in material science, to understand the optical property of materials.  Absorption cross section use in dye sensitized solar cell to define, how dye will absorb light in a specific area of the photosensitized film of TiO2.  In Nanoscience absorption cross section use to define the optical property of particles, like absorption and scattering.
  8. 8. Surface Coverage  Surface coverage is the number of moles of sensitizer per squarre cm of projected surface area of the film. Surface coverage is calculated by using the formula- Г = A(λ)/σ(λ) where, A(λ) is absorbance at given wavelength, σ(λ) is absorption cross section in units cm2/mol.  Surface coverage depend on, how the dye attach to the TiO2 electrode. Molecular structure and the adsorption condition (i.e. immersing solvent and time) will have a large impact on the molecular packing, geometry, and aggregation of the porphyrin molecules on the TiO2 electrodes.  The photocurrent density (Jsc) of the solar cell is related to the amount of sunlight which can absorbed by the cell. This amount of sunlight depends on the fraction of the dye which is adsorbed on the surface of TiO2 and the extinction coefficient of the dye. The surface coverage value is also increased with increasing the immersion time to become saturated.
  9. 9. Application of Surface Coverage Surface coverage value gives the idea, which solvent is beneficial for our solar cell performance.  For good surface coverage value, electrons should be injected efficiently into the semiconductor by the photo-excited dye molecules, and for this reason it is necessary to form a monolayer of dye on the TiO2 surface.  For good surface coverage value, electrons should be injected efficiently into the semiconductor by the photo-excited dye molecules, and for this reason it is necessary to form a monolayer of dye on the TiO2 surface.
  10. 10. Langmuir Adsorption Isotherm Model Surface binding was monitored spectroscopically by measuring the change in film and solution absorbance after soaking the film for 1h by varying immersion solutions with known concentration of sensitizers. The equilibrium binding for PPIX was well described by the Langmuir adsorption isotherm model from which surface binding constant (Kad) were calculated using equation- Where, [PPIX]eq is equilibrium sensitizer concentration, Kad is surface binding constant, Г0 is saturation surface coverage, Г is equilibrium surface coverage, Plots of [PPIX]eq/Г vs [PPIX]eq were fitted linearly to obtain the binding constants Kad and surface coverage Г0.
  11. 11.  The performance of the DSSCs is strongly dependent on the quality of the adsorption of the dye molecules on the surface of TiO2 in three different ways- 1. The photocurrent density (Jsc) of the solar cell is related to the amount of sunlight which can absorbed by the cell. This amount of sunlight depends on the fraction of the dye which is adsorbed on the surface of TiO2 and the extinction coefficient of the dye. 2. Electrons should be injected efficiently into the semiconductor by the photo-excited dye molecules, and for this reason it is necessary to form a monolayer of dye on the TiO2 surface. 3. Dye molecules should be regenerated by the redox mediator and not with electrons from the semiconductor. This latter recombination process can be avoided by formation of a monolayer of the dye that behaves like a blocking layer.  The Langmuir isotherm connects the surface coverage of adsorbate to the concentration of a medium. The Langmuir equation has three assumptions; (i) the ability of adsorption for all sites are equal. (ii) only a monolayer of adsorbed molecule will be formed on the surface. (iii) adsorbed molecules have no interaction with neighboring adsorbents.
  12. 12. Intramolecular Charge Transfer  Intramolecular charge transfer referred to the migration of electron in which migrating group never leaves the molecule during migration (Intramolecular charge transfer). While, when migrating group may leave the molecule during migration i.e., the migrating group first gets completely detached from the molecule, subsequently it gets reattached at some other reactive site of the molecules, called as intermolecular charge transfer.  The appearance of the ICT emission depends on the electron donating and -accepting properties of groups within or attached to the fluorophore through “bond”. In ICT, photo induced charge transfer take place from electron donating group such as amino, alcohols, sulfides, esters, nitriles and carboxyl group to electron accepting group such as nitro, cyanide and aldehyde through bond.  If ICT occur in polar solvent, emission spectra show the spectral shift with increase in polarity.
  13. 13.  Figure shows the emission spectra of a Bodipy fluorophore that has been substituted with a dimethyl amino group. In a low polarity solvent the usual narrow emission is seen with a small Stokes shift. In slightly more polar solvents a new longer wavelength emission is seen that is due to an ICT state.
  14. 14. Application Of Intramolecular Charge Transfer (ICT)  ICT has been well established that organic π-conjugated donor-acceptor (D-A) molecules have potential applications in electronics such as electro optic devices.  Intramolecular charge transfer (ICT) in organic system have been widely investigated in order to understand the factors controlling the charge separation and charge recombination, it highly affect the DSSC performance.
  15. 15. Photo-induced Electron Transfer  Photoinduced electron transfer has been extensively studied to understand quenching and to develop photovoltaic devices.  Photoinduced electron transfer is mostly used to describe the redox potentials of the fluorophore and quencher. Studies of PET are usually performed in polar solvents.  Redox potential- The redox potential is a measure (in volts) of the affinity of a substance for electrons, its electro negativity — compared with hydrogen (which is set at 0). Substances more strongly electronegative than (i.e., capable of oxidizing) hydrogen have positive redox potentials. Substances less electronegative than (i.e., capable of reducing) hydrogen have negative redox potentials. When electrons flow "downhill" in a redox reaction, they release free energy. We indicate this with the symbol ΔG (delta G) preceded by a minus sign. If it requires an input of free energy to force electrons to move "uphill" in a redox reaction. We show this with ΔG preceded by a plus sign.
  16. 16.  Upon excitation the electron donor transfers an electron to the acceptor with a rate kP(r), forming the charge-transfer complex [DP +AP –]*. This complex may emit as an quenched and return to the ground state. The important part of this process is the decrease in total energy of the charge transfer complex. The energy decreases because the ability to donate or accept electrons changes when a fluorophore is in the excited state. Excitation provides the energy to drive charge separation.
  17. 17.  DP and AP do not form a complex when both are in the ground state because this is energetically unfavorable. The energy released by electron transfer can also change if the ions become solvated and/or separated in a solvent with a high dielectric constant.  Why is the energy of the charge-transfer state lower than the energy before electron transfer? This can be understood by considering the energy required to remove an electron completely from the electron donor, which is the energy needed to ionize a donor fluorophore. When the fluorophore is in the excited state the electron is at a higher energy level than a ground-state electron. Hence it will require less energy to remove an electron from the S1 (LU) state then from the S0 (HO) state. This means the donor fluorophore in the S1 state has a higher propensity to donating an electron. Now consider a quencher that is an electron acceptor. The energy released on binding the electron is larger if the electron returns to the S0 state than to the S1 state. The electron can return to the lowest orbital of the quencher because the donor–acceptor complex is momentarily an excited-state complex. When the electron acceptor is in the excited state there is a place for the electron to bind to the lowest orbital.
  18. 18. Conclusion  By optimizing the surface concentration value or high extinction coefficient, high light harvesting efficiency can achieve.  The amount of sunlight absorbed depends on the cross section area of the dye, state of target molecule, which is adsorbed on the surface of TiO2 and the extinction coefficient of the dye.  Surface coverage value of PPIX/TiO2 film in DMF solution is 2.36 X 10-9 cm2/mol, THF solution 4 X 10-9 cm2/mol and in Mixture solvent 5 X 10-8 cm2/mol. This is mainly because DMF and THF both are non-protic solvent, but DMF have high dipole moment and high dielectric constant then THF. So, surface coverage is the important factor to increase the DSSC performance efficiency.  By using appropriate donor-acceptor combination, we can increase the ICT efficiency, by which DSSC efficiency can increase.
  19. 19. Investigation of extinction coefficient ( ) of PPIX film in DMF, THF, Mix solvents.  Measure absorbance of PPIX film in DMF, THF, Mix solvent (at 1 X 10-4M) and calculate the surface coverage value in respective solvent (Immersion solvent).  Measure absorbance of PPIX film in DMF (1,3,5 X 10-5 M), THF ((1,3,5 X 10-5 M)), Mix solvent ((1,3,5 X 10-5 M)) and calculate the Kadsorption value in respective solvent.  Measure absorbance of PPIX in DMF solvent (concentration 1 X 10-4M ) 1hour, 3 hour, 5 hour (Immersion Time).  To study the change in light harvesting efficiency (L.H.E) with immersion solvent, immersion time, immersion concentration respectively.  To study the change in surface coverage value ( ) with immersion solvent, immersion time, immersion concentration respectively. To study the effect of different solvent to control the aggregation and adsorption kinetics for increase the efficiency of DSSC performance.  To make the highly transparent TiO2/PPIX films..  To study the electron injection and charge recombination study by using Steady state and time resolved fluorescence spectroscopy. Scope of Project
  20. 20. Reference

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