1. Light Harvesting Efficiency, Absorption Cross Section, Surface Coverage, Langmuir
Adsorption Isotherm, Intramolecular Charge Transfer, Photo-induced Electron Transfer
Dhruv Sharma

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. 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.  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. 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. 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. 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. 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. 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. 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.  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.

13. 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.

14.  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.

15. 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.

16. 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.

17.  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.

18.  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.

19. 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.

20. 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

21. Reference