Light Harvesting Efficiency, Absorption Cross Section, Surface Coverage, Langmuir

Adsorption Isotherm, Intramolecular Charge Transfer, Photo-induced Electron Transfer

Dhruv Sharma

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

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.

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

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.

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)

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

Reference