dye-sensitized

solar cells

By: Heba Bakry

Outline

1. Introduction.

2. Historical background .

3. Mechanism of DSSC .

4. Composition of DSSC.

5. How does DSSC work?

6. Operation principle of the dye-sensitized nano-crystalline solar cell (DSC).

7. Present DSC research and development.

dye-sensitized solar cells are a generation

photovoltaic (solar) cell that converts any visible

light into electrical energy.

invented by Brian o’ Regan and Michael Gratzel .

DSSC is a disruptive technology that can be used

to produce electricity in a wide range of light

conditions, indoors and outdoors, enabling the

user to convert both artificial and natural light into

energy to power a broad range of electronic

devices.

Introduction…

Historical background…

The first panchromatic film, able to render the image of a scene realistically into black and white, followed on the work of Vogel in Berlin after 1873, in which he associated dyes with the halide semiconductor grains. The first sensitization of a photo-electrode followed shortly thereafter . And their operating mechanism is by injection of electrons from photo-excited dye molecules into the conduction band of the n-type semiconductor substrates .

date to the 1960s the idea developed that the dye could function most efficiently if chemisorbed on the surface of the semiconductor . The concept emerged to use dispersed particles to provide a sufficient interface , then photo-electrodes where employed .

Titanium dioxide became the semiconductor of choice. The material has

many advantages for sensitized photochemistry and photo-electrochemistry:

it is a low cost , widely available, non-toxic and biocompatible material, and

as such is even used in health care products as well as domestic

applications such as paint pigmentation. The standard dye at the time was

tris(2,2-bipyridyl-4,4-carboxylate) ruthenium(II), the function of the

carboxylate being the attachment by chemisorption of the chromophore to

the oxide substrate.

in1991 of the sensitized electrochemical photovoltaic device with a

conversion efficiency at that time of 7.1% under solar illumination, was

incremental, a synergy of structure, substrate roughness and morphology,

dye photo-physics and electrolyte redox chemistry. That evolution has

continued progressively since then, with certified efficiency now over 10%.

Mechanism of DSSC

Mechanism of DSSC

The incident photon is absorbed by dye complex

photosensitizers adsorbed on the TiO2 surface.

The photosensitizers are excited from the ground state (S) to

the excited state (S∗). The excited electrons are injected into

the conduction band of the TiO2 electrode. This results in the

oxidation of the photosensitizer (S+).

S + hν → S∗

S∗ → S+ + e− (TiO2)

The injected electrons in the conduction band of TiO2 are

transported between TiO2 nanoparticles with diffusion toward

the back contact (TCO). And the electrons finally reach the

counter electrode through the circuit

he oxidized photosensitizer (S+) accepts electrons from the I− ion

redox mediator leading to regeneration of the ground state (S), and

the I− is oxidized to the oxidized state, I3−

S+ + e− → S

he oxidized redox mediator, I3−, diffuses toward the counter

electrode and then It is reduced to I− ions.

I3− + 2 e− → 3 I−

The efficiency of a DSSC depends on four energy levels of the

component: the excited state (approximately LUMO) and the

ground state (HOMO) of the photosensitizer, the Fermi level of the

TiO2 electrode and the redox potential of the mediator (I−/I3−) in the

electrolyte.

DSSC Structure

The DSSC device consists of :

Glass sheet

transparent conducting (ITO or FTO)

semiconducting electrode

n-type TiO2 and p-type NiO

Dye-sensitizer

Light harvesting and electronic transition

Redox mediator

I- / I3- or CoII / CoIII complexes

Counter electrode

Carbon or Pt

semiconducting electrode…

In the old generations of photo-electrochemeical solar cells (PSC)

photo-electrodes were made from bulky semiconductor materials such

as Si, GaAs or CdS .

However, these kinds of photo-electrodes when exposed to light they

undergo photocorrosion that results in poor stability of the photo-

elctrochemical cell .

The use of sensitized wide bandgap semiconductors such as TiO2, or

ZnO resulted in high chemical stability of the cell due to their

resistance to photo corrosion .

The problem with bulky single or poly-crystalline wide band gap is the

low light to current conversion efficiency mainly due to inadequate

adsorption of sensitizer because of limited surface area of the

electrode.

semiconducting electrode…

One of the important factors that affect the cell's efficiency is the

thickness of the nanostructured TiO2 layer which must be less than 20

nm to ensure that the diffusion length of the photoelectrons is greater

than that of the nanocrystalline TiO2 layer.

TiO2 is the most commonly used nanocrystalline semiconductor oxide

electrode in the DSSC as an electron acceptor to support a molecular

or quantum dot QD sensitizer is TiO2 (Gratzel, 2003).

Other wide band gap semiconductor oxides is becoming common is

the zinc oxide ZnO. ZnO possesses a band gap of 3.37 eV and a

large excitation binding energy of 60 meV.

Dye-sensitizer…

Absorb all light below a threshold

wavelength of about 920 nm.

Contain attachment such as Carb-

oxylate or Phosphonate group for

better attachment with semiconductor oxide.

Energy level of the excited state should be well matched to the

lower bound of the conduction band of the oxide to minimize

energetic losses during the electron transfer reaction.

Stable enough to sustain about 10^8 turnover cycles

corresponding to about 20 years of exposure to natural light.

Redox electrolyte…

Electrolyte containing I-/I3- redox ions is used in DSSC to

regenerate the oxidized dye molecules and hence completing

the electric circuit by mediating electrons between the

nanostructured electrode and counter electrode.

NaI, LiI and R4NI (tetraalkylammonium iodide) are well

known examples of mixture of iodide usually dissolved in

nonprotonic solvents such as acetonitrile, propylene

carbonate and propionitrile to make electrolyte.

Cell performance is greatly affected by ion conductivity in the

electrolyte which is directly affected by the viscosity of the

solvent.

Counter electrode…

How does DSSC work? The dye is the photoactive material of DSSC, and can produce electricity

once it is sensitized by light .

The dye catches photons of incoming light (sunlight and ambient artificial

light) and uses their energy to excite electrons, behaving like chlorophyll in

photosynthesis

The dye injects this excited electron into the Titanium Dioxide (a white

pigment commonly found in white paint)

The electron is conducted away by nano-crystalline titanium dioxide .

A chemical electrolyte in the cell then closes the circuit so that the

electrons are returned back to the dye

It is the movement of these electrons that creates energy which can be

harvested into a rechargeable battery, super capacitor or another electrical

device.

The voltage generated under illumination corresponds to the

difference between the Fermi level of the electron in the solid

and the redox potential of the electrolyte.

Energy Level Diagram

Efficiency of DSSC…

Electrical power generated = Jsc x Voc

Voc ~ 0.7 (greater than normal Silicon cells)

Isc for DSSC ~ 20 mA/cm^2, but Silcon cells ~ 35 mA/cm^2

Efficiency

Peak conversion Efficiency achieved ~ 11 %

Max Peak conversion Efficiency ~ 15 %

The End