Dye-sensitized solar cells (DSSCs) are a type of solar cell that uses dye molecules to absorb sunlight and convert it to electrical energy. They were invented in 1991 by Brian O'Regan and Michael Grätzel. DSSCs consist of a photo-sensitized anode, an electrolyte containing a redox couple, and a cathode. When light is absorbed by the dye, electrons are injected into the conduction band of the semiconductor and transported through the external circuit to be collected at the cathode, while the dye is regenerated through the redox shuttle. DSSCs offer advantages such as low cost, flexibility in design, and the ability to work in low light conditions. Recent research aims to

1. dye-sensitized
solar cells
By: Heba Bakry

2. 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.

3.  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…

4. 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 .

5.  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%.

6. Mechanism of DSSC

7. 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

8.  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.

9. 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

10. 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.

11. 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.

12. 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.

13. 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.

14. Counter electrode…

15. 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.

16.  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

17. 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 %

19. The End