2. It is estimated that the global energy consumption will increase from
13.5 TW (in 2001) to 27- 41 TW (by 2050).
MAJOR SOURCES OF ENERGY
steam & water.
i.e. 119 kJ/g.
5. It is non toxic.
6. Can be used in
industry, for the
for use in fuel
of energy in the
5. PEC technology is based on solar energy, which is a
perpetual source of energy and water, which is a
PEC technology is environmentally safe, with no
PEC technology may be used on both large and small
PEC technology is relatively uncomplicated.
6. PHOTO- ELECTROCHEMISTRY OF WATER
The principle of photoelectrochemical water decomposition is based
on the conversion of light energy into electricity within a cell
involving two electrodes(or three), immersed in an aqueous
electrolyte, of which at least one is made of a semiconductor
exposed to light & able to absorb light. This electricity is then used
for water electrolysis.
7. The performance of PECs is considered in terms of:
Excitation of electron – hole pairs in
photo – electrodes.
Charge separation in photo electrodes.
Electrode processes & related
charge transfer within PECs
Generation of PEC voltage
required for water decomposition
10. ENERGY BAND IN SEMICONDUCTOR
Consist of a large number of closely spaced energy levels.
Bands are made up large number of atomic orbitals and the
difference in energy between adjacent orbitals within a given
energy band is so small so that band can be considered a
continuum of energy levels
11. VALENCE BAND & CONDUCTION BAND
Energy band: highest occupied energy level is called the
valence band and the lowest unoccupied energy level is
called the conduction band.
12. •The band gap, Eg, is the smallest energy difference between
the top of the valence band and the bottom of the conduction
•The required band gap of the semiconductor for water splitting
should be 1.8 – 2.2 eV.
•Large band gap semiconducting oxides are stable in aqueous
electrolyte but absorb in UV region which is only about 4% of
the solar spectrum, whereas small band gap semiconductor &
optimum band gap semiconductor have the potential to absorb
visible part of solar spectra but corrode when dipped in
BAND GAP (Eg)
13. BAND EDGES
A semiconductor capable of spontaneous water splitting must have
conduction band energy (EC) higher and valence band energy (EV) lower
than that of reduction potential Ered (H2/H+) & oxidation potential Eox(OH-
/O2) of water respectively.
O2 / H2O
Eg ≥ 2eV
V vs NHE
Ideal straddling condition of Conduction & valence band edges of a
14. Energy level Diagram
Band positions of several semiconductor materials in contact
with aqueous electrolyte at pH 1
It is imp. because when a reference electrode is used to make
measurements, it compares Ef of semiconductor with its own
unchanging Fermi level.
FERMI ENERGY (EF)
In an extrinsic semiconductor:
n – type
p – type
In an intrinsic semiconductor:
It is the energy level where probability of occurrence of an electron is half.
n – type p – type
BAND BENDING :-
EF > Eredox - e- will be transferred from
electrode into the solution and the there
is a positive charge associated with the
space charge region.
EF < Eredox – e- must transfer from the
solution to the electrode to attain
equilibrium and generates a negative
charge in the space charge region.
It is the difference between the potential at the surface and potential in the
bulk of the semiconductor.
The electric field that is formed in space charge region results in bending of
Band bending acts as a barrier for the recombination of charge carriers.
Band bending becomes zero only at flat band potential(Vfb)
17. Also called as depletion region.
An insulating region within a
conductive, doped semiconductor
material where the charge carriers are
diffused or forced away by an electric
It is called so because it is formed
from a conducting region by removal
of all charge carriers leaving none to
carry a current.
n – type
p – type
SPACE CHARGE REGION :-
18. The flat band potential corresponds to the externally applied potential
for which there is no band bending at the semiconductor surface.
This potential is equal to the curvature of the bands in the absence of
any potential applied to the interface.
Photo-cells equipped with a photo-anode made of materials with
negative flat-band potentials (relative to the redox potential of the
H+/H2 couple, which depends on the pH) can split the water molecule
without the imposition of a bias.
If Vfb is positive, then more electrons are attracted towards space
charge region, hence this region decreases and it leads to increase in
the recombination of charge carriers.
If Vfb is negative, then space charge region becomes broadened as a
result there is decrease in recombination of charge carriers.
FLAT BAND POTENTIAL :-
19. BENCHMARKS EMERGE TO MAKE PEC
Band gap energy around 2 eV
Strong optical absorption
High electron mobility
Long life time of charge carriers
Must straddle with redox potential of water
Stability in strong electrolytes
Good catalytic properties
Conversion Efficiency- 10%
Current Density (JPC) – 10-15
Material Durability ˃2000 h
•Extends absorption in
• Increases the lifetime
of photo generated
• Improvement in
absorption of solar
•Enhanced photo -
response with dyes
• Modify electronic
improve visible light
•Referred as ‘second
SWIFT HEAVY ION
•For modification in
surface properties of
resulting in alteration
in the photo response of
the material in PEC cell.
• Broad absorption
• Good charge transportation
• Reduced recombination rate
• Inbuilt electric field at the
23. X- Ray Diffractometer
• For measurement of Phase & Particle size
• Scherrer’s Equation (for crystallite size): 0.9λ/β cos θ
UV – Vis Spectrophotometer
• For the measurement of band gap.
• For direct- indirect, allowed or forbidden transition.
Atomic Force Microscope (AFM)
• To determine surface morphology of hetero- junction
Potentiostat – For PEC study.
(Scanning Electron Microscope)
Scans the surface of the sample by releasing electrons and making
the electrons bounce or scatter upon impact. The machine collects the
scattered electrons and produces an image.
Information on the
sample’s surface and its
Shows the sample bit by
bit as area where the
sample is placed can be
rotated in different
3D image Resolution-0.4nm
25. PHOTOCURRENT – VOLTAGE CHARACTERISTICS
• They are the useful tool in determining the operating characteristics
of a device by showing its possible combinations of current &
voltage. As a graphical aid visually understand better what is
• These curves show the relationship between the current flowing
through an electrical or electronic device & the applied voltage across
V < 0, Cathodic current in forward bias
V > 0, Anodic current in reverse bias
26. PARAMETERS OBTAINED FROM THE I-V PLOT:
1. Photocurrent Density: Difference of light current- dark current/area of
More photocurrent density, more is hydrogen production.
2. Open Circuit Voltage (Voc): The voltage between the terminals when no
current is drawn (infinite load resistance)
In Electron recombination kinetics, high Voc - low recombination rate
and high photocurrent density.
3. Short circuit current (Isc) : The current when the terminals are connected
to each other (zero load resistance)
Isc increases with light intensity, as higher intensity means more
photons, which in turn means more electrons