Energy conversion materials

o The Fermi level is an energy pertaining to electrons in a semiconductor... c Under light, photoelectrons enter the conduction band; the band bending is reduced and a photovoltage is ge

Energy Conversion Materials

The solar spectrum

oAbout 46% of the spectral energy

is distributed in the visible region

oAbout 49% in near IR

oAbout 3% in UV region and rest in far IR region

Solar energy conversion devices

Methods of tapping solar energy

Plants Flat plate, tube p/n Si, a-Si, GaAs

(Visible light ) (IR radiation) (Visible light)

η = 2-4% η = 12-26%

D.1 Biomimetism Mimicking Photosynthesis via chemicals

D.2 PEC cells

a LJSC

(i) Sc/Elect/M η= 13-14%

(ii) Photogalvanic cells M/Elect/M η= 0.01%

b Photoelectrosynthesis (PES) cells

(i) Photoassisted electrolysis cells η= 13.3%

(ii) Photoassisted electrosynthesis cells

eg CO2 CH3OH N2 NH3

Drawback Of The Present Devices

electron-hole pair or breaking of the chemical bond

radiation cannot be used

laboratory scale) but it is not completely realised

Photovoltaics &

Photoelectrochemical cells (PEC)

o In semiconductor physics, thedepletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an electric field

o The only elements left in the depletion region are ionized donor or acceptor impurities.

o The Fermi level is an energy pertaining to electrons in a semiconductor

Devices of solar energy conversion

1 Photovoltaic cells

2 Photoelectrochemical cells

4 Solar thermal (eg water heater)

5. Dye sensitized solar cells

The semiconductor / electrolyte interface

Relation to Fermi level

Ec

Ev semiconductor electrolyte

D inversion layer

conduction band E

valence band

Ec Ef

Eredox semiconductor electrolyte

+

+ + +

-+

+ + +

+

+

-

-

-+

+ + +

-+

+ + +

-+

-conduction band electrons

positive charge carriers electrolyte anions

Photoelectrochemical cells

in contact with an electrolyte

semiconductor, produces electron - hole pairs

oxidation and reduction reactions in the system.

Photoelectrochemical effect at semiconductor - redox electrolyte interface.

a) On contact the Fermi level of the n-type semiconductor equilibrates with that of the metal and with the redox couple of the

electrolyte.

b) After charge (electron) transfer, a band bending is established as in the case of the previous solid-state junctions, with establishment of the depletion zone.

c) Under light, photoelectrons enter the conduction band; the band bending is reduced and a photovoltage is generated between the semiconductor Fermi level and the redox potential of the electrolyte - equivalent to the potential of the metal counter-electrode

d)

Regenerative photoelectrochemical cells

Classification of Photoelectrochemical cells

 PEC cells are Classified into two types according to their application

1 Liquid Junction Solar Cell (LJSC) –

This cell is used to convert solar energy into electrical energy

2 Photoelectrosynthesis (PES) cells –

Easy junction formation (mere dipping of the SC

electrode in the electrolyte).

 In-situ water electrolysis is possible.

 Efficiencies of polycrystalline bulk and thin film

electrodes are comparable to those of single

crystal electrodes.

 Novel reaction products are possible and catalytic

effects (Photocatalysis) can be induced on the SC

surfaces.

 Particulate systems can be used.

Major advantages of PEC cells over photovoltaic cells

Mechanism of Liquid Junction Solar Cells

 The simplest LJSC consists of two electrodes (one of them a SC and the other a metal) dipped in an electrolyte containing a redox couple

 Both the electrodes must be inert, i.e., the electrode material itself should not take part in the electro- chemical reactions

 One of the important requirements for the operation of an LJSC is the presence of depletion layer at the surface of the SC electrode

 For this, the initial Fermi level of the SC should be above (in the case of n-type semiconductors) the Eredox.

Cell : n-CdSe / Na2S + S + NaOH / Pt

At the anode:

S2- + 2h+ -> S2

At the cathode:

S2 2- + 2e- - > +

Net reaction : Nil

Liquid Junction Solar Cell (LJSC)

Energetics of LJSC

Energetics of LJSC

 Energy band representation of the

operation of PAE cell

(a) in dark, after

Cell : n-SrTiO3 / NaOH / Pt

Conditions for Efficient Solar Energy Conversion – Electrodes

 The requirements for the electrode materials are:

(1) Band gap should be optimum (see section on

efficiency considerations).

(2) The doping level should be optimum so that there will be a good spatial separation of the

photo-generated carriers and hence, high quantum efficiency.

Conditions for Efficient Solar Energy Conversion – Redox couple

 The following are the requirements for the redox couple:

(i) The electrolyte should have a value of Eredox

(ii) Eredox should be in such a position that the

electrode decomposition reactions are not kinetically favoured

(iii) The reactions at the two electrodes should be perfectly reversible

(iv) Solution should have adequate transparency

(v) There should be low ohmic resistance (in order to minimise the internal resistance of the cell).

Why Silicon

Silicon is a very common element abundant

in nature

(it is the main element in sand and quartz)

Silicon is considered as the most suitable

material for solar energy conversion

because of

1 its abundance

2 Optimum band gap of 1.23 eV at 300K

3 Cost effectiveness

Production of Silicon

1 Metallurgical Grade Silicon:

SiO2 +2C Si +2CO

 Sand (SiO2) is heated with carbon in an electric furnace to reduce it.

 The silicon thus obtained is 99% pure and is called metallurgical grade silicon.

 This is purified further to reduce levels of impurities to make it suitable for use in devices.

2 Semiconductor Grade Silicon:

Si + 3HCl SiHCl3 + H2

 The product trichlorosilane (SiHCl3) is liquid at room temperature

 It is fractionally distilled to remove chlorides of dopants and of other impurities, such as iron and copper and also SiCl4.

Single crystal growth of silicon Czochralski Technique

The principle of this technique is growth of single crystal by a gradual layer-by-layer

condensation of the melt.

Essential parts of the apparatus are:

(i) A crucible to hold the melt

(ii) Heater to heat the crucible

(iii) A seed crystal

(iv) A crystal holder and a mechanism to raise and rotate the crystal and for necking

(v) A sealed enclosure to maintain suitable atmosphere for crystal growth.

• High-purity, semiconductor-grade silicon is melted in a crucible, usually made of quartz Dopant impurity atoms such as boron or phosphorus can be added to the molten silicon in precise amounts to dope the silicon, thus changing it into p-type or n-type silicon This influences the electronic properties of the silicon A precisely oriented rod-mounted seed crystal is dipped into the molten silicon The seed crystal's rod is slowly pulled upwards and rotated simultaneously By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible

to extract a large, single-crystal, cylindrical ingot from the melt Occurrence of unwanted instabilities in the melt can be avoided by investigating and visualizing the temperature and velocity fields during the crystal growth process This process is normally performed in an inert atmosphere, such as argon, in an inert chamber, such as quartz

Purification of Silicon Single crystal

Zone Refining Technique

A parameter, called the distribution coefficient is defined as k = CS/CL

• Zone melting (or zone refining or floating zone process) is a group of similar methods of

purifying crystals, in which a narrow region of a crystal is molten, and this molten zone is moved along the crystal (in practice, the crystal is pulled through the heater) The molten region melts impure solid at its forward edge and leaves a wake of purer material solidified behind it as it moves through the ingot The impurities concentrate in the melt, and are moved to one end of the ingot Zone refining was developed by William Gardner Pfann in Bell Labs as a method to prepare high purity materials for manufacturing transistors Its early use was on germanium for this purpose, but it can be extended to virtually any solute-solvent system having an appreciable concentration difference between solid and liquid phases at equilibrium This process is also known as the float zone process, particularly in semiconductor materials processing

Photochemical cells used for the photoassisted electrolysis of H2O

1. Sunlight energy (photon of light) passes through the titanium dioxide layer and strikes electrons within the adsorbed dye molecules Electrons gain this energy and become excited because they have the extra energy

2 The excited electrons escape the dye molecules and become free electrons These free electrons move through the titanium dioxide and accumulate at the -ve plate (dyed TiO2 plate)

3 The free electrons then start to flow through the external circuit to produce an electric current This electric current powers the light bulb

4 To complete the circuit, the dye is regenerated The dye regains its lost electrons from the iodide electrolyte Iodide (I-) ions are oxidised (loss

of electron) to tri-iodide (I3-) The free electrons at the graphite plate then reduce the tri-iodide molecules back to their iodide state The dye molecules are then ready for the next excitation/oxidation/reduction cycle.

Dye Sensitization - Grätzel cell

Photoelectrochemical processes in a dye-sensitized solar cell.

In a molecular system such as the dye, the gap between the highest occupied molecular orbital and the lowest unoccupied level (HOMO-LUMO gap) is analogous to the conduction band - valence band gap in a semiconductor.

Mediator

Red Ox

Cathode

Maximum Voltage

h ν

e - e

-The Grätzel Cell

Prepared Gr ä tzel cell

Construction of Grätzel cell

o In Grätzel cell a range of organic dyes are used

o Examples: ruthenium- Polypyridine, Indoline dye & metal free organic dye

o These dyes are extractable from simple foods such as hibiscus tea, tinned summer fruits, blackberries

o A dye is then adsorbed onto the TiO2 layer by immersing the plate into a dye solution for 10 min.(approx.)

o The plates are then carefully sandwiched together and secured using a paperclip

o To complete the cell a drop of iodide electrolyte is added between the plates

o Figure shows a Grätzel cell prepared from hibiscus tea

o The upper plate is the TiO2 plate, dyed with hibiscus tea and the lower plate is coated with graphite.

Working Principle of Grätzel Cell

o Sunlight energy passes through the titanium

dioxide layer and strikes electrons within the

adsorbed dye molecules

o Electrons gain this energy and become excited

o The excited electrons escape from the dye

molecules to become free electrons

o These free electrons move through theTiO2 and

accumulate at the –ve plate (dyed TiO2 plate)

o The free electrons then start to flow through the

external circuit to produce an electric current

o This electric current powers the light bulb

o To complete the circuit, the dye is regenerated

o The dye regains its lost electrons from the iodide

electrolyte

o Iodide (I-) ions are oxidised to tri-iodide (I3 -)

o The free electrons at the graphite plate then reduce

the tri-iodide molecules back to their iodide state

o The dye molecules are then ready for the next

excitation/oxidation/reduction cycle

Characteristics of Semiconductor electrodes

 Oxidic semiconductors (OS) such as TiO2, ZrO2, etc are being widely used as electrodes for

a) photoelectrochemical (PEC) conversion of solar energy

b) as photocatalysts for decomposition of toxic pollutants and

c) for preparation of the practically important catalysts

for the last 25 years.

 To improve photochemical properties of the OS at λ = 400 nm, doping of the OS matrix with transition metal ions was usually applied.

 It should be mentioned that influence of various metal dopants on the OS properties is rather well known, whereas peculiarities of their structure are studied poorly.

TiO2 based cells

A. Structure of the Doped Polycrystalline TiO2

.

o The samples of the ceramic polycrystalline TiO2 doped electrodes were prepared by elaborate mixing the precise amounts of specially purified TiO2,

V2O5, Cr2O3 or Nb2O5 powders, pressed into bricks and heated in air at 1200 °C for 2 h in inert atmosphere (He).

o. Then the stuffs were ground and treated at 1200 °C for 2 h in inert atmosphere.

o. Samples set 1 contained in their matrix uncontrolled amount of oxygen vacancies

o. The samples of set 2 were additionally treated at 900 0C in air for 2 h to obviate these vacancies.

o. The X -ray analysis showed that all mixtures had the rutile structure.

o. The bricks of this modified TiO2 were cut to plates of 1.0 mm thickness & both faces were polished.

o. The back side was covered by In or Cu using the vacuum-deposition technique, to make the electrical contact.

Photoelectrochemical properties of doped TiO2

Fig 1 presents the photocurrent

Spectra of polycrystalline Ti1-xVxO2

electrodes at different x values.

Similar ones have been obtained

for Ti1-xCrxO2 Samples

Although there is a strong increase

of the visible light absorption at

x > 0.01 there is a tenfold (Fig 1)

drop of the photocurrent with

increasing of x (Fig 2).

For better understanding of the

causes of this drop the Spatial

organization of the doped OS on

a molecular level has been studied

Basic principles of the photoconductive effect

 Directly beneath the conduction band of the CdS crystal is a donor level and there is an acceptor level above the valence band In darkness, the electrons and holes in each level are almost crammed in place in the crystal and the photoconductor is at high resistance.

 When light illuminates the CdS crystal and is absorbed by the crystal, the electrons in the valence band are excited into the conduction band This creates pairs of free holes in the valence band and free electrons in the conduction band, increasing the conductance.

 Furthermore, near the valence band is a separate acceptor level that can capture free electrons only with difficulty, but captures free holes easily This lowers the recombination probability of the electrons and holes and increases the number for electrons in the conduction band for N-type conductance.