Unit II - Study Materials on Energy Conversion Materials

8/7/2019 Unit II - Study Materials on Energy Conversion Materials

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UNIT II - ENERGY CONVERSION MATERIALS

1. Production of metallurgical Grade SiliconThe starting material is pure sand, which is available in plenty on earth's crust. Sand

(SiO2) is heated with carbon in an electric furnace to reduce it according to the reaction:

SiO2 +2C-------------------- Si +2CO

The silicon thus obtained is of 99% purity and is called metallurgical grade silicon. This

has to be purified further to a very low level of impurity content to make it suitable for

use in devices.

2. Semiconductor Grade Silicon

The semiconductor grade silicon has only a fee parts per billion of impurities.

Starting from the metallurgical grade, it can be produced by the zone refining process.

The other common method is a chemical process. The metallurgical grade silicon is

dissolved in HCl :

Si + 3HCl ------------- SiHCl3 + H2

The product trichlorosilane (SiHCl3) is a liquid at room temperature. The fractional

distillation of this liquid removes chlorides of dopants and of other impurities, such as

iron and copper and also SiCl4. A mixture of the purified SiHCl3 and H2 is then

evaporated and passed through a reactor which contains "slim rods" of high purity

silicon. The gaseous mixture now undergoes the reverse reaction:

SiHCl3 + H2 ---------------Si + 3HCI

Solid silicon is deposited on the heated slim rods, which grow radially. Rods of

semiconductor-grade, polycrystalline silicon up to 200 mm in diameter and 2-3 m long

can be produced by this process

3. Single Crystal Growth:


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Single crystals of semiconductor grade silicon are grown either by the Czochralski (CZ)

method or by the float zone (FZ) method.

Semiconducting devices like diodes, transistors, solar cells, etc., are made of single crystals of

semiconductor. The crystals have to be perfect single crystals without any defects, the

crystals must be extremely pure. There should not be any impurities other than the one

deliberately added, such as donor and acceptor impurities. One of the standard methods of

growing single crystals of semiconductors is the czochralski technique. The method used for

purifying semiconductors is called the zone refining technique.

Czochralski Technique

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

condensation of the melt. The material is melted in a crucible and a small seed crystal isimmersed into the surface of the melt, such that only a small part of the crystal is inside the

me1t. The temperature gradient at the crystal-melt interface is critically maintained such that

at the melt, the temperature is just above the melting point and at the crystal it is just below

the melting point. The seed crystal is gradually pulled upwards maintaining a critical pulling

rate. The temperature at the crystal-melt interface is controlled by the temperature gradient

and the pulling rate of the crystal. Asthe crystal is pulled, the melt condenses on the crystal

and thus the crystal grows. The schematic diagram of the Czochralski apparatus is shown in

below fig.

The essential parts of the apparatus are:


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(i) A crucible to hold the melt.

The crucibles are cylindrical shaped and are usually made of carbon. For

GaAs crystals, silica crucible are used.

(ii)Heated to heat the crucible.

The most commonly used heating method is resistance-heating or rf induction-heating.

For resistance heating SiC is used as the heating element

(iii) A seed crystal.

The seed crystal is cut from a previous crystal. When such a crystal is not available,

the growth can be initiated on a thin rod or wire. From the

crystal grown on the rod, a small seed crystal may be cut.

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

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

Procedure: Pure material is taken in the crucible and the temperature is raised just above the

melting point. The seed crystal is introduced into the melt by means of a crystal holder. A

small amount of seed material is initially dissolved. The temperature is then adjusted suitably

to promote the growth of the seed crystal and the crystal is gradually withdrawn maintaining

the crystal-melt interface near the surface of the melt. Initially the diameter of the crystal is

large and the crystal may not be perfect. The pulling rate is then increased to reduce the

diameter. This is called necking. At this stage a goodsingle crystal will be formed and then

again by gradually reducing the pulling rate and simultaneously controlling the temperature,

the diameter of the crystal can be increased. The crystal is rotated as it is being pulled, to

promote stirring of the melt and to average out slight temperature gradients.

The atmosphere of crystal growth is controlled by enclosing the unit in an evacuated

chamber. This avoids contamination and oxidation. If necessary the chamber may be filled

with hydrogen or an inert gas such as nitrogen, at suitable pressures to maintain equilibrium

vapour pressure. For GaAs, arsenic atmosphere is used.

By this technique, large single crystals of about 15 cm diameter arid 10 cm length can be

grown. To grow p-type or n-type crystals, the impurities are added to the melt in suitable

proportions.

Zone Refining Technique


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The zone refining technique is used for purification of semiconducting materials. The

principle used in this technique is that when a solid is partly melted, creating a solid-melt

interface, most of the impurities tend to diffuse towards the liquid phase.

A solid-melt interface is created by melting a small portion or a zone of the material (which

is in the form of a long rod) by means of a short induction heater. The molten zone is swept

from one end of the rod to the other. This causes the impurities to get collected at one end of

the rod, thus leaving the rest of the solid relatively purer. The zone refining apparatus is

schematically shown in below Fig.

The material is taken in the form of a long rod or in a long boat-like crucible. Several

heaters are arranged the length to create a number of moIten zones. The most commonly

used heating method is rf induction heating. The crucible is moved past the heater, making

the molten zones pass through the solid.

When a liquid-solid interface is created, at equilibrium, the impurity has a greater

concentration ill the liquid phase than in the solid phase. A parameter, called the distribution

coefficient is defined as

k= CS/CL

where Cs is the concentration of the impurity in the solid phase and CL is the con-

centration of the impurity in the liquid phase. The distribution coefficient is characteristic of

a given impurity in the solid. For most impurities, kis less than one (0.1-0.00 I). So, if the

molten zone is swept across the solid, the impurity atoms tend to concentrate in the liquid

zone as it passes through the solid. Several passes of the molten zone may be made across

the rod to get good purification. In practice, the solid is passed once through several heaters


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as shown in Fig. 8.6.

Zone refining technique will be more effective if the ratio of the width of the molten zone

to the length of the ' rod is less than 10 %. Purity levels of better than 99.9999% can be

achieved by this technique.

Zone melting:The principle of the zone melting can also be used to grow single crystals.

This is called Zone technique. In this technique, a seed crystal is inserted at the starting end

of the crucible containing a polycrystalline sample. A molten zone is created in the rod at a

distance farther from the end containing the seed. The zone is then backed upto the seed is

wetted; then the zone is slowly moved through the specimen to get a single crystal in the

shape of a rod.

Zone leveling: Zone melting can also be used to get uniform doping of impurities in

semiconducting crystals and the technique is calledzone levelling. The crystal is first purified

by zone refining; then a large concentration of the desired impurity is introduced into the

molten zone. If the impurity concentration is large, and the zone is swept through, as many

impurity atoms are trapped during freezing as are removed by melting. This helps in getting a

uniform distribution of the impurities in the crystal.

Floating zone method: In this method the polycrystalline rod is held vertically between two

chucks and a single molten zone is passed from one end of the rod to the other by moving the

heater. The molten zone is held in position between the two solid portions by the surfacetension of the liquid. The advantage of this method is that no crucible is required and thus

crucible contamination is eliminated. Here the diameter of the rod has to be small, so that the

surface tension forces may hold the liquid in position.

SEMICONDUCTOR BASED

PHOTOELECTROCHEMICAL CELLS

1. SOLAR SPECTRUMSolar energy, as received on the earths surface, represents a clean, non-poluting,

abundant and relatively free energy. The solar radiation ranges from 200-2000nm in the

electromagnetic spectrum with a distribution shown in Fig.1.1. About 45% of the spectral

energy is distributed in the visible region, about 52% in near IR, about 3% in UV region and

the rest in far IR regions. This spectral distribution of the solar radiation is modified due to


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atmospheric extinction and selective absorption by CO2, O3 and water vapour and scattering

due to clouds, dust and water droplets. Thus, solar radiation reaching the earth is either direct

or diffused depending on the path length and transparency of the atmosphere.

2. DEVICES FOR SOLAR ENERGY CONVERSIONThere are various ways of harnessing solar energy. Among the systems converting solar

energy into electrical/chemical energy, the most important ones are:

(1)Photovoltaic cells (p-n junction and Schottky junction, which use solid statejunction),

(2)Photoelectrochemical cells (the Photo assisted electrolysis cells and liquid junctionsolar cells) and]

(3)Photo galvanic cells (in which light is absorbed by dye molecules)(4)Solar thermal

Since these devices are quantum converters, in which a photon is absorbed resulting in an

electron-hole pair or breaking of the chemical bond, these can use only the relatively high

energy photons and considerable portion of the IR radiation cannot be used. The photovoltaic

technology is well-established and very high efficiencies of the order of 26% (on a laboratory

scale) have been realized. In addition, several other techniques exist for the conversion of

solar energy into other forms of energy. (Table 1.1)

3. PHOTOELECTROCHEMICAL (PEC) CELLSThe semiconductor based photo electrochemical (PEC) cells form an important class

of the direct conversion systems. These cells can be classified into two types according to

their application. When the cell is used for the conversion of solar energy into electrical

energy, it is called the Liquid Junction Solar Cell (LJSC). In the second class of cells, namely

the Photoelectrosynthesis (PES) cells, solar energy is converted into chemical energy in the

form of fuels. The most attractive application of the latter class of cells is the Photoassisted

electrolysis is a valuable fuel and energy carrier. It is non-polluting, renewable, inexhaustibleand very flexible with respect to conversion to other forms of energy (like heat via

combustion or electricity via fuel cells). Also, the storage of hydrogen is much easier than the

storage of other forms of energy like heat or electricity. Finally, hydrogen is a valuable basic

chemical feedstock used in large quantities for NH3 synthesis and petroleum refining. The

major breakthrough in the study of Photoelectrochemistry came in the early 1970s, when


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Fujishima and Honda (1972) demonstrated the potential application of Photoelectrochemical

systems for solar energy conversion and storage. Fujishima and Honda used an n-type TiO 2

electrode for the photoelectrodecomposition of water.

4. ADVANTAGES AND DISADVANTAGES OF PHOTOELECTROCHEMICAL(PEC) CELLSPhotoelectrochemical (PEC) cell is a device in which a photoactive semiconductor

material is in contact with an electrolyte (Fig. 1.2). Irradiation of the SC/electrolyte

junction with light of energy > Eg, the band gap of the semiconductor, produces electron-

hole pairs. The electron-hole pairs are spatially separated (due to the junction potential) to

drive oxidation and reduction reactions in the system.

The major advantages of PEC cells over photovoltaic cells are:

(i) Easy junction formation (mere dipping of the SC electrode in the electrolyte).(ii) In-situ water electrolysis is possible.(iii) Efficiencies of polycrystalline bulk and thin film electrodes are comparable to

those of single crystal electrodes.

(iv) Novel reaction products are possible and catalytic effects (Photocatalysis) can beinduced on the SC surfaces.

(v) Particulate systems can be used.The major advantages of PEC cells is the photocorrosion of the semiconductors (esp.

conventional semiconductors like Si, GaAs, CdS etc.) in strong aqueous electrolytes.


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5.


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5. ENERGETICS OF THE SEMICONDUCTOR/ELECTROLYTE INTERFACE

The photoelectrochemical effect is based on the formation of a semiconductor/electrolyte

junction when a semiconductor is immersed in a suitable electrolyte. The junction is

characterized by a space charge layer (also called as the depletion layer) in the

semiconductor adjacent to the electrolyte. The space charge layer is formed due to the

equilibration of the chemical potentials of the SC and the electrolyte whenever they are

different initially. For semiconductors, the chemical potential is the Fermi level and for

the electrolyte it is the redox potential of the redox couple present in the electrolyte.

When the initial Fermi level of an n-type semiconductor is above the redox potential

of the electrolyte, equilibration of the chemical potentials occurs by the transfer of

electrons from the SC to the electrolyte. This results in a positive space charge layer in the

SC adjacent to the electrolyte (Fig. 1.3). Since the region is depleted of the majority

carriers, it is also called the depletion region or layer. As a result, the conduction and

valence bands in the SC are bent such that a potential barrier is developed which prevents

further transfer of majority carriers to the electrolyte.

For p-type semiconductors having the Fermi level below the redox potential of the

electrolyte, the inverse but analogous situation occurs.

The width of the space charge layer in the semiconductor depends on the carrier

concentration, dielectric constant of the SC. The typical widths vary from 100A& to

several m .

A charged layer also exists in the electrolyte the well-known Helmholtz layer. This layer

consists of charged ions from the electrolyte adsorbed on the SC electrode. The width of

the Helmholtz layer is of the order of a fewA& .

6. CLASSIFICATION OF PEC CELLSA PEC cell consists of a semiconductor in contact with a transparent electrolyte. The

photosensitivity of the cell is mainly determined by the interband transitions of SC. When

light (photons) of energy hvEg falls on the SC/electrolyte interface, electron-hole pairs

are produced and these electron-hole pairs are used to carry out electrochemical reactions.

The electric field of the depletion layer enables the effective spatial separation of the


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electron-hole pairs and thus, prevents their recombination. Depending on the nature of the

reactions occurring at the SC and counter electrodes. PEC cells can be classified into two

types.

If the electrochemical reaction at the SC electrode is exactly reversed at the counter

electrode, then the overall composition of the electrolyte remains unchanged and the net

result of the photoprocess is the flow of current in the external circuit. Such a cell, which

is analogous to the solid state photovoltaic cell, is called the Liquid Junction Solar Cell

(LJSC).

e.g., n-CdSe/Na2S+S+NaOH/Pt cell

At the anode: Sx2-

+S2-

+2h+

---> Sx+12-

At the cathode: Sx+12-

+2e-

--- > Sx2-

+S2-

If two different reactions occur at the two electrodes of the cell, then the result of the

photoprocess is the storage of energy in the form of chemical energy of the products. The

energy stored is the difference in free energy between the two products. This class of the

PEC cells is called the Photoelectrosynthetic (PES) cells. Photoassisted electrolysis (PAE)

cells fall under this category.

e.g., n-SrTiO3/NaOH/Pt

At the anode : 2OH-+ 2h

+--- > 1/2O2 + H2O

At the cathode: 2H2O + 2e-

--- > H2 + 2OH-

Net reaction : H2O --- > H2 + H2O2

Which is nothing but electrolysis of water.

If the energetic are favorable, it is possible to electrolyse water without applied bias, as

has been demonstrated in the case of SrTiO3 (Wrighton et al, 1976a; Mavroides et al,

1976) and Zn2TiO4 (Matsumoto et al, 1986).

7. LIQUID JUNCTION SOLAR CELLS (LJSC)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 electrochemical 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 Fig.1.12 (a). For this, the initial Fermi level of the SC

should be above (in the case of n-type semiconductors) the Eredox.


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The non-equilibrium electrons in the valence band produced by illumination with light of

energy hvEg are spatially separated by the field in depletion layer (Fig.1.12 (b)). The

minority carriers (h+

in the n-type SCs) are transferred to the surface where they are used

up for oxidation and the electrons are transferred to the surface where they are used up for

reduction and the electrons are transferred to the bulk, then via the external circuit to the

counter electrode, where they are used up for the reverse reaction (reaction). Thus, the

main difference between a PEC cell and the photovoltaic cell is that in the former the

photogenerated carriers transfer the stored energy from light to chemical species in the

solution whereas in the later, there is no such process.

LJSCs with p-type SCs function in a similar fashion. In this case, the condition for

formation of the depletion layer becomes Eredox > EF.SC should be below Eredox initially).LJSCs with two Sc electrodes (one of them being n-type and the other p-type) are also

known. In this case,

Voc(max)= )()( pFnF EE

For efficient solar energy conversion, 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 W> ( )AD NorNW /1/1

1

. This isnecessary for good spatial separation of the photogenerated carriers and hence, high

quantum efficiency.

(3)Large values of . This is usually found for direct band gap SCs.The following are the requirements for the redox couple:

(i) The electrolyte should have a value of Eredox so as to give a large value ofFEredox

E ,

(ii) Eredox should be in such a position that the electrode decomposition reactions arenot 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).


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PEC cells used for water electrolysis consist of a SC electrode in contact with an

electrolyte which contains two redox couples, namely, H+/H2 and O2/H2O. the energetics

of such cells is shown in Fig.1.13 (for an n-type SC).

Since there are two redox couples present, the electrochemical potential can be anywhere

depending upon the initial relative concentration of H2 and O2 in the electrolyte. In (a), the

initial electrochemical potential is arbitrarily drawn just above the O2/H2O redox level. In

the dark (b), the Fermi level of the SC equilibrates with the electrochemical potential of

the electrolyte, producing a band bending.

Upon illumination, the Fermi level of the SC rises towards Vfb, producing a photovoltage,

Vph, Vph can be measured between the SC and the counter electrodes. However, the value

of Vph depends upon the initial Fermi level of the metal counter electrode which in turndepends on the initial concentrations of H2 and O2 in the electrolyte.

Summarizing, apart from electrode stability, the essential requirements for achieving

photoelectolysis of water are:

(i) The width of the band gap of the SC should exceed the difference between theelectrochemical potentials for the hydrogen and oxygen electrode reactions in

water

(1.23 eV).

ii) The flat band potential for an n-type semiconductor should be more negative than the

hydrogen evolution potential (or, for a p-type semiconductor should be more positive than the

oxygen evolution potential). This is essential for photoelectrolysis without any external bias.


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Semiconductor electrodes

Oxidic semiconductors (OS) such as TiO2, ZrO2, etc. were widely used as electrodes forphotoelectrochemical (PEC) conversion of solar energy, as photocatalysts for

decomposition of toxic pollutants and for preparation of the practically important

catalysts during the last 25 years.

To improve photochemical properties of the OS at . * 400 nm, doping of the OS matrixwith transition metal ions was usually applied.

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

Structure and Photoelectrochemical Properties of the Doped Polycrystalline TiO2

The samples of the ceramic polycrystalline TiO2 doped electrodes have been prepared byelaborate mixing the precise amounts of specially purified TiO2, V2O5, Cr2O3 or Nb2O5

powders, pressed into bricks and heated in air at 1200 C during 2 h in inert atmosphere

(He).

Then the stuffs were grinded and treated at 1200 C during 2 h in inert atmosphere. Samples of such a set (V-I) contained in their matrix the uncontrolled amount of oxygen

vacancies.

The samples of the second set V-2 were additionally treated at 900 0C in air during 2 h toobviate these vacancies.

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

The bricks of the modified TiO2 were cut to plates with thickness of 1.0 mm, both faceswere polished.

The back side was covered by In or Cu using the vacuum-deposition technique, then theohmic contact was made.


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CONSTRUCTION CHARACTERISTICS OF CDS AND CDSE CELLS

Photosensitive devices can be divided into photovoltaic devices and photo emissive

devices. CdS cells are a type of photoconductive device. They are semiconductor sensors that

utilize the photoconductive effect in which light entering the photoconductive surface reduces

the resistance. A voltage is applied to both ends of a CdS cell and the change in resistance due

to light is output as a current change signal. Despite of small size, the output current per

photoconductive surface area is large enough to drive relays directly. For this reason, CdS

cells are used in a wide variety of fields. Here we will explain briefly the basic operating

principles, fabrication and structure of CdS cells.


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This l/d is an important factor in designing the electrode configuration. In other words

the shorter distance between the electrodes and the greater the electrode length, the

higher the sensitivity and the lower the cell resistance. Thus, the electrode patterns for

high-sensitivity CdS cells consist of many zig-zags.

STRUCTURE

The manufacturing process of the photoconductive layer into the sintered type, single

crystal type and evaporated type can divide CdS cells. Of these different types the sintered

type offers high sensitivity areas, a large mass production effect and relatively superior

production profitability. so AGI uses the sintering film fabrication method.

Here is the process for making sintered CdS cells. Impurities and a fusing agent for

encouraging crystal growth are added to highly pure CdS crystal power and this mixture is

dissolved in water. The resulting solution is applied to CdS ceramic substrate and dried, and

then it is sintered in a high-temperature oven to form multiple crystals. In this way, a thick

layer with the photoconductive effect is formed.

Then, lead terminals are introduced to the CdS substrate and the CdS is packaged

(Figure 4 and photo I shows an example of the structure of a plastic-coated CdS cell.)

CHARACTERISTICS

In the selection of a suitable CdS cell, the characteristics required by the functions of

the circuit in which the CdS cell is to be used are important; in general, there are analog uses

such as light measurement and digital uses such as on-off switching.

Use in digital circuits such as switching requires a fast response and a high ratio between

illuminated resistance and daIk resistance. For illumination and exposure meter and other

devices that measure brightness, the sensitivity, the slope of the

resistance vs. illuminant (gamma), and the spectral response characteristic are important

Therefore, understanding the various characteristics of CdS cells presented below is

important for selecting the night CdS cell for your application.


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SENSITIVITY

i Spectral Response Characteristic

The relative Sensitivity of a CdS cell is dependent on the wavelength of the incident

light. The sensitivity as a function of wavelength is called the spectral response

characteristic. Fundamentally, the maximum sensitivity wavelength (or peak wavelength) for

CdS cell is 515 nm, but by controlling the composition ratio of CdS to CdSe, the maximum

sensitively can be optimized at a wavelength between 515 and 730nm. So, photoconductive

cells with spectral response close to that of the human eye are available. Figure 6 shows

these relationships. In general, CdS Cd (SiDSe), and CdSe cells are all often called iCdS

cell i. This catalog also uses this terminology.

By using a CdS cell with a spectral response similar to the human eye, it can be widely and

easily used in applications as sensors substituting for the human eye.

Dye sentisized solar cells

General Operating Principles

Conventional solar cells convert light into electricity by exploiting the photovoltaic effectthat exists at semiconductor junctions. They are thus closely related to transistors and

integrated circuits.

The semiconductor performs two processes simultaneously: absorption of light, and theseparation of the electric charges ("electrons" and "holes") which are formed as a

consequence of that absorption.


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However, to avoid the premature recombination of electrons and holes, thesemiconductors employed must be highly pure and defect-free.

The fabrication of this type of cell presents numerous difficulties, preventing the use ofsuch devices for electricity production on an industrial scale.

In contrast, the dye sensitized solar cells work on a different principle, whereby theprocesses of light absorption and charge separation are differentiated. Due to their simple

construction, the cells offer hope of a significant reduction in the cost of solar electricity

Light absorption is performed by a monolayer of dye (S) adsorbed chemically at thesemiconductor surface.

After having been excited (S*) by a photon of light, the dye - usually a transition metalcomplex whose molecular properties are specifically engineered for the task - is able to

transfer an electron to the semiconductor (TiO2) (the process of "injection").

The electric field inside the bulk material allows extraction of the electron.

Positive charge is transferred from the dye (S+) to a redox mediator ("interception")present in the solution with which the cell is filled, and then to the counter electrode.

Through this last electron transfer, in which the mediator is returned to its reduced state,the circuit is closed.

The theoretical maximum voltage that such a device could deliver corresponds to thedifference between the redox potential of the mediator and the Fermi level of the

semiconductor.

The maximal voltage corresponds to the difference between the redox potential of themediator and the Fermi level of the semiconductor.

Figure 1 presents a cartoon of the make-up of the present generation of dye-sensitizedphotoelectrochemical cells based on nanocrystalline films of TiO2.


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Description of Dye sensitized Solar cell

The solar cell consists of two conducting glass electrodes in a sandwich configurationwith a redox electrolyte separating the two.

On one of these electrodes a few micron-thick layer of TiO2 is deposited using a colloidalpreparation of monodispersed particles of TiO2.

The compact layer is porous with a high surface area allowing monomoleculardistribution of dye molecules.

After appropriate heat treatment to reduce the resistivity of the film, the electrode with theoxide layer is immersed in the dye solution of interest (typically 2xl0 -4M in alcohol) for

several hours.

The porous oxide layer acts like a sponge and there is very efficient uptake of the dye,leading to intense coloration of the film.

Molar absorbances of 3 and above are readily obtained within the micron-thick layer witha number of Ru-polypyridyl complexes.

The dye-coated electrode is then put together with another conducting glass electrode andthe intervening space is filled with an organic electrolyte (generally a nitrile) containing a

redox electrolyte (I-E/EI3-).

A small amount of Pt (5-10 glcm2) is deposited to the counter-electrode to catalyze thecathodic reduction of triiodide to iodide.

After making provisions for electrical contact with the two electrodes, the assembly issealed.

The absorption of light by a monolayer of dye is always destined to be weak.


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A respectable photovoltaic efficiency cannot therefore be obtained by the use of a flatsemiconductor surface but rather by use of a porous, nanostructured film of very high

surface roughness.

When light 'penetrates the photosensitized, semiconductor "sponge", it crosses hundredsof adsorbed dye monolayers.

The nanocrystalline structure equally allows a certain spreading of the radiation.

The end result is a greater absorption of light and its efficient conversion into electricity.

Despite the heterogeneous nature of the semiconducting material, the diffusion ofelectrons in the bulk matter towards the supporting conductor occurs with almost no

energy loss.

The recombination between the electron which is injected into the conduction band of thesemiconductor, and the hole that remains on the oxidized dye is effectively very slow,

compared to the reduction of the latter by the mediator in solution.

Furthermore, electron-hole recombination in the semiconductor which seriously affectsthe efficiency of classic photovoltaic cells does not occur in this case, due to the fact that

there is no corresponding hole in the valence band for the electron in the conduction band.

As a result, the efficiency of the cell is not impaired by weak illumination, e.g. under acloudy sky, in contrast to what happens with classical systems.

Assembly of the Cell

Adsorption of the photosensitizer onto nanostructured TiO2 film is performed by simpleimmersion in a dye solution.

The counter electrode is then deposited facing the photoanode and separated by a thinspacer.

The gap between the electrodes is then filled with a low-volatility electrolyte (such as amolten salt) containing the redox mediator.


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To date, the mediator having given the best efficiency is the iodide-triiodide couple(I

-/ I

-3).

The construction is completed by hermetic sealing of the whole assembly. No other complicated procedures are necessary and production costs are thus minimized.

The Counter-Electrode

The counter-electrode is composed of glass covered with a conducting oxide layer.

A tiny amount of platinum (5-10 mg cm2) is deposited at the surface in order to catalyzethe reduction of mediator (I3+ 2e

- - -->3I-)

A new procedure for platinization developed in our laboratory results in a surfacepossessing a remarkable electrocatalytic activity unaffected by the anodic corrosion from

which conventional galvanostatically-deposited and sputtered surfaces suffer.

Documents

Unit II - Study Materials on Energy Conversion Materials