Nanomaterials in Energy Conversion devices

The solar spectrum

oAbout 46% of the spectral energy is distributed in the visible regionoAbout 49% in near IR

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

Solar energy conversion devicesMethods of tapping solar energy

A. Photosynthesis B. Water heaters C. Photovoltaic cells D. Chemical routes 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 Devices1. These devices are quantum converters, in which a

photon is absorbed resulting in an electron-hole pair or breaking of the chemical bond

2. These can use only the relatively high energy photons and considerable portion of the IR radiation cannot be used.

3. The photovoltaic technology has very high efficiencies of the order of 26% (on a laboratory scale) but it is not completely realised.

Terminology

o In semiconductor physics, the depletion 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 cells2. Photoelectrochemical cells3. Photogalvanic cells4. Solar thermal (eg. water heater)5. Dye sensitized solar cells

Photovoltaics

Photovoltaics & Photoelectrochemical cells (PEC)

Photovoltaics• Conversion solar energy to electricity – Photovoltaic effect• Observed by French physicist Edmund Becquerel• First observed in 1870s with solid selenium • Selenium solar cell ( 1% efficiency), costly• Highly pure crystalline silicon (Czochralski method) – 1950s• Silicon photovoltaic cells( 4% efficiency) – 1954• Bell labs improved to 11 % efficiency• Need of highly crystalline materials

Photovoltaic cells A solar cell is a device that converts the energy of sunlight

directly into electricity by the photovoltaic effect.

The photovoltaic effect involves creation of a voltage (or a corresponding electric current) in a material upon exposure to electro-magnetic radiation.

Though the photovoltaic effect is directly related to the photoelectric effect, the two processes are different

In the photoelectric effect electrons are ejected from a material's surface upon exposure to radiation of sufficient energy.

Semiconductors for Solar 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 abundance2. Optimum band gap of 1.23 eV at 300K3. Cost effectiveness

Effects of light on siliconLight induced Electron – hole pair generation is Si crystal

Separation of holes and electronsBy potential barrier

Separated charges are less likelyTo recombine

Creation of potential barrier• Potential barrier separates the electrons and holes• This can be created by altering the crystal structure• Internal potential barrier can be created by addition of

dopants• p ( free holes, B) and n (free electrons, P) type dopants

Junction• The line dividing n and p type Si establishes the potential barrier• The area of contact between n and p type semiconductor is important

Creation of electron hole pair, creating voltage

Silicon

Advantages of Nanocrystals and Quantum dots

• Size tuning result in bandgap variation. Blue shift in the wavelength of absorption with size reduction

• Size reduction reduces the scattering of light• Quantum dots has less reflective losses in

comparison to bulk semiconductors• Increase in surface area gives better

absorption efficiency• Better separation of photo generated carriers

Production of bulk 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.

SiHCl3 + H2 Si + 3HCl

A mixture of purified SiHCl3 and H2 is evaporated and passed through a reactor which contains "slim rods" of high purity silicon.

Solid silicon is deposited on the heated “slim rods”.

Single crystal growth of silicon Czochralski Technique

The principle of this technique is growth of single crystal by a gradual layer-by-layercondensation of the melt.

Essential parts of theapparatus 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.

Purification of Silicon Single crystal Zone Refining Technique

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

Nanostructured materials for photovoltaics

• Recently semiconductor Quantum dots, Nanowires, nanorods were intensively studied for photovoltaics application

• Fine tunable heterostructures and composition• Core-shell heterostructures• Easy separation of photogenerated charge carriers• Strong light confinement, higher optical absorption

and conversion efficiency• Morphology dependent light absorption

Silicon nanowire photovoltaics

(a) Schematics of two distinct motifs for nanowire photovoltaics where the single p -type/intrinsic/ n -type ( p-i-n ) diodes are synthetically integrated in (top) axial and (bottom) core/shell structures. (b) Scanning electron microscopy (SEM) images of p-i-n silicon nanowires. (top) As-grown nanowire with nanocluster catalyst on right tip of nanowire. (bottom) Dopantselective etched nanowire highlighting the distinct p-, i-, and n- type regions with lengths consistent with growth times.

Silicon nanowire

(c) SEM images of a p-i-n coaxial silicon nanowire at different magnifications. Images were recorded with the electron beam (left) perpendicular to the nanowire axis and (right) nearly end. Schematic and SEM image of device fabrication with single core-shell nanowire for photovoltaics

Silicon nanowire photovoltaics• In the axial structure, the active region of the device is located at the

position of the p-i-n modulation, while in the core/shell radial nanowire, this active p-i-n interface extends along the entire length of the nanowire

• The core/shell geometry enables collection of photo-generated charge carriers on a much shorter (radial) length-scale than in the axial structure, which, in principle, can lead to higher efficiencies

Binary phase diagram of axial and radialSi nanowire growth

Single nanowire solar cell

Cross-sectional schematics of four distinct core/shell diode geometries investigated as standalone single nanowire solar cells. The core in all structures is p -type. (d) Normalized (photocurrent/short-circuit photocurrent) light I–V characteristics of single nanowire solar cells corresponding to the four distinct diode geometries

Homo and heterojuctions

• Bandgap modulation in heterojunction and improvement in efficiency

Novel silicon nanowire photovoltaics

Schematic of novel tandem silicon single nanowire assemblies for improving the solar cell efficieny

Silicon nanowire biosensor, pH sensor self powered by silicon nanowire photovoltaics

Photoelectrochemical cells

• Photoelectrolysis• Photoelectrochemical power generation

Photoelectrochemical cellsPhotoelectrochemical (PEC) cell is a device in which a

photoactive semiconductor material as photoelectrode is in contact with an electrolyte .

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.

Heterogenous charge transfer and the kinetics are limited by the materials and their properties

The semiconductor / electrolyte interfaceRelation to Fermi level

C. depletion layersemiconductor electrolyte

conduction band E

valence band

EcEf Eredox

Ev

conduction bandE

valence band

EcEf Eredox

A. flat band potential

Ev

semiconductor electrolyte

+

B. accumulation layer

conduction band

Evalence band

EcEf Eredox

Ev

semiconductor electrolyte

D. inversion layer

conduction band E

valence band

EcEf

Eredox

semiconductor electrolyte

++++

++

--

- -

--

+++++

++

--

- -

--

+++++

++

--

- ---

+++++

++

--

- ---

-+-

conduction band electrons

positive charge carriers

electrolyte anions

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) Minority carriers - holes - are then available for an oxidation reaction with the electrolyte at the Semiconductor photoanode. A reduction reaction takes place at the cathodic counter electrode.

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 – In this class of cells, solar energy is converted into

chemical energy in the form of fuels.

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.

Working of LJSC

The non-equilibrium electrons in the valence band are produced by illumination of light with energy hν ≥ Eg.

The minority carriers (h+ in the n-type SC’s) 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

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).

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

At the anode: Sx

2- + S2- + 2h+ ---> Sx+12-

At the cathode:

Sx+12- + 2e- --- > Sx

2- + S2-

Net reaction : Nil

Liquid Junction Solar Cell (LJSC)

Energetics of LJSC

Energetics of LJSC

Energetics of LJSC

Energy band representation of the operation of PAE cell

(a) in dark, after equilibration

(b) under illumination without applied bias

(c) under illumination with applied bias to

effect electrolysis

Cell : n-SrTiO3 / NaOH / Pt

At the anode :

2OH- + 2h+ ½ O2 + H2O

At the cathode:

2H2O + 2e- H2 + 2OH-

Net reaction :

2H2O 2H2 + ½ O2

Photo-assisted electrolysis (PAE) cells

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

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

electrode decomposition reactions are not kinetically favoured

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

(ii) Solution should have adequate transparency

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

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.

MediatorRed Ox

Cathode

MaximumVoltage

h

S/S+

S*

Diffusion

ElectrolyteDyeTiO2

Conducting glass

Injection

E vsNHE(V )

1.0

0.5

0

-0.5

e - e -

The Grätzel Cell

Upper Plate : Dye coated TiO2

Plate (-Ve)

Lower Plate : Graphite coated

conductor (+Ve)

Prepared Grätzel 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. Construction:o Two transparent glass plates are perforated on one side with a

transparent thin layer of a conducting material.o Onto the conducting sides, one plate is coated with graphite and

the other plate is coated with titanium dioxide (TiO2). 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 Cello 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 energyb) as photocatalysts for decomposition of toxic pollutants

and c) for preparation of the practically important catalystsfor 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 cellsA. 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.

Photoconductivity

In certain materials, there is an increase in electrical conductivity which results from increase in the number of free charge carriers generated when photons are absorbed.

Note: The photons must have quantum energy sufficient to overcome the band-gap in the material in question.

Nanostrucutred electrodes for photoelectrodes

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.

Cadmium Sulphide (CdS) CellThe processes of making the photoconductive layer

a) sintered type b) single crystal typec) evaporated type

Sintered type offers high sensitivity, a large mass production effect and relatively superior production profitability.

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.

Then it is sintered in a high-temperature oven to form multiple crystals. Thus, a thick layer with the photoconductive effect is formed.

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

CdS Cell

Working process of CdS cells

Spectral response of CdS Cells The relative Sensitivity of a CdS cell depends on the wavelength of

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

response characteristic. 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. By using a CdS cell with a spectral response similar to the human

eye, it can be widely and easily be used in applications as sensors

substituting for the human eye.

Spectral Response Characteristics of CdS cell

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.

CdS cells Vs Human Eye

Examples of CdS cell configurations

Plastic coated CdS cells