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PRAXIS BUSINESS SCHOOL

USE OF NANOTECHNOLOGY TO IMPROVE THE UTILIZATION OF

SOLAR ENERGY THE VISUALISATION OF GENERATION-III CELL

A REPORT

SUBMITTED TO

Prof. Prithwis Mukherjee

BUSINESS INFORMATION SYSTEMS

ON 7TH NOV 2010

BY

PUNAM GARU


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Abstract

This project discusses the application of nanotechnology to improve the utilisation of solar energy.

The project starts with the concept of nanotechnology and why is energy a concern for all of us today.

Then the prospect of using nanotechnology in the generation of solar power to make it more efficient

and economical is being discussed. The presently used Generation-I cells and Generation-II cells for

the utilisation of solar energy is being discussed and then the concept of a Generation-III cell for the

future is being discussed. The advantages of a Generation-III cell and the challenges we would face

for its formation is being discussed. The projects aims at understanding the technology involved in

generation of solar power and how nanotechnology can be used to make this process more

economical and efficient.


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What is Nanotechnology?

Nanotechnology is the manipulation or self-assembly of individual atoms, molecules, or molecular

clusters into structures to create materials and devices with new or vastly different properties.

Nanotechnology can work by reducing the size of the smallest structures to the nanoscale e.g.

photonics applications in nanoelectronics and nanoengineering or it can work by manipulating

individual atoms or molecules into nanostructures and this more closely resembles chemistry or

biology.

The definition of nanotechnology is based on the prefix nano which means dwarf. In more technical

terms the word nano means 10^-9, or one billionth of something. The word nanotechnology is

generally used when referring to materials with the size of 0.1 to 100 nanometres, however it is also

important that these materials display different properties from bulk (or micrometric and larger)

materials as a result of their size. These differences include physical strength, chemical reactivity,

electrical conductance, magnetism, and optical effects.

Why is the issue of energy being addressed?

Energy needs have been growing exorbitantly across the globe and across the sectors. The energy

needs of the 21st

century are going to be definitely higher than ever. But at the same time, they have

to be met with qualitatively different sources and processes. The reason is that majority of the present

sources of energy are neither clean nor renewable and are depleting progressively. Many of those

processes cause heavy damage to the environment. There have been efforts to get energy through

cleaner and renewable sources (hydro and wind); but they too have locational and mechanical

limitations. Moreover the entire demand-supply chain in the energy sector is quite capital intensive,

monopolistic and hence cumbersome.

Nanotechnology has shown the possibility of getting cheap and clean energy through its strategic

applications. Its intersection with energy is going to change the way energy is being generated,

stored, transmitted, distributed and managed. Nanotechnology has the potential to particularly

revolutionize the solar energy sector. The advantage of solar energy is that it can be produced by

every house hold on their roof top or farm. We consume only a fraction of the energy delivered by the

Sun. Theoretically, it is possible to convert the rest into usable and transportable energy. If we take

into account the potential of energy generated by Suns invisible rays (infrared), then the potential is

immense. Moreover solar energy is cleaner and sustainable than any of the other energy streams.

Because of this reason, nowdays not only energy but clean energy is becoming a huge business in

itself. Many of the developments in clean energy revolve around strategic applications of

nanoscience. Nanotechnology offers more possibilities in solar energy sector than they have been

experimented today. Hence nanotechnology offers immense potential to harness solar energy.


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Nanotechnology In Improvement Of Utilization Of Solar Energy

The recent advances in nanotechnology can be used to make generation of solar thermal energy

more efficient and economical with reduced infrastructure costs. The heat transfer fluids that containsthe nano particles can be used for this purpose. There can be significant improvements in thermal

conductivity, boiling point, freezing point, time stability and other physical properties based upon the

properties of the nano particles. Altogether these factors can lead to the reduction of cost per watt

criterion that is essential for implementation of any renewable energy source. Apart from this nano

engineered thermoelectric materials can be used for the efficient conversion of thermal energy to

electricity.

A solar cell is a thin wafer of semiconductor which is placed between two conducting terminals.

Voltage gets developed across the cell when light strikes on the cell an passes into the

semiconductor. A wire is placed across the terminals which allows the voltage to drive a current thatcan power a load.

The photovoltaic material used in the first modern solar cell was crystalline silicon. These Generation-

I cells which are now made out of crystalline silicon wafers are the most popular cells in todays

market due to their efficiency. The Generation-I cells have a 25 percent power-conversion efficiency

which implies that it can convert 25 percent of the incident solar power to electric power. The installed

solar panels which are hundreds of Generation-I cells wired together with a power-conversion

efficiency of 15 percent receives 1000 watts of solar power per square meter when the sun is

overhead. This implies that solar panels covering 30 square meters which would be a portion of a

typical roof would produce enough power which would be around 5 kilowatts to run a small

household. But then Generation-I panels are very costly. The solar panels or a 5- kilowatt household

costs above $15,000 and the entire system, including storage batteries for standalone systems or

power connections for the electric grid would cost above $30,000. The high cost of the solar panels of

the Generation-I cells is due to the crystalline silicon wafers. Elaborate processes and lots of energy

are required to grow large, perfect single-crystal ingots of high-purity silicon, which are then sliced into

paper-thin (200-micron-thick) single-crystal wafers (a micron is a millionth of a meter). 200 microns

contains 500,00 atoms which is very thick by industry standards and also very expensive to produce.

The cost of Generation-II cells is being reduced by discarding the thick single-crystal wafer and

instead using thin films of different kinds of photovoltaic material like amorphous (noncrystalline)

silicon or cadmium telluride. By discarding the thick single-crystal wafer in each cell the fabrication

cost is also being reduced but then this decreases the efficiency of the cells. The cells would either

absorb less sunlight or they are less efficient at transporting the charge carriers to the terminals.

Hence even though the popularity of solar power is growing but as long as electric power from solar

photovoltaics remains many times more costly than power from fossil fuels, solar photovoltaics will

likely remain a minor player in the energy sector. Therefore the challenge is to identify an approach


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that is both cheap and efficient. To overcome this challenge intensive research is being conducted to

create higher efficiency and cheaper Generation-III solar cells. There is a possibility of creating

photovoltaic cells with efficiency above 20 percent and a price cheap enough to replace fossil fuels.

To make this dream a reality nanotechnology is being put to use.

[This diagram shows: (A) Sunlight contains photons with a spectrum of colors or energies. (B) A high-

efficiency tandem solar cell developed by NREL which has three thin layers of different

semiconductors. As shown, each starts absorbing light at a different wavelength (colour). Together

they operate like batteries in a series (the same current flows through each, and the total voltage

equals the sum of their separate voltages). The cell operates at more than 40 percent power-

conversion efficiency, but the expense of producing the thin-film layers limits use of the tandem solar

cell to satellites and other niche applications.

Source: www.lanl.gov/science/1663/june2010/story2.shtml ]


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For this purpose semiconductor nanostructures, especially nanocrystal quantum dots can be used as

they are cheap to make and thats because they can be grown and processed in solution. Quantum

dots are tiny specks of matter that each contain from 100 to 10,000 atoms and extend only 2 to 20

nanometers (billionths of a meter) in each direction. The properties exhibited by nanocrystal quantum

dots fit rightly with the needs of photovoltaics. Firstly they are easy to grow and process. With

research the process of growing nanocrystals in solution has become more refined hence achieving

excellent uniformity in their size and surface and allowing the fluid to spread and dry. Such films can

be much cheaper to fabricate than ordinary semiconductor thin films, which require expensive

deposition techniques.

Secondly, a thin film of nanocrystals (50 to 100 nanocrystals thick) can absorb as much light as the

standard single-crystal wafer, which is 1000 times thicker.

Thirdly, nanocrystals can be grown to many different sizes with which the energy gap can be varied.

To understand this property of nanocrystals we got to first consider how a voltage develops when light

strikes a solar cell made of bulk silicon. Individual photons (quantum particles of light) are absorbedby individual electrons that are bound to atoms in the semiconductors crystal lattice. The solar

photons have the right amount of energy to free the electrons from their bound positions and give

them a certain voltage. Now these freed (conduction) electrons collect at the negative terminal of the

solar cell and flow as current when the cell is placed in a circuit. Now to understand the right amount

of energy to free the electrons we got to understand that a semiconductors electrons exist on an

ascending hierarchy (ladder) of energy levels (allowed energy states). The ladder is divided into the

valence band, for bound electrons, and the conduction band, for conduction electrons, with an energy

gap between the two bands.


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[ This diagram shows the concept of energy gap where in a semiconductor, the electrons (blue dots)

fill up the ladder of allowed energy states to the top of the valence band. An energy region with no

energy states (the energy gap) separates the highest rung of the valence band from the bottom rung

of the empty conduction band. This gap has an energy value denoted by Egap.

Source: https://www.lanl.gov/science/1663

/june20

10/story

2a.shtml]

The bound electrons on the highest rung of the valence band can move to the lowest rung of the

conduction band (becoming conduction electrons) only by absorbing enough energy to jump the gap,

which is energy equal to or greater than the amount of energy represented by the energy gap. Solar

photons have energies in this range, therefore a bound electron that absorbs a solar photon can

overcome this energy gap hence becoming a conduction electron. But when the electron jumps the

energy gap to form a conduction electron it leaves behind a vacancy in the valence band. This hole

which is formed in the crystal lattice acts as a positive charge, attracting the new conduction

electrons negative charge. Together, the two called an exciton. Electricity is created when an electric

field causes the conduction electrons and holes to move to opposite terminals.

[ This diagram shows the process of creating excitons where an electron absorbs a photon with

energy Egap and jumps across the energy gap to the conduction band, where it becomes a negative-

charge carrier. Therefore the electron leaves a vacancy, a hole that looks like a positive-chargecarrier. The electron and hole are slightly bound together, and the pair is called an exciton.

Source: https://www.lanl.gov/science/1663/june2010/story2a.shtml]


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In macroscopic samples of a given semiconductor, the energy gap is always the same and is

independent of the sample size, but in tiny nanocrystals, quantum effects causes the energy gap to

vary with size i.e. larger the nanocrystal smaller is its energy gap. Therefore solution chemistry can be

used to grow nanocrystals of a precise size, thus tuning the energy at which the crystals begin

absorbing photons. To leverage this fact we could layer three nanocrystal thin films, each with

progressively smaller nanocrystals and therefore larger energy gaps. The smallest nanocrystals (in

the top layer) would selectively convert the highest-energy photons into charge carriers at a voltage

about equal to the energy gap. Lower-energy photons would pass through to the second and third

layer, where they would produce charge carriers at successively lower voltages. The total voltage

would equal the sum of the voltages derived from the three layers. Thus, the layering would allow a

higher voltage to be extracted from the solar spectrum and result in higher power output than is

obtained from a single layer. Potentially, this three-layer device could perform as efficiently as the

very costly tandem solar panels used power satellites. Research is being done to exploit a number of

novel nanoscale phenomena that could raise the maximum possible power-conversion efficiency of a

single-layer device, improve its light-absorption properties and circumvent present difficulties oftransporting the charge carriers to the terminals.

The first challenge is to maximise the efficiency for a single-layer device. In a macroscopic

semiconductor, each photon with energy equal to or greater than energy gap gets absorbed, hence

freeing one electron. If the energy of the photon equals to the energy gap then it boosts an electron to

the bottom rung of the conduction band, where it becomes a negative-charge carrier with a voltage

equal to energy gap. But then when a photon with more energy than the energy gap boosts an

electron to a higher conduction rung, it does not result in a higher-voltage electron and potentially

higher power-conversion efficiency (electric power is equal to current times voltage). Instead, this

electron immediately begins to loose energy to heat through collisions with the atoms of the crystallattice. In energy space like the operation of a pinball machine, each collision sets off a lattice

vibration (phonon), causing the electron to lose energy and descend another rung of the energy

ladder until its energy (voltage) is energy gap, which implies that until the electron is sitting on the

bottom rung of the conduction band. The descent takes a few picoseconds (trillionths of a second) at

most. Thus, irrespective of the of the photon energy absorbed, the generated photovoltage (and

electric power) in a semiconductor photocell is the same, and a single semiconductor layer can never

have 100 percent power-conversion efficiency. In fact, the maximum power-conversion efficiency. In

fact, the maximum power-conversion efficiency is 31 percent. But then nanocrystals can beat this

limit. This would be possible because when we will confine the electrons to small nanocrystals, all of a

sudden, for each high-energy photon absorbed, we can free not just one but two or more negative-

charge carriers with voltage of energy gap. And if we can transport these carriers to the terminals then

we can get higher electric current. This phenomenon of creating two free electrons with one high

energy photon is called carrier multiplication and this phenomenon was first measured in nanocrystals

in 2004. The original motivation for searching for this effect in nanocrystals traces back to the concept

of a phonon bottleneck. The energy ladder of a nanocrystal is different from one in a macroscopic

sample; namely, the rungs are so widely spaced that the energy difference between rungs is much


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larger than the energy it takes to create a single phonon. An electron boosted to a high-energy rung in

a nanocrystal must create several phonons at once to descend to the next rung down, but that

process is much slower (has lower probability) than creating a single phonon. Therefore, the hung-

up electron with its extra energy courses back and forth through the nanocrystal and is more likely to

strike a bound electron, boosting the latter up to the conduction band and bumping itself down to a

lower rung. Therefore absorption of a single high-energy photon could create two electrons in the

conduction band instead of one. But then while energy loss through phonon emission is likely

suppressed to bulk solids, experiments indicate that other energy-loss processes compete with carrier

multiplication. Understanding these other ways of energy-loss processes is another challenge to this

concept. If the phenomenon of carrier multiplication worked perfectly, then the extra current created

would raise the power output, thereby increasing the maximum possible power-conversion efficiency

for a single layer device to about 41 percent. At present two and one-half to three energy gaps of

photon energy is required to create two excitons. It would be our aim to reduce this threshold energy

to the theoretical limit given by energy conservation, which is two energy gaps for two excitons.

[This diagram shows: (A) Nanocrystal quantum dots which are surrounded by a layer of organic

molecules (surfactants) that allows precise size control, prevent conduction electrons from getting

trapped at the surface, and make nanocrystals soluble. (B) The process of carrier multiplication - the

freeing of two electrons (or creation of two excitons) with one high-energy photon is depicted two

ways: on the energy ladder and within the crystal lattice. In the latter, the valence electrons orbit theatomic nuclei, and the conduction electrons move freely.



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Another way of exceeding the 31 percent limit is to increase the voltage by harvesting the hot (high-

energy) electrons directly, before they either lose their energy to phonons or create multiple carriers.

In particular, a phonon bottleneck could be used to maintain the photogenerated electrons in the hot

state for times sufficient to collect those electrons at the negative terminal and send them down a

wire. The hot electrons would drive a voltage that is greater than the energy gap, resulting in

increased power output. The novel nanoscale materials that exhibit a pronounced phonon bottleneck

and also efficient schemes for harvesting hot electrons are yet to be developed.

But then whether solar photons create a single conduction electron, multiple electrons or hot

electrons, the real challenge for nanocrystal devices is transporting or conducting the charge carriers

to the terminals.

As each electron is not completely free but instead slightly bound to a hole, forming an exciton,

therefore these two charge carriers must be separated and transported very quickly, otherwise, the

exciton will decay in less than a microsecond through recombination and the electron will fall back into

the hole hence emitting the absorbed solar energy as a photon.

In Generation-I cells, charge separation and transport happen readily. Firstly , the silicon wafer is

doped with impurities that provide ready-made charge carriers (conduction electrons and holes) and

therefore a high conductivity. Secondly, the doping creates an interface with the electron-donating

impurities on one side (the n layer) and hole-donating impurities on the other (the p layer).

Electrons migrate across this interface, or p-n junction, setting up a permanent electric field that

separates electron-hole pairs as soon as they are created, and helps transport the electrons with their

negative charges to the negative terminal and the holes with their positive charges to the positive

terminal.


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[ This diagram shows the process of harvesting charge carriers. Here a crystalline silicon solar cell

quickly separates the excitons electrons and holes because the cell contains two silicon layers: a p-

doped layer containing acceptor atoms, which tend to accept extra electrons, and an n-doped layer

containing donor atoms, which tend to give away their electrons. When these layers are brought into

contact, they form a p-n junction electrons are exchanged, and ionized donors and acceptors create

strong electric field. It is this electric field that drives the electrons and holes apart as soon as sunlightcreates them, thereby preventing recombination (in which the electron falls back into the hole, and

solar energy is re-emitted as a photon). The charge carriers collect at opposite conducting terminals

and flow around a circuit to power, for example, an electric motor.

Source: https://www.lanl.gov/science/1663/june2010/story2a.shtml]

This method of charge separation and transport does not apply in nanocrystal thin films. Nanocrystals

tend to expel impurities during crystal growth, making it difficult to create well-defined p-n junctions.

Furthermore, nanocrystals are surrounded by insulating organic molecules and the charge carriers,

once created, must tunnel through that layer and then hop from nanocrystal to nanocrystal to reach

the terminals. All of this is inefficient. Charge carriers can be in the insulating layers or lost through

recombination with an oppositely charged carrier as they hop to the terminals. Recombination

becomes a very serious and immediate problem when carrier multiplication creates two excitons.

These two excitons can interact and share energy so that one electron gets boosted back up to a

higher energy, and the other electron recombines with its hole. This process is called Auger

recombination and happens in tens of picoseconds (trillionths of a second). To take advantage of the


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carrier-multiplication effect, the carriers have to be extracted from the nanocrystal more quickly than

Auger recombination can occur. Finding ways to do this is another challenge to this concept.

One way would be to give the electrons someplace to go to get away from the holes. This can be

done by growing projections (arms) on the nanocrystals. These arms are bits of semiconductor that

can act as electron acceptors. Each time a photon generates an electron-hole pair, the electronrapidly separates from the hole (it stays in the central core) by travelling to the arms. This separation

buys time because it reduces the chance that the electron will recombine with a hole before being

extracted from the nanostructure. Therefore, if we can assemble a nanocrystal thin film with the arms

forming a continuous network and the nanocrystals and arms are embedded in a material that

conduct holes, then charge transport and charge collection might happen quite easily which is infact a

key to efficient photovoltaics.

[ This diagram shows electron-accepting arms as shown in the electron micrograph.


An interesting solution to harvesting current avoids the need for having charge carriers to hop

between nanocrystals. Instead, the exiton in one nanocrystal daecys ( electron and hole recombine ),

and the energy emitted does not go into a photon but into an electric field that acts like a photon,

exciting an exciton in a neighbouring nanocrystal. This transport of energy between nanocrystals

repeats until the exciton reaches a p-n junction, where it is split into an electron and a hole by an

electric field.


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For the purpose of understanding this energy-transport process, we will consider that at a distance

the excitons electron-hole pair looks like a positive charge and a negative charge separated by a very

small distance, this is known also as an electric dipole. The dipole produces a characteristic electric

field that extends well beyond the nanocrystal. The dipole field puts one nanocrystal in contact with

another and allows nanocrystals to transport energy from one to the other.

Because the energy emitted when an exciton decays is lower than the energy absorbed to create it

and the energy-transport process is especially favoured from a small nanocrystal with a large energy

gap to a larger energy nanocrystal with a smaller gap. Therefore in this case, each energy exchange

produces an exciton with slightly less energy than its predecessor. Hence, more of these different

mechanisms of charge and energy transport needs to be explored in the hope that one or more

mechanisms will work out thereby dramatically increasing the efficiency of harvesting usable

electricity from nanocrystal thin-film solar cells.

[ This diagram shows the process of energy transport in a layering of nanocrystal thin films that have

progressively larger quantum dots. The top layer, the smallest quantum dots, will absorb, say, a blue

photon and create an exciton. The exciton moves from smaller to larger quantum dots through a two-

step sequence of decay and excitation (arrows) and then to the p-n junction, where the electric field


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splits the exciton into an electron and a hole that travel to different terminals. This process eliminates

charge hopping.


The real challenge is to integrate these processes into a working device. The Los Alamos team along

with NREL is trying fabricate and test nanocrystal thin-film devices and is dealing with issues of

practical photovoltaics. The center is also developing novel materials that can find immediate use in

photovoltaics. One example is inexpensive germanium nanocrystals that can be made in different

sizes and applied in inexpensive multilayered devices that absorb energy from the infrared to the

ultraviolet. Hence the new physics being developed at the centre will eventually lead to real life

devices capable of replacing bulky and expensive silicon solar panels.

Hence the usage of nanotechnology to improve the generation of solar power is a feasible idea in the

near future. Nanotechnology would make the generation of solar power more economical and far

more efficient hence generating more electricity with the absorbed solar power. Hence the cost

reduction which will be facilitated through the use of nanotechnology can make the generation of

electricity through solar power a feasible business idea. In countries such as India where electricity is

still unable to reach the rural hinterlands and even the availability of electricity is accompanied with

innumerable power cuts, the generation of a clean, renewable, sustainable and relatively cheap

source of energy would be an ultimate solution to all the energy problems. This would also break the

monopoly which is enjoyed in the market by the limited number of power suppliers and would open

doors to several industrialists to generate this alternative source to energy so as to benefit himself

and the society.


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Bibliography

1) https://www.lanl.gov/science/1663/june2010/story2.shtml

2) http://www.thegovmonitor.com/world_news/europe/new-laser-nanofabrication-technology-

reduces-production-costs-30372.html

3) https://www.lanl.gov/science/1663/june2010/story2a.shtml

4) http://www.nanotech-now.com/columns/?article=206

5) http://www.nanoforum.org/dateien/temp/nanotechnology%20in%20agriculture%20and%20food.pdf

6) ftp://ftp.cordis.europa.eu/pub/nanotechnology/docs/elsa_governance_nano.pdf

7) http://ourworld.unu.edu/en/putting-nanotech-to-green-use/

8) http://en.wikipedia.org/wiki/List_of_nanotechnology_applications

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