Science Snippets- 2

In the previous article - Snippets-1, some notable advances in the cancer area

were highlighted. Here we shall examine some impacting developments in

nanomaterials and nanotechnology that may interest the layman reader.

The three topics chosen for presentation are 2D nanomaterials,

nanogenerators and new generation solar cells. In a follow-up article, two

futuristic concepts - quantum computers (devices based on quantum mechanical

phenomena such as superposition and entanglement to perform operations on data) and

claytronics (a futuristic simulation system concerned with programmable matter that

can morph nearly any object imagined into another object with different size, shape,

colour and function) –will be discussed.

Nanomaterials such as colloidal metal particles, metal oxides, nano-structured

conducting polymers and carbon nanotubes have recently attracted much

interest owing to their applications in nano-scaled devices, sensors and

detectors. The defining characteristic of nanomaterials is their size in the range

of 1-100 nanometers (nm). One nanometer, i.e. 10-9 m, spans 3-5 atoms lined

up in a row. It is to be understood that “nanomaterials are not simply

another step in miniaturization, but a different arena entirely; the nanoworld

lies midway between the scale of atomic and quantum phenomena, and the

scale of bulk materials” {Ref: http://www.csa.com/ discoveryguides

/nano/overview.php}.

A) Two-Dimensional Nanomaterials: Graphene & Molybdenum

Disulphide

Arguably, the two most exciting nanomaterials discovered in recent times are

2D graphene, a one-atom-thick sheet of carbon atoms arranged hexagonally,

discovered in 2004 by Nobel Laureate Andre Geim at the University of

Manchester, and 2D molybdenum disulphide (MoS2), discovered in 2011 by

two groups of scientists, one at the Swiss Federal Institute

of Technology Lausanne {Ref: Andras Kis et al, Nature Nanotechnology, 2011, 6:

147-150} who produced a transistor on the material and the other at MIT

where the researchers succeeded in making a variety of electronic components

from it, including an inverter, a NAND gate, a memory device and a ring

oscillator { Ref: Han Wang et al, Ref: Nano Lett. 2012, doi:10.1021/nl302015v}

The MIT researchers claim the extremely thin and transparent 2D MoS2 could

help usher in radically new products, including in combination with other 2D

materials, from light-emitting devices that allow whole walls to glow to

clothing with embedded electronics that constitute the circuitry of a cell phone

to glasses with built-in display screens.

Both materials have excellent electronic and optoelectronic applications that

potentially can exceed those of silicon. Graphene is effectively the thinnest

material that we can make out of atoms. Surprisingly it is also very strong,

thanks to a lack of crystal boundaries to break and the very strong bonds

between the carbon atoms that make up its honeycomb lattice. The electronic

properties of graphene, which has zero band gap, are rather unusual. The

interaction between the electrons and graphene’s honeycomb atomic

structure causes the electrons to behave as if they have absolutely no mass,

and because of this the electrons are governed by the Dirac equation – the

quantum mechanical description of electrons moving relativistically – and are

therefore called Dirac fermions. The electrons in graphene travel large

distances without being scattered and at speeds 300 times less than the speed

of light in vacuum enabling relativistic effects to be observed without using

particle accelerators! Because the Dirac fermions in graphene carry one unit of

electric charge, they can be manipulated using electromagnetic fields, an

important consideration for applications in modern electronics.

2D Molybdenum disulphide, on the other hand, is a layered semiconductor

material possessing a band gap, a key property that makes it possible to create

transistors, the basic component of logic and memory circuits, and

furthermore the magnitude of the band gap which is 1.8 eV gives it an

advantage over silicon in suppressing the source-to-drain tunnelling at the

scaling limit of transistors. Silicon chips now have features as small as 22 nm

but silicon technology is susceptible to oxidation which reduces performance

and causes energy losses. This portends opportunities for MoS2 and graphene

{Ref: http://www.eurekalert.org/pub_releases/2009-06/dbnl-bgg060809.php}.

A material’s band gap dictates the minimum energy an outer shell (valence)

electron needs to escape an atom and become a mobile charge carrier;

likewise, the band gap will prevent electrons with too much energy from

joining the atom. The band gap generally refers to the energy difference

(in electron volts) between the top of the valence band and the bottom of

the conduction band in insulators and semiconductors. By manipulating the

band gap, scientists can indirectly control the photons produced or absorbed

when electrons undergo energy changes. A non-zero band gap can be achieved

with a graphene bilayer by having electrical gates on both the top and bottom

layers; the average effect of the displacement fields in each layer breaks the

bilayer's inversion symmetry and hence gives rise to a non-zero band gap

which is tunable from 0 to 0.25 eV by varying the voltage applied to the gate

electrodes. The gated bilayer device, which is a field-effect transistor (FET) (see

Fig.1) is built on a silicon substrate (the bottom gate) and contains a thin

insulating layer of silicon dioxide between the substrate and the stacked

graphene layers. There is a transparent layer of sapphire (aluminium oxide)

over the graphene layers and on top of this, the top gate, made of platinum

{Ref. Feng Wang et al, Nature, 2009, 459: 820-823}. Applications made possible by

this breakthrough of electrical doping are new kinds of nanotransistors and –

because of its narrow band gap – nano-LEDs and other nanoscale optical

devices in the infrared range. Chemical doping of one of the layers with

adsorbed metal atoms was previously attempted by these researchers, but

such doping being uncontrolled is not compatible with device applications.

Fig.1: Dual-gate bilayer graphene FET

2D materials are fundamentally different from the normally encountered 3D

materials. Their planar geometry makes it easier to fabricate circuits and

complex structures by tailoring 2D layers into desired shapes. A number of

well-known forms of carbon such as carbon nanotubes and Buckminster

fullerene (C60) derive from graphene (see Fig.2 below).

Fig.2: (2D) Graphene (top left) consists of 20 hexagonal lattice of carbon atoms. Each C atom

has 4 valence electrons, one is left free- allowing graphene to conduct electricity. Other

well-known forms of carbon derive from graphene: (3D) graphite is a stack of graphene

layers (top right); (1D) carbon nanotubes are rolled-up cylinders of graphene (bottom left);

and a (0D) Buckminster fullerene (C60) molecule consists of graphene balled into a sphere

by introducing some pentagons as well as hexagons into the lattice (bottom right).

{Ref: A. H. Castro Neto, F. Guinea, and N. M. R. Peres, Physics World, Nov. 2006}

Carbon nanotubes, for example, which have garnered much attention for their

strength and such fascinating potential applications as faster computer chips,

better solar cells and better capacitors as replacement for batteries, can be

considered as rolled-up graphene sheets. These nanotubes have diameters of

few nanometers but have the potential to have lengths over one million times

more than their diameter. A single-walled carbon nanotube (SWNT) consists of

a single graphene cylinder whereas a multi-walled carbon nanotube (MWNT)

comprises of several concentric graphene cylinders. Although carbon

nanotubes have never been fabricated from graphene directly, the possibility

of achieving this by twisting a graphene nanoribbon has been shown to be a

tenable proposition from quantum molecular-dynamics simulations and

classical continuum-elasticity modelling studies {Ref: P.Koskinen et al, Phys Rev B

,2012, 85, 085428}. The arc-evaporation method, which produces the best

quality nanotubes, involves passing a current of about 50 amps between two

graphite electrodes in an atmosphere of helium. This causes the graphite to

vaporise, some of it condensing on the walls of the reaction vessel and some of

it on the cathode. It is the deposit on the cathode which contains the carbon

nanotubes. Single-walled nanotubes are produced when Co and Ni or some

other metal is added to the anode.

Single and double layered MoS2-coated multi-walled carbon nanotubes have

also been successfully prepared by pyrolyzing (NH4)2MoS4-coated multi-walled

carbon nanotubes in an H2 atmosphere at 900℃ {Ref: Xu Chun Song et al, Chinese

Chemical Letters, 2004, 15: 623-626}.

A single molecular layer of MoS2 is built up of Van der Waals - bonded S-Mo-S

units comprised of a layer of Mo atoms sandwiched between two layers of

sulphur atoms (Fig.3). The strong intra-layer covalent bonds confer MoS2

crystals excellent mechanical strength, thermal stability up to 1090C in an inert

environment, and a surface free of dangling bonds. The weak inter-layer Van

der Waal’s force allows single- or few-layer MoS2 thin films to be created

through micro-mechanical cleavage technique and through anisotropic 2D

growth by chemical vapour deposition. This unique property of MoS2, and 2D

materials in general, enables the creation of atomically smooth material sheets

and the precise control on its number of molecular layers.

Fig.3: 2D Molybdenum Disulphide. Mo atoms are shown in teal, and S atoms in yellow.

{Source: Han Wang et al, Ref: Nano Lett. 2012, doi:10.1021/nl302015v}

“Nano-electronic devices built on 2D materials offer many benefits for further

miniaturization beyond Moore’s Law and as a high-mobility option in the

emerging field of

large-area and low-cost electronics that is currently dominated by low-mobility

amorphous silicon and organic semiconductors”, notes Han Wang, a lead

researcher in the MIT team. But the lack of a reliable large-scale production

method, transcending the currently common mechanical exfoliation technique

based on bulk samples, is an inhibiting issue for their practical applications.

A bottom-up process to make gram scale quantities of graphene was recently

reported by Australian scientists starting from completely non-graphitic

precursors – ethanol and sodium. The approach simply involves reacting the

two components together under pressure to produce a white powder than

turns black when heated. This material is made up of fused carbon sheets that

can be broken down into single sheets of carbon using mild sonication {Ref:

Mohammad Choucair, P. Thordarson & J. A. Stride Nature Nanotechnology, 2009, 4:

30 - 33}.

However, recent research suggests that increasingly efficient and scalable

methods for the fabrication of monolayer and few-layer graphene and MoS2

may no longer be elusive. Among encouraging reports in this direction are a

scalable chemical vapour deposition process described by Ajayan, Jun Lou and

co-workers at Rice University {Ref: Small, 2012, 8: 966-971} for atomic-layered

MoS2 synthesized directly on SiO2 substrates, and a ball milling-cum-sonication

technique reported by Yao et al {Ref: J. Mater. Chem., 2012, 22: 13494-13499}

for fabricating nanosheets of graphene and MoS2.

B) Nanogenerators

Ever wondered whether you could squeeze a flexible computer chip between

your fingers and convert that mechanical motion into electrical energy that

can recharge an iPod or a pacemaker or achieve the same aim with your mere

footsteps with the chip now inside the sole of your shoe? How about

powering an implanted insulin pump by one’s own heart beat? Far-fetched

ideas, you might think, but not so according to Georgia Tech’s Professor

Zhong Lin Wang. He and his colleagues at Georgia Tech have built a

nanometer-scale generator based on arrays of nanowires grown on a rigid

substrate and topped with a metal electrode. Later versions embedded both

ends of the nanowires in polymer and produced power by simple flexing.

Bending of the zinc oxide nanowire arrays produces an electric field by

the piezoelectric properties of the material. The semiconductor properties of

the device create a Schottky barrier, that is, a rectifying barrier for electrical

conduction across the semiconductor-metal interface. The generator is

estimated to be 17% to 30% efficient in converting mechanical motion into

electricity. As reported in Science Daily {Ref: http://www.sciencedaily.com/

releases/2010/11/101108151416.htm}, “Wang and his team in 2010 were able

to produce 3 volts of potential and as much as 300 nanoamperes of current,

an output level 100 times greater than was possible a year earlier, from an

array measuring about 2 cm by 1.5 cm. First, they grew arrays of a new type

of nanowire that has a conical shape. These wires were cut from their growth

substrate and placed into an alcohol solution. The solution containing the

nanowires was then dripped onto a thin metal electrode and a sheet of

flexible polymer film. After the alcohol was allowed to dry, another layer was

created. Multiple nanowire/polymer layers were built up into a kind of

composite, using a process that Wang believes could be scaled up to industrial

production.” Continuing research is very likely to result in more spectacular

results to serve the power needs of very small devices that can be used in

applications such as health care, environmental monitoring and personal

electronics if the way to power them can be simultaneously addressed.

C) New generation Organic Polymer based and Inorganic Quantum

Dot based Solar Cells (with explanatory notes)

Research on photovoltaic cells (solar cells) has received much impetus in

recent times as part of the relentless global efforts to seek alternative and

renewable energy sources to fossil fuels which are dwindling in reserves.

Semiconductors play a central role in solar cells whose efficiency is defined

as the electrical power it puts out as percentage of the power in incident

sunlight. A solar cell is essentially a p-n junction with a large surface area.

The generation of electric current happens inside the depletion zone of the

p-n junction.

One of the most fundamental limitations on the efficiency of a solar cell is

the band gap of the semi-conducting material used in conventional solar

cells which is the energy required to promote an electron from the bound

valence band into the mobile conduction band.

The band gap represents the minimum energy difference between the top

of the valence band and the bottom of the conduction band. However, the

top of the valence band and the bottom of the conduction band are not

generally at the same value of the electron momentum. In a direct band

gap semiconductor, the top of the valence band and the bottom of the

conduction band occur at the same value of momentum, whereas in

an indirect band gap semiconductor, the maximum energy of the valence

band occurs at a different value of momentum to the minimum in the

conduction band energy, as in the schematics given below {Ref:

http://www.doitpoms.ac.uk/tlplib/semiconductors/direct.php}:

When an electron is knocked loose from the valence band by the incident

light photon, it goes into the conduction band as a negative charge, leaving

behind a ‘hole’ of positive charge. The incident light thus produces

electron-hole pairs on both sides of the p-n junction, that is, in the n-type

emitter and p-type base. The generated electrons from the base and holes

from the emitter then diffuse to the junction and are swept away by the

junction’s electric field, but in opposite directions. If the solar cell is

connected to an external circuit, an electric current is generated. If the

circuit is open, then an electrical potential or voltage is built up across the

electrodes. This is how a solar cell functions.

Each photon of energy E has momentum, p= E/c, where c is the velocity of

light. An optical photon has energy of the order of 10–19 J, and,

since c =3 × 108 ms–1, a typical photon has a very small amount of

momentum. A photon of energy Eg, where Eg is the band gap energy, can

produce an electron-hole pair in a direct band gap semiconductor quite

easily, because the electron does not need to be given very much

momentum. However, an electron must also undergo a significant change

in its momentum for a photon of energy Eg to produce an electron-hole pair

in an indirect band gap semiconductor. This is possible, but it requires such

an electron to interact not only with the photon to gain energy, but also

with a lattice vibration called a phonon in order to either gain or lose

momentum.

Interactions among electrons, holes, phonons and photons and other

particles are required to satisfy conservation of energy and crystal

momentum (i.e., conservation of total k-vector) {Ref: http://

en.wikipedia.org/wiki/Direct_and_indirect_band_gaps}.

One consequence of the top of the valence band and the bottom of the

conduction band occurring at the same value of momentum in a direct

band gap semiconductor (the electron and the hole sharing the same k-

vector) is electron-hole annihilation with release of energy or radiative

recombination. Radiative recombination is a much slower process in an

indirect band gap material because of the involvement of the phonon to

carry away the difference in the momenta of the electron and the hole. This

is why light-emitting and laser diodes are almost always made of direct

band gap materials, and not indirect band gap ones like silicon.

The exact reverse of radiative recombination is light absorption. For the

same reason as above, light with a photon energy close to the band gap can

penetrate much farther before being absorbed in an indirect band gap

material than in a direct band gap one (at least insofar as the light

absorption is due to exciting electrons across the band gap). This is an

important fact to note in solar cells. Silicon is the most common solar-cell

material, despite the fact that it is an indirect band gap material and,

therefore, does not absorb light very well.

In a solar cell, photons with less energy than the band gap slip right through

the semiconductor material without being absorbed, while photons with

energy higher than the band gap are absorbed, but their excess energy is

wasted, and dissipated as heat. How does this happen? The higher energy

photons excite the electrons to energy levels higher than those associated

with the semiconductor’s conduction band. These “hot electrons” at

picosecond levels can tunnel out of the semiconductor material—instead of

recombining with a hole or being conducted through the material to a

collector. This results in increased current leakage and concomitant heating

of the device. Because “hot electrons” generally give off their excess energy

as phonons, heating of the semiconductor device is to be expected.

Solutions to tapping the “hot electrons” for their energy before they are

irrevocably lost as heat have been presented by a number of investigators

in the recent literature, all aimed at realising an ‘ultimate solar cell’. There

are three critical steps involved in this effort, according to Professor

Xiaoyang Zhu, a lead researcher in this field at Texas University, these

being: firstly, a slowing down or cooling of the “hot electrons”; secondly, a

means of transferring the “hot electrons” to an electron conductor; and

thirdly, drawing them out to an electrical conducting wire without heating

up the wire. The first step has been demonstrated to be achievable with

semiconductor nanocrystals (quantum dots; more on this in later

paragraphs). The second step was demonstrated by Zhu when he

discovered that hot electrons from photo-excited lead selenide

nanocrystals can be transferred to an electron conductor made of titanium

dioxide {Ref: X.-Y.Zhu, Science, 2010, 328(5985): 1543-1547}.

The third step still awaits experimentation as this calls for adjusting the chemistry at the interface of the conducting wire with the electron conductor that will not lead to heat dissipation in the conducting wire.

An alternative approach to solar cells that is being widely looked at is to use

organic semiconductors that can efficiently absorb the phonons released by

the “hot electrons”. Each phonon absorbed by the organic semiconductor, a

typical example of which is a thin film of polycrystalline pentacene, results

in the creation therein of a singlet exciton (a bound electron-hole pair) of

high binding energy. The singlet exciton undergoes a rapid internal

conversion into a dark state of multi-exciton character that efficiently splits

into two triplet excitons from which an additional electron can be

harvested. This way, a photon provides double the electrons. In general, to

enable multiple exciton generation (MEG), the photons have to have

energies at least twice the band gap (to obey the law of conservation of

energy). Exploiting the dark state can potentially increase solar cell

efficiency to 44%.

Organic semiconductors are gaining a lot of attention due to their potential

to be fabricated at low cost onto lightweight and flexible substrates. The

mechanism of light‐to‐electric energy conversion in organic solar cells is

different than in common inorganic solar cells. As opposed to crystalline

inorganic materials, light absorption does not directly create free charge

carriers in bulk organic materials. Instead, the photoexcited electron and

hole attract each other through Coulomb interaction. The binding energy of

these electron‐hole pairs (excitons) is typically 0.2–0.8 eV. The excitons are

strongly bound in these materials as a consequence of the low dielectric

constants in the organic components, which are insufficient to affect direct

electron–hole dissociation. Exciton dissociation occurs almost exclusively at

the interface between two materials of differing electron affinities (and/or

ionization potentials): the electron donor and the electron acceptor. To

generate an effective photocurrent in these organic solar cells, an

appropriate donor–acceptor pair and device architecture must thus be

selected.

Three types of organic solar cells can be discriminated: dye‐sensitized,

small‐molecule‐, and polymer‐based solar cells.

The dye‐sensitized solar cell (DSSC) was introduced by O’Regan and Grätzel

in 1991 {Ref: B. O’Regan, M. Gratzel, Nature, 1991, 353: 737}, and is now

considered a cost‐effective alternative for silicon solar cells. A typical dye‐

sensitized solar cell is comprised of a ruthenium dye with π‐conjugated

ligands having anchoring groups that bind to TiO2, adsorbed at the surface

of a high surface area nanoparticulate TiO2 electrode.

An example of a small-molecule solar device is one consisting of intrinsic

copper phthalocyanine as the electron donor and intrinsic perylene

tetracarboxylic derivative as the electron acceptor.

Research on polymer solar cells based on π‐conjugated polymers has made

rapid progress since the discovery in 1977 of the conductive properties of

such polymers by Shirakawa and co-workers {Ref: C. K. Chiang, C. R. Fincher, Y.

W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, A. G.MacDiarmid, Phys.

Rev. Lett. 1977, 39: 1098}.

The photoactive layers of the most efficient polymer solar cells to-date

constitute of phase separated blends of electron donor and acceptor

materials consisting of domains with nanometer dimensions. Two main

approaches have been explored in the effort to develop viable devices: the

donor–acceptor bilayer, commonly achieved by vacuum deposition of

molecular components, and the so-called bulk heterojunction (BHJ) (see

Fig.4). The state-of-the-art in the field of organic photovoltaics is currently

represented by bulk heterojunction (BHJ) solar cells based on poly(3-

hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric

acid methyl ester (PCBM), with reproducible efficiencies approaching 5 % {

Ref: B.C.Thompson and J.M.J. Frechet: Angew. Chem. Int. Ed. 2008, 47, 58 – 77,

and references therein}.

Fig.4: BHJ Solar Cell {Source: http://www.oe-chemicals.com/ dictionaryM-Z.html}

The bulk heterojunction concept was first introduced by Sariciftci and co-

workers by mixing a semiconducting conjugated polymer as the donor

material with fullerene C60 as the electron acceptor {Ref: N. S. Sariciftci, L.

Smilowitz, A. J. Heeger, F. Wudl, Science 1992, 258: 1474}. Such a blend can be

deposited from solution on a transparent conducting electrode [often

indium tin oxide (ITO), coated with a conductive polymer layer] and capped

with a metal electrode to obtain working solar cells.

Among more recent examples of heterojunction polymer solar cells are

those based on the prototypical p- and n-type organic semiconductors

pentacene (P5) and fullerene (C60) {Ref: P.Sullivan and T.S Jones, Organic

Electronics, 2008, 9:658-660}. Pentacene, which is a polycyclic aromatic

hydrocarbon with five linearly-fused benzene rings, has the distinction of

being the first individual molecule whose 3D image has been captured. This

was accomplished by researchers from IBM in 2009 using an atomic force

microscope; the molecule is just 1.4 nm long.

Fig.5 below shows a schematic of a solar cell device structure {Ref: Bernard

Kippelen et al, spie.org/x8867.xml?pf=true&ArticleID=x8867} with purified P5

and C60 deposited successively on indium tin oxide (ITO) conductive glass

substrate in vacuum by thermal evaporation using shadow masking.

Aluminium electrodes are the deposited as top electrodes. A bathocuproine

(BCP) layer is used as a passivation layer to prevent excitons generated in

the C60 layer from being quenched at the organic/metal interface, and to

protect the acceptor layer during metal deposition. A typical device area is

ca. 0.1 cm2. Over the wavelength range 400-700 nm studied, a peak

“external” quantum efficiency of 69% was observed at λ = 668 nm.

Fig.5: Device structure of a pentacene/C60 solar cell.

Quantum efficiency refers to the percentage of photons that are converted

to electric current (i.e., collected carriers) when the cell is operated under

short circuit conditions. Some of the light striking the cell is reflected, or

passes through the cell (transmitted); external quantum efficiency is the

fraction of incident photons that are converted to electric current. Not all

the photons captured by the cell contribute to electric current; internal

quantum efficiency is the fraction of absorbed photons that are converted

to electric current.

Solar cells operate as quantum energy conversion devices, and are

therefore subject to the "thermodynamic efficiency limit" which is the

absolute maximum theoretically possible conversion efficiency of sunlight

to electricity. Its value is about 86%, which is due to the Carnot limit.

% Efficiency of a photovoltaic cell = Power output of PV cell Power input from the sun

The power input from the sun is taken as 1000 watts/m2 on a sunny

summer day, 900 watts/m2 on a sunny autumn or spring day, 700 watts/m2

on a sunny winter day. These values are less than that of the solar constant

(the amount of the sun’s energy that reaches the edge of Earth’s

atmosphere) which has the average value of 1,368 watts/m2. The Shockley-

Queisser (SQ) Efficiency Limit calculation is based on "standard test

conditions" (STC). The STC conditions approximate solar noon at the spring

and autumn equinoxes in the continental United States with the surface of

the solar cell aimed directly at the sun. The modern SQ Limit calculation is

a maximum efficiency of 33% for any type of single junction solar cell. In

practice, the best achievable is about 25%. The first practical photovoltaic

cell developed in 1954 at Bell Laboratories by Chapin, Fuller and Pearson

which was based on a diffused silicon p-n junction had an efficiency of 6%.

Since then, solar cells containing different types of inorganic

semiconductors have been made, using various device configurations and

employing single‐crystalline, poly‐crystalline, and amorphous thin‐film

structures. First generation PV devices based on crystalline silicon currently

dominate the PV market with a 90% market share. The dominance stems

mainly from the wide availability of silicon and the reliability of the devices,

as well as from knowledge and technology borrowed from microelectronics

industry.

It is possible to improve on the efficiency of a single-junction solar cell by

stacking materials with different band gaps together in what are called

tandem or multi-junction cells. Stacking dozens of different layers together

can increase efficiency theoretically to greater than 70%. But this results in

technical problems such as strain damages to the crystal layers. The most

efficient multi-junction solar cell reported to-date is one that has three

layers: gallium indium phosphide/gallium arsenide/germanium

(Ga0.5In0.5P/GaAs/Ge) with band gaps of 1.8, 1.4, and 0.7 eV, respectively,

made in 2001 by the National Center for Photovoltaics in the US, and

reaches power conversion efficiency of 32.0% under I sun (defined as 1000

W/m2).

Similar attempts to improve the absorption of organic solar cells by using

multi-junction structures have been made. Blom and co-workers {Ref: A.

Hadipour, B. de Boer, P.W.M. Blom, Organic Electronics, 2008, 9: 617–624), for

example, have reported on the improved performance characteristics of a

tandem organic solar cell based on a 250 nm blend of regioregular poly(3-

hexylthiophene) (rr-P3HT) and the fullerene derivative [6,6]-phenyl-C61-

butyric acid methyl ester (PCBM) for the bottom cell and a 80 nm blend of

poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-

PPV) and PCBM for the top cell. An optical spacer with a thickness of 190

nm was used to separate the sub cells.

An interesting new development is the discovery by Professor Saki Sonoda

and his research group at Kyoto Institute of Technology of a single-

junction PV cell that is capable of generating electricity not only from

visible light, but from ultraviolet and infrared light as well. This new PV

cell was made by 'doping' a wide band gap transparent composite

semiconductor - in this case, gallium nitride- with a 3d transition metal

such as manganese or cobalt{Ref: http://phys.org/ news 188637189.html }.

Infrared (IR) photovoltaic cells per se – which transform infrared light into

electricity - are also attracting much attention, since about half the Sun's

energy arrives at near infrared frequencies. Photovoltaic cells that respond

to infrared – ‘thermovoltaics’ - can even capture radiation from a fuel-fire

emitter; and co-generation of electricity and heat are said to be quiet,

reliable, clean and efficient. A 1 cm2 silicon cell in direct sunlight will

generate about 0.01W, but an efficient infrared photovoltaic cell of equal

size can produce theoretically 1W in a fuel-fired system {Ref: http://www.i-

sis.org.uk/QDAUESC.php}.

One development that has made IR photovoltaics attractive is the

availability of light-sensitive conjugated polymers - polymers with

alternating single and double carbon-carbon (sometimes carbon-nitrogen)

bonds – which with chemical doping manifests increased electronic

conductivity.

In order to make conjugated polymers work in the infrared range,

researchers at the University of Toronto led by Professor Edward Sargent

combined infrared-sensitive nanocrystals of lead sulphide (PbS) with a

conjugated polymer - poly[2-methoxy-5-(2'-ethylhexyloxy-p-

phenylenevinylene)] (MEH-PPV) {Ref:. E.H.Sargent, Nature Photonics, 2009,

3:325-331}. Such nanocrystals or quantum dots have quantum optical

properties that are absent in the bulk material due to the confinement of

electron-hole pairs (called excitons) on the particle. The researchers used

around 90% nanocrystals by weight, dissolving the two components in

chloroform before spin-coating the material onto a substrate to create a

film that was 100-150 nm thick. By changing the size of the nanocrystals,

they were able to tune the nanocrystals to be sensitive to infrared

wavelengths of around 980 nm, 1200 nm or 1355 nm. In the absence of

nanocrystals, the MEH-PPV polymer reacted to wavelengths between

around 400 and 600 nm - i.e. visible light. The devices consisted of a glass

substrate, an indium tin oxide layer, a poly(p-phenylenevinylene) (PPV)

coating, an MEH-PPV/PbS nanocrystal blend and an upper magnesium

metal contact. Upon photoexcitation and placing the device under forward

bias, electrons were extracted through the Mg contact and holes through

the PPV/ITO contact. Since this first successful report, several groups have

since improved on this prototype, including the University of Toronto

researchers themselves {Ref: Jiang Tang and Edward H. Sargent, Advanced

Materials, 2011, 23: 12-29}.

Besides, lead sulphide, other quantum dot materials (QDs) that have been

investigated include lead selenide (PbSe) and titanium oxide (TiO2).

The outstanding advantage that QDs bring to solar cell technology is their

tunable band gap, that is, the wavelength at which they will absorb or emit

radiation can be adjusted at will: the larger the size, the longer the

wavelength of light absorbed or emitted. The greater the band gap, the

more energetic the photon absorbed and the greater the output voltage.

On the other hand, a lower band gap results in the capture of more photons

including those in the IR, resulting in higher output of current but at lower

voltage.

Multi-junction solar cells made from a combination of colloidal quantum

dots (CQDs) of differing sizes and thus differing band gaps are a promising

means by which to increase the energy harvested from the Sun's broad

spectrum. The first efficient solution processed tandem solar cell based on

CQDs was reported by Professor Edward Sargent and his University of

Toronto research group in 2011 using the size-effect tuning of a single CQD

material, PbS. They used “a graded recombination layer (GRL) to provide a

progression of work functions from the hole-accepting electrode in the

bottom cell to the electron-accepting electrode in the top cell, allowing

matched electron and hole currents to meet and recombine” (see Fig.6). The

tandem solar which efficiently connects the bottom VIS cell with the top IR

cell had an open-circuit voltage of 1.06 V, equal to the sum of the two

constituent single-junction devices, and a solar power conversion efficiency

of up to 4.2% {Ref: E.H.Sargent, et al: Nature Photonics, 2011,5:480–484}.

Fig.6: Lead Sulphide Colloidal Quantum Dot Tandem Solar Cell

A second advantage with QDs is that they can be moulded into a variety

of different forms, in sheets or 3-D arrays. They can be easily combined

with organic polymers, dyes or made into porous films. In the colloidal form

suspended in solution, they can be processed to create junctions on

inexpensive substrates such as plastics, glass or metal sheets. Indeed,

quantum dot based photovoltaic cells based around dye-sensitized

colloidal TiO2 films were investigated as early as 1991. An enhancement in

dye-sensitized solar cells overall conversion efficiency was recently

observed for the photoanode consisting of nanosized TiO2 single crystals

with higher percentage of exposed (001) facets, increasing from 7.47%,

8.14% to 8.49% for the TiO2 single crystals with ca. 10%, 38%, and 80%

percentage of exposed (001) facets {Ref: Gao Qing (Max) Lu, et al: Advanced

Functional Materials, 2011, 21: 3982–3989}. Titanium dioxide (TiO2)

nanocrystals are known to absorb UV light. But their spectral absorption

capability has recently been enhanced by introducing disorder in the

surface layers of nanophase TiO2 through hydrogenation which yields a

black TiO2 {Ref: Samuel S Mao, et al: Science. 2011, 331(6018):746-50}.

A third advantage with QDs is multiple exciton generation (MEG), a

phenomenon that does not readily occur in bulk semiconductors where the

excess energy simply dissipates away as heat before it can cause other

electron-hole pairs to form. The first evidenced report of MEG in QDs was

by Schaller and Klimov from the Los Alamos National Laboratory New

Mexico who observed this phenomenon with PbSe nanoparticles of less

than 10 nanometers in diameter {Ref: R.D.Schaller and V.I.Klimov, Physical

Review Letters 2004, 92:186601-1}. The Los Alamos scientists, however, did

not build a solar cell. The first report of MEG in a quantum dot solar cell

based on PbSe with external photocurrent quantum efficiency exceeding

100% is credited to Arthur Nozik who observed an external quantum

efficiency that peaked at 114 ± 1% in the best device measured. The

associated internal quantum efficiency (corrected for reflection and

absorption losses) was 130% {Ref: Arthur J Nozik et al, Science, 2011;

334(6062):1530-3}.

The optimism is widespread that solar cells based on quantum dots

theoretically could convert more than 65% of the sun’s energy into

electricity, approximately doubling the efficiency of present day solar cells

{Ref: www.solterrasolarcells.com/downloads/quantumdots.pptx}. By the turn of

this decade significant technological progress can be anticipated on solar

cells based on quantum dots. Among other reasons quantum dot-based

solar cells are being looked at as a next-generation photovoltaic is that they

can be deposited in roll-to-roll method using technologies similar to printing

paper, making such modules much cheaper to produce than silicon-based

PV. The world in the meanwhile still awaits a cheaper and efficient proven

technology for solar conversion; the market for solar electric energy has

grown by 20%–25% per year over the past 10 years.

vg kumar das (21 September 2012)

vgkdasorig@gmail.com