science snippets- 2 · claytronics (a futuristic simulation system concerned with programmable...
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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)