Science Snippets- 2. phenomena such as superposition and entanglement to perform operations on data) and

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1 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 nanometers (nm). One nanometer, i.e 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: 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 (MoS 2 ), 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: } who produced a transistor on the material and the other at MIT

2 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: /nl302015v} The MIT researchers claim the extremely thin and transparent 2D MoS 2 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 MoS 2 and graphene {Ref:

3 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: }. 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

4 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

5 tenable proposition from quantum molecular-dynamics simulations and classical continuum-elasticity modelling studies {Ref: P.Koskinen et al, Phys Rev B,2012, 85, }. 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 MoS 2 -coated multi-walled carbon nanotubes have also been successfully prepared by pyrolyzing (NH 4 ) 2 MoS 4 -coated multi-walled carbon nanotubes in an H 2 atmosphere at 900 {Ref: Xu Chun Song et al, Chinese Chemical Letters, 2004, 15: }. A single molecular layer of MoS 2 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 MoS 2 crystals excellent mechanical strength, thermal stability up to C 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 MoS 2 thin films to be created through micro-mechanical cleavage technique and through anisotropic 2D growth by chemical vapour deposition. This unique property of MoS 2, 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: /nl302015v}

6 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 MoS 2 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: } for atomic-layered MoS 2 synthesized directly on SiO 2 substrates, and a ball milling-cum-sonication technique reported by Yao et al {Ref: J. Mater. Chem., 2012, 22: } for fabricating nanosheets of graphene and MoS 2.

7 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: releases/2010/11/ 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.

8 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:

9 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 J, and, since c = ms 1, a typical photon has a very small amount of momentum. A photon of energy E g, where E g 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 E g 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: en.wikipedia.org/wiki/direct_and_indirect_band_gaps}.

10 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

11 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): }. 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 ev. The excitons are

12 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 dyesensitized solar cell is comprised of a ruthenium dye with π conjugated ligands having anchoring groups that bind to TiO 2, adsorbed at the surface of a high surface area nanoparticulate TiO 2 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-

13 hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C 61 -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: dictionarym-z.html} The bulk heterojunction concept was first introduced by Sariciftci and coworkers by mixing a semiconducting conjugated polymer as the donor material with fullerene C 60 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 (C 60 ) {Ref: P.Sullivan and T.S Jones, Organic Electronics, 2008, 9: }. 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

14 and C 60 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 C 60 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 cm 2. Over the wavelength range nm studied, a peak external quantum efficiency of 69% was observed at λ = 668 nm. Fig.5: Device structure of a pentacene/c 60 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.

15 % 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/m 2 on a sunny summer day, 900 watts/m 2 on a sunny autumn or spring day, 700 watts/m 2 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/m 2. 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 (Ga 0.5 In 0.5 P/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

16 reaches power conversion efficiency of 32.0% under I sun (defined as 1000 W/m 2 ). 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: ), 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-C 61 - 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 singlejunction 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: news 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 cm 2 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: 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

17 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-pphenylenevinylene)] (MEH-PPV) {Ref:. E.H.Sargent, Nature Photonics, 2009, 3: }. 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 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 (TiO 2 ). 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

18 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: }. 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

19 colloidal TiO 2 films were investigated as early as An enhancement in dye-sensitized solar cells overall conversion efficiency was recently observed for the photoanode consisting of nanosized TiO 2 single crystals with higher percentage of exposed (001) facets, increasing from 7.47%, 8.14% to 8.49% for the TiO 2 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: }. Titanium dioxide (TiO 2 ) 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 TiO 2 through hydrogenation which yields a black TiO 2 {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: }. 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: 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

20 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