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Nanotechnologies for Energy Gehan Amaratunga Engineering Dept., University of Cambridge, UK Public Lecture, Yunnan, China, April 2013.

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Presentation on theme: "Nanotechnologies for Energy Gehan Amaratunga Engineering Dept., University of Cambridge, UK Public Lecture, Yunnan, China, April 2013."— Presentation transcript:

1 Nanotechnologies for Energy Gehan Amaratunga Engineering Dept., University of Cambridge, UK Public Lecture, Yunnan, China, April 2013

2 Context of Lecture Electricity: fastest growing form of energy – Over 10 trillion kWh generated/year Over half this energy is currently ‘wasted’ – e.g. Incandescent lamps ~5% efficient All electronic products, mobiles to data centres, need a converter – Powered or charged Off the Mains Line Efficient power conversion is the key to: – Green Electricity Generation and Energy Savings - lower carbon emissions Intelligent power saves natural resources – Coal/oil/gas AND…steel, copper, plastics… Saved Energy is “Free & Clean” Energy harvesting can offset gird power for electronics - Requires development of new energy storage technologies

3 Small Matters ‘ No one could make a greater mistake than he who did nothing because he could only do a little’ – Edmund Burke Viral energy generation and saving – ‘Trillions of micro is mega’ In 2003 the number of Si transistors manufactured (10 18 ) exceeded the planets ant population by 100 X

4 Example: Drive to lower costs of solar cells has led to the development of several new technologies. Some of the directions being pursued are: Reduction in the use of materials – i.e. thinner solar cells Reduction in the electronic quality of materials – use of lower cost, lower purity materials Use of solution processable (e.g. printable) materials which enable high volume, low cost roll-to-roll processing Improved structural and optical design to allow the above developments to maintain sufficient efficiencies Additionally, several other “features” obtainable with non traditional materials have allowed development of other technologies, initially for consumer electronics The possibility of semi-transparent solar cells Mechanical flexibility/conformability (e.g. backpack integration, BIPV) Lower weight and packaging requirements Technologies Driven by Economics

5 Surface area Flexibility Heterostructures Optical Effects Printability Quantum Effects Why Nanomaterials?

6 Use of Nanomaterials so far…

7 TiO 2 : mesoporous for greater surface area to attach dye porosity > 50% nanoparticles ~20 nm other semiconductors TiO 2 easy to synthesize, abundant inexpensive Electrolyte : usually iodide/tri-iodide couple reduces dye after injection to TiO 2 new research in gel electrolyte Dye: usually ruthenium based Electrodes: SnO 2 thin film and Pt thin film Case 1:Dye Sensitized Solar Cells (DSSCs)

8 Nanocrystalline oxide photoanode mesoscopic TiO 2 film conductive SnO 2 (F) current collector Advantage of nanocrystalline Oxides electrodes: 1) translucent electrode - avoids light scattering losses 2) Small size is within minority carrier diffusion length, the valence band holes reach the surface before they recombine. Consider a one micron (10 -6 m ) layer of particles with a diameter of 20 nm and a porosity of 50% spread on a 1 cm 2 flat electrode Volume occupied by spheres is 0.5  10 -4 cm 3 Since A/V = 3/r A = 3V/r = 3  0.5  10 -4 / 10 -6 = 150 cm 2 The internal area is 150 times higher than the geometric area nanotechnology

9 y M. Gratzel

10 Random bulk heterojunctions allow much larger contact area between the two types of molecules, increasing charge collection efficiency and useful area. Ideal mixing conditions allow the average distance for exctions to travel before reaching a boundary to be in the order of 10nm Case 2: Ordered Charge Collection

11 M.D.McGeHee MRS Bulletin 30 (2005) Controlled dimensions (exciton diffusion distance) No dead ends in structure (min recombination) Ordered structure (high  )

12 ZnO NW - SWNT TF OPVs Substrate 100 mW/cm 2

13 Indium Tin Oxide (ITO) traditional transparent conductor. But indium becoming scarce/limited supply Crystalline nature leads to poor mechanical performance (flexibility) due to cracking Vacuum deposition A solution nanowires Silver nanowires or carbon nanotubes form an excellent flexibility tolerant alternative to ITO Case 3: Flexibility – Transparent Conductors Ag C

14 Case 4: Printability

15 refractive indexes N0 N1 N2 IR Xo Si coating air A method of enhancing the generation rate in the Si is to have an anti-reflection coating on the surface of the Si Fig. 8.3 - Anti-reflection coating This gives the condition that, when Therefore R( ) = 0 when. For Si at 0.6  m (near peak of solar spectrum) giving as the optimum condition for minimising the reflectivity. The thickness of the anti-reflective coating is mm Case 5: Optical Properties (Antireflection)

16 M.A. Green: ‘Solar Cells’

17 Graded Refractive Index

18 Antireflection Coatings Orthogonal photon absorption and carrier collection Reduced optical reflection Enhanced absorption (Light trapping) Enhanced carrier collection (carrier collection distance comparable to minority carrier diffusion length) Higher surface/interface recombination Fan et al. Nano Res 2 (2009) 829

19 Increasing the Optical Path Length Surface Texturing Rear Reflectors A thinner solar cell which retains the absorption of the thicker device may have a higher V oc In the case of ideal lambertian light trapping the path length is effectively increased by 4n 2 For silicon with a refractive index of 3.5, light trapping increases the path length by a factor of ~50

20 Plasmonics QD solar cells Case 6: Physics M. D. Brown et al., Nano letters, vol. 11, no. 2, pp. 438-45, Feb. 2011.

21 Carrier Transportation path Minimize carrier diffusion length Light Trapping Enhanced surface area a-Si Photovoltaics

22 Fabrication Patterned carbon nanotube arrays with 638 nm spacing. Patterned carbon nanotube arrays coated with 150 nm amorphous silicon layer. Carbon nanotube arrays coated with 250 nm a- Si and 80 nm ITO layer. Cross sectional view of the lower left sample. H. Zhou et al Adv. Mats. 2009

23 Ⅰ Ⅱ Characterisation H. Zhou et al., Adv. Mater 2010


25 In most cases: ↑ surface area = ↑ surface defects = ↑ recombination = ↓ Performance Exotic materials/structures not necessarily environmentally stable Some fancy results, but new problems – E.g. Transparent conductors for Nanowire solar cells Complex architectures tend to be difficult to manufacture and not cost effective. Fundamental Problems

26 As we realise the challenges nanomaterials pose, we are better situated to tackle those challenges Selected cases can be chosen in which these hurdles are not an issue, then the nanostructures may be used beneficially – AR coatings (not active material) – Transparent conductors (e.g. graphene) – Photoelectrochemical cells – Energy storage On the bright side…

27 Photoelectrochemical Cells

28 Energy Storage

29 EDLC Overview Electrochemical Capacitors Mechanism – Electrochemical Double layer The use of very high surface area materials combined with the small distance between the positive and negative charge in the Helmholtz layer (~1nm) results in an extremely large capacitance value. Source: Research Physics VI, Universitat Wurzburg

30 Carbon Nanomaterials in supercapacitors Activated carbon Carbon nanotubes Carbon nanohorns Carbon nano-onions Graphene Aerogels

31  Simple, low temperature solution deposition  Flexible and conducting  High surface area  Length and density can be easily and accurately controlled SWCNT Thin filmsAligned MWCNT Forests CNTs – Versatile material

32 Transferred CNT films

33 50 mV/s Cyclic Voltagram  Growth on Si substrates allows for use of optimum temperatures and very high growth rates  Shear transfer process allows the use of plastic substrates  High conductivity and alignment allows for use as charge collector However:  Two step process Transferred CNT EDLCs

34 Stretchable Capacitors

35 Graz, I.M.,Cotton, D.P.J, Lacour, S.P Stretchable organic thin film transistor Applied Physics Letters. What is Stretchable Electronics? - Conformable A composition of electronic materials and/or components formed across a substrate in a manner to allow the overall substrate to repeatedly deform >>5% without electrical failure.

36 Conformability The Morph concept Thin, Compliant, Transparent

37 Electrical Characteristics under Strain37 220um length CNTs 130um length CNTs M. Cole et al. Journal of Nanomaterials

38 Stretchable Supercapacitor Construction Elastomer Shear transferred CNTs Stretchable separator – e.g. lycra + electrolyte Shear transferred CNTs Elastomer Rigid Cu Current Collectors

39 Supercapacitor Performance 100%70% 50% 30%20% 10%

40 Cycling at 100% stretch 40

41 Circuit Embedded Packaging

42 FPC Integrated packaging

43 Flex tests 43 43

44 Flexible batteries

45 …PET-sheet …Carbon electrode …Mixture of MnO 2 & SWNTs (cathode) …solid electrolyte (no separator required) …Zn-foil ( anode ) …Al (or Cu) -connector Cathode contents MnO 2 : SWNTs Electrolyte contents NH 4 Cl : ZnCl : PEO : TiO2 Nanoparticle s

46 Hiralal et al ACS Nano, 2010

47 Li foil – CNH/CNT battery Li foil not used as secondary batter due to dendrite growth during charging – short circuit current and explosions. Can be overcome with a solid polymer electrolyte

48 Li foil – CNH/CNT battery CNT/CNH CNT only Specific capacity as function of specific current at 10mA/g, 100 mA/g and 200 mA/g for battery with CNTs (square ■ ) and with aligned CNTs combined with CNHs (circle ●).

49 Conclusions Nanotechnologies open up new horizons for energy generation and storage Initial applications and learning will be at ‘small scale’ specially for consumer electronics A major problem lies in surfaces – higher surface areas = high recombination. Interfaces need to be studied The technically simplest approaches with tangible gains are the ones most likely to be adopted in the short term.

50 Acknowledgements Cambridge Pritesh Hiralal, Haolan Wang, Emrah Unalan, Tim Butler, Hang Zhou, Sai Siva Reddy, Younjin Choi, Chih Tao Chien, Yuhao Sun, Wengpeng Deng, Caston Urayi Nokia Research Centre, Cambridge Markku Rouvala, Di Wei, Yingling Liu, Alan Colli, Piers Andrew, Tapani Ryhanen, Alan Colli Tokyo Institute of Technology Kenichi Suzuki, H. Matsumoto, Akihiko Tanioka FEI Ioannis Alexandrou Aixtron-Nanoinstruments Nailn Rupesinghe, Ken Teo Asylum Research Financial Support Nokia – Cambridge Strategic Research Alliance in Nanotechnology Dyson Research, Intel, Samsung

51 Thank you!

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