nanotechnologies for energy gehan amaratunga engineering dept., university of cambridge, uk public...
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Nanotechnologies for Energy
Gehan AmaratungaEngineering Dept., University of Cambridge, UK
Public Lecture, Yunnan, China, April 2013
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
Small Matters
‘No one could make a greater mistakethan he who did nothing because hecould only do a little’ – Edmund Burke
Viral energy generation and saving – ‘Trillions of micro is mega’
In 2003 the number of Si transistors manufactured (1018) exceeded the planets ant population by 100 X
http://www-g.eng
.cam.ac.uk/cnt/
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
Surface areaFlexibility
HeterostructuresOptical Effects
PrintabilityQuantum Effects
Why Nanomaterials?
Use of Nanomaterials so far…
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.cam.ac.uk/cnt/
TiO2 : mesoporous for greater surface area to attach dye
• porosity > 50%• nanoparticles ~20 nm• other semiconductors• TiO2 easy to synthesize, abundant
inexpensive Electrolyte : usually iodide/tri-iodide couple
• reduces dye after injection to TiO2
• new research in gel electrolyte Dye: usually ruthenium based Electrodes: SnO2 thin film and Pt thin film
Case 1:Dye Sensitized Solar Cells (DSSCs)
Nanocrystalline oxide photoanode
mesoscopic TiO2 film
conductive SnO2(F)
current collector
Advantage of nanocrystallineOxides electrodes: 1) translucent electrode - avoids light scattering losses2) Small size is within minority carrier diffusion length, the valence band holes reach the surface before they recombine.
Consider a one micron (10 -6m) layer of particles with a diameter of 20 nm and a porosity of 50% spread on a 1 cm2 flat electrode
Volume occupied by spheres is 0.5 10-4 cm3
Since A/V = 3/r
A = 3V/r = 3 0.5 10-4 / 10-6 = 150 cm2
The internal area is 150 times higher than the geometric area
nanotechnology
y
• M. Gratzel
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
M.D.McGeHee MRS Bulletin 30 (2005)
Controlled dimensions (exciton diffusion distance) No dead ends in structure (min recombination) Ordered structure (high )
ZnO NW - SWNT TF OPVs
• Substrate
-0.2 0.0 0.2 0.4 0.6-3
-2
-1
0
1
2 SWNT-ZnO light SWNT-ZnO dark
Cur
rent
Den
sity
(m
A/c
m2 )
Voltage (V)
Voc = 460mVIsc = -2.31FF ~ 0.6
Eff. ~ 0.64
• 100 mW/cm2
• 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
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refractiveindexes
N0
N1
N2
I R
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
01
21
212
10
101
212
22
1
212
22
1 2,,,
cos21
cos2 xn
nn
nnr
nn
nnr
rrrr
rrrrR
This gives the condition that, when 2
202
1
202
1min01 4
nnn
nnnRxn
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
201 nnn
911 n
9140
600
x
832 n
m
Case 5: Optical Properties (Antireflection)
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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
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Increasing the Optical Path LengthSurface Texturing
Rear Reflectors
A thinner solar cell which retains the absorption of the thicker device may have a higher Voc
In the case of ideal lambertian light trapping the path length is effectively increased by 4n2
For silicon with a refractive index of 3.5, light trapping increases the path length by a factor of ~50
• Plasmonics
• QD solar cells
Case 6: Physics
M. D. Brown et al., Nano letters, vol. 11, no. 2, pp. 438-45, Feb. 2011.
http://www.grc.nasa.gov/WWW/RT/RT2001/5000/5410bailey1.html
• Carrier Transportation path• Minimize carrier diffusion length• Light Trapping
Enhanced surface area a-Si Photovoltaics
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
• Ⅰ • Ⅱ
Characterisation
H. Zhou et al., Adv. Mater 2010
• 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
• 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…
Energy Storage
EDLC OverviewElectrochemical 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.
Very large A and very low d =VERY LARGE C!d
AC
Source: Research Physics VI, Universitat Wurzburg
Carbon Nanomaterials in supercapacitors
• Activated carbon• Carbon nanotubes• Carbon nanohorns• Carbon nano-onions• Graphene• Aerogels
Simple, low temperature solution deposition
Flexible and conducting
High surface area Length and density can
be easily and accurately controlled
SWCNT Thin films Aligned MWCNT Forests
CNTs – Versatile material
Transferred CNT films
Growth time
Active mass (1)
Capacitance (mF/cm2)
Capacitance (F/g) ESR(Ω)
1 min 0.6 mg 3.85 mF/cm2 25.6F/g 1043
5 min 1.2 mg 12.4 mF/cm2 44.0 F/g 92
10 min 2.3 mg 26 mF/cm2 45.2 F/g 43.8
• 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
Stretchable Capacitors
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.
Conformability The Morph concept Thin, Compliant, Transparent
Electrical Characteristics under Strain
37
220um length CNTs130um length CNTs
M. Cole et al. Journal of Nanomaterials
Stretchable Supercapacitor Construction
Elastomer
Shear transferred CNTs
Stretchable separator – e.g. lycra+ electrolyte
Shear transferred CNTs
ElastomerRigid Cu Current Collectors
Supercapacitor Performance
100%70%50%
30%20%10%
0% 5% 10% 15% 20% 30% 50% 70% 80% 90% 100%
40
50
60
70
80
90
100
Capacitance (m
F)
ES
R (
Strain
5
10
15
20
25
30
35
40
45
0 5 10 15 20-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4 No stretch 5% 10% 15% 20% 30% 70% 100%
Vol
tage
(V)
Time (s)
Cycling at 100% stretch
40
0 200 400 600 800 1000 1200 1400 1600 18000
2
4
6
8
10
12
14
Cap
acita
nce
(mF
)
Cycle Number
100% Stretch
0 200 400 600 800 1000 1200 1400 1600 180020
30
40
50
60
70
80
90
100
110
100% Stretch
ES
R ()
Cycle Number
Circuit Embedded Packaging
FPC Integrated packaging
Flex tests
• 43
VIDEO0043.3gp
-2 0 2
-1.5
-1.0
-0.5
0.0
0.5
1.0
Nor
mal
ised
cur
rent
without bending 2.5 cm curvature diameter 1.5 cm curvature diameter 0.5 cm curvature diameter straight after 50 times bending
Voltage (V)
43
Flexible batteries
RESULT - FLEXIBLE ZN-C BATTERY
…PET-sheet
…Carbon electrode…Mixture of MnO2 & SWNTs (cathode)…solid electrolyte (no separator required)…Zn-foil (anode)
…Al (or Cu) -connectorCathode contents MnO2 : SWNTsElectrolyte contents NH4Cl : ZnCl : PEO : TiO2 Nanoparticle s
Hiralal et al ACS Nano, 2010
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
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 ).
http://www-g.eng
.cam.ac.uk/cnt/
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.
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.cam.ac.uk/cnt/
AcknowledgementsCambridgePritesh Hiralal, Haolan Wang, Emrah Unalan, Tim Butler, Hang Zhou, Sai Siva Reddy,
Younjin Choi, Chih Tao Chien, Yuhao Sun, Wengpeng Deng, Caston UrayiNokia Research Centre, CambridgeMarkku Rouvala, Di Wei, Yingling Liu, Alan Colli, Piers Andrew, Tapani Ryhanen, Alan ColliTokyo Institute of TechnologyKenichi Suzuki, H. Matsumoto, Akihiko TaniokaFEIIoannis AlexandrouAixtron-NanoinstrumentsNailn Rupesinghe, Ken TeoAsylum Research
Financial SupportNokia – Cambridge Strategic Research Alliance in NanotechnologyDyson Research, Intel, Samsung
Thank you!