Nanotechnologies for Energy
Gehan Amaratunga
Engineering 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 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 (1018) exceeded the
planets ant population by 100 X
Technologies Driven by Economics
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
Why Nanomaterials?
Surface area
Flexibility
Heterostructures
Optical Effects
Printability
Quantum Effects
Use of Nanomaterials so far…
Case 1:Dye Sensitized Solar Cells (DSSCs)
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
Nanocrystalline oxide photoanode
nanotechnology
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
conductive SnO2(F)
current collector mesoscopic TiO2 film
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.
y
•M. Gratzel
Case 2: Ordered Charge Collection
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
Controlled dimensions
(exciton diffusion distance)
No dead ends in structure
(min recombination)
Ordered structure
(high )
M.D.McGeHee MRS Bulletin 30 (2005)
ZnO NW - SWNT TF OPVs
2
Current Density (mA/cm )
•100 mW/cm2
2
Voc = 460mV
Isc = -2.31
FF ~ 0.6
1
SWNT-ZnO light
SWNT-ZnO dark
Eff. ~ 0.64
0
-1
-2
-3
-0.2
0.0
0.2
Voltage (V)
•Substrate
0.4
0.6
Case 3: Flexibility – Transparent Conductors
Ag
•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
C
Case 4: Printability
Case 5: Optical Properties (Antireflection)
A method of enhancing the generation rate in the Si is to have an anti-reflection coating on the
surface of the Si
I
refractive
indexes
R
N0
air
Xo
N1
coating
Si
N2
Fig. 8.3 - Anti-reflection coating
r1  r2  2r1r2 cos 
2
R
2
1  r1 r2  2r1r2 cos 
2
2
, r1 
This gives the condition that, when n1 x0   4
n0  n1
2n1 x0
n  n2
, r2  1
, 
n0  n1
n1  n2

R min
 n1 2  n0 n2 

  2

 n1  n0 n2 
2
Therefore R() = 0 when n1  n0 n.2 For Si at 0.6 m (near peak of solar spectrum) n2  3  8 giving
n1  1  9 as the optimum condition for minimising the reflectivity. The thickness of the anti-reflective
06
coating is
m
x0 
0  4  1 9
M.A. Green: ‘Solar Cells’
Graded Refractive Index
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
Increasing the Optical Path Length
Surface Texturing
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 4n 2
For silicon with a refractive index of 3.5, light trapping increases the path length by a factor of ~50
Rear Reflectors
Case 6: Physics
• Plasmonics
M. D. Brown et al., Nano letters, vol. 11, no. 2, pp. 438-45, Feb. 2011.
• QD solar cells
http://www.grc.nasa.gov/WWW/RT/RT2001/5000/5410bailey1.html
Enhanced surface area a-Si
Photovoltaics
•Carrier Transportation path
•Minimize carrier diffusion length
•Light Trapping
H. Zhou et al Adv. Mats. 2009
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 aSi and 80 nm
ITO layer.
Cross
sectional
view of the lower
left sample.
Characterisation
•Ⅰ
H. Zhou et al., Adv. Mater 2010
•Ⅱ
Fundamental Problems
• 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.
On the bright side…
• 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
Energy Storage
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
A
C
d
Carbon Nanomaterials in
supercapacitors
•
•
•
•
•
•
Activated carbon
Carbon nanotubes
Carbon nanohorns
Carbon nano-onions
Graphene
Aerogels
CNTs – Versatile material
SWCNT Thin films
Simple, low temperature
solution deposition
 Flexible and conducting
Aligned MWCNT Forests
 High surface area
 Length and density can be
easily and accurately
controlled
Transferred CNT films
Transferred CNT EDLCs
 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
•50 mV/s Cyclic Voltagram
Growth
time
•However:
 Two step process
Active Capacitance
mass (1)
(mF/cm2)
Capacitanc
e (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
Stretchable Capacitors
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.
Graz, I.M. ,Cotton, D.P.J , Lacour, S.P Stretchable organic thin film transistor Applied Physics Letters.
Conformability
The Morph concept
Thin, Compliant, Transparent
Electrical Characteristics under
Strain
220um length CNTs
130um length CNTs
M. Cole et al. Journal
of Nanomaterials
3
7
Stretchable Supercapacitor Construction
Elastomer
Shear transferred CNTs
Stretchable separator – e.g. lycra
+ electrolyte
Shear transferred CNTs
Elastomer
Rigid Cu Current Collectors
Supercapacitor Performance
10%
20%
30%
100
50%
0.3
Voltage(V)
0.2
0.1
0.0
40
80
35
30
70
25
60
20
15
50
10
40
-0.1
5
-0.2
0% 5% 10% 15% 20% 30% 50% 70% 80% 90% 100%
-0.3
Strain
-0.4
-0.5
0
5
10
Time (s)
15
20
Capacitance (mF)
No stretch
5%
10%
15%
20%
30%
70%
100%
0.4
90
100%
ESR (
70%
45
Cycling at 100% stretch
110
100% Stretch
14
100
90
10
80
ESR ()
Capacitance (mF)
12
8
6
4
50
30
0
200 400 600 800 1000 1200 1400 1600 1800
Cycle Number
4
0
60
40
2
0
70
20
100% Stretch
0
200 400 600 800 1000 1200 1400 1600 1800
Cycle Number
Circuit Embedded Packaging
FPC Integrated packaging
Flex tests
Normalised current
1.0
0.5
0.0
-0.5
without bending
2.5 cm curvature diameter
1.5 cm curvature diameter
0.5 cm curvature diameter
straight after 50 times bending
-1.0
-1.5
-2
4
3
0
Voltage (V)
2
•43
Flexible batteries
…Carbon electrode
…Mixture of MnO2 & SWNTs
(cathode)
…solid electrolyte (no separator
required)
…Zn-foil (anode)
…PET-sheet
…Al (or Cu) -connector
Cathode contents
MnO2 : SWNTs
Electrolyte 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
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 ●).
CNT/CNH
CNT only
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.
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
Thank you!