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“Nanotechnology, a Driver for
Developing Future Solar Cells,
Supercapacitors, and Sensors”
R.P.H. Chang
Northwestern University
R.P.H. Chang-Northwestern
2
Outline of the talk
Introduction
3rd Generation hybrid solar cells
Integrating supercapacitors for
energy storage
Plasmonic sensors in the IR
Conclusion
Introduction
Why is nanotechnology urgently needed to solve global
problems?
Three examples of current research in my lab: Solar-
electricity conversion; integrating supercapacitors for
energy storage; and sensors based IR plasmonics
Technology alone will not solve our global problems
quickly. The need for world-wide research collaboration
and education is of paramount importance!
Silicon
Semiconductor
Compound
Semiconductor
New
Solar Cell
Crystalline
Amorphous
single
poly (bulk)
poly (thin film)
II-VI (CdS, CdTe)
Organic Solar Cell
Dye-sensitized Solar Cell
Perovskite -based Solar Cell; 2012
III-V (GaAs, InP)
I-III-VI (CuInSe2)
Types of solars
Solar cell--basic device operation
Issues: 1. sensitizer absorption spectrum and efficiency 2. diffusion length of charges in the material (defects & scattering) 3. interfacial compatibility (band alignment and chemistry) 4. efficiency of electron and hole transport layers 5. workfunction and interface chemistry at the electrode contacts 6. photon confinement and optical index matching 7. others
Dye TiO2
e-
FT
O
FT
O
Pt
3I-
I-
I3-/I-
D+/D
∆V e-
e-
hν
TiO2
Photo
electrode
I-/I3- redox
Mediator
Dye-sensitized (Grätzel-type) Solar Cell
S + light → S*
S* + TiO2 → S- + e- (TiO2)
e- (TiO2) + C.E → TiO2 + e- (C.E) + electrical energy
S+ + 3/2 I- → S + ½ I3-
½I3- + e- (C.E) → 3/2 I- + C.E
Dye-sensitized solar cells mimic the photosynthetic process by using a sensitizer dye
to harvest light energy to generate electrical power.
- 4.2
- 3.64
- 6.01
Liquid Electrolyte
TCO
TiO2
hopping
Dye
Organic HTM Au
h+
e– e–
3.2eV
전해질
TiO2 Nanoparticle
hv
Co
nd
uc
tive
Gla
ss
dye
light Au
Organic HTM
NN
H3CO
OCH3
H3CO
H3CO
NN
OCH3
H3CO
OCH3
OCH3
• Contact to TiO2 Electrode
• Low Ionic Conductivity
• Penetration Problems • Not enough to absorb light
( due to thin layer of TiO2)
• Limited carrier recombination
~ 5% of Conversion Efficiency
Bach et al., Nature, 395,583 (1998)
2,2’,7,7’-tetrakis(N,N-di-p-methoxy-
Phenyl amine)9,9’-spirobifluorene
Solid State DSSCs with organic electrolyte
All-solid-state dye-sensitized solar cells with high efficiency
Nature 485,486–489 (24 May 2012)
SnIx based Perovskite – as a Hole Transporting Material
Figure. Crystal structure and optical and
electrical transport properties of CsSnI3.
a, Distorted three-dimensional perovskite structure
of CsSnI3 at RT. Red polyhedron, [SnI3]−: yellow,
Cs. b, Sharp absorption edge at 1.3 eV of CsSnI3
is shown. c. A typical ingot of CsSnI3 grown in a
Bridgman furnace. d. Temperature dependence of
electrical conductivity (■) and Seebeck coefficient
(●).
CsSnI3 , a p-type direct bandgap
semiconductor with energy gap of
1.3 eV was used.
High hole mobility of μh=585 cm2 V-1 s-1
at RT
Figure. Cross-sectional electron microscopy image of
CsSnI3 /TiO2 cell (thickness ~ 10 μm) on Si. a, Cross-
sectional SEM image. b, Elemental mapping by energy
dispersive spectroscopy for the rectangular area indicated by a
blue square in a, showing homogeneous infiltration of CsSnI3
into nanoporous TiO2. c, Back scattering cross-sectional SEM
image, showing no discernible contrast difference. d, Cross-
sectional HRTEM image. e, Left panel: experimental electron
diffraction pattern indicating TiO2 (ring patterns) and CsSnI3
(spots indicated by white arrows between ring patterns); right
panel: theoretical calculation of anatase TiO2.
Figure . Energy levels of the components of
CsSnI3 solid-state solar cell.
The valence band maximum (orange color)
and the conduction band minimum (blue color)
of TiO2 and CsSnI3 are represented in eV,
along with the energy difference between the
edges. The ground (orange) and excited states
(blue) of N719 dye is also shown. The energy
scale is referenced to vacuum level.
CsSnI3 and TiO2
overlapped
unclear lattice
fringes
TiO2
Code
Impedance analysis JV characteristics
R1
(Ω)
C1
( 10-5 F)
R2
(Ω)
C2
( 10-3 F)
Rtotal
(Ω)
VOC
(V)
Jsc
(mA/cm2)
FF
(%)
EFF
(%)
(a) CsSnI 17.9 1.54 101.9 1.6 122.0 0.638 8.82 66.1 3.72
(b) CsSnI2.95F0.05 5.72 1.04 55.7 4.0 64.0 0.649 12.2 70.7 5.62
(c) CsSnI2.95F0.05 + 0.02g SnF2 5.07 3.32 42.3 4.4 49.6 0.666 15.7 65.2 6.81
(d) CsSnI2.95F0.05 + 0.05g SnF2 5.01 1.64 31.9 2.2 37.3 0.688 16.3 69.4 7.78
(e) CsSnI2.95F0.05 + 0.10g SnF2 6.03 0.99 59.2 3.9 67.8 0.654 13.6 61.4 5.46
Figure. a, The incident photon to current efficiency
(IPCE) spectrum and b, Optical absorbance spectra
of the devices
-55
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
Z"
(oh
m)
140120100806040200
Z' (ohm)
(a) CsSnI (b) CsSnIF
(c) CsSnIF + 0.02g SnF2
(d) CsSnIF + 0.05g SnF2
(e) CsSnIF + 0.1g SnF2(a)
(b)(c)(d)
(e)
18
16
14
12
10
8
6
4
2
0
Ph
oto
cu
rre
nt
de
ns
ity
(m
A/c
m2)
0.700.600.500.400.300.200.100.00
Photovoltage (V)
(a) CsSnI (b) CsSnIF
(c) CsSnIF + 0.02g SnF2
(d) CsSnIF + 0.05g SnF2
(e) CsSnIF + 0.10g SnF2
Progress in solid-state DSSCs
1998 2000 2002 2004 2006 2008 2010 2012 2014 20160
2
4
6
8
10
12
14
16
18
20
Orgainic absorber
Perovskite absorber
Po
wer
Co
nvers
ion
Eff
icie
ncy (
%)
Year
N719 dye / CsSnI3,
CH3NH3PbI3-xClx / spiro-
OMeTAD
CH3NH3PbI3 / PTAA
CH3NH3PbI3 / spiro-OMeTAD
Y-TiO2 / CH3NH3PbI3-xClx /
spiro-OMeTAD
CH3NH3PbI3 / Iodide Liquid electrolyte
CH3NH3PbI3 / Iodide Liquid electrolyte
Indoline dye / spiro-
OMeTAD with Ag
Z907 / spiro-
OMeTAD
N719 / CuSCN
N719 / CuI
(Source: Wikipedia)
Perovskite material
N719 dye
Si
Ge
The Shockley-Queisser limit Theoretical Solar Cell Efficiency
• Shockley–Queisser limit 33% at
1.34eV bandgap,
• 1.1eV Si can achieve 29%
• Eg ≈ 1.3 ~ 1.5 eV for optimum
efficiency
Bandgap and Theoretical Solar Cell Efficiency
Generic formula: ABX3
A B X
Organic–inorganic hybrid perovskite compounds based on metal halides adopt the
ABX3 perovskite structure.
large cation
(Cs, CH3NH3+)
metal cation
(Pb, Sn)
halide
(Cl-,Br-,I- )
Perovskite Crystal Structure
Organic/inorganic perovskites are hybrid layered materials typically with an
ABX3 structure, with A being a large cation, B smaller metal cation and X an anion
from the halide series. They form an octahedral structure of BX6, which forms a three
dimensional structure connected at the corners
Organohalide lead perovskites based solar cell
FTO
Blocking TiO2 layer
Perovskite / TiO2 + HTM
HTM (Spiro-OMeTAD)
Metal contact
- 4.3
FT
O G
lass
TiO
2 fil
m
- 4.0
Pe
rovs
kit
e
-3.93
- 5.43 HTM
- 5.22
e–
Perovskite Solar Cell Architecture
Burschka, J.; Pellet, N.; Moon, S.-J.; Gao, P.; Nazeeruddin, M. K.; Gratzel, Nature 2013, 499 (7458), 316-319.
General methods to prepare perovskite active layers
One-Step Deposition Sequential Deposition
(Spin coating)
Vapor Deposition
Inorganic film
Organic vapor
Zhou, H. et al. Science 345, 542-546, doi:10.1126/science.1254050 (2014).
Interface engineering of highly efficient Lead-based perovskite solar cells
Air stability issue
Toxic Pb
Perovskite-based hybrid solar cells
Our R & D Goals for Perovskite-solar cells focus on:
Use of benign materials: choice SnIx based perovskite
Need ambient stability during processing
Large area and low temperature processing conditions
Low cost materials
Device reliability and high efficiency
To this end we are also studying Cs2SnI6 as a hole transporter or sensitizer This is an air stable compound!!
Cs2SnI6 belongs to the class of perovskites
(a defect variant of the AMX3 structure type)
In Cs2SnI6 half of the octahedral Sn atoms are missing
creating discrete [SnI6]2- octahedra. The compound is
therefore a molecular salt and contains Sn4+ rather than
Sn2+ in CsSnI3. This accounts of the air stability and high
solubility of the material.
Figure . Structure of Cs2SnI6
Material properties of Cs2SnI6
10
8
6
4
2
0
A (α
S-1
)
2.82.42.01.61.20.8 Energy (eV)
100
80
60
40
20
Refle
cta
nc
e (%
)
Eg = 1.3eV
4.3
FT
O G
lass
TiO
2 fil
m
4.2
Cs
2S
nI 6
4.14
5.44
CH
3N
H3P
bI 3
3.93
5.43
Cs
Sn
I 3
3.62
4.92
(a) (b)
Figure. (a) Optical absorption (black) and Diffuse reflectance spectra (blue) (b) Energy diagram of Cs2SnI6
TiO2 Film
FTO glass
at 500oC, 30 min
Post treatment
TiO2 Film
FTO glass
Spin-coating Dye - 2000rpm for 60 sec
Ru
HO
O
OH
O
NN C
CS
S
Ru
HO
O
OH
O
NN C
CS
S
FTO glass
Dye/ TiO2 /Cs2SnI6
TiO2 / Dye TiO2 / Dye/
Cs2SnI6
Post heating HTM coating - Heating at 130oC for 10min
Experiment
Figure. (a) SEM image and XPS spectra of Cs2SnI6 coated TiO2 film; (b) Ti 2p;
(c) O 1s; (d) Cs 3d; (e) Sn 3d; (f) I 3d
TiO2 Sphere / Cs2SnI6
(~ 4.42μm)
Cs2SnI6 layer (~ 0.7μm)
20
18
16
14
12
10
8
6
4
2
0
Ph
otc
cu
rre
nt
de
ns
ity
(m
A/c
m2)
0.90.80.70.60.50.40.30.20.10.0
Photovoltage (V)
N719 YD2-oC8 YDD6 Triple Mixture Triple Mixture w 3D PhC
Cell
JV characteristics
VOC (V) Jsc (mA/cm2) FF (%) EFF (%)
Single dye
N719 0.836 15.9 70.4 9.35
YD2-oC8 0.769 14.3 67.5 7.45
YDD6 0.687 6.42 68.7 3.03
Triple Dye
YDD6/YD2/N719 0.818 18.6 72.8 11.2
(w 3D PhC) 0.816 20.3 72.7 12.1
Mixed Triple Dye from CY Yeh 90
80
70
60
50
40
30
20
10
0
IPC
E (
%)
850800750700650600550500450400350
Wavelength (nm)
N719 YD2-oC8 YDD6 Triple Mixture
Figure. Crystal structure, XRD pattern, optical absorption
and photoluminescence spectra, conductivity and Seebeck
coefficient of CH3NH3SnI3 perovskite. a, Perovskite crystal
structure of the CH3NH3SnI3-xBrx absorber materials. b,
Experimental (red) and simulated (black) X-ray diffraction
pattern for CH3NH3SnI3. c,d, Optical absorption and
photoluminescence spectra (c) and conductivity and Seebeck
coefficient (d) as a function of temperature for a
sample of CH3NH3SnI3 prepared using the solution method
SnIx based Solar Cell – as a Sensitizer
Nature Photonics 8, 489–494 (2014)
doi:10.1038/nphoton.2014.82
Conclusion
We have identified Cs2SnI6 as an air stable and Pb free
compound for perovskite based solar cell.
We have shown by electro-spray, large area and
low temperature processing conditions can be achieved.
We are working towards low cost, high efficiency, and
reliable devices.
R.P.H. Chang-Northwestern 26
Acknowledgments
Byunghong Lee (GS)
Feng Hao (PD)
Dana Cao (GS)
Nanjia Zhou (GS)
Constantinos C. Stoumpos (R Associate)
Mercouri G. Kanatzidis (Professor of Chemistry)
Funding from DOE and NSF
Hierarchical Design for
Fabricating Cost-Effective High
Performance Supercapacitors
Nam Dong Kim, D. Bruce Buchholz, Gilberto Casillas,
Miguel José-Yacaman, R.P.H. Chang
Introduction and rationale
The need for efficient renewable energy sources and energy storage systems demands the design and fabrication of ever higher charge-density nanoparticles (HCDN). This implies: design a nanostructure with a large surface area for a
given total volume or mass.
equally important is the consideration of surface reactivity and the availability of nanoscale channels for atomic/charged species to access internal/external surface sites of the nanostructure.
Work reported in Advanced Functional Materials DOI:10.1002/adfm.201304130
Some challenges
The big challenge is in the nanostructure architecture where all of these parameters need to be optimized simultaneously.
For the case of a supercapacitor, the nanostructure material is used as the framework to store and transport charges between two parallel electrodes.
In addition, electrochemical storage can take place at the interfaces, due to the redox reactions which contribute to the pseudo-capacitance of the device
Our approach
Our hierarchical design and assembly of the HCDN is based on vertically integrating optimal materials-performance at each length scale, ranging from the atomic to micrometers in length:
At the atomic level, an abundance of active sites for chemical activity and charge accumulation are needed.
At the sub-nano level, abundant channels for efficient mass/charge transport of electrolyte species are needed within the nano-structure.
At the nano-structure level, optimum packing is needed for charges and good electrical contact among the HCDN particles for rapid charge transport.
Finally, at the micron level, the HCDN must be assembled and mechanically compacted between the two electrodes to deliver the maximum power density. Thus the elastic properties of the HCDN were part of our design consideration..
Electron energy loss spectroscopy used to map the
locations of B, N, and C in the individual particles
After annealing to remove the amorphous portion of
the nanoparticle composite
(a) dark-field image; (b) the carbon skeleton consists of nanohorns &
nano graphene sheets; ( c ) diffraction patterns: left, as grown; right, after
annealing
Compression experiment of a single BN-1H
nanoparticle: a) Zero loading on the particle. (b) 530, (c) 924, and (d) 1278 nN loading.
0 2000 4000 6000 8000 10000
0
20
40
60
80
100C
ap
acita
nce r
ete
ntio
n (
%)
Number of cycles
Capacitance retention
Conclusion
We have reported a cost-effective approach to fabricate high performance supercapacitors
Taken together our integrated approach provided
us with a power density of nearly 4.58 kW cm-3
The described fabrication process and supercapacitor can be easily integrated with the solar cells described above
Plasmonics
• Coupling between light wave and plasmon
• Plasmon: A quantum of collective motion of electrons
• Localized surface plasmon resonances(LSPR)
• High Field
• d << λ
(small modal volume)
40
|E|/|E0|
E
Spectrum of LSPR
Extinction spectra of 50 nm silver nanospheres in air, water, and
silica. As the refractive index of the medium increases, the
nanoparticle spectrum shifts to longer wavelengths.
•
41
Applications of Plasmonics
Optoelectronics, Lithography
Surface Enhanced Raman/Vibration Spectroscopy
Fang, Science (2005)
43
Motivation
Extend the spectrum region (to Infrared)
Vibrational absorption generally occurring
at infrared
As does telecommunication wavelengths
Indium-tin-oxide (ITO)
nanorod
Boltteseva and Atwater (2011)
Basic properties of ITO
EF
Eg > 3.2 eV
Degenerated semiconductor (1019 to 1021 cm-3)
Wide band gap
No phonon modes < 30 μm
Tunable carrier concentration
Defect Chemistry
Low ε”/ε’
45
Fabrication of the array
S.Q. Li ACS Photonics (2014)
46
S.Q. Li ACS Photonics (2014)
ITO film Deposition
Spin-coating PMMA Resist
E-Beam Lithography
1.Deposition of gold film
2. Lift-off of PMMA resist
ITO Nanorod growth
YSZ (100)
2 um 6 um 8 um Height Control
300 nm
1 μm
Lattice Spacing Variation Morphology and Composition Mapping
S. Q. Li et al, ACS Nano (2011)
ITO Nanorod Arrays
47
Longitudinal LSPR of ITO Nanorod
k
Height: 500 nm
Cross-section:150 nm x 150 nm
ωp=2.1 eV; γ = 0.062 eV
Extinction Cross-section
Polarizability of dipole
48
E
Wavevector:
Resonance Frequency:
Resonance Strength:
Damping factor:
σe
Advantage of Nanorod
Arrays
Tunable Longitudinal Modes
Far field coupling strong
Ordered
h
w: 185 nm
w
Shifted by 1 for clarity
Simulation
Experiment
49
Plasmonic-Photonic
Coupling
Fano resonance
– Interaction between broad
(strong) and sharp (weak)
resonance
– Photonic coupling
• Coupling between plasmon
and photonic (grating) mode
• Zou and Schatz(2004)
Sharp Broad
weak strong
Photonic Plasmonic
51
Photonic Coupling in an
ITO nanorod array
Principle and Calculation
a
b
Extinction Cross-section
k
52
Using ITO array for femto second light
modulation with pump-probe.
For optical communication and remote sensing
Acknowledgement
• Mike Li
• Peijun Guo
• Dr. Bruce Buchholz
• Dr. Daniel Tice
Collaborators • Dr. Leo E. Ocola (ANL)
• Professor Tamar Seideman
• Professor Kazuaki Sakoda (NIMS)
• Professor Emily Weiss
• Dr. Wei Zhou (Odom group)
• Dr. Lingxiao Zhang (Seideman group)
• Yi Hua (Odom group)
• Xing Liao (Mirkin group)
Professor John B. Ketterson
Professor Teri W. Odom
Professor George Schatz
56