“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!

R.P.H. Chang

Our global challenges

Top 10 global issues facing the 21st century

4

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

Typical dye materials for DSSC

Black dye

N719 dye

N3 dye

N719 dye Black dye N3 dye

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

SnIx as Sensitizer

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

Large area solar cell

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..

Detail processing steps for fabricating SC

Morphology of the as arc synthesized B-N

co-doped HCDN

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.

The isotherms are Type II IUPAC classification

without adsorption hysteresis

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

Murray and Barnes (2007) 42

Nanotechnology since

middle-age

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

Dielectric Sensing with

LSPR of ITO Nanorods

ACS Nano (2011)

50

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

Plasmonic Resonance with

High Q

Q = 200

Freestanding hole array: Q ~ 197, Yanik et al PNAS(2011)

53

4321

log(E2)

Near Field Mapping

Freestanding dipole Monopole on gold

54

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

Conclusion

We have shown the importance of

nanotechnology to provide solutions to

global problems

Examples were given in solar energy

generation, capacitors to store energy, and

the use of IR plasmonic effects for remote

sensing and high speed optical

communication.