Synthesis of ZnO Nano/Microspheres and Development of ... · Synthesis of ZnO Nano/Microspheres and...

Synthesis of ZnO Nano/Microspheres

and Development of Organic Solar

Cells

Gon Namkoong

ARC-Old Dominion University

Outline

• Synthesis of ZnO nano/microspheres Control of ZnO morphologies

Effect of structure direct agents on ZnO morphologies

Control of uniformity, distribution, and size of ZnO spheres

• Organic solar cells Recombination process of organic solar cells

Degradation mechanisms of organic solar cells

Simulation of 3D organic morphologies

• Power of Words

Outline







• Power of His Words

ZnO morphologies

Materialtoday, Vol 7,

p 26 (2004)

Unique optical,

electrical, and

structural properties

Many applications

Missing morphology

ZnO sphere

ZnO structures

a=3.247 Å

b=5.207 Å Nuclear Instruments and Methods in Physics

Research Section B. Vol. 281, pp 77 (2012)

Noncentrosymetric ZnO

structure

6.1a

b

ZnO structures (Cont.)

ZnO has a

noncentrosymmetric

crystal structure

Scientific reports,

Vol 2, pp 587


ZnO has a

noncentrosymmetric

crystal structure

Strong spontaneous

polarization

Scientific reports,

Vol 2, pp 587


ZnO has a

noncentrosymmetric

crystal structure

Strong spontaneous

polarization

Surface charge on the

(0001) plane

Scientific reports,

Vol 2, pp 587

Hydrothermal synthesis of ZnO

Zinc acetate ))(( OHCOOCHZn 223 2

Ammonia hydroxide

OHNH4

Autoclave reactor


Scientific reports,

Vol 2, pp 587

2

43)(NHZn2+ 2+

Zn cation Zn anion

2

43)(NHZn2

43)(OHZn


Preferred growth of ZnO

Nature nanotechnology, Vol. 6, pp 103 (2011)

Novel approach for ZnO spheres 1. Control of cation species adjustment of pH

Namkoong et al,

Thin Solid Films, Vol. 534,

pp 76 (2013)

ZnO polar surface preferential growth

ZnO with different pH values Zinc acetate

Novel approach for ZnO spheres

Structure directing

agents (SDA)– Urea

and ethanol

SDA will passivate

ZnO that suppress

further nucleation

Suppression of (0001)

growth

2. Passivate the polar surface using SDA

Urea

Control of Zn(NH3)42+ nucleation species

Spherical shape

Effect of SDA on ZnO morphologies

Urea

ethanol

Temporal evolution of ZnO




SDA

Control of size and distribution of ZnO

spheres Urea Ethanol

Urea (1):Ethanol (1) Urea (1):Ethanol (1.25)

Urea

ethanol

Xray diffraction measurement

(b)

Zn

O (

10

0)

Zn

O (

10

1)

Zn

O (

10

2)

Zn

O (

11

0)

Zn

O (

20

0)

Zn

O (

10

3)

(a)

(b)

Inte

ns

ity

(a

rb. u

nit

s) (002)

Confocal PL of ZnO spheres

5μm 400 500 600 700

ZnO121-b

Laser : DPSS 355nm

Grating : 150 g/mm-500 nm Blz

Int.time : 500 ms

Temp. : RT

Peak at : 387 nm

Inte

nsity (

a.u

.)

Wavelength (nm)

No defects are observed

ZnO

Confocal PL

Summary

ZnO polar surface was responsible for

preferential growth

Structure directing agents (SDA) effectively

passivated the ZnO polar surface, leading to

balanced vertical and lateral growth rate

Careful combination of SDA allowed for the

control of both size and size distribution of

ZnO spheres

Outline




Synthesis of ZnO nano/microspheres




• Power of His Words

Cost

Eff

icie

nc

y

Current state-of-the art solar cells

Semiconductor

PV (GaAs,

Si, GaN….) Thin Film PV

(CIGS, a-Si,

CdTe….)

New generation

PV (Organic,

Inorganic, …)

Organic solar cells

Transparent solar cells Absorption of polymer

350 400 450 500 550 600 650 700 750

Ab

so

rpti

on

(a

rb.

un

its

)

Wavelength (nm)

PTB7Transparent

Fabrication of organic solar cell

OMe

O

PC71BM

Polymer PCBM fullerene

Donor Acceptor

PCDTBT:PCBM solar cells

-

+

Heeger et al, Nature photonics, Vol 3,

pp 297 (2009)

Recombination processes

Light

-n-contact

p-contact +

exciton

photons

5

Exciton generation

exciton=~ nanoseconds

-n-contact

p-contact

+

bound e/h

photons

5


Light

nm

DL excitonexciton

10~

exciton=~10 ns

Exciton diffusion length

Heterojunction bipolar

Hetero-interface

Dissociation center

Nanoscale morphologies

- n-contact

p-contact

+

free electron

free hole

photons

5


Light

- - n-contact

p-contact

+

exciton bound e/hfree electron

photons

5

+


Light

Langevin recombination

)( 2

ir nnpkR

)( pnr

qk μμ

ε

Namkoong et al, Organic electronics, Vol. 14, pp 74 (2013)

Degradation of

organic solar cells

Lifetime > 20 years Lifetime < 6 years

Degradation mechanisms of

organic solar cells Degradation processes of PPV(polyphenylene

vinylene) polymer

Photo-oxidation

Light (hv)

scission

phenylene

phenylene

phenylene

Reducing charge transport efficiency

Creating defects and trap centers

Influence of photo-oxidation on

charge transport

Photo-oxidation

trapped

trapped X

Degradation of organic solar cells

OMe

O

PC71BM

Polymers PCBM

PTB7

PTB7 PCBM

Degradation of organic solar cells

0 2 4 6 8 100.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

(a)

Norm

ali

zed

Eff

icie

ncy

Time (Days)

PTB7/PC71

BM

With TiOx

Without TiOX

~50%

~25%

Role of TiO2 for organic solar cells

+O2

CO2 (gas) H2O(gas)

Sol-Gel

processed

TiOx

TiO2 Redox

process

Effect of sealing of organic solar cells

on degradation Al/glass

ITO/glass

PEDOT:PSS

Organic blends

TiO2

Optical adhesive 32%

41%

99%

0 2 4 6 8 10 12 14 16 18 20-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Norm

ali

zed

Eff

icie

ncy

Time (Days)

PTB7/PC71

BM/TiOx

With sealant glass in Glove box

With sealant glass in air

Without sealant glass in air

PTB7:PCBM/TiOx

Sealed, in glove box

Sealed, in air

Unsealed, in air

Comparison of UV-VIS absorption

350 400 450 500 550 600 650 700 750

(b)

Ab

sorp

tion

(arb

.un

its)

Wavelength (nm)

PTB7/PC71

BM/TiOx

With sealant glass in air

As cast

After 10 days

After 20 days

Sealed PTB7/PCBM in air Not sealed PTB7/PCBM in air

350 400 450 500 550 600 650 700 750

(c)

PTB7/PC71

BM/TiOx


As cast

After 10 days

After 20 days

Ab

sorp

tion

(arb

.un

its)

Wavelength (nm)

Absorption of PTB7 and PCBM

Not sealed PTB7/PCBM in air

350 400 450 500 550 600 650 700 750

(c)

PTB7/PC71

BM/TiOx


As cast

After 10 days

After 20 days

Ab

sorp

tion

(arb

.un

its)

Wavelength (nm)

350 400 450 500 550 600 650 700 750

Ab

so

rpti

on

(a

rb.

un

its

)

Wavelength (nm)

PTB7

PCBM

OMe

O

PC71BM

PCBM

P3HT

Sn

S

S

S

S

O OR

F

n

OR

OR

R= 2-ethylhexyl

PTB7

PTB7

Degradation mechanism for organic

solar cells

PTB7 PCBM

Sealed PTB7/PCBM in air

Degradation mechanism for organic

solar cells

PTB7 Defective PCBM

Not sealed PTB7/PCBM in air

defect states

Namkoong et al, unpublished work (2014)

Summary

Degradation of organic solar cells is due to

chemical degradation in the presence of

oxygen

Longer exposure to oxygen will create many

defects and trap centers that will force

organic solar cells to reduce lifetime

The degradation of organic solar cells is

governed by the degradation of PCBM rather

than organic polymer

Simulation of

organic morphologies

Simulation of organic morphologies

(a) (b)AFM image of organic surface Semiconductor surface

Simulation of organic morphologies

Polymer: Fullerene 1 : 1

P3HT

Sn

S

S

S

S

O OR

F

n

OR

OR

R= 2-ethylhexyl

PTB7

OMe

O

PC71BM

7 days mixing

Effect of uniform morphologes

300 400 500 600 700 800

0.10

0.15

0.20

0.25

0.30

0.35

0.40

5 days

7 days

1 day

Y A

xis

Title

X Axis Title

3 days

-0.2 0.0 0.2 0.4 0.6-12

-10

-8

-6

-4

-2

0

2

4

7 days mixing

J(m

A/c

m2)

V(Volt)

1 day mixing

Light absorption JV characteristics

Phase separation of organics

• Spinodal decomposition

• Binodal decomposition

Spatial coordinate

Co

mp

os

itio

n

Spatial coordinate

Co

mp

os

itio

n

Phase separation

TdSdHdG

G: Gibbs free energy

H: Enthalpy

S: Entropy

Fre

e e

ne

rgy

Composition of polymer

0 1CA CBCx

Spontaneous process

Reactants

Products

Enthalpy (DH)<0

Enthalpy (H) defines system

energy. Entropy (S) measures disorders

of systems

DS>0

0< TdSdHdG

Phase separation Polymer: Fullerene 1 : 1

TdSdHdG

G: Gibbs free energy

H: Enthalpy

S: Entropy

0dG <Spontaneous

process

0dG nonspontaneous

process

dS>0

Fre

e e

ne

rgy


0 1CA CBCx

Flory-Huggins/Allen-Cahn

)lnln( BAABB

B

BA

A

A

site

CCCm

CC

m

C

v

RTf

Flory-Huggins type of free energy

)( CkC

fM

t

C 222

C: concentration a: solution parameter M: diffusivity of the phase k: gradient energy coefficient

Allen-Cahn Equation

Fre

e e

ne

rgy


0 1CA CBCx

Numerical simulation of partial

differential equations

• Finite different method

2

2

x

f

t

f

)( 2

2

11

2

2 2D

D

Offf

x

f iii

''

''

''

''

''

n

n

n

n

nn

nnn

f

f

f

f

f

f

f

f

ff

bc

cbc

abc

abc

ab

1

3

2

1

1

3

2

1

111

333

222

11

Not suitable for higher

order differential equation

Memory issues

Large truncated errors

Convergence issues

Finite different vs. Spectral method

Finite different method

Spectral method

M. Mehra et al, Comparison between different

numerical methods for discretization of PDEs.

Spectral methods

)(xk polynomials

)sin()cos( kxikxeikx

ikx

k ex )( Fourier spectral method

: interpolating function

-1 -0.5 0 0.5 1-1

-0.5

0

0.5

1

1.5polynomial fitting

max error = 5.9001

-1 -0.5 0 0.5 1-1

-0.5

0

0.5

1

1.5trigonometric fitting

max error = 0.017523

f(x) f(x)

Polynomial fitting Trigonometric fitting

1D Allen-Cahn equation

32

uux

u

t

u

32

kkkk uuuik

t

u

)(

kj uuFFT

)(

)/)((

)()/(

hik

uhuu k

nnkn

k1

112

31

)(uiFFTu

Inverse FFT

3121

knn

knk

nk

nk uuuik

h

uu)()(

3D Allen-Cahn Equations

)( CkC

fM

t

C 22

)lnln( BAABB

B

BA

A

A

site

CCCm

CC

m

C

v

RTf

Flory-Huggins type of free energy

Allen-Cahn Equation

)( CkC

fM

t

C

2

222

)(

)cos(

x

ki

D

FFT

Simulated organic morphologies

Summary

Spectral method has been used to

numerically solve higher order differential

equations

Flory-Huggins and Allen-Cahn equations

were used to simulate 3D organic

morphologies

Outline




Synthesis of ZnO nano/microspheres




• Power of Words

Experiment of the power of words

Words 1 In the beginning was the Word, and the Word was with

God, and the Word was God. John 1:1

1In the beginning God created the heavens and the

earth. 3 And God said, “Let there be light,” and there was

light. Genesis 1:1,3.

12 For the word of God is alive and active. Sharper than

any double-edged sword. Hebrew 4:12

Idiom and proverb

Birds hear what is said by day, and rats hear what is said by night

Korean proverb

Ground

Warmer

Cooler (dense air)

Prove

Ground

Warmer

Cooler (dense air)

Birds hear what is said by day Rats hear what is said by night

Conclusion

Research History

Conclusion

Re-search His story

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