supra-hierarchical nano-structured organic thin …supra-hierarchical nano-structured organic thin...

Supra-hierarchical nano-structured organic thin film solar cell

Institute of Advanced EnergyKyoto University

Susumu Yoshikawa

3rd Japan-German Bilateral Workshop on Molecular Electronics

21-23, Jan., 2009, 烏丸 Kyoto Hotel, Japan

Contents□ Insertion of TiO2 ETL in polymer solar

cell of P3HT:PCBM □ Preparation of PEDOT:PSS polymer

brush as HTL in polymer solar cell□ Concept of Supra-hierarchical

nano-structured cell(Hybrid solar cell）

ITO

Glass

Al

V

Heterojunction(Tang Cell)

ITO

Glass

Al

n-type semiconductor

p-type semiconductor

V

Bulk heterojunction(Sariciftci Cell)with C60

Metal

Glass

Al

V

Schottky junction(Calvin Cell)

organic semi-conductor

Biography of Organic Solar Cell & Novel Architecture of OSC with Supra-Hierarchical Nano-Structure

Supra-Hierarchical Nano-Structure (SHNS)

Glass

V

Al

CCH2

H 3C CO

OS

S

S

S

CHCH 2

O3S

Device architecture of OSC with SHNS

n-type (Acceptor)

p-type (Donor)

2006 supra-hierarchical nano-structured cell (Yoshikawa)2004 tandem heterojunction photovoltaic cell (Forrest,Uchida)2000 bulk MDMOPPV/PCBM heterojunction PV cell (Brabec)1996 C60-linked molecular type PV cell (Imahori)1995 bulk MEHPPV/PCBM heterojunction PV cell (Heeger)1995 bulk polymer/polymer heterojunction PV cell (Friend)1994 bulk polymer / C60 heterojunction PV cell （Heeger）1993 polymer/ C60 heterojunction PV cell (Sariciftci)1991 dye-sensitized TiO2 PV cell (Graetzel)1991 bulk dye/dye heterojunction PV cell (Hiramoto)1990 tandem PV cell (Hiramoto)1986 heterojunction PV cell (Tang)1958 photo-induced current with MgPor (Calvin)

η < 0.01%η = 1%

η = 4%η > 7%

TiO2

Donor/Acceptor

PEDOT

ITO

OSC for Next-Generation

n-type semi-conductor

p-type semi-conductor

Topics are focused to improvements in device structures.

IntroductionIntroduction

Organic Solar Cells

Dye Sensitized

Cells

Organic TF Cells

Small Molecule

Polymer PV

Advantages of polymer PV・ Fabrication under ambient atmosphere

・ Potential low-cost manufacturing

Problems of Organic PV・ Low efficiency

・ Low durability

Current status of polymer solar cell ・ Recently high efficiencies over 5% are reported.

・ Poly(3-hexylthiophene)/PCBM is commoly used as a high carrier mobility system.

・ Bulk heterojunction is important for highly efficient cell-structure.

Organic Semi-conductorsComparison, organic and inorganic meterials

~100 times

few times

anisotropy

van derWaals

~10Organic

Chemical bonding~1000Inorganic

CohesionForce

mobility[cm2/Vs]

organic materials:origin of electric conduction ⇒ planer π-conjugated molecule

Fundamental Limitations to Organic Solar Cells Fundamental Limitations to Organic Solar Cells

The exciton and carrier diffusion bottleneckSince LD is short & μ is little, there exists a trade off in thickness.

Maximum: exiciton collection & Minimum series resistance thin film

Maximum: absorption thick film

How to solve these problems?Using a bulk heterojunction

Using tandem cells (capture more light in thin layer

Using 1D nanostructured array for carrier path

（Supra-hierarchical nano-structured cell)

Using material with long range order

Using thin HTL and ETL with EBN and HBN

Problems

Enhanced efficiency and stability in P3HT:PCBM bulk heterojunction solar cell by using TiO2 as electron transport layer

Organic Thin Film solar Cell

□ Insertion of electron transport layer (ETL) in polymer solar cell・ P3HT: PCBM with TiO2 layer

Necessity of hole blocking layer in polymer cell

ITO glassTCOHTLLAL

anodeHTL

PEDOT:PSSP3HT:PCBM

AlNothing (LiF)

To achieve highly efficient charge transfer and charge collection

Poly(3,4-ethylenedioxythiophene)-poly(styrensulfonate)[PEDOT:PSS]

[6,6]-phenyl C60-butyric acid methyl ester [PCBM]

Poly(3-hexyl thiophene)

[P3HT]

←TiOx層

Conditions for HBL

n-type semiconductor

LUMO is between Al and PCBM

A wide band gap4.24.2

7.4

3.7

6.1

Al

TiO₂PCBM

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

-1.0 -0.5 0.0 0.5 1.0 1.5

Voltage (V)

Cur

rent

den

sity

(mA

/cm2 )

in illumination

in darkUnder illumination, I-V curve shows a less rectification.

Increase in a back electron transfer

Au/P3HT/Al

In dark, P3HT/Al interface forms a Schottky barrier to prevent a back electron transfer and shows a favorable rectification.

Necessary to introduce a new barrier

Fig.3 I-V curve of Au/P3HT/Al in dark and under illumination.

Necessity of HBL (hole blocking layer)Necessity of HBL (hole blocking layer)

CO

C

CO

C

O

O

O

O

NTCDA as HBLAg/C60/CuPc/ITO2μm

Hiramoto, Appl. Phys. Lett., 86, 063509 (2005).

Hal et al. reported

Ti-alkoxide room temperature over night

• TiO₂ (７7～91%)

UV-Vis and cyclic voltammetry suggest that band gap is 3.7 eV、

HOMO is 8.1 eV、

LUMO is 4.4 eV

ITO

4.8PEDOT

5.0

P3HT

5.2

3.3

PCBM

6.1

3.7 Al

4.3

TiO

₂

4.4

8.1

TiO2 prepared by this method has an amorphous structure

Fig.8 Flat band potentials of ITO/PEDOT/P3HT:PCBM/Al.

Energy diagram of TiOEnergy diagram of TiO2 2

P.V.A. Hal et. al., J. Adv. Mater. 2003, 15, No.2

Use of TiOx layer as optical spacer,K. Lee, et. al., Adv. Mater. 2006, 18, 572

Fig.10 Effect of TiO₂ on Efficiency (a), FF (b), Voc (c), Isc (d).

(a) (b)

(c) (d)

Effects of Effects of TiOTiO22 film thicknessfilm thickness on photovoltaic propertieson photovoltaic properties

0.0

1.0

2.0

3.0

4.0

0 10 20 30thickness (nm)

effic

ienc

y (%

)

0.3

0.5

0.7


FF0.2

0.3

0.4

0.5

0.6


Voc

(V)

5.0

6.0

7.0

8.0

9.0

10.0


Isc

(mA

/cm

2 )

オプティカルスペーサではない

s

T

oc

T

prev

psj R

VV

VRjR

RjV

≈=∂∂

=

exp10

＋

＋

pps

T

ssc

T

prev

psV RRR

VRj

VRj

RR

jV

≈≈−

=∂∂

= ＋

＋

＋

)exp(10

100100max ×⋅⋅

=×=light

scoc

light PFFJV

PP

η

scocscoc JVJV

JVPFF

⋅⋅

=⋅

= maxmaxmax

I-V curve of solar cells.

Simulated sun light of A.M. 1.5G 100mW/cm² was illuminated onto the cell.

Voltage (V)Vmax

Voc

Jmax

Jsc

Rs

jsc

j

jD VVD

jRp

Equivalent circuit of solar cells.

Series resistance （Rs）and parallel resistance (Rp)

Evaluation method of organic solar cellEvaluation method of organic solar cell

Rp

dv/dj = Rp

Rp

Rs

Reduce in an internal resistance → decrease in Rs→ increase in Isc

Improve of carrier selectivity → increase in Rp→ increase in Voc

Improve of carrier selectivity

→ increase in Rp

Reduce in an internal resistance → decrease in Rs

Insertion of TiO2 layer induced a marked increase in Rp

→ Improve of carrier selectivity

① Series resistance （Rs） and parallel resistance （Rp）

Rp and Rs of devices containing TiO₂ layer with several thicknesses.

soc

RVdJ

dV=

)(

pRdJ

dV=

)0(

Method of calculating Rs and Rp.

RpRp incleaseinclease and and RsRs decrease by decrease by TiOxTiOx insertioninsertion

Rp x Rs

0

200

400

600

800

1000

1200

1400

0 10 20 30

Thickness

Rp

02468101214

Rs

2V

-2V Voltage

Current density

I(-2V)

I(2V)

(RR) = -I(-2V)

I(2V)

Improvement of rectification by insertion of TiO2

layer → TiO2 layer blocks hole carriers.

② Change in rectification ratio (RR)

Fig.13 The ratio of the current density of devices with several thickness TiO₂ at ＋2V an －2V in dark.

0.0001

0.001

0.01

0.1

1

0 10 20 30

Thickness (nm)

Rec

tific

atio

n R

atio

Fig.14 Method of calculating RR.

Rectification ratioRectification ratio

Improved cell structure with TiO2 layer on active layer (P3HT:PCBM) is quite promising.

TiO2 layer acts as a hole blocking layer.

Optimal 4% conversion efficiency was obtained with very high fill factor under ambient atmospheric condition without sealing.

Isc of the device with TiO2 decreased only 6% after 100 hour illumination, showing high durability under ambient atmospheric condition.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

300 400 500 600 700 800

Wave length (nm)

IPC

E

0

2

4

6

8

10

12

-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Voltage (V)

Cur

rent

den

sity(

mA

/cm2 )

EffIsc

Voc

FF

= 4.05 (%)=9.72 (mA/cm2)= 0.60 (V)= 0.70

Fig.22 I-V curve of the best cell. Fig.23 IPCE spectrum of the best cell.

Conclusions Conclusions

Organic Thin Film solar Cell

□ Preparation of hole transport layer (HTL) using PEDOT:PSS polymer brash

Rolls of Hole Transporting Layer (HTL)

PEDOT/PSS

Organic Thin-Film Solar Cells

●PEDOT:PSS films help to planarize the ITO surface●PEDOT:PSS films appear to make the ITO surface more uniformly electroactive●PEDOT:PSS layer reduces the electrode surface polarity, making it more compatible with

nonpolar components of OPVs●PEDOT:PSS layer appears to increase the effective work function of the resulting substrate

ITO cathode

Metal anode (Al)

Hole Transporting Layer （HTL)

Electron Transporting Layer (ELT)

Light Absorption Layer (LAL)

The HTL plays a key role in the OPVs.

highly rectification (electron blocking) ability (large carrier mobility and/or conductivity), transparency, low resistance at the interface, and so on.

PEDOT:PSSPoly(3,4-ethylenedioxythiophene)-poly(styrensulfonate）

●Low solubility→ Hard to be processed→ Unsuitable for thin-film-making●Dispersed in water → Water is unsuitable for electronic manufacture such as coating●Impossible to be coated on the substrate such as glass or electrode without the

binder→ Lack of adhesion to the substrate●High acidity, corrosive, absorbent material→ ITO is eroded●No durability against water, other solvents, and scratching

●Neutralized and dispersed in organic solvents →No corrosive for ITO and no desiccant

●Directly polymerized from the glass or electrode without the binder

●High density without pin-hole →Decrease the resistance at the interface and enhance the physical stability of the film

We need novel materials for HTLs.

PEDOT/PSS: What is the problem?

Doping of PSS cuts the π conjugation of PEDOT and binds H+ from PSS.

Isolated π electron migrates along the PEDOT chain shows high conductivity.

PEDOT:PSSPoly(3,4-ethylenedioxythiophene)-poly(styrensulfonate）

Polymer brush

［PSS brush：chemical oxidation, PSSEt brush：electrochemical polymerization］

Si wafer or ITO

C OC

O(CH2)nSi

H3C CH3

Br

CHCH2

O3S

HCCH2

SO3

SO

O

SO

O

S

O

O

SO

O

-2 -1 0 1 25

0

-5

-10

-15

-20

-25

-30

Curr

ent

/ m

A

Potential / V)

ITOのみBHE固定化PSSEtブラシ

重合膜の形成

Control of ATR polymerization of SSNa, SSEt on ITO

Highly dense polymer brushes were obtained.PSSNabrush：surface density ３７％, PSSEt brush：３８％（grafted density：0.26 chains/nm2）

Highly expanded and oriented polymer brush in water and/or CH3CN

In situ polymerization of EDOT

220 oC

0 1 2 3 4 5 6 7 80

10

20

30

d/ n

m

Mn / 104

1.0

1.2

1.4

1.6

M w/M

n

Prof. Y. Tsujii, et al, Institute for Chemistry, Kyoto Univ.

C OC

O(CH2)nSi

H3C CH3

Br

C OC

O(CH2)nSi

H3C CH3

BrSSEtSSEt

CHH2C

SO3Et

①① グラフト重合グラフト重合 ②②inin--situsitu重合重合

EDOTEDOTOO

S

③脱保護③脱保護

in AcCN △ 220℃

CHCH2

SO3H

PSSEt brush PEDOT/PSSEt PEDOT/PSS

PSSPSS

Highly ratio of PEDOTcomponent

0 20 40 60 80 1000.0

0.5

1.0

1.5

PEDOT:PSSEt Thickness / nm

Con

vers

ion

Effic

ienc

y / %

18nm (very thin!)

graft polymerization

in situ polymerization

- Et

Polymer brush

Enhancement through deprotection of Et!!

Prof. Y. Tsujii, et al, Institute for Chemistry, Kyoto Univ.

Hybrid solar Cell

□ Consept of supra-hierarchical nano-structured cell

1st Generation 2nd Generation 3rd Generation

Introduction of 1D nano-materials for facile carrier path in the bulk hetero-junction

TiO2 Nanotube Arrays

ZnO nanorod Arrays TiO2 nanotube Arrays

• Fabricating TiO2 nanotube array from ZnO nanorod array templateHigh Crystallinity

TiO2 nanotube Arrays

1μm1μm

ZnO Nanorod TiO2 Nanotube

Enhance the performance of the nanorod structure of ZnO/ polymer hybrid solar cell by modifying the metal oxide surface with various dyes

N719Ruthenium complex

NKX-2677Coumarin dye

D149Indoline dye

Eosin-YXanthene dye

Concentration : 0.5 mM

0

0.2

0.4

0.6

0.8

1

1.2

1.4

350 400 450 500 550 600 650 700nm

Abs.


• The peaks of each dye can be observed before polymer deposition• After dye treatment and polymer deposition, we can’t observed any peaks of dye treatment

ZnO/ dye/ polymer

ZnO/ polymer

ZnO/ dye

From UV result, the amount of coated active film after surface modification by dye was different to un-modified surface because the contact angle was changed after surface treatment


0.0

2.0

4.0

6.0

8.0

10.0

12.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Voltage [V]

Cur

rent

Den

sity

[mA/

cm2 ]

Eosin-Y

D149

NKX-2677

N719

Without dye

• The significantly improve in performance of device after dye treatment • Surface treatment with ruthenium dye (N719) show the low FF value compared to other dyes• Surface treatment with D149 show the high performance in Jsc of 9.87 mA/cm2, Voc of 0.58V, FF of 0.47, and η of 2.71%

5.29 0.54 0.37 1.06Without dye

9.680.550.42.13N719

9.570.580.482.63NKX-2677

9.870.580.472.71D149

9.280.570.462.43Eosin-Y

Jsc[mA/cm2]Voc[V]FFn [%]dyes

Polymer : P3HT:PCBM (30:18 mg/ml)Speed of spinning : 1000 rpmAnealed : 140 oC 5 minElectrode : Ag

Structure : FTO/ZnO/P3HT:PCBM/Ag

Orientation controlled graft polymerization of thiophenes from methanofullerene on TiO2 electrode

（initiator or linker-D-A type）methanofullerene

nanopillar TiO2

Assembly of OSC with Supra-Hierarchical Nano-Structure

O-

O

Binder & DyeInitiator

TiO2

ITO

hole

electron

S

C6H13

n

S

OO

n

（proposal１）（proposal ２）

（prospects for upconversion）

η = Voc × Jsc × FF = 4.1% × 0.8/0.6 × 13/10 × 0.7/0.7 > 7.1%

• Charge separation efficiency → up• Lifetime of exciton → long• Charge transport efficiency → up• Large absorption coefficient →IPCE

up

Power conversion efficiencies

ΔEg ：HOMO (Donor) and LUMO (Acceptor) offset μ：carrier mobility

L：exciton diffusion length ε ：absorbance λ：wavelength

η ∝ ΔEg × μ × L ×∫ε dλ

ηEQE = ηA × ηED × ηCT × ηCC

Device architecture of OSC with SHNS

Supra-hierarchical nano-structured cell

New MaterialEntering

Battery Back-up

/7 Yen/kWh

2002 2007 2010 2020 2030

Very-Thin Cell/Multi-junction

14 Yen/kWh

Elec

tric

ity C

ost

50 Yen/kWh

Bulk Si &Thin Film Si/ Compound

Active Grid Control

23 Yen/kWh

30 Yen/kWh

New MaterialEntering

Battery Back-up

/7 Yen/kWh

2002 2007 2010 2020 2030

Very-Thin Cell/Multi-junction

14 Yen/kWh

Elec

tric

ity C

ost

50 Yen/kWh50 Yen/kWh

Bulk Si &Thin Film Si/ Compound

Active Grid Control

23 Yen/kWh

30 Yen/kWh30 Yen/kWh

Japanese PV Roadmap until 2030Japanese PV Roadmap until 2030

Present utility cost:23 yen/kWh (private household)15 yen/kWh (industry)

Electricity generation cost:5-6 yen/kWh (Nuclear)9 yen/kWh (hydro)

100GW

1US$= 113 JPY

1THB= 3 JPY

Research Target：Organic Solar Cell for Roof Top Module

100

80

60

40

20

0

Efficiency ％

50,00040,00030,00020,00010,0000

Cost，Yen/ｍ２

１００円/Ｗ

１５０円/Ｗ

７５円/Ｗ

２０円/Ｗ５０円/Ｗ

２５０円/Ｗ

Current Organic SC第１世代（Si他【現状】）

2nd Generation （low cost） for Market Application3rd Generation (high efficient ) for Long Target

1st Target【Year 2015】Module effic.：１０％Cell Cost： 75Yen／WPower Cost： 14Yen／kWh

第２世代(薄膜型）

第３

世代

supra-hierarchical nano-structured organic thin …supra-hierarchical nano-structured organic thin...

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