Supra-hierarchical nano-structured organic thin film solar cell I - - PowerPoint PPT Presentation

Supra-hierarchical nano-structured

• rganic thin film solar cell

I nstitute of Advanced Energy Kyoto University

Susum u Yoshikaw a

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)

• rganic

semi- conductor

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

Supra-Hierarchical Nano-Structure (SHNS)

Glass V Al

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.

Introduction Introduction

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. ・ Bu Bulk h lk hetero rojunction

• n is

is im important f for h r high ghly ly efficie cient ce cell- ll-stru ructure.

Organic Semi-conductors

Comparison, organic and inorganic meterials ~100 times few times anisotropy van der Waals ~10 Organic Chemical bonding ~1000 Inorganic Cohesion Force mobility [cm2/Vs]

• rganic materials:
• rigin of electric conduction ⇒

planer π-conjugated molecule

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

The exciton and carrier diffusion bottleneck Since 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 glass TCO HTL LAL anode HTL PEDOT:PSS P3HT:PCBM Al Nothing (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 gap

4.2 4.2 7.4 3.7 6.1

Al TiO₂ PCBM

• 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) C urrent density (m A /cm

2)

in illumination in dark

Under 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)

C O C C O C O O O O

NTCDA as HBL Ag/C60/CuPc/ITO

2μm

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

Hal et al. reported Ti-alkoxide room temperature

• ver 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

4.8

PEDOT

5.0

P3HT

5.2

3.3

PCBM

6.1

3.7

Al

4.3

TiO₂

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 TiO Energy 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 TiO TiO2

2 film thickness

film thickness on photovoltaic properties

• n photovoltaic properties

0.0 1.0 2.0 3.0 4.0

10 20 30

thickness (nm) efficiency (%)

0.3 0.5 0.7 10 20 30 thickness (nm) FF

0.2 0.3 0.4 0.5 0.6 10 20 30 thickness (nm) Voc (V) 5.0 6.0 7.0 8.0 9.0 10.0 10 20 30 thickness (nm) Isc (mA/cm2)

オプティカルスペーサではない

s T

• c

T p rev p s j

R V V V R j R R j V ≈ = ∂ ∂

=

exp 1 ＋ ＋

p p s T s sc T p rev p s V

R R R V R j V R j R R j V ≈ ≈ − = ∂ ∂

=

＋ ＋ ＋ ) exp( 1

100 100

max

× ⋅ ⋅ = × =

light sc

• c

light

P FF J V P P η

sc

• c

sc

• c

J V J V J V P FF ⋅ ⋅ = ⋅ =

max max max

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

R s jsc j jD V VD jRp Equivalent circuit of solar cells.

Series resistance （Rs） and parallel resistance (Rp)

Evaluation method of organic solar cell Evaluation 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.

s

• c

R V dJ dV = ) (

p

R dJ dV = ) (

Method of calculating Rs and Rp.

Rp Rp inclease inclease and and Rs Rs decrease by decrease by TiOx TiOx insertion insertion

Rp x Rs

200 400 600 800 1000 1200 1400 10 20 30 Thickness Rp

2 4 6 8 10 12 14 Rs

2V

• 2V

Voltage

Curren t 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 10 20 30 Thickness (nm) Rectification Ratio

Fig.14 Method of calculating RR.

Rectification ratio Rectification 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) IPCE 2 4 6 8 10 12

• 0.1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 Voltage (V) Current density(mA/cm

2)

Eff Isc 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:PSS Poly(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:PSS Poly(3,4-ethylenedioxythiophene)-poly(styrensulfonate）

Polymer brush

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

Si wafer or ITO

C O C O (CH2)n Si H3C CH3 Br

CH CH2 O3S HC CH2 SO3 S O O S O O S O O S O O

• 2
• 1

1 2 5

• 5
• 10
• 15
• 20
• 25
• 30

Current / mA 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

1 2 3 4 5 6 7 8 1 2 3 d / nm Mn / 104 1 .0 1 .2 1 .4 1 .6 M w / Mn

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

C O C O (CH2)n Si H3C CH3 Br C O C O (CH2)n Si H3C CH3 Br

SSEt SSEt

CH H2C SO3Et

① ① グラフト重合 グラフト重合 ②in in-

• situ

situ重合 重合

EDOT EDOT

O O S

③脱保護 ③脱保護

in AcCN △ 220℃

CH CH CH CH2 SO3H

PSSEt brush PEDOT/PSSEt PEDOT/PSS

PSS PSS Highly ratio of PEDOT component

20 40 60 80 100 0.0 0.5 1.0 1.5

PEDOT:PSSEt Thickness / nm Conversion Efficiency / %

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 Introd

• duct

uction of

• n of 1

1D na nano-m no-materia erials fo ls for fa r facile cile ca carr rrier p ier path in th the bu bulk h hete tero-juncti tion

TiO2 Nanotube Arrays

ZnO nanorod Arrays

TiO2 nanotube Arrays

• Fabricating TiO2 nanotube array from ZnO nanorod array template

High Crystallinity TiO2 nanotube Arrays

1μm 1μm

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

N719 Ruthenium complex NKX-2677 Coumarin dye D149 Indoline dye Eosin-Y Xanthene dye Concentration : 0.5 mM

0.2 0.4 0.6 0.8 1 1.2 1.4 350 400 450 500 550 600 650 700

nm Abs.

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

• The peaks of each dye can be observed before polymer

deposition

• After dye treatment and polymer deposition, we can’t
• bserved 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

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

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]

Current D ens ity [m A/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 1.06 Without dye 9.68 0.55 0.4 2.13 N719 9.57 0.58 0.48 2.63 NKX-2677 9.87 0.58 0.47 2.71 D149 9.28 0.57 0.46 2.43 Eosin-Y

Jsc[mA/cm2] Voc[V] FF n [%] dyes

Polymer : P3HT:PCBM (30:18 mg/ml) Speed of spinning : 1000 rpm Anealed : 140 oC 5 min Electrode : 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 & Dye Initiator

TiO2 ITO

hole electron

S C6H13 n S O O 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)

• ffset

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 Material Entering Battery Back-up /

7 Yen/kWh

2002 2007

2010

2020

2030

Very-Thin Cell/ Multi-junction 14 Yen/kWh Electricity Cost 50 Yen/kWh Bulk Si & Thin Film Si/ Compound Active Grid Control

23 Yen/kWh

30 Yen/kWh New Material Entering Battery Back-up /

7 Yen/kWh

2002 2007

2010

2020

2030

Very-Thin Cell/ Multi-junction 14 Yen/kWh Electricity Cost 50 Yen/kWh 50 Yen/kWh Bulk Si & Thin Film Si/ Compound Active Grid Control

23 Yen/kWh

30 Yen/kWh 30 Yen/kWh

Japanese PV Roadmap until 2030 Japanese 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

Efficiency ％

50,000 40,000 30,000 20,000 10,000

Cost，Yen/ｍ

２

１００円/Ｗ １５０円/Ｗ ７５円/Ｗ ２０円/Ｗ ５０円/Ｗ ２５０円/Ｗ

Current Organic SC

第１世代（Si他【現状】）

2nd Generation （low cost） for Market Application 3rd Generation (high efficient ) for Long Target

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

第２世代(薄膜型） 第３世代