polym. nano mater. lab efficient photovoltaic devices with solution process, stamping transfer...

Polym. Nano mater. Lab

Efficient Photovoltaic Devices with Solution Process, Stamping Transfer

Technique and Controlled Nano-structure

O Ok Park1,*, Dong Hwan Wang1, Hang Ken Lee1, Jong Hyoek Park2

Sang Hyuk Im3, Dae Geun Choi4, Ki Joong Lee4

1Department of Chemical & Biomolecular Engineering, KAIST

& 2Sungkyunkwan University & 3Korea Research Institute of Chemical Technology,

& 4KIMM(Korea Institute of Machinery & Materials)

at Seoul National University

-2010. 3. 29 (Mon.)- ( pm : 5:00)


Contents

Introduction Introduction 11

Part 3. Efficient PV Devices with Controlled Nano-StructurePart 3. Efficient PV Devices with Controlled Nano-Structure44

AcknowledgementsAcknowledgements55

Part 2. Novel Fabrication by Stamping Transfer TechniquePart 2. Novel Fabrication by Stamping Transfer Technique33

Part 1. Photovoltaic Devices Fabricated by Solution ProcessPart 1. Photovoltaic Devices Fabricated by Solution Process22

- Enhanced high temp. long-term stable PV devices with thermally stable TiOx

- Concentration graded P3HT/PCBM bilayer using solvent swelling effect

- Effect of ordered 2D-dot nano-patterned anode

- Enhanced charge collection via nano porous morphology

- Buffer layer modification by UV irradiation

- Active layer transfer by printing process

- Ratio controlled single active layer versus double layer


Earth uses 13 terawatts(TW) of power a year Sun deposits 120,000 TW of energy a year

Need for solar cells !!


What is solar cell ?

A solar cell : Light energy(photons) Electrical energy(electrons)

Photovoltaic (PV) effect : When sunlight is absorbed by some materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage) is called the PV effect.

n-type

p-type

valence band

or

Light absorption

- +

1. Photo-generation of charge carriers

2. Separation of charge carriers

electron

hole

Conduction band


Classification of solar cell Inorganic Solar Cells 1. Silicon Semiconductor 1) Crystalline 2) Amorphous

2. Compound Semiconductor 1) II-VI (CdS, CdTe) 2) III-V (GaAs, InP) 3) I-III-VI (CuInSe2)

< Basic Organic Solar Cell structure>

Organic Solar Cells 1. Polymer Solar Cell 2. Dye-sensitized Solar Cell 3. Nano Hybrid Solar Cell

<composite film>


Properties of Polymer Solar Cell

Flexibility Semi-transparent Low cost process-Plastic substrate

-Unbreakable device-Room Temp processing-applicable to window -solution processing

-Mass production

Advantages

Low Efficiency (%) Durability

Disadvantages


Mechanism of Polymer Solar Cell

1. Light absorption

2. Exciton (e-h pair) creation

3. Exciton diffusion

4. Charge separation

5. Charge transport

6. Charge collection

Electron acceptor

Hole acceptor

CathodeAnode

HOMO

HOMO

LUMO

LUMO

h+

e–

e–


< P3HT/PCBM blend >

Active layer structure of polymer solar cells

(A) P3HT/PCBM Bilayer layer (B) P3HT/PCBM <Bulk-heterojunction structure>

BHJ1. Large D-A interfacial area2. Low transport with long distance

Bilayer1. Low D-A interfacial area2. High transport with short distance

Intermixing zone zoneHybrid type

< Large D-A interfacial area >< High transport >


Contents

Polymer Solar Cell

ITO or IZO (Anode)

Cathode (Al)

Glass

Buffer layer(PEDOT:PSS)

Active layer(Model case : P3HT/PCBM)

7. Modified PEDOT:PSS by UV irradiation.

5. 2D-dot patterned anodeITO or IZO (Patterned Anode)

1. Linear polymeric TiOx

2. Concentration graded spin-coated bilayer (solvent swelling)3. Single layer stamping active layer (BHJ)

4. Bilayer stamping active layer (BHJ)

6. Spin-coated nano-porous active layer (BHJ)

< Current >


Photovoltaic Devices Fabricated by Spin Coating

- Enhanced high temperature stable PV devices with TiOx

- Concentration graded P3HT/PCBM bilayer(intermixing layer) vs BHJ Cell

Part 1


Research Background

Use of TiOx in polymer solar cells1. Optical spacer

2. Hole blocking and electron transport layer

3. Barrier to physical damage and chemical degradation

Redistributing the light intensity inside the device

- Adv. Mater, 18, 572 (2006)

Blocking hole transport to metal cathode and facilitate the electron transport.

- Adv. Mater, 19, 2445 (2007)

- Appl. Phys. Lett., 90, 163517 (2007)

- Appl. Phys. Lett., 92, 243308 (2008)

- Sol. Energy Mater. Sol. Cells, 92, 1476 (2008)

- Appl. Phys. Lett., 90, 163517 (2007)

1. Enhanced high temp. long-term stable PV devices with thermally stable TiOx


Polymeric TiOx 1. Methanol+ Deionized-water + Polymeric precursor ( Titanium isopropoxide + Acetic acid (CH3COOH) )

TiOx

Polymeric

: Hydrolysis and condensation sol-gel reaction

2. The reaction mixture was stirred by magnetic bar for 24 hours

Conventional TiOx

1. Titanium isopropoxide(TIP) + Isopropanol

2. Hydrolysis & Condensation in air

Ti-OiPr + AcOH -> iPrOAcE + Ti-OH (Hydrolysis)

Ti-OiPr + Ti-OH -> iPrOH + Ti-O-Ti (Condensation)

Objective : Fabrication of high efficient, cost effective, and durable polymer solar cells using thermally stable polymeric TiOx protecting layer.

TiOx

TiTi

OTi

OO

OTi

O

O….. …..

…..

…..

- -

++ +x

+

- Adv. Mater, 18, 572 (2006) -



Features of polymeric TiOx : Synthesized by simple sol-gel process, Stable for the air condition, Polymeric precursor (TTIP with acetic acid) can enhance the thermal

stability. ※FT-IR spectroscopy of the polymeric TiOx

: The absorption bands at 550, 660, 1030, 1425 and 1542cm -1 =>Stretching and Vibrations of Ti-O-Ti bond

TiOx is grown to linear polymer bridged by bidentate acetate ligand and shows thermally stable property.

Typical Synthesis

: Hydrolysis and condensation sol-gel reaction under presence of acetic acid from titanium tetra-isopropoxide

< Conventional TiOx : Thermally degraded (2.76% to 0.93% ) >

< Polymeric TiOx : Higher performance (2.30% to 3.02%) >

※Efficiencies depending on annealing Temp.



: The performance decreased due to unwanted morphology change and exciton doesn’t diffuse to Al : Aggregation of nano-particles (Trap sites)

Morphologies of conventional or polymeric TiOx films (By AFM)Conventional TiOx films (TTIP+Isopropanol)

Aggregated Nano-particles

Annealing

: The roles of TiOx interlayer (electron transporting and hole blocking) are not affected by operating temperature of the device.

Polymeric TiOx films (TTIP+Acetic acid+Methanol+Water)

Annealing

The devices with polymeric TiOx showed thermally

stable property from the continuous heating

< J. Phys. Chem. C, 113, 17268 (2009) >

※Efficiencies as a function of storage time in 80



Summary

1. Enhanced high temp. long-term stable PV devices with linear TiOx

Thermally stable TiOx protecting layer was introduced for durable and efficient

polymer solar cells by using simple sol-gel reaction.

=> TiOx develops into a linear polymer structure due to acetic acid (CH3COOH)

which acts as an excellent protecting layer from oxygen and humidity.

=> The device with thermally stable polymeric TiOx layer has high efficiency due

to enhancing interfacial contact b/w active layer and Al electrode by post

annealing process.

< J. Phys. Chem. C, 113, 17268 (2009) >


(a)

PEDOT:PSS

ITOGlass

PCBM / Intermixing zone / P3HTAl

PCBM rich in DMIntermixed zoneP3HT rich in CB

<Schematic diagram of device structure >

(b)

< SEM image of dissolved PCBM from dichloro-methane>

BHJ

1. Large D-A interfacial area

2. Low transporting property

Bilayer

1. Low D-A interfacial area

2. High transporting property

Intermixing zone

Hybrid type

< Large D-A interfacial area >< High transporting property >

Objective : Fabrication of solution processable bilayer polymer solar cells with large charge generation regions and high transporting property.

2. Poly(3-hexylthiophene) / [6,6]-phenyl C61-butyric acidmethyl ester bilayer

using solvent swelling effect


< Schematic diagram of intermixed zone >

SEM images and Auger spectroscopy of the concentration graded bilayers

< Depth profile of P3HT/PCBM bilayers (Auger spectroscopy)>

1. PCBM was well dissolved in DM and the layer can partially swell the bottom P3HT layer.

2. The -S- signal : Sharp increased from 60nm region -> Concentration gradient regions of the intermixed zone created form two different solvent

P3HT rich in CB100nm

PCBM rich in DM100nm

PEDOT:PSS

ITO

Intermixed zone

60nm

< Auger Electron Spectroscopy >

-S- signal




AFM/ SEM images of P3HT line pattern before and after solvent swelling



The P3HT/PCBM bilayer polymer solar cells with a concentration gradient can be successfully fabricated by simple spin-coating of two different solvent (chlorobenzene & dicholormetane) with swelling effect.

(a) (b)

(c) (d)

- SEM (a) and AFM (c) image of P3HT pattern without solvent contact. (73nm).

- SEM (b) and AFM (d) image of P3HT pattern after dichloromethane spin-coating with waiting time for 2min. (41nm)


< 2.3×10-6 torr >Area(cm2)

Voc Jsc FFEff. (%)

Reference (BHJ) 0.050 0.56 7.98 0.58 2.59Controlled

Cell(Bilayer)0.049 0.58 9.35 0.48 2.64

Jsc (from 7.98 to 9.35)-> charge carrier harvesting : P3HT rich and PCBM rich region will collect electron and hole more efficiently.

Concentration gradient regions of the intermixed zone -> Efficient Charge generation and transportation rather than single BHJ active layer

Device performance and band diagram of the bilayer device

< Appl. Phys. Lett. 95, 043505 (2009) >




Summary

The P3HT/PCBM bilayer polymer solar cells with a concentration gradient can be

successfully fabricated by simple spin-coating process using solvent swelling effect,

then the intermixed zone was detected by depth profile of Auger Electron Microscope(AES).

=> Polymer PVs with concentration graded bilayer showed better Jsc and the performances

compared to the device with a single BHJ due to efficient charge generation and transporting.

< Appl. Phys. Lett. 95, 043505 (2009) >




1. 2.


Novel Fabrication by Stamping Transfer Technique

- Active layer transfer by printing process

- Ratio controlled Single active layer versus double layer

Part 2


1. Spin-coated BHJ active layer in polymer solar cells

Motivation

Charge separation view : Easy process and high efficiency with low cost fabrication

Alan J. Heeger., Science., 317, 222 (2007) Alan J. Heeger., nat. photonics., 3, 297 (2009) O Ok Park., Sol. Energy Mater. Sol. Cells 92, 1181 (2007) O Ok Park., Appl. Phys. Lett., 92, 143504 (2008)O Ok Park., J. Phys. Chem. C., 113, 17268 (2009)

2. Spin-coating process have some drawbacks

It is not proper to fabricate photovoltaic modules with large areas

and not to deposit the BHJ active layer to specific regions of substrate.

Spin-coating process wastes a lot of solution materials and it takes

much times to figure out optimum donor and acceptor blending ratio.

3. Active layer transfer by printing process


3. BHJ PVs by imprinting transfer technique with UV-cured resin coated film

Previous research

K. C. Ho., J. Mater. Chem., 19, 4077 (2009)

: Fabrication of device through a stamping technique with PDMS stamp (The polymer film was completely transferred to the target substrate.)

: However, the mold need to non-destructive solvent treatment method or plasma to the top of PDMS surface. => To change the surface energy and cleaning the PDMS surface.

< Time consuming and complex process >

The objective :

1. Designing novel process : Imprinting transfer technique

2. The device with enhanced eff (%) : Insertion of polymeric TiOx b/w BHJ and Al electrode to increase charge transporting property.

BHJ PVs can be prepared via a simple imprinting transfer technique by using Norland Optical Adhesive 63 UV-curable resin based stamp without any surface treatment.

※ Advantages of imprinting technique : Simple and Cost effective process : Large active layer : Fabricate tandem or cascade polymer LED



(a) UV-curable resin coated PC film (b) Fabrication of device by stamping transfer technique

Features of NOA 63- Viscosity (25 ) : 2000cps- Refractive index of cured polymer : 1.56- Fast cure / Long shelf-life- Strong bonds to glass, metal, ceramics and plastics- Low shrinkage - low stress - Gap filling properties

Experimental Scheme

Enhancing EFF(%)


Novel process Design


2.2cm

2.2cm

Large area by

Stamping transferred BHJ

on the ITO substrate

Result and Discussion



Voc Jsc FFEff.(%)

Spin-coated BHJ 0.57 8.58 0.51 2.51

Spin-coated BHJwith TiOx

0.62 9.14 0.52 2.98

Imprinted transferred BHJ

0.60 7.17 0.52 2.22

Imprinted transferred BHJ

with TiOx0.57

10.34

0.54 3.19

J – V curves of devices with spin-coated BHJ(or with TiOx interlayer) and imprinted transferred BHJ

(or with TiOx interlayer).

The Rms value of the imprinted BHJ is much lower as 2.2nm : Jsc reduced from 8.58 to 7.17

Voc was increased from 0.57 to 0.60 due to better interfacial contact effect b/w electrode and BHJ from regular force of rubbing process.

After inserting of TiOx, the imprinted BHJ device shows higher Jsc and eff. Compared to the spin-coated BHJ with TiOx( At 1.6×10-6 torr )

Result and Discussion



Rms : 2.2nm Rms : 5.0nmITO glass

PEDOT:PSS

resin-coatedPC film

Active Layer

Hot Plate(90 )

Rubbing process(regular force )AFM images of the printed active layer morphology

++

+x+

--

Flat surface would help the formation of more uniform thickness of the TiOx interlayer (Stronger effect of TiOx interlayer)

: Maximize the Jsc and FF due to electron transporting and hole blocking property of TiOx interlayer.

< Imprinted transferred BHJ > < Imprinted transferred BHJ with TiOx >

< Org. Electron. In press (2010) >



4. Ratio controlled Single active layer versus double layer

Lift-off

Experimental Scheme

Motivation : To solve the low eff(%) problem of transfer-printed bilayer organic PVs by determining the optimum

blend ratio for each layer with a different P3HT / PCBM blend ratio

Contact Angle : 28°

UV-curable resin coated PC-film

BHJ active materialsBHJ active materials

[PDMS mold]

Contact Angle : 39°

Contact angle measurement (b/w PDMS mold and UV-PC film)

< UV-PC film has a better organic solvent wettability compared to PDMS surface. >

[ ]


Fabrication of the concentration graded BHJ bilayer polymer solar cells with P3HT/PCBM ratio control using printing transfer technique.


PCBM rich region

P3HT rich region

ITO glass

PEDOT:PSS

TiOx

Al

Ratio controlledPrinting transferred BHJ Bilayer device

< Device Scheme >


(A) The printed double BHJ with TiOx devices were well-worked without destroying preceding underlayers and showed high efficiency due to proper ratio control and intermixed zone.

(B) The device with printed bilayer from 8:2+2:8+TiOx showed the best performance of 3.24% rather than the single BHJ with TiOx (3.01%) after post annealing process due to efficient charge generation region.

(C) The optimum annealing temperature showed 150 for 30min (Higher than Tg of P3HT) (190 : Degradation of device / 70 and 110 : insufficient for reorganizing the active polymer )

P3HT:PCBM=2:8 in CBIntermixed zone

P3HT:PCBM=8:2 in CBSingle BHJ with TiOx

Fabrication of the concentration graded BHJ bilayer polymer solar cells with P3HT/PCBM

ratio control using printing transfer technique.


< Langmuir, In revision (2010) >


Summary

1. BHJ polymer solar cells can be made by an imprinting transfer technique

(novel process) with the help of UV-curable resin coated polycarbonate film (PC-film).

=> This imprinting transfer technique can easily fabricate the multilayer organic

optoelectronics without destroying the preceding underlayers.

=> The transferred BHJ active layer from UV-PC film was perfectly retained at 220nm

thick and crystalline structures on top of the PEDOT-PSS-coated ITO without any

pre-treatment. (cost-effective process)

Uniform surface morphology of the ratio-controlled double layer prepared from the

imprinting transfer technique can have more positive characteristics of enhancing the Jsc

and FF when the technique is combined with intermixed zone and TiOx systems.

< Langmuir, In revision (2010) >

<Novel Fabrication by Stamping Transfer Technique> - Active layer transfer by printing process - Ratio controlled Single active layer versus double layer

< Org. Electron. In press (2010) >


1. 2.

3. 4.


Efficient PV Devices with Controlled Nano-Structure

- Effect of ordered 2D-dot nano-patterned anode

- Enhanced charge collection via nano porous morphology

- Buffer layer modification by UV irradiation

Part 3


5. Effect of ordered 2D-dot nano patterned anode for polymer solar cell

Objective : To fabricate efficient polymer solar cell introducing well-ordered structure using simple imprint technique Increase in the interfacial area

Effective charge harvesting

※ Indium zinc oxide : High mechanical robustness, High conductivity, Low temperature

proccesability, and available for flexible solar cells electrode

Carrier collection loss : Separated charge carriers(electron) at D/A interface are collected at active/metal electrode interface. However, large amount of carrier loss is occurred during this process.

※Schematic of the fabrication sequence

PUA : Poly(Urethane Acrylate) UV-Curing


SEM images of patterned PUA film, IZO (anode), and Active layer

(c) 50nm pattern-IZO


(e) 50nm pattern-IZO-PEDOT:PSS-Active

(f) 200nm pattern-IZO-PEDOT:PSS-Active

Embossing structures were retained after spin coating process

Patterned PUA film

Glass

IZO

(a) 50nm pattern-PUA

(b) 200nm pattern-PUAIZO (300nm)

Patterned PUA filmGlass

(d-1) Cross-section of 200nm height pattern-IZO

(d) 200nm pattern-IZO



The enlarged contact area between the active layer and the embossing pattern assist an efficient carrier collection : Photocurrent density (Jsc increases from 7.03 to 8.98 mA/cm )

Well-ordered embossing anode structures can assist with extraction of electrons and holes as it will in some regions reduce the distance they have to travel to the electrode.

J-V curve of the devices and Diffuse reflectance spectra

< Org. Electron. 11, 285-290 (2010) >

Voc also related to the contact area b/w active and Al. (Influenced by the aspect ratio of pattern)

2


Summary


Highly ordered 2D-dot nano-patterned anode can enhance the device performance due

to the large interfacial area between both of the electrodes and the active layer.

=> Nano-patterned structures can efficiently harvest electrons and holes.

=> Increased optical absorption due to light trapping or scattering of reflected light

by the imprinted well-ordered pattern structures.

< Org. Electron. 11, 285-290 (2010) >


6. Enhanced charge collection via nano porous morphology

Motivation : Carrier collection at Cathode/active layer interface

Separated charge carriers(electron) at D/A interface are collected at active/electrode interface.

However, large amount of carrier loss is occurred during this process. Typically, buffers(interlayer) are used to solve this problems,

-Anode/active : PEDOT:PSS (Adv. Mater. 14,662,2002)

: Fluorene based polymer (Appl. Phys. Lett. 92023504,2008)

--Cathode/active : PEO (Adv. Mater. 20,2376,2008)

TiOx(Adv. Mater. 18,572,2006)

ZnO (Appl. Phys. Lett. 91,113520,2007)

Larger interfacial area between active/metal electrode interface can reduce this carrier loss. introduction of nano-pores on active layer surface

ITO

Cathode

Glass

Buffer layer

Active layer



Materials

PCBM

Electron Donor

P3HT

Electron Acceptor

CH3 C

CH3

CN

N N C

CH3

CN

CH3 2CH3 C

CH3

CN

. + N2

AIBN

AIBN : 2,2’-azobisisobutyronitrile

- widely used radical initiator- initiation Temp. : 65- decompose Temp : 107 - half life time

Ex.) 70 -> 4.8h 105 -> 3min

P3HT : poly(3-hexylthiophene)

PCBM : [6,6]-phenyl-C-61-buytyric acid methyl ester

PEDOT:PSS -> Poly(3,4-ethlyene dioxythiophene):poly(styrene sulphonate)



Experimental procedure

Encapsulation&

Evaluation

Pre-annealing105 10min, N2



Result and discussion

-0.2 0.0 0.2 0.4 0.6 0.8

-8

-6

-4

-2

0

2

Cu

rre

nt(

mA

/cm

2)

Bias(V)

P3HT: PCBM = 1 : 0.6 P3HT: PCBM: AIBN = 1 : 0.6 : 0.05 P3HT: PCBM: AIBN = 1 : 0.6 : 0.1 P3HT: PCBM: AIBN = 1 : 0.6 : 0.3

FF & Jsc: Increased interfacial area between active layer and metal electrode -> carrier loss -> carrier collection -> FF and Jsc

However, when AIBN is mixed over 0.1wt%, Jsc and FF is decreased. -> residual AIBN contents in the active layer hinder the carrier transport

I-V curve

Glass

PED

OT

:PSS

Metal C

athodeM

etal Cathode

Metal C

athodeM

etal Cathode

Incident light

Diffused light

Diffuse reflectance spectra

Lower reflectivity indicates stronger absorption of incident light

Reflectivity difference is negligible

Change in Light absorption is not a reason of FF and Jsc increase

∵ Scattering media size is too small than wavelength of light.

400 500 600 7000

5

10

15

20

25

30

Refle

ctan

ce(%

)

Wavelength(nm)

P3HT:PCBM = 1:0.6

P3HT:PCBM:AIBN = 1:0.6:0.1



Result and discussion : Effect on charge transporting- XRD & PL spectra

500 550 600 650 700 750

P3HT P3HT:AIBN=1 : 0.05 P3HT:AIBN=1 : 0.1 P3HT:AIBN=1 : 0.3

Inte

ns

ity

(a. u

.)Wavelength(nm)

- Crystallinity of the active layer is decreased as AIBN contents as increased.

- 2θ ≈ 5°intensity Crystallinity

Carrier transport

- P3HT interchain stacking PL self-quenching PL intensity (Nat. Mater. 5,197(2006) J. Mater. Chem. 18,306,(2008))


Metal


< Appl. Phys. Lett. Accepted (2010) >

TEM Image and Summary

< 2~5nm nano-pores produced on active layer surface >

1. Polymer solar cell with nano-porous active surface was fabricated.

2. Nano pore on photo-active layer enlarge the surface area between active layer and metal cathode (no change in light absorption, decrease in charge transport)

charge collection enhancing

Jsc , FF

overall PCE was increased.


7. Buffer layer modification by UV irradiation

Motivation : Carrier collection at anode/active layer interface

ITO

Cathode

Glass

Buffer layer

Active layer

* PEDOT:PSS -> Poly(3,4-ethlyene dioxythiophene):poly(styrene sulphonate)

* DMSO -> dimethyl sulfoxide

PEDOT:PSS is the most famous buffer layer on modifying the interface

between ITO and active layer because of its high transparency in the visible

range, excellent thermal stability, and processibility in aqueous solution.

However, some of the holes nonetheless cannot be transported to the anode

owing to the high bulk resistance and/or improper contact condition of the

PEDOT:PSS.

- PEDOT:PSS + EG (Adv. Funct. Mater. 15, 203, 2005 )- PEDOT:PSS + DMSO (Synth. Met. 126,311,2002)

- PEDOT:PSS + Mannitol (Appl. Phys. Lett. 90, 063509, 2007)

- ITO /AgOx / PEDOT:PSS (Appl. Phys. Lett. 92, 013306, 2008)



Buffer layer modification : Conformation change of PEDOT : PSS can be induced by external energy.

UV source : Output power density of 544μW/cm2

centered at 365nm

1. UV-Ozone : degradation of the aromatic rings2. O2 plasma : etching of the film surface3. UV : no degradation and etching

a. Device resistanceb. Work function

-> Effect of UV irradiation

Structure of PEDOT:PSS

-Work function : 4.8~5.2ev-Boling point : approximately 100

* PEDOT:PSS -> Poly(3,4-ethlyene dioxythiophene):poly(styrene sulphonate)



Device performance

- UV irradiation -> lower Rs -> Jsc, FF increase

- The effect of UV irradiation is saturated after 40min

Rs,total = Rs,bulk + Rs,interf

0.0 0.2 0.4 0.6

-10

-8

-6

-4

-2

0

Cur

rent

(mA

/cm

2)

Bias(V)

Pristine PEDOT:PSS with UV 20min with UV 40min with UV 60min

Voc Jsc FF η

1 0.608±0.003

9.10±0.16 54±1.2 3.05±0.04

2 0.614±0.007

9.20±0.11 57±1.0 3.29±0.08

3 0.612±0.006

9.89±0.28 56±1.5 3.46±0.04

4 0.616±0.004

9.83±0.17 57±1.4 3.50±0.03



Effect of UV irradiation on PEDOT:PSS

Benzoid

(1445cm-1)

Quinoid

(1422cm-1)

Coil-structureExpanded-coil of linear

structure

-> lower surface resistance

Raman spectroscopy

1. Reduction of bulk resistance Structure of buffer layer(PEDOT:PSS) can be changed by UV exposure

Surface resistance decreased (0.72MΩ/sq -> 0.39 MΩ/sq)

1400 1450 1500

Ranam shift(cm-1)

Pristine PEDOT:PSS UV irradiated PEDOT:PSS



Effect of UV irradiation on PEDOT:PSS 2. Reduction of interfacial resistance

-> interface resistance between PEDOT:PSS and active layer is reduced

(a) AC impedance spectroscopy

Re of devices is decreased : 0.55MΩ -> 0.21MΩInterfacial resistance

device resistance

Jsc

0.0 0.2 0.4 0.60.0

0.1

0.2

0.3

-Z'' (M

)

Z'(M)

Pristine PEDOT:PSS with UV 20min with UV 40min with UV 60min

3. Work function-> work function of PEDOT:PSS is increased about 0.25ev.

0 20 40 60 80 100

5.00

5.05

5.10

5.15

5.20

5.25

Wor

k fu

nctio

n(eV

)

UV irradiation time(min)

P3HT

5.2

4.8

≒5.2

-Work function change of PEDOT:PSS layer does not make the energy barrier of hole transport from active layer to buffer layer



1. PEDOT:PSS buffer layer is modified by UV irradiation.

2. Polymer solar cells(PSCs) used modified buffer layer showed enhanced Jsc, FF due to decrement

of bulk and interfacial resistance of PEDOT:PSS by UV irradiation.

4. Work function change of buffer layer dose not effect on the performance of PSCs because it

doesn’t make energy barrier for hole transport.

3. Surface roughness change was negligible by UV irradiation

Jsc, FF change depend not on roughness effect but on device

resistance reduction

Summary

< Org. Electron. 10, 1641-1644 (2009) >


Conclusion

Polymer Photovoltaic Cell

ITO or IZO (Anode)

Cathode (Al)

Glass

Buffer layer(PEDOT:PSS)

Active layer(Model case : P3HT/PCBM)

7. Modified PEDOT:PSS by UV irradiation.

5. 2D-dot patterned anodeITO or IZO (Patterned Anode)

1. Linear polymeric TiOx

2. Concentration graded spin-coated bilayer (solvent swelling)3. Single layer stamping active layer (BHJ)

4. Bilayer stamping active layer (BHJ)

6. Spin-coated nano-porous active layer (BHJ)

< Current >


Center for Advanced Flexible Display Convergence supported by NRF

Brain Korea 21 Project supported by Ministry of Education, Science and Technology

Center for World Class University supported by NRF

Acknowledgements


Thank you for your attention !Thank you for your attention !

Polymer Nano-materials Lab.

Research area

http://stereo.kaist.ac.krProf. J.H. Park Dr. S.H. Im

- Polymer Solar Cell & PLED

- Nano-patterning

& Soft-lithography

- Metal Nano-particles (Au, Ag)

- Colloidal Self Assembly

- Flexible Display

- Polymer Nano-compositse


1. 2.

3. 4.

5. 6.


Thank you for your Thank you for your attention!!attention!!

polym. nano mater. lab efficient photovoltaic devices with solution process, stamping transfer...

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