Semitransparent Organic Solar CellsTam Hoi Lam, Wu Zhenghui and Furong Zhu

Department of Physics and Institute of Advanced M t i lMaterials

Hong Kong Baptist University

UNSW, 30 Aug, 2012

Outline� A glimpse of photovoltaic technologies

Outlineg g

� ITO-based transparent cathode

� Optical enhancement in semitransparent

organic solar cells

� Applications of semitransparent OSCs

� Summary

2

Photovoltaic technologiesPhotovoltaic technologiesc-Si, poly c-SiIII-V semiconductor (GaAs InP

BulkBulkIII V semiconductor (GaAs, InP

Thin film c-Si, a-Si, hybrid SiII-VI semiconductor film (CdTe)

Solarcells

Thin FilmThin Film( )

CuInSe2, Cu(InGa)Se2

cellsOrganicOrganic

Solid dye-sensitized solar cellsOrganic/inorganic hybridOrganic (polymer & small molecule)

NanoNano--t t dt t d

Nano-particle, nano-wire & quantum dot-based solar cellsstructuredstructured based solar cells

3

Si & thin film solar cellsSi & thin film solar cells

4

Other PV technologiesg

TypeBenefit or intended benefit (all are light weight

d t ili )

Efficiency at which it is i bl

Challenges

compared to silicon) viableCIGS High efficiency at low cost, long life,

transparent , no disposal problems10-15% i.e. now Price of indium

F i l hi h ffi i (b l CIGS 10 15% i Cd i i

CdTe

Fairly high efficiency (below CIGS,above others). Well

10-15% i.e. now for buildings

Cd is a poison, controlled disposal only, so not allowed in consumer goods

t t h t i tetc, tough to print,supply of Te?

DSSCTolerant of polarized light, low level light, transparent, extreme angle of

10-15% i.e. now LiquidsPrice of ruthenium?DSSC

incoming light, choice of colors, no disposal problems

OSCLow cost, huge materials varieties, tunable electronic/optical properties

8-10% Cost, efficiency, stability narrow

5

OSC tunable electronic/optical properties stability, narrowspectrum

Limitations in OSCsNarrow absorption band

0.8

P3HT:PCBM (1:0 8)

Solar spectrum and available photocurrent

0.4

0.6

orpt

ion

(a.u

.)

P3HT:PCBM (1:0.8)

300 400 500 600 700 8000.0

0.2

Abs

o

• It limits the absorption and thus short circuit current density• Low carrier mobility ( 10-6 to 10-3 cm2/Vs)

Wavelength (nm)

• Low carrier mobility (~ 10-6 to 10-3 cm2/Vs)– Active layer cannot be too thick <300nm– Fill factor affected due to the large series resistance

6

Limitations in OSCsLimitations in OSCsInefficient utilization of the solar energy by current available materials.

4

6

Outx 10

-18 )

0.15

0.20

t

FF < 0.7 Voc < 0.65 VMe

O

O

2

4

Elec

trons

O

Phot

ons

In (x

0.05

0.10

Pow

er O

u

Pow

er In

14 3% ~ 5 %P3HT

PCBMS n

500 1000 1500 2000 2500 3000 35000

Wavenlength (nm)500 1000 1500 2000 2500 3000 3500

0.00

Wavelength (nm)

14.3% 5 %

Possible approaches:• Low band gap photoactive materials • Tandem cells

7

Tandem cells• Light in-coupling features , light harvesting

Advantages of OSCsAdvantages of OSCs

• Solution processable, lightweight, translucent & flexible, potential low cost Power generation windows

• Roll to roll coating technologies could realize scalable production processscalable production processwith high productivity

• Less energy consumptionArchitectural surfaces

• Less energy consumptionduring fabrication process, less CO2 emission.

8 Flexible, transparent & decorative

Motivation and challenges• Perceived benefits of OSC technologies

L hi h l fl ibili li h i h

ot at o a d c a e ges

– Lower cost, higher volumes, greater flexibility, light weight,

size and shape

– New device concepts inverted translucent– New device concepts, inverted, translucent

– The best research OSC efficiency approaching 10%

• Technical challengesTechnical challenges– New semiconducting and conducting materials with tunable

electronic and optical properties

– Efficient charge collection properties at organic/electrode interface

– light harvesting solution processing stability thin filmlight harvesting, solution processing stability, thin film encapsulation…

9

Conversion stepsConversion step Incident photons Loss mechanism

- Reflection, Transmission- Spectral mismatch

- Exciton recombination

Light absorption

E citon generation Exciton recombination

- Excition transfer with recombinations - No charge separation and subsequent

bi ti f it

Exciton generation

Exciton diffusionrecombinations of excitons

- Recombination of charges- Limited mobility of charges

Charge separationy g

- Recombination near electrodes- Energy barrier at electrodes

Charge transport

Separated charges at electrodes10

OSC is a thin film systemOSC is a thin film systemJ=2

N2=n2-ik2E1+

E +

J=3N3=n3-ik3

E +

J=M-1NM-1=nM-1-ikM-1

E + E +

J=2N2=n2-ik2E1+

E +

J=3N3=n3-ik3

E +

J=M-1NM-1=nM-1-ikM-1

E + E +Incident

E1-E2+

E2-

E3+

E3-

E3+

E3-

EM+

EM-E1-E2+

E2-

E3+

E3-

E3+

E3-

EM+

EM-

light

J=1N1=n1-ik1

OPV

J=MN=n0

J=1N1=n1-ik1

N=n0

P3HT:PCBM

Cathode (Ca/Ag)

A

V

PEDOT:PSS

P3HT:PCBM

Cathode (Ca/Ag)

A

V

PEDOT:PSS HT

T:PS

S

O

Cat

hode

(Ca/

Ag)

4.2eV

3.7eV

3.0eVe-

HT

T:PS

S

O

Cat

hode

(Ca/

Ag)

4.2eV

3.7eV

3.0eVe-

Glass substrate AirAir

Schematic representation of a thin film OSC

GlassITO

Light

GlassITO

Light4.8eV

P3H

PCB

M

PED

OT

ITO

5.2eV4.9eV

4.2eV

6.1eV

P3H

PCB

M

PED

OT

ITO

5.2eV4.9eV

4.2eV

6.1eV

OCH3

e-

S n

3

O

P3HT PCBM

11

Optical constants

1.55

1.60

0.08

0.10E

xtincex, n

n_PEDOT k_PEDOT

Optical constantsAirAir

1.45

1.50

0 02

0.04

0.06

ction coefficienRef

ract

ive

ind

300 400 500 600 700 800 900 10001.40 0.00

0.02 nt, k

Wavelength (nm)

1 0 1 0

SubstrateSubstrate

2.0

2.2

0.6

0.8

1.0

Extinctionin

dex,

n

n_P3HT:PCBMk_P3HT:PCBM

2.4

2.8

0.6

0.8

1.0

Extinction

e in

dex,

n

n_CuPc:C60 k_CuPc:C60

1.6

1.8

0.2

0.4n coefficient, k

Ref

ract

ive

i

1.6

2.0

0 0

0.2

0.4

n coefficient, k

Ref

ract

ive

12

300 400 500 600 700 800 900 10000.0

k

Wavelength (nm)

300 400 500 600 700 800 900 10000.0

Wavelength (nm)

Interlayer:Interlayer:1) Thin metal layer Catho

deEBL

• Discontinuous and high reflectivity in NIR

2) Oxide/organic interlayer

Anode contact

SC Blend

) O de/o ga c e aye

• Limited lateral conductivity

3) TCO electrode is suited for application in Transparent SubITO

PedotSC Blend

RZ

3) TCO electrode is suited for application in

• Organic, inorganic and hybrid solar cells

• Simple, effective and low cost technology

• Greater fabrication flexibility

13

High performance TCO (ITO)• Low resistance, high transparency with smooth surface

C ibili f d i f b i i i h d h

High performance TCO (ITO)

• Compatibility of device fabrication with no damage to the

underlying functional organic layers

• Excellent delineation and adhesion capabilities

• Size, shape and substrate flexibility

• Lower cost and higher volumes

• Emerging new applications:Emerging new applications:

Flexible electronics including FOLED displays, lighting,

OSC organic sensorsOSC, organic sensors …

14

Low processing temperature ITOp g p

5

6

6

7 Sheet resistivity Mobility Carrier concertration 70

3)

3

4

4

5

resi

stiv

ity(E

-04O

.cm

)

40

50

60

Mobility(cm

2/V.son

certr

atio

n(E+

20/c

m

1

2

0 1 2 3 4 52

3

hydrogen partial pressure(E 3Pa)

shee

t r

20

30

s) c

arrie

r co

hydrogen partial pressure(E-3Pa)

80

100

(%)

Cut off has a blue shift

20

40

60

Tran

smia

ttanc

e

H2 0 Pa (Eg: 3.87eV)

H2 1.1 x10-3Pa (Eg:3.92eV)

H 2 0 x10-3 Pa (E : 4 01eV)

Cut-off has a blue shift

15

200 400 600 800 1000

0

W avelength (nm)

H2 2.0 x10 Pa (Eg: 4.01eV)

H2 2.6 x10-3 Pa (Eg: 4.01eV)

Low processing temperature ITO• Deposition at a temp < 60OC

Low processing temperature ITO

• TCO (ITO, AZO, IZO)/PC & PET etc.

• T(� ) over 85%

R ~ 25 / ~130 f ITO• Rs~ 25 �/sq, ~130nm, for ITO

• Smooth surface, rms < 1.0 nm

• Low stress• Low stress

• High etching rate

• No damage to the underlying organic/polymerNo damage to the underlying organic/polymer materials

• Applications: top-emitting OLEDs, FOLED, t d & it tOSC i t t d

16

tandem & semitransparent OSCs, integrated org electronic devices …

St k d OSC 1 8 2 0Stacked OSCs1.01.21.41.61.8

1 0

1.5

2.0

coef

ficie

nt (a

.u.)

nce

(Wm

-2nm

-1)

AM1.5 Global P3HT:PCBM CuPc:C60Top Cell

Bottom Cell

Top cell

Cathode

0 00.20.40.60.8

0 0

0.5

1.0

rmal

ized

abs

orpt

ion

Spec

tral I

rradi

an

Bottom Cell

Interlayer300 400 500 600 700 800 90010001100

0.0 0.0

No

W avelength (nm)

SubstrateITO

� An increased Voc, photocurrent matching required

� Thinner active layer for better charge transport

E i l

17

� Extensive spectral response

Integrated absorptancein

nm

e in n

m

e in n

m

e in n

m

e

Absorptance contour mapping

60e (%

)

3%

ITO/PEDOT:PSS/P3HT:PCBM/Ca/Ag

teg ated abso pta ceer

thi

ckne

ss

Abs

orpt

anc

er t

hick

ness

Abs

orpt

anc

er t

hick

ness

Abs

orpt

anc

er t

hick

ness

Abs

orpt

anc

40

50

abso

rpta

nce

Ca/AgCa/Ag40%

53%

Laye

Laye

Laye

Laye

Wavelength (nm)

10

20

30

grat

ed li

ght P3HT:PCBM

PEDOT:PSS

ITO

Glass

P3HT:PCBM

PEDOT:PSS

ITO

GlassIntegrated absorptance of P3HT:PCBM layer:

0 50 100 150 200 250 300

10

Inte

g

P3HT:PCBM thickness (nm)���

��

����

dF

dFAA PCBMHTP

PCBMHTP)(

)()()( :3

:3

• Maximum absorption for P3HT:PCBM based OSCs occurs at 75nm, 200nm and 250nm.

• Oscillation absorption behavior � interference effect• Oscillation absorption behavior � interference effect.

18

Light distribution1.4

PSS

rAlBM

Os(%)

0.8

1.0

1.2

PED

OT:

P

AiA

P3H

T:PC

B

ITO

Gla

ss

nsity

, |E

|2

Interference40

50

60ab

sorp

tanc

e 53%

40% Cat

hode

0 2

0.4

0.6

Opt

ical

den

Interferenceeffect dominates

10

20

30

egra

ted

light

1.41.6

Air

Gla

ss Al

CB

M

T:PS

S

ITO 1.6

1.8

AirAl

CB

M

T:PS

S

ITO

Gla

ss

-100 0 100 200 300 400 500 6000.0

0.2O

75nm0 50 100 150 200 250 300Inte

P3HT:PCBM thickness (nm)

0.81.01.21.4 G

P3H

T:PC

PED

OT

tens

ity, |

E|2

0 81.01.21.4

P3H

T:PC

PED

OTG

200250nm

Cat

hode

Cat

hode

0.20.40.6

Opt

ical

int

0.20.40.60.8 200nm250nm

19 Appl. Phys. Lett. 90 (2007) 103505.

-100 0 100 200 300 400 500 6000.0

Position, x (nm)-100 0 100 200 300 400 500 600

0.0

Semitransparent OSCsSemitransparent OSCs- two competing performance indexes: T(�) & A(�)

Trans 50 50

In

P3HT:PCBM

interlayerITO

�40

45

50

40

45

50 tegrated lignce

(%)

Total transmittance Integrated absorptance

Glass

ITOPEDOT:PSS

P3HT:PCBM �P3HT:PCBM

30

35

30

35

ght absorptTran

smitt

a

0 20 40 60 80 100 120 140 16020

25

20

25

tance (%)

T

ITO cathode thickness (nm)

• ITO acts as refractive index matching layer to enhance the T(�)

• ITO improves the lateral conductivity of the top cathode

ITO cathode thickness (nm)

20

Appl. Phys. Lett. 90 (2007) 103505.

Semitransparent OSCsDevice characteristics

15

cm2 ) 200nm P3HT:PCBM (A)

75nm P3HT:PCBM (B)Semitransparent OPV (C)

60

7063%

- Device characteristics

10

ensi

ty (m

A/c Semitransparent OPV (C)

30

40

50

40%

59%

PC

E %

0

5

Cur

rent

De

0

10

20

30

200nm P3HT:PCBM (A)75nm P3HT:PCBM (B)Semitransparent OPV (C)

IP

0.0 0.1 0.2 0.3 0.4 0.5 0.60

Voltage (V)

Devices V (V) J (mA/cm2) FF% PCE%

300 400 500 600 7000

Wavelength (nm)

Devices Voc (V) Jsc (mA/cm2) FF% PCE%A 0.57 14.77 34 2.8B 0.57 8.22 43 2.0C 0 54 6 67 51 1 83C 0.54 6.67 51 1.83

21

Semitransparent OSCs: ITO/Z P C /C /BPh /A /ITO(40 )ITO/ZnPc:C60/C60/BPhen/Ag/ITO(40nm)

(b)

10

20 Dark Semitransparent cell Control cell

cm2 )

50

60

70%

)

Calculated Measured

(a)

IEEE J, Selected Topics in Quantum 10

0

10

t den

sity

(mA

/c

20

30

40

50

ansm

ittan

ce (%

Electronics, 16 (2010) 1685.

LiF

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-20

-10

Cur

rent

Voltage (V)300 400 500 600 700 8000

10

Tra

Wavelength (nm)

Device�VOC��

(V)

FF��

(%)

JSC

(mA/cm2)

��

(%) 1.0

1.5LiFBphen

(V)� (%)� (mA/cm ) (%)

Control�device� 0.56� 60�±1� 11.8 ±�0.3 4.0�±�0.2

Semitransparent�

photovoltaic�cell�0.55� 57�±�1� 9.6�±�0.2� 3.0�±�0.1�

0.5

TOc:C

60

|E|2

220 50 100 150 200 250 300

0.0 Top

I

Ag

C60

ZnP

c

ITO

Position (nm)

Semitransparent OSCs: integrated OSC/OLEDSemitransparent OSCs: integrated OSC/OLED

Transparent cathode

TOPDOLED

TOLED

transparent cathode

TOPD

Reflective cathode

Transparent cathode

interlayerOLED

OSC

Al

0 8

1.0

unit) OPV IPCE

MOLED ELa)

60

80ITO anode

Transparent substrate

ITO anodeTransparent substrate

ITO anode

Type 1 Type 2

ITO/glassOSC

0.2

0.4

0.6

0.8 IPC

E (%

)

D in

tens

ity (a

rb.

20

40

60Type-1 Type-2

TOLED

Vs

OPD

Vs

OSCTOLED

ZnPc:C60-OSC/interlayer/Alq3:C545-OLED

300 400 500 600 700

0.0OLE

DWavelength (nm)

0

TOLED

RVIn

OPD

TR

VInT

TOLED

60 y

23

Org. Electron. 12 (2011) 1429

ITO/OSC-ZnPc:C60/interlayer/MOLED-Alq3:C545ITO/OSC ZnPc:C60/interlayer/MOLED Alq3:C545

Device EL peak position (nm)

FWHM(nm)

Luminous efficiency (cd/A)

Control OLED 526 32 10.5 (@ 4.5V)

OSC/OLED 521 33 11 5 (@ 4 5V)OSC/OLED 521 33 11.5 (@ 4.5V)

Device VOC (V) FF (%) JSC (mA/cm2) PCE (%)

Control OSC 0.56 58.9 11.4 3.76

OSC/OLED 0 55 57 2 10 5 3 30

Org. Electron. 12 (2011) 1429

24

OSC/OLED 0.55 57.2 10.5 3.30

SummarySummary• Semitransparent OSCs

– Optical admittance analysis

– Transparent electrode and optical coupling

M l i j i OSC WOLED li h i– Multi-junction OSCs, WOLED lighting,

optical sensors etc.

A li ti• Applications– Power generation windows

A hit t l f– Architectural surfaces

– Car windshield

Tension membrane

25

– Tension membrane

Thank you

26