Plastic Solar Cells: current status and future

prospects

Bernard Kippelen, Neal R. Armstrong, and Seth Marder

Optical Sciences Center, and Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA

Alvin Kwiram Symposium

Seattle, June 24, 2003

NREL, ONR, NSF

Collaborators

Kippelen Group Armstrong Group Marder Group

Benoit Domercq Britt A. Minch Steve Barlow

Seunghyup Yoo Wei Xia Yadong Zhang

Carrie Donley

Chet Carter

Prof. David O’Brien, deceased

Metal deposition on plastics from solution, micro-size Metal deposition on plastics from solution, micro-size features using soft lithography and transfer features using soft lithography and transfer

Low temperature processing Low temperature processing of organic semiconductors, of organic semiconductors, metals and dielectrics on metals and dielectrics on flexible substrates: low cost flexible substrates: low cost ($0.01)($0.01)

Organic ElectronicsOrganic Electronics

A Complementary Material PlatformA Complementary Material Platform

OLEDsOLEDs

Light emission

High luminescence efficiency

Low mobility OK

Photo-stability

Light-weight flexible substrate, barrier to O2 and H2O

OFETsOFETs PVPV

High mobility

Very thin OK

No condition on optical absorption

High mobility

High photo-generation efficiency

Absorb visible spectrum

Photo-stability

Memories

Electro-optics

Lasers and amplifiers

A convergence of new material technologies for the A convergence of new material technologies for the development of Application Specific Integrated Plastic Chips development of Application Specific Integrated Plastic Chips

(ASIPC)(ASIPC)

Light-weight, high versatility, low cost, large area

Technology OpportunitiesTechnology Opportunities

Efficiency

Low cost

Flexibility

Military

Consumer

Com

mer

cial

Low cost scanners

Optical isolators

Devices that take advantage of the integration of photodetectors on light-weight flexible substrates

Outline

• Introduction to photovoltaic technologies

• Organic excitonic solar cells

• Requirements for conversion with high efficiency

• An approach based on self-assembly

HOMO

LUMO

Electrode

Electrode

Semiconductor

I

V

max SC OC

solar solar

P I VFF

P P

max max

SC OC

I VFF

I V

Solar cell parameters

short-circuit current ISC

open circuit voltage VOC

fill factor FF

Evolution of PV Technologies

AM 1.5 G, 25 C, 1 sun = 100mW/cm2[A.M.: air mass; G: global, direct + scattered; angle of 48.2, zenith angle (sec(48.2) = 1/cos(48.2) = 1.5)]

SiliconOrganic (Tang)

Organic (Forrest)

Organic Graetzel

Polymer (Shaheen)

Hybrid nanorods

Open circuit voltage (Voc; V) 0.7 0.45 0.5 0.721 0.82 0.7Short circuit current density (Jsc; mA/cm2) 43 2.3 18 20.53 5.25 5.7Voc x Jsc (mW/cm2) 30.1 1.035 9 14.80213 4.305 3.99Fill factor (FF) 0.8 0.65 0.4 0.704 0.61 0.4

Illumination intensity (mW/cm2) 100 75 100 100 106.2 96.41 sun = 100 mW/cm2

Conversion efficiency (%) 24.08 0.9 3.6 10.4 2.5 1.7

State-of-the-art in organic photovoltaics

Grätzel cell (liquid electrolyte, solid)

Small molecules (bi-layers)

Polymer blends (interpenetrated networks)

Hybrid approaches (Inorganic sc doped in organic matrix)

500 1000 1500 2000 25000

20

40

60

80

Maximum Current Density Availablein 1 Sun @ AM 1.5G

J

( m

A/c

m2 )

Wavelength ( nm )

Jtotal current density integrated from 0 to maximum available value, assuming 100% EQE.

0

1

2

3

4

5020403a

Sp

ect

ral P

ho

ton

Flu

x D

en

sity

( 1

014 p

ho

ton

s/se

c-cm

2 )

The ChallengeThe Challenge

Harvesting the solar spectrum and…

optimize absorption, charge generation, charge collection: photocurrent

optimize relative energy levels: built-in voltage

optimize electrical characteristic: fill factor

…maintain simultaneously high open circuit voltage and high fill factor

Step #1: Achieve efficient dissociation of excitons in organic materials

Overcome exciton binding energy

vacuum

electrode

electrode

HOMO

LUMO

Single layer

vacuum

Double layer

Ansatz: the maximum value for Voc is the smallest band gap he maximum value for Voc is the smallest band gap minus the exciton binding energy (0.5 eV)minus the exciton binding energy (0.5 eV)

500 1000 1500 2000 25000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

897

Maximum Available Jsc

Voc

Productfor a given cut-off wavelength

( 1.38 eV )

J sc/1

00

(m

A/c

m2 ),

no

rma

ilize

d J

sc V

oc

Cut-off Wavelength (nm)

Jsc

xVoc

normalized by

100mW/cm2

Jsc

available between 0 and (nm)

0.0

0.5

1.0

1.5

020603c

Voc available for Eg corresponding to Minimum band offset 0.5eV assumed at the junction V

oc

( V

)

Optimum harvesting:

400 – 900 nm (1.4 eV)

Maximum efficiency:

= 28% x FF

500 1000 1500 2000 25000

5

10

15

20

25

30

021003b

Effi

cien

cy (

%)

Cut-off Wavelength (nm)

FF = 1.0 FF = 0.8 FF = 0.7 FF = 0.6 FF = 0.5 FF = 0.4

4 3 2 1 0.5

Energy Gap ( eV )

High Efficiency: Maximize Light Harvesting and Fill High Efficiency: Maximize Light Harvesting and Fill FactorFactor

Equivalent Circuit Model

)(1)exp(

1

10 AR

VJ

nV

AJRVJ

RRJ

Pph

T

S

PS

ARJ

V

J

JVnV

P

OCphTOC

00

1ln

00

)1(1lnJ

J

R

R

J

J

A

nVRJ SC

P

SphTSSC

OPEN-CIRCUIT VOLTAGE

(J=0)

SHORT-CIRCUIT CURRENT DENSITY

(V=0)

Understanding Key Factors for Efficient Organic Photovoltaic Cells

Finite conductance of materials and contact resistance : nonzero Rs

Leakage path : finite Rp

What determines the fill factor ?

0.0 0.2 0.4 0.6 0.8

-30

-20

-10

0

10

20020603b

0.71,14.5%

0.81,16.8%

0.60,12.3%

FF=0.42,=8.3%

J (

mA

/cm

2 )

Voltage ( V )

RpA = 50 cm2

100 200 500 5000 50000 500000

Jph

= 26 mA/cm2, J0 = 30 pA/cm2

n = 1.5, under 1 Sun ( AM 1.5G )

Effects of RpEffects of Rp

102 103 104 105 1060.95

0.96

0.97

0.98

0.99

1.00

021003c

Effect of Rp on V

oc at Different Photocurrents

Jph

= 26mA/cm2

Jph

= 2.6mA/cm2

J0 = 30pA/cm2, n=1.5

Voc

normalized by the value corresponding to R

p=infinite

No

rma

lize

d V

oc(R

p)

RpA ( cm2 )

0.0 0.2 0.4 0.6 0.8-35

-30

-25

-20

-15

-10

-5

0

5020603a

0.75,15.6%

0.64,13.3%

0.54,11.1 %

FF=0.34,=7.1%

0.81,16.8%

J0 = 30 pA/cm2

Jph

= 26 mA/cm2

n = 1.5

Cur

rent

Den

sity

( m

A/c

m2 )

Voltage ( V )

J(RsA=0 cm2)

J(RsA=2 cm2)

J(RsA=4 cm2)

J(RsA=6 cm2)

J(RsA=8 cm2)

J(RsA=10 cm2)

J(RsA=20 cm2)

Effects of REffects of Rss

A need for high mobility materials

Self-Assembly: a Path for Controlled Morphology in Self-Assembly: a Path for Controlled Morphology in Wet Processed MaterialsWet Processed Materials

Adapted from D. Haarer

Existing other approachesC12H25

C12H25

C12H25

C12H25

C12H25

C12H25

NN

O

O

O

O

K. Müllen, R. Friend et al. Science, 293, 1119, (01)

Mixtures of HBC and perylene

Voc = 0.69 V; FF = 0.4; Jsc = 30 A/cm2

Saturation for illumination > 1 mW/cm2

Choice of hexabenzocoronene (HBC) driven by large core that

can lead to large mobility

2max 3 exp( 83/ ) cm / Vsn

Number of atoms (C,O,N) in the core

John Warman, Adv. Mater. 13, 130 (01)

Our material’s choice: phthalocyanines

NH

NN

NH

N N

N

N

Good Thermal stabilityGood Thermal stability

Strong Molar absorptivityStrong Molar absorptivity

Good Light stabilityGood Light stability

NH

NNNH

N N

N

N

O

O

OO

OO

O

O

Skoulios et al. J. Am. Chem.Soc. 1982, 104, 5245-5247

e-

e-

e-

e--e

-e

-e

Alkoxy substituted Pc known to form discotic hexagonally ordered mesophases.

Problems: KI > 350°C, difficult to align, no photocurrent when combined with PTCDI

N

NNN

N N

N

N

O

O

O

O

OO

OO

OO

OO

OO

OO

Cu

Molecular design

Core:

Metal:

Arms:

Tuning of spectroscopic and electronic properties

Provides large core for strong -orbitals coupling and cohesive forces through Van der Waals interactions

Influence the solid-to-mesophase (KDh) and mesophase-to-isotropic liquid (DhI) transition temperatures

NNN

N

N N

N

N

S

O

S

O

S

O

S

O

SO

SO

SO

SO

CuN

NNN

N N

N

N

O

O

O

O

O

O

O

O

OO

OO

OO

OO

Cu

S-Et-O-Bz CuPcO-Et-O-Bz CuPc

Molecular optimization

KDh 111°C; DhI > 400°C; difficult to process

KDh 134°C; DhI 320°C; easy to process into thin films by spin-coating (chloroform)

Optical properties

400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Opt

ical

Den

sity

Wavelength (nm)

Film

0.0

2.0x105

4.0x105

6.0x105

8.0x105

Solution

Mol

ar E

xtin

ctio

n C

oeffi

cien

t

Material CharacterizationMaterial Characterization

3 4 5 6 70

2000

4000

6000

8000

10000

12000

14000020303a

XRD of Spin-Coated Films on ITO

Co

un

ts/S

ec

2 theta

20nm 20nm Annealed ITO BK

d

Substrate

d

Substrate

Small-angle X-ray scatteringSmall-angle X-ray scattering

d

Substrate

Data show that Pc form three different types of crystalline phases; dependent on surface treatment

Before annealing

After annealing

AFM studies

Spin-coated at 4000 rpm on PEDOT:PSS/ITO; 180°C for two hrs.

Possibility to form nanostructured surfaces by thermal annealing to create high area networks for improved exciton dissociation

Device Configuration

DLC-CuPc

C60

* Energy scale in eV w.r.t vacuum

Experimental Results

-0.4 -0.2 0.0 0.2 0.4-1.0

-0.5

0.0

0.5

1.0

Cur

rent

Den

sity

(m

A/c

m2 )

Voltage (V)

as-apun annealeddark light

Annealing of DLC-CuPc film resulted in 3.7-fold increase in Jsc.

Estimation of RsA values by inverse slopes of J-V curves at V »Voc suggests increase of mobility in annealed device.

Reduction of Voc is considered due to creation of pinholes in DLC-CuPc film caused by dewetting while being annealed.

Jsc

(mA/cm2)FF Voc (mV)

RsA

(cm2)

Not Annealed 0.12 0.32 317 6.1

Annealed 0.44 0.39 222 1.6

Result for device with ITO/PEDOT:PSS (30nm) /DL-CuPc (20nm)/C60 (40nm)/BCP (10nm)/Al, under 50mW/cm2 (AM1.5Direct illumination)

e-

e-

e-

e--e

-e

-e

Self-assembled electron transport materialsSelf-assembled electron transport materials

ONN

O

NN

O

NN

OC8H17

OC8H17

C8H17O

OC8H17

OC8H17

OC8H17C8H17O

C8H17O

C8H17O

“Star-like” discotic LC oxadiazole materials with

good electron mobility

TOF experimentsTOF experiments

2

Vv E

L

L L

v V

NN22 laser, 337 nm, 6 ns laser, 337 nm, 6 ns

R = 10R = 1022 –10 –1044 , C = 10 pF, RC , C = 10 pF, RC << <<

TOF mobility results at TOF mobility results at room temperatureroom temperature

Cyclicvoltammetry of Discotic LCs

-6 10-6

-4 10-6

-2 10-6

0

2 10-6

4 10-6

-2000-1600-1200-800-4000

Triazine versus Fc

potential, mV

-1.5 10-5

-1 10-5

-5 10-6

0

5 10-6

1 10-5

-2000-1500-1000-5000

DLCOX5 versus Fc

potential, mV

ON

N

O

NNO

N N

R2

R1 R3

R2

R1

R3

R2R1

R3 N

NN

ON

N

R2

R1 R3

O

NN

R2

R1

R3

O

N N

R2R1

R3

0.6 volt shift in reduction potentional

OE Testing Facilities

Fully automated high vacuum deposition system with four organic sources and two high power sources for metals and oxides (co-deposition capabilities). Integrated with double glove box (one dry and one wet with integrated spin-coater).

Conclusions and future work

Transport properties of organic semiconductors often limit power conversion efficiency in organic solar cells. High mobility required in materials that can be processed from solution.

DLC-CuPc is solution-processible, and we demonstrated that its transport property can be improved in the discotic liquid crystalline phase.

Photocurrents reaching mA/cm2, significant improvement over HBC-based devices Development of discotic electron-transport oxadiazole-based materials.

Optimization of parameters will require control of interfaces, relative orbital energies, control of morphology through use of self-assembly.

Additional supporting informationAdditional supporting information

e-/h+

Inte

rfac

ial

char

ge

inje

ctio

n

Ho

le a

nd

ele

ctro

ntr

ansp

ort

Ch

arg

eR

eco

mb

inat

ion

Excited

statefo

rmatio

n

En

ergy tran

sfer

En

erg

y tr

ansf

er

Exc

ited

sta

tefo

rmat

ion

Ch

arge sep

aration

Interfacia l ch

ar ge

collectio

n

Ho

le and

electron

transp

ort

h

h

Emission

Excitatio

n

Stabilization of geometry and patterning

O

O

O

O

254 nm

Photo-crosslinking between adjacent side chains through cyclobutane links; > 50 % conversion of styryl groups

70nm

0nm

100m

8 m2m8 m

a b

150m

0nm

100nm100nm

150m

bare Si

0.0 0.2 0.4 0.6 0.8-35

-30

-25

-20

-15

-10

-5

0

5020603a

0.75,15.6%

0.64,13.3%

0.54,11.1 %

FF=0.34,=7.1%

0.81,16.8%

J0 = 30 pA/cm2

Jph

= 26 mA/cm2

n = 1.5

Cur

rent

Den

sity

( m

A/c

m2 )

Voltage ( V )

J(RsA=0 cm2)

J(RsA=2 cm2)

J(RsA=4 cm2)

J(RsA=6 cm2)

J(RsA=8 cm2)

J(RsA=10 cm2)

J(RsA=20 cm2)

0.0 0.2 0.4 0.6 0.8

-30

-20

-10

0

10

20020603b

0.71,14.5%

0.81,16.8%

0.60,12.3%

FF=0.42,=8.3%

J (

mA

/cm

2 )Voltage ( V )

RpA = 50 cm2

100 200 500 5000 50000 500000

Jph

= 26 mA/cm2, J0 = 30 pA/cm2

n = 1.5, under 1 Sun ( AM 1.5G )

Effects of REffects of Rss and R and Rpp on Fill Factor in High on Fill Factor in High

Photocurrent RegimePhotocurrent Regime

Minimize RMinimize Rss and Maximize R and Maximize Rpp

Excitonic Solar Cells: Energy Level EngineeringExcitonic Solar Cells: Energy Level Engineering

Vacuum level

EA

IpE

EHTL

ETL

A) B)

C) D)

Band offset < exciton binding energy

Band offset > exciton binding energy

Working hypothesis: the maximum value for Vthe maximum value for Vococ is the smallest is the smallest

band gap minus the exciton binding energy (0.5 eV)band gap minus the exciton binding energy (0.5 eV)

10 100 10000.6

0.7

0.8

0.9

1.0Effect of J

0 on PV Performance

Jph

= 26 mA/cm2, n = 1.5

Vo

c (

V )

, F

F

J0 ( pA/cm2 )

Voc

FF

12

14

16

18

20021003e

Eff

icie

ncy

(%)

Efficiency