UV-vis and Transport Characterization of Degradation in Polymer Blend Photovoltaics

UV-vis and Transport Characterization of Degradation in Polymer Blend Photovoltaics

American Physical Society March Meeting 2011

Dallas, TX

Emilee Sena, Justin Peel, Shreya Nathan,  Devin Wesenberg, Marianne Wallis,

  Thorsteinn Adalsteinsson, Brian McNelis, Richard Barber

Santa Clara University

Overview•Description of organic photovoltaics

•How the devices work

•Device fabrication

•Device measurement

•Active layer molecules

•UV-vis spectroscopy

What Are Organic Photovoltaic Devices?

O

O

SS

H3C(H2C)4H2C

CH2(CH2)4CH3

n

A

Multijunction

+

Charge Generator

Charge Acceptor

-

A

e-

Multijunction

ħω

Bulk Heterojunction

What Are Organic Photovoltaic Devices?

ħω

O

O

SS

H3C(H2C)4H2C

CH2(CH2)4CH3

n

A

ħω

+

Charge Generator

Charge Acceptor

-

A

e-

Multijunction

ħω

Bulk Heterojunction

What Are Organic Photovoltaic Devices?

ħω

SS

H3C(H2C)4H2C

CH2(CH2)4CH3

n

e-

A

ħω

+

Charge Generator

Charge Acceptor

-

A

e-

Multijunction

Bulk Heterojunction

What Are Organic Photovoltaic Devices?

ħω

O

O

SS

H3C(H2C)4H2C

CH2(CH2)4CH3

n

e-

A

ħω

+

Charge Generator

Charge Acceptor

-

A

e-

Multijunction

Bulk Heterojunction

What Are Organic Photovoltaic Devices?

ħω

O

O

SS

H3C(H2C)4H2C

CH2(CH2)4CH3

n

e-

e-

A

+

Charge Generator

Charge Acceptor

-

A

e-

Multijunction

Bulk Heterojunction

ħω

Why Study Them?•Advantages over inorganic (silicon) solar cells:

•Inexpensive

•Flexible

•Lightweight

•Manufacturable

•Why aren’t they used now?

Research Goals•Understand and improve

•Device lifetime

•Degradation

•Power conversion efficiency

The ProjectPhysics:

•Sample Fabrication

•Measurement of Device Performance

Organic Chemistry:

•Molecule Synthesis

Physical Chemistry:

•Spectroscopic Analysis

How Are the Devices Made?

•Indium-Tin-Oxide (ITO)•Indium-Tin-Oxide (ITO): “transparent” conductor

(Cross section)

How Are the Devices Made?

•Indium-Tin-Oxide (ITO)

(Cross section)

How Are the Devices Made?

SO3H

nPSS

(poly(styrenesulfonate))

O O

S nPEDOT (poly(3,4-

ethylenedioxythiophene))

•Indium-Tin-Oxide (ITO)

•PEDOT:PSS

•Indium-Tin-Oxide (ITO)

•PEDOT:PSS

How Are the Devices Made?

SO3H

nPSS

(poly(styrenesulfonate))

O O

S nPEDOT (poly(3,4-

ethylenedioxythiophene))

•Indium-Tin-Oxide (ITO)

•PEDOT:PSS

How Are the Devices Made?

Ph O(CH2)17CH3

O

P3HT (poly(3-hexylthiophene))

PCBOD ([6-6]-phenyl C61 butyric acid octadecyl ester)

•Indium-Tin-Oxide (ITO)

•PEDOT:PSS

•Active layer

PCBM ([6-6]-phenyl C61 butyric acid methyl ester)

O

O

(or)

How Are the Devices Made?

•Indium-Tin-Oxide (ITO)

•PEDOT:PSS

•Active layer

•LiF

How Are the Devices Made?

•Indium-Tin-Oxide (ITO)

•PEDOT:PSS

•Active layer

•LiF

•Al

How Are the Devices Measured?

V varied & recorded

A

I recorded

Solar simulator illuminates from below

and I-V curves are generated

Device Performance

|I*V|MAX

(V,I)

Power Conversion Efficiency:

ɳ = [(IV)max/area]/(Pin/area)

Pin = 100 mW/cm2

Device Degradation

Device Degradation

The Active Layer•Electron donor and acceptor molecules

•Light-induced electron transfer

SS

H3C(H2C)4H2C

CH2(CH2)4CH3

n

Ph O(CH2)17CH3

O

+

P3HT

PCBOD

PCBM

O

O

(or)

Meet the Molecules!•PCBM: [6,6]-phenyl-C61-butyric-acid-methyl-ester

•PCBOD: [6,6]-phenyl-C61-butyric-acid- octadecyl-ester

•Alkyl chain improves solubility, lifetime

PCBODPCBMC60

O

O

O

O

UV-Vis Spectroscopy: Why Do We Use It?

•Insight into:

•Absorption of light energy to excite electrons

•Changes in chemical environment

•Absorption changes due to degradation

How Does a Typical Spectrum Look?

•Ultraviolet and visible range

•Differential spectrum

•Time evolution

•MATLAB for analysis

300 nm 750 nm

How Does a Typical Spectrum Look?

0 1 2 3 4 5 6 7 80

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02x = 0.403 PCBOD:P3HT Blend, Annealed at 220C

Time (days)

Dif

fere

nti

al A

bso

rban

ce (

a.u

.)

554 nm

Finding the Characteristic Time (τ)

0 1 2 3 4 5 6 7 8 9 10-10

-9

-8

-7

-6

-5

-4

-3

Time (days)

ln [

Am

ax -

A(t

)]

x = 0.403 PCBOD:P3HT Blend, Annealed at 220C

y = - 0.57*x - 4

554 nm linear

Exponential Fit, Solve for Characteristic Time (τ):

Ln [Amax – A (t) ] = -t / τ + constant

Result for 554 nm: τ = 1/0.57 = 1.75 days

IV data: τ = 1/0.56 = 1.79 days

Variable Factors•Weight percent

•Blend stoichiometry

•Annealing temperature

•Most significant improvement in device performance

Annealing

Anneal on hotplate

(up to 380°C)

Continue fabrication process with LiF and Al layers

(HEAT)

Effects of Annealing: Device Performance

0.4 Molar Fraction PCBOD:P3HT Blend

O

O

0.16 Molar Fraction PCBM:P3HT Blend

300 350 400 450 500 550 600 650 700 750-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

Wavelength (nm)

Ab

sorb

ance

(a.

u.)

x=0.159 PCBM:P3HT Blend, Annealed at 220C

300 350 400 450 500 550 600 650 700 750-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

Ab

sorb

ance

(a.

u.)

Wavelength (nm)

x=0.159 PCBM:P3HT Blend, Not Annealed

300 350 400 450 500 550 600 650 700 750-0.04

-0.035

-0.03

-0.025

-0.02

-0.015

-0.01

-0.005

0

0.005

Wavelength (nm)

Dif

fere

nti

al A

bso

rban

ce (

a.u

.)

x=0.159 PCBM:P3HT Blend, Not Annealed

300 350 400 450 500 550 600 650 700 750-0.04

-0.035

-0.03

-0.025

-0.02

-0.015

-0.01

-0.005

0

0.005

x=0.159 PCBM:P3HT Blend, Annealed at 220C

Dif

fere

nti

al A

bso

rban

ce (

a.u

.)

Wavelength (nm)Dif

fere

nti

al S

pec

tru

mIn

itia

l S

pec

tru

m

Not Annealed; PCEmax = 0.64 % Annealed at 220°C; PCEmax = 1.17%

450 nm

555 nm

λmax = 555 nm

O

O

Effects of Annealing: Hypotheses

• Decreased quenching

• Morphological changes

• Crystallite formation

• Aggregation

• More stable structure

Summary•Organic photovoltaics use light energy for electron transfer to produce current

•UV-vis spectroscopy can be used to indicate changes in structure and chemical environment, how absorption changes with degradation

•Annealing at high temperatures improves device performance

Funding

Santa Clara University's BIN-REU - UCSC BIN-RDI, NASA Grant NNX09AQ44A

SCU, Science Technology and Society Grants

Grant from IntelliVision Technologies

AcknowledgementsMichael Sena