How Molecular Structure Influences Device Performance in Organic Solar Cells

Fullerene Derivatives

Kirsten Parratt, Loo Lab, 11/9/2010

How it works

• Photons absorbed by the organic compounds in the active layer create an exciton which diffuses randomly

• Upon reaching the acceptor and donor interface, the electron dissociates from the hole

• Both electron and hole are transported to their respective electrode

Al

ITO ITO ITO

AlAl

Why Organic Solar Cells?

An alternative to silicon solar cells:• Easier manufacturing• Low temperature processing• Solution processing • Lower costs• Flexible substrates

• P3HT/PCBM cells currently have one of the highest efficiencies (~5-6%)

• PCBM: [6,6]phenyl-C61-butyric acid methyl ester, acceptor small molecule

• P3HT: Poly(3-hexylthiophene), donor polymer

P3HT

PCBMITO

Al

3.7 eV

5.1 eV

PCBM

Electron Acceptor and Donor

Al

ITO

P3HT

LUMO

HOMO

Light

• P3HT/PCBM cells currently have one of the highest efficiencies (~5-6%)

• PCBM: [6,6]phenyl-C61-butyric acid methyl ester, acceptor small molecule

• P3HT: Poly(3-hexylthiophene), donor polymer• Charge transport through pi orbitals

P3HT

PCBMITO

Al

3.7 eV

5.1 eV PCBM P3HT

Electron Acceptor and Donor

Light

• P3HT/PCBM cells currently have one of the highest efficiencies (~5-6%)

• PCBM: [6,6]phenyl-C61-butyric acid methyl ester, acceptor small molecule

• P3HT: Poly(3-hexylthiophene), donor polymer

P3HT

PCBMITO

Al

3.7 eV

5.1 eV

Electron Acceptor and Donor

PCBM P3HT

• P3HT/PCBM cells currently have one of the highest efficiencies (~5-6%)

• PCBM: [6,6]phenyl-C61-butyric acid methyl ester, acceptor small molecule

• P3HT: Poly(3-hexylthiophene), donor polymer

P3HT

PCBMITO Al

3.7 eV

5.1 eV

Electron Acceptor and Donor

PCBM P3HT

Overview of Morphology-Length Scales

Molecular ordering

Crystal size

Phase separation

• Systematically altered fullerene for better packing• How the molecules pack effects device

performance

CF3-TNPS-Tet-Fu TNPS-Tet-Fu TES-Tet-Fu

Structure/Function Relationship

J. Anthony

Large

Side group

Small

Side group

Desired Stacking

• Contact between fullerenes should have better charge transfer

• Fullerene-acene contact will be worse• Best packing comes from the closest fullerenes

J. Anthony

Bad transfer

Good transfer

Stacking

CF3-TNPS-Tet-Fu TNPS-Tet-Fu TES-Tet-Fu

J. Anthony

Good

Transport

Bad

Transport

Bad transfer

Good transfer

Single Carrier Diodes

• Composed of only a fullerene• No photocurrent generation• Measure the transport of

charge through the active layer

FullereneITO

PedotAl

ue= (J0.5/V)2* L3*e0*er*8/9e0-permitivity of free space = 8.85418782 × 10-12 m-3 kg-1 s4 A2

er-dielectric constant = 3.9

- Measure of how fast charges can transport through the layer

Mobility

0 1 2 3 40.0

0.1

0.2

0.3

0.4

0.5

0.6[J

(mA

/cm

^2)]^

0.5

Voltage (V)

CF3-TNPS-Tet-Fu TNPS-Tet-Fu Tes-Tet-Fu

Efficiency

-0.75 -0.50 -0.25 0.00 0.25-0.4

0.0

0.4

0.8

1.2

1.6

2.0

J (m

A/c

m^2

)

Voltage (V)

Bilayer

Voc

Jsc

Maximum power

Efficiency = max power

100 mW/cm2

Bilayer Comparison• Jsc shows same trend as mobilities in SCD• CF3-TNPS-Tet-Fu shows worst Jsc and device performance

-0.9 -0.6 -0.3 0.0 0.3

-0.06

0.00

0.06

0.12

0.18

0.24

J (m

A/c

m^2

)

Voltage (V)

CF3-TNPS-Tet-Fu TNPS-Tet-Fu Tes-Tet-Fu

Efficiency (%)

3.3E-2

1.6E-3

4.77E-5

Conclusion• The observed mobilities and efficiencies show the

same trends• Most likely this trends correlates to the size of the

side group

CF3-TNPS-Tet-Fu TNPS-Tet-Fu TES-Tet-Fu

Large Side group

Low efficiency

Small Side group

High efficiency

Future WorkCrystallized derivatives would allow us to determine

if the molecules are packing as planned– More through testing of solvent vapor and thermal

annealling– Thermal evaporation of the fullerene layer

• Professor Loo • Stephanie Lee• Loo lab• PEI

Acknowledgements