Thin Film Fabrication and Morphology Studies

Kate Dickinson

Mentor: Mike Brady

PI: Dr. Michael Chabinyc

Introduction

•  Organic photovoltaics (OPVs) aim to provide simple and efficient sources of clean energy

•  OPVs have a maximum efficiency of ~5%

•  Multiple methods of thin film fabrication

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Introduction

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World Record: 8.3% Efficiency (Roll-to-Roll Printing Method)

Introduction

ITO Substrate

PEDOT:PSS

P3HT:PCBM Active Layer

Aluminum Electrode

P3HT:PCBM Photovoltaic

Courtesy of A. Muñoz

Fabrica'on  techniques  determine  morphology  and  cell  efficiency  

ITO Electrode

Aluminum Electrode

1)  Light Absorption

2)  Exciton Diffusion

3)  Charge Separation

4)  Charge Transport

5)  Clean Energy

Introduction OPVs use a electron-hole (exciton) system to transport charge

Courtesy of A. Ostrowski and B. Walker

Materials •  P3HT: Electron Donor

Poly(3-hexylthiophene) p-type semiconductor

•  PCBM: Electron Acceptor [6,6]-phenyl C61 butyric acid methyl ester n-type semiconductor

General Research Focus

•  How can OPV technology be improved?

•  How can fabrication techniques be improved without losing efficiency?

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•  Parameter Relationships: How are blade coating parameters related?

•  Film Morphology: How does morphology change for different blade coating parameters?

•  Crystallinity: How can P3HT:PCBM crystallinity be controlled and enhance charge mobility?

RET I Focus

Spin Coating: Spreads film across substrate while controlling rpm and time

Thin Film Fabrication Methods

Deposition Drying Film

Substrate

Spin

Courtesy of A. Muñoz

Blade Coating: Spreads film across substrate while controlling temperature, speed and blade height

Thin Film Fabrication Methods

Blade

Heat Plate, T(°C)

BASE

Substrate Solution

θ Velocity

Heat Plate, T(°C)

BASE

V

Heat Plate, T(°C)

BASE

Film

Courtesy of A. Muñoz

Thin Film Fabrication Methods

P3HT:PCBM Photovoltaics

Spin  Coated  OPV   Blade  Coated  OPV  

Spin Coated Blade Coated

Parameters •  Time •  RPM

•  Blade speed and height •  Substrate temperature

Efficiency •  2% (as-cast) •  5% (annealed)

•  3-4%

Pros •  Higher efficiency •  Extremely simple

•  Greater parameter control •  Industrially more practical •  Less wasted materials

Cons •  Industrially impractical •  Limited parameter control •  Wasteful 

•  Too many parameters •  Lower efficiency

Thin Film Fabrication Methods

PS:tol Study

0  

200  

400  

600  

800  

1000  

30  25  20  15  10  5  

Film

 Thickne

ss  (n

m)  

Blade  Speed  (mm/s)  

50°C   60°C   70°C  

0  

100  

200  

300  

400  

500  

600  

15  10  5  

Film

 Thickne

ss  (n

m)  

Blade  Speed  (mm/s)  

10  mg/mL   20  mg/mL   30  mg/mL  

Thickness Measurements 20 mg/mL PS:tol

Thickness Measurements 10-30 mg/mL PS:tol

5 mg/mL P3HT:tol Study

P3HT:tol at 5 mm/s

P3HT:tol at 10 mm/s

0  

50  

100  

150  

15  10  5  

Film

 Thickne

ss  (n

m)  

Blade  Speed  (mm/s)  

60°C   70°C  

Thickness Measurements AFM Roughness Measurements

30 mg/mL P3HT:CB Study

0  

200  

400  

600  

800  

1000  

1200  

1400  

15  10  5  

Film

 Thickne

ss  (n

m)  

Blade  Speed  (mm/s)  

80°C  

Thickness Measurements AFM Roughness Measurements

P3HT:CB at 5 mm/s Annealed at 150°C for 20 min

P3HT:CB at 15 mm/s Annealed at 150°C for 20 min

5  

2.5  

-­‐2.5  

5  

0  

90 mg/mL P3HT:CB Study Thickness Measurements AFM Phase Comparison

0  

500  

1000  

1500  

2000  

2500  

3000  

3500  

4000  

4500  

5000  

25  20  15  10  2.5  

Film

 Thickne

ss  (n

m)  

Blade  Speed  (mm/s)  

80°C  

P3HT:CB at 25 mm/s

P3HT:CB at 25 mm/s

Melted at 250°C and cooled

30 mg/mL P3HT:PCBM Study

0  

100  

200  

300  

400  

500  

600  

700  

800  

25  20  15  10  5  

Film

 Thickne

ss  (n

m)  

Blade  Speed  (mm/s)  

80°C  

Thickness Measurements

P3HT:PCBM at 5 mm/s

P3HT:PCBM at 5 mm/s

Melted at 250°C and quenched

AFM Phase Comparison

Conclusion  •  Parameter Relationships: Film thickness is directly

proportional to substrate temperature, blade speed and solution concentration increase

•  Film Morphology: Concentration dependent; slower blade speeds decrease roughness; annealing continues to decreases film roughness

•  Crystallinity: Blade coating forms clustered P3HT pockets; melting and rapid quenching produces longer, more evenly distributed fibrils throughout P3HT:PCBM bilayer

Next Steps  

Acknowledgments Mike Brady

Justin Cochran

Chris Shuttle

Dr. Michael Chabinyc Dr. Alan Heeger

Dr. Frank Kinnaman

UCSB Materials Research Lab

California NanoSystems Institute

National Science Foundation