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Sam SprawlsSURES ProgramMentors: Dr. Ina Martin, Prof. Ken Singer, PhysicsCase Western Reserve UniversityUndecided MajorJuly 31, 2013

EXPLORATION OF SILANES AND POLYMERS AS

INTERFACIAL LAYERS IN ORGANIC PHOTOVOLTAICS

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Energy

• Huge part of our daily lives– American economy– American culture

• What comprises American energy?

2011 total US energy consumption by fuel type

eia.gov/totalenergy/data/annual

Renewablesources

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Photovoltaics

• Not just renewable• They do have their challenges – Materials– Disposal– Lifetime/Durability

• How can solar panels become competitive?

Efficiency

StabilityCost2011 total US energy

consumption by fuel typeeia.gov/totalenergy/data/annual

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• Electron-blocking material• PEDOT:PSS (standard) is

acidic, corrodes ITO• Research questions:

1. Can silanes be used as a substitute for PEDOT:PSS?

2. Can adding a silane layer between the PEDOT:PSS and TCO minimize the TCO degradation?

Organic Photovoltaics

TCO Layer (ITO)

Electron-blocking material

Bulk-heterojunction (P3HT:PCBM) (active layer)

Hole-blocking material (LiF)

Metal Cathode (Al)

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Silane Chemistry

• Silane hydrogen bonds with hydroxyl group• Condensation reaction leads to covalent bond• Clean transparent conductive oxide (TCO)

surface has OH (hydroxyl) groups• Water hydrolyzes the ethoxy group

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Silane Chemistry

APDMES film APTES film

4. R. Wieringa. Surface-Grafted Polyglutamate Films with Reaction-Induced Polar Order; Ph.D. Thesis: University of Groningen, 2000.

APDMES APTES

SiO2 SiO2

3-aminopropyldimethylethoxysilane 3-aminopropyltriethoxysilane

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Characterization of Silanes on Silica and TCOs

• Ellipsometry and scanning electron microscope– APTES showed rougher multilayer– APDMES showed smoother sub monolayer

• Contact angles

people.eku.edu/ritchisong/554notes1.html

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Contact Angle

0 5 10 15 20 25 30 35 40 450

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20

30

40

50

60

70

80

Bare SilicaAPTESAPDMES

Thickness (Å)

Cont

act A

ngle

• Contact angles confirm silane modification

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Solar Cell Fabrication

• Silanize TCO (chemical bath deposition)

• (Spin coat PEDOT:PSS)• Spin coat P3HT:PCBM

absorber layer• Vapor deposit lithium

fluoride, then aluminum

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Solar Cell Characterization: IV Curves

Short circuit current (ISC)

Open circuit voltage (VOC)

Voltage at max power point (VMPP)

Current at max power point (IMPP)FF = Vmpp * Impp / (ISC * VOC)

Curr

ent (

mA)

Voltage (V)

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Solar Cells EfficienciesComparison of Silane vs. PEDOT:PSS EBL

Silane PEDOT:PSS η (% efficiency) Number of cellsNone Y 4.5 ± 0.1 9APDMES N 0.24 ± 0.01 3APTES N 0.01 ± 0.01 3None N 0.4 ± 0.3 3

• Silanes are not an effective substitute for PEDOT:PSS as an EBL

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Solar Cells EfficienciesComparison of Silane/PEDOT:PSS vs. PEDOT:PSS EBL

Silane PEDOT:PSS η (% efficiency) Number of cellsNone Y 4.5 ± 0.1 9APDMES Y 4.1 ± 0.2 6APTES Y 3.4 ± 0.5 2

• Both silanes result in decreased efficiency • APDMES (the thinner layer) has a smaller effect

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Conclusions

• APDMES had a 8 ± 6% drop from the control• APTES had a 20 ± 10 % drop from the control• Thin silane layer has moderate effects• To improve cell value, silanes need to increase

cell lifetime

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Future Work

• Lifetime/durability studies of APTES on TCOs (in progress in Prof. French’s Research Group)

• Additional silane characterization• Additional silanes (different functional groups)• Additional polymers– PVP (polyvinylpyrrolidone)

• Aging study of cell performance• Additional TCOs– Aluminum doped zinc oxide

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Acknowledgements

• My mentors, Prof. Kenneth Singer and Dr. Ina Martin

• My fellow researchers, Rosemary Bramante and Robert Minkebige

• Dominion• SURES, Sheila Pedigo and Bethany Pope

MORE Center

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SEM Images

APDMES APTES