An Inorganic/Organic Hybrid Coating for Low Cost Metal Mounted Dye Sensitized

Solar CellsNeil Vyas, Cecile Charbonneau, Matthew Carnie, David Worsley and Trystan Watson

SPECIFIC, College of Engineering Swansea University, United Kingdom

email: 581895@swansea.ac.uk

Presentation Outline• Dye-sensitised Solar cells

• The Aim and Challenges

• Materials Selection

• Titanium Nitride & Polyimide

• Development of TiN – PI coatings

• Electrical Characterization

• Surface Morphologyi. Profilometryii. SEM Analysis

• Photovoltaic Performance i. J-V Characteristicsii. Electrochemical Impedance Studiesiii. Optoelectronic Transient Studies

• Conclusions

Dye-sensitized Solar Cells

Dye-sensitized Solar Cells

• Dye-sensitized solar cells (DSC) are a new generation of photovoltaic device which principally contains a photosensitive dye adsorbed onto the porous network of a wide band-gap semiconductor material.

• Direct conversion of light energy into electrical energy takes place in a DSC through a photoelectrochemical process.

• Photoexcitation of the adsorbed dye results in electron injection into the conduction band of the semiconductor material (e.g. TiO2) deposited onto the FTO glass electrode. Injected electrons are then collected at the glass working electrode and transported to the counter electrode (typically a Pt-coated FTO glass) through an externally attached circuit.

• A redox mediator at the counter electrode collects the electrons and re-generates the dye ground state through an oxidation-reduction cycle.

The Aim

The Aim

The aim of this research is to develop novel electrically conducting coatings for dye solar cell application.

The Aim

The aim of this research is to develop novel electrically conducting coatings for dye solar cell application.

These coatings will be used to coat electrically non-conducting as well as electrically conducting substrates to facilitate DSC fabrication on them.

Challenges

Electrical conductivity of the coatings should be equivalent to TCO

coated glass substrates at high temperature (450oC)

Challenges

Electrolyte corrosion

resistance Electrical conductivity of the coatings should be equivalent to TCO

coated glass substrates at high temperature (450oC)

Challenges

Electrolyte corrosion

resistance Electrical conductivity of the coatings should be equivalent to TCO

coated glass substrates at high temperature (450oC)

Coatings should be durable and stable under environmental conditions

Materials selection

Coatings High

temperature

stability

Electrical

conductivity

Engineering polymers• PEEK• Poly-sulfone• Polyimide

Conducting fillers•Titanium nitride•Tungsten carbide•Tin•Cobalt

TiN and Polyimide

Some Significant Properties of Titanium Nitride• Chemically inert• Corrosion resistant• Electrically conducting, 1.3 x 106 Ω.m• Density, 5.43 g/cm3

• Molar mass, 61.87 g/mol• Melting point, 2930 oC

NaCl type TiN crystal

TiN and Polyimide

Some Significant Properties of Titanium Nitride

Some significant Properties of Polyimide

• Chemically inert• Corrosion resistant• High temperature resistant• Electrical insulator• Density, 1.43 g/cm3

• Glass transition temperature, >400oC

• Chemically inert• Corrosion resistant• Electrically conducting, 1.3 x 106 Ω.m• Density, 5.43 g/cm3

• Molar mass, 61.87 g/mol• Melting point, 2930 oC

NaCl type TiN crystal

Imidization of an Anhydride and an Amine to Polyimide

Development of TiN – PI coatings• TiN (of particle size < 3 µ, Sigma-

Aldrich) at various wt% (30 – 100%) was blended with a polyimide (PI) precursor (Poly (pyromellitic dianhydride-co-4, 4′-oxydianiline), amic acid, Sigma Aldrich) to develop the TiN/PI composite coatings.

• The composition was then hand blended in a glass jar for approximately 30 minutes to obtain a mix of homogeneously distributed TiN particles in the polyimide matrix.

• Bulk resistivity values of the developed coatings were measured using Jandel multipurpose four point probe system (RM3000) and the coating with lowest possible resistivity value (90%TiN added PI) was used to coat the metallic substrates for dye-sensitized solar cell application.

TiN – Polyimide Coated Substrates =

[Where ϕ = volume fraction of TiN into the polymer, Mpolymer = molecular weight of polyimide, Mpigment = molecular weight of TiN, ρpigment = density of TiN, ρpolymer = density of polyimide]

%TiN in PI

70 80 90 100

Volume Fraction,

ø

0.12

0.14 0.15

0.17

Electrical Characterization

Electrical Sheet Resistance Measurement at Curing Temperature

30% 50% 70% 90% 110%1

10

100

wt% TiN

Rs, Ω/

□ • Electrical sheet resistance of the cured coatings was found to be decreasing with increase in TiN concentration into the polyimide matrix.

Electrical Characterization

Electrical Sheet Resistance Measurement at Curing Temperature

Electrical Sheet Resistance Measurement at Sintering Temperature

0 5 10 15 20 25 30 350

10

20

30

40100% TiN-PI

90% TiN-PI

80% TiN-PI

70% TiN-PI

Time, min

Rs, Ω/

□

30% 50% 70% 90% 110%1

10

100

wt% TiN

Rs, Ω/

□ • Electrical sheet resistance of the cured coatings was found to be decreasing with increase in TiN concentration into the polyimide matrix.

• Prolonged heat treatment at 450oC caused sheet resistance to increase steadily with time.

Surface Morphology (Profilometry)

0 3000 6000 9000 120000

30000

60000

90000

Micrometer, μm

Nanomete

r, nm

90% TiN – PI Coating at 350oC 90% TiN – PI Coating at 450oC

• Surface profile data suggest that even after heat treatment at 450oC; 90%TiN added polyimide coatings do not exhibit any changes in the coating thickness. However, average roughness value was slightly increased from 0.67 µ to 0.75 µ.

Coated ECCS

Uncoated ECCS

Surface Morphology (SEM Analysis)

350◦C heat treated coating

TiN Particle

PI matrix

Surface Morphology (SEM Analysis)

350◦C heat treated coating 450◦C heat treated coating

TiN Particle

PI matrix

Surface Morphology (SEM Analysis)

350◦C heat treated coating 450◦C heat treated coating• SEM images of cured and sintered coatings display no

significant changes. However, better interconnection between TiN particles can be observed in case of the sintered coating.

TiN Particle

PI matrix

Photovoltaic Performance (J-V Characteristics)

0 0.25 0.5 0.75

-5

-3

-1

1

3

5

7

9 TitaniumGlassTitanium nitride

Voltage (V)

Curr

ent de

nsity

Jsc, m

A/cm2

Cells Voc (V) Jsc

(mA/cm2)FF % η %

Glass 0.70 6.84 69.57 3.33Ti 0.70 5.57 73.97 2.91TiN 0.69 4.31 66.48 2.00

• TiN based DSC exhibited lower Jsc and photoconversion efficiency values but still remained comparable to the other cells.

Photovoltaic Performance (EIS Studies)

0.4 0.5 0.6 0.7 0.8 0.91

10

100

1000Glass-RrcTiN-RrcTi-Rrc

Voltage (V)

Recomb

inatio

n resi

stance

(Rrc)

• EIS studies show that TiN plays no significant role in catalysing recombination of electrons in the device.

• Charge transfer resistance at the counter electrode of the TiN-PI based cell was almost equal to the cells based on Ti metal and FTO glass.

0.4 0.5 0.6 0.7 0.8 0.91

10

100

1000Glass-RceTiN-RceTi-Rce

Voltage (V)

Char

ge t

ransfe

r resi

stance

(Rce)

, Ohms

Photovoltaic Performance (Optoelectronic Transient Studies)

0.01

0.1Glass TiNTi

Current density (Jsc)

Life

time

• Short circuit transient measurements indicate that electron transport is much slower in TiN device which is probably the reason for lower photocurrents and has been attributed to poor surface roughness of the TiN substrate (Ra = ~ 0.6 µm).

0 10 20 30 40 50 60 70 800

0.10.20.30.40.50.60.70.8

TiN TitaniumGlass

Time (s)

Volt

age (V

)

• It has been observed that photovoltage decay rates in TiN and Ti metal based cells are slower than the glass based DSC which shows that both TiN and Ti has some blocking effect on electron recombination which inhibits faster reduction of the redox electrolyte.

Conclusions

• This study confirms that low cost ferrous alloys such as ECCS can be used as DSC substrates if coated with TiN incorporated polyimide coating.

Conclusions

• This study confirms that low cost ferrous alloys such as ECCS can be used as DSC substrates if coated with TiN incorporated polyimide coating.

• TiN/PI coated ECCS substrates demonstrate comparable photoconversion as well as charge transfer properties to the Ti metal and glass based DSCs.

Conclusions

• This study confirms that low cost ferrous alloys such as ECCS can be used as DSC substrates if coated with TiN incorporated polyimide coating.

• TiN/PI coated ECCS substrates demonstrate comparable photoconversion as well as charge transfer properties to the Ti metal and glass based DSCs.

• TiN/PI displayed excellent blocking effect against the charge recombination at the TiO2/electrolyte interface which proves the suitably of TiN/PI as a novel coating for metal mounted dye-sensitized solar cells.

Interesting Facts…

• Established in 1920, located in Wales, United Kingdom.

• The University has achieved the highest growth in world-leading and international quality research activity in the UK.

• Civil Engineering ranked second in the UK, General Engineering ranked fifth.

• Medicine ranked seventh in the UK.

• Within Wales, out of 31 subject areas, Swansea ranked first in 17 areas, and first or second in 24 areas.