Dye Sensitized Solar Cells

a review and commentary

by

Adil HassibPhysics &Astronomy Dept. – King Saud University

Addison: “There is a better way. Find it!!”

Bohr: “ Never express yourself more clearly than you are able to t o think.”

Aristotle: “The essentials of a phenomenon are best understood if one tries to explore their rise from the very beginnings.”

1839 Edmund Becquerel, a French physicist observed the photovoltaic effect.

(E. Becquerel,"Mčmoire sur les effets électriques produits sous l'influence des rayons solaires", C. R. Acad. Sci. Paris, 1839, 9, 561-567)

1914 The existence of a barrier layer in photovoltaic devices was noted.

1916 Robert Millikan provided experimental proof of the photoelectric effect.

The PhenomenonInteractions of Photons and Electrons

History

Photovoltaics Bequerel, 1839. Electrochemical Fritts, 1883. Selenium cells RCA, 1950 –1960 p-n junctions SC.

Today's commercially available silicon solar cells have efficiencies of about 18-19%.

Photoelectrics Einstein 1904-5, photon theory

Progress in solar cell efficiencies (1976 to 2003) for various research or laboratory devices. All these cell efficiencies have been confirmed and were measured under standard reporting conditions.Source:Thomas Surek, National Renewable Energy Laboratory

2. SOLAR RADIATION AND AIR MASS

2.1. RADIATION PROPERTIES OF THE ATMOSPHERE

P=1.367kWm-2 - the solar constant – solar radiation power outside the Earth’s atmosphere

Taken from: S. M. SZE; Physics of Semiconductor Devices; Second Edition; John Wiley & Sons;New York; 1981

2.3. SOLAR SPECTRAL IRRADIANCE

3. THE PHOTOVOLTAIC CHARACTERISTICS OF THE SOLAR CELLS

How the device works?

.

The p-n junction under illumination (on the right). A photon induced hole-electron pair is separated by the local field of the junction. Taken from: F. C. TREBLE (Editor); Generating Electricity from the Sun; Pergamon Press, Inc.;New York; 1991

3.1. Simplified equivalent circuit of a solar cell

IG: generated current

IJ: juncktion current

Introduction

Main Theme:

How (& why) can biopolymers (melanins) be used to harvest photons and

produce photoelectrons across the UV and Visible, and do they have

the potential to be viable solar photovoltaic biomaterials?

What Makes a Good Solar Photovoltaic Material?

1. High photon capture cross-section in the UV and Visible

2. Efficient production of photoexcited charges

3. Efficient transfer of photoexcited charges to some external circuit (i.e. able to be integrated into a suitable PV device platform)

4. Cheap, biocompatible (preferably bioavailable), processable, stable (chemically and photochemically), etc.

“The perfect world”: ex=1, low $/watt, short payback, positive

environmental impact

IntroductionMelanin BasicsEumelanins: Basic Physical & Chemical Properties vs. PV Requirements

Photon capturePhotoexcited charge generationPhotoexcited charge collection and transfer to the external circuit

Eumelanins: Basic Technological Considerations vs. PV Requirements

Synthesis & bioavailabilitySolid thin film fabrication & processabilitySuitable electrode surfaces (device engineering)

Challenges (current and future work)

Summary & Acknowledgements

Melanin Basics

• Specific class of polycyclic biopolymer related to the humic acids & found throughout nature – very important in humans

• Biological roles: photoprotectants, pigments, free radical scavengers, antioxidants, charge transport mediators

• Implicated in melanoma skin cancer and Parkinson’s disease

• Chemically stable, redox active, strong chelating power

• Random heteropolymers of indolequinones

• Often found intimately associated with “melanoproteins”

• Important – biologically unique set of solid state properties:

N

O

O

H

H

H

NO

O

H

2e,2H+

NO

O

H

Monomer Redox Cycling

Photoexcited Charge Collection

• The photogenerated electron (or hole) must be able to diffuse to a suitable electrode surface in order to be transferred into an external circuit - this requires:

o adequate electron / hole mobility within the photoactive material (i.e. a reasonable electrical conductivity)

o efficient coupling of the photoactive material to some host electrode

o (plus - for regenerative devices - the usual establishment of an intrinsic electric field / charge separation mechanism, and the efficient transport of the electron / hole through the electrode to the external circuit)

• Melanin electrical conductivity ()

– pressed pellets of synthetic or natural material ~10-8Scm-1

– molecularly continuous thin films (electropolymerised) ~10-5-10-6Scm-1

dependent upon doping, water content and polymer composition (MW)

• Coupling melanin to a host electrode

– possible to functionalise the polymer in order to manufacture a strong covalent coupling to the host electrode

Photoexcited Charge Collection – Coupling to a Suitable Electrode (e.g. TiO2)

• Photoelectrochemical technology approach (Gratzel) – regenerative test device:– cheap, biocompatible materials– nano structuring to increase total surface

area for photon absorption– melanin provides sensitisation of titania

(photoanode) into visible (in theory)

Nano structured photoanode(titanium dioxide + photosenstising biopolymer)

h

I

TCETCE

Redox Electrolyte (Liquid)

Carbon / platinum counter cathode

S0/S+

S*

mediator

red

oxsensitiser

photoanode

e-E

Ec

Ev

Schematic energy level diagramfor a sensitised PEC solar cell

Photoexcited Charge Collection – Coupling to a Suitable Electrode (e.g. TiO2)

0.00

0.01

0.10

1.00

10.00

100.00

1000.00

300 350 400 450 500 550 600

Wavelength (nm)

Log

Nor

mal

ised

Pho

tocu

rren

t

TiO2

TiO2 + Melanin

TiO2 + AnthoCyanine

Photoaction spectra show:- expected UV photo-response for TiO2

- evidence of visible light photoresponse in melanin sensitised system- evidence of photoexcited carrier injection into the titania after absorption of a visible photon by the melanin- demonstration of coupling of the melanin to a suitable electrode

PLUS

Regenerative PV action:- small but measurable- I-V characterisation yields a visible light power conversion efficiency of 0.1%

Evidence for photo-excited carrier injection

Technological Considerations - Biopolymer Design, Synthesis &

Availability

OHNH2

COOH

OH

OH

NH2

COOH COOH

NH2

O

O

NHOH

OH

COOH

O

COOH

NOHNHOH

OH

NHOH

OH

COOH

tyrosine dopa dopaquinone

leucadopachromedopachrome5,6, dihydroxyindole

(DHI)

5,6-dihydroxyindole-2-carboxylic acid(DHICA)

Melanins

Goals:• Controlled composition DHI/DHICA polymers• Ready supply of natural material – various sources

Technological Considerations - Biopolymer Thin Film Fabrication & Processing

• Thin films of DHI / Dopa melanin have been synthesised by electropolymerisation from precursor solutions onto conducting glass substrates

Melanin thin films prepared from dl-Dopaby electropolymerisation

fracturesurface

film surface

Chemical analysis vs. Dopa melanin-solid state nmr-XPS-elemental analysisConfirms that the material is melanin

Physical analysis-XRD-SEM-TEM-ConductivityStructurally continuous, amorphoussemiconducting, free standing thin film

Challenges

Current

• Produce good quality material – powders, composites, molecularly continuous thin films (of controlled composition & MW)

• Better chemical characterisation

• Understand electronic and optical properties (especially band structure and charge transport) – are they really condensed solid state amorphous semiconductors?

• Learn how to modify electronic properties (band gap, conductivity) – Cu doping

• Understand how to maximise photogeneration, collection, and coupling to suitable electrodes (minimise phonon related de-excitation pathways)

• Maximise efficiency in two regenerative PV platforms (Gratzel and p-i-n all soft solid state) – goal 1% in Gratzel platform by end 2003

Future

• Other photonic, optoelectronic, electronic device & sensor applications (e.g. broad band photodetectors, humidity sensors)

• Interactions of pigment and host melanoproteins

• Studies on malignant melanoma pigment (early cancer detection?)

Summary & conclusions

• Melanins are a class of biopolymers with unique physical and chemical properties

• They possess the prerequisite properties to act as broad band solar photon harvesting materials and PV materials in regenerative devices

• They offer the added advantages of being soft solids, biocompatible and bioavailable (also relatively easy to produce synthetic analogues)

• Its by no means clear if they will prove viable as PV materials, but they are certainly interesting from a technological perspective

• Many challenges – understand / manipulate mesoscopic properties, and maximise efficiency within a regenerative PV platform

“For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled”

Dick Feynman

Absorbance:• Broadband

• Increases exponentially towards UV.

350 400 450 500 550 600 650

0.050.100.150.200.250.300.350.400.450.500.550.600.650.700.750.800.850.900.95

Absorb

ance (

cm

-1)

Wavelength (nm)

0.001% 0.0025% 0.005%

Absorbance of Synthetic Melanin Solutions at Various Concentrations

PL: Re-absorption Correction

350 400 450 500 550 600 650-50000

0

50000

100000

150000

200000

250000

300000

350000

400000

Pumping at 380nm

After Re-absorption Correction

Em

issi

on

(cp

s)

Wavelength (nm)

0.001% 0.0025% 0.005%

350 400 450 500 550 600 6500

50000

100000

150000

200000

250000

300000

350000

400000

Em

issi

on (

cps)

Wavelength (nm)

0.001% 0.0025% 0.005%

Before Re-absorption Correction

Pumping at 380nm

• Strong re-absorption.• Strong, broad emission.• Emission increases linearly with concentration.• Fit to two gaussians (in frequency space)

- Two dominant transitions

•Peaks do not shift with concentration (yay!).• Peaks do shift with pump wavelength (multiple dominant transitions).

PLE:

280 300 320 340 360 380 400 420 440 460 480 500 520 540 560 5800

200000

400000

600000

800000

1000000

1200000

1400000

1600000

1800000

Em

issi

on (

cps)

Wavelength (nm)

477nm 485nm 546nm 578nm

PLE detecting at various wavelengths

0.005% SyntheticMelanin Solution

Re-absorption Corrected

•PLE profiles • Change shape when detect at different wavelengths.• Fit to three gaussians (in frequency space).

- Three dominant excitation wavelengths.

Summary of Learnings from Steady State Spectroscopy

• Very strong absorber– Must (and can!) correct emission measurements for re-

absorption.

• Strong emission!– Once re-absorption corrected shows expected trends.

– Broad, fits to two gaussians (two dominant transitions?).

– PL peaks shift with pump wavelength (multiple transitions).

– PLE profiles fit three gaussians (three dominant excitation wavelengths).

• Error analysis – Errors due to re-absorption correction process very small in

comparison to error from PL and PLE scans themselves.

Synthesis of the Nanotitanium Suspension

Procedure:• Add 9 ml (in 1 ml increments) of

nitric or acetic acid (ph3-4) to six grams of titanium dioxide in a mortar and pestle.

• Grinding for 30 minutes will produce a lump free paste.

• 1 drop of a surfactant is then added ( triton X 100 or dish washing detergent).

• Suspension is then stored and allow to equilibrate for 15 minutes.

Coating the Cell• After testing to determine which

side is conductive, one of the glass slides is then masked off 1-2 mm on THREE sides with masking tape. This is to form a mold.

• A couple of drops if the titanium dioxide suspension is then added and distributed across the area of the mold with a glass rod.

• The slide is then set aside to dry for one minute.

Calcination of the Solar Cells

• After the first slide has dried the tape can be removed.

• The titanium dioxide layer needs to be heat sintered and this can be done by using a hot air gun that can reach a temperature of at least 450 degrees Celsius.

• This heating process should last 30 minutes.

Dye Absorption and Coating the Counter Electrode

• Allow the heat sintered slide to cool to room temperature.

• Once the slide has cooled, place the slide face down in the filtered dye and allow the dye to be absorbed for 5 or more minutes.

•While the first slide is soaking, determine which side of the second slide is conducting.•Place the second slide over an open flame and move back and forth.•This will coat the second slide with a carbon catalyst layer

Dye Absorption and Coating the Counter Electrode

• Allow the heat sintered slide to cool to room temperature.

• Once the slide has cooled, place the slide face down in the filtered dye and allow the dye to be absorbed for 5 or more minutes.

•While the first slide is soaking, determine which side of the second slide is conducting.•Place the second slide over an open flame and move back and forth.•This will coat the second slide with a carbon catalyst layer

Assembling the Solar Cell• After the first slide had

absorbed the dye, it is quickly rinsed with ethanol to remove any water. It is then blotted dry with tissue paper.

• Quickly, the two slides are placed in an offset manner together so that the layers are touching.

• Binder clips can be used to keep the two slides together.

•One drop of a liquid iodide/iodine solution is then added between the slides. Capillary action will stain the entire inside of the slides

How Does All This Work?1. The dye absorbs

light and transfers excited electrons to the TiO2.

2. The electron is quickly replaced by the electrolyte added.

3. The electrolyte in turns obtains an electron from the catalyst coated counter electrode.

TiO2=electron acceptor; Iodide = electron donor;Dye = photochemical pump

Ionic Liquids in General

• Ionic liquids are liquids formed only of ions.

• Ionic liquids do not have a solvent component.

• Ionic liquids have a low vapor pressure so they are non-volatile.

Picture taken from Chem. World. June 2004, V:1(6)Martin Earle-Quill

Room Temperature Ionic Liquids (RTILs)

• A RTIL is an ionic material that is liquid at room temperature.

•They are based primarily on asymmetric organic cations paired with inorganic anions.

•By changing the ions, a countless variety of RTILs can be obtained.

•The asymmetric shape of the cations prevents the packing and the formation of solids at RT.

•Air and water resistant.

•Can be hydrophilic or hydrophobic.

Possible Applications of Ionic Liquids

• Applications as Solvent: They are non volatile and recyclable. Suitable for extraction processes without loss of solvent.

• ‘Chemically Active’ Solvent: Catalytic activity.• New Propellants and Fuels: High energy density

that may be used in propulsion.• Application as electrolyte in Solar Cells: non-

leaking and good charge transport properties

Dye-sensitized solar cells: Dye-sensitized solar cells: ComponentsComponents

1. Granular TiO2 forming a nanoporous structure.

2. A dye, which is a light sensitive substance spread on the TiO2 surface.

3. A redox couple (I-/I3-), located in the space between the dye and the cathode.

4. A solvent for the redox couple, e.g. a Room temperature Ionic Liquid.

TiO2

Electrolyte

Relevant issues concerning RTILs

• RTIL in confined geometries• RTIL in the presence of charged walls

Dye-sensitized solar cells: OperationDye-sensitized solar cells: Operation

1. Dye electrons are excited by

solar energy absorption.

2. They are injected into the conduction band of TiO2.

3. Get to counter-electrode (cathode) through the external circuit.

4. : Redox regeneration at the counter-electrode (oxidation).

5. : Dye regeneration reaction (reduction).

6. Potential used for external work:

--3 I32I e

e2II3 -3

-

redoxFext VEV

Red=I-

Ox =I3-

Ionic Liquid [bmim]+ I -

The Solar Constant

Check yourself: Does everyone know what a watt (W) is? A milliwatt (mW)?

We call this number “The Solar Constant” and designate it by the

Greek letter sigma ().

When we measure the midday intensity of sunlight at the Earth’s

surface, we find that about 136.7 mW fall on every square centimeter.

At 1 A.U.: = 136.7 mW/cm2.

An A.U. is the average Earth-Sun separation, ~ 150,000,000 km.

1 A.U.

IV

Dark Characteristic Light Characteristic

I

V Power Generating Region

Power Dissipating

Region

Power Dissipating Region

The VI characteristic of a solar cell is usually displayed like this:

V

IV

I

The coordinate system is flipped around the voltage axis.

Set #1: ISC , PMAX , VOC

(0.5V, 0 mA) V I = 0 mW

(0.43 V, 142 mA) V I = 61 mW

ISC

VOC

PMA

X(0V, 150 mA)

V I = 0 mW

Some typical values

Nanocrystalline Solar Cells: The MaterialsMaterials:

1. (2) F-SnO2glass slides

2. Iodine and Potassium Iodide

3. Mortar/Pestle4. Air Gun5. Surfactant (Triton X

100 or Detergent)6. Colloidal Titanium

Dioxide Powder7. Nitric Acid8. Blackberries,

raspberries, green citrus leaves etc.

9. Masking Tape10. Tweezers11. Filter paper12. Binder Clips13. Various glassware14. Multi-meter

Preparation of Nanotitanium and Electrolyte Solution

Nanotitanium1. Add 2-ml of 2,4 – Pentanedione (C5H8O2) to 100-ml of anhydrous

isopropanol [ (CH3)2CHOH ] and stir covered for 20 minutes.

2. Add 6.04-ml of titanium isopropoxide (Ti[(CH3)2CHO]4 to the solution and stir for at least 2 hours.

3. Add 2.88-ml of distilled water and stir for another 2 hours.4. The solution must then age for 12 hours at room temperature.5. Since you now have a collodial suspension, the solvent must be

evaporated off in an oven to collect the powder.

Electrolyte solution1. Measure out 10-ml of ethylene glycol

2. Weigh out 0.127-g of I2 and add it to the ethylene glycol and stir.

3. Weigh out 0.83 g of KI and add it to the same ethylene glycol.4. Stir and sore in a dark container with a tight lid.

Silicon Crystals Comparison

Type Efficiency in Lab Efficiency in Production

Monocrystalline 24 14-17

Polycrystalline 18 13-15

Amorphous 13 5-7

Source: Solar Server.de

Solar Energy Spectrum

• Power reaching earth 1.37 KW/m2

Solar cell – Working Principle

Operating diode in fourth quadrant generates power

CdTe/CdS Solar Cell

• CdTe : Bandgap 1.5 eV; Absorption coefficient 10 times that of Si

• CdS : Bandgap 2.5 eV; Acts as window layer

Limitation :

Poor contact quality with p-CdTe (~ 0.1 cm2)

PV: The Technology

$

coal nuclear gas oil wind solar

2.1 ¢ 2.3 ¢3.6 ¢ 3.9 ¢ 5.5 ¢

22 ¢

Nuclear Energy Institute, American Wind Energy Association, American Solar Energy Society

Today: Production Cost of Electricity

By Sylvia Tulloch

Actual or forecast market acceptance to market decline for the important PV technologies

Timing

PV Technology PVGeneration

Forecast time from marketacceptance to decline

Silicon Crystalline 1 1970 - 2020Silicon Amorphous 2 1983 - 2025Silicon Thin Film 2 2001 - 2050CdTe 2 1995 - 2010CIS/CIGS other 3/5, 2/4/6 2 2000 - 2050DSC 3 2003 - 2055DSC - hybrid 3+ 2015 - 2100Organic - hybrid 3+ 2015 - 2100Biological 4 2030 - 2100+

Gratzel Cell

• Photo-electrochemical Cell

– Artificial photosynthesis

• Solid State Analogue

SnO2

SnO2

Electrolyte

TiO2 10m

Platinum

Dye/biopolymer/quantum dots

Indium Tin OxideOn Glass Support

Polymer Quantum DotMatrix

Aluminium

Energy Regan and Gratzel, Nature 353 (1991) p737

Huynh et al. Science 29 295 (2002) p2425

DYESMain Theme:

How (& why) can biopolymers (Dyes) be used to harvest photons and

produce photoelectrons across the UV and Visible, and do they have

the potential to be viable solar photovoltaic biomaterials?

What Makes a Good Solar Photovoltaic Material?

1. High photon capture cross-section in the UV and Visible

2. Efficient production of photoexcited charges

3. Efficient transfer of photoexcited charges to some external circuit (i.e. able to be integrated into a suitable PV device platform)

4. Cheap, biocompatible (preferably bioavailable), processable, stable (chemically and photochemically), etc.

Solar spectrumNatural Dye Absorption

Photosynthetic Process

• Sunlight is trapped by chloroplasts

• Water is transported from soil to leaf

• Carbon dioxide enters through stomata

• Water and light combine to form chemical energy

• Chemical energy and carbon dioxide rearrange to form carbohydrates and oxygen

• Sugar is stored in plant and oxygen is released through stomata

Operationally

Basics IMelanin: Basic Physical & Chemical Properties vs. PV Requirements

Photon capture

Photoexcited charge generation

Photoexcited charge collection and transfer to the external circuit

melanins: Basic Technological Considerations vs. PV Requirements

Synthesis & bioavailability

Solid thin film fabrication & processability

Basics IIPhoton Capture (Solar)

• Very strong, broad band UV & visible absorption – eumelanins are black

• Monotonic behaviour vs. • Fits a simple exponential:

o extended conjugation & phonon coupling: continuum of states within the and * manifolds

o an amorphous semiconductor displaying an exponential Urbach tail near the absorption edge?:

• Consistent with photoprotective function, and potentially useful for solar photon harvesting

EEE 00

Tauc Plot: 50.LinearE.vsE

Band StructureNarrow Bandgap (1.4eV) Amorphous Semiconductor

(Condensed Solid State)?

• Amorphous model for melanin powders:– narrow bandgap consistent with

broad band UV & Vis absorption

– high density of states at the Fermi level & sub band

– two activation energies (0.1 and 0.78eV) below and above 311K respectively (lower activation energy related to localised density of states at Fermi Level)

– thermopower measurements: p-type conductivity for T~293K and n-type for T~325K !

• Consistent with structural observations – no crystallinity

EF

EV

EC

E

N(E)

1.40eV

0.78eV

0.2eV

{From DC conductivity measurementsof amorphous powders}

Amorphous electronic behaviour?

*

Conduction

Photoexcited Charge Generation• Melanins photoconduct with a

photo-action spectrum which matches their absorption spectrum

Photoactivity of Thin Film Dopa Melanin (Electropolymerised)

20

25

30

35

40

45

50

0 30 60 90 120 150 180 210 240 270 300 330

Time (s)

Re

sist

ance

(M

Oh

ms)

Lamp OnLamp Off

• Rapid “light induced” decrease in resistance (ambient conditions, 19V bias voltage, illumination by a Hg-vapour lamp ~300mW/cm2)

Photoactivity of Thin Film Dopa Melanin (Electropolymerised): +19V Bias

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 30 60 90 120 150 180 210 240 270 300 330

Time (s)

Ph

oto

curr

ent

(mic

roA

)

Lamp OffLamp On

Charge couplingPhotoexcited Charge Collection – Coupling to a Suitable Electrode (e.g. TiO2)

• Photoelectrochemical technology approach (Gratzel) – regenerative test device:– cheap, biocompatible materials– nano structuring to increase total surface

area for photon absorption– melanin provides sensitisation of titania

(photoanode) into visible (in theory)

Nano structured photoanode(titanium dioxide + photosenstising biopolymer)

h

I

TCETCE

Redox Electrolyte (Liquid)

Carbon / platinum counter cathode

S0/S+

S*

mediator

red

oxsensitiser

photoanode

e-E

Ec

Ev

Schematic energy level diagramfor a sensitised PEC solar cell

Challenges ( short range?)

Better chemical characterisation

Understand electronic and optical properties (especially band structure and charge transport) – are they really condensed solid state amorphous semiconductors?

Learn how to modify electronic properties (band gap, conductivity) – e.g. via doping

Understand how to maximise photogeneration, collection, and coupling to suitable electrodes (minimise phonon related de-excitation pathways)

Produce good quality, suitable materials – powders, composites, molecularly continuous thin films (of controlled composition & MW)

Interactions of pigment and host