Nanomaterials for Organic Solar Cells

Rochester Institute of Technology

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2007

Nanomaterials for Organic Solar Cells

Annick Anctil

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Abstract

Organic solar cells performances are _mainly limited _by photon absorption due to the

mismatch between _{the conducting} polymer and the solar _spectrum, as well as, exciton diffusion

sincetheexcitons are not_efficientlydissociatedintofreecharges. Nanomaterialssuch as_fullerenes,

quantum _dots, and carbon nanotubes were investigated in combination with conjugated polymers to allow for a broader _harvesting of the solar spectrum. For P3HT:PCBM solar _cells, optimal

efficiencies were obtained _using a combination of _{annealing before,} as well _as, after contact deposition. _Usingtheconceptsdeveloped for_P3HT,theeffectof_solvent,molecular_weight, andthe

type of fullerenes was _{investigated using MEH-PPV. Efficiencies} were increased from 0.57% to

2.06% under AMI. 5 _{by optimizing} the polymer molecular _{weight, the} device thickness and the

fullerene derivative. Three types of quantum dots were investigated to enhance photoconversion

below the polymer absorption: _CdSe, InP and InAs QDs. _Sub-bandgap photoconversion was

observed around 1.3eVforInAs QDs. _Finally, both SWNT and WMNT were investigated for improved charge dissociation and transport in organic solarcells. The maximum _loadingof carbon

nanotubes was increased _by using cut carbon nanotubes as well as _{by using} an intrinsic layer.The

most _promising option for _using carbon nanotubes in polymeric solar cells was found to be the

Table

Contents

LISTOF FIGURES I

CHAPTER1: INTRODUCTION 1

1.1 Conjugated Polymers 4

1.2 Fullerene Derivatives 7

1.3 Quantum Dots 8

1.4 Carbon Nanotubes 10

CHAPTER2: FULLERENES 11

2.1 Experimental 11

2.2 _{Poly(3-alkylthiophenes)} 12

2.3.1 Poly(3-alkythiophenes)/C60 PCBMcomposites 14

2.3.2 _Annealingand molecular weight 15

2.3 MEH-PPV 20

2.3.1 Chainorientation andthickness 20

2.3.2 Fullerenesoptimization 24

CHAPTER 3: QUANTUM DOTS 27

3.1 Experimental 27

3.1.1 Covalentsynthesis_{of CdSe QDs} 27

3.1.2 Non-covalentsynthesis_{of InP}nanocrystals 28

3.1.3 Covalentsynthesis_{of InAs}nanocrystals 28

3.2 CdSe Quantumdots 29

3.3 Indium Phosphide 32

3.4 IndiumArsenide 35

CHAPTER 4: CARBON NANOTUBES 40

4.1 Experimental 40

4.1.1 Intrinsic Layer Fabrication 41

4.1.2 Aligned Arrays 42

4.2 Laserproduced carbon nanotubes composites 42

4.3 Intrinsiclayer 46

4.4 Aligned Arrays 48

CONCLUSION 51

APPENDIX 53

Appendix A: Photovoltaiccharacterization 53

List

Figures

Figure 1: Photovoltaiceffect in organic solar cells5 l

Figure 2: Conditionsfor charge transfer 2

figure 3: amoandam 1.5spectrumwith various polymer absorption 3

Figure4:Chemicalstructures ofMEH-PPVandP3HT 5

Figure 5:_(a)Polythiopheneand_(b)the4possible triads resulting from combination of the3

possible diads12 5

Figure6:Chemicalstructure ofPPVandMEH-PPV 6

FIGURE 7: C60,C60PCBMANDC70PCBM19 7

Figure 8: Bandgaptuning with particle sizefor various semi-conductors 8

Figure 9: Schematiclayout ofa bulk heterojunction 11

Figure 10: Effectof the addition ofC60PCBMon the_{current-voltage(I-V)}curve forP3HT 12

Figure 11:Spectralresponseof the composite and absorption of each component 13

Figure12: Energylevels for different_{poly(3-alkylthiophenes)29, 30} 13

Figure 13: ComparisonbetweenP3HT: PCBM andP30T:PCBM _{(1:1 ratio) Under AMO} 14

Figure 14:_(a)Curent Densitymeasurements for different ratiosofP3BT:P30Tbycomparison to

P3HT 15

Figure15: Energylevel forC60and70withP3HT 17

Figure 16: ComparisonbetweenC60andC70PCBMwithP3HT_(1:1)UnderAMO 17

Figure 17: Spectralresponse of devices made usingC60 PCBMandC70PCBMwithP3HT(1:1

w/w). In onset,theresponsefrom theC70is enhanced for the lower energy 18

Figure18:P3HT:PCBM(1:0.8)INDCBWITH PRE-ANNEALING AT110CFOR30MINUTES under argon

AND POST-ANNEALINGON A HOT PLATE IN THE GL0VEB0X AT140CFOR30MINUTES.TESTED

UNDERAM 1.5 19

FIGURE 19:MEH-PPVCHAIN CONFORMATION WHENDEPOSITEDFROM_(A)A SOLVENT CONTAININGABENZENE

RING AND_(B),WITHOUT BENZENE RING 21

Figure 20:Current-densitymeasurementsofMEH-PPV:PCBM_(1:4)spin-coated froma solution

OF O-XYLENE OR CHLOROFORM 21

Figure 21: J-VComparisonofMweffect on theperformanceofMEH-PPV:PCBMorganic solar

with a4:1weight ratio(TestedUNDERAMI.₅₎ 22

Figure 22:l-Vcurves ofMEH:PPV-PCBM(1:4by_weight)spin-coated from o-xylene at different

speeds 23

Figure23: Effectof spin-speed onExternalQuantum Efficiency_(E.Q.E.)forMEH-PPV:PCBM (1:4

Figure24: l-Vcurves forMEH-PPVwithC60 PCBMorC70 PCBM(1:4by weight).Testedunder

AM1.5 24

Figure 25:_(a)l-Vcurves for differentC70ratioinMEH-PPV:PCBMdevices_(AM1.5)and_(b)

spectral response 25

Figure 26: MEH-PPV/C70optimal spectral response overlaid with theAM 1.5spectrum 26 Figure 27: Energylevel diagram forCdSe QD: MEH-PPVcomposites 30

Figure28: l-Vcurves forMEH-PPVwithCdSe QDsandC60 PCBMin different ration.Tested

underAMO 31

Figure 29: UV-visspectra of theCdSe QDsused for theMEH-PPV:CdSe QDscomposites with the External Quantum Efficiency_(E.Q.E.)of the different composites 32

Figure 30: Temporalevolution spectra ofInPQD 33

Figure 31: Fluorescenceand absorption ofInPQDsin hexanes after30minutes growth 34 Figure 32:(a)InP QDscoated by the myristic acid_(b) l-Vcurves ofP3HT:InPQDscomposite under

AMO 34

Figure 33:_(a)UV-Visspectra depicting temporal evolution ofInAs QDssynthesized using a(1.1):1 ratioand_(b)optical spectra ofInAs QDsfor different precursor ratios AFTER1HOUR

GROWTH 35

Figure34: UV-Visabsorption spectra forInAs QD:MEH-PPVcomposites with increasing w/w LOADING OFQDS. THEDASHED LINE REPRESENTS THE ABSORPTION SPECTRUM FOR THE STARTING

QDs 36

Figure 35:l-Vcharacteristicfor(MEH:PPV:InAsQDs:PCBM,by_weight)under simulatedAMI. 5

ILLUMINATION 37

Figure 36: ExternalQuantum Efficiency_(EQE)of(MEH-PPV:InAsQD:PCBM,by_weight)devices and an expanded region of the spectral response data below the polymer bandgap.the

inset shows the typicalplspectra forinasqdswith a bandgap of~1.3 ev 37

Figure37: Energylevel diagram for the composite devices(literaturevalues extracted forInAs

qds45 meh-ppvandpcbm46). thedashed lines show device recombination pathways 38 Figure 38: Exampleof purifiedSWNTsbefore and after the cutting in a piranha solution 40

Figure39:PPVcuring 41

Figure 40: Sublimationprocessof pentacene onITOand device structure fabricated using

MEH-PPV:PCBM(1:4by_WEIGHT) 41

Figure41:ConditionforMWNTaligned arraysgrowth using_CVD,Scanningelectron

micrographof chemical vapor deposited multi wall carbon nanotube array on indium

tinoxide coated glass and photovoltaic structure 42

Figure42:_(a)% SWNTin1:1:PCBMunder simulatedAMOand_(b)energy diagram of the

COMPOSITE WITH POSSIBLE RECOMBINATION PATHWAYS 43

Ill

Figure 44:_(a)PolymerComparisonMEH-PPV:PCBM _(1:4)- P3HT: PCBM

(1:1)and_(b)effectof the increasing ofSWNTin MEH-PPVdevices forMEH-PPV: PCBM (1:4by_weight) 44

Figure 45: Energydiagram for different conjugated polymers withSWNTandPCBM 45

Figure 46:_(a)l-Vcurves forMWNTcompared toSWNTin MEH-PPV: PCBM (1:4by_weight)and

(b)spectral response 45

Figure47:_(a)Structureof the device with the intrinsicPPVlayer and_(b)current-density

MEASUREMENTS WITHHlPCOSWNT 46

Figure 48: Energylevel diagram forPPV/P3HT:PCBM:SWNTdevices(literaturevalues for

SWNT30) 47

Figure 49: l-Vfor1%carbon nanotubes(SWNTor_MWNT)with and without thePentacenelayer

INMEH-PPV:C60PCBM(1:4BYWEIGHT) 47

Figure 50: Energydiagramof pentacene intrinsic layerwithP3HT/SWNT/PCBMcomposite

(LITERATUREVALUEFORPENTACENE48 48

Figure 51: Nomarskimicrograph_(10X)ofMWNTaligned arrayscovered with P3HT:PCBM_(1:1)

ACTIVE LAYER.THEGREEN COLOR REPRESENTMWNTGOINGTHROUGHTHE POLYMER 49

Figure 52:Schematicdevice diagram and current-voltage measurements(AM_1.5) 49

Figure 53: Energylevel inMWNTaligned array withP3HT:PCBM composite 50

Figure 54:Current-Voltage_(l-V)curvesforan organic solar cell 53

Chapter 1 : Introduction

I

Chapter 1:

Introduction

Currently there exists a large need for inexpensive renewable _{energy sources;}

particularly low-cost photovoltaic devices. For this reason, organic solar cells have been

receiving an _increasing amount of attention over the last decade. These devices offer the

potential for low cost _{processing, flexibility,} and light weight that could meet the needs of a

number of applications. _{Not only} are organic semiconductors cheaper than their inorganic

counterparts, but due to the means _by which _they are _fabricated, their properties can be

controlled_duringchemical synthesis.

Organic semiconductor development started in the 1970s, but the interest in these

materialsincreased _dramaticallyafter_Tangcreatedthefirst homojunction in 1986.1

However, a

major milestone in organic photovoltaics was achieved _by Yu et al. in 1995 in which both

electron donorand acceptormaterialswere blendedintoa single layerand created whatis now

called a bulk

heterojunction.2

Blending had the advantage of _increasing the interfacial area between the donor and _{acceptor, resulting in considerably} higher photocurrent density.

Recentlyefficiencies _up to 5% have been reported _byoptimizingthe _{nanomorphology}ofthese

heterojunctions using fullerenes derivatives in particular in combination with conjugated

polymers.3,4

In ordertoget a _{better understanding}ofthe limitationsand current challenges of

organic solar _cells, it is essential to understand the photogeneration process in a typical bulk

heterojunction asillustrated in Figure 1.

6

*d> _J7I

Glass+ITO

PEDOTPSS

_,

"

#^^LI]rjr _{.J Bulk heterojunction}

* L*%% ?* _*

m Aluminum

Figure 1:Photovoltaiceffectinorganic solar cells

1. Photonabsorption

2. Excitongeneration

3. Excitondiffusion

4. Exciton dissociation

5. Carriertransport

Chapter 1 : Introduction

I

Accordingto Figure _1, an exciton isfirstgenerated _during photon absorption dueto the

electronic transition from HOMO to LUMO levels in the donor material. Photon absorption

depends on the optical absorption coefficient as well as the thickness of the donor material.

Thisphotoexcitation results inthe generationof an electron-hole pair which is inan excited but

neutral state with limited lifetime.The electron and the holeare bound _byan _energy which is

smallerthan the _{energy gap} between the permitted bands and formwhat is called an exciton.

The exciton _{binding energy is} around 0.1 to 0.2 eV for organic

materials.6

Due to its limited

lifetime, the exciton needs to diffuse fast enough to ajunction before recombination can take

place in ordertocontributeto the photocurrent.Thislimited diffusion length severelylimitsthe

thickness of these devices. In the presence of a _{sufficiently strong internal} electric _field, the

exciton can split into a free hole and an electron-. This internal field can be provided at the

donor-acceptor _interface, ifthe difference in electron affinities is suitable. It is estimated that

only 10% ofphotoexcitations lead tofree charge carriers in conjugated

polymers.7

Blending of

conjugated polymerswith electron acceptors such as_fullerenes, is a_veryefficient_wayto break

apart photoexcited excitons into free charge carriers. Once the charges are _{free, they} travel

towards the electrodes _using the classic _hopping mechanism in organic

materials.8 However,

their _mobilitycan bereduced _bytraps.

A gradient in the chemical potentials of electrons and holes is built _{up in} a

donor-acceptorjunctionwhich is determined _bythe differencebetween the HOMOlevel ofthe donor

and the LUMO levels of the

acceptor.7

This internal electrical field determines the maximum

open circuit voltage _(Voc) and contributes to the field-induced drift of charge carriers. Finally,

the charges can be collected at the electrodes as _long as: _{(Ef)cathode<(ELUMo)acceptor} and

(Ef)anode>(EHOMo)donor(seeFigure 2).

Energy(eV)

<J>*

* HOMOHOMO

Donor

HOMO

Acceptor

Chapter 1 : Introduction

I

In order to obtain an efficient solar _cell, all the steps discussed above have to be

optimized. _First, to optimize the photon _absorption, the optical losses have to be minimized.

Considering that the refractive index of organic materials _{is relatively low,} the reflection

coefficient is also _{relatively low}(ft/4%)6 and therefore is notthe main factor. The poor photon

absorption _{is mainly} due to the mismatch between the solar spectrum and the absorption

spectrum of the _conducting polymer. In _theory, to have an optimal solar cell, the acceptor

bandgapshould bearound 1.4eVforwhichthemaximum _efficiencywould be31%under1sun

AM1.5.9

The majorityof_{semiconducting} polymers have bandgaps higherthan 2 eV (620 nm),

which limits the possible theoretical maximum photovoltaic conversion _efficiency to less than

30%10

-AMO

-AM1 5

-PV

MEH-PPV

P3HT

Pentacene 2500

2000

E ₁₅₀₀

0)

|

500

0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Energy(eV)

Figure 3: AMOandAM1.5spectrum with various polymer absorption

Photon absorption can also be maximized _byoptimizing the devicethickness. Asample

too thin causes part ofthe incident radiationto be unabsorbed, resulting in these photons not

contributing to the charge generation. _Conversely, a sample too thick causes series resistance due to the exciton generation zone _being too far from an interface. _{Subsequently,}

recombination occurs before dissociation of the excitons. Since polymer absorptions

coefficients _may reach _105cm"1, a 100 nm thickness _{is generally} enough to absorb most ofthe

Chapter 1 : Introduction

I

Anothermain challenge is related to theexciton diffusion due to conjugated _polymers,

which exhibit poor chargetransport in bulk heterojunctions. In orderto increase the_efficiency,

the donor and acceptor phases should be separated _by a distance of 10 to 20 nm and the

interfacial area should be increased. The use ofdifferent work-function _electrodes, as well as

concentration _gradients, can contribute to the charge transport toward the electrodes. The

donorand acceptor phases mustforman _{interpenetrating} networkto efficientlydissociate and

transport the charges. This nanoscale _morphology has been shown to be _strongly affected _by

the _{processing conditions,} such as the deposition _technique, the _solvent, the _processing

temperatures,thedonor/acceptorratio,thesolution_{concentration,}andthethermalannealing.

Therefore, to improve efficiency, an improved _harvesting of light could be achieved

through the use of various nanomaterials which will contribute through energy-transfer. The

focus of this dissertation will be on the investigation of nanomaterials _{including fullerenes,}

quantum_dots, and carbon nanotubestodeterminetheseeffects on the production of_{energy in}

various conjugated polymers solar cells. Since the _morphology of the active layer has a direct

impact on the performance ofthe organic solar _cells, various processing studies will also be

presented _{(solvent, annealing,} components _ratio, molecular weight). With this objective in

mind,thevarious materialsthatwerestudied aredescribed below.

1.1 Conjugated Polymers

Theconjugated polymersbackbone iscomposed of_alternatingsingleand doublebonds.

Eachcarbon bindsonlyto threeadjacent _{atoms, resulting in} one electron per carbon leftinthe

Pzorbital._{The overlap}ofthese Pzorbitals causestheformation of rebondsalongthe_backbone,

and derealization ofthere electrons. Polymerswith a delocalized re electron system can absorb

sunlight, create photogenerated charge carriers and transport these charge carriers. The

absorption of conjugated polymers ranges from yellowto red. The absorption for one type of

polymer can alsobechanged _bytheadditionof various pending

Chapter 1 : Introduction

I

Theprincipal conjugated polymerscanbe divided in 3families7 :

I. Derivativesof phenylene vinylenebackbones

(poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-l,4-phenylevinylene _(MEH-PPV)

II. Derivativesofthiopheneschains_{(poly(3-hexylthiophene)(P3HT)}

III. Derivatives of fluorine backbones

(poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl-l,4-phenylenediamine) PFB

Thetwoprincipal conjugated polymersinvestigated forthis project were

Poly[2-methoxy-5-(2-ethylhexyloxy)-l,4-phenylenevinylene] ( MEH-PPV) and

(Poly(3-hexylthiophene-2,5-diyl))(P3HT)

S \ //

MEH-PPV P3HT

Figure 4: Chemicalstructures ofMEH-PPVandP3HT

The poly(3-alkyls) (P3AT) family of polymer is comprised of derivatives of thiophenes

with different regioregularity and _pending chains. All polythiophenes have similar optical

bandgaps inthe order of 1.9eV. As thechain length is _increased,the electrochemical _bandgap

also increases. P3AT are polythiophenes with three asymmetric substituted _thiophenes,

resulting in 3 possible couplings between the 2 and 5 positions. The head-head configuration

(HH:2-2 coupling) causes the thiophene _group to be out of phase, which reduces the

conjugation andthe_conductivityofthepolymer.

^h

(a)

Chapter 1 : Introduction

I

The record power_{efficiency devices have been fabricated using poly(3-hexylthiophene)}

with high regioregularity. High _{regioregularity}optimizes thechain _packing,which improvesthe

optical and transport properties of the P3HT:PCBM blend upon

annealing.13 Microcrystalline

lamellar stacking is _generally accepted as the main factor for high _efficiency devices. This

structure can be improved through thermal annealing14

and combined thermal and external voltage

annealing.15,16

The optimal _{annealing temperature is} related to the glass transition

temperature which varies with molecular weight and polydispersity. _Annealing causes

recrystalization, resulting in improved charge transfer and a reduced _density of defects at the interfaces.7

Poly(p-phenylene _{vinylene) (PPV)} is a conductive polymer with limited interest for

organic solar cells due to its absorption around 2.5 eV. _{By modifying} this basic structure to poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-l,4-phenylevinylene _(MEH-PPV), the band _gap has

shifted to lower _energy (~2.1 _eV), which is an _{improved energy} absorption for the solar

spectrum.

Poly(p-phenylene vinylene) PPV

Poly[2-((2-ethylhexyl)oxy-5-methoxy-p-phenylene)vinylene]

MEH-PPV

Figure 6: Chemicalstructure ofPPVandMEH-PPV

The addition of branched side chains also improves the solubility of the derivative

compared to PPV that allows solution _processing and _{easy blending} of polymers for the

preparationofbulkheterojunction.11 The _{hole-conductivity}for MEH-PPV is lowerthan P3HT. In

order to obtain an efficientcharge separation, a 4:1 (PCBM: _MEH-PPV) weight ratio is _typically

used. Due to the high efficiencies obtained with _P3HT, MEH-PPV has not been a focus of research. However with an open-circuit voltage of _{approximately 0.8V,} compared to 0.6V for bulk heterojunctions with _C60PCBM, MEH-PPV represents an _interesting alternative for

increasing efficiencies. At this point, the highest _efficiency reported is 2.07% underAMI. 5 (80

Chapter 1 : Introduction

I

annealing and optimal _processing conditions has been done and this has resulted in lower

efficienciesforthispolymer-fullerene system.

1.2 Fullerene Derivatives

Carbon fullerene _typically refers to a molecule _{containing 60} carbon atoms with

icosahedralsymmetry. Italso includes largermolecular weightfullerenessuch as _{C70, C76,} C78

18 C80that possessdifferentgeometric structures .

Figure 7:_C60,C60 PCBM andC70 PCBM

C60 or _{buckminsterfullerene,} is an electron acceptor that can be electrochemically

reduced_upto6electrons. _{Its solubility is limited,}thereforea soluble derivative called PCBM

(1-(3-methoxycarbonyl)propyl-l-phenyl[6,6]C61) which was first synthesized _by Wuld et al., is

mainlyusedtomake solar cells7.

The size and dispersion ofthe PCBM nanophases have a direct impact on the devices'

performances. The most _widely used composition ratio of PCBM:P3HT is 1:1. Forthis ratio, a

sudden _drop in efficiency is observed after short _annealing; an effect associated to the

formation of large PCBM aggregates. _{By lowering}the PCBM:P3HT ratioto _0.8:1, the _annealing

temperature has been increased from 110C to 150C, resulting in higher efficiencies. The

higher annealing temperature improves the recrystalization. The lower PCBM content reduce

the sizeofthe PCBM aggregates_{causing degradation,} allowing longerannealingtime. Sincethe

structureis closertoa equilibriummixture,degradation ofthecompleteddeviceisalso slower.

Only recently has this ratio been reduced furtherto 0.6:1 making the first 6% efficient device

Chapter1 : Introduction

I

1.3 Quantum Dots

Generally, polymer photovoltaic devices _{rely solely} on photon absorption _by the

conducting polymer andare _bandgap limited_(typically >_{2 eV) in} regardsto the solarspectrum.

Quantum dotsare capable of size-dependent optical _absorption, exciton dissociation centersin

conjugated polymers, and a meansfor carriertransport._Therefore, the combination oforganic

and inorganic semiconductors offer the _possibility of _absorbing a larger portion of the solar

spectrum. _Byvaryingthesize andthe material ofthe quantum_dots, the band _gapcan betuned

to thedesired range.

0

2.5 2 1.5 1

QD _{Eg Energy(eV)} ill'

1.7 1.421.271.12 0.36 BuIkE _(eV)

g

Figure8:_{Bandgaptuning}with particle sizeforvarious semi-conductors

UsingQDs isconsidered a _promisingoptionsinceinorganicsemiconductornanoparticles

can have high absorption coefficients and high photoconductivity compared _{to many} organic

semiconductor materials. Excitonswhich are created upon photo-excitation are separated into

free charges _efficiently at interfaces between organic semiconductors and inorganic

semiconductors. Hybrid solar cells have been demonstrated in conjugated polymer blends

Chapter 1 : Introduction I 9

Compared to dye _{molecules, appropriately} synthesized semiconductorQDs offer wider

tunabilitywith narrower and more stablefluorescence. QDsare also _particularlysuited forthe

near-IR region where the increased absorption is needed because organic dyes have poor

emission _efficiencyin thatregion.20

The synthesis of colloidal nanocrystals is a special case of crystallization that yields

massivenumber of crystalsin solution with identical dimensions.The interest in QDsstarted in

the _1980s, but thefirst real success in their production was obtained _using an organometallic

approach _{in coordinating} solvent in the _early 1990s.21 This approach originated from the

utilization of organometallic compounds in the formation of _{extremely high quality} quantum

wells through metal organic chemical vapor deposition _(MOCVD) methods. Organometallic

reactions are _extremely sensitive to airand moisture due to _{high reactivity} ofthe precursors.

The coordinatingsolvents are used to balancethe _reactivityofthe

precursors.22 Toobtain high

qualitycrystalline_{QDs, it is generally}establishedthat hightemperatureis required.

To _date, most of the research on QDs for solar cell composites has focused on CdSe

since their synthesis is relatively straightforward and allows narrow distribution of QDs with

specific absorption.Otherresearchers havetried tofabricatecomposite devices _{using MEH-PPV}

or P3HT. Their low efficiencies _(<0.05%) can be mostly attributed to device fabrication.23,24 In

order to _develop a general method for device preparation and characterization, CdSe QDs has

been investigated in combination with MEH-PPV. _However, sincethe absorption ofthese QD is

similarto the polymer_{bandgap, only}a _relativelyminorimprovement _bycomparisonto thepure

polymer can beexpected.

Agreaterimprovementofthe photovoltaicperformance could beachieved through the

use oflll-V _QDs, which havethe appropriate _bandgap in the desired absorption. _{The quality} of

QDs forlll-Vsemiconductors as well as theirsynthesis have not reached the same level as the

CdSe. Indium phosphide QDs wasthefirst lll-Vsystem to be synthesized through non-covalent

approaches._However, this system _{is extremely} sensitive to air and moisture as well as to the

fatty

acids'

chain length. The non-coordinating approach has been studied _{thoroughly by}

various groups for _InP, but only by Battaglia et al. for

InAs.25

Organometallic approaches in

Chapter 1 : Introduction

I

extreme _toxicity, a special _handling precaution hasto be taken.22

Although the_viability of

QD-based polymer solar cellshas been described inthe_literature, InAs QDshaveonly recentlybeen

shown to exhibitaphotoresponse inconjunctionwith nanocrystalline_Ti02films.26 InP and InAs

QDs will be investigated in organic solar cells. Due to the absorption peak of _InAs, a _sub-gap

conversion _mayoccur.

1.4 Carbon Nanotubes

Dueto their unique properties, carbon nanotubes can be envisaged forvarious roles in

photovoltaic devices. Single wall carbon nanotubes _(SWNT) have _received, so _far, the most

interestdue to their_{semi-conducting} properties. SWNT's have been used aselectron acceptor

and electron _extracting electrode in bulk heterojunctions.27

Multi-wall carbon nanotubes have

been used as the _{hole-extracting} electrode, which has resulted in a reduction of series

resistance and an increase infillfactor.28

Growth of aligned arrays of carbon nanotubes on _{ITO may} also be used as

three-dimensional hole_collectingelectrodes. Bulkheterojunctions have partiallyresolvedthe issue of

the short exciton diffusion length in organic_{PV, however,}thelowcharge_mobilityof conjugated

polymers remains a _limiting effect. This low charge _{mobility may} be improved _by the

interpenetrating electrode, which extracts the charges more efficiently. _{Only recently} have

aligned arraysof carbon nanotubesbeen used as largeelectrodesfororganic

Chapter 2 : Fullerenes 11

Chapter

2:

Fullerenes

2.1 Experimental

Two types offullerenes _derivatives, C60 PCBM and C70 _PCBM, were investigated as

electron acceptorsin bulk heterojunction. Bulk heterojunction nanostructureshave a direct

influence on device performance. For this reason, the effect of _spin-speed, the type of

solvent, annealing(pre-postannealing), PCBM ratio and polymermolecularweighthas been

studiedfor MEH-PPVand P3HT.

Bulk heterojunction solar cells were fabricated _following the sandwich structure

ITO/PEDOT:PSS/Active layer/AI illustrated in Figure 9. The glass substrate is coated with

indiumtinoxide_(ITO) (Delta 7-10_Q/sq), which is cleaned_usinga 2stage ultrasonic_cleaning

procedurewith acetone andisopropanol anddried.

Figure 9: Schematic layoutof abulkheterojunction

Poly(ethylene-dioxythiophene) doped with polystyrene-sulfonic acid _(PEDOT:PSS)

wasthen spin-coated at3500-4500rpmtoobtainathicknesslayeraround 60to 80nm.The

PEDOT:PSS layer improved the surface _quality as well as facilitated the hole

injection/extraction. It_{is necessary}to_drythe Pedot:PSSlayerat110Cfor1hourto remove

anywaterand reducefuture degradation.7

The active layer components are sonicated and mixed in the appropriate solvent

(DCB for P3HT and o-xylene for _MEH-PPV) before _spin-coating on _top of the PEDOT:PSS

Chapter 2 : Fullerenes

I

device degradation. For selected _{studies, post-annealing} was performed on completed

devices in a glovebox _using a hot plate. The back electrical contact was 1000 A of

evaporated aluminum andtheactiveareawas calculatedforcurrent_densitymeasurement.

The current-voltage curves were obtained _using a _Keithley 237 source-measure unit

and a Newport Oriel Instrument lightsource, calibrated underair mass 1.5(AMI.₅₎orAMO.

Spectral response was performed _using an Optronic Laboratories instrument, calibrated

with a UV enhanced silicon detector. Refer to Appendix A forcharacterization calculations

and performance measurements.

2.2

Poly

(3-alkylthiophenes)

Poly(3-hexylthiophenes)composites with C60 PCBMare responsibleforthe recorded

power

efficiency.14

Figure10showsthedramaticeffect oftheaddition ofPCBMtoP3HTon

thedevicesperformances,which increasestheopencircuitvoltage and currentdensity.

V =519mV V =200mV

10"

10"

|

S? io5

Q *^

0 10

3 o

10-7

10*

I =5.34mA/arT I =1.51uA/cnf

sc sc f

FF=_26% FF=39%

1

PCBM-P3HT

Pure P3HT

\ ^

-1.0 -0.5 0.0 0.5 1.0

Voltage(V)

Figure 10: Effectoftheaddition ofC60 PCBMon_{the current-voltage(l-V)}curvefor P3HT

Spectral responseisusefulto investigate theabsorptionlimitation ofthedevices. From

Chapter 2 : Fullerenes

I

eV which isconsistent withtheabsorption of each component.This isnot anidealcombination

consideringthat theoptimal_{energy is 1.4}eV.

LU

a

PEDOTPSS

ITO Glass 8_c

CO -Q j

0.5 1.0 1.5 2.0 2.5 3.0 3.5

Energy (eV)

Figure 11: Spectralresponse ofthecomposite and absorption of each component

Oneapproachtoobtaining an improved spectrum_matchingwould includethe use of

a conjugated polymer with a largerabsorption inthe near-IR. The electrochemical _bandgap

for poly(3-alkythiophenes) (P3AT) is slightly larger with an increased side chain

length7

(Figure ₁₂₎and therefore other P3ATs such as _{Poly(3-buthylthiophenes)(P3BT)} and

Poly(3-octylthiophenes) (P30T)have been comparedto P3HT.

Energy(eV)

<fA1 =_4.3

P3BT P3HT P30T PCBM

Chapter 2 : Fullerenes

I

2.3.1 Poly(3-alkythiophenes)/C60 PCBMcomposites

As hypothesized _by the _{bandgap increase,} the open circuit voltage of the P30T

devices compared to P3HT is higher. _However, the current _density of the P30T devices is

significantly less (Figure 13). Since all devices were made _using the same _annealing

conditions (110C for 1 hour), it is possible these conditions were not optimal for P30T.

Longer chains increase the bandgap, while _loweringthe absorption coefficient. _Therefore,

the increased chain length produces a change in the optimal thickness and microstructure

neededtoabsorb mostofthelight7thatshould beoptimized.

10"

kp

10-E o

^

55 c <u Q

10-a> b

10"*

10'

PC

PC 3M-P3HT

BM-P30T

Jsc Voc F F

(mA/cm2) (mV) [<

P30T 0.00243 637 14

P3HT 5.34 519 26

-1.0 -0.5 0.0 0.5 10

Voltage _(V)

Figure13: ComparisonbetweenP3HT: PCBM andP30T:PCBM (1:1_{ratio) Under AMO}

As an alternative, poly(3-butylthiophenes) (P3BT) was considered in combination

with P30T. The _bandgap of P3BT is lower than P3HT, therefore _reducing theVoc. However

this reduced open circuit voltage may be increased by combining the P3BT with different

ratios of P30Tsuchas 1:1, 1:3 or3:1._{The annealing}timewas lowered to30minutes based

on published studies _showing a faster degradation for P3BT than for P3HT at high

temperature.29

The mostefficient P3HT:PCBM devicewas obtained usingthe30minutes _annealing

time, however, all the devices hada lower efficiencythan the devicesmade _{using P3HT. In}

Chapter 2 : Fullerenes

I

containing a larger portion of P30T have a higher open circuit voltage than the 3:1

P3BT:P30Twhich isconsistent withthe_{energy band}alignment.

12.0 8.0 4.0 E E $ on </S 0.0 C (U a o 4.0 -8.0 -12.0 P3HT 1:1(P3BT:P30T) 3:1 (P3BT:P30T) 1:3(P3BT:P30T)

Iff

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.E 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Voltage_{(V) Energy (eV)}

(a) (b)

Figure 14:_(a)Current_Densitymeasurementsfor differentratios ofP3BT:P30T_bycomparisontoP3HT. (1:1 P3AT:PCBMratiokeptconstant)underAMOand _(b)spectral responseforthesame ratios.

Devices made of a combination of P3BT and P30T have better efficienciesthan the

pure P30Tdevices(Figure 14). Forthe composite_containingthehigherpercentage ofP30T

(1:3 _P3BT:P30T), the current _density was 5.9

mA/cm2

compared to 2.43

uA/cm2 for the

P30T alone. On the other _hand, the open circuit voltage decreased from 0.637 V to 0.4 V

dueto the addition ofP3BT.

2.3.2 _Annealingandmolecular weight

Amongthe numerous research teams _working on organicsolar _{cells, there} does not

appearto be an agreed standard for theoptimal _annealingconditionsfor P3HT:PCBM bulk

heterojunctions. The lack of agreement is caused _by the wide variance in the polymer

regioregularity13 and chain

length29

which affects its charge _mobility and _annealing

Chapter 2 : Fullerenes 16

Low Mw P3HT has a limited photoresponse due to confinement ofthe conjugation

length. For Mw around _{10,000 g/mol, P3HT} forms crystalline _nanorods, which exhibit low

charge _mobility due to poor long-range _{interconnectivity} of polymer chains. When P3HT

Mw is higher than 30,000 g/mol, it forms nano fibrils which are connected over _long

distance (ca. _{500 nm)}which leadto an enhanced

mobility.29,31

Dueto itsrecord efficiencies, P3HThas becomethemoststudied polymer insolarcell

studies. A recent_{study comparing}the effect ofmolecular weight and performance of solar

cells suggestedtheobserved increasedhole _{mobility in P3HT}comparedto _{P3BT may}be due

to different molecularweight and _{polydispersity}ofthe P3BTsamples. Both P3BT and P3HT

have the same the re- _re

stacking distance between their crystallized _chains, however P3BT

packs more closely compared to P3HT across its interdigitated alkyl chains (1.26 nm and

1.64 nm respectively). _{Systematic study} of the optimal _annealing temperature for P3HT

samples with various molecular weight and _{polydispersity} has shown that the overall

efficiency, as well as the optimal _{annealing temperature,} increases with _increasing

molecular

weight.29

Therefore, the synthesis and characterization of other

poly(3-alkylthiophenes) such as, P30T and P3BT, is not as optimal asfor P3HT. Alotof attention

has been givento the regioregularityofthe polymer, but onlylimited studies on the effect

of the molecular weight. Actual molecular weight _may be the most important factor in

controlling annealingconditions and _improvingchargetransfer.

The most typical effect of degradation in organic solar cells is the overgrowth of

PCBM crystals.3

Properties of P3AT diminish after over annealing due to a complete

segregation of P3AT into unconnected nanorods for polymers of lower masses. The

diminishing properties_mayalso have been caused _byelectrode degradation due the higher

temperature _annealing required forhigh molecular weight materials. Polydisperse samples

enhanced the ability of the polymers to self-organize on _annealing at optimized

temperatures.29

Studying the glass transition temperature of higher molecular weights P30T and

P3BT samples _may give insight to the preparation conditions and optimal _annealing

Chapter 2 : Fullerenes 17

Due to the higher devices performances with the _{P3HT system,} a side-by-side

comparison of C60 and C70PCBM was conducted _using this polymer. _According to the

energy level alignment, using the C70 instead of the C60 creates more allowed

HUMO-LUMO transitionsandthereforeincreasesthecurrent

density.32 Energy(eV) Fullerenes 2.96 ~ i :4 Oito=4.7 : :5 :6 -3.7 339 t I t I <x !

CH-fj

Oai=_4.3

P3HT C60PCBM C70 PCBM

Figure 15:_Energylevelfor C60and70withP3HT

UsingtheC70PCBM resulted in increases inthecurrent _densityandthe open circuit

voltage. Thisresult is consistentwith the different_energy level ofC70and C60(Figure 15).

Annealingseemsto havethe same effect on both fullerenessincethefill factor is thesame in bothcases.

o E o < -0.8 -1.6 in -3.2 c <D Q 13 o -4 -4.8 -5.6 -[C70]PCBM-P3HT [C60]PCBM-P3HT

Jsc Voc FF

(mA/cm2) (mV) (%)

C60 4.91 478 42

C70 6.41 511 41

0.50

Voltage_(V)

Chapter 2 : Fullerenes

I

Thespectral response showsan enhanced quantum_efficiencyintheregionbetween

1.6and 1.8eV as well as abroadershoulderin the_{high energy}region(Figure 17).

C60-P3HT

C70-P3HT

0.5 1.0 1.5

"5B

Energy (eV)

Figure 17: Spectral response ofdevices made _{using C60} PCBM andC70 PCBM with P3HT(1:1 w/w). In

onset, theresponsefromtheC70isenhancedforthe_{lower energy}

Published times for _annealing conditions _{vary considerably} between groups as a

result of _processing conditions and _starting materials. Layer thickness of the active layer

variesfrom80 nmto 175 nm. _{Similar efficiency}wasobtained when _{temperature annealing}

rangesfrom 110Cto 158Cwithtime_varying between 2 minutes and 2hours.3,4'33There is no common agreement _regarding what time interval the _annealing should be performed.

Annealing priorto the aluminum deposition _{(pre-annealing)} creates an even surfaceforthe

contacts. On theother _hand, sincetheorganic layer is not protected yet _by the _aluminum,

degradationmaybe fasterthan afterthemetal deposition. Insome_{case, pre-annealing may}

not be _necessary due to the equipment used for the contacts evaporation. _Depending on

the equipment and the _throwing distance, some evaporators will generate more heat and

therefore some _{pre-annealing may happen in} the evaporator. _Annealing afterthe contacts

deposition _{(post-annealing)} is generally performed at highertemperature since the goal is

the recrystalization of the

polymer.33

Since these conditions _vary between groups, both

techniques were _{investigated using} a 0.8: 1 PCBM/P3HT ratio based on the improved

stabilityand high _{reproducibility}attained _byMaet

al.3

Chapter 2 : Fullerenes

I

from Rieke metals, and _processing conditions described previously, the best-reward

combination included _{pre-annealing} at 110C for 30 minutes under argon and

post-annealingonahotplatein a gloveboxat140Cfor30minutes.

PEDOT:PSS

ITOGlass

Jsc (mA/cm2)

Voc (mV)

FF _n

(%i

Noannealing 2.22 385 48.5 0.41

Pre-annealing 4.05 452 47.0 0.86

Pre+ post anneal_ing 5.06 570 51.4 . -:

E

o

-1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Voltage(V)

Figure 18: P3HT:PCBM(1:0.8) in DCBwith _{pre-annealing}at110Cfor 30minutes under argon and post-annealingon ahotplateintheglovebox at140Cfor 30minutes.TestedunderAM 1.5

Theoretically, the Voc should decrease with annealing. _{During annealing} the n-n

orbital interactions _{increase, resulting in heighted HOMO levels,} and a reduced bandgap.29

In this study, the Vocwas increased _during the first annealing as compared to the

non annealed devices. _Annealing at this lowertemperature _may have reduced the defects

attheinterfacewiththecontactsand provided a superior metal deposition. _{Post-annealing}

was also_necessarytoachieve highercurrent_densityaswellas a higheropencircuitvoltage.

Thefill factor_(FF) value indicates the internal resistance towards chargesflow. The

improved FF with pre and post _annealing suggest a reduction in defects and the formation

offibrils_during recrystalization,allowingtheformationofpathwayforcharge

transport.31

Since annealingcauses phase segregation, a lower PCBM:P3HT ratio was chosen. In

theory in order to obtain an enhanced _{crystallinity,} annealing should be performed higher

than the glasstransition temperature of 110C for P3HT.34

The improved _{crystallinity may}

Chapter 2 : Fullerenes

I

decreasewhilethefeaturesrelated tothepolymerincreases.Witha lowerratio,the carrier

transportbecame more efficient and carrier_trappingissuppressed.35

In orderto further improve the performance of _{poly(alkyl-thiophenes) devices,} the

effect of PCBM ratio in combination with _annealing temperature should be investigated.

Since crystallization has a dramatic impact on charge _transport, techniques such as _XRD,

AFM and other characterization methods_maybe usedto relatenanostructure_orderingwith

annealing. Charge carrier _mobility could also be studied _usingfield effecttransistors _(FET),

or other techniques to further investigate the effect of molecular weight and

regioregularity. P3ATswith longer alkyl chainsand highermolecularweight should beused

in ordertoincreasetheopen circuit voltagewhich_currently limitsthepower efficiency.

2.3 MEH-PPV

Dueto the high _efficiency devices obtained _usingP3HT, little workhas been doneon

MEH-PPV. Based on the knowledge developed with _P3HT, the same concepts of molecular

weight,solvent effects and thicknessoptimization were appliedforMEH-PPV.

2.3.1 Chain orientation andthickness

The increased electrical properties of devices after_annealing ofthe P3HT is due to

the enhanced _packing parallel to the substrate after recrystalization which is observed for

regioregular

samples.13

Two factors strongly influence chains _{packing in MEH-PPV:} the

solvent and the molecular weight ofthe polymer. When the polymeris spin-coated froma

solvent_containingbenzene rings,thechains will orient such astheside chains pointinwards

toward each other (Figure 19). Such orientation creates a _highly ordered system. In

non-cyclic _{solvents, the} chains will have a twisted conformation which does not favor close

Chapter 2 : Fullerenes

I

(a) (b)

Figure19: MEH-PPVchain conformation whendeposited from_(a)a solventcontaininga_{benzene ring}and (b),without_{benzene ring}

The improved nanostructure created _byusing a cyclicsolventhasa direct impacton

device performance as characterized _by current-voltage measurements (Figure 20).

o-xylene was chosen as the aromatic solvent since it has been shown that it results in

better polymer chain _{stackingresulting}in betterelectrical conduction 36

H I Cl

ci

Chloroform

4.0

Chloroform

-N . Xylene

I

E

f

c CD

Q

o

--4.0

--0.8 -0.4 0.0 0.4

Voltage (V)

Chapter 2 : Fullerenes

I

A second parameter _influencing the orientation of the chains is the molecular

weight. Most low molecular weight MEH-PPV have a random orientation of their chain

segments, whilethe highest Mwhave most oftheirchains parallel tothefilm plane. Similar

to _P3HT, the parallel orientation of chains _{may benefit} charge transport. The refractive index and the absorption coefficients are also increased due to the parallel chain

orientation.37

Using the same conditions with a 4:1 PCBM:MEH-PPV weight ratio, the efficiency obtained with higher molecular weight is more than double the _efficiency

obtained fromthe lowestmolecular weight combination.

i2 55 0 n. d) Q -2 Low Mw Medium Mw " High Mw VI- ^--^^^ III -0.8 Mw Jsc (mA/cm2) Voc (mV) FF (%) 1 (%) Low

Mw: 40,000-70,000

2.07 747 36.0 0.57

Medium

Mw:70,000-_100,000 1.88 808 37.4 0.57

High Mw: 150,000-250,000

3.89 835 38.5 1.25

0.8 -0.4 0.0 0.4

Voltage_(V)

Figure 21: J-V Comparison ofMweffect on theperformance ofMEH-PPV:PCBM organic solar with a4:1

weight ratio(Testedunder_AM1.5)

The same _annealing was performed on all devices at low temperature. _Annealing has been shownto deterioratethedevices' performances attemperatureaslowas50Cfor

30 minutes.Sincetheglasstransition temperaturefor MEH-PPV is 65 C38 _bycomparisonto

110C for _P3HT, lowertemperatureannealingshould beused.

Since the absorption coefficient is affected _by the molecular _weight, the optimal

thickness for effective chargetransport was studied. _Spin-coatingspeed has a directeffect

on the performance of the devices due to the limited hole _mobility in organic

semiconductors. From the current-voltage characteristic _{graph, the} open circuit voltage

Chapter 2 : Fullerenes

I

However, the current densities as well as the FF increase substantially with higher

speed, which correspondsto lowerthickness. The maximum _efficiency was obtained when

thethicknesswas 130 nmas measured _usinga contact profilometer.

3.0 2.0 OJ E o 5 1.0 E.

If

750 230 1.29 775 27.4 0.27

1500 200 1.72 S02 32 6 0.45

2000 130 1.88 808 37.4 :.

V--1.0 -0.5 0.0 0.5 1.0

Voltage_(V)

Figure 22: l-Vcurves ofMEH:PPV-PCBM(1:4_byweight)spin-coatedfromo-xylene atdifferentspeeds.

The improved chargetransferinthe polymer can alsobe observed _usingthespectral

response_data, where the peak associated with the polymer (around _{2.4 eV) increases} with

a decrease inthickness.

Z3 750rpm i

4

T

-gl5

\\

LU a uj 10 -l\ 5 n .. f^

1.0 1.5 2.0 2.5 3.0 3.5

Energy(eV)

4.0

Figure 23: Effect of spin-speed on External Quantum _{Efficiency (E.Q.E.)} for MEH-PPV:PCBM (1:4 _by

Chapter 2 : Fullerenes

I

This enhanced response from the polymer illustrates the holes mobility limitation.

For comparison, the featuresassociated with C60PCBM (around 3.5 eV) increase from 750

rpmto1500rpm but_stayconstant whenthespeed isfurtherincreased.

Thehigherefficiencies are obtained with a thicknessof130 nm. _{Since annealing} has

aneffectonthedistribution ofPCBMaggregates within the polymer,athickerfilmcould be

possible withoptimized PCBM ratio and_{annealing temperature.}

2.3.2 Fullerenesoptimization

Deviceswerefabricated _{using C70 PCBM instead} oftheC60 PCBM. Thiswasdoneto test

ifthe _{improved efficiency}would be similar towhatwas _previously observed with P3HT. In

the case of _MEH-PPV, the open-circuit voltage stayed _constant, while the current _density

was _{considerably increased by}the use ofC70 PCBM instead ofthe _{C60. The efficiency}from

C60toC70rosefrom 1.25%to2.06%which_{is substantially}greaterthan the P3HTresult.

w c Q c 0)

O 6

C60

4 C70

2 //

0

2 >w

4 >s

6

i i i i 1 i i ii

Jsc Voc FF n

(mA/cm2) (mV) (%) (%)

C60 3.89 835 38.5 1.25

C70 5.17 835 47.7 2.06

-0.8 -0.4 0.0 0.4 08

Voltage_(V)

Figure24: l-Vcurvesfor MEH-PPVwithC60PCBMorC70 PCBM(1:4_byweight).TestedunderAM1.5

Based on the metal-insulator model, the addition of C70 should result in an

increased open circuit voltage compared to the C60 basedontheir respective_energylevels

Chapter 2 : Fullerenes

I

energy of such systems should be investigated to understand the energytransfer process and the possible efficiency.

The effect ofPCBM ratio was also considered in orderto optimize the performance

of MEH-PPV devices. The 1:4 weight ratio between MEH-PPV and PCBM is considered standard for most groups.17,39,40

Based on the recent improved _efficiency of P3HT devices due to lower PCBM content, the ratio was _{slightly lowered from} 3 to 4 in order to find

optimal conditions. From Figure _25, the current _density is almost the same for a 3.5 or 4 ratio, while it is _{significantly lowered for} a ratio of _{3. The Voc steadily} decreases when

loweringthe PCBMratio.Theoptimal conditions arethen thestandard 1:4ratio. Additional

research would be required_usingvarious_annealingconditionstooptimizethePCBMratio.

1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Voltage_(V)

Energy (eV)

(a) (b)

Figure 25: _(a) l-V curves for different C70 ratio in MEH-PPV:PCBM devices _(AM1.5) and _(b) spectral

response

4.0

From the spectral _response, the PCBM peak (~_{3.4 eV)} decreases with _increasing

PCBM ratio. The polymer peak (~2.5 _eV) follows a different trend since it reaches a

maximumforthe 3.5 ratio. More _interestingthan theobserved maximumofthe polymer is

LU

6

LU

Chapter 2 : Fullerenes 26

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Energy (eV)

Figure 26: MEH-PPV/C70optimal spectral response overlaid withtheAM 1.5spectrum

The effect of temperature on MEH-PPV has not been investigated to the same

extent as on P3HT. _Preliminarydata on MEH-PPV indicateda decrease in performance and

degradation after_{only 30}minutes at80 C. Howeverwith a higher molecular weight

MEH-PPV, an _{annealingstudy}could be beneficial and allow higher efficiencydevices with higher

Chapter 3 : Quantum Dots 27

Chapter

3: Quantum Dots

3.1 Experimental

3.1.1 Covalentsynthesis _ofCdSe QDs

CdSe quantum dots are the most _widely known and studied system due to their

relatively simple synthesis and characterization. For this _{synthesis, 25.68 mg (0.2mmol)} of

cadmium oxide was added with _{227.6 mg} of stearic acid (0.08 _mmol) and 2.54 mL of

1-octadecene_(ODE)toathreeneckflaskconnectedtoaSchlenkline_followingtheestablished

protocol.41

Thesolution was pumped and purged 3 timesbefore _raisingthe temperature to

200 Cto obtain a clear solution.Theflaskwas cooled to room temperature and _{1.5 g} (5.5

mmol)of octadecylamine_(ODA) and _{0.5 g}oftriocylphosphine oxide _(TOPO)were addedto

thesystem and _thoroughlydegassed.Thereaction mixture wasraised to280Cwhere2mL

ofcold stock solution was injected (1 _{M TOPSe previously} prepared in the glovebox). The

reaction temperaturewas reduced to 250 C and holded forone hourto obtain largeQDs.

Theproduct was _repeatedlywashedin a_separatoryfunnel usinghexaneand methanol until

a bright_toplayerwasobtained.

Since cappingagentsmaysuppressthechargetransferprocess, ligandexchangewas

performed on the QDs to replace the organic _{capping covering} the nanoparticles with

pyridine.7, 22

Replacing the _long chain _fatty acid for a shorter _element, such as _pyridine,

allows asuperior contact between the donor and acceptormaterial.Afterprecipitation and

removal of _{the supernatant,} a small amount of pyridine was added to the _QDs, and the

mixture was sonicated for 10 minutes prior to precipitation _by hexanes. The ligand

exchange was repeated 3 times before _drying the QDs at 70 C. Thesame ligand exchange

processwasusedon allQDssystems priortodevice fabrication.

Deviceswerefabricated usinga 1:2 _{(MEH-PPV:CdSe)}weight ratio ino-xylene. Forall

QDs, absorption spectra were performed on a Perkin Elmer Lamda 900 UV-VIS-NIR

Chapter 3 : Quantum Dots

I

Fluorometer in orderto obtain information aboutthe electronic transitions _(bandgap) and

thesizedistributionof nanocrystals.

3.1.2 Non-covalentsynthesis _{of InP}nanocrystals

The colloidal synthesis was based on the non-covalent synthesis of lll-V QDs

developed _byPeng.25

InP QDsarecurrently themostsensitivesystem to theligand lengthas

well asto the presence of air and oxygen. In a typical reaction, 29.2 mg of indium acetate

(0.1 _mmol) was added to 68.5 mg of myristicacid (0.3 mmol) and 5 mL ofODE in a

three-neckflaskand heatedto 120C. Thesolution was pumped atthis temperaturefor 1 hourto

remove_any moisture or airinthesystem.Afterrepeated _pumpingandpurging,thereaction

flask was further heated to 300C under argon. At this temperature, a 2 mL solution

containing tris(trimethylsilyl)phosphine _(P(TMS)3) was injected (12 ul of _PTMS3 + 2 mL

anhydrous ODEwas prepared in the glovebox).Thetemperaturewas reducedto 270 C for

crystal growth and maintained for 30 minutes. The product was cleaned through a similar

technique usedfor_CdSe, while _usingamixture of methanoland acetone _(1:9)to precipitate

and hexaneto resuspend theQDs. Deviceswere prepared afterligand exchange_{using P3HT}

with a1:1weight ratioin DCB.

3.1.3 Covalentsynthesis _ofInAsnanocrystals

InAs QDs were synthesized _following established procedures _using

tris(trimethylsilyl)arsine with lnCI3. The synthesis of the tris(methylsilyl)arsine _(As(TMS)3)

was adapted from

literature.42

In atypical synthesis, 1.7 g of sodium (73.5 _mmol)was_finely

cut and added with 29.4 mmol of arsenic and 70 mL of anhydrous toluene into a 3-neck

flask. The solution was heated to reflux while _stirring for 48 hours. After cooling to room

temperature, 73.5 mmolofchlorotrimethylsilanewas added _slowlyto the reactionflaskand

Chapter 3 : Quantum Dots 29

After_{filtration, the resulting} liquid wasdistilled to removethe excess tolueneor _any

othervolatile byproducts. Dueto the _{high reactivity}of_{the precursors,}theQDsynthesis was

conducted inthe glovebox. InAsQDswere prepared _{by injecting}a _lnCI3solution mixed with

As(TMS)3 into Trioctylphosphine _(TOP) at 300 C, and the reaction flask was maintained at

260 C for growth43. Aliquots were taken at regular intervals and quenched in anhydrous

tolueneforcharacterization (fluorescence and absorption).The product was cleaned _usinga

toluene/methanol precipitation technique followed _by resuspension. The ln:As ratio was

varied _during the _{initial mixing} before injection. The photovoltaic devices were made _by

dissolvingthe InAs QDs in asmall amountof 1,2-dichlorobenzene_(DCB) before_beingadded

in different mass ratiosto the MEH-PPVand the PCBM. The reference MEH-PPVdevicewas

made_{using 7.5} mg/mLMEH-PPV in o-xylene.Theweightratios ofMEH-PPVand PCBM were

kept constant at 1:4 while different ratios of InAs QDs were added to the composite. An

annealing stepat 110Cfor1hourunder argonfollowedthecomposite spin-coating.

3.2 CdSe Quantum dots

Due to their simple synthesis and focused size _{distribution,} the first quantum dots

investigated were CdSe. The principal difference between bulkcrystals with an infinite size

and quantum dotswith afinal size is the surface_atoms, which possess_dangling bondsthat

must be stabilized. In colloidal _{synthesis, the} nucleation takes place _immediately after

injection and continues until the temperature andthe monomerconcentration _drop below

a critical threshold. The nucleation kinetics is difficult to study. _{After nucleation,} there are

two distinct kinetic regimes. _{First, during} the _focusing stage, the average nanocrystal size

increases relatively rapidly and the size distribution is focused. When all the monomer in

solution has been used for the QDs growth, the defocusing regime begins. The monomer

concentration is lower than the nanocrystals concentration. _Therefore, the smaller

nanocrystals, which have the lower solubility, start _shrinking and _releasing monomer

through Ostwald ripening.Thebiggernanocrystalsusethe monomerinsolutionto continue

their growth, which results in a broadened size distribution. To minimize this effect,

Chapter 3 : Quantum Dots

I

The biggest QDs that could be prepared _{using this technique} had an absorption

around 2.1 eV. The repeated _cleaning process has the advantage of _removing most ofthe

smaller QDs from the solution therefore _contributing to a smaller size distribution and

enhanced fluorescence. MEH-PPV was chosen as the conjugated polymer dueto its lower

absorption compared to P3HTwhich _mayallowtheobservation of_sub-bandgap absorption

around 2.1 eV. _{The energy} levels ofCdSe QDs andthe polymer are well _{aligned, therefore,}

thedevices' performancesshould be improved_bytheaddition oftheQDs.

Energy (eV)

2.26

4 Oito=4.7

4.94

~\

3.32

*a,

5.5- 5.78

MEH-PPV CdSe QD

Figure 27:Energylevel diagram for CdSe QD: MEH-PPVcomposites

Theaddition ofCdSe QDs increasesthecurrent _densitywhen comparedto thepure

polymer. _However, the performance ofthe reference _device, a typical MEH-PPV:PCBM (1:4

weightratio), is still superiorin comparison to thecomposite _containingtheQDs. _Currently

onlytwo groups have published results for CdSe: MEH-PPV composite blends. In one _case,

with a 90% wt of _CdSe, the current _density was approximately

2uA/cm2

with a Voc of

0.72V,24

while the other _group used 80% wt of _CdSe, and obtained current _density of 2.6

uA/cm2

with a Voc of

0.58V.23

In both cases, the performances of the devices are

E o

E

w c Q) o

c

I

O

-1.1

-1.8

Chapter 3 : Quantum Dots

I

MeH-PPV

MEH-PPV+QD(1:2)

MEH-PPV+PCBM _(1:4)

0.0 0.2 0.4 0.6 0.8

Voltage_(V)

10 1.2

Figure 28: l-V curves for MEH-PPV with CdSe QDs and C60 PCBM in different ration. Tested

underAMO

Theaddition oftheCdSe QDsto thepure polymerincreased boththecurrent_density

andthe open circuit-voltage.The current_densitydifference between thedifferentgroupsis

most _likelydue toefficientchargetransfer which is affected _by ligand exchange. In _theory,

the Vocshould be similar sincethe open circuit voltage should be a function oftheenergy

level ofthe materials in the composite3. _Using differentsizes of quantum dots may slightly

change the quantum dots bandgap, therefore _changing the Voc but not _sufficiently to

explain a difference of 0.3 V between our measurements and those reported earlier.

Surface defects or partial _shunting ofthe devices mayalso lower the open circuit voltage.

Considering that both the current _density and the open circuit voltage were _{significantly}

higherthanwhat was published _byothergroups, indicates betterprocessingconditionsthat

wouldtherefore beappliedforother quantum dotscomposites.

The QDs' sizeis closeto the absorption edge ofthepolymer which results inthe

sub-bandgapabsorption not ableto be_directlyobserved in thespectral response. _However, the

QDs improve the charge dissociation since the entire spectral response curve is increased

and not _onlythe region between 2 and 2.2 eV. In _{particular, the} region between 3 and 3.8

Chapter 3 : Quantum Dots

I

2 2.5 3

Energy (eV)

Figure 29: UV-visspectra oftheCdSe QDsused fortheMEH-PPV:CdSe QDscomposites withthe

ExternalQuantum _{Efficiency(E.Q.E.)}ofthedifferentcomposites.

UsingQDs with lower energy absorption could result _{in sub-bandgap} absorption. In

order to do so, new injection of precursors at low temperature would be _necessary to

increasethe monomer concentration and therefore allowfurthergrowth oftheQDs. Since

the growth temperature is considerably lower than the injection _temperature, this

techniquecan be appliedforCdSe withoutformationof new nuclei.

These devices were _primarily prepared to establish a fabrication procedure for

composites devices made with QDs. This objective was_successfullyaccomplished sincethe

addition ofCdSe QDsafterligandexchangedemonstratedenhanced performance.

3.3 Indium Phosphide

Since indiumphosphideQDssynthesis issimplerthanotherlll-Vsystemssuch as InSb

or _{GaSb, they} were synthesized first in preparation for the InAs QDs using a

non-coordinating solvent approach. Forthis technique, the _reactivity of the monomers can be

tuned _by varying the ligand concentrations in the non-coordinating solvents. This tunable

reactivity provides a _way to balance nucleation and growth. This cannot be controlled _by

Chapter 3 : Quantum Dots 33

temperaturefor lll-Vto provide high qualitycrystalline QDs.The optimal reaction involvesa

short and fast nucleation followed _by a growth stage without new nucleation or ripening.

Too _longof a nucleation period will resultin insufficient monomer left in solution toallow

sufficient QDs growth and therefore result in unstable

nanocrys