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Research ArticleOrganic Solar Cells with Boron- or Nitrogen-Doped CarbonNanotubes in the P3HT : PCBM Photoactive Layer

Godfrey Keru,1 Patrick G. Ndungu,1,2 Genene T. Mola,1

Ana F. Nogueira,3 and Vincent O. Nyamori1

School o Chemistry and Physics, University o KwaZulu-Natal, Westville Campus, Private Bag X, Durban , South AricaDepartment o Applied Chemistry, University o Johannesburg, P.O. Box , Doornontein, Johannesburg , South Arica

Chemistry Institute, State University o Campinas, P.O. Box , Campinas, SP, Brazil

Correspondence should be addressed to Vincent O. Nyamori; [email protected]

Received September ; Revised December ; Accepted December

Academic Editor: Christian Brosseau

Copyright Godrey Keru et al. Tis is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Either boron- or nitrogen-doped carbon nanotubes (B- or N-CNs) were incorporated in bulk heterojunction organic solar cellsphotoactive layer composed o poly(-hexylthiophene) (PH) : (,)-phenyl-C

61-butyric acid methyl ester (PCBM). Te physical

and chemical properties were investigated using different spectroscopic techniques. Te cell perormance was ollowed rom theircurrent-voltage (-) characteristics. Recombination dynamics o the photo-generated ree charge carriers were investigated using

micro- to milliseconds transient absorption spectroscopy (AS). ransmission electron microscopy (EM) images revealed thepresence o cone structures and bamboo compartments in B-CNs and N-CNs, respectively. X-ray photoelectron spectroscopy(XPS) revealed very little boron was substituted in the carbon network and presence o pyrrolic, pyridinic, and quaternary specieso nitrogen in N-CNs.-characteristics were ound to be similar or the devices with B- and N-CNs even though boron- andnitrogen-doped CNs are known to have different properties, that is, p-type and n-type, respectively. AS results showed that alldevices had long lived ree charge carriers but the devices with B- or N-CNs had low power conservation efficiency and voltage.

1. Introduction

Te demand on energy supply has become a key concern withthe advancement and development o nations. Te currenttrend suggests that the uture global economy is likely to

consume even more energy. At the moment, the main sourceo energy is still ossil uels (oil, coal, and gas). However, thedwindling size o global ossil uel resources compoundedwith unavourable climatic changes due to emission o oxideso carbon by the use o these uels calls or the need to lookor an alternative source o energy. Renewable sources whichare environmentally riendly and inexhaustible are the best

alternative [, ]. Among the renewable resources o energy,solar energy is one o the most abundant resources andcan easily be converted to electricity by use o photovoltaiccells []. Bulk heterojunction organic solar cells (OSCs)whose photoactive layers are composed o p-type conjugatedpolymer and ullerene derivatives are the most promising

device structure or solar energy conversion. Te advantageso using OSCs over others include low device manuacturingcost using solution processing to ultrathin lm and exibleand light weight substrates [, ]. Recently, power conversionefficiencies above % have been reported or OSCs [],

which is the minimum value required or commercializationo OSC in the energy market [].

One o the major limitations o OSCs is low diffusionlength o the excitons that ranges between and nmwhich causes high recombination and low power conversion[]. Tis difficulty has been eased by the introduction obulk heterojunction (BHJ) design in which the donor andacceptor molecules are blended in the photoactive medium.Such mixture o donor-acceptor molecules increases theefficient dissociation o excitons in the medium by creatinglocal donor/acceptor interaces. Although ree charge carriergeneration is improved using BHJ, high efficiency cannot beachieved due to the disordered nature o the medium that

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2016, Article ID 5923402, 11 pageshttp://dx.doi.org/10.1155/2016/5923402

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Journal o Nanomaterials

prevents the smooth transport o charges. Instead chargesare orced to use percolation pathsvia a hoping mechanismto reach electrodes [, ]. Low charge carrier mobility inthe bulk o organic materials can be addressed by the incor-poration o one-dimensional nanostructures such as carbonnanotubes (CNs) which provide a high mobility pathway or

charge carriers within the semiconducting materials [].CNs in the photoactive layer o OSC improve not onlycarrier mobility but also mechanical exibility o the polymermaterials and are also compatible with throughput solutionprocessing []. Despite such attractive advantages o CN,their incorporation in the photoactive layer o OSCs ofenhas resulted in poor perormances than those without CNs[]. Derbal-Habak et al. [] investigated the cell efficiencyby varying wt.% SWCNs, that is, between and . wt.%o ester unctionalized HiPCo SWCNs. Teir ndings indi-cated that the reduction was directly proportional to thepercentage weight ratio o SWCNs, with . wt.% having thehighest reduction. Even when CNs in the photoactive layerare purported to enhance efficiencies, values reported arestill very low or commercialization. For instance, Stylianakisand Kymakis [] reported an improvement rom . to.% in addition to .% unctionalized single-walled CNs(SWCNs). Tis enhancement was attributed to efficientexcitons dissociation and improved hole mobility. Khatriet al. [] reported enhanced photovoltaic properties byintroducing a combination o SWCNs and unctionalizedmultiwalled CNs (MWCNs) in poly(-octylthiophene)(PO)/n-silicon hybrid heterojunction solar cells. SWCNsassisted in excitons dissociation and electron transer whileMWCNs in hole transer. Kuila et al. [] incorporated CNsin the polymer matrix by grafing PH on the walls o CNsvia ester bonds and an efficiency o .% was obtained orthis cell.

On the other hand, there have been reports that suggestthere are positive effects with the introduction o either B-CNs or N-CNs in the photoactive layer o OSCs [, ].Te possible reason or this enhancement can be due to theB- or N-CNs serving very specic role with regard to thecharge transer in the medium. B-CNs usually behave asp-type semiconductors with high work unction (. eV)that matches well with the highest occupied molecularorbital (HOMO) o the donor polymer (PH) (. eV)and make them selective to hole transer []. However, N-CNs behave as n-type semiconductor with work unction(. eV) close to the lowest occupied molecular orbital

(LUMO) o the receiver materials, which are commonlyullerene derivatives such as -(-methoxycarbonyl)-propyl--phenyl-[6,6]C

61(PCBM) (. eV) []. It is energetically

avourable or electrons to move rom the LUMO o theacceptor to N-CNs; thereore, they are electron selec-tive. For example, Lee et al. [] reported % increase inefficiency with the incorporation o N-CNs in the pho-toactive layer poly(-hexylthiophene) : indeneC

60bisadduct

(PH : ICBA/N-CNs) rom . to .%. Te same grouphad earlier reported high efficiency by incorporating . wt.%o either B- or N-CNs in the photoactive layer o the cellPH : PCBM which resulted in an efficiency o .% and.% or B- and N-CNs, respectively. Tese high efficiencies

were attributed to selectivity o charge carrier transport in themedium.

In this paper, we investigated how doping o CNswith either boron or nitrogen affects the perormance oOSCs when incorporated in the photoactive layer o thecell composed o either PH/B- or N-CNs : PCBM. X-

ray photoelectron spectroscopy (XPS) was used to analysethe elemental composition as well as the chemical bondingenvironment o boron or nitrogen. Atomic orce microscope(AFM) and scanning electron microscope (SEM) were usedto determine the morphology o the photoactive lm layer.Ultraviolet-visible (UV-Vis) and photoluminescence (PL)were used to determine absorbance and emission, respec-tively. ransient absorption spectroscopy (AS) was used todetermine the lie-time o dissociated charge carriers andthe recombination dynamics. AS is an excellent techniqueor investigating recombination dynamics in the photoactivelayer o a photovoltaic device. o the best o our knowledge,this is the rst report on reasons why despite enhancingproperties o CNs by doping with either boron or nitrogen,their perormance in OSCs was opposite o the expectation.

2. Experimental

.. Materials and Methods. Chemicals and solvents usedin this study were o analytical grade and used as receivedunless otherwise stated. Benzyl alcohol (%), thionyl chlo-ride (%), poly(-hexylthiophene),-dyl (PH) (%),-(-methoxylcarbonyl)propyl--phenyl-[6,6] C

61 (PCBM)

(%), and poly(,-ethylenedioxythiophene) : polystyrenesulphonate [PEDO : PSS] (%) were purchased romMerck, Germany. ,-Dichlorobenzene (%) and isopropyl

alcohol (%) were sourced rom a different company, thatis,Vetec Qumica Fina Ltda., Brazil.

B-CNs weresynthesized usinga modiedmethod previ-ously reported in the literature []. Briey, a mixture o er-rocene (. wt.%) as the catalyst, triphenylborane (. wt.%)as the boron source, and toluene (to make wt.%) asthe carbon source was pyrolysed at C in a quartzreactor tube in a muffle urnace. A mixture o wt.%hydrogen gas in argon was used as a carrier gas as wellas reducing agent. N-CNs were synthesized as reportedearlier by our group []. Briey, . wt.% o (-{[(pyridin--yl)methylidene]amino}phenyl)-errocene catalyst was dis-solved in acetonitrile solvent to make wt.% solution. Tis

wasollowedby pyrolysisat

Cinaquartzreactortubeinamuffle urnace. As-synthesized B- and N-CNs were puriedand unctionalized by reuxing with M nitric acid or hr.o enhance the CNs dispersion properties in solvents, theywere urther unctionalized with benzyl alcohol as illustratedin Scheme .

.. Preparation o B- and N-CNs/PH : PCBM Solution.Solution or the devices was prepared using a modiedmethod previously reported in the literature []. Briey,.wt.% o B- or N-CNs to the weight o PH wasweighed and dispersed in . g o ,-dichlorobenzene andbath-sonicated by using ultrasonic cleaner model UDSH

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C

C

CC

O

O

O

O

OH OH

OH

HO

C

C

CC

O

O

O

O

Cl Cl

Cl

Cl

C

C

CC

O

O

O

O

O

O

O

OH

Reux DMF, heat

Heat,

O

24 hrs

SOCl2

24 hrs

6 M HNO3

S : Functionalization o B- and N-CNs with benzyl alcoholviaester bond.

operating at kHz or hr. o this mixture, mg o PHand mg o PCBM were added to the dispersion and urthersonicated or hr. Te mixture was then transerred to astirring plate and stirred or hr at C, in the dark. A

solution o PH : PCBM without CNs was also preparedin a similar manner.

.. Device Preparation. Devices were prepared on indiumtin oxide (IO) coated substrates (Lumtec, ). Part o theIO was etched by putting a paste o zinc metal on the partthat needed to be etched then put in a M hydrochloric acidsolution. Te etched substrate was cleaned by sequentiallysonicating with a detergent solution, distilled water, acetone,and isopropyl alcohol, or min in each case. Clean sub-strates were dried in a stream o dry nitrogen gas. A L

o PEDO : PSS was spin coated on the cleaned substratesat rpm or s and then annealed or min at C.A L photoactive layer B- or N-CNs/PH: PCBM wasspin-coated at rpm or s and then annealed in thedark or min at C. A thin buffer layer o lithiumuoride (. nm) and nm thick aluminium electrodes were

vacuum-evaporated at . 6 mbars. Tin lm o thephotoactive layer was spin-coated rom the same solutionand similar conditions on a glass substrate without IOor AS and AFM characterization. A similar solution oPH : PCBM without CNs was also prepared to makedevices or comparative purposes. In all the devices, spincoating o the solutions was carried outin ambient conditions

and then quickly transerred into a glove box or annealingand characterization.

.. Characterization echniques. As-prepared B- or N-CNs structures were determined with transmission electronmicroscopy (EM), JEOL JEM transmission electronmicroscope,at kV. Images were captured using Megaview camera and then analysed using iEM sofware. Sampleswere dispersed in ethanol by sonication and then depositedon carbon coated copper grids. Morphology o the thinlm was determined with AFM, Agilent Pico Scan microscope in a tapping mode. Images were acquired usingNanosur Easyscan . For SEM, a JEOL-JSM- LV operat-ing at kV was used. Images were acquired using JSM-LV sofware.

Te elemental composition, as well as the chemicalbonding environment o boron- and nitrogen-doped in theCNs, was analysed with X-ray photoelectron spectroscopy(XPS), Termo VG Scientic Sigma Probe instrument. Temonochromatic Al KX-ray source (. eV) was ocusedon the sample (size m). Te highresolution spectra werecollected by passing energy o eV at energy step size o. eV with ood gun turned on to eliminate surace charge.

Absorption o the PH : PCBM lms and or those withB- or N-CNs wasdetermined with UV-Vis, model HpAdiode array spectrometer equipped with Hp UV-Vis Chem-station sofware. Photoluminescence quenching o PH byeither B- or N-CNs was determined using a dilute solution

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Tick wall

Cone structure

100 nm

(a)

Tin wall Bamboocompartment

100 nm

(b)

F : Structure o (a) B-CNs with cone shaped structures and thick walls and (b) N-CNs with bamboo compartments and thin walls.

o PH/B- or N-CNs in , dichlorobenzene with a ISS-PC spectrouorometer (using Vinci sofware), at nmexcitation wavelength. Solution or photoluminescence wasprepared as reported in the literature []. In brie, mg o

PH was dissolved in ,-dichlorobenzene to make mLsolution. For solutions with B- or N-CNs, . wt.% to theweight o PH o B- or N-CNs was added and then bath-sonicated or one hour. Necessary equivalent dilutions weremade to record spectra.

Charge recombination dynamics and the yield o reecharge carrier pairs that were generated in the photoactivelms layer (B- or N-CNs/PH : PCBM and PH : PCBM)were determined using microsecond to millisecond laser-based AS ollowing photoexcitation o the lms. AS kinetictraces were taken using a simultaneous pump and probesetup. A pulsed nitrogen laser (Photon echnology Interna-tional GL-) was used to excite a dye laser (Photon ech-

nology International GL-), producing emission at nmwhich was directed to the sample, and this induced electronicexcitation. A quartz halogen lamp (Bentham IL) and pho-todiode were used to measure the absorption characteristicso the samples at a typical probe wavelength o nm.Monochromators (Photon echnology International) wereemployed to rene the probe wavelength.

Hall Effect measurements were carried out using EcopiaHall effect measurements system, model HMS equippedwith HMS VER .. sofware in the presence oa . permanent magnet. A thin lm o B- or N-CNs was deposited on a glass substrate by drop-castingrom ,-dichlorobenzene solution. Aluminium metal was

thermoevaporated in van der Pauw geometry on top othe lm to provide our probes contacts. Current-voltagecharacteristics o the devices were determined using Keithley,model source meter, with a xenon lamp illumination at mW cm2.

3. Results and Discussion

.. Structure o B- and N-CNs. Pristine CNs usually havetubular structures which are hollow and can be modied withthe introduction o heteroatoms such as boron or nitrogeninto the carbon network []. Figure presents typicalEM images o the B-CNs and N-CNs. From the gure,

: Peak range or each and quantity (at.%) o boron, nitrogen,oxygen, and carbon in B- and N-CNs.

ype o CNs Peak position (eV) Element Atomic %

B-CNs.. B s ... C s .

.. O s .

N-CNs

.. C s .

.. N s .

.. O s .

we observed cone shaped structures and thick walls ( nm)or B-CNs whereas N-CNs had bamboo compartmentsand thin walls ( nm). Formation o cone shaped structuresand bamboo compartment were preliminary indictors o

incorporation o boron and nitrogen heteroatoms in thehexagonal carbon network []. B-CNs were also observedto have kinks and twisted compared to N-CNs whichwere relatively straight. Te kink and twist observed can beattributed to the size o B-C bonds which are larger than C-Cbond by about .% [].

Incorporation o boron and nitrogen in the carbonnetwork was urther assessed using XPS. For B-CNs, threepeaks were identied, that is, a peak at eV assigned to

B s,. eV assigned to C s rom sp2 hybridised carbon,and . eV or O s. For the N-CNs, three peaks werealso identied and assigned as ollow: at. eV or C s, at eV or N s, and at. eV or O s. able illustrates

the atomic % o each and peak ranges as determined by XPS.Te oxygen detected could have originated rom two

sources; that is, rstly, it could be introduced during acidpurication and, secondly, it could be rom physisorptionprocess through oxygen-doping on the CNs walls []. odetermine the elemental composition as well as the chemicalbonding environment o boron and nitrogen in B- and N-CNs, B s and N s peaks weredeconvoluted. Te B s peak at. eV wasdeconvoluted into peaks (Figure ). Lowerpeaks ( eV) areassociated with B-Cbonding while theones at high energy levels ( eV) are associated withoxidised species o boron and correspond to BC

2O, BCO

2,

and B2O

3[].

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186 188 190 192 194

Co

unts

Binding energy (eV)

BC2O (191.5 eV)

BCO2 (193.1 eV)B substituted C (189.5 eV)B4C (187.5 eV)

F : Deconvoluted B s peaks showing the bonding environ-ment o boron in B-CNs.

396 398 400 402 404 406 408

Counts

Binding energy (eV)

Pyrrolic N (400.8 eV)

Pyridinic N (398.9 eV)

Quaternary N (401.8 eV)

Molecular N (404.8)

F : Te our peaks o N s afer deconvolution.

From Figure , substituted boron peak was the smallestindicating that very little boron was incorporated into the

hexagonal network o carbon. B-CNs with low boronconcentration exhibit metallic character since insertion oboron in hexagonal carbon network results in large numbero acceptors near the Fermi level []. Effect o this in thephotoactive layer o OSCwill be discussed laterin Section ..

Te N s peak at .. eV was also deconvolutedinto peaks (Figure ) which corresponds to the differentbonding environments o nitrogen in a hexagonal carbonnetwork. From Figure , the highest peak obtained was thato pyrrolic nitrogen, ollowed by pyridinic and quaternary,and lastly the smallest represented the molecular nitrogen.Te carbon-nitrogen (C-N) bonds in quaternary or pyrrolic

environsprovidedelocalised electrons intothe sp2 hybridized

: Hall effects measurements and mobility o thin lm o B-and N-CNs.

ype oCNs

Bulk concentration(cm3)

Sheet concentration(cm2)

Mobility(cm2V1s1)

N-CNs .21 .16 .1

B-CNs .19 .14 .

carbon lattice. Tis induces sharp localised states above theFermi level due to extra electrons, and this results in the N-CNs behaving like n-type semiconductor or donor states.However, pyridinic type, where nitrogen is coordinated bytwo carbon atoms, induces localised states below the Fermilevel. Tese make N-CNs that were semiconducting to showmetallic (p-type) behaviour depending on the level o doping[].

Effect o this on the perormance in OSCs will bediscussed later in Section .. B- or N-CNs were mixed withpolymer PH to orm nanocomposites that were eventually

mixed with PCBM to orm part o the photoactive layer inOSC.

Behaviour o B-CNs as p-type and N-CNs as n-type semiconductors was urther supported by Hall effectmeasurements (able ). Te negative value or bulk con-centration in N-CNs and positive value in B-CNs werean indicator that they were behaving as n-type and p-type,respectively, semiconductors. Baumgartner et al. [] studieda lm o MWCNs using Hall effect measurements and, intheir ndings, a positive Hall coefficient value was obtainedand they concluded that the lm was predominantly a holeconductor (p-type).

.. Effect o B- or N-CNs on the Spectroscopic Proper-ties o the PH : CNs Blends. Photoluminescence o thenanocomposite (Figure ) was obtained rom a solution oPH/B- or N-CNs in ,-dichlorobenzene beore PCBMwas added. From the gure, we observed % quenchingo the emission spectra with the inclusion o N-CNs. Tiswas an indication o charge transer between N-CNs andPH. We attribute this quenching to the position o PHLUMO (. eV) and work unction o N-CNs (. eV)which are close to each other []. Tis makes it energeticallyavourable or electrons transer rom PH LUMO to N-CNs. Our observations differ with those o Lee et al. []who reported insignicant quenching or a solution o PH

and N-CNs. Tey attributed this to energy barriers betweensemiconductor (PH) and conductor (N-CNs). However,B-CNs spectrum was almost a replica to that o PH.Te possible reason or this was that B-CNs do not acceptelectrons rom the PH due to the mismatch between thework unction o B-CNs (. eV) and the LUMO o PH(. eV); hence, quenching was not possible.

Absorbance o the nanocomposite was measured afermixing PH/B- or N-CNs with PCBM and then spin-coating thin lms on a glass substrate and annealing at Cor min (Figure ).

From Figure , we noted a red shif or the absorbance oPH when B- or N-CNs (rom . nm to . nm and

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: Photovoltaic characteristics o the devices.

OSC OC(V) SC(mA/cm2) FF (%) (%)

PH : PCBM .. .. . ..

PH/B-CNs : PCBM .. .. .. .

PH/N-CNs : PCBM . .. .. ..

550 600 650 700 750

Intensity(a.u.)

Wavelength (nm)

P3H

P3H/B-CNs

P3H/N-CNs

F : Photoluminescence spectra or pristine PH (blackcurve), overlapped by PH/B-CNs (red curve) and quenching byPH/N-CNs (blue curve).

. nm or B-CNs and N-CNs), respectively, were addedto the polymer. Similar observations were made by Karim[], who attributed this to enhanced ground interactionbetween CNs and the polymer. Due to this interaction,we expected CNs to create a good percolation pathway orthe photo-generated charge carriers and eventual efficientcollection at the electrodes. A maximum absorption (max) o. nm, . nm, and .nm or PH, PH/B-CNs,and PH/N-CNs was observed, respectively. Te slight redshif omax or the lms with doped CNs was attributedto polymer wrapping on the CNs suraces which increasedconjugation length as a result o- interactions []. Temaxobserved or thin lm was more red-shifed compared

to that o a solution o PH in ,-dichlorobenzene (notshown here) which was nm. Tis red shif can be dueto enhanced polymer structure ordering by annealing [].Additionally, annealing increases - overlap in the polymerchains and causes phase segregation between the donorpolymer and acceptor PCBM which increases max o thepolymer [].

.. Photovoltaic Properties. Bulk heterojunction organicsolar cells were produced in a sandwich type device structureconsisting o different layers o materials (Figure ). Tephotoactive layer was composed o PH/B-CN/PCBM orPH/N-CN/PCBM.

400 600 800

Absorbance(a.u.)

Wavelength (nm)

P3H/N-CNs : PCBM

P3H/B-CNs : PCBM

P3H : PCBM

F : UV-Vis absorbance o thin lms black pristine PH, redPH/B-CNs, and blue PH/N-CNs; all the lms had PCBM.

Te current-voltage (-) curves given in Figure aretypical representatives o the OSC diodesunder investigation.According to the cell parameters provided in able , theperormance o the devices did not improve as expected aferblending the photoactive layer with B- or N-CNs.

Te device without doped CNs had an efficiency o.% while the ones with B-CNs and N-CNs had .%and .%, respectively. Despite the act that the two typeso CNs have different electrical properties (i.e., the N-CNsbehaving like n-type and B-CNs as p-type semiconductors),both have exhibited similar behaviour in the devices. Byintermixing the PH donor, PCBM electron acceptor witheither B- or N-CNs was expected to improve the efficiencyo charge separation at the D-A interaces. Tis was to take

place in the photoactive mediumby a way o enhanced chargetranser as a result o high mobility in doped CNs [].However, the samples with doped CNs exhibited drasticreduction in the values o the OC and FF which mightbe attributed to high recombination in the devices. Similarbehaviourwas maniested in both types o doped CNs whichcould also be associated with the metallic character inducedby low level o boron doping in B-CNs and high levelo pyridinic nitrogen in N-CNs. In all the devices, it wasanticipated to have almost the same OCas it mainly dependson the difference between the HOMO level o the donor andthe LUMO level o the acceptor [, ]. Te observed lowOC in the solar cells with doped CNs can be attributed

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Al

P3H/B- or N-CNs/PCBM

IO coated glass

CH2(CH2)4CH3

n

S

O60

OCH3

PEDO : PSS

F : Schematic diagram o the device.

0.0 0.2 0.4 0.6 0.8 1.0

V(v)

B-CN0

5

5JSC

(mA/cm

2)

5.2 eV

LUMO

HOMO

PCBM

6.1 eV

3.7 eV

N-CN

4.2 eV

LUMO

HOMO

P3H

5.1 eV

3.2 eV

P3H/N-CNs : PCBM

P3H/B-CNs : PCBMP3H : PCBM

F : I-V curves o devices with and without doped CNsand the energy band diagram showing the positions o HOMO andLUMO o donor and acceptor as well as work unctions o dopedCNs.

to the recombination processes and low shunt resistances(., . or devices with B-CNs and N-CNs, resp.,as compared to . or the device without CNs) dueto possible short circuits and leakage current caused bythe presence o out-protruding CNs in the photoactivelayer. Derbal-Habak et al. [] reported that the presence ometallic CNs in the photoactive layer avours recombination

o ree charge carriers. Additionally, Holt et al. [] usedtime-resolved microwave conductivity to show that metallicSWCNs in PH blend were unavourable to photovoltaicperormance.

According to the results given in able , the devices hadsimilar SC values which are known to depend on devicequantum efficiency (QE). Te QE o a device is proportionalto light absorption by the photoactive layer ollowed byexcitons dissociation at the D-A interaces as well as theefficiency o charge collection. Te results in able suggestthat the collection o the current rom all the devices arealmost the same but differ signicantly in the values o theopen circuit voltages which is responsible or the variationsin the power conversion efficiencies. Tese results indicatethat the lowOC mainly comes rom high leakage currentin the devices due to the existence o doped CNs in thesample. High leakage current ofen results in low shuntresistance which severely affects the value o the open circuit

voltage. However, Lee and coworkers [, ] have reportedimpressive power conversion efficiency o .% and .%using N-CN and B-CNs, respectively. Te difference indevice perormances is due to the act that our samples wereprepared in ambient environment and the doping level in theCNs used was different.

.. Morphology o the Films. Morphology and size o pho-toactive layer lms o the devices were analysed using AFMand SEM. Figure shows AFM images o these lms. Fromthe images, it was observed that lms with N-CNs were

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(m

)

254.5

200.0

150.0

100.0

0.25

0.00

50.0

0.0

(nm)

y:10.0mx:

10.0m

(a)

114.2

80.0

60.0

40.0

20.0

0.0

(nm)

y:10.0m

x:

10

.0m

114

0

(nm)

(b)

278.6

0.0

0.00

0.28

50.0

100.0

150.0

200.0

(nm)

y:10.0m x:10.0 m

(m)

(c)

F : AFM lm images o (a) PH : PCBM, (b) .nm thick lm o PH/N-CNs : PCBM, and (c) . nm thick lm o PH/N-CNs : PCBM.

rougher compared to the one without doped-CNs. Tisroughness decreased as the lm thickness increased, that is,rom . nm to . nm. Similar observations were madeor nm and nm thick lms with B-CNs. However,specic RMS roughness values were ound to be dependent

on lm thickness that is ., ., and .nm or .,., and . nm thick lms, respectively. Te average sizeo the photoactive layer was nm. We observed sharppointed laments protruding on the surace o the lmswith doped-CNS. Tese laments could be the cause o

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Doped CNs

2 m

F : SEM image showing dispersion o doped CNs. Telength o the doped CNs is in microns scale.

short circuiting o these devices. Tis has also been suggestedby Berson et al. [] who attributed reduction in OC andFF in their devices PH/CN/PCBM to the ormation olamentary electrical short circuiting which was as a result

o CNs extending across the photoactive layer.Te SEM image in Figure shows that the doped CNs

were well dispersed in the lm; thereore, we ruled outbundling o CNs in these devices. Hence, we attribute thegood dispersion to covalent unctionalization o our dopedCNs with benzyl alcohol via ester groups. Tis has beenreported by other authors where the covalent unctionaliza-tion o CNs with ester groups promotes dispersion in thepolymer matrix [].

From Figure , the size o the doped CNs was biggercompared to the thickness o the lm nm. Hence, thereis a possibility that the doped-CNs were extending acrossthe photoactive layer.

.. Recombination Dynamics in the Film Blends. Althoughintermixing o polymer donor and ullerene acceptor resultsin signicant enhancement o photo-induced charge gen-eration, it can also increase the interacial area availableor bimolecular recombination. ransient absorption spec-troscopy (AS) was used to study recombination dynamicsin PH : PCBM, PH/B- or N-CNs lms blend. AS wasused to provide inormation on the yield o photo-generatedcharges (polarons). Tis was achieved by monitoring opticalabsorption which directly relates to the yield o photo-generated charges []. Figure shows AS signal or theblend lms at nm pump wavelength and nm probe

wavelength at s time delay. Te probe wavelength was set at nm in order to correspond with transient absorption oPH positive polarons (PH+). Additionally, low intensityexcitation was employed to ensure photo-generated chargecarriers were comparable with those generated under solarillumination.

We observed the lms transient signal decay corre-sponding to power law decay (OD) suggesting trap-limited recombination extending to millisecond time scale[, ]. Tis long-lived power law decay is a characteristico bimolecular charge recombination o dissociated chargedspecies []. AS excitation was undertaken in a nitrogenatmosphere (lms were put in a culvert and then lled with

0.01

0.0

0.1

0.2

0.3

0.4

ime (sec)

1E 31E 41E 5

OD

P3H/N-CNS : PCBM

P3H/B-CNS : PCBM

P3H : PCBM

F : Te transient absorption decay kinetics o aPH: PCBM ( : ) blend lm and a similar blend containing. wt.% o either B- or N-CNs obtained as a unction o laserexcitation density with a pump wavelength o nm and a probewavelength o nm.

nitrogen gas). o rule out the ormation o triplet state, thesame experiment was also done under oxygen atmosphereand it was observed that the signal was not quenched. Tishas also been observed by Ohkita et al. [].

Despite the act that the blend with N-CNs had thehighest percent o photo-generated charge carriers whichalso had the longest lie time (average time a ree chargecarrier lives beore recombining nongeminate), this did notresult into a good photovoltaic output. We anticipated goodefficiency rom this device due to high yield o long-liveddissociated polarons. Te AS results o high yield o photo-generated ree charge carriers or PH/N-CNs : PCBMlm correlate well with photoluminescence results, whereexcitons quenching was observed and this is an indicationthat N-CNs provided extra excitons dissociation sites. TeAS results observed lead us to a conclusion that B- and N-CNs in this study were acting as generated charge carrierstraps which encouraged bimolecular recombination instead

o acting as pathway to the electrodes. Te possible reasonor this can be due to the metallic nature o the doped-CNsused as a result o low substitutional boron concentrationin B-CNs and high percentage o pyridinic nitrogen inN-CNs. Additionally, the length o doped CNs (microns) as compared to the thickness o photoactive lm( nm) encouraged shunting by providing alternativepath or electrons. Tis caused energy loss and a decreaseo OC o the devices. Other authors, such as Liu et al.[], have analysed intensity dependent photocurrents inPH/SWCNs : PCBM and attributed the poor peror-mance o their device to the bimolecular recombinationwhich was due to presence o metallic SWCNs. Hence,

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metallic CNs do encourage bimolecular recombination andthis can be the major reason or efficiency deterioration ordevices with CNs.

4. Conclusion

Te OSC devices with B- or N-CNs, in the photoactivelayers, were successully abricated. Te devices recorded lowcell efficiency compared to a standard PH : PCBM device.Te devices behaved in a similar manner despite the actthat B- and N-doping made properties o photoactive layerdifferent. Te doped-CNs were metallic in nature as a resulto low concentration o substitutional boron in B-CNs andpyridinic-nitrogen species in N-CNs. Hence, this is one othe main reasons or similar behaviour within the devices.Hence, rom the ndings, it is important to have a verycontrolled heteroatom doping in CNs. In the case o N-CNs, one should strive to eliminate unwanted species onitrogen, that is, pyridinic nitrogen, whilst in B-CNs one

should increase the amount o substitutional boron. Also, thelength oB- orN-CNs was estimated tobe between and microns, and thus some extended across and even protrudedon both sides o the photoactive layer. Te length o CNs tobe incorporated should be o the right size to improve theirimplementation in the photoactive layer o OSCs.

Conflict of Interests

Te authors declare that there is no conict o interestsregarding the publication o this paper.

Acknowledgments

Te authors thank Nanotechnology and Solar cell Laboratory,Chemistry Institute, University o Campinas, where themajor part o this work was undertaken through the cour-tesy o Proessor Dr. Ana Flavia, Brazilian NanotechnologyNationalLaboratory (LNNano) or the XPS acility (proposal,XPS-), the University o KwaZulu-Natal (UKZN), theNational Research Foundation (NRF), and the India, Brazil,and South Arica (IBSA) energy project or nancial assis-tance. G. Keru also thanks the UKZN College o Agriculture,Engineering andScience oraward o a postgraduate bursary.

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