Organic Electronics 9 (2008) 809–815

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Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

Photoinduced charge separation in poly(1,4-phenylenevinylene)derivatives studied by electron paramagnetic resonance

E.A. Thomsen a, D.J. Keeble b,*, B. Lochab c, P.L. Burn c,1, H. El-Mkami a, I.D.W. Samuel a

a Organic Semiconductor Centre, School of Physics and Astronomy, SUPA, University of St. Andrews, St. Andrews, KY16 9SS, UKb Carnegie Laboratory of Physics, School of Engineering, Physics and Mathematics, University of Dundee, Dundee DD1 4HN, UKc Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA, UK

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 April 2008Received in revised form 20 May 2008Accepted 23 May 2008Available online 4 June 2008

PACS:36.20.Kd76.30.Pk78.66.Qn84.60.Jt

Keywords:PPVsExciton dissociationElectron paramagnetic resonance

1566-1199/$ - see front matter � 2008 Elsevier B.Vdoi:10.1016/j.orgel.2008.05.020

* Corresponding author. Tel.: +44 (0) 1382381382388313.

E-mail address: d.j.keeble@dundee.ac.uk (D.J. Ke1 Present address: Centre for Organic Photonics and

Molecular and Microbial Sciences, University of Que4072, Australia.

We report a study of photoinduced charge separation in poly(1,4-phenylenevinylene)(PPV) derivatives with enhanced exciton dissociation using light-induced electron para-magnetic resonance (EPR). Four polymers were studied; all contained alkoxy and fluorenylside chain units, and two of them also contained a nitro group on the 7-positon of the flu-orenyl moieties. Pulsed EPR detected a light induced polaron for all the polymers with theintensity significantly higher for those with nitro groups. A second center was observed forthese materials and spectra could be fitted assuming a radical anion was localized on thenitro groups. The fraction of radical anion and polaron centers was comparable, consistentwith intramolecular exciton dissociation for the PPV derivatives containing the nitrogroups.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Organic solar cells rely on charge carrier generationfrom exciton dissociation, and on the efficient collectionof these separated carriers at the electrodes. The dominantapproach to achieve this in solid-state solar cells is to blendconjugated polymers with fullerenes [1,2]. While steadyimprovements in device performance have been madewith a small number of intensively studied materials therehas not been the rapid improvement in efficiency seenwith organic light-emitting diodes. This suggests that

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eble).Electronics, School ofensland, Queensland

new concepts in polymer design need to be explored tosee whether each of the steps in the photovoltaic process(light absorption, charge separation, and charge transport)can be optimized. The work described here focuses on thefirst two of these processes, namely light absorption andcharge separation. For polymer:fullerene bulk heterojunc-tion devices, once the polymer absorbs a photon there israpid and efficient intermolecular electron transfer onto afullerene molecule, resulting in a radical anion on the ful-lerene and a positive polaron on the polymer. Steady statelight-induced electron paramagnetic resonance (LEPR) al-lows long lived paramagnetic centers to be observed, andinsight on their molecular environment to be obtained.For conjugated polymer: fullerene blends both the radicalanion and the positive polaron are clearly resolved [3].Alternative approaches have been used to obtain chargeseparation, including bulk heterojunctions formed fromblends of two conjugated polymers with differing electron

810 E.A. Thomsen et al. / Organic Electronics 9 (2008) 809–815

affinities [4], and hybrid blends of polymers with inorganicmaterials [5].

Devices formed from blending materials give good effi-ciencies, but controlling the morphology in a reproducibleway during processing and long-term device operationpresents challenges [6]. Indeed the best bulk heterojunc-tion devices are comprised of poly(3-n-hexylthiophene):fullerene blends in which complicated solvent and anneal-ing processes are used to achieve the optimized perfor-mance. One strategy that has been studied to improvethe stability of the active film is to covalently tether the do-nor and acceptor [7–9], although there is no strong evi-dence that this leads to improved device performance.The materials studied here differ from these tethered do-nor and acceptors, which are mostly connected by non-conjugated linkers and hence considered as individualchromophores, in that the donor and the acceptor are inconjugation. Although there have been polymers that haveelectron donating and electron withdrawing units withinthe polymer main chain these latter polymers have beensynthesized to have a low optical transition energy [10–15]. In contrast, the polymers for this work have been de-signed to enhance intramolecular exciton dissociation. Thepreparation of poly(1,4-phenylenevinylene)s (PPVs) thathave electron donating and withdrawing groups in a paraarrangement on the phenyl ring of the polymer backboneand their use in simple photovoltaic devices have beenpreviously reported [16]. With these new polymers itwas found that the presence of the electron donating andwithdrawing groups in conjugation led to a quenching ofthe luminescence and here we use LEPR to unambiguouslyidentify the products of photoinduced charge separation.The electron accepting unit was a nitro group attached tothe 7-position of the fluorenyl side group. In addition, con-trol polymers without the nitro group were also studied. Inthe materials containing a nitro group, light induced EPRshowed that photoexcitation resulted in positive polaronson the polymer backbone and the generation of localizedradical anions centered on the nitro group. In contrast, onlya weak positive polaron center was observed for the poly-mers without the nitro groups. This indicates that havingelectron donating and withdrawing groups in conjugationcan give an intramolecularly charge separated state.

Electron magnetic resonance methods can identifyparamagnetic electronic states, and may provide detailedinformation on the local structure. For the S = 1/2 centersrelevant to this work, spectra can be analyzed using thespin-Hamiltonian [17],

H ¼ lBB � g � SþX

i

ðS � A � Ii þ gn;ilNB � IiÞ ð1Þ

which includes the electronic Zeeman term, hyperfineinteractions, and nuclear Zeeman interactions, respec-tively, and where lB is the Bohr magneton and lN the nu-clear magneton, and the sum includes relevant magneticneighbor nuclei. The sample is exposed to highly mono-chromatic microwave photons, typically in the 9–10 GHzrange, and an external magnetic field is swept. Performingthe experiment at higher microwave frequencies increasesthe field values required to observe resonance and resultsin an improved ability to resolve different g-values. Split-

tings in the spectrum arising from the hyperfine interac-tion between the electron spin and magnetic nuclei arenormally independent of the microwave frequency used.

Previous EPR and electron nuclear double resonancestudies on PPV have established the existence of a centerassociated with delocalized p orbitals on the polymerchain [18,19]. Similar light induced spectra assigned topolarons have been observed for modified PPV materials,for example MEH-PPV [20], CN-PPV [20], and MDMO-PPV[3]. Measurements at 9.5 and 25 GHz on partially orientedPPV [19], and at 95 GHz on MDMO-PPV [3], have reportedthe polaron center to be axial with gjj ¼ 2:0034 andg? ¼ 2:0024, giving giso = 2.0027 (giso = 1/3(gx + gy + gz)).The g-values provide characteristic information about theparticular paramagnetic center. The deviation from thefree-electron value of ge = 2.0023 depends on the detail ofthe electronic state containing the unpaired spin density,including the admixture of excited states, and the degreeof spin-orbit coupling. The nature and number of magneticnuclei (for example I = 1/2 proton or I = 1 nitrogen) in thelocal environment of the unpaired electronic spin, alongwith the degree of coupling between the electronic spinand the magnetic nuclei, can result in hyperfine splittingof the EPR line. If this is unresolved it contributes to linebroadening. The EPR linewidths for the polaron have beenshown to be consistent with those expected from unre-solved proton hyperfine splitting given the degree of local-ization on vinyl and phenyl sites [20]. A radical anion EPRcenter has been unambiguously identified only for the con-jugated polymer: fullerene blends. In this case localizationwithin the distorted fullerene gives characteristic rhombicsymmetry g-values with an isotropic value slightly lessthan the free electron value [3].

2. Experimental

In this study four PPV derivatives were investigated(Fig. 1). All the derivatives contained fluorenyl and alkoxygroups in a para arrangement across the phenyl ring of thepolymer backbone, and two of the polymers had the flu-orenyl units modified by the attachment of a nitro groupin the 7-position. The materials were poly{2-[(9,9-di-n-propyl-9H-7-nitrofluoren-2-yl)]-5-(2-ethylhexyloxy)-1,4-phenylenevinylene} (NDPFEH-PPV) and NDPF2EH-PPV,where an additional fluorenyl unit is added to make a long-er side chain. The control materials without the nitro groupwere poly{2-[(9,9-di-n-propyl-9H-fluoren-2yl)-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene]} (DPFEH-PPV) andDPF2EH-PPV for the polymer with the extra fluorenyl moi-ety in the sidechain.

Optical absorption measurements were performed onthin film samples using a Cary Varian UV–vis absorptionspectrometer. Photoluminescence quantum yield (PLQY)measurements followed the method of Greenham andcoworkers. [4], and used a 407 nm GaN laser as the illumi-nation source.

Continuous wave (CW) 9.5 GHz EPR experiments used aBruker EMX with a super-high-Q resonator (ER4122SHQ)with ER072 electromagnet. Field swept, echo-detected,pulsed EPR measurements were performed with a Bruker

O

n*

*

O

(c)

(d)

n n

O

O2N

**

NO2

O

n

(a)

(b)

n n

n

n

n

Fig. 1. Chemical structures for the materials studied, (a) NDPF2EH-PPV,(b) NDPFEH-PPV, (c) DPF2EH-PPV, and (d) DPFEH-PPV.

Fig. 2. Absorption spectra for DPFEH-PPV (solid), NDPFEH-PPV (dash),DPF2EH-PPV (dash-dot), and NDPF2EH-PPV (dash-dot-dot).

Table 1Solution photoluminescence quantum yield (PLQY) results for the fourpolymers, NDPFEH-PPV, NDPF2EH-PPV, DPFEH-PPV, and DPF2EH-PPV,studied

NDPFEH NDPF2EH DPFEH DPF2EH

PLQY (%) 3 ± 1 6 ± 1 19 ± 2 30 ± 2

E.A. Thomsen et al. / Organic Electronics 9 (2008) 809–815 811

ELEXSYS E580 spectrometer at 9.8 and 33.7 GHz usingdielectric ring resonators (ER4118X-MD5-EN andEN5107, respectively). Field swept measurements weremade at 20 K. All light induced spectra used a 407 nm,4 mW, GaN laser. Reference measurements were madeon the two spectrometers using 1.6 mm diameter Coaland DPPH standard samples. The Hall field of the ER072magnet was calibrated using an NMR magnetometerplaced at the resonator position using two crossed laserbeams. Both thin cast films and solution samples werestudied, but were found to give identical dark and light-in-duced EPR spectra, the results presented here are from�5 mg/mL frozen solutions. The solvent used was choloro-benzene. Simulation of the spectra using Eq. (1) were madewith the package EasySpin [17].

3. Results and discussion

The UV–visible absorption spectra for the four PPVderivatives are shown in Fig. 2. The spectra show broadabsorption over the wavelength range 200–525 nm. In

general for PPV based polymers there are two mainabsorption bands. The short wavelength region at around210 nm corresponds to localized p–p* transitions, andthe longer wavelength absorptions are due to delocalizedp–p* transitions. In DPFEH-PPV, the absorption maximumat 209 nm is due to the localized p–p* transitions, those at280 nm and 313 nm correspond to the chromophore con-taining the phenyl ring of the polymer backbone and thefluorenyl unit, and the broad absorption around 455 nmis due to the delocalized p–p* transitions of the polymerbackbone. The optical absorption spectra of NDPFEH-PPValso shows components due to the localized p–p* transi-tions (202 nm), the chromophore comprising the phenylring and fluorenyl unit (359 nm), and the delocalized p–p* transitions (454 nm). The chromophore peak is redshifted compared to DPFEH-PPV due to the presence ofthe nitro group. DPF2EH-PPV also shows absorption fea-tures due to localized (213 nm) and delocalized (460 nm)p–p* transitions, and has a peak at 337 nm assigned tothe chromophore comprising the phenyl unit and fluorenylmoieties. This peak is red shifted compared to that ofDPFEH-PPV due to the extra fluorene unit. NDPF2EH-PPVfollows the same trend, with the chromophore peak redshifted relative to DPF2EH-PPV due to the presence of thenitro group.

The photoluminescence quantum yield (PLQY) resultsfrom thin films given in Table 1 shows the luminescencefor nitro group containing polymers was suppressed by

812 E.A. Thomsen et al. / Organic Electronics 9 (2008) 809–815

approximately a factor of five compared to those withoutthe electron accepting unit. The reduction in PLQY insolution could be either due to rapid loss of energy vianon-radiative pathways such as energy dissipation byvibrational modes or photoinduced charge separation forNDPFEH-PPV and NDPF2EH-PPV. The PLQY values for thematerials containing the longer side chain were a factorof two greater than the shorter side chain polymers, sug-gesting a decreased charge separation for the materialswith longer side chains. It is interesting to note that thePLQY for both nitro substituted polymers is only �20% oftheir unsubstituted counterparts.

Fig. 3 shows the light and dark 9.8 and 33.7 GHz echo-detected field swept pulsed EPR spectra for the nitro groupcontaining NDPFEH-PPV in frozen solution. The light in-duced spectra for DPFEH-PPV (no nitro groups) at approx-imately the same concentration are shown in Fig. 4, withthat for DPF2EH-PPV (an extra fluorenyl moiety but no ni-tro groups) included as an insert. Fig. 5 shows the light in-duced spectra for NDPF2EH-PPV (which contains the extra

Fig. 3. Field swept pulsed EPR spectra for NDPFEH-PPV at 9.7 and 33.7 GHz. Todifference spectra and simulation (dashed line).

fluorenyl moiety and nitro group in the sidechains). A nar-row Gaussian line attributed to positive polarons was ob-served in the light induced spectra for all samples, withthe signal intensity much greater for the polymers withthe nitro groups with respect to their non-nitrated equiva-lents. The spectra from the nitro group containing poly-mers, NDPFEH-PPV and NDPF2EH-PPV, also showed anadditional broad component, see Figs. 3 and 5.

The LEPR spectra from DPFEH-PPV and DPF2EH-PPV (nonitro groups) at 9.8 and 33.7 GHz were simulated assum-ing a single S = 1/2 center using a Gaussian lineshape.Simultaneous fitting spectra recorded at the two micro-wave frequencies allowed both the frequency dependentg-value anisotropy, gjj � g?, and the frequency indepen-dent contributions to be determined. The g-value anisot-ropy for DPFEH-PPV and DPF2EH-PPV was found to be�0.0005, see Fig. 4 and Table 2, and is slightly less than val-ues obtained for PPV materials in previous studies [3,19].The isotropic g-value for the polaron center observed forNDPFEH-PPV was determined to be 2.0031 ± 0.0002, simi-

p panels dark and 407 nm illuminated spectra (grey line), bottom panel

Fig. 4. Light induced EPR spectra for DPFEH-PPV at 9.7 and 33.7 GHz,simulation (dashed line). Insert shows the spectrum for DPF2EH-PPV.

Fig. 5. Light induced EPR spectra for NDPF2EH-PPV at 9.7 and 33.7 GHz,simulation (dashed line).

Table 2Simulation parameters for light induced EPR spectra, g-values andlinewidths

Center gx gy gz DB (mT)

NDPFEHPolaron 2.0028 2.0028 2.0033 0.55Radical anion 2.0051 2.0032 2.0027 0.80

NDPF2EH

Polaron 2.0028 2.0028 2.0034 0.55Radical anion 2.0050 2.0035 2.0035 0.80

DPFEH

Polaron 2.0028 2.0028 2.0033 0.75

E.A. Thomsen et al. / Organic Electronics 9 (2008) 809–815 813

lar values were observed for the four derivatives studiedhere (see Table 2). The experimental linewidth,0.80 ± 0.05 mT at 9.7 GHz, is dominated by unresolved pro-ton hyperfine structure fitted with a frequency indepen-dent value of 0.75 mT. This is slightly larger than valuesreported by Kuroda et al. [20] of 0.45 ± 0.01 and0.66 ± 0.02 mT for CN-PPV and MEH-PPV, respectively,but similar to those given by De Cuester et al. [3] forMDMO-PPV (0.8 mT for the parallel component, and0.6 mT for the perpendicular component).

For the non-nitrated derivatives (DPFEH-PPV andDPF2EH-PPV) the polaron spectra were fitted using thesame g-value anisotropy as for the nitro group containingmaterials (NDPFEH-PPV and NDPF2EH-PPV), but with asmaller frequency independent width of 0.55 mT, as shownin Table 2. This quite marked reduction in the linewidthindicates a reduction in the contribution of unresolved pro-ton hyperfine coupling. It has been reported that the dom-

inant contributions to this are from the backbone vinylenesites [20]. The presence of the side chain nitro group altersthe distribution of the polaron spin density.

814 E.A. Thomsen et al. / Organic Electronics 9 (2008) 809–815

The LEPR spectra for NDPFEH-PPV and NDPF2EH-PPVcould be satisfactorily simulated using two centers; the po-laron, and a radical center mainly localized on nitrogen, seeFigs 3 and 5 and Table 2. Initial attempts to simulate thecomplete spectrum by assuming the broad componentwas due to proton hyperfine splittings or by assuming asingle delocalized center with spin density on a nitrogenatom were unsuccessful [21,22]. The fits at the two fre-quencies shown assumed the broad component was dueto a second nitro group radical center with 14N hyperfinetensor principle values of 25, 15 and 64 MHz, giving an iso-tropic hyperfine constant of 35 MHz. This value is consis-tent with previous reports of radical anion centers

Fig. 6. Normalized double integral area of continuous wave LEPR spectrafor NDPFEH-PPV, (a) against temperature, (b) against time from the startof the 407 nm illumination, the time at which the illumination wasturned off is marked.

containing a nitro group, which are typically in the range28–35 MHz [21,22]. The simulations shown in Figs. 3 and5 required a 55% contribution from the nitrogen radical an-ion center. That is, for each positive polaron created on thepolymer backbone a radical anion was formed. The factthat these measurements were carried out in solutionunambiguously identifies that the quenching of the lumi-nescence is due to intramolecular charge separation. Inter-estingly the same results were observed for films of thepolymers indicating that intermolecular interactions inthe solid state has only a minor if any effect on the chargeseparation processes in either the non-nitrated or nitratedpolymers.

Fig. 3 also shows the presence of a dark spectrum fromNDPFEH-PPV, and a comparable signal was observed forNDPF2EH-PPV (not shown). No dark spectrum was re-solved for the control polymers DPFEH- and DPF2EH-PPV.The microwave frequency dependence shows that thespectral width is dominated by hyperfine interactions.The large magnitude of these interactions required to sim-ulate the spectrum are not consistent with p–electron pro-ton coupling expected in PPV materials [23]. Theseobservations suggest the dark paramagnetic centers areassociated with the nitro groups. A possible explanationcould be the existence of different resonance contributions,including one where there is a radical cation on the nitro-gen and a radical on an oxygen. It should be noted that thelight induced centers are generated only in the illuminatedsurface layer, while the dark signal is detected from the to-tal bulk volume of the sample and could be due to a lowlevel impurity phase.

The decrease in the double integrated CW LEPR spec-trum area with temperature shown in Fig. 6a forNDPFEH-PPV is consistent with the expected Curie behav-ior. The linewidth remained constant through the temper-ature range studied. No additional physical processes arerequired to account for the temperature dependence, forexample enhanced recombination. Fig. 6b shows the timeevolution of the LEPR signal after the start of illumination,and after it was terminated. The LEPR signal was observedto reach a maximum after approximately 40 min. At 20 Kthere is evidence for a small, approximately 5%, drop inintensity immediately after the illumination is turned offbut the majority of the signal is persistent due to deeplytrapped carriers. These observations are similar to thosereported for CN-PPV and MEH-PPV [20].

4. Conclusions

In conclusion, functionalizing PPV derivatives with a ni-tro group results in intramolecular charge separation of theexciton. This is shown by the light-induced electron para-magnetic resonance results which detected both the posi-tive polaron, and a radical anion mainly localized on thenitro group of the modified side chain. Approximatelyequal amounts of the two centers were observed to resultfrom the photoexcitation. The control PPV derivatives,DPFEH- and DPF2EH-PPV, showed only the polaron centerbut with a markedly reduced intensity. The reduction inthe EPR linewidth for the polaron centers in NDPFEH-PPV

E.A. Thomsen et al. / Organic Electronics 9 (2008) 809–815 815

and NDPF2EH-PPV compared to the control polymers pro-vides evidence that the presence of the nitro group altersthe polaron spin density distribution. The PPV derivativesNDPFEH- and NDPF2EH-PPV provide examples of an alter-native method of charge separation for organic photovol-taic applications.

Acknowledgements

We thank Research Councils UK Basic Technology Pro-gramme (GR/S85726/01) and EPSRC for financial support,EAT acknowledges an ORS award.

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