University of Groningen

Delocalisation softens polaron electronic transitions and vibrational modes in conjugatedpolymersKahmann, Simon; Loi, Maria A.; Brabec, Christoph J.

Published in:Journal of Materials Chemistry C

DOI:10.1039/c8tc00909k

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Citation for published version (APA):Kahmann, S., Loi, M. A., & Brabec, C. J. (2018). Delocalisation softens polaron electronic transitions andvibrational modes in conjugated polymers. Journal of Materials Chemistry C, 6(22), 6008-6013.https://doi.org/10.1039/c8tc00909k

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6, 6008

Delocalisation softens polaron electronictransitions and vibrational modes in conjugatedpolymers†

Simon Kahmann, ab Maria A. Loi *b and Christoph J. Brabec*ac

In this work we study the photoinduced signatures of polarons in conjugated polymers and the impact

of charge carrier delocalisation on their spectra. The variation of film crystallinity for two prototypical

systems – blends of the homopolymer P3HT or the donor–acceptor polymer PCPDTBT with PCBM – allows

probing changes of the polaron absorption in the mid infrared spectral region. Increased polaron delocalisa-

tion entails a shift of the electronic transition to lower energy in both cases. Also, infrared active vibrations

soften due to a higher polymer chain order. Our findings help in providing a more complete understanding

of polaron properties in conjugated materials and bring the application of the polaron absorption spectrum as

an indicator for the environment on a more thoroughly studied foundation.

1 Introduction

Charge carriers in conjugated polymers affect the energetics of theirenvironment and lead to the formation of strongly coupled polarons.Although recently contested,1 this mechanism is generally assumedto be governed by their strong electron–phonon coupling with thepolymer backbone. Based on the Holstein model for molecularpolarons,2 the relaxation is commonly assumed to shift the polaronenergy levels into the fundamental band gap of the material – themagnitude of which is termed the relaxation energy. Although thedetails of level occupation and position are still debated,3 there is ageneral consensus that this process gives rise to two new opticaltransitions (classically termed P1 and P2) in the mid (MIR) and nearinfrared (NIR) spectral region (see Fig. 1(a) for a sketch of a positivepolaron).

Studies involving crystalline polymers furthermore revealedadditional absorption bands to form when polarons delocalisenot only along the polymer backbone (intrachain), but also overadjoining chains (interchain). These transitions are denoted

DP4 (sometimes CT,4 consider Fig. 1(a) for a scheme). The polarondelocalisation has consequently been identified as a crucial factorfor the shape, position and strength of the polaron absorptionsignature in the mid infrared spectral region.6 Whilst amorphousmaterials allow polarons to only reside localised on a short unit(depicted in Fig. 1(b) (top)), the planarisation of the polymerbackbone allows delocalisation in the intrachain direction(middle panel). The presence of aligned chains, finally, enablesthe necessary polaron delocalisation perpendicular to the back-bone (bottom). Theoretical studies generally suggest a shift tolower energy of the absorption band for a larger delocalisation.7

It was furthermore proposed that the concept of two independenttransitions P1 and DP1, although descriptive, was oversimplifiedand the shape of the absorption band was formed by a complexinterplay based on the extent of polaron delocalisation in the intra-and interchain direction.6

In addition to the broad electronic absorption bands, polaronspectra also exhibit narrow and strong vibrational signals,commonly referred to as infrared active vibrations (IRAVs). Theseare traditionally explained through the infrared activation ofRaman modes in the presence of the carrier’s electric field –discussed in the frameworks of the effective conjugation coordi-nate (ECC)8 or the amplitude mode model (AM).9 These materialspecific vibrations can be used as a fingerprint for the environ-ment of the charge carriers. Also this classical explanation hasrecently been contested and it was suggested that pronouncedvibrational signals should be considered as related to polaronsinstead of the ground state of the polymer.10,11

The low energy signatures of polymer polarons have latelymoved back into the focus of research interest as a means to extractinformation about the chain conformation in the environment of

a Institute for Materials for Electronics and Energy Technology (i-MEET),

Friedrich-Alexander Universitat Erlangen-Nurnberg, Martensstraße 7,

91058 Erlangen, Germanyb Photophysics and OptoElectronics Group, Zernike Institute of Advanced Materials,

University of Groningen, Nijenborgh 4, 9747 AG, The Netherlands.

E-mail: m.a.loi@rug.nl; Tel: +31 50 363 4119c Bavarian Center for Applied Energy research (ZAE-Bayern), Immerwahrstraße 2,

91058 Erlangen, Germany

† Electronic supplementary information (ESI) available: Additional absorptiondata of P3HT films, MIR PIA spectra including o-IDTBR blended with P3HT, NIRPIA spectra of selected P3HT blends and their IR transmission, and the PIAspectrum of neat PCPDTBT. See DOI: 10.1039/c8tc00909k

Received 22nd February 2018,Accepted 8th May 2018

DOI: 10.1039/c8tc00909k

rsc.li/materials-c

Journal ofMaterials Chemistry C

PAPER

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the charge carrier. The groups of Schwartz12 and Salleo,13 forexample, studied the spectra of doped films of P3HT to investigatethe location of doping-induced charge carriers. Simultaneously, westudied the spectra of polymer-wrapped single walled carbonnanotubes and assessed the location of carriers in these hybridsystems to find that wrapping polymers actually carry polaronsdespite nominally unfavourable energy levels.14

Experimental studies evaluating the impact of the carrierdelocalisation on the polaron signature so far either consideredthe two extreme cases of amorphous and highly crystallinematerials – making it difficult to predict the effect of subtlechanges on polaron delocalisation – or tested the impact of themolecular weight as a means to change the chain order – whichrequires using different batches of material.4,5,15 Missing still isan appropriate assessment of incremental morphology changesof the same material as well as the consideration of state-of-the-art donor–acceptor type (DA) polymers.

Since dopants affect the delocalisation of polarons in polymerfilms and can also corrupt the morphology,13 we study thephotoinduced absorption of films of two prototypical conjugatedpolymers under variation of their crystallinity. The homopolymerP3HT is employed in its regiorandom (RRa) and regioregular (rr)variant and the degree of chain order of the latter is variedthrough thermal annealing. Additionally, the DA polymerPCPDTBT is investigated when cast in the presence of solventadditives to enhance the polymer crystallinity. Using these twowell-studied systems, we are able to carefully vary the polymerchain order and observe the effect on the polaron signatures.

For both polymers, we find a significant red-shift of theelectronic absorption bands in the MIR with increasing crystal-linity. In accordance with theoretical propositions, we ascribethis effect to the extended delocalisation of polarons in moreordered films. A close consideration of the position of polaronIRAVs furthermore reveals that also these features undergosoftening, i.e. a reduction in energy, for more extendedpolarons.

2 Experimental

RRa-P3HT was purchased from Rieke Specialty Polymer, rr-P3HTfrom Sigma Aldrich, PCPDTBT from 1-material and PC60BM fromSolenne. All materials were used as received. P3HT-related materialswere dissolved in chloroform at 60 1C and PCPDTBT-relatedmaterials in chlorobenzene at 80 1C. Solutions were prepared at20 mg mL�1 concentration and stirred overnight. Blends withPCBM were mixed at a ratio of 1 : 1 (polymer : PCBM) in the caseof P3HT and at a ratio of 1 : 2 for PCPDTBT. Additives were usedat 3 vol% concentration. Films were spin-cast at 1200 rpm onquartz or ZnSe substrates.

Absorption spectra of films on quartz slips were recorded ona Shimadzu 3600 UV-vis-NIR spectrometer.

For PIA measurements at low energy, the samples weremounted into a cryostat without being exposed to air andbrought into the beam path of a Bruker Vertex 70 FTIR spectro-meter. As discussed in detail in our previous work,16 we carriedout at least 1024 measurement cycles of ‘‘light on’’/‘‘light off’’.

Quasi-steady-state PIA studies in the NIR were performed byexciting the sample using a 2.3 eV laser, chopped at 141 Hz, andprobing with the continuous spectrum of a Xe lamp. Thetransmitted light is dispersed by a 1200 lines mm�1 gratingmonochromator (iHR320, Horiba) and detected by a siliconphotodetector. Additional measurements with a blocked Xelamp account for the sample PL.

3 Results and discussion

Fig. 1(c) shows the materials studied in this investigation. Thepolythiophene derivative P3HT was used with a random (RRa)and highly regular (rr) head-to-tail configuration. The former isknown to generate highly disordered, i.e. amorphous, films and thelatter generally forms films, which exhibit both crystalline andamorphous regions. The crystallinity can be increased upon post-deposition exposure to elevated temperatures – thermal annealing.17

Fig. 1 Classical schematic of polaron energy levels and allowed optical transitions (a) along with the mechanisms for wavefunction delocalisation inconjugated polymers with different degrees of disorder (b). The materials used in this study are depicted in (c), where RRa- and rr-P3HT denote theregiorandom and regioregular variants of poly-(3-hexylthiophene), DIO denotes 1,8-diiodooctane and ODT denotes 1,8-octanedithiol.

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We here use the well-established connection between the degreeof P3HT film crystallinity and its absorption spectrum to assessthe chain order in films.18,19 Ascertaining the degree of crystal-linity is especially important, as previous studies on the effect ofchain order unfortunately lacked such information.20

The absorbance spectra of the corresponding films are givenin Fig. 2(a). The amorphous RRa-variant exhibits a broad andunstructured absorption band around 2.8 eV. When casting amixture with the electron acceptor PCBM, this band becomesnarrower and the absorption increases at high energy due tothe contribution of PCBM. As expected, thermal annealing doesnot affect this material (neither neat or when blended, Fig. S1,ESI†), as the steric hindrance of the side chains is too large foran appropriate alignment. For neat films of the rr-variant, wefind the absorption band to peak at 2.4 eV. Additionally, thebroad band exhibits a strong substructure, which is due to thevibrational levels participating in the transitions. Surprisingly,despite casting from volatile CF at 60 1C, we could not achievefurther improvement in the chain order of the neat polymer

through thermal annealing (Fig. S2(a), ESI†). For the blend ofrr-P3HT with PCBM, finally, the polymer absorption peaksaround 2.5 eV and the degree of chain order lies between thetwo aforementioned cases. Importantly, the disorder due to thepresence of PCBM can be reduced through thermal annealing,as shown by the enhanced vibrational substructure in Fig. 2(a).Spectroscopy of photoconductors fabricated under the sameconditions (Fig. S2(b), ESI†) and IR transmission spectra(Fig. S4, ESI†) furthermore support this assessment.

Turning to the photoinduced absorption spectra, we onlyconsider the results for blends with the as-cast RRa-variant andthe two cases of rr-P3HT in the main text. Additional data canbe found in the ESI.† We note that in contrast to earlierreports,4 it was not possible for us to detect a PIA signal forneat RRa-P3HT. We assume this to be due to a higher purity ofour material, since defects are considered to be sites of excitondissociation.

Fig. 2(b) shows the three relevant spectra over the entirein-gap energy range. As introduced above, two distinct regions

Fig. 2 The normalised direct absorbance spectra of P3HT containing films display a broad and unstructured band peaking around 2.3 eV in the case ofan amorphous film and an increasingly pronounced vibrational substructure as well as a red-shifted peak for more crystalline samples (a). Thephotoinduced absorption spectra of selected films include two pronounced regions of absorption in the NIR and MIR respectively (b). Amorphous P3HTexhibits an absorption band around 0.5 eV, whilst more crystalline films peak at lower energy. A close-up of the low energy region (c) reveals a red-shiftupon increasing the polymer crystallinity – a similar softening process can also be observed for narrow vibrations (d).

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of absorption can be identified in the NIR and MIR spectralregion. The amorphous RRa-P3HT:PCBM (Fig. 1(b) top) displaysa broad and featureless absorption peaking at 1.5 eV in the NIR(the signal magnitude in (b) was doubled to allow for a bettercomparison). In the low energy region, a PIA band forms around0.5 eV, which is generally attributed to the intrachain polarontransition P1.4,5

The two spectra involving rr-P3HT, on the other hand,display a more structured absorption in the NIR with one peakbetween 1.2 and 1.3 eV and another at 1.85 eV. Whilst PCBM�

gives rise to absorption at 1.23 eV, this signal is generally weakand eclipsed in blend with P3HT. The observed absorptionband at this energy can thus safely be attributed to the P2

transition of P3HT. In the MIR spectral region, no band formsaround 0.5 eV for either sample, but both spectra display abroad and structured absorption at lower energy superimposedwith the aforementioned IRAVs.

A closer look at the respective normalised spectra, as shownin Fig. 2(c), reveals that the maximum of the electronic transitionshifts from approximately 0.1 to 0.08 eV upon annealing therr-variant. This trend agrees well with the propositions by theSpano group that predicted a red-shift in the MIR upon largerpolaron delocalisation.6 A similar trend has also been observedupon increasing the molecular weight of P3HT chains.15 Thesimple polaron model introduced above assigns the strong absorp-tion peaking around 0.1 eV to the delocalised interchain transition,whereas Fig. 2(b) shows the intrachain transition in this case toform a shoulder between 0.3 and 0.4 eV (red-shifted due to alarger delocalisation along the backbone compared to RRa-P3HT,Fig. 1(b)). Whilst these assignments are in agreement with theemergence of the second polaron absorption peak at 1.85 eV –consequently identified as DP2 transition4 – this discussion isoversimplified and the interplay of intra- and interchainpolaron delocalisation contributes in a complex way to theshape of the electronic absorption band.6

The close-up in (c) furthermore shows a finite, albeit weak,absorption band of the RRa variant between 0.1 and 0.2 eVunderneath the strong IRAVs. This weak contribution is unexpectedfrom the classical description and has recently been connected tothe third type of polaron transition termed P0.6

In contrast to the extreme comparison between RRa- andrr-P3HT, considering the latter upon annealing allows theobservation that the red-shift of the PIA spectra in the MIR isalso accompanied by a blue shift of the NIR contribution for theannealed film (also see Fig. S3 for additional spectra in the NIR,ESI†). As discussed above, the P1 transition energy is consideredto scale with the polaron reorganisation energy – a lowerreorganisation energy in more crystalline samples leads to theobserved trend and thereby underlines the descriptive power ofthe classical model.

As a side note, we also investigated the MIR signature ofrr-P3HT when blended with the non-fullerene electron acceptoro-IDTBR. This material has recently attracted considerableattention since it outperforms the fullerenes in solar cells.21

The data are shown in Fig. S3 (ESI†) for an as-cast film andcompared with neat rr-P3HT as well as annealed rr-P3HT:PCBM

(in p-type polymers, negative charges do not contribute to thephotoinduced signature22). The spectrum exhibits a maximumat low energy indicating highly ordered polymer chains andthereby proving the favourable morphology to be one reason forthe attractive performance of this blend.

Finally, we consider the infrared active vibrations at lowenergy, as shown in Fig. 2(d). Molecular vibrations give rise to aplethora of narrow features superimposed on the electroniccontributions. Due to the interplay with the electronic back-ground, these can manifest as peaks or as dips. Whilst some ofthem can be traced back to ground state modes, the distinctassignment to polaron-related modes has recently beeninvoked for the most prominent features.10,11,16 Of the smallerfeatures, some shift to lower energy (e.g. at 0.102 eV). This modewas previously identified as the QC–H out of plane bendingmode in IR transmission and to be a strong indicator for thepolymer conformation.23 In crystalline phases, this mode leadsto absorption at 101.7 meV (820 cm�1) and in disorderedphases the peak shifts to 103–104 meV (830–840 cm�1). Thistrend is similarly observed in the transmission data given inFig. S4 (ESI†). The red-shift is thus attributed to the changesin the ground state energetics of the polymer (values are givenin Table 1). Notably, other modes remain unaffected by thepolaron chain order, e.g. the indicated peak at 0.173 eV, whichcorresponds to another IR active mode of the ground state,whose position remains constant also in direct transmission.More importantly, all three depicted spectra include prominentpeaks between 0.13 and 0.16 eV – the region predicted tocontain the most prominent vibrational modes of P3HTpolarons.10 All peaks of this class undergo softening uponincreasing the polaron delocalisation.

Concluding the data obtained for P3HT, we find a red-shiftof the low energy absorption of the polaron upon slightlyincreasing the chain order. This observation is accompaniedby a blue-shift in the NIR spectral region underlining the viabilityof the concept of the polaron reorganisation energy. IRAVs in theMIR can be divided according to their origin. Those related tothe ground state behave according to the impact of chainconformation on the ground state mode, but general softeningis observed for modes attributed to the polaron for extendedcarrier delocalisation.

The neat DA polymer PCPDTBT (Fig. 1(c)) can form apartially crystalline film. In the presence of PCBM, however,

Table 1 Position of the most relevant IRAVs for P3HT:PCBM as discussedin the main text

RRa rr as-cast rr annealed

meV cm�1 meV cm�1 meV cm�1

131.2 (1058) 129.6 (1045) 128.6 (1037)138.0 (1113) 137.2 (1107) 136.7 (1103)141.3 (1140) 139.1 (1122) 138.9 (1120)

145.6 (1174) 145.6 (1174)148.3 (1196) 147.3 (1188) 147.3 (1188)158.1 (1275) 154.0 (1242) 153.7 (1240)160.9 (1298) 156.4 (1261) 156.1 (1259)172.0 (1387) 172.0 (1387) 172.0 (1387)

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the chains are highly disordered and films are consideredamorphous.24 In contrast to P3HT, the degree of chain ordercan be affected through the addition of small amounts ofsolvents at a high boiling point, which selectively dissolve PCBM(most commonly 1,8-octanedithiol or 1,8-diiodooctane), insteadof thermal annealing. Doing so leads to a high degree of chainconformation with a pronounced p-stacking of approximately3.8 Å separation between the chains.24,25

This alignment again manifests as a pronounced substructurein the absorption spectra.26 The relevant curves are plotted inFig. 3(a), showing the two absorption bands typical for DApolymers around 1.6 and 3 eV. The low energy band is virtuallyunstructured for the amorphous PCPDTBT:PCBM film, butexhibits two subpeaks at 1.61 and 1.76 eV in the neat case. Filmdeposition with either additive also gives rise to this vibrationalstructure and shifts the subpeaks towards lower energy – therebyindicating an even higher degree of order than found for theneat film.

Fig. 3(b) displays an overview of the photoinduced spectra inthe MIR region. As discussed before,16 the electronic P1 transitionof PCPDTBT:PCBM consists of a substructure already in thedisordered film – illustrating the limits of applicability of thepolaron scheme in Fig. 1(a). In the absence of solvent additives,two broader peaks are found at 0.33 and 0.25 eV and a sharp peakis observed around 0.12 eV in the region of IRAVs. The spectraobtained for films with either additive display a red-shift of thebroader peaks to approximately 0.31 and 0.23 eV – the emergenceof an identical signature for both additives also rules out thepossible observation of signals from these molecules. Also the neatpolymer exhibits the same energies (Fig. S5, ESI†).

Again, as for P3HT, the larger delocalisation, enabled by thegreater crystallinity, leads to a reduction of what is commonlydescribed as P1 transition energy. It is striking, though, that thechange in the electronic signature is distinctly small – theshape remains largely unaffected and only shifts by approxi-mately 0.02 eV. Given the formed extended crystals in thepresence of additives, these results thus show that the polarondelocalisation is rather limited in this system and their spectralsignature is mostly governed by partial alignment alreadyfound in the amorphous case.16 Nonetheless, the results con-firm delocalisation to reduce the electronic transition energiesof polarons in the MIR.

Fig. 3(c) shows a close-up of the region below 0.2 eV containingthe pronounced vibrational structure. In all cases, a strong absorp-tion around 0.12 eV dominates the spectra. From our previous work,using DFT calculations, we know that polarons give rise to strongvibrational modes in this region.16 Importantly, also these vibrationsshift to lower energy for more delocalised polarons in films pro-cessed with additives. This is in agreement with above observationsfor P3HT and calculations by Anderson et al.11 Notably, the extendedpolymer order also leads to an increased and broad absorptionbelow 0.12 eV, which cannot be accounted for by a narrow vibra-tional mode. This effect is indicated by an arrow and supports ourprevious claim of an additional electronic contribution (DP1) at thisenergy.16 Extended crystallinity leads to a larger signal at lowerenergy, which again proves the red-shift of this electronic transition.

Similar to P3HT above, many of the minor vibrational featuresmanifested as dips are not affected by the polaron delocalisation.The three peaks between 0.15 and 0.18 eV, however, exhibit ameasurable red-shift as well (see Table 2). Again, these modes havebeen attributed to polaron modes before.16

Fig. 3 Crystalline films of PCPDTBT produce a red-shift of the low energyabsorption band as well as a more pronounced vibrational substructure (a). Thecorresponding PIA spectra show similar polaron signatures for all samples (b),which shift to lower energy for more ordered films. A close-up of the lowenergy region indicates softening also of the vibrational modes and a broadsignal around 0.1 eV, associated with the DP1 electronic transition (c).

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4 Conclusions

We investigated the photoinduced absorption spectra of twodifferent conjugated polymers whilst varying the degree of chainorder. We chose the greatly studied homopolymer P3HT and theprototypical donor–acceptor polymer PCPDTBT and their blendswith the electron acceptor PCBM, for which the film morphologycan be adjusted through post-deposition thermal annealing orthrough the use of solvent additives. A more crystalline morphologyenables a larger delocalisation of the polaron leading to a signifi-cant red-shift of the electronic transition energy in the mid infraredspectral region (P1 and DP1). Simultaneously, polaron inducedvibrational modes (IRAVs) furthermore undergo softening for moredelocalised charge carriers.

These investigations show the great power of using the lowenergy polaron signatures to identify and study the materialscarrying the electric charge, as well as to study their localenvironment. The MIR PIA spectra can thus not only be usedas a fingerprint for specific polymers, but also offer informationabout the chain alignment in the vicinity of charge carriers.

The careful changes in the chain confirmation investigated herehelp to prospectively enable a more thorough understanding of theinvolved energy states that give rise to the observed transitions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

S. K. acknowledges the Ubbo Emmius Foundation and theResearch Training Group GRK 1896 of the German ResearchFoundation for funding his doctoral work.

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Table 2 Position of relevant PCPDTBT:PCBM IRAVs affected by thepolaron delocalisation. The energy is reduced for more crystalline films

No additive ODT DIO

meV cm�1 meV cm�1 meV cm�1

120.5 (972) 119.1 (961) 119.1 (961)127.7 (1030) 124.5 (1004) 124.6 (1005)151.6 (1223) 150.6 (1215) 150.9 (1217)160.2 (1292) 159.7 (1288) 159.7 (1288)171.7 (1385) 169.5 (1367) 169.8 (1370)

Paper Journal of Materials Chemistry C