Vibrational Spectroscopy of a Low-Band-Gap DonorâAcceptor

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Vibrational Spectroscopy of a Low Band Gap Donor-Acceptor Copolymer and Blends Franziska Fuchs, Simon Schmitt, Christof Walter, Bernd Engels, Eva M. Herzig, Peter Muller-Buschbaum, Vladimir Dyakonov, and Carsten Deibel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03429 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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The Journal of Physical Chemistry

Vibrational Spectroscopy of a Low Band Gap Donor-Acceptor Copolymer and Blends Franziska Fuchs,† Simon Schmitt,† Christof Walter,‡ Bernd Engels,‡ Eva M. Herzig,¶,§ Peter Müller-Buschbaum,k Vladimir Dyakonov,†,⊥ and Carsten Deibel∗,# †Experimental Physics VI, Julius Maximilian University of Würzburg, D-97074 Würzburg, Germany ‡Institute for Physical and Theoretical Chemistry, Julius Maximilian University of Würzburg, D-97074 Würzburg, Germany ¶Herzig Group, Munich School of Engineering, Technische Universität München, D-85747 Garching, Germany §Dynamics und Structure Formation, Fachbereich Physik, Universität Bayreuth, Universitätsstraße 30, Bayreuth 95440, Germany kLehrstuhl für funktionelle Materialien, Physik Department, Technische Universität München, D-85748 Garching, Germany ⊥Bavarian Centre for Applied Energy Research (ZAE Bayern), D-97074 Würzburg, Germany #Institut für Physik, Technische Universität Chemnitz, 09126 Chemnitz, Germany E-mail: [email protected]

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Abstract The molecular vibrations of the polymer PCDTBT are examined with resonant Raman spectroscopy accompanied by DFT calculations. With the comparison of the building blocks (DTBT and carbazole), the monomer unit and the polymer, most of the strongest vibrations can be identified and assigned. This systematic study reveals that the polymer vibrations are dominated by the monomer modes. Blending with PC71 BM at varying fullerene load exhibits no influence on the Raman signature of PCDTBT. This interesting finding is discussed in the context of yet changing morphology, which is investigated by atomic force microscopy and grazing incidence wide angle x-ray scattering measurements, and solar cell performance. Therefore, Raman spectroscopy may not be generally suitable to study the molecular order in very amorphous low band gap polymers, as shown here for PCDTBT.

Introduction For optimization of organic photovoltaic (OPV) devices, the understanding of the underlying links between physical properties on a molecular scale and the macroscopic device performance are crucial. 1 A lot of effort has led from initial power conversion efficiencies of a few percent using PPV to the current record efficiencies of about 11% with new low band gap materials. 2 The molecular order plays a distinctive role, and to investigate it, also Raman spectroscopy has been proposed. 3–5 Up to now, several publications have emerged, mainly focusing on P3HT 3,4,6–9 or PPVs. 10–12 For the more efficient low band gap copolymers such as F8BT 13 or PCDTBT, 5 research in this direction is still at an early stage. Especially the connection of Raman signatures with morphological properties or molecular order in organic solar cells is not yet fully elucidated. Therefore, we investigated this connection, using neat PCDTBT and blended PCDTBT:PC71 BM, having almost twice the efficiency of P3HT:PCBM devices. 14 First, the Raman spectra of the polymer building blocks, accompanied by DFT calculations of the re2


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spective molecular vibrations, were measured. They allow assigning certain vibrations of the polymer spectrum. Second, by comparing Raman spectra of the blend with different ratios with morphological changes revealed by AFM and GIWAXS measurements, it becomes clear that the different techniques show complementary aspects of the blend material.

Methods Sample Preparation The polymer’s building blocks, DTBT and Carbazole, were synthesized at and provided by the group of Prof. Scherf at the Bergische Universität Wuppertal. The polymer PCDTBT (Poly[N-9′ -heptadecanyl-2,7-carbazole-alt-5,5-(4′ ,7′ -di-2-thienyl-2′ ,1′ ,3′ -benzo-thiadiazole) was purchased from 1-Material. The fullerene derivative PC71 BM ([6,6]-phenyl-C71-butyric acid methyl ester) was obtained from Solenne. No additional purification was performed. All materials were dissolved in chlorobenzene. Sample preparation was done in nitrogen atmosphere. To come as close as possible to solar cell working conditions, a 30 nm to 50 nm thick layer of PEDOT:PSS from Clevios (PVP Al 4083) was spin-coated on top of the Si substrate. The substrate was then annealed for 10 min at 130◦ C before deposition of the PCDTBT and PC71 BM. All thin films were manufactured by spin coating for 60 s with 1000 rpm on pre-cleaned silicon wafer substrates, resulting in an active layer thickness in the range of 50 nm to 200 nm. The addition of PCBM was able to quench the photoluminescence signal—strongly overlaying the Raman intensities—of the material under investigation without disturbing the Raman signal.

Raman Spectroscopy For the Raman measurements, a confocal micro-Raman spectrometer LabRAM HR800 from Horiba was used with excitation by a frequency-doubled Nd:YAG laser (532 nm, ca. 40 mW laserpower on the sample). The used objective has 10x magnification, and the backscattered 3



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light is, after passing the Czerny-Turner monochromator, detected with a CCD camera. Acquisition time was 15 seconds. For spectra recording and background subtraction, the measurement software LabSpec from Horiba was employed. All measurements were recorded at room temperature, with the samples being in a home-built nitrogen flow box with optical access to avoid degradation.

Computational Details For the calculation of vibrational spectra, the TURBOMOLE 6.3 program package 15 was used. Geometry optimizations as well as frequency calculations were performed on the density functional level using the B3LYP functional 16–19 and the cc-pVDZ basis sets. 20 Preliminary test calculations using the cc-pVTZ basis sets have shown that the cc-pVDZ basis is sufficiently converged. Raman scattering activity coefficients were converted into Raman intensities.

Morphological Characterization The atomic force microscopy (AFM) images were recorded with a Dimension Icon microscope from Veeco/Bruker using the tapping mode. The used cantilever (AC240TS from Olympus) has an eigenfrequency of about 70 kHz and the attached AFM tip a tip radius of 9 nm. GIWAXS measurements were performed to examine the internal film morphology. 21 The experiments were carried out at beamline P03, Desy at an x-ray energy of 11.4 keV with a 1M Pilatus detector from Dectris and a sample detector distance of 19.7 cm. 22 GIWAXS data was collected for neat PCDTBT and the 1:4 PCDTBT:PC71 BM sample. All samples were prepared on PEDOT:PSS covered Si substrates. The reference of PEDOT:PSS on Si was also measured. Due to the weak scattering of the samples in comparison to the background signal all samples were measured at different incident angles. Varying the incident angle gives depth sensitivity by measuring below the critical angles of all materials, between the critical angles of the organic materials and the substrate as well as above all critical angles. This 4


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way the scattering contribution of the substrate is distinguished from that of the polymer and polymer blend films. To aid peak identification and 2d display of the data, planarization of the data is achieved via subtraction of the featureless surface information. The conversion into q-space was done using the program GIXSGUI. 23

Results Raman Spectroscopy The measured Raman spectra of the building blocks carbazole and DTBT (see solid lines in figure 1a) show several vibrational lines, especially in the region between (1000-1600) cm-1 . This is expected due to the size of the molecules and their organic nature, but makes a detailed assignment of a Raman peaks to a certain vibrational mode more complex. Artifact peaks originate from the substrate, namely the Si Raman peak at 521 cm-1 , as well as the PCBM added as photoluminescence quencher. Without this quencher, the molecules’ photoluminescence overlaid the Raman peaks by orders of magnitude. The artifacts were assigned in comparison to Si wafer and to pure PCBM, and are denoted in the molecules’ spectra with Si for silicon and ∗ for PCBM, respectively. The calculated vibrations are shown in figure 1a as black strokes. Here, not only the positions could be simulated, but— by calculating the Raman scattering activity coefficients—the relative intensity of each mode could be found. The comparison of calculated and measured spectra shows good resemblance, so that especially the most prominent vibrations of the molecules could be assigned to certain modes. The carbazole unit has the strongest vibrations at (1420-1480) cm-1 and 1623 cm-1 . The PCBM peaks are higher in comparison to the building block DTBT with PCBM, so that we expect that the carbazole vibrations are not as Raman active as the DTBT vibrations. For the latter molecule the highest Raman intensities can be seen at 1272 cm-1 , (1340-1390) cm-1 and 1527 cm-1 . All these vibrations are of the stretching or breathing type. Going from the building blocks to the monomer unit, no new modes occur 5



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(see figure 1b). Some modes are clearly due to a specific building block; such as 1272 cm-1 , (1340-1390) cm-1 (DTBT) or (1420-1480) cm-1 and 1623 cm-1 (carbazole). In contrast, other modes are mixed vibrations, where both building blocks vibrate together, such as the 1375 cm-1 and 1527/1542 cm-1 modes. For the monomer CDTBT, calculations of the vibrational spectra for different conformations were carried out (see table S1 and figure S1 in the supporting information). They are slightly different, but none of them matches the measured spectrum exactly. This shows that the sample consists of various, statistically distributed molecular conformations and the measured spectrum is an average over these. The polymer spectrum looks very similar to the monomer spectrum, although some modes are strongly suppressed, such as the carbazole 1430 cm-1 and 1464 cm-1 peaks. That means the polymer vibrations are dominated by the monomer vibrational modes. Since the most prominent vibrations have been assigned, it is interesting to consider the blend material PCDTBT:PC71 BM as well, which is commonly used as active layer in organic solar cells. In figure 1b, the pure PCDTBT spectrum is depicted in comparison to the PCDTBT:PC71 BM blend ratios 1:1 and 1:4. Remarkably, the spectra do not change, except for the more pronounced PCBM peaks ∗, in proportion to the increasing PCBM fraction.

Morphological characterization The findings from Raman spectroscopy are in sharp contrast to the morphological characterization: The AFM phase images (see figure S2 in the supporting information) for PCDTBT:PC71 BM 1:1 and 1:4 show that the 1:1 blend is quite uniform, whereas the 1:4 blend is more coarse with a phase separation being visible in the image. This clearly indicates a change in morphology, at least at the surface, where AFM probes the topography. The change in morphology at different blend ratios can also be seen in TEM measurements. 24 Looking into more detail into the best performing sample (1:4) helps to understand this discrepancy. Since the 2D GIWAXS data show background contribution from the substrate it can be concluded that the signal from ordered (crystalline) material present in the thin 6


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films is very weak. As a consequence, the degree of crystallinity in the films is low. GIWAXS measurements were carried out at different incident angles to gain depth dependent information on the film structure. Below the critical angle of the organic thin films the GIWAXS data yield information on the near-surface crystalline structures which are present in the top few nanometers of the film. No Bragg peaks are identifiable for any of the samples for the data measured below the critical angle. Therefore, the near-surface region of the films contains amorphous PCDTBT and PC71 BM. Bragg peaks that can be associated with the materials in question only start occurring for values of the angles of incident above the critical angle of these materials, i.e. as soon as the X-rays are penetrating the entire film. At an incident angle of 0.13◦ we measure the bulk of the organic films but still have total reflection off the inorganic substrate, while at higher incident angles the strong substrate signal starts dominating the collected GIWAXS data. The raw data is shown as figure S3 in the supporting information. To make the material signatures visible to the eye, the featureless surface information at low incident angle (0.1◦ ) is used to planarize the data. The resulting 2D data and the plot of the vertical sector profiles (figure 2) clearly show a weak ordering typical for PEDOT:PSS thin films. 25 The neat PCDTBT film on top of PEDOT:PSS shows a combination of the PEDOT:PSS rings at q=12.2 and 17.7 nm-1 and oriented contributions of Bragg peaks typical for PCDTBT at q=4.2 and 15.4 nm-1 , respectively, in accordance with literature values. 26–29 The detected PCDTBT signal is very weak indicating that only a small fraction of the PCDTBT is crystalline. Therefore the majority of the PCDTBT is present in an amorphous state. Upon the addition of PC71 BM, contributions of ordered PCDTBT are no longer detectable but are replaced with the typical powder rings for PC71 BM at q=6.3, 13.5 and 18.1 nm-1 in good agreement with literature, 30 indicating that the weak ordering of PCDTBT is even suppressed by the addition of PC71 BM. The GIWAXS results explain why the Raman scattering, a method that is sensitive to the immediate nanoscale environment of the molecules and therefore also nanoscale ordering,

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is not resulting in any changes in the standard Raman signal. While GIWAXS is only sensitive to crystalline material, it does not collect information on the disordered, amorphous part of the bulk film. This is in contrast to Raman spectroscopy which collects signal strength from all molecules within the probed volume, independent of the state of order of the present molecules. For such weak ordering as observed here in the described samples the amorphous Raman signal dominates the collected signal and consequently there are no changes discernable between the samples in Raman spectroscopy. As a result, it is important to carefully choose the appropriate combination of observation methods to allow the correct conclusions on the nanomorphology of thin film samples. Apart from using complementary techniques such as scattering, it can be beneficial for some material systems to consider more sensitive Raman methods by using for example resonance Raman scattering to exploit minor changes in excitation energies 31,32 or charge transfer processes in mixed polymer systems. 33 The morphological information observed by GIWAXS is significant for the solar cell performance, as first shown by Park et al. 24 for the different blend ratios of PCDTBT:PC71 BM. The best ratio turned out to be 1:4 with a power conversion efficiency of 6.1%.

Conclusions In this work, we linked the results from Raman simulations and measurement to first identify and assign the PCDTBT molecular vibrations, that are dominated by the monomer vibrational modes. Subsequently, we examined the influence of fullerene load in the PCDTBT:PC71 BM blend. Although AFM and GIWAXS measurements show a clear change in morphology towards a higher order for a higher fullerene loading, this change is not detected by Raman measurements. While GIWAXS relies on scattering by crystalline material, Raman spectroscopy is sensitive to the immediate nanoscale environment of the molecules irrespective of the degree of crystallinity. That means the amorphous Raman signal dominates the overall

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signal for the weak ordering found in the PCDTBT based thin films. Therefore we conclude that in the case of very amorphous low band gap polymers, Raman spectroscopy is not a straight-forward technique to study the molecular order of conjugated polymers.

Supporting Information Available The following file is available free of charge. • fuchs2017raman - supporting information.pdf: contains a table of some prominent vibrations of PCDTBT and its building blocks, the influence of molecular conformation on calculated Raman spectra of PCDTBT, as well as AFM phase images of the blend films and the raw 2D GIWAXS data.

Acknowledgement The authors thank Hannes Kraus for fruitful discussions. The work was supported by the Bavarian Collaborative Research Project “Solar Technologies go Hybrid” (SolTech) and the DFG Research Unit FOR1809. We are grateful for the supply of the carbazole and DTBT molecules from the group of Prof. Scherf in Wuppertal. We thank the LRZ for computation time.

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Graphical TOC Entry Carbazole DTBT CDTBT PCDTBT PCDTBT:PCBM 1:1 PCDTBT:PCBM 1:4 1000

1200 1400 Raman shift [cm-1]

1600

TOC Graphic

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(a)

(d)

(b)

(e)

(c)

(f)

Figure 2: Left: GIWAXS 2 D data planarized at incident angle 0.13◦ and right: vertical sector cuts at angle of incidence indicated in the legend of planarized data, (a) & (d) PEDOT:PSS on silicon, (b) & (e) PCDTBT and (c) & (f) 1:4 PCDTBT:PC71 BM.

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Vibrational Spectroscopy of a Low-Band-Gap DonorâAcceptor

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Vibrational Spectroscopy of a Low-Band-Gap DonorâAcceptor