Influence of Processing Solvents on Optical Properties and

Article pubs.acs.org/Macromolecules

Influence of Processing Solvents on Optical Properties and Morphology of a Semicrystalline Low Bandgap Polymer in the Neutral and Charged States F. S. U. Fischer,† K. Tremel,† A.-K. Saur,† S. Link,† N. Kayunkid,‡ M. Brinkmann,‡ D. Herrero-Carvajal,§ J. T. López Navarrete,§ M. C. Ruiz Delgado,§ and S. Ludwigs*,† †

IPOC-Functional Polymers, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany Institut Charles Sadron (UPR22), CNRS 23 rue du Loess, 67034 Strasbourg Cedex 2, France § Department of Physical Chemistry, University of Málaga, Campus de Teatinos s/n, Málaga 29071, Spain ‡

S Supporting Information *

ABSTRACT: We present a systematic study of the morphology and absorption properties of a typical donor− acceptor polymer (PCPDTBT) with semicrystalline behavior in solution and in thin films. In-situ spectroelectrochemical data give information about the evolution of the absorption spectra from neutral to charged species. The experimental data are supported by theoretical calculations in the framework of the density functional theory (DFT). Regarding thin film structures, we show that the choice of the solvent has significant influence on the morphology in thin films: whereas CS2 and CHCl3 give rather structureless (amorphous) morphologies, films from 1-CN exhibit a clear crystalline nanofiber morphology. Accompanying UV/vis/NIR spectra of films are highly dependent on the morphology and therefore on the choice of the processing solvent. The absorption of fiber morphologies is strongly red-shifted compared to the structureless films.

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INTRODUCTION Donor−acceptor polymers with low bandgaps have become high potential donor materials for organic solar cells since they were first introduced in the early 1990s.1 Compared to the conventionally used donor material poly(3-hexylthiophene) (P3HT), they show desirable properties for polymer bulk heterojunction solar cells with the fullerene derivative [6,6]phenyl-C61/71-butyric acid methyl ester (PCBM) such as a smaller band gap and lower lying HOMO/LUMO levels which results in higher quantum efficiencies.2,3 Beside solar cells, other applications of low bandgap polymers e.g. in supercapacitors are discussed in the literature.4 The absorption spectra of donor−acceptor polymers typically show two absorption maxima: one at high and one at low energies, the low-energy one being the most dominant. While the low-energy absorption is generally attributed to an intramolecular charge transfer (CT) transition from the donor to the acceptor unit within the polymer backbone, the highenergy peak is correlated to a more localized π−π* transition.5 Density functional theory (DFT) calculations resulted in a localized LUMO on the acceptor blocks and a delocalized HOMO over the whole backbone.6,7 The interest in such kind of materials and their photophysical properties has continuously increased over the past years. It was shown for example that by changing either the ratio of acceptor and donor blocks6 or the chemistry of the acceptor block itself the absorption maxima of the low-energy transition could be strongly shifted.8 © XXXX American Chemical Society

One outstanding donor−acceptor polymer is PCPDTBT (poly{[4,4-bis(2-ethylhexyl)cyclopenta(2,1-b;3,4-b′)dithiophen]-2,6-diyl-alt-(2,1,3-benzothiadiazol)-4,7-diyl}). This material was first introduced by Brabec et al. in 2006; efficiencies of up to 7% after optimization were expected.2 Today, power conversion efficiencies of 4.5−5.5% are reported.9−12 Optimizations of efficiency were mainly obtained by morphology improvement using solvent additives such as diiodooctane or alkanedithiols during the production process.12−14 Bazan et al. reported for example high efficiencies in PCPDTBT/PCBM organic solar cells of films spin-coated from chlorobenzene (CB) with diiodooctane as additive.11,15 Film absorption maxima up to 1.6 eV/800 nm for the low-energy transition were characteristic for the polymer in these blend films. Another method to improve PCPDTBT solar cells was shown by von Hauff et al.16 Chemical doping of PCPDTBT with the strong acceptor molecule F4TCNQ results in improved charge carrier mobilities of the pure polymer and an increase of the Jsc (short circuit current) and the open circuit voltage in PCPDTBT/PCBM solar cells. Received: May 5, 2013 Revised: May 29, 2013

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dx.doi.org/10.1021/ma400939z | Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Molecular structure of PCPDTBT (top) and Mülliken atomic charge distribution of (CPDTBT)3 oligomer in the neutral ground state (bottom). (b) UV/vis spectra of 0.01 mg/mL solutions of PCPDTBT in CS2 (black), 1-CN (red), CB (blue), CHCl3 (green), and THF (purple); the spectra are normalized with respect to the low-energy maxima. (c) TD-DFT calculated chain length dependence of the absorption maxima for (CPDTBT)n oligomers (with n = number of repetition units, from n = 1 to n = 5), using the B3LYP and CAM-B3LYP functional as a function of 1/ n. Experimental polymer value in CB solution is shown as a square; solid lines are Kuhn fits.23 (d) DFT//B3LYP/6-31G** molecular orbitals involved in the S0 → S1 (low-energy band) and S0 → S9 (high-energy band) transitions for (CPDTBT)3 oligomer. Here, n = 3 has been shown as a representative case; the other cases (different n values or different functional) show similar trends.

In the literature we found only very few studies13,15,17−20 that investigate the morphology and absorption properties of the pure homopolymer, i.e., without any additives such as haloalkanes/alkyl thiols or PCBM as acceptor in blends. The optical properties of the homopolymer were investigated in chlorobenzene solutions by Bazan et al.15 and Lanzani et al.17 showing the following absorption characteristics: one main absorption at low energies of around 1.72 eV/720 nm accompanied by a shoulder at 1.85 eV/670 nm and one highenergy absorption peak at around 3.02 eV/410 nm. The lowenergy peak was assigned to a HOMO → LUMO transition from a delocalized HOMO to a LUMO localized on the benzothiadiazol (BT) units, suggesting a strong charge-transfer excited state character. The shoulder of the low-energy band at 1.85 eV/670 nm was assigned to a vibronic mode and the highenergy band to a S0 to Sn transition.17 In thin films the two main maxima appear at 3.02 eV/410 nm and around 1.61 eV/770 nm accompanied by a pronounced shoulder at 1.71 eV/725 nm.15,17 The main difference in solution and film spectra is the broadening of the spectra and the red-shift of up to 40−50 nm for the main low-energy transition. Further shifting of the low-energy absorption up to 785 nm in pure films and 800 nm in PCBM blend films typically connected to the presence of the above-mentioned

solvent additives and their role in promoting PCPDTBT crystallization.15,17 Regarding morphology of the homopolymer films, Russel et al.19 studied spin-coated films from CB solutions with grazing incidence X-ray diffraction. The films were described as featureless after spin-coating. An edge-on orientation of the chains was suggested, Chassé and co-workers made a similar statement based on NEXAFS measurements.18 A fibrillar morphology in blends of PCPDTBT/PCBM was reported when spin-coating from high boiling point additives as 1,8diiodooctane by Russel et al. The main focus of this study is to correlate thin film morphologies of the homopolymer with processing conditions. While in the literature mainly chlorobenzene is used for film preparation, we can show that the choice of the solvent and the preparation conditions have a huge influence on both morphology and optical properties of spin-coated homopolymer films. Before we concentrate on the thin film properties of the polymer, (i) the solution properties of PCPDTBT as a function of the solvent polarity/vapor pressure are discussed with the aim to get a deeper insight into the electronic and optical properties associated with the intrachain polymer backbone and (ii) in-situ spectroelectrochemistry in solution and in films is further shown to characterize the charged species and clarify B


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(Figure S2). Figure 1c shows the vertical transition energies associated with the low-energy band of the (CPDTBT)n oligomers as a function of 1/n, which allows for the extrapolation to the infinite conjugated polymer. The lowenergy band calculated at the CAM-B3LYP level for the infinite polymer (1.83 eV) is in very good agreement with the experimental data (1.68 eV in CB) while that calculated at the B3LYP level (1.02 eV) is strongly underestimated; this result highlights the importance of the use of a range-separated DFT functional (i.e., CAM-B3LYP) for a better evaluation of the optical polymer-limit properties in the neutral state. Looking at the DFT//B3LYP/6-31G** topologies of the frontier molecular orbitals of the CPDTBT trimer taken as a model (see Figure 1d), we note that: (i) The HOMO is delocalized through the whole conjugated backbone of the polymer whereas the LUMO is principally localized on the BT units; thus, the low-energy band (S0 → S1 transition) calculated at 831 nm (1.49 eV, f = 2.20) for (CPDTBT)3 which is associated with a HOMO → LUMO transition has a significant charge-transfer character. (ii) The LUMO+3 orbital spreads not only over the CPDT units but also over the thiadiazole unit of BT; therefore, the high-energy band (S0 → S9 transition) calculated at 463 nm (2.68 eV, f = 1.11) and which is associated with a HOMO → LUMO+3 transition also has certain chargetransfer character. This charge-transfer character of both lowand high-energy bands is also supported by the very similar excited state dipole moments calculated using the RhoCI density for S1 (10.36 D) and S9 (8.43 D) excited states (i.e., associated with the low- and high-energy bands, respectively), which are significantly larger than those found for the ground state S0 (3.41 D). These results are in good accordance with the experimental data that evidence shiftings of both bands when changing the polarity of the solvent. To gain a deeper understanding of the influence of the solvent on the absorption properties of the polymer, optimization of the ground state in different solvents has been performed for (CPDTBT)3 using the PCM approach; TDDFT-PCM vertical electronic excitation energies were computed on these previously optimized geometries, and excited state dipole moments were calculated using the RhoCI density. In accordance with the experimental data, both lowand high-energy bands are blue-shifted when increasing the polarity of the solvent (i.e., in passing from CS2 to THF), with the shifting being less pronounced for the low-energy band (Figure S3). Our calculations reveal that a more polar solvent (THF versus CS2) slightly increases the charge-transfer character of the ground state (the dipole moment of the S0 state slightly increases from 3.83 D in CS2 to 4.04 D in THF) while the excited state becomes less polar (the dipole moment of the S1 state decreases from 9.63 to 9.03 D and the S9 state from 7.99 to 7.68 D, which amounts to 0.60 and 0.31 D, respectively in going from CS2 to THF) (Table S2). Note that the same trend is obtained when a very high polar solvent such as DMSO is considered (i.e., the dipole moment of S0 increases by 0.29 D while that of the excited S1 and S9 excited states decrease by 1.07 and 0.43 D, respectively, in going from CS2 to DMSO). In-Situ Spectroelectrochemistry. Chemical doping of PCPDTBT was studied by von Hauff and co-workers, who reported the influence and improvement of F4TCNQ pdopants on charge transport mobilities and efficiencies in organic solar cells.16 Results by Da Como et al. on pump− probe (pump 400 nm/3.1 eV) experiments of chemically

the absorption spectra evolution as a function of the doping degree. All these experimental observations are complemented by density functional theory (DFT) calculations of their neutral and oxidized species.

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RESULTS AND DISCUSSION Absorption Spectroscopy in Different Solvents. The polymer used in this study was PCPDTBT (poly{[4,4-bis(2ethylhexyl)cyclopenta(2,1-b;3,4-b′)dithiophen]-2,6-diyl-alt(2,1,3-benzothiadiazol)-4,7-diyl}, chemical formula Figure 1a), with a Mw of 23 kg/mol and PDI of 1.7 as evidenced by hightemperature SEC. Five solvents were chosen which differ in polarity and boiling point. All are good solvents for PCPDTBT. For the solution spectra 0.01 mg/mL solutions of the polymer in CS2 (carbon disulfide, bp = 46 °C, ε = 2.6), 1-CN21 (1chloronaphthalene, bp = 259 °C, ε = 5.04), CB (chlorobenzene, bp = 132 °C, ε = 5.6), CHCl3 (bp = 61 °C, ε = 4.6), and THF (bp = 66 °C, ε = 7.4) were studied22 (Figure 1b). Solutions with concentrations up to 3 mg/mL showed no changes in the UV/vis spectra with the exception of THF, which showed a small aggregation shoulder around 800 nm above 1 mg/mL (see Figure S1). The solution spectrum in CS2 (black curve) reveals two absorption maxima at 426 nm (2.91 eV) and 743 nm (1.67 eV), the second being accompanied by a shoulder at 660 nm (1.88 eV). The CB spectrum is identical to spectra from the literature.15,17 Compared to the spectrum in CS2, all other solution spectra show a hypsochromic shift for the low- and high-energy transitions. Note that the spectrum recorded in the most polar solvent THF shows the largest blue-shifts (i.e., ∼30 nm/0.07 eV for the low-energy band and ∼18 nm/0.13 eV for the high-energy band). When shifting all spectra along the xaxis in such a way that all low-energy maxima appear at the same wavelength, the shapes of the spectra perfectly match each other, which is a strong indication that the shifts are not due to aggregation effects. It is interesting to note that the Mülliken atomic charge distribution in the neutral ground state shows a small polarization of the charge over the cyclopentadithiophene (CPDT) and BT units (i.e., along the longitudinal polymer backbone) while a strong charge polarization is found within the BT unit (with the external N−S−N group of the thiadiazole bearing a charge ∼−0.61 e, whereas the charge over its benzyl group amounts to ∼+0.67 e) (Figure 1a); therefore, the external more polarized N−S−N unit of the BT groups is expected to play a significant role in the electrostatic interactions with the surrounding solvent molecules. In order to better understand the optical and electronic properties of the PCPDTBT polymer, its molecular and electronic structure was studied at the DFT level using an oligomer approach.23 Simulated absorption spectra for the (CPDTBT)n oligomers with n = 1−5 were determined with TD-DFT using the B3LYP and CAM-B3LYP functionals (Figure S2 and Table S1). The simulated absorption spectra reveal two absorption bands, which is in good accordance with the experimental data and previously reported theoretical data.7,17 The empirically found relative intensities of the lowenergy band with respect to the high-energy band is very well captured for the calculated absorption spectral profile starting from the CPDTBT trimer, namely (CPDTBT)3. With the extension of the backbone length, a significant red-shift of the low-energy band is found, with the shift leveling off considerably on the extension to the trimer and tetramer C


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Figure 2. In-situ spectroelectrochemical data of the oxidation of PCPDTBT shown in (a) and (b) in chlorobenzene/THAPF6 (0.2 M) under thin layer conditions at 10 mV/s with a PCPDTBT concentration of 1.1 mg/mL and as film in (c) and (d) in acetonitrile/TBAPF6 (0.1 M) at 10 mV/s; film spin-coated from a 3 mg/mL chlorobenzene solution. Marked potentials in the cyclic voltammograms in (a) and (c) correspond to the respective neutral and doped states in the absorption spectra in (b) and (d).

Figure 3. (a) DFT//B3LYP/6-31G** overall excesses of Mülliken atomic charges for each oxidized form of (CPDTBT)3 with respect to the neutral species. (b) Vertical excited state transitions (solid bars) calculated at the TDDFT//B3LYP/6-31G** level for (CPDTBT)3 in the neutral, radical cation, and dication states. The vertical dashed lines are the experimental data. (c) SOMO and LUMO of the radical cation of (CPDTBT)3.

organic solar cells.24 Here, we conduct in-situ spectroelectrochemistry of PCPDTBT films and solutions to follow in-situ

(F4TCNQ) doped PCPDTBT showed an increased density of photogenerated polarons and an improved charge separation in D


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transitions are calculated for the radical cation of the CPDTBT trimer at 1890 nm (0.66 eV, f = 0.62), 1033 nm (1.20 eV, f = 2.57), and 654 nm (1.89 eV, f = 0.20). The most intense one observed at 1033 nm is attributed to a transition between the singly occupied molecular orbital (SOMO) and the LUMO, which is in good agreement with the broad experimental band observed at 1280 nm. As depicted in Figure 3c, the SOMO is delocalized over the whole conjugated polymer backbone (with strong contribution of the CPDT units), whereas the LUMO is mainly localized on the benzothiadiazole (BT) units; therefore, the SOMO → LUMO transition reveals certain charge-transfer character which explains in turn the broadness of the experimental band. Calculations of the CPDTBT trimer dication show one prominent transition at 1067 nm (1.16 eV, f = 4.42); again, the experimental spectra show a similar trend. Comparison of the spectroelectrochemical data of the films with respect to the chlorobenzene solutions reveals the following characteristics. In the neutral state, the films show two bands at 412 and ∼727 nm accompanied by a new maximum at 776 nm. The band at 727 nm is probably the same transition as the 720 nm band in CB solution with a slight redshift. We attribute the 776 nm absorption to interchain interactions and aggregation in the films. Upon oxidation to the radical cation, a broad band around 1100 nm appears. Again, a complete disappearance of the neutral state only occurs when the dicationic state is reached. This state displays a broad band above 1000 nm and a maximum at 813 nm which is not visible in the solution spectra. The inset in Figure 2d highlights the development of the band at 776 nm with respect to the 727 nm band. The data suggest that the 776 nm band is decreasing faster than the 727 nm band. One potential explanation of this behavior is the aggregation of the chains by interchain interactions due to the semicrystalline nature of the polymer in the solid state. In the case of poly(3-hexylthiophene) Pielichowski et al.28 reasoned that the charging of aggregated polymer chains is favored because of an extension of the conjugated system over several polymer chains. In the following film absorption spectra are discussed in further detail. UV/vis/NIR Spectroscopy of Different Film Morphologies. Figure 4 shows AFM height images of thin films of PCPDTBT spin-coated from CS2 (a), CHCl3 (b), CB (c), THF (d), and 1-CN (e). Spin-coating conditions and solution concentrations are indicated in the figure caption. The corresponding phase images are added to the Supporting Information Figure S8. Two extremely different morphologies are obtained from CS2 and 1-CN. For the low boiling point solvent CS2 a rather disordered featureless structure is visible (Figure 4a). The film spin-coated from 1-CN, on the other hand, exhibits a fiberlike structure with fiber lengths up to 1 μm and average widths of around 40 nm (Figure 4e). A comparison of the vapor pressures of these two solvents suggests the following explanation; caused by the low vapor pressure of 1-CN, the polymer chains are given much more time to arrange during film formation compared to the low boiling point solvent CS2. Drying of the film takes ∼20 times longer for 1-CN than for CS2 in the experiment. Figure 4f shows a characteristic TEM image of a film prepared from 1-CN. The fibers of Figure 4e are visible; additionally, a zigzag-pattern of ∼40 nm features can be resolved. Interestingly, TEM allows to see clearly a Bragg

changes in the absorption (400−1600 nm) of the polymer during oxidation/doping of the polymer and to allow allocation of the spectral bands of the doped species. Figures 2a and 2c show cyclic voltammograms (CV) of the oxidation cycles of PCPDTBT in chlorobenzene solutions and as films, respectively. A reduction cycle of the film CV is available in the Supporting Information Figure S4. From the oxidation and the reduction onsets (oxidation: Eonset = 0.07 V; reduction: Eonset = −1.71 V) of the film CV HOMO and LUMO levels could be calculated as −4.87 and −3.09 eV, respectively, using a value of −4.8 eV relative to the vacuum energy level.25 Thus, an electrochemical bandgap of 1.78 eV was determined. Turner et al. reported similar HOMO and LUMO values of −5.00 and −3.19 eV, respectively, for PCPDTBT films.26 The corresponding UV/vis/NIR spectra during the electrochemical oxidation of PCPDTBT in chlorobenzene solution are represented in Figure 2b. The absorption maxima of the neutral polymer with wavelengths at 411 and 720 nm decrease continuously upon increase of the potential. Spectra at approximately 0.3 V vs Fc/Fc+ with a broad band at around 1280 nm can be correlated with the first oxidation of the polymer to the radical cation. Further increase of the potential gives indication of the dicationic state. Only after oxidation to the dication, the absorption of the neutral species is completely vanished. Figure 3 summarizes computational data of the optimized geometries and the Mülliken atomic charges of the neutral, cation, and dication states of CPDTBT trimer. As seen in Figure 3a, the excess of positive charge of the cation and dication is distributed over the whole π-conjugated backbone, with the charge storage over the CPDT unit being twice as large as that located over the BT unit in accordance with the donor character of the CPDT group. In the same line, the geometric relaxation upon oxidation is delocalized over the whole conjugated backbone with slightly larger geometrical changes localized on the CPDT units (Figure S5). When a comparison is made between the B3LYP and CAM-B3LYP results, as expected, CAM-B3LYP localizes the charge defect in a greater extent than B3LYP does (Figure S5).27 The favorable tendency of the PCPDTBT polymer backbone toward charge delocalization is also well captured by the potential energy surfaces of the monomer as a function of the torsional angle (θ) between the BT and CPDT units (Figure S6). Two well-defined minimum energy conformers (syn and anti) with nearly coplanar π-conjugated backbones and very similar energies are found, being separated by a moderate energy barrier of 5.8 kcal/mol for the neutral state that corresponds to the perpendicular (θ = 90°) conformation. This energy barrier significantly increases up to 10.5 and 30.6 kcal/ mol for the radical cation and dication species, respectively, in accordance with its more quinoid-like structure upon oxidation (i.e., the C−C bond between the BT and CPDT unit decreases from 1.457 Å in the neutral state to 1.434 and 1.392 Å in the cation and dication states, respectively); the rigid structure attained upon charging should in turn facilitate the charge delocalization over the polymer backbone. TD-DFT vertical transitions of the radical cation and dication species for the CPDTBT trimer are shown in Figure 3b. The B3LYP results are found to match the experimental data better than CAM-B3LYP does (Figure S7); this might be related with the strong localization of the charge defect predicted with CAM-B3LYP (Figure S5). Three electronic E


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fast growth direction and (ii) that the chain axis direction is inclined to the edge of the lamellae. Preliminary electron diffraction experiments suggest that the long axis of the fibers corresponds to the π-stacking direction (ED results published elsewhere). We believe therefore that in a first step small units of around ∼40 nm are formed which later form superstructures in the form of fibers. In the case of 1-CN this seems to happen during the spin-coating process. The films spin-coated from the other three solvents CHCl3, CB, and THF show intermediate structures (Figure 4b,c,d). Randomly distributed ∼40 nm features are visible from THF solutions. For further information about potential aggregation phenomena, UV/vis/NIR absorption spectroscopy was conducted for the films studied with AFM. Figure 4g summarizes in one graph the spectra of the films spin-coated from the five different solvents. The broadening of the film compared to the solution spectra is typical for semicrystalline polymer films due to an increase in conjugation length distribution.30 The maximum absorption in the low-energy regions of the films spin-coated from the five different solvents are 735 nm (1.69 eV) for CS2, 760 nm (1.63 eV) for CHCl3, 773 nm (1.60 eV) for CB, 780 nm (1.59 eV) for 1-CN, and 784 nm (1.58 eV) for THF. All spectra further show a shoulder around 713−740 nm which is in the region of the maximal absorption in the respective solution spectra and which can probably be attributed to amorphous parts of the films. The shoulder intensity varies depending on the solvent which could be correlated to the different extent of crystallinity. The absorption maxima of 780 and 784 nm for films spin-coated from 1-CN and THF, respectively, are rather remarkable. In the literature values between 780 and 790 nm (1.59 and 1.57 eV) are only reported when ∼2% DIO as solvent additive was added to the processing solvent CB for the homopolymer films.11,15 As side experiment we have also studied solutions and spin-coated PCPDTBT films from 2% w/w DIO solutions of THF, CB, and 1-CN (Figure S9). The solution spectra (Figure S9a) show the same solvent dependency as the spectra without DIO. For the films we detect 789 nm (1.57 eV) for THF/DIO, 790 nm (1.57 eV) for 1-CN/DIO, and 793 nm (1.56 eV) for CB/DIO; i.e., the absorption increases by an additional 5 nm (THF), 13 nm (1-CN), and 20 nm (CB). DIO seems to have the largest influence on the films processed from CB. Coming back to the films from the pure solvents without any solvent additives, we can state that the careful choice of the processing solvent leads to maxima up to 780 nm (1-CN) and 784 nm (THF), which implies better crystallinity in films processed from 1-CN and THF than in CB.

Figure 4. (a−d) AFM (height) images are shown after spin-coating (1500 rpm, 3 mg/mL) from different solvents (glass substrates, phase images are shown in Figure S8; inlays show phase image in higher resolution for films prepared on Si wafers). (e) AFM (height) and (f) TEM image of a film spin-coated from 1-CN. The red lines in the TEM image highlight the characteristic zigzag pattern. (g) UV/vis/ NIR spectra of the films shown in (a) to (f) from CS2 (black), CHCl3 (green), CB (blue), THF (purple), and 1-CN (red).

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CONCLUSIONS We presented a systematic investigation of the absorption properties of solutions and thin films of PCPDTBT in the neutral and charged states by UV/vis/NIR spectroscopy and insitu spectroelectrochemistry. As solvents we used CS2, 1-CN, CHCl3, CB, and THF. By performing quantum chemical calculations, we have been able to (i) predict with accuracy the polymer-limit optical properties in the neutral state and (ii) interpret and assign the electronic absorption spectra evolution upon doping in solution. Interestingly, in the literature mainly films spin-coated from CB and from CB-solutions in the presence of solvent additives

contrast between crystalline (dark) and amorphous (brighter) alternation as in the case of epitaxied P3HT films.29 The ratio between dark and brighter areas gives a rough indication of the crystalline fraction in the sample (around 50% from a threshholding of the TEM BF image). This zigzag pattern is similar to that observed in the lamellar structures of polyethylene and also P3HT.29 It hints at (i) an intrinsic tendency of twinning inside the crystalline domains along the F


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transmission UV/vis/NIR spectroscopy during cyclic voltammetry the cell was placed in a self-made cuvette holder using fiber optics. The CVs were measured with a Metrohm Autolab potentiostat (PGSTAT 101, Software: Nova 1.8), and the corresponding UV/vis/ NIR spectra were obtained on a Zeiss spectrometer (CLH600, MCS621 VIS II and MCS611 NIR 2.2 μm) using fiber optics. All experiments were carried out under argon atmosphere at scan rates of 10 mV s−1 using 0.2 M THAPF6 or 0.1 M TBAPF6 as electrolyte in chlorobenzene and MeCN for solution and thin film measurements, respectively. As an internal standard the redox couple ferrocene/ferrocenium was used. The exact conditions are given in the main text. Theoretical Investigations. PCPDTBT has been modeled via an oligomer approach,23 thus considering n oligomer units, namely (CPDTBT)n with n ranging from 1 to 5, by means of a density functional theory (DFT) approach. Geometry optimizations of the neutral ground state (S0) were performed for the entire series. Optimized geometries and Mülliken atomic charges in the cation and dication states were performed for the monomer, trimer, and pentamer (n = 1, 3, and 5, respectively). The calculations were performed by using the hybrid, generalized gradient approximation (GGA) functional B3LYP33,34 and the range-separated CAM-B3LYP35 functional in conjunction with the 6-31G** basis set.36,37 All alkyl chains along the conjugated backbone were truncated to methyl groups to ease the computational cost. All geometrical parameters were allowed to vary independently apart from planarity of the rings unless otherwise stated. The frequency analysis was followed to ensure that the optimized structures were stable states. Tight self-consistent-field and geometry optimization convergence criteria and ultrafine integration grids are employed. The rotational barriers for (CPDTBT)1 in the neutral, cation, and dication states have been computed at the B3LYP/6-31G** level by spanning the torsional angle between the planes of the CPDT and BT rings (θ) in steps of 10°. A flexible rotor approximation was used in all cases, the molecular geometry of each point of the rotational profile being optimized at a fixed θ value. Time-dependent DFT (TDDFT)38−40 calculations (in vacuo) were performed to assess the excited state vertical transition energies. In addition, optimization of the ground state geometry and calculation of the vertical transition energies have been performed for the CPDTBT trimer in different solvents (carbon disulfide, chlorobenzene, chloroform, THF, and DMSO) using the PCM model at the B3LYP level.41,42 Absorption spectra were simulated through convolution of the vertical transition energies and oscillator strengths with Gaussian functions. All calculations were performed with Gaussian 09 (Revision B.01),43 and molecular orbital contours were plotted using Molekel 4.3.44

such as DIO are discussed. The highest reported absorption maxima amount to 785 nm for homopolymer films, the strong red-shift being connected to the presence of the solvent additive during deposition. We observe absorption maxima up to 780 and 784 nm when studying films spin-coated from THF and 1-CN. These strong red-shifts are also accompanied by fiberlike patterns, particularly distinct in films from 1-CN. We want to point out that the choice of the solvent has a huge influence on both the morphology and the absorption properties of PCPDTBT. The studies shown in this publication are in our opinion relevant to the whole class of low bandgap polymers which have typically semicrystalline character.

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EXPERIMENTAL SECTION

Materials and Film Preparation. The PCPDTBT (poly{[4,4bis(2-ethylhexyl)cyclopenta-(2,1-b;3,4-b′)dithiophen]-2,6-diyl-alt(2,1,3-benzothiadiazol)-4,7-diyl}) polymer was purchased from 1material. To analyze the purity of the polymer, a 1H NMR in CDCl3 (250 MHz) was measured; no impurities could be detected. The molecular weight of the polymer was determined to Mw = 23 kg/mol with a PDI of 1.7 (HT-SEC, 160 °C, trichlorobenzene, against PS). All solvents (CS2: carbon disulfide; 1-CN: 1-chloronaphthalene;21 CHCl3: chloroform; CB: chlorobenzene; THF: tetrahydrofuran) used were purchased from Sigma-Aldrich (p.a. grade). Film Processing. The polymer was dissolved in the respective solvents by continuously stirring a 3 mg/mL solution under heating for 1−2 h at elevated temperatures (35−80 °C) (always at least 10 °C below the boiling point). The films were prepared by spin-coating at 1500 rpm for 30−600 s within 24 h after preparation of the solutions. The films were vacuum-treated to remove remaining solvent in the film. As substrates 1 × 1 cm2 microscope glass slides (for absorption and AFM height measurements), silicon wafers (for the AFM phase in Figure 4), and silicon wafers with a 230 nm thick SiOx-layer (for TEM) were used. The substrates were cleaned with a CO2 snow jet followed by exposure to oxygen plasma (Diener Femto 100 W) for at least 300 s. Both solution and film preparations were done under a nitrogen atmosphere. Structural Characterization. AFM was performed on a Dimension Icon AFM from Bruker operating in tapping mode. For TEM analysis, the PCPDTBT films on SiOx were coated with a thin amorphous carbon film and removed from the substrate by floating on a diluted aqueous HF solution (10 wt %) and subsequent recovery on TEM copper grids. TEM was performed at the Institut Charles Sadron in bright field, high resolution, and diffraction modes using a CM12 Philips microscope equipped with a MVIII (Soft Imaging System) charge coupled device camera. UV/vis/NIR spectroscopy was performed on a Zeiss spectrometer (light source: CLH600, detector: MCS621 VIS II and MCS611 NIR 2.2 μm) in transmission mode using fiber optics. In-Situ Spectroelectrochemistry. The spectroelectrochemical experiments in solution were performed under thin layer conditions in a self-made cell with a standard three-electrode setup similar to the setup described by Geskes and Heinze.31 As working electrode and mirror for reflection absorption spectroscopy a polished platinum disk (d = 4 mm) sealed in glass was used. The distance between the bottom of the cell and working electrode could be adjusted by a micrometer screw and was set to 20 μm for all experiments assuring finite diffusion conditions. A silver wire served as pseudoreference electrode and a platinum wire as counter electrode. The spectroelectochemical experiments of thin films were performed with a three-electrode setup in a self-made apparatus. The working electrode was an indium tin oxide-coated glass on which the polymer film was spin-coated. As counter electrode a platinum wire was used, and a Ag/AgCl-coated silver wire served as the reference electrode. The cell was made of borosilicate glass and had an intergrated column similar to the setup of Kiesele.32 Before each measurement the cell was dried under vacuum, and the solvent was filtered through the internal aluminum oxide column (Sigma-Aldrich, activated, neutral, Brockmann I). For

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S9 and Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank B. Omiecienski for her work and help with the in-situ spectroelectrochemistry setups and measurements. This work was funded by the DFG through the priority program SPP1355 and the Emmy Noether program. S.L. and M.B. thank the DFG for further support by IRTG-1642. The work at the University of Málaga was supported by the Ministerio de Ciencia e Innovación (MICINN) through project reference CTQ2012G


Macromolecules

Article

33733 and by the Junta de Andaluciá (project PO9-FQM4708). M.C.R.D. thanks the MICINN for a “Ramón y Cajal” Research contract.

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