Charge-Transfer–Solvent Interaction Predefines Doping Efficiency in p

Article pubs.acs.org/cm

Charge-Transfer−Solvent Interaction Predefines Doping Efficiency in p‑Doped P3HT Films Lars Müller,*,†,‡,§ Diana Nanova,†,‡,§,□ Tobias Glaser,†,§,◆ Sebastian Beck,†,§ Annemarie Pucci,†,§,∥ Anne K. Kast,†,⊥ Rasmus R. Schröder,†,∥,⊥ Eric Mankel,†,# Patrick Pingel,○ Dieter Neher,∇ Wolfgang Kowalsky,†,‡,§ and Robert Lovrincic*,†,‡ †

InnovationLab, Heidelberg 69115, Germany Institute for High-Frequency Technology, TU Braunschweig, Braunschweig 38106, Germany § Kirchhoff Institute for Physics, Heidelberg University, Heidelberg 69117, Germany ∥ Centre for Advanced Materials, Heidelberg University, Heidelberg 69117, Germany ⊥ CellNetworks, BioQuant, Heidelberg University, Heidelberg 69117, Germany # Materials Science Department, Surface Science Division, TU Darmstadt, Darmstadt 64289, Germany ○ Fraunhofer Institute for Applied Polymer Research, Potsdam 14476, Germany ∇ Institute of Physics and Astronomy, University of Potsdam, Potsdam 14476, Germany ‡

S Supporting Information *

ABSTRACT: Efficient electrical doping of organic semiconductors is a necessary prerequisite for the fabrication of high performance organic electronic devices. In this work, we study p-type doping of poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) spin-cast from two different solvents. Using electron diffraction, we find strong dopant-induced π−π-stacking for films from the solvent chloroform, but not from chlorobenzene. This image is confirmed and expanded by the analysis of vibrational features of P3HT and polaron absorptions using optical spectroscopy. Here, a red-shifted polaron absorption is found in doped films from chloroform, caused by a higher conjugation length of the polymer backbone. These differences result in a higher conductivity of films from chloroform. We use optical spectroscopy on the corresponding blend solutions to shed light on the origin of this effect and propose a model to explain why solutions of doped P3HT reveal more aggregation of charged molecules in chlorobenzene, whereas more order is finally observed in dried films from chloroform. Our study emphasizes the importance of solvent parameters exceeding the bare solubility of pure dopant and host material for the preparation of highly conductive doped films.

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INTRODUCTION Molecular doping of solution processable conjugated polymers is widely applied to increase conductivities and improve the performance of various organic electronic devices.1−4 A variety of small molecule dopants, tailored for different host materials, has been investigated.5,6 For doping of conjugated polymers, the dopant is usually dissolved in organic solvents and thereafter mixed with the polymer solution to be deposited onto a substrate. In contrast to inorganic semiconductors, dopants for organic materials oxidize (p-doping) or reduce (ndoping) the host molecules in an electrochemical charge transfer process. In order to allow for efficient charge transfer (CT) in the case of p-type doping, the lowest unoccupied molecular orbital (LUMO) of the dopant molecule has to be lower than the highest occupied molecular orbital (HOMO) of the matrix molecule. The material system poly(3-hexylthio© 2016 American Chemical Society

phene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) as dopant fulfills this criterion very well with the P3HT HOMO at around 5 eV compared to the LUMO of F4TCNQ at 5.24 eV.7 In accordance with this energetic matching, Pingel and Neher reported integer charge transfer upon contact of F4TCNQ with P3HT.8,9 At high dopant loadings of 10−20 wt %, maximum conductivities of 0.1−10 S/cm can be achieved.2,10,11 However, the fraction of dopants that actually contribute to the electrical conductivity by generating a mobile hole (i.e., the doping efficiency) was determined to be only 5%, implying that a large amount of charges remains bound after charge transfer.8 Received: April 22, 2016 Revised: June 1, 2016 Published: June 1, 2016 4432

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F4TCNQ doped P3HT is very sensitive to a variety of parameters, and interactions of host and dopant molecules already in solution can alter thin film properties. It was shown that the temperature of the blend solution determines the thinfilm conductivity.12 By varying the dopant concentration, Duong et al. concluded on two different doping regimes: a weak and a strong one.10 The transition to the strong regime is accompanied by the formation of a new crystalline phase in films, interpreted as F4TCNQ incorporated into P3HT crystallites.10 Recent publications aim at a comprehensive picture of how molecular order influences doping of conjugated polymers. The goal is an ideal nanomorphology that ensures a maximum of free charge carriers contributing to the conductivity and also provides continuous pathways for efficient hole conduction, e.g., via tie chains. Recent studies by Gao et al. emphasize the need for ordered P3HT-domains to efficiently separate charge carriers and hence to efficiently dope.13−15 By comparing regiorandom with regioregular P3HT13 or artificially grown J-aggregate nanofibers with polymorphic H-aggregate nanofibers with less order,14 they conclude on the need for high purity and order of P3HT for intrachain polaron delocalization, resulting in efficient charge separation. The deliberate change of order in pure polymer films has been investigated in the past to quite a large extent, particularly for P3HT.16−19 It is well-known for solution processed polymers that solvent−solute interactions as well as solvent characteristics such as the boiling point regulate packing and ordering in dried films.20,21 Films of pure P3HT show a higher amount of π−π-stacking from high-boiling point solvents, which allow the polymer chains to rearrange due to the increased drying time.21 The role of solvent for doping of conjugated polymers was not yet investigated thoroughly, although observations do not completely match for films prepared from different solvents as recently remarked by Salzmann et al.6 Screening existing literature for doping of P3HT with F4TCNQ reveals that mainly two solvents were used: chloroform8,9,22 and chlorobenzene.10,12,13,15 Although these two solvents differ in terms of boiling point (chloroform, 64 °C; chlorobenzene, 131 °C) and polarity, there is no direct comparison of films produced from these two solvents to date. In the following we show that p-doped films from chloroform exhibit a significant, dopant-induced increase of order in terms of π−π-stacking. This leads to an extended conjugation length of the P3HT backbone and results in a higher conductivity of doped films spin-cast from chloroform. This increased order is not observed in doped films from chlorobenzene, which furthermore exhibit two crystalline phases. However, doped films from chloroform only show one distinct π−π-stacking distance, which can be attributed to F4TCNQ-intercalated P3HT crystallites. We therefore find that for p-doped P3HT the boiling point of the solvent is not the predominant parameter to induce order in dried films as it is the case for pure P3HT. In the doped case, we suggest the solvent polarity to dominate the ordering process and predefine order in thin films already in the solution state. Beyond this mechanistic insight, our results allow for an evaluation of existing literature, in which either chloroform or chlorobenzene was used as solvent. We provide an insight in how films from these two solvents differ.

Article

MATERIALS AND METHODS

Materials. P3HT (Mw = 31 kg/mol, regioregularity >93%) from Merck and F4TCNQ (Sigma-Aldrich) were used as received without further purification. Doped solutions for films were prepared by dissolving and stirring P3HT (20 mg/mL) and F4TCNQ (0.5 mg/ mL) separately for 15 h before mixing them. Comparing the neat F4TCNQ solutions, we observed a solvatochromism, in detail a hypsochromic shift (blue shift) from the less polar solvent chlorobenzene to the more polar solvent chloroform. The blend solutions were stirred for 3 h prior to spin coating. We used anhydrous chloroform and chlorobenzene as solvents and prepared all solutions at 50 °C under inert N2 atmosphere. The doping ratio for all measurements shown here was 1:10, meaning one F4TCNQ molecule per 10 thiophene units (corresponding to a dopant molar fraction of 0.091).10 Sample Fabrication. All films were prepared by spin coating (doped) solutions (T = 50 °C) in inert N2 atmosphere. For conductivity, IR, and UV−vis measurements, films with equal thicknesses were prepared at 4000 rpm spin coating speed with an acceleration of 4000 rpm/s for 30 s in the case of chloroform and 400 rpm with 200 rpm/s for 30 s in the case of chlorobenzene, resulting in film thicknesses of ∼21 nm. For TEM measurements, films were spincast at 800 rpm for 30 s, yielding film thicknesses of ∼21 nm for chlorobenzene and ∼65 nm for chloroform. For TEM samples, P3HT:F4TCNQ was spin-cast on ITO-glass, covered with poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). PEDOT:PSS was processed as follows: spin coating at 1000 rpm with 4300 rpm/s for 10 s followed by 4300 rpm with 4300 rpm/s for 30 s and thermal annealing at 140 °C for 30 min. After spin coating the polymer layer, deionized water was used to dissolve the PEDOT:PSS and float the films off the glass substrates. Hereafter, we skimmed the films with a copper grid (200 mesh) coated with a holey carbon film QUANTIFOIL (3.5/1). Conductivity Measurements. In the case of pure P3HT, we measured the conductivity laterally between two gold electrodes with a distance of 24 μm on a glass substrate. The presented conductivities are averages for 14 devices each. For doped P3HT layers, we performed four-point probe measurements to exclude the contact resistance. Here, the presented data are averages of five measurements each. TEM Measurements. Electron diffraction was performed with a Libra 200 MC KRONOS, Carl Zeiss Microscopy, at 60 keV acceleration voltage. A total electron dose of ∼670 electrons per nm2 was applied for diffraction measurements. In order to ensure reproducibility, measurements have been done at several positions for each sample. IR Spectroscopy. IR transmission spectra of pure and doped P3HT layers were measured with an FT-IR spectrometer Vertex 80v from Bruker. For each spectrum 200 scans measured with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector were averaged, and a resolution of 4 cm−1 was used. During the measurements the spectrometer was evacuated (3 mbar) to prevent absorptions from ambient air. All spectra shown here are relative transmission spectra as the polymer films on silicon substrates were measured relative to the cleaned bare silicon wafer. UV−Vis Absorption Spectroscopy. Measurements were performed using an AvaLight-DHS-Bal (Balanced Deuterium-Halogen light source (200−2000 nm)). We prepared thin films on glass with parameters described above. Measurements are performed in transmission mode. For measurements on solutions, we prepared solutions identical to these for films and diluted them directly before measurement from 3.05 to 0.012 mg/mL. The temperature of the blend was ∼50 °C to ensure comparability to solutions used for spin coating.

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RESULTS AND DISCUSSION Electron Diffraction on Thin Films. We investigated thin films of pristine and doped P3HT spin-cast from chlorobenzene and chloroform solutions at 50 °C. For all measurements

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looks very similar to the sequentially coated films reported by Scholes et al., which was shown to exhibit an exceptionally good order.25 This view will be further substantiated by our following discussion of the polaron features seen in the NIR− UV−vis spectra below. In addition to the different intensities of the (010) peak, there is also a shift of peak positions. Doped films from chloroform (chlorobenzene) show a spacing of 1.86 nm (1.96 nm) in the lamellar stacking direction compared to 1.71 nm (1.64 nm) for pristine P3HT. The spacing in the π−π-stacking direction in chloroform-cast films decreases from 0.386 nm (q = 2.588 nm−1) for pristine P3HT to 0.371 nm (q = 2.696 nm−1) for P3HT:F4TCNQ. However, films from chlorobenzene exhibit a double peak, meaning two species of differently ordered domains. Such a double peak was already observed for films from chlorobenzene and high dopant loadings (dopant molar fraction > 0.03).10 In their work, Duong et al. assign the general change in stacking distance and the occurrence of the double peak to the incorporation of F4TCNQ into ordered P3HT domains. Our measurements do not allow for an exact determination of the position of the first feature of this double peak. We therefore cannot clearly deduce if there are pure P3HT domains and mixed crystallites or if two new phases without any pure P3HT domains form. However, the study of Duong et al. points toward the latter option since in their work both distances do not perfectly match the stacking that is observed in films of pure P3HT. Aggregates of pure F4TCNQ can be excluded by IR measurements as discussed subsequently. All measured stacking distances for the two characteristic peak positions are summarized in Table S3 of the Supporting Information. UV−Vis Absorption Spectroscopy on Thin Films. Macroscopic morphology properties of undoped and doped P3HT layers can be deduced from optical absorption spectroscopy.8,15,21,26 UV−vis absorption spectra are shown in Figure 2. Absorbance spectra of pure P3HT are composed of a contribution from crystalline regions within the films at low energies and disordered polymer chains at higher energies. The crystalline P3HT chains form weakly interacting H-aggregate states.21 For P3HT doped with F4TCNQ, additional polaron absorptions at around 1.3 and 1.7 eV as well as contributions of charged F4TCNQ molecules at ∼1.5 eV (double-peak structure) and ∼3 eV add to the spectrum.27 Chlorobenzenecast films of undoped P3HT show a clear vibronic structure and red-shifted absorption compared to films cast from chloroform, pointing to more ordered films from chlorobenzene. A more quantitative analysis confirms this impression: On the basis of the analysis of order in P3HT in literature,21,28 the spectra shown in Figure 2 are normalized to the 0−1 transition peak at ∼2.2 eV. Making use of the weakly interacting H-aggregate model developed by Spano,26,29 the ratio of 0−0 to 0−1 transition peak can be used to estimate the degree of excitonic coupling within aggregates, which is related to conjugation length and quality of crystallites.21,26,28−30 In films of pure 0−1 is larger for films cast from the high P3HT, the ratio I0−0 a /Ia boiling point solvent chlorobenzene compared to chloroform. This is exactly what literature suggests: For pure P3HT, more order in thin films is expected from high boiling point solvents like chlorobenzene compared to low boiling point solvents like chloroform.21 In the case of p-doped P3HT, we no longer observe this pronounced vibronic structure in chlorobenzenecast films. Although the intensities at the energy of the 0−0

shown in this work, the doping ratio is 1:10, meaning one F4TCNQ molecule per 10 thiophene units. We performed electron diffraction in a transmission electron microscope (TEM). This method allows direct access to structural information such as characteristic distances in crystallites and gives a general impression on the overall order in the thin films. Figure 1 shows profiles that were obtained by integration over

Figure 1. Radial profiles of electron diffraction patterns of thin films made of pristine P3HT and P3HT:F4TCNQ spin-cast from chlorobenzene (top) and chloroform (bottom). The dashed lines show characteristic distances for pristine P3HT and P3HT:F4TCNQ.

the measured diffraction patterns. Exemplary diffraction patterns and bright-field images are shown in the Supporting Information. In all profiles of doped and pure P3HT, two distinct features are observed, originating from characteristic stacking directions of crystallites within the films. The (100)-orientation (interlamellar stacking) corresponds to a spacing of 1.68 nm, whereas the (010)-orientation (π−π-stacking) forms with a distance of around 0.38 nm.10,23,24 For the (100)-orientation (interlamellar stacking), there is a significant increase in intensity for chlorobenzene-cast layers upon doping. In the case of chloroform-cast layers, only a broadening of this peak is observed. In contrast, the (010)-peak (π−π-stacking) shows a strong intensity increase upon doping of the chloroform-cast layers. Films cast from chlorobenzene do not show a comparable increase in intensity of this π−π-stacking peak. This noticeable dopant-induced increase of the π−π-stacking peak for doped chloroform-cast films can either stem from an overall increased degree of order (i.e., an increase of the number of aggregated polymer chains or longer planarized P3HT chain segments) in the π−π-stacking direction or from a variation of the aggregate orientation with respect to the substrate plane upon doping. In fact, an increased isotropy in orientation has been reported at low dopant concentrations; however, this isotropy decreases again for high concentrations in films spin-cast from o-dichlorobenzene.25 At this point, we speculate that the observed increase of the π−π-stacking signal is due to an increased order in the doped chloroform-cast layers. The electron diffraction profile of chloroform-cast films 4434

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Strong vibrational modes below 1500 cm−1 and a broad absorption band centered at around 4000 cm−1 (0.5 eV) arise in the spectra of both doped P3HT layers compared to pure P3HT independent of the solvent. Both kinds of features reveal successful doping and have been observed before for doped P3HT layers.8,27,31,32 Successful doping was furthermore confirmed by photoemission spectroscopy, shown in Figure S1 of the Supporting Information. The strong vibrational modes below 1500 cm−1 are the characteristic IR-activated vibrations (IRAV) of the positively charged P3HT. The broad absorption can be attributed to the lowest optical transition P1 of 1D hole polarons in P3HT that form due to doping with F4TCNQ.8,27,31,32 1D in this case means intrachain delocalization, whereas 2D is usually used to describe interchain coupling along the lamellar stacking direction.31 We do not observe significant contribution of this 2D polaron in films from both solvents, which is in accordance with the findings of Wang et al.27 In doped films spin-cast from chloroform compared to chlorobenzene, the maximum of this polaron absorption is redshifted by 850 cm−1 to an energy of 3900 cm−1. An explanation for such a red-shift in films from chloroform can be given by a stronger delocalization of the polaron on the P3HT, especially in the case of 1D polarons.33 A higher delocalization in turn requires a higher conjugation length that is directly connected to the degree of order in aggregates/crystallites, fitting well to the absolute increase of π−π-stacking in films from chloroform, which we observed with electron diffraction. This conclusion gives further evidence for a higher amount of order in films of doped P3HT spin-cast from chloroform. Regardless of the shift, films from the two solvents both exhibit the spectral shape of 1D polarons, although TEM diffraction measurements indicate a clearly higher amount of lamellar stacking in films from chlorobenzene. In addition to the absorption bands of P3HT, also Fano-type features from the CN-stretching vibrations34,35 of the F4TCNQ

Figure 2. Absorbance spectra of pure P3HT (top) and P3HT:F4TCNQ (bottom) with a doping concentration of 1:10 spin-cast from chlorobenzene (CB, blue) and chloroform (CF, red) normalized to the 0−1 transition peak at ∼2.2 eV. The dashed line at 2.04 eV marks the 0−0 transition, which is associated with conjugation length and crystallinity of P3HT.

transition peak are higher for doped chloroform-cast films, we note that a detailed analysis is not possible due to the lack of clearly resolved vibronic features. Hence, we refer to the subsequent analysis of IR spectra for doped films. IR-Spectroscopy on Thin Films. In Figure 3 IR transmission spectra of pure and doped P3HT layers spincast from chlorobenzene and from chloroform are shown.

Figure 3. Relative transmittance at normal incidence of P3HT, F4TCNQ, and P3HT:F4TCNQ with a doping concentration of 1:10 spin-cast from chlorobenzene (top) and chloroform (bottom) on a Si substrate with natural oxide. The broad absorption band (a) is caused by the polaron absorption from positively charged P3HT, and its maximum position is marked with a dashed line. The absorption bands in (b) are evoked by the CN-stretching vibration of F4TCNQ. The dashed lines at 2169 and 2194 cm−1 mark the fitted excitation energies of the CN-stretching vibrations of the charged F4TCNQ in doped P3HT layers. 4435

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well to the lower boiling point of chloroform compared to chlorobenzene, which usually leads to less order in dried films.21 In contrast to this finding, doped layers show the opposite behavior with a significantly higher conductivity of 4.5 S/m for films spin-cast from chloroform compared to 0.3 S/m for films from chlorobenzene. A detailed comparison of absolute measured values for the conductivity with literature proves to be difficult when taking into account that not only the solvent but further processing parameters like the temperature of the blend solution12 before spin coating and material parameters such as the regioregularity of P3HT39 have significant impact on the conductivity. However, the overall magnitude of a few S/m for the given concentration of one F4TCNQ molecule per ten P3HT monomer units fits to the values that would be expected from literature.10 UV−Vis Absorption Spectroscopy on Solutions. With the solvent as the only parameter that was changed, the degree of order within thin films of doped P3HT could be already predefined in the solution state by interaction of charged molecules with the solvent. To assess the mixture of dopant and host materials in solution, we measured UV−vis absorbance spectra of solutions of P3HT:F4TCNQ in chloroform and chlorobenzene. The measured spectra as well as a fit and its components can be seen in Figure 5.

molecule can be observed, see Figure 3b. Neutral F4TCNQ layers show a CN-stretching peak at 2228 cm−1 (b1u) and a smaller contribution at 2214 cm −1 (b 2u ); 35 but in P3HT:F4TCNQ even the signal at 2228 cm−1 is not visible. This finding indicates that the number of uncharged F4TCNQ molecules within the films is below the detection limit. This furthermore excludes the existence of isolated F4TCNQ aggregates. In the doped layers, both vibrational modes are strongly red-shifted and show a Fano-type asymmetric line shape, especially for these modes in the film from chloroform. The asymmetric line shape originates most likely from an interference of the discrete CN-stretching vibration and the broad electronic excitation centered at ∼0.5 eV, as it has already been described by Ö sterbacka et al. for the intramolecular interference of the polaron excitation and the IRAV modes in P3HT.31 In order to extract meaningful oscillator data, a careful analysis of these vibrational lines was conducted. The spectra were fitted using a second order polynomial baseline (dashed, green line in Figure 3b) combined with two Fano-type oscillators. Details on this analysis are provided in the Supporting Information. According to the Fano effect, the different asymmetries indicate differently strong coupling between the CN vibrations of the charged F4TCNQ and the polaron on the P3HT. So, considering the differently asymmetric line shapes, the excitation energies of the two vibrational modes shift close to 2194 and 2169 cm−1, respectively, for both solvents. The positions of these CN stretching vibrations have successfully been used as an indicator for the degree of charge transfer.22,36−38 Comparing the measured positions of this vibrational mode to previously published values, different degrees of charge transfer can be ruled out and the integer charge transfer as found by Pingel et al. is confirmed for films from both solvents.8 Conductivity. Figure 4 shows conductivity measurements on thin films of pure and doped P3HT. It can be seen that in the case of pure P3HT, films from chloroform exhibit a slightly lower conductivity than films that were prepared from chlorobenzene. This result corresponds

Figure 5. Absorbance spectra of P3HT:F4TCNQ dissolved in chlorobenzene (top) and chloroform (bottom) (black curves). The continuous red line represents a fit that is composed of the curves in dotted lines. Pure P3HT (orange) and F4TCNQ (green) spectra were measured in solution and used as input for the fit. The polaron (purple) and aggregate (cyan) contributions are fitted with a Gaussian, and the F4TCNQ anion spectrum (blue) was extracted from Pingel et al.8 and Wang et al.27

As has been proposed by Wang et al.,27 we decompose our spectra into contributions from neutral disordered P3HT and F4TCNQ (both reference spectra have been separately measured in both solvents), charged F4TCNQ (reference spectrum taken from the literature8,27), aggregated P3HT chains, and charged P3HT (for simplicity and in order to

Figure 4. Lateral conductivity obtained from four-point probe (doped P3HT) and IV (pure P3HT) measurements on thin films spin-cast from chloroform and chlorobenzene. 4436

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Chemistry of Materials reduce the amount of fit parameters, the last two contributions are approximated by wide Gaussian-shaped features). Note that we cannot clarify the nature of the charges on the P3HT chains. The charged-P3HT feature might be due to polarons or bipolarons, as has been suggested as equilibrium charge storage configuration in doped P3HT solutions.27,40 Aggregates are thought to be composed of charged P3HT since neutral P3HT does not form aggregates. Jacobs et al. presented evidence that in solution the dopant ions surround such aggregates instead of intercalating them, leading to isolated polymer domains in thin films, which are detrimental for charge transport due to the lack of tie chains.41 Our fits to the spectra clearly reveal a stronger signal from such aggregates at 2.2 eV and F4TCNQ anions around 1.4−1.6 eV as well as stronger polaron absorption in solutions of chlorobenzene. Gao et al. were able to show that the F4TCNQ anion peaks in solutions of chlorobenzene only occur for regioregular P3HT, which is known to aggregate and develop ordered structures.13 The planarization of the P3HT backbone upon charge transfer leads to the combined effect of stabilizing the polaron as well as the doping, which results in the observed stronger polaron and F4TCNQ anion absorption. This result in addition to the higher P3HT aggregate signal in chlorobenzene solution implies that P3HT:F4TCNQ in chlorobenzene already forms ordered domains in solution, while this formation is significantly suppressed in chloroform. In solutions of pure P3HT no signature of aggregates is observed. The individual parameters that emerged from the fits are listed in the Supporting Information. We note that the general effect of aggregation of P3HT:F4TCNQ in solution was already observed previously.41 From Blend Solutions to Thin Films. The question why more aggregation in chlorobenzene solutions is observed and how this can lead to more order in doped thin films from chloroform is not easy to answer. In the following we suggest an explanation for this phenomenon. P3HT and F4TCNQ are known to dissolve in chlorobenzene10,13,15,27 and chloroform.8,9,12,13,21,32 Pure P3HT dissolved in chloroform or chlorobenzene also does not show aggregation behavior as it can be seen from UV−vis absorption spectra of the pure material (Figure 5). Here, the feature assigned to P3HT aggregates at 2.2 eV does not appear, in contrast to solutions of P3HT:F4TCNQ. One possible mechanism to drive aggregation can be the solubility. Cook et al., for example, were able to control the degree of aggregation within pure P3HT by adding a solvent that poorly dissolves P3HT.20 The fact that we observe these aggregates only in doped solutions implies that the crucial factor is not given by the solubility of the neutral molecules, but the solubility of the charged molecules or even P3HT:F4TCNQ pairs. For charged molecules, a better solubility would be expected in polar solvents. In the case of the two solvents used for this study, chloroform (ENT (CF) = 0.259) is more polar than chlorobenzene (ENT (CB) = 0.188).42 Hence, charged molecules (such as P3HT:F4TCNQ chargetransfer pairs) are expected to dissolve better in chloroform than in chlorobenzene. This prerequisite together with the finding that low solubility induces the formation of aggregates leads to the prediction that more aggregates should occur in chlorobenzene solutions. This is exactly what we find from UV−vis absorption spectra on solutions of P3HT with F4TCNQ (Figure 5). Since the density of charged molecules or complexes depends on the dopant concentration, a decreasing influence of solvent polarity is expected with lower

F4TCNQ content, until the behavior reaches that of pristine P3HT. For the formation of well-ordered thin films, these aggregates seem to be detrimental as deduced from the shown measurements. The question if charge transfer in chloroform solutions generally occurs or if it takes place only during drying of the films can be answered by the simple observation that the solution immediately changes its color upon mixing P3HT and F4TCNQ. This implies an immediate charge transfer reaction upon mixing both materials as an equilibrium of charged and neutral molecules establishes. It is known that the backbone planarity of P3HT chains significantly increases upon contact and charge transfer with the F4TCNQ molecule.15 This in turn facilitates stacking along the π−π direction.43,44 The results of the UV−vis spectra of solutions (Figure 5) point to a more homogeneously distributed F4TCNQ in chloroform solutions compared to chlorobenzene where aggregates form. This, together with the induced higher backbone planarity, enables the formation of well-ordered thin films in terms of π−πstacking from chloroform. In chlorobenzene solutions, however, the already aggregated and thus not homogeneously dispersed molecules in turn seem to hinder the formation of such a high degree of order in doped thin films. This is in excellent agreement with the observation of a clearly defined π−π-stacking diffraction peak in films from chloroform (Figure 1) in contrast to the observed double peak in films from chlorobenzene. Our observations are summarized in a schematic in Figure 6. It is clear that the observed effects

Figure 6. Schematic drawing and summary of the observed effects from the solution state to dried thin films of P3HT:F4TCNQ.

could vary with different molecular weights of the used P3HT. Furthermore, the regioregularity plays an important role for the ability to form aggregates, which was shown to impact doping of P3HT with F4TCNQ.13 Nevertheless, our P3HT sample has rather high molecular weight and regioregularity, such as those P3HT batches that are usually used in well-performing devices.

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CONCLUSIONS In this work on F4TCNQ-doped P3HT, we provide a direct comparison of doped P3HT films that differ only in the amount of order. We find a dopant-induced increase of order in terms of π−π-stacking for doped films from the solvent chloroform compared to films of pure P3HT via electron diffraction. In 4437

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ACKNOWLEDGMENTS We acknowledge the German Federal Ministry of Education and Research (BMBF) for financial support within the InterPhase project (FKZ 13N13656, 13N13657, 13N13658, 13N13665).

contrast, no significant dopant-induced change in terms of π−π-stacking was observed from chlorobenzene. The increased π−π-stacking in chloroform-cast films leads to more delocalized charge carriers by increasing the conjugation length of the polymer. This is shown with optical spectroscopy by a redshifted polaron absorption peak for films from chloroform in the infrared region and supported by analysis of P3HT absorption features in the visible range. As a consequence, these differences lead to a significantly increased lateral conductivity of doped thin films from chloroform compared to films from chlorobenzene. Optical spectroscopy on solutions reveals more aggregation of molecules already in the solution state for chlorobenzene compared to chloroform. We propose the polarity of the solvent to be the driving force for the solubility of the doped and hence charged molecules. This solubility, which regulates the aggregation behavior in solution, predefines the degree of order in thin films. The aggregation in the chlorobenzene blend solution turns out to be detrimental for the order within thin films and hence the doping efficiency. We therefore provide a hitherto unnoticed insight in how the amount of order in doped polymer films is governed by the choice of solvent. For this approach it is of great importance not to rely on the solvent’s boiling point as it is necessary for pure P3HT, but to take into account the charge transfer that takes place already in solution. Existing studies of F4TCNQ-doped P3HT in many cases only use one of the here investigated solvents: chloroform or chlorobenzene. As Salzmann et al. previously noted in their review on doping of organic semiconductors, the processing parameters and especially the choice of solvent might be a major reason for some apparent inconsistencies between existing studies.6 Hence, this work can at least partly fill this gap and facilitate the comparison of published experiments. Beyond this, our results demand for new processing guidelines regarding the choice of solvent for the preparation of highly conductive doped polymer films since the driving force for the formation of this comparably high amount of order is clearly different compared to pure P3HT.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01629. Fit-parameter for UV−vis spectra of solutions; photoemission spectra of thin films (doped and undoped); fitparameters for IR spectra (Fano-type lineshapes); TEM PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +49 6221 5419 123. E-mail: [email protected]. *E-mail: [email protected]. Present Addresses □

(D.N.) IBM Research & Development, Schönaicher Str. 220, 71032 Böblingen, Germany. ◆ (T.G.) Bruker Optik GmbH, Rudolf-Plank-Str. 27, 76275 Ettlingen, Germany. Notes

The authors declare no competing financial interest. 4438

DOI: 10.1021/acs.chemmater.6b01629 Chem. Mater. 2016, 28, 4432−4439

Article

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