Controlling the Polarity of Fullerene Derivatives to Optimize

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Controlling the Polarity of Fullerene Derivatives to Optimize Nanomorphology in Blend Films Fukashi Matsumoto,* Toshiyuki Iwai, Kazuyuki Moriwaki, Yuko Takao, Takatoshi Ito, Takumi Mizuno, and Toshinobu Ohno* Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan S Supporting Information *

ABSTRACT: Developing a design strategy to establish the compatibility of acceptor materials with donor materials is important for the rational development of organic solar cells. We synthesized 2,6-dimethoxyphenyl methanofullerene derivatives to realize an enhanced open-circuit voltage, and we investigated polarities and their effects on the film morphology of the active layer. The polarities of the synthesized fullerene derivatives were affected significantly by the presence of functional groups, such as methoxy, ether, and ester groups. Macro/nanoscopic morphological investigation and spectroscopic analysis of the blend films of the poly(3hexylthiophene)(P3HT)/fullerene derivatives showed that a balanced polarity between materials results in the formation of optimized nanomorphology without grains and robust phase separation. Measurements of the device performance of the photovoltaic cells composed of P3HT and the fullerene derivatives confirmed the same tendency as that shown in the morphological analysis. This finding enables us to obtain an improved power conversion efficiency because of the enhanced open circuit voltage derived from the fullerene derivatives. KEYWORDS: organic photovoltaics, fullerene derivative, surface free energy, morphology, open circuit voltage

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INTRODUCTION Organic photovoltaics (OPVs)1 have drawn remarkable attention recently because of their several advantages, such as low weight, low fabrication cost, and suitability for the development of flexible devices. The most prominently reported material system for bulk heterojunction (BHJ) OPVs is a mixture of poly(3-hexylthiophene) and [6,6]phenyl-C61-butyric acid methyl ester (P3HT:PCBM).2 The recent development of low-band gap polymers has provided a drastic improvement in the efficiency of OPVs;3−7 however, research to develop novel C60-based acceptor materials that can replace PCBM has not been successful until now.8−10 We previously reported on the modification of fullerene derivatives and investigated their properties and performance in photovoltaic cells.11−14 Recently, a number of bisadduct derivatives, which have two substituents on the fullerene cage, exhibited promising performance when applied with P3HT because of the enhanced open circuit voltage (VOC) derived from the shallow lowest unoccupied molecular orbital (LUMO) energy.15 Zhao et al. achieved an enhanced VOC of 0.84 V and a power conversion efficiency of 6.5% by using indene-C60 bisadduct.16 Cheng et al. achieved a VOC of 0.87 V and an efficiency of 5.2% by applying di(4-methylphenyl)methano-C60 bisadduct.17 However, these new acceptors do not always exhibit superior performance compared with PCBM, 18 particularly when they are combined with low-band gap polymers, such as poly(N-9″-heptadecanyl-2,7-carbazole-alt© XXXX American Chemical Society

5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (PCDTBT). These results indicate that acceptors are not always compatible with the huge diversity of donor polymers available, even if they achieve a good match with P3HT. In designing acceptor materials, achieving compatibility with the donor materials is essential to obtain a good result; however, the most effective strategy to achieve this compatibility is yet to be developed. In this study, we aimed to investigate the morphological effect of the molecular structures of acceptors in a blend with P3HT. The active layer morphology critically affects exciton diffusion.19 Excitons cannot reach a charge dissociation interlayer and be stabilized to the ground state in a robust BHJ network because the typical exciton diffusion length in donor polymers is limited to ∼10 nm. Donor and acceptor materials should form a nanoscale interpenetrating bicontinuous network to ensure the efficient dissociation of excitons. Recent studies have shown that demixing of polymer/fullerene blends can play an important role in organic solar cell degradation.20−22 Therefore, controlling nanoscale morphology also provides a pathway to stabilize device performance. The active layer morphology can be controlled via several approaches, such as the use of additives23 and the selection of an appropriate solvent.24 The geometrical structure of Received: November 18, 2015 Accepted: February 1, 2016

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DOI: 10.1021/acsami.5b11180 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

gel permeation chromatography provided purified products, and the structures of the obtained fullerenes 4 were confirmed with 1H and 13C NMR, IR spectroscopy, and MALDI TOFMS. We estimated the solubilities of the obtained derivatives from the concentration of a saturated solution in dichloromethane, which was chosen as the solvent because fullerene derivatives have a modest solubility in it. Otherwise, a high polar solvent, such as 1,2-dichlorobenzene, makes the measurement difficult because it dissolves a fullerene derivative almost infinitely. Changing the alkyl substituents did not result in a significant difference in product yields, but it affected the solubility of the methanofullerenes (Table 1). The solubilities

acceptor molecules can be assumed to affect the nanomorphology of BHJ structure. Troshin et al. investigated the relation between the solubility of fullerene derivatives and their photovoltaic performance, and the authors reported that derivatives with moderate solubilities result in an appropriate film morphology and performance.25 Still, the compatibility between donor and acceptor materials has not been explained merely by solubility. On the other hand, we deduced that the miscibility of donor and acceptor materials in the active layer is affected by the intermolecular forces between them. Therefore, we investigated the polarity of acceptors as an index of compatibility with donor materials to clarify and visualize the effect of intermolecular forces. Although some studies have already been performed on the relation between the polarity of donor materials and film morphology,26,27 only a few reports are available on acceptors.28 We synthesized fullerene derivatives systematically and investigated the effect of molecular polarity on film morphology and device performance. As a basic design of fullerene derivatives, PCBM-like monosubstituted methanofullerenes were selected instead of bisadduct derivatives, which are difficult to purify because of their structural mixtures and lack of reproducibility as materials. As mentioned previously, VOC can be enhanced by a high-lying LUMO energy level. In a previous study, we investigated the effect of donor substituents on the LUMO energy and found that substitution at the ortho position of the phenyl group of PCBM significantly affects the LUMO energy.29 Hereby, we designed 2,6-dimethoxyphenyl methanofullerene as a key structure.

Table 1. Product Yield and Solubility Dataa

a

compounds

yield (%)

solubility (g/L)

PCBM DMPCEP (4a) DMPCHp (4b) DMPCN (4c) DMPCMEM (4d) TMPCMEM (4e) DMPCBM (4f)

41 50 51 43 28 44

16.0 6.82 3.66 9.82 7.23 8.10 5.84

Solubility in dichloromethane at room temperature.

of 4c, 4d, and 4e, which have a long alkyl chain or ether moieties, are relatively high. Unfortunately, the solubility of any of methanofullerenes 4 cannot exceed that of PCBM. The reduction potentials of the methanofullerenes were measured with cyclic voltammetry under argon atmosphere at room temperature. All compounds exhibited three reversible electroreductions in the potential range from −1.0 to −2.5 V relative to a ferrocene/ferrocenium (Fc/Fc+) internal reference (Figure S1 in Supporting Information). The LUMO energy of the obtained methanofullerenes was calculated with the following equation:

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RESULTS AND DISCUSSION The methanofullerene derivatives were synthesized according to Scheme 1. 1,3,5-Trimethoxybenzene or 1,3-dimethoxybenScheme 1. Synthesis of Fullerene Derivatives 4

E = −(4.80 + Ered1)(eV)

(1)

where is the first half-wave reduction potential of the fullerene derivatives relative to Fc/Fc+. The redox potential of the Fc/Fc+ couple has an absolute energy level of −4.80 eV relative to vacuum. To avoid any experimental errors, the measurements of the reduction potentials were repeated three times. The average values of the redox potentials and the LUMO energies of each compound are summarized in Table S1. As depicted on the left axis of Figure 1, all the obtained methanofullerenes (4a−f) showed a higher LUMO energy level than PCBM because of the electron-donating effect of the methoxy substituents. A comparison of the LUMO energies of PCBM and DMPCBM (4f) shows that each methoxy group substituted at the ortho position of the phenyl ring stabilized the LUMO energy by 20 meV. Interestingly, the methoxy group placed at the para position in TMPCMEM (4e) had no effect on increasing its LUMO energy to be higher than that of DMPCMEM (4d). The alkyl-substituted derivatives (4a−c) exhibited higher LUMO energies than the fullerenes (4d−f) probably because polar groups, such as the ether and ester groups, behaved as electron inductors toward C60, although they were connected through a saturated alkyl chain. The significant stabilization of the LUMO energy could result from a “through-space” effect between electrons in the oxygen lone pairs and π-electrons on the C60 cage; this effect was discussed in detail in our previous report.29 Ered1

DCB: 1,2-dichlorobenzene.

zene was acylated with the use of various acid chlorides. Addition of an n-butyllithium solution to 1,3-dimethoxybenzene, followed by treatment with copper(I) bromide and acid chlorides, resulted in 2,6-dimethoxyphenyl carbonyls (2a−d, 2f). After the conversion of these carbonyls to the corresponding tosylhydrazones 3, [6,6]methanofullerene derivatives 4 were prepared by the reaction of C60 with 3 in the presence of KOtBu, followed by the photoisomerization reaction of the resulting [5,6]fulleroids. Recycle preparative B


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Figure 1. Plots for LUMO energy level (left axis) and interfacial free energy (IFE) (right axis) of the fullerene derivatives along with their chemical structures. Error bars represent the standard deviation of the mean. The lines connecting the data serve as guides to the eye.

Table 2. Surface Free Energya and Interfacial Free Energy (IFE)b Data for Fullerene Derivatives and P3HT Films

Surface free energy is defined as the excess energy at the surface of a material compared with the bulk and is assumed to result from several forces because of various molecular interactions, such as electrostatic, donor−acceptor, dispersion, dipole−dipole, and hydrogen bond interactions. Therefore, we consider surface free energy as a suitable parameter to discuss the intermolecular forces between donor and acceptor materials. The surface free energies of the fullerene derivatives and P3HT were evaluated with the contact angles of water droplets and hexadecane on the spin-coated films. The contact angle data of the droplets are summarized in Table S2. The correlations between the contact angle and surface free energy of a solid surface and liquid were determined with Young’s equation: γS = γSL + γL cos θ

2( γLdγSd +

(4)

γLpγSp ) = γLtotal(1 + cos θ )

γ1pγ2p )

γtotal (mJ/m2)

γinter (mJ/m2)

PCBM DMPCEP (4a) DMPCHp (4b) DMPCN (4c) DMPCMEM (4d) TMPCMEM (4e) DMPCBM (4f) P3HT

27.4 27.2 27.3 27.3 27.4 27.5 27.4 24.0

3.7 0.5 0.9 1.2 6.3 5.2 5.8 0.2

31.1 27.7 28.2 28.5 33.7 32.7 33.2 24.2

2.3 0.2 0.4 0.5 4.4 3.5 4.0

state. This result was depicted in Figure 1 (right axis), along with the structure of the fullerene derivatives. Blend films composed of P3HT and fullerene derivatives were investigated to verify the effect of fullerene polarity on film morphology. The blend films were prepared on PEDOT:PSS/glass substrates by spin-coating of a solution of P3HT and the fullerene derivatives in chlorobenzene (CB) or 1,2-dichlorobenzene (DCB). The thickness of the films cast from CB and DCB was around 100 and 80 nm, respectively. Figure 2A shows the optical microscopy images of the blend films cast from a CB solution. Clearly, large grains were formed in some films. Campoy-Quiles et al. suggested that these grains are indeed rich in fullerene, and the authors confirmed this finding through Raman scattering measurements.33 We could not obtain a homogeneous solution of DMPCHp (4b) in CB because of its low solubility (3.66 g/L, Table 1), and, thus, the grains observed in the blend film should be those of the insoluble fullerenes. On the other hand, derivatives 4d−f also led to the formation of grains despite being completely soluble in CB. This result indicates that grain formation occurred because of phase separation with P3HT during the spin-coating process, regardless of the solubility of the fullerene derivatives. The area of each grain and the number of grains were calculated with an image analysis software program (Table S3), and the mean grain area was plotted as a function of the IFE in Figure 3A. The grain size tends to be large and correlates with the difference in polarity between P3HT and the fullerene derivatives. The blend films prepared from DCB did not exhibit clear differences among themselves (Figure S2, Table S3), whereas those prepared from CB did. In Figure 2B, atomic force microscopy (AFM) images were shown for the nanoscopic morphological investigation. The measurement

The superscripts d and p denote the dispersive and polar components, respectively, of γtotal. The interfacial free energies (IFE) between P3HT and the fullerene derivatives were calculated to obtain direct information about the difference in material polarities. IFE γinter was calculated with the following equation:32 γ inter = γ1total + γ2total − 2( γ1dγ2d +

γp (mJ/m2)

Dispersive (γd) and polar (γp) components of surface free energy. γtotal = γd + γp. bIFE (γinter) between P3HT and each fullerene derivative. γinter = γ1+ γ2 − 2(γd1γd2)1/2 − 2(γp1γp2)1/2.

where γ denotes the surface free energy, θ is the contact angle, and subscripts S and L denote the solid and liquid interfaces, respectively. We obtained surface free energy γS according to Kaelble−Uy theory30,31 by using the known parameter values of γL for the probe liquids: (3)

γd (mJ/m2)

a

(2)

γStotal = γSd + γSp

compounds

(5)

where subscripts 1 and 2 denote P3HT and the fullerene derivatives, respectively. The calculated surface free energies and IFEs of each material are summarized in Table 2. Comparison of the IFEs of PCBM and DMPCBM (4f) showed that the presence of methoxy groups was clearly responsible for the high polarity of the material. The difference in alkyl chains resulted in drastic changes in material polarity. DMPCMEM (4d) and TMPCMEM (4e) showed high IFEs because the polar ether moieties and alkyl chains of 4a−c significantly decreased their polarities. We obtained slightly higher IFEs as the length of the alkyl chain increased. The surface molecular ordering and geometric configuration of the functional groups might affect material polarity in the solid C


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Figure 2. (A) Optical microscopy images (500 × 500 μm) and (B) AFM phase images (500 × 500 nm) of P3HT:fullerene derivative blend films cast from CB solution.

Figure 3. Plots for (A) mean grain size (μm2) in the images of Figure 2A and (B) mean crystal size of P3HT (nm2) in Figure 2B as a function of the IFE. Error bars represent the standard error. Lines are provided as guides to the eye.

was performed on the area without grains in each film. The obtained phase images represent bright chainlike features of P3HT crystallites and P3HT/fullerene mixture in the dark phase.11 The mean area of the P3HT crystallites in each image was also analyzed (Table S3) and is depicted in Figure 3B. The size of the crystal tends to increase relative to the IFE, that is, phase separation seems to be prominent in the case of highpolarity materials (4d−f). The films consisting of PCBM and 4a−c exhibited fine fibril structures in accordance with the results of the optical observations. We investigated the films precisely by subjecting them to spectroscopic measurements. Figure 4 shows the normalized absorption spectra of the blend films cast from CB and DCB without annealing. The absorption peak around 2.5 eV is generally known to be derived from P3HT, and its occurrence is almost independent of the presence of the fullerene derivatives.34 The lower energy part of the spectrum is composed of the absorbance of P3HT crystalline regions, which form weak interchain H-aggregates.35 Spano et al. developed a model to describe the absorption of P3HT aggregates A as a function of photon energy ℏω (eq 6):36

Figure 4. Normalized UV−vis absorption spectra of P3HT:fullerene derivative blend films cast from (A) CB and (B) DCB. The spectra are normalized at about 2.4 eV. The aggregate P3HT absorbance fitted by eq 6 for the P3HT/PCBM blend film and the vibrational transitions, labeled A0−0−0−4, are shown as dashed and dotted lines, respectively.

∑n≠mSn/n!(n−m), and n is the vibrational quantum number. We applied the same Gaussian line width σ for each vibronic transition, as well as the Huang−Rhys factor S as unity. By fitting eq 6 to the spectra, we extracted exciton bandwidths W and plotted them in Figure 5 as a function of the IFE. The fitting parameters are summarized in Table S4. The best fits for the P3HT/PCBM blend films and the individual vibrational

⎞2 −S ⎛ e − S S m ⎞⎛ W e Gm⎟⎟ Γ(ℏω − E0 − 0 − mEp) A∝ ∑⎜ ⎟⎜⎜1 − 2Ep ⎝ m ! ⎠⎝ ⎠ m=0 (6)

Figure 5. Plots for the exciton bandwidths (W) for the P3HT obtained by the fits of absorption spectra in (A) Figure 4A (cast from CB) and (B) Figure 4B (cast from DCB) as a function of the IFE. Error bars represent the standard deviation. Lines are provided as guides to the eye.

where m is the vibrational level, S is the Huang−Rhys factor, W is the exciton bandwidth of aggregates, Ep is the energy of the CC stretch (0.18 eV),37 Γ is the Gaussian line shape, and E0−0 is the 0−0 transition energy. Gm is a constant, Gm = D


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consisting of 4a−c, which had low IFEs, exhibited highly quenched spectra compared with those of PCBM. By contrast, the high IFEs of 4d−f led to large PL intensities. These results agree well with the formation of grains and P3HT crystallites in the blend films (Figure 2). Altering the casting solvent from CB to DCB reduced the dependence of the PL intensities on the IFE. On the basis of these observations, we concluded that the polarity of a fullerene derivative significantly affects film morphology. The high polarity of the fullerene derivatives causes phase separation between P3HT and fullerenes because of the difference in aggregation behavior, which in turn results in the formation of large grains and the high crystallinity of P3HT (shown in Figures 2 and 4). Generally, annealing P3HT/PCBM films induces significant phase separation accompanied by the further crystallization of P3HT.24 The annealing process is essential for the device fabrication of the P3HT/PCBM system. In this procedure, the diffusion of PCBM in the polymer matrix is considered as a key to achieving the optimized BHJ structure for effective carrier transport.40 We will discuss the details in annealed blend films and diffusion behavior as a function of the acceptor structure in our future work by using molecular dynamics calculation and thermal analysis. Photovoltaic cells were prepared with the fullerene derivatives described above. The typical derivatives with low (DMPCEP, 4a) and high (DMPCBM, 4f) polarities were chosen for a simple comparison because they formed relatively homogeneous films with P3HT when DCB was used as a cast solvent (Figure S2). The cells were fabricated with the same procedure and under the same conditions as those used for PCBM cells. The properties of the resulting ITO/ PEDOT:PSS/P3HT:fullerene/TiOx/Al devices were measured under a simulated AM1.5G condition (Figure 8, Table S5).

transitions are also depicted in Figure 4. Spano’s theory indicates that exciton bandwidth W is inversely proportional to the conjugation length and degree of intrachain order in the P3HT aggregates.38 The blend films consisting of the relatively low-polarity derivatives (4a−c) exhibited large W values. Furthermore, the blend films consisting of high-polarity derivatives (4d−f) showed low W values, a result suggesting the formation of a significant number of P3HT aggregates.36 From these observations, the polarity of the fullerene derivatives can be concluded to affect the crystallinity of P3HT. With the values reported for the pristine P3HT films cast from various solvents considered (∼120 meV),36 the fullerene derivatives that had compatible polarities with P3HT interfered with P3HT aggregation, and this phenomenon led to the formation of amorphous-like films. When DCB was used as the cast solvent, the dependence of the W values on the IFE values decreased, but the lower W values than those for the films cast from CB explain the high crystallinity of P3HT in the blend films. Figure 6 shows the photoluminescence (PL) spectra of the blend films cast from CB and DCB solution. As is well-known,

Figure 6. Photoluminescence spectra of P3HT:fullerene derivative blend films (A) cast from CB and (B) DCB.

the PL of P3HT is strongly quenched in the presence of neighboring fullerene derivatives.39 The intensity of the remaining PL allows us to recognize the presence of isolated crystalline P3HT formed by phase separation with the fullerene derivatives. The integrated PL intensities of the blend films are plotted as a function of the IFE in Figure 7. The films

Figure 8. Current density−voltage curves measured under simulated AM1.5G solar illumination at 100 mW/cm2 intensity for ITO/ PEDOT:PSS/P3HT:fullerene/TiOx/Al devices. The curves were averaged from three or four devices, as summarized in Table S5.

Despite the quite similar structure of DMPCBM to PCBM, the power conversion efficiency (PCE) of the device based on the high-polarity derivative (DMPCBM, 4f) was quite low probably because of phase separation in the BHJ structure, as discussed above; this situation resulted in prompt carrier recombination. On the other hand, DMPCEP (4a), which maintains a balanced polarity with P3HT, exhibited almost the same values of short circuit current density (JSC) and fill factor (FF) as PCBM did. An improved PCE was obtained because of the enhanced VOC by 80 mV, which was also higher in accordance with the higher LUMO energy level than that of PCBM (Figure 1). These results indicate that establishing a balance between donor and

Figure 7. Plots for the integrated areas of the photoluminescence spectra in (A) Figure 6A (cast from CB) and (B) Figure 6B (cast from DCB) as a function of the IFE. The area was integrated from 1.48 to 2.10 eV. Lines are provided as guides to the eye. E


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temperature and then centrifuged. The solution was filtered through a 0.45 μm PTFE syringe filter. A 1.0 mL sample of this saturated solution was diluted to 50 mL with dichloromethane. The solubility values of the fullerene derivatives were calculated by the absorbance value of the solution at 430 nm with the use of the absorbance of a 0.1 g/L solution of the fullerene derivatives in dichloromethane. Morphological and Spectroscopic Characterization. The thin films for morphological investigation were spin-coated from chlorobenzene or 1,2-dichlorobenzene solutions (30 mg/mL) of P3HT and the fullerene derivatives (1:1 by weight for PCBM; the ratio was modified in accordance with the molecular weight of the fullerene derivatives) at 1000 rpm on the PEDOT:PSS films formed on cleaned glass plates. The films were dried under vacuum for 30 min and kept in a dark and inert atmosphere until they were subjected to optical measurements. AFM phase images were taken at a scan rate of 0.5 Hz in the tapping mode. The absorption spectra of thin films were measured with a UV/vis spectrometer in a standard double-beam transmission alignment with the use of a PEDOT:PSS film as a reference. Photoluminescence spectra were recorded on a steady-state spectrofluorometer with the same slit widths, integration time, and excitation at 500 nm wavelength. Device Fabrication. The photovoltaic devices were made with the following structure. Glass/ITO/PEDOT:PSS/P3HT:acceptor/TiOx/ Al ITO substrates were first cleaned by sonication sequentially in detergent, water, acetone, and methanol, followed by treatment with UV-ozone. The ITO glass plates were modified with a PEDOT:PSS thin film, which was spin-coated from a PEDOT:PSS aqueous solution (Clevious P VP AI 4083). The thickness of the PEDOT:PSS layer was about 30 nm. The substrates were dried at 150 °C for 20 min in air and transferred to a nitrogen-filled glovebox. The 1,2-dichlorobenzene solutions composed of P3HT (1 wt %) and fullerene derivatives (0.8 wt %) were spin-cast at 800 rpm on top of the PEDOT:PSS layer. Then, the TiOx layer was spin-coated from a 1 wt % methanol solution of titanium(IV) isopropoxide at 2000 rpm, and the substrates were heated at 150 °C for 5 min. Thermal evaporation and a shadow mask were used to deposit the Al top electrode (100 nm). Device performance was measured with a source meter under simulated AM1.5G irradiation (100 mW/cm2). A mask was used to define the device illumination area of 4.0 mm2.

acceptor polarities is essential to obtain an appropriate BHJ morphology and device performance, and that we can predict the compatibility of new acceptor materials by using the surface free energy as an index before tremendous efforts are made to optimize device fabrication procedure. Further investigation of photovoltaic cells on the basis of the newly developed derivatives, along with the properties observed during the annealing process, will be reported in future works.

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CONCLUSIONS We prepared dimethoxyphenyl-substituted methanofullerenes and investigated their structural effect on the morphology of P3HT/fullerene blend films by using the surface free energy as an index of material compatibility. The obtained fullerene derivatives exhibited a higher LUMO energy level than PCBM because of the methoxy groups placed in proximity to the C60 cage.29 Polar substituents, such as ether and ester groups, provided a significant enhancement in the surface free energy of the fullerene derivatives. We found that the deviation in the surface free energy between P3HT and the fullerenes led to a large phase separation in the BHJ structure, and making the polarity of the fullerene derivatives match with that of the donor polymer provided the optimized morphology. The photovoltaic cell prepared with the high-polarity fullerene derivative (DMPCBM (4f)) showed lower performance than the cell using PCBM because of an inappropriate morphology. Instead, the PCE and VOC values obtained with the low-polarity derivative of DMPCEP (4a) as an acceptor were better than those obtained using PCBM in accordance with the high LUMO energy level and the optimized morphology. This report shows that the design of the acceptor materials, in which the interactions with donor materials are also considered, is quite important. A balance between the surface free energies of donor and acceptor materials is the key to achieving an appropriate BHJ morphology and high performance without deteriorating the pristine properties of the acceptor materials. The surface free energy is a useful tool to design fullerene derivatives compatible with the latest low-band gap polymers.

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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/acsami.5b11180. Additional figures and tables, synthetic details, and figures giving 1H NMR, 13C NMR, and HRMS spectra of new compounds. (PDF)

EXPERIMENTAL SECTION

General Methods. Reduction potentials were determined by cyclic voltammetry with the use of a platinum working electrode, platinumwire counter electrode, and Ag/Ag+ reference electrode. Measurements were performed under Ar gas; a 1,2-dichlorobenzene solution containing tetrabutylammonium perchlorate (0.1 M) was used as a supporting electrolyte, and the scan rate was 20 mV/s at room temperature. Contact angle measurements of the solid films with the use of a contact angle meter were performed with water and hexadecane as the probe liquids. The sample films were made on cleaned glass by spin coating, typically from 1 wt % solution at 800 to 1000 rpm. The solvent was optimized according to the materials to form the flat film with the use of chloroform, chlorobenzene, and 1,2dichlorobenzene. 1H NMR and 13C NMR spectra were recorded with 270 or 300 MHz instruments in deuterated solvents with tetramethylsilane as an internal reference. Mass spectra were obtained with matrix-assisted laser desorption ionization time-of-flight mass spectrometry. PCBM was synthesized in accordance with the methods described in the literature.41 Dichloromethane, tetrahydrofuran, and 1,2-dichlorobenzene, as used in the reactions, were purified and freshly distilled by standard procedures. All the other solvents and materials are commercially available and were used as received. Solubility Measurements. The solubility of the fullerene derivatives was determined with the following process. An excess amount of fullerene derivatives was mixed in 1.5 mL dichloromethane. The mixture was subjected to sonication for 15 min at room

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +81-6-6963-8057. *E-mail: [email protected]. Tel.: +81-6-6963-8016. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank A. Hirai and Dr. K. Hida for their help in synthesizing compounds and collecting data. This research was supported by Core Research for Evolutional Science and Technology of the Japan Science and Technology Agency (CREST, JST).

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ABBREVIATIONS NMR nuclear magnetic resonance IR infrared ITO indium tin oxide F


Research Article


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PEDOT poly(3,4-ethylenedioxythiophene) PSS poly(styrenesulfonate) MALDI TOF-MS matrix-assisted laser desorption ionization time-of-flight mass spectrometry

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H


Controlling the Polarity of Fullerene Derivatives to Optimize

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