NIR-Absorbing Merocyanine Dyes for BHJ Solar Cells - Chemistry of

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NIR-Absorbing Merocyanine Dyes for BHJ Solar Cells André Zitzler-Kunkel,† Martin R. Lenze,‡ Nils M. Kronenberg,‡,§ Ana-Maria Krause,† Matthias Stolte,† Klaus Meerholz,*,‡ and Frank Würthner*,† †

Institut für Organische Chemie & Center for Nanosystems Chemistry, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany ‡ Department of Chemistry, Universität zu Köln, Luxemburger Straße 116, 50939 Köln, Germany S Supporting Information *

ABSTRACT: We have synthesized a series of new, polymethine chain extended merocyanine dyes 1−4 bearing varied acceptor units and an aminothiophene donor moiety. The optical and electronic properties of these new merocyanines have been studied in comparison with their corresponding lower homologues 5−8, which contain two methine groups less, by UV−vis and electrooptical absorption (EOA) spectroscopy and cyclic voltammetry. The absorption spectra of π-extended merocyanines are markedly redshifted, and their extinction coefficients are significantly increased compared to those of their lower homologues. The photovoltaic characteristics of these dyes have been explored in devices using them as donor and PC61BM fullerene as acceptor materials. Our detailed studies reveal that, despite more favorable absorption properties, the π-extended merocyanines exhibit lower short-circuit current densities (JSC) as well as decreased open-circuit voltages (VOC) and power conversion efficiencies (PCE) compared with those of their respective lower homologues. The unexpected decreased JSC values could be explained in terms of looser packing features of π-extended chromophores in the solid state as revealed by single-crystal X-ray analysis of two pairs (1/5 and 4/8) of these dyes. By optimization of device setup PCE of 2.3% has been achieved with the π-extended donor material 4.

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INTRODUCTION In view of the rapidly growing world population the ongoing global efforts for economic prosperity will lead to a rapid depletion of fossil fuel resources, and thus the challenge of securing a sustainable energy supply will arise in the foreseeable future. The production of renewable energy is an encouraging approach to face this challenge. Among the available renewable energy sources, solar energy appears to be the most promising primary energy source of the future as more power of the sun strikes the Earth’s surface within 1 h than being consumed over a whole year.1,2 Conventional silicon-based solar cells, despite their high power conversion efficiencies (PCE) of up to ca. 25%,3 come along with many disadvantages such as high production costs, high weight, and rigidity of the panels. Over the last two decades, organic solar cells have become increasingly more attractive. This new generation of photovoltaic modules excel with low production costs and the ability to manufacture lighter, highly flexible4−7 and semitransparent devices,8,9 which open avenues for new application areas such as building integrated photovoltaics10 for sun shading and electricity generating glass facades. In the initial stage of organic photovoltaics significant efforts have been devoted to solutionprocessed bulk heterojunction (BHJ)11 solar cells composed of polymeric poly(3-hexylthiophene) (P3HT) as an electron donor and a soluble fullerene derivative, typically [6,6]phenyl-C61-butyric acid methyl ester (PC61BM), as an electron acceptor material. Despite enormous research efforts devoted © 2014 American Chemical Society

to the development of such solar cell devices, their performance remains still unsatisfactory.12,13 One of the reasons for that might be the spectral mismatch of solar cell absorption spectra with the solar photon flux, which culminates at around 700 nm.12 Over time further advances in the field have been realized by utilizing low-bandgap polymers14,15 that allow a broader spectral coverage and hence resulted in PCE exceeding 9%.16,17 Another current approach to generate efficient solar cells is the replacement of polymeric material by a discrete π-conjugated molecular donor.18−21 Small molecules have several advantages compared with their polymeric pendants, in particular, their intrinsic monodispersity and their facile synthesis and purification as well as easy tailoring of their absorption and electronic properties. For discrete photovoltaic donor materials various organic dyes such as oligothiophene,22,23 triarylamine,24,25 BODIPY,26,27 diketopyrrolopyrrole (DPP),28 isoindigo,29 dipolar D−A dyes,30,31 and benzodithiophene32,33 have been employed, reaching power conversion efficiencies beyond 8%.33 However, even for these state-of-the-art donor materials most of the solar photons with energies less than ∼1.8 eV, which display almost 50% of the sunlight intensity, are not absorbed. This illustrates the persistent demand for new nearinfrared (NIR) absorbing dyes for organic solar cell Received: June 13, 2014 Revised: August 2, 2014 Published: August 12, 2014 4856

dx.doi.org/10.1021/cm502302s | Chem. Mater. 2014, 26, 4856−4866

Chemistry of Materials

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Chart 1. Chemical Structures of Merocyanine Dyes 1−4 with π-Enlarged Polymethine Chain (n = 1) and Their Lower Homologues 5−8 (n = 0) Investigated in This Work

Scheme 1. Synthesis of the π-Enlarged Merocyanine Dyes 1−4a

a

AH2 denotes the respective CH-acidic compounds: 1-benzyl-6-hydroxy-4-methyl-2-oxo-1,2-dihydropyridine-3-carbonitrile for 1, 2-(4-phenyl-5Hthiazol-2-ylidene)-malononitrile for 2, 2-(4-tert-butyl-5H-thiazol-2-ylidene)-malononitrile for 3, and 2-(3-oxo-indan-1-ylidene)-malononitrile for 4

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RESULTS AND DISCUSSION Synthesis. The novel π-enlarged merocyanine dyes 1−4 were synthesized by a Knoevenagel condensation reaction of the respective CH-acidic compounds (AH2) with 2-aminothiophene-5-acrylaldehyde 11 in 19−32% yields (Scheme 1). The aldehyde 11 was synthesized from aminothiophenecarbaldehyde 951 by a Wittig olefination reaction using acetalprotected phosphonium salt 10 according to the literature.52 The reference merocyanine dyes with shorter polymethine chain 746 and 846 are literature known, while 551 and 646 were synthesized according to the procedures reported for similar compounds. Synthetic details are given in the Experimental Section. The new compounds were characterized by 1H NMR and 13C NMR (see Figures S1−S14 in the Supporting Information), high resolution mass spectrometry (HRMS), and elemental analysis (for 1, 3, 4, and 6). The π-enlarged merocyanines 1 and 4 as well as the lower homologue 5 were additionally characterized by single-crystal X-ray diffraction analysis. Optical and Electrochemical Properties. To get insight into the impact of polymethine chain length on optical and electrochemical properties of merocyanine dyes, we have performed UV−vis and cyclic voltammetry studies of πextended dyes 1−4 and their lower homologues 5−8 in CH2Cl2. As shown in Figure 1, almost all of the investigated merocyanine dyes exhibit sharp and intense absorption bands. Only for dye 8 a broader absorption profile is observed with a relatively low maximum absorption coefficient of εmax = 66000 L mol−1 cm−1. A comparison of the absorption maxima within the dye series 1−4 and 5−8, respectively (see Table 1), clearly reveals an influence of the different acceptor units (A) on the absorption properties of these merocyanines. Thus, the dyes with pyridone acceptor 1 and 5 absorb at the shortest

applications. There are, however, a few dyes that possess intense absorption in the NIR region (λmax of thin films >700 nm) and could be identified as promising donor materials for photovoltaics. The reported examples comprise phthalocyanines,8,34 aza-dipyrromethenes,35,36 squaraines,37−39 cyanine salts,9,40,41 and dithienosiloles.42,43 Our groups have shown that dipolar merocyanine dyes are very promising donor materials for solar cell applications.44−47 We have also reported that merocyanines bearing a strong electron-withdrawing acceptor unit, particularly 2-oxo-5-dicyanomethylene-pyrrolidine, can be used as NIR absorber materials in BHJ solar cells, however, with unsatisfactory PCE.48 It is generally known that the extension of πconjugation of chromophores leads to a red-shift of the absorption maximum49,50 and thus may lead to a better solar cell performance. Keeping this in mind, we wanted to explore whether the extension of polymethine chain in merocyanine dyes would possibly provide more appreciable materials. Therefore, we have synthesized π-extended merocyanine dyes 1−4 bearing different acceptor units and an aminothiophene group as donor moiety and studied their optical and electronic properties as well as their photovoltaic performance in BHJ solar cells in comparison with their lower homologues 5−8 having two methine groups less (for structures, see Chart 1). To our knowledge, such a comparative study on the impact of polymethine chain length on photovoltaic properties is not reported to date. Our detailed studies disclose that, despite their advantageous absorption properties, the higher homologues 1−4 exhibit lower short-circuit current densities (JSC) and PCE compared to those of the corresponding lower homologues 5−8. This unexpected observation is rationalized on the basis of better packing of the chromophores with shorter polymethine chain in the solid state. 4857



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cyanine-type conjugation made possible by two additional methine units of the thiazole-based acceptor. Comparison of the optical properties of π-extended dyes 1− 4 with those of their lower homologues 5−8 clearly demonstrates two effects. First, the extension of the polymethine chain by two methine carbon atoms (in 1−4) results in a bathochromic shift of the absorption band of more than 100 nm compared to that of the lower homologues 5−8. Accordingly, the π-extended dyes 1−4 can be considered almost as cyanine-type.49 Second, the chromophores 1−4, with the exception of 3, exhibit significantly higher molar extinction coefficients compared to those of the references 5−8 (Table 1). Since the investigated merocyanine dyes display similar sharp absorption profiles as evident from their fwhm values (Table 1), the increase of εmax is directly reflected in an increased tinctorial strength of the π-enlarged dyes. Consequently, for chromophores 1−4 the determined absorption densities (μ2ag M−1)46 are markedly increased compared to those of their reference dyes 5−8 (see Table 1). These advantageous optical properties of 1−4 should be beneficial for their application in BHJ solar cells. Next, the effect of chain length variation on the redox properties of chromophores 1−4 and 5−8 has been studied by cyclic voltammetry (CV) in dry CH2Cl2 using ferrocene as an internal standard. As a representative example for the comparison of corresponding homologues, the cyclic voltammograms of the pair 1 and 5 are depicted in Figure 2a (for the cyclic voltammograms of the dyes, see Figure S15 in the Supporting Information). The electrochemical data for merocyanine dyes 1−8 are compiled in Table 1. These data reveal that all these dyes exhibit one reversible oxidation and one irreversible reduction wave within the accessible potential range, respectively. Compared to references 5−8, the reversible oxidation waves of π-extended chromophores 1−4 are shifted by 0.20−0.28 V to lower potentials, whereas their irreversible (and accordingly less accurate) reduction potentials experience an opposite trend and exhibit more positive reduction potentials. In compliance with the bathochromic shift of the absorption band, these opposing effects result in a significant reduction of the band gap for the π-extended merocyanine dyes 1−4. The observed oxidation potentials for 1−8 were transformed into HOMO energy levels vs vacuum according to the equation EHOMO = −5.15 eV − e·Eox. Although the LUMO levels could be calculated with the same equation, we

Figure 1. UV−vis absorption spectra of π-extended (1−4; solid lines) and corresponding reference (5−8; dashes lines) merocyanine dyes in CH2Cl2 (c ≈ 1 × 10−5 M; T = 25 °C; 1 and 5: green, 2 and 6: red, 3 and 7: blue, 4 and 8: black) and solar photon flux at AM1.5 conditions (black).

Table 1. Optical and Electrochemical Properties of the Merocyanine Dyes 1−8 in CH2Cl2 at 298 K dye

λmax [nm]

εmax [L mol−1 cm−1]

μ2ag M−1a [D2 mol g−1]

fwhm [cm−1]

Eredb [V]

Eox [V]

1 5 2 6 3 7 4 8

656 540 764 659 753 651 691 580

219000 159000 171000 126000 144000 142000 181000 66000

0.31 0.21 0.33 0.23 0.32 0.24 0.37 0.23

875 872 1457 941 952 794 1079 2202

−1.36 −1.67 −1.13 −1.27 −1.22 −1.39 −1.32 −1.53

0.24 0.52 0.20 0.41 0.17 0.37 0.27 0.55

a For the determination of the transition dipole moment μag by integration of ε(ṽ), see the Supporting Information. bPeak potential of irreversible redox process.

wavelengths of 656 and 540 nm, respectively, in the present series of dyes. The use of 2-(3-oxo-indan-1-ylidene)-malononitrile acceptor unit in 4 and 8 induces a red shift of the absorption maxima of 35−40 nm compared to those of 1 and 5. More interestingly, 2-(5H-thiazol-2-ylidene)-malononitrile acceptor in merocyanines 2, 3, 6, and 7 attains further bathochromic shifts of 60−80 nm compared to those of 4 and 8. This distinct red-shift is a consequence of an extended

Figure 2. (a) Cyclic voltammograms of the homologous pair of merocyanine dyes 1 and 5 in CH2Cl2 (c ≈ 2.5 × 10−4 M; vs Fc/Fc+; 100 mV s−1; NBu4PF6 (0.1 M)). (b) Impact of polymethine chain length on the FMO energy levels and band gap of dyes 1−8 and their relative position to the LUMO of PC61BM (EHOMO = −5.15 eV − e·Eox; ELUMO = EHOMO + hc/λmax). 4858



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and 5 in Figure 3 (for the EOA spectra of dyes 2, 3, 4, and 6, see the Supporting Information Figures S17−S19). The

decided to take an alternative approach due to the irreversibility of the reduction waves for our dyes and the fact that only HOMO level calculations are in accordance with Koopmans theorem. Thus, the LUMO energy levels were estimated from the optical band gap in CH2Cl2 by using the equation ELUMO = EHOMO + hc/λmax. We like to note that by this means we neglect the exciton binding energy which is, however, compensated by the fact that we used the energies of the absorption maxima of the dyes in solution that are 0.2−0.5 eV larger than those obtained for thin films (see below). The frontier molecular orbital (FMO) energy levels and band gaps resulted from this analysis are schematically illustrated in Figure 2b. A comparison of the determined FMO energies for the different homologous dye pairs clearly reveals that the π-extension induces an increase in the HOMO energy levels by ca. 0.2 eV, concomitant with a slight decrease of the LUMO energy levels. A similar trend in the FMO energy level alignment was obtained by DFT calculations (B3-LYP) that were performed to attain a more comprehensive picture (for details, see Figure S16 and Table S1 in the Supporting Information). The calculations reveal that both HOMO and LUMO are distributed over the entire πconjugated path of the investigated merocyanines. This is in accordance with the experimental results as the π-extension leads to an increase of the HOMO and decrease of the LUMO energies. As an important outcome of this analysis we can conclude that the minor decrease of the LUMO levels upon extension of the polymethine chain should not prohibit electron transfer to the fullerene manifold in BHJ solar cells (vide inf ra) because this process is still exergonic by ∼0.3 eV for all investigated dyes. Electro-Optical Absorption Measurements. To obtain reliable information on the ground (μg) and excited (μa) state dipole moments as well as the dipole moment change Δμ = μa − μg upon optical excitation, electro-optical absorption (EOA) measurements 53,54 were performed for the π-extended merocyanine dyes 1−4 in dioxane solution. In EOA experiments the influence of an external electric field E0 is determined on the optical density of a dye solution. Measurements for two different linear polarizations of the incident light, parallel (φ = 0°) and perpendicular (φ = 90°) to the applied electric field, are performed to determine their influence on the molar decadic extinction coefficient ε. The change of ε in an electric field is mainly caused by a partial alignment of the dipolar dye molecules and a redistribution of charge density. A quantitative determination of these effects can be achieved by a numerical band shape analysis, which provides the desired ground (μg) and excited (μa) state dipole moments and the dipole moment change Δμ. The obtained Δμ values in combination with the transition dipole moment μag, which is accessible by UV−vis spectroscopy, can then be applied to classify the dyes using the resonance parameter c2 (eq 1) as polyene-type (c2 ≈ 0; Δμ > 0), cyanine-type (c2 ≈ 0.5; Δμ ≈ 0), or betaine-type (c2 ≈ 1; Δμ < 0). ⎛ 1⎜ c = ⎜1 − 2⎜ ⎝ 2

⎞ ⎟ 2 2 ⎟ 4μag + Δμ ⎟⎠

Figure 3. UV−vis absorption spectra (dashed lines) and electrooptical absorption measurements for merocyanine dyes 1 (black) and 5 (red) in 1,4-dioxane solution at T = 298 K. Electro-optical data points are shown for parallel (○: φ = 0°) and perpendicular polarization (•: φ = 90°) of the incident light relative to the applied external electric field, and corresponding nonlinear regression curves are shown as solid lines.

obtained dipole moments and subsequently calculated c2 parameters are compiled in Table 2 for the chromophores Table 2. Electro-Optical Properties of the Chromophores 1− 8 from EOA and UV-Vis Measurements in 1,4-Dioxane Solution at 298 K dye

μag [D]

μga [D]

μaa [D]

1 5 2 6 3 7b 4 8b

11.8 9.7 11.7 10.0 11.4 9.9 11.7 9.8

13.4 13.6 15.0 14.3 13.3 13.1 9.1 8.6

18.0 15.0 16.7 15.4 18.2 15.6 16.0 12.7

Δμa [D] 4.6 1.4 1.7 1.1 4.9 2.5 6.9 4.1

c2 0.40 0.47 0.46 0.47 0.40 0.44 0.36 0.40

a

Dipole moments were calculated for the gas-phase by solvent correction within the approximation of the Onsager Continuum Model.58 bThe values are taken from ref 46 and shown for the purpose of comparison.

studied here. For comparison, the characteristics for the lower homologues are given as well.46 Chromophore 1 displays increased absorbance in the case of parallel polarization (φ = 0°) of the incident light relative to the applied external electric field (open symbols in Figure 3), whereas a decreased absorbance is observed when the light is perpendicularly polarized to E0 (filled symbols). Accordingly, this dye shows a positive electrochroism, which is also found for all other merocyanines in this study. This finding reveals that the ground state dipole moment μg and the transition dipole moment μag are oriented almost parallel to each other.55 Our experiments further disclose that the EOA spectra for parallel polarized light (φ = 0°) are bathochromically shifted compared to the UV−vis spectrum, indicating that the optical excitation is associated with an increase in the dipole moment (Δμ > 0). The dipole moments deduced from electro-optical absorption spectroscopy summarized in Table 2 reveal that the πextended chromophores 1−4 possess large ground state dipole

Δμ

(1)

The c2 parameter is a measure for the charge transfer from the donor to the acceptor subunit of the chromophore, which increases with increasing c2. The typical UV−vis absorption and EOA spectra obtained for these dyes in dioxane are depicted for the homologues 1 4859



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Figure 4. J−V response of optimized merocyanine:PC61BM solar cells using the electron donor materials (a) 1 (60 wt % PC61BM), 2 (50 wt % PC61BM), 3 (70 wt % PC61BM), and 4 (50 wt % PC61BM) and (b) analogous reference dyes 5 (70 wt % PC61BM), 6 (50 wt % PC61BM), 7 (75 wt % PC61BM), and 8 (60 wt % PC61BM). (c) EQE of the devices with dyes 1−5 and 8 and (d) IQE spectra of the solar cells for the homologue dye pair 4/8 calculated using the absorption from refection data.

moments μg in the range of 13 D (1,2,3) and 9 D (4). In accordance with the bathochromic shift of their EOA compared to their UV−vis spectrum, these dyes display positive Δμ values between Δμ = 1.7 D (2) up to Δμ = 6.9 D (4). These Δμ values are caused by a significant dipole moment of the respective excited state μa, which reaches values up to μa = 18.2 D (3). The resonance parameters c2 of these merocyanine dyes with values of 0.40 (3), 0.36 (4), 0.40 (1), and 0.46 (2) are lower than the cyanine limit (c2 = 0.5), attributing these chromophores with a partial polyene-type character. A comparison of the determined electro-optical parameters for π-extended chromophores 1−4 with the EOA characteristics of their reference dyes 5−8 should be of interest as it would unveil the effect of π-extension on the push−pull character of this class of dyes. It is a common perception that an extension of the conjugated bridge for a given donor− acceptor pair results in a more polyene-type character of a dye molecule,57 hence leading to smaller c2 values. Indeed, the ground state dipole moments of the corresponding chromophores are quite similar and only marginally increased for the πextended dyes. In contrast, the dipole moment of the excited state μa increases upon π-extension, resulting in a distinct increase of the Δμ parameter. As a consequence the c2 resonance parameters for homologues dye pairs 1/5, 2/6, 3/ 7, and 4/8 (see Table 2) disclose smaller c2 values for the πextended merocyanines (1−4). Similar electro-optical effects have previously been observed for elongated donor−acceptor polyenes.59 It has to be noted at this point, however, that these dipole moments and c2 values given in Table 2 correspond to isolated molecules in the gas phase. More polarizing environments such as the bulk state or a polar solvent will polarize dipolar molecules easily by 0.1 to 0.2 c2-units toward a more zwitterionic structure.56 This polarizing effect is clearly revealed by single crystal X-ray analysis (see below) which show almost

equal bond lengths of the polymethine chain for all four crystallized compounds. Therefore, we can assume that the electron distribution of all eight investigated merocyanine dyes is close to the cyanine limit in the bulk state. BHJ Solar Cells. The present two series of dyes 1−4 and 5−8 offer an excellent opportunity as model systems to investigate the impact of polymethine chain length as well as the applied acceptor units on photovoltaic performance. In order to assess the suitability of π-extended dyes 1−4 as electron donor materials in BHJ solar cells, we prepared devices with these compounds as donor materials and PC61BM fullerene as an acceptor. For comparison, we fabricated similar BHJ cells with the lower homologues 5−8. The general device architecture of these cells was glass/ITO/PEDOT:PSS (35 nm)/dye:PC61BM (∼50 nm)/Ca (4 nm)/Ag (150 nm). A detailed description of device fabrication and characterization as well as the thin film spectra of optimized devices are provided in the Supporting Information. The J−V characteristics of both series of investigated merocyanine:PC61BM BHJ solar cells are depicted in Figure 4a,b, and the photovoltaic characteristics are summarized in Table 3. All devices were optimized in regard to the active layer thickness and dye:fullerene ratio. A comparison of the PCEs within the two different series of devices reveals lowest efficiencies (0.45−1.12%) for solar cells with dyes bearing a thiazole acceptor unit (2, 3 and 6, 7), which originates both from low open-circuit voltages (VOC) and short-circuit current densities (JSC). The use of the pyridone acceptor in dyes 1 and 5 afforded slight improvement in the PCE due to better VOC and JSC values. The best solar cells in this series were achieved with the dyes 4 and 8 that incorporate the 2-(3-oxo-indan-1ylidene)-malononitrile acceptor unit reaching efficiencies of 1.50% and 2.01%, respectively. In particular, the obtained higher JSC values for 4 (5.11 mA cm−2) and 8 (6.07 mA cm−2) 4860



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the π-extended dyes 1−4 in solution (see Table 1) and a favorable broadening as well as red shift (30−60 nm for dyes 1−4 compared to 15−30 nm for dyes 5−8) of the absorption bands in the thin films (Figure S20), the solar cells prepared with these donor materials show significantly lower JSC values than those of lower homologues 5−8. As shown in Figure 4c, the bathochromic shift in absorption is reflected in the external quantum efficiency (EQE) spectra. For the π-extended chromophores 1−4 substantially lower values are achieved compared to their lower homologues (22%/42% for dyes 1/5 and 31%/49% for dyes 4/8, respectively). The internal quantum efficiency (IQE) incorporates the absorption of the active layer and therefore allows for comparing the effectiveness of differently absorbing solar cells. IQE was approximated by normalizing EQE to the absorption of the whole device measured in reflection mode. This is exemplarily shown for solar cells with 4 and 8 (Figure 4d). The resulting IQE spectra reveal an average IQE of ca. 35% for the π-extended dye 4 compared to ca. 45% IQE for its lower homologue 8. This difference of ca. 30% agrees reasonably well with the difference in JSC for the two devices (compare Table 3). We note that the use of reflection data leads to an underestimation of IQE and potential alteration of the spectral shape.60 From our experience with other merocyanine dyes, we estimate the true IQE to be ca. 15−20% higher. Since this argument applies similarly to both cells, the above conclusion remains unchanged. Similar results are expected for the other homologue pairs. Although we could only estimate the LUMO levels from cyclic voltammetry data (Figure 2) which do not take into account the influence of the polarizing environment or the orientation of the dyes at the heterojunction interface,61,62 we hesitate to attribute the reduced JSC and EQE values for devices based on π-extended materials 1−4 to a lack of driving force for charge carrier separation at the heterojunction interface, since the LUMO levels of these dyes are well in the range of the anticipated necessary offset of 0.3−0.4 eV63 above the LUMO level of PC61BM. Importantly, the LUMO levels of these πextended chromophores are stabilized only by 0.05−0.13 eV compared to the LUMOs of corresponding reference dyes (Figure 2) which suggests that the driving force for exciton dissociation for the homologue dye pairs at the PC61BM interface is comparable. Therefore, the reduction of JSC values might be attributed to a deterioration of the exciton and/or charge transport properties in the manifold of π-extended merocyanines 1−4. To verify this hypothesis, the packing arrangement of a selection of present merocyanine dyes was investigated in the solid state, since a dense packing of the chromophores would result in a strong overlap of adjacent π-systems to facilitate the formation of efficient percolation paths for exciton and charge carrier transport.22,64 Fortunately, we were able to grow suitable crystals of π-extended merocyanines 1 and 4 and the corresponding lower homologue of 1, that is 5, for singlecrystal X-ray analysis.65 The crystal structure of 8, which is the corresponding lower homologue of 4, has been reported previously.46 Thus, the packing features of two homologue dye pairs 1 and 4 (Figure 5) and 5 and 8 (Figure 6) can be discussed in a comparative manner. The molecular structures of the π-extended dyes 1 and 4 and their lower homologues 5 and 8 show for merocyanines typical, highly conjugated π-systems as revealed by a significant bond

Table 3. Absorption Maxima in Thin Films and Characteristic Solar Cell Parameters of Solution-Processed BHJ Solar Cells Based on π-Extended Dyes 1−4 or Their Lower Homologues 5−8 with the General Device Setup Glass/ITO/PEDOT:PSS/Dye:PCXBM/Ca/Ag under Simulated AM1.5G Solar Irradiation of 100 mW cm−2 Intensityd dye 1 5 2 6a 3 7a,b 4 8 3c 4c

λmax [nm] (thin film) 691 558 796 691 794 682 742 595 794 742

wt % PCXBM 60 (X 70 (X 50 (X 50 (X 70 (X 75 (X 50 (X 60 (X 70 (X 50 (X

VOC [V]

JSC [mA cm−2]

FF

PCE [%]

0.66

3.21

0.31

0.66

0.80

4.19

0.36

1.22

0.53

2.20

0.39

0.46

0.71

2.80

0.37

0.70

0.51

2.96

0.30

0.45

0.72

4.50

0.35

1.10

0.73

5.11

0.42

1.50

0.90

6.07

0.37

2.01

0.62

5.59

0.41

1.40

0.74

6.65

0.47

2.34

= 61) = 61) = 61) = 61) = 61) = 61) = 61) = 61) = 71) = 71)

a

Solar cells with Al top electrode (120 nm). bThe values are taken from ref 46. cSolar cells with MoO3 hole collecting contact instead of PEDOT:PSS. dThe data shown are averaged values. Standard deviations are typically ±0.01 for VOC and FF, between ±0.1 and ±0.3 for JSC, and between 0.05 and ±0.1 for PCE. For number of devices and more details on the standard deviations the reader is referred to Table S2 in the Supporting Information section.

account for the increase of PCE values. The utilization of MoO3 and PC71BM resulted in a further improvement of the BHJ solar cells. Based on these optimization steps, with dyes 3 and 4 as donor materials PCEs of up to 1.40% and 2.34%, respectively, were achieved (Table 3). In the standard cell architecture (hole collecting contact: PEDOT:PSS; acceptor: PC61BM), all devices based on πextended merocyanines 1−4 show significantly lower efficiencies than similar cells with their lower homologues 5−8 (see Table 3). This finding can be attributed to several factors: On the one hand, the extension of the polymethine chain by two methine groups in chromophores 1−4 leads to an increase in the HOMO energy levels by about 0.20 eV (compare Figure 2b). The resulting smaller band gap between the dyes’ HOMO levels and the LUMO level of PC61BM leads to a reduced, maximum achievable VOC, which can be nicely observed for all four dye pairs (Table 3). Thus, the raise of the HOMO energy levels upon π-extension of the polymethine chain is directly reflected in about 0.20 V reduced VOC values of fabricated solar cells with dyes 1−4 compared to those of their smaller homologues 5−8. On the other hand, next to the VOC, the JSC values of the fabricated devices are distinctly influenced by the polymethine chain length. The JSC values strongly depend on the ability of dyes to absorb light, transport the generated excitons to the heterojunction interface, and accomplish charge separation and transportation of the charge carriers to the electrodes, respectively. Despite the higher absorption densities found for 4861



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Figure 5. X-ray crystal structure analysis of (a-d) the π-extended merocyanine 1 and (e-h) its lower homologue 5. (a,e) Molecular structures in the crystal with indicated bond lengths given in 10−12 m. (b,f) Top view of the dimer synthon in the π-stack of 1 and 5 in the crystal lattice. (c,d,g,h) Different side views of the π-stack with antiparallel orientation of the chromophores (ellipsoids at the 50% probability level, H atoms and the butyl side chains on the thiophene donor moiety are not shown for clarity; gray: C, blue N, red O, yellow: S).

structural units are illustrated for the chromophore pairs 1/5 and 4/8, respectively, for comparison. In the crystal lattice of these dyes, the molecules are stacked in parallel planes. In such a stacked column of chromophores each molecule possesses one proximal and one distal antiparallel neighbor, thus forming centrosymmetric dimers with different intermolecular distances. It is notable that for the smaller homologue reference dyes 5 and 8 very similar π−πdistances (3.52 and 3.63 Å for 5; 3.48 and 3.60 Å for 8) are observed between the different dimer units in the columnar stack, while these π−π-distances significantly differ from those for π-extended dyes 1 and 4 (see Figures 5 and 6). Thus, chromophores 5 and 8 feature a compact π-stack of closely

length equilibration along the conjugated path (Figure 5a,e and Figure 6a,e). Furthermore, the chromophores exhibit a nearly coplanar π-scaffold in which the mean planes of the acceptor heterocycles are only 8° for 1, 13° for 4, 6° for 5, and 13° for 846 twisted with respect to the mean plane of the thiophene donor. As a general structural motif in the solid state packing of the investigated dyes, an antiparallel dimer unit is formed due to the strong and directional dipole−dipole interactions between the highly dipolar chromophores, which can be defined as supramolecular synthon66 of the crystal structures. In Figures 5 and 6 such dimer units and the π-stacking beyond these 4862



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Figure 6. X-ray crystal structure analysis of (a-d) the π-extended merocyanine 4 and (e-h) its lower homologue 8. The crystal structure for 8 has been reported previously.46 (a,e) Molecular structures in the crystal with indicated bond lengths given in 10−12 m. (b,f) Top view of the dimer synthon in the π-stack of 4 and 8 in the crystal lattice. (c,d,g,h) Different side views of the π-stack with antiparallel orientation of the chromophores (ellipsoids at the 50% probability level, H atoms and the butyl side chains on the thiophene donor moiety are not shown for clarity; gray: C, blue N, red O, yellow: S).

packed chromophores with a large contact area of adjacent πsystems, as also only small longitudinal displacements occur between the molecules. However, for the π-extended merocyanines 1 and 4 a larger transversal displacement to the next dimer synthon is observed, whereby the π−π-contact between adjacent molecules is reduced, despite a shorter distance between the chromophore planes. Thus, due to their better packing in the solid state with lower longitudinal and transversal offsets between adjacent chromophores, the lower homologues 5 and 8 could form more efficient percolation paths compared to those of their π-extended counterparts (1 and 4) in bulk materials. These molecular packing features in the solid state are in our opinion the most likely reason for the

inferior JSC values of the π-extended merocyanines compared to those of their lower homologues.

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CONCLUSIONS Our detailed comparative studies with four different pairs 1/5, 2/6, 3/7, and 4/8 of merocyanine dyes have shown that the elongation of the polymethine chain is a fruitful approach to improve the optical properties on the molecular level. However, despite more advantageous absorption properties of π-extended chromophores 1−4, their solar cell efficiencies are inferior to those of their lower homologues 5−8. This surprising finding can be rationalized on the basis of a reduced effective photovoltaic gap, i.e. the difference between HOMO of the donor and LUMO of the acceptor, and looser packing feature 4863



Article

(m, 4H), 1.01 (t, J = 7.3 Hz, 6H). 13C NMR (101 MHz, CD2Cl2): δ 178.8, 172.4, 171.2, 145.5, 144.3, 142.5, 133.9, 130.8, 130.2, 129.1, 128.7, 128.2, 119.42, 119.39, 117.1, 112.1, 55.5, 29.6, 20.5, 13.9. HRMS (ESI, pos. mode): m/z 472.1749 [M]+, calcd for C27H28N4S2: 472.1750. UV−vis (CH2Cl2): λmax (ε) = 764 nm (171000 L mol−1 cm−1). CHNS (%): calcd for C27H28N4S2: C, 68.61; H, 5.97; N, 11.85; S, 13.57; found: C, 68.91; H, 6.02; N, 11.77; S, 13.23. CV (CH2Cl2, 0.1 M NBu4PF6, Fc/Fc+): Epred = −1.13 V, E1/2ox = 0.20 V. Synthesis of Dye 3. Dye 3 was synthesized according to the general procedure using 300 mg (1.13 mmol) of 11 and 232 mg (1.13 mmol) of acceptor 2-(4-tert-butyl-5H-thiazol-2-ylidene)-malononitrile in Ac2O (3.5 mL). Yield: 125 mg (276 μmol, 25%), green solid. Mp.: 208−211 °C. 1H NMR (400 MHz, CD2Cl2): δ 7.69 (d, J = 13.0 Hz, 1H), 7.31 (d, J = 13.1 Hz, 1H), 7.31 (d, J = 4.8 Hz, 1H), 6.18 (t, J = 12.8 Hz, 1H), 6.17 (d, J = 4.8 Hz, 1H), 3.46 (d, J = 7.8 Hz, 4H), 1.75− 1.66 (m, 4H), 1.49 (s, 9H), 1.46−1.36 (m, 4H), 0.99 (t, J = 7.3 Hz, 6H). 13C NMR (101 MHz, CD2Cl2): δ 184.4, 178.9, 169.9, 143.5, 140.8, 128.8, 127.0, 118.5, 118.1, 116.2, 109.4, 101.4, 55.0, 38.4, 31.5, 29.6, 29.0, 20.5, 14.0. HRMS (ESI, pos. mode): m/z 452.2065 [M]+, calcd for C25H32N4S2: 452.2063. UV−vis (CH2Cl2): λmax (ε) = 753 nm (144000 L mol−1 cm−1). CV (CH2Cl2, 0.1 M NBu4PF6, Fc/Fc+): Epred = −1.22 V, E1/2ox = 0.17 V. Synthesis of Dye 4. Dye 4 was synthesized according to the general procedure using 300 mg (1.13 mmol) of 11 and 219 mg (1.13 mmol) of acceptor 2-(3-oxo-indan-1-ylidene)-malononitrile in Ac2O (3 mL). For column chromatography CH2Cl2/n-hexane = 9:1 and for recycling-HPLC CH2Cl2/n-hexane = 3:1 was used as eluent. Yield: 160 mg (362 μmol, 32%), green solid. Mp.: 203−205 °C. 1H NMR (400 MHz, CD2Cl2): δ 8.51−8.48 (m, 1H), 8.32 (d, J = 12.5 Hz, 1H), 8.05 (t, J = 12.2 Hz, 1H), 7.70−7.66 (m, 1H), 7.63−7.54 (m, 2H), 7.47 (d, J = 13.6 Hz, 1H), 7.38 (br, 1H), 6.21 (d, J = 4.8 Hz, 1H), 3.48 (t, J = 7.8 Hz, 4H), 1.76−1.67 (m, 4H), 1.47−1.37 (m, 4H), 1.00 (t, J = 7.4 Hz, 6H). 13C NMR (CD2Cl2, 101 MHz, δ): δ 190.3, 170.6, 160.4, 148.1, 146.9, 144.3, 140.5, 137.5, 133.8, 133.1, 127.1, 124.5, 122.4, 117.7, 117.21, 117.17, 109.6, 61.9, 54.9, 29.6, 20.5, 14.0. HRMS (ESI, pos. mode): m/z 441.1866 [M]+, calcd for C27H27N3OS: 441.1869. UV−vis (CH2Cl2): λmax (ε) = 691 nm (181000 L mol−1 cm−1). CHNS (%): calcd for C27H27N3OS: C, 73.44; H, 6.16; N, 9.52; S, 7.26; found: C, 73.61; H, 6.19; N, 9.53; S, 7.00. CV (CH2Cl2, 0.1 M NBu4PF6, Fc/Fc+): Epred = −1.32 V, E1/2ox = 0.27 V. Synthesis of Dye 5. A 2.0 mL solution of 5-(dibutylamino)thiophene-2-carbaldehyde (9; 120 mg, 499 μmol) and 120 mg (501 μmol) of acceptor 1-benzyl-6-hydroxy-4-methyl-2-oxo-1,2-dihydropyridine-3-carbonitrile was heated to 90 °C for 30 min. After having been cooled to room temperature, i-PrOH and n-hexane were added, and the precipitate was filtered off and washed with n-hexane. The obtained crude product was purified by column chromatography (silica gel, CH2Cl2/MeOH = 100:1). Subsequent precipitation from a CH2Cl2/n-hexane mixture afforded the pure target compound. Yield: 95 mg (206 μmol, 41%), red solid. Mp.: 202−203 °C. 1H NMR (400 MHz, CD2Cl2): δ 7.61 (s, 1H), 7.58 (d, J = 5.4 Hz, 1H), 7.40−7.36 (m, 2H), 7.32−7.19 (m, 3H), 6.41 (d, J = 5.2 Hz, 1H), 5.16 (s, 2H), 3.54 (t, J = 7.7 Hz, 4H), 2.50 (s, 3H), 1.77−1.68 (m, 4H), 1.47−1.36 (m, 4H), 0.99 (t, J = 7.3 Hz, 6H). 13C NMR (101 MHz, CD2Cl2): δ 176.6, 163.7, 162.6, 158.7, 152.8, 142.4, 138.4, 128.60, 128.59, 127.4, 124.9, 117.7, 111.4, 107.2, 94.8, 54.3, 43.1, 29.7, 20.5, 19.1, 13.9. HRMS (ESI, pos. mode): m/z 462.2208 [M+H]+, calcd for C27H32N3O2S: 462.2210. UV−vis (CH2Cl2): λmax (ε) = 540 nm (159000 L mol−1 cm−1). CV (CH2Cl2, 0.1 M NBu4PF6, Fc/Fc+): Epred = −1.66 V, E1/2ox = 0.52 V. Synthesis of Dye 6. A 6.0 mL Ac2O solution of 5-(dibutylamino)thiophene-2-carbaldehyde (9; 1.44 g, 6.00 mmol) and 1.35 g (6.00 mmol) of acceptor 2-(4-phenyl-5H-thiazol-2-ylidene)-malononitrile was heated to 90 °C for 30 min. The precipitate was filtered of and washed with isopropyl alcohol and n-hexane. Yield: 1.66 g (3.72 mmol, 62%), green solid. Mp.: 280−283 °C. 1H NMR (600 MHz, DMSOd6): δ 8.03 (d, J = 5.3 Hz, 1H), 7.98 (s, 1H), 7.70−7.66 (m, 2H), 7.60−7.54 (m, 3H), 6.97 (d, J = 5.3 Hz, 1H), 3.72 (t, J = 7.5 Hz, 4H), 1.76−1.69 (m, 4H), 1.43−1.35 (m, 4H), 0.96 (t, J = 7.4 Hz, 6H). 13C NMR (151 MHz, DMSO-d6): δ 177.3, 175.7, 169.7, 150.7, 133.7,

of chromophores 1−4 compared to 5−8 resulting in a decreased VOC and JSC, respectively. Nevertheless, based on thorough optimization using MoO3 as interlayer and PC71BM as an acceptor, a power conversion efficiency of 2.3% could be achieved for the π-extended dye 4, which renders these new materials promising complementary NIR absorbers for tandem solar cells. Further investigations in this direction are underway.

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EXPERIMENTAL SECTION

Synthesis of (E)-3-(5-(Dibutylamino)-thiophene-2-yl)-acrylaldehyde (11). To a cooled (ice bath) suspension of (1,3-dioxolan-2ylmethyl)-triphenylphosphonium bromide (10; 2.39 g, 5.57 mmol) in THF (20 mL) was added potassium tert-butoxide (605 mg, 5.39 mmol) under magnetical stirring. After 30 min, a solution of 5(dibutylamino)-thiophene-2-carbaldehyde (9; 500 mg, 2.09 mmol) in THF (50 mL) was added dropwise over a period of 10 min. The resulting solution was stirred overnight at 25 °C and subsequently hydrolyzed by addition of aqueous oxalic acid (4 g in 40 mL of H2O). After having been stirred for an additional 24 h, the mixture was extracted with diethyl ether (3 × 50 mL). The combined organic extracts were washed with saturated aqueous NaHCO3 and dried over sodium sulfate. The solvent was removed by distillation under reduced pressure, and the obtained crude product was purified by column chromatography (silica gel, n-hexane/ethyl acetate = 1:1). Yield: 519 mg (1.96 mmol, 94%), yellow oil. 1H NMR (400 MHz, CD2Cl2): δ 9.39 (d, J = 7.9 Hz, 1H), 7.39 (d, J = 14.8 Hz, 1H), 7.10 (d, J = 4.3 Hz, 1H), 5.97 (dd, J = 14.8 Hz, J = 7.7 Hz, 1H), 5.85 (d, J = 4.3 Hz, 1H), 3.31 (t, J = 7.7 Hz, 4H), 1.68−1.59 (m, 4H), 1.42−1.32 (m, 4H), 0.96 (t, J = 7.4 Hz, 6H). 13C NMR (101 MHz, CD2Cl2): δ 192.0, 163.8, 146.0, 137.4, 121.6, 119.7, 102.9, 53.8, 29.5, 20.6, 14.0. HRMS (ESI, pos. mode): m/z 264.1419 [M−H]+, calcd for C15H22NOS: 264.1417. General Procedure for the Synthesis of π-Extended Merocyanine Dyes 1−4. A solution of (E)-3-(5-(dibutylamino)thiophene-2-yl)-acrylaldehyde (11) in acetic anhydride and one equivalent of the respective acceptor building block 1-benzyl-6hydroxy-4-methyl-2-oxo-1,2-dihydropyridine-3-carbonitrile,67 2-(4phenyl-5H-thiazol-2-ylidene)-malononitrile,68 2-(4-tert-butyl-5H-thiazol-2-ylidene)-malononitrile,46 or 2-(3-oxo-indan-1-ylidene)-malononitrile69 was heated to 90 °C for 1 h. After having been cooled to room temperature, the solvent was distilled off under reduced pressure. The obtained crude product was prepurified by column chromatography (silica gel, CH2Cl2/MeOH = 99:1). The analytically pure target compounds were obtained after recycling-HPLC purification (normal phase column, CH2Cl2/MeOH = 99:1) and subsequent precipitation from a CH2Cl2/n-hexane mixture. Synthesis of dye 1. Dye 1 was synthesized according to the general procedure using 300 mg (1.13 mmol) of 11 and 270 mg (1.13 mmol) of acceptor 1-benzyl-6-hydroxy-4-methyl-2-oxo-1,2-dihydropyridine-3carbonitrile in Ac2O (3 mL). Yield: 168 mg (344 μmol, 30%), blue solid. Mp.: 209−210 °C. 1H NMR (400 MHz, CD2Cl2): δ 8.00 (br, 1H), 7.45−7.33 (m, 5H), 7.31−7.26 (m, 2H), 7.24−7.19 (m, 1H), 6.24 (d, J = 4.9 Hz, 1H), 5.14 (s, 2H), 3.48 (t, J = 7.8 Hz, 4H), 2.46 (s, 3H), 1.75−1.66 (m, 4H), 1.46−1.36 (m, 4H), 0.98 (t, J = 7.4 Hz, 6H). 13 C NMR (101 MHz, CD2Cl2): δ 171.5, 163.7, 162.6, 158.2, 152.2, 149.5, 144.8, 138.7, 128.6, 128.3, 127.3, 118.8, 117.7, 112.0, 110.2, 95.4, 55.0, 43.2, 29.6, 20.5, 19.0, 13.9. HRMS (ESI, pos. mode): m/z 487.2286 [M]+, calcd for C29H33N3O2S: 487.2288. UV−vis (CH2Cl2): λmax (ε) = 656 nm (219000 L mol−1 cm−1). CHNS (%): calcd for C29H33N3O2S: C, 71.43; H, 6.82; N, 8.62; S, 6.58; found: C, 71.81; H, 6.88; N, 8.71; S, 6.25. CV (CH2Cl2, 0.1 M NBu4PF6, Fc/Fc+): Epred = −1.36 V, E1/2ox = 0.24 V. Synthesis of Dye 2. Dye 2 was synthesized according to the general procedure using 430 mg (1.62 mmol) of 11 and 365 mg (1.62 mmol) of acceptor 2-(4-phenyl-5H-thiazol-2-ylidene)-malononitrile in Ac2O (3.5 mL). Yield: 148 mg (313 μmol, 19%), green solid. Mp.: 238−240 °C. 1H NMR (400 MHz, CD2Cl2): δ 7.71−7.67 (m, 2H), 7.54−7.45 (m, 3H), 7.36 (d, J = 12.7 Hz, 1H), 7.32 (d, J = 4.7 Hz, 1H), 7.22 (d, J = 13.3 Hz, 1H), 6.23 (d, J = 4.7 Hz, 1H), 6.23 (dd, J = 13.3 Hz, J = 12.7 Hz, 1H), 3.49 (t, J = 7.6 Hz, 4H), 1.78−1.68 (m, 4H), 1.48−1.38 4864



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(14) Zhou, H.; Yang, L.; You, W. Macromolecules 2012, 45, 607. (15) Facchetti, A. Chem. Mater. 2011, 23, 733. (16) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Nat. Photonics 2012, 6, 591. (17) Liao, S.-H.; Jhuo, H.-J.; Cheng, Y.-S.; Chen, S.-A. Adv. Mater. 2013, 25, 4766. (18) Walker, B.; Kim, C.; Nguyen, T. Q. Chem. Mater. 2011, 23, 470. (19) Mishra, A.; Bäuerle, P. Angew. Chem., Int. Ed. 2012, 51, 2020. (20) Lin, Y.; Li, Y.; Zhan, X. Chem. Soc. Rev. 2012, 41, 4245. (21) Chen, Y.; Wan, X.; Long, G. Acc. Chem. Res. 2013, 46, 2645. (22) Fitzner, R.; Mena-Osteritz, E.; Mishra, A.; Schulz, G.; Reinold, E.; Weil, M.; Körner, C.; Ziehlke, H.; Elschner, C.; Leo, K.; Riede, M.; Pfeiffer, M.; Uhrich, C.; Bäuerle, P. J. Am. Chem. Soc. 2012, 134, 11064. (23) Li, Z.; He, G.; Wan, X.; Liu, Y.; Zhou, J.; Long, G.; Zuo, Y.; Zhang, M.; Chen, Y. Adv. Energy Mater. 2012, 2, 74. (24) Roquet, S.; Cravino, A.; Leriche, P.; Aleveque, O.; Frere, P.; Roncali, J. J. Am. Chem. Soc. 2006, 128, 3459. (25) Shang, H.; Fan, H.; Liu, Y.; Hu, W.; Li, Y.; Zhan, X. Adv. Mater. 2011, 23, 1554. (26) Rousseau, T.; Cravino, A.; Ripaud, E.; Leriche, P.; Rihn, S.; De Nicola, A.; Ziessel, R.; Roncali, J. Chem. Commun. 2010, 46, 5082. (27) Bura, T.; Leclerc, N.; Fall, S.; Lévêque, P.; Heiser, T.; Retailleau, P.; Rihn, S.; Mirloup, A.; Ziessel, R. J. Am. Chem. Soc. 2012, 134, 17404. (28) Walker, B.; Tamayo, A. B.; Dang, X. D.; Zalar, P.; Seo, J. H.; Garcia, A.; Tantiwiwat, M.; Nguyen, T. Q. Adv. Funct. Mater. 2009, 19, 3063. (29) Mei, J.; Graham, K. R.; Stalder, R.; Reynolds, J. R. Org. Lett. 2010, 12, 660. (30) Chen, Y.-H.; Lin, L.-Y.; Lu, C.-W.; Lin, F.; Huang, Z.-Y.; Lin, H.-W.; Wang, P.-H.; Liu, Y.-H.; Wong, K.-T.; Wen, J.; Miller, D. J.; Darling, S. B. J. Am. Chem. Soc. 2012, 134, 13616. (31) Leliège, A.; Régent, C.-H. L.; Allain, M.; Blanchard, P.; Roncali, J. Chem. Commun. 2012, 48, 8907. (32) Zhou, J.; Wan, X.; Liu, Y.; Zuo, Y.; Li, Z.; He, G.; Long, G.; Ni, W.; Li, C.; Su, X.; Chen, Y. J. Am. Chem. Soc. 2012, 134, 16345. (33) Zhou, J.; Zuo, Y.; Wan, X.; Long, G.; Zhang, Q.; Ni, W.; Liu, Y.; Li, Z.; He, G.; Li, C.; Kan, B.; Li, M.; Chen, Y. J. Am. Chem. Soc. 2013, 135, 8484. (34) Martinez-Diaz, M. V.; de la Torre, G.; Torres, T. Chem. Commun. 2010, 46, 7090. (35) Meiss, J.; Holzmueller, F.; Gresser, R.; Leo, K.; Riede, M. Appl. Phys. Lett. 2011, 99, 193307. (36) Leblebici, S. Y.; Catane, L.; Barclay, D. E.; Olson, T.; Chen, T. L.; Ma, B. ACS Appl. Mater. Interfaces 2011, 3, 4469. (37) Silvestri, F.; Irwin, M. D.; Beverina, L.; Facchetti, A.; Pagani, G. A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 17640. (38) Mayerhöffer, U.; Deing, K.; Gruss, K.; Braunschweig, H.; Meerholz, K.; Würthner, F. Angew. Chem., Int. Ed. 2009, 48, 8776. (39) Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R. Adv. Energy Mater. 2011, 1, 184. (40) Bouit, P.-A.; Rauh, D.; Neugebauer, S.; Delgado, J. L.; Piazza, E. D.; Rigaut, S.; Maury, O.; Andraud, C.; Dyakonov, V.; Martin, N. Org. Lett. 2009, 11, 4806. (41) Véron, A. C.; Zhang, H.; Linden, A.; Nüesch, F.; Heier, J.; Hany, R.; Geiger, T. Org. Lett. 2014, 16, 1044. (42) Takacs, C. J.; Sun, Y.; Welch, G. C.; Perez, L. A.; Liu, X.; Wen, W.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2012, 134, 16597. (43) Coughlin, J. E.; Henson, Z. B.; Welch, G. C.; Bazan, G. C. Acc. Chem. Res. 2014, 47, 257. (44) Kronenberg, N. M.; Deppisch, M.; Würthner, F.; Lademann, H. W. A.; Deing, K.; Meerholz, K. Chem. Commun. 2008, 6489. (45) Steinmann, V.; Kronenberg, N. M.; Lenze, M. R.; Graf, S. M.; Hertel, D.; Meerholz, K.; Bürckstümmer, H.; Tulyakova, E. V.; Würthner, F. Adv. Energy Mater. 2011, 1, 888. (46) Bürckstümmer, H.; Tulyakova, E. V.; Deppisch, M.; Lenze, M. R.; Kronenberg, N. M.; Gsänger, M.; Stolte, M.; Meerholz, K.; Würthner, F. Angew. Chem., Int. Ed. 2011, 50, 11628.

132.8, 130.1, 129.5, 128.6, 128.5, 126.4, 120.7, 115.1, 54.6, 47.2, 28.6, 19.1, 13.1. HRMS (ESI, pos. mode): m/z 446.1589 [M]+, calcd for C25H26N4S2: 446.1593. UV−vis (CH2Cl2): λmax (ε) = 659 nm (131000 L mol−1 cm−1). CHNS (%): calcd for C25H26N4S2: C, 67.23; H, 5.87; N, 12.54; S, 14.36; found: C, 67.10; H, 5.84; N, 12.57; S, 14.53. CV (CH2Cl2, 0.1 M NBu4PF6, Fc/Fc+): Epred = −1.27 V, E1/2ox = 0.41 V.

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ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR spectra; cyclic voltammograms; determination of transition dipole moment μag; DFT calculations; EOA measurements; crystal structure determination; solar cell device fabrication and characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.M.). *E-mail: [email protected] (F.W.). Present Address

§ School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, Scotland.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Hannah Bürckstümmer and Dr. Elena Tulyakova for the synthesis of merocyanine dyes 7 and 8. Financial support by the DFG within the priority program “Elementary Processes of Organic Photovoltaic” (WU 317/10 and ME 1246/19) and the BMBF within the MEDOS project (FKZ: 03EK3503C) is gratefully acknowledged.

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ABBREVIATIONS PCE, power conversion efficiency; BHJ, bulk heterojunction; EOA, electro-optical absorption; DPP, diketopyrrolopyrrole; JSC, short-circuit current density; NIR, near-infrared; HRMS, high resolution mass spectrometry; fwhm, full-width at halfmaximum; FMO, frontier molecular orbital; VOC, open-circuit voltage; FF, fill factor

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REFERENCES

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