Influence of Merocyanine Molecular Dipole Moments on the Valence

Article pubs.acs.org/JPCC

Influence of Merocyanine Molecular Dipole Moments on the Valence Levels in Thin Films and the Interface Energy Level Alignment with Au(111) Stefan Krause,*,† Matthias Stolte,‡ Frank Würthner,‡ and Norbert Koch†,§ †

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Bereich Solarenergieforschung, Albert-Einstein-Strasse 15, 12489 Berlin, Germany ‡ Institut für Organische Chemie and Center for Nanosystems Chemistry, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany § Institut für Physik, Humboldt-Universität zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany S Supporting Information *

ABSTRACT: We study the effect of increasing permanent dipole moment of four dipolar merocyanine dyes on the energy level alignment with Au(111) and the energy levels in thin films with ultraviolet (UV) photoemission spectroscopy. Preceding X-ray diffraction experiments showed that these dipolar molecules form centrosymmetric dimeric units in their bulk crystal structure. The effect of dimerization was also seen in a splitting of the highest occupied level of bilayers and thin films of all four molecules. This split into two components due to intermolecular electronic coupling between two merocyanines is estimated to be 0.2−0.3 eV. Monolayer films on Au(111) did not exhibit such splitting due to a different adsorption structure. A significant ultraviolet radiation sensitivity of the dyes near the metal surface was noticed. Unlike other radiation-sensitive compounds, the reason for molecule damage is related to the secondary electrons produced in the metal substrate during photoelectron spectroscopy measurements and not to the UV radiation itself.

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INTRODUCTION Merocyanine dyes belong to the class of push−pull chromophores1 with a donor (D) and an acceptor (A) moiety connected by a π-conjugated bridge. This architecture gives rise to a permanent ground-state dipole moment as well as a strong absorbance in the visible region. Such dipolar molecules with low molecular symmetry are generally avoided in organic semiconductor research as low charge carrier mobility is then assumed in thin films.2 However, recently, Huang et al. reported hole mobility values of up to 0.18 cm2 V−1 s−1 in wellordered thin films of highly dipolar merocyanines similar to 4 (chemical structure shown in Figure 1) in organic thin-film transistors (OTFTs).3 Furthermore, the high absorption cross section in the visible make merocyanines good electron-donor candidates in solutionas well as vacuum-processed bulkheterojunction solar cells with, for example, phenyl-C61-butyric acid methyl ester (PCBM) or C60 as acceptor. The chemical structures of the merocyanine molecules discussed in this work (1−4) are shown in Figure 1. For 3 and derivatives thereof, high power conversion efficiencies (PCEs) of up to 6.1% were reported for solution- and vacuum-processed solar cells.4−6 Further device concepts are currently being discussed since the absorption range of merocyanines can easily be tailored to cover the entire visible spectral range up to the near-infrared7 with a combination of several merocyanine dyes, or one may © 2013 American Chemical Society

even use their tendency to align their dipoles for the formation of supramolecular structures.8 However, crucial information for a reliable design of photovoltaic cells or thin-film transistors with these molecules, such as the energy position of the highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) levels with respect to electrodes, is so far mostly estimated from solutionbased methods, such as cyclic voltammetry (CV) or ultraviolet−visible spectroscopy (UV−vis). Therefore, the measured energy levels are those of individual molecules rather than those in thin films. Furthermore, the sample work function (Φ) and possible structure-dependent ionization potential (IP) and electron affinity (EA) values9,10 are not accessible via these methods. For a systematic approach to study the influence of the dipolar character and low symmetry of merocyanines on their IP, EA, and Φ in contact with a Au(111) model electrode, the set 1−4 with increasing dipole moment was chosen. Their structure as well as their experimentally determined dipole moments can be found in Figure 1. They all consist of the same indoline electron-donor group (+) with varying electronReceived: June 19, 2013 Revised: August 7, 2013 Published: August 8, 2013 19031

dx.doi.org/10.1021/jp4060395 | J. Phys. Chem. C 2013, 117, 19031−19037

The Journal of Physical Chemistry C

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Photoelectron spectroscopy was performed in an Omicron Multiprobe system with separate preparation and analysis chambers. It features a SPECS Phoibos 100 electron energy analyzer and a UV lamp for our ultraviolet photoelectron spectroscopy (UPS) measurements. To measure the secondary electron cutoff (SECO), a bias voltage of −10.00 V was applied to the sample with a stabilized voltage supply. In UPS, we give an error bar of ca. 30 meV for relative energy positions, as estimated from a fitting routine of a straight line through the inflection point of a valence peak or SECO (described in ref 15). However, for absolute values, we estimate the error to be 100 meV due to the uncertainty of exactly locating a peak onset (as described in detail in ref 9). The instrumental resolution is approximately 150 meV, as measured at the Fermi edge of a clean gold sample. The roughness of the substrates used, Au(111) single crystals, is a rather local parameter on the many-micrometer scale, and varies between many 100 nm wide atomically smooth terraces to a few nanometer corrugation at step-bunches (as confirmed by STM measurements of the bare substrates). Because the surface area of terraces outweighs that of steps and step-bunches by far, we expect no significant influence of these on molecular order. Thin films of 1−4 were produced on sputter-cleaned and annealed Au(111) via organic molecular beam deposition with a temperature-controlled commercial triple evaporation source under ultra-high-vacuum conditions and the substrate at room temperature. The deposition rate was controlled with a quartz microbalance for thickness monitoring and was set to ca. 1 Å/ min. The base pressure of the system was better than 5 × 10−10 mbar. Ground-state dipole moments (μg) of chromophores were determined by means of electrooptical absorption (EOA) spectroscopy of 10−6 mol/L 1,4-dioxane solutions. By this method, the difference of absorption of a solution with and without an externally applied electric field E is measured with light polarized parallel and perpendicular to the direction of E.16,17 The dichroism induced in a solution by both an alternating and a constant electric field of about 3·106 V m−1 depends on the orientational order of the molecules due to their μg, the shift of the absorption band [proportional to the dipole moment difference Δμ = μe − μg (μe: excited-state dipole moment)], and on the electric field dependence of the electric transition dipole moment μeg(E). Cyclic voltammetry (CV) was performed with a standard commercial electrochemical analyzer (EC epsilon; BAS Instrument, U.K.) in a three-electrode single-compartment cell under argon. Dichloromethane (HPLC grade) was obtained from J. T. Baker (Mumbai, India) and dried over calcium hydride and degassed prior to use. The supporting electrolyte tetrabutylammonium hexafluorophosphate (TBAHFP) was synthesized according to the literature,18 recrystallized from ethanol/water, and dried in high vacuum. The measurements were carried out at a concentration of 10−4 mol/L with ferrocene as an internal standard for the calibration of the working electrode potential. The setup consists of a working electrode (Pt disc), a reference electrode (Ag/AgCl), and an auxiliary electrode (Pt wire). HOMO levels were calculated from CV measurements at scan rates of 100 mV/s (half-wave potential E1/2ox for reversible processes and peak potentials Ep for irreversible processes) calibrated against the ferrocene/ferrocenium couple (Fc/Fc+, −5.15 eV)19,20 as an internal standard.

Figure 1. Structures of the four investigated merocyanine dyes. All have the same donor part with varying acceptor moieties. Their ground-state dipole moments, as determined by electrooptical absorption spectroscopy, are given below the structures. The investigated molecules are the following: 1: 2-[2-(1-butyl-3,3dimethyl-1,3-dihydro-indol-2-ylidene)-ethylidene]-indan-1,3-dione; 2: 2-{2-[2-(1-butyl-3,3-dimethyl-1,3-dihydro-indol-2-ylidene)-ethylidene]-3-oxo-indan-1-ylidene}-malononitrile; 3: 1-butyl-5-[2-(1-butyl3,3-dimethyl-1,3-dihydro-indol-2-ylidene)-ethylidene]-4-methyl-2,6dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile; and 4: 2-{4-tert-butyl5-[2-(1-butyl-3,3-dimethyl-1,3-dihydro-indol-2-ylidene)-ethylidene]5H-thiazol-2-ylidene}-malononitrile.

acceptor moieties (−) (see Figure 1). Their ground-state dipole moment was determined by electrooptical absorption spectroscopy in 1,4-dioxane at 298 K. Next to the molecular structures, the ground-state dipole moment is given in Figure 1, covering a wide range from 3.8 to 13.3 D. Au(111) was used as the model electrode due to its well-ordered surface and low chemical reactivity that allows one to study thin films with minimal influence by the substrate. Gold is also a commonly used electrode material in OTFTs and inverted photovoltaic cells, such as dye-sensitized solar cells, for which the merocyanines are considered good candidates.11 In this work, we investigate the energy level alignment of 1− 4 with Au(111) and the valence electronic structure as a function of film thickness by ultraviolet photoelectron spectroscopy (UPS). One focus lies on the effect of dimerization of the molecules in their bulk crystalline structure as found in preceding X-ray diffraction measurements.3,6,12 We derive a good estimate of the magnitude of the HOMO splitting due to the intermolecular electronic coupling between the two adjacent merocyanines in a dimeric unit found in the bilayer and the thin-film spectra.

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EXPERIMENTAL SECTION The syntheses and basic characterization of 25 and 313,14 were reported previously. The synthetic procedures and characterization of the merocyanine dyes 1 and 4 are given in the Supporting Information. UV−vis measurements of thin films were conducted with a PerkinElmer Lambda 950 spectrometer. Samples for UV−vis studies were prepared via spin-coating from chloroform solution onto a quartz substrate. For comparison, optical gaps for the molecules in 10−5 mol/L solution were also obtained in dichloromethane as well as in 1,4-dioxane with no apparent differences. 19032



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RESULTS AND DISCUSSION A molecular layer thickness-dependent series of UPS spectra were recorded for 1−4 in order to assess the energy level alignment at the interface to Au(111) as well as the electronic structure of thick films. For 1−3, it was possible to grow closed multilayer films with the substrate at room temperature (RT), which was deduced from the continuous attenuation of the substrate signal until its disappearance around the nominal two to three layer coverage. A detailed investigation with X-ray photoelectron spectroscopy (XPS) to obtain a more precise picture of the growth mode was not possible due to the observed severe radiation damage of the merocyanines by the X-rays. 4 grew in a layer-plus-island fashion, which was deduced from the fact that the gold valence states are only weakly attenuated with higher film thickness and the quartz microbalance continued to show a deposition rate. Our first observation in the thickness-dependent valencelevel spectra was an energy shift between the HOMO of the first and the second layer. This shift occurred for all molecules and was on the order of 0.3−0.5 eV. As an example, for 3, the spectra of the (nominal) monolayer, bilayer, and multilayer films are shown in Figure 2a. A clear shift toward higher

binding energy between the monolayer and the multilayer peak maxima is visible. The 9 Å spectrum even exhibits a double peak structure, that is, a superposition of contributions from the monolayer and multilayer, with the same maxima positions as monolayer and multilayer. This energy shift was also found for the other molecules (Figure 2b). Its magnitude is very similar for 4 (∼0.5 eV) and less for 1 and 2 (∼0.3 eV). One origin of this shift is screening of photoelectrons by the metal substrate, which depends on the distance of the molecule from the metal. However, screening-related shifts typically appear smooth in thickness-dependent studies with a magnitude of 0.2−0.4 eV, as reported in the literature,21,22 which is not sufficient, at least for 3 and 4. A second possible mechanism is a different orientation of molecules in the monolayer as opposed to multilayers, which most likely involves dimerization of molecules with an antiparallel orientation of their dipoles. X-ray diffraction measurements3,12,23 on merocyanine crystals support this supposition. The driving force for this is the electrostatic interaction between the two molecular dipole moments that want to minimize the resulting dipole in their dimer. This means that the resulting dipole moment of the dimer will be very low or even zero. Such a dimerization will lead to an intermolecular electronic coupling of the two molecules and to a split of the electronic states, which was reported for other molecular semiconductors based on measurements as well as modeling before.24−27 The magnitude of this splitting depends on molecular distance and position with respect to each other. Furthermore, an associated orientational change of the molecule can also account for the observed energy shift between mono- and multilayer.10 Therefore, a dimerization that starts in the second layer, most likely including an orientation change, would satisfactorily explain the observed abrupt energy shift between mono- and multilayer. More insight into the dimer formation and hints for the orientation of the molecules can be gained from an analysis of the peak full width at half-maximum (FWHM, width in the following) of the HOMO for the different thicknesses. The multilayer width is rather large with 0.6 eV compared to the monolayer width of 0.3−0.4 eV, and the peak maxima appear flat. Following the proposed splitting of the HOMO due to the formation of molecular dimers with antiparallel dipole orientation, we fitted these spectral features by two peaks to quantify the magnitude of the splitting. A set of two peaks with reasonable widths of 0.45−0.50 eV (compare refs 9, 28) in a distance of ca. 0.3 eV perfectly reproduce these HOMO features. As an example, the HOMO feature of 3 (18 Å

Figure 2. (a) Valence-level spectra for 3, 9, and 18 Å of 3 on Au(111). These spectra correspond to about one monolayer (3 Å), about two ML (9 Å), and a multilayer film of approximately 4−5 layers (18 Å). A clear energy shift between the first and the second layer is visible, which could also be found for all other molecules. Additionally, two possible subpeaks were fitted to the 18 Å HOMO with a width of 0.45 eV and a distance of 0.28 eV (in red for better visibility). (b) The HOMO region of 1−4 for a nominal coverage of 1−3 layers (except 60 Å for 4) in which two features are distinguishable. The separation of the two peaks is noted for each molecule.

Table 1. Summary of Solution and Thin-Film Parameters Measured for the Four Investigated Merocyaninesb Φ/eV IPUPS/eV EGap, opt, s/eV E1/2/mV (ox./red) IPCV/V EGap, opt, l/eV DAu(111)/eV shiftfirst to second layer/eV shiftML to multilayer/eV

1

2

3

4

4.32 5.3 2.24 607a/−1935 5.76 2.50 1.08 0.34 0.64

4.43 5.4 1.90 648/−1515 5.80 2.15 0.97 0.28 0.49

4.15 5.3 2.09 673/−1748 5.82 2.35 1.25 0.47 0.47

4.25 4.9 1.75 434/−1421 5.58 1.99 1.15 0.53 0.53

a

Peak potentials Ep for irreversible process. bThe Au(111) work function was measured to be 5.40 eV on a freshly cleaned crystal. The molecules are ordered from low to high dipole moment from left to right (s: solid = thin film; l: liquid = in solution). 19033



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coverage) fitted with these parameters is shown in Figure 2a (fitted signals in red). Overall, we thus estimate the dimerization-related HOMO splitting to be approximately 0.2−0.3 eV. However, without precise structural data, we cannot exclude peak broadening by film inhomogeneity,9,28 and the estimate rather reflects the upper limit. From the small HOMO width of the first layer, we can, therefore, deduce that the molecules do not yet dimerize. The work function change (ΔΦ) of 1.0−1.3 eV does not show a dependence on the dipole moment of the molecules (compare Table 1). Since a dimerization in the first layer would also explain such a dipole moment independence, we need both facts to propose that the first layer lies flat on the gold surface; that is, the dipole moments cannot contribute to ΔΦ. The magnitude of this work function change is close to reported values of 1 eV for, for example, pentacene/Au,29 that can be explained by a simple “push-back” of the electron density above the gold surface. The peak width analysis corroborates and extends the model to lying molecules in the first layer and formation of dimers, including orientational change in the subsequent layers. The thin film (ca. 30 Å nominal coverage) valence spectra and corresponding SECO spectra (including work function values) of all four merocyanines are shown in Figure 3. The

acceptor strength, which also correlates with the magnitude of the molecular dipole moment. The detailed evolution of Φ and HOMO-onset position with nominal film thickness for all four molecules is summarized in Figure 4. Please note that the closed circles represent data of

Figure 4. Work function (Φ) and HOMO position for merocyanines 1−4 depending on the thin film thickness on Au(111). Zero thickness corresponds to the clean Au(111) with a work function of 5.40 eV. The full circles correspond to measurements with optimal He I conditions in the lamp, whereas the open circles are data points measured with a filter in front of the lamp (attenuation factor of 100). The HOMO scale is about half the energy width of the Φ scale.

measurements taken with a standard UV lamp, and the open circles were measured with an attenuation filter (factor 100) in front of the lamp on separate samples. A clear difference between these measurements can be seen beyond the monolayer (at ca. 3−4 Å). Whereas the data points measured with standard high photon f lux (marked “w/o filter”) continue shifting sturdily with nominal film thickness, the ones with the 100 times lower photon f lux (marked “w/filter”) exhibit a smaller shift, which also clearly saturates at a film thickness of about 30 Å. 1 and 2 show an additional shift of 0.2−0.3 eV for the third layer. 3 and 4 complete their energy shift after the second layer (measured with filter), which can be seen by comparing the shift of second to monolayer to the one of mono- to multilayer in Table 1. Since the photon-flux-dependent shifts in Figure 4 appear to be thickness-dependent, we additionally investigated the UV radiation exposure time dependence. This is necessary to distinguish between charging and concomitant sample damage in poorly conducting films and a preferential alignment of molecular dipoles building up an electric field within the film. For instance, continuous thickness-dependent shifts induced by, even minimal, preferential dipole orientation were reported for Alq3 by Ishii et al.30 We measured the SECO before and after 15 min of UV exposure. The SECO was chosen due to its high signal intensity and thus short data acquisition time for a better temporal resolution. Figure 5 shows the SECO for a fresh 50 Å thin film of 3 on Au(111), which shows a distinct shift of 140 meV toward lower kinetic energy after 15 min of exposure to UV radiation without the attenuation filter. Subsequently, the filter was inserted and a clear shift back to higher kinetic energy by 50 meV occurred (gray curve). Furthermore, the SECO became steeper and less broad. This behavior is clearly related to radiation damage and charging (plus damage-induced charging) of the thin films and not to preferential dipole orientation.

Figure 3. Valence band spectra and secondary electron cutoff (SECO) for 1−4. For 4, the 60 Å spectra was chosen to see the multilayer HOMO and still contains the Au valence states.

spectra for 1−3 are very similar, whereas the one of 4 still contains gold substrate contributions, as evidenced by the clearly visible Fermi edge. The spectral resemblance is expected and originates from the molecular structure. As mentioned above, all molecules have the same donor moiety, and one known property of such push−pull molecules is that the HOMO is more localized on the donor part, at least for the less polar derivatives.1 The work function (Φ) of pristine Au(111) (5.40 eV) was significantly reduced by 1.0−1.3 eV upon merocyanine deposition. The magnitude of the Φ reduction did not show a dependence on the dipole moment. Thus, the resulting IP values (obtained by adding the low binding energy HOMO-onset position to Φ; summarized in Table 1) are rather similar as well. For the optical gap (measured by UV− vis, Table 1) of thin films on quartz, we see a reasonable trend with increasing acceptor strength; that is, the stronger the acceptor, the smaller the optical gap. As the low binding energy HOMO-onset is essentially the same for all four merocyanine multilayer films on Au(111), this implies that the LUMO level moves closer to the substrate Fermi level with increasing 19034



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binding energy, which can range from 0.1 to 1.0 eV,9,36,37 depending on the specific material under consideration. Returning to the IP values, we note that there are various ways of deducing the HOMO level energy from CV data, which depend on the value used for the ferrocene standard (e.g., ref 38). There are cases where IP values from UPS and CV experiments agree fairly well (e.g., D’Andrade et al.39); however, molecular-orientation-dependent IP changes10 can never be inferred from CV. Because of these discrepancies in calculating IP values from CV data, the original E1/2/*Ep (E1/2 in the following) values are also given in Table 1. Not predictable from individual material parameters is the actual energy level alignment at the molecule/metal interface. Molecule-deposition-induced Φ changes (ΔΦ) shift the molecular levels (e.g., HOMO and LUMO) with respect to the metal Fermi level. As mentioned above, no apparent relation between molecular dipole and ΔΦ (see Table 1) was found in our work. We note in the context of device fabrication that ΔΦ values strongly depend on the detailed structure and composition of the electrode (crystal face, morphology, contamination) as exemplified by Hwang et al.37 Therefore, the values reported here for Au(111) are rather the upper limit for ΔΦ on other gold electrodes.

Figure 5. SECO of a 50 Å thick film of 3 on Au(111) measured at three different times. Freshly prepared film, then after the VB was measured for about 15 min with a shift of 140 meV to lower KE, and finally with the factor 100 filter in the UV light beam and a shift of 50 meV back to higher KE.

Noteworthy, no such shifts were found for 1−4 deposited on indium tin oxide and TiO2, even for films as thick as 130 Å.31 This leads us to the conclusion that charging due to poor thinfilm conductivity as well as UV radiation damage intrinsic to merocyanines32,33 does not play a role, as we find the SECO shifts only for the metal substrate. The underlying mechanism, therefore, has to be linked to the Au substrate, which is known to have high photoionization cross sections for its valence states in the UV regime that we use for UPS. The Au 5d states photoionization cross section is about 10 times higher than, for example, that of the Ti 3d states for He I light.34 Many organic compounds are sensitive to lowenergy electrons, which makes experimental techniques, such as inverse photoemission spectroscopy (IPES) or low-energy electron diffraction (LEED), so challenging. Therefore, we propose that the photoelectrons emitted from the metal substrate into the organic film are responsible for the radiation intensity and exposure time dependence of the SECO shifts (as exemplarily shown in Figure 5). The metal photoelectrons induce radiation damage in the merocyanines, which consequently exhibit increased charging during UPS measurements. Sanche35 reported an elaborate study on the effect of low-energy electrons on organic compounds. In this publication, it was shown that the compounds decompose and negatively charged ions may leave the film, which might also contribute to positive charging of the thin film. Besides radiation intensity dependence, no aging effects were observed during the experiments, that is, on the time scale of a few hours. Annealing effects were not studied on purpose, because the weak interaction of all compounds with the inert Au(111) surface most often leads to film dewetting upon annealing, which would render the experiments not very useful. In a previous study of these materials, annealing led to a significant increase of the crystallinity in films; however, severe roughening of the surface was observed at the same time.3 All key parameters obtained from our experiments are summarized in Table 1. Systematic differences between data obtained for thin films [by UV−vis (s) and UPS] compared to solution-based methods [CV and UV−vis (l)] persist, asin partexpected. The IP values in the solid (UPS) are lower by 0.4−0.6 eV than in the liquid (CV), and the optical gaps are smaller as well (by ∼0.25 eV). We refrain from estimating the electron affinity of the merocyanines, as subtracting the optical gaps from IP values must still be corrected by the exciton

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CONCLUSIONS Four merocyanine dyes with increasing permanent dipole moment were investigated on Au(111). We correlated molecular geometry of the same donor moiety in the push− pull compound with the small differences in the valence-level shape. The HOMO levels were all found between 1.0 and 1.2 eV relative to the gold Fermi level. The HOMO binding energy of 1 and 2 shifts toward higher binding energy by 0.2−0.3 eV up to a nominal film thickness of ca. 30 Å. This was explained by a thickness-dependent change in molecular orientation from flat-lying to vertically inclined. For merocyanines 1−4, the molecular dipole moment increases, and the optical gap decreases accordingly with increasing strength of the acceptor subunit. The HOMO staying almost at the same energy with respect to the substrate Fermi level means that the LUMO is moving closer to it with increasing acceptor strength. The second observation is that all molecules tend to form dimers with opposing dipole direction from the bilayer onward. Two distinct HOMO peaks were found that showed a shift of 0.3−0.5 eV between the first and the second layer. The huge width of the multilayer HOMO peak suggested the existence of two components and a continued dimer formation in the multilayer. This dimerization allowed us to study the effect of intermolecular electronic coupling, and we could quantify it to about 0.2−0.3 eV. The interface dipole at the Au(111)/molecule interface, however, did not show any dependence on the molecular dipole moment. Since the dipole is in the plane of the molecule, we conclude on lying molecules (at least in the first layer). This conclusion is corroborated by the analysis of the HOMO peak width of monolayer and multilayer, in which the small monolayer peak width indicates no dimerization in the first layer. Such a dimerization would also explain the lack of molecular dipole moment dependence. All compounds were very sensitive to X-ray and UV radiation. For UPS, the use of an attenuation filter was found to be indispensable for correct interface energy level alignment measurements. Without the filter, Φ and HOMO shift 19035



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continuously with increasing film thickness and measurement time. This behavior is due to the high yield of photoelectrons emitted from the metal substrate during UPS measurements; these electrons damage the molecules, which induces charging of the molecular films.

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Relationships to Design Rules. Angew. Chem., Int. Ed. 2009, 48 (14), 2474−2499. (12) Ojala, A.; Bürckstümmer, H.; Hwang, J.; Graf, K.; von Vacano, B.; Meerholz, K.; Erk, P.; Würthner, F. Planar, Bulk and Hybrid Merocyanine/C-60 Heterojunction Devices: A Case Study on Thin Film Morphology and Photovoltaic Performance. J. Mater. Chem. 2012, 22 (10), 4473−4482. (13) Würthner, F. DMF in Acetic Anhydride: A Useful Reagent for Multiple-Component Syntheses of Merocyanine Dyes. Synthesis 1999, 12, 2103−2113. (14) Würthner, F.; Sens, R.; Etzbach, K.-H.; Seybold, G. Design, Synthesis, and Evaluation of a Dye Library: Glass-Forming and SolidState Luminescent Merocyanines for Functional Materials. Angew. Chem. Int. Ed. 1999, 38 (11), 1649−1652. (15) Eich, D.; Ortner, K.; Groh, U.; Chen, Z. H.; Becker, C. R.; Landwehr, G.; Fink, R.; Umbach, E. Band Discontinuity and Band Gap of MBE Grown HgTe/CdTe(001) Heterointerfaces Studied by kResolved Photoemission and Inverse Photoemission. Phys. Status Solidi A 1999, 173 (1), 261−267. (16) Beckmann, S.; Etzbach, K.-H.; Krämer, P.; Lukaszuk, K.; Matschiner, R.; Schmidt, A. J.; Schuhmacher, P.; Sens, R.; Seybold, G.; Wortmann, R.; Würthner, F. Electrooptical Chromophores for Nonlinear Optical and Photorefractive Applications. Adv. Mater. 1999, 11 (7), 536−541. (17) Liptay, W. Excited States; Academic Press: New York, 1974; Vol. 1. (18) Fry, A. J. Laboratory Techniques in Electroanalytical Chemistry, 2nd ed.; Marcel Dekker Ltd: New York, 1996. (19) Zacharias, P.; Gather, M. C.; Rojahn, M.; Nuyken, O.; Meerholz, K. New Crosslinkable Hole Conductors for Blue-Phosphorescent Organic Light-Emitting Diodes. Angew. Chem., Int. Ed. 2007, 46 (23), 4388−4392. (20) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Electrochemical Considerations for Determining Absolute Frontier Orbital Energy Levels of Conjugated Polymers for Solar Cell Applications. Adv. Mater. 2011, 23 (20), 2367−2371. (21) Helander, M. G.; Greiner, M. T.; Wang, Z. B.; Lu, Z. H. Effect of Electrostatic Screening on Apparent Shifts in Photoemission Spectra near Metal/Organic Interfaces. Phys. Rev. B 2010, 81 (15), 153308. (22) Hill, I. G.; Makinen, A. J.; Kafafi, Z. H. Initial Stages of Metal/ Organic Semiconductor Interface Formation. J. Appl. Phys. 2000, 88 (2), 889−895. (23) Würthner, F.; Meerholz, K. Systems Chemistry Approach in Organic Photovoltaics. Chem.Eur. J. 2010, 16 (31), 9366−9373. (24) da Silva Filho, D. A.; Kim, E. G.; Brédas, J. L. Transport Properties in the Rubrene Crystal: Electronic Coupling and Vibrational Reorganization Energy. Adv. Mater. 2005, 17 (8), 1072−1076. (25) Kwon, S.; Wee, K.-R.; Kim, J. W.; Pac, C.; Kang, S. O. Effects of Intermolecular Interaction on the Energy Distribution of Valance Electronic States of a Carbazole-Based Material in Amorphous Thin Films. J. Chem. Phys. 2012, 136 (20), 204706. (26) Ueno, N.; Kera, S. Electron Spectroscopy of Functional Organic Thin Films: Deep Insights into Valence Electronic Structure in Relation to Charge Transport Property. Prog. Surf. Sci. 2008, 83 (10− 12), 490−557. (27) Yoshida, H.; Sato, N. Crystallographic and Electronic Structures of Three Different Polymorphs of Pentacene. Phys. Rev. B 2008, 77 (23), 235205. (28) Krause, S.; Schöll, A.; Umbach, E. Interplay of Geometric and Electronic Structure in Thin Films of Diindenoperylene on Ag(111). Org. Electron. 2013, 14 (2), 584−590. (29) Koch, N.; Kahn, A.; Ghijsen, J.; Pireaux, J. J.; Schwartz, J.; Johnson, R. L.; Elschner, A. Conjugated Organic Molecules on Metal versus Polymer Electrodes: Demonstration of a Key Energy Level Alignment Mechanism. Appl. Phys. Lett. 2003, 82 (1), 70−72. (30) Ishii, H.; Hayashi, N.; Ito, E.; Washizu, Y.; Sugi, K.; Kimura, Y.; Niwano, M.; Ouchi, Y.; Seki, K. Kelvin Probe Study of Band Bending

ASSOCIATED CONTENT

S Supporting Information *

Synthesis of the molecules 1 and 4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel:+49 30 8062 15148. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank P. Zybarth for support with UV−vis measurements and J. P. Rabe for providing access to a UPS setup. F.W. and M.S. gratefully acknowledge financial support by the Bavarian State Ministry of Science, Research, and the Arts within the Collaborative Research Network ‘‘Solar Technologies go Hybrid’’. S.K. and N.K. acknowledge support by the SPP1355 of the DFG and the Helmholtz-Energie-Allianz “Hybrid-Photovoltaik”.

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REFERENCES

(1) Meier, H. Conjugated Oligomers with Terminal Donor− Acceptor Substitution. Angew. Chem., Int. Ed. 2005, 44 (17), 2482− 2506. (2) Coropceanu, V.; Cornil, J.; da Silva, D. A.; Olivier, Y.; Silbey, R.; Bredas, J. L. Charge transport in Organic Semiconductors. Chem. Rev. 2007, 107 (4), 926−952. (3) Huang, L.; Stolte, M.; Bürckstümmer, H.; Würthner, F. HighPerformance Organic Thin-Film Transistor Based on a Dipolar Organic Semiconductor. Adv. Mater. 2012, 24 (42), 5750−5754. (4) Steinmann, V.; Kronenberg, N. M.; Lenze, M. R.; Graf, S. M.; Hertel, D.; Bürckstümmer, H.; Würthner, F.; Meerholz, K. A Simple Merocyanine Tandem Solar Cell with Extraordinarily High OpenCircuit Voltage. Appl. Phys. Lett. 2011, 99 (19), 193306. (5) Kronenberg, N. M.; Deppisch, M.; Würthner, F.; Lademann, H. W. A.; Deing, K.; Meerholz, K. Bulk Heterojunction Organic Solar Cells Based on Merocyanine Colorants. Chem. Commun. 2008, 48, 6489−6491. (6) Bürckstümmer, H.; Kronenberg, N. M.; Gsänger, M.; Stolte, M.; Meerholz, K.; Würthner, F. Tailored Merocyanine Dyes for SolutionProcessed BHJ Solar Cells. J. Mater. Chem. 2010, 20 (2), 240−243. (7) Steinmann, V.; Kronenberg, N. M.; Lenze, M. R.; Graf, S. M.; Hertel, D.; Meerholz, K.; Bürckstümmer, H.; Tulyakova, E. V.; Würthner, F. Simple, Highly Efficient Vacuum-Processed Bulk Heterojunction Solar Cells Based on Merocyanine Dyes. Adv. Energy Mater. 2011, 1 (5), 888−893. (8) Zitzler-Kunkel, A.; Lenze, M. R.; Meerholz, K.; Würthner, F. Enhanced Photocurrent Generation by Folding-Driven H-Aggregate Formation. Chem. Sci. 2013, 4 (5), 2071−2075. (9) Krause, S.; Casu, M. B.; Schöll, A.; Umbach, E. Determination of Transport Levels of Organic Semiconductors by UPS and IPS. New J. Phys. 2008, 10 (8), 085001. (10) Duhm, S.; Heimel, G.; Salzmann, I.; Glowatzki, H.; Johnson, R. L.; Vollmer, A.; Rabe, J. P.; Koch, N. Orientation-Dependent Ionization Energies and Interface Dipoles in Ordered Molecular Assemblies. Nat. Mater. 2008, 7 (4), 326−332. (11) Mishra, A.; Fischer, M. K. R.; Bäuerle, P. Metal-Free Organic Dyes for Dye-Sensitized Solar Cells: From Structure: Property 19036



Article

at Organic Semiconductor/Metal Interfaces: Examination of Fermi Level Alignment. Phys. Status Solidi A 2004, 201 (6), 1075−1094. (31) Winkler, S. Private communication. (32) Koch, E. E. Photoemission from Organic Molecular Solids and Organometallic Compounds. Phys. Scr. 1987, 1987 (T17), 120. (33) Koch, N.; Pop, D.; Weber, R. L.; Bowering, N.; Winter, B.; Wick, M.; Leising, G.; Hertel, I. V.; Braun, W. Radiation Induced Degradation and Surface Charging of Organic Thin Films in Ultraviolet Photoemission Spectroscopy. Thin Solid Films 2001, 391 (1), 81−87. (34) Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization CrossSections and Asymmetry Parameters: 1 ≤ Z ≤ 103. At. Data Nucl. Data Tables 1985, 32 (1), 1−155. (35) Sanche, L. Irradiation of Organic and Polymer Films with LowEnergy Electrons. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 208 (0), 4−10. (36) Hill, I. G.; Kahn, A.; Soos, Z. G.; Pascal, R. A. Charge-Separation Energy in Films of p-Conjugated Organic Molecules. Chem. Phys. Lett. 2000, 327 (3−4), 181−188. (37) Hwang, J.; Wan, A.; Kahn, A. Energetics of Metal−Organic Interfaces: New Experiments and Assessment of the Field. Mater. Sci. Eng., R 2009, 64 (1−2), 1−31. (38) Weil, K. G.; Sawyer, D. T.; Roberts, J. L., Jr. Experimental Electrochemistry for Chemists. Ber. Bunsen-Ges. Phys. Chem. 1975, 79 (9), 842−843. (39) D’Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.; Thompson, M. E. Relationship between the Ionization and Oxidation Potentials of Molecular Organic Semiconductors. Org. Electron. 2005, 6 (1), 11−20.

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Influence of Merocyanine Molecular Dipole Moments on the Valence

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