Light Absorption in Organic Thin Films: The Importance of Oriented

Article pubs.acs.org/JPCC

Light Absorption in Organic Thin Films: The Importance of Oriented Molecules Olga Guskova,†,* Christoph Schünemann,†,‡ Klaus-Jochen Eichhorn,† Karsten Walzer,§ Marieta Levichkova,§ Steffen Grundmann,§ and Jens-Uwe Sommer†,§ †

Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Str. 6, 01069 Dresden, Germany Heliatek GmbH, Treidlerstr. 3, 01139 Dresden, Germany ‡ Institut für Energietechnik, Technische Universität Dresden, Helmholtzstr. 14, 01069 Dresden, Germany § Instutut für Theoretische Physik, Technische Universität Dresden, Zellescher Weg 17, 01069 Dresden, Germany §

S Supporting Information *

ABSTRACT: In this work we apply a joint experimental and theoretical approach to investigate thin films of side chain substituted dicyanovinyl quaterthiophenes (DCV4T-Et2) and DCV4T-Et2:C60 blends, prototypic absorbers for small molecule organic solar cells. Structural characterization of the morphology of thin films thermally deposited at different substrate temperatures on a silica surface was performed by variable angle spectroscopic ellipsometry, grazing incidence X-ray diffraction, and atomic force microscopy measurements. These methods, combined with full-atomistic molecular dynamic (MD) simulation, provide detailed information about thin film morphology, namely about molecular orientation, absorption, phase separation, and crystallinity, i.e., factors that affect the efficiency of organic solar cells. Using molecular dynamics simulation, we can constitute why the DCV4T-Et2 molecules arrange strongly tilted in pristine (69° to 70° tilt angle to the substrate normal) and DCV4T-Et2:C60 blend films (tilt angle of 65° to 69°).

1. INTRODUCTION In recent years, research on organic solar cells has shown impressive progress. In particular, the speed of power conversion efficiency improvement for small molecule solar cells is formidable. In the beginning of 2013, a power conversion efficiency of 12%, comparable to that of inorganic thin film solar cells, was reached.1 The photoactive region of such organic solar cells typically consists of a donor−acceptor bilayer or donor−acceptor blend layer embedded in between ndoped and a p-doped charge extraction layers.2 This heterojunction architecture is crucial because Frenkel excitons (tightly bound electron/hole pairs), generated by light absorption in the donor or acceptor material, can only be separated at the interface between donor and acceptor. For efficient exciton separation the donor−acceptor blend layer architecture is preferred in comparison to the bilayer structure since excitons in organic materials typically show a diffusion length of only 10 nm.3 For both, bilayer and donor−acceptor blend layer, the film thickness is limited to several tenths of nanometers to guarantee efficient charge extraction and exciton separation.4 Thus, a major challenge of organic solar cells is to guarantee that most of the sunlight in the absorber films is absorbed in films thinner than 50 nm. For this purpose, the molecular orientation or more precisely the orientation of the molecular transition dipole moment μ in the absorber film is essential. To maximize thin film absorption (under perpendicular illumination) μ of the molecule must be aligned parallel to the thin film surface and thus to the electric field of the incident © XXXX American Chemical Society

beam. Typically, for planar organic molecules, the transition dipole moment is oriented within the molecular plane. Thus, a flat-lying molecular orientation (with respect to the film surface) is optimal to achieve maximum light absorption in solar cells.5 However, organic molecules without any functional groups (only consisting of carbon and hydrogen atoms) typically tend to grow upright standing in organic thin films leading to low absorption.5 Most experimental work dealing with adsorption of small conjugated molecules, including DCV4T-derivatives, is primarily concentrated on the “chemical structure/surface forces” relation that determines the way of molecular self-organization in thin films, whereas some computer simulations appeared that studied only the bulk morphology excepting the impact of the surface. For instance, a series of side chain substituted dicyanovinyl oligothiophenes has been modeled with respect to structural properties and molecular packing in the crystalline homophase.6−9 It was shown that even small structural variations in molecular geometry of DCV4T (the length and position of the side chains) can change the strength and pattern of intermolecular interactions (nonbonding short contacts, π−π interaction, weak hydrogen bonding) in crystals and packing behavior and lead to remarkable differences in solid-state organization. The latter factor is crucial for exciton diffusion Received: May 16, 2013 Revised: July 16, 2013

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dx.doi.org/10.1021/jp4048083 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Incidence X-ray Diffraction (GIXRD), detailed information about thin film morphology could be achieved: molecular orientation, absorption, phase separation, and crystallinity. In modeling, the well-established multiscale approach is used to characterize the intermolecular and donor/substrate interactions and their impact on the molecular orientation of DCV4TEt2 in the thin films.

length and charge transport and , as a consequence, affects the performance of photovoltaic devices.8,10 In this work, the correlation of the microscopic charge dynamics and degree of conformational and energetic disorder in crystalline, amorphous, and smectic mesophases of DCVTs was described by combination of density-functional theory, atomistic MD simulations, and kinetic Monte Carlo algorithm, which allowed evaluation of the charge percolating networks in model systems. In the present work, we apply a joint experimental and theoretical approach to investigate (i) how the chemical structure influences the molecular orientation in thin films; (ii) how a nearly flat-lying orientation in pristine and blended films with C60 acceptor can be reached; and (iii) what mechanism is responsible for the desired flat-lying molecular orientation, which is important to further improve the light absorption and thus the solar cell power conversion efficiency. For experimental characterization, two pristine DCV4T-Et2 (dicyanovinyl end-capped quaterthiophene with ethyl side chains, molecular structure in Figure 1) and two blend DCV4T-Et2:C60 (blend ratio 1:1 (vol%)) films are deposited on heated (substrate temperature Tsub = 70 °C) and unheated substrates (Tsub = 30 °C), since the substrate temperature changes the thin film crystallinity. Combining the Variable Angle Spectroscopic Ellipsometry (VASE) and Grazing

2. EXPERIMENTAL SECTION All materials are thermally evaporated at a base pressure of (10−7...10−8) mbar. DCV4T-Et2 (Heliatek GmbH, Dresden, Germany) is used as synthesized. The acceptor material C60 (Creaphys, Dresden, Germany) is purified three times by vacuum sublimation before being used for thin film preparation. Pristine DCV4T-Et2 and blend DCV4T-Et2:C60 films are deposited via thermal evaporation in a UHV chamber (Creaphys, Germany) using a deposition rate of 4 Å/s to avoid spontaneous molecular crystallization. The film thickness is controlled by quartz crystal microbalances. Twenty × 20 mm2 sized silicon wafers precovered with a 950 nm thin SiO2 film are used as substrates to guarantee similar growth conditions in simulation and experiment. Out-of-plane grazing incidence measurements are performed using a Bruker D8 Discover diffractometer at the Fraunhofer Center Nanoelectronic Technologies (Dresden, Germany). The device setup and the measurement routine can be found elsewhere.4 The incident angle is fixed at ω = 0.18° (all X-ray patterns are not background corrected). Atomic force microscopy (AFM) measurements are done in the peak force tapping mode using a Dimension ICON (Bruker-Nano, U.S.). As tip silicon nitride sensors SCANASYST-AIR (Bruker, U.S.) with a nominal spring constant of 0.4 N/m and a tip radius of 2 nm are used. Thin film absorbance is determined from transmission measurements using a two beam spectrometer UV-1800 (Shimadzu Corporation). The uniaxial anisotropic optical constants of the sample are estimated by VASE. This method measures the polarization change of the reflected light for different incident angles and varied wavelength. In order to derive precise results, we use interference enhanced substrates (silicon wafer with an additional 950 nm thin film of SiO2). VASE measurements are done with an M2000 UI ellipsometer (J.A. Woollam Co., Inc.), covering the wavelength range of 245 to 1680 nm. The quality of the fit is estimated by the uniqueness of the modeled parameters and the mean squared error (MSE) according to ref 11. From the experimental ψ and Δ spectra, the optical constants are estimated by modeling ψ and Δ with several Gaussian oscillators (details in Figure S1 of the Supporting Information (SI)). For the pristine DCV4T-Et2 films, an MSE of 25 is achieved, whereas for the DCV4TEt2:C650 blend films an MSE of 18 is achieved. The measurement routine and evaluation are the same as used in Wynands et al.11 In sum, four samples are morphologically characterized by GIXRD, VASE, and AFM measurements. All measurements are carried out on nonencapsulated pristine and blend films. First-principles calculations are performed using the Gaussian package.12 Equilibrium values of bond lengths, angles, and torsions are taken from the vacuum-optimized geometry which are obtained by DFT calculations at B3LYP/6-31G(d) level (see Table S1 of the SI). The potential energy surfaces of side chain rotation in DCV4T-Et2 are evaluated by performing constrained geometry optimizations (relaxed rotor approximation). The ZINDO/S CI (single excitation) method is used to

Figure 1. (a) Molecular structure, dimensions, and (b) optimized geometry of the DCV4T-Et2 molecule (top and side view, carbon is shown in green, nitrogen in blue, sulfur in yellow, and hydrogen in white); the value and direction of both ground state dipole moment μGS and transition dipole moment μTD for the first excited state are shown; (c) thin film absorbance spectra of a pristine DCV4T-Et2 and a blend DCV4T-Et2:C60 (1:1 vol%) film deposited on unheated quartz substrates. B



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Figure 2. GIXRD patterns of (a) 100 nm thin pristine DCV4T-Et2 and (b) 100 nm thin DCV4T-Et2:C60 (1:1 vol%) blend films deposited onto unheated and 70 °C heated SiO2−Si-wafers. The 50 nm thin pristine C60 film in (b) is implemented to correlate the peaks obtained in DCV4TEt2:C60 patterns. The inset in (a) illustrates a possible molecular tilt angle of the DCV4T-Et2 molecules of 70° in the pristine film resulting from the length of the linear molecule (2.4 nm) and the lattice plane distance of 0.8 nm belonging to the Bragg reflection at 2θ = 10.8°.

attached to four covalently bonded thiophene rings. The molecular structure and dimensions of DCV4T-Et2 are depicted in Figure 1a and the three-dimensional optimized geometry imaging the fractional differences in chemical bond order is shown in Figure 1b. The thiophene monomer units show all-trans conformation. The molecule has an almost planar conformation (all details are summarized in Table S1 of the SI) because of delocalization of π-electrons along the conjugated backbone. When deposited in thin films, the DCV4T-Et2 show strong absorption, as depicted in Figure 1c for a pristine and a DCV4T-Et2:C60 (1:1 vol%) blend film. In both, pristine and blend, the broad absorption band of the DCV4T-Et2 molecule is located between 450 and 700 nm. Thus, DCV4T-Et2 is a green donor absorber and complementary to the absorption bands of C60 at wavelengths smaller than 450 nm (see Figure 1c). The morphology of the pristine DCV4T-Et2 and the mixed DCV4T-Et2:C60 films is investigated by GIXRD, VASE, and AFM. GIXRD measurements reveal detailed information about the intermolecular order (crystallinity) and molecular orientation in thin films. However, this information can only be extracted from crystalline regions of the film. In contrast, VASE can also probe the orientation of amorphous films by measuring the anisotropic optical constants. 3.1.1. GIXRD and AFM Measurements. In Figure 2a, the GIXRD pattern of the pristine DCV4T-Et2 films shows an intense Bragg reflection at 2θ = 10.8° and several very small reflections at larger 2θ angles. Accordingly, the DCV4T-Et2 films grow highly crystalline with preferred orientation. Introduction of thermal energy during thin film growth by applying higher Tsub = 70 °C results only in a weak rise of the Bragg reflection height at 2θ = 10.8°. This demonstrates that the growth of the film is highly crystalline even for unheated substrates (Tsub = 30 °C) and cannot be significantly improved by higher Tsub. Using the Bragg equation, the lattice plane distance corresponding to the Bragg reflection at 2θ = 10.8° is calculated to be 0.81 nm. This lattice plane is aligned nearly parallel to the substrate surface. More precisely, the lattice plane distance is tilted 5.3° with respect to the substrate surface since the incident angle is fixed at ω = 0.18° and the detector angle is 2θ = 10.8° (the corresponding bisecting line of incident and refracted X-ray beam is tilted 5.3° toward the substrate normal). Considering that the molecule is 2.4 nm long, the Bragg reflection at 2θ = 10.8° can be originated by DCV4T-Et2 molecules which are 70° tilted with respect to the substrate

calculate the transition dipole moments between the ground and the first ten excited states. Atomistic MD simulation of the dynamic growth of layers on the amorphous silica is performed using the LAMMPS simulation package13 in NVT ensemble (Nosé-Hoover thermostat, τT = 30 fs) and reparameterized version of polymerconsistent class II force field (PCFF).14 The atom-based summation method is used to calculate the nonbonded van der Waals interactions with a cutoff distance of rl = 15 Å. The standard Ewald method is used for evaluating Coulomb interactions. The partial atom charges of DCV4T-Et2 are assigned according to the PCFF force field, the silica bulk and surface charges are taken from ref 15 (Tables S2 and S3 of the SI). The procedure of the system preparation included several steps. First of all, to construct the substrate, the annealed amorphous silica surface with dangling oxygen atoms16 is replicated along the principal axis to obtain a periodic lattice of size 57.02 × 57.02 Å in the x and y direction. In order to generate the hydroxylated (hydrophilic) silica surface, free oxygen atoms are saturated by hydrogen atoms on both faces.17 The height of simulation cell is selected to provide a sufficient space to dispose the oligomer chains of DCV4T-Et2 (Δz = 50 Å is added to the simulation cell in z-direction17). Since physical adsorption is not altering the geometry of the substrate surface, the atoms of the substrate are kept fixed in their position, while oligomer atoms are allowed to move. The film growth is microscopically simulated in a quasi-static way by adding one molecule at a time to the pre-equilibrated aggregate.18 Thereby the vapor deposition of oligomer molecule on silica is modeled by inserting DCV4T-Et2 into the simulation box one by one with an interval 2 ns (τadd = 2 ns, T = 350 K). As a new molecule is placed in the box, the system is equilibrated to observe an adsorbed state of the molecule. At the last stage of preparation, the final relaxation of the adsorption layer (the total number of the molecules was Nmol = 20) at T = 300 K during 4 ns is performed. Equilibration time is chosen so that potential energy, orientational order parameters, and several other characteristics become stable within this time. The production run is then extended up to 5 ns, over which the snapshots of the system are stored every 10 ps. Technical details can be found in the SI.

3. RESULTS AND DISCUSSION 3.1. Experimental Investigations. In the molecule DCV4T-Et2, two cyanovinyl groups and two ethyl groups are C



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normal and thus nearly flat-lying (see inset of Figure 2a). Another possible orientation of DCV4T-Et2 molecules leading to this Bragg reflection might be a parallel alignment of the molecular long axis with the substrate surface, but with an upright standing molecular short axis which is 0.8 nm long (see Figure 1a). A clear distinction between both orientations can only be made if the crystallographic phase would be known. Unfortunately, no single crystal data is available for this molecule. However, from the position of the Bragg reflection, we can assume that the DCV4T-Et2 molecules are oriented nearly flat-lying with tilt angles larger than 70° in the crystalline film regions. For efficient organic solar cells, the problem of exciton separation due to their low diffusion length can be overcome by enhancing the interface between donor and acceptor. This can be achieved by coevaporating the donor molecule DCV4T-Et2 with the acceptor molecule C60 which leads to a formation of phase separated C60 and DCV4T-Et2 domains. For the case of mixing both molecules, the crystalline growth of the DCV4TEt2 molecules is suppressed in the blend films, as shown in Figure 2b. For DCV4T-Et2:C60 blends deposited on unheated substrates, no Bragg reflection is obtained. Instead, only two broad peaks at 2θ = 10° and 20° are visible. The peak at 2θ = 20° is originated by short-range ordered C60 molecules, while the other peak at 2θ = 10° can be attributed to short-range ordered DCV4T-Et2 and/or C60 molecules.4 Thus, although the blend film is amorphous, we find that the film is weakly phase separated due to the detected short-range order of C60 and DCV4T-Et2 molecules in its domains. This phase separation can be improved by applying a higher Tsub during codeposition of C60 and DCV4T-Et2. In Figure 2b, the blend film deposited at Tsub = 70 °C indicates a significant increase of the peak at 2θ = 10.5° revealing that small crystallites of DCV4T-Et2 molecules are formed. The Bragg reflection at 2θ = 10.5° is not expected to be caused by C60 crystallites because for crystalline C60, the peak at 2θ = 20° would be separated into two Bragg reflections as depicted for the GIXRD pattern of the pristine C60 film in Figure 2b. The process of phase separation of blended donor-C60 films is discussed in detail in ref 4. However, C60 seems to remain amorphous in its domains, since the peak at 2θ = 20° does not split into the Bragg reflections present in the pristine C60 film (see Figure 2b). This development is different from previous investigations made for zinc-phthalocyanine:C60 blend films where C60 domains started to crystallize for higher Tsub while the planar zinc-phthalocyanine remained amorphous in its domains.4 However, in this work C60 crystallites are detected first at Tsub = 100 °C. Hence, probably the applied temperature regime (Tsub = 70 °C) is slightly below for C60 to crystallize in its domains of the DCV4T-Et2:C60 blend film. In general, an improved phase separation in substrate heated DCV4TEt2:C60 blend films is obtained due to the formation DCV4T-Et2 nanocrystallites indicating more long-range ordered domains. The topography of the same samples investigated by GIXRD is probed by AFM. Due to the crystalline growth of the pristine DCV4T-Et2 films the surface of the film exhibits a grainy structure as shown in Figure 3. Heating the substrate leads to a significant increase in grain size and consequently in surface roughness (3.1 nm for Tsub = 30 °C to 4.0 nm for Tsub = 70 °C). In contrast, the topography of the amorphous DCV4TEt2:C60 blend film (Tsub = 30 °C) shows a very smooth and homogeneous surface (roughness 0.4 nm). Heating the

Figure 3. 2 × 2 μm2 sized AFM images of the 100 nm thick pristine DCV4T-Et2 and DCV4T-Et2:C60 (1:1 vol%) blend films deposited at different substrate temperatures on SiO2−Si-wafers. σRMS corresponds to the RMS-roughness of the film surface.

substrate leads to a slight increase in film roughness (0.6 nm) and formation of small grains. Accordingly, the results of GIXRD and AFM measurements confirm each other for all four samples. 3.1.2. VASE Measurements. While GIXRD reveals information about molecular arrangement and orientation in crystalline regions of thin films, VASE can probe the mean orientation of all molecules in the organic thin film. In Figure 4, the spectra of n (index of refraction) and k (extinction coefficient) of the pristine DCV4T-Et2 and mixed DCV4T-Et2:C60 (1:1 vol%) films are shown, which are extracted by modeling the measured ψ and Δ spectra with a Gaussian oscillator layer (see Figure S1 of the SI). In the following discussion, we will focus on the extinction coefficient k of the films. All investigated thin films exhibit a strong uniaxial anisotropy of the optical constants where the in-plane component of absorption (kin‑plane) is larger than the out-of-plane component (k out‑of‑plane ). In the denotation of Figure 4a, kin‑plane corresponds to the absorption for perpendicular light incidence as typically used for organic solar cell characterization. The pristine DCV4T-Et2 films are highly anisotropic since kin‑plane is up to four times larger than kout‑of‑plane and Δn (λ = 1200 nm) = nin‑plane − nout‑of‑plane = 0.3. From the differences of the maximum of kin‑plane and kout‑of‑plane, the mean tilt angle of the molecular transition dipole moment μ can be estimated.19 For DCV4T-Et2, we found that the transition dipole moment is aligned parallel to the molecular long axis (Figure 1b, Table S4 of the SI). Hence, the mean tilt angle of μ is equal to the mean molecular tilt angle α. In pristine DCV4T-Et2 films, the molecular tilt angle α is in the range of 70° (with respect to the substrate normal) and nearly independent of Tsub. This nearly flat-lying orientation is appropriate to induce high light absorption since μ is only tilted by 20° (in mean) to the electric field vector of the photon in the case of perpendicular illumination. A molecular tilt angle of at least 70° is also found by GIXRD (Figure 2a) for DCV4T-Et2 present in crystallites of the film and thus confirms the results of the VASE measurements. D



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Figure 4. Anisotropic optical constants (a) 100 nm thin pristine DCV4T-Et2 and (b) 100 nm thin DCV4T-Et2:C60 (1:1 vol%) blend films deposited onto unheated and 70 °C heated SiO2−Si-wafers. Mean molecular orientations (tilt angle α) of the DCV4T-Et2 molecules in the films are calculated from the maxima of kin‑plane to kout‑of‑plane (at fixed wavelength) as explained in ref 19.

In DCV4T-Et2:C60 blend films, the optical anisotropy is less pronounced (Δn(λ = 1200 nm) = nin‑plane − nout‑of‑plane = 0.1 and kin‑plane/kout‑of‑plane = 2 at λ = 530 nm) than in the pristine DCV4T-Et2 (Figure 4b). This is expected because 50 vol% of the mixed film consist of isotropic C60 molecules resulting in isotropic thin film properties. However, the obtained anisotropy illustrates that even in the amorphous DCV4T-Et2:C60 blend film on unheated substrate the DCV4T-Et2 molecules are not randomly oriented in its domains but grow in a preferred orientation. From kin‑plane and kout‑of‑plane (at the absorption maximum at λ = 530 nm) the molecular tilt angle of the DCV4T-Et2 molecules in its domains is determined to α = 65° (unheated) and 62° (Tsub = 70 °C). Compared to the molecular orientation in pristine DCV4T-Et2 film, α is slightly smaller in the blend films. However, the molecules remain in a strongly tilted orientation even in the amorphous DCV4TEt2:C60 blend at Tsub = 30 °C, comparable to the pristine films. In sum, we found that DCV4T-Et2 grows highly crystalline in a nearly flat-lying orientation in pristine thin films leading to strong thin film absorption. In DCV4T-Et2:C60 blend films,

the crystalline growth of DCV4T-Et2 is strongly suppressed but the molecules remain preferred oriented in its domains. In the second part of this work, we will focus on the reason for this molecular oriented crystalline growth by simulations. 3.2. Theoretical Investigations. 3.2.1. DFT Calculations. In the current study, full geometry optimization of DCV4T-Et2 in vacuum is performed with DFT/B3LYP and 6-31G(d) basis set. All wave functions are stable and the obtained geometries are in a minimum after frequency analysis. As a result, the thiophene monomer units show all-trans conformation with very small deviation of the backbone from planarity. The bond lengths, valence angles, and dihedrals are summarized in Table S1 of the SI. In conjugated molecules, almost all changes in geometry lead to the distortion of π-electron delocalization. Since dicyanovinyl-groups are also involved in the conjugation (see Figures 1b, S2, and S3 of the SI), one can expect that torsional angles are correlated with one another, i.e., the rotation of one such angle will affect the geometric arrangement of the rings along the oligomer backbone. Therefore, in order to define the torsional energy profile of dicyanovinyl group, all internal E



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coordinates except the dihedral angle of interest (C18−C17− C1−S1, Figure 1b) are not fixed, when the latter is kept rigid at every Θ-value. This is so-called relaxed rotor approximation. The calculated energy profile for rotation of one DCV group which is shown in Figure 5 has two minima corresponding to

(pKa is found to be 4−5.5 on quartz22 and 6.1 in amorphous silica23) to interact favorably with molecules having some basicity, i.e., in this particular case, the acid−base interactions as well as the H-bonds can be realized. DCV4T-Et2 molecule having four cyano-groups can be considered as proton acceptor molecule because of the lone pair of nitrogen atoms, so it can play a role of nucleophile in hydrogen bonding. To analyze the preferred orientations of DCV4T-Et2 on silica, we performed a series of MD runs that imitated the adsorption of a single molecule. Figure 6 illustrates the final

Figure 6. Snapshots of a single DCV4T-Et2 molecule in three typical orientations with respect to the silica surface. Here z is surface normal, α is the tilt angle between C17C21-vector of thiophene backbone (for details see Figure 1b) and the surface normal z. The bending angle β is the average angle C17C9C21 (C17C8C21). White stars indicate the nitrogen atoms participating in H-bonds with silanols. Panels a and b show the tilted molecules with α = 65.8 ± 2.5° and 53.4 ± 1.9°, respectively, that form only one H-bond with the substrate. The panel c illustrates the situation, where the molecule is bound to the substrate by means of two hydrogen bonds between one −CN- group of two DCV-terminal groups. The bent angle β = 157.2 ± 1.1°.

Figure 5. Calculated dicyanovinyl-group torsional energy profile of DCV4T-Et2 molecule.

cis-(Θ = 0°) and trans-conformer (Θ = 180°). The potential has rather high barrier at Θ = 90°, which makes the rotation around C1−C17 and C16−C21 bond hindered. The gas phase optimized geometry has been used as the basis for excited state calculations. The Table S4 of the SI contains the excitation energies, the magnitudes of transition dipole moments, and x, y, and z components associated with the corresponding dipole and transition moments. In the above representation (Figure 1b), the molecule lies in the xy plane and is elongated along the x axis, hence the z component only makes a very slight contribution toward the overall direction. In case of the ground state dipole moment μGS, the direction of dipole moment vector is perpendicular to the molecular plane (Figure 1b). In contrast, the transition dipole moments μTD show that the only significant contribution lies along the x axis (first excitation). This is indicative of charge transfer along the extended aromatic system and consistent with the classical description of these excitations.20 3.2.2. All-Atom MD Simulations. We first will discuss in some detail the adsorption behavior of a single DCV4T-Et2 molecule. This system provides information about both molecule/solid interactions and strong surface effect that defines the orientation of the molecules with respect to the substrate. There are several important factors that determine the structure of thiophene-based organic/inorganic interfaces:21 the most obvious one is surface chemistry and the nature and position of side chains and terminal functional groups, the second one is the existence of an epitaxial relationship between the crystal lattices of the substrate and organic molecular layer. The silica chemical surface can be described as composed of silanol groups −SiOH and Si−O−Si siloxane linkages (Figure S4 of the SI). As shown in Figure S4 of the SI, this amorphous surface is characterized by regions that are hydrophilic and hydrophobic on nanometer length scale, depending on the presence of silanols. The hydrophobic areas have the average diameter 2.083 ± 0.201 nm. Silanol groups have enough acidity

MD snapshots. In all three alignment motifs, the formation of hydrogen bonds was observed: the tilted molecules (Figure 6a, b) form only one H-bond with the substrate, the average length −CN···H is 3.17 ± 0.13 Å and hydrogen bond angle is 149.8°, which corresponds to weak H-bonding following the classification of Jeffrey.24 Here, we should stress that these results are obtained in force field-based MD simulation. Since the interaction type that dominates in formation of weak Hbonds is electrostatic and dispersion forces, the magnitude of such interaction also depends on partial charges and the electrostatic potential of the molecule. In this study, we applied the PCFF-tabulated partial charges that are a reasonably good approximation of the real charge distribution, but still do not perfectly represent the nature of the conjugated molecules. At the level of isolated molecules, ab initio methods can provide the information on the length of H-bond in silanol/1,1dicyanoethene and silanol/2-dicyanovinylthiophene pairs. For this purpose, Møller−Plesset (MP2) perturbation theory with 6-311+G(d,p) basis set has been used to characterize the intermolecular interactions between the molecule and functional group of the solid (Figure S5 of the SI). The population analysis is done applying NBO-method.25 In H-bonding, the interaction strength depends on proton donor ability and on the hydrogen-bond acceptor ability: for the same electrophile (silanol) the H-bond length is a little bit shorter in the case of acetonitrile (1.85 Å as compared to 2.055 Å or 2.057 Å for 1,1dicyanoethene or silanol/2-dicyanovinylthiophene, respectively), because the lone electron pair of nitrogen in 1,1dicyanoethene or silanol/2-dicyanovinylthiophene molecule is participating in a longer conjugation path, electrons are more delocalized, consequently the −CN group is characterized by lower basicity. In both cases, the bond length is in the borderline region between moderate and weak class (the F



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Figure 7. Adsorption onto hydrophilic silica (top and side view); surface area is 3251.28 Å2, the surface thickness is 16.4 Å (oxygen atoms are shown in red, silicon tetrahedrons in beige, carbon in green, nitrogen in blue, sulfur in yellow, and hydrogen in white). Nmol is the number of DCV4T-Et2 molecules. The organic layer thickness is approximately 14.3 Å.

penalty is ∼12 kJ/mol per one terminal functional group (i.e., such an orientation of the molecule is energetically unfavorable). To conclude this part, it is necessary to place an emphasis on the fact that the orientational effect of the substrate is already present at the single-molecule level before molecular aggregation to form quasi-2D nuclei or thin films. Having described which factors govern the orientation of isolated DCV4T-Et2 molecule on silica, the influence of intermolecular oligomer/oligomer interactions on the possibility of spontaneous self-organization was estimated in simulation of several molecules (Nmol = 2−20) with random initial configurations. Figure 7 collects the snap-shots from MD simulations. The analysis of trajectories shows that in all cases the molecules are strongly tilted toward the substrate surface with average tilt angle 60.1 ± 6.5° (the average tilt angle for the molecules like in Figure 6a,b) and tend to adopt the orientation as depicted in the Figure 6a. The average H-bond length is 3.73 ± 0.22 Å. A few molecules with orientation as depicted in the Figure 6b form a stack via π−π interaction; the direction of πstacking is parallel to the surface (Figure 7, Nmol = 7). In the many-chain system, only two molecules adopted the orientation with tilt angle α which was close to 90° (Figure S6 of the SI). As the first adsorption layer is completed (starting from Nmol = 9, the average distance from the silica surface is equal to 4.9

hydrogen bond angle is 178.4° in silanol/1,1-dicyanoethene complex and 164.1° in silanol/2-dicyanovinylthiophene pair). The tilted configurations that were observed in MD simulation of isolated DCV4T-Et2 molecule on silica differ from one another in conjugated backbone orientation: in the first case, the plane is oriented to be flat-lying (Figure 6a), in the second, to be standing upright (Figure 6b). In both cases, the longest molecular axis is oriented along the substrate, with some tilt angle. The molecular tilt angles α = 65.8 ± 2.5° (Figure 6a) and 53.4 ± 1.9° (Figure 6b) are in a good agreement with the experimental results. The second angle is lower due to the presence of the ethyl side groups which prevent the closer contact with the substrate. There is the second population, where the tilt angle α is close to 90° (Figure 6c). In this case, the flat-lying molecule is bound to the substrate by means of two H bonds (one −CN group from every terminal group). The molecule bridges two opposite sides of hydrophobic silica regions (the molecule length is 2.4 nm, the hydrophobic silica area is 2.083 ± 0.201 nm in diameter). The gain in hydrogen bonding (electrostatic) energy is compensated by energy loss caused by bending of the conjugated backbone (the bent angle β = 157.2 ± 1.1°). The average dihedral angle Θ of both dicyanovinyl-groups corresponds to the value 33.7 ± 8.1°, which means the energy G



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Å), the system clearly illustrates the initial ordering of the molecules in monolayer, which results in formation of several subclusters with aligned oligomers, but there is no common orientation of the subclusters and formation of registered states because of the amorphous nature of the substrate. For all subclusters, the nematic order parameter26 ⟨P2⟩ = ⟨3/2 cos2 γ − 1/2⟩, where γ is the angle formed by vector of the molecule (C17C21) and the average direction of this vector, is defined. The averaging is performed over time and over all molecules in a cluster. The calculated ⟨P2⟩ parameter within subcluster is relatively high, being typically between 0.64 and 0.85. The second layer with the average distance of the molecules from the silica surface 8.2−10.0 Å is characterized by flat-lying tilted molecules that may interact via π−π interaction (the distance between molecules in adjacent layers is 3.3−5.1 Å, which means that the second layer is more disordered and not so well-defined, so it is difficult to draw the line between two layers). For the second adsorption layer the driving force for such orientation is the interaction between conjugated systems, i.e., it is much less affected than the first layer by the underlying substrate and the interaction with silanols. Although we did not detect the formation of well-ordered truly crystalline films (or layers) of DCV4T-Et2 on silica in the simulations due to the low number of molecules contained in the system, such subclusters possibly are prototypes of ordered stacks perpendicular to the surface, which were obtained in our experiments.

Article

ASSOCIATED CONTENT

S Supporting Information *

Measured and modeled ψ and Δ spectra for varied incident angles of a pristine DCV4T-Et2 and a DCV4T-Et2:C60 blend film deposited on unheated substrates (VASE measurements) and technical details of simulations (MD and DFT calculations). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +49-351-4658-746; fax: +49-351-4658-752; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Free State of Saxony and the European Regional Development Fund under contract number 100108773. The authors also thank Dr. Lutz Wilde for performing GIXRD and Andreas Janke for AFM measurements.

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REFERENCES

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4. CONCLUSIONS Different experimental and theoretical techniques were applied to study thin films of side chain substituted dicyanovinyl quaterthiophenes and DCV4T-Et2:C60 blends on silica surfaces. We found that DCV4T-Et2 grows highly crystalline in a nearly flat-lying orientation in pristine thin films, leading to strong thin film absorption. In DCV4T-Et2:C60 blend films, the crystalline growth of DCV4T-Et2 is strongly suppressed, but the molecules remain preferred oriented in its domains. The simulation results confirm that the driving force of orientation of the DCV4T-Et2 oligomers on silica is controlled by the chemical nature of the substrate surface and the character and position of functional groups on the conjugated molecule. Single-molecule simulations clearly demonstrate hydrogen-bonding between cyano-groups and silanols. In general, two different orientations were observed: the first one is tilted configuration, the second one includes almost flatlying molecules. In the latter case, the molecule/substrate interactions slightly change the rotational angle of the terminal functional groups and can lead to the small bending of the conjugated backbone if both functional groups of symmetrically decorated thiophene backbone are participating in H-bond formation with the substrate. The simulations of several DCV4T-Et2 molecules show the tendency to form the subclusters with aligned (primary quasi-2D geometry26,27) tilted molecules. Comparing our results to the one made for similarly shaped DCV molecules like DCV4T by Koerner et al.,28 DCV6T by Wynands et al.,11 or DCV5T compounds by Fitzner et al.,29 our findings can explain the obtained molecular alignment as well, highlighting the assignability and universality of our work. Considering the high power conversion efficiency (4.8−6.9%)29 of DCV5T-Me:C60 solar cells strengthens the necessity of understanding why such molecules are oriented nearly flatlying. H



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