Aqueous Route to PhthalocyanineâFullerene Composites with

Article pubs.acs.org/JPCC

Aqueous Route to Phthalocyanine−Fullerene Composites with Regular Structure Nicholas Yu. Borovkov* and Arkadiy M. Kolker G. A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences, 1 Akademicheskaya St., 153045 Ivanovo, Russia S Supporting Information *

ABSTRACT: Random mixtures of phthalocyanines (Pcs) with fullerene C60 are currently exploited as active layers in organic solar cells. Therefore, spatially regular Pc−C60 heterostructures, not designed so far, are of great interest as molecularly mixed donor−acceptor phases for further improving the device performance. Here we investigate the prospects to create such heterostructures by fine intermixing of π-rich Pcs with fullerene C60. Specifically, this work reports on the mixing behavior of cobalt tetrakis(3-amino-5-tert-butyl)phthalocyanine (CoPc#) with fullerene C60. A composite film (CoPc#/C60 = 1:2, mol) was prepared by the Langmuir−Schaefer technique from a floating layer with doubledecker architecture, where the CoPc# monolayer was interlinked with the C60 submonolayer by the clustered water. This film was structurally analyzed and compared with the CoPc# film used as a reference. The latter film is found to be constructed from the 2-stack dye aggregates, which are assembled in the χ-phase mode, randomly oriented, and loosely packed. In the composite film, the C60 molecules tend to intercalate into the interaggregate voids, making the aggregates consolidate and align in the vertical direction. Accordingly, the delamination process, which freely occurs in the pristine dye film, is inhibited in the composite. All aggregative features of CoPc# indicate that both structure and arrangement of the dye aggregates are determined by water-assisted H bonding via the primary amine groups. This kind of interaction is of use for tailoring Pc-based frameworks capable to host the C60 molecules.

1. INTRODUCTION Over the last century, organic photovoltaics had passed a thorny way from fundamental researches into photoconductivity of natural dyes to laboratory-scale conversion of the incident light to electricity.1 Nowadays its further progress is boosted by a head-to-head race going on between two kinds of composite photovoltaic materials, polymer−fullerene and phthalocyanine−fullerene ones.2 The former composites are structurally versatile and easy to prepare by wet-processing techniques,3,4 whereas their donor−acceptor interface is tunable by synthetic and physicochemical tricks.5 A technical advantage of the latter composites is determined by excellent photo- and electrophysical properties of phthalocyanines (Pcs).6,7 Nonsubstituted Pcs used in such composites are poorly soluble; hence, the materials are prepared by precise vacuum sublimation.8 A sore point of Pc-based photovoltaics is the low power conversion efficiency, which drastically mismatches the bright physical properties of the dyes.9 Recently there were persistent attempts to improve the Pc−C60 materials by expanding the interface geometry,10−12 creating the vertical concentration gradient,13 changing the size,14 the shape,15 and the orientation16 of the Pc aggregates. Despite some generally positive effects, no technical breakthrough was achieved so far. Functional properties of the Pc−C60 composites are severely restricted by interfacial compatibility between the donor and acceptor components.17 In the sublimed Pc−C60 composites, © 2014 American Chemical Society

the Pc molecules are believed to be assembled into nanoscale 1stack aggregates (26 molecules, ca. 10 nm in length), which are randomly oriented in space.18 A good orientation favoring a close contact between the face of the Pc macrocycle and the C60 molecule is achieved only via an external structure-forming factor.16 Moreover, the typical Pc−C60 heterojunction shows a fast initial degradation due to the light-induced interfacial disordering.19 This fact indicates that the disordered Pc−C60 interface is energetically favorable and hardly may be much improved in a straightforward way. So, other Pc/C 60 intermixing algorithms are desired to substitute the primitive forced blending of the two components.18 A new approach to polymer-based photovoltaic materials is a design of bimolecular crystals with definite composition, stable structure, and tunable physical properties.20 No wonder a keen interest in wet-processable binary Pc−C6021−23 and ternary Pc− C60−polymer24,25 composites was sparked some years ago. In line with this trend, we undertook a search for soluble C60specific Pcs, which may be used to modify the classical Pc−C60 interface,10 to better disperse fullerenes in polymer matrixe,24 and, perhaps, to construct a molecularly mixed phase with fullerene C60. As a result, well-shaped Pc−C60 microparticles Received: April 10, 2014 Revised: June 10, 2014 Published: June 11, 2014 14403

dx.doi.org/10.1021/jp503520a | J. Phys. Chem. C 2014, 118, 14403−14409

The Journal of Physical Chemistry C

Article

3. RESULTS AND DISCUSSION 3.1. Langmuir−Schaeffer Films. The CoPc# dye is an airstable amorphous material well soluble in organic solvents. Its dark-green color is determined by the Q-band located at 758 nm (Figure 1, curve 1). In the cast films, the Q-band is centered

were prepared by the Langmuir−Schaefer (LS) technique and drop-casting.26 The formerly used Pc was equipped with two different peripheral functionalities, the primary amine and phenoxy groups, and, hence, had the π-rich and sterically flexible molecule. The present work is supposed to provide a further insight into the Pc−C60 interaction from the standpoint of materials chemistry. Specifically it seeks to answer two questions. Whether π-rich but sterically rigid Pcs are suitable for constructing the molecularly mixed phase with fullerene C60. If not, whether the traditionally disordered structure of the Pc− C60 composites may be refined using mild wet-processing techniques.

2. EXPERIMENTAL SECTION Materials. Phthalocyanine CoPc# was synthesized from cobalt tetrakis(3-nitro-5-tert-butyl)phthalocyanine (CoPc*)27 by quantitative reduction with sodium disulfide.28 It was twice purified by column chromatography (both as a nitrosubstituted precursor and after reduction) on silica L 100/160 (Chemapol, Czech Republic) using benzene as an eluent. Fullerene C60 (99.5%) was obtained from Carbon Technologies and Materials Co. (Russia). Cyclohexene (purum, Fluka) was distilled on an efficient Vigreux column before using. Chloroform (HPLC-grade, Sigma−Aldrich) was used as obtained. Procedures. The dilute CoPc# solution (0.3 mM) in chloroform and the saturated C60 solution (0.9 mM) in cyclohexene were used to deposit the substances onto the water surface. The mixed layer was prepared by a two-step procedure.26 First, an appropriate amount of the CoPc# solution was spread over the triply distilled water surface from a Hamilton 10 μL syringe, and the solvent was allowed to evaporate. Thus, the CoPc# monolayer with the preset initial surface concentration was created. Then an appropriate amount of the C60 solution was spread over the dye-covered water surface to obtain the mixed monolayer with the preset molar ratio CoPc#/C60 of 1:2.2. After evaporation of the solvent, the layer was compressed with the rate of 5 cm2/min. The wellcompressed layer was transferred onto a quartz glass slide cleaned with hydrogen peroxide. The dipping was performed at the surface pressure maintained constant during the dipping process. The slide was repeatedly lowered in a nearly horizontal position until it came in contact with the layer, and after a pause of ca. 5 s, it was lifted with a speed of 2 mm/min. The film was blown with a stream of nitrogen after each stroke. Instrumentation. A rectangular Teflon trough (NT− MDT, Russia) was used to prepare the floating layers for film deposition. Optical spectra were measured on a PC-controlled Specord M400 spectrophotometer fitted out with a glass polarizer (Carl Zeiss, Germany). The differential absorption technique29 was used to minimize the contribution of light scattering in optical extinction of the films. Small-angle X-ray scattering (SAXS) from the films was studied on the AMUR-K diffractometer, using the Ni-filtered Cu Kα radiation.30 Optical microscopy images were taken with an Altami Polar 312 microscope (Altami, Russia). Height images were taken with a Solver P47 PRO atomic force microscope (NT−MDT) operated in the semicontact mode under ambient conditions. Silicon cantilevers NSG010 (NT-MDT) with the 6 nm curvature radius of the tip and the 150 kHz resonant frequency were used.

Figure 1. Optical spectra of CoPc#: 1, benzene solution; 2, LS film, 90 layers; 3, LS film, 30 layers, fresh; 4, the same film, aged. Curve 1 is normalized; curves 2−4 are shown as recorded.

at the same wavelength, indicating a lack of the intermolecular charge-transfer caused by the π−π coupling. Structural data on CoPc# (the Supporting Information, Part I) show that the CoPc# molecules are assembled into nanoscale aggregates, which are reminiscent of the β-phase of unsubstituted Pcs. Comparative analysis of CoPc# and CoPc* reveals that the CoPc# molecules are packed more tightly than the CoPc* ones, but at the same time, the CoPc# aggregates are aligned far poorer. Taken on the whole, these facts indicate that the aggregation behavior of CoPc# is determined by an attraction of the van der Waals nature, most probably by intermolecular H bonding. To find out how intimately the nearly planar CoPc# molecules may be intermixed with the spherical C60 ones, we prepared the ideal floating layers, where neither π-stacked dye aggregates nor fullerene “grapes” occurred. The relevant experimental data and analysis thereof are given in the Supporting Information (Part II). Then two pristine dye films of different thickness were prepared using the ideal CoPc# monolayer as a starting material. The thicker 90-layer film exhibited a smooth optical spectrum with a high baseline and an IR band at 840 nm (Figure 1, curve 2). Such a band is a wellknown feature of the χ-like phase in which the stacked molecules are staggered but not shifted.31 A spectrum of the freshly deposited 30-layer film was noisy and exhibited a surprisingly intense band at ca. 740 nm (curve 3), which experienced a red shift to ca. 820 nm and a nearly 3-fold loss in intensity upon natural aging (cf. curves 3 and 4). Probing the 90-layer film with SAXS revealed a structure with interlayer periodicity of 4.5 nm (Supporting Information, Figure S11), whereas the maximal molecular dimension of CoPc# is only ca. 2.5 nm. This fact indicates that the dye aggregates possess supramolecular 2-stack architecture. Such a feature was originally revealed for an octopus-like Pc with eight terminal benzyl groups.32 That structure was stabilized by entrapped water but, in contrast with ours, irreversibly decayed upon film dehydration. For this reason, we consider stability of the 2-stack 14404



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aggregates of CoPc# as a sound evidence for interstack H bonding in the solid state. In order to prop up an idea of supramolecular H bonding, the bimolecular CoPc# aggregates were simulated semiempirically (Supporting Information, Part III). According to the quantum-chemical data, the optimized aggregate configurations are staggered in such a way that the primary amine groups of two molecules are finely fitted to one another. Next we prepared a composite LS film with the CoPc#/C60 ratio of ca. 1:2, mol. The preparation of the mixed floating layer is detailed in the Supporting Information (Part II). A unique feature of this layer was its double-decker architecture; namely, the layer was a combination of the CoPc# monolayer and the C60 submonolayer33 interlinked by the clustered water (Supporting Information, Figure S14). The double-decker layer was transferred onto glass by 30-fold dipping, and the composite LS film was studied on a subject of homogeneity and structural ordering. The 90-layer pristine dye film said above was used as a reference. The effect of fullerene C60 on the dye aggregation was studied using a combination of SAXS and polarized optical spectroscopy. Diffraction pattern of the composite film (Figure 2) shows a sharp Bragg peak corresponding to the interlayer

Figure 3. Normalized polarized optical spectra of the composite CoPc#−C60 film (1:2, mol; 30 layers). Italic numbers show the angle of incidence.

These data prove definitely that CoPc# is a structure-forming component, whereas fullerene C60 acts as a structural modifier. The pristine dye film is constructed from the nanoscale aggregates oriented randomly in the vertical direction. Probably the film is structurally inhomogeneous and there exist some regions where the aggregates are more or less aligned, as the low-intense but fairly sharp Bragg peak indicates. Fullerene C60 favors alignment of the dye aggregates in the vertical direction and, thus, imparts additional ordering to the film structure. To reveal a mechanism, through which the C60 molecules affect the spatial arrangement of the CoPc# aggregates, a thorough morphological study of the LS films was performed. Because of intense optical scattering, the films were well observable under natural light. The pristine dye film (Figure 4, top) is continuous in contrast with the solution-cast one (Supporting Information, Figure S3). Its surface looks uneven due to numerous blobs (lighter than background), which tend to merge into shapeless agglomerates (darker than background). Figure 4 shows a rare location, where the agglomerates are ample enough to assemble into a ring-shaped microstructure. This feature indicates that the film was being formed from the colloidal particles, nanosuspension or hydrogel, upon evaporation of dye-entrapped water. The composite film (Figure 4, bottom) looks glossy and exhibits no dye particles. Instead, there are C60-rich pools of 1−5 μm in diameter, which occupy ca. 1% of the film surface. In view of the drastic inhomogeneity of fullerene composites,3,21,26,34 such a surface seems practically uniform, indicating that the CoPc#− C60 system is highly resistant to the phase separation. Besides, there is a network of artificial grooves caused by a shear stress applied to the film during deposition. Finally the LS films were observed by atomic force microscopy (AFM). Figure 5 shows two representative locations in the pristine dye film. The film surface exhibits giant 3D protrusions (the top image) being 80−120 nm in height and ca. 500 nm in diameter. A typical protrusion (the bottom image) has a carved apex and is ornamented with vertical striations, which are believed to be vestiges of crystallinity.35 This kind of a nanostructure is well reproducible: it is encountered throughout the whole surface, while its habit shows no variations of note (Supporting Information, Figure S19). Also, two representative locations in the composite film are shown in Figure 6. The film surface consists of spacious lowland regions imprinted with the grooves and cramped

Figure 2. Small-angle X-ray diffraction pattern of the composite CoPc#−C60 film (ca. 1:2, mol; 30 layers). Italic decimal shows the interlayer periodicity (nm). The insert illustrates the data point scattering around 4°, where the second order reflection is located.

periodicity of 4.5 nm2, which is equal to one in the reference film. Using the Scherrer relation, the correlation length was estimated as ca. 40 nm, i.e., nine 2-stack units. Comparative inspection of the SAXS data on the two LS films reveals a pair of features associated with fullerene C60: the enhanced intensity of the Bragg peak and an extra peak resulting from the secondorder reflection (Figure 2, insert). Figure 3 shows the polarized optical spectra of the composite film. At the zero angle of incidence, the spectrum is close to one of the pristine dye films (Figure 1, curve 2). However, when the film-covered slide is being inclined, the IR-band shows a progressing blue shift and broadening. Initially the spectrum profile becomes structureless but then retrieves its ordinary one-band profile. At the limiting angle of 60°, the IR-band is centered at ca. 760 nm, i.e., at the same wavelength as the Qband of the dye molecule (Figure 1, curve 1). Thus, the χ-like phase of CoPc# exhibits two transition moments crossing at the nearly right angle, one of which (840 nm, intermolecular) is parallel to the film surface, while the other (760 nm, intramolecular) stands almost vertically. 14405



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Figure 6. Perspective AFM images of the composite film shown in Figure 4 (bottom image). The top image presents a general view; the bottom image displays nanoscale grooves.

Figure 4. Optical microscopy images of the LS films: top, the pristine CoPc# film (90 layers); bottom, the composite CoPc#−C60 film (ca. 1:2, mol; 30 layers). Here microscale morphological defects, agglomerates and grooves, are displayed.

observe a rich set of variously matured morphological elements needed to reconstruct the prehistory of the composite film. The reconstruction is as follows. The mixed floating layer used for the film deposition is a laminated hydrogel, where the interfacial water is jammed between the CoPc# and C60 monolayers. Therefore, the freshly deposited film is soft and responds to the applied shear stress with a change in morphology from continuous to broken. After the stress is removed, the film regains its consistency very fast, as the steep grooves’ banks indicate. At the same time, the film retains enough water to continue its shear-triggered structural evolution in the vertical direction. In accordance with the concept,36 such a behavior may be rationalized by the thixotropic hydrogel nature of the ternary system CoPc#− C60−H2O in which fullerene C60 creates soft junction points between the hydrated CoPc# aggregates, thus acting as a gelating agent. Now observing the grooves more closely (Figure 6, bottom), one may note that the banks are covered with creases oriented perpendicular to the shear direction. The creases tend to merge into buckles, which serve as 3D nuclei for the cone-shaped protrusions. As the cones are growing, the original creases are being smoothed out and finally obliterated. Thus, striations on giant protrusions should be considered as residual defects rather than symptoms of molecular ordering. As for the carved apex noted above, it plainly indicates that giant protrusions arise upon fusion of cone-shaped ones (Supporting Information, Figure S19). Accordingly, the visually observed mesoscale blobs result from fusion of neighboring giant protrusions. The 3D protrusion is known to be a classic morphological feature of polymer films prepared by wet-processing techniques.37,38 Recently a comprehensive work39 reported on a delamination mechanism of the film buckling phenomena. Our AFM images allow directly observing, seemingly for the first time, how this universal colloidal mechanism works at the nanolevel. 3.2. Mechanistic Remarks. An underlying objective of this work was to find out the kind of Pc−C60 interaction, which

Figure 5. Perspective AFM images of the pristine dye film shown in Figure 4 (top image). The top image presents a general view; the bottom image displays a giant protrusion.

highland ones built from giant protrusions (the top image). Taking into account the impact of fullerene C60 on the film morphology (Supporting Information, Part IV), we made out the latter regions as C60-poor and focused our attention on the lowlands (the bottom image). The nanoscale grooves have nearly vertical banks, where small cone-shaped protrusions are located. Some cones rest also on grooves’ beds. This morphology is highly irregular and, hence, allows one to 14406



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photovoltaic nanomaterial being a spatially regular framework of the columnar tetrabenzoporphyrin aggregates intercalated with a fullerene C60 derivative. Though porphyrins are poorer semiconductors and light-absorbers than Pcs, the solar cell45 showed the power-conversion efficiency of 5.2%, a value being twice as large as one of the cells based on the Pc−C60 composite with “ideal” interdigitated architecture,10 thus emerges a fatal feature of all Pcs used currently in wetprocessable photovoltaics:21,22 because of the conventional aggregation mechanism, these dyes have no propensity to selfassemble into a regular framework, whereas any chaotic structural pattern is intrinsically inefficient. In our opinion, novel Pcs with tailor-made molecular geometry are needed to fabricate photovoltaic materials of next generations. For instance, much promise shows the highly symmetrical octopus-like Pc said above.32 If the terminal benzyl groups of its molecule were equipped with the primary amine functionality, a kind of Pc-based framework, outwardly similar to the columnar liquid-crystalline packing46 but also capable of hosting the C60 molecules, might be prepared.

might be of use to construct the molecularly mixed donor− acceptor phase. Therefore, we chose CoPc#, whose aggregation is driven by relatively weak van der Waals forces rather than the charge-transfer π−π coupling. The binary system CoPc#−C60 was prepared under conditions as mild as possible, fully preventing the premature homoaggregation of both components. All the same, no structural manifestations of the direct CoPc#−C60 interaction were revealed in the solid state. When assembled in an aqueous medium, the CoPc # aggregates are structured in the χ-phase mode. The χ-like phase is not available from organic media because the πstacking interaction is too weak to surmount the energy barrier between the lower aggregated species and the most stable phase.31 This obstacle is easily overcome via hydration, a powerful factor promoting aggregation of hydrophilic Pcs.40 Additionally the χ-phase aggregates are stabilized by interstacked H bonding, as their architecture indicates. A smooth fusion of the 2-stack aggregates is sterically hindered and interaggregated H bonding expands further in a random manner. Thus, upon full film dehydration, an amorphous material with spacious interaggregated voids arise (“brick-pile” in Figure 7). Because the amorphous structure is also inherent

4. CONCLUSIONS 1) The binary LS film composed of aminophthalocyanine CoPc# and fullerene C60 was prepared, structurally analyzed, and compared with the pristine dye film used as a reference. All aggregative features of CoPc#, especially the stable 2-stack architecture of the aggregates, indicate that both structure and arrangement of the aggregates are determined by water-assisted H bonding via the primary amine groups. Fullerene C60 is unable to interfere with the aggregate formation but tends to intercalate into the interaggregate voids, making the dye aggregates align in the vertical direction. 2) A dominant morphological feature of the CoPc# film is a giant 3D protrusion made from the spatially disordered dye aggregates. It arises because the freshly prepared film is colloidal and, therefore, suffers pronounced delamination. In the binary LS film, delamination phenomena are inhibited by fullerene C60.

Figure 7. Schematic of the pristine (top) and C60-intercalated (bottom) systems of the CoPc# aggregates. Color code: the 2-stack aggregates of CoPc#, green; the C60 molecules, black.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

General data on CoPc#; a helpful procedure for extracting physicochemical information from featureless compression curves; simulation data on the CoPc# aggregates; images of the LS films with various C60 content. This material is available free of charge via the Internet at http://pubs.acs.org.

in simple Pcs, whose molecules bear no substituents except for the primary amine group,41 interaggregated H bonding should be recognized as a general driving force of aggregation of all amino-substituted Pcs. The C60 molecule is unable to hook the CoPc# partner directly but is well compatible with the clustered water.33 Therefore, when placed into the colloidal system CoPc#−H2O, fullerene C60 does not interfere with the aggregate formation but tends to intercalate into the waterfilled interaggregated voids. Patching the voids consolidates the aggregates, and the vertically ordered nanostructure arises (“brick-work” in Figure 7). Figure 7 implies that fullerenes may be rationally used as nanofillers irrespectively of their customary photovoltaic function. Indeed, we found out at least three works42−44 reporting on the improved electrophysical behavior of organic semiconductors doped with fullerene C60 or organic derivatives thereof. Moreover, Figure 7 clears the way for a new Pc/C60 intermixing algorithm. Recently researchers45 presented a

Corresponding Author

*(N.Y.B.) E-mail: [email protected]. Phone: 7(4932)351545. Fax: 7(4932)336246. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the Russian Foundation for Basic Research (Grants 12-03-97515 and 14-03-00176) and partially by the Russian Academy of Sciences (Program “New chemical substances and materials”). The authors thank Dr. L. A. Valkova for film preparation and Dr. I. V. Kholodkov for film characterization. 14407



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