Self-Assembly of a Fullerene Poly(3-hexylthiophene) Dyad

Solution-processed polymer bulk heterojunction (BHJ) solar cells exhibit potential advantages of competitive cost, large-scale roll-to-roll fabrication and mechanical flexibility compared to existing silicon solar cells.1 For most BHJ solar cell devices, the conjugated polymer and the electron acceptor such as [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) are randomly distributed throughout the film. Such disordered phase separation leads to localized trapping of charge carriers in isolated donor/acceptor domains that restrict the efficient charge transport to electrodes.2 To overcome this problem, intense efforts have been directed to optimize the morphology of BHJ solar cells. These include thermal/solvent annealing, certain additives and addition of “nonsolvents”.3 On the other hand, donor–acceptor (D–A) molecules consisting of fullerene and oligothiophene or oligo(p-phenylenevinylene) have been synthesized for morphology control.4 The device efficiency involving these molecules, however, remains rather low. Similarly, D–A block copolymers consisting of poly(alkoxyphenylenevinylene) (PPV)5 or poly(3-hexylthiophene) (P3HT)6 in one block and pendant fullerenes in the other show less competitive efficiency compared to that of the state-of-the-art P3HT/PCBM blend systems.

Here we report a new D–A polymer-fullerene adduct, originally designed to provide improved control of the morphology of polymer BHJ solar cells. The target molecule, PCB–P3HT (Figure1), consists of a linear P3HT as the electron donor that is terminated at one end by a fullerene as the electron acceptor. Different from previous D–A block copolymers,5, 6 the single fullerene moiety may not hinder the self-assembly of P3HT, once the chain length of P3HT is properly chosen. At the same time, the Flory–Huggins interaction between P3HT and fullerene can still induce the phase separation. While we have carried out independent studies on the synthesis and characterization of PCB–P3HT, very recently Lee et al.7 reported the same molecule and used it as a compatibilizer for P3HT/PCBM BHJ solar cells.

Figure 1. A) Molecular structure of PCB–P3HT. B) Molecular structure of P3HTOH. C) Digital photographs of C₆₀ (0.5 mg mL⁻¹), C₆₀/P3HTOH (2:5, wt.), PCB–P3HT (1.2 mg mL⁻¹), and C₆₀/PCB–P3HT at different weight ratios in CS₂/THF (1:3, v/v).

Despite the enormous advances in the synthesis of fullerene-based dyads8 as well as the self-assembly of amphiphilic fullerene derivatives,9 we note that only few fullerene-capped conjugated polymers have been synthesized so far,10 largely due to the lack of conjugated polymers with well-defined end groups. One exceptional example is the chain-growth polymerization of poly(3-alkylthiophenes) which allows chain-end functionalization with different chemical moieties.11 Recently, Hillmyer and coworkers synthesized regioregular P3HT with both ends capped by methylfulleropyrrolidine (denoted as C₆₀–P3HT–C₆₀).12 Nevertheless, no long-distance ordering of phase separation was present in thin films of C₆₀–P3HT–C₆₀, compared to those of pristine P3HT.

To the best of our knowledge, however, the self-assembly behavior of these C₆₀-capped P3HT dyads remains unreported, either in solution or in thin films. Here we present results to illustrate this aspect. The main focus of this study is to examine factors such as solvents that affect the self-assembly of these dyad molecules in thin films from solution-processing. The ultimate goal is to achieve long-range order of well-defined D–A phase separation in thin films using these dyad molecules in order to improve the efficiency of polymer BHJ solar cells.

The synthesis of PCB–P3HT started from PCBM which, after being converted to its acid (PCBA, Scheme S1, Supporting Information) form using a literature procedure,13 was linked to one end of P3HT (number-averaged molecular weight, M_n = 4000; ratio of weight-averaged to number-averaged molecular weights, M_w/M_n = 1.4; DP ≈ 30, where DP represents the number average degree of polymerization.) via an esterification reaction with the hydroxyl group at one end of each P3HT polymer chain (denoted as P3HT-OH). The synthetic details and characterization are presented in the Supporting Information.

The reaction of P3HT-OH with PCBA through a Steglich reaction14 gave the target product that is easily soluble in chloroform, tetrahydrofuran (THF), chlorobenzene (CB) and ortho-dichlorobenzene (o-DCB). Compared to the poor solubility of PCBA in each of these solvents, the enhanced solubility of PCB–P3HT is evidence for the covalent linkage of the C₆₀ to the end of the P3HT chains. In addition, the covalent linkage of the C₆₀ moiety to one end of the P3HT molecule leads to 44% quenching of the PL intensity of the oligothiophene compared to that of P3HT-OH (Figure S5). In contrast, only 7% of the PL intensity is quenched in P3HT-OH mixed with free PCBM even when the fraction of C₆₀ in this mixture is higher than that in the PCB–P3HT molecule.

Experiments to investigate the role of PCB–P3HT as a “surfactant” to transfer pristine fullerenes into THF, known to be a good solvent for P3HT, while a poor solvent for fullerenes, are described in the following sections. This is followed by a description of results of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM).

In order to know whether PCB–P3HT could serve as a “surfactant” to enhance the solubility of pristine C₆₀, we performed a series of solubility tests. In this experiment, solid P3HT-OH, C_60, C₆₀/P3HT-OH and C₆₀/PCB–P3HT were first dissolved in CS₂ (1.0 mL, a good solvent for C₆₀) in an ultrasonicator bath (130 W) for 10 min, followed by addition of THF (3.0 mL) in one portion. Figure 1 shows the photographs of these samples after standing in capped vials overnight under ambient conditions. It is clear that C₆₀ shows relatively low solubility in CS₂/THF (1:3, v/v). Solid particles precipitated from the solution immediately after addition of THF to the solution of C₆₀ in CS₂. After standing overnight, this mixture resulted in a light purple solution with significant amounts of solids settling to the bottom of the vial. Similar precipitation of C₆₀ was observed in the mixture of P3HT-OH (1.2 mg mL⁻¹) and C₆₀ (0.5 mg mL⁻¹). This result suggests that P3HT-OH did not enhance the solubility of C₆₀ in CS₂/THF (1:3, v/v).

In contrast, the mixture containing PCB–P3HT (1.2 mg mL⁻¹) and C₆₀ (0.5 mg mL⁻¹) remained clear after addition of THF to the solution in CS₂. This mixture and PCB–P3HT (1.2 mg mL⁻¹) itself remained stable (i.e. no precipitation) after standing up to 3 days under ambient conditions. At a constant concentration of PCB–P3HT (1.2 mg mL⁻¹), the mixture of C₆₀/PCB–P3HT remained clear and stable when the concentration of C₆₀ increased to 1.0 mg mL⁻¹ but turned turbid when the concentration of C₆₀ reached 1.2 mg mL⁻¹. This result implies a saturation limit of 1.0–1.2 mg mL⁻¹ of C₆₀ under these conditions.

The deeper color of C₆₀/PCB–P3HT in CS₂/THF (1:3, v/v), compared to independent P3HTOH and C₆₀ with matched concentration in the same solvent mixture, clearly indicates the enhanced solubility of C₆₀ in the presence of PCB–P3HT.

Considering the selective solubility of P3HT in THF, we expected that PCB–P3HT could self-assemble into micelle structures in which the PCB aggregates in the micelle core, which is surrounded by P3HT chains that provide the colloidal stability in this solvent. To test this hypothesis, a solution of PCB–P3HT in THF (2.0 mg mL⁻¹) was drop-cast on a carbon-coated copper grid, dried in air and characterized under TEM. Figure2A shows a bright-field TEM image of this sample. Visible are well-dispersed particles with an average diameter of 130 ± 40 nm. Each particle, appearing as dark spots in the image, is surrounded by a light-gray corona. This core-shell feature can be seen more clearly under high-magnification (inset of Figure 2A). We believe that the dark spots shown in Figure 2A correspond to PCB aggregates, based on the relatively high electron density in comparison with P3HT, while the corona of light gray corresponds to a shell of P3HT chains. A schematic representation of these P3HT-stabilized C₆₀ micelles is shown in Figure 2E. In addition, one can see some micellar aggregates fused together, in which the coalescence of several micelle cores occurred (Figure S6, Supporting Information).

Figure 2. Representative TEM (bright-field) (A), SEM (top view, on Si/SiO₂) (B,C), and AFM (height, on Si/SiO₂) images of PCB–P3HT from THF (2.0 mg mL⁻¹) (D). C) A high-resolution SEM (side view, 40^o) of the same sample shown in Figure B. E) A schematic presentation of the P3HT-stabilized fullerene micelles formed by PCB–P3HT in THF. The inset of A shows a high-resolution TEM image of a single micellar particle of PCB–P3HT.

In order to obtain 3-D information of these PCB–P3HT aggregates formed in THF, we further characterized the same sample as shown in Figure 2A using SEM and AFM. Figure 2B shows a top-view SEM image of PCB–P3HT solution (2.0 mg mL⁻¹, THF) drop-cast on a Si/SiO₂ substrate followed by drying at room temperature. One can see particles with diameters varying from ca. 100 nm to 1.5 μm. Most of them appear spherical, while a few (labeled by arrows in Figure 2B) are slightly elliptic. The side-view of one particle under high magnification (Figure 2C) clearly shows the 3-D structure of these particles.

Figure 2D shows the AFM image of PCB–P3HT films dried on a glass slide from a solution (2.0 mg mL⁻¹) in THF. Again, globular objects with an average height of 150–250 nm were observed. The diameter of these aggregates varies from 100 nm to 1 μm which is typically larger than the actual diameter due to the convolution effect of the AFM tip. Nevertheless, the overall morphology and the height measured by AFM are consistent with the results obtained by SEM (Figure 2B).

Comparing the TEM image (Figure 2A) and the SEM image (Figure 2B), one can see that the core size of each micelle is relatively uniform, whereas there is a large variation for the whole size of each globular aggregate. Such discrepancy may be rationalized by the formation of large compound micelles in which multiple daughter micelles coalesce into a larger one. Such coalescence can be evidenced from the TEM image shown in Figure 2A, where the formation of micelle clusters with multiple cores and fused shells can be discerned. A referee suggested light scattering confirmation of the micelles in solution. Although we saw intense light scattering from the solution, suggesting the presence of large aggregates (several hundreds of nanometeres), it was extremely difficult to obtain a valid autocorrelation function, presumably due to interference from the light absorption and emission of P3HT. Similar large compound micelles have been demonstrated by Eisenberg and co-workers in a diblock copolymer system.15

To understand whether the incorporation of extra pristine C₆₀ affects the size and the shape of the PCB–P3HT micelles, a series of solution samples (shown in Figure 1C) containing different ratios of C₆₀/PCB–P3HT were examined by SEM and TEM. The results are presented in the Supporting Information (Figure S7–9). At a constant concentration (1.2 mg mL⁻¹) of PCB–P3HT in CS₂/THF (1:3, v/v), the average diameter of the micellar aggregates increased from 0.47 ± 0.11 μm, to 0.95 ± 0.44 μm at a weight ratio of 1:2.5 of C₆₀/PCB–P3HT. When the weight ratio of C₆₀/PCB–P3HT increased to 2:2.5, the average size of the aggregates increased further, but accompanied by significant interparticle coalescence which makes the size analysis of individual particles impossible. The size of the aggregates did not increase further as the weight ratio of C₆₀/PCB–P3HT was increased to 1:1. Again, flocculation of particles led to large aggregates with irregular shapes and sizes.

To examine the effect of solvent on the self-assembly of PCB–P3HT in solution, we next characterized the morphology of PCB–P3HT in o-DCB in which C₆₀ shows much higher solubility than in THF. In this experiment, a solution of PCB–P3HT (1.0 mg mL⁻¹, in o-DCB) was drop-cast on a carbon-coated copper grid and dried in air at room temperature over three days. Figure3A shows a representative bright-field TEM image of this sample. Interestingly, one can see long interpenetrating fibrous networks. These fibers appear uniform in diameter (15–20 nm), whereas the length varies, with some extending to micrometers. It should be noted that the white spots in Figure 3A are due to the pin holes from the supporting carbon film. We observed similar fibrous network structures by TEM when the concentration of PCB–P3HT is increased to 5.0 mg mL⁻¹ (Figure S10, Supporting Information). Considering the relatively good solubility of C₆₀ in o-DCB, we believe that the formation of these fibrous nanostructures of PCB–P3HT is driven by the local crystallization of regioregular P3HT of this dyad, while the PCBs form a monolayer shell around these fibrils, as shown in Figure 3B. Again, we employed AFM to further characterize the drop-cast films of PCB–P3HT (1.0 mg mL⁻¹, in o-DCB) dried on a glass slide. Figure 3C,D shows similar interpenetrating fibrous structures under this condition. The comparison of the TEM (Figure 3A) and the AFM (Figure 3C,D) results suggests that the formation of these nanofibrous structures is independent of substrate.

Figure 3. A) A bright-field TEM image of PCB–P3HT (1.0 mg mL⁻¹ in o-DCB) dried on a carbon-coated copper grid at room temperature. B) A schematic presentation of the fibrous self-assembly formed by PCB–P3HT in o-DCB. C,D) AFM images (C: height, D: phase) of PCB–P3HT (1.0 mg mL⁻¹ in o-DCB) deposited on glass substrates by drop-casting followed by drying at room temperature.

To understand the role of crystallization of P3HT in the formation of these fibrous structures in PCB–P3HT, we investigated the morphology and the structure of P3HTOH in a thin film prepared by drop-casting a dilute solution (1.0 mg mL⁻¹, in o-DCB) in a manner described above. The TEM image (Figure S11, Supporting Information) indicates the formation of fibrous structures with an average diameter of ca. 10 nm that is smaller than that of PCB–P3HT formed under similar conditions. In addition, the long-distance ordering of the P3HTOH film appears higher than that in the film of PCB–P3HT.

We further examined the phase separation of PCB–P3HT in relatively thick films. In this experiment, a solution of PCB–P3HT (5.0 mg mL⁻¹, in o-DCB) was spin-cast (700 rpm) on a Si/SiO₂ substrate. Figure4 shows the morphologies of the film before (A, B) and after (C, D) thermal annealing. Compared to the thin films drop-cast from a more dilute solution of PCB–P3HT (1.0 mg mL⁻¹, in o-DCB; Figure 3C,D), the film spin-cast from 5.0 mg mL⁻¹ solution in o-DCB shows a higher surface-coverage on the substrate before thermal annealing. The phase image (Figure 4B) still shows interpenetrating networks of fibrous structures. Interestingly, thermal annealing at 100 °C for 5 min led to enlargement of phase separation both in plane (horizontal) and out of plane (vertical). The bicontinuous network structure can be discerned clearly both from the height (Figure 4C) and the phase (Figure 4D) images.

Figure 4. AFM images of PCB–P3HT deposited on Si/SiO₂ substrates by spin-coating (700 rpm) from o-DCB (5.0 mg mL⁻¹) before (A, B) and after (C, D) thermal annealing at 100 °C for 5 min. The scale bar at the bottom of the images is 400 nm.

In summary, we have synthesized a C₆₀-capped P3HT dyad molecule (PCB–P3HT). This dyad molecule could serve as a compatibilizer in BHJ7 and as a “surfactant” to enhance the solubility of fullerenes in poor solvents such as THF, known to be a good solvent for P3HT.

Moreover, the phase separation of PCB–P3HT in thin films leads to well-defined nanostructures, where the solvent plays a vital role in determining the morphology and the structure of these self-assembling structures. PCB–P3HT from THF solution tends to form spherical aggregates in which the methanofullerene moieties aggregate in the micelle core stabilized by P3HT chains that extend to the solvent medium. In contrast, PCB–P3HT from o-DCB solution forms fibrous structures with a uniform diameter of 15–20 nm and varying lengths up to micrometers. In these fibrous structures, P3HT chains crystallize to form the core which is surrounded by a monolayer of C₆₀ moieties. Such interpenetration fibrous structures still exist in relatively thick films (ca. 50 nm in thickness). A thermal annealing of these films resulted in enhancement of phase separation both in plane (horizontal) and out of plane (vertical).

We expect that these different self-assembled structures of PCB–P3HT dyads could be interesting model systems for study of photoinduced charge transfer. For example, in the spherical self-assemblies formed from THF solution, photo­induced electrons after the exciton dissociation at PCB/P3HT interfaces would be localized in the micellar core, while photoinduced holes would be localized in the corona. Nevertheless, in the elongated one-dimensional fibrous self-assemblies, we could imagine both holes and electrons tend to be delocalized in the P3HT phase and in the methanofullerene sheath, respectively.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

We thank Konarka Technologies for support. We also thank Dr. Yanming Sun for help in thin-film preparation. We are grateful to Dr. Stephan Kraemer and Mr. Mark Cornish for help in electron microscopy. M. Wang thanks NSERC Canada for the support through a Postdoctoral Fellowship.