Interfacial self-assembly of nanoporous C₆₀ thin films

Abstract

Nanoporous surface morphologies often perform better than their planar or bulk counterparts in phenomena such as catalysis and charge transport. In organic photovoltaics, for example, fullerene films structured on a scale corresponding to the exciton diffusion length, typically a few tens of nanometers, could improve charge separation. Existing methods to form such films, however, typically do not reach features smaller than 100 nm. Here, we propose a simple method based on interfacial nucleation and growth to produce large and flexible 2D percolating assemblies of C₆₀ nanoparticles with diameters below 50 nm that cover up to 60% of the surface. These films can be rendered insoluble by photopolymerization and infiltrated with other functional molecules. We illustrate the benefit of such stabilized nanoporous films on the example of organic solar cells of the architecture ITO/TiO₂/C₆₀/P3HT/MoO₃/Ag. Structured C₆₀ films with interconnected morphological features perform better than planar C₆₀ films by 50% on average, reaching a power conversion efficiency of 1.3%. Due to the ease of film fabrication and small dimensions of the C₆₀ morphological features, these results may find application beyond organic photovoltaics, in a variety of fields where an enhanced interface with a fullerene surface is needed.

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Introduction

The properties of nanomaterials in general, and porous materials in particular, differ from those of their bulk counterparts because surface and interface phenomena play an important role on the nanometer scale.¹ Enhanced interfaces compared to planar counterparts increase the efficiency of nanoporous materials in processes like catalysis,² gas storage,³ molecular recognition,⁴ separation,⁵ and solar energy conversion.^6,7

Carbon-rich materials such as fullerenes have played a fundamental role in the development of modern nanoscience and technology.^8,9 Nanoporous C₆₀ films are for example used in catalytic methanol oxidation¹⁰ or as electrode materials in aqueous solutions.¹¹ Beyond these two examples, the outstanding opto-electronic properties of C₆₀ associated with a versatile chemistry makes fullerenes interesting for optics¹² or biology¹³ and essential for the entire organic photovoltaic (OPV) field where fullerenes are widely used as electron acceptor materials.^14–18 For such an application, nanoporous C₆₀ thin films would be highly desirable because excitons are split into free charge carriers at molecular heterojunctions.⁷ In many cases, active materials have been deposited as bilayers,^14,15,19,20 or interdigitated heterojunctions, usually using soluble fullerene derivatives^21–23 rather than pristine C₆₀.²⁴ Nanoporous C₆₀ films with morphological features in the range of the typical exciton diffusion length, ∼10–20 nm in comparable systems,^16,25,26 would offer an attractive alternative to bilayers in bi-component systems that do not phase-separate spontaneously to form an optimal bulk heterojunction morphology.

The fabrication of porous two-dimensional films of fullerenes on a large scale remains challenging. Some attempts were made by sacrificial templating,²⁷ electrodeposition from colloids,^28,29 or self-assembly of synthesized C₆₀ nanoparticles.³⁰ These methods remain limited by the difficulty to prepare C₆₀ nanoparticles smaller than 100 nm in diameter.^30–32 Interfacial self-assembly has emerged as a promising method for the fabrication of free-floating films that can be readily integrated into functional devices.^33,34 In particular, progress in the field of metallic nanoparticle assembly³⁵ inspired the interfacial technique that we present here to form single-layer or multilayer two-dimensional C₆₀ films having morphological features below 50 nm in diameter. Rapid solvent evaporation on an air/water interface is responsible for the formation of 2D interdigitated networks, typical of diffusion-limited colloid aggregation (DLCA).³⁶ The increase of interface area resulting from these nanostructures exceeds 30%. Importantly, the films are floating on a liquid surface and are flexible, allowing further transfer onto any substrate.

Pristine C₆₀ is known to photo-polymerize in the solid state in a 2 + 2 cycloaddition process.^37–40 This polymerization has for example been used to pattern fullerene films,⁴¹ but was also identified as a degradation pathway that limits long term OPV cell performance.^40,42 Here, we use C₆₀ photopolymerization to stabilize optimal fullerene morphologies against further solvent processing, similarly to results obtained by chemically modifying the fullerene.^26,43–45 Nanoporous films were stabilized, infiltrated with a well-known electron-donor material, poly(3-hexylthiophene-2,5-diyl) (P3HT) – resulting in finely inter-penetrating C₆₀/P3HT domains – and implemented as photoactive layer in C₆₀/P3HT solar cells. By adding a nanoporous C₆₀ layer to the bilayer configuration, the enhanced interface and the film morphology adapted to the exciton diffusion length resulted in an improvement of 50% on average and up to 80% in the power conversion efficiency (PCE), reaching η = 1.3%.

Experimental

Formation of C₆₀ films

5 mg of C₆₀ (Aldrich, 99.9%) were ground to fine powder in a 5 mL glass vial. 5 mL of chloroform (Sigma Aldrich, >99.8%) were added, the mixture was sonicated 30 minutes and left overnight to settle. C₆₀ films were formed at the interface between water and air, in a 10 mL round poly(tetrafluoroethylene) (PTFE) beaker with a diameter of 2.1 cm by pouring dropwise 5 to 500 μL of saturated C₆₀ solution on the convex water meniscus formed upon contact with PTFE. The films formed a thin brown and immobile layer on top of the water meniscus within a few minutes. Optionally, the C₆₀ solution was deposited under reduced pressure. If not stated otherwise, 50 μL of solution were deposited at a pressure of 500 mbar. To upscale the film area to several cm² for solar cell fabrication on 2.5 × 2.5 cm² large substrates, 100 μL of saturated C₆₀ solution were deposited drop wise in a 3.8 cm wide PTFE beaker without applying vacuum.

Transfer to substrate

The films were transferred to Si/SiO₂ or glass by dipping the substrate vertically in the beaker and retracting it through the floating film. No double layers were formed by this process. To form multilayers, the same substrate was dipped and retracted through multiple freshly assembled films.

Stabilization and infiltration

C₆₀ films were stabilized by light exposure (1 sun) overnight under inert atmosphere, at least 6 h are necessary according to previously published results.⁴⁰ The solubility of stabilized films was tested by spin-coating (1000 rpm, 60 s) pure chloroform (1 mL on 4 cm²). Regioregular P3HT (Rieke Speciality Polymers) was dissolved in chloroform (1.25 to 10 mg mL⁻¹) and spin-coated (1000 rpm, 30 s) on stabilized C₆₀ films.

Solar cell fabrication

Except for nanoporous C₆₀ film deposition, all device fabrication steps were done in a glove box under nitrogen (H₂O < 1 ppm, O₂ < 10 ppm). ITO substrates (Geomatec, sheet resistance 20 ohm square⁻¹) were cleaned and coated with TiO₂ (50 nm).⁴⁶ To this end, 0.95 g of Ti-(O-iC₃H₇)₄ (Aldrich, 99.9%) were diluted in 10 mL of ethanol and stirred at 0 °C. A solution consisting of concentrated HCl (Sigma-Aldrich 0.03 g), water (0.135 g) and ethanol (10 mL) was added while stirring. The mixture was stirred 30 min at 0 °C before spin-coating (step 1: 5 s, 300 rpm s⁻¹ to 1000 rpm; step 2: 60 s, 3000 rpm s⁻¹, 4000 rpm). The films were annealed to 460 °C in 3 h, kept at 460 °C for 2 h, and cooled to room temperature. TiO₂ films were heated to 120 °C on a hot plate under nitrogen (10 min) before depositing any further layer. C₆₀ nanostructured films were deposited on TiO₂ directly or on a 30 nm thick evaporated C₆₀ layer. The films were dried on a hotplate (80 °C) for 6 h under nitrogen to remove water and exposed to 1 sun illumination overnight under nitrogen to photo-polymerize C₆₀.⁴⁰ Planar C₆₀ films were deposited by thermal evaporation at <5 × 10⁻⁶ mbar. The active layer was covered with 10 nm of thermally evaporated MoO₃ (Sigma Aldrich, 99.99%) as a buffer layer and 70 nm of Ag (Cerac, 99.99%) as an electrode. The active area of the cells was 0.031 or 0.071 cm².

Characterization

Scanning electron microscopy (SEM) was performed on a Hitachi SU8000 at a typical voltage of 1.5 kV. Images were analyzed using ImageJ. To evaluate lateral dimensions of the assemblies, SEM images of 1.2 × 1.2 μm² were binarized using an Otsu threshold. The line plot of such an image shows maxima at the averaged position of interfaces. Scanning force microscopy (SFM) was performed on an MFP-3D (Asylum Research) with Olympus AC160TS-R3 cantilevers. TEM was performed on a Philips CM30 operated at a voltage of 300 kV. Devices were transferred into an air-tight transfer box for J–V characteristics measurement under a solar simulator delivering a light intensity of 100 mW cm⁻². Photoluminescence was measured on a Fluorolog (Horiba Jobin-Yvon).

Results & discussion

Nanoporous C₆₀ thin films

Two-dimensional nanostructured thin films of C₆₀ were formed by evaporation of C₆₀-saturated chloroform solutions at the convex interface formed between water and air in a PTFE beaker (Fig. 1a). The films obtained are percolating networks of C₆₀ particles forming a single layer (Fig. 1b), covering surfaces up to several square centimeters. A 400 μm² area is shown in Fig. S1, ESI.† The films are floating on the water meniscus, allowing their transfer onto diverse substrates including glass, silicon, or metals. An example of a single layer 2D film deposited on silicon is shown in Fig. 1b and c. Multilayer nanoporous films could be produced by successive deposition of films on the same surface (Fig. 1d). The nanoporous films exhibit astonishing mechanical flexibility and can, for example, effectively cover pronounced geometrical kinks (Fig. S2, ESI†). This flexibility can be explained by the film morphology. SFM revealed that the C₆₀ film is composed of agglomerated nanoparticles (Fig. 2a and b). The typical height of the C₆₀ particles is 20–30 nm (Fig. 2c). The total C₆₀ surface coverage reached up to 60%. Transmission electron microscopy (TEM) showed that the films are indeed composites of crystalline C₆₀ nanoparticles between 10 and 50 nm bound together by amorphous C₆₀ (Fig. S3, ESI†). The film shown in Fig. 2 has a 36% increased surface area compared to a plain C₆₀ film, considering the C₆₀ particles as spheres.

Fig. 1 Fabrication of nanoporous C₆₀ films. (a) Sketch of experimental setup comprising a saturated C₆₀/CHCl₃ solution and water as the subphase in a PTFE beaker; optionally, vacuum was applied around the beaker to tune the film morphology; (b and c) SEM images of a single-layer C₆₀ film; (d) SEM image of a triple layer C₆₀ film.

Fig. 2 SFM characterization of a nanoporous C₆₀ film. (a) 25 μm² scan showing 2D percolating structures typical of DLCA; (b) 1 μm² zoom showing that the film is composed of C₆₀ nanoparticles with an average diameter of 32 ± 10 nm; (c) profile plot extracted from (a) along the red line.

A similar convex interface was already used to self-assemble 2D films of metallic nanoparticles.³⁵ Dushkin et al. measured and modeled the self-assembly of latex particles on water meniscus.⁴⁷ They found that latex colloidal crystals nucleate at the center of the meniscus and grow radially under capillary action until they cover most of the surface. This comparison, along with our morphological observations allows us to interpret the C₆₀ film formation as follows: as the chloroform solution is suddenly brought into supersaturation under solvent evaporation, multiple nucleation events occur at the air/solvent interface. The mobile C₆₀ clusters thus formed may in parallel either grow radially or aggregate on other clusters, forming morphologies typical of diffusion-limited colloid aggregation with multiple seeds.³⁶ This model was tested by varying the amount of solution deposited on the water meniscus and the surrounding pressure.

Tuneable film properties

Varying the volume of C₆₀ solution deposited on the water meniscus allows tuning the total surface coverage with little variations of the lateral dimensions of the C₆₀ structures (Fig. 3a). The surface coverage could be varied between 37% and 60% by depositing 25 to 200 μL of C₆₀-saturated chloroform solution on 3.5 cm² of interface (Fig. 3a). Below 25 μL, the film only partially covered the water meniscus. Above 250 μL, large C₆₀ crystals formed at the air/water interface, leading to film inhomogeneity and no increased coverage (Fig. S4, ESI†). In order to probe the effect of solvent evaporation time, 50 μL of saturated chloroform were deposited under pressures varying from 300 to 1000 mbar (Fig. 3b). The surface coverage did not vary during this experiment. Lowering the surrounding pressure leads to the formation of smaller lateral dimensions of the assemblies, reaching 35 nm at 300 mbar. The lower pressure limit is given by the boiling point of the carrier liquid water. In summary, C₆₀ coverage is mainly controlled by the volume of deposited solution, i.e. the number of nucleation events per unit surface, and the size of the resulting features is mainly controlled by the particle growth time, when solution and stable nuclei coexist. These two parameters, evaporation time and volume of deposited C₆₀ solution allow tuning the morphological outcome of C₆₀ thin films self-assembled on an air/water interface. The method was scaled up for film sizes over 11 cm² (see Experimental section). Films obtained on such a large scale showed particles that were smaller than 25 nm with a surface coverage above 50% (Fig. 4).

Fig. 3 Evolution of the film morphology with experimental parameters. (a) Effect of the volume of deposited C₆₀ solution on the lateral dimensions of C₆₀ features and on the total surface coverage; (b) effect of the chamber pressure on the lateral dimensions of the C₆₀ features (drop volume 50 μL in either case).

Fig. 4 Stabilization of the C₆₀ nanoporous morphology on glass against solvent rinsing. (a) Flat C₆₀ after photo-polymerization and solvent rinsing; (b) 500 nm large zoom of the same sample, the inset shows a flat C₆₀ film directly after deposition; (c) nanostructured C₆₀ film after photo-polymerization and before solvent rinsing; (d) same area imaged after solvent rinsing, the inset maps the difference in height between (c) and (d); (e) representative profiles extracted from (c) and (d).

Morphology stabilization and P3HT infiltration

For the versatile use of nanostructured C₆₀ films, the ability to freeze their optimal morphology against further processing or ageing is appealing.^26,43–45 It is known that C₆₀ photo-polymerizes in the solid state, yielding poorly soluble species.⁴⁰ This reaction is used here to stabilize nanoporous C₆₀ films against solvent processing. Solvent rinsing was carried out in a way that mimics the conditions under which films were treated during solar cell processing. The insolubility of flat polymerized C₆₀ films reached almost 100% (Fig. 4a and b). For nano-structured C₆₀ films, SFM measurements (Fig. 4c–e) of the same area before and after solvent rinsing showed an overall insolubility of 84 ± 15% (average of 5 samples). A reference SEM image demonstrating the pronounced solubility of a C₆₀ film that was not polymerized is shown in Fig. S5, ESI.† We found a similar level of stability against solvent rinsing for as-prepared films, and for films used for solar cell fabrication that were annealed and dried before photo-stabilization.

The insolubility of stabilized nanoporous C₆₀ films allows infiltrating them with a second material from the same casting solvent. P3HT coated from chloroform was taken as an example to illustrate the concept (see Fig. S6, ESI†). The concentration of P3HT was optimized to completely cover the C₆₀ nanostructures. The peak-to-valley amplitude was reduced from 72 nm to 3.3 nm by increasing the P3HT concentration up to 10 mg mL⁻¹. Films coated at 5 mg mL⁻¹ resulted in an optimal morphology and were chosen for the rest of the study. High-resolution SFM (see Fig. S7, ESI†) and SEM (Fig. 5) were used to prove that P3HT chains effectively infiltrate the nanoscopic voids in C₆₀ films.

Fig. 5 Use of nanoporous C₆₀ films (S-C₆₀) in solar cells. (a) SEM cross-section image of the S-C₆₀ cell without top contact; (b) SEM cross section of the S-C₆₀/F-C₆₀ cell.

Use in solar cells

The merits of a C₆₀ small-scale stabilized particle network can be demonstrated via improved charge separation in organic photovoltaics.⁷ Devices with the configuration ITO/TiO₂/C₆₀/P3HT/MoO₃/Ag were fabricated to measure the effect of C₆₀ nano-morphology on cell performance. The C₆₀ layer was either flat (40 nm evaporated C₆₀, named F-C₆₀), or nanostructured as described above (S-C₆₀) or a stack of both structures, S-C₆₀ on 30 nm F-C₆₀. 30 nm F-C₆₀ films were chosen in this case to keep the film absorption constant. Removing water from S-C₆₀ films before illumination was done by thermally annealing the samples under nitrogen (6 h, 80 °C). Drying induced a slight coarsening of the morphology. The change in morphology and the insolubility of films that were dried on a hotplate before polymerization are shown in Fig. S8, ESI.† Solar cell performance data are summarized in Table 1. First, C₆₀ stabilization was essential to device performance as pristine C₆₀ is largely dissolved when attempting to infiltrate P3HT from a chloroform solution. Consequently, the performance of non-stabilized F-C₆₀/P3HT cells was poor (Table 1, entry a). Stabilized F-C₆₀/P3HT cells reached on average η = 0.67% with J_SC = 2.5 mA cm⁻², V_OC = 0.48 V, FF = 56.8% (Table 1, entry b). S-C₆₀ alone, even when stabilized, performed less than F-C₆₀ (Table 1, entry c and d). This can be ascribed to the lower electron acceptor content in a S-C₆₀ film, as compared to F-C₆₀, and P3HT contacting directly the TiO₂ layer. These two adverse effects were reduced by stacking two S-C₆₀ layers on top of each other (Table 1, entry e), and suppressed by depositing S-C₆₀ on top of F-C₆₀ (Table 1, entry f). In the latter case, and for a constant total cell absorption, the average cell performance was increased by ∼50% compared to the flat film. Adding a further S-C₆₀ layer increased the best device performance further (Table 1, entry g).

Table 1 Summary of solar cell performance data^a

Entry	Structure of C60 layer	VOC [V]	JSC [mA cm^−2]	FF [%]	η [%]	ηmax [%]
a F-C60 denotes a planar C60 film, S-C60 a nanostructured film. Solar cell performance parameters are the average and standard deviation (s.d) values of a batch of more than 11 identically processed devices. VOC denotes the open-circuit voltage, JSC the short-circuit current, FF the fill factor, and η the power conversion efficiency.
a	F-C60 no light	0.29 (0.10)	1.3 (0.4)	29.6 (9.5)	0.13 (0.1)	0.27
b	F-C60 light	0.48 (0.02)	2.5 (0.2)	56.8 (2.3)	0.67 (0.1)	0.71
c	1 layer S-C60 no light	0.16 (0.04)	0.43 (0.1)	30.8 (3.0)	0.02 (0.01)	0.03
d	1 layer S-C60 light	0.42 (0.08)	0.92 (0.3)	45.9 (5.8)	0.18 (0.1)	0.27
e	2 layers S-C60 light	0.48 (0.02)	1.50 (0.7)	46.4 (2.2)	0.32 (0.1)	0.41
f	1 layer S-C60 on F-C60 light	0.52 (0.004)	3.3 (0.3)	59.0 (1.6)	1.0 (0.1)	1.14
g	2 layers S-C60 on F-C60 light	0.47 (0.09)	3.4 (0.9)	53.2 (5.4)	0.87 (0.3)	1.28

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Cross-section SEM images of S-C₆₀ and S-C₆₀ on F-C₆₀ active layers are shown in Fig. 5a and b, respectively, and a cross-section SEM image of F-C₆₀ active layer is shown in Fig. S9, ESI.† The SEM images reveal an enhanced interface after adding S-C₆₀ on F-C₆₀. From SFM data shown earlier, an increase of the interface area of ∼18% can be calculated by considering each C₆₀ feature as half-spherical, and the interface increase is about 36% for spherical structures. From this, it is clear that the 50% average increase in the cell efficiency and in particular the 32% increase in J_SC upon adding one S-C₆₀ layer onto F-C₆₀ is mainly due to an increase of the C₆₀/P3HT interface. This hypothesis is also supported by photoluminescence experiments. The difference between P3HT emission on F-C₆₀ and on S-C₆₀/F-C₆₀ is close to 30% (Fig. S10, ESI†). Internal photon-to-current conversion efficiency (IPCE) data from F-C₆₀ and 2 × S-C₆₀ on F-C₆₀ devices are shown in Fig. S11, ESI.† The IPCE maximum value was 30% for F-C₆₀ and 42% for 2 × S-C₆₀ on F-C₆₀.

Reports on C₆₀/P3HT solar cells are scarce, because of the low solubility of C₆₀ in organic solvents and because the two components do not spontaneously phase-separate to form an optimal bulk-heterojunction morphology, limiting drastically the cell performance.²⁴ One solution found by Geiser et al. was to anneal the whole device to force the morphology to evolve favorably, reaching over 2% in PCE.²⁴ Even if the final PCE shown here does not reach that of bulk-heterojunction fullerene/P3HT devices, the solution proposed, using a C₆₀ film stabilized in an optimal morphology, can be seen as rather universal and opens important improvement possibilities for other working systems.

Conclusions

In this article we have shown a new method based on interfacial nucleation-and-growth to form nanoporous C₆₀ films. They can be described as 2D percolating networks of C₆₀ nanoparticles with diameters below 50 nm and as small as 10 nm. The particles are the smallest and most monodisperse reported to date. Both single-layer and multilayer films could be readily fabricated, stabilized against further solvent processing, and the process was up-scaled to several square centimeters. The lateral dimensions of C₆₀ domains below 50 nm make such thin film morphologies highly adapted for any physical or chemical process where an enhanced interface is advantageous. We chose to illustrate this by the example of increased performance in organic P3HT-based solar cells, accomplished by an enhanced C₆₀/P3HT interface and adapted donor/acceptor morphology. However, we believe that our assembly method may be extended beyond fullerenes and OPV, to other functional molecules and fields where enhanced interface matters, such as catalysis, sensing or fuel cells.

Acknowledgements

The authors thank the staff of the BRNC, Rüschlikon for support, J. E. Quinsaat (Empa Dübendorf) for TEM imaging, M. Makha (Empa Dübendorf) for providing TiO₂ substrates, C. Ruiz-Vargas, N. Mojarad (ETH) for helpful comments, and F. Nüesch (Empa Dübendorf) for support.

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Footnote

† Electronic supplementary information (ESI) available: SEM images of large area nanostructured C₆₀ films, SEM images of S-C₆₀ deposited on a 90° geometric kink, a TEM image of S-C₆₀, an SEM image of large C₆₀ crystals, SEM images showing the solubility of S-C₆₀ films that were not illuminated, SFM images of P3HT infiltration, an SFM study of S-C₆₀ films upon drying, an SEM side-image of a F-C₆₀/P3HT solar cell, P3HT photoluminescence data, and IPCE measurements of characteristic solar cells. See DOI: 10.1039/c6ra02720b

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