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. 2026 Sep 10;11(37):55685–55691. doi: 10.1021/acsomega.6c04877

Facile Polymer-Assisted Assembly of Fullerenes into Ultrathin Crystalline Nanosheets with Enhanced Electron Transport Properties

Anjali Bindu Ramesan †,‡, Sandeepa Kulala Vittala †,‡, Vibhu Darshan †, K N Narayanan Unni †,‡,*, Joshy Joseph †,‡,*
PMCID: PMC13613396  PMID: 42798994

Abstract

Ordered assembly of fullerenes is of significant importance in organic electronic applications, as charge carrier mobility in fullerene-based materials is highly sensitive to structural order. However, commonly employed solution-processing techniques often suffer from strong aggregation tendencies or uncontrolled crystallization of fullerenes during film formation. In contrast, advanced nanofabrication techniques that enable precise control over fullerene morphology at the nanoscale are typically complex and costly. In this work, we propose a facile supramolecular strategy based on the polymer-assisted assembly of a pyridinium-functionalized fullerene amphiphile (FPy) to achieve ordered fullerene architectures, thereby enhancing charge carrier mobility through reduced energetic disorder. A commercially available anionic polymer, polystyrenesulfonate (PSS), was employed to assemble the cationic fullerene derivative (FPy) primarily via electrostatic interactions, further stabilized by π–π stacking between fullerene units. The interaction of FPy with PSS was confirmed by UV–visible absorption spectroscopy, while TEM, SEM, and AFM analyses revealed the formation of ultrathin two-dimensional crystalline nanosheets with highly ordered fullerene packing. X-ray diffraction (XRD) analyses clearly demonstrate a transition from polycrystalline FPy to a highly ordered and uniform face-centered cubic (FCC) lattice in the PSS/FPy hybrid, confirming the anionic polymer-induced switching toward a more ordered nanoassembly. Space-charge-limited current (SCLC) measurements further revealed an order-of-magnitude enhancement in electron mobility (0.92 × 10–3 cm2 V–1 S–1) compared to FPy alone, which can be correlated with the reduced energetic disorder in the FCC lattice structure of the PSS/FPy hybrid assembly. Overall, this study highlights the significance of polymer-assisted assembly of rationally designed amphiphilic fullerenes as a general and scalable strategy to improve electron transport in a wide range of optoelectronic devices.

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1. Introduction

Fullerenes, because of their unique molecular structure and exceptional electronic properties, have emerged as outstanding electron acceptors and transport materials in organic electronic applications. Their widespread use in organic electronic devices arises from reduced reorganization energy, efficient forward and reverse electron-transfer processes compared to planar electron acceptors, and high charge-carrier mobility. − Among fullerene derivatives, C60 is the most extensively studied and employed material in organic electronics and exhibits one of the highest reported mobilities, attributed to its high molecular symmetry and deep-lying LUMO level. , However, the practical application of C60 is limited by its poor solubility and strong aggregation tendency in common solvents, which adversely affect the film morphology and device stability. Consequently, chemically modified fullerene derivatives have been extensively explored, offering improved solubility, enhanced solution processability, and tunable electronic energy levels.

PCBM, a methanofullerene derivative, is widely employed as an electron acceptor material in electronic devices due to its excellent electron-extraction capability and high electron mobility. − Despite these advantages, the long-term operational stability of PCBM-based active layers remains a significant bottleneck, limiting their practical deployment. To address this challenge, various strategies have been explored, including the incorporation of suitable additives − and the optimization of solution-processing and deposition techniques. − The chemical structure of PCBM also permits further functionalization, enabling the development of a library of derivatives with reduced aggregation tendencies and enhanced device performance. − For example, Yang et al. synthesized two highly soluble methanofullerenes by introducing a branched alkoxy side chain onto the phenyl ring and demonstrated the impact of carboxylic acid incorporation and electron-donating substitution on charge transport, achieving electron mobilities in the range of 10–2–10–3 cm2 V–1 s–1.

In a related approach, Thelakkat and coworkers reported PCBM-grafted copolymers as electron transport layers, exhibiting an electron mobility of approximately 1 × 10–4 cm2 V–1 s–1, determined using the space-charge-limited current (SCLC) method. While these studies highlight the effectiveness of chemical modification strategies, they also underscore the strong dependence of charge transport on molecular packing and structural order.

Indeed, numerous studies have demonstrated that an ordered molecular arrangement within the active layer is crucial for achieving efficient charge transport and enhanced performance in OFETs and photovoltaic devices. − In this context, Huang and coworkers systematically examined the relationship between structural disorder, energetic disorder (i.e., energy-level broadening), and open-circuit voltage in fullerene-based electron-transport layers for perovskite solar cells. Their findings showed that a simple solvent-annealing treatment significantly reduces structural disorder in PCBM assemblies by promoting the formation of a face-centered cubic (FCC) lattice. This improved structural ordering effectively suppresses energetic disorder, leading to enhanced electron transport and an increased open-circuit voltage. Lei and coworkers’ pioneering investigation of fullerene ammonium iodide (PCBANI) established an n-doping mechanism in which partial electron transfer from iodide ions to the fullerene core enhances electron transport. Subsequently, they further improved the electron-transport properties of PCBANI by inducing enhanced lamellar ordering through solvent-mediated assembly.

Covalent cross-linking is a well-accepted strategy for enhancing the structural robustness of fullerene-based devices by suppressing excessive fullerene molecular aggregation or crystallization. For example, Hsu and coworkers reported improved power conversion efficiencies in inverted polymer solar cells using thermal cross-linking of a dendron-functionalized fullerene derivative bearing two styryl groups. Other reactive functionalities, such as epoxides, , halides, , and azides, , have also been employed to enable cross-linking, contributing significantly to the advancement of fullerene-based solar cells. Recently, our group introduced a cross-linking strategy to fabricate solvent-resistant adhesive films by designing a fullerene-benzoxazine derivative for facilitating electron transport in inverted polymer solar cells. We also proposed a rational molecular design strategy for developing new polyfullerenes via the oxidative polymerization of an aniline-appended fullerene derivative. However, most of these approaches necessitate the use of millimolar concentrations of fullerenes to attain the desired electron mobility, and the associated processes are often labor-intensive and energy-consuming. Therefore, the development of simpler and more efficient strategy is highly desirable.

Previously, we demonstrated the DNA-assisted assembly of a cationic pyridinium-appended fullerene amphiphile, FPy, into ultrathin fullerene nanosheets through electrostatic interactions between the cationic pyridinium moieties and the negatively charged phosphate backbone of DNA. These types of highly ordered DNA nanostructures are particularly attractive in the context of DNA nanoelectronics since the structure of DNA provides opportunities for further functionalization through sequence-specific design and chemical modification. Besides these advantages of multifunctional DNA nanostructures, their integration into large-scale electronic devices may be limited by cost and processing considerations. Thus, we have extended the work to investigate whether a commercially available synthetic polyelectrolyte could direct a similar fullerene organization while offering greater practical relevance to organic electronic devices. In this work, we exploit the electrostatic interactions between FPy and the anionic polymer, polystyrenesulfonate (PSS), commonly used with PEDOT as a solubilizing and charge-transport component in photovoltaic devices, to generate long-range-ordered fullerene assemblies using only micromolar concentrations of the fullerene. As we previously demonstrated in DNA-assisted FPy assemblies, the electrostatic interactions between FPy and PSS, coupled with π–π stacking interactions among the fullerene cores, govern the hierarchical organization of the components into ultrathin crystalline nanosheets. XRD analysis demonstrates the preferential organization of FPy into an FCC lattice structure upon assembly with PSS. This enhanced long-range crystalline order leads to significantly improved electron mobility in the PSS/FPy hybrid compared to pristine FPy.

To further establish the importance of the amphiphilic design of FPy and the role of polymer-assisted assembly in modulating electron transport, we conducted a comparative study using PCBM and the PSS/PCBM hybrid system. Overall, this study highlights the potential of a facile polymer-assisted assembly strategy for improving electron transport in fullerene-based layers while minimizing the material usage and processing complexity.

2. Materials and Methods

2.1. Materials

All reagents and solvents used for the synthesis were purchased from commercial suppliers, including Sigma-Aldrich, Alfa Aesar, and Spectrochem, and used without further purification, unless otherwise specified. Poly­(sodium 4-styrenesulfonate) (PSS) with an average molecular weight of 70,000 was obtained from Sigma-Aldrich. The detailed synthesis and characterization of the amphiphilic fullerene derivative, FPy were reported previously.

2.2. Interaction Studies of FPy with PSS

For interaction studies between FPy and PSS, all experiments were carried out in a 10% DMSO-water mixture. The fullerene derivative (FPy) was initially dissolved in DMSO and subsequently diluted to the desired concentration using a 10% DMSO-water solvent system. UV–vis absorption measurements were performed using a Shimadzu UV-2600 PC spectrophotometer with a quartz cuvette of 1 cm path length. A 3 μM solution of FPy was placed in the cuvette, and incremental aliquots of PSS were added from a 1 mg mL–1 stock solution while maintaining the FPy concentration constant. The changes in absorbance were recorded after each addition to monitor the interaction between FPy and PSS.

2.3. Characterization of PSS/FPy Hybrid

Transmission electron microscopy (TEM) samples were prepared by mixing PSS (10 μg/mL) and FPy (3 μM) in a 10% DMSO-water mixture, followed by incubation for 30 min. 20 μL of the resulting solution was cast onto a carbon-coated copper grid and dried under ambient conditions. HRTEM measurements were performed using an FEI Tecnai G2 30 microscope equipped with EDAX and operated at an accelerating voltage of 300 kV. Images were recorded using a Hamamatsu ORCA CCD camera. Atomic force microscopy (AFM) samples were prepared by dropcasting the PSS/FPy mixture (10 μg/mL: 3 μM; incubated for 30 min in 10% DMSO-water) onto a freshly cleaved mica surface and subsequently dried under ambient conditions. AFM measurements were carried out on a Bruker MultiMode AFM operating in tapping mode, using microfabricated TiN cantilever tips (NT-MDT NSG series) with a resonance frequency of ∼299 kHz and a spring constant in the range of 20–80 N m–1. Section analysis of AFM images was performed offline using NanoScope Analysis 1.5 software.

Scanning electron microscopy (SEM) samples were prepared similarly to the AFM samples and analyzed using a JEOL JSM-35 Scanning Electron Microscope at an accelerating voltage of 15 kV. X-ray diffraction (XRD) measurements were conducted using a Xeuss SAXS/WAXS system equipped with a Genix microsource (Xenocs), operated at 50 kV and 0.6 mA. The Cu Kα radiation (λ = 1.54 Å) was collimated using a FOX2D mirror and two pairs of scatterless slits (Xenocs). Two-dimensional WAXS patterns were collected using a Mar345 image plate detector and processed with Fit2D software.

2.4. Space-Charge-Limited Current (SCLC) Measurements

For SCLC measurements, fullerene solutions (FPy or PSS/FPy) prepared in a 10% DMSO-water mixture were drop-cast onto precleaned indium tin oxide (ITO) substrates. The coated ITO substrates were dried on a hot plate at 70 °C to form the active layer. The thickness (L) of the films was optimized with the aid of a Dektak XT stylus profilometer and is mentioned in the corresponding graphs, with a standard deviation of ±10 nm. Subsequently, an Al electrode was deposited by thermal evaporation under vacuum. The devices were encapsulated immediately after fabrication to minimize degradation of the active layer. Control devices based on PCBM and PSS/PCBM were fabricated following an identical procedure, using PCBM solutions prepared in a 10% THF–water mixture.

The charge-carrier mobility was obtained by fitting the J–V characteristics in the space-charge-limited current regime using the Mott–Gurney equation:

J=9ε0εrμV28L3

where ε0 is the permittivity of free space (8.854 × 10–12 F/m), εr is the dielectric constant of the active layer (∼3, a value commonly reported for fullerene-based organic semiconductors and PCBM-derived electron-transport materials), , μ is the electron mobility, V is the voltage drop across the device, and L is the active layer thickness. The fitting was performed in the quadratic region of the J–V curve, where J showed a V2 dependence. Mobility values are reported as mean ± standard deviation obtained from measurements on at least five independent samples.

3. Results and Discussion

3.1. Molecular Structure and Interaction of FPy with PSS

The cationic amphiphilic fullerene, FPy (Figure A), was designed to bind with anionic polymers through electrostatic interactions and to promote the formation of ordered fullerene nanostructures via subsequent π–π interactions between fullerene moieties. In this structure, the fullerene is functionalized with a cationic pyridinium unit linked through a triethylene glycol spacer, which enhances its solubility in aqueous solvent systems and facilitates supramolecular organization. In our earlier work, we demonstrated that FPy interacts electrostatically with various oligonucleotides, resulting in the formation of ultrathin nanosheets with highly ordered fullerene assemblies. In this regard, polystyrenesulfonate (PSS, Figure B), an anionic polymer with sulfonate groups, serves as a suitable analog to DNA due to its flexible polyanionic backbone.

1.

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Molecular structures of (A) FPy and (B) PSS and (C) changes in the absorption spectra of FPy with the addition of PSS.

The interaction between FPy and PSS was first examined by UV–visible absorption spectroscopy (Figure C). Gradual addition of PSS to a solution of FPy in a 10% DMSO-water mixture led to a decrease in the characteristic fullerene absorption bands at ∼260 nm and ∼334 nm, indicating the onset of aggregation of FPy in the presence of PSS. The apparent increase in absorption near 260 nm at higher PSS concentrations arises from the intrinsic absorption of PSS itself. The strong electrostatic attraction between the pyridinium moieties of FPy and the sulfonate groups of PSS effectively screens the Coulombic repulsion among the cationic fullerene molecules, thereby facilitating their closer packing and assembly. This screening effect facilitates closer packing of FPy units, ultimately promoting π–π stacking interactions and the formation of ordered fullerene assemblies.

3.2. Morphological Analysis of PSS/FPy Hybrid

The morphology of the PSS/FPy hybrids was systematically investigated by SEM, TEM, and AFM and compared with that of PSS alone, prepared in a 10% DMSO–water mixture. SEM images of the PSS/FPy hybrid revealed the formation of micrometer-sized, sheet-like architectures with well-defined and sharp edges. These features are distinctly different from the comparatively amorphous, sheet-like morphology observed for pristine PSS (Figure A,B). The coassembly of PSS and FPy within the hybrid structure is further confirmed by EDS elemental mapping of C, N, and S (Figure S1), indicating the uniform distribution of both components across the nanosheets.

2.

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SEM images of (A) PSS alone and (B) PSS/FPy hybrid; TEM images of (C) PSS alone and (D) PSS/FPy hybrid; and the (E) magnified TEM image of the PSS/FPy hybrid (inset: SAED pattern).

TEM images of the PSS/FPy hybrid demonstrate clear lattice features, suggesting that the nanosheet crystallinity arises from an ordered arrangement of fullerene units (Figure C–E). The corresponding selected-area electron diffraction (SAED) pattern further confirms the crystalline nature of the hybrid nanosheets, displaying well-defined diffraction spots indicative of highly ordered molecular packing. In contrast, TEM images of PSS alone show no discernible crystalline features, highlighting the structural ordering induced by FPy incorporation. AFM analysis also differentiates the hybrid from pristine PSS (Figure S2). PSS alone forms relatively larger nanosheets with lateral dimensions of approximately 1–2 μm and a thickness of ∼2.5 nm, exhibiting limited lateral interconnection (Figure S2A). In comparison, the PSS/FPy hybrid displays micrometer-scale nanosheets formed via the lateral association of smaller nanosheets (100–500 nm), while maintaining a comparable thickness of ∼2.5 nm (Figure S2B,C). The corresponding height profiles (Figure S2D,E) confirm similar thickness values for both systems, suggesting that the assembly process promotes controlled lateral growth rather than uncontrolled vertical stacking or random aggregation. The morphological analyses collectively suggest that the anionic polymer promotes the assembly of fullerenes through multivalent interactions between the polymer backbone and cationic pyridinium moieties, thereby bringing the fullerene moieties together. Subsequently, π–π stacking and van der Waals interactions between the fullerene amphiphiles facilitate structural ordering in the hybrid assemblies.

To further support the concept, we investigated the interaction of FPy with a neutral polymer, polyvinylpyrrolidone (PVP, Figure S3A), and an anionic small molecule, 4,5-dihydroxy-1,3-benzenedisulfonic acid (DHBS, Figure S3B). TEM analysis of PVP/FPy hybrid showed that PVP can also induce ordered fullerene assemblies due to strong ion–dipole interactions between the pyridinium cations of FPy and the carbonyl groups of PVP (Figure S3Cand D). Meanwhile, the anionic small molecule DHBS fails to produce ordered assemblies of fullerenes (Figure S3E). These observations indicate that polymer-assisted multivalent interactions are capable of directing the assembly of FPy and that polymer architecture is essential for directing the assembly.

3.3. XRD Analysis

To gain deeper insight into the structural organization of fullerenes within the nanosheets, XRD analysis of the PSS/FPy hybrid was performed and compared with that of pristine FPy under identical conditions (Figure A). The diffraction pattern of bare FPy exhibits multiple reflections that can be indexed to different close-packed fullerene arrangements, including face-centered cubic (FCC), octahedral, and triclinic phases. Miller indices 110, 112, and 121 correspond to triclinic phases, whereas 230 and 140 correspond to octahedral lattices. All other peaks in bare FPy indicate an FCC lattice, which is displayed in bold in Figure A. − This multiplicity of reflections indicates polymorphic disorder in the pristine FPy sample. The XRD pattern of PSS exhibits diffraction features within the same angular region as FPy. However, the diffraction profile of the PSS/FPy hybrid differs significantly from those of both pristine FPy and PSS, exhibiting selective enhancement and retention of specific reflections together with suppression of several reflections observed in pristine FPy and PSS, as shown in Figure S4. The reflections retained in the XRD pattern of the PSS/FPy hybrid can be indexed exclusively to the FCC lattice of fullerene (Table ). This observation suggests that PSS selectively directs the organization of fullerene units into a well-defined FCC-packed structure, suppressing the formation of triclinic and octahedral domains, as shown in Figure B. The preferential stabilization of the FCC phase highlights the structure-directing role of the polymer during assembly. A detailed summary of the XRD data, including the peak positions (2θ), d-values, and corresponding diffraction planes for FPy and PSS/FPy, along with their corresponding lattice assignments, is provided in Table S1. Peak assignments were made by comparison with reported diffraction patterns of fullerene crystalline phases in the literature. −

3.

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(A) XRD spectra of FPy alone and PSS/FPy hybrid; (B) schematic representation of PSS-assisted assembly of FPy to an FCC lattice structure.

1. Assignment of XRD Diffraction Planes Observed for FPy and PSS/FPy Hybrid, with Corresponding Lattice Structures (FCC, Octahedral, and Triclinic) − Identified for Each Reflection.

Peaks in FPy
 Peaks in PSS/FPy
FCC Octahedral Triclinic FCC
220 230 110 220
311 140 112 222
222   121 331
331     133
133     420
420     422
422     333
333     511
511      
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3.4. Electron Mobility Measurements by SCLC Method

The electron mobility of the PSS/FPy hybrid nanosheets was evaluated using the space-charge-limited current (SCLC) method. Compared to pristine FPy, a substantial enhancement in electron mobility was observed upon assembly with PSS, which can be attributed to the formation of an ordered FCC lattice in the hybrid nanosheets. The FCC packing motif of fullerenes is particularly favorable for electron-transport layers, as it minimizes energetic disorder within the close-packed structure. FPy alone exhibited an electron mobility of 1.2 (±0.5) × 10–4 cm2 V–1 s–1, with an active-layer thickness of 120 ± 10 nm, whereas the PSS/FPy hybrid showed nearly an order-of-magnitude enhancement to 0.92 (±0.2) × 10–3 cm2 V–1 s–1 with a comparable active-layer thickness of 150 ± 10 nm (Figure and Figure S6A). This improvement clearly highlights the beneficial effect of polymer-directed structural ordering on charge transport.

4.

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(A) Device structure for electron mobility measurements by the SCLC method, and (B) mobility measurements of FPy and PSS/FPy with comparable active-layer thicknesses of 120 ± 10 nm and 150 ± 10 nm, respectively.

To further establish the importance of amphiphilic molecular design of FPy and its ordered assembly with PSS, control experiments were conducted using PCBM under identical fabrication conditions to eliminate processing-related variations (Figure S5 and Figure S6B). Under similar fabrication conditions, FPy alone displays an electron mobility comparable to that of standard PCBM, with values of 3.8 (±0.4) × 10–4 cm2 V–1 s–1 (L = 175 ± 10 nm) and 4.9 (±0.5) × 10–4 cm2 V–1 s–1 (L = 230 ± 10 nm), respectively. However, upon incorporation of PSS, the electron mobility of PCBM decreases to 1.5 (±0.04) × 10–4 cm2 V–1 s–1 (L = 175 ± 10 nm). Unlike FPy, PCBM lacks ionic functionalities, resulting in disrupted molecular packing due to random aggregation in the presence of PSS, as shown in Figure S7. In contrast, the electrostatic interaction between FPy and PSS promotes organized fullerene packing into an FCC lattice, leading to a marked enhancement in electron mobility to 1.8 (±0.6) × 10–3 cm2 V–1 s–1 (L = 200 ± 10 nm) (Table S2). Collectively, these results underscore the critical role of ionic amphiphilic design and polymer-assisted structural ordering in achieving superior electron-transport performance.

4. Conclusions

In summary, we demonstrate that a rationally designed cationic fullerene derivative, FPy, can be organized into a highly ordered crystalline assembly through electrostatic interaction with the commercially available anionic polymer polystyrenesulfonate (PSS). Morphological analyses of the PSS/FPy hybrid confirm the formation of ultrathin nanosheets, while SAED and XRD results reveal the preferential organization of fullerene units into a face-centered cubic (FCC) lattice in the presence of PSS. Such polymer-assisted assembly is particularly advantageous for electron transport layers, as FCC packing effectively reduces energetic disorder in fullerene assemblies. Electron mobility measurements of pristine FPy and the PSS/FPy hybrid, evaluated using the space-charge-limited current (SCLC) method, reveal an approximately 1 order of magnitude enhancement in electron mobility for the PSS/FPy hybrid compared to pristine FPy. Notably, this improvement is achieved using micromolar concentrations of fullerene, without the need for covalent cross-linking or postdeposition treatments. Furthermore, comparative studies with PCBM confirm that the enhanced charge transport originates from the synergistic effect of the amphiphilic molecular design and polymer-directed structural ordering. Overall, this work presents a simple and efficient supramolecular strategy to enhance charge-carrier transport in fullerene-based layers for electronic device applications.

Supplementary Material

ao6c04877_si_001.pdf (629KB, pdf)

Acknowledgments

Financial support from the DST Nano Mission Project (DST/NM/TUE/EE-02/2019-1), the CSIR-NIIST DCPC Centre of Excellence (PC-II-25014/3/2022-PC II-CPC, FTS: 3019123), and the Anusandhan National Research Foundation (ANRF) under the CRG scheme (CRG/2022/005710) is gratefully acknowledged. A.B.R. acknowledges the University Grants Commission (UGC), Government of India, for the award of a research fellowship. This work was partially supported by the National Laboratories Scheme under the ULIP [Grant Number MLP002633].

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c04877.

  • EDS elemental mapping and AFM images of PSS, FPy, and PSS/FPy hybrid; TEM images of PVP/FPy and DHBS/FPy hybrids; XRD spectra of PSS, FPy, and PSS/FPy hybrid; mobility measurements of PCBM, PSS/PCBM, FPy, and PSS/FPy; log I–log V plots for A) FPy and PSS/FPy B) PCBM, PSS/PCBM, PSS/FPy, and FPy; TEM images of the PSS/PCBM hybrid; assignment of XRD diffraction planes observed for FPy and PSS/FPy hybrid; mobility measurements of FPy, PCBM, PSS/PCBM, and PSS/FPy by the SCLC method (PDF)

The authors declare no competing financial interest.

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