A porphyrin-fullerene dyad with a supramolecular "double-cable" structure as a novel electron acceptor for bulk heterojunction polymer solar cells

Direct Observations of Uniform Bulk Heterojunctions and the Energy Level Alignments in Nonfullerene Organic Photovoltaic Active Layers

State-of-the-art organic photovoltaic (OPV) cells rely on the engineering of the energy levels of the organic molecules as well as the bulk-heterojunction nanomorphology to achieve high performance. However, both are difficult to measure inside the active layer where the electron donor and acceptor molecules are mingled. While the energy level alignments of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) between the electron donors and acceptors may be altered in the mixed active layer compared to their pure forms, the nanomorphology of the donor and acceptor molecular domains is mostly studied in indirect means. Here, we present the direct observations of the nanomorphology of the molecular domains as well as the energy level alignments in the active layer of a nonfullerene-based OPV (donor: PBDB-T-2F and acceptor: IT-4Cl) using cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S). It is revealed that (1) the bulk-heterojunction (BHJ) structures are homogeneous and uniform throughout the ∼1.2 μm thick active layer; (2) the energy alignments between the donor-rich and acceptor-rich domains are directly observed; (3) there exist the intermixing domains at the boundaries of the donor-rich and acceptor-rich domains with thickness in the nm scale; (4) the exciton binding energies in PBDB-T-2F and IT-4Cl are estimated to be 0.74 and 0.32 eV, respectively; and (5) there is an ∼0.7 V loss in the open circuit voltage. The results provide a nanoscale understanding of the OPV active layers to guide further improvement of the OPV performance.

Self-Assembled Materials Incorporating Functional Porphyrins and Carbon Nanoplatforms as Building Blocks for Photovoltaic Energy Applications

As a primary goal, this review highlights the role of supramolecular interactions in the assembly of new sustainable materials incorporating functional porphyrins and carbon nanoplatforms as building blocks for photovoltaics advancements.

Host-guest chemistry of giant molecular shape amphiphiles based on POSS-PDI conjugates

Giant shape amphiphiles (GSA) are giant molecules made with nano-building blocks that have distinct shapes. The incompatible packing behaviors of the nano-building blocks of GSA could create cavities within certain conformers of the GSA, but the host-guest chemistry of GSA has not been explored yet. In this study, POSS-PDI-POSS (PPP), which is made by connecting two nano-cubes, isobutyl-polyhedral oligomeric silsesquioxanes (POSS), to a conjugated π-conjugated core, perylene diimide (PDI), is demonstrated as a novel acyclic synthetic host. In its bent conformer, PPP shows a cavity next to its PDI core. Via forming host-guest complexes with π-conjugated guests such as pyrene and perylene, PPP is found to transform from the bent-conformer into the extended-conformer, creating the steric features to accommodate guest molecules. Subsequent thermal annealing of the host-guest complexes removes the π-conjugated guests and restores the bent conformation and photophysical properties of PPP, which verifies that PPP, as a novel acyclic host, is capable of dynamic host-guest assembly. Moreover, the results prove that cavities at the molecular level can be created by connecting nano-building blocks with distinct shapes. This finding may inspire developments in the host-guest chemistry of GSA and nanomaterial innovation.

Glassy Porphyrin/C60 Composites: Morphological Engineering of C60 Fullerene with Liquefied Porphyrins

Morphological control of C₆₀ fullerene using liquefied porphyrins (1 and 2) as the host matrices was explored. Slow evaporation of the solvent of the equimolar mixture of porphyrin and C₆₀ in toluene afforded the porphyrin/C₆₀ composite with a 3:1 molar ratio. The stoichiometric binding behaviors suggest that specific porphyrin-C₆₀ interactions operate the formation of the porphyrin/C₆₀ composites, as corroborated by spectroscopic and thermal properties, and glazing-incidence wide-angle X-ray diffraction. Under the bulk conditions, the conventional thermodynamic advantage of multiple binding cooperativity for molecular recognition is unlikely to explain the stoichiometric binding behaviors. Instead, we propose a size-matching effect on the porphyrin-C₆₀ interaction in the bulk porphyrin matrices, i.e., "supramolecular solvation". The glassy nature of the porphyrin matrices was transmitted to C₆₀ through the specific interaction, and the porphyrin/C₆₀ composites adopted glassy states at room temperature.

Tuning fullerene miscibility with porphyrin-terminated P3HTs in bulk heterojunction blends

Understanding and manipulating the miscibility of donor and acceptor components in the active layer morphology is important to optimize the longevity of organic photovoltaic devices and control power conversion efficiency. In pursuit of this goal, a "porphyrin-capped" poly(3-hexylthiophene) was synthesized to take advantage of strong porphyrin:fullerene intermolecular interactions that modify fullerene miscibility in the active layer. End-functionalized poly(3-hexylthiophene) was synthesized via catalyst transfer polymerization and subsequently functionalized with a porphyrin moiety via post-polymerization modification. UV-vis spectroscopy and X-ray diffraction measurements show that the porphyrin-functionalized poly(3-hexylthiophene) exhibits increased intermolecular interactions with phenyl-C61-butyric acid methyl ester (PCBM) in the solid state compared to unfunctionalized poly(3-hexylthiophene) without sacrificing microstructure ordering that facilitates optimal charge transport properties. Additionally, differential scanning calorimetry revealed porphyrin-functionalized poly(3-hexylthiophene) crystallization decreased only slightly (1-6%) compared to unfunctionalized poly(3-hexylthiophenes) while increasing fullerene miscibility by 55%. Preliminary organic photovoltaic device results indicate device power conversion efficiency is sensitive to additive loading levels, as evident by a slight increase in power conversion efficiency at low additive loading levels but a continuous decrease with increased loading levels. While the increased fullerene miscibility is not balanced with significant increases in power conversion efficiency, this approach suggests that integrating non-bonded interaction potentials is a useful pathway for manipulating the morphology of the bulk heterojunction thin film, and porphyrin-functionalized poly(3-hexylthiophenes) may be useful additives in that regard.

Engineering self-assembly of giant molecules in the condensed state based on molecular nanoparticles

In biological systems, it is well-known that the activities and functions of biomacromolecules are dictated not only by their primary chemistries, but also by their secondary, tertiary, and quaternary hierarchical structures. Achieving control of similar levels in synthetic macromolecules is yet to be demonstrated. Most of the critical molecular parameters associated with molecular and hierarchical structures, such as size, composition, topology, sequence, and stereochemistry, are heterogenous, which impedes the exploration and understanding of structure formation and manipulation. Alternatively, in the past few years we have developed a unique giant molecule system based on molecular nanoparticles, in which the above-mentioned molecular parameters, as well as interactions, are precisely defined and controlled. These molecules could self-assemble into a myriad of unconventional and unique structures in the bulk, thin films, and solution. Giant molecules thus offer a robust platform to manipulate the hierarchical structures via precise and modular assemblies of building blocks in an amplified size level compared with small molecules. It has been found that they are not only scientifically intriguing, but also technologically relevant.

Regioisomeric Tandem Triblock Shape Amphiphiles Based on Polyhedral Oligomeric Silsesquioxanes

We report the design, synthesis and self-assembly of a series of tandem triblock shape amphiphile regioisomers composed of one rod-like fluorinated polyhedral oligomeric silsesquioxane (FPOSS) cage tethered with two spherical tert-butyl-POSS (tBPOSS) cages in para-, meta-, and ortho-configurations. Precision syntheses are achieved through sequential "click" reactions and the compounds are thoroughly characterized by combined techniques of NMR, FT-IR and MALDI-TOF MS spectroscopy. Only the ortho-isomer forms an ordered lamellae structure induced by the crystallization of perfluoroalkyl-chains and the other two remain amorphous due to frustrated packing by unfavorable regio-configuration. The distinct assembly behaviors of these three regioisomers reflect the delicate influence of a minute structural difference on precision macromolecular assembly.

Fullerene size controls the selective complexation of [11]CPP with pristine and endohedral fullerenes

The ability of the carbon nanoring [11]cycloparaphenylene ([11]CPP) for coordinating fullerenes has been tested using a series of hosts, including the pristine fullerenes C60, C70, C76 and C78, the clusterfullerene Sc3N@C80, monometallic endofullerenes Y@C82 and Tm@C82, and dimetallic endofullerenes Y2@C82 and Lu2@C82. A systematic theoretical study employing dispersion corrected density functional methods has been carried out in order to explore the characteristics of the complexes and the strength of the interaction. Depending on the dimer, complexation energies span from around -36 kcal mol-1 with C60 to -53 kcal mol-1 with the C82 derivatives. Dispersion is the main stabilizing contribution in these dimers, so the molecules arrange to maximize the number of close interatomic contacts. Since most fullerenes can properly fill the cavity of the nanoring the stability of the complexes is pretty similar, with the exception of the smallest fullerenes. The complexes with endohedral fullerenes show similar stabilities in all cases studied, with no noticeable dependence on the nature of the endohedral species. The results obtained suggest that fullerenes larger than C76 could be selectively encapsulated by [11]CPP compared to smaller fullerenes.

Assessment of electronic transitions involving intermolecular charge transfer in complexes formed by fullerenes and donor–acceptor nanohoops

Inserting an anthraquinone or tetracyanoanthraquinone unit in cycloparaphenylene nanohoops facilitates intermolecular electron transfer to a fullerene guest.

Conjugated systems of porphyrin–carbon nanoallotropes: a review

This review summarizes the synthesis and applications of various porphyrin–carbon nanoallotrope conjugates.

Transition from disordered aggregates to ordered lattices: kinetic control of the assembly of a computationally designed peptide

Natural biomolecular self-assembly typically occurs under a narrow range of solution conditions, and the design of sequences that can form prescribed structures under a range of such conditions would be valuable in the bottom-up assembly of predetermined nanostructures. We present a computationally designed peptide that robustly self-assembles into regular arrays under a wide range of solution pH and temperature conditions. Controling the solution conditions provides the opportunity to exploit a simple and reproducible approach for altering the pathway of peptide solution self-assembly. The computationally designed peptide forms a homotetrameric coiled-coil bundle that further self-assembles into 2-D plate structures with well-defined inter-bundle symmetry. Herein, we present how modulation of solution conditions, such as pH and temperature, can be used to control the kinetics of the inter-bundle assembly and manipulate the final morphology. Changes in solution pH primarily influence the inter-bundle assembly by affecting the charged state of ionizable residues on the bundle exterior while leaving the homotetrameric coiled-coil structure intact. At low pH, repulsive interactions prevent 2-D lattice nanostructure formation. Near the estimated isoelectric point of the peptide, bundle aggregation is rapid and yields disordered products, which subsequently transform into ordered nanostructures over days to weeks. At elevated temperatures (T = 40 °C or 50 °C), the formation of disordered, kinetically-trapped products largely can be eliminated, allowing the system to quickly assemble into plate-like nanostructured lattices. Moreover, subtle changes in pH and in the peptide charge state have a significant influence on the thickness of formed plates and on the hierarchical manner in which plates fuse into larger material structures with observable grain boundaries. These findings confirm the ability to finely tune the peptide assembly process to achieve a range of engineered structures with one simple 29-residue peptide building block.

Synthesis and Self-Assembly of Shape Amphiphiles Based on POSS-Dendron Conjugates

Shape has been increasingly recognized as an important factor for self-assembly. In this paper, a series of shape amphiphiles have been built by linking polyhedral oligomeric silsesquioxane (POSS) and a dendron via linkers of different lengths. Three conjugates of octahedral silsesquioxanes (T₈-POSS) and dendron are designed and synthesized and are referred to as isobutyl T₈-POSS gallic acid derivatives (BPOSS-GAD-1, BPOSS-GAD-2, BPOSS-GAD-3). These samples have been fully characterized by ¹H-NMR, ¹³C-NMR, Fourier transform infrared (FT-IR) spectroscopy and matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry to establish their chemical identity and purity. Driven by different interactions between POSS and dendron, ordered superstructure can be found upon self-assembly. The stabilities and structures of these samples are further studied by using differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and molecular simulations. The results show that their melting points range from 74 °C to 143 °C and the molecular packing schemes in the assemblies can form lamellar structure of BPOSS-GAD-3 as determined by the different linkers.

Controlling the enthalpy–entropy competition in supramolecular fullerene liquid crystals by tuning the flexible chain length

The supramolecular fullerene liquid crystal phase and structure can be regulated by controlling the enthalpy–entropy competition by tuning the flexible chain length.

A theoretical study of complexes between fullerenes and concave receptors with interest in photovoltaics

The proper combination of host and guest allows controlling the stability and charge transfer capability of fullerene–concave receptor complexes.

Photochemical Isomerization and Topochemical Polymerization of the Programmed Asymmetric Amphiphiles

For the advancement in multi-stimuli responsive optical devices, we report the elaborate molecular engineering of the dual photo-functionalized amphiphile (abbreviated as AZ₁DA) containing both a photo-isomerizable azobenzene and a photo-polymerizable diacetylene. To achieve the efficient photochemical reactions in thin solid films, the self-assembly of AZ₁DA molecules into the ordered phases should be precisely controlled and efficiently utilized. First, the remote-controllable light shutter is successfully demonstrated based on the reversible trans-cis photo-isomerization of azobenzene group in the smectic A mesophase. Second, the self-organized monoclinic crystal phase allows us to validate the photo-polymerization of diacetylene moiety for the photo-patterned thin films and the thermo-responsible color switches. From the demonstrations of optically tunable thin films, it is realized that the construction of strong relationships between chemical structures, molecular packing structures and physical properties of the programmed molecules is the core research for the development of smart and multifunctional soft materials.

Conformational Preferences and the Phase Stability of Fullerene Hexa-adducts

Molecular conformation and the assembly structure determine the spatial arrangements of the constituent units and the functions of a molecule. Although, fullerene hexa-adducts (FHAs) have been known as functional materials with great versatility, their conformational preferences and phase stability remain a complicate issue. By choosing bithiophene (T2 ) and dodecyl bithiophene (C12 T2 ) as the peripheral units of FHA, and using microscopic, scattering and diffraction characterizations, our study reveals how the intramolecular interaction and environmental stimulus affects the conformational preferences and phase stability of FHAs.

Artificial hemes for DSSC and/or BHJ applications

In this review paper a summary of our studies is presented concerning the power conversion efficiency of DSSC and BHJ based on porphyrin hybrid materials.

New conjugated molecules with four DPP (diketopyrrolopyrrole) moieties linked by [2,2]paracyclophane as electron acceptors for organic photovoltaic cells

Two conjugated molecules were investigated as electron acceptors for OPVs and it was observed that their power conversion efficiency can reach 1.84% after blending with P3HT.

Porphyrin-incorporated 2D D-A polymers with over 8.5% polymer solar cell efficiency

A copolymerization strategy is developed to utilize porphyrin as a complementary light-harvesting unit (LHU) in D-A polymers. For polymer solar cells (PSCs), the presence of LHUs increases the short-circuit current density (Jsc ) without sacrificing the open-circuit voltage (Voc ) and fill factor (FF). Up to 8.0% power conversion efficiency (PCE) is delivered by PPor-2:PC71 BM single-junction PSCs. A PCE of 8.6% is achieved when a C-PCBSD cathodic interlayer is introduced.

Multiply biphenyl substituted zinc(II) porphyrin and phthalocyanine as components for molecular materials

The preparation and physico-chemical characteristics of zinc(II) porphyrin and phthalocyanine derivatives with biphenyl units are reported. These compounds have been prepared as components for molecular electronics systems and rotor-based molecular machines, where the biphenyl units can act as paddles because they are oriented quasi-perpendicularly to the plane of the aromatic macrocycles which would be coordianted through the transition metal ion by an axial ligand. The minimalist design along with the absence of solubilizing groups leads to a low solubility of the compounds in organic solvents; the phthalocyanines is only sparingly soluble while the porphyrin is more easily manipulated, but in any case the concentration of both compounds is sufficient for surface deposition. The luminescence of the compounds is characteristic of the central unit, although it is clear in the absorption spectra that the phthalocyanine derivative has a particularly strong tendency to aggregate non-specifically. The porphyrin forms microcrystals while the phthalocyanines which bears eight biphenyl units forms amorphous aggregates from 1,2-dichlorobenzene reminiscent of glasses of other biphenyl derivatives, which is interesting for the preparation of amorphous materials for optics applications.

[60]Fulleropyrrolidines bearing π-conjugated moiety for polymer solar cells: contribution of the chromophoric substituent on C₆₀ to the photocurrent

Two fullerene-terthiophene dyads without hexyl chains (3T-C₆₀) and with hexyl chains (3TH-C₆₀) on the terthiophene substituent are synthesized by 1,3-dipolar cycloaddition of corresponding azomethine ylides to C₆₀. The cyclic voltammetry studies indicate no apparent electronic communication between the terthiophene pendent group and the fulleropyrrolidine core in the ground state. However, a significant florescence quenching is observed for 3T-C₆₀ and 3TH-C₆₀, compared to their fluorescent terthiophene (3T) and 3TH precursors, respectively, suggesting the occurrence of strong intramolecular electron/energy transfers in the photoexcited state. Furthermore, these new fulleropyrrolidine derivatives are applied as electron acceptors to fabricate poly(3-hexylthiophene) (P3HT) based bulk heterojunction solar cells. The incident photon-to-current efficiency (IPCE) value of the P3HT/3T-C₆₀ device is significantly higher than that of the P3HT/PCBM cell in wavelengths of 350-420 nm. This finding provides direct evidence for the contribution of 3T excitons to the photocurrent. Replacing 3T-C₆₀ with 3TH-C₆₀ effectively improves the morphology of the photoactive layer and widens the window of optimal D/A ratios, raising the power conversion efficiency (PCE) from 2.14% to 2.54%. Importantly, these devices exhibit superior stability of PCE against high-temperature aging.