Dopant-Stabilized Assembly of Poly(3-hexylthiophene).

Polymer self-assembly is a powerful approach for forming nanostructures for solution-phase applications. However, polymer semiconductor assembly has primarily been driven by solvent interactions. Here, we report poly(3-hexythiophene) homopolymer assembly driven and stabilized by oxidative doping with iron (III) p-toluenesulfonate in benzonitrile. By this improved method, dopant mol % and addition temperature determine the size and morphology of oxidized polymer nanostructures. The dopant counterion provides colloidal stability in a process of dopant-stabilized assembly (DSA). Each variable governing polymer assembly is systematically varied, revealing general principles of oxidized nanostructure assembly and allowing the polymer planarity, optical absorption, and doping level to be modulated. Oxidized nanostructure heights, lengths, and widths are shown to depend on these properties, which we hypothesize is due to competing nanostructure formation and oxidation mechanisms that are governed by the polymer conformation upon doping. Finally, we demonstrate that the nanoparticle oxidative doping level can be tuned post-formation through sequential dopant addition. By revealing the fundamental processes underlying DSA, this work provides a powerful toolkit to control the assembly and optoelectronic properties of oxidatively doped nanostructures in solution.

Poly(ionic liquid) Gating Materials for High-Performance Organic Thin-Film Transistors: The Role of Block Copolymer Self-Assembly at the Semiconductor Interface.

The widespread realization of wearable electronics requires printable active materials capable of operating at low voltages. Polymerized ionic liquid (PIL) block copolymers exhibit a thickness-independent double-layer capacitance that makes them a promising gating medium for the development of organic thin-film transistors (OTFTs) with low operating voltages and high switching speed. PIL block copolymer structure and self-assembly can influence ion conductivity and the resulting OTFT performance. In an OTFT, self-assembly of the PIL gate on the semiconducting polymer may differ from bulk self-assembly, which would directly influence electrical double-layer formation. To this end, we used poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)) as a model semiconductor for our OTFTs, on which our PILs exhibited self-assembly. In this study, we explore this critical interface by grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) of P(NDI2OD-T2) and a series of poly(styrene)-b-poly(1-(4-vinylbenzyl)-3-butylimidazolium-random-poly(ethylene glycol) methyl ether methacrylate) (poly(S)-b-poly(VBBI+[X]-r-PEGMA)) block copolymers with varying PEGMA/VBBI+ ratios and three different mobile anions (where X = TFSI-, PF6-, or BF4-). We investigate the thin-film self-assembly of block copolymers as a function of device performance. Overall, a mixed orientation at the interface leads to improved device performance, while predominantly hexagonal packing leads to nonfunctional devices, regardless of the anion present. These PIL gated OTFTs were characterized with a threshold voltage below 1 V, making understanding of their structure-property relationships crucial to enabling the further development of high-performance gating materials.

High-Throughput Screening of Antisolvents for the Deposition of High-Quality Perovskite Thin Films.

One-step solution deposition of high-quality perovskite thin films relies heavily on a small number of antisolvents. Here, we design a simple minimum volume colorimetric solution assay to screen over 100 different solvents. We correctly identify 14 previously reported antisolvents and predict 20 novel candidates. We then refine the assay through analysis of screening results, available solvent properties, and qualitative evaluation of films cast using 50 candidates. Using the refined findings, we successfully demonstrated 15 different antisolvents for characterization and evaluation in inverted devices, including six previously unreported candidates. All candidates produced power conversion efficiencies comparable to chlorobenzene controls without any additional optimization. This work presents the largest scope of antisolvents reported, can be easily adapted to other perovskites, and opens the door to selecting antisolvents based on a wide range of desirable properties including efficiency, usability, safety, and industrial viability.

Oxidation promoted self-assembly of π-conjugated polymers.

Self-assembly is an attractive strategy for organizing molecules into ordered structures that can span multiple length scales. Crystallization Driven Self-Assembly (CDSA) involves a block copolymer with a crystallizable core-forming block and an amorphous corona-forming block that aggregate into micelles with a crystalline core in solvents that are selective for the corona block. CDSA requires core- and corona-forming blocks with very different solubilities. This hinders its use for the self-assembly of purely π-conjugated block copolymers since blocks with desirable optoelectronic properties tend to have similar solubilities. Further, this approach is not readily reversible, precluding stimulus-responsive assembly and disassembly. Here, we demonstrate that selective oxidative doping of one block of a fully π-conjugated block copolymer promotes the self-assembly of redox-responsive micelles. Heteroatom substitution in polychalcogenophenes enables the modulation of the intrinsic polymer oxidation potential. We show that oxidized micelles with a narrow size distribution form spontaneously and disassemble in response to a chemical reductant. This method expands the scope of π-conjugated polymers that can undergo controlled self-assembly and introduces reversible, redox-responsive self-assembly of π-conjugated polymers.

Helicoidal Patterning of Gold Nanorods by Phase Separation in Mixed Polymer Brushes.

The spatial distribution of polymer ligands on the surface of nanoparticles (NPs) is of great importance because it determines their interactions with each other and with the surrounding environment. Phase separation in mixtures of polymer brushes has been studied for spherical NPs; however, the role of local surface curvature of nonspherical NPs in the surface phase separation of end-grafted polymer ligands remains an open question. Here, we examined phase separation in mixed monolayers of incompatible polystyrene and poly(ethylene glycol) brushes end-capping the surface of gold nanorods in a good solvent. By varying the molar ratio between these polymers, we generated a range of surface patterns, including uniform and nonuniform polystyrene shells, randomly distributed polystyrene surface patches, and, most interestingly, a helicoidal pattern of polystyrene patches wrapping around the nanorods. The helicoidally patterned nanorods exhibited long-term colloidal stability in a good solvent. The helicoidal wrapping of the nanorods was achieved for the mixtures of polymers with different molecular weights and preserved when the quality of the solvent for the polymers was reduced. The helicoidal organization of polymer patches on the surface of nanorods can be used for templating the synthesis or self-assembly of helicoidal multicomponent nanomaterials.

Applying Heteroatom Substitution in Organic Photovoltaics.

Poly(3-alkylthiophene) (P3AT) has been a central focus of research on organic photovoltaics (OPVs) for well over a decade. Due to their controlled synthesis P3ATs have proven to be a vital model system for developing an understanding of the effects of polymer structure on optoelectronic properties and blend morphology in bulk heterojunction OPVs. Similar to their thiophene counterparts, selenophene and tellurophene can be polymerized in a controlled manner. As single atom substitution results in significant differences in absorption, charge transport and self-assembly these model systems provide a unique opportunity to probe fundamental structure-property relationships. In this account, we provide an overview of our work on copolymers of thiophene and selenophene and examine how the optoelectronic and morphological behavior of these materials can be strategically adjusted through polymer design. We also highlight recent developments on poly(3-alkyltellurophene) and comment on its future in fundamental and applied studies.

Poly(3-alkylthiophene)- block-poly(3-alkylselenophene)s: Conjugated Diblock Co-polymers with Atypical Self-Assembly Behavior.

Understanding self-assembly behavior and resulting morphologies in block co-polymer films is an essential aspect of chemistry and materials science. Although the self-assembly of amorphous coil-coil block co-polymers is relatively well understood, that of semicrystalline block co-polymers where each block has distinct crystallization properties remains unclear. Here, we report a detailed study to elucidate the rich self-assembly behavior of conjugated thiophene-selenophene (P3AT- b-P3AS) block co-polymers. Using a combination of microscopy and synchrotron-based X-ray techniques, we show that three different film morphologies, denoted as lamellae, co-crystallized fibers, and patchy fibers, arise from the self-assembly of these block co-polymers over a relatively narrow range of overall degrees of polymerization (30 < N < 90). Crystallization-driven phase separation occurs at a very low N (<35), and lamellar films are formed. Conversely, at medium N (50-60) and high N (>80), the thiophene and selenophene blocks co-crystallize into nanofibers, where medium N leads to much more mixing than high N. The overall tendency for phase separation in these systems follows rather different trends than phase separation in amorphous polymers in that we observe the greatest degree of phase separation at the lowest N. Finally, we demonstrate how each morphology influences transport properties in organic thin-film transistors comprised of these conjugated polymers.

Self-Organization and Charge Transport Properties of Selenium and Tellurium Analogues of Polythiophene.

A series of conjugated polymers comprising polythiophene, polyselenophene, and polytellurophene with branched 3,7-dimethyloctyl side chains, well-matched molecular weight, dispersity, and regioregularity is synthesized. The ionization potential is found to vary from 5.14 to 5.32 eV, with polytellurophene having the lowest potential. Field-effect transistors based on these materials exhibit distinct hole transport mobility that varies by nearly three orders of magnitude, with polytellurophene having the highest mobility (2.5 × 10-2 cm² V-1 s-1 ). The large difference in mobility demonstrates the significant impact of heteroatom substitution. Although the series of polymers are very similar in structure, their solid-state properties are different. While the thin film microstructure of polythiophene and polyselenophene is identical, polytellurophene reveals globular features in the film topography. Polytellurophenes also appear to be the least crystalline, even though their charge transport properties are superior to other samples. The torsional barrier and degree of planarity between repeat units increase as one moves down group-16 elements. These studies show how a single atom in a polymer chain can have a substantial influence on the bulk properties of a material, and that heavy group-16 atoms have a positive influence on charge transport properties when all other variables are kept unchanged.

Synthesis of fused-ring aza-dipyrromethenes from aromatic nitriles.

Aza-dipyrromethenes are ligands for a number of useful fluorescent dyes and electronic materials. They have high absorption in the red and NIR range. Fused-ring aza-dipyrromethenes cannot be synthesized through the same methods used to make aza-dipyrromethenes, which are common commercial dyes. The current synthesis is limited to treating a phthalonitrile-based electrophile with a Grignard reagent followed by reduction with formamide at high temperature. In this work, we introduce a new fused-ring aza-dipyrromethene synthesis from ortho-lithiated aromatic nitriles. This method allows for a much wider range of functional groups, less complicated starting materials, and yields that are consistent with the highest reported for aza-dipyrromethenes of any kind.

Patchy Nanofibers from the Thin Film Self-Assembly of a Conjugated Diblock Copolymer.

An unexpected morphology comprising patchy nanofibers can be accessed from the self-assembly of an all-conjugated, polyselenophene-block-polythiophene copolymer. This morphology consists of very small (<10 nm), polythiophene- and polyselenophene-rich domains and is unprecedented for both conjugated polymers and diblock copolymers in general. We propose that the patchy morphology occurs from the enhanced miscibility of the blocks arising from the longer alkyl chains in comparison to similar block copolymers with shorter alkyl chains, which fully phase separate, as well as the difference in rigidity between the polythiophene and polyselenophene blocks. This work demonstrates a facile way to tune the self-assembly behavior of conjugated block copolymers by modification of the side chain substituents.

Increasing Polymer Solar Cell Fill Factor by Trap-Filling with F4-TCNQ at Parts Per Thousand Concentration.

Intrinsic traps in organic semiconductors can be eliminated by trap-filling with F4-TCNQ. Photovoltaic tests show that devices with F4-TCNQ at parts per thousand concentration outperform control devices due to an improved fill factor. Further studies confirm the trap-filling pathway and demonstrate the general nature of this finding.

Synthesis of Macrocyclic Poly(3-hexylthiophene) and Poly(3-heptylselenophene) by Alkyne Homocoupling.

Here we report the synthesis of cyclic samples of poly(3-hexylthiophene) (P3HT, degrees of polymerization = 25, 40, and 75) and poly(3-heptylselenophene) (P37S, DP = 30). Cyclization was accomplished using a mild alkyne-alkyne homocoupling procedure. Alkyne-terminated poly(ethylene glycol) was then coupled to residual uncyclized polymers, which were subsequently removed by column chromatography, enabling isolation and characterization of pure cyclic polymers. Cyclization was confirmed by the disappearance of terminal alkyne protons, the decrease in hydrodynamic radius [measured by size exclusion chromatography (SEC)], and the observed identical molecular weight distribution [measured by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry]. The lower weight macrocyclic polymers have decreased self-assembly as measured by optical absorption and transmission electron microscopy. The highest weight macrocycles were imaged using scanning tunneling microscopy. Cyclic polymers adopted a tightly bent conformation, while their linear analogues assembled as fully extended chains. Our method of cyclization and purification is broadly applicable to conjugated polymers (CPs) and will enable the development of novel optoelectronic materials.