Optical Properties of Benzotriazole-Based Conjugated Polyelectrolytes

A series of conjugated polyelectrolytes (CPEs) based on an electron-deficient polybenzotriazole backbone with various pendant ionic functionalized side chains were synthesized directly from the corresponding ionic monomers. Of particular interest was to use the different chemical structures to understand how the optical features are influenced by the ionic side chains. We found that interchain aggregation is favored in low dielectric constant solvents for cationic CPEs. Moreover, aggregated species absorb at longer wavelengths and exhibit higher fluorescence quantum yields

Ternary D1–D2–A–D2 Structured Conjugated Polymer: Efficient “Green” Solvent-Processed Polymer/Neat‑C₇₀ Solar Cells

In contrast to the great efforts on developing novel donor (D)–acceptor (A) copolymers, research on investigating the backbone composition of conjugated polymer is rare. In this contribution, we disclose the design and synthesis of a ternary D1–D2–A–D2 structured conjugated polymer PBSF. Compared to the typical D–A polymer with fixed D/A moiety number, the ternary structure can tune the optical and electrical properties more comprehensively and delicately. Precisely control of the ternary fragments relative to the backbone vector was achieved, further promoting sufficient planar structure, strong intermolecular packing, and excellent charge transport. Finally, the additive and annealing-free polymer solar cells based on PBSF and phenyl-C₇₁-butyric acid methyl ester ([70]­PCBM; PCE = 7.4%) or cheap, nonfunctionalized C₇₀ (PCE = 5.3%) demonstrate excellent performance using either chlorinated or nonhalogenated “green” solvent. We believe that this novel and efficient ternary structure may spark future polymer design to achieve sustainable-processed photovoltaic devices for practical mass production

End-Group-Mediated Aggregation of Poly(3-hexylthiophene)

The solid-state microstructure of semiconducting polymers is known to influence properties relevant for their function in optoelectronic devices. While several strategies exist in the literature for controlling desired morphological organization, preaggregation in solution via polymer chain end-functionalization remains relatively unexplored. In this work, we synthesized two poly­(3-hexylthiophene) (P3HT) derivatives with different end-groups by using click chemistry. End-groups chosen for this study were derivatives of sulforhodamine 101 dye and phenyl-C₆₁-butyric acid methyl ester (PC₆₀BM). The chemical functionality of the end-group is found to influence the aggregation properties as a function of solvent quality. End-group-induced preaggregation ultimately impacts the optical properties and molecular orientation in the solid state. These results suggest future applications of this structural modification strategy to modulate device characteristics

Chiroptical Properties of a Benzotriazole–Thiophene Copolymer Bearing Chiral Ethylhexyl Side Chains

Conjugated polymers containing alternating thiophene units and benzotriazole structural units bearing either chiral (<i>S</i>)-2-ethylhexyl (PBTz-Th*) or racemic 2-ethylhexyl side chains (PBTz-Th) were synthesized. Characterization by optical absorption spectroscopy of both PBTz-Th* and its racemic counterpart reveal aggregated chains, even at dilute concentrations in good solvents. The presence of a chiral substituent permits characterization via circular dichroism (CD) spectroscopy. CD spectra provide evidence of chiral aggregates of PBTz-Th* chains even at 0.01 mg/mL in dichlorobenzene. When PBTz-Th* solutions are diluted with PBTz-Th, the resulting CD spectrum suggests that PBTz-Th* chains are chiral in the aggregate. Chiral ordering is also found to translate from aggregates in solution to the solid state

High Thermal Stability Solution-Processable Narrow-Band Gap Molecular Semiconductors

A series of narrow-band gap conjugated molecules with specific fluorine substitution patterns has been synthesized in order to study the effect of fluorination on bulk thermal stability. As the number of fluorine substituents on the backbone increase, one finds more thermally robust bulk structures both under inert and ambient conditions as well as an increase in phase transition temperatures in the solid state. When integrated into field-effect transistor devices, the molecule with the highest degree of fluorination shows a hole mobility of 0.15 cm²/V·s and a device thermal stability of >300 °C. Generally, the enhancement in thermal robustness of bulk organization and device performance correlates with the level of C–H for C–F substitution. These findings are relevant for the design of molecular semiconductors that can be introduced into optoelectronic devices to be operated under a wide range of conditions

Solution-Processed pH-Neutral Conjugated Polyelectrolyte Improves Interfacial Contact in Organic Solar Cells

The intrinsic acidic nature of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole-transporting layer (HTL) induces interfacial protonation and limits the device performance in organic solar cells based on basic pyridylthiadiazole units. By utilizing a pH neutral, water/alcohol soluble conjugated polyelectrolyte CPE-K as the HTL in p-DTS(PTTh₂)₂:PC₇₁BM solar cells, a 60% enhancement in PCE has been obtained with an increased <i>V</i>_bi, reduced <i>R</i>_s, and improved charge extraction. These effects originate from the elimination of interfacial protonation and energy barrier compared with the PEDOT:PSS HTL

Anisotropic Thermal Transport in Thermoelectric Composites of Conjugated Polyelectrolytes/Single-Walled Carbon Nanotubes

We report a method to determine the thermal conductivities of polymer composites with single-walled carbon nanotubes (SWNTs) using time-domain thermoreflectance. Both through-plane and in-plane thermal conductivities were determined. Two types of CPEs used in these studies are of the same conjugated backbone but with either cationic (CPE-PyrBIm₄) or anionic (CPE-Na) pendant functionalities. The CPE-Na/SWNT composites are p-type conductors, whereas the CPE-PyrBIm₄/SWNT counterparts exhibit n-type charge transport. The CPE/SWNT films were prepared through a filtration method that preferentially aligns the SWNTs in the in-plane direction. Attaching the composites onto glass substrates with a precoated heat transducer allows one to measure the through-plane thermal conductivity of materials with rough surfaces. The in-plane thermal conductivity can be measured by embedding thick samples into epoxy followed by microtoming to expose the relatively smooth cross sections. The thermal conductivity along the in-plane direction is found to be higher than that along the through-plane direction. Indeed, the anisotropy factor of thermal conductivity in these composites is approximately an order of magnitude, favoring in-plane direction