Professor Thuc-Quyen Nguyen and her team from the University of California, Santa Barbara, have achieved a groundbreaking breakthrough in their latest research. They have successfully synthesized a series of conjugated polyelectrolytes (CPE) containing cyclopentadithiophene-substituted-benzothiadiazole. The alkyl chain lengths of these synthesized compounds vary between 2 and 5 carbon atoms, with the aim of investigating how this length variation affects their optical, electrochemical, and morphological properties.

As these conjugated polyelectrolytes are mixed conductors, they can serve as the active layer of accumulation mode organic electrochemical transistors (OECTs). The conductivity, volume capacitance, and ion and electron conduction rates of these polyelectrolytes are all influenced by the length of their alkyl chains.

Furthermore, the computational results of density functional theory (DFT) have helped explain why the doping levels of these molecules vary with structural changes.

This study reveals that the conductivity, volume capacitance, and ion and electron conductivity of CPEs are affected by the length of their alkyl chains. This finding holds significant implications for the photovoltaic industry as it may pave the way for the development of more efficient and tunable photovoltaic devices.

Additionally, the team utilized density functional theory (DFT) calculations to elucidate the impact of molecular structure on doping propensity. This insight holds immeasurable value in the design and fabrication of organic electronic devices, including organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and bioimaging devices.

The research team stated, “We conducted a systematic study on the structure-property relationship of conjugated polyelectrolytes (CPEs) with varying side-chain lengths. Prior to investigating this particular series of CPEs, we hypothesized that as the side-chain length decreases, the negative charge of stable free electrons (polarons) would be closer, thus increasing the ease of doping. However, the trend observed from the UV-Vis spectroscopy measurements contradicts this assumption.”

Results from differential pulse voltammetry (DPV) of cyclopentadithiophene derivatives and CPE films demonstrate that an increase in side-chain length leads to a decrease in oxidation potential. This reduction in oxidation potential can be explained by the increased ease of free electron formation. Data from grazing-incidence wide-angle X-ray scattering (GIWAXS) indicates that an increase in side-chain length enhances crystallinity, suggesting higher hole mobility. Finally, density functional theory (DFT) studies reveal that systems with longer side chains have lower formation energies.

Preliminary structure-property characterization indicates that the sulfonic acid moiety exerts an electron-attracting effect on the cyclopentadithiophene ring, rendering the oxidation and doping of the polymer more challenging. Moreover, the flexibility of longer alkyl chain lengths may contribute to the proximity between the sulfonic acid group and free electrons, leading to improved stability of free electrons and increased doping levels. This emphasizes the importance of conducting systematic studies on conjugated polyelectrolytes to aid in the design of superior materials.

In conclusion, shorter alkyl chain lengths are unfavorable for the initiation of oxidation in our system, thereby reducing the doping level of the conjugated polyelectrolyte.