PDI-Based Oligomers and Polymers for Organic Electronic and Photonic Devices

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Feb 22 2022

In an article recently published in the open-access journal Accounts of Materials Research, researchers discussed the recent development of oligomers and polymers based on perylene diimide as well as their applications for organic optoelectronics.

Study: Perylene Diimide-Based Oligomers and Polymers for Organic Optoelectronics. Image Credit: Cavan-Images/Shutterstock.com

Background

As a traditional dye, perylene diimide (PDI) has several advantages, including structural variety, tunable optical and electrical properties, high electron affinity, strong light absorption, stability, and good electron-transporting properties.

In organic electronics and photonic devices, PDI-based polymers and oligomers are promising candidates for n-type semiconductors. A polymer solar cell (PSC) is a promising clean and renewable energy technology with several advantages, including ease of preparation and lightweight, low cost, semi-transparent, and flexible construction.

Fullerene derivatives such as [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) have long been the most common electron acceptors in the PSCs active layer. PCBM, on the other hand, has some drawbacks, such as poor absorption, significant energy waste, and an unstable morphology.

In comparison to PCBM, PDI-based materials have numerous advantages. As a result, PDI-based polymers and oligomers are commonly used in the active layers of PSCs as electron acceptors. In addition, light-emitting diodes (LEDs), organic field-effect transistors (OFETs), lasers, optical switches, and photodetectors all utilize PDI-based oligomers and polymers as n-type semiconductors.

About the Study

In the present study, the authors presented a brief overview of recent advances in PDI-based polymers and oligomers, as well as their applications in organic electrical and photonic devices, particularly FETs and solar cells. To overcome excess crystallization and PDI aggregation, as well as to achieve suitable donor/acceptor (D/A) miscibility and phase separation, the authors developed star-shaped PDI trimers, linear-shaped PDI dimers with varied bridges, and PDI polymers in 2007.

Molecular design strategies were created to promote the backbone’s planarity and a lower lowest unoccupied molecular orbital (LUMO) level in PDI polymers, which is helpful for OFETs' strong intermolecular stacking, mobility, and air stability. The potential of PDI polymers towards the two-photon absorption (2PA) and perovskite solar cells in addition to PSC and OFET applications was demonstrated. Future research topics for PDI oligomers and polymers performance optimization were also suggested.

The achievements in PDI-based oligomers and polymers, mainly from the Zhan group, and their potential applications in organic electrical and photonic devices, particularly solar cells and FETs, were summarized in this study. The logical design of PDI-based star-shaped and linear polymers and oligomers was examined along with the structure-property interactions. In addition, major research directions for the near future were suggested.

Observations

In this study, the researchers observed that the early bulk-heterojunction PSCs based on PDI acceptors had a relatively low power conversion efficiency, despite the fact that parent PDI dyes have significant absorption, huge electron affinity, and high electron mobility (ca. 0.1%). Strong intermolecular stacking, severe D/A phase separation, and wide crystalline domains caused by the parent PDI's extremely planar shape resulted in low exciton dissociation efficiency and poor device performance.

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Furthermore, it was observed based on some studies that at 1520 nm, an alternating copolymer based on dithienocoronene diimide (DTCDI) and porphyrin (POR) connected by ethynylene (P-(DTCDI-E-POR)) has a larger 2PA cross-section of 7809 GM/repeat unit. The enhanced electron mobility of 0.06 cm² V^-1 s^-1 and better air stability of the n-channel OFETs based on P(PDI-dithienothiophene) were attributed to the high-quality dielectric/polymer interface formed during the lamination process.

It was also determined that in organic electrical and photonic devices, such as PSCs and OFETs, a variety of linear-shaped PDI dimers, star-shaped PDI trimers, and linear-shaped PDI polymers were developed and used as n-type semiconductors.

The configuration of these PDI-based materials was closely related to the stiffness and conjugation of their molecular backbones and could have a major impact on their absorption, energy levels, intermolecular interactions, and charge transport. It was also observed that increased molecular backbone rigidity could lead to wave function coherence, electron delocalization along the backbone, and greater transition dipole moments.

Furthermore, based on many studies, it was observed that by increasing the planarity of molecular backbones, the absorption of PDI-based oligomers/polymers could be red-shifted. Charge generation and JSC enhancement in PSCs benefited from the stronger and broader absorption. By increasing the planarity of molecular backbones, the LUMO energy levels of PDI-based oligomers/polymers could be upshifted and downshifted, respectively.

It was demonstrated in some studies that controlling the arrangement of molecular backbones could alter the intermolecular interactions of PDI-based oligomers/polymers.

Conclusions

In conclusion, this study elucidated the optoelectronic capabilities of PDI-based materials. The researchers also highlighted that the isomer-pure PDI-based materials must be manufactured since regio-isomers have a considerable impact on optoelectronic characteristics. They also concluded that PSCs require PDI acceptors with near-infrared (NIR) absorption and very high absorption coefficients since the solar spectrum's highest photon density is in the NIR region.

The authors believe that PSCs require improved molecular backbone planarity to establish a fine balance between charge transport and donor/acceptor phase separation. It was also determined that the high mobility of n-channel OFETs is dependent on the planarity of molecular backbones. This work also demonstrated that weak chemical connections can be built between the heteroatoms/functional groups on the PDI-based electron-transporting layer (ETLs) and perovskites in high-performance perovskite solar cells.

The authors emphasized that some significant research concerns demand further attention in the future to improve the optoelectronic properties of PDI-based materials.

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