Photopolymerization of Patterned and Multilayer Thin Films for Organic Optoelectronics

Significance 

Vacuum deposition is currently the dominant industrial technique used to fabricate many organic electronic devices. Given the disadvantages of this method in terms of material waste and cost, many other scalable and cost-effective alternatives, such as spin coating and inkjet printing, have been proposed. However, the commercial adoption of these promising technologies is still limited, as it is challenging to deposit successive layers without dissolving the previous one, reducing the efficiency of the device.

Using solution processing to optimize layer thickness is also challenging because defects like pinholes reduce film uniformity and device efficiency. Direct growth of polymer brushes from electrode surfaces has emerged as a promising alternative for addressing these challenges. Polymer brushes allow for low-cost production of organic devices and show promise for preparation of charge-transporting thin films.

Combining digital projection lithography and surface-initiated-photoinduced electron transfer-reversible addition-fragmentation chain transfer (SI-PET-RAFT) may be a useful technique for the low-cost patterning and synthesis of multilayer films for organic electronics. Many organic light emitting diodes (OLEDs) produced in recent years have used thermally activated delayed fluorescence (TADF) to achieve high quantum efficiencies and improved devices lifetime. Nevertheless, more experimental evidence is still required to validate these assertions.

On this account, Dr. Jade Poisson, Dr. Alexander Polgar and Professor Zachary Hudson from The University of British Columbia in collaboration with Dr. Michele Fromel and Professor Christian Pester from The Pennsylvania State University developed a new oxygen-tolerant SI-PET-RAFT approach to prepare patterned and multilayer thin films for organic electronics. This approach was used to fabricate triblock copolymer films possessing the trilayer architecture adopted in optoelectronic devices. In these multilayer architectures, the authors also incorporated emitting layer materials demonstrating both state-of-the-art thermally assisted fluorescence (TAF) and TADF. The original research article – labeled a “hot paper” by the journal – is currently published in Angewandte Chemie International Edition.

Their approach demonstrated oxygen tolerance and provided a high degree of control over the layer thickness without requiring an inert environment like other methods. By modifying the polymerization time, the polymer brush thickness could be tunned as desired. Other benefits included high chain-end fidelity and remarkable grafting density. With the ability to rinse the substrate and reinitiate the polymer brush growth, it was possible to achieve larger polymer brush thicknesses of above 100 nm. Furthermore, combining digital projection lithography and photomasks enabled spatial control at the microscale resolution, allowing readily customizable patterns with both complex topography and sharp edges to be created.

In summary, the researchers successfully demonstrated the applicability of oxygen tolerant SI-PET-RAFT methodology for producing patterned multilayer thin films of organic semiconductor polymers. The experimental set-up was extremely cost-efficient as it only required a white lamp as the source of irradiation and a glass coverslip to prevent the evaporation of the solvent during fabrication. The polymer brushes exhibited high electrochemical stability, suggesting that the materials could be used for optoelectronic devices. In a statement to Advances in Engineering, Professor Zachary Hudson said their findings create a low-cost approach for fabricating complex films for organic electronics, which could lead to new technologies for manufacturing of display panels in cellular phones, televisions, and solid-state lighting.

About the author

Jade Poisson completed her B.Sc. in biological chemistry at the University of Ontario Institute of Technology in 2018. She is currently pursuing her Ph.D. at the University of British Columbia with Prof. Zachary Hudson. In 2019, she held a MITACS Globalink exchange fellowship to the Department of Chemical Engineering at the Pennsylvania State University with Prof. Christian Pester. Her research focuses on the preparation of materials for optoelectronics using surface-initiated polymerization.

Reference

Poisson, J., Polgar, A., Fromel, M., Pester, C., & Hudson, Z. (2021). Preparation of Patterned and Multilayer Thin Films for Organic Electronics via Oxygen‐Tolerant SI‐PET‐RAFTAngewandte Chemie International Edition, 60, 19988-19996.

Go To Angewandte Chemie International Edition

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