Controlling the orientation of hole transport molecules in bottlebrush polymers for enhanced OLED performance
Year: 2022
Type: article
Abstract: Organic light-emitting diodes (OLEDs) have been widely applied in lighting and display technologies. The performance of OLED devices depends on not only material composition but also the packing and morphology of each functional layer. Much effort has been devoted to control the alignment of charge transport functionalities to achieve a higher OLED efficiency by lowering the hole-injection barrier.[1] Recently, Wooley and co-workers reported an innovative method to control the orientation of hole-transporting (HT) moities in macromolecular charge transport materials.[2] Although aligning HT moieties parallel to the substrate has been achieved for vapor-deposited small molecules, it remains a longstanding challenge for solution-processable polymer systems.[3] Leveraging controlled polymerization and block copolymer design, Wooley and colleagues successfully synthesized unique bottlebrush architectures where the polymer backbones could vertically “stand” on the substrate while the pendant grafts were extended parallel to the substrate (Figure 1). Key to their successs is the strategic placement of surface-energy reducing fluorocarbon moieties in these bottlebrush polymers, which led to vertical brush alignement on the substrates and unprecedented high degrees of anisotropic “face-on” orientation of triphenylamine (TPA) moities. By comparing the photophysical and electronic properties of polymers with varying pendant grafts, characterizing the thin films on different substrates, and evaluating the performance of hole-only and OLED full devices, the authors demonstrated that the “face-on” packing of TPA moieties within the HT layer results in devices with improved luminance, quantum efficiency, efficiency roll-off, and transient electroluminescent decay, when compared to isotropic counterparts. This work should inspire researchers to develop new chemical strategies to assemble solution-processable macromolecules in thin films with high levels of orientational control. Future efforts should be focused on investigating on how alignment of functional moieties affects the performance of individual OLED components (e.g., hole injection, hole transport, emissive, and electron transport layers), as well as understanding any synergistic effects on overall device performance. The authors declare no conflict of interest.
Source: Aggregate
DOI: 10.1002/agt2.259
Authors: Ho Fung Cheng, Jeremiah A. Johnson
Institution: Massachusetts Institute of Technology
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,000
OpenAlex ID: W4290975529