Chain conformation control of fluorene-benzothiadiazole copolymer light-emitting diode efficiency and lifetime
File(s)
Published version
Author(s)
Wang, Bingjun
Ye, Hao
Riede, Moritz
Bradley, Donal DC
Type
Journal Article
Abstract
The β-phase, in which the intermonomer torsion angle of a fraction of chain segments approaches ∼180°, is an intriguing conformational microstructure of the widely studied light-emitting polymer poly(9,9-dioctylfluorene) (PFO). Its generation can in turn be used to significantly improve the performance of PFO emission-layer-based light-emitting diodes (LEDs). Here, we report the generation of β-phase chain segments in a copolymer, 90F8:10BT, containing 90% 9,9-dioctylfluorene (F8) and 10% 2,1,3-benzothiadiazole (BT) units and show that significant improvements in performance also ensue for LEDs with β-phase 90F8:10BT emission layers, generalizing the earlier PFO results. The β-phase was induced by both solvent vapor annealing and dipping copolymer thin films into a solvent/nonsolvent mixture. Subsequent absorption spectra show the characteristic fluorene β-phase peak at ∼435 nm, but luminescence spectra (∼530 nm peak) and quantum yields barely change, with the emission arising following efficient energy transfer to the lowest-lying excited states localized in the vicinity of the BT units. For ∼5% β-phase chain segment fraction relative to 0% β-phase, the LED luminance at 10 V increased by ∼25% to 5940 cd m–2, the maximum external quantum efficiency by ∼61 to 1.91%, and the operational stability from 64% luminance retention after 20 h of operation to 90%. Detailed studies addressing the underlying device physics identify a reduced hole injection barrier, higher hole mobility, correspondingly more balanced electron and hole charge transport, and decreased carrier trapping as the dominant factors. These results confirm the effectiveness of chain conformation control for fluorene-based homo- and copolymer device optimization.
Date Issued
2021-01-20
Date Acceptance
2021-01-01
Citation
ACS applied materials & interfaces, 2021, 13 (2), pp.2919-2931
ISSN
1944-8244
Publisher
American Chemical Society
Start Page
2919
End Page
2931
Journal / Book Title
ACS applied materials & interfaces
Volume
13
Issue
2
Copyright Statement
© 2021 The Authors. Published by American Chemical Society. This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes.
Identifier
https://www.ncbi.nlm.nih.gov/pubmed/33411508
Subjects
chain conformation
copolymers
polyfluorenes
polymer light-emitting diodes
β-phase
Publication Status
Published
Coverage Spatial
United States
Date Publish Online
2021-01-07