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.
Bibliographical noteKAUST Repository Item: Exported on 2021-01-13
Acknowledged KAUST grant number(s): 121053645
Acknowledgements: B.W. and D.D.C.B. thank the China Scholarship Council (CSC, No. 201700260029), Pacific Alliance Group (PAG), China Oxford Scholarship Fund (COSF), and King Abdullah University of Science and Technology (KAUST, No. 121053645) for studentship support. H.Y. and D.D.C.B. thank the Jiangsu Industrial Technology Research Institute (JITRI) and the JITRI-Oxford IMPACT Institute for project
funding (R57149/CN001). The authors further thank Professors Henry Snaith and Stephen Morris for access to facilities and Drs. Chen Sun and Pascal Kaienburg for fruitful discussions.