Wang Bingjun, Ye Hao, Riede Moritz, Bradley Donal D C
Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, UK.
Physical Science and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.
ACS Appl Mater Interfaces. 2021 Jan 20;13(2):2919-2931. doi: 10.1021/acsami.0c18490. Epub 2021 Jan 7.
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, 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.
β相是广泛研究的发光聚合物聚（9,9-二辛基芴）（PFO）的一种有趣的构象微观结构，其中一部分链段的单体间扭转角接近180°。其生成反过来可用于显著提高基于PFO发射层的发光二极管（LED）的性能。在此，我们报告了在一种共聚物90F8:10BT中生成β相链段，该共聚物含有90%的9,9-二辛基芴（F8）和10%的2,1,3-苯并噻二唑（BT）单元，并表明对于具有β相90F8:10BT发射层的LED，性能也有显著改善，这推广了早期PFO的结果情况。通过溶剂蒸汽退火以及将共聚物薄膜浸入溶剂/非溶剂混合物中均可诱导出β相。随后的吸收光谱显示在约435nm处有芴β相的特征峰，但发光光谱（峰值约530nm）和量子产率几乎没有变化，其发射是在有效能量转移到位于BT单元附近的最低激发态之后产生的。相对于0%β相，当β相链段分数约为5%时，10V下LED的亮度增加了约25%，达到5940cd/m²，最大外量子效率提高了约61%，达到1.91%，并且操作稳定性从运行20小时后64%的亮度保持率提高到90%。针对潜在器件物理的详细研究确定，降低的空穴注入势垒、更高的空穴迁移率、相应更平衡的电子和空穴电荷传输以及减少的载流子俘获是主要因素。这些结果证实了链构象控制对于基于芴的均聚物和共聚物器件优化的有效性。
