The controlling of molecular orientation and structural ordering of organic semiconductors is crucial to achieve high performance electronic devices. In this work, large‐area highly oriented and ordered films of an excellent electron transporter Poly{[N,N′‐bis(2‐octyldodecyl)‐1,4,5,8‐naphthalenedicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2)) are achieved by improved solution‐cast in high magnetic field. Microstructural characterizations reveal that the chain backbones of P(NDI2OD‐T2) are highly aligned along the applied magnetic field in the films. Based on the synchrotron‐based X‐ray diffraction analysis of the polymer films cast from different solvents, a mechanism which controls the alignment process is proposed, which emphasizes that molecular aggregates of P(NDI2OD‐T2) preformed in the solution initiate magnetic alignment and finally determine the degree of film texture. Furthermore, the time‐modulated magnetic field technique is utilized to effectively control the orientation of π‐conjugated plane of the backbones, thus the degree of face‐on molecular packing of P(NDI2OD‐T2) is enhanced significantly. Thin film transistors based on the magnetic‐aligned P(NDI2OD‐T2) films exhibit an enhancement of electron mobility by a factor of four compared to the unaligned devices, as well as a large mobility anisotropy of seven.
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