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Molecular dynamics (MD) simulations are employed to study biaxial stretch-induced crystallization of polymers, during which the individual roles of chain conformation and orientation on crystal nucleation and growth are clarified. Systems with different stiffness and orientations are constructed by changing the stretch ratios of the x-and y-axis, which allow us to figure out the individual contributions of chain conformation and orientation to flowenhanced nucleation. The results show that nucleation occurs in areas with high segment orientation, and the higher orientation corresponds to the shorter nucleation induction period. The relationship between the nucleation induction period and orientation is quantitatively expressed, which indicates that orientation plays a dominant role in flow-enhanced nucleation. On the other hand, the results show that chain stiffness exhibits a negative correlation with nucleation in biaxial stretch, supporting that conformational entropy reduction is not the main driving force in flow-induced crystallization of polymers. With the secondary nucleation model, the crystal growth rates in different directions correlate well with the orientation at the growth front of the clusters, further confirming the decisive role of orientation in crystal nucleation and growth. Finally, crystal cluster merging is proposed to be a way to form shish structures in highly oriented melts.

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Foldable solar cells have attracted increasing attention for portable and wearable applications. The challenge to achieve foldability is free from the formation of cracks under large strain. Our mechanical simulations suggests that a reduction in substrate thickness from 150 to 25 μm is a means to mitigate the strain in polymer solar cells under folding with extremely small curvature radius of sub‐millimeter. The polymer solar cells are experimentally prepared on ultrathin 25 μm cellophane substrates with ZnO/ultrathin Ag/ZnO (OMO) transparent electrodes, instead of conventional ITO, composing a typical device structure, cellophane/OMO/ZnO/PTB7‐Th:PC71BM/MoO3/Al. The solar cells exhibit a power conversion efficiency of 5.94% and a power weight ratio of 2.11 W g−1. After folding the solar cells for 35 cycles, the conversion efficiency still maintained 92% of the initial value, which is mainly ascribed to the synergistic effects of ultrathin substrates and the OMO electrodes, as inferred from the stable resistance of the cellophane/ZnO/ultrathin Ag/ZnO even after 50 folding cycles. This work paves the way toward realizing superior foldability not only in solar cells, but also in other optoelectronic devices.