Thin-film polymer solar cell consisting of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) demonstrates elastic stretchability with the aid of a high boiling point additive, 1,8-diiodooctane (DIO). The usage of DIO not only helps to form uniformly distributed nanocrystalline grains, but may also create free volumes between the nano-grains that allow for relative sliding between the nano-grains. The relative sliding can accommodate large external deformation. Large dichroic ratios of the optical absorption of both PC71BM and PTB7 were observed under large-strain deformation, indicating reorientation of the nanocrystalline PC71BM and PTB7 polymer chains along stretching direction. The dichroic ratio decreases to nearly 1.0 as the blend was relaxed to 0% strain. Therefore, the nanometer-size grain blending morphology provides an approach to impart stretchability to organic semiconductors that are otherwise un-stretchable. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 814-820