Thermal Expansion Behavior of PM6 Studied Using In Situ Wide‐Angle X‐Ray Scattering

Abstract In organic photovoltaics, even small morphological changes on the sub‐nanometer scale can significantly impact the device's performance. Such structural changes intrinsically occur due to the thermal response of a material, making a fundamental understanding of these processes essential. In this study, the thermal expansion of the high‐performance donor material PDBD‐T‐2F (PM6) is characterized using in situ transmission and grazing incidence wide‐angle X‐ray scattering (WAXS and GIWAXS) over the temperature range of 30–300 °C. By systematically increasing the complexity of the sample, multiple influencing factors are deciphered. Significantly, distinct structural differences and temperature expansion behavior is observed for the face‐on and edge‐on crystallite fraction. In particular, the initial lamellar stacking d ‐spacings differ for the two fractions and moreover, the face‐on fraction exhibits significantly lower thermal expansion in lamellar direction compared to the edge‐on fraction. In situ GIWAXS measurements performed during solution coating show that these differences are formed during film formation. Additionally, an impact of the thermal expansion mismatch between the substrate and the sample is observed, limiting the in‐plane π–π stacking expansion. The results emphasize the interplay between molecular packing, substrate interaction, and thermal response.