Investigation of Nanomorphology Change in Bulk Heterojunction Films using Synchrotron X-ray Diffraction

Here we report the in-situ measurement of the nanomorphology (nanostructure) change in blend films of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C-61 (PCBM), which is one of the standard bulk heterojunction combinations for organic solar cells. The time-dependent variation of the nanostructures at a fixed temperature was monitored with a grazing angle incident X-ray diffraction system empowered by a synchrotron radiation source. In order to understand the influence of film thickness, we fabricated blend films with two different thicknesses (75 nm and 270 nm) using the same substrates coated with a hole-collecting buffer layer as used for the corresponding solar cells. Results uncovered that the nanomorphology was not monotonically changed but fluctuated up to 30 min, followed by stabilization in the presence of continuous small variations upon further annealing. This fluctuation was similarly observed for both 75 nm and 270 nm thick blend films, even though the thicker films exhibited slightly delayed behavior to reach a well-defined alignment of the P3HT chains.