Freezing of Rotation of P3HT Crystallites by PCBM Molecules during Thermal Relaxation of the Blend Thin Films

Abstract

Poly(3-hexylthiophene), or P3HT, is a semicrystalline polymer with charge conduction anisotropy with respect to its different axes, which calls for gaining better control of texture for targeted device applications. A blend system of P3HT and [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) is studied for understanding the role of PCBM in texture control of P3HT crystallites. 2D grazing incidence diffraction (2D-GIXD) combined with high resolution grazing incidence X-ray diffraction (1D-GIXD) is performed on thin films of pristine P3HT and blend (P3HT/PCBM) thin films prepared by spin coating. Thermal annealing at 110 °C is used for relaxing the films. The lamellar P3HT crystallites preferably orient along the surface plane with lamellar stacking in a vertical direction and have a distribution of orientation with a limiting angle of 44°. In the pristine P3HT system, the rotations of crystallites are confirmed by the retreat of the limiting angle. The observed intensity cross-over for precursor and annealed samples is again the signature of the rotations of crystallites. For the blend system, the intensity cross-over is absent and there is no sign of retreat of the limiting angle, which explicitly confirms the blocking of rotation lamellar crystallites. This study provides, for the first time, the clear evidence with unambiguous analysis of blocking of rotation of crystallites due to presence of PCBM, which is explained with an analogy of the "ball-and-stick" mechanism. Atomic force microscopy (AFM) studies of surface morphology correlates with the above findings.