Understanding the crystallization process of a diketopyrrolopyrrole‐based conjugated polymer in blend films

Abstract

AbstractThe film morphology of fullerene and diketopyrrolopyrrole‐based conjugated polymers (PDPPs) blends largely influences the device performance in organic solar cells. It is critical to control the morphology of blend films, which usually requires investigations of the crystallization of PDPP‐based thin films. Here, we study the influence of marginal solvent additive 1,2‐dichlorobenzene (ODCB) and non‐solvent additive 1,8‐diiodooctane (DIO) on the crystallization of poly[2,5‐bis(2‐octyldodecyl)pyrrolo‐[3,4‐c]pyrrole‐1,4(2H,5H)‐dione‐alt‐2,2′: 5′,2″: 5″,2″′‐quaterthiophene] (PDQT). The blends formed fibril structures in thin films, as revealed by transmission electron microscopy. The fibril density increased and the width decreased with the ODCB amount. The critical ODCB content to achieve constant fibril width is almost proportional to the concentration of PDQT. Higher ODCB content also results in higher fibril density in pure PDQT films. In contrast, the amount of DIO has a negligible influence on the fibril width and density of thin films. Moreover, novel dendritic fibrils were formed in PDQT films upon addition of ODCB. A model based on nucleation and growth is proposed to explain these findings. The heterogeneous nucleation was dominant with the presence of ODCB, while the homogeneous nucleation was prevailing when DIO was used. The results show that initial nucleation density and growth direction are key factors determining the fibril width.