(A) study on mechanical and electronic properties of naphthalene diimide based polymer blend films

Abstract

The growing demand for commercialization of stretchable and flexible organic electronics is creating a shift in research interest to investigating mechanical properties of conjugated polymers. The superior mechanical stability and good electrical performance of conjugated polymers are mutually exclusive, because the former needs to amorphous regions enabling large plastic deformation and the latter requires high crystallinity for facile charge transport. It has been well known that the high crystallinity of rigid backbone structure are much inferior to ductility and toughness. Therefore, the general and common approach resolving this apparent contradiction is finding the best compromise between mechanical and electrical properties through the molecular engineering. However, very limited efforts has been devoted to investigate the intrinsic mechanical properties of n-type conjugated polymers compared to their p-type counterparts. Since the n-type conjugated polymers are usually applied as a thin film state with thickness ~ 100 nm in organic electronics, the understanding on the mechanical properties of n-type conjugated polymers in thin film state is highly urgent. Moreover, typical organic electronics such as organic field-effect transistors (OFETs) and polymer solar cells (PSCs) consisting of various layers inevitably have weak interfaces between materials, thereby it is essential to understand fracture behavior occurred in interfaces. In particular, it is well known for classic polymer that the molecular weight of polymer should be above the critical molecular weight where the polymer chains are highly entangled each other to acquire superior mechanical properties, but there was almost no previous researches about critical molecular weight of n-type conjugated polymer, thereby the systematic investigation on molecular weight dependence of mechanical compliance of n-type conjugated polymer is highly desirable. For application in wearable electronics, PSCs should be able to stand tensile strain of at least 20~30%, which occurs by human body activity and delicate muscle movements. In addition, it is suggested that the PSCs with a $G_c$ value below 5 J m^{-2}$ are often difficult to withstand mechanical stresses from manufacturing and operation. Therefore, the development of the BHJ active materials with intrinsically high mechanical robustness (i.e., strain at fracture (> 20-30%) and a Gc value (> 5 $J m^{-2}$)) is urgent and important to promote long-term device stability, high production yield through large-scale manufacturing, as well as their use in the wearable and portable device applications. Herein, we thoroughly investigated the intrinsic mechanical properties of naphthalene diimide (NDI) based n-type semicrystalline conjugated polymer (P(NDI2OD-T2) pristine films via controlling the number-average molecular weights ($M_n$). Furthermore, we expanded our findings into all-polymer solar cells (all-PSCs) system. Firstly, the all-PSCs exhibit much superior mechanical robustness compared to PSC comprised of small molecule acceptors (i.e., PCBM and ITIC). Most importantly, while the overall mechanical properties such as cohesion energy, strain at fracture, and toughness increase in terms of the $M_n$, the significant enhancements in the properties can be achieved only when the $M_n$ of PAs is higher than the critical molecular weight ($M_c$). Therefore, it is expected that this work can provide the important clues to the design n-type electron accepting materials for producing wearable and stretchable power generators that require high performance with excellent mechanical deformability.