Investigating the effect of cosolvents on P3HT/O-IDTBR film-forming kinetics and film morphology

Abstract

The morphology of active layer in bulk heterojunction (BHJ) organic solar cells is decisive to the device performance. Previous works have shown that the solvent engineering is an effective method to optimize the morphology of active layer. However, screening the proper solvent is a tedious task, and we know very little about how to select a proper solvent for a particular system, especially for polymer/non-fullerene blend systems. Here, we combined the spectroscopic analysis in various solvent mixtures during film-forming process to reveal the relationship among the cosolvent characteristics, film-forming kinetics and film morphology. In this article, P3HT/O-IDTBR blend was selected as model system due to being facile synthesized under a large-scale. Chlorobenzene (CB) was selected as main solvent, and the cosolvents were grouped into three categories according to its boiling point (bp) compared to CB. The cosolvents with lower bp, like chloroform (CF), can facilitate a faster film-forming process, reducing the domain size but sacrificing the crystallinity of both components. For the cosolvents with higher bp, like o-dichlorobenzene (DCB) and 1,2,4-trichlorobenzene (TCB), the self-organization process of P3HT and O-IDTBR is separated and its duration was extended, constructing highly crystalline nano-interpenetrating network. However, the cosolvents with very high bp, such as chlornaphthalene (CN), would residue in film and keep P3HT and O-IDTBR self-organizing for longer time, leading to larger phase separation. This work systematically investigated the effect of cosolvent on the film-forming kinetics, and proposed a guideline of how to select a proper cosolvent according to the crystallinity and domain size of active layer.