In situ studies of the molecular packing dynamics of bulk-heterojunction solar cells induced by the processing additive 1-chloronaphthalene

Abstract

Processing additives have been widely utilized for high-performance organic bulk-heterojunction (BHJ) photovoltaic devices. However, the role of processing additives remained unclear due to the limited information relying on the final BHJ film state rather than the intermediate film state during solvent evaporation. Here, by using in situ GIWAXS measurements on the intermediate BHJ film, we propose a possible phase separation mechanism in efficient BHJ solar cells consisting of a narrow band gap polymer (P1) and PC71BM via the use of 1-chloronaphthalene (1-CN) as a processing additive. We found that adding small amounts of an additive, 1-CN, with a high boiling point and a high PC71BM solubility can prolong the solvent evaporation time and dissolve many PC71BM molecules, promoting the strong P1 polymer:solvent and PC71BM:solvent interaction to produce pure domains of each component. Thus, the bi-continuous networks for both P1 and PC71BM and their enlarged interfacial area are well fabricated in the BHJ films, inducing balanced photo-charge carrier densities for the electrons and holes and improving the overall photovoltaic performance. Therefore, our findings elucidate the kinetic motions of two organic phases affected by the physical properties of the solvents in the process of film formation and establish criteria for BHJ systems.