Improved storage stability of conjugated polymer solutions with a versatile Non-halogenated solvent for organic solar cells

Abstract

Progress towards the realization of organic solar cells (OSCs) as alternative energy sources has made tremendous strides in recent years. The solution processability of the active layers endows OSCs with several advantages, and concomitantly, it is a bottleneck to their commercial application owing to the instability of conjugated polymer solutions in ambient environments. Herein, the key factors dominating the solution stability of a conjugated polymer processed with non-halogenated solvents, such as toluene and 1,2,4-trimethylbenzene (TMB), were investigated using a prototypical polymer donor PM6. The analysis of solvent parameters showed that the relatively larger polarity and molar volume of TMB weakened the aggregation tendency of PM6 molecules by strengthening the solvation effect, resulting in superior storage stability compared with the toluene-processed solution. It was found that the unstable toluene-processed PM6 solution led to an inappropriate phase separation and reduced uniformity of the blend film, thus weakening the power conversion efficiency (PCE) of devices with the sequentially deposited PM6/L8-BO active layers. The TMB-processed PM6 solution exhibited excellent storage stability of up to 60 days, and the device performances based on these aged solutions were comparable with those of the devices manufactured with fresh solutions. This study provides guidance for solvent selection in the industrial production of OSCs, which will benefit the preparation, storage, and processing of active layer solutions, resembling commercially used PEDOT:PSS inks.