Green Solvent-additive ink formulations based on Hansen Solubility Parameters for up-scaling of organic solar cells

The upsurge of oil price and the rapid drain of its sources has driven the scientific community to prioritize the lookout for cost-effective, renewable and green technologies for energy production. Organic Solar Cells (OSC) position themselves as an attractive alternative due to their light weight, flexibility, semitransparency, low production costs, and the potential easy upscaling processability. One of the most important parameters within an OSC is the solid-state morphology of its photoactive layer, which is mainly dictated by the solvent evaporation process taking place during the deposition of the inks. A common way to manipulate the morphology is by the employment of solvent additives. Hansen Solubility Parameters (HSP) is a theory employed to predict the interactions between two materials based on dispersion interactions (δD), permanent dipolar molecular interactions (δP), and hydrogen bonding interactions (δH). A great challenge for up-scaling this technology is the usage of halogenated and toxic solvents, therefore, employing the HSP theory we selected four possible green solvent-additive systems (o-xylene as host and limonene, tetralin, veratrole and DPE as additives) and built, tested and characterized inverted architecture devices employing them as medium for (1:1) and (1:5) donor-acceptor blends via blade-coating methods. Proportional blend devices were able to reach a PCEmax value of 8,63% with 2.5% by volume of veratrole; while donor diluted blends were able to reach a PCEmax value of 6.74% with 1% by volume of limonene. Simultaneously, it was possible to observe that in proportional blend devices, employing donor-akin additives resulted in smaller FF percentages (at least by 10%) and slightly higher Voc values when compared with acceptor-akin additives; while in donor-diluted devices the opposite phenomenon took place. On another note, a direct correlation between boiling point and superficial donor material percentage was found: if the former is high, so will be the latter and vice versa. This finding seems to work the other way around for 1:1 blend. Finally, higher boiling point additives demonstrated to perform the best for donor-diluted inverted architecture systems, since high superficial donor percentages were observed.