Green Inks for the Fabrication of Organic Solar Cells: A Case Study on PBDTTPD:PC$_{61}$BM Bulk Heterojunctions

Nonhalogenated ecofriendly solvents are an important asset to avoid costly safety precautions during the fabrication of organic solar cells by printing. Yet, in the past, the quest for suitable nontoxic solvents has widely used empirical approaches. Herein, a comprehensive solubility study is rolled out embracing Hansen solubility parameters (HSPs), tailoring of binary solvents and rational choices of solvent additives, identifying ecofriendly solvents or solvent combinations for the deposition of poly-benzodithophene-thienopyrroledione (PBDTTPD)/fullerene thin-film blends. A particular challenge is the low polymer solubility even in common halogenated solvents. Following the HSPs, initially, a list of suitable solvent candidates is identified which are tested toward their applicability in solar cell fabrication. Among the shortlisted solvents, significant differences between p-xylene and o-xylene are observed, which can be compensated using solvent additives. The ecofriendly green solvent eucalyptol in combination with benzaldehyde and p-anisaldehyde in a ternary solvent mixture gives rise to decent solar cell performances. Solar cells are produced with power conversion efficiencies matching those conventionally fabricated from state-of-the-art halogenated solvents comprising chlorobenzene and chloronaphthalene. ... mehrNotably, the Hansen solubility approach provides an initial choice of solvents, but comes to its limits in predicting the best micromorphology formation, or if solvents react with the organic semiconductors.