Green Inks for the Fabrication of Organic Solar Cells: A Case Study on PBDTTPD:PC₆₁BM Bulk Heterojunctions

Abstract

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. Notably, 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.

Advances in the device design and printing technology for eco-friendly organic photovoltaics

“…Several groups have used HSPs to screen non-halogenated solvents for the preparation of high-performance OSCs. 37–45 Yang et al 37 successfully fabricated ternary eco-friendly OSCs based on PTB7-Th:DR3TSBDT:PC71BM with a PCE of 12.3% by assaying the solubility of the active materials in different solvents and quantifying the solvent-active material interactions using HSPs. First, Yang et al measured the solubility of each active material in green solvents and mapped Hansen spheres (Fig.…”

Gas-assisted blade-coating of organic semiconductors: molecular assembly, device fabrication and complex thin-film structuring

“…15 Modifying the formulation of solutions necessitates labour-intensive and material-inefficient trials which are further restricted by the increasing emphasis placed on the use of non-toxic, 'green' solvents. 17,18 Moreover, given that microstructure formation is highly sensitive to material-specific solubilities and miscibilities, such solution optimisation remains, essentially, empirical. 19,20 An alternative approach to accelerating the film formation kinetics for deposition by blade-coating involves in situ external gas drying.…”

High-performing organic electronics using terpene green solvents from renewable feedstocks

“…However, the carbon emissions of halogenated solvents, in addition to their toxicological effects such as reproductive hazards and cancer, pose a serious risk to human safety and the environment, rendering them unsustainable for high-throughput fabrication of solution-based electronic devices 6,7 . Recent strategies to transition towards environmentally friendly solvents require either the modification of OSC sidechains to improve solubility [8][9][10] , the utilization of hydrocarbon solvents along with additives like diphenyl ether to boost performance [11][12][13] or the calculation of Hansen solubility parameters (HSP) to methodical filter potential solvent candidates for processing [14][15][16][17] . Yet, these strategies were limited to five standard hydrocarbon-based solvents: xylene, toluene, trimethylbenzene, o-methylanisole and tetrahydrofuran.…”