A tool for identifying green solvents for printed electronics

Printed electronics is poised for large-scale, cost-efficient fabrication of electronic and photonic devices using solution-processed inks. Device performance strongly depends on ink properties (solute solubility, viscosity, wettability, film formation, vapor pressure, shelf life). A critical, often overlooked issue is the sustainability of solvents used, many of which pose health, safety, and environmental risks—especially problematic in open fabrication environments exposing workers to vapors. The research question is how to systematically identify greener solvents that maintain functional equivalence (dissolution performance and processing properties) to currently used, less sustainable solvents. The authors aim to develop and demonstrate a practical tool that organizes solvents by Hansen solubility parameters (HSPs), key ink properties, and sustainability descriptors to guide replacement with greener alternatives, and validate it via a case study on light-emitting electrochemical cells (LECs).

Green solvent assessment frameworks such as the Globally Harmonized System (GHS) hazard and precautionary statements inform solvent sustainability evaluations. Several organizations and companies (ACS GCI Pharmaceutical Roundtable, IMI CHEM21, Pfizer, Sanofi, GSK) have created solvent selection guides that quantitatively rank solvents on health, safety, environmental impact, and other factors, with reviews finding broad consistency among guides. This work adopts the updated GSK solvent sustainability guide, which aggregates 10 sub-categories into four category scores (Health, Safety, Environment, Waste) and a composite score G (1–10; higher is better). For dissolution functionality, the Hansen method partitions cohesive energy into dispersion (δD), polar (δP), and hydrogen-bonding (δH) parameters and defines a distance Ra in 3D HSP space to quantify similarity in solubility behavior. These established approaches underpin the tool’s solvent ranking by functionality and sustainability.

Tool design and data: The tool compiles 132 solvents with their Hansen solubility parameters (δD, δP, δH), physical properties (boiling point, viscosity, surface tension), identifiers (CAS), and sustainability data (GSK category and composite scores; specific hazard information). Solvent sustainability is summarized via GSK scoring: Health (H = sqrt(Health Hazard × Exposure Potential)), Safety (S = sqrt(Flammability & Explosion × Reactivity & Stability)), Environment (E = Air × Aqueous impacts), Waste (W = sqrt(Incineration × Recycling × Bio Treatment × VOC)), and composite G = H × S × E × W (normalized to 1–10). If subcategory data are incomplete, GSK downgrades the score. Functional solvent ranking: The dissolution capacity similarity is quantified using the Hansen distance Ra^2 = 4(ΔδD)^2 + (ΔδP)^2 + (ΔδH)^2. The web tool allows users to: (i) choose one or more known functional solvents for a given solute; the tool then ranks all solvents by increasing Ra relative to the selected solvent(s) or their mean HSPs; or (ii) input the solute’s own HSPs to rank solvents by proximity. Users can refine candidates by setting acceptable ranges for boiling point, viscosity, and surface tension. Case study protocol (LEC ink): The authors applied the tool to replace chlorobenzene in a multi-solute active-material ink for high-performance LECs requiring ~30 g L−1 total solute concentration. Starting from chlorobenzene as the known functional solvent, the tool ranked solvents by Ra and filtered by sustainability (G ≥ 7). The seven closest sustainable candidates were identified. Experimental validation assessed solubility at target concentration and device performance. Experimental procedures: Master inks were prepared by dissolving PVK (Mw 1.1×10^6 g mol−1), OXD-7, Ir(R-ppy) emitter, and THABF in candidate solvents (chlorobenzene, ethoxybenzene, anisole) at 70 °C with stirring. Active inks were blended at mass ratio PVK:OXD-7:Ir(R-ppy):THABF = 32.3:32.3:29.0:6.4 and stirred at 70 °C. Spin-coated devices used ITO/PEDOT:PSS substrates, with active layers dried at 70 °C to 120 nm thickness; Al top electrodes were thermally evaporated. For spin coating, solute concentration and spin speed were tuned per solvent (chlorobenzene: 30 g L−1, 2000 rpm; ethoxybenzene: 46.5 g L−1, 3000 rpm; anisole: 40 g L−1, 2000 rpm). Bar-coated devices used PET/ITO with bar-coated PEDOT:PSS and active-material inks at 35 g L−1; resulting active layer thicknesses were ~160–220 nm depending on solvent. Devices were driven at constant current density j = 77 A m−2; luminance and voltage were recorded, and EL spectra measured. Storage stability was tested with inks stored 30 days before device fabrication. At least six devices per condition were fabricated; reported device data represent typical performance.

- Tool capability: Interactive ranking of 132 solvents by Hansen distance to known functional solvents or solute HSPs, with filtering by physical properties and sustainability (GSK composite score). Visualization with size/color coding (green G ≥ 7, yellow/orange G = 5–6, red G ≤ 4) highlights overall sustainability. - Case study—candidate identification: Using chlorobenzene (G = 5.4) as the known solvent for an LEC multi-solute ink, the closest sustainable (G ≥ 7) candidates ranked by Ra were: ethoxybenzene (Ra = 2.3, G = 7.2), anisole (Ra = 5.5, G = 7.4), cyclohexanone (Ra = 5.7, G = 7.2), methyl oleate (Ra = 6.2, G = 7.5), 2-ethylhexyl acetate (Ra = 7.2, G = 7.7), pentyl acetate (Ra = 7.7, G = 7.2), n-butyl acetate (Ra = 7.7, G = 7.5). - Solubility screening: Ethoxybenzene and anisole dissolved the multi-component solute at target concentration. Cyclohexanone also dissolved but was not pursued due to no sustainability improvement and more hazard statements. Acetates and methyl oleate did not dissolve at required concentration, suggesting a dissolution boundary around Ra > 6 for this system (not a sharp threshold). - Device performance (spin-coated fresh inks): All three solvents (chlorobenzene, ethoxybenzene, anisole) yielded uniform films (120 nm), identical EL spectra (green, λpeak ≈ 525 nm), fast turn-on to >1000 cd m−2 (<2 s). Peak luminance and current efficiency were slightly higher with chlorobenzene (≈3100 cd m−2, 39.9 cd A−1) than with ethoxybenzene/anisole (≈2540 cd m−2, ≈32.9 cd A−1); operational lifetimes were essentially the same. - Storage stability (30 days): Devices from stored chlorobenzene ink showed ~20% drop in peak performance; ethoxybenzene and anisole inks were more robust. OXD-7 crystallization occurred in chlorobenzene upon storage, inhibited in ethoxybenzene and absent in anisole, indicating closer HSP match of OXD-7 with anisole. - Scalable bar coating: Uniform films and bright, uniform emission were achieved for all three solvents; bar-coated devices showed performance close to spin-coated devices. Slightly lower luminance and higher voltage for bar-coated devices were attributed to slightly thicker active layers. Ethoxybenzene and anisole successfully replaced chlorobenzene in both spin and bar coating. - Sustainability metrics: Chlorobenzene had low category scores for Health (H = 4.0) and Environment (E = 3.7), composite G = 5.4. Ethoxybenzene and anisole showed higher composite scores (G = 7.2 and 7.4, respectively), with ethoxybenzene’s Health score (4.9) limited by incomplete data in GSK, likely underestimating its true value.

The study addresses the challenge of replacing hazardous solvents in printed electronics without compromising ink function. By combining Hansen solubility parameters with key processing properties and a comprehensive sustainability scoring (GSK), the tool enables targeted selection of greener solvents with similar dissolution behavior to known functional solvents. In the LEC case study, the approach identified ethoxybenzene and anisole as viable replacements for chlorobenzene, maintaining essential ink processability and device performance (fast turn-on, comparable lifetime) while improving sustainability profiles. Improved storage stability with anisole/ethoxybenzene further underscores better solute–solvent compatibility. The tool’s integration of functionality and sustainability, coupled with interactive filtering, provides practical guidance for both laboratory and industrial settings to reduce exposure to harmful solvents and environmental impact without sacrificing device quality.

The work introduces an open-access green-solvent selection tool that organizes solvents by Hansen solubility parameters, physical properties, and GSK-based sustainability metrics to rationally identify functional, safer alternatives. Demonstration on a high-performance LEC ink replaced chlorobenzene with ethoxybenzene and anisole, preserving device performance in both spin- and bar-coating processes and enhancing sustainability and storage robustness. This approach supports broader adoption of greener solvent systems in printed electronics. Future work could expand solvent and material databases, integrate additional process-relevant properties, refine solute HSP inputs, and validate across diverse device classes and multi-solvent formulations.

- Sustainability data completeness: Some solvents (e.g., ethoxybenzene) have incomplete subcategory data in the GSK guide, potentially underestimating their composite sustainability score. - Ra threshold variability: The Ra boundary for dissolution is system-specific and not sharp; observed non-solubility beyond Ra ≈ 6 in this multi-solute system may not generalize. - Case specificity: Experimental validation focused on one device type (LEC) and a particular multi-solute ink; broader testing across materials and processes would further establish generality. - High-concentration solubility: Meeting high total solute concentrations (~30–46.5 g L−1 depending on solvent) can limit candidate solvents even if Ra is modest; physical property constraints (viscosity, boiling point, surface tension) also affect applicability.