The field of printed electronics is rapidly expanding, offering cost-effective fabrication of electronic and photonic devices. This growth is driven by significant research and development focusing on ink properties like solubility, viscosity, and wettability, as these directly impact device performance. However, the sustainability of the solvents used is a critical, often overlooked aspect. Many currently employed solvents pose significant health, safety, and environmental risks, especially given the open environments in which ink deposition and solvent evaporation frequently occur. Worker exposure to solvent vapors necessitates identifying functional "green" solvent replacements that maintain ink performance. This paper addresses this challenge by presenting a freely available online tool designed to streamline the identification of suitable, environmentally friendly solvent alternatives. The tool leverages Hansen solubility parameters, key ink properties, and sustainability descriptors to systematically suggest greener options with similar dissolution capabilities to existing, less sustainable solvents. The tool's utility is illustrated through a case study involving a multi-solute ink for high-performance light-emitting electrochemical cells (LECs), where a problematic solvent is successfully replaced by two environmentally benign alternatives. The development and implementation of this tool represent a significant step toward promoting sustainability within the printed electronics industry.

The literature review highlights the significant research efforts dedicated to improving printed electronics inks, emphasizing the importance of various ink properties for optimal device functionality. However, the review points to a lack of focus on the sustainability of solvents used in the process. Existing studies discuss the need for greener alternatives but lack a comprehensive, readily accessible tool for identification. The authors mention the work of several organizations (e.g., the American Chemical Society Green Chemistry Institute Pharmaceutical Roundtable, IMI: CHEM21) and companies (Pfizer, Sanofi, GlaxoSmithKline) which have contributed to the quantitative classification and ranking of solvent sustainability. These efforts, though differing slightly in their weighting of properties, demonstrate consistent overall solvent ranking, providing a foundation for the development of a unified approach to solvent selection. The review also briefly touches upon the Hansen solubility parameters and their role in predicting solvent efficacy for specific solutes, a core component of the new tool.

The green solvent selection tool developed in this study utilizes a combination of techniques to identify suitable replacements for existing solvents. First, it employs the Hansen solubility parameters (HSPs) to predict the dissolution capacity of a solvent. The HSP method breaks down the total cohesion energy of a solvent into three parameters: dispersion energy (δD), polar energy (δP), and hydrogen-bonding energy (δH). The tool calculates the distance (Ra) between the HSPs of a target solvent and potential replacement solvents, with smaller Ra values indicating greater similarity in solubility capacity. This calculation is based on equation (1) where the dispersion term is weighted four times more than other terms due to its relative importance. The tool incorporates the GSK solvent sustainability guide, which quantitatively assesses solvents across ten subcategories grouped into four main categories (health, safety, environment, and waste disposal). Each subcategory receives a score (1-10), and a composite score (G) is calculated to provide an overall sustainability rating. High G scores (7 and above) indicate more sustainable solvents. The tool integrates these sustainability scores with the HSP-based solubility prediction. The user selects a known functional solvent or provides the HSPs of the solute(s) of interest. The tool then ranks all solvents by their Ra value relative to the selected solvent or solute HSPs. Users can further refine results by defining functional ranges for other key ink properties like boiling point, viscosity, and surface tension, excluding solvents that fall outside these ranges. The tool's functionality is demonstrated in a case study involving the replacement of chlorobenzene in a high-performance LEC ink, which involves four different solutes with a high total solute concentration(~30g/L). The authors initially select chlorobenzene, a known functional solvent in the case study, and rank all 132 solvents by their Ra value. They further refine this selection, filtering for solvents with G>7. Potential replacements are experimentally tested for their ability to dissolve the multicomponent solute. LEC devices are fabricated using inks with the chosen green solvents and chlorobenzene as a control. Device performance parameters, including luminance, voltage, and operational lifetime, are measured and compared for both fresh and stored inks using both spin coating and bar coating deposition methods.

The study's key findings center around the successful development and application of a green solvent selection tool for printed electronics. The tool effectively integrates Hansen solubility parameters with a sustainability assessment based on the GSK solvent sustainability guide. The tool's utility is validated through a case study focused on the replacement of chlorobenzene, a harmful solvent used in a high-performance LEC ink. The tool identified ethoxybenzene and anisole as promising green replacements based on their close proximity to chlorobenzene in the Hansen solubility space and high sustainability scores. Experimental validation confirmed that both solvents successfully dissolved the multi-component solute mixture, creating uniform and pinhole-free active-material thin films. LEC devices fabricated with inks using ethoxybenzene or anisole exhibited performance comparable to those using chlorobenzene, achieving similar luminance, efficiency, and operational lifetime. In terms of device performance metrics, the turn-on time to reach a luminance of >1000 cd m−2 was fast and comparable across all three inks (chlorobenzene, ethoxybenzene, and anisole). While peak luminance and efficiency were slightly higher for chlorobenzene (3100 cd m−2 and 39.9 cd A−1, respectively), those of ethoxybenzene and anisole were comparable (~2540 cd m−2 and ~32.9 cd A−1). Importantly, operational lifetime was essentially identical across all three ink solvents. The study further investigated ink storage stability, revealing that inks based on ethoxybenzene and anisole exhibited better long-term stability compared to chlorobenzene ink, likely due to differences in solute solubility. Finally, the successful implementation of bar coating—a more scalable fabrication method—with all three inks demonstrates the viability of these green solvents for large-scale production. The researchers found uniform thin films with all three active material inks using bar coating, and these bar coated films could be employed in LEC devices. The slightly lower luminance and higher drive voltage of the bar coated LECs compared to spin coated LECs is possibly due to the active material thickness in the former being slightly larger than the optimum 120 nm. Overall, the results highlight the ability of the tool to effectively identify functional and sustainable solvent replacements for complex multi-solute inks, paving the way for greener manufacturing processes in printed electronics.

The results of this study demonstrate the effectiveness of the developed green solvent selection tool in addressing the critical need for sustainable solvents in printed electronics. The successful replacement of chlorobenzene with ethoxybenzene and anisole in the LEC ink showcases the tool's practical applicability. The comparable performance of LECs fabricated with these green solvents confirms their functionality as direct replacements. The improved long-term storage stability of inks containing ethoxybenzene and anisole highlights an additional benefit of using these greener alternatives. Furthermore, the successful bar coating fabrication demonstrates the feasibility of these green solvents for scalable manufacturing processes, which is crucial for industrial adoption. The open-access nature of the tool will facilitate its wider adoption by researchers and industries, accelerating the transition to more sustainable practices in printed electronics. Future research can focus on expanding the solvent database, incorporating additional sustainability metrics, and refining the prediction models to enhance the accuracy of solvent recommendations. This could also include incorporating more data on the lifecycle assessment of the solvents, allowing for a more comprehensive sustainability analysis. The study's success suggests that systematic approaches to solvent selection, combined with readily accessible tools, can significantly contribute to the creation of environmentally friendly manufacturing processes across multiple industries.

This research successfully developed and validated a user-friendly, open-access tool for identifying green solvents in printed electronics. The tool effectively combines Hansen solubility parameters with sustainability metrics to suggest functional and environmentally friendly alternatives. A case study demonstrated the successful replacement of a harmful solvent in light-emitting electrochemical cell ink with two suitable green alternatives, without compromising device performance. This tool is poised to facilitate a significant transition towards more sustainable manufacturing practices within the printed electronics industry and beyond. Future work could involve expanding the solvent database, improving prediction models, and incorporating a broader range of sustainability criteria.

While the tool successfully identified and validated green solvent replacements in a specific case study, its generalizability to other inks and applications needs further investigation. The accuracy of solvent predictions relies on the accuracy of the input data (HSPs and sustainability scores). Incomplete or inaccurate data might lead to less reliable predictions. The tool's current focus on solubility and sustainability metrics could be expanded to include other important factors such as toxicity, cost, and availability, which can impact the overall suitability of a solvent. Finally, while bar coating was demonstrated, the study focused primarily on spin coating, which is less scalable. Further validation across a broader range of printing techniques is warranted.