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

The paper addresses the challenge of replacing toxic, high–carbon footprint, petrochemical halogenated solvents in organic electronics with renewable, low-toxicity alternatives while retaining high device performance. Organic electronics generally have lower carbon emissions than inorganic counterparts, but state-of-the-art materials and processing still rely heavily on halogenated solvents. The purpose is to demonstrate that terpene solvents from renewable feedstocks can serve as viable green solvents for high-performing OPVs, OLEDs, and OFETs. The study establishes a Hansen solubility parameter (HSP)-guided framework to formulate inks and control film formation dynamics to achieve optimal morphology and charge transport, thereby enabling sustainable, scalable manufacturing aligned with circular carbon economy principles.

Prior work to green organic semiconductors has explored biomass-derived monomers and modified synthetic routes, but halogenated solvents continue to pose health and environmental risks (including reproductive hazards and carcinogenicity). Strategies to transition to greener processing have included modifying OSC sidechains to improve solubility, using hydrocarbon solvents with performance-boosting additives (for example, diphenyl ether), and employing Hansen solubility parameters to filter solvent candidates. However, past efforts were largely limited to a narrow set of non-halogenated hydrocarbon/aromatic solvents (xylene, toluene, trimethylbenzene, o-methylanisole, tetrahydrofuran) and struggled to match solubility and morphology to achieve on-par performance with conventional halogenated solvents. The broader industrial use of HSP for solvent selection in other sectors suggests it could systematically guide biosolvent selection for OSCs by balancing solubility, evaporation behavior, toxicity, boiling point, and renewability.

• Solvent selection via HSP: Determined Hansen solubility spheres (SS) and parameters for OPV donor PM6 and NFA BTP-eC9 using a binary solvent gradient method and group contribution approaches. Screened >10,000 industrial solvents (HSPiP) and ranked by LD50 and boiling point; extended the GSK solvent selection guide to include terpenes with waste/health/safety/environmental metrics.
• Terpene formulations: Identified renewable terpene solvents eucalyptol (Eu), d-limonene (Lim), β-pinene (Pin), and l-menthone (Men) and formulated binary inks by linear HSP additivity with higher-boiling co-solvents: Eu:Tetralin (Eu:Tet), Lim:Indane (Lim:Ind), Pin:Ethyl phenyl sulfide (Pin:EPS), Men:Tetralin (Men:Tet). Optimized volumetric ratios to minimize RED to PM6 (donor-limited solubility).
• Film formation analysis: Defined solubility affinity (Asol) in HSP space to predict component saturation/crystallization during drying. Measured evaporation rates and two-stage/constant-rate behavior via thermogravimetric analysis (10 µl droplets, 70 °C, N2 flow). Conducted in situ UV–vis absorbance during blade coating to track emergence and growth of PM6 (~625 nm) and BTP-eC9 (~830 nm) peaks and derived peak formation speed profiles.
• Device fabrication and characterization:
  • OPVs: Blade-coated PM6:BTP-eC9 (1:1.2, 10 mg ml−1) active layers from the four terpene-based inks; optimized blade speed and temperature. Conventional architecture: ITO/PEDOT:PSS/PM6:BTP-eC9/PDINO/Ag. Inverted and large-area/module devices also fabricated with ZnO and MoOx stacks. JV (AM 1.5G, LED simulator), EQE, light-intensity dependence of Voc and Jsc, SCLC mobilities (hole-only and electron-only), series resistance, and stability (shelf-life in N2 and ISOS-D-1; encapsulated outdoor).
  • Morphology: AFM (RMS roughness), GIWAXS (IP/OOP line-cuts; lamellar 100 and π–π stacking 010 peak areas), surface energy (contact angles) to assess vertical composition.
  • Universality: Extended Eu:Tet processing to other OPV blends (P3HT:O-IDTBR, PTB7-Th:IEICO-4F, PM6:IT-4F, PM6:PY-IT, and ternary PM6:(PY-IT:BTP-eC9)).
  • OLEDs: Super Yellow and green Livilux polymer OLEDs processed from Eu-based inks; measured JV, luminance, current efficiency, EL spectra.
  • OFETs: Bottom-contact top-gate devices using O-IDTBR and 2PyDPP-2CNTVT processed from Eu; measured transfer/output curves and mobilities in linear/saturation regimes.

• Green solvent identification and properties: Terpene solvents (Eu, Lim, Pin, Men) from renewable feedstocks have low toxicity (LD50 > 2,500 mg kg−1; comparable to table salt) and lower GWP than halogenated solvents (e.g., limonene ~0.4 vs chloroform ~3.4 kg CO2-eq kg−1). Potential for carbon-negative production via biorefineries.
• OPV performance (PM6:BTP-eC9, normal architecture, blade-coated in air):
  • CF:DIO control: PCE 15.6 ± 0.39% (PCEmax 15.9%), Jsc 25.9 mA cm−2, Voc 0.84 V, FF 72%.
  • Eu:Tet: PCE 15.1 ± 0.37% (PCEmax 15.7%), Jsc 25.7 mA cm−2, Voc 0.82 V, FF 72% — on par with CF:DIO; EQE ~80%; lower series resistance (3.82 Ω) than CF:DIO (5.2 Ω).
  • Lim:Ind: PCE 9.8 ± 0.65% (max 10.5%); Jsc 22.8 mA cm−2, Voc 0.81 V, FF 50%.
  • Pin:EPS: PCE 10.3 ± 0.84% (max 11.9%); Jsc 24.2 mA cm−2, Voc 0.82 V, FF 54%.
  • Men:Tet: PCE 9.7 ± 0.21% (max 10.3%); Jsc 20.8 mA cm−2, Voc 0.79 V, FF 58%.
  • Recombination analysis: Slopes of Voc vs ln(Illuminance) close to kBT/q for CF:DIO (1.02), Eu:Tet (1.07), Pin:EPS (1.03) indicating dominant bimolecular recombination; higher slopes for Lim:Ind (1.22) and Men:Tet (1.62) imply more trap-assisted recombination. Jsc vs light intensity exponents α ~0.96 (Eu:Tet, Pin:EPS, CF:DIO), indicating weak bimolecular recombination.
  • Charge transport: SCLC mobilities show Eu:Tet with more balanced μh/μe ratio (1.66) compared to CF:DIO (0.98); Lim:Ind and Pin:EPS have μh/μe >6 (imbalanced).
• Morphology: Eu:Tet films exhibit fibrillar morphology and low RMS roughness (2.73 nm) similar to CF:DIO (1.36 nm), with strong IP (100) and OOP (010) GIWAXS features, indicating balanced donor/acceptor crystallization and favorable vertical stratification. Men:Tet films show large RMS (29.66 nm) and visible phase separation.
• Drying kinetics: Formulations with larger BP differences (Pin:EPS, Eu:Tet) show two-stage evaporation; others (Lim:Ind, Men:Tet) show constant rates. In situ absorbance reveals earlier onset of BTP-eC9 solidification as terpenes evaporate; Eu:Tet and CF:DIO show slower, synchronized crystallization of both components leading to better morphology.
• Scalability and stability:
  • Large-area single cells (2.4 cm2, inverted): PCE up to 12.4% (avg 11.4 ± 0.8%).
  • Five-cell module (12 cm2, inverted): PCE 9.1% (avg 8.3 ± 0.7%).
  • Stability: Retained ~92% (N2) and ~60% (ambient ISOS-D-1) after 1,000 h; encapsulated outdoor devices maintained performance for up to 90 days (Feb–May 2022) in Thuwal, Saudi Arabia.
• Universality across materials/devices (Eu:Tet inks):
  • OPVs: PM6:PY-IT (15.7 ± 0.3%, max 15.9%); ternary PM6:(PY-IT:BTP-eC9) 16.3%; PTB7-Th:IEICO-4F 9.8 ± 0.3% (max 10.6%); P3HT:O-IDTBR 5.1 ± 0.2% (max 5.3%); PM6:IT-4F 8.2 ± 0.8% (max 9.2%).
  • OLEDs (Eu): Super Yellow CEmax 5.1 cd A−1, Lmax 9,000 cd m−2; Green Livilux CEmax 1.4 cd A−1, Lmax 4,144 cd m−2 (comparable to CF/Tol benchmarks).
  • OFETs (Eu vs CB): O-IDTBR μe up to 0.91 cm2 V−1 s−1 (Eu) vs 0.57 (CB); μh up to 0.37 (Eu) vs 0.22 (CB); 2PyDPP-2CNTVT μe 0.48 (Eu) vs 0.21 (CB), μh 0.32 (Eu) vs 0.15 (CB); Ion/Ioff >105.

The study demonstrates that renewable terpene solvents, selected and formulated via a Hansen solubility framework, can replace halogenated/aromatic solvents without sacrificing device performance or scalability. By tuning solubility affinity and evaporation profiles through binary formulations (e.g., Eu:Tet), the drying pathway synchronizes donor and acceptor crystallization, yielding favorable nanoscale morphology, balanced charge transport, reduced recombination, and high FF—comparable to CF:DIO controls. Morphological (AFM, GIWAXS) and electrical analyses (SCLC, light-intensity dependence) corroborate that Eu:Tet promotes fibrillar interpenetrating networks and balanced μh/μe, leading to PCEs up to 15.7% in PM6:BTP-eC9 devices. The approach generalizes to multiple OPV blends, OLEDs, and OFETs, and scales to modules with competitive efficiencies and encouraging stability, supporting broader adoption in sustainable manufacturing. Furthermore, terpenes provide benefits in toxicity and carbon footprint, aligning with circular carbon economy goals.

Terpene solvents from renewable biosources offer a low-toxicity, potentially carbon-negative alternative to conventional halogenated solvents for high-performing organic electronics. An HSP-guided binary solvent formulation strategy, emphasizing balanced solubility affinity and an acceptor-leaning high–boiling point co-solvent, enables controlled film formation, favorable morphology, and competitive device metrics across OPVs, OLEDs, and OFETs. The method is scalable to large-area devices and modules with stable outdoor operation. The publicly available green solvent library supports broader materials adoption; highlighted candidates for future studies include carvone, linalool, anisole, and γ-valerolactone. Future work may optimize formulations across more material systems, further enhance large-area performance, and conduct comprehensive life-cycle assessments to solidify environmental benefits.

• Performance depends strongly on formulation and drying kinetics; some terpene combinations (Lim:Ind, Pin:EPS, Men:Tet) show lower PCEs, higher recombination, or morphological phase separation, requiring careful optimization of co-solvent ratios and coating conditions.
• Large-area and module efficiencies, while competitive (12.4% single-cell, 9.1% module), are lower than small-area champion cells and may need further process optimization.
• Stability was demonstrated up to 1,000 h (lab) and 90 days (outdoor) for the reported systems; longer-term field data and broader environmental stress testing are still needed.
• Universality was shown across several material systems, but not all blends were fully optimized (some noted as first-run results) and broader materials validation is warranted.
• Accurate HSP determination and access to co-solvents with suitable toxicity/boiling points are prerequisites; results may vary with material batches and processing equipment.