The Hansen solubility approach towards green solvent processing: n-channel organic field-effect transistors under ambient conditions

Abstract

The adoption of green solvents is of utmost importance for the solution-based fabrication of semiconductor thin films and for the commercialization of (opto)electronic devices, especially in response to evolving regulatory mandates for handling organic materials. Despite the increasing interest in this area, the scarcity of green solvent-processed n-channel OFETs, especially functioning under ambient conditions, highlights the need for further research. In this study, we demonstrated the Hansen solubility approach to study the solubility behavior of an ambient-stable n-type semiconductor, 2,2′-(2,8-bis(3-dodecylthiophen-2-yl)indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile (β,β′-C₁₂-TIFDMT), and to analyze potential green solvents for thin-film processing. The Hansen solubility parameters were determined to be δ_D = 20.8 MPa^1/2, δ_P = 5.8 MPa^1/2, and δ_H = 5.5 MPa^1/2 with a radius (R₀) of 8.3 MPa^1/2. A green solvent screening analysis based on the minimal distance constraint and quantitative sustainability score identified ethoxybenzene, anisole, 2-methylanisole, and 2-methyltetrahydrofuran as suitable green solvents (R_a′s = 5.17–7.93 MPa^1/2 < R₀). A strong thermodynamic correlation was identified between the solubility and the semiconductor–solvent distance in the 3D Hansen solubility space, in which the maximum solubility limit could be estimated with the enthalpy of fusion (ΔH_fus) and melting temperature (T_mp) of the semiconductor. To the best of our knowledge, this relationship between the maximum solubility limit and thermal properties has been established for the first time for organic semiconductors. Bottom-gate/top-contact OFETs fabricated by spin-coating the semiconductor green solutions exhibited μ_es reaching ∼0.2 cm² V⁻¹ s⁻¹ (I_on/I_off ∼10⁶–10⁷ and V_on ∼0–5 V) under ambient conditions. This device performance, to our knowledge, is the highest reported for an ambient-stable green solvent-processed n-channel OFET. Our HSP-based rational approach and unique findings presented in this study can shed critical light on how green solvents can be efficiently incorporated in solution processing in organic (opto)electronics, and whether ambient-stable n-type semiconductors can continue to play an important role in green OFETs.