Performance enhancement of air-stable thieno[2,3-b]thiophene organic field-effect transistors via alkyl chain engineering

Abstract

In this study, four novel thieno[2,3-b]thiophene (TT) small molecules, 2,5-bis((5-octylthiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (1), 2,5-bis((5-(2-ethylhexyl)thiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (2), 3,4-dimethyl-2,5-bis((5-octylthiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (3), and 2,5-bis((5-(2-ethylhexyl)thiophen-2-yl)ethynyl)-3,4-dimethylthieno[2,3-b]thiophene (4), were synthesized and explored as channel layers for organic field-effect transistors (OFETs). Conjugated triple bonds and flexible alkyl side chains were strategically integrated into the TT core to promote efficient carrier transport. The compounds were characterized using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV-visible spectroscopy (UV-vis), and cyclic voltammetry (CV) to evaluate their thermal stability, optical properties, and electrochemical behavior. Organic thin films were prepared through solution shearing, and their surface morphology and microstructure were analyzed using atomic force microscopy (AFM) and X-ray diffraction (XRD). Among the four, compounds 1–3 showed p-channel activity. Notably, compound 1, which possesses linear alkyl side chains, demonstrated decent electrical performance under ambient conditions, achieving a hole mobility of 0.42 cm² V⁻¹ s⁻¹ and a current on/off ratio exceeding 10⁸. These results reveal that appropriate alkyl chain engineering enhances molecular packing and crystallinity, thereby improving device performance. Furthermore, devices based on compound 1 maintained stable operation upon 90-day storage, demonstrating excellent air stability.

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1. Introduction

Organic semiconductors (OSCs) have emerged as attractive candidates for future optoelectronic technologies due to their intrinsic flexibility, low cost, tunable electronic structures, and excellent solution processability. These characteristics render them suitable for a broad spectrum of electronic applications, including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), sensors, and memory devices.^1–9

Compared to polymeric semiconductors, π-conjugated small molecules offer several distinct advantages, such as superior electrical performance due to ordered molecular packing, precise tunability of electronic properties through molecular design, enhanced solubility, and a straightforward synthesis and purification process.^10–12

Among the various chemical moieties investigated as OSCs, thienothiophenes (TTs), the simplest form of fused thiophenes, have garnered significant attention as small-molecule p-type OSCs due to their excellent charge transport capabilities. These favorable properties are largely attributed to strong π–π interactions facilitated by sulfur atoms and an inherently planar molecular structure.^13–18 Thienothiophenes exist in four isomeric forms: thieno[2,3-b]thiophene, thieno[3,2-b]thiophene, thieno[3,4-b]thiophene, and thieno[3,4-c]thiophene (Fig. 1(a)).^19,20 Among them, thieno[2,3-b]thiophene uniquely features a cross-conjugated structure, in contrast to the conjugated systems of the other isomers. This cross-conjugation restricts effective conjugation, leading to a lower HOMO energy level, thereby increasing the ionization potential. These electronic properties suppress oxidation under ambient conditions, positioning them as promising candidates for air-stable OSCs.^21–24 Nevertheless, the nature of cross-conjugation may hinder charge carrier mobility and negatively affect device performance, highlighting the need for further investigation to overcome this limitation.

Fig. 1 (a) Chemical structures of thienothiophene isomers, (b)–(d) examples of thieno[2,3-b]thiophene-based small-molecular OSCs.

To improve electrical performance, considerable research has focused on developing novel π-conjugated backbones with extended conjugation, and representative OFET performances from previous studies are summarized in Table S1 (ESI†). For instance, Liu et al. reported BTTB, a benzothieno[2,3-b]thiophene-based semiconductor in which a benzene ring is fused to the thieno[2,3-b]thiophene. The resulting OFET exhibited promising electrical properties, achieving a hole mobility of 0.46 cm² V⁻¹ s⁻¹ (Fig. 1(b)).²⁵ In another study, Shi et al. enhanced π-conjugation in a thieno[2,3-b]thiophene system by integrating an additional thiophene unit and a double bond. The sulfur atom of the additional thiophene unit enhanced the π-conjugation, while the double bond facilitated compact molecular packing. As a result, the device exhibited a hole mobility of 2.0 cm² V⁻¹ s⁻¹ (Fig. 1(c)).²⁶ In this context, our group previously developed an OFET employing a thieno[2,3-b]thiophene core extended via a triple bond to a thiophene unit. The resulting device demonstrated a hole mobility of 0.074 cm² V⁻¹ s⁻¹ with superior air stability (Fig. 1(d)).²⁷ The incorporation of triple bonds to aromatic rings not only enhances π-conjugation but also effectively reduces steric repulsion and promotes molecular planarity, leading to ordered film morphology and improved device performance.^28–30

For solution-processed OSCs, introducing alkyl chains into π-conjugated systems has become a common approach to improve solubility in organic solvents and to promote favorable molecular packing. Although these alkyl substituents do not directly influence the intrinsic optoelectronic properties, they play a crucial role in enabling efficient solution processing and enhancing charge transport in devices.^31–34 Importantly, variations in alkyl chain length, degree of branching, and substitution position can significantly influence the solubility, thin-film morphology, and carrier transport characteristics of OSCs, highlighting the critical role of side-chain engineering.^35–37 For example, Sawamoto et al. revealed that attaching a single branched alkyl chain along the longitudinal axis of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) effectively enhanced solubility without compromising the semiconducting performance. In contrast, the incorporation of two branched alkyl chains disrupted molecular packing, which led to a decline in device performance.³⁸ In another study, Zhang et al. examined how linear alkyl chain length influenced the morphology and electrical properties of 2,6-di(4-alkyl-phenyl)anthracene (C_n-Ph-Ant, with n denotes alkyl chain length) derivatives. A clear trend of increased charge mobility with shorter alkyl chains was observed, which was attributed to improved film microstructure, including enhanced crystallinity and grain size.³⁶ In contrast, Hu et al. reported that within an n-type OSC, incorporating a branched alkyl group with increased chain length enhanced OFET performance by affording larger grain sizes.³⁹

Building upon these findings, we synthesized four novel, solution-processable thieno[2,3-b]thiophene derivatives: 2,5-bis((5-octylthiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (1), 2,5-bis((5-(2-ethylhexyl)thiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (2), 3,4-dimethyl-2,5-bis((5-octylthiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (3), and 2,5-bis((5-(2-ethylhexyl)thiophen-2-yl)ethynyl)-3,4-dimethylthieno[2,3-b]thiophene (4) (Fig. 2). To promote π-conjugation and ensure molecular planarity, each compound was designed by attaching additional thiophene rings to the thieno[2,3-b]thiophene core through a triple bond. Alkyl side chains were grafted onto the α-position of the appended thiophene rings to improve solubility in organic solvents. To investigate the influence of alkyl chain architecture on charge transport behavior, linear and branched alkyl chains were incorporated in compounds 1 and 2, respectively. Compounds 3 and 4 are structural analogs of compounds 1 and 2, respectively, with two additional methyl groups incorporated into the core to study the effects of methyl substitution.

Fig. 2 Chemical structures of novel synthesized TT-based compounds 1–4.

The thermal stability, optical properties, and electrochemical behavior were characterized using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV-visible spectroscopy (UV-vis), and cyclic voltammetry (CV), providing a comprehensive understanding of the physicochemical behavior of the developed compounds. The frontier molecular orbital energy levels and electronic distributions were further estimated through density functional theory (DFT) calculations. Bottom-gate/top-contact OFETs were fabricated via solution shearing (SS) with designed organic semiconductors as the channel layer. Atomic force microscopy (AFM) and X-ray diffraction (XRD) were used to analyze the surface morphology and crystallinity of the thin films. Finally, electrical performance was evaluated to elucidate the influences of alkyl side chains, triple bonds, and methyl substituents on charge transport properties. The OFETs based on compounds 1–3 showed p-channel behavior under ambient conditions. Notably, compound 1 achieved a remarkable hole mobility of 0.42 cm² V⁻¹ s⁻¹ and a current on/off ratio exceeding 10⁸, due to superior film texture and morphology. These results demonstrate that extending π-conjugation via triple bond, combined with the incorporation of appropriately designed alkyl side chains, offers a reliable route to improve the performance of organic semiconductors. Furthermore, devices incorporating compound 1 exhibited excellent air stability, maintaining consistent electrical characteristics after 90 days of ambient storage.

2. Experimental details

2.1 Materials and methods

Air and moisture-sensitive reactions were performed in a nitrogen atmosphere using oven-dried glassware and anhydrous solvents. All reagents were purchased from commercial suppliers and used without further purification unless otherwise specified. 2,5-Diiodothieno[2,3-b]thiophene (1a) and 2,5-diiodo-3,4-dimethylthieno[2,3-b]thiophene (1b) were synthesized employing the reference method.^40,41 2-iodo-5-octylthiophene and 2-(2-ethylhexyl)-5-iodothiophene were synthesized employing the reference method.^41,42 Solvents were either freshly distilled or dried by filtration through an alumina column before use. General analytical techniques for the synthesized compounds followed established procedures, with full experimental details available in (ESI†).²⁷

2.2 Synthesis

2,5-Diiodothieno[2,3-b]thiophene (1a) and 2,5-diiodo-3,4-dimethylthieno[2,3-b]thiophene (1b) were synthesized as described in the reference methods.^40,41 Sonogasira coupling of deprotected compound 1c with 2-iodo-5-octylthiophene and 2-(2-ethylhexyl)-5-iodothiophene afforded 2,5-bis((5-octylthiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (1) and 2,5-bis((5-(2-ethylhexyl)thiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (2), respectively. Sonogasira coupling of deprotected compound 1d with 2-iodo-5-octylthiophene and 2-(2-ethylhexyl)-5-iodothiophene afforded 3,4-dimethyl-2,5-bis((5-octylthiophen-2-yl)ethynyl)thieno[2,3-b]thiophene (3) and 2,5-bis((5-(2-ethylhexyl)thiophen-2-yl)ethynyl)-3,4-dimethylthieno[2,3-b]thiophene (4), respectively, as shown in Scheme 1. The detailed synthesis procedures, along with ¹H and ¹³C NMR spectra for compounds 1–4, are provided in the ESI.†

Scheme 1 Synthetic scheme of compounds 1–4.

2.3 Device fabrication

Bottom-gate/top-contact OFETs were fabricated with the synthesized compounds to evaluate charge transport properties. Heavily n-doped silicon wafers coated with a thermally grown 300 nm SiO₂ (capacitance per unit area, C_i = 11.4 nF cm⁻²) served as both the substrate and the gate dielectric. Before film deposition, Si/SiO₂ substrates were cleaned sequentially by ultrasonication in acetone and isopropanol for 10 min each, followed by oxygen plasma treatment for 5 min (Harrick plasma, PDC-32G, 18 W). To render the substrate surface hydrophobic and suitable for solution shearing, a hydroxyl-functionalized polystyrene brush (M_w: 32 000 g mol⁻¹) dissolved in toluene (0.5 wt%) was applied as a surface modifier.⁴³ Organic semiconductor films were deposited from toluene solutions using a solution-shearing technique. The optimized solution concentrations were 4 mg mL⁻¹ for compounds 1 and 2 and 2 mg mL⁻¹ for compounds 3 and 4. Shearing speeds ranging from 2 to 40 mm min⁻¹ were tested and optimized to achieve uniform film morphology. Post-deposition, the films were annealed at temperatures between 40–90 °C for 2 h to eliminate residual solvent. Finally, gold source and drain electrodes were thermally evaporated through a shadow mask (deposition rate: 0.2 Å s⁻¹, thickness: 30 nm), defining a channel width (W) of 500 μm and a length (L) of 100 μm.

2.4 Device and film characterization

The electrical characteristics of the OFETs were measured at room temperature in ambient air using a Keithley 4200 semiconductor parameter analyzer. Charge carrier mobility in the saturation region (μ_sat) was calculated according to the equation μ_sat = (2I_DSL)/[WC_i(V_G − V_th)²], where I_DS is the source–drain current, V_G is the gate voltage, and V_th represents the threshold voltage. The structural and morphological characteristics of the organic thin films were carried out using an atomic force microscope (AFM, Park System, XE7) operated in non-contact mode and an X-ray diffractometer (XRD, Bruker, D8 Discover).

3. Results and discussion

3.1 Thermal, optical, and electrochemical characteristics

The thermal stability of the compounds was analyzed using TGA and DSC (Fig. S1, S2 and Table 1). The temperatures corresponding to 5% weight loss (T_d) observed in compounds 1–4 were recorded at 388, 325, 405, and 402 °C, respectively. Compounds 3 and 4, incorporating methyl substituents on the TT core, demonstrated higher T_d than compounds 1 and 2, which do not contain methyl substituents. This behavior could be attributed to the incorporation of methyl substituents, which might restrict the rotational freedom of the molecular backbone, promote intramolecular interactions with adjacent π-systems, and reinforce the structural rigidity of the molecules.^44–46 In addition, the C–C linkages arising from methyl substitution are likely to require higher thermal energy for cleavage, contributing to the enhanced thermal stability.^47,48 Notably, all compounds retained residual mass even after heating to 800 °C. This behavior, commonly observed in triple-bond-containing organic semiconductors, may originate from triple bonds within the molecular structures, which tend to yield thermally stable carbonaceous residues during thermal decomposition.^49–52 In the DSC measurements, sharp endothermic peaks were detected, with the respective melting temperatures (T_m) for compounds 1–4 recorded at 112, 85, 64, and 44 °C, respectively. The decreasing trend in melting temperatures from compounds 1 to 4 is likely associated with a progressive weakening of intermolecular interactions, primarily caused by steric hindrance arising from branched alkyl chains and the incorporation of two additional methyl groups.^44,53,54

Table 1 Physicochemical and theoretical properties of TT derivatives

	T d (°C)	T m (°C)	λ max (nm)	E g (eV)	E ^onsetox (V)	E HOMO (eV)	E LUMO (eV)	E HOMO (eV)	E LUMO (eV)
a Temperature corresponding to 5% weight loss. b Melting point. c Maximum absorption wavelength obtained from UV-vis spectroscopy. d Energy levels from cyclic voltammetry, ELUMO = EHOMO + Eg. e Energy levels from theoretical calculation.
1	388	112	349	3.20	1.26	−5.11	−1.91	−5.08	−1.55
2	325	85	350	3.20	1.29	−5.14	−1.94	−5.09	−1.57
3	405	64	340	3.22	1.20	−5.05	−1.83	−5.04	−1.44
4	402	44	341	3.23	1.20	−5.05	−1.82	−5.07	−1.44

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To characterize the optical properties, UV-vis absorption spectra were measured in chloroform solution and as thin films on glass substrates (Fig. 3 and Table 1). In chloroform solution, the maximum absorption peaks appeared at 349, 350, 340, and 341 nm, respectively. The observed blue shift for compounds 3 and 4, compared to 1 and 2, can be ascribed to the introduction of methyl groups at the β-position of each thiophene core. These substituents induce steric hindrance, resulting in a twisted molecular geometry that reduces electronic conjugation, which in turn results in a shift of the maximum absorption to shorter wavelengths.^55,56 The band gaps (E_g) of compounds 1–4 were estimated from the onset wavelengths in the UV-vis spectra using Planck's relation. The calculated values were 3.20, 3.20, 3.22, and 3.23 eV, respectively. Notably, the wide band gaps exceeding 3 eV for compounds 1–4 imply that these compounds are not readily oxidized, which may contribute to their enhanced stability in air (vide infra).^57,58 For the thin film absorption spectra, compounds 1–4 exhibited red-shifted absorption relative to solution-state spectra, with maximum peaks at 365, 358, 351, and 345 nm, respectively. In addition to the red shift, distinct shoulder peaks were observed in the thin-film spectra of compounds 1–4, which were absent in their corresponding solution spectra. These spectral features can be attributed to backbone planarization and molecular aggregation in the solid-state films, which promote enhanced intermolecular interactions such as π–π stacking.^59–61 Notably, the red shift was most pronounced in compound 1, but gradually diminished in compounds 2 and 3, and was least significant in compound 4. This trend suggests that molecular aggregation is increasingly suppressed by the presence of branched alkyl chains and methyl substituents, which hinder close packing in the solid state.^62–64

Fig. 3 UV-vis absorption spectra of compounds 1–4 (a) in chloroform solutions (5.0 × 10⁻² mM) and (b) as thin films on glass substrate.

Finally, CV was performed at room temperature to investigate the electrochemical characteristics of compounds 1–4 (Fig. 4 and Table 1). The oxidation onset potentials were observed at 1.26, 1.29, 1.20, and 1.20 V, respectively. Based on these values, the HOMO energy levels were calculated based on the equation presented below,

Fig. 4 Cyclic voltammograms of compounds 1–4 in dichloromethane solutions (3.5 mM).

The HOMO energy levels of compounds 1–4 were determined to be −5.11, −5.14, −5.05, and −5.05 eV, respectively. The corresponding LUMO levels were estimated by subtracting the optical band gaps derived from UV–vis absorption spectra from the HOMO values, yielding −1.91, −1.94, −1.83, and −1.82 eV for compounds 1–4, respectively. The elevated HOMO and LUMO levels in compound 3 and 4, relative to compounds 1 and 2, likely result from the electronic perturbation and steric effects introduced by electron-donating methyl substituents.

The electron-donating methyl substituents enhance localized electron density within the π-conjugated backbone, which reduces the stabilization of the frontier molecular orbitals and elevates energy levels.^65,66 Furthermore, the steric hindrance introduced by methyl groups distorts the thiophene rings, thereby disrupting the π-conjugation along the backbone. This structural deformation reduces orbital delocalization and further elevates the frontier energy levels.^44,65,67

3.2 Theoretical calculation

DFT calculations were performed to examine the optimized molecular structures and frontier molecular orbital distributions. Based on the optimized molecular geometries, the theoretical lengths of compounds 1–4 were 3.26, 2.31, 3.04, and 2.29 nm, respectively (Fig. S3, ESI†). Although compounds 1 and 3 possess the same linear alkyl chain, and compounds 2 and 4 share the same branched alkyl chain, compound 3 exhibited a shorter molecular length than compound 1, and compound 4 was shorter than compound 2. This deviation may result from stereochemical distortion induced by the methyl substituents on the TT core, leading to molecular asymmetry and backbone bending.^68,69 This structural distortion was further evaluated by analyzing the dihedral angles between the TT core and the adjacent thiophene units bearing the alkyl chains. In compounds 1 and 2, which lack methyl substitution, both sides exhibit relatively small and symmetric dihedral angles (12°/10° for compound 1 and 12°/12° for compound 2). In contrast, compounds 3 and 4 show increased asymmetry and backbone bending, with dihedral angles of 25°/11° and 6°/15°, respectively. As shown in Fig. S4 (ESI†), the HOMO orbitals were delocalized across the entire conjugated backbone in all cases, while the LUMO orbitals were predominantly confined to the π-system. Notably, as previously mentioned, the presence of alkyl side chains had a negligible impact on the distribution of the frontier orbitals. The computed energy levels for the HOMO and LUMO orbitals of compounds 1–4 were −5.08/−1.55, −5.09/−1.57, −5.04/−1.44 eV, and −5.07/−1.44 eV, respectively. The corresponding theoretical band gaps were 3.53, 3.52, 3.60, and 3.63 eV, respectively. These theoretical values followed a similar trend to those derived experimentally from CV and UV-vis spectroscopy (Fig. 5 and Table 1).

Fig. 5 Schematic representation of HOMO/LUMO energy levels for compounds 1–4 (a) experimentally determined values, (b) theoretically calculated values.

3.3 Thin-film microstructure and morphology

AFM and XRD analyses were conducted to analyze the morphology and microstructure of the organic semiconductor thin films. First, the surface morphology was characterized using AFM. As illustrated in Fig. 6(a) and (b), both compounds 1 and 2 formed terrace-like structures, although with noticeable differences. The step height measured for compound 1 thin films was approximately 3.3 nm, aligning well with its calculated molecular length of 3.26 nm, and the films displayed relatively large crystalline grains (Fig. 6(e)). In comparison, thin films of compound 2 exhibited a step height of approximately 1.7 nm, which is smaller than the calculated molecular length of 2.31 nm, with correspondingly smaller grain domains (Fig. 6(f)). Compounds 3 and 4 exhibited island-like surface configurations, even when processed under mild thermal conditions (40 °C) (Fig. 6(c) and (d)). This behavior can be attributed to their intrinsically asymmetric and bent molecular structure, as previously confirmed by dihedral angle analysis, which hinders efficient molecular packing and film uniformity. In addition, the relatively low melting points of the compounds may further contribute to this behavior by promoting irregular molecular packing.^68,69 Second, the microstructures of the films were investigated using XRD (Fig. 7). The XRD pattern of the thin film derived from compound 1 revealed pronounced (001) diffraction peaks, indicative of high crystallinity and dense molecular packing. By contrast, compound 2 exhibited noticeably weaker (001) reflections, implying a less ordered molecular arrangement within the film.^70,71 For compounds 3 and 4, no distinct diffraction peaks were observed. The first diffraction peaks for compounds 1 and 2 appeared at 2θ = 2.7° and 2θ = 5.2°, which translated to d-spacings of 3.28 nm and 1.7 nm, respectively. Overall, these XRD results correlate well with the AFM analysis. The consistency between the d-spacing (or step height) and the molecular length observed for compound 1 implies an edge-on molecular arrangement with molecular backbones aligned almost vertically relative to the substrate. In contrast, the smaller d-spacing (or step height) compared to the molecular length observed for compound 2 suggests a relatively tilted molecular packing.^72–74

Fig. 6 AFM images (5 μm × 5 μm) of thin films formed from compounds 1–4 (a) 1, (b) 2, (c) 3, and (d) 4. Step-height profiles are shown for (e) compound 1 and (f) compound 2.

Fig. 7 X-ray diffraction profiles obtained from thin films of compounds 1–4.

3.4 Field-effect transistor characterization

Bottom-gate/top-contact OFETs were fabricated using the solution-shearing method to assess the charge transport characteristics of the novel materials. The channel layers were deposited under optimized conditions, such as solution concentration, annealing temperature, and shearing speed, to promote the formation of well-ordered thin films. Electrical characterizations were performed under ambient conditions using a probe station. Among the four compounds, compounds 1–3 showed p-type field-effect behavior with stable operation in air, whereas compound 4 did not display any noticeable field-effect response. After identifying the compounds that exhibited field-effect characteristics, eight OFETs were fabricated and measured for each compound under optimized processing conditions to assess device-to-device reproducibility. The electrical performance parameters, including average charge carrier mobility, current on/off ratio, and threshold voltage, are summarized in Table 2. Notably, OFETs based on compound 1 achieved the highest performance, with a hole mobility of 0.42 cm² V⁻¹ s⁻¹ and a current on/off ratio exceeding 10⁸ (Fig. 8). The superior performance of compound 1 is likely attributed to the strategic incorporation of alkyl side chains, which enhance solubility in organic solvents and promote the formation of uniform thin films.^75,76 This is supported by AFM and XRD analyses, which reveal a homogeneous and highly crystalline film that enables efficient charge transport, thereby contributing to the enhanced hole mobility.^72,77 In contrast, compounds 2 and 3 showed markedly reduced performance, with mobilities of 0.0034 cm² V⁻¹ s⁻¹ and 0.00017 cm² V⁻¹ s⁻¹, respectively (Fig. S5, ESI†). These results are primarily attributed to steric hindrance, although the origins of these effects differ among the compounds. In compound 2, the branched alkyl group at the α-position of thiophene induces substantial steric hindrance, which disrupts efficient π–π stacking and thereby overrides the solubility advantages typically associated with alkyl side chains.^78,79 In the case of compound 3, the methyl substitution on the TT core likely induces steric hindrance, interfering with π–π stacking and distorting the molecular backbone. This distortion leads to backbone bending and molecular asymmetry, which may hinder intramolecular charge transport by perturbing the conjugation pathway.^38,80,81 The absence of field-effect characteristics in compound 4 may be attributed to the combined impact of both steric effects.

Table 2 Summary of electrical characteristics for OFETs based on compounds 1–4. (μ: charge carrier mobility, I_on/I_off: current on/off ratio, V_th: threshold voltage)^a

Compound	μ sat, avg (μsat, max) (cm^2 V^−1 s^−1)	I on/Ioff	V th (V)
a Average values were based on the measurements of 8 devices under ambient conditions.
1	0.33 ± 0.09 (0.42)	(1.5 ± 0.6) × 10^8	−19 ± 6
2	0.0024 ± 0.001 (0.0034)	(1.1 ± 0.7) × 10^6	−21 ± 4
3	0.00012 ± 0.00005 (0.00017)	(1.0 ± 0.4) × 10^5	−18 ± 4
4	—	—	—

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Fig. 8 Transfer (a) and output (b) characteristics of the OFETs based on compound 1.

To assess the long-term air stability of the OFETs, the electrical characteristics of compound 1 were re-examined after 90 days of ambient storage under dark conditions, at ∼40% relative humidity, without electrical bias. As depicted in Fig. 9, compound 1 preserved transfer behavior with a mobility of 0.36 cm² V⁻¹ s⁻¹, retaining over 85% of its initial performance. The device also maintained a current on/off ratio above 10⁸, confirming the superior stability in air.

Fig. 9 Variation in the transfer characteristics of compound 1-based OFETs after 90 days of ambient storage.

4. Conclusion

To conclude, we designed four thieno[2,3-b]thiophene-derived organic semiconductors that are compatible with solution-based processing. Their thermal behavior was assessed through TGA and DSC. The frontier orbital energy levels and band gaps were extracted from UV-vis spectra and CV. The experimental values showed good agreement with DFT-calculated data, confirming the reliability of the theoretical predictions. All synthesized compounds showed wide band gaps above 3.0 eV, which may be the result of the intrinsic electronic stability of the thieno[2,3-b]thiophene core. This feature highlights their suitability for application in air-stable organic semiconducting devices. AFM and XRD analyses revealed that steric hindrance from methyl groups and branched alkyl chains disrupted molecular packing and reduced thin-film crystallinity. In contrast, compound 1, which incorporates a linear alkyl chain and lacks a methyl group, exhibited a highly ordered film microstructure. As a result, OFETs based on compound 1 revealed outstanding electrical performance with superior air stability. Under ambient conditions, compound 1 achieved a hole mobility of 0.42 cm² V⁻¹ s⁻¹ with a current on/off ratio greater than 10⁸. Overall, this study confirms that the strategic incorporation of triple bonds and linear alkyl chains is an effective approach to enhancing the performance of thieno[2,3-b]thiophene derivatives. The synthesized compounds demonstrate considerable potential as air-stable p-type OSCs and represent attractive materials for use in future small-molecule optoelectronic applications.

Author contributions

Seongjin Oh: conceptualization, investigation, data curation, writing – original draft, and visualization. Hyowon Kang: investigation, data curation, writing – original draft, and visualization. Choongik Kim: conceptualization, writing – review & editing, supervision, and funding acquisition. SungYong Seo: conceptualization, writing – review & editing, supervision, and funding acquisition.

Data availability

Data for this article are available at the following link: https://www.dropbox.com/scl/fo/or3u8d4unnmn9iib7si2t/AEyl7AvVEMifScoDtwHvPBc?rlkey=lwuf0sc512kyc92nno5wgo7o4&st=yrqqb3f0&dl=0.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (RS-2024-00455440 and RS-2024-00449127).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc01512j

‡ These authors contributed equally to this work.

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