Diselenophene‐Dithioalkylthiophene Based Quinoidal Small Molecules for Ambipolar Organic Field Effect Transistors

Abstract

This work presents a series of novel quinoidal organic semiconductors based on diselenophene‐dithioalkylthiophene (DSpDST) conjugated cores with various side‐chain lengths (‐thiohexyl, ‐thiodecyl, and ‐thiotetradecyl, designated DSpDSTQ‐6, DSpDSTQ‐10, and DSpDSTQ‐14, respectively). The purpose of this research is to develop solution‐processable organic semiconductors using dicyanomethylene end‐capped organic small molecules for organic field effect transistors (OFETs) application. The physical, electrochemical, and electrical properties of these new DSpDSTQs are systematically studied, along with their performance in OFETs and thin film morphologies. Additionally, the molecular structures of DSpDSTQ are determined through density functional theory (DFT) calculations and single‐crystal X‐ray diffraction analysis. The results reveal the presence of intramolecular S (alkyl)···Se (selenophene) interactions, which result in a planar SR‐containing DSpDSTQ core, thereby promoting extended π‐orbital interactions and efficient charge transport in the OFETs. Moreover, the influence of thioalkyl side chain length on surface morphologies and microstructures is investigated. Remarkably, the compound with the shortest thioalkyl chain, DSpDSTQ‐6, demonstrates ambipolar carrier transport with the highest electron and hole mobilities of 0.334 and 0.463 cm² V⁻¹ s⁻¹, respectively. These findings highlight the excellence of ambipolar characteristics of solution‐processable OFETs based on DSpDSTQs even under ambient conditions.

A series of novel quinoidal organic semiconductors based on diselenophene‐dithioalkylthiophene (DSpDST) conjugated cores with various side‐chain lengths are designed for solution‐processable organic field effect transistors (OFETs). Remarkably, the compound with the shortest thioalkyl chain, DSpDSTQ‐6, demonstrates ambipolar carrier transport with the highest electron and hole mobilities of 0.334 and 0.463 cm² V⁻¹ s⁻¹, respectively.

1. Introduction

The pursuit of air‐stable, solution‐processable organic semiconductors has garnered significant attention in the development of high‐performance organic field effect transistors (OFETs) for a wide range of applications, including memory devices, smart cards, radio‐frequency identification tags, transparent circuits, electronic papers, flexible displays, and sensors.^{[ 1} , ² , ³ , ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹² , ¹³ , ^{14 ]} These devices offer attractive features such as desirable electronic properties, flexibility, cost, versatile functionalization, and processability. Among the various parameters defining OFET performance, the charge‐carrier mobility (μ) is a critical factor that quantifies the average drift velocity of the charge carriers (holes or electrons) per unit electric field. The realization of high‐performance OFETs necessitates the utilization of organic semiconductor materials with high charge carrier mobility, good environmental stability, and ease of processability.^{[ 15} , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ^{20 ]} Such novel semiconductor materials can be obtained via advancements in material science and device fabrication techniques.^{[ 21} , ^{22 ]} In particular, thiophene‐based compounds such as oligothiophenes and fused thiophene aromatics have demonstrated exceptional performance in this regard.^{[ 23} , ²⁴ , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ^{29 ]} Notably, these organic semiconductors rely on weak intermolecular interactions, such as Van der Waals, S–S, π–π, and C–H interactions, rather than the strong covalent bonds present in inorganic semiconductors.^{[ 30 ]} Consequently, enhancing these intermolecular interactions emerges as a promising avenue for improving the OFET performance.

Dicyanomethylene (DCN)‐end‐capped quinoidal structures have emerged as a promising platform for organic semiconductors due to their unique electronic properties, which can be attributed to several factors. Firstly, the presence of π‐electron conjugation along the alternating C =  C/C− C bonds of the quinoid backbone leads to enhanced delocalization,^{[ 31 ]} and this intrinsic aromaticity provides a relatively low reorganization energy compared to that of other aromatic compounds.^{[ 32 ]} In addition, the potent alkyl‐acceptor moieties of the quinoidal compounds create a highly hybridized frontier molecular orbital system, thus leading to a deep lowest unoccupied molecular orbital (LUMO) and an enhanced highest occupied molecular orbital (HOMO). This energy level modulation favors the development of high‐mobility donor‐acceptor compounds based on a variety of donors and acceptors that exhibit effective charge carrier transport.^{[ 33 ]} Thus, the addition of two strongly electron‐withdrawing groups (DCN) affords low‐lying LUMO energy levels, thereby leading to facile electron injection and improving the ambient stability of the OFET.^{[ 34} , ^{35 ]}

In view of the above considerations, various approaches have been explored for the development of high‐performance quinoidal oligothiophene‐based semiconductors, with a focus on achieving high planarity, close π–π stacking, and strong intermolecular or noncovalent interactions, in combination with heteroatom effects and alkyl chain modifications.^{[ 36} , ³⁷ , ³⁸ , ^{39 ]} For example, the energy‐level modification of a thienoquinoidal semiconductor with electron‐withdrawing DCN‐end‐capping units, along with suitably solubilizing alkyl chains for increased film processability and the introduction of hetero‐atoms in the side chain to promote molecular orbital delocalization, has been shown to improve the device performance via strong intramolecular/conformational locking.^{[ 34} , ⁴⁰ , ⁴¹ , ⁴² , ⁴³ , ^{44 ]} In addition, soluble dithioalkyl‐substituted organic semiconductors have been reported previously by the author's research group,^{[ 35} , ^{43 ]} wherein the sulfur atoms of the thioalkyl chains provided intramolecular locks with the sulfur atoms of the thiophene (S···S), thereby enhancing the molecular planarity and charge transport efficiency.^{[ 45 ]} For example, thioalkyl side chains were successfully used to fabricate solution‐processable n‐type quinoidal organic semiconductors such as the tetra thioalkyl‐substituted TSBTQ in Figure  1a, which exhibited an electron mobility of 0.18 cm² V⁻¹ s⁻¹. Notably, the selection of a suitable thioalkyl side chain leads to the potential for an environmentally stable OFET.^{[ 46 ]}

Figure 1.

Chemical structures of the reported a) Thioalkyl/Se‐containing small molecules with dicyanomethylene end‐capping units, and b) Se‐containing fused small molecules for OFETs. c) Pseudo‐pentaheteroacene‐like structure of DSpDST core and the chemical structure of quinoids discussed in this study. μ _h and μ _e denote maximum hole and electron mobilities. The symbols (sc), and (s) denote spin coating and solution‐processed semiconductor films.

Selenium heterocycles, such as selenophene, possess distinct characteristics compared to their sulfur‐containing counterparts. Specifically, they are larger in size, more polarizable, and less electronegative.^{[ 47} , ^{48 ]} In the solid state, selenium‐containing compounds demonstrate enhanced intermolecular interactions, including partial Se‐Se interactions, which are absent from their sulfur‐containing analogs. These Se‐Se interactions contribute to improved molecular ordering and unique solid‐state organization, often leading to excellent charge transport properties.^{[ 49} , ^{50 ]} Thus, selenophene has been extensively utilized in the development of p‐type OFETs with outstanding performance.^{[ 50} , ⁵¹ , ⁵² , ⁵³ , ^{54 ]} For example, by taking advantage of Se in the intramolecular lock series, the author's group has recently achieved significant advancements, with the selenotetradecyl‐substituted molecule DDTT‐SeBT exhibiting the highest hole mobility thus far reported for all/fused thiophene semiconductors (>4.0 cm² V⁻¹ s⁻¹).^{[ 55 ]} Furthermore, chemical structures of the reported Se‐containing small molecules for OFETs have been given in Figure 1a,b. To investigate the extent of intermolecular overlap, selenophenes were introduced as replacements for thiophenes, and the resulting spin‐coated thin films of the quinoidal mixed oligomer DCMT‐Se‐derivative were shown to exhibit ambipolar carrier transport, with electron and hole mobilities of 0.016 and 0.007 cm² V⁻¹ s⁻¹, respectively.^{[ 56 ]} Furthermore, Marder et al. reported the charge transport properties of DSP derivatives to achieve electron mobilities of up to 0.00087 cm² V⁻¹ s⁻¹ in spin‐cast layers,^{[ 57 ]} while Takimiya et al. reported hole mobilities of up to 0.70 cm² V⁻¹ s⁻¹ for single‐crystal OFETs based on the small molecule BSBS.^{[ 58 ]} Moreover, the latter research group also investigated β‐MS‐BDS, which is the only known methylseleno‐substituted small molecular semiconductor, and achieved a hole mobility of 0.45 cm² V⁻¹ s⁻¹.^{[ 59 ]} Furthermore, Okamoto et al. reported an impressive charge‐carrier mobility of 11 cm² V⁻¹ s⁻¹ for the alkyl‐substituted selenium‐bridged V‐shaped organic semiconductor C₁₀‐DNS‐VW.^{[ 60 ]}

The incorporation of selenophene into organic semiconductors has shown promising results in OFETs. Nevertheless, it is necessary to investigate the structure‐property relationship of the selenophene‐containing semiconducting materials.^{[ 61 ]} For example, the present authors have previously reported the synthesis of conjugated tetracyanoquinodimethane‐substituted quinoidal dithioalkylterthiophene (DSTQ) compounds such as DSTQ‐14 (Figure 1c), which features a dithiotetradecyl side chain and exhibits a superior electron mobility of 0.77 cm² V⁻¹ s⁻¹.^{[ 46 ]} Hence, to further explore the structure‐property relationship and its influence on the OFET performance, the thiophene units of DSTQ‐14 compound are replaced with selenophene to obtain novel quinoidal compounds that incorporate distinct thioalkyl chains, namely DSpDSTQ‐6 (1), with a dithiohexyl side chain (SR = SC₆H₁₃), DSpDSTQ‐10 (2), with a dithiodecyl side chain (SR = SC₁₀H₂₁), and DSpDSTQ‐14 (3), with a dithiotetradecyl side chain (SR = SC₁₄H₂₉). Further, to investigate the intramolecular lock property and intermolecular interactions, the optical, electrochemical, and thermal properties of the newly synthesized DSpDSTQs are analyzed and compared. A crystal structure analysis of the DSpDSTQ‐14 reveals a planar DSpDST core with highly ordered molecular packing, strong π–π interactions, intramolecular charge transfer between S(thioalkyl) and Se moieties, and intermolecular (N−H, and Se−Se) interactions among the face‐to‐face molecular layers. Bottom‐gate top‐contact (BGTC) OFET devices are then fabricated using both solution‐sheared and spin‐coated films to evaluate the charge transport characteristics. The results indicate that the DSpDSTQ‐6‐based solution‐sheared OFETs, featuring a dithiohexyl side chain, exhibit ambipolar charge‐carrier transport, along with the highest electron and hole mobilities of 0.334 and 0.463 cm² V⁻¹ s⁻¹, respectively. This is attributed to the formation of aligned films with larger crystallites along the shearing direction and close molecular packing facilitated by noncovalent interactions, as revealed by the morphological and microstructural analyses. Furthermore, the air‐stable ambipolar characteristics of the DSpDSTQ‐based OFETs afford great potential for application as low‐cost, solution‐processable, ambipolar OFETs.

2. Results and Discussion

2.1. Synthesis

The synthetic route to quinoids 1−3 is shown in Scheme  1 . Compounds 4–7 were synthesized according to the previously reported procedure.^{[ 46} , ^{62 ]} Briefly, the central dithioalkylated thiophene unit (6) was obtained by initial dilithiation of 3,4‐dibromothiophene (4) using n‐BuLi, followed by the reaction with sulfur and subsequent dialkylation with the appropriate alkyl bromide. Subsequent dibromination of 6 using n‐bromosuccinimide (NBS) provided the corresponding intermediate 7, which was then subjected to Stille coupling with 2‐(tributylstannyl)‐selenophene in the presence of a palladium catalyst to afford compound 8. Further treatment of 8 with NBS yielded the dibrominated intermediate 9. Finally, the desired quinoids DSpDSTQ(1−3) were obtained via Takahashi coupling of the compound 9 with malononitrile in the presence of tetrakis(triphenylphosphine)palladium, followed by oxidation using a saturated solution of bromine in water. The as‐synthesized quinoids exhibited good solubility in common organic solvents and were suitable for thin‐film solution processing. The chemical structures of all target compounds were confirmed by ¹H NMR, ¹³C NMR, and mass spectrometry. Detailed information regarding the synthetic procedures and characterization can be found in the Supporting Information.

Scheme 1.

Synthetic route to quinoidal semiconductors DSpDSTQ (1‐3).

2.2. Physical Characterization

The thermal properties of the as‐synthesized DSpDSTQ (1–3) are revealed by the differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) results in Figure S1 and S2 (Supporting Information), respectively, and are summarized in Table  1 . The DSC scans clearly indicate distinct phase transitions for each of the compounds, characterized by sharp endothermic peaks above 196 °C. Notably, the melting points of the DSpDSTQ molecules are seen to decrease with the decreasing length of the alkyl side chain, i.e.: 1 (224 °C) > 2 (209 °C) > 3 (196 °C). This trend is also reflected in the melting temperatures, i.e.: 1 (229 °C) > 2 (214 °C) > 3 (209 °C). Further, the TGA analysis demonstrates that all three DSpDSTQ molecules exhibit high thermal stability, with an initial weight loss of ≈ 5% occurring in the temperature range of 291–307 °C.

Table 1.

Thermal, Optical, and Electrochemical properties of all three Compounds.

Compound	T d a) [°C]	T m b) [°C]	λ max(sol) c) [nm]	λ max(film) d) [nm]	E ox e) [V]	HOMO f) [eV]	E red e) [V]	LUMO f) [eV]	ΔE g g [eV]
DSpDSTQ‐6 (1)	307	224	707	713	1.40	−5.60	0.02	−4.22	1.38
DSpDSTQ‐10 (2)	291	209	708	713	1.40	−5.60	0.02	−4.22	1.38
DSpDSTQ‐14 (3)	295	196	707	726	1.40	−5.60	0.01	−4.21	1.39

^a) Decomposition temperatures were determined from TGA;

^b) Melting temperatures were determined from DSC;

^c) Absorption spectra were measured in o‐C₆H₄Cl₂;

^d) Thin films were solution‐sheared onto a quartz glass;

^e) By DPV in o‐C₆H₄Cl₂ at 25 °C. All potentials are reported with reference to an Fc/Fc⁺ internal standard (at +0.6 V);

^f) Using HOMO/LUMO = −(4.2 + E _ox/E _red);

^g) The energy gap was calculated from the difference between HOMO and LUMO measured by DPV.

The UV‐vis absorption spectra of the DSpDSTQ compounds in diluted o‐dichlorobenzene solution and solution‐sheared films are presented in Figure  2 . Here, the solution‐state spectra are seen to be very similar, with a maximum absorption wavelength (λ_max ) centered at ≈707 nm (Figure 2a). This suggests that the thioalkyl side chain substituent does not exert a significant influence on the π‐conjugated quinoidal backbone of each compound, and is consistent with previous studies.^{[ 63} , ^{64 ]} Notably, all three solution‐sheared films exhibit substantial bathochromic shifts compared to the solution phase with Δλ values of 6, 5, and 19 nm for DSpDSTQ‐6, DSpDSTQ‐10, DSpDSTQ‐14, respectively. These shifts can be attributed to both the different extent of aggregation behavior and distinct electronic coupling and packing modes of the DSpDSTQ molecules in solution‐sheared films.^{[ 65} , ^{66 ]} The intense, low‐energy absorption bands in the range of 600–800 nm are assigned to π–π* transitions with intramolecular charge transfer character between the electron‐rich and electron‐deficient moieties, while the weaker high‐energy bands in the range of 350–450 nm are ascribed to localized electronic transitions of the quinoidal backbone.^{[ 67 ]} The broader absorption profiles of the solution‐sheared DSpDSTQ films relative to those obtained in the solution state can also be explained by molecular aggregation.^{[ 68 ]}

Figure 2.

UV–vis absorption spectra of DSpDSTQs in a) dilute o‐dichlorobenzene and b) solution‐sheared film state.

The electrochemical properties of the three DSpDSTQs are revealed by the differential pulse voltammetry (DPV) results in Figure  3 and Table 1. Here, all of the quinoidal molecules exhibit similar oxidation and reduction potentials, thereby indicating that the alkyl substituent has little effect on the electrochemical properties of the DSpDSTQ. Consequently, by using the equation HOMO/LUMO = −(4.2 + E _ox/E _red), and assuming an internal standard ferrocene/ferrocenium (Fc/Fc⁺) oxidation at −4.8 eV, the LUMO and HOMO values of all three compounds are calculated to be ≈ −4.2 and −5.6 eV, respectively. Moreover, the electrochemically derived HOMO‐LUMO energy gap (E _g) of each compound is as low as ≈1.38 eV, thereby demonstrating that the DSpDSTQs are favorable as a new class of semiconducting materials for use in OFETs. Furthermore, all three DSpDSTQs exhibit a LUMO energy of ≈ −4.2 eV, thereby indicating their suitability for efficient charge transfer and the development of air‐stable, solution‐processable OFETs.

Figure 3.

a) DPV of DSpDSTQs in o‐dichlorobenzene. b) DPV‐derived HOMO and LUMO energy levels. All potentials reported are referenced to an Fc/Fc⁺ internal standard (at +0.6 V).

2.3. Theoretical Calculations

The frontier molecular orbitals of the DSpDSTQ compounds are elucidated by the results of electronic structure calculations at the B3LYP/6‐311(d,p) level of density functional theory (DFT) in Figure S3 (Supporting Information). Despite the varying lengths of the thioalkyl chains, all of the calculated compounds exhibit similar electron density distributions, with HOMO and LUMO being extensively delocalized over the entire quinoidal backbone and sulfur atoms. This significant delocalization of the quinoidal structure may partly explain the observed ambipolar behavior. Moreover, ambipolar materials typically possess a small bandgap that matches the required HOMO/LUMO levels for efficient injection of electrons and holes from a single electrode material.^{[ 69} , ^{70 ]} All three DSpDSTQ derivatives meet the requirement with a low bandgap of 1.56 eV. The exceptional LUMO level of –4.44 eV stems from the stronger electron‐withdrawing nature of the cyano group, which provides better resistance to water and oxygen, making it a potential candidate for environmentally stable OFETs.^{[ 71} , ⁷² , ⁷³ , ^{74 ]} Further, the DFT calculations for the DSpDSTQ core indicate high planarity, with a skeletal distortion of <8° (Figure S3a, Supporting Information), which is beneficial for charge transport by promoting efficient delocalization of the π−electrons and facilitating π−π overlap and intermolecular interactions. This result also confirms the presence of the intramolecular locks provided by the S···Se interactions,^{[ 43 ]} with a shorter Se(core)··S(R) distance of 3.08 Å, and a larger selenophene distortion angle of 7.31°, compared to 3.12 Å and 2.89° for the previously‐reported S···S interaction and thiophene distortion, respectively.^{[ 46 ]} This result can be attributed to the greater steric bulk of the selenium atom, which also leads to larger polarizability, thereby enhancing the charge transfer.^{[ 75} , ^{76 ]} The energy difference between the aromatic and quinoid forms (ΔE = E _quinoid− E _aromatic, kcal mol⁻¹) for selenophene is relatively smaller than for thiophene.^{[ 77 ]} This indicates that selenophenes have a more quinoidal character than their thiophene counterparts, which increases the efficiency of conjugation through the backbone and lowers the reorganization energy.^{[ 36 ]} Additionally, this effect promotes the destabilization of the ground state of the molecule toward a quinoidal configuration, which effectively lowers the band gap in the conjugated system.^{[ 78 ]}

2.4. Single Crystal Structure

As a representative example, the diffraction‐derived single crystal structure of the DSpDSTQ‐14 is presented in Figure  4 and Figure S4 (Supporting Information), and summarized in Table S1 (Supporting Information). These results indicate that the DSpDSTQ‐14 crystallizes in the triclinic space group P1 and adopts a face‐to‐face slipped π‐‐π stacking arrangement with a stacking distance of 3.50 Å. This stacking configuration is believed to be highly conducive to efficient charge transport.^{[ 40} , ^{46 ]} Within the crystal structure, the shortest intramolecular distance between S(thioalkyl) and Se(selenophene) is measured as 3.09 Å, while the shortest distance between S(thioalkyl) and S(thioalkyl) is found to be 3.34 Å (Figure 4a,d). The front and side views of two stacked DSpDSTQ molecules with a short packing distance of 3.50 Å are depicted in Figure 4b,c. The shortest intermolecular N···H and Se···Se distances among the face‐to‐face layers of the DSpDSTQ‐14 molecules are found to be 2.71−2.72 Å (Figure S4b,c, Supporting Information) and 3.73 Å (Figure 4e) respectively. These distances are significantly smaller than the sum of the Van der Waals radii of the respective atoms, thereby indicating the presence of strong intra‐ and intermolecular interactions. The molecular packing arrangement of the DSpDSTQ molecules exhibits slipping angles of 21.8° and 77.8° along the long and short molecular axes, respectively (Figure 4e,f). Based on these results, it can be concluded that the DSpDSTQ‐14 quinoids possess planar molecular cores with short packing distances, thereby suggesting favorable intra‐ and intermolecular non‐bonded contacts. These characteristics are expected to minimize the occurrence of morphological defects and enhance charge transport in the OFETs.

Figure 4.

Single‐crystal structure of DSpDSTQ‐14 (3) in stick models (a−c) and space‐filling packing models (d−f). a) Top view of 3 with intramolecular S(thioalkyl)−Se distance of 3.09 Å and the molecular length of 15.69 Å. b) Front view of two stacking DSpDSTQ molecules. The interplanar distance is ∼3.50 Å. c) Side view of 3. d) Short intramolecular S(thioalkyl)−S(thioalkyl) distance of 3.34 Å and S(thioalkyl)−Se is 3.09 Å. (e−f) Molecular packing arrangement of DSpDSTQ molecules with a face‐to‐face layer stacking distance of 3.50 Å and exhibiting the slipping angles of 21.8° and 77.8°. The shortest intermolecular Se−Se distance of 3.73 Å. The selenium, sulfur, and nitrogen atoms are specified in purple, red, and blue color, respectively. The alkyl chains are omitted for clarity.

2.5. Charge Transport Measurements

To obtain a comprehensive understanding of the impact of the semiconducting properties of DSpDSTQ compounds with different lengths of thioalkyl chains, bottom‐gate top‐contact (BGTC) OFETs were fabricated using both solution‐shearing and spin‐coating methods for comparison, as detailed in the Supporting Information. The solution‐shearing method allows for the aligned growth of semiconductor crystals by controlling various processing parameters such as shearing speed, substrate temperature, solvent choice, and semiconductor concentration.^{[ 79} , ⁸⁰ , ⁸¹ , ^{82 ]} This method enables the formation of an optimized film with well‐aligned structures under the influence of the shearing force. It is worth noting that DSpDSTQ derivatives exhibit great solubility in ortho‐dichlorobenzene (o‐DCB), making them suitable for solution processing. Detailed information on OFET device fabrication and measurement procedures can be found in the Experimental Section. Representative transfer and output curves of the as‐fabricated OFETs are presented in Figure  5 . Here, the transfer characteristics revealed by the drain current‐gate voltage (I _d−V _g) curves clearly indicate the ambipolar behavior of all three DSpDSTQ compounds (Figure 5a–c). This indicates that the substitution of the larger and more polarizable Se atom has a positive influence on efficient carrier injection from gold electrodes, resulting in balanced ambipolar mobilities at room temperature.^{[ 83 ]} Moreover, the drain current‐drain voltage (I _d−V _d) curve of the DSpDSTQ‐6 compound reveals a significant increase in the channel output current at low V _g and high V _d values (Figure 5d). This behavior is a characteristic of ambipolar charge transport,^{[ 84} , ⁸⁵ , ^{86 ]} indicating the ability of this compound to support both electron and hole transport.

Figure 5.

Representative transfer characteristics of solution‐sheared OFETs based on a) DSpDSTQ‐6, b) DSpDSTQ‐10, c) DSpDSTQ‐14 thin films (under a constant V _d of 100 V for n‐type and –100 V for p‐type measurements); d) Output characteristics of solution‐sheared OFETs based on DSpDSTQ‐6 thin films. All the OFETs were measured in the glove box at room temperature.

The threshold voltage (V _th) is extracted from the slope and intercept of $\sqrt{I_{\mathrm{d}}}$ versus V _g in the saturation regime, and the charge carrier mobilities (μ) are calculated according to gradual‐channel approximation in the saturation region by the equation below:

$$\mu =\frac{2L}{CW}{\left(\frac{\partial \sqrt{I_{\mathrm{d}}}}{\partial V_{\mathrm{g}}}\right)}^2$$ (1)

where C is the capacitance per unit area of the dielectric layer, L is the channel length (25 µm), and W is the channel width (1500 µm). The device parameters, including maximum mobility (μ _max), average mobility (μ _avg), V _th, and current ON/OFF ratios (I_ON/I_OFF) are summarized in Table  2 . Notably, the solution‐shearing devices exhibit clear thioalkyl side chain‐dependent and electrical anisotropy. Thus, as the side chain length increases, the mobility decreases, with the DSpDSTQ‐6, DSpDSTQ‐10, and DSpDSTQ‐14 exhibiting electron μ_avg values of 0.182 ± 0.083, 0.101 ± 0.046, and 0.079 ± 0.042 cm² V⁻¹ s⁻¹, respectively, and hole μ _avg values of 0.203 ± 0.098, 0.095 ± 0.028 and 0.065 ± 0.023 cm² V⁻¹ s⁻¹, respectively. Hence, the DSpDSTQ‐6 exhibits the highest electron and hole μ _max values of 0.334 and 0.463 cm² V⁻¹ s⁻¹, respectively. Additionally, it is observed that the parallel direction shows a consistently higher μ compared to the perpendicular direction, indicating a preferable charge transport by shearing‐induced crystal growth.

Table 2.

Summary of OFET Parameters Based on Solution‐Sheared DSpDSTQ Thin Films, as Measured in the glove box at Room Temperature.

Compound	Shearing Direction	n‐channel			p‐channel
		μ avg a) (μ max b) ) [cm2 V−1 s−1]	V th [V]	I ON/I OFF [‐]	μ avg a) (μ max b) ) [cm2 V−1 s−1]	V th [V]	I ON/I OFF [‐]
DSpDSTQ‐6 (1)	parallel	0.182 ± 0.083 (0.334)	28.2 ± 4.2	102–103	0.203 ± 0.098 (0.463)	−39.0 ± 10.5	102–104
	vertical	0.153 ± 0.052 (0.241)	42.1 ± 10.9	102–103	0.197 ± 0.133 (0.343)	−55.5 ± 9.3	102
DSpDSTQ‐10 (2)	parallel	0.101 ± 0.046 (0.163)	27.6 ± 2.9	102–103	0.095 ± 0.028 (0.144)	−44.1 ± 13.3	102–103
	vertical	0.067 ± 0.042 (0.137)	28.2 ± 1.7	102–103	0.045 ± 0.010 (0.061)	−28.9 ± 12.7	102–103
DSpDSTQ‐14 (3)	parallel	0.079 ± 0.042 (0.156)	35.3 ± 10.2	102–103	0.065 ± 0.023 (0.100)	−15.5 ± 5.6	102–104
	vertical	0.044 ± 0.019 (0.072)	33.4 ± 6.7	102–103	0.054 ± 0.017 (0.085)	−23.4 ± 10.5	102–105

^a) The average TFT characteristics were obtained from >10 devices originating from 3 to 4 semiconductor depositions;

^b) Maximum mobility.

The relatively low ON/OFF ratio can be attributed to the thicker semiconductor film (70–120 nm) produced by solution‐shearing, which specifically leads to a higher OFF current causing the unideal ON/OFF ratio. Meanwhile, the large V_th and switch‐on voltage indicate the existence of deep trap states at the interface between the semiconductor film and the dielectric.^{[ 87} , ^{88 ]} Mobile carriers may interact with shallow trap states in the vicinity of a chemical impurity, allowing the capture of the electron into a deeply trapped state.^{[ 89} , ^{90 ]} Consequently, the present results are comparable with those of previous studies, where transistors based on the majority of organic semiconductor crystals with ambipolar characteristics by using air‐stable electrode materials such as gold, would have required relatively high operating voltages for the simultaneous injection and transport of the two opposite carriers. The fact can be attributed to high ohmic contact in the crystal/metal interface, which indicates the existence of an injection barrier.^{[ 91} , ⁹² , ^{93 ]}

For comparison, representative transfer curves of the spin‐coated DSpDSTQ‐6‐based devices are presented in Figure S5 (Supporting Information). There, typical ambipolar characteristics are again observed. However, the manifestation of p‐type behavior occurred at high V_g , where the hole mobility cannot be calculated accurately under this circumstance. The extracted electron mobility of the spin‐coated DSpDSTQ‐6 OFETs is 9.84 × 10⁻³ cm² V⁻¹ s⁻¹, which is over one order of magnitude lower than the solution‐sheared counterparts.

2.6. Thin Film Morphological and Microstructural Analysis

To understand the effects of side‐chain engineering on the OFET performance of the DSpDSTQ thin films, the morphologies are examined via polarized optical microscopy (POM; Figure  6a‐c) and by atomic force microscopy (AFM; Figure 6d‐f). Herein, the POM images reveal large crystalline domains with millimeter‐scale dimensions along the shearing direction. Further, DSpDSTQ‐10 and DSpDSTQ‐14 films exhibit randomly distributed spherulitic precipitates with a cross structure. This phenomenon is driven by strong solvent evaporation at the nucleation sites, which generates a concentration gradient and induces supersaturation, thereby initiating the crystallization process near the aggregation points.^{[ 94 ]} Meanwhile, AFM topographic images present that DSpDSTQ‐6 contains well‐interconnected crystals with a terrace‐like morphology, while both DSpDSTQ‐10 and DSpDSTQ‐14 show more cracks and grain boundaries. The presence of grain boundaries can create an energy barrier and limit the charge carrier mobility in organic semiconductor films. Moreover, the root mean square roughness (R _rms) of the thin film increases from 3.4 nm for the DSpDSTQ‐6 to 8.6 and 9.3 nm for the DSpDSTQ‐10 and DSpDSTQ‐14, respectively, due to the increase in side‐chain length. The smoother surface of DSpDSTQ‐6 allows for more efficient charge transport with fewer barriers and grain boundaries, which is consistent with its better electrical performance reported in Table 2. By contrast, the longer insulating alkyl chains of the DSpDSTQ‐10 and DSpDSTQ‐14 may hinder the charge transport efficiency, thereby causing a lower field effect mobility. For comparison, the OM and POM images of the spin‐coated DSpDSTQ‐6 OFETs are presented in Figure S6 (Supporting Information), where the fast and random growth of crystals has resulted in a discontinuous and non‐oriented crystalline morphology. Overall, the above thin film morphological analysis reveals the influence of side chain length on the crystal structure, surface roughness, and interconnectivity of the DSpDSTQ films, which ultimately impacts the charge transport properties in the OFETs.

Figure 6.

POM images of solution‐sheared a) DSpDSTQ‐6, b) DSpDSTQ‐10, and c) DSpDSTQ‐14; AFM images of solution‐sheared d) DSpDSTQ‐6, e) DSpDSTQ‐10, and f) DSpDSTQ‐14.

In addition, the molecular packing structures of the DSpDSTQ compounds are revealed by the 2D grazing‐incidence wide‐angle X‐ray scattering (GIXRD) patterns obtained with the incident beam parallel and perpendicular to the shearing direction in Figure  7 . The unit cell parameters are defined in Figure S7 (Supporting Information), where the a, b, and c axes represent the π–π interaction distance, the main core length, and the length of the alkyl side chain, respectively. The various parameters were determined from the change in incident beam direction, as shown in Figure S8 (Supporting Information), where the in‐plane (q _xy) direction represents the π–π interaction distance parallel to the shearing direction and the main core length perpendicular to the shearing direction, while the out‐of‐plane direction (q _z) reflects the alkyl side‐chain length regardless of the incident beam direction. The (00l) lamellar diffractions in the out‐of‐plane (q _z) direction were clearly detected for all films, indicating a dominant edge‐on molecular orientation with the side chains aligned perpendicular to the substrate. The lamellar distance (d ₀₀₁) values are determined from the primary peaks as 23.6, 26.6, and 29.7 Å for the DSpDSTQ‐6, DSpDSTQ‐10, and DSpDSTQ‐14, respectively. This is consistent with the increase in alkyl chain length (Table S2, Supporting Information).

Figure 7.

2D GIXRD patterns of solution‐sheared DSpDSTQ‐6, DSpDSTQ‐10, and DSpDSTQ‐14 films with an incident beam along the a) parallel (//) and b) perpendicular (⊥) directions with respect to the shearing direction.

A comparison between the GIXRD patterns in Figure 7a,b indicates that all of the DSpDSTQs exhibit more and stronger diffraction peaks in the perpendicular direction than in the parallel direction. This indicates a greater degree of orientation in the microstructure induced by solution‐shearing method. Among all the compounds, DSpDSTQ‐6 exhibits the sharpest diffraction peaks in both parallel and perpendicular directions, indicating the most ordered layered structure and highest crystallinity. Furthermore, only DSpDSTQ‐6 film shows the (02l) diffractions in the parallel direction, representing the periodic structure along the conjugated backbone in b‐axis. Meanwhile, the (10l), (20l), (30l), and (40l) diffraction peaks in perpendicular direction correspond to the 𝜋‐𝜋 stacking of the conjugated backbones along a‐axis, which provides a more efficient pathway for charge carriers to transport. This is the reason why the DSpDSTQ films display higher mobilities along the parallel direction than the perpendicular direction. In the case of the spin‐coated films (Figure S9, Supporting Information), only the DSpDSTQ‐6 exhibits a clear (001) diffraction peak, while the other films show amorphous GIXRD patterns. This confirms that the solution‐shearing method directly affects the anisotropic behavior of the DSpDSTQ film microstructure.

Based on the abovementioned GIXRD diffraction patterns and single‐crystal data, the molecular packing behavior in a thin‐film state can be proposed. A comparison of the length of the c‐axis from single‐crystal data and lamellar distance from GIXRD indicates that the DSpDSTQ molecules are aligned with the c‐axis, as shown in Figure S8 (Supporting Information). The AFM height profile in Figure S10 (Supporting Information) provides some information on the surface crystallographic plane of extended growth terraces. In comparison with the DFT calculation, the GIXRD results and height profile data demonstrate a longer side chain length. This can be attributed to the solution‐shearing process, inducing lattice distortion and elongating the c‐axis. Moreover, the step height of the DSpDSTQ‐6 (2.4 nm) is consistent with the GIXRD‐derived single layer (2.38 nm), thus suggesting the formation of a monolayer on the terrace structure.

The coherence length (L_c ), which provides information about the grain size, can be determined from the GIXRD data. Specifically, after determining the full width at half maximum (FWHM) by integrating the entire primary peak, the L_c value is given by the Scherrer equation, Equation (2):

$$L_c=\frac{K\lambda }{\beta \cos \theta }$$ (2)

where K is the constant of integrated FWHM (K = 0.89), λ is the wavelength of XRD radiation, β is the FWHM in radians, θ is the Bragg diffraction angle.^{[ 95 ]} The FWHM values were found to be 0.18, 0.23, and 0.32 Å⁻¹ for DSpDSTQ‐6, DSpDSTQ‐10, and DSpDSTQ‐14 films, respectively, resulting in L_c values of 296.0 Å, 248.0 Å, and 178.0 Å. Larger grain sizes indicate fewer grain boundaries and better charge transport. Therefore, the inferior charge transport and lowest mobility of the DSpDSTQ‐14 film can be explained by its smallest grain size and the presence of cracks in the crystals, as imaged by AFM. Conversely, the DSpDSTQ‐6 provides the highest OFET mobility because it has the largest L_c value, along with a highly ordered, highly crystalline structure, as indicated by its strong diffraction.

2.7. Device Stability Measurement

Finally, the environmental stabilities of the OFETs are evaluated by examining their performances under ambient conditions over an extended period. In the DSpDSTQ compounds, the introduction of strong electron‐withdrawing groups such as cyano groups, results in a deep‐lying LUMO level below –4.0 eV. This modification helps minimize water and oxygen‐induced trapping and improves the intrinsic environmental stability of the organic semiconductors. In the stability test, non‐encapsulated/non‐passivated DSpDSTQ‐6 OFETs with top gold electrodes were monitored for hole and electron mobility over a period of 28 days under ambient conditions (35‐45% relative humidity and room temperature). The results in Figure  8 demonstrate that all of the devices exhibit good performance throughout the test duration, particularly in terms of stable electron transport. This indicates that the DSpDSTQ‐6 provides good air stability, making it a promising candidate for application in low‐cost and solution‐processable ambipolar OFETs with the capacity for long‐term device operation without significant performance degradation. In brief, by tailoring the chemical structure and optimizing the LUMO level, the organic semiconductor material can be provided with greater stability, which is a crucial factor for practical OFET applications.

Figure 8.

Ambient stability of DSpDSTQ‐6 OFETs stored at a relative humidity of 35–45% and room temperature.

Developing ambipolar materials that can effectively inject both electrons and holes from a single electrode material is crucial for streamlining circuit design in real‐world applications. Common organic semiconductors often exhibit unipolar behavior and have rather broad band gaps, typically between 2–3 eV. One efficient way to reduce bandgap is to construct donor‐acceptor moieties, in which strong electron acceptors are co‐polymerized with electron‐rich donor monomers. In order to facilitate electron transport, a low LUMO molecule design strategy has been employed because the work function of common electrode materials is more aligned to HOMO than LUMO. Since the hybridization of the molecular orbitals, LUMO is dominated by the electron‐accepting unit. This work is dedicated to molecular design for DSpDSTQ, adding strong electron‐withdrawing functional groups such as the cyano group can greatly lower LUMO level and boost air stability simultaneously.

3. Conclusion

A novel series of small‐molecule quinoidal semiconductor materials based on diselenophene‐dithioalkylthiophene (designated DSpDSTQ) were developed herein for application in organic field‐effect transistors (OFETs). By varying the thioalkyl side chain length, compounds with distinct morphologies and microstructures were obtained, thereby modulating the electrical performance. In particular, the compound with the shortest side chain, designated DSpDSTQ‐6, exhibited the highest maximum electron and hole mobilities of 0.334 and 0.463 cm² V⁻¹ s⁻¹, respectively, as evidenced by optical absorption, electronic/crystal structure, surface morphology, and solid‐state molecular packing analyses. The superior performance of the DSpDSTQ‐6 was ascribed to the formation of aligned films with larger crystallites along the shearing direction, as evidenced by charge transport anisotropy measurements. Furthermore, the number of grain boundaries was reduced due to the presence of well‐connected semiconductor domains, thereby facilitating efficient charge transport, as confirmed by the largest coherence length and closely packed molecular stacking. Overall, the DSpDSTQ‐based OFETs exhibited excellent ambipolar characteristics and stable charge transport, making them promising solution‐processable and air‐stable organic semiconductor candidates for various OFET applications. This research not only advances the field of organic electronics but also underscores the potential of DSpDSTQ‐based compounds as valuable building blocks in the design and development of high‐performance organic electronic devices.

4. Experimental Section

General Procedures for Final Target Compounds (1–3)

Malononitrile (4 equiv.) was added into a solution of sodium hydride (10 equiv.) in dry THF (20 mL) at 0 °C, and the mixture was warmed to room temperature and stirred for 30 min. 5,5′‐(3,4‐bis(alkylthio)thiophene‐2,5‐diyl)bis(2‐bromoselenophene) (9) (1 equiv.) and tetrakis(triphenylsphosphine)palladium (0.2 equiv.) were then added. The mixture was refluxed for 12 h, and then saturated bromine water was added slowly at 0 °C. The mixture was extracted with CH₂Cl₂, washed with brine, dried over Na₂SO₄, and evaporated. The residue was purified by column chromatography using dichloromethane/hexane followed by recrystallization from toluene yielding the desired quinoidal compound was obtained as a blue‐green solid.

Synthesis of DSpDSTQ‐6 (1)

¹H NMR (500 MHz, CDCl₃): δ (ppm) 7.66 (d, J = 5.9 Hz, 2H), 7.44 (d, J = 5.9 Hz, 2H), 3.05 (t, J = 7.5 Hz, 4H), 1.71‐1.65 (m, 4H), 1.46‐1.40 (m, 4H), 1.33‐1.25 (m, 8H), 0.9‐0.87 (m, 6H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 176.47, 150.94, 143.42, 142.35, 134.94, 133.25, 114.67, 113.15, 73.71, 38.66, 31.20, 29.63, 28.48, 22.48, 13.97. HRMS (MALDI, [M]⁺) calcd. for C₃₀H₃₀N₄S₃Se_2: 701.9963, Found: 701.9958.

Synthesis of DSpDSTQ‐10 (2)

¹H NMR (500 MHz, CDCl₃): δ (ppm) 7.66 (d, J = 6.0 Hz, 2H), 7.44 (d, J = 6.0 Hz, 2H), 3.05 (t, J = 7.4 Hz, 4H), 1.71‐1.65 (m, 4H), 1.45‐1.39 (m, 4H), 1.29‐1.25 (m, 24H), 0.89‐0.86 (m, 6H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 176.46, 150.91, 143.40, 142.32, 134.93, 133.24, 114.67, 113.13, 73.73, 38.61, 31.87, 29.66, 29.50, 29.47, 29.27, 29.03, 28.80, 22.67, 14.10. HRMS (MALDI, [M]⁺) calcd. for C₃₈H₄₆N₄S₃Se_2: 814.1215, Found: 814.1210.

Synthesis of DSpDSTQ‐14 (3)

¹H NMR (500 MHz, CDCl₃): δ (ppm) 7.66 (d, J = 5.9 Hz, 2H), 7.45 (d, J = 6.0 Hz, 2H), 3.05 (t, J = 7.5 Hz, 4H), 1.71‐1.65 (m, 4H), 1.45‐1.39 (m, 4H), 1.31‐1.25 (m, 40H), 0.89‐0.86 (m, 6H). ¹³C NMR (125 MHz, CDCl₃): δ (ppm) 176.45, 150.91, 143.37, 142.30, 134.92, 133.25, 114.67, 113.12, 73.76, 38.61, 31.92, 29.67, 29.56, 29.47, 29.35, 29.05, 28.81, 22.68, 14.10. HRMS (MALDI, [M]⁺) calcd. for C₄₆H₆₂N₄S₃Se_2: 926.2467, Found: 926.2462.

Device Fabrication and Measurement

The fabrication of the OFET devices followed a standard bottom‐gate top‐contact (BGTC) structure. Highly n‐doped silicon wafers with a thickness of 300 nm of thermally grown SiO₂ were subjected to sonication for 5 min each in acetone and isopropanol. The wafers were then dried with nitrogen gas and treated with UV ozone plasma for 5 min to ensure surface cleanliness. To passivate the dielectric surface, a self‐assembly monolayer (SAM) was formed using (2‐phenylethyl) trichlorosilane (PETS) from a 1 mM solution in toluene. The silicon substrates were immersed in the PETS solution at a temperature of 55 °C for 1 h, allowing the SAM formation. The DSpDSTQ compounds were dissolved in o‐chlorobenzene at a concentration of 4 mg mL⁻¹ and used as the active material. The solution was deposited on the PETS‐treated substrates using the solution‐shearing method. This involved placing ≈15 µL of the solution on a heated platform (80–110 °C) and controlled shearing at a rate of 40–80 µm s⁻¹ using an upper shearing plate modified with octadecyltrichlorosilane (ODTS). Additionally, spin‐coated DSpDSTQ films were prepared as a comparison, using a concentration of 10 mg mL⁻¹ and a spin rate of 2000 rpm for 60 s. Following deposition, all samples were thermally annealed at 60 °C for 1 h under vacuum. For the formation of source and drain electrodes, a 60 nm layer of Au was thermally evaporated onto the samples using a shadow mask. The channel length and width were set at 25 and 1500 µm, respectively. The electrode patterns were designed to be oriented along both the parallel and perpendicular directions with respect to the shearing direction. The electrical characteristics of the fabricated OFETs were measured by a Keithley 4200‐SCS semiconductor parameter analyzer at room temperature within an N₂‐purged glove box.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Velusamy A., Chen Y.‐Y., Lin M.‐H., Afraj S. N., Liu J.‐H., Chen M.‐C., Liu C.‐L., Diselenophene‐Dithioalkylthiophene Based Quinoidal Small Molecules for Ambipolar Organic Field Effect Transistors. Adv. Sci. 2024, 11, 2305361. 10.1002/advs.202305361

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.