High-mobility and nonhalogenated-solvent-processable n-type organic semiconductors enabled by alkyl-side-chain engineering

Abstract

Organic semiconductors possessing both green-solvent processability and superior efficiency are highly desirable for green electronics. A simple but effective strategy to improve the solubility of high-performance organic semiconductors in nonhalogenated solvents by tuning the branch length of the alkyl side chain is presented herein. We synthesize two new thiophene-diketopyrrolopyrrole-based quinoidal molecules (TDPPQs) bearing alkyl chains with varied side branch length but the same branching position at the third carbon, which are soluble in nonhalogenated solvents such as 2-methyltetrahydrofuran (2-MTHF) and acetone (AT). Based on the TDPPQ compounds, we have carried out systematic studies on the relationship between film microstructure and charge transport in their organic field-effect transistors (OFETs). It is found that the branch length of alkyl chains, the processing solvents, and the processing methods have obvious effects on the thin-film microstructure and lead to remarkable changes in OFET performance. Notably, TDPPQ-B exhibits an exciting electron mobility of up to 2.17 cm² V⁻¹ s⁻¹ in OFETs processed from 2-MTHF solution by edge-casting, which is the first report on nonhalogenated-solvent-processed OFETs with electron mobility above 1 cm² V⁻¹ s⁻¹. This work sheds light on the design of high-performance n-type organic semiconductors processable with environmentally benign solvents.

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1. Introduction

Organic semiconductors have attracted considerable industrial and academic interest owing to their great potential for next-generation electronic devices, such as organic solar cells (OSCs), organic light-emitting diodes (OLEDs), organic thermoelectrics (OTEs), and organic field-effect transistors (OFETs).^1–5 Solution processability is one of the most distinctive features of organic semiconductors, which provides tremendous potential for low-cost and large-area device fabrication.⁶ In the past few decades, solution-processed OFETs have seen remarkable advances due to many research efforts, such as material development and device fabrication.^7–11 However, most of the high-mobility organic semiconductors are only dissolved in halogenated solvents such as chloroform, chlorobenzene, and ortho-dichlorobenzene, which are among the most toxic and environmentally damaging solvents.^12–14

To overcome human health and environmental risks, and to lower the barrier to industrial feasibility, great endeavors have been dedicated to the design and synthesis of organic semiconductors that can be processed by environmentally benign solvents.^15–20 Recent results indicate that high hole mobilities well over 1.0 cm² V⁻¹ s⁻¹ can be achieved by many OFETs based on nonhalogenated-solvent-processable p-type small molecular and polymeric semiconductors.^21–27 In comparison, only a few n-channel OFETs processed from nonhalogenated solution exhibited mobilities >0.20 cm² V⁻¹ s⁻¹.^28,29 For example, O-IDTBR, an n-type organic semiconductor typically used as an acceptor in bulk-heterojunction OSCs, showed electron mobilities of 0.37–0.91 cm² V⁻¹ s⁻¹ in OFETs processed from a terpene solvent.²⁸ Overall, the development of nonhalogenated-solvent-processable n-type organic semiconductors has largely lagged behind their p-type counterparts. This uneven development makes the exploration of high-performance n-type semiconductors for nonhalogenated-solvent-processed n-channel OFETs a key issue in organic electronics.

Side-chain engineering, the installation of appropriate side-chain substituents to π-conjugated backbones, is an effective approach to develop excellent solution-processable organic semiconductors. It's well known that the length, position, and branching point of side chains significantly affect intermolecular interaction, molecular packing, and device performance.^30–32 With optimized side chains, dicyanomethylene-substituted n-type organic semiconductors based on some conjugated cores (e.g. thienoisoindigo (TII), dithieno[3,2-b:2′,3′-d]thiophene (DTT), naphthalene diimide (NDI), diketopyrrolopyrrole (DPP), etc.) exhibit excellent electron mobilities and ambient stability in OFETs (Fig. 1).^11,33–40 However, they were generally processed from halogenated solvents.

Fig. 1 Chemical structures and OFET performances of (a)–(i) some typical n-type organic semiconductors incorporating dicyanomethylene end groups.

In this paper, we present a simple yet effective approach for developing high-performance dicyanomethylene-substituted n-type organic semiconductors that can dissolve in nonhalogenated solvents by tuning the branch length of the alkyl side chain with the branching point at the third carbon position, as illustrated in Fig. 2. New thiophene-diketopyrrolopyrrole-based quinoidal molecules (TDPPQs), TDPPQ-A and TDPPQ-B (see Fig. 2), become processable with nonhalogenated solvents like 2-methyltetrahydrofuran (2-MTHF) and acetone (AT) after incorporating shorter branches (b-C₁₃H₂₇ and b-C₁₇H₃₅) compared to TDPPQ-3 (b-C₂₃H₄₇, Fig. 1i), a material we reported previously.⁴⁰ The OFET results reveal that a thin film of TDPPQ-B exhibits an exciting electron mobility of up to 2.17 cm² V⁻¹ s⁻¹ in air when processed with 2-MTHF, a solvent that can be derived from renewable resources.^41,42 To our knowledge, this is the highest electron mobility for n-channel OFETs processed from a nonhalogenated solvent to date.

Fig. 2 Molecular structures of TDPPQ-A and TDPPQ-B.

2. Results and discussion

2.1 Materials and characterization

The syntheses of TDPPQ-A and TDPPQ-B are shown in the ESI.† Starting from the corresponding precursors, the TDPPQs were synthesized through a palladium-catalyzed coupling reaction, followed by oxidation with saturated bromine water. The alkyl side chains of TDPPQ-A and TDPPQ-B are branched at the third carbon away from the π-conjugated backbone with different branch length. Their chemical structures were fully characterized by nuclear magnetic resonance spectroscopy (NMR) and high-resolution mass spectrometry (HRMS). Thermogravimetric analysis (TGA) showed that the thermolysis onset temperatures for TDPPQ-A and TDPPQ-B were 312 and 319 °C, respectively, and their melting points obtained by differential scanning calorimetry (DSC) were 170 and 175 °C, respectively (Fig. S1 and S2, ESI†). The thermal analysis results suggest the high thermal stability of TDPPQs.

The solubilities of TDPPQs were determined by the procedure reported previously,^21,25 and the results are collected in Table S1 (ESI†). At room temperature, the solubilities of TDPPQ-A in 2-MTHF and AT were estimated to be 11.1 and 6.3 mg mL⁻¹ respectively, while the solubilities of TDPPQ-B in these two solvents are 9.2 and 4.1 mg mL⁻¹ respectively. Although chloroform (CF) is found to be the best solvent for TDPPQ compounds, the solubilities of TDPPQ-A and TDPPQ-B in nonhalogenated solvents 2-MTHF and AT are sufficient for OFET fabrications.

The solubilities of TDPPQ-A and TDPPQ-B can be qualitatively explained by using Hansen solubility parameters (HSPs), which are estimated as the sum of the contributions from dispersive interactions (δ_D), dipole molecular interactions (δ_P), and hydrogen bonding interactions (δ_H).^14,43 It is well known that a compound tends to dissolve more readily in solvents with HSPs that are closer to its own. TDPPQ molecules with different alkyl chains could have similar δ_P because of their symmetric quinoidal structures. Moreover, δ_H for TDPPQ compounds could also be similar, because their intermolecular hydrogen bonding exists between the nitrogen on the end-capped dicyanomethylenes and the β hydrogen of the thiophene ring,⁴⁴ which can be hardly influenced by alkyl side chains. Thus, the difference in HSPs for TDPPQ compounds can be mainly determined by that in δ_D values. As shown in Table S1 (ESI†), δ_D values of 2-MTHF and AT are lower than that of CF. The decrease in the branch length of the alkyl side chains is expected to lower the δ_D values of TDPPQ-A and TDPPQ-B, which become closer to those of 2-MTHF and AT by comparing with that of TDPPQ-3. As a result, TDPPQ-A and TDPPQ-B become more soluble in 2-MTHF and AT than TDPPQ-3.

The comparison of the absorption spectra of TDPPQ-A and TDPPQ-B in solution and the thin-film state are illustrated in Fig. 3, and corresponding data are summarized in Table S2 (ESI†). The two compounds showed similar absorption spectra in 2-MTHF and AT solutions with the maximum absorption peaks at 635–638 nm, indicating that variation of the alkyl chains has an almost negligible effect on the photophysical properties of the conjugated backbone. For each TDPPQ compound, the absorption spectra of the thin films prepared from the solutions in 2-MTHF and AT exhibited similar bands, which are broadened and red-shifted relative to the corresponding solution spectra because of J-aggregation. In the thin films of TDPPQ-A, the 0-0 vibrational peaks in the absorption bands of the thin films are red-shifted to 729 nm by about 90 nm. In comparison, the 0-0 vibrational peaks of TDPPQ-B in the thin film spectra were significantly red shifted to 757 nm by about 120 nm, suggesting that the intermolecular packing in the films of TDPPQ-B is much stronger than that of TDPPQ-A, an encouraging indication for more favorable charge transfer in the thin-film devices of TDPPQ-B relative to those of TDPPQ-A.

Fig. 3 Absorption spectra of TDPPQ-A and TDPPQ-B in (a) solution and in (b) thin films on quartz substrates.

Cyclic voltammetry (CV) measurements were conducted to investigate the electrochemical properties and energy levels of TDPPQ-A and TDPPQ-B. As shown in Fig. S3 (ESI†), the two compounds show similar redox behavior with two well-resolved reversible reduction peaks, but no oxidation peak is observed. The lowest unoccupied molecular orbital (LUMO) energies calculated from the data of CV and absorption spectra are −4.51 eV for both TDPPQ-A and TDPPQ-B (Table S2, ESI†), which are deep enough for air-stable n-channel OFETs. The calculated electron density distributions on the HOMO and LUMO for TDPPQ compounds are shown in Fig. S4 (ESI†). The highly delocalized conjugated systems may improve their intermolecular couplings and facilitate charge transport.

2.2. OFET Performance

The charge transport properties of TDPPQ-A and TDPPQ-B were investigated by fabricating OFETs in a top-gate/bottom contact (TG/BC) configuration. The SiO₂/Si substrates for device fabrication were modified with cured benzocyclobutene (BCB) films. Thin films were deposited on the substrates by spin-coating the solutions of TDPPQ compounds, and then thermally annealed. Typical transfer and output curves are displayed in Fig. 4 and Fig. S5 (ESI†), respectively. All devices exhibited typical n-channel charge transport characteristics in air. The device characteristics of TDPPQs, including the electron carrier mobility, threshold voltage, and current on/off ratios are summarized in Table 1. It was found that the optimized performance of TDPPQs was achieved at 150 °C with quite different electron mobilities. Based on the respective transfer curves of the spin-coated devices fabricated with AT and 2-MTHF (Fig. 4), the optimum electron mobilities of TDPPQ-A were estimated to be 0.29 and 0.37 cm² V⁻¹ s⁻¹, respectively, while those of TDPPQ-B were 0.46 and 0.62 cm² V⁻¹ s⁻¹, respectively. As can be concluded from the above-mentioned results, the devices fabricated with 2-MTHF show superior mobilities relative to those fabricated with AT for each TDPPQ compound. Moreover, a trend of alkyl chain branch-dependent mobility can be clearly observed, in which the devices based on TDPPQ-B show better performance compared to those based on TDPPQ-A. As references, the OFET devices spin-coated from chloroform (CF) solutions were also measured. As a result, the optimum electron mobilities of TDPPQ-A and TDPPQ-B in CF spin-coated devices were 0.39 and 0.65 cm² V⁻¹ s⁻¹ respectively (Table 1 and Fig. S6, ESI†), which were only slightly higher than those of 2-MTHF spin-coated devices, suggesting that 2-MTHF is a superior nonhalogenated solvent for processing our new compounds.

Fig. 4 Transfer curves of the OFET devices based on (a) and (d) AT spin-coated films, (b) and (e) 2-MTHF spin-coated films, and (c) and (f) 2-MTHF edge-cast films of (a)–(c) TDPPQ-A and (d), (e) and (f) TDPPQ-B, respectively. (g) Schematic of the edge-casting method for thin film preparation. The domains grow along the direction of solvent evaporation (yellow arrow). (h) Photograph of a film being processed by edge-casting. (i) Optical microscope images of the OFET device based on edge-cast films.

Table 1 Maximum electron mobility (μ_e), current on/off ratio (I_on/I_off), and threshold voltage (V_Th) for OFETs based on TDPPQ-A and TDPPQ-B films

Material	Solvent	Method	μ e (cm^2 V^−1 s^−1)	I on/Ioff^a	V Th (V)
a The device characteristics were obtained from 15 devices, and all devices were measured under ambient conditions.
TDPPQ-A	AT	Spin-coating	0.29 (0.23 ± 0.03)	10^5–10^6	−14 to −10
	2-MTHF	Spin-coating	0.37 (0.31 ± 0.04)	10^5–10^6	−18 to −13
	CF	Spin-coating	0.39 (0.33 ± 0.03)	10^5–10^6	−17 to −11
	2-MTHF	Edge-casting	0.96 (0.84 ± 0.09)	10^6	−32 to −22
TDPPQ-B	AT	Spin-coating	0.46 (0.42 ± 0.02)	10^5–10^6	−27 to −22
	2-MTHF	Spin-coating	0.62 (0.56 ± 0.03)	10^5–10^6	−35 to −29
	CF	Spin-coating	0.65 (0.59 ± 0.04)	10^4–10^5	−35 to −30
	2-MTHF	Edge-casting	2.17 (2.04 ± 0.11)	10^5–10^6	−39 to −27

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To enhance the device performance further, thin films of TDPPQ compounds were prepared for the OFET devices by edge-casting instead of spin-coating. Edge-casting is a solution-crystallizing method that has been reported to form highly-oriented crystalline films.^45–47 Compared to other processing methods (e.g. bar-coating, solution shearing, inkjet printing, etc.), edge-casting is an easier fabrication method because it does not need any machine. However, edge-casting seems to have relatively more limitations in solvent choice, e.g. AT and CF are found not to be applicable to edge-casting of TDPPQ compounds because they evaporate too fast due to their low boiling points (56 °C and 61 °C). In comparison, 2-MTHF with a much higher boiling point of 80 °C works as a good edge-casting solvent. In our typical edge-casting procedure, a droplet of the solution of TDPPQ-A or TDPPQ-B in 2-MTHF was placed at the edge of a sustaining piece on an inclined BCB-modified SiO₂/Si substrate (Fig. 4g and h). Along the evaporation direction of 2-MTHF, the domains of TDPPQs grew on top of the substrate in the direction of the OFET channel (Fig. 4i), allowing efficient electron transport. Although TDPPQ compounds did not form a single-crystal film with the edge-casting process, the OFET devices based on the edge-cast films did show excellent electron transport properties with maximum electron mobilities of 0.96 and 2.17 cm² V⁻¹ s⁻¹ for TDPPQ-A and TDPPQ-B, respectively, which were much higher than those of the spin-coated films (Table 1). To the best of our knowledge, the above results represent the best performance for n-channel OFETs processed with nonhalogenated solvents to date. Moreover, these high-performance devices exhibited almost no change in mobility after being stored under ambient conditions for 4 weeks (Fig. S7, ESI†), suggesting the excellent air stability.

2.3. Thin film characterization

Atomic force microscopy (AFM) was utilized to investigate the morphology of the TDPPQ films, which were processed from AT, 2-MTHF, and CF, respectively. As shown in Fig. S8 (ESI†), the films spin-coated from 2-MTHF and CF solutions have comparable domain sizes, which are larger compared to AT-processed films. Meanwhile, comparing the domain size of the two TDPPQs with the same processing solvents, TDPPQ-B thin films with longer branch length exhibit larger domain size than TDPPQ-A films. Notably, the edge-cast film of TDPPQ-B (Fig. S9, ESI†) seems to have even larger domain size and fewer grain boundaries compared to all the spin-coated films of TDPPQ-B, indicating more favorable charge carrier transport.

Fig. 5a–f presents the two-dimensional (2D) grazing incidence wide-angle X-ray scattering (GIWAXS) images of TDPPQ films processed from AT solutions (spin-coating) and 2-MTHF solutions (spin-coating & edge-casting). Line profiles of the patterns in the out-of-plane and in-plane directions are shown in Fig. 5g, h, i and j, respectively. Along the out-of-plane direction, both TDPPQ-A and TDPPQ-B have long-range ordered edge-on lamellar packings in films. TDPPQ-B films show higher order with six diffraction peaks (100), (200), (300), (400), (500) and (600), while only five diffraction peaks are observed for TDPPQ-A films, suggesting that TDPPQ-B has a more ordered edge-on molecular packing than TDPPQ-A. The values for the lamellar d-spacing of the (100) peaks in the TDPPQ-A and TDPPQ-B films are 22.8 and 27.6 Å, respectively, which correlate well with the branch length of their alkyl chains. In the in-plane direction, TDPPQ-B films show extremely strong π-stacking (010) diffractions, while the (010) peaks for TDPPQ-A films are very weak, indicating that TDPPQ-B has much stronger π-stacking interaction. Moreover, the π–π stacking distance in the TDPPQ-B films (3.19 Å) is shorter than that in the TDPPQ-A films (3.25 Å), which suggests that TDPPQ-B possesses closer molecular backbone packing in comparison to TDPPQ-A. These differences could be mainly responsible for the observation that OFET devices based on TDPPQ-B display higher electron mobilities than TDPPQ-A. The 2D-GIWAXS images of the CF spin-coated films are presented in Fig. S10 (ESI†) for comparison. For the films of each TDPPQ compound, the intensity of the (100) diffractions decreases following the trend of 2-MTHF edge-cast > CF spin-coated ≈ 2-MTHF spin-coated > AT spin-coated film (Fig. 5g and h and Fig. S10c and d, ESI†), suggesting that the edge-cast films exhibit a more ordered film structure compared to the spin-coated films. A similar trend was observed in the crystal coherence length (L_c) of these films (Table S3, ESI†), which reflects the domain sizes.⁴⁸ Determined by GIWAXS data utilizing the Scherrer equation, the evolution trend of L_c values was largely consistent with the analysis on AFM morphology. Thus, the 2-MTHF edge-cast films of TDPPQ-B with the highest degree of crystalline order and largest domain size among all the TDPPQ films achieve the highest electron mobility in OFETs.

Fig. 5 2D-GIWAXS patterns of (a) and (d) AT spin-coated films, (b) and (e) 2-MTHF spin-coated film, and (c) and (f) 2-MTHF edge-cast films of (a)–(c) TDPPQ-A and (d), (e) and (f) TDPPQ-B, respectively. The corresponding GIWAXS line-cut profiles of (g) and (i) TDPPQ-A and (h) and (j) TDPPQ-B films along the (g) and (h) out-of-plane and (i) and (j) in-plane directions.

3. Conclusion

In conclusion, we have developed two new n-type organic semiconductors (TDPPQ-A and TDPPQ-B) for high-performance n-channel OFETs processed from nonhalogenated solvents. Tuning the branch length of the alkyl chain branched at the third carbon position remarkably boosts the solubility of TDPPQ compounds in nonhalogenated solvents such as 2-MTHF and AT. Through the combinatorial analysis of OFET performance and thin-film characterization, we found that TDPPQ-B with longer branch length has a higher degree of order and a shorter π–π stacking distance in the film compared with TDPPQ-A, which are well correlated with its superior OFET performance. TDPPQ-B film prepared from 2-MTHF solution by edge-casting exhibits an electron mobility as high as 2.17 cm² V⁻¹ s⁻¹, representing the highest electron mobility to date for nonhalogenated-solvent-processed OFETs. These findings demonstrate that the fine-tuning of branched side chains can provide an effective approach to achieve high-performance organic semiconductors processable with green solvents and also helps in-depth understanding of structure–property relationships.

4. Experimental section

4.1. OFET fabrication and measurement

The devices were fabricated in the bottom-gate/top-contact configuration. The precleaned SiO₂/Si substrate was covered with a BCB insulating layer by spin-coating of a 5 vol% BCB solution in mesitylene followed by thermal annealing at 250 °C for 2 h in a glovebox to eliminate charge traps. The total capacitance of the BCB layer and SiO₂ was 9.7 nF cm⁻². TDPPQ film was prepared on the BCB-treated substrate by spin-coating a 4 mg mL⁻¹ solution in 2-MTHF or AT at 1000 rpm for 40 s, followed by thermal annealing at 150 °C for 20 min. The edge-casting was performed on an inclined BCB-modified substrate under ambient conditions. The solution of TDPPQ-A or TDPPQ-B in 2-MTHF was dropped on the edge of a sustaining piece. After slow solvent evaporation, the edge-cast film was formed, and then it was annealed at 150 °C for 10 min to remove solvent residue. Au (40 nm) source and drain contacts were deposited on the organic layer sequentially by vacuum evaporation through a metal shadow mask under the pressure of 10⁻⁴ Pa. The channel width and length were 1000 and 250 μm, respectively. The measurement of OFET devices was conducted on a Keithley semiconductor parameter analyzer in air. The effective charge carrier mobility was calculated according to the previous report.⁴⁹

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work is supported by the Shanghai Leading Talent Program.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc00388a

‡ These authors contributed equally as first authors.

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