π-Conjugated dithieno[3,2-b:2′,3′-d]pyrrole (DTP) oligomers for organic thin-film transistors

Abstract

Two new molecules with acceptor–donor–donor–acceptor (A–D–D–A) configuration bearing coplanar electron-donating dithieno[3,2-b:2′,3′-d]pyrrole (DTP) as the donor unit and the electron-withdrawing dicyanovinylene as the acceptor block, DTP-L and DTP-S, were synthesized. The introduction of two branched alkyl chains with different lengths at the N-position of DTP led to different transport properties with the longer alkyl chains (DTP-L) showing hole mobility of up to 0.12 cm² V⁻¹ s⁻¹ with on/off ratios of 10⁶ without being subjected to annealing, and the one with short alkyl chains (DTP-S) exhibiting very poor hole mobility of 7.0 × 10⁻⁴ cm² V⁻¹ s⁻¹. The poor performance of DTP-S films was mainly caused by a less ordered film and low crystallinity.

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Introduction

Organic field-effect transistors (OFETs) have been extensively explored on the grounds of their potential applications in radio frequency identification tags, large-area displays, sensors, and low-cost logic circuits.¹ The intensification of research on OFETs has boosted the vigorous development of organic semiconductors.^2,3 Numerous small molecules and polymers have been synthesized and fabricated in OFET devices, which feature p-channel, n-channel or ambipolar transport. These small molecules and polymers are usually composed of fused aromatic rings such as thienothiophene, cyclopentanedithiophene (CPDT), naphthodithiophenediimide (NDTI), and benzodithiophene (BDT) as the donors in conjunction with acceptors such as dicyanomethylene group,^4,5 diketopyrrolopyrrole⁶ and benzothiadiazole^7,8 to tune optical and electronic properties.^9–12 These fused aromatic structures may enhance π–π stacking and hence induce higher molecular ordering, which may lead to improved device performance.^10,13–15

Dithieno[3,2-b:2′,3′-d]pyrrole, serving as donor building block^16,17 for functional π-conjugated systems across the fused ring has been developed for organic photovolatics (OPVs)¹⁸ and organic field-effect transistors (OFETs).^9,18–20 Large alkyl chains attaching to N-position of DTP serving as an effective way to improve solubility, coupling with rigidifying the aromatic rings, the bridging DTP structure can thus be expected to serve to improve semiconducting characteristics and hole-transport properties of the materials in which it functions as a donor block.^6,21 A substantial number of DTP-based semiconductors, both small molecules and polymers, have been synthesized and fabricated in OFET devices. For instance, Marder et al. presented a series of D–A–D and A–D–A small molecules with benzothiadiazole (BTD) as the acceptor and DTP as the donor.¹⁷ However, the average hole mobility of 5.9 × 10⁻³ cm² V⁻¹ s⁻¹ is really moderate. Also, Jenekhe et al. reported donor–acceptor copolymers consisting of DTP as the donor and benzobisthiazole as the acceptor and the highest hole mobility of those copolymers is 5.9 × 10⁻⁴ cm² V⁻¹ s⁻¹ with on/off ratios of 10³.¹⁶ The relative high-lying HOMO levels (above −4.8 eV) of those materials create a large injection barrier between the HOMO level and the high work function of electrodes. McCullough et al. reported D–A copolymers PBTzDTPs consisting of bithiazole as the acceptor and DTP as the donor and achieved field effect mobilities as high as 0.14 cm² V⁻¹ s⁻¹ with current on/off ratios up to 10⁶.¹⁵ McCullough et al. also reported other copolymers using strong electron-deficient diketopyrrolopyrrole (DPP) as the acceptor and an high average hole mobility of 0.29 cm² V⁻¹ s⁻¹ was obtained.⁶ All these high performance copolymers have suitable HOMO energy levels at −5.2 eV. Therefore, suitable HOMO energy level that reduces injection barrier from the work function of electrodes is needed to achieve high carrier mobility. The incorporation of dicyanomethylene (DCM) groups at both termini has been used extensively as a strategy to adjust energy levels and band gap due to its strong electron affinity.^18,20

Herein, we report two DTP-based small molecules in which DTP dimer core is connected to dicyanovinyl, DTP-L with long branched alkyl chain attaching to N-position of DTP and DTP-S with short alkyl chain attaching to N-position of DTP. The combination of coplanar electron-rich and extended conjugation length DTP dimer and strong electron affinity of dicyanomethylene groups to form A–D–D–A structure can induce the desirable changes in electronic structures of the materials.¹¹ While DTP-S-based device exhibited very poor hole mobilities of 7.0 × 10⁻⁴ cm² V⁻¹ s⁻¹, DTP-L-based device displayed hole mobilities of up to 0.12 cm² V⁻¹ s⁻¹ with on/off ratios of 10⁶, which, to the best of our knowledge, is one of the highest values yet obtained for DTP-based small molecules p-type OFETs.

Results and discussion

Synthesis

The synthesis of DTP dimer was carried out according to the similar procedures established in the literature,^20,21 as shown in Fig. 1. Formyl groups were attached to DTP dimer via Vilsmeier–Haack reaction in high yield. Dicyanovinyl groups were shaped on DTP dimer via Knoevenagel condensation using malononitrile in the presence of N,N-diisopropylethylamine (DIPEA) and the reaction could be accelerated with a catalytic amounts of β-alanine in reasonable yield (see Experimental section and ESI† for details).

Fig. 1 Synthesis of DTP-L and DTP-S.

Optical and electrochemical properties

UV-Vis absorption spectra of DTP-L and DTP-S were recorded in DCM solution and as thin films in order to understand optical properties. As shown in Fig. 2, these two compounds exhibited almost same and maxima absorption at about 597 nm in solution, and have similar band gap of around 1.93 eV, estimated from the absorption edge. As expected, the introduction of dicyanovinyl groups into DTP dimer backbone greatly decreases the band gap (band gap of DTP dimer is estimated at about 2.64 eV).²¹ The alternating length of alkyl chains produced a negligible influence on the photo physical properties of the conjugated backbone. The absorption spectrum of both compounds became wider and bathochromic spectral shifts were observed in the film state. In particular, the absorption spectrum of DTP-L film broadly spanned the long wavelength region (450–800 nm), which indicated strong intermolecular interactions in DTP-L film.

Fig. 2 UV-Vis absorption spectra of DTP-L and DTP-S in DCM solution and in films.

Electrochemical properties of DTP-L and DTP-S were characterized by cyclic voltammetry (CV) with tetrabutylammonium hexafluorophosphate as an electrolyte in DCM solution at a scan rate of 100 mV s⁻¹. The cyclic voltammograms of both molecules were presented in Fig. 3. The onset of potential waves were used to estimate the HOMO and LUMO energy levels of each compound. HOMO energy levels were calculated at −5.31 and −5.30 eV for DTP-L and DTP-S according to the onset points of the oxidation waves of 0.91 and 0.90 V. All the optoelectronic parameters such as optical band gap, absorption maximum, HOMO level, and LUMO level were summarized in Table 1. DFT calculation revealed that the LUMO orbitals well delocalized over the conjugated chain, and the HOMO orbitals mainly localized on donor units (see Fig. S2 in ESI†).

Fig. 3 Cyclic voltammogram of DTP-L and DTP-S in DCM/0.1 M nBu₄NPF₆.

Table 1 Optical and electrochemical properties data

Compd	E^onsetox (V)	λmax^a (nm)	λonset^a (nm)	E^optg^b (V)	HOMO^c (eV)	LUMO^d (eV)
a Determined from UV-Vis absorption spectra.b Estimated from the absorption edge using the formula E^optg = 1240/λonset.c Calculated from the onset point of the oxidation wave using the formula HOMO = −(4.4 + E^onsetox).d Calculated from the HOMO values and the optical band gap using the formula LUMO = HOMO + E^optg.
DTP-L	0.91	596	642	1.93	−5.31	−3.38
DTP-S	0.90	597	642	1.93	−5.30	−3.37

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OFET device properties and characterization

To investigate the charge transport properties of the new small molecules, we fabricated bottom-gate/top contact (BGTC) OFET devices. The thin films of DTP-L and DTP-S were deposited on top of the octadecyltrimethoxysilane (OTS)-treated SiO₂/Si substrates by spin coating of a chloroform solution and were annealed at 90, 120, 150 °C. The gold source/drain electrodes were adopted and deposited on the semiconductor layer by thermal evaporation through a shadow mask. Both the thin film devices were fabricated and measured under ambient conditions, and the mobility values were calculated from the transfer characteristics in the saturation regime. Typical transfer and output curves are displayed in Fig. 4 and the OFET results are collected and summarized in Table 2. FET devices based on DTP-L displayed highest hole mobilities of 0.12 cm² V⁻¹ s⁻¹ with I_on/I_off value of 10⁶ without being subjected to annealing. With the annealing temperature increasing, the mobility met a decline. By contrast, the thin films of DTP-S exhibited very poor hole mobilities of 7.0 × 10⁻⁴ cm² V⁻¹ s⁻¹ when annealed at 90 °C with three orders of magnitude lower than that of DTP-L.

Fig. 4 Output and transfer characteristics of OFETs based on (a and b) DTP-L and (c and d) DTP-S films.

Table 2 OFET devices characteristics for DTP-L and DTP-S

Compound	T^a (°C)	μh^b (cm^2 V^−1 s^−1)	VT^b (V)	Ion/Ioff^b
a Annealing temperature.b At least ten times tests of each material devices to form characteristics data under ambient conditions.
DTP-L	RT	0.11 ± 0.01	−8 to −10	10^6
	90	0.033 ± 0.004	−5 to −12	10^5
	120	0.023 ± 0.001	−7 to −11	10^5
	150	0.016 ± 0.001	−13 to −18	10^5
DTP-S	RT	(1.9 ± 0.2) × 10^−4	−30 to −40	10^3
	90	(6.0 ± 1.0) × 10^−4	−35 to −40	10^3
	120	No FET
	150	No FET

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Thin film morphology and crystallinity

To demonstrate the impact of annealing temperature on performance and the great discrepancy of charge-transport behavior between DTP-L and DTP-S-based OTFTs, we utilized atomic force microscopy (AFM) to investigate film morphologies. Fig. 5 shows the AFM images of spin-coated films annealed at different temperatures (from 90 to 150 °C). The DTP-L-based films as cast appeared very good crystalline with large and obvious domains, which is in agreement with good FET performance, where hole mobility of 0.12 cm² V⁻¹ s⁻¹ was obtained. When the annealing temperature increased, there was a slight decrease in the root-mean-square (RMS) roughness from 11.0 to 10.0 nm along with the decrease of grain size of the DTP-L films. Note that flake grains were developed when annealing temperature reached at 150 °C. However, there was no improved FET performance to be obtained in DTP-L films. Although DTP-S films were more continuous and relatively uniform with RMS roughness of 2.6 nm, they appeared very poor crystalline with small domains compared with DTP-L thin films. It may help to explain the poor FET performance of DTP-S films. With the increase of annealing temperature, DTP-S thin films met an increase in the RMS roughness. When DTP-S films annealed at 120 and 150 °C, they showed very discontinuous surface morphology with large RMS roughness. The great enlargement of grain boundaries and disruption of grain connectivity led to no FET performance.

Fig. 5 AFM images of solution-processed films on OTS-modified Si/SiO₂ substrate annealed at (a and e) as cast; (b and f) 90 °C; (c and g) 120 °C; (d and h) 150 °C. Films of (a–d) are based on DTP-L and (e–h) based on DTP-S.

The crystallinity and molecular organization of thin films were also investigated by X-ray diffraction (XRD). Fig. 6 shows XRD patterns of the spin-coated thin films of DTP-L and DTP-S annealed at different temperatures. It is obviously that DTP-L films demonstrate better crystallinity with obvious peak at 5.0° referred to as (100) corresponding to d-spacing of 17.6 Å. The intensity of diffraction peaks became more distinctive and new peaks referred to as (200) and (300) arose when the films were subjected to growing annealing temperature, indicating an enhanced crystallinity. Unlike DTP-L films, there are almost no diffraction peaks emerging in DTP-S films, indicating largely amorphous morphology. Although DTP-S films showed more uniform surface, the very poor crystallinity with small grains helped to understand that DTP-S films had poor FET performance. The first-order diffraction peak of DTP-L films appears diffraction signals at 2θ = 5.0° with a d-spacing of 17.6 Å, which is larger than that of DTP-S. The increase in alkyl chain length brings about an increase of d-spacing from 12.5 to 17.6 Å. Considering that both molecules have the same π-conjugated skeleton length, the increase of d-spacing might be ascribed to the extension of the branched alkyl chain which are perpendicular to the π-conjugated skeleton of molecules. Therefore, we speculate that the molecules may adopt end-to-end molecular packing configuration on the surface of the dielectric substrate along the alkyl stretching direction.⁵ Through figures of XRD and AFM, the mobility of DTP-L films exceeding that of DTP-S-based films by a wide margin and annealing temperature not serving to improve charge carrier transport could be understandable.

Fig. 6 1D-GIXRD patterns of thin films of (a) DTP-L and (b) DTP-S annealing at different temperatures.

Conclusions

In summary, we have designed, synthesized, and characterized two DTP-based derivatives (DTP-L and DTP-S) for application in solution-processed FETs. Variation of alkyl chain length, a simple strategy, is used effectively to tune charge transport properties. The introduction of dicyanovinyl groups into DTP dimer backbone resulted in a reduced band gap and a relatively suitable HOMO level at −5.3 eV which may facilitate the hole injection from Au electrodes. Thermal annealing of OFET devices is also found to have great effects on charge transport with a direct correlation to crystallinity and morphology of the films according to AFM and XRD. FET devices based on DTP-L displayed highest hole mobilities of 0.12 cm² V⁻¹ s⁻¹ without being subjected to annealing. These results indicate that the appropriate modification of DTP dimer serving as donor building block in extended functional π-conjugated systems can improve charge transport and may trigger new molecular engineering methods for designing more promising DTP-based small molecules for potential applications in OFETs and OSCs.

Experimental section

Chemicals and reagents were purchased from commercial sources such as Sigma-Aldrich, Acros and Alfa Aesar without further purification unless otherwise stated. Two dithieno[3,2-b:2′,3′-d]pyrrole compounds with different branched alkyl chains were purchased from Solarmer Materials (Beijing) Inc. and THF were dried over sodium sand using benzophenone as an indicator and freshly distilled prior to use.

DTP-L: in a one-necked 100 mL round-bottom flask, 4,4′-bis(2-hexyldecyl)-4H,4′H-[2,2′-bidithieno[3,2-b:2′,3′-d]pyrrole]-6,6′-dicarbaldehyde (200 mg, 0.232 mmol) and malononitrile (61.3 mg, 0.928 mmol) were dissolved in chloroform (20 mL). A catalytic amounts of β-alanine (8.91 mg, 0.10 mmol) and a drop of DIPEA were added and stirred for 48 h at room temperature. Solvent was evaporated by rotary evaporation. The crude product was purified by column chromatography on silica gel using dichloromethane as the eluent to give a dark green solid (195 mg, 87.8% yield) ¹H NMR (400 MHz, CDCl₃) δ 7.72 (s, 2H), 7.59 (s, 2H), 7.13 (s, 2H), 4.10 (d, J = 7.2 Hz, 4H), 2.01 (m, 2H), 1.41–1.23 (m, 48H), 0.88–0.84 (m, 12h); ¹³C NMR (101 MHz, CDCl₃) δ 150.89, 150.45, 145.77, 142.22, 133.16, 125.82, 115.06, 114.73, 114.51, 107.55, 72.71, 52.07, 39.03, 31.86, 31.71, 31.60, 29.85, 29.51, 29.46, 29.25, 26.39, 22.65, 22.59, 14.09, 14.06. Anal. calcd for C₆₆H₈₈N₆O₂S₄: C 70.25, H 7.58, N 8.78. Found: C 70.14, H 7.61, N 9.03.

DTP-S was synthesized by following a similar procedure as that for DTP-S but by starting from 4,4′-bis(2-ethylhexyl)-4H,4′H-[2,2′-bidithieno[3,2-b:2′,3′-d]pyrrole]-6,6′-dicarbaldehyde as a dark green solid (121 mg, 92% yield). ¹H NMR (400 MHz, CDCl₃) δ 7.73 (s, 2H), 7.61 (s, 2H), 7.14 (s, 2H), 4.11 (d, 4H), 1.96 (m, 2H), 1.41–1.23 (m, 16H), 0.97–0.88 (m, 12h); ¹³C NMR was difficult to obtain because of relatively poor solubility. Anal. calcd for C₆₆H₈₈N₆O₂S₄: C 65.54, H 5.50, N 11.46. Found: C 65.57, H 5.54, N 11.48.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21290191, 21333011), Chinese Ministry of Science and Technology (2011CB808401), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12000000). This paper was grateful to the NSFC for support of this research under grant 21002127, 212111120.

References

1. L. Torsi, M. Magliulo, K. Manoli and G. Palazzo, Chem. Soc. Rev., 2013, 42, 8612–8628.
2. A. Pron, P. Gawrys, M. Zagorska, D. Djurado and R. Demadrille, Chem. Soc. Rev., 2010, 39, 2577–2632.
3. W. Wu, Y. Liu and D. Zhu, Chem. Soc. Rev., 2010, 39, 1489–1502.
4. C. Zhang, Y. Zang, E. Gann, C. R. McNeill, X. Zhu, C. A. Di and D. Zhu, J. Am. Chem. Soc., 2014, 136, 16176–16184.
5. Y. Qiao, Y. Guo, C. Yu, F. Zhang, W. Xu, Y. Liu and D. Zhu, J. Am. Chem. Soc., 2012, 134, 4084–4087.
6. T. L. Nelson, T. M. Young, J. Liu, S. P. Mishra, J. A. Belot, C. L. Balliet, A. E. Javier, T. Kowalewski and R. D. McCullough, Adv. Mater., 2010, 22, 4617–4621.
7. S. Zhang, Y. Guo, H. Fan, Y. Liu, H.-Y. Chen, G. Yang, X. Zhan, Y. Liu, Y. Li and Y. Yang, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 5498–5508.
8. Y. Lu, H. Chen, X. Hou, X. Hu and S.-C. Ng, Synth. Met., 2010, 160, 1438–1441.
9. X. Zhang, J. W. Shim, S. P. Tiwari, Q. Zhang, J. E. Norton, P.-T. Wu, S. Barlow, S. A. Jenekhe, B. Kippelen, J.-L. Brédas and S. R. Marder, J. Mater. Chem., 2011, 21, 4971.
10. H.-I. Lu, C.-W. Lu, Y.-C. Lee, H.-W. Lin, L.-Y. Lin, F. Lin, J.-H. Chang, C.-I. Wu and K.-T. Wong, Chem. Mater., 2014, 26, 4361–4367.
11. S. Kumar, K. R. Justin Thomas, C.-T. Li and K.-C. Ho, Org. Electron., 2015, 26, 109–116.
12. X. Zhan, Z. A. Tan, E. Zhou, Y. Li, R. Misra, A. Grant, B. Domercq, X.-H. Zhang, Z. An, X. Zhang, S. Barlow, B. Kippelen and S. R. Marder, J. Mater. Chem., 2009, 19, 5794.
13. S. C. Rasmussen and S. J. Evenson, Prog. Polym. Sci., 2013, 38, 1773–1804.
14. J. Kesters, P. Verstappen, W. Vanormelingen, J. Drijkoningen, T. Vangerven, D. Devisscher, L. Marin, B. Champagne, J. Manca, L. Lutsen, D. Vanderzande and W. Maes, Sol. Energy Mater. Sol. Cells, 2015, 136, 70–77.
15. J. Liu, R. Zhang, I. Osaka, S. Mishra, A. E. Javier, D.-M. Smilgies, T. Kowalewski and R. D. McCullough, Adv. Funct. Mater., 2009, 19, 3427–3434.
16. E. Ahmed, S. Subramaniyan, F. S. Kim, H. Xin and S. A. Jenekhe, Macromolecules, 2011, 44, 7207–7219.
17. L. E. Polander, L. Pandey, S. Barlow, S. P. Tiwari, C. Risko, B. Kippelen, J.-L. Brédas and S. R. Marder, J. Phys. Chem. C, 2011, 115, 23149–23163.
18. C.-L. Chung, C.-Y. Chen, H.-W. Kang, H.-W. Lin, W.-L. Tsai, C.-C. Hsu and K.-T. Wong, Org. Electron., 2016, 28, 229–238.
19. E. Zhou, Q. Wei, S. Yamakawa, Y. Zhang, K. Tajima, C. Yang and K. Hashimoto, Macromolecules, 2010, 43, 821–826.
20. A. Yassin, T. Rousseau, P. Leriche, A. Cravino and J. Roncali, Sol. Energy Mater. Sol. Cells, 2011, 95, 462–468.
21. A. Yassin, P. Leriche and J. Roncali, Macromol. Rapid Commun., 2010, 31, 1467–1472.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis of intermediate products, NMR spectra, DFT calculation results. See DOI: 10.1039/c5ra24845k

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