Thienoisoindigo-based copolymer with fused thieno[3,2-b]thiophene as a donor in thin film transistor applications with high performance

Abstract

A new thienoisoindigo (TIG)-based copolymer with thieno[3,2-b]thiophene as a donor is synthesized. The copolymer with a short π–π stacking distance of 3.43 Å exhibits an excellent hole mobility up to 0.69 cm² V⁻¹ s⁻¹ under optimum conditions. This performance is the record value among reported TIG-based transistors.

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Recently, donor–acceptor (D–A) alternating copolymers have been investigated for polymer thin film transistor (PTFT) applications due to their excellent field-effect mobilities.¹ Using the D–A structural approach, charge delocalization in the polymer backbone can be improved by the formation of a quinoidal structure, which is driven by the intramolecular charge transfer (ICT) between the D–A moieties, facilitating intrachain charge transport.² Besides, the intermolecular π–π stacking distance can be shortened by the polar interaction of the intermolecular D–A moieties, which is advantageous for interchain charge transport.³ Hence D–A copolymers manifest a relatively high mobility compared to conventional thiophene-based polymers.⁴ Isoindigo (IG) is an acceptor molecule consisting of two indolin-2-one units with strong electron-withdrawing characteristics. It has a hole mobility (μ_h) in the range of 10⁻³ to 1.0 cm² V⁻¹ s⁻¹ when incorporated into copolymers.⁵ However, the structural repulsion between the protons on the phenyl rings and the oxygen atoms of the ketopyrrole moieties in IG not only results in a reduction of coplanarity but also interferes in the stacking interaction of neighboring polymer chains, which is unfavourable for charge transport.⁶ To alleviate this structural drawback, a novel thienoisoindigo (TIG) acceptor has been proposed where the outer phenyl rings of IG were replaced by thiophene units.⁷ Due to the reduction in steric hindrance between the sulfur atoms in the thiophene units and the carbonyl oxygen atoms, the structural planarity of TIG can be improved.^6–8 In addition, the interchain packing can also be promoted by the π-conjugated backbone and the polar functionality, resulting in better charge carrier transport.^7,8 However, the mobilities of TIG-based copolymers only reach the range of 10⁻³ to 0.20 cm² V⁻¹ s⁻¹, but with a lower on/off current ratio (I_on/I_off < 10³) and a higher threshold voltage (V_th > 5.9 V).^7,8

In recent reports, fused thieno[3,2-b]thiophene (TT) with a more coplanar structure and more electron-donating properties has been employed as a donor in D–A copolymers to enhance the charge carrier mobilities of PTFTs, where a mobility up to 1.9 cm² V⁻¹ s⁻¹ has been achieved.^5,9 This high mobility can be ascribed to the enhanced interchain π orbital overlap due to the extension of the π-conjugation in the backbone and the shortened π–π stacking distance caused by the strengthened interchain interaction of the D–A moieties.⁹ This result suggests that TT is a promising donor for D–A type copolymer semiconductors. Therefore, in this work, fused TT is introduced as a donor moiety in the synthesis of a new TIG-based copolymer (PTIG-co-TT) to further enhance the mobility in TIG-based transistors. Furthermore, thiophene molecules are used as end-cappers for the copolymer to eliminate the influence of end group defects in the performance of the transistor, such as disturbance of the chain packing, charge carrier traps and a rise in the off current.¹⁰

The synthetic procedure for PTIG-co-TT was via a Stille coupling polycondensation reaction between (E)-2,2′-dibromo-4,4′-bis(2-octyldodecyl)-[6,6′-bithieno[3,2-b]pyrrolylidene]-5,5′(4H,4′H)-dione and 2,5-bis(trimethylstannyl)-thieno[3,2-b]thiophene, and then the reaction was terminated by thiophene molecules (Scheme 1). The monomer was synthesized according to the reported procedure⁷ and the details are described in the ESI.† The copolymer, with a number average molecular weight (M_n) of 90.6 kD and a corresponding polydispersity index (PDI) of 2.92, was evaluated by gel permeation chromatography (GPC) using THF as the eluent (Fig. S1 in the ESI†). The copolymer showed excellent thermal stability with a decomposition temperature over 437 °C (Fig. S2 in the ESI†).

Scheme 1 Synthetic route for the PTIG-co-TT copolymer. (i) Pd₂(dba)₃, P(o-Tol)₃, toluene, 110 °C; (ii) 2-tributylstannylthiophene, then 2-bromothiophene.

The UV-vis-NIR absorption spectra of PTIG-co-TT in 1,2,4-trichlorobenzene (TCB) solution and as a thin film annealed at 200 °C display dual absorption band characteristics at high and low energy regions (Fig. 1), which are similar to those of TIG-based copolymers reported in the literature.^7,8 In TCB solution, the maximum absorption peak (λ_max) is at 952 nm. Compared to the solution spectrum, the spin-cast film shows a slight red-shift of 35 nm in λ_max (987 nm), but exhibits a significant red shift of 70 nm in the absorption onset (from 1230 nm to 1300 nm). This implies that the copolymer has a more ordered molecular organization upon film formation. The optical band gap (E^opt_g) of PTIG-co-TT, calculated from the absorption onset (1300 nm) of the thin film, is 0.95 eV, which is lower than the TT-containing copolymer with an IG acceptor (ref. 5, E^opt_g = 1.55 eV) for 0.6 eV. The smaller optical band gap of PTIG-co-TT illustrates that the TIG acceptor is able to form more effective π-conjugation compared to IG in a copolymer. The highest occupied molecular orbital (HOMO) level of the PTIG-co-TT film was measured by ultraviolet photoelectron spectroscopy (UPS) and was estimated to be −5.15 eV (Fig. S3 in the ESI†). This indicates that it is beneficial for charge injection from gold electrodes to form a p-channel transport, relative to −5.70 eV for the analogous IG-based copolymer.⁵ Using the HOMO level and the optical band gap, the lowest unoccupied molecular orbital (LUMO) level of PTIG-co-TT was calculated to be −4.20 eV.

Fig. 1 Normalized absorption spectra of PTIG-co-TT in 1,2,4-trichlorobenzene solution and as a thin film annealed at 200 °C.

Bottom-gate/bottom-contact (BG/BC) transistors were fabricated for investigating the charge transport behaviour of PTIG-co-TT. Thin films of PTIG-co-TT were deposited on top of n-octadecyltrimethoxysilane (OTS) treated Si/SiO₂ substrates by spin-coating the polymer solution in TCB. Subsequently, the devices were annealed and measured under nitrogen. Fig. 2(a) presents the transfer characteristics of the PTIG-co-TT-based devices at a drain voltage (V_DS) of −100 V before and after heat treatment. All devices exhibited typical p-channel transfer characteristics with a linear trend in the square root of the drain current (I_DS^1/2) versus the gate voltage (V_GS) plot above the V_th, indicating that the mobilities are independent of the V_GS bias in operation. This implies that charge traps from electronic defects are absent in the channel.¹¹ The field-effect mobilities were extracted by the linear fitting of I_DS^1/2versus V_GS in the saturation regime. All results are summarized in Table 1. The device with the as-spun film has a mobility of 0.12 cm² V⁻¹ s⁻¹ with an I_on/I_off value of 1.7 × 10⁵ and a V_th value of −0.6 V. After thermal treatment, the mobilities were improved compared to that of the pristine film. The annealed device at 200 °C yielded the highest mobility, with a value of 0.23 cm² V⁻¹ s⁻¹, which is about 2 times better than that of the as-spun device. Up to 0.69 cm² V⁻¹ s⁻¹ with a current density over 0.01 A cm⁻¹, an I_on/I_off value over 10⁴ and a V_th value of 1.5 V was obtained for the device with a channel length (L) of 20 μm and a channel width (W) of 10 000 μm (Fig S4 in the ESI†). When the annealing temperature was further raised to 300 °C, the mobility decreased to 0.14 cm² V⁻¹ s⁻¹, which can be attributed to a change in the structural order of the polymer chains as discussed below. In addition, all the devices had close I_on/I_off values and V_th values regardless of whether the film was or was not thermally treated, suggesting that PTIG-co-TT possesses an outstanding thermal stability in devices without rise in the off current and V_th shift in comparison with some other D–A copolymers.^7,8,12Fig. 2(b) and (c) show the output characteristic curves of the PTIG-co-TT-based devices with the as-spun film and the film annealed at 200 °C. In the device annealed at 200 °C, the saturation drain current goes up to 3.33 mA at a V_GS value of −100 V. This is more than a 3 times improvement over the as-spun device (1.1 mA), implying that a better performance can be obtained after thermal annealing.

Fig. 2 (a) Transfer characteristics at a fixed V_DS of −100 V for PTIG-co-TT TFTs based on as-spun and annealed (100, 200 and 300 °C) films. The output characteristics of PTFTs based on PTIG-co-TT with (b) the as-spun film and (c) the annealed film at 200 °C.

Table 1 Electric performance of PTFT devices based on PTIG-co-TT thin films before and after thermal treatment

Heat treatment temp. [°C]	μ h [cm^2 V^−1 s^−1]		I on/Ioff	V th [V]
	Maximum	Average
a Maximum and average hole mobilities of the PTFTs. The average value was calculated from at least three devices (L = 10 μm and W = 5000 μm). b Different device geometry (L = 20 μm and W = 10 000 μm) and dielectric thickness (200 nm).
As-spun	0.12	0.10	1.7 × 10^5	−0.6
100	0.18	0.14	1.3 × 10^5	0.1
200	0.23	0.21	2.0 × 10^5	−0.5
200^b	0.69	0.59	6.4 × 10^4	1.5
300	0.14	0.13	2.0 × 10^5	−1.6

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To investigate the effects of thermal treatment on the charge carrier mobility, PTIG-co-TT thin films were characterized by grazing incidence X-ray diffraction (GI-XRD) analysis to explore the chain packing and molecular orientation (Fig. 3 and 2D patterns in Fig. S5 in the ESI†). A primary diffraction peak (100) at a 2θ value of 3.50° and a weak diffraction peak (010) at a 2θ value of 22.40° were observed in the out-of-plane direction and the in-plane direction, respectively, corresponding to aligned PTIG-co-TT chains with a d-spacing of 21.82 Å and a π–π stacking distance of 3.43 Å. The annealed films exhibited more intense diffraction peaks, indicating a more ordered crystalline structure in the annealed PTIG-co-TT film device for efficient charge carrier transport. The results above clearly demonstrate that PTIG-co-TT chains take on a predominantly ordered lamellar edge-on packing structure relative to the substrate. This orientation is conducive to achieving a high carrier mobility because of the direction of the charge carrier transport through the π–π stacking, which is parallel to the channel between source and drain electrodes.¹³ The remarkably small π–π stacking distance (3.43 Å) of PTIG-co-TT is the shortest distance among reported TIG-based copolymers and consequently this results in a higher mobility compared to other reported TIG-based copolymers.^7,8 However, when the annealing temperature was increased to 300 °C, both the intensity of the diffraction peak (010) in the out-of-plane direction (Fig. 3(a)) and the intensity of the diffraction peak (100) in the in-plane direction (Fig. 3(b)) obviously increased. This implies that PTIG-co-TT at this annealing temperature has a critical alignment transition to form more PTIG-co-TT chains with a face-on configuration as shown in Fig 3(c), leading to a decrease in the mobility (0.14 cm² V⁻¹ s⁻¹) relative to the highest mobility (0.23 cm² V⁻¹ s⁻¹) with an optimal packing structure at 200 °C for PTFT performance.¹⁴ Additionally, the thin films of PTIG-co-TT were also characterized by atomic force microscopy (AFM) to better understand the relationships between the morphology and device performance. As shown in Fig. 4(a), the as-spun PTIG-co-TT film has highly independent granules on the OTS-modified substrate. In contrast to the as-spun film, the annealed film at 200 °C consists of uniformed polymer nanofibrils with interconnected networks (Fig. 4(b)), resulting in an enhanced mobility due to more efficient pathways for charge carrier transport.¹⁰

Fig. 3 Grazing incidence X-ray diffraction measurements with (a) out-of-plane and (b) in-plane scattering geometry for PTIG-co-TT thin films on OTS-treated Si/SiO₂ substrates before and after thermal treatment (the insets show magnification in the range of 10 to 30 degrees); (c) schematic illustration showing the annealing temperature dependent arrangement of the PTIG-co-TT chains on the OTS-treated Si/SiO₂ substrates from fully edge-on orientation to partial edge-on with some face-on configuration (the dark green cuboids represent the polymer backbones).

Fig. 4 AFM tomography images of PTIG-co-TT on OTS-treated Si/SiO₂ substrates with (a) the as-spun film and (b) the annealed film at 200 °C.

Conclusions

In conclusion, we have presented the synthesis of a new TIG-based copolymer, PTIG-co-TT, with fused thieno[3,2-b]thiophene as a donor and thiophene molecules as end-cappers for use in PTFTs. The shallow HOMO level of −5.15 eV is ideal for the injection of hole carriers from gold electrodes. The polymer chains of PTIG-co-TT form an ordered lamellar edge-on orientation with the shortest π–π stacking distance (3.43 Å) among reported TIG-based copolymers. However, the copolymer has an alignment transition of chain packing at a higher post-annealing temperature (300 °C). The annealed PTIG-co-TT film possesses a reticular structure consisting of intertwined nanofibrils compared to a nodular structure in the as-spun film. Using optimal heat treatment and a proper device geometry for the best chain packing and morphology, an excellent hole mobility up to 0.69 cm² V⁻¹ s⁻¹ can be achieved. Besides, PTIG-co-TT shows an outstanding thermal stability in device performance, providing further progress towards the use of TIG-based copolymers in PTFT applications.

Acknowledgements

This work was supported by the Ministry of Education and the Ministry of Science and Technology with financial aid through projects NSC-101-2120-M-007-004, NSC-102-2633-M-007-002 and NSC-102-2221-E-007-131.

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, TFT fabrication, UPS, AFM and GI-XRD. See DOI: 10.1039/c4tc02355b

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