Ambipolar charge transport of TIPS-pentacene single-crystals grown from non-polar solvents

Abstract

Charge transport of solution-grown TIPS-pentacene single-crystals depends on the polarity of the used solvents. Crystals grown from non-polar solvents exhibit ambipolar transport (μ_e 0.020 cm² V⁻¹ s⁻¹; μ_h 5.0 cm² V⁻¹ s⁻¹), while polar solvents suppress electron transport. This work indicates that appropriate selection of solvents should further harvest the n-type behaviors of organic semiconductors.

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Conceptual insights

Ambipolar organic field-effect transistors (OFETs) are essential for various electronic applications such as organic complementary circuits and light-emitting transistors. Although organic semiconductors should, in principle, provide an equal opportunity for both holes and electrons, electron transport is practically less-frequently observed. Currently, it is widely believed that the limited electron transport is associated with high electron injection barriers and the electron traps at the semiconductor–dielectric interface. Here, we demonstrate an ignored alternative source to suppress the electron transport. We show that trace amounts of polar solvent residues from crystal growth of organic semiconductors ruin n-channel conductance. In contrast, the non-polar solvents do not. Grown from various non-polar solvents, single-crystals of 6,13-bis(triisopropyl-silylethynyl)pentacene (TIPS-pentacene), a standard p-channel material, conduct electrons in FETs using Au as both source and drain electrodes. As hexane was used, we achieved an electron mobility as high as 0.020 cm² V⁻¹ s⁻¹ as well as a hole mobility of 5.0 cm² V⁻¹ s⁻¹. Hence, this work should further harvest the n-type behaviors of organic semiconductors.

Organic field-effect transistors (OFETs) are promising for a variety of low-cost, light-weight and flexible electronic applications.^1,2 Specifically, ambipolar OFETs in which both holes and electrons are mobile have been attracting increasing attention for their applications in light-emitting transistors^3–6 and complementary circuits.^7–11 Fundamentally, organic semiconductors should allow both holes and electrons to transport.^11–14 But in practice, unipolar transport dominates. Initially, ambipolar OFETs were realized by combining the electronic functions of both p- and n-channel components in the active layers in the forms of bi-layers,^15–18 blends^19,20 and co-crystals.^21–24 Although it is difficult to achieve ambipolar transport from single-component active layers mainly due to the lack of electron conductance,^14,25,26 a few examples were, subsequently, proved to conduct both holes and electrons.¹¹ The obstacles to electron conductance have been found to be associated with high electron injection barriers^3,14 and the electron traps at the semiconductor–dielectric interface.¹³ By reducing the electron injection barrier using either narrow band-gap semiconductors^10,11,27,28 or low work-function electrodes,^9,29–33 ambipolar OFETs were greatly developed. Furthermore, eliminating the electron traps (such as hydroxyl groups) on the dielectrics resulted in the harvest of n-type behaviors.^{3,12,13,25,34} Currently, OFETs with the highest mobilities are fabricated from a low-band gap polymer with a hole mobility of 8.84 cm² V⁻¹ s⁻¹ and an electron mobility of 4.34 cm² V⁻¹ s⁻¹.²⁸ The progress of ambipolar OFETs highlights the effects of the electrode/semiconductor and semiconductor/dielectric interfaces on the charge transport. However, we envision that another possible source to reduce electron transport induced during device fabrication might be neglected. Solution processing^35–38 and solvent annealing^39,40 are widely used to fabricate high-performance OFETs and solvent residues are inevitably introduced to the semiconducting layers. These solvent molecules might potentially affect the charge transport. In this work, FETs were fabricated based on single-crystals of 6,13-bis(triisopropyl-silylethynyl)pentacene (TIPS-pentacene)⁴¹ grown from a variety of solvents to examine solvent effects on the device performance. We found that, for this widely recognized p-channel material,^42–47 ambipolar transport in FETs using Au as both source and drain electrodes could be realized as various non-polar solvents were used.

TIPS-pentacene is soluble in various solvents,⁴¹ and thus was selected in this work to study the solvent effect. Single-crystals of TIPS-pentacene were prepared from six solvents including hexane, heptane, cyclohexane, dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃) and tetrahydrofuran (THF), using the droplet-pinned crystallization (DPC) method.^48,49 Divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB)-covered SiO₂/Si substrates were used to grow the crystals as BCB was found to efficiently eliminate the electron traps arising from the surface hydroxyl group on SiO₂.¹³ In all cases, well-aligned single-crystals were obtained, as shown by the optical microscopy (OM) images with crossed polarizers (Fig. 1A and B and S1†). The single-crystallinity was reconfirmed by selected-area electron diffraction (SAED) patterns. Similar SAED patterns with single sets of diffraction spots were found in the crystals from all the six solvents (Fig. 1E–G), indicative of the same crystallographic structure. The millimeter-long ribbon shaped crystals were 35–173 nm high and 3.3–43 μm wide. Steps were observed on the flat terraces (Fig. 1C and D), with a step height of 1.63 nm that is close to 1.65 nm of the (001) d-spacing. Nanoparticles, likely from impurities expelled by the growing crystals,⁵⁰ were found on the crystal surfaces, especially at the step edges (Fig. 1D). With these well-aligned crystals, FETs were constructed by depositing Au source and drain electrodes in a top-contact, bottom-gate configuration (Fig. 2C, inset), with a channel length (L) of 50 μm and width (W) of 1 mm. The charge transport characteristics were investigated in a N₂ atmosphere and the saturation regime mobility was calculated. As the crystals did not cover the whole channel region, the real W/L value was measured (Fig. 2F, inset) to calculate the mobility values.

Fig. 1 Morphologies and crystalline structures of well-aligned TIPS-pentacene crystals. (A–D) Morphologies of the crystals grown from hexane solutions: (A) an OM image of the crystals; (B) an OM image of the crystals between crossed-polarizers with uniform brightness throughout the crystals, indicative of single-crystallinity; (C and D) atomic force microscopy (AFM) images of the crystals. (E) An SAED pattern of the crystals grown from heptane. (F and G) SAED patterns of the crystals grown from CH₂Cl₂: (F) as-grown; (G) after being heated.

Fig. 2 FET characteristics of the crystals grown from hexane solutions. (A–D) Typical transfer and output characteristics of the FETs in operation modes of p-channel (A and C) and n-channel (B and D). Inset: a schematic representation of the FET configuration, where S is the source, D the drain and G the gate. (E and F) Histograms of the hole and electron mobility of 50 FETs. Inset: an OM image showing the method of W/L measurement. L was measured from the real channel length and W was measured from the contacting area of the crystals that cross the source (S) and drain (D) electrodes. W was calculated by the equation: W = ∑(W₁ + W₂)/2.

We first examined the FETs based on crystals grown from hexane. The typical transfer and output characteristics are shown in Fig. 2A–D, wherein ambipolar behavior is surprisingly observed. The typical output characteristics in both p-channel (Fig. 2C) and n-channel (Fig. 2D) operation modes exhibited excellent gate modulation. Histograms of both hole and electron mobility for 50 devices from 3 substrates are shown in Fig. 2E and F. For the hole transport, an average hole mobility (μ_h) of 2.2 ± 1.2 cm² V⁻¹ s⁻¹ (range: 0.41 to 5.4 cm² V⁻¹ s⁻¹, Fig. 2E), on-to-off current ratios (I_on/I_off) > 10³, and threshold voltages (V_T) between −66 and −87 V were obtained. For the electron transport, an average electron mobility (μ_e) of 5.3 × 10⁻³ ± 7.2 × 10⁻³ cm² V⁻¹ s⁻¹ (range: 5.5 × 10⁻⁵ to 2.4 × 10⁻² cm² V⁻¹ s⁻¹, Fig. 2F), I_on/I_off > 6, and V_T between 44 and 92 V were achieved. The large V_T is similar to the previous reports on ambipolar OFETs^10,28,29,33 and suggests the existence of traps. The variations of μ and V_T are mainly associated with two factors: first, the varied crystal thickness and width that will induce different contact resistances; second, the slightly different orientation of the crystals with respect to the electrodes that leads to anisotropic transport. For a single device, mobilities of 5.0 cm² V⁻¹ s⁻¹ for hole transport and 0.020 cm² V⁻¹ s⁻¹ for electron transport were obtained. The hole mobility obtained here is among the highest values for unstrained TIPS-pentacene crystals,^42,44,50,51 while the electron transport has not been previously observed as Au is used as both source and drain electrodes. This observation is obviously different from the n-channel behavior reported before where the crystals were interfaced with a low-work-function metal electrode.⁵² One possible explanation for this unprecedented electron transport is that the TIPS-pentacene crystals might be n-doped by unknown impurities (e.g., from solvent, chemical or substrate).^52–54 However, as n-doped,⁵² the crystals exhibited electron transport but their hole transport was suppressed remarkably. The emergence of the electron transport with a concomitant unsuppressed hole mobility in the current work implies a different mechanism that is further studied below.

Another possible explanation for the electron transport in hexane-grown crystals is solvent polarity. Polar molecules were previously shown to interfere with the charge transport,^55–58 while the non-polar ones were not.^44,56,59 We hypothesized that the non-polar nature of hexane was associated with the emergence of electron transport. Accordingly, we examined two more non-polar solvents and three polar solvents. 50 devices based on crystals grown from heptane, a non-polar solvent, were studied in the n-channel operation mode. Clear electron transport was observed from both the transfer and output characteristics (Fig. 3A and C). A μ_e of 1.8 × 10⁻² ± 1.6 × 10⁻² cm² V⁻¹ s⁻¹ (range: 4.0 × 10⁻⁴ to 7.3 × 10⁻² cm² V⁻¹ s⁻¹, Fig. 3E), I_on/I_off > 10, and V_T between 64 and 90 V were obtained. Similarly, FETs based on crystals grown from cyclohexane, also a non-polar solvent, exhibited electron transport (Fig. 3B and D). A μ_e of 3.2 × 10⁻² ± 2.9 × 10⁻² cm² V⁻¹ s⁻¹ (range: 3.3 × 10⁻³ to 1.3 × 10⁻¹ cm² V⁻¹ s⁻¹, Fig. 3F), I_on/I_off > 10, and V_T between 53 and 90 V were obtained, as summarized in Table 1. In addition to the electron transport, these devices still maintained their hole transport properties (Fig. S2 and S3†).

Fig. 3 FET characteristics of the crystals grown from heptane (A, C and E) or cyclohexane (B, D and F) solutions. (A–D) Typical transfer (A and B) and output characteristics (C and D) of the FETs in the n-channel operation mode. (E and F) Histograms of the μ_e of 50 FETs. Typical transfer, output characteristics in the p-channel operation mode and histograms of the μ_h of these FETs are shown in Fig. S2 and S3.†

Table 1 Electron transport performance of TIPS-pentacene^a

Solvents	μ e (cm^2 V^−1 s^−1)		I on/Ioff	V T (V)
	Average	Max
a Measured in a glovebox. As the devices were exposed to air, the electron transport behavior disappeared because the electrons were trapped by oxygen and moisture.
Hexane	5.3 × 10^−3 ± 7.2 × 10^−3	2.4 × 10^−2	>6	44 to 92
Heptane	1.8 × 10^−2 ± 1.6 × 10^−2	7.3 × 10^−2	>10	64 to 90
Cyclohexane	3.2 × 10^−2 ± 2.9 × 10^−2	1.3 × 10^−1	>10	53 to 90

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In sharp contrast to the three non-polar solvents, FETs based on the crystals grown from CH₂Cl₂, a polar solvent, showed no gate modulation in the n-channel operation mode (Fig. 4A and C, arrows). Similarly, crystals grown from the other two polar solvents, CHCl₃ and THF, exhibited no electron transport properties except that a very small number of devices (3 out of 50 devices) for the CHCl₃-grown crystals showed an electron mobility of ∼1.0 × 10⁻³ cm² V⁻¹ s⁻¹. As expected, these devices still worked in the p-channel operation mode (Fig. 4A, C and E and S4†). The distinct electron transport properties of the crystals grown from the six solvents indicate a significant solvent effect on charge transport. Clearly, electron transport in the crystals grown from polar solvents is suppressed. Because the same device configuration and the same semiconducting layer were used, the solvent effect should have originated from the solvent molecules themselves. We believe that the solvents affect the charge transport through their trace amount of residues in the crystal or at the crystal/substrate interface. Random electric fields associated with the residual polar solvent molecules introduce energetic disorder and positional disorder⁵⁵ in the TIPS-pentacene crystals and reduce their charge transport performance. The negative effect of polar molecules on p-channel FETs was observed previously^56,60 and was not appreciated for n-channel FETs. In order to prove the existence of the trace amount of solvents, we compared the FET performance between the as-prepared and heat-treated CH₂Cl₂-grown crystals. 31 devices were heated and tested after cooling down. More than half (17 out of 31) of the devices exhibited electron transport with the highest μ_e of 6.3 × 10⁻³ cm² V⁻¹ s⁻¹ (Fig. 4F). For a single device, their transfer and output characteristics before and after heat treatment clearly indicated a transition from an electron inactive channel to an electron transporting channel (Fig. 4A–D). In addition to the emergence of electron transport, the hole mobility increased for each heated device (Fig. 4E). Also, instead of heat treatment, keeping in a glovebox at room temperature for a week also provided the devices with originally non-existent electron transport ability. Because the heat treatment did not change the molecular arrangement in the crystals as shown by SAED patterns (Fig. 1F and G), the emergence of electron transport is attributed to the partial removal of polar solvent residues during heating or lastingly staying, and indirectly proves the existence of the trace amount of solvent molecules. In addition, the crystals were grown from mixed solvents of hexane and CH₂Cl₂ in different volume ratios. The electron transport was found to be gradually suppressed as the concentration of CH₂Cl₂ increased consistently with the proposed negative effect of polar solvent residues on the electron transport (Fig. S5†).

Fig. 4 FET characteristics of the as-prepared (A and C) and heat-treated (B, D and F) crystals grown from CH₂Cl₂ solutions. (A and B) Typical transfer characteristics of the FETs in the n-channel operation mode. (C and D) Output characteristics of the FETs in both p- and n-channel operation modes. (E) Histogram of the μ_h of 50 FETs without heat treatment (bottom) and the μ_h of 31 FETs with heat treatment (top). Before heating, a μ_h of 0.40 ± 0.30 cm² V⁻¹ s⁻¹ (range: 0.11 to 1.2 cm² V⁻¹ s⁻¹), I_on/I_off > 10³, and V_T between −41 and −77 V were obtained. After heating, a μ_h of 1.0 ± 0.50 cm² V⁻¹ s⁻¹ (range: 0.29 to 2.2 cm² V⁻¹ s⁻¹), I_on/I_off, > 10⁴, and V_T between −41 and −85 V were obtained. (F) Histogram of the μ_e of 17 FETs with heat treatment. A μ_e ranging from 3.3 × 10⁻⁵ to 6.3 × 10⁻³ cm² V⁻¹ s⁻¹, I_on/I_off > 10, and V_T between 62 and 90 V were obtained.

Conclusions

In summary, we demonstrated a solvent effect on the electron transport properties of TIPS-pentacene. As grown from non-polar solvents including hexane, heptane and cyclohexane, TIPS-pentacene crystals exhibited electron transport with an electron mobility up to 0.13 cm² V⁻¹ s⁻¹ in addition to hole transport. In contrast, the crystals grown from polar solvents (CH₂Cl₂, CHCl₃, THF) showed only hole transport except that a very small number (3 out of 150) of devices exhibited electron transport (mobility ∼ 1.0 × 10⁻³ cm² V⁻¹ s⁻¹). This discrepancy between polar and non-polar solvents suggests a dipolar effect of the residual solvents in the crystals on their charge transport. As such, this work provides an alternative source for charge transport reduction. Together with the widely used strategies to promote electron transport including reducing the charge injection barrier³ and eliminating the charge trapping,¹³ appropriate selection of solvents and/or additives should further harvest the electron behaviors of solution-grown organic thin films and crystals.

Acknowledgements

This work was supported by the 973 Program (2014CB643503), National Natural Science Foundation of China (51222302, 51373150, 51461165301, 91233114), Zhejiang Province Natural Science Foundation (LZ13E030002), and Fundamental Research Funds for the Central Universities. HLX is supported by the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract no. DE-AC02-98CH10886. YJL thanks China Scholarship Council (CSC) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4mh00211c

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