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Enhancing the Thermal Stability of Organic Field-Effect Transistors by Electrostaticaly Interlocked 2D Molecular Packing Zhi-Ping Fan, Xiang-Yang Li, Geoffrey E Purdum, Chen-Xia Hu, Xian Fei, ZiFa Shi, Chun-Lin Sun, Xiangfeng Shao, Yueh-Lin Loo, and Hao-Li Zhang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01579 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Chemistry of Materials

Enhancing the Thermal Stability of Organic Field-Effect Transistors by Electrostaticaly Interlocked 2D Molecular Packing Zhi-Ping Fan,† Xiang-Yang Li,† Geoffrey E. Purdum, ‡ Chen-Xia Hu,† Xian Fei,† Zi-Fa Shi,† Chun-Lin Sun, † Xiangfeng Shao,† Yueh-Lin Loo,*,§ and Hao-Li Zhang*,†,║ †

College of Chemistry and Chemical Engineering, Lanzhou University, State Key Laboratory of Applied Organic Chemistry (SKLAOC); Key Laboratory of Special Function Materials and Structure Design (MOE), Lanzhou, 730000, P. R. China. ‡ Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ 08544 , USA. § Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ 08544, USA. ║ Tianjin Key Laboratory of Molecular Optoelectronic Sciences, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Department of Chemistry, Tianjin University, Tianjin, 300072, P. R. China. ABSTRACT: Thermal instability is one of the major obstacles for the application of organic devices. We report herein for the first time that an interlocked two dimensional (2D) cross-stacking molecular arrangement can significantly enhance the thermal stability of solution processed organic field-effect transistor (OFETs) from small molecules. The 2,8-bis(butyl(methyl)amino)-indeno[1,2b]fluorene-6,12-dione (BMA-IFD) can form an unique 2D cross-stacking motif in the β-phase polymorph, in which the molecules are interlocked by the multiple intermolecular electrostatic interactions and steric hindrance from the terminal butyl moieties. Single-crystal OFETs from the β-phase polymorph exhibits maximum hole mobility up to 1.26 cm2V−1s−1, much higher than that of the α-phase crystals possessing one dimensional (1D) lamellar stacking motif (0.21 cm2V−1s−1). Thermal stability tests showed that the OFETs comprising β-phase crystals can maintain high mobility up to 140°C, which is in contrast to the low thermal stability of the α-phase devices. This work reveals that designing organic semiconductors with interlocked 2D cross-stacking molecular arrangement could be a new and efficient strategy to elevate the carrier mobility and thermal stability of solution-processed organic electronic devices.

Organic field-effect transistor (OFET) is one of the most important elements for future organic electronics, including various logic circuits1,2 and electronic skins.3 OFETs exhibit many attractive features, particular the capability to be fabricated via solution process atop large-area, flexible substrates.4,5 However, solution-processed OFETs generally degrade quickly at high temperatures due to several factors, such as thermally induced oxidation, changes in the crystal structure, change in film morphology, and mechanical stresses due to thermal expansion mismatches.6 Rational design of new heat-tolerant organic semiconductors plays the central role for the realization of thermally-stable OFETs. A common approach to enhance thermal stability of OFETs involves the use of highly conjugated rigid frameworks with shorter, or no, alkyl chains to strengthen the intermolecular interactions and raise the melting point.7-9 For instance, the maximum working temperature of the OFETs based on dinaphthothienothiophene (DNTT) derivatives can raise from 70°C to near 250°C by replacing the decyl with phenyl moieties on the core.7 However, an obvious limitation of such strategy is that these molecules with rigid framework generally possess poor solubility in common solvents and cannot be solution processed. To date, nearly all the small molecule OFETs reported to be thermally stable at temperature above 100°C were fabricated via vacuum deposition or sublimation methods (Table S1, Supporting Information).6-13 We propose that rational design of molecules to adopt interlocked packing

motifs could lead to soluble molecular semiconductors for thermal stable OFETs. To achieve this goal, deep insight into the correlation between the molecular packing of semiconductors and the thermal stability of devices is needed. Single-crystal field-effect transistors (SCFETs) comprised of different polymorphs provide an essential opportunity to quantitatively investigate the effects of molecular packing on device performance,14-16 since many interference factors like different molecular structures, grain boundaries and trap states can be largely ruled out.17,18 However, such research remains very challenging because the fabrication of SCFET devices from the specific polymorphs of a semiconductor is very difficult.19,20 For instance, although two polymorphs of the tetrathiafulvalene (TTF) was known since 1970’s,21 its phasedependent SCFET properties were only obtained recently.22 Many extensively studied organic semiconductors, such as several pentacene23-25 and benzothienobenzothiophenes (BTBT) derivatives,26-28 have been known to be polymorphic for quite long time, but their phase-dependent charge transport properties have yet been reported in SCFET devices.

Scheme 1. The chemical structure of the BMA-IFD.

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Herein, we report an interlocked 2D cross-stacking molecular arrangement to dramatically improve the thermal stability of solution processed SCFETs. Our investigation was based on the SCFETs comprising two distinct polymorphs of 2,8bis(butyl(methyl)amino)indeno[1,2-b]fluorene-6,12-dione (BMA-IFD, Scheme 1). The BMA-IFD has a rigid and highly conjugated core, which is chemically very stable under ambient conditions. Importantly, the molecule was designed with a donor-acceptor structure consisting of two electron donating butyl(methyl)amino groups and two electron withdrawing carbonyl groups, which form two positive and two negative electrostatic centers, respectively. The unique molecular structure not only enable the BMA-IFD to be processed in most organic solvents, but also allow it to assemble into either 1D lamellar stacking or interlocked 2D cross-stacking polymorphs depending on the solution processing conditions. Compared to the 1D lamellar stacking polymorph, the interlocked 2D crossstacking polymorph can significantly enhance the hole mobility and thermal stability of the SCFETs.

tal, which belongs to the triclinic system with P-1 space group, was assigned as the α-phase. The molecules in the α-phase crystal adopt a 1D lamellar-stacking (Figure 1c) with the π-π distance of 3.245 Å between the IFD planes (Figure 1d). In contrast, the flake-shaped crystal, which belongs to the monoclinic system with P21/c space group, was assigned as the βphase. As shown in Figure 1g and 1h, the molecules in the βphase crystal adopt a 2D crossed-stacking with the π-π distance of 3.306 Å between the IFD planes. It is noted that the alkyl moieties adopt different conformation in the two different polymorphs. In the α-phase crystal, the butyl chains are extended within the plane of the IFD core, providing little steric hindrance to prevent molecules slipping along their longitudinal direction (Figure 1d, Figure 2c). On the contrary, in the 2D cross-stacking β-phase crystal, the butyl chains stretch out of the IFD plane, giving significant steric hindrance to prohibit the molecules slipping along the longitudinal direction of the IFD skeleton (Figure 1h, Figure 2d). Importantly, the molecules in the β-phase crystal are interlocked by intermolecular electrostatic interaction. Figure 2a shows the electrostatic potential (ESP) calculated by density function theory (DFT) using Gaussian 09 package.30,31 The ESP result reveals that the negative and positive potentials are concentrated on the carbonyl and butyl(methyl)amino groups, respectively (Figure 2a and 2b). In the α-phase crystal, each molecule interacts with two adjacent molecules along the πstack direction (Figure 2c). The molecules slip along their long axis to optimize the electrostatic attractions between the carbonyl and butyl(methyl)amino groups and therefore form a 1D slipped-stacking motif. In contrast, in the β-phase crystal, the BMA-IFD molecules take two different orientations and adopt a 2D cross-stacking motif. In the 2D cross-stacking motif, each molecule interacts with four adjacent molecules with favorable multiple electrostatic attractions between the carbonyl and butyl(methyl)amino groups (Figure 2d). In addition to the steric hindrance effects discussed above, the multiple electrostatic attractions in the β-phase crystal significantly strengthen the interlocking effect in the 2D cross-stacking.

Figure 1. (a) The optical image and (b, c, d) crystal structure of the α-phase crystal. (e) The optical image and (f, g, h) crystal structure of the β-phase crystal. The molecules in c, g, h are colored differently only for clarity purpose.

The BMA-IFD was synthesized following our previously reported procedures;29 and the detailed synthesis (Scheme S1), optical and electrochemical properties (Figure S1) are available in the Supporting Information. The BMA-IFD possesses a band gap of 1.6 eV, and the highest occupied molecular orbital (HOMO) energy level of -5.0eV (Figure S1), which may allow efficient hole injection in device.29 When crystallized from different solutions by slow evaporation of solvents, interestingly, some ribbon or flake shaped single crystals were obtained (Figure 1a and 1e). The ribbon-shaped crystal was obtained from chloroform solution. In contrast, the flakeshaped crystal was obtained from xylene solution with concentration of 0.2 mg/ml. However, a mixture of the ribbon and flake shaped crystals are easy to obtain from xylene solution with concentration above 0.5 mg/ml. The ribbon-shaped crys-

Figure 2. (a) The calculated electrostatic potential of BMA-IFD; (b) a cartoon of the BMA-IFD compound with the main positive and negative electrostatic centers and side groups. Cartoon diagrams for the electrostatic interactions corresponding to the molecular packing of the (c) α-phase crystal and the (d) β-phase crystal.

Moreover, the stability of the two polymorphs was theoretically investigated by comparing their cohesive energies.32 The cohesive energy is the energy gained for per molecule by arranging the molecules in a crystalline state, as compared with


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Chemistry of Materials that in the gas state. Our calculations indicate that the α-phase and β-phase crystals possess cohesive energies of 442.1 kcal/mol and 504.5 kcal/mol, respectively. The significantly higher cohesive energy of the β-phase crystal can be attributed to that the interlocked crossed-stacking stabilizes the crystal structure. Therefore, it is reasonable to expect that the thermal induced structure distortion to the β-phase crystals shall be strongly inhibited than that in the α-phase crystals.

Figure 3. (a) The DSC curves of the two polymorphs with heating rates of 5°C/min. (b) XRD patterns of the α-phase polymorph as a function of annealing temperature between RT and 190°C. The insitu temperature-dependent Raman spectra of (c) the α-phase and (d) β-phase crystals.

The thermal stabilities of the two polymorphs were investigated by the differential scanning calorimetry (DSC). The αphase polymorph shows two exothermic peaks around 157°C and 168°C during heating and an endothermic peak around 66°C during cooling (Figure 3a). In contrast, the DSC of the β-phase polymorph exhibits an exothermic peak around 170°C during heating and an endothermic peak around 60°C during cooling. Figure 3b shows the X-ray diffraction (XRD) patterns of α-phase polymorph after being heated to different temperatures for six hours and then cooled to room temperature. The XRD patterns reveal that the α-phase polymorph is stable upon heating at 140°C or below, but transforms into the β-phase at temperatures above 155°C. Assisted by the XRD results, we assign the phase transition peak near 157°C on the DSC of the α-phase polymorph to the α-to-β polymorphic transformation. Meanwhile, the β-phase polymorph transforms into an unresolved meta-stable polymorph upon heating above 168°C, while the meta-stable polymorph reversibly transforms back into the β-phase polymorph when cooled below 66°C. Temperature-dependent in-situ Raman spectroscopy was utilized to study the changes in the BMA-IFD crystals during thermal treatment. The carbonyl Raman peak near 1700 cm-1 was selected to monitor the thermal stability of the crystals, in which the peak intensity at room temperature was used as the reference. For the α-phase crystal, the peak intensity quickly decreased to about 69% when the temperature was raised to 100°C (Figure 3c), indicating low crystal stability upon heating. In contrast, the intensity of carbonyl peak of the β-phase

crystal, at 1702.71 cm-1, only decreased 16% when heated to 100°C (Figure 3d), indicating good crystal stability. The microcrystals of the two polymorphs were synthesized atop silicon wafers for convenience of device fabrication (Figure S2). The transmission electronic microscopy (TEM) and corresponding selected-area electron diffraction (SAED) were performed to determine the molecular orientation in the microcrystals. As shown in Figure 4a and 4b, the α-phase crystal mainly grows along the (001) direction to form the nanoribbons, while the β-phase crystal mainly grows along both (001) and (010) directions to form the microsheets.

Figure 4. The SAED and TEM images of (a) the α-phase and (b) the β-phase crystals (the scale bar is 5µm). The transfer integrals of (c) the α-phase and (d) the β-phase crystals along the (001) directions. The molecules in e and f are colored differently only for clarity purpose. The optical image of the SCFET device based on (e) the α-phase and (f) the β-phase crystals.

To gain insights to the charge transport ability of the two polymorphs along the π-stack directions, we calculated the transfer integral between neighboring molecules in the two polymorphs by the Amsterdam Density Functional (ADF) software.33,34 The α-phase crystal shows a 1D charge transport pathway along the (001) direction, with transfer integral of 13.4 meV (Figure 4c). In contrast, the β-phase crystal exhibits a 2D charge transport network along the (001) direction, in which charge carriers can transport from one molecule to the two adjacent molecules (Figure 4d). The transfer integrals of the two transport channels along the (001) direction of the βphase crystal are calculated to be 10.4 and 10.3 meV, respectively. Although the transfer integrals of each transport channel in β-phase crystal is slightly smaller than that of the αphase crystal, the combination of two channels could facilitate charge transport better than the case with single channel. Bao et al have also indicated that the essentially equivalent electron couplings along two charge transport pathways could facilitate charge transport in OFETs.35 Therefore, the β-phase crystal is expected to possess higher charge transport ability than the α-phase crystal. The SCFETs comprising the two polymorphs were fabricated by the “organic ribbon mask” method.36 The optical images of the representative devices are shown in Figure 4e and 4f.



The SCFETs were measured in air, and the representative transfer and output curves are shown in Figure S3. The αphase crystals show the maximum and average hole mobilities of 0.21 cm2V−1s−1 and 0.10 cm2V−1s−1 for 25 devices, respectively. In contrast, the β-phase crystals exhibit the best hole mobility of 1.26 cm2V−1s−1, while the average hole mobility is about 0.43 cm2V−1s−1 for 40 devices.

phase crystal consists of molecules stacked in 1D lamellar structure, which exhibits moderate hole mobility and degraded quickly upon heating, just like most conventional organic semiconductors. In contrast, the β-phase crystal structure consists of a 2D cross-stacking structure, in which the molecules are interlocked by both steric hindrance and multiple electrostatic interactions. The interlocked 2D cross-stacking in β-phase results in a much higher cohesive energy, and helps to restrain molecular motion and prevent structural degradation during heating. SCFET measurements showed that the β-phase crystals exhibited much higher hole mobility than that of the αphase. Importantly, the β-phase crystal can maintain high hole mobility in air with operation temperature up to 140°C, in great contrast to the fast mobility drop of the α-phase crystal during temperature rising. Moreover, the hole mobility of βphase crystal even increases with temperature in the range below 90°C, confirming that the charge transport follows a thermal activated hopping mechanism. Our results reveal that designing interlocked 2D cross-stacking packing motif can be an efficient strategy to elevate the carrier mobility and thermal stability of solution-processed organic electronic devices.

ASSOCIATED CONTENT Supporting Information Figure 5. The transfer curves of the (a) α-phase and (b) β-phase SCFETs through the in-situ temperature-dependent device measurement. The relative hole mobility of the (c) α-phase and (d) βphase SCFETs measured at different temperatures. The lines serve only as guides to the eyes.

The temperature-dependent hole mobility of the two polymorphs were measured in-situ during the heating of the transistors. The transfer curves of the devices working at different temperatures are shown in Figure 5a and 5b, while the corresponding mobility change as a function of temperature are summarized in Figure 5c and 5d. It can be seen that the mobility of the SCFETs based on α-phase crystals showed an initial increase when the temperature raised from 25°C to 40°C and then a sharp decrease at temperatures above 40°C. For instance, relative to the hole mobility measured at room temperature, the hole mobility decreased to about 40% at 70°C, and 10% at 100°C. The dramatic mobility decrease is mainly attributed to the structural instability of the α-phase crystals, as well as the deterioration in electrical contact. In contrast to the fast mobility drop of the α-phase crystals, heating of the transistors from β-phase crystals significantly raised the carrier mobility. For instance, the hole mobility of the βphase SCFETs increased to 157% of the initial value at 90°C. Such mobility increase upon heating is attributed to a thermally activated hopping mechanism, in which higher temperature facilitate faster charge transport.37 Though further heating the device resulted in a mobility drop below the maximum, the hole mobility remained 70% of the initial value even when the temperature reaches 140°C. The experimental results confirm that the interlocked cross-stacking structure significantly enhance the thermal stability of the SCFETs from the β-phase polymorph, providing a wide working temperature range. In summary, our investigation on the temperature-dependent properties of the two polymorphs from molecule BMA-IFD has revealed the dramatic effect of molecular packing on the thermal stability of the corresponding OFET devices. The α-

The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental details, characterization data, NMR spectra, Figure S1−S3, and Table S1 (PDF) Crystallographic data in CIF format (ZIP)

AUTHOR INFORMATION Corresponding Author *Hao-Li Zhang, Email: [email protected] * Yueh-Lin Loo, Email: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (NSFC. 51733004, 51525303, 21572086, 21702085, 21673106, 21602093), Ministry of Science and Technology of China (2017YFA0204903), and 111 Project. Fundamental Research Funds for the Central Universities (lzujbky-2017-11 ， lzujbky-2017-109). The authors thank beam line BL14B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

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