Effects of p-(Trifluoromethoxy)benzyl and p-(Trifluoromethoxy

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Effects of p-(trifluoromethoxy)benzyl and p-(trifluoromethoxy)phenyl Molecular Architecture on the Performance of Naphthalene Tetracarboxylic Diimide-Based Air Stable n-Type Semiconductors Dongwei Zhang, Liang Zhao, Yanan Zhu, Aiyuan Li, Chao He, Hongtao Yu, Yaowu He, Chaoyi Yan, Osamu Goto, and Hong Meng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04753 • Publication Date (Web): 29 Jun 2016 Downloaded from http://pubs.acs.org on July 4, 2016

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ACS Applied Materials & Interfaces

Effects of p-(trifluoromethoxy)benzyl and p(trifluoromethoxy)phenyl Molecular Architecture on the Performance of Naphthalene Tetracarboxylic DiimideBased Air Stable n-Type Semiconductors Dongwei Zhanga, Liang Zhaob, Yanan Zhua, Aiyuan Lia, Chao Hea, Hongtao Yub, Yaowu Hea, Chaoyi Yana, Osamu Gotoa,*, Hong Menga,* a

School of Advanced Materials, Peking University Shenzhen Graduate School, Peking

University, Shenzhen, 518055, China. E-mail: [email protected] b

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China KEYWORDS Organic thin film transistor, air stable, n-type organic semiconductor, naphthalene diimide, trifluoromethoxy

ACS Paragon Plus Environment

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ABSTRACT

N,N’-bis(4-trifluoromethoxyphenyl) naphthalene-1,4,5,8-tetracarboxylic acid diimide (NDIPOCF3) and N,N’-bis(4-trifluoromethoxybenzyl) naphthalene-1,4,5,8-tetracarboxylic acid diimide (NDI-BOCF3) have similar optical and electrochemical properties with deep LUMO level of approximately 4.2 eV, but exhibit significant differences in electron mobility and molecular packing. NDI-POCF3 exhibits non-detectable charge mobility. Interestingly, NDIBOCF3 shows air stable electron transfer performance with enhanced mobility by increasing the deposition temperature onto the octadecyltrichlorosilane (OTS)-modified SiO2/Si substrates and achieves electron mobility as high as 0.7 cm2V-1s-1 in air. The different mobilities of those two materials can explain by several factors including thin film morphology and crystallinity. In contrast to the poor thin film morphology and crystallinity of NDI-POCF3, NDI-BOCF3 exhibits larger grain sizes and improved crystallinities due to the higher deposition temperature. In addition, the theoretical calculated transfer integrals of intermolecular lowest unoccupied molecular orbital (LUMO) of the two materials further show that large intermolecular orbital overlap of NDI-BOCF3 can transfer electron more efficiently than NDI-POCF3 in thin film transistors. Based on the fact that the theoretical calculations consistent well with the experimental results, it can be concluded that p-(trifluoromethoxy) benzyl (BOCF3) molecular architecture on the former position of naphthalene tetracarboxylic diimides (NDI) core provides more effective way to enhance the intermolecular electron transfer property than p(trifluoromethoxy) phenyl (POCF3) group for the future design of NDI related air stable n channel semiconductor.


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1. Introduction Organic thin film transistors (OTFTs) have attracted great attentions due to their considerable potential applications in organic electronics such as complementary circuits1,2, sensors3 and display backplanes4. In fact, the hole mobilities of p-channel organic semiconductors have been significantly improved5 beyond the hydrogenated amorphous silicon6,7. However, the development of n-type materials was far behind the p-type materials due to the difficulty of strong electron withdrawing group development8 and the instability in ambient air9-13. To design a molecule with high electron mobility and excellent air stability is still challenging for n-type organic semiconductors.

Naphthalene tetracarboxylic diimides (NDIs) is one of the typical imide-based core structures for the utilization of n channel organic semiconductors due to the simplicity of synthesis and high electron affinity14-16. Up to now, the highest electron mobility of NDIs has reached 6.2 cm2V-1s-1 in Ar17 and 3.5 cm2V-1s-1 in ambient air18. For the rational design of air stable n-type semiconductors based on NDI core, many efforts have been made such as investigating alkyl chain lengths18,19, and introducing fluoro19 and oxygenated groups20. The trifluoromethoxy group (OCF3) is considered to be a strong electron withdrawing and π donating substituent based on three fluorine atoms and oxygen lone pair21. As a practical matter, the introduction of p(trifluoromethoxy) benzyl (BOCF3) in NDI (Figure 1) can form air stable n-type semiconductor material with mobility of 3.58 x 10-2 cm2V-1s-1 without surface treatment in the air20,22. However, detailed studies on the effects of benzyl and phenyl groups have not been elucidated.


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Figure 1: Chemical structures of NDI-POCF3 and NDI-BOCF3 In this study, we intend to modify the molecular packing of NDI-BOCF3 by replacing the BOCF3 substituent with p-(trifluoromethoxy) phenyl (POCF3) substituent at the former position of NDI to obtain a new compound of NDI-POCF3 (Figure 1), and investigate the differences of those two materials systematically. Interestingly, the substituent variety slightly affects the electrochemical and optical properties (Table 1) of those two materials, but significantly changes the charge transfer and molecular packing characteristics. The OTFT performance of NDIBOCF3 could be improved drastically by increasing the deposition temperature during vacuum evaporation and by using octadecyltrichlorosilane (OTS)-modified SiO2/Si substrate. The highest electron mobility of 0.7 cm2V-1s-1 was obtained in air, which is approximately 20 times higher than previously reported data22. However, the OTFTs for NDI-POCF3 did not work. The reason was clarified by the analysis of thin films using X-ray diffraction (XRD) and atomic force microscope (AFM), and the calculation of intermolecular transfer integral of LUMO based on the single crystal data using amsterdam density functional (ADF).


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2. Experimental Section Synthesis. The detailed systhsis routes and chemicophysical characterizations of NDI-POCF3 and NDI-BOCF3 are decribed in the supporting information. OTFT Device Fabrication. Oganic thin-film transistors with top contact/bottom gate configuration were fabricated by the vapor deposition process. The heavily doped silicon wafers with oxide thickness of 300 nm were firstly cleaned with the acetone, deionised water and isopropanol in the ultrasonic bath, respectively. These substrates were then dried by the nitrogen gas and further cleaned in oxygen plasma for 15 min. It was then treated in 0.03 g/ml concentration of octadecyltrichlorosilane (OTS) solution in toluene at 60 oC for 20 min to form self-assembled monolayer. The OTS-modified SiO2/Si substrates were rinsed with fresh toluene in the sonic bath for 1 min and dried by the nitrogen gas. The organic active layer was then evaporated onto these substrates to thickness of approximately 35 nm under the pressure of 1 × 10-4 Pa for each substrate temperature condition. Gold source and drain electrodes (W/L = 10) were deposited on the organic active layer using a shadow mask. Electrical characteristics of the OTFT devices were measured by Agilent B1500A in the ambient air. The electron mobility (µe) was obtained from the saturation regime using the following equation23: I = C µ W⁄2L V − V

(1)

In the above equation, Ids, W, L, Vg Vth adn Ci represent the drain-source current, chanel width, chanel length, gate voltage, threshold voltage and the capacitance per unit area of the gate dielectric layer, respectively.


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3. Results and Discussion The electrochemical property and UV-vis absorption spectra of those two materials were analyzed in dichloromethane solution and in thin-film/solution (Figure 2). The related results are summarized in Table 1, and similar optical and electrochemical characteristics were observed. The absorption edge of NDI-BOCF3 and NDI-POCF3 were measured to be 416 nm and 411 nm in thin film and 390 nm and 389 nm in solution (Figure 2b). According to the optical characteristics of thin film, the band gap was calculated to be 2.98 eV and 3.02 eV for NDIBOCF3 and NDI-POCF3, respectively. Based on the calibration of redox potentials of an internal ferrocene reference (Fc/Fc+), the thermally evaporated thin films of those two materials on GCE electrode were prepared for the cyclic voltammograms (CV) measurements. According to the onset of CV reduction peaks, the deep LUMO levels of NDI-BOCF3 and NDI-POCF3 were estimated to be -4.22 eV and -4.17 eV, respectively. The low LUMO energy levels promise the candidates for the application of

n

channel OTFTs with excellent air stability24. Based on the

LUMO level and energy band gap confirmations, the highest occupied molecular orbital (HOMO) levels of NDI-BOCF3 and NDI-POCF3 were obtained to be -7.20 eV and -7.19 eV, respectively. In addition, the thermogravimetric analysis (TGA) shown good thermal stability with onset decomposition temperature up to 352°C and 394°C for NDI-BOCF3 and NDI-POCF3, seen in Figure S1.


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Figure 2: The electrochemical (a) and UV-vis absorption spectra (b) of NDI-POCF3 and NDIBOCF3 Table 1: Electrochemical and optical properties of NDI-POCF3 and NDI-BOCF3

1

E[onset, red vs. Fc/Fc+]

ELUMO

EHOMO

λthin-film

λsol

Eg

(V)1

(eV)2

(eV)3

(nm)4

(nm)5

(eV)6

NDI-BOCF3

-0.88

-4.22

-7.20

416

390

2.98

NDI-POCF3

-0.93

-4.17

-7.19

411

389

3.02

Estimated from electrochemical determination versus Fc/Fc+. 2ELUMO= -(E [onset, red vs. Fc/Fc+] + 5.1)

(eV)25. 3EHOMO= (ELUMO - Eg). 4Estimated from thin film absorption spectra. 5Solution absorption spectra in chloroform. 6Energy band gap calculated from the absorption edge of the as-cast thinfilm.


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Figure 3: Typical OTFT characteristics (transfer curve and output curve) for NDI-BOCF3 thin films grown on the OTS-modified SiO2/Si substrates. a) and b) were obtained at Tsub=RT. c) and d) were obtained at Tsub= 70oC. Interestingly, benzyl and phenyl related NDI-BOCF3 and NDI-POCF3 have similar optical and electrochemical characteristics, but presented significant differences in intermolecular electron transport. Figure 3 shows the typical transfer and output curves of OTFTs for NDI-BOCF3 deposited at substrate temperature of room temperature (Figure 3a and b) and 70 oC (Figure 3c and d), respectively. The detailed device performances were summarized in Table 2. As can be seen, the electron mobility of NDI-BOCF3 increased from 0.23 to 0.70 cm2V-1s-1 as the substrate temperature increased from RT to 70 °C, but decreased to 0.16 cm2V-1s-1 after the substrate


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temperature increased to 100 °C. In contrast, NDI-POCF3 based OTFTs was showed nondetectable charge mobility in spite of having the similar properties as NDI-BOCF3 (Table 1). Table 2. Device performance of NDI-POCF3 and NDI-BOCF3 Polycrystalline Thin film Device Tsub (oC)

Ave. Mobility (cm2/Vs)

Max. Mobility (cm2/Vs)

Ion/Ioff

Vth (V)

RT

0.22 ± 0.01

0.23

1.5x106

-4.4

50

0.28 ± 0.05

0.37

4.2x105

18.9

70

0.62 ± 0.06

0.70

3.9x106

21

100

0.15 ± 0.01

0.16

2.4x106

-2.1

RT, 70

—

—

—

—

NDI-BOCF3

NDI-POCF3

Ave.

In order to clarify the differences of the OTFT characteristics between NDI-BOCF3 and NDIPOCF3, we first analyzed the temperature-dependent crystal phase evolutions using thin-film Xray scattering (XRD). Figure 4a shows XRD patterns of NDI-BOCF3 thin films deposited on the OTS-modified SiO2/Si substrates at different temperatures. Sharp Bragg reflections up to third order were clearly seen, suggesting high degrees of crystallinity for NDI-BOCF3. Based on the single crystal data, the peak at 2θ = 4.54°corresponds to (100) plane and the other two peaks are successive orders of reflections, (200) and (400) planes, respectively. Thus, NDI-BOCF3 thin film has the d spacing distance (d100) of 1.95 nm, which is close to both of interplane distances of 2.09 nm in a-axis direction of the single crystal and the step height of approximately 2.0 nm obtained from AFM images. This result indicates that the a-axis direction of NDI-BOCF3 molecules (Figure 6a) were aligned nearly perpendicular to OTS-modified SiO2/Si substrate.


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Furthermore, the intensity of each peak increased gradually as the substrate temperature increased. This result suggests that the crystalline quality was improved, which is consistent with OTFT mobility results increase from room temperature to 70°C as shown in Table 2. In contrast, NDI-POCF3 thin film deposited at substrate temperature of 70°C showed two diffraction peaks at 2θ = 9.9° and 18.4° corresponds to (001) and (002) plane with weak intensity (Figure 4b), suggesting the poor crystallinity of the thin film in which results in non-detectable mobility of NDI-POCF3. Besides, the corresponding d spacing distance (d001) of the primary diffraction peak is calculated to be 0.98 nm, which is equal to the interplane distances of 0.98 nm in c-axis direction of the single crystal. This result may suggest that the c-axis direction of NDI-POCF3 (Figure 6c) crystallites in the thin film is perpendicular to the OTS-modified SiO2/Si substrate.

Figure 4: XRD patterns for (a) NDI-BOCF3 and (b) NDI-POCF3 polycrystalline thin films deposited on the OTS-modified SiO2/Si substrates by vacuum evaporator with various substrate temperatures.


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Temperature-dependent thin-film morphology of two materials was analyzed by AFM and exhibited quite disparate results. Figure 5a-d show NDI-BOCF3 thin films deposited at the substrate temperature of RT, 50, 70, and 100°C, respectively. Rod liked grains were observed for the NDI-BOCF3 thin films at all deposition temperatures. The grain size was increased from several hundred nanometers to approximately 2 µm by increasing the substrate deposition temperature from RT to 100 °C. This result supports our OTFT mobility observations (Table 2). In general, well defined thin film with large grain size and high crystallinity provides positive effect on the carrier mobility26-30, suggesting that the highest mobility should be obtained at substrate temperature of 100°C. However, the device performance decreased when the temperature was increased to 100°C. Based on the XRD and AFM results, the reason is possible that the larger grains with highly crystallinity formed at higher substrate temperatures impinged upon each other and generated voids which affects the continuity of the thin film31. In contrast, a completely different phenomenon was observed in the AFM images of NDI-POCF3 thin films, as shown in Figure 5e-f. The grain size did not change after increasing the substrate temperature. Besides, a higher RMS roughness ~ 15 nm (Figure 5e) was detected which is approximately five times larger than that for NDI-BOCF3 thin film at the same substrate temperature (Figure 5a). The thin-film with smaller grain size and large RMS roughness were found to be detrimental to the device performances26,30. The XRD and AFM results suggest that NDI-POCF3 thin films have a different crystal structure from NDI-BOCF3 thin films. The electron mobility of NDIBOCF3 is expected to be higher than that of NDI-POCF3 due to the well-defined thin film morphologies with enhanced crystallinities.


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Figure 5: AFM images of NDI-BOCF3 (a-d) and NDI-POCF3 (e-f) based polycrystalline thinfilm on the OTS-modified SiO2/Si substrate at various substrate temperatures. In order to further understand the effect of BOCF3 and POCF3 molecular architecture on the performance of NDI based semiconductors, the intermolecular transfer integrals were processed by importing the single crystal data in a modelling software, namely, amsterdam density functional (ADF)32 which uses density function theory for the calculation of principle electronic structure. The electronic couplings (V)33 between neighbouring molecules in the single crystal can be calculated to predict the benefit of BOCF3 and POCF3 substitutions at former positions of NDI. It was performed by generalized gradient approximation with the PW91 functional (GGA:PW91) and the basis set of triple-Z 2 plus polarization functions (TZ2P) in the ADF program32,34,35. However, the single crystal data of NDI-POCF3 and NDI-BOCF3 were required


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for the calculation of transfer integrals of LUMO. Thus, the single crystals of those two materials were grown by the physical vapour transport method36 and analyzed by the x-ray crystallographic. The detailed molecular packing of those two materials was shown in Figure 6.

As expected, significant differences of molecular packing and intermolecular relationship are observed for those two materials. For the NDI-BOCF3, the interplane distances of a = 20.905 Å, b = 4.649 Å and c = 14.258 Å were demonstrated by the x-ray crystallographic. The short interplane distances provide strong face-to-face intermolecular interactions and large transfer integral of LUMO. Therefore, the strongest transfer integral of LUMO was calculated to be 79.7 meV in b-axis direction for NDI-BOCF3 (Figure 6b). Besides, the opposite directions between two neighbouring molecules with dihedral angle of approximately 95° were investigated and caused the herringbone-liked packing structure (Figure 6b). The intermolecular orbital overlaps of LUMO between two opposite neighbouring molecules in this direction were calculated to be 14.1 meV (Figure 6a,b), which indicates a one-dimensional (1D) electronic structure for NDIBOCF3. In contrast, an approximately parallel molecular orientation in different molecules (Figure 6d) was demonstrated for NDI-POCF3. The interplane distances of NDI-POCF3 were found to be a = 5.122Å, b = 24.503 Å and c = 9.786 Å, respectively. The largest transfer integral of NDI-POCF3 in a-axis direction was calculated to be 42.9 meV, which is smaller than NDIBOCF3. Large intermolecular orbital overlaps are good for the efficient carrier transport in OTFTs37. In addition, the transfer integrals of LUMO in other directions were close to 0 meV (Figure 6c and e), which leads to highly 1D electronic structure. However, highly 1D electronic structure of active materials is not preferred in OTFTs due to the arbitrary orientations of the crystalline grains in the thin-film and the incapability of the highly interactive electronic


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structure over the thin film38,39. Based on the thin film characteristics and the calculated transfer integral of LUMO, we can conclude that NDI-BOCF3 can transport electron more efficiently than NDI-POCF3 in OTFTs. The results are consistent with our measurements of OTFT devices.

NDI-BOCF3 based OTFTs exhibited excellent device performances in the air. The BOCF3 group not only protects the molecular center from oxygen and moisture20 due to the perfluorinated groups, but also provides a strong electron withdrawing and π donating substituent based on three fluorine atoms and oxygen lone pair21. In addition, it also provides more effective way to control overall thin film morphology, crystalline quality and molecular packing than POCF3 group in NDI, leading to an excellent electron mobility in n-channel OTFTs. Moreover, the introduction of POCF3 group into NDI decreases the intermolecular transfer integral of LUMO, which should be the main factor causing the lack performance of NDI-POCF3. In special, the calculated transfer integrals of LUMO are negligible (~ 0 meV), except in the direction of close face-to-face intermolecular interaction. It not only causes highly 1D electronic structure, but also leads to inefficient electron transport in OTFTs. Furthermore, highly 1D electronic structure will further leads to arbitrary orientation of crystallites in thin film, which results in negative effects on the film morphology and device performance37,39. The results indicate the great potential of BOCF3 group on the future design of NDI related air stable n channel semiconductors.


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Figure 6: The molecular packing structure and intermolecular transfer integrals of LUMO in the single crystal of NDI-BOCF3 (a-b) and NDI-POCF3(c-e).

4. Conclusions In this study, the effect BOCF3 and POCF3 molecular architecture on the performance of NDI based n channel semiconductor has been systemically studied. The substituent variety of those two materials only has a slight effect on the electrochemical and optical properties, but significant changed the intermolecular electron transport and molecular packing behaviors. NDIPOCF3 thin films showed poor film morphology and crystallinity with non-detectable mobility.


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In contrast, NDI-BOCF3 thin films exhibited good film morphology with increased grain size and crystalline quality by increasing the substrate temperature. In particular, the excellent electron mobility of up to 0.7 cm2V-1s-1 has been achieved through the deposition of NDI-BOCF3 on OTS-treated substrate with temperature of 70 °C. Furthermore, the electronic structure in solid state can efficiently explain the differences in device characteristics. The theoretical calculations based on single crystal data revealed that the intermolecular orbital overlaps of NDIBOCF3 were larger than NDI-POCF3. From the results, we concluded that BOCF3 substitution at former position of NDI are a type of promising molecular modification for the further development of NDI related materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.” The systhsis routes of NDI-POCF3 and NDI-BOCF3, the chemicophysical characterizations including 1H,

13

C NMR spectroscopy, EI-mass spectroscopy, elemental analysis and TGA

measurements.

AUTHOR INFORMATION Corresponding Author *Hong Meng. Email: [email protected] *Osamu Goto. Email:[email protected]


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ACKNOWLEDGMENT This work is supported by Shenzhen Key Laboratory of Shenzhen Science and Technology Plan (ZDSYS20140509094114164), Guangdong Talents Project, NSFC (51373075), National Basic Research Program of China (973 Program, No. 2015CB856505; 2015CB932200), The National Research

Foundation

(20133221110004),

The

Shenzhen

Peacock

Program

(KQTD2014062714543296), Provincial Science and technology project of Guangdong Province (2015B090914002; 2014B090914003; 2013B090400016).

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Effects of p-(Trifluoromethoxy)benzyl and p-(Trifluoromethoxy

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