Thermally stable organic thin film transistors based on 2-(anthracen-2-yl)tetracene

Article abstract of DOI:10.1016/j.orgel.2020.105787

Organic semiconductors with high mobility and high thermal stability are of great importance for practical application of organic electronics. To explore new semiconductors by taking advantage of the intrinsic properties of tetracene molecule, herein, we report the design and synthesis of a novel p-type tetracene derivatives, 2-(anthracen-2-yl)tetracene (TetAnt). Top contact organic thin-film transistors (OTFTs) based on TetAnt show a hole mobility of up to 0.79 cm² V^?1 s^?1. In addition, a high mobility of ~0.4 cm² V^?1 s^?1 is maintained even after thermal stressed to a high temperature of 290 °C, indicating the excellent thermal stability of TetAnt.

Full text of DOI:10.1016/j.orgel.2020.105787

Journal Pre-proof
Thermally stable organic thin film transistors based on 2-(anthracen-2-yl)tetracene
Huinan Yu, Yang He, Yuting Wu, Yaowu He, Chao He, Hong Meng
PII:
S1566-1199(20)30173-7
DOI:
https://doi.org/10.1016/j.orgel.2020.105787
ORGELE 105787
Reference:
To appear in: Organic Electronics
Received Date: 8 February 2020
Revised Date: 25 April 2020
Accepted Date: 25 April 2020
Please cite this article as: H. Yu, Y. He, Y. Wu, Y. He, C. He, H. Meng, Thermally stable organic
thin film transistors based on 2-(anthracen-2-yl)tetracene, Organic Electronics (2020), doi: https://
doi.org/10.1016/j.orgel.2020.105787.
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Thermally Stable Organic Thin Film Transistors Based on
2-(anthracen-2-yl)tetracene
Huinan Yua, Yang Hea, Yuting Wub, Yaowu Heb, Chao Heb,* and Hong Meng b,*
a Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials
(IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials
(SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing
211816, P. R. China.
bSchool of Advanced Materials, Peking University Shenzhen Graduate School,
Shenzhen 518055, P. R. China.
*Corresponding Author: hechao@pkusz.edu.cn; menghong@pku.edu.cn
KEYWORDS: Tetracene; Anthracene; Organic thin film transistor; Thermal stability
ABSTACT: Organic semiconductors with high mobility and high thermal stability
are of great importance for practical application of organic electronics. To explore
new semiconductors by taking advantage of the intrinsic properties of tetracene
molecule, herein, we report the design and synthesis of a novel p-type tetracene
derivatives, 2-(anthracen-2-yl)tetracene (TetAnt). Top contact organic thin-film
transistors (OTFTs) based on TetAnt show a hole mobility of up to 0.79 cm2 V-1 s-1. In

addition, a high mobility of ~0.4 cm2 V-1 s-1 is maintained even after thermal stressed
to a high temperature of 290 °C, indicating the excellent thermal stability of TetAnt.
Graphical abstract

1. Introduction
Organic thin film transistors (OTFTs) have drawn much attention due to their
low cost, low-temperature fabrication, compatibility with flexible substrates, and the
ability to fabricate large area devices. The performance of OTFTs has been
continuously improved in recent years as a result of dedicated efforts [1-5]. Numerous
researches have been devoted to develop high mobility organic semiconductors.
Among them, organic semiconductors based on benzothieno [3,2-b] benzothiophene
(BTBT) [6-8] and polyacene compounds [9] such as anthracene [10-14], tetracene
[15-18] and pentacene [19-21] have been extensively investigated for p type organic
semiconductor materials.
Extension of π-conjugation and introduction of alkyl chains are two important
methods to tune the properties of organic semiconductors and boost the performance
of resulting devices. Among the most widely studied acene based compounds,
anthracene offers a wide band gap, high fluorescence quantum yield and high mobility.
Moreover, anthracene is a typical p-type organic semiconductor with a low carbon to
hydrogen ratio, a highly conjugated plane structure, good thermal stability. Aromatic
hydrocarbons such as thiophene, phenyl, naphthalene, and bithiophene are typically
introduced to connect with anthracene to expand the π-conjugated system and induce
potential versatility. The introduction of an aromatic substituent at the 2-position [22,
23] or 2,6-position [24, 25] can effectively improve the degree of conjugation and
planarity, thereby enhancing the device performance based on corresponding
compounds. For acene based compounds, it is found that as the number of benzene
ring repeating units increases, the highest occupied molecular orbital (HOMO) level
energy increases, which is beneficial for efficient hole injection [26]. That is partly the

reason why the pentacene and its derivatives show better semiconducting properties,
but their stability is inferior to tetracene and anthracene based counterparts [27].
Extensive efforts are also devoted to develop high mobility semiconductors based on
tetracene core. For instance, Meng et al reported that the hole transport in OTFT
devices can be improved by introduction of phenyl into tetracene, which results from
the increased electronic coupling between two neighboring molecules. Two p-type
tetracene derivatives, namely 2-(4-dodecyl-phenyl)-tetracene (C12-Ph-TET) and
2-phenyl-tetracene (Ph-TET) with hole mobilities of up to 1.80 cm2 V-1 s-1 and 1.08
cm2 V-1 s-1 were demonstrated, respectively, compared to only 0.12 cm2 V-1 s-1 for
tetracene [28].
Along with mobility, the device thermal stability is another concern for OTFTs,
which is of vital importance for their practical application. The thermal stability of
OTFTs is highly dependent on the thermal stability of the used organic
semiconductors. There are a few reports on the thermal stability of OTFTs containing
thermally stable organic semiconductors realized via oligomerization of appropriate
aromatic units [25, 29-31]. For example, Yokota et al reported a highly thermally
stable flexible OTFT based on a thermally stable organic semiconductor, DPh-DNTT,
which maintains 80% its original mobility upon heating to 250 °C [30].
Based on the above studies, we expect that bonding anthracene and tetracene
together may result in a high mobility and thermally stable p-type semiconductor
material. Considering all these aspects, herein, we design and synthesize a new
compound, namely 2-(anthracene-2-yl)tetracene (TetAnt). Its thermal, photochemical,
electrochemical, and charge transport characteristics as well as consequent impact on
the thermal stability of resulting OTFTs are systematically investigated. Modest

mobility approaching 1 cm2 V-1 s-1 and high device thermal stability are demonstrated
for the OTFTs based on TetAnt.
2. Results and discussion
2a. Synthesis
The synthetic route of TetAnt is depicted in Scheme 1. Phenylboronic acid and
naphthalen-2-ylboronic acid were bought from Energy Chemistry. 2-bromoanthracene
and 2-(anthracene-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane were synthesized
according to literature [32], and the 2-bromo-tetracene was synthesized according to a
delivered patent [33]. At last, TetAnt compound was obtained by Pd-mediated Suzuki
coupling and pure compound was gained by vacuum sublimation. The chemical
structure of TetAnt was confirmed by elemental analysis and mass spectrometry.
Scheme 1. Synthetic routes of compound TetAnt
2b. Thermal properties
The thermal properties of TetAnt were firstly examined by thermogravimetric
analysis (TGA). The results are shown in Fig. 1. TGA was measured under N2
atmosphere with a scanning rate of 10 °C/min. The decomposition temperature
(temperature corresponding to a 5% weight loss) is determined to be 428 °C.
Differential scanning calorimetry (DSC) analysis under N2 showed a reversible

process of melting and recrystallization, but unfortunately no endothermic or
exothermic peaks were found for TetAnt during scan between 25 °C and 380 °C even
after several trials (not shown here). The TGA and DSC results indicate the excellent
thermal stability of TetAnt, indicating that the introduction of tetraphenyl group
improves the thermal stability.
Fig. 1. Thermogravimetric analysis of TetAnt.
2c. Photochemical and electrochemical properties
The photochemical and electrochemical properties of TetAnt were investigated by
UV-Vis absorption spectroscopy and cyclic voltammetry (CV). The absorption spectra
of TetAnt in 1, 2-dichlorobenzene solution and in thin film are shown in Fig. 2a. As
seen from the figure, the absorption spectra of TetAnt in solution and in thin film have
similar triple finger shape with the onset of 556 nm in solution and 566 nm in thin
film, respectively. The redshift of 10 nm and an enhanced ratio of the first vibration
peak intensity compared with the second one indicate clearly J-aggregation in the thin
film. The absorption onset of 566 nm for TetAnt in thin film gives an estimated
optical bandgap of 2.19 eV.

CV measurement of TetAnt thin film was performed to calculate its ionization
potentials. Since the irreversible reduction behavior (Fig. 2b), only HOMO level
could be estimated. According the onset of first oxidation potential of TetAnt, HOMO
level is calculated to be -5.51 eV. Combined the values of optical band gap and the
HOMO level, the LUMO level is calculated to be -3.32 eV for TetAnt. The
introduction of tetraphenyl groups has undoubtedly narrowed the bandgap and raised
the HOMO level.
Fig. 2. (a) Normalized UV-vis absorption spectra of TetAnt in solution and thin film; (b) CV curves of
TetAnt. The oxidation potential is obtained at the onset of the cyclic voltammogram.
2d. Single crystal structure and calculated mobility
High quality single crystals of TetAnt were grown by physical vapor transport
method for single crystal structure analysis. The single crystal structure of TetAnt
was determined by X-ray diffraction. X-ray crystallographic results (CCDC 1982781)
reveal that TetAnt crystals belong to P1 space group with crystal parameters of a=
5.9760(2) Å, b= 7.4945(2) Å, c= 21.8325(7) Å and α= 83.770(3)°, β= 89.241(3)°,
γ =89.720(3)°. The molecular length of TetAnt is 20.495 Å (Fig. 3a). The torsion
angle between the central anthracene and tetracene rings is below 1°. The TetAnt
molecules adopt a typical arrangement of herringbone packing (Fig. 3b). Moreover,
several C-H-π interactions with distance ranging from 2.963 Å to 3.152 Å are

observed between each TetAnt molecule and its nearest six neighbor molecules (Fig.
3c). High charge carrier mobility is expected from such a dense packing structure. So,
we calculated the mobility of TetAnt using Gaussian 09 D01 version software, with
B3LYP(D3) functional and 6-311G(d, p)2 basis set, based on the single crystal
structure. When one TetAnt molecule is chosen as the center, the charge transfer
integrals for the surrounding six molecules are between 34.21 meV and 68.26 meV
(Fig. 3b). In addition, the reorganization energy of TetAnt is 92.7 meV. The
calculated hole mobility of TetAnt is 1.22 cm2 V-1 s-1, much higher than that of
2-anthryl-2-anthracence (2Ant, 0.52 cm2 V-1 s-1) as listed in Table S1 [34].
Fig. 3. (a) Molecular length of TetAnt, (b) herringbone packing and (c) short contacts in TetAnt.
2e. Charge transport properties
We fabricated bottom-gate top-contact OTFT devices based on TetAnt by
thermal evaporation under N2 to evaluate their charge transport properties. Different
substrate temperatures (Tsub) were chosen to optimize the device performance. The
fabrication and characterization details are descripted in the Experiment part. The
device parameters, including mobility (μ), threshold voltage (VTH), and the on/off
current ratio (Ion/Ioff) are summarized in Table 1. Representative transfer and output
curves are shown in Fig. 4.

Table 1. Performance of OTFTs based on TetAnt deposited at different substrate temperatures.
Typical p-type transport properties are observed for all devices. As seen from
Table 1, increasing the substrate temperature helps a lot to improve the device
performance, with the mobility increases by twenty times from Tsub of 30 ºC to Tsub of
120 ºC (the highest value ~0.8 cm2 V-1 s-1). A modest mobility value higher than 0.15
cm2 V-1 s-1 even maintains at Tsub of 150 ºC. The saturation current increases with the
mobility as evidenced by the transfer and output curves shown in Fig. 4. Negative
differential resistance are observed in the saturation part of some output curves, which
probably results from hole traps induced by penetration of moisture to the conducting
channel during measurement. The on/off is high than 105 for all devices thanks to the
suppression of the off current. However, the threshold voltages are not satisfactory,
which probably due to the mismatch of the HOMO level of TetAnt (-5.51 eV) with
the work function of gold (5.1 eV).
Fig. 4. (a-e) Representative transfer curves of TetAnt OTFTs deposited at 30 °C, 60 °C, 90 °C, 120 °C,
150 °C, respectively; (f-j) corresponding output curves.
2f. Morphological characterization

To understand the relationship between the device performance and the
morphology, atomic force microscope (AFM) and X-ray diffraction (XRD) were
performed to analyze TetAnt thin films with a thickness of ~40 nm deposited on the
octyltrichlorosilane (OTS)-treated n+-Si/SiO2 substrate at different substrate
temperatures. The AFM images as shown in Fig. 5 reveal that the grain size increases
with the substrate temperature. Thin films deposited at Tsub of 120 ºC (Fig. 5d) show
obviously larger grains (~1 μm) than those deposited at lower substrate temperature.
Noncontinuity and large surface roughness (RMS = 35.5nm) are observed for Tsub of
150 ºC which severely degrade the device performance. The AFM results indicate that
increasing the substrate temperature is beneficial for the surface morphology and
grain size of TetAnt thin films, which is consistent with the device performance.
However, the grain sizes is not large enough compared to BTBT derivatives. And
large surface roughnesses are observed in all TetAnt thin films, which should be the
main reasons for the relative low mobility of TetAnt based OTFTs. Since the initial
growth mode of the first several layers are important for the charge transport of
OTFTs, we also measured the AFM images of TetAnt layer with a thickness of ~3 nm
deposited at Tsub of 30 ºC and 120 ºC. The AFM images and the cross-section profiles
imply a layer by layer growth mode, but the high density of nuclei limits the size of
grains [35].

Fig. 5. (a~e) AFM height images of thin films (5μm x 5μm) of TetAnt deposited on OTS-treated
Si/SiO2 substrates at different substrate temperature.
Thin films of TetAnt with a thickness of ~40 nm were further investigated by
XRD to reveal their crystallinity and molecular orientations. As shown in Fig. 6, high
order reflection peaks assignable to (00l) are observed for all TetAnt thin films. The d
spacing value is ~2.22 nm, which is consistent with the molecular length of TetAnt
and c value of the single crystal parameters. This indicates an edge-on structure on the
substrate, which is beneficial for carrier transport. Furthermore, the position of
reflection peaks does not change much when the substrate temperature increases,
implying that increasing the substrate temperature does not severely change the
molecular packing. The intensity of diffraction peaks in XRD patterns of different
substrate temperatures increases with the substrate temperature below 120 ºC,
indicating an improvement in crystallinity. Both AFM and XRD results justify that
increasing the substrate temperature is beneficial for OTFT devices based on TetAnt
thin films.

Fig. 6. XRD patterns of TetAnt thin films deposited at different substrate temperatures.
2g. Thermal stability of OTFT devices.
We have performed thermal stress test using devices of TetAnt (Tsub = 120 ºC) to
evaluate the device thermal stability. The device was put on the heater all the time
during the measurement in air with a temperature increasing rate of 20 ºC/min.
Transfer curves were taken in situ 2min after the corresponding temperature was
reached. For organic semiconductors, prolonged heating, especially heating above the
glass transition temperature or the melting point, usually leads to morphology changes
and device performance degradation. According to TGA and DSC, TetAnt compound
has good thermal stability. Good thermal stability is also expected for its OTFT
devices. As shown in Fig. 7a, device based on TetAnt maintains half of its initial
value even at a high temperature of 290 ºC, though it shows a decrease tendency as a
function of the temperature. Transfer curves shift to positive direction as shown in Fig.
7b. Devices show no performance when the temperature is higher than 290 ºC due to
the serious channel damage caused by continuous annealing (Fig. S2).

Fig. 7. (a) Mobility of OTFT devices based on TetAnt in situ measured at different temperatures; (b)
Transfer curves of TetAnt based OTFTs in situ measured at different temperatures.
3. Conclusion
To conclude, we have designed and synthesized a novel p-channel tetracene
derivative, TetAnt. Increasing the substrate temperature is beneficial for OTFTs based
on TetAnt and a high hole mobility up to 0.79 cm2 V-1 s-1 is demonstrated at Tsub of
120 ºC. Moreover, OTFTs based on TetAnt show high thermal stability up to 290 ºC.
Our results here indicate TetAnt is a promising candidate for practical applications in
future organic electronics.
4. Experimental section
4a. Synthesis and characterization
2-Bromoanthraquinone [32] In a 250 ml round-bottomed flask, anhydrous
acetonitrile (75 mL), copper (II) bromide (15.00 g, 67.1 mmol), isoamyl nitrite (9 mL,
67.1 mmol) were added at 0 °C and stirred for 20 minutes. Then the mixture was
warmed to RT and stirred for one hour. And then cooled to 0 °C,
2-aminoanthraquinone (7.5 g, 33.5 mmol) dissolved in THF (100 mL) was added
quickly to the resulting solution and the mixture was stirred at 0 °C for 2 h. After the
reaction was completed, the solvents were removed by rotary evaporation and dark

brown solid was obtained and then was purified using column chromatography to
obtain 2-bromoanthraquinone as a yellow solid (yield: 30%, 3.00 g).
2-Bromoanthracene A part NaBH4 (4.69 g, 123.9 mmol) was added to a solution of
2-bromoanthraquinone (5.95 g, 20.72 mmol) in a mixed solvent of isopropanol and
THF (1: 1, 150 mL) at 0 °C and the mixture was stirred for 3 hours. Then the mixture
was warmed to RT and another NaBH4 (2.34 g, 62.3 mmol) was added. The mixture
was stirred at RT overnight, and then DI water (7 mL) was added. The resulting
mixture was stirred for another 12 hours. After the solvents were removed under
reduced pressure, HCl (3M) was slowly added the obtained residue until foaming
ceased. The resulting solution was stirred under reflux for 6 hours. The resulting
suspension was filtered, and the residue was purified using column chromatography to
obtain the title compound (yield: 21%, 1.5 g).
2-(Anthracene-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane A 500 mL round
bottom flask was charged with (12.10 g, 36 mmol) 2-dibromoanthracene, (10.98 g,
43.2 mmol) bis(pinacolato)diboron, (7.21 g, 73.2 mol) potassium acetate and 2% mol
[1,1'-bis(diphenylphosphino)ferrocene]dichloropalladium(II);
complex
with
dichloromethane. Then 250 mL dimethyl sulfoxide was added to the mixture and the
suspension was purged with nitrogen for 15 minutes. The resulting mixture was
stirred at 95 °C for 12 hours. The mixture was extracted with dichoromethane and the
organic layer was dried with anhydrous MgSO4 after poured into 500 mL ice water.
The residue obtained by evaporated the solvent was purified by column
chromatography to give bright yellow solid. (yield: 50%, 9.76 g).
4-Bromo-1,2-bis(dibromomethyl)benzene Under UV light, bromine (2.73 mL, 53
mol) was slowly added to 4-bromo-o-xylene (2.50 g, 9.0 mol) which was dissolved in

30 mL CCl4 in a two-necked round bottom flask. Then the reaction mixture was
stirred at RT for one hour. After the reaction was completed, the reaction mixture was
washed with water and concentrated under reduced pressure. The precipitate was
filtered off and washed with hexane and get a white solid after dried in
drying box (2.68 g, 56%)
a vacuum
8-Bromotetracene-5,12-dione [33] In
a
500 ml round bottom flask,
4-bromo-1,2-bis-dibromomethylbenzene (10.00 g, 20 mol), 1,4-naphthoquinone
(3.15g, 20 mol), and NaI (34.40 g, 230 mmol) in 150 ml DMAc were added. The
mixture was refluxed overnight. After cooling down, the reaction mixture was poured
into 250 ml water. The precipitate was filtered off and washed with MeOH twice and
then purified by vacuum sublimation to give the pure product as a yellow solid (1.80 g,
26%)
2-Bromotetracene Al wire (6.57 g, 240 mmol), HgCl2 (0.13 g, 480 mmol),
cyclohexanol (150 ml) and CBr4 (0.64 g, 1.90 mmol) were added into a 300 ml flask.
The mixture was purged with nitrogen for 15 minutes. The reaction was initiated by
heating and when the reaction is too intense, one can cool it to slow down the reaction,
and then completed after refluxing for
4
hours. Then added
8-bromo-naphthacene-5,12-dione (9.54 g, 24 mmol) into the mixture and was
refluxed for 2 days. After cooling down the mixture was poured into MeOH/H2O/conc.
HCl solution (1/1/1, 400 ml). The precipitate was filtered off and washed with
MeOH/H2O/conc. HCl and methanol. The crude product was purified by vacuum
sublimation to give pure product as an orange solid. (6.78 g, 78%)
2-(Anthracene-2-yl)tetracene Refer to the synthesis steps of 2,2'-bianthracene, Yield:
40%. Because of the low solubility in solvent, the NMR was not measured. HRMS.

(MALDI) m/z: calcd for C32H20 [M+H]+ 404.51, found 404.6796. Anal. Calcd for
C32H20; C, 95.05; H, 4.95;. Found C, 93.04; H, 5.41.
4b. General procedures and experimental details
Fabrication and characterization of OTFT devices. Organic thin film transistors
were fabricated based on n+-Si/SiO2 (300nm) substrate with a top-contact structure.
The substrates were firstly ultrasonically washed with acetone, deionized water and
isopropanol for 15min, sequentially. After dried with N2 flow, the substrates were
exposed to UV/Ozone treatment for 15min and then treated with OTS in N2 golvebox.
For OTS treatment, the substrates were immersed in the OTS solution (25mg/mL in
toluene) for one and a half hours at 60 °C, and then washed with pure toluene.
Organic semiconductor layer (~40 nm) was deposited on the OTS treated n+-Si /SiO2
substrates by thermal evaporation in high vacuum at various substrate temperatures,
and then the gold source and drain (~40 nm) were thermally deposited on the organic
semiconductor layer under a pressure of about 6×10-5 Pa through shadow masks. The
channel width to length ratio is 10. The electrical characterization of the OTFTs was
performed by Keysight B1500A parameter analyzer on a probe station in ambient air.
The hole mobility was extracted from the equation (VDS = −40V), IDS = (W/2L) μ Ci
(VGS-VTH)2, where IDS is the source-drain current, W is the channel width and L is the
channel length, Ci is the capacitance per unit area of the SiO2 gate dielectric (1.12×10−4
F/m2), VGS is the gate voltage and VTH is threshold voltage.
Cyclic voltammetry TetAnt thin films were deposited onto platinum electrode by
thermal evaporation. To detect the HOMO and LUMO levels, 0.1 mol L-1

tetrabutylammonium hexafluorophosphate (Bu4NPF6) anhydrous acetonitrile (CH3CN)
solution was prepared. The CV data were acquired using Ag/Ag+ as the reference
electrode (+0.08 V vs. Ag/Ag+ nonaqueous reference electrode) at a scan rate of 100
mV s-1 under nitrogen atmosphere. The Ag/Ag+ electrode was prepared by soaking Ag
wire into a 0.01 mol L-1 AgNO3 solution in dry CH3CN and 0.1 mol L-1
tetrabutylammonium hexafluorophosphate (Bu4NPF6).
Acknowledgements
This work was financially supported by Postgraduate Research & Practice
Innovation Program of Jiangsu Province, Shenzhen Science and Technology research
grant
(JCYJ20170412151139619),
Shenzhen
Peacock
Plan
(KQTD2014062714543296), Guangdong Talents Project.
Appendix A. Supplementary data
Supplementary data related to this article can be found at XXX.
Notes
The authors declare no competing financial interest.
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Highlights:
ò
A high hole mobility of 1.22 cm2 V-1 s-1 is calculated and hole mobility up to 0.79
cm2 V-1 s-1 is demonstrated experimentally for TetAnt.
ò
Thin film morphology and performance of organic transistors based on TetAnt are
very sensitive to the substrate temperature.
Half of the original mobility is maintained even after thermal stressed up to 290 .
ò
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: