Supplementary Materials for · measurements of the physicochemical properties and device...

advances.sciencemag.org/cgi/content/full/4/2/eaao5758/DC1

Supplementary Materials for

Wafer-scale, layer-controlled organic single crystals for high-speed

circuit operation

Akifumi Yamamura, Shun Watanabe, Mayumi Uno, Masato Mitani, Chikahiko Mitsui, Junto Tsurumi,

Nobuaki Isahaya, Yusuke Kanaoka, Toshihiro Okamoto, Jun Takeya

Published 2 February 2018, Sci. Adv. 4, eaao5758 (2018)

DOI: 10.1126/sciadv.aao5758

This PDF file includes:

section S1. Materials

section S2. Details of transport measurements

section S3. Characterization of single crystals

section S4. Details of high-frequency measurements

fig. S1. Scheme for the synthesis of C8-DNBDT-NW.

fig. S2. Phase transition temperature and melting point of C8-DNBDT-NW.

fig. S3. Structure, transfer integrals, and effective masses of Cn-DNBDT.

fig. S4. Crystal packing structure of C8-DNBDT-NW.

fig. S5. Effect of thermal annealing on the transfer characteristics of 2L C8-

DNBDT-NW FET.

fig. S6. Device characteristics of 1L C8-DNBDT-NW FET.



fig. S9. Estimation of contact resistance by the gFPP method.

fig. S10. Transfer characteristics of 2L-OFETs from TLM measurements.

fig. S11. Transfer characteristics of 3L-OFETs from TLM measurements.

fig. S12. Channel length dependence of two-terminal mobility in 2L- and 3L-

OFETs.

fig. S13. Typical example of a wafer-scale C8-DNBDT-NW single crystal.

fig. S14. Observation of molecular step at domain boundaries.

fig. S15. Raman spectroscopy measurements of 1L, 2L, and 3L crystals of C8-

DNBDT-NW.

fig. S16. Simulation of Raman peaks for crystals of C8-DNBDT-NW.

fig. S17. Sample preparation for TEM measurements.

fig. S18. SAED patterns taken for 1L crystalline domains at different positions.

fig. S19. SAED patterns taken for 2L crystalline domains at different positions.

fig. S20. Device characteristics of a short-channel device.

fig. S21. Dynamic response of a short-channel 2L-OFET.

table S1. Solubility of Cn-DNBDT.

table S2. Phase transition temperature and melting points of Cn-DNBDT.

table S3. Crystal data for C8-DNBDT-NW.

table S4. Comparison of cutoff frequency.

References (38–48)

section S1. Materials

Synthesis of C8–DNBDT–NW

3,11-Diiododinaphtho[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene (I–DNBDT) as a key

precursor was synthesized from commercially available 2,7-dibromo-naphthalene by a 5-step

process, as illustrated in fig. S1. A Negishi coupling reaction was used with I–DNBDT treated

with n-octyl zinc chloride in the presence of a palladium catalyst to afford the target

compound C8–DNBDT–NW as a yellow solid in 87 % yield. C8–DNBDT–NW was purified

by multiple recrystallization and sublimation procedures to obtain a device-grade sample.

fig. S1. Scheme for the synthesis of C8-DNBDT-NW.

Zinc chloride in tetrahydrofuran (THF; 1.0 M, 900 L, 0.900 mmol, 3.0 mol amt.) and lithium

chloride in THF (0.5 M, 1.80 mL, 0.900 mmol, 3.0 mol amt.) were successively added to n-

octylmagnesiumbromide (2.0 M in Et2O, 450 L, 0.900 mmol, 3.0 mol amt.) in toluene (15

mL) at 0°C and stirred for 10 min. 3,11-diiododinaphtho[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-

b’]thiophene (193 mg, 0.300 mmol) and [1,1’-bis(diphenylphosphino)ferrocene]

dichloropalladium(II) dichloromethane adduct (9.8 mg, 0.012 mmol, 4 mol%) were then

added to this solution. After stirring at 110°C for 12 h, the reaction mixture was cooled to

room temperature to form a yellow precipitate, which was collected by filtration. After

dissolving the organic material in 1,2-dichlorobenzene at 130°C, the solution was passed

through a short pad of silica gel and Celite® to remove the inorganic materials. After removal

of the solvent, the crude material was purified by recrystallization from o-dichlorobenzene

(oDCB) to afford a yellow precipitate (161 mg, 0.261 mmol, 87 % yield). Prior to

measurements of the physicochemical properties and device fabrication, the obtained product

was further purified by repeated recrystallization from 1,2-dichlorobenzene and sublimation.

Yield: 87 %. Yellow solid. m.p.: > 300°C. 1H NMR (400 MHz, CDCl2CDCl2, 100°C): 0.86

(t, J = 6.8 Hz, 6H, CH3), 1.26-1.44 (m, 20H, (CH2)5), 1.73 (quin, J = 7.6 Hz, 4H, ArCH2CH2),

2.79 (t, J = 7.6 Hz, 4H, ArCH2), 7.37 (d, J = 8.8 Hz, 2H, ArH), 7.65 (s, 2H, ArH), 7.94 (d, J =

8.8 Hz, 2H, ArH), 8.19 (s, 2H, ArH), 8.60 (bs, 4H, ArH). 13C NMR was not recorded due to

poor solubility. HRMS (APCI+): Calcd for C42H47S2 [M+H] 615.3119, found, 615.3099.

Anal. Calcd for C42H46S2: C, 82.03; H, 7.54. Found C, 81.92; H, 7.29.

Solubility test

50 L of 3-chlorothiophene was repeatedly added to approximately 1 mg of sample. The

resulting suspension was shaken and sonicated at 60°C. The total amount of solvent (mL) was

converted into solubility in wt%. The results are summarized in table S1. A 20 % increase in

solubility for C8–DNBDT–NW compared to the previously synthesized compound, C10–

DNBDT, was advantageous for control of the number of layers in film preparation, as

discussed in the main text.

table S1. Solubility of Cn-DNBDT.

Cn–DNBDT Solubility (wt%)*

C8 0.027 C10 0.022

∗ in 3-chlorothiophene at 60ºC

Thermal analyses

Differential scanning calorimetry (DSC; Rigaku Thermo Plus EVO IIDSC 8231)

measurement was performed by placing the sample in an aluminum pan and heating at the rate

of 5°C min1 under N2 purge at a flow rate of 100 mL min1 (fig. S2). Al2O3 was used as a

reference material. DSC measurements provide phase-transition data for the endothermic

process (either from solid to liquid or from solid to liquid crystal).

fig. S2. Phase transition temperature and melting point of C8-DNBDT-NW. DSC trace of

C8–DNBDT–NW upon heating in the range of 50 to 350°C in a flow of nitrogen gas (scan rate:

5°C min1, N2 purge: 100 mL min1).

table S2. Phase transition temperature and melting points of Cn-DNBDT.

Cn–DNBDT Tphase (°C) m.p. (°C)

C8 256, 283 > 350

C10 213, 266 344

Single crystal analyses Single crystals were obtained by recrystallization from oDCB. Cn–DNBDT–NW was grown by gradual cooling of hot oDCB solution. Single-crystal X-ray diffraction data were collected on

an imaging plate diffractometer (Rigaku R-AXIS RAPID II) with Cu K radiation. The molecular packing structure (ellipsoid type) of Cn–DNBDT with the intermolecular interactions and short contacts are shown in fig. S3.

fig. S3. Structure, transfer integrals, and effective masses of Cn-DNBDT.

table S3. Crystal data for C8-DNBDT-NW.

Entry C8–DNBDT–NW

formula C42H46S2

FW 614.95

T / K 296

wavelength /Å 1.54187 (CuK)

color yellow

crystal size / mm 1.500 x 1.000 x 0.005

crystal system monoclinic

space group P21/c a / Å

b / Å

c / Å

35.6179(7)

7.86754(15)

6.10980(11)

/ deg 90

/ deg 90.262(6)

/ deg 90

V / Å3 1712.10(6) Z 2

DX / g cm3 1.193

µ mm1 1.605

reflections collected 18970

unique reflections 3066

refined parameters 199

GOF on F2 1.503

R1 [I > 2(I)]a 0.0500

wR2 (all data)b 0.1379

min,max / e Å3 0.25, 0.19

CCDC number 1565819

section S2. Details of transport measurements

Materials and sample preparation for field-effect transistor (FET) measurements

All measurements were performed with our benchmarked organic semiconducting molecule,

C8–DNBDT–NW (fig. S4A), which was synthesized and purified in-house (see Section S1).

Although the -conjugated core is extended, decent solubility in common organic solvents

allows for the controllable production of single-crystalline thin films (see more details in fig.

S2 and table S1). The crystal structure of a C8–DNBDT–NW bulk single crystal grown via

recrystallization from 15 mg mL1 oDCB solution was determined by single-crystal X-ray

diffraction to be a herringbone structure, as shown in fig. S4B. Transfer integrals calculated

based on the crystal packing structure were estimated to be 49.2 and 48.0 meV along the

column and transverse directions; these are ideally balanced, and thus advantageous for two-

dimensional carrier transport. Crystal growth during continuous edge-casting occurs along the

c-axis.

fig. S4. Crystal packing structure of C8-DNBDT-NW. (A), Molecular structure of C8–

DNBDT–NW. (B), In-plane packing structure of C8–DNBDT–NW. Transfer integrals

calculated using Gaussian 09 with the PBEPBE functional and 6-31G(d) basis set. (C), View from the

b*-axis, which is perpendicular to the channel direction.

FETs for gated four-point-probe (gFPP) and transmission line method (TLM) measurements

were fabricated on a Si substrate with a 100 nm thick silicon dioxide layer. The Si/SiO2

substrate was cleaned by sonication in acetone and 2-propanol for 10 min each. The substrate

was further cleaned by oxygen plasma for 30 min. A self-assembled monolayer (SAM) of 2-

(phenylethyl)trimethoxysilane (-PTS) was then deposited on the surface by vapor deposition

at 120ºC for 3 h. After the formation of -PTS, the surface was cleaned with toluene and 2-

propanol in an ultrasonic bath. A single-crystal of C8–DNBDT–NW was grown directly on

the top of the -PTS treated substrate from purified 0.02 wt% 3-chlorothiophene (PI-

CRYSTAL Inc.) solution using the continuous edge-casting technique described in the main

text (Fig. 1A). Although C8–DNBDT–NW can be dissolved in common organic solvents such

as oDCB (b.p. 180ºC), anisole (b.p. 153ºC), and tetralin (b.p. 207ºC), their boiling points are

higher than that of 3-chlorothiophene (b.p. 133ºC). Thus, 3-chlorothiophene is more suitable

for the room-temperature processes. In our meniscus-driven solution-process method, the

precipitation rate of solute is important both to grow the large crystal and to control the

thickness. After optimization process, it was found that 3-chlorothiophene is the best solvent

particularly for C8–DNBDT–NW because of its well-balanced boiling point and solubility.

The substrate was sheared at a constant rate of 20 m s1 and heated up to 60 – 70ºC, and the

blade to hold the solution was fixed at 100 m above the substrate. The thickness of the

crystalline film was controlled by adjustment of the substrate temperature. After annealing the

substrate at 80ºC in vacuum to remove residual solvent, F4–TCNQ and Au were subsequently

deposited through a metal mask to form source/drain electrodes. C8–DNBDT–NW layers

were patterned by dry-etching processes with a YAG laser (V-Technology Co., Ltd., Calisto

( = 266 nm)).

fig. S5. Effect of thermal annealing on the transfer characteristics of 2L C8-DNBDT-NW

FET. (A), Transfer curves of a bilayer C8–DNBDT–NW FET before (square) and after (circle)

post-annealing treatment. (B), VG-dependence of the two-terminal mobility (2T) estimated

from the slopes of each transfer curve. Transfer characteristics were measured at VD = 3 V

with devices fabricated for TLM-measurements, where the channel length (L) and width (W)

were 140 m and 463 m, respectively.

FET measurements

FET measurements were conducted using semiconductor parameter analyzer (Keithley 4200-

SCS). Two-terminal mobility (2T) was calculated from the equation 2T =

(L/W)(1/CiVD)(∂ID/∂VG), where L, W, and Ci are the channel length, channel width, and the

capacitance per unit area of gate insulator, respectively. In the present device, Ci is extracted

from the dielectric constant value of 3.9. Note that the device performance for samples

without post-annealing was non-ideal, where superlinear behavior was observed in the

transfer (ID vs. VD) curve in the linear region (fig. S5A). The resulting mobility estimated from

the slope of the transfer curve was significantly dependent on the applied gate voltage (fig.

S5B). Such non-ideal behavior of the transfer characteristics often causes significant

overestimation of the mobility (38,39). After annealing at 100ºC for 10 h in vacuum, the

inflection of the transfer curve was completely disappeared, and the mobility was determined

to be almost VG-independent in the high VG region. It is thus considered that the post-

annealing process restrains the strong VG-dependence of the contact resistance (39). The

observed VG-independent mobility i.e., the clear plateau in mobility in the high VG region,

indicates that charge carriers in the present organic single crystals are likely to undergo band-

like transport due to a low density of trap states (7,40,41). All the measurements shown in the

main text were performed using cured devices that were subjected to optimized annealing

treatment.

Figures S6 to S8 show the transistor performance of mono-, bi-, and tri-layer (abbreviated as

1L, 2L, and 3L) C8–DNBDT–NW single-crystalline thin films. Both 2L- and 3L-OFETs show

textbook-like performance, whereas the transfer curve of the 1L-OFET shows a nonlinear

increase of drain current (ID) with gate voltage (VG). This results in a monotonic increase of

the mobility because charge traps can hinder the band-like transport.

fig. S6. Device characteristics of 1L C8-DNBDT-NW FET. (A), Transfer curve in the linear

region. (B), VG-dependence of the mobility determined from the slope of the transfer-curve in

(A). (C), Transfer curve in the saturation region. (D), VG-dependence of the mobility

determined from the slope of the transfer-curve in (C). (E), Output characteristics. Channel

length and width are 150 m and 36 m, respectively.

· ·

Contact resistance evaluation by the gFPP method

For gFPP measurements, the four-terminal architecture shown in Figs. 2A-C of the main text

was employed. The distances between the source and two longitudinal probes along the

channel direction (defined as x1 and x2 in fig. S9A) were designed to be 50 m and 100 m.

Four-terminal sheet conductivity, 4T was extracted from the following equation

σ4T = (𝐼D

𝑉2 − 𝑉1) (

𝑥2 − 𝑥1

𝑊) (1)

where V1 and V2 are the potentials at the voltage probes located at x1 and x2. The potential

profile from the source to the drain along the channel direction is linear; therefore, the voltage

drop at the source and the drain (Vcs and Vcd) should be extracted by extrapolation of the

potential profile shown in fig. S9. When ohmic contact is established, the drain current

increases linearly with respect to Vcs; therefore, it is possible to estimate the contact

resistance from the slope of the ID-Vcs plot. For the 3L-OFET, nonlinear behavior in the ID-

Vcs plot was observed, as discussed in the main text; therefore, the contact resistance is

variant with respect to Vcs. Nevertheless, the width-normalized contact resistance at the

source electrode (Rcs·W) of the 2L-OFET was apparently lower than that of the 3L-OFET by

comparison of both the Rcs·W values extracted from the common Vcs of 0.1 V (fig. S9B).

The contact resistance estimated from gFPP method was found to be higher than that from

TLM (shown in the main text). In gFPP method, channel potentials monitored with voltage

probes are likely to suffer from microscopic cracks along the channel direction, albeit

invisible in eyes, which may overestimate the contact resistance. In addition, the contact

resistance evaluated from TLM is inherently an ensemble value among 10 transistors with

different channel lengths. Thus, we believe that TLM can evaluate accurate contact resistance.

Contact resistance evaluation by TLM

All the two-terminal device characteristics of the 2L- and 3L-OFETs from TLM

measurements are summarized in figs. S10 and S11. For the 2L-OFETs, clear plateaus are

observed for the VG-dependence of the two-terminal mobility in the high VG region, even for

all the devices with different channel lengths. On the other hand, the 3L-OFETs typically had

an overshoot in the transfer curves, which is more apparent in devices with short channel

lengths and results in a non-ideal inflection of the VG-dependent mobility, even after post-

annealing treatment. This indicates that the 3L-OFETs suffer from strong VG-dependence of

the contact resistance due to non-ohmic carrier injection. Figure S12 shows that 2T for 2L-

OFETs is less sensitive to the channel length than that of 3L-OFETs.

fig. S9. Estimation of contact resistance by the gFPP method. (A), Schematic illustration

of the gFPP measurement. Voltage drops at the source and drain electrodes are estimated by

extrapolation of the potential profile. (B), VG-dependent contact resistance at the source of the

2L- and 3L-OFETs, where Vcs was fixed at 0.1 V.

fig. S10. Transfer characteristics of 2L-OFETs from TLM measurements. Transfer curves

in the linear region, where channel lengths were designed to be different. VD was fixed at 3 V.

fig. S11. Transfer characteristics of 3L-OFETs from TLM measurements. Transfer curves

in the linear region, where channel lengths were designed to be different. VD was fixed at −3 V.

fig. S12. Channel length dependence of two-terminal mobility in 2L- and 3L-OFETs. All

mobilities in the linear region are determined from the slopes of the transfer curves in the high

VG region.

section S3. Characterization of single crystals

Surface images

fig. S13. Typical example of a wafer-scale C8-DNBDT-NW single crystal. (A),

Photograph of the C8–DNBDT–NW crystal fabricated on 4-inch Si wafer. (B),

Optical microscopy image of C8–DNBDT–NW crystals.

AFM and SEM measurements

For atomic force microscopy (AFM; SII NanoTechnology Inc. SPA400) and

scanning electron microscopy (SEM; Hitachi High Technologies Inc. SU8200)

measurements, a single- crystalline thin film of C8–DNBDT–NW was grown on a

-PTS treated Si/SiO2 substrate using the continuous edge-casting technique. AFM

measurements revealed a clear step-and-terrace structure at the boundaries between

mono- and bilayers, where the observed molecular step (3.6 nm) equaled the

monolayer height (shown in fig. S14A). SEM measurements were conducted using

an acceleration voltage below 0.3 kV to avoid any damage to the organic crystals.

Figure S14B shows an SEM image taken from the boundary between 1L and 2L

crystalline domains. The contrast observed is due to a difference in the thickness.

Raman spectroscopy

For Raman spectroscopy, 1L, 2L, and 3L single-crystal thin films of C8–DNBDT–

NW were grown on a -PTS treated Si/SiO2 substrates. Raman spectra were

measured using a spectrophotometer system (Horiba LabRAM HR Evolution)

with a 532 nm laser as the excitation source. A laser light from the polarization

angle parallel to the crystal growth direction was irradiated onto selective

domains with different thicknesses. The background spectrum from a Si/SiO2 substrate was subtracted from the measured spectra. Figures S15A and B show

Raman spectra of the 1L, 2L, and 3L C8–DNBDT–NW crystals. The peak

intensity increases linearly with respect to the thickness of the crystals over a

broad wavenumber. The integrated intensity at a representative peak (ca. 1380

cm1) shown in fig. S15B, which is assigned to an intramolecular vibrational

mode described in fig. S16B according to density functional theory (DFT)

calculation, increases linearly with the number of layers (shown in fig. S15B).

This result implies that Raman spectroscopy can be utilized for inspection of the

thickness of organic crystals in a similar way to graphene (42).

fig. S14. Observation of molecular step at domain boundaries. (A), AFM

image of the C8–DNBDT–NW crystal including a step-and terrace structure. (B),

SEM image of the boundary between 1L- and 2L- domains of C8–DNBDT–NW

crystals.

fig. S15. Raman spectroscopy measurements of 1L, 2L, and 3L crystals of C8-DNBDT-

NW. (A), Raman spectra of C8–DNBDT–NW thin films with 1L-, 2L-, and 3L-domains.

(B), Magnified spectra (1100-1600 cm1). (C), Layer-dependence of the integrated intensity

derived from the peak marked in (B).

fig. S16. Simulation of Raman peaks for crystals of C8-DNBDT-NW. (A), Experimental

data for the 3L-domain and DFT calculation results based on the bulk crystal structure

shown in fig. S4. Theoretical calculations were carried out using Gaussian 09 with the

UB3LYP functional and the 6-31G(d) basis set. A scale factor of 0.9613 (43) was applied

to the calculated Raman spectra. (B), Intramolecular vibrational mode corresponding to the

peak marked in (A).

Sample preparation for TEM measurements

Thin films for TEM measurements were fabricated by the following process. A 20 wt%

aqueous solution of poly(acrylic acid) (PAA; Sigma-Aldrich, MW ~ 5,000) was spin-coated

on a Si/SiO2 substrate at 3000 rpm for 30 s. The substrate was annealed on a hotplate at 80 ˚C

for 1 h to remove residual water. An insulating polymer, parylene (diX-SR, Daisan Kasei Co.,

Ltd.), was deposited on the top of the PAA layer via chemical vapor deposition to a thickness

of 40 nm. 1L and 2L C8–DNBDT–NW thin films were grown on the top of the parylene layer

by continuous edge casting with the same conditions as described previously. The sample was

then annealed at 80 ˚C for 10 h in vacuum. After cutting the edge of the parylene film, a few

droplets of water were placed on the edge to dissolve the PAA layer. The floating thin film

was then transferred onto a TEM grid. Figure S17B shows a typical sample prepared for TEM

observation.

fig. S17. Sample preparation for TEM measurements. (A), Schematic illustration of a

prepared sample. (B), Photograph of a TEM grid after the transfer of a thin film sample.

Cross-polarized microscopic images of a 1L C8–DNBDT–NW sample (C), before and (D),

after transfer. Red circles in both images show the same particle of organic material.

TEM measurements

TEM (Jeol Ltd. JEM-2100F) measurements were performed at an acceleration voltage of 120

kV. The sample was cooled down to 100 K to prevent degradation under irradiation. The

direction of the electron beam was parallel to the a-axis of the C8–DNBDT–NW crystal

structure shown in the previous section. The lattice constants assumed from the diffraction

pattern shown in Fig. 2 in the main text were determined to be b = 8.31 Å and c = 6.28 Å for

the 1L crystal, and b = 8.20 Å and c = 6.25 Å for the 2L crystal, which are almost identical to

those in bulk C8–DNBDT–NW. Figures S18 and S19 show selected-area electron diffraction

(SAED) patterns for 1L and 2L C8–DNBDT–NW single-crystalline thin films taken at different

positions, from which the crystal structure of C8–DNBDT–NW is assigned to a herringbone

structure with the space group P21/c. Although no significant difference was observed in the

SAED patterns of 1L and 2L C8–DNBDT–NW, either doubly overlapped patterns or arc-

elongated diffraction spots are often observed only in 1L crystals, as shown in fig. S18, which

suggests that a finite fraction of structural defects and disorder are distributed in 1L crystals.

During crystal growth of a bilayer crystal, a sufficient amount of C8–DNBDT–NW can be

supplied, which possibly relaxes the defects in the first layer. Another possibility to account for

the imperfections of the first layer is strong interaction between the first layer and the SAM

functionalized substrate. The C8–DNBDT–NW molecules in the first layer are anchored to the

substrate, whereas the opposite side is not stabilized. Hence, the first layer tends to be

influenced by the underlying substrate, which can cause lattice mismatches or unfavorable

disorder in the 1L crystals. The susceptibility of the substrate to such can be relieved by the

deposition of a secondary layer to stabilize the exposed side.

fig. S18. SAED patterns taken for 1L crystalline domains at different positions. The

white scale bars represent 5 nm1. Doubly overlapped patterns or broader diffraction spots are

observed.

fig. S19. SAED patterns taken for 2L crystalline domains at different positions. The white

scale bars represent 5 nm1.

section S4. Details of high-frequency measurements

Fabrication of short-channel devices

An Eagle XG (Corning, Inc.) glass substrate was cleaned by sonication in acetone and 2-

propanol for 10 min each, and then annealed on a hotplate at 200ºC. The surface of the

substrate was treated with a silane coupling agent (KBM-903, Shin-Etsu Chemical Co., Ltd.)

to prevent delamination of the upper gate electrodes. After the substrate was exposed to the

silane coupling agent vapor at 90ºC for 1 h, it was cleaned and heated under the same

conditions as mentioned previously. A 45 nm thick layer of silver was deposited by thermal

evaporation and patterned by the following photolithographic processes. Positive photoresist

(OFPR-800LB, Toyko Ohka Kogyo Co., Ltd.) was spin-coated at 3000 rpm for 40 s, and then

heated on a hotplate at 90ºC for 90 s. The substrate was exposed to ultraviolet (UV) light at 36

mJ cm−2 and then immersed in developer (NMD-3, Toyko Ohka Kogyo Co., Ltd.) for 30 s.

After heating the substrate at 110ºC for 2 s, the substrate was immersed in an Ag etchant

(SEA-1, Kanto Chemical Co., Inc.) for 20 s. The residual etchant was thoroughly removed by

deionized water. Finally, the substrate was immersed in 1-methyl-2-pyrrolidone and 2-

propanol for 1 min to strip the photoresist, and then dried on a hotplate at 90ºC for 5 min.

A 100 nm thick layer of aluminum oxide as the gate dielectric layer was deposited by an

atomic layer deposition (ALD) technique. The capacitance of the aluminum oxide measured at

a frequency of 1 kHz was 60.3 nF cm−2. The surface was treated with 2-

(phenylhexyl)phosphonic acid SAM by immersing the substrate into the 0.2 mM solution for

13 h. 2L C8–DNBDT–NW single crystals were grown by the continuous edge casting method.

Even though an aluminum oxide gate dielectric was employed, the phenyl-terminated SAM

can adjust the surface energy to be identical to that on the -PTS treated Si/SiO2. Thus, the

organic crystals were successfully grown at almost the same condition as described in section

2. F4–TCNQ and Au were subsequently vacuum-deposited onto the crystals to form the

patterns of source and drain electrodes. The patterning was conducted using a fluorinated

photoresist, which does not damage the organic semiconductors (30). A fluorine negative

photoresist (OSCoR4001, Orthogonal, Inc.) was spin-coated at 1500 rpm for 30 s. The

substrate was heated in an oven at 60ºC for 30 min and then exposed to the UV light at a power

of 75 mJ cm−2. The substrate was immersed in NovecTM 7300 (3M Japan, Ltd.) for 3.5 min to

develop. The Au layer was then etched using an iodine etchant (AURUM S-50790, Kanto

Chemical Co. Inc.). The residual etchant was rinsed from the substrate with deionized water for

15 min. The photoresist was then removed by soaking the substrate in NovecTM7100 (3M

Japan, Ltd.) and then heated to 80ºC for 4.5 min. Finally, the substrate was annealed in a

vacuum at 80ºC for 10 h. Figure S20 shows the transistor performance of the fabricated short

channel devices. Generally, short-channel OFETs are likely to be influenced by a short channel

effect (44). However, the present short-channel OFET of C8–DNBDT–NW shows ideal

transfer and output characteristics, which is confirmed by a good agreement of linear and

saturation mobilities.

In the main text, two theoretical limits of rectifying frequency frectify were estimated. Here, we

explain more details. Firstly, the maximum frectify in the absence of gate-dependent mobility

was estimated as follows. frectify is defined as that Vout in a low-frequency limit, which is 4.7 V

in the present device, is decreased by a factor of 3 dB, thus frectify is defined as the frequency

at which Vout reaches 3.3 V. Because in the present diode-connected OFET, Vin with an

amplitude of 8 V is applied both to VG and VD, and Vin with am amplitude of 3.3 V outputs at

the source electrode, the maximum input voltage is estimated to be Vin Vout, i.e., 8 V

V4.7 V. From fig. S20, the effective mobility eff at VG = 4.7 is determined to be

2.1 cm2 V−1 s−1 (indicated as an arrow in fig. S20D), resulting in frectify = 37 MHz (see equation

(3) in the main text). Secondly, because eff is apparently reduced at lower VG regime due to its

gate dependence, as shown in fig. S20D, the reduction in eff was taken account into frectify. To

do so, μeffave that is averaged over the low voltage regime is estimated by taking an integral on a

closed interval [t1, t2] as follows

μeffave =

1

𝑡2 − 𝑡1∫ μeff(𝑉G(𝑡))𝑑𝑡

𝑡2

𝑡1

(2)

where eff (VG) is the gate dependent mobility, VG = V0sint + Vout. Assuming that the diode-

connected OFET is in the saturation regime, and VG dependent mobility that is measured in a

dc condition (shown in fig. S20D) is applicable to higher frequency bands, effave is

determined to be 1.8 cm2 V−1 s−1, resulting in frectify is estimated to 31 MHz. This is indeed in

a good agreement with experiments shown in the main text.

fig. S20. Device characteristics of a short-channel device. (A), Transfer curve in the linear

region. (B), VG-dependence of the mobility determined from the slope of the transfer curve in


determined from the slope of the transfer-curve in (C).

Measurements of the cut-off frequency

To estimate the cut-off frequency fT, of the fabricated short-channel 2L-OFET, the dynamic

responses of the gate and drain current were recorded by application of an ac voltage with dc

voltage offset to the gate electrode. Figure S21A shows a typical result measured at a

frequency of 7 MHz. An ac component of the gate and drain currents (ig(t) and id(t)) can be

expressed as

𝑖g(𝑡) = 𝐶i𝑊(𝐿 + 𝐿C)𝑑𝑣g(𝑡)

𝑑𝑡 (3)

and

𝑖d(𝑡) = 𝜇eff 𝐶i𝑊𝑉D

𝐿 𝑣g(𝑡) (4)

where Ci is the capacitance per unit area, W is the channel width, vg(t) is an ac component of

the applied gate voltage, and Vth is the threshold voltage. Note that these equations predict that

ig(t) has a phase shift of π/2 compared to id(t), which is consistent with the experimental

result in fig. S21A. This consistency validates the present measurements. The amplitudes of

ig(t) and id(t) (IG and ID) were extracted by fitting the output current signals with a

sinusoidal wave. Figure S21B shows the frequency dependence of IG and ID. The cut-off

frequency, which is defined as a characteristic frequency where IG is identical to ID, was

estimated to be 20.0 MHz.

fig. S21. Dynamic response of a short-channel 2L-OFET. (A), ig(t) and id(t) measured at a

frequency of 7 MHz. (B), Frequency dependence of IG and ID.

Table S4 provides a summary of the cut-off frequency, fT (45). Although cut-off frequencies

of more than 20 MHz have been already reported, it is reasonable to compare the cut-off

frequency normalized with respect to the applied voltage because it is scaled with the input

voltage. The highest voltage-normalized fT was recorded for the C10–DNTT transistor, where

fT was estimated to be 19 MHz at an applied voltage of 10 V. The cut-off frequency

accomplished with the 2L-OFET is slightly higher than the best value, even though it has a

longer channel length, which indicates that 2L crystals are potential candidates for high-speed

operation.

table S4. Comparison of cutoff frequency.

Material Input voltage Effective mobility Channel length Cut-off frequency

(V) (cm2 V−1 s−1) (m) (MHz)

C60 (46) 20 2.22 2 27.7

rubrene (47) 15 4.0 4.5 25

C10–DNTT (48) 20 0.4 2 20

P(NDI2OD–T2) (45) 30 0.9 1.75 20

C10–DNTT (30) 10 2.5 2 19

pentacene (46) 20 0.73 2 11.4

C8–DNBDT–NW 10 2.7 3 20

Supplementary Materials for · measurements of the physicochemical properties and device...

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