A dendritic oligoarylamine-substituted benzimidazole derivative as a useful n-type dopant

Article abstract of DOI:10.1039/c8tc01280f

4-(N,N-Dimethylamino)phenyl-substituted 1,3-dimethyl-2,3-dihydro-1H-benzimidazole (N-DMBI-H) has been reported as a useful solution-processable n-type dopant to control electrical conductivity over a wide range in PCBM ([6,6]-phenyl-C₆₁-butyric acid methyl ester) thin films. An approach for increasing the chemical stability of the DMBI-H dopant molecule in solution phase by attachment of dendritic oligoarylamine at the 2-position of the benzimidazole ring (N3-DMBI-H) has been explored in this study, and moreover the efficacy of the solution-processable n-type dopant has been clarified together with the electronic properties of N3-DMBI-H. On the basis of electrochemical, spectroelectrochemical, and quantum chemical calculations, it has been deduced that one-electron-oxidized species for N3-DMBI-H as well as N-DMBI-H are readily converted into diamagnetic cationic species by releasing a hydrogen atom.

Full text of DOI:10.1039/c8tc01280f

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A dendritic oligoarylamine-substituted
benzimidazole derivative as a useful n-type
Cite this:DOI: 10.1039/c8tc01280f
dopant†
a
Masashi Uebe,
Yuji Yoshihashi,b Kei Noda, *b Masayuki Matsubaraa and
a
Akihiro Ito
*
4-(N,N-Dimethylamino)phenyl-substituted 1,3-dimethyl-2,3-dihydro-1H-benzimidazole (N-DMBI-H) has
been reported as a useful solution-processable n-type dopant to control electrical conductivity over a
wide range in PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) thin films. An approach for increasing
the chemical stability of the DMBI-H dopant molecule in solution phase by attachment of dendritic
oligoarylamine at the 2-position of the benzimidazole ring (N3-DMBI-H) has been explored in this study,
and moreover the efficacy of the solution-processable n-type dopant has been clarified together with
the electronic properties of N3-DMBI-H. On the basis of electrochemical, spectroelectrochemical, and
quantum chemical calculations, it has been deduced that one-electron-oxidized species for N3-DMBI-H as
well as N-DMBI-H are readily converted into diamagnetic cationic species by releasing a hydrogen atom.
Received 16th March 2018,
Accepted 19th May 2018
DOI: 10.1039/c8tc01280f
rsc.li/materials-c
substances like alkali and alkaline earth metals. As an effective
workaround for the difficulty in use, application of air-stable salts
Introduction
Efficient and stable doping at specific areas in organic electronic or hydride-reduced forms of p-conjugated organics as precursors
devices such as organic light-emitting diodes (OLEDs), organic of n-type dopants is considered to be a strong approach,
solar cells (OSCs) and organic field-effect transistors (OFETs) has as evidenced by Leo’s and Bao’s groups.22,24–28 Recently, the
attracted growing interest in recent years to improve their device derivatives of 1,3-dimethyl-2,3-dihydro-1H-benzimidazole [DMBI-H
behavior, performance, and stability.1,2 It has been shown that in Scheme 1] have been employed as organic n-type dopants, not
contact-area-limited doping achieves ohmic-like contacts for only in OFETs, but also in OLEDs,29 OSCs,30 and thermoelectric
enhancing the charge carrier injection for OLED and OFET materials.31–33
devices.3–18 Channel-area doping in OFETs also provides an
In addition to the importance of fine-controlling the transistor
effective way to fine-tune the device performance, to control the behavior by organic doping, solution processing of organic electronic
threshold voltage, and to remove the unwanted trap states in the materials is becoming indispensable to device fabrication in
channel, thereby leading to the stability of the transistors.16,19–22
In addition, control of the energy-level alignment and the open-
circuit voltage in planar heterojunction OSCs has been demon-
strated with the aid of molecular doping.23
Concomitantly with the development of doping technology
in organic devices, the creation of molecular p- and n-type dopants
suited for versatile application has become a primary focus in the
field of organic electronics. In particular, the design of n-type
dopants is far more challenging due to the requirement of low
ionization energies, inevitably leading to highly air-sensitive
a Department of Molecular Engineering, Graduate School of Engineering, Kyoto
University, Nishikyo-ku, Kyoto 615-8510, Japan. E-mail: aito@scl.kyoto-u.ac.jp
b Department of Electronics and Electrical Engineering, Keio University,
3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan.
Scheme 1 Molecular structures of DMBI-H derivatives as organic hydride
E-mail: nodakei@elec.keio.ac.jp
donor n-dopants.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8tc01280f
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view of realization of low-cost, flexible, and large-area electronics. As shown below, the present molecular design gave rise to a new
In this regard, Bao and co-workers first demonstrated that DMBI-H-type n-dopant with chemical stability, N3-DMBI-H.
4-(N,N-dimethylamino)phenyl-substituted DMBI-H (N-DMBI-H
in Scheme 1) was useful as a solution-processable n-type dopant
to control electrical conductivity and electron transport over a
Results and discussion
Synthesis of N3-DMBI-H and its stability in solution
wide range in PCBM ([6,6]-phenyl-C61-butyric acid methyl ester)
based OFETs.22 Herein, we report an approach for increasing
the stability of DMBI-H dopant molecule in solution phase by The preparation of the target benzimidazole derivative, N3-DMBI-H,
attachment of dendritic oligoarylamine (N,N-bis[4-(di-4-anisyl- was carried out by following the synthetic procedure shown in
amino)phenyl]amine: 1 34–37) at the 2-position of the imidazole Scheme 2. The acetic acid catalyzed cyclic condensation reaction
ring (N3-DMBI-H in Scheme 1), and show the efficacy of the between N,N0-dimethyl-o-phenylenediamine and amino-substituted
solution-processable n-type dopant together with the electronic benzaldehyde 2, which was obtained by using the Buchwald-
properties of N3-DMBI-H. The further improvement of solubility Hartwig amination reaction38,39 between 4-bromobenzaldehyde
in common organic solvents, chemical instability in solution, and dendritic triamine 1,34 gave N3-DMBI-H as an air-stable
and the prevention of dopant diffusion in host materials can be yellow solid in 21.6% yield. For comparison, we also prepared
anticipated due to the introduction of the bulky dendritic N-DMBI-H according to the literature procedure.40
oligoarylamine 1. More importantly, in addition to the simple
The chemical stability of N3-DMBI-H in air-equilibrated
kinetic stabilization due to the bulky dendritic oligoarylamino chlorobenzene and CH2Cl2 was examined under ambient light
group, it can be expected from DFT calculations (Fig. S1, ESI†) (Fig. 1). For comparison, we also checked the chemical stability
that the introduction of the dendritic oligoaryamino group in of N-DMBI-H in solution phase under the same conditions. As
place of the dimethylamino group in N-DMBI-H changes the is apparent from Fig. 1a and b, the solutions of N-DMBI-H
character of the HOMO, from the 1H-imidazole-localized one gradually decomposed with appearance of a new absorption band
in N-DMBI-H to the dendritic oligoarylamine-localized one (at around 3.5 eV) in the lower energy region. This new absorption
in N3-DMBI-H, thus leading to the chemical stability of the band probably indicates the generation of the N-DMBI cation
1H-benzimidazole moiety as a key component of DMBI-H-type (N-DMBI+) as a result of the oxidation of N-DMBI-H and the
n-dopant molecules. Furthermore, in hydrogen-atom-removed subsequent follow-up hydrogen-atom-transfer reaction, as observed
diamagnetic cationic species, N3-DMBI+, as the final product in in Fig. 3a and d. However, the degradation mechanism remains
the n-doping process, the resulting cationic species can also be obscure at the present stage.41 In stark contrast to N-DMBI-H, no
stabilized due to the charge delocalization over the dendritic noticeable change was observed for N3-DMBI-H in chlorobenzene
oligoarylamino group as a result of decreasing dihedral angle solution (Fig. 1c).42 This observation indicates the preponderance of
between the p-plane defined by the benzimidazole p-plane N3-DMBI-H in solution-processability under ambient conditions, as
and the dendritic oligoarylaminophenyl group (Fig. 5a and d). compared to N-DMBI-H.
Scheme 2 Synthetic route for N3-DMBI-H.
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Fig. 1 Change of absorption spectra of N-DMBI-H in (a) chlorobenzene (1 ꢂ 10ꢀ4 M) [as-prepared solution (blue line); after 1 day (broken line); after
4 days (red line)] and (b) CH2Cl2 (1 ꢂ 10ꢀ4 M) [as-prepared solution (blue line); after 1 day (broken line); after 4 days (red line)], and of N3-DMBI-H in
(c) chlorobenzene (1 ꢂ 10ꢀ4 M) [as-prepared solution (blue line); after 1 day (broken line); after 4 days (red line)] and (d) CH2Cl2 (1 ꢂ 10ꢀ4 M) [as-prepared
solution (blue line); after 1 day (broken line); after 4 days (red line)], at room temperature under ambient light and air.
Electrochemistry, spectroelectrochemistry, and DFT calculations and its oxidized states.40,43 In the case of solution-processed
N-DMBI-H-doped PCBM thin film, it was speculated that the
As shown in Scheme 3, the oxidation process of DMBI-H
n-doping process was initiated by the C–H bond homolysis of
derivatives as hydride-donor dopant molecules remains elusive
N-DMBI-H in the annealing step for the deposited film.22
due to intricate pathways including hydride (Hꢀ), a hydrogen
atom (Hꢁ), a proton (H+), and electron transfer from DMBI-H
Fig. 2 Cyclic voltammograms of N-DMBI-H and N3-DMBI-H in CH2Cl2
at 298 K, measured with 0.1 M nBu4NBF4 as the supporting electrolyte at a
scan rate of 100 mV sꢀ1
Scheme 3 Oxidation pathways of DMBI-H derivatives as hydride-donor
dopant molecules.
.
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Shortly afterwards, it was recognized that DMBI-H dopants act for N-DMBI-H (ꢀ0.05 V (1e) vs. Fc/Fc+) and N3-DMBI-H (+0.03 V
first to initiate either hydride or hydrogen atom transfer in (1e) vs. Fc/Fc+), and the third reversible oxidation process for
solution.44 In the present study, although the actual n-doping N3-DMBI-H (+0.22 V (1e) vs. Fc/Fc+) correspond to oxidation
reaction rules out the initial electron transfer process, the from the substituted aminophenyl groups.
electrochemical behaviors of N-DMBI-H and N3-DMBI-H were
Spectroelectrochemical measurements were also performed
investigated by cyclic voltammetry (CV) measurements to gain in CH2Cl2 to characterize the one-electron-oxidized products
insight into the follow-up process after one-electron oxidation of both N-DMBI-H and N3-DMBI-H, from the viewpoint of
of these DMBI-H derivatives, from the viewpoint of fundamental fundamental oxidation chemistry (Fig. 3). Note again that
oxidation chemistry (Fig. 2). Note that hence the electrochemical the present spectroelectrochemical studies have nothing to do
process studied here has nothing to do with the real n-doping with the real n-doping process. During the electrochemical
process. The first oxidation for both compounds was an oxidation of N-DMBI-H, a lowest energy band appeared at 3.54 eV
irreversible process, thus suggesting the follow-up reaction (350 nm), and this band continued to grow up until the one-electron
takes place immediately after the one-electron oxidation from oxidation was completed (Fig. 3a). Further electrochemical oxidation
the benzimidazole moiety. The oxidation peak potentials for of one-electron-oxidized species resulted in the decrease of
N-DMBI-H and N3-DMBI-H were located at ꢀ0.15 and ꢀ0.11 V absorbance for all the observed optical bands, signifying the
vs. Fc/Fc+, respectively. The second reversible oxidation process decomposition of oxidized species. A similar spectral change was
Fig. 3 UV/vis-NIR optical absorption spectra of stepwise electrochemical oxidation (a) from the neutral (blue line) to one-electron-oxidized states (red
line) of N-DMBI-H, (b) from the neutral (blue line) to one-electron-oxidized states (blue broken) of N3-DMBI-H, and (c) from the one-electron-oxidized
(blue broken line) through dicationic (blue line) to tricationic states (red line) of N3-DMBI-H, in CH2Cl2 (at 1 ꢂ 10ꢀ4 M) with 0.1 M nBu4NBF4 at 298 K; the
blue, black, green, and red sticks represent the DFT-computed lower transition energies and their oscillator strengths for the neutral, radical cation,
hydrogen-atom-removed neutral radical, and hydrogen-atom-removed cation species, respectively, of (d) N-DMBI-H and (e) N3-DMBI-H.
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also observed for the electrochemical oxidation of N3-DMBI-H,
although the newly observed lowest-energy band was discerned
as a shoulder band at around 3.0 eV (Fig. 3b). In contrast to
N-DMBI-H, additional two-stage one-electron oxidation processes
were detected (Fig. 3c), and these spectral changes closely
resembled the spectral changes from neutral through the radical
cation to the closed-shell dication of dendritic oligoarylamine
1,35,36 thus corresponding to the oxidation processes for the
substituted dendritic oligoarylamine moiety of N3-DMBI-H.
According to the possible oxidation pathways of DMBI-H
derivatives (Scheme 3), the oxidized species of N-DMBI-H and
N3-DMBI-H characterized by the lowest-energy absorption band
+
can be regarded as either radical cation (DMBI-Hꢁ ), proton-
removed neutral radical (DMBIꢁ), or hydrogen-atom-removed
diamagnetic cation (DMBI+). To ascertain the origin of the
oxidized species, we carried out DFT calculations, and firstly,
the ground-state structures for all the possible three states were
optimized at the (U)B3LYP/6-31G* level of theory (Fig. 4 and 5).
As shown in Fig. 4a, the imidazole ring in neutral N-DMBI-H
adopted a puckered non-planar structure with the puckering
Fig. 5 DFT-optimized structures, selected bond lengths (Å), and puckering
+
and dihedral angles for (a) N3-DMBI-H, (b) N3-DMBI-Hꢁ , (c) N3-DMBIꢁ, and
+
(d) N3-DMBI+; (e) the computed spin density distributions for N3-DMBI-Hꢁ
and N3-DMBIꢁ, at the (U)B3LYP/6-31G* level of theory.
Fig. 4 DFT-optimized structures, selected bond lengths (Å), and puckering
+
and dihedral angles for (a) N-DMBI-H, (b) N-DMBI-Hꢁ , (c) N-DMBIꢁ, and
(d) N-DMBI+; (e) the computed spin density distributions for N-DMBI-Hꢁ
+
and N-DMBIꢁ, at the (U)B3LYP/6-31G* level of theory.
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+
angle of 17.21. However, in one-electron-oxidized form, N-DMBI-Hꢁ ,
the puckering angle considerably decreased so as to ensure the
planarity of the imidazole ring (Fig. 4b). For both N-DMBI-H and
Table 1 TD-DFT//(U)B3LYP/6-31G* vertical excitation energies calculated
for N-DMBI-H and N3-DMBI-H, and their oxidized species, that are related
to the lowest energy optical absorption bands observed experimentally for
N-DMBI-H and N3-DMBI-H, and their electrochemically oxidized species
+
N-DMBI-Hꢁ , the benzimidazole moiety was perpendicular to
Compound
hn ( f )a [eV]
Major contributionb
the N,N-dimethylaminophenyl group substituted at the 2-position
of the imidazole ring. Once the proton or the hydrogen atom was
abstracted from N-DMBI-Hꢁ+, the dihedral angle between the
p-planes defined by the benzimidazole moiety and the N,N-
dimethylaminophenyl group decreased for both N-DMBIꢁ and
N-DMBI+, indicating an extension of p-conjugation (Fig. 4c and d).
This p-extension was confirmed also by the difference in spin density
N-DMBI-H
3.90 (0.001)
4.51 (0.15)
5.16 (0.31)
5.24 (0.32)
3.50 (0.061)
2.07 (0.17)
3.55 (0.71)
3.43 (0.60)
4.77 (0.30)
3.28 (0.03)
3.40 (0.76)
3.43 (0.29)
3.47 (0.19)
3.87 (0.11)
3.89 (0.20)
0.86 (0.54)
1.47 (0.13)
2.05 (0.27)
2.44 (0.14)
1.70 (0.30)
2.90 (0.45)
H - L
H - L+2
Hꢀ1 - L+1
Hꢀ2 - L
N-DMBI-Hꢁ+
N-DMBIꢁ
a-Hꢀ4 - a-L
a-Hꢀ1 - a-L
b-H - b-L, a-H - L+3
H - L
N-DMBI+
+
distribution between N-DMBI-Hꢁ and N-DMBIꢁ, as shown in Fig. 4e.
Hꢀ2 - L, H - L+2
N3-DMBI-H
H - L
For the optimized structures of neutral N3-DMBI-H and their
oxidized species, the same trend was seen, although a decrease of
the puckering angle of the imidazole ring was not so remarkable
H - L+2
H - L+1
H - L+3
H - L+5
+
for the one-electron-oxidized form, N3-DMBI-Hꢁ (Fig. 5b). This
H - L+6
result was closely related to the fact that the HOMO of N3-DMBI-H
was distributed mainly on the dendritic oligoarylamine substituent,
whereas that of N-DMBI-H was mainly on the benzimidazole moiety
+
N3-DMBI-Hꢁ
N3-DMBIꢁ
N3-DMBI+
b-H–1 - b-L
b-H–3 - b-L
a-H - a-L+3
b-H - b-L, a-H - a-L+9
H - L
+
(Fig. S1, ESI†). Hence, the spin density in N3-DMBI-Hꢁ was not
localized on the imidazole ring, but on the dendritic oligoarylamine
substituent (Fig. 5e).
Hꢀ2 - L
a
b
Oscillator strength ( f ) values are given in parentheses. HOMO and
The diversity of structural features optimized for the possible
three oxidized species of N3-DMBI-H and N3-DMBI-H strongly
suggests that the lower-energy electronic transitions vary signifi-
cantly according to their oxidized species, whereby leading to
the assignment of the oxidized species observed in absorption
spectra (Fig. 3). Hence we performed time-dependent DFT
(TD-DFT) calculations at the optimized geometries of three
possible oxidized species (TD-DFT//(U)B3LYP/6-31G*), and the
computed vertical excitation energies and oscillator strengths
are listed in Table 1. Compared with these results (Fig. 3d and e),
the observed lowest-energy bands are explainable by the existence
of hydrogen-atom-removed diamagnetic cations (N-DMBI+ and
N3-DMBI+). The electrochemically generated one-electron-
LUMO are abbreviated as H and L, respectively.
+
+
oxidized species N-DMBI-Hꢁ and N3-DMBI-Hꢁ turned out to
readily release a hydrogen atom, leading to generation of the
diamagnetic cationic species (N-DMBI+ and N3-DMBI+).
++
Fig. 6 (a) Observed and (b) simulated X-band ESR spectra of N3-DMBIꢁ
in CH2Cl2 (1.0 mM) at 293 K (oxidant: AgSbF6).
Electron spin resonance (ESR) study for polycationic species of
N3-DMBI-H
In the previous section, it has been shown that the generated
++
radical cations of N-DMBI-H and N3-DMBI-H underwent fast the spin density at the central nitrogen atom in N3-DMBIꢁ is
hydrogen-atom transfer to give their diamagnetic cationic forms, diminished, probably due to the influence of Coulombic repulsion
N-DMBI+ and N3-DMBI+, respectively. Although polycationic from the charged benzimidazolium moiety, while the spin density
+
species of N3-DMBI-H do not directly relate to the n-type doping in the reference radical cation N3ꢁ is mainly distributed on the
process, they are worth a mention in passing. When N3-DMBI-H central nitrogen atom (Fig. 7).35 As expected from the ESR-activity of
was treated with 2 equiv. of AgSbF6 in CH2Cl2, the generated the dicationic species, the tricationic species generated by treatment
dicationic species was ESR-active (Fig. 6), providing strong evidence of 3 equiv. of AgSbF6 in CH2Cl2 was ESR-silent, thereby indicating
that the dicationic species is N3-DMBIꢁ++. The observed multiplet the formation of N3-DMBI3+.
hyperfine structure of N3-DMBIꢁ++ can be explained by the presence
of two sets (1N; 2N) of equivalent nitrogen nuclei, four sets (2H; 2H;
Doping effect and n-channel OFET device performance with
N3-DMBI-H as an n-type dopant
2H; 8H) of equivalent hydrogen nuclei, and the contributions from
the remaining hydrogen nuclei with negligibly small hyperfine To evaluate the doping effect of N3-DMBI-H, undoped and
coupling constants, which are incorporated in the line width N3-DMBI-H-doped PCBM OFET devices were fabricated with
(0.09 mT) of the spectrum (Fig. 6). Interestingly, it is noted that a bottom-gate/top-contact structure. A heavily doped n-type
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14
++
+
Fig. 7 (a) 1H and N hyperfine coupling constants for N3-DMBIꢁ and the reference compound N3ꢁ (see ref. 30), and (b) the DFT-computed spin
++
density distribution for N3-DMBIꢁ (UB3LYP/6-31G*).
silicon substrate covered with a 200 nm thick thermally grown recognized, which might be attributed to phase separation
oxide dielectric layer acted as a gate electrode. PCBM semi- or inhomogeneous distribution of the dopants in the PCBM
conductor thin film layers were formed by (i) drop-casting from thin films.
chlorobenzene solutions of PCBM and N3-DMBI-H at various
We firstly measured two-terminal current–voltage (I–V) char-
doping concentrations (0, 0.5, 2.0, and 5.0 wt%) and (ii) annealing acteristics of the PCBM thin films on the fabricated devices. As
at 100 1C for 10 h after drying in an N2-filled glovebox. Top-contact shown in Fig. 9, an appreciable increase in current values upon
source-drain electrodes (25 nm) were deposited by Au evaporation doping was observed by more than five orders of magnitude
through a shadow mask with L/W = 50/1000 mm. The electrical compared to those of the undoped PCBM thin film, though the
characterization was carried out under vacuum (1 ꢂ 10ꢀ2 Pa). To values were leveling off at doping concentrations larger than
check the influence of the dopant molecules on the morphology of 2 wt%. This result indicates that the electric resistance of PCBM
PCBM thin film, we performed atomic force microscopy (AFM) thin films is considerably decreased by N3-DMBI-H doping,
observations on the PCBM thin film surfaces (Fig. 8). In a previous probably due to carrier (electron) doping effects. Note that the
study on N-DMBI-H-doped PCBM thin films,22 the aggregation carrier doping effect of N3-DMBI-H was comparable to that of
behavior of N-DMBI-H dopants in the films was confirmed at N-DMBI-H; the two-terminal current–voltage (I–V) characteristics of
a higher doping ratio. In our N3-DMBI-H-doped samples, undoped and N-DMBI-H-doped PCBM thin films measured under
aggregation of the dopants did not appear obviously and typical the same conditions are provided as ESI† (Fig. S2). Additionally, in
root-mean-square surface roughnesses for the three samples the optical absorption spectrum for the 5 wt% doped PCBM thin
were similar, as seen in Fig. 8. However, in comparison between film deposited on a quartz substrate, the lowest-energy band
the undoped (Fig. 8a) and doped samples (Fig. 8b and c), some originating from the generation of radical anion species of PCBM
small differences in the fine features of the topography can be was confirmed at around 1030 nm (1.20 eV), also supporting n-type
Fig. 8 AFM images of (a) undoped, (b) 2 wt% N3-DMBI-H-doped, and (c) 5 wt% N3-DMBI-H-doped PCBM thin films (scan area: 3 mm ꢂ 3 mm). Typical
root-mean-square surface roughnesses for the three samples were estimated to be 0.28, 0.25, and 0.23 nm, respectively.
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significantly improved the chemical stability in solution phase,
as compared to N-DMBI-H as one of the few solution-processable
organic n-type dopants. Although the electrochemical process
does not have directly to do with the real n-doping mechanism, it
has been deduced from electrochemical, spectroelectrochemical,
and computational studies that the one-electron-oxidized species
of both N-DMBI-H and N3-DMBI-H readily lose a hydrogen atom
to create diamagnetic cationic species N-DMBI+ and N3-DMBI+,
thus shedding some light on understanding of the elusive
oxidation process of DMBI-H derivatives. In addition to the
preeminent solution-processability of N3-DMBI-H, the n-type
doping ability has also been demonstrated by the electrical
measurements of N3-DMBI-H-doped PCBM thin films, providing
the possibility of n-type doping in air.
Experimental section
Fig. 9 Two-terminal I–V characteristics of undoped and N3-DMBI-H-
doped PCBM thin films.
Electrochemical measurements
The redox properties were evaluated by cyclic voltammetry (CV)
in CH2Cl2 solution at 298 K with 0.1 M tetra-n-butylammonium
tetrafluoroborate (TBABF4) as supporting electrolyte (scan rate
100 mV sꢀ1) using an ALS/CHI electrochemical analyzer model
612A. A three-electrode assembly was used, which was equipped
with a platinum disk (2 mm2), a platinum wire, and Ag/0.01 M
AgNO3 (acetonitrile) as the working electrode, the counter electrode,
and the reference electrode, respectively. The redox potential were
referenced against a ferrocene/ferrocenium (Fc0/+) redox potential
measured in the same electrolytic solution.
doping by N3-DMBI-H (Fig. S3, ESI†). From the transfer length
method (TLM) analysis,45,46 the conductivities of the present doped
samples were estimated to be 5.4 ꢂ 10ꢀ4 and 6.4 ꢂ 10ꢀ4 S cmꢀ1
,
again indicating an increase of conductivity of more than five orders
of magnitude, compared to that of the undoped PCBM thin film
(6.6 ꢂ 10ꢀ9 S cmꢀ1). For comparison, the electrical conductivity of
the N-DMBI-H-doped PCBM thin film (at 10 wt%) was estimated to
be 1.0 ꢂ 10ꢀ3 S cmꢀ1, based on the I–V characteristics shown in
Fig. S2 (ESI†). According to the previous literature,22 the maximum
value of the conductivity of the N-DMBI-H-doped PCBM thin film
was reported to be 1.9 ꢂ 10ꢀ3 S cmꢀ1. In terms of these conductivity
measurements, we clearly ascertained that N3-DMBI-H can work as
a solution-processable donor dopant like N-DMBI-H.
Spectroelectrochemical measurements
UV/vis-NIR optical absorption spectra were obtained with a Perkin-
Elmer Lambda 950 spectrometer. Spectroelectrochemical
measurements were carried out with a custom-made optically
transparent thin-layer electrochemical (OTTLE) cell (light pass
length = 1 mm) equipped with a platinum mesh, a platinum
coil, and a silver wire as the working electrode, the counter
electrode, and the pseudo-reference electrode, respectively. The
potential was applied with an ALS/CHI electrochemical analyzer
model 612A.
Next, information about the performance of the present
N3-DMBI-H-doped PCBM OFET devices was obtained from
the transfer and output curves in Fig. 10. Consequently, the
0.5 wt% doped PCBM device exhibited typical behavior for
n-channel FET devices, and the electron field-effect mobility
(me) from the saturation regime was evaluated to be 2.3 ꢂ
10ꢀ3 cm2 Vꢀ1 ꢀ1, together with the threshold voltage (Vth) of
s
ESR measurements
7.5 V (Fig. 10a and b). In contrast to the doped PCBM thin film,
we could not observe any FET characteristics for the undoped
PCBM thin film (Fig. 10c), although pristine PCBM is considered as
a native n-type semiconductor.16 This is probably because hydroxyl
groups generated on the UV/ozone-treated silicon oxide surface
inevitably acted as electron trap sites, thus leading to no observation
of n-channel FET behavior.47 Accordingly, n-type doping by
N3-DMBI-H enhanced both the decrease of the electron trap sites
and the increase of electron density in the PCBM thin film, causing
the FET characteristics of the doped PCBM thin film.
ESR spectra were recorded with a JEOL JES-TE200 X-band ESR
spectrometer, in which temperature was controlled by a JEOL
ES-DVT3 variable-temperature unit in the range of 120–300 K.
A Mn2+/MnO solid solution was used as a reference.
Quantum chemical calculations
Quantum chemical calculations were performed using a hybrid
Hartree–Fock/density functional theory (HF/DFT) method
((U)B3LYP).48 Full geometrical optimization of N-DMBI-H,
+
+
N-DMBI-Hꢁ , N-DMBIꢁ, N-DMBI+, N3-DMBI-H, N3-DMBI-Hꢁ
,
N3-DMBIꢁ, N3-DMBI+, and N3-DMBIꢁ were carried out, and
++
Conclusions
furthermore, their local minimum structures were confirmed
The introduction of dendritic oligoarylamine groups at the 2-position by performing subsequent frequency analyses. Excitation energies
of 1,3-dimethyl-2,3-dihydro-1H-benzimidazole (N3-DMBI-H) has were computed on the basis of the time-dependent density
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Fig. 10 Bottom-gate/top-contact structured OFET device performance: (a) output and (b) transfer characteristics of 0.5 wt% N3-DMBI-H-doped PCBM
thin film, and (c) output curves of undoped PCBM thin film, which were fabricated by drop-casting from chlorobenzene solution.
functional method (TD-DFT)49 by using the (U)B3LYP/6-31G*
Synthesis of dendritic triamine aldehyde 2. A mixture of 1 34
optimized structures. All the computations employed the 6-31G* (1.25 g, 2 mmol), p-bromobenzaldehyde (0.556 g, 3 mmol),
basis set.50 All these computational approaches are implemented NaOtBu (0.288 g, 3 mmol), Pd(dba)2 (58 mg, 0.1 mmol), and
in the Gaussian 09 package of ab initio MO calculation.51
P(tBu)3HBF4 (58 mg, 0.2 mmol) in toluene (15 mL) was refluxed
under Ar atmosphere for 24 h. The resulting mixture was cooled
down to room temperature and washed with brine. The organic
Synthesis and characterization
All the purchased reagents were of standard quality, and used layer was separated and dried over Na2SO4. After evaporation of
without further purification. N-DMBI-H was prepared by following the solvent, the crude product was chromatographed on silica
the reported method.40 All the purchased solvents were purified, gel (n-hexane : ethyl acetate = 1 : 1 as eluent) to afford 2 as
dried, and degassed by standard procedures. Column chromato- a yellow solid (515 mg, 35.4%); m.p. 102–104 1C; 1H NMR
graphy was carried out with silica gel (Kanto Chemical Co., Inc., (400 MHz, CDCl3) d 9.74 (s, 1H), 7.62 (d, J = 8.8 Hz, 2H), 7.07 (d,
1
silica gel 60N, spherical neutral). H and 13C NMR spectra were J = 9.0 Hz, 8H), 6.98 (d, J = 8.5 Hz, 4H), 6.92 (d, J = 8.8 Hz, 2H),
measured with
a JEOL JNM-AL400 FT-NMR spectrometer. 6.88 (d, J = 8.8 Hz, 8H), 6.83 (d, J = 9.0 Hz, 4H), 3.79 (s, 12H);
Chemical shifts of NMR spectra are determined relative to the 13C NMR (100 MHz, CDCl3): d 189.9, 155.8, 153.8, 146.1, 140.6,
internal tetramethylsilane (TMS) standard (d), and are given in parts 138.0, 131.2, 127.6, 127.2, 126.5, 121.1, 116.9, 114.6, 55.4; ESI
per million (ppm). High resolution (HR) electrospray ionization HRMS: m/z calcd for C47H41N3O5: 727.3041 [M+]; found 727.3031.
(ESI) mass spectra were recorded using a mass spectrometer
(Thermo Fisher EXACTIVE). Elemental analysis was performed by phenylenediamine35 (2 mmol, 272 mg) in methanol (2 mL),
Synthesis of N3-DMBI-H. To a solution of N,N0-dimethyl-o-
the Center for Organic Elemental Microanalysis, Kyoto University.
2 (1 mmol, 727 mg) was added at room temperature, and then a
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Journal of Materials Chemistry C
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1 day, the resulting mixture was evaporated. The crude product
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2 : 1 as eluent) to afford N3-DMBI-H as a yellow solid (182.9 mg,
1
21.6%); m.p. 168–169 1C; H NMR (400 MHz, CDCl3) d 7.35 (d,
J = 8.5 Hz, 2H), 7.04 (d, J = 8.8 Hz, 8H), 7.04 (d, J = 9.0 Hz, 2H),
6.96 (d, J = 9.0 Hz, 4H), 6.86 (d, J = 9.0 Hz, 4H), 6.81 (d,
J = 8.8 Hz, 8H), 6.70 (m, 2H), 6.41 (m, 2H) 4.78 (s, 1H), 3.78 (s,
12H), 2.57 (s, 6H); 13C NMR (100 MHz, CD2Cl2): d 156.0, 149.7,
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120.8, 119.3, 114.9, 105.8, 94.0, 55.9, 33.4; ESI HRMS: m/z calcd
for C55H51N5O4: 844.3857 [M ꢀ H]+; found 844.3845; elemental
analysis (%) calcd for C55H51N5O4: C, 78.08; H, 6.08; N, 8.28;
found: C, 77.87; H, 6.13; N, 8.13.
¨
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Bottom-gate/top-contact (BG/TC) OFETs were constructed on
heavily doped n-type silicon wafers (o0.02 O cm; size 1.5 cm ꢂ
1.5 cm) covered with thermally grown SiO2 (200 nm), which
were cleaned by ultrasonication in EtOH, vapor degreasing, and
then treatment in an UV-ozone cleaner (Filgen UV253E) for
30 min. The undoped and doped PCBM solutions (0, 0.5, 2.0,
and 5.0 wt%) were drop-cast onto the substrates as active layers,
and then annealed at 100 1C for 10 h in order to activate
N3-DMBI-H molecules as precursors of n-type dopants. BG/TC
OFETs were completed by forming top-contact source-drain
electrodes (25 nm) by Au evaporation under vacuum (1 ꢂ 10ꢀ4 Pa)
using a shadow mask with L/W = 50/1000 mm. All the processes were
performed in an N2-filled glovebox. The electrical measurements of
the fabricated devices were conducted under vacuum (1 ꢂ 10ꢀ2 Pa)
using a Hewlett Packard HP4155 semiconductor parameter analyzer.
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thin films was observed by tapping mode AFM with an Innova
AFM (Bruker).
´
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Conflicts of interest
21 S. Olthof, S. Singh, S. K. Mohapatra, S. Barlow, S. R. Marder,
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23 N. Shintaku, M. Hiramoto and S. Izawa, J. Phys. Chem. C,
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There are no conflicts to declare.
Acknowledgements
This work was supported by the Project of International Standards
Development on Energy Conservation of Ministry of Economy,
Trade, and Industry of Japan (METI). Numerical calculations were
partly performed at the Supercomputer System of Kyoto University
(Japan) and the Research Center for Computational Science in
Okazaki (Japan). M. U. thanks the JSPS Research Fellowship for
Young Scientists.
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41 A reviewer has remarked that the degradation of N-DMBI-H
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inside the glove box. Instead of spincoating, we chose the
drop-casting method for sample preparation. In the case of
drop-casting of PCBM solution in chlorobenzene, the sub-
strate surface should be hydrophilic, otherwise continuous
(uniform) film cannot be deposited at all. However, the SAM
modification and organic coating make the SiO2 surface
hydrophobic. Therefore, we were forced to use UV-ozone-
treated silicon oxide as the dielectrics. Probably long (30 minutes)
UV-ozone-treatment for SiO2 caused the abnormal OFET
behavior shown in Fig. 10c. We did not intend to magnify
the n-doping effect artificially at all. In relation to the
present surface treatment for SiO2, the output characteristics
(Fig. 10a) still suffered from the effects of electron traps
existing at the interface between the SiO2 gate insulator and
the PCBM film. These electron traps mainly derive from
hydroxyl groups on the SiO2 surface and can be suppressed
by electron doping in the transistor channel region with
N3-DMBI-H dopants. However, we think that electron
accumulation and transport in the channel under gate bias
were still hindered to a certain extent due to the remaining
electron traps, even after using a 0.5 wt% N3-DMBI-H-doped
PCBM layer as an active layer of the transistor.
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