Development of low bandgap polymers for red and near-infrared fullerene-free organic photodetectors

Article abstract of DOI:10.1039/d1nj01694f

Two photoconductive conjugated polymers (PDTPTT and PCPDTTT) were synthesized to be utilized in red and near-infrared (NIR) organic photodetectors (OPDs). The low bandgap was achieved by stabilizing the quinoidal structure of the conjugated backbone, and both donor polymers showed strong red and NIR absorption in the range of 500-900 nm. To enhance the exciton separation and intensify the red and NIR absorption, p-n bulk heterojunction OPDs were fabricated by blending a PDTPTT (or PCPDTTT) and a low bandgap nonfullerene acceptor (IDIC). The PCPDTTT:IDIC devices showed excellent OPD performances with a detectivity (D*) of 1.14 × 10¹²Jones and a ?3 dB bandwidth (f_?3dB) of 211.7 Hz at ?1 V, whereas the PDTPTT:IDIC devices were not successful due to the high dark current density (J_D) at negative bias. The interfacial energies of the PDTPTT:IDIC and PCPDTTT:IDIC blends were calculated by measuring the solvent contact angles and we found that the lower interfacial energy of the PCPDTTT:IDIC blends could make a well-mixed nanomorphology in the blend films, resulting in superior OPD properties. On the other hand, the shallow HOMO energy level (?4.66 eV) of PDTPTT could make substantialJ_D, which showed suboptimal OPD performances.

Full text of DOI:10.1039/d1nj01694f

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Development of low bandgap polymers for red
and near-infrared fullerene-free organic
photodetectors†
Cite this: New J. Chem., 2021,
45, 10872
WonJo Jeong,‡a Jinhyeon Kang,‡ab Moon-Ki Jeong,c Jong Ho Wonb and
a
In Hwan Jung
*
Two photoconductive conjugated polymers (PDTPTT and PCPDTTT) were synthesized to be utilized in
red and near-infrared (NIR) organic photodetectors (OPDs). The low bandgap was achieved by
stabilizing the quinoidal structure of the conjugated backbone, and both donor polymers showed strong
red and NIR absorption in the range of 500–900 nm. To enhance the exciton separation and intensify
the red and NIR absorption, p–n bulk heterojunction OPDs were fabricated by blending a PDTPTT (or
PCPDTTT) and a low bandgap nonfullerene acceptor (IDIC). The PCPDTTT:IDIC devices showed excellent
OPD performances with a detectivity (D*) of 1.14 ꢀ 1012 Jones and a ꢁ3 dB bandwidth (fꢁ3dB) of 211.7 Hz
at ꢁ1 V, whereas the PDTPTT:IDIC devices were not successful due to the high dark current density (JD)
at negative bias. The interfacial energies of the PDTPTT:IDIC and PCPDTTT:IDIC blends were calculated by
measuring the solvent contact angles and we found that the lower interfacial energy of the PCPDTTT:IDIC
blends could make a well-mixed nanomorphology in the blend films, resulting in superior OPD properties.
On the other hand, the shallow HOMO energy level (ꢁ4.66 eV) of PDTPTT could make substantial JD,
which showed suboptimal OPD performances.
Received 7th April 2021,
Accepted 12th May 2021
DOI: 10.1039/d1nj01694f
rsc.li/njc
is utilized in color-selective OPDs.16,17 Currently, color-selective
photoconductive materials for red, green, blue and near-infrared
Introduction
Polymer-based organic photodetectors (OPDs) have received (NIR) OPDs have been developed and several synthetic strategies
great attention due to their light weight, solution processability, were reported to increase the responsivity (R) and detectivity (D*)
strong light-absorption coefficient and color tunability of the in the devices.18–20 R and D* are the figures of merit used to
photoconductive layer.1–7 However, due to the strong exciton evaluate OPD performance. The R is expressed as the photo-
binding energy of organic materials, the photoconductive layer current density (Jph) per input light power (Pin), which indicates
is commonly required to make bulk heterojunction binary how many photons are changed into current. However, Jph
blends composed of a polymer donor and an electron acceptor includes both photocurrent and dark current densities (JD), and
to increase exciton diffusion and separation.8–11 The combination thus R values vary depending on the charge transport mechanism
of a donor and an acceptor having different absorption areas and operating voltage.4,21 Thus, D* considering both the R value
makes broad absorption which is the basic strategy to build and noise spectral density is more important to judge photo-
panchromatic absorption OPDs.12–15 On the other hand, detecting properties.
matching of the absorption area between a donor and an acceptor
In the case of blue selective OPDs, high resonance energy
benzene rings are commonly introduced in the conjugated
backbone not to decrease the bandgap of a polymer donor.
However, because it is difficult to make blue selective electron
acceptors, Schottky diodes are commonly reported for blue
selective OPDs.22–24
a Department of Organic and Nano Engineering, and Human-Tech Convergence
Program, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763,
Republic of Korea. E-mail: inhjung@hanyang.ac.kr
b Department of Chemistry, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu,
Seoul 02707, Republic of Korea
In the case of green selective OPDs, the combination of
benzene and thiophene rings in the conjugated backbone is
highly efficient to make the middle bandgap polymer donor
and the representative examples are benzo[1,2-b:4,5-b0]
dithiophene-based polymers.16,25 As an acceptor, fullerene
derivatives such as [6,6]-phenyl C70 butyric acid methyl ester
c School of Chemical and Biological Engineering, Seoul National University,
1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
† Electronic supplementary information (ESI) available: Experimental details,
GPC and 1H NMR spectra of the synthesized polymers. See DOI: 10.1039/
d1nj01694f
‡ W. Jeong and J. Kang contributed equally to this work.
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(PC70BM) are commonly used2 and short p-conjugated small- length alternation (BLA) of the conjugated ring system.44
molecule acceptors are recently reported.26
The reduction of BLA energy indicates low bandgap
a
In the case of red selective OPDs, highly p-conjugated back- polymer.45 The synthesized PDTPTT and PCPDTTT donors
bone structures are required and thus both low bandgap showed broad absorption in the range of 400–1000 nm, and
donors and acceptors have been extensively studied. Pecunia the maximum absorption peak appeared at around 700 nm. To
et al. made p–n bulk heterojunction OPDs composed of a intensify the red and NIR absorption, a 2,20-[(4,4,9,9-tetrahexyl-
squaraine-based small-molecule donor and a polycyclic aromatic 4,9-dihydro-s-indaceno[1,2-b:5,6-b0]-dithiophene-2,7-diyl)bis[methyl-
nonfullerene acceptor (NFA). Red selective OPDs were achieved idyne(3-oxo-1H-indene-2,1(3H)-diylidene)]]bis-propanedinitrile
with a D* of 4.70 ꢀ 1011 Jones without any bias and a full width (IDIC) NFA acceptor46 was blended with the synthesized donor
at half maximum (FWHM) of 186 nm. By replacing a donor with polymers, and the p–n bulk heterojunction red and NIR
a hole transporting polymer, D* could be increased up to 1.42 ꢀ photodetectors were successfully developed.
1013 Jones at 0 V and an FWHM of 141 nm.27 Kim et al.
developed
By calculating the interfacial energy between a polymer
diketopyrrolopyrrole-based red polymer, and donor and a NFA acceptor and between organic materials and
a
reported Schottky OPDs with a D* of 4.63 ꢀ 1012 Jones and an a processing solvent (chloroform), we found that the low
FWHM of 148 nm.28 Dong et al. reported a squaraine-based interfacial energy between PDTPTT and CHCl3 could result in
small molecule and achieved a D* of 3.2 ꢀ 1012 Jones at ꢁ2 V uniform and smooth film surfaces, and the low interfacial
with an FWHM of 80 nm via a vacuum-deposited Schottky energy between PCPDTTT and IDIC could make a well-mixed
OPD.29 Luszczynska et al. blended a hexafluoroquinoxaline- bulk heterojunction layer with low bimolecular recombination
based donor and an NFA, and reported a D* of 2 ꢀ 1013 Jones and fast signal response in OPDs. Notably, we suggest that the
at ꢁ2 V in the red area.30
shallow highest occupied molecular orbital (HOMO) energy
In the case of NIR OPDs, broad-band absorption OPDs level of PDTPTT and the deep lowest unoccupied molecular
covering the red and NIR areas have been developed.31–33 orbital (LUMO) energy level of IDIC can generate substantial JD
Nguyen et al. developed a cyclopentadithiophene-based small- at negative bias, whereas PCPDTTT maintaining a suitable
molecule acceptor and reported a D* of around 1012 Jones at HOMO level was effective to reduce JD in OPDs. As a result,
ꢁ2 V for the NIR spectral region (up to 1010 nm) by blending PCPDTTT:IDIC devices showed promising photodetecting per-
with a low bandgap polymer donor.34 Maes et. al. reported bay- formance with a D* of 1.14 ꢀ 1012 Jones and a ꢁ3 dB bandwidth
annulated indigo-based NIR absorption copolymers and ( fꢁ3dB) of 211.7 Hz at ꢁ1 V.
showed a D* of 1012 Jones at a bias of ꢁ2 V within the spectral
window of 600–1100 nm.35 Someya et al. used an
indacenodithiophene-based low bandgap polymer and devel-
Results and discussion
Synthesis and characterization of the polymers
oped ultra-flexible NIR-responsive OPDs with an outstanding
cut-off response frequency over 1 kHz at ꢁ3 dB.36 Ng et al.
developed shortwave infrared OPDs with photoresponse up to The synthetic route for the PDTPTT and PCPDTTT is shown in
1400 nm, and showed a D* of around 1011 Jones at 0 V.37 Huang Scheme 1. 2-Ethylhexyl 4,6-dibromo-3-fluorothieno[3,4-b]thio-
et al. reported self-filtering narrowband NIR OPDs with an phene-2-carboxylate (compound 5) was purchased from
FWHM of B50 nm via manipulating the dissociation of Frenkel SunaTech Inc., and 3,30-dibromo-2,20-bithiophene and 4H-
excitons and showed a high D* of 1.2 ꢀ 1013 Jones at 860 nm cyclopenta[2,1-b:3,4-b0]dithiophene were synthesized from the
under ꢁ0.1 V.38
previously reported literature.39,43
Compound 1 was synthesized by the Buchwald–Hartwig
amination of 3,30-dibromo-2,20-bithiophene and 2-ethylhexylamine,
In this study, we developed two low bandgap polymers,
poly[(4-(2-ethylhexyl)-4H-dithieno[3,2-b:20,30-d]pyrrole-2,6-diyl)-
alt-(2-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate 4,6- and compound 3 was obtained by the substitution of two
diyl)] (PDTPTT) and poly[(4,4-bis(2-ethylhexyl)-4H-cyclopenta 2-ethylhexyl side chains on 4H-cyclopenta[2,1-b:3,4-b0]
[2,1-b:3,4-b0]dithiophene-2,6-diyl)-alt-(2-ethylhexyl-3-fluorothieno dithiophene. The final monomers 2 and 4 were synthesized
[3,4-b]thiophene-2-carboxylate 4,6-diyl)] (PCPDTTT), by Stille from compound 1 and compound 3, respectively, by the con-
coupling of distannylated dithieno[3,2-b:20,30-d]pyrrole (DTP) tinuous process of lithiation using BuLi and distannylation
monomer (or distannylated cyclopenta[2,1-b:3,4-b0]dithiophene using trimethyltin chloride. The synthesized monomers were
(CPDT) monomer) and dibrominated thieno[3,4-b]thiophene used directly for polymerization.
(TT). DTP39–41 and CPDT42,43 are well-known strong electron-
PDTPTT and PCPDTTT were obtained by microwave-assisted
donating (SD) moieties. The fused ring system of two thiophene Stille polycondensation of compounds 2 and 5, and compounds
rings offers SD properties and the organic soluble alkyl side 4 and 5, respectively. Both polymers exhibited good solubility in
chains can be introduced at the bridging nitrogen or carbon tetrahydrofuran (THF), dichloromethane, chloroform, etc.
ꢀ
ꢁ
atom. The resonance effect of the bridging nitrogen makes the
The number average molecular weight Mn of polymers
DTP ring more electron-donating than the CPDT ring. To make was determined by gel permeation chromatography (GPC), and
low bandgap polymers, a fluorinated TT ring was copolymerized those of PDTPTT and PCPDTTT were 4 kDa (Ð = 1.2) and
with DTP or CPDT. TT rings can stabilize the quinoidal structure 6.2 kDa (Ð = 1.4), respectively. We tried to increase the
of the conjugated polymer backbone, which facilitates the bond molecular weight of the polymers using a microwave reactor,
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film states, and thus both polymers have similar absorption
patterns in film states. The optical bandgaps of PDTPTT and
PCPDTTT were estimated from the absorption onset wave-
lengths (lonset) of 975 nm and 940 nm, respectively, in film
states, and the corresponding bandgaps were 1.27 eV and
1.32 eV, respectively. The bandgap of PDTPTT was lower than
that of PCPDTTT, which is because the DTP moiety in PDTPTT
has stronger electron-donating properties than the CPDT unit
in PCPDTTT.
The HOMO and LUMO energy levels of the polymers were
estimated from cyclic voltammetry and shown in Fig. 1b.47
The oxidation onset potentials of PDTPTT and PCPDTTT were
ꢁ0.07 and 0.40 V, respectively, which correspond to ꢁ4.66 eV
and ꢁ5.14 eV, respectively. The shallow HOMO energy level of
PDTPTT indicates its strong electron-donating characteristics.
The reduction onset potentials of PDTPTT and PCPDTTT
similarly appeared near ꢁ1.2 V, which correspond to ꢁ3.5 eV.
The identical electron-withdrawing TT moieties on both
Scheme 1 Synthetic route for poly[(4-(2-ethylhexyl)-4H-dithieno
[3,2-b:20,30-d]pyrrole-2,6-diyl)-alt-(2-ethylhexyl-3-fluorothieno[3,4-b]thio- polymers resulted in similar LUMO energy levels. As a result,
phene-2-carboxylate 4,6-diyl)] (PDTPTT) and poly[(4,4-bis(2-ethylhexyl)-
the electron-donating properties of DTP and CPDT cores greatly
contributed to the determination of the HOMO energy levels of
the polymers, whereas the electron-withdrawing TT moiety
4H-cyclopenta[2,1-b:3,4-b0]dithiophene-2,6-diyl)-alt-(2-ethylhexyl-3-fluorothieno
[3,4-b]thiophene-2-carboxylate 4,6-diyl)] (PCPDTTT).
contributed to the determination of the LUMO energy levels
but it was not successful probably due to the impurity in the of the polymers.48 The energy level diagram is described in
distannylated monomers 2 and 4. However, both polymer Fig. 1c, and the optical and electrochemical properties are
solutions make viscous flows and thus we could obtain uniform summarized in Table 1.
and well-coated polymer films enough for the study on the
photodetecting properties. The 1H NMR spectra of the
Photodetecting properties
monomers and polymers and GPC spectra were recorded and Bulk heterojunction OPDs were fabricated with a device structure
are shown in the ESI† (Fig. S1–S6).
of ITO/ZnO/PEIE/PDTPTT:IDIC or PCPDTTT:IDIC/MoOx/Ag
The UV-vis absorption spectra of PDTPTT and PCPDTTT in (Fig. 2a). A collimated 680 nm red LED (Thorlabs, M680L4) was
solution and film were recorded and are shown in Fig. 1a. Both used as a light source and the OPD properties were measured as a
polymers showed strong red and NIR absorption in the range of function of light intensity. The current density–voltage ( J–V)
550–900 nm. The maximum absorption peak of PDTPTT and characteristics of the PDTPTT:IDIC and PCPDTTT:IDIC devices
PCPDTTT was 667 and 660 nm, respectively, in chloroform, and are shown in Fig. 2b and c. Because OPDs require fast response
678 and 696 nm, respectively, in film. PCPDTTT showed a blue- time to be utilized in image sensors, we studied their OPD
shifted absorption peak compared to PDTPTT in solution, but properties in the photoconductive mode (ꢁ1 V and ꢁ2 V) which
the absorption peak of PCPDTTT was strongly red-shifted in are summarized in Table 2.
Fig. 1 (a) Absorption spectra of PDTPTT (black line) and PCPDTTT (red line) in solution (dotted line) and film (solid line); the blue line indicates IDIC
absorption. (b) Cyclic voltammogram of PDTPTT, PCPDTTT, IDIC and ferrocene; the green line indicates ferrocene reference (the halfway potential (E1/2
between ferrocenium (Fc+) and ferrocene (Fc) peaks is estimated to be ꢁ4.8 eV). (c) Energy level diagram of PDTPTT, PCPDTTT and IDIC.
)
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Table
1
Optical and electrochemical properties of the synthesized
PDTPTT:IDIC devices was more realistic to judge the OPD
properties. D* is expressed as eqn (1), and determined by the
noise spectral density and R value of the devices.
PDTPTTP and PCPDTTT
lmax (nm)
CV
HOMO
CV
LUMO
Eox (V)/E
Ere (V)/E
ECgV
pffiffiffiffiffiffiffiffiffi
Solutiona Filmb (eV)c
(eV)d
(eV)e
Â
Ã
ADf
NEP
R
Dꢂ ¼
¼
cmHz0:5
W
ꢁ1; jones
(1)
pffiffiffiffiffiffiffiffiffi
PDTPTT
PCPDTTT 660
IDIC
667
678
696
707
ꢁ0.07/ꢁ4.66
0.40/ꢁ5.14
0.99/ꢁ5.72
ꢁ1.25/ꢁ3.48
ꢁ1.26/ꢁ3.47
ꢁ0.81/ꢁ3.92
1.22
1.63
1.78
2qJd
—
Due to the low JD values of the PCPDTTT:IDIC devices, the
D* values of the PCPDTTT:IDIC devices were at least 10 times
higher than those of the PDTPTT:IDIC devices (Table 2). In
addition, as shown in Fig. 3a, the D* values of the PCPDTTT:
IDIC devices were much higher than those of the PDTPTT:IDIC
devices at all the wavelength area, and the maximum D* value
of the PCPDTTT:IDIC device at 710 nm was 20 times higher
than that of the PDTPTT:IDIC device.
a
b
Maximum absorption in chloroform solution. Maximum absorption
in film states fabricated by spin-coating of polymer solution.
c
Oxidation potential (Eox) and the corresponding HOMO energy level.
Reduction potential (Ere) and the corresponding LUMO energy level.
Electrochemical bandgap from CV measurement.
d
e
The ratio between Jph and JD is called the on/off ratio (Jph/JD),
which is one of the most important parameters to determine
photodetecting properties. As shown in Fig. 2b and c, and
Table 2, the on/off ratios of PDTPTT:IDIC and PCPDTTT:IDIC
devices were 3.88 and 3.56 ꢀ 103, respectively, at ꢁ2 V under
the light irradiation of 5 mW cmꢁ2. The PCPDTTT:IDIC device
showed 1000 times higher on/off ratio than the PDTPTT:IDIC
device, indicating superior photodetecting properties of the
PCPDTTT:IDIC device.
Responsivity (R) indicates how efficiently photons are changed
into current density in the devices, and was calculated from the
Jph per Pin (Table 2). The R value of the PDTPTT:IDIC device was
0.192 A Wꢁ1 at ꢁ2 V under 5 mW cmꢁ2 light power, but its value
was significantly increased as the LED power density was
decreased. As a result, the R value of the PDTPTT:IDIC device
reached 2.44 A Wꢁ1 at ꢁ2 V under 0.010 mW cmꢁ2. However, its
high R value was not obtained from the photocurrent density, but
mostly from the JD in OPDs. Thus, we could understand that
evaluating the device performance by R alone was meaningless. In
the case of PCPDTTT:IDIC devices, the R values were relatively
constant depending on the light intensity: 90 mA Wꢁ1 at a light
intensity of 0.1 mW cmꢁ2 and 64 mA Wꢁ1 at 5 mW cmꢁ2. Thus,
we suggest that R values of OPDs are only reliable when the on/off
ratio differed by more than 100.
The ꢁ3 dB bandwidth (fꢁ3dB) was obtained by confirming
the frequency response which corresponds to ꢁ3 dB from the
initial value at ꢁ1 V of the bias voltage as shown in Fig. 3b. The
f
was 100.3 Hz for the PDTPTT:IDIC devices and 211.7 Hz
ꢁ3dB
for the PCPDTTT:IDIC devices, which implied that the
PCPDTTT:IDIC devices had a faster signal response than the
PDTPTT devices. As a result, both the static and dynamic
properties of the PCPDTTT:IDIC devices were superior to those
of the PDTPTT:IDIC devices.
Morphological and charge transport characteristics
The high JD of the PDTPTT:IDIC devices at negative bias was
closely related to the energy level difference between the HOMO
energy level of the donor (EHOMO,D) and the LUMO energy level
of the acceptor (ELUMO,A).49 The low energy gap (EHOMO,D
ꢁ
ELUMO,A) can generate electrical conductivity in the p–n junction
diode,49 which makes JD in the devices. In our study, PDTPTT
had a shallow HOMO energy level of ꢁ4.66 eV, which was 0.48 eV
higher than that of PCPDTTT (ꢁ5.14 eV). As shown in Fig. 2b
and 3c, JD of PDTPTT was significantly increased depending on
the negative bias and was dominant at low light irradiation. This
indicates the induced electric conductivity by applying for the
negative bias. On the other hand, PCPDTTT had a much deeper
Notably, the JD values of the PCPDTTT:IDIC devices were
effectively suppressed in the photoconductive mode, whereas
those of the PDTPTT:IDIC devices were not suppressed.
Thus, the comparison of D* between PCPDTTT:IDIC and
HOMO energy level and a larger energy gap (EHOMO,D ꢁ ELUMO,A
)
of 1.2 eV than PDTPTT, thus it was more difficult to make JD in
the PCPDTTT:IDIC devices. As shown in Fig. 3c, the low JD of the
Fig. 2 (a) OPD structures prepared using ITO/ZnO/PEIE/photoconductive layer/MoOx/Ag. J–V characteristics of the (b) PDTPTT:IDIC and
(c) PCPDTTT:IDIC devices as a function of flux under collimated 680 nm red LED illumination.
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Table 2 Summary of the photodetecting properties of PDTPTT:IDIC and PCPDTTT:IDIC devices
Bias (V)
Pin (W cmꢁ2
)
JD (A cmꢁ2
)
Jph (A cmꢁ2
)
On/off ratio
R (A Wꢁ1
)
D* (Jones)
PDTPTT:IDIC
PCPDTTT:IDIC
PDTPTT:IDIC
PCPDTTT:IDIC
ꢁ2
1.02 ꢀ 10ꢁ4
5.00 ꢀ 10ꢁ4
1.00 ꢀ 10ꢁ3
5.00 ꢀ 10ꢁ3
9.9 ꢀ 10ꢁ5
5.03 ꢀ 10ꢁ4
1.00 ꢀ 10ꢁ3
5.00 ꢀ 10ꢁ3
1.02 ꢀ 10ꢁ4
5.00 ꢀ 10ꢁ4
1.00 ꢀ 10ꢁ3
5.00 ꢀ 10ꢁ3
9.9 ꢀ 10ꢁ5
5.03 ꢀ 10ꢁ4
1.00 ꢀ 10ꢁ3
5.00 ꢀ 10ꢁ3
2.48 ꢀ 10ꢁ4
2.48 ꢀ 10ꢁ4
2.48 ꢀ 10ꢁ4
2.48 ꢀ 10ꢁ4
8.95 ꢀ 10ꢁ8
8.95 ꢀ 10ꢁ8
8.95 ꢀ 10ꢁ8
8.95 ꢀ 10ꢁ8
5.48 ꢀ 10ꢁ5
5.48 ꢀ 10ꢁ5
5.48 ꢀ 10ꢁ5
5.48 ꢀ 10ꢁ5
1.17 ꢀ 10ꢁ8
1.17 ꢀ 10ꢁ8
1.17 ꢀ 10ꢁ8
1.17 ꢀ 10ꢁ8
2.48 ꢀ 10ꢁ4
2.94 ꢀ 10ꢁ4
3.65 ꢀ 10ꢁ4
9.62 ꢀ 10ꢁ4
8.92 ꢀ 10ꢁ6
4.11 ꢀ 10ꢁ5
7.76 ꢀ 10ꢁ5
3.19 ꢀ 10ꢁ4
6.49 ꢀ 10ꢁ5
1.11 ꢀ 10ꢁ4
1.72 ꢀ 10ꢁ4
5.17 ꢀ 10ꢁ4
6.92 ꢀ 10ꢁ6
3.15 ꢀ 10ꢁ5
5.85 ꢀ 10ꢁ5
2.22 ꢀ 10ꢁ4
1.00
1.19
1.47
3.88
9.97 ꢀ 10
4.59 ꢀ 102
8.67 ꢀ 102
3.56 ꢀ 103
1.19
2.03
3.13
2.44
2.73 ꢀ 1011
6.61 ꢀ 1010
4.10 ꢀ 1010
2.16 ꢀ 1010
5.32 ꢀ 1011
4.83 ꢀ 1011
4.58 ꢀ 1011
3.77 ꢀ 1011
1.52 ꢀ 1011
5.31 ꢀ 1010
4.10 ꢀ 1010
2.47 ꢀ 1010
1.14 ꢀ 1012
1.03 ꢀ 1012
9.57 ꢀ 1011
7.25 ꢀ 1011
5.89 ꢀ 10ꢁ1
3.65 ꢀ 10ꢁ1
1.92 ꢀ 10ꢁ1
9.01 ꢀ 10ꢁ2
8.17 ꢀ 10ꢁ2
7.76 ꢀ 10ꢁ2
6.37 ꢀ 10ꢁ2
6.37 ꢀ 10ꢁ1
2.22 ꢀ 10ꢁ1
1.72 ꢀ 10ꢁ1
1.03 ꢀ 10ꢁ1
6.99 ꢀ 10ꢁ2
6.27 ꢀ 10ꢁ2
5.85 ꢀ 10ꢁ2
4.43 ꢀ 10ꢁ2
ꢁ2
ꢁ1
ꢁ1
9.44
5.93 ꢀ 102
2.70 ꢀ 103
5.01 ꢀ 103
1.90 ꢀ 104
Fig. 3 (a) Detectivity of PDTPTT:IDIC and PCPDTTT:IDIC devices at a bias of ꢁ2.0 V as a function of wavelength. (b) The frequency response curve of
OPDs at a bias of ꢁ1.0 V. (c) The Jph as a function of light power density under 680 nm collimated red LED illumination at ꢁ2 V.
PCPDTTT:IDIC devices was advantageous to detect low power microscopy (AFM) (Fig. S7, ESI†). The root-mean-square
light (0.010 mW cmꢁ2) signals. In addition, the excellent linear roughness (Rq) values of the PDTPTT:IDIC and PCPDTTT:IDIC
relationship between Pin and Jph (Pin = (Jph)a, a = 0.999) indicates surfaces were 1.10 and 2.63 nm, respectively. The PDTPTT:IDIC
the minimized bimolecular recombination process, facilitating blend film showed a more uniform and smooth film surface
charge transfer and fast signal response in OPDs.50–52
than the PCPDTTT:IDIC blend film.
The surface morphologies of the PDTPTT:IDIC and
To understand the morphological characteristics of these
PCPDTTT:IDIC films were evaluated by using atomic force blended films, the water and isopropyl alcohol contact angles
were measured on the pristine PDTPTT, PCPDTTT and IDIC
films using the sessile drop method (Fig. 4). From the contact
angles, the surface free energies of PDTPTT, PCPDTTT and
IDIC were calculated using Young’s equation (gLV cos y = gSV
+
53
gSL
)
and Owens and Wendt’s geometric mean equation.54
ꢂ
ꢃ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffiffiffiffi
gSL ¼ gLV þ gSV ꢁ 2
gdLVgdSV þ gpLVgp
(2)
SV
The contact angles and surface energies of PDTPTT,
PCPDTTT and IDIC are summarized in Table S1 (ESI†) and
Table 3, respectively. Finally, we could obtain the interfacial
energies between PDPTTT (or PCPDTTT) and IDIC and between
PDPTTT, PCPDTTT or IDIC and a processing solvent (chloroform)
using their surface energies. As shown in Table S2 (ESI†), the
Fig. 4 Wetting angle measurement by water on (a) PDTPTT, (b) PCPDTTT
and (c) IDIC films, and by isopropyl alcohol on (d) PDTPTT, (e) PCPDTTT
and (f) IDIC films.
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Table 3 Calculated dispersive (gdSV), polar (gSpV) and total (gSV) surface free
energies and polarity of each substance
mixture was poured into methanol, and the precipitate was
dissolved in chloroform and filtered through Celite to remove
the metal catalyst. The polymer fibers were washed by soxhlet
extraction with methanol, acetone and chloroform. The final
polymer was obtained after reprecipitation with methanol.
PDTPTT: 106.2 mg, 54.3%.
Surface energy (mJ mꢁ2
)
Polymer
gSV
gdSV
gpSV
Polarity
PCPDTTT
PDTPTT
IDIC
22.84
22.96
24.74
18.32
19.39
14.78
4.52
3.57
9.96
0.1977
0.1556
0.4026
Synthesis of PCPDTTT
PCPDTTT was synthesized with an identical procedure to
PDTPTT. Compound 4 (200 mg, 0.275 mmol) and compound
5 (130 mg, 0.275 mmol) were used to obtained the final
PCPDTTT (132.0 mg, 67.2%).
interfacial energy (1.25 mJ mꢁ2) between PCPDTTT and IDIC was
smaller than that (1.91 mJ mꢁ2) between PDTPTT and IDIC. This
indicates better mixing in the PCPDTTT:IDIC blend film, which
probably resulted in the faster signal response of the PCPDTTT:
IDIC devices. On the other hand, the interfacial energy
(0.862 mJ mꢁ2) between PDTPTT and chloroform was smaller
than that (1.394 mJ mꢁ2) between PCPDTTT and chloroform.
Thus, the smooth and uniform surface of the PDTPTT:IDIC blend
film would have originated from this low interfacial energy
between PDTPTT and chloroform.
Device fabrication
Indium-doped tin oxide-coated glass (ITO-coated glass) was
used as the substrate. Each ITO substrate was sonicated for
20 min in acetone, distilled water (D.W), and ethanol. Then, the
cleaned ITO-coated glasses were subjected to UV-O3 treatment
for 20 min. The fabricated photodetector was composed of ITO/
ZnO/photoconductive layer/MoOx/Ag. First, the ZnO layer was
used as the electron transporting layer (ETL) via a sol–gel
process. The precursor solution for the ZnO layer was prepared
by dissolving 0.45 M zinc acetate dehydrate (Zn(Ac)2ꢃH2O, Alfa
Aesar) and 0.45 M ethanolamine (NH2CH2CH2OH, Sigma
Aldrich) in 2-methoxyethanol (Alfa Aesar) for 3 h at 60 1C.
The prepared ZnO precursor solution was spin-coated onto the
UV-O3-treated ITO substrates at 3000 rpm for 30 s. The ZnO-
coated ITO substrates were placed onto a 200 1C hot plate and
annealed for 10 min. Polyethylene imine (PEIE, Sigma Aldrich,
polyethyleneimine, 80% ethoxylated solution, 37 wt% in H2O)
was used for an ETL buffer layer. 0.1 wt% PEIE solution was
prepared by diluting the purchased PEIE stock solution in
2-methoxyethanol. Then, the PEIE solution was spin-coated
onto the ZnO film at 5000 rpm for 60 sec. After spin-coating,
the coated substrates were thermally annealed at 150 1C for
10 min. For the photoconductive layer, PCPDTTT and PDTPTT
were used as donor materials, and IDIC was used as an acceptor
material. Donor and acceptor materials are prepared by stirring
with chloroform at a total concentration of 20 mg mlꢁ1 for at
least 3 hours. At this time, the ratio of the donor and acceptor
materials was 1 : 1 by weight ratio. The blended solution was
spin-coated onto the prepared ZnO/ITO substrates. All procedures
for the BHJ active layer were carried out in an N2-filled glove box.
A MoOx (B8 nm) layer and an Ag (B120 nm) layer were deposited
by thermal evaporation under a vacuum of B10ꢁ6 Torr for the
hole transporting layer (HTL) and electrode, respectively. The
calculated active layer was 0.0707 cm2.
Experimental
4-(2-Ethylhexyl)-2,6-bis(trimethylstannyl)-4H-dithieno[3,2-
b:20,30-d]pyrrole (2)
Compound 1 (0.55 g, 1.89 mmol) was dissolved in anhydrous
THF (15 mL). 2.5 M n-BuLi in hexanes (3.0 mL, 7.50 mmol) was
added dropwise to the solution at ꢁ78 1C and stirred for 2 h.
After the solution was stirred 3 h at room temperature, the
solution was cooled down to ꢁ78 1C again, and 1.0 M trimethyltin
chloride in THF (10 mL, 10 mmol) was added dropwise and
stirred overnight under N2 atmosphere. The reaction mixture was
then extracted with diethyl ether, washed with water, and dried
over MgSO4. After that, the solvent was removed to afford
1
compound 2 as a brown oil (0.817 g, 70.7%). H-NMR (400 MHz,
CDCl3, d): 6.96 (s, 2H), 4.06 (m, 2H), 1.98 (br, 1H), 1.31(m, 8H), 0.90
(t, J = 6.4 Hz, 6H), 0.40 (s, 18H)
(4,4-Bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]dithiophene-
2,6-diyl)bis(trimethylstannane) (4)
Compound 4 was synthesized with a similar procedure to
compound 2. Compound 3 (1.0 g, 2.48 mmol), 2.5 M n-BuLi
in hexanes (4.0 mL, 10 mmol) and 1.0 M trimethyltin chloride
in THF (12.4 mL, 12.4 mmol) were used to afford compound 4
1
as a brown oil (0.850 g, 47.1%). H-NMR (400 MHz, CDCl3, d):
6.94 (s, 2H), 1.83 (m, 4H), 0.98–0.83 (m, 18H), 0.73 (t, J = 6.4 Hz,
6H), 0.58 (t, J = 8.4 Hz, 6H), 0.36 (s, 18H).
Synthesis of PDTPTT
Compound 2 (200 mg 0.324 mmol), compound 5 (153 mg,
0.324 mmol), Pd2(dba)3 (9 mg, 0.01 mmol) and p-(o-toyl)3
Conclusions
(10 mg, 0.03 mmol) were mixed together in a microwave tube. PDTPTT and PCPDTTT, two low bandgap polymers, were
After that, degassed anhydrous toluene (2.0 mL) was added to synthesized via the Stille coupling of distannylated DTP or
the mixture. The reaction condition was programmed as CPDT monomer and dibrominated TT comonomer, respectively.
follows: 80 1C for 10 min, 120 1C for 10 min and 150 1C for Both donor polymers showed strong red and NIR absorption in
60 min in a microwave reactor. After polymerization, the the range of 500–900 nm, and a good spectral overlap with an
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Paper
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IDIC acceptor. p–n bulk heterojunction OPDs composed of
PDTPTT:IDIC and PCPDTTT:IDIC blend films were fabricated,
and the PCPDTTT:IDIC devices showed superior photodetecting
properties with a D* of 1.14 ꢀ 1012 Jones and an fꢁ3dB of 211.7 Hz
at ꢁ3 dB at ꢁ1 V compared to those of the PDTPTT:IDIC devices
(D* = 1.52 ꢀ 1011 Jones, f
= 100.3 Hz). By measurement of
ꢁ3dB
water and isopropyl alcohol contact angles on the PDTPTT,
PCPDTTT and IDIC films, their interfacial energies were
calculated and we found that PCPDTTT and IDIC can make
better mixing in blend films. In addition, the shallow HOMO
energy level (ꢁ4.66 eV) of PDTPTT could make substantial dark
current density, which resulted in a poor on/off ratio (Jph/JD) and
low D* in OPDs.
Author contributions
Prof. I. H. Jung planed and supervised this study. W. Jeong and
M.-K. Jeong synthesized monomers and polymers. W. Jeong
characterized all the materials. J. Kang fabricated and analyzed
the OPDs. Prof. J. H. Won studied the surface energy and
miscibility of the materials. Prof. I. H. Jung, W. Jeong, and
J. Kang wrote the manuscript.
Conflicts of interest
There are no conflicts to declare.
20 H. Ren, J.-D. Chen, Y.-Q. Li and J.-X. Tang, Adv. Sci., 2021,
8, 2002418.
21 D. Yang and D. Ma, Adv. Opt. Mater., 2019, 7, 1800522.
22 H. Guo, Y. Wang, R. Wang, S. Liu, K. Huang, T. Michinobu
and G. Dong, ACS Appl. Mater. Interfaces, 2019, 11,
16758–16764.
Acknowledgements
This study was supported by the National Research Foundation
of Korea (2016R1A5A1012966, 2019R1A2C1089081 and
2020M3H4A3081816). We appreciate Prof. Dae Sung Chung
and Ms Juhee Kim from the Pohang University of Science and
Technology for studying the dynamic characteristics of OPDs.
23 S. Yoon, Y.-H. Ha, S.-K. Kwon, Y.-H. Kim and D. S. Chung,
ACS Photonics, 2018, 5, 636–641.
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27 K. Xia, Y. Li, Y. Wang, L. Portilla and V. Pecunia, Adv. Opt.
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