Alcohol-soluble conjugated oligomers as the cathode interfacial layer in polymer solar cells

Article abstract of DOI:10.1080/15421406.2018.1456050

Two easily accessible fluorene-based conjugated oligo-electrolytes (COEs) FTF- and FBF-NBr have been developed as the cathode interfacial layer (CIL) in inverted type polymer solar cells (iPSCs). CILsare interpretative to improving the power conversion ef

Full text of DOI:10.1080/15421406.2018.1456050

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Alcohol-soluble conjugated oligomers as the
cathode interfacial layer in polymer solar cells
Youn Hwan Kim, Mutia Anissa Marsya, Nadhila Sylvianti, Dong Geun Kim,
Hee Lack Choi, Chan Young Park, Gun Dae Lee & Joo Hyun Kim
To cite this article: Youn Hwan Kim, Mutia Anissa Marsya, Nadhila Sylvianti, Dong Geun Kim, Hee
Lack Choi, Chan Young Park, Gun Dae Lee & Joo Hyun Kim (2018) Alcohol-soluble conjugated
oligomers as the cathode interfacial layer in polymer solar cells, Molecular Crystals and Liquid
Crystals, 660:1, 60-65, DOI: 10.1080/15421406.2018.1456050
To link to this article: https://doi.org/10.1080/15421406.2018.1456050
Published online: 02 May 2018.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Alcohol-soluble conjugated oligomers as the cathode interfacial
layer in polymer solar cells
Youn Hwan Kima, Mutia Anissa Marsyaa, Nadhila Sylviantia, Dong Geun Kima,
Hee Lack Choib, Chan Young Parka, Gun Dae Leec, and Joo Hyun Kima
aDepartment of Polymer Engineering, Pukyong National University, Busan, Korea; bDepartment of Materials
Science & Engineering, Pukyong National University, Busan, Korea; cDepartment of Industrial Chemistry,
Pukyong National University, Busan, Korea
ABSTRACT
KEYWORDS
Cathode interfacial layer;
fluorene-based conjugated
oligomer; inverted polymer
solar cell
Two easily accessible fluorene-based conjugated oligo-electrolytes
(COEs) FTF- and FBF-NBr have been developed as the cathode interfa-
cial layer (CIL) in inverted type polymer solar cells (iPSCs). CILsare inter-
pretative to improving the power conversion efficiency (PCE) and long-
term stability of the polymer photovoltaic cell that utilizes a high work
function cathode. Compared to the reference devices without inter-
layer, FBF- and FTF-NBr exhibit significant improvements of the device
parameters by reducing the work function of indium thin oxide (ITO).
Conjugatedoligo-electrolytes in this work havelow lying HOMO levels
−5.54 eV for FTF-NBr and −5.77 eV for FBF-NBr which are beneficial to
hole-blocking ability. The iPSCs with FBF- and FTF-NBr as the inter layer
at the cathode side were fabricated to investigate the effect of CIL on the
photovoltaic properties. As a result, the PCE of 7.89% with FBF-NBr and
8.05% with FTF-NBr as the CIL has been achieved. Thus, the results indi-
cate that fluorene-based COEs are potential CIL materials for the iPSCs.
1. Introduction
Polymer solar cells (PSCs) have attracted attention even if they have drawbacks such
as low efficiency, and lack of long term stability because of several advantages such
as felxible, lightweight, inexpensive processing cost. To improve the power conversion
efficiency (PCE) of PSCs, new conjugated polymers and conjugated small molecules
have been synthesized as donor materials [1–4]. These new materials have appropriate
bandgap and good solubility in common organic solvent, resulting in high PCE. Espa-
cially, poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl-alt-ethylhexyl-
3-fluorothieno[3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7) and its derivatives, which are
consist of thieno[3,4-b]thiophene and benzo[1,2-b:4,5-b’]dithiophene units, are widely used
in PSCs [5]. Alternatives of fullerene acceptor also have been developed because of low
absorption in visible spectrum, limited energy level variability and high cost. The new accep-
tors should have a complementary absorption wavelength with the donor materials and be
well soluble in common organic solvents to provide a good blending system [6–9]. As another
CONTACT Prof. Joo Hyun Kim
jkim@pknu.ac.kr
Yongdang-Dong, Nam-Gu, Busan -, Korea (ROK).
Department of Polymer Engineering, Pukyong National University,
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
61
Figure . (a) Scheme of inverted PSCs structure. (b) Energy level diagram of ZnO, FBF-NBr, and FTF-NBr.
method for increasing the PCE is by introducing interfacial layer materials such as conjugated
polymer or oligo-electrolytes [10–14].
In this article, we fabricated inverted PSCs (iPSCs) (as shown in Fig. 1(a)) with the cath-
ode interlayer based on fluorene-based conjugated oligo-electrolytes (COEs), 6,6ꢀ,6ꢀꢀ,6ꢀꢀꢀ-(1,4-
phenylenebis(9H-fluorene-9,9,2-triyl))tetrakis(N,N,N-trimethylhexan-1-aminium) bromide
(FBF-NBr) and 6,6ꢀ,6ꢀꢀ,6ꢀꢀꢀ-(thiophene-2,5-diylbis(9H-fluorene-9,9,2-triyl))tetrakis(N,N,N-
trimethylhexan-1-aminium) bromide (FTF-NBr) (Scheme 1). FBF-NBr and FTF-NBr have
cationic quaternary ammonium pendants on the conjugated polymer backbone. Cationic
quaternary ammonium pendants induce favorable interface dipole which means that the thin
layer of COEs make efficient charge collection from the active layer to the cathode. As a result,
short-circuit current (Jsc)improves due to favorable interface dipole, leading to an increase in
PCE of device (Fig. 1(b)).
Scheme . Synthesis of conjugated oligomers (FBF- and FTF NBr).

62
Y. H. KIM ET AL.
2. Experimental
2.1. Materials
Chemicals were purchased from Alfa Aesar and were used as received unless otherwise
described. PTB7 (Cat. No. OS0007) and PC71BM (Cat No. nano-cPCBM-SF) were purchased
from 1-materialCo. and nano-C, Inc., respectively.
... Synthesis of ,ꢀ,ꢀꢀ,ꢀꢀꢀ-(phenyl-,-diylbis(H-fluorene-,,-triyl))tetrakis(N,N,N-
trimethylhexan--aminium) bromide (FBF-NBr)
This oligomer was synthesized by the Suzuki coupling reaction. The benzeneboronic ester
(0.33 gr, 1 mmol), the 2-bromo-9,9-bis-(6-bromo-hexyl)-9H-fluorene (1.43 g, 2.5 mmol), 5%
mol of tetrakis(triphenylphospine) palladium [Pd(PPh3)4] (0.058 g, 0.050 mmol), and sev-
eral drops of aliquat 336 in 20 mL of degassed 1:1 (by volume) mixed solvent of toluene and
2M K2CO3 aqueous was stirred for 72 hours at 110°C under the N2. A portion of 100 mL
of water was added the mixture and allowed to cool to room temperature. The mixture was
extracted with methylene chloride (MC) and the extracted organic layer was dried over anhy-
drous MgSO4. The organic solvent was removed by using a rotary evaporator. The crude prod-
uct was purified by column chromatography on silica gel using MC/n-hexane. Then, the ionic
FBF salt (FBF-NBr) was obtained by treating 2,5-bis-[9,9-bis-(6-bromo-hexyl)-9H-fluoren-
2-yl]-benzene with trimethylamine in tetrahydrofuran (THF) for 6 days, followed by extrac-
tion with water. The water was then removed under reduced pressure. The trimer was dried
1
in vacuum overnight and was obtained as a yellow solid. H NMR (600 MHz, CD3OD) δ
7.88∼7.84 (m, 2H), 7.81∼7.78 (d, J = 7.6 Hz, 1H), 7.79∼7.75 (m, 2H), 7.72∼7.66 (m, 2H),
7.48∼7.40 (m, 1H), 3.24∼3.16 (m, 4H), 3.02 (s, 18H), 2.23∼2.03 (m, 4H), 1.61∼1.46 (m, 4H),
1.19∼1.08 (m, 8H), 0.72∼0.56 (m, 4H).
... Synthesis of ,ꢀ,ꢀꢀ,ꢀꢀꢀ-(thiophene-,-diylbis(H-fluorene-,,-triyl))tetrakis
(N,N,N-trimethylhexan--aminium) bromide (FTF-NBr)
A mixture of 2,5-bis-tributylstannanyl-thiophene (0.4 mmol) and 2-bromo-9,9-bis-(6-
bromo-hexyl)-9H-fluorene (1 mmol) were dissolved in degassed toluene. [Pd(dppf)Cl2]
(22 mg) were added to the mixture as catalyst. The reaction mixture was stirred and heated
to reflux for 48 h under N2 atmosphere. A portion of 100 mL of water was added the mixture
and allowed to cool to room temperature. The mixture was extracted with MC and the
extracted organic layer was dried over anhydrous MgSO4. The organic solvent was removed
by using a rotary evaporator. The crude product was purified by recrystallization using
methanol. Then, the ionic FTF salt (FTF-NBr) was obtained by treating 2,5-bis-[9,9-bis-(6-
bromo-hexyl)-9H-fluoren-2-yl]-thiophene with trimethylamine in THF for 6 days, then the
solvent was removed. The trimer was dried in vacuum overnight and was obtained as a red
solid. 1H NMR (600 MHz, CD3OD) δ 7.78∼7.71 (m, 2H), 7.69∼7.67 (m, 1H), 7.44∼7.27 (m,
4H), 3.19∼3.08 (m, 4H), 2.99 (s, 18H), 2.16∼1.98 (m, 4H), 1.47∼1.42 (m, 4H), 1.18∼1.02
(m, 8H), 0.59∼0.52 (m, 4H).
2.2. Fabrication of PSCs
In the iPSCs with the structure of ITO/ZnO (40 nm)/FBF or FTF-NBr/active layer
(PTB7:PC71BM) (70 nm)/MoO3 (10 nm)/Ag(100 nm) fabrication process, ITO-coated glass

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
63
substrates were washed by ultrasonic agitation in detergent, deionized water, methanol, ace-
tone, and isopropanol respectively. After UV ozone treatment for 5 min, the ZnO layer was
spin-cast on the pre-cleaned ITO at 2000 rpm for 1 min by using the sol-gel process. The sol-
gel solution was prepared with zinc acetate dehydrate (0.164 g), and ethanolamine (0.05 ml)
dissolved in 2-methoxyethanol (1 ml). The mixture was stirred at 60°C for overnight prior to
carrying out the film deposition. The thin film of ZnO precursor was annealed at 200°C for
10 min in air.FBF- and FTF-NBr was dissolved in methanol to 1 mg/ml. The solution was
spin cast on the ZnO layer at a speed of 4000 rpm for 1 min. The BHJ active layer was spin-
cast in a N2 filled glove box from the blend solution of PTB7: PC71BM (1:1.5 weight ratio)
in chlorobenzene with 3% v/v % 1,8-diiodooctane (DIO) at 1800 rpm for 120 s. Prior to spin
coating, the active solution was filtered through a 0.2 µm membrane filter. Finallythe MoO3
and Ag layer were deposited sequentially at 2 × 10−6 Torr by thermal evaporation through a
shadow mask with a device area of 0.13 cm2.
2.3. Characterization
The current density–voltage of the solar cells were measured by KEITHLEY Model 2400
source-measure unit under 1.0 sun (100 mW/cm2) condition from a 150 W Xe lamp with an
AM 1.5G filter. A calibrated Si reference cell with a KG5 filter certified by National Institute of
Advanced Industrial Science and Technology was used to confirm 1.0 sun condition. Kelvin
probe microscopy (KPM) measurements (KP technology Ltd. Model KP020) were performed
to confirm the work function of the cathode with or without interlayer, which are estimated
by the contact potential difference between the sample and the KP tip. The KP tip work func-
tion was calibrated by a standard gold with a work function of 5.1 eV. A thin film of interlayer
(ca. 5 nm) was prepared by spin-coating from the solution in methanol (MeOH) on the sur-
face of the ZnO under ambient condition to measure contact potential difference by KPM and
investigate the effective work function of interlayer coated substrate.
3. Results and discussion
Prior to the device fabrication, optical and electro chemical properties of oligomers were
investigated by UV-vis absorption spectroscopy and cyclic voltammetry (CV). As shown
in Fig. 2(a), FBF- and FTF-NBr solutions in methanol showed an absorption maximum
Figure . (a) UV-vis absorption spectra in solution and film of FBF- and FTF NBr. (b) CV spectra of FBF-
and FTF NBr.

64
Y. H. KIM ET AL.
Figure . (a) Current density–voltage curves of PSCs under AM .G simulated illumination with an intensity
of  mW/cm (inset: under the dark condition). (b) IPCE spectra of the devices with and without cathode
interfacial layer.
(λabs,max) at 381 and 330 nm, respectively. As for the thin film of COEs, λabs,max are red-shifted
than those of solutions, which appeared at 392 and 340 nm for FBF- and FTF-NBr, respec-
tively. The HOMO and LUMO levels of for the oligomers were obtained from the onset of the
oxidation and reduction potential of cyclic voltammograms, which were obtained in 0.1 M
of Bu4NPF4 in an acetonitrile solution. The HOMO energy level of FBF- and FTF-NBr were
calculated to be − 5.54 and − 5.77 eV, respectively. The LUMO energy level of the FBF- and
FTF-NBr were estimated to be − 3.29 and − 3.19, respectively.
We fabricated iPSCs with device structure of ITO/ZnO/FBF or FTF NBr/active layer
(PTB7:PC71BM)/MoO3/Ag. The current density-voltage (J-V) characteristics were mea-
sured under an AM 1.5G simulated illumination with a 100 mW/cm−2 and the dark con-
dition showed in Fig. 3(a). The Jsc of the device with ZnO/FBF- and FTF-NBr exhibit
15.08 and 15.64 mA/cm2 respectively, which are higher than the device based on ITO/ZnO
(14.65 mA/cm2). However, the fill factor (FF) of the devices with CILs are almost identical to
that of the reference device. The device with FTF-NBr as the CIL shows the PCE of 8.05%
and the PCE of the device with FBF-NBr also has a higher than reference device. It is indi-
cating that the FBF- and FTF-NBr layer induce favorable interface dipole between ZnO and
active layer. Comparing the FBF- and FTF-NBr layer, the Jsc of the FTF-NBr is measured
higher because the FTF-NBr has a thiophene ring, which acts as an electron donor, it makes
better electron collection efficiency than that of FBF-NBr. The incident photon-to-current
efficiency(IPCE) spectra of the devices are presented in Fig. 3(b). The IPCE values of the
FTF-NBr device is higher than those of the other devices. IPCE results are well correlate with
the Jsc of the devices.
Table . Photovoltaic performances of devices with and withoutcathode interfacial layer.
Jsc
Voc
(V)
FF
(%)
PCE
(%)
Work
Function (eV)
Buffer layer
(mA/cm)
ZnO/MeOH
ZnO/FBF-NBr
ZnO/FTF-NBr
.
(.)
.
(.)
.
.
(.)
.
(.)
.
.
(.)
.
(.)
.
.
(.)
.
(.)
.
− .
− .
− .
(.)
(.)
(.)
(.)

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
65
Kelvin Probe Microscopy (KPM) was used to investigate the work function of ZnO and
oligomer coated ZnO layer. The work function of FBF- and FTF-NBr coated ZnO are mea-
sured to be −4.14 and −4.15 eV, which are higher than that of ZnO (−4.4 eV) (Table 1) due to
formation of favorable interface dipole. Even though, FBF-NBr coated ZnO is almost identi-
cal to that of FTF-NBr the work function, the performances of the device with FTF-NBr are
better that those of the device based FBF-NBr. This is presumably due to that FTF-NBr has
electron rich thiophene ring.
4. Conclusion
We have successfully synthesized and characterized two easily accessible FBF- and
FTF-NBr as the CIL in iPSCs. The Jsc of the device with ZnO/FBF- and FTF-NBr exhibit
15.08, 15.64 mA/cm2 respectively, which are higher than the device based on ZnO/MeOH
(14.65 mA/cm2). Accordingly, the PCE of the device with FBF- and FTF-NBr exhibit 7.89%,
and 8.05% respectively, which are higher than the device with ZnO/MeOH (7.44%) due to
formation of favorable interface dipole. These results demonstrate that introducing COE is a
good strategy for obtain high photovoltaic performances of device.
Funding
This research work was supported by a grant of Pukyong National University (year 2017).
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