Phosphine oxide and Amino N-oxide functionalized phenylquinoline-based small molecules: New cathode interfacial layers for high-performance inverted organic solar cells

Article abstract of DOI:10.1016/j.orgel.2018.04.011

Two novel small-molecules PO-PhQ and PO-PhQNO, in which both polar phosphine oxide (PO) and N–oxide (NO) groups are incorporated in phenyl quinolone core unit were synthesized as efficient cathode interfacial layers (CILs) for inverted organic solar cells (IOSCs). Because of the phenylquinoline (PhQ) group, both PO-PhQ and PO-PhQNO are endorsed with high electron mobility. Due to the presence of polar P=O and N-oxide groups in their molecular structure, PO-PhQ and PO-PhQNO possess good solubility in polar solvents which make them as suitable candidates for interfacial modification of solution processed multilayer IOSCs. As a result, the power conversion efficiency (PCE) of the inverted devices based polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7):(6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) active layer with PO-PhQ and PO-PhQNO as CILs achieve PCE values of 9.03 and 8.53%, respectively, with 10–16% improvement than that of the control device with ZnO. In addition, both PO-PhQ and PO-PhQNO make certain the IOSC devices with long-term stability. To the best of our knowledge, this is the first time that cathode interfacial materials based on the combination of PhQ and P=O groups is reported. The effective application of these alcohol solution processed CILs indicate that the lending P=O to PhQ could be very promising strategy in developing high performance and eco-friendly solar cells. Our findings would contribute to enhancing the OSC device performances from synthetically view point of designing new materials.

Full text of DOI:10.1016/j.orgel.2018.04.011

Accepted Manuscript
Phosphine oxide and Amino N-oxide functionalized phenylquinoline-based small
molecules: New cathode interfacial layers for high-performance inverted organic solar
cells
Nallan Chakravarthi, Ho-Yeol Park, Um Kanta Aryal, Junyoung Kim, Yeong-Soon Gal,
Myungkwan Song, Young-Rae Cho, Sung-Ho Jin
PII:
S1566-1199(18)30167-8
10.1016/j.orgel.2018.04.011
ORGELE 4619
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 29 January 2018
Revised Date: 4 April 2018
Accepted Date: 4 April 2018
Please cite this article as: N. Chakravarthi, H.-Y. Park, U.K. Aryal, J. Kim, Y.-S. Gal, M. Song, Y.-R. Cho,
S.-H. Jin, Phosphine oxide and Amino N-oxide functionalized phenylquinoline-based small molecules:
New cathode interfacial layers for high-performance inverted organic solar cells, Organic Electronics
(2018), doi: 10.1016/j.orgel.2018.04.011.
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ACCEPTED MANUSCRIPT
Graphical Abstract

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Phosphine oxide and Amino N-oxide functionalized Phenylquinoline-based small
molecules: New cathode interfacial layers for high-performance inverted organic solar cells
Nallan Chakravarthi a,1, Ho-Yeol Park a,1, Um Kanta Aryal a, Junyoung Kim a, Yeong-Soon Gal
b, Myungkwan Song c, Young-Rae Chod,*, Sung-Ho Jin a,*
a Department of Chemistry Education, Graduate Department of Chemical Materials, Institute for
Plastic Information and Energy Materials, Pusan National University, Busandaehak-ro 63-2,
Busan 46241, Republic of Korea
b Polymer Chemistry Laboratory, College of Engineering, Kyungil University, Gyeongsan, 712-
701, Republic of Korea
c Advanced Functional Thin Films Department, Surface Technology Division, Korea Institute of
Materials Science (KIMS), 797 Changwondaero, Sungsan-Gu, Changwon, Gyeongnam 642-831,
Republic of Korea
d Division of Materials Science and Engineering, Pusan National University, Busandaehak-ro
63-2, Busan 46241, Republic of Korea
* Corresponding authors.
E-mail addresses: yescho@pusan.ac.kr (Y.-R. Cho), shjin@pusan.ac.kr (S.-H. Jin).
N. Chakravarthi, H.-Y. Park, U. K. Aryal made equal contribution for this article.

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ABSTRACT
Two novel small-molecules PO-PhQ and PO-PhQNO, in which both polar phosphine oxide (PO)
and N–oxide (NO) groups are incorporated in phenyl quinolone core unit were synthesized as
efficient cathode interfacial layers (CILs) for inverted organic solar cells (IOSCs). Because of the
phenylquinoline (PhQ) group, both PO-PhQ and PO-PhQNO are endorsed with high electron
mobility. Due to the presence of polar P=O and N-oxide groups in their molecular structure, PO-
PhQ and PO-PhQNO possess good solubility in polar solvents which make them as suitable
candidates for interfacial modification of solution processed multilayer IOSCs. As a result, the
power conversion efficiency (PCE) of the inverted devices based polythieno[3,4-b]-thiophene-
co-benzodithiophene (PTB7):(6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) active layer
with PO-PhQ and PO-PhQNO as CILs achieve PCE values of 9.03 and 8.53%, respectively, with
10-16 % improvement than that of the control device with ZnO. In addition, both PO-PhQ and
PO-PhQNO make certain the IOSC devices with long-term stability. To the best of our
knowledge, this is the first time that cathode interfacial materials based on the combination of
PhQ and P=O groups is reported. The effective application of these alcohol solution processed
CILs indicate that the lending P=O to PhQ could be very promising strategy in developing high
performance and eco-friendly solar cells. Our findings would contribute to enhancing the OSC
device performances from synthetically view point of designing new materials.
Keywords: Cathode interfacial layers, Phenylquinoline, Phosphine oxide and N–oxide, Long-
term stability

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1. Introduction
The recent advances in molecular engineering have revealed a series of organic solar cell
(OSC) potential advantages that may ultimately outbalance the benefits of silicon based solar
cells [1-3]. Flexible OSCs have received huge consideration owing to their potential applications
as renewable and eco-friendly energy sources and high chances of their large area installation
[4,5]. Remarkable progress has been realized for OSCs in achieving power conversion efficiency
(PCE), due to the significant accomplishments made in the development of semiconducting π-
conjugated materials containing donor-acceptor type polymers and small molecules, and
anode/cathode interface engineering [6-8]. Among various key components present in the bulk
heterojunction (BHJ) device structure, interfacial layers (ILs) situated between the photoactive
layer and charge collecting electrodes, considerably improve the PCE of the corresponding OSCs
[9-13]. These ILs act as versatile materials that behaves as electrode surface modifiers/charge
injection layers, in such a way that the ILs can control charge recombination at the photoactive
layer/electrode interface by blocking the charges to travel in undesired paths [14,15]. But, the
lack of key comprehension of the cathode/anode interface modification and surface features
restrict their requirement and development [16,17]. Therefore, expanding the class of new, yet
efficient ILs and examining the interfacial property are very important for improving OSC
device performance.
Various types of ILs made of neutral organic small molecules, alcohol/water-soluble
conjugated/non-conjugated polymer electrolytes (CPEs), have been utilized to increase the PCEs
of the OSC devices [18-21]. It has been well proven that the phosphine oxide (P=O) and N-oxide
end groups in the molecular framework generates encouraging interface dipoles, sequentially
reducing the energy barrier for the charge transport via altering the work function of the cathode

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[13,22,23]. Additionally, conjugated/non-conjugated polymers with ionic end groups also reduce
the work function of the cathode due to the formation of interfacial dipole. However, when
compared to that of the polymeric interfacial materials, organic small molecules possess various
benefits including synthetic ease, high purity and no problem in batch-to-batch differences.
Generally, phenylquinoline (PhQ) derivatives are used as active electron transporting
components for organic light emitting diode applications [24]. These PhQ derivatives, especially
poly-(phenylquinoline) are highly thermally robust, photochemically very stable, and shown to
have excellent electron accepting properties with high electron affinity, which made them very
interesting for OLED applications as an efficient electron transporting materials [25]. On the
other hand, P=O based compounds are known to be effective electron transport materials for
OLEDs, due to the high electronegativity of the oxygen atom that makes the P=O group highly
polar and also show electron withdrawing character [26]. Thus, combining P=O group to PhQ
and PhQN-oxide (PhQNO) moieties is an interesting approach to realize new and conceivably
better cathode interfacial layers (CILs) with better electron transporting nature. Despite the great
achievement in the OSC research field, the cathode interfacial materials explored until now for
photovoltaic applications is restricted to a few families. In this scenario, it is of no surprise to
mention that the organic small molecules based on the combination of P=O/N-oxide and PhQ
groups exemplify a completely unexplored class of materials that could potentially be employed
as effective CILs. This class of PhQ-based small molecules offers simple and therefore low-cost
synthesis, good solubility in various polar solvents.
Having the idea to expand the family of new CILs, we synthesized two new π-conjugated
small-molecules,
(2,4-difluoro-4'-(4-phenylquinolin-2-yl)-[1,1'-biphenyl]-3-
yl)diphenylphosphine oxide (PO-PhQ) and 2-(3'-(diphenylphosphoryl)-2',4'-difluoro-[1,1'-

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biphenyl]-4-yl)-4-phenylquinoline 1-oxide (PO-PhQNO), based on phenyl quinoline moiety and
used as CILs for inverted organic solar cells (IOSCs). The molecular structures of PO-PhQ and
PO-PhQNO contains P=O and P=O/N-oxide groups respectively in their molecular framework,
as shown in Scheme 1. Under standard illumination conditions, optimized IOSCs exhibit a
maximum PCE value of 9.03% (PO-PhQ) and 8.53% (PO-PhQNO), which is quite higher than
that of the control devices without CILs. The stability test in case of PTB7:PC71BM IOSCs
reveal that the existence of PO and PhQ based CILs leads to considerably improved operating
lifetimes.
2. Experimental Section
2.1. Materials and measurements
All reagents were purchased from Sigma-Aldrich (Korea) and Solarmer (China) and used
without further purification. Moisture-sensitive reactions were conducted in a N2 atmosphere.
1
The other materials were common chemicals and were used as received. H NMR spectra were
recorded on a Varian Mercury Plus 300 MHz spectrometer (USA) in CDCl3 using
tetramethylsilane as an internal reference. The chemical shifts were recorded in ppm related to
the singlet of CDCl3 at 7.26 for 1H NMR spectroscopy. The UV-visible absorption spectra were
recorded on a JASCO V-570 spectrophotometer (USA). Thermal gravimetric analysis was
carried out on a Mettler Toledo TGA/SDTA 851e (Switzerland) under N2 atmosphere at a
heating rate of 10 °C min−1 and differential scanning calorimetry (DSC) 822e analyzer under N2 at a
heating rate of 10 °C min−1.
2.2. Device characterization of IOSCs and PSCs
The performances of IOSCs, and PSCs were measured using a calibrated air mass (AM)
1.5 G solar simulator (Oriel Sol3A Class AAA solar simulator, models 94043A (Newport

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Stratford, Inc.,USA)) with a light intensity of 100 mW/cm2 adjusted using a standard PV
reference cell (2 cm × 2 cm monocrystalline silicon solar cell, calibrated at NREL, Golden, CO)
and a computer-controlled Keithley 2400 (Keithley Instruments, Inc. USA) source measure unit.
The external quantum efficiency (EQE) spectrum was measured using an Oriel IQE-200
(Newport Stratford,Inc.,USA) equipped with a 250-W quartz tungsten halogen lamp as the light
source and a monochromator, an optical chopper, a lock-in amplifier, and a calibrated silicon
photodetector. While measuring the J–V curves for the OSC devices, a black mask was used and
only the effective area of the cell was exposed to light irradiation. Atomic force microscopy
(AFM) was used to measure film thickness, roughness, and surface morphologies in tapping
mode acquired with a XE-100 (Park System Corp, Korea). All photoemission measurements
were carried out in a PHI-5000 Versa Probe II (ULVAC-PHI, Inc. Japan) ultrahigh vacuum
surface analysis system equipped with a He-discharge lamp (21.22 eV) and a monochromatic Al
kα X-ray. All spectra were measured at a pressure of 1 × 10−6 Pa. Water contact angles were
measured using a contact angle 101 measuring system (Plasma systems and materials, Korea).
2.2.1. Fabrication of IOSCs
Indium tin oxide (ITO)-coated glass substrates were cleaned by the process of sonication
in acetone, deionized water and IPA sequentially and then dried with N2 flow. Clean dried
samples were treated UV-plasma for 20 minutes. ZnO layer (using ZnO sol-gel precursor) was
spin coated with 2,000 rpm for 40 sec (obtained thickness ~45 nm) on the ITO substrate and
annealed at 160 °C for 15 min. CILs PO-PhQ and PO-PhQNO were spin coated over ZnO layer.
CILs were dissolved in 2-methoxy ethanol (2-MOE) with varying concentrations. PTB7:PC71BM
(8:12 mg in 1 mL CB with 3% DIO additive) as photoactive layer, was spin coated with rpm
1000 for 40 s and kept for slow dry in covered glass petri dishes at room temperature for 20-30

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min. The photoactive layer thickness measured about 100 nm. Finally, MoO3 (6 nm) layer and
Ag (100 nm) layer were thermally evaporated through a shadow mask providing a device active
area of 0.11 cm2.
2.3. 2-(4-Bromophenyl)-4-phenylquinoline (1)
A mixture of 4-bromoacetophenone (3 g, 15 mmol), 2-aminobenzophenone (3 g, 18 mmol),
diphenyl phosphate (3.8 g, 18 mmol), and m-cresol (40 mL) was flushed with N2 while stirring at
room temperature for 30 min and then refluxed for 12 h at 140 °C. The reaction mixture was
distilled to remove m-cresol, residue was diluted with MC and washed with 10% sodium
hydroxide solution. The reaction mixture was washed with sodium chloride solution and the
resulting organic layer was dried over anhydrous MgSO4 and evaporated the solvent. The
residue was then purified by column chromatography on silica gel (hexane:ethyl acetate (EA),
1
9:1 v/v) to furnish an off-white powder 1 (3.9 g, 54%). H NMR (300 MHz, CDCl3, δ): 8.21
(d, J = 8.4 Hz, 1H), 8.06 (d, J = 8.4 Hz, 2H), 7.88 (d, J = 8.4 Hz, 1H), 7.75 (s, 1H), 7.71 (dd, J =
7.6, 7.6 Hz, 1H), 7.62 (d, J = 8.4 Hz, 2H), 7.52-7.49 (m, 5H), 7.46 (dd, J = 7.6, 7.6 Hz, 1H). 13C
NMR (300 MHz, CDCl3, δ): 155.7, 149.6, 148.9, 138.6, 138.4, 132.1, 130.3, 129.9, 129.7, 129.3,
128.8, 128.7, 126.7, 126.0, 125.9, 124.1, 119.0.
2.4. 2-(2',4'-Difluoro-[1,1'-biphenyl]-4-yl)-4-phenylquinoline (2)
Toluene (20 mL), ethanol (10 mL), and 2M aqueous Na2CO3 (12 mL) were added to a mixture
of 2-(4-bromophenyl)-4-phenylquinoline (1.5 g, 4 mmol), 2,4-difluorophenylboronic acid (0.65
g, 5 mmol), and tetrakis(triphenylphosphine)palladium (0.125 g, 0.2 mmol). The mixture was
refluxed for 12 h under a N2 atmosphere. After cooling to room temperature, the mixture was
extracted with dichloromethane and dried over anhydrous Na2SO4. The solvent was removed
under reduced pressure and the residue was purified by column chromatography on silica gel

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using dichloromethane (MC)/hexane (4:1) as the eluent to produce a white solid. Yield: 55%. 1H
NMR (300 MHz, CDCl3, δ): 8.36 (m, 3H), 8.02 (d, 1H), 7.95 (s, 1H) 7.83 (m, 1H), 7.74 (d, 2H),
13
7.64 (m, 7H), 7.07 (m, 2H). C NMR (300 MHz, CDCl3, δ): 105.14, 105.48, 105.81, 111.60,
111.65, 111.88, 111.94, 119.66, 119.72, 119.77, 121.33, 121.38, 121.48, 126.03, 129.10, 129.46,
133.69, 133.72, 133.86, 151.51, 151.66, 161.22, 161.39, 163.85, 164.01, 164.74, 164.90, 167.24,
173.49.
2.5. (2,4-Difluoro-4'-(4-phenylquinolin-2-yl)-[1,1'-biphenyl]-3-yl)diphenylphosphine oxide (PO-
PhQ)
In a 50-mL three-neck flask, 2-(2',4'-difluoro-[1,1'-biphenyl]-4-yl)-4-phenylquinoline (1 g, 2.5
mmol) was dissolved in 30 mL of THF and the temperature of the reaction was cooled down to
−78 °C. To the reaction mixture, lithium diisopropylamide (2 M, 1.6 mL) was added dropwise,
and the temperature of the reaction was maintained at −78 °C for another 1 h. Then, 0.54 mL of
chlorodiphenylphosphine (0.67 g, 2.8 mmol) was added to the mixture and the solution was
stirred overnight. The reaction was quenched with water, extracted with EA, and dried over
anhydrous Na2SO4. After removing the solvent, the residue was diluted with 30 mL of MC and
25 mL of hydrogen peroxide solution (30%, w/w), and then stirred at room temperature for 3 h.
The crude product was extracted with chloroform (CF) and purified by column chromatography
on silica gel using pure EA as an eluent to afford PO-PhQ as white solid. The overall yield was
58%. 1H NMR (300 MHz, CDCl3, δ): 8.24 (m, 1H), 7.94 (d, 1H), 7.85-7.74 (m, 7H), 7.60-47 (m,
13
14H), 7.04 (m, 1H). C NMR (300 MHz, CDCl3, δ): 112.77, 113.09, 119.19, 125.70, 125.83,
125.99, 126.20, 126.54, 127.73, 128.53, 128.65, 128.70, 129.48, 129.51, 129.57, 129.68, 130.11,
131.13, 131.27, 132.18, 132.21, 133.68, 134.99, 136.08, 138.24, 139.33, 148.78, 149.32, 156.00,
158.83, 164.64, 171.25.

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2.6. 2-(3'-(Diphenylphosphoryl)-2',4'-difluoro-[1,1'-biphenyl]-4-yl)-4-phenylquinoline-N-oxide
(PO-PhQNO)
PO-PhQ (1 g, 1.6 mmol) was dissolved in CF (25 mL) and 0.45 g of m-CPBA was slowly
added, and stirring was continued overnight at room temperature. Then the reaction mixture was
basified with an aqueous KOH solution. The organic layer was separated and dried over
anhydrous Na2SO4. The solvent was removed under pressure and purified by column
chromatography on silica gel using EA/hexane (4:1) as the eluent to produce a yellow solid.
1
Yield: 61%. H NMR (300 MHz, CDCl3): 8.96 (1H, d), 8.08-8.05 (2H, m), 7.97-7.94 (1H, d),
13
7.84-7.82 (5H, m), 7.75–7.48 (16H, m), 7.06 (1H, m). C NMR (300 MHz, CDCl3, δ): 120.58,
123.43, 126.63, 128.01, 128.53, 128.65, 128.70, 128.82, 128.94, 129.63, 129.89, 130.57, 131.10,
131.24, 132.21, 132.18, 133.20, 133.65, 135.21, 136.07, 137.05, 138.13, 142.27, 143.76.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic routes for the intermediates, PO-PhQ and PO-PhQNO are shown in Scheme S1,
and the complete procedures for the synthesis of the CILs are described in the Supporting
Information. Initially, 4-bromoacetophenone was treated with 2-aminobenzophenone in the
presence of m-cresol to get intermediate 1. Upon treating 2,4-difluorobenzeneboronic acid with
compound 1, the difluorophenyl substituted intermediate 2 was obtained. Then the compound 2
was reacted with lithium diisopropylamide and chlorodiphenylphosphine to generate an
intermediate which was further used without purification. Upon further oxidation of the
unpurified intermediate using hydrogen peroxide, PO-PhQ was attained. Further, PO-PhQ was
treated with m-CPBA at room temperature to get PO-PhQNO. Owing to the polar P=O and N-
oxide groups, both PO-PhQ and PO-PhQNO are soluble in high polar organic solvents such as

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methanol, isopropanol and 2-methoxymethanol. On the other hand, PO-PhQ and PO-PhQNO are also
soluble in common organic solvents such as tetrahydrofuran (THF), chloroform (CF), and
chlorobenzene (CB). Further, all the intermediates and final products were purified by simple
column chromatography, and the chemical structures of the products were characterized by
1
spectroscopic methods. The H-NMR spectra of PO-PhQ and PO-PhQNO complies with the
chemical structure, as shown in Fig S1 and S2. The FT-IR spectra of PO-PhQ and PO-PhQNO,
shown in Fig. S3a, clearly show a sharp characteristic absorption at 1192 cm−1 for the P=O
group.
3.2. Thermal, optical and electrochemical properties
The thermal properties of the PO-PhQ and PO-PhQNO were measured by thermal gravimetric
analysis (TGA) at a heating rate of 10 °C/min (Fig. 1a). The onset of decomposition temperature
(Td, corresponding to 5% weight loss) of PO-PhQ and PO-PhQNO was 320 and 130 °C,
respectively. The difference in the decomposition temperatures of PO-PhQ and PO-PhQNO can
be well correlated to their molecular structure. The only structural difference in PO-PhQ and PO-
PhQNO is that the former compound contains only P=O group and the latter compound contains
both P=O and N-oxide groups in the molecular framework. The Td of PO-PhQNO with single N-
oxide group is in agreement with the water soluble naphthalene diimide derivative (NDIO) that
possess two terminal N-oxide groups [23]. As shown in Fig. 1b, clear glass transition
temperatures (Tg) can be seen from the differential scanning calorimetry (DSC) graphs emerged
at approximately 96 and 116 °C for PO-PhQ and PO-PhQNO, respectively. It is worth stating
that the Tg of new CILs is significantly higher than that of well-known electron-transporting
material such as bathophenanthroline with Tg value of 66 [13].

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The UV−vis absorption spectra of PO-PhQ and PO-PhQNO in diluted CHCl3 solution
and film state are shown in Fig. 1c and d and the absorption maxima values are listed in Table 1.
PO-PhQ and PO-PhQNO showed similar absorption profiles both in solution and film states. The
absorption maximum of PO-PhQ in film/solution states were found to be at shorter wavelength
compared to that of PO-PhQNO, which may be due to relatively less strong withdrawing nature
of PO-PhQ. The absorption maxima of PO-PhQ and PO-PhQNO in solution state were observed
at 276 and 294 nm, respectively, whereas the absorption maxima of PO-PhQ and PO-PhQNO in
film state were found at 286 and 302 nm, respectively. The optical band gap (Egopt) of PO-PhQ
and PO-PhQNO was estimated from the onset absorption edge of the thin film absorption spectra
and found to be 3.34 and 2.98, respectively. The highest occupied molecular orbital (HOMO)
and energy levels of PO-PhQ and PO-PhQNO were determined using cyclic voltammetry (CV),
as shown in Fig. S3b. The HOMO levels of PO-PhQ and PO-PhQNO were observed at -6.08 and
-5.73 eV, respectively. The lowest unoccupied molecular orbital (LUMO) levels are calculated
from Egopt and the HOMO energy levels and were found to be -2.74 and -2.75 eV, respectively.
3.3. Photoelectron spectroscopy measurements
Ultraviolet photoelectron spectroscopy (UPS) was used to illustrate the interfacial dipole
moment at the ZnO layer surface due to incorporation of PO-PhQ and PO-PhQNO layers. Fig. 2a
shows the shift in the secondary cut-off of the ZnO layer after spin-coating the PO-PhQ and PO-
PhQNO layers. The schematic diagram of the generation of interfacial dipole due to
incorporation of CILs is shown in Fig. 2b. The pristine ZnO film shows a photoemission cutoff
at 17.09 eV, which corresponds to a work function (WF) of 4.02 eV. After spin-casting the PO-
PhQ and PO-PhQNO layers, the WF value decreases to 3.41 and 3.71 eV respectively. A
significant shift in the secondary cut-off of UPS spectra indicates a lowering of the WF of ZnO,

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leading to a downward vacuum level shift. This point out that the interfacial dipole moment
induced between the ZnO and the photoactive layer increased upon incorporation of PO-PhQ or
PO-PhQNO CILs. It is of very important that the Ohmic contact formed at the cathode interface
possess much high JSC, therefore the JSC is profoundly linked with the work function change. In
the case of PO-PhQ and PO-PhQNO CILs, the improvement in JSC compared to that of the
pristine ZnO based devices point out that the effective formation of Ohmic contact at the cathode
interface. The WF value of the ZnO layer after insertion of PO-PhQ and PO-PhQNO was 0.61
and 0.31 eV lower than that of the ZnO layer, respectively, indicating that the WF of the ZnO
layer after PO-PhQ and PO-PhQNO treatment was shifted by 0.61 and 0.31eV toward the
vacuum level. This showed that the interfacial dipole moment formed between the ZnO and the
photoactive layer augmented upon the incorporation of PO-PhQ and PO-PhQNO CILs.
3.4. Photovoltaic performance of IOSCs with PO-PhQ and PO-PhQNO CILs
To examine the interfacial modification ability of the new CILs, poly[[4,8-bis[(2-
ethylhexyl)oxy]benzo[1,2-b:4,5-b″]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]-
thieno[3,4-b]thiophenediyl]] (PTB7):[6,6]-phenyl-C71–butylic acid methyl ester (PC71BM) based
IOSCs were fabricated with a device configuration of ITO/ZnO/CIL (PO-PhQ or PO-
PhQNO)/PTB7:PC71BM/MoO3/Ag. The PO-PhQ and PO-PhQNO films were prepared by spin
casting the 2-methoxyethanol (2-MOE) on the surface of the photoactive layer. The current
density–voltage (J–V) characteristics for IOSCs were measured under AM 1.5G irradiation (100
mW cm−2) and are shown in Fig. 3a. The corresponding photovoltaic parameters, including
open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and PCE, estimated
from the J−V curves are summarized in Table 2. Various concentrations of CIL were also
scrutinized and the optimized one was found to be 0.5 mg/mL for both PO-PhQ and PO-PhQNO.

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There are few reports stating that due to the solvent treatment at the interface of the photoactive
layer/cathode, there is a chance for the improvement of PCE in IOSCs [27]. In order to avoid any
misleading due to the CIL processing solvent (2-MOE), the ZnO layer in the IOSC device was
treated with 2-MOE. The control device based on ZnO exhibited a PCE of 7.76%, with VOC of
0.73 V, JSC of 15.67 mA/cm2, and FF of 67.66%. When the PTB7:PC71BM film was treated with
2-MOE, the PCE was increased from 7.76% to 7.95%. On the other hand, after incorporating a
thin layer of CILs film between the ZnO and photoactive layer, the photovoltaic parameters
including FF, JSC, VOC were consecutively improved, in turn giving higher PCE value compared
to the pristine ZnO electrode. IOSCs with PO-PhQ and PO-PhQNO revealed the improved PCEs
of 8.53% and 9.03%, respectively. It was very clear from Table 1, the PCE increases in the order:
ZnO < ZnO/2-MOE < ZnO/PO-PhQNO < ZnO/PO-PhQ. Further to check the reproducibility,
ten IOSC devices using the ZnO/PO-PhQ and ZnO/PO-PhQNO bilayer as the CIL were
investigated under the same fabrication conditions. The PCE improvement of the devices with
PO-PhQ and PO-PhQNO CILs was mostly attributed to the enhanced JSC and FF values (from
15.67 mA/cm2, 67.66% (ZnO) to 16.25 mA/cm2, 69.57% (ZnO/PO-PhQNO) and 16.41 mA/cm2,
72.98% (ZnO/PO-PhQ), indicating that both PO-PhQ and PO-PhQNO CILs reduced the
interfacial energy barrier by adjusting the interfacial dipole moment between the ZnO and
photoactive layers. As shown in the Fig. S4, the PL quenching was dramatically increased due to
the incorporation of PO-PhQ and PO-PhQNO CILs, irrespective of no variation in the absorption
spectra of the BHJ blend with ZnO and ZNO/CILs. Hence, it could be confirmed that ZnO/PO-
PhQ and ZnO/PO-PhQNO cathode bilayer framework improves the charge transport ability
when compared to that of pristine ZnO layer. We fabricated the PSC devices with various
concentrations of 0.3-0.8 mg/mL to examine the dependency of PCE on concentration of PO-

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PhQ and PO-PhQNO. Interestingly, the PCE of the PO-PhQ and PO-PhQNO based IOSC
devices remained at high values of 8.28% and 8.07%, respectively (Table S1 and Fig. S5).
The external quantum efficiency (EQE) spectra of ZnO, ZnO/PO-PhQ, and ZnO/PO-
PhQNO based IOSCs are shown in Fig. 3b. All the devices exhibited similar EQE spectra over
the wavelength range from 400 to 800 nm. The integrated JSC values estimated from the EQE
curves were 15.15, 15.13, 15.36, 16.22 mA/cm2 for the pristine ZnO, ZnO/2-MOE, ZnO/PO-
PhQ, and ZnO/PO-PhQNO-based IOSC devices, respectively. The integrated JSC values are very
well matched with JSC obtained from the J−V measurements. The EQE of the device with the
ZnO/PO-PhQ layer showed a noticeable enhancement in the wavelength region ranging from
550 to 700 nm in comparison with the pristine ZnO device, which resulted in higher JSC. As
shown in Table 2, the ZnO/CIL-based device has lower series resistance (Rs) than the pristine
ZnO or 2-MOE-treated devices. These results clearly confirmed that the PO-PhQ and PO-
PhQNO based device possessed the best Ohmic contact at the photoactive layer/cathode
interface.
The surface morphologies of the ZnO surface covered with PO-PhQ and PO-PhQNO
CILs were investigated using tapping-mode atomic force microscopy (AFM). The root-mean-
square (RMS) roughness of the pristine ZnO layer, ZnO/PO-PhQNO and ZnO/PO-PhQ layers
were 4.62, 4.32, and 3.98 nm, respectively as shown in Fig. S6. The RMS values point out that
the ZnO surface was slightly smoothened due to the presence of PO-PhQ and PO-PhQNO. On
the other hand, the surface of the PTB7:PC71BM layer with PO-PhQ and PO-PhQNO layers
showed well reduced roughness compared to that of the ZnO on the surface of the photoactive
layer. The smoothly modified surfaces of ZnO and PTB7:PC71BM by PO-PhQ and PO-PhQNO,
improves the affinity via establishing good contact between the stacked layers (i,e., ZnO

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/photoactive layer and photoactive layer/cathode), thus improving the contact. Thus it could be
anticipated that the beneficial contacts between the stacked layers enhances the charge transport
at the cathode interface (ZnO/photoactive layer) in the IOSC devices to afford a superior PCE for
PO-PhQ and PO-PhQNO on the ZnO surface.
We calculated the contact angles and surface energies (γs) of the pristine ZnO film and of
the ZnO/PO-PhQ and ZnO/PO-PhQNO layers. It is very imperative to know the γs values to
comprehend the wettability and adhesion properties of an interface. The γs values were
calculated using the Owens and Wedt geometric mean equation generally used to calculate the
surface energy between water and diiodomethane [28,29]. The water contact angles and γs values
are shown in Table 3. As shown in Fig. 4, the deposition of PO-PhQNO and PO-PhQ layers on
the pristine ZnO layer increased the water contact angle from 30.1° to 36.1° and 44.4°, signifying
that the ZnO/PO-PhQ and ZnO/PO-PhQNO surfaces became more hydrophobic than the surface
of pristine ZnO. The hydrophilic P=O and N-oxide groups of the PO-PhQ and PO-PhQNO layers
were seemingly arranged at the ZnO interface, while the hydrophobic conjugated phenyl units
containing aromatic framework was present at the photoactive layer surface. The water contact
angle results recommended that PO-PhQ and PO-PhQNO layers generated an interfacial dipole
moment at the surface of the ZnO layer. Besides the increase in hydrophobicity, the γs value of
the pristine ZnO layer decreased upon deposition of additional PO-PhQ and PO-PhQNO layers,
from 67.3 to 63.1 and 58.2 mN/m, respectively. From the γs values, it can be inferred that the
electrons that were generated by photoexcitation can be easily moved and ejected from the
photoactive layer to the cathode through the PO-PhQ and PO-PhQNO layers. We checked the
long-term stability of the IOSC devices for 1000 hours without encapsulation (Fig. S7). The high
stability of the PO-PhQ/ZnO and PO-PhQNO/ZnO devices over pristine ZnO may be due to the

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superior hydrophobic properties of the CILs, which stopped further entry of water and oxidation
into the surface of photoactive layer.
The electron mobility was calculated by using space charge limited current (SCLC)
measurements performed in electron-only devices the following diode structure
ITO/Al/ZnO/CIL/LiF/Al [30]. The dark J-V curves of the electron-only devices are shown in
Fig. S8. The respective electron mobility was calculated according to the Mott-Gurney equation,
J = (9/8) εrε0µ (V2/L3) where, εr is the dielectric constant (εr = 3), ε0 is permittivity of free space.
L is the thickness of the ZnO and CILs, µ is the charge mobility, and V is the voltage drop across
the device. The ZnO/PO-PhQ and ZnO/PO-PhQNO-based device showed µSCLC,e of 8.15 × 10-5
cm2 V-1 s-1 and 7.42× 10-5 cm2 V-1 s-1, respectively which is two times higher than the µSCLC,e of
ZnO with a value of 3.72 × 10-5 cm2 V-1 s-1. The µSCLC,e results are well lined up with higher FF
and the improved dark current (forward bias), as shown in Fig. 4a. The mobility results specify
that the incorporation of PO-PhQ and PO-PhQNO layers on ZnO surface developed the
interfacial contact between the ZnO and the photoactive layer, thus increasing the PCE in the
ZnO/PO-PhQ and ZnO/PO-PhQNO-based devices.
The dark J−V characteristics of PTB7:PC71BM blends with or without CILs on ZnO
surface were shown in Fig. 5a. The series resistance (RS) for the corresponding devices are
shown in Table 2. The incorporation of CILs on the ZnO surface reduced RS for PO-PhQ from
2.01 to 1.48 Ωcm2 and for PO-PhQNO from 2.01 to 1.76 Ωcm2. The Rs results could give a valid
reason for the experimental enhancement in JSC, from 15.67 to 16.41 mA/cm2 for PO-PhQ and
from 15.67 to 16.25 mA/cm2 for PO-PhQNO. The RSH values of pristine ZnO, ZnO/PO-PhQ, and
ZnO/PO-PhQNO were 5.46 × 103, 2.12× 104 and 1.12 × 104 Ωcm2, respectively, demonstrating
that the ZnO combined with PhQ based CILs had a lower leakage current and excellent diode

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characteristics. These features elevated the FF of pristine ZnO from 67.66 to 72.98% and
69.57%, respectively for ZnO/PO-PhQ, and ZnO/PO-PhQNO bilayers. The surface modification
by PO-PhQ and PO-PhQNO improves hydrophobicity of ZnO film (Fig. 4a) owing to the
aromatic hydrocarbon moieties out of ZnO surface, leading to a better contact to photoactive
active layer, which is consistent with electrical impedance spectrum (EIS) results (Fig. 5b). The
extracted contact resistance of PO-PhQ or PO-PhQNO modified ZnO (ZnO/PO-PhQ and
ZnO/PO-PhQNO) film is found to be remarkably reduced, facilitating electron transport to ITO
electrode. This result in conjunction with the abovementioned increased electron mobility will be
remarkable significant for the increase of photocurrent in IOSC devices.
4. Conclusion
In summary, we developed two novel PhQ-based alcohol-soluble π-conjugated organic
small molecules, PO-PhQ and PO-PhQNO as an efficient CIL to modify the ZnO surface in BHJ
IOSCs. The UPS studies revealed that the polar groups (P=O and N-oxide) in the new CIL
molecules play a critical role in reducing the energy barrier by generating interfacial dipole at the
cathode interface. The contact angle and surface energy results reveal both PO-PhQ and PO-
PhQNO reduced the surface energy and increased the hydrophobic nature thus improving both
interfacial contacts and wettability between the ZnO and photoactive layer. The CILs enhanced
the electron collection efficiency of the IOSC devices and modified the rough ZnO surface to a
smoother surface, thus enriched the PCE and stability. Even though the donor-acceptor type
polymers were incorporated to enable the impact of PO-PhQ and PO-PhQNO as CILs in BHJ
IOSCs, it is anticipated that these CILs could deliver extraordinary PCE in perovskite solar cells
in various types of device configurations via solution process. Further, the research outcome

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reported in this work could motivate additional interest in this type of organic small molecules
based on PhQ for optoelectronic applications.
Acknowledgments
This work was supported by National Research Foundation (NRF-2016R1E1A1A01942593) by
the Ministry of Science, ICT & Future Planning, Republic of Korea.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/
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Scheme Captions
Scheme 1. Design strategy for the design of phenylquinoline-based new interlayers (PO-PhQ,
PO-PhQNO).
Figure Captions
Fig. 1. (a) TGA curves of PO-PhQ and PO-PhQNO, (b) DSC curves of PO-PhQ and PO-
PhQNO, (c) UV−vis absorption spectra of PO-PhQ and PO-PhQNO in CF solution, and (d) thin
film state.
Fig. 2. (a) UPS spectra of the ZnO layer, before and after deposition of PO-PhQ and PO-
PhQNO, (b) The schematic energy diagram of interfacial dipole of the ZnO and the ZnO/CIL
thin layers.
Fig. 3. (a) J−V and (b) EQE spectra of PTB7:PC71BM-based IOSCs of the ZnO and PO-
PhQ/ZnO, and ZnO/PO-PhQNO thin layers, respectively, under AM 1.5 G irradiation (100 mW
cm−2).
Fig. 4. Water and diiodomethane contact angles of ZnO, ZnO/PO-PhQ, and ZnO/PO-PhQNO
surfaces.
Fig. 5. (a) J−V characteristics of PTB7:PC71BM-based PSCs on top of the ZnO and PO-
PhQ/ZnO, and ZnO/PO-PhQNO thin layers in the dark and (b) Nyquist plots of devices
with/without CILs under the AM 1.5 illumination.

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Scheme 1. Design strategy for the design of phenylquinoline-based new interlayers (PO-PhQ,
PO-PhQNO).

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Fig. 1. TGA curves of PO-PhQ and PO-PhQNO, (b) DSC curves of PO-PhQ and PO-PhQNO,
(c) UV−vis absorption spectra of PO-PhQ and PO-PhQNO in CF solution, and (d) thin film state.

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Fig. 2. a) UPS spectra of the ZnO layer, before and after deposition of PO-PhQ and PO-PhQNO,
(b) The schematic energy diagram of interfacial dipole of the ZnO and the ZnO/CIL thin layers.

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0
-4
80
(a)
(b)
60
ZnO
ZnO/2-MOE
ZnO/PO-PhQNO
ZnO/PO-PhQ
-8
ZnO
ZnO/ 2-MOE
ZnO/ PO-PhQNO
ZnO/ PO-PhQ
40
20
0
-12
-16
0.0
0.2
0.4
0.6
0.8
400
500
600
700
800
Wavelength (nm)
Voltage (V)
Fig. 3. (a) J−V and (b) EQE spectra of PTB7:PC71BM-based IOSCs of the ZnO and PO-
PhQ/ZnO, and ZnO/PO-PhQNO thin layers, respectively, under AM 1.5 G irradiation (100 mW
cm−2).

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Fig. 4. Water and diiodomethane contact angles of ZnO, ZnO/PO-PhQ, and ZnO/PO-PhQNO
surfaces.