Small-molecule electrolytes with different ionic functionalities as a cathode buffer layer for polymer solar cells

Article abstract of DOI:10.1039/d0tc02513e

In polymer solar cells (PSCs), electrolytes as the cathode interlayer are responsible for generating an interface dipole, which reduces the charge collection barrier at the cathode interface. In this study, small-molecule electrolytes were designed and synthesized as the cathode interlayer in inverted PSCs to overcome high processing costs caused by complicated synthesis procedures. Moreover, the proposed design strategy allows changing the ion size and number of ionic functionalities to tune the magnitude of the interface dipole. The structure of ((benzene-1,2,4,5-tetrayltetrakis(methylene))tetrakis(triphenylphosphonium) bromide) (4PBr) comprises a phenyl core with four benzyl triphenylphosphonium salts and (N,N′-(1,2-phenylenebis(methylene))bis(N,N-diethylethanaminium) bromide) (2EBr) has two benzyl trialkyl ammonium salts. The device based on 4PBr exhibited a power conversion efficiency of 10.63% (Jsc = 20.2 mA cm-2, Voc = 0.79 V, and FF = 66.6%), which is superior to that based on 2EBr (PCE = 9.56%, Jsc = 19.7 mA cm-2, Voc = 0.80 V, and FF = 61.1%). Thus, we demonstrated the possibility of maximizing performances without complicated synthesis procedures. This journal is

Full text of DOI:10.1039/d0tc02513e

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DOI: 10.1039/D0TC02513E
ARTICLE
Small-molecule electrolytes with different ionic functionality as
cathode buffer layer for polymer solar cells
Mijin Jeonga, Doo Kyung Moonb, Hyun Sung Kim*,c, Joo Hyun Kim*,a
Received 00th January 20xx,
Accepted 00th January 20xx
In polymer solar cells (PSCs), electrolytes as the cathode interlayer are responsible for generating an interface dipole, which
reduces the charge collection barrier at the cathode interface. In this study, small-molecule electrolytes were designed and
synthesized as the cathode interlayer in inverted PSCs to overcome high processing costs caused by complicated synthesis
procedures. Moreover, the proposed design strategy allows the ion size and number of ionic functionalities to tune the
DOI: 10.1039/x0xx00000x
magnitude
of
the
interface
dipole.
The
structures
of
((benzene-1,2,4,5-
tetrayltetrakis(methylene))tetrakis(triphenylphosphonium) bromide) (4PBr) comprises a phenyl core with four benzyl
triphenylphosphonium salts and (N,N'-(1,2-phenylenebis(methylene))bis(N,N-diethylethanaminium) bromide) (2EBr) has
퐽푠푐
two benzyl trialkyl ammonium salts. The device based on 4PBr exhibited a power conversion efficiency of 10.63% (
=
2
2
푉표푐 = 0.79 V, FF = 66.6%), which is superior to that with 2EBr (PCE = 9.56%, 퐽푠푐 = 19.7 mA/cm , 푉표푐 = 0.80 V,
20.2 mA/cm ,
FF = 61.1%). Thus, we demonstrated the possibility of maximizing performances without complicated synthesis procedures.
size of the counter anion36 and cation46 of organic electrolytes
on the photovoltaic properties. The results suggested that a
lager counter ion leads to a larger interface dipole, indicating its
Introduction
Over the last decade, there have been several approaches in
developing highly efficient polymer solar cells (PSCs) due its
processability, flexibility, and low-cost fabrication in large-area
applications.1–4 Outstanding improvements with power
conversion efficiencies (PCE) of up to 16%5 have been achieved
by developing efficient photoactive materials6–16 and interface
engineering the electrode.17–37 Among these, tuning the energy
offset at the cathode interface is an important issue that should
be addressed to enhance electron collection ability.
ZnO has been commonly used as the electron transport layer in
inverted PSCs. To maximize its electron collection ability, there
have been several approaches in changing its layer properties,
such as inserting conjugated/non-conjugated polymer
electrolytes17–30 as the interlayer and self-assembled
monolayer31–34 treatments. Thus, better PCEs was achieved by
reducing the energy offset at the cathode interface and
enhancing the charge collection capability.
influence on the energy offset at the cathode interface. Lee et
al. and Jo et al. also reported that the number of ionic
functionalities on electrolytes affect the performance of
PSCs.47–49 This suggests that the magnitude interface dipole
depends on the number of ionic functionalities. Moreover, the
size of ions and number of ionic functionalities have been
observed to efficiently reduce the charge collection barrier at
the interface, thereby improving device performances.
With hese, we designed and synthesized (N,N'-(1,2-
phenylenebis
bromide)
(methylene))bis(N,N-diethylethanaminium)
and ((benzene-1,2,4,5-tetrayltetrakis
(2EBr)
(methylene))tetrakis (triphenylphosphonium) bromide) (4PBr)
(shown in Figure 1a). Both 2EBr and 4PBr have the advantage
of allowing mass production without a complex synthetic
procedure and low product yield. The dipole moment of the
ionic compound is proportional to the distance between two
charges. Therefore, the dipole moment of triphenyl
phosphonium salt would be larger than that of triethyl
ammonium salt. In addition to this, 4PBr is expected to have a
higher dipole moment than 2EBr due to the number of ionic
functionalities. The device based on a bulk-heterojunction
structure (Figure 1b) composed of poly([2,6′-4,8-di(5-
ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoro-
2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7-
Th) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM)
(Figure 1c) as the active layer enhances the PCE based on ZnO
Conjugated polymer electrolytes (CPEs) have been widely used
as the cathode interlayer. However, CPEs have complicated
synthetic procedures and extremely low product yield, limiting
their commercial large-area application. In terms of processing
cost, the development of small-molecule organic electrolytes38–
45
with simple synthetic procedures can attract attention in
large-area application. Recently, we reported the effect of the
a. Department of Polymer Engineering, Pukyong National University, Busan 48513,
Korea
from 8.75% (short circuit current 푠푐 = 17.5 mA/cm2, open circuit
퐽
b. Division of Chemical Engineering, Konkuk University, Seoul 05029, Korea
c. Department of Chemistry, Pukyong National University, Busan 48513, Korea
Electronic Supplementary Information (ESI) available: Experimental details including
materials, synthesis, measurements, and fabrication of devices
* corresponding Authors. Kimhs75@pknu.ac.kr, jkim@pknu.ac.kr
푉
퐽
voltage 표푐 = 0.80 V, fill factor FF = 62.5%) to 10.63% (
= 20.2
푠푐
2
푉
mA/cm , 표푐 = 0.79 V, FF = 66.6%), by introducing 4PBr as the
interlayer. Moreover, the device based on 4PBr hasa more
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ARTICLE
Journal Name
characterized by the 1H, 13C NMR, and elemental analysis, which
View Article Online
DOI: 10.1039/D0TC02513E
are provided in the Supplementary Information.
We fabricated the inverted type PSCs (illustrated in Figure 1b)
based on ZnO with and without small-molecule electrolytes as
the electron transporting layer and investigated the effect of
ionic functionality on the photovoltaic properties. Detailed
procedures for the fabrication of PSCs are described in the
Supplementary Information and the photovoltaic parameters
with different thickness of the cathode interlayer are
summarized in Table S1. The optimum thickness of the
interlayer was determined to be 5⁓6 nm. As shown in Figure
S1, small-molecule electrolytes do not tend to aggregate on the
surface of ZnO. The surface roughness of the ZnO surface with
2EBr and 4PBr were 0.58 and 0.60 nm, respectively, which are
comparable to the surface roughness of ZnO surface (0.55 nm).
Figure 2a shows the current density–voltage (J–V) curves of
PSCs with and without interlayer under illumination while the
photovoltaic performances are summarized in Table 1.
Significant enhancements are observed in devices based on ZnO
Figure 1. Chemical structure of (a) small-molecule electrolytes and (b)
PTB7-Th and PC71BM. (c) Device structure of PSC used in this study.
with an interlayer. The PCEs of devices with 2EBr and 4PBr are
퐽
superior PCE than that based on 2EBr (PCE = 9.56%,
= 19.7
푠푐
2
퐽
푉
9.56% ( 푠푐 =19.7 mA/cm , 표푐 = 0.80 V, FF = 61.1%) and 10.63%
2
푉표푐
mA/cm ,
= 0.80 V, FF = 61.1%). This could be mainly attributed to
퐽푠푐
2
퐽
푉
(
= 20.2 mA/cm , 표푐 = 0.79 V, FF = 66.6%), while that based on
ZnO without interlayer was limited to 8.75% ( 푠푐 =17.5 mA/cm2,
푠푐
the improved
by the reduced Schottky barrier at the cathode
퐽
interface.
푉
퐽
표푐 = 0.80 V, FF = 62.5%). Noticeably, the enhanced 푠푐 was the
main contributor of the improved PCE.
Kelvin probe microscopy (KPM) is a common powerful tool for
determining the effective work function of a metal or metal
Results and Discussion
2EBr and 4PBr were synthesized by simple substitution reaction
between 1,2-bis(bromomethyl)benzene and triethylamine in
toluene and acetonitril, respectively. All compounds were well
oxide. Thus, KPM measurements were conducted to investigate
21–23,26,27,36,37,42-46,48
퐽
how the interlayer affects the changes of
.
푠푐
A larger Schottky barrier at the interface interrupts charge
collection capability.
Figure 2. (a) J–V curves and (b) energy level diagram of the materials and (c) incident photon to converted
electron (IPCE) curves of the PSCs at the optimum processing condition.
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0TC02513E
Table 1. Photovoltaic parameters of PSCs based on ZnO/interlayer. The averages (10 devices) are shown in parentheses.
a
b
c
퐽푠푐
(mA/cm2)
푉표푐
푉
( )
퐽푠푐,푐푎푙
푅푠
푅푠ℎ
FF
PCE
%
( )
Interlayer
-
%
(
)
(mA/cm2)
(Ω∙cm2)
(kΩ∙cm2)
17.5
(17.3)
19.7
(19.6)
20.2
(20.1)
0.80
(0.80)
0.80
(0.80)
0.79
62.5
(62.7)
61.1
(60.4)
66.6
8.75
(8.69)
9.56
(9.41)
10.63
(10.29)
17.1
3.29
893
2EBr
4PBr
19.1
19.6
2.60
2.28
645
2533
(0.79)
(65.2)
a Calculated from IPCE curves. b,cSeries and shunt resistance data are calculated from the device showing the best PCE.
Thus, the transition from a Schottky to Ohmic contact at the
퐽
interface is a crucial factor to achieve high 푠푐. As shown in
Figure 2b, the work function of ZnO with 2EBr and 4PBr is −4.29
and −4.18 eV, respectively, while that of ZnO was −4.4 eV. The
change in magnitude of the interface dipole depended on
changing the size of the cation and number of ionic
functionalities. Reducing the Schottky barrier height results to a
퐽
high 푠푐, which is strongly related to the change of work
퐽
function. Herein, the improved
indicates the formation of
푠푐
favorable interface dipole with its magnitude depending on the
퐽
푠푐
ion size and number of ionic functionalities. The calculated
data from IPCE curves (Figure 2c) showed an extremely good
퐽푠푐
correlation with the
data of devices under a simulated
푅
illumination of 1.0 sun. The series resistance ( 푠) and shunt
푅
resistance ( 푠ℎ) were calculated from the inverse slope near the
high current regime and the slope near the lower current region
푅
in the dark J−V curves. The
data also showed its coherence
푠
퐽
푅
with the 푠푐 of PSCs. The
data of the device based on pure
푠ℎ
Figure 3. Current density–voltage curves of electron-only device with a
configuration of ITO/ZnO (25 nm)/interlayer/PC71BM (60 nm)/Al (100
nm). (inset: with fitted line,V: applied voltage, Vbi: built-in voltage.
ZnO, and ZnO with 2EBr, and 4PBr were 893, 645, and 2553
kΩcm2, respectively, which are well correlated with their FF.
Interestingly, the 푠ℎof the device based on 2EBr was smaller
푅
than that without interlayer.
To understand the charge transport and collection properties of
To elucidate the effect of the interlayer on the electron
transporting/collecting property of ZnO, we fabricated and
tested the electron devices with a structure of ITO/ZnO (25
nm)/with or without interlayer (60 nm)/PC71BM/Al (100 nm)
(Figure 3). Figure 3 shows the space-charge limited current
(SCLC) characteristic of the current density and voltage. The
electron mobilities of the devices were calculated by the SCLC
method expressed by the Mott-Gurney Law. The electron
mobilities of the devices based on 2EBr and 4PBr were 9.2×10−4
and 1.2×10–3 cm2/Vs, respectively, which are comparable to the
device based on ZnO without interlayer (7.4×10–3 cm2/Vs).
However, the turn-on voltages of the devices with 2EBr and
4PBr, which was estimated according to Parker,49 were 1.26 and
0.94 V, respectively, while that based on ZnO without interlayer
was limited to 1.65 V. Thus, the interlayer facilitates the
electron collection capability from PC71BM layer to ZnO layer,
owing to the introduced interlayer that reduced the Schottky
barrier between them. Moreover, 4PBr has easier electron
collection ability than 2EBr. These results are well coherent with
퐽
the devices, we analyzed the photocurrent density ( 푝ℎ) as a
푉
function of the effective voltage ( 푒푓푓) by:
퐽푝ℎ
퐽
퐽
–
퐿 퐷
=
푉푒푓푓
푉
푉
–
0 푎
=
퐽
푠푐
the improvement of
.
Figure 4. Jph as a function of Veff curves the PSCs.
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퐽
퐽
푘 푇 푞 퐼
, , , and are the Boltzmann constant,VtieewmAprtieclreaOtnulirnee
where 퐿 is the measured current density under illumination,
where
퐷
is the measured current density without illumination, 0 is the in Kelvin, electron charge, and illDuOmI:in10a.t1i0o3n9/D0inTtCe0n2s5i1t3yE,
푉
퐽
푉
푠
voltage at 푝ℎ=0, and 푎 is the applied voltage.
respectively. When bimolecular recombination is dominated,
퐽
푉
푒푓푓
As shown in Figure 4, the log ( 푝ℎ) as a function of log (
)
is approximately equal to 1 and reaches 2 in the case of
푉
푠
showed a linear relationship at low
and saturated at the dominant trap-assisted recombination. The values of the
푒푓푓
푉
푉
high
regime. The
value of the devices reaching the device based on pure ZnO, with 2EBr, and with 4PBr were 1.78,
푒푓푓
푒푓푓
푉
saturated photocurrent ( 푠푎푡) increased in the following order: 1.71, and 1.29, respectively. Therefore, the lowest trap-assisted
≅
4PBr (0.17 V) < 2EBr (0.31 V) ZnO (0.34 V). Hence, a smaller recombination behavior of the device based on 4PBr can justify
푉
퐽
and FF values. Overall, most device parameters
푠푐
푠푎푡 denotes faster transition from the space-charge-limited to its highest
퐽
푠푐
the saturation regime. The results followed the trend of the
and PCE of the devices.
related to the charge extraction and recombination
characteristics of the devices are continuously improved by
퐽
푉
푠
We examined the
and
of the devices as a function of introducing the interlayer. In addition, the change in the
푠푐
표푐
illumination intensity50–52 to understand the charge values of the devices agree well with the PCE trend of PSCs.
recombination kinetics at the interfaces. The relationship To further observe the carrier recombination and transport
퐽
퐼
between 푠푐 and illumination intensity is generally defined by mechanism, we examined the electrical impedance spectra (EIS)
퐽푠푐 ∝ 퐼훼
. When the device exhibits a completely bimolecular of the PSCs under dark conditions. EIS measurements were
α
recombination under short-circuit condition,
is unity. As performed at 0 V with a frequency range of 1–1.0 MHz.
α
shown in Figure 5a, the of the devices based on ZnO with 2EBr
and 4PBr are both 0.97, which is comparable to that based on
ZnO (0.93). This indicates that the devices exhibited the
푉
suppression of undesirable bimolecular recombination.
is
표푐
defined by
푉표푐 ∝ 푠푘푇/푞 ∙ 푙푛 (퐼)
Figure 6. (a) EIS spectra at 0.8 V under dark condition, inset shows
the equivalent circuit for analysis of EIS spectra (Rs: Ohmic resistance
including the electrodes and bulk resistance, R: resistance associated
with the interface charge transport, C: capacitance) and (b) Bode
plot.
Figure 5. (a) Jsc – light intensity and (b) Voc – light intensity plots of
the PSCs.
4 | J. Name., 2012, 00, 1-3
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5
J. Yuan, Y. Zhang, L. Zhou, H.-L. Yip, T.-K. Lau, X. Lu, C. Zhu, H. Peng, P.
Figure 6a shows the Nyquist plots of the devices at 0.8 V. The
View Article Online
A. Johnson, M. Leclerc, Y. Cao, J. Ulanski, Y. Li, and Y. Zou, Joule, 2019,
EIS spectra were linearly fitted to estimate the recombination
DOI: 10.1039/D0TC02513E
3, 1140–1151.
푅
resistance ( 푟푒푐). A single semi-circle without a transmission line
6
7
8
9
X. Li, X. Liu, W. Zhang, H.-Q. Wang, and J. Fang, Chem. Mater., 2017,
29, 4176–4180.
was observed in the Nyquist plots of each devices. Its diameter
is assigned as the charge transfer resistance.53,54 The size of the
Z. Wu, C. Sun, S. Dong, X.-F. Jiang, S. Wu, H. Wu, H.-L. Yip, F. Huang,
and Y. Cao, J. Am. Chem. Soc., 2016, 138, 2004–2013.
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and X. Chen, Adv. Mater., 2013, 25, 6889–6894.
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T. Rispens, L. Sanchez, and J. C. Hummelen, Adv. Funct. Mater., 2001,
11, 374–380.
푅
EIS semi-circle reflects the extent of 푟푒푐; thus, depending on
the extent of the charge recombination PSCs. The magnitude of
푅
kΩ
the 푟푒푐 of the devices were 1.51, 0.836, and 1.97
for pristine
ZnO, with 2EBr, and with 4PBr, respectively. Notably, the device
푅
based on ZnO with 4PBr exhibited the highest 푟푒푐, indicating
푅
푟푒푐
the lowest reduced interfacial recombination. The trend of
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퐹퐹
푅
followed those of the
and 푠ℎ of PSCs. As shown in Figure 6b,
퐹퐹
the peak frequency of the devices followed the trend of the and
푠ℎ. This indicates that the interlayer also modify the electron
푅
τ
life time. Moreover, the electron lifetime ( ) can be estinated
τ = 1/2π푓푚푖푑
푓푚푖푑
using the equatrion
,
where
is the 13 M. H. Hoang, G. E. Park, D. L. Phan, T. T. Ngo, T. V. Nguyen, C. G. Park,
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midfrequency peak of the Bode plot in the impedance
spectrum.53 The estimated of the devices based on pristine
τ
μs
,
ZnO, ZnO/2EBr, and ZnO/4PBr were 25, 13, and 32
respectively. This result supports the EIS analysis results.
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Conclusions
Small-molecule electrolytes, 2EBr and 4PBr, have been successfully
synthesized. The PSC with 2EBr enhanced PCE, reaching up to 9.56%.
PCE was further enhanced by replacing 4PBr as the interlayer due to the
increased ion size and number of ionic functionalities. The enhanced
PCE can be mainly attributed to the improvement of Jsc. In addition, the
trend of the FF followed that of PCEs. The charge-generation, charge-
transport, and charge-recombination characteristics also supported the
PCE enhancement. As a result, we demonstrated the possibility to
maximize performances without complicated synthesis procedures.
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Conflicts of interest
There are no conflicts to declare.
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and B. Song, Macromol. Rapid Commun., 2018, 39, 1700828
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32 S.-H. Liao, H.-J. Jhuo, Y.-S. Cheng, and S.-A. Chen, Adv. Mater., 2013,
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This research work was supported by the New & Renewable
Energy Core Technology Program of the Korea Institute of
Energy Technology Evaluation and Planning (KETEP) granted
financial resource from the Ministry of Trade, Industry &
Energy, Republic of Korea (20193091010110) and was
supported by Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded by the
Ministry of Education (2019R1A2C1002585).
33 H. Yang, T. Wu, T. Hu, X. Hu, L. Chen, and Y. Chen, J. Mater. Chem. C,
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Journal of Materials Chemistry C
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DOI: 10.1039/D0TC02513E
Small-molecule electrolytes with diverse ionic functionality as the
cathode buffer layer for polymer solar cells
Mijin Jeonga, Doo Kyung Moonb, Hyun Sung Kim*,c, Joo Hyun Kim*,a
Table of Contents Entry
Small-molecule electrolytes were designed and synthesized as the cathode interlayer in inverted polymer solar
cells.