A water/alcohol-soluble conjugated porphyrin small molecule as a cathode interfacial layer for efficient organic photovoltaics

Article abstract of DOI:10.1039/c6ta05726h

A water/alcohol soluble conjugated porphyrin small molecule, FNEZnP-OE, in which two amino-functionalized fluorenes (9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorenes) are linked to a porphyrin core substituted with two polar 3,4-bis-[2-(2-methoxy-ethoxy)-ethoxy]-phenyls by ethynylene linkages, is designed and synthesized as a cathode interfacial material (CIM) for bulk heterojunction organic solar cells. The PTB7/PC₇₁BM- and PTB7-Th/PC₇₁BM-based devices with FNEZnP-OE as the electron transport layers (ETLs) exhibit power conversion efficiencies (PCEs) of 8.52% and 9.16%, respectively, which are increased by 47% and 41% compared to the devices with no ETL (5.78% and 6.50%, respectively). Most significantly, these PCEs are increased by 13.6% and 8%, respectively, over those with the widely used polymer ETL PFN (poly[(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctylfluorene)]). The outstanding performance of FNEZnP-OE CIM is contributed by not only the polar groups of 9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorenes and 3,4-bis-[2-(2-methoxy-ethoxy)-ethoxy]-phenyls but also the porphyrin-relating π-conjugated backbone of FNEZnP-OE induced by the ethynylene linkages, demonstrating that functionalized porphyrins are very promising interfacial materials.

Full text of DOI:10.1039/c6ta05726h

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DOI: 10.1039/C6TA05726H
A water/alcohol soluble conjugated porphyrin small
molecule as a cathode interfacial layer for efficient organic
photovoltaics
Lin Zhang, Chang Liu, Tianqi Lai, Huadong Huang, Xiaobin Peng*, Fei Huang, Yong Cao
State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology, 381 Wushan
Road, Guangzhou 510640, China. *Email: chxbpeng@scut.edu.cn
Abstract: A water/alcohol soluble conjugated porphyrin small moleculeFNEZnPꢀOE, in
which two aminoꢀfunctionalized fluorene 9,9ꢀbis(30ꢀ(N,Nꢀdimethylamino)propyl)ꢀ2,7ꢀ
fluorenes are linked to a porphyrin core substituted with two polar 3,4ꢀbisꢀ[2ꢀ(2ꢀmethoxyꢀ
ethoxy)ꢀethoxy]ꢀphenylsby ethynylene linkages, is designed and synthesized as a cathode
interfacial material (CIM) for bulk heterojuction organic solar cells. The PTB7/PC71BM– and
PTB7ꢀTh/PC71BM–based devices with FNEZnPꢀOE as the electron transport layers (ETLs)
exhibit power conversion efficiencies (PCEs) of 8.52% and 9.16%, respectively, which are
increased by 47% and 41% compared to the devices with no ETL (5.78% and 6.50%),
respectively. Most significantly, these PCEs are increased by 13.6% and 8%, respectively,
than those with widely used polymer ETLs PFN (poly[(9,9ꢀbis(30ꢀ(N,Nꢀ
dimethylamino)propyl)ꢀ2,7ꢀfluorene)ꢀaltꢀ2,7ꢀ(9,9ꢀioctylfluorene)]).
performance of FNEZnPꢀOE CIM is contributed by not only the polar groups of 9,9ꢀbis(30ꢀ
(N,Nꢀdimethylamino)propyl)ꢀ2,7ꢀfluorenes and 3,4ꢀbisꢀ[2ꢀ(2ꢀmethoxyꢀethoxy)ꢀethoxy]ꢀ
The
outstanding
phenyls but also the porphyrinꢀrelating πꢀconjugated backbone of FNEZnPꢀOE induced by
the ethynylene linkages, demonstrating that functionalized porphyrins are very promising
interfacial materials.
Keywords: water/alcohol soluble; porphyrin; small molecule; electron transport layer
1

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1. Introduction
Bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted ever increasing
interest due to lightꢀweight, mechanical flexibility, and the potential of developing largeꢀarea
devices through lowꢀcost solution processing.1,2 Also, the power conversion efficiencies
(PCEs) more than 10% have been achieved in the past few years.3,4 Among several factors
that contributed to this advance, cathode interfacial materials (CIMs)ranging from inorganic
metal compounds to organic polymeric/small molecules have been proven effective in
improving OSC performance5ꢀ16 because an effective electron transport layer (ETL) between a
metal cathode and an active layer can decrease the high work function (WF)of a bare metal
cathode to improve the ohmic contact of the metal with the electron acceptor material and
thus to enhance the openꢀcircuit voltage (VOC) of OSCs.16ꢀ18
Among the numerous CIMs, water/alcoholꢀsoluble conjugated polymers are very
promissing in optoelectronic devices since they are processed from environmentally friendly
solvents of alcohol, which is important for future industrial applications and can prevent
solvent erosion during the multilayer device fabrication since active layers are usually not
soluble in alcohol. Especially, the water/alcoholꢀsoluble conjugated CIM poly[(9,9ꢀbis(30ꢀ
(N,Nꢀdimethylamino)propyl)ꢀ2,7ꢀfluorene)ꢀaltꢀ2,7ꢀ(9,9ꢀioctylfluorene)] (PFN) has been
widely used in organic photovoltoics (OPVs),4,19ꢀ21 and the aminoꢀfunctionalized fluorene
moiety plays very important roles for the excellent performance.
Compared to polymers, organic small molecule (SM) –based CIMs are attracting
attentions recently due to their intrinsic advantages such as wellꢀdefined chemical structure,
simple synthesis and purification and good batchꢀtoꢀbatch reproducibility. However, the
reports on SM CIMs are very limited.11,15, 22ꢀ27 Therefore, it is very important to develop new
and better CIMs. As the analogues of chlorophylls, porphyrins and their derivatives have been
intensively investigated as electronic materials due to the big πꢀconjugated planes, high molar
2

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absorption coefficients and easy chemical modifications of the peripheries to improve the
intramolecular charge transport and intramolecular electron transfer. We employed
ethynylene linkages to connect electron acceptor moieties such BT and DPP with porphyrin
core to synthesize conjugated porphyrin donor materials for OSCs. Contributed by the
ethynylene linkages, these molecules are planar and the backbones are fully conjugated,
significantly enhancing their charge mobilities and OPV performance compared to the
corresponding materials without ethynylene linkages.28ꢀ30 Very recently, mesoꢀtetraꢀ
pyridiniumꢀ and mesoꢀtetraꢀphenyl–derived porphyrins were reported as CIMs with improved
OPV performance. However, it is noted that these compounds are not conjugated due to the
perpendicular conformation between the pyridiniums or phenyl groups and the porphyrin
core,31ꢀ33 which can limit the charge mobilities/transportation between the cathode and the
active layers even though the selfꢀassembled porphyrin can enhance the electron
transportation to some extent.31
In this study, using ethynylene linkages, we connect a very typical CIM block 9,9ꢀ
bis(30ꢀ(N,Nꢀdimethylamino)propyl)ꢀ2,7ꢀfluorene (FN) to a porphyrin core substituented with
two polar 3,4ꢀbisꢀ[2ꢀ(2ꢀmethoxyꢀethoxy)ꢀethoxy]ꢀphenyls to synthesize a conjugated
porphyrin small molecule FNEZnPꢀOE (Figure 1a). Contributed by the polar groups of two
FN moieties and two 3,4ꢀbisꢀ[2ꢀ(2ꢀmethoxyꢀethoxy)ꢀethoxy]ꢀphenyls, FNEZnPꢀOE shows
good solubility in alcohol and is also soluble in pure water. The BHJ OSCs using FNEZnPꢀ
OE as the ETL between Al cathode and the active layers of PTB7/ PC71BM and PTB7ꢀ
Th/PC71BM
(Figure1,
PTB7:
poly[[4,8ꢀbis[(2ꢀethylhexyl)oxy]benzo[1,2ꢀb:4,5ꢀb′]
dithiopheneꢀ2,
6ꢀdiyl]
[3ꢀfluoroꢀ2ꢀ[(2ꢀethylhexy)carbonyl]thieno[3,4ꢀb]thiophenediyl]],
PTB7ꢀTh: poly[[2,6′ꢀ4,8ꢀdi(5ꢀethylhexylthienyl)benzo[1,2ꢀb;3,3ꢀb] dithiophene] [3ꢀfluoroꢀ
2[(2ꢀethylhexyl)carbonyl]thieno[3,4ꢀb]thiophenediyl]] and PC71BM: [6, 6]ꢀphenyl C71 butyric
acid methyl ester) exhibit PCEs of 8.52 and 9.16%, which are increased by 47% and 41%
compared to the corresponding devices without any interlayer (5.78% and 6.50%),
3

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respectively. Most significantly, these PCEs are even higher than those with the widely used
polymer PFN ETLs with 13.6% and 8% improvement, respectively.
Figure 1. Molecular structures of a) FNEZnPꢀOE and c) PTB7 and PTB7ꢀTh, and b) device
architecture.
2. Experimental section
OPV Fabrication: ITO coated glass substrates were cleaned prior to device fabrication
by sonication in acetone, detergent, distilled water, and isopropyl alcohol. After treated with
an oxygen plasma for 4 min, 40 nm thick PEDOT:PSS (Bayer Baytron 4083) layer was spinꢀ
coated on the ITOꢀcoated glass substrates at 2500 rpm for 30 s, the substrates were
subsequently dried at 150 oC for 10 min in air and then transferred to a N2ꢀglovebox. Then, an
active layer was prepared on the top of PEDOT: PSS layer by spin coating at 1400 rpm for 40
s from a chlorobenzene solution of PTB7 (or PTB7ꢀTh): PC71BM (1:1.5 w/w, total
concentration of 25 mg/ml) with 3 vol% 1, 8ꢀdiiodoctane. 0.4 mg/ml FNEZnPꢀOE methanol
solution was then spin coated onto the active layer with a speed of 2000 rpm for 30 s (10 nm
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thick). For PFN interlayers, methanol solution with concentrations of 0.2 mg/ml was spin
coated onto the active layer with a speed of 2000 rpm for 30 s. Finally, Al (~90 nm) was
evaporated in a highꢀvacuum chamber with base pressure <3×10ꢀ6 mbar as the top electrode.
Masks made from laser beam cutting technology with a wellꢀdefined area of 0.16 cm2 were
attached to define the effective area for accurate measurement.
Thickness measurment: The thicknesses of films more than 100 nm were measured by
a profile. But for the very thin films, their thicknesses were calculated according to the
absorption and thickness values of thick films and the absorption values of very thin films.
3. Results and discussion
3.1.Synthesis, and Optical and Electrochemical Properties
As shown in Scheme S1, FNEZnPꢀOE was obtained from the coupling of 5,15ꢀ
diethynylꢀ10,20ꢀbis(3,4ꢀbisꢀ[2ꢀ(2ꢀmethoxyꢀethoxy)ꢀethoxy]phenyl)porphyrin zinc with 2ꢀ
bromoꢀ9,9′ꢀbis(3″ꢀ(N,Nꢀdimethylamino)propyl)ꢀfluorene, which were both synthesized and
purified according to reported procedures.28,34 Contributed by the two FN moieties and two
3,4ꢀbisꢀ[2ꢀ(2ꢀmethoxyꢀethoxy)ꢀethoxy]ꢀphenyl groups, FNEZnPꢀOE shows excellent
solubility in polar solvents such as H2O, alcohol, DMF, and dimethyl sulfoxide (DMSO)
without using any acid. Furthermore, FNEZnPꢀOE also shows good thermal stability with
decomposition temperature higher than 300 °C (Figure S5).
As seen in Figure 2a, FNEZnPꢀOE exhibits a typical porphyrin Soret band at 454 nm
and a Q band at 665 nm in CH3OH. Since none conjugated zinc porphyrins usually exhibit
Soret and Q bands at about 420 and 550 nm, respectively,31, 35 the significant redshifts of the
Soret and Q bands are attributed to the enhanced intramolecular charge transfer (ICT)
between the fluorene units and the porphyrin core due to the πꢀconjugation of the backbone
induced by the ethynylene linkages. Furthermore, though the Q band intensity of FNEZnPꢀOE
is much weaker than the Soret band, it is still much stronger than the very weak Q bands of
5

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nonꢀconjugated zinc porphyrins, giving a more evidence of the conjugation between FN units
and the porphyrin core.
The quantum chemical calculations further confirm the πꢀconjugation of FNEZnPꢀOE
backbone. Density functional theory (DFT) calculations at the B3LYP/(6ꢀ
31G(D)+LANL2DZ) level show the optimum geometry is planar(Figure S2), enabling the
whole molecule conjugated. And the πꢀelectrons at the highest occupied molecular orbital
(HOMO) are delocalized at both the porphyrin core and the two fluorene moieties while those
at the lowest unoccupied molecular orbital (LUMO) are mainly localized at the porphyrin
core, which is beneficial for the ICT as revealed by its absorption spectrum.
In film, these bands redꢀshift to 470 and 683 nm, respectively, with the enhanced
intensity of the Q band, and the absorption edge is seen at 713 nm, from which the optical
band gap (Egopt) was calculated to be 1.74 eV. The HOMO energy level (EHOMO) of FNEZnPꢀ
OE was measured by cyclic voltammetry (CV). Since the oxidation onset potential (Eox) of
Fc/Fc+ was measured to be 0.36 V with respect to the Ag/AgCl reference electrode under the
same experimental conditions, the EHOMO of FNEZnPꢀOE was estimated to be −4.99 eV from
its onset oxidation potential (Figure S3) and according to the formula of EHOMO = – (Eox +
4.80 – 0.36) (eV).36 From the Egopt and EHOMO, itsLUMO energy level (ELUMO) was calculated
to be −3.25 eV, which is higher than those of PTB7 and PTB7ꢀTh as illastrated in the energy
level diagram of Figure 2b.27, 37,38
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a)
1.0
0.8
0.6
0.4
0.2
0.0
Film
CH3OH
300
400
500
600
700
Wavelength (nm)
Figure 2. (a) Absorption spectra of FNEZnPꢀOE in CH3OH solution and in thin film, and (b)
energy level diagram of the relating OPV materials.
3.2.Photovoltaic Properties
To evaluate the interfacial performance of FNEZnPꢀOE, itwas spin coated on top of the
active layers from methanol solution as an ETL with the conventional BHJ OSC architecture
of ITO/ PEDOT:PSS/active layer/FNEZnPꢀOE/Al (ITO: indium tin oxide, PEDOT:PSS:
poly(3,4ꢀethylenedioxythiophene):poly(styrenesulfonate)) (Figure 1b) with PTB7:PC71BM
or PTB7ꢀTh:PC71BM as the active layers. For comparison, devices with no and PFN ETLs
were also fabricated at the same condition. And the typical current density–voltage (J–V)
characteristics and the corresponding EQE curves of the solar cells are shown in Figure 3
with the OPV characteristics of the short circuit current (Jsc), open circuit voltage (VOC), fill
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factor (FF) and power conversion efficiency (PCE) summarized in Table 1. As seen in
Figure 3 and Table1, without any ETL, the OSCs with PTB7:PC71BM as the active layers
show a mediate PCE of 5.78% with a JSC of 15.96 mA cm−2, a VOC of 0.62 V and a FF of
59.48%. When the very typical polymer CIM of PFN was employed, the VOC, JSC, and FF
values are increased simultaneously to 16.37 mA cm−2, 0.72 V and 63.61%, respectively,
leading to a much improved PCE of 7.50%. When PFN ETL is replaced by FNEZnPꢀOE, the
PCE can be further enhanced to 8.52% with a significantly enhanced FF to 70.31%, a slightly
improved VOC of 0.74 V and the almost same JSC of 16.38 mA cm−2. The PCE of 8.52% is
increased by 13.6% and 47.4% compared to those devices with PFN and no ETL, respectively.
For the devices based on PTB7ꢀTh:PC71BM as the active layers, the solar cells with
FNEZnPꢀOE ETLs show a JSC of 16.47 mA cm−2, a VOC of 0.81 V and a FF of 68.69%,
leading to a PCE of 9.16%, which is increased by 8% and 41% compared to those devices
with PFN and no ETL, respectively.We noted that some groups have already reported high
PCE around 9% and the PCEs optimized here are only mediate for devices based on PTB7ꢀ or
PTB7ꢀTh:PC71BM active layers and PFN ETLs,27 which may be ascribed to the material
batch differences.
The JSC values calculated from the external quantum efficiency (EQE) spectra (Figure
S4), of the devices without and with ETLs are listed in Table 1, which agree well with the
measured ones. It is noted that FNEZnPꢀOE ETLs can enhance the JSC, VOC and FF
simultaneously though only slightly for JSC but significantly for both the VOC and
FFcompared to those with no ETL for both PTB7ꢀTh:PC71BM– and PTB7:PC71BM–based
devices. Even compared to the devices with the widely used PFN ETLs, the OSCs with
FNEZnPꢀOE ETLs show slightly improved JSC and VOC, and enhanced FF in different extent,
indicating that FNEZnPꢀOE performs excellent as a CIM.
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b)
a)
5
0
-5
0
Bare Al
PFN
FNEZnPꢀOE
Bare Al
PFN
FNEZnPꢀOE
-5
-10
-15
-20
-10
-15
-20
-0.2
0.0
0.2
0.4
0.6
0.8
-0.2
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Voltage (V)
Figure3. JꢀV curves of OSCs with/without ETLs using a) PTB7:PC71BM and b) PTB7ꢀ
Th:PC71BM as the active layers.
Table 1. Device characteristics of the OSCs.
cal
VOC
JSC(JSC
)
FF
PCE
Rs
Active layer
ETL
(mA cmꢀ2)
(V)
(%)
(%) max(ave)
(ꢁ· cm2)
no
15.96 (15.19)
0.62
59.48
5.78 (5.53± 0.25)
7.73
PTB7:PC71BM
PFN
FNEZnPꢀOE
no
16.37 (15.27)
16.38 (15.82)
16.07 (14.97)
16.35 (15.29)
16.47 (16.05)
0.72
0.74
0.71
0.79
0.81
63.61
70.31
57.00
65.70
68.69
7.50 (7.45± 0.05)
8.52 (8.39± 0.13)
6.50 (6.40± 0.10)
8.48 (8.42± 0.06)
9.16 (9.13± 0.03)
6.83
4.76
9.90
7.09
5.38
PTB7ꢀ
PFN
Th:PC71BM
FNEZnPꢀOE
3.3. Work Functions of the Modified Cathodes
Since it has been reported that the VOC enhancement by an ETL could be attributed to the
WF decreaseof a cathode,25 we employed Kelvinꢀprobe system under N2 atmosphere to
measure the effective WF values of the Al electrodes bared and modified with the two CIMs
of PFN and FNEZnPꢀOE to clarify the effect of the FNEZnPꢀOE ETL. As listed in Table S2,
the WF values of bared, FNEZnPꢀOE and PFN modified Al electrodes were measured to be
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3.9, 3.4 and 3.5 eV, respectively.39 The WF of FNEZnPꢀOE modified Al electrode is even
slightly smaller than PFN modified on, indicating the good electron injection ability of
FNEZnPꢀOE.
The electron transport from the active layer to the Al cathode was not very efficient in
devices with bare Al cathode because of the Schottkyꢀbarrier effect caused by the high contact
resistance between the high workꢀfunction Al cathode and the LUMO of PC71BM.40 However,
aminoꢀfunctionalized materials modified metal cathodes (such as Al) could increase builtꢀin
potential of the OSCs through the surface dipole interaction between the pendant amino
groups and the cathodes, leading to increased VOC values.41,42 Since it also has been proved
that polar side groups can induce a CIM withexcellent interfacial modification capability to
enhance the charge collection from an active layer to a electrode,43 the excellent electron
transport and extraction abilities of FNEZnPꢀOE can be ascribed to not only the aminoꢀ
functionalized side groups but also the polar 3,4ꢀbisꢀ[2ꢀ(2ꢀmethoxyꢀethoxy)ꢀethoxy]ꢀphenyl
groups, which plays very important roles for the reduction of the WF of the Al electrode due
to the large interfacial dipoles.44ꢀ46
3.4 Dark J–V Characteristics of the Devices
Compared to devices with PFN ETLs, the performance enhancement of FNEZnPꢀOE–
based ones is mainly contributed by the improvement of FF especially for PTB7:PC71BM
active layer devices. The dark J–V characteristics of the devices shown in Figure 4 can
provide useful information for the FF improvement. In dark, the reverse currents of the OSCs
in the region from −2 to 0 V are the largest suppressed by FNEZnPꢀOE ETL among the three
devices, suggesting the best charge transport and extraction abilities to reduce the hole and
electron recombination near the FNEZnPꢀOE modified Al cathode.25 These results are also
consistent with the decreased series resistances (Rs) by FNEZnPꢀOE ETLs as summarized in
Table 1 since reduced RS values have been reported to be important for achieving higher FF
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values in OSCs.47 In addition, the reduced leakage currents and Rs in devices are also
beneficial for obtaining a higher VOC due to the reduced potential drop across the devices.48
b)
a)
103
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
10-7
103
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
10-7
Bare Al
PFN
FNEZnP-OE
Bare Al
PFN
FNEZnP-OE
-2
0
2
-2
0
2
Voltage (V)
Voltage (V)
Figure 4. Dark J–V characteristics of the devices with a) PTB7:PC71BM and b) PTB7ꢀ
Th:PC71BM as the active layers.
3.5 Electron mobilities
Electron mobilities by the space charge limited current (SCLC) method for the electronꢀ
only devices based on the architectures of ITO/ ZnO/PTB7:PC71BM/interlayer/Al provide us
more information for the enhanced FF by FNEZnPꢀOE ETL.49 As calculated from Figure 5,
the electron mobilities for the devices with no, PFN and FNEZnPꢀOE ETLs are 1.92×10ꢀ5,
2.30×10ꢀ4 and 3.05×10ꢀ4 cm2 Vꢀ1 sꢀ1, respectively. While the electron mobilities of the both
devices with PFN and FNEZnPꢀOE ETLs show more than 10ꢀfolds enhancement compared
to that with the bare Al, the mobility of FNEZnPꢀOE modified devices is even larger than
that of PFN modified ones, demonstrating the better electron transporting of FNEZnPꢀOE.
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104
103
102
PFN
FNEZnPꢀOE
Bare Al
2.4
3.2
Voltage(V)
Figure 5. JꢀV curves of the electronꢀonly devices with the device structure of
ITO/ZnO/PTB7:PC71BM/interlayer/Al.
4. Conclusions
In summary, a new conjugated water/alcoholꢀsoluble small molecule FNEZnPꢀOE,
consisting of two aminoꢀfunctionalized fluorence units and a porphyrin core with the polar
3,4ꢀbisꢀ[2ꢀ(2ꢀmethoxyꢀethoxy)ꢀethoxy]ꢀphenyls connected by ethylene linkages, is developed
as a cathode interface material for OSCs. The BHJ solar cells with FNEZnPꢀOE–modified Al
cathodes, and PTB7:PC71BM and PTB7ꢀTh:PC71BM as the active layers show power
conversion efficiencies of 8.52% and 9.16%, which are increased by 47% and 41% compared
to the devices with no ETLs (5.78% and 6.50%), respectively.More importantly, FNEZnPꢀOE
performs even better than a widely used polymer CIM PFN with 13.6% and 8% PCE
enhancement, respectively. One the one hand, the two aminoꢀfunctionalized fluorence units
and the two polar 3,4ꢀbisꢀ[2ꢀ(2ꢀmethoxyꢀethoxy)ꢀethoxy]ꢀphenyls at porphyrin core play very
important roles for the more favorable WF of FNEZnPꢀOE modified cathode to increase the
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builtꢀin potential. On the other hand, the extended πꢀconjugation backbone of FNEZnPꢀOE by
the ethylene linkages is also beneficial for the enhanced electron transport. These preliminary
results demonstrate that suitably functionalized porphyrins and their analogues are very
promising cathode interfacial materials.
Acknowledgements
This work was financially supported by the grants from International Science and
Technology Cooperation Program of China (2013DFG52740, 2010DFA52150), and the
National Natural Science Foundation of China (51473053, 51073060).
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Graphical Abstract:
A water/alcohol soluble conjugated porphyrin small molecule exhibits very good
ability as a cathode interfacial material in organic solar cells.