New solution-processable small molecules as hole-transporting layer in efficient polymer solar cells

Article abstract of DOI:10.1039/c3ta12935g

Small molecules TPDA, TPDB, and TPDH, comprising the same backbone TPD (N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4, 4′-diamine) and different carboxyl side chains were designed and synthesized as hole-transporting materials in polymer solar

Full text of DOI:10.1039/c3ta12935g

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New solution-processable small molecules as hole-transporting layer in
efficient polymer solar cell
s
Kunyuan Lu, Jianyu Yuan, Jun Peng, Xiaodong Huang, Linsong Cui, Zuoquan Jiang, Hai-Qiao Wang*
and Wanli Ma*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Small molecules TPDA, TPDB, and TPDH, comprising the same backbone TPD (N,N'ꢀdiphenylꢀN,N'ꢀ
bis(3ꢀmethylphenyl)ꢀ(1,1'ꢀbiphenyl)ꢀ4,4'ꢀdiamine) and different carboxyl side chains were designed and
synthesized as holeꢀtransporting materials in polymer solar cells. These small molecules demonstrated
10 improved solubility in polar solvents by introduction of the weakly acidic carboxyl groups. The length of
the side chains can also influence the film forming ability. Compared to conventional PEDOT:PSS, these
small molecules showed higher transmittance in visible range as revealed by UVꢀvis measurement.
Desirable energyꢀlevel alignment for efficient holeꢀtransporting and electronꢀblocking ability was
indicated by their absorption and UPS spectra. Without applying any postꢀtreatment to these buffer layers,
15 comparable and improved device performances were achieved compared with PEDOT:PSS buffered
control devices. High power conversion efficiency of 6.51% was realized by TPDB buffered device
employing a low band gap polymer PBDTTPD as the active material, which showed ~15% efficiency
enhancement over the control devices. Equally important, better device stability was demonstrated by
using TPDB as the new holeꢀtransporting layer.
DITPYꢀPh4,29 TAT and TMTI.30 Besides which, inorganic
materials such as graphene/graphene oxides,31ꢀ34 semiconducting
20 1 Introduction
37
metal oxides including NiO,35 MoO3,36 WO3 and others38,39
Bulk heteroꢀjunction (BHJ) polymer solar cells (PSCs) have
attracted remarkable attention during the past ten years as their
wellꢀknown advantages of low cost, light weight, mechanical
flexibility and easy fabrication.1ꢀ8 Very recently, the power
25 conversion efficiency (PCE) exceeding 10% for single junction
PSC has been reported.9ꢀ11 The impressive progresses in the
development of new conjugated polymers have contributed a lot
to the fast increase of device performance.2, 12ꢀ14 In which, the
have also been reported as other classes of HTMs. Although
50 moderate efficiencies have been achieved for the devices buffered
by most of these materials, challenges are still there. For
example, waterꢀsoluble materials generally contain strong acidic ꢀ
SO3H group, and as to crosslinkable materials and metal oxides,
complicated deposition processes or extra postꢀtreatments are
55 always required. Furthermore, for solutionꢀprocessed organic
HTMs, few of them have shown better efficiency than
PEDOT:PSS in PSCs. In this work, we designed and developed
new HTMs with the purpose to achieve high efficiency and good
stability for PSCs with easy fabrication process.
interfacial layers have also played an important role in achieving
15, 16
30 highꢀquality devices.10,
In traditional PSCs, the holeꢀ
transporting layer (HTL) has been typically used to improve the
compatibility of the transparent indium tin oxide (ITO) anode and
the active layer, adjust the energy levels for efficient hole
collection, minimize the contact resistance and thus improve the
35 device performance.17ꢀ19
60
Three small molecules TPDA, TPDB, and TPDH with
different side chains including acetic acid, butyric acid or
hexanoic acid were designed and prepared by modifying N,N'ꢀ
diphenylꢀN,N'ꢀbis(3ꢀmethylphenyl)ꢀ(1,1'ꢀbiphenyl)ꢀ4,4'ꢀdiamine
(TPD), an archetypical holeꢀtransporting material used in organic
Commercial
poly
(3,4ꢀethylenedioxythiophene):poly
(styrenesulfonate) (PEDOT:PSS) is currently the mostly popular
holeꢀtransporting material (HTM) in PSCs. However its intrinsic
65 lightꢀemitting diodes (OLEDs).40,41 TPDꢀbased compounds have
been used or further modified as efficient HTMs in organic solar
cells due to their high transmittance, suitable highest occupied
molecular orbital (HOMO) and good electronꢀblocking ability.26ꢀ
21
deficiencies, such as acidic and hygroscopic properties,20,
40 electrical inhomogeneity22 and insufficient electronꢀblocking
ability23, still limit its further application. Therefore, a number of
other materials have been reported to replace PEDOT:PSS,
30, 42ꢀ44
However, most of these TPDꢀtype films have been
70 deposited by thermal evaporation or prepared in a complex way.
In this work, new TPDꢀtype HTMs were designed to possess
suitable energy levels and good solubility in polar solvents, thus
can be applied in PSCs by a solution process. The optimized
including waterꢀsoluble polymers such as PSSAꢀgꢀPAIN and
25
SPDPA,24,
crosslinkable materials such as TPDSi2,26
45 PTPDSi2,27 and IDTꢀBTPA,28 and small molecules such as
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films exhibit high transmittance in the visible range and provide a
favorable interface demonstrated by atomic force microscopy
(AFM) and contact angle measurement. To investigate the
applicability of these new TPDꢀtype materials as HTLs in PSCs,
(3×5 ml), and then transferred to a 500 ml conical flask
containing 200 ml of water and a stir bar. The solution was stirred
and back extracted by diethyl ether (2×100 ml). Then the
aqueous phase moderate HCl (1 M) was added. The mixture was
5
prototype polymer poly(3ꢀhexylꢀthiophene) (P3HT) and another 60 then extracted with diethyl ether (3×125 ml). The solvent was
efficient polymer donor poly(benzo[1,2ꢀb:4,5ꢀb']ꢀdithiopheneꢀ
thieno[3,4ꢀc]pyrroleꢀ4,6ꢀdione) (PBDTTPD)45ꢀ48 were chosen as
the active materials to fabricate the photovoltaic devices, with
the PEDOT:PSS HTL buffered ones as the control samples. The
removed under reduced pressure, the residual solvent was
removed in vacuo to give a pale yellow solid of TPDA (0.26 g,
67%). H NMR (400 MHz, (CD3)2CO) δ (ppm): 7.40 (d, J = 9.1
1
Hz, 4H), 7.14 (m, 4H), 6.96 (m, 4H), 6.90 (m, 8H), 6.84 (dd, J =
10 best PCE of 6.51% was achieved when TPDB was applied in 65 11.2, 4.3 Hz, 4H), 4.60 (s, 4H). 13C NMR (300 MHz, (CD3)2CO)
PBDTTPDꢀbased device with a postꢀtreatmentꢀfree process,
showing a ~15% enhancement in comparison with the control
device. Equally important, the devices with TPDBꢀHTL exhibited
better stability than the control devices buffered by PEDOT:PSS
15 under ambient conditions.
δ (ppm): 168.90, 154.40, 146.48, 140.52, 133.53, 128.59,
126.60,122.78, 122.18, 121.72, 115.09, 64.20. MS EI (m/z): [M+]
calcd 636.23; found 636.25. Anal. Calcd for C40H32N2O6: C,
75.46; H, 5.07; N, 4.40; found: C, 74.71; H, 4.90; N, 4.37.
70 Synthesis of TPDBM Synthesis of TPDBM was performed by
employing the procedure of TPDAM described above using
methyl 4ꢀbromobutyrate under reflux condition for 12 h. The
product was purified by column chromatography over silica using
a ethyl acetate:hexane mixture (1:5) as the eluent to give a pale
2 Experimental section
2.1 Materials
All chemicals, unless otherwise noted, were purchased from 75 yellow solid of TPDBM(0.74 g, 61%). 1H NMR (400 MHz,
SigmaꢀAldrich or J&K Chemicals then used as received.
20 PBDTTPD was synthesized according to the previous literature.47
N,N'ꢀdiphenylꢀN,N'ꢀbis(4ꢀhydroxyphenyl)ꢀ(1,1'biphenyl)ꢀ4,4'ꢀ
diamine (TPDOH) was prepared according to the literature.50,51
CDCl3) δ (ppm): 7.42 (d, J = 8.9 Hz, 6H), 7.25(m, 8H), 7.09 (dd,
J = 18.7, 8.6 Hz, 8H), 6.86 (m, 6H), 4.02 (s, 4H), 3.72 (s, 6H),
2.57 (t, J = 7.3 Hz, 4H), 2.14 (m, 4H). 13C NMR (300 MHz,
CDCl3) δ (ppm): 172.63, 128.00, 126.03, 121.81, 114.29, 65.84,
80 50.62, 29.52, 23.64. MS EI (m/z): [M+] calcd 720.32; found
720.29.
2.2 Instrumentation
1H NMR and 13C NMR spectra were recorded on a Varian Unity
25 Inova 400 spectrometer or Varian Unity Inova 300 spectrometer.
Mass spectra were recorded on a Thermo ISQ mass spectrometer
using electron impact (EI) ionization technique. Elemental
analyses of C, H, and N were carried out on organic elemental
analyzer Vario EL Ⅲ (German ELEMENTAL Corp.).UVꢀVis
30 absorption spectra were carried out with a Perkin Elmer. Lambda
750 spectrophotometer. UPS analyses were obtained from an
unfiltered He I (21.2 eV) gas discharge lamp and a hemispherical
analyzer. The contact angles were measured using contact angle
system OCA. Atomic force microscopy (AFM) phase images
35 were recorded on Veeco Multimode V instrument.
Synthesis of TPDB Synthesis of TPDB was performed by
employing the procedure of TPDA described above using
TPDBM to obtain light yellow solid (0.60 g. 79%). 1H NMR (400
85 MHz, (CD3)2CO) δ (ppm): 10.40 (s, 1H) 7.37 (d, J = 8.5 Hz, 4H),
7.13 (t, J = 7.7 Hz, 4H), 6.95 (d, J = 8.8 Hz, 4H), 6.92 (m, 8H),
6.82 (d, J = 8.7 Hz, 4H), 3.93 (t, J = 6.2 Hz, 4H), 2.39 (t, J = 7.3
Hz, 4H). 13C NMR (300 MHz, (CD3)2SO) δ (ppm): 174.95,
155.81, 148.11, 147.18, 140.42, 133.66, 130.05, 128.06, 127.60,
90 123.32, 122.82, 116.29, 67.46, 30.83, 25.00. MS EI (m/z): [M+]
calcd 692.29; found 692.26. Anal. Calcd for C44H40N2O6: C,
76.28; H, 5.82; N, 4.04; found: C, 75.82; H, 5.94; N, 4.05.
Synthesis of TPDHM Synthesis of TPDHM was performed by
employing the procedure of TPDBM described above using
95 methyl 6ꢀbromohexanoate. The product was purified by column
chromatography over silica using a ethyl acetate:hexane mixture
2.3 Synthesis
Synthesis of TPDAM TPDOH (0.89 g, 1.71 mmol), methyl
bromoacetate (0.36 ml, 3.76 mmol), potassium carbonate (4.73 g,
34.21mmol), and acetone (18 ml) were added into a 50 ml roundꢀ
40 bottom flask. The reaction was stirred at room temperature for 3
h, resulting in a white suspension. After water (15 ml) was added
to the mixture, the product was extracted with diethyl ether (3×20
ml). Then the solvent was removed under reduced pressure. The
residue was purified by column chromatography over silica using
45 a ethyl acetate:hexane mixture (1:4) as the eluent to give a pale
white solid of TPDAM (0.97 g, 85%). 1H NMR (400 MHz,
CDCl3) δ (ppm): 7.41 (d, J = 9.1 Hz, 6H), 7.24 (m, 8H), 7.08 (m,
8H), 6.86 (m, 6H), 4.63 (s, 4H), 3.83 (s, 6H). 13C NMR (300
MHz, CDCl3) δ (ppm): 169.77, 129.45, 127.50, 123.52, 115.97,
50 66.06, 52.53. MS EI (m/z): [M+] calcd 664.26; found 664.23.
Synthesis of TPDA To a 50 ml roundꢀbottom flask, TPDAM
(0.40 g, 0.60 mmol), potassium hydroxide (0.80 ml, 18 M) and
THF (5 ml) were added. Then the reaction was refluxed for 9 h.
A white precipitate developed as the reaction proceeded. The
55 solid was collected by vacuum filtration and rinsed with THF
1
(1:5) to give a pale yellow solid of TPDHM (0.40 g, 41%). H
NMR (400 MHz, CDCl3) δ (ppm): 7.39 (s, 6H), 7.23 (m, 8H),
7.07 (m, 8H), 6.84 (s, 6H), 3.94 (s, 4H), 3.68 (s, 6H), 2.36 (t, J
100 =7.5 Hz, 4H), 1.83 (m, 4H), 1.74 (m, 4H), 1.54 (m, 4H). 13C
NMR (300 MHz, CDCl3) δ (ppm): 173.03, 127.87, 125.99,
121.68, 114.14, 66.77, 50.22, 32.94, 27.96, 24.65, 23.64. MS EI
(m/z): [M+] calcd 776.38; found 776.39.
Synthesis of TPDH The synthesis of TPDH was performed by
105 employing the procedure of TPDH to obtain the light yellow solid
1
(0.25 g, 87%). H NMR (400 MHz, (CD3)2CO) δ (ppm): 10.35
(1H, s, 1H), 7.40 (d, J = 8.5 Hz, 4H), 7.15 (t, J = 7.4 Hz, 4H),
6.94 (d, J = 8.8 Hz, 4H), 6.92 (m, 8H), 6.82 (d, J = 8.9 Hz, 4H),
3.90 (t, J = 6.4 Hz, 4H), 2.21 (t, J = 7.3 Hz, 4H), 1.68 (m, 4H),
110 1.54 (m, 4H), 1.43 (t, J = 6.8 Hz, 4H). 13C NMR (400 MHz,
(CD3)2SO) δ (ppm): 174.48, 155.65, 147.43, 146.50, 139.60,
132.94, 129.37, 127.39, 126.90, 122.62, 122.11, 115.57, 67.51,
33.65, 28.53, 25.22, 24.31. MS EI (m/z): [M+] calcd 748.35;
2
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found 748.32. Anal. Calcd for C48H48N2O6: C, 76.98; H, 6.46; N,
3.74; found: C, 76.50; H, 6.42; N, 3.69.
2.4 Device fabrication and characterization
The properties of these TPDꢀtype HTMs were studied in the
devices with a structure of ITO/HTMs/Polymer:PC61BM/LiF/Al.
The ITOꢀcoated glass substrate was cleaned stepwise in acetone,
detergent, water, isopropyl alcohol and acetone under ultrasonic.
After an oven drying, the ITO was treated in an ultravioletꢀozone
chamber for 30 min. The PEDOT:PSS layer was spinꢀcoated onto
5
10 the ITO surface at 4500 rpm for 40s and subsequently baked at
150 oC for 10 min. The optimized TPDꢀtype HTLs were prepared
on the ITO substrates by spinꢀcoating (3000 rpm) TPDA
isopropanol (IPA) solution (1 mg mLꢀ1), TPDB IPA solution (1
mg mLꢀ1), and TPDH dimethylformamide (DMF) solution (0.5
15 mg mLꢀ1) respectively for 40s. Subsequently, the active layer was
Scheme 1 Synthetic route to TPDA, TPDB and TPDH.
spinꢀcoated at 1200 rpm for 30s from
a solution of
P3HT:PC61BM (11:9, w/w) containing 1% 1,8ꢀdiiodooctane
(DIO) (v/v) or PBDTTPD:PC61BM (10:10, w/w) containing 2%
DIO (v/v) in chloroform in a glove box with nitrogen atmosphere.
20 Finally, 0.6 nm of LiF and 100 nm of Al were deposited on the
photoactive layer in a vacuum chamber by thermal evaporation at
a pressure lower than 1×10ꢀ6 Torr. The active area of the device
was 7.25 mm2. Current density–voltage (JꢀV) characteristics of
the devices were measured in glove box using a programmable
25 Keithley 2400 source meter under simulated AM 1.5G solar
irradiation at 100 mW cmꢀ2 (Newport, Class AAA solar
simulator, 94023AꢀU). The light intensity was calibrated by a
certified Oriel Reference Cell (91150V). The external quantum
efficiency (EQE) was recorded using a certified IPCE instrument
30 (Zolix Instruments, Inc, Solar Cell Scan100).
Fig. 1 Optimized geometries and molecular orbital surfaces of the HOMO
60 and LUMO of TPDA, TPDB and TPDH, obtained at the DFT/B3LYP/6-31G(d)
Level.
ꢀ35.86o), TPDB (39.75o and ꢀ35.56o) and TPDH (39.58o and ꢀ
35.39o) suggest that the increase in the chain length contributes
little to the distortion of the molecule structure. The electron
65 density of the HOMO is mainly delocalized through the πꢀorbitals
of the triphenylamine units. And as expected, the alkoxy chain
contributes to the delocalization of HOMO but less influences the
lowest unoccupied molecular orbital (LUMO) electron density.
This will be helpful in tuning the HOMO levels.
3 Results and discussion
3.1 Design and synthesis of HTMs
In order to improve the solubility of TPD, weakly acidic carboxyl
groups were introduced to the TPD moiety via alkoxy chain. Here
35 the alkoxy chain is used as we believe it could balance the
electronꢀwithdrawing effect of carboxyl group by its electronꢀ
donating property, which then guaranties the holeꢀtransporting
ability of the resultant molecules. Furthermore, the chain length
of the alkoxy chain can be further adjusted to improve the
40 solubility and the filmꢀforming ability of the molecule.49 The
synthetic procedures of TPDA, TPDB, TPDH are shown in
70 3.3 Physical properties
UV−vis spectroscopy was used to evaluate the absorption of the
solutionꢀprocessed thin films of TPDA, TPDB and TPDH as
depicted in Fig. 2(a). All the films show similar absorption
spectra mainly in the ultraviolet region, suggesting big optical
75 bandgaps (Egopt) which can be calculated from the absorption
onset (λonset). Fig. 3 shows the optical transmission spectra of the
optimized films of TPDA, TPDB, TPDH, along with the
spectrum of PEDOT:PSS film used as a reference. Consistently,
all the TPDꢀbased HTMs exhibit better transmittance in the
80 visible range over PEDOT:PSS, resulting in less interference with
the absorption of the active materials, hence benefiting light
harvesting in the PSCs.
Ultraviolet photoelectron spectroscopy (UPS) was utilized to
investigate the energy level alignment of the HTMs films, as
85 displayed in Fig. 2(b). The ionization potential (IP) was estimated
from the measured cutꢀoff energy of the secondaryꢀelectron and
the onset energy of the HOMO level. The LUMO levels of the
Scheme 1. The common precursor TPDOH was prepared
according to previous reports.50,
The TPDꢀtype ester
51
compounds were synthesized via Williamson ether synthesis with
45 good yields. Further hydrolysis of these esters afforded the target
compounds, whose chemical structures were confirmed by 1H
NMR, 13C NMR, and Mass spectra (see Experimental Section for
details).
3.2 Theoretical calculation
50 The geometrical and electronic properties of TPDA, TPDB and
TPDH were revealed by the Gaussian 09 program suite. The
density functional theory (DFT) calculation was optimized at the
B3LYP/6ꢀ31G(d) level. Molecular orbitals were visualized using
Gaussview. As displayed in Fig. 1, the slight difference of the
55 dihedral angles between different benzenes in TPDA (40.61o and
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100
(a)
1.0
0.8
0.6
0.4
0.2
0.0
TPDA
TPDB
TPDH
98
96
94
92
90
TPDA
TPDB
TPDH
PEDOT:PSS
300
350
400
450
500
300
400
500
600
700
800
Wavelength (nm)
Wavelength(nm)
Fig. 3 Optical transmittance of the TPDA, TPDB, TPDH films and the
PEDOT:PSS layer on the quartz.
(b)
TPDA
TPDB
TPDH
35 and PEDOT:PSS respectively indicate that the surfaces of TPDꢀ
based HTLs are more hydrophobic than the surface of
PEDOT:PSS films (Fig. S2, see the ESI†), which may benefit the
wetting of the polymer:fullerene solution and the consequent
forming of the active film on these TPDꢀbased HTLs.53
40 3.4 Photovoltaic properties
To investigate the effect of these HTMs, PSCs were fabricated
with the configuration of ITO/HTMs/P3HT:PC61BM/LiF/Al (Fig.
4(a)). For comparison, control devices were fabricated with
PEDOT:PSS as the HTL. The detailed device results were
16 14 12 10
8
6
4
2
0
Binding Energy (eV)
45 summarized in Table
2 (more details about the device
optimization under different conditions can be found in Fig. S3,
see the ESI†). Excellent capabilities as HTLs in PSCs are
confirmed for both TPDA and TPDB by their comparable device
performances (PCE 2.92% and 3.03%) with that of the control
50 device (PCE 2.94%), while TPDH exhibited a relatively lower
PCE due to a smaller Jsc and FF, which could be attributed to its
relatively longer side chain. The long and insulating side chain
leads to increased series resistance (Table 2) and reduced
photovoltaic performance, even though TPDH could form a
55 smoother and more uniform film than TPDA and TPDB.
Fig. 2 (a) Normalized absorption spectra of solution processed films of
TPDA, TPDB and TPDH on quartz. (b) UPS spectra of TPDA, TPDB and
TPDH thin films on the ITO glasses.
5
opt
HTMs can be calculated by using the corresponding IP and Eg
(See Table 1).51 Together with the reported electronic energy
levels of PEDOT:PSS,35 P3HT,17 PBDTTPD,47 and PC61BM,17
the energy diagram of the materials are presented in Fig. 4 (b).
10 TPDA, TPDB, and TPDH show similar HOMO level very close
to that of PEDOT:PSS, suggesting the potential as efficient hole
collecting materials. Furthermore, the significantly higher LUMO
levels (ꢀ2.19 to ꢀ2.13 eV) of these HTMs relative to those of most
polymers endow them good electronꢀblocking capability.
15
The atomic force microscope (AFM) images of the HTLs
involved in this work were taken to study the morphologies of
these films (Fig. S1, See the ESI†). Given that TPDA and TPDB
are easily soluble in isopropanol (IPA) (>10 mg mlꢀ1) while
TPDH is soluble in dimethylformamide (DMF) (>10 mg mlꢀ1),
20 these films were prepared from the corresponding solvents with
the optimized concentrations. Their surface roughnesses (in Root
Mean Square, RMS) are 4.3 nm, 4.1 nm and 2.9 nm for TPDA,
TPDB and TPDH film respectively, indicating that the film
surface gets smoother with increased chain length, which is in
25 accordance with the general understanding that long side chain
could improve the film quality of small molecules.49 In addition,
the interface between the active layer and HTL can usually be
influenced by their specific adhesion, which lies on molecules’
affinity for water or oil. The measurements of contact angle to
30 water were carried out to study these properties. The measured
angles of 62.9o, 61.9o, 66.8o and 5.2o for TPDA, TPDB, TPDH
Fig. 4 (a) Device structure of the PSCs. (b) Schematic energy diagram of
the materials involved in the PSCs. (c) The molecular structures of TPDA,
TPDB, TPDH, P3HT, PBDTTPD, and PC61BM.
4
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Table 1 Photophysical properties of TPDA, TPDB, TPDH
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(a)
opt
HTM
λmax
λonset
Eg
IP
LUMO
(eV)
(nm)
(nm)
(eV)
(eV)
TPDA
TPDB
TPDH
356
358
362
402
403
406
3.08
3.08
3.05
5.25
5.21
5.24
-2.17
-2.13
-2.19
a Footnote text.
TPDB
PEDOT:PSS
0
-2
TPDA
TPDB
TPDH
300
400
500
600
700
PEDOT:PSS
PEDOT:PSS
/ TPDB
Wavelength (nm)
25
-4
(b)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-6
-8
(a)
-10
0.0
0.1
0.2
Voltage(V)
0.3
0.4
0.5
0.6
TPDB
PEDOT:PSS
0
-2
TPDA
TPDB
TPDH
PEDOT:PSS
PEDOT:PSS
/ TPDB
300
400
500
600
700
-4
Wavelength (nm)
Fig.
6
EQE spectra of the devices with configuration: (a)
ITO/HTMs/P3HT:PC61BM/LiF/Al (b) ITO/HTMs/PBDTTPD:PC61BM/LiF/Al.
-6
-8
polymer P3HT was used, slightly increased series resistances
30 were recorded for the TPDꢀHTM buffered devices compared to
the control device (Table 2). We consider this evidenced the
improved contact between the TPDꢀHTL and the PBDTTPD
layer, which is consistent with the enhanced interfacial molecular
interaction. As a result, increased Jsc (10.86, 11.51 and 10.89 mA
35 cmꢀ2 respectively) and largely enhanced FF (constantly 65%)
were observed for all the PBDTTPDꢀbased devices using TPDꢀ
type materials as the HTLs, while the reference device with
PEDOT:PSS HTL showed Voc, Jsc, and FF values of 0.91 V,
10.76 mA cmꢀ2, and 58% respectively, corresponding to a PCE of
40 5.68%, as shown in Table 3 and Fig. 5 (b). The increase of both
Jsc and FF should be attributed to the enhanced holeꢀextraction
from the polymer to the anode. The highest PCE of 6.51% was
achieved by the optimized device using TPDB as HTL with Jsc up
to 11.51 mA cmꢀ2 and FF up to 65%, which were much higher
45 than those of the control device.
To further understand the effect of these new HTMs on the
modification of the interface, we deposited PEDOT:PSS and
TPDB successively as the HTL in devices adopting the same
device structure. According to the results in Table 2 and Table 3,
50 the PCE of 3.16% for the P3HTꢀbased device and 5.93% for the
PBDTTPDꢀbased device were slightly higher over the
corresponding control devices. This could be attributed to the
improved surface hydrophobic property and the electronꢀblocking
ability of the PEDOT:PSS film by the top TPDB layer. In
-10
-12
(b)
0.0
0.2
0.4
Voltage(V)
0.6
0.8
5
Fig. 5 Current density-voltage characteristics of the optimized devices
with configuration: (a) ITO/HTMs/P3HT:PC61BM/LiF/Al (b)
ITO/HTMs/PBDTTPD:PCBM/LiF/Al.
It would be more attractive if these new TPDꢀtype HTLs can
be effectively applied in various polymer solar cells, especially in
10 those highly efficient semiconducting polymerꢀbased PSCs. And
it is known that the effect of the interfacial layer had a reliance on
the chemical structures of the polymer donors.15 With this in
consideration, a low bandgap polymer PBDTTPD containing the
thieno[3,4ꢀc]pyrroleꢀ4,6ꢀdione unit with alkoxy side chains was
15 employed as the donor. With which, we believe that OꢀH and Oꢀ
O interactions tend to be formed between the side chains of the
HTM and the alkoxy side chains of active polymer and hence
good contact tends to be achieved between HTM and PBDTTPD,
which will help to improve the device performance.15,54,55 As
20 shown in Table 2 and Table 3, obviously decreased series
resistances (Rs) were observed (Table 3) for the TPDꢀHTM
buffered devices (7.5, 4.9 and 9.9 ꢁ cm2) compared to the
PEDOT:PSS buffered device (15.9 ꢁ cm2) when the polymer
PBDTTPD was used (Table 3). However, in contrast, when the
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Table 2 Photovoltaic performances of P3HT:PC61BM based devices with different HTLs.
Jsc
Rs
HTL
Voc (V)
FF (%)
PCE (%)
(mA cm-2)
(Ω cm2)a
PEDOT:PSS
TPDA
TPDB
0.55
0.56
0.55
0.55
0.53
8.21
8.42
8.62
8.20
8.90
65
62
64
57
67
2.94
2.92
3.03
2.57
3.16
8.8
9.7
9.5
17
TPDH
PEDOT:PSS/TPDB
7.1
a Series resistance deduced from the inverse slope near Voc in the J–V curve.
Table 3 Photovoltaic performances of PBDTTPD:PC61BM based devices with different HTLs.
JSC
Rs
HTL
VOC (V)
FF (%)
PCE (%)
(mA cm-2)
(Ω cm2)a
PEDOT:PSS
TPDA
TPDB
0.91
0.87
0.87
0.83
0.91
10.76
10.86
11.51
10.89
11.04
58
65
65
65
59
5.68
6.14
6.51
5.88
5.93
15.9
7.5
4.9
9.9
15.1
TPDH
PEDOT:PSS/TPDB
a Series resistance deduced from the inverse slope near Voc in the J–V curve.
5
addition, the effects of various solvents, solution concentrations 20 unencapsulated solar cells based on both P3HT:PC61BM and
of TPDB on the device performance were also examined (Table
S1, See the ESI†).
The external quantum efficiency (EQE) curves of the devices
were presented in Fig. 6. The P3HT:PC61BMꢀbased solar cells
PBDTTPD:PC61BM in ambient conditions (Fig. 7) and in glove
box with nitrogen atmosphere (Fig. S4, See the ESI†). The
devices with TPDB HTL exhibited comparable stability in an
inert atmosphere like the corresponding PEDOT:PSS buffered
10 with TPDB and PEDOT:PSS as HTLs showed the maximum 25 PSCs. However, when stored in the laboratory ambient
EQE values of 54% and 51% at ~510 nm with theoretical Jsc
values 7.78 mA cmꢀ2 and 7.18 mA cmꢀ2, respectively. For
PBDTTPD:PC61BMꢀbased devices, the maximum EQE values of
61% and 54% at ~620 nm with theoretical Jsc values 10.09 mA
conditions, the TPDB buffered devices showed obvious slower
degradation than the control devices in the parameters Jsc, FF and
PCE, with an almost unchanged Voc. This could be explained by
the well improved quality of TPDB with much decreased acidic
15 cmꢀ2 and 9.11 mA cmꢀ2 were recorded when TPDB and 30 and hygroscopic properties compared to PEDOT:PSS.56
PEDOT:PSS applied as the HTL respectively. These results are in
good agreement with the device J–V characteristics (Fig. 5, Table
4 Conclusions
2, 3) and the transmittance spectra of TPDB and PEDOT:PSS
In summary, we have demonstrated efficient and stable PSCs
(Fig. 3). The stability measurements were carried out for
1.0
0.8
1.0
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
Voc
Jsc
Voc
Jsc
Voc
Jsc
Voc
Jsc
-0.2
-0.4
-0.6
-0.2
-0.4
-0.6
FF
PCE
PEDOT:PSS
FF
PCE
TPDB
FF
PCE
PEDOT:PSS
FF
PCE
TPDB
PBDTTPD
P3HT
(b)
(a)
0
2
4
6
8
10
12
14
16
0
1
2
3
4
5
Time (hour)
Time (hour)
35 Fig. 7 The stability of device (P3HT:PC61BM) characteristics (a) and device (PBDTTPD:PC61BM) characteristics (b) in air under ambient conditions without
encapsulation.
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55 9 R. F. Service, Science, 2011, 332, 293.
10 Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat. Photonics,
2012, 6, 591ꢀ595.
11 M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop,
Prog. Photovoltaics Res. Appl., 2013, 21, 1ꢀ11.
using solutionꢀprocessable new TPDꢀtype materials as the holeꢀ
transporting layer by spinꢀcoating method without any post
treatment. All films based on these TPDꢀtype materials exhibited
good transmittance and suitable energy levels. Film quality could
5
be tuned in some extent by adjusting the length of the side chain 60 12 C. Cabanetos, A. El Labban, J. A. Bartelt, J. D. Douglas, W. R.
Mateker, J. M. J. Fréchet, M. D. McGehee and P. M. Beaujuge, J.
Am. Chem. Soc., 2013, 135, 4656ꢀ4659.
13 D. Qian, W. Ma, Z. Li, X. Guo, S. Zhang, L. Ye, H. Ade, Z. a. Tan and
J. Hou, J. Am. Chem. Soc., 2013, 135, 8464ꢀ8467.
65 14 J. Yuan, Z. Zhai, H. Dong, J. Li, Z. Jiang, Y. Li and W. Ma, Adv.
Funct. Mater., 2013, 23, 885ꢀ892.
15 Z. He, C. Zhang, X. Xu, L. Zhang, L. Huang, J. Chen, H. Wu and Y.
Cao, Adv. Mater., 2011, 23, 3086ꢀ3089.
16 Z. He, C. Zhong, X. Huang, W.ꢀY. Wong, H. Wu, L. Chen, S. Su and
Y. Cao, Adv. Mater., 2011, 23, 4636ꢀ4643.
17 H.ꢀL. Yip and A. K.ꢀY. Jen, Energy Environ. Sci., 2012, 5, 5994ꢀ6011.
18 R. Po, C. Carbonera, A. Bernardi and N. Camaioni, Energy Environ.
Sci., 2011, 4, 285ꢀ310.
19 H. Ma, H.ꢀL. Yip, F. Huang and A. K.ꢀY. Jen, Adv. Funct. Mater.,
2010, 20, 1371ꢀ1388.
of small molecules, consequently influence the device
performance. As the most efficient HTM, TPDB with butyric
acid side chain provided comparable performance for the
P3HT:PC61BMꢀbased solar cells compared to the control device.
10 While with PBDTTPD:PC61BM as the active layer, device with
TPDB HTL exhibited higher PCE up to 6.51% with an increase
of ~15% in comparison with the control device, due to the
improved Jsc and FF. Equally significant, the unencapsulated
TPDBꢀbuffered devices showed better stability than the
15 PEDOT:PSS control devices in ambient conditions. All these
results suggest that TPDB is a very promising holeꢀtransporting
material for polymer solar cells.
70
75
20 P. Škraba, G. Bratina, S. Igarashi, H. Nohira and K. Hirose, Thin Solid
Films, 2011, 519, 4216ꢀ4219.
21 M. P. de Jong, L. J. van Ijzendoorn and M. J. A. de Voigt, Appl. Phys.
Lett., 2000, 77, 2255ꢀ2257.
80 22 L. S. C. Pingree, B. A. MacLeod and D. S. Ginger, J. Phys. Chem. C,
2008, 112, 7922ꢀ7927.
23 S. K. Hau, H.ꢀL. Yip, J. Zou and A. K.ꢀY. Jen, Org. Electron., 2009,
10, 1401ꢀ1407.
Acknowledgements
This work was supported by the National High Technology
20 Research and Development Program of China (863 Program)
(grant no. 2011AA050520), the National Natural Science
Foundation of China (grant no. 61176054), the Priority Academic
Program Development of Jiangsu Higher Education Institutions.
This is also a project supported by the Fund for Innovative
25 Research Teams of Jiangsu Higher Education Institutions.
24 J. W. Jung, J. U. Lee and W. H. Jo, J. Phys. Chem. C, 2010, 114, 633ꢀ
85
637.
25 C.ꢀY. Li, T.ꢀC. Wen and T.ꢀF. Guo, J. Mater. Chem., 2008, 18, 4478ꢀ
4482.
26 A. W. Hains, J. Liu, A. B. F. Martinson, M. D. Irwin and T. J. Marks,
Adv. Funct. Mater., 2010, 20, 595ꢀ606.
Notes and references
90 27 A. W. Hains, C. Ramanan, M. D. Irwin, J. Liu, M. R. Wasielewski and
T. J. Marks, ACS Appl. Mater. Interfaces, 2009, 2, 175ꢀ185.
28 Y. Sun, S.ꢀC. Chien, H.ꢀL. Yip, Y. Zhang, K.ꢀS. Chen, D. F. Zeigler,
F.ꢀC. Chen, B. Lin and A. K.ꢀY. Jen, Chem. Mater., 2011, 23, 5006ꢀ
5015.
Jiangsu Key Laboratory for CarbonꢀBased Functional Materials &
Devices, Institute of Functional Nano & Soft Materials (FUNSOM),
Soochow University, Suzhou, Jiangsu 215123, China. Eꢀmail:
30 hqwang@suda.edu.cn; wlma@suda.edu.cn
95 29 S. Berny, L. Tortech, M. Véber and D. Fichou, ACS Appl. Mater.
Interfaces, 2010, 2, 3059ꢀ3068.
30 S. W. Shelton, T. L. Chen, D. E. Barclay and B. Ma, ACS Appl. Mater.
Interfaces, 2012, 4, 2534ꢀ2540.
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
35 ‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
31 S. S. Li, K. H. Tu, C. C. Lin, C. W. Chen and M. Chhowalla, ACS
100
Nano, 2010, 4, 3169ꢀ3174.
32 K.ꢀH. Tu, S.ꢀS. Li, W.ꢀC. Li, D.ꢀY. Wang, J.ꢀR. Yang and C.ꢀW. Chen,
Energy Environ. Sci., 2011, 4, 3521ꢀ3526.
33 Q.ꢀD. Yang, T.ꢀW. Ng, M.ꢀF. Lo, N.ꢀB. Wong and C.ꢀS. Lee, Org.
Electron., 2012, 13, 3220ꢀ3225.
1 L. Dou, J. You, J. Yang, C.ꢀC. Chen, Y. He, S. Murase, T. Moriarty, K.
40
Emery, G. Li and Y. Yang, Nat. Photonics, 2012, 6, 180ꢀ185.
2 Y. Liang, Z. Xu, J. Xia, S.ꢀT. Tsai, Y. Wu, G. Li, C. Ray and L. Yu,
Adv. Mater., 2010, 22, E135ꢀE138.
3 G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science,
1995, 270, 1789 ꢀ1791.
45 4 G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery and Y.
Yang, Nat. Mater., 2005, 4, 864ꢀ868.
5 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger and
G. C. Bazan, Nat. Mater., 2007, 6, 497ꢀ500.
105 34 G. H. Jun, S. H. Jin, B. Lee, B. H. Kim, W.ꢀS. Chae, S. H. Hong and S.
Jeon, Energy Environ . Sci., 2013, DOI: 10.1039/c3ee40963e.
35 J. R. Manders, S.ꢀW. Tsang, M. J. Hartel, T.ꢀH. Lai, S. Chen, C. M.
Amb, J. R. Reynolds and F. So, Adv. Funct. Mater., 2013, 23, 2993ꢀ
3001.
110 36 Z. Tan, D. Qian, W. Zhang, L. Li, Y. Ding, Q. Xu, F. Wang and Y. Li,
J. Mater. Chem. A, 2013, 1, 657ꢀ664.
37 H. Choi, B. Kim, M. J. Ko, D.ꢀK. Lee, H. Kim, S. H. Kim and K. Kim,
Org. Electron., 2012, 13, 959ꢀ968.
38 R. Lampande, G. W. Kim, J. Boizot, Y. J. Kim, R. Pode and J. H.
6 S. Günes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 2007, 107,
50
1324ꢀ1338.
7 L.ꢀM. Chen, Z. Hong, G. Li and Y. Yang, Adv. Mater., 2009, 21, 1434ꢀ
1449.
8 W. Ma, C. Yang, X. Gong, K. Lee and A. J. Heeger, Adv. Funct.
Mater., 2005, 15, 1617ꢀ1622.
115
Kwon, J. Mater. Chem. A, 2013, 1, 6895ꢀ6900.
39 H.ꢀQ. Wang, N. Li, N. S. Guldal and C. J. Brabec, Org. Electron.,
2012, 13, 3014ꢀ3021.
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 7

Journal of Materials Chemistry A
Page 8 of 9
View Article Online
DOI: 10.1039/C3TA12935G
40M. Stolka, J. F. Yanus and D. M. Pai, J. Phys. Chem., 1984, 88, 4707ꢀ
4714.
41 B. E. Koene, D. E. Loy and M. E. Thompson, Chem. Mater., 1998, 10,
2235ꢀ2250.
5
42 S. Feser and K. Meerholz, Chem. Mater., 2011, 23, 5001ꢀ5005.
43 M. Shin, H. Kim, S. Nam, J. Park and Y. Kim, Energy Environ. Sci.,
2010, 3, 1538ꢀ1543.
44 G. Zhang, W. Li, B. Chu, L. Chen, F. Yan, et al. Appl. Phys. Lett.,
2009, 94, 143302
10 45 G. Zhang, Y. Fu, Q. Zhang and Z. Xie, Chem. Commun., 2010, 46,
4997ꢀ4999.
46 Y. Zhang, S. K. Hau, H.ꢀL. Yip, Y. Sun, O. Acton and A. K.ꢀY. Jen,
Chem. Mater., 2010, 22, 2696ꢀ2698.
47 C. Piliego, T. W. Holcombe, J. D. Douglas, C. H. Woo, P. M.
15
20
Beaujuge and J. M. J. Fréchet, J. Am. Chem. Soc., 2010, 132, 7595ꢀ
7597.
48 Y. Zou, A. Najari, P. Berrouard, S. Beaupré, B. Réda Aïch, Y. Tao and
M. Leclerc, J. Am. Chem. Soc., 2010, 132, 5330ꢀ5331.
49 Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian, J. You, Y. Yang
and Y. Chen, Adv.Energy Mater., 2011, 1, 771ꢀ775.
50 B. A. Kamino, B. Mills, C. Reali, M. J. Gretton, M. A. Brook and T. P.
Bender, J. Org. Chem., 2012, 77, 1663ꢀ1674.
51 C.ꢀH. Ku, C.ꢀH. Kuo, C.ꢀY. Chen, M.ꢀK. Leung and K.ꢀH. Hsieh, J.
Mater. Chem., 2008, 18, 1296ꢀ1301.
25 52 S. Cho, J. Lee, M. Tong, J. H. Seo and C. Yang, Adv. Funct. Mater.,
2011, 21, 1910ꢀ1916.
53 H. Sirringhaus, N. Tessler and R. H. Friend, Science, 1998, 280, 1741ꢀ
1744.
54 C. He, C. Zhong, H. Wu, R. Yang, W. Yang, F. Huang, G. C. Bazan
30
and Y. Cao, J. Mater. Chem., 2010, 20, 2617ꢀ2622.
55 L. Zhang, C. He, J. Chen, P. Yuan, L. Huang, C. Zhang, W. Cai, Z.
Liu and Y. Cao, Macromolecules, 2010, 43, 9771ꢀ9778.
56 M. Jørgensen, K. Norrman and F. C. Krebs, Sol. Energy Mater. Sol.
Cells, 2008, 92, 686ꢀ714.
8
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New TPD-type hole-transporting materials containing carboxyl groups with different chain
length as HTLs in efficient P3HT and PBDTTPD-based polymer organic solar cells.