Efficient inorganic-organic hybrid perovskite solar cells based on pyrene arylamine derivatives as hole-transporting materials

Article abstract of DOI:10.1021/ja410659k

A set of three N,N-di-p-methoxyphenylamine-substituted pyrene derivatives have successfully been synthesized and characterized by ¹H/ ¹³C NMR spectroscopy, mass spectrometry, and elemental analysis. The optical and electronic structu

Full text of DOI:10.1021/ja410659k

Communication
pubs.acs.org/JACS
Efficient Inorganic−Organic Hybrid Perovskite Solar Cells Based on
Pyrene Arylamine Derivatives as Hole-Transporting Materials
Nam Joong Jeon,† Jaemin Lee,† Jun Hong Noh,† Mohammad Khaja Nazeeruddin,‡ Michael Gratzel,‡
̈
and Sang Il Seok*,†,∥
†Division of Advanced Materials, Korea Research Institute of Chemical Technology, 141 Gajeong-Ro, Yuseong-Gu, Daejeon 305-600,
Republic of Korea
‡Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Science, Swiss Federal
Institute of Technology, CH-1015 Lausanne, Switzerland
∥Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea
S
* Supporting Information
transporting material (HTM), a 15.0% power conversion
efficiency (η) was achieved under 1 sun (100 mW/cm2)
ABSTRACT: A set of three N,N-di-p-methoxyphenyl-
illumination.4f Snaith and co-workers4g showed a maximum
amine-substituted pyrene derivatives have successfully
been synthesized and characterized by 1H/13C NMR
spectroscopy, mass spectrometry, and elemental analysis.
performance of η = 15.4%, with an open-circuit voltage (Voc) of
1.07 V and a short-circuit current density (Jsc) of 21.5 mA cm−2,
The optical and electronic structures of the pyrene
derivatives were adjusted by controlling the ratio of N,N-
di-p-methoxyphenylamine to pyrene, and investigated by
UV/vis spectroscopy and cyclic voltammetry. The pyrene
derivatives were employed as hole-transporting materials
(HTMs) in fabricating mesoporous TiO2/CH3NH3PbI3/
HTMs/Au solar cells. The pyrene-based derivative Py-C
exhibited a short-circuit current density of 20.2 mA/cm2,
an open-circuit voltage (Voc) of 0.886 V, and a fill factor of
69.4% under an illumination of 1 sun (100 mW/cm2),
resulting in an overall power conversion efficiency of
12.4%. The performance is comparable to that of the well-
studied spiro-OMeTAD, even though the Voc is slightly
lower. Thus, this newly synthesized pyrene derivative
holds promise as a HTM for highly efficient perovskite-
based solar cells.
based on CH3NH3PbI3−xClx deposited as thin layers with spiro-
OMeTAD as the hole conductor. In mesoscopic solar cells using
perovskites, several conducting polymers such as poly(3-
hexylthiophene) (P3HT), poly[N-9-heptadecanyl-2,7-carb-
azole-alt-3,6-bis(thiophen-5-yl)-2,5-dioctyl-2,5-dihydropyrrolo-
[3,4]pyrrole-1,4-dione] (PCBTDPP), poly[2,1,3-benzothia-
diazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-
b′]dithiophene-2,6-diyl]] (PCPDTBT), poly[[9-(1-octyl-
nonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzo-
thiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT), and poly-
(triarylamine) (PTAA) have been used as HTMs.4d,e,h However,
the high cost of HTMs and/or low performance hinders the
advancement of cost-effective and practical perovskite solar cells.
As mentioned above, it has been demonstrated that spiro-
OMeTAD is an effective small-molecule HTM for perovskite
solar cells. However, the spirobifluorene core in the spiro-
OMeTAD molecule is relatively expensive due to extensive
synthetic processes for its preparation. So it would be of
importance to develop a cheaper alternative to spiro-OMeTAD
for successful commercialization of hybrid perovskite solar cells.
The pyrene core deserves attention in that regard, because it can
be isolated from coal tar or produced in a wide range of
combustion conditions. Pyrene has fast charge-transport ability
due to its strong electron delocalization character that is a result
of the four fused benzene rings of an alternate polycyclic aromatic
hydrocarbon. Because pyrene can be substituted with a range of
functional groups, pyrene-core derivatives are useful as HTMs.5
However, the synthesis of N,N-di-p-methoxyphenylamine-
substituted pyrene derivatives and their application as HTMs
in perovskite-based solar cells have not yet been reported.
In this work, we report the synthesis and characterization of
three pyrene-core arylamine derivatives, and their application in
perovskite-based solar cells. The spirobifluorene core of spiro-
OMeTAD was replaced with a pyrene core. The role of the
ossil fuel consumption results in greenhouse gas production,
Fwhich in turn causes global warming; therefore, the demand
for carbon-free energy sources has increased. Solar energy is the
largest source of carbon-free energy that can be converted into
other forms of energy, such as heat and electricity. The use of
solar cells is the most effective way to convert sunlight directly
into electricity. Even though single-crystalline silicon-based solar
cells are still used, great efforts to fabricate cost-effective and
high-efficiency solar cells are being investigated. Novel
approaches include solid-state dye-sensitized solar cells,1 organic
solar cells,2 and inorganic−organic hybrid heterojunction solar
cells using inorganic nanocrystalline semiconductors and
quantum dots.3
Very recently, methylammonium lead halide (CH3NH3PbX3,
where X corresponds to halogens) perovskites were introduced
as promising light harvesters in solar cells.4 When CH3NH3PbI3
(= MAPbI3) was loaded on a mesoporous (mp)-TiO2 electrode
in conjunction with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-
amine)-9,9′-spirobifluorene (spiro-OMeTAD) as the hole-
Received: October 18, 2013
Published: December 6, 2013
© 2013 American Chemical Society
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Communication
Figure 2. (a) UV/vis absorption spectra of Py-A, Py-B, Py-C, and spiro-
OMeTAD in chlorobenzene. (b) Cyclic voltammograms of Py-A, Py-B,
Py-C, and spiro-OMeTAD. Square wave voltammetry (SWV) was
carried out at a step potential of 4 mV, square wave amplitude of 25 mV,
and square wave frequency of 15 Hz, at a scan rate of 40 mV/s with
ferrocene as the internal standard. Scan rate was 100 mV/s.
Figure 1. (a) Structures of spiro-OMeTAD. (b) Synthetic routes for
pyrene arylamine derivatives.
methoxy group in the N,N-di-p-methoxyphenylamine substitu-
ent is to adjust the electronic properties of the hole conductor.
The cells, fabricated using one of the pyrene derivatives as a
HTM, showed a PCE of 12.4% under AM 1.5 G illumination at
100 mW/cm2; this is one of the highest PCE values reported to
date for mp-TiO2/CH3NH3PbI3 perovskite/small-molecule
HTM with the exception of spiro-OMeTAD.
The procedures for the synthesis of the pyrene arylamine
derivatives are shown in Figure 1, and experimental details are
given in the Supporting Information (SI).
were consistent with the proposed structures. The three
compounds have good solubility in all common organic solvents.
The solubility of the pyrene arylamine derivatives decreased with
an increase in the number of di-p-methoxyphenylamino
substituents.
The UV/vis absorption spectra of Py-A, Py-B, and Py-C in
chlorobenzene are shown in Figure 2a. All derivatives show
absorption bands in the visible region, in contrast to spiro-
OMeTAD that shows an absorption band in the UV region. The
absorption maxima of Py-A, Py-B, and Py-C are centered at 420,
470, and 492 nm, respectively. This is due to the electron-
donating effect of the diphenylamine group, which is directly
bonded to the pyrene core and increases the electron density in
the pyrene core. Thus, the increased electron density modifies
the position of the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO), and the
gap between them. The energy level of the compounds was
characterized by cyclic voltammetry (CV, see Figure 2b). All CV
measurements were carried out in freshly distilled 1,2-dichloro-
benzene using 1 mM tetrabutylammonium hexafluorophosphate
(TBAPF6) as electrolyte in a three-electrode system, and each
solution was purged with N2 prior to measurement. The working
electrode, the reference electrode, and the counterelectrodes
were a Pt disk, Ag/AgCl, and Pt rods, respectively. All
measurements were carried out at room temperature using a
μAUTOLAB Type III potentiostat, employing the electro-
chemical software GPES. The optical properties and electro-
chemical data of Py-A, Py-B, and Py-C with regard to spiro-
OMeTAD were obtained by UV/vis spectroscopy and CV
measurement; the results are summarized in Table 1.
A triarylamine unit containing a para-substituted methoxy
functional group was synthesized by the Buchwald−Hartwig
amination reaction,6 carried out between various bromopyrene
derivatives and dimethoxydiphenylamines under the modified
reaction conditions. Mono-, tri-, and tetra-substituted pyrenes
were then successfully purified by column chromatography and
recrystallization. The new pyrene arylamine derivatives 5, 6, and
7 (denoted as Py-A, Py-B, and Py-C, respectively) were fully
characterized by 1H/13C NMR spectroscopy, mass spectrometry,
and elemental analysis (see the SI). It was observed that the 1H
NMR spectrum of the tetra-substituted Py-C is very simple due
to its symmetric structure. The Py-C spectrum show two singlets
at δ = 7.88 and 7.43 ppm, corresponding to the pyrene ring
protons at positions 4, 5, 9, and 10, and positions 2 and 7 in a 2:1
ratio, respectively. A pair of doublets in 1:1 ratio at δ = 6.85 and
6.67 ppm for the aromatic protons and a singlet at δ = 3.71 ppm
for the eight methoxy group protons are also observed. Py-A and
1
Py-B structures were also confirmed by H NMR via proton
integration and chemical shift identification of the substituted
pyrenes. In addition, the structures of Py-A, Py-B, and Py-C were
further established by mass spectrometry and their respective
molecular ion ([M+]) peaks: 429 for Py-A, 883 for Py-B, and
1110 for Py-C. Finally, we confirmed the structures by elemental
analysis, where the experimental ratios of nitrogen, carbon, and
proton agreed well with the expected ratios. All the analytical data
We compared the photovoltaic performance of the perovskite-
based solar cells fabricated using the three types of pyrene
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Table 1. Summary of the Optical and Electrochemical
a
Properties
a
EHOMO and ELUMO values obtained by comparison with spiro-
b
c
OMeTAD. From absorption at λmax. From intersection of absorption
spectra. From CV measurements and referenced to ferrocene. ELUMO
= EHOMO + Egap
d
e
.
derivatives (Py-A, Py-B, Py-C), and the spiro-OMeTAD as
HTMs, with tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-
cobalt(III) tris(bis(trifluoromethylsulfonyl)imide)] (FK209),
lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI), and 4-
tert-butylpyridine (tBP) as additives, dissolved in toluene. Here,
FK209 (co-complex) was used because it increased the
performance of the perovskite-based solar cell.7 The concen-
tration of HTMs was also adjusted in the range of 12.5−25.0 mM
to control the overlayer thickness on the perovskite. Finally, gold
as a top electrode for each cell was chosen and deposited by
thermal evaporation because its work function is close to the
HOMO of the pyrene derivatives. As shown in Figure 3a, the
Figure 4. (a) Current density−voltage (J−V) curves for TiO2/MAPbI3/
HTMs/Au fabricated with Py-A, Py-B, Py-C, Sp-A, and Sp-B as HTMs,
and without HTM. (b) The efficiency dependence on the overlayer
thickness with Py-C and spiro-OMeTAD; inset is dark J−V curves for
Py-C and Sp-B devices.
solution concentration for HTMs. The gold contact on top of
this organic conductor is also observed.
Figure 4a shows current density−voltage (J−V) curves for
TiO2/MAPbI3/HTMs/Au solar cells fabricated using Py-A, Py-
B, and Py-C as HTMs. The photovoltaic parameters for these
devices are summarized in Table 2. For comparison, cells without
Table 2. Summary of Short-Circuit Current Densities, Open-
Circuit Voltages, Fill Factors, and Overall Conversion
a
Efficiencies Obtained from the Device Shown in Figure 4a
HTM
Jsc (mA cm−2
)
Voc (V)
FF (%)
η (%)
Rs (Ω)
without
Py-A
Py-B
17.7
10.8
20.4
20.2
20.2
21.0
0.80
0.89
0.95
0.89
0.83
1.01
56.5
34.6
63.7
69.4
61.8
59.5
8.0
3.3
65.6
402.2
64.3
51.6
64.6
85.9
12.3
12.4
10.3
12.7
Py-C
Sp-A
Sp-B
a
Masks (0.092 cm2) made of thin metal were attached to each cell
before measurement.
Figure 3. (a) Energy level diagram of the corresponding materials used
in our devices. (b) Cross-sectional structure of the representative device.
Scale bar = 500 nm.
a HTM and with spiro-OMeTAD were also fabricated. In the
case of spiro-OMeTAD, ∼70 and ∼180 nm overlayer thicknesses
were compared, denoted as Sp-A, and Sp-B, respectively). The
device comprising mp-TiO2/MAPbI3/Au without a HTM gives
Voc = 800 mV, Jsc = 17.7 mA cm−2, a fill factor (FF) of 56.5%, and
an overall conversion efficiency η = 8.0%. These values are similar
to those recently reported by Etgar et al.8 In contrast, mp-TiO2/
MAPbI3/HTMs/Au devices exhibit dramatically enhanced
performance, depending on the HTMs, with the exception of
the cell fabricated with Py-A as HTM. The low performance
shown by Py-A is mainly related to the insufficient driving force
for hole injection owing to a low offset in the energy level
between MAPbI3 and Py-A (see Table 2). In addition, the cell
fabricated with Sp-A exhibits a low Voc = 0.83 V relative to those
of Py-B and Py-C showing a same thickness as that of Sp-B.
HOMO levels of Py-A, Py-B, and Py-C are ∼−5.41, −5.25, and
−5.11 eV, respectively. The band alignment in TiO2, MAPbI3,
and pyrene derivatives is such that exciton dissociation and
charge transfer at the interface are energetically favorable. Hence,
a driving force may possibly be generated for hole transfer from
the MAPbI3 into the pyrene derivatives. Figure 3b compares
cross-sectional images, as observed by scanning electron
microscopy, of the representative solar cell device. The image
shows a device thickness of ∼670 nm, consisting of a MAPbI3-
deposited mesoporous layer (∼600 nm) that is uniformly capped
by a HTM overlayer of pyrene arylamine derivative (∼70 nm).
Here, the thickness of HTMs can be controlled by adjusting the
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Figure 4b shows the PCEs for the cells fabricated as a function
of overlayer thickness for Py-C and spiro-OMeTAD. The cells
fabricated with Py-C show a maximum PCE at a relatively thinner
overlayer thickness than 100 nm. On the other hand, spiro-
OMeTAD has shown a maximum efficiency in relatively thicker
cells than 180 nm in overlayer thickness. The cells fabricated
using the thicker spiro-OMeTAD are similar with the average
performance reported by M. Gratzel et al.4f The values of Jsc for
the cells fabricated with Py-B, Py-C, Sp-A, and Sp-B are 20.4,
20.2, 20.2, and 21.0 mA cm−2, respectively. This indicates that
there is a small difference in the charge collection of the HTMs
(Py-B, Py-C, Sp-A, and Sp-B). We also see the enhancement of
the Voc values in the sequence of Sp-A, Py-C, Py-B, and Sp-B. The
difference in Voc for the cells fabricated with Py-B and Py-C as
HTMs agrees with the relative difference in the HOMO levels of
the two HTMs because the photovoltage is determined by the
difference in the quasi-Fermi level of the electrons in the TiO2
and the quasi-Fermi level of the holes in the hole conductor. The
higher Voc value observed in Sp-B might be attributed to the
effective movement of electrons from the HOMO of spiro-
OMeTAD to the Co-complex, which is used as dopant, inducing
a lower shift in the Fermi level to the HOMO of spiro-
OMeTAD.7 In addition, the Voc may be attributed the charge
recombination kinetics. Diode-like current−voltage (I−V)
characteristics in the dark can give an evident insight into the
charge recombination in the real device. As shown in the inset of
Figure 4b, the cell with Py-C has a relatively lower dark current
and an upper shift in the onset, compared to Py-C. This implies
that Py-C has an electron-blocking ability superior to that of
spiro-OMeTAD. Thus, it would be effective to reduce the use of
expensive HTMs. However, the dependence of the Voc on the
HTMs may be attributed to several factors and will require
additional study. Figure 4a and Table 2 show that comparable
performance of Py-C and Sp-B as HTMs is due to the enhanced
FF despite the difference in their Jsc’s and Voc’s. The FF is related
to series resistance (Rs) and shunt resistance (Rsh). In order to
obtain a high FF, the solar cells should have a low Rs and a high
Rsh. In other words, the HTM layer should effectively block the
electron and transport the hole from MAPbI3 to the Au
electrode. The main difference between the cells studied here is
the HTM, as the configurations of the cells are similar with regard
to the thickness of the overlayers below the Au electrode (see
Figure 3b). This means that the FF value can partly be explained
by the increased hole-transporting and electron-blocking ability
of the HTMs, which would decrease the possibility for the
photogenerated charges to recombine before they reach the
contacts. From the J−V curves under a condition of 1 sun, the Py-
B- and Py-C-based cells showed Rs = 64.26 and 51.57 Ω,
respectively, whereas the spiro-OMeTAD had Rs = 85.87 Ω.
Therefore, the pyrene-core HTM, especially Py-C, exhibited a
better FF than the spiro-OMeTAD.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and additional figures. This material is
■
S
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
seoksi@krict.re.kr (seoksi@skku.edu)
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was supported by the Global Research Laboratory
(GRL) Program and the Global Frontier R&D Program on
Center for Multiscale Energy System funded by the National
Research Foundation under the Ministry of Science, ICT &
Future, Korea, and by a grant from the KRICT 2020 Program for
Future Technology of the Korea Research Institute of Chemical
Technology (KRICT), Republic of Korea.
REFERENCES
■
(1) (a) Burschka, J.; Dualeh, A.; Kessler, F.; Baranoff, E.; Cevey-Ha, N.-
L.; Yi, C.; Nazeeruddin, M. K.; Gratzel, M. J. Am. Chem. Soc. 2011, 133,
18042. (b) Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G.
Nature 2012, 485, 486. (c) Zhang, W.; Zhu, R.; Li, F.; Wang, Q.; Liu, B. J.
Phys. Chem. C 2011, 115, 7038.
(2) (a) Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.;
Heeger, A. J. Nat. Mater. 2012, 11, 44. (b) Li, G.; Zhu, R.; Yang, Y. Nat.
Photon. 2012, 6, 153. (c) Zhou, J.; Zuo, Y.; Wan, X.; Long, G.; Zhang, Q.;
Ni, W.; Liu, Y.; Li, Z.; He, G.; Li, C.; Kan, B.; Li, M.; Chen, Y. J. Am.
Chem. Soc. 2013, 135, 8484.
(3) (a) Sangtra, P. K.; Kamat, P. V. J. Am. Chem. Soc. 2012, 134, 2508.
(b) Lee, Y. H.; Im, S. H.; Rhee, J. H.; Lee, J.-H.; Seok, S. I. ACS Appl.
Mater. Interfaces 2010, 2, 1648. (c) Chang, J. A.; Rhee, J. H.; Im, S. H.;
Lee, Y. H.; Kim, H.-J.; Seok, S. I.; Nazeeruddin, M. K.; Gratzel, M. Nano
Lett. 2010, 10, 2609. (d) Im, S. H.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.;
̈
Maiti, N.; Kim, H.-j.; Nazeeruddin, Md. K.; Gratzel, M.; Seok, S. I. Nano
̈
Lett. 2011, 11, 4789. (e) Chang, J. A.; Im, S. H.; Lee, Y. H.; Kim, H.-j.;
Lim, C.-S.; Heo, J. H.; Seok, S. I. Nano Lett. 2012, 12, 1863. (f) Im, S. H.;
Kim, H.-j.; Kim, S. W.; Kim, S.-W.; Seok, S. I. Energy Environ. Sci. 2011, 4,
4181.
(4) (a) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J. Am. Chem.
Soc. 2009, 131, 6050. (b) Lee, M. M.; Teuscher, J.; Miyasaka, T.;
Murakami, T. N.; Snaith, H. J. Science 2012, 338, 643. (c) Kim, H.-S.;
Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.;
Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Gratzel, M.; Park, N.-G.
Sci. Rep. 2012, 2, 591:1-7. (d) Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal,
T. N.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Kim, H.; Sarkar, A.;
̈
Nazeeruddin, M. K.; Gratzel, M.; Seok, S. I. Nat. Photon. 2013, 7, 486.
̈
(e) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Nano Lett.
2013, 13, 1764. (f) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker,
R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Nature 2013, 499, 316.
̈
(g) Liu, M.; Johnston, M. B.; Snaith, H. J. Nature 2013, 501, 395.
(h) Cai, B.; Xing, Y.; Yang, Z.; Zhang, W.-H.; Qiu, J. Energy Environ. Sci
2013, 6, 1480.
(5) (a) Haedler, A. T.; Misslitz, H.; Buehlmeyer, C.; Albuquerque, R.
In summary, we synthesized pyrene-core arylamine derivatives
that were applied in solar cells as HTMs. On the basis of the CV
measurements and UV/vis spectra, the HOMO/LUMO of the
pyrene-core arylamine derivatives were characterized for use as
HTMs. The derivatives were used to fabricate feasible perovskite
solar cells. Energy conversion efficiencies of 12.4% and 12.7%
were measured for the devices with Py-C and spiro-OMeTAD as
HTM. We believe that pyrene-core arylamine derivatives can be
used as HTMs for the fabrication of efficient and cost-effective
photovoltaic devices.
Q.; Kohler, A.; Schmidt, H.-W. ChemPhysChem 2013, 14, 1818.
̈
(b) Anant, P.; Lucas, N. T.; Ball, J. M.; Anthopoulos, T. D.; Jacob, J.
Synth. Met. 2010, 160, 1987.
(6) Thomas, K. R.; Lin, J. T.; Tao, Y.-T.; Ko, C.-W. J. Am. Chem. Soc.
2001, 123, 9404.
(7) Noh, J. H.; Jeon, N. J.; Choi, Y. C.; Nazeeruddin, M. K.; Gratzel, M.;
Seok, S. I. J. Mater. Chem. A 2013, 1, 11842.
(8) Laban, W. A.; Etgar, L. Energy Environ Sci. 2013, 6, 3249.
̈
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