O-methoxy substituents in spiro-OMeTAD for efficient inorganic-organic hybrid perovskite solar cells

Article abstract of DOI:10.1021/ja502824c

Three spiro-OMeTAD derivatives have been synthesized and characterized by ¹H/¹³C NMR spectroscopy and mass spectrometry. The optical and electronic properties of the derivatives were modified by changing the positions of the two methoxy substituents in each of the quadrants, as monitored by UV-vis spectroscopy and cyclic voltammetry measurements. The derivatives were employed as hole-transporting materials (HTMs), and their performances were compared for the fabrication of mesoporous TiO₂/CH ₃NH₃PbI₃/HTM/Au solar cells. Surprisingly, the cell performance was dependent on the positions of the OMe substituents. The derivative with o-OMe substituents showed highly improved performance by exhibiting a short-circuit current density of 21.2 mA/cm², an open-circuit voltage of 1.02 V, and a fill factor of 77.6% under 1 sun illumination (100 mW/cm²), which resulted in an overall power conversion efficiency (PCE) of 16.7%, compared to ～15% for conventional p-OMe substituents. The PCE of 16.7% is the highest value reported to date for perovskite-based solar cells with spiro-OMeTAD.

Full text of DOI:10.1021/ja502824c

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Communication
o-Methoxy Substituents in Spiro–OMeTAD for Efficient
Inorganic–Organic Hybrid Perovskite Solar Cells
Nam Joong Jeon, Hag Geun Lee, Young Chan Kim,
Jangwon Seo, Jun Hong Noh, Jaemin Lee, and Sang Il Seok
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja502824c • Publication Date (Web): 16 May 2014
Downloaded from http://pubs.acs.org on May 21, 2014
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o-Methoxy Substituents in Spiro–OMeTAD for Efficient Inor-
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Nam Joong Jeon,†,§ Hag Geun Lee,†,§ Young Chan Kim,† Jangwon Seo,† Jun Hong Noh,† Jaemin Lee,*,
† and Sang Il Seok*,†,
‡
†Division of Advanced Materials, Korea Research Institute of Chemical Technology, 141 GajeongꢀRo, YuseongꢀGu,
Deajeon 305ꢀ600, Republic of Korea
‡Department of Energy Science, Sungkyunkwan University, Suwon 440ꢀ746, Korea
Supporting Information
addition of 4–tert–butyl pyridine (tꢀBP) and lithium
ABSTRACT: Three spiro–OMeTAD derivatives have been
1
bis(trifluoromethanesulfonyl) imide (LiTFSI) in spiro–OMeTAD,
which resulted in an enhanced PCE of 2.56% at one sun condiꢀ
tion.12 This system was further optimized and a PCE of 7.2% was
reported by increasing the hole mobility of spiro–OMeTAD by
more than one order of magnitude by doping it with a Co(III)
complex and by using a high absorption coefficient organic dye.13
synthesized and characterized by H/13C–NMR, and mass specꢀ
trometry. The optical and electronic properties of the derivatives
were modified by changing the position of the two p–methoxy (–
OMe) substituents in each of the quadrants, followed by UV–Vis
spectroscopy and cyclic voltammetry measurements. The derivaꢀ
tives were employed as hole–transporting materials (HTMs) and
their performances were compared for the fabrication of mesopoꢀ
rous (mp)–TiO2/CH3NH3PbI3/HTMs/Au solar cells. Surprisingly,
the cell performance was dependent on the position of the p–, m–,
and o–OMe substituents. The o–OMe substituents showed highly
improved performance by exhibiting a short–circuit current densiꢀ
ty (Jsc) of 21.2 mA/cm2, an open–circuit voltage (Voc) of 1.02 V,
and a fill factor (FF) of 77.6 % under illumination of 1 sun (100
mW/cm2), which resulted in an overall power conversion efficienꢀ
cy (PCE) of 16.7 %, compared to PCE (~15%) of conventional p–
OMe positions. The PCE (16.7 %) is the highest value reported
for perovskite–based solar cells to date with spiro–OMeTAD.
Several attempts were made to develop alternative organic
HTMs for sensitized solar cells. Recently, we successfully develꢀ
oped an efficient inorganic–organic hybrid solar cell using
poly(3–hexylthiophene) (P3HT) and poly[N–9–hepta–decanyl–
2,7ꢀcarbazole–al–3,6–bis(thiophen–5–yl)–2,5–dioctyl–2,5–diꢀ
hydropyrrolo[3,4–]pyrrole–1,4–dione] (PCBTDPP) as effective
hole conductors and Sb2S3 or Sb2Se3 as sensitizers.14ꢀ16 Thus,
P3HT, PCPDTBT and poly–triarylamine (PTAA) were employed
for the fabrication of hybrid halide perovskite solar cells.3 Howꢀ
ever, most of the HTMs are made of polymers whose synthesis
and purification is difficult, with the possible disadvantage of
incomplete filling of the mesoporous TiO2 with spiro–OMeTAD.
Small molecular HTMs such as N,N′–dialkyl perylenediimide
(PDI) and N,N′–bis(3ꢀmethylphenyl)–N,N′–diphenylbenzidine
(TPD) were also tried to increase the open circuit voltage of perꢀ
ovskite solar cells.10 Recently, we synthesized pyrene arylamine
derivatives and applied them as HTMs in perovskite–based solar
cells, but their performance was still poor when compared to spiꢀ
ro–OMeTAD.17
Very recently, hybrid halide perovskites (e.g, methylammonium
lead halide: CH3NH3PbX3, where X corresponds to halogens)
were introduced as promising light harvesters in solar cells.1ꢀ5
Mesoporous (mp) TiO2 or Al2O3 was used as the scaffold for perꢀ
ovskite absorbers in the perovskite–based solar cells with hole–
transporting materials (HTMs). In these cells, excited carriers can
be injected into hole and electron transporting material for collecꢀ
tion at the electrodes and the perovskite itself can behave as an
electron or hole transporter, depending on the scaffold used.1,3
Recently, planar heterojunctions of a bulk layer of perovskite
acting as both the absorber and long range transporter of charged
species has shown the highest efficiencies.6ꢀ8 Currently, most
state–of–the–art perovskite–based devices utilize spiro–OMeTAD
From the perspective of material’s chemical structure, it is noteꢀ
worthy that methoxy groups (–OCH3: –OMe) were introduced to
spiro–OMeTAD in order to control the oxidation potential of the
materials. The –OMe group is electron withdrawing by inductive
effect, but it can also exhibit electron–donating behavior under
resonance stabilization. Depending on the substitution position,
Hammett has reported two opposite effects of the –OMe substituꢀ
ent in the aromatic ring i.e., electron–donating for para (p) and
electron–withdrawing for meta (m).18 In addition to such electronꢀ
ic effects, substitution at the ortho (o) position can also influence
the oxidation potential by steric effect. Therefore, a simple and
effective strategy to fine–tune the electronic properties is to
change the substitution position of the –OMe group in arylamine–
based HTMs. There are no reports on the application of o–, m–
derivatives in spiro–OMeTAD as HTMs in perovskite–based solar
cells. Hence, we decided to investigate the effect of replacing p–
with o– and m– derivatives (see supplementary Figure S1 for the
molecular structures) in spiro–OMeTAD.
((2,2′,7,7′–tetrakis(N,N–di–p–methoxyphenylamine)
–9,9′–
spirobifluorene)) the as HTM in their structures,1,2,4ꢀ8 although
other types of HTMs were reported by several groups.3,9,10 For
example, Graetzel et al. reported a 15.0% power conversion effiꢀ
ciency (PCE) for np–TiO2/CH3NH3PbI3/spiro–OMeTAD/Au.5
Snaith et al.6 showed a maximum efficiency of 15.4% with a Voc
of 1.07 V and a Jsc of 21.5 mA cm–2, based on the CH3NH3PbI3ꢀ
xClx deposited as thin layers with spiro–OMeTAD as the hole
conductor. Initially, spiro–OMeTAD was used as a solid hole
conductor in dye–sensitized solar cells (DSSCs) and it gave
0.74% PCE under full sunlight.11 The PCE was increased by the
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In this study, we systematically changed the position of –OMe
estingly, the change of substitution position from p–OMe to m–
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substituents of spiro–type arylamine HTMs from p to m and o,
and investigated their structure–property relationship in perovꢀ
skite–based hybrid solar cells. Thus, we report the synthesis and
characterization of three spiro–OMeTAD derivatives and their
application in perovskite–based solar cells. The position of one of
the two p–OMe substituents in each of the quadrants of spiro–
OMeTAD was replaced with m– and o–OMe. The role of the –
OMe group in the spiro–OMeTAD substituent is to adjust the
electronic properties of the hole conductor. Surprisingly, the cell
using o–derivative in spiro–OMeTAD showed better performance
compared to p– and m–derivatives with 16.7% overall power
conversion efficiency (PCE) under AM 1.5G illumination at 100
mW/cm2; this is one of the highest PCE values reported to date
for mp–TiO2/CH3NH3PbI3 perovskite/spiro–OMeTAD.
OMe resulted in an increased oxidation potential. This is because
the m–OMe group exerts an electron–withdrawing effect while
the p–OMe group exerts an electron–donating effect, as previousꢀ
ly reported by Hammett.18 However, contrary to m–substitution,
substitution of the –OMe group at the o–position did not show
noticeable changes in the oxidation potential, which means that
the electron–donating property of the –OMe group is maintained
in o–substitution. Based on the reported highest occupied molecuꢀ
lar orbital (HOMO) energy level values (5.22 eV) of pp–spiro–
OMeTAD,2 HOMO energy levels of pm– and po–spiro–
OMeTAD were calculated to be –5.31 and –5.22 eV, respectively.
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pp
pm
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Wavelength (nm)
ꢀ2.18eV
ꢀ2.31eV
ꢀ2.28eV
(c)
E (eV)
Au
HTM
-3.93
-4.0
pp
po
pm
Figure 1. Synthetic routes for spiroꢀOMeTAD derivatives.
Au
-5.1
The procedures for the synthesis of the spiro–OMeTAD derivaꢀ
tives are shown in Figure 1 and experimental details are explained
in the supporting information (SI). Spiro–OMeTAD derivatives
were synthesized by the Buchwald−Hartwig amination reaction
between 2,2′,7,7′–tetrabromo–9,9′–spirobifluorene (1) and the
respective dimethoxydiphenylamines containing p–OMe, m–OMe
or o–OMe substituents (2, 3 and 4, respectively). Compound 5,
which contains two pꢀOMe substituents in each of the quadrants,
is the well–known spiro–OMeTAD, and in order to clearly define
the substitution position, it is denoted as pp–spiro–OMeTAD in
this study. The other two spiro–type arylamine derivatives with
different substitution positions are denoted as pm–spiro–
OMeTAD (6) and po–sprio–OMeTAD (7). All the three spiro–
OMeTAD compounds were purified by column chromatography
FTO
ꢀ5.22eV
ꢀ5.22eV
-5.44
MAPbI3
ꢀ5.31eV
TiO2
1 µm
(d)
Figure 2. (a) UV/Vis absorption spectra of ppꢀ, pmꢀ, and poꢀ
spiroꢀOMeTAD in chlorobenzene. b) Cyclic voltammograms of
ppꢀ, pmꢀ, and poꢀspiroꢀOMeTAD. (c) Energy level diagram of the
corresponding materials used in our devices. (d) Crossꢀsectional
structure of the representative device.
1
followed by recrystallization. They were characterized by H/13C
NMR spectroscopy, and mass spectrometry (see the Supporting
Information). All the analytical data were consistent with the proꢀ
posed structures. The three compounds have good solubility in all
the common organic solvents.
Figure 2c shows the energy level diagram of the three spiro deꢀ
rivatives. The lowest unoccupied molecular orbital (LUMO) enꢀ
ergy levels were calculated by adding the optical band–gap enerꢀ
gies to the HOMO energy levels. The electronic effects of the
spiro derivatives are observed to change substantially depending
on the substitution position. Compared to the p–OMe, the m–OMe
primarily decreased the HOMO energy level (ca. 0.09 eV). This is
mainly due to the electron–withdrawing effect of the m–OMe
substitution that does not contribute to resonance stabilization. On
the other hand, the o–OMe increased the LUMO energy level (ca.
0.10 eV), compared to p–OMe mainly due to steric effect, i.e., the
band–gap energy increased conceivably because of increased
dihedral angle between the o–OMe–phenyl group and the fluorene
ring, which results in a decrease of the effective conjugation of
the molecule. The electron–donating effect of o– and p–OMe was
almost the same as revealed by CV measurements.
The UV/vis absorption spectra of the three spiro–OMeTAD deꢀ
rivatives in the film state are shown in Figure 2a. All the derivaꢀ
tives showed absorption bands in the UV region, and the absorpꢀ
tion maxima of pp–, pm–, and po–spiro–OMeTAD are centered at
387, 387, and 381 nm, respectively. The absorption onset waveꢀ
length that reflects the optical band–gap energy of the material
helps in identifying the influence of the substitution position. The
absorption onset of the three derivatives moved to shorter waveꢀ
length from pp– (423 nm) to pm– (413 nm), and further to po–
spiro–OMeTAD (409 nm), which implies that the newly syntheꢀ
sized pm– and po–spiro–OMeTAD have increased band–gap enꢀ
ergies compared to the well–known pp–spiro–OMeTAD.
In order to study the electrical properties of these compounds,
we investigated the electrochemical properties by cyclic voltamꢀ
metry (CV). As shown in Figure 2b, all the three compounds
showed similar cyclic voltammograms in the oxidation scan.19
The first two overlapped oxidation peaks can be attributed to the
two consecutive oxidation reactions of the molecule to generate a
monocation radical and a quinoidal dication, respectively.20 Interꢀ
We compared the similarities and differences in photovoltaic
performance of the three spiro–OMeTAD derivatives as HTMs by
fabricating the perovskite–based solar cells. For cell fabrication,
we used a solvent mixture of γ–butyrolactone and dimethyl sulꢀ
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foxide to deposit a uniform CH3NH3PbI3 (=MAPbI3) layer.21 The
OMeTAD devices exhibit dramatically enhanced performance
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cell was fabricated by spin coating a layer of MAPbI3 onto the
mp–TiO2 surface, followed by drying at 100 °C. Spin coating of
the pp–, pm–, and po–spiro–OMeTAD as HTMs was accomꢀ
plished with Li–bis(trifluoro–methanesulfonyl)imide (Li–TFSI),
and 4–tert–butyl pyridine (t–BP) dissolved in toluene. In this
study, we did not incorporate hole dopants like tris[2–(1H–
and the performance of cells fabricated from pm–spiro–OMeTAD
is inferior to that of pp–spiro–OMeTAD. In Figure 3a, a small
difference in Jsc and Voc among pp–, pm–, and po–spiro–
OMeTAD derivatives with a large difference in FF is observed.
Generally, in solar cells, series resistance (Rs) reduces the FF, but
does not affect Voc and Jsc, if it is not excessively high values. A
straightforward method of estimating the Rs from a solar cell is to
find the slope of the JꢀV curve near the open–circuit voltage. As
shown in the Rs determined from the slope of Table 1, po–spiro–
OMeTAD–based cells showed lowest Rs of 32.96 ꢁ, whereas the
pp– and pm–spiro–OMeTAD showed Rs of 45.37 ꢁ and 64.42 ꢁ,
respectively. Besides Rs, the FF is also related to shunt resistance
(Rsh). In perovskite–based solar cells consisting of mp–TiO2–
perovskite composite/perovskite layer/HTM/Au, the HTM funcꢀ
tions as both hole–transporting and electron–blocking layer.3 po–
spiro–OMeTAD having the highest LUMO will block electron
flow into the metal electrode, which is responsible for increased
Rsh. Therefore, po–spiro–OMeTAD effectively blocks the elecꢀ
tron and transports the hole from MAPbI3 to the Au electrode.
Accordingly, the superior performance of po–spiro–OMeTAD
may be rationalized in terms of the increased FF value through a
low Rs and a high Rsh. However, further studies are needed to
quantify the extent of this effect among pp–, pm–, and po–spiro–
OMeTAD. Figure 3b shows monochromatic incident photon conꢀ
version efficiency (IPCE) spectrum for the cells fabricated using
pp–, pm–, po– and commercial pp–spiro–OMeTAD, and the inteꢀ
grated photocurrent densities are in good agreement with the valꢀ
ues measured in J–V curves, although the IPCE spectra differs
slightly with wavelength.
pyrazol–1–yl)–4–tert–butylpyridine)cobalt(III)
tris(bis
(tri–
fluoromethylsulfonyl)imide)] (FK209) because we can fabricate
the cells with a very thin layer of spiro–OMeTAD. The same
concentration of spiroꢀOMeTAD derivatives/toluene solutions for
HTMs were used to keep very similar thickness them as overlayer
on the perovskite. Finally, Au electrode was deposited on the top
by thermal evaporation to complete the solar cell structure. Figure
2d shows cross–sectional image of the representative solar cell
device fabricated in this work, as observed by a scanning electron
microscopy (SEM). As can be seen, the MAPbI3 perovskite forms
a thin capping layer of about 350 nm with full infiltration into ~
250ꢀnmꢀthick mp–TiO2 and a spiro–OMeTAD overlayer (~70
nm). The thickness of The thicknesses of the pp, pm, and po overꢀ
layers were almost similar, as estimated by same molecular
weight. The gold contact on top of this organic conductor is also
observed. In Figure 2c, the band alignment in TiO2, MAPbI3, and
spiro–OMeTAD derivatives can be observed. The band alignment
is such that the exciton dissociation and charge transfer at the
interface are energetically favorable with possible driving force
for a hole transfer from the MAPbI3 into the spiro–OMe derivaꢀ
tives. Therefore, free charge carriers (or excitons) generated from
the MAPbI3 layer can be extracted (or dissociated) by transferring
an electron to the underlying mp–TiO2 layer through hole transfer
to the spiro–OMeTAD HTM. However, there was a large hystereꢀ
sis and distortion in the J–V curves in the reverse (from the open
circuit voltage (Voc) to the short circuit current (Isc)) and forward
(i.e. from Isc to Voc) modes under standard air–mass 1.5 global
(AM 1.5G) illumination for the perovskite–based solar cells, esꢀ
pecially when it is measured at relatively short delay times.21 Such
hysteresis and discrepancy leads to over– or under–evaluated
performance which induces a considerable error in measuring the
cell efficiency. This is because reliable solar cell J–V measureꢀ
ments should exhibit coincident curves from the forward and reꢀ
verse directions. Figure S2 shows the variation of energy converꢀ
sion efficiency for representative solar cell (using poꢀspiroꢀ
OMeTAD as HTM), measured by forward and reverse scans with
10 mV voltage steps and a delay time of 100 ms. Serious hystereꢀ
sis is observed between both the scan directions with different
performance; the efficiency in the reverse scan is 17.6%, while it
is 14.1% in the forward scan. The efficiency measured by both the
scan directions decreases and increases symmetrically with inꢀ
creasing delay time and then matched at 1000 ms, but average
value remains almost the same after 300 ms (see supplemental
Figure S3). Therefore, in this work, we showed current density–
voltage (J–V) curves by averaging J–V curves obtained in both the
scan directions with a delay time of 300 ms, because an excesꢀ
sively long measurement time is impractical.
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Table 1. Average photovoltaic parameters for reverse and forꢀ
ward scanedꢀMAPbI3 perovskite solar cells using commercial,
pmꢀ, poꢀ, and ppꢀspiroꢀOMeTAD as HTMs
100
(a)
(b)
20
)
80
60
40
20
0
15
(
Commercial
Commercial
pp
pm
po
pp
pm
po
10
5
0
0.0
0.2
0.4
0.6
0.8
1.0
300
400
500
600
700
800
Wavelength (nm)
Voltage (V)
With this in mind, we compared the photovoltaic performance of
the cells fabricated using pp–, pm–, and po–spiro–OMeTAD deꢀ
Figure 3. (a) Current density–voltage (J–V) curves for
TiO2/MAPbI3/HTMs/Au fabricated with ppꢀ, pmꢀ, poꢀ and comꢀ
mercial ppꢀspiroꢀOMeTAD as HTMs. (b) the corresponding IPCE
spectra.
rivatives.
Figure
3a
shows
J–V
curves
for
TiO2/MAPbI3/HTMs/Au solar cells fabricated using three HTMs
and the photovoltaic parameters of these devices are summarized
in Table 1. In all the cases, overlayer thickness of ~70 nm was
observed in the SEM measurement (see Figure 2d). The device
comprising of mp–TiO2/MAPbI3/pp–Spiro–OMeTAD gave an
open–circuit voltage (Voc) of 1.00 V, a short–circuit current densiꢀ
ty (Jsc) of 20.7 mA cm–2, a fill factor (FF) of 71.1 %, and an overꢀ
all conversion efficiency (η) of 14.9 %. These values are compaꢀ
rable and the cell performance is similar to that of the cells fabriꢀ
cated with commercial pp–spiro–OMeTAD (Merck), as reported
by Gratzel et al.5 In contrast, the cells fabricated with po–spiro–
In summary, we synthesized pp–, pm– and po–spiro–OMeTAD
derivatives that were applied as HTMs to the MAPbI3 pervoskite–
based solar cells. Based on the CV measurements and UV–Vis
spectra, the HOMO/LUMO energy levels of each derivative were
characterized. The derivatives were used to fabricate feasible
perovskite–based solar cells. Energy conversion efficiencies of
16.7% were obtained, which is the highest value reported till date
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(17) Jeon, N. J.; Lee, J.; Noh, J. H.; Nazeeruddin, M. K.; Grätzel, M.;
for perovskite–based solar cells with spiro–OMeTAD. The supeꢀ
rior performance of po–spiro–OMeTAD was mainly attributed to
the increased FF, owing to its low Rs and high Rsh. We believe
that this study will open new avenues for the development of
HTMs for perovskite–based solar cells for the fabrication of effiꢀ
cient and cost–effective photovoltaic devices.
Seok, S. I. J. Am. Chem. Soc. 2013, 135, 19087.
1
2
3
4
(18) Hammett, L. P. J. Am. Chem. Soc. 1937, 59, 96.
(19) Planells, M.; Abate, A.; Hollman, D. J.; Stranks, S. D.; Bharti, V.;
Gaur, J.; Mohanty, D.; Chand, S.; Snaith, H. J.; Robertson, N. J.
Mater. Chem. A. 2013, 1, 6949.
5
(20) Seo, E. T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy, D.
6
W.; Adams, R. N. J. Am. Chem. Soc. 1966, 88, 3498.
7
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(21) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu S.; Seok, S. I.
submitted.
ASSOCIATED CONTENT
Supporting Information
Details of experimental and additional supplementary figures.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*
Eꢀmail: seoksi@krict.re.kr (seoksi@skku.edu) and
jminlee@krict.re.kr
Author Contributions
§ N.J.J. and H.G.L. contributed equally.
ACKNOWLEDGMENT
This work was supported by the Global Research Laboratory
(GRL) Program, the Global Frontier R&D Program on Center for
Multiscale Energy System funded by the National Research
Foundation in Korea, and by a grant from the KRICT 2020 Proꢀ
gram for Future Technology of the Korea Research Institute of
Chemical Technology (KRICT), Republic of Korea.
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TOC GRAPHICS
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3
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16.7
5
6
7
8
H3CO
H3CO
OCH3
OCH3
H3CO
H3CO
N
N
N
N
OCH3
OCH3
14.9
13.9
po-spiro-OMeTAD
9
pp-spiro-OMeTAD
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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44
45
46
47
48
49
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pm-spiro-OMeTAD
65.2
71.1
77.6
Fill Factor (%)
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