Nanostructured TiO2/CH3NH3PbI3 heterojunction solar cells employing spiro-OMeTAD/Co-complex as hole-transporting material

Article abstract of DOI:10.1039/c3ta12681a

For using 2,2′,7,7′-tetrakis(N,N′-di-p- methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) as a hole conductor in solar cells, it is necessary to improve its charge-transport properties through electrochemical doping. With the aim of fabricating efficient mesoscopic TiO₂/CH₃NH₃PbI₃ heterojunction solar cells, we used tris[2-(1H-pyrazol-1-yl)-4-tert- butylpyridine)cobalt(iii) tris(bis(trifluoromethylsulfonyl) imide)] (FK209) as a p-dopant for spiro-OMeTAD. The mixture of spiro-OMeTAD, FK209, lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI), and 4-tert-butylpyridine (TBP) exhibited significantly higher performance than mixtures of pristine spiro-OMeTAD, spiro-OMeTAD, and FK209, and spiro-OMeTAD, Li-TFSI, and TBP. Such a synergistic effect between the Co-complex and Li-TFSI in conjunction with spiro-OMeTAD effectively improved the power conversion efficiency (PCE) of the fabricated solar cells. As a result, we achieved PCE of 10.4%, measured under standard solar conditions (AM 1.5G, 100 mW cm^-2).

Full text of DOI:10.1039/c3ta12681a

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PAPER
Nanostructured TiO2/CH3NH3PbI3 Heterojunction Solar Cells
Employing Spiro-OMeTAD/Co-complex as Hole-Transporting Material
Jun Hong Noh,a Nam Joong Jeon,a Yong Chan Choi,a Md. K. Nazeeruddin,b Michael Grätzel,b Sang Il
Seoka,c,*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
For using 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenyl amine)-9,9′-spirobifluorene (spiro-OMeTAD) as a
hole conductor in solar cells, it is necessary to improve its charge-transport properties through
electrochemical doping. With the aim of fabricating efficient mesoscopic TiO2/CH3NH3PbI3
10 heterojunction solar cells, we used Tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III)
tris(bis(trifluoromethylsulfonyl) imide)] (FK209) as a p-dopant for spiro-OMeTAD. The mixture of spiro-
OMeTAD, FK209, lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI), and 4-tert-butylpyridine (TBP)
exhibited significantly higher performance than mixtures of pristine spiro-OMeTAD, spiro-OMeTAD,
and FK209, and spiro-OMeTAD, Li-TFSI, and TBP. Such a synergistic effect between the Co-complex
15 and Li-TFSI in conjunction with spiro-OMeTAD effectively improved the power conversion efficiency
(PCE) of the fabricated solar cells. As a result, we achieved PCE of 10.4%, measured under standard solar
conditions (AM 1.5G, 100 mW/cm2).
materials in DSSCs. Very recently, Kanatzidis et al.8 achieved a
Introduction
PCE of 8.5% for a ssDSSC employing CsSnI3, a p-type inorganic
semiconductor, as a hole-transporting material (HTM). On the
other hand, the efficiency of spiro-OMeTAD-based ssDSSCs was
50 increased dramatically through dye development and TiO2
modification, as well as through the addition of ionic compounds
such as lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI) and
4-tert-butylpyridine (TBP) to spiro-OMeTAD. Grätzel et al.
reported further improvement in the performance of spiro-
55 OMeTAD-based ssDSSCs achieved using tris(2-(1H-pyrazol-1-
yl)pyridine)cobalt(III) (FK102) as a chemical dopant. They
achieved the highest recorded efficiency of 7.2% for spiro-
OMeTAD-based DSSCs by using an organic dye with a high
molar extinction coefficient9.
Sun is the greatest energy source for the world toward solving the
20 serious issue of global warming due to excessive CO2 generation.
Solar cells have been regarded as an effective means for directly
converting solar energy to electricity. The best commercial
silicon solar cells available today have efficiencies up to 20% but
are still relatively expensive to produce in terms of both materials
25 and techniques. Therefore, new solar cell technologies are needed
for relatively low-cost conversion of solar energy to electricity so
that solar cells can achieve cost-parity with fossil fuels.
Since Grätzel et al.1 reported the fabrication of relatively
cheap solar cells—called dye-sensitized solar cells (DSSCs)—
30 using titania nanoparticles, organic dyes, and liquid electrolytes,
extensive studies have been carried out to increase the
performance of these cells. In addition to high performance, long-
term stability is one of the key requirements of photovoltaic
60
Recently, perovskite-based methylammonium lead halide
(CH3NH3PbX3, X = halogen; CH3NH3: MA) and its mixed-halide
crystals, with three-dimensional perovskite structures, have been
used as light harvesters for solar cells10-14. Snaith et al.11 and Park
et al.12 independently demonstrated highly efficient solar cells
devices employing DSSCs. Although
a power conversion
35 efficiency (PCE) of more than 12% has recently been achieved
for DSSCs2, sealing issues due to volatile electrolytes have to be
addressed for achieving long-term durability. With the aim of
developing all-solid-state dye-sensitized solar cells (ssDSSCs) for
overcoming such leakage problems, Bach et al.3 in 1998 reported
40 for the first time ssDSSCs fabricated using 2,2′,7,7′-tetrakis(N,N-
65 using CH3NH3PbI2Cl and CH3NH3PbI3, respectively, as a light
harvester; both of them used spiro-OMeTAD as a hole conductor.
However, the fill factors (FFs) of these cells were low; this have
been due to the high series resistance caused by the relatively
poor charge transport of spiro-OMeTAD13. Therefore, it would be
70 important to fabricate the cells with higher hole conductors.
In this work, we fabricated heterojunction solar cells using
di-p-methoxyphenylamine)-9,9′-spirobifluorene
(spiro-
OMeTAD) as an organic hole conductor, recording 0.74% in its
overall efficiency. Thereafter, several materials such as CuSCN4,
P3HT5, poly(3,4-ethylenedioxythiophene) (PEDOT),6 and
45 polyaniline,7 have been studied for use as solid hole-conducting
sprio-OMeTAD/
Tris[2-(1H-pyrazol-1-yl)-4-tert-
butylpyridine)cobalt(III) tris(bis(trifluoromethylsulfonyl)imide)]
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DOI: 10.1039/C3TA12681A
(FK209) composite as a hole conductor with CH3NH3PbI3
perovskite on a mesoporous (mp) TiO2 layer. The cells consisted
of the following components: FTO (F-doped tin oxide)/blocking
consecutively carrying out spin coating at 2000 rpm for 60 s and
at 3000 rpm for 60 s; the substrate was then dried on a hot plate at
100 °C for
60 (Merck)/chlorobenzene
complex(FK209)/acetonitrile
5
min.
A
50 mM spiro-OMeTAD
layer
(bl)-TiO2/mp-TiO2/CH3NH3PbI3/spiro-OMeTAD
+
solution
and
a
Co(III)-
5
FK209/Au. Interestingly, the use of the sprio-OMeTAD/FK209
composite greatly increased the overall efficiency of the cells
through a simultaneous increase of both open circuit voltage (Voc)
and FFs. One of the cells fabricated in this study showed a PCE
solution
with
designed
concentrations in the range 4.15–33.24 mM were prepared to
obtain the mixed solution to be used for spin-coating the spiro-
OMeTAD:FK209 layer. In addition, 37.5 µl Li-
of 10.4% under AM 1.5G illumination at 100 mW/cm2; this value 65 bis(trifluoromethanesulfonyl) imide (Li-TFSI)/acetonitrile (170
10 is among the highest reported PCE values to date for mp-
TiO2/CH3NH3PbI3 perovskite/small-molecule spiro-OMeTAD
solar cells.
mg/1 ml) and 17.5 µl TBP were added as additives. Each HTM
solution was spin-coated on CH3NH3PbI3/mp-TiO2/bl-
a
TiO2/FTO substrate at 3000 rpm for 30 s. Finally, a Au
counterelectrode was deposited by thermal evaporation. The
70 active area of this electrode was fixed at 0.16 cm2.
Experimental
Characterization
Synthesis of Tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)co-
15 balt(III) tris(bis(trifluoromethylsulfonyl)imide)) (FK209).
To measure the conductance of spiro-OMeTAD films, we
thermally grew a 300-nm-thick SiO2 layer on a heavily doped n-
type (100) Si wafer (0.05 ꢀ cm) to be used as a substrate. After
75 UV/ozone treatment of the SiO2/Si wafer, we spin-coated each
HTM solution on the wafer at 3000 rpm for 120 s. Two Au (60
nm) electrode films were deposited by thermal evaporation
through a shadow mask with a channel length of 50 ꢁm and a
channel width of 3000 ꢁm. I–V curves between the two Au
80 electrodes were recorded using a semiconductor parametric
analyzer and a source meter (Keithley 2400). The absorption
spectra for diluted spiro-OMeTAD solutions were measured
using a UV-Vis spectrometer (Hitachi U-3300). EQE was
measured using a power source (Newport 300 W Xenon lamp,
85 66920) with a monochromator (Newport Cornerstone 260) and a
multimeter (Keithley 2001). The J–V curves were measured using
a solar simulator (Newport, Oriel Class A, 91195A) with a source
meter (Keithley 2420) at 100 mA/cm2 illumination AM 1.5G and
a calibrated Si-reference cell certificated by NREL. The J–V
90 curves for all devices were measured by masking the active area
with a metal mask 0.096 cm2 in area.
3.45
g (17.1 mmol, 3 eq) of 2-(1H-pyrazol-1-yl)-4-tert-
butylpyridine was suspended in a 2:1 mixture of water (120 mL)
and methanol (60 mL) and heated to 70 °C. Then 1.36 g (5.7
mmol, 1 eq) of CoCl2*6H2O was added and the mixture was
20 stirred at the same temperature for 10 minutes. 10 mL of H2O2
(30%) and 10 mL of HCL (25 %) were added to oxidize the
cobalt and the mixture was further heated to 70 °C for 2 h. Then a
concentrated solution of 8.18 g (28.5 mmol, 5 eq) Lithium
bis(trifluoromethanesulfonyl)imide in water was added slowly at
25 50 °C to precipitate the product. The mixture was allowed to cool
to room temperature and the orange solid was collected on a
sintered glass-frit and washed with water. The solid was dried in
air and then in vacuo to obtain the pure product as an orange
solid. Yield: 7.76 g (5.16 mmol, 91%). 1H NMR (400 MHz,
30 Acetone-D6): δ 9.87–9.64 (m, 3H, ArH), 8.79–8.65 (m, 3H,
ArH), 8.00–7.79 (m, 6H, ArH), 7.78–7.62 (m, 3H, ArH), 7.37–
7.17 (m, 3H, ArH), 1.72–1.15 (m, 27H, tBut).
Solar cell fabrication
A 60-nm-thick dense blocking layer of TiO2 (bl-TiO2) was
35 deposited onto a F-doped SnO2 (FTO, Pilkington, TEC8)
substrate by spray pyrolysis deposition carried out using a 20 mM
titanium diisopropoxide bis(acetylacetonate) solution (Aldrich) at
450 °C to prevent direct contact between FTO and the hole-
conducting layer. 750-nm-thick mesoporous TiO2 (mp-TiO2)
40 films were screen-printed onto the bl-TiO2/FTO substrate using a
paste that was prepared according to a reported method15, and the
films were then calcined at 500 °C for 1 h to remove the organic
part. The films were then immersed in a 40 mM TiCl4 aqueous
solution at 60 °C for 1 h and were then heat-treated at 500 °C for
45 30 min to improve interfacial contact with nanocrystalline TiO2.
CH3NH3I were synthesized from 30 mL hydroiodic acid (57% in
water, Aldrich), by reacting 27.86 mL methylamine (40% in
methanol, Junsei Chemical Co., Ltd.) in a 250-mL round-
bottomed flask at 0 °C for 2 h with stirring. The precipitates were
50 recovered by evaporating the solutions at 50 °C for 1 h. The
products were dissolved in ethanol, recrystallized using diethyl
ether, and finally dried at 60 °C in a vacuum oven for 24 h. The
CH3NH3PbI3 solution (40 wt%) was prepared by reacting the
synthesized CH3NH3I powder and PbI2 (Aldrich) in γ-
55 butyrolactone at 60 °C for 12 h. The CH3NH3PbI3 solution was
then coated onto the mp-TiO2/bl-TiO2/FTO substrate by
Results and discussion
Fig. 1a shows the energy level diagram for the different solar cell
components, depicting the conduction band of TiO2, conduction
95 and valence bands of CH3NH3PbI3 perovskite, and position of the
highest occupied molecular orbital (HOMO) of spiro-OMeTAD
as HTM and the Co-complex (FK209). In this scheme, the
electrons and holes generated in CH3NH3PbI3 can be
energetically injected into the mp-TiO2 and HTM phases,
100 respectively. The HOMO energy of the Co-complex is around
340 meV lower than that of spiro-OMeTAD.9 The HOMO
energies of the Co-complex and spiro-OMeTAD were derived
from the oxidation potential (1.06 V vs. normal hydrogen
electrode (NHE)) of the Co-complex and the first oxidation
105 potential (0.72 V vs. NHE) of spiro-OMeTAD9, respectively.
This is because HOMO energy is dependent on the oxidation
potential (i.e., removal of an electron from the HOMO), as well
as on the oxidation state of the compound. The movement of
electrons from the HOMO of sprio-OMeTAD to FK209 induces
110 the creation of hole carriers in spiro-OMeTAD, leading to an
increase in its conductance. Fig. 1b shows a comparison of the
charge conductance of pristine spiro-OMeTAD (=HTM A), spiro-
2
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DOI: 10.1039/C3TA12681A
OMeTAD with Li-TFSI and TBP (=HTM B), spiro-OMeTAD
with 7.7 mol% FK209 (=HTM C), and spiro-OMeTAD with 7.7
mol% FK209, Li-TFSI, and TBP (=HTM D). For measuring the
Moreover, additional increase in Jsc and Voc was observed and
this might also be attributed to the improved charge extraction
owing to the smaller resistance of HTM B. Similarly, the use of
conductance, a 200-nm-thick film of each of the above four 40 HTM C leads to an increase in all the performance parameters: Jsc
5
mixtures was deposited on a SiO2/Si substrate and a Au electrode
was deposited by thermal evaporation through a shadow mask
(channel length: 50 ꢁm; channel width: 3000 ꢁm). The addition
of Li-TFSI (TBP) to spiro-OMeTAD increases the conductance
of 18.1 mA/cm2, Voc of 0.80 V, and FF of 55.6%, resulting in a
remarkable increase in PCE from 5.0% to 8.1%. Here, it is
notable that Voc of the device fabricated using HTM C was
greatly increased to 0.80 V from 0.70 V (for the device
to be more than 100 times that of pristine spiro-OMeTAD, in 45 fabricating using HTM A), even though the Jsc, FF, and serial
10 accordance with previously reported behavior16. Moreover, the
resistance values of this device are comparable with the
corresponding values for the device fabricated using HTM B.
This indicates that FK209 as a dopant for spiro-OMeTAD plays
an important role in increasing Voc because the Voc value of HTM
conductance of pristine spiro-OMeTAD was greatly increased
(~280 times) upon the addition of 7.7 mol% FK209. In particular,
the simultaneous addition of FK209 and Li-TFSI (TBP) to spiro-
OMeTAD increases the conductance 780 times, indicating that 50 B was slightly increased to 0.72 V from 0.70 V for HTM A.
15 the co-addition of Li-TFSI (TBP) and FK209 is highly effective
for reducing the charge transport resistance of spiro-OMeTAD.
Generally, Voc is determined by the difference in the Fermi levels
of TiO2 and HTM. Therefore, the higher Voc value of HTM C was
assumed to be due to the lower Fermi level of HTM. Further
improvement in the performance was achieved by using HTM D,
55 which consisted of a mixture of spiro-OMeTAD, 7.7 mol%
FK209, Li-TFSI, and TBP. The values of FF and Voc were the
highest for this device at 63.6% and 0.85 V, respectively. This
may be attributed to better hole doping of spiro-OMeTAD and
lower series resistance in this case. The increase in Voc observed
60 in HTM C and D will again be discussed later.
20
18
)
16
a
14
12
(
10
8
Pristine spiro
Sprio+Li,tbp
6
4
Spiro:FK209
Spiro:FK209+Li,tbp
2
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Voltage (V)
)
0.0
b
-0.5
(
Fig. 1. (a) Energy-level diagram for each component of devices and (b)
relative conductance of corresponding 200-nm-thick spiro-OMeTAD
-1.0
-1.5
-2.0
20 films deposited on a SiO2/Si substrate. Conductance values are
normalized on the basis of conductance of pristine spiro-OMeTAD film.
Fig. 2a shows current density–voltage (J–V) curves for
TiO2/CH3NH3PbI3/HTMs/Au solar cells fabricated using HTM A,
HTM B, HTM C, and HTM D. The photovoltaic parameters for
25 these devices are summarized in Table 1. For the reference device
fabricated using HTM A, the following values were obtained: Jsc
of 15.8 mA/cm2, Voc of 0.70 V, and FF of 45.4 %, yielding an
overall PCE of 5.0%. A low FF that can be attributed to the high
charge transport resistance of this device limits its performance.
30 On the other hand, the device fabricated using HTM B exhibited
dramatically enhanced performance: Jsc of 17.7 mA/cm2, Voc of
0.72 V, FF of 56.9 %, and overall PCE of 7.3 %. The high values
of Jsc and FF mainly contribute to the high PCE. As expected
from Fig. 1b, the increased conductance of spiro-OMeTAD with
35 ionic additives decreases the series resistance from 137.2 to 97.2
Ω, corresponding to the increase in FF from 45.4% to 56.9%.
-0.5
0.0
0.5
1.0
Voltage (V)
Fig. 2. (a) J–V characteristics measured under AM 1.5G solar irradiance
(100 mW/cm2) for devices fabricated using HTM A, HTM B, HTM C,
and HTM D as HTM layers, (b) Corresponding dark J–V curves for
65 devices.
The concentration of oxidized spiro-OMeTAD, i.e., hole
doping, plays a very important role in determining the charge
transport resistance of spiro-OMeTAD and the final photovoltaic
performance of solar cells17. It was reported that in ionic-doped
70 spiro-OMeTAD systems17, Li-TFSI plays the role of a catalyst in
the formation of oxidized spiro-OMeTAD via a photoinduced
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DOI: 10.1039/C3TA12681A
reaction. Oxidized spiro-OMeTAD can be easily distinguished
from ground-state spiro-OMeTAD by UV-Vis absorption
induce a shift in the Fermi level to the HOMO level due to
increased hole concentration21. Consequently, in the cell
spectroscopy because the former has an absorption band at 520 35 fabricated using HTM C, the Fermi level of spiro-OMeTAD can
nm18.
be lowered by increasing the hole concentration when the charge
transfer interaction occurs. This would lead to an expansion of
the gap between the quasi-Fermi level of TiO2 and the Fermi
level of spiro-OMeTAD that in turn would result in higher Voc
40 values.9 However, the increased Voc value may be attributed to
several factors. In addition to the more negative Fermi level and
the lower resistance of the charge transport complex, the diode-
like current–voltage (I–V) characteristics in the dark can give an
insight into the charge recombination in real devices. Fig. 2b
45 shows dark J–V data for the devices fabricated using HTM A,
HTM B, HTM C, and HTM D. In comparison with the reference
device fabricated using HTM A, the device fabricated using HTM
D showed the lowest dark current and a 150-mV shift in the onset
of the dark current despite the highest relative conductance of this
50 HTM film (Fig. 2b). Because the difference on the dark current
results from an electron flow from TiO2/CH3NH3PbI3
nanocomposite to different-type HTMs in the devices, the lowest
dark current for the device fabricated using HTM D reveals that
HTM D has a superior electron blocking ability22. In addition, the
55 TiO2/CH3NH3PbI3 has different features in the device structure
compared with conventional solid-state sensitized solar cells.
CH3NH3PbI3 as light harvester is fully deposited into mp-TiO2
electrode with overlayers that are densely formed on top of the
mp-TiO213. This structure can prevent the direct contact between
60 TiO2 nanoparticles and HTM, resulting in the reduction of an
interfacial charge recombination between TiO2 and spiro-
OMeTAD. Consequently, the addition of FK209 and Li-TFSI +
TBP to spiro-OMeTAD (HTM D) has a synergistic effect on the
photovoltaic performance of the solid-state mesoscopic
65 CH3NH3PbI3/TiO2 heterojunction solar cell in terms of hole
transport and electron blocking ability.
5
Table 1. Photovoltaic parameters of devices shown in Fig. 2a
.
Jsc
Voc
FF
η
Rs
(mA/cm2)
(V)
(%) (%)
(Ω)
Pristine Spiro
(HTM A)
15.8
17.7
18.1
18.1
0.70 45.4 5.0
0.72 56.9 7.3
0.80 55.6 8.1
0.85 63.6 9.7
137.2
97.2
Spiro+Li,TBP
(HTM B)
Spiro:FK209
(HTM C)
103.3
72.0
Spiro:FK209+Li,TBP
(HTM D)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.08
0.07
0.06
0.05
0.04
400
500
600
700
Wavelength (nm)
Pristine spiro
Sprio+Li,tbp
Spiro:FK209
Spiro:FK209+Li,tbp
0.0
400
500
600 700 800
Wavelength (nm)
20
10 Fig. 3. Absorption spectra for HTM A, HTM B, HTM C, and HTM D.
)
18
Inset shows a magnified image of oxidized spiro-OMeTAD peak at 520
nm.
16
14
(
Fig. 3 shows the UV-Vis absorption spectra for HTM A,
15 HTM B, HTM C, and HTM D. The UV-Vis absorption spectra
for HTM C and D clearly show a new absorption band with the
peak maxima located at 520 nm regardless of the presence of Li-
TFSI. According to previously reported UV-Vis absorption
spectra for FK209, the absorption at 520 nm does not correspond
20 to FK209 itself.9 Therefore, this new absorption band results from
the formation of oxidized spiro-OMeTAD cation radical,
indicating the occurrence of charge transfer interaction between
FK209 and spiro-OMeTAD19,20. Because the solutions were kept
in the dark prior to UV-Vis spectroscopy, the photoinduced
25 oxidization of spiro-OMeTAD by the addition of Li-TFSI and
TBP is not expected, and therefore, the spiro-OMeTAD with Li-
TFSI and TBP does not show any absorption band at 520 nm.
Therefore, the new absorption band at 520 nm reveals that the
addition of FK209 to spiro-OMeTAD can lead to the formation of
30 oxidized spiro-OMeTAD via a charge transport reaction between
spiro-OMeTAD and FK209 even under dark conditions. The
12
Blank
2.0 %
10
8
6
4
2
0
4.0 %
7.7 %
14.3 %
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Voltage (V)
Fig. 4. J–V characteristics measured under AM 1.5G solar irradiance (100
mW/cm2) for devices containing spiro-OMeTAD:FK209 charge transport
70 layer and added Li-TFSI + TBP as a function of FK209 concentration.
It is believed that FK209 generates additional charge carriers for
spiro-OMeTAD, leading to an increased conductivity of HTM.
Thus, we investigated the J–V characteristics of the fabricated
devices as a function of FK209 concentration in the range
increase in the concentration of oxidized spiro-OMeTAD can 75 0(blank)–14.3 mol%, based on same Li-TFSI + TBP as HTM D.
4
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DOI: 10.1039/C3TA12681A
The corresponding I–V curves under simulated AM 1.5G solar
irradiance (100 mW cm-2) are shown in Fig. 4 and the results are
summarized in Table 2. The device fabricated using the spiro-
OMeTAD (92.3 mol%):FK209 (7.7 mol%) charge transport
complex showed the best performance. The Voc and FF values for
this device increased from 0.72 V to 0.84 V and from 58.3% to
65.3% while the Jsc value was maintained constant until the
FK209 concentration of 11.9 mol%, yielding an overall PCE of
9.9%. However, at higher concentrations (14.3 mol%) of FK209,
a
20
)
18
16
14
J : 18.3 mA cm-2
5
(
12
10
8
SC
V
: 865 mV
OC
6
FF : 66.0 %
: 10.4 %
4
10 Voc and FF values decreased. This reduction in the performance
at higher FK209 concentrations might be attributed to the
increased recombination due to excessive hole density or
inhomogeneity in the charge transport complex. Consequently,
the optimum concentration of FK209 for the spiro-
15 OMeTAD:FK209 charge transport complex is 7.7 mol% in the
experimental range. However, further study is needed to
conclusively establish the effect of recombination kinetics and
charge transport in spiro-OMeTAD:Co-complex composites on
the HTM layer behavior in mesoscopic CH3NH3PbI3/TiO2
20 heterojunction solar cells.
η
2
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Voltage (V)
0.9
b
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Table 2. Photovoltaic parameters of devices shown in Fig. 4.
Jsc
(mA/cm2)
18.0
Voc
FF
η
Rs
(V)
(%)
(%)
7.6
8.7
9.0
9.9
9.2
(Ω)
300
400
500
600
700
800
Blank
2.0 %
4.0 %
7.7 %
14.3 %
0.72
0.76
0.78
0.84
0.80
58.3
62.8
63.4
65.3
62.1
94.7
71.8
71.5
68.5
65.8
Wavelength (nm)
18.0
50 Fig. 5. (a) J–V characteristics measured under AM 1.5G solar irradiance
(100 mW/cm/2) for a champion device fabricated using HTM D, (b)
corresponding external quantum efficiency (EQE) spectrum as a function
of monochromatic wavelength.
18.0
18.2
18.3
25
Fig. 5a and b show the I–V curve and external quantum
efficiency (EQE) spectrum, respectively, for a champion device
containing 7.7 mol% FK209. The device showed Jsc of 18.3
mA/cm2, Voc of 865 mV, and FF of 66% under AM 1.5G solar
irradiance (100 mW cm-2), yielding a PCE of 10.4%. The EQE
55 Acknowlegement
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
60 grant from the KRICT 2020 Program for Future Technology of
the Korea Research Institute of Chemical Technology (KRICT),
Republic of Korea.
30 spectrum shows a maximum value of 0.78 at 540 nm and overall
high values of more than 0.60 up to 750 nm, yielding an
integrated Jsc value of 18.1 mA/cm2. The excellent photovoltaic
performance with PCE greater than 10% obtained in this study
reveals that the composite charge transport complex containing
35 spiro-OMeTAD and Co-complexes can be used to increase the
photon-to-electron conversion efficiency of recently developed
solid-state mesoscopic lead-halide heterojunction solar cells.
Notes and references
aDivision of Advanced Materials, Korea Research Institute of Chemical
65 Technology, 141 Gajeong-Ro, Yuseong-Gu, Daejeon 305-600, Korea.
Fax:+82-42-861-4251; Tel:+82-42-860-7314; E-mail:seoksi@krict.re.kr
bLaboratory for Photonics and Interfaces, Institute of Chemical Sciences
and Engineering, School of Basic Science, Swiss Federal Institute of
Technology, CH-1015 Lausanne, Switzerland
Conclusions
We have fabricated highly efficient mesoscopic
40 TiO2/CH3NH3PbI3 heterojunction solar cells by using a mixture
of spiro-OMeTAD, FK209, Li-TFSI, and TBP as HTM. The
performance of the devices was substantially equivalent to the
charge conductance of HTM themselves, responsible for series
resistances. In particular, the addition of FK209 (Co-complex) to
45 spiro-OMeTAD increased Voc, mainly due to the reduced carrier
recombination and lower Fermi level of HTM. Finally, we
achieved an PCE of 10.4% for a solar cell using spiro-OMeTAD
as HTM under standard test conditions (AM 1.5G, 100 mW/cm2).
70 cDepartment of Energy Science, Sungkyunkwan University, Suwon 440-
746, Korea
1
2
B. O’Regan, M. Grätzel, Nature, 1991, 353, 737..
A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, Md. K.
Nazeeruddin, E. W.-G. Diau, C.-Y.Yeh, S. M. Zakeeruddin, M.
Grätzel, Science, 2011, 334, 629.
75
3
U. Bach, D. Lupo, P. Compte, J. E. Moser, F. Weissörtel, J. Salbeck,
H. Spreitzer, and M. Grätzel, Nature, 1998, 395, 583.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5

Journal of Materials Chemistry A
Page 6 of 6
View Article Online
DOI: 10.1039/C3TA12681A
4. I. Yafit, N. Olivia, P. Miles, H. Gary, J. Phys. Chem. C 2009, 113,
42546.
5. W. Zhang, R. Zhu, F. Li, Q. Wang, B. Liu, J. Phys. Chem. C 2011,
115, 7038.
5
10
15
20
25
30
35
40
6. J. Xia, N. Masaki, M. Lira-Cantu, Y. Kim, K. Jiang, S. Yanagida, J.
Am. Chem. Soc. 2008, 130, 1258.
7. S. X. Tan, J. Zhai, M. X. Wan, L. Jiang, D. B. Zhu, Synthetic Metals,
2003, 137, 1511.
8. I. Chung, B. Lee, J. He, R. P. H. Chang, M. G. Kanatzidis, Nature,
2012, 485, 486.
9. J. Burschka, A. Dualeh, F. Kessler, E. Baranoff. N.-L. Cevey-Ha, C.
Yi, M. K. Nazeeruddin, and M. Gratzel, J. Am. Chem. Soc., 2011,
133, 18042.
10. A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc.
2009, 131, 6050.
11. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith,
Science 2012, 338, 643.
12. H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A. Marchioro,
S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E. Moser, M. Grätzel,
N.-G. Park, Sci. Rep. 2012, 2, 591.
13. J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C.-S. Lim, J. A. Chang,
Y. H. Lee, H. Kim, A. Sarkar, M. K. Nazeeruddin, M. Grätzel, and
Seok, S. I. Nature Photonics, 2013, 7, 486.
14. J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, and S. I. Seok, Nano
Lett., 2013, 13, 1764.
15. J. A. Chang, J. H. Rhee, S. H. Im, Y. H. Lee, H. Kim, S. I. Seok, Md.
K. Nazeeruddin, and M. Grätzel, Nano Lett., 2010, 10, 2609.
16. A. Abate, T. Leijtens, S. Pathak, J. Teuscher, R. Avolio, M. E. Errico,
J. Kirkpatrik, J. M. Ball, P. Docampo, I. McPhersonc, H. J. Snaith,
Phys. Chem. Chem. Phys., 2013, 15, 2572.
17. U. B. Cappel, T. Daeneke, and U. Bach, Nano Lett. 2012, 12, 4925.
18. U. Bach, Y. Tachibana, J.-E. Moser, S. A. Haque, J. R. Durrant, M.
Gratzel, D. R. Klug, J. Am. Chem. Soc., 1999, 121, 7445.
19. D. Y. Chen, W. -H. Tseng, S. -P. Liang, C. -I. Wu, C. -W. Hsu, Y.
Chi, W. -Y. Hung, P. -T. Chou, Phys. Chem. Chem. Phys., 2012, 14,
11689.
20. E. F. Aziz, A. Vollmer, S. Eisebitt, W. Eberhardt, P. Pingel,D. Neher,
and N. Koch, Adv. Mater., 2007, 19, 3257.
21. R. Scholin, M. H. Karlsson, S. K. Eriksson, H. Siebahn, E. M. J.
Johansson, and H. Rensmo, J. Phys. Chem. C, 2012, 116, 26300.
22. N. Li, B. E. Lassiter, R. R. Lunt, G. Wei, and S.R. Forest, Appl. Phys.
Lett., 2009, 94, 023307.
6
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This journal is © The Royal Society of Chemistry [year]