Efficient planar-heterojunction perovskite solar cells achieved via interfacial modification of a sol-gel ZnO electron collection layer

Article abstract of DOI:10.1039/c4ta03954h

The importance of interfacial engineering as a new strategy for improving the power conversion efficiencies (PCEs) of planar-heterojunction (PHJ) perovskite solar cells is highlighted in this study. With our optimized interfacial modification, we demonstrated efficient PHJ perovskite solar cells with a high PCE of 12.2% using a sol-gel-processed ZnO ECL modified by [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM). This journal is

Full text of DOI:10.1039/c4ta03954h

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Efficient Planar-Heterojunction Perovskite Solar Cells
Achieved via Interfacial Modification of Sol-gel ZnO
Electron Collection Layer
Cite this: DOI: 10.1039/x0xx00000x
a
b
c
a
Received 00th January 2014,
Accepted 00th January 2014
Junghwan Kim, Geunjin Kim, Tae Kyun Kim, Sooncheol Kwon, Hyungcheol
b a c b *a,b,c
Back, Jinho Lee, Seoung Ho Lee, Hongkyu Kang and Kwanghee Lee
DOI: 10.1039/x0xx00000x
www.rsc.org/
The importance of interfacial engineering as a new strategy
Recently, studies on planar-heterojunction (PHJ) perovskite
for improving the power conversion efficiencies (PCEs) of solar cells, sandwiched between the hole transport layer (HTL)
planar-heterojunction (PHJ) perovskite solar cells is and the electron collection layer (ECL) without mesoporous
highlighted in this study. With our optimized interfacial scaffolds, have attracted much attention due to their simplified
modification, we demonstrated efficient PHJ perovskite solar fabrication process and compatibility with roll-to-roll
1
, 8
cells with a high PCE of 12.2% using a sol-gel-processed ZnO processing on flexible substrates.
To realize the PHJ
ECL modified by [6,6]-phenyl C61 butyric acid methyl ester perovskite solar cells, several nano-sized TiO2 or ZnO
(
PCBM).
nanoparticles (NPs) have been used as compact ECLs in the
devices. However, the NPs are usually synthesized by
3
The recent tremendous progress in thin-film solar cell
technology using solution-processable organic-inorganic hybrid
relatively complex procedures including long reaction time,
1
3
precipitation, washing, and dilution processes. In addition,
ligand-linked NPs need a high temperature annealing process,
which is incompatible with flexible substrates, and ligand-free
perovskites-such as CH NH PbX (X = Cl, Br, I)-is an
3
3
3
1
-6
important milestone in the field of renewable solar devices.
1
4, 15
Because hybrid organic-inorganic methylammonium lead
halide perovskites exhibit excellent semiconducting properties,
NPs usually suffer from poor dispersion stability.
In
contrast, sol-gel processed metal oxides have many advantages
of simple synthesis, long-term stability, solution, and low
temperature processability and are more suitable for printable
2
-1 -1
such as high charge-carrier mobility (~10 cm V s ), a direct
band gap (~ 1.5 eV), a broad absorption range, a small exciton
binding energy (~ 20 meV), and a long-range exciton diffusion
length (100 ~ 1000 nm), the new family of solar cells
incorporating hybrid light-harvesting materials is expected to
1
6
photovoltaics.
Nevertheless, there are few reports to successfully
achieve power conversion efficiencies (PCEs) of up to 20% in demonstrate efficient PHJ perovskite solar cells by
4
,
6
the near future.
Motivated by the great potential of incorporating the sol-gel metal oxides. In addition, although the
organometal halide perovskites, intensive studies have been interfacial engineering of the interface between electrodes and
5, 6
conducted on the charge dynamics of perovskites, synthesis the light absorber films has been one of the most important
7
of new materials, optimization of perovskite absorber layers by strategies for improving the PCEs of thin-film solar cells, only
4
, 8
9
increasing crystallinity and surface coverage, and realization few reports on this method in the field of PHJ perovskite solar
1
0-12
of low-temperature processed devices
years.
over the past several cells have been published. Therefore, the aim of this work was
to replace the complicated NPs with simple sol-gel compact
ECLs and to study the effect of the interfacial modification of
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the sol-gel processed ECLs on device performance by
To selectively extract electrons from the CH NH PbI
3 3 3
analyzing the electronic structures of the interfacial layers and absorber, a 30-nm-thick sol-gel-processed ZnO layer was
perovskite absorber for optimizing the device performance of deposited onto the ITO electrode as the ECL. The sol-gel ZnO
PHJ perovskite solar cells.
film offers several advantages including easy solution
processability, air stability, excellent optical transparency, good
film formation, and relatively low-temperature processability.
Furthermore, because the sol-gel ZnO has much higher electron
mobility than that of titanium oxide, it is an ideal material as an
In this communication, we successfully demonstrate efficient
PHJ perovskite solar cells with a sol-gel processed ZnO ECL
modified by a thin organic surface modifier, [6,6]-phenyl C61
1
6
butyric acid methyl ester (PCBM). By depositing PCBM onto
ECL for the PHJ perovskite solar cells.
For the interfacial
the sol-gel ZnO, the electronic structure of the ZnO could be
modulated, and the Voc values of the devices containing the
ZnO/PCBM were enhanced compared to those of the devices
containing only ZnO. In addition, the interfacial modification of
ZnO was observed to significantly reduce trap-assisted charge
recombination at the interface of ZnO as well as in the bulk of
the perovskite absorber, thereby improving the device
performance. With our optimized interfacial modification, we
demonstrated efficient PHJ perovskite solar cells with a PCE of
modification of the top surface of the ZnO layer, PCBM were
spin-coated onto the ZnO film and then annealed at 80 °C for 5
min. Because the fullerene derivative of the PCBM is an
excellent electron acceptor having favorable electronic structure
with respect to the perovskite absorber, it is expected that the
modifiers can effectively modulate the interfacial properties of
1
7
ZnO. Scanning electron microscopy (SEM) and atomic force
microscopy (AFM) measurement show that the ZnO and the
ZnO/PCBM films have
a very smooth and compact
12.2% using a sol-gel-processed ZnO/PCBM ECL.
morphology (Figure S3 and S4).
a
In our perovskite solar cells, we utilized a new class of
PBDTTT-type low-band-gap conjugated polymer composed of
benzodithiophene (BDT) and thieno[3,4-b]thiophene (TT) units,
<
Interfacial layers>
Ag
PTB7-Th
CH NH PbI
MoO
3
O
OMe
18
3
3
3
PTB7-Th, as a HTL. Because the PTB7-Th layer has an
Sol-gel ZnO
ITO
Glass
-2
2
-1 -1
excellent hole mobility of 1.5 x 10 cm V s as well as an
appropriate oxidation potential of 5.2 eV, it is expected to
function efficiently as a HTL in perovskite solar cells (see
supporting information S5). More detailed information
regarding the preparation of the perovskite absorber, sol-gel
ZnO, and PTB7-Th and the device fabrication is described in
the experimental section.
PCBM
b
c
e-
3
.5 eV
e-
3.6 eV
3
.69 eV
E´
F
4
.6 eV
Ag
CH
3
NH
3
PbI
3
ΔE´off
E
F
ΔEoff
ITO
ZnO
5.1 eV
3 3 3
CH NH PbI
5.2 eV
ITO
ZnO
The energy-level diagrams of the component materials and
the effect of ZnO interfacial modification are illustrated in
Figure 1b and c, respectively. The valence band maximum
5
.6 eV
h+
6
.89 eV
Figure 1. (a) The device structure of the planar-heterojunction perovskite
solar cells and the molecular structures of the interfacial layers PCBM. A
cross-sectional SEM image of the device is also shown. Energy-level
diagram of the components of the perovskite solar cells (b) without and (c)
with the PCBM. The energy-level offset (ΔEoff) is the energy-level difference
(
VBM) values of the ZnO and the perovskite films were
determined by ultraviolet photoelectron spectroscopy (UPS),
and the conduction band minimum (CBM) values were
calculated by adding the optical band gap (E ) derived from the
g
between the quasi Fermi levels of the electron collection layer and the hole UV-visible absorption onset (Figure S6). The spectroscopic
transport layer.
measurements show that the ZnO film has a VBM of 6.89 eV
and a CBM of 3.69 eV, respectively. The electronic structure of
PTB7-Th was obtained from the literature. Because the CBM
1
8
Figure 1a illustrates the device architecture of the PHJ (3.60 eV) and VBM (5.20 eV) of the perovskite film are close
perovskite solar cells and the chemical structures of the organic to those of the ZnO and PTB7-Th layers, respectively, the
interfacial modifier PCBM used in this work. The device efficient charge extraction (or dissociation) of free charge
structure is ITO/ZnO/CH NH PbI /PTB7-Th/MoO /Ag, where carriers (or excitons) generated in the perovskite film is
3
3
3
3
the perovskite film (CH NH PbI ) was grown using a expected, as shown in Figure 1b. It is noteworthy that a stable
3
3
3
sequential deposition method to achieve a good film coverage molybdenium oxide (MoO ) thin film was deposited on top of
x
2
and crystallization (see supporting information S1). The results the PTB7-Th HTL in our perovskite solar cells. Because the
of a structural investigation of PbI and CH NH PbI using a TMO interlayer helps to extract holes more efficiently, we
2
3
3
3
selected area electron diffraction (SAED) clearly show that the obtained dramatically enhanced device performance by
phase of PbI2 (hexagonally closed-packed layer) was introducing the MoO interlayer (see supporting information
successfully transformed into the perovskite structure of Figure S7). The role of MoO layer is to reduce contact
CH NH PbI (see supporting information S2).
resistance at the interface of HTL and Ag electrode by hole-
x
x
3
3
3
1
9, 20
doping of the organic HTL with the MoOx.
Therefore, we
2
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expect that various new types of organic materials can be an interfacial dipole shifting the vacuum level upward, and the
successfully applied to perovskite solar cells as efficient HTLs WF decreases to 3.79 eV. The photoemission onset of the ZnO
by combining them with a MoO interlayer.
film further shifts toward higher binding energy upon the
deposition of the PCBM, decreasing the WF to 3.74 eV. In
contrast, the valence band edge (Evalence) of the ZnO film shifts
toward lower binding energy with interfacial modification,
shifting the energetic position of the VBM upward with respect
to the Fermi level (EF).
x
To enhance the efficiency of perovskite solar cells,
improving electron transport/extraction is also very important.
Therefore, we intended to modify the surface of the ZnO film
by incorporating PCBM for better electron transport/extraction
between the ZnO and the perovskite absorber. In addition, as
shown in Figure 1c, because the Voc of the ‘p-i-n’ perovskite
solar cells is usually determined by the energy-level offset
a
(
ΔE ) between the quasi Fermi levels of the ECL and the HTL
off
4
2
1, 22
adjacent to the perovskite absorber,
it is expected that an
ZnO
ZnO/PCBM
enhancement in the Voc values is possible by changing the
electronic structure of the ZnO film using surface modifier,
thereby improving device performance.
0
-4
-8
a
He I
-12
ZnO/Perovskite
ZnO/PCBM
ZnO
-16
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
Voltage (V)
ITO
0
2
4
6
0
5
10
15
20
16
18
b
Binding Energy (eV)
80
ZnO
b
Evac
ZnO/PCBM
E
vac
6
0
WF~ 4.38 eV
5.20 eV
3.60 eV
6
.89 eV
3
3.69 eV
3
.74 eV
3.44 eV
4.03 eV
.79 eV
40
20
0
E
CB
E
CB
E
F
E
F
ITO
5.24 eV
E
VB
E
VB
ZnO
ZnO/PCBM
Perovskite
4
00
500
600
700
800
Wavelength (nm)
Figure 2. (a) UPS spectra of ITO, ITO/ZnO, ITO/ZnO/PCBM, and
ITO/ZnO /perovskite films. (left) the valence band edge, (middle)
whole UPS photoemission spectra, and (right) magnified section of
the photoemission onset. (b) Energy-level diagrams of the various
Figure 3. (a) Current density-voltage characteristics of the planar-
heterojunction perovskite solar cells incorporating ITO/ZnO and
ITO/ZnO/PCBM as cathodes measured under AM 1.5G
irradiation at 100 mW cm . (b) EQE spectra of the perovskite
solar cells.
-2
films resulting from the UPS studies. The Fermi level (E ) values of
F
each film are aligned due to the thermodynamic equilibrium that
arises when they are joined together.
The electronic structures of the ITO, ZnO, ZnO/PCBM, and
ZnO/perovskite films deduced from the UPS measurements are
illustrated in Figure 2b. The E values of each film are aligned
F
due to thermodynamic equilibrium when they are joined
together; band bending is not shown. Because the CBM of the
ZnO is lower than that of perovskite, the efficient charge
extraction (or dissociation) of free charge carriers (or excitons)
generated in the perovskite film is expected. For the
ZnO/PCBM ECL, because the CBM of ZnO/PCBM is located
between that of ZnO and that of perovskite, much more
favorable energy-level alignment occurs at the interfaces,
facilitating cascade charge extraction. In addition, the high
voltage output of the perovskite solar cell with the ZnO/PCBM
layer is expected due to the large energy-level offset between
To investigate the electronic structures of the ZnO and
ZnO/PCBM, we carried out ultraviolet photoelectron
spectroscopy (UPS) measurements. Figure 2a shows the UPS
spectra of the ITO, ZnO, ZnO/PCBM, and ZnO/perovskite
films. The Photoemission parameters obtained from the UPS
measurements are summarized in Table S1. The high binding
energy onset of bare ITO is 16.82 eV, yielding a work function
(
WF) of 4.38 eV. The deposition of a ZnO film on the ITO
substrate leads to a shift in the photoemission onset toward
higher binding energy (17.41 eV), indicating the formation of
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the quasi Fermi levels of the ECL and the HTL adjacent to the
perovskite absorber relative to that afforded by ZnO alone.
a
1.0
ZnO
To explore the effects of interfacial modification on device
performance, we fabricated PHJ perovskite solar cells with a
structure of ITO/sol-gel ZnO/CH NH PbI /PTB7-Th/MoO /Ag
ZnO/PCBM
0
.9
.8
3
.49 K T/q
B
3
3
3
3
0
and ITO/sol-gel ZnO/PCBM/CH NH PbI /PTB7-Th/MoO /Ag.
3
3
3
3
The current density-voltage (J-V) characteristics of the devices
under AM 1.5 G irradiation with an irradiation intensity of 100
0.7
4.10 K T/q
B
-
2
mW cm and external quantum efficiency (EQE) spectra are
shown in Figure 3a and b, respectively. The photovoltaic
parameters of these devices are summarized in Table 1. As
shown in Figure 3a, the device with a sol-gel ZnO film
exhibited a PCE of 8.37% with a short-circuit current density
0.6
1
10
100
-2
Light intensity (mW cm )
-
2
(
Jsc) of 14.31 mA cm , an open circuit voltage (V ) of 0.86 V,
oc
b
and a fill factor (FF) of 68%. The performances are comparable
to those of perovskite solar cells incorporating ZnO NPs which
were synthesized by the previously reported method. (see
4
10
ZnO
ZnO/PCBM
3
supporting information Figure S8)
3
2
1
10
10
10
On the other hand, the perovskite solar cells containing sol
gel ZnO/PCBM showed remarkably enhanced device
performance compared with that of the ZnO devices, resulting
-
2
in the best PCE of 11.04% with J ~ 15.10 mA cm , V ~ 1.03
sc
oc
V, and FF ~ 71%. Notably, the Voc and FF values were further
enhanced relative to those of the of ZnO devices. As indicated
by the electronic structure of the films (see Figure 2), the
enhancement in the device performance can be attributed to the
following factors: the larger energy-level offset between the
quasi Fermi levels of the ECL and the HTL adjacent to the
perovskite absorber and the more favorable energy-level
alignment between the CBM of ZnO/PCBM and the perovskite
absorber for efficient electron collection, resulting in a high FF.
When considering the perovskite solar cells incorporating only
PCBM on ITO substrate exhibited poor device performances
0
.0
0.2
0.4
0.6
0.8
Voltage (V)
Figure 4. (a) Light-intensity dependence of the Voc of the perovskite
solar cells with ZnO and ZnO/PCBM. The lines represent the best
fits to the data, for which the slope is indicated. (b) Recombination
resistance at different applied biases extracted from IS
characterization of the perovskite solar cells.
semi-logarithmic plot of Voc as a function of light intensity for
the devices and the slopes derived from the lines fitted linearly
to the data. As shown in Figure 4a, the device with only ZnO
shows a strong dependence of Voc on the light intensity, with a
slope of 4.10 k T/q, where k is Boltzmann’s constant, q is the
elementary charge, and T is temperature in Kelvin. However,
when the ZnO is modified with PCBM, the device shows a
lower slope of 3.49 k T/q, indicating reduced trap-assisted
(
see supporting information Figure S9), the synergic effect of
ZnO and PCBM is crucial for improving device performances.
Table 1. Photovoltaic parameters under AM 1.5G solar
B
B
-
2
spectrum with a light intensity of 100 mW∙cm .
Jsc
2
Voc
Cathode
mean ± s.d.)
PCE (%)
(Best)
[a]
-2
FF
R (Ω∙cm )
(
s
(
V)
B
(
mA cm )
1
7, 25
recombination in the devices.
The slope of our perovskite
0
0
.78 ±
.05
7.65 ± 0.47
(8.37)
ITO/ZnO
14.54 ± 0.16
0.68 ± 0.01
9.0 ± 0.87
solar cells is similar with previously reported values of
CH NH PbI perovskite devices but slightly higher than that of
3
3
3
1
.02 ±
.01
10.87 ±
26-28
mixed halide (CH NH PbI Cl ) perovskite solar cells.
speculate that the differences can be related with perovskite
We
3
3
3-x
x
ITO/ZnO/PCBM
14.73 ± 0.23
0.73 ± 0.02
7.4 ± 0.25
0
0.13 (11.04)
[
a] The R
s
of the perovskite solar cells were calculated using composition, because the exciton diffusion length and charge
−1
(
dJ/dV) at V = Voc.
dynamics were significantly affected by doping Cl ions into
6
, 29
CH NH PbI perovskite.
3
3
3
To further investigate the role of interfacial modification in
enhancing the performance of the fabricated devices, we
It is noted that the slopes of the CH NH PbI -based PHJ
3
3
3
measured the light-intensity dependence of Voc for the perovskite solar cells are relatively larger than those of polymer
perovskite solar cells with ZnO and ZnO/PCBM ECLs, which solar cells (PSCs) (~ k T/q) and solid-state, dye-sensitized solar
B
describes the order of recombination processes in solar devices cells (SDSSCs) (~ 1.7 k T/q); this finding suggests that the
B
2
3, 24
(
see supporting information Figure S10).
Figure 4a shows a dominant charge recombination mechanism of the perovskite
4
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0
solar cells at the open-circuit voltage is monomolecular
recombination, which is distinguished from bimolecular
recombination in PSCs and a combination of monomolecular
and bimolecular recombination in SDSSCs at the open-circuit
80
60
40
20
0
-4
Voc: 1.00 V
Jsc: 18.18 mA cm
FF: 0.67
-2
3
0, 31
voltage.
-8
Monomolecular recombination, also referred to as Shockley-
Read-Hall (SRH) recombination, is a first-order recombination
process in which electrons and holes recombine through trap
PCE: 12.2%
-
12
4
00
600
800
Wavelength (nm)
3
1, 32
sites in the bulk of an absorber or at interfaces in devices.
-16
By surface modification using the fullerene derivative, we
expect that the trap sites in the ZnO film were significantly
reduced, thereby enhancing the device performance. Although
the trap-filling mechanism is not clearly understood at this
moment, we speculate that there are some charge transfer
reactions that occur from the PCBM to ZnO due to the
difference in the WF or a chemical interaction that fills the trap
0
.0
0.2
0.4
Voltage (V)
Figure 5. (a) Current density-voltage characteristics of the best planar-
heterojunction perovskite solar cells with structure of
ITO/ZnO/PCBM/CH NH PbI /spiro-OMeTAD/MoO /Ag measured
0.6
0.8
1.0
a
3
3
3
x
-
2
1
8, 33
under AM 1.5G irradiation at 100 mW cm . EQE spectrum of the
device is shown in the inset.
sites in the ZnO.
To further reveal the trap-filling
mechanism is beyond the scope of this work, and study of the
mechanism is underway in our laboratory.
ITO/ZnO/PCBM/CH NH PbI /sprio-OMeTAD/MoO /Ag,
3
3
3
3
under AM 1.5 G irradiation with irradiation intensity of 100
The charge recombination processes were also investigated
-
2
mW cm and the corresponding external quantum efficiency
EQE) spectrum (inset). As shown in Figure 5, the PHJ
perovskite solar cell showed a very high PCE of 12.2% with a
by extracting the recombination resistance (R ) from
rec
(
impedance spectroscopy (IS) measurements of the perovskite
solar cells. Generally, the features observed in the low-
frequency region in the IS spectra are closely related with
sc
-2
J
of 18.18 mA cm , V of 1.00 V, and FF of 67%. The
oc
outperforming efficiency of the spiro-OMeTAD device
compared to the efficiency of the PTB7-Th device of PTB7-Th
charge recombination processes, and the R is obtained by
rec
3
4, 35
fitting the Nyquist plots in this region.
Figure 4b shows the
is attributed to the higher J , which is mainly derived from
sc
Rrec values of the perovskite solar cells with ZnO and
ZnO/PCBM at different applied voltages under 1-sun
illumination. As shown in Figure 4b, the device with
ZnO/PCBM exhibited higher Rrec values (lower recombination
rate) compared to those of the device containing only ZnO at
low applied bias, indicating that surface charge recombination
efficient light harvesting with EQE values approaching 80%
over the wavelength range of 400 ~ 650 nm, as shown in the
inset of Figure 5. It is noteworthy that the results obtained for
our optimized device are among the best reported for state-of-
the-art low-temperature solution-processed photovoltaics.
3
6
was significantly suppressed by the interfacial modification.
Conclusions
Interestingly, the interfacial modification of ZnO also decreased
the extent of bulk charge recombination, which corresponds to
In conclusion, we successfully demonstrated efficient PHJ
perovskite solar cells with a high PCE of 12.2% by fine-tuning
the interfacial properties of a sol-gel-processed ZnO ECL using
PCBM as an organic surface modifier. By depositing PCBM
onto the ZnO film, the Voc values of the devices with
ZnO/PCBM substrates increased to ~ 1.03 V compared to the
Voc of ~ 0.83 V measured for devices with only ZnO due to the
modified electronic structure of the ZnO layer, as evidenced by
the results of photoelectron spectroscopy analysis. In addition,
trap-assisted charge recombination at the interface of ZnO and
in the bulk of the perovskite absorber was effectively
suppressed by the interfacial modification, thereby improving
the performance of the devices. These results highlight the
importance of interfacial engineering as a new strategy for
improving the PCEs of planar-structured perovskite solar cells.
By further optimization and by pursuing a rational interfacial
engineering design, we expect that greater performance
enhancements can be achieved in the near future.
3
7
the higher Rrec values obtained at high applied bias. We
speculate that the reduced bulk charge recombination is
attributed to an increased built-in potential, due to the modified
electronic structure of the ZnO/PCBM substrate, which
facilitated more efficient sweeping of photo-generated charge
1
7
carriers. Therefore, based on the light-intensity dependence of
Voc and the IS measurements, it is confirmed that the
interfacial modification of the ZnO substrate with PCBM
significantly reduced trap-assisted charge recombination at the
interfaces as well as in the absorber of the devices, thereby
enhancing the device performance.
To further optimize the perovskite solar cells, we
incorporated 2,2′,7,7′-tetrakis(N,N- di-p-methoxyphenylamine)-
,9′-spirobifluorene (spiro-OMeTAD) as the HTL which is the
9
most widely used organic transport layer in SDSSCs and
perovskite solar cells due to its excellent optical transparency in
3
8, 39
the visible range.
Figure 5 shows the current density-
voltage (J-V) characteristics of the devices with a structure of
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Journal of Materials Chemistry A
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COMMUNICATION
Journal Name
Acknowledgements
This work was supported by the Core Technology Development
16. Y. Sun, J. H. Seo, C. J. Takacs, J. Seifter and A. J. Heeger, Adv.
Mater., 2011, 23, 1679-1683.
Program for Next-Generation Solar Cells of the Research Institute 17. A. K. K. Kyaw, D. H. Wang, V. Gupta, J. Zhang, S. Chand, G. C.
for Solar and Sustainable Energies (RISE) at GIST and the National Bazan and A. J. Heeger, Adv. Mater., 2013, 25, 2397-2402.
Research Foundation (NRF) of Korea (NRF- 18. S.-H. Liao, H.-J. Jhuo, Y.-S. Cheng and S.-A. Chen, Adv. Mater.,
014R1A2A1A09006137). We also acknowledges the support 2013, 25, 4766-4771.
provided by the National Research Foundation of Korea (NRF) grant 19. M. C. Gwinner, R. D. Pietro, Y. Vaynzof, K. J. Greenberg, P. K. H.
2
funded by the Korea government (MSIP) (No. 2008-0062606,
CELA-NCRC).
Ho, R. H. Friend and H. Sirringhaus, Adv. Funct. Mater., 2011, 21,
1432-1441.
2
0. M. Kröger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky and A. Kahn,
Notes and references
Appl. Phys. Lett., 2009, 95, 123301-1-3.
a
Department of Nanobio Materials and Electronics, Gwangju Institute of 21. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J.
Science and Technology, Gwangju 500-712, Republic of Korea.
Snaith, Science, 2012, 338, 643-647.
b
School of Materials Science and Engineering, Gwangju Institute of 22. S. Ryu, J. H. Noh, N. J. Jeon, Y. C. Kim, W. S. Yang, J. W. Seo and
Science and Technology, Gwangju 500-712, Republic of Korea.
S. I. Seok, Energy & Environmental Science, 2014.
c
Heeger Center for Advanced Materials & Research Institute for Solar 23. L. J. A. Koster, V. D. Mihailetchi, R. Ramaker and P. W. M. Blom,
and Sustainable Energies, Gwangju Institute of Science and Technology,
Gwangju 500-712, Republic of Korea.
Appl. Phys. Lett., 2005, 86, -.
24. M. Lenes, S. W. Shelton, A. B. Sieval, D. F. Kronholm, J. C.
Hummelen and P. W. M. Blom, Adv. Funct. Mater., 2009, 19, 3002-
3007.
†
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.
25. S. R. Cowan, W. L. Leong, N. Banerji, G. Dennler and A. J. Heeger,
Adv. Funct. Mater., 2011, 21, 3083-3092.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See 26. D. Bi, S.-J. Moon, L. Haggman, G. Boschloo, L. Yang, E. M. J.
DOI: 10.1039/c000000x/ Johansson, M. K. Nazeeruddin, M. Gratzel and A. Hagfeldt, RSC
Advances, 2013, , 18762-18766.
7. D. Bi, G. Boschloo, S. Schwarzmuller, L. Yang, E. M. J. Johansson
and A. Hagfeldt, Nanoscale, 2013, , 11686-11691.
J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. 28. J. You, Z. Hong, Y. Yang, Q. Chen, M. Cai, T.-B. Song, C.-C. Chen,
Nazeeruddin and M. Gratzel, Nature, 2013, 499, 316-319. S. Lu, Y. Liu and H. Zhou, ACS Nano, 2014, , 1674-1680.
D. Liu and T. L. Kelly, Nat. Photon., 2014, , 133-138. 29. N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu and S. I. Seok,
Nat. Mater., 2014, advance online publication
3
2
1
2
.
.
M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, 395-398.
5
8
3
4
.
.
8
P.-W. Liang, C.-Y. Liao, C.-C. Chueh, F. Zuo, S. T. Williams, X.-K.
Xin, J. Lin and A. K. Y. Jen, Adv. Mater., 2014, n/a-n/a.
.
30. H. J. Snaith, L. Schmidt-Mende, M. Grätzel and M. Chiesa, Phys.
Rev. B, 2006, 74, 045306.
5
6
.
.
G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S.
Mhaisalkar and T. C. Sum, Science, 2013, 342, 344-347.
31. S. R. Cowan, A. Roy and A. J. Heeger, Phys. Rev. B, 2010, 82
,
S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P.
245207.
Alcocer, T. Leijtens, L. M. Herz, A. Petrozza and H. J. Snaith, 32. D. Credgington, F. C. Jamieson, B. Walker, T.-Q. Nguyen and J. R.
Science, 2013, 342, 341-344. Durrant, Adv. Mater., 2012, 24, 2135-2141.
J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal and S. I. Seok, Nano 33. H. Ma, H.-L. Yip, F. Huang and A. K. Y. Jen, Adv. Funct. Mater.,
Lett., 2013, 13, 1764-1769. 2010, 20, 1371-1388.
Q. Chen, H. Zhou, Z. Hong, S. Luo, H.-S. Duan, H.-H. Wang, Y. Liu, 34. V. Gonzalez-Pedro, E. J. Juarez-Perez, W.-S. Arsyad, E. M. Barea, F.
7
8
9
1
.
.
.
G. Li and Y. Yang, J. Am. Chem. Soc., 2013, 136, 622-625.
G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely and H. J.
Snaith, Adv. Funct. Mater., 2014, 24, 151-157.
Fabregat-Santiago, I. Mora-Sero and J. Bisquert, Nano Lett., 2014, 14
888-893.
,
35. A. Dualeh, T. Moehl, N. Tétreault, J. Teuscher, P. Gao, M. K.
0. J. T.-W. Wang, J. M. Ball, E. M. Barea, A. Abate, J. A. Alexander-
Nazeeruddin and M. Grätzel, ACS Nano, 2013, 8, 362-373.
Webber, J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H. J. Snaith 36. E. J. Juarez-Perez, M. Wuβler, F. Fabregat-Santiago, K. Lakus-
and R. J. Nicholas, Nano Lett., 2013, 14, 724-730.
Wollny, E. Mankel, T. Mayer, W. Jaegermann and I. Mora-Sero, The
Journal of Physical Chemistry Letters, 2014, , 680-685.
37. B. Suarez, V. Gonzalez-Pedro, T. S. Ripolles, R. S. Sanchez, L. Otero
1
1
1
1
1
1. K. Wojciechowski, M. Saliba, T. Leijtens, A. Abate and H. J. Snaith,
5
Energy & Environmental Science, 2014,
2. J. M. Ball, M. M. Lee, A. Hey and H. J. Snaith, Energy &
Environmental Science, 2013, , 1739-1743.
7, 1142-1147.
and I. Mora-Sero, The Journal of Physical Chemistry Letters, 2014, 5,
6
1628-1635.
3. W. J. E. Beek, M. M. Wienk, M. Kemerink, X. Yang and R. A. J. 38. J. H. Noh, N. J. Jeon, Y. C. Choi, M. K. Nazeeruddin, M. Gratzel and
Janssen, The Journal of Physical Chemistry B, 2005, 109, 9505-9516. S. I. Seok, Journal of Materials Chemistry A, 2013, , 11842-11847.
4. Z. E. Allouni, M. R. Cimpan, P. J. Høl, T. Skodvin and N. R. Gjerdet, 39. D. Bi, L. Yang, G. Boschloo, A. Hagfeldt and E. M. J. Johansson,
1
Colloids and Surfaces B: Biointerfaces, 2009, 68, 83-87.
5. M. R. Arefi and S. Rezaei-Zarchi, International Journal of Molecular
Sciences, 2012, 13, 4340-4350.
The Journal of Physical Chemistry Letters, 2013, 4, 1532-1536.
6
| J. Name., 2012, 00, 1-3
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Page 7 of 7
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C4TA03954H
Graphics for Contents
Efficient Planar-Heterojunction Perovskite Solar Cells Achieved
via Interfacial Modification of Sol-gel ZnO Electron Collection
Layer
a
b
c
a
b
Junghwan Kim, Geunjin Kim, Tae Kyun Kim, Sooncheol Kwon, Hyungcheol Back,
a
c
b
*a,b,c
Jinho Lee, Seoung Ho Lee, Hongkyu Kang and Kwanghee Lee
The importance of interfacial engineering as a new strategy for improving the power
conversion efficiencies (PCEs) of planar-heterojunction (PHJ) perovskite solar cells is
highlighted by incorporating sol-gel ZnO modified with PCBM.