High-performance hole-extraction layer of sol-gel-processed nio nanocrystals for inverted planar perovskite solar cells

Article abstract of DOI:10.1002/anie.201405176

Hybrid organic/inorganic perovskite solar cells have been rapidly evolving with spectacular successes in both nanostructured and thin-film versions. Herein, we report the use of a simple sol-gel-processed NiO nanocrystal (NC) layer as the hole-transport layer in an inverted perovskite solar cell. The thin NiO NC film with a faceted and corrugated surface enabled the formation of a continuous and compact layer of well-crystallized CH₃₃NH₃PbI ₃ in a two-step solution process. The hole-extraction and -transport capabilities of this film interfaced with the CH₃NH₃₃PbI ₃ film were higher than those of organic PEDOT:PSS layers. The cell with a NiO NC film with a thickness of 30-40 nm exhibited the best performance, as a thinner layer led to a higher leakage current, whereas a thicker layer resulted in a higher series resistance. With the NiO film, we observed a cell efficiency of 9.11%, which is by far the highest reported for planar perovskite solar cells based on an inorganic hole-extracting layer.

Full text of DOI:10.1002/anie.201405176

Angewandte
Chemie
DOI: 10.1002/anie.201405176
Perovskite Solar Cells
Hot Paper
High-Performance Hole-Extraction Layer of Sol–Gel-Processed NiO
Nanocrystals for Inverted Planar Perovskite Solar Cells**
Zonglong Zhu, Yang Bai, Teng Zhang, Zhike Liu, Xia Long, Zhanhua Wei, Zilong Wang,
Lixia Zhang, Jiannong Wang, Feng Yan, and Shihe Yang*
Abstract: Hybrid organic/inorganic perovskite solar cells have
been rapidly evolving with spectacular successes in both
nanostructured and thin-film versions. Herein, we report the
use of a simple sol–gel-processed NiO nanocrystal (NC) layer
as the hole-transport layer in an inverted perovskite solar cell.
The thin NiO NC film with a faceted and corrugated surface
enabled the formation of a continuous and compact layer of
well-crystallized CH3NH3PbI3 in a two-step solution process.
The hole-extraction and -transport capabilities of this film
interfaced with the CH3NH3PbI3 film were higher than those of
organic PEDOT:PSS layers. The cell with a NiO NC film with
a thickness of 30–40 nm exhibited the best performance, as
a thinner layer led to a higher leakage current, whereas
a thicker layer resulted in a higher series resistance. With the
NiO film, we observed a cell efficiency of 9.11%, which is by
far the highest reported for planar perovskite solar cells based
on an inorganic hole-extracting layer.
15%.[4] However, in all these high-efficiency cells, costly
spiro-OMeTAD is used for the hole-transport layer, which
hampers the commercialization of perovskite solar cells.[5]
Recently, an inverted thin-film perovskite solar cell was
reported in which the perovskite layer was sandwiched
between a planar layer of the electron acceptor and the
transparent hole acceptor poly(3,4-ethylenedioxythiophe-
ne):polystyrene sulfonate (PEDOT:PSS), which is widely
used in organic photovoltaic devices.[6] However,
PEDOT:PSS is less than ideal owing to its acidity, tendency
to absorb water, and inability to block electrons.[7]
tantalizing possibility is to use p-type inorganic materials
(NiO, CuSCN, etc.) to replace the PEDOT:PSS layer.[8]
A
A
salient problem with the use of a NiO hole-transport layer in
perovskite solar cells is the difficulty for it to support
a sufficiently thick perovskite film (typically < 60 nm),
which has so far limited the cell efficiency.[9] Thus, elaboration
of the NiO layer is required to enhance its anchoring
capability. An ideal p-NiO film for high photovoltaic (PV)
performance should 1) have good optical transparency,
2) prevent electron leakage, 3) have appropriate energy
levels, and 4) support a high-quality and sufficiently thick
perovskite film.[10] The last point is of particular interest and
constitutes the theme of the present study.
We synthesized a layer of well-joined nickel oxide nano-
crystals (NCs) for hole extraction and transport, each with
a size of 10–20 nm, on fluorine-doped tin oxide (FTO)
substrates. This NiO NC layer exhibited a higher transparency
than that of conventional NiO thin films, thus saving the
incident light for the active absorber. Also controlled
aggregation of the faceted NiO nanocrystals resulted in
a corrugated surface that could support an approximately
300 nm thick film of cubic CH3NH3PbI3 crystals with good
coverage and interconnectivity. These advantageous features
of the NiO NC layer led to a high efficiency of the inverted
perovskite solar cells of over 9.11%. Photoluminescence (PL)
studies revealed a higher hole-extraction ability of the NiO
NC layer than those of PEDOT:PSS and NiO thin films.
We used atomic force microscopy (AFM) and trans-
mission electron microscopy (TEM) to characterize the
morphology of the NiO NC films. The AFM image in
Figure 1A shows that high-quality NiO nanocrystals with
a size of 10–20 nm were uniformly deposited and well-
connected after spin coating and annealing of the sol–gel
precursor. For a typical synthesis of the NiO NC film, the sol
solution was prepared by dissolving nickel(II) acetylaceto-
nate in diethanolamine. The precursor was heated at 1508C to
form the sol suspension, which was then spin coated on to the
FTO, followed by annealing at 5008C. Figure 1B shows the
H
ybrid organic/inorganic perovskite materials are currently
among the most competitive absorbers for efficient solar
cells.[1] Since they were first reported, perovskite-based solar
cells have reached an efficiency of almost 16%,[2] owing to the
advantageous characteristics of perovskite materials: an
appropriate direct band gap, a high absorption coefficient,
and a propensity to form thin films with excellent carrier-
transport properties and an apparent tolerance of defects.[3]
The design versatility of perovskite solar cells has allowed
either a mesoporous metal-oxide scaffold or simply a thin-
film structure to be adopted, both with high efficiency over
[*] Z. L. Zhu,[+] T. Zhang, Prof. S. H. Yang
Nano Science and Technology Program
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong (China)
E-mail: chsyang@ust.hk
Z. L. Zhu,[+] Y. Bai,[+] T. Zhang, Dr. X. Long, Z. H. Wei, Z. L. Wang,
Prof. S. H. Yang
Department of Chemistry
The Hong Kong University of Science and Technology
L. X. Zhang, Prof. J. N. Wang
Department of Physics
The Hong Kong University of Science and Technology
Dr. Z. K. Liu, Prof. F. Yan
Department of Applied Physics and Materials Research Centre
The Hong Kong Polytechnic University, Hong Kong (China)
[+] These authors contributed equally.
[**] This research was supported by the HK-RGC General Research
Funds (HKUST 606511 and 605710).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201405176.
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did not exhibit any X-ray diffraction (XRD) peaks (Fig-
ure 1D), thus indicating that the NiO is either amorphous or
too weakly crystalline. In contrast, for the NiO NC film,
several clear peaks belonging to the cubic NiO crystal
structure in the space group Fm3m (NaCl-type) were
observed, including (111), (200), and (220).
Ultraviolet photoelectron spectroscopy (UPS) was used
to probe the work functions (F) of the NiO thin film,[11] the
NiO NC layer, and the PEDOT:PSS film (Figure 2A,B; see
Figure S2 for the full UPS spectra). The F value of the NiO
NC layer was 5.36 eV, thus indicating that the electron-donor
level of NiO NC layer matches well with the energy level of
the valence-band (VB) edge (ꢀ5.4 eV) of CH3NH3PbI3 for
facile hole extraction. However, the F values of the NiO thin
film (5.12 eV) and the PEDOT:PSS film (5.21 eV) are both
higher than the VB edge of CH3NH3PbI3: a situation
unfavorable for hole extraction. The appropriate energy-
level alignment with the NiO NC layer bodes well for the
performance of the corresponding solar-cell devices.
We further studied the NiO NCs and NiO thin film by X-
ray photoelectron spectroscopy (XPS) to gain information
about their structure and composition.[12] Figure 2C,E shows
the Ni 2p3/2 XPS spectra. The first of the two common peaks
observed, the left peak at the binding energy of 861.3 eV, is
ascribed to a shakeup process in the NiO structure, and the
second, right peak at 855.5 eV is due to the Ni2+-vacancy-
induced Ni3+ ion or nickel hydroxides (NiIIOH) and oxy-
hydroxides (NiIIIOOH). However, there is an additional
intense peak at 853.7 eV for the NiO NC film. This peak
corresponds to the Ni2+ in the standard Ni O octahedral
ꢀ
bonding configuration in cubic rock-salt NiO. This assignment
was verified by the O 1s XPS spectra in Figure 2D,F.
Although the peaks of NiOOH (531.9 eV) and NiO
(529.2 eV) appear in both samples, the huge difference in
the intensity ratio of the peaks indicates that the NiO thin film
has not been completely transformed from the precursor into
well-crystallized NiO. Thus, the XPS results proved that our
Figure 1. A) AFM roughness image of a NiO nanocrystal film on FTO.
B) Bright-field TEM image of a sample of NiO NCs with the corre-
sponding diffraction pattern (inset). C) Optical transmission spectra
and D) XRD patterns of NiO NC films grown on quartz and annealed
at different temperatures.
ꢀ
NiO NC film chiefly adopts the Ni O octahedral bonding
configuration, whereas in the NiO thin film, there exists
a large amount of NiIIO and NiIIIOOH owing to the low-
temperature process.
TFM image, which confirms the size of the NiO nanocrystals
at 10–20 nm. As a result of the heating process, the NiO NCs
aggregated together and became well-connected, which was
expected to be beneficial for hole transfer through the NiO
film. The electron-diffraction pattern (inset in Figure 1B)
shows a polycrystalline NiO structure, and from a high-
resolution TEM (HRTEM) image (see Figure S1 in the
Supporting Information), the outer d spacing of the nano-
crystals was measured to be 0.21 nm, which matches well with
the interplanar spacing of (200). We determined the optical
properties as well as the crystal and electronic structures of
the thin films (Figure 1C,D). First of all, our NiO NC film
(5008C) showed higher transparency than the low-temper-
ature NiO film (2758C; Figure 1C), which is highly desirable
for solar-cell devices because it preserves the incoming
photons for the neighboring absorber layer. The higher
transparency of our NiO NC film (5008C) is clearly due to
the annealing-induced crystallization, which eliminates defect
states and impurities. Indeed, the low-temperature NiO film
The relevant energy levels for the solar-cell device,
including the valence-band (VB) and conduction-band (CB)
edges of each layer, are shown in Figure 3A. On light
absorption, excitons are generated in the CH3NH3PbI3 layer
and then dissociated, followed by transfer of the electron to
the phenyl-C61-butyric acid methyl ester (PCBM) layer and
the hole to the NiO layer. For the cubic structure of NiO
shown in Figure 3B, the VB edge (ꢀ5.5 eV) of NiO is very
close to the VB edge of CH3NH3PbI3 (ꢀ5.4 eV), and the
Fermi level (Ef) of the NiO NCs is even higher (ꢀ5.36 eV),
thus making hole extraction favorable from CH3NH3PbI3 to
NiO NCs.
Figure 3C shows the cross-sectional-view SEM image of
a glass/FTO/NiO/CH3NH3PbI3 film. To prepare this film, we
applied the two-step CH3NH3PbI3-deposition method of
Grꢀtzel and co-workers.[4b,13] The CH3NH3PbI3 perovskite
layer can be readily distinguished with a thickness of 250 nm,
and the layer of NiO NCs has a thickness of about 40 nm. The
2
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thin films, the coverage of
CH3NH3PbI3 is much poorer than
on the layer of NiO NCs, and there
are bare areas without any perov-
skite coating. Comparison of the
cross-sectional-view images between
Figure 3C and Figure S4D (see the
Supporting Information) shows that
a thicker layer of closely joined,
larger cubic CH3NH3PbI3 crystals
could be formed on the NiO NC
layer. Clearly, the roughness of the
surface of the NiO film plays a large
role in directing the overgrowth of
the perovskite layer, especially for
two-step deposition. Understand-
ably, the roughness causes PbI2 to
be deposited in a relatively thick and
well-connected layer by high-speed
spin coating. Owing to this well-
crystallized, interconnected, and
thick CH3NH3PbI3 layer, our perov-
skite solar cell based on a NiO NC
layer could deliver higher efficiency,
as a result of a higher photocurrent
and voltage, as discussed in more
detail below.
Typical curves of photocurrent
density (J) versus voltage (V) for
solar cells with different NiO films
are shown in Figure 4A, and the
parameters for PV performance are
summarized in Table 1. To examine
the thickness effect, we fabricated
a series of perovskite solar cells with
NiO NC layers of different thick-
nesses. The champion turned out to
be the perovskite solar cell with
a NiO NC layer with a thickness of
40 nm. This solar cell exhibited the
highest power-conversion efficiency
(PCE) of 9.11%, with a short-circuit
Figure 2. Photoelectron spectroscopy of the various films studied. A,B) UPS spectra (He I radia-
tion<) of the NiO NCs, the NiO thin film, and PEDOT:PSS in the cutoff (A) and onset regions (B).
C–F) XPS spectra at the Ni 2p3/2 and O 1s core levels of a NiO NC film (C,D) and a NiO thin film
(E,F).
top-view image (Figure 3D) shows that a well-crystallized
cubic phase of the CH3NH3PbI3 perovskite layer was formed
on top of the layer of NiO NCs. For comparison, top-view and
cross-sectional images (see Figure S4) of CH3NH3PbI3 on
PEDOT:PSS and on a NiO thin-film layer formed by low-
temperature solution processing showed no clear cubic phase
of CH3NH3PbI3 deposited on PEDOT:PSS and inadequate
coverage of perovskite on the polymer surface, although the
films were formed by the same two-step deposition method. It
is possible that the N,N-dimethylformamide and isopropanol
would dissolve part of the PEDOT:PSS during spin coating of
the PbI2 precursor and dip coating in CH3NH3I solution.
Similarly, the SEM images of the low-temperature NiO thin-
film/perovskite composite (see Figure S4C,D) explain why
these films also show lower cell performance than the NiO
NC layer, as described in more detail below. To put it simply,
because of the smoother surface of the low-temperature NiO
current (Jsc) of 16.27 mAcmꢀ2, an open-circuit voltage (Voc) of
0.882 V, and a fill factor (FF) of 0.635. The perovskite solar
cell with a 20 nm layer of NiO NCs showed a PCE value of
6.04% with Jsc = 13.64 mAcmꢀ2, Voc = 0.704 V, and FF =
0.630, and the perovskite solar cell with a 70 nm NiO NC
layer showed a PCE value of 5.58% with Jsc = 10.72 mAcmꢀ2,
Voc = 0.851 V, and FF = 0.612. The performance of the
PEDOT:PSS-based perovskite solar cell is also plotted in
Figure 4. We found that at a 40 nm thickness of the NiO NC
layer, the Voc and Jsc values were highest. The improvement in
Voc and Jsc in going from the 20 nm to the 40 nm film is
ascribed to the improved electron blocking and the reduced
charge recombination and leakage current, as observed
previously by other research groups.[14] On the other hand,
for the film-thickness increase from 40 to 70 nm, the decrease
in photocurrent and voltage is most likely due to the increased
series resistance and photoabsorption of the NiOx NC film.[15]
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Figure 4. Typical J–V curves and IPCE spectra of the perovskite solar
cells. A) Photocurrent density versus applied voltage. B) IPCE spectra
of perovskite solar cells with different NiO hole-transport layers and
PEDOT:PSS.
We performed steady-state photoluminescence (PL) and
time-resolved photoluminescence (TRPL) measurements to
study exciton diffusion in the hole-transport layers. The
normalized steady-state PL spectra in Figure 5A show that
the PL quenching with the NiO NCs is the most dramatic
among all of the samples. This result shows that the NiO NC
layer is the most effective hole extractor owing to the high
interfacial film quality with intimate contact with the
perovskite and the high mobility. The TRPL profiles in
Figure 5B show that the PL decay rate is also highest for the
NiO NC layer. We fitted the PL decay curve to a biexponential
function [Eq. (1); see the Supporting Information for the
fitting result].
Figure 3. A) Energy-level-alignment diagram of the device components
(relative to the vacuum level). The dashed line shows the Fermi level
of NiO NCs from the UPS measurement. B) Crystal structure of NiO.
C) Cross-sectional-view and D) top-view SEM images of a typical
device based on the glass/FTO/NiO/CH3NH3PbI3 structure.
In comparison, the performance of the perovskite solar
cells based on the low-temperature NiO thin film and
PEDOT:PSS was generally low. The very low Voc (0.647 V)
and Jsc values (9.82 mAcmꢀ2) for the cell based on the NiO
thin film can be explained by the following factors: 1) the
higher work function, as discussed above along with the UPS
results, 2) the poorly crystallized and interconnected
CH3NH3PbI3 particles, and 3) the insufficient thickness of
the absorber layer. For the PEDOT:PSS-based cell, on the
other hand, the very large bare area not covered by
CH3NH3PbI3 appears to be the main reason for the low
efficiency. We also examined the incident-photon-to-current
efficiency (IPCE; Figure 4B). The integrated photocurrent
from the IPCE is consistent with the
ꢀ
ꢁ
ꢀ
ꢁ
t
t1
t
t2
IðtÞ ¼ A1 exp
þ A2 exp
ð1Þ
Typically, the initially photogenerated excitons diffuse
into defects, and the small time constant t1 reflects this early
event. It is the large time constant t2 that is associated with the
Table 1: Parameters for the photovoltaic devices with different transport layers.
measured Jsc value. We note that the
IPCE is similar to those reported
previously by others for inverted
perovskite solar cells, and the small
dip at 650 nm could result from
reduced absorption of the perov-
skite.[16]
NiO thickness (device)
V
OC [V]
JSC
FF
PCE [%]
Rs
Work
function [eV]
[mAcmꢀ2
]
[Wcmꢀ2
]
20 nm (NCs)
40 nm (NCs)
70 nm (NCs)
40 nm (thin film)
PEDOT:PSS
0.704
0.882
0.851
0.647
0.613
13.64
16.27
10.72
9.82
0.630
0.635
0.612
0.591
0.533
6.04
9.11
5.58
3.75
2.70
12.6
17.4
22.8
21.6
24.1
5.36
5.36
5.36
5.12
5.21
8.25
4
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processable perovskite solar cell based on a NiO nanocrystal
hole-extraction layer with an inverted planar structure rather
than a mesoporous structure. It opens the way for the design
of hybrid tandem solar cells, in which other layers could be
made, for example, on the basis of existing copper indium
sulfide or c-Si technologies.
Received: May 11, 2014
Published online: && &&, &&&&
Keywords: energy conversion · inorganic hole-extraction layers ·
.
NiO nanocrystals · organic–inorganic hybrid composites ·
perovskite solar cells
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Figure 5. A) Steady-state PL spectra (excitation at 514.5 nm (20 mW))
and B) transient PL decay (excitation at 400 nm (1 Wcmꢀ2)) of NiO
NCs (40 nm), PEDOT:PSS, and a NiO thin film interfaced with
a CH3NH3PbI3 film. The bare CH3NH3PbI3 film was used as a refer-
ence.
exciton lifetime of CH3NH3PbI3. Thus, a small t2 value will
indicate a fast exciton- or hole-diffusion process. Indeed, of all
the samples we studied, our NiO NC layer exhibited the
smallest t2 value, which corresponds to the highest hole-
extraction and -transport efficiency. These high hole-extrac-
tion and -transport rates contribute significantly to the high Jsc
value for the perovskite solar cell with a NiO NC layer.
In conclusion, we have used a sol–gel process to fabricate
a thin NiO nanocrystalline film for the development of a NiO/
CH3NH3PbI3 solar-cell device. A key finding is that the rough
NiO film surface formed by the aggregation of faceted NiO
nanoscale monocrystals permits the formation of an intimate,
large interfacial-area junction with the CH3NH3PbI3 film,
thereby dramatically enhancing the cell efficiency. More
specifically, the rough NiO surface is beneficial for the
deposition of PbI2 in the first step, and during the dipping
process, CH3NH3PbI3 experiences unconstrained crystallite
growth that occurs in the absence of a mesoporous scaffold.
Large crystallites of CH3NH3PbI3 accrete as guided by the
rough NiO surface, thus leading to a highly textured
CH3NH3PbI3/PCBM interface with dimensions commensu-
rate with the electron–hole diffusion lengths. The crystallites
also act as efficient light-scattering centers, which increase the
effective optical path length of the device. The combination of
these effects as well as the suitable energy levels of the NiO
NC layer yielded a PCE of up to 9.11%, which, to our
knowledge, is the highest reported for inverted planar
CH3NH3PbI3 solar-cell devices with an inorganic hole trans-
porter. Thus, this study has demonstrated a simple and readily
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Communications
Perovskite Solar Cells
The advantages are crystal clear: A
transparent layer of NiO nanocrystals
fabricated by a simple sol–gel processing
method acted as an efficient hole-trans-
port layer in an inverted perovskite solar
cell. With the NiO film (see picture), a cell
efficiency of 9.11% was observed: by far
the highest for a planar perovskite solar
cell based on an inorganic hole-extraction
layer.
Z. L. Zhu, Y. Bai, T. Zhang, Z. K. Liu,
X. Long, Z. H. Wei, Z. L. Wang,
L. X. Zhang, J. N. Wang, F. Yan,
S. H. Yang*
&&&& — &&&&
High-Performance Hole-Extraction Layer
of Sol–Gel-Processed NiO Nanocrystals
for Inverted Planar Perovskite Solar Cells
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!