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Ming-Wei Lin, Kuo-Chin Wang, Jeng-Han Wang, Ming-Hsien Li, Yu-Ling Lai, Takuji Ohigashi,
Nobuhiro Kosugi, Peter Chen,* Der-Hsin Wei, Tzung-Fang Guo, and Yao-Jane Hsu*
costs play an important role in conventional thin-film technologies. The aim
for power conversion efficiency (PCE)
comparable with that of an organic solar
cell is generally limited by the energetic
loss involved in separating or transferring the bound charges from the absorber
following photoexcitation. A way to
overcome this limitation is to employ
solution-processable thin-film absorbers,
in which the photoinduced electrons
and holes can separate with minimal
cost of energy. As an emerging solutionprocessable material, an organic−inorganic perovskite hybridized compound(CH3NH3PbI3, also denoted as MAPbI3)
attracts intensive attention for optoelectronic applications.[1–9] Early work focusedon utilizing small MAPbI3 nanoparticles
to sensitize TiO2 with an iodide liquid
electrolyte, and produced a PCE below 7% because of the instability of the perovskite in a highly polar liquid electrolyte.[2,3]A remarkable advance for perovskite solar cells (PSC) was
obtained on replacing the liquid hole-transporting medium
with a solid-state organic hole-transport material (HTM) composed of 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′bifluorene (spiro-OMeTAD).[6] Through controlling the formation of the perovskite layer and the contact electrode materials, PSC at the state of the art have a remarkably improved
PCE over 19%.[10,11] PSC employing inorganic HTM (i.e.,nickel oxide (NiO), copper iodine (CuI), copper(I) thiocyanate(CuSCN), and graphene oxide) currently attract much attention
because of their greater mobility, effective transparency in thevisible region, and satisfactory chemical stability.[12–16] Amongthese inorganic HTM, NiO is promising to be a cathode interlayer candidate for PSC because of its wide bandgap and electronic properties, such as control of Ni:O stoichiometry andappropriate alignment of the band-edge position with thatof perovskite. The first report of PSC based on a metal-oxidep-type semiconductor of nickel-oxide (NiOx) as hole-transportlayer delivered PCE = 7.8% using a structure similar to that oforganic photovoltaics.[17] Since then, PSC employing NiO asa p-type contact have been widely investigated, with remarkable progress in photovoltaic performance. A comprehensive
review summarizes the recent progress of NiO p-type-contactbased PSC in detail.[15] A perovskite device at the state of theOrganometal-trihalide-perovskite-based solar cells have exhibited highefficiencies when incorporated into mesoscopic NiO (NiOnc) hole-transportlayers. The integration of a NiOnc-perovskite heterojunction provides an inorganic alternative as a p-type contact material with efficient hole extraction forperovskite-based solar cells. Herein the origin of such highly efficient carriertransport is studied in terms of electronic, chemical and transport properties of a NiOnc-perovskite heterojunction with X-ray photoelectron spectra,
ultraviolet photoelectron spectra, near-edge X-ray absorption fine structure
spectra, a scanning transmission X-ray microscope, and calculations ofelectronic structure. A pronounced chemical redox reaction is found at an
NiOnc-perovskite heterojunction such that PbI2 is oxidized to PbO with subsequent formation of hole-dopant CH3NH3PbI3–2δOδ at the heterojunction. The
generation of hole-doping CH3NH3PbI3–2δOδ induced by the redox reaction
at the NiOnc/perovskite heterojunction plays a significant role to facilitate thecarrier transport, and thus enhances the photovoltaic efficiencies.

1. Introduction
Solution-processable organic photovoltaic cells with merits ofmass production, versatile functionality, and low production
Dr. M.-W. Lin, Y.-L. Lai, Dr. D.-H. Wei, Dr. Y.-J. Hsu
National Synchrotron Radiation Research Center
Hsinchu 30076, Taiwan, Republic of China
E-mail: yjhsu@nsrrc.org.tw
Dr. K.-C. Wang, Dr. M.-H. Li,
Dr. P. Chen, Prof. T.-F. Guo
Department of Photonics
National Cheng Kung University
Tainan 70101, Taiwan, Republic of China
E-mail: petercyc@mail.ncku.edu.tw
Dr. J.-H. Wang
Department of Chemistry
National Taiwan Normal University
Taipei 10610, Taiwan, Republic of China
Dr. T. Ohigashi, Prof. N. Kosugi
UVSOR Synchrotron
Institute for Molecular Science
Myodaiji, Okazaki 444-8585, Japan
Dr. P. Chen, Prof. T.-F. Guo, Dr. Y.-J. Hsu
Research Center for Energy Technology and Strategy (RCETS)
National Cheng Kung University
Tainan 70101, Taiwan, Republic of China

DOI: 10.1002/admi.201600135

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Improve Hole Collection by Interfacial Chemical Redox
Reaction at a Mesoscopic NiO/CH3NH3PbI3 Heterojunction
for Efficient Photovoltaic Cells

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art using pulsed-laser deposition of a high-quality nanostructured p-type NiO electrode achieved PCE = 17.3% which is near
the best inverted p-i-n PSC (18.1%) using PEDOT:PSS.[18,19]
These results demonstrate the promising potential of replacingorganic HTM or a hole extraction layer with inorganic NiO.
Since that hole extraction is highly dependent on offsets
of energy levels at the perovskite/HTM interfaces.[20–25] NiO
exhibits an appropriate alignment of the valence-band (VB)
edge with that of perovskite, which is favorable for efficienthole extraction as demonstrated from the photoluminescence(PL) and transient absorption spectra,[13,14,26] but these reported
energy levels −5.2 eV (VB of NiO) and −5.4 eV (VB of perovskite) are generally measured with pristine materials. The real
chemical environment at the real NiO/perovskite heterojunction remains unknown. The aim of our present work is to scrutinize the interfacial chemical properties with various surfacesensitive spectral techniques and calculations of electronic
structure. We found that a chemical redox reaction occurredat the NiO/perovskite heterojunction, which might modify theenergy levels at the interface and contribute some impact on
the photovoltaic performance.

2. Results and Discussion
The related current–voltage (I–V) characteristics of photovoltaic
devices made of mesoscopic NiOnc/perovskite/PC61BM heterojunction solar cell have been published in our previous
work.[13] The best-performing cell using PC61BM delivered highVoc of 1040 mV with short-circuit photocurrent density (Jsc) of
13.24 mA cm−2 and 69% fill factor, leading to an overall PCE
of 9.51%. Herein we focus on the origin of such highly efficient carrier transport in terms of electronic, chemical, andtransport properties of a NiOnc-perovskite heterojunction. We
applied deposition in two steps to prepare the MAPbI3 perovskite. In this process, PbI2 dissolved in N,N-dimethylformamide
was spin-coated onto a NiOnc substrate; the substrate was thendipped into a CH3NH3I (MAI) solution that was dissolved in
2-propanol to produce MAPbI3. Figure 1a shows the depth
profile of a survey X-ray photoelectron spectra (XPS) for ITO/
NiOnc/MAPbI3 etched with sputtered Ar ions. In the figure, ablue line, representing ITO/NiOnc/MAPbI3 before etching, displays several evident signals for carbon 1s (C 1s) and nitrogen
1s (N 1s), lead 4f (Pb 4f), and iodine 3d (I 3d) that reveal characteristic signals of perovskite MAPbI3. The oxygen (O) andnickel (Ni) that are features of NiOnc were not observed becauseof the surface sensitivity of XPS. For the surface sputtered with
Ar ions for 90 min, the red spectrum displays clear evident Ni
2p, O 1s, Pb 4f, and I 3d features. The broad emission locatedat 250 eV is assigned as Ar 2p1/2 and Ar 2p3/2. Besides, the peaklocated at around 320 eV is assigned as Ar 2s. These signals are
arising from Ar sputter. As the atomic sensitivity factors[27,28] of
C 1s and N 1s are a tenth those of Pb 4f and I 3d, the intensities
of C 1s and N 1s are small in the MAPbI3 and greatly decreased
after ion sputtering. To examine the distribution of composition near the interface between perovskite and bottom NiOnc,
we estimated the stoichiometry of perovskite at the top and at
the NiOnc/MAPbI3 interface with an intensity ratio of Pb 4f andI 3d, described by
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Figure 1. a) XPS depth profile of survey spectra for an ITO/NiOnc/
MAPbI3 perovskite sample etched with sputtered Ar ions. The blue line
represents before etching, and red line is the surface sputtered by Ar ionsfor 90 min. The ITO/MAPbI3 in the black line is fabricated for comparison.
The incident synchrotron photon energy is 1100 eV. b) Schematic of the
ITO/NiOnc/MAPbI3 structure. The scheme illustrates a gradient structureof a NiOnc/MAPbI3 perovskite layer. Pristine MAPbI3 is on top followedby an interface layer near the NiOnc-MAPbI3 perovskite heterojunction.

 I I 3d/ASFI 3d  4f IPb /ASFPb 4f 

(1)

We found that the value varied from the top perovskite layer(≈4) to near the interface layer (≈3), which implies that a composition gradient exists in the perovskite film, especially at theinterface region. Compared with ITO/MAPbI3 (black line in
Figure 1a), the spectrum demonstrates a value ≈4, which differs from sputtered samples. The results indicate that NiOnc is
involved in an interfacial reaction at the heterojunction. On thewhole, the analysis of the depth profile of the NiOnc/MAPbI3
perovskite layer discloses that the composition is not homogeneous as expected, but exhibits a distribution dependent on
depth, especially near the NiOnc-perovskite interface. Figure 1b is
a schematic illustration of a varied structure of a NiOnc/MAPbI3
perovskite layer; the pristine MAPbI3 is at the top, followed by aninterface layer near the NiOnc-MAPbI3 perovskite heterojunction.
To clarify the composition of the interface layer, we inspected
the chemical states and electronic structures of the perovskite
layer with synchrotron XPS and near-edge X-ray absorption© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 2. XPS of a) Pb 4f7/2, b) I 3d5/2, c) Ni 2p3/2, and d) O 1s of ITO/NiOnc/PbI2, ITO/NiOnc/MAPbI3, with ITO/NiOnc and ITO/PbI2 samples forcomparison.

fine structure (NEXAFS) spectra. XPS of Pb 4f7/2 for ITO/
NiOnc/PbI2 and ITO/NiOnc/MAPbI3 are shown in Figure 2a.
The Pb 4f7/2 spectrum displays binding energy (BE) 138.2 eV
for ITO/NiOnc/PbI2 (as seen in Figure 2a-top), but smaller BE
137.9 eV for ITO/NiOnc/MAPbI3 (as seen in Figure 2a-middle).
These Pb XPS results show a negative BE shift 0.3 eV when
ITO/NiOnc/PbI2 further reacted with the MAI solution.[29] The

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spin-orbit splitting between Pb 4f7/2 and Pb 4f5/2 is 4.9 eV
for all cases (Pb 4f5/2 not shown herein), which agrees satisfactorily with the literature.[29–31] On spectral deconvolution, the ITO/NiOnc/PbI2 sample exhibits three features at BE
138.2, 137.8, and 136.7 eV, which are attributed separately to
PbI2, PbO,[32] and Pb. The Pb 4f7/2 spectrum of ITO/NiOnc/
MAPbI3 is composed of three signals at BE 137.9, 137.5, and© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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136.7 eV, which are assigned to MAPbI3, MAPbI3–2δOδ, andPb, respectively. Relative to the Pb 4f7/2 signal of ITO/PbI2 in
Figure 2a-bottom, similar signals of PbI2 at 138.2 eV and Pb
at 136.7 eV are simultaneously detected for ITO/PbI2 and ITO/
NiOnc/PbI2, but the PbO component is not found for the ITO/
PbI2 sample that has a narrower Pb 4f7/2 signal. From an analysis of the composition, we found a new feature of MAPbI3–2δOδ
in ITO/NiOnc/MAPbI3 and PbO in ITO/NiOnc/PbI2.
Figure 2b shows an I 3d5/2 signal at binding energy 618.9 eV
for ITO/NiOnc/PbI2 and 618.6 eV for ITO/NiOnc/MAPbI3 with
spin-orbit splitting 11.5 eV for I 3d3/2 for all samples.[29,31] Withdeconvolution, we found that the ITO/NiOnc/PbI2 spectrum wascomposed of two clear features at BE 619.9 and 618.9 eV, which
are assigned to I2 and PbI2, respectively. From the corresponding
I 3d5/2 of reference sample ITO/PbI2 (in Figure 2b-bottom), we
observed a major signal at BE 618.9 eV for PbI2 and a weak I2
feature at 619.9 eV. The residual I2 is more noticeable for ITO/
NiOnc/PbI2 than for ITO/PbI2, which might be caused by the
reduction of NiO to metallic Ni (detailed discussion follows).
After the ITO/NiOnc/PbI2 sample was immersed in MAI solution,
the I 3d signal comprised three discernible features at 619.9 eV
for I2, 619.6 eV for MAPbI3–2δOδ, and 618.6 eV for MAPbI3.
Ni 2p3/2 spectra are shown in Figure 2c. For the ITO/NiOnc
sample (Figure 2c-bottom), two features at 854.1 and 855.7 eV
are attributed to NiO and Ni2O3 states, respectively.[33–35] The
peak located at 861 eV is attributed to satellite feature.[36] After
deposition of PbI2 on NiOnc (Figure 2c-top), an additional feature appearing at smaller binding energy, 853.0 eV, is assigned
to metallic Ni.[33–35] On the ITO/NiOnc/MAPbI3 surface(Figure 2c-middle), not much variation after MAI deposition on
the ITO/NiOnc/PbI2 surface was observed, except a decreased
intensity of metallic Ni at 853.0 eV. This result indicates thatthe signal of metallic Ni at 853.0 eV originated from reductionof NiO interacting with PbI2. The metallic Ni may effectively
decrease the contact resistance of anode/active layer interface,which is favorable for hole extraction from active layer to theanode.[37] On the other hand, from the thermal dynamic pointof view, we cannot exclude the formation of NiI2 as NiO reactedwith PbI2, because the peaks of Ni 2p and I 3d for NiI2 arealmost overlapped with Ni 2p of Ni2O3 and I 3d of PbI2 in XPS.
As a result, the metallic Ni or NiI2 was formed at the interface.
Corresponding O 1s spectra appear in Figure 2d. The
features of Ni2O3, NiO, and PbO are clearly seen at 531.4,
529.6, and 528.1 eV after PbI2 deposition on NiOnc; shown in
Figure 2d-top, the former two signals are at the same position
with ITO/NiOnc (Figure 2d-bottom). As a result, the formationof metallic nickel and PbO indicates an interfacial redox reaction when NiOnc was processed with PbI2. Through pouring
an MAI solution on ITO/NiOnc/PbI2 to form the perovskite,
PbO became involved to form MAPbI3–2δOδ at 527.8 eV. In
consequence, the XPS indicate that a chemical redox reaction
occurred at the heterojunction of NiOnc/MAPbI3. To explorein further detail beneath the NiOnc, we recorded NEXAFS
spectra with Auger electron yield (AEY) and total electron
yield measurements (TEY). Because NEXAFS directly probes
the unoccupied electronic states, it is powerful to examine theredox species from the interface near the NiOnc substrate.
Figure 3a,b shows NEXAFS spectra of the Ni L3-edge for
ITO/NiOnc, ITO/NiOnc/PbI2, and ITO/NiOnc/MAPbI3 in TEY
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and Auger electron spectra (AES) modes, respectively. BothTEY spectra and AES are indicated to show a gradual shift
toward smaller photon energy for the Ni L3-edge of ITO/NiOnc/
PbI2 (red line) and ITO/NiOnc/MAPbI3 (blue line) relative
to ITO/NiOnc (green line). The smaller energy shift for ITO/
NiOnc/PbI2 originated mainly from Ni reduction at the NiOnc/
PbI2 heterojunction. On inspection of ITO/NiOnc/MAPbI3(processed with MAI), the spectrum is similar to that of ITO/
NiOnc/PbI2 because the Ni reduction occurred at the NiOnc/
PbI2 interface. The spectral line width is noticeably smaller for
the AES than the TEY mode because AES probes the surface
near the interface, but TEY detects simultaneously both surface
and bulk information of NiOnc.
The corresponding O K-edge NEXAFS spectra of ITO/NiOnc,
ITO/NiOnc/PbI2, and ITO/NiOnc/MAPbI3 in TEY spectra and
AES are recorded in Figure 3c and 3d, separately. The TEY
spectrum of ITO/NiOnc (green line in Figure 3c) exhibits threemajor features at 532.1, 537.4, and 540.4 eV, which are assigned
to band transitions mainly from O 1s to Ni 3d-O 2p hybridized
orbitals for the first line and Ni 4s4p-O 2p for the other twolines. This result indicates that the spectrum of pristine NiO
agrees satisfactorily with previous data.[38–44] The O K-edge TEY
spectra of ITO/NiOnc/PbI2 (red line) and ITO/NiOnc/MAPbI3(blue line) are similar to those of ITO/NiOnc (green line). As aresult, the O K-edge in the TEY mode illustrates that the variation caused by the redox reaction at the heterojunction of PbI2
and MAPbI3 on NiOnc is limited. In contrast, the AEY spectra
shown in Figure 3d demonstrate an additional distinct feature at 534.5 eV for ITO/NiOnc/PbI2, and at 535.1 eV for ITO/
NiOnc/MAPbI3 (also seen in the inset), beyond three major signals specified above, O 1s to Ni 3d-O 2p at 532.1 eV and to Ni
4s4p-O 2p at 537.4 and 540.4 eV. These additional features areapparently produced by the contact reaction at the heterojunction of NiOnc/PbI2 and NiOnc/MAPbI3. The intensity of thesignal at 532.1 eV gradually decreased, but the other features
about 535–545 eV became broader because of a new shoulder
feature emerging at a greater photon energy for NiOnc/PbI2 andNiOnc/MAPbI3.
For the ITO/NiOnc sample, the feature at 532.1 eV is a transition of the O 1s electron to unoccupied Ni 3d-O 2p hybridized states that has a main character of Ni 3d. The shape andintensity of the feature hence depend on the Ni site symmetry,the occupation number of the d levels and the O Ni bondlength.[45–47] Upon deposition of PbI2, the decreased intensity of the feature at 532.1 eV reveals unambiguously that the
3d8(Ni2+) – 2p6(O2−) mixing is weakened through the interfacial
redox reaction. The reduction of Ni2+ to Ni0 leads to a decreased
number of 3d vacancies of nickel, and hence results in adecreased hybridization of O 2p states with the Ni 3d states.[48]The other signals of pure NiOnc about 535–545 eV are transitions to O 2p containing a mixed character of O 2p, Ni 4s,
and 4p states.[46] Similarly, upon PbI2 deposition, the variationof the number of 4s and 4p vacancies also leads to a changeof 4s4p(Ni2+) – 2p(O2−) mixing, and hence to a variation of thesignal intensities. As a result, the interfacial redox reaction
diminishes the Ni site symmetry and distorts the local structurearound the O or Ni atom, resulting in an increased degree ofdisorder in the NiOnc system, and hence altering the intensities
and shapes of the scattering resonances at 537.4 and 540.4 eV.

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Figure 3. NEXAFS spectra of the Ni L3-edge in a) TEY and b) AES modes, and O K-edge spectra in c) TEY and d) AES modes for ITO/NiOnc, ITO/
NiOnc/PbI2, and ITO/NiOnc/MAPbI3. The green line represents ITO/NiOnc, the red line ITO/NiOnc/PbI2, and the blue line ITO/NiOnc/MAPbI3.

Mesoporous NiO is effectively used as an anode material
to grow CH3NH3PbI3 perovskite for a highly efficient organic
solar cell. The redox reaction of PbI2 with a mesoporous NiOelectrode determined by XPS, NEXAFS spectra in AES and TEY
modes can be described as follows.

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NiO + PbI2 → PbO + Ni + I2 orPbO + NiI2

(2)

PbI2 + CH3NH3I → CH3NH3PbI3(3)

PbO + CH3NH3I → CH3NH3PbI3– 2δ Oδ (MAPbI3– 2δ Oδ )(4)

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As shown in Equation (2), an interfacial redox reaction
occurs between PbI2 and NiOnc; PbI2 and NiOnc reacted to form
PbO when PbI2 was spin-coated on NiOnc. In addition to theformation of perovskite from PbI2 and CH3NH3I, the consequent PbO reacts with CH3NH3I to produce oxygen-substituted
perovskite, MAPbI3–2δOδ. It is worth noting that the formationof PbO, MAPbI3–2δOδ, I2, and Ni is clearly observed in XPS and
AEY spectra as mentioned above (shown in Figures 2 and 3).
The oxygen in CH3NH3PbI3 perovskite thus leads to a doping
effect, hence influencing the transport of charge carriers to
the mesoporous NiO electrode. In the literature, some authors

predicted that the defects induced on substitution of Pb or I
in CH3NH3PbI3 might produce new intraband states in the
bandgap to attract an electron or hole.[49–51] Based on their
theoretical work, Yan and co-workers proposed that interfaces
between the perovskite absorbers and some contact oxides
might form a new state to transfer an electron or hole, so toaffect the device performance. Although speculation explained
the improved large PCE, experimental evidence regarding
interfacial states is still unproved. Our results strongly andclearly demonstrate that interfacial reactions induce iodine substitution by O in the formation of perovskite. We expect that

Figure 4. OD images of a) O K-edge at 532.1 eV, b) Pb N-edge at 654 eV, and c) O K-edge at 534.5 eV for SiN/NiOnc/PbI2. The micro-NEXAFS spectraof d) O K-edge and e) Pb N-edge are extracted from Figure a and b, respectively. The inset in Figure d shows the O K-edge NEXAFS of pristine NiOnc.

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www.advmatinterfaces.decharacteristic absorption feature of the O K-edge at 532.1 eV
for NiOnc. The Pb OD map of the SiN/NiOnc/PbI2 as shown in
Figure 4b was recorded at 654 eV. The brighter contrast in these
two images represents greater O and Pb absorption intensities. The image mappings of O and Pb of SiN/NiOnc/PbI2 show
a reverse contrast but a similar distribution in morphology
and size. When the photon energy was tuned to 534.5 eV(Figure 4c), which marks the characteristic O signal of PbO
observed in the AES spectra as mentioned above, the mapping
also showed a distribution of morphology and size similar tothat at 532.1 eV except a darker contrast in the central spot andFigure 5. OD images of a) O K-edge at 532.1 eV, b) Pb N-edge at 654 eV, and c) O K-edge at 535.1 eV for SiN/NiOnc/MAPbI3. The micro-NEXAFS
spectra of d) O K-edge and e) Pb N-edge are extracted from Figure a and b, respectively.

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the substituent doping effect, like MAPbI3–2δOδ, might function as a bridge to influence the charge and thus modify theshort-circuit photocurrent (Jsc) and large filling factor (FF) of
the solar cell.
To examine further the interfacial redox reaction at theNiOnc and CH3NH3PbI3 heterojunction, we undertook
chemical mapping of elemental distributions within microaggregates of SiN/NiOnc/PbI2 and SiN/NiOnc/MAPbI3 with ascanning transmission X-ray microscope (STXM), shown in
Figures 4 and 5, respectively. Figure 4a shows STXM imagesof SiN/NiOnc/PbI2 from optical-density (OD) mapping of theFULL PAPER

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a brighter contrast in the vicinity. The brighter central spot isthe absorption of a larger cluster in pristine NiOnc, which wasused for image tracking during the STXM measurements. The
contrast weaker for 532.1 eV but brighter for 534.5 eV in thevicinity indicates that Ni 3d-O 2p mixing is involved in an interfacial redox reaction at the heterojunction and thus decreases
the absorption intensity of NiOnc and increases the intensity of
associated PbO. As a result, Pb is apparently bonded to O andlocated on the mesoporous NiOnc.
The micro-NEXAFS spectra extracted from the cross marks
depicted in Figure 4a and 4b are shown in Figure 4d and 4e,
respectively. The blue- and red-cross marks represent greaterand less absorption intensity for O mapping in NiOnc and viceversa for Pb mapping in PbI2. The NEXAFS spectrum corresponding to pristine NiOnc is shown in the inset of Figure 4d.

As mentioned above, the O K-edge NEXAFS spectra of
ITO/NiOnc/PbI2 exhibit a decreased intensity for the band transition of Ni 3d-O 2p and become broader for the Ni 4s4p-O 2ptransition when the interfacial redox reaction occurs. Figure 4dshows the micro NEXAFS spectrum from the blue and red
marks in Figure 4a that express a similar variation for the darkarea (red mark) but the same curve as pristine NiOnc for thebright area (blue cross). The mark with greater Pb content (redcross in Figure 4b) exhibits a large decreased intensity of theNi 3d-O 2p transition as shown in Figure 4d. Hence the greater
is the extent to which the redox reaction occurs, the less is theintensity of the feature at 532.1 eV, agreeing satisfactorily withthe results of the AES NEXAFS spectra.
The O and Pb images of SiN/NiOnc/MAPbI3 that were prepared on soaking the SiN/NiOnc/PbI2 film in MAI solution

Figure 6. a), b), and c) are Pb, I, and O partial DOS, respectively. d) Total DOS of MAPbI3 and MAPbI3–2δOδ. a–d) The black line represents MAPbI3,
the blue line represents MAPbI3–2δOδ. e,f) Calculated band structures for MAPbI3 and MAPbI3–2δOδ. The band edges are 1.94 and 1.67 eV for MAPbI3
and MAPbI3–2δOδ, respectively.

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www.advmatinterfaces.dein Figure 5a and 5b are displayed in Figure 5d and 5e for theO K-edge and the Pb N-edge, respectively. In Figure 5d, bothblue and red marks present decreased intensity for the transition Ni 3d-O 2p at 532.1 eV. The small intensity differenceof the Ni 3d-O 2p transition of SiN/NiOnc/ MAPbI3 reveals asmaller contribution from pristine NiOnc, which indicates thatthe redox reaction occurred at the NiOnc-PbI2 heterojunction.
The micro-NEXAFS spectrum of the Pb N-edge in Figure 5ealso shows a similar shape and intensity. These findings fromthe microstructures coincide with XPS and NEXAFS spectra.

Figure 7. a) Schematic illustration depicting the substitution of two iodine atoms with one O atom. MAPbI3–2δOδ facilitates hole transfer after light
extraction from MAPbI3 (Pb: tan balls; I: green balls; N: blue balls; C: cyan balls; H: white balls; O: red balls). b) Band offset of NiOnc: CH3NH3PbI3
heterojunction. The upper VB in CH3NH3PbI3–2δOδ (upper shift 0.10 eV) indicates a facilitated hole injection from CH3NH3PbI3 (−0.94 eV) to NiOnc(−0.84 eV).

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are shown in Figure 5. The O mapping obtained at 532.1(Figure 5a) and 535.1 eV (Figure 5c), the characteristic signals of NiOnc and MAPbI3–2δOδ observed in AES spectra as
mentioned above, displays variations similar to that of SiN/
NiOnc/PbI2. The Pb mapping obtained at 654 eV (Figure 5b)
demonstrates a reverse contrast for two large clusters and abrighter dense packing for the others relative to O mapping.
All results indicate the growth of perovskite, which is locatedon the NiOnc mesoporous networks after MAI treatment. The
corresponding micro-NEXAFS spectra denoted by cross marks

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Our results indicate that a redox reaction occurred at theNiOnc/PbI2 heterojunction and formed MAPbI3–2δOδ in a subsequent reaction.
To explore the role of MAPbI3–2δOδ in the relation of chargetransfer, we calculated the electronic structure (band structure and density of states) based on the perovskite unit cell
containing four MAPbI3 units with and without O doping.
In perovskite MAPbI3, MA and Pb donate one and two electrons, respectively, to three I ions, forming a band gap 1.94 eV
between unoccupied Pb p orbital and occupied I p orbital (seeblack lines in Figure 6a,b,d,e). MA lacks a significant contribution around the band edge but donates one electron to the Pb-I
framework. The minimum of the conduction band (CBM) of
MAPbI3 is mainly contributed by the Pb p orbital; the valenceband edge is mostly occupied by an I p orbital as the MA has
a limited effect in the band gap, consistent with recent investigation.[52,53] In MAPbI3–2δOδ, on replacing two I with one O,
MA and Pb also donate one and two electrons, respectively, but
to both I and O that results in a band gap 1.67 eV. The corresponding density of state (DOS) is shown in the blue lines
in Figure 6a–d; the band structure is shown in Figure 6f. The
value is 13.9% less than that of pristine MAPbI3. In contrast,
the VB and CBM are both near the Fermi edge. In consideration of the variation of the VB, the upper VB in MAPbI3–2δOδ(upper shift 0.10 eV) indicates a facilitated hole injection from
CH3NH3PbI3 (−0.94 eV) to NiOnc (−0.84 eV). To further confirm the effect of MAPbI3–2δOδ on facilitating charge transfer,the valence band edges for NiOnc, NiOnc/PbI2, and NiOnc/
MAPbI3 on ITO were determined by ultraviolet photoelectron spectra (UPS) as shown in the Supporting Information.
It clearly shows the energy matching of the NiOnc/MAPbI3
interface due to the valence band edge of NiOnc/PbI2 is shifted
toward valence band edge of perovskite. From both aspects on
modified NiOnc and interfacial redox, it clearly demonstrates
the energy matching facilitates the hole transporting from
MAPbI3 to NiOnc.
The redox behavior in a perovskite-mesoporous NiO solar
cell is schematically illustrated in Figure 7a. The dissolving
Pb2+ ions from PbI2 are oxidized to PbO when PbI2 is in contact with NiOnc. The consequent PbO then reacts with MAI toproduce hole-doping MAPbI3–2δOδ, which functions as a gap
state between the MAPbI3 and NiOnc. The band offset about
the NiO:CH3NH3PbI3 heterojunction is drawn in Figure 7b.
The reacted MAPbI3–δOδ makes feasible an opportunity totune the injection level and thus to influence the hole transport after excitation of MAPbI3. This interfacial hole doping
induced by a chemical redox reaction leads to a large Jsc anda large FF performance of PSC. Compared to the planar heterojunction of NiOx/perovskite, the NiOnc/perovskite behaves
like a bulk-heterojunction as NiOnc thoroughly contacts with
PbI2 in the process to form NiOnc/CH3NH3PbI3 interface dueto the large surface area of nanocrystalline. It is anticipatedthat the effect of CH3NH3PbI3–δOδ on facilitating the holetransport is more apparent in nanocrystalline NiOnc/perovskite than in planar NiOx/perovskite. In summary, our results
indicate that an interfacial optimized hole-transport material
for a perovskite solar cell including controlling the perovskite/ hole-transport material is essential for a highly efficientsolar cell.
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3. Conclusion
In summary, we present the electronic and chemical structuresof the heterojunction for MAPbI3 perovskite on mesoporous
NiOnc. A depth-profiling study of NiOnc/perovskite indicatesthat a chemical redox reaction occurred at the heterojunctionof NiOnc/PbI2. We found that PbO forms after PbI2 is coatedon NiOnc, which then reacts with MAI to produce MAPbI3–2δOδ
during the growth of the perovskite. Based on calculationsof electronic structure, the formation of MAPbI3–2δOδ at theNiOnc/MAPbI3 junction is favorable for hole transport from
MAPbI3 to NiOnc. This interfacial chemical redox reaction plays
a key role in achieving a large Jsc and a large FF performanceof a PSC that should be taken into consideration in designinghighly efficient PSC devices. Our results provide an improved
understanding of the interfacial chemical properties of theNiO/perovskite heterojunction. With appropriate surface engineering and control of the energetic profile, we expect highly
efficient inverted perovskite solar cells employing NiO to beachievable in the near future.

4. Experimental Section
The mesoporous NiO solution applied to form a mesoscopic layer for
spin-coating was made from an NiO slurry and anhydrous ethanol inmass ratio 1:7. The NiO slurry was prepared from NiO nanopowder(Inframat, 3 g) dissolved in ethanol (80 mL), to which was subsequently
added ethyl cellulose (15 g, 10 mass% in EtOH) and terpineol (10 g). The
solution was stirred, dispersed with an ultrasonic horn and concentrated
with a rotary evaporator to remove ethanol to 23 mbar.
CH3NH3I was synthesized according to a method described in ourprevious publication: methylamine (CH3NH2) (13.5 mL, 40 mass%in aqueous solution, Alfa Aesar) and hydriodic acid (HI) (15.0 mL,
57 mass% in water, Alfa Aesar) were reacted together in a roundbottomed flask (250 mL) at 0 °C for 2 h with stirring. After the reaction,
the solvent was evaporated with a rotary evaporator. Product methyl
ammonium iodide (CH3NH3I) was formed from the above procedure.
The precipitate was washed three times with diethyl ether (SigmaAldrich), and dried in a vacuum oven at 60 °C overnight.[5,6,17]
The mesoscopic layer of NiOnc (nc: nanocrystal) was coated on thesubstrates through spin-coating at 4000 rpm for 30 s. After annealing at
400 °C for 30 min, a p-type mesoporous network was formed, of thickness
250 nm. A sequential deposition method was adopted to fabricate an
NiOnc/MAPbI3 layer. Before deposition of PbI2, the substrates werepreheated at 70 °C. PbI2 solution (1 m in N,N-dimethylformamide) was
spun onto the mesoporous NiO film at 6500 rpm for 90 s, followed byannealing at 70 °C for 30 min. After cooling to room temperature, theresulting films were immersed in 2-propanol to pre-wet the substrates
for 5 s, dipped into CH3NH3I solution (10 mg mL−1 in 2-propanol) for
40 s at room temperature, and annealed at 70 °C for 30 min to complete
the perovskite crystallization.
Photoemission spectra and NEXAFS spectra were recorded at
U5-spectroscopy beamline 09A2 in National Synchrotron Radiation
Research Center (NSRRC) in Taiwan. To characterize the chemical andelectronic structures, test samples with configurations ITO/NiOnc/
PbI2 and ITO/NiOnc/MAPbI3 were prepared; ITO/PbI2, ITO/MAPbI3,
and ITO/NiOnc samples were fabricated as references for comparison.
The dimensions of each sample for XPS, UPS, and measurement of
NEXAFS were about 1.0 × 1.0 cm2. Images from an O K-edge and Pb
N-edge STXM were obtained at BL4U in UVSOR Synchrotron in Japan.
In the STXM mapping measurements, a monochromatic X-ray beam
was focused with a Fresnel zone plate to a 30 nm spot on the sample,
which was raster-scanned with synchronized detection of transmitted
X-rays to generate a sequence of images (stacks) over the range of© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Supporting Information
Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements
Ministry of Science and Technology (MOST 103-2112-M-213-007-MY3
and 103-2221-E-006-029-MY3) and National Synchrotron Radiation
Research Center provided financial support of this work. P.C. appreciates
financial support from the Top-Notch Project under the Headquartersof University Advancement at National Cheng Kung University, whichis sponsored by Taiwan Ministry of Education. The authors also thankthe financial support from the UVSOR-RIKEN collaboration program for
STXM experiments.
Received: February 18, 2016
Revised: May 13, 2016
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