The electronic structure of metal oxide/organo metal halide perovskite junctions in perovskite based solar cells

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SUBJECT AREAS:

SOLAR CELLS

ELECTRONIC PROPERTIES AND

MATERIALS

Received
1 September 2014

Accepted
2 February 2015

Published
3 March 2015

Correspondence and

requests for materials

should be addressed to

[L.E. (lioz.etgar@mail.](mailto:lioz.etgar@mail.huji.ac.il)

[huji.ac.il)](mailto:lioz.etgar@mail.huji.ac.il)

- These authors

contributed equally to

this work.

## The electronic structure of metal oxide/ organo metal halide perovskite junctions in perovskite based solar cells

Alex Dymshits 1 *, Alex Henning 2 *, Gideon Segev 2, Yossi Rosenwaks 2 & Lioz Etgar 1

1The Hebrew University of Jerusalem, Institute of Chemistry, Casali Center for Applied Chemistry, Jerusalem 91904, Israel,
2Department of Physical Electronics, School of Electrical Engineering, Tel Aviv University, Ramat Aviv 69978, Israel.

Cross-sections of a hole-conductor-free CH3NH3PbI3 perovskite solar cell were characterized with Kelvin
probe force microscopy. A depletion region width of about 45 nm was determined from the measured
potential profiles at the interface between CH3NH3PbI3 and nanocrystalline TiO2, whereas a negligible
depletion was measured at the CH3NH3PbI3/Al2O3 interface. A complete solar cell can be realized with the
CH3NH3PbI3 that functions both as light harvester and hole conductor in combination with a metal oxide.
The band diagrams were estimated from the measured potential profile at the interfaces, and are critical
findings for a better understanding and further improvement of perovskite based solar cells.

rgano lead halide perovskite is a promising material in photovoltaic solar cell research. During the past
five years, the power conversion efficiency of perovskite based solar cells drastically increased, exceeding
energy conversion efficiencies of 20.1% 1,2,5,8 . A long diffusion length of electrons and holes was recently
# O
observed in organo metal halide perovskite and is one of the main reasons for the high solar cell efficiency 3,4 .
Several techniques are used for the perovskite deposition including solution-processed (one-step and two-step)
deposition, evaporation method, and vapor assisted solution process (VASP) 5–9 . Solar cells with a high open
circuit voltage of more than 1.3 V were demonstrated using CH3NH3PbBr3 as the light harvester 10,11 .

Despite the rapid increase in the photovoltaic performance of perovskite based solar cells, the operation
mechanism of the metal oxide/perovskite junction is still under debate. In a recent work, the junctions of a
CH3NH3PbI32xClx based solar cell were investigated by electron beam-induced current (EBIC) and a p-i-n
junction was suggested 12 . An indication for carrier accumulation in the perovskite absorber material was observed
by impedance spectroscopy measurements 13 . Kelvin probe force microscopy (KPFM), a scanning force microscopy based technique, was recently used to characterize the perovskite top layer 14,15 where the presence of a small
potential barrier at the grain boundaries (GBs) was found. Bergmann et al. have recently measured cross-sections
of a complete perovskite based solar cell under illumination with KPFM and showed that the potential is similar to
a p-i-n type junction 16 . KPFM is based on measuring and compensating the electrostatic forces between a sample
and an AFM tip to obtain the contact potential difference (CPD) 17,18, which is a measure of the Fermi level energy
if the electron affinity is known.

In this work, we use KPFM to measure the potential along the metal oxide/CH3NH3PbI3 interface for two
different nanocrystalline (nc) metal oxides, TiO2 which functions as an electron acceptor due to the possibility for
electron injection, and porous Al2O3 which acts as a scaffold 19 . In the current solar cell configuration the
CH3NH3PbI3 functions both as a light harvester and a hole conductor, therefore the solar cell operates without
an additional hole transporting material. The chosen device structure facilitates the preparation of smooth crosssections and simplifies the KPFM measurement and its interpretation. Our high-resolution KPFM measurements
in the dark provide insight into the interface electronic structure. A surface space charge region of ca. 45 nm is
determined for the nc-TiO2/Perovskite junction and is expected to enhance charge carrier separation and solar
cell performance. The free carrier density was estimated from the band bending at the perovskite grain boundaries
induced space charge region.

Results and Discussion
Kelvin probe force microscopy on cross-sections of hole-conductor-free perovskite based solar cell in dark
(Fig. 1a) was used to determine the electronic structure of the interfaces that are critical for the performance

(a) (b)

Pt coated tip

Si substrate

CH3NH3PbI3

porous Al2O3

FTO

Vac +
VCPD

Ground

Contacts

TiO2/CH3NH3PbI3
TiO2/CH3NH3PbI3

Al2O3/CH3NH3PbI3

Figure 1 | (a) Schematic illustration of the KPFM measurement setup. The contact potential difference (CPD) is determined by compensating the
electrostatic forces between the tip and sample where nc MeOx stands for nanocrystalline metal oxide. (b) HR-SEM images of the hole-conductor-free ncAl2O3/MAPbI3 perovskite solar cell.(c) Current-voltage curves of the studied solar cells and (d) the corresponding IPCE spectra.

of these solar cells. Perovskite solar cell cross-sections were prepared
in a nitrogen glovebox by cleavage. Capillary forces were minimized
allowing a small tip-surface distance hence better lateral resolution
compared to an AFM setup in ambient. A typical structure of the
hole-conductor-free perovskite solar cell is shown in the high resolution scanning electron microscopy (HR-SEM) micrograph
(Fig. 1b). The MAPbI3 perovskite is deposited on top of the metal
oxide using the two-step deposition process described earlier 6,20 .
Table 1 and figure 1c show the photovoltaic parameters and the
current voltage curves of the hole-conductor-free MAPbI3
(CH3NH3 5 MA) solar cells using nc-TiO2 or nc-Al2O3 as the metal
oxide. Figure 1d presents the incident photon to current efficiency
(IPCE) of the corresponding cells. For both cells, the response covers
the whole visible range. However, for the nc-TiO2/MAPbI3 cell, the
IPCE reaches 75%, while for the nc-Al2O3/MAPbI3 cell, 35% IPCE is
achieved in good agreement with the current density-voltage (J-V)
characteristics measured under a solar simulator. Upon visible-light
excitation of the TiO2/MAPbI3/Au cell, electrons and holes are generated inside the perovskite layer. The electrons are then injected into
the conduction band of the TiO2 and driven towards the FTO,
whereas holes are transported through the perovskite layer to the
Au contact.

For the Al2O3/MAPbI3/Au configuration, injection of electrons
into the nanoporous Al2O3 is energetically not allowed; therefore,

the photogenerated charge carriers are transported through the
MAPbI3 film to the appropriate contacts.

The KPFM cross-section images of the two different cells in
dark, nc-TiO2/MAPbI3 and the nc-Al2O3/MAPbI3, are presented
in Figure 2a and 2b. The high spatial resolution was achieved by
maintaining the tip-sample distance below 5 nm at a slow scan
rate of 0.1 Hz. The perovskite penetrates through the nc-Al2O3,
sintered to the grounded contact; consequently, the Fermi level is
assumed to be aligned and equal in all three layers. This allows a
quantitative interpretation of the measured potential for both
solar cell structures. Figure 2c and 2d show the CPD statistical
distribution for the two interfaces; A CPD variation of 90 mV was
determined for both metal oxides and a CPD variation of 65 mV
was found for the perovskite layer. These variations are mainly
due to the nanostructure and inhomogeneous surface of the measured materials entail a high amount of defect states. The measured average potential difference, DCPD, at the interface between
nc-TiO2 and MAPbI3, as well as between the nc-Al2O3 and
MAPbI3 interface is 0.155 V and 0.12 V, respectively. Although
the nc-TiO2/MAPbI3 based solar cell has a higher efficiency, the
potential difference between the two interfaces in the dark
(35 mV) is negligible small. Some of the perovskite grains have
a lower CPD and are expected to be less active in the solar cell, as
previously observed with EBIC measurements 15 .

Table 1 | Photovoltaic parameters of the studied hole-conductor-free perovskite solar cells

Jsc (mA/cm 2 ) FF Voc (V) Efficiency (%)

nc-TiO2/MAPbI3 12.56 56 0.86 6.1
nc-Al2O3/MAPbI3 7.25 48.6 1.0 3.9

### (a) (b)

CH3NH3PbI3

CPD (V)
0.25

TiO2 Al2O3

0
#### Si Si

-0.3

### (c) (d)

∆CPD = 0.155 V ∆CPD = 0.12 V

CH3NH3PbI3

Figure 2 | KPFM images of solar cell cross-sections showing (a) the nc-TiO2/MAPbI3 and (b) the nc-Al2O3/MAPbI3 interface.The white line indicates the
CPD profiles shown in figure 4. (c) CPD statistical distribution for the CPD image of the nc-TiO2/MAPbI3 interface (d) CPD statistical
distribution for the CPD image at the Al2O3/MAPbI3 interface. The dashed line indicates the CPD profile along a grain boundary shown in figure 3b.

The CPD is higher (more positive) at the grain boundaries
between adjacent perovskite crystals by ca. 25 mV (see figure 3b),
implying a minor effect on the perovskite solar cell performance as
this potential barrier is around the thermal voltage at room temperature. Figure 3a shows a schematic band diagram for the case of
positive charge accumulation (hole traps) at the grain boundary. It
is possible to calculate the width of the barrier potential at the grain
boundaries using known barrier models 21,22 provided the grain
boundary is of negligible width compared to the grain size. This

(a) (b)

Grain boundary

Grain 1 Grain 2

wGB

model was used to analyze grain boundaries in p-type polycrystalline
silicon, II-VI semiconductors and chalcopyrite materials.

The electrostatic screening length, LS (within Ls charges are electrostatically screened; beyond this length charges are not affected by other
charges) is extracted by fitting the measured potential profile across the
GB (Fig. 3b) with an exponential function and is approximately 6.5 nm,
which is much smaller than the average grain size of the perovskite
(grains of around 200 nm can be observed in figures 2a and 2b). The
dopant concentration, Nd, can be estimated with equation 1 22,

≈ 25 mV

ECB

EVB

VGB

Figure 3 | (a) Electronic band structure at the grain boundaries where the potential is slightly higher (more p-type) compared with the grain body due to
hole traps. VGB-potential height at the grain boundaries. WGB-width of potential barrier. (b) CPD profile along the grain boundary between
two adjacent perovskite grains indicated in the CPD image of figure 2a

Figure 4 | (a) Three dimensional representation of the measured CPD and the nc-TiO2/MAPbI3 and (b) the nc-Al2O3/MAPbI3 interface. (c) CPD profile
and the corresponding electric field, dCPD/dx, for the nc-TiO2/MAPbI3 (d) for the nc-Al2O3/MAPbI3 interface. The depletion width at the
perovskite is defined as Wp.

Figure 4c and 4d show the CPD and the electric field, E 5 dCPD/dx,
across the interface for the nc-TiO2/MAPbI3 and the nc-Al2O3/
MAPbI3 cell structure, respectively. Accordingly, the depletion width
for the nc-TiO2/MAPbI3 heterojunction is <45 nm with a maximum electric field of E < 9?10 4 V/cm. In the case of the ncAl2O3/MAPbI3 interface, there is no electron injection from the
MAPbI3 into the nc-Al2O3, since it functions rather as a scaffold.
The electric field profile for the nc-Al2O3/MAPbI3 interface is narrower, showing a depletion (space charge region) width of around
Wp < 10 nm in the perovskite side of the junction (figure 4d). This
corresponds to the depletion region width at the perovskite side at
the nc-TiO2/MAPbI3 interface.

LS~

sf f i f f i f f i f f i f f

ekbT
q 2 Nd

ð1Þ

where e 5 e0ep (ep < 20 23 is the dielectric constant of the perovskite and e0 is the permittivity), kb is the Boltzmann constant, T
is the temperature and q is the elementary charge. By using
extracted LS in equation 1, the dopant density (Nd) was estimated
to be 7 3 10 17 cm 23, which is in a good agreement with reported
values observed by Guerrero et al. 24

The CPD profiles of the metal oxide/MAPbI3 interfaces, shown in
figures 4a–d, provide estimation for the junction depletion width.

ECB

EVB

(a) nc-TiO2 CH3NH3PbI3 (b)

χ2 = 3.9 eV

Φ2 = 4.8 eV

Eg2 = 1.55 eV

CH3NH3PbI3

nc-Al2O3

χ1 = 2 eV

Φ1= 4.7 eV

χ2 = 3.9 eV

= 4.1 eV

Φ1

χ1 = 4.1 eV
Φ2 = 4.8 eV

e -

= 4.65 eV

e -

Eg1 = 3.2 eV

Eg2 = 1.55 eV

h +

EVac

ECB

EVB

EVac

ECB

EVB

Figure 5 | (a) and (b) Band diagram of the TiO2/MAPbI3 and Al2O3/MAPbI3 interfaces, respectively, calculated using the KPFM measurements. The
conduction and valence bands position are according to Ref 18 and 26

Table 2 | Depletion region width, carrier density and maximum electric field at the junction

Sample Depletion width Wp/Wn (nm) Nd (Perovskite) (cm 23 ) (calculated from the grain boundaries) DCPD (V) Electric field max. (V/m)

nc-TiO2/MAPbI3 10/35 7 3 10 17 0.155 9 3 10 6

nc-Al2O3/MAPbI3 Wp < 10 7 3 10 16 0.12

Figures 5a and 5b show the suggested energy band diagrams for
the nc-TiO2/MAPbI3 and the nc-Al2O3/MAPbI3 interfaces using the
measured work functions in dark. As opposed to the nc-Al2O3, the
nc-TiO2 has an active role in the electron transport. For the nc-TiO2/
MAPbI3 interface (Fig. 5a) electron transport takes place mainly
through the nc-TiO2 since the electron injection is favorable. The
measured space charge region at this interface assists in the charge
separation and inhibits recombination which assist to the PV
performance.

For the nc-Al2O3/MAPbI3 interface, an oxide-semiconductor
band structure is proposed with a relatively small band bending at
the perovskite side. Since the 80 nm thin Al2O3 layer is nanoporous,
the perovskite penetrates through the Al2O3 enabling electron transport to the contacts. We note that experiments with an Al2O3 layer
thickness .80 nm drastically reduced the solar cell performance.

The long diffusion length assists in the charges transport to the
corresponding contacts.

Table 2 summarizes the parameters obtained in this study. The
carrier density of the perovskite was calculated using the electronic
barrier model; the depletion region width and the electric field were
estimated from the cross section KPFM measurements.

Conclusions
Cross-sections of hole-conductor-free perovskite solar cells were
measured using KPFM. The measured potential differences between
the nanoporous metal oxide and the perovskite in dark are consistent
for both nc-TiO2 (DCPD 5 0.155 V) and nc-Al2O3 (DCPD 5
0.12 V) based cells and were used to estimate the band diagrams.
Moreover, the measured width (wp 5 10 nm) of the depletion region
at the perovskite side is the same. However, while the measured band
bending at the nc-Al2O3/MAPbI3 side is negligible small, we measured a 45 nm depletion region width at the TiO2 side of the ncTiO2/MAPbI3 interface. This depletion region contributes to the
separation of photogenerated charge carriers and thus a higher performance of the TiO2 based cells. The measured potential difference
of 25 mV at the perovskite grain boundaries reveals hole accumulation at the GBs and plays a minor role for the solar cell performance.
The results shed additional light on the electronic structure of these
highly efficient perovskite based solar cells.

Experimental
Material synthesis. TiO2 paste DSL 90-T composed of 20 nm particles was
purchased from DYESOL. The TiO2 paste was diluted with ethanol in ratio of 154 by
weight and deposited by spin coating at 2000 r.p.m. for 10 sec. The TiO2 film was
annealed at 500uC for 30 min. The nc-Al2O3 20 wt.% in isopropanol (,50 nm
particle size) was purchased from Sigma-Aldrich. The nc-Al2O3 was diluted in
isopropanol in ratio of 1512 by weight. The deposition and the annealing conditions
were the same as for TiO2.

CH3NH3I was synthesized as described previously 25, by reacting 30 mL of
methylamine (40% in methanol, TCI) and 32.3 mL of hydroiodic acid (57 wt% in
water, Aldrich) in a 250 mL round bottom flask at 0uC for 2 h with stirring. The
precipitate was recovered by putting the solution on a rotavap and carefully removing
the solvents at 50uC. The yellowish raw product of methylammonium iodide
(CH3NH3I) was washed with ethanol by stirring the mixture for 30 min. Then the
mixture was filtered and washed three times with diethylether. After filtration, the
solid was collected and dried at 70uC in a vacuum oven for 24 h.

Device fabrication. The substrate of the device was a SNO25F (FTO) conducting
glass (15 V?cm 21 ), Pilkington). A blocking layer was deposited on the FTO glass
using a solution of titanium diisopropoxidebis(acetylacetonate) (TiDIP, 75% in
isopropanol, Aldrich) in ethanol. The TiDIP solution was spin coated and then
annealed at 450uC for 35 min The TiO solution or the Al O solution were spin

coated and annealed at 500uC for 30 min subsequent to TiCl4 treatment for 30 min at
70uC and annealing at 500uC for 30 min.
The synthesis of the CH3NH3PbI3 on the TiO2 surface was carried out by a twostep deposition technique.

First, PbI2 was dissolved in DMF and dropped onto the TiO2 film and spin coated,
followed by annealing at 70uC for 30 min. In the second step, the cell was dipped into
methylammonium solution. Following the dipping step, the samples were annealed at
70uC for another 30 min. Finally, the back contact was deposited by evaporating
50 nm of gold under pressure of 5*10 26 Torr. The active area was 0.09 cm 2 .
The KPFM cross sections samples were prepared on a crystalline silicon substrate.
The Si wafer was etched from native oxide by hydrofluoric acid (48 wt% in water,
Aldrich) for 15 min and cleaned by oxygen plasma for 1 min.

Photovoltaic characterization. Photovoltaic measurements were made on a New
Port system, composed of an Oriel I–V test station using an Oriel Sol3A simulator.
The solar simulator is class AAA for spectral performance, uniformity of irradiance,
and temporal stability. The solar simulator is equipped with a 450 W xenon lamp.
The output power is adjusted to match AM1.5 global sunlight (100 mWcm 22 ). The
spectral match classifications are IEC60904-9 2007, JIC C 8912, and ASTM E927-05.
I-V curves were obtained by applying an external bias to the cell and measuring the
generated photocurrent with a Keithley model 2400 digital source meter. The voltage
step and delay time of photocurrent were 10 mV and 40 ms, respectively. Oriel IQE200 was used to determine the monochromatic incident photon-to-electric current
conversion efficiency. Under full computer control, light from a 150 W xenon arc
lamp was focused through a monochromator in the 300–1800 nm wavelength range
onto the photovoltaic cell under test. The monochromator was incremented through
the visible spectrum to generate the IPCE (l) as defined by IPCE (l) 5 12,400 (Jsc/l
Q), where l is the wavelength, Jsc is the short-circuit photocurrent density (mA
cm 22 ), and Q is the incident radiative flux (mWcm 22 ). Photovoltaic performance was
measured by using a metal mask with an aperture area of 0.09 cm 2 .

High Resolution Scanning Electron Microscopy (HR-SEM). SIRIONHR SEM was
performed with FEI (Field Emission Instruments), The Netherlands. The
measurement conditions were 5 kV at a magnification of 39,000.

Kelvin probe force microscopy (KPFM). Various locations of the cleaved crosssections were measured by KPFM and consistent CPD profiles with same variations
were obtained for each solar cell type. The measured CPD was constant over a day
range. Topography and CPD were measured with a slow scan rate (0.1 Hz) and in the
attractive force regime (force-distance curve) in order to avoid tip-sample crashes.
Amplitude modulation KPFM was carried out with a commercial AFM (Dimension
Edge, Bruker Inc.) inside a nitrogen glove box with less than 1 ppm H2O at room
temperature. The CPD was measured simultaneously with the topographic signal at
an effective tip sample distance of 5 nm during scanning. The topographic height was
obtained by maintaining the amplitude of the first cantilever resonance (f1st <
75 kHz) at a predefined amplitude set point of approximately 10 nm. The CPD was
determined by compensating the ac component of the electrostatic force at angular
frequency v with an applied dc voltage (5jCPDj) in a feedback control loop. To
separate topographic from CPD signal, increase the sensitivity, and minimize probesample convolution effects, the AC electrostatic force component was generated at
the second resonance 26, f2nd < 450 kHz, of the cantilever by applying an ac voltage of
about 500 mV. Highly conductive cantilevers with Pt/Ir coating (PPP EFM,
Nanosensors) were used for KPFM.

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Acknowledgments
We would like to thank the Israel Alternative Energy Foundation (I-SAEF) that financed
this research, the Ministry of Industry Trade and Labor Office of the Chief Scientist Kamin
project No.50303, and the Tashtiot project of the Office of the Chief Scientist.

Author contributions

A.D. prepared the cells and characterized them, analyzed the data and took part in the
discussion. A.H. performed the KPFM measurements and took part in the discussion and
the preparation of the paper. Y.R. is the supervisor of A.H. and he participated in the
discussion. L.E. is A.D.’s supervisor; L.E. wrote the paper and planned its experiments. G.S.
did the simulation of the band diagram and contribute in the discussion part. The paper was
edited and approved by all authors.

Additional information
Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Dymshits, A., Henning, A., Segev, G., Rosenwaks, Y. & Etgar, L. The
electronic structure of metal oxide/organo metal halide perovskite junctions in perovskite
based solar cells. Sci. Rep. 5, 8704; DOI:10.1038/srep08704 (2015).

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