This document is the Accepted Manuscript version of a Published Work that appeared in final form in
Chem. Mater. 2016, 28, 207-213 copyright © American Chemical Society after peer review and technical editing
by the publisher. To access the final edited and published work see
http://pubs.acs.org/doi/abs/10.1021/acs.chemmater.5b03902

Decreasing Charge Losses in Perovskite Solar Cells Through
the mp-TiO2/MAPI Interface Engineering.
Jose Manuel Marin-Beloqui,1 Luis Lanzetta1 and Emilio Palomares*1,2
1Institute

of Chemical Research of Catalonia ( ICIQ). The Barcelona Institute of Science and Technology. Avda. Països
Catalans, 16. Tarragona. E-43007. Spain
2ICREA. Passeig Lluís Companys, 23. Barcelona. E-08010. Spain.
KEYWORDS: Pervoskite, Solar cells, charge transfer, interface

Based on our experience on controlling the recombination kinetics in Dye Sensitized Solar Cells (DSSC) through the modification of the mesoporous TiO2 (mp-TiO2) interface we have carried out the modification of the methyl ammonium lead
iodide (MAPI) /mp-TiO2 interface with a nanoscopic layer of insulating Al2O3. The effects over the device efficiency, Voc
device reproducibility and the relationship between the observed increase in open-circuit voltage (Voc) and the presence
of the Al2O3 layer is thoroughly discussed and explained. In contrast with the experimental observation in DSSC of TiO2
conduction band edge shift for Al2O3 coated mp-TiO2 films, in MAPI perovskite solar cells the charge vs voltage measurements carried out under sun-simulated irradiation conditions results in a negligible shift of the exponential charge distribution either measured using PICE (Photo Induced Charge Extraction) or PIDC (Photo Induced Differential Charging).
Furthermore, an important decrease on the charge recombination lifetime is measured for the Al2O3 treated samples
which lead to an improvement of the overall device efficiency due to the slower rate in the back-electron transfer reactions
.

Introduction.
The use of methyl ammonium lead iodide (MAPI) perovskite-type semiconductor material has been the focus
of increase interest for the development of efficient solar
cells1-3. In less than 5 years solar-to-electrical conversion
efficiency have been increased from η=3.8% 4efficiencies
to near 20%5, exceeding the top best efficiencies measured for other so-called third-generation solar cells such
as DSSC6 (Dye Sensitized Solar Cells), OSC7 (Organic Solar
Cells) and QDSC8 (Quantum Dot Solar Cells). The easy-tofabricate procedures, materials low cost and past experience on the above mentioned solar cell technologies have
paved the way for MAPI solar cells to become a hot spot in
materials science for energy conversion devices such as
solar cells and light-emitting diodes9. Moreover, the recent discovery of the use of MAPI as semiconductor for
solar cells, has also presented novel scientific challenges
as for example, the explanation of the unusual large
amount of measured charge on MAPI solar cells10, the

presence of hysteresis effects11-14, the bi-exponential
nature of the registered small-perturbation based Voc
decays15, 16 and/or the presence of interfacial dipoles17, 18.
Whilst, the increase in solar cell efficiency has been spectacular, with a learning-curve never seen for other solar
cell technologies alike CdTe and CIS, among others19, the
knowledge, however, on the detailed mechanisms that
allows the efficient conversion of sun-light into electrical
current have been less explored with remarkable exceptions16, 20-23 and, moreover, there is still the scientific
challenge to approach MAPI solar cells to their maximum
theoretical efficiency through the reduction of nonradiate charge recombination losses24, better spectral
response25 and , moreover, increasing operational stability26.
One particular issue of MAPI perovskite solar cells is
that the semiconductor thin film can be used in different
device configurations as for example: (a) the use of mesoporous metal oxide scaffolds alike Al2O3, TiO2 or ZnO27

as contacts, (b) without the mesoporous scaffold and (c)
using organic materials as selective contacts28. Nonetheless, it is worthy to mention that the best certified devices
have been described, to the best of our knowledge, with
the archetypal device configuration5 of FTO/d-TiO2/mpTiO2/MAPI/HTM/Au where FTO is the fluorine doped tin
oxide semiconductor layer deposited on glass, d-TiO2 is a
thinner (~50nm) dense layer of TiO2, mp-TiO2 is a thin
mesoporous layer (~450nm) and the HTM is the organic
hole transport material. The choice of metal contact is
usually gold (Au) on this class of device architecture too.
For the reasons given below the above described device configuration is the standard used in this work unless otherwise stated: On the one hand, our experience on
the deposition of the TiO2 metal oxide29, its characterization and previous encouraging results in relation with
this work using this device structure15 and, on the other
hand, to clarify further the role on the mp-TiO2 layer in
the MAPI perovskite solar cell.
It is well known that reducing the charge losses due to
inconvenient interfacial charge recombination reactions
under operation is paramount to increase the solar cell
efficiency30. Learning from past studies in DSSC, we have
observed that direct back-electron transfer from the photo-injected electrons at the mp-TiO2 to the oxidized electrolyte was a major issue to overcome to increase the
solar cell efficiency. Indeed, reducing this particular
charge recombination reaction through better dye design
and rational interface engineering lead to better and
more reproducible DSSC31.
In our first communication using the MAPI perovskite
semiconductor in solar cells15, we hypothesized that several interfacial and band to band recombination pathways can be found in MAPI/mp-TiO2/HTM sample as
illustrated in Scheme 1. For one, after electron injection
from the MAPI to the TiO2 conduction band (reaction 3,
Scheme 1) subsequent interfacial back electron transfer
may takes place (reaction 6, Scheme 1). For another, the
photo-injected electrons at the TiO2 can also recombine
with holes at the oxidized HTM (analogously to the reaction occurring in solid-state DSSC but it his case cannot be
a direct recombination as there is a thin layer of MAPI
perovskite between the TiO2 and the HTM) and as measured by Moser and co-workers using laser transient absorption spectroscopy20, ( reaction 5, Scheme 1). Moreover, electrons at the MAPI perovskite can also recombine
with the holes at the oxidized HTM (reaction 4, Scheme
1). Last but not least, band to band non-radiate and radiate charge recombination reactions at the MAPI perovskite (reaction 2 and 1, Scheme 1) have to be also considered as well as possible charge recombination reactions
between electrons at the MAPI and surface states at the
TiO2 (reaction 7, Scheme 1).
Hence, the modification of the mp-TiO2/MAPI perovskite interface may affect at least one of the recombination pathways that limit the solar cell performance.
The use of conformally deposited insulating overlayers
onto the TiO2 nanocrystals surface have been previously
explored by our group and others in DSSC32-34. Indeed, it
has been demonstrated that the interfacial engineering
onto the semiconductor nanocrystals, either TiO2 or ZnO,
leads to a substantial decrease of the charge losses due to
interfacial recombination processes. In fact, a recent publication35 by Jung and co-workers describes the use of

MgO onto mp-TiO2 in MAPI solar cells, however, the device electronic characterization to understand the changes in solar cell Voc remained unclear.
In the present work we have fabricated MAPI perovskite solar cells with the archetypal structure mentioned
above as well as MAPI perovskite solar cells with an Al2O3
insulating thin layer onto the nanocrystalline TiO2 particles that conform the mp-TiO2 layer. The former will be
described along this work as MAPI perovskite solar cells,
while the latter will be designed as Al2O3-coated perovskite solar cells. Moreover, we have used advanced photo-induced characterisation techniques, namely photoinduced transient photo-voltage (PIT-PV), photo-induced
differential charging (PIDC) and photo-induced transient
photo-current (PIT-PC) to thoroughly analyse the differences in charge density (defined as the total accumulated
charge in the device at a given bias), the charge recombination lifetime and the relationship between both mentioned parameters. The findings described below are key
to (a) understand further the device charge recombination processes that hamper the solar-to-electrical conversion efficiency and (b) to increase the device fabrication
reproducibility through interface engineering.

Scheme 1. The photo-induced interfacial charge transfer
recombination reactions in MAPI perovskite solar cells;
(1) band to band radiate recombination, (2) band to band
non radiate recombination, (3) interfacial electron transfer from MAPI conduction band (CB) to TiO2 and (4) interfacial electron transfer from the MAPI CB to the HTM,
(5) interfacial electron transfer from TiO2 CB to the HTM,
(6) interfacial electron transfer from the TiO2 CB to the
MAPI and (7) interfacial electron transfer from the MAPI
CB to TiO2 surface states (s.s.).

Experimental Section.
Device preparation.
The FTO (Fluorine doped Tin Oxide) coated glasses (resistance= 8Ω/cm2) were etched using Zn powder (Alfa
Aesar 98%) and a 2 M solution of HCl according to the
desired pattern. Afterwards, the substrates were cleaned
for 15 minutes in an ultrasound bath with de-ionized
water with Hellmax soap, with de-ionized water and

finally with ethanol. The substrates were dried and an
UV/Ozone treatment was performed for 20 minutes.
A solution of 0.65 mL of Ti (IV) isopropoxide (Sigma
Aldrich 97%) and 0.38 mL of acetylacetone (Sigma Aldrich) were mixed in 5 mL of ethanol. This solution was
spun-casted at 3000 rpm for 60s over the FTO. The substrates were calcined at 500oC for 30 min to obtain the
titanium oxide dense layer. Afterwards, titanium oxidecoated substrates were immersed in a 40 mM TiCl4 solution at 70ºC for 30 minutes. Then, substrates were
cleaned with water and ethanol and heated at 390oC for
20 minutes.
A mixture of TiO2 paste (Ti Nanoxide HT/SP Solaronix)
and ethanol 2:5 (w:w) was spin-coated at 5000 rpm for
30 seconds. The substrates were heated at 325o C for 30
min, 375oC for 5 min, 450oC for 15 min and 500oC for 30
min. The substrates were heated at 500 oC for 20-30
minutes prior to their use for further steps.

to-induced transient Photovoltage), PIT-PC ( photoinduced transient photocurrent) and PIDC ( photoinduced differential charging) are described with system diagrams at the Supplementary Information.

Results and Discussion.
Device characterization.
Figure 1 illustrates the current density vs voltage measurements (IV curves) for archetypal MAPI perovskite solar
cells and the Al2O3 coated ones under standard sunsimulated irradiation condition (100mW/cm2 @1.5 AM G).

The alumina coating step was followed as previously
reported by Palomares et al., in brief 3 mL of 3 M
Al(secbut)3 (Sigma Aldrich©) and 17 mL of dry iPrOH
(Sigma Aldrich ©) were mixed in order to get a 0.15 M
solution in a glove box, and it was stirred and heated in
the glove box at 70oC for 20 minutes. Then, the mp-TiO2
coated substrates were immersed in the solution at 70oC
for 20 minutes and the substrates were rinsed with
iPrOH. Afterwards, substrates were heated up at 435oC
for 30 minutes.
The methylammonium iodide was synthesized as described previously.36
For the perovskite deposition, a 3:1 molar ratio solution of methylammonium iodide and PbCl2 (Sigma Aldrich
© 98%) in DMF (dimethyl formamide) was prepared. The
perovskite precursor was deposited by spin-coating over
the mp-TiO2 layer at 2000 rpm for 1 min. The asdeposited substrates were heated at 120oC for 30
minutes, and their colour changed from yellow to black.
The spiro-OMeTAD (1-Material ©), the HTM, was dissolved in chlorobenzene (CBZ) to reach a 70 mg/mL.
Also, 28.8 µl of tBuPyr ( tert-butylpyridine) and 17.5 µl of
a 520 mg/mL of a LiTFSI solution in acetonitrile were
added to the OMeTAD solution as additives. The HTM
layer was deposited by spin-coating the solution onto the
perovskite at 2000 rpm for 1 minute.
Finally, 80 nm of gold metal was evaporated as the anode by thermal evaporation at a pressure not higher than
1x10-6 mbar.
All the steps for the alumina coating and the perovskite
and HTM deposition were carried out inside the glove
box to avoid humidity ([O2] < 100 ppm and [H2O] < 0.1
ppm).
Current vs voltage ( JV) measurements.
The solar cells performances were measured with a
Sun 200 solar simulator (150 W, ABET Technologies)
with the appropriate filters to simulate the AM 1.5G solar
spectrum. A silicon diode was used to set the illumination
intensity to 100mWm-2. The applied potential and cell
current were measured with a Keithley © 2400 digital
source meter.
Time Resolved Photo-induced measurements.

All photo-induced measurement techniques such as
PICE ( Photo-induced charge extraction, PIT-PV ( pho-

Figure 1. Several MAPI perovskite solar cells with and
without the Al2O3 modified mp-TiO2/MAPI interface
under 1 sun illumination and the corresponding dark
curves (dashed lines).

As can be seen in Figure 1, solar cells using Al2O3
mp-TiO2 coated MAPI perovskite solar cells show an
increase in Voc and photocurrent leading to an average efficiency of ηAl2O3=12% in contrast to standard
MAPI perovskite solar cell that show a standard average efficiency of η=10.2%. Table 1 lists all the relevant
parameters for the measured cells in this work.
Table 1. Most relevant parameters obtained from
the IV curves illustrated in Figure 1.
Device
number

Voc
(mV)

Jsc
(mA/cm2)

FF(%)1

η

Al2O3
coated

Cell 1

920

18.6

67.8

11.6

yes

Cell 2

960

20.1

66.1

12.7

yes

Cell 3

939

19.3

65.1

11.8

yes

Cell 4

870

17.6

67.9

10.4

no

Cell 5

870

17.0

72.3

10.7

no

Cell 6

860

15.0

73.6

9.5

no

1device fill factor. Solar cell area of 0.25cm2. All solar cells
were fabricated and measured under the same conditions at
forward bias.

Further measurements of solar cells (Figure 2) fabricated in different days and conditions, as the devices depicted
and listed in Figure 1 and Table 1 respectively, but always
keeping identical measurement and fabrication conditions
within each set of samples of Al2O3 coated mp-TiO2 and the
corresponding MAPI perovskite solar cells as control show
identical trend with the Al2O3 modified mp-TiO2/MAPI
interface having higher Voc and, in average, higher solarto-energy conversion efficiency.

have previously demonstrated40. The use of PIDC is particularly necessary when the PICE decay has a longer lifetime
than the measured PIT-PV decay at 1 sun (as for MAPI
perovskite solar cells in Figure 3).

Figure 3. PICE decay (black line) for a MAPI perovskite solar
cell and the PIT-PV transient (blue line) upon sun-simulated
illumination equivalent to 1 sun. Notice the different time
scales for the PICE (bottom axis) and the PIT-PV (top axis).

To estimate the photo-generated charge stored at the
MAPI perovskite solar cell we must assume that (a) the
solar cell has not charge losses at short circuit and (b) the
PIT-PC under 1 sun and in the dark are similar. In other
words, the charge generation efficiency is similar at shortcircuit and at open circuit solar cell conditions. Figure 4
shows the PIT-PC measurements for both types of MAPI
perovskite solar cells.

Figure 2. Solar cell open-circuit voltage (Voc) distribution for 28 Al2O3 coated mp-TiO2 devices (grey) and 27
MAPI perovskite solar cells (red). Top, devices measured
under reverse bias. Bottom, devices measured under
forward bias.

In order to understand the effect of the Al2O3 onto the
mp-TiO2 and the overall solar cell performance and compare to what has been observed in DSSC we have carried
out different experiments detailed below. The use of photo-induced differential charging (PIDC) has been very useful to analyse the accumulated charge at the solar cell under different light-bias (solar cell voltage induced by different illumination intensities) in DSSC37, QDSC38, OSC39
and more recently in MAPI solar cells16. Moreover, it allows
comparing the photo-generated charge density, at the
same Voc, for different solar cells and, furthermore, is a
key measurement in order to fairly compare charge recombination lifetime between different types of solar cells.
Photo-Induced Differential Charging (PIDC).
The details for the PIDC technique can be found at the
Supplementary information of this manuscript. In brief, the
charge is measured using PIT-PV and PIT-PC (photoinduced transient photo-current) as our group and others

Figure 4. PIT-PC decays measured for (top) mpTiO2/MAPI
perovskite and (bottom) Al2O3/mpTiO2/ MAPI perovskite
solar cells.

In contrast to previous measurements on DSSC with the
Al2O3 coating, there is no sensible and reproducible shift
between the measured exponential curves41. In DSSC with
the mp-TiO2 conformally coated with Al2O3 a shift of the
measured charge vs voltage exponential curve is observed
and assigned to a shift of the TiO2 conduction band edge
with concomitant shift of the quasi-Fermi level for the
electrons at the mp-TiO2. The mp-TiO2 conduction band
shift plus the measured slower interfacial recombination
kinetics between the electrons at the mp-TiO2 and the
oxidized electrolyte, in DSSC, lead to a measured increase
in the open circuit voltage of the DSSC. Thus, next is to
measure the PIT-PV in the MAPI perovskite solar cells and
see if there is correlation with the measured higher open
circuit voltage.
Photo-Induced Transient Photovoltage (PIT-PV).
The PIT-PV technique has been widely used to correlate
the measured charge, at a given voltage, with the charge
recombination lifetime. In a seminal paper42, Bisquert and
Zaban defined the Voc decay technique for DSSC and later
our group and others used a modified version of the technique (see Supplementary Information), which is based in
small perturbation of the open-circuit cell voltage using a
fast light pulse. The fast pulse disrupts the equilibrium
raising the quasi-Fermi level for the electrons at the mpTiO2, which is restored after the pulse finishes to the original energy level and, thus, restoring the initial open-circuit
voltage. In DSSC, the voltage decay promoted by the fast
light pulse results in a mono-exponential decay with an
amplitude always smaller than 10mV to ensure a minor
perturbation of the cell open-circuit voltage.
Figure 6 illustrates the PIT-PV decays for our MAPI perovskite solar cells.

As can be seen in Figure 4, either the PIT-PC under 1 sun or
under dark for both type of solar cells are quite close. Hence,
we carried out the PIDC for both type of solar cell devices.
Figure 5 illustrates the charge measured from the PIDC vs
solar cell voltage.

Figure 6. Measured PIT-PV for a MAPI perovskite solar cell
(black) and an Al2O3-coated MAPI perovskite solar cell (red)
at 100mW/cm2 light irradiation. The inset shows the fastest
component of the PIT-PV.

Figure 5. Measured charge using PIDC at different solar cell
voltage from MAPI perovskite solar cells and Al2O3–coated
perovskite solar cells. The Cell 3A has been removed for clarity read-out of the data in the figure.

The first difference observed in comparison to the PITPV decays in DSSC is the bi-exponential nature of the decays measured either with the Al2O3 coating or the MAPI
perovskite as control. As shown in Figure 6 both samples
show bi-exponential decays with τ1 = 175 µs (20%) and
τ2= 3 µs (80%) for Al2O3 coated solar cells and τ1= 73µs
(20%) and τ2= 0.8 µs (80%) for MAPI perovskite solar cells

under similar charge density ~35nC/cm2. This result is in
good agreement with previous data16, 23.
In MAPI perovskite solar cells, there is still far from clear
the origin of the large photo-voltage measured that in
some cases is overpassing 1V. Moreover, the large differences in charge density measured using PICE (see Supplementary Information) and PIDC and the different chemical
nature that both types of charges may have, tking into
account the differences in the kinetics from the biexponential PIT-PV decay, makes even more challenging to
assign the processes that gives rise to the accumulated
charges measured. Thus, it may well be that the measured
transient photo-voltage does not correspond to a charge
recombination process (charge losses) but to, for example,
a reorganization of dipoles at the MAPI that leads to a
change in voltage or to a combination of both processes.
Nevertheless, we have studied in detail the PIT-PV and we
have proposed that the fast component of the PIT-PV decay
is the product of the electron-hole recombination process
and hence the photo-induced charge recombination lifetime16. Hence, based in our past experience we have also
used for comparison purposes between MAPI solar cells
the charge measured by PIDC and the fastest component of
the PIT-PV decay.
Charge recombination lifetime vs charge density.
It is important to notice that , for the comparison, we use
the measured charge from PIDC, instead of the measured
cell Voc at different light intensities (so called light bias)
because it is well established for other solar cells such as
DSSC, OPV and QDSC that it may well be that at the same
Voc different solar cells have very different amount of
stored charge and thus, the compared charge recombination lifetime will not be meaningful and appropriate. Figure 7 illustrates the carrier recombination lifetime vs electrical charge for both type of solar cells.

Figure 7. Non-radiate carrier recombination lifetime (fast
component PIT-PV decay) vs solar cell electrical charge measured by PIDC. The numbers correspond to the recombination
order factor (OF)

As illustrated in Figure 7 the use of Al2O3 conformal coating leads to slower interfacial charge recombination lifetimes when compared to the standard mpTiO2 film.
We also would like to emphasize that although the slowest component of the PIT-PV decay, either with the charge

measured using PICE or the charge measured using PIDC,
cannot reproduce fairly the Jrec (data shown in the Supplementary Information, Table S1) and, thus, seems that is not
related to the “electrical charge” (understanding as electrical charge electrons and holes) at the MAPI perovskite still
accounts for at least 20% of the PIT-PV decay and, interestingly the PIT-PV slow component of the decay is slower for
Al2O3 . A comparison of both types of solar cells, the Al2O3
coated and the MAPI perovskite device, is illustrated in
Figure 8.

Figure 8. The slow component for the PIT-PV decays vs solar
cell measured charge by PIDC at different light bias. The numbers correspond to the recombination order factor (OF)

Conclusions
We have used Al2O3 conformal coating from solutionprocessed methods onto mesoporous TiO2 electrodes
(mpTiO2) used in the preparation of efficient MAPI perovskite solar cells. The use of Al2O3 coating leads to an improvement in the solar cell efficiency mainly due to a higher open circuit voltage (Voc). The increase in Voc is systematic and reproducible. Moreover, we have studied the
interfacial charge recombination processes using photoinduced time resolved methods in complete devices under
sun-simulated irradiation conditions. Using PIDC we have
not observed a clear shift on the exponential charge distribution as it was demonstrated for Al2O3/mpTiO2 coated
DSSC.
The analysis of the PIT-PV decays shows two different
time components, in good agreement with previous measurements with other MAPI perovskite solar cells. In fact,
for Al2O3 coated solar cells both time components present
at the PIT-PV decay are slower in comparison with the
standard MAPI perovskite solar cell. A careful comparison
of both charge recombination lifetime components at the
same charge density further confirms that the recombination lifetime for Al2O3 treated solar cells is significantly
slower.
Hence, it seems clear that the slower charge recombination kinetics in the Al2O3 coated MAPI perovskite solar
cells can be directly correlated with the solar cell Voc improvement. However, unlike the case of DSSC, it was unclear why both PIT-PV time components are affected by

the presence of the Al2O3 coating onto the mesoporous
TiO2. A feasible hypothesis in the light of the results obtained in this work is that both PIT-PV components can be
assigned to two different charge recombination processes;
one process related to the charge transfer of electrons
from the TiO2, upon electron transfer from the MAPI perovskite under illumination, with the holes at the HTM and
a second, and faster charge recombination process, between electrons at the TiO2 and holes at the MAPI perovskite. In both cases the Al2O3 will act as a physical barrier
(alike in the case of DSSC) decreasing the rate of the back
electron transfer reaction upon photo-induced electron
injection from the MAPI perovskite to the TiO2 CB.

Acknowledgements.
The authors would like to thank ICIQ and ICREA for the economical support and the Spanish MINECO for projects
CTQ2013-47183-R and the Severo Ochoa Excellence Accreditation 2014.2018(SEV-2103-0319).

ASSOCIATED CONTENT
Supporting information contains PICE, voltage stability and
PIT-PV measurement. A detailed description of the PICE and
PIT-PV systems and, moreover, a HRTEM image of the coated
and uncoated Al2O3 nanoparticles. This material is available
free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION
Corresponding Author
*epalomares@iciq.es

Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of
the manuscript.

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