Efficient organic-inorganic hybrid perovskite solar cells processed in air

Article abstract of DOI:10.1039/c4cp03726j

Organic-inorganic hybrid perovskite solar cells with fluorine doped tin oxide/titanium dioxide/CH₃NH₃PbI_3-xCl_x/poly(3-hexylthiophene)/silver were made in air with more than 50% humidity. The best devices showed an open circuit voltage of 640 mV, a short circuit current density of 18.85 mA cm^-2, a fill factor of 0.407 and a power conversion efficiency of 5.67%. The devices showed external quantum efficiency varying from 60 to 80% over a wavelength region of 350 nm to 750 nm of the solar spectrum. The morphology of the perovskite was investigated using scanning electron microscopy and it was found to be porous in nature. This study provides insights into air-stability of perovskite solar cells. This journal is

Full text of DOI:10.1039/c4cp03726j

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Efficient Organic Inorganic Hybrid Perovskite
Solar Cells Processed in Air
Cite this: DOI: 10.1039/x0xx00000x
a
b
c
Madhu Seetharaman S , Puvvala Nagarjuna , P Naresh Kumar , Surya
b
c
a*
Prakash Singh , Melepurath Deepa , Manoj A G Namboothiry
Received 00th January 2012,
Accepted 00th January 2012
Abstract: Organic-inorganic hybrid perovskite solar cells with: fluorine doped tin oxide/ titanium
dioxide / CH NH / poly (3-hexylthiophene)/silver were made in air with more than 50%
DOI: 10.1039/x0xx00000x
3
3 x
PbI3-xCl
humidity. The best devices showed an open circuit voltage of 640mV, a short circuit current
www.rsc.org/
2
density of 18.85 mA /cm , a fill factor of 0.407 and a power conversion efficiency of 5.67%. The
devices showed external quantum efficiency varying from 60 to 80 % over a wavelength region of
3
50 nm to 750 nm of the solar spectrum. The morphology of perovskite was investigated using
scanning electron microscopy and it was found to be porous in nature. This study provides
insights for air-stability of perovskite solar cells.
Introduction
The mixed halide CH NH Pb3ꢀxCl perovskite was found to
3 3 x
be more stable and at the same time, have better carrier
transport properties than the CH NH PbI perovskite material
Organicꢀinorganic hybrid perovskite materials are of great
interest for optoelectronic applications. Recently, light emitting
diodes and field effect transistors based on perovskite have
been reported, where the perovskite was synthesized by mixing
methyl ammonium iodide (CH NH I) and lead iodide in
organic solvents followed by annealing [1,2] . This was
followed by reports of photovoltaic properties of the same
material in dye sensitized solar cells [3ꢀ6]. Perovskite materials
based on methyl ammonium iodide mixed with lead iodide are
widely used as an active material in perovskite solar cells as it
can be solution processed and are cost effective. Over the last
few years, this material and its derivatives have delivered
power conversion efficiency (PCE) exceeding 19%, [7] which
generated a lot of interest in the solid state dye sensitised
photovoltaic research community.
3
3
3
[
15]. Furthermore, CH NH Pb Cl perovskite was found to
3 3 3ꢀx x
exhibit ambipolar transport behaviour, which aided in
developing simple planar structured cells, with an extremely
thin absorber (ETA) layer. Simultaneously, a solid state hole
transporting (HTM) material, SpiroꢀMeOTAD (2,2’,7,7’ꢀ
tetrakis(N,Nꢀdiꢀpꢀmethoxyphenylamine)ꢀ9,9’ꢀspirobifluorene)
was used in these cells as it can penetrate through the porous
perovskite layer and provide a better contact for charge
transportation [3,9,16,17]. Other alternatives and less costly
organic HTM materials like Poly triꢀaryl amine (PTAA), Poly
3
3
(
3ꢀhexylthiopheneꢀ2,5ꢀdiyl) (P3HT) and inorganic materials
such as Copper Iodide (CuI) have also been used in the past
18ꢀ25 and high performance devices were achieved. One of
[
]
the challenges in making solution processed devices is their fast
degradation in the presence of moisture in the atmosphere.
In this article, an organic inorganic mixed halide perovskite
solar cell with CH NH PbI Cl as an extremely thin absorber
Even though CH NH PbI based cells have shown promise
3
3
3
and very high PCE’s, they are found to be unstable and tend to
undergo fast degradation in performance in the presence of an
electrolyte [3,8]. It is reported that the hygroscopic property of
methyl ammonium cation makes it difficult to process solar
cells in ambient air conditions using this material, without the
use of an inert atmosphere. Additionally, these cells require a
3
3
3ꢀx
x
material and P3HT as a HTM over a thin film layer of dense
TiO as an electron transport material (ETM) is reported. The
2
devices were made in ambient air conditions with humidity
greater than 50%. Most of the reported high efficiency solution
processed perovskite based photovoltaic devices were
fabricated in controlled atmosphere conditions with water and
oxygen contents less than 1 ppm levels. An undoped P3HT was
used as a hole transporting layer as our recent modelling studies
have revealed that such an undoped hole transporting layer can
enhance the photovoltaic properties of perovskite solar cells.
nano structured or a mesoporous layer of TiO for electron
2
extraction resulting in a high performance [5,8ꢀ10]. A new
mixed halide perovskite material, CH NH PbI Cl , used an
3
3
3ꢀx
x
Al O scaffold instead of a mesoporous TiO layer, which gave
2
3
2
high performances with a boost in open circuit voltage resulting
in a PCE above 10%.[9,11ꢀ14]
This journal is © The Royal Society of Chemistry 2013
J. Name., 2013, 00, 1-3 | 1

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4CP03726J
DOI: 10.1039Jo/Curnal Name
Experimental Methods
software, ImageJ [28]. UVꢀVis absorption spectra were
acquired using a SHIMADZU UVꢀ3600 spectrophotometer.
The currentꢀvoltage characteristics of the devices were
Device Fabrication
Fluorineꢀdoped tin oxide (FTO) glasses (Pilkington) were measured with a Keithley 6430 source meter in dark and under
2
cleaned by ultrasonication in ethanol for 30 minutes and treated an illumination of 1000W/m AM1.5G spectrum using an Oriel
in a UV Ozone cleaner for 30 min. The cleaned FTO glasses 3A solar simulator tested with a NREL calibrated silicon solar
were coated with 0.15
M
titanium diisopropoxide cell. The external quantum efficiency (EQE) measurements
bis(acetylacetonate) (75% by wt in isopropanol, Aldrich) in 1ꢀ were done using a lockꢀin technique with a 250 W xenon lamp
butanol (Aldrich) solution by the spinꢀcoating method at 2000 coupled with a Newport monochromator and chopped at 40 Hz
o
rpm for 60 s, followed by heating at 125 C. The films were with a light chopper blade as light source. A lockꢀin amplifier
cooled down to room temperature and a 0.3 M titanium (SRS 830, Stanford Research Systems Inc. USA) was used to
diisopropoxide bis(acetylacetonate) solution in 1ꢀbutanol was measure currents and a NIST calibrated silicone photodiode
spin coated. The procedure was repeated twice to make a pin was used to find the incident power spectral response of the
hole free dense TiO film [8]. The substrates were annealed in a incident light. The measurements were done using shadow
2
o
box furnace at 450 C for 60 minutes. The perovskite sensitizer masks to avoid edge effects and proper mismatch factor was
CH NH I was prepared according to the reported procedure taken to square off the spectral mismatch for calculating PCE
3
3
[
26]. A mixture of hydroiodic acid (57 wt.% in water, Aldrich) and EQE [29, 30].
and methylamine (40% in methanol, Aldrich) were stirred in an
o
ice bath for 2 hours. After stirring at 0 C for 2 hours, the Results and Discussion
o
resulting solution was evaporated at 50 C for 1 hour and
Mixed halide perovskite was spin coated in ambient
methyl ammonium iodide was obtained. The precipitate was
washed three times with diethyl ether and dried under vacuum
in dark conditions.
conditions, on a plane glass plate and was observed. The film
was transparent before annealing and turned to dark brown
upon annealing as shown in Figure 1. Unlike CH NH PbI , the
3
3
3
For
preparing
the
CH NH PbI3ꢀxClx
layer,
3
3
mixed halide perovskite remained in the same state without any
change for more than one month when coated with a layer of
P3HT and is confirmed using the UVꢀVis studies of bilayer
CH NH PbI Cl /P3HT structures. The UVꢀVis absorbance
methylammonium iodide (CH NH I) and lead (II) chloride
3
3
(
PbCl , Aldrich) were dissolved in anhydrous N,Nꢀdimethyl
2
formamide (DMF, Aldrich) mixed in a 3:1 molar ratio of
CH NH I to PbCl , to make the 30% and 40% by weight
3
3
3ꢀx
x
3
3
2
spectrum of the bilayer CH NH PbI Cl /P3HT, single layers
3
3
3ꢀx
x
precursor solutions. The 40 wt% and 30 wt% solutions were
spin coated at 2000 rpm and 1500 rpm respectively for 45 s to
make perovskite films of uniform thickness for device
performance comparison. The substrates were subsequently
of CH NH PbI Cl and P3HT of similar thickness as used in
3
3
3ꢀx
x
optimized device structure is as shown in Figure 2(a). The
absorption of CH NH PbI Cl /P3HT bilayer of film thickness
3
3
3ꢀx
x
similar to that of the actual device shows features of both P3HT
and perovskite. P3HT has an absorption edge at wavelength (λ)
640 nm with shoulder at λ ~ 600 nm and a peak at λ ~ 520
o
heated at 100 C on a hot plate, under dark conditions, for 90
minutes [27]. The hole transporting layer of Poly (3ꢀ
hexylthiopheneꢀ2,5ꢀdiyl) (P3HT) was prepared by spin coating
a 15 mg/mL solution in chlorobenzene at 1500 rpm for 2 min.
The substrates were loaded into a vacuum thermal evaporator
and a 120 nm thick layer of silver (Ag) was thermally
~
nm. The perovskite showed a broad absorption with an edge at
λ ~ 800 nm and a peak at λ ~ 450 nm. Even though a perovskite
layer of thickness 250 nm should absorb all the light incident
5
ꢀ1
on it due to an absorption coefficient of ~ 10 cm [31], poor
coverage of perovskite films make light to pass through it and
results in absorption contribution from P3HT in the bilayer
structure. The SEM images (Figure 2(c) and Figure 2(d)) of
mixed halide perovskite show the porous nature of the
CH NH PbI3ꢀxClx film validating the reason for P3HT
ꢀ
6
evaporated as counter electrode at a pressure of 6 x 10 mbar.
A number of devices were made for making a statistical
comparison. The thickness of each layer was measured using a
Dektak 6M stylus profilometer. The thickness of each layer of
the optimized device with a correction factor of ± 5 nm is as
3
3
follows TiO blocking layer ~ 120 nm, Perovskite layer ~ 250
2
contribution in the UVꢀVis spectrum of bilayer structure. The
needle structure of the perovskite as seen from the SEM image
can be attributed to the increased absorbance of the perovskite
film in the longer wavelength region due to increased internal
scattering in such kind of structures.[25]
The absorption data of perovskite/P3HT bilayer coated on a
plane glass plate of similar thickness as in the actual device
made at the time of device fabrication) is compared with
nm, P3HT layer ~ 100 nm and Ag layer ~ 120 nm. The active
2
area of the device is 0.09 cm .
Measurements and characterizations:
Xꢀray diffraction pattern of perovskite was measured on Xꢀ
Pert Pro Analytical diffractometer.
Morphology and
microstructure characterizations were performed using a field
emission scanning electron microscope (Nova NanoSEM NPE
(
absorption spectra of actual device FTO/TiO /Perovskite/
2
2
06 high resolution SEM). The coverage of the perovskite was
P3HT/Ag (absorption were taken on device where Ag was not
coated) after one month and of that after 6 months as shown in
Figure 2b to study the stability of perovskite layer. The
calculated by processing scanning electron microscope image
of the film using greyscale threshold method in open source
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4CP03726J
absorption spectra of the bilayer after one month shows a morphology without any change, when stored for a period of
change in the spectral envelope in the spectral region attributed more than a month After confirming the stability of the
to contribution from P3HT (450 nm < λ < 600 nm) showing perovskite by different methods, photovoltaic devices were
degradation of P3HT over time. The spectral envelope remain made with the device structure: FTO/ TiO / CH NH PbI
3ꢀ
.
2
3
3
Cl /P3HT/Ag. A schematic of device structure and energy
x
x
level diagram of different material used in the device is as
shown in Figure 3a and Figure 3b respectively.
A TiO dense layer with a thickness of about 120 nm was
2
used as an electron transporting, hole blocking layer. It has
been reported earlier that the mixed perovskite diffusion lengths
for electrons and holes are typically, in the order of 1 micron,
with a good ambipolar behaviour.[15] The optimal thickness of
perovskite was 250 nm. Our recent dark IꢀV modelling study
revealed that an undoped hole transporting layer is better for
reducing the space charge effects [32]. Hence, one of the
commonly used pꢀtype polymers, P3HT, was employed herein,
as the hole transporting layer in the devices. The P3HT film
thickness was optimized at 100 nm for highest efficiency
devices. All these layers were solution processed in air; they
were spin coated under ambient conditions with humidity levels
in excess of 50%. One of the main factors affecting the
performance of the perovskite solar cell is the thickness and the
film quality of the active layer. The formation of pinꢀholes can
cause the hole transport material and electron transport material
to come in contact, resulting in the formation of parallel paths,
causing high shunt resistance and recombination of electrons
and holes. Even though better coverage is achieved over TiO2
by increasing the thickness of the TiO2 layer and the
concentration of the perovskite precursor solution, the
maximum coverage of the perovskite film is around 95% of the
surface area only in literature [12, 26]. The SEM images of the
perovskite active layer show decent coverage over the dense
Figure.1ꢀ CH
3 3 x
NH PbI3ꢀxCl film (a) before and (b) after annealing in ambient
atmospheric conditions, the film which was initially transparent was found to
change into a dark grey colour, indicating the formation of CH
after 45 minutes.
3 3 x
NH PbI3ꢀxCl ,
TiO layer and about 75% coverage at higher magnification
2
with needle like structures Figure 2(c). The film was found to
/P3HT bilayer; (b) comparison of UV-Vis be porous in nature at higher magnification. (Figure 2(d))
Figure.2 (a) comparison of UV-Vis absorption spectra of CH
P3HT layer and CH NH PbI3-xCl
absorption spectrum of CH NH PbI3-xCl
the actual device fabrication, with UV-Vis absorption spectra of actual device
FTO/TiO /CH NH PbI3-xCl /P3HT/Ag after 1 month and device after 6 months.
Absorption cross section is selected in such a way that there is no Ag coating on
3 3 x
NH PbI3-xCl layer,
3
3
x
3
3
x
/P3HT bilayer made on a glass along with
2
3
3
x
(
top of P3HT) (c) Scanning electron microscopy (SEM) images of CH
perovskite film and (d) film at higher magnification.
3 3 x
NH PbI3-xCl
unchanged in the spectral window attributed to perovskite (650
nm < λ < 800 nm and λ < 450 nm) and can be attributed to a
stable perovskite layer. Further storage of the device in ambient
condition over a period of 6 month showed degradation of
perovskite layer and is attributed to the disappearance of the
spectral features in the λ region from 650 nm to 800 nm. The
Xꢀray diffraction analysis of the mixed halide perovskite shows
o
o
characteristic peaks at 14.1 and 28.5 (Figure 3(c)). The Xꢀray
analysis of the perovskite films, even after one month (Figure
3
(d)) showed the same peaks with unaltered peak positions,
thus illustrating that there is no significant degradation of the
perovskite film with time. However, the films after 6 months,
Figure 3. (a) Schematic device structure; (b) Energy level diagram of the different
materials used in the device; (c) X-ray Diffraction (XRD) analysis plot of
shows a change in peak position (Figure 3(e)), with a prominent CH NH PbI3-xClx films at initial stage, (d) that after one month storage and
3
3
o
(
e)after 6 months storage.
peak at 12.7 , indicating the presence of PbI , due to the
2
degradation of perovskite.[27] The film retained the same
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Journal Name
The devices were made with different concentrations of nearꢀ infrared region, to a wavelength of 800 nm. Comparison
CH NH PbI Cl . The dark current characteristic shows a of EQE of the device and absorption spectra of CH NH PbI
3
3
3ꢀx
x
3
3
3ꢀ
diode like behaviour. The JꢀV (current density Vs voltage)
Cl /P3HT bilayer shows that the EQE of the device is resultant
x
x
characteristics were recorded under A.M.1.5G, 1Sun of photocurrent generation in P3HT and perovskite and the
illumination (Figure 4(a)). The devices with 30% (by wt.) effective transport of free carriers to the electrode through these
solution of the perovskite precursor gave a PCE of 4.79% with layers. It is to be noted that P3HT shows an excitonic feature
2
short circuit current density (J ) of 14.98 mA/cm , open circuit resulting in the formation of excitons (bound electron hole pair)
sc
voltage (V ) of 781 mV and fill factor (FF) of 0.409. In order as the primary photoexcited species. But in perovskite, due to
oc
to enhance the solar cell performance, the concentration of the its high dielectric constant [33], free electrons and holes are
perovskite solution was increased to 40% (by wt.). When the created upon photoexcitation. In the λ ranging from 800 nm to
process was repeated with 40% (by wt.) solution of the 650 nm the photocurrent is due to that of perovskite. The
perovskite precursor, the PCE was observed to increase to electrons and holes which are formed in the perovskite,
2
5
0
.67% with Jsc of 18.85 mA/cm , V of 640mV and FF of especially near the CH NH PbI Cl /P3HT interface, are well
oc
3
3
3ꢀx
x
.407. The J was found to increase when the concentration of transferred to the electrodes through perovskite and hole
sc
the solution was increased, the Voc was found to decrease and transporting P3HT. There is a significant light harvesting in this
the FF was found to be almost similar. The increase in Jsc is λ region with a reduced ratio of EQE ratio numbers of peaks at
attributed to improvement in the absorption of perovskite layer λ ~520 nm to that at 750 nm compared to the ratio of
made of 40 wt % solutions due to its better surface coverage. absorbance of biꢀlayer at these respective λ’s, This can be
The perovskite films prepared from 40 wt% solution has a attributed to good electronic transport of perovskite. The peak
surface coverage of 75% and that made from 30 wt% solutions at λ ~ 640 nm can be due to the increased absorption of the
has a surface coverage of 63%. The Voc decrease can be bilayer with P3HT contributing to the photocurrent by
attributed to the intercalation of different layers into each other producing free charge carriers due to dissociation of exciton
due to the porous nature of the active layer.
created in P3HT (at absorption edge of P3HT) at the
P3HT/Perovskite interface. EQE for λ < 640 nm is a net result
of photocarriers created in perovskite and P3HT and its
effective transport across the device. Deviation of EQE spectra
from absorption spectra for λ < 620 nm can be attributed to
reduced contribution from P3HT due to the creation of charge
transfer excitons in P3HT. High energy photons are needed to
dissociate this charge transfer excitons and this results in a EQE
peak at λ ~ 485 nm, which is blue shifted compared to the
bilayer absorption peak. In the shorter wavelength region
perovskite absorption prevails and it follows the absorption
spectra. The observed increase in EQE of solar cells (in the λ
range 400 nm to 650 nm) made with 40 wt% solution compared
to that of 30 wt% solution can be due to increased absorption of
perovskite layer made with 40 wt% solution. Most of the light
in this spectral region have a penetration depth which is less
than the thickness of the perovskite film due to an absorption
5
ꢀ1
coefficient of 10 cm . It has been observed that the poor
coverage of the perovskite film reduces light absorption in this
spectral region. Hence the enhancement of EQE in this spectral
Figure 4. (a) J-V Characteristic of the devices made with 30% and 40% of
CH
with 30% and 40% of CH
absorbance spectra of CH NH
3
NH
3
PbI3-xCl
x
(by wt.) precursor solution (b) EQE spectrum of solar cells made
NH PbI3-xCl (by wt.) precursor solution and UV-Vis
PbI3-xCl /P3HT bilayer.
3
3
x
3
3
x
The PCE values are comparable to the values in literature
obtained for solution processed perovskite solar cells which are
made in controlled atmosphere. The EQE spectra (Figure 4(b))
showed the incident photon to current conversion efficiency
varying from 60 to 80 % over a wavelength region of 350 to
Figure 5. Histogram of number of devices with the corresponding power
conversion efficiencies obtained.
7
50 nm of the solar spectrum with an edge extending to the
4
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DOI: 10.1039/C4CP03726J
region is due to the better coverage of perovskite films made
from 40 wt% solution compared to that made from 30 wt%
solutions. A histogram of PCE values of 50 different cells
obtained in our lab is given for consistency and reproducibility
in Figure 5.
c
Department of Chemistry, Indian Institute of Technology Hyderabad,
Ordnance Factory Estate, Yeddumailaramꢀ502205, Telangana, India
.
(
1) J. H. Im, C. R. Lee, J. W Lee, S. W. Park and N. G .Park, Nanoscale
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(
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sc
oc
5
This shows the degradation of the device in humid air
conditions. Furthermore, reduction in the performance of the
device can be attributed to the silver electrodes which were
found to have deteriorated with time, in accordance with the
observation by Snaith et al.[34] and due to degradation of
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halide ion in the perovskite can result in the degradation of the
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(
(
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3
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3ꢀx
x
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398
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perovskite due to its poor coverage. The XRD and UV Vis
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Ferro, T. Besagni, A. Rizzo, G. Calestani, G. Gigli, F. De Angelis
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Organic -Inorganic hybrid perovskite solar cells based on CH
thiophene) as the hole transporting layer fabricated in ambient air conditions by solution processing
2x6mm (300 x 300 DPI)
3 3 x
NH PbI3-xCl and undoped Poly (3-hexyl
1