Structure and function relationships in alkylammonium lead(II) iodide solar cells

Article abstract of DOI:10.1039/c4ta06174h

Alkylammonium lead(ii) iodide materials (APbI₃), based on the general formula of CH₃-(CH₂)_n-NH₃PbI₃, may lead to a monumental leap in developing affordable photovoltaics. Herein, we correlate the structure and function relationships of alkylammonium lead(ii) iodide in solar cells. We investigated changes in the structure of APbI₃ materials by varying the alkylammonium cations in their structure. As the size of the alkylammonium cation increased, the crystallographic unit cell increased in size and yielded lower symmetry crystals. High symmetry materials, those with cubic symmetry, showed the highest conductivity, the smallest bandgap, and produced the best performing solar cells. Structural changes were investigated by X-ray crystallography, X-ray powder diffraction, and Raman scattering.

Full text of DOI:10.1039/c4ta06174h

View Article Online
View Journal
Journal of
Materials Chemistry A
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: M. Safdari, A.
Fischer, B. Xu, L. Kloo and J. M. Gardner, J. Mater. Chem. A, 2014, DOI: 10.1039/C4TA06174H.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/materialsA

Page 1 of 7
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C4TA06174H
Journal Name
RSCPublishing
ARTICLE
Structure and Function Relationships in
Alkylammonium Lead(II) Iodide Solar Cells
Cite this: DOI: 10.1039/x0xx00000x
Majid Safdaria, Andreas Fischera, Bo Xub, Lars Klooa and James Gardnera*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
Alkylammonium lead(II) iodide materials (APbI3), based on the general formula of CH3ꢀ
(CH2)nꢀNH3PbI3, may lead to a monumental leap in developing affordable photovoltaics.
Herein, we correlate the structure and function relationships of alkylammonium lead(II) iodide
in solar cells. We investigated changes in the structure of APbI3 materials by varying the
alkylammonium cations in their structure. As the size of the alkylammonium cation increased,
the crystallographic unit cell increased in size and yielded lower symmetry crystals. High
symmetry materials, those with cubic symmetry, showed the highest conductivity, the smallest
bandgap, and produced the best performing solar cells. Structural changes were investigated by
Xꢀray crystallography, Xꢀray powder diffraction, and Raman scattering.
www.rsc.org/
methylammonium lead(II) halide, as light absorber, in combination
Introduction
with SpiroꢀOMeTAD, as a hole transport material, have shown
12, 17
remarkable performance.7,
To further improve on solar cells
Solidꢀstate dyeꢀsensitized solar cells (ssDSCs) are making
substantial progress as an alternative to conventional dyeꢀsensitized
solar cells. The highest recorded efficiency for ssDSCs utilizing an
organic photosensitizer and the hole transporting material Spiroꢀ
OMeTAD (2,2',7,7'ꢀtetrakis(N,Nꢀdiꢀpꢀmethoxyphenylamino)ꢀ9,9'ꢀ
spirobifluorene) was 7.2%.1 Though impressive, this result is still
lower than record for the conventional DSCs based on a liquid
electrolyte.2 A revolution in solidꢀstate solar cells was initiated by
employing methylammonium lead(II) halides (MAPbX3) as
photosensitizers and holeꢀtransporting materials in mesoscopic solar
cells.3, 4 The MAPbX3 materials have several properties that make
them interesting for use as light harvesting and chargeꢀtransporting
components in solar cells, e.g direct band gap, large extinction
based on alkylammonium lead(II) halides, APbX3, it is necessary to
understand the fundamental properties from which they derive their
function. In the present work, we investigate the effect of crystal
structure on the electronic properties of the materials by varying the
alkylammonium cation size. As mentioned elsewhere,18 ideal
perovskites are metal oxides with a cubic structure and a formal
composition AMX3, where
A and M are cations in two
crystallographically different positions, A positioned in the unit cell
cube corners and M positioned in the cube center, and X is
positioned at the center of each unit cell edge. Methylammonium
lead(II) halides have a perovskite crystal structure. In the perovskites
of interest to solar cells, A is an arbitrary monovalent cation, M is a
mainꢀgroup element, such as tin or lead, and X is a halogen. Many of
the materials of interest also significantly deviate from the ideal
perovskite structure in terms of both structure and composition; in
the latter case, typically the amount of X is higher giving rise to
layered, perovskiteꢀlike structures of the Mitzi type.19ꢀ21 The
perovskite structure can be regarded as consisting of layers along
different crystallographic directions, and several possible
orientations can be considered, eg. <100>, <110> and <111>.
Perovskiteꢀlike structures typically consist of layers of cornerꢀ
sharing metal halide octahedra. The structure of these AMX3
materials represents an interesting platform for structureꢀproperty
modifications, where the layered structure motif offers sufficient
flexibility for modifications and can be amended through the
changing of the composition of the different components in the
precursor solutions. One can change the halide anion, which leads to
coefficients,5 high carrier mobility,4,
and the ease of solution
6,
7
processing. Another inherent key property of the MAPbX3 materials
is most likely related to their tendency to generate a material with
few bulk and surface defects, even inside a mesoporous metal oxide
host.8, 9 Finding other solution processed materials that can improve
upon present material properties and show reasonably decent
function in combination with other components in the solar cell
represents a challenge in this field.10, 11 Mesoscopic solar cells based
upon methylammonium lead(II) iodide as a light absorber have
shown major progress during the last two years and are presently
over 16% power conversion efficiency.3, 4, 7, 12ꢀ14 Prior to their use in
ssDSC, MAPbX3 quantum dots were utilized as light absorbing
materials in liquidꢀbased DSC.15, However, these types of solar
16
cells showed low stability due to the dissolution of
methylammonium iodide into the electrolyte solution resulting in
rapid loss in solar cell function. Changing from liquid electrolyte to
solid hole transporting material was a very significant development
in this field. Recently, perovskite thin film solar cells based on
different materials with a range of bandgaps, although with a
23
conserved perovskite crystal structure.22,
On the other hand, as
shown in this work, changing the organic cation has remarkable
This journal is © The Royal Society of Chemistry 2013
J. Name., 2013, 00, 1-3 | 1

Journal of Materials Chemistry A
Page 2 of 7
ARTICLE
VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4TA06174H
effect on the structure of the material, which would be beneficial for
Samples were ground in a mortar before collecting powder Xꢀray
increasing our knowledge about the structure and function diffraction patterns. We investigated the recovered powders of the
relationship of these materials. In this work, we have prepared three APbI3 materials by Xꢀray diffraction, PANalyticalꢀX’Pert PRO
APbI3 materials and varied the alkylammonium as cation chain diffractometer using CuꢀKα radiation, and scanned over a 2θ range
length. We fabricated methylammonium iodide, ethylammonium of 10° to 40°.
iodide and propylammonium iodide and used them to prepare APbI3
Raman spectra of wellꢀground APbI3 powders were acquired on a
materials with the respective alkylammonium cations. The
BioRad FTS 6000 spectrometer equipped with a Raman accessory
using a quartz beamsplitter, 4 cmꢀ1 resolution and a nitrogen cooled
synthesized materials were methylammonium lead(II) iodide
(MAPbI3), ethylammonium lead(II) iodide (EAPbI3) and
Ge detector.
propylammonium lead(II) iodide (PAPbI3). It is proposed that the
In order to measure the electrical conductivity, samples were
organic cation in this structure does not have a major role in the
prepared as follows and as shown in Figure 1. A 600 nm layer
of mesoporous TiO2 was deposited onto a nonꢀconductive glass
substrate, APbI3 materials were then spin coated into the TiO2
layer and lastly, on the top, two thin layers of silver 2 cm wide
were deposited. The photocurrent was measured when a bias
voltage from ꢀ1 V to 1 V was applied across the two silver
layers. The inverse of the slope obtained from the currentꢀ
voltage curve is the film resistance, R.1 The electrical
conductivity (σ) was calculated by using the following equation
(1):
electronic band structure and strictly maintains the neutrality of the
crystal lattice, while the band structure is dependent on the lead
halide anion structure.24 However, the size of cation is very
important in determining the threeꢀdimensional structure of the
material. We have focused on the effect of this change in order to
obtain fundamental information about the materials and connect it to
the performance of the solar cells based on these materials.
Experimental
Material synthesis: All chemicals were purchased from Sigmaꢀ
Aldrich unless otherwise stated. Alkylammonium lead(II) iodide
materials (APbI3) were prepared according to the procedure used
ꢀ
σ = ꢁ.ꢂ.ꢃ (1),
16
elsewhere3, and are summarized briefly below. To prepare each
alkylammonium iodide, hydroiodic acid (15 mL of 57 wt. % in
water) was mixed and stirred with the relevant alkylamine (13.9 mL
of 40% methyl amine in methanol, 10.8 mL of 70% ethylamine in
water, and 11.2 mL of propylamine >99%) at 0°C for two hours. The
resulting solution was evaporated and washed three times with
diethyl ether and then dried under vacuum for 12 hours at 60°C.
where W is the width between adjacent silver channels, d is the
depth of the APbI3/TiO2 film corrected for the TiO2 volume, and L is
the length Ag film. The TiO2 films have 60% porosity. To correct for
the volume of the TiO2, the thickness of the materials (d) was
multiplied by 60%.
For the synthesis of APbI3, 1:1 molar ratios of the
alkylammonium salt and PbI2 (99.999%) at a concentration of 0.013
M were dissolved in dimethylformamide (anhydrous, 99.8%) at 60
°C overnight. The solution was filtered (if any precipitation
occurred) through a PTFE syringe filter 0.45 ꢁm pore size. Sample
powder was recovered by evaporation of the solvent at 50 °C. All of
the synthesis processes were performed under ambient air.
a.
Silver top layer
TiO2 + Alkylammonium lead iodide
Glass
b.
Characterizations: A 40 wt % solution of the APbI3 materials
was used to spin coat the materials onto a microscope slide covered
by a 2ꢁm layer of TiO2. This was followed by heating at 100 °C.
UVꢀVisible absorption spectra were recorded using UVꢀVisible Cary
300 spectrophotometer.
Figure 1. Schematic a. crossꢀ
sectional view and b. top view diagram of the devices used for the
conductivity measurement.
Solutions containing 400 mg/ml of the APbI3 samples in γꢀ
Butyrolactone (Reagent Plus® >99%) were used for single crystal
growth. The solutions were heated to 100 °C and were kept at this
temperature for 30 minutes. The solutions were then cooled very
slowly (3 °C/h). Crystal growth started at 75 °C. For MAPbI3
samples, crystals were stable at temperature higher than 55 °C due to
its phase transition at 55 °C from cubic (at higher than 55 °C) to
tetragonal (at lower than 55 °C). For PAPbI3, stable crystals were
obtained at room temperature.18 Single crystals of methylammonium
lead(II) iodide and propylammonium lead(II) iodide (shown using a
compound optical microscope in Figure 3) were used for the
collection of data at 323 K. Diffraction data were collected on a
BrukerꢀNonius KappaCCD diffractometer. Absorption corrections
based on multiple scans (SADABS) were applied.25 The structures
were solved using direct methods and refined on F2 with anisotropic
thermal parameters for all nonꢀH atoms.26 H atoms were refined on
calculated positions using a riding model. Table 1 summarizes the
results of the structure determinations. Despite our best efforts,
single crystals of EAPbI3 could not be grown in our hands.
Solar cell preparation: The FTOꢀglass was cleansed by washing in
an ultrasonic bath containing a solution of 2 wt% detergent.
Cleaning was continued in an ultrasonic bath containing ethanol and
then acetone. To make a compact TiO2 blocking layer, the cleansed
FTO substrates were heated to 500°C, then a solution of 2 M
titanium isopropoxide in propanol was spray pyrolyzed onto the
FTO. The mesoporous TiO2 layer was prepared by spin coating of
TiO2 on the substrate at 3000 rpm spinning rate. Films were then
sintered at 500 °C for 30 minutes. Sintered films were immersed into
a 40 mM solution of TiCl4 in water for 30 minutes at 70 °C. Lastly
the TiO2 films were sintered at 500°C for 30 minutes. DMF
solutions with 40 wt % of APbI3 samples were used. Materials were
spin coated (at 1500 rpm spinning rate) into the TiO2 mesoporous
layer followed by annealing to 100 °C for 5 minutes. The hole
transporting material (HTM) solution contained 0.08 M Spiroꢀ
OMeTAD (2,2',7,7'ꢀtetrakis(N,Nꢀdiꢀpꢀmethoxyphenylamino)ꢀ9,9'ꢀ
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 3 of 7
Journal Name
Journal of Materials Chemistry A
View Article Online
ARTICLE
DOI: 10.1039/C4TA06174H
spirobifluorene, 99.99%, Lumtec), 0.03 M bis(trifluoromethane) that by changing to a bulkier cation, from methyl to propyl, the
4ꢀ
sulfonimide lithium salt (LiTFSI, 99%, Ioꢀliꢀtec) and 0.198 M 4ꢀtertꢀ distance between PbI6 structural units increases; this leads to
butylpyridine (TBP, 99%). A mixture of 10 wt % acetonitrile decreased orbital overlap and lower absorption in the visible region
(99.8%) and 90 wt % chlorobenzene (99.8%) was used as solvent for of the spectrum.
hole transporting material solution. Spin coating of the HTM
Powder X-ray diffraction: The obtained diffractograms are
shown in Figure 3. The indices for MAPbI3 and EAPbI3 were taken
from literature values and matched well with our data.18, 28, 29 The
indices for PAPbI3 were taken from our single crystal data (shown
below in Figure 6). The powder patterns show that all of the APbI3
materials are polycrystalline and are a single phase. When
comparing the three APbI3 materials, there does not appear to be a
strong correlation between the calculated Miller indices for the
powder patterns and the peak positions. There is substantial
anisotropy in the powder pattern for PAPbI3; many of the peaks
predicted from single crystal data are supressed. Since the powders
more closely relate to materials present in the solar cells, this bias in
orientation may be present in solar cells made using PAPbI3 as well.
However, since the high Zꢀnumber elements Pb and I dominate the
powder Xꢀray pattern, the patterns only produce indirect information
on the less massive alkylammonium groups. To further refine these
conclusions, we grew single crystals of MAPbI3 and PAPbI3.
solution was performed at 2000 rpm. Finally, a 200 nm thick layer of
silver was deposited on top of the HTM layer as a counter
electrode.3, 7 All of the solar cell fabrication steps were performed in
ambient air and at ambient temperature, except for the final silver
evaporation process to generate the counter electrode.
Results and discussion
UV-Visible measurement: All of the samples showed absorption
edges in either the nearꢀIR and visible regions that correspond to the
energy gap between the valence band and conduction band, Figure 2.
a.
b.
400
500
600
700
800
1,5
2,0
2,5
Wavelength (eV)
Wavelength (nm)
Figure 2. a. Absorption spectra and b. Tauc plot of the ꢀꢀꢀ
(CH3NH3)PbI3,ꢀꢀꢀ(CH3CH2NH3)PbI3,and ꢀꢀꢀ (CH3CH2CH2NH3)PbI3.
For the calculation of the bandgap the Tauc equation was used,
equation (2).27
(αhν)1/n = A(hν ꢀ Eg) (2),
Table 1 Calculated bandgap for the APbI3 materials using Tauc
equation for direct bandgap transitions.
Figure 3. Powder Xꢀray diffraction patterns of ꢀꢀꢀ
(CH3NH3)PbI3,
ꢀꢀꢀ
(CH3CH2NH3)PbI3,
and
ꢀꢀꢀ
APbI3
Bandgap(eV)
(CH3CH2CH2NH3)PbI3.
CH3NH3PbI3
1.6
2.2
2.4
CH3CH2NH3PbI3
CH3CH2CH2NH3PbI3
Single-crystal X-ray diffraction: The crystallographic data for the
refined structures are presented in Table 2
.
b.
a.
Where h is Planck constant, ν is the frequency, α is the
absorption coefficient, Eg is the bandgap and A is a normalization
constant. For a direct electronic transition, as is observed in the
APbI3 materials, the bandgap is calculated to be the abscissa
intercept on a plot of (αhν)1/2 vs hν. For MAPbI3 a band edge was
detected at 785 nm; this is equivalent to a bandgap of 1.6 eV, which
is in agreement with other studies.18 While for EAPbI3 this value has
been shifted to 568 nm, corresponding to a bandgap of 2.2 eV. For
PAPbI3, a higher shift was detected to 523 nm, corresponding to a
bandgap of 2.4 eV. As it is shown in the Figure 2, the APbI3
materials containing bulkier alkylammonium groups have absorption
onsets that are shifted to shorter wavelengths. It can be concluded
Figure 4. Compound optical microscope images of (a)
methylammonium lead (II) iodide, MAPbI3 and (b)
propylammonium lead(II) iodide, PAPbI3 crystals used for single
crystal Xꢀray diffraction.
The refined crystal structures of the samples are presented in
Figures
5 and 6 for methylammonium lead(II) iodide and
propylammonium lead(II) iodide, respectively. The data for crystal
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 3

Journal of Materials Chemistry A
Page 4 of 7
View Article Online
ARTICLE
Journal Name
DOI: 10.1039/C4TA06174H
structure of the ethylammonium lead(II) iodide was extracted from a units has proved that upon changing the cation from
report by Park et al.29
methylammonium to the bulkier propylammonium cation the
interaction between the main structural units is reduced. The bigger
cation occupies a larger space between the lead iodide units and is an
obstacle to forming cornerꢀsharing units in all three dimensions. As
it can be seen in the Figure 6, the structure of PAPbI3 is a one
dimensional PbI3 chain due to the occupation of a larger volume by
the cation. This bigger space between lead iodide units even
facilitates solvent incorporation into the structure. As mentioned
above, the unit cells for PAPbI3 incorporate γꢀbutyrolactone as a
structural feature.
Table 2 Refined structural details of methylammonium lead(II)
iodide and propylammonium lead(II) iodide. Ethylammonium
lead(II) iodide structural details were reported by Park et al.29
29
MAPbI3
EAPbI3
PAPbI3 + C4H6O2
Space
group
P m ꢀ3 m
Pmmn
C c
Cell
lengths
(Å)
a 6.284
b 6.284
c 6.284
α 90.00
β 90.00
γ 90.00
a 8.742
b 8.147
c 30.310
α 90.00
β 90.00
γ 90.00
248.15
1
a 17.770
b 12.380
c 8.071
α 90.00
β 116.77
γ 90.00
Cell
angles
Figure 5. Methylammonium lead(II) iodide (MAPbI3) crystal
structure shown at a 50% probability level.
Cell volume (Å3)
2158.78
1585.26
4
Z
8
In agreement with literature, MAPbI3 has a cubic structure with a
space group of Pmꢀ3m.18 The exact positions of the atoms in the
methylammonium group was not obtained from the crystal structure
determination because of a combination between the large difference
in electron density with respect to lead and iodine, and because of
dynamic or static disorder in the crystal lattice. In this structure, the
distances between lead atoms are equal and unit cells are corner
sharing in all three dimensions, Figure 5. The distance between lead
atoms is 6.284 Å in all three dimensions with an ideal angle of 180°
for PbꢀIꢀPb and 90° between IꢀPbꢀI. Park et al29 report on the
structural analysis of EAPbI3 and declare an orthorhombic structure
in the space group of Pmmn. The single crystals of PAPbI3 reveal a
solvate that incorporates a single GBL molecule into the structure. A
monoclinic structure in the space group of Cc was found for the
PAPbI3 solvate. For EAPbI3 and PAPbI3, the bulkier cation leads to
a structure that is not cornerꢀsharing, but is instead faceꢀsharing in
only one dimension. A one dimensional chain was found for both
lead iodide units in EAPbI3 and PAPbI3. A larger distance between
structural lead iodide units was observed as compared to MAPbI3.
For EAPbI3 the distances between the lead atoms within one chain
are observed in the range of 3.648 to 4.499 Å and between the chains
they are observed at 8.804 Å. For PAPbI3 lead atomꢀatom distances
Figure 6. Propylꢀ
ammonium lead(II)
iodide shown as ballꢀ
andꢀstick models for
simplicity along the three different axes
.
As shown from the crystal structure of the materials, the cation is
embedded in the void between the lead iodide structural units, which
are sharing octahedral faces. The bulkiness of the cations clearly
within the chain and between the chains are 4.036 Å and 10.829 Å, increases the distance between lead iodide structural units in two out
respectively. In EAPbI3, the IꢀPbꢀI angles varies between 172.02 and
178.83 degrees and in PAPbI3 same angles fluctuates from 175.88 to
178.42. Unlike in MAPbI3, which has linear PbꢀIꢀPb angles, the
angles for PbꢀIꢀPb in PAPbI3 are between 77.04 and 77.85 degrees.
This change in PbꢀIꢀPb angle is due to the shift from cornerꢀsharing
octahedra in MAPbI3 to faceꢀsharing octahedra in PAPbI3. The Pb to
Pb distance through PbꢀI bonds in EAPbI3 varies between 6.050 and
6.858 Å, which is on average longer than the distances between Pb
of three dimensions, and it significantly changes the structure of the
materials as was shown by Mitzi and coworkers for related
materials.19, 21
Raman spectroscopy: The obtained Raman spectra are shown
in Figure 7. For all samples, an intense peak was observed at ~120
cmꢀ1, which is attributable to the PbꢀI bond vibration.30 In the ideal
atoms in MAPbI3 (6.284 Å). The same phenomenon is observed for and threeꢀdimensional case, the fundamental vibrational mode that is
PAPbI3, where the distance varied between 6.423 and 6.479 Å. The
noticeable increase of the distances between lead iodide structural
responsible for the strongest Raman signal originates from the triply
degenerate (along the three crystallographic directions in the cubic
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 5 of 7
Journal Name
Journal of Materials Chemistry A
View Article Online
ARTICLE
DOI: 10.1039/C4TA06174H
AMI3 structure), asymmetric IꢀPbꢀI (or PbꢀIꢀPb) stretch. For devices and incident photonꢀtoꢀcurrent conversion efficiency (IPCE)
measurements.
MAPbI3, EAPbI3, and PAPbI3 samples this peak was observed at
131 cmꢀ1, 113 cmꢀ1, and 115 cmꢀ1 respectively. In EAPbI3 and
PAPbI3, the IꢀPbꢀI angle is less than 90 degrees and the PbꢀI
distances are greater than in MAPbI3. This qualitatively matches
well with Raman data, which indicate that the PbꢀI bond order
decreases in EAPbI3 and PAPbI3. We attribute this as the main
reason for the shifting the PbꢀI vibration to the lower frequency. All
of the PbꢀI bond lengths in the cubic MAPbI3 are 3.14 Å. This can be
compared to EAPbI3, which has PbꢀI bond lengths in the range of
3.025 to 3.429 Å (average 3.246Å). For PAPbI3, PbꢀI bond lengths
are in the range of 3.20 Å to 3.25 Å (average 3.228Å) (Table S3). It
is notable that the largest difference is seen when going from the MA
to the EA cation. Of course, because of the difference in linkage
between the PbI6 octahedra in the three different compounds, also
differences in effective mass can play a role.
a
.
b.
d.
c.
0.020
90
80
70
60
50
40
30
20
10
0
0.015
0.010
0.005
0.000
Concluded from single crystal data of the PAPbI3, it was
discussed that due to bulky size of cation the solvent is present in the
crystal structure. Sample powders were recovered from
dimethylformamide solution of the compounds. By comparing the
Raman spectra of PAPbI3 and dimethylformamide,31 it is clear that
dimethylformamide is present in the sample powder of PAPbI3.
Detected peaks at 667 cmꢀ1, 866 cmꢀ1, and 1413 cmꢀ1 are marked by a
red arrow in the spectrum of PAPbI3. These peaks may be weakly
observed in the EAPbI3 spectrum and are not present in the spectrum
Jsc x 100
IPCE x 10
Jsc x 10
IPCE x 100
0.0
0.2
0.4
V (v)
0.6
0.8
400
600
800
Wavelength (nm)
Figure 8. a. Representative photographs of TiO2ꢀsubstrates spinꢀ
coated by the three different alkylammonium lead(II) iodides, APbI3
b. Schematic device architecture for the APbI3ꢀbased solar cells c. IV
of MAPbI3.
Observed peaks between 1400 cmꢀ1 to 1700 cmꢀ1 are
and d. IPCE curves obtained for
ꢀꢀꢀ (CH3NH3)PbI3, ꢀꢀꢀ
attributed to the δ(CH2) and δ(CH3). Primary amine peaks related to
vibrations of NꢀH bond have been observed at 3000 cmꢀ1. These
peaks have been identified in Raman spectra of the alkylammonium
salt as well as in spectra of alkylammonium lead(II) iodide (See
Figures S1, S2 and S3).
(CH3CH2NH3)PbI3, and ꢀꢀꢀ(CH3CH2CH2NH3)PbI3 based solar cells.
IV characterization was done under 1 sun AM1.5G illumination.
Table 3 Photovoltaic parameters (Average values of 5 devices) of
the MAPbI3, EAPbI3 and PAPbI3 devices. 600 nm thick TiO2 films
were used as substrate, and SpiroꢀOMeTAD was used as the HTM.
A 200 nm layer of silver was used as back contact.
MAPbI3
EAPbI3
PAPbI3
Jsc
mAcmꢀ2
16.29±1.69
0.77±0.14
0.075±0.021
0.0
100
200
300
Wavenumber (cm-1
)
Voc (V)
FF
0.784±0.026
0.580±0.011
7.4±0.59
0.662±0.040
0.521±0.051
0.26±0.025
2.83
0.564±0.034
0.372±0.034
0.016±0.004
0.18
η (%)
APCE
78.35
0.0
1200
Wavenumber (cm-1)
2400
Band
gap(eV)
1.6
2.2
2.4
Figure 7.
a. Raman spectra of ꢀꢀꢀ (CH3NH3)PbI3, ꢀꢀꢀ
(CH3CH2NH3)PbI3, and ꢀꢀꢀ (CH3CH2CH2NH3)PbI3b. Magnification
of the low frequency vibrations attributed to PbꢀI structural units.
For the solar cells based on the MAPbI3 as the photosensitizer, a
power conversion efficiency of 7.4% was obtained (in our hands).
For the EAPbI3 and PAPbI3 sensitized solar cells a dramatic drop in
conversion efficiency was observed, 0.26% and 0.02%, respectively.
In an effort to reduce the well documented effects of hysteresis,32 we
have scanned all IV curves from positive to negative potentials at a
scan rate of 10 mV/s. The IPCE onset for EAPbI3 and PAPbI3 based
Solar cell data: Solar cells were prepared based on the three
APbI3 materials as photosensitizers; all of the other components
were same for the solar cells. Figure 8 shows the related curves of
current densityꢀvoltage (IV) average values for five individual
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5

Journal of Materials Chemistry A
Page 6 of 7
VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C4TA06174H
solar cells were observed at 550 nm and 525 nm respectively, which decreased in efficiency as the length of the alkylammonium cation
are in agreement with the UVꢀvisible spectra. The decrease in
efficiency of the EAPbI3 and PAPbI3 solar cells is mainly due to the
very low photocurrent of the solar cells. Current density declined
from 16.3 mA cmꢀ2 for MAPbI3ꢀbased solar cell to 0.8 mA cmꢀ2 for
EAPbI3ꢀbased solar cell and 0.075 mAcmꢀ2 for PAPbI3ꢀbased solar
cell. Part of this can be attributed to the increasing bandgap of these
materials; the bandgap of MAPbI3, EAPbI3, PAPbI3 were determined
to 1.6, 2.2, and 2.4 eV, respectively. However, the Absorbed Photonꢀ
size increased.
We have provided insight into the key structural properties and
their major influences on the physical properties of the APbI3
perovskiteꢀtype materials. We have proved that dimensionality is an
important factor in obtaining desirable properties from these
materials for photovoltaic applications. It has a major effect on the
toꢀcurrent Conversion Efficiency (APCE) for each APbI3 material absorption and conductivity, which is highly influential on the
decreases as the alkylammonium cation increases in size, which
suggests that the chargeꢀinjection or chargeꢀcollection is the main
limiting factor in these solar cells. Dimensionality may play a
significant role as well. EAPbI3 and PAPbI3, which are oneꢀ
dimensional materials, rather than threeꢀdimensional as MAPbI3, the
charge transport ability is much more restricted (as is reflected in
their lower electrical conductivities). The lower conductivity
exhibited by EAPbI3 and PAPbI3 leads to longer electron transport
times in the film and a significantly larger risk of charge
recombination for the injected electrons in the conduction band of
these materials resulting in decreased charge collection. The APCE
values drastically decrease from 78% for MAPbI3 to 3% for EAPBI3
and furthermore to 0.2% for PAPbI3. The APCE decreasing trend is
further evidence for the lower efficiency of the solar cells based on
the EAPbI3 and PAPbI3. In perovskiteꢀbased solar cells, it has been
functionality of these materials. Crystallographic data of PAPbI3 has
been analysed. We have investigated the absorption and conductivity
properties of the alkylammonium lead(II) iodide. We have shown
that these properties are severely influenced by the structure of these
materials. For the first time, EAPbI3 and PAPbI3 have been used in
the solid state solar cells. To further improve upon perovskiteꢀlike
solar cells, this study may offer guidance in the further search to
develop new leadꢀfree materials. Further investigations are presently
underway to evaluate how the structure of leadꢀfree perovskiteꢀlike
materials affects their photovoltaic properties.
Acknowledgements
The authors would like to acknowledge the generous support from
reported that electrons will be carried along the structural lattice of the Swedish Government through “STandUP for ENERGY”, The
the perovskite material before injection to the oxide semiconductor.4
Swedish Energy Agency, The Swedish Research Council, and Knut
There is no reason to believe that another working mechanism is
& Alice Wallenberg Foundation.
predominant for EAPbI3 or PAPbI3, when compared to the better
performing MAPbI3ꢀbased solar cells. Previously, EAPbI3 has been
Notes and references
used as a quantumꢀdot photosensitizer in liquid DSC.29 The low
conductivity is likely to not have played a substantial role in such
solar cells, since charge transport distances in the EAPbI3 were on
the order of the quantumꢀdot (nanometers) distances instead of 100’s
of nanometers as in solidꢀstate DSCs. The shorter charge transport
distance is likely to lead to a lower risk of charge recombination.
This could lead to a significant drop in photocurrent density and the
resulting power conversion efficiency.
a
Division of Applied Physical Chemistry, Department of Chemistry, KTH
Royal Institute of Technology, SEꢀ100 44, Stockholm, Sweden.
b
Organic Chemistry, Center of Molecular Devices, Department of
Chemistry, KTH Chemical Science and Engineering, 10044, Stockholm,
Sweden
† Electronic Supplementary Information (ESI) available: Tables S1, S2,
and S3 including detailed crystal structural information of the materials
and Figures S1ꢀS3 comparing Raman data for the three APbI3 materials.
CCDC 1012805, contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12
Union Road, Cambridge CB2 1EZ, UK; fax:441223336033; eꢀmail:
deposit@ccdc.cam.ac.uk)
Conclusions
In order to investigate the structure and function relationships for
alkylammonium lead(II) iodides (APbI3), three different APbI3
materials were synthesized with varying bulkiness in the
alkylammonium cation (A). From the spectral and structural
characterization, we have concluded that by introducing a bulkier
cation to the APbI3 materials that the material dimensionality is
reduced and that the distance between PbꢀI structural units on
average increases, both factors result in a substantially lower
electrical conductivity. The increase in average distances between
leadꢀiodide structural units is expected to dramatically reduce orbital
overlap in EAPbI3 and PAPbI3. The absorption spectra indicate that
by moving from MAPbI3 to PAPbI3 that the electronic transitions are
shifted to higher energy, which results in weaker absorbance at
longer wavelengths. As a consequence, and as expected, an increase
in the bandgap of the compounds from 1.6 eV for MAPbI3, 2.2 eV
for EAPbI3, and 2.4 eV for PAPbI3 was observed. For PAPbI3 the
bulkiness of the cation facilitates the incorporation of solvent
molecules into to the crystalline structure. The observed changes in
the conductivity of the APbI3 materials are in logical agreement with
the structural observations. Solar cells based on these three materials
1. J. Burschka, A. Dualeh, F. Kessler, E. Baranoff, N.ꢀL. CeveyꢀHa, C.
Yi, M. K. Nazeeruddin and M. Grätzel, Journal of the
American Chemical Society, 2011, 133, 18042ꢀ18045.
2. S. Mathew, A. Yella, P. Gao, R. HumphryꢀBaker, F. E.
CurchodBasile, N. AshariꢀAstani, I. Tavernelli, U.
Rothlisberger, K. NazeeruddinMd and M. Grätzel, Nat Chem
,
2014, , 242ꢀ247.
6
3. H.ꢀS. Kim, C.ꢀR. Lee, J.ꢀH. Im, K.ꢀB. Lee, T. Moehl, A. Marchioro,
S.ꢀJ. Moon, R. HumphryꢀBaker, J.ꢀH. Yum, J. E. Moser, M.
Gratzel and N.ꢀG. Park, Sci. Rep., 2012, 2.
4. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J.
Snaith, Science, 2012, 338, 643ꢀ647.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 7 of 7
Journal Name
Journal of Materials Chemistry A
View ArticAle ROnTliInCeLE
DOI: 10.1039/C4TA06174H
5. K. Akihiro, I. Masashi, T. Kenjiro and M. Tsutomu, Chemistry 32. E. L. Unger, E. T. Hoke, C. D. Bailie, W. H. Nguyen, A. R. Bowring,
Letters, 2012, 41, 397.
T. Heumuller, M. G. Christoforo and M. D. McGehee, Energy
Environ. Sci., 2014, , 3690ꢀ3698.
6. C. R. Kagan, D. B. Mitzi and C. D. Dimitrakopoulos, Science, 1999,
286, 945ꢀ947.
7
7. J. Burschka, N. Pellet, S.ꢀJ. Moon, R. HumphryꢀBaker, P. Gao, M. K.
Nazeeruddin and M. Gratzel, Nature, 2013, 499, 316ꢀ319.
8. S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P.
Alcocer, T. Leijtens, L. M. Herz, A. Petrozza and H. J. Snaith,
Science, 2013, 342, 341ꢀ344.
9. G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S.
Mhaisalkar and T. C. Sum, Science, 2013, 342, 344ꢀ347.
10. F. Hao, C. C. Stoumpos, D. H. Cao, R. P. H. Chang and M. G.
Kanatzidis, Nat Photon, 2014, 8, 489ꢀ494.
11. C. C. Stoumpos, C. D. Malliakas and M. G. Kanatzidis, Inorganic
Chemistry, 2013, 52, 9019ꢀ9038.
12. M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, 395ꢀ398.
13. N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu and S. I. Seok,
Nat Mater, 2014, 13, 897ꢀ903.
14. J.ꢀH. Im, I.ꢀH. Jang, N. Pellet, M. Grätzel and N.ꢀG. Park, Nat Nano
2014, , 927ꢀ932.
,
9
15. A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, Journal of the
American Chemical Society, 2009, 131, 6050ꢀ6051.
16. J.ꢀH. Im, C.ꢀR. Lee, J.ꢀW. Lee, S.ꢀW. Park and N.ꢀG. Park,
Nanoscale, 2011, 3, 4088ꢀ4093.
17. H. Zhou, Q. Chen, G. Li, S. Luo, T.ꢀb. Song, H.ꢀS. Duan, Z. Hong, J.
You, Y. Liu and Y. Yang, Science, 2014, 345, 542ꢀ546.
18. T. Baikie, Y. Fang, J. M. Kadro, M. Schreyer, F. Wei, S. G.
Mhaisalkar, M. Graetzel and T. J. White, Journal of Materials
Chemistry A, 2013, 1, 5628ꢀ5641.
19. D. B. Mitzi, M. Yuan, W. Liu, A. J. Kellock, S. J. Chey, V. Deline
and A. G. Schrott, Advanced Materials, 2008, 20, 3657ꢀ3662.
20. D. B. Mitzi, C. A. Feild, Z. Schlesinger and R. B. Laibowitz, Journal
of Solid State Chemistry, 1995, 114, 159ꢀ163.
21. D. B. Mitzi, Journal of Materials Chemistry, 2004, 14, 2355ꢀ2365.
22. E. Edri, S. Kirmayer, D. Cahen and G. Hodes, The Journal of
Physical Chemistry Letters, 2013, 4, 897ꢀ902.
23. J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal and S. I. Seok, Nano
Letters, 2013, 13, 1764ꢀ1769.
24. I. Borriello, G. Cantele and D. Ninno, Physical Review B, 2008, 77
,
235214.
25. G. M. Sheldrick, University of Göttingen, Germany, 2003.
26. G. M. Sheldrick, Acta Crystallographica Section A, 2008, 64
.
27. J. Tauc, Grigorovici, Radu. and Vancu, Anina, Phys. Status Solidi B
1996, 15, 10.
,
28. L. Etgar, P. Gao, Z. Xue, Q. Peng, A. K. Chandiran, B. Liu, M. K.
Nazeeruddin and M. Grätzel, Journal of the American
Chemical Society, 2012, 134, 17396ꢀ17399.
29. J.ꢀH. Im, J. Chung, S.ꢀJ. Kim and N.ꢀG. Park, Nanoscale Research
Letters, 2012, 7, 353.
30. C. Quarti, G. Grancini, E. Mosconi, P. Bruno, J. M. Ball, M. M. Lee,
H. J. Snaith, A. Petrozza and F. D. Angelis, The Journal of
Physical Chemistry Letters, 2013, 5, 279ꢀ284.
31. S. Michel, A. Courjaud, E. Mottay, C. Finot, J. Dudley and H.
Rigneault, BIOMEDO, 2011, 16, 021108ꢀ021108ꢀ021104.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7