Effect of guanidinium on the optical properties and structure of the methylammonium lead halide perovskite

Article abstract of DOI:10.1016/j.jallcom.2017.12.177

The stability and performance of perovskite-based solar cells can be improved by changing the nature of the organic cation. Herein, mixed methylammonium–guanidinium perovskites (MA_1-xGA_xPbI₃) are structurally and optically characterized. The Pawley fit method confirmed the formation of the iodide halide GAPbI₃ tetragonal phase (P42_NMC). Up to 20% of the guanidinium cation was incorporated in the methylammonium lead iodide perovskite, producing a lattice enlargement, which was investigated studying the shift of the diffraction peaks of the MAPbI₃ (I4_CM) tetragonal lattice. Long-term stability was tested, resulting in improved mixed perovskites with a low GA content. The bandgap shifted to lower energies. The absorption bandgap diminished slightly when the GA cation substituted up to 20% of MA in MAPbI₃, degrading when the GA amount in the mixed perovskite is larger. FESEM morphological analysis was performed showing that a uniform thin film was deposited. PL studies showed that only shallow defects had been introduced.

Full text of DOI:10.1016/j.jallcom.2017.12.177

Accepted Manuscript
Effect of guanidinium on the optical properties and structure of the Methylammonium
lead halide perovskite
Erika Vega, Miguel Mollar, Bernabé Marí
PII:
S0925-8388(17)34373-6
10.1016/j.jallcom.2017.12.177
JALCOM 44263
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 21 October 2017
Revised Date: 13 December 2017
Accepted Date: 16 December 2017
Please cite this article as: E. Vega, M. Mollar, Bernabé. Marí, Effect of guanidinium on the optical
properties and structure of the Methylammonium lead halide perovskite, Journal of Alloys and
Compounds (2018), doi: 10.1016/j.jallcom.2017.12.177.
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ACCEPTED MANUSCRIPT
Effect of Guanidinium on the Optical Properties and Structure of the
Methylammonium Lead Halide Perovskite
Erika Vega, Miguel Mollar, Bernabé Marí
Institut de Disseny i Fabricació (IDF) – Universitat Politècnica de València, València, Spain. Camino de
Vera s/n 46022.
{
ervefl,mmollar,bmari}@upv.es
Corresponding author: Erika Vega Fleitas
Abstract
The stability and performance of perovskite-based solar cells can be improved by
changing the nature of the organic cation. Herein, mixed methylammonium–
guanidinium perovskites (MA1-xGA
x
PbI
3
) are structurally and optically characterized.
tetragonal
The Pawley fit method confirmed the formation of the iodide halide GAPbI
3
phase (P42_NMC). Up to 20% of the guanidinium cation was incorporated in the
methylammonium lead iodide perovskite, producing a lattice enlargement, which was
3
investigated studying the shift of the diffraction peaks of the MAPbI (I4_CM)
tetragonal lattice. Long-term stability was tested, resulting in improved mixed
perovskites with a low GA content. The bandgap shifted to lower energies. The
absorption bandgap diminished slightly when the GA cation substituted up to 20 % of
3
MA in MAPbI , degrading when the GA amount in the mixed perovskite is larger.
FESEM morphological analysis was performed showing that a uniform thin film was
deposited. PL studies showed that only shallow defects had been introduced.
Keywords: Thin film perovskite, Mixed halide perovskite, Gap energy, Chemical
composition, Photoluminescence.
1

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1
. Introduction
The methylammonium lead halide perovskite (CH
3 3 3
NH PbX , X= halogen;
CH NH = MA) and its mixed-halide crystals have been studied in depth in recent years,
3 3
showing many optimal optoelectronic features of an ideal solar cell absorber. Among
these properties, a favorable bandgap, high absorption coefficient, excellent charge
mobility, high photoluminescence quantum efficiencies, small exciton binding energy,
tunable bandgap and a reasonably defect tolerance [1-6] can be highlighted. These
features have given perovskites an important role as an absorber in photovoltaic cells
[
7-11]. Indeed, in a relatively short period of time, devices based on perovskite
materials have achieved a performance of certified 22.1% PCE [12], starting from a
conversion efficiency of only 3.9% in 2009 [1] and being, at the same time, more cost-
effective than the silicon solar ones.
When it comes to the synthesis of the perovskite, a wide variety of procedures
has been employed such as vacuum deposition [13], vapor-assisted solution processing
[
14], atomic layer deposition [15], solution processing in one or two steps [16,17] etc.
However, MAPbI perovskite materials are still not stable enough in the long term to
become fully affordable and scalable. MAPbI perovskite materials have been proven
to degrade under humid conditions [18], and several techniques have been applied in
order to improve stability [19-21]. In this context, it has been found that MAPbBr is
less sensitive to humidity [22,23], but at the expense of losing absorption properties
3
3
3
[
24]. Some researchers focused on mixed halide perovskites MAPbI3−xBr
x
and MAPbI3–
x
x
Cl as the substitution of iodine by chloride or bromide atoms leads to a significant
improvement of perovskite stability [25,26].
Defect energy levels have been shown to be situated relatively shallow [27,28]
and although it had been hypothesized that grain boundaries do not contribute
significantly to recombination losses in perovskites, it has been shown that they are
much more harmful than previously assumed [29,30]. This means that if it were
possible to mitigate nonradiative carrier losses, the efficiency of perovskite solar cells
could be substantially improved. In order to successfully mitigate such charge carrier
losses, recombination pathways within the perovskite and device have been
suppressed. Employing additives in perovskite methylammonium lead iodide seems to
2

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be an efficient method to tune its properties such as defect density and charge carrier
lifetime. Regarding this topic, under-coordinated ions that may exist at grain
boundaries and surfaces that can act as charge carrier trap/recombination centers
[
31,32] have been described.
Different passivation methods have been employed up to now. For example, an
iodopentafluorobenzene (IPFB) post-treatment has been used to passivate under-
coordinated iodine [31], thiophene and pyridine have been employed to passivate
under-coordinated Pb ions [32] and, recently, Br and Cl have been suggested to assist
in suppressing recombination and decoupling electron−hole pairs [33,34].
+
In order to improve performance, the guanidinium organic molecule CH
6
N (GA)
3
has been investigated as an additive for dye-sensitized solar cells (DSSCs), in which it
serves to passivate TiO surfaces. Also, it has been studied as an additive in MAPbI in
2
3
order to improve structure, stability and performance. GA has a larger size (278 pm)
than MA (217 pm), which can lead to different perovskite structures and/or behaviors
[
37,38], therefore it is necessary to investigate the consequences of the GA addition in
detail. Some authors suggest that bias-induced motion and hysteric effects might be
influenced by the zero-dipole moment of GA [35,36].
Very recently, a passivation effect through partial GA incorporation in perovskite
films has been demonstrated [35], obtaining a significant mitigation of nonradiative
decay and an enhanced carrier lifetime, high open circuit voltages and improved
device performances.
This study presents the structural and optical effects of partial GA incorporation in
MAPbI
3
perovskite thin films with the aim of increasing and improving the stability of
solar cells.
2
. Experimental procedure
2
.1 Perovskite fabrication
Guanidinium iodine (CH
6 3
N I, GAI) was purchased from Sigma Aldrich. Crystalline
methylammonium iodide (CH NH I, MAI) powder was synthesized by reacting 0.3 mol
3
3
of concentrated hydriodic acid (Sigma Aldrich) with 0.3 mol of methylamine (CH NH )
3
2
solution (33% in methanol, Sigma Aldrich) whilst being stirred for 2 h in a 250 mL
3

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round-bottom flask kept in an ice bath (0°C). The solvent was evaporated at 50 °C in a
rotary evaporator. Then, the precipitate was filtered and recrystallized by solving it in
absolute ethanol and precipitating it with the addition of diethyl ether. After this
process, pure MAI was obtained.
The films were deposited onto the indium tin oxide (ITO) substrates previously
cleaned with soap (2%, Hellmanex) and rinsed with deionized water. Then, the
substrates are rinsed with ethanol, acetone and isopropyl alcohol for 15 min by
sonication. After that, the substrates were dried with argon blow.
The MA GA PbI films were prepared by mixing a stoichiometric ratio of GAI, MAI
1
-x
x
3
and PbI (99.9%, Sigma Aldrich). The precursors were dissolved at 45% wt in
2
dimethylformamide (DMF) under inert atmosphere in an argon glove box. The
solutions were kept under magnetic stirring at 60°C for 30 minutes. The films were
deposited, inside the globe box, using the spin coating process at 5000 rpm for 20 s,
then dried and annealed at 100°C for 1 h, for the MAPbI
PbI samples for 15 minutes. These samples were kept in the glove box
before analysis.
3
samples and at 135°C for the
MA1-xGA
x
3
2
.2 Thin film characterization
X-Ray Diffraction:
X-Ray diffraction patterns of perovskite films were obtained by a RIGAKU Ultima
IV diffractometer in the Bragg-Bentano configuration using Cu K
Å).
α
radiation (λ = 1.54060
Absorption Spectroscopy:
Absorption spectra of perovskites were calculated from the transmittance
measurements obtained with an Ocean Optics HR4000 spectrometer equipped with a
Si-CCD detector and an integrating sphere to collect specular and diffuse transmittance
at room temperature.
Photoluminescence (PL):
4

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Photoluminescence spectra were recorded at low temperature using an HE-closed
cryostat. A 325 nm He-Cd laser was used as PL excitation source. Photoluminescence
data were recorded by a Hammamatsu Si-based back-thinned CCD detector.
Field-Emission Scanning Electron Microscopy:
Surface morphology and elemental composition measurements of perovskite film
samples were taken using an environmental scanning electron microscope FESEM
(
Quanta 200 − FEI).
3
. Results and Discussion
3
.1 X-Ray Diffraction
x 3
Fig. 1a presents the diffractograms (XRD) of MA1-xGA PbI for different x values.
The MAPbI
8.4° (2 ), corresponding to planes 100 and 200, respectively. These peaks can be
observed for the MAPbI and MA GA PbI samples (0.1 ≤ x ≤ 0.2) with a GA content of
3
perovskite characteristic peaks are located at approximately 14.1° and
2
θ
3
1-x
x
3
up to 20%. When the GA content is above 20%, the 8.4° and 11.2° peaks,
corresponding to the GA tetragonal phase become visible, indicating the solubility limit
has been reached.
To determine the lattice parameter of GAPbI , a Pawley refinement of the XRD
3
spectra was carried out. A tetragonal lattice type (space group P42_NMC) was found
when neglecting the ITO peaks. Fig. 1b shows a good fit between the simulated and
the experimental diffractograms.
In order to gain further insight into the GA incorporation, Pawley refinements of
x 3
the MA1-xGA PbI (x = 0, 0.1, 0.2) XRD patterns were carried out obtaining a tetragonal
lattice type (space group I4_CM). Experimental and simulated data for the 100 peak
are shown in Fig. 1c. The addition of GA produces a slight shift toward a lower angle of
the characteristic 100 and 200 MA perovskite peaks, located at 14.1° and at 28.4°,
which can be attributed to the fact that GA has a larger volume than MA and the
partial substitution produces an increase in the lattice parameter. Table 1 summarizes
the results of the Pawley refinements.
5

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Fig. 1d shows the evolution of the lattice parameters vs GA content x. As can be
seen, the cell parameter a varies almost linearly, following Vegard’s law. However,
parameter c slightly increases with a GA content of up to 10%. The abrupt decrease for
higher concentrations than x = 0.15 indicates a probable lattice change.
Figure 1. (a) X-ray diffractograms of MA1-xGA
for pure GAPbI corresponding to the space group tetragonal P42_NMC. (c) XRD patterns of the MAPbI
and MA1-xGA PbI with x = 0.1, 0.2. (d) Composition dependent evolution of lattice parameters as a
function of GA content.
x 3
PbI (0 ≤ x ≤ 1), by the spin coating process. (b) XRD spectra
3
3
x
3
Table 1. Refinement factors and refined cell parameters using the Pawley method.
Sample id.
MAPbI
a = b (Å)
8,87659
8,90393
8,93066
11,97202
c (Å)
Rwp
3
12,65859
12,66932
12,61813
20,86364
4,42%
4,21%
3.95%
5,52%
MA0.9GA0.1PbI
3
3
MA0.8GA0.2PbI
GAPbI
3
In order to evaluate the stability of mixed perovskite samples, diffractograms of
PbI thin films were recorded immediately after sample
preparation. These samples were kept at room temperature and humidity for eight
MAPbI
3
and MA1-xGA
x
3
6

ACCEPTED MANUSCRIPT
months before a control diffractogram was made. Fig. 2 shows diffractograms for
MAPbI and MA GA PbI before and after eight months aging, denoted (A) and (B),
3
0.9
0.1
3
respectively. As can be seen, the aged pure methylammonium perovskite shows a
2.6° peak, corresponding to PbI , evidencing perovskite decomposition. Nevertheless,
1
2
the diffractogram of the mixed methylammonium-guanidinium perovskite sample
remains unaltered. Samples with a greater GA contents show PbI and/or GA peaks
2
that evidence decomposition or guanidinium perovskite segregation, as can be seen in
Fig. S1.
3 3
Figure 2. X-ray diffractograms of MAPbI and MA0.9GA0.1PbI before and after eight months.
3
.2 Absorbance
Fig. 3a displays the absorption spectra for different GA values. An integrating
sphere was used to collect both, specular and diffuse transmittance to reduce the
effect of light scattering originated from refraction and reflection phenomena on the
perovskite crystals.
x 3
The GA fraction contained in MA1-xGA PbI affects the optical properties. It can be
appreciated easily that an increasing amount of GA results in front edge diminution,
and, therefore, a reduction of the absorbance.
Fig. 3b provides Tauc plots showing variations in the optical bandgap. It can be
observed that for the samples between 0 and 20%, the bandgap evolves very slightly
towards lower energies. In Fig. 3a, a decrease of the slope for a GA contents above
2
0% can be appreciated, indicating a degradation of the bandgap. This result agrees
with the previously suggested lattice change. Fig. 3c shows a change of color varying
from black for MAPbI to yellow for GAPbI
3
3
.
7

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Table 2 shows the estimated bandgap (Eg) of MA GA PbI for samples between 0
1
-x
x
3,
and 20% GA contents obtained from Fig. 3b. It can be seen that the bandgap remains
almost constant, the overall variation between all the samples being 1.59 to 1.60 eV,
corresponding to absorption edges spanning 775−780 nm. The inset of Fig. 3b shows
the estimated bandgap (Eg) for the GAPbI sample being 2.62 eV, corresponding to an
3
absorption edge of 470 nm.
Figure 3. (a) UV-Vis absorption spectra of MA1-xGA
x 3
PbI for different GA contents. (b) Tauc plots
showing variations in optical bandgap. (c) Thin film images of MA1-xGA PbI for x = 0, 0.05, 0.1, 0.15, 0.2,
x
3
0
.3, 0.4, 0.5, 1, from left to right.
Table 2. Band gap variation depending on GA content.
Absorption edge at Room T
Sample Composition
(
eV)
(nm)
775
780
780
780
780
470
MAPbI
3
1.60
1.59
1.59
1.59
1.59
2.62
MA0.95GA0.05PbI
MA0.9GA0.1PbI
MA0.85GA0.15PbI
MA0.8GA0.2PbI
3
3
3
3
GAPbI
3
3
.3 Photoluminescence
The photoluminescence study at different temperatures illustrates charge carrier
dynamics as well as the quality of the material emission. Fig. 4a shows the PL results of
8

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MA GA PbI (x = 0, 0.15, 0.5, 1) at 11 K. Intensities have been normalized to clarify the
1
-x
x
3
shift of the emission peaks. It can easily be observed that the peak, about 800 nm for
the MA-GA-based samples, shifts slightly to higher wavelengths as the GA
concentration increases. This behavior could be explained by the bandgap defects
introduced by the GA doping. Further research is required to clarify this behavior.
Moreover, a pure GAPbI perovskite shows a single peak located at 565 nm.
3
For a GA content of 50%, both peaks, at 570 and 810 can be observed, which is
indicative of the two phases being present, as shown in Fig. S2, being in agreement
with previous results for a GA content above 20%.
PL intensity of the MAPbI and MA0.85GA0.15PbI samples at 11 K is shown in Fig. 4b.
3
3
MA0.85GA0.15PbI shows a large rise in PL intensity, more than three times than that of
3
the pure MA samples, as has been described in ref [35]. This improvement in PL could
be due to the fact that the incorporation of GA reduces the amount of non-radiative
levels, and, then the recombination takes place through radiative paths, resulting in
the PL improvement.
Table 3 shows that the calculated bandgap at different temperatures obtained
from PL measurements (Fig S3) increased as the temperature increased, as can be
inferred from the evolution of the peaks towards lower wavelengths.
From this table it can be seen that the gap energy at 11 K for MA0.85GA0.15PbI
3
is
inferior to the gap for MAPbI , which may be due to the incorporation of GA,
3
producing the aforementioned supplementary energy levels above the valence band or
conduction band, and the reduction of the gap, as a consequence.
λex= 325 nm; PL recorderd at 11K
λex= 325 nm; PL recorderd at 11K
MA0.85GA0.15
1,0
0,8
0,6
0,4
0,2
0,0
MA0.85GA0.15
MAPbI3
MA0.5GA0.5
MAPbI3
GAPbI3
400
500
600
(nm)
700
800
900
600
700
800
λ (nm)
900
λ
Figure 4. (a) Normalized photoluminescence emissions of MA1-xGA PbI for x = 0, 0.15, 0.5, 1 at 11 K. (b)
x
3
3 3
Photoluminescence emissions of MAPbI and MA0.85GA0.15PbI recorded at 11 K.
9

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Table 3. Band gap variation according to temperature and GA content.
Sample
PL peak position at 11 K PL peak position at 20 K PL peak position at 70 K PL peak position at 100 K
Composition
(
eV)
(nm)
787
827
807
565
(eV)
1.58
1.51
1.53
2.21
(nm)
785
818
807
560
(eV)
1.58
1.52
1.56
2.22
(nm)
782
814
795
558
(eV)
1.58
1.53
1.56
-
(nm)
782
809
795
-
MAPbI
3
1.57
1.49
1.53
2.19
MA0.85GA0.15PbI
3
MA0.5GA0.5PbI3
GAPbI
3
3
.4 FESEM Analysis
SEM analysis was conducted in order to examine the impact of GA
incorporation on the morphology. Fig. 5 presents top view SEM images of the MA1-
GA PbI perovskite thin films deposited on ITO substrates for x = 0, 0.15, 0.5, 1. As the
x
x
3
amount of GA increases, the layer becomes more homogeneous, although the
absorption edges shift to higher energies and then the overall absorption decreases.
These results are in agreement with previous results [35].
MA0.85GA0.15PbI needle-shape layers can be appreciated, having a better-covered and
3
smoother surface. The size of the needle-shape diminishes, increasing the
homogeneity. Although the increased gap to be appreciated in Fig. 3c produces a
decrease in the harvesting capacity, the improved homogeneity of the layer is able to
compensate for this loss.
1
0

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3 3
Figure 5. FESEM micrographs of perovskite thin films of (a) MAPbI , (b) MA0.85GA0.15PbI , (c)
3 3
MA0.5GA0.5PbI and (d) GAPbI .
4
. Conclusions
This work reports the synthesis of MA1-xGA
inert atmosphere, deposited on ITO substrate by spin coating. X-Ray diffraction
analysis showed that the MAPbI perovskite characteristic peaks were only found in
x 3
PbI for different x values under an
3
x
the MA1-xGA PbI
3
samples (0.1 ≤ x ≤ 0.2), indicating the GA incorporation in the MAPbI
3
lattice.
Tetragonal lattice type, space groups P42_NMC and I4_CM, were found for GAPbI
and MAPbI , respectively. When the GA content exceeds 20%, both MAPbI
GAPbI peaks were present, indicating the solubility limit was reached.
3
3
3
and
3
Absorption spectra show that an increase in the GA content affects the optical
properties, reducing the absorbance and, therefore, the light harvesting capability.
Samples with a GA content between 0 and 20% showed a very slight shift towards
lower energies, and samples with a GA content above 20% exhibited a decrease of the
slope and, therefore, a degradation of the bandgap. The estimated bandgap (Eg) for
the samples with a GA content between 0 and 20% decreased from 1.60 to 1.59 eV,
corresponding to absorption edges spanning 775−780 nm.
1
1

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PL results of MA GA PbI for different values of x at different temperatures show
1
-x
x
3
that the peaks of the GA-based samples shift slightly to higher wavelengths as the GA
concentration increases. Energy peak decreases can be explained by the bandgap
defects introduced by the substitution of MA by GA. Above a GA content of 20%, peaks
at 570 and 810 nm can be observed, confirming that both phases are present.
SEM analysis images of the MA GA PbI perovskite thin films deposited on ITO
1
-x
x
3
substrate show that grain continuity seems to have improved.
5
. Acknowledgments
Funding: This work was supported by Ministerio de Economía y Competitividad
ENE2016-77798-C4-2-R] and Generalitat Valenciana [Prometeus 2014/044].
[
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Highlights:
•
•
Research on mixed methylammonium-guanidinium perovskites thin films is
reported.
x 3
Optimal GA content for MA1-xGA PbI is set for long-term stability and
performance.
•
•
•
Solubility limit of GA content has been established at 20%.
Bandgap tuning results for GA content below solubility limit are presented.
Detailed structural and optical characterizations are reported.
1