CHME 498
Undergraduate Research
New Mexico State University
Crystallization Kinetics of Organic-Inorganic Tri-halide
Perovskites and
The Role of the Lead Anion in Crystal Growth
Hilda Fontes
800580794
May 13th, 2016
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SUMMARY
Looking for the improvement of efficiency and lower cost in solar
cells, many research reported the device performance of organicinorganic tri-halide perovskites (CH3NH3)PbI3 founding that planar
devices are the most efficient for solar cells because of the final film and
crystal morphology (the grain size and orientation).
Based on the research done, they are looking for a device that can
improve the efficiency of the solar cells at a lower price, elucidate the
crystallization process of the organic-inorganic halide perovskites and
improve the crystal growth dynamics.
They assume the huge improvement on efficiency created by
abundant constituents at a lower price and then began the “future live”
with the energy obtained by perovskite solar cells.
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TABLE OF CONTENTS
Introduction……………………………………………………………………3
Data Obtained…………………………………………………………………4
Kinetic Modeling of the Precursor-to-Perovskite
Transformation……………………………………………………………….5
Dependency of the Activation Energy on the Lead
salt………………………………………………………………………………….6
Precursor Phase and the Role of the spectator salt…………6
Disadvantages…………………………………………………………………7
Conclusion………………………………………………………………………7
Key concepts…………………………………………………………………..8
References………………………………………………………………………9
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INTRODUCTION
Kinetics study involves quality and quantity measure of the
transformation and ending points in a reaction. On this study, they just
distinct three different structures that occur in a solid state: a
metastable, crystalline intermediate (precursor), the desired perovskite
and the third one the PbI2 decomposition product. Working in different
parameters (different temperature, different deposition method and
changes to the lead salt reagent) they conclude that always will get the
same evolution: precursor  perovskite PbI2.
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DATA OBTAINED
 Perovskite and PbI2 key peaks are located at q = 10nm-1 and q = 9nm-1
(where q represent the magnitude of the scattering vector defined as q =
4∏ sin θ/λ where θ is half of the total scattering angle and λ is the X-ray
wavelength)
The WAXS data give important data about the experiments, the direct measure
of the quantity of diffracting planes with a specific orientation (used to determine
x(t).)
After some calculations they conclude with the following definition for x(t):
x(t)= A10(t)/ A10(tend). That give them the conclusion that x(0)=0 and x(tend)=1.
WAXS images
40 wt %
solution of
PbCl2:(MA)I in a
1:3 molar ratio
on Si, thermally
annealed from
ambient
temperature to
100 °C
"Crystallization
Kinetics of OrganicInorganic Trihalide
Perovskites and the
Role of the Lead
Anion in Crystal
Growth." [Ithaca,
New York] 27 Jan.
2015: 2350-358.
Print.
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KINETIC MODELING OF THE PRECURSOR-TOPERVSKITE TRANSFORMATION
After proving the measure for x(t) been valid, the next step is work on the
mathematical analysis of kinetic data to prove the variety of growth and
transformation in perovskite. First, they used the following formula for isothermal
data:
Ln (tx2 – tx1) = Ea/RT - ln k0 + ln( βx2 – βx1 )
Where:
-txn = Time at which the transformed fraction is xn.
-Ea = Effective activation energy
-R = Gas constant
-T = Temperature
-k0 = Rate constant pre-factor
-βxn = State property that is invariant to the time/temperature path
Once they got the data, they created Ln (tx2 – tx1) vs 1/RT plot with the slope
line taken of Ea. Isothermal data was extracted from 80,90,100 and 110°C and x(t)
from WAXS data. They pick 80°C as a started point because previous experiments
showed that crystallization doesn’t occur below 80°C and morphological change
takes place at 130°C. After data collected, they conclude that the rate of the
transformation increases at higher temperatures, the hottest sample (110°C)
shown good UV-vis absorption, data clearly proved that they choose an
appropriate methodology.
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DEPENDENCY OF ACTIVATION ENERGY ON
THE LEAD SALT
Once the kinetics study was done, they choose to focus on the dependency
of activation energy on the lead salt because it showed that increased the device
performance.
The next experiments are based on four different systems, using lead
acetate, lead iodide (single temperature), lead chloride and nitrate. They used a
spectator salt ((MA)Sp) when all or some of the lead salt is not incorporated into
the perovskite. Also, the iodide films were prepared in a solution using 3:1 molar
ratio of (MA)I:PbI2 and annealed at a temperature of 150⁰C. They conclude that
kinetics of the acetate are faster than those on the chloride system.
To conclude, activation energy (Ea) it is one of the most important factors
that has the ability to manipulate the crystallization process.
PRECURSOR PHASE AND THE ROLE OF THE
SPECTATOR SALT
The precursor is one of the most important steps on the process
transformation to create perovskite, it has the ability to change the structural
information and compositional ambiguity. During this study, they had the kinetic
analysis performed giving us a way to distinguish the precursor from the final
perovskite structure.
The final step of this experimental data analysis was the use of extracted Ea
(activation energy), this change trying to get a different crystallization path and get
some answer about other results obtained by previous studies.
The most important things that we can identify because of the precursor are
the kind of process that can be evaporation of the solvent, diffusion of the excess
spectator salt out of the precursor structure, removal of the spectator salt from
the film, and finally the removal of stoichiometric (MA)I from the perovskite lattice.
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DISADVANTAGES OF PEROVSKITE SOLAR CELLS
Although perovskite have shown an amazing improvement in efficiency
compared to the other materials already used in solar cells, it still have big
disadvantages in real life application. After some experiments, the lead content of
the material have shown a negative environmental impact, they are working on
remove the requirement for lead. Also, organic solar cells shows a deficiency in
stability term because it’s really sensitive to moisture, just a few drops of water can
completely destroy the material and obviously drops its efficiency to 0.
CONCLUSION
After a lot of samples and experimental data, they determined the
appropriate kinetics model on each case, then they applied their knowledge to
extract the activation energy (Ea) for the precursor-to-perovskite transition in thin
films. They conclude that the better way to obtain better crystal growth is
changing the lead salt, it also showed an improvement in the effects on the film
morphology.
To conclude, by the end of the 2014 (received article date) they have
observed a crystalline structure prior to perovskite formation, then the
decomposition of lead iodide. They found that the precursor material is a mixture
of more than one materials and at least one of them are lead halide clusters that
help to begin the formation in the solution. Then they created a model to extract
the activation energy for the precursor-to-perovskite transition, and conclude that
the change in lead salt allow a better crystal growth and has measurable and
advantageous effects on the film morphology.
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KEY CONCEPTS
X-ray diffraction analysis
 Structural Analysis  X-ray diffraction provides most definitive structural
information ¬Interatomic distances and bond angles
 X-rays To provide information about structures we need to probe atomic
distances - this requires a probe wavelength of 1 x 10-10 m ~Angstroms
WAXS (Wide angle X-ray scattering)
 WAXS data are often used to test structural models, identify structural
similarities and characterize structural changes. WAXS is highly
complementary to crystallography and NMR.
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REFERENCES
Cornell University, Department of Materials Science and Engineering, Cornell High
Energy Synchtron Source (CHESS), Clarendon Laboratory University of Oxford, and
Kavli Institute at Cornell for Nanoscale Science Cornell University. "Crystallization
Kinetics of Organic-Inorganic Trihalide Perovskites and the Role of the Lead Anion
in Crystal Growth." [Ithaca, New York] 27 Jan. 2015: 2350-358. Print.
Loye, Hanno Zur. "X-Ray Diffraction: HOW IT WORKS WHAT IT CAN AND WHAT IT
CANNOT TELL US." University of South Carolina, 2013. Web.
<http://www.chem.sc.edu/faculty/zurloye/xrdtutorial_2013.pdf>.
Major, Jon. "Revolutionary Perovskite Solar Cells Could Be a Game Changer, but
Questions Remain." The Conversation. Academic Rigor, Journalistic Flair, 15 Aug.
2014. Web. 11 May 2016. <http://theconversation.com/revolutionary-perovskitesolar-cells-could-be-a-game-changer-but-questions-remain-30497>.