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Controlling structure and photophysical properties of bi triple perovskite through substitutional series Cs₃Bi₂Br₍₉₋ₓ₎Iₓ

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Controlling structure and photophysical properties of bi triple perovskite through substitutional series Cs₃Bi₂Br₍₉₋ₓ₎Iₓ

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Content Controlling Structure and Photophysical Properties of Bi Triple Perovskite
Through Substitutional Series Cs
3
Bi
2
Br
(9−x)
I
x
by
Bethany L. Seckman
A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
Master of Science
(CHEMISTRY)
December 2018
Copyright 2018 Bethany L. Seckman
Acknowledgments
First off, I want thank my advisors, Professor Brent C. Melot and Mark E.
Thompson. Working on this project was challenging at times, but I knew that
there was always help available from the both of them if I just asked.
A huge thank you to both of my groups. You all were so nice to me the last
few years and I could not have asked for a better group of people for this journey.
Everyone had so much knowledge to share and where one group’s knowledge left
off the other one picked up. I would not have made it this far without you guys.
Taylor Hodgkins, my project buddy, thank you. Our discussions about science
were where I learned so much during this degree. When I was bouncing ideas
off of you, you never made me feel like I was less than for having a terrible idea
while running on no sleep and too much caffeine. Those gentle nudges back
in the right direction kept everything on track and kept me sane in the process.
Thank you for all the laughs and the insider knowledge on the good taco stands.
Ariel Nessl, you always knew when I needed a break. Thank you for dragging
me out to get coffee when the refinements weren’t working out. Your advice on
what to do when times got tough was exactly what I needed to hear even if I
didn’t know it at the time. Thank you for being my closest friend while I was
here at USC and running with my to Trader Joes for every group meeting and
the occasional Pokemon raid on the way. I’ll remember this time with fondness
because of how you helped me through.
JoAnna Milam-Guerrero thank you for all the advice and encouragement that
there are many paths to success and happiness and that what was right for me
may not be what I expected. I don’t know who’s going to message me with a
ii
kitten sneezing emoji every time I sneeze now, but I entrust the emoji collection
to you. I know it’s in good hands.
Erica Howard thank you for always checking on me, exchanging cat pictures
with me, and saving various parts of my backpack from being chewed on. Thanks
for accepting me into the Ohio group even though I only lived there, not attended
undergrad. And let’s not forget that Andy was a crucial part of this thesis so give
him a treat for me.
Nick Bashian, thank you for providing a laugh with the obvious answer to any
question. You always had answers to my glovebox questions and were always
willing to run the odd errand. I appreciate those things more than you know.
Also thank you for not putting bananas on my desk, they probably would have
gotten lost among the papers, let’s be real.
Then there’s my high schooler who isn’t a high schooler anymore. Juliane
Oberstien, thank you for cleaning so many, many glass slides even though i was
your least favorite job. Teaching you everything you know about lab work was
a joy. You were always so willing and eager to learn and you helped with so
much of the data acquisition for this thesis you don’t even know. I’m so glad you
decided to go into chemistry and I wish you all the luck in your undergraduate
career.
Thank you to the Scanlon group for running calculations for this work. With-
out your contributions I would not have had any place to start with the refine-
ment comparison, and so I am forever grateful.
A huge thank you to Judy Fong, Magnolia Benitez, and Michele Dea for help-
ing me with all the non-science problems that grad school brings. The depart-
ment would fall apart without you guys.
iii
Thank you to my family. Thank you to my grandpa for all the support
throughout my education. It meant the world to me to know you always sup-
ported me. And no, I still don’t know how to play the Old Cat even with a fancy
degree. Maybe one day I’ll figure it out. Also thank you to my Aunt Steph, my
one and only and of course best Aunt. Thank you for always telling me I was
smart even when I didn’t feel like it. I know you’re still hoping I move closer
after this, but thank you for supporting my move clear out to California in the
first place.
Perhaps my most important thank you is my Mom. I would not be in grad
school without you, let alone finishing up with a masters. I don’t know where
I would be. Thank you so much for helping me get to this point in my life.
Thank you for taking all of my phone calls. Thank you for listening to me when
I needed it most. Thank you for telling me it was ok to take the path I wanted.
Even though I have to sleep on the couch when you visit, they were still some
of the best times I had in LA. Wherever I end up next I know you will be right
behind me with support and I cannot tell you how much that means to me. If I
had to pick one person to dedicate this to it would be you. Thank you for being
the best mom I could have ever had.
iv
Contents
List of Tables vi
List of Figures vii
1 Introduction 1
1.1 The Importance of Solar Energy and Current Generation Technology 1
1.2 Next Generation Solar Technology . . . . . . . . . . . . . . . . . 5
2 CompositionalControlovertheElectronicandCrystallographicStruc-
ture of Bi-based Triple Perovskites 10
2.1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.2 Computational . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3 Summary and Future Directions 22
Reference List 23
v
List of Tables
2.1 Bandgap and Structure of Series . . . . . . . . . . . . . . . . . . 20
vi
List of Figures
1.1 a) Typical device structure of silicon based solar cells.[1] b) Assem-
bly of a full photovoltaic unit for grid use.[2] c) The decrease in
power output for silicon cells versus angle of incidence is marked,
losing ten percent of its efficiency by 20 degrees. . . . . . . . . . 4
1.2 a) A perovskite with the formula ABX
3
b) A double perovskite with
the formula AB
3+
B
1+
X
6
c) A triple perovskite with the formula
A
3
B
3
X
9
d) A 1D perovskite with connectivity only along a 1D chain
of atoms. e) A 2D perovskite with connectivity along a sheet of
atoms. f) A 3D perovskite with connectivity in all three planes. . 7
1.3 MAPbI
3
has a structure of corner sharing octahedra in all direc-
tions, allowing it to carry charge easily through its bonds. . . . . 8
2.1 Room temperature crystal structure of Cs
3
Bi
2
Br
9
and Cs
3
Bi
2
I
9
, where
Bi is represented bu the dark grey spheres, the light grey spheres
represent Bi, the light blue spheres I, and the black spheres Cs. . 11
2.2 Powders of substitutions starting with x=0 on the left and contin-
uing to x=9 on the right. . . . . . . . . . . . . . . . . . . . . . . . 13
vii
2.3 (a)Results of Cs
3
Bi
2
Br
9
Rietveld refinement. R
B
ragg=5.6% (b)
Results of Cs
3
Bi
2
Br
3
I
6
Rietveld refinement using the Cs
3
Bi
2
Cl
9
CIF.
R
B
ragg=5.0% . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4 (a) Lattice parameter a increasing with increasing I. (b) Lattice
parameter c increasing with increasing I. (c) Shows the peaks in
the pattern shifting to the left indicating the d spacing increasing
with increasing I. . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.5 a) Cs
3
Bi
2
Br
9
structure with no octahedral rocking b) Cs
3
Bi
2
Cl
9
structure with octahedral rocking . . . . . . . . . . . . . . . . . . 16
2.6 CIF output of Cs
3
Bi
2
Br
9
refined Cs
3
Bi
2
Br
6
I
3
showing the lack of
bonds to the Cs1 and Br2 site. . . . . . . . . . . . . . . . . . . . . 17
2.7 The first equivalent of I drops the bandgap dramatically while
each subsequent addition of I lowers the bandgap in a more linear
manner. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.8 Tauc plot of the compositional series from x=0 to x=9 shows the
magnitude of the exicton peak decreases with increasing I concen-
tration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
viii
Controlling Structure and Photophysical Properties of Bi Triple Perovskite
Through Substitutional Series Cs
3
Bi
2
Br
(9−x)
I
x
by
Bethany L. Seckman
ix
Abstract
In response to the structural shortcomings of Cs
3
Bi
2
I
9
and the sub-optimal
bandgap and high exciton binding energy of Cs
3
Bi
2
Br
9
our group set out to create
a compound which had a lower bandgap while maintaining a structure more
suited to charge transfer. In our studies we substituted in I for Br in Cs
3
Bi
2
Br
9
to
create the substitutional series Cs
3
Bi
2
Br
(9−x)
I
x
in an attempt to maintain the more
advantageous structure while red shifting the bandgap back to a more useful
level. In this study we found that the substitutional series shifts structure from
the P3m1 of the Cs
3
Bi
2
Br
9
to a lower symmetry structure P321 similar to that of
Cs
3
Bi
2
Cl
9
. This lower symmetry structure differs in that the octahedra rock with
respect to the c-axis. This rocking does not change the corner sharing nature of
the octahedra, therefore the structure is still advantageous for charge transport.
Diffuse reflectance of the substitutional series showed a marked red shift in the
band gap from 2.6 eV to 2.0 eV for the substitutional series. Examining the
exciton peak in the spectra also showed a noticeable decrease in its magnitude
across the series. Maintaining a 2D corner sharing structure while lowering the
bandgap and exicton binding energy with increasing I concentration show that
the aim of the study was successful. Therefore, our results indicate that further
studies on the materials should be conducted, testing their efficiency in a device
setting to elucidate how the structural changes improve the performance.
x
Chapter 1
Introduction
1.1 The Importance of Solar Energy and Current
Generation Technology
All of the hottest years on record have been in the last 20 years.[3] With tem-
peratures predicted to increse even more, making weather patterns more unsta-
ble and sea levels rise, thus eroding at the coasts and a great deal of our major
cities[4], it is clear that something must be done to stop this temperature swell.
This large temperature surge is almost unanimously attributed to the marked
rise is the Earth’s greenhouse gases in the last hundred years, the most impor-
tant of which is CO
2
.[5, 6] The path to curbing the increasing temperatures is
clear, reducing the production of carbon dioxide. One of the major producers of
CO
2
in the world is electricity generation.[4] To this end, efforts into exploring
renewable, sources of energy which do not produce CO
2
have taken off in the
past few decades.
Much of this attention has focused on solar energy because of the abundance
of light and energy that the sun’s rays deliver to the Earth’s surface on a daily
basis. As a whole the sun delivers more energy in one hour than is used by
society in a year.[7] Capturing even a percentage of this output would exceed
the world’s energy needs without the potential damage to wild life that wind
and hydroelectric power hold.[7]
1
Solar cells use semiconducting materials to capture energy from the sun.[8]
These materials have an energy gap between their conduction and valence band
where electrons are forbidden to reside, called the band gap.[9] In order to
bridge this gap and conduct current, electrons must have a certain amount of
energy to bridge this gap. The larger the band gap, the more energy is needed to
cross this gap.[10] This band gap energy is dependent on the material and if it
has any dopant molecules contained within.[11] For solar applications it would
seem that the lower the bandgap, the better, however, this is not so. There
are other considerations like radiative and internal resistance that come into
play. In a paper Shockley and Queisser went through the basic considerations for
efficiency in a solar material and calculated that the optimal band gap was 1.34
eV.[12]
Having an optimal band gap in a material is not the only thing that deter-
mines the efficiency of light conversion. There are other considerations like
charge mobility and non-radiative recombination that must be considered that
the Shockley-Quisser limit did not take into consideration.[13] Calculating the
efficiency of a solar device uses the open circuit voltage (V
OC
), the short circuit
current (J
SC
), and fill factor (FF). These measurements bear out the other losses
in efficiency that a solar cell experiences. The equation used to calculate the
efficecy of a cell is:
η=
V
OC
J
SC
FF
P
in
whereη is efficency and P
in
is the power in to the device.[14] V
OC
and J
SC
can be
directly measured on a device while FF can be calculated one of several different
ways using the measured quantities of V
OC
and J
SC
.[15] In order for solar to be
a viable power source this efficiency must be as high as can be achieved, and
while modern technology has made great strides forward, it is not perfect yet.
2
Currently most of the solar cells manufactured are based on silicon, whether
single crystalline, polycrystalline, or amorphous.[16, 17] These devices are p-n
juncton based systems wherein the electrons generated by the sun migrate to the
n layer and holes to the p layer, creating a net flow of electricity. Typical device
structure can be seen in Figure 1.1a. Single crystalline cells are by far the most
efficient with current commercial models achieving a solar conversion efficiency
of over 20 % for the most advanced cells, and average cells reaching around 16
%.[18] Silicon has a good band gap for a photovoltaic device, 1.1 eV, that is near
the Shockley-Queisser optimal band gap of 1.34 eV.[12, 19] This gives silicon a
32 % theoretical efficiency, which can be increased by the use of multi-junction
cells and concentrators if necessary. This efficiency is not as high as some other
semiconductors, such GaAs which has a bandgap of 1.43 eV that is closer to the
ideal 1.34 eV than silicon; however silicon does not have the issues with cost
that GaAs does, as Ga is very rare, which is part of the reason why silicon is
the main commercial material used.[20] Thus to be a viable solar material the
constituents must be Earth abundant. As silicon makes up almost 30 % of the
Earth’s crust, it meets this criteria but falls short in other areas.
Along with its slight deviation from the ideal bandgap for maximum effi-
ciency, silicon has other major issues such as its indirect bandgap. Being an
indirect bandgap semiconductor means the generation of a useable electron-hole
pair is dependent on the light absorbed coupling with a photon, which is statisti-
cally unlikely.[21] In order to create a cell with any measureable efficiency, thick
layers of silicon have to be employed in order to increase the chance of light
being absorbed for charge generation. Since such thick layers of material are
required, this increases the weight of the cells by a great deal. With the other
components necessary to complete the assembly of a commercial cell, shown in
3
Figure 1.1b, the weight of a single residential panel is upwards of 40 pounds
and a full array weighs almost a ton.[22] This weight prevents cells from being
installed in places that cannot handle the weight, and also prevents the use of
the cells in transparent applications like windows. Even if the thickness did not
cause problems for window applications, for silicon cells to work at an optimum
level the angle of incidence must be as close to 0 as possible. This is impracti-
cal for most home installations where the panels remain fixed in angle.[23] For
other types of solar cells, such as organic photovoltaics, this strong dependence
on the angle of incident light is reduced, allowing the cells to lose less of their
efficiency as fixed cells than traditional silicon.[24]
All of these problems for silicon cells are taken into consideration in the
search for a next generation material. Future materials must be Earth abun-
dant, should have a direct band gap that is as close as possible to 1.34 eV, and
be light weight, but most importantly it must be low cost. While silicon is Earth
a)
b)
c)
Figure 1.1: a) Typical device structure of silicon based solar cells.[1] b) Assembly
of a full photovoltaic unit for grid use.[2] c) The decrease in power output for sil-
icon cells versus angle of incidence is marked, losing ten percent of its efficiency
by 20 degrees.
4
abundant, it exists mostly in the form of silica and other silicates.[25] In order
to purify silicon, silica is reacted in a carbothermic reactor to generate metallic
grade silicon. This takes 11 kWh/kg of material that must be further purified
to electrical grade silicon by reacting the resulting silicon with HCl at 400
◦
C
which is then distilled. The last step finally produces electrical grade silicon by
heating the resultant trichlorosilane to 1100
◦
C for days at a time to form sin-
gle crystal silicon.[26] This process consumes a huge amount of energy, between
120-180 kWh/kg, and therefore increases the cost of any silicon cell despite the
abundance of the starting materials.[26] In order for solar power to reach the
tipping point and truly take the place of fossil fuels this processing cost must
be decreased as much as possible. While economics of scale have helped in the
decrease of price, the cost per watt generated of solar cells is still aobve 2 dollars
versus the current cost of less than 0.70 cents per watt because of this large pro-
cessing cost.[27] In this vein, for solar energy to be commercially viable research
must focus on bringing down cost and mitigating the other problems inherent in
the use of silicon as the commercial solar absorber.
1.2 Next Generation Solar Technology
In order to meet the challenges presented, development of next generation
solar materials must have a structure suited to charge transfer in order to obtain
good efficiency. Materials with a great deal of connectivity are suited solar
applications.[28, 29, 30] Bonds allow charge to charge to flow easily through
the material. Materials with connectivity in all 3 planes is more likely to con-
duct better than a material with 0D dimers with no connectivity.[31, 32, 33] The
choosing a versatile structure that can be tuned readily also holds advantages
5
in being able to use it in a larger array of solar applications. Picking the right
structure for continued research is paramount to the success of finding the next
commercial solar material quickly enough to help stop Earth’s warming before
irreversible damage is done.
Perovskites are a very versatile crystal structure.[34, 35, 36, 37] The typical
formula is ABX
3
but there are also double perovskites A
2
B
2
X
6
and triple per-
ovskites A
3
B
3
X
9
.[38, 39, 40] These differing structures can be seen in Figure
1.2a-c respectively. Perovskites also can crystalize in 3D, 2D, and 1D crystal struc-
tures shown in Figure 1.2d-f.[41, 42, 43] Access to so many derivative structure
leads to a large number of compositions that can crystallize in the perovskite crys-
tal system. Access to so many possible compounds within the same system allows
for more rational design principles, allowing for research for the next big solar
material to progress more quickly. Another plus of the perovskite system lies in
one of the hallmarks of the structure: its corner sharing octahedra. Corner shar-
ing octahedra allow for the easy flow of charge through the bonds of the material
instead of charges having to hop from molecule to molecule.[44] Perovskites are
also easy to synthesize; solvothermal and calcination are two popular synthesis
routes requiring a limited amount of steps and little purification.[45] Compared
to organic absorbers which may take many steps and a great deal of purification,
perovskite synthesis is more accessable.[46, 47] These perks have led to a large
body of research on perovskites, including recently perovskite solar materials.
Methylammonium lead iodide (MAPbI
3
) is one such material.
MAPbI
3
is a perovskite with the structure ABX
3
, a structure that is particu-
larly suited to charge transport as it is made up of corner sharing octahedra in
all 3 dimensions, as can be seen in Figure 1.3. MAPbI
3
garnered a great deal of
attention for its meteoric rise in its power conversion efficiency, from around 4 %
6
to over 23 % in less than ten years.[48] This efficiency rivals silicon cells while
still being made from earth abundant materials, thus making it a very attrac-
tive alternative. On top of rivaling silicon in efficiency, it has a direct bandgap
and therefore it does not need large layers of material to absorb light, there-
fore making cells lighter in weight. Futhermore these materials may be solu-
tion processed, which is one of the least costly processing methods for solar cell
fabrication.[49, 50, 45] If mass produced, solution processed cells could rival
the price per watt of fossil fuels, which would be a huge step forward towards
energy independence.
However, MAPbI
3
has problems which can also be improved upon. The com-
pound is moisture sensitive and upon sitting in humid air will degrade back into
a) b)
c)
d) e) f)
Figure 1.2: a) A perovskite with the formula ABX
3
b) A double perovskite with
the formula AB
3+
B
1+
X
6
c) A triple perovskite with the formula A
3
B
3
X
9
d) A 1D
perovskite with connectivity only along a 1D chain of atoms. e) A 2D perovskite
with connectivity along a sheet of atoms. f) A 3D perovskite with connectivity in
all three planes.
7
its starting materials, MAI and PbI
2
.[51] This degradation severely limits the
efficiency of the devices. In order to be a commercially viable material, devices
need to have mostly stable conversion efficiency for two decades or more, yet
MAPbI
3
degrades within less than 24 hours in air.[52] Encapsulation fixes some
of this problem, but cells are still not as stable as desired, still degrading in the
timescale of days.[53] Considering there are other contributions to the decompo-
sition of the material, including heat and continuous illumination, much research
has gone into fixing all of these problems so that MAPbI
3
can become a viable
solar material.[54]
Even if these stability concerns were addressed there would still be an obsta-
cle in the way of MAPbI
3
becoming a commercial material; it contains Pb. With
the toxicity of Pb being a major concern, especially to children, manufactur-
ing solar cells containing Pb could be risking substantial Pb contamination of
the land and water around the solar installations. Reports have compared the
amount of Pb that would be used to generate all power in the United States with
Figure 1.3: MAPbI
3
has a structure of corner sharing octahedra in all directions,
allowing it to carry charge easily through its bonds.
8
MAPbI
3
would be more than both current electricity generation and metal pro-
cessing, on the order of thousands of tons of Pb released per year.[55] This pollu-
tion potential has led some manufactures to be wary of producing the material,
thus limiting production output and therefore viability of MAPbI
3
as a commer-
cial material.
In response to this Pb toxicity issue, research has branched into trying to find
new Pb free perovskite alternatives.[56, 57, 58] The first metal replacements that
were considered were metals in the same family, primarily Sn and Ge, however,
these were more unstable and less efficient than the lead compounds.[59, 60, 61]
After metals in the same oxidation state were exhausted, other metals were con-
sidered such as Bi. Bi was chosen because it is similar in size to Pb
2+
and is
isoelectronic. However, Bi
3+
with its higher oxidation cannot access the typi-
cal ABX
3
structure, which requires a metal in in the 2+ oxidation state.[62, 63]
However, it can crystallize in other perovskite systems like double and triple per-
ovskites where the excess charge can be balanced. Double perovskites allow
for mixed metals on the B site to fulfill the 4+ charge on the site, such as
Bi
3+
and Ag
+
.[58] Triple perovskites allow for the access of stable 2D defect
ordered structures such that one of the B sites remains vacant for charge bal-
ancing purposes.[64] This tunability of these two crystal structures offer a large
breadth of possible compounds to be investigated in the search for a practical
lead free perovskite solar material.
9
Chapter 2
Compositional Control over the
Electronic and Crystallographic
Structure of Bi-based Triple
Perovskites
Perovskites have drawn much attention in recent years in the continued
search for a viable next generation solar material. These materials have advan-
tages over traditional silicon cells in that they are solution processable, lighter,
and have the potential to be cheaper than silicon cells if produced on a large
scale.[49, 50, 65, 66, 67, 68, 69] Recent research into these materials has
focused on finding less toxic derivatives than the record setting methylammo-
nium lead iodide (MAPbI
3
)[70] as concern over potential lead pollution has
led to concerns over the viability of wide scale use.[55] Other than being
toxic, MAPbI
3
also suffers from stability problems in humid air, leading to the
decomposition into the starting materials and severely reducing any photovoltaic
performance.[71, 72] Replacing the Pb with other metals in the same oxidation
state results only in more unstable and far less efficent compounds than the
original MAPbI
3
.[73, 60, 74] Thus, research has branched out to other types of
perovskites including double and triple perovskites.
10
The defect ordered triple perovskite Cs
3
Bi
2
I
9
drew early attention because of
its suitable bandgap.[31, 75, 76] With a bandgap around 2 eV, it had potential to
be a replacement for MAPbI
3
, however, when employed in devices the efficiency
was less than one percent.[31] Further investigation into the cause of such poor
results returned deep defects within the material that led to non-radiative recom-
bination as well as high carrier masses.[75] These effects are mainly due to the
crystal structure of the material shown in Figure 2.1a below. The Bi atoms form
a face-sharing dimer with I on the corners of the bi-octahedra. These dimers
are not ideal for charge diffusion as the charge has to hop from dimer to dimer
in order to travel through the material instead of flowing freely through bonds
within the material. This mechanism means that most of the generated charges
recombine before being able to be collected, making the material a poor solar
absorber even with its bandgap of 2.0 eV.
b) a)
Figure 2.1: Room temperature crystal structure of Cs
3
Bi
2
Br
9
and Cs
3
Bi
2
I
9
, where
Bi is represented bu the dark grey spheres, the light grey spheres represent Bi,
the light blue spheres I, and the black spheres Cs.
11
In order to shift the structure into one more suitable to charge collection,
compositional changes are the most straightforward place to start as composi-
tional tuning has already been demonstrated to tune the bandgap of other per-
ovskite materials.[41] Recently our group has published on Cs
3
Bi
2
Br
9
,[64] mak-
ing Br substitution the obvious choice for substitution. Changing the halide from
I to Br shifts the structure of the material from Bi dimers to 1D corner sharing
chains, which are much more conducive to transporting charge, shown in Figure
2.1b. However, upon the first investigation of this compound the bandgap was
found to be 2.6 eV, which is not suitable for single-junction solar applications
as not enough of the visible light spectrum is absorbed to produce an efficient
cell. Through other investigations of halide perovskites it has been shown that
bandgap tuning is possible with the substitution of larger halide ions. There-
fore substituting in I atoms for Br could lower the bandgap to more acceptable
levels for a solar absorber. However, the question remained on whether the
structure would remain the same upon substitution. Halide perovskites substi-
tutional series have been shown to adopt the structure of one or both of the
ends of the series[68, 77, 78], or even form an amorphous phase at some levels
of substitution.[65] To our knowledge no series where the structure of the end
compounds is different than that of the substitutions has been reported. In this
paper we present a substitutional series from Cs
3
Bi
2
Br
9−x
I
x
x=1-6 wherein the
Bi octahedra remain in the preferred chain configuration in a different crystal
structure than both end points, while the bandgap of the fully I compound is
matched.
12
2.1 Methods
2.1.1 Experimental
Cs
3
Bi
2
Br
9−x
I
x
x=1-6 powders were made using stoichiometric amounts of
CsI, CsBr, BiI
3
, BiBr
3
dissolved in methanol heated to 80
◦
C in a sand bath and
stirred for an hour. For Cs
3
Bi
2
Br
9
and Cs
3
Bi
2
I
9
, the solvent was changed to their
respective halide acid. After heating, the samples were allowed to cool slowly
to room temperature. This yielded a color spectrum ranging from yellow of the
completely Br compound darkening to orange upon substitution, continuing to
darken with increasing I concentration until the dark red of the fully I compound
was reached, as can be seen below in Figure 2.2. The powders of these com-
pounds were then vacuum filtered, washed with methanol, and dried by pulling
air over the samples.
From the resulting powders, films were spun using a 30 weight percent solu-
tion of the powder in dimethyl sulfoxide. Using dynamic deposition 100 μL of
the solution was injected onto the substrate when the desired 2500 rpm speed
Figure 2.2: Powders of substitutions starting with x=0 on the left and continuing
to x=9 on the right.
13
was met. 4 seconds later two drops of toluene were added to assist with solvent
evaporation to ensure the crystallites had better contact. The films were then
annealed in air at 100
◦
C for 5 minutes.
Lab x-ray diffraction was carried out on a Bruker D8 diffractometer with a
Cu
Kα
source (λ
1
=1.5406 Åλ
2
=1.5444 Å). High resolution characterization was
carried out at the Advanced Photon Source at Argonne National Lab on beam-
line 11BM. The compounds were run in 2 separate batches due to dilution issues
with average wavelengths of 0.414613 Å, for x=0,4,5,6 and 0.412602 Å, for
x=1,2,3,9 respectively. Discrete detectors covering an angular range from -6 to
16
◦
2θ were scanned over a 34
◦
2θ range, with data points collected every 0.001
◦
2θ and a scan speed of 0.01
◦
s
−1
. These resulting patterns were then refined by
the Rietveld method with the FullProf software.[79] Starting lattice parameters
for the substitutions were determined using the Dicvol program within the Full-
Prof suite.
Spectral measurements were carried out on a Perkin-Elmer UV-Vis-NIR Lamda
950. Measurements on the powders were taken using diffuse reflectance geome-
try using the device’s integrating sphere. The samples were prepared by grinding
the compounds with BaSO
4
to create a 3 percent by weight solid solution. Film
samples were run in transmission geometry and those signals were converted to
absorbance using the Kubelka-Munk transformation for direct transitions.
2.1.2 Computational
Density Functional Theory (DFT) calculations were performed in order to
determine the effect of iodine substitution on the electronic structure. Projec-
tions of the atomic orbital character on the bands show that the top of the valence
band for the substituted compounds consists of Bi s, I p, and Br p orbitals. The
14
percentage of I p character increases 5-25% with increased substitution, while
there is a much smaller decrease in the Bi s orbital character of 1-3%.
2.2 Results and Discussion
The results of the refinements indicate that all the materials made were phase
pure, showing no extraneous peaks upon close examination as shown in Figure
2.3.
The refinements also show that with increasing concentration of I the lattice
parameters of the substitutions also increases as shown in Figure 2.4a,b, as is
expected with I being a bigger atom than Br. This increase in lattice parameters
is shown as a leftward shift in the peaks in the x-ray pattern indicating that the
d spacing is increasing with increasing atom size as shown in Figure 2.4c.
During the course of the refinements while refining the thermal parameters
the Br1 site, Wyckoff position 3e, consistently returned larger than expected val-
ues. Switching to anisotropic mode for all sites alleviated some of this problem,
a) b)
Figure 2.3: (a)Results of Cs
3
Bi
2
Br
9
Rietveld refinement. R
B
ragg=5.6% (b)
Results of Cs
3
Bi
2
Br
3
I
6
Rietveld refinement using the Cs
3
Bi
2
Cl
9
CIF. R
B
ragg=5.0%
15
a)
b) c)
Figure 2.4: (a) Lattice parameter a increasing with increasing I. (b) Lattice
parameter c increasing with increasing I. (c) Shows the peaks in the pattern
shifting to the left indicating the d spacing increasing with increasing I.
however the site in question would not refine anisotropically without the thermal
parameter going negative. It was determined that there was too much electron
density on the site and and this disparity was distorting the parameter. In order
to lower these numbers a trial was conducted wherein some of the I occupancy
was shifted to the Br2 site. This shift lowered the thermal parameters by an
a) b) b)
Figure 2.5: a) Cs
3
Bi
2
Br
9
structure with no octahedral rocking b) Cs
3
Bi
2
Cl
9
struc-
ture with octahedral rocking
16
average of 3 and put them back into a more acceptable range for a disordered
system.
DFT calculations showed that some of the compounds with higher I concen-
trations preferred to crystallize in the lower symmetry P321 instead of P3m1.
However, this preference is governed by only a small 20 meV difference in sta-
bility. Therefore, in order to confirm the findings of the calculations, the refine-
ments of x=1-6 the compounds were matched to CIFs of Cs
3
Bi
2
Cl
9
for the lower
symmetry as well as to a CIF for Cs
3
Bi
2
Br
9
. Since these two systems are differ-
entiated in crystal structure only by a slight rocking of the Bi octahedra in the
c-axis direction, shown in Figure 2.5 it was possible to refine each compound to
both CIFs easily with little difference in appearance.
In order to fully investigate which crystal structure was preferred the statistics
were compared. For the x=1 compound comparing the R-Bragg yielded 3.75
percent for the P3m1 matched refinement and 3.64 percent for the P321 matched
pattern, indicating a preference for the lower symmetry structure with only the
Figure 2.6: CIF output of Cs
3
Bi
2
Br
9
refined Cs
3
Bi
2
Br
6
I
3
showing the lack of bonds
to the Cs1 and Br2 site.
17
addition of one equivalent of I. This trend continues for the x=2 sample and
on through the rest of the series. Furthermore for the x=3-6 samples when
examining the structure outputs of the refinements they showed that for the
P3m1 refined samples had nonsensical structures, shown below in Figure 2.6,
where the the Cs1/1a site atom is missing all its bonds, and the Cs2/2d site is
missing all bonds to the Br 6g site and many of the bonds to the 3e sites as well.
The disappearance of these bonds indicates that the model has made the bonds
too long to fit within the unit cell. Upon examination, this break occurred at the
start of the profile match when the lattice parameters were refined and nothing
could be done to fix this issue. This doubly confirms that the substitutions prefer
the lower symmetry P321 structure.
With a bandgap of 2.66 eV Cs
3
Bi
2
Br
9
, has limited solar applications with such
a large bandgap. In other halide perovskites, substituting in I has lowered the
bandgap via stronger orbital overlap due to its larger atomic radii. In the sub-
stitutional series, the bandgap dropped significantly upon the addition of one
equivalent of I from 2.66 eV to 2.34 eV. In the Cs
3
Bi
2
Br
9
the conduction band
Figure 2.7: The first equivalent of I drops the bandgap dramatically while each
subsequent addition of I lowers the bandgap in a more linear manner.
18
Figure 2.8: Tauc plot of the compositional series from x=0 to x=9 shows the
magnitude of the exicton peak decreases with increasing I concentration.
maximum and valence band minimum both lie on the Bi atoms. The large drop
in the bandgap corresponds to I states being added which in turn lowered the
conduction band, therefore shrinking the bandgap. After the first substitution
the decrease in bandgap becomes smaller, but more consistent in magnitude.
The resulting downward trend can be seen in Figure 2.7. At x=6 the bandgap
settles at 2.05 eV, only 0.01 eV above the fully I compound, indicating that while
the higher substitutions can be made, for bandgap tuning purposes there is no
benefit. Exact bandgap values for each compound can be found in Table 1.
Another issue that was partially solved by substituting in I was that of the
strong exciton coupling in the Cs
3
Bi
2
Br
9
. As seen in Figure 2.8 as the I concentra-
tion increases, the magnitude of the exciton decreases, flattening out to a small
bump in the x=6 compound’s spectra. This decrease in the exciton indicates that
there is a decrease in the exciton binding energy due to the I states delocalizing
the exciton from the Bi atoms, allowing the exciton to be more easily separated
into an electron and hole such that current can be collected from the material,
19
Table 2.1: Bandgap and Structure of Series
Compound Bandgap (eV) Space group
Cs
3
Bi
2
Br
9
2.66 P3m1
Cs
3
Bi
2
Br
8
I 2.34 P321
Cs
3
Bi
2
Br
7
I
2
2.30 P321
Cs
3
Bi
2
Br
6
I
3
2.22 P321
Cs
3
Bi
2
Br
5
I
4
2.19 P321
Cs
3
Bi
2
Br
4
I
5
2.14 P321
Cs
3
Bi
2
Br
3
I
6
2.05 P321
Cs
3
Bi
2
I
9
2.04 P63/mmc
allowing the exciton to be more easily separated into an electron and hole such
that current can be collected from the material. Therefore I substitution not only
narrowed the bandgap to values more useful in the solar field, but enabled easier
charge collection.
2.3 Conclusions
Creating a substitutional series,Cs
3
Bi
2
Br
(9−x)
I
x
, allows for the best qualities of
both ends of the series, a structure more suited to charge transport and a lower
bandgap. The addition of I to the fully Br structure immediately shifted the struc-
ture from the higher symmetry P3m1 to the lower symmetry P321, agreeing with
DFT calculations that predicted this shift. The substitution of I also lead to the
decrease of the bandgap from 2.66 eV to 2.05 eV. This bandgap is statistically
equivalent with the fully I compound, Cs
3
Bi
2
I
9
, but still retains the corner shar-
ing chains of octahedra, instead of the isolated dimers found in the I compound.
Thus structure and bandgap considerations are in balance in the end of the sub-
stitutional series. The substitution also decreases the exciton binding, allowing
20
the compounds to charge separate more readily, indicating the higher substitu-
tions with smaller bandgaps and smaller exciton binding show more promise as
solar materials than the fully Br compound or the fully I.
21
Chapter 3
Summary and Future Directions
Finding the next generation solar materials is one of the most important
research goals facing the world at the moment. In order to accomplish this
goal, understanding how structural tuning can affect the structure and proper-
ties of a material is crucial in further rational design of better compounds. In our
study we found that by substituting I into Cs
3
Bi
2
Br
9
to create the substitutional
series Cs
3
Bi
2
Br
(9−x)
I
x
we could tune the bandgap while maintaining a preferred
2D structure. With a bandgap almost equal to that of Cs
3
Bi
2
I
9
while avoiding
the 0D structure of the fully I compound, Cs
3
Bi
2
Br
3
I
6
shows the most poten-
tial to improve on efficiency as a solar material. With the substitutional series
showing a reduction in the large exciton binding energy of Cs
3
Bi
2
Br
9
the series
mitigates problems from both ends of series. With these improvements further
work must be done on making devices to see if the improvements in the struc-
ture, bandgap, and exciton binding energy translate into gains in the efficiency in
device infrastructure. While testing these cells efforts could be made into pairing
the substitutions with a near IR organic absorber. This could have potential as an
efficient tandem cell. These trials will show the merit of compositional studies to
advancing our understanding of solar materials as a whole.
22
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Abstract (if available)

Abstract In response to the structural shortcomings of Cs₃Bi₂I₉ and the sub-optimal band gap and high exciton binding energy of Cs₃Bi₂Br₉ our group set out to create a compound which had a lower bandgap while maintaining a structure more suited to charge transfer. In our studies we substituted in I for Br in Cs₃Bi₂Br₉ to create the substitutional series Cs₃Bi₂Br₍₉₋ₓ₎Iₓ in an attempt to maintain the more advantageous structure while red shifting the bandgap back to a more useful level. In this study we found that the substitutional series shifts structure from the P3m1 of the Cs₃Bi₂Br₉ to a lower symmetry structure P321 similar to that of Cs₃Bi₂Cl₉. This lower symmetry structure differs in that the octahedra rock with respect to the c-axis. This rocking does not change the corner sharing nature of the octahedra, therefore the structure is still advantageous for charge transport. Diffuse reﬂectance of the substitutional series showed a marked red shift in the band gap from 2.6 eV to 2.0 eV for the substitutional series. Examining the exciton peak in the spectra also showed a noticeable decrease in its magnitude across the series. Maintaining a 2D corner sharing structure while lowering the bandgap and exicton binding energy with increasing I concentration show that the aim of the study was successful. Therefore, our results indicate that further studies on the materials should be conducted, testing their efﬁciency in a device setting to elucidate how the structural changes improve the performance.

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Creator Seckman, Bethany L. (author)

Core Title Controlling structure and photophysical properties of bi triple perovskite through substitutional series Cs₃Bi₂Br₍₉₋ₓ₎Iₓ

School College of Letters, Arts and Sciences

Degree Master of Science

Degree Program Chemistry

Publication Date 10/08/2018

Defense Date 08/10/2018

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag lead free perovskite,OAI-PMH Harvest,perovskite,photovoltaic,solar absorber

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark E. (committee chair), Melot, Brent C. (committee member), Ravichandran, Jayakanth (committee member)

Creator Email inkheart9459@yahoo.com,seckman@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-73933

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Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

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