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Optical properties of lead and bismuth halide perovskites for photovoltaic applications

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Optical properties of lead and bismuth halide perovskites for photovoltaic applications

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University of Southern California

Optical Properties of Lead and Bismuth Halide Perovskites for
Photovoltaic Applications

Kelsey Bass

Submitted to the Department of Chemistry
in partial satisfaction of the requirements
for the degree of Master of Science

August 9, 2016

i

Abstract

Since the first use of a perovskite material in a solar cell device, this class of materials
has been widely studied. The best performing perovskite, CH
3
NH
3
PbI
3
, has allowed solution
processed solar cell devices to achieve power conversion efficiencies approaching those of
commercial silicon devices. The research focus has been largely on improving device
performance; however, of late, interest in studying the fundamental properties of CH
3
NH
3
PbI
3

and other halide perovskites has increased. In an effort to study the CH
3
NH
3
PbI
3
and
CH
3
NH
3
PbBr
3
phases in as pristine a state as possible, we found that preparation of these
compounds in an inert atmosphere resulted in poorly crystalline powders and films. Upon
exposure of these powders and films to moisture the crystallinity and photoluminescent lifetime
increased. We conclude that synthetic conditions influence the crystallization of these perovskite
phases. Although these lead phases result in high-performing photovoltaic devices, the presence
of lead is a concern for potential commercialization. Further research focused on the study of the
structure-property relationship on a non-toxic bismuth phase, Cs
3
Bi
2
Br
9
. We found that this
material displayed strong exciton-phonon coupling and that annealing Cs
3
Bi
2
Br
9
films in a
bromine-rich atmosphere enhanced the vibronically structured emission profile. These results
indicate that future work on structural or compositional modifications to reduce the large exciton
binding energy in the bismuth-phases is necessary in order to develop these materials as light
absorbing layers in photovoltaic devices.

ii

Contents

Abstract i

Contents ii

List of Figures iii

Chapter 1: Introduction to Perovskites 1
1.1 Photovoltaic Applications 1
1.2 Perovskite Structure-Property Relationship 2

Chapter 2: Influence of Moisture on the Preparation, Crystal Structure, 6
and Photophysical Properties of Organohalide Perovskites
2.1 Introduction 6
2.2 Experimental 6
2.3 Results and Discussion 7

Chapter 3: Vibronic Structure in the Room Temperature Photoluminescence 14
of the Halide Perovskite Cs
3
Bi
2
Br
9

3.1 Introduction 14
3.2 Experimental 16
3.3 Results and Discussion 17

Summary and Future Directions 25

Acknowledgements 26

References 27

iii

List of Figures

1.1 Crystal structure of traditional perovskite and structural derivatives 3

2.1 Powder diffraction data for CH
3
NH
3
PbBr
3
and CH
3
NH
3
PbI
3
8

2.2 Results of Rietveld refinement for CH
3
NH
3
PbBr
3
9

2.3 Results of Rietveld refinement for CH
3
NH
3
PbI
3
10

2.4 Optical emission spectra of CH
3
NH
3
PbBr
3
films 12

2.5 Time-resolved photoluminescence spectra of CH
3
NH
3
PbBr
3
films 13

3.1 Room temperature crystal structure of Cs
3
Bi
2
Br
9
15

3.2 X-ray diffraction pattern and absorption-emission spectra of film of 18
Cs
3
Bi
2
Br
9
after annealing in a Br
2
atmosphere

3.3 X-ray diffraction pattern and absorption-emission spectra of film of Cs
3
Bi
2
Br
9
19
as cast before annealing in a Br
2
atmosphere

3.4 Tauc plot comparing the diffuse reflectance from powders of Cs
3
Bi
2
Br
9
and films 20

3.5 Comparison of the emission spectra for the as-cast and annealed films 21

3.6 DFT band structure of the room temperature form of Cs
3
Bi
2
Br
9
22

3.7 Phonon band structure and local bismuth coordination environment for the room 23
temperature structure of Cs
3
Bi
2
Br
9

1

Chapter 1

Introduction to Perovskites

1.1 Photovoltaic Applications

As global energy consumption increases, renewable energy sources have become
increasingly studied. The abundance of solar energy makes the study of low-cost and efficient
solar cell devices worthwhile. Traditional silicon solar cells have decreased in cost, but more
recently studied materials show promise for even lower cost and higher efficiency solar cells.
The perovskite class of materials has emerged as a foremost contender for an alternative solar
cell material.
1

The perovskite compositions that first attracted attention for photovoltaic applications
were CH
3
NH
3
PbI
3
and CH
3
NH
3
PbBr
3
. These halide perovskites have low-cost precursor
materials and solution processability, which reduces the difficulty and cost of device fabrication.
Miyasaka et al. first reported on using CH
3
NH
3
PbI
3
as a light absorbing layer in the dye-
sensitized solar cell architecture with a power conversion efficiency of 3.81%.
2
Over the next
several years, the efficiency of CH
3
NH
3
PbI
3
devices increased as device architecture was
improved. Most recently, the Grätzel group has reported a large-area device with 20.5%
efficiency.
3
Research efforts focus largely on improvement of device performance, especially for
lead-based phases, but a fundamental understanding of the photophysical properties will be
invaluable to further improvement and engineering of perovskite photovoltaic devices.
4

2

1.2 Perovskite Structure-Property Relationship

The perovskite structure has a general formula of ABX
3
, in which A and B are cations and
X is an anion. The B cation forms BX
6
corner-sharing octahedra with the X anion, with the 12-
coordinate A cations between the octahedra. This structure is close-packed and the ideal space
group is cubic. The sizes of the constituent ions greatly influence the symmetry and extent of
distortion, which can be described by the Goldschmidt tolerance factor (eq. 1).

(1)

In this equation r
A
, r
B
, and r
X
are the ionic radii of the A, B, and X ions respectively. The
tolerance factor is also calculated on the assumption that all ionic radii in the structure are
touching. A tolerance factor of 1 indicates a stabilized cubic structure, with lower symmetries
stabilized as the tolerance factor decreases. Flexible bond angles allow distortions such as
octahedral rotation and tilting, which contribute to lowered symmetry (e.g. tetragonal and
orthorhombic). Reduced symmetry can also occur from ions that are too small for sites, such as
off-centering of a too small B cation in the octahedra.
5

Structural derivatives of the ABX
3
perovskite occur when ions of lower or higher valences
are introduced. Halide perovskites, which are common in photovoltaic studies, contain
monovalent A and divalent B cations. A double perovskite structure, ABB’ X
6
, is twice the
traditional perovskite unit cell with two cations on the B site. The total valence of these B site
cations is four in a halide structure. The ordering of the cations forms a rock salt type structure in
which each B site atom sits in every other octahedron. The triple perovskite structure, A
3
B
2
X
9
, is
a tripling of the perovskite unit cell and is vacancy-ordered in order to accommodate the charge
3

of the B cation, which is trivalent in a halide composition. The B site vacancies are ideally
ordered in such a way as to produce corrugated octahedral layers, resulting in a two-dimensional
structure.

Fig. 1.1 Crystal structures of (a) the traditional ABX
3
perovskite where the blue spheres are A cations, black spheres
are B cations, and orange spheres are X anions; (b) ABB'X
6
double perovskite where green spheres are A cations,
purple spheres are B cations, magenta spheres are B' cations, and brown spheres are X anions; (c) A
3
B
2
X
9
vacancy-
ordered triple perovskite structure where purple spheres are A cations, gray spheres are B cations, and blue spheres
are X anions.

The ability of the perovskite structure to accommodate a large variety of ions, which can
cause a variety of distortions, creates structures with interesting properties for numerous
applications. Physical properties that have attracted interest in perovskites include
superconductivity, ferroelectricity, ionic conductivity, and other dielectric properties. However,
for photovoltaic applications, the ability to tune the band gap, broad and strong absorption, high
charge mobilities, and long charge diffusion lengths of the halide phases have made these widely
studied.
4

The best suited band gap for solar light absorption that optimizes voltage and reduces
collection of extra current is 1.4 eV. The perovskite band gap can be decreased with increase
dimensionality, less electronegative anions, and a smaller electronegativity difference between
the B and X ions. The A cation has some effect on the band gap by affecting the B-X bond length
due to lattice expansion or contraction. The perovskite electronic structure is largely affected by
the B and X ions, as the valence band maximum consists of B s and X p orbitals and the
conduction band minimum consists of B p and X p orbitals. Iodine-containing compositions have
smaller band gaps due to the decreased electronegativity of iodine with respect to bromine and
chlorine. A smaller electronegativity difference between iodine and tin causes these
compositions to have smaller band gaps compared to their lead counterparts.
6
However, other
factors such as stability and device performance also must be considered in tuning the band gap.
As the composition of interest has turned to non-lead phases, such as bismuth or
antimony, the structural and electronic changes induced by substituting in other metal cations can
present challenges for potential device performance. The excitons in MAPbI
3
are the more
delocalized Wannier type, thus charges are more easily separated. As the dimensionality of the
structure decreases the excitons are the more tightly bound Frenkel type due to quantum
confinement effects.
4
In order to produce more practical alternatives for lead perovskite phases, a
way must be found to decrease the exciton binding energy in non-lead, two-dimensional triple
perovskite structures.

5

Chapter 2

Influence of Moisture on the Preparation, Crystal Structure, and
Photophysical Properties of Organohalide Perovskites
†

In order to improve and fully understand perovskite device performance, exploration of
the fundamental properties of the chosen perovskite material is necessary. The improvement of
CH
3
NH
3
PbI
3
devices has been a driven effort and device efficiencies have risen to over 20%.
7
In
an effort to synthesize and study the most pristine CH
3
NH
3
PbBr
3
and CH
3
NH
3
PbI
3
powders and
films, we discovered that moisture has a large effect on the crystallinity of both phases and
photoluminescent lifetime of the bromide phase. In this chapter, the effect of preparing lead-
based organohalide perovskites under inert conditions has been investigated. We find that when
prepared under anhydrous conditions, only poorly crystalline powders were obtained and upon
exposure to small amounts of moisture a rapid crystallization into the expected cubic unit cell for
CH
3
NH
3
PbBr
3
and tetragonal cell for CH
3
NH
3
PbI
3
was observed. While the as-prepared iodide
phase is non-emissive, the lifetime of the emission for the bromide is found to be much longer
when prepared under atmospheric conditions.

†
Chapter content from Bass, K. K., McAnally, R. E., Zhou, S., Djurovich, P. I., Thompson, M. E., & Melot, B. C.
Influence of moisture on the preparation, crystal structure, and photophysical properties of organohalide perovskites.
Chem. Commun. 50, 15819-15822 (2014).
6

2.1 Introduction

Organohalide perovskites have attracted considerable attention for their use in solution
processable solar cells with power conversion efficiencies of over 15%.
2,5,8–25
Most reports have
focused on improving device performance whereas the fundamental nature of the perovskite
phases used in the devices is less explored. While it has been acknowledged that devices should
be prepared in conditions with less than 1% humidity,
17
few reports have been made on the
influence of the atmosphere on the resulting product. Niu et al. performed experiments to
compare the structure and performance of CH
3
NH
3
PbI
3
photovoltaic devices before and after
exposure to 60% humidity, but prepared all of their samples in ambient conditions.
26
To better
understand the role of moisture in the preparation of these phases, we have synthesized
CH
3
NH
3
PbBr
3
and CH
3
NH
3
PbI
3
in an air free environment and studied the structure and
photophysical properties of the air free and air exposed powders and films.

2.2 Experimental

All reagents were purchased fresh from the respective supplier and kept inside an Ar-
filled glove box with less than 0.1 ppm of moisture and approximately 0.30 ppm of oxygen.
CH
3
NH
3
Br and CH
3
NH
3
I were prepared by mixing 40% CH
3
NH
2
in water (Spectrum Chemical
Mfg. Corp.) with concentrated HBr (Sigma Aldrich, 48%) or HI (Sigma Aldrich, 57%) as
described previously.
27
Prior to use, CH
3
NH
3
I was purified further by sublimation to remove a
yellow-orange coloration that was likely the result of I
2
present in the HI. Powdered reagents like
Pb(NO
3
)
2
(Acros Organic), CH
3
NH
3
Br, and CH
3
NH
3
I were weighed and sealed inside of the
7

glove box before moving to a Schlenk line for the reaction. Ground glass fittings were preferred
over rubber septa due to the reactivity of the septa with the fumes of the concentrated acids. A
typical synthesis was performed under a constant positive pressure of nitrogen with
stoichiometric amounts of Pb(NO
3
)
2
and CH
3
NH
3
X in concentrated HX. The solution was heated
to 100 °C until the powders were fully dissolved before cooling to room temperature. The
resulting orange (bromide) and black (iodide) mixture of crystals and powder were washed with
anhydrous diethyl ether and dried under vacuum.

2.3 Results and Discussion

X-ray diffraction patterns collected on a Bruker D8 powder diffractometer equipped with
a Lynxeye detector and Co-Kα tube are shown in Fig. 2.1. The pristine patterns were collected
using an air-tight stainless steel cell equipped with a 25 mm Be window that has been described
previously.
28
Very weak reflections are seen in the powders prior to exposure to atmosphere for
either the CH
3
NH
3
PbBr
3
[Fig. 2.1 (a)] or the CH
3
NH
3
PbI
3
[Fig. 2.1 (b)] powders. Upon opening
the cell and allowing it to sit on the bench top for only a few minutes, peaks which could be
indexed to a cubic unit cell were found for the bromide and a tetragonal cell for the iodide. The
difference in quality of the patterns is because the iodide had to be scanned at a much faster rate.
Allowing the black powder to sit for more than a few minutes in air resulted in the powder taking
on a brown tint, indicating a small conversion to the yellow hydrated phase. No diffraction peaks
were seen for the hydrated phase, however, so the reaction likely occurred primarily on the
surface without much penetration into the bulk.

8

Fig. 2.1 Comparison of laboratory powder diffraction data for (a) CH
3
NH
3
PbBr
3
and (b) CH
3
NH
3
PbI
3
materials
prepared/characterized under inert atmosphere (blue line) and exposed to ambient atmosphere (red lines). The panels
on the right emphasize the lowest angle peak which splits on distortion from the cubic to tetragonal phase. The
asterisks denote reflections from the Be window of the air-free XRD cell.

To mitigate the conversion to the hydrated phase and obtain a more accurate picture of
the structure, the as-prepared powders were briefly exposed to ambient atmosphere before
sealing in kapton capillaries and sending for synchrotron diffraction experiments at the 11-BM
beam line at Argonne National Lab. The results of the Rietveld refinement of CH
3
NH
3
PbBr
3
and
CH
3
NH
3
PbI
3
structures against the synchrotron data are shown in Fig. 2.2 and 2.3 respectively.
The high intensity and resolution of the 11-BM data shows extremely phase-pure powders with
no sign of any secondary phases. Anisotropic atomic displacement parameters were refined for
both phases and show a significant degree of disorder in the halide ions. This is most likely
related to rotational disorder associated with the methylammonium groups which are known to
9

be highly disordered at room temperature.
10,29,30
The tetragonal distortion in the iodide is
attributed to a slight ordering of these groups, and results in an alternating 16° rotation of the
corner-sharing octahedra down the length of the c axis of the unit cell which generates new
reflections at 6.3° and 6.5° in 2θ. There is simultaneously a compression of the c/a ratio from the
ideal 1.50 to 1.43 which splits the [100] Bragg reflection as emphasized in the inset.

Fig. 2.2 Results from the Rietveld refinement of the cubic structure of CH
3
NH
3
PbBr
3
at room temperature against
high resolution synchrotron diffraction data obtained from the 11-BM beam line at Argonne National Laboratory on
powders which were exposed to the atmosphere. R
Bragg
= 7.48%,R
f
= 3.19%, χ
2
= 1.38. The inset on the left
illustrates the resulting structure with atoms visualized at the 95% probability level. The inset on the right
emphasizes the 100 Bragg reflection of the cubic perovskite.

10

Fig. 2.3 Rietveld refinement of the tetragonal structure of CH
3
NH
3
PbI
3
at room temperature against 11-BM data for
powders which were exposed to the atmosphere. R
Bragg
= 5.23%, R
f
= 5.39%, χ
2
= 1.09. The inset on the left
illustrates the resulting structure with atoms visualized at the 95% probability level. The inset on the right
emphasizes the splitting of the 100 peak into the 002 and 110 planes of the tetragonal structure.

To distinguish the effect of oxygen from atmospheric moisture, dry O
2
and moist N
2
were
separately passed over a small amount of the pristine bromide powder. Importantly, exposure to
the anhydrous gases had absolutely no effect on the observed diffraction patterns, while the
moist nitrogen was found to rapidly trigger the crystallization into the cubic phase. It is not
immediately clear how moisture promotes this rapid crystallization. However, considering these
organohalide perovskites are extremely soluble in aqueous solutions, it is possible that humidity
in the air triggers nucleation and crystallization as has been seen in amorphous lactose
particles.
31,32
Similar effects also occur with organic solvents inducing crystallization of
Na
3
Au(SO
3
)
2
and tris(8-hydroxy-quinoline)aluminum films.
33,34

11

Since the as-prepared powders are non-emissive, the role of moisture on the
photophysical properties was explored by dissolving powders which had never been exposed to
moisture in anhydrous DMF at a concentration of 10 wt %. Two sets of films were then cast; one
inside an Ar-filled glove box (less than 0.01 ppm of moisture and 23.5 °C) and the other in open
air (58.2% humidity and 22.6 °C). X-ray diffraction patterns confirmed that films cast under
anhydrous conditions remained poorly crystalline and that exposure to moisture resulted in rapid
crystallization to highly oriented crystallites which predominantly showed diffraction peaks
associated with [00l] Bragg planes much like in the films cast in air. Analysis using scanning
electron microscopy demonstrated that the morphology of the films cast under Ar versus those
cast in air is different. The films cast in air exhibit smaller particle sizes, which is counter-
intuitive to the enhanced photoluminescence lifetime. Thus, particle size is irrelevant in these
films and does not explain the differences in lifetimes. The higher crystallinity of the particles in
the films cast in air seems to allow the films to exhibit increased lifetimes. We note that similar
problems with obtaining complete wetting of the substrate were encountered as previously
reported,
26
but additives to improve coverage were avoided to reduce any potential impact on the
performance.
In order to confirm that there were no major differences between the poorly crystalline
and crystalline phases, the optical emission spectra for both phases were collected. Aside from a
slight difference in peak width, all three films were confirmed to display emission at the
previously reported wavelength of 540 nm for the bromide phase (Fig. 2.4), albeit with very low
efficiency (Φ < 0.01). In contrast, no emission was observed from films prepared from the iodide
powders around the expected wavelength of 770 nm. Very weak emission was observed for the
iodide closer to 520 nm, but this is believed to be associated with a small impurity related to the
12

yellow hydrated form. This observation is in keeping with what has previously been reported for
materials prepared using solution deposition techniques.
35

Fig. 2.4 Optical emission spectra for pristine films of CH
3
NH
3
PbBr
3
before (filled blue circles) and after exposure to
atmosphere (unfilled red squares) as well as those cast in air (unfilled green triangles).

Time-resolved emission data were recorded using a time-correlated single photon
counting (TCSPC) technique. Fig. 2.5 shows the resulting transient decays for a film cast in air-
free conditions, the same film which was then exposed to the atmosphere, and the one cast in air.
The difference between films cast in air and under inert atmosphere is striking. Films exposed to
air or cast in atmospheric conditions were found to show increasingly long luminescent lifetimes.
Given that there are no substantial morphological differences between the films, this is likely a
result of differences in the crystallinity of each film. The pristine films are poorly crystalline and
show the most rapid decay in the emission lifetime. It is not surprising that only after the films
are crystalline that emission lifetimes begin to reflect what has been reported in the literature.
36,37

The loss of long-range periodicity in the poorly crystalline films appears to provide new routes
13

by which the excited state can decay at a significantly accelerated rate. Our results then appear to
indicate that the presence of a small amount of moisture is absolutely necessary in order to
produce the highly crystalline films which exhibit the long-lived excited states that have been
seen in the literature, although it is still not exactly clear how moisture promotes crystallization.

Fig. 2.5 Time-resolved photoluminescence emission spectra for pristine films (filled blue circles) and films which
were exposed to ambient atmosphere (unfilled red squares) for the CH
3
NH
3
PbBr
3
. Note the exceptionally longer
lifetimes when exposed to air and recall that all iodide-based phases failed to show any sign of luminescence.

In summary, our results demonstrate that the synthetic conditions under which
organohalide perovskites are prepared plays a critical role in promoting crystallization. Further
studies are needed to understand why exposure to moisture triggers the crystallization of the
perovskite phase, but it is clear from the photoluminescence data that the excited state in the
amorphous films is quenched at a significantly faster rate than in those which were fully
crystallized.

14

Chapter 3

Vibronic Structure in the Room Temperature Photoluminescence of the
Halide Perovskite Cs
3
Bi
2
Br
9
‡

As focus in the perovskite photovoltaic community has begun to move to non-toxic, lead-
free phases, we moved to studying the structure-property relations of bismuth perovskites,
Cs
3
Bi
2
Br
9
in particular, with regard to their optical properties. The Cs
3
Bi
2
Br
9
electronic
structure, determined from Density Functional Theory calculations, shows the top of the valence
band and bottom of the conduction band minimum are, unusually, dominated by Bi-s and Bi-p
states respectively. This produces a sharp exciton peak in the absorption spectra with a binding
energy that was approximated to be 940 meV, which is substantially stronger than values found
in other halide perovskites and instead more closely reflects values seen in alkali halide crystals.
This large binding energy is indicative of a strongly localized character and results in a highly
structured emission at room temperature as the exciton couples to vibrations in the lattice.

3.1 Introduction

Interest in developing new solution processable light absorbing layers for solar panels has
surged since the first reports on the perovskite CH
3
NH
3
PbI
3
.
2
Despite the extremely high
photovoltaic efficiency these materials have demonstrated,
7,38,39
the potential environmental and

‡
Contributions to the work have been made from Laura Estergreen, Stephen Bradforth, Christopher Savory, John
Bruckeridge, David Scanlon, Peter Djurovich, Mark Thompson, and Brent Melot.
15

public-health consequences that could result from the wide-scale distribution of solar panels
based on water soluble compounds containing Pb presents a huge hurdle to the eventual
commercialization of perovskite solar cells.
40
To address these concerns, perovskites based on
Sn
2+
have been suggested as less-toxic alternatives, but in most cases these analogues have
proven to be too unstable towards oxidation to be realistic replacements.
41

More recently, emphasis has shifted away from maintaining a perfectly isostructural
framework towards simply identifying new materials that can efficiently convert sunlight into
electrical charge. In this respect, bismuth halides offer a promising new avenue because their
strong absorption coefficients make them very effective at capturing the solar spectrum.
42

Unfortunately, the trivalent oxidation state of Bi ions prohibit their direct incorporation into the
prototypical ABX
3
perovskite structure. Instead, structural derivatives like double and triple
perovskites, which partially substitute some of the bismuth sites with lower valent metals or
vacancies, offer the best opportunity to evaluate the photophysical properties of Bi-based
absorbing layers.
43–46

Fig. 3.1 Room temperature crystal structure of Cs
3
Bi
2
Br
9
. The gray atoms are Bi, blue atoms are Br, and purple
atoms are Cs.
16

We investigate the optical properties of Cs
3
Bi
2
Br
9
, a layered form of the vacancy-ordered
perovskites, which can be viewed as a tripling of the traditional perovskite unit cell with only
two thirds of the octahedral positions fully occupied. The remaining octahedral sites remain
vacant and segregate so as to produce corrugated layers of corner-sharing BiBr
6
octahedra as
illustrated in Fig. 3.1. The resulting local coordination environment of the octahedra is irregular
and exhibits a trigonal distortion that produces three short and three long Bi-Br bonds with the
shorter linkages shared between neighboring octahedra in each layer. Previous work on
Cs
3
Bi
2
Br
9
found no evidence for photoluminescence above liquid helium temperatures aside
from a weak red emission that quenched above 160 K. The authors attributed this signal to small
amounts of oxygen defects resulting from the use of a Bi
2
O
3
precursor.
47,48

3.2 Experimental

Bulk powder of Cs
3
Bi
2
Br
9
was prepared in solution. A stoichiometric ratio of CsBr (Alfa
Aesar) and BiBr
3
(Sigma Aldrich) was dissolved in 5 mL of 48 wt % HBr (Sigma Aldrich). The
solution was heated at 80°C for 1 h and slowly cooled to RT. The resulting yellow powder was
filtered, washed with ethanol, and dried under vacuum. Thin films were prepared by spin casting
a 40 wt % Cs
3
Bi
2
Br
9
in dimethylsulfoxide (DMSO) solution onto a quartz substrate at 2500 rpm
for 45 s. The films were annealed in a Parr autoclave with several drops of bromine at 100°C for
2 h. A one inch high quartz tube was used to raise the films from the bottom of the Teflon liner.
Films were stored in a desiccator.
Periodic DFT calculations were performed using the Vienna Ab Initio Simulation
Package (VASP).
49–52
PBEsol
53
was used as the exchange correlation functional for geometric
17

optimization and phonon calculations while the hybrid functional HSE06, incorporating 25%
Hartree Fock exchange and a screening parameter of ω= 0.11 bohr
-1
,
54
with the addition of spin-
orbit effects (HSE06+SOC) was used to calculate the electronic band structure. Valence and core
electron interactions were accounted for within the projector-augmented wave (PAW) method,
55

with the inclusion of relativistic effects for bismuth. A cutoff energy of 560 eV and a k-mesh of
3 3 3 were used in all calculations and the structure was considered converged when forces did
not exceed 0.01 Å
-1
on each atom.
Phonon calculations were performed using the PHONOPY package, with phonon
frequencies determined using the finite differences method.
56,57
Additionally, splitting between
transverse and longitudinal optical modes was found using a non-analytical correction term,
58–60

with high frequency dielectric constant and Born effective charges obtained using Density
Functional Perturbation Theory as available in VASP.
61
PBEsol was used for these calculations
as it has been found to provide accurate results for a range of semiconductors.
62

3.3 Results and Discussion

Considering the recent reports that CH
3
NH
3
PbI
3
and other halide perovskites tend to
exhibit large concentrations of vacancies in halide sublattice,
63
the as-cast films were
subsequently annealed in a Br
2
-rich atmosphere in order to mitigate any influence from defects
on the intrinsic optical properties. The annealed films show a significant increase in the
diffracted intensity compared with the as-cast films [see Fig. 3.2 (a) and Fig. 3.3 (a)], which
suggests a substantial improvement to crystal quality with no appreciable change to particle
shape or size during the process as determined by scanning electron microscopy.
18

Fig. 3.2 (a) X-ray diffraction pattern and (b) absorption-emission (λ
exc
=330 nm) spectra of film of Cs
3
Bi
2
Br
9
after
annealing in a Br
2
atmosphere.
19

Fig. 3.3 (a) X-ray diffraction pattern and (b) absorption-emission (λ
exc
=330 nm) spectra of film of Cs
3
Bi
2
Br
9
as cast
before annealing in a Br
2
atmosphere.

The X-ray diffraction pattern of the annealed films indicates the crystallites are highly
oriented with the (00l) facets exposed parallel to the plane of the substrate. Films prepared in this
way exhibited a strong absorption and, in contrast to previous reports, a weak blue emission at
room temperature as shown in Fig. 3.2 (b) and (c). The onset of optical absorption for the films
agrees well with the diffuse reflectance data collected on the polycrystalline powder, which show
a direct band gap absorption at 2.65 eV (468 nm) [Fig. 3.4]. The annealed films display a sharp
exciton peak (λ
max
=440 nm) when measured in transmission [Fig. 3.2 (b)], similar in shape to
features observed in layered organic-inorganic hybrid perovskites.
64–66
To a first approximation,
the binding energy of the electron-hole pair can be estimated as the difference between the
maxima of the exciton peak and the leading edge of the subsequent plateau, which for Cs
3
Bi
2
Br
9

20

is roughly 940 meV. Compared to the 50 meV exciton binding energy for CH
3
NH
3
PbI
3
67
or the
200-300 meV seen for similar layered perovskites,
68,69
this represents a very strongly localized
state that is more commonly found in wide bandgap alkali halides like NaCl.
70

Fig. 3.4 Tauc plot comparing the diffuse reflectance from powders of Cs
3
Bi
2
Br
9
and films showing a consistent
optical band gap of 2.65 eV.

Although the photoluminescence was very weak, effectively invisible to the naked eye, it
still proved possible to resolve five well-defined peaks at 463 nm, 468 nm, 473 nm, 481 nm, and
492 nm [Fig. 3.2 (c)]. It is highly unusual to observe this kind of structured emission at room
temperature for non-molecular solids, but it is well known to occur at liquid helium temperatures
in high purity semiconductors.
71
At these low temperatures, the thermal energy available is
insufficient to dissociate the electron-hole pair and, as a consequence, the exciton must couple to
phonons in the lattice in order to recombine.
As seen from Fig. 3.5, the phonon replicas in the emission spectra show no systematic
spacing in energy and therefore seem to suggest that the exciton couples to more than one
21

phonon. Similar luminescent properties have been reported when small concentrations of
absorbing ions containing a pair of s
2
electrons (such as Bi
3+
, Sb
3+
, or Pb
2+
) were substituted into
non-absorbing matrices like Cs
2
NaYCl
6
.
72
In this dilute limit, the vibronic structure was
attributed to a pseudo Jahn-Teller distortion where the Bi ions shifted away from the center of
the regular octahedra to create one short and one long axial bond length.
73
These previous reports
found several sharp emission lines on top of a broader band, much like in the spectra seen here
for Cs
3
Bi
2
Br
9
, albeit only at liquid helium temperatures.

Fig. 3.5 Comparison of the emission spectra for the as-cast and annealed films. Note the non-uniform spacing of the
emission lines, which suggests that emission is from an indirect rather than a direct gap.

Density Functional Theory (DFT) calculations were employed to further elucidate the
nature of the electronic excitations. The band structure, shown in Figure 3.6, reveals the
existence of a low lying 2.52 eV indirect transition from the Γ point to A as well as a slightly
22

larger direct gap of 2.64 eV. The shape of the absorption spectra suggests that, despite the
presence of a lower lying indirect excitation, the primary mechanism for optical excitation occurs
at the Γ-point. The weak emission intensity seems consistent with relaxation from the indirect
gap and is expected as consequence of the excitons coupling to one or more phonons to emit a
photon. Projecting the atomic orbital character onto the bands reveals that the top of the valence
band consists of an admixture of Br 4p and Bi 6s orbitals with the bottom of the conduction band
consisting primarily of Bi p. Thus it appears the excited electron-hole pair is strongly localized
on Bi centers in the lattice, and has a pronounced Frenkel (localized) character, rather than the
typical Wannier (delocalized) behavior typical of semiconductors.

Fig. 3.6 HSE06+SOC DFT band structure of the room temperature form,

, of Cs
3
Bi
2
Br
9
.

The calculated phonon dispersion also offers insight into why recombination of the
exciton leads to vibronically structured emission. As seen in Fig. 3.7 (a), two soft modes
associated with a rocking motion of the BiBr
6
octahedra suggest the structure is unstable with
23

respect to an energy-lowering structural distortion. This kind of dynamical instability is in
keeping with previous temperature-dependent structural studies that identified a phase transition
below 95 K, which distorts the structure into a monoclinic phase.
74
The symmetries of these
unstable modes (E
g
and A
1u
) are consistent with a distortion from the trigonal to monoclinic form
that results from a rocking motion of bromine ions to produce an off-centering of the Bi within
the octahedral cage as illustrated in Fig. 3.7 (b) and (c). Therefore, it appears that optical
excitation of the films triggers a similar Jahn-Teller distortion of the BiBr
6
octahedra in a similar
manner to previous observations for isolated Bi ions.
73

Fig. 3.7 (a) Phonon band structure for the room temperature trigonal structure of Cs
3
Bi
2
Br
9
. Local bismuth
coordination environment in the room temperature (a) and low temperature (b) structure form of Cs
3
Bi
2
Br
9
.

Unlike other halide perovskites, Cs
3
Bi
2
Br
9
exhibits unusually strong exciton-phonon
coupling. In this aspect, the strongly localized electron-hole pairs created in Cs
3
Bi
2
Bi
9
are similar
to excitons found in organic chromophores.
75
This difference offers a unique counterexample to
24

the weakly bound excitons in CH
3
NH
3
PbI
3
that are easily dissociated into free carriers in
photovoltaic cells.
76
The unusual optical properties of Cs
3
Bi
2
Br
9
make it an interesting new
platform to study the interplay of excitons with lattice vibrations in this new class of light-
absorbing materials and offers fundamental insight into how to design new, less-toxic solution
processable halide perovskites. In order for materials like Cs
3
Bi
2
Br
9
to find utility in photovoltaic
devices, future work will focus on developing structural or compositional modifications that
decrease the exciton binding energy and reduce the band gap in order to increase the amount of
light collected in the solar spectrum.

25

Summary and Future Directions

The study of the fundamental properties of CH
3
NH
3
PbI
3
and other halide perovskites is
important for further improvement and design of light absorbing materials for photovoltaic
devices. In our study of the CH
3
NH
3
PbI
3
and CH
3
NH
3
PbBr
3
phases, we found that preparation of
these compounds in an inert atmosphere resulted in poorly crystalline powders and films, which
demonstrated an increase in crystallinity and photoluminescent lifetime upon exposure to
moisture. Thus, these results indicate that the crystallinity of these phases is affected by synthetic
conditions. We also studied the electronic structure and optical properties of the non-toxic
bismuth phase Cs
3
Bi
2
Br
9
, which displays strong exciton-phonon coupling. Additionally,
annealing in a bromine-rich atmosphere is shown to have an effect on the vibronic structure of
the emission. A reduction in the exciton binding energy and band gap in the bismuth-phases will
improve the potential for these materials to be used as light absorbers. Future work with non-
lead perovskite phases will involve modifications of the anion and/or cation lattice of the
bismuth perovskite structure, such as with iodine or antimony substitution, in order for these
materials to be used effectively in solar cell devices.

26

Acknowledgements

I would like to thank Profs. Brent Melot and Mark Thompson for acting as my research
advisors for my work at USC and for their support. I would also like to thank Dr. Peter
Djurovich for assistance with photophysical measurements, analysis, and discussion, and the
members of the Melot and Thompson groups for their support and help along the way. In
particular, I would like to thank Shiliang Zhou for showing me the ropes in my first year and for
his continued help with anything lab related, and Eric McAnally for helping me get started with
my project and instructing me in how to use various instruments.
The contributions of groups from USC and other institutions have made the work on
Cs
3
Bi
2
Br
9
possible. Thank you to Christopher Savory, Dr. John Bruckeridge, and Dr. David
Scanlon from University College London for performing the DFT calculations, without which
we could not have studied the electronic and phonon structure of Cs
3
Bi
2
Br
9
; and to Laura
Estergreen and Prof. Stephen Bradforth from USC for assistance with obtaining absorption and
emission spectra, discussions, and other measurements that aided in studying Cs
3
Bi
2
Br
9
.

27

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Abstract (if available)

Abstract Since the first use of a perovskite material in a solar cell device, this class of materials has been widely studied. The best performing perovskite, CH₃NH₃PbI₃, has allowed solution processed solar cell devices to achieve power conversion efficiencies approaching those of commercial silicon devices. The research focus has been largely on improving device performance

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Creator Bass, Kelsey (author)

Core Title Optical properties of lead and bismuth halide perovskites for photovoltaic applications

School College of Letters, Arts and Sciences

Degree Master of Science

Degree Program Chemistry

Publication Date 07/27/2016

Defense Date 06/24/2016

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