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DEGGENDORF INSTITUTE OF TECHNOLOGY
Faculty of Electrical Engineering , Media Technology
and Information Technology
Metal Halide Double Perovskite as Solar Cell
Absorber
Examination Paper
Name: Jerome Michael Issak
Matriculation Number: 681094
Course : Masters in Electrical Engineering and Information Technology
Examiner: Prof. Dr.-Ing.Günter Keller
Subject: Renewable Energy
Semester: Winter Semester 2017/18
Date of Submission : 29.12.2017
Declaration of Authorship
Hereby I declare, that this examination paper is my own work and I didn’t use other
aids as referenced. All parts taken by other sources by sense or wordings are marked
and the sources are referenced. This include figures, diagrams, sketches or similar
and also internet sources.
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Table of Contents
1. Introduction …………………………………………………………………………………………. 3
2. Rapid Evolution of Perovskite Cell Efficiencies ……………………………………… 4
3. Lead Free Metal Halide Perovskite ………………………………………………………… 5
4. Overview of Halide Double Perovskite …………………………………………………… 7
5. Characteristics of Halide Double Perovskite ………………………………………….. 9
5.1 Photoluminescence Lifetime ……………………………………………………………….. 9
5.2 Thermal Stability ……………………………………………………………………………… 10
5.3 Environmental Stability ……………………………………………………………………… 11
5.4 Defect Tolerance ……………………………………………………………………………… 12
6. Alloying of Halide Double Perovskite …………………………………………………… 13
6.1 Dilute Alloying …………………………………………………………………………………. 13
6.2 Trivalent Metal Alloying …………………………………………………………………….. 14
7. Thin Film Preparation of Halide Double Perovskite ………………………………. 16
7.1 General Description …………………………………………………………………………. 16
7.2 Substrate Preparation ………………………………………………………………………. 17
7.3 Perovskite Film Preparation ………………………………………………………………. 17
7.4 Fabrication ……………………………………………………………………………………… 17
7.5 Characterization ………………………………………………………………………………. 17
8. Photovoltaic Applications …………………………………………………………………… 18
9. Summary ……………………………………………………………………………………………. 18
10. References ……………………………………………………………………………………….. 19
Keywords
1. Lead Free Halide Perovskite
2. Halide Double Perovskite – Bi and Ag
3. Tunability of Bandgap
4. Power Conversion Efficiency
5. Photoluminescence Lifetime
6. Defect Tolerance
7. Optical Absorption
8. Thermal and Environmental Stability
9. Trivalent Metal Alloying
10. Flash Evaporation Printing
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1. Introduction
During recent years there is a tremendous growth in solar energy sector due to their
low cost effectiveness and environment friendliness offering an answer to the
increasing concern for global warming and greenhouse gases by fossil fuels. Initially,
silicon solar cells have been used for photovoltaic technologies in solar cell industry
for better cost of production and high efficiency. Nowadays, different types of solar
cells are being used in the solar industry for achieving high efficiency. One of the
significant light absorbing materials in photovoltaics industry are “Perovskite Solar
Cells”. The active layers in PV modules of perovskites deliver high open circuit voltages
under sun illumination which then converts the incident energy into electrical power.
The energy of photons which are equal to bandgap in PV cell are only absorbed in
solar cells 1.
The recent progress on the research of perovskite solar cells attained significant
achievements due to their high power conversion efficiency. For fabrication of dye-
sensitized solar cells, an organic metal halide perovskite CH3NH3PbI3 is used as a
sensitizer and it attains power conversion efficiency of 3.8%. The efficiency of
perovskite decreases due to the use of liquid electrolyte as perovskite gets dissolved
easily in liquid electrolyte . The solid electrolyte like 2,2′,7,7′-tetrakis(N,N-p-dimethoxy-
phenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD) are used for the replacement for
liquid electrolyte which causes increase in efficiency of 9.7% and also the stability of
the perovskite is improved. The further research and development of perovskite solar
cells helps in attaining power conversion efficiency of 21.1% 2.
Lead based halide perovskites has been the fast growing materials in perovskite
optoelectronics for obtaining high power efficiency. The solution-processable
perovskites are considered for the next generation solar cells for photovoltaic
applications. However, the amount of toxicity level present in lead causes stability
issues and environmental impact issues. The homovalent substitution of lead in
perovskite affects the optoelectronic properties with increase in band gaps and
effective masses. Hybrid lead halide perovskites can be easily degraded due to
moisture and heat upon prolonged exposure to light. One of the possible way for better
performance is to replace lead by heterovalent substitution of noble metals that is the
formation of double perovskites. Tunability of band gap of oxide perovskite is possible
using double perovskite structure. This structure exhibits good ferroelectric and
ferromagnetic properties. The structural refinement in double perovskite is determined
by X-ray diffraction. The indirect band gap and photoluminescence peak are obtained
by optical absorption spectrum. In order to replace lead, several combination of
elements have been substituted and mostly it leads to the rapid degradation of halide
perovskites. Finally, the double perovskites are taken which are occupied by one
monovalent and one trivalent cation to help in maintaining charge neutrality. Bismuth
and Silver are chosen for the formation of halide double perovskite as it is best suitable
for optoelectronic applications 3.
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2. Rapid Evolution of Perovskite Cell Efficiencies
Halide Perovskites are the emerging materials for low cost and cost effectiveness solar
cells. The power conversion efficiency of the perovskite solar cells increased from
3.8% in 2009 to 22.1% in 2016. The efficiency evolution is shown in Fig. 1 4.
Fig. 1: Rapid Evolution of Power Conversion Efficiency of Perovskite Solar Cells 4.
The two organic-inorganic hybrid halide perovskites CH3NH3PbBr3 and CH3NH3PbI3
gained power conversion efficiency of 3.13% and 3.81% respectively. The high
efficiency of 9.7% is achieved when perovskite absorbers are used as the primary
photoactive layer for fabrication of solid-state meso-superstructured perovskite solar
cells. The deposition of dense and uniform perovskite films is significant for high
efficiency and optical absorption. The crystallization of FAPBI3 is done by direct
intermolecular exchange of dimethyl sulfoxide (DMSO) molecules in PbI2 with
formamidinium iodide which is required for deposition of high quality FAPBI3 thin films.
This high quality thin films gains a power conversion efficiency greater than 20%.
Crystallization and efficiencies of FAPBI3 are shown in Fig. 2 4.
Fig. 2: a. FAPBI3 perovskite crystallization 4.
b. Efficiencies of FAPBI3 based perovskite cells 4.
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3. Lead Free Metal Halide Perovskite
In solution based photovoltaics, the growth transition of lead based perovskites is
improved by reaching over 20% efficiency. However, the main drawback is the toxicity
caused by lead which is the substantial hurdle in many perovskite based photovoltaics
applications. This fact set up a new research field in lead free metal halide perovskites
and the possible way to replace lead. It is considered as the potentially low cost
alternative to silicon based photovoltaics as it can be prepared in both vacuum and
solution based techniques due to their facile low temperature solution processability.
Perovskites come up with ABX3 structure. Monovalent organic or inorganic cations
occupies A site while B site is occupied by divalent metal cation and X site by halide.
The charge neutrality of anions and cations defines the formability of metal halide
perovskites. The structure of perovskite is given in the Fig. 3 5.
Fig. 3: Crystal structure of ABX3 – type metal halide perovskites 5.
The octahedral factor µ is used to define the stability of BX6 octahedra. The octahedral
factor µ has to be between 0.442 and 0.895 for obtaining stability of metal halide
perovskites. The octahedral factor is the ratio of radii of B site cation and halide
counterion which is given by 5
µ = rB
rX . (1)
where, rB is radii of B site cation.
rX is radii of halide counterion 5.
The Goldschmidt tolerance factor is used for explaining the heterovalent and
homovalent substitution in metal halide perovskites. It can predict the novel lead free
perovskite compounds for photovoltaic applications based on ionic radii of the involved
ions. The tolerance factor is given by 5
t = (rA + rX)
?2(rB + rX) . (2)
where, rA is radii of A site cation 5.
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Different substitution of lead can be possible by replacement of lead by group-14
elements, alkaline earth metals, transitions metals and lanthanides. The suitable
replacement for lead can be group-14 elements like Tin and Germanium because of
their good optoelectronic applications. The most common lead-free tin halide
perovskites is CH3NH3SnI3 and can be prepared in mesoporous Titanium dioxide
(TiO2) due to short range carrier diffusion lengths of tin halide perovskite. It has
relatively a low band gap of 1.23 eV but in CH3NH3SnBr3, the optical band gap is 2.2
eV and can be processed by vapour deposition methods. A mixed iodide bromide
perovskite semiconductor CH3NH3SnIBr2 has optical band gap of 1.75 eV with
efficiency of 5.73% in meso-structured perovskites. When the band gap becomes
wider, there will be increase in open circuit voltage Voc and decrease in current density
JSC .The properties of CH3NH3Sn(IBr) perovskites are shown in Fig. 4 5.
Fig. 4: A. Absorption Properties B. Energy Level Diagram C. J-V curves 5.
Even though Sn perovskites have shown long diffusion length, superior electron
mobility and narrow optical band gap, the main drawback is that it gets oxidizes at its
+4 stable oxidation state from +2 when exposed to ambient air so that the fabrication
has to be done at the inert gas atmosphere. Later, group-15 elements Bismuth(Bi) and
Antimony(Sb) based perovskites has come into the research field as it exhibits identical
electronic configuration and comparable ionic radii of that of lead. The ternary bismuth
halide absorbers Cs3Bi2I9 and MA3Bi2I9 emerged recently as potential candidate due to
its non-toxicity and high moisture stability. It exhibits optical band gap of 2.2 eV and
2.1 eV. MA3Bi2I9 has long term stability compared to MAPbI3 as the colour of MA3Bi2I9
didn´t change when exposed to relative humidity of 61%. Later then formation of double
perovskites has been considered for research. The possible heterovalent substitution
of Pb2+ by trivalent metals(Bi,Sb) in combination of monovalent metals like Silver(Ag),
gold(Au) and copper(Cu) forming double perovskites molecular structures of A2MM´X6
where A=Cs+, MA+, M=Bi3+,Sb3+ , M´=Ag+, Au+, X=I-,Br-,Cl-. The most common double
perovskites are Cs2BiAgBr6 and Cs2BiAgCl6 reported band gap of 2.19 eV and 2.77 eV
obtained from diffuse reflectance spectroscopy and DFT calculations 6.
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4. Overview of Halide Double Perovskite
A promising avenue for addressing both stability and toxicity is by the formation of
halide double perovskites Cs2BB’X6. Among all the possible double perovskites
Cs2BiAgBr6 and Cs2BiAgCl6 are demonstrated by solid state reactions and solution
processing. Both the compounds absorb light in the visible region of the solar
spectrum, crystallize in the elpasolite structure and exhibit impressive stability under
ambient conditions. Optical measurements suggests that both Cs2BiAgBr6 and
Cs2BiAgCl6 indirect band gap semiconductors but they are sizeable optical band gaps.
The band gap range from 1.83 eV to 2.19 eV for Cs2BiAgBr6 and 2.2 eV to 2.77 eV for
Cs2BiAgCl6 .The differences in the band gaps are due to the measurement techniques
like optical absorption and photoluminescence vs diffuse reflectance. The electronic
properties of these compounds are calculated by using the principles of density
functional theory (DFT) and local density approximation (LDA). Spin orbit coupling is
used for description of band gaps and effective masses of halide perovskites. The
formation of isolated conduction band in the middle of scalar relativistic band gap is
possible only by spin orbit coupling (SOC) 7.
Fig. 5: DFT-LDA band structure Cs2BiAgBr6 (b) and Cs2BiAgCl6 (a) 7.
Blue lines in the Fig. 5 describes the calculations with spin orbit coupling and red lines
describes without spin orbit coupling. The light blue shading highlights the width of the
lowest conduction band 0.6 eV for Cs2BiAgCl6 and 0.9 eV for Cs2BiAgBr6 7.
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The rock salt structure of the double perovskite is Cs2BBiX6 where B+ and Bi3+ occupy
octahedral center while Cs+ stand in the center of cuboctahedron. The partial charge
densities of conduction band maximum (CBM) and valence band maximum (VBM) are
used for describing the distribution of photo-induced electron-hole pairs in bismuth
based halide double perovskites. The conduction band maximum stems from Bi 6p
and Ag 5s while Bi 6s and Ag 4d contributes to the valence band maximum as the
stereo-chemical lone pair electrons of Bi 6s plays significant role in stabilization of rock-
salt perovskites. The unusual electronic and optical properties of double perovskites
are related by lone-pair behaviour of Bi 6s orbital like the organolead halide hybrid
perovskites where Pb 6s plays a significant role for unusual electronic and optical
properties. The formation of CBM and VBM does not include Cs+ while Bi 6s and halide
p orbitals constitute the VBM states. This confirms that Bi halide rock salt double
perovskites is suitable for lead free perovskites 8.
The curvature of energy dispersion curve around the bottom of conduction bands and
top of the valence bands is used for the calculation of effective masses (m*) of
electrons and holes. For computing effective masses we use the equation 8
m* = ?2 ?2?(?)
??2
-1
. (3)
where, ?(?) is band edged eigen values
? is the wave vector
m* is the effective mass 8.
Ag based halide perovskites has larger band gap than Cu based halide perovskites
while Br based halide perovskites possess lower band gap than Cl based halide
perovskites. The bandgap and effective masses of bismuth halide double perovskites
are listed in a table below 8.
Double
Perovskite
Band Gap (eV) Effective Masses
me mh me* mh*
Cs2AgBiBr6 1.10 0.38 0.35 0.37 0.14
Cs2AgBiCl6 1.71 0.43 0.44 0.53 0.15
Cs2CuBiBr6 0.51 0.35 0.42 – –
Cs2CuBiCl6 0.83 0.47 0.56 – –
Table 1: Band gaps and effective masses of double perovskites 8.
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5. Characteristics of Halide Double Perovskite
5.1 Photoluminescence Lifetime
The fundamental photoluminescence lifetime of Cs2BiAgBr6 is ca. 660 ns suitable for
photovoltaic applications. Comparison between single crystal and power PL decay
shows high defect tolerance. The lifetime of Cs2BiAgBr6 is much higher than the
recombination lifetime of high quality (MA)PbBr3 films(170 ns) and (MA)PbI3 films(736
ns to1 µs). PL decay curves shows that majority of carrier recombination is possible
with only 6% loss in moving from single crystal to powder. The perovskite shows an
indirect bandgap with shallow absorption region at 1.8 eV and with increase in
absorption near 2.1 eV shown in Tauc Plot of indirect transition in Fig. 6A. The linear
regions of the plot shows the phonon-assisted processes with transitions at 1.83 eV
and 2.07eV in absorption and emission of phonon. At room temperature, perovskite
shows weak PL at 1.87 eV and PL at 23 K is more intense with peak centred at 1.98
eV shown in Fig. 6B. Analysis of time trace involves short lifetime process, inter-
mediate life time process and long time component as shown in Fig. 6C. In analysis of
short lived process lifetime, the magnitude of this process(PL intensity x Time) is larger
in powder(5%) than in single crystal(0.02%). The intermediate life time is shorter in
powder(54 ns) than single crystal(145 ns) as powders have more defects and surface
states than single crystal, this implies that short and intermediate lifetime process may
originate from trap or surface-state emission. PL decay curves defines the sum of
radioactive and non-radioactive recombination rates. Long carrier recombination
lifetime defines the good photovoltaic performance. Long lived recombination lifetimes
emit 85% of excited carriers in crystal compared to 80% of the powder 9.
Fig. 6: A . Absorbance Spectrum of powder. Inset: Tauc Plot characteristics of
indirect band gap 9.
B . Steady State room temperature photoluminescence spectrum of powdered
sample upon 500nm excitation . Inset: low temperature PL spectrum 9.
C . Time resolved room temperature PL and fits for the PL decay time in powder and
single crystal samples 9.
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5.2 Thermal Stability
The stable chemical potential region of pure Cs2BiAgBr6 is narrow, so that the chemical
conditions have to be controlled to avoid all impurity phases. For stabilizing Cs2BiAgBr6
, the thermodynamic equilibrium should be reached 10
2 ?µCs + ?µAg + ?µBi + 6?µBr = ?HCs2AgBiBr6 = – 11.36 eV . (4)
where, ?HCs2AgBiBr6 is the formation of enthalpy of Cs2BiAgBr6. To avoid the formation
of competing secondary phases including CsBr, CsBr3, AgBr, BiBr3, CsAgBr2,
Cs2AgBr3 and Cs3Bi2Br9 the following conditions must be satisfied 10
?µAg + ?µBr ? ?HAgBr = – 0.65 eV. (5)
?µAg + 3?µBr ? ?HAgBr3 = – 4.37 eV . (6)
?µCs + ?µAg + 2 ?µBr ? ?HCsAgBr3 = – 4.67 eV . (7)
2?µCs + ?µAg + 3?µBr ? ?HCs2AgBr3 = – 8.51 eV . (8)
3?µCs + 2?µAg + 9?µBr ? ?HCs3Bi2Br9 = – 19.17 eV 10. (9)
Fig. 7: a. Polyhedron Structure 10.
b. Slices at ??Ag= 0 eV 10.
c. Slices at ??Ag= – 0.4 eV 10.
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The polyhedron structure (Fig. 7a) exhibits a long but narrow shape so that the
chemical conditions are carefully controlled to obtain single phase Cs2BiAgBr6 and
avoid impurity phases. The higher side (ABCD) of the polyhedron indicates Cs-, Ag-
and Bi-rich/Br-poor chemical conditions (Fig. 7b) while the lower side (OPQR) of the
polyhedron indicates Cs-, Ag- and Bi-poor/Br-rich chemical conditions. The HIJK side
(Fig. 7c) have all positive intrinsic defects. The stability of Cs2BiAgBr6 can be obtained
by calculated decomposition energy. The calculated decomposition enthalpies (?Hd)
are 0.64, 0.24, 0.16 and 0.11 eV per formula unit shows that Cs2BiAgBr6 will not
decompose spontaneously. For MAPbI3 , ?Hd values are smaller and even negative
indicating that it exhibits intrinsic thermal instability while Cs2BiAgBr6 is larger than
MAPbI3 and the double perovskite exhibits better ambient and thermal stabilities than
lead halide perovskites 10.
5.3 Environmental Stability
The sample Cs2BiAgBr6 is exposed to ambient atmosphere in both light and dark
conditions for obtaining the stability of the compound. The sample was loaded into flat
sample holder then it is exposed to ambient air for X-ray Power Diffraction
measurements (XPRD). The sample is kept in dark for two weeks and it shows no
change in visual inspection. The sample is kept next to a large glass window exposing
to ambient atmosphere and visible light. The darkening of exposed surface of
Cs2BiAgBr6 is obtained and the sample received six hours of direct exposure each day.
The sample is analysed by UV-Vis diffuse reflectance spectroscopy and XPRD after
two and four weeks exposure to sunlight. The reflectance spectrum for Cs2BiAgBr6
changes after two weeks exposure to light and ambient air. This shows the degradation
of double perovskite phase over time when exposed to light and moisture 11.
Fig. 8: UV-Vis Diffuse Spectra showing light instability of Cs2BiAgBr6 11.
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5.4 Defect Tolerance
The intrinsic point defects of Cs2BiAgBr6 includes four vacancies (VCs, VAg, VBi, VBr),
four interstitials (Csi, Agi, Bii, Bri), six cation-on-cation antisites (CsAg, CsBi, AgCs, AgBi,
BiCs, BiAg), three cation-on-anion antisites (CsBr, AgBr, BiBr) and three anion-on-cation
antisites (BrCs, BrAg, BrBi). The electrical and photovoltaic properties are affected by the
low ?H values of all the vacancies. VAg acts as shallow acceptor as it has the lowest
?H and the transition level ?(01-?) at 0.04 eV above the valance band maximum
(VBM). VAg is good for p-type conduction. A strong antibonding coupling between Ag
4d and Br 4p is created by the shallowing nature of VAg as it pushes the VBM to a high
energy level. VCs is also a shallow acceptor as the transition level (0/1-) lies at 0.07 eV
above VBM. Due to the high iconicity of Cs+, it has higher ?H compared to VAg and
contributes less to the p-type conduction. VBr is a shallow donor as the transition level
(0/1-) is at 0.03 eV below conduction band maximum (CBM). Agi and Bri have low low
?H values among all the interstitials. Agi is a shallow donor as the transition level is at
0.05 eV below CBM. Bri is a deep acceptor as the transition level (0/1-) is at 0.26 eV
above VBM. The B-site cation-on-cation antisites AgBi and BiAg have low ?H values
which have an important influence on electrical properties. AgBi is a dominant defect
that deteriorates photovoltaic performance. The intrinsic defects with high ?H values
have a little influence on the electrical and photovoltaic properties of Cs2BiAgBr6. The
transition energy levels are shown in Fig. 9 10.
Fig. 9: Transition energy levels of a. Intrinsic acceptors of Cs2BiAgBr6 10.
b. Intrinsic Donors of Cs2BiAgBr6 10.
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6. Alloying of Halide Double Perovskite
6.1 Dilute Alloying
Normally halide double perovskites have larger bandgaps that afford weak visible light
absorption. The well known halide double perovskite evaluated as an absorber
Cs2BiAgBr6 has a larger bandgap of 1.95 eV. The dilute alloying decreases Cs2BiAgBr6
bandgap by ca. 0.5 eV. The alloyed halide double perovskite shows comparable band
gap energy and carrier lifetime to those of (CH3NH3)PbI3. Indirect band gap of
Cs2BiAgBr6 provides inferior light absorption compared to lead based perovskites.
Lead filled 6s orbitals at the valence band maximum and empty 6p orbitals at the
conduction band maximum plays a significant role in strong direct-gap absorption of
lead based perovskites. The inclusion of Ag s and Ag d orbitals in Cs2BiAgBr6 shifts
conduction and valance band maximum to indirect band gap . The incorporation of Tl+
as a dilute impurity to Cs2BiAgBr6 make it suitable as a direct band gap absorption 12.
Fig. 10: Dilute Alloying of Cs2BiAgBr6 12.
Tl content can be tuned across the series Cs2(Ag1-aBi1-b)TlxBr6 resulting in 0.03-0.75
atom % Tl. X-ray absorption near-edge structure (XANES) measurement at Tl-L3 edge
determines the oxidation state of Tl. The energy loss or gain of Tl substitution at both
Ag+ and Br3+ sites can be calculated by the formula 12.
?E = Edopped – Eundopped . (10)
where, Edopped is the dopped energy level.
Eundopped is the undopped energy level.
?E is the change in energy level 12.
The energy difference of Tl substitution of Ag and Bi sites decreases when ?E(Bi3+) =
0.1 eV and ?E(Ag+) = – 0.01 eV. The substitution of Tl+ for Ag+ is energetically
favourable when ?E= – 0.05 eV. Tl+/Ag+ substitution occurs in concentrated Tl
regime by alloying Ag into Tl-Bi perovskite. All the measurements indicate that Tl can
substitute at either Ag or Bi sites by dilute alloying 12.
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The reflectance spectra of Cs2BiAgBr6 and Cs2(Ag1-aBi1-b)TlxBr6 are converted to
pseudo absorbance spectra for examining the optical effects of Tl alloying. The
extrapolated band gaps are from ?r vs photon energy (E) plots where r = 0.5 and 2 for
indirect and direct band gaps and ? is the pseudo absorption coefficient. At x = 0.010,
the band gaps energy drops from 1.95 eV in Cs2BiAgBr6 (indirect) to 1.72 eV (direct)
or 1.57 eV(indirect) in Cs2(Ag1-aBi1-b)TlxBr6. At the highest alloying level x = 0.075, the
band gap decreases more slowly and reaches 1.40 eV (indirect) and 1.57 eV (direct).
The Cs2BiAgBr6 bandgap is brought within and ideal range by dilute Tl alloying and
making it suitable for applications like single-junction photovoltaic absorber 12.
Fig. 11: A. XANES of Cs2(Ag1-aBi1-b)TlxBr6 12.
B. Absorbance spectra of 1d 12.
C. Extracted Band gaps 12.
6.2 Trivalent Metal Alloying
The bandgap of Cs2BiAgBr6 is generally large, indirect or direct forbidden and not
suitable for high efficiency photovoltaic applications. Eventhough, the dilute alloying of
Tl have comparable band gap and carrier life time identical to lead based halide
perovskite, the toxicity of Tl also remains a problem. Atomic substitution is found to the
best method for bandgap engineering for halide double perovskite (A2MIMIIIX6) where
MIII provides the most degrees of freedom so that trivalent metal alloying is done by
replacing BiIII lattice site with SbIII and InIII making it suitable for optoelectronics.
Powdered X-ray diffraction (PXRD) measures the lattice constants of alloyed
compounds Cs2Ag(Bi1-xMx)Br6 (M = In, Sb) where the measured values decrease with
increasing x. InIII can replace 75% of the BiIII and SbIII replaces 37.5%. The diffuse
reflection band gap values increases in energy with InIII incorporation and decreases
with SbIII substitution. Broad photoluminescence peak ranges from 500 to 900 nm
depending on alloy type and concentration 13.
15
From the Tauc plots, direct band gaps from 2.15 eV to 2.41 eV and indirect band gaps
from 1.86 eV to 2.27 eV in Cs2Ag(Bi1-xMx)Br6 (M = In, Sb) alloys are detected. As the
x value increases from 0 to 0.75, the bandgap of Cs2AgBi1-xInxBr6 increases from 2.12
eV to 2.27 eV. As x increases from 0 to 0.375, the bandgap of Cs2AgBi1-xSbxBr6
decreases from 2.12 eV to 1.86 eV. The relationship between the bandgap and alloying
level can be obtained by using the formula 13
E(x) = E(CABB) + E(CAIB) – E(CABB) – b x + bx2 . (11)
where, E(x) is the bandgap of Cs2AgBi1-xInxBr6 .
E(CAIB) is the bandgap of Cs2AgInBr6 .
E(CABB) is the bandgap of Cs2BiAgBr6 .
b is the bowing parameter 13.
Fig. 12: a. Indirect Tauc plot of Cs2AgBi1-xInxBr6 13.
b. Indirect Tauc plot of Cs2AgBi1-xSbxBr6 13.
c. Measured direct and indirect bandgaps of In and Sb 13.
d. Quadratic fit of indirect band gap of Sb and In. 13.
e. Photographs of both Sb and In samples 13.
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7. Thin Film Preparation of Halide Double Perovskite
7.1 General Description
The low solubility precursors have been considered for the fabrication of high quality
films.. Light absorption measurements and resolved PL measurements shows that this
films exhibit identical absorption properties of lead based perovskites. Film synthesis
conditions are used to improve the optoelectronic properties of Cs2BiAgBr6 making it
suitable for photovoltaic applications. All synthesis steps are conducted under ambient
conditions while hole transporting layer is performed in nitrogen-filled glove box.
Dimethylsulfoxide(DMSO) has the highest ability to dissolve precursors like AgBr, CsBr
and BiBr3. Cs2BiAgBr6 films is prepared by spin-coating a dimethylsulfoxide-based
precursor solution on top of a substrate. The “preheating step” involves the heating of
substrate and precursor solution to 75 ºC and improves the surface coverage and film
quality. After the spin-coating procedure, the “annealing step” is performed at a
temperature of 250 ºC and this high annealing temperature are required to convert
precursors into Cs2BiAgBr6 revealed by power X-ray diffraction measurements. It is
also used to remove the side phases which is found during the thin film formation of
double perovskite .Preheating of both the substrate and solution increases the quality
of double perovskite films and improvement of surface coverage. The vanishing of side
phases is not caused by the removal of metal solvent intermediates which is shown by
PXRD measurements 14.
Fig. 13 : Schematic synthesis route for Cs2BiAgBr6 thin films 14.
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7.2 Substrate Preparation
Fluorine-doped tin oxide (FTO)-coated glass sheets are prepared with zinc powder and
3 M HCL. They are cleaned by using 2% Hell manex solution and rinsed by using
deionized water and ethanol. Titanium dioxide(TiO2) was deposited using spin-coating
a sol-gel precursor solution at 2000 rpm for 45 s and annealing at 500 ºC for 45 min.
Oxygen plasma treatment is used for removing the organic residues. Preparation of
sol-gel solution involves addition of HCL solution in 2-propanol and stirred solution of
titanium isopropoxide in 2-propanol. Mesoporous-TiO2 of 800 nm thick, is applied by
spin coating 100 µL of TiO2 nanoparticle paste diluted in absolute ethanol onto the
TiO2 layer at 2500 rpm for 30 s and annealing is held at 500 ºC for 15 min under
ambient conditions 14.
7.3 Film Preparation
Preparation of precursor involves dissolving 268 mg BiBr3 ,112.8 mg of AgBr and 254
mg CsBr in dimethylsulfoxide. Before spin-coating both the solution and substrates are
preheated at 75 ºC. The hot precursor solution of 100µL is spin-coated into TiO2
covered substrate at 2000 rpm for 30 s. Under ambient conditions, the substrates are
annealed at 285 ºC for 5 min for double perovskite formation. The temperature
variation of both heating and annealing is used for identifying the optimal conditions of
phase pure films 14.
7.4 Fabrication
The films are covered by a hole transporting layer (HTL) of 2,2′,7,7′-tetrakis-9,9′-
spirobi-fluorene.The preparation of HTL solution is done by dissolving 73 mg of spiro-
OMeTAD in chlorobenzene. After filtration, the solution is mixed with 4-tert-
butylpyridine and 173 mg m/L bis-sulphonamide lithium salt solution in acetonitrile.
Spin-coating of that solution is done at 1500 rpm for 45 s. For drying the solvent, the
sample rotation is done at 2000 rpm for 5 s. The top layer of the device is thermally
deposited by 40 nm thick gold electrodes under high vacuum 14.
7.5 Characterization
Thin films are performed by Powder X-ray Diffraction(PXRD) measurements using a
Brucker D8 Discover X-ray diffractometer operating at 40 kV and 30 mA comprising
Ni-filtered Cu K? radiation and a position-sensitive detector. The formation of phase
pure films are assured by PXRD measurements. The films were prepared on glass
substrates or on the mp-TiO2 substrates for UV-vis measurements. The double
perovskite films are prepared on 800 nm thick mp-TiO2 or mp-Al2O3 films for PL
measurements. Steady-state absorption spectra is acquired by using a Lambda 1050
UV-vis spectrophotometer. Fluotime 300 spectrofluorometer is used for conducting
steady state and time resolved PL measurements 14.
18
8. Photovoltaic Applications
The quality of printed films cannot be compared with thin films prepared by the physical
vapor deposition (PVD) methods due to the high purity in this process. The combination
of compact PVD process with printed electronics is possible by flash-evaporation
methodology (FEP). The flash evaporator is made by cross-stacked super-aligned
carbon nanotube (CNT) films for high temperature response. A flash evaporation
transfer ribbon is made possible by coating of the target material on the flash
evaporator. The printing of the target material on the carbon nanotube flash evaporator
is done by gas-phase transportation. The flash evaporation methodology is one of the
suitable printing solution for perovskite thin films. The perovskite thin films are
synthesized by the two-step deposition method. Nanoporous perovskite thin films are
printed on substrates as precursors by flash-evaporation methodology. The properties
and morphology of the thin films are measured and shown by X-ray diffraction,
ultraviolet-visible absorption spectroscopy and scanning electron microscopy. The
perovskite thin films which are integrated into photovoltaic devices by flash-
evaporation spectroscopy shows a power conversion efficiency of 10.3%. Flash
evaporation technique is an asset to printable electronics and flexible electronics due
to their cost effectiveness and environmental friendliness. This thin films produced by
flash evaporation technique are used as photodetectors 15.
X-ray detection plays a significant role in medical diagnosis and scientific research
applications. Compared to indirect X-ray detection, direct X-ray detection enables high
spatial resolution. Organic-inorganic hybrid lead halide perovskites explored for X-ray
detection shows large X-ray attenuation coefficient, high carrier drift length per unit
electric field and low cost solution for growth of single crystals. Even though as lead is
the core component it causes serious damage to the body such as several brain related
symptoms. X-ray detectors with low detection limit is considered for imaging
applications as it minimizes the lateral spreading of charges and improves the spatial
resolution. Therefore, lead free perovskites single crystals can be used for X-ray
detection with better sensitivity and detection limits. Cs2BiAgBr6 single crystals is
considered as sensitive X-ray detectors with low detection limits. The low noise level
is the key for achieving a high signal current for obtaining low detection limit.
Cs2BiAgBr6 contains heaviest stable element Bi and has high atomic number than
other lead based perovskites and allows more efficient X-ray attenuation and improved
signal current. Cs2BiAgBr6 has high resistivity than other lead based perovskites so
that it allows low dark current and reduced noise current. The suppressed field-driven
ionic migration is exhibited by Cs2BiAgBr6 single crystals and large external bias is
obtained with improved signal current and low noise current. The high resistivity, low
ionization energy, high average atomic number and high sensitivity of Cs2BiAgBr6
collectively makes the double perovskite Cs2BiAgBr6 single crystals suitable for
sensitive X-ray detection used in photovoltaic applications 16.
19
9. Summary
This technical paper explains about the metal halide double halide perovskite solar
cells, their characteristics and their photovoltaic applications . Perovskite solar cells
has been considered as the fast growing high efficiency solar cells compared to silicon
solar cells. Various types of perovskites has been emerged and play a vital role in the
photovoltaic applications like lead based perovskites, organic-inorganic hybrid
perovskites but double perovskites has more specific advantages over the other types
of perovskites . Cs2BiAgBr6 is considered as the good visible light absorber and as the
best lead free metal halide double perovskite for optoelectronic applications like
photodetectors. Even though this double perovskites has indirect band gap, they have
good photoluminescence life time compared to other perovskites. Research is still
going on for finding the best combination of double perovskites for direct band gap and
for obtaining a better optical absorption properties. Thus perovskites solar cells has
evolved as the promising materials for achieving high efficiency in the solar industry.
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