investigation regarding the role of chloride in organic-inorganic

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Investigation Regarding the Role of Chloride in Organic-InorganicHalide Perovskites Obtained from Chloride Containing Precursors

M. Ibrahim Dar, Neha Arora, Peng Gao, Shahzada Ahmad,Michael Grätzel, and Mohammad Khaja Nazeeruddin

Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl503279x • Publication Date (Web): 13 Nov 2014

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Investigation Regarding the Role of Chloride in

Organic-Inorganic Halide Perovskites Obtained from

Chloride Containing Precursors

M. Ibrahim Dar,*a Neha Arora,

a Peng Gao,

a Shahzada Ahmad,

b Michael Grätzel*

a and

Mohammad Khaja Nazeeruddin*a

aLaboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss

Federal Institute of Technology (EPFL), Station 6, 1015 Lausanne, Switzerland

bAbengoa Research, C/ Energía Solar nº 1, Campus Palmas Altas-41014, Sevilla, Spain

ABSTRACT: As the photovoltaic performance of a device is strongly influenced by the

morphology of perovskite, achieving precise control over the crystal formation of organic-

inorganic halide perovskites synthesized in the ambience of chloride ions has garnered much

attention. Although the resulting morphology dictates the performance of the device

considerably, the understanding of the role of chloride ions has been scant. To unravel this

mystery, we investigated three different organic-inorganic halide perovskite materials grown

from the chloride-containing precursors under different but optimized conditions. Despite the

presence of chloride ions in the reaction mixture, scanning transmission electron microscopy-

energy dispersive spectroscopy (STEM-EDS) reveals that the CH3NH3PbI3 perovskites formed

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are chloride-free. Moreover bright field transmission electron microscopy indicates that chloride

ions effect the growth of the CH3NH3PbI3.

KEYWORDS Perovskite, CH3NH3PbI3, Photovoltaic, Electron microscopy, STEM

Organic-inorganic halide perovskites have recently gained prominence due to the development

of high efficiency solar cells.1,2 Because of their structural and compositional diversity, these

materials have been identified as candidates in a myriad of applications such as photovoltaics

and luminescent displays.3,4,5,6 The properties of organic-inorganic halide perovskites are a

function of dimensions, morphology, phase, and chemical composition which, together,

eventually define the performance of the solar cell based on these materials.7,8,9 Therefore,

gaining a rational control over the phase as well as the morphology of organic-inorganic halide

perovskites has been pursued intensively.7,8

In controlling the growth of the perovskite layer, solution-based bottom-up approaches provide

more flexibility than other methods employed for the fabrication of high-efficiency devices. By

adopting ‘‘solution chemistry’’ for the synthesis of various organic-inorganic mainly two

dimensional lead/tin-based halide perovskites, Mitzi and co-workers pioneered the two-step

approach.5,6 Earlier, the fabrication of such devices employed a single-step deposition of the

CH3NH3PbI3 layer10,11 which was succeeded by the sequential, two-step conversion method for

the deposition of the CH3NH3PbI3 layer onto a mesoporous TiO2 film which provided a better

control over the growth of perovskite structures.1,12,13 Two-step methodology involves the

deposition of PbX2 (X= Cl, Br, I) onto a substrate and the resulting film is subsequently

transformed into CH3NH3PbX3 by dipping into a solution of CH3NH3X3 whereas, single step

methodology involves the deposition of mixture of PbX2 and CH3NH3X3 onto a substrate.14,11

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In addition to the development of fabrication methods for the preparation of perovskites, there

has been an upsurge of interest in understanding their composition. This is primarily because

composition dictates the property of the perovskite material significantly which, in turn,

determines the performance of the resulting device.15 In principle, the desired properties of a

hybrid organic-inorganic halide perovskite can be achieved by appropriately changing the

organic cation, central metal ion, or the halide ion.7,9,15,16,17,18 In addition to other compositions,

CH3NH3PbI3 and CH3NH3PbI3−xClx have largely been explored as the light-harnessing material

in perovskite solar cells.1,2 One of the most remarkable properties associated with these

perovskites is that the excitons in the solar cell can dissociate either in the bulk or at the donor-

acceptor interface, and owing to the long carrier lifetime and good carrier transport

characteristics, charge extraction remains efficient.19,20 Although the presence of the chloride ion

supposedly changes the properties of the resulting perovskites, i.e., CH3NH3PbI3−xClx, questions

surround the presence and understanding the role of the chloride. To answer these questions, we

have selected three perovskite systems synthesized by different approaches under optimized

conditions. We have exploited electron microscopy to determine the content and distribution of

the constituents, particularly the chloride ions in the perovskites. Using scanning transmission

electron microscopy energy-dispersive spectroscopy (STEM-EDS), we conclude that the

presence of the chloride ion as a substitutional dopant in the perovskite is not detectable; indeed

it was found in the form of unconverted PbCl2. To the best of the authors’ knowledge, so far such

an investigation has been scantly carried out on CH3NH3PbI3 materials. Such a technique, i.e.,

STEM-EDS exhibits several advantages over other techniques used for chemical analysis such as

X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM)-EDS. Both

XPS and SEM-EDS characterization techniques analyze the samples at bulk scale whereas

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STEM-EDS can bring out information locally, i.e., at sub nanometer (nm) level. In addition to

chemical analysis, we also carried out extensive morphological and structural characterization

using transmission electron microscopy (TEM) in bright field-TEM (BFTEM) and electron

diffraction (SAED) mode. Our unprecedented study indicates that the chloride ions influence the

growth of the CH3NH3PbI3 significantly without entering into its lattice.

Based on the experimental conditions that were optimized for the synthesis of perovskite

materials, four different samples were obtained as summarized in Table 1. The resulting

perovskite structures were thoroughly examined for their morphology, size, composition,

crystallinity, and dispersion by electron microscopy.

Table1. Summary of the experimental conditions used for the synthesis of different perovskite

structures.

*(MAI, MACl and IPA are the acronyms for methyl ammonium iodide, methyl ammonium chloride and isopropyl alcohol , respectively).

Morphological Characterization. To analyze the gross morphology of perovskite structures

deposited on TiO2 photoanode, field emission scanning electron microscopy (FESEM) was used.

The morphology obtained by SEM of the three different samples prepared in the presence of

Sample

code

Precursors used Approach Dipping time Annealing

condition

P1 PbI2, MAI:MACl (95:5) wt% in (8 mg in 1 mL of IPA)

Two-step 4 min 110 °C for 30 min

P2 PbCl2 and MAI (8 mg in 1 mL of IPA)


P3 PbCl2: MAI= 1:3 Single-step ------ 110 °C for 30 min

P4 PbI2 and MAI (8 mg in 1 mL of IPA)


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chloride ion is shown in Figure 1. The micrograph of P1 displays the formation of perovskite

crystals which are polydisperse (Figure 1a). In addition to structures measuring ~100-300 nm in

diameter, well–faceted, elongated perovskite structures (Figure 1b) with a high aspect ratio were

also observed.

Figure 1. SEM analysis displaying the top view of perovskite structures; Low-magnification

and high-magnification SEM micrographs of samples; P1 (a, b), P2 (c, d), and P3 (e, f).

To understand the formation of such elongated structures with an average length of ~1000 nm

(as evident from high-magnification cross sectional SEM, see S.I.), we synthesized CH3NH3PbI3

using pure MAI, i.e., no MACl was used (P4). Expectedly, we did not observe the formation of

any elongated structures under similar conditions (see S.I.), suggesting that chloride ions from

the MACl precursor might promote growth along a particular crystallographic direction.

Nevertheless, prolonged dipping of PbI2 film into pure MAI solution leads to the evolution of

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large and faceted CH3NH3PbI3 structures (see S.I.). Formation of larger perovskite crystals could

be explained by evoking the phenomenon of ripening. Low-magnification SEM micrograph of

P2 (Figure 1c) displays the formation of highly dense, uniformly distributed perovskite

structures. High-magnification SEM imaging (Figure 1d) discriminated between asymmetric and

elongated perovskite structures. Although P1 and P2 were obtained using a two-step method

involving chloride content, structures in P1 are well faceted and more elongated than in P2,

which possibly explains the dominant role of the chloride ions present in the solution in orienting

the growth of perovskite structures.

In contrast to P1 and P2, the perovskite (P3) was deposited using a single-step approach,

which evidently leads to inhomogeneous deposition of perovskite. Low-magnification SEM

(Figure 1e) reveals the formation of interlinked structures, whereas high-magnification SEM

(Figure 1f) ostensibly brings out that the mesoporous TiO2 substrate is partially covered with a

perovskite overlayer. Such an observation is consistent with the literature where it has been

documented that a slowly evaporating solvent such as DMSO reduces the surface coverage of

perovskite.21 Employing a solvent like DMF was not considered as an alternative because PbCl2

shows lower solubility in it.22 From the comparative SEM analysis of P1, P2, and P3, it could be

contended that the sequential technique provides superior control over the growth of perovskite

and that crystal formation might occurs under near-equilibrium conditions, as the resulting

CH3NH3PbI3 structures are well-formed.

Structural Characterization. The dimensions, crystallinity, and morphology of the perovskite

structures were examined in detail by transmission electron microscopy (TEM) (Figure 2). The

bright-field TEM (BFTEM) micrograph obtained from P1 (Figure 2a) shows the formation of a

well-faceted, elongated crystal with an average diameter of 400 nm and length 1000 nm,

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corroborating SEM analysis. Selected-area electron diffraction (SAED) shows a spot pattern

(Figure 2b), evidencing the single-crystalline nature of the perovskite structure; the pattern could

be indexed to the tetragonal phase of CH3NH3PbI3, in agreement with the literature.23 High-

resolution TEM (HRTEM) (Figure 2c) reveals lattice fringes with d spacing of 6.0 Å, indexable

to the (002) planes of the tetragonal phase of CH3NH3PbI3, consistent with the fast Fourier

transform (FFT) pattern in the inset.

Figure 2. TEM analysis of perovskites; BFTEM, SAED patterns and HRTEM of P1 (a-c), P2

(d-f), and P3 (g-i) samples (inset: FFT patterns obtained from the corresponding HRTEM).

It is to be noted that CH3NH3PbI3 formation takes place along the [001] direction which leads

to the formation of elongated, well-faceted structures. The formation of elongated structures is

not trivial, as tetragonal phase might exhibit shape anisotropy. Moreover, the presence of well-

formed crystals of CH3NH3PbI3, as evident from SAED and HRTEM images, indicates that

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growth takes place under near-equilibrium conditions. In other words, oriented growth occurs

through the attachment of nuclei along a particular crystallographic orientation. Such a

phenomenon of oriented attachment is well established in entirely inorganic nanostructures

grown from solutions under optimized conditions.24 Therefore, the growth of CH3NH3PbI3

structures can be explained by invoking the phenomena of oriented attachment, as amply

supported by electron microscopy. We also conclude that under optimized conditions, sequential

deposition method offers a rational control over the growth of such perovskite crystals.

Likewise sample P2 was examined by TEM (Figure 2d-f). BFTEM micrographs (Figure 2d)

show the presence of asymmetric structures along with elongated rods (inset; Figure 2d). The

average dimension of asymmetric structures is ~400 nm and that of the elongated rod was

estimated to be ~600 nm along the extension (Figure 2d; inset). The SAED displays spot pattern

(Figure 2e) evidences that the CH3NH3PbI3 asymmetric structures are single-crystalline. This

was further confirmed by the presence of clear lattice fringes in the corresponding HRTEM

image of the structure (Figure 2f).

The BFTEM image of P3 reveals the presence of aggregated structures (Figure 2g); such

morphology could result from the coalescence of structures during the growth that is expected

while using a single-step method for the deposition of perovskites. The SAED pattern and

HRTEM image show that perovskite structures are crystalline in nature (Figure 2h, i).

Elemental Analysis. Although electron microscopy provided ample evidence for the formation

of CH3NH3PbI3 structures under different conditions, it is well known that composition strongly

dictates the properties of perovskites.9,13 Specifically, the pronounced diffusion length in

perovskite-based devices has been ascribed to the presence of chloride ions.17 Therefore,

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examination of composition and atomic distribution becomes imperative and was carried out by

performing EDS mapping in the STEM mode on all the three samples.

Figure 3. STEM-EDS analysis of individual CH3NH3PbI3 crystal; (a) STEM bright-field

image, (b) STEM-EDS spectrum, (c) Pb map, (d) I map, and (e) N map.

Figure 3a shows the bright-field STEM image of a well-faceted perovskite crystal. It is to be

noted that SAED confirmed that it is a single crystal. Energy-dispersive X-ray spectroscopy in

the STEM mode (STEM-EDS, Figure 3b) shows that Pb, I, and N are present in a perovskite

single crystal. However, to within the detection limit of EDS (~1000 ppm) no feature

corresponding to Cl was detected. It is to be noted that the reaction mixture contained 5 mol % of

MACl. Colella et al.25 showed that in iodide-based perovskites; less than 3-4% of chloride

content present in the reaction mixture could incorporate into the lattice. Thus, the concentration

of chloride ions used here i.e., 5% mol MACl, would have easily led to their detection in our

sample, if present. Moreover, Colella et al. documented that the considerable difference in the

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ionic radii limits the formation of a continuous solid solution.25 Furthermore, “STEM area

mapping” of a single elongated perovskite crystal (Figure. 3c-e) reveals homogeneous

distribution of the constituent atoms, i.e., Pb, I, and N, throughout the dimension of perovskite

crystal. As we could not surprisingly, detect any signal corresponding to chlorine, the case for

chloride ions possibly acting as growth directing agents becomes stronger.

Further we analyzed P2 samples using STEM. Figure 4a shows the bright-field STEM image

of asymmetric perovskite crystals. In addition to Pb, I, and N, STEM-EDS (see S.I.) reveals the

presence of Cl in the sample. To examine the distribution of constituent elements further,

“STEM area mapping” was carried out. Such mapping provides direct evidence of distribution of

Pb throughout the CH3NH3PbI3 crystal (Figure. 4c). However, the distribution of Cl is found to

be inhomogeneous (Figure 4f). The question that therefore arises is whether the Cl is present in

the form of unconverted PbCl2 or in the mixed perovskite, i.e., CH3NH3PbClxI3-x.

Figure 4. STEM-EDS analysis of CH3NH3PbI3 asymmetric crystals; (a) STEM bright field

image (b) STEM-EDS spot spectrum (c) Pb map, (d) N map, (e) I map, and (f) Cl map.

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To probe the source of Cl, we analyzed a single elongated perovskite rod using STEM-EDS.

As evident from SEM analysis, these rods were grown along with the chloride-containing

asymmetric structures (Figure 1a-b). Expectedly, STEM-EDS (see S.I.) confirmed the absence of

Cl in the rods and area mapping (see S.I.) reveals a homogeneous distribution of the constituent

atoms, i.e., Pb, I, and N throughout the perovskite rod (see S.I.). Furthermore, spot EDS

spectrum under the STEM mode (Figure 4b) obtained from chloride containing region shows

strong Cl signal but no feature corresponding to N was observed. Therefore, the chloride content

in the asymmetric perovskite structures (Figure 4a) could be attributed to the presence of PbCl2,

which apparently remained unconverted underneath the perovskite structure. Although

CH3NH3PbI3 was obtained using PbCl2, i.e., in the presence of a high concentration of Cl, it

appears that, under given conditions, the probability of incorporation of Cl in the final

CH3NH3PbI3 material is negligible.

Figure 5. STEM-EDS analysis of aggregated CH3NH3PbI3 structures; (a) STEM bright-field

image, (b) STEM-EDS spectrum, (c) Pb map, (d) N map, and (e) I map.

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Using a single step approach, Tidhar et al.22 reported that the addition of PbCl2 affects the

crystal formation kinetics without being a part of the final product except in trace amount. Our

observation is in agreement with their study as we also found that PbCl2 in two-step

methodology improves the surface coverage of perovskite without being part of it (detection

limit of characterization technique ~1000 ppm). However, we noted that the rate of conversion

into CH3NH3PbX3 was found to be slower while using PbCl2 in comparison to PbI2.

Figure 5a shows the bright-field STEM image of aggregated perovskite (P3) structures,

complementing the BFTEM analysis. The STEM-EDS spectrum (Figure 5b) brings out the

presence of Pb, I, and N in P3. However, no peak corresponding to Cl was detected in the

aggregated perovskite structures. Furthermore, STEM-EDS area mapping (Figure 5c-e)

evidences a homogeneous distribution of constituent atoms, i.e., Pb, N, and I within the

aggregated perovskite structures. Surprisingly, we could not detect any signal corresponding to

chlorine; it is to be noted that the reaction mixture contained PbCl2: MAI in the molar ratio of

1:3.

Understanding the role and confirming the presence of Cl in perovskites is of great scientific

interest and the studies carried out in this direction have not been conclusive so for. Nanova et

al.26 documented excellent studies based on analytical transmission electron microscopy for the

visualization of the mesoporous absorber layer in a perovskite solar cell. Docampo et al.27

reported that addition of chloride in the immersion solution leads to the enhancement of short-

circuit current of solution-processed planar heterojunction solar cells. However, authors’ were

unable to detect any feature corresponding to chloride using EDX and electron energy loss

spectroscopy. Using depth-profile XPS, Yu at al.28 detected negligible amount of Cl content in

the fully annealed perovskite films. Grancini et al.29 concluded that the Cl induces a preferred

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orientation in the crystalline grains and observed that the mesoporous metal oxide scaffold

retains a negligible amount of chlorine. Furthermore on the basis of DFT calculations, as

compared to bulk perovskite, preferential position of chloride was found to be the TiO2

interface.30 In the formation of organo-lead halide perovskite films, Williams et al.31 elucidated

the structural role played by the chloride but to within the detection limit of XPS, the feature

characteristic of Cl 2p core electrons was found to be absent.

Although on the basis of some elemental analysis techniques, presence of negligible amount of

chloride have been reported32 however, it becomes imperative to determine the nature of chloride

present in the samples, i.e., whether Cl is present in the form of PbCl2 or mixed halide

perovskite. We investigated the chemical composition of three different CH3NH3PbI3 perovskites

materials fabricated using different sources of chloride ions and methods. To within the detection

limits, STEM-EDS reveals clearly that the CH3NH3PbI3 perovskite formed are chloride-free.

Even though EDS signal corresponding to Cl was detected in the samples prepared by the

sequential method with pure PbCl2 as the lead precursor, careful examination shows that this is

attributable to unconverted lead chloride. In summary, our distinctive study indicates that under

given conditions the chloride ions influence the growth of the CH3NH3PbI3 significantly without

occupying the lattice.

Experimental Section

Photoanode preparation. A commercial paste (Dyesol 18 NRT) was spin-coated onto a

precleaned TCO glass substrate (NSG 10, Nippon sheet glass, Japan) containing a ~70 nm-thick

TiO2 blocking layer, followed by a series of sintering steps (325 °C for 5 min with 15 min ramp

time, 375 °C for 5 min with 5 min ramp time, 450 °C for 15 min with 5 min ramp time, and 500

°C for 15 min with 5 min ramp time), and the sintered films were used as the photoanode. Prior

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to the deposition of TiO2 blocking layer, the FTO substrate was cleaned with a detergent, rinsed

with water and ethanol, and then treated with a UV/O3 cleaner for 15 min.

Deposition of perovskite using sequential method (P1). A 1.25 M PbI2 solution was prepared

in N,N-dimethylformamide (DMF) solvent by constant stirring at 70 °C. The TiO2 photoanode

films were then coated with PbI2 by spin-coating at 3000 rpm for 30 s, and dried at 70 °C for

30 min. After cooling to room temperature, the films were dipped in a solution containing a

mixture of CH3NH3I:CH3NH3Cl (95:5) wt% in 2-propanol (8 mg mL-1) for 4 min at 60 °C,

rinsed with 2-propanol for 2 s and annealed at 110 °C for 30 min.

Deposition of perovskite using sequential method (P2). A 1.25 M PbCl2 solution in dimethyl

sulfoxide (DMSO) was prepared by constant stirring at 70 °C. The TiO2 photoanode films were

then coated with PbCl2 by spin-coating at 3000 rpm for 30 s, and dried at 70 °C for 30 min. After

cooling to room temperature, the films were dipped in a solution of CH3NH3I in 2-propanol (8

mg mL-1) for 4 min, rinsed with 2-propanol for 2 s and dried at 110 °C for 30 min.

Deposition of perovskite using single-step method (P3). One mole of PbCl2 (278.1 mg) and

three moles of CH3NH3I (158.9 mg) were dissolved in 1 mL of dimethyl sulfoxide solvent. After

heating at 70 °C for 30 min the reaction mixture was deposited onto TiO2 photoanode films by

spin-coating at 1500 rpm for 30 s, and annealed at 110 °C for 30 min.

Deposition of CH3NH3PbI3 using sequential method (P4). A 1.25 M PbI2 solution in N,N-

dimethylformamide (DMF) was prepared by constant stirring at 70 °C. The TiO2 photoanode

films were then coated with PbI2 by spin-coating at 3000 rpm for 30 s, and dried at 70 °C for

30 min. After cooling to room temperature, the films were dipped in a solution of CH3NH3I in 2-

propanol (8 mg mL-1) for 4 min, subsequently rinsed with 2-propanol for 2 s and annealed at 70

°C for 30 min. All the samples were prepared in a controlled atmosphere, with humidity < 2%.

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Materials characterization. The CH3NH3PbI3 structures were examined by Osiris field-emission

transmission electron microscope operating at an accelerating voltage of 200 kV, equipped with

an Oxford energy-dispersive X-ray (EDS) detector. The TEM specimens of CH3NH3PbI3 were

prepared by slow evaporation of diluted solutions, obtained by dispersion of powders by

sonication in toluene, and deposited on a formvar-coated holey carbon copper grid under dry

conditions (humidity <2%). The CH3NH3PbI3 powder was obtained after scratching the film

from the substrate. A field-emission scanning electron microscope (FESEM, Merlin) was

employed to analyze the morphology of the samples. An electron beam accelerated to 3 kV was

used with an in-lens detector.

ASSOCIATED CONTENT

Supporting Information. SEM and STEM data of perovskite structures. This material is

available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected], [email protected], [email protected]; Fax:

(+) 41-216936100

Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS

M.I.D and N.A. gratefully acknowledges financial support from the Swiss confederation under a

Swiss Government Scholarship Programme. M.K.N and M.G acknowledge the European Union

Seventh Framework Programme under grant agreement number: 309194 “GLOBASOL” and

Grant 308997 “NANOMATCELL”. Authors’ would like to thank Prof. S. A. Shivashankar

(CeNSE Department, IISc) and Dr. Duncan (CIME, EPFL) for helpful discussions.

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Table of Contents Graphic and Synopsis

Scanning transmission electron microscopy energy-dispersive spectroscopy (STEM-EDS)

confirmed that the presence of the chloride ion as a substitutional dopant in the perovskite is not

detectable although Cl was found in the form of unconverted PbCl2.

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investigation regarding the role of chloride in organic-inorganic

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