Temperature Dependent Photo-induced Reversible Phase Separation

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Temperature Dependent Photo-induced reversible Phase Separation in Mixed Halide Perovskite Pronoy Nandi, chandan giri, Diptikanta Swain, U Manju, Subhendra D. Bhanu Mahanti, and Dinesh Topwal ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00587 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 22, 2018

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ACS Applied Energy Materials

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Temperature Dependent Photo-induced Reversible

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Phase Separation in Mixed Halide Perovskite

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Pronoy Nandi†,‡, Chandan Giri†, Diptikanta Swain§, U. Manju¶, Subhendra D. Mahanti⊥, Dinesh Topwal†,‡,∗ †

Institute of Physics, Sachivalaya Marg, Bhubaneswar-751005, Odisha, India.

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‡

Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai 400085,

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India

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§

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru-560012, India

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¶

CSIR -Institute of Minerals and Materials Technology, Bhubaneswar - 751013, India

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⊥

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48824, USA

Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan

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AUTHOR INFORMATION

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Corresponding Author

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*Address correspondence to this author [email protected], [email protected]

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ACS Paragon Plus Environment

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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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Even though tandem solar cells comprising of mixed halide perovskites CH3NH3Pb(I1-xBrx)3

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were expected to have much higher efficiency, the observation that they undergo photoinduced

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phase separation/demixing put forth a limitation to their possible utility. Herein, using

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temperature dependent photoluminescence studies, we show that the stated photoinduced phase

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separation occurs only in a narrow temperature range and above a particular bromine

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concentration. Our observation of disappearance of phase separation at elevated temperatures

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suggests the possibility that these tandem solar cells may indeed work better at elevated

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temperatures. Further, we provide the first experimental proof for the demixing transition

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temperature as predicted by Bischak et al. and also observe that demixing and remixing

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temperatures are pinned to crystallographic phase transition temperatures. Longer carrier lifetime

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of iodide-rich clusters is observed confirming the strong electron-phonon interaction (polaronic

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effect) which is absent in the initial mixed states.

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TOC GRAPHICS

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KEYWORDS: Mixed halide perovskite, Phase separation, Photoluminescence spectroscopy,

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Time resolved spectroscopy, Photo induced effect, Solar cell materials

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Lead halide hybrid perovskite solar cell materials have generated a surge of excitement in

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recent years as their power conversion efficiencies increased from 3 to 22% with just 5 years of

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research.1-6 Additionally, these materials have potential applications in a wide variety of high-

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performance optoelectronic devices including bright light emitting diodes,6-8 photodetectors,9-11

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color imaging devices,12 phototransistors,13 lasers,14-15 etc. due to their favorable material

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properties, which include high absorption coefficient with a sharp absorption edge,16 high

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photoluminescence quantum yield,17-18 long carrier diffusion lengths,19 high defect tolerance20

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and low surface recombination velocity21. At the same time, easy solution processability and

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completely tunable optical bandgap from blue to red regions of wavelength just by changing the

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halide ion concentration7-8 makes the family of mixed halides pervoskites, MAPb(I1-xBrx)3 (MA

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= CH3NH3, 0 ≤ x ≤ 1) potential candidates for application in multijunction/tandem solar cells22-28

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and tunable LEDs.6-8 It was prediced that coupling of perovskite material with commercially

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available Si,22-26 Germanium27 and Copper indium gallium selenide (CIGS) cells28 could increase

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the cell efficiency compared to the respective single junction cells. However, when mixed-

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halides perovskite was employed as solar cell absorber layers by replacing some I with Br, the

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increase in bandgap (of the perovskite material) did not yield a corresponding increase in open

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circuit voltage. Subsequently, Hoke et. al. reported that mixed I/Br pervoskites underperforms as

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illumination induces a strong and reversible band gap feature (at lower band gap) which

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disappeared after several minutes in dark (referred as Hoke effect). Such insatiability arises due

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to light induced halide phase separation that leads to the formation of smaller-bandgap “trap”


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states.29-38 However, the mechanism driving phase separation and remixing were not well

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understood by the Hoke effect.33-35 D. W. de Quilettes et al. also observed a strong correlation

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between redistribution of iodine away from illuminated region and an increase of

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photoluminescence (PL) emission peak intensity with illumination time for MAPbI3 films which

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produced a strong visual demonstration of light induced halide migration.34

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Recently, by combining cathodoluminescence (CL) with scanning electron microscope (SEM)

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studies Bischak et al. showed that small clusters enriched with one halide species get localized

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near the grain boundaries after prolonged illumination35 and ascribed this to photogenerated

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charge carriers and their accompanying lattice distortion, polarons. They argued that

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photoinduced nanoscale phase separation was mediated through the strain generated by the

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polaron and provided a rather convincing support for this picture by carring out Molecular

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Dynamics (MD) simulations both in the absence and in the presence of hole polarons. In

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addition, they also corroborated this using a Landau theory of atomic phase separation (Br-rich

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and I-rich) in the presence of excited charge carriers. It was predicted that after continuous

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illumination, the halide ions get excited and come closer to Pb atoms and a demixed trap state

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was formed only after applying certain thermal energy. Their MD simulations also suggested that

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when photoinduced charged excitations generate sufficient local strain, the solid solution phase

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gets destabilized and a demixed phase appears containing nanodomains, rich in one of the halide

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ions. The trapped electronic states associated with these nanodomains (due to dependence of

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bandgaps on the alloy concentration) play an important role in stabilizing this phase separation.

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It must here be noted that an actual experimental verification of the critical temperature at which

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demixing and mixing occurs is due. Since the lifetime of the trapped carriers are strongly

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temperature dependent, we believe that a temperature dependent study of light induced phase


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separation in mixed halide perovskite is essential for future advances to mitigate photoinduced

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effects in these devices. In addition, such a study can provide new opportunities for expanding

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the functional applications like, sensing, switching and optical memory.

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In the present work, we have focussed on understanding the temperature dependence of

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photoinduced phase separation of MAPb(I1-xBrx)3 (for x = 0.15, 0.24 and 0.27) by employing

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steady state photoluminescence. We find that the phase separation scenario is valid only in a

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narrow temperature window and above a certain concentration of Br. Further, by optimizing the

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film preparation we could enhance the phase separation time, suggesting that larger grains and

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lower vacancy concentrations may improve phase stability and effectively eliminate phase

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segregation.

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Conventional method to prepare MAPb(I1-xBrx)3 thin films is by dissolving constituent

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elements (PbI2, MAI, MABr and PbBr2) in appropriate ratio in either Dimethylformamide

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(DMF) or gamma-Butyrolactone (GBL). However, maintaining homogeneity and precise

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stoichiometry of films prepared by this method is tricky as solubility of PbI2+MAI is greater in

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GBL and solubility of PbBr2+MABr is more in DMF than in other solvents, which may affect

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the final outcome. Alternatively, to fabricate high quality large-grained perovskite thin films

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with an improved phase purity and better reproducibility, single-crystals of MAPb(I1-xBrx)3 were

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dissolved in GBL and were used as a precursors.39-42 Stoichiometric ratio of halides in single

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crystals and thin films were confirmed by single crystal and powder XRD using different

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electron cloud density of I and Br. Besides this, careful temperature dependent powder X-ray

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diffraction measurements and Rietveld refinements were performed to understand the crystal

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structure and crystallographic phase transition temperatures.


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Figure 1. X-ray diffraction (XRD) patterns of MAPb(I1-xBrx)3 (x=0.15, 0.24, 0.27) powders (a) and thin films (b) collected at room temperature (300 K). Temperature dependent XRD patterns of x = 0.24 shown for some selective temperatures: 100, 200, 250 and 300 K across the crystallographic phase transition (c). SEM images of the perovskite films on FTO glass prepared by the solution prepared from single crystals of MAPb(I0.76Br0.24)3 (d) and MAI+MABr+PbI2 (e).

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Figure 1a shows the room temperature powder X-ray diffraction patterns of single crystal sample

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(crushed in powder form before measurements) for three different compositions of MAPb(I1-

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xBrx)3

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made from these crystals. Observation of sharp well defined peaks indicate that the samples are

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highly crystalline in nature43. As expected for other members of the organic-inorganic hybrid

for x = 0.15, 0.24 and 0.27, while Figure 1b shows the XRD patterns of the thin films


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perovskite family, mixed halides also undergo crystallographic phase transitions from cubic to

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tetragonal to orthorhombic phase with the decrease of temperature due to the reduction of MA

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orientation inside the PbX6 octahedra.18,44,45 Literature suggests that MAPbI3 and MAPbBr3

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transform from cubic to tetragonal phase at 330.8 K and 237.1 K and tetragonal to orthorhombic

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phase at 161.8 K and 149.4 K, respectively. Temperature dependent x-ray diffraction

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measurements were performed on the mixed halide samples and the results of the same are

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presented in figure 1c (some representative XRD patterns) where for x=0.24 sample, transition

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temperature from cubic to tertagonal and tertagonal to orthorombic phase was found to be around

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280 K and 200 K, respectively. We also report scanning electron microscopy (SEM) images of

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the perovskite films prepared from two different methods (i) by dissolving MAPb(I0.76Br0.24)3

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single crystals and (ii) by conventional one step deposition route (MAI+ MABr+ PbI2) as shown

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in figures 1(d) and 1(e), respectively. SEM images clearly show large and improved grain size

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for the thin films synthesized in the first case as compared to the one derived from MAI+

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MABr+ PbI2 precursor solution.39-42 Hence, crystal dissolved thin films were selected for further

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experiments.

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To analyze photo-induced phase separation, temperature dependent PL measurements were

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carried out after different light soaking times ranging from 0 s to 4 hours as depicted in figures 2

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and 3. Substitution of Br for I in MAPbI3 lattice results in a gradual increase in the band gap.

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However, Hoke et al. found that for Br content x > 0.2, there is an inherent instability in the

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presence of light with the disappearance of the initial PL peak intensity and corresponding rise of

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a low-energy peak with exposure time. They argued that iodine and bromine ions were

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segregated into higher bandgap Br rich and lower bandgap I rich domains upon illumination.29

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Using density functional theory (DFT) a miscible gap of Helmholtz free energy as a function of


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halide composition at room temperature was observed by Brivio et al., which suggests that

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illumination supplies the required energy to overcome the kinetic barrier to enter into the

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metastable state and cause phase segregation.46 However, this theory fails to explain the

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reversible behavior upon the removal of illumination. As pointed out earlier in this paper, both

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molecular dynamics (MD) simulation and Landau theory studies by Bischak et al. show that due

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to the interplay between electronic and elastic properties, photoexcited state phase structure of

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mixed halide perovskite is different from that in the absence of photoexcitation, because of the

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polaronic strain associated with the excited charge carriers.35 They show that in the absence of

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photoexcited carriers, mixed I/Br perovskites exhibited demixing transitions as a function of

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temperature with a critical temperature of 190 K. In the presence of photoexcited charge carriers

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which behave like polarons, the demixing to mixing transition temperature increases and the

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mixing critical temperature is 343 K. Experimentally a careful temperature dependent study of

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the demixing transition both in the absence and presence of photoexcitation is not yet available.

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To fulfill this gap and to clarify the nature of phase separation and redistribution of halide ions in

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case of MAPb(I1-xBrx)3 alloys, we have carried careful temperature dependent PL studies on

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three samples with varying x ( x= 0.15, 0.24 and 0.27). In addition, to understand the underlying

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dynamics, we have also carried out time resolved measurements of the initial and photoexcited

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emission states.


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Figure-2. (a) Schematics of the experiment protocol for studying temperature dependence of photo-induced phase separation (PS). Temperature dependent PL spectra of (b) x= 0.15; (c) x= 0.24; and (d) x= 0.27 thin films collected after 0 s and 4 hrs of light soaking time, represented by black and red sold lines. Here the spectra taken at different temperatures are shifted vertically and are normalized for help in viewing.

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Figure 2a shows schematics of the experiment protocol. Sample was first cooled from 300 K to a

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specified temperature in the absence of light (step 1) and PL spectrum corresponding to 0 s

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soaking time was recorded (step 2) as represented by solid black line (in figures 2b, 2c and 2d).

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Thereafter, sample was subjected to prolonged illumination and PL spectra was collected at

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intermediate stages (figure 3), PL data corresponding to 4 hour soaking time (step 3) is

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represented by solid red line (in figures 2b, 2c and 2d). Finally, sample was heated back to 300 K


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(step 4), for x = 0.15 and 0.24 sample, it did not matter if step 4 was carried out in the presence

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or absence of light. Steps 1 to 4 were repeated for multiple temperatures. Results of temperature

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dependent PL experiments after normalization (for clarity) are summarized in figures 2b, 2c and

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2d which correspond to composition x = 0.15, 0.24 and 0.27 respectively. For the x=0.15 sample,

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the peak position and the full width half maximum (FWHM) of 0 s and 4 hrs soaking time

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emission spectra remained same throughout the temperature regions between 77 K and 300 K as

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shown in figure 2(b) indicating that no phase separation occurs for x = 0.15 (low Br

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concentration) sample under illumination, even at low temperatures. This result is consistent with

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the room temperature reports where it was claimed that photo-induced phase separation was

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observed only when Br concentration is greater than 0.2.29,30 Sample with Br concentration x =

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0.15 is hence below the percolation threshold (x=0.2)29, where in the sea of I rich domains, Br

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rich domains are randomly distributed and do not form long range connectivity even in the

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presence of light to give rise to phase separation.

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Figure 2c shows results from x=0.24 thin film; it exhibits similar PL spectra for pre (0 s soaking

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time) and post illumination (4 hrs soaking time, result is same even for 10 hours soaking time)

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between 77 K and 175 K. However, at 200 K, in addition to the initial emission peak at 716 nm,

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a new emission peak appears at 746 nm, intensity of which gradually increases with time and

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saturates after continuous illumination. It is interesting to note that at 200 K the intensities of

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both 716 nm and 746 nm peaks are nearly same which suggests that at the stated temperature,

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charge excitations generate sufficient lattice strain to destabilize I/Br solid solution to initiate

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photo-induced phase separation. Further, in the temperature range 225 K to 275 K, intensity of

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peak corresponding to 746 nm increases rapidly with time (figure 3(a)) and overshadows 716 nm

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peak after 4 hours of continuous illumination. More strikingly, it is observed that at 300 K and


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above, complete annihilation of the secondary peak corresponding to 746 nm occurs, as pre-and

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post-illumination emission spectra look the same suggesting no phase separation due to

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illumination.

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Our temperature dependent PL studies suggest that demixing of halide ions due to polaronic

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strain is initiated at around 200 K, is consistent with the mean field theory temperature-

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composition phase diagram in the photoexcited state by Bischak et al.35 Their simulation

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suggests that mixed I/Br perovskite undergoes demixing transition with a critical temperature of

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190 K and we provide a direct experimental proof of temperature induced demixing, validating

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their theoretical claims. Further, disappearance of the photo-induced phase separation (or

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remixing) is observed at an elevated temperature (300 K and above), which can be explained

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from the dynamic behavior of electron-phonon interactions. At higher temperatures electron

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phonon interaction results in decreased electron hole mobility thereby hindering ion migration

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and formation of phase separated domains. Also, theoretical calculations revealed that above a

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critical temperature (343 K) solid solutions of hybrid perovskites are stable for any composition,

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which supports our observation of an upper limit of the critical temperature for I/Br solid

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solutions.46 Surprisingly, the crystallographic phase transition temperatures of x = 0.24

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composition i.e. 200 K and 280 K (from orthorhombic to tetragonal and from tetragonal to cubic

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phase), are in close proximity to the formation and annihilation of photo-induced phase

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separation temperatures. Similar behavior was also observed for the x=0.27 composition, which

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is depicted in figure 2(d). Here the only difference is that the demixing and remixing (of halide

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ions) transition temperatures are slightly offset as evident from the emergence and disappearance

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of the light induced secondary peak in the PL spectra owing to different crystallographic phase

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transition temperatures. As a result, light induced phase separation still persists at room


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temperature and it disappears at an elevated temperature around ~ 325 K. A more detailed and

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careful study (which is beyond the scope of the present work) is required to pin down the origin

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of the correlation between crystallographic phase transition and photo-induced phase separation,

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if any. It may be noted that most of the mixed halide samples studied in the literature are with

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composition x ≥ 0.3 for which photo-induced phase separation is observed at room temperature

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and remixing temperature is expected at much higher temperature (≥ 325K).29-30 Through our

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careful experiments and judicious choice of composition we are able to catch the demixing and

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remixing transition as a function of temperatures for these mixed halide systems. Based on the

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above observations, we propose that tandem solar cells using mixed halides may work more

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efficiently at elevated temperatures where there is no photo-induced phase separation.

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To gain further insight about the dynamic behavior of carrier-strain interaction, i.e. formation

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and break down of iodide rich clusters, the time evolution of the PL spectra both in the presence

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and absence of light at 225 K are presented in figure 3. In this figure we see that for 0 s soaking

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time only one emission peak (peak 1) is seen, 716 nm for x = 0.24 and 699 nm for x = 0.27. It

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was found that this original peak disappears and a new emission peak associated with the iodide-

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rich region gradually appears at 746 nm (peak 2), as a function of time. Growth of integrated

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intensity of peak-2 continues with time and saturates around 4 hours after which no further

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iodide-rich cluster formation takes place, as shown in the inset of figures 3(a) and (c). We further

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investigated the sample by turning off the continuous illumination (after 5 hours of soaking time)

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and recording the PL spectra at regular intervals (figures 3(b) and (c)). Photo induced peak

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(peak-2) intensity does not fall to zero as soon as the light is turned off; it decreases gradually

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and the spectra corresponding to 0 s soaking time is recovered after 6 to 10 hours, confirming

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the reversibility of photo-induced phase separation. Further, to understand the dependence of


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photo induced phase separation on the wavelength and intensity of the incident light, PL

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experiments were performed with five different wavelengths (325 nm, 350 nm, 400 nm, 450 nm

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and 500 nm) with ~1.5 mW/cm2 power on the sample and with five different light intensities (1,

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1.5, 2, 2.5 and 3 mW/cm2) at 325 nm at three different temperatures i.e. 77 K, 225 K and 300 K

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for x= 0.24 sample. No photo-induced phase-separation (i.e. evolution of PL intensity with

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illumination time) was observed by changing either excitation intensity or wavelength of the

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excited light at 77 K and 300 K. However, there was a systematic evolution of 746 nm peak

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intensity with time at 225 K, which is depicted in figures 3(e) and 3(f) and is fitted by mono-

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exponential time constant.47 Fitting yielded a time constant of 2.07, 3.58, 5.01, 7.32 and 10.58

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hrs for 325 nm, 350 nm, 400 nm, 450 nm and 500 nm illumination, respectively. Gradual

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increase of time constant or slowing down of photo induced phase separation with increased

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wavelength may be due to the dependence of absorption coefficient on wavelength.47

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Additionally, evolution of PL peak with time by changing the intensity/power of the excitation

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light (325 nm) is depicted in figure 3(f) and the extracted time constants by fitting mono-

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exponential behaviour are 4.2, 2.07, 1.54, 0.95 and 0.43 hrs for 1, 1.5, 2, 2.5 and 3 mW/cm2

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illumination, respectively; as expected higher power of incident light resulted in faster phase

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separation.33 Literature suggests that the formation of demixed state takes fraction of minutes for

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thin films35 and hours for bulk samples/crystals under illumination while the remixing time after

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the illumination is turned off, which is also the annihilation time of the iodide-rich clusters, is in

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hours.48 Our results are consistent with the literature reports with the only exception that it takes

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much longer time for our thin film samples to phase separate indicating better quality of the

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samples, with lesser defects49 and large crystal domains50 (with lesser grain boundaries) which

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kinetically limit the Hoke effect and effectively reduce the phase segregation.51 This observation


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supports the idea that continuous illumination generates a finite steady state concentration of

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charged carriers near the grain boundaries which by coupling to the local strain are responsible

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for polaronic cluster formation and phase separation.35,52 Hence the grain size of mixed halide

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perovskites play a crucial role in forming the light induced phase separation or formation of

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metastable states.

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Figure-3: Evolution of PL spectra at 225 K in the presence (a, c) and absence (b, d) of light for the compositions as marked in the figure. Inset shows integrated intensity of peak 1 and peak 2 as a function of time. Evolution of PL peak intensity at 225 K by varying the (e) excitation light wavelength and (f) light intensity/power (λ=325 nm).

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To know more about the real-time dynamics of the photo-excited charge carriers, life time

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measurements of pre-illuminated (black dots) and photo-excited demixed states (blue dots) for x


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= 0.24 composition were performed at 225 K. The results, presented in figure 4 are quite striking.

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Lifetime of pre-illuminated peak shows a single decay with carrier lifetime of 0.46 ns, whereas

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the lifetimes of carriers associated with photo-excited demixed state (after 5 hours of continuous

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illumination) shows two characteristic decay times namely one 0.43 ns with 44.8% weight and a

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second, 11.79 ns with 55.2% weight.53-54 In the mixed or pre-illuminated case where the system

282

is homogeneous, light absorption creates an electron–hole pair which quickly recombine to

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produce single native emission (corresponding to life time of 0.46 ns). Weakly bound electron-

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hole can also rapidly dissociate, travel long distances, deforming the surrounding lattice through

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electron-phonon coupling and induce halide anion rearrangement. Upon prolonged illumination,

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generated polarons funnel into the reduced bandgap I-rich domains driving the perovskite to

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form microscopic phase segregation into iodide- and bromide-rich domains. In tandem, electron

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hole pairs can also recombine reversing the phase separation process. Observation of two types

289

of carrier decay times in phase separated samples can be interpreted as one arising from quick

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recombination of electron–hole pair as soon as they are formed (0.43 ns) reversing the effect of

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phase separation and the other from the iodide-rich domains (11.79 ns). The reason the latter has

292

a longer life time (~25 times larger) can be ascribed to the strong polaronic effect. This is also

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referred to as emission from the “trap” state confined to the iodide-rich clusters. To identify the

294

carrier lifetime of photogenerated charge carriers at an elevated temperature, lifetime

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measurement of peak 2 was also performed at 275 K and a comparative study between 225 K

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and 275 K is shown in the inset of figure 4. Enhanced lifetime at higher temperature is consistent

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with the longer life time of polarons under photoirradiation.55 It is believed that halide migration

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in perovskites occurs through halogen vacancies,43 and ion mobility in these materials are more

299

facile at grain boundaries compared to the bulk material.44 Therefore, reducing defect density


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and the number of grain boundaries should decrease the rate at which phase segregation occurs.

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Through our carefully controlled sample preparation process we ensured higher-quality films

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with larger grain size and lower defect densities (figure 1(d)) resulting in longer time for phase

303

separation to occur (4-5 hours) compared to the conventionally grown films (grown by one step

304

process) which has smaller grain sizes (figure 1(e)) and photo induced phase separation occurs

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much faster ( ~2.5 hours).

306 307 308 309 310

Figure 4. Lifetime measurement of peak 1 corresponding to 716 nm (black dots) and photoexcited peak 2 (746 nm) (blue dots) with corresponding exponential fit (red solid line) for x = 0.24 sample. Inset shows comparison of life time measurements spectra of peak 2 at 225 K and 275 K with corresponding fit.

311 312

In conclusion, we have successfully demonstrated temperature dependent nature of photo-

313

induced phase separation in the mixed halide perovskites. PL studies revealed that demixing of


16

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314

halide ions into iodine rich and bromine rich domains occurred above a critical temperature and

315

concentration of Br, in a narrow temperature window. Above this temperature window, i.e at

316

elevated temperature, the electron-phonon interaction results in a decreased electron hole

317

mobility and they recombine to form mixed state. Our findings may hence pave a relook at

318

further optimization of tandem solar cell comprising of mixed halides for higher temperature

319

applications, where there is no photo-induced phase separation, than at room temperature. Our

320

observation also suggests that demixing and remixing temperatures are pinned to

321

crystallographic phase transition temperatures hence providing an ample playground for

322

fundamental research as well. Further, appearance of longer lifetime of carriers in the phase

323

separated state confirms the presence of strong electron-phonon interaction while longer

324

saturation time of the cluster formation confirms that grain boundary/sample quality plays a

325

crucial role in phase separation.

326 327

328

Experiment:-

329

Materials: - Hydroiodic acid (HI) (57 wt.% in water), hydrobromic acid (HBr) (48 wt.% in

330

water), methylamine (CH3NH2) (40 wt.% in water), PbBr2 (99.99%), PbI2 (99.99%), gamma-

331

Butyrolactone (GBL, 99.9%) were purchased from Sigma-Aldrich and used as received without

332

further purification.

333

Synthesis of MAPb(I1-xBrx)3 Crystals/thin films: Single crystals of MABr (MAI) was

334

synthesized by reacting CH3NH2 and HBr (HI) in molar ratio of 1.2:1. The HBr (HI) was added

335

dropwise into the CH3NH2 in a flask under nitrogen atmosphere in an ice bath for 3 hrs, the


17


Page 18 of 28

336

resulting solution was evaporated at 60° C in a rotary evaporator to remove the unreacted solvent

337

and white crystals of CH3NH3X (X= Br, I) were obtained. To grow single crystals of MAPb(I1-

338

xBrx)3

339

1 M GBL. Temperature of MAPb(I1-xBrx)3 solution was increased to 95°C and maintained at the

340

temperature until desired MAPb(I1-xBrx)3 single crystals formed. Thin films were fabricated on

341

FTO glass from single crystals by dissolving them in GBL (40% wt.) and by spin-coating at

342

2000 rpm for 60 seconds.

343

Single crystal/powder XRD and SEM: -

344

Crystals were mounted on a Hampton cryoloop for single crystal XRD measurements on an

345

Oxford Xcalibur (Mova) diffractometer equipped with an EOS CCD detector using Mo Kα

346

radiation (λ= 0.71073Å). Data collection, data reduction and numerical absorption corrections

347

were performed using the programs present in the CrysAlisPro software suite. Crystal structures

348

were refined by using SHELXL-97 program present in WinGX suit. Powder X-ray diffraction of

349

bulk and thin film samples were performed in Bruker D8 Advance X-ray diffractometer using

350

CuKα radiation (λ= 1.5406Å) equipped with Phenix Oxford closed cycle helium cryostat for

351

temperature dependent studies. The grain sizes of the thin film prepared from single crystal

352

dissolved solution and MAI+MABr+PbI2 precursor solution were investigated using a field-

353

emission scanning electron microscope (FESEM; Neon 40 cross-beam system, MS Carl Zeiss

354

GmbH).

precursor solution with 1 M PbI2, (1-x) M CH3NH3I and x M CH3NH3Br was prepared in

355

Temperature dependent photoluminescence: - Steady state photoluminescence spectra were

356

acquired using an Edinburgh FLS920 spectrometer, using excitation energy of 325 nm from

357

Xenon lamp with light intensity 1.5 mW/cm2, while the sample was mounted on Oxford


18

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358

Instruments OptistatDN cryostat (vacuum~ 4.3 X 10-3 mbar) for temperature dependent studies.

359

Further, excitation wavelength and light intensity dependent PL measurements were performed

360

using Xenon lamp with different wavelengths and light intensity. Time resolved PL experiment

361

was performed on the same FL 920 spectrometer by means of time correlated single photon

362

counting (TCSPC) method using a pulsed diode laser.

363

AUTHOR INFORMATION

364

Corresponding Author

365

*Email: [email protected], [email protected]

366

ORCID

367

Pronoy Nandi: 0000-0002-3229-5080

368

Chandan Giri: 0000-0001-7026-6320

369

U Manju: 0000-0001-9842-6648

370

Dinesh Topwal: 0000-0003-1486-8348

371

Present Address

372

(C.G.) Universidad Autónoma de Madrid, Department of Organic Chemistry, Francisco Tomás

373

y Valiente 7, Cantoblanco, 28049, Madrid.

374

Notes

375

The authors declare no competing financial interests.

376

ACKNOWLEDGMENT

377

SDM would like to acknowledge Institute of Physics for kind hospitality. The authors

378

would also like to thank Prof. P. V. Satyam (Institute of Physics, Bhubaneswar) and his

379

student Puspendu Guha for their support in carrying out FESEM measurements.


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