Variations in the Composition of the Phases Lead to the Differences in

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C: Energy Conversion and Storage; Energy and Charge Transport

Variations in the Composition of the Phases Lead to the Differences in the Opto-electronic Properties of MAPbBr Thin Films and Crystals 3

Qi Shi, Supriya Ghosh, Pushpendra Kumar, Laura Christina Folkers, Suman Kalyan Pal, Tõnu Pullerits, and Khadga Jung Karki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06937 • Publication Date (Web): 03 Sep 2018 Downloaded from http://pubs.acs.org on September 3, 2018

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The Journal of Physical Chemistry

Variations in the Composition of the Phases Lead to the Differences in the Opto-electronic Properties of MAPbBr3 Thin Films and Crystals Qi Shi†,#, Supriya Ghosh‡,#, Pushpendra Kumar†, Laura C. Folkers§, Suman Kalyan Pal‡, Tõnu Pullerits†, and Khadga J. Karki†* †

The Division of Chemical Physics and NanoLund, Lund University, Box 124, 22100 Lund, Sweden

‡

School of Basic Sciences and Advanced Material Research Center, Indian Institute of Technology Mandi, Kamand, 175005 HP, India §Centre for Analysis and Synthesis, Lund University, Box 124, 22100 Lund, Sweden

AUTHOR INFORMATION Corresponding Authors * [email protected]

+46 46 222 83 40

Author Contributions #

Q. S. and S. G. contributed equally.

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Abstract Photoluminescence (PL) spectra from thin films (TFs) and bulk crystals (BCs) of hybrid organo-halide perovskites are significantly different, the origin of which and their impact on the efficiency of the perovskites based photoactive devices have been debated. We have used two photon photoluminescence (2PPL) to study the temperature dependent changes in the spectra of the TFs and the BCs of methylammonium lead bromide (MAPbBr3) perovskites in order to clarify the origin of the differences. Our results show that the differences in the spectra are due to the variation in the phase composition. At room temperature the tetragonal (TE) phase is dominant in the BCs while the orthorhombic (OR) phase is dominant in the TFs. The PL spectra of the TFs also show discernible contributions from the TE and the cubic (CU) phases. At lower temperatures, the increase in excitonic recombination causes a red shift of the PL from the TFs while a phase transition from the TE to the OR phase results in a blue shift of the PL from the BCs. The temperature dependent narrowing of the PL line widths show a stronger coupling between the longitudinal optical (LO) phonons and the free carriers in the OR phase as compared to the TE phase implying a reduced carrier mobility. However, as the OR phase is metastable at the room temperature, the slow phase transition to the TE phase should improve the photocurrent yield in the TFs provided that the sample is shielded from other types of degradation.


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Introduction Recently, hybrid organic-inorganic perovskites have been widely investigated for their use in optoelectronics. MAPbX3 perovskites, as a group of popular perovskites, have high optical absorption coefficients resulting from band to band transition, high carrier mobilities, long carrier recombination time and long electron-hole diffusion distance, all of which are important in photovoltaics and lighting applications1–6. It is well known that the photovoltaic performance of the perovskite solar cells depend on the preparation conditions. However, less is known about the underlying physical and optical properties that influence the overall functionality of the photoactive devices7–17. Regarding this, the obvious differences in the optical properties and the photocurrent yields in the perovskite TFs and BCs have remained a topic of intense debate7,18–23. Three different phases are known to exist in the bulk crystals of MAPbBr3. Measurements of the PL spectra at room temperature shows a dominant at 2.22 eV that originate from the TE phase24. At temperatures below 160 K, the peak position shifts to 2.24 eV, which corresponds to the PL from the OR phase25. The free carriers and the excitons in the solution processed perovskites can also be trapped and as a result they can affect the PL26–31 and the electrical properties32–34 of the perovskites. A number of work on phase transitions in MAPbX3 (X=Cl, Br, I) BCs at different temperatures25,35–39 highlight the significant impact of the different phases on the optical properties. Apart from the different phases, the lattice vibrations have large influence on charge carrier diffusion lengths6,10,40, charge carrier mobilities41,42, and thermal conductivity43. Charge carrier cooling is also affected by charge-phonon interaction44. The free carriers and the excitons in perovskites can interact with two types of lattice vibrations, namely the acoustic43,45, and the LO phonons46,47. Recent reports have demonstrated great significance of phase transition and lattice vibrations in the photovoltaic behavior of MAPbBr3 perovskites6,10,25,35–48. However, it has not been shown that the 3 ACS Paragon Plus Environment


differences in the opto-electronic properties of the TFs and the BCs at the room temperature is due to the difference in the phase composition. Here, we present a detailed analysis of the 2PPL spectra and their temperature dependence elucidating the importance of the composition of the phases on the carrier-phonon interaction, and its effects on the opto-electronic properties of the TFs and the BCs. We have used two-photon excitation mainly because the penetration depth of one-photon excitation in the direct band-gap MAPbBr3 is only tens of nanometers. Thus compared to the one-photon excitation, the two-photon excitation allows us to probe the PL from the bulk. Experimental Methods MAPbBr3 bulk crystals and thin film synthesis and characterization. MAPbBr3 perovskite material was synthesized following the procedure described by Kojima et al49. For a typical synthesis, 21.12 g (~0.01 mol) Methylammonium bromide (Dyenamo ＞ 98%) and 3.67g (~0.01 mol) PbBr2 (Sigma-Aldrich ≥98%) were dissolved in 10 ml N, NDimethylformamide (Sigma-Aldrich ≥99%) to prepare the mixture. The glass substrates were carefully washed by acetone (Ultrasound machines). About 100 µL mixture solution was dropped onto the glass substrate by a volumetric auto-pipette and annealed at 80 C until the complete evaporation of the solvent resulting in the formation of bulk crystals. For the MAPbBr3 thin film preparation, a 5 µL mixture solution was spin coated on a glass substrate at 4000 rpm. Spectroscopic Techniques. The 2PPL optical setup has been described elsewhere26,29,50,51. In brief, a pulsed Ti-Sapphire oscillator (Synergy, Femtolasers) centered at 800 nm with pulse duration of about 10 fs was used as the excitation light source. Typically, the average power used for the two-photon excitation was of about 15 mW. A dichroic mirror with edge at 700 nm was used to couple the 4 ACS Paragon Plus Environment

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laser to an inverted microscope (Nikon Ti-S). The focus spot size of the laser beam was about 2 µm. The PL was collected with the same objective in the epi-direction and recorded by a spectrometer. A temperature controlled stage (Linkam Scientific Instruments, LTS420E-P) was used to vary the temperature of the sample in the range of 77 K to 298 K. Results and discussion

Figure 1. Left (a), SEM images of a MAPbBr3 thin film (TF) composed of nano- and microcrytallites and, right (b), a bulk crystal (BC). Down (c), powder-XRD diffraction pattern of MAPbBr3 TFs (marked as blue) and BCs (marked as red) deposited on a glass substrate. Figure 1a and 1b show a typical scanning electron microscopy (SEM) micrographs of the MAPbBr3 TF and the BC, respectively. The TF consists of nano- and micro-crystallites that stand individually on the substrate. The size of the crystals varies from about 100 nm to 1.5 5 ACS Paragon Plus Environment


µm. The size of the BC is about 200 µm × 200 µm. Figure 1c shows the powder-XRD pattern with the diffraction peaks at 12.36°, 14.90°, 21.07°, 30.06°, 33.70°, 37.11° and 45.82°, which correspond to the lattice planes of (011), (001), (110), (200), (210), (220) and (300) respectively. The crystalline structure of the MAPbBr3 BCs belongs to the 4 noncentrosymmetric TE phase24,52. Different from the powder-XRD patterns of the BCs, the diffraction peak of the TFs at 12.36°, which is a typical peak of the OR phase53, clearly shows its presence at the room temperature. It should be noted that the powder-XRD measurements have been acquired for a limited range of θ values between 10—50 degrees. As the spectra are not averaged over all the possible orientations of the sample, the peak positions in the measurements only help us to identify the different phases present in the system. In order to quantify the relative composition of the different phases, we analyze the PL spectra from the samples.


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Figure 2. (a) Temperature dependent PL spectra of the MAPbBr3 TF. (b) Temperature dependent PL spectra of the MAPbBr3 BC. (c) and (d) show normalized PL spectra of MAPbBr3 TF and BC, respectively, at some selected temperatures. Figures 2a and 2b show the temperature dependent PL spectra of MAPbBr3 TFs and BCs, respectively. Figures 2c and 2d show the normalized PL spectra of the TFs and the BCs at selected temperatures. The spectra of the two systems show clear differences. The major peaks of the spectra are at 544 nm (2.28 eV) and 558 nm (2.22 eV) in the TFs and the BCs at 7 ACS Paragon Plus Environment


room temperature, respectively. While the PL spectrum of the BCs show only one peak with a broad tail spanning till 580 nm, a shoulder on each side of the major peak, corresponding to two additional minor peaks, is evident in the TFs. The three peaks, which are centered at 2.22 eV, 2.28 eV and 2.31 eV (see the supporting information figure S3a for the results of the curve fitting), correspond to the emissions from the TE, the OR and the CU phases, respectively. Evidently, emissions in the TFs arise from all the three phases that can be identified in the powder-XRD shown in Figure 1c. Importantly, the OR phase component centered at 2.28 eV occupies 63% of the PL spectrum of the TFs at the room temperature, which shows its predominance (Figure S3a). In contrast, most of the emission from the BC at the room temperature is from the TE phase (Figure S3b). Note that previous reports have shown that the emission from the OR phase in the BCs is centered at 552 nm (2.246 eV)25, which differs from our observation by about 34 meV. This discrepancy can be explained by the fact that the value we have observed is at the room temperature, while others have only reported the measurements done at the low temperatures. The peak position of the PL from the OR phase at the room temperature has not been determined previously. The measurements of the PL spectra from the BCs and the TFs we have done at 79 K (figure S3c and S3d) show that the peak position corresponding to the OR phase is at 2.246 eV, which agrees with the earlier reports. The blue shift in the spectra at the room temperature is due to the increase in the fractional contribution of the carrier-carrier recombination with respect to the excitonic recombination. Thus, in order to compare the emissions from the different phases at different temperatures, we should also take into account the change in the population of the excitons and the free carriers, as both of them contribute to the emissions. The populations of the free carriers and the excitons in a semiconductor at any given temperature is usually approximated by the Saha-Langmuir equation (1)14,48,54,55: 8 ACS Paragon Plus Environment

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=

,

(1)

where x is the ratio between the free charge carriers over the total excitation density, n is the excitation density, h is the Planck constant, Eb is the exciton binding energy, T is the temperature, kB is the Boltzmann constant and µ is the reduced mass of the exciton. For a typical excitation density of 1017 cm-3, one expects the population of the free carriers to dominate. The Saha-Langmuir equation shows that the population of the excitons increases with the lowering of the temperature, which also leads to the red shift in the PL spectrum as observed in our measurements. This is different from the red shift caused by the re-absorption of the emission in perovskite micro-crystals56,57 when the PL is collected in the forward direction. In our measurements, the emission is collected in the epi-direction. Careful analysis of the peak position in Figure 2c shows that the peak position shifts from 544 nm to 552 nm14, from which we estimate the binding energy of the excitons to be about 34 meV. This value is close to the binding energy of about 25 meV measured by using magneto optical absorption spectroscopy25. In contrast to the TFs, we observe an abrupt blue shift from 560 nm to 552 nm in the PL from the BCs at about 150 K as shown in Figures 2b and d indicating a phase transition from the TE to the OR phase, which is consistent with previous reports58. In order to further investigate the effects of the different phases on the opto-electronic properties, we analyze the linewidth of the PL as a function of temperature. As shown in Figure 2, spectra from both the samples show narrowing of the linewidth with cooling. The narrowing has been discussed previously in relation to the carrier-phonon and exciton-phonon coupling44,59,60. Here we use a similar analysis based on the full width at half maximum (FWHM) of the spectra, shown in Figure 3, to assess the carrier-phonon and exciton-phonon couplings in both the samples. The relationship between the PL linewidth and phonon scattering can be described by the following equations (2 and 3), 9 ACS Paragon Plus Environment


= + + ! + +"#$ ,

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(2) '

= + % + % ! & ! + %"#$ (

,

(3)

where Γ0 is the temperature independent inhomogeneous broadening resulting from the disorder and imperfections61,62, Γac and ΓLO are homogeneous broadening due the acoustic and the LO phonons with the corresponding coupling strength % and % ! , respectively61,62, and inhomogeneous broadening due to the ionized impurities (Γimp)63,64. The carrier phonon coupling is proportional to the occupation of the respective phonons (NLO), which can be described by the Bose-Einstein distribution functions (4), +,

& ! = 1/* − 1. ,

where 0

!

(4)

is the energy of LO phonons, and kB is the Boltzmann constant.

It is well known that Fröhlich interaction with the LO phonon are mainly responsible for the broadening of the PL spectra in polar semiconductors at the room temperature44,60. In order to extract the coupling parameters with the LO phonons and the acoustic phonons, we separately fit the data at two different temperature regions. Such separate regions are justified by the fact that energies of the two different types of phonons are very different. The acoustic phonons have energies 012

Variations in the Composition of the Phases Lead to the Differences in

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