Optical and Scanning Probe Identification of Electronic Structure and

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Optical and Scanning-Probic Identification of Electronic Structure and Phases in CHNHPbBr Crystal 3

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Hye Ri Jung, Gee Yeong Kim, Bich Phuong Nguyen, Hye-Jin Jin, William Jo, Trang Thi Thu Nguyen, Seokhyun Yoon, Won Seok Woo, Chang Won Ahn, Shinuk Cho, and Ill Won Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06765 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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The Journal of Physical Chemistry

Optical and Scanning-Probic Identification of Electronic Structure and Phases in CH3NH3PbBr3 Crystal Hye Ri Jung1, Gee Yeong Kim1, Bich Phuong Nguyen1, Hye-Jin Jin1, William Jo1*, Trang Thi Thu Nguyen1, Seokhyun Yoon1, Won Seok Woo2, Chang Won Ahn2, Shinuk Cho2, and Ill Won Kim2 1

Department of Physics, Ewha Womans University, Seoul, 120-750, Korea

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Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan,

680-749, Korea

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ACS Paragon Plus Environment


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ABSTRACT Hybrid lead halide perovskite possesses attractive properties and outstanding performance. Electronic structure of hybrid lead halide perovskite single crystal can be used to comprehend its intrinsic properties. In this study, the crystal structure of CH3NH3PbBr3 perovskite crystal was elucidated by transmission electron microscopy. Phonon modes and CH3NH3+ rocking modes were analyzed. Band gaps of the crystal were obtained by temperature-dependent Raman scattering spectroscopy and photoluminescence measurement. Surface potential measurement for a (001) plane exhibited a work function of the material and other phases of PbBr2 and CH3NH3Br. From experimentally determined values, an electronic structure of CH3NH3PbBr3 was built. It showed a p-type characteristic.

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INTRODUCTION Organic-inorganic hybrid perovskite is one of the most promising materials not only for the absorber layer of a solar cell, but also for light emitting diodes (LED), photodetectors, and sensors.1,2 Its usefulness in various applications is due to its superior properties, including easy manufacture at a low cost and outstanding electrical and optical properties as a high carrier mobility and absorption.3-5 Perovskite solar cell has demonstrated an unparalleled technology development speed, achieving a high efficiency of over 22% in a brief time period due to its properties. Solar cell material generally has a high open circuit voltage with high external quantum efficiency (EQE). In other words, excellent solar cell material has a great probability to be a LED. Perovskite is commensurate with notable carrier transport ability, realizing about 1.1 V of open circuit voltage which is close to 1.25 V, the theoretical maximum open circuit voltage. Moreover, perovskite LED has high color purity, tunable band gaps with adjustable composition, and brilliant color reproduction with a narrow full width half maximum in the photoluminescence (PL) spectrum.6 In previous reports, CH3NH3PbBr3 thin film has attained over 8% EQE while CH3NH3PbBr3 crystallite has attained 9.3% EQE with a green light.7-9 However, further studies are required to compensate its disadvantages, specifically toxicity of lead, stability problems that reduce efficiency, deposition methods for large areas, and ferroelectricity.10–12 Fundamental details of the material need to be understood first to resolve these issues. Although many studies have addressed these issues, most studies are focused on thin films. Grain boundary issue in a surface of a material affects the grain growth behavior which is influenced by compositions and textures. It even serves as a trapping source of ionic migrations.13,14 Single crystal is an ideal form of perovskite material which is akin to an enlarged unitary grain on 3



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thin film. Therefore, research on perovskite single crystal without grain boundaries is important to understand the intrinsic properties of perovskite material used as absorption layer in solar cells. However, few studies have determined the properties of single crystal. Even fewer studies have described the properties of perovskite single crystal. Single crystal could be an efficient element of a successful device model by controlling its optical and electronical transport properties.15-18 Method of producing single crystals and investigation on their fundamental properties are required. The general formulation of the perovskite structure is ABX3. We focused on hybrid lead tribromide perovskite consisting of Atom A (an organic material with methyl-ammonium located at the corner of the cubic structure), atom B (lead located at the body centered position), and X (halide materials with Br located at the face-centered position). When the structure is strictly cubic, CH3NH3PbBr3 perovskite is stable at room temperature. However, when the symmetry is broken, tilting and distortion will occur between the atoms and the structure will become tetragonal or orthorhombic. Previous studies on the structure of CH3NH3PbI3 which is similar to perovskite structure have identified correlation between phase transition and properties.19,20 In this paper, we analyzed representative structures of CH3NH3PbBr3 using transmission electron microscopy (TEM). Our results confirmed the uniform plane and the tetragonal phase of the perovskite material. Structural analysis using TEM seemed to show diverse phases caused by external factors such as high energy of electron beam.21 We also verified its structural characteristics using Raman scattering spectroscopy. Its surface characteristics were derived by integrating surface potential and deconvolution of calculated work function using atomic force microscopy (AFM).22 Finally, we described band diagram of each material and combined band gap energies examined by PL spectra with work function by Kelvin probe force microscopy (KPFM). 4


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Table 1. Reported characteristics of the crystal structure of CH3NH3PbBr3

EXPERIMENTAL METHODS Growth of crystals began with the production of seed crystals. Seed crystals were prepared in a saturated solution that was stirred for two hours at 50 °C. The solution was filtered (0.2 mm) and kept in an oil bath on a 90 °C hot-plate for several days. The size of the crystal can be controlled according to the immersion time of the seed crystal in the saturated solution. The longer the seed crystal is submerged in the solution, the larger the crystal becomes. Prepared seed crystals can grow to various sizes from 1 to 10 mm for single crystals. An inset picture (Figure 1a) showed that crystals used in this study grew in small sizes of 5 mm x 5 mm. Crystalline formation had a transparent orange color and rectangular cuboid-shapes bound with (100) and (010) planes. The (001) plane was observed in our studies. To verify crystalline orientation, structure, and atomic ratio of the crystal structure, the material was examined through TEM and energy dispersive X-ray spectrometry (EDS). TEM was used to characterize the crystallinity and microstructure of the perovskite crystal. Focused ion beam (FIB, NOVA 600 Nanolab) milling was used to prepare TEM. Samples were about 10 µm in length and 5 µm in width. They were measured along the [001] zone axis. The accelerating voltage of TEM (JEM-2100F JEOL) was 200 keV and point resolution was 0.23 nm. EDS mapping was carried out with JEOL JEM-2100F microscope. 5



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To measure optical and surface properties of each material, Raman scattering spectroscopy and AFM were used. Raman scattering spectroscopy was performed to identify phase formation of the material. We employed three different excitation wavelengths (488, 514.5, and 532 nm) by using an Ar+ ion laser and a DPSS laser. All Raman scattering spectra were measured depending on temperature variation. PL spectra were used to characterize band gap energies. A 532 nm laser was used to achieve excitation of electrons. To determine surface characteristics, AFM was employed. In particular, KPFM was used to investigate surface potential and analyze work functions. AFM measurements were conducted in a glove box filled with nitrogen atmosphere. Humidity and temperature were retained under 0.5 ppm and 22.9 °C. Pt/Ir-coated tips were used for all AFM measurements. The surface potential with topography under non-contact mode was examined using KPFM, with AC voltage amplitude of 1 V and a frequency of 102 kHz to obtain sufficient signals. A lock-in amplifier was controlled with a sensitivity of 100 m/nA. The resonant frequency of probe for non-contact mode was about 52.53 kHz.

RESULT AND DISCUSSION

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Figure 1. (a) Cross-sectional view of the crystal prepared by a focused ion beam for transmission electron microscope. Inset shows crystal shape and color with crystal axes. (b) Fourier transformed pattern showing tetragonal crystal structure.

TEM image sampling for focused ion beam (FIB) milling was prepared before TEM measurement (Figure 1a). The crystal flake manufactured by FIB was separated into two different layers. One was the cover layer for FIB processing while the other was perovskite crystal flake. The flake of perovskite crystal except the covering layer appeared to have a generally uniform shape with 7



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a flat surface. Fast Fourier transformation (FFT) result of high resolution-TEM (HRTEM) image is shown in Figure 1b. Each spot in the figure was indexed as (020) and (200) planes in the crystal structure. The angle between patterns in the reciprocal lattice was 90°. Indexing was estimated to be I4/mcm, confirming the space group for single-phase with β-tetragonal symmetry. These results are summarized in Table 1, consistent with results of previous work.23,24 The zone axis was [001], corresponding to the observed plane. The lattice showed a few smudge spots which originated from the breaking down of crystal structures due to the high-energy electron beam. When the electron beam hit the sample, the structure of the surface was deformed. This was verified from the creation of grains inside the uniform crystal observed before measurement. Scattered spots in FFT patterns seemed to be created by concentrated high-energy. In particular, dots distribution in an uncertain direction might be due to more than one secondary substances.

Figure 2. Temperature-dependent Raman scattering spectra of CH3NH3PbBr3. Numbers designate main Raman peaks.

Raman scattering spectra of each crystal were used to confirm crystal phases (Figure 2). In 8


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terms of energy, there is clear distinction between phonon modes associated with organic atoms and inorganic atoms, respectively, due to atomic mass difference. Phonon modes associated with inorganic atoms are located below 200 cm-1. For example, vibrations associated with Pb-Br2 at 99 and 134 cm-1 are not shown here. Modes associated with organic atoms are observed at above 200 cm-1. A He-Ne laser with an excitation wavelength of 632.8 nm was used to obtain CH3NH3PbBr3 spectra. When the temperature went down, the phase transition behavior from cubic to orthorhombic phase could be clearly observed by monitoring changes in phonon modes. In the organic part, modes associated with NH3+ bending located at 1425, 1476, and 1592 cm-1 are numbered in peaks 5, 6, and 7, respectively. Peak 1 located at 326 cm-1 is associated with NH3+ torsion. It showed increasing intensity and sharpening of linewidth. Peaks 2 and 4 located at 919 and 1257 cm-1, respectively, are related to CH3NH3+ rocking. They exhibited decreasing intensity and broadening of linewidth with lower temperatures. Additionally, peaks associated with CH3NH3+ located over 2800 cm-1 are numbered in peaks 8 and 9. Peak 3 located at 974 cm-1 describes C-N stretching. Leguy et al. have described phonon modes and their relations with phase transitions of Raman peaks using 785 nm of exciton wavelength. Niemann et al. have reported those at 1064 nm with temperature dependence. Our results showed similar tendencies compared to those previous studies.25,26

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Figure 3. Temperature dependent PL spectra of CH3NH3PbBr3 with structural phase transition. Inset shows room temperature PL spectrum which indicates band gap energy of CH3NH3PbBr3.

PL spectra depending on temperature variations were used to confirm band gap energies, some of the most intrinsic properties of the material (Figure 3). Depending on structure transition, band gap energy varied. Particularly, the band gap energy for CH3NH3PbBr3 was about 2.27 eV at room temperature. If CH3NH3PbBr3 is applied to LED, it could be used for green light with wave length of 545 nm which can be turned on by 2.27 eV of band gap energy.

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Figure 4. (a) Topography image, (b) Surface potential distribution by KPFM, and (c) Work function calculation with deconvolution of CH3NH3PbBr3.

The work function of the material can be evaluated from surface potential mapping by using KPFM. Surface potential images of CH3NH3PbBr3 with topography were obtained by KPFM (Figure 11



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4a and 4b). The work function of CH3NH3PbBr3 was found to be 4.56 eV. This was consistent with our earlier results showing that the work function of CH3NH3Pb(I,Br)3 thin film was 4.7 eV.27,28 Deconvolution of work function revealed two different secondary peaks (Figure 4c). These variations in values might be due to compositional variations of CH3NH3Br or PbBr2. Such phenomenon in perovskite material has been reported in the case of CH3NH3PbI3.29-31 Huang et al. reported that work functions of CH3NH3Br and PbBr2 were shifted down and up, respectively, based on stable perovskite surface.25

Figure 5. The proposed electronic structure of CH3NH3PbBr3 based on measurement results obtained in this study. The electronic affinity was based on results of Ref. 31. The electron affinity used to draw the band diagram was 3.36 eV in CH3NH3PbBr3, similar to that in a previous study.32 The electron affinity was combined with work function of both materials. 12


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Through expected band diagram, p-type semiconducting characteristics were obtained (Figure 5). There are some alternations in n-/p-semiconducting type by controlling doping. Even surface segregation can lead to type change.33–35 Thus, unintentional or intentional doping might also affect the crystal depending on surrounding conditions.

CONCLUSIONS Fundamental properties of perovskite crystals were investigated in this study. The electron beam used in TEM had sufficient energy to transform the crystal structure of perovskite. Therefore, many phases were observed concurrently with degradation materials including PbBr2 and CH3NH3Br. The component ratio of Br was evidently low in EDS, causing smudgy second phase spots in FFT patterns. Our results confirmed the structure and electron diffraction pattern of hybrid lead bromide perovskite crystals and band gap energy of perovskite crystals by comparison with previous research studies. We also investigated its work function. By integrating the observed fundamentals, we predicted intrinsic band diagrams of these crystals. These diagrams showed different tendencies. However, its properties can be controlled by doping and growth processes. We established intrinsic properties and band diagram of perovskite crystals. We also demonstrated the ability to control semiconducting type by managing growth processes and phase transition.

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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Notes The authors declare that they have no competing financial interests.

ACKNOWLEDGMENT This research was supported by grants (2014R1A1A4A01004404, 2016R1D1A1B01009032, and 2017R1D1A1B03034293) of the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Education, Republic of Korea. This work was also supported by the Priority Research Centers Program (2009-0093818) through the National Research Foundation (NRF) funded by the Ministry of Education, Republic of Korea.

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TOC Graphic

19



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Table 1. Reported characteristics of the crystal structures of CH3NH3PbBr3. 521x125mm (96 x 96 DPI)


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Figure 1. (a) Cross-sectional view of the crystal prepared by a focused ion beam for transmission electron microscope. Inset shows the crystal shape and color with the crystal axes. (b) Fourier transformed pattern showing the tetragonal crystal structure. 142x261mm (96 x 96 DPI)



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Figure 2. Temperature-dependent Raman scattering spectra of CH3NH3PbBr3. Numbers designate main Raman peaks 292x117mm (96 x 96 DPI)


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Figure 3. Temperature dependent PL spectra of CH3NH3PbBr3 with structural phase transition. Inset shows room temperature PL spectrum which indicates band gap energy of CH3NH3PbBr3. 252x161mm (96 x 96 DPI)



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Figure 4. (a) Topography image, (b) Surface potential distribution by KPFM, and (c) Work function calculation with deconvolution of CH3NH3PbBr3. 109x262mm (96 x 96 DPI)


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Figure 5. The proposed electronic structure of CH3NH3PbBr3 based on the measurement results in this study. The electronic affinity was used from the results of Ref. 29. 122x131mm (96 x 96 DPI)



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Table of Contents (TOC) Image 49x37mm (220 x 220 DPI)


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Optical and Scanning Probe Identification of Electronic Structure and

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