Room-temperature Cubic Perovskite Thin Films by Three-step All

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Room-temperature Cubic Perovskite Thin Films by Three-step All-vapor Conversion from PbSe to MAPbI3 Jijun Qiu, Lance L. McDowell, and Zhisheng Shi Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00142 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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Crystal Growth & Design

Room-temperature Cubic Perovskite Thin Films by Three-step All-vapor Conversion from PbSe to MAPbI3 Jijun Qiu*, Lance L. McDowell, Zhisheng Shi*

School of Electrical and Computer Engineering, University of Oklahoma, Norman, Oklahoma 73019, USA

ABSTRACT: Device performance of organic-inorganic halide solar cells significantly depends on the quality of the perovskite absorber films, which is dominated by synthesis techniques. Here, we demonstrated a three‐step all-vapor conversion route method to deposit uniform, pinhole-free CH3NH3PbI3 perovskite films with high crystallinity and purity. Notably, stable cubic-phase CH3NH3PbI3 converted at high temperature (≥160 oC) could survive at room temperature, which is desired for high efficiency and long-term stability of organic-inorganic halide-based opt-electric

ACS Paragon Plus Environment

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Crystal Growth & Design 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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devices. Additionally, this method has the potential for large-scale production due to its low cost, high throughput, large-area uniformity and good reproducibility.


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I Introduction Organic-inorganic halide perovskite films, such as CH3NH3PbX3 (X= I−, Br−, Cl−), have received considerable attention due to their surprisingly excellent photo-electro properties and simple, lowcost fabrication process. High optical absorption 1, tunable direct band gap 2, and long-range carrier diffusion length 3 make organic-inorganic halide perovskite films extremely promising candidates for thin-film solar cells, light-emitting diodes (LEDs) and x-ray detection. For example, power conversion efficiency (PCE) of perovskite solar cells have increased from 3.8% 4 to 23.7% 5 in recent 9 years, approaching their theoretical PCE limitation based on the Shockley-Queisser (SQ) radiative efficiency 6. The perovskite-inserted hybrid cell also achieved a promising efficiency of 28% by combining with Si cells 7. The fabrication cost of halide perovskite solar cells is 1/5-1/8 of silicon solar cells with similar PCE. LEDs based on Perovskite (PLED) have achieved an external quantum efficiency of 20.7% and an energy-conversion efficiency of 12%, exceeding the best performing OLED 8. SAIT researchers have also fabricated halide perovskite X-ray detectors with different structures that, in addition to being significantly lower in radiation, is 20 times higher in sensitivity for X-rays, as well as cheaper in price compared to conventional flat panel detectors 9.

By now, two easy deposition technologies, one-step and two-step processes, have been developed to fabricate and tailor high-quality organic-inorganic halide perovskite films. The one-step techniques

10-13,

including spin-coating, doctor-blade coating, spray coating, inkjet printing and

slot die coating were adopted widely in the lead halide perovskite solar cells due to its easier operation and low cost. However, a general disadvantage of one-step technology is the difficult control on perovskite stoichiometry, crystallinity and surface morphology, which highly affect the performance and reproducibility of PCE

14.

Therefore, the two-step sequential methods were


3


exploited, including solution-based

15, 16

and vapor-based processes

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17-21.

Although the two-step

fabrication method becomes more complicated, the stoichiometry, morphology, and crystallinity of perovskite films could be precisely tailored by controlling the procedure parameters in either step, showing a great potential to improve the performance of perovskite solar cells. However, the first-step spin-coating approach, which is employed in two-step sequential techniques to form PbI2 films, is difficult for large-area and large-scale production. Therefore, a lot of efforts have been focusing on improving two-step technologies for future commercialization and higher PCE. A three-step technique was first exploited by Sargent in 2015 22. Firstly, lead sulfide (PbS) was deposited by atomic layer deposition, and then converted into PbI2 and MAPbI3 by exposure to iodide (I2) gas and CH3NH3I vapor, respectively. Subsequently, Somobrata Acharya has successfully realized the direct conversion of ultrathin two-dimensional (2D) PbS nanocrystals (NCs) into uniform rectangular shaped hybrid perovskite NCs retaining 2D morphology by successive iodine vapor and MABr exposures 23. Then José Maria Clemente da Silva Filho

24

modified this technique by using sputtering assisted solution processing. They

deposited PbS films via sputtering and converted PbS into perovskite by immersing the PbS films in an iodine atmosphere at room temperature, followed by dipping in a solution of methylammonium iodide (CH3NH3I). The solid conversion step from PbS to PbI2 is helpful to form more compact and pinhole-free perovskite films by eliminating the large amount of colloid suspension and needle-shaped solvation intermediates induced by solvent evaporation

25, 26, 27,

resulting in an enhanced optical absorption in the visible spectrum. Compared with PbI2


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deposition, more synthesis strategies, which are compatible with large scale production, could be employed to deposit PbS or other candidates. In this work, we propose and demonstrate an alternative three-step all-vapor conversion route for producing high quality perovskite films. The setup and fabrication procedure are schematically illustrated in Figure 1. First, Lead Selenide (PbSe), rather than PbS, is employed as the firstprecursor film for post-conversion into perovskite films. PbSe has the benefit of having a lower Se vapor pressure than S 28, better thickness control 29, 30 and faster conversion efficiency to PbI2 31. PbSe thin films with good homogeneity have been successfully deposited by molecular beam epitaxy (MBE)

32,

physical vapor deposition (PVD)

33

and chemical bath deposition (CBD)

34

method in previous works. Herein, PVD was adopted to fabricate PbSe films in this work due to its advantage for large-scale implementation. Second, PVD-PbSe precursor films were converted into PbI2 by exposure to the sublimed I2 vapor. Thermodynamic studies have shown that the conversation from PbSe to PbI2 could easily achieved even under trace amounts of iodide assisted by a carrier gas 35, 36. Third, in situ closed-space vapor transport (ITCSVT) has been proposed as a substitute for the solutionprocess for the conversion of PbI2 to MAPbI3. Along with the typical advantages of the traditional vapor-based process, ITCSVT has been proven to be a scalable, low-cost and high-throughput technique, largely in part of not having the need for a glove box and/or vacuum instruments, and the dramatically decreased consumption of organic materials 37. Furthermore, it is already a wellestablished

industrial

closed-space

technology

for

cost-competitive,

commercial-scale

manufacturing of polycrystalline CdTe solar cells. The whole manufacturing cycle will be extended duo to the introduction of deposition and conversion PVD-PbSe precursor films, however, the engineering in morphology, crystal and


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stability could be designed and realized flexibly for high performance devices. Here, the advantages of the three-step all-vapor conversion process in terms of purity, crystal phase, crystal grain size, crystallographic orientation, homogeneity and compactness were verified by comparing with the MAPbI3 perovskites synthesized from the solution-based process. It is also advantageous for such three-step all-vapor conversion processes to produce perovskite alloys with the tailorable composition, which are believed to be the most promising solution to the notorious stability issue 38-40,

such as Pb-Sn and Pb-Cs by using co-depositing techniques, or even more complex structures

by using mixed halide carrier gas (iodine and bromine) and organic cation (MA+ or FA+) combination. Therefore, this method is particularly versatile for the fabrication of multi-layer stacked perovskite tandems and integration onto cadmium telluride (CdTe), copper-indiumgallium-selenide (CIGS) and textured silicon (Si) heterojunction cells. Furthermore, this technique could offer the potential to grow engineered thin-film Quantum Well or imbedded Quantum Dot perovskite structures with precise control of composition in different layers. II Method All chemicals used in the experiment were analytic grade reagents without further purification. The glass substrates were cleaned by ultrasonication in water, acetone and ethanol for 15 minutes each. 2.1 Fabrication of perovskite films 2.1.1 Step-one: growth of PbSe films by PVD. PbSe films were obtained by physical vapor deposition in high vacuum. High purity 99.99% PbSe source was deposited on the cleaned glass substrates at 250 oC with a base pressure of 2×10-4 Pa. The deposition rate was controlled at 2.0 nm/min to get a mirror-like opaque grayish film.


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2.1.2 Step-two: conversion of PbSe into PbI2 films The PbSe films were placed inside a quartz tube furnace with two blocks, as shown in Figure 1. The sublimated iodine gas (I2) was introduced into the tube by the nitrogen (N2) carrier-gas with 5 sccm flow rate from one of the connectors. Although conversion from PbSe to PbI2 can occur at room temperature, it would take days for complete conversion. For this reason, prepared films were catalyzed at 200 oC for 15 mins in order to reduce the conversion time. During the conversion process, the exhaust gas was introduced into a 5 mol/L sodium hydroxide (NaOH) solution to get rid of the residual I2 gas. To avoid oxidation, 20 sccm ultrahigh purity N2 carrier-gas was purged into the tube for 5 mins at room temperature before introducing I2 gas. 2.1.3 Step-three: conversion of PbI2 into MAPbI3 films by ITCSVT The methylammonium iodide (MAI) organic source was placed in a home-made flat-bottom glass boat with a diameter of 20 mm. Subsequently, the obtained PbI2 films were placed over the boat with the PbI2 surface facing down. A spacing of 1.0 mm, which was dominated by the boat height, was used in our process. The boat was then heated to 120-180 oC at a rate of 10 oC/min at ambient pressure. This temperature was maintained for 120 mins before cooling down to room temperature. The solution-based conversion step consisted of dipping the PbI2 thin film into a solution of 10 mg/ml of MAI in isopropyl alcohol heated to 50 °C for 10 minutes. After conversion, the films were crystalized at 110 °C for 30 minutes in air. During all syntheses, the relative humidity was in the 50% range. 2.2 Characterization of precursor and perovskite films The morphology and thickness of the films were studied by field-emission scanning electron microscopy (FESEM, Zeiss Neon-40 EsB, Japan). Phase characterization was performed by X-ray


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diffraction method (Bruker D8 X-ray Diffractometer, USA) utilizing Cu Kα radiation. Raman scattering was carried out in a confocal micro-Raman configuration (XploRA, Horiba), in the wavenumber range 50 to 500 cm−1. Optical properties were obtained using Fourier transform infrared spectroscopy (FTIR) in transmittance mode, in the wavenumber range 20000 to 10000 cm−1 (Bruker, IFS 66/S). III Results and Discussion 3.1 Structural and Morphological properties The typical crystallization evolution for all step of the conversion process were evaluated by XRD, as shown in Figure 2. The XRD pattern of the PVD-PbSe film displays one strong peak and three weak peaks, which are assigned to (200), (220), (311) and (400) planes of cubic PbSe. The large intensity ratio of (200)/(220) peaks indicates the film deposited at 250 oC is polycrystalline with a highly preferred [100] growth orientation. After exposure in the diluted I2 vapor at 200 oC for 15 min, the PbSe peaks vanish completely, supplemented by a strong peak at 12.76 o and four weak peaks at 38.66, 39.53, 41.68, 52.40 o, which are all in accord with standard PbI2 powder diffraction as catalogued by JCPDS 07-0235. These peaks are identified as the planes of (001), (003), (110), (111) and (004), respectively, and indicates a high [001] textured hexagonal polycrystal-structure. No existing PbSe peak contributions confirm that [001] oriented PbSe was completely converted into [001] textured PbI2 polycrystal film in 15 mins, indicating a high conversion efficiency by the chemical reaction as follows: PbSe + I2 → PbI2 + Se (↑). After the ITCSVT process, the resulting MAPbI3 perovskite film shows several peaks at 14.14, 20.01, 24.54, 28.38, 31.80, 34.92, 40.52, 43.08, 50.14, 52.32, 54.60, 58.56, 60.70 and 62.46 o,


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indexed by the planes (100), (110), (111), (200), (210), (211), (220), (221), (222), (320), (321), (400), (410) and (430) of cubic Pm-3m phase. Generally, cubic MAPbI3 has a tendency to transform into a tetragonal I4/mcm structure at temperatures lower than 54 oC, and then orthorhombic Pnma at -111 oC 41 - 43. In our case, however, the MAPbI3 cubic structure survived at room temperature and is confirmed from three aspects. First, iconic (211) peak at 23.6 o, which only exists in the tetragonal phase, is not detected in the XRD pattern. Second, no obvious peak splitting or shoulders are observed at (100) and (200) peak positions, whereas split peaks of (002)(110) and (004)-(220) could be observed in the tetragonal phase, as shown in the enlarged XRD patterns. Third, based on Bragg’s law, the calculated lattice constant a of the cubic phase is 6.258 Å, which agrees well with the theoretical value of Pseudo-cubic MAPbI3 at room temperature calculated by Baikie

44.

Subsequent experiments proved that cubic phase survived at room

temperate as a result of the higher growth temperature. As a comparison, the XRD pattern of solution-based MAPbI3 film was added. Consequently, the two peak-splitting around 14.26 and 28.54 o, along with the presence of the (211) peak showing up at 23.68 o, certainly reveals the formation of tetragonal phase MAPbI3. In addition, a tiny PbI2 (001) peak at 12.76 o remained, indicating an incomplete conversion. The amount of residual lead iodide is small relative to the amount of perovskite due to the XRD intensity ration between PbI2(001) and MaPbI3-(110). This incomplete conversion is unavoidable in the solution-based conversion process, which has been attributed to the quick growth of a thick shell of perovskite when using the dipping technique in MAI-isopropyl alcohol solution, hindering total PbI2 conversion

45, 46.

The smaller intensity and wider FWHM shown by the XRD pattern of the

solution-based MAPbI3 film indicates a possible advantage to the ITCSVT process given the potential to obtain perovskite films with high crystal quality.


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Figure 3 shows a typical morphological evolution duration of three-step all-vapor conversion process. The PbSe film deposited at 250 oC shows densely packed grains with a vertical columnar structure and smooth top surface. The grain size (column width) is about 100∼110 nm, and is the same as the column length (thickness), indicating a cubic growth habit. This agrees well with the [100] growth orientation from XRD pattern. After converting into PbI2, the film shows a rough surface covered with hexagonal platelet-shape crystals. The average diameter and thickness of PbI2 crystals are ∼200 and ∼50 nm, respectively. This architecture is as a result of the hexagonal and layered crystal structure, where I–Pb–I atom layers are covalently bonded and stacked by Van der Waals force in the c-axis 47, 48. Although most of the hexagonal platelets lie low with the hexagonal face parallel to the substrate, a small number of platelets randomly shoot out from the crystal boundary at the bottom PbI2 layer with various angles relative to the substrate, leaving small voids among the platelets. The total thickness of the PbI2 layer is about ∼210 nm, approximately twice the thickness of the PbSe film. This change in ratio of thicknesses after conversion from PbSe to PbI2 matches well with the calculated value of 2.1, declaring a complete conversion from PbSe to PbI2. After ITCSVT process, pinhole-free MAPbI3 films with large crystal domains and uniform surface are obtained, as shown in Figure 3 (g) to (j). The large crystal domain size is larger than 1.5 µm, suggesting good crystallinity

49,

which is confirmed by the high intensity and narrow

FWHM of the XRD measurement. The irregular shaped, large crystal domains with round and smooth surface result in flexible interconnections, forming smooth, crack-free boundaries, resulting in a much smoother surface with full surface coverage. Side-view FESEM shown in Figure 3 (h), reveals the entire perovskite film is composed of a single layer of large crystals, where the measured height of ∼400 nm determines the thickness. Large crystal domains originate from the decreased reaction rate in diluted MAI vapor, and the increased diffusion rate and depth of


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MAI molecules diffusing into PbI2 crystals under high temperature. At the same time, in situ annealing process in MAI environment promotes the further recrystallization and restrains the degradation of the MAPbI3 crystals, resulting in both high crystallinity and full surface coverage. However, the solution-based MAPbI3 film exhibits a coarse surface with local compact grain stacking and an incomplete surface coverage, as see in Figure 3 (e) and (f). These typical morphological traits are typical for the solution-based perovskite films. Most of perovskite crystals have similar cuboid shapes, except for few tetragonal rods protruding from the surface. The size of these cuboids is broadly distributed from 100 to 200 nm. It is worth noting that the thickness of the solution-based perovskite film is only ∼ 250nm, less than the calculated value of ∼400 nm based on the increased volume of unit cell 22. This is attributed to dissolution during the MAPbI3 crystal growth. The reactions involved in this step are believed to be as follows 46, 50: CH3NH3PbI3 (s) + I-→ CH3NH3- + PbI42PbI2(s) + 2I- → PbI42A high number of voids could be observed at the bottom of the solution-based perovskite film from the cross-sectional FESEM image, as shown in Figure 3 (f), further confirming the dissolution process. Shorter growth time could effectively hinder the dissolution process, however, the conversion efficiency of PbI2 will be restrained, giving rise to an impurity problem. Compared with the solution-based method, the three-step all-vapor conversion route shows advantages in fabricating a uniform, pinhole-free oriented-growth perovskite film, with high crystal quality and large crystal domains, offering great potential for significant improvement of solar cells. The scalability of the tree-step all-vapor conversion technique, along with its relatively


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low-cost and high-throughput technique provides additional advantages given the potential to scale-up perovskite solar cell production. The conversion of PbSe to MAPbI3 was also investigated using Raman spectroscopy, as shown in Figure 4. The PbSe Raman spectrum in the 50~500 cm-1 range shows a prominent peak at 135.4 cm-1, which is associated with the first-order longitudinal optical phonon (LO(Γ)). The weak humps at 76.9 and 250.6 cm-1 correspond to the second-order transverse-optical phonon 2TO(Γ) and the overtone 2LO(Γ), respectively. These peaks are in agreement with the previous report 51. After step-two of the conversion process, converting PbSe into PbI2, five distinct peaks erect at 72.8, 94.6, 110.9, 166.4, and 214.9 cm−1. The two main Raman bands at 95 and 74 cm-1 are assigned to A1g vibrational modes due to PbI2 breathing deformation and the degenerated Eg band owing to the shearing motion of two I ion layers, respectively. The low density broad peak 214 cm-1 is about twice the energy of 109 cm-1 absorptions, which are considered to be the second order band A2u (2LO) and Au (LO) mode

52 - 55,

respectively. The absence of any extra Raman

peaks contributions from the previous step, along with the sharpness of the PbI2 peaks, indicates good crystallinity in the PbI2 conversion process. After reacting with MAI at high temperature, obvious peak shifts are observed from the MAPbI3 Raman spectrum. Most notably, is the dominant peak shift from 94.1 to 142.1 cm-1, which is assigned to MA+ rotation in the cage formed by 4 PbI6 octahedral, indicating a fully crystalline MAPbI3 is formed 56. The MAPbI3 Raman spectrum also reveals a board band at 270 ~ 300 cm-1, which may be ascribed to higher ordering of the MA+ cations due to a frequency doubled 140 ~ 150 cm-1 band. Being consistent with theoretically predicated bands, the bands falling at 70.1 and 86.5 cm-1 also correspond to the rigid-body motion of the MA+ ions, meanwhile, the 60.6 and 81.1 cm-1 bands are assigned to the Pb-I stretching and I-Pb-I bending modes

57.

The disappearance of the unperturbed Pb–I breathing modes further


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indicates the formation of MAPbI3 crystalline phase by intercalating MA+ ions into the PbI2 framework. The transformation of PbSe to MAPbI3 by all-vapor phase reactions was also probed by UV-visible transmission spectra, as shown in Figure 5. No transmittance spectrum was observed for the PbSe film due to the opaque characteristic of the glass slide in the mid-infrared range (3-5 µm). The (001) textured PbI2 film shows an onset of absorption at 500 nm, corresponding to the optical band gap of 2.5 eV, as shown in Figure 6. This transmittance spectrum with obvious absorption features indicates that the PbI2 film converted from PbSe has a very smooth surface and uniform thickness. After the conversing to perovskite, the absorption edge shifts to 725 nm, corresponding to a decreased band-gap of 1.65 eV. This is similar to what has been reported for other MAPbI3 films prepared from CVD method 58, 59, but is larger than the value of 1.51 eV reported for MAPbI3 films and single crystal fabricated from solution-based processes. In addition, the optical property change can be easily observed by color changes in each step of the conversion process, which change from shining-grey to bright-yellow, then to dark-brown, shown in the inset of Figure 6. 3.2 Phase and Morphology control for MAPbI3 synthesized from three-step all-vapor conversion route It has been proven that special morphological features, such as large grain size, high crystal orientation, pinhole-free could increase the PCE of perovskite solar cells. For example, large crystal grains were found to increase the PCEs by motivating the exciton formation 60, 61. Pinholefree films also can also dramatically enhance Jsc of PSCs by suppressing the surface charge recombination resulting from the contact of electron and hole transport layers (ETL and HTL) 62. The crystallographic orientation of perovskite films is found to have a significant impact on the open circuit voltage Voc due to different electronic trap site densities on these surfaces 63 - 65.


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The three-step all-vapor conversion route shows flexibility in the control of both crystal structure and morphology for perovskite films. Figure 7 shows morphology control by adjusting the growth temperature. Clearly, lots of pinholes are observed from the MAPbI3 film grown under 140 oC. With increasing temperature, the average size of crystal domains undergoes a slight decrement, and then a distinct increment after the temperature is increased more than 160 oC, with further increase up to ~2 µm at 180 oC. Meanwhile, the surface become much smoother with increasing temperature. The size decrement along with a rougher surface when increasing temperature from 120 to 140 oC, is mainly attributed to an increase in reaction rate, resulting in degradation of crystallinity. This is further evidenced by the lower XRD density and wider FWHM peak at 140 oC,

shown in Figure 8. Further increasing the temperature results in an even faster reaction rate,

however, the MAI diffusion is enhanced, the decomposition-recrystallization process becomes more frequent due to the higher the migration kinetic energy, resulting in larger crystal domains and much smoother surfaces with higher crystal quality. This is further confirmed by the crystal fusion observed from the single-crystal layer compared with the double-stacked layer, as shown in Figure 7 (f) and (d), respectively. One of goals in the perovskite solar cell community has been focusing on fabricating large-grain size, pin-hole free perovskite films by using synthesis strategy

66

and solvent-engineering

67, 68,

expecting to enhance the performance and device stability by reducing grain boundaries and trap-state density and prolonging carrier lifetime. As a technique that also can grow more than micro-scale crystalline grains, in addition to the merits of vapor-based methods, the three-step allvapor conversion technique can also precisely tailor the grain size by simply adjusting the growth


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temperature, showing good control and repeatability as compared with the solution-based hotcoating technique 66 and the solvent-engineering 67, 68. Furthermore, an obvious crystalline transformation from tetragonal to cubic phase is observed at the 160 oC. Although peak splitting is not observed from the XRD patterns, the appearance of the (211) planes confirm the MAPbI3 films converted under 160 oC formed a tetragonal structure. At the same time, an obvious peak shift to lower angle is observed with increasing temperature from 120 o to 160 oC, indicating lattice dilation, which is in accord with the lattice changed trend for MAPbI3 changing from tetragonal to cubic phase 44. The lattice dilation is also in line with the lattice change trend derived from the increasing temperature powder X-ray diffraction experiments 44.

It has been proven that the lattice dilation is associated with two factors in our case: (1) the high

(001) orientation of PbI2 films converted from (001) oriented PVD-PbSe, and (2) the high migration energy and high MAI concentration at higher temperature. Since MAI is polar and has low symmetry of C3v, its orientation should be disordered to satisfy the Oh symmetry in the cubic phase 69. To construct a highly symmetric cubic structure, therefore, iodine ions are also correspondingly disordered to form the PbI6 octahedral, where every MA+ ion is caged in the space made by four order-disordered PbI6 octahedra. With decreasing temperature, iodine ions incline to ordered via the R4+ rotational distortion mode, resulting in cubic to tetragonal transition with lower symmetry 70. As the temperature further decreases, MA+ ions start ordering in the unit cell by a combination of the R4+ and M3+ distortion modes, further reducing the symmetry to form an orthorhombic phase. However, the increase of the growth temperature inevitably leads to a higher concentration and a higher migration energy of MA+ and I- ions, contributing to the stoichiometric composition. In other words, high temperature could significantly decrease the iodine vacancies (Vi··) concentration 71. There is also the possibility of


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weaken Van der Waals interaction existing among MA+, while strengthened bonds exist between the amino group and the halide ions resulting from higher concentration and higher migration energy of MA+ and I- ions. The weakened Van der Waals interaction existing among MA+ allow for every MA+ ion move more freely inside the cage at high temperature, keeping its disorder for Oh symmetry. Meanwhile, the strengthened bonds between the amino group and the halide ions effectively suppress the out-of-phase PbI6 rotation, avoiding the ordering of I- ions that occurs at decreasing temperature, which protects the cubic structure at room temperature. A stable MAPbI3 cubic structure at room temperature is desired for solar cells. It not only can conduce to photoelectrical devices 72, but also can improve the thermal and long-term stability by eliminating the undesired lattice distortion and strain caused by the cubic-tetragonal transition. The morphological changes from growth temperatures leads to the difference in transmission spectra, as shown in Figure 9. The transmittance decreases with increasing the growth temperature, indicating an increased absorption coefficient in the 0.5~1.0 µm range. This is ascribed to improved crystallinity and full surface coverage with increasing growth temperature. Compared with ITCSVT-MAPbI3, the film synthesized from solution-based process shows a lower transmittance in the wavelength higher than 0.725 µm. However, it does not mean a stronger light harvesting capacity due to higher crystallinity. This is mainly caused by the strong scattering due to small grain size, coarse surface and voids due to the dissolution, shown in Figure 3 (f). The calculated band-gap of solution-based MAPbI3 film is about 1.58 eV, smaller than the Eg (1.65 eV) of samples from vapor-based process. IV Conclusion


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In summary, we have demonstrated a new three-step all-vapor conversion route to produce MAPbI3 films. The first step involves (001) oriented PbSe thin films deposited by PVD method. Subsequently, PbSe films are converted into PbI2 through annealing in sublimated I2 gas. The PbI2 film is then converted into a MAPbI3 perovskite film by In-situ closed-space vapor transport method at high growth temperature. As an alternative to the conventional chemical bench approach, the proposed route shows obvious advantages in morphology control, improved crystallinity and purity, and phase stability. Being an easy, cost-effective, and well-known technique for thin film deposition, the three-step all-vapor conversion route has the potential to scale-up the large-area perovskite solar cell production.


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FIGURES

Figure 1 Setup and Schematic diagram for three-step all-vapor conversion route from PbSe to MAPbI3. Step-one: PbSe films obtained by physical vapor deposition (PVD); Step-two: the conversion of PbSe to PbI2 by exposing PbSe films in sublimated I2 vapor at high temperature, and Step-three: the conversion of PbI2 to MAPbI3 by in situ closed-space vapor transport (ITCSVT).


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Figure 2 Corresponding XRD pattern for the sequential conversion from PbSe to MAPbI3, including ITCSVT (red) and solution-based process (pink). The inset figure focus on the characteristic peaks around 14 o shows the peak splitting and peak shifting for tetragonal (pink) phase. No shifting is observed from the peak marked in gray frame, which originate from the substrate.


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Figure 3 The morphological evolution for the sequential conversion from PbSe to MAPbI3. (a) plan- and (b) cross-view FESEM image of PbSe film by PVD method. (c) plan- and (d) cross-view FESEM images for PbI2 film by exposure in I2 vapor at 200 oC for 15 mins; (e) plan- and (f) crossview of MAPbI3 film by conversion from PbI2 in MAI- isopropyl alcohol solution at 50 oC for 15 mins; (g), (i), (j) plan-view FESEM images with various magnification and (h) cross-view FESEM images for MAPbI3 film obtained from ITCSVT at 160 oC for 2 hours.


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Figure 4 Raman spectra for the sequential conversion from PbSe to MAPbI3 in the 50 to 500 cm−1 wavenumber range.


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Figure 5 Transmission spectra for PbI2 and MAPbI3 films


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Figure 6 Band-gap calculation for the films at each conversion (As a reference, the band-gap of PbSe was calculated from our previous sample growth on the CaF2 substrate which is transparent in the mid-infrared wavelength range).


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Figure 7 Morphology control for MAPbI3 films by adjusting ITCSVT conversion temperatures. (a) and (b), (c) and (d), (e) and (f) plan- and cross-view FESEM images of MAPbI3 films converted at 120, 140 and 180 oC for 2 hours, respectively.


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Figure 8 Phase control for MAPbI3 films by adjusting ITCSVT conversion temperatures. (a) Overall XRD diffraction patterns of MAPbI3 films converted at 120, 140 and 180 oC for 2 hours, respectively. (b) Normalized high-resolution characteristic peaks ranging from 13.5 to 14 o, showing a shift to small angle with increasing temperature. (c) Zoomed characteristic peaks ranging from 19-25 o, showing disappearance of (211) plane after temperature higher than 160 oC. No shifts are observed from the peak coming from substrate, marked with gray frame.


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Figure 9 Optical properties for MAPbI3 films obtained from ITCSVT process and solution-based process. The inset shows the calculated Eg difference between the two processes.


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AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected] and [email protected]

Author Contributions J. J. Qiu and Z. S. Shi contributed to the design and implementation of the process. J. J. Qiu carried out all the experiments of sample synthesis and most of measurements. J. J. Qiu and Z. S. Shi analyzed all of the results and wrote the manuscript. L. L. McDowell assisted with the XRD and Raman measurements and edited the manuscript. Z. S. Shi supervised the project. All the authors discussed the results and approved the final manuscript for submission.

Notes The authors declare that they have no competing interests.

ACKNOWLEDGMENT


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We thank Prof. Binbing Weng (University of Oklahoma, School of Electrical and Computer Engineering) for assistance with the FTIR and Raman data collection, and Dr. Preston Larson (University of Oklahoma, Samuel Roberts Noble Microscopy Laboratory) for the FESEM measurement.

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For Table of Contents Use Only

Room-temperature Cubic Perovskite Thin Films by Three-step All-vapor Conversion from PbSe to MAPbI3

Jijun Qiu*, Lance L. McDowell, Zhisheng Shi*

Large-crystal size, pinhole free CH3NH3PbI3 films made of stable cubic phase at room temperature are directly synthesized by a three-step all-vapor conversion route, which has the potential for large-scale production due to its low cost, high throughput, large-area uniformity and good reproducibility.


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Room-temperature Cubic Perovskite Thin Films by Three-step All

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