Understanding the Formation of Vertical Orientation in Two

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Understanding the Formation of Vertical Orientation in Two-dimensional Metal Halide Perovskite Thin Films Alexander Z. Chen, Michelle Shiu, Xiaoyu Deng, Mustafa Mahmoud, Depei Zhang, Benjamin J. Foley, Seung-Hun Lee, Gaurav Giri, and Joshua J. Choi Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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Chemistry of Materials

Understanding the Formation of Vertical Orientation in Twodimensional Metal Halide Perovskite Thin Films Alexander Z. Chen1, Michelle Shiu1, Xiaoyu Deng1, Mustafa Mahmoud1, Depei Zhang2, Benjamin J. Foley1, Seung-Hun Lee2, Gaurav Giri1, Joshua J. Choi1,*

1Department 2Department

of Chemical Engineering, University of Virginia, Charlottesville, VA, 22904, USA of Physics, University of Virginia, Charlottesville, VA, 22904, USA

ABSTRACT: Metal halide perovskites have demonstrated strong potential for optoelectronic applications. Particularly, twodimensional (2D) perovskites have emerged to be promising materials due to their tunable properties and superior stability compared to their three-dimensional counterparts. For high device performance, 2D perovskites need a vertical crystallographic orientation with respect to the electrodes to achieve efficient charge transport. However, the vertical orientation is difficult to achieve with various compositions due to a lack of understanding of the thin film nucleation and growth processes. Here we report a general crystallization mechanism for 2D perovskites, where solvent evaporation and crystal growth compete to influence the level of supersaturation, and a low supersaturation is necessary to crystallize vertically oriented thin films starting from nucleation at the liquid-air interface. Factors influencing the supersaturation and crystallization dynamics, such as choices of organic spacers, solvents and solvent drying rate, have a strong influence on the degree of crystallographic orientation. With this understanding of crystallization mechanism, we demonstrate direct crystallization of thin films with strong vertical orientation using three different organic spacers without any additives, and the vertically oriented 2D perovskites result in efficient and stable solar cell operation.

1. Introduction Metal halide perovskites have emerged as promising next generation optoelectronic materials, with their extraordinary performance breakthroughs in solar cells1-5, light emitting diodes6-9, photodetectors10-14, radiation detectors15-17 and lasers18-20. Its exceptional optoelectronic performance21-25, combined with low cost solution processability, has attracted enormous research attention. Recently, two-dimensional (2D) perovskites have garnered great interest due to their superb long-term stability, tunable properties and excellent device performance26-28. 2D perovskites have the chemical formula of B2An-1MnX3n-1, where B is the organic spacer cation R-NH2+, A is small cation CH3NH3+ (MA+), HC(NH2)2+ (FA+), Cs+ or Rb+, M is metal Pb2+ or Sn2+, X is the halide Cl-, Br- or I-. The number of octahedra layer, n, can be tuned by changing the stoichiometry. The introduction of organic spacers in 2D perovskites improves the material stability by the hydrophobicity of the spacers and increased formation energy29. In 2D perovskite based optoelectronic applications, a crystallographic orientation perpendicular to the electrodes is needed for efficient device performance30. In device architecture typically used in solar cells and light-emitting diodes wherein 2D perovskite thin film is sandwiched by

electrodes on the top and the bottom, a strong vertical orientation is desired. Due to the charge insulating nature of the bulky organic spacers sandwiching the inorganic metal halide slabs, a vertical orientation of the layered structure allows direct charge transport pathway between top and bottom electrodes, while randomness in crystallographic orientation leads to poorly performing devices26, 30. Reports in literature use different methods to fabricate 2D perovskite thin films that result in various degrees of vertical orientation26, 31-37, without a clear strategy for the different choices of compositions and processing conditions. Although several studies reported vertical orientation with specific processes26, 31-32, its formation mechanism is not well understood and a general strategy to controllably obtain vertical orientation is not yet available. A deeper understanding of the 2D perovskite crystallization process is necessary to develop vertically oriented 2D perovskites with new structures and compositions to achieve higher performance in solar cells, light emitting diodes and photodetectors. In this study we show the crystallization mechanism for vertical orientation in 2D perovskite. Our previous work demonstrated that the vertical orientation in 2D perovskite thin films can originate from nucleation at the liquid-air interface30. However, the subsequent crystal growth is largely unknown, making it difficult to fully control the degree of orientation.

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Figure 1. Illustration of the crystallization mechanism of vertically oriented 2D perovskite. After the formation of a vertically oriented crust at the liquid-air interface, (a), continuous crystal growth of a vertically oriented thin film at low supersaturation, provided by a slow solvent removal rate, results in a vertically oriented thin film (not to scale). (b) Additional nucleation events, due to high supersaturation with fast solvent removal rate, introduce orientation randomness (not to scale). (c-f), GIWAXS patterns with increasing solvent removal rate, including slow (c), medium (d), fast (e) and fastest (f). The results show that faster solvent removal rate results in a lower degree of vertical orientation. Here we focus on the crystal growth process and show that a continuous crystal growth without additional nucleation is necessary to produce vertically oriented 2D perovskite thin films. Therefore, to achieve strong vertical orientation, a low supersaturation condition is crucial to promote crystal growth from the crust at liquid-air interface while suppressing additional nucleation. It is proposed that solvent evaporation and crystal growth competes to influence the level of supersaturation. We demonstrate the generality of this crystallization mechanism with three different organic spacers (iso-butylammonium, n-butylammonium and phenylethylammonium) and three different solvent choices (gamma-butyrolactone, dimethylacetomide and 2methoxyethanol). Based on this crystallization mechanism, we consistently found that slower solvent evaporation rate is beneficial for generating higher degree of vertical orientation for all three organic spacers. Choice of organic spacers also has an impact on the degree of orientation, with isobutylammonium (isoBA) having stronger vertical orientation formation ability than n-butylammonium (nBA). With these new insights we were able to directly crystallize vertically oriented phenylethylammonium (PEA) based 2D perovskite thin films, which were shown to have advantageous optoelectronic properties38 but are more difficult to form vertical orientation with. Solar cells fabricated with strong vertical orientation based on the organic spacers were found to exhibit excellent performance and stability. 2. Nucleation and growth mechanism of vertical orientation It was recently shown that vertical orientation in 2D perovskites can originate from the initial nucleation of a

vertically oriented crust at the liquid-air interface30 as illustrated in Figure 1a. For the vertical orientation to form across the entire film thickness, according to classical crystallization theory39, the formation of the top-crust needs to be followed by continuous crystal growth without additional nucleation events (Figure 1a). This uninterrupted growth requires a low supersaturation environment approaching the meta-stable zone, which promotes growth and suppresses nucleation. At the presence of high supersaturation, nucleation is favored against growth, leading to more additional nucleation39. As shown in Figure 1b, these additional nuclei can form within the bulk liquid phase as well as the substrate-liquid interface. The liquid-substrate interface typically induces horizontal orientation due to the van der Waals attraction between 2D crystal plates and the substrate40. Homogeneous nucleation within the bulk liquid phase often leads to random orientation due to the isotropic environment. As a result, high supersaturation during 2D perovskite crystallization should lead to a more randomly oriented crystals underneath the vertically oriented thin crust. To verify the proposed crystallization mechanism, we tuned the level of supersaturation via controlling the solvent removal rate from the solution precursor solution during thin film crystallization at the same temperature. The degree of orientation was characterized with grazing incidence wide angle X-ray scattering (GIWAXS). As demonstrated previously30, the initial crystallization at the liquid-air interface is confirmed with the crust scraping test as shown in Figure S13. For results shown in Figure 1c-f, the precursor solution consisted of lead iodide (PbI2), methylammonium iodide (MAI) and n-butylammonium iodide (nBAI) at molar ratio of 2:3:4 dissolved in gamma-butyrolactone to yield a precursor solution

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for compositionally 4-layer 2D perovskite nBA2MA3Pb4I13 at 0.75 M Pb concentration. The precursor solution was spread onto clean glass substrates via a brief spincoating step (see Methods section for details). The initial brief spincoating was terminated before crystallization initiates, followed by treatment of four different solvent removal conditions at which thin film crystallization commences. In the four different conditions, the level of supersaturation was controlled by varying the rate of solvent removal at the same temperature (Figure S4). In "slow" condition, the spincoating is stopped and the wet thin film is left to dry being stationary at room temperature, which takes around 150 s to fully crystallize after terminating the spincoating (Figure S5). It has the lowest level of supersaturation. In "medium" condition, the spincoating continues for about 50 s until perovskite has completely crystallized. The convection provided by spincoating elevates the rate of solvent evaporation and level of supersaturation. In "fast" condition, a room temperature N2 flow of approximately 9 m/s normal to the substrate surface is introduced, in addition to the spincoating. The wet liquid film crystallizes a few seconds after introducing the N2 flow (Figure S5). The airflow further increases convection and solvent evaporation rate during crystallization. In "fastest" conditions, 100 µL of toluene is dropped onto the spinning slides to extract the solvent. This rapid anti-solvent dripping results in an instant solvent extraction and crystallization, which has the highest level of supersaturation. The time of crystallization was monitored by optically tracking the opaqueness of the thin film. The schematics of these processes and time-evolution of opaqueness is shown in Figure S4 and S5.

The GIWAXS characterization of the degree of crystallographic orientation in different solvent removal conditions is shown in Figure 1c-f. X-ray diffraction intensity of slow and medium films concentrates into spots, indicating the grains are highly oriented with respect to the substrate. Since the long range peaks at Qz = 0 Å are not present in the GIWAXS pattern possibly due to the intrinsic material diffraction behavior or the sample itself blocking these diffraction peaks30, we indexed the pattern using the rest of the diffraction peaks. After comparing the indexing with different structures including n = 1-4 layer 2D perovksite as well as tetragonal (110) oriented MAPbI3 structure41-43, we found the pattern is best indexed by vertically oriented 4-layer (n=4) nBA2MA3Pb4I13 reported in literature42 (Figure S6). As usually shown in the literature7, 9, 37, 44-45, impurities other than n = 4 2D perovskite exist in our samples to varying degrees which can be detected in absorbance spectra (Figure S14). Nonetheless, our GIWAXS and indexing results suggest that 4-layer 2D perovskite is the dominant component of the crystalline species in our samples (Figure S6). The degree of vertical orientation was determined to be 99% for both “slow” and “medium” drying rate samples based on a fitting method described in the Method section. The diffraction intensity smears into rings for fast and fastest conditions, indicating a large fraction of randomly oriented grains, with the degrees of vertical orientation calculated to be 53% and 10%, respectively. Our results show that faster solvent removal rate results in a lower degree of preferential vertical orientation, which is consistent with our proposed crystallization mechanism. Therefore, a key to obtaining high degree of preferential vertical orientation, which is critically important for high device performance, is to suppress additional nucleation after formation of initial vertically oriented layers at the liquidair interface. 3. Effect of experimental parameters on degree of orientation

Figure 2. (a, c), Degrees of vertical orientation with different solvent removal rate, solvent choices and organic spacers. GIWAXS patterns of the most vertically oriented 2D perovskite films based on isoBAI (b) and nBAI (d) spacers.

To achieve a vertical orientation across the entire film thickness after the formation of vertically oriented crust, the growth rate needs to be fast enough to consume the precursor concentration build-up due to solvent removal. Using different solvents and organic spacers will influence these processes through different solvent removal rate, growth reaction rate and precursor diffusion rate. In addition, the different solvents and organic spacers will alter other parameters such as solubility, surface energy, etc. Therefore, we sought to test the generality of the proposed crystallization mechanism with various solvents and organic spacers while investigating any differences. We chose 3 solvents which have different solvent evaporation rates (Vevap, as labelled on the material safety data sheets, evaporation rate of n-butylacetate reference is set as 1), gamma-butyrolactone (gBL, Vevap = 0.03), dimethylacetamide (DMAc, Vevap = 0.17) and 2-methoxyethanol (2ME, Vevap = 0.5). Two organic spacers, n-butylammonium iodide (nBAI) and isobutylammonium iodide (isoBAI), were studied26, 46. Thin films were fabricated at different solvent removal rates as described



above and GIWAXS was used to characterize the degree of vertical orientation. As shown in Figure 2, for all organic spacer and solvent combinations, conditions with faster solvent removal rate result in a lower degree of preferential vertical orientation. This shows the generality of our proposed crystallization mechanism across different solvents and organic spacers. The GIWAXS patterns for each condition are shown in Figure S7-8. The patterns of the most strongly oriented thin films with isoBAI and nBAI based 2D perovskites are shown in Figure 2b and d. In the literature, antisolvent dripping method has been demonstrated to yield smooth and uniform thin films, which is one of the most popular methods for fabricating 3D perovskite based devices4, 47 and often in 2D perovskites8-9, 48. However, our results with the fastest drying condition indicate that it is the least effective method to achieve a vertical orientation, with degree of orientation less than 20% in all organic spacers and solvents tested. This suggests that much higher performance in device applications is possible with a more vertically oriented thin film. Our results show that isoBAI consistently has a stronger vertical orientation forming ability compared to nBAI, with the latter being the most commonly used in literature. Out of the 12 crystallizing conditions, isoBAI has 6 conditions toward the slower solvent removal regime that achieve an almost complete vertical orientation, with degree of orientation higher than 95%, while nBAI only has 3 conditions that do so. The stronger orientation formation ability of isoBAI allows fabrication of near-completely oriented thin films with new solvent 2ME with faster solvent removal rate, which could not be achieved with nBAI. This opens up new processing windows for fabricating 2D perovskite thin films with different film morphology, thickness or special synthesis conditions that can ultimately benefit manufacturing of 2D perovskite devices.

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supersaturation is then consumed by crystal growth, which involves solute diffusion to the solid-liquid boundary layer and reaction to form the crystal. One possible explanation for the different degrees of orientation is that isoBAI has a faster crystal growth rate compared to nBAI, allowing a rapid crystal growth that effectively consumes the increasing precursor concentration to maintain a low level of supersaturation and prevent additional nucleation. Another possibility is that nBAI and isoBAI have different diffusion coefficients in the precursor solution. A higher diffusion coefficient of solutes could facilitate the diffusion-reaction process of the crystal growth, as well as alleviate any concentration gradient that could cause local high supersaturation. However, we did not find a significant difference in the diffusion coefficient of nBAI and isoBAI in the precursor solution, with diffusion coefficient measured to be 7.6×10-10 m2/s for both using nuclear magnetic resonance spectroscopy. The diffusion coefficient measurement results are shown in Figure S9. This result suggests that the crystal growth process is reaction-limited rather than diffusion limited and points to a possibility that isoBAI causes faster crystal growth reaction rate compared to nBAI. Moreover, the inter-connection between the solvent and the organic spacer will also likely influence the degree of orientation. Different interactions among molecules and at interfaces could result in various degrees of preferential orientation as well, similar to what was observed in the organic semiconductor field49. Further studies are required to determine precise mechanism that governs impact of different surface ligands on 2D perovskite crystallization. The different choices of solvents also have a strong effect on the degree of orientation, with gBL (Vevap = 0.03) producing the strongest orientation, followed by DMAc (Vevap = 0.17), and 2ME (Vevap = 0.5) producing the lowest degree of vertical orientation in general with same processing conditions. These observations indicate that solvents with slower evaporation rate generally result in a higher degree of orientation. However, there are also multiple other factors in play. Different solvents not only have different evaporation rates, they also have different solubility for the precursor chemicals, as well as different crystallization dynamics and interaction with the precursor45, 50. It is also difficult to deconvolute the effects of different processing conditions for different solvents. Regardless, our results show that solvents with a slower evaporation rate are likely to better maintain a low supersaturation needed to achieve high degree of vertical orientation.

Figure 3. Proposed crystal growth mechanism. Solvent evaporation increases the supersaturation at the crystal growth front, while crystal growth consumes the supersaturation buildup. The crystal growth is a two step process including solute diffusion to the crystal-liquid boundary layer and reaction to form the crystal. To explain the difference in degrees of orientation with isoBAI and nBAI, we proposed a crystal growth mechanism illustrated in Figure 3. After the formation of the oriented thin crust, solvent evaporation provides the supersaturation at the liquid-air interface that drives the crystal growth. The


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Figure 4. (a) Degrees of orientation in PEAI based 2D perovskites, PEA2MA3Pb4I13, with different solvent choices and drying rates. (b) GIWAXS pattern of the most vertically oriented PEA2MA3Pb4I13 film. We applied this understanding to other organic spacer systems that have previously been challenging to achieve high degree of vertical orientation with. Phenylethylammonium (PEA) has recently been shown to have less electron-phonon coupling than nBA, which alleviates non-radiative decay and leads to higher photoluminescence quantum yield (PLQY). However, it is more difficult to achieve vertical orientation with PEA spacer. Studies with vertically oriented PEA 2D lead perovskite thin films almost exclusively introduced significant amount of NH4SCN32, 51. To the best of our knowledge, strong vertical orientation in compositionally 4 layer (n = 4) 2D lead perovskite with PEA spacers without any additives has not yet been reported. Here, by applying the insights from this work on 2D perovskite crystallization, we are able to directly crystallize vertically oriented PEAI based 2D perovskite thin films without any additives. As shown in Figure 4, combinations of different solvent choice and processing conditions result in varying degrees of vertical orientation. The GIWAXS patterns are shown in Figure S10. The same trend in nBAI and isoBAI based 2D perovskite is also observed in PEAI based perovskite, where faster solvent evaporation results in less oriented thin films, with gBL generating the highest degree of orientation and 2ME generating the lowest with same processing conditions. This further demonstrates the generality of our 2D perovskite thin film crystallization mechanism. There is only one experimental condition resulting in a high degree of vertical orientation of more than 95%, less than the 3 conditions for nBAI and 6 for isoBAI, indicating the vertical orientation formation ability is the lowest for PEAI. The difficulty in forming vertical orientation with PEA partly explains the lack of previous reports with vertically oriented high layer number 2D lead perovskite based on PEA without additives. The most vertically oriented PEAI perovskite was achieved with the slowest solvent removal condition and gBL solvent, which has a 96% degree of vertical orientation, with the GIWAXS pattern shown in Figure 4b. X-ray diffraction (XRD) patterns are shown in Figure S11. Although horizontally oriented impurities are observed in GIWAXS pattern and the film morphology is not perfect for photovoltaic applications (Figure S12), this result demonstrates that there is no fundamental limit to achieve vertical orientation in PEAI based 2D perovskites and more strongly oriented thin films is possible through further optimizing the experimental parameters. 4. Device performance and stability Being able to achieve near-complete vertical orientation with the different organic spacers allows the comparison of their photovoltaic performance. The solar cell structure illustrated in Figure 5a employs a mesoporous TiO2 substrate, since the topdown crystallization mechanism of 2D perovskite allows the deposition on rough substrates30. This is confirmed by the vertical orientation of these 2D perovskites on mesoporous

substrates (Figure S13). Experimental conditions for solar cell fabrication are selected to simultaneously achieve nearcomplete vertical orientation and the best film morphology for solar cell fabrication. The chosen conditions were medium drying rate with gBL for nBAI based 2D perovskites and medium drying rate with DMAc for isoBAI based 2D perovskites. The selected fabrication methods result in highly reproducible solar cells with nBAI and isoBAI, with efficiency statistics and the best solar cell performance shown in Figure 5b and c. IsoBAI based 2D perovskite solar cells outperforms nBAI based ones, with the highest efficiency of 11.04%, mainly from having higher short circuit current (Jsc) and open circuit voltage (Voc). We attribute the higher Jsc to more light absorption in isoBAI thin films, as shown in Figure S14. The higher vertical orientation forming ability of isoBAI allows the adoption of DMAc which results in thicker isoBAI based thin films, compared to the best performing nBAI based thin films with gBL solvent (Figure S15). Generally higher Voc observed in isoBAI based devices indicates higher film quality, less electronic defects and shunting. It is also possible that the different organic spacers could affect the electronic properties of the thin film38, thus resulting in different Voc.

Figure 5. Device performance with different organic spacers. The device structure is illustrated in (a). (b) Efficiency statistics of solar cells fabricated with 2D perovskites with isoBAI or nBAI spacers. (c) J-V curves of the best performing solar cells in the statistics test. (d) Stability of devices with different organic spacers kept in ambient environment (rH < 25% ). Device stability is one of the most important parameters for perovskite solar cells, which is a major reason for investigating 2D perovskite devices. We conducted stability tests of our solar cells in ambient environment (rH < 25%) with unencapsulated devices. Au electrodes were used for longer term stability testing. The results are shown in Figure 5d. Both nBAI and isoBAI achieved long term stability over 1000 hours while maintaining more than 92% of initial efficiency, demonstrating the commercial potential of 2D perovskite photovoltaics. 5. Conclusion



In conclusion, we reveal a crystallization mechanism for vertical orientation in 2D perovskite thin films, where the formation of a strong vertical orientation requires a continuous crystal growth from the liquid-air interface, which can be provided by a low supersaturation during crystallization to promote growth and suppress nucleation. Choice of organic spacers also has a strong impact on thin film degree of orientation by changing the crystallization dynamics, with isoBA found to have the strongest orientation forming ability. 2D perovskite thin films with strong vertical orientation were fabricated with isoBA, nBA and PEA without any additives. Efficient solar cells based on 2D perovskites with minimal degradation after 1000 hours were demonstrated. 6. Experimental Section Materials and preparations Indium tin oxide (ITO) substrates (15 Ω cm−2) were purchased from Kintec. Titanium diisopropoxide bis(acetylacetonate) (Ti(acac)2OiPr2) 75 wt% in isopropanol, anhydrous 2ME, anhydrous DMAC, anhydrous gBL, anhydrous chlorobenzene, anhydrous butanol, anhydrous ethanol, anhydrous toluene, 4-tert-Butylpyridine (tBP, 96%), anhydrous acetonitrile, bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI, 99.95%) and FK 209 Co(III) TFSI salt (Co TFSI) were purchased from Sigma-Aldrich. PbI2 was purchased from TCI America. MAI, nBAI, isoBAI, PEAI, 18NR-T transparent titania paste were purchased from Greatcell Solar. Ag and Au evaporation pellets were purchased from Kurt Lesker. Ti(acac)2OiPr2 was diluted to 0.15 M with butanol and filtered with 0.2 µm filter before use. 18NR-T transparent titania paste was diluted with ethanol and stirred overnight to make the mesoporous TiO2 suspension. Perovskite precursor solutions with 4-layer composition BA2MA3Pb4I13 at 0.75 M Pb were made by dissolving MAI, PbI2, and different organic spacers including nBAI, isoBAI or PEAI in different solvents, and stirred for 2 hours to fully dissolve. 72.3 mg SpiroOMeTAD was dissolved in chlorobenzene and 28.8 µL tBP, 17.5 µL 520 mg/mL Li-TFSI / acetonitrile, 29 µL 300 mg/ml Co-TFSI / acetonitrile were added to the solution. Perovskite film fabrication The four solvent removal conditions, slow, medium, fast and fastest, all went through the same initial spincoating process where the solution was dynamically spin-casted at 2000 rpm for a certain time period which was different depending on the choice of solvents. The initial brief spincoating was terminated before crystallization, followed by treatment of four different solvent removal conditions where thin film crystallizes. In "slow" condition, the spincoating was stopped and the wet slide was left to dry being stationary at room temperature. In "medium" condition, the spincoating continued until perovskite is completely crystallized. In "fast" condition, a room temperature N2 flow of approximately 9 m/s normal to the substrate surface was introduced, in addition to the spincoating. The flow rate of N2 was provided by an airflow generator positioned 15 cm above the spinning slide and measured by an anemometer (Proster PST-TL017). In "fastest" conditions, 100

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µL of toluene was dropped onto the spinning slides to extract the solvent. All slides were annealed at 100 °C for 10 min. Solar cell fabrication and characterization ITO slides were cleaned by sequentially sonication in soap water, deionized water and ethanol, then treated with ozone plasma for 10 minutes before use. Ti(acac)2OiPr2 solution was spin-casted onto ITO slides in ambient condition at 4000 rpm for 60 seconds followed by annealing at 500 °C for 5 minutes to yield a compact TiOx layer. Mesoporous TiO2 suspension solution was statically spin-casted at 4000 rpm for 60 seconds followed by annealing at 500 °C for 30 minutes to form a mesoporous TiO2 layer. The slides were then taken into the glovebox for subsequent fabrication steps. For isoBAI based 2D perovskite, DMAc was used as solvent. For nBAI based 2D perovskites, gBL was used a solvent. For fabrication of nBAI devices, the precursor solution was spin-casted at 2000 rpm for 3 minutes, where the thin film turned dark completely. For isoBAI devices, the precursor solution was spin-casted at 2000 rpm for 2 minutes and the film turned dark completely. After crystallization, the slides were annealed at 100 °C for 10 min. After the slides cooled down, Spiro-MeOTAD was spin-casted at 4000 rpm for 60 seconds and annealed at 60 °C for 5 minutes. 50 nm Ag or Au electrode was then thermally evaporated in ultra-high vacuum ( 175 mum in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347 (6225), 967-70. 23. Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.; Zhang, X.; Dowben, P. A.; Mohammed, O. F.; Sargent, E. H.; Bakr, O. M. Solar cells. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 2015, 347 (6221), 519-22. 24. Yin, W. J.; Shi, T.; Yan, Y. Unique properties of halide perovskites as possible origins of the superior solar cell performance. Adv Mater 2014, 26 (27), 4653-8. 25. Chen, T.; Chen, W. L.; Foley, B. J.; Lee, J.; Ruff, J. P. C.; Ko, J. Y. P.; Brown, C. M.; Harriger, L. W.; Zhang, D.; Park, C.; Yoon, M.; Chang, Y. M.; Choi, J. J.; Lee, S. H. Origin of long lifetime of band-edge charge carriers in organic-inorganic lead iodide perovskites. Proc Natl Acad Sci U S A 2017, 114 (29), 7519-7524. 26. Tsai, H.; Nie, W.; Blancon, J. C.; Stoumpos, C. C.; Asadpour, R.; Harutyunyan, B.; Neukirch, A. J.; Verduzco, R.; Crochet, J. J.; Tretiak, S.; Pedesseau, L.; Even, J.; Alam, M. A.; Gupta, G.; Lou, J.; Ajayan, P. M.; Bedzyk, M. J.; Kanatzidis, M. G. High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells. Nature 2016, 536 (7616), 312-6. 27. Zhang, X.; Ren, X.; Liu, B.; Munir, R.; Zhu, X.; Yang, D.; Li, J.; Liu, Y.; Smilgies, D.-M.; Li, R.; Yang, Z.; Niu, T.; Wang, X.; Amassian, A.; Zhao, K.; Liu, S. Stable high efficiency two-dimensional perovskite solar cells via

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cesium doping. Energy & Environmental Science 2017, 10 (10), 2095-2102. 28. Grancini, G.; Roldan-Carmona, C.; Zimmermann, I.; Mosconi, E.; Lee, X.; Martineau, D.; Narbey, S.; Oswald, F.; De Angelis, F.; Graetzel, M.; Nazeeruddin, M. K. OneYear stable perovskite solar cells by 2D/3D interface engineering. Nat Commun 2017, 8, 15684. 29. Quan, L. N.; Yuan, M.; Comin, R.; Voznyy, O.; Beauregard, E. M.; Hoogland, S.; Buin, A.; Kirmani, A. R.; Zhao, K.; Amassian, A.; Kim, D. H.; Sargent, E. H. LigandStabilized Reduced-Dimensionality Perovskites. J Am Chem Soc 2016, 138 (8), 2649-55. 30. Chen, A. Z.; Shiu, M.; Ma, J. H.; Alpert, M. R.; Zhang, D.; Foley, B. J.; Smilgies, D. M.; Lee, S. H.; Choi, J. J. Origin of vertical orientation in two-dimensional metal halide perovskites and its effect on photovoltaic performance. Nat Commun 2018, 9 (1), 1336. 31. Soe, C. M. M.; Nie, W.; Stoumpos, C. C.; Tsai, H.; Blancon, J.-C.; Liu, F.; Even, J.; Marks, T. J.; Mohite, A. D.; Kanatzidis, M. G. Understanding Film Formation Morphology and Orientation in High Member 2D Ruddlesden-Popper Perovskites for High-Efficiency Solar Cells. Advanced Energy Materials 2018, 8 (1), 1700979. 32. Zhang, X.; Wu, G.; Fu, W.; Qin, M.; Yang, W.; Yan, J.; Zhang, Z.; Lu, X.; Chen, H. Orientation Regulation of Phenylethylammonium Cation Based 2D Perovskite Solar Cell with Efficiency Higher Than 11%. Advanced Energy Materials 2018, 8 (14), 1702498. 33. Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.; Karunadasa, H. I. A layered hybrid perovskite solar-cell absorber with enhanced moisture stability. Angew Chem Int Ed Engl 2014, 53 (42), 11232-5. 34. Soe, C. M. M.; Stoumpos, C. C.; Kepenekian, M.; Traore, B.; Tsai, H.; Nie, W.; Wang, B.; Katan, C.; Seshadri, R.; Mohite, A. D.; Even, J.; Marks, T. J.; Kanatzidis, M. G. New Type of 2D Perovskites with Alternating Cations in the Interlayer Space, (C(NH2)3)(CH3NH3)nPbnI3n+1: Structure, Properties, and Photovoltaic Performance. J Am Chem Soc 2017, 139 (45), 16297-16309. 35. Liao, J.-F.; Rao, H.-S.; Chen, B.-X.; Kuang, D.-B.; Su, C.-Y. Dimension engineering on cesium lead iodide for efficient and stable perovskite solar cells. Journal of Materials Chemistry A 2017, 5 (5), 2066-2072. 36. Zou, W.; Li, R.; Zhang, S.; Liu, Y.; Wang, N.; Cao, Y.; Miao, Y.; Xu, M.; Guo, Q.; Di, D.; Zhang, L.; Yi, C.; Gao, F.; Friend, R. H.; Wang, J.; Huang, W. Minimising efficiency roll-off in high-brightness perovskite lightemitting diodes. Nat Commun 2018, 9 (1), 608. 37. Milot, R. L.; Sutton, R. J.; Eperon, G. E.; Haghighirad, A. A.; Martinez Hardigree, J.; Miranda, L.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Charge-Carrier Dynamics in 2D Hybrid Metal-Halide Perovskites. Nano Lett 2016, 16 (11), 7001-7007. 38. Gong, X.; Voznyy, O.; Jain, A.; Liu, W.; Sabatini, R.; Piontkowski, Z.; Walters, G.; Bappi, G.; Nokhrin, S.; Bushuyev, O.; Yuan, M.; Comin, R.; McCamant, D.; Kelley, S. O.; Sargent, E. H. Electron-phonon interaction in efficient perovskite blue emitters. Nat Mater 2018, 17 (6), 550-556.


Page 9 of 10 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


39. Markov, l. V., World Scientific: Singapore, 2004; p 548. 40. Ha, S. T.; Su, R.; Xing, J.; Zhang, Q.; Xiong, Q. Metal halide perovskite nanomaterials: synthesis and applications. Chem Sci 2017, 8 (4), 2522-2536. 41. Billing, D. G.; Lemmerer, A. Synthesis, characterization and phase transitions in the inorganicorganic layered perovskite-type hybrids [(CnH2n+1NH3)2PbI4], n = 4, 5 and 6. Acta Crystallogr B 2007, 63 (Pt 5), 735-47. 42. Stoumpos, C. C.; Cao, D. H.; Clark, D. J.; Young, J.; Rondinelli, J. M.; Jang, J. I.; Hupp, J. T.; Kanatzidis, M. G. Ruddlesden–Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors. Chemistry of Materials 2016, 28 (8), 2852-2867. 43. Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and nearinfrared photoluminescent properties. Inorg Chem 2013, 52 (15), 9019-38. 44. Liu, J.; Leng, J.; Wu, K.; Zhang, J.; Jin, S. Observation of Internal Photoinduced Electron and Hole Separation in Hybrid Two-Dimentional Perovskite Films. J Am Chem Soc 2017, 139 (4), 1432-1435. 45. Quintero-Bermudez, R.; Gold-Parker, A.; Proppe, A. H.; Munir, R.; Yang, Z.; Kelley, S. O.; Amassian, A.; Toney, M. F.; Sargent, E. H. Compositional and orientational control in metal halide perovskites of reduced dimensionality. Nat Mater 2018, 17 (10), 900-907. 46. Chen, Y.; Sun, Y.; Peng, J.; Zhang, W.; Su, X.; Zheng, K.; Pullerits, T.; Liang, Z. Tailoring Organic Cation of 2D Air-Stable Organometal Halide Perovskites for Highly Efficient Planar Solar Cells. Advanced Energy Materials 2017, 7 (18), 1700162. 47. Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent engineering for highperformance inorganic-organic hybrid perovskite solar cells. Nat Mater 2014, 13 (9), 897-903. 48. Yang, X.; Zhang, X.; Deng, J.; Chu, Z.; Jiang, Q.; Meng, J.; Wang, P.; Zhang, L.; Yin, Z.; You, J. Efficient green light-emitting diodes based on quasi-two-dimensional composition and phase engineered perovskite with surface passivation. Nat Commun 2018, 9 (1), 570. 49. Hiszpanski, A. M.; Khlyabich, P. P.; Loo, Y.-L. Tuning kinetic competitions to traverse the rich structural space of organic semiconductor thin films. MRS Communications 2015, 5 (03), 407-421. 50. Foley, B. J.; Girard, J.; Sorenson, B. A.; Chen, A. Z.; Niezgoda, J. S.; Alpert, M. R.; Harper, A. F.; Smilgies, D. M.; Clancy, P.; Saidi, W. A.; Choi, J. J. Controlling nucleation, growth, and orientation of metal halide perovskite thin films with rationally selected additives. Journal of Materials Chemistry A 2017, 5 (1), 113-123. 51. Zhang, X.; Wu, G.; Yang, S.; Fu, W.; Zhang, Z.; Chen, C.; Liu, W.; Yan, J.; Yang, W.; Chen, H. Vertically Oriented 2D Layered Perovskite Solar Cells with Enhanced Efficiency and Good Stability. Small 2017, 13 (33), 1700611.

52. Jiang, Z. GIXSGUI: a MATLAB toolbox for grazing-incidence X-ray scattering data visualization and reduction, and indexing of buried three-dimensional periodic nanostructured films. Journal of Applied Crystallography 2015, 48 (3), 917-926.



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Understanding the Formation of Vertical Orientation in Two

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