Dynamical Transformation of 2D Perovskites with Alternating Cations

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Dynamical Transformation of 2D Perovskites with Alternating Cations in the Interlayer Space for High-Performance Photovoltaics Yalan Zhang, Peijun Wang, Ming-Chun Tang, Dounya Barrit, Weijun Ke, Junxue Liu, Tao Luo, Yucheng Liu, Tianqi Niu, Detlef-M. Smilgies, Zhou Yang, Zhike Liu, Shengye Jin, Mercouri G. Kanatzidis, Aram Amassian, Shengzhong (Frank) Liu, and Kui Zhao J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Journal of the American Chemical Society

Dynamical Transformation of 2D Perovskites with Alternating Cations in the Interlayer Space for High-Performance Photovoltaics Yalan Zhang, Peijun Wang, Ming-Chun Tang, Dounya Barrit, Weijun Ke, Junxue Liu, Tao Luo, Yucheng Liu, Tianqi Niu, Detlef-M. Smilgies, Zhou Yang, Zhike Liu, Shengye Jin, Mercouri G. Kanatzidis,* Aram Amassian,* Shengzhong (Frank) Liu,* Kui Zhao* Y. Zhang, T. Luo, Y. Liu, T. Niu, Prof. Z. Yang, Prof. Z. Liu, Prof. K. Zhao, Prof. S. (F.) Liu Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China. Email: [email protected] M.-C. Tang, D. Barrit, Prof. A. Amassian King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC) and Physical Science and Engineering Division (PSE), Thuwal 23955-6900, Saudi Arabia. Prof. A. Amassian Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC, 27695, USA. Email: [email protected] Dr. W. Ke, Prof. M. G. Kanatzidis Department of Chemistry, Northwestern University, Evanston, IL 60208, USA. Email: [email protected] Dr. D.-M. Smilgies Cornell High Energy Synchrotron Source, Cornell University, Ithaca, NY 14850, USA. P. Wang, Dr. J. Liu, Prof. S. Jin, Prof. S. (F.) Liu Dalian National Laboratory for Clean Energy; iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China. Email: [email protected] KEYWORDS: ACI perovskites, Phase transition, in-situ GIWAXS, Solar cells, High-performance. ABSTRACT: The two-dimensional (2D) perovskites stabilized by alternating cations in the interlayer space (ACI), define a new type of structure with different physical properties than the more common Ruddlesden-Popper (RP) counterparts. However, there is a lack of understanding of material crystallization in films and its influence on the morphological/optoelectronic properties and the final photovoltaic devices. Herein, we undertake in situ studies of the solidification process for ACI 2D perovskite (GA)(MA)nPbnI3n+1 (=3) from ink to solid-state semiconductor, using solvent mixture of DMSO:DMF (1:10 v/v) as solvent, and link this behavior to solar cell devices. The in situ grazing-incidence x-ray scattering (GIWAXS) analysis reveals a complex journey through disordered sol-gel precursors−intermediate phases−and ultimately to ACI perovskites. The intermediate phases, including a crystalline solvate compound and the 2D GA2PbI4 perovskite, provide a scaffold for the growth of the ACI perovskites during thermal annealing. We identify 2D GA2PbI4 to be the key intermediate phase, which is strongly influenced by the deposition technique and determines the formation of the 1D GAPbI3 byproducts and the distribution of various n phases of ACI perovskites in the final films. We also confirm the presence of internal charge transfer between different n phases through transient absorption spectroscopy. The high quality ACI perovskite films deposited from solvent mixture of DMSO:DMF (1:10 v/v) deliver a record power conversion efficiency of 14.7% in planar solar cells, and significantly enhanced long-term stability of devices in contrast to the 3D MAPbI3 counterpart.

Introduction

Metal halide perovskites have attracted tremendous interest in

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properties, and the final photovoltaic performance. Here, we focus on the ACI perovskite (GA)(MA)nPbnI3n+1 (=3, denoted as ACI). We applied so-called anti-solvent engineering and hot-casting techniques to control the kinetic transformation from disordered sol-gel colloidal precursor to the crystalline phase and investigated how this dynamical process influences the resulting thin film and its photovoltaic performance. The use of in situ GIWAXS revealed the formation of the intermediate phases, including an intermediate crystalline solvate and the 2D GA2PbI4 perovskite, followed by transformation to the desirable ACI 2D perovskites. A schematic model is proposed to illustrate how the ACI perovskites are formed from DMSO/DMF solvent mixture. The distribution of the various n phases and internal charge transfer were probed by transient absorption spectroscopy. The influences of dynamical transformation on morphological and optoelectronic properties were further investigated. Finally, we achieved a high power conversion efficiency (PCE) of 14.7% for solar cell based on high-quality ACI perovskite film, highlighting the importance of temporally controlling the intermediate phases for high quality thin films and high performance photovoltaic devices.

recent years because of their remarkable optoelectronic properties, including high optical absorption,1,2 long carrier lifetime,3 and large carrier mobility,4 and defect tolerance despite being amenable to solution processing. This unique combination of properties enables perovskite materials to be excellent candidates in next-generation thin film photovoltaics (PVs),2,5 light-emitting devices,6 lasers, and highly sensitive photodetectors.7-10 Two-dimensional (2D) perovskites, which can be derived from the 3-dimensional (3D) ABX3 structural framework, feature a remarkable structural flexibility and tunability.11-14 This provides a fertile “playground” for the preparation of structures with desirable physical and optoelectronic properties. The 2D structures can be divided into several categories, based on the different cuts from the 3D framework, including the ⟨100⟩-, ⟨110⟩-, and ⟨111⟩-oriented families as well as more exotic possibilities.15,16 Among the ⟨100⟩oriented families, the Ruddlesden-Popper (RP) organicinorganic halide perovskites are by far the most common structural type and have attracted attention as a means to address long-term stability for perovskite solar cells.17-20 Recently, other ⟨100⟩-oriented types, including the Dion−Jacobson (DJ)21-23 have also emerged in the halide perovskite family. In pioneering work, hybrid 2D lead iodide perovskites with two different alternating cations in the interlayer space (ACI) —derived from the oxide perovskites— have recently become another focus of interest due to their unprecedented structure.24 This new type of 2D structure, described by the formula (GA)(MA)nPbnI3n+1 (GA = guanidinium, MA = methylammonium), is stabilized by the ordering of two different small cations (GA and MA) in the interlayer space, which results in closer [PbnI3n+1] slab contacts and different physical properties in comparison to the RP perovskites.24 Importantly, these ACI perovskites exhibit potential as light-harvesting materials for solar cells, with a power conversion efficiency (PCE) of ~7.3% and a high opencircuit voltage of ~1 V recorded for (GA)(MA)nPbnI3n+1 (n = 3) based solar cells.24 For the RP perovskites, much has been achieved towards forming a detailed understanding of the underlying kinetics of film formation and its impact on the solid outcome and device performance.17,25-27 For example, previously we identified the key role played by the intermediate solvate during phase conversion in determining the orientation and thickness distribution of quantum wells using time-resolved grazingincidence wide angle x-ray scattering (GIWAXS).26 The choice of large cation and film deposition method was also reported to account for the variations in the thickness distribution of quantum wells.27 Guided by this understanding, the PCE of the RP perovskite-based solar cells has been propelled to the current record of 15.4%.28 For the hybrid DJ perovskite, the influence of crystal structure on the optical and electronic properties was demonstrated, with the PCE reaching 7.32%.23 For the hybrid ACI perovskites, there is not yet a detailed understanding of the film formation or how to control the quality of the thin film in terms of microstructure and morphology. Meanwhile, the mechanism by which these ACI perovskites assemble remains to be understood because the deposition technique, the solvent used and dynamical transformation can have important consequences in the phase distribution, the energy landscape homogeneity, optoelectronic

Results and discussion Morphology and texture The hybrid ACI 2D perovskite (GA)(MA)nPbnI3n+1 ( = 3, n was set by the ratio of precursors used for a given solution) films were fabricated from a solvent mixture of DMSO:DMF (1:10 v/v), followed by thermal annealing at 80 °C for 15 min. Briefly, the spin coating was performed using anti-solvent (chlorobenzene) engineering or hot-casting techniques (see details in Experimental section). For comparison, a film fabricated without anti-solvent engineering or hot-casting was also prepared and is defined here as the control film. We first examined the role of the deposition technique on the nature of solid film. Figure 1a-c shows scanning electron microscopy (SEM) images of the control, anti-solventprocessed and hot-cast films. We observed elongated grains with micrometer length in all ACI films, distinct from the 3D MAPbI3 polycrystalline film, where granular grains with sizes of hundreds of nanometers are densely packed (Figure S1).29,30 Pinholes were observed in the control film (Figure 1a), which makes it challenging to fabricate high-performance planar solar cells. The anti-solvent-processed film shows substantially improved coverage and uniformity in comparison to the control one (Figure 1b). The hot-cast film looks less reflective in contrast to the other two, suggesting poor uniformity. Indeed, we observed phase segregation consisting of long micrometer length rods and large grains of several micrometers in the hot-cast film (Figure 1c). The rod-like morphology was recently observed in a presumed 3D “MA1– xGAxPbI3” film containing ˃25% GA and ascribed to the 1D GAPbI3 perovskite byproduct.31 The phase segregation is attributed to incompatibility between 1D and 2D packing of perovskites. Note that such phase segregation was not observed for the RP perovskite films fabricated using the hotcasting technique.17,19 This hints at a specific nature of the crystallization mechanism for the ACI perovskite, which is distinct from what has been reported for the RP or 3D perovskites,30,32,33 and will be discussed in the next section.


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(a)

(b)

(c)

1 μm

1 μm

1 μm

(d)

(f)

(e)

ACI perovskite c-TiO2 FTO

25

(h) )

20

qz (nm

-1

(020)

10

15

10

10

5

5

5 -25 -20 -15 -10-1 -5 0 qxy (nm )

Intensity (a.u.)

(020)

5

Control Anti-solvent Hot-cast

10 15 q (nm-1)

20

(k)

-25 -20 -15 -10-1 -5 0 qxy (nm )

-25 -20 -15 -10-1 -5 0 qxy (nm ) Control Anti-solvent Hot-cast

900

Intensity (a.u.)

(j)

(i)

15

qz (nm

15

20

-1

)

20

25

(g)

-1

)

25

qz (nm

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


800

(l) I H N C Pb

700

b

600 500

100 120 140 160 Azimuth (degrees)

c

Figure 1. (a-c) Top-view and (d-f) cross-sectional SEM images of the control, anti-solvent-processed (chlorobenzene), and hot-cast perovskite films, respectively. (g-i) Ex-situ GIWAXS patterns of the control, anti-solvent-processed, and hot-cast films, respectively. (j) The corresponding intensity versus q for the diffraction features of the three films. (k) The azimuth angle of the (020) diffraction, showing the differences in crystal orientation of the three films. (l) Crystal structure of the n = 3 ACI 2D perovskite (GA)(MA)3Pb3I10. The cross-sectional SEM shows small domains and rough surface in the control film (Figure 1d). The anti-solventprocessed case nonetheless exhibits well-connected grains and seamless grain boundaries in the perpendicular direction (Figure 1e), all of which are favorable for charge transport. By contrast, the hot-cast film indicates large holes at the perovskite-TiO2 contact, and consequently, poor compactness and uniformity (Figure 1f). The GIWAXS data also shows

that the orientation of the 2D crystallites relative to the substrates is far more random than reported for 2D RP perovskite films.17,26 These observations further point to a specific crystallization mechanism for the ACI perovskite that is highly influenced by the deposition techniques. The highquality anti-solvent-processed film is therefore highly desirable for achieving solar cells with high PCEs. The textures of the ACI perovskite films were investigated



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favored in the out-of-plane direction which is primarily expressed in the anti-solvent-processed sample. We illustrate the crystal structure in Figure 1l for the desired n = 3 ACI perovskite (GA)(MA)3Pb3I10. The GA and MA ordering leads to a doubling of the basic perovskite unit cell.24 Note that the crystal distortions originated from the GA cations are stabilized at the neighboring cavities where 2 MA cations localize, preserving the layered structure.

using GIWAXS measurements. The 2D GIWAXS patterns for the control, anti-solvent-processed, and hot-cast films are shown in Figure 1g-i, respectively. The intensity vs. q curves (integrated over all angles) are plotted in Figure 1j. We observe three dominant scattering rings appearing at q~10.1010.30, ~17.27-17.70 and ~19.80-20.40 nm-1 representing the ACI perovskite phase with orthorhombic symmetry (Figure 1g-i).24,34 We note the discrete Bragg spots at q = 8.44 and 6.29 nm-1 for the hot-cast film (Figure 1i). This diffraction feature was previously observed in the “MA1-xGAxPbI3” film31 and GAPbI3 film,35 and was intimately linked to the 1D GAPbI3 phase. The observed X-ray features are in line with the morphological observation (Figure 1c), supporting the conclusion of phase segregation to layered ACI, 1D GAPbI3 and possibly 3D MAPbI3 phases. The perovskite orientation with respect to the substrate was further analyzed. Figure 1k plots the azimuth angular distribution of the (020) diffraction at q = ~10.10-10.30 nm-1. The control film shows a broad orientation distribution of the (020) diffraction with a weak peak at 135°, indicating random orientation of the ACI perovskite crystals. The anti-solventprocessed film exhibits a dominant peak at 90°, a signature of out-of-plane orientation with respect to the substrate. In contrast, two dominant peaks appear at 108° and 135° for the hot-cast film, indicating two dominant crystalline orientations. One can therefore expect that charge transport should be

Crystallization mechanism Next we sought to probe the mechanism of the kinetics transformation during film deposition by investigating the structural evolution in situ using time-resolved GIWAXS.30,32,33,36-38 Figure 2a-c illustrates the time evolutions of the diffraction features vs. scattering vector q and time for the control, anti-solvent-processed, and hot-cast samples, respectively. Representative 2D GIWAXS snapshots taken at the end of spin coating are illustrated in Figure 2d-f. We observed a scattering halo at low q values (~2-7 nm-1) when spin-coating starts, which corresponds to the disordered colloidal sol-gel precursor and was also previously observed for MAPbI3 perovskite and PbI2.30,32,36 As the solvent evaporates, the scattering halo at low q dynamically vanishes, giving way to the formation of intermediate phases made up of PbI2, GA2PbI4, and a precursor-solvent crystalline solvate.

Figure 2. (a-c) In situ GIWAXS analysis showing the dynamical transformation from disordered precursor to perovskite for the control, anti-solvent-processed, and hot-cast samples, respectively. (d-f) 2D GIWAXS snapshots taken at the end of spin coating showing different textures for the control, anti-solvent (chlorobenzene)-processed, and hot-cast films, respectively. The red


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horizontal line is caused by the detector. (g) and (h) The corresponding intensity versus q for the diffraction features of the MAPbI3, control, anti-solvent-processed, and hot-cast films. (i-k) Schematic model illustrating the self-assembly of the ACI perovskite. with the aid of a schematic model (Figure 2i-k). The observation of the intermediate solvate and the perovskite phases during film formation is attributed to the intricate interactions between the solvent, lead halide, and organic components. Lead halide (PbI2 in this report) prefers to form Lewis adducts with solvents with Lewis base groups,39 which competes with its interaction with organic cations (MA+ and GA+ in this report). The intricate interactions between the three components lead to the formation of the intermediate solvate (Figure 2j) and perovskite phase (Figure 2k). After deposition, the intermediate phases disintegrate partially during thermal annealing to provide a framework for the nucleation and growth of the ACI perovskite (inside the red rectangle in Figure 2k).26,32,40 Faster solvent extraction leads to stronger interaction between lead iodide and organic cations, and therefore preferential assembly of the crystalline perovskite phase, including the GA2PbI4 phase (inside the dark rectangle in Figure 2k), which is highly magnified in the hotcast case. Correlating changes in the kinetics with the film morphology suggests that the presence of pinholes in the control film is ascribed to the formation of PbI2 during spin coating, as demonstrated previously for the 3D MAPbI3 case.36 Meanwhile, the appearance of the 1D GAPbI3 phase in the hot-cast film is attributed to the preferential crystallization of the 2D GA2PbI4 during film formation, which then interacts with MA+ and PbI2 during thermal annealing to form the ACI perovskite and the GAPbI3 byproduct.

The three ACI perovskite samples exhibit distinct dynamics of structural evolution from disordered colloidal sol-gel to perovskite phase. The control film case exhibits solidification at ca. 60 s, along with the appearance of textured phases (Figure 2a). The scattering features located at q = 4.69, 5.13, 6.55 and 8.41 nm-1 correspond to the precursor-solvent intermediate crystalline solvate.30,33,36 The weak features at q = 9.20, 9.53 and 10.32 nm-1 are assigned to PbI2,33 the (110)oriented 2D GA2PbI435 and the ACI perovskite,24 respectively. The faster solvent extraction decreases the duration of the transformation from disordered sol-gel precursor to textured phases to about 40 s and 22 s for the anti-solvent-processed and hot-cast films (Figure 2b, 2c), respectively. Faster solvent extraction via anti-solvent dripping or hot-casting suppresses the formation of PbI2, and results in an unexpected enhancement in the intensity of the intermediate solvate in stark contrast to the 3D MAPbI3 case.33,36,37 In contrast to the control film, in the anti-solvent-processed case we observed the ACI and the 2D GA2PbI4 phases. To our surprise, in the hot-cast case, the GA2PbI4 phase dominates and the ACI phase is nearly absent, indicating the GA+ is playing a critical role in the interaction between organic cations and lead halide during film formation. These dynamical observations indicate that the ACI phase formation depends on deposition conditions and is a complex journey through disordered colloidal solgel−intermediate phases−and ultimately to the ACI perovskites. The intermediate phases, including crystalline solvate and the 2D GA2PbI4, provide a scaffold for the growth of the desirable ACI perovskite during thermal annealing. This material-related structural evolution highlights the specific nature of the crystallization mechanism, which appears to be distinct from those of the 3D and the 2D RP cases. The representative 2D GIWAXS snapshots taken at the end of spin coating contribute to the understanding of the crystallization mechanism of the ACI perovskite (Figure 2df). The control film exhibits diffraction rings with stronger intensities along certain segments for the intermediate solvate and the ACI perovskite (Figure 2d), indicating orientation randomness to some extent. In contrast, sharp and discrete Bragg spots were observed for the 2D GA2PbI4 structure (Figure 2d, 2f), which is highly prominent for the hot-cast film. We further plotted the intensity vs. q of the three GIWAXS snapshots in Figure 2g and 2h for the intermediate solvate and perovskite phases, respectively. These plots reveal a diffraction shift from 4.49 to 4.71 nm-1 for the intermediate solvate in the control sample with respect to the 3D MAPbI3 counterpart (Figure 2g). The shift to higher q value suggests a unit cell contraction, which may originate from less solvent trapped inside the solvated phase, or a coexistence of two different cations, such as MA+ and GA+. The diffraction peak of the intermediate solvate continuously shifts to 4.80 and 4.85 nm-1 for the anti-solvent-processed and hot-cast films, respectively. The diffraction of the 2D GA2PbI4 phase remains at q = 9.53 nm-1 for the three cases. Interestingly, we also observed a continuous diffraction shift to higher q for the ACI structure with a faster solvent extraction (Figure 2h), which may originate from the distortion of the Pb-I inorganic framework due to the GA+ incorporation. The crystallization mechanisms deduced here are illustrated

Photophysical properties Our series of photophysical measurements indicate process dependent kinetics-controlled variations in the distribution of the various n phases and the presence of internal photoexcited charge transfer. Figure 3a shows the normalized absorption spectra. The anti-solvent-processed and hot-cast films exhibit an appreciably enhanced absorption in the red part of the spectrum compared to the control film. The band edge determined from Tauc plots (Figure S2) red-shifts from 1.83 to 1.75 and 1.61 eV for the control, anti-solvent-processed and hot-cast films, respectively. Additional excitonic peaks typically assigned to n = 1-3 were also observed.24 The valence band maxima (VBM) in the materials were measured experimentally using photoelectron spectroscopy in air and were determined to be 5.50, 5.66, and 5.54 eV for the control, anti-solvent-processed and hot-cast films (Figure S3a-b), with the conduction band minimum (CBM) of 3.67, 3.91, and 3.93 eV, respectively. Apparently, the control film has largest energy mismatch for electron extraction in comparison to the other two cases (Figure S3d). The photoluminescence (PL) spectra for the control, antisolvent-processed, and hot-cast films exhibit a red-shift from 751 to 756 and 766 nm (Figure 3b), respectively. The redshifts observed in both absorption and emission spectra imply the formation of narrower bandgaps or higher n values of the 2D ACI perovskites for the anti-solvent-processed and hotcast films compared to the control one, and the difference is more prominent for the hot-cast film, with the band edge and PL emission peak position close to 3D MAPbI3.3 The presence of different n values is further reflected by the variation of charge-carrier lifetimes. The lifetime values determined from



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whereas the shorter lifetimes and larger variations for the other two cases are possibly due to lower uniformity of the films or/and the possible type-II band alignment of the GAPbI3 byproduct.41

the time-resolved PL (TRPL) spectroscopy (Figure 3c) are 11.3 ± 2.3, 36.9 ± 1.5 and 15.8 ± 5.6 ns for the control, antisolvent-processed and hot-cast films, respectively. The longer lifetime for the anti-solvent-processed case indicates fewer traps, which is expected to yield slower recombination,

Figure 3. (a) UV-Vis absorption spectra (b) steady-state PL spectra and (c) TRPL spectra of the control, anti-solvent-processed, and hot-cast films were measured from film side. (d-f) Transient absorption (TA) spectra at different delay times for the three films under back-side excitation. (g) TA kinetics probed at n =1, 2, 3 and n ≈ ∞ bands under back-side excitation for the representative anti-solvent-processed film. (h) Schematic model showing internal electron transfer from low-n to high-n phases. Schematic energy diagram of different pure n phases as determined in accordance with previous reports.24 We further performed ultrafast transient absorption (TA) spectroscopy to probe the distribution of the various n phases for the three types of ACI films. The films were excited with a femtosecond laser pulse, and the photo-induced changes in the absorption (ΔA) spectra were probed with a time-delayed laser-generated white light probe pulse.42,43 Figure 3d-f illustrates the TA spectra at various delay times for the control, anti-solvent-processed, and hot-cast films, respectively. The bleached excitonic transitions lead to negative features, while the positive features are due to blueshifting of the exciton peak, which is caused by hot carriers and body effects at early delay times.44,45 The control film

features dominant bleach peaks for low-n (n = 1-3) phases with a vanishingly small amplitude for the = ∞ (between 710-750 nm). The bleach peak for the = ∞ (at ~730 nm) is slightly enhanced for the anti-solvent-processed film, whereas it dominates all perovskite phases for the hot-cast film (at ~750 nm). We further fitted this exciton bleach peak and found a larger full-width-at-half-maximum (FWHM) value for the anti-solvent-processed film compared to the hot-cast one. This indicates the formation of hierarchical phases from low-n to high-n for the anti-solvent-processed film. By contrast, the hot-cast film mainly consists of 3D MAPbI3 and n = 1-3 phases.


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Journal of the American Chemical Society uniformity and non-hierarchical phases in the hot-cast film.

Previous work on films found a spontaneous charge transfer between phases with different n values for the RP perovskites.42-44 Here we also confirm the presence of charge transfer in the phase-mixed ACI perovskite film. Figure 3g illustrates the dynamical evolution of the bleaching recovery for the representative anti-solvent-processed film (see Figure S4 for the control and hot-cast films). The bleaching for the n = 1 exciton resonance appears instantaneously at ca. 0.1 ps, followed by an ultra-fast decay at ca. 0.5 ps. We also observed a similar fast buildup for the n = 2-3, but with a slower decay than for the n = 1. Notably, the bleach recovery kinetics for low-n is accompanied by the formation of the n = ∞ bleaching. The buildup of the n = ∞ bleaching is significantly delayed to ca.1 ns, closely matching the decay onset for the n = 3 phase. We speculate that this evolution in the TA spectra and kinetics is due to the consecutive electron transfer from low-n to highn phases, which is principally accompanied by hole transfer in the opposite direction, as shown in Figure 3h. This result is analogous to the previous observations for the films of phasemixed RP perovskites.42-44 Meanwhile, the hot-cast film exhibits an obviously delayed buildup for the n=∞ bleaching signature with respect to the control and anti-solventprocessed cases, which is possibly attributed to lower

Electronic properties and photovoltaic performance The trap state density and charge mobility in the vertical direction of the ACI films were evaluated to assess the key impact of morphological changes. The electron-only devices were fabricated with the architecture FTO/cTiO2/perovskite/phenyl-C61-butyric acid methyl ester (PCBM)/Ag, as shown in Figure 4a (top). The dark I-V characteristics are shown in Figure 4b for the representative anti-solvent-processed sample and in Figure S5a,b for the control and hot-cast films, respectively. The electron mobilities were estimated to be 0.05 ± 0.01, 0.63 ± 0.02, and 0.09 ± 0.07 cm2V-1s-1 for the control, anti-solvent-processed and hot-cast films, respectively. The corresponding trap state densities were estimated to be 12.9 ± 1.3, 0.7 ± 0.3, and 4.6 ± 3.2 × 1016 cm-3 (Figure 4c). Although overall these are low mobilities we can see that comparatively better carrier mobility and lower trap state density are achieved in the antisolvent-processed film than the two other films, which is expected to benefit charge collection in a complete solar cell device.



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Figure 4. (a) Electron-only device architecture (top) and solar cell device architecture (bottom). (b) Dark current-voltage curve of the electron-only device for the representative anti-solvent-processed (chlorobenzene) sample. (c) Trap state density and electron mobility for the three ACI films. (d) PCE histograms for the three ACI devices. (e) J–V curves of the three champion devices. (f) The stabilized power outputs and current densities of the champion devices measured at a fixed maximum power point (MPP) voltage as a function of time. (g) The slopes of the normalized J-V curves at each voltage for the three devices. (h) The influences of the drip time for the anti-solvent-processed case (top) and the preheated stage temperature for the hot-cast case (bottom) on photovoltaic performance. (i) J–V curves and PCEs of the anti-solvent-processed cell before and after 240 days ambient exposure (~30-40% RH, ~25 °C, dark) without encapsulation. Inset showing photographic photos of the anti-solvent-processed film before and after 240 days ambient exposure. Table 1. Summary of the photovoltaic parameters of the devices based on the three ACI films. Parameters Samples Control Anti-solvent Hot casting

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

average

1.08±0.01

17.3±0.32

63.3±0.91

12.5±0.19

max

1.09

17.9

66.1

12.91

average

1.13±0.01

18.0±0.38

65.6±1.01

14.1±0.21

max

1.15

18.8

67.8

14.69

average

0.97±0.02

19.0±0.55

63.2±1.90

12.5±0.78

max

1.00

20.7

66.3

13.87

uniformity, the presence of low-n phases including 2D ACI perovskite and 1D GAPbI3 byproducts which lead to a high recombination loss. Furthermore, the morphological changes may lead to variations in the charge collection within the cells, which can be experimentally verified by performing a detailed analysis of the J-V curves (Figure 4g and Figure S6b).36,46,47 The slopes of the light J-V characteristics as a function of applied voltage exhibit mild S-shaped curves near the Voc for the control and hot-cast devices, indicating the presence of an energetic barrier for charge collection, as reported previously.46,47 This would cause an energy loss of carriers through recombination and reduces charge collection efficiency at low fields. The anti-solvent-processed case, with a lower energy barrier, exhibits a charge collection efficiency of 8.6 at Voc, higher than the control (5.3) and hot-cast (6.7) cases. The present study also found that the drip time for the antisolvent-processed case and the stage temperature for the hotcast case led to variations in photovoltaic performance (Figure 4h). The variations in the solution deposition hold the key to controlling the kinetics of film formation, which ultimately enables the tunability of the formation of the intermediate solvate, PbI2, (GA)(MA)nPbnI3n+1 ACI and GA2PbI4 perovskite phases. Different crystallization mechanisms in turn lead to the variations of the morphological properties and the final photovoltaic performance. Finally, we demonstrated an excellent ambient stability of the ACI perovskite solar cells. Figure 4i shows the J-V curves and PCEs of non-encapsulated champion cell before and after 240 days ambient exposure (humidity and temperature of ~30-40% RH and 25 ºC, respectively) in the dark. The aged ACI perovskite solar cell retains 88% of its initial PCE (14.69% vs. 12.95%), which significantly surpasses the 3D MAPbI3 one under the same condition (Figure S7a). In other word, the incorporation of GA+ into MA-based 3D framework could enhance the crystal stability (Figure S7b), which is analogous with the previous observation for a 3D MA1–xGAxPbI3 film and ascribed to the decreased formation enthalpy of perovskite structures.31 As expected, the heat stability of the ACI 2D perovskite also outperforms the 3D MAPbI3 one under the same condition (Figure S8).

The ACI perovskite films were incorporated into planar solar cells with the structure of FTO/cTiO2/perovskite/2,2’,7,7’ -tetrakis-(N,N-di-pmethoxyphenylamine)-9,9’-spirobiﬂuorene (spiroOMeTAD)/Au (Figure 4a, bottom). The average and champion photovoltaic figures of merit for the three devices are listed in Table 1. The PCE distributions of the devices are compared in Figure 4d. We achieved PCEs of 14.1 ± 0.21% for the anti-solvent-processed devices, higher than 12.5 ± 0.19% for the control and 12.5 ± 0.78% for the hot-cast cases. Note that these PCEs are significantly higher than the previous record of ~7.3% for ACI-type perovskite solar cells.24,25 The higher PCEs obtained for the anti-solvent-processed devices are likely the result of greater uniformity and lower recombination losses. The larger PCE distribution for the hotcast device is possibly correlated with the presence of phase separation of the 1D GAPbI3 from the 2D ACI perovskites. The highest PCE reaches 14.69% with a short-circuit current density (Jsc) of 18.8 mA cm−2, an open-circuit voltage (Voc) of 1.15 V, and a fill factor (FF) of 67.8% for the anti-solventprocessed sample (Figure 4e), while PCEmaxs of 12.91% and 13.87% were obtained for the control and hot-cast samples, respectively. External quantum efficiency (EQE) spectra for the three devices are illustrated in Figure S6a, with negligible mismatch between the integrated current and the Jsc. Figure 4f illustrates the stabilized power output measured under ambient conditions (25 °C; relative humidity of 40%) and continuous AM 1.5-G, 1-sun illumination at a fixed maximum power point (MPP) voltage of 0.78 V for 300 s. All devices exhibit a slow rise in the first ~10-30 s to peak values -- suggesting trapping is playing a role -- followed by an excellent photostability for the remainder of the 300 s. Correlating changes in device parameters indicates that the enhanced PCE for the anti-solvent-processed case originates mainly from the Voc improvement compared to the control and hot-cast ones. We identified key roles played by the morphological properties in photovoltaic performance. The low band edge is expected to yield a high Voc for the ACI films. However, large voltage loss is observed for both the control and the hot-cast cases, which is more prominent for the latter. The voltage loss is mainly attributed to the poor


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Journal of the American Chemical Society Steady-state photoluminescence (PL) (excitation at 510 nm, front-side excitation) and time-resolved photoluminescence (TRPL) (excitation at 510 nm and emission at 806 nm, frontside excitation) were measured with a PicoQuant FT-300. Transient Absorption Measurements. Femtosecond pumpprobe transient absorption (TA) measurements were performed at three different power densities (i.e. 30, 70 and 120 μJ/cm2). The femtosecond laser pulse was generated by a Ti:sapphire femtosecond regenerative amplifier with 800 nm wavelength and 1 kHz repetition rate (Coherence) and served as both pup and probe beams. The pump pulse with a wavelength of 400 nm and duration of 50 fs generated via a second harmonic generator (SHG) was used to excite all the samples, and the probe beam was detected by a high-speed spectrometer (HELIOS, Ultrafast Systems). The wavelength range of the detector was set from 400 to 850 nm, and the spot size of the TA is approximately 0.5 mm2 as evaluated by imaging the laser spot. All experiments were carried out at room temperature (i.e. T=300K). Electronic microscopy. The surface morphologies of the perovskite films were characterized by SEM (FE-SEM; SU8020, Hitachi) at an acceleration voltage of 3 kV and by atomic force microscope (AFM, Dimension ICON). Photoelectron Spectroscopy. The energy of the valence band maximum was measured using a Riken Keiki AC-2 photoelectron spectrometer in air (PESA) at room temperature. The measurement was carried out with UV photons emitted from a deuterium lamp which was then monochromated by a grating spectrometer and focused on the sample. Then, photoelectrons emitted from the sample were counted by an open counter. The UV light intensity was 10 nW. The measurement ranged from 4.2 to 6.8 eV with a step of 0.1 eV and a counting time of 10 s. Grazing-incidence wide angle X-ray scattering measurements. GIWAXS measurements were performed at D-line of the Cornell High Energy Synchrotron Source (CHESS). The wavelength of the X-rays was 0.972 Å with a bandwidth ∆λ/λ of 1.5%. The scattering signal was collected by a Pilatus 200K detector, with a pixel size of 172 µm by 172 µm placed 184.0066 mm away from the sample position. The incidence angle of the X-ray beam was 0.50°. Spin-coating experiments were conducted using a custom-built spin-coating stage with splashing of solvent shielded using Kapton tape and controlled from a computer outside the hutch. The exposure time was kept at 0.2 s to obtain the detailed information of the process. Ambient conditions at CHESS were approximately 23 °C and ca. 30% relative humidity. Device characterization. The J-V performance of the perovskite solar cells was analyzed using a Keithley 2400 SourceMeter under ambient conditions at room temperature, and the illumination intensity was 100 mW cm-2 (AM 1.5G Oriel solar simulator). The scan rate was 0.3 V s-1. The delay time was 10 ms, and the bias step was 0.02 V. The power output of the lamp was calibrated using an NREL-traceable KG5-filtered silicon reference cell. The device area of 0.09 cm2 was defined by a metal aperture to avoid light scattering from the metal electrode into the device during the measurement. The EQE was characterized on a QTest Station 2000ADI system (Crowntech Inc., USA), and the light source was a 300 W xenon lamp. The monochromatic light intensity for the EQE measurement was calibrated with a reference silicon photodiode.

Conclusion In summary, our study presented a significant advance in the understanding of both the dynamical transformation during film formation and the control of the morphology, distribution of the various n phases, optoelectronic properties, and final photovoltaic performance for the 2D ACI perovskites. The surprisingly complex journey through disordered sol-gel precursors−intermediate phases−and ultimately ACI perovskites demonstrates the importance of temporally controlling intermediate phases during film formation. The formation of intermediate phases is driven by intricate interactions between the lead halide, organic cations, and solvent. Moreover, it highlights the importance to control the distribution of the various n phases and optoelectronic properties via tuning the intermediate phases because of several direct kinetically arrested phases get involved. These insights will be useful towards a achieving more desirable final outcomes and increases in photovoltaic performance. Experimental section Solution preparation. The perovskite solution (1.2 M) was comprised of GAI (99.5%, Alfa Aesar.), MAI (99.5%, pOLED.) and PbI2 (99.9985%, Alfa Aesar) (1:3:3 molar ratio) in 1 mL of DMF (99.8%, Aladdin) and 100 μL of DMSO (99.9%, Aladdin) mixed solvents. Chlorobenzene (99.8%) was purchased from Sigma-Aldrich. The Spiro-OMeTAD solution was prepared by dissolving 90 mg Spiro-OMeTAD, 22 µL lithium bis(trifluoromethanesulfonyl) imide (99%, Acros Organics, 520 mg mL−1 ) in acetonitrile (99.7+%, Alfa Aesar), and 36 µL 4-tert-butylpyridine (96%, Aldrich) in 1 mL chlorobenzene. All the solutions were prepared inside a nitrogen glove box. Device fabrication. The FTO-coated glass (2.5 cm × 2.5 cm) was cleaned by sequential sonication in acetone, isopropanol, and ethanol for 30 min each and then dried under N2 flow and treated by ozone plasma for 15 min. The TiO2 was prepared by chemical bath deposition with the clean substrate immersed in a TiCl4 (CP, Sinopharm Chemical Reagent Co., Ltd) aqueous solution with the volume ratio of TiCl4:H2O equal to 0.0225:1 at 70 °C for 1 h. The spin-coating was accomplished under inert atmosphere inside a nitrogen glove box. For directspinning film preparation, the prepared solution was spincoated on FTO/TiO2 substrates at a low speed of 500 rpm for 3 s and followed by a high speed of 4000 rpm for 60 s without any anti-solvent. The substrates were then annealed at 80 °C for 15 min. For anti-solvent-processed film preparation, the asprepared solution was spin-casted on FTO/TiO2 substrates with the same speed as before. At ≈ 45 s before the last spincoating step, 300 µL of neat chlorobenzene was dropped onto the substrate. The films were then annealed at 80 °C for 15 min. For hot-cast films, the substrates were preheated at 100 °C for 10 min, and the perovskite layer was deposited by spincasting 60 μL of the as-prepared solution on the substrates at 4000 rpm for 25 s. The substrates were then annealed at 80 °C for 10 min. Subsequently, the hole transfer materials (HTMs) were deposited on the top of the perovskite by spin-coating at 4000 rpm for 15 s and followed by evaporation of a 100 nm thick gold electrode on the top of each cell. Optical metrology. UV-Visible absorption spectra were acquired on a PerkinElmer UV-Lambda 950 instrument.



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(4) Sadhanala, A., Ahmad, S., Zhao, B. D., Giesbrecht, N., Pearce, P. M., Deschler, F., Hoye, R. L., Godel, K. C., Bein, T., Docampo, P., Dutton, S. E., De Volder, M. F. and Friend, R. H. Blue-Green Color Tunable Solution Processable Organolead Chloride-Bromide Mixed Halide Perovskites for Optoelectronic Applications. Nano Lett. 2015, 15, 6095-6101.

Carrier mobility measurements. Electron-only devices (glass/FTO/c-TiO2/perovskites/phenyl-C61-butyric acid methyl ester (PCBM)/Ag) were fabricated to measure the electron mobilities of the devices. The dark J-V characteristics of the electron-only devices were measured using a Keithley 2400 SourceMeter. The mobility was extracted by fitting the J-V curves in the space-charge-limited-current (SCLC) regime with the Mott-Gurney equation. The trap state density was determined by the trap-filled limit voltage using the equation given in the supporting information.

(5) Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. and Snaith, H. J. Efficient Hybrid Solar Cells Based on MesoSuperstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. (6) Abdi-Jalebi, M., Andaji-Garmaroudi, Z., Cacovich, S., Stavrakas, C., Philippe, B., Richter, J. M., Alsari, M., Booker, E. P., Hutter, E. M., Pearson, A. J., Lilliu, S., Savenije, T. J., Rensmo, H., Divitini, G., Ducati, C., Friend, R. H. and Stranks, S. D. Maximizing and Stabilizing Luminescence from Halide Perovskites with Potassium Passivation. Nature 2018, 555, 497-501.

ASSOCIATED CONTENT Supporting Information SEM image of the MAPbI3 film, Tauc-plot of UV-Vis absorption, Photo-electron spectroscopy, TA kinetics for the control and hotcast films, Dark current-voltage measurements, External quantum efficiency (EQE), and comparison of long-term and thermal stability of devices. This material is available free of charge via the Internet at http://pubs.acs.org.

(7) Liu, Y. C., Zhang, Y. X., Yang, Z., Feng, J. S., Xu, Z., Li, Q. X., Hu, M. X., Ye, H. C., Zhang, X., Liu, M., Zhao, K. and Liu, S. Z. F. Low-Temperature-Gradient Crystallization for Multi-Inch HighQuality Perovskite Single Crystals for Record Performance Photodetectors. Materials Today 2018, 10.1016/j.mattod.2018.04.002.

AUTHOR INFORMATION

(8) Wei, W., Zhang, Y., Xu, Q., Wei, H. T., Fang, Y. J., Wang, Q., Deng, Y. H., Li, T., Gruverman, A., Cao, L. and Huang, J. S. Monolithic Integration of Hybrid Perovskite Single Crystals with Heterogenous Substrate for Highly Sensitive X-Ray Imaging. Nat. Photonics 2017, 11, 315-321.

Corresponding Author *[email protected]

(9) Liu, Y. C., Zhang, Y. X., Yang, Z., Yang, D., Ren, X. D., Pang, L. Q. and Liu, S. Z. F. Thinness- and Shape-Controlled Growth for Ultrathin Single-Crystalline Perovskite Wafers for Mass Production of Superior Photoelectronic Devices. Adv. Mater. 2016, 28, 92049209.

*[email protected] *[email protected] *[email protected]

(10) He, Y. H., Matei, L., Jung, H. J., McCall, K. M., Chen, M., Stoumpos, C. C., Liu, Z. F., Peters, J. A., Chung, D. Y., Wessels, B. W., Wasielewski, M. R., Dravid, V. P., Burger, A. and Kanatzidis, M. G. High Spectral Resolution of Gamma-Rays at Room Temperature by Perovskite CsPbBr3 Single Crystals. Nat. Commun. 2018, 9, 1609.

ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (2017YFA0204800, 2016YFA0202403), National Natural Science Foundation of China (61604092, 61674098, 91733301), DNL Cooperation Fund CAS (DNL180311), National University Research Fund (GK201802005), the 111 Project (B14041), the National 1000 Talents Plan program (1110010341), and the King Abdullah University for Science and Technology (KAUST). CHESS is supported by the NSF Award DMR-1332208. The work at Northwestern University was supported in part by the LEAP Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award DE-SC0001059 (film fabrication and characterization). PESA measurements were carried out with equipment acquired by ONR grant N00014-18-1-2102.

(11) Mao, L. L., Stoumpos, C. C. and Kanatzidis, M. G. TwoDimensional Hybrid Halide Perovskites: Principles and Promises. J. Am. Chem. Soc. 2018, 10.1021/jacs.8b10851. (12) Jiang, Y. Z., Yuan, J., Ni, Y. X., Yang, J. E., Wang, Y., Jiu, T. G., Yuan, M. J. and Chen, J. Reduced-Dimensional α-CsPbX3 Perovskites for Efficient and Stable Photovoltaics. Joule 2018, 2, 1356-1368. (13) Rong, Y. G., Hu, Y., Mei, A. Y., Tan, H. R., Saidaminov, M. I., Seok, S. I., McGehee, M. D., Sargent, E. H. and Han, H. W. Challenges for Commercializing Perovskite Solar Cells. Science 2018, 361, 1214. (14) Hu, Y., Zhang, Z. H., Mei, A., Jiang, Y. Y., Hou, X. M., Wang, Q. F., Du, K., Rong, Y. G., Zhou, Y. H., Xu, G. Z. and Han, H. W. Improved Performance of Printable Perovskite Solar Cells with Bifunctional Conjugated Organic Molecule. Adv. Mater. 2018, 30, 1705786.

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Dynamical Transformation of 2D Perovskites with Alternating Cations

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