Ligand-Size Related Dimensionality Control in Metal Halide

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Ligand-Size Related Dimensionality Control in Metal Halide Perovskites Peirui Cheng,† Peijun Wang,‡ Zhuo Xu,† Xuguang Jia,§ Qilin Wei,∥ Ningyi Yuan,§ Jianning Ding,§ Ruipeng Li,⊥ Guangtao Zhao,† Yingchun Cheng,∥ Kui Zhao,*,† and Shengzhong Frank Liu*,†,‡

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†

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, P. R. China ‡ Dalian National Laboratory for Clean Energy, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, P. R. China § School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, P. R. China ∥ Key Laboratory of Flexible Electronics and Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, P. R. China ⊥ NSLS II, Brookhaven National Lab, Upton New York 11973, United States S Supporting Information *

ABSTRACT: Low-dimensional organic−inorganic hybrid perovskites have triggered many fundamental research studies due to their intrinsic tunable photovoltaic properties, technologically relevant stability, and promising efficiency. However, there is limited information on how ligand size influences inherent structural and electronic properties of perovskites. To gain deeper understanding of ligand-size related structural and film properties, we fabricated a series of (L)2(MA)n‑1PbnI3n+1 materials by introducing organic spacer ligands of n-CH3CH2NH3 (EA), n-CH3(CH2)2NH3 (PA), and nCH3(CH2)3NH3 (BA) into the three-dimensional (3D) methylammonium (MA) lead iodide (MAPbI3) system with the same inorganic layer thickness (average ⟨n⟩ = 4). We demonstrate that the increased number of carbon atoms on ligands affects compatibility of ligands with the 3D [PbI6]4− framework, leading to different structural dimensionality and crystal orientation, largely explaining different electronic properties, crystal stability, and the consequent device performance of solar cells. This work provides key missing information on how ligand size influences structural dimensionality and desirable electronic properties for future stable and efficient solar cells.

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Low-dimensional Ruddlesden−Popper (RP) layered perovskites have attracted attention due to their superior phase stability and moisture resistance. These layered perovskite show unprecedented physical properties and technologically relevant stability in contrast to their 3D counterparts, and huge potential in photovoltaic applications,15−19 light-emitting devices (LEDs),20−22 and field-effect transistors (FETs).23 These layered perovskites have the general chemical formula of (L)2(A)n−1MnX3n+1 (n = 1−∞), where L+ refers to a large aliphatic cation or an aromatic alkylammonium cation acting as

rganic−inorganic metal halide perovskite compounds with the general chemical formula of AMX3 (A = CH3NH3+, HC(NH2)2+; M = Sn2+, Pb2+, Ge2+; X = F−, Cl−, Br−) have triggered tremendous scientific efforts and revolutionized many achievements in photovoltaic applications in the latest period.1−5 Particularly, three-dimensional (3D) halide perovskites, including methylammonium based MAPbI3 and formamidinium based FAPbI3 as light absorbing materials, have demonstrated excellent intrinsic properties such as tunable direct bandgap,6 long exciton and charge diffusion lengths,7−11 and high absorption coefficients.2,12,13 Recently, intensive efforts have revolutionized the certified power conversion efficiency (PCE) of the perovskite solar cells closed to 23.7%.14 © XXXX American Chemical Society

Received: May 21, 2019 Accepted: July 8, 2019 Published: July 8, 2019 1830

DOI: 10.1021/acsenergylett.9b01100 ACS Energy Lett. 2019, 4, 1830−1838

Letter

Cite This: ACS Energy Lett. 2019, 4, 1830−1838

Letter

ACS Energy Letters

Figure 1. Schematic structure of MAPbI3 and (L)2(MA)n‑1PbnI3n+1 perovskites (⟨n⟩ = 4) with EA, PA, and BA as ligands.

Figure 2. Grazing-incidence wide-angle X-ray scattering (GIWAXS) images of (a) MAPbI3, (b) EA intercalated perovskite, (c) PA intercalated perovskite, and (d) BA intercalated perovskite. (e) Intensity versus q for all perovskite samples. (f) Azimuth angle of the (111) peak for all perovskite samples.

inorganic perovskite frameworks of corner-sharing [MX6]4− metal halo-generated octahedrals. 25 The corner-sharing [MX6]4− octahedral can be separated by large organic cations, due to the fact that large organic cations do not fit into such an octahedral structure. The introduction of large organic cations

an organic spacer; the inorganic perovskite framework comprises A cations, M cations, and X anions; and the integer n refers to the number of inorganic perovskite layers.1,24 The framework has a unique configuration comprising organic spacers of bilayer chains of large alkylammonium cations and 1831

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nm−1 corresponding to the (110) and (220) diffractions (Figure 2a), respectively.43,44 Similar diffraction patterns are observed for the EA based material (Figure 2b). Note that we did not observe additional diffraction peaks at q < 10 nm−1, suggesting an insufficiently crystallographic feature of the layered structure when intercalating the EA into the inorganic [PbI6]4− frameworks. The theoretical calculation reported previously has identified the EA as the closest candidate for 3D compatible perovskites because of a small energy loss of less than 0.1 eV per EA molecule for the MA/EA exchange.45 Our observations also experimentally confirm the high compatibility of the EA with a 3D structure. Further increasing carbon atoms leads to a distinct diffraction feature. The PA material shows discrete sharp diffraction dots (Figure 2c), indicating a textured polycrystalline film.46,47 In addition to the main Bragg diffraction of the (111) plane at q = 10 nm−1, we also observed diffraction dots at q = 2.5, 5.0, and 7.6 nm−1 (Figure 2c,e). This indicates appearance of n < 4 phases,48 and suggests a broad thickness distribution of the inorganic layers in the PA intercalated perovskite, even though the stoichiometry of the precursors is intended to prepare a single phase. By contrast, diffraction features at q < 10 nm−1 were not observed for the BA case, which exhibits a main Bragg diffraction dot at q = 10 nm−1 assigned to the (111) plane of n ≥ 4 phases (Figure 2d,e).30 This observation suggests less coherent crystallographic regions of n < 4 phases for the BA material in contrast to the PA one. The perovskite orientation with respect to the substrate was further analyzed. Figure 2f plots the azimuth angular distribution of the main diffraction feature at q = 10 nm−1 for the four materials. The MA and EA films exhibit a broad orientation distribution at all angles without noticeable peaks, indicating orientation randomness of perovskite crystals. While the PA and BA films show a dominant peak at 90°, a signature of out-of-plane orientation with respect to the substrate for n ≥ 4 phases. The ligand-size related structural evolution indicates a definitive restriction when intercalating ligands into inorganic framework. This is quite important, as large ligands must fit into an area defined by the terminal halides from four adjacent corner-sharing octahedra.49 Given the Goldschmidt’s Tolerance Factor t = 1 and Shannon ionic radii of Pb and I (RPb = 1.19 Å and RI = 2.20 Å), the limit size of ligands is reported be ca. 2.6 Å when fitting into [PbI6]4− octahedral.50 The EA (radii of ca. 2.7 Å) features are close to the value of the limit size and have similar electron negativity to the MA, and exhibits the potential to be compatible with a 3D framework. The PA and BA are larger in contrast to the MA, rendering steric hindrance for the intercalation into the [PbI6]4− octahedral and causing separate [PbI6]4− octahedrals with the consequence of observed layered structure. In addition to steric hindrance, there are also changes of the bonding/coordination preferences between the metal ions and different ligands,49 which can impact the structural chemistry. For example, ligands with different alkyl length would cause different surface energetics with a consequence of different thickness distribution of the inorganic layer. Ligand chemistry could also alter the nucleation mechanism, leading to orientation preferences in the PA and BA based materials in contrast to the 3D MAPbI3.30 This ligand-size related structural evolution is further reflected from the perspective of photophysical properties.

chains can afford superior hydrophobicity to the perovskite structure. Meanwhile, the dielectric constant of the [MX6]4− octahedral (≥6) differentiates it from organic cations (∼2.4),26 leading to the formation of periodic arrays of quantum wells.27 The unique physical properties of RP perovskites have triggered intensive research in photovoltaic applications. Tsai et al. demonstrated a hot-casting strategy to obtain vertically aligned (BA)2(MA)n‑1PbnI3n+1 (BA = C4H9NH3, n = 3 and 4) quantum wells with respect to the substrate.16 The dynamical phase evolution was further investigated,28−30 which highlights the key role of the solvent in controlling orientation of quantum wells. The charge dynamics of these RP perovskites were also investigated by different groups.31−35 Recently, a series of novel RP perovskites based on large aliphatic or aromatic ligands, such as n-butylammonium,16,28,36 phenethylamine,17,37,38 and thiophenemethylammonium39 were developed. So far, the majority of the reported studies focus on synthesis, film formation control, and morphological properties for the n = 3−5 RP perovskite family and their influence on photovoltaic performance. Existing research for the n = 1 perovskites has shown that ligand size could possibly induce different corner- and face-sharing of [PbI6]4− octahedral and the consequence of varied atomic and electronic structures and optoelectronic properties.40,41 However, there is no detailed understanding about the effect on atomic and electronic structures and the consequent optoelectronic properties for the n = 3−5 RP perovskite family, of the systematic modification of the organic spacer ligands in the same family of homologous materials, and among the same inorganic layer thickness under identical experimental conditions. Herein, we introduce organic spacer ligands of nCH 3 CH 2 NH 3 (EA), n-CH 3 (CH 2 ) 2 NH 3 (PA), and nCH3(CH2)3NH3 (BA) into the 3D MAPbI3 system, and evaluate their inherent structural and electronic properties with same inorganic layer thickness (⟨n⟩ = 4) as a function of the number of the carbon atoms of organic ligands. The results indicate that increasing carbon atoms from EA to PA, and BA leads to lower compatibility between the ligands and the 3D [PbI6]4− framework, with a consequence of different perovskite assembly in terms of dimensionality, distribution, and orientation of quantum wells, which result in different charge transfer kinetics between quantum wells, optoelectronic properties, crystal stability, and photovoltaic performance. The increased understanding of the effect of ligand size enables a level of control for the dimensionality and physical and optoelectronic properties of perovskites for future stable and high-performance solar cells. Figure 1 illustrates the structure of 3D MAPbI3 (labeled as MA) and those intercalated with large ligands of nCH3CH2NH3 (labeled as EA), n-CH3(CH2)2NH3 (labeled as PA), and n-CH3(CH2)3NH3 (labeled as BA). The ligandintercalated systems exhibit a chemical formula (L)2(MA)n‑1PbnI3n+1 (⟨n⟩ = 4), where the inorganic [PbI6]4− frameworks are sandwiched by ligands EA, PA, and BA with gradually increased numbers of carbon atoms in contrast to the MA molecule, and ⟨n⟩ represents the number of the inorganic layers (i.e., layer thickness). We acquired a series of highquality films using hot-casting technique for the EA, PA, and BA-intercalated perovskites.16 The MA film was fabricated using antisolvent (chlorobenzene) engineering.42 The resulting materials were characterized using 2D grazing incidence wide-angle X-ray scattering (GIWAXS). The MA film exhibits nearly isotropic Bragg rings at q = 10 nm−1 and 20 1832

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Figure 3. (a) Absorption spectra, (b) photoluminescence (PL) spectra, and (c) time-resolved PL (TRPL) spectra of all samples. TA spectra at various delay times of (d) EA, (e) PA, and (f) BA intercalated perovskite films. TA dynamic kinetics plots of (g) EA, (h) PA, and (i) BA intercalated perovskite films.

the charge-carrier state in these crystals. The excitonic energy of the 3D bulk PL emission shows a slight shift from 1.61 eV (768 nm) to 1.64 eV (757 nm) with addition of ligands (Figure 3b), suggesting the distortion of the lattice with the intercalation of larger organic cations into the 3D framework.51,52 We also observed stronger PL emission for the low-n phases within PA- and BA-based films from the bottom-side excitation in contrast to the front-side, indicating a higher concentration of low-n phases near the substrate (Figure S3). Figure 3c shows the TRPL spectra for the four samples on glass. The average carrier recombination lifetime (τave) was determined using a biexponential fitting:53−56

We further investigated photophysical properties of four perovskites. Figure 3a shows the UV−vis absorption spectra of perovskite films. The MA film exhibits band edge absorption at ca. 780 nm (1.58 eV). The addition of the EA leads to decreased absorption intensity in the visible range (∼500−780 nm) compared to the MA one. Both the PA and BA films exhibit distinct absorption features, where additional absorption peaks were observed at ca. 573 nm (2.16 eV), ca. 607 nm (2.04 eV), and ca. 644 nm (1.93 eV), corresponding to n = 2, 3, and 4 phases (see Figure S1 for n = 1 phase), respectively. The additional absorption feature at lower wavelength is due to different sets of electron negativities and, consequently, charge localization in the PA and BA-intercalated crystals.49 This further suggests a 3D compatible structure for the EA film, along with the formation of a layered structure in the PA and BA films, in line with GIWAXS observations. The optical results further indicate the formation of bulk crystals, as the band gap Eg determined using the Tauc-plot only shows a slight variation from 1.58 to 1.65, 1.63, and 1.60 eV for the MA, EA, PA, and BA films, respectively (Figure S2). The larger Eg in the EA film is ascribed to expanded and distorted [PbI6]4− octahedrals as reported previously,45 due to the slightly larger values of the EA features than the limited size when fitting into [PbI6]4− octahedral. The steady-state photoluminescence (PL) and time-resolved PL (TRPL) spectroscopies were further performed to probe

f (t ) = A1exp( −t /τ1) + A 2 exp(−t /τ2) + B

(1)

where τ1 and τ2 are the lifetimes for the fast and slow recombination, respectively, A1 and A2 are the relative decay amplitude, and B is a constant. The average lifetimes determined are 35.1, 5.0, 17.7, and 15.9 ns for the MA, EA, PA, and BA films, respectively. The ligand-intercalated films exhibit shorter lifetime, suggesting higher recombination losses in contrast to the MA case. We also observed ligand-dependent carrier concentration, electron mobility, and PL quantum yield (PLQY) in thin films (Figure S4). Interestingly, the EA case exhibits lower electron mobility and higher carrier concentration with a consequence of higher PLQY in contrast to other three films, in line with 1833

DOI: 10.1021/acsenergylett.9b01100 ACS Energy Lett. 2019, 4, 1830−1838

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Figure 4. (a) Plan-view of scanning electron microscopy (SEM) and (b) atomic force microscopy (AFM) images of the MAPbI3 and ligandintercalated films.

Figure 5. (a) J−V curves of the solar cells based on the MAPbI3, and ligand-intercalated perovskite films. (b) Comparison of photovoltaic parameters of all devices. (c) EQE and integrated short-circuit current of the champion cells for the four films. (d) PCE degradation of solar cells under ambient exposure for 120 h.

bleaching in their respective charge filling.31 The TA spectra at various delay times were shown in Figure S6 for the MA film and Figure 3d−f for the EA, PA, and BA films, respectively. The MA film exhibits a single bulk peak at ca. 756 nm (Figure S6a), which also dominates the EA film along with a weak bleach at ca. 644 nm (n = 4 phase) (Figure 3d).45 This weak bleach clearly features an existence of quantum wells for the EA material, even though the largest EA tends to form the 3D bulk structure when fitting into the [PbI6]4− octahedral. By contrast, we observed a strong bleach feature corresponding to n = 2−5 phases in the PA and BA films (Figure 3e,f), suggesting a broad thickness distribution of the inorganic layers which could be attributed to ligand-induced change of

shorter lifetime observed from TRPL. The exciton binding energy for the ligand-intercalated perovskites is slightly higher that of the MA-based one (50−60 vs 30 meV) (Figure S5), suggesting that charge-carrier transport is expected to be similarly dominated by free carriers as that in the MA-based perovskite.16 To gain a deeper understanding of the ligand-size related compositional distribution and charge dynamics, we further performed ultrafast transient absorption (TA) spectroscopy.57 The films were excited with a femtosecond laser pulse, and the photoinduced changes in the absorption (ΔA) spectra were then probed with a time-delayed laser-generated white light probe pulse.57 The time-delayed exciton accumulation within the films can be probed from photoinduced 1834

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ACS Energy Letters Table 1. Statistical Analysis of Photovoltaic Parameters of MAPbI3, EA, PA, and BA Intercalated Perovskite Devices MA Jsc (mA cm−2) Voc (V) FF (%) PCEave (%) PCEmax (%)

20.6 ± 1.06 ± 75.2 ± 16.2 ± 16.92

0.36 0.01 0.44 0.47

EA

PA

BA

14.2 ± 1.57 1.06 ± 0.02 62.2 ± 1.3 9.9 ± 0.54 10.61

16.6 ± 0.83 1.09 ± 0.02 47.2 ± 0.75 8.6 ± 0.37 9.05

18.9 ± 0.7 1.1 ± 0.01 53.8 ± 0.86 11.5 ± 0.24 11.93

surface energetics.30 Increasing the carbon number on ligands from EA to PA and BA leads to a higher concentration of quantum wells. We further confirmed the presence of spontaneous charge transfer in the EA, PA, and BA films. The EA film exhibits an instantaneous fast buildup of the n = 4 exciton resonance at ca. 0.5 ps, followed by a decay during 1−30 ps (Figure 3g). We observed similar buildup for the n = 2−4 exciton resonances in the PA and BA films (Figure 3h,i). Note that the bleach recovery for the n = 2−4 phases is accompanied by the formation of bulk bleaching. These results imply electron transfer from lower-n to higher-n and 3D bulk phases accompanied by hole transfer in the opposite direction, consistent with previous findings for RP perovskites.31,34,58 We sought to determine how ligand size influences morphological properties. We observed relatively faster color change from pale yellow to dark brown for the PA and BA films during film formation, suggesting faster crystallization in contrast to the EA case. The morphological difference can be verified from plan-view scanning electronic microscopy (SEM) and atomic force microscopy (AFM) images (Figure 4). The MA film exhibits ca. 200−300-nm-sized and densely packed 3D crystals and a root-mean-square (RMS) of 8.9 nm, analogous with previous observations.43,54 The EA film shows quite high rough surface (RMS = 106.0 nm) while without noticeable small grains, which is in stark contrast to the MA films. By contrast, the PA and BA cases show densely stacked ∼2−3-μm-sized 2D planes suggesting an anisotropic growth of crystals, and low RMS values of 2.1 and 8.6 nm, respectively. Such a smooth surface is beneficial for the charge extraction when compared to the EA case. This morphological evolution indicates that larger ligand size is playing a role in anisotropic growth of perovskite, with the consequence of the formation of 2D planes. We proceeded to evaluate ligand-related photovoltaic performance. We incorporated these perovskite films into ni-p architecture of FTO/c-TiO2/Perovskite/Spiro-OMeTAD/ Au. Figure 5a shows the current density−voltage (J−V) curves for each champion cells, and the detailed parameters are shown in Table 1. We achieved PCEs of 16.2 ± 0.47%, 9.9 ± 0.54%, 8.6 ± 0.37%, and 11.5 ± 0.24% for the MA, EA, PA, and BA devices, respectively, with the highest PCE achieving 16.92%, 10.61%, 9.05%, and 11.93% (Figure 5b). The lower PCEs obtained for the EA devices are due to lower short-circuit current density (Jsc) (20.99 vs 15.80 mA cm−2) and fill factor (FF) (75.14% vs 62.20%), and are likely the results of higher charge recombination losses, while the lower PCEs for the PA and BA films are more correlated with decreased FF (75.14% vs 47−54%). Higher PCE obtained for the BA-based devices is attributed to relatively lower trap density and higher mobility in contrast to the EA- and PA-based ones (Figure S4). External quantum efficiency (EQE) spectra for the four devices are illustrated in Figure 5c, within a 5% mismatch between the integrated current and the Jsc. Furthermore, the ambient

stability of four devices exposed to air without encapsulation for 120 h was also tested (humidty and temperature of ca. 40% RH and 25 °C, respectively), the results of which are shown in Figure 5d. The MA device exhibits ca. 20% loss of its initial efficiency after ambient exposure. By contrast, the ligandintercalated devices show PCE losses of ca. 10%, ca. 5%, and ca. 2% for the EA, PA, and BA based materials, respectively. As shown in Figure S8, the stable photocurrent density and PCE of the MA-, EA-, PA-, and BA-based solar cells measured under the standard 1-sun illumation at a fixed maximum power point (MMP) demonstrate excellent photostability. This observation indicates an enhanced device stability with increasing number of carbon atoms on ligands, which motivates us to undertake further analysis on the ligand-size related crystal stability. Perovskite crystal stability tests were performed for nonencapsulated perovskite films under ambient conditions (humidity and temperature of ca. 40% RH and 25 °C, respectively) in the dark. X-ray diffraction (XRD) patterns were periodically recorded for two months and illustrated in Figure 6a−d. We observed an appearance of PbI2 diffraction (at 2θ = 11.9°) within 5 days for the MA case, suggesting crystal degradation.59 By contrast, the PbI2 feature is not observable even after 60 days ambient exposure for the EA, PA, and BA cases. This observation indicates an intrinsic instability of MAPbI3 crystal, which can be significantly improved when intercalating large ligands. We performed first-principles density functional theory (DFT) calculations to probe the origin of the enhanced stability of the ligand-intercalated crystals. The intercalation of large ligands into the 3D framework leads to an extrusion force to the surface of the perovskite structure, with the consequence of changed distance between the Z coordinate of the C−N bond (denoted as A) and the Z coordinate of the Pb−I bond (denoted as B) (Figure S7). The A−B value, a signature of extrusion force, was evaluated and illustrated in Figure 6e for the MAPbI3, and the ligand-intercalated layered structure (⟨n⟩ = 4). The A−B value decreases gradually from 3.41 to 3.39, 3.36, and 3.32 Å when intercalating MA, EA, PA, and BA into the [PbI6]4− framework. The decrease in the A−B values with increased numbers of carbon atoms suggests larger extrusion effect and a unit cell contraction, which should lead to higher thermodynamic stability. The thermodynamic stability, corresponding to formation energy of the target materials, was further analyzed and illustrated in Figure 6f for the four cases. The formation energy corresponds to the free energy difference between the target materials and possible decomposed products. Higher formation energy suggests higher thermodynamic stability of the material. We observed the formation energy of perovskite gradually increasing from 13.5 to 15.8, 18.0, and 20.3 eV when intercalating ligands MA, EA, PA, and BA into the [PbI6]4− framework, suggesting gradually suppressed phase instability with longer ligands. These results indicate the key role of ligand length to the thermodynamic stability of perovskite and the consequence of device stability. 1835

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size related dimensionality control will play an essential role in further development of stable and high performance lowdimensional perovskite optoelectronics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b01100. Additional data and figures and a detailed description of the experimental methods (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K. Z.). *E-mail: [email protected] (S. L.). ORCID

Yingchun Cheng: 0000-0002-8495-9184 Kui Zhao: 0000-0002-9512-0405 Shengzhong Frank Liu: 0000-0002-6338-852X Author Contributions

P.C. performed most of the experiments. K.Z. conceived the project. K.Z. and S.L. supervised the project. P.W. and Z.X. helped with the TRPL test. G.Z. helped with SEM measurement. Q.W. and Y.C. helped with the DFT calculation. X.J., N.Y., and J.D. helped with the TA measurements. R.L. helped with the GIWAXS measurement. All contributed to the writing of the Letter. Notes

Figure 6. (a−d) The XRD patterns showing crystal stability during ambient exposure for MAPbI3, and EA, PA, and BA intercalated perovskites recorded for 60 days. * shows perovskite diffraction and # shows PbI2 diffraction. (e) A−B value showing the distance between the C−N bond (denoted as A) and the Z coordinate of the Pb−I bond (denoted as B) for the MAPbI3 and EA, PA, and BA intercalated perovskites. (f) Formation energy for the MAPbI3 and EA, PA, and BA intercalated perovskites.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2017YFA0204800, 2016YFA0202403), National Natural Science Foundation of China (61604092, 61674098, 91733302, 91833302), National University Research fund (Grant No. GK201802005, Grant No. GK201803038), the 111 Project (B14041), and National 1000-talent-plan program (1110010341). This research used CMS beamline of the NSLS II, a U.S. Department of Energy (DOE) Office of Science User Facility operated by Brookhaven National Lab under Contract No. DESC0012704.

In summary, we discovered that the inherent atomic and electronic structures and the consequent optoelectronic properties can be significantly impacted by the number of carbon atoms on ligands for (L)2(MA)n‑1PbnI3n+1 perovskites (⟨n⟩ = 4). The EA intercalated perovskite retains preferential formation of 3D structure and orientation randomness of crystals, along with weak features of quantum wells. By contrast, further increasing carbon number leads to preferential formation of a layered structure for the PA and BA intercalated materials, with highly oriented crystals. The observed difference in perovskite dimensionality can be attributed to the compatibility between ligands and 3D [PbI6]4− framework. We also found a higher concentration of low-n quantum wells with increasing carbon number on ligands, and the spontaneous charge transfer from lower-n to higher-n phases in the EA, PA, and BA films. Furthermore, we achieved PCEs of 16.92% for MAPbI3 solar cells, and 10.61%, 9.05%, and 11.93% for the (L)2(MA)n‑1PbnI3n+1 solar cells (⟨n⟩ = 4) with EA, PA, and BA as ligands, respectively. The addition of ligands leads to PCE drop in contrast to the MAPbI3 solar cell, due to higher recombination losses for the EA case and larger exciton binding energy for the PA and BA cases, which however promotes the long-term device stability due to the fact that the larger ligand size favors higher thermodynamic stability of the perovskite structure. We believe the understanding of ligand-

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