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Letter 2

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Highly Efficient Ruddlesden–Popper Halide Perovskite PAMAPbI Solar Cells Peirui Cheng, Zhuo Xu, Jianbo Li, Yucheng Liu, Yuanyuan Fan, Liyang Yu, Detlef-M. Smilgies, Christian Müller, Kui Zhao, and Shengzhong (Frank) Liu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01153 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Energy Letters

Highly

Efficient

Ruddlesden–Popper

Halide

Perovskite PA2MA4Pb5I16 Solar Cells Peirui Cheng,1 Zhuo Xu,1 Jianbo Li,1 Yucheng Liu,1 Yuanyuan Fan,1 Liyang Yu,2 Detlef-M. Smilgies,3 Christian Müller,2 Kui Zhao1,* and Shengzhong (Frank) Liu1,4,* 1

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. 2

Department of Chemistry and Chemical Engineering, Chalmers University of Technology,

Göteborg, Sweden 3

Cornell High Energy Synchrotron Source, Cornell University, Ithaca, NY 14850, USA.

4

Dalian National Laboratory for Clean Energy, iChEM, Dalian Institute of Chemical Physics,

Chinese Academy of Science, Dalian, 116023, P. R. China Corresponding Authors *Kui Zhao: [email protected]. (K. Z.) *Shengzhong (Frank) Liu: [email protected]. (S. L.)

ACS Paragon Plus Environment

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Abstract

Two-dimensional (2D) Ruddlesden-Popper (RP) organic-inorganic perovskites have emerged as promising candidates for solar cells with technologically relevant stability. Herein, a new RP perovskite, the fifth member ( = 5) of the (CH3(CH2)2NH3)2(CH3NH3)n-1PbnI3n+1 family (abbreviated as PA2MA4Pb5I16), was synthesized and systematically investigated in terms of photovoltaic application. The obtained pure PA2MA4Pb5I16 crystal exhibits a direct band gap of Eg = 1.85 eV. Systematic analysis on the solid film highlights the key role of the precursorsolvent interaction on the quantum well orientation, phase purity, grain size, surface quality and optoelectronic properties, which can be well-tuned with addition of dimethylsulfoxide (DMSO) into the N,N-dimethylformamide (DMF)-precursor solution. These findings present opportunities for designing a high quality RP film with well-controlled quantum well orientation, micrometersized grains and optoelectronic properties. As a result, we achieved power conversion efficiency (PCE) up to 10.41%.

TOC GRAPHIC


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ACS Energy Letters

Hybrid lead halide perovskites with a general formula of AMX3 have attracted intense research interest in the last few years due to their excellent properties such as high absorption coefficient,1-2 long exciton diffusion length,3-4 and tunable direct bandgap,5 making it the leading contender for photovoltaic applications. The class of three-dimensional (3D) perovskites, such as HC(NH2)2PbI3 or (CH3NH3)PbI3 based perovskites, have revolutionized the field of photovoltaics by achieving the certified power conversion efficiency (PCE) of up to 23.3%.6 However, their environmental instability still presents a major challenge to commercializing this technology.7-8 This challenge has propelled tremendous efforts to develop novel intrinsically stable materials.9 In particular, lead-based Ruddlesden-Popper (RP) layered perovskites have emerged, which promise high performance and exhibit technologically relevant stability for photovoltaic (PV) devices,10-12 light emitting diodes (LEDs),13-16 and field-effect transistors (FETs).17 RP perovskites have a general formula of (RNH3)2(A)n−1MnX3n+1, where RNH3+ = organic spacer, A+ = Cs, CH3NH3, HC(NH2)2, M2+ = Pb, Sn, Ge, and X− = Cl, Br, I, and n is an integer between 1 and ∞.18 To successfully implement two-dimensional (2D) RP perovskites in solar cells, recent reports have capitalized on the tunability of chemical structure.19-20 To date, mainly two kinds of 2D RP perovskites have been used in solar cells: n-C4H9NH3 (n-BA)

10-11, 21-23

and

C6H5C2H4NH3 (PEA)-based devices,20, 24-26 which mostly emphasize the n = 4 members.12, 27 Recently, IC2H4NH3,28 iso-C4H9NH3 (iso-BA),29 n-C6H13NH3,30 and polyethylenimine (PEI)based RP perovskites were also explored for solar cells.31 These large organic spacers between conductive inorganic frameworks inhibit charge transport between neighboring inorganic layers, leading to quantum well formation. Perovskite based on quantum-well structure exhibits high environmental tolerance due to inhibited reaction between oxygen, moisture and semiconducting


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inorganic frameworks.18, 32 Meanwhile, the adjacent layers are held together by weak Van der Waals forces, ensuring promising structure stability. In series of these compounds, it has been shown that increasing the number of perovskite layers (n in the chemical formula) improves the photovoltaic performance. Specifically, great success in solar cells has been achieved for the BA-based = 4 member (n-BA2MA3Pb4I13). For example, a 12.52% PCE was demonstrated when casting solution on the hot substrate, which ensures an efficient charge transport along the out-of-plane direction.11 We demonstrated a remarkable PCE of 13.7% from Cs-doped nBA2MA3Pb4I13 solar cells with negligible hysteresis and good photo- and environmental stability.9 Further investigations using time-resolved grazing incidence wide-angle X-ray scattering

(GIWAXS)

investigations

revealed

formation

of

(BAI)·(MAI)·(PbI2)·N,N-

dimethylformamide (DMF) or (BAI)·(MAI)·(PbI2)·dimethylsulfoxide (DMSO) intermediate solvates when casting BA2MA3Pb4I13 precursors from DMF or DMSO at room temperature.33 However, the light absorption of the = 4 member is still low because of the relatively wide band gaps. Therefore, gaining absorption in the visible region requires investigation of higher-nvalue members, such as the = 5 member. Early trials based on the = 5 member was reported by Kanatzidis’s group using n-BA as organic spacer.34-35 However, the performance was relatively poor (˂ 10%) and lags far behind the n= 4 member. The performance difference between the = 4 and = 5 members points out the unique and material-specific challenges of translating optoelectronic properties between different n members. With this in mind, it is highly imperative to develop a high quality = 5 member RP perovskite, and uncover how the film-formation properties can be tuned for high performance solar cells. In this work, a new RP perovskite, the = 5 member of the (CH3(CH2)2NH3)2(CH3NH3)n1PbnI3n+1

family with n-CH3(CH2)2NH3 (PA) spacer, which is shorter than n-BA, was synthesized


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ACS Energy Letters

and investigated in terms of photovoltaic application. The PA2MA4Pb5I16 crystals were synthesized, which exhibit a direct band gap of 1.85 eV. The thin films were fabricated with controlled quantum well orientation, phase purity, grain size, film quality and optoelectronic properties by tuning precursor-solvent interaction. We applied these films into solar cells, and achieved a high PCE of 10.41% with open-circuit voltage (Voc) of 1.11 V, short-circuit current density (Jsc) of 18.89 mA cm-2, and fill factor (FF) of 49.53%. The PA2MA4Pb5I16 crystals were prepared by properly tuning PbO, MAI, and n-propylamine stoichiometric ratios in aqueous hydriodic acid (HI). Briefly, plate-like crystals were isolated when slowly cooling the liquor from boiling temperature to room temperature. As shown in Figure 1a, PA2MA4Pb5I16 incorporates five [Pbl6]4- octahedra in the unit cell, which share corners along the direction perpendicular to the layers. The individual [Pb5l16]6- slabs are separated by the large organic ligands (PA2)2+. Similar to BA2MA3Pb4I13,36 PA2MA4Pb5I16 also crystallizes in an orthorhombic space group, with a unit cell of a = 8.624 Å, b = 72.53 Å, and c = 9.297 Å. The X-ray diffraction data exhibits the sharp Bragg peaks at 2θ = 13.88º and 28.32º (Figure 1b), assigned to the (111) and (202) peaks with d-spacings of 6.37 Å and 3.15 Å, respectively.34 Note that the undergrown phases of other n members were not observed from the X-ray features. This suggests high phase purity for the obtained PA2MA4Pb5I16 crystals. Figure 1c illustrates the scanning electron microscopy (SEM) images of a few typical crystals. The crystals grow in the form of thin and square platelets with thickness of ca. 2 µm and size of tens of micrometers. Some platelets grow vertically, leading to extensive twinning. These observations indicate both platelet and vertical growth habits for the PA2MA4Pb5I16 crystals, while the former seems more appreciable. This is in contrast to the lower-n members such as n = 1 BA2PbI4 reported previously.34 The BA2PbI4 crystals grew freely along the plane of the


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inorganic layers but only slowly along the vertical stacking direction.34 Density functional theory (DFT) calculation was further performed to investigate the electronic density of states (DOSs) (Figure 1d, see Figure S1 for details). The PBE method without consideration of spin-orbitalcoupling effect gives better description on the electronic structure with respect to the experimental values, consistent with the previous report. The partial density of states is consistent with the BDCD analysis. The PDOS clearly shows that the main contribution to the VBM originates from the 5p orbitals of I atoms, while the CBM is mainly composed of 5p orbitals of Pb atoms (Figure S1).37 The band gap (Eg) of the PA2MA4Pb5I16 determined with the GGA-PBE method shows Eg = 1.85 eV. This result suggests the PA2MA4Pb5I16 to be a promising candidate for solar cells, although the band gap of the PA2MA4Pb5I16 is slightly higher than the traditional 3D MAPbI3 (1.85 eV vs. 1.59 eV).38

Figure 1. (a) Schematic view of the PA2MA4Pb5I16 structure, which includes two organic spacers and inorganic layers. (b) X-ray powder diffraction pattern of PA2MA4Pb5I16 crystals. (c)


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ACS Energy Letters

SEM images of the obtained PA2MA4Pb5I16 platelet crystal. (d) Electronic density of states (DOS) of the PA2MA4Pb5I16 perovskite.

We further fabricated thin films using hot-casting strategy.11 The morphology, optoelectronic properties and photovoltaic performance were assessed. Our in situ X-ray observations of solution-casting = 4 BA2MA3Pb4I13 indicate the formation of BA2MA3Pb4I13-solvent intermediate phase during solution-drying.33 The formation of intermediate phase arises from strong interactions between solvents and precursors. Motivated by this, we tune the film quality of the PA2MA4Pb5I16 by solvent-engineering and understand the underlying factors to improve microstructure and optoelectronic properties. The films were fabricated from neat DMF, DMF: DMSO = 7: 3 (v/v), DMF: DMSO = 1: 1 (v/v), DMF: DMSO = 3: 7 (v/v) and neat DMSO, respectively. Figure 2a illustrates the XRD features of these thin films. All films exhibit two distinct and dominant (111), (202) Bragg reflections, similar to that of single crystals. This indicates a preferential orientation of crystal domains in bulk films. The solvent effect plays a critical role in the PA2MA4Pb5I16 crystalline behavior. The Full width at half maximum (FWHM) values of the (111) peak significantly decreases from ca. 0.34º to ca. 0.15º for the neat DMF and DMSOderived films (Figure 2b), respectively. These observations suggest that the introduction of DMSO enhances the crystal sizes or crystal quality for the bulk film.39 Similar phenomenon was also found recently in the case of n-BA based = 4 member films.35 One can therefore deduce from these observations that the perovskite crystallization from metal halide and organic ligand is sensitive to the solvent nature.


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Figure 2. (a) X-ray diffraction patterns of the PA2MA4Pb5I16 films cast from different solvents. (b) Full width at half-maximum (FWHM) of the (111) diffraction. Grazing-incidence wide-angle X-ray scattering (GIWAXS) data of thin films cast from (c) pure DMF, and (d) 1: 1 DMF:


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ACS Energy Letters

DMSO solvents. (e) Intensity versus q for the films cast from different solvents. (f) Azimuth angle of the (111) peak for the PA2MA4Pb5I16 films cast from different solvents.

Phase purity was further evaluated. Diffraction peaks appear in the low-angle region (2θ ˂ 13º) when the DMSO is introduced, indicating formation of lower-n members. This can be further confirmed using grazing-incidence wide-angle X-ray scattering (GIWAXS) diagnostics. The GIWAXS patterns were illustrated in Figure 2c, d for the representative DMF and DMF: DMSO (1: 1) films and in Figure S2 for the left films. These films all exhibit dominant Bragg reflection at q = 10.0 nm-1. Remarkably, the DMSO-derived films show additional Bragg spots at q = 2.5 nm-1, 5.0 nm-1 and 7.6 nm-1 (Figure 2e), corresponding to the n ˂ 5 phases. We postulate that the introduction of the DMSO yields stronger interactions between solvent and lead halide through formation of Lewis acid-adduct,40 and hydrogen bond between solvent and n-PA cation. Precursors preferentially crystallize into n ˂ 5 members due to lower nucleation barrier. This feature can also be found for the n-BA-based counterparts.26 For example, both for the n-BAbased = 4 and = 5 members, appearance of lower-n members can be found with addition of the DMSO solvent into the DMF solution.28,

33

Based on these observations, we

identified the precursor-solvent interaction as a key issue for the phase purity of RP perovskites even though the molar ratio of used precursors is intended for a single phase. Figure 2f illustrates the pole figures of the Azimuth angle for the (111) peak of all films. A dominant peak at Pole figure 90º along with broad peak from 120º to 180º is observed for the DMF case. While the DMSO-derived films mainly exhibit a dominant peak at 90º. This observation indicates orientation randomness while preferential out-of-plane orientation of the [(MA)n-1PbnI3n+1]2- quantum wells with respect to the substrate for the DMF film and DMSO-


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derived films, respectively.38 Apparently, the quantum well orientation can be tuned by the choice of processing solvent. Note that charge transport in the bulk film is highly dependent on the quantum well orientation. The DMSO-induced preferential out-of-plane orientation of quantum well is highly required to ensure efficient charge transport to the electrodes. Therefore, optimization of solution-processing for the vertical growth of the inorganic slabs is essential for RP perovskite solar cells.

Figure 3. (a) Plan-view scanning electron microscopy (SEM), and (b) atomic force microscopy (AFM) images of the films casting from DMF, 1: 1 DMF: DMSO, and DMSO solvents. (c) Cross-section SEM images of the PA2MA4Pb5I16 perovskite solar cell devices prepared from DMF, 1: 1 DMF: DMSO, and DMSO solvent systems.


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ACS Energy Letters

The introduction of the DMSO solvent prolongs the perovskite crystallization kinetics during solution-casting process. The neat DMF sample turns from yellow to dark brown immediately within spinning time of 3 seconds, indicating crystallization occurs and fast solvent removal. The crystallization kinetics is significantly slowed for the DMSO-derived cases due to high boiling point of DMSO (189 ºC vs. 153 ºC). Upon this interesting scenario, quite distinct morphology is expected. The film morphology was assessed using scanning electron microscopy (SEM) and atom force microscopy (AFM). The plan-view SEM images are illustrated in Figure 3a for the representative DMF, DMF: DMSO (1: 1) and DMSO films, and in Figure S3 for the left ones. The DMF cast film exhibits 2-3 µm sized protuberances on the surface, where gaps with width of ~500 nm are observed. The surface coverage is superior for the DMSO-derived films. The large protuberances are eliminated. We ascribe the improved surface to the prolonged crystallization kinetics. More in details, AFM images reveal average root-mean-square (RMS) roughness of ca. 38.9 nm for the DMF-cast film, which is significantly decreased for the DMSO-derived films (ca. 20 nm). Meanwhile, the DMF-cast film exhibits plane size of 200-500 nm (Figure 3b), which dramatically increases to 1-2 µm for the DMSO-derived films (Figure S3). The increased domain size is expected to minimize charge loss between grains for solar cells.41 The crosssectional SEM images illustrated in Figure 3c show thickness of ca. 1 µm and densely packed small grains for the DMF-cast sample. Perpendicular growth of the PA2MA4Pb5I16 planes is observed for the DMSO-derived films. Note that the thickness of films exhibits a continuous decrease with increasing the DMSO ratio (e.g., 0.8 µm for the film cast from neat DMF, 0.5 µm for the film cast from DMF: DMSO (1:1) and only 0.2 µm for the film cast from neat DMSO) (Figure 3c and Figure S4). In sharp contrast to three-dimensional (3D) perovskites, thickness is more prominent in the RP perovskite solar cells. This is because charge carriers need to


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surmount increased potential barriers before extraction at the charge transport layer, especially for the case that quantum wells are not well perpendicularly aligned with respect to the substrate. Therefore, the charge recombination in device would get higher as the RP perovskite thickness increases. Based on these observations, charge collection is expected to be more efficient for the DMSO-derived solar cells. The morphological changes motivated further study into optoelectronic properties. Steady-state absorption, photoluminescence (PL) and time-resolved PL (TRPL) measurements were carried out and shown in Figure 4a-c. Distinct absorption features were observed between the DMF-cast film and DMSO-derived films (Figure 4a). For example, in addition to the lowest bandgap absorption, higher energy absorption peaks at ca. 567 nm, 604 nm and 641 nm corresponding to the n = 2, n = 3 and n = 4 phases are more prominent for DMSO-derived films in contrast to the DMF-cast film.42 This result is highly consistent with the GIWAXS observation, confirming phase impurity again. Meanwhile, the band gap Eg values determined using Tauc-plot are 1.66 eV for the DMF-cast film (Figure S5), while 1.62-1.63 eV for the DMSO-derived films.43 These Eg values are consistent with the UPS results (Figure S6), while are lower in contrast to the theoretical calculation. This difference hints the role played by the formation of higher-n phases in the film. The addition of DMSO solvent narrows the Eg value of the PA2MA4Pb5I16 film to a great extent, which corresponds to the conception that RP films have the tunability of optical properties. Apparently, the DMSO-derived films with smaller Eg are expected to yield higher short-circuit current (Jsc) in solar cell.


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(b) n=3 n=2 n=4

0.5

n≈∞ 0.0 500

1.5

1.0

DMF 7:3 DMF:DMSO 1:1 DMF:DMSO 3:7 DMF:DMSO DMSO 1.60 eV

1.65 eV

0.5

0.0

600 700 800 Wavelength (nm)

1.5

1.6 1.7 Energy (eV)

1.8

(d) pure DMF 7:3 DMF:DMSO 1:1 DMF:DMSO 3:7 DMF:DMSO pure DMSO

0.1

0.01

0

100

200 300 Time (ns)

-3

1

6

15

(c)

DMF 7:3 DMF:DMSO 1:1 DMF:DMSO 3:7 DMF:DMSO DMSO

Normalized PL

1.0

Trap density (×10 cm )

Normalized absorbance

(a)

Normalized intensity

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ACS Energy Letters

4

2

400

DMF 7:3

1:1

3:7 DMSO

Figure 4. (a) Absorption spectra, (b) photoluminescence (PL) spectra, (c) Time-resolved PL (TRPL) spectra, and (d) Trap density of the perovskite films cast from different solvents.

Figure 4b illustrates PL spectra for all films. The PL peak, corresponding to the excitonic contribution of the bulk film, decreases from 1.65 eV to 1.60-1.62 eV for the DMF-cast film and DMSO-derived films, respectively. This phenomenon is in good agreement with the aforementioned X-ray observations (Figure 2a) that crystallinity and grain size were dramatically enhanced for the films cast from DMSO-contained solutions. This observation


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therefore implies that the optoelectronic properties of PA2MA4Pb5I16 films are related with crystal quality. The linewidth of the PL peak was further determined to verify this conception. The linewidth value decreases from 88.4 meV to 81.7 meV, 82.7 meV, 78.7 meV and then 74.9 meV for the neat DMF, DMF: DMSO (7: 3), DMF: DMSO (1: 1), DMF: DMSO (3: 7), and neat DMSO-cast films, respectively. The decrease of the linewidth in the DMSO-derived films confirms superior optoelectronic properties from high quality crystals.44 Notably, these PL spectra are very well resolved with a near Gaussian-type emission, without obvious low-energy tail which can be found in other low-n RP perovskites such as BA2MAPb2I7.34 The low-energy tail is characteristic of RP perovskites and has been attributed to trap states.45 Our observation therefore indicates that the PA2MA4Pb5I16 films tend to exhibit superior optoelectronic properties. TRPL spectroscopy was used to investigate the carrier lifetime of the films on glass (Figure 4c). The correlated parameters were fitted using a bi-exponential equation:46-47 () = exp (−/ ) + exp (−/ ) +

(1)

where τ1 and τ2 are slow and fast decay time constants, respectively, A1 and A2 are their corresponding decay amplitude, and B is a constant. TRPL displays lifetimes (τave) from 0.64 to 49.8, 55.4, 43.1, and 41.3 ns for the DMF, DMF: DMSO (3: 7), DMF: DMSO (1: 1), DMF: DMSO (7: 3) and DMSO films, respectively. The lifetime of carriers in all DMSO-derived films is significantly longer in contrast to the neat DMF film, showing that the reduction of recombination rate leads to longer time the carriers existing in the films.46 The trap state density was further determined from the dark current-voltage characteristics (Figure S7). The trap state density was determined by the trap-filled limit voltage using following equation:9, 48


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ACS Energy Letters

=

(2)

where VTFL refers to the trap-filled limit voltage (the voltage value of the pink point between the Ohmic period and the TFL period), εr is the relative dielectric constant and the value taken here is 28.8,49 ε0 is the vacuum permittivity, q is the electron charge, and L is the thickness of the perovskite film. We found that the trap densities decrease from 6.0×1015 cm-3 to 2.1-2.8×1015 cm-3 for the neat DMF and DMSO-derived films (Figure 4d), respectively.50 This result proves that defects are remediated in the DMSO-derived films, which leads to decreased charge recombination and should increase fill factor (FF) in the corresponding solar cells.


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Figure 5. (a) Schematic structure of the planar solar cells. (b) J-V curves of the solar cells based on the PA2MA4Pb5I16 films cast from different solvents. (c) The comparison of photovoltaic parameters. (d) The external quantum efficiency (EQE) of all devices.

We proceeded to evaluate whether the PA2MA4Pb5I16 is capable of producing highly efficient solar

cells.

Solar

cells

were

fabricated

with

n-i-p

architecture

of

FTO/c-

TiO2/PA2MA4Pb5I16/Spiro-OMeTAD/Au (Figure 5a). The current density-voltage (J-V) characteristics are presented in Figure 5b and the statistical analysis containing average values and standard deviations of photovoltaic parameters are compiled in Supporting Information Table S1. The device based the neat DMF-cast film exhibits an average PCE of 5.62± 0.49%, with the highest values of an open-circuit voltage (Voc) of 1.13 V, a Jsc of 11.12 mA cm-2, a FF of 48.46% and an overall PCE of 6.11%. All the DMSO-derived devices generally exhibit superior performance in contrast to the neat DMF device due to significant increased Jsc. For example, the highest Jsc reaches 18.89 mA cm-2 for the DMF: DMSO (1: 1) based device, leading to a PCEmax of 10.41% (PCEave = 10.05± 0.20%). To our knowledge, this performance is the best among all n = 5 member RP perovskite based devices reported so far.34-35, 44, 51-52 The excellent performance for the DMSO-derived devices is ascribed to the superior morphology, crystal quality and optoelectronic properties. The best performance obtained for the DMF: DMSO (1: 1) based device is due to proper thickness (ca. 0.5 µm), which reaches a balance between light absorption and charge transport. Figure 5d shows the external quantum efficiency (EQE) data of all devices. Good photocurrent generation in the visible region with a band edge near 785 nm can be observed. The integrated Jsc values match well with that obtained by light J-V curves. The devices based on neat DMF and DMSO-cast films exhibit relatively lower EQE especially in the


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ACS Energy Letters

range of 500-750 nm, which is consistent with the lower Jsc for these two devices. Note that the FF value for the PA2MA4Pb5I16 based devices is much lower than the conventional 3D MA or FA based ones. This is reasonable because exciton binding energy for the RP perovskites is relatively larger than 3D ones,44 and the charge transport within the RP perovskite bulk films have to conquer the energy barriers to be extracted. Judging from the relatively low value of FF, we expect the device efficiency to be further improved in the future with further optimization in charge recombination within the bulk film and at the perovskite-contact interface. The stable output photocurrent density and PCE of these cells recorded under the standard 1 sun illumination indicates exhibit excellent photo-stability (Figure S8). Furthermore, the ambient stability of these cells exposure to air (in the dark, relative humidity ~50-60% RH, room temperature 25 ºC) without encapsulation was also tested, which is shown in Figure S9. The negligible loss of device performance after 500 hours exposure to air indicates excellent ambient stability of devices which originates from high resistance to moisture attack.11 In conclusion, this is the first time to report synthesis of the = 5 member RP perovskite PA2MAn-1PbnI3n+1 crystal and its application in solar cells. It is found that the precursor-solvent interaction plays a key role in the quantum well orientation, phase purity, grain size, surface quality and optoelectronic properties of the films. Addition of DMSO into the DMF solvent enhances the film crystallinity, quantum well orientation, and improves surface quality and charge carrier lifetimes. The phase purity in solid film is highly influenced by the nucleation barrier, which can be tuned by the Lewis-base adduct and hydrogen bond between precursor and solvent. As a result, highly efficient solar cells have been fabricated with PCE as high as 10.41%. The work demonstrates effective control of crystallinity, orientation, and optical properties in the


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= 5 member RP perovskite, making it possible to optimize layered perovskite for high solar cell performance.


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ACS Energy Letters

ASSOCIATED CONTENT Supporting Information The Supporting Information is available of charge on the ACS Publications website at DOI: Additional data and figures and a detailed description of the experimental methods included in the SI.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (K. Z.) *E-mail: [email protected] (S. L.) Author Contributions P. C. performed most of the experiments, K. Z and S. L. supervised the overall project, Z. X. helped the DFT calculation, J. L. helped SEM test, Y. L. and Y. F. helped TRPL test, L. Y., D. S. and M. C. helped the GIWAXS measurement, K. Z., P. C. and S. L. contributed to the writing of the paper. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work was supported by the National Key Research and Development Program of China (2017YFA0204800, 2016YFA0202403), National Natural Science Foundation of China (61604092, 61674098), National University Research Fund (Grant Nos. GK201802005), the 111 Project (B14041), and National 1000-talent-plan program (1110010341). CHESS is supported by the NSF Award DMR-133208.

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