A New Organic Interlayer Spacer for Stable and Efficient 2D

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A New Organic Interlayer Spacer for Stable and Efficient 2D Ruddlesden-Popper Perovskite Solar Cells Zhimin Li, Ning Liu, Ke Meng, Zhou Liu, Youdi Hu, Qiaofei Xu, Xiao Wang, Shunde Li, Lei Cheng, and Gang Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b01652 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 1, 2019

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A New Organic Interlayer Spacer for Stable and

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Efficient 2D Ruddlesden-Popper Perovskite Solar

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Cells

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Zhimin Li †,‡,#, Ning Liu‡,#, Ke Meng*,†, Zhou Liu†, Youdi Hu†, Qiaofei Xu†, Xiao Wang†,

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Shunde Li†, Lei Cheng† and Gang Chen*,†,§

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†School

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China

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‡School

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Technology, Shanghai 200093, China

of Physical Science and Technology, ShanghaiTech University, Shanghai 201210,

of Environment and Architecture, University of Shanghai for Science and

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§Shanghai

Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese

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Academy of Sciences, Shanghai 201204, China

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ABSTRACT: Two-dimensional (2D) perovskite materials have exhibited great possibilities

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towards the fabrication of highly efficient and stable solar cell devices. The large degree of

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structural versatility due to the viable choices of organic interlayer spacers promises new and

ACS Paragon Plus Environment

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valuable 2D perovskite species. Herein, phenyltrimethylammonium (PTA+) is successfully

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employed as the organic interlayer spacer to prepare the 2D Ruddlesden-Popper perovskite films

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that exhibit exceptional optoelectronic properties. By adding Cl- ions during film growth, the

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(PTA)2(MA)3Pb4I13 (MA = methylammonium) perovskite films are effectively prepared with

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tunable crystal orientation and film morphology. The optimized devices fabricated with the

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assistance of Cl- ions deliver the power conversion efficiency up to 11.53%, which is ascribed to

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the simultaneous reductions of charge transfer resistance and defect-induced charge

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recombination. Moreover, the PTA-based 2D perovskite solar cells demonstrate remarkable

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environmental and thermal stabilities.

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KEYWORDS: 2D perovskite, solar cell, phenyltrimethylammonium, device stability

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Organic-inorganic hybrid perovskite materials, with the general chemical formula of ABX3 (A =

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methylammonium (MA), formamidinium (FA), Cs; B = Pb, Sn; X = Cl, Br, I), have exhibited

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great potential towards the realization of highly-efficient, cost-effective photovoltaic devices.1-4

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The power conversion efficiency (PCE) of solar cells featuring these three-dimensional (3D)

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perovskites has surpassed 23% due to their remarkable optoelectronic properties.5-6 Despite all

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the astonishing progresses made in the last decade, the intrinsic environmental instability issue of

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3D perovskites has hindered the further development and commercialization of perovskite solar

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cells.7-9 Two-dimensional (2D) perovskites, represented by the Ruddlesden-Popper (RP) type,

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have recently attracted enormous attention since they provide a much higher degree of structural


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versatility thus promise more attractive properties, such as good resistance to moisture and

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heat.10-13 Conceptually, 2D perovskites consist of the negatively charged Pb-I slabs of various

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layer numbers and the intercalated long-chain organic cations (interlayer spacers) to balance

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charges and maintain structural integrity. The structural stability is ensured by the van der Waals

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forces between unit layers. The chemical formula of 2D perovskites can be written as (A’)m(A)n-

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1BnX3n+1,

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Pb-I slabs. Here, the size and shape of the spacer cation A’ effectively determine the relative

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rotation and tilting of the Pb-I octahedra in the inorganic slabs thus alter the optoelectronic

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properties of 2D perovskites.10,14

where A’ represents the long-chain organic cation and n indicates the layer number of

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To date, the most commonly employed organic spacer cation A’ includes the hydrophobic

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aliphatic alkylammonium cation of n-C4H9NH4+ (BA+) and benzene based aromatic

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alkylammonium cation of C6H5C2H4NH4+ (PEA+).15-19 Solar cells employing these two cations

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have achieved good efficiency with long-term stability through the careful control of crystal

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orientation and film morphology, demonstrating the great potential of 2D perovskites in

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photovoltaic applications.11,20-21 Recently, the A’ spacer cations are tailored by either varying the

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organic chain length/geometry or adding new functional groups to achieve further performance

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enhancement and possible new features.20,22-25 For instance, n-propylammonium cation (PA+)

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with shorter alkyl chain length was introduced to prepare PA2MA4Pb5I16 films for better charge

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transport and extraction properties. A promising PCE of 10.41% was obtained while maintaining

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good device stability.23 To the same end, butylammonium ion with a shorter branched-chain (iso-

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BA+) other than its linear analogue was employed to obtain (iso-BA)2(MA)3Pb4I13 perovskites

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which delivered an impressive PCE of 10.63%.24 By decorating additional functional groups,

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iodoethylammonium22 or 2-thiophenemethylammonium20 ions were also incorporated as


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interlayer spacers in RP perovskite films that show encouraging device performances. These

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achievements affirm that the structural versatility of 2D perovskites originating from the wide

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choices of A’ cations could provide great opportunities for screening 2D perovskite materials

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with outstanding optoelectronic properties and directing further device performance

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enhancement. Currently, the A’ cations in 2D perovskites involve primary ammonium ions

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(RNH3+, R is an alkyl or aryl group), where their high degree of substitution for the sites

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anchoring to Pb-I octahedra is largely overlooked. It was evidenced that quaternary ammonium

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ions (R4N+) could effectively passivate the charge defects in 3D perovskite films.5,26-27 In

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addition, the deduction of the hydrogen bonds (NH2-H···Pb-I) in R4N+ may considerably affect

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the optoelectronic properties of the resultant perovskites.12 These merits drive us to exploit

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quaternary ammonium ions as organic spacers for a new class of 2D perovskites.

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Herein, a new organic spacer cation, phenyltrimethylammonium (PTA+), is employed to

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prepare the RP type 2D perovskite films. PTA+ has a molecular structure reminiscent of PEA+,

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with each of the N-H hydrogen atoms substituted by a -CH3 group, which constitutes a R4N+ ion.

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The perovskite films based on (PTA)2(MA)n-1PbnI3n+1 with a series of n values are prepared,

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which show distinct 2D characteristics both optically and crystallographically. The

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(PTA)2(MA)3Pb4I13 film is further evaluated and optimized by adding various ratios of Cl- ions

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during film growth. Remarkably, grazing-incidence X-ray diffraction (GI-XRD) patterns and

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scanning electron microscopy (SEM) images show that the 2D (PTA)2(MA)3Pb4I13 perovskite

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films prepared with the assistance of Cl- ions have highly-oriented crystal structures and uniform

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surface morphology. These features endow the films with superior charge transport ability and

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low density of defect states. The optimized device exhibits a PCE up to 11.53% with good

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resistance to moisture and heat, making it suitable for practical applications.


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The PTA based 2D perovskite films were prepared using a one-step spin-coating method.

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Specifically, phenyltrimethylammonium iodide (PTAI), methylammonium iodide (MAI) and

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lead iodide (PbI2) were dissolved in dimethyl sulfoxide (DMSO) at a Pb2+ concentration of 0.8

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M, with the molar ratio of 2 : (n-1) : n for PTAI : MAI : PbI2 in preparing (PTA)2(MA)n-1PbnI3n+1

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for n = 1-5, respectively (n is set by the ratio of the precursors). The PTA molecule and the

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corresponding 2D perovskite (n = 4) are schematically shown in Figure 1a. The GI-XRD

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patterns, ultraviolet–visible (UV-vis) absorption and photoluminescence (PL) spectra of the

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PTA-based 2D perovskites are presented in Figures S1-2. The typical XRD peaks in the low

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wavevector transfer (q) range and UV-vis characteristics in the short wavelength region exhibit

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the 2D features of these films. The PL peaks gradually red shift with increasing n values for n

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equal to 1-3, which correlates well with the reported bandgap-variation property of 2D

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perovskites.10 For higher n values of 4 and 5, the PL peak positions remain almost the same,

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indicating there are quantum well (QW) thickness distributions in these films other than the pure

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phase product.28 We subsequently focus on the n = 4 species (PTA)2(MA)3Pb4I13 (set by the ratio

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of precursors) for further film characterization and device application, since it delivers better

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photovoltaic performance in our initial assessment (Figure S3) and the successful deployment of

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the n = 4 class of 2D perovskite with other organic spacers in the literatures.11, 24. The UV-vis

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absorption spectrum of the n = 4 film is presented in Figure 1b. The absorption features at 345,

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425 and 475 nm in the short wavelength region could be attributed to the PTA-based 2D

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perovskites with low n values and an optical bandgap of 1.61 eV is estimated from the

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absorption edge (Figure S4). The PL peak of the (PTA)2(MA)3Pb4I13 film locates at 766 nm. It

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shows an asymmetric Gaussian shape with a high-energy tail, likely due to the 2D perovskite

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species with lower n values.28 The XRD pattern of the (PTA)2(MA)3Pb4I13 film shows the typical


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2D features with the diffraction peaks at the lower q values of 0.51, 0.56, 0.6, 0.71 and 0.81 Å-1

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originating from the (0k0) planes and a dominant Bragg reflection peak at q of 1 Å-1 attributing

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to the (111) plane of Pb-I slabs (Figure 1c). The optical and XRD analyses reveal the distribution

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of QW thickness in the (PTA)2(MA)3Pb4I13 film and confirm the high crystal coherency of the

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2D species.28

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Figure 1. (a) Schematic illustrations of the PTA+ ion (blue, white and purple balls represent C, H

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and N atoms, respectively) and the (PTA)2(MA)3Pb4I13 crystal structure. (b) UV-vis and PL

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spectra and (c) XRD pattern of the (PTA)2(MA)3Pb4I13 film.


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In order to gain more crystallographic information of the PTA-based perovskite film, the 2D

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GI-XRD pattern of the (PTA)2(MA)3Pb4I13 species is recorded as shown in Figure 2a. The nearly

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isotropic diffraction rings (especially the one at q = 1 Å-1) indicate the small size and random

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orientation of the 2D perovskite crystals, which is adverse for charge transport within the

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perovskite film.11 We subsequently introduce Cl- as additive to assist the growth of the n = 4

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PTA-based 2D perovskite films. Different molar ratios of methylammonium chloride (MACl)

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are added in the starting precursors to reach a molar ratio of MACl: MAI = x, where x are 0.05,

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0.1, 0.2 and 0.5, respectively. The 2D GI-XRD patterns of the resultant films denoted as 0.05 Cl-,

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0.1 Cl-, 0.2 Cl- and 0.5 Cl- are presented in Figures 2b-e. Upon the addition of Cl-, discrete Bragg

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spots emerge in the 2D GI-XRD patterns and become sharper with less dispersive edges for

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higher Cl- concentrations. This indicates Cl- could effectively increase the crystallinity of the

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film and regulate the crystal orientations. More interestingly, at the low Cl- concentration (0.05),

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the parallel alignment of the diffraction spots of the (111), (002), (2100) planes and the (311),

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(004) planes (highlighted by dotted circles) confirms the preferably vertical orientation of the 2D

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perovskite QWs. As previously reported, the vertically oriented 2D perovskites, which has the

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inorganic Pb-I slabs perpendicular to the substrate, could facilitate charge carrier transportation

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and enhance the photovoltaic performance of solar cell devices.11,29 For films employing higher

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Cl- concentrations (represented by 0.5 Cl- in Figure 2e), the parallelly aligned diffraction spots

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remain sharp (dotted circles) accompanied by the emergence of vertical Bragg spot arrays

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(dashed rectangles). This indicates the coexistence of vertically and horizontally grown 2D

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perovskite crystals in the films using 0.1 0.2 and 0.5 Cl- as additives. It was experimentally

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verified that the incorporation of additives could affect the crystal orientation in the resultant 2D

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perovskite films.15,20 We speculate that the orientation regulating ability of Cl- ions originates


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from its interaction with Pb2+ which changes the precursor availabilities for vertically and

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horizontally oriented Pb-I slabs during solvent evaporation. We also find there are 2D species

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with multiple n values in these films as indicated by the features in the low q range (smaller than

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1) and their higher order diffraction signals. It is noteworthy that these features also change from

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diffraction rings for the pristine 2D film to diffraction spots for the films prepared with Cl-

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additives, which further demonstrates the orientation regulation capability of Cl-. The GI-XRD

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studies on the 2D perovskites adding different amounts of Cl- indicate the film employing 0.05

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Cl- provides the optimal crystal orientation for optoelectronic applications.

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Figure 2. 2D GI-XRD patterns of the (PTA)2(MA)3Pb4I13 films prepared with the MACl:

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MAI ratios of (a) 0, (b) 0.05, (c) 0.1, (d) 0.2 and (e) 0.5. (f-j) SEM images of the

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corresponding films.

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The morphological influence of Cl- on the 2D perovskite films is further investigated by SEM.

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The (PTA)2(MA)3Pb4I13 film prepared without Cl- additive shows low crystallinity with a bumpy


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surface consisting of randomly distributed nanocrystals (Figure 2f). Upon the addition of 0.05 Cl-

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, the film becomes dense and flat with the emergence of micron scale crystal grains (Figure 2g).

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The enhancement in crystallinity upon the Cl- addition is in agreement with the GI-XRD results.

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Further increasing the Cl- concentration leads to distinct film morphologies. For the 0.1 Cl- case,

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discrete polygonal grains emerge from the continuous perovskite film (Figure 2h). As the Cl-

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concentration further increases as for the 0.2 and 0.5 Cl- cases, the emerged grains become larger

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and show higher surface coverage, however, considerable amounts of voids and pinholes exist in

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the resultant films. The discontinuousness of the 2D perovskite films employing higher Cl-

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concentrations may cause shunts between the charge transport layers and form recombination

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centers that impair the resultant device performance. The SEM results demonstrate that the Cl-

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additive could effectively alter the surface morphology, crystallinity and grain sizes of the 2D

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PTA-based perovskite films.

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To evaluate the influence of Cl- additive on the film optoelectronic properties, we subsequently

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carried out UV-vis and PL measurements on the Cl- incorporated 2D perovskite films (Figure

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S5). There is no new feature observed in the absorption spectra upon the addition of Cl- ions,

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while the PL spectra show a side peak at 640 nm next to the original peak at 766 nm for higher

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Cl- concentrations. We rule out the possibility of phase separation since X-ray photoelectron

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spectroscopy (XPS) results indicate there is negligible Cl in the final perovskite films (Figure

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S6). Previously, Cl- ions have been successfully introduced to assist the growth of high-quality

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3D perovskite films.7,30-31 It was concluded that the complex interaction between the Cl- and Pb2+

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ions during the film formation process could alter the crystal nucleation and growth rate thus

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affect the final morphology. The MACl was further sublimated, resulting in the formation of

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pure-phase 3D perovskites. We propose that the addition of Cl- ions during the preparation of the


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PTA-based 2D perovskite could simultaneously alter the crystal orientation and morphology of

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the films through the ionic interactions between Pb2+ and Cl- in precursor solutions. The Cl-

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species are also eliminated during the film crystallization process. The variations in the optical

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spectra may originate from the different levels of defects or a possibly altered distribution of QW

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thicknesses. The high concentrations Cl- ions (0.1, 0.2 and 0.5 Cl-) could adversely alter the

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crystal formation energy and in turn affect the film crystal orientation and morphology. The fine

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tuning of the Cl- additive concentration is required to form high-quality 2D PTA based

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perovskite films.

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We proceed to investigate the influence of the Cl- ions on the photovoltaic behavior of the PTA-

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based 2D perovskite films. Solar cells are fabricated based on the (PTA)2(MA)3Pb4I13 films

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prepared with 0, 0.05, 0.1 and 0.2 Cl-, containing indium doped tin oxide (ITO) as substrate,

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SnO2 nanoparticle film as electron transport layer (ETL), 2D perovskite as light-absorbing layer,

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spiro-OMeTAD as hole transport layer (HTL) and Ag as metal electrode, as schematically shown

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in Figure 3a. The typical cross-sectional SEM image of the 2D perovskite solar cell device based

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on (PTA)2(MA)3Pb4I13 with 0.05 Cl- is also presented in Figure 3a. The thicknesses of the active

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layers are about 40, 200, 150 and 60 nm for the ETL, 2D perovskite, HTL and Ag, respectively.

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The uniform and continuous cross-sectional view of the perovskite film with few grain

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boundaries manifests its high quality. The SnO2 nanoparticle film is adopted due to its proper

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energy level alignment with respect to the 2D perovskite and remarkable charge

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extraction/transportation abilities (Figure 3b). The energy level parameters in Figure 3b are

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obtained from UPS and UV-vis results as presented in Figure S7 and summarized in Table S1.

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The PCE distribution and best J-V performance of the devices based on 0, 0.05, 0.1 and 0.2 Cl-

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are presented in Figures 3c and d. The performance trend is in good accordance with the


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crystallographic and morphological study results. Specifically, the device based on

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(PTA)2(MA)3Pb4I13 film prepared without Cl- exhibit an average PCE of 5.05 % with an open

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circuit voltage (Voc) of 0.93 V, a short circuit current (Jsc) of 12.41 mA/cm2 and a fill factor (FF)

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of 43.79 %. Upon the addition of 0.05 Cl-, the average device PCE boosts to 10.02 % with

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overall improved Voc, Jsc and FF of 1.05 V, 14.07 mA/cm2 and 67.61 %, respectively (Table S2).

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The dramatically enhanced performance is associated with the superior crystallinity, crystal

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orientation, morphology and optoelectronic properties of the Cl- incorporated perovskite films.

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Further elevating the concentration of Cl- to 0.1 and 0.2 considerably deteriorates the device

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performance, leading to the average PCEs of 7.01 % and 4.47 %, respectively. This underscores

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the importance of the fine tuning of Cl- concentrations to attain the optimal 2D perovskite film

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quality. Figure 3e presents the external quantum efficiency (EQE) curves of the best devices

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featuring various Cl- concentrations, where the integrated Jsc are in good agreement with the

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values obtained from the J-V study. It is noteworthy that the features (peaks and dips) in the EQE

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curve directly depict the light absorption and charge transportation processes in the device,

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which are associated with the overall film qualities. Specifically, the dips at ~ 425 nm of the

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EQE spectra match well with the absorption features of the corresponding films (Figure S2),

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which may originate from the non-photoactive 2D species with low n values in the samples.


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Figure 3. (a) Schematic illustration of the 2D perovskite solar cell device architecture

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and the cross-sectional SEM image of a typical device employing (PTA)2(MA)3Pb4I13 with

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0.05 Cl- as the light absorbing layer. (b) Energy level diagram with the values obtained

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from UPS and UV-vis measurements (dashed lines show the energy levels of the 2D

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perovskite prepared with 0.05 Cl-). (c) The device PCE distributions as a function of the


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Cl- concentration. (d) The J-V curves and (e) EQE curves of the best (PTA)2(MA)3Pb4I13

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based solar cell devices prepared with various concentrations of Cl-.

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The optimized (PTA)2(MA)3Pb4I13 based 2D perovskite solar cell with 0.05 Cl- exhibits a PCE

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of 11.53% which is 70% higher than that without Cl- additive. The best device shows a high Voc

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of 1.09 V, Jsc of 14.33 mA/cm2, FF of 73.91% and the stabilized efficiency of 10.18% with small

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hysteresis (Figure S8). To rationalize the outperformance of the devices with Cl- ions, we

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perform light intensity dependent J-V measurements on devices based on 0.05 Cl- and 0 Cl-,

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respectively. The Voc of both devices show linear relationship corresponding to the light intensity

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in log scale (Figure 4a). The fitted slope for the two cases are 1.57 and 1.80 kT/e (k is

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Boltzmann’s constant, T is temperature and e is elemental charge) respectively for 0.05 Cl- and 0

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Cl-, indicating the lower rate of trap-induced recombination in the former case.32-33 The lower

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combination rate in the 0.05 Cl- device contributes to the higher Voc. Figure 4b shows Jsc versus

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light intensity in a double logarithmic scale. The slopes reflecting the space charge effects in the

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devices are close to 1 for both cases, suggesting a low rate of non-geminate recombination.34-35

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The open circuit voltage decay (OCVD) measurement is further introduced to evaluate the

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overall recombination behaviors in the 2D perovskite devices. OCVD is a convenient technique

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to investigate the dynamic processes in photovoltaic devices and has been widely applied in

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sensitized solar cells.36-37 The results in Figure 4c indicate that the Voc decay of the cell with 0.05

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Cl- is much slower than that without Cl-. The lower Voc decay is associated with the longer

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charge carrier lifetime, which is in accordance with the lower rate of charge recombination

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observed.38


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Figure 4. The optoelectronic performances for solar cells with the (PTA)2(MA)3Pb4I13

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films prepared with 0 Cl- and 0.05 Cl-. (a) Voc versus light intensity on a semi-natural

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logarithmic scale. (b) Jsc versus light intensity on a double-logarithmic scale. (c) Open-

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circuit voltage decay curves. (d) Experimental and simulated Nyquist plots (inset shows


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the equivalent circuit) of the devices under illumination at Voc. (e) Steady-state PL

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spectra and (f) Time-resolved PL decay curves of the 2D perovskite films.

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The electrical impedance spectroscopy (EIS) is further introduced to elucidate the charge carrier

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transport and recombination processes in the devices. The Nyquist plots for the 0.05 Cl- and 0 Cl-

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devices are shown in Figure 4d, where the cells are illuminated under the open circuit condition.

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The two semi-circles presented in the Nyquist plots could be attributed to the charge transfer and

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charge recombination processes respectively.39-40 The Nyquist plots are fitted using the

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equivalent circuit given in the inset of Figure 4d. The simulated charge transfer resistance Rct and

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charge recombination resistance Rrec of the 0 Cl- device are 9.6 and 19.1 Ω·cm2, while the lower

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Rct and Rrec values of 4.6 and 9.1 Ω·cm2 are obtained for the 0.05 Cl- device. The lower

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resistance values in the 0.05 Cl- device are associated with the enhanced charge transfer and

13

reduced charge recombination, demonstrating the high quality of the perovskite film.40

14

We further introduce the steady-state PL and time-resolved photoluminescence (TRPL)

15

techniques to exam the quality of the 2D perovskite films prepared with 0.05 Cl- and 0 Cl- (on

16

quartz substrates) as shown in Figures 4e and f. Upon the addition of Cl-, the steady-state PL

17

intensity of the film is increased by ~5 times (Figure 4e). The largely enhanced PL intensity

18

could be attributed to a significantly elongated PL lifetime (Figure 4f). The TRPL curves are

19

fitted with a bi-exponential function of time t, f (t) = A1exp(−t/τ1) + A2exp(−t/τ2) + B, where τ1

20

and τ2 represent the time constants of the fast and slow decay processes respectively, A1 and A2

21

are the corresponding decay amplitudes, B is a constant. The fast decay component τ1 is

22

associated with the trap states formed due to vacancies or interstitials, while the slow component


15

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τ2 is assigned to the free carrier radiative recombination. The average recombination lifetime

2

(τavg) associated with the overall recombination process could also be estimated from the TRPL

3

data.41 All the fitted TRPL parameters for the 0.05 Cl- and 0 Cl- films are listed in Table S3. The

4

average lifetime of the 2D perovskite films prepared with 0 Cl- (2.2 ns) is considerably prolonged

5

to 12.4 ns in the 0.05 Cl- samples, indicating the Cl- treatment could effectively reduce defect

6

states and increase charge carrier lifetime.

7

The encouraging performance of the PTA based 2D perovskite could be attributed to the unique

8

molecular structure of the organic spacer with four methyl groups anchoring on the ammonium

9

N atom. To verify this, we further employ phenylammonium (PA, primary ammonium), N-

10

phenylmethylammonium (PMA, secondary ammonium) and N,N-dimethylphenylammounium

11

(DPA, tertiary ammonium) as organic spacers to prepare 2D perovskites. The chemical structures

12

of these organoammonium ions are presented in Figure 5a. The n = 4 perovskite films are

13

prepared employing the corresponding organoammonium iodide salts with 0.05 Cl- as additive

14

(the synthesis and sample preparation details are given in the supporting information). The PA,

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PMA and DPA based solar cells show the average PCEs of 3.9%, 7.8% and 4.3%, respectively

16

(Figure 5b). As shown in Figure 5c, the PTA based PSC delivers the best efficiency with the

17

overall higher Voc and FF values. The higher Voc and FF values of the device are generally

18

associated with reduced charge recombination and elongated charge carrier lifetime,41 which

19

implies that the PTA organic spacer with three methyl groups could most effectively eliminate

20

defects/traps in 2D perovskite films. The relatively lower Jsc of the PTA based device, as we note

21

above, could be due to the presence of the non-photoactive 2D species in the PTA based films.

22

As previously reported, a subtle modification on the chemical structure of the organic spacer

23

could cause significant change to the properties of the resultant 2D perovskite, possibly due to


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altered formation energy.42 The methyl groups simultaneously alter the rigidity, size and

2

dielectric constant of the organic interlayer spacer, which could indirectly yet significantly affect

3

the optoelectronic properties of the resultant perovskites.43

4

5 6

Figure 5. (a) Chemical structures of the PA, PMA, DPA and PTA organic spacers. (b) PCE

7

distributions. (c) J-V curves of the best devices featuring the different organic spacers.

8 9

We track the XRD patterns of the (PTA)2(MA)3Pb4I13 based 2D perovskite films (0.05 Cl-)

10

which are stored under different conditions to investigate their long-term stability. The films are


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placed at 30 ± 10% relative humidity (RH) for moisture stability test and at 85 oC under N2 for

2

thermal stability test, respectively, with the 3D MAPbI3 films taken as references. According to

3

the results presented in Figure 6a, the XRD patterns of the 2D perovskite films show no

4

noticeable change either at the ambient condition (30 ± 10% RH) after 60 days or at 85 oC for 20

5

days, indicating the remarkable stability of the PTA-based 2D perovskites, whereas the 3D

6

MAPbI3 films start to decompose to PbI2 in both conditions (Figure 6b). We further investigate

7

the stability of the unencapsulated PTA-based device by storing it in N2. The 2D perovskite

8

based device retained 90% of its initial PCE after 1600 h, whereas the reference 3D solar cell

9

shows significant degradation (Figure 6c). The devices are also aged at 30 ± 10% RH or 50 oC to

10

test the environmental and thermal stabilities. Although the performances of both the 2D and 3D

11

devices decay faster than those in the N2 condition possibly due to the instability of dopants in

12

HTM and the diffusion of metal electrodes, the 2D (PTA)2(MA)3Pb4I13 based devices show

13

overall better stabilities as compared with the MAPbI3 based devices (Figure S9). We attribute

14

the better stabilities of the 2D perovskite solar cells to the low volatility and the hydrophobic

15

nature of the PTA+ organic spacer. We also compare the stability of the quaternary ammonium

16

PTA based 2D perovskite with its primary ammonium PA based counterpart. The film XRD and

17

device PCE evolution results (Figure S10) show the superior stabilities of the PTA based film

18

and device. There are multiple factors, such as the incorporation of lipophilic methyl groups, that

19

could benefit the stability of the PTA based 2D perovskites, which further emphasizes the

20

importance of molecular designs for the interlayer organic spacers in 2D perovskites.


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Figure 6. Environmental and thermal stability tests of (a) 2D (PTA)2(MA)3Pb4I13 and (b)

3

3D MAPbI3 films by tracking their XRD patterns. (c) Device stability of the 2D and 3D

4

perovskite solar cell devices in N2.

5 6

In summary, PTA+ is successfully employed as a new organic interlayer spacer to prepare the

7

2D Ruddlesden-Popper perovskites with remarkable optoelectronic properties. The Cl- ions with

8

the optimized concentration of 0.05 M (with respect to MAI) could effectively regulate the

9

crystallization of the PTA-based 2D perovskite through their interaction with Pb2+, which

10

produces high-quality (PTA)2(MA)3Pb4I13 films with vertically aligned crystal orientations, large

11

grain sizes, pinhole-free morphology and suitable energy levels. The PTA-based 2D perovskite

12

solar cell devices deliver the best PCE of 11.53%, mainly ascribed to the good charge transfer


19

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capability and the low defect-induced recombination rate. In addition, the PTA-based 2D

2

perovskite films and solar cell devices demonstrate extraordinary resistances to moisture and

3

heat. In view of their exciting properties, the PTA-based 2D perovskites are expected to be a new

4

source for high-performance optoelectronic devices with technologically relevant long-term

5

stability.

6 7

ASSOCIATED CONTENT

8

Supporting Information. The Supporting Information is available free of charge on the

9

ACS Publications website.

10

Experimental section; GI-XRD results, PL and UV-vis spectra of the

11

(PTA)2(MA)n-1PbnI3n+1

12

(PTA)2(MA)n-1PbnI3n+1 films with n = 3-5; UV-vis reflection absorption

13

spectrum and Tauc plots of the 2D (PTA)2(MA)3Pb4I13 film; PL and UV-vis

14

spectra of the (PTA)2(MA)3Pb4I13 films prepared with different MACl: MAI

15

ratios; XPS, UPS, UV-vis spectra and Tauc plots of the (PTA)2(MA)3Pb4I13

16

films prepared with 0, 0.05 and 1 Cl-; J-V hysteresis behavior and stabilized

17

PCE of the optimized device; additional stability test of the 2D perovskite

18

solar cell devices; stability tests of the PA and PTA based films and devices;

19

energy levels, photovoltaic performance and charge carrier lifetime parameters of

20

the 2D perovskite films (PDF)

films;

PCE

distributions

and

J-V

curves

for


20

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AUTHOR INFORMATION

3

Corresponding Author

4

*E-mail: (K.M.) [email protected].

5

*E-mail: (G.C.) [email protected].

6

Author Contributions

7

#

8

Notes

9

The authors declare no competing financial interest.

Z.L. and N.L. contributed equally to this work.

10

ACKNOWLEDGMENT

11

This work was financially supported by National Natural Science Foundation of China

12

(U1632265, 51802194) and the Science and Technology Commission of Shanghai

13

Municipality (17YF1412000, 19ZR1476900). The authors thank all the team members at

14

beamline BL14B1 of SSRF.

15

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Figure 1. (a) Schematic illustrations of the PTA+ ion (blue, white and purple balls represent C, H and N atoms, respectively) and the (PTA)2(MA)3Pb4I13 crystal structure. (b) UV-vis and PL spectra and (c) XRD pattern of the (PTA)2(MA)3Pb4I13 film. 123x120mm (300 x 300 DPI)


Nano Letters 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

Figure 2. 2D GI-XRD patterns of the (PTA)2(MA)3Pb4I13 films prepared with the MACl: MAI ratios of (a) 0, (b) 0.05, (c) 0.1, (d) 0.2 and (e) 0.5. (f-j) SEM images of the corresponding films. 225x91mm (300 x 300 DPI)


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Figure 3. (a) Schematic illustration of the 2D perovskite solar cell device architecture and the cross-sectional SEM image of a typical device employing (PTA)2(MA)3Pb4I13 with 0.05 Cl- as the light absorbing layer. (b) Energy level diagram with the values obtained from UPS and UV-vis measurements (dashed lines show the energy levels of the 2D perovskite prepared with 0.05 Cl-). (c) The device PCE distributions as a function of the Cl- concentration. (d) The J-V curves and (e) EQE curves of the best (PTA)2(MA)3Pb4I13 based solar cell devices prepared with various concentrations of Cl-. 157x162mm (300 x 300 DPI)


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Figure 4. The optoelectronic performances for solar cells with the (PTA)2(MA)3Pb4I13 films prepared with 0 Cl- and 0.05 Cl-. (a) Voc versus light intensity on a semi-natural logarithmic scale. (b) Jsc versus light intensity on a double-logarithmic scale. (c) Open-circuit voltage decay curves. (d) Experimental and simulated Nyquist plots (inset shows the equivalent circuit) of the devices under illumination at Voc. (e) Steady-state PL spectra and (f) Time-resolved PL decay curves of the 2D perovskite films. 160x179mm (300 x 300 DPI)


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Figure 5. (a) Chemical structures of the PA, PMA, DPA and PTA organic spacers. (b) PCE distributions. (c) J-V curves of the best devices featuring the different organic spacers. 172x124mm (300 x 300 DPI)


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Figure 6. Environmental and thermal stability tests of (a) 2D (PTA)2(MA)3Pb4I13 and (b) 3D MAPbI3 films by tracking their XRD patterns. (c) Device stability of the 2D and 3D perovskite solar cell devices in N2. 160x102mm (300 x 300 DPI)


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TOC figure 158x60mm (300 x 300 DPI)


A New Organic Interlayer Spacer for Stable and Efficient 2D

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