Time-Resolved Electrical Scanning Probe Microscopy of Layered

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Time-Resolved Electrical Scanning Probe Microscopy of Layered Perovskites Reveals Spatial Variations in Photoinduced Ionic and Electronic Carrier Motion Rajiv Giridharagopal, Jake T. Precht, Sarthak Jariwala, Liam Collins, Stephen Jesse, Sergei V. Kalinin, and David S Ginger ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08390 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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Time-Resolved Electrical Scanning Probe Microscopy of Layered Perovskites Reveals Spatial Variations in Photoinduced Ionic and Electronic Carrier Motion

Rajiv Giridharagopal,1 Jake T. Precht,1 Sarthak Jariwala,1 Liam Collins,2 Stephen Jesse,2 Sergei V. Kalinin,2 David S. Ginger1* 1 Department 2Center

of Chemistry, University of Washington, Seattle, WA, 98195, United States

for Nanophase Materials Science, Oak Ridge National Lab, Oak Ridge, TN, 37830, United States *Corresponding Author: [email protected]

TABLE OF CONTENTS GRAPHIC

Keywords: layered perovskites, time-resolved electrostatic force microscopy, G-Mode, Kelvin probe force microscopy, big data microscopy, Ruddlesden-Popper

ABSTRACT We study light-induced dynamics in thin films comprising Ruddlesden-Popper phases of the layered 2D perovskite (C4H9NH3)2PbI4 (BAPI). We probe ionic and electronic carrier dynamics


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using two complementary scanning probe methods, time-resolved G-mode Kelvin probe force microscopy (G-KPFM) and fast free time-resolved electrostatic force microscopy (FF-trEFM), as a function of position, time, and illumination. We show that the average surface photovoltage sign is dominated by the band bending at the buried perovskite-substrate interface. However, the film exhibits substantial variations in the spatial and temporal response of the photovoltage. Under illumination, the photovoltage equilibrates over hundreds of microseconds, a timescale associated with ionic motion and trapped electronic carriers. Surprisingly, we observe that the surface photovoltage of the 2D grain centers evolves more rapidly in time than at the grain boundaries. We propose that the slower evolution at grain boundaries is due to a combination of ion migration occurring between PbI4 planes, as well as electronic carriers traversing grain boundary traps, thereby changing the time-dependent band unbending at grain boundaries. These results provide a model for the photoinduced dynamics in 2D perovskites and are a useful basis for interpreting photovoltage dynamics on hybrid 2D/3D structures.

Organic-inorganic hybrid halide perovskites are widely-studied materials for thin film optoelectronics, including photovoltaics and light-emitting diodes.1-4 Conventionally, films for photovoltaic applications have been made from the typical 3D perovskite ABX3, where A = a monovalent (often organic) cation, B = a divalent (typically inorganic, Pb2+) cation, and X = a monovalent halide anion; the canonical halide perovskite structure used for solar cells being CH3NH3PbI3 (MAPI). However, ion motion, stability, and defect formation are factors that still need to be understood and controlled.5-7


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In an effort to improve moisture-related stability issues that can affect 3D perovskites, several groups have reported the use of reduced dimensional phases, including 2D layered Ruddlesden-Popper systems.8-14 Lower-dimensional perovskite materials also exhibit attractive benefits in light-emitting applications15-19 given their high (100-300 meV)20-24 exciton-binding energies compared to the small ~7 meV binding energies of conventional perovskites.25 Recent efforts have shown improvements in the performance in these materials, in particular with purported 2D/3D combinations that tune the benefits of 3D perovskites with enhanced stability due to the spacer cations.9, 11, 12, 16, 26 While the hydrophobic surfaces and general reduction in water absorption likely play a role in the longer lifetimes of the 2D perovskites,8, 12 the exact origin of their increased stability, yet mixed performance, compared to their 3D parent structures is still unclear. Ion motion, perhaps dominated by grain boundaries, perhaps by halide vacancies or intrinsic defects,6 is currently believed to be a major source of instability in 3D hybrid perovskites and their derivative structures.5, 6, 27 In addition to being the likely source of hysteresis in many devices, ion motion can, in conjunction with charge injection, lead to formation of non-radiative defect centers,28 and has been proposed as a dominant source of loss and instability threatening the long-term success of hybrid perovskite-based technologies.27, 29 In this context, one hypothesis put forward to explain the improved stability of the layered 2D hybrid perovskites relative to their 3D analogs is a reduction in ion mobility in the 2D phase due to the planes of (typically) hydrophobic aliphatic chains parallel to the layering axis.30, 31 On the other hand, ion motion across the layered planes has been demonstrated, even in single crystals.32 In order to better understand these phenomena, we study local photoinduced ion motion in 2D Ruddlesden-Popper phases.


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Ruddlesden-Popper layered films are formed by using bulky organic cations that cannot intercalate within the perovskite layers. One common example of Ruddlesden-Popper halide perovskites occurs with the use of butylammonium cations in butylammonium/methylammonium lead iodide, which has the general form (C4H9NH3)2(CH3NH3)n-1PbnI3n+1. In the limit of n=1 these films consist of a single PbI4 layer separated by butylammonium cations (Fig. 1A) (BAPI). Single layer films like BAPI provide a limiting case from which processing can be optimized to improve, for instance, luminescence quantum yield15 in LEDs or to tailor cations to enhance PCE in solar cells.33 Furthermore, while early reports tended to focus on spatially-averaged photoluminescence properties,34,

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the more recent trend towards mixed 2D/3D systems has renewed interest in

understanding nanoscale electronic structure-function relationships. Beyond those systems, the wide tunability of 2D systems has encouraged intense research in studying their basic properties14 and in other applications like transistors.36, 37 Here, we show that layered n=1 BAPI films exhibit highly heterogeneous dynamics across multiple timescales in response to photoexcitation, with grain boundaries exhibiting slower photoinduced charge buildup relative to grain interiors. We use a combination of two dataintensive electrical scanning probe methods, time-resolved EFM (FF-trEFM)38-44 and GeneralMode KPFM (G-KPFM), to locally probe photoinduced dynamics at microsecond timescales. We propose that these dynamics are related to a combination of ion motion and relatively slow carrier motion through energetic traps at these boundaries, given the ~hundreds of microseconds timescales observed. Together, these data also demonstrate the utility of modern “big data” processing methods for scanning probe information.45


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Fig. 1. Representative schematic and structure of (C4H9NH3)2PbI4 (BAPI) (A) Structure of the 2D Ruddlesden-Popper BAPI system, of general form (C4H9NH3)2(CH3NH3)n-1PbnI3n+1 where n=1 here. (B) Representative UV-Vis and photoluminescence spectra of BAPI films. (C) Representative topography of BAPI on ITO showing grains of several microns.

RESULTS AND DISCUSSION We study BAPI, (BA)2(MA)n−1PbnI3n+1 with n=1, as the simplest representative member of the layered Ruddlesden-Popper lead halide perovskites with butylammonium cations. Fig. 1 shows the crystal structure (Fig. 1A), optical characterization (Fig. 1B) (UV-Vis and PL), and AFM topography (Fig. 1C) of the BAPI (n=1) films typical of those we used for this study. The


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data in Fig. 1 are characteristic of BAPI thin films, and consistent with the reported bandgap (~2.24 eV8, 46) (Fig. 1B) as well as exciton absorption at ~510 nm reported previously8, 33, 46 that is ndependent,47 though not layer-dependent.48 BAPI films on ITO tend to form grains with scale of several microns, (Fig. 1C) with the PbI4 planes parallel to the surface,8 and a roughness in the grains of ~15 nm with some voids between crystals. On TiO2 we observe similar structures with albeit slightly smaller crystal sizes (~1-2 μm) (Fig. S1A). Using the preparation described in the Methods section, we yield films with ~500 nm thickness and grains of >1 µm diameter. X-ray diffraction data (Fig. S1) are consistent with those results found in similar n=1 materials for the (00l) locations.8, 49, 50

Fig. 2. Photoluminescence mapping of BAPI films. (A) Confocal photoluminescence mapping with λ=470 nm (9 μJ/cm2 per pulse) excitation over a representative BAPI film on glass. (B) Local excitation and widefield collection of photoluminescence. Here, the lifetime is short enough (~1 ns, Fig. S2, S3) that only the area adjacent the laser spot is observed +/- ~1 μm, in contrast to the case with 3D perovskites. The beam diameter, measured as ~300 nm, is shown as a red circle in the center of each excitation spot. The intensity variation in (A) is due to photodegradation over


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the course of the scan under high fluence, which is common for the n=1 films even when imaged in dry N2.

We first study these films with conventional confocal photoluminescence (PL). These images show that that the emission properties of the film are heterogeneous, with brighter areas exhibiting greater PL intensity (see also Fig. 2).51 However, the short lifetime of

Time-Resolved Electrical Scanning Probe Microscopy of Layered

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