Unveiling Structurally Engineered Carrier Dynamics in Hybrid Quasi

Letter pubs.acs.org/JPCL

Unveiling Structurally Engineered Carrier Dynamics in Hybrid QuasiTwo-Dimensional Perovskite Thin Films toward Controllable Emission Qiuyu Shang,†,# Yunuan Wang,†,‡,# Yangguang Zhong,†,§ Yang Mi,§ Liang Qin,§ Yuefeng Zhao,*,‡ Xiaohui Qiu,§ Xinfeng Liu,§ and Qing Zhang*,†,∥ †

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China School of Physics and Electronics, Shandong Normal University, Jinan 250014, China § CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China ∥ Research Center for Wide Gap Semiconductor, Peking University, Beijing 100871, China ‡

S Supporting Information *

ABSTRACT: Quasi-two-dimensional Ruddlesden−Popper perovskites driving carrier selfseparation have rapidly advanced the development of high-performance optoelectronic devices. However, insightful understanding of carrier dynamics in the perovskites is still inadequate. The distribution of multiple perovskite phases, crucial for carrier separation, is controversial. Here we report a systematic study on carrier dynamics of spin-coated (C6H5CH2CH2NH3)2(CH3NH3)n−1PbnI3n+1 (n = 3 and 5) perovskite thin films. Efficient electrons transfer from small-n to large-n perovskite phases, and holes transfer reversely with time scales from ∼0.3 to 30.0 ps. The multiple perovskite phases are arranged perpendicularly to substrate from small to large n and also coexist randomly in the same horizontal planes. Further, the carrier separation dynamics is tailored by engineering the crystalline structure of the perovskite film, which leads to controllable emission properties. These results have important significance for the design of optoelectronic devices from solar cells, light-emitting diodes, lasers, and so forth.

O

perovskite, combining advantages of 2D and 3D perovskites, has received increasing attention and has been studied extensively recently.28,30−33 A quasi-2D perovskite is also called a Ruddlesden−Popper (RP) perovskite,3 which has a chemical formula of (A, R)2(MA)n−1MnX3n+1, where A+ is a large aliphatic or R+ aromatic alkylammonium cation, MA+, M2+, and X− are the CH3NH3+ cation, metal cation, and halide anion, respectively, and n is the layer number of the 3D perovskite embedded between two layers of A+ cations. In the quasi-2D structures, the organic cation layer owning a large electronic band gap serves as a barrier layer, and the 3D perovskite layers function as well layers. Therefore, quasi-2D perovskites naturally form quantum well structures with an atomically sharp interface between “barriers” and “wells”. The optical and electronic properties can be easily tailored by wells thickness, herein quantized as the layer number n of 3D perovskites. In particular, a recent in-depth study on the structure and photophysics of quasi-2D perovskites found that the quasi-2D perovskites are not single-phase but rather consist of a collection of phases exhibiting a variety of n values even

rganic−inorganic hybrid perovskites are a kind of new material that is self-assembled by organic and inorganic components at the molecular scale. They combine the advantages of organic and inorganic components and have great potential in the field of photovoltaic applications. The photon energy conversion efficiency of a solution-processed three-dimensional (3D) perovskite thin film solar cell has been enhanced from 3.8 to 22.1% in a few years.1−6 The superior photovoltaics of perovskites are attributed to their long charge carrier diffusion length,7−13 bipolar carrier transport,12−14 large optical absorption coefficient,12−14 high charge mobility,12,13 low trap states,14,15 and so forth. Furthermore, as a direct-bandgap semiconductor, their structure and energy band are both designable and adjustable, and therefore perovskites can be widely used in photodetectors, single photons, lasers, and other fields.16−28 However, these 3D lead halide perovskites show poor stability to atmospheric moisture, which would extensively hinder their applications. As a contrast, a two-dimensional (2D) perovskite with atomically sharp quantum well structure exhibits better environmental stability; besides, the quantum confinement effect, flexible composition, and structure homogeneity have led to successful applications in emitting diodes and light-harvesting devices.3 However, due to their large exciton binding energy (hundreds of meV),4,29 a 2D perovskite is not an ideal material for electronic devices in great need of free carriers. To solve this problem, a quasi-2D © 2017 American Chemical Society

Received: July 18, 2017 Accepted: August 28, 2017 Published: August 28, 2017 4431

DOI: 10.1021/acs.jpclett.7b01857 J. Phys. Chem. Lett. 2017, 8, 4431−4438

Letter

The Journal of Physical Chemistry Letters

Figure 1. Structure of spin-coated quasi-2D RP perovskites (n = 3). (a) Schematics of the crystalline structure of quasi-2D RP perovskites. (b) Cross section SEM image of an as-grown quasi-2D RP perovskite film spin-coated on an ITO glass substrate (n = 3). The scale bar is 200 nm. (c) XRD of as-grown lead halide perovskites.

though they were intended to be grown as single-phase.4,34,35 With this distinct stacking geometry, free carriers, including electrons and holes, can be effectively separated in RP perovskites. Due to their good environmental stability, high quantum yield, and carrier separation efficiency, quasi-2D perovskites have been widely used in LED devices with external quantum efficiencies (EQEs) up to 11.7%.35 Despite these remarkable successes, there are still two important issues needing further study. First, the crystalline structure of quasi2D perovskites, in particular, the distribution and alignment of multiple phase perovskites, is still controversial, which is critical for carrier transfer dynamics among multiphase perovskites. For instance, Yuan et al. reported that the multiple phases in (C6H5CH2CH2NH3, PEA)2(MA)n1PbnI3n+1 are arranged randomly on the substrate,4 while Wang et al. considered that in (C4H9NH3, BA)2(MA)n−1PbnI3n+1 the multiple phases are arranged from small to large n along the direction vertical to the substrate.35 The controversy may be due to the difference of the large aliphatic cation. Compared with insightful understanding of carrier dynamics in (BA)2(MA)n−1PbnI3n+1 thin films, the study of carrier properties of (PEA)2(MA)n−1PbnI3n+1 is still inadequate.34,36 The second issue arises on how to control carrier and exciton dynamics through structural engineering of quasi-2D perovskites and then improve performance of optoelectronic devices.37−39 For instance, photoelectric conversion devices, that is, LEDs and solar cells, benefit from efficient free carrier separation, whereas excitons are preferred in photonic and excitonic devices in need of high luminous efficiency or stable exciton states. Herein, charge carrier dynamics in (PEA)2(MA)n1PbnI3n+1 RP perovskite films has been studied using transient absorption spectroscopy (TAS), low-temperature photoluminescence (PL) spectroscopy, and Kelvin-probe force microscopy (KPFM). Fast and efficient carrier transfer occurs between the multiple phases of the perovskite with a rate of sub-1 ps and efficiency near 100%.34 The multiple phases of perovskites are arranged along the direction vertical to the substrate from small to large n and also coexist randomly in the same horizontal planes. Besides, carrier transfer could be tailored by engineering the crystalline structure of perovskites so that efficient charge transfer occurs in films with low density of pinholes. These results have important applications for the development of perovskite light conversion and detection devices. Figure 1a shows schematics of the crystalline structure for quasi-2D perovskites (n = 3), which consist of an inorganic layer constructed by a top-connected [PbI6]2− octahedron and

an organic layer of the phenethylamine cation (C6H5CH2CH2NH3+, PEA+). The inorganic and organic layers are alternately arranged in space to form a layered structure, which naturally forms quantum well structures with an atomic sharp interface. If there is only a PEA+ cation, it is a 2D perovskite (n = 1); meanwhile, it is a 3D perovskite (n = ∞) when there is only MA+; the remainings are quasi-2D RP perovskites (n = 2, 3, 4, 5, ...). The quasi-2D perovskites were prepared by the solution-processed spin-coated method (Figure S1a). The morphology and crystalline structures of as-grown quasi-2D perovskite thin films are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). Figure 1b shows a cross-sectional SEM image of an n = 3 perovskite film on an ITO glass substrate, which suggests that the as-grown perovskite film is a dense, pinhole-free film with a thickness of ∼250 nm. The top-view SEM and AFM images (Figure S1b,c, prepared as n = 3) along with corresponding cross section SEM image show that the film has a smooth and uniform surface coverage with a root-mean-square (rms) of ∼7.8 nm. Room-temperature XRD spectra of 2D, quasi-2D (n = 2, 3, 5), and 3D perovskite films are shown in Figure 1c from the lower to upper panels, respectively. The diffraction of (110) and (220) planes for the 3D perovskite at 14.6 and 28.9° is consistent with that from previous reports.40 However, the sharp diffraction peaks of the (00l, l = 2, 4, 6, 8, ...) planes at 5.8, 11.3, 16.8, 22.3, and 27.7° observed in the 2D perovskite indicate that 2D perovskite may be a split of 3D material in the direction of the (110) plane. These sharp diffraction peaks were also observed in the n = 2 perovskite film, which shows that the organic−inorganic layer grows along the direction perpendicular to the substrate. For the n = 3 perovskite film, in addition to these diffraction peaks of (00l, l = 2, 4, 6, 8, ...) planes, the diffraction peak of the (111) plane at 14.5° was also observed as the result of competition between PEA+ ions and MA+ ions, considering that PEA+ ions limit the growth of the film in the plane and MA+ ions tend to make the material grow toward the outside of the plane. Also, it is more apparent for the n = 5 perovskite film. Steady-state optical spectroscopy was first conducted to explore exciton features of as-grown quasi-2D RP perovskites. Figure 2a shows room-temperature absorption spectra on the log scale of quasi-2D RP perovskites with different n values (n = 1, 2, 3, 5, ∞). Only one single sharp exciton absorption peak at 516 nm (E1) is observed for the n = 1 (2D) perovskite film, which suggests that the exciton is stable at room temperature 4432


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corresponding PL peak is gradually red-shifted, locating at ∼708 and ∼731 nm, which is consistent with the absorption spectrum in which large n value phases exhibit smaller intrinsic exciton absorption energy. Notably, the emission peaks for n = 3 and 5 are not located near their intrinsic exciton absorption peaks at ∼609 and ∼680 nm, which may be due to the fact that the major photogenerated carriers’ population travels to the edges of the crystal and efficiently emits photons at a lower energy.41 Meanwhile, the emission peak is ∼764 nm for n = ∞, and the corresponding exciton absorption peak is 753 nm. Therefore, by modulating the composition ratio of PEA+ and MA+, the quasi-2D perovskite films could exhibit tunable luminescence from green (520 nm) to red (764 nm). In Figure 2c, the intrinsic exciton absorption energy deduced from experiment (solid green dots, approximating the n = ∞, dashed lines) is always bigger than that calculated by the quantum well model (solid red line) simply adopting a well layer thickness of n multiples of 3D perovskite thickness d3D. When the temperature decreases from 295 to 80 K, only one peak is observed in PL spectroscopy (Figure S3) for n = 1, 3, and 5 perovskite films, suggesting (1) low density of bound exciton states due to defects and (2) that the carriers’ selfseparation from small to large n perovskite phases is still efficient and a main channel in the exciton decay process even that the exciton is much more stable at low temperature. The emission peak is red-shifted, which is similar to other 3D pure inorganic and 2D lead halide perovskites reported in previous works.42 The PL intensity increases when the temperature decreases from 295 to 110 K for n = 5 (Figure S3c). With further decreasing temperature from 110 to 80 K, the PL intensity decreases. Similar temperature-dependent behaviors are observed at 160 and 140 K for n = 1 and 3. The fwhm of the emission peak significantly broadens when the temperature increases from 80 to 295 K (i.e., fwhm from 57.66 to 71.42 nm for n = 5), possibly due to enhanced phonon scattering at higher temperature. The temperature-dependent emission intensity of n = 1, 3, and 5 perovskite films above the phase transition point are plotted in Figure 2d, and it can be fitted using the following formula10

Figure 2. Steady-state absorption and PL spectroscopy of spin-coated quasi-2D RP perovskite thin films. (a) Absorption spectra on the log scale of as-grown quasi-2D RP perovskite films spin-coated on an ITO glass substrate. (b) Normalized PL spectra of as-grown quasi-2D RP perovskites under front-excitation with a continuous wave (CW) 405 nm laser. (c) n dependence of absorption peaks extracted from (a); the dashed line indicates the absorption peaks of the n = ∞ perovskite phase. Dashed line: band gap energy for pure 3D perovskite. Green dots: exciton absorption energy out of the absorption spectra checked with references. Red curve: calculated n-dependent fundamental emission energy based on the quantum wells model. (d) Temperature dependence of the integrated PL intensity for n = 1 (red dots), 3 (yellow dots), and 5 (green dots). The solid lines are fitted curved using the Arrhenius equation.

for n = 1. As for n = 2, new absorption peaks beyond the E1 peak appear at 567 nm (E2), 609 nm (E3), 640 nm (E4), and 680 nm (E5), which is consistent with previous reports for different n phases.34,41 With a further increase of [PbI6]2− layers (n = 3, 5), the E1, E2, and E3 peaks become weaker due to the lesser composition of these phases. Particularly, E1 is hard to resolve when n = 5, suggesting the small composition of the n = 1 phase. Apart from the aforementioned exciton absorption peaks, the peak showing up at around 700 nm which is close to 3D perovskite (n = ∞), may result from a group of perovskite phases with large n values. For 3D perovskite, the sharp excitonic peaks could not easily be resolved and the absorption edge appears at around ∼753 nm.4,35,41 Figure 2b shows steady-state PL of as-grown perovskites with different n values (n = 1, 2, 3, 5, ∞). Compared with multiple peaks observed in absorption spectroscopy, only one emission peak is generally identified by emission spectroscopy for each perovskite film. The emission peak is 520 nm for n = 1, which corresponds to the E1 exciton absorption peak at ∼516 nm. The 4 nm red shift is due to Stokes shifts because of exciton− phonon scattering. For n = 2, the emission peak moves toward ∼573 nm, which corresponds to the E2 exciton absorption peak at ∼567 nm. Compared with the E1 exciton peak with a full width at half-maximum (fwhm) of ∼15 nm, the E2 exciton has a lower energy and larger fwhm (∼32 nm), which is due to the increasing quantum well thickness (two [PbI6]2− layers). With an increase of the [PbI6]2− layer number for n = 3 and 5, the band gap gradually becomes smaller, and therefore, the

I (T ) =

I(0) 1 + c1 exp( −E B /κBT )

(1)

in which κB, EB, I(0), and c1 are Boltzmann’s constant, the exciton binding energy, the PL intensity at 0 K, and a dimensionless constant, respectively. The evaluated exciton binding energies of n = 1, 3, and 5 perovskites are 191.5, 136.5, and 113.5 meV, respectively, which are close to those in the previous reports.4,42 It could be understood that the wave functions of electrons and holes are more overlapped in the wells with smaller thickness, leading to larger exciton binding energy. To clearly elucidate the interaction mechanism between different n value phases in as-grown quasi-2D RP perovskite films, the dynamics of the charge carrier was investigated using femtosecond TAS excited at different wavelengths and directions. There are two excitation methods in which the laser beam (400 nm) first impinging the perovskite (or the ITO glass substrate) is defined as the front (or back) excitation. In TAS experiments (instrument response function (IRF) = 150 fs, Figure S2b), the lead halide perovskite film is first excited under back-excitation with a femtosecond 400 nm laser pulse (power: 2 μJ/cm2) to conduct TAS spectroscopy, and 4433


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Figure 3. TAS for spin-coated quasi-2D perovskite films (∼250 nm thickness, prepared as n = 3 and 5). (a) Schematic of carrier transfer in the n = 3 perovskite film. The electron transfers from small-n to large-n perovskite phases, and the hole transfers from large-n to small-n perovskite phases. The transfer times are extracted from the TA kinetics (c). (b) Photoinduced changes in TAS (ΔA) of the n = 3 perovskite film at selected probe delay times under back-excitation at 400 nm, which shows photobleaching at PB1 (2.40 eV), PB2 (2.19 eV), PB3 (2.04 eV), and PBn>3 (1.85 eV). (c) TA kinetics probed at 2.40, 2.19, 2.04, and 1.85 eV for the n = 3 perovskite film extracted from TAS (b). Solid lines are fitting results of the kinetics by a multiexponential function. The time constants are indicated along with the curves in the same color. (d) Schematic of carrier transfer in the n = 5 perovskite film, indicating that there is little n = 1 phase in this thin film. (e) TAS for the n = 5 perovskite film at different probe delay times under back-excitation at 400 nm, which shows photobleaching at PB2 (2.19 eV), PB3 (2.04 eV), and PBn>3 (1.70 eV). (f) TA kinetics probed at PB2 (2.19 eV), PB3 (2.04 eV), and PBn>3 (1.70 eV) for the n = 5 perovskite film. The time constants are indicated along with the curves in the same color.

of the n = 2 perovskite phase shows an ultrafast dominant decay of around 0.5 ps, which also matches the formation time of the PB3 formation time (∼0.5 ps). The larger PB formation time, that is, the electrons transfer time to n > 3 perovskite phases (∼30.0 ps), is proof for the population of the carriers transferring to large n phases. These results indicate that a substantial portion of photogenerated electrons from n = 1, 2, and 3 perovskite phases will be localized to n > 3 phases within 30.0 ps. Besides, the bleach recovery kinetics times become slower with increasing n value, and a rising kinetics that appears for large-n perovskite phases also suggests the cascade carriers’ accumulation on large-n perovskite phases. A similar trend is also observed in the n = 5 perovskite film, as shown in Figure 3d−f. Due to tiny amounts of the n = 1 perovskite phase (Figures 2b and S6f), excitons are mainly formed in the perovskite phase with the largest band gap (567 nm, 2.19 eV, PB2), and with increasing decay time, PB3 (609 nm, 2.04 eV) and PBn>3 (∼730 nm, 1.70 eV) phases’ bleach and grow in sequence, which is the result of electrons transferring from small-n to large-n perovskite phases. Time traces at selected probe wavelengths (567, 609, and 730 nm) are shown in Figure 3f. The PB2 and PB3 show an ultrafast dominant decay with time constants of around 0.4 and 1.0 ps, respectively. The PB2 decay time constant is well matched with the fast PB formation time of n = 3 (∼0.4 ps, 609 nm, 2.04 eV). Also, the PB formation time for n > 3 perovskite phases is 26.6 ps (∼730 nm, around 1.70 eV), indicating that a substantial portion of photogenerated electrons localize to n > 3 phases within 26.6 ps. Compared to the n = 3 film, the n = 5 film exhibits a faster

photoinduced changes in absorption spectrum (ΔA) are then probed with a time-delayed laser-generated white light pulse. To exclude the higher-order process, the excitation fluence was controlled at 2 μJ/cm2 throughout the whole experiment (Figure S2a). It is noted that the TAS of as-prepared thin films shows no variation during the TAS experiment, suggesting the good stability of the film under excitation of the femtosecond laser. Figure 3a shows the schematic of carrier transfer in the n = 3 multiple-phase perovskite film. Photoinduced electrons are transferred downstream from small-n phases to large-n perovskite phases, while holes are transferred upstream from large-n phases to small-n perovskite phases, which will be demonstrated later. Figure 3b shows the TAS results of the n = 3 perovskite film. Excitons are first primarily formed in the 2D perovskite phase (n = 1), which manifests a fast buildup of the photobleaching (PB) at the exciton absorption peak (516 nm, 2.40 eV, PB1), corresponding to the n = 1 perovskite phase. With increasing decay time, PB2 (567 nm, 2.19 eV), PB3 (609 nm, 2.04 eV), and PBn>3 (670 nm, 1.85 eV) of larger-n perovskite phase resonance grow one after another gradually. The evolution of TA spectra from 0 to 1000 ps clearly shows that carriers reach large-n perovskite phases from small-n perovskite phases. Time traces at selected probe wavelengths (516, 567, 609, and 670 nm) are shown in Figure 3c. To evaluate the formation time of PBn, especially for PBn>3, the kinetics are fitted by a multiexponential function.34 The PB1 of the n = 1 perovskite phase shows an ultrafast dominant decay with a time constant of around 0.3 ps, which is the same as the formation time of the PB2 formation time (∼0.3 ps). The PB2 4434


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Figure 4. Multiwavelength excitation TAS and low-lying excitation TAS for the n = 5 perovskite film. (a) Multiwavelength excitation TAS for the n = 5 perovskite film under back-excitation at 400, 530, 580, and 640 nm. The time constants are indicated along with the curves in the same color. (b) TAS of the n = 5 perovskite film (∼250 nm thickness) under front-excitation at 720 nm. PB at 567 and 612 nm is assigned to the n = 2 and 3 perovskite phases.

Figure 5. Distribution of as-grown multiple-phase quasi-2D perovskite films. (a) TAS at different delay times for the n = 5 perovskite film (∼250 nm thickness) under front-excitation at 400 nm. (b) Time traces (after subtracting the photoinduced absorption signal) at PB2 (2.19 eV), PB3 (2.04 eV), and PBn>3 (1.70 eV) extracted out from TAS (a). The time constants are indicated along with the curves in the same color. (c) Upper panels: AFM images of as-grown RP perovskite films, n = 3 (left) and 5 (right). Lowers panels: surface potential distribution of n = 3 (left) and 5 (right) measured by a Kelvin probe force microscope. (d) Schematic diagram of the arrangement of the multiphase perovskite film.

decay time and formation time, which can be attribute to the small portion of n = 1 and smoother phase structure. The efficient electrons transmitted from the small-n perovskite phases to the large-n perovskite phases could undisputedly account for the results of the TA spectrum and steady-state PL spectroscopy. Except for electron transfer, carrier transportation from small-n to n ≈ ∞ perovskite phases could be also attributed to energy transfer without depopulation and population between multiple phases. To confirm it is charge transfer dynamics in the multiphase quasi-2D RP perovskite films, multiwavelength excitation TAS and low-lying excitation TAS were conducted. Figure 4a shows the TAS for the n = 5 perovskite film under back-excitation at different wavelengths (400, 540, 580, and 640 nm probed at 720 nm). The delay times of the photobleach at 720 nm, that is, the electron transfer time to larger n, turns out to be shorter with lengthening of the excitation wavelength, indicating that there is electrons transfer to n ≈ ∞ perovskite

phases, which is in good agreement with previous results. Meanwhile, to confirm the direction of hole transfer, low-lying excitation TAS for the n = 5 perovskite film is conducted under front-excitation at 720 nm (power: 2 μJ/cm2), which could exclude the possibility of directly exciting the n = 1, 2, and 3 perovskite phases. In Figure 4b, two small PB peaks located at 567 and 609 nm are observed due to PB of n = 2 and 3 perovskite phases, which increases gradually with increasing decay time. The observation suggests that holes transfer from the n ≈ ∞ perovskite phase to n = 3 and then to n = 2 perovskite phases. The TAS PB signals for hole transfer are relatively lower than electron transfer, which is partially due to the less efficient hole transfer compared to electron transfer.34,43−45 Further, detailed multiwavelength excitation TAS for n = 3 (Figure S5) and n = 5 (Figure S6) perovskite films clearly illustrates that there is carrier transfer among the multiphase perovskite films, that is, electrons transfer from small-n to large-n perovskite phases and holes transfer reversely. 4435


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The Journal of Physical Chemistry Letters To determine the arrangement of multiphase perovskites, the back-excitation PL for n = 3 (Figure S4c) and n = 5 (Figure S6f) perovskite films was conducted to compare with the frontexcitation PL in Figure 2b. The back-excitation PL shows emission peaks located at n = 1, 2, and 3 perovskite phases in addition to the dominant emission from near n ≈ ∞ phases, indicating that the small-n perovskite phases should be primarily located at the bottom surface and the large-n perovskite phases should be mainly located at the upper surface of the film. To further demonstrate this hypothesis, the TAS results of the n = 5 perovskite film were collected under front-excitation at 400 nm (Figure 5a,b). The TAS after subtracting the photoinduced absorption signal from the n ≈ ∞ phase located in the positive ΔA region is shown in Figure S6f, showing clear PB peaks attributed to n = 2 and 3 perovskite phases. Similar to back-excitation results, the front-excitation TAS shows a dominant photobleaching (PBn>3) at around 711 nm (large-n perovskite phases); however, the relative intensities of PB2 and PB3 at 567 and 609 nm (small-n perovskite phases) under back-excitation are greater than that under frontexcitation, which is identical to the previous work.34 The difference in the TA spectra between back- and front-excitation implies that the small-n perovskite phases should be primarily distributed on the substrate surface and the large-n perovskites phases should be primarily distributed on the surface of the film. The decay times of n = 2, 3, and >3 phases are 0.3, 0.7, and 20.8 ps (Figure 5b), which are faster than those of backexcitation conditions (Figure 3f). The relatively slower decay rate of the absorption peak under back-excitation is related to the laser beam directly impinging on the small-n perovskite phases, supporting that multiphase perovskites are aligned from small to large n along the direction perpendicular to the substrate. Further, KPFM was carried out to measure the surface potential to confirm the hypothesis of arrangement of as-grown quasi-2D RP perovskite films, as shown in Figure 5c. The upper panels show AFM images of as-prepared perovskite films with a composition ratio of n = 3 (left) and 5 (right), and the lowers present the surface potential distribution of the two types of perovskite films, respectively. The average surface potential of n = 5, ranging from −784 to 173 mV, is relatively lower than that of n = 3, ranging from 125 to 568 mV, respectively, which suggests that the Fermi level of n = 5 perovskites lies lower than that of the n = 3 film from the vacuum level. On the other hand, the surface potential span over a wide range suggests the coexistance of multiple phases in the horizontal planes on the surface. To summarize, as shown in Figure 5d, the multiple phases of perovskites are not arranged vertically with the substrate from small-n to large-n phases, but they are aligned parallel to the substrate randomly, as previously reported.34,35 Due to the multiple phases of as-grown quasi-2D perovskite films, the emission properties for n = 3 and 5 perovskite films are highly dependent on the crystalline structure of the films. In high-quality RP films with a low density of pinholes, only one emission peak is observed at the lowest exciton peak position even though there are many absorption peaks due to multiphase perovskites. However, for samples with low crystalline quality, the carrier transfer is not efficient, which leads to several emission peaks assigned to individual exciton recombination. In order to investigate the effect of film quality on carrier transfer, samples with different quality were prepared by changing the preparation conditions. As shown in Figure 6a, the n = 3 film prepared with a spin-coating speed of 4000

Figure 6. PL and TAS for n = 3 perovskite films with different crystalline quality. (a) SEM images of as-grown quasi-2D perovskite films with rapid spin coating of 4000 rps (S1, ∼250 nm thickness) and 6000 rps (S2, ∼200 nm thickness). The scale bar is 200 nm. (b) PL spectra of as-grown RP perovskite films under front-excitation with a CW 405 nm laser. (c) TAS of as-grown RP perovskite films under back-excitation at 400 nm.

rounds per second (rps) (upper panel, S1) shows better crystal quality and a flatter surface than that at 6000 rps (bottom panel, S2). As shown in Figure 6b, The PL spectra of quasi-2D RP perovskite films under front-excitation at 405 nm show one emission peak at ∼708 nm (n > 3 perovskite phases); however, more emission peaks at 520, 573, and 618 nm ascribed to n = 1, 2, and 3 appear in the film with low crystalline quality. To further understand the crystalline structure-dependent emission properties, TAS probed at 2.5 ps under back-excitation at 400 nm is conducted and is shown in Figure 6c. For S1 with good crystalline quality, strong PB in the wavelength range of 650− 750 nm (n > 3 perovskite phases) indicates carrier accumulation on this perovskite phase. As a contrast, in samples with poor crystalline quality, individual strong PB1 and PB2 bleaching peaks dominate; however, PBn>3 becomes much weaker, suggesting little carrier transfer from small-n (n = 1, 2, 3) to large-n (n > 3) perovskite phases. Both steady-state PL and TAS prove that efficient carrier transfer exists in films with a low density of pinholes; otherwise, the carriers will recombine with multicolor emission. In summary, carrier dynamics of (PEA)2(CH3NH3)n−1PbnI3n+1 2D RP perovskites is systematically studied using steady-state PL and TAS. In solutionprocessed spin-coated perovskite films with a low density of pinholes, efficient electrons transfer from small-n value perovskite phases toward large-n perovskite phases, and holes transfer in the opposite direction from room temperature to 80 K, as observed within 30 ps. Besides, the multiple phases of perovskites were naturally arranged in the order of n from small to large along the direction vertical to the substrate, and different n perovskites also distribute in the same planes parallel to the substrate, which was demonstrated by steady-state PL, TAS, and KPFM. These results would be helpful for development of quasi-2D perovskites-based photovoltaics and other optoelectronics devices.

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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/acs.jpclett.7b01857. Materials synthesis procedure and structural characterization, instrument response function and linearity of TAS, detailed temperature dependence of PL spectroscopy, front- and back-excitation transient absorption and time-resolved PL spectroscopy of n = 3, excitation wavelength dependence of TAS of the n = 3 and 5 4436


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perovskites, steady-state PL spectroscopy of the n = 3 and 5 perovskite phases (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q. Zhang). *E-mail: [email protected] (Y. Zhao). ORCID

Xinfeng Liu: 0000-0002-7662-7171 Qing Zhang: 0000-0002-6869-0381 Author Contributions #

Q.S. and Y.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Q.Z. acknowledges support of start-up funding from Peking University, the 1000 Talent Programs of China, and the Open Research Fund Program of the State Key Laboratory of LowDimensional Quantum Physics. Q.Z. also acknowledges funding support from the Ministry of Science and Technology (2017YFA0205700; 2017YFA0304600). X.F.L acknowledges support from the Ministry of Science and Technology (No. 2016YFA0200700 and 2017YFA0205004), the National Natural Science Foundation of China (No. 21673054), the Key Research Program of Frontier Science, and CAS (No. QYZDB-SSW-SYS031).

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