Unveiling Structurally Engineered Carrier Dynamics in Hybrid Quasi

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Unveiling Structure Engineered Carrier Dynamics in Hybrid QuasiTwo-Dimensional Perovskites Thin Film Towards Controllable Emission Qiuyu Shang, Yunuan Wang, Yangguang Zhong, Yang Mi, Liang Qing, Yuefeng Zhao, Xiaohui Qiu, Xinfeng Liu, and Qing Zhang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01857 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on August 28, 2017

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The Journal of Physical Chemistry Letters

Unveiling Structure Engineered Carrier Dynamics in Hybrid Quasi-Two-Dimensional Perovskites Thin Films Towards Controllable Emission Qiuyu Shang,1# Yunuan Wang,1,2# Yangguang Zhong,1,3 Yang Mi,3 Liang Qin,3, Yuefeng Zhao,2,* Xiaohui Qiu,3 Xinfeng Liu,3 Qing Zhang1,4,* 1

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China 2 School of Physics and Electronics, Shandong Normal University, Jinan 250014, China 3 CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China 4 Research Center for Wide Gap Semiconductor, Peking University, Beijing 100871, China # Qiuyu Shang and Yunuan Wang contributed equally to this work. * Corresponding: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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Absstract Quaasi two-dim mensional Ru uddlesden-P Popper pero ovskites driv ving carrierr self-separaation has rapidly advvanced the developmeent of high performan nce optoelecctronic devices. wever, insigghtful undeerstanding of carrier dynamics in i the peroovskites is still How inaddequate. Thhe distributtion of muultiple perovskite phasses, cruciall importantt for carrrier separatiion, is in controversy c y. Here we report a sy ystematic sttudy on caarrier dynnamics of sppin-coated (C ( 6H5CH2C CH2NH3)2(C CH3NH3)n-1Pb P nI3n+1 (n = 3 and n = 5) peroovskite thinn films. Effficient electr trons transfeer from small-n to larg rge-n perovsskite phases and hooles transferr reversely with time scales from ~ 0.3 too 30.0 ps. The mulltiple perovvskites phasses are arrannged perpeendicularly to substratee from smaall to largge n, and alsso co-exist randomly inn the same horizontal planes. Furrther, the caarrier sepaaration dynaamics is tailored by enngineering crystalline c sttructure of pperovskite film, f whiich leads to t controllable emisssion properrties. Thesee results hhave imporrtant signnificance foor the desig gn of optoeelectronic devices from m solar celll, light emittting diodde, laser, etcc. TOC C：

2


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Organic-inorganic hybrid perovskite is a kind of new material which is self-assembled by organic and inorganic components at molecular scale. It combines the advantages of organic and inorganic components and has a great potential in the field of photovoltaic applications. The photon energy conversion efficiency of solution processed three-dimensional (3D) perovskite thin film solar cell is enhanced from 3.8 to 22.1 % in 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 etc. Furthermore, as a direct-band gap semiconductor, its structure and energy band are both designable and adjustable and therefore perovskites can be widely used in photodetector, single photon, lasers and other fields.16-28 However, these 3D lead halide perovskites show poor stability to atmospheric moisture, which would be extensively hinder their applications. As a contrast, two-dimensional (2D) perovskite with atomically-sharp quantum well structure exhibits better environment stability, besides, the quantum confinement effect, flexible composition and structure homogeneity have led to successful applications in emitting diode and light-harvesting devices.3 However, due to their large exciton binding energy (hundreds of meV),4,29 2D perovskite is not an ideal material for electronic device in great need of free carriers. To solve this problem, quasi-2D perovskite, combining advantages of 2D and 3D perovskites, has received increasing attentions and been studied extensively recently.28,30-33 Quasi-2D perovskite is also called as Ruddlesden-Popper (RP) perovskite,3 3



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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+, X- the CH3NH3+ cation, metal cation and halide anion, respectively, and n is the layer number of 3D perovskite embedding between two layers of A+ cations. In the quasi-2D structures, organic cation layer owning large electronic band gap serves as barrier layer and 3D perovskite layers is functioned as well layer. Therefore, quasi-2D perovskites naturally form quantum well structures with atomically sharp interface between “barriers” and “wells”. The optical and electronic properties can be easily tailored by wells thickness, herein, quantized as layer number n of 3D perovskites. In particular, the recent in-depth study on 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 exhibit a variety of n values even though it was intended to be grown as a single-phase.4,34,35 With this distinct stacking geometry, free carriers, including electron and holes, can be effectively separated in RP perovskites. Due to its good environmental stability, high quantum yield and carrier separation efficiency, quasi-2D perovskites have been widely used in LED devices with external quantum efficiency (EQE) up to 11.7%.35 Despite these remarkable successes, there are still two important issues needing further study. Firstly, the crystalline structure of quasi-2D perovskites, in particular the distribution and alignment of multiple phases perovskites is still controversy, which is critical important for carrier transfer dynamics among multi-phase perovskites. For instance, Yuan et al. reported that the multiple phases in (C6H5CH2CH2NH3, PEA)2(MA)n—1PbnI3n+1 are arranged randomly 4


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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 substrate.35 The controversy may be due to the difference of large aliphatic cation. Compared

with

insightfully

(BA)2(MA)n-1PbnI3n+1thin

film,

understanding the

study

of on

carrier carrier

dynamics

in

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 structure engineering of quasi-2D perovskites and then improve performance of optoelectronic devices.37-39 For instance, photoelectric conversion devices, i. e. LED and solar cells, are benefited 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 carriers transfer occurs between the multiple phases of perovskite with rate of sub 1 ps and efficiency near to 100%.34 The multiple phases of perovskites are arranged along the direction vertical to the substrate from small to large n, and also co-exist randomly in the same horizontal planes. Besides, carriers transfer could be tailored by engineering the crystalline structure of perovskites that efficient charges transfer occurs in films with low density of pinhole. These results have important applications for the development of perovskite light conversion and detection devices. 5



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Figure 1a shows scheematics of ccrystalline structure forr quasi-2D pperovskites (n = 3), w which consiists of inorg ganic layer cconstructed d by top-con nnected [PbII6]2- octahedron and organic layer l of phenethylam p mine cation n (C6H5CH H2CH2NH3+ , PEA+). The inorrganic and organic lay yers are allternately arranged a in space to fform a lay yered struucture, whicch naturally forms quanntum well structures s with w atomic sharp interfface. If tthere is onlly PEA+ cation, it iss 2D perov vskite (n = 1), meanw while, it iss 3D peroovskite (n = ∞) wheen there is only MA A+, the rem mainings aree quasi-2D D RP peroovskites (n = 2, 3, 4, 5…). The qquasi-2D perovskites were w prepar ared by solu ution proccessed spinn-coated method m (Fiigure S1a). The morphology aand crystaalline struuctures of ass-grown qu uasi-2D peroovskite thin n films are characterizzed by scan nning elecctron microoscopy (SEM M), atomic force micrroscopy (AFM) and xx-ray diffracction (XR RD).

Figuure 1. Structure of sp pin-coated qquasi-two dimensional d l (2D) Rudddlesden-Popper (RP P) perovskittes (n = 3)). (a) Schem matics of crystalline c structure oof quasi-2D D RP peroovskites. (bb) Cross-section scannning elecctron micro oscope (SE EM) imagee of as-ggrown quasii-2D RP perrovskite film m spin-coatted on ITO glass g substrrate (n = 3). The scalle bar is 2000 nm. (c) X--ray diffracttion of as-grown lead halide h perovvskites. Figure 1bb shows cro oss-sectionaal SEM imaage of n = 3 perovskiite film on ITO glasss substratee, which suggests s thhat the as-grown perovskite film m is a deense, 6


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pinhole-free film with a thickness of ~ 250 nm. Top-view SEM and AFM images (Figure S1b and c, prepared as n = 3) along with corresponding cross-section SEM image show that the film has a smooth and uniform surface coverage with root-mean-square (r. m. s) 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 bottom to upper panel, respectively. The diffraction of (110) and (220) planes for the 3D perovskite at 14.6o and 28.9o is consistent with previous reports.40 However, the sharp diffraction peaks of the (00l, l = 2, 4, 6, 8…) planes at 5.8o, 11.3o, 16.8o, 22.3o, 27.7o observed in the 2D perovskite indicates that 2D perovskite may be a split of 3D material in a direction of (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, diffraction peak of (111) plane at 14.5o 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 towards outside of the plane. And it is more apparent for 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 in 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 n = 1 (2D) perovskite film, which suggests that exciton is stable at room temperature for n 7



= 1. As for n = 2, new absorption peaks beyond E1 peak appear at 567 nm (E2), 609 nm (E3), 640 nm (E4) and 680 nm (E5) are observed, which is consistent with previous reports for different n phases.34,41 As further increasing of [PbI6]2- layers (n = 3, 5), the a

b 3D (n=∞) n=5 n=3 n=2 2D (n=1)

c

520 585 650 715 780 Wavelength (nm)

2.4

quasi-2D

2

3D

35 ∞

600 700 800 Wavelength (nm)

d

EB = 113.5 meV

Ttrans

EB = 136.5 meV

Intensity (a.u.)

2.0 1.8

0

1

500

Eabs

2.2

1.6

PL Intensity (a.u.)

Absorbance (a.u.)

2D

Energy (eV)

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

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E~n-2 2 4 6 8 Layer Numbers

EB = 191.5 meV

Ttrans Ttrans n=5 n=3 n=1

10

75

150 225 300 Temperature (K)

Figure 2. Steady-state absorption and photoluminescence (PL) spectroscopy of spin-coated quasi-2D RP perovskite thin films. (a) Absorption spectra in log scale of as-grown quasi-2D RP perovskite films spin coated on ITO glass substrate. (b) Normalized PL spectra of as-grown quasi-2D RP perovskites under front-excitation with continuous wave (CW) 405 nm laser. (c) n-dependence of absorption peaks extracted out from (a), the dashed line indicates the absorption peaks of n = ∞ perovskite phase. The dashed line: band gap energy for pure 3D perovskite. Green dots: exciton absorption energy out from absorption spectra checked with references. Red curve: calculated n-dependent fundamental emission energy based on quantum wells model. (d) Temperature dependence of integrated PL intensity for n = 1 (red dots), 3 (yellow dots), 5 (green dots). The solid lines are fitted curved using Arrhenius equation. E1, E2, E3 peak becomes weaker due to the less composition of these phases. Particularly, E1 is hard to be resolved when n = 5, suggesting the little composition of 8


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n = 1 phase. Apart from the aforesaid exciton absorption peaks, the peak showing up 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 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 redshift is due to Stokes shifts because of exciton-phonon scattering. For n = 2, the emission peak moves towards ~ 573 nm, which corresponds to E2 exciton absorption peak at ~ 567 nm. Compared with E1 exciton peak with full width at half maximum (FWHM) ~ 15 nm, E2 exciton has lower energy and larger FHMW (~ 32 nm), which is due to the increasing of quantum well thickness (two [PbI6]2- layers). With increase of [PbI6]2- layer number for n = 3 and 5, the band gap gradually becomes smaller and therefore the corresponding PL peak is gradually red shifted locating at ~ 708 nm and ~ 731 nm, which is consistent with the absorption spectrum that 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 peak at ~ 609 nm and ~ 680 nm, which may be due to that 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 9



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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 quantum well model (solid red line) simply adopting well layer thickness of n multiples of 3D perovskite thickness d3D. When temperature decreases from 295 K to 80 K, only one peak is observed in PL spectroscopy (Figure S3) for n = 1, 3 and n = 5 perovskite films, suggesting that 1) low density of bound exciton states due to defects and 2) carriers’ self-separation from small to large n perovskite phases is still efficient and play a main channel in exciton decay process even that the excitons is much more stable at low temperature. The emission peak is red-shifted, which is similar with other 3D, pure inorganic and 2D lead halide perovskites reported in previous works.42 The PL intensity increases when temperature decreases from 295 to 110 K for n = 5 (Figure S3c). With further decreasing temperature from 110 K to 80 K, the PL intensity decreases. Similar temperature dependent behaviors are observed at 160 K, 140 K for n = 1, 3. The FWHM of emission peak significantly broadens with temperature increasing from 80 K to 295 K (i. e., FWHM from 57.66 nm to 71.42 nm for n = 5), possibly due to enhanced phonon scattering at higher temperature. The temperature-dependent emission intensity of n = 1, n = 3 and n = 5 perovskite films above phase transition point are plotted in Figure 2d, and it can be fitted using the following formula:10

10


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I (T ) =

I (0) 1 + c1 exp(− EB / κ BT )

[1]

In which, κB, EB, I(0), c1 is Boltzmann’s constant, exciton binding energy and PL intensity at 0 K, a dimensionless constant, respectively. The evaluated exciton binding energy of n = 1, 3 and 5 perovskites are 191.5 meV 136.5 meV, and 113.5 meV, respectively, which is close to the previous report.4,42 It could be understood that the wave functions of electrons and holes is 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 charge carrier was investigated using femtosecond TAS excited at different wavelengths and direction. There are two excitation method that the laser beam (400 nm) firstly 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), 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 photo-induced 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 experiments (Figure S2a). It is noted that the TAS of as-prepared thin films shows no variation during TAS experiment, suggesting the good stability of the film under excitation of femto-second laser. Figure 3a shows the schematic of carriers transfer in n = 3 multiple phases 11



perovskite film. Photo-induced electrons are transferred downstream from small-n phases to large-n perovskite phases while holes transferred upstream from large-n phases to small-n perovskite phases which will be demonstrated later. Figure 3b shows the TAS results of n = 3 perovskite Film. Excitons are firstly primarily formed in 2D perovskite phase (n = 1), which manifests a fast build-up of the photobleaching (PB) at the exciton absorption peak (516 nm, 2.40 eV, PB1) corresponding to n = 1 b

a

0.5 ps

30.0 ps

n = 3 2.7 ps

Ex

PL

Ground state

0.76 ps 0.9 ps 10 ps 100 ps 1000 ps

PB1 PB2 525 600 675 Wavelength (nm)

e

d

n=2

0.4 ps

PL

Ground state h+

-0.016 -0.024

0.00 1

0.032 0.024

0.9 ps 1 ps PB2 PB3 10 ps 100 ps 1000 ps

525

600 675 Wavelength (nm)

Delay time (ps)

10

τ1 = 0.4 ps PB 2 τ1 = 1.0 ps PB 3 τet = 26.6 ps PB n>3

0.016

-0.008 0.76 ps

n = 3 1.0 ps

0.01

f

0.000

26.6 ps

τ1 = 2.7 ps PB3 τet = 30.0 ps PBn>3

0.02

750

n = 5-back

e-

Excited state

Ex

PB3 PB n>3 1 ps

-0.02

h+

n=1

0.03

0.00

−ΔA

n=2

τ1 = 0.3 ps PB1 τ1 = 0.5 ps PB2

0.04

−ΔA

n=1

0.3 ps

ΔA

Excited state

c n = 3-back

e-

ΔA

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

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0.008

PBn>3 750

0.000 1

Delay time (ps)

10

Figure 3. Transient absorption spectra (TAS) for spin-coated quasi-2D perovskite films (∼ 250 nm thickness, prepared as n = 3 and n = 5). (a) Schematic of carriers transfer in n = 3 perovskite film. The electron transfers from small-n to large-n perovskite phases and hole transfer from large-n to small-n perovskite phases. The transfer times are extracted out from the TA kinetics (c). (b) Photo-induced changes in TAS (ΔA) of 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 eV, 2.19 eV, 2.04 eV and 1.85 eV for n = 3 perovskite film extracted out from TAS (b). Solid lines are fitting results of the kinetics by multi-exponential function. The time constants are indicated along with the curves in the same color. (d) Schematic of carriers transfer in n = 5 perovskite film, indicating there are little n = 1 phase in this thin film. (e) TAS for 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) 12


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TA kinetics probed at PB2 (2.19 eV), PB3 (2.04 eV) and PBn>3 (1.70 eV) for n = 5 perovskite film. The time constants are indicated along with the curves in the same color. perovskite phase. With the increasing of 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 phases resonance grow one after another gradually. The evolution of TA spectra from zero to 1000 ps clearly shows that carriers reach large-n perovskite phases from small-n perovskite phases. Time traces at selected probe wavelengths (516 nm, 567 nm, 609 nm 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 multi-exponential function.34 The PB1 of n = 1 perovskite phase shows an ultrafast dominant decay with a time constant of around 0.3 ps, which is as the same as the formation time of PB2 formation time (~ 0.3 ps). The PB2 of n = 2 perovskite phase shows an ultrafast dominant decay of around 0.5 ps, which also matches the formation time of PB3 formation time (~ 0.5 ps). The larger PB formation time, i. e., the electrons transfer time to n > 3 perovskite phases (~ 30.0 ps) is a 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, 3 perovskite phases will be localized to n > 3 phases within 30.0 ps. Besides, the bleach recovery kinetics times become slower with the increasing of n value and a rising kinetics appears for large-n perovskite phases also suggests the cascade carriers’ accumulation on large-n perovskite phases. The similar trend is also observed in n = 5 perovskite film, as shown in Figure 3d-f. Due to tiny amounts of n = 1 perovskite phase (Figure 2b, Figure S6f), excitons are mainly formed in the perovskite phase 13



with largest bandgap (567 nm, 2.19 eV, PB2), and with the increasing of decay time, PB3 (609 nm, 2.04 eV) and PBn>3 (~ 730 nm, 1.70 eV) phases bleach 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 nm, 609 nm and 730 nm) are shown in Figure 3f. The PB2 and PB3 show an ultrafast dominant decay with a time constant of around 0.4 ps 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). And the PB formation time for n > 3 perovskite phases is 26.6 ps (~ 730 nm, around 1.70 eV) indicating that the substantial portion of photogenerated electrons localize to n > 3 phases within 26.6 ps. Compared to n = 3 film, the n = 5 film exhibits faster decay time and formation time, which can be attribute to the few 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 undisputed account for the results of TA spectrum and steady-state PL spectroscopy. a n = 5-back

τet = 25.7 ps τet = 24.5 ps τet = 23.3 ps τet = 20.7 ps

0 1

0.008

400 nm 530 nm 580 nm 640 nm

10 100 Delay time (ps)

n = 5-front

0.000

PB2 PB3

-0.008 0.76 ps 0.9 ps -0.016 2 ps 10 ps -0.024 100 ps 1000 ps

ΔA

1

b

Intensity

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

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1000

525

720 nm signal

600 675 Wavelength (nm)

750

Figure 4. Multi-wavelength excitation TAS and low lying excitation TAS for n = 5 perovskite film. (a) Multi-wavelength excitation TAS for n = 5 perovskite film under back-excitation at 400 nm, 530 nm, 580 nm and 640nm. 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. Photobleaching at 567 nm and 612 nm are assigned to the n = 2 and 3 perovskite phases. 14


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Except electrons 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 multi-phase quasi-2D RP perovskite films, multi-wavelength excitation TAS and low lying excitation TAS were conducted. Figure 4a shows the TAS for n = 5 perovskite film under back-excitation at different wavelength (400 nm, 540 nm, 580 nm and 640, probed at 720 nm). The delay times of the photobleach at 720 nm, i.e., the electron transfer time to larger n, turns shorter as lengthening the excitation wavelength, indicating that there are 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 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, 3 perovskite phases. In Figure 4b, two small PB peaks locating at 567 nm and 609 nm are observed due to the PBs of n =2, and 3 perovskite phases, which increases gradually with increasing of decay time. The observation suggests that holes transfer from 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 holes transfer comparing to electron transfer.34,43-45 Further, detailed multi-wavelength excitation TAS for n = 3 (Figure S5) and n = 5 (Figure S6) perovskite films clearly illustrate that there are carriers transfer among the multi-phases perovskite film, i.e. electrons transfer from small-n to large-n perovskite phases and holes transfer reversely. 15



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To determ mine the arraangement off multi-phase perovskites, the bacck-excitation n PL for n = 3 (Figgure S4c) an nd n = 5 (F Figure S6f)) perovskitee films werre conducteed to mpare with the front-eexcitation P PL in Figurre 2b. The back-excitaation PL sh hows com emiission peakss locating at a n = 1, 2, 3 perovskitte phases in n addition tto the domiinant emiission from near n ≈ ∞ phases, inddicating thatt the small n perovskitee phases sh hould majorly locate at the botto om surface aand the large n perovsk kite phases m mainly locaate at

d of as-grow wn multiple phases quaasi-2D perovvskite filmss. (a) Figuure 5. The distribution TAS S at different delay tim mes for n = 5 perovsk kite film (∼ ∼ 250 nm thhickness) under u fronnt-excitationn at 400 nm m. (b) Time ttraces (afterr subtracting photoinduuced absorp ption signnal) at PB2 (2.19 eV), PB P 3 (2.04 eeV) and PB Bn>3 (1.70 eV V) extractedd out from TAS (a). The time constants are a indicateed along wiith the curv ves in the same colorr. (c) Uppper panels: atomic forcce microscoopy images of as-grown n RP perovvskite films,, n = 3 (leeft) and 5 (rright). Lowers panels: surface potential distribution of n = 3 (left) and a 5 (righht) measureed by Kelv vin potentiaal force miccroscope, reespectively.. (d) Schem matic diaggram of arraangement off multi-phasse perovskite film. the upper surfaace of the fillm. To furthher demonsttrate this hy ypothesis, thhe TAS of n = 5 peroovskite film m were colleected underr front-excittation at 400 nm (Figuures 5a and 5b). Thee TAS after subtracting g the photoiinduced absorption sig gnal from thhe n ≈ ∞ phase 16


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located at 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) around 711 nm (large-n perovskite phases); however, the relative intensities of PB2 and PB3 at 567 nm and 609 nm (small-n perovskite phases) under back-excitation are greater than that under front-excitation, which is identical to the previous work34. The difference in the TA spectra between back- and front-excitation implies that the small-n perovskites phases should majorly distributed on the substrate surface and the large-n perovskites phases majorly distributed on the surface of the film. The decay time of n = 2, 3 and n > 3 phases is 0.3, 0.7 and 20.8 ps (Figure 5b), which is faster than those of back-excitation condition (Figure 3f). The relatively slower decay rate of absorption peak under back-excitation is related to the laser beam directly impinging the small n perovskite phases, supporting the multi-phase perovskites are aligned from small to large n along the direction perpendicular to the substrate. Further, KPFM were 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 composition ratio of n = 3 (left) and 5 (right), and the lowers present surface potential distribution of the two types of perovskite film, 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 Fermi level of n = 5 perovskites lies lower than n = 3 film from the vacuum level. On the other hand, the 17



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surfface potentiial span in a wide rangge suggestss the coexissting of muultiple phasees in the horizontal planes p on th he surface. To be shorttly concluded, as show wn in Figuree 5d, v too substrate from f the multiple phhases of perrovskites iss not either arranged vertically smaall-n to larrge-n phasees, but alsso aligned parallel to o substratee randomly y, as prevviously repoorted.34,35

Figuure 6. PL annd TAS for n = 3 peroovskite filmss with different crystallline quality y. (a) SEM M images of o as-grown n quasi-2D pperovskite films f with spin s coatingg rapid of 4000 4 rounnd per secoond (r. p. s)) (S1, ~ 2500 nm thickn ness) and 6000 6 r. p. s (S2, ~ 200 0 nm thicckness), resppectively. The T scale bbar is 200 nm. (b) PL L spectra oof as-grown n RP peroovskite film ms under fro ont-excitatioon with CW W 405 nm laser. l (c) TA AS of as-grrown RP pperovskite films f underr back-excitaation at 400 0 nm. Due to thee multiple phases p of ass-grown qu uasi-2D pero ovskite film ms, the emisssion propperties for n = 3, 5 perrovskites fillms are high hly dependeent on crysttalline struccture of thhe films. Inn high qualiity RP filmss with low density of pinhole, p onlly one emisssion peakk is observved at low west excitonn peak position even n though thhere are many m absoorption peaaks due to multi-phasse perovskittes. Howev ver, for sam mples with low crysstalline quallity, the carrriers transfeer is not effiicient, which leads to sseveral emisssion peakks assignedd to individu ual exciton recombinattion. In ord der to investtigate the effect of ffilm qualityy on carrierr transfer, ssamples witth different quality weere prepared d by channging the preparation conditions. c As shown in i Figure 6aa, n = 3 film m prepared with 18


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spin-coating speed of 4000 round per second (r. p. s.) (upper panel, S1) shows better crystal quality and flatter surface than 6000 r. p. s. (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 nm, 573 nm, 618 nm ascribed to n = 1, 2, 3 appear in the film with low crystalline quality. To further understanding of crystalline structure dependent emission properties, TAS probed at 2.5 ps under back-excitation at 400nm is conducted and shows in Figure 6c. For S1 with good crystalline quality, strong PB in wavelength range of 650-750 nm (n > 3 perovskite phases), indicating carrier accumulation on this perovskite phase. As a contrast, in samples with poor crystalline quality, individual strong PB1 and PB2 bleaching peak are dominated; however, PBn>3 becomes much weaker, suggesting little carriers transfer from small-n (n = 1, 2, 3) to large-n (n >3) perovskite phases. Both of the steady-state PL and TAS prove that efficient carrier transfer exists in films with 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 solution processed spin coated perovskite films with low density of pinholes, efficient electrons transfer from small n values perovskite phases towards large n perovskite phases and holes transfer in the opposite direction from room temperature to 80 K was 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 19



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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. Acknowledgements Q.Z. acknowledges the support of start-up funding from Peking University, one-thousand talent programs of China, open research fund program of the state key laboratory of low-dimensional quantum physics. Q.Z. also thanks funding support from the Ministry of Science and Technology (2017YFA0205700; 2017YFA0304600). X.F.L thanks the support from the Ministry of Science and Technology (No. 2016YFA0200700 and 2017YFA0205004), National Natural Science Foundation of China (No. 21673054), Key Research Program of Frontier Science, CAS (No. QYZDB-SSW-SYS031). Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Materials synthesis procedure and structural characterization. Instrument response function and linearity of transient absorption spectroscopy. Detailed temperature dependence of PL spectroscopy. Front and back excitation transient absorption and time resolved PL spectroscopy of n = 3. Excitation wavelength dependence of transient absorption spectroscopy of n = 3 and 5 perovskite. Steady-state PL spectroscopy of n = 3, 5 perovskite phase. 20


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Notes

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Unveiling Structurally Engineered Carrier Dynamics in Hybrid Quasi

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