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Engineered Directional Charge Flow in Mixed TwoDimensional Perovskites Enabled by Facile Cation-Exchange Junhui Wang, Jing Leng, Junxue Liu, Sheng He, Yu Wang, Kaifeng Wu, and Shengye Jin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08535 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017

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The Journal of Physical Chemistry

Engineered Directional Charge Flow in Mixed Two-Dimensional Perovskites Enabled by Facile Cation-Exchange Junhui Wang, Jing Leng, Junxue Liu, Sheng He, Yu Wang, Kaifeng Wu*, Shengye Jin*

State Key Laboratory of Molecular Reaction Dynamics and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China, 116023.

AUTHOR INFORMATION Corresponding Author [email protected] Tel.: +86-411-84379313

[email protected] Tel.:+86-411-84379805

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ABSTRACT

Two-dimensional (2D) organolead halide perovskites are promising candidates for many optoelectronic applications in view of their improved moisture-resistance and tunability in bandgap energy as compared to their 3D counterparts. Herein, we demonstrate the use of cation exchange between A and B in (A)2(B)n-1PbnI3n+1 (A = CH3(CH2)3NH3+, B = CH3NH3+) to fabricate mixed 2D perovskites with well-controlled n-values of perovskite constituentsas well as their relative amounts.We find that the perovskite components in thin films can be systematicallytuned from pure 3D to 2D mixture(containing multiple 2D components), and to pure single-phase 2D (n = 1)by simply adjusting the cation-exchange time. Extensive static and transient spectroscopic measurements show the internal electron-holeseparation in the mixed2D perovskite filmdue to the ordered band alignment between different perovskite components. This engineered directional charge flow can be particularly helpful for the photovoltaic application of 2D perovskites.


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INTRODUCTION

Due to theunique combination of facile synthesis and superior optoelectronic properties, organolead halide perovskitesMAPbX3(MA = CH3NH3+, X = I-, Br- or Cl-) have

achieved

tremendous

success

in

photovoltaic

and

optoelectronic

applications.1-14However, one challenge that hampers the application of perovskites in real-life devices is their chemical stability. Although several factors including heat and light exposure influence the stability of perovskite, decompositioninduced by water exposure is one of the more pressing issues.15-19Recently, the development of two-dimensional (2D) layered perovskites provides a route to mitigate the solubility issue of the organic cation in the perovskite structure. 2D perovskites have a general chemical formula of (RNH3)2(MA)n-1PbnX3n+1, where RNH3+ is the large aliphatic oraromatic ammonium cation and n represents the number of inorganic perovskite layers. In this notation, the limit n = ∞ corresponds to cubic 3D MAPbI3 perovskites.The

larger

hydrophobic

functional group

(RNH3) protects the

metal-halide layers from water infiltration, which has led to 2D perovskite solar cells with significantly improved moisture resistance.20-23In addition to the improved chemical stability, compared with the 3D perovskites the exciton binding energy in 2D perovskites is much larger due to geometric and dielectric contrast effects, and the tunability in exciton absorption/emission energy can be achieved by changing the number of n through quantum confinement effect. Therefore, additional associated opportunities are the applications of 2D perovskite materials in light-emitting diodes with wide color tunability.24-27 3 ACS Paragon Plus Environment


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2D perovskite films in devices typically containbutylammonium (BA)20, 21, 24, 28 and 2-phenylethylammonium (PEA)22,

23, 25, 26, 29

as the hydrophobic cations and

wereusually fabricated via a hot-casting process,20-26with the n value of 2D perovskite controlled by the stoichiometric ratio between PbI2, MAI and BAI (PEAI).However, recent studies have established that although the films were nominally prepared as with a single n value, they contain multiple perovskite components with the n = 2, 3, 4… to ≈ ∞.25,

27, 30

Interestingly, this mixed-dimensionalityfeature of the 2D

perovskite films leads to spontaneous charge carrier accumulation at specific perovskite components via energy transfer25,

27, 31

and/or internal electron-hole

separation between different perovskite components via charge transfer.30These are properties enabling the improved performance of 2D perovskite films in LEDs, solar cells and other optoelectronic devices.25-27, 32-35However, the underlying mechanism responsible for the formation of the mixed dimensionality structure remains unknown. Moreover, while the spontaneous formation of the mixedstructure is interesting, it on the other hand limits the controllability and tunability on then values of perovskite components as well as their relative amounts and thus consequent carrier dynamics in the mixed2Dfilms.

A unique opportunity for controlling the properties of lead halide perovskites is the versatile tunability on their elemental component and thus optical band gap through simple ion exchange reactions,34, 36-42 which can be harnessed to create mixed structures with engineered energy/charge flow directions. Intuitively, mixed perovskite structures can be produced through either anion or cation exchanges 4 ACS Paragon Plus Environment

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reactions. The former has been demonstrated, for example, in our recent work that charge carriers can be funneled over a distance of several micrometers in perovskite nanowires with built-in halide gradient generated by facile anion exchange.38 For 2D perovskites, cation exchange can work as effectively as anion exchange because according to their general formula of (RNH3)2(MA)n-1PbnX3n+1, exchange reactions between RNH3+ and MA+ can be used to tune the thickness (or n) and hence band gap of the quantum confined layers. In related studies,the facile conversion from pure n=1 2D perovskite film ((IC2H4NH3)2PbI4) to mixed-dimensional perovskites by cation exchange was reported by Mhaisalkaret al.34Furthermore,Zhao et al. reported the fabrication of high quality 3D MA1-xFAxPbI3 (FA = CH(NH2)2) from 2D perovskites by cation exchange43and effective conversions from 3D to single phase 2D perovskites were also reported by Ogaleet al. and Ptasinskaet al.44, 45However, cation exchange has yet to be utilized to prepare mixed 2D perovskites with different n–value components aligned in the same film for engineered directional energy/charge flow. Herein, we report the effective, programmable and reversible conversion between 3D MAPbI3 and 2D (BA)2(MA)n-1PbnI3n+1perovskites by cation-exchange reactions in BAI (3D to 2D) and MAI (2D to 3D) solutions. We found that by simply adjusting the cation-exchange time the perovskite components in thin films could be tuned from pure 3D to mixed-dimensional2D (containing n =1, 2, 3 and ∞), and finally to pure single-phase 2D (n = 1). Using static and transient absorption and photoluminescence (PL) techniques, we also observedthat electrons and holes could be separated, i.e., flowed along opposite directions, in the mixed2D 5 ACS Paragon Plus Environment


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perovskite filmdue to the band alignment between different perovskite components. This internal electron-hole separation enabled by facile cation exchange reaction is particularly attractive for the implementation of 2D perovskites into photovoltaic devices.

EXPERIMETNAL SECTION

Preparation of 3D MAPbI3 thin films.The precursor methylamine iodide (MAI) was synthesized by mixing 61 mL of methylamine (33 wt% in absolute ethanol) and 65 mL of HI (57 wt% in water by weight) in a 250 mL flask in an ice bath at 0 °C for 2 h with stirring. CH3NH3I was precipitated as the solvent was carefully removed using a rotate evaporator (Dragon Laboratory Instruments Limited RE100-PRO, China) at 50 °C. The white CH3NH3I powder was then washed for three times with diethyl ether, and dried at 80 °C in a vacuum oven for 24 h. 461 mg of PbI2 (99.999%, Sigma-Aldrich), 159 mg of CH3NH3I, and 162 mg of dimethylsulfoxide (DMSO) (molar ratio 1:1:2) was mixed in 1200 mg of anhydrous N,N-dimethylformamide (DMF) solution at room temperature with stirring for 1 h in order to prepare a CH3NH3I•PbI2•DMSO adduct solution. This precursor solution was stored under a dry nitrogen atmosphere.

Glass (or FTO) slides were cleaned using an ultra-sonication bath in soap water and rinsed progressively with distilled water, isopropyl alcohol and acetone, and finally treated with oxygen plasma for 20 min. To deposit perovskite films, the CH3NH3PbI3 6 ACS Paragon Plus Environment

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precursor solution (60 µL) was first dropped onto the cleaned glass substrate in a nitrogen-filled glovebox. The substrate was then spun at 4000 rpm for 40s and 0.25 mL of anhydrous chlorobenzene was quickly dropped onto the center of the rotating substrate in 6 seconds before the surface color changed from transparent to light brown. After spin-coating, the substrate was annealed at 100 °C for 10 min on a hot plate in order to form a dense 3D MAPbI3 thin film. These 3D MAPbI3 thin films were then stored in the golvebox before cation-exchange treatment.

Reversible conversion of 3D MAPbI3to2D (BA)2(MA)n-1PbnI3n+1 thin films. The butylammonium iodide (BAI) was synthesized from the reaction of n-butylamine with hydriodic acid (HI) (47wt% in water) at 0 °C. The crude product was obtained by slowly evaporating the solvent under reduced pressure. Then the white precipitate was dissolved in ethanol and recrystallized by adding diethyl ether. The small crystals were further washed with diethyl ether several times before drying them in vacuum oven. After drying overnight, the white crystalline powder was sealed under nitrogen and transferred into a glovebox for further use.

To transform the 3D MAPbI3to 2D perovskitethin film, 80 µL BAI-isopropanol (BAI-i-PrOH) solution with a concentration of 0.1 M was dropped onto the center of the pre-prepared MAPbI3 film at a time. In order to obtain two-dimensional (2D) perovskite thin films with different extent of cation-exchange, the dropped BAI-i-PrOH solution stood on the on theMAPbI3 filmsfor different amount of time (3 s, 10 s, 30 s, 60 s and 90 s) before spin-coating at 5000 rpm for 40 s to remove the



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remaining solutions. To reversibly transform the 2D perovskite back to 3D films, the MAI-n-butyl alcoholsolution with a concentration of 0.06 Mwas dropped on the 2D perovskite films and remained for 2 min before spin-coating at 5000 rpm for 40s. All the experiments were conducted in a nitrogen-filled glovebox.

Preparation of 2D layered (BA)2(MA)n-1PbnI3n+1 single crystals. The 2D layered (BA)2(MA)n-1PbnI3n+1 single crystals with different n values were synthesized followingthe liquid phase crystallization method reported previously28. Lead(II) iodide (99%), hydriodic acid (57 wt %, stabilizer free in water), n-butylamine (99.5%), and methylamine solution (33 wt % in ethanol) were purchased from Sigma-Aldrich and diethyl ether (BHT stabilized) was purchased from Tianjin Kemiou Chemical Reagent Co.. At 110 °C, stoichiometric quantities of lead(II) iodide, n-butylamine, and methylammonium iodide were dissolved in a minimum volume of hydriodic acid for the growth of (BAI)2(MAI)n−1(PbI2)n with n = 1, 2, 3, 4 and 5. For each single crystal sample, plate-like crystals with a size of a few millimeters were obtained after the solution was slowly cooled to −10 °C. The crystals were rinsed with cold diethyl ether and dried at 60 °C under vacuum for 24 h before exfoliation. We mechanically exfoliated each crystal and transferred the flakes onto clean fused silica for linear absorption measurements.

Ultrafast transient absorption spectroscopy measurement.The femtosecond transient absorption setup is based on a regenerative amplified Ti:sapphire laser system from Coherent (800 nm, 35 fs, 6 mJ/pulse, and 1 kHz repetition rate),


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nonlinear frequency mixing techniques and the Helios spectrometer (Ultrafast Systems LLC). Briefly, the 800 nm output pulse from the regenerative amplifier was split in two parts with a 50% beam splitter. The transmitted part was used to pump a TOPAS Optical Parametric Amplifier (OPA) which generates a wavelength-tunable laser pulse from 250 nm to 2.5 µm as pump beam. The reflected 800 nm beam was split again into two parts. One part with less than 10% was attenuated with a neutral density filter and focused into a 2 mm thick sapphire window to generate a white light continuum (WLC) from 450 nm to 800 nm used for probe beam. The probe beam was focused with an Al parabolic reflector onto the sample. After the sample, the probe beam was collimated and then focused into a fiber-coupled spectrometer with CMOS sensors and detected at a frequency of 1 KHz. The intensity of the pump pulse used in the experiment was controlled by a variableneutral-density filter wheel. The delay between the pump and probe pulses was controlled by a motorized delay stage. The pump pulses were chopped by a synchronized chopper at 500 Hz and the absorbance change was calculated with two adjacent probe pulses (pump-blocked and pump-unblocked). All experiments were performed at room temperature.

RESULTS AND DISCUSSION

We developed a facile strategy for thereversibletransformation between 3D MAPbI3 perovskite and layered 2D perovskites by a cation-exchange reaction (see Figure 1). We first prepared dense and pinhole-free 3D MAPbI3 perovskite thin film 9 ACS Paragon Plus Environment


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using a stoichiometric [MAI+PbI2] mixture precursor solution. 80 µLBAI isopropanol (i-PrOH) solution (0.1 M) was then dropped onto the center of the MAPbI3 film. After waiting for controlled timesfor cation-exchange reaction, the substrate was spin-coated to remove the remaining solutions (see Supporting Information for details). The 3D MAPbI3 perovskite films quickly turned from dark brownto orange-yellow,

indicating

the

formation

of

(BA)2(MA)n-1PbnI3n+12D

perovskitescontaining multiple perovskite phases (with n = 1, 2, 3…to n≈∞). These phases are aligned in the order of their n values along the direction perpendicular to the substrate (as confirmed later), forming a step-like bandgapenergyband alignment(Figure 1) inside the film and thus enabling electron (from small-n to large-n) and hole (from large-n to small-n) transfers in opposite direction. A similar procedure is also applicable for the reversed 2D-to-3D conversion (Figure 1) by dropping theMAIn-butyl alcohol (n-BuOH) solution(0.06 M)onto the 2D perovskite films. After a cation-exchange reaction for 2 min, the 2D thin film returned to dark brown, indicative of the successfultransformation from 2D back to 3D perovskite films.


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Figure1.Scheme of charge flow engineering in mixed-dimensional 2D perovskites enabled by facile cation exchange. (a) The cation exchange reaction allows for reversible conversion between 3D MAPbI3 and layered 2D (BA)2(MA)n-1PbnI3n+1 perovskites. By controlling the cation exchange time, we fabricate perovskite films containing 2D perovskite components with differentn values (n =1, 2, 3 and ∞). (b) Due to the ordered distribution of various n-value components inside the film, the band alignment drives directional electron flow from small-n to large-n perovskites and hole flow in the opposite direction and thus very efficient charge separation. The reversible 3D-to-2D transformations of perovskites are confirmed by their UV-vis absorption spectra before and after the 3D-to-2D cation exchange reactions with different reaction times (see Figure 2a). Right after 3 seconds of cation exchange, the perovskite film shows clear absorption peaks at ~520 nm (2.38 eV) and 575 nm (2.16 eV) in addition to theabsorption of 3D perovskites at ~750 nm (1.65 eV), which can be assigned to the absorptions of 2D perovskite components with n = 1, and 2, respectively,by comparing with the absorption of (BA)2(MA)n-1PbnI3n+12D perovskite single crystals (Figure S1).Photoluminescence and transient absorption spectra of the film also indicate the formation of n = 3 component in the film (see later) although it is very weak on the UV-vis spectra. As the cation exchange reaction time increases, the absorption of 3D perovskite component diminishes quickly, and the perovskite film becomes dominated by 2D perovskite components with n = 1 and 2, and eventually turns into single-phase 2D perovskite with n = 1 after 90 s of cation exchange reaction. The reversible transformation of these 2Dfilms back to 3D perovskites by cation-exchange with MAI solution is also confirmed by the UV-vis and PL spectra (Figure S2), XRD patterns (Figure S3) and TA spectra (Figure S4), where spectroscopic and structural characters of all 2D perovskite components disappeared. 11 ACS Paragon Plus Environment


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The X-ray diffraction (XRD) patterns of the perovskite films(Figure 2b and S3) before and after the cation-exchange reaction further confirms the 3D-to-2D transformation.In the perovskite film with a cation-exchange reaction of 10 s(Figure 2b), the characteristic XRD peaks of 3D perovskite disappear,21 accompanied by the appearance of 2D (BA)2(MA)n-1PbnI3n+1 perovskite peaks[PbI4 (002), P2I7 (040, 111) and Pb3I10 (222)],20indicating the formation ofn = 1, 2 and 3 2D perovskite components.In order to understand the growth orientation of these 2D components the XRD pattern of the 2D perovskite powder (by scratching the perovskite materials off the film) is also compared in Figure 2b. The absence of (0k0) plane diffraction in 2D perovskite films indicates the vertical orientation (perpendicular to the substrate) of the 2D perovskite layers,20suggesting that the mixed2D perovskite films fabricated by cation-exchange have the same growth orientation as in those by the one-step hot-casting process.25, 27, 30In the inset of Figure 2b, we also compare the SEM images of the perovskite films before and after the 3D-to-2D transformation. The surface of 2D perovskite film after cation-exchange reaction becomes rougher than that of 3D perovskite, which is probablyindicative of the inclusion of larger cations (BA+) in the crystal lattice. Furthermore, the cross-section SEM images (Figure S5) of the films also show almost un-changed thickness before and after the cation-exchange. These results indicate that the cation-exchange reaction is a robust method for fabricating high quality 2D perovskite films, which allows for systematical tuning from pure 3D perovskites to mixed 2D and to pure single-phase perovskites by adjusting the cation-exchange time. 12 ACS Paragon Plus Environment

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Figure 2. (a) The UV-vis absorption spectra of the perovskite film before (3D) and after the cation-exchange reaction with different reaction times (3s to 90s). The dashed lines indicate the absorption peak positions of n = 1, 2, 3, 4 and ∞ (3D) perovskites. (b) The XRD patterns of the perovskite film before (3D-film) and after (2D-film) a cation-exchange reaction of 10 seconds. The XRD pattern of the 2D perovskite powder (2D-powder) obtained by scratching the perovskites off the film is also plotted for comparison. The insets show the SEM images of the perovskite films before and after the cation-exchange reaction. The scale bar is 5 µm. In the transformation from 3D to 2D perovskites, the cation exchange reaction starts from the upper surface of the perovskite film, which should result in a ordered distribution of different perovskite components in the final mixed 2D perovskite films before complete transformation to pure n = 1 perovskites. To examine such distribution, the PL spectra of the 2D perovskite film were collected under two excitation configurations. The configuration with the laser beam (405 nm) impinging the perovskite (or the glass substrate) is defined as the front (or back) excitation (see Figure 3a). In Figure 3b, we compare the PL spectra of the 2D perovskite films under 13 ACS Paragon Plus Environment


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the two excitation configurations. For the mixed2D films with cation-exchange reaction times of 3 and 10 seconds, their PL spectra exhibit multiple emission peaks, which are recognized as the 2D perovskites components with n = 1, 2 and 3 and ∞(or 3D). This mixed-dimensionality feature is consistent with the observation in the UV-vis absorption spectra. However, the two excitation configurations result in very different relative emission intensity between the 2D (n = 1, 2 and 3) and 3D components. The front (or back) excitation leads to relatively higher emission intensity from 2D (or 3D) components. This is consistent with our expectation that in the mixedperovskite films the 2D perovskite components are mainly located at the upper surface of the film, and the 3D component remains at the bottom (glass) side when the cation exchange reaction time is short and the BA+ cations have not yet penetrated through the entire film thickness. With increasing cation exchange time (≧ 30s), the PL spectra of perovskite films become dominated by the emission peak from n = 1 perovskite.


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Figure 3. (a) The two excitation configurations (back exc. and front exc.) in the PL and transient absorption measurements. (b) PL spectra of the perovskite films before (3D) and after the cation-exchange reactions with different reaction times under both front- (blue squares) and back- (red circles) excitations. The dashed lines indicate the emission peak positions of n =1, 2, 3 and 3D perovskites. Previous studies have reported the observations of internal carrier transportations by energy or charge transfer between different perovskite components in the mixed2D perovskite films.25, 27, 30, 31We also examined the charge carrier dynamics inside the perovskite films after cation-exchange reaction by using femtosecond transient absorption (TA) spectroscopy (See SI for details). To further confirm the ordered distribution of different perovskite components in the films, the pump pulse also excited the perovskite films in two configurations (front and back), and the induced absorption changes (∆A) as functions of both wavelength and time were recorded. In Figure 4 a, c and e, we show the TA spectra of the 2D perovskite films fabricated after 3, 10 and 30 seconds of cation-exchange reactions (see Figure S5 for perovskite 15 ACS Paragon Plus Environment


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films with longer cation-exchange times) under front-excitation at 400 nm. Consistent with their UV-vis absorption spectra, the TA spectra of these films exhibit multiple bleach bands at each perovskite components with n = 1, 2, 3 and 3D due to charge carrier filling upon excitation. The TA spectra of these films under back-excitation at an early delay time (2 ps) are also shown in these figuresfor comparison,where the amplitude of the TA bleaches from larger-n perovskite components are relatively larger than those under front-excitation. This difference between the two excitation configurations agrees with the observation in the PL spectra, and further confirms that the different perovskite components should distribute in the order of their n values in the films (small-n component at the upper surface and large-n component at the bottom surface). We believe that the ordered distribution of different perovskite components in the mixed2D film is formed because the extent of cation-exchange (MA+ to BA+) decreases from the upper surface to the bottom surface when reaction time is short.

Previously we reported that the 2D perovskite films fabricated by one-step spin-coating process contained multiple perovskite phases (with n = 2, 3, 4…to n≈∞) arranged in the order of their n values along the direction perpendicular to the substrate.30 Such an arrangement forms a step-like band alignment (Figure 1) inside the film and thus results in electron (from small-n to large-n) and hole (from large-n to small-n) transfers in opposite direction. We therefore expect that such directionalcharge flow should also occur in the mixed2D perovskite films fabricated by cation-exchange reaction having similar mixed-dimensionalarchitecture. Figure 4b, 16 ACS Paragon Plus Environment

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d and f show the TA kinetics (with front-excitation at 400 nm) probed at each bleach band for the TA spectra in Figure 4a, c and e, respectively. For the film with cation-exchange time of 3s (Figure 4b), the TA kinetics decay slower as the n value increases, and for the n = 3 and 3D (n = ∞) perovskite components the rising kinetics appears (after 1 ps) upon the initial amplitude generated by direct excitation. Moreover, the rising time becomes longer for larger-n perovskite component. Similar trend in TA kinetics is also observed for perovskite films with 10 and 30 seconds cation-exchange reaction times (see Figure 4d and f). These results indicate the consecutive carrier transportation from small-n to large-n perovskite components, and can be attributed to internal electron transfer processes according to our previous report (see ref. 30). Such electron transfer is not observed in the 2D perovskite films with 60 and 90 s of cation-exchange times (see Figure S6), likely because that the film is dominated by the n = 1 component.



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Figure 4. The TA spectra at the indicated delay times (a, c and e) and the corresponding kinetics probed at different absorption bands (b, d and f) of the perovskite films after the 3s (a, b), 10s(c, d) and 30s (e, f) of cation-exchange reaction. The perovskite films are under front-excitation at 400 nm (power = 1.4 µJ/cm2). For comparison, a TA spectrum at the delay time of 2ps under back-excitation with the same excitation wavelength and power (black circles) are also shown for each film in panel a, c and e. The solid lines in panel b, d and f are the fits of the kinetics by a multi-exponential function with the fitting parameters listed in Table S1. Directionally opposite to the electron transfer, the hole transfer from large-n to small-n perovskite components is also expected to occur in the mixedperovskite filmsaccording to the band alignment (Figure 1b). To probe the hole transfer, we 18 ACS Paragon Plus Environment

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selectively excited only the perovskite component with the largest n value in the film under back-excitation. Figure 5a and c show the TA spectra of the mixed2D perovskite films with cation-exchange reaction times of 3 and 10 seconds with the excitation wavelength at 760 nm. In addition to the broad photoinduced absorption (PIA) from the excited 3D perovskite component, a small bleach peak is observed at n = 2 absorption band. Because the films contain relatively less amount of n = 3 perovskite component, a very weak bleach at n = 3 band is only observable at later delay times when the PIA signal becomes small. The TA kinetics probed at n = 2 and n = 3 absorption bands (Figure 5b and d) after subtracting the PIA contribution clearly confirm the gradual formation of their bleach peaks. These kinetics show an initial amplitude right after the pump, which is likely due to the direct excitation at 760 nm by two-photon absorption. The kinetics at n = 2 and n = 3 bands should thus include contributions from i) intrinsic carrier (generated by the direct excitation) recombination and electron transfer to the 3D perovskite component, and ii) hole transfer from the 3D perovskite component. The former (latter) should results in the decay (increase) of the carrier density at the n = 2 and 3 bands. To estimate the contribution of the intrinsic carrier decay in kinetics, the TA kinetics from n = 2 and 3 2D perovskite single crystals (SC) (excited by two-photon absorption) are also shown in Figure 5b and d for comparison. Noticeably, under back-excitation at 760 nm the TA kinetics probed at n = 2 and 3 bands in the mixed2D perovskite films exhibit a rising process (except the one probed at n = 2 in Figure 5d), followed by a carrier decay that is much slower than the intrinsic carrier recombination in single crystals. 19 ACS Paragon Plus Environment


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This result suggests the carrier populating process to the n = 2 and 3 bands, which can only be attributed to hole transfer from 3D component to the n = 2 and 3 components in the mixed perovskite films. Because of the smaller contribution of hole filling to the TA bleach signal46, 47

and the presence of possible hole trapping states,48, 49 the amplitude of bleach peaks

generated by hole transfer is much smaller than that by electron transfer, which hinders the clear observation of hole transfer kinetics. For the perovskite film with cation-exchange time of 30 s, excitation at n = 3 or n = 2 perovskite component does not lead to the observable hole transfer from n =3 to n = 2 or n = 2 to n = 1 component (Figure S7). This is likely because of the less amount of n = 3 perovskite component in the film and/or the large n = 2 bleach signal generated by direct excitation that overwhelms the bleach formation process by hole transfer.


Back-exc. @ 760 nm

a ∆ A (mOD)

1.2

0 (ps) 1.4~2.8 103~162 344~542 1041~1636 4940~7762

3s

0.8

0.4

n=2 n=3

3s

0.08

0.04

0.00

n=2 n=3 560

Back-exc. @ 760 nm

b 0.12

0.0 640

720

0

800

2000

Wavelength (nm) 0 (ps) 1.1~2.3 132~208 599~942 1809~2843 4940~7762

10 s 0.8

0.4

4000

6000

8000

Delay Time (ps)

d

n=2 n=3

10 s 0.08

−∆Α (mOD)

c ∆A (mOD)

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−∆Α (mOD)

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0.04

0.0 0.00

n=2 n=3 560

640

720

Wavelength (nm)

800

0

400

800

2000

4000

6000

8000

Delay Time (ps)

Figure 5. The TA spectra at the indicated delay times (a, c) and the corresponding kinetics probed at different absorption bands (b, d) of the perovskite films after the 3s (a, b) and 10s (c, d) of cation-exchange reaction. In panel b and d, the TA kinetics of n = 2 and 3 single crystals (SCs) (grey triangles) are also shown for comparison with the kinetics in the perovskite films probed at the bands of the same n values. The perovskite films are under back-excitation at 760 nm (power = 21 µJ/cm2). The solid lines in panel b and d are the fits of the kinetics by a multi-exponential function with the fitting parameters listed in Table S2. To estimate the internal charge transfer times and the lifetimes of separated electron and hole, the TA kinetics in Figure 4 and 5 are fitted by a multi-exponential function. The fitting parameters are listed in Table S1 and Table S2. In the mixed2D perovskite film of ~290 nm thickness, the electron transfer time is estimated to be ~17 to 127 ps between different components in the mixed2D perovskite films with different cation-exchange times. The hole transfer time is estimated to occur on ~160ps to nanosecond time scales. Importantly, by comparing the TA kinetics of the perovskite components in the mixed-dimensionalfilms with the intrinsic carrier 21 ACS Paragon Plus Environment


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kinetics in the corresponding (with the same n value) single crystals (see Figure S8 and Figure 5b and d), we observed that the lifetimes of separated electrons and holes were extended to>8 ns from ~2 ns in single crystals.

In contrast to the weak electron-hole binding energy in 3D perovskitesthat allows for excitons dissociation into free electrons and holes, the electron-hole bindingis strongly enhanced in 2D perovskites, as partially reflected from their much shorter intrinsic charge carrier lifetime (a few nanoseconds) than that (as long ashundreds of nanoseconds to microseconds) in the 3D perovskites.50-52 Therefore, how to dissociate the excitons in 2D perovskites is important particularly for their applications in photovoltaic devices. Recently, Mohiteet al. reported the existence of a low energy state in both 2D perovskite single crystals and thin films, where the excitons accumulates and are protected from nonradiative recombination to achieve longer carrier lifetimes.53However, the mechanism of producing such low energy states is still unclear. Alternatively, in this work, by creating the multiple perovskites components in the mixed2D perovskite films using cation-exchange, the excitons can also dissociated by charge transfer into separated electrons and holes with longer lifetimes.This self-charge-separation property should be highly desirable for photovoltaic applications. Furthermore, the cation-exchange reaction also provides the tunability in components and corresponding carrier dynamics by simply adjusting the cation-exchange

time.

This method should

thus be

useful in

preparing

mixed-dimentional2D perovskite films with desired perovskite components and energy band alignment. 22 ACS Paragon Plus Environment

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As mentioned earlier in this paper, other research groups have reported energy transfer, rather than electron or hole transfer, from small-n to large-n perovskite components in the mixed2D film with a thickness of

Engineered Directional Charge Flow in Mixed Two ... - ACS Publications

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