Supporting Information for. Long-Distance Charge Carrier Funneling in Perovskite Nanowires Enable by Built-in Halide Gradient

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1 Supporting Information for Long-Distance Charge Carrier Funneling in Perovskite Nanowires Enable by Built-in Halide Gradient Wenming Tian, Jing Leng, Chunyi Zhao and 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, 457 Zhongshan Rd., Dalian, China, Methods: Synthesis of MAPbI 3, MAPbBr 3 and mixed halide MAPbBr x I 3-x single-crystalline NWs Synthesis of CH 3 NH 3 I. The CH 3 NH 3 I 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 flask in an ice bath at 0 C for 2 h with stirring. The methylammonium iodide (CH 3 NH 3 I) was achieved as the solvent was carefully removed using a rotate evaporator (Dragon Laboratory Instruments Limited RE100-PRO, China) at 50 C. The white CH 3 NH 3 I powder was washed with diethyl ether three times. The final product was collected by filtration and dried at 80 C in a vacuum oven for 24 h. Synthesis of CH 3 NH 3 Br. The CH 3 NH 3 Br was synthesized by mixing 30.5 ml of methylamine (33 wt% in absolute ethanol) and 28 ml of HBr (48 wt% in water by weight) in a flask in an ice bath at 0 C for 2 h with stirring. The methylammonium bromine (CH 3 NH 3 Br) was achieved as the solvent was carefully removed using a rotate evaporator (Dragon Laboratory Instruments Limited RE100-PRO, China) at 50 C. The white CH 3 NH 3 Br powder was washed with diethyl ether three times. The final product was collected by filtration and dried at 80 C in a vacuum oven for 24 h. Growth of CH 3 NH 3 PbI 3 NWs. To synthesis single-crystal CH 3 NH 3 PbI 3, we smeared appropriate amount of 100 mg/ml PbAc 2 3H 2 O dimethylsulfoxide (DMSO) solution on a glass slide to form a thin film and dried the glass slide for 30 min at 65 C to evaporate off the solvent. We S1

2 then immersed the PbAc 2 glass slide in 30 mg/ml CH 3 NH 3 I isopropanol solution for ~24 h at room temperature to grow the NWs. After reaction, the sample was rinsed with isopropanol to remove the residual salt on the film, and then dried under a stream of nitrogen flow. Growth of large CH 3 NH 3 PbBr 3 single crystals. We synthesized large (size in millimeter) MAPbBr 3 single crystals by anti-solvent vapor-assisted crystallization. PbBr 2 and MABr (1/1 by molar, 0.2 M) were dissolved in N,N-dimethylformamide (DMF), then we immersed the beaker (containing the above DMF solution) into anti-solvent dichloromethane (DCM). MAPbBr 3 single crystals were grown at the bottom and the wall of the beaker, with slow diffusion of the anti-solvent dichloromethane (DCM) in to the solution. Growth of evenly halide-mixed MAPbBr x I 3-x NWs For synthesis of the MAPbBr x I 3-x nanowires, appropriate amount of 100 mg/ml PbAc 2 3H 2 O dimethylsulfoxide (DMSO) solution was smeared on a glass slide to form a thin film and dried the glass slide for 30 min at 65 C to evaporate off the solvent. We then immersed the PbAc 2 glass slide in 1 ml of mixed CH 3 NH 3 I and CH 3 NH 3 Br in IPA solution with a concentration ratio of 16:4, 12:4, 10:6 and 8:6 mg/ml, respectively. After a reaction time of ~24 h, the glass slide was taken out, and rinsed with isopropanol to remove the residual salt on the film, then dried under a stream of nitrogen flow. PL measurement: We used a home-built photoluminescence (PL)-scanned imaging microscope coupled with a time-correlated single photon counting (TCSPC) module to map the PL kinetics within the perovskite NW. Excitation of the sample is achieved with a supercontinuum white-light laser (500 nm with 1 MHz repetition rate and ~6 ps pulse width, SC400-PP, Fianium, UK). For wide-field illumination, the laser beam is defocused before a 100 air objective lens (NA = 0.95) to form an excitation spot of ~23 μm in diameter, which ensures the NW is homogeneously excited. For partial excitation measurement, the same excitation laser beam was fixed at specific position of the NW with the spot radius of ~0.6 μm through the same objective lens. PL image measurement was collected by fast rotation of the galvanometer mirror and using a high speed detector (HPM , Hamamatsu, Japan) with a 708 nm long pass filter. S2

3 Each scanning image contains pixels. A 0.5 mm pinhole was placed before the detector to ensure that only PL from a diffraction-limited spot was observed. The magnification, M, from the sample plane into the pinhole is M = 20.8 M lens = 2080 (M lens is the magnification of the objective lens, 20.8 is magnification in the scan head). The theoretical PL collection spot diameter (d PL ) was calculated by dividing the pinhole size with the magnification d PL = d pinhole /M = 240 nm, which was smaller than the radius of the Airy disc (r Airy disc = 0.61λ/N.A. = /0.95 = 455 nm with emission spectrum centered at 708 nm). To obtain steady state fluorescence emission spectra, a setup which mainly consists of a monochromator (SpectraPro-2300i, Acton Research Co., USA) and an intensified charge coupled device (ICCD) camera (PI-MAX:1024HB, USA) was used, sharing the same excitation source and microscope objective for signal collection. The slit in the monochromator is 0.02 mm, which ensure the PL collection area is small enough. Carrier transportation model in halide-gradient perovskite NWs The PL kinetics at different positions in the halide-gradient perovskite NW shown in Figure 3d can be described by the following 1-D carrier transportation model, which includes both carrier diffusion and energy-driven carrier transportation processes: 2 (z,t) (z,t) D μe 2 t z (z,t) z 2 ( z) k1(z,t) k2(z,t) (S1) I PL ( z, t) ( z, t) 2 (S2) where ϕ(z,t) is the concentration of charge carriers at time t at the position z (0 z L, L is NW length); D is the diffusion coefficient; is the carrier mobility; E(z) is the local energy potential caused by the built-in halide gradient in the NW; k 1 and k 2 are the mono-molecular and bi-molecular carrier recombination rate constants, respectively. We assumed that E(z) is evenly distributed in the NW and E(z)=ΔE/L. ΔE is the bandgap energy difference between two ends of the NW and can be estimated from the amount of shift between the local PL emission spectra at two ends S3

4 of the NW (see Figure S6). Because the change of bandgap energy between the Br-rich and I-rich regions are majorly due to the change of VB band edge, the measured here should be close to the hole mobility. Using above equations, the carrier density and PL kinetics at a delay time t at any position z in the NW can be simulated. For pure MAPbI 3 NWs, the diffusion of the carriers can be also described by Eq. S1 and S2 with E(z) = 0. The boundary condition for the diffusion equation discussed above is defined as: ( L, t) z (0, t) z 0 The + and represent the forward and backward first-order differential, indicating the carriers in NWs do not diffusion out of its boundary. Under homogeneous excitation condition, the initial (t=0) carrier concentrations at different positions in the NW are the same (e.g. the NWs in Figure 3), while under local excitation condition (e.g. the NWs in Figure 4) the initial distribution of carriers in the NW can be described by: 2 ( z z0) ( z 0, ) ( z0 0, )exp( 2 ) 2 (S4) r where z 0, ) is the initial carrier density at the center of excitation position; r is the ( 0 radius of the excitation spot which is measured to be ~0.5 μm. Because Eq. S1 does not have a resolution, we perform the simulation of the carrier diffusion and transport process in perovskite NWs using a home-built program. The length (L) of the NW and excitation position (z 0 ) were determined from the optical and PL intensity images. From this simulation, the change of carrier density as a function of delay time at any position (within a circle area of ~0.5 μm in radius) in the NW is determined, and is used to fit the experimental PL kinetics. Using the parameters obtained by fitting the PL kinetics at different positions, we can also obtain the total carrier density (z) at any position z according to the following equation: t S4 (S3) (z) max (z,t)dt (S5) 0

5 where t max is the whole PL decay time recorded in our experiments. This equation is applied to fit the data in Figure 4c. Determination of carrier recombination constants in perovskite NWs The carrier recombination in perovskite can be described by the following rate equation: d( t) dt k (S6) ( t) k2( t) - k3( t) where (t) is carrier density at time t; k 1, k 2 and k 3 denote the mono-molecular (defect trapping), bi-molecular, and tri-molecular recombination rate constants respectively. The PL intensity I PL (t) is proportional to ϕ(t) 2. PL kinetic traces measured under different excitation power (Figure S8) were globally fit by Eq. S6, yielding the recombination rate constants. S5

6 Figure S1. SEM image of the as-grown MAPbI 3 perovskite single-crystalline NWs. The sale bar is 10 μm. Figure S2. Schematic presentation to illustrate the fabrication of halide-gradient MAPbBr x I 3-x NWs by a solid-to-solid halide exchange reaction. S6

7 Figure S3. SEM and EDS mapping measurements of single crystalline (a) MAPbI 3 and (b) MAPbBr 3 NWs. The scale bars are 2 µm. Figure S4. Additional examples of the EDS and local spectroscopy measurements of halide-gradient MAPbBr x I 3-x NWs. The scale bars are 2 µm. S7

8 Figure S5. Time-resolved and PL-scanned imaging microscopy for the PL intensity imaging and local kinetics measurements on the perovskite NWs. Figure S6. Local emission spectra for the halide-gradient MAPbBr x I 3-x NW shown in Figure 3 in the main text. Based on the shift of the spectra, the maximum bandgap energy difference between the bromide-rich region and the iodide-rich region is calculated to be ~0.07 ev. S8

9 Figure S7. PL spectra, PL lifetime, SEM and EDS mapping measurements of the evenly halide-mixed MAPbBr x I 3-x NWs. The scale bars are 2 µm. These NWs show blue-shifted spectra relative to the pure MAPbI 3 NWs. The extent of shift depends on the concentration of bromide ions. The lifetime of these NWs are all a few hundreds of nanoseconds. S9

10 Figure S8. Excitation-power-dependent PL decays of (a) a MAPbI 3 NW (b) an evenly Br-doped MAPbBr x I 3-x NW (fabricated by solid-to-solid ion exchange reaction). N 0 is the initial photogenerated carrier density under different excitation power. The global fit (solid lines) of these kinetics by Eq.S6 yields the recombination rate constants. The two types of NWs show similar k 1 and k 2 rate constants, indicating that the ion-exchange reaction does not induce significant additional defects into the Br-doped NWs. Figure S9. Local emission spectra for the halide-gradient MAPbBr x I 3-x NW shown in Figure 4 in the main text. Based on the shift of the spectra, the maximum bandgap energy difference between the bromide-rich region and the iodide-rich region is calculated to be ~0.06 ev. S10

11 Figure S10. Local PL kinetics in the (a) halide-gradient MAPbBr x I 3-x NW in Figure 4a and the (b) MAPbI 3 NW in Figure 4b. The solid lines are the global fits of the kinetics by Eq. S1 and S2, yielding the diffusion coefficient (D) and charge mobility (μ) of the NWs (See Table S1). Using these fitting parameters, the data in Figure 4c is fit by Eq. S5. Table S1. Fitting parameters for the PL results shown in Figure 3d and Figure S10 by Eq. S1.The fitting parameters for the PL results in Figure S10 is used to fit the data in Figure 4c by Eq. S5. D (cm 2 s -1 ) μ (cm 2 v -1 s -1 ) k 1 ( 10 6 s -1 ) k 2 (cm 3 s -1 ) * NW in Figure 3d 1.17 ± ± NW in Figure 4a and S10a 1.04 ± ± NW in Figure 4b and S10b 1.22 ± ± * The fitting of the PL kinetics does not yield a reliable k 2 value because of the low carrier density. S11

12 Figure S11. PL intensity images of a gradient MAPbBr x I 3-x NW and a MAPbI 3 NW with a focused laser excitation at different positions (indicated as the red circle). The red arrow indicates the direction of carrier transportation. In the gradient MAPbBr x I 3-x NW the carrier transportation from iodide-rich region to bromide rich region is forbidden no matter where the NW is excited. In the MAPbI 3 NW, carriers diffuse to wherever the density is low, lacking the directionality. The scale bars are 2 µm. S12

13 Figure S12. Plots of the emission peak wavelength as a function of environment temperature (examined after 30 minutes) for four different gradient MAPbBr x I 3-x NWs. The almost constant shift between Br-rich and I-rich regions indicates that the halide gradient is stable within the examined temperatures from 25 o C to 100 o C. S13

14 Figure S13. (a, b and c) Plots of the emission peak wavelength at different examination days after synthesis for gradient MAPbBr x I 3-x NWs. (d) PL intensity image (homogeneous excitation) of the gradient MAPbBr x I 3-x NW shown in panel (a) 55 days after synthesis. The scale bar is 2 µm. S14