Cation-Dependent Light-Induced Halide Demixing. in Hybrid Organic-Inorganic Perovskites

Transcription

1 Supporting Information for Cation-Dependent Light-Induced Halide Demixing in Hybrid Organic-Inorganic Perovskites Carolin M. Sutter-Fella #*, Quynh P. Ngoǁ#, Nicola Cefarin ^, Kira L. Gardner, Nobumichi Tamura, Camelia V. Stan, Walter S. Drisdell, Ali Javeyǁ#, Francesca M. Toma and Ian D. Sharp +* Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, US # Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, US ǁ Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, US ^ Department of Physics, Graduate School of Nanotechnology, University of Trieste, Trieste, Italy Cyclotron Road, Lawrence Berkeley National Laboratory, Berkeley, California 94720, US 1

2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, US Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States + Walter Schottky Institut and Physik Department, Technische Universität München, Garching, Germany Corresponding Author * Carolin M. Sutter-Fella: csutterfella@lbl.gov Ian D. Sharp: sharp@wsi.tum.de (previously Lawrence Berkeley National Laboratory) 2

3 Experimental CH 3NH 3Pb(I 1-xBr x) 3 (MAPb(I 1-xBr x) 3) film fabrication: Solution process MAPb halide films were fabricated on glass substrates via spin-coating in a fume hood. For that purpose, two precursor solutions with 1 M MAI and 1 M PbI 2 (Alfa Aesar, %) and 1 M MABr and 1 M PbBr 2 (Sigma-Aldrich %) were dissolved in DMF and filtered with a 0.2 µm polytetrafluorethylene filter. The methyl ammonium halides were synthesized as described in References. To obtain mixed I/(I+Br)-halide films, the two precursor solutions were mixed in 1,2 the desired ratio. The precursor solution, as well as the substrates, were pre-heated to 110 C on a hot plate before the precursor was spun onto the substrate at 500 / 2000 rpm for 5 / 45 s. The MAPb(I 1-xBr x) 3 films were dried on a hot plate at 110 C under N 2 flow for 45 min. CH 3NH 3Pb(I 1-xBr x) 3 (MAPb(I 1-xBr x) 3) film fabrication: Low-Pressure Vapor-Assisted Solution Process (LP-VASP) Pinhole free MAPb-halide thin films with controlled Br concentration were synthesized by a two-step low pressure vapor-assisted solution process (LP-VASP). First, the mixed lead halide 1 3 precursor, comprising 0.8 M PbI 2 (Alfa Aesar, %) and 0.2 M PbBr 2 (Sigma-Aldrich, %) dissolved in N,N dimethylformamide (Sigma-Aldrich, 99.9%) and filtered with a 0.45 μm syringe filter, was spin-coated onto the glass substrate and dried at 110 C for 15 min in N 2 atmosphere. Spin coating of the mixed lead halide film was conducted in air at 500 rpm for 5s, followed by 2000 rpm for 3 min. Second, the precursor film was annealed in a test tube with a total of 0.1 g MAI/MABr at 120 C for 2 h at a pressure of 0.4 Torr. The Br content is tuned by the ratio of MAI to MABr. After conversion of the PbI 2/PbBr 2 precursor to MAPb-halide the sample 3

4 was rinsed in isopropyl alcohol and dried with N 2. For the purpose of PL analysis, it was not possible to obtain reliable measurements of the solution-processed MA samples prior to the occurrence of demixing. For this reason, prior data of VASP samples were used in Figures 4b, S4 and S7. (CH(NH 2) 2) 0.83Cs 0.17Pb(I 1-xBr x) 3 (FA 0.83Cs 0.17Pb(I 1-xBr x) 3) film fabrication: Antisolvent process Fabrication of FACsPb halide films was performed on glass substrates via spin-coating in an inert N 2 glovebox environment. 1 M stoichiometric precursor solutions were prepared in a one pot synthesis by dissolving FAI (Dyesol), CsI (Sigma-Aldrich, 99.9% trace metals), PbBr 2 (TCI America), and PbI 2 (TCI America, 99.99%). The molar ratio of formamidinium to cesium (FA:Cs) cations was held at 83:17 and various solutions were made with a range of bromide to iodide (Br:I) 4 ratios. All solutions were dissolved in anhydrous DMF (Acros Organics) and DMSO (Sigma- Aldrich) at a volume ratio of 80:20 and filtered through a 0.2 µm polytetrafluorethylene filter. The precursor solution was then spin-coated at 4000 rpm for 60 s and a chlorobenzene anti-solvent was dispensed over the film at exactly 15 s prior to the end of the spin process to aid in perovskite crystallization. Films were annealed on a hot plate at 100 C for 30 min in the N 2 glovebox at < 5 ppm O 2 and < 1 ppm H 2O. For all samples, substrates were cleaned with successive ultrasonic baths in detergent, deionized water, acetone, and isopropanol before use. After deposition of perovskite films, samples were spin-coated with PMMA as a protection layer against moisture and beam damage. 4

5 Table S1. Additional samples prepared with extended cation and halide (including Cl) compositions. Sample [FAI] [CsI] [PbBr2] [PbI2] XRD pure black PL halide Remarks (M) (M) (M) (M) perovskite phase demixing No No Yes No Eg (PL) = 1.64 ev Yes Yes Yes Yes No did not dissolve No Yes Yes Sample [FAI] [CsI] [PbCl2] [PbI2] XRD pure PL halide Remark (M) (M) (M) (M) perovskite phase demixing No No decreased solubility Synchrotron X-ray diffraction measurements: Measurements were performed at beamline of the Advanced Light Source, Lawrence Berkeley National Laboratory. Samples were placed under continuous N 2 flow to prevent degradation associated with X-ray irradiation in the presence of ambient atmosphere (further details on beam damage prevention were discussed in Ref. ) and were oriented at a 5 angle with 5 respect to the incident beam, in grazing incidence geometry. The detector (DECTRIS Pilatus 1 M) was located at 40 with respect to the direct beam and approximately 164 mm from the sample. The detector geometry was calibrated using a finely ground Al 2O 3 reference sample. Measurements were performed at 8 kev ( Å), with an estimated flux of 10 photons/s on an exposed area 10 5

6 of about 230 µm for a total of 30 or 60 min (MAPb halide or FACsPb halide, respectively). The 2 covered 2θ range was from The grazing-angle geometry, in combination with an area detector, enabled complete XRD patterns to be recorded within only 60 s. Measurements were collected before illumination, during illumination with a Xe lamp, and ~7 min after illumination. The excitation power was measured by a ThorLabs photodiode power meter (S120C). Figure S1. Synchrotron X-ray diffraction data zooming in at 12.7 corresponding to PbI 2 phase to illustrate the influence of constant photon exposure during XRD measurement on film degradation/decomposition. Degradation under X-ray exposure would result in an increase in PbI 2 phase which was not observed here. 6

7 Photoluminescence quantum yield measurements: Calibrated photoluminescence quantum yield measurements on PMMA coated lead halide perovskite thin films were obtained with a home-built micro-pl setup using the nm line of an Ar ion laser. The excitation power was measured by a ThorLabs photodiode power meter and the actual laser intensity hitting the sample was corrected for the absorption in the film. To calibrate the system sensitivity function of a Lambertian reflector, a Spectralon sample was measured under white light illumination. The external quantum yield was obtained by dividing the photoluminescence signal by the incident laser power. The effect of halide demixing was considered here in the following way: The QY was calculated from PL spectra that do not show any signs of halide demixing i.e. spectral shifts and/or change in emission energy. This can be achieved experimentally by limiting the illumination density and reducing the illumination duration. Urbach tail fitting from PL: The optical emission rate is related to the absorption coefficient as demonstrated by van Roosbroeck and Shockley. 6 The relationship between absorption coefficient α(hν) and the photoluminescence spectrum P(hν) can be expressed as α(hν) P(hν) * +, -./0 1 2 (34) 2 e(34/6 7)/6 9 in the case of thin samples. 7 With the photon energy hν, the Boltzmann constant k, the absolute temperature T and the refractive index n. E > is the Urbach tail parameter given by the slope at the absorption band edge. 7

8 a b c d e f Figure S2. The homogeneity of the FACsPb(I 1-xBr x) 3 films was investigated by steady-state spotto-spot PL measurements. All spectra show some fluctuations with respect to the emission wavelength, of up to 10 nm, as well as variation in intensity. This inhomogeneity is independent of the halide composition and is likely linked to the radially-driven perovskite recrystallization induced by the spun-on anti-solvent method (see Experimental Section above). (a)-(e) Point-topoint PL spectra of FA 0.83Cs 0.17Pb(I 1-xBr x) 3 samples with x = 0.1, 0.2, 0.4, 0.5, and 0.6. (f) Bandgap versus composition as extracted from PL peak maxima (point-to-point measurements with corresponding standard deviation) and EDX measurements (taken at two independent spots and rounded to one digit after the coma), respectively. 8

9 Figure S3. Fitting of the low energy tail to extract the Urbach energy given by the inverse slope. Figure S4. Urbach energies of FACsPb- and MAPb-halide perovskites extracted from the lowenergy PL tail as a function of the bandgap. Urbach energies for MAPb-halide perovskites are shown for two processes: synthesized with a two-step low pressure vapor-assisted solution process (LP-VASP) and via solution process (extracted from Hoke et al. ). 8 9

10 a b c Figure S5. MAPb-halide samples were processed by solution process as described above. (a) Position of the (100) x-ray diffraction peak of MAPb-halide films as a function of Br content, x (black line: linear fit to the data; the linear relationship has been reported by Noh et al. ). The fit 9 is used to extract halide composition (red vertical lines) for the two lowest Br-content samples corresponding to the measured XRD peak position. (b) The (100) x-ray diffraction peak position and (c) XRD FWHM as a function of Br content, x, in FACsPb- and MAPb-halide films. a b c Figure S6. Illumination time-dependent steady-state PL spectra of (a) FACsPb(I 0.6Br 0.4) 3, (b) FACsPb(I 0.5Br 0.5) 3, and (c) FACsPb(I 0.4Br 0.6) 3 thin films taken at ~50 mw/cm illumination at roughly 2 1 min increments. The dotted lines show PL spectra obtained after relaxation in darkness for 5 min. 10

11 MAPb(I 1-x Br x ) 3 FACsPb(I1 1-x Br x ) 3 x = 0.7 x = 0.6 Figure S7. Photo-induced PL change of MAPb- and FACsPb-halide perovskites with high Brcontent measured at ~0.6 suns and ~0.5 suns, respectively. MAPb-halide perovskites were synthesized with a two-step low pressure vapor-assisted solution process (LP-VASP). Figure S8. Illumination-induced changes of the Urbach energies of FACsPb(I 1-xBr x) 3 samples with x = 0.2, 0.5, and 0.6 (time evolution of the spectra under nm laser illumination are shown in Figure 2). In red, the initial Urbach energy before illumination (514.5 nm at ~50 mw/cm ) and in grey after 10, 17, and 20 min of illumination, respectively. The upper x-axis 2 11

12 indicates the emission wavelength that was fitted by a Gaussian fit to extract the Urbach energies after illumination. Figure S9. Comparison of PL emission for different FACsPb halides. The light induced I-rich emission is indicated by the arrows. The black curve is given as a reference for x =

13 Figure S10. Xe lamp spectrum used as white light source during synchrotron X-ray diffraction measurements. a MAPb(I 0.4 Br 0.6 ) 3 FA 0.83 Cs 0.17 Pb(I 0.4 Br 0.6 ) 3 b Figure S11. Full synchrotron diffraction patterns of a) MAPb(I 0.4Br 0.6) 3 and b) FA 0.83Cs 0.17Pb(I 0.4Br 0.6) 3 samples before illumination, after extended white light illumination of 30 min (a) and 60 min (b), and after ~7 min of relaxation in darkness. 13

14 MAPb(I 0.5 Br 0.5 ) 3 MAPb(I 0.4 Br 0.6 ) 3 a b MAPb(I 0.3 Br 0.7 ) 3 c Figure S12. Synchrotron diffraction patterns of a) MAPb(I 0.5Br 0.5) 3, b) MAPb(I 0.4Br 0.6) 3 with zoom in, and c) MAPb(I 0.3Br 0.7) 3 samples after various dark, illuminated, and dark relaxed conditions. All samples show some PbI 2 phase at ~12.7 ; the peak intensity of PbI 2 phase does not change under illumination. Since halide demixing and mixing kinetics are highly asymmetric due to large activation energies for halide ion migration in the dark, complete relaxation in the dark was not 10,11 evaluated. For example, Figure S12c shows peak broadening under illumination within one minute while relaxation in the dark takes > 20 min. Note, halide demixing is not only a function of x but also of the illumination density as shown in Ref. 2 14

15 Figure S13. Synchrotron diffraction patterns for FA 0.83Cs 0.17Pb(I 1-xBr x) 3 with x = 0.6 and 0.7 taken in the dark, after 60 min of illumination, and in the dark after relaxation for about 7 mins. Almost invariant peak shapes are observed for both compositions, which indicate absence of shear strain, diffuse scattering, and changes in lattice d-spacing. Figure S14. (100) peaks of MAPb-halides with Br compositions x = 0.6 and 0.7. The inset shows the (100) diffraction angle versus Br composition together with a linear fit (the linear 15

16 relationship was previously shown by Noh et al. 9 ) to extract the (100) position for x = 0.2 at 2 Theta of about a b c d e Figure S15. (a-e) Photoluminescence quantum yield of FA 0.83Cs 0.17Pb(I 1-xBr x) 3 samples with x = 0.1, 0.2, 0.4, 0.5, and 0.6 versus the injected number of suns-equivalent illumination density. Two independent points where measured to confirm representative QY values given the spot-to-spot variation in PL as shown in Figure S2. 16

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