High Photoluminescence Quantum Yield in Band. Gap Tunable Bromide Containing Mixed Halide

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1 Supporting Information for High Photoluminescence Quantum Yield in Band Gap Tunable Bromide Containing Mixed Halide Perovskites Carolin M. Sutter-Fella, Yanbo Li ǁ, Matin Amani, Joel W. Ager III #, Francesca M. Toma ǁ, Eli Yablonovitch, Ian D. Sharp ǁ*, and Ali Javey * Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, United States Materials Sciences Division, Joint Center for Artificial Photosynthesis, and ǁ Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Materials Science and Engineering, University of California, Berkeley, California 94720, United States Methods Synthesis of CH 3 NH 3 I: A 250 ml round bottom flask was charged with 24 ml of a 33% solution of methylamine (Sigma-Aldrich) in absolute ethanol, 10 ml of a 57% solution of hydrogen iodide (Sigma-Aldrich) in water, and 100 ml of ethanol under nitrogen atmosphere, and left stirring for 2 h at room temperature. The solvent was then removed under reduced

2 pressure at 50 C, and a white precipitate formed. The product was collected, thoroughly dried, and finally recrystallized from ethanol (Sigma-Aldrich 99.5%). The solid was then dried again at 60 C for 24 h to yield CH 3 NH 3 I. Synthesis of CH 3 NH 3 Br: A 250 ml round bottom flask was charged with 28 ml of a 33% solution of methylamine (Sigma-Aldrich) in absolute ethanol, 10 ml of a 48% solution of hydrogen bromide (Sigma-Aldrich) in water, and 100 ml of ethanol under nitrogen atmosphere, and left stirring for 2 h at room temperature. The solvent was then removed under reduced pressure at 50 C, and a white precipitate formed. The product was collected, thoroughly dried, and finally recrystallized from ethanol (Sigma-Aldrich 99.5%). The solid was then dried again at 60 C for 24 h to yield CH 3 NH 3 Br. Perovskite synthesis. Perovskite films with increasing Br concentration were fabricated over the full spectrum ( ev) by a modified two-step low pressure vapor-assisted solution process (LP-VASP). For the synthesis of mixed CH 3 NH 3 PbI 3-x Br x (0 < x <3), a mixed solution of 0.8 M lead iodide and 0.2 M lead bromide in DMF was used as the lead halide precursor. Lead halide precursor film was spin-coated on a glass substrate at a spin speed of 1500 rpm in air. The precursor film was dried on a hotplate under N 2 flow at 110 C for 15 min. The sample was then transferred to a test tube charged with a 0.1 g mixture of CH 3 NH 3 I and CH 3 NH 3 Br. The ratio between the CH 3 NH 3 I and CH 3 NH 3 Br was adjusted (9:1, 8:2, 7:3, 6:4, 5:5, 3:7) to achieve different Br concentration in the final perovskite film (as determined by energy dispersive X-ray spectroscopy (EDX) measurements, see below). The tube was evacuated with a rotary pump before immersing into a silicone oil bath. The mixed lead halide film was annealed in the mixed CH 3 NH 3 I/CH 3 NH 3 Br vapor at 120 C for 2 h under a pressure of ~0.4 Torr to form mixed I/Br perovskite (CH 3 NH3PbI 3-x Br x ) compositions. For the synthesis of pure CH 3 NH 3 PbI 3 perovskite, 1 M PbI 2 and pure CH 3 NH 3 I were used as the precursor solution and vapor source, respectively.

3 For the synthesis of pure CH 3 NH 3 PbBr 3 perovskite, 0.8 M PbBr 2 and pure CH 3 NH 3 Br were used as the precursor solution and vapor source, respectively. The LP-VASP was conducted under the same process as the mixed I/Br perovskites. After the vapor annealing process, the perovskite films were washed with IPA and a PMMA layer was spin-coated on as a protection layer before further optical characterization. Perovskite material characterization. XRD patterns of the samples were measured with a Rigaku SmartLab X-ray diffractometer using Cu K α radiation at 40 kv and 40 ma. Scanning electron micrographs were taken on a Zeiss Gemini Ultra-55. The film composition was extracted from EDX measurements at 7 kv acceleration voltage on the aforementioned Zeiss tool. Transmittance (T%) and Reflectance (R%) measurements were taken at room temperature with a SolidSpec-3700 UV-VIS-NIR spectrophotometer (SHIMADZU). Diffuse reflectance was measured with BaSO 4 as baseline and transmittance with PMMA/glass as baseline. Absorption A% was calculated via A% = 100% - R% - T% and the absorption coefficient α was calculated using α = ln (100 R% T% ) t where t is the film thickness. Steady-state photoluminescence. Calibrated luminescence efficiency measurements were obtained with a home-built micro-pl setup using either the nm or 488 nm line of an Ar ion laser. Measurements were conducted in ambient conditions (20 to 21 C, 40 to 60% relative humidity). More information on the PL setup and calibration can be found in Ref. 1 The laser power was varied by applying neutral density filters. The excitation power was measured by a ThorLabs photodiode power meter (S120C). For powers < 500 pw, lock-in detection from the output of a calibrated photodiode was used. The laser intensity hitting the sample was corrected for the actual absorption in the perovskite film at the respective wavelength. The measurements were conducted by focusing the laser beam onto the sample using an 80 objective (MD plan, NA = 0.9, ~3 μm 2 spot size) and the PL signal was collected through the same objective and

4 detected by a Si CCD camera (Andor idus BEX2-DD). Before each measurement the background was taken and subtracted from the actual PL measurement. The Measurements were performed starting at the lowest pump-power and were terminated when sample degradation was observed as the pump-power was increased. The illumination duration was reduced subsequently when increasing the pump-power; typically starting with 120 s and finishing with 1 s at the highest pump-power. The system sensitivity function of a Lambertian reference was obtained by measuring a Spectralon sample illuminated by a white light source. The external QY was obtained by dividing the photoluminescence by the pump-power. The internal QY (iqy) was extracted from the external QY by correcting it by 1/4n 2, where n is the refractive index of the perovskite. 2 PL imaging was performed on a fluorescence microscope (100 objective) with a green LED centered at 470 nm (intensity 550 nw) for excitation and a CCD detector (Andor Luca) for image collection. Time-resolved photoluminescence. A supercontinuum pulsed laser source (Fianium WhiteLase SC-400; with ps pulse width) with a repetition rate of 2 MHz was used for TRPL measurements. An excitation wavelength of either 514 nm or 488 nm (2 nm bandwith) was selected using a double monochrometer. The laser light was focused by an 80 objective (MD plan, NA = 0.9, ~3 μm 2 spot size) and PL was detected by an avalanche photodiode operating in single counting mode (IDQuantiqe). The instrument response time is 0.3 ns as extracted from a single exponential decay fit (Figure S8d).

representing the full compositional range under investigation.

a) Absorption coefficient of CH 3 NH 3 PbI 3-x Br x samples extracted from

5 Fig. S1. Film morphology. Top view scanning electron microscope images of CH 3 NH 3 PbI 3-x Br x samples representing the full compositional range under investigation. The images show pin-hole free and compact films. Fig. S2. a) Absorption coefficient of CH 3 NH 3 PbI 3-x Br x samples extracted from transmittance and diffuse reflectance measurements. b) Band gap (extracted from PL measurements) versus Br concentration x (from EDX spectroscopy) with empirical quadratic fit resulting in a bowing parameter b = 0.34 ev.

2 measured directly after fabrication and 9 weeks later (storing the

the PbI 2 phase over time (dashed line).

6 Fig. S3. XRD and iqy over time. (a) XRD pattern of a sample with composition CH 3 NH 3 PbI 2.8 Br 0.2 measured directly after fabrication and 9 weeks later (storing the samples in a N 2 box under room light conditions) showing the appearance of the PbI 2 phase over time (dashed line). (b) iqy for the same sample measured directly after fabrication, 6 and 9 weeks later. The increasing iqy over time is possibly related to the evolving PbI 2 phase seen in (a) which was reported to occur dominantly at grain boundaries where it could passivate defects and help to improve charge carrier collection. 3

this study. The illumination duration at the highest illumination power shown here is for 1s.

7 Fig. S4. Maximum illumination intensity with respect to Br concentration before illumination induced modifications appear under illumination durations used for this study. The illumination duration at the highest illumination power shown here is for 1s. Red squares: spectral red shift and blue squares: spectral blue shift. Fig. S5. Influence of illumination power on PL spectra. (a) PL spectra with increasing pump power. (b) PL peak position shift after exposure to too high laser intensity accompanied by intensity drop. (c,d) normalized PL spectra analyzed for extracting the QY (only black curves).

Red curves depict PL spectra shift upon high laser illumination and are not

Influence of high power illumination on PL spectra.

increasing power) from black (7600 suns), red (16595 suns), green (34570 suns)

The inset depicts the PL spectra obtained at 14 suns, each spectrum measured

8 Red curves depict PL spectra shift upon high laser illumination and are not considered for the QY analysis. Fig. S6. Influence of high power illumination on PL spectra. PL spectra with increasing pump power (arrow points in the direction of increasing power) from black (7600 suns), red (16595 suns), green (34570 suns) to blue (67736 suns). The inset depicts the PL spectra obtained at 14 suns, each spectrum measured after the high power measurement. Fig. S7. Influence of constant illumination but increasing exposure time. (a) A sample with composition CH 3 NH 3 PbI 2.8 Br 0.2 was held under constant illumination at 2 and 90 suns intensity

for 500 s. No PL peak shift was observed during constant illumination. Under 2 suns exposure, the intensity remained stable for 500 s, and, under 90 suns, it remained stable up to 400 s.

4 under constant illumination at 2 suns shows a peak shift to higher wavelength after less than 10s that remains at this position for more than 120 s.

9 for 500 s. No PL peak shift was observed during constant illumination. Under 2 suns exposure, the intensity remained stable for 500 s, and, under 90 suns, it remained stable up to 400 s. (b) A sample with composition CH 3 NH 3 PbI 1.6 Br 1.4 under constant illumination at 2 suns shows a peak shift to higher wavelength after less than 10s that remains at this position for more than 120 s. (c) A sample with composition CH 3 NH 3 PbI 1 Br 2 under constant illumination at 0.6 suns intensity. With increasing illumination duration, the intensity of the main peak at 630 nm drops while simultaneously a second peak at 740 nm appears. The illumination induced phase separation is reversible. After leaving the sample 2 min in the dark the initial PL peak at 630 nm is observed in absence of the signal at 740 nm (inset). Fig. S8. Photoluminescence and iqy. (a) Photoluminescence signal versus generation rate with corresponding power-law fits, and (b) the corresponding iqy curves for samples with various Br concentrations. (c) Spot-to-spot variation of the iqy for the pure Iodine sample (x = 0) and a sample with composition CH 3 NH 3 PbI 1.9 Br 1.1.

10 Fig. S9. Temperature dependence of the PL signal. The measurements were performed in vacuum at ~60 suns (6000 mw/cm 2 ). Heating a sample with composition CH 3 NH 3 PbI 2.7 Br 0.3 from room temperature to 410 K results in an invariant PL shape with an intensity drop to 65% at 400 K. This finding supports the superior sample stability. Cooling to 170 K results in an intensity increase by 40% that translates to an QY and iqy increase of 40%. The lower bound of the temperature dependent measurements is set by a phase change, from a tetragonal to an orthorhombic crystal system at 150 K 4,5 and the higher bound is limited by the PMMA glass transition. Transient Photoluminescence Pulsed laser excitation is investigated to extract the minority carrier lifetimes. The time-resolved PL (TRPL) decays strongly depend on the pump-power and follow a single exponential decay for low laser fluences (corresponding to an initial carrier concentration of cm -3 or 4 nj cm -2 ) and a double exponential decay for higher laser fluences (Figure S8). Similar behavior was previously reported for CH 3 NH 3 PbI 3-x Cl 6,7 x and CH 3 NH 3 PbI 3. 7

11 The combination of steady-state and time-resolved PL measurements relates the QY to the actually measured lifetime τ by using the inverse of the radiative and non-radiative lifetimes, τ r and τ nr, respectively, representing the probability of a radiative transition 1/τ r and the probability of a non-radiative transition 1/τ nr in the following way: QY = 1/τ r 1/τ r +1/τ nr = τ τ r. Thus, very high QY implies τ r τ nr (i.e. the probability for a non-radiative decay goes to zero). For the above presented results this means that the non-radiative lifetime increases with increasing pump-power given that the radiative lifetime is an intrinsic material specific quantum mechanical parameter. This indicates that processes that lead to non-radiative recombination such as traps in the band gap are effectively suppressed by optical doping i.e. filling of intra-gap states with free carriers. A model recently presented by Stranks et al. 6 is used to model both, the TRPL and steady-state QY data (Figure S9).

12 Fig. S10. Time-resolved luminescence. Luminescence decay of CH 3 NH 3 PbI 3-x Br x samples with (a) x = 0, (b) with x = 0.1, (c) with x = 1.2, and (d) and x = 3 at different pump densities n 0 = 10 15, 10 16, cm -3 and exponential decay fits to the data. Please note the different time scale in (d). IRF represents the instrument response function.

13 Modeling Despite the hybrid organic-inorganic nature of these materials charge transport is reported to mainly proceed via free charge carriers (at 300K, 100 mw/cm 2 ). 5,6,8,9 This can be partially explained by the very low exciton binding energies of mev reported for CH 3 NH 3 PbI 3 and CH 3 NH 3 PbI 3-x Cl 5,7,10 x and mev reported for CH 3 NH 3 PbBr 3. 11,12 Assuming the traditional model that describes free carriers in inorganic semiconductors where the total recombination R is given by R = An + Bnp + Cn 2 p, with A being the Shockley-Read-Hall (SRH) recombination rate, B the radiative recombination rate, C the Auger recombination rate, and n and p the concentration of charge carriers. With the QY given by the radiative recombination over the total recombination Bnp/R it was not possible to reproduce the iqy curves. However, our iqy as well as the PL decay data for Br-containing mixed halide perovskites can be modelled by a kinetic recombination model that was recently presented by Stranks et al. 6 (see Figure S9). Their model predicts a dynamic equilibrium of free carriers and excitons in the presence of sub-bandgap trap states. Applying this kinetic model, modified by adding an Auger recombination term and removing the saturation parameter (sample CH 3 NH 3 PbI 2.9 Br 0.1 ), we find that the trap density is cm -3, the trap population rate is cm 3 s -1, and the depopulation rate is cm 3 s -1. The extracted Auger coefficient is cm 6 s -1.

Fig. S11 Modeling of iqy and TRPL data of the CH 3 NH 3 PbI 2.9 Br 0.

b) TRPL decay with different carrier injection.

Literature overview on reported (electrical) V oc s measured in

14 Fig. S11 Modeling of iqy and TRPL data of the CH 3 NH 3 PbI 2.9 Br 0.1 sample. a) iqy and fit from the model (solid line). b) TRPL decay with different carrier injection. Dotted lines are fits from the model. Fig. S12 Electrical V oc. Literature overview on reported (electrical) V oc s measured in perovskite solar cells. Dashed line: band gap, solid line: band gap less entropy term. Noh et al.; 13 Liang et al.; 14 Suarez et al. 15

15 AUTHOR INFORMATION Corresponding Author * (I.D.S.): * (A.J.): Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. C.M.S.-F. and Y.L. contributed equally. References (1) Amani, M.; Lien, D.-H.; Kiriya, D.; Xiao, J.; Azcatl, A.; Noh, J.; Madhvapathy, S. R.; Addou, R.; Kc, S.; Dubey, M.; Cho, K.; Wallace, R. M.; Lee, S.-C.; He, J.-H.; Ager, J. W.; Zhang, X.; Yablonovitch, E.; Javey, A. Science 2015, 350, (2) Yablonovitch, E.; Cody, G. D. IEEE Trans. Electron Devices 1982, 29 (2), (3) Chen, Q.; Zhou, H.; Song, T.-B.; Luo, S.; Hong, Z.; Duan, H.-S.; Dou, L.; Liu, Y.; Yang, Y. Nano Lett. 2014, 14, (4) Mashiyama, H.; Kurihara, Y.; Azetsu, T. J. Korean Phys. Soc. 1998, 32, (5) D Innocenzo, V.; Grancini, G.; Alcocer, M. J. P.; Kandada, A. R. S.; Stranks, S. D.; Lee, M. M.; Lanzani, G.; Snaith, H. J.; Petrozza, A. Nat. Commun. 2014, 5, (6) Stranks, S. D.; Burlakov, V. M.; Leijtens, T.; Ball, J. M.; Goriely, A.; Snaith, H. J. Phys. Rev. Appl. 2014, 2, (7) Saba, M.; Cadelano, M.; Marongiu, D.; Chen, F.; Sarritzu, V.; Sestu, N.; Figus, C.; Aresti, M.; Piras, R.; Geddo Lehmann, A.; Cannas, C.; Musinu, A.; Quochi, F.; Mura, A.; Bongiovanni, G. Nat. Commun. 2014, 5, (8) Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D.-D.; Higler, R.; Hüttner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; Atatüre, M.; Phillips, R. T.; Friend, R. H. J. Phys. Chem. Lett. 2014, 5, (9) Manser, J. S.; Kamat, P. V. Nat. Photonics 2014, 8, (10) Miyata, A.; Mitioglu, A.; Plochocka, P.; Portugall, O.; Wang, J. T.-W.; Stranks, S. D.;

16 Snaith, H. J.; Nicholas, R. J. Nat. Phys. 2015, 11, (11) Tanaka, K.; Takahashi, T.; Ban, T.; Kondo, T.; Uchida, K.; Miura, N. Solid State Commun. 2003, 127, (12) Galkowski, K.; Mitioglu, A.; Miyata, A.; Plochocka, P.; Portugall, O.; Eperon, G. E.; Wang, J. T.-W.; Stergiopoulos, T.; Stranks, S. D.; Snaith, H. J.; Nicholas, R. J. arxiv: (13) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Nano Lett. 2013, 13, (14) Liang, P.-W.; Chueh, C.-C.; Xin, X.-K.; Zuo, F.; Williams, S. T.; Liao, C.-Y.; Jen, A. K.- Y. Adv. Energy Mater. 2015, 5, (15) Suarez, B.; Gonzalez-Pedro, V.; Ripolles, T. S.; Sanchez, R. S.; Otero, L.; Mora-Sero, I. J. Phys. Chem. Lett. 2014, 5,

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