Supplementary Information

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1 Supplementary Information Photo-induced Emissive Trap States in Lead Halide Perovskite Semiconductors Silvia Motti a,b, Marina Gandini a,b, Alex Barker a, James M. Ball a, Ajay Ram Srimath Kandada a, Annamaria Petrozza a a Center for Nano Science and Istituto Italiano di Tecnologia, via Giovanni Pascoli 70/3, Milano, Italy. b Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci, 32, Milano, Italy. Experimental section Sample preparation Lead(II) bromide (PbBr 2, 98%), N,N-dimethylformamide (DMF, anhydrous, 99.8%), Chlorobenzene (anhydrous, 99.8%), and dimethyl sulfoxide (DMSO, anhydrous, 99.9%) were purchased from Sigma-Aldrich; methylammonium bromide (MABr) and methylammonium iodide (MAI) were purchased from Dyesol; and lead (II) iodide (PbI 2, %, CAS No ) was purchased from Alfa Aesar. All chemicals were used without any further purification. Glass substrates were cleaned in acetone and isopropyl alcohol (IPA) for 10 minutes by sonication. The cleaned glass substrates were treated with Oxygen plasma for 10 minutes before any further deposition.

2 MAPbBr 3 thin films. Different deposition procedures where tested, providing always similar results. Procedure a) A hot solution of MABr and PbBr 2 in DMF (molar ratio 1:1, 30 wt% concentrated, stirred at 70 C) was spin-coated on a clean glass substrate at 3000 rpm for 60 seconds. The samples were baked at 100 C for 15 minutes. Procedure b) adapted Nanocrystal-Pinning technique [Ref. Cho H. et al., Science (2015), , ]. In this case, two steps of spin-coating speed were used (500 rpm for 7 seconds, 3000 rpm 90 seconds). After spin-speed acceleration, a solution of MABr and PbBr 2 (molar ratio 1.05:1) in DMSO was spin-coated onto the clean glass substrate. After 60 seconds, the pinning occurred by dropping 300 µl of chlorobenzene on the spinning sample. The samples were then baked at 90 C for 10 minutes. Thin films were obtained with different thickness (300nm-500nm range) and both with and without a polymethyl methacrylate (PMMA) capping layer, which did not have any effect on the reported results. CsPbBr 3 thin films. A hot solution of CsBr and PbBr 2 in DMSO (molar ratio 1:1, concentration 10 wt%, stirred at 100 C for at least 4 hours) was spin-coated on a clean glass substrate at 3000 rpm for 120 seconds. The samples were annealed at 100 C for 5 minutes. MAPbI 3 thin films. The procedure used is based on quenching of the precursor solution with an antisolvent during spin coating [M. Xiao et al., Angewandte Chemie vol. 126, p10056 (2014)], in a nitrogen filled glovebox. A 1.45 M precursor solution of PbI 2 :MAI:DMSO in a molar ratio of 1:1:1 was prepared in DMF. This solution was spin coated onto the glass substrate at 4000 rpm, with an acceleration of 4000 rpm/s, for

3 15 s. After 6 s toluene, an antisolvent to the precursor solution, was dropped onto spinning sample by pipette. The samples were then annealed at 100 C for 10 minutes. Isolated, micrometer sized, MAPbBr 3 crystallites. MAPbBr 3 crystals were formed by instantaneous precipitation in chlorobenzene. 150 µl of a solution of MABr and PbBr 2 in DMF (molar ratio 1:1, 30 wt% concentrated) was added to 3 ml of chlorobenzene. As soon as the perovskite solution was injected, micrometer-sized crystals started to precipitate. The crystals were then drop-casted on a clean glass substrate, and left to dry at room temperature. Steady-state PL The excitation source was provided by a Q-switched Ti:Sapphire based regenerative amplifier (Coherent RegA 9000) operating at 250KHz seeded by a mode-locked Ti:Sapphire oscillator (Coherent Micra-18) operating at 80 MHz. The beam was focused onto a BBO crystal generating second harmonic at 400 nm and pulses of ~100 fs duration. The PL was collected in reflection geometry, perpendicular to the excitation line, and focused into a fiber coupled to a spectrometer (Ocean Optics Maya Pro 2000). For relative PLQY, the integrated PL was measured at varying excitation intensities and plotted as: Relative PLQY = PL/I pump Measurements in vacuum were performed under active pumping and pressure bellow 10-5 mbar. Dry atmosphere was obtained from a cylinder of dry air (Nordival, purity 5.5). Time-resolved Photoluminescence

4 The thin film samples were mounted in a chamber in vacuum (under 10-5 mbar) that was vented with room atmosphere for comparison of dynamics. PL was collected in reflection geometry, perpendicular to the excitation line. Samples were excited from both sides of the sample (glass substrate or sample film), without any relevant difference between them. Time-resolved PL measurements on MAPbBr 3 films were performed using a femtosecond laser source and a streak camera detection system (Hamamatsu C5680). The laser source was a Q-switched Ti:Sapphire based regenerative amplifier (Coherent RegA 9000) operating at 250KHz seeded by a mode-locked Ti:Sapphire oscillator (Coherent Micra-18) operating at 80 MHz. Pulses of~100fs duration of the second harmonic at 400 nm were obtained by focusing on a BBO crystal and directed to the sample. The photoluminescence was collected and focused onto a spectrometer coupled to the detection system, and time-resolved with a linear voltage sweep module. Time-resolved PL measurements on MAPbI 3 films were performed using a Time Correlated Single Photon Counting setup. The output of An unamplified tunable Ti:Sapphire laser (Coherent Chameleon Ultra II, temporal and spectral bandwidths of ~140 fs and ~5 nm, respectively) was tuned for central wavelengths of 700 nm and then directed to the sample. The measurements were performed with excitation at 4 MHz, obtained using an acousto-optical modulating pulse picker (APE Pulse Select). The PL was collected and focused onto a spectrometer coupled to a N 2 cooled photomultiplier. Confocal Microscopy The excitation source is provided by a supercontinuum laser (SuperK Extreme, NKT Photonics), which is spectrally filtered by an acousto-optic modulator (SuperK Select, NKT Photonics). The selected output at 480 nm was directed to a home built

5 transmission microscope in confocal configuration. The sample is placed on top of a piezoelectric translation stage. Photoluminescence was measured with a 500 nm long pass filter on the detection line. The transmitted excitation or photoluminescence signal is collected and focused into a fiber coupled to a photodiode and a Lock-in amplifier. Transient Absorption Spectroscopy Thin film samples were mounted in a chamber in vacuum (under 10-5 mbar) that was vented with room atmosphere for comparison of dynamics. Transient absorption signal was collected in transmission geometry. An amplified Ti:sapphire laser (Quantronix Integra-C) generates pulses of ~100 fs centered at 800 nm. A broadband white light probe is generated by focusing the pulses into a thin sapphire plate. 355 nm pump light is obtained from a Q-switched Nd:YVO 4 laser (Innolas Picolo) which is electronically triggered and synchronized to the Ti:sapphire laser via an electronic delay. The pump pulses have a width of ~700 ps FWHM. Including jitter, the system has time resolution and jitter of approximately 1 ns. After interaction with the sample, a homebuilt prism spectrometer disperses the probe light on to a fast CCD array, enabling broadband shot-to- shot detection. Supplementary figures

6 Figure S1. Relative PLQY of MAPbBr 3 thin film in vacuum. The order of measurement goes from blue to red. All curves were taken from the same spot, measured from high to low excitation density, increasing the start point for each curve, and showing the hysteretic behavior even at low light intensities. Figure S2. Integrated PL intensity of MAPbBr3 thin film over time. Sample was stabilized under illumination and active vacuum for 30 minutes and the chamber was filled with either dry air or room atmosphere.

7 Figure S3. MAPbBr 3 PL spectra at 77 K, 117 K and 275 K.

8 Figure S4. PL decays of MAPbI 3 thin film with increasing (a) and decreasing (b) excitation density in vacuum, and (c) in air. Figure S5. Integrated PL of CsPbBr 3 thin film over time. Sample is in dark before t=0, after illumination we see a quench in PLQY, followed by an enhancement when the sample is exposed to air. Figure S6. PL decays of CsPbBr 3 thin film under active vacuum (blue) and air (red).

9 Figure S7. PL spectrum of CsPbBr 3 thin film in vacuum, at excitation density of cm -3 after illumination for at least 30 minutes. Figure S8. Partial reversibility of PL quenching in vacuum. Excitation light is unblocked at t=0 and the integrated PL intensity was measured at excitation density of 1016 cm-3. Excitation was blocked again and the sample was left in dark for one hour, after which the signal was seen to partially recover.

10 Figure S9. (a) MAPbBr 3 XRD characterization, (b) SEM of MAPbBr 3 thin film cross section, (c) SEM top view of MAPbBr 3 thin film and (d) microscope image of µm size MAPbBr 3 crystals.