Supporting Information. Highly Efficient Cs-based Perovskite Light-Emitting Diodes Enabled by Energy Funnelling

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1 Electronic Supplementary Material (ESI) for Chemical Communications. This journal is The Royal Society of Chemistry 2017 Supporting Information Highly Efficient Cs-based Perovskite Light-Emitting Diodes Enabled by Energy Funnelling Yan Fong Ng, abc Sneha A. Kulkarni, a Shayani Parida, d Nur Fadilah Jamaludin, abc Natalia Yantara, a Annalisa Bruno, a Cesare Soci, e Subodh Mhaisalkar ac and Nripan Mathews* ac a. Energy Research NTU (ERI@N), Research Techno Plaza, X-Frontier Block, Level 5, 50 Nanyang Drive, Singapore b. Interdisciplinary Graduate School, Nanyang Technological University, 50 Nanyang Avenue, Singapore c. School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore d. Indian Institute of Science, CV Raman Road, Bengaluru, Karnataka , India e. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore

2 Experimental Section Device fabrication: The perovskite based LED (PeLED) devices were fabricated on indiumdoped tin oxide (ITO, 7 Ω sq 2 ) coated glass substrates. The substrates were cleaned by successive bath sonication in decon soap, deionized water, acetone, isopropanol and ethanol. They were then dried under N 2 and treated in oxygen plasma for 20 min. Hole transporting layer (HTL), poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) or PEDOT:PSS (Aldrich, highconductivity grade) was spin-coated on the cleaned substrates at 4000 rpm for 60 s and annealed at 200 C for 40 s in air to remove water from the PSS shell of the PEDOT:PSS grains. The standard cesium lead bromide (CsPbBr 3 ) perovskite solution was prepared as per our earlier reported method, 1 where excess CsBr precursor was added to suppress formation of surface Pb defects. 2 In brief, cesium bromide (CsBr) (Aldrich, 99.9%) and lead bromide (PbBr 2 ) (TCI Chemicals) were mixed in a 2:1 mole ratio (0.5 M) in dimethyl sulfoxide (DMSO) solvent. Thus mixed solution was stirred overnight at room temperature and subsequently, filtered with a 0.25 µm PVDF filter. The PEABr precursor of known amount was prepared separately in dimethyl formamide (DMF). The CsPbBr 3 perovskite solution with excess PEABr addition were prepared by mixing the two solution in different mole ratios i.e. PEABr:CsPbBr 3 = 0:1, 0.05:1, 0.1:1, 0.2:1, 0.3:1, 0.5:1, 0.8:1 and 1:1. The solutions were stirred for a few hours at room temperature before spin-coating in an argon-filled glove box at 5000 rpm for 30 s to form a thin film on the PEDOT:PSS coated ITO substrate. Thereafter, the films were left to dry for 30 mins at ambient temperature before placing in a thermal evaporator. The electron transporting layer (ETL) (1,3,5- Tris(1-phenyl-1-H-benzimidazol-2-yl)benzene or TPBi (LumTec, sublimed > 99.5%)) (45 nm) and cathode (Ca (8 nm) / Al (100 nm)) were thermally evaporated at a base pressure of < Torr to complete the process of device fabrication. The devices were finally encapsulated inside the glove box. The final cell size was measured to be 8 mm 2. Device Characterization: All devices were tested at ambient conditions. The characteristic current density voltage luminance (J-V-L) were recorded with a Keithley 2612B source meter and an OceanOptics QE Pro spectrometer connected to an integrating sphere and operated using Ciemo LabVIEW software. Optical absorption spectroscopy: The absorption spectra of perovskite films (on glass) were recorded using UV-Vis-NIR spectrophotometer (UV-3600, Shimadzu) equipped with an integrating sphere. 2

3 Steady-state photoluminescence (PL): The PL spectra were measured using Fluoromax-4 (Horiba Jobin Yvon) spectrofluorometer at an excitation wavelength of 350 nm and the intensities were corrected against the number of absorbed photons at the same excitation wavelength. The PL excitation spectra were measured with the detector fixed at the strongest PL peak position (3D emission) while the samples were photo-excited across a range of wavelengths. Time-resolved photoluminescence (TRPL) measurements: TRPL is conducted using a Picoquant PicoHarp 300 time correlated single photon counting (TCSPC) system in a micro-pl setup with Nikon microscope objective (20x magnification, NA = 0.3). A ps-pulsed laser diode emitting at 405 nm with 40 MHz repetition rate (Picoquant P-C-405B) is used to excite the sample. The output signal is then fiber coupled to Acton SP-2300i monochromator (300 mm focal length) for spectral selection of the emission light. Another optical fiber connected to the output of the monochromator is used to couple spectral separated output light to an avalanche diode that is synchronized with the excitation laser via the TCSPC electronic. Overall, the FWHM of the system instrument respond function is expected to be at 50 ps. Field emission scanning electron microscopy (FESEM): FESEM imaging of perovskite films (on ITO/PEDOT:PSS substrates) was performed using a JEOL JSM-7600F scanning electron microscope at 5-10 kv with a working distance of ~8 mm. X-ray diffraction (XRD): The XRD measurements were performed with Bruker D8 Advance X-ray diffractometer using Ni-filtered Cu Kα radiation. 3

4 Figure S1: UV-visible absorption spectra of CsPbBr 3 perovskite films prepared with different mole ratio of PEABr:CsPbBr 3. * represents the absorption onset for 3D CsPbBr 3 perovskite while lower wavelength excitonic peaks corresponding to the 2D and quasi-2d formation at different n values are represented by red dots ( ). 4

5 Figure S2: Photoluminescence (PL) spectra of perovskite films ( exc = 350 nm) prepared using different PEABr:CsPbBr 3 mole ratio of 0:1, 0.05:1 0.1:1, 0.2:1, 0.3:1, 0.5:1, 0.8:1, and 1:1. The perovskite film formed using the PEABr:CsPbBr 3 ratio of 0.8:1 shows remarkably high PL intensity as compared to other ratios. The PL measurements of all samples were carried out at identical measurement conditions i.e. under same excitation wavelength ( ex = 350 nm) and emission silt width. 5

6 Figure S3: Normalized PL spectra of CsPbBr 3 perovskite films prepared using different mole ratio of PEABr:CsPbBr 3 mole ratio of 0:1, 0.1:1, 0.2:1, 0.3:1, 0.5:1, 0.8:1 and 1:1. Figure S4: Digital photographs of CsPbBr 3 perovskite films formed with different PEABr:CsPbBr 3 mole ratios under UV illumination. 6

7 Table S1: TRPL lifetimes and decay parameters CsPbBr 3 perovskite films prepared with different mole ratio of PEABr:CsPbBr 3 (0:1, 0.3:1 and 0.8:1) at different excitation wavelength. The PL decay curves are fitted using a bi-exponential decay function: t τ t I(t) = A f e f τ + A s e s, where A f and A s refer to the decay amplitudes (i.e., weighting factors) of the fit and τ f and τ s refer to the decay time constants of the fast and slow components τ respectively. Average lifetime τ avg = A f τ f + A s τ s avg was calculated by A f + A s. PEABr:CsPbBr 3 ratio 0:1 (pristine CsPbBr 3 ) Wavelength (nm) A f τ f (ns) A s τ s (ns) τ ave (ns) : :

8 Figure S5: Field emission scanning electron microscopic (FESEM) top-view images of CsPbBr 3 perovskite films prepared using different mole ratio of PEABr:CsPbBr 3, showing reduction in grain size with increasing compactness and smoothness of the film with increasing PEABr content. 8

9 Figure S6: FESEM cross-section images of CsPbBr 3 perovskite films prepared using different mole ratio of PEABr:CsPbBr 3, showing smoother and flatter films with increasing PEABr content. Thicknesses of all films are closely similar ( 85 nm). 9

10 Figure S7: Superimposed X-ray diffraction (XRD) patterns of perovskite films prepared using different mole ratio of PEABr:CsPbBr 3. (a) As prepared films after vacuum drying and (b) thicker films prepared using drop-casting and subsequent annealing at 100 C for 20 mins. 10

11 Table S2: Summary of device performance of CsPbBr 3 light-emitting diodes (PeLEDs) with different mole ratio of PEABr:CsPbBr 3. Figure S8: Normalized EL spectra of all devices (in (a) linear scale and (b) log scale) under an applied bias of 6 V, showing single emission for all PEABr:CsPbBr 3 compositions. 11

12 Figure S9: EL spectra of 0.8:1 PeLED device under different applied biases from 3 7 V, showing only a single emission peak even at high voltages. To evaluate the effect of PEABr on the thermal stability of CsPbBr 3, thermogravimetric analysis (TGA) was performed (Figure S10). Since PEABr is an organic compound, it is expected that by incorporating it into the CsPbBr 3 perovskite framework, the temperature tolerance will be lower than pristine CsPbBr 3. Indeed, the results show that the mixed-dimensional perovskite underwent two different weight loss transitions, firstly occurring at a lower temperature of ~251 C, associated to the decomposition of PEABr, and secondly at a higher temperature of ~567 C, associated to the decomposition of CsPbBr 3. This implies the thermal stability is slightly compromised with the addition of PEABr and the material starts to decompose at a lower temperature. However, it is noted that the initial weight loss at 251 C was a mere 10%. 12

13 Furthermore, this temperature is also higher than the decomposition temperature of MAPbBr 3, which starts at ~220 C, 3 which means that overall, its thermal stability is still relatively robust. Figure S10: Thermogravimetric analysis (TGA) of standard CsPbBr 3 (black line), PEABr (pink line), and mixed-dimensional perovskite with PEABr:CsPbBr 3 ratio of 0.8:1 (orange line). (Note: Initial ~5% weight loss for black and orange curves is due to removal of residual DMSO solvent since these two samples were drop-casted from solution.) References 1. N. Yantara, S. Bhaumik, F. Yan, D. Sabba, H. A. Dewi, N. Mathews, P. P. Boix, H. V. Demir and S. Mhaisalkar, J. Phys. Chem. Lett., 2015, 6, H. Cho, S.-H. Jeong, M.-H. Park, Y.-H. Kim, C. Wolf, C.-L. Lee, J. H. Heo, A. Sadhanala, N. Myoung, S. Yoo, S. H. Im, R. H. Friend and T.-W. Lee, Science, 2015, 350, M. Kulbak, S. Gupta, N. Kedem, I. Levine, T. Bendikov, G. Hodes and D. Cahen, J. Phys. Chem. Lett., 2016, 7,