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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 Mhaisalkarac and Nripan Mathews*ac

a. Energy Research Institute @ NTU (ERI@N), Research Techno Plaza, X-Frontier Block, Level 5, 50 Nanyang Drive, Singapore 637553b. Interdisciplinary Graduate School, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798c. School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798d. Indian Institute of Science, CV Raman Road, Bengaluru, Karnataka 560012, Indiae. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371

Electronic Supplementary Material (ESI) for Chemical Communications.This journal is © The Royal Society of Chemistry 2017

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Experimental Section

Device fabrication: The perovskite based LED (PeLED) devices were fabricated on indium-

doped 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 N2 and treated in oxygen plasma for 20 min. Hole transporting layer

(HTL), poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) or PEDOT:PSS (Aldrich, high-

conductivity 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 (CsPbBr3) 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 (PbBr2) (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 CsPbBr3 perovskite solution with excess PEABr addition were prepared

by mixing the two solution in different mole ratios i.e. PEABr:CsPbBr3 = 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 < 3 ×

10−6 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 mm2.

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.

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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.

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Figure S1: UV-visible absorption spectra of CsPbBr3 perovskite films prepared with different

mole ratio of PEABr:CsPbBr3. * represents the absorption onset for 3D CsPbBr3 perovskite

while lower wavelength excitonic peaks corresponding to the 2D and quasi-2D formation at

different n values are represented by red dots ().

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Figure S2: Photoluminescence (PL) spectra of perovskite films (exc = 350 nm) prepared using

different PEABr:CsPbBr3 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:CsPbBr3 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.

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Figure S3: Normalized PL spectra of CsPbBr3 perovskite films prepared using different mole

ratio of PEABr:CsPbBr3 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 CsPbBr3 perovskite films formed with different

PEABr:CsPbBr3 mole ratios under UV illumination.

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Table S1: TRPL lifetimes and decay parameters CsPbBr3 perovskite films prepared with

different mole ratio of PEABr:CsPbBr3 (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:

, where Af and As refer to the decay amplitudes (i.e., weighting 𝐼(𝑡) = 𝐴𝑓𝑒‒ 𝑡 𝜏𝑓+ 𝐴𝑠𝑒

‒ 𝑡 𝜏𝑠

factors) of the fit and and refer to the decay time constants of the fast and slow components 𝜏𝑓 𝜏𝑠

respectively. Average lifetime was calculated by .𝜏𝑎𝑣𝑔𝜏𝑎𝑣𝑔=

𝐴𝑓𝜏𝑓+ 𝐴𝑠𝜏𝑠𝐴𝑓+ 𝐴𝑠

PEABr:CsPbBr3

ratioWavelength

(nm) Afτf

(ns) Asτs

(ns)τave

(ns)496 0.845 1.532 0.176 4.225 1.9962

516 0.640 1.769 0.390 5.420 3.15140:1

(pristine CsPbBr3)526 0.502 2.767 0.485 8.449 5.5591

436 0.632 0.493 0.102 1.561 0.6414

466 0.865 0.659 0.142 2.843 0.967

496 0.523 1.807 0.477 6.805 4.191

516 0.467 2.100 0.515 11.298 6.9238

0.3:1

526 0.465 2.117 0.516 9.812 6.1645

436 1.02 0.437 –– –– 0.437

466 0.729 0.711 0.258 4.144 1.6084

496 0.503 2.481 0.504 7.947 5.2167

516 0.369 2.099 0.632 8.761 6.3052

0.8:1

526 0.272 3.142 0.680 14.714 11.408

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Figure S5: Field emission scanning electron microscopic (FESEM) top-view images of CsPbBr3

perovskite films prepared using different mole ratio of PEABr:CsPbBr3, showing reduction in

grain size with increasing compactness and smoothness of the film with increasing PEABr

content.

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Figure S6: FESEM cross-section images of CsPbBr3 perovskite films prepared using different

mole ratio of PEABr:CsPbBr3, showing smoother and flatter films with increasing PEABr

content. Thicknesses of all films are closely similar ( 85 nm).

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Figure S7: Superimposed X-ray diffraction (XRD) patterns of perovskite films prepared using

different mole ratio of PEABr:CsPbBr3. (a) As prepared films after vacuum drying and (b)

thicker films prepared using drop-casting and subsequent annealing at 100 °C for 20 mins.

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Table S2: Summary of device performance of CsPbBr3 light-emitting diodes (PeLEDs) with

different mole ratio of PEABr:CsPbBr3.

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:CsPbBr3 compositions.

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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 CsPbBr3, thermogravimetric analysis

(TGA) was performed (Figure S10). Since PEABr is an organic compound, it is expected that by

incorporating it into the CsPbBr3 perovskite framework, the temperature tolerance will be lower

than pristine CsPbBr3. 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 CsPbBr3. 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%.

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Furthermore, this temperature is also higher than the decomposition temperature of MAPbBr3,

which starts at ~220 °C,3 which means that overall, its thermal stability is still relatively robust.

Figure S10: Thermogravimetric analysis (TGA) of standard CsPbBr3 (black line), PEABr (pink

line), and mixed-dimensional perovskite with PEABr:CsPbBr3 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.)

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