Efficient perovskite light-emitting diodes featuring

ARTICLES PUBLISHED ONLINE: 16 JANUARY 2017 | DOI: 10.1038/NPHOTON.2016.269

Efficient perovskite light-emitting diodes featuring nanometre-sized crystallites Zhengguo Xiao1, Ross A. Kerner1, Lianfeng Zhao1, Nhu L. Tran2, Kyung Min Lee1, Tae-Wook Koh1, Gregory D. Scholes2 and Barry P. Rand1,3* Organic–inorganic hybrid perovskite materials are emerging as highly attractive semiconductors for use in optoelectronics. In addition to their use in photovoltaics, perovskites are promising for realizing light-emitting diodes (LEDs) due to their high colour purity, low non-radiative recombination rates and tunable bandgap. Here, we report highly efficient perovskite LEDs enabled through the formation of self-assembled, nanometre-sized crystallites. Large-group ammonium halides added to the perovskite precursor solution act as a surfactant that dramatically constrains the growth of 3D perovskite grains during film forming, producing crystallites with dimensions as small as 10 nm and film roughness of less than 1 nm. Coating these nanometre-sized perovskite grains with longer-chain organic cations yields highly efficient emitters, resulting in LEDs that operate with external quantum efficiencies of 10.4% for the methylammonium lead iodide system and 9.3% for the methylammonium lead bromide system, with significantly improved shelf and operational stability.

H

ybrid organic–inorganic perovskites are emerging as a new generation of low-cost, solution-processed semiconducting materials with favourable optoelectronic properties such as strong absorption coefficients, tunable bandgap1,2, large and balanced electron and hole mobilities, long carrier diffusion lengths, small exciton binding energy3,4 and unique defect properties with only shallow point defects formed5. These characteristics have allowed for consistent and rapid progress in solar cell efficiency, which has risen dramatically from 3.8% to over 22% in less than five years due to efforts on perovskite film morphology optimization, interface engineering, and so on6–17. Owing to their facile solution processing, high colour purity and tunable bandgap18–22, hybrid perovskites are also promising for LEDs. Nevertheless, the small exciton binding energy in 3D perovskites (methylammonium lead iodide, MAPbI3 , and methylammonium lead bromide, MAPbBr3) result in small electron–hole capture rates for radiative recombination18. Therefore, ultrathin perovskite layers and/or small perovskite grain size have been employed to spatially confine electrons and holes to promote bimolecular radiative recombination18–20. Recently, an external quantum efficiency (EQE) of 8.5% for MAPbBr3-based LEDs has been demonstrated through the formation of MAPbBr3 nanograins with an average size of 100 nm, and reduction of metallic lead impurities19. However, due to the rapid crystallization speed of 3D perovskites, grain sizes are generally hundreds of nanometres in size from either one-step or two-step solution processes13,23,24, with a resulting large surface roughness. Here, we report a solution process to form highly uniform and ultra-flat perovskite films with nanometre-sized grains that allow us to demonstrate highly efficient LEDs. The addition of longchain ammonium halides (for example, n-butylammonium halides (BAX, X = I, Br)) in the 3D perovskite precursor solution impedes the growth of 3D perovskite grains and dramatically decreases film roughness to 1 nm. The nanometre-sized grains feature reduced dimensionality, starting a transition from 3D to layered (so-called Ruddlesden–Popper) perovskite structures,

manifesting in stronger and blue-shifted photoluminescence (PL) and electroluminescence (EL) compared with the emission from the 3D perovskite. Notably, the EQE of iodide perovskite (I-perovskite, BAI:MAPbI3) LEDs increased from 1.0% to 10.4% due to the incorporation of BAI. Similarly, the EQE of bromide perovskite (Br-perovskite, BABr:MAPbBr3) LEDs increased from 0.03% to 9.3% following the incorporation of BABr. The power efficiency (PE) and current efficiency (CE) reached 13.0 lm W–1 and 17.1 cd A–1, respectively, for Br-perovskite LEDs. Furthermore, the shelf stability of unencapsulated I- and Br-perovskite LEDs in N2 was dramatically improved after adding long-chain halides. Specifically, LEDs without BAX degrade to 60–70% of their initial value within 2 days whereas LEDs with BAX (X = I, Br) show no degradation after storage in the glove box for more than 8 months.

Perovskite film formation and characterization

To deposit uniform pin-hole-free perovskite films with small grain size, toluene was dropped on the spinning film at an appropriate time to extract the processing solvent (dimethylformamide, DMF), thus halting the morphological evolution and maintaining small crystallite size25,26. To ensure consistency with the device structure, the optical, morphological and structural properties of I- and Br-perovskite films were probed on poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine] (poly-TPD) and poly(Nvinylcarbazole) (PVK), respectively. The perovskite thickness is approximately 80 nm for I-perovskite films and 70 nm for Br-perovskite films. Atomic force microscope (AFM) images of MAPbI3 (Fig. 1a) and MAPbBr3 (Fig. 1g) films without annealing show that both are very uniform. The root mean square (r.m.s.) roughness is 4.9 nm for the MAPbI3 film and 3.4 nm for the MAPbBr3 film, among the most flat and uniform perovskite films reported. Long-chain ammonium ions cannot fill the corner of PbX4 (X = I, Br, Cl) octahedral layers and therefore induce the formation of layered perovskites27. Notably, when BAX (X = I, Br) is added into the MAPbX3 precursor solution, the growth of 3D perovskite grains during the film formation process is dramatically impeded.

1

Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA. 2 Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA. 3 Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, USA. * e-mail: [email protected] NATURE PHOTONICS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturephotonics

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Figure 1b–f and h–l show AFM images of I- and Br-perovskite films prepared from different molar ratios of BAI:MAPbI3 and BABr: MAPbBr3 in the precursor solution, respectively. As shown in Fig. 1, when the molar ratio of BAX:MAPbX3 (X = I, Br) increases to 10:100, films feature smaller grain size and therefore reduced roughness. As the BAX concentration increases, grain size continues to decrease and films become smoother, decreasing to 0.6 nm r.m.s. for I-perovskite films and 1.0 nm r.m.s. for Br-perovskite films, for a molar ratio of BAX:MAPbX3 (X = I, Br) of 40:100. In fact, the morphologies of these films are similar to solution-processed polymers, where grain sizes are too small to be resolved by the AFM tip. When the BAX:MAPbX3 molar ratio is increased to larger than 60:100, larger grains start to form with the roughness of the film 2

simultaneously increasing, due to the formation of layered perovskite phases. X-ray diffraction (XRD) was conducted to determine the composition and crystallinity of the films. As shown in Fig. 1m,n, no diffraction peaks corresponding to layered perovskite grains are observed when the BAX molar ratio is smaller than 40:100 for I-perovskite and 20:100 for Br-perovskite films, indicating that the as-formed films are composed of grains that are nearly indistinguishable from the 3D perovskite crystal structure, with BA cations self-assembled at the grain surfaces. It should also be noted that the full-width at half-maximum (FWHM) of the diffraction peaks after adding BAX increases, indicating that crystallite size decreases. The crystallite size of MAPbI3 calculated from the XRD

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data applying a Scherrer analysis (Supplementary Information) is 16 and 13 nm, respectively, when the BAI:MAPbI3 ratio is increased from 10:100 to 20:100, which is approximately 25 and 20 PbI4 layers (considering the lattice constants of a = b = 8.86 Å, c = 12.65 Å for the tetragonal structure)6. Similarly, the crystallite size of the MAPbBr3 is calculated to be 16 and 11 nm for BABr: MAPbBr3 loadings of 10:100 to 20:100, which is approximately 27 and 19 PbBr4 layers (considering the lattice constants of cubic MAPbBr3: a = b = c = 5.9 Å). In agreement with the AFM and XRD results, the small grain size of the perovskite film after incorporation of BAX is confirmed by scanning electron microscopy (SEM) as shown in Supplementary Fig. 1. The crystallite of

MAPbX3 with BAX self-assembled at the surface can also be considered within the unit cell of layered BA2MAn–1PbnX3n+1 Ruddlesden–Popper perovskite phases where n equals the number of PbX4 octahedral layers in the crystallite. The discrepancy between the n values calculated above and the n values expected from the BAX and MAX molar ratios in the precursor solution may result from the different reaction speed between MAX and BAX with PbX2 (X = I, Br). Herein, we use the term ‘3D perovskite crystallites’ in the discussion of the abovementioned films for simplicity, even though we recognize that these particles possess reduced dimensionality from n = ∞. For loadings beyond these threshold values, BAX is no longer playing the role of surfactant



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and crystallite terminator, but rather participates in crystallization. When this transition occurs, grain size begins to increase again and films become rougher. For instance, when the molar ratio is further increased to 60:100 for BAI:MAPbI3 or 40:100 for BABr: MAPbBr3 , layered perovskite diffraction peaks start to appear. Ultimately, when the BAX ratio reaches 100:100, films become dominated by layered perovskite grains, which resemble the BA2MAn–1PbnX3n+1 perovskite, with n = 2 (Supplementary Fig. 2). The absorption and PL spectra of I- and Br-perovskite films with different molar ratios of BAX:MAPbX3 (X = I, Br) in the precursor solution are shown in Fig. 2a–d. With increased BAX loading, the absorption edge blue-shifts, and excitonic absorption peaks corresponding to layered perovskite grains emerge when the BAX ratio is increased to 60:100 for I-perovskite or 40:100 for Br-perovskite, in line with our observations from XRD as discussed above. In turn, the PL peaks of BAX:MAPbX3 films gradually blue-shift with increasing BAX content from 0:100 to 40:100. The PL quantum yield (QY) concomitantly increases from 0.2 to 0.9, 1.9 and 6.6% for I-perovskite, and from 0.2 to 0.5, 7.0 and 40.1% for Br-perovskite films with BAX molar ratio increasing from 0:100 to 10:100, 20:100 and 40:100, respectively. The time-resolved PL measurement showed that the average decay time is increased from 8.8 ns (14.3 ns) to 11.4 ns (26.2 ns) with the BAI (BABr) molar ratio increasing from 0:100 to 20:100 (Fig. 2e–h and Supplementary Table 1). The increased and blue-shifted PL emission is due to the reduced 3D perovskite 4

crystallite dimensionality to n < 30 surrounded by BA cations, inducing a disorder effect when the lattice periodicity is interrupted28. When the BAX (X = I, Br) ratio is further increased to 60:100, layered perovskite grains with excitonic absorption form, inducing significant blue-shifts of absorption and PL29, as shown in Fig. 2b,d. The blue-shift accompanying the reduction in crystallite size is also suggested by the evolution of the absorption and PL spectra due to thermal annealing (Supplementary Fig. 3). Thermal annealing drives grain growth and increases grain size as confirmed by both AFM (Supplementary Fig. 4) and XRD (Supplementary Fig. 5) for both I- and Br-perovskite films. The FWHM of the (110) and (220) diffraction peaks for I-perovskite and (100) and (200) diffraction peaks for Br-perovskite reduce after thermal annealing for various molar ratios of BAX:MAPbX3 in the precursor solution.

Hybrid perovskite LEDs The LED structures are: ITO (150 nm)/HTL (30 nm)/I-perovskite (80 nm) (or Br-perovskite, 70 nm)/TPBi (60 nm)/LiF (1.2 nm)/Al (100 nm) (ITO, indium tin oxide; HTL, hole-transporting layer; TPBi, 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)). This structure is shown schematically in Fig. 3a, along with energy diagrams in Fig. 3b,c. Supplementary Fig. 6 shows cross-sectional SEM images of I-perovskite devices, which confirm film thicknesses and the small grain size after BAI incorporation. The



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Figure 4 | Perovskite LED performance characterization. a–c, I-perovskite LEDs with different BAI:MAPbI3 molar ratios, showing EL (open circles) and PL (solid lines) spectra (a), J–V–L curves (b) and EQE curves (c). It should be noted that BAI:MAPbI3 (60:100) LED has higher luminance than 20:100 due to the EL spectrum that has shifted closer to peak eye response. d–f, Br-perovskite LEDs with different BABr:MAPbBr3 molar ratios, showing EL (open circles) and PL (solid lines) spectra (d), J–V–L curves (e) and EQE curves (f). The scanning rate for all devices is 0.1 V s–1.

HTL utilized for I-perovskite-based LEDs was poly-TPD with a highest occupied molecular orbital (HOMO) of 5.6 eV whereas PVK with a deeper HOMO level of 5.8 eV was used for Br-perovskite-based LEDs owing to better energy level alignment for hole injection and electron blocking. The TPBi layer serves as an electron-injecting and hole-blocking layer for both I-perovskite and Br-perovskite LEDs. It should be noted that the energy levels of the perovskites may change as a result of the BAX incorporation. However, this change is expected to be small (approximately 60 meV for I-perovskite and 80 meV for Br-perovskite calculated from the PL peak shift) such that it has a minimal effect on charge injection and blocking. Figure 4a,d show the normalized EL spectra of I- and Br-perovskite LEDs, respectively. Similar to the PL, EL is blue-shifted with increasing molar ratio of BAX:MAPbX3 (X = I, Br). The small blue-shift of EL peak position compared with PL (solid lines) for some devices may result from a distribution of crystallite sizes and corresponding n values, with the smaller crystallites the more Table 1 | Perovskite LED performance metrics. BAX:MAPbX3

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easily electrically populated30. The EL of I-perovskite LEDs with molar ratio of 100:100 and Br-perovskite LEDs with molar ratio of 60:100 and 100:100 are too weak to be measured, possibly the result of the increasing population of layered perovskite phases that have been reported to have strong electron–phonon coupling that inhibits efficient EL31. The current density–voltage–luminance (J–V–L) curves of I- and Br-perovskite LEDs and corresponding EQE curves are shown in Fig. 4b,c and Fig. 4e,f, respectively. The angular spectra and intensity profiles are shown in Supplementary Fig. 7. The EQE of LEDs without the addition of BAX are low, possibly a result of larger grain size and consequently reduced electron– hole capture rate. With increasing molar ratio of BAX:MAPbX3 to 20:100 in the precursor, the EQE of the device dramatically increases. The highest EQEs reach 10.4% for 20:100 I-perovskite and 9.3% for 20:100 Br-perovskite LEDs. Notably, the EQE of the perovskite LEDs are relatively flat over the range of the measurement, in contrast to recent work showing that EQE can increase substantially with current density20. We believe this to be primarily due to the low leakage currents in our devices, at approximately 10−6 mA cm–2, indicating very few parasitic leakage pathways exist between electrodes. Also, the fact that the EQE versus current density slope reduces with the addition of BAX indicates that, even at low current density, non-radiative recombination is slower than radiative bimolecular recombination, in agreement with the increased QY and longer PL lifetimes of films with BAX. It should be noted that the EQE has a higher value than PLQY for some devices, an aspect due to the nearly two-orders-of-magnitude higher charge density associated to a current of 10–20 mA cm–2 compared with the charge density generated by the optical generation of the PLQY measurement. The PE and CE curves of both I- and Br-perovskite LEDs are shown in Supplementary Fig. 8. The peak PE and CE for Br-perovskite LEDs reach 13.0 lm W–1 at 1,200 cd m–2 and 17.1 cd A–1 at 2,900 cd m–2, respectively, while PE and CE of the I-perovskite devices are relatively low due to the near-infrared light emission (Fig. 4a). While EQE reaches a maximum within the range of 5.0–6.5 V, corresponding to 15–50 mA cm–2, there is not a strong roll-off at higher voltages, although devices consistently



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fail at voltages of 8–10 V (0.2–1 A cm–2) due to Joule heating (Supplementary Fig. 9). For either the I- or Br-perovskite LEDs, when the BAX ratio increases beyond 40:100, the device performance decreases. Detailed performance metrics of both I- and Br-perovskite devices are provided in Table 1. It has recently been reported that excess MABr in MAPbBr3 can reduce the content of metallic lead and increase the EQE of MAPbBr3-based LEDs19. As a comparison, we fabricated both I- and Br-perovskite LEDs with MAX:MAPbX3 (X = I, Br) molar ratio of 5:100 and 20:100 (MAX:PbX2 molar ratio of 105:100 and 120:100), with EQE curves shown in Supplementary Fig. 10. 6

While excess MAX is effective in increasing EQE for both I- and Br-perovskite LEDs (to 3.6 and 2.1%, respectively), we find that the incorporation of bulky ammonium halide ligands provide greater efficiency gains because they are effective in allowing for smaller grain size, and thin films with low roughness. We note that excess MAX can also effectively decrease crystallite size by forming many stacking faults within a grain32. A high density of stacking faults can serve the same crystal termination effect as the long-chain alkylammonium halides used herein, although less efficiently. As further evidence of the versatility of this approach, another type of large-group cation, phenylethylammonium iodide



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(PEAI), was incorporated with PEAI:MAPbI3 molar ratio of 20:100. As shown in Supplementary Fig. 10, the highest EQE reaches 9.6% after the addition of PEAI in the MAPbI3 precursor solution. Recently, efficient I-perovskite LEDs using stoichiometric PEA2MAn–1PbnX3n+1 solutions with EQE up to 8.8% for n = 5 have been reported33. The non-stoichiometric solution in our work resulted in higher EQE that might be due to the passivation effect of the extra organic cations.

Hysteresis and stability of hybrid perovskite LEDs The photocurrent hysteresis of perovskite solar cells is thought to be due to ion migration and associated traps34–36, an aspect that complicates accurate efficiency measurement. Hysteresis has also been previously reported to exist in the J–V–L response of perovskite LEDs18, and can lead to inaccurate performance characterization due to its influence on the steady-state EL emission and angular profile measurements. And because ion migration primarily occurs along grain boundaries34, we have investigated the influence of BAX additives on LED hysteresis. By measuring the J–V–L characteristics with various scanning directions and rates, as shown in Fig. 5a–f, it can be seen that all devices show some level of hysteresis for both J and L. Nevertheless, the devices with BAX:MAPbX3 (X = I, Br) molar ratio of 20:100 exhibit reduced hysteresis. Supplementary Fig. 11 shows the scanning-rate-dependent EQE for both I- and Br-perovskite LEDs, demonstrating that LEDs without BAX are more scanning-rate dependent. We believe that the longchain BA cations that self-assemble at the crystallite surface either impede ion motion or prevent ions from within crystallites from participating in ion migration processes. In essence, the BA cation, being larger than MA, decreases the cation’s thermodynamic activity at the surface (that is, by reducing the equilibrium vapour pressure at the surface). To ensure that devices are able to be reliably characterized, the steady-state EQE was measured at a constant voltage of 5.0 V. As shown in Fig. 5g,h, the EQE of the device without BAX showed a strong poling effect, while the devices with BAX rapidly stabilize. The initial increase of the EQE in films without BAX might result from trap filling such that injected charges have a higher radiative recombination rate, while the decrease of the EQE after a few seconds might be due to ion-migration-induced degradation of the perovskite layer. Stability is of critical importance for perovskite LEDs, and thus the shelf stability of unencapsulated LEDs stored in N2 were evaluated over time. As shown in Fig. 5i,j, the EQE of the control devices without BAX in the precursor solution decreases to 60–70% of their initial value after 2 days, whereas the EQE of an I-perovskite LED with BAI:MAPbI3 molar ratio of 20:100 is stable without degradation after storage for more than 8 months. Similarly, the Br-perovskite LEDs also show no degradation after storage for 4 months. The improved stability of the perovskite LEDs with BAX can be attributed to the ultra-smooth, pinhole-free and compact perovskite films as well as the bulky nature of the long alkyl chains that stabilize the crystallite surfaces. The stabilized nanocrystallite perovskite films also enable improved operational stability, shown in Supplementary Fig. 12, an aspect that is critical for display and lighting applications. One possible reason for the operational instability is that ion migration induces degradation and sites for non-radiative recombination in the perovskite layer as the LEDs work at high voltages. This is confirmed by the decreased PL intensity of the LED after operating at 5 V for 5 min (Supplementary Fig. 13). Another possible reason is thermal degradation caused by Joule heating. We thus propose three approaches to continue to improve the operational stability of perovskite LEDs: (1) use more bulky organic ligands to stabilize the surface and impede ion migration; (2) decrease turn-on voltage through further interface engineering such that devices operate at lower voltages; and (3) at least partially replace MA cations with formamidinium ammonium or Cs to increase thermal stability.

In conclusion, we have shown that the addition of long-chain ammonium halides in the precursor solution can dramatically impede the grain growth of 3D perovskite crystallites and decrease film roughness. The nanometre-sized 3D perovskite grains induce enhanced and blue-shifted PL and EL emission, with the longchain ammonium cations stabilizing the grain surface. As a result, perovskite LEDs made from BAX-incorporated layers showed greatly improved device performance, shelf stability and operational stability. It is also worth investigating whether the incorporation of long-chain ammonium halide ligands within perovskite films can benefit other devices such as photodetectors, solar cells and lasers.

Methods Methods and any associated references are available in the online version of the paper. Received 11 April 2016; accepted 6 December 2016; published online 16 January 2017

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Acknowledgements We acknowledge a DARPA Young Faculty Award (award no. D15AP00093) for research funding. B.P.R. acknowledges the support of a DuPont Young Professor Award for research funding. R.A.K. acknowledges support from the National Science Foundation Graduate Research Fellowship under grant no. DGE 1148900.

Author contributions T.-W.K. and Z.X. designed the device structure. Z.X. performed the AFM, PL and absorption measurements, and fabricated the LEDs. R.A.K. developed the surfacted perovskite processing protocol, synthesized the precursors and helped to calculate grain size. L.Z. conducted the XRD and SEM measurements. N.L.T. and G.D.S conducted the TRPL and QY measurements. K.M.L. assisted with LED characterization. B.P.R. supervised the work. Z.X. and B.P.R. wrote the manuscript. All authors read and commented on the manuscript.

Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to B.P.R.

Competing financial interests

The authors declare no competing financial interests.



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DOI: 10.1038/NPHOTON.2016.269

Methods

Materials synthesis. MAI, BAI and PEAI were synthesized by mixing methylamine, butylamine and phenylethylamine (Sigma Aldrich) with equimolar amounts of aqueous HI (Sigma Aldrich, stabilized) at 0 °C with constant stirring under N2. MABr and BABr were synthesized with analogous procedures. The organoammonium halides (MAI, BAI, PEAI, MABr and BABr) were washed with an ethanol:ether mixture and rotovaped several times to remove the HI stabilizer. Perovskite film deposition and characterization. Poly-TPD (or PVK) was dissolved in chlorobenzene at a concentration of 6 mg ml–1. Poly-TPD (or PVK) was spin-coated on ITO at 1,000 r.p.m. for 60 s, followed by thermal annealing at 150 °C (or 120 °C for PVK) for 20 min. The poly-TPD layer was treated by O2 plasma for 2 s to improve wetting. The MAPbI3 or MAPbBr3 precursors were dissolved in DMF (Sigma Aldrich, 99.8% anhydrous) at concentrations of 0.43 M and 0.3 M, respectively. Then, different molar amounts of BAI and BABr were added into the MAPbI3 and MAPbBr3 solutions, respectively, before spin-coating. The spin-coating rate for the I-perovskite was 6,000 r.p.m., and toluene was dropped on the spinning substrate at 5 s. The spin speed for the Br-perovskite was 4,500 r.p.m., and the toluene was dropped on the spinning substrate at 4 s. The AFM measurements were conducted in the tapping mode in a N2 filled glove box (Veeco di Innova). The SEM measurements were conducted with an FEI XHR (extreme high resolution) SEM (Verios 460) using immersion mode. XRD measurements were performed with a Bruker D8 Discover X-ray diffractometer with Bragg-Brentano parallel beam geometry, a diffracted beam monochromator and a conventional Cu target X-ray tube set to 40 kV and 40 mA. Absorption spectra were measured using a Cary 5000 UV-Vis-NIR system (Agilent). The PL spectra were

measured using an FLS980 spectrometer (Edinburgh Instruments) with an excitation wavelength of 380 nm. PLQY measurement and calculation. The QY of the I- and Br-perovskite films was measured using a PTI QuantaMaster 400 Steady State Fluorometer with Petite Integrating Sphere. The excitation wavelengths for I- and Br-perovskite are 450 and 400 nm, respectively, with excitation intensities of 2.2 and 2.1 mW cm–2, respectively. Time-resolved PL measurement. Time-resolved PL measurements were taken using a Horiba DeltaFlex time-correlated single-photon counting system. The samples were excited by a pulsed laser diode (DeltaDiode-Horiba) with a centre wavelength of 406 nm, an excitation intensity of ∼4 mW cm–2 and a repetition rate that is less than the reciprocal of the measurement range. The time resolution was determined to be ∼100 ps from the instrument response function. All decay traces were taken at 5 nm increment and with constant run time of 10 s. The emission bandpass was 16 nm for all samples except for BABr:MAPbBr3 (20:100 and 40:100) samples where it was set to 1 nm to protect the detector from oversaturation. Device fabrication and characterization. All perovskite films were dried at 70 °C for 5 min following spin-coating to fully dry the film and ensure the full reaction of the precursors without affecting the morphology and grain size substantially. TPBi, LiF and Al layers were sequentially thermally deposited on top of the perovskite film to thicknesses of 60, 1.2 and 100 nm, respectively. The device area was 0.1 cm2. LEDs were measured in N2 using a homemade motorized goniometer set-up consisting of a Keithley 2400 sourcemeter unit, a calibrated Si photodiode (FDS-100CAL, Thorlabs), a picoammeter (4140B, Agilent) and a calibrated fiber optic spectrophotometer (UVN-SR, StellarNet).

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