Proton-transfer-induced 3D/2D hybrid perovskites suppress ion migration and reduce luminance overshoot

Perovskite light-emitting diodes (PeLEDs) based on three-dimensional (3D) polycrystalline perovskites suffer from ion migration, which causes overshoot of luminance over time during operation and reduces its operational lifetime. Here, we demonstrate 3D/2D hybrid PeLEDs with extremely reduced luminance overshoot and 21 times longer operational lifetime than 3D PeLEDs. The luminance overshoot ratio of 3D/2D hybrid PeLED is only 7.4% which is greatly lower than that of 3D PeLED (150.4%). The 3D/2D hybrid perovskite is obtained by adding a small amount of neutral benzylamine to methylammonium lead bromide, which induces a proton transfer from methylammonium to benzylamine and enables crystallization of 2D perovskite without destroying the 3D phase. Benzylammonium in the perovskite lattice suppresses formation of deep-trap states and ion migration, thereby enhances both operating stability and luminous efficiency based on its retardation effect in reorientation.

Supplementary Figure 2 Fluctuations of total energy as the evolution of simulation time and the snapshots of atomic configurations after the first-principles molecular dynamics (MD) simulations (3.5 ps) with a time step of 1.2 fs at the temperature of 300 K. a and b for the 2D perovskites formed by protonated BnA (BnA + ) and unprotonated BnA, respectively. First-principles molecular dynamics (MD) simulations presented the importance of proton-transfer. We utilized a 2√2 × 2√2 × 1 supercell to compare thermal fluctuation behaviours of perovskite lattice including protonated or unprotonated BnA at 300K. The atoms in the 2D perovskite formed by the protonated BnA (BnA + ) only slightly vibrate around their equilibrium positions and show steady energy fluctuations, which suggests that the 2D structure is well maintained. On the contrary, the 2D perovskite formed by the unprotonated BnA showed highly significant oscillations of the fluctuations of total energy, which indicates the intrinsic instability of the system. This result supports the importance of the protonation which enables BnA + to form a strong bond in the lattice and implies that the incorporation of the protonated BnA (BnA + ) can lead to effective passivation since it can maintain the 3D/2D hybrid structure without forming defects in the lattice.
Supplementary Figure 3 The 1 H MAS NMR spectra for the solid-state perovskites with varying a BnA concentration (0, 2.4, 30, 50 and 100%) and b ANI concentration (0, 2.4, and 100%) under a fast spinning speed of 40 kHz at 14.1 T. The pristine sample showed two dominant 1 H peaks at 6.36 ppm and 3.30 ppm which can be assigned to the hydrogens in NH3 (H-NH3) and CH3 (H-CH3) of MA + , respectively. 1 With the addition of BnA, the spectrum had two more peaks that arose from the hydrogens bonded to the benzene ring (H-B) and in CH2 (H-CH2) of BnA + . The positions of H-B varied in the range of approximately 6.7 to 8 ppm and the peak of H-CH2 was at around 4.8 ppm. 2 The 1 H peak intensity of H-CH3 decreased with an increase in BnA concentration because MA + is deprotonated to be methylamine which can be easily evaporated during the film annealing process. Meanwhile, the peak intensities of H-B, H-NH3, and H-CH2 gradually increased confirming that the protonated BnA composes the solid-state crystalline perovskite. Upon the addition of ANI, in contrast, the peak positions of H-NH3 and H-CH3 were invariant indicating ANI did not participate in the formation of perovskite.  Supplementary Figure 10 Integrated PL as a function of the excitation density of 3D and 3D/2D hybrid perovskites.
Supplementary Figure 11 PL emission of 3D perovskite, MAPbBr3 according to temperature from 70 K to 100 K. The peaks were deconvoluted into band-edge emission and trap-mediated emission.   It is worth noting that the smaller d-spacing (1.15 nm) than that obtained by grazing-incident X-ray diffraction (GIXD) analysis (1.66 nm) can be attributed to transient lattice contraction due to the highly strong beam intensity. 3,4 Interestingly, the highly periodic 2D perovskite was only observable at the shell region of the grain. Because the 2D perovskite has lower surface energy than the 3D perovskite due to its fewer surface dangling bonds and surface relaxation, growth of the 2D perovskite most possibly occurs on the 3D perovskite grains and it also makes an interface with grain boundary which has the highest surface energy, thereby lowering the total potential energy of the system. 3

Half lifetime t 1/2 [h]
This work [6] [7] [9] [10]   Table 2 Analysis of the X-ray diffraction spectra in Supplementary Figure 4. The lattice constants of the perovskite crystal were calculated using the Bragg diffraction equation: 2dsinθ = nλ (n = 1, 2, 3•••), where d is the crystal plane distance, θ is the diffraction angle at the XRD spectra, n is the order, and λ = 1.54 Å. The lattice constant of the 3D (n = ∞) perovskite phase is calculated by (100) peak, resulting in d = 5.89 Å of a cubic MAPbBr3 lattice. The d values of 2D perovskite (n = 1 and n = 2) phases were calculated by additional peaks that emerged at 5.32° and 3.97°, which marked with asterisks in Figure 4h. where ′10 . ′ denotes the equilibrium constant of protonation between MA and MAH + . 11 The equilibrium constant of the proton transfer between MA and aniline (ANI) is given by Thus, the protonated BnA (i.e., BnAH + ) predominates over the protonated ANI with the standard Gibbs freeenergy change Δ = -6.51 kcal•mol -1 at 300 K. This result thermo-dynamically supports our conclusion in the main text that the protonation tendency of BnA is much (> 10 4 times) stronger than that of ANI.
Supplementary Note 2: Improved PL of the 3D/2D hybrid perovskites owing to the energy band alignment Alignment of energy bands contributes to the improvement in PL properties of the 3D/2D hybrid perovskite film. Ultraviolet photoemission spectroscopy (UPS) analysis of each 3D MAPbBr3 and 2D BnA2PbBr4 film confirmed the type-I quantum well band alignment of 3D/2D hybrid perovskite (Supplementary Figure 8 and Supplementary  Table 4). 12,13 In this quantum well, the excited charge carriers can be efficiently confined within the 3D phase ( Supplementary Figure 9), enabling efficient radiative recombination in 3D/2D hybrid perovskite without a shift of emission wavelength. 14 In addition, the 2D perovskite reduces the defect density by passivating dangling bonds on 3D perovskite grains, which can act as charge trap sites leading to the increased rate of non-radiative recombination.