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Suppressing part disproportionation in quasi-2D perovskite light-emitting diodes

Blocking ionic transport by ligand design

Section disproportionation of intermediate n-values is spontaneous for typical 2D perovskites as a result of the n = 1 part is enthalpically favored and a combination of n-phases is entropically favored18. Since mass transport (ion diffusion) should occur between layers throughout part disproportionation, we hypothesized that the ligands may management the kinetics by modulating both the interlayer diffusion barrier or the variety of interfacial defects via which ion diffusion can happen17. Current works have demonstrated that conjugated ligands (e.g., thiophenylethylammonium, TEA, Fig. 1a) can inhibit ion diffusion inside and between 2D perovskite heterostructures22,23,24, which supplies the technique of testing the above speculation. Right here, we additional designed and synthesized two novel natural conjugated ligands, PPT’ (2-(5-(3′,5′-dimethyl-[1,1′-biphenyl]−4-yl)thiophen-2-yl)ethyl-1-ammonium iodide) and PPT (2-(5-(2,2′-dimethyl-[1,1′-biphenyl]−4-yl)thiophen-2-yl)ethyl-1-ammonium iodide) (Fig. 1a). Their synthesis procedures are detailed in Supplementary Data25. The design of those ligands was motivated by their comparatively giant π-systems, which is already a longtime issue for suppressing ion-diffusion, and their distinct cross-sections as a result of elevated steric barrier related to the phenyl-phenyl dihedral of PPT. This latter issue could result in higher resolution processability and weird part management behaviors, which is but to be explored.

To look at these hypotheses, we synthesized n = 1 lead iodide-based 2D perovskite bulk crystals for PPT’ and PPT, specifically (PPT’)2PbI4 and (PPT)2PbI4. The highest views of their single crystal constructions are proven in Fig. 1b–e with beforehand reported (BA)2PbI426 and (TEA)2PbI427 as references (see Supplementary Figs. 1, 2 and Supplementary Desk 1 for extra particulars). The TEA natural layer displays barely elevated floor protection on the PbI42− inorganic octahedron layers relative to BA, whereas the PPT’ and PPT layers present considerably higher protection. To check their stability towards ion diffusion, n = 1 2D perovskite skinny movies have been handled with hydrobromic acid vapor. The PL picture confirmed that the emission of (BA)2PbI4 fully transformed from inexperienced to purple (Fig. 1f and Supplementary Fig. 3a), implying that Br simply penetrates the BA ligand layer and substitutes the I within the inorganic layer. Within the case of (TEA)2PbI4, the movie exhibited blended inexperienced and blue emissions on account of partial substitution of I by Br (Fig. 1g and Supplementary Fig. 3b). Surprisingly, neither the (PPT’)2PbI4 or (PPT)2PbI4 movie exhibited a PL colour change (Fig. 1h, i and Supplementary Fig. 3c, d), suggesting minimal Br penetration. Be aware, there’s much less blue shift within the PL spectra of (PPT)2PbI4 in contrast with (PPT’)2PbI4 after therapy (Supplementary Fig. 3c, d), indicating barely higher safety by PPT ligand. The general ligand-dependence of ion penetration agrees with the crystal-based floor protection analyses.

To offer molecular perception into these distinct behaviors, molecular dynamics (MD) simulations have been used to check the free power profiles of interlayer I diffusion in perovskites substituted with completely different ligands. Section disproportionation essentially entails the alternate of small A-cations, halide anions, and lead cations between layers, both via defects, layer edges, or via direct diffusion throughout the natural ligand layer. The activation energies of the latter mechanism are in contrast as a place to begin for deciphering the distinct behaviors of those ligands. Umbrella sampling was used to drag an I out of its crystal lattice to diffuse via the natural ligand layer of mannequin n = 1 (BA)2PbI4, (TEA)2PbI4, (PPT’)2PbI4, and (PPT)2PbI4 techniques. In every simulation, periodic boundary circumstances have been used to successfully simulate stacked 2D perovskites with an instance proven in Supplementary Fig. 4. The crystal constructions of (BA)2PbI4 and (TEA)2PbI4 turned distorted throughout the pulling course of (Fig. 1j, okay, additionally see prime view in Supplementary Fig. 5), leading to a number of neighboring ions being liberated to diffuse between layers (brown spheres in Fig. 1j). In distinction, (PPT’)2PbI4 and (PPT)2PbI4 retained their crystal constructions throughout ion diffusion (Fig. 1l, m, additionally see prime view in Supplementary Fig. 5). This comparability supplies qualitative proof that the bulkier ligands stabilize the perovskite lattice even within the presence of ion defects. The free power barrier for diffusion of I via the ligand will increase with size and π-conjugation such that BA <TEA <PPT’ ~ PPT (Fig. 1n). The free energies required for PPT’ and PPT are twice these of BA and TEA, indicating that these cumbersome natural conjugated ligands can successfully inhibit direct interlayer ion diffusion and probably additionally hinder diffusion restricted part disproportionation. Notably, the smaller interstitial cations and Pb2+ additionally must bear interlayer diffusion throughout the part disproportionation of high-n phases. Though we count on comparable tendencies for these ions, the simulations of n > 1 techniques are at present past the scope of this work. Regardless, the n = 1 simulations might present a rationale for kinetically restricted disproportionation through ligand design.

Section distribution management and mechanistic understanding

To evaluate the influence of those ligands on part distribution, they have been used to manufacture lead iodide-based quasi-2D perovskite (n > 1) skinny movies that have been subjected to multi-mode characterizations. As proven in Fig. 2a, the skinny movies produced from BA and TEA ligands utilizing a stoichiometric ratio of nominal <n > = 3 (i.e., L2FA2Pb3I10 in precursor resolution, L = BA or TEA right here) present absorption peaks at 572 and 776 nm, which correspond to n = 2 and n ~ ∞ (3D) phases, respectively. The PPT’-based movie displays distinct absorption peaks at 572, 633, and 682 nm comparable to n = 2, 4, and 5 phases, respectively. The PPT-based movie displays absorption peaks solely at 633 and 682 nm, suggesting predominant n = 4 and 5 phases with a narrower part distribution. The photoluminescence (PL) of every movie was additionally characterised (Fig. 2b), the place a number of broad emission peaks from n = 2 and different high-n phases are evident within the BA, TEA, and even PPT’ movies. Apparently, the PL emission profile of the PPT movie is well-defined and dominated by n ≈ 4–6 phases. The part distributions of every movie have been estimated by linear superposition (Fig. 2c) utilizing the absorption coefficient of every n-phase (Supplementary Fig. 6). The slender part distribution within the PPT movie results in blue shift of the primary emission peak from close to infrared to purple (Fig. second) and improved PL quantum yield (PLQY) (Supplementary Fig. 7).

Fig. 2: Section distribution management and power switch dynamics of quasi-2D perovskite skinny movies.
figure 2

a, b UV-vis (a) and PL (b) spectra of the quasi-2D perovskite movies. From left to proper: the vertical sprint traces in (a) point out the absorption peaks of the n = 2, n = 5, and n ~ ∞ (3D) phases. All of the movies have been fabricated from the precursor options with a stoichiometric ratio of nominal <n > = 3. c The relative contents of various n phases for perovskite movies. The relative contents have been estimated from the absorption spectra based mostly on the linear superposition of the corresponding absorption coefficient of every n-phase. This may be additional verified by the GISAXS and TA information proven beneath. d Images of quasi-2D perovskite movies with completely different natural ligands below ultraviolet lamp irradiation. eh GISAXS sample of (e) BA, (f) TEA, (g) PPT’, and (h) PPT based mostly movies. qxy and qz symbolize the in-plane and out-of-plane scattering vectors, respectively. The dominated n phases within the movies have been decided by referring to their corresponding single-crystal constructions. il Pseudo colour maps of TA spectra and mp, corresponding kinetics curves at two completely different wavelengths of (i, m) BA, (j, n) TEA, (okay, o) PPT’, (l, p) PPT based mostly movies. The power switch time fixed is fitted to be 0.38 ± 0.05, 0.43 ± 0.03, 0.30 ± 0.05, and 0.22 ± 0.04 ps for BA, TEA, PPT’, and PPT movie, respectively.

Grazing incidence small-angle X-ray scattering (GISAXS) was employed as a structural probe to quantify the completely different n phases within the movies. Primarily based on the out-of-plane scattering patterns, the BA and TEA movies are primarily composed of a high-n (close to 3D) part accompanied by a smaller fraction of low-n phases (Fig. 2e, f). The PPT’ movie primarily consists of median-n phases starting from n = 2 to eight (Fig. 2g), whereas the PPT movie is dominated by a narrower part distribution starting from n = 2 to six (Fig. 2h). These information comport with the optical measurements and collectively present sturdy proof of efficient part distribution management utilizing the PPT ligand, and to a lesser diploma the PPT’ ligand. This interpretation that suppressing off-target disproportionation by PPT’ and PPT ligands can also be corroborated by different scattering and microscopy characterizations reminiscent of powder X-ray diffraction in Supplementary Fig. 8 and Kelvin probe power microscopy (KPFM) contact potential distinction (CPD) photos28 in Supplementary Fig. 9. As a further demonstration, perovskite skinny movies have been fabricated with a collection of different natural conjugated ligands (reminiscent of PEA, 2 P, 2 T, and three T, see Supplementary Fig. 10), all of which have been confirmed to exhibit broad n distributions which might be unsuitable for environment friendly and colour tunable LEDs. These observations spotlight the distinctive function of the newly developed ligands in controlling skinny movie progress and part formation.

To visualise the part disproportionation course of and examine the underlying mechanism, ex-situ PL throughout spin-coating, in-situ PL throughout thermal annealing and the corresponding GISAXS measurements have been carried out. It’s revealed that n ~ 3 phases kind on the preliminary spinning stage for all circumstances (Supplementary Figs. 11, 12), however BA and TEA cations result in part disproportionation to low- and high-n phases throughout late-stage of spinning, whereas PPT’ and PPT movies retain n ~ 3 part all through the spinning course of (Supplementary Fig. 11). Then, a short-term thermal annealing (100 °C for 10 min) was utilized and extra thorough part disproportionation happens in BA and TEA based mostly movies, whereas PPT’ and PPT based mostly movies are comparatively secure (see Supplementary Figs. 1317 for extra detailed discussions). Nevertheless, upon heating the PPT movie at 100 °C for a protracted interval (> 5 h), small quantities of n = 2 and n ~ ∞ phases appeared (Supplementary Fig. 18), validating that the median-n phases are kinetic merchandise and that they’ll ultimately disproportionate, albeit with a better activation power than the opposite movies. These outcomes are additionally in good settlement with that of different characterizations, reminiscent of ex-situ UV-vis (Supplementary Fig. 19) and in-situ grazing incidence wide-angle X-ray scattering (GIWAXS) (Supplementary Fig. 20). We additional carried out disproportionation response kinetics research on the spin-coated moist movies by following a reported methodology29. The activation power retrieved from this research (Supplementary Figs. 2125) is according to the pattern of simulated free power for ion diffusion (Fig. 1n). These observations will be summarized as a crystal formation mannequin for quasi-2D perovskites, the place ligands play an necessary function in regulating interlayer ion-transport and in the end the kinetics of part disproportionation (Supplementary Fig. 26). Apparently, the management over the n-phase supplied by the PPT ligand allows us to tune the PL emission wavelength from 610 to 720 nm by adjusting the stoichiometric ratio of precursor options (Supplementary Figs. 27, 28) or annealing temperature (Supplementary Fig. 29), making them helpful in secure and color-tunable LEDs.

Transient absorption (TA) spectroscopy measurements have been then carried out to research the power switch dynamics between completely different n phases. The distinctive picture bleach peaks have been assigned to completely different n-phases within the quasi-2D perovskite movies (Fig. 2i–l), which once more aligns with our evaluation on part distribution. The corresponding TA dynamics display the power switch from low-n to high-n phases amongst all of the movies, by evaluating the rising time of extracted kinetic curves at completely different wavelengths (Fig. 2m–p). Accordingly, the power switch time fixed within the PPT movie is measured to be ~0.2 ps, which is quicker than that in different movies (~0.3 to 0.4 ps)30. The sooner power switch processes could also be related to a mix of fewer switch steps (higher part purity) and diminished carrier-trapping (decrease defect density) within the PPT movie10.

Improved optoelectronic properties by part management

Reducing defect density and suppressing non-radiative recombination pathways are critically necessary for LEDs. Determine 3a illustrates cascade power switch between completely different n-phase domains in perovskite movies. The power switch is normally accompanied with trap-related nonradiative losses and undesired radiative loss if inter-phase power switch is much less environment friendly. This implies that slicing down switch steps will probably be helpful to scale back these losses, thus boosting the PLQY. By steadily altering the excitation wavelength to decrease power (longer wavelength) to lower the absorption of the low n phases, we clearly noticed enhanced PLQY for all movies (Fig. 3b). We then carried out ultraviolet photoemission spectroscopy (UPS) measurement (Supplementary Figs. 30, 31). The outcomes point out that BA and TEA are extra n-type, i.e., the Fermi power is nearer to the conduction band, which can be attributed to the presence of extra defect states. Time-resolved PL (TRPL) (Supplementary Fig. 32) qualitatively revealed remarkably diminished lure states with PPT. Such a decrease lure density considerably results in a a lot increased PLQY (as much as 94%) of PPT movie in comparison with that of BA, TEA, and PPT’ circumstances below all excitation circumstances (Fig. 3b, Supplementary Fig. 7).

Fig. 3: Improved optoelectronic properties with higher part management.
figure 3

a Schematic illustration of cascaded power switch between completely different n-phases. The power switch (EnT) in quasi-2D perovskites is normally accompanied with nonradiative losses and undesired radiative recombination losses if with out efficient part distribution management. b Excitation wavelength dependent PLQY of BA (black), TEA (purple), PPT’ (blue), and PPT (inexperienced) based mostly quasi-2D perovskite movies. cj, Present density versus voltage curves for hole-only (cf) and electron-only (gj) gadgets below darkish circumstances. Inset reveals the machine constructions. ITO, indium tin oxide; Poly-TPD, Poly(N,N’-bis-4-butylphenyl-N,N’-bisphenyl)benzidine); CBP, 4,4’-Bis(N-carbazolyl)−1,1’-biphenyl; TPBi, 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene. SCLC fittings (stable traces) reveal the trap-filling restrict voltage (VTFL) for gap and electron, respectively, exhibiting diminished lure densities of quasi-2D perovskites comprised of PPT ligand.

We additional constructed gap (h)-only (inset in Fig. 3c) and electron (e)-only (inset in Fig. 3g) gadgets for electrical transport measurements to quantify the lure densities. Utilizing a space-charge-limited present (SCLC) mannequin, we estimated the lure state densities from: Nt(e/h) = 2εrε0VTFL(e/h) / (qd2), the place Nt is the lure state density, VTFL is the trap-filled restrict voltage, d is the space between the electrodes, q is the elementary cost, and ε0 and εr are the vacuum permittivity and relative permittivity, respectively. It was discovered that the VTFL for each electron and gap are within the order of BA > TEA > PPT’ > PPT (Fig. 3c–j). The outlet lure densities for BA, TEA, PPT’, and PPT-based gadgets have been decided to be 4.4 × 1016, 4.3 × 1016, 3.7 × 1016, and a pair of.1 × 1016 cm−3, respectively. The electron lure densities have been decided to be 5.0 × 1016, 4.8 × 1016, 4.2 × 1016, and three.5 × 1016 cm−3, respectively. The diminished defect density together with improved PLQY make PPT-based quasi-2D perovskite skinny movies promising for environment friendly LEDs.

LED machine traits

Machine structure is proven in Fig. 4a31,32. Poly-TPD was used as hole-transporting layer and TPBi was used as electron-transporting layer. An ultrathin polyvinylpyrrolidone (PVP) interlayer was inserted in between Poly-TPD and perovskite layer to enhance the wettability and mitigate the interfacial loss12,33,34. The thicknesses of Poly-TPD, perovskite and TPBi layers are round 30, 20, 60 nm, respectively, as decided by optical profilometer. The scanning electron microscope (SEM) photos (Supplementary Fig. 33) of quasi-2D perovskites present uniform movies with full floor protection freed from pinholes. Atomic power microscope (AFM) measurements present that the as-prepared PPT-based skinny movie has a root-mean-square (r.m.s.) roughness of ~0.67 nm, which is decrease than skinny movies ready with different natural ligands (Supplementary Fig. 9a–d) and favorable for decreasing present leakage in LEDs. The present density-voltage-radiance (JVR) and EQE-current density (EQE-J) curves (Fig. 4b, c) of LEDs exhibit completely different EL behaviors. The turn-on voltage for radiance considerably decreases from 4.0 V (BA) to 2.8 V (PPT), which confirms the PPT movie has diminished defect densities. To this finish, the ensuing EQEs for various ligands are in good settlement with their part distribution evaluation and PLQY information. Excitingly, the PPT-based machine delivers a peak EQE of 26.3%, representing probably the most environment friendly purple LEDs based mostly on quasi-2D perovskite reported to date (Supplementary Desk 2). The luminance curves and present effectivity for PPT’ and PPT gadgets are plotted in Supplementary Fig. 34. Extra particulars on machine optimizations and corresponding machine performances will be present in Supplementary Figs. 3539.

Fig. 4: Machine efficiency of distribution managed quasi-2D perovskite LEDs.
figure 4

a Machine structure. PVSK signifies the quasi-2D perovskites investigated on this work. b, c Present density-voltage-radiance (JVR) curves (b) and EQE attribute (c) of quasi-2D perovskite LEDs. d EL spectra below ahead biases of 4, 5, 6, 7 and eight V. Inset is {a photograph} of a working machine. e Histogram of EQEs measured from 70 gadgets, which provides a median worth of twenty-two.9% and a relative commonplace deviation of 6.9%. f Simulated fractional energy distribution within the LED construction as a operate of emission wavelength. g Operational stability measurement of quasi-2D perovskite LEDs based mostly on completely different ligands with out encapsulation, carried out in nitrogen stuffed glove field at a relentless present density of 0.1 mA/cm2. h Operational stability of PPT machine below numerous present densities.

Determine 4d reveals the EL spectra of our champion machine below completely different driving voltages and an EL picture of an working machine (Fig. 4d, inset). The EL spectra centered at 700 nm corresponds to a deep-red emission, which is distinct from the near-infrared emission of the BA, TEA, and PPT’ gadgets (Supplementary Fig. 40). As well as, the EL spectra don’t shift with growing the voltage, indicating improved colour stability. An EQE histogram for 70 gadgets reveals a median EQE of twenty-two.9% with a low relative commonplace deviation of 6.9% (Fig. 4e), demonstrating good reproducibility. The machine traits relating to JVR and EQE-J curves of all 70 gadgets are included in Supplementary Fig. 41. The champion machine was cross-checked at Nationwide Cheng Kung College (NCKU, Taiwan), which reveals a peak EQE of twenty-two.6% (Supplementary Fig. 42). We additionally carried out an optical simulation on our LEDs to make clear the outcoupling efficiencies by utilizing a classical dipole mannequin with self-retrieved permittivity for every layer (Supplementary Fig. 43). The consequence suggests a lightweight outcoupling effectivity of 35.3% for the champion machine (Fig. 4f), which can result in a theoretically predicted most EQE of 33% when contemplating a PLQY of 94%. Additional optimizations will be carried out together with passivating defects and controlling the orientation of quasi-2D perovskites35, which shall push the EQE to an excellent increased degree.

Ion transport not solely impacts the part distribution in quasi-2D perovskite, but in addition limits the machine stability below electrical bias. PPT’ and PPT-based gadgets exhibit negligible hysteresis within the ahead and reverse JV scans in contrast with BA, and TEA-based gadgets (Supplementary Fig. 44a–d). This implies efficient immobilization of ions below electrical operation by our designed ligands. We additional employed a capacitance spectroscopy method to research the ion migration within the working gadgets (Supplementary Fig. 44e). The relative magnitudes of capacitance at low-frequency area (<103 Hz) present outstanding improve, which is attributed to the buildup of digital and ionic prices on the machine interfaces and perovskite grain boundaries36. Particularly, the rise of relative capacitance magnitude on the low-frequency area is diminished following the order of BA > TEA > PPT’ > PPT. The correlation between construction of the natural ligands and ionic transport within the perovskites was additional verified utilizing galvanostatic assessments (Supplementary Figs. 44f–h, 45)37. In keeping with different principle and related measurements, the perovskites with BA and TEA certainly present a comparatively increased ionic conductance, whereas the PPT’ and PPT integrated perovskites present a a lot decrease worth.

We examined operational stability for all 4 forms of gadgets initially at a comparatively low fixed present density of 0.1 mA cm−2 to higher analyze the degradation processes (Fig. 4g). The EQE of BA based mostly machine overshot to 162% of the preliminary worth, then dropped to 50% after a time of T50 = 16.7 min. The principle causes behind this overshoot are nonetheless unclear within the area however are normally attributed to the ion migration and accumulation at two interfaces of the perovskite layer. Such ion accumulation may gain advantage the cost injection by a robust native electrical area constructed on the interfaces38, thus resulting in an effectivity improve in a recent machine for a brief interval. The TEA-based machine reveals a diminished EQE overshoot as 129% and an elevated T50 of ~50 min. In distinction, the PPT’ machine displays an enhanced operational lifetime as T50 = ~30 h with a small overshoot of 102%. For PPT machine, a remarkably elongated T50 of ~220 h with none overshoot have been noticed. The EL spectra of PPT machine didn’t present apparent shift throughout this long-term operation (Supplementary Fig. 46). Subsequent, we evaluated the PPT machine lifetime at elevated present densities (Fig. 4h). The gadgets exhibit T50 of 30.9, 7.1, 2.8 h at a relentless present density of 1, 5, 12 mA/cm2 with a corresponding preliminary luminance of 10, 35, 100 cd/m2; respectively. These outcomes are corresponding to and even higher than the prevailing quasi-2D perovskite LEDs (Supplementary Desk 2). We additional characterised degraded gadgets utilizing ToF-SIMS. The PPT-based gadgets exhibit considerably suppressed iodine diffusion in contrast with the BA-based gadgets (Supplementary Figs. 47, 48). We acknowledge that extra efforts are nonetheless required to spice up the operational stability to the extent of commercialization. For example, different points by way of cost injection stability, electrochemical redox reactions in perovskites, in addition to joule heating impact that will result in the degradation of perovskite LEDs additionally have to be significantly thought-about and addressed sooner or later39.

Wavelength tunability and spectral stability

With higher management over part distribution, right here we tune the EL emission throughout a variety of spectra from 666 to 740 nm (Fig. 5a) using precursor options with assorted <n> numbers and blended halide contents (Supplementary Desk 3). The Fee Internationale de l’Eclairage (CIE) coordinates of various emission wavelengths are plotted in Fig. 5b. Amongst them, high-purity purple emission was obtained with a peak EQE of twenty-two.1% (Fig. 5c) and a (CIE) coordinates of (0.70, 0.29), which matches effectively with the ITU-R Suggestion BT.2100 colour house of pure purple colour. The machine efficiency of quasi-2D perovskite LEDs with completely different emission wavelengths are proven in Supplementary Figs. 49, 50 with their peak EQEs summarized in Fig. 5c. The completely different EQE values at numerous emission wavelengths are in all probability on account of distinct power switch pathways and assorted defect densities amongst completely different n-phases, highlighting the vital function of part distribution management. Importantly, the time evolution monitoring of EL spectra (Fig. 5d, e) present extraordinary spectral stability by way of suppressed Br/I segregation and part disproportionation. That is considerably improved in contrast with different LEDs based mostly on quasi-2D or blended halide perovskites40,41.

Fig. 5: Wavelength tunability and spectral stability.
figure 5

a, b EL spectra (a) of PPT based mostly quasi-2D perovskite LEDs with completely different compositions and the corresponding CIE coordinates (b). Sometimes, <n > = 3 represents that the stoichiometric ratio of precursor resolution follows the final chemical method of (PPT)2FAn−1PbnI3n+1 with a nominal n = 3. c The connection of EQEs towards EL peaks. d, e Time evolution monitoring of EL spectra for LEDs fabricated from <n> = 3 with 10% Br + 90% I (d), and <n> = 5 with 100% I (e) qausi-2D perovskites. The present density was maintained fixed at 1 mA cm−2 throughout the measurement. The extraordinary spectral stability, the place the EL spectra didn’t present apparent shift for each circumstances below long-term operation, demonstrates the nice suppression of halide segregation and part disproportionation, respectively.

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