Editor's recommendation: Quasi-two-dimensional perovskite light-emitting diodes exhibit high outer quantum efficiency. In this paper, the authors used a synergistic dual additive strategy to prepare perovskite films with low defect density and high environmental stability, and achieved a record value of 28.1% EQE.
Quasi-2D perovskites have long been considered to have good "energy funnel/cascade" structures and excellent optical properties compared to 3D perovskites. However, due to Auger recombination caused by high current density, most quasi-two-dimensional perovskite light-emitting diodes (PeLEDs) exhibit high outer quantum efficiency (EQE) but are not stable in operation.
Here, researchers at Southeast University et al. used a collaborative dual additive strategy to prepare perovskite films with low defect density and high environmental stability by using 18-crown-6 and poly (glycol) methyl ether acrylate (MPEG-MAA) as additives. The double additive containing C-O-C bonds can not only effectively reduce perovskite defects, but also destroy the self-aggregation of organic ligands, inducing the formation of perovskite nanocrystals with quasi-nucleus/shell structures. After thermal annealing, MPEG-MAA with a C=C bond can polymerize to obtain a comb polymer, further protecting the passivated perovskite nanocrystals from water and oxygen. Finally, the most efficient green PeLED was achieved, with a normal EQE of 25.2%, a maximum EQE of 28.1%, and the service life of the device in an air environment (T50) was increased by more than ten times, which provided a novel and effective strategy to manufacture High Efficiency and Long Life PeLED. The paper was published in Adv. entitled "Perovskite Light-Emitting Diodes with EQE Exceeding 28% through a Synergetic Dual-Additive Strategy for Defect Passivation and Nanostructure Regulation." Mater.
Thesis Link:
https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202103268
Perovskites are becoming promising luminescent materials due to their low cost, narrow half-peak width (FWHM) emission, and panchromatic tunableness. However, the low exciton binding energy of 3D perovskites means that most excitons can thermally dissociate into free charge. Therefore, for a long time, the performance of perovskite light-emitting diodes (PeLED) based on 3D perovskites has been unsatisfactory. To improve the efficiency and stability of PeLED, several strategies have recently been adopted, such as the formation of a quasi-nucleus/shell structure by anti-solvent crystallization, passivation of perovskite defects by small molecules or polymers, and the construction of quasi-two-dimensional perovskites by reducing crystal size. These methods have successfully improved the external quantum efficiency (EQE) of PeLED, emitting up to 23.4% of green light and up to 21.6% of near-infrared light.
While the above strategy has significantly improved efficiency, there are still many unresolved issues. The anti-solvent method can easily limit the grain size of 3D perovskite nanocrystals, but can lead to a sharp decrease in reliability and repeatability. Commonly used small molecules or polymer additives containing hydrophilic groups such as hydroxyl, carboxyl, and carbonyl groups can be coordinated with the unpaired Pb2+ in perovskites to reduce the density of defective states; however, as a high polar organic material with strong water absorption, they are not conducive to improving the oxidation resistance and water resistance of perovskite films. Compared with 3D perovskite nanocrystals, quasi-2D perovskites not only inherit good thin film morphology and high moisture resistance, but also inherit high photoluminescent quantum yield (PLQY) due to the "energy funnel/cascade" structure from low n phase to high n phase.
Quasi-two-dimensional perovskites based on large steric hindrance ligands have achieved high EQE emissions of red, green, and blue light; however, as many recent studies have reported, their operating life is much shorter than that of 3D perovskite-based devices. The main reason is that the aggregated ligand causes strong internal Joule heating and Auger recombination at the high current density of PeLED. At the same time, organic ligands such as phenethyl ammonium bromide (PEABr) will undergo thermal decomposition during device operation, resulting in increased perovskite defects. To solve this problem, the use of small cation KBr instead of insulated long-chain PEABr spacers resulted in a 3.75-fold increase in stability (T50) at 100 cd m-2; however, the device efficiency was significantly reduced (7.7% vs 16.2%) compared to devices using PEABr. Therefore, how to improve device stability and maintain high efficiency at the same time is challenging.
Figure 1. Photophysical properties of PEABr:CsPbBr3 perovskite films, no (w/o) or with (w/) crown and crown:MPEG-MAA additives. a) UV visible absorption. b) Graph of the absorption coefficient (α) used to extract Urbach energy and the photon energy. c) Statistical PLQY. The illustration shows photographs of different perovskite films under ultraviolet irradiation. d-f) 2D profile image of TRPL. g-i) Confocal PL intensity plot under 410 nm photoexcitation.
Figure 2. a,b) 1H NMR spectra before and after MPEG-MAA heat treatment (a) and MPEG-MAA, crown, original perovskite, MPEG-MAA treated perovskite, crown treated perovskite, and crown: MPEG-MAA treated perovskite (b). c) Schematic diagram of crystal structure changes and defect passivation of crown and MPEG-MAA.
Figure 3. a-c) 2DGIWAXS profile and d) XRD spectra of PEABr: CsPbBr3 perovskite films, free of additives, containing crown and crown: MPEG-MAA. e,f) (100) curves of perovskite films with crown (e) and crown:MPEG-MAA (f) in GIWAXS with different instrument Ψ values (0−40°). The illustration depicts crystal structure changes with the introduction of crown and crown:MPEG-MAA.
Figure 4. Perovskite films placed at humidity > 50% stability in the air. a) Ultraviolet-visible absorption spectrum of perovskite films containing and without additives (solid lines represent fresh films, dotted lines represent films exposed to air). b-d) 3D view of the PL spectrum of perovskite films with and without additives over time.
Figure 5. Device characteristics of PeLED. a,b) Cross-section STEM image of PeLED without additives (a) and w/crown: MPEG-MAA(b). c) J-V-L, d) LE-J and e) EQE-J characteristics. f) EL spectrum of PeLED. The illustration shows a photo of the device at a 4 V offset. g) 25 EQE histograms of individual PeLED with crown:MPEG-MAA additives. The fitted curve is a distribution function curve. h) Half-life curve in unpackaged ambient air at 100 cd m-2 brightness.
In summary, the authors demonstrate a synergistic dual-additive strategy to alter the crystal structure of perovskites and passivate the surface defects of high-efficiency PeLED. By introducing thermally polymerizable MPEG-MAA, water resistance and oxygen barrier can be improved. At the same time, it can also cause compressive stress in perovskite crystals, which is the reason for enhancing the stability of perovskites. Compared to quasi-two-dimensional PEABr:CsPbBr3 perovskites with easy self-aggregation PEABr, quasi-core/shell perovskite structures containing dual additives have evenly distributed PEA cations and fewer defects. As a result, PeLED achieved a maximum EQE of 28.1% and a normal EQE of 25.2%. In addition, 4.04 hours of operating T50 life is achieved at an initial brightness of 100 cd m-2, which is 10.36 times that of the reference device, even in ambient air without packaging. This work provides new insights into the regulation and defect passivation of perovskite structures to overcome existing EL efficiency and working life improvement limitations through synergistic additive strategies. (Text: None)
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