In the field of perovskite electroluminescent devices (PeLEDs), the performance of blue electroluminescent devices lags behind other similar devices due to the lack of fabrication methods. Here, researchers from Beijing Institute of Technology, Dalian Research Institute of Chemical Physics, Chinese Academy of Sciences, and Shanghai Institute of Applied Physics, Chinese Academy of Sciences used 2-phenylethylamine bromide (PEABr) and 3,3-diphenylpropylamine bromide (DPPABr). ) of mixed ligands to prepare CsPbClBr2 nanocrystalline films in situ. Mixing the two ligands together resulted in a strong blue light emission at 470 nm with a photoluminescence quantum yield as high as 60% due to the formation of a narrow quantum well width distribution. On this basis, a highly efficient blue perovskite device with a maximum external quantum efficiency of 8.8% was obtained at 473 nm.
The related paper was published in the journal Nature Communication with the title "Dimension control of in situ fabricated CsPbClBr2 nanocrystal films toward efficient blue light-emitting diodes".
Perovskite light-emitting diodes (PeLEDs) have emerged as an emerging display technology due to their high color purity, high external quantum efficiency (EQE), and solution processability. Taking advantage of the ionic properties of metal halide perovskites, PELEDs can be directly fabricated by an in-situ fabrication technique of spin-coating perovskite precursor solutions on target substrates. Since room-temperature-operating perovskite electroluminescence (EL) devices were first reported in 2014, green, red, and near-infrared PeLEDs have achieved maximum EQEs of over 20%, comparable to organic light-emitting diodes and quantum dot light-emitting diodes. However, the performance of blue PeLEDs still lags behind their green, red, and near-infrared light-emitting diodes, especially for display applications in the pure blue region (455–475 nm), which is an obstacle to the development of full-color display technologies.
In general, spectral modulation of perovskite-type emitters can be achieved by tuning composition, size, and/or size. By reducing the size of bulk perovskites or introducing mixed halides, three-dimensional perovskite nanocrystals with blue emission were successfully prepared. However, the efficiency and stability issues of blue electroluminescent devices based on such small-sized perovskite nanocrystals are mainly due to complicated purification and phase separation.
Another strategy to achieve high-efficiency blue PeLEDs is to construct quasi-two-dimensional (quasi-2D) perovskite structures with multiple quantum wells. The photoluminescence (PL) properties of these quasi-2D perovskites are closely related to the energy transfer from small to large n domains. It is found that a flat quasi-2D perovskite quantum well width distribution (QWD) is essential for facilitating carrier transport and reducing additional energy loss for realizing high-performance photovoltaic devices. However, the effect of QWD on EL devices is less studied.
It is known that QWD can be controlled by adjusting the ratio of precursor mixtures or by ligand engineering. Here, it is demonstrated that the use of dual ligands is an effective strategy to control the QWD of CsPbClBr2 nanocrystalline films prepared in situ. 2-Phenylethylamine bromide (PEABr) is an efficient ligand for forming small n domains, while 3,3-diphenylpropylamine bromide (DPPABr) is an efficient ligand for forming large n values. A judicious choice of the ratio of the two ligands can narrow the QWD with a central domination of n = 4.
This efficient size control facilitates efficient energy transfer, resulting in strong blue light emission at 470 nm wavelength with PL quantum yield (PLQY) as high as 60%. Utilizing dual ligands with a propensity to form small n domains and large n domains is a versatile strategy to achieve narrow QWD for enhanced PL properties. Based on the optimized thin films prepared by mixing PEABr and DPPABr, a high-efficiency blue electroluminescence device with a maximum EQE of 8.8% was obtained at a wavelength of 473 nm. (Text: Aisin Gioro Star)
Fig. 1 Structural characteristics of CsPbClBr2 nanocrystalline thin films. Schematic diagram of the in-situ preparation process of CsPbClBr2 nanocrystalline thin films. The relationship between the integral intensity q of the GIWAXS pattern of CsPbClBr2 nanocrystalline films with different ratios of DPPABr and PEABr was studied.
Fig. 2 Optical measurements of CsPbClBr2 nanocrystalline thin films. Steady-state photoluminescence spectra, absorption spectra and b-PLQYs of CsPbClBr2 nanocrystalline films with different ratios of DPPABr and PEABr were studied.
Fig. 3 The effect of QWD on its carrier dynamics. a, b Peak FWHM evolution extracted from broad bleached peaks (425–470 nm) of D0P8, D4P4 and D8P0 samples. c Schematic illustration of the carrier behavior after excitation. The carrier recombination process can be divided into five stages: I, carrier formation; II, exciton transfer; III, charge transfer; IV, reverse charge transfer; V, continuous charge transfer and recombination.
Figure 4 Blue perovskite device features. Energy level diagram of an electroluminescent device. Cross-sectional TEM image of a multilayer electroluminescent device. c EL spectra at 3.6, 4.4 and 5.2V forward bias. d Current density-brightness-voltage characteristics of the best performing device. EQE – Voltage characteristics of optimal performance equipment. f Maximum EQE histogram of 28 devices.
