Abstract
1. Introduction
The metal halide perovskites have a general chemical formula of ABX3: A is a monovalent organic or inorganic cation, e.g., CH3NH3+(MA+), CH(NH2)2+(FA+), Cs+, Rb+; B is a divalent metal cation, e.g., Pb2+, Sn2+; X is a halide anion, e.g., Cl–, Br–, I–. The perovskites have attracted tremendous interests due to their excellent properties such as easy solution-based fabrication process, high absorption coefficient, tunable energy bandgaps, narrow emission linewidths, and long exciton diffusion length[
In 2014, the first bright and room-temperature operative Pero-LEDs were fabricated using MAPbI3–xClx (near-infrared) and MAPbBr3 (green) as emitting layers, which obtained an external quantum efficiencies (EQEs) of 0.76% and 0.1%[
In this review, the recent progress of blue Pero-LEDs, especially for the strategies of preparing the blue perovskite emitting layer, are summarized. We categorize the strategies into two: compositional engineering and size controlling of the emitting units. Compositional engineering, including A-, B-, and X-site doping and lead-free perovskites, is the simplest and most effective strategy to tune the bandgap (emitting color) of perovskites[
2. The strategies to prepare blue-emitting perovskites
To prepare the blue-emitting perovskites, researchers have proposed a mass of methods. Fundamentally, these methods could be categorized into two basic aspects. One is enlarging the energy bandgap by replacing the elements of perovskite crystals. Due to the excellent tolerance of perovskite crystals, lots of elements could be used to form the stable perovskite crystals[
2.1. Compositional engineering
Generally, according to the tight-binding approximation, the bandgap of crystals will get widened if the elements are replaced by the ones in the smaller period of the same family[
Figure 1.(Color online) Blue-emitting perovskites prepared by composition engineering. (a) Normalized absorbance and (b) photoluminescence of MAPb(Br1−
However, using the simple halogen doping method to prepare the Pero-LEDs, the problem of phase segregation will be inevitable when the devices operated at high voltage. Hence, researchers developed alternative methods by A- or B- site doping to control the bandgap of perovskites and stabilize the crystalline phases. Todorovic´ et al.[
Compared to the regulation of A- and X-site of perovskite crystals, B-site doping, and lead-free perovskites could own a stronger ability to adjust the bandgap and achieve deep-blue emission. Leng et al.[
According to the summaries in Table 1, it is obvious that few papers have reported high EQE by using the strategy of compositional engineering, even though the PL emission of perovskites could easily be adjusted by this method. In other words, it is hard to fabricate the spectrally stable and high efficiency deep blue-emitting Pero-LEDs only by compositional engineering.
2.2. Size control of the emitting units
As the particle size decreases to the nanoscale, the continuous energy levels near the Fermi energy level will be divided[
The 2D/quasi-2D perovskites could have a general formula of L2(ABX3)n–1BX4, where L is a monovalent long alkyl chain, and n is the number of stacking perovskite units. The bandgap of perovskites could be tuned by controlling the n values. Liang et al.[
Figure 2.(Color online) Blue-emitting perovskites prepared by forming the 2D and quasi-2D structure. (a) Crystal structure of 2-phenylethylammonium lead bromide, (PEA)2PbBr4, which is a 2D layered perovskite, and (b) the corresponding PL and EL peaks located at 407 and 410 nm, respectively. The weak EL peak at 375 nm is from TPBi, consistent with its PL (gray curve). Reproduced with permission from Ref. [
Although some progress has been made in phases stability and performance enhancement of Pero-LEDs based on quasi-2D perovskites, the efficient pure and deep blue-emitting devices are still difficult to fabricate owning to the poor electrical conductivity caused by the excess organic ligands adding. To overcome this obstacle, the as-synthesized QDs, NPs and ultra-thin quasi-2D perovskites are reported. With these strategies, the size of the perovskite emitting units could also be reduced to the nanoscale, and the bandgap will be enlarged due to the quantum confined effect. In 2015, Song et al.[
Figure 3.(Color online) Blue-emitting perovskites prepared by controlling the size of perovskite crystals. (a) Size-dependent PL spectra and photographs of monodisperse perovskite CsPbBr3 QDs. Reproduced with permission from Ref. [
However, there are still many problems for QDs, NPs, and ultra-thin quasi-2D perovskites, such as current leakage caused by the low coverage of the emitting layer. To fabricate efficient and color stable blue-emitting Pero-LEDs, we should combine the several methods of preparing blue-emitting perovskites. Firstly, the halogen doped method could be used in the synthesis of QDs and NPs to improve the ability of bandgap adjustment. Song et al.[
Figure 4.(Color online) Blue-emitting perovskites prepared by applying several methods simultaneously. (a) Composition-tunable PL spectra of perovskite CsPbX3 QDs by adding the different halides. Reproduced with permission from Ref. [
The summaries in Table 1 show that researchers have started to combine the strategies of compositional engineering and size controlling of emitting units. Pero-LEDs with high EQEs were fabricated based on the perovskites prepared by the comprehensive strategies. However, we believe that a more effective combination of the strategies could be carried out to fabricate the blue-emitting Pero-LEDs with spectrally stable emission and high EQE.
3. Challenges and future outlook
Although great progress has been achieved in the blue-emitting Pero-LEDs fabrication in the past few years, the high-efficiency deep-blue-emitting (440–470 nm) devices are still lacking in demonstrations. We list and draw an EQE evolution curve in Fig. 5 using the reported EQEs of blue-emitting Pero-LEDs, most EQEs of Pero-LEDs with the emission peak at less than 480 nm are no more than 3%, and that ones with the emission peak at less than 440 nm are no more than 1%. Besides, improving the EL spectra stability and operational stability of the blue-emitting Pero-LEDs are still big challenges. To overcome these obstacles, we need to utilize the two strategies (compositional engineering and size controlling of the emitting units) comprehensively and optimize the device structure of Pero-LEDs.
Figure 5.(Color online) The recorded EQEs of blue-emitting Pero-LEDs in recent years.
3.1. Developing spectrally stable deep-blue emission
Considering the side effects of using a single method to tune the emission of Pero-LEDs to deep-blue gamut, the strategies using several methods simultaneously were studied. Shang et al.[
3.2. Improving efficiency and operation stability
To fabricate the high-efficiency and operational stable blue-emitting Pero-LEDs, only improving the performances of perovskites is not enough. It is also critical to improve the charge injection ability and control the injection balance of electrons and holes by optimizing the structure of Pero-LEDs. Gangishetty et al.[
4. Conclusion
We have summarized the strategies of preparing blue-emitting perovskites and discussed their advantages and disadvantages. The compositional engineering strategies could precisely adjust the bandgap of perovskites. However, the drawbacks (e.g., phase segregation and poor quality of film morphology) could not be ignored. More environmentally stable blue-emitting perovskites could be obtained by size controlling of perovskite crystals. However, the size is hard to be controlled precisely and tending to be inhomogeneous. Hence, the strategies should be utilized comprehensively to improve the performances of blue-emitting perovskites. Moreover, we also discussed the challenges of fabricating high-efficiency stable blue-emitting Pero-LEDs. The most urgent challenge is increasing the EQE and keeping the deep blue emission at the same time. Researchers should then pay more attention to the spectral stability and the long operational lifetime of blue-emitting Pero-LEDs. We believe that the high-efficiency stable blue-emitting Pero-LEDs will be fabricated by improving the performances of perovskites and optimizing the architecture of Pero-LEDs.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos. 51802102, 21805101 and 51902110).
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