Fig. 1. (Color online) Blue-emitting perovskites prepared by composition engineering. (a) Normalized absorbance and (b) photoluminescence of MAPb(Br_1−xCl_x)₃ (0 ≤ x ≤ 1). Reproduced with permission from Ref. [11]. Copyright 2015, American Chemical Society. (c) The curves of electroluminescence of Pero-LEDs based on MAPb(Br_1−xCl_x)₃ (0 ≤ x ≤ 1). Reproduced with permission from Ref. [12]. Copyright 2015, American Chemical Society. (d) UV–vis absorption and steady-state PL spectra of PEA₂(Rb_xCs_1−x)₂Pb₃Br₁₀ (0 ≤ x ≤ 1) perovskites. (e) The PL spectra evolution of PEA₂(Rb_0.6Cs_0.4)₂Pb₃Br₁₀ perovskites after continuous thermal treatment (100 °C) for different times. (f) The EL spectra of Pero-LEDs based on PEA₂(Rb_0.6Cs_0.4)₂Pb₃Br₁₀ perovskites at different voltage bias. Reproduced with permission from Ref. [26]. Copyright 2019 Springer Nature.

Fig. 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. [15]. Copyright 2016, American Chemical Society. (c) The EL spectra of Pero-LEDs based on the quasi-2D perovskites of (EA)₂MA_n−1Pb_nBr_3n+1 (MA : EA = 1 : 0, 1 : 1, and 1 : 1.3 respectively). Reproduced with permission from Ref. [37]. Copyright 2017, American Chemical Society. (d) Schematic of charge carrier cascade in the quasi-2D perovskite of PA₂(CsPbBr₃)_n−1PbBr₄ MQWs, and (e) the EL spectra of corresponding Pero-LEDs under different voltage bias. Reproduced with permission from Ref. [29]. Copyright 2018, Elsevier Ltd. (f) The stable EL spectra of Pero-LED based on quasi-2D perovskite of PEA₂A_n−1Pb_nBr_3n+1 under different voltage bias. Reproduced with permission from Ref. [30]. Copyright 2018 Springer Nature. (g) The diagram of carriers transfer between perovskite quantum wells (2D) and bulk perovskite part (3D), the Cs₄PbBr₆ facilitate carriers centralization. (h) The stability test under 10 mA/cm² of the device with different amounts of Cs₄PbBr₆ additive and the traditional MAPbBr₃ devices, and the EL spectra curves of 0 and 12 h are almost completely coincident. Reproduced with permission from Ref. [31]. Copyright 2018, WILEY-VCH.

Fig. 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. [13]. Copyright 2015, WILEY-VCH. (b) Photographs of CsPbBr₃ NPs dispersion obtained at different temperatures and corresponding UV–vis absorption and PL emission spectra. Reproduced with permission from Ref. [32]. Copyright 2018, Elsevier Ltd. (c) PL (solid lines) and absorption (dashed lines) spectra of CsPbBr₃ NPs colloids for varying NPs thickness. Reproduced with permission from Ref. [14]. Copyright 2018, American Chemical Society. (d) The STEM-HAADF image of a cross-sectional Pero-LEDs based on the ultra-thin perovskite of PBABr_y(Cs_0.7FA_0.3PbBr₃). (e) The corresponding EQE and (f) EL spectra with the operation voltage increasing. Reproduced with permission from Ref. [17]. Copyright 2019, Springer Nature.

Fig. 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. [13]. Copyright 2015, WILEY-VCH. The EL spectra of Pero-LEDs based on the perovskites of (b) (Rb_0.33Cs_0.67)_0.42FA_0.58PbBr₃ and (c) (Rb_0.33Cs_0.67)_0.42FA_0.58PbBr_1.75Cl_1.25. Reproduced with permission from Ref. [33]. Copyright 2019, The Royal Society of Chemistry. (d) The luminance-bias and (e) EQE-current density curves of CsPbBr₃ : PEACl (1 : 1) devices with different ratios of YCl₃. And (f) the EL spectrum stability test of a Pero-LED based on CsPbBr₃ : PEACl : 2%YCl₃ with continuous bias of 3.2 V for 120 min. Reproduced with permission from Ref. [9]. Copyright 2019 Springer Nature.

Fig. 5. (Color online) The recorded EQEs of blue-emitting Pero-LEDs in recent years.

Table 1. Performance summary of blue-emitting perovskites and the corresponding Pero-LEDs.