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对于应用于白光LED的无机钙钛矿而言,首先需要具备高的发光效率。然而,无机钙钛矿特别是量子点在制备过程中不可避免地会引入一定的缺陷,其中包括卤素空位、Pb空位和Pb间隙等。这些缺陷将会成为非辐射复合中心,使光生激子和光生载流子发生淬灭,从而极大地降低无机钙钛矿的发光效率[16]。除此之外,无机钙钛矿还容易受到水、热、光照和极性溶剂的影响,造成其稳定性降低,发光效率出现衰减。因此,需要通过包括包覆、表面配体交换、嵌入等在内的一系列策略减少其缺陷密度,同时优化表面形貌,达到提升无机钙钛矿发光性能和稳定性的作用[17-19]。
此外,为了满足白光发射光谱的需要,发光材料也必须具有较宽的发光光谱。但是由于无机钙钛矿的结构较为稳定,离子键结合能较强,因此其发光光谱较窄(半高宽一般仅有10~20 nm),无法满足宽光谱白光的需要。因此,还需要通过掺杂等方法在无机钙钛矿中引入新的宽发光峰,或者通过与其他的发光材料以一定比例混合形成多个不同峰位的发光峰,以获得宽光谱的白光发射。近年来一些典型的无机钙钛矿白光LED的优化工艺和器件参数如表1所示。
表 1 一些典型的无机钙钛矿白光LED的优化工艺和器件参数
Table 1. Optimized technologies and device parameters of typical inorganic perovskite white LEDs
Emitting materials CIE coordinates CCT
/KCRI Luminous efficiency
/lm·W−1Gamut
NTSCRef. Photoluminescent WLEDs based on inorganic lead halide perovskites CsPbBr3/CsPbBr1.2I1.8 (0.33, 0.30) 120% [20] CsPbBr3/CsPbBrxI3−x (0.31, 0.34) [21] CsPbBr3/red phosphors (0.334, 0.362) 5447 93.2 [22] CsPbBr3/red phosphors (0.33, 0.33) 5569 18.9 126% [23] CsPbBr3/red phosphors (0.32, 0.30) 98 130% [24] CsPbBr3 nanocrystal and nanosheet/CsPbBr1.5I1.5 (0.33, 0.34) 123% [25] CsPbBr3/CsPb(Br/I)3 (0.33, 0.33) 61.2 [26] CsPbBr3/Ag-In-Zn-S (0.404, 0.411) 3689 91 40.6 [27] CsPbBr3/red phosphors (0.351, 0.346) 4743 64 [28] CsPbBr3/CdSe (0.30, 0.32) 63 138% [29] CsPb(BrCl)3/CsPbBr3/CsPb(BrI)3 (0.31, 0.38) [30] CsPbBr3/CsPb(Br0.4,I0.6)3 (0.38, 0.37) 3876 114% [31] CsPbBr3/CsPb(Br0.4I0.6)3 (0.24, 0.28) 30 113% [32] Zn:CsPbCl3/CsPbBr3/CsPbI3 (0.321, 0.296) 6285 86.3 67.5 118% [33] Al:CsPbBr3/CsPbBr3/CdSe@ZnS (0.32, 0.34) 21.6 116% [34] Nd:CsPbBr3/CsPbBr3/CsPbI3 (0.34, 0.33) 5310 122% [35] Sn:CsPbBr3/CsPbBr3/Ag-In-Zn-S (0.41, 0.48) 3954 89 43.2 [36] Mn:CsPb(Br/Cl)3/CsPbBr3 3857 91 68.4 [37] Ce3+/Mn2+: CsPbClxBr3−x (0.32, 0.29) 89 51 [38] Photoluminescent WLEDs based on inorganic lead-free perovskites CsCu2Cl3/Cs3Cu2Cl5/red phosphors (0.37, 0.338) 5285 94 [39] Cs4MnBi2Cl12/green and blue phosphors (0.32, 0.30) [40] Pb: Cs3Cu2Br5 (0.333, 0.341) 5469 98 [41] Sb3+/Bi3+: Cs2NaInCl6 [42] Cs2(Ag0.6Na0.4)InCl6 (0.396, 0.448) 4054 [43] -
对于无机钙钛矿而言,可以通过在制备过程中改变工艺参数以控制其组分与形貌,从而得到不同发光峰位的无机钙钛矿。2016年,南京理工大学曾海波课题组[20]就利用常温法制备出不同卤素组分的CsPbX3纳米晶,随后他们将绿色的CsPbBr3和红色的CsPbBr1.2I1.8两种纳米晶滴在发光在460 nm的蓝光芯片上,制备了白光LED器件。通过调整红绿两种钙钛矿纳米晶的比例,所获得的白光色温可以在2500~11500 K范围内移动,即分别对应纯白光(色坐标在(0.33, 0.33)处的白光)和暖白光。随后,王恺等[21]将弱极性的低沸点低毒性溶剂加入到前驱体中,在常温下制备了不同卤素组分的CsPbX3。以异丙醇为代表的弱极性溶剂可以有效地促进前驱体的定向生长,最后可以得到CsPbX3纳米线。同时,与传统常温法制备的CsPbX3纳米晶相比,这种改进的方法可以显著地提高纳米晶的稳定性和发光效率,所得到的CsPbBr3与CsPbBrxI3−x在蓝光LED的激发下可以发射出明亮的白光,其色坐标为(0.31, 0.34)。
对于热注入法而言,加热反应的温度可以直接决定无机钙钛矿最终的尺寸和形貌[22]。在油酸作为表面配体辅助生长的过程中,当温度低于80 ℃时,所得到的CsPbBr3纳米晶尺寸仅有3.2 nm,并且其中混有部分中间产物。随着热注入温度的升高,纳米晶尺寸不断增大,并且形貌由低温下的球形变化到高温下的立方体,发光峰位也会随之发生移动,如图1(a)~(c)所示。将得到的绿光CsPbBr3纳米晶(荧光量子产率(PLQY)为72%)和红色荧光粉一起滴在蓝色芯片上就可以形成白光LED器件,其色坐标为(0.334, 0.362),显色指数(color rendering index,CRI)高达93.2, 色温(correlated color temperature,CCT)为5447 K。
图 1 (a) 80 ℃, (b) 120 ℃和(c) 140 ℃下热注入得到的CsPbBr3纳米晶的形貌[22];(d) CsPbX3纳米晶的PLQYs与ZnX2加入量的关系[24];基于CsPbBr3与CsPbBr1.5I1.5的(e) 白光LED光谱和(f) CIE色坐标和色三角[25]
Figure 1. Morphologies of CsPbBr3 NCs synthesized at (a) 80 ℃, (b) 120 ℃ and (c) 140 ℃[22]; (d) Variations of the PLQYs depending on the added amounts of ZnX2 of CsPbBr3 NCs[24]; (e) Spectra of white LED and (f) CIE color coordinates and color triangle based on CsPbBr3 and CsPbBr1.5I1.5[25]
对于无机钙钛矿特别是纳米晶而言,由于纳米晶的比表面也很大,同时纳米晶的表面存在着大量的缺陷,会极大地降低其发光效率,因此对纳米晶进行一定的表面修饰是提高其发光效率的一个有效的策略。在纳米晶的合成过程中,一般使用油酸和油胺作为其表面配体,但是油酸和油胺在无机钙钛矿纳米晶表面的结合能很低,很容易在纳米晶生长和后期处理的过程中从表面脱离,造成的结果是表面缺陷增多,同时纳米晶极易团聚。针对这一问题,研究人员提出对纳米晶表面进行配体优化,旨在减少纳米晶表面缺陷以降低团聚。臧志刚等[44]利用含有两个支链的己基癸酸代替传统的油酸,将其加入热注入的前驱体溶液中,制备了表面为己基癸酸的CsPbBr3纳米晶。与油酸作为表面配体的CsPbBr3纳米晶相比,含有新配体的纳米晶其PLQY和稳定性均有所提高。褚君浩等[23]在常温法中采用4,4'-偶氮双(4-氰基戊酸)作为表面配体,也制备出了PLQY高达72%的CsPbBr3纳米晶。将其与红色荧光粉一起滴在蓝色芯片上,就可以制备出白光LED,其色坐标为(0.33, 0.33),CCT为5569 K,流明效率为18.9 lm/W。
除了表面配体优化之外,针对纳米晶表面的缺陷类型,研究人员进一步提出了利用卤离子化合物直接钝化其表面的卤离子空位缺陷,达到减少表面缺陷密度的作用。夏志国等[24]利用后处理的方式,直接将溶于正己烷中的ZnX2溶液加入热注入法合成的相应卤素的CsPbX3纳米晶中。如图1(d)所示,经过一定时间的常温搅拌之后,CsPbX3的PLQY上升,并且会随着ZnX2溶液加入量的增加而继续上升,说明纳米晶表面与卤离子相关的缺陷受到了钝化,这种后处理的方法可以在常温和较短时间内实现对纳米晶发光效率的提升。唐志康等[25]进一步采用KBr作为表面钝化剂作用于CsPbBr3纳米晶中,同时起到了钝化纳米晶表面Cs空位和Br空位的效果,实现了荧光效率和稳定性的大幅度提升。他们将得到的CsPbBr3纳米晶与红光CsPbBr1.5I1.5按照一定的比例混合,再使用蓝光芯片进行激发,制备了白光LED器件,如图1(e)所示。所得到的白光色坐标为(0.33, 0.34),并且其色域是NTSC(National Television System Committee)标准的123%,是Rec. 2020标准的92%,显示出较宽的色域优势,如图1(f)所示。
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对于商用的白光LED而言,发光材料的稳定性是决定白光器件使用寿命的关键。虽然无机钙钛矿相比有机杂化钙钛矿的稳定性高,但是其仍然易受到水、热、紫外光和极性溶剂的影响而出现分解,造成发光性能衰减。包覆是一种常见的提高材料稳定性的方法,一般采用稳定性较好的材料进行包覆,从而起到保护其内部来达到提高稳定性的效果。
无机氧化物作为一种十分稳定的材料,非常适合于作为包覆层以稳定其中的无机钙钛矿。W. Yu等[26]在2016年就提出了利用(3-苯胺基)-三乙基硅烷作为包覆前驱体,在常温下水解形成SiO2以包覆CsPbX3纳米晶。他们分别将SiO2包覆的绿色CsPbBr3和红色CsPb(Br/I)3混合滴在蓝光芯片上形成白光LED。器件的色坐标在(0.33, 0.33),流明效率为61.2 lm/W,器件在连续工作10 h之后发光未出现较大的衰减,说明通过包覆起到了提升器件发光性能和稳定性的作用。臧志刚等[27]将(3-苯胺基)-三乙基硅烷直接加入CsPbBr3纳米晶的前驱体中,在常温下合成纳米晶的同时,利用包覆剂的快速水解SiO2层完成了对纳米晶的包覆。由于包覆剂水解和常温纳米晶形核长大的时间基本一致,因此所得到的是单包覆的纳米晶,即一个SiO2层中只包覆一个纳米晶,如图2(a)所示。SiO2层的引入可以在一定程度上起到钝化纳米晶表面缺陷的作用,因此随着所加入的包覆剂量的增加,纳米晶表面的SiO2层的厚度不断增加,其PLQY也随之不断上升。但是当过量的包覆剂水解后,其PLQY会出现下降。除此之外,在包覆后的纳米晶中加入极性溶剂乙醇30 min后,纳米晶的发光仍然可以保持在初始值的80%以上,而没有包覆的纳米晶的发光则基本完全淬灭,这说明了SiO2包覆对纳米晶稳定性的显著提升作用。包覆后的CsPbBr3纳米晶与红色的Ag-In-Zn-S纳米晶在蓝色芯片激发下发射出明亮的暖白光,器件的CCT为3689 K,CRI为91,流明效率为40.6 lm/W,显示出了较好的照明特性。另外,他们还在CsPbBr3纳米晶表面包覆了ZrO2,包覆后的纳米晶的PLQY和稳定性也得到了一定程度的提高[28]。他们将ZrO2包覆的CsPbBr3纳米晶与红色荧光粉分别沉积在蓝色芯片上,制备了白光LED,其流明效率进一步增加到了64 lm/W。
图 2 (a) SiO2包覆CsPbBr3纳米晶的TEM图[27];(b) Janus结构纳米晶SiO2/CsPbBr3 TEM图;(c) 生长Janus结构纳米晶CsPbBr3/SiO2 示意图[29];(d) 利用两步法生长CsPbX3-zeolite-Y示意图;基于(e) CsPb(Br,I)3钙钛矿量子点和(f) CsPb(Br,I)3-zeolite-Y的白光LED随着驱动电流增加的光谱变化[31] ;(g) 基于多孔SiO2的CsPbX3纳米晶的白光LEDCIE色坐标和色三角[32]
Figure 2. (a) TEM image of SiO2-coating CsPbBr3 NCs[27]; (b) TEM image of SiO2/CsPbBr3 NCs with Janus structure; (c) Schematic of the whole formation process of CsPbBr3/SiO2 NCs with Janus structure[29]; (d) Schematic of the two-step synthesis of CsPbX3-zeolite-Y composites; Spectra change of white LEDs composed of (e) CsPb(Br,I)3 perovskite QDs and (f) CsPb(Br,I)3-zeolite-Y composites with increase of currents[31]; (g) CIE color coordinate and color triangle of white LEDs based on mesoporous silica CsPbBr3 NCs[32]
除了完全包覆在纳米晶表面形成核壳结构外,无机氧化物包覆纳米晶的部分表面后,同样也可以起到类似的作用。张桥等[29]利用Cs4PbBr6在水中转化为CsPbBr3的特点,在Cs4PbBr6的正己烷分散液中,加入作为包覆前驱体的四甲基硅烷和水。水的加入一方面导致Cs4PbBr6转化为CsPbBr3,另一方面引起了四甲基硅烷分解为SiO2,并在水与正己烷的界面处形成了包覆纳米晶一个面的Janus结构,如图2(b)所示。此外,也可以通过同样的方法在纳米晶表面形成CsPbBr3/Ta2O5 Janus结构,如图2(c)所示。与无机氧化物形成的Janus结构可以有效地提高CsPbBr3纳米晶的发光稳定性,由CsPbBr3/ SiO2纳米晶作为绿光成分并结合红光的CdSe纳米晶,在蓝色芯片的激发下可以发射出明亮的白光,器件的色坐标为(0.30, 0.33),CRI为63。
同样,有机聚合物也可以作为无机钙钛矿纳米晶的包覆剂,并且其本身带有的特殊官能团能够与纳米晶紧密结合,有利于促进包覆的进行。L. Manna等[30]利用双亲聚苯乙烯-聚丙烯酸聚合物作为包覆剂,将其直接加入至纳米晶前驱体溶液中。在纳米晶生长的过程中聚合物中的聚丙烯酸一端接在纳米晶表面,而另一端聚苯乙烯则成为支链。在加入甲醇之后,聚苯乙烯由于溶解度降低而逐渐发生聚集,从而形成一层较为致密的聚合物包覆壳层。经过这种方法包覆后的CsPb(BrCl)3、CsPbBr3和CsPb(BrI)3纳米晶在一定比例混合后可以制备白光LED,其色坐标为(0.31, 0.38),在四个周老化测试后白光性能几乎没有出现衰减,说明有机聚合物包覆对器件稳定性具有较为明显的提升作用。
另外一种包覆的方式是利用多孔或框架材料内部存在空间和活性位点的特点,将无机钙钛矿嵌入其中而实现对纳米晶的保护。华南理工大学研究人员[31]在含有Na+离子的多孔状沸石中加入CsBr并在60 ℃下进行搅拌,其中的Na+就会被Cs+取代。如图2(d)所示,此时沸石将与PbX2在130~170 ℃下继续反应,生成嵌入在沸石中的CsPbX3纳米晶。通过改变加入的PbX2的卤素组分可以得到不同发光的无机钙钛矿固体。他们将嵌入沸石中的绿色CsPbBr3与红色CsPb(Br0.4,I0.6)3共同滴在蓝色芯片上,制备了白光LED,器件的色坐标为(0.38, 0.37)。如图2(e)和(f)所示,其中作为红光成分的CsPb(Br0.4I0.6)3纳米晶在大电流下下降至初始值的82%,而嵌入沸石中的CsPb(Br0.4I0.6)3在同样的器件电流下则下降至67%,显示出与沸石形成的复合结构对器件稳定性的提升。
刘如熹等[32]在含有CsPbX3纳米晶的正己烷溶液中加入孔径为12~14 nm的多孔二氧化硅,从而使纳米晶可以进入其多孔位置,形成了类似包覆的结构,提高纳米晶的稳定性。他们将不同卤素组分的红光和绿光纳米晶混合后滴在蓝光芯片上,制备了白光LED,白光对应的色坐标在(0.24, 0.28),流明效率为30 lm/W。与传统的基于荧光粉和Cd基纳米晶的两种白光LED相比,基于无机钙钛矿纳米晶的白光LED色域分别是NTSC和Rec 2020标准的113%和85%,如图2(g)所示,显示出较宽的色域和优异的照明能力。
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作为一种典型的半导体材料,无机铅卤钙钛矿的性能可以通过掺杂这种重要的方法来进行调控。具体而言,对无机铅卤钙钛矿的掺杂主要目的是提高其发光效率,以及通过掺杂引入新的发光峰。对用于白光LED的无机铅卤钙钛矿而言,通过掺杂来对其进行性能调控也是提高白光器件性能的一个重要的策略。
N. Lee等[33] 在热注入制备CsPbCl3纳米晶过程中加入了ZnCl2作为掺杂前驱体,在210 ℃下实现了Zn的掺杂。掺杂进入晶格的Zn2+会替代部分的Pb2+,而由于Zn-Cl键的键长短于Pb-Cl键,导致晶格形成能增加。所以,Zn掺杂后虽然CsPbCl3纳米晶的PL峰没有出现明显的移动,但是其PLQY却会增加至85%以上。这样高发光效率的Zn: CsPbCl3纳米晶可以作为白光LED的蓝光成分,并与绿光CsPbBr3和红光CsPbI3共同组成白光LED的发光层。在紫外芯片的激发下,他们所制备的白光LED的色坐标位于(0.321, 0.296),CCT为6285 K,CRI为86.3,而流明效率可以高达67.5 lm/W,显示出良好的亮度性能。
相比之下,大多数掺杂离子进入铅卤钙钛矿中后,由于其离子半径和价态与Pb2+不同,会引起晶格出现变形,从而改变铅卤钙钛矿的发光峰位。研究人员[34]在热注入前驱体中加入了AlBr3作为Al3+的掺杂剂,在150 ℃下实现了Al3+掺杂CsPbX3纳米晶。研究发现,Al2Br6二聚体中的Al-Br键的键长与Pb-Br键长十分接近,因此有利于Al3+以二聚体的形式代替原来的Pb-Br八面体,实现Al3+的掺杂,如图3(a)所示。Al3+的s轨道会与Br的p轨道以及Pb的p轨道杂化而形成一个新的能级,从而导致发光峰位出现蓝移。B. Monserrat等[35]将Nd3+掺入到CsPbBr3纳米晶中,由于Nd3+的掺入使得禁带发生展宽,所以纳米晶的发光峰随着Nd3+掺入量的增加而不断蓝移。他们制备的白光LED以Nd3+掺杂的CsPbBr3、无掺杂的CsPbBr3和CsPbBr1.2I1.8分别作为蓝、绿和红光成分,得到的白光色坐标为(0.34, 0.33),CCT为5310 K。
图 3 (a) Al3+以二聚体形式完成掺杂的过程[34];随着Sn2+比例的增加,纳米晶的(b) PL光谱和(c) PLQY的变化[36];在基于Mn2+掺杂CsPb(Br/Cl)3的白光LED在不同的驱动电流下(d)发光光谱和(e)CIE色坐标的变化[37]
Figure 3. (a) Schematics showing the Al3+ doping in dimer form[34]; Variety of (b) PL spectra and (c) PLQY of NCs with increase of doping ratio of Sn2+[36]; Evolution of (d) EL spectra and (e) CIE color coordinates of white LEDs based on Mn2+-doping CsPb(Br/Cl)3[37]
臧志刚等[36]在常温下将不同比例的Sn2+掺入CsPbBr3纳米晶中,他们发现随着Sn2+比例的不断增加,CsPbBr3纳米晶会首先出现发光峰的蓝移,同时PLQY也会上升。当Sn2+的比例超过20%之后,随后继续增加Sn2+,纳米晶发光峰位开始逐渐红移,并且PLQY不断下降,如图3(b)和(c)所示。除了光学性能的改善之外,20%Sn2+掺入后还会引起纳米晶热稳定性的上升,这都与Sn2+对纳米晶表面的钝化作用有关。他们随后将20%Sn2+掺杂的CsPbBr3纳米晶和红色Ag-In-Zn-S纳米晶一起滴在蓝色芯片上形成了白光LED,其色坐标位于(0.41, 0.48),CCT为3954 K,而CRI为89,流明效率为43.2 lm/W。唐孝生等[37]利用常温法在CsPb(Br/Cl)3纳米晶中掺杂Mn2+,由于Mn2+在纳米晶中引入了新的能级,因此掺杂后出现了发光峰在607 nm的宽的黄光发射。他们将掺杂后的CsPb(Br/Cl)3纳米晶与CsPbBr3纳米晶混合后滴在蓝光芯片上,制备了白光LED。如图3(d)所示,随着器件驱动电流的增加,白光光谱的强度也出现上升,并且对应于红、绿和蓝(由CsPb(Br/Cl)3纳米晶本征发光贡献)三色的发光峰也同时上升。但是,由于电流的增加加剧了量子点的热淬灭,因此白光器件的CCT逐渐向蓝光区域移动,如图3(e)所示。
宋宏伟等提出利用共掺杂的策略在无机铅卤钙钛矿中掺杂两种不同种类的离子,实现对其多重性能调控的效果。他们[45]首先研究证明当掺杂8.7%的Bi3+和2.5%的Mn2+之后,可以得到发光性能较好的白光LED,而调整其中的Bi3+/Mn2+的比例则能够有针对性地调节器件白光的色温(2750~19000 K内变化)。随后,他们将Bi3+、Mn2+、Eu3+、Sm3+和Ce3+离子分别组合对CsPbClxBr3-x纳米晶进行共掺杂[38]。其中,Ce3+/Mn2+共掺杂时所得到的纳米晶PLQY最高, Ce3+中的4 f-5 d跃迁能级会引起纳米晶中出现434 nm的蓝色发光峰,而Mn2+中的4T1-6A1能级跃迁则会引入一个位于592 nm的宽的发光峰。另外,在共掺杂中当固定Ce3+的浓度后,随着掺入的Mn2+浓度的增加,Mn2+对应的592 nm处的发光强度将不断增强,最后过量的Mn2+掺入会引起发光出现略微的下降。而相比之下,Ce3+对应的发光峰强度则会由于出现能量转移而随着Mn2+浓度的增加不断下降。他们利用共掺杂后纳米晶中出现多个发光峰的特点,直接用蓝色芯片激发掺杂2.9% Ce3+和9.1% Mn2+的CsPbCl1.8Br1.2,制备了色坐标位于(0.33, 0.29),CRI为89,而流明效率为51 lm/W的白光LED。白光器件在日常环境下20 h后,发光光谱未发生较大的变化,说明白光器件具有较好的稳定性。
Research progress in inorganic perovskites white LEDs and visible light communication (Invited)
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摘要: 在现代社会中,白光发光二极管(LED)在照明和显示背板等诸多领域都有着重要的基础性作用。为了获得具有优异性能的白光LED,首先需要获得满足白光LED发光需要的高性能的发光材料。而作为一类新兴的半导体材料,无机钙钛矿(CsPbX3, X = Cl, Br, I)由于其具有的高发光量子产率、发光波长可调、色纯度高和稳定性好等优点,在发光应用特别是白光LED领域展现出了巨大的潜力。文中将首先分别从光致白光LED和电致白光LED两个方面出发,综述近期在基于无机钙钛矿的白光LED方面所取得的研究进展,随后分别介绍以上两个体系中改性后的无机钙钛矿发光材料与其他发光材料复合形成白光以及无机钙钛矿单组分白光的代表性成果。最后,对钙钛矿白光LED在可见光通信方面所取得的最新进展进行介绍,并且对白光LED以及可见光通信的研究发展趋势与挑战进行了总结和展望。Abstract: In the modern world, the white light-emitting diodes (LEDs) play an important and basic role on various fields, including illumination and display back board. To obtain excellent performance white LEDs, the emissive materials needed to be studied. As a kind of promising semiconductor, inorganic perovskites (CsPbX3, X=Cl, Br, I) exhibit prominent potentials in the white LEDs, due to their high photoluminescence quantum yields, tunable emission wavelength, high color purity and excellent stability. In this review, the research progresses in the electroluminescent and photoluminescent white LEDs based on inorganic perovskites were introduced. Then, the represent achievements on the white LEDs of the mix of improved inorganic perovskites and other emissive materials, as well as the single components of inorganic perovskites, could be exhibited. In the end, the employment of the white LEDs on visible light communication was highlighted. In addition, the development trends of the white LEDs and visible light communication were summarized and prospected.
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图 1 (a) 80 ℃, (b) 120 ℃和(c) 140 ℃下热注入得到的CsPbBr3纳米晶的形貌[22];(d) CsPbX3纳米晶的PLQYs与ZnX2加入量的关系[24];基于CsPbBr3与CsPbBr1.5I1.5的(e) 白光LED光谱和(f) CIE色坐标和色三角[25]
Figure 1. Morphologies of CsPbBr3 NCs synthesized at (a) 80 ℃, (b) 120 ℃ and (c) 140 ℃[22]; (d) Variations of the PLQYs depending on the added amounts of ZnX2 of CsPbBr3 NCs[24]; (e) Spectra of white LED and (f) CIE color coordinates and color triangle based on CsPbBr3 and CsPbBr1.5I1.5[25]
图 2 (a) SiO2包覆CsPbBr3纳米晶的TEM图[27];(b) Janus结构纳米晶SiO2/CsPbBr3 TEM图;(c) 生长Janus结构纳米晶CsPbBr3/SiO2 示意图[29];(d) 利用两步法生长CsPbX3-zeolite-Y示意图;基于(e) CsPb(Br,I)3钙钛矿量子点和(f) CsPb(Br,I)3-zeolite-Y的白光LED随着驱动电流增加的光谱变化[31] ;(g) 基于多孔SiO2的CsPbX3纳米晶的白光LEDCIE色坐标和色三角[32]
Figure 2. (a) TEM image of SiO2-coating CsPbBr3 NCs[27]; (b) TEM image of SiO2/CsPbBr3 NCs with Janus structure; (c) Schematic of the whole formation process of CsPbBr3/SiO2 NCs with Janus structure[29]; (d) Schematic of the two-step synthesis of CsPbX3-zeolite-Y composites; Spectra change of white LEDs composed of (e) CsPb(Br,I)3 perovskite QDs and (f) CsPb(Br,I)3-zeolite-Y composites with increase of currents[31]; (g) CIE color coordinate and color triangle of white LEDs based on mesoporous silica CsPbBr3 NCs[32]
图 3 (a) Al3+以二聚体形式完成掺杂的过程[34];随着Sn2+比例的增加,纳米晶的(b) PL光谱和(c) PLQY的变化[36];在基于Mn2+掺杂CsPb(Br/Cl)3的白光LED在不同的驱动电流下(d)发光光谱和(e)CIE色坐标的变化[37]
Figure 3. (a) Schematics showing the Al3+ doping in dimer form[34]; Variety of (b) PL spectra and (c) PLQY of NCs with increase of doping ratio of Sn2+[36]; Evolution of (d) EL spectra and (e) CIE color coordinates of white LEDs based on Mn2+-doping CsPb(Br/Cl)3[37]
图 4 Cs3Cu2I5的(a)晶体结构,(b)PL和PLE光谱以及(c)光激发态重组后的构型坐标示意图[46];(d)不同温度合成的Cs3Cu2Cl5和CsCu2Cl3的PL光谱[39];(e) Cs4MnBi2Cl12在不同激发功率下的PL光谱变化;(f)基于Cs4MnBi2Cl12的白光LED光谱[40]
Figure 4. (a) Crystal structure, (b) PL and PLE spectra, (c) schematic configuration coordinate for the excited-state reorganization of Cs3Cu2I5[46]; (d) PL spectra of Cs3Cu2Cl5 and CsCu2Cl3 synthesized at different temperatures[39]; (e) Power-dependent PL spectra of Cs4MnBi2Cl12; (f) Spectra of white LEDs based on Cs4MnBi2Cl12[40]
图 5 Pb2+掺杂Cs3Cu2Br5的(a)晶体结构和(b)在310 nm下激发的PL光谱图[41];(c) Sb3+/Bi3+共掺Cs2NaInCl6的白光PL光谱[42];(d) Cs2AgInCl6和Cs2Ag0.6Na0.4InCl6的光学吸收和PL光谱图[43];(e) Cs2AgIn0.6Bi0.15La0.25Cl6和 Cs2AgIn0.8Bi0.2Cl6的PL光谱[47];(f)在Cs2AgBi0.01In0.99Cl6引入Li+与K+合金后的CIE色坐标[48]
Figure 5. (a) Crystal structure and (b) PL spectra excited under 310 nm of Pb2+-doped Cs3Cu2Br5[41]; (c) White-light PL spectra in Sb3+/Bi3+-codoped Cs2NaInCl6[42]; (d) Optical absorption and PL spectra of Cs2AgInCl6 and Cs2Ag0.6Na0.4InCl6[43]; (e) PL spectra of Cs2AgIn0.6B0.15La0.25Cl6 and Cs2AgIn0.8Bi0.2Cl6[47]; (f) CIE color coordinates of Li+/K+ alloyed Cs2AgBi0.01In0.99Cl6[48]
图 6 以CsPbBrxCl3−x混合MEH: PPV为发光层的白光LED的(a)能带结构图和(b)EL光谱与混合比例的关系[50];α/δ-CsPbI3混合相白光LED的(c)器件结构、(d)EL光谱、(e)电流-电压和光亮度-电压的曲线以及(f)外量子效率-电流和电功率-电流曲线[53]
Figure 6. (a) Schematic of band structure and (b) variety of EL spectra with the changes of blend ratio of white LED in CsPbBrxCl3−x/MEH:PPV as luminous layer[50]; (c) Device structure, (d) EL spectrum, (e) current density-voltage and luminance-voltage curves, (f) EQE-current density and current efficiency-current density curves of white LEDs mixed with α/δ-CsPbI3 emitting layers[53]
图 7 (a) CsCu2I3@Cs3Cu2I5薄膜的制备方法示意图;(b) 不同比例前驱体制备得到CsCu2I3@Cs3Cu2I5薄膜的吸收和PL光谱[54];(c)未加“吐温”和(d)加了“吐温”的CsCu2I3/Cs3Cu2I5薄膜的时间分辨掠入射宽角X射线散射图;经过“吐温”处理后的CsCu2I3/Cs3Cu2I5白光LED的(e)不同电压下的器件EL光谱图和(f)外量子效率-电流密度曲线[55]
Figure 7. (a) Schematic diagram of the preparation process of CsCu2I3@Cs3Cu2I5 composites; (b) Absorption and PL spectra of the CsCu2I3@Cs3Cu2I5 composites with varied CsI/CuI molar ratios precursor[54]; Time-resolved GIWAXS profiles of CsCu2I3/Cs3Cu2I5 composites (c) without Tween and (d) with Tween; (e) EL spectra of the device under different voltages and (f) curve of EQE versus current density[55] of white LED after CsCu2I3/Cs3Cu2I5 processed by Tween
图 8 (a) 在不同的传输速率下的比特误码率[56];(b) 典型的可见光通信测试系统示意图[58];基于ZrO2/CsPbBr3白光LED的可见光通信中不同电频率下(c)电-光-电频率响应和(d)接受信号的信噪比[28];(e) 在不同的驱动电流下白光LED的频率响应[59];(f) 基于Cs3Cu2Cl5纳米晶白光LED的可见光通信星座图[60]
Figure 8. (a) Bit-error rates(BERs) at different data rates[56]; (b) Typical schematic diagrams of VLC test system[58]; (c) Electrical-optical-electrical frequency response and (d) received signal-to-noise ratio of white LEDs signal in VLC system based on ZrO2/CsPbBr3[28]; (e) Frequency response of white LEDs at different current densities[59]; (f) Constellation diagrams for white LEDs in VCL based on Cs3Cu2Cl5 NCs[60]
表 1 一些典型的无机钙钛矿白光LED的优化工艺和器件参数
Table 1. Optimized technologies and device parameters of typical inorganic perovskite white LEDs
Emitting materials CIE coordinates CCT
/KCRI Luminous efficiency
/lm·W−1Gamut
NTSCRef. Photoluminescent WLEDs based on inorganic lead halide perovskites CsPbBr3/CsPbBr1.2I1.8 (0.33, 0.30) 120% [20] CsPbBr3/CsPbBrxI3−x (0.31, 0.34) [21] CsPbBr3/red phosphors (0.334, 0.362) 5447 93.2 [22] CsPbBr3/red phosphors (0.33, 0.33) 5569 18.9 126% [23] CsPbBr3/red phosphors (0.32, 0.30) 98 130% [24] CsPbBr3 nanocrystal and nanosheet/CsPbBr1.5I1.5 (0.33, 0.34) 123% [25] CsPbBr3/CsPb(Br/I)3 (0.33, 0.33) 61.2 [26] CsPbBr3/Ag-In-Zn-S (0.404, 0.411) 3689 91 40.6 [27] CsPbBr3/red phosphors (0.351, 0.346) 4743 64 [28] CsPbBr3/CdSe (0.30, 0.32) 63 138% [29] CsPb(BrCl)3/CsPbBr3/CsPb(BrI)3 (0.31, 0.38) [30] CsPbBr3/CsPb(Br0.4,I0.6)3 (0.38, 0.37) 3876 114% [31] CsPbBr3/CsPb(Br0.4I0.6)3 (0.24, 0.28) 30 113% [32] Zn:CsPbCl3/CsPbBr3/CsPbI3 (0.321, 0.296) 6285 86.3 67.5 118% [33] Al:CsPbBr3/CsPbBr3/CdSe@ZnS (0.32, 0.34) 21.6 116% [34] Nd:CsPbBr3/CsPbBr3/CsPbI3 (0.34, 0.33) 5310 122% [35] Sn:CsPbBr3/CsPbBr3/Ag-In-Zn-S (0.41, 0.48) 3954 89 43.2 [36] Mn:CsPb(Br/Cl)3/CsPbBr3 3857 91 68.4 [37] Ce3+/Mn2+: CsPbClxBr3−x (0.32, 0.29) 89 51 [38] Photoluminescent WLEDs based on inorganic lead-free perovskites CsCu2Cl3/Cs3Cu2Cl5/red phosphors (0.37, 0.338) 5285 94 [39] Cs4MnBi2Cl12/green and blue phosphors (0.32, 0.30) [40] Pb: Cs3Cu2Br5 (0.333, 0.341) 5469 98 [41] Sb3+/Bi3+: Cs2NaInCl6 [42] Cs2(Ag0.6Na0.4)InCl6 (0.396, 0.448) 4054 [43] -
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