Quantum confinement of 2D perovskite nanoplatelets for light emission

“钙钛矿”一词是指一类具有共同晶体结构和通式ABX3的化合物。典型的金属卤化物钙钛矿（MHP）在a位具有一价阳离子，在B位具有二价金属阳离子，在X位具有卤化物阴离子。金属卤化物钙钛矿的晶体结构为BX64-八面体，其中B元素位于中心，卤化物阴离子占据八面体的位置。a位阳离子被困在八面体之间，这对于钙钛矿晶体结构的形成很重要。MHPs由于其优异的光电性能，在光电应用领域引起了广泛的研究兴趣。其中一种主要的MHP材料是卤化铅钙钛矿，即铅（Pb）作为B阳离子。由于其带隙可调、生产方便、缺陷容忍度高，在光伏和光发射应用中具有很大的潜力。基于mhp的光电子器件具有良好的性能，如发光二极管（led）的外量子效率（EQE）为20.3%，单结太阳能电池的记录功率转换效率（PCE）为25.2%。虽然大块钙钛矿薄膜已经显示出巨大的潜力，纳米级钙钛矿材料往往表现出其他有趣的性质。为了探索钙钛矿材料的更多潜在应用，人们对纳米级钙钛矿的形貌进行了广泛的研究。图4显示了钙钛矿纳米材料已被合成为纳米立方（NCs），纳米血小板（NPLs），纳米棒（NRs），以及量子点（QDs）根据其大小和形状分类。在纳米级半导体材料中，量子约束通过精细控制带隙对光电性能起着决定性作用。在经典量子阱模型中，半导体形成“阱”，外部环境充当“壁”。因此，能级上升到一个更高的水平。因此，在相同的组合物中，钙钛矿发光体可以通过纳米尺度的尺寸调整提供不同的发射波长，为目标发射波长提供了可选的选择。钙钛矿纳米颗粒的典型边长为8-15 nm，呈立方形状，是钙钛矿材料中研究最多的一种形式。CsPbX3 nc提供尺寸和成分调谐，涵盖400-710 nm的发射波长。计算出CsPbBr3材料的激子玻尔半径为3.5 nm。钙钛矿纳米片（NPLs）由几层钙钛矿晶格组成，这些NPLs表现出很强的量子约束效应，相对于大块钙钛矿，在高达0.6 eV （~100 nm）的发射波长上有显著的蓝移。二维结构可以用它们自己的分子组成来表示，例如单层的Cs2PbBr4和双层的CsPb2Br5。二维受限结构增加了激子结合能，并通过厚度控制实现了发射调谐。由于接触面积大，堆积不良贷款之间的电荷转移增强。由于配体覆盖率大，不良贷款被认为比nc对环境湿度更稳定。然而，不良贷款在溶液中往往不太稳定，因为它们容易聚集，其独特的性质受到损害。与纯绿色和红色LED相比，蓝色发光存在寿命短、发光量子产率低、效率低等问题，是LED照明和显示行业面临的一个具有挑战性的课题。蓝色LED是照明和显示应用的重要组成部分，因为三基色以最通用的方式组成白光。

The term "perovskite" refers to a family of compounds with a common crystal structure and the general formula ABX3. Typical metal halide perovskites (MHP) have a monovalent cation at the A site, divalent metal cation at the B site, and halide anion at the X site. The crystal structure of metal halide perovskite is characterised by the BX64- octahedra, where the B element sits at the centre and halide anions occupy the octahedral sites. The A-site cation is caged in between the octahedra, which is important for enabling the perovskite crystal structure to form. MHPs have attracted significant research interest for optoelectronic applications due to their outstanding optoelectronic properties. One of the main MHP materials is lead halide perovskite, i.e., lead (Pb) as B cation. They have great potential in photovoltaic and light emission applications due to a tuneable band gap, facile production, and high defect tolerance. MHP-based optoelectronic devices have promising performances, for example, external quantum efficiency (EQE) of 20.3% in light-emitting diodes (LEDs) and record power conversion efficiency (PCE) of 25.2% in a single junction solar cell.While bulk perovskite films have been showing great potential, nanoscale perovskite materials often exhibit other interesting properties. In order to explore more potential applications of perovskite material, nanoscale perovskite morphologies have been investigated extensively. Figure 4 shows perovskite nanomaterials that have been synthesized as nanocubes (NCs), nanoplatelets (NPLs), nanorods (NRs), as well as quantum dots (QDs) classified according to their size and shape. In nanoscale semiconductor materials, quantum confinement plays a decisive role in the optoelectronic properties by finely manipulating the band gap. The semiconductor forms the "well" while the outer environment serves as the "wall" in the classic quantum well model. Therefore, the energy levels are upshifted to a higher level. As a result, within the same composition, perovskite light emitters can provide different emission wavelengths by nanoscale size tuning, providing alternative choices for targeted emission wavelengths. Perovskite NCs have typical edge length of 8-15 nm in cubic shape, and is the mostly studied form of perovskite material due to well-established protocols. CsPbX3 NCs offers both size and compositional tuning, covering emission wavelengths from 400-710 nm. It is noted that the exciton Bohr radius for CsPbBr3 materials is calculated as 3.5 nm. Perovskite nanoplatelets (NPLs) consist of few-layer perovskite lattices and these NPLs been shown to exhibit strong quantum confinement effect, with a significant blue-shift in emission wavelength up to 0.6 eV (~100 nm) relative to bulk perovskite. 2D structures can be represented by their own molecular compositions, e.g., single layer Cs2PbBr4 and double layer CsPb2Br5. The 2D confined structure brings increased exciton binding energy, and it allows emission tuning by thickness control. Charge transfer between stacked NPLs is enhanced due to large contact area. NPLs are proposed to be more stable than NCs against environmental moisture due to large ligand coverage[47]. However, NPLs tends to be less stable in solution as they agglomerate easily, having its unique properties compromised.Blue emission is a challenging topic for LED lighting and display industry as it suffers from many problems, such as shorter lifetime, lower luminescence quantum yield, and lower efficiency compared with pure green and red LEDs. Blue LED is an essential composition of lighting and display applications as the three primary colour compose white light in the most versatile way.