Quantum confinement of 2D perovskite nanoplatelets for light emission

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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.