NSRRC Activity Report 2023

Soft Matter 039 T he development of mixed-cation-based metal-halide perovskite nanocrystal (PeNC) systems 1 provides a promising avenue for enhancing device stability and charge transport properties, overcoming the limitations associated with the bulk polycrystalline nature of methylammonium (MA), formamidinium (FA), and Cesium (Cs) systems. 2 Controlled modulation of hybrid MA + , FA + , and Cs + cations, along with the desired stoichiometry, allows for precise adjustments to the structural and chemical properties of PeNCs, further elevating device performance. For example, introducing a known amount of Cs + cations into unstable MAPbBr 3 or FAPbBr 3 NCs not only enhances their air stability but also induces strong entropic stabilization due to the smaller ionic radius of Cs (1.81 Å) compared to MA (2.70 Å) and FA (2.79 Å). However, the insertion of Cs + into FA-perovskite causes lattice distortion, leading to reduced polaron formation and shortened photoluminescence (PL) lifetimes. 3 To address this challenge, Hao et al. demonstrated that doping FA into CsPbI 3 promotes orbital overlap, thereby facilitating easier polaron formation, enhancing tolerance factor stability, prolonging carrier lifetimes, minimizing trap-assisted recombination, and boosting PCE up to 16.6%. 4 Given the similarities between CsPbI 3 and CsPbBr 3 , FA-doping into CsPbBr 3 was predicted to regulate Pb-Br-Pb bond length and angle, thus significantly impacting perovskite lattice stability to enhance light-emitting diode (LED) performance. Chi-Ching Kuo (National Taipei University of Technology) and his collaborators presented novel post-synthesis Cs/ FA mixed-cation PeNCs possessing high structural and optoelectronic properties to achieve high exciton recombination efficiency in LEDs. First, they illustrated that the three-dimensional network of cubic Cs 1− x FA x PbBr 3 hybrid perovskite NCs (HPNC) featured a corner-sharing PbBr 6 4− octahedral structure with Cs + /FA + cations interspersed between the PbBr 6 octahedra, as shown in Fig. 1(a) . Notably, the incorporation of FA + cations into the CsPbBr 3 structure results in a redshift Mixed-Cation Hybrid Perovskite Nanocrystals High-purity Cs 1−x FA x PbBr 3 hybrid perovskite nanocrystals are achieved using a straightforward hot-injection method combined with cation compositional engineering, providing a practical pathway for versatile optoelectronic and photonics applications. Fig. 1 : (a) Crystal structure of Cs 1-x FA x PbBr 3 HPNCs. (b) XRD patterns of Cs 1-x FA x PbBr 3 HPNCs with various Cs/FA ratios. (c−f) Synchrotron 2D GIWAXS pattern of CsPbBr 3 , Cs 0.7 FA 0.3 PbBr 3 , Cs 0.5 FA 0.5 PbBr 3 , and Cs 0.3 FA 0.7 PbBr 3 , respectively. (g−i) 1D GIWAXS profile of Cs 1-x FA x PbBr 3 with (110) diffraction peaks of (h) in-plane line cuts. (i) Corresponding azimuthally integrated scattering intensity of the (100) plane as indicated in (c−f). [Reproduced from Ref. 5]