NSRRC Activity Report 2023

040 NSRRC ACTIVITY REPORT 2023 of the band gap. This shift is linked to a reduction in octahedral tilting and an increase in Br−Pb−Br angles, contributing to an enhanced Pb−Br orbital overlap. Furthermore, the conduction band comprises antibonding combinations of Pb (6p) and Br (4p) orbitals, thus confining the exciton recombination luminescence within the PbBr 6 4− octahedron. Secondly, grazing-incidence wide-angle X-ray scattering (GIWAXS) and grazing-incidence small-angle X-ray scattering (GISAXS) at TLS 23A1 were used to systematically analyze the crystal orientation of perovskite thin films. The 2D GIWAXS patterns 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 perovskite films are depicted in Figs. 1(d)–(f) . As expected, the 1D GIWAXS profiles ( Fig. 1(g) ) exhibit diffraction peaks corresponding to the (100), (110), and (200) planes at positions consistent with the XRD spectra ( Fig. 1(b) ). The azimuthally integrated scattering intensity along the ring at the q = 10.9 nm −1 (100) plane was further investigated, as shown in Fig. 1(i) . Remarkably, Cs 0.3 FA 0.7 PbBr 3 and Cs 0.5 FA 0.5 PbBr 3 exhibited strong peak intensity at 90° to the azimuth with uniform orientation and well-defined spots compared to CsPbBr 3 and Cs 0.7 FA 0.3 PbBr 3 ( Figs. 1(d)–(f) ). Additionally, the mixed-cation Cs 1− x FA x PbBr 3 films in the range of 0−0.7 exhibit diffraction peaks at q xy = 15 nm −1 corresponding to the (110) crystal planes, slightly shifted toward the low- q region from 15.16° to 14.98° ( Fig. 1(h) ). This confirms that suitable A-site cation exchange between CsPbBr 3 and FAPbBr 3 not only enhances the tolerance factor but also imparts high crystallinity and a well-ordered crystal structure to the perovskite films. Additionally, the corresponding Cs 1−x FA x PbBr 3 HPNCs were analyzed using GISAXS spectra ( Figs. 2(a)–(e) ). The 1D GISAXS profiles for 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 exhibit scattering peaks at Q m = 0.06 Å −1 and 0.045 Å −1 . The domain spacings of Cs 1− x FA x PbBr 3 with various compositions ranging from 0 to 0.7 were estimated to be approximately 11 and 14 nm using the formula d = 2 π / Q m . In summary, the implementation of GIWAXS and GISAXS has significantly enhanced our understanding of the crystal orientation at the intra-grain scale of Cs 1− x FA x PbBr 3 HPNCs. Synchrotron-radiation-based GIWAXS and GISAXS were employed to investigate the microstructure and crystal orientation of perovskite polycrystalline films through A-site cation exchange between CsPbBr 3 and FAPbBr 3 . Notably, the stacking pattern of crystal planes in the hybrid perovskite thin film transforms with varying amounts of FA + cation doping into the CsPbBr 3 structure, which indicates complete replacement of initial Cs + cations in the PeNCs by FA + ions. The larger ionic radius of FA + (2.79 Å) compared to Cs + (1.81 Å) plays a pivotal role in modifying the Pb−Br bond lengths and angles within the PbX 6 4− octahedron. The ability to probe the crystal orientation of mixed-cation-based PeNC systems underscores the importance of synchrotron-based soft X-ray microscopy and spectroscopy, offering valuable insights into the relationship between multiple ordered crystal orientations and optoelectronic properties. (Reported by Fang-Cheng Liang, National Taipei University of Technology) This report features the work of Chi-Ching Kuo and his collaborators published in Adv. Mater. 35 , 2207617 (2023). Fig. 2 : (a) 1D GISAXS profile of Cs 1-x FA x PbBr 3 HPNCs. (b−e) Synchrotron 2D GISAXS pattern of CsPbBr 3 (b), Cs 0.7 FA 0.3 PbBr 3 (c), Cs 0.5 FA 0.5 PbBr 3 (d), and Cs 0.3 FA 0.7 PbBr 3 (e), respectively. [Reproduced from Ref. 5]