2020同步年報

Physics and Materials Science 015 The authors demonstrated a precise control of the forma- tion of a perovskite film in a two-step spin-coating process on performing sequential A-site doping of Cs + and GA + cations in FAPbI 3 -based perovskites. 7 As is well known, the growth of the films involves manifold crystallization paths that thereby lead to distinct film morphologies, as unveiled by GIWAXS measurements in situ . In an undoped film, the two-step process was found to decouple effectively the crystal nucleation and the crystal growth, as the hexagonal PbI 2 formed in the first step essentially provides the nu- cleation sites ( Fig. 2 ) for further growth of the perovskite α -phase in the second step ( Fig. 3 ). Remarkably, the results also showed that the doping of Cs + cations induces the formation of a δ-CsPbI 3 phase and restrains the crystalliza- tion of PbI 2 . In addition, the authors undertook calculations with density-functional theory (DFT) that indicated that the transition to the α -phase is preferentially initiated from the δ-CsPbI 3 phase instead of PbI 2 , as the reaction enthalpy in a Cs-rich environment gets reduced. The δ-CsPbI 3 phase thus offers an alternative phase-transition path to the perovskite α -phase: although the sparse nucleation sites of δ-CsPbI 3 cause the growth of large grains, it also leads to the forma- tion of many pin-holes caused by the non-uniform nucleation. The authors showed, however, that this problem could be solved on doping GA + cations in the second step, which noticeably accelerated the crystal growth and resulted in large grains free of voids via Ostwald ripening and en- hanced grain-boundary migration ( Fig. 3 ). This observation was also rationalized with DFT results, which showed that GA + balances the bonding stress caused by the Cs doping, through an increased bonding strength with Pb–I octa- hedral frameworks. Most importantly, the authors also showed that, using the perovskites fabricated through the sequential doping of Cs + and GA + , device efficiencies over 23% were achieved, through the improved morphology, enhanced crystallinity and fewer defects in the films. 7 The present work thus revealed an unprecedented advan- tage of the two-step process over the one-step process: the two-step process decouples nucleation and crystal growth and offers sequential doping for a precise control of the growth of a perovskite film toward highly efficient devices. (Reported by Ashish Chainani) This report features the work of Shuxia Tao, Xinhui Lu and their coworkers published in Adv. Mater. 32 , 2004630 (2020). TLS 23A IASW – Small/Wide Angle X-ray Scattering • Time-resolved Grazing-incidence Wide-angle X-ray Scat- tering In Situ • Materials Science, Chemistry, Condensed-matter Physics, Environmental and Earth Science References 1. A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 , 6050 (2009). 2. Z. Xiao, C. Bi, Y. Shao, Q. Dong, Q. Wang, Y. Yuan, C. Wang, Y. Gao, J. Huang, Energy Environ. Sci. 7 , 2619 (2014). 3. J. H. Im, I. H. Jang, N. Pellet, M. Gratzel, N. G. Park, Nat. Nanotechnol. 9 , 927 (2014). 4. N. Ahn, D. Y. Son, I. H. Jang, S. M. Kang, M. Choi, N. G. Park, J. Am. Chem. Soc. 137 , 8696 (2015). 5. Q. Jiang, Y. Zhao, X. Zhang, X. Yang, Y. Chen, Z. Chu, Q. Ye, X. Li, Z. Yin, J. You, Nat. Photonics 13 , 460 (2019). 6. P. Gratia, I. Zimmermann, P. Schouwink, J. H. Yum, J. N. Audinot, K. Sivula, T. Wirtz, M. K. Nazeeruddin, ACS Ener- gy Lett. 2 , 2686 (2017). 7. M. Qin, H. Xue, H. Zhang, H. Hu, K. Liu, Y. Li, Z. Qin, J. Ma, H. Zhu, K. Yan, G. Fang, G. Li, U.-S. Jeng, G. Brocks, S. Tao, X. Lu, Adv. Mater. 32 , 2004630 (2020).