Johnny C. HO

Flexible Thin Film Electronics


Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong SAR 999077, China








Dr. Johnny Ho is an Associate Head and a Professor in the Department of Materials Science and Engineering as well as a core member in the State Key Laboratory of Terahertz and Millimeter Waves at the City University of Hong Kong. He also serves as an Acting Director of Knowledge Transfer Office at the City University of Hong Kong. He got his BS, MSc and PhD degrees at the University of California, Berkeley, USA, in 2002, 2005 and 2009, respectively. Dr. Ho was a post-doctoral researcher in the Nanoscale Synthesis and Characterization group (Materials Science Division) at Lawrence Livermore National Laboratory, California, USA (2009-2010). His current research interests focus on the synthesis, characterization and applications of nanoscale materials for electronic, optoelectronic, sensor and energy harvesting devices. Over the years, Dr. Ho has won a number of awards, including President’s Awards & Outstanding Research Awards at the City University of Hong Kong, World Cultural Council Special Recognition Award, Hong Kong Research Grants Council (RGC) Fellowship Award, and recently elected to be a Founding Member in the Young Academy of Sciences of Hong Kong (YASHK), and Fellow member of the Royal Society of Chemistry (FRSC), etc. To date, Dr. Ho has published more than 210 journal articles with an H-index of 58 and a total citation of over 13750 (Google Scholar May 2022). Visit his web for more information ( 





Abstract for Presentation

 Photoelectric Property, Mechanical Flexibility and Stability Enhancement of Quasi-2D Halide Perovskites


Three-dimensional (3D) organic-inorganic halide perovskite MAPbI3 has attracted significant research attention because of its high performance in optoelectronic devices. However, the stability of conventional 3D halide perovskite is somewhat poor. One effective way to tackle this stability issue is to insert large-scale organic molecules into the perovskites to form the two-dimensional (2D) or quasi-2D Ruddlesden-Popper phase. Although the inserted large-scale organic molecules of Ruddlesden-Popper perovskites (RPPs) can considerably improve their ambient stability, these organic molecules would inevitably also serve as insulating barriers, substantially degrading their optoelectronic device performance and thermal stability. In this view, ethylenediammonium (EDA) can be an ideal organic molecule here, with almost the shortest alkyl chain separating the metal halide octahedral layers. Notably, since there are two NH4+ in each EDA cation, one EDA layer is sufficient to function as the spacer cations rather than two BA or two PEA layers (i.e., organic bilayers) required in the conventional RPPs, where this kind of 2D perovskites are called Dion-Jacobson phase-layered perovskites (DJPs). This kind of DJPs can also overcome the poor flexibility of RPPs thanks to the absence of the weak van der Waals interaction in the lattice. At the same time, utilizing the unique nature of RPPs, high-quality RPP films with gradient band alignments are successfully synthesized by a simple one-step spin-coating process. It is observed that the graded RPP phases distribute sequentially with the increasing n values towards the film surface. This way, by constructing the proper bottom and top contact electrodes, vertical-structure photodetectors based on the graded RPP phases can be readily achieved. These photodetectors exhibit self-powered characteristics with superior performance and enhanced flexibility. All these findings can demonstrate the promising potential of using quasi-2D halide perovskites for next-generation flexible optoelectronic devices. 












[1]   Lai Z., Wang F., Meng Y., Bu X., Kang X., Quan Q., Wang W., Yip S.P., Liu C., Ho J.C. "Solution-Processed Lead-Free Double Perovskite Microplatelets with Enhanced Photoresponse and Thermal Stability", SCIENCE CHINA Materials, 65, 1313, 2022.
[2]   Lai Z., Wang F., Meng Y., Bu X., Kang X., Quan Q., Wang W., Liu C., Yip S.P., Ho J.C. "Superior Performance and Stability of 2D Dion–Jacobson Halide Perovskite Photodetectors Operated under Harsh Conditions without Encapsulation", Advanced Optical Materials, 9, 2101523, 2021.
[3]    Lai Z., Meng Y., Zhu Q., Wang F., Bu X., Li F., Wang W., Liu C., Wang F., Ho J.C. "High-Performance Flexible Self-Powered Photodetectors Utilizing Spontaneous Electron and Hole Separation in Quasi-2D Halide Perovskites", Small, 17, 2100442, 2021.
[4]    Lai Z., Dong R., Zhu Q., Meng Y., Wang F., Li F., Bu X., Kang X., Zhang H., Quan Q., Wang W., Wang F., Yip S.P., Ho J.C. "Bication-Mediated Quasi-2D Halide Perovskites for High- Performance Flexible Photodetectors: From Ruddlesden−Popper Type to Dion−Jacobson Type", ACS Applied Materials & Interfaces, 12, 39567-39577, 2020.
[5]    Dong D., Lan C., Li F., Yip S.P., Ho J.C. "Incorporating Mixed Cations in Quasi-2D Perovskites for High-Performance and Flexible Photodetectors", Nanoscale Horizons, 4, 1342-1352, 2019.