  Twin boundaries (TBs) were identified to show conflicting positive/negative
effects on the physical properties of CH3NH3PbI3 perovskite, but their roles on
the mechanical properties are pending. Herein, tensile characteristics of a
variety of TB-dominated bicrystalline CH3NH3PbI3 perovskites are explored using
molecular simulations. TB-contained CH3NH3PbI3 are classified into four types
from their tensile ductile detwinning characteristics. Type I is characterized
by smooth loading flow stressstrain responses, originating from relatively
uniform stress distribution induced gradual amorphization at TB region. Types
II and III are represented by sudden drop of loading stresses but then distinct
ductile flow stress-strain curves, resulting from limited and large-area
amorphizations of TB-involved structures, respectively. However, Type IV is
highlighted by double apparent peaks in the loading curve followed by ductile
flow response, coming from stress-concentration of
localization-to-globalization at TB structure, as well as amorphization. This
study provides critical insights into mechanics of CH3NH3PbI3 perovskites, and
offers that TB engineering is a promising strategy to design mechanically
robust hybrid organic-inorganic perovskites-based device systems
