Fig. 1. (Color online) (a) Crystal structure for perovskite materials. Reproduced with permission^[3], Copyright 2019, the Royal Society of Chemistry. (b) Calculated transition energy levels of different defects in MAPbI₃. The corresponding formation energies are shown in parentheses. Reproduced with permission^[4], Copyright 2014, Wiley-VCH. (c) Charge recombination pathways in perovskite films. Reproduced with permission^[5], Copyright 2018, the Royal Society of Chemistry.

Fig. 2. (Color online) (a) Pb-I antisite defects passivated by PCBM. Reproduced with permission^[7], Copyright 2015, Nature Publishing Group. (b) Uncoordinated Pb²⁺ passivated by thiophene. Reproduced with permission^[9], Copyright 2014, American Chemical Society. (c) Hydrogen bondings (O–H···I and N–H···I) between 4-ABPACl and perovskite film. Reproduced with permission^[18], Copyright 2015, Nature Publishing Group. (d) Halogen bonding between IPFB and halogen anion. Reproduced with permission^[21], Copyright 2014, American Chemical Society. (e) Halogen vacancy passivated by K⁺. Reproduced with permission^[28], Copyright 2018, Nature Publishing Group. (f) PEAI post-treatment on perovskite film. Reproduced with permission^[29], Copyright 2019, Nature Publishing Group. (g) MA and I vacancies passivated by 3-(decyldimethylammonio)-propane-sulfonate zwitterion. Reproduced with permission^[31], Copyright 2018, Wiley-VCH.

Fig. 3. (Color online) (a) SEM image for perovskite film containing PbI₂ phases. (b) Type-I energy level alignment. Reproduced with permission^[34], Copyright 2014, American Chemical Society. (c) PbI₂ as charge-transport barrier at grain boundaries. Reproduced with permission^[35], Copyright 2016, American Chemical Society. (d) 2D perovskites establish type-I heterojunction. Reproduced with permission^[36], Copyright 2017, Nature Publishing Group. (e) DFT calculation for trap states on the surface of different crystallographic facets. Reproduced with permission^[40], Copyright 2020, Nature Publishing Group.