Rui Wang, Jintao Wang, Shaun Tan, Yu Duan, Zhao-Kui Wang, Yang Yang Opportunities and Challenges of Lead-Free Perovskite Optoelectronic Devices  Rui Wang, Jintao Wang, Shaun Tan, Yu Duan, Zhao-Kui Wang, Yang Yang  Trends in Chemistry  DOI: 10.1016/j.trechm.2019.04.004 Copyright © 2019 Terms and Conditions

Figure 1 An Energy Diagram of Common Lead-Free Perovskites. The diagram shows electron-transporting layers (ETLs) and hole-transporting layers (HTLs). Energy values shown are in electron volts. Trends in Chemistry DOI: (10.1016/j.trechm.2019.04.004) Copyright © 2019 Terms and Conditions

Figure 2 Structures of Lead-Free Perovskites. (A) 3D crystal structure of CH3NH3SnI3-xBrx [24]. (B) 2D layered structure of Cs3Sb2I9 [23]. (C) Local structure of the Bi2I93- anion (left); cation and anion positions in the unit cell (right) [12]. (D) The crystal structure of Cs2AgBiBr6 [28]. Trends in Chemistry DOI: (10.1016/j.trechm.2019.04.004) Copyright © 2019 Terms and Conditions

Figure 3 Fabrication Processes of the Lead-Free Perovskite Solar Cells (PSCs). (A) Summary of the spin-coating and evaporation process. (B) Low-pressure-assisted fabrication process [32]. (C) Schematic of the vapor-assisted solution process (VASP) and low-temperature VASP (LT-VASP) fabrication processes [36]. Trends in Chemistry DOI: (10.1016/j.trechm.2019.04.004) Copyright © 2019 Terms and Conditions

Figure 4 Stability of Lead-Free Perovskite Solar Cells under Different Environmental Conditions. These include encapsulated (E), unencapsulated (UE), ambient conditions (air), glove box (N2), and heat. Trends in Chemistry DOI: (10.1016/j.trechm.2019.04.004) Copyright © 2019 Terms and Conditions

Figure I Lead Toxicity and Associated Symptoms. Trends in Chemistry DOI: (10.1016/j.trechm.2019.04.004) Copyright © 2019 Terms and Conditions