**4. Conclusions**

In summary, we have given an overview of the efforts focused on the ZnO of ETLs. Their doping effect and interface modification between the ETL and perovskite layer have been developed and applied in PSCs. Because charge extraction, transfer, and recombination mainly occur at the interfaces of a cell, the interfacial layer between the perovskite layer and the bottom electrode needs to be closely aligned by introducing efficient interfacial materials. For conventional PSCs, a ZnO ETL, good electron-transport ability and a low recombination rate at the interface, works well for high-performance PSC.
