**Conflict of interest**

transport the electron and hole efficiently to ETL and HTL, respectively, has been discussed thoroughly. Therefore, selecting the appropriate materials for the carrier transport layer is considered as a primary issue to prevent charge recombination and diminish energy loss at the interface between the perovskite active layer and ETL or HTL. In recent studies, the electron transport materials and the hole transport materials have been explored to construct an

**Figure 9.** Energy band diagram of different electron and hole transport layers with respect to perovskite-structured

A good electron-transporting material should exhibit (1) good electron mobility to facilitate electron collection, (2) wide band gap for not hindering the absorption behavior of the perovskite active layer, and (3) both the valence band/conduction band should be lower than that of perovskite-structured material to promote electron migrate to ETL and block the hole to trans-

owing to their electrical and optical properties [31]. A good hole-transporting material should exhibit (1) efficient hole mobility to promote hole collection, (2) the valence band/conduction band both should be higher than the perovskite-structured active layer to promote hole migration to HTL and further transport to the electrode, and (3) there should be photochemical stability. The most widely-explored materials for HTL has been divided into 3 groups: (1) small molecules (e.g., spiro-OMeTAD), (2) inorganic materials (e.g., CuI, CuSCN, NiO, etc.), and (3) conducting polymers (e.g., P3HT, PEDOT, PTAA, etc.) [32, 33]. **Figure 9** examines the proposed band alignment for the commonly used materials for ETL and HTL which is helpful for understanding interface properties and constructing high-efficiency perovskite-structured photovoltaic devices.

Perovskite-structured photovoltaic materials attract a lot of interest due to its facile fabrication process and rapid progress in device performance. A series of perovskite-structured

, are widely applied as ETLs, especially TiO2,

, ZnO, and SnO2

optimal band alignment for achieving high efficiency [28–30].

port. Metal oxides, such as TiO2

photovoltaic devices.

88 Solar Panels and Photovoltaic Materials

**5. Conclusion**

There are no conflicts of interest to declare.
