**2. Electron transporting layer in perovskite solar cells**

The electron-transporting layer (ETL), one of the most important components in the PSCs for highly efficient performance, plays an essential role in extracting and transporting photogenerated electrons. Simultaneously, it also serves as a hole-blocking layer to suppress carrier recombination. The characteristics of the ETL, especially its carrier mobility, energy band alignment, morphology, trap states, and related interfacial properties are major factors to determine the device behavior and photovoltaic performance of PSCs [17]. A relatively high electron mobility is desirable for ETLs to efficiently transport and collect electrons transport, contributing to the increase of short-circuit current density (Jsc), and fill factor (FF). The better matching energy level between ETLs and the perovskite layer can facilitate electron extraction and transport. Furthermore, the open-circuit voltage (Voc) can be determined by the energy level differences between the Fermi levels (EF) of the ETL and EF of the hole-transporting layer (HTL) [18–20]. Hence, the energy-level engineering is widely used to improve the Voc of a photovoltaic device. Trap states in the ETLs also play important roles in charge transport. Therefore, improving interface contact between ETLs and the perovskite layer is an efficient method to optimize device performance and enhance charge transport. Morphologies of the ETL are also considered to enhance its contact with the perovskite layer for achieving better device behavior.

To date, TiO2 materials have been used as ETLs in most frequently reported PSCs. The electron injection rates between the perovskite absorber and TiO2 ETLs are very fast, but the high electron recombination rates are also seen due to the low electron mobility and transporting properties [21]. In addition, a high-temperature process was required for high-quality mesoscopic TiO2 layer [16, 22]. Hence, these characteristics of TiO2 materials may act as impediments to improve device performance and their further application for developing low-cost perovskite solar cells on various flexible substrates [23, 24]. On the other hand, ZnO is a widebandgap semiconductor of the II-VI semiconductor group, which has several favorable properties, including good transparency, high electron mobility, wide bandgap, and strong room-temperature luminescence. Several types of morphologies in ZnO, such as thin film, single-crystal, nanowire, and nanorod has been found and made using the low-temperature solution processes. The native doping of the semiconductor due to oxygen vacancies or zinc interstitials is n-type [25]. Moreover, ZnO is a well-known material that has similar energy level as TiO2 but has better electron mobility (bulk mobility: 200–300 cm2 /V s [25–27]) than TiO2, which lets it an ideal candidate for a low-temperature processed electron-selective contact for transparent electrodes, organic solar cell, thin-film transistors, and light-emitting diodes.
