**3. ZnO film/nanostructure as electron transporting layer**

**Figure 4.** Structure diagram of (a) mesoscopic perovskite solar cell and (b) planar perovskite solar cell.

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

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 perov-

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

device behavior.

206 Nanostructured Solar Cells

In 2013, ZnO was firstly applied as ETL of PSCs. Kumar et al. [28] reported flexible PSCs employing the ZnO compact layer as a hole-blocking layer and a ZnO nanorods mesoscopic scaffold layer as an electron transporter. The ZnO compact layer was formed by electrodeposition and ZnO nanorods grown by chemical bath deposition, which allow the processing of low-temperature, solution-based ETLs. The planar device, which only uses the ZnO compact layer can also be made, but they presented lower Jsc and FF than nanorod-based devices. Conversion efficiencies of 8.90% were achieved on rigid substrates, while the flexible ones yielded 2.62%. In the same year, Bi et al. [29] used well-aligned ZnO nanorod arrays as ETLs. They consider that the perovskite material has better solar cell stability and is therefore more suited as a sensitizer for ZnO nanorod arrays. Therefore, their results showed the ZnO nanorod-based PSCs had a good long-term stability of PSCs. It was found that the electron transport time and lifetime vary with the ZnO nanorod length, a trend which is similar to that in DSSCs, suggesting a similar charge transfer process in the ZnO nanorod array/ CH3NH3PbI3 interface as in conventional DSSCs. However, a solar cell efficiency of only 5.0% was achieved under AM 1.5 G illumination due to more recombination losses than TiO2 devices. The reason indicated that the ZnO nanorod array grown by different processes may affect the PSC performance.

A breakthrough came in the end of 2013, Liu and Kelly [30] reported that a room temperature solution-processed thin compact ZnO ETL was used to fabricate a highly efficient planar perovskite solar cell with a champion efficiency of 15.7%, an average efficiency of 13.7%, and the flexible ones yielded 10.2%, in which ZnO prepared by a co-precipitation method had superior electron mobility, and the ETLs were fabricated without any sintering steps, as shown in **Figure 5**. Besides solution-based co-precipitation ZnO ETLs, a sol-gel solutionprocessed ZnO ETLs were reported by Kim et al. [31] in 2014. Moreover, a vacuum-processed ZnO ETL has been prepared for high-efficiency PSCs, such as an atom layer deposition (ALD) [32] or sputtering method [33, 34]. Several types of ZnO nanostructures have been studied to replace the mesoporous TiO2 nanostructures in the conventional PSCs [35–38].

On the other hand, the electrical characteristics of ZnO can be increased by extrinsically doping a small amount of impurities [39, 40], such as Al. The ionic radius of Al3+ is 0.54 Å, which is smaller than that of Zn2+ (0.74 Å). Therefore, Al3+ can replace Zn2+ in the lattice point and acts as a donor to increase the conductivity of ZnO [41, 42], and at the same time, it remains the excellent transparence in the visible-light region. This is why a high-quality Al-doped ZnO (AZO) thin film can also be used as a transparent conductive oxide (TCO) electrode, just like other IIIA elements (B, Ga, and In)-doped ZnO [41]. Al-doped ZnO thin film, which was deposited using the electrospraying method, was also used as ETLs in PSCs to suppress charge recombination at the ZnO/perovskite interface, resulting in better efficiency than pure ZnO devices [43]. The charge recombination of the ZnO-based device was also suppressed by appropriate Mg-doping. It mainly attributed to the conduction band offset at the interface between ZnO ETL and perovskite layer [44].

**Figure 5.** J–V curves of devices on (a) glass and (b) flexible PET substrates in the illumination (red line) and dark (black line) for the highest-performing ITO/ZnO/CH3NH3PbI3/spiro-OMeTAD/Ag devices in reference [31]. (c) Photograph of a device prepared on a flexible PET substrate.

The electron extraction by the ETL in perovskite cell strongly depends on the work function (WF) of the ETL. An energy barrier mismatch (a Schottky barrier) between the WF of the ETL and the lowest unoccupied molecular orbital (LUMO) of perovskite absorber could lead to inefficient electron extraction. Therefore, matching the WF of ETL with the absorber could reduce a Schottky barrier or form an Ohmic contact [45, 46] to facilitate the electron injection or collection [47]. Nitrogen-doped ZnO electron materials combined with a dipole layer can increase electron concentration to improve perovskite infiltration and reduce the work function [38]. Above all, doping is an effective way to modify the electrical properties of ZnO.

Self-assembled monolayer (SAM) is well-known that surface treatments can decrease the number of charge carrier traps and tune the surface WF of ETLs. Modification of the interface of solar cells using functional SAMs is an effective method to improve device performance. Modifying the ZnO ETL with 3-aminopropanoic acid monolayer can improve the interfacial energy level alignment due to the formation of permanent dipole moments, which decreased the WF of ZnO from 4.17 to 3.52 eV, and help to obtain highly crystalline perovskite layer with reduced pin-hole and trap-state density [48]. The stoichiometry of ZnO thin film was also affected the photovoltaic device performance. Tseng et al. [33] used sputtered ZnO thin films, which stoichiometry was controlled by adjusting the ratio of working gases (Ar and O2) during radio frequency (RF) magnetron-sputtering process, as an ETL in PSCs. As mentioned earlier, the native doping of the ZnO due to oxygen vacancies shows n-type semiconductor. The surface conductivity of ZnO film was affected by the presence of oxygen vacancy of the lattice, which will show even more accentuated variations of the electrical behavior in a thin film. ZnO film with more oxygen vacancies has higher surface conductivity; therefore, device based on ZnO using pure Ar deposition has smaller series resistance. Furthermore, ZnO using pure Ar deposition has lower WF of 4.33 eV than that using Ar/O2 mixed gas deposition (4.48) but both have a same bandgap. Therefore, ZnO using pure Ar deposition lower conduction band level (down-shift) than that using Ar/O2 mixed gas deposition to increase the driven force of electron injection (or charge separation) from perovskite (or ZnO/perovskite interface) and lower valance edge can block the hole more efficiently. Both better conductivity and lower conduction band level of ZnO result in high charge extraction efficient; therefore, the corresponding device has high Jsc. (**Figure 6a** and **b**) The hole-blocking ability of ZnO film using pure Ar deposition was also supported by the dark current of the corresponding device illustrated in **Figure 6c**. Cell-based ZnO-Ar electron-transporting layer has smaller dark current indicated that ZnO-Ar has better hole-blocking ability when other components in the cell are supposed to be the same.

On the other hand, the electrical characteristics of ZnO can be increased by extrinsically doping a small amount of impurities [39, 40], such as Al. The ionic radius of Al3+ is 0.54 Å, which is smaller than that of Zn2+ (0.74 Å). Therefore, Al3+ can replace Zn2+ in the lattice point and acts as a donor to increase the conductivity of ZnO [41, 42], and at the same time, it remains the excellent transparence in the visible-light region. This is why a high-quality Al-doped ZnO (AZO) thin film can also be used as a transparent conductive oxide (TCO) electrode, just like other IIIA elements (B, Ga, and In)-doped ZnO [41]. Al-doped ZnO thin film, which was deposited using the electrospraying method, was also used as ETLs in PSCs to suppress charge recombination at the ZnO/perovskite interface, resulting in better efficiency than pure ZnO devices [43]. The charge recombination of the ZnO-based device was also suppressed by appropriate Mg-doping. It mainly attributed to the conduction band offset at the interface

**Figure 5.** J–V curves of devices on (a) glass and (b) flexible PET substrates in the illumination (red line) and dark (black line) for the highest-performing ITO/ZnO/CH3NH3PbI3/spiro-OMeTAD/Ag devices in reference [31]. (c) Photograph of

The electron extraction by the ETL in perovskite cell strongly depends on the work function (WF) of the ETL. An energy barrier mismatch (a Schottky barrier) between the WF of the ETL and the lowest unoccupied molecular orbital (LUMO) of perovskite absorber could lead to inefficient electron extraction. Therefore, matching the WF of ETL with the absorber could reduce a Schottky barrier or form an Ohmic contact [45, 46] to facilitate the electron injection or collection [47]. Nitrogen-doped ZnO electron materials combined with a dipole layer can increase electron concentration to improve perovskite infiltration and reduce the work function [38]. Above all, doping is an effective way to modify the electrical properties of ZnO.

Self-assembled monolayer (SAM) is well-known that surface treatments can decrease the number of charge carrier traps and tune the surface WF of ETLs. Modification of the interface of solar cells using functional SAMs is an effective method to improve device performance. Modifying the ZnO ETL with 3-aminopropanoic acid monolayer can improve the interfacial energy level alignment due to the formation of permanent dipole moments, which decreased the WF of ZnO from 4.17 to 3.52 eV, and help to obtain highly crystalline perovskite layer with reduced pin-hole and trap-state density [48]. The stoichiometry of ZnO thin film was also affected the photovoltaic device performance. Tseng et al. [33] used sputtered ZnO thin films, which stoichiometry was controlled by adjusting the ratio of working gases (Ar and O2) during

between ZnO ETL and perovskite layer [44].

208 Nanostructured Solar Cells

a device prepared on a flexible PET substrate.

**Figure 6.** (a) I-V curves, (b) illustration of the frontier orbitals energy levels for ZnO prepared using pure Ar and 20% O2, and (c) Dark current curves of perovskite solar cell using pure Ar, 10%, and 20% Ar/Ar + O2 ratio mixed gas as the electron transport layer. The illustration of the frontier orbitals energy levels for ZnO-Ar, ZnO-20%, and perovskite.

Despite the excellent characteristics of ZnO, in 2015, Yang et al. [49] found the thermal instability of PSCs fabricated using ZnO ETLs. They show that the basic nature of the ZnO surface leads to proton-transfer reactions at the ZnO/CH3NH3PbI3 interface, which results in decomposition of the perovskite film PbI2, as shown in **Figure 7**. The decomposition process is accelerated by the presence of surface hydroxyl groups and/or residual acetate ligands. To reduce the decomposition, Cheng et al. [50] introduced a buffer layer in between the ZnO-NPs and perovskite layers. They found that a commonly used buffer layer with small molecule like [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) can slow down but cannot avoid the decomposition completely. On the other hand, a polymeric buffer layer using poly(ethylenimine) (PEI) can effectively separate the ZnO-NPs and perovskite, which allows larger crystal formation with thermal annealing. Today, thermal instability of PSCs using ZnO ETLs remains the major challenge limiting their further application and device performance.

**Figure 7.** (a) Optimized geometrical structure of the ZnO/CH3NH3PbI3 interface. The inset shows a magnified view of the deprotonated methylammonium cations. (b) Photographs of CH3NH3PbI3 films deposited on ZnO layers and heated to 100°C with different times (from a to g).
