**Author details**

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

**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 heat-

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

ed to 100°C with different times (from a to g).

**4. Conclusions**

210 Nanostructured Solar Cells

the major challenge limiting their further application and device performance.

Lung-Chien Chen\* and Zong-Liang Tseng

\*Address all correspondence to: ocean@ntut.edu.tw

Department of Electro-Optical Engineering, National Taipei University of Technology, Taipei, Taiwan
