Tandem solar cells which combines several solar cells in series have been investigated to achieve high efficiency solar cells. Recently, perovskite/crystalline silicon tandem solar cells have attracted much attention as promising tandem structure because these solar cells have high external quantum efficiency in different wavelength range. There is a report on simulated efficiency of two terminal perovskite/crystalline silicon tandem solar cells. In that report, projected efficiency of over 30% is suggested by using inverted pyramid structure. However, most of perovskite solar cells are fabricated by solution process which is not suitable way for textured substrates, indicating that it is difficult to achieve such structured tandem solar cells in current conventional process. To overcome this problem, we focused on sputtering process. Sputtering can achieve homogeneous films even the substrate has large roughness such as textured substrates. Therefore, it is expected that sputtering is suitable method to achieve high efficient tandem solar cells. In addition, there are no reports on sputtered perovskite films which are used for solar cells. In this work, we fabricated perovskite films by sputtering and investigated the characteristics.

In this work, we tried to fabricate CH₃NH₃PbI₃ perovskite films which is commonly used as light absorber. The perovskite films were obtained from sputtered PbI₂ films. PbI₂ sputtering target was fabricated from PbI₂ powder using a pressing under 10 MPa on stainless plate. Then, PbI₂ films were deposited on glass substrates by RF sputtering using the PbI₂ target. The sputtering condition was Ar atmosphere, under 1 Pa at room temperature. Perovskite films were obtained by reacting sputtered the PbI₂ films and CH₃NH₃I gas in petri dish. The obtained perovskite films were characterized by X-ray diffraction (XRD) measurement and scanning electron microscopy (SEM) observation.

 Figure 1 shows XRD pattern of a perovskite film obtained from sputtered PbI₂ film. Sputter processed perovskite films showed typical diffraction patterns of CH₃NH₃PbI₃ perovskite films and had similar peaks to solution processed perovskite films.

Figure 2 shows a cross sectional SEM image of a PbI₂ film sputtered on fluorine-doped tin oxide (FTO) and TiO₂ coated glass substrate. This image clearly indicates that homogeneous PbI₂ and/or perovskite films could be fabricated by sputtering even if the substrates have large roughness. This result suggested sputtering is promising process to achieve high efficiency tandem solar cells.