Chemical state of chlorine in perovskite solar cell and its effect on the photovoltaic performance

Inorganic–organic hybrid perovskite solar cells have become of interest due to their high solar-to-electric power conversion efficiency and low fabrication costs. Recently, it is reported that chlorine doping is an effective means of improving the conversion efficiency and stability of these cells. However, the role played by chlorine and the chemical state of chlorine in the MAPbI 3− x Cl x perovskite is still the subject of much debate. In the present work, we fabricate Cl-containing perovskite films by spin coating a mixed solution of CH 3 NH 3 Cl and CH 3 NH 3 I on PbI 2 films in air. The chemical state of Cl in these films is investigated by X-ray diffraction and X-ray photoelectron spectroscopy. It is found that the Cl is present in different chemical states depending on the concentration of CH 3 NH 3 Cl used in the original synthesis. XPS depth profile results demonstrate that Cl is present throughout the samples rather than solely at the perovskite/TiO 2 interface as has been reported previously. However, the Cl located at the interface is more stable during thermal treatment of the films. Photovoltaic characterization reveals that strong relationship between the chemical state of chlorine and photovoltaic performance exists.