Localized incorporation of cesium ions to improve formamidinium lead iodide layers in perovskite solar cellsElectronic supplementary information (ESI) available. See DOI: 10.1039/c8ra04742a

For the perovskite solar cells with formamidinium lead iodide (FAPbI 3 ) as a light harvester, cesium ions (Cs + ) can be used to stabilize the perovskite crystal structure of FAPbI 3 . However, the incorporation of Cs + ions usually reduces the grain size and degrades the crystallization of FAPbI 3 layers, and this is harmful to the photovoltaic performance of solar cells. In this work, we incorporate Cs + ions into FAPbI 3 layers using the interfacial doping method, which is different from the mixed solution doping method in previous reports. Elemental analysis indicates that Cs + dopants cannot be detected at the outer surfaces of perovskite layers, and the majority of Cs + dopants should be localized in the vicinity of TiO 2 /perovskite interfaces, which is remarkably different from the distribution of Cs + dopants in the perovskite layers prepared using the mixed solution doping method. It is found that interfacial doping method can avoid the blue shift of the light absorption edge and can improve the crystallization of FAPbI 3 layers. For the optimized conditions, Cs x FA 1− x PbI 3 solar cells prepared using the interfacial doping method achieve a power conversion efficiency (PCE) of 17.1%, which is better than the PCE of Cs x FA 1− x PbI 3 devices prepared using the mixed solution doping method. An interfacial doping method leads to a localized profile of dopants at interfaces, which results in improved photovoltaic performance.