Perovskite Solar Cells with > 19% Efficiency Achieved by an Advanced Three-Step Method Using Additional HC(NH2)(2)I-Nal Spin-Coating

Recently, a doping of sodium ion (Na+) into the perovskite solar cells has attracted much attention. Supposing that the Na+ ions substitute the A-site cation of the perovskite structure, the Na+ doping is expected to enlarge a bandgap of perovskite layer. Additionally, the Na+ doping may increase the grain size and decrease the trap density. We recently reported a new 3-step method using additional spin-coating of a HC(NH2)(2)I (FAI) solution on a 2-step prepared CH3NH3PbI3 (MAPbI(3)) layer containing an unreacted PbI2, which produced a multiple bandgap structure by forming a FA-rich layer with a smaller bandgap near the electron transport layer. Here, we attempted to form the top layer with larger bandgap by adding a small amount of NaI into the FAI solution for the third step and investigated the effect of NaI post-treatment on the perovskite layer. The NaI addition slightly enlarged the bandgap of surface of the perovskite layer. Additionally, it accelerated the grain growth and effectively decreased grain boundaries, while the Na+ tended to increase nonradiative recombination. As a result, the NaI addition enhanced the power conversion efficiency from 18.12% (FAI only) to 19.07% (with NaI addition) without degrading stability in air. The appropriate amount of NaI addition on the FAI solution is beneficial to boost the photovoltaic performance.