Perovskite Solar Cells Prepared By Advanced Three-Step Method Using Additional Hc(Nh2)(2)I Spin-Coating: Efficiency Improvement With Multiple Bandgap Structure

In the conventional two-step prepared perovskite solar cells, the CH3NH3PbI3 (MAPbI(3)) film usually contains an unreacted PbI2 at the interface between an electron transport layer (ETL) and a perovskite active layer. To reduce the unreacted PbI2 in the two-step prepared MAPbI(3) film, we have recently reported a new three-step method, which was realized by an additional MA(I,Br) spin-coating. Here, we propose an advanced three-step method, viz., an additional HC(NH2)(2)I (FAI) spin-coating on the two-step prepared MAPbI(3) film. The additional FAI spin-coating formed a FA(x) MA(1-x)PbI(3) solid solution by the incorporation of FA ion into MAPbI(3). Also, the additional FAI spin-coating yielded a FA(y)MA(1-y)PbI(3) layer (y > x) by converting the unreacted PbI2, which resulted in the layered structure with different FA concentrations and hence, with the multiple bandgap structure. The best PCE of 18.1% was achieved by optimizing the FAI spin-coating process.