Effect of perovskite overlayers on TiO₂ electrodes in perovskite-sensitized solar cells

In this paper, we have studied the effect of the thickness of a CH₃NH₃PbI₃ perovskite overlayer on mesoporous TiO₂ electrodes in perovskite solar cells. The overlayers were prepared by spin coating PbI₂ films on the electrodes, which were subsequently exposed to a CH₃NH₃I/2-propanol solution. We controlled the thickness of the perovskite overlayer by changing the PbI₂ solution concentration. The thicknesses of the overlayers spin-coated from 0.5, 0.75, 0.9, and 1 M PbI₂ solutions were approximately 179, 262, 316, and 341 nm, respectively. Perovskite solar cells with an approximately 316-nm-thick overlayer showed the highest efficiency of 9.11%. We conclude that optimization of the perovskite overlayer thickness in the solar cell structure is necessary to improve the cell efficiency.