In order to figure out the function of tiny PbI2 in perovskite
solar cell, an illustration on the correlation between the morphology
and the energy band gap diagram of the PbI2/perovskite film
is demonstrates in Fig. 3d. In the path 1, the bottom PbI2 block
the electron injection from perovskite to TiO2, as the conduction
band of PbI2 is located higher than that of the perovskite. But in
the path 2 and 3, the remnant PbI2 acts as a blocking layer between
the TiO2 semiconductor and the perovskite, reducing the probability
of back electron transfer (charge recombination). Synthetically
balance thedouble-edgedeffect of PbI2,itisnecessary to controlthe
trace amount PbI2 in order to improve perovskite solar cell device
performance [17].