Publication date: 1st July 2014
In order to analyse the crystal transformation from hexagonal PbI2 to CH3NH3PbI3 by sequential (two-steps) deposition process, the perovskite CH3NH3PbI3 layers were deposited on flat and/or porous TiO2 layers. Although the narrower pores using small nanoparticle prohibited the effective transformation, the porous-TiO2 matrix can help the crystal transformation of PbI2 to CH3NH3PbI3 by sequential two-steps deposition. The resulting PbI2 crystal in porous TiO2 electrodes didn’t deteriorate the photovoltaic effects. Moreover, it is confirmed that the porous TiO2 electrode had a function to prohibit the short circuit between working and counter electrodes in perovskite solar cells. Using flat-TiO2 substrate, the PbI2 precursor layer can’t be completely converted to CH3NH3PbI3 crystal by the two-steps method in spite of thin PbI2 layer (200 nm) (Fig. 1). On the other hand, the PbI2 precursor in porous TiO2 layer (d = 90 nm) with 1-μm thickness can be completely converted to CH3NH3PbI3 by the two-steps method (Fig. 2). Therefore, it was considered that the enhancement of crystal transformation from PbI2 to CH3NH3PbI3 in the porous TiO2 layer using the two-steps method was attributed to the interface between TiO2 and PbI2 in the porous TiO2 substrate.