Nevertheless, the pin-holes are appearing on perovskite film for
MAPbI3-0.4prefilm sample. As shown in the high magnification in
Fig. 2f, the TiO2 substrate can be seen clearly. The light-generated
electron and hole can recombine easily before collection, due to
the direct contact between TiO2 and spiro-OMeTAD [25]. It may
be the main factor for worsening perovskite solar cell performance
of MAPbI3-0.3prefilm and MAPbI3-0.4prefilm. Through controlling
the roughness, thickness, and coverage of MAPbI3 film, the best
photovoltaic device based on MAPbI3-0.2prefilm is obtained.
XRD analysis was conducted to verify the phase composition of
the x-prefim, PbI2-xprefilm and the final MAPbI3 layer on mesoporous
TiO2 films. As seen in Fig. 3a, MAPbI3 and PbI2 both exits
in the x-prefilm. MAPbI3 is polar tetragonal material at room temperature,
and the MAPbI3 films usually demonstrate the preferred
orientation of (1 1 0) peak at 14.82◦ [26]. And the PbI2 can be easily
certified via the (0 0 1) at 12.56◦ [27]. Furthermore, the peak
intensity of MAPbI3 is stronger with the solution concentration
increasing, and the 0.4-prefilm show the highest peak. It may be
ascribed to the amount or the crystallinity of MAPbI3 enhancement.
After the formation of x-prepared film, the PbI2 solution was
spin-coated on it, and MAPbI3 is resolved into PbI2 and MAI [28].
In Fig. 2b, there is no MAPbI3 peak any more, leaving the PbI2 peak
exists, and PbI2-0.2prefilm has the highest peak intensity. But it is
hard to detect the trace amount of MAI through XRD characterization.
In Fig. 2c, beside MAPbI3 (1 1 0) peak mentioned above,