Figure 4. (a) Mesoscopic perovskite solar cell with mesoporous TiO 2
layer and (b) planar structure without a mesoporous TiO 2 layer. Thin
fi lm (TF) on fl uorine doped tin oxide (FTO) is an n-type semiconductor.
HTM stands for hole transporting material. In the mesoscopic structure,
electrons can be collected directly and/or via TiO 2 layer.