J–V curves for the perovskite solar cells fabricated with different thicknesses of β-TiOPc: (1) 20 nm, (2) 40 nm, (3) 60 nm, (4) spiro-OMeTAD (40 nm) without dopping;
(b) IPCE spectra of FTO/TiO2/CH3NH3PbI3/HTM/Au solar cells (HTMs (40 nm)): (1) β-TiOPc, (2) α-TiOPc, (3) spiro-OMeTAD.