The hole-transporting material (HTM) consisted of 0.315 M ofSpiro OMeTAD, 0.15 M of tert-butylpyridine and 32 mM of lithium
bis(trifluoromethylsyfonyl)imide salt. The HTM solution was
deposited over the PbeSbeS-coated TiO2 electrode and spun at a
rate of 2000 rpm for 30 s. A gold film of thickness ~100 nm was
evaporated on top of the HTM layer via sputtering to complete the
solar cell fabrication. Fig. 1(b) displays an SEM image of a completed
solar cell, which had the following structure: FTO/blocking layer
(30 nm)/mp-TiO2þPbeSbeS þ Spiro OMeTAD (1.34 mm)/Spiro
OMeTAD overlayer (210 nm)/Au electrode (100 nm), where the
thickness of each layer is indicated in brackets.