Surface modification of sputtered NiO_x hole transport layer for CH₃NH₃PbI₃ perovskite solar cells

The modification of the sputtered NiOx (x & GE; 1)/CH3NH3PbI3 interface by 2-(3,6-dimethoxy-9H-carbazol-9-yl) ethyl] phosphonic acid (MeO-2PACz) considerably enhances the power conversion efficiency of perovskite solar cells whose structure is ITO/NiOx/CH3NH3PbI3/[6,6]-phenyl C-61 butyric acid methyl ester (PCBM)/aluminum-doped zinc oxide (AZO)/Ag. In devices without MeO-2PACz, the internal quantum efficiency (IQE) above 450 nm increases with the increase in NiOx thickness from 4 to 53 nm, although even in the thickest case, the IQE never reaches 90%. On the other hand, devices with MeO-2PACz modified NiOx show thickness-insensitive IQE of ca. 90%. We propose that (1) MeO-2PACz effectively fills the pinholes in thinner NiOx and (2) it passivates the carrier trapping/recombination defects at the NiOx/perovskite interface.