Self-Doped Conducting Polymer as a Hole-Extraction Layer in Organic-Inorganic Hybrid Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells are fabricated using a water-soluble, self-doped conducting polyaniline graft copolymer based on poly(4-styrenesulfonate)-g-polyaniline (PSS-g-PANI) as an efficient hole-extraction layer (HEL) because of its advantages, including low-temperature solution processability, high transmittance, and a low energy barrier with perovskite photoactive layers. Compared with conventional poly(3,4-ethylenedioxythiop hene): poly(styrene sulfonate) (PEDOT: PSS) dispersed in water solution, PSS-g-PANI molecules are dissolved in water because of the polymeric dopant covalently bonded with PANI, and can steadily remain as an initial solution during long-term storage and over a wide pH range to fabricate a HEL with fewer surface defects. The built-in potential and device characteristics are substantially improved because of the surface energy state of PSS-g-PANI below Fermi-energy level. Moreover, the PSS-g-PANI mixed with electron-withdrawing perfluorinated ionomer (PFI) exhibits a higher work function (5.49 eV) and deeper surface energy state below the Fermi level; thus, an ohmic contact at the HEL/methylammonium lead iodide perovskite interface is obtained. Finally, the power conversion efficiency was increased from 7.8% in the perovskite solar cells with PEDOT: PSS to 12.4% in those with the PSS-g-PANI: PFI.