In situ construction of gradient heterojunction using organic VO_x precursor for efficient and stable inverted perovskite solar cells

Inverted perovskite solar cells (PSCs) have attracted tremendous attention recently but the energy levels between the perovskite absorber and conventional hole transport layers (HTL) are mismatch, resulting in the lower open-circuit voltages (V_oc) than that of regular PSCs. Herein, a gradient heterojunction (GHJ) based on poly (3,4−ethylenedioxythiophene: polystyrenesulphonate) (PEDOT:PSS)/PEDOT:PSS-VO_x was constructed in situ by low-temperature annealing and used as HTL of the inverted PSCs. This GHJ structure fabricated conveniently by doping a small amount of triisopropoxyvanadium oxide isopropyl alcohol solution into the PEDOT:PSS solution during spin-coating can efficiently facilitate charge separation and improve charge extraction efficiency, leading to significantly improved PSC performance with V_oc up to 1.02 V and power conversion efficiency (PCE) to 18.0%. More impressively, owing to the more hydrophobic surface and lower acidity than the PEDOT:PSS layer after the formation of high work function VO_x mainly on the surface of HTL, the GHJ-based PSCs show excellent long-term stability, which retain over 80% or 70% of their initial PCEs after exposure to full spectrum illumination in N₂ for 750 h or in air for 175 h, respectively. These results illustrate the significant advantages of the in situ formed VO_x-modified HTLs in gradient structures using organic VO_x precursors, providing important clues in constructing GHJ for inverted PSCs with high efficiency and stability.