Nickel Oxide/Cobalt Phthalocyanine as a Hole Transport Bilayer for Efficient and Stable Inverted Perovskite Solar Cells

This study demonstrates that the use of bilayer films based on nickel oxide (NiO_x;) and cobalt phthalocyanine (CoPc) represents a promising hole transport layer (HTLs) for inverted perovskite solar cells (PSCs). NiO_x; films are fabricated using the spin-coating method from a sol-gel solution. Films (CoPc_evap) and nanowires (CoPc_nws) on the NiO_x; surface are produced by thermal sputtering and physical gradient-temperature vapor deposition. It is demonstrated that PSCs with a NiO_x; layer exhibit a power conversion efficiency (PCE) of only 18,1%. The incorporation of a CoPc_evap intermediate layer between NiO_x; and the perovskite increases the PCE to 19.1%. The highest PCE, reaching 20.7%, is achieved with a bilayer HTLs based on NiO_x;/CoPc_nws. Analysis of the PSC impedance spectra shows that the CoPc_nws intermediate layer reduces the HTLs resistance and increases the recombination resistance at the perovskite/HTLs interface, which extends the effective lifetime of charge carriers. The stability of NiO_x;-based PSCs is 48%, while PSCs with bilayer HTLs based on NiO_x;/CoPc_nws and NiO_x;/CoPc_evap exhibits higher stability of 71% and 90% over 600 hours. The results demonstrated that solar cells based on NiO_x;/CoPc inhibit the perovskite degradation process and reduce charge recombination, thereby improving the performance and stability of the inverted PSCs.