Asymmetrically Substituted Phthalocyanines as Dopant-Free Hole Selective Layers for Reliability in Perovskite Solar Cells

Dopant-free metal phthalocyanines are viable alternatives to the classical 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) in perovskite solar cells (PSCs), due to their appealing optoelectrical properties and chemical stability. However, low carrier concentration, transportability, and narrow band gap limit their application. Here, we designed and investigated six innovative asymmetrically substituted metal phthalocyanines (MPcs, M = Zn or Cu), and established the correlation among the electronic structure, charge carrier transfer parameter, and core metal/substitutions in MPcs by transient absorption spectroscopy and electron paramagnetic resonance. We probed the charge transport properties of ZnPcs including their carrier lifetime, diffusion coefficient, and diffusion length by transient absorption spectroscopy. We noted that ZnPcAE presents a longer diffusion length (1.94 nm) than the control ZnPcTB4 (0.80 nm), which is advantageous for reducing charge recombination and gives a higher power conversion efficiency in the fabricated PSCs. Importantly, the devices with MPcs yielded improved stability under multistress conditions. Our work provides a molecular guideline for designing MPcs and their application as dopant-free hole-transporting materials for perovskite solar cell fabrication.