Molecular Design Strategy in Developing Titanyl Phthalocyanines as Dopant-Free Hole-Transporting Materials for Perovskite Solar Cells: Peripheral or Nonperipheral Substituents?

We demonstrate a molecular design strategy to enhance the efficiency of phthalocyanine (Pc)-based hole-transporting materials (HTMs) in perovskite solar cells (PSCs). Herein, two titanyl phthalocyanine (TiOPc) derivatives are designed and applied as dopant-free HTMs in planar n-i-p-structured PSCs. The newly developed TiOPc compounds possess eight n-hexylthio groups attached to either peripheral (P-SC6-TiOPc) or nonperipheral (NP-SC6-TiOPc) positions of the Pc ring. Utilizing these dopant-free HTMs in PSCs with a mixed cation perovskite as the light-absorbing material and tin oxide (SnO2) as the electron-transporting material (ETM) results in a considerably enhanced efficiency for NP-SC6-TiOPc-based devices compared to PSCs using P-SC6-TiOPc. Hence, all of the photovoltaic parameters, including power conversion efficiency (PCE), fill factor, open-circuit voltage, and short-circuit current density, are remarkably improved from 5.33 ± 1.01%, 33.34 ± 3.45%, 0.92 ± 0.18 V, and 17.33 ± 2.08 mA cm-2 to 15.83 ± 0.44%, 69.03 ± 1.59%, 1.05 ± 0.01 V, and 21.80 ± 0.36 mA cm-2, respectively, when using the nonperipheral-substituted TiOPc derivative as the HTM in a PSC. Experimental and computational analysis suggests more compact molecular packing for NP-SC6-TiOPc than P-SC6-TiOPc in the solid state due to stronger π-π interactions, leading to thin films with better quality and higher performance in hole extraction and transportation. PSCs with NP-SC6-TiOPc also offer much higher long-term stability than P-SC6-TiOPc-based devices under ambient conditions with a relative humidity of 75%.