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

摘要： 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 non-peripheral (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 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 non-peripheral 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%.