Isomeric Carbazole-Based Hole-Transporting Materials: Role of the Linkage Position on the Photovoltaic Performance of Perovskite Solar Cells

Two structural isomers of carbazole decorated with triarylamine have been designed and synthesized with a facile synthetic procedure. The impact of triarylamine substitution on the isomeric structural linkage of carbazole on the optical, thermal, electrochemical, and photovoltaic properties has been extensively studied by combining experimental and simulation methods. Car[2,3] showed a red shift in the absorption maximum compared to that of Car[1,3], indicating the linear conjugation along the 2,7-position of carbazole in the former. The high thermal decomposition temperature (>420 °C) of these compounds could be attributed to the rigid structure of the carbazole core. Perovskite solar cells fabricated with Car[2,3] as the hole transporting material (HTM) displayed the highest power conversion efficiency (PCE) of 19.23%. It can be attributed to the suitable energy alignment of the highest occupied molecular orbital (HOMO) of HTM with the adjacent perovskite valence band energy level, which results in efficient hole transport. Furthermore, the molecular dynamic simulation demonstrates that the triphenylamine substitution on the 2,3,6,7 positions of Car[2,3] results in a more planar molecular alignment on top of the perovskite surface, promoting an efficient hole extraction. Essentially, when Car[1,3] and Car[2,3] were applied in perovskite solar cells, they showed enhanced long-term stability by retaining >80% of their initial PCEs after 1000 h of continuous illumination.