Comparative Evaluation of Semiconjugated Cyclic Oligomer and Polymer of Octyltriphenylamine as Hole Transporting Material for Perovskite Solar Cell

While conventional linear semiconducting polymers generally possess terminal groups that lead to spatial and dynamical heterogeneity inappropriate in charge transporting process, it is considered that cyclic oligomers have several advantages as charge transporters due to the well-defined molecular structure without end groups and homogeneous electron distribution in frontier orbitals. Here, we prepared 4-octyltriphenylamine (OTPA)-based cyclic oligomeric mixture (m-COTPA) consisting of 5, 6, and 7-mers and a linear polymeric analogue (POTPA) via a one-pot reaction. Optical analyses revealed that POTPA exhibited slightly red-shifted absorption maximum, broader absorption profile and smaller Stokes shift compared with m-COTPA. Space-charge-limited current measurements demonstrated that m-COTPA exhibited superior hole mobility compared to POTPA. A perovskite solar cell utilizing m-COTPA as hole transporting layer achieved a power conversion efficiency of 10.60%, significantly outperforming POTPA-based device (7.67%). Cyclic oligomers are promising candidates for developing high-performance organic semiconducting materials even without time-consuming isolation into each oligomer.