P-Dopant with Spherical Anion for Stable n-i-p Perovskite Solar Cells

Li-TFSI/t-BP is the most widely utilized p-dopant for hole-transporting materials (HTMs) in state-of-the-art perovskite solar cells (PSCs). However, its nonuniformity of doping, along with the hygroscopicity and migration of dopants, results in the devices exhibiting limited stability and performance. This study reports on the utilization of a spherical anion derived from the p-dopant, regulated by its radius and shape, as an alternative to the linear TFSI^- anion. The theoretical and experimental results reveal that the spherical anion significantly increases the doping effect of HTMs due to an enhanced electron transfer from larger dipole moments. The enhanced transfer leads to a shift in the Pb-6p defect orbitals, resulting in shallower trap states. Moreover, compared to the linear structure of the TFSI^- anion, the anion of sodium tetrakis[3,5-bis(trifluoro methyl)phenyl]borate (Na-TFPB) with a larger van der Waals radius and spherical shape offers increased hydrophobicity and migration barriers, which can protect the perovskite crystal and facilitate stable p-doping of HTMs. The use of Na-TFPB results in enhanced thermal and ambient stability of PSCs. The devices fabricated with the shape- and radius-regulated p-dopant achieve remarkable efficiencies of 24.49 % and 24.31 % for CJ-01 and spiro-OMeTAD, respectively, representing the highest efficiency values for organic dopants to date. This study underscores the ingenious design of spherical anions of p-dopants in contrast to the conventional linear anions.