Enhanced Hole Mobility of p-Type Materials by Molecular Engineering for Efficient Perovskite Solar Cells

Star-shaped triazatruxene derivative hole-transporting materials (HTMs), namely, 3,8,13-tris(4-(8a,9a-dihydro-9H-carbazol-9-yl)phenyl)-5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (TAT-TY1) and 3,8,13-tris(4-(8a,9a-dihydro-9H-carbazol-9-yl)phenyl)-5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (TAT-TY2), containing electron-rich triazatruxene cores and donor carbazole moieties, were synthesized and successfully used in triple-cation perovskite solar cells. All the HTMs were obtained from relatively inexpensive precursor materials using well-known synthesis procedures and uncomplicated purification steps. All the HTMs, including the 5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (TAT-H) main core, had suitable highest occupied molecular orbitals (HOMOs) for perovskite (TAT-H: -5.15 eV, TAT-TY1: -5.17 eV, and TAT-TY2: -5.2 eV). Steady-state and time-resolved photoluminescence results revealed that hole transport from the valence band of the perovskite into the HOMO of the new triazatruxene derivatives was more efficient than with TAT-H. Furthermore, the substitution of n-hexylcarbazole and 9-phenylcarbazole in triazatruxene altered the crystalline nature of the main core, resulting in a smooth and pinhole-free thin-film morphology. As a result, the hole mobilities of TAT-TY1 and TAT-TY2 were measured to be one order of magnitude higher than that of TAT-H. Finally, TAT-TY1 and TAT-TY2 achieved power conversion efficiencies of up to 17.5 and 16.3%, respectively, compared to the reference Spiro-OMeTAD. These results demonstrate that the new star-shaped triazatruxene derivative HTMs can be synthesized without using complicated synthesis strategies by controlling the intrinsic morphology of the TAT-H main core.