Molecular Electronic Study of Spiro-[cyclopenta[1,2-<i>b</i>:5,4-<i>b</i>′]dithiophene-4,9′-fluorene] Derivatives: Route to Decent Hole-Transporting Materials

Spiro-type molecules have been extensively explored as hole-transporting materials (HTMs) in perovskite solar cells (PSCs). However, a closer look at the rule of design of such molecules is missing, and a combined experimental and theoretical study is needed. In this work, three spiro-[cyclopenta[1,2-b:5,4-b′]dithiophene-4,9′-fluorene] (SDTF) derivatives decorated with triphenylamine have been designed and synthesized. The thermal and optoelectronic properties, single-crystal structures, and charge-transport properties have been extensively investigated to reveal the impact of the linkage position of triphenylamine groups by combining experimental and theoretical methods, and the results are consistent with each other. Moreover, transfer integrals and reorganization energies in these molecules are also calculated to predict hole mobility. Our comprehensive studies show that SDTF-2 with a suitable energy alignment, good hole injection/transportability, and high theoretical hole mobility can be employed for efficient perovskite solar cells, achieving a high power conversion efficiency of 19.3%. In addition, first-principle calculations reveal that the strong interactions between SDTF-2 and the perovskite surface can facilitate fast hole extraction. Our way of study has provided useful information as part of the Materials Genome Initiative. We hope our investigation will offer a reliable path to predict or find the potential of novel spiro-type HTMs to become the alternative choice of spiro-OMeTAD.