Interfacial and Molecular Engineering of a Helicene-Based Molecular Semiconductor for Stable and Efficient Perovskite Solar Cells

Due to the attractive optoelectronic properties and extensiveapplicationin solar cells, the design of new helicene-type molecular semiconductorsis of remarkable importance for development of high-efficiency hole-transportingmaterials (HTMs). In this work, three helicene HTMs are constructedon the basis of the molecular conformation of experimentally reportedT5H-OMeDPA by replacing the thia[5]helicene unit with a more curved & pi;-linker of dithia[6]helicene and considering the effects offluorine substitution. The electronic and optical properties, holetransport, and interfacial property are studied with quantum chemistrymethods, and the results show that new tailored HTMs perform muchbetter than T5H-OMeDPA in terms of the calculated HOMO levels, holemobility, optical adsorption, solubility and stability, interfacialstability, and hole transfer. The more stable HOMO levels of new HTMs(SM31-SM33) will be beneficial for the regulation of interfacialenergy alignment. Importantly, the hole mobilities of the designedHTMs are also distinctly improved due to the enhanced electronic couplingand lowered hole reorganization energies. The fluorine substitutioncan clearly heighten the orbital overlapping via compact moleculepacking. In the meantime, dithia[6]helicene can lower the reorganizationenergy although the more curved & pi;-electron system often disturbsthe intermolecular & pi;-& pi; stacking. Moreover, ourresults also reveal that additional Pb-S interactions can effectuallypromote the interface adsorption and hole extraction. In sum, ourstudy shows the validity of fluorine substitution and introduces helicenewith a more extended structure, and three potential helicene-typeHTMs are proposed.