D-pi-D molecular semiconductors for perovskite solar cells: the superior role of helical versus planar pi-linkers

Controlling the mode of molecular packing and the size of molecular aggregates is of fundamental importance for high-performance charge transport materials in next-generation optoelectronic devices. To clarify the peculiar role of helicene as a kernel block in the exploration of unconventional organic semiconductors, in this work thia[5]helicene (T5H) is doubly aminated with electron-donating dimethoxydiphenylamine to afford T5H-OMeDPA, which is systematically compared with its perylothiophene (PET) congener (PET-OMeDPA). On the basis of the quantum theory of atoms in molecules and energy decomposition analysis of single crystals, it is surprisingly found that while pi-pi stacking of planar PET is stronger than that of helical T5H, this desirable effect for the charge transport of organic semiconductors is completely lost for donor-pi-donor (D-pi-D) type PET-OMeDPA but is retained for T5H-OMeDPA to a large extent. Consequently, the T5H-OMeDPA single-crystal presents about 5 times higher theoretical hole-mobility than PET-OMeDPA. More critically, the solution-processed racemic glassy film of T5H-OMeDPA displays a 3 times higher hole-mobility in comparison with the PET-OMeDPA counterpart, due to a larger domain of molecular aggregates. With respect to PET-OMeDPA, there is a weaker electronic coupling of helical T5H-OMeDPA with perovskites, leading to reduced interfacial charge recombination. Due to reduced transport resistance and enhanced recombination resistance, perovskite solar cells with T5H-OMeDPA exhibit a power conversion efficiency of 21.1%, higher than 19.8% with PET-OMeDPA and 20.6% with the spiro-OMeTAD control.