Nitrogen substitution improves the mobility and stability of electron transport materials for inverted perovskite solar cells

A suitable electron transport material (ETM) plays key roles in efficient perovskite solar cells (PSCs), because it is beneficial for exciton dissociation and charge transport at the interface thus increasing the short circuit current density. Based on the experimentally reported efficient electron transport molecule 10,14-bis(5-(2-ethylhexyl)thiophen-2-yl)-dipyrido[3,2-a:2', 3'-c][1,2,5]thiadiazolo[3,4-i]phenazine (TDTP), we theoretically design a set of new ETMs (TDTP-1, TDTP-2a, TDTP-2b, TDTP-3a, and TDTP-3b) by introducing a nitrogen atom into the thiophene ring or replacing a hydrogen atom on the methyl with an amino group. Quantum-chemical calculations reveal that the designed molecules behave much better than TDTP in terms of electron mobility, air stability, and solubility, where the electron mobility of TDTP-3b is two orders of magnitude higher than that of TDTP owing to the extra S center dot center dot center dot N interactions in TDTP-3b that lead to the quasi two-dimensional p packing motif which facilitates electron transport evidently. Moreover, we find that the substitution effect of the nitrogen atom strongly depends on the position, where the nitrogen atom at the beta-position of the thiophene ring (TDTP-2b and TDTP-3b) is more conducive to electron transport. Importantly, our calculations show that the ETM/ perovskite interface interaction is enhanced after the introduction of the nitrogen atom and amino group thanks to the added N center dot center dot center dot Pb interaction, which favors electron transport with the newly designed ETMs. Our results not only report a set of novel promising ETMs, but also provide a useful design strategy for efficient ETMs.