Computational design of dopant-free hole transporting materials: achieving an optimal balance between water stability and charge transport

Hole transporting materials (HTMs) play a crucial role in the performance and stability of perovskite solar cells (PSCs). The interaction of HTMs with water significantly affects the overall stability and efficiency of these devices. Hydrophilic HTMs or those lacking adequate water resistance can absorb moisture, leading to degradation of both the HTM and the perovskite layer. In this study, we employed a proof-of-principle approach to investigate the effect of various chemical modifications on a promising HTM candidate, 8,11-bis(4-(<i>N</i>,<i>N</i>-bis(4-methoxyphenyl)amino)-1-phenyl)-dithieno[1,2-<i>b</i>:4,3-<i>b</i>]phenazine (TQ4). Using molecular dynamics simulations, we examined the collective behavior of chemically modified TQ4 molecules in the presence of water at different concentrations. To ensure that enhanced water resistance did not compromise the desirable electronic properties of the HTM, we analyzed both the individual and collective electronic structures of the HTM molecule and its molecular crystal. Additionally, we calculated the charge transport rate in different directions within the HTM crystal using Marcus theory. Our findings indicate that chemical modifications at the periphery of TQ4, particularly the symmetric addition of two F-chains, result in the optimal combination of electronic, crystal structure, and water-resistant properties. HOMO shape analysis reveals that the HOMO does not extend onto the added F-chains, reducing the maximum predicted hole mobility relative to TQ4 by an order of magnitude. Despite this, a hole mobility of 2.8 × 10^-4 cm² V^-1 s^-1 is successfully achieved for all designed HTMs, reflecting a compromise between stability and charge transport. This atomistic insight into the collective behavior of chemically modified HTMs and its effect on hole transport pathways paves the way for designing more effective HTMs for PSC applications.