Effects of Substituents on Transport Properties of Molecular Materials for Organic Solar Cells: A Theoretical Investigation

We present a theoretical investigation of the effects of substitution of the triphenylamine (TPA) block on the overall properties of materials based on small push–pull molecules designed as donors for organic photovoltaics (OPV). In particular, we exploit modern computational techniques such as density functional theory (DFT), time-dependent DFT, molecular dynamics, and the Marcus theory to analyze the charge and exciton transport properties in crystalline and amorphous phases of four compounds in which one phenyl ring of the TPA block of 2-[(5-{4-[methyl(phenyl)amino]phenyl}thiophen-2-yl)methylene]malononitrile is replaced with a methyl, an α-naphthyl, and a β-naphthyl. Our calculations unveil the molecular rationale behind the different transport properties observed in the experiments. We show that, although the effects of the substituents on the electronic and optical properties are negligible, they have an impact on the molecular packing of the crystalline structure, thus explaining the different macroscopic transport properties (observed and calculated). In particular, the substitution of a phenyl with a methyl favors face-to-face π–π packing in the crystal structure and allows a good π-orbital overlap and high hopping rates. On the other hand, the introduction of an α-naphthyl group generates a steric hindrance that negatively affects the transport properties. Moreover, the investigated substitutions do not significantly influence the degree of local order in the amorphous bulks displaying complete disorder and low hole mobilities. These results, in agreement with the experimental findings, suggest that our computational approach is able to account for the macroscopic effect of subtle transformations of a molecular structure on transport properties and thus can be further employed to obtain valuable insights into the molecular design of optimized active materials for OPV.