Charge Transport and Rectification in Donor−Acceptor Dyads

Organic, conjugated donor−acceptor (D−A) systems are essential components of photovoltaic devices. Design and optimization of D−A systems is typically based on trial-and-error experimentation methods that would benefit from fundamental physical insights on structure−function relationships at the molecular level. Here, we implement a nonequilibrium Green’s function methodology at the density functional theory (DFT) level and examine charge-transport and rectification properties of a series of conjugated D−A systems. We investigate 42 molecular junctions formed by D− A dyads bridging model gold electrodes, showing clearly how transport properties are determined by chemical composition, symmetry of frontier orbitals, and molecular conformation. Key properties are compared to experimental data. Notably, an inverse correlation between conductance and rectification is found, with relatively large rectification ratios caused by the asymmetry of frontier orbitals near the Fermi level. We discuss design principles that should be valuable for the rational design of molecular D−A systems with appropriate transport properties.