Assessment of density-functional models for organic molecular semiconductors: The role of Hartree-Fock exchange in charge-transfer processes

The optimization of the parameters controlling the migration of holes and electrons in charge-transport processes has recently attracted much attention for the field of organic electronics. A large set of relevant results for molecular design has been largely assisted by computational modeling tools. However, the limit of applicability of approximate computational models, whose paradigm is often referred as density-functional theory (DFT), might soon be reached due to accuracy matters. The large dependence of these parameters on the chosen theoretical method is demonstrated here by systematic application of a large variety of DFT methods. They are found to lead to highly accurate results if and only if a delicate balance between exact-like and approximate DFT exchange effects is achieved; thus, predictions might strongly depend on the nature of the method being applied. This situation is solved by providing a new set of optimized exchange-correlation functionals as a good enough automated approximation for the study and design of new π-conjugated charge-carrier materials.